Colorants and Auxiliaries Vol 1

iii

Colorants and auxiliariesORGANIC CHEMISTRY AND APPLICATION PROPERTIES

Second Edition

Volume 1 – Colorants

2002

Society of Dyers and Colourists

Edited by John Shore

Formerly of BTTG/Shirley and ICI Dyes (now DyStar), Manchester, UK

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iv

ISBN 0 901956 77 5

Copyright © 2002 Society of Dyers and Colourists. All rights reserved. No part of this publicationmay be reproduced, stored in a retrieval system or transmitted in any form or by any means withoutthe prior permission of the copyright owners.

Published by the Society of Dyers and Colourists, PO Box 244, Perkin House, 82 Grattan Road,Bradford, West Yorkshire BD1 2JB, England, on behalf of the Dyers’ Company PublicationsTrust.

This book was produced under the auspices of the Dyers’ Company Publications Trust. The Trustwas instituted by the Worshipful Company of Dyers of the City of London in 1971 to encouragethe publication of textbooks and other aids to learning in the science and technology of colour andcoloration and related fields. The Society of Dyers and Colourists acts as trustee to the fund.

Typeset by the Society of Dyers and Colourists and printed by Hobbs The Printers, Hampshire, UK.

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Contributors

John ShoreFormerly of BTTG/Shirley and ICI Dyes (now DyStar), Manchester, UK

David PattersonFormerly senior lecturer, Department of Colour Chemistry and Dyeing, University of Leeds, UK

Geoff HallasFormerly senior lecturer, Department of Colour Chemistry and Dyeing, University of Leeds, UK

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Contents

Preface ix

CHAPTER 1 Classification and general properties of colorants 1

1.1 Introduction 11.2 Development of colorant classification systems 21.3 Colour Index classification 41.4 Chemical classes of colorants 51.5 Colour and chemical structure 141.6 Application ranges of dyes and pigments 181.7 Colorants and the environment 33

References 42

CHAPTER 2 Organic and inorganic pigments; solvent dyes 45

2.1 Pigments 452.2 Dyes converted into pigments 482.3 Azo pigments 532.4 Phthalocyanine pigments 672.5 Quinacridone pigments 712.6 Isoindolinone pigments 732.7 Dioxazine pigments 732.8 Diketopyrrolopyrrole pigments 732.9 Fluorescent pigments 742.10 Inorganic pigments 752.11 How pigments act as colorants 822.12 Solvent dyes 862.13 Conclusion 86

References 87Bibliography 88

CHAPTER 3 Dye structure and application properties 89

3.1 Dye characteristics and chemical structure 893.2 Dyeability of fibres in relation to dye structure 1163.3 Application properties and chemical structure 134

References 176

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CHAPTER 4 Chemistry of azo colorants 180

4.1 Introduction 1804.2 Mechanism of diazotisation and coupling 1804.3 Diazo components and diazotisation methods 1824.4 Preparation and use of coupling components 1864.5 Structure of azo dyes 1934.6 Preparation and importance of naphthalene intermediates 1964.7 Schematic representation of coupling 2044.8 Sulphonated azo dyes 2044.9 Unsulponated monoazo dyes 2114.10 Basic azo dyes 2184.11 Azoic diazo and coupling components 2204.12 Stabilised diazonium salts and azoic compositions 2234.13 Azo pigments produced by final coupling 2254.14 Implications of new technology in diazotisation and coupling 227

References 228

CHAPTER 5 Chemistry and properties of metal-complex and mordant dyes 231

5.1 Introduction 2315.2 Fundamental concepts 2335.3 Electronic structure of transition-metal ions 2355.4 Structural characteristics necessary for complex formation 2405.5 Preparation of metal-complex colorants 2485.6 Isomerism in metal-complex dyes 2605.7 Stability of metal-complex dyes 2615.8 Chromium-related problems in the mordant dyeing of wool 268

References 277

CHAPTER 6 Chemistry of anthraquinonoid, polycyclic and miscellaneouscolorants 280

6.1 Anthraquinone acid, disperse, basic and reactive dyes 2806.2 Polycyclic vat dyes 2946.3 Indigoid and thioindigoid dyes 3166.4 Sulphur and thiazole dyes 3216.5 Diarylmethane and triarylmethane dyes 3276.6 Miscellaneous colorants 344

References 353

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CHAPTER 7 Chemistry of reactive dyes 356

7.1 Introduction 3567.2 Reactive systems 3587.3 Monofunctional systems 3617.4 Bifunctional systems 3857.5 Chromogens in reactive dyes 4007.6 Stability of dye–fibre bonds 4107.7 Reactive dyes on wool 4157.8 Reactive dyes on silk 4207.9 Reactive dyes on nylon 4247.10 Novel reactive dyeing processes 426

References 440

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ix

Preface to Volume 1

This Second Edition of a textbook first published in 1990 forms part of a series on colourand coloration technology initiated by the Textbooks Committee of the Society of Dyers andColourists under the aegis of the Dyers’ Company Publications Trust ManagementCommittee, which administers the trust fund generously provided by the WorshipfulCompany of Dyers.

The initial objective of this series of books has been to establish a coherent body ofexplanatory information on the principles and application technology of relevance forstudents preparing to take the Associateship examinations of the Society. This particularbook has been directed specifically to the subject areas covered by Section A of Paper B: theorganic chemistry and application of dyes and pigments and of the auxiliaries used withthem in textile coloration processes. However, many qualified chemists and colouristsinterested in the properties of colorants and their auxiliaries have found the First Editionuseful as a work of reference. For several reasons it has been convenient to divide thematerial into two separate volumes: 1. Colorants, 2. Auxiliaries. Although fluorescentbrighteners share some features in common with colorants, they have been treated asauxiliary products in this book.

This first volume of the book is concentrated on the chemical characteristics of dyes andpigments, with emphasis on attempts to interpret their colouring and fastness properties interms of the essential structural features of colorant molecules. This Second Edition hasbeen extensively updated and greater attention has also been given to factors associatedwith the potential impact of colorants and their metabolites on the environment. Allchapters have been affected by these changes, but the concluding chapter on reactive dyescontains more new material than the others. Rationalisation of the global dyemakingindustry during the 1990s means that many of the traditional commercial names of dyes andpigments have disappeared. For this reason Part 2 of the Colorants Index has beeneliminated and colorants have been specified almost always by their CI Generic Names. Thefundamental value of the unique Colour Index International to colorant makers and users isrecognised worldwide.

Chapters 4 and 7 in the First Edition were written by Vivian Stead and Chapter 5 byFrank Jones. Sadly, Frank died in 1989 and Vivian in 1996, but my co-authors and myselfwould like to record our tribute for the major contributions to this volume by our formerfriends and colleagues. We have tried to preserve their original style intact during thenecessary updating process. Our grateful thanks are due to John Holmes and CatherineWhitehouse for their patient copy editing and to the publications staff of the Society,especially Carol Davies, who have prepared all the material in this new edition forpublication.

JOHN SHORE

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Chapters in Volume 2

Chapter 8 Functions and properties of dyeing and printing auxiliaries

Chapter 9 The chemistry and properties of surfactants

Chapter 10 Classification of dyeing and printing auxiliaries by function

Chapter 11 Fluorescent brightening agents

Chapter 12 Auxiliaries associated with main dye classes

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1

1

CLASSIFICATION AND GENERAL PROPERTIES OF COLORANTS

CHAPTER 1

Classification and general properties ofcolorants

John Shore

1.1 INTRODUCTION

It is important to distinguish clearly between dyes, pigments and colorants. Such terms aresometimes incorrectly used in various major scientific languages, as though they weresynonymous [1]. All dyes and pigments are colorants: when present on a substrate theyselectively modify the reflection or transmission of incident light. During application to asubstrate, a dye either dissolves or passes through a state in which its crystal structure isdestroyed. It is retained in the substrate by adsorption, solvation, or by ionic, coordinate orcovalent bonding. A pigment, on the other hand, is insoluble in and unaffected by thesubstrate in which it is incorporated. These inherent characteristics mean that dyes andpigments have quite different toxicological and environmental profiles [1].

Synthetic dyes and pigments have been available to the colorant user since the mid-nineteenth century. The important naturally occurring substrates of pre-industrial societies(cotton, linen, silk, wool, leather, paper, wood) share certain similarities, since they are allessentially saccharidic or peptide polymers. They could thus be coloured using a relativelyshort range of dyes and pigments, also of natural origin. An early objective of plannedresearch on synthetic dyes, therefore, was to replace the leading natural extracts (alizarinand indigo) by their synthetic equivalents. Simultaneously with this diligent and ultimatelysuccessful effort, other chemists were discovering totally new chromogens unknown innature: azine, triarylmethane and others from arylamine oxidation, azo colorants from thediazo reaction, and eventually azo–metal complexes and phthalocyanines. Building onsuccess with indigo and anthraquinone derivatives, the systematic approach led on torelated but new chromogens with outstanding properties: vat dyes and novel pigments.

Linked to this research by a common interest in certain versatile intermediates and asimilar urge to extend the limited range of natural substrates, a new breed of organicchemist, the polymer specialist, was vigorously developing novel regenerated and syntheticfibres, plastomers, elastomers and resins. Most of these differed markedly in structure andproperties from natural polysaccharides or polypeptides. Particularly in the mid-twentiethcentury, urgent demands arose for special new colorants and application techniquesdesigned to colour these substrates. Disperse dyes for ester fibres, modified basic dyes foracrylic fibres and pigments for the mass coloration of fibres and plastics are typical examplesof the response of the colour chemist. Natural fibres also gained from this broad wave ofresearch: reactive dyes for cellulosic and protein fibres, and fluorescent brighteners forundyed textiles, paper and detergent formulations were discoveries stemming essentiallyfrom this exceptionally active period.

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2 CLASSIFICATION AND GENERAL PROPERTIES OF COLORANTS

In the closing decades of the twentieth century, the emergence of an unknown substratebecame a rare event. The rate of introduction of radically new colorants, auxiliary productsand processes fell markedly. An increasingly adverse balance arose between the escalatingcosts of the research effort and of much more stringent hazard testing, as against thediminishing value of marginal technical or economic improvements to existing ranges ofcolorants on standard substrates. Many of the pathways of colorant research have turnedaway from conventional outlets for dyes and pigments towards more esoteric applications[2–7]. Although colorants of these types are unlikely to match the traditional textile dyes interms of total sales value, their unit prices and profit margins can often be exceptionallyhigh.

Many specialised applications of colorants are related to the way in which they absorband emit light. The ability of a dye molecule to absorb depends critically on its orientationwith respect to the electrical vector of the incident light, i.e. the polarisation of absorption.In recent years this has become of practical significance in the field of liquid crystal displays[8]. Colorants exhibiting high absorption of infrared light have found many diverseapplications, ranging from solar energy traps to laser absorbers in electro-optical devices[9,10]. Dye lasers are based on dyes that fluoresce with high quantum efficiency. They mustshow good photostability and be marketed in a state of high purity, thus commanding a highunit price. Fluorescent dyes are also used in biochemical and medical analysis whereextremely low detection limits are required. Polymeric colorants have been developed aspotential food colourings [11], since chemicals of relative molecular mass greater than about20 000 cannot be absorbed into the gastro-intestinal tract. Such colorants should pose notoxicological problems as food additives.

The chemical or photochemical activity of dyes forms the basis of many of theirinnovative uses. Indicator systems and lactone colour formers exploit reversible colourchanges. Thermochromism is applied in novelty inks, temperature sensors and imagingtechnology. Photosensitising cyanine dyes are used to transfer absorbed light energy to silverhalides in photography. Certain dyes are effective sensitisers of free-radical reactions,thereby initiating the crosslinking or photodegradation of polymers on exposure to light.Photochromic colorants have been employed in light monitors, reversible sun screens,optical data recording and novelty surface coatings.

1.2 DEVELOPMENT OF COLORANT CLASSIFICATION SYSTEMS

A major objective of this chapter is to outline the principal system by which colorants areclassified, namely the widely accepted Colour Index classification. After tracing thedevelopments from which this system has evolved [12,13], the distribution of existing dyesand pigments among the various classes listed therein will be introduced. Each of theseclasses will be discussed in turn, illustrated by structural formulae.

The earliest comprehensive alphabetical listing [14] of synthetic products used in thecoloration industry was published in 1870. The beginnings of systematic classification basedon chemical structure, with subgrouping according to hue, were first seen in the 1880s. Atypical presentation of this period [15] listed about 100 ‘coal-tar dyes’ in hue order. It isinteresting that 50% of them were acid dyes and 20% basic dyes, about 40% being placed inthe ‘red’ category. Undoubtedly the most successful of these early systems were the famous‘Farbstofftabellen’ of Gustav Schultz, which ran through seven editions between 1888 and

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1932. The number of chemical entries rose from about 280 to nearly 1500 over these years.The later editions of this work pioneered many of the features eventually adopted in theColour Index.

The Society of Dyers and Colourists embarked on the First Edition of the Colour Index in1921 as a series of monthly issues that were first offered as a bound volume in 1924. Therewere over 1200 entries for synthetic colorants, as well as sections on natural dyes andinorganic pigments. Updating was discontinued in 1928, so that by 1945 the need for aSecond Edition had become urgent. Much detailed information on the products of Germanmanufacturers became available following the Second World War. Collaboration with theAmerican Association of Textile Chemists and Colorists resulted in the four-volume SecondEdition published in 1956–58. This contained about 3600 colorants differing in constitutionand an especially useful innovation was the separate listing of commercial names (31,500)under equivalent CI generic names (4600 entries).

This edition and the completely revised five-volume Third Edition (1971) established theColour Index as the leading reference work for the classification of colorants, fully justifyingthe cognomen International belatedly added in 1987. The fourth revision (1992) of theThird Edition consisted of nine volumes. The original data on technical properties (Volumes1–3) and chemical constitution (Volume 4) was supplemented (Volumes 6–9) at roughlyfive-year intervals.

The latest revision of the Colour Index has become an electronically searchable databaseavailable on CD-ROM as well as the traditional book form, providing improvedfunctionality and better value for money. Chemical constitutions, indexes of commercialnames and lists of manufacturers have been computerised for ease of reference and searchpurposes. The commercial listing function Volume 5 was detached in 1997 to form a newannual publication, the SDC Resource File. The aim of this novel concept was to providecolorant users with the latest comprehensive information on relevant products and services.This is provided by suppliers to the colour-using industries and coordinated by the SDCthrough its Colour Index organisation [13]. In 1998 a new edition covering pigments andsolvent dyes designed explicitly for the pigment industry was published [16], the technicaland scientific content of the material being upgraded [17].

This divergence is a response to certain problems that have arisen, particularly in relationto commercial product listings. As non-traditional suppliers based in low-cost countrieshave taken a greater share of world trade in colorants, the attitude of established Europeanand Japanese producers towards disclosure of information has changed. When suchcompanies have already expended substantial resources on research, development andhazard testing to launch a new product, they are understandably reluctant to surrendercommercially sensitive data into the public domain and thus give their competitors a headstart. Colorant users rely on the equivalence of CI generic names of commodity products asa basis of comparison between suppliers, but the long-established dyemakers are wary of thisequivalence for novel products because it offers low-cost competitors an easy entry intotraditional markets [13].

The Colour Index has become a standard reference for customs and importing authoritiesin many countries. Health and safety inspectorates have used CI designations in dealingwith colorant manufacturers notifying hazard testing data for their products. As with someother European Union initiatives, administration of legislation governing the notification ofcommercial chemicals for hazard control purposes has generated problems for suppliers,

DEVELOPMENT OF COLORANT CLASSIFICATION SYSTEMS

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users and enforcing authorities. Distinguishing between ‘existing’ (i.e. notified in 1981) and‘new’ products is not as easy as it sounds. Organic colorants present special difficultiesbecause of their complex structures and chemical nomenclature, variations of counter-ionswith the same ionic dye, obsolete or confusing identities of ‘existing’ products, and, notleast, the ubiquitous marketing of mixed colorants to match specific colours or technicalproperties [18].

1.3 COLOUR INDEX CLASSIFICATION

Most organic colorants in the Colour Index, including many of those not assigned a specificchemical constitution number, are placed in one of 25 structural classes according to theirchemical type (where this is known). The largest class, azo colorants, is subdivided into foursections depending on the number of azo groups in the molecule. Metal-complex azocolorants are designated separately by description but are classified together with theirunmetallised analogues in the same generic class. Excluding the colorant precursors, such asazoic components and oxidation bases, as well as the sulphur dyes of indeterminateconstitution, almost two-thirds of all the organic colorants listed in the Colour Index belongto this class, one-sixth of them being metal complexes. The next largest chemical class isthe anthraquinones (15% of the total), followed by triarylmethanes (3%) andphthalocyanines (2%). No other individual chemical class accounts for more than 1% of theColour Index entries.

The distribution of each chemical type between the major application groups of colorantsis far from uniform (Table 1.1). Stilbene and thiazole dyes are almost invariably direct dyes,also containing one or more azo groups. Acridines and methines are usually basic dyes,

Table 1.1 Percentage distribution of each chemical class between major application ranges

Distribution between application ranges (%)

Chemical class Acid Basic Direct Disperse Mordant Pigment Reactive Solvent Vat

Unmetallised azo 20 5 30 12 12 6 10 5Metal-complex azo 65 10 12 13Thiazole 5 95Stilbene 98 2Anthraquinone 15 2 25 3 4 6 9 36Indigoid 2 17 81Quinophthalone 30 20 40 10Aminoketone 11 40 8 3 8 30Phthalocyanine 14 4 8 4 9 43 15 3Formazan 70 30Methine 71 23 1 5Nitro, nitroso 31 2 48 2 5 12Triarylmethane 35 22 1 1 24 5 12Xanthene 33 16 9 2 2 38Acridine 92 4 4Azine 39 39 3 19Oxazine 22 17 2 40 9 10Thiazine 55 10 10 25

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whereas nitro, aminoketone and quinophthalone derivatives are often disperse dyes. Metal-complex azo and formazan types are mainly acid dyes but phthalocyanines are important forreactive dyes. Indigoids are predominantly vat dyes but anthraquinones retain considerablesignificance as acid, disperse or vat dyes. The data in this table are given in percentagesbecause the actual numbers of dyes recorded gradually increase as new products are added.They relate to all those dyes listed where the chemical class is known, including products nolonger in commercial use.

There are nineteen generic name groups (application ranges) listed in the Colour Index.CI Acid dyes constitute the largest range, with about 55% of them still in commercial use.Next come CI Direct dyes (40% still active) and CI Disperse dyes (60% active). CI Reactivedyes (75% active), CI Basic dyes, CI Solvent dyes and CI Pigments (all 60% active)continue to progress, but CI Vat dyes (45% active) and CI Mordant dyes (33% active) are indecline [19]. CI Sulphur dyes can be regarded as a distinct chemical class as well as anapplication range, although a few vat dyes are manufactured in a similar way. Essentially CIFood dyes and CI Leather dyes are selections from the larger ranges of textile dyes. Severalof these application types are represented in the CI Natural dyes and pigments category. Thegroupings according to generic name also include colorant precursors (CI AzoicComponents and CI Developers, CI Ingrain dyes, CI Oxidation Bases) and uncolouredproducts (CI Fluorescent Brighteners and CI Reducing Agents) associated with textilecoloration.

Chemists concerned with innovative applications for specialised colorants havehighlighted the need for an independent directory covering all such products and uses thatdo not fit easily into the above textile-oriented categories [5]. Quite often these researchprojects signal a need for a specific combination of relatively unorthodox properties, such asfluorescence, infrared absorption or photosensitivity combined with solubility in an unusualsolvent. Often this has involved a time-consuming search through the hardcopy ColourIndex and other more specialised catalogues. Now that a searchable CD-ROM version hasbecome available, together with the annual SDC Resource File in which to locate suitablesuppliers, it has become easier for less conventional demands on the database to beadequately met [13].

1.4 CHEMICAL CLASSES OF COLORANTS

A brief description of each class of colorant is given below, in order to show how theycontribute to the overall distribution outlined in Table 1.1. For further details the reader isreferred to the later Chapters 2 to 7. Many of the lesser-known chemical classes are morefully described in Chapter 6.

The order of discussion of the chemical classes included here differs somewhat from thatin the Colour Index. Thiazole dyes are dealt with immediately after the stilbene class becauseboth of these, like the polyazo types, contribute notably to the range of direct dyes. Theanthraquinone, indigoid, quinacridone, quinophthalone, benzodifuranone and aminoketoneclasses form another series with certain structural similarities and important applications invat or disperse dyes and pigments. Phthalocyanine and formazan are stable metal-complexchromogens. The remaining seven categories included are already less important and stilldeclining in commercial significance. The arylmethane, xanthene, acridine, azine, oxazineand thiazine chromogens share a limited degree of resemblance in structural terms.

CHEMICAL CLASSES OF COLORANTS

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1.4.1 Azo colorants

The presence of one or more azo (–N=N–) groups, usually associated with auxochromicgroups (–OH or –NH–), is the characteristic feature of this class. Hydroxyazo dyes exhibitbenzenoid–quinonoid tautomerism with the corresponding ketohydrazones [4,20,21]. Atleast half of all commercial azo colorants belong to the monoazo subclass, that is, they haveonly one azo group per molecule. This proportion is considerably higher among the metal-complex azo dyes. Direct dyes represent the only application range where monoazocompounds are relatively unimportant; disazo and trisazo dyes are preferred in order toconfer higher substantivity for cellulose. The numerous ways in which diazo and couplingcomponents can be used to assemble azo colorants for many purposes are discussed inChapter 4. Yellow azo chromogens are occasionally linked to blue anthraquinone orphthalocyanine structures in order to produce bright green colorants.

1.4.2 Thiazole dyes

The characteristic chromogen of this class is the thiazole ring itself, normally forming part ofa 2-phenylbenzothiazole grouping. Most are yellow direct dyes of the azophenylthiazole (1.1)type, but a minority are simple basic dyes with an alkylated thiazolinium group (1.2), such asThioflavine TCN (CI Basic Yellow 1) shown. The thiazole ring enhances substantivity forcellulose and thus has been incorporated into certain anthraquinonoid and sulphurised vatdyes. Several important blue basic dyes are 2-phenylazo derivatives of 6-alkoxybenzothiazolinium compounds (1.3). A typical red disperse dye for cellulose acetate inthis class is the 6-methoxybenzothiazole CI Disperse Red 58 (1.4).

N

S

N

1.1

N

N+

SCH3

N(CH3)2

CH3 X1.2

_

CI Basic Yellow 1

N+

SRO

N

R X

N NR2

1.3

_

N

S

N

NH3CO N(CH2CH2OH)2

1.4

CI Disperse Red 58

1.4.3 Stilbene dyes and fluorescent brighteners

Stilbene dyes are mixtures of indeterminate constitution resembling polyazo direct dyes intheir application properties. They result from the alkaline self-condensation of 4-nitrotoluene-2-sulphonic acid (1.5) or its initial condensation product (1.6; X = NO2),

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either alone or with various arylamines. The characteristic chromogens are azo- orazoxystilbene groupings (1.7). As with sulphur dyes, the CI generic names of stilbene dyesrefer not to specific chemical entities but to mixtures of related compounds with closelysimilar dyeing and fastness properties. Almost all of them are yellow to brown direct dyes forcellulosic fibres and leather.

O2N CH3

SO3H

1.5

X CH

SO3H

CH X

HO3S1.6

CH CH N N

(O)

1.7

Approximately 75% of fluorescent brighteners belong to the stilbene class. These arealmost invariably derived from 4,4′-diaminostilbene-2,2′-disulphonic acid (1.6; X = NH2),often condensed with cyanuric chloride to take advantage of the further contribution of thes-triazine rings to substantivity for cellulose.

1.4.4 Anthraquinone colorants

Strictly speaking, the characteristic chromogen of these should be anthraquinone (1.8) itself,but the term, ‘anthraquinonoid’, is frequently extended, in the Colour Index as elsewhere, toinclude other polycyclic quinone structures. These are often synthesised fromanthraquinone derivatives and most of them, including dibenzopyrenequinone (CI VatYellow 4), pyranthrone (CI Vat Orange 9), isoviolanthrone (CI Vat Violet 10) andviolanthrone (CI Vat Blue 20), are strongly coloured even in the absence of auxochromes.Indanthrone (1.9; CI Vat Blue 4), the first polycyclic vat dye to be discovered, resulted froman unsuccessful attempt to link two anthraquinone nuclei via an indigoid chromogen.Polycyclic pigments are dealt with in Chapter 2 and the many derivatives of anthraquinoneapplicable as acid, basic, disperse, mordant, reactive and vat dyes are discussed in Chapter 6.

O

O1.8

Anthraquinone

O

O

HN

NH

O

O

1.9

Indanthrone


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1.4.5 Indigoid colorants

This substantial class of vat dyes and pigments has declined markedly in importance relativeto the anthraquinone derivatives. The still pre-eminent representative is indigo (1.10; CIVat Blue 1), the dye most consistently in demand of all time. Originally obtained fromnatural sources, indigo was probably the decisive impetus for the early development of thesynthetic dye industry [22]. In indigo and thioindigo (1.11; CI Vat Red 41) the chromogenicsystem is symmetrical. These dyes can exist in both cis and trans forms; the latter is the morestable form that predominates in the solid state. Unsymmetrical indigoid dyes are alsoknown, in which the two halves of the molecule united by the central C=C bond differ insubstitution pattern, heteroatom or orientation of the hetero ring, as in the monobrominatedindolethianaphthene analogue (1.12).

NH

OHN

O1.10

Indigo

S

S

O

O1.11

Thioindigo

O

S

O

NH

Br

1.12

1.4.6 Quinacridone pigments

This chromogen (1.13) is somewhat reminiscent of the indigoid and anthraquinone typesbut it has not yielded useful vat dyes. Bluish red pigments of the quinacridone class areespecially important in violet and magenta colours or for deep reds in admixture withinorganic cadmium scarlets.

N

N

O

OH

H

1.13Quinacridone

1.4.7 Benzodifuranone dyes

The discovery in 1979 of the benzodifuranone chromogen (1.14) and its exploitation in reddisperse dyes for polyester fibres [23,24] emerged from ICI research towards newchromogens of high colour value, brightness and substantivity to overcome the relativeweakness of anthraquinones and dullness of monoazo alternatives in the red disperse dyearea. A striking improvement in build-up properties was found by introducing asymmetry

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into the dye molecule, especially where the substituents R are different. Benzodifuranonederivatives are unlikely to yield useful water-soluble dyes for cotton or wool, since thelactone rings in the chromogen are readily hydrolysed. In fact this property is utilised toadvantage when these disperse dyes are applied to polyester, the dyeings produced beingreadily cleared with alkali [25].

O

O

R

O

O

R1.14

1.4.8 Quinophthalone dyes

The name of this structural class (‘quinoline’) in the Colour Index is not ideal becausequinoline derivatives feature in other related classes, such as the methine basic dyes with aquinolinium cationic group. The class is more precisely associated with quinophthalone(1.15), the characteristic chromogen derived by condensation of quinoline derivatives withphthalic anhydride. This small class of yellow compounds contributes to the disperse, acid,basic and solvent ranges of dyes.

NH

O

O

1.15

Quinophthalone

1.4.9 Aminoketone and hydroxyketone dyes

This small group of hydroxyquinone (1.16), arylaminoquinone (1.17) and aminophthalimide(1.18) derivatives has contributed a few members of some application ranges, mainly yellowdisperse dyes and reddish brown vat dyes, but there are superior alternatives available fromthe major chemical classes.

HO C

OHHO

O

CH3

1.16

NH

NH

O

O

1.17

N

O

O

1.18


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1.4.10 Phthalocyanine colorants

Substituted derivatives of metal-free phthalocyanine (CI Pigment Blue 16) and a series ofmetal complexes, notably copper phthalocyanine (1.19; M = Cu; CI Pigment Blue 15),contribute brilliant blue and green colours to several application ranges [26]. Pigments andreactive dyes are especially dependent on this chemical class, but examples exist in all theimportant ranges except disperse dyes. These colorants are discussed in more detail insections 2.4, 5.4.3 and 7.5.10. More recently, owing to their complex molecular structureand high electron-transfer ability, phthalocyanine derivatives are being used increasingly innon-coloration applications such as catalysis, optical recording, photoconductive materials,photodynamic therapy and chemical sensors [27].

N

N

NN

N

N

NN

M

1.19

1.4.11 Formazan dyes

This small class of blue copper-complex dyes has made a significant contribution to the acidand reactive ranges in recent years (sections 5.4.2, 5.4.3 and 7.5.8). The essentialchromogen is the bicyclic 1:1 chelated grouping illustrated (1.20). Trivalent metals such aschromium, nickel or cobalt will give tetracyclic 1:2 complexes with a central metal atom,analogous to conventional 1:2 metal-complex azo dyes.

N

O

N N

N

Cu

1.20

1.4.12 Cyanine colorants

Dyes in this category, classified as methines (1.21) and polymethines (1.22) in the ColourIndex, characteristically contain a conjugated system through one or more methine (–CH=)groups terminating with heterocyclic atoms, usually nitrogen as in the quinoline (1.21) and

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trimethylindoline (1.22) types [4]. The most important function of cyanine colorants is assensitisers in photography. One or more methine groups may be replaced by nitrogen atoms,as in the azacyanines. Many methine compounds intended for textiles are yellow or red basicdyes, but uncharged yellow methine disperse dyes (1.23) and azomethine chromium-complex solvent-soluble dyes (1.24) are also significant.

N+

CH N R

R X–

1.21

N+

CH3 X–

H3C CH3

CH

N

H3C CH3

CH3

1.22

CH CH

CH C R

CN

R2N

1.23

CH

O

N

OCr

1.24

1.4.13 Nitro and nitroso colorants

Nitro dyes exhibit benzenoid-quinonoid tautomerism (1.25) and their colour is attributedmainly to the o-quinonoid form, since this can be stabilised by hydrogen bonding. Thetautomeric o-nitrosonaphthols (1.26) readily form chelate complexes with metals. A fewyellow nitro disperse dyes, including CI Disperse Yellow 1 (1.25), and brown acid dyesremain of significance. The remaining nitro and nitroso colorants, such as (1.26) and its 1:3iron (II) complex (1.27), are no longer of commercial interest.

NO2N

N

H

OH

N

N O

H

OH

O2N

O O

O

1.25CI Disperse Yellow 1

NaO3SNaO3S

1.26

O

NO

H NO

H

O


chpt1(1).pmd 15/11/02, 14:4111


1.4.14 Diphenylmethane and triarylmethane colorants

Although treated as separate classes in the Colour Index, these structural types are closelyrelated and the few diphenylmethane dyes such as auramine (1.28; CI Basic Yellow 2) arenow of little practical interest. Commercial usage of the triarylmethane dyes and pigmentshas also declined considerably in favour of the major chemical classes. They were formerlynoteworthy contributors to the acid, basic, mordant and solvent ranges, primarily in theviolet, blue and green sectors. Numerous structural examples are recorded in the ColourIndex. The terminal groupings can be amine/quinonimine, as in auramine and crystal violet(1.29; CI Basic Violet 3), hydroxy/quinone, or both. The aryl nuclei are not alwaysbenzenoid (section 6.5).

1.27

N

NaO3S

O

O

Fe

SO3Na

N

O

O

SO3Na

NO

O

–

Na+

CI Acid Green 1

C

NH2 X

N(CH3)2(CH3)2N

+ 1.28_

Auramine

C

N(CH3)2 X(CH3)2N

N(CH3)2

+

1.29

_

Crystal violet

1.4.15 Xanthene colorants

Structurally related to the triarylmethanes is the xanthene chromogen (1.30), in which twoof the aryl nuclei are linked by an oxygen atom to form a pyrone ring. Similar terminalgroupings (amino, hydroxy, or both) are usually present. Xanthene dyes have mainly

chpt1(1).pmd 15/11/02, 14:4112

13

contributed red members to the acid, basic, mordant and solvent ranges, but there has beenslow decline in their commercial importance.

O NR2 X

1.30

+ _

1.4.16 Acridine dyes

Acridine derivatives, such as acriflavine (1.31), can be regarded as relatives of thediphenylmethane class in which the two benzene nuclei are linked by nitrogen to form apyridine ring. Their insignificance nowadays resembles that of their relatives, but they wereformerly useful mainly as orange or yellow basic dyes [28].

N NH2 X

CH3

H2N

1.31

+ _

Acriflavine

1.4.17 Azine, oxazine and thiazine colorants

These three classes are treated separately in the Colour Index but it is useful to comparethem in view of their structural similarity. Their chromogenic groupings differ only in thebridging link of the central pyrazine (1.32), oxazine (1.33) or thiazine (1.34) ring. Azine dyesfrom a wide variety of structural subclasses (quinoxalines, eurhodines, safranines,aposafranines, indulines, nigrosines) are illustrated in the Colour Index. Their commercialimportance, mainly as red to blue basic dyes, blue acid dyes and blue or black solvent-solubledyes, has declined markedly. Thiazine dyes, such as methylene blue (1.34; CI Basic Blue 9),were never of much real significance. Only the oxazine class has retained its standing, notonly for long established products such as CI Basic Blue 3 (1.33), but also as bright bluemembers of the direct and reactive dye ranges containing the triphenodioxazinechromogenic system (1.35) [29]. Bluish violet pigments of exceptionally high tinctorialstrength have also been derived from this chromogen.

N

NR2N NR2 X+

1.32

_

N

O N(CH2CH3)2 X(CH3CH2)2N

1.33

+ _

CI Basic Blue 3


chpt1(1).pmd 15/11/02, 14:4113


1.4.18 Indamine, indophenol and lactone dyes

These three chemical classes listed in the Colour Index are no longer of any practicalsignificance. They are mentioned here only for their resemblance to quinonoid and ketonedyes already discussed. The chromogens are quinonimine (1.36), with amino or hydroxyauxochromes respectively in the indamines and indophenols, and a lactone ring (1.37) witha hydroxy auxochrome.

N

S(CH3)2N N(CH3)2 X+

1.34

_

CI Basic Blue 9

N

O

O

N

1.35

Triphenodioxazine

N N

1.36

O C

O

1.37

1.5 COLOUR AND CHEMICAL STRUCTURE

Before moving on to a description of the application ranges of dyes and pigments, it isappropriate to trace briefly the developments in understanding of the relationship betweencolour and chemical constitution. This subject has been reviewed most thoroughlyelsewhere [30–33] and the intention here is only to outline the basic principles so that thereader can appreciate the need for such a variety of structural types of colorant. Therequirements of colour and application are often in conflict and this forms a major part ofthe subject matter in succeeding chapters.

The first general theory relating depth of colour to molecular structure was made by Witt(1876), who recognised that all dyes then known contained aryl rings bearing unsaturatedgroups, such as =C=O, –N=O or –N=N–, which he termed ‘chromophores’. Intensecolour could be developed from such a ‘chromogenic’ grouping by attaching weakly basicsubstituents such as –OH or –NH– groups, called ‘auxochromes’, to the aryl ring.Subsequent developments recognised the particularly strong chromophoric effect of chargedcentres, as in the triarylmethane dyes that lack conventional uncharged chromophores.Although no longer tenable, the quinonoid theory of Armstrong (1888) helped to explainthe intense colour of these and related basic dyes so prevalent at that time. The influence ofmultiple auxochromic substitution on colour was examined by Kauffmann (1904), whoshowed that the deepest (most bathochromic) colour was obtained with auxochromes in the2,5-positions relative to the chromophore, as in the dihydroxyazobenzene anions 1.38(orange) and 1.39 (blue).

Improved understanding of the interaction of visible and ultraviolet radiation withorganic structures aroused interest in the tautomeric capabilities of dye molecules such asbenzenoid-quinonoid tautomerism in azo (1.40), anthraquinone (1.41) and triarylmethane(1.42) systems. According to Watson (1913), if a dye could show a quinonoid structure in all

chpt1(1).pmd 15/11/02, 14:4114

15

of its possible tautomeric forms, then it would be deeply coloured, however small itsmolecular size. For example, the blue indamine (1.43) may be contrasted with the yellow4,4′-diaminoazobenzene (1.44), even though the latter has the greater degree ofconjugation. Watson and Meek (1915) suggested that the oscillation between tautomericforms corresponded to the reversal of double and single bonds along the conjugated chain ofthe molecule: the longer the chain, the slower the period of vibration. This accordedqualitatively with the relation between conjugation and absorption.

N

O

O N

1.38

_

_N

O

O

N

1.39

_

_

O

NN

O

N

H

N

H

1.40

NC C N

1.42

+

NHN NH2

1.43

NH2N N NH2

1.44

Mathematical elucidation of the principles of quantum mechanics in the 1920s, leadingto the concept of molecular orbitals and a much more precise understanding of the nature ofchemical bonding in the 1930s, provided the first opportunity for chemists to developqualitative predictive methods of relating colour to chemical constitution. The equationsinvolved are so complicated, however, that approximation methods had to be employed,particularly for calculating transition energies in organic molecules. Three distincttheoretical approaches were adopted: the valence bond, the free electron and the molecularorbital methods.

Valence bond theory was initially the most widely accepted approach, probably because itdepended on familiar concepts of mesomeric effects in conjugated systems. The theoryassumed that the true wave function for the mesomeric state of a molecule is a linear sum ofthose of the contributing canonical forms. The technique was never successful forquantitative calculation of the absorption spectra of dyes, however, because of thedifficulties encountered when introducing the numerous canonical structures necessary forcomputational precision.

COLOUR AND CHEMICAL STRUCTURE

O–

O– O

O

1.41

chpt1(1).pmd 15/11/02, 14:4115


The free electron (FEMO) theory had its origins in work on the conduction electrons ofmetals in the 1940s, when several workers independently recognised the close analogybetween these and the delocalised π-electrons of polyene dyes. The method was extended tomany other classes of dyes, notably by Kuhn in the 1950s, but it has not found generalacceptance for spectroscopic calculations, since it lacks adaptability by simple parameteradjustment.

The molecular orbital (MO) approach developed more slowly but was ultimately muchmore successful than the other approaches. Dewar (1950) was able to predict absorptionmaxima for various types of cyanine dyes in excellent agreement with experiment. A majoradvance was made in 1953 when a self-consistent molecular orbital method specificallytaking into account antisymmetrisation and electron repulsion effects was developed byPariser, Parr and Pople. The PPP-MO method established itself over the next two decades asthe most useful and versatile technique for colour prediction, especially when the micro-electronics revolution provided facilities for overcoming the complexity of the necessarycalculations.

Limitations of space preclude mention of more than a handful of recent examples wherethe PPP-MO method has been employed to predict the absorption properties of dyes fromvarious classes. Simulation of the change in molecular geometry on excitation by iterationwithin the framework of the PPP-MO procedure has been used to calculate the fluorescencemaxima of dyes with satisfactory accuracy to permit reliable predictions [34]. A techniquefor predicting absorption bandwidths has been devised, based on the linear relationshipbetween the fluorescence Stokes shift of a dye and the absorption half-bandwidth.Theoretical Stokes shifts were computed using PPP-MO parameters for the various types ofbands encountered in dye spectra. The predictive value of the method was tested on dyesfrom various chemical classes. The correlation between calculated and experimentalbandwidths was good enough to predict brightness as well as colour [35]. Modification ofthe parameters for protonated N atoms enabled the visible spectral shift attributable toformation of the azonium cation (1.45) from N,N-diethylaminoazobenzene to be predictedreliably. Using these new parameters, halochromism could also be quantified in this way formore complex bis-azonium ions with insulated chromogens [36].

N N(CH2CH3)2 N NH(CH2CH3)2N N

X

+

1.45

_

HX

A striking feature of disperse dye development in recent decades has been the steadygrowth in bathochromic azo blue dyes to replace the tinctorially weaker and more costlyanthraquinone blues. One approach is represented by heavily nuclei-substituted derivativesof N,N-disubstituted 4-aminoazobenzenes, in which electron donor groups (e.g. 2-acylamino-5-alkoxy) are introduced into the aniline coupler residue and acceptor groups(acetyl, cyano or nitro) into the 2,4,6-positions of the diazo component. A PPP-MO study ofthe mobility of substituent configurations in such systems demonstrated that coplanarity ofthe two aryl rings could only be maintained if at least one of the 2,6-substituents was cyano.Thus much commercial research effort was directed towards these more bathochromic o-cyano-substituted dyes.

chpt1(1).pmd 15/11/02, 14:4116

17

A more important trend, however, has been the development of intensely bathochromicmonoazo blues by replacing either one of the aryl rings in the aminoazobenzene chromogenby a heterocyclic ring, such as thiazole or thiophene. Study of a series of thiazolyl- andthienylazo dyes of this kind using the PPP-MO method and its CNDO/S refinement showedthat their highly bathochromic intensity is not attributable to the 3d atomic orbitals on thesulphur atom of the heterocyclic ring [37]. New parameters for S-containing heterocyclicsystems enabled good agreement between experimental and calculated spectral features tobe obtained for a series of monoazo dyes derived from N-phenylpyrrolidine [38]. Spectraldata for dyes prepared by coupling diazonium salts with 2-thioalkyl-4,6-diaminopyrimidineand with 3-cyano-1,4-dimethyl-6-hydroxypyrid-2-one indicated that the yellow to orangepyrimidine dyes exist in the azo tautomeric form (1.46), whereas the brighter yellowpyridone dyes exist as ketohydrazones (1.47). The spectral behaviour was predicted well byPPP-MO calculations, although steric effects intervened when a substituent in the diazocomponent was ortho to the azo group of a diaminopyrimidine dye [39].

N

N

S

H2N

N

H2N

NAr R

1.46

N

CH3

CNH3C

O

O

N

N

H

Ar

1.47

The peak wavelengths of some arylated diaminoanthrarufin disperse dyes calculated bythe PPP-MO route agreed well with the observed values. The 1-arylamino-3-aryl-5-aminoderivatives (1.48; X = Ar, Y = H) showed better dichroism and improved solubility in liquidcrystals than their 1-amino-5-arylamino analogues (1.48; X = H, Y = Ar). The tris-arylatedderivatives (1.48; X = Y = Ar) also had satisfactory dichroism and solubility. The presenceof 1-arylamino and 3-aryl substituents in these deeply coloured anthraquinone dyes resultedin favourable orientation in liquid crystals, producing the desired properties [40]. A range ofmetal-free phthalocyanine colorants and their metal complexes were prepared and theirspectral characteristics in solution were predicted by PPP-MO calculation. The influence ofthe central metal atom, substituents on the aryl rings and the type of solvent was examined.The nature of the electronic excitation processes was interpreted in terms of the calculatedchanges in π-electron charge density. Good agreement with experimental absorptionmaxima was obtained, except in the case of a tetranitro derivative [41].

HO

NH

HN

OH

Ar

O

OY

X

1.48

One of the most noteworthy consequences of the refinement of these quantitativetreatments of colour-structure relationships has been the stimulus it has given to the search

COLOUR AND CHEMICAL STRUCTURE

chpt1(1).pmd 15/11/02, 14:4117


for radically new chromogenic systems. The benzodifuranone chromogen (1.14) is aparticularly interesting example. One of the remarkable properties of this system is thesensitivity of the absorption band to the strength of the electron-donating groups R. Thusthe unsubstituted chromogen (1.14; R = H) absorbs at 460 nm, whereas the dimethoxyderivative (1.14; R = OCH3) absorbs at 540 nm [23].

Another novel chromogen of high bathochromic intensity is the dihydrobenzothiophene-1,1-dioxide nucleus with an arylmethine group in the 2-position and a dicyanovinyl residuein the 3-position (1.49). This provides brilliant blues with molar absorption coefficients inthe 70 000 region. Analogous dyes with carbonyl in place of the sulphonyl group are lessintense, raising speculation about other applications of the sulphone acceptor group as partof a heterocyclic ring system.

S

CH

NC CN

1.49OO

1.6 APPLICATION RANGES OF DYES AND PIGMENTS

As mentioned earlier, the nineteen application categories forming the respective series of CIgeneric names are not restricted to colorants. In the following discussion, however, thecolorant ranges are emphasised because this is where the major variations in chemical classare found. Thus it is necessary to depart from the alphabetical order of listing in the ColourIndex. The application ranges of most interest for dyeing cellulosic fibres (vat, sulphur,reactive and direct dyes) are discussed first, followed by those for synthetic fibres and wool(disperse, basic, acid and mordant dyes) and finally the non-textile colorants (leather dyes,food dyes, solvent dyes and pigments). The colorant precursors and uncoloured productslisted form more homogeneous groups that are best discussed where relevant in theappropriate chapters of Volumes 1 and 2. There is much variety of chemical structure amongfluorescent brighteners, but the characteristic classes represented are mostly different fromthose in colorants.

1.6.1 Vat dyes

A vat dye is a water-insoluble colorant containing two or more keto groups. It can thus bebrought into aqueous solution by a reduction process (vatting), which converts the vat dyeinto its alkali-soluble enolic (leuco) form. As the soluble sodium enolate the leuco vat dyehas substantivity for cellulose. The application of vat dyes to cellulosic fibres (virtually theonly fibre type on which their outstanding fastness properties can be exploited) thusproceeds in four stages:(a) reduction and dissolution,(b) absorption by cellulose,(c) reoxidation and(d) association of the vat dye molecules within the fibre (Scheme 1.1).

chpt1(1).pmd 15/11/02, 14:4118

19

Sodium dithionite is the traditional reducing agent used for vat dyeing but concern aboutenvironmental problems has aroused interest in less damaging alternatives, such as thioureadioxide and the biodegradable and sulphur-free hydroxyacetone (section 12.9.1). Recycling isworthy of consideration for vat dyebaths because the redox process is reversible and only thedye absorbed by the cellulosic fibres is oxidised at a later stage of processing [42]. One of thedyehouses in North Carolina, USA, has been successfully recycling indigo dyebaths for manyyears [43].

Vat dyes are used to colour both components in pale depths on polyester/cellulosic fibreblends [44] but coloration of the polyester component in this case is more closely analogousto disperse dyeing (section 1.6.5). Anthraquinone disperse dyes resemble those vat dyes thatare substituted anthraquinone derivatives and in both instances it is exclusively the virtuallywater-insoluble keto form that is absorbed by the polyester fibre.

Approximately 80% of all vat dyes belong to the anthraquinonoid class (see Table 1.2) andthis certainly includes the leading products in all sectors of the colour gamut. Indigo and itsderivatives as well as the sulphurised vat types contribute mainly to the navy blue sector,whereas thioindigoid and indolethianaphthene derivatives occupy the red to brown region. Inthis and succeeding tables, the figures again relate to all dyes of known chemical type listedand are given in percentage terms to permit easier comparison between the distributions indifferent hue sectors. The total number of dyes in each of the primary sectors (yellow, red orblue) is, of course, almost always larger than those in the non-primary sectors.

The vat dyes section of the Colour Index incorporates a subgroup called solubilised vatdyes. These are sodium salts of sulphuric acid esters of the parent leuco vat dyes, such as CISolubilised Vat Blue 6 (1.50). In contrast to the leuco compounds, the vat leuco estersdissolve readily in water at neutral pH. They have relatively low substantivity for celluloseand thus have been used mainly in continuous dyeing and printing. In the presence of anoxidant in mineral acid solution (sodium nitrite and sulphuric acid, for example) the leucoester is rapidly decomposed and the insoluble vat dye regenerated. Thus application of a vatleuco ester represents a simpler (but more costly and less versatile) alternative toconventional dyeing methods via the alkaline leuco compound.

O

O

O

O

O

Odyebath cellulose

(a) (b) (c)

O

O

O

On

(d)

Scheme 1.1

APPLICATION RANGES OF DYES AND PIGMENTS

chpt1(1).pmd 15/11/02, 14:4119


1.6.2 Sulphur dyes

These resemble the vat dyes in certain ways, although they are of indeterminate constitutionand usually mixtures of different chemical species (section 6.4). The characteristicdisulphide group (D-S-S-D in Scheme 1.2) is always present in the insoluble form of asulphur dye, which is brought into aqueous solution by reduction to the alkali-soluble(leuco) form (D-S–). The soluble sodium thiolate form of the leuco sulphur dye hassubstantivity for cellulose. Thus the application of sulphur dyes to cellulosic fibres is a three-stage process (Scheme 1.2) broadly similar to that already outlined for vat dyes.

Table 1.2 Percentage distribution of chemical classes in vat dye hue sectors

Distribution in hue sector (%)% of allvat

Chemical class Yellow Orange Red Violet Blue Green Brown Black dyes

Anthraquinonoid 94 92 65 67 62 90 83 90 79Indigoid 11 17 2 5Thioindigoid 4 15 14 4 4Indole-thianaphthene 2 14 5 2 10 4Aminoketone 3 5 5 9 3Sulphurised vat 3 11 5 3Miscellaneous 4 2 5 5 2

N

N

NaO3SO

NaO3SO

Cl

OSO3Na

OSO3Na

Cl

1.50

CI Solubilised Vat Blue 6

D S S D D S

dyebath cellulose

D S D S S D

(a) (b) (c)Scheme 1.2

In the Colour Index the sulphur dyes are classified into four subgroups. Conventionalinsoluble disulphide brands, taking the generic CI Sulphur dye designation, are converted tothe leuco compound by adding sodium sulphide to a boiling aqueous dispersion of the parent

chpt1(1).pmd 15/11/02, 14:4120

21

dye. CI Leuco Sulphur dyes are marketed as concentrated solutions (or dispersions) of thepre-reduced dye with a small excess of the reducing agent. They can be added directly to adyebath prepared with additional reducing agent and are particularly convenient forcontinuous dyeing. CI Solubilised Sulphur dyes are sodium salts of thiosulphato derivativesof the parent dyes. Like the CI Solubilised Vat dyes they are highly soluble in water but lowin substantivity for cellulose. Addition of alkali and reducing agent converts them to theleuco sulphur form. The Colour Index refers to a fourth subgroup, the CI Condense Sulphurdyes, but these are only of historical interest. They were water-soluble alkyl- orarylthiosulphato derivatives, mostly of phthalocyanine dyes. In the presence of an alkalinereducing agent (sodium sulphide or polysulphide) they underwent a polycendensationreaction to give an insoluble coloured polymer within the fibre, the chromogenic groupingsbeing linked together via disulphide bridges as in conventional sulphur dyeings.

Improvements in dye synthesis, formulation and application in recent years have led tomodern ranges of sulphur dyes with notable advantages: freedom from heavy metals, AOXor carcinogenic arylamines, high exhaustion and insolubility after reoxidation.Developments in application methods include exhaust dyeing under a nitrogen atmosphere,indirect electrochemical reduction and pad–steam dyeing with fixation in a flash ager [45].

Sulphur dye effluent from traditional dyeing systems contains sulphides and thiosulphatesas well as the residual unfixed dyes. The discharge of sulphide liquors to drain is notnormally permissible because of the toxicity of hydrogen sulphide vapour that would bereleased under acidic conditions. Corrosion of the sewerage system and damage to thetreatment works would also occur. Stringent standards are required for consent and sulphidewaste is normally treated by oxidation or precipitation separately from other effluent streams[46]. Sulphur-free biodegradable products such as glucose or hydroxyacetone have beenevaluated as alternative reducing agents for sulphur dyes but they are more costly and lessversatile than the traditional sulphide or hydrosulphide.


1.6.3 Reactive dyes

These dyes are capable of reacting chemically with a substrate under suitable applicationconditions to form a covalent dye-substrate bond.

The characteristic structural feature is thus the possession of one or more reactivegroupings of various kinds (section 7.2). Almost always these may be categorised as either:(a) an activated unsaturated vinyl group that reacts with cellulose by addition to the

double bond, or(b) an activated halogeno substituent (C1 or F) that undergoes nucleo-philic substitution

by a cellulosate anion (section 7.3.1).

A marked trend in recent years has been the growth in availability and variety of ranges ofdyes containing two or more reactive groups per molecule (section 7.4). Although morecostly to manufacture, such structures react with greater efficiency, so that less dye is lost byhydrolysis.

The introduction of such a specific means of ensuring dye fixation freed the colourchemist from many of the structural limitations implicit in the design of most other ranges ofdyes, so that virtually any chemical class may be employed as a chromogen [47]. In practice,however, the reactive dye chemist has relied heavily on the azo chromogen (see Table 1.3),

chpt1(1).pmd 15/11/02, 14:4121


usually unmetallised but sometimes as metal complexes in the duller hues, such as navy,brown and black. Anthraquinone, phthalocyanine, formazan and triphenodioxazinechromogens have made notable contributions in blue and turquoise hues but in all otherhue sectors the azo dyes account for more than 95% of reactive dye structures.

Several instances of respiratory sensitisation resulting from exposure to reactive dyes havebeen reported. If an individual becomes sensitised to reactive dyes, it is essential that anyfuture contact with these dyes or other respiratory allergens be avoided. Subsequentexposures may cause anaphylactic shock and can progress into convulsions, coma and death.As yet, there is no animal test that can be used to predict reliably the potential of a reactivedye to cause respiratory sensitisation [1].

Reactive groups in dye molecules that fail to react with the substrate are hydrolysedduring dyeing or discharge of the residual dyebath. Recycling is therefore not a viable optionin the case of reactive dyeings [42]. Bioelimination (the sorptive removal of dyes duringbiological treatment of effluent) is also ineffective for reactive dyes, which show littleadsorption in this way. This behaviour is independent of the degree of sulphonation or theease of hydrolysis of the reactive dye molecules. The use of cationic flocculants for removalof reactive dye hydrolysates is of considerable interest [48].

By far the most important application segment for reactive dyes is the dyeing and printingof cellulosic fibres. Their impact in wool dyeing has been less dramatic but the exceptionallyhigh wet fastness requirements of machine-washable knitwear, for example, do justify theuse of reactive dyes, in spite of their higher cost relative to conventional dyes for wool.Reactive dyes are suitable for dyeing glove leather, silk and similar materials whereresistance to water and cleaning aids is essential.

1.6.4 Direct dyes

These are defined as anionic dyes with substantivity for cellulosic fibres applied from anaqueous dyebath containing an electrolyte. The forces that operate between a direct dye andcellulose include hydrogen bonding, dipolar forces and non-specific hydrophobic interaction,depending on the chemical structure and polarity of the dye. Apparently multipleattachments are important, since linearity and coplanarity of molecular structure seem to bedesirable features (section 3.2.1). The sorption process is reversible and numerous attemptshave been made to minimise desorption by suitable aftertreatments (section 10.9.5). Thetwo most significant non-textile outlets for direct dyes are the batchwise dyeing of leatherand the continuous coloration of paper.

Table 1.3 Percentage distribution of chemical classes in reactive dye hue sectors

Distribution in hue sector (%)% of allreactive


Unmetallised azo 97 90 90 63 20 16 57 42 66Metal-complex azo 2 10 9 32 17 5 43 55 15Anthraquinone 5 34 37 3 10Phthalocyanine 27 42 8Miscellaneous 1 1 2 1

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23

In every hue sector, at least 70% of direct dyes are unmetallised azo structures (see Table1.4). In contrast to all other application ranges, the great majority of them are disazo orpolyazo types, the former predominating in the brighter yellow to blue sectors and the latterin the duller greens, browns, greys and blacks. The copper-complex direct dyes are mostlyduller violets, navies and blacks derived from the disezo and polyazo subclasses. Stilbene andthiazole dyes, the only non-azo contributors in the yellow to red sector, bear certainsimilarities to the disazo and polyazo structures (sections 1.4.1 to 1.4.3). The dioxazine andphthalocyanine chromogens are represented in bright blue direct dyes of high light fastness.

Table 1.4 Percentage distribution of chemical classes in direct dye hue sectors

Distribution in hue sector (%)% of alldirect


Monoazo 4 7 14 4 8 3 5Disazo 58 53 71 83 52 21 22 23 49Polyazo 8 16 10 3 31 64 67 68 33Copper-complex azo 3 9 12 4 1 8 5Stilbene 17 23 2 1 7 1 5Thiazole 13 1 1Dioxazine 1 3 1Phthalocyanine 2 2 1

As there is no chemical change of direct dyes during orthodox application, their exhaustdyebaths are eminently suitable for recycling. Membrane processes have been usedsuccessfully to remove direct dyes from dyehouse effluents. There are possible cost savingsassociated with reuse of the electrolyte, depending on the rejection properties of themembrane [42]. High adsorption of direct dyes occurs during biological treatment ofdyehouse waste liquors containing them. This effect is not dependent on the degree ofsulphonation of the direct dye molecules [48]. Oxidation with ozone or precipitation usingcationic flocculants are effective ways of eliminating direct dye residues.

1.6.5 Disperse dyes

These dyes have affinity for one or, usually, more types of hydrophobic fibre and they arenormally applied by exhaustion from fine aqueous dispersion. Although pure disperse dyeshave extremely low solubility in cold water, such dyes nevertheless do dissolve to a limitedextent in aqueous surfactant solutions at typical dyeing temperatures. The fibre is believedto sorb dye from this dilute aqueous solution phase, which is continuously replenished byrapid dissolution of particles from suspension. Alternatively, hydrophobic fibres can absorbdisperse dyes from the vapour phase. This mechanism is the basis of many continuousdyeing and printing methods of application of these dyes. The requirements and limitationsof disperse dyes on cellulose acetate, triacetate, polyester, nylon and other synthetic fibreswill be discussed more fully in Chapter 3. Similar products have been employed in thesurface coloration of certain thermoplastics, including cellulose acetate, poly(methylmethacrylate) and polystyrene.


chpt1(1).pmd 15/11/02, 14:4123


Yellow disperse dyes are known from a wide range of chemical classes (see Table 1.5) butazo dyes derived from heterocyclic coupling components are particularly important foreconomic reasons. The monoazo subclass dominates throughout the disperse dye range andthere are no metal-complex disperse dyes designed for the ester fibres, although they doexist as a subgroup of ‘acid dyes’ for wool. Anthraquinone derivatives traditionallydominated the bright red to bright green series but here again cost considerations havefavoured the introduction of new monoazo dyes, often with heterocyclic amines as diazocomponents, for the dyeing of polyester fibres [49]. Browns and blacks on polyester andacetate fibres have always been provided almost entirely by azo disperse dyes. The disperserange contains relatively few homogeneous green, brown or black dyes, however. Many ofthe commercial navy, green, brown and black products are in fact mixtures of two or morecomponents (section 3.2.3).

Disperse dyes from the monoazo and anthraquinone classes have been implicated in casesof contact dermatitis. Circumstances common to such cases appear to be heavy depths ofthese dyes on nylon rather than polyester and occurring in articles of clothing that are indirect contact with the skin, often in areas that are likely to become moistened byperspiration. Hosiery, socks, blouses and close-fitting athletic or fashion wear, such as velvetleggings, are representative of the types of garment where this problem has arisen [1].

There is no chemical change of disperse dyes during orthodox application, so theirexhaust dyebaths are suitable for recycling. Such dyes were successfully removed from aneffluent stream by a microfiltration membrane module on an industrial scale [50]. In thistrial the permeate was reused but not the dyes, although other work has demonstrated thatrecycling of disperse dyes is possible [51]. Moderate to high adsorption of disperse dyes takesplace during biological treatment of dyehouse effluent. Although dispersible but insoluble inwatercourses, disperse dyes released to the aquatic environment have shown no evidence ofbioaccumulation in fish [48].

1.6.6 Basic dyes

Controversy has arisen at times [52] regarding the apparent synonymity of the terms ‘basicdye’ and ‘cationic dye’. The Society of Dyers and Colourists defines a basic dye as‘characterised by its substantivity for the acidic types of acrylic fibres and for tannin-mordanted cotton’, whereas a cationic dye is defined as one ‘that dissociates in aqueous

Table 1.5 Percentage distribution of chemical classes in disperse dye hue sectors

Distribution in hue sector (%)% of alldisperse


Azo 48 92 73 47 27 30 100 100 59Anthraquinone 6 2 25 53 72 65 32Nitro 16 3 3Aminoketone 8 2 1 1 5 2Methine 14 2Quinophthalone 4 1Miscellaneous 4 1 1 1

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solution to give a positively charged coloured ion’. Since the second of these describes achemical feature rather than an application property, the term ‘basic dyes’ is adopted in thisbook for this range, as in the Colour Index. This category includes those weakly basic dyes(1.51) that are essentially uncharged under neutral or alkaline conditions and only form acation by protonation in acidic solution, such as chrysoidine (CI Basic Orange 2). Thepowerful dye-fibre forces responsible for the excellent exhaustion and high wet fastness ofbasic dyes on acrylic fibres are believed to include electrostatic attraction to anionic sites inthe fibre and non-specific hydrophobic interaction.

N N

H2N

NH3 X N N NH2 + HX

H2N

+

1.51

_

Chrysoidine

Many brilliantly coloured and tinctorially strong basic dyes for silk and tannin-mordantedcotton were developed in the early decades of the synthetic dye industry. Most of thesebelonged to the acridine, azine, oxazine, triarylmethane, xanthene and related chemicalclasses; their molecules are usually characterised by one delocalised positive charge. Thus incrystal violet (1.29) the cationic charge is shared between the three equivalent methylatedp-amino nitrogen atoms. A few of these ‘traditional basic’ dyes are still of some interest inthe dyeing of acrylic fibres, notably as components of cheap mixture navies and blacks, butmany ‘modified basic’ dyes were introduced from the 1950s onwards for acrylic andmodacrylic fibres, as well as for basic-dyeable variants of nylon and polyester [44].

Most of these products are azo or anthraquinone types, often with a localised quaternaryammonium group isolated from the chromogen by a saturated alkyl chain, as in CI Basic Red18 (1.52). Such products often exhibit higher light fastness than the traditional delocalisedtypes. Improved azomethine, methine and polymethine basic dyes of good light fastness arealso available. In contrast to the more specialised traditional classes, the azo and methinedyes have contributed to the basic dye range across the entire spectrum of hues (see Table1.6) and now account for a clear majority of all basic dyes listed in the Colour Index.

Basic dyes do not undergo chemical change during dyeing, but the proportion remaining inthe exhausted dyebath is low (typically 2–3%) and scarcely justifies isolation for reuse.Recycling of the process water, however, may allow recovery of the inorganic salts and otherauxiliary chemicals present [42]. There is normally a high degree of sorptive removal ofresidual basic dyes during biological treatment of effluent. This is important, because basic dyestend to exhibit toxicity to aquatic organisms. In a survey of 3000 dyes in common use, 98% of

O2N N

Cl

N N

CH2CH3

CH2CH2N(CH3)3 X+

1.52

_

CI Basic Red 18


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them showed low toxicity to fish (LC50 > 1 mg/l). In 27 instances, however, 16 of them basicdyes and 10 of these triphenylmethane types, the LC50 was of the order of 0.05 mg/l [53]. Astudy by the American Dye Manufacturers’ Institute (ADMI) of the effects of 56 dyes on thegrowth of green algae, 15 of them inhibited algal growth and 13 of these were basic dyes. Inscreening tests designed to determine whether dyes have an adverse effect on waste waterbacteria and hence on the operation of effluent treatment plants, only 18 of the 202 dyesexamined gave an LC50 value less than 100 mg/l and all of these were basic dyes [53].

1.6.7 Acid dyes

These are defined as anionic dyes characterised by substantivity for protein fibres. Levellingacid dyes and 1:1 metal-complex dyes are normally applied to wool from strongly acidicsolutions, but the 1:2 metal complexes and many milling acid dyes have considerablesubstantivity even from neutral dyebaths (section 3.2.2). Wool, silk and nylon contain basicgroups and the uptake of levelling acid dyes by nylon at acidic pH can usually be related tothe amine end group content of the fibre. Under neutral dyeing conditions, however, non-specific hydrophobic interaction makes a considerable contribution, reinforcing theelectrostatic bonding between such fibres and acid dyes of higher wet fastness.

Azo acid dyes represent the biggest single segment in the Colour Index, about two-fifths ofthem being metal-complex types (see Table 1.7). Monoazo dyes are supreme in all huesectors of the azo acid range except the unmetallised browns and blacks, where disazostructures are more numerous. Anthraquinone and triarylmethane acid dyes traditionallyprovided most of the brilliant violet to green members of this range but demand for themdeclined, especially the triarylmethane types, as bright azo alternatives were developed. Thesignificance of nitro yellows and browns, xanthene reds and violets and the phthalocyanineblues has always been marginal in the acid dye range.

The first acid dye, Orange I (1.53; CI Acid Orange 20), was discovered in 1876. All but ahandful of the acid dyes developed since then were evaluated initially with wool dyeing inmind. In terms of adaptability to the coloration of other substrates, however, acid dyes haveproved pre-eminent. This is the main reason for their number and variety. As well as thedyeing and printing of nylon and protein fibres, acid dyes are important for the coloration ofleather, paper, jute, wood and anodised aluminium. Most of the permitted dyes for food and

Table 1.6 Percentage distribution of chemical classes in basic dye hue sectors

Distribution in hue sector (%)% of allbasic


Azo 33 51 55 38 34 12 100 60 43Methine 41 13 23 21 2 10 17Triarylmethane 5 18 23 56 11Acridine 12 36 7Anthraquinone 5 15 5Azine 6 13 5 20 5Oxazine 11 20 3Xanthene 11 5 3Miscellaneous 14 10 22 6

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cosmetics were originally marketed long ago for the dyeing of wool. Some solvent-solubledyes are formed by co-precipitation of acid dyes and basic dyes. Concentrated monoazo aciddyes, often unmetallised bright yellows and reds, can be precipitated as their water-insolublemetal salts for use as pigments (section 2.2.1).

Acid dyes are highly suitable for reuse because they do not undergo chemical changeduring dyeing. Removal of acid dyes from dyehouse effluent has been achieved by membraneprocessing on an industrial scale. The process water and auxiliary chemicals are also suitablefor recycling to the dyebath [42]. The sorptive removal of acid dyes during biologicaltreatment of effluent varies with their degree of sulphonation. Levelling acid dyes of highsolubility exhibit low sorption, whereas the more hydrophobic neutral-dyeing dyes arebioeliminated to a much greater extent [48]. As in the case of reactive dyes, flocculationwith cationic agents is an effective method of removing the highly sulphonated acid dyes.Electrochemical coagulation of acid dyes is a viable alternative, with values of 55–85%colour removal from carpet dyeing waste waters being reported [54]. Reuse of the permeatefrom a reverse-osmosis membrane process on dyebath effluent containing premetallised aciddyes has been achieved [50].

1.6.8 Mordant dyes

The somewhat ambiguous term ‘mordant dye’ is defined as a dye that is fixed with a‘mordant’. A mordant is itself defined as ‘a substance, usually a metallic compound, appliedto a substrate to form a complex with a dye which is retained by the substrate more firmlythan the dye itself’. Unfortunately, the mordant dyes category in the Colour Index follows

Table 1.7 Percentage distribution of chemical classes in acid dye hue sectors

Distribution in hue sector (%)% of allacid


Unmetallised azo 65 55 64 22 17 15 79 47 48Metal-complex azo 31 42 29 44 21 39 13 46 31Anthraquinone 2 18 36 22 3 3 10Triarylmethane 12 16 16 5Xanthene 1 1 4 3 2Azine 1 1 1 4 1 1Phthalocyanine 5 1 1Nitro 1 1 5 1Miscellaneous 2 1 7 3 1

NaO3S N

N

H

O

1.53

CI Acid Orange 20


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the more restricted convention of excluding both chelatable direct dyes capable ofcomplexing with copper salts and those traditional basic dyes that were formerly applied totannin-mordanted cotton, both of which subgroups conform to the above less than precisedefinition. A more logical generic name for those chelatable anionic dyes capable of beingfixed to wool by aftertreatment with a dichromate would be the more popular term ‘chromedyes’ or ‘chrome mordant dyes’, but the designation CI Mordant dye is well entrenched.Since the range has been steadily declining for many years, the ambiguity is of decreasingsignificance too. In all hue sectors except violet and blue, where triarylmethane,anthraquinone and oxazine dyes make notable contributions, 80% or more of chrome dyesare of the o,o′-dihydroxymonoazo type (1.54; CI Mordant Violet 5). Although most chromedyes are only of interest for wool dyeing, a minority are applicable to most of those othersubstrates where acid dyes are found useful.

NaO3S

N

O

N

OH

1.54

H

CI Mordant Violet 5

The complex formed when a mordant dyeing is aftertreated in a dichromate solution isretained by the wool in preference to the unmetallised mordant dye, which may desorb tosome extent during the treatment. The latter is rather unstable in an oxidising solution andquinonoid by-products are often formed. If the chromium complex of the dye is formed fromthe desorbed dye in solution, this will further complicate the composition of theaftertreatment liquor. Thus reuse of mordant dyeing and aftertreatment baths is not anoption. Furthermore, 100% rejection of dichromate ions would be required if the permeateof a membrane process treating the effluent was to be recycled [42].

1.6.9 Leather dyes

The application range designated by this generic name in the Colour Index incorporatesthose acid, direct and mordant dyes with substantivity for leather and satisfactory fastnesson that substrate [55]. It is a commercially important sector, the number of products listedbeing exceeded only by the complete acid or direct dye ranges. As expected from the sourcesof this selection, about 85% of leather dyes are azo compounds (35% disazo, 30% monoazo,20% metal-complex monoazo) and the remainder are mainly yellow to orange stilbene dyesand anthraquinone or triarylmethane types in the violet to green sectors.

There is growing concern over the potential risks to human health and the environmentarising from leather goods. Dye manufacturers and tanneries are concerned about effluent,air pollution, containers and packaging [56]. In the light of the relatively importantcontribution to leather dyeing of azo dyes that can yield hazardous arylamines on reduction,careful guidance on the selection of dyes for leather is essential, with emphasis onprocurement from reliable sources and the utilisation of liquid formulations to minimise

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effluent treatment [57]. Based on theoretical considerations of dye structure and applicationproperties, a range of compatible dyes that give deep colours of good fastness on leather hasbeen developed [58].

For the reasons already outlined, only selected dyebath wastes from leather dyeing will besuitable for recycling (providing mordant dyes are absent). Bioelimination of the disazodirect dyes and monoazo metal-complex dyes will be relatively high. Flocculation usingcationic precipitants should be an effective way of dealing with the residual dyes remainingin leather dyeing effluents. It is also necessary to consider methods for the disposal of leathergoods at the end of their useful life. Dyed leather in itself, however, is not regarded as posinga hazard to humans, nor is it considered harmful to the environment [56].

1.6.10 Food dyes

Increasing legislation over the last fifty years has been imposed to control additives such ascolorants that might enter foodstuffs directly or by migration from food containers orpackages. Similar criteria cover the composition of drugs and cosmetics, but the cosmeticsregulations tend to be less stringent in terms of chronic toxicity because skin represents aneffective barrier to many chemicals.

Freedom from toxicity is clearly the primary consideration for dyes used in foods, drugsand cosmetics, followed by high solubility and chemical stability in the appropriate mediumof incorporation. The possibility that they may affect consumers adversely has causedgrowing concern [59,60]. Legislation over many years has increasingly restricted the usage ofsynthetic colorants to certain permitted products that have shown no harmful effects whentested rigorously.

There is as yet no agreed international list of permitted food colours. Thus a food dyethat is permitted in one country may be considered unacceptable in another. The syntheticfood colorants permitted in the European Union are listed in Table 1.8 [60]. All wereoriginally introduced as acid dyes for wool many years ago. Furthermore, more than thirtycolorants of natural origin are permitted in most countries. The natural carotenoid dyes areof outstanding importance for colouring edible fats and oils. These yellow to red methinedye structures occur in many families of plants and animals, including vegetables, berries,


Table 1.8 Synthetic colorants permitted for use in food in the European Union (EU)

EU No Colorant CI Food CI Acid Chemical class

E102 Tartrazine Yellow 4 Yellow 23 MonoazoE104 Quinoline Yellow Yellow 13 Yellow 3 QuinophthaloneE110 Sunset Yellow Yellow 3 MonoazoE122 Carmoisine Red 3 Red 14 MonoazoE123 Amaranth Red 9 Red 27 MonoazoE124 Ponceau 4R Red 7 Red 18 MonoazoE127 Erythrosine Red 14 Red 51 XantheneE131 Patent Blue V Blue 5 Blue 1 TriarylmethaneE132 Indigo Carmine Blue 1 Blue 74 IndigoidE142 Green S Green 4 Green 50 TriarylmethaneE151 Black PN Black 1 Disazo

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fungi, insects, bacteria, feathers and egg yolks [61]. In general, the regulations governing theuse of permitted additives such as those added directly in food manufacture are appliedsimilarly in different member countries of the EU. There is greater variation, however, withregard to the indirect contamination of food by transfer of additives from packagingmaterials.

1.6.11 Solvent dyes

Solvent dyes are characterised by their solubility in one or more organic solvents. Numerousmedia are of interest in practice, the most significant being alcohols, ethers, esters, ketones,chlorinated solvents, hydrocarbons, oils, fats and waxes. The many practical applications inwhich the specific solubilities of these dyes are exploited have been outlined elsewhere(section 2.12). More than half of the solvent dyes listed in the Colour Index areunsulphonated azo compounds (27% monoazo, 18% monoazo metal-complex, 9% disazo)and a further 7% are complexes formed by co-precipitation of acid dyes with basic dyes,mostly yellows and reds. Unsulphonated anthraquinones lead the field in the bright violet togreen sectors (17%). Xanthene reds (5%) and triarylmethane blues (5%) are the othernoteworthy segments represented, together with a miscellany (12%) drawn from almost allother chemical classes. Some simple yellow solvent dyes used widely in the past, such as 4-aminoazobenzene (CI Solvent Yellow 1), are now known to be carcinogenic.

1.6.12 Pigments

In contrast to solvent dyes, pigments are substances in particulate form. They are essentiallyinsoluble in the media into which they are incorporated, and are mechanically dispersedtherein in order to modify the colour and/or light-scattering properties of such media. Thesecharacteristics of low solubility in water and organic solvents, which are obligatory for thesatisfactory technical performance of pigments, also mean that their bioavailability isunusually low. This is certainly the prime reason for the remarkably low toxicity of organicpigments [1]. In many cases, the solubilities in n-octanol and water are so low that theexperimental determination of solubility poses serious difficulties, so that calculationprocedures are adopted in assessing the toxicological hazards of pigments [62].

The complete range of pigments may be conveniently divided into three main groups onthe basis of their chemical constitution and mode of preparation for use:(a) Organic pigments (approximately 60% of the total number of products) are

concentrated nonionic colorants from various chemical classes,(b) Water-soluble dyes (approximately 20%) that are rendered insoluble by various

techniques of precipitation,(c) Inorganic pigments (approximately 20%) are coloured insoluble materials from

inorganic sources.

Monoazo and disazo compounds form by far the largest subgroup among the organicpigments, mainly occupying the yellow-orange-brown-red sectors of the colour gamut. Thisclass may be subdivided further into metal-free structures with inherently negligiblesolubility in water and those containing potentially ionisable groups that must be convertedto their heavy-metal salts (section 2.3). Anthraquinonoid pigments are analogous to vat

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dyes in their keto forms (section 1.6.1) and they provide mostly red-violet-blue colours ofhigh light fastness. Further chromogens represented include isoindolinones (yellow-orange),quinacridones (red-violet) and phthalocyanines (blue-green).

Conventional basic dyes (section 1.6.6) can be rendered insoluble by precipitation usingphosphomolybdic or phosphotungstic acids, or alternatively copper(II) hexacyanoferrate.These complexes exhibit higher light fastness than the parent basic dyes on a traditionaltannate mordant. Concentrated anionic monoazo dyes (mainly yellows, oranges andespecially reds) can be precipitated as their insoluble metal salts, typically those of barium,aluminium or calcium. These processes of converting soluble dyes into pigments byprecipitation, often in the presence of an inert substrate such as the so-called aluminahydrate, are now of declining importance.

The inorganic group of pigments represents the naturally occurring coloured mineralsthat have been widely used throughout recorded history to colour ceramics, glass and manyother artefacts. This class includes coloured metal salts such as lead chromate (CI PigmentYellow 34), complex salts such as Prussian blue (CI Pigment Blue 27) and ultramarine (CIPigment Blue 29), metal oxides such as titanium dioxide (CI Pigment White 6) and freeelements such as carbon black (CI Pigment Black 6).

1.6.13 Azoic components and compositions

These are the only ranges of precursor products in the Colour Index that are stillcommercially significant. Azoic dyes have a close formal relationship to those monoazopigments derived from BON acid or from acetoacetanilides (section 2.3.1) and some arechemically identical with them, although they are used in a totally different way. Azoiccomponents are applied to produce insoluble azo dyes within the textile substrate, which isalmost always cotton. Corresponding azoic components for the dyeing of cellulose acetate,triacetate and polyester fibres were once commercially important, but are now obsoletebecause of environmental hazards and the time-consuming application procedure.

In dyeing with azoic components, an insoluble azo compound is produced within the fibreby the coupling of an azoic diazo component (a diazotised arylamine) with an azoic couplingcomponent (usually an arylide of BON acid or a related naphthol). Azoic dyes provide onlyrelatively dull violets and navy blues, costly greens, and browns or blacks of limitedversatility, but are tinctorially strong, bright and economical in the orange to red sectors.Some of the diazotisable amines of former importance in this range, such as 4-chloro-2-methylaniline (CI Azoic Diazo Component 11) or 3,3′-dimethoxybenzidine (CI Azoic DiazoComponent 48) are now known to be carcinogenic.

The two types of component form separate generic series in the Colour Index. There isalso a third series, designated CI Azoic compositions. Now obsolescent, these preparedmixtures of an azoic coupling component and a stabilised diazo component were intendedmainly for textile printing. The stabilisation prevented coupling until the printed fabric wassteamed under appropriate conditions to regenerate the active diazonium salt and promoteformation of the azoic dye. When dyeing with azoic components it is difficult to preventsome desorption of coupling component into the developing bath containing the diazo salt.The latter is also inherently unstable, releasing nitrogen to leave the phenolic analogue ofthe original arylamine. Thus recycling is not a realistic option for residual azoic dyebathsbecause of their complex composition [42].


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1.6.14 Developers

This short range of simple coupling components, comprising phenols, naphthols and 1-phenyl-3-methylpyrazol-5-one, is seldom used nowadays. Developers were once used asaftertreatments to enhance the wet fastness of certain direct dyes for cellulosic fibres ordisperse dyes for cellulose acetate. Each of these dyes contained a primary arylaminegrouping that was capable of diazotisation and coupling to the developer molecule. Thisapplication technique was prolonged, however, and the improvement in fastnessperformance was disappointingly slight. The usage of developers in this way has fallen tonegligible levels.

1.6.15 Oxidation bases

Like azoic components, these are precursors of a distinct chemical class of colorant as well asan application category, but they are too restricted in attainable hues to be described as a‘range’. This cheap but unpleasant method of continuous coloration, once popular forcotton printing in visually attractive illuminated resist styles under aniline black or paraminebrown colours, has been superseded by environmentally more acceptable and controllablealternative processes. Oxidation dyeing was simply the in situ formation of a polymeric blackor dark brown azine colorant of indeterminate constitution by acidic oxidation of a simplearylamine such as aniline or p-phenylenediamine on the fibre, using a chlorate ordichromate as oxidant and a copper salt catalyst. Although no longer significant in textiledyeing or printing, coloration techniques of a similar kind have remained the primarymethod of colouring human hair, feathers and natural furs.

1.6.16 Ingrain dyes

This is another somewhat ambiguous category in the context of the Colour Indexclassification, but fortunately it is now merely of historical interest. An ‘ingrain’ dyeingprocess is defined as the formation of a colorant ‘in situ in the substrate by the developmentand coupling of one or more intermediate compounds’. This imprecise description clearlyembraces azoic dyeing and the application of oxidation bases, but the generic term ‘CIIngrain dye’ is limited to a third small group of now obsolete colorant precursors, again toorestricted in hue to be regarded as a ‘range’.

They were introduced in the late 1940s for the textile printing of cellulosic fabrics underthe trade names Alcian (ICI) and Phthalogen (BAY). Both types resulted in theinsolubilisation of copper phthalocyanine or related pigments within the fibre, although therespective application techniques differed considerably. Reactive phthalocyanine dyes in the1960s superseded these early approaches to the attainment of fast bright blues andturquoises on cellulosic fabrics [63].

1.6.17 Fluorescent brightening agents

These essentially colourless compounds have been defined as substances ‘that, when addedto an uncoloured or a coloured substrate, increase the reflectance of the substrate in thevisible region by converting UV radiation into visible light and so increase the whiteness orbrightness of the substrate’. The air of uncertainty in the final clause of this definition

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reflects the general indecision among practitioners in this field about whether to call them‘fluorescent brightening agents’ (or FBAs), as in the Colour Index, ‘fluorescent whiteners’(FWs) or ‘optical whiteners’ (OWs).

Since their commercial introduction during the 1940s as components of proprietarydetergents and laundry preparations, these products have found extensive usage in thewhitening of paper and textile materials. Disperse FBAs are available for whiteninghydrophobic fibres and solvent-soluble FBAs impart fluorescence to oils, paints, varnishesand waxes. Approximately 75% of commercially established FBAs are stilbene derivativeswith inherent substantivity for paper and cellulosic textiles, but the remainder come fromabout twenty different chemical classes. These include aminocoumarins (6%),naphthalimides (3%), pyrazoles and pyrazolines (each about 2%), acenaphthenes, benzidinesulphones, stilbene-naphthotriazoles, thiazoles and xanthenes (each about 1%). FBAs ofthese and other chemical types are discussed in detail in Chapter 11 of Volume 2.

1.6.18 Reducing agents

As noted earlier, the specific inclusion in the Colour Index of a group of textile auxiliariesunder this generic name seems anomalous, if not aberrant. The justification claimed is thatthey are indispensable for the application of vat dyes and in discharge printing or strippingprocesses. A similar level of interdependency could be claimed for certain other classes ofauxiliaries, as is discussed particularly in Chapters 10 and 12 of Volume 2. One has only tovisualise textile printing without thickeners or emulsifying agents to realise this. In futureplans for further development of the Colour Index, however, there seems to be no scope forextending coverage to other groups of auxiliary products used in colorant applicationprocesses [l3].

1.7 COLORANTS AND THE ENVIRONMENT

Health and safety were absent from the list of priorities in the early decades of the syntheticdyes industry. Practical experience in the primitive working conditions of the time [64] nodoubt made workers aware of the more obvious dangers, such as corrosive acids, flammablesolvents and potentially explosive nitro compounds. Accidents must have occurredfrequently, reminding victims and supervisors alike of the penalties suffered if hazardouschemicals were handled carelessly.

More insidious, however, were the potential health risks resulting from longer-termexposure to certain organic and inorganic chemicals. Operatives were expected to provideand wash their own working clothes. These must have become heavily contaminated withchemical stains, putting the worker’s family also at risk of exposure to hazardous vapours[65]. It was not until the 1940s that the connection was made between the high incidenceof bladder cancer in employees of dye-making and dye-using firms and their exposure tocertain arylamines, following epidemiological studies of individuals who had spent theirworking lives in these industries. Compounds now recognised as highly carcinogenic,notably 2-naphthylamine and benzidine, had been widely used as dye intermediates sincethe 1880s. Even if such causal links had been suspected over the intervening decades, it isdifficult to see how they could have been demonstrated in any other way.

Following a period of unprecedented growth and optimism in the chemical industries

COLORANTS AND THE ENVIRONMENT

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worldwide during the 1950s and 1960s, a sociopolitical reaction set in during and after theoil crises of the 1970s. Public demand, often fuelled by media misinformation andspeculation [60,66], became insistent that the manufacture and use of industrial chemicalsshould be more closely regulated and monitored. A series of tragic incidents occurred overthis period, involving cyclohexane at Flixborough (UK), dioxins at Seveso (Italy), mercurycompounds at Minamata (Japan) and methyl isocyanate at Bhopal (India). These werefollowed in 1986 by the Chernobyl (USSR) nuclear explosion that polluted much ofNorthern Europe and the Schweizerhalle (Switzerland) fire that polluted the Rhine basin inthe heart of Europe, reinforcing this trend towards a political climate of reform and control.

1.7.1 Risk evaluation and prevention

Indispensable to the management of risk by reduction or avoidance is a knowledge of therisk and the controlling factors determining its magnitude. Risk in this context is a functionof the potentially harmful effects arising from inherent toxicological properties of thechemical and the extent of its bioavailability to the organism exposed. Risk is also a functionof degree of exposure and the probability of its occurrence. Obviously, the risk ofexperiencing harmful effects can be lowered by limiting the degree of exposure and thisapproach affords a means of improving safety [67]. Industry has a substantial interest inhelping the risk assessment approach to work. Failure to do so will encourage regulationbased on hazard considerations alone. There is already a tendency in favour of theprecautionary principle and the introduction of various ‘black-lists’. Such discriminatoryactions undermine the agreed basis for chemicals control and warrant an unreservedrejection by the chemical industry [68].

Thus two components, exposure and hazard, must be evaluated together in determiningthe level of risk posed by a given colorant or other chemical. Risk management maytherefore be regarded as a series of interdependent steps:(1) Exposure assessment(2) Hazard assessment(3) Risk evaluation(4) Risk prevention

The process of risk evaluation for personnel working with dyes and textile chemicals hasbeen discussed in detail [69]. The more extensive the database covering toxicological,physical, chemical and application properties of the product, the easier it is to assess therisks involved.

Although exposure levels are just as important as hazard potential for the risk assessment,the quality of the exposure data is often the weak point in the data available. Consequently,in many instances it is not possible to be fully confident of the reliability of the riskassessment, which may tend to be in error on the side of overestimation. Particular attentionshould be given to improving aspects of exposure assessment (occupational exposure,consumer exposure, environmental release) [70]. The widespread use of colorants creates agreat diversity of exposure situations. The most serious exposure potential exists foroperatives in colorant manufacture and those employees of dye user firms engaged inweighing and dispensing dyes. Dust particles less than 7 µm in size can gain access into thelungs and pose the greatest problem. It is not feasible to market all colorants in liquid form

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and considerable efforts have been devoted to the development of low-dusting solid forms.The monitoring of amine excretion in the urine of individuals exposed to arylamines, or dyesexpected to be metabolised in the body to such amines, offers a possibility of checking theadequacy of safety precautions [67].

The conventional classification of organic colorants into broad chemical classes orapplication ranges is of limited help in hazard assessment. It is not possible to generaliseabout the toxicological properties of entire groups like these. Biological activity can varydramatically in spite of close structural relationships. Nevertheless, an observed toxic effectcan often be attributed to a specific structural feature within a narrow subclass of colorants,or to a specific metabolite produced from them as in the formation of benzidine from itsparent disazo dyes. Organic colorants generally exhibit relatively low acute toxicity. This isespecially true of organic pigments because of their extremely low bioavailability. Of majorconcern, however, are the potential carcinogenic and allergic effects.

As a basis for the determination of risk it must be assumed that the colorants are properlyhandled and applied. It is not appropriate to estimate risk primarily on the basis of exposurevalues obtained under improper working conditions, or where appropriate plant andequipment are not available. Ensuring satisfactory operating conditions and training ofoperatives to handle products correctly is essential nowadays for technological success aswell as for health and safety requirements. In this way, exposure levels can be kept below thethreshold of unacceptable risk. It is reasonable to accept that for practical purposes levels ofexposure exist below which the risk becomes trivial [67].

The various measures to reduce risk are an integral part of risk management. A state of‘zero risk’ cannot be reached, but efforts to maintain exposure levels below the threshold ofunacceptability must be unremitting, in order to increase the margin of safety. An essentialprerequisite for effective risk control is the provision of readily accessible hazard informationon computer disc, in safety data sheets and on warning labels. It is prudent to minimiseexposure to all chemicals through good working practice. Respiratory protection byapproved equipment must be worn wherever dusts or aerosols are being generated ordisturbed. A constructive approach to reducing risk is the replacement of hazardousproducts by safer ones. This cannot be achieved quickly in most instances, because of thecomplex profile of technical and economic requirements that governs selection of a colorantfor a specific purpose [71].

Over recent years there has been an extraordinary growth of ecolabelling schemesworldwide, but notably in Europe. The term ‘ecolabel’ has been defined as an EU scheme topromote products with reduced environmental impact during their life cycle [72]. Theseschemes build upon increasing public awareness of environmental issues, offering anopportunity to increase and sustain consumer interest in reducing the environmentalimpact, as well as opening up new marketing opportunities. The chemical industry’s stancehas been supportive of these objectives but somewhat sceptical and concerned at themultitude of different schemes [68,70].

As a basis for evaluating individual schemes, the Ecological and Toxicological Associationof the Dyestuffs Manufacturing Industry (ETAD) concluded that a satisfactory schemeshould meet the following criteria:(1) Objective criteria must be provided(2) The approach should be risk-based, not hazard-based(3) Consumer goods should be specified, rather than the colorants used to colour them


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(4) Dye selection should only be necessary if there is real environmental benefit(5) European or international schemes are preferable, rather than a proliferation of

national ecolabels.

The main deficiency of existing schemes is that they focus on the exclusion of certainhazardous dyes but do not take into account that it is the conditions of manufacture andapplication that are the most important determinants of environmental impact [68,70].

1.7.2 Sensitisation, toxicity and carcinogenicity

Allergic contact dermatitis or skin sensitisation by dyes or other chemicals manifests itself asa persistent irritating rash. The presence of certain dye stains on the skin has been known toaccelerate the reddening effect of sunlight exposure (erythema). The causation of skinsensitisation by dyes, both in animal tests and in exposed workers or the general public, hasbeen reviewed [73,74]. Cases of occupational skin sensitisation attributable to dyes areuncommon [75], even in those workplaces where inadequate handling precautions havebeen taken. Occasionally, reports of organic pigments causing skin sensitisation have arisenbut such cases appear to be due to the presence of soluble impurities. Instances of contactdermatitis linked to the wearing of close-fitting garments often seem to be associated withnylon dyed with certain disperse dyes of low fastness in full depths (section 1.6.5). A list ofnine such dyes that may be sensitising has been drawn up: CI Disperse Yellow 3, Orange 3,37 and 76, Red 1 and Blue 1, 35, 106 and 124. Garments containing any of these dyesshould carry a hazard warning label and for contact clothing such as hosiery they should notbe used. The possible mechanism of sensitisation in the case of azo dyes is thought to be theproduction of the quinonimine derivative by reduction and oxidation [75].

Sensitisation of the respiratory tract by inhaling dust particles from various chemicals hasfrequently been reported in industry. It is likely to result in symptoms of respiratory diseaseor distress when the sensitised individual is exposed to a specific allergen. Respiratory allergyis the clinical manifestation of this state, with bronchial asthma or allergic rhinitis(resembling hay fever) constituting typical disease symptoms. It is believed that relativelyhigh exposure levels are important in the induction phase. There is also evidence that apredisposition to respiratory allergy may be caused by genetic or other factors.

Instances of severe sensitisation to the dust from reactive dyes have been reported [76].These prompted the UK Health and Safety Executive to initiate a study involving about 440workers in 51 dyehouses who were in contact with reactive dye powders. About 15% ofthem showed work-related respiratory or nasal symptoms. In 21 individuals their allergicreactions could be attributed to contact with one or more specific reactive dyes [77].Reactive dyes are able to react with amino, hydroxy and thiol groups in proteins. Such areaction seems to be the initial step of the sensitisation process. The reactive dye may reactwith human serum albumin (HSA) to form a dye-HSA conjugate, which behaves as anantigen. This in turn gives rise to specific antibodies and these, through the release ofmediators such as histamine, produce the allergic symptoms [67,77].

Acute toxicity refers to effects that occur within a brief time after a short-term exposure,such as a simple oral administration. The generally low acute oral toxicity of colorants iswell-established [78–81]. This is normally expressed in terms of the LD50 value, a

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statistically derived dose that is expected to cause death in 50% of treated animals (typicallyrats) when administered over a prescribed period in the test. In 1974 ETAD began aprogramme to generate a systematic toxicological database. More than 80% of commercialdyes have an LD50 value (rat, oral) greater than 5000 mg/kg.

In response to an EEC Council Directive of 1979 (6th amendment) regarding thelabelling of dangerous substances, ETAD in 1986 decided to publish a list of twelvecolorants that have been classified as toxic on the basis of their acute peroral LD50 values[82]. These varied within the range 25 mg/kg (CI Basic Red 12) to 205 mg/kg (CI BasicBlue 81) and the list included six basic dyes, three azoic diazo components, two acid dyesand one direct dye. Although such data provide an essential basis for advice on safehandling procedures, long-established experience indicates that dyes, and even more soorganic pigments, present few acute toxicological risks providing good working practices arefollowed.

In the debate about the toxic effects of dyes and chemicals, there is no doubt thatcarcinogenic effects are perceived by the general public as the most threatening. Chemicalsremain a focus for this concern in spite of the weight of evidence that they make only aminor contribution to the incidence of cancer [60,67,83]. The generally accepted estimateof cancer causation, based on mortality statistics, indicates that only 4% of all cancer deathsare attributable to occupational exposure. Another 2% are considered to arise fromenvironmental causes and 1% from other forms of exposure to industrial products.

As by far the largest chemical class, it is perhaps not surprising that azo dyes haveattracted most attention with regard to carcinogenicity. Some structure-carcinogenicitytrends for azo dyes and their metabolites have been discussed with a view to the predictionof dye carcinogenicity [84,85]. If an azo dye is carcinogenic and is relatively stable in thehydroxyazo tautomeric form, the dye itself is likely to be the active carcinogen. In contrast,those dyes that exist predominantly in the ketohydrazone form are more readily reduced tometabolites. In this case, the pro-carcinogen is likely to be an arylamine and the ultimatecarcinogenic potential can then be deduced from the availability of a suitable active site onthe metabolite. Azo pigments, because of their extreme insolubility and low bioavailability,are unlikely to be metabolised even if they exist preferentially in the hydrazone form [84]. Alarge majority of water-soluble azo dyes do not form carcinogenic arylamines whenreductively cleaved. In most cases, the reduction products are arylaminesulphonic acids,which have little or no carcinogenic potential.

Epidemiological studies first alerted the colorants industries to the causal links betweencertain manufacturing operations and an increased risk of bladder cancer among workers[1,60,86]. Regulations were passed in the 1960s that placed a virtual ban on the importationand use of certain dye intermediates, such as benzidine and 2-naphthylamine, and certainprocesses, including auramine (1.28) manufacture [87]. Most responsible colorantmanufacturers in Europe, Japan and the USA ceased production of benzidine-based dyes inthe early 1970s due to inability to ensure their safe handling in dyehouses. However, owingto the attractive economic and technical merits of such dyes on leather and cellulosic fibres,manufacture continued in other parts of the world (for example, Latin America, India andthe Asia Pacific region). The risk to workers in manufacturing and dyeing plants in theseareas is of concern because of the poor conditions of occupational hygiene that often exist[1].


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1.7.3 Colorants in waste waters

The high visibility of water-soluble dyes released to the environment ensures that onlyextremely low concentrations in watercourses would not be noticed. A typical visibility limitin a river would be about 0.1 to 1 mg/l, but this varies with the colour, illumination anddegree of clarity of the water. The human eye can detect a reactive dye concentration as lowas 0.005 mg/l in pure water, particularly in the red to violet hue sector [88]. There isconsiderable debate, however, about what level of environmental hazard is represented per seby colour in effluent. The view has been expressed that dyestuffs should not be regarded aswater pollutants because at concentrations of the same order of magnitude as these visibilitylimits their harmful effects are negligible [89]. Nevertheless, even though this colourproblem is mainly if not entirely an aesthetic one, the fact is that the general public will nottolerate coloured amenity water and the problem therefore has to be addressed and rectified[90,91,92].

The main consideration regarding the environmental impact of dye residues is concernedwith toxicity to aquatic organisms. This is normally expressed in terms of the LC50 value,which represents the concentration of the substance under test that is required to kill 50%of the organisms exposed. With the exception of a small minority (about 2%, mainly basicdyes), organic dyes generally show only low toxicity to fish and other organisms such asDaphnia magna [48]. Bioaccumulation is also important, defined as the factor F = Ca/Ce,where Ca is the concentration of the pollutant in the fish species and Ce that for the generalenvironment [93]. The partition coefficient (P) of the colorant in an n-octanol/watermixture is used as an indicator of bioaccumulation. If P is less than 1000 it can be predictedthat F in fish will be less than 100, at which level no problems are foreseen. Water-solubledyes do not bioaccumulate and even those disperse dyes and pigments that give P valuesabove 1000 still show no evidence of bioaccumulation in fish [48].

Algae are an important part of the aquatic ecosystem, with algal photosynthesis a criticalsource of oxygen. The adverse effects of dyes in inhibiting the growth of green algae do notparallel the effects on fish, so that no conclusions about the one can be drawn from theother. Nevertheless, those basic dyes that yield low LC50 values in fish toxicity tests also tendto inhibit algal growth at concentrations as low as 1 mg/l. It is also true that a majority ofbasic dyes have an inhibitory effect on waste water bacteria, having LC50 values of less than100 mg/l, so that they tend to have a deleterious effect on the operation of effluenttreatment plants (section 1.6.6).

In view of the good to excellent fastness of most colorants, it is not surprising that theyare not readily biodegradable. Biodegradability may be defined as the degree ofdecomposition of an organic contaminant after biological treatment under specifiedconditions [94]. With the brief retention times normally prevailing in effluent plants, thereis practically no evidence of biodegradation of colorants under aerobic conditions [53].Bioelimination includes removal of the colorant by adsorption on the biomass as well as thatundergoing biochemical decomposition. A large majority of dyes are adsorbed by thebiomass to the extent of 40–80%. High adsorption occurs with basic dyes, direct dyes,disperse dyes and most of the premetallised and milling acid dyes [48,95,96]. The only dyetypes that are not substantially adsorbed during biological treatment are the highly solublemultisulphonated levelling acid dyes and virtually all reactive dyes, which share similarcharacteristics.

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1.7.4 Hazardous arylamines

Anaerobic conditions, such as apply when digesting sewage sludge or when residualcolorants are present in river sediments, favour biodegradative reactions. Under thesecircumstances the biodegradation of dye chromogens is a primary cause of colour removal.Dyes are quite readily absorbed by sludge, suspended solids or sedimental matter. With azocolorants these conditions render the azo group susceptible to reductive cleavage, giving riseto arylamines as breakdown products [97,98]. There is concern that arylamine metabolitesformed under such anaerobic conditions could be desorbed later into the aquatic aerobicenvironment and thus represent a hazard. The arylamines, however, are generallysusceptible to aerobic degradation. Aniline and its monosubstituted derivatives, such asanisidines, phenetidines and toluidines, are readily biodegraded. Diaminobiphenyls, namelybenzidine, dianisidine, dichlorobenzidine and tolidine, are more resistant but still inherentlybiodegradable [99]. A wider selection of arylamine metabolites from azo dyes, includingarylaminesulphonic acids, gave broadly similar results [100].

The voluntary cessation of manufacture of benzidine and the dyes derived from it bymajor dyemakers in the early 1970s created a series of research targets to find replacementswith corresponding technical properties. Alternative non-mutagenic diamines were soughtand found to yield dyes exhibiting satisfactory performance [101]. Unfortunately, these werealmost always substantially less cost-effective than the analogous benzidine-based productsthey were intended to replace and which were still commercially available from non-traditional suppliers. Following the emergence in the early l990s of conclusive evidence ofanimal carcinogenicity from CI Acid Red 114 (1.55) derived from o-tolidine and CI DirectBlue 15 (1.56) derived from o-dianisidine, several dye manufacturers ceased production ofthese and other dyes made from these two diamines.

H3C SO2

N

CH3

N

H

N

H3C

SO3Na

NaO3S

O

O

N

1.55

CI Acid Red 114

N

N N

H

N

NaO3S

NH2

O H

H3CO OCH3

O

H2N

SO3Na

NaO3SSO3Na1.56

CI Direct Blue 15


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The German government issued in July 1994 an amended regulation concerningconsumer goods involved in direct body contact, including clothing, shoes and bedlinen.This banned the use of azo colorants that could be reduced to give any of twenty specifiedarylamines. These amines have been classified by the German MAK commission assubstances that have been unequivocally proved to be carcinogenic. This regulationembraces the concept that an azo dye that can cleave to yield a carcinogenic arylamine isitself a carcinogen [70]. It is estimated [1] that about 150 commercial azo colorants arecarcinogenic according to this definition. Since the German ban, ecolabelling schemeswithin the EU have adopted the same list of twenty arylamines and added two more thathave been classified by the EU as Category 2 carcinogens [102]. A full list of these specifiedarylamines is given in Chapter 4 (Table 4.1).

Since the implications of this unilateral ban by the German authorities began to berealised, it seems to have developed into a prime example of how not to enact legislation insuch a commercially and technically complex area. There was an immediate and substantialimpact on certain sectors of industry, notably textiles and leather. The globalisation of tradein the l990s ensured that there were many repercussions both inside and outside the EU. Norisk analysis had been carried out and there was no consultation with interested partiesoutside Germany before the ban, in spite of notification procedures required by EUregulations [103]. The Netherlands later enacted a similar ban and several other EUmember states [70] are intending to introduce regulations concerning consumer goodsinvolved in direct body contact.

Although the 1994 regulation specified consumer goods, suppliers were asked by Germanretailers to guarantee that all materials supplied were free from the banned dyes. Thuscompanies higher up the supply chain became involved, if their goods containing such dyeswere to be imported into Germany. A major stumbling-block was the lack of an official listof dyes to be banned [104]. Member firms of ETAD quickly undertook to inform theircustomers and other interested parties if they supplied any of the azo dyes implicated in thislegislation [105]. This still left dye users and others uncertain about supplies from elsewhere,especially from colorant merchants who do not manufacture the products that they sell.Until 1997, there was no officially recognised test method [106] to isolate and identify thespecified amines from textiles or leather.

Analysis was sometimes undertaken by organisations lacking the necessary skills orexpertise. It was not surprising, therefore, that spurious results (false positives) weresometimes obtained [103,105]. The early tests carried out with alkaline dithionite attemperatures above 70 °C could give rise to banned amines by reactions other than azoreduction, including reduction of nitro groups [107], hydrolysis of amides anddesulphonation of water-soluble amines [108]. In order to minimise the misleading resultsobtained under these conditions, methods of reduction using buffered dithionite at 70 °C orlower temperatures were developed [108,109].

1.7.5 Halogenated colorants

During the l990s, several environmental agencies and activist groups have argued that thebanning of chlorine and all chlorinated organic chemicals will be necessary to protect theenvironment. The impact of such a ban would be immense, particularly for those organic dyesand pigments that are predominantly dependent on chlorine-containing intermediates leading

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to their manufacture. Approximately 40% of all organic pigments contain chlorine in thepigment itself, although this corresponds to only 0.02% of total chlorine usage. Interestingly,organofluorine compounds do not fall into the AOX classification since the fluoride ionliberated as soluble silver fluoride according to the test protocol is not detected [25].

In the EU and Japan, controls on the discharge of absorbable organohalogen compoundsare becoming increasingly severe. Insect-proofing agents for wool, trichlorobenzene carriersfor polyester dyeing and reactive dyes of the chloroheterocyclic types certainly fall into thiscategory [65,110]. It seems likely that reactive dyes containing vinylsulphone orfluoroheterocyclic groups will become increasingly important [25]. Many direct, disperse andvat dyes also contain chloro-substituted aryl nuclei [111]. It is to be hoped that rationalevaluation of the available evidence will convince regulatory authorities that the merepresence of an inert chloro substituent in a molecule does not mean that it will pose anenvironmental risk [1].

1.7.6 Heavy-metal contaminants

Heavy metals are widely used as catalysts in the manufacture of anthraquinonoid dyes.Mercury is used when sulphonating anthraquinones and copper when reacting arylamineswith bromoanthraquinones. Much effort has been devoted to minimising the trace metalcontent of such colorants and in effluents from dyemaking plants. Metal salts are used asreactants in dye synthesis, particularly in the ranges of premetallised acid, direct or reactivedyes, which usually contain copper, chromium, nickel or cobalt. These structures aredescribed in detail in Chapter 5, where the implications in terms of environmental problemsare also discussed. Certain basic dyes and stabilised azoic diazo components (Fast Salts) aremarketed in the form of tetrachlorozincate complex salts. The environmental impact of theheavy metal salts used in dye application processes is dealt with in Volume 2.

The toxic effects of trace metals towards animals or aquatic life are highly dependent onthe physical and chemical form of the contaminant [112]. For example, dissolved copper(II)or chromium(III) ions are highly toxic, whereas the same atoms coordinated within stableorganic ligands such as dye molecules are not harmful. Unfortunately this is not widelyacknowledged in setting limits for consent conditions, where the total metal content is oftenspecified rather than the forms in which it is present. The permitted levels for trace metalsin dyehouse effluents vary from one country to another and even between different areas inthe same country [65,94,113]. Improved dyeing methods that have been developed tominimise release of residual chromium when applying chrome dyes to wool are outlinedelsewhere (section 5.8.2).

When restrictions on the contamination of effluent by chromium residues were imposedin the 1970s, the initial reaction in the wool dyeing industry was to predict the rapid demiseof chrome dyes. This forecast decline did not materialise because of the outstanding fastnessof chrome dyeings and the efforts made to minimise effluent pollution [25,65]. In 1996member firms of the GuT carpet ecolabelling scheme in the EU introduced a voluntary banagainst the use of metal-complex dyes on certain nylon floorcoverings. Extension of this banto all carpets made from nylon, wool or their blends has been predicted [104]. Ecolabellingschemes covering apparel and household textiles must also take account of the presence ofpremetallised dyes because of the obvious risk that extraction into perspiration or saliva cantake place from dyeings of inadequate wet fastness [114].


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1.7.7 Natural and synthetic dyes

In Germany during the 1970s there was a growing demand by supporters of the Greenmovement for greater use of natural dyes of vegetable origin to dye natural fibres such aswool, silk and cotton. This trend has been taken up enthusiastically in tropical countrieswith climates suitable for growing such dye-yielding plants [115,116], including woad [117],lac dye [118], lichens [119] and even tea plants [120,121]. Research has been carried outwith a view to minimising the amounts of mordanting chemicals necessary to apply naturaldyes [122–124], in order to offset criticism that such processes would be as polluting as theuse of premetallised synthetic dyes.

Contamination of effluent streams with residual heavy metals from mordanting [125,126]is by no means the only drawback of a return to colouring methods that prevailed before thediscovery of synthetic dyes [127,128]. Natural dyes are tinctorially weaker and duller, areoften difficult to dye level and show inferior fastness to light and washing compared withtheir synthetic counterparts. Although applicable to natural fibres with the aid of mordantchemicals, important synthetic polymers such as polyester or acrylic fibres cannot be dyedusing these products [126,129]. Calculations show that about 400 kg of cultivated dye plantsare required to dye to the same depth as given by 1 kg of synthetic dye on cotton or wool, ata cost ratio of about 100:1. Furthermore, if the worldwide consumption of dyed cotton werecoloured with natural vegetable dyes rather than synthetic ones, approximately 30% of theworld’s agricultural land would be needed for the cultivation [125,127]. This is more than13 times the area currently in use to grow the cotton and does not take into account whatwould be required if the other textile fibres, paper and leather were also to be coloured inthe same way.

The extraction of natural dyes from animal sources is just as wasteful of resources, time-consuming and by no means environmentally friendly. To obtain 1 kg of cochineal scarletrequires the harvesting of 150 000 insects reared on cactus plants. The living insects areswept off the leaves into bowls or cloths and executed by immersion in steam or hot water,then dried by long exposure to sunlight. If production of the classical dye Tyrian purple wereto be restored on a large scale, isolation of only 1 kg of this colorant would demand theslaughter of about 10 million specimens of a Mediterranean mollusc (Murex brandaris – nowrare). Vast quantities of these discarded shells that develop an obnoxious odour whenexposed to the sun would become unsightly spoilheaps, extremely offensive to the coastalenvironment [104,127].

REFERENCES 1. E A Clarke and D Steinle, Rev. Prog. Coloration, 25 (1995) 1. 2. P F Gordon and P Gregory, Organic chemistry in colour, (Berlin: Springer-Verlag, 1983). 3. P F Gordon and P Gregory, Developments in the chemistry and technology of organic dyes, Ed J Griffiths (Oxford:

Blackwell Scientific Publications, 1984) 66. 4. H Zollinger, Color chemistry 2nd Edn, (Weinheim: VCH Publishers, 1991). 5. J Griffiths, J.S.D.C., 104 (1988) 416. 6. Colour chemistry: Design and synthesis of organic dyes and pigments, Eds A T Peters and H S Freeman (Glasgow:

Blackie, l991). 7. Modern colorants: Synthesis and structure, Eds A T Peters and H S Freeman (Glasgow: Blackie, 1994). 8. G W Gray, Chimia, 34 (1980) 47. 9. M Okawara et al., Organic colorants: A handbook of data of selected dyes for electro-optical applications, (Oxford:

Elsevier, 1988). 10. Infrared absorbing dyes, Ed M Matsuoka (New York: Plenum, l990).

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11. J T Guthrie, Macromolecular dyes in Encyclopaedia of polymer science and engineering, Vol 5 (1986) 277. 12. B C Burdett, J.S.D.C., 98 (1982) 114. 13. P M Dinsdale, J.S.D.C., 112 (1996) 38. 14. J W Slater, Manual of colours and dyewares, (London: 1870). 15. W R Richardson, J.S.D.C., 1 (1885) 251. 16. Colour Index International, Pigments and solvent dyes Edn (Bradford: SDC 1998). 17. A Abel, J Oil Colour Chem. Assoc., 81 (1998) 77. 18. H Wolfson, J.S.D.C., 112 (1996) 347. 19. N Ward, private communication. 20. K H Saunders and R L M Allen, Aromatic diazo compounds, 3rd Edn (London: Edward Arnold, 1985). 21. G Hallas and A H M Renfrew, J.S.D.C., 112 (1996) 207. 22. J Shore, Textilveredlung, 12 (1986) 207. 23. C W Greenhalgh, J L Carey and D F Newton, Dyes and Pigments, 1 (1980) 103. 24. C W Greenhalgh et al., J.S.D.C., 110 (1994) 178. 25. D M Lewis, Rev. Prog. Coloration, 29 (1999) 23. 26. F H Moser and A L Thomas, Phthalocyanine compounds, (New York: Reinhold, 1963). 27. N Sekar, Colourage, 45 (Aug 1998) 47. 28. A Albert, The acridines, 2nd Edn. (London: Edward Arnold, 1967). 29 A H M Renfrew, Rev. Prog. Coloration, 15 (1985) 15. 30. J Griffiths, Colour and constitution of organic molecules, (London: Academic Press, 1976). 31. J Fabian and H Hartmann, Light absorption of organic colorants, (Berlin: Springer-Verlag, 1980). 32. J Griffiths, Rev. Prog. Coloration, 11 (1981) 37; 14 (1984) 21; Chem. Brit, 22 (1986) 997. 33. M Matsuoka, J.S.D.C., 105 (1989) 167. 34. W Fabian, Dyes and Pigments, 6, No 5 (1985) 341. 35. C Lubai et al., Dyes and Pigments, 10, No 2 (1989) 123. 36. J Griffiths, J Hill and B Fishwick, Dyes and Pigments, 15, No 4 (1991) 307. 37. Y Kogo, Dyes and Pigments, 6, No 1 (1985) 31. 38. G Hallas and R Marsden, Dyes and Pigments, 6, No 6 (1985) 463. 39. C Lubai et al., Dyes and Pigments, 7, No 5 (1986) 373. 40. S Yasui, M Matsuoka and T Kitao, Dyes and Pigments, 11, No 2 (1989) 81. 41. R M Christie and B G Freer, Dyes and Pigments, 24 (1994) 113, 259; 27 (1995) 35. 42. C Diaper, V M Correia and S J Judd, J.S.D.C., 112 (1996) 273. 43. J J Porter, Am. Dyestuff Rep., 22 (Jun 1990) 21. 44. J Shore, Blends Dyeing, (Bradford: SDC, 1998). 45. O Annen, Melliand Textilber., 79 (1998) 752. 46. James Robinson Ltd., J.S.D.C., 111 (1995) 172. 47. I D Rattee, J.S.D.C., 85 (1969) 23. 48. I G Laing, Rev. Prog. Coloration, 21 (1991) 56. 49. J F Dawson, Rev.Prog.Coloration, 14 (1984) 90; J.S.D.C., 99 (1983) 183; 107 (1991) 395. 50. C A Buckley, Water Sci. Tech., 25 (1992) 203. 51. J J Porter and G A Goodman, Desalination, 49 (1984) 185. 52. H Zollinger, J.S.D.C., 88 (1972) 447. 53. R Anliker, IFATCC Congress, Budapest (Jun 1981) lecture. 54. W C Tincher, Text. Chem. Colorist, 21 (1989) 33. 55. K Eitel, E Rau and J Westphal, Rev. Prog. Coloration, 14 (1984) 119. 56. A G Puntener, J. Soc. Leather Tech. Chem., 82 (1998) 1. 57. H Gardere, J. Amer. Leather Chem. Assoc., 93 (1998) 215. 58. J Berenguer, M Dien and J Rocas, J. Soc. Leather Tech. Chem., 82 (1998) 154. 59. Developments in food colours, Ed.J Walford (London: Elsevier) Vol 1 (1980) Vol 2 (1984). 60. A J Hinton, Rev. Prog. Coloration, 18 (1988) 1. 61. N Sekar, Colourage, 45 (Oct 1998) 55. 62. R Anliker and P Moser, Ecotox. Environ. Safety, 13 (1987) 43. 63. J Shore, Dyer, 167 (Jun 1982) 28. 64. M R Fox, Dyemakers of Great Britain 1856–1976, (Manchester: ICI, 1987). 65. D M Lewis, J.S.D.C., 105 (1989) 119. 66. R H Horning, Text. Chem. Colorist, 18 (Jan 1986) 17. 67. R Anliker and D Steinle, J.S.D.C., 104 (1988) 377. 68. E A Clarke, J.S.D.C., 114 (1998) 348. 69. J A LoMenzo, Risk assessment’s role in dye safety, Proc. AATCC Symp., (Mar 1984) 18. 70. H Motschi and E A Clarke, Rev. Prog. Coloration, 28 (1998) 71. 71. J Park and J Shore, Rev. Prog.Coloration, 12 (1982) 1.

REFERENCES

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72. B J McCarthy and B C Burdett, Rev. Prog. Coloration, 28 (1998) 61. 73. F Klaschka, Melliand Textilber., 75 (1994) 193. 74. K L Hatch, Text. Chem. Colorist, 30 (Mar 1998) 22. 75. V A Shenai, Colourage, 45 (Oct/Nov 1998) 62. 76. J M Wattie, J.S.D.C., 103 (1987) 304. 77. Clinical and immunological investigations of respiratory disease in workers using reactive dyes (London: HSE, Aug

1986). 78. E A Clarke and R Anliker, The handbook of environmental chemistry, Vol 3A, Ed O Hutzinger (Berlin: Springer-

Verlag, 1980) 181. 79. E A Clarke and R Anliker, Rev. Prog. Coloration, 14 (1984) 84. 80. R Anliker in Toxic hazard assessment of chemicals, Ed M L Richardson (London: Royal Society of Chemistry, 1986)

116. 81. V A Shenai, Toxicity of dyes and intermediates, (Bombay: Mirachem Industries, 1996). 82. R Anliker et al., J.S.D.C., 104 (1988) 223. 83. J H Duffus, Cancer and workplace chemicals, (Leeds: H & H Scientific Consultants, 1995). 84. P Gregory, Dyes and Pigments, 7, No 1 (1986) 45. 85. K Hunger, Chimia, 48 (1994) 520. 86. C E Searle, Chem. Brit., 22 (1986) 211. 87. Carcinogenic Substances Regulations 1967 (London: HMSO, 1967). 88. J H Pierce, J.S.D.C., 110 (1994) 131. 89. W Beckmann and U Sewekow, Textil Praxis, 46 (1991) 346. 90. P Cooper, J.S.D.C., 109 (1993) 97. 91. Colour in dyehouse effluent, Ed P Cooper (Bradford: SDC, 1995). 92. Environmental chemistry of dyes and pigments, Eds A Reife and H S Freeman (New York: Wiley-Interscience, 1996) 93. F Moriarty, Chem. Ind., (1986) 737. 94. J Park and J Shore, J.S.D.C., 100 (1984) 383. 95. H R Hitz, W Huber and R H Reed, J.S.D.C., 94 (1978) 71. 96. A J Greaves, D A S Phillips and J A Taylor, J.S.D.C., 115 (1999) 363. 97. K Wührmann, K Mechsner and T Kappeler, Eur. J. Appl. Microbiol. Biotechnol., 9 (1980) 325. 98. V A Shenai, Colourage, 44 (Dec 1997) 41. 99. D Brown and P Laboureur, Chemosphere, 12 (1983) 405.100. D Brown and B Hamburger, Chemosphere, 16 (1987) 1539.101. V G Yadav, Colourage, 45 (Jan 1998) 53.102. P A Turner, J.S.D.C., 111 (1995) 53.103. K T W Alexander, J.S.D.C., 112 (1996) 341.104. B C Burdett, Dyer, 180 (Sep 1995) 16.105. ETAD, J.S.D.C., 111 (1995) 97.106. K Hübner, E Schmelz and V Rossbach, Melliand Textilber., 78 (1997) 720.107. C T Page and J Fennen, J. Soc. Leather Tech. Chem., 82 (1998) 75.108. W B Achwal, Colourage, 44 (May 1997) 29.109. F Planelles et al, J. Soc. Leather Tech. Chem., 82 (1998) 45.110. B M Müller, Rev. Prog. Coloration, 22 (1992) 14.111. W D Kermer, Melliand Textilber., 69 (1988) 586.112. G M P Morrison, G E Batley and T M Florence, Chem. Ind., (1989) 791.113. W D Kermer and I Steenken-Richter, Melliand Textilber., 76 (1995) 433.114. U Sewakow, Text. Chem. Colorist, 28 (1996) 21.115. S I Ali, J.S.D.C., 109 (1993) 13.116. P M Chan, C W M Yuan and K W Yeung, Text. Asia, 29 (May 1998) 59.117. M Ryder, Wool Record, 157 (May 1998) 41.118. Eco-Fab, Colourage, 45 (May 1998) 79.119. K D Casselman, Wool Record, 157 (Oct 1998) 57.120. P M Chan, C W M Yuan and K W Yeung, Text. Asia, 28 (Oct 1997) 58.121. H T Deo and B K Desai, J.S.D.C, 115 (1999) 224122. G Dalby, J.S.D.C., 109 (1993) 9.123. U Sewekow, Melliand Textilber., 76 (1995) 330.124. H Bürger, Melliand Textilber., 76 (1995) 910.125. B Glover, J.S.D.C., 114 (1998) 4.126. V A Shenai, Colourage, 45 (Jan 1998) 19.127. B Glover and J H Pierce, J.S.D.C., 109 (1993) 5.128. B Glover, Text. Chem. Colorist, 27 (Apr 1995) 17.129. W B Achwal, Colourage, 45 (Jan 1998) 45.

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45

CHAPTER 2

Organic and inorganic pigments; solvent dyes

David Patterson

2.1 PIGMENTS

2.1.1 General introduction – what are pigments?

The word ‘colorant’ is defined in dictionaries as ‘a substance used for colouring’ and it cantherefore be used to describe both dyes and pigments. Since this chapter is concerned onlywith pigments it is necessary to find a criterion for deciding whether a given colorant shouldbe treated as a pigment or a dye. In practice this is not as simple as it might seem. Onewidely used criterion is that of the solubility of the colorant in the material which it is beingused to colour. If it is insoluble, it is a pigment; if soluble, it is a dye.

The criterion of solubility is insufficient in itself, however. The same colorant cansometimes be a dye and, in other applications, a pigment. Vat dyes are an example of this.They were originally developed as dyes for textiles, but later were prepared in a differentphysical form for use as pigments. Indeed, it may be argued that since vat dyes are water-insoluble and to get them to diffuse into fibres they must first be reduced, they should not beconsidered as dyes at all, but as pigments. Again, the aqueous solubility of many dispersedyes is so low that various devices have to be used to induce them to diffuse into polyesterfibres. From all of this, it is fair to say that there is no infallible simple criterion for decidingwhether a colorant is a dye or a pigment. It is always necessary to take into account theactual use to which the colorant is being put.

A further, and most important, difference between pigments and dyes is that pigments areused as colorants in the physical form in which they are manufactured. Here, physical formmeans both the crystal structure and the particle size distribution of the pigment. Thephysical form of dyes is becoming of increasing importance as methods of handling thembecome more automated. In most dyeing processes, however, the dyes are first dissolved inwater and their physical form is thereby destroyed. Thus physical form is not generally ofsuch overriding importance as it is in the case of pigments.

Because of the requirements of insolubility in water, in organic solvents and in themedium that it is being used to colour, the application processes for using pigments are quitedifferent from those for dyes. Coloration with pigments is essentially a process of dispersionof solid particles of the pigment in a semi-solid medium.

Entire technologies are involved in each of the three largest applications of pigments,which are as colorants in paints, plastics and printing inks [1]. Each of these media iscomposed mainly of organic constituents and the broad general rule that ‘like dissolves inlike’ gives rise to considerable technical problems in some cases, particularly where entirelycovalent organic pigments are being used. However, since some coloured pigments are

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46 ORGANIC AND INORGANIC PIGMENTS; SOLVENT DYES

inorganic, as are all white pigments, many problems of migration and colour bleeding can besolved by changing from organic to inorganic pigments.

The chemical nature of most organic pigments is closely similar to that of the syntheticdyes that have been discovered during the past 150 years. In fact, with the exception of thephthalocyanines, almost every chemical class of pigments has been developed first fordyestuff use. There are some signs that this may not continue to be the case, with thedevelopment of some new organic pigments for specialised uses.

Finally, passing mention must be made of the two most important organic pigments in ourworld, both natural products. These are chlorophyll and haemoglobin, which are absolutelyvital in the strict meaning of the word, but only chlorophyll has found a commercial use as acolorant in food preparation.

2.1.2 The background history of pigments

The discoveries of archaeologists indicate that the first pigments used by mankind, mainly todecorate both himself and his possessions, were the earth pigments. These are widelydistributed throughout the world, are highly resistant to decomposition by heat, light andweathering, and indeed without these properties would not have survived through thecenturies.

Earth pigments were probably first recognised simply because their colour stood out whenhard lumps of rock were examined. Such rocks were broken up and the desirable colouredbits picked out. The coloured pigments were then ground to a fine powder and blown ontothe painting surface using a hollow tube, or mixed with fatty materials to form a kind ofcrude paint that was applied with the fingers or a reed. The prehistoric cave paintings foundin parts of Spain and France were made in this way.

Examples of such earth pigments are the bright red pigment vermilion (mercurysulphide), the yellow orpiment (arsenic trisulphide), the green malachite (basic coppercarbonate) and the blue lapis lazuli (natural ultramarine). There are many natural sources ofwhite pigments such as chalk and kaolin, while black pigments could be obtained ascharcoal from incompletely burnt wood and as soot in smoke from burning oils. The ColourIndex gives more examples, as do Kearton [2] and Skelton [3].

For many centuries, pigments have been derived from the colouring matters foundoccurring naturally in many plants and even in some animals. Examples are the red pigmentmadder and the blue indigo, both extracted from plants, cochineal and lac lake both frominsects, the much-prized Tyrian purple derived from certain shellfish, logwood extractsranging in colour from red to brown or black, depending on the precipitant, and finally sepiaobtained from cuttlefish [4].

The methods used for making pigments from these and other natural dyes were more likerecipes than scientific procedures and were probably derived from the work of alchemistsand herbalists. The former spent their lives trying to prepare gold by dissolving all kinds ofcheap substances in acids and then re-precipitating them (hence their discovery of manyprecipitants), while the latter sought to extract compounds of medicinal value from plantsand some of their extracts must have included natural dyes.

Among the precipitants employed were tannic acid, tartar emetic, rosin soaps, fatty acid(stearic, oleic) soaps, sulphonated oils (Turkey red oil), ‘earth lakes’ (mixed naturalsilicates), phosphates, casein and arsenious acid. The fastness properties of these pigments

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47

were relatively poor, both to water and to light. Furthermore, the colour of the pigmentsmade by these methods largely depended on the particular precipitant or combination ofprecipitants used, a fact also exploited by medieval dyers to extend the range of colours thatthey could produce from the very limited number of natural dyes available to them. Many ofthe methods for making these pigments [5] are of historical interest only, except for a fewwhich are mentioned later.

2.1.3 More recent developments

The industrial revolution brought about in the 19th century the rise of the chemicalindustry, which is such an important part of the world economy today. The discovery in the1850s of the first synthetic dye by William Perkin and of the diazo reaction by Peter Griessstarted the search for other synthetic colorants, leading eventually to their almost completemarket dominance. Many of what are now the largest international chemical companiesbegan their activities in this area, later expanding into plastics and pharmaceuticals. Forover a hundred years research aimed at synthesising new organic colorants was activelypursued. Patents were filed and new products appeared on the market. Thousands of newcompounds were prepared and tested so that tens of new colorants, generally dyes, could beintroduced commercially. Some of these were also tested for suitability as pigments, but fewhad all the properties required. Only the organic phthalocyanine and the inorganic titaniumdioxide pigments have gone into large-scale production directly as pigments without beingfirst used as dyes.

Certain commercial developments in the last twenty years or so have had importanteffects on the pattern of pigments production. One is the rapid growth of textile productionin the Asia Pacific region and another is the development of automated dyeing methods,coupled with instrumental methods of colour measurement and of computerised colourrecipe prediction.

Colorimetric prediction is equally applicable to pigment applications and this has directedattention to the possibility of rationalising the ranges of dyes and pigments needed. Intheory, it should always be possible to match any shade by mixing three or four suitablecolorants. This is probably one of the reasons for the reduction in the number of pigmentscommercially available. For example, the number of different yellow pigments allocatedColour Index numbers and listed in the 1982 edition of Pigments and Solvent Dyes was 166,but this had fallen to 121 in the 1997 edition. Of these, no less than 26 are inorganic,including 8 new entries. These are mixed oxides of various metals including titanium, zincand vanadium intended for ceramic coloration and are mentioned later.

Other factors, such as the increasing use of pigments in textile printing, to such an extentthat pigments now comprise nearly half of the colorants used for this purpose, means thatthe textile industry is an important market sector for pigments. The geographical shift of thetextile industry has resulted in extensive reorganisation of some of the largest internationalcompanies that in the past have dominated the manufacture of colorants and some long-established commercial names have disappeared as a result.

Within the pigments industry itself, there has been a growing realisation that the physicalstate of pigments is of the greatest importance to pigment users. Most suppliers now offerseveral products based on the same chemical structure and the physical form in which it issupplied is chosen to suit the intended application. Thus, the 1997 edition of Pigments and

PIGMENTS

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Solvent Dyes shows for each product, not only the application but also in which of eightdifferent physical forms it is supplied. These forms are: powder, presscake, granule, chip orflake, liquid dispersion, paste, master batch, flush colour. Not all pigments are offered in allthese physical forms, since for a particular application this may be neither possible nornecessary. To assist users, manufacturers frequently give a separate commercial name toeach form in which they supply the pigment. With the exception of the powder form, thismeans that they will generally contain auxiliaries such as dispersing agents, chosen to assistin the process of incorporating them into the particular medium for which they areintended.

The increase in colour printing of newspapers and periodicals, in colour printing onvarious plastics for the packaging of foods sold in supermarkets, as well as the introductionof colour printers for use with personal computers, have all presented many technicalproblems in formulating coloured inks for these purposes. In Pigments and Solvent Dyes, CIPigment Yellow 1, which has been in commercial production for a century, now has no lessthan 96 entries of commercial products based on it. This illustrates its widespread use inprinting as well as in other applications, but also is a symptom of the tendency to contractthe range of chemically different pigments in production.

Other recent developments, including fluorescent pigments, polymeric pigments and theproblems which have arisen concerning health hazards from the use of benzidine-basedintermediates in making certain pigments, are mentioned later in this chapter.

It should also be mentioned that the structural formulae of the azo pigments discussed inthis chapter now take account of the investigations by Whitaker [6] into the crystalstructures of these pigments. These have shown the importance of hydrogen bondingbetween the pigment molecules in the solid state. The molecular formulae and the chemicalsynthesis stages of the preparation of these pigments remain unchanged.

2.2 DYES CONVERTED INTO PIGMENTS

2.2.1 Dyes converted by precipitation on substrates – ‘lakes’

With the introduction of synthetic dyes, attempts were made to prepare pigments from themby methods based on those that had been used with the natural dyes. Many soluble azo dyescan be rendered insoluble by precipitating them as the salts of heavy metals in the presenceof so-called alumina hydrate. This method will be treated in detail later and here only themaking of pigments from acid dyes and basic dyes will be mentioned. The use of aluminahydrate as a basic substrate for making pigments from dyes is now, however, of diminishingcommercial importance.

Alumina hydrate is made by slowly adding a 10% solution of sodium carbonate to a well-stirred 10% solution of aluminium sulphate, thus producing a white gelatinous precipitate ofaluminium hydroxide. This is colloidal in nature and even prolonged washing does notremove all the sulphate ions adsorbed on it, but by careful control a consistent product witha high absorption power for precipitated dyes can be made. The precipitants used wereantimony, barium and aluminium salts in the presence of rosin. In the USA alone, some 400tons p.a. of methyl violet (CI Basic Violet 3) were precipitated by these methods for use oncopying paper and in typewriter ribbons. The advent of computers with printers and wordprocessing facilities has made such products obsolescent.

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49

Other dyes that were extensively precipitated for use in printing inks were erioglaucine(2.1) and eosine (2.2) but their light fastness is too low to be acceptable nowadays. Theprocess was called ‘laking’ and the products were termed ‘lakes’.

CH2

N

H3CH2C

NaO3S

C N

CH2CH3

CH2

SO3NaSO3

–2.1

+

CI Acid Blue 9

O

Br

HO

Br Br

Br

O

COOH

2.2

CI Acid Red 87

2.2.2 Dyes converted by precipitation with complex acids – ‘toners’

In the early years of the 20th century efforts to improve the properties of pigments preparedfrom basic dyes took a fresh direction: precipitation of the pigments using the mordants thatwere employed when the same substances were being used as dyes. The light fastness of theresulting pigments remained poor, however.

Another approach was much more successful. It had been noted in biochemical workthat some amines could be precipitated by treatment with acid in the presence of sodiumphosphate and sodium tungstate or molybdate. When these precipitants were tried on basicdyes, pigments were produced with a higher light fastness than that of the parent dyes. Thisimportant discovery was patented by the Bayer company in Germany just before the FirstWorld War, but the shortage of tungsten and molybdenum resulting from the demand forthese metals for use in armoured steel during the war delayed the exploitation of thisdiscovery until peace returned.

Between the two world wars it was found that there was no need to isolate the complexacids used as precipitants; they could be used in solution. The use of mixed solutions ofphosphotungstic and phosphomolybdic acids was also studied, as well as the effects ofchanging the pH of the solutions used and of various heat treatments after precipitation onthe colour of the pigments produced.

The Second World War again resulted in a shortage of tungsten and molybdenum and inan effort to overcome this the so-called ‘copper toners’ were made by precipitating basicdyes with copper(II) hexacyanoferrate. The resulting products were less brilliant in colour

DYES CONVERTED INTO PIGMENTS

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than their predecessors and lower in strength and light fastness. Despite being cheaper, theyfound only limited use when supplies of tungsten and molybdenum resumed after the warhad ended.

At that time, manufacturers typically produced ranges of about fifteen pigments of thischemical type, comprising blues, greens, yellows and reds. Some were made by precipitatingmixtures of dyes; for example, blue pigments of increasingly greener hue could be preparedby adding minor but increasing amounts of a yellow dye to a blue dye. Commerciallysignificant differences in technical properties, as well as price, could also be achieved bychanging the ratio of tungsten to molybdenum in the complex acids.

The chemistry of these complex phosphotungstic (PTA), phosphomolybdic (PMA) andmixed (PTMA) acids was not fully understood until X-ray diffraction methods were used todetermine their structures in the early 1950s. This work has shown that the structure ofthese compounds is built up from WO6 octahedra. By sharing two oxygen atoms a W2O10unit (which does not exist independently) can be formed. In the phospho-12-tungstic acidthe sharing process is continued so that there are four W3O9 (hypothetical) units clusteredaround the central PO4 group in the form of a tetrahedron.

The overall structure of the anion is shown in formula 2.3. Since the phosphorus atomhas five positive charges, the overall negative charge on the ion is three, i.e. [P(W3O10)4]3–.The hydrated free acid is thus phospho-12-tungstic acid, i.e. H3[P(W3O10)4].14H2O. Thereare other complex phosphotungstic acids known with different phosphorus:tungsten ratios,but only the 1:12 series has found use in pigment making. Since tungsten and molybdenumare isomorphous in these compounds, the use of any ratio of these two metals is possibletheoretically in preparing the complex acids. In practice, ratios of 85:15 or 60:40 seem tohave been found satisfactory for most purposes. Molybdenum is less expensive than tungstenand thus it might seem advantageous to use lower ratios, but the pigments thus producedtend to darken on exposure to light, an undesirable property. The ratio of dye cations tocomplex anions in the pigments is 4:1. The only examples of pigments of this type currentlymade are CI Pigment Blue 1, Greens 1 and 2, Violets 1, 2 and 3, and Red 81.

O

W

O

OO

W

WO

O

O

O

OO

P OO WW

WO

O

WO

O

O

O

O

O

O

O O

O

WW O

O

O

O

O

O

W

W

O

O O

O

OO

O

O

O

2.3

O

W

Phospho-12-tungstic acid ion

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2.2.3 Vat dyes converted into pigments

Although vat dyes may be regarded as pigments in view of their water insolubility, they werefirst used as dyes. When prepared for dyeing, their particle size is an important technicalfactor that influences their rate of reduction (section 3.1.4). Particle size was found to beeven more important when attempts were made to use vat dyes as pigments. Commercialsuccess was only achieved when methods for further decreasing and controlling the particlesize were found, so that bright and tinctorially strong pigments resulted.

The impetus for these developments came mainly from the need for pigments of highlight fastness in car finishes, particularly when light-coloured cars became fashionable in the1950s. The necessary white pigment (titanium dioxide) was available, but to make thedesired off-white paints small amounts of coloured pigments had to be mixed with thetitanium dioxide. In such formulations many pigments, particularly azo pigments, showmuch diminished light fastness, from ISO ratings of 7 to ratings of 3 or 4.

The light fastness of vat dyes is generally good, so they were tried out for this purpose. Inthe physical form available at the time the dullness and poor tinctorial strength shown bymany vat colours was unacceptable. Work was put in hand to alter the purity, particle sizedistribution and in some instances the crystal modification to make them more suitable forpigmentary use. Vesce [7] of Harmon Colors and Gaertner [8] of Ciba-Geigy have givenaccounts of these developments.

The chemical classes of vat colours from which suitable pigments have been developedare anthraquinones, perinones, perylenes and thioindigos. Many vat dyes have been tested,but only about a dozen have met the stringent fastness standards demanded. Seven of thosefound suitable in all respects are shown in Table 2.1 and their chemical structures are givenas 2.4 to 2.10.

The methods used to convert these vat dyes into a suitable physical form (and in somecases, crystal structure) for use as pigments have been carefully guarded industrial secrets,revealed only in patents. The general principles are clear, however. One method is to reducethe vat dye in the usual manner to bring it into solution and then to re-precipitate it undervery carefully controlled conditions. The other is to subject the dye to a fine grindingoperation. Whichever approach is used, the aim is to reduce the mean particle size to below1 µm (1000 nm).

Table 2.1 Typical pigments prepared from vat dyes

Structure No Common name CI Vat CI Pigment

2.4 Flavanthrone Yellow Yellow 1 Yellow 242.5 Anthanthrone Orange Orange 3 Red 1682.6 Anthrapyrimidine Yellow Yellow 20 Yellow 1082.7 Indanthrone Blue Blue 4 Blue 60a

2.8 Isoviolanthrone Violet Violet 1 Violet 312.9 Perinone Orange Orange 7 Orange 43b

2.10 Perylene Maroon Red 23 Red 179

a α-form onlyb trans form only

DYES CONVERTED INTO PIGMENTS

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Dry grinding can be carried out in two ways. The first comprises dry grinding in thepresence of an inert substance such as salt, which can be removed by aqueous washing whenthe particle size of the dye has been sufficiently decreased. Alternatively, grinding in thepresence of an organic solvent can be used. It is possible that local heating under the intenseshearing conditions in the mill causes the dye particles to pass temporarily into solution. On

N

N

O

O2.4

CI Pigment Yellow 24

O

Br

O

Br

2.5

CI Pigment Red 168

O

NN

N

HO

O

2.6

O


O

O

N

NH

H

O

O

2.7

CI Pigment Blue 60

O

O

2.8

Dichloro derivative of

CI Pigment Violet 31

N

N

O

N

N

O

2.9CI Pigment Orange 43

N N CH3H3C

O

OO

O

2.10

CI Pigment Red 179

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53

release from the local high-temperature zone the solubility limit is exceeded and the dyecomes out of solution at a smaller particle size and in some cases with a different crystalstructure. Sometimes hybrid processes of grinding in the presence of both salt and anorganic solvent have been used. More detailed information has been published about thedetails of the dry grinding method in relation to phthalocyanine and quinacridone pigments,rather than vats, and this will be mentioned later.

The development of effective grinding processes for pigments, where the required meanparticle size is much smaller than the wavelength of visible light (400–650 nm), was delayedbecause visible light microscopy could not be used. The desired size of the particles is belowthe resolving power of these instruments. The increased resolving power of ultravioletmicroscopes was of some help, but still not sufficient for effective monitoring of thecomminution processes. Centrifugal and surface area measurements were also useful, butlaser light scattering, and in particular scanning electron microscopy, finally enabled theprocesses to be fully monitored. Although instruments for carrying out this research areexpensive, together with X-ray diffraction they enabled both the size and shape of theparticles and their crystal structure to be defined. The importance of pigment particles ofsubmicron size is considered later (section 2.11).

2.3 AZO PIGMENTS

Azo pigments comprise by far the largest chemical class of compounds from which pigmentsare made, reflecting the wide range of aromatic amines that can be diazotised and thenumerous compounds to which the resulting diazo compounds can be coupled, not all ofcourse yielding pigments. The hues of the azo pigments range from red, yellow and orange toblue, in descending order of numbers of each hue.

The diazotisation and coupling reactions were exploited by the firm of Read Holliday in1880 with the discovery of Vacanceine red, the first of the ‘ice colours’. Cotton pretreatedwith 2-naphthol was immersed in a solution of diazotised 2-naphthylamine, prepared in thepresence of crushed ice. Para red (2.11), however, patented by the same company in 1887,achieved much more success. This was applied to cotton in the same way using 2-naphtholand diazotised p-nitroaniline. Para red was also manufactured and marketed as a useful azopigment.

N

N

OH

O2N

2.11

CI Pigment Red 1 Para Red

Since the chemistry of the reactions involved in making both azo dyes and azo pigmentsis the same, discussion of these details is confined to Chapter 4. Only the structures of azopigments and the special features of their preparation, largely concerned with preparing thepigments in their optimum physical form appropriate to their end use, are considered here.To do this, a classification of azo pigments is needed, as shown in Figure 2.1.

AZO PIGMENTS

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The primary division separates azo pigments into those that are metal-free and those thatcontain a metal atom in their structure. This means that those in the former group areentirely covalent in nature while those in the latter are not. An entirely covalent moleculepossesses one property that is highly desirable in a pigment: insolubility in water. At thesame time, such molecules are to some extent soluble in the organic media and solvents inwhich these pigments are generally used – paints, printing inks and plastics. The result isthat unmetallised azo pigments have a tendency to diffuse in these media, giving rise tocolour changes and poor fastness properties.

The need to make azo pigments that are metal-containing arises from the requirement toreduce the aqueous solubility of some azo colorants to a sufficiently low level to make themusable as pigments. The problem arises when either the amine or the coupling component(or both) from which the colorant is prepared contain substituents such as hydroxy or acidicgroups. Depending on their number and location in the molecule, these can confer somedegree of aqueous solubility on the resulting azo compound. The means adopted to reducethis solubility to an acceptable level depends on its degree, causing the pigment to fall intoeither group 2a or group 2b of the classification shown in Figure 2.1. All the pigments ingroup 2b are made from azo dyes. Those in group 3, of which there are only very few, aremetal chelates.

2.3.1 Metal-free azo pigments

Pigments in group 1 made from 2-naphthol

Para red (2.11), which has already been mentioned, was the earliest pigment of this type.Toluidine red (CI Pigment Red 3) was first made in 1905. To make it, the amine m-nitro-p-toluidine (called MNPT in the pigments industry) is diazotised and coupled to 2-naphthol,the same coupling component that was used to make Para red.

There is probably more published information about the making of CI Pigment Red 3than any other pigment, since in BIOS report 1661 [9] no less than six detailed works

Azo pigments

Metal-free

Water-insoluble(group 1)

Water-soluble(group 2)

Low solubility(group 2a)

High solubility(group 2b)

Chelates(group 3)

Metal-containing

Figure 2.1 Classification of azo pigments

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55

processes are described. While all these processes yielded pigments with the same chemicalstructure, the hue and other properties of the resulting products differed sufficiently forthem to be sold under different trade names.

Since one of the recurrent problems of the industrial production of azo pigments is tokeep the coloration properties of the various batches constant within very close limits, it isuseful to see the effects of changes in the process variables. They are set out in Table 2.2[10]. In order to see which of these many variables had the greatest effect on the pigmentproperties, Hildreth and Patterson [11] investigated the effects of changing the followingvariables on the colour of the resulting pigments:(a) concentration of coupling component(b) pH of coupling(c) temperature of coupling(d) effect of the use of dispersing agent in the coupling bath.

The effects of the changes in the preparation methods were assessed by measuring thecolour of the pigments in draw-downs and the particle size distributions of the powders. Itwas found that changes in all the listed variables could produce changes of several NBS

Table 2.2 Batch processes for preparation of Toluidine Red (CI Pigment Red 3)

Helio Lithol Fast Lithol Fast Lithol Fast HansaRed RBL Scarlet GRN Scarlet RB Scarlet RN Red B

1. Diazotisationm-nitro-p-toluidine suspn. (mol/l) 1 1.25 3.3 1.25 1.25Temperature (°C) 2–5 0 0 0 0Diazo conc. (mol/l) 0.59 0.71 1.1 0.71 0.71Carbon addition No No Yes No No

2. Coupling componentConc. of soln. of 2-naphthol in caustic soda (mol/l) 0.36 0.25 0.35 0.252-naphthol, whether precipitated Yes (HCl) No Yes (HCl) No Yes (HCl)2-naphthol conc. at 0.21 0.061 0.245 0.082 coupling (mol/l) suspn. soln. suspn. soln. suspn.pH at coupling 5–8 10 8 10 –

3. Coupling conditionsTemperature (°C) 40 10 25 25 36Method of adding below above above above below diazo component surface surface surface surface surfaceTime taken (hours) 2 2.5 1 2.5 3

4. Subsequent treatmentHeating after coupling (°C) 50 none 90 95 (30 min) 70 (2.5 hr)Washing water (2 hr) water none water water

Drying temp (°C) 50–55 50–55 (paste made) 45 50–55

AZO PIGMENTS

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units of colour difference (one unit being about the commercial limit of acceptabilitybetween pigment batches). The tinctorially strong pigments were those with the largestproportion of particles below 500 nm in size and the largest surface area. The need for thestrictest possible control of process variables to reduce batch-to-batch variability was clearlyshown, but the variables needing the closest control were not the same for all the pigmentsinvestigated.

Both Para red and CI Pigment Red 3 are prepared using 2-naphthol as the couplingcomponent. Other pigments made from the same coupling component, all discovered before1910, are compounds 2.12 to 2.14 (Table 2.3). All these pigments, except CI PigmentOrange 5, are made by direct diazotisation of the appropriate amine using sodium nitrite andhydrochloric acid at temperatures around 0 °C, then adding the diazotised amine (the‘diazo’) to an alkaline solution of the coupling component. There may be problems incarrying out diazotisation because of the hydrophobic nature of many aromatic amines. Twomethods of overcoming this are frequently used. Either the hydrochloride of the amine isfirst formed, by stirring it with concentrated hydrochloric acid, or the particle size of theamine is reduced by dissolving it in concentrated sulphuric acid and then re-precipitating.Both methods aim to increase the rate of diazotisation, which tends to be slow on account ofthe low temperature and the small surface area for chemical attack, particularly if the amineparticles have aggregated because of unfavourable storage conditions or other reasons.

Another technical problem in diazotising these hydrophobic amines is how to decidewhen diazotisation is complete, especially when the diazo component is sparingly soluble

Table 2.3 Typical group 1 azo pigments made from 2-naphthol

Structure No Commercial Name CI Pigment

2.12 Fire Red Red 42.13 Red Toner Red 62.14 Permatone Orange Orange 5

N

H

Cl

N

O2N

O

2.12

CI Pigment Red 4

N

H

NO2

N

Cl

O

2.13

CI Pigment Red 6N

H

NO2

N

O2N

O

2.14

CI Pigment Orange 5

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57

and therefore appears in the form of a suspension. It is essential that the amine should becompletely diazotised, because any unreacted amine will remain in the precipitated pigmentwith adverse effects on the colour and fastness properties. Diazotisation is complete if a testshows that free nitrous acid is present after sufficient reaction time has elapsed, as indicatedby experience. A further test is carried out a few minutes later to confirm that a small excessstill remains. The presence of nitrous acid produces an immediate and intense dark bluecolour on starch-iodide paper when the diazo solution is spotted onto it. If this colour is notobtained the cause may be lack of acid or of nitrite. The amount of acid is increased if a testwith pH paper shows insufficient to be present. Retesting with starch-iodide paper will thenindicate whether more sodium nitrite is needed or not.

The amine that has to be diazotised to make CI Pigment Orange 5, namely 2,4-dinitroaniline, cannot be diazotised by the usual direct method. Nitrosylsulphuric acid(HSO4.NO) is needed and this is so corrosive that lead-lined vessels must be used forhandling it. Even though this pigment has an unusually high light fastness in reduction (5–6at 1/25 International Standard Depth, which is two units higher than most of the others inthis group), the high costs of its production have led to a decline in its use.

Pigments in group 1 made from acetoacetanilides

In the early 1900s several yellow pigments were made by coupling substituted nitroanilinesto acetoacetanilides. They are commonly known as the Hansa Yellows (2.15–2.18); thosethat are greener in hue are identified by the increasing number before the ‘G’ in the suffix totheir names (Table 2.4). The G (CI Pigment Yellow 1) and 10G (CI Pigment Yellow 3)pigments are the most widely used, the latter being much greener than the former buthaving only about one-third the tinctorial strength. The light fastness of all these pigmentsis 7 in full strength but falls to 4 in 1/25 International Standard Depth, with the exception ofCI Pigment Yellow 3 which is rated 6+. This high light fastness in reductions makes CIPigment Yellow 3, in the words of one of its many manufacturers: ‘an inexpensive pigmentwith good to very good light fastness’. It is available in many physical forms and findsextensive use in paints and printing inks.

Disazo pigments made from substituted benzidines

Benzidine is an aromatic diamine that can be tetrazotised and coupled to two molecules of acoupler such as acetoacetanilide. This gives a yellow disazo pigment that can be regarded asa double molecule of the monoazo pigments described above. The possibilities of making a

Table 2.4 Typical group 1 azo pigments made fromacetoacetanilides

Structure No Commercial Name CI Pigment

2.15 Hansa Yellow G Yellow 12.16 Hansa Yellow 3G Yellow 62.17 Hansa Yellow 5G Yellow 52.18 Hansa Yellow 10G Yellow 3

AZO PIGMENTS

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range of colorants were well researched and both dyes and pigments of this general structurewere produced. Of the commercial pigments, most were made from 3,3′-dichlorobenzidineor 3,3′-dimethoxybenzidine (o-dianisidine) but this is not the case with some of the disazodyes.

The hues of these yellow disazo pigments are generally redder than those of thecorresponding monoazo pigments made from the same coupling components, but their mainadvantage is that their tinctorial strength is three or four times greater. This is particularlyuseful in printing ink applications, where the flow properties of the inks depend on theloading of pigments in the formulations.

Disazo yellows have another advantage over monoazo yellows in that they show lessblooming in plastics. Blooming is a tendency of pigment molecules to migrate to the surfaceof the plastic articles that they are used to colour. It can occur in moulding processes andalso more slowly when in use. In either case it leads to colour loss and sometimes tocontamination of articles coming into contact with the surface. The reason for the betterperformance of the disazo pigments is that their doubled molecular size leads to slowerdiffusion in plastics, but the matter is complicated by the presence of organic plasticisers andother additives in some types of plastics. On the other hand, the light fastness of the disazoyellow pigments is usually lower than that of the monoazo types.

The most widely used pigment of this type is CI Pigment Yellow 12 (2.19), made by directtetrazotisation of 3,3′-dichlorobenzidine as the hydrochloride, followed by coupling of onemole of the product to two moles of acetoacetanilide. During the tetrazatisation reaction thegreen solution becomes yellowish brown. The analogous structure CI Pigment Yellow 13(2.20) is made by tetrazotising 3,3,’-dichlorobenzidine and coupling to acetoacet-m-xylidide.It has found wide use in printing inks, including water-based formulations, and opaque andtransparent types are available.

NO2

H

N

O

N

N

H

O

H3C

H3C

2.15

CI Pigment Yellow 1

NO2

H

N

O

N

N

H

O

H3C

Cl

2.16

CI Pigment Yellow 6

NO2

H

N

O

N

N

H

O

H3C

2.17

CI Pigment Yellow 5

NO2

H

N

O

N

N

H

O

H3C

Cl Cl

2.18

CI Pigment Yellow 3

chpt2(1).pmd 15/11/02, 14:4258

59

The influence of mixed coupling on the properties of CI Pigment Yellow 12 has beenstudied recently [12]. Carboxy- or sulpho- substituted derivatives of acetoacetanilide wereevaluated as co-coupling components and analysis revealed that the state of the crystal andthe particle size were changed and new diffraction peaks were observed. When thesemodified pigments were treated with a fatty amine such as stearylamine, the hydrocarbonchains enclosed the anionic groups in the co-coupler so that properties such as flowability,wettability and dispersibility in nonpolar solvents were greatly improved.

The undoubted technical merits of these pigments have been overshadowed, however, bythe fact that they are simple derivatives of benzidine and therefore possibly carcinogenic. In1971 the major European companies voluntarily ceased manufacture of dyes based onbenzidine. Some of the subsequent results were unforeseen. As part of their consumerprotection laws, the German government in 1994 framed some regulations in which thewording was not very precise, in particular on the point about whether the regulations applyto pigments as well as to dyes. There have been some amendments to clear up this point anda 1996 amendment excluded poorly soluble pigments with a relative molecular mass greaterthan 700. There is now a list (Table 4.1) of 22 different amines (including the diamine usedfor the preparation of CI Pigment Yellows 12 and 13). If a colorant, on reduction by astandard test method, can be shown as a possible source of any of these amines, then its useis prohibited. As with other apparently simple applications of scientific testing to legalmatters (such as vehicle driving with a blood alcohol value above the legal limit) thesensitivity of the test method used and the test errors can give rise to problems. The wholematter is by no means finally settled, but remains important because Germany is both withinthe European Union and an important market for colorant producers throughout the world.The above restrictions on the usage of colorants yielding the banned amines apply equally toimported supplies.

N

O

H

O

N

N

H

Cl

CH3

Cl

N

H O

N

N

H

O

H3C

2.19


N N

N H

H N

CH3

O

N

H

H3C

H3C

O

Cl

N

H

O

H3C

O

CH3

CH3

Cl

2.20


AZO PIGMENTS

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Pigments made from derivatives of 3-hydroxy-2-naphthoic acid (BON acid)

In about 1910, problems were encountered when pigments made using BON acid as thecoupling component were reduced with alumina hydrate to make printing inks. Theattractive bluish red of the pigments turned brown, according to Zitscher [13]. The difficultywas overcome by using anilides of BON acid instead of the acid itself. Several new pigmentswere introduced, the so-called Permanent Reds, only one of which, CI Pigment Red 2(2.21), is currently made. The anilide of BON acid was marketed in 1912 as Naphtol AS (CIAzoic Coupling Component 2) for use in azoic dyeing and printing.

In the 1920s the hue range of pigments with this type of chemical structure was furtherextended by using other Naphtols as coupling components. These pigments have a lightfastness of about 7 in full strength. Being rather complex hydrophobic structures they tendto have poor solvent fastness properties; this is a considerable disadvantage in theformulation of paints for vehicles, which are applied by spraying methods and are thinnedwith solvents. Two pigments discovered at this time and still in production are CI PigmentBrown 1 (2.22) and the blue disazo pigment CI Pigment Blue 25 (2.23), made by couplingtetrazotised o-dianisidine to Naphtol AS.

N

H

Cl

N

O

NH

O

Cl

2.21

CI Pigment Red 2

N

H

Cl

N

O

NH

O

Cl

2.22

CI Pigment Brown 1

H3CO OCH3

N

H

O

N

N

HO

H3CO

OCH3

H

N

N

O N

O

H

2.23

CI Pigment Blue 25

Azo condensation pigments

Although metal-free azo pigments have good chemical resistance their completely covalentnature leads to a tendency to bleed in organic solvents and to migration and blooming in

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plastic formulations. It was found that the problems of migration and blooming decreasedwhen the overall size of the molecule was increased. Attention turned to the possibility ofdoubling the size of the pigment molecules by first condensing one mole of benzidine withtwo moles of BON acid to form a dinaphthol and then coupling this to two moles ofdiazotised aromatic amine, as shown in scheme 2.1. Difficulties in the synthesis were soonencountered, because after one molecule of diazotised amine has coupled the solubility ofthe product in water is so low that it comes out of solution and does not couple with thesecond molecule of diazotised amine.

H2N NH2COOHHO HOOC OH

OHHNOCHO CONH

OHHNOCHO CONH

NN N N

N N+

dinaphthol

1. condense

++

2. coupleto two molesof diazotisedamine

2

Scheme 2.1

The solution to this difficulty, discovered by Schmid of Ciba in the early 1950s, is toreverse the order of the two steps. The diazotised amine is first coupled with BON acid andthen two moles of this intermediate are condensed with one mole of benzidine in the form ofits hydrochloride. The condensation is carried out in a high-boiling solvent such asnitrobenzene, when yields of 95% can be attained. These pigments have been called ‘azocondensation pigments’ in view of their method of synthesis.

AZO PIGMENTS

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Gaertner [8] has given some interesting insights into the problems of developing newproducts in a field of organic synthesis that has been the subject of well-funded andintensive research for nearly one hundred years. He stated that the Ciba company had usedtheir new method to prepare well over 10 000 speculative samples. Those selected fordevelopment were given the commercial name of Cromophtals and initially eleven newpigments were marketed. Not all pigments now termed Cromophtals are of this chemicaltype, however, and allowing for this there are now about ten azo condensation pigmentsbeing sold. This illustrates the large expense involved in introducing new technical productsand the difficulty of finding a place for them in a very competitive market, particularly astheir initial selling price must be fixed at a level high enough to recoup their developmentcosts. New pigments made from intermediates that are already in large-scale productionseem to have the best chance of competing with existing products.

The small range of azo condensation pigments has become established only because theycombine outstanding fastness to light, heat, solvents and chemicals, finding acceptance foruse in paints, printing inks and the coloration of plastics and some synthetic fibres.

Benzimidazolone pigments

A further solution to the problem of reducing the reactivity of the carboxyl group in BONacid, when this compound is used as a coupling component in the preparation of azopigments, was found in 1964 when the Hoechst company introduced their benzimidazolonepigments. A series of new Naphtol AS derivatives was made by condensing 5-aminobenzimidazolone (2.24) with BON acid. Other new coupling components have beenmade by condensing this compound with acetoacetanilides, which are used as couplingcomponents for the Hansa Yellow pigments mentioned earlier.

NH

CO

HN

H2N

2.24

5-Aminobenzimidazolone

Two examples of benzimidazolone pigments are CI Pigment Yellow 120 (2.25) made bydiazotising dimethyl 5-aminoisophthalate and coupling with N-(2-oxobenzimidazol-5-yl)acetoacetamide and CI Pigment Brown 25 (2.26) made from diazotised 2,5-dichloroanilineand 2-hydroxy-3-N-(2′-oxobenzimidazol-5′-yl)naphthalamide. Benzimidazolone pigments havegiven improved migration fastness and lower bleed in paint and PVC formulations, possiblybecause of hydrogen bonding between the carbonamide groups in adjacent molecules in thecrystal lattice of such compounds.

Pigments made from pyrazolones

The last group of metal-free azo pigments are the pyrazolone compounds, the mostcommonly used examples being made from the coupling component 1-phenyl-3-methylpyrazol-5-one, also used in making azo dyes. The two most important pigments are

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63

the reddish yellow monoazo Hansa Yellow R (2.27; CI Pigment Yellow 10), in which theamine diazotised is 2,5-dichloroaniline, and the disazo pigment CI Pigment Orange 13(2.28), made from 3,3′-dichlorobenzidine. The latter pigment comes into the same categoryas the other disazo pigments mentioned earlier, as regards precautions in its use. In makingboth of these pigments the coupling component must be dissolved in concentrated causticsoda solution before diluting it prior to the coupling stage itself.

N

H O

N

N

H

O

N

N

H3COOC

H3COOC

H

H

O

H3C

2.25


NH

OH

N

N

Cl

Cl

N

N

H

H O

O

2.26

CI Pigment Brown 25

N

H

Cl

NN

N

O

Cl

H3C

2.27


N

Cl

N

N

CH3

O

N

N

H

H

N

N

N

O

CH3

Cl

2.28

CI Pigment Orange 13

2.3.2 Metal-containing azo pigments

As already mentioned, certain azo colorants have low aqueous solubility but not sufficientlylow for them to be usable as pigments. These azo colorants are prepared from amines andcoupling components containing hydroxyl or carboxyl groups in their sodium salt form as aresult of alkaline coupling conditions.

AZO PIGMENTS

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In many instances the aqueous solubility can be reduced to acceptably low levels byreplacing sodium with a heavier metal, most commonly calcium, barium or manganese.These colorants fall into group 2a of the classification of azo pigments shown in Figure 2.1.When the solubility of the sodium-containing azo colorants is higher, so that they can beused and regarded as azo dyes, it may be possible to form heavy-metal salts only in thepresence of basic substrates, usually alumina hydrate. These pigments consequently fall intogroup 2b of the classification. Whether a particular pigment falls into group 2a or 2bdepends on the method used to make it in the form of these heavy-metal salts. The hues ofthe products formed using various heavy-metal precipitants are often so different that theyare sold as separate commercial brands.

Pigments in group 2a

The first pigments in this group appeared about 1900 and were termed the Lithols. LitholRed (2.29; CI Pigment Red 49) is made by diazotising Tobias acid (2-naphthylamine-1-sulphonic acid) and coupling it to 2-naphthol. Diazotised Tobias acid is zwitterionic andtherefore sparingly soluble, forming a dispersion rather than a solution. This gives rise todifficulties in deciding when the diazotisation reaction is completed, which can be overcomeby the method described in section 2.3.1. The product of coupling is the sodium salt, whichis yellowish red with a solubility just low enough for it to be used as a pigment in printinginks. The pigment found wider use when converted to its barium and calcium salts, whichare of a bluer hue.

O

HN

N

SO3Na

2.29

CI Pigment Red 49

Lake Red C (2.30; CI Pigment Red 53), made by diazotising CLT acid (5-amino-2-chlorotoluene-4-sulphonic acid) and coupling to 2-naphthol, is a pigment that has alsofound considerable use in printing inks. The sodium salt gives prints with a strong bronzesurface finish, which seems to depend on the presence of three moles of water ofcrystallisation to each mole of pigment. If these are driven off by strong heating the bronzingeffect is lost and cannot be regained. The barium salt found wider pigment applications.

Tobias acid diazotised and then coupled to BON acid has been used as the manganesesalt (CI Pigment Red 63) to give a bordeaux shade, but two other pigments (2.31; CIPigment Red 48, and 2.32; CI Pigment Red 57) made from the same coupling componentare of much greater current importance. The first is made from 2B acid (4-amino-2-

chpt2(1).pmd 15/11/02, 14:4264

65

chlorotoluene-5-sulphonic acid) and the second from 4B acid (4-aminotoluene-3-sulphonicacid). They are called 2B and 4B toners, as might be expected, whilst compound 2.32 is alsonamed Lithol Rubine. The 2B toner is converted into strontium, barium, calcium andmanganese salts, whilst the 4B toner is used as its calcium and barium salts. The principaluse for both pigments was originally in printing inks, particularly in oil-based types.Nowadays, however, numerous specially prepared commercial brands are available with theirphysical properties adjusted for use in plastics, various types of paints and even, in the caseof CI Pigment Red 57, as colours for cosmetics and pharmaceuticals. The specifications ofthese products, particularly as regards the absence of heavy metals, is exceptionallystringent. Specially designed plant, used only for pigment intended for these purposes, isneeded.

Rosination

Long before such specialised uses of pigments in group 2a were considered, the process ofrosination played an important part in their manufacture. In the preparation of all thesepigments in the form of their heavy-metal salts, the method of metal exchange is to heat thesodium salt with a solution of the chloride of the required heavy metal. Outstandingtechnical advantages result if this metal exchange is carried out in the presence of rosinsoap. The resulting pigments are more easily dispersed, of greater tinctorial strength andhigher brilliance, imparting a glossier appearance to printing inks. Rosination is therefore amost important part of pigment technology.

Rosin is a natural product derived, along with turpentine, from the resin collected fromcertain American pine trees. It is graded by colour and the grade WW (water white) is usedin the pigments industry. The acidic components originally contain about 40% L- and D-pimaric acid but during processing these are converted to abietic acid (2.33). Neoabietic,dehydroabietic and dihydroabietic acids are also present, according to Harris [14]. Rosinsoap is made by dissolving the rosin in a hot solution of sodium hydroxide maintained at aslow a temperature as possible consistent with keeping the soap in solution – it is almostinsoluble in cold water. The hot solution, containing also the dissolved heavy metal chloride,is added to a dispersion of the sodium salt of the pigment. The whole is brought to the boiland, at a temperature between 80 and 90 °C, a marked colour change indicates that metal

O

HN

N

NaO3S

CH3

Cl

2.30

CI Pigment Red 53

O

HN

N

2.31

NaO3S

COONa

Cl

CH3

CI Pigment Red 48

O

HN

N

COONa

2.32

SO3Na

CH3

CI Pigment Red 57

AZO PIGMENTS

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exchange has occurred. The amount of rosin soap used comprises some 15–30% of the massof the final pigment.

Rosin acts as a non-diluting substrate and has the effect of increasing the proportion ofparticles in the pigment with a size below 250 nm, but there is no evidence of chemicalcombination. Attempts to fractionate rosin to identify any specially active constituent havebeen unsuccessful, as have attempts to find better general dispersing agents, althoughvarious manufacturers have produced ranges of easily dispersible pigments containing otheradditives. Preparations of crude CI Pigment Yellow 13 (2.20), the recrystallised material andthe rosin-coated pigment were compared recently using solid state NMR spectroscopy andpowder X-ray diffraction. The primary effect of rosination with either abietic acid or itsderivative staybelite was to increase the crystalline order of the pigment [15].

Pigments in group 2b prepared from azo dyes

Many azo dyes prepared as sodium salts can be made into pigments by precipitating them asthe salts of heavy metals, but only in the presence of basic substrates. The most commonlyused substrate is alumina hydrate, as mentioned in section 2.2.1. The pigment is prepared byrunning solutions of the dye and a salt of the precipitating metal, generally barium oraluminium, into a suspension of alumina hydrate. An intimate mixture of the precipitatedheavy-metal salt and alumina hydrate is formed and filtered off. The colloid chemistry ofsuch systems, along with the technical and colour properties of the resulting pigments, iscomplex and depends on the dye: alumina hydrate ratio and the pH of the mixture. Weiserand Porter [16] studied the preparation of pigments from the dye Orange II (2.34; CI AcidOrange 7).

Many other dyes, including tartrazines and eosines, have been made into pigments in thepast for use in printing inks, in which their soft texture and bright hues were much valued.Their light fastness was generally only 1 on the ISO scale and they are now obsolete.

CH

H3C

COOHH3C

CH3

CH3

2.33

Abietic acid

HO3S

N

O

2.34

N

H

CI Acid Orange 7

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67

2.4 PHTHALOCYANINE PIGMENTS

Phthalocyanine was the first new chromogenic type to be introduced into the field of thecolour chemistry of organic compounds concerned with pigments. Before this discovery, allthe known organic pigments had been developed either by making dyes insoluble or bysynthesising new insoluble azo compounds. This development is therefore interesting bothtechnically and scientifically.

It began in 1928 when a blue impurity was found in phthalimide during its manufactureat Scottish Dyes (later part of ICI). The method of preparation being used was to passammonia through molten phthalic anhydride in an iron vessel [17]. It is now known thatthe blue impurity was iron phthalocyanine. This was found to be exceptionally stable andsteps were taken to determine its structure. It is of interest that another important bluepigment (ultramarine; section 2.10.2) was also first synthesised as the result of investigatinga blue impurity. This was in an industry not directly related to colour making, namely,sodium carbonate manufacture at St Gobain in France.

With hindsight it can be guessed that other ways of solving the impurity problem inmaking phthalimide, such as the use of a ceramic vessel, might well have solved the initialdifficulty but would have resulted in missing the discovery of what has turned out to be amost important chromogen. Two other preparations of phthalocyanine had already beenreported in the chemical literature but did not lead to the recognition of its usefulness, thefirst in 1907 [18] and the second in 1927 [19]. Linstead and his co-workers investigated thechemical structure of the blue impurity and its novel structure was finally established byRobertson [20] using X-ray diffraction. Copper phthalocyanine (2.35; CI Pigment Blue 15)first appeared commercially in 1935 under the ICI trade name of Monastral Blue B.

N

N

NN

N

N

NN

Cu

2.35

3

4 5

6

Copper phthalocyanine

2.4.1 Manufacture of copper phthalocyanine [21,22]

Two processes have been used for making copper phthalocyanine, but neither yields a formsuitable for immediate use as a pigment since the physical form and size distribution of theresulting particles are far from the optimum. The product is generally referred to as ‘crudeblue’ and requires purification and a sometimes complex series of finishing processes beforeit is in a form ready for use as a pigment.

PHTHALOCYANINE PIGMENTS

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The solvent process

The reactants are phthalic anhydride, urea and copper(II) chloride, which are heated in ahigh-boiling aromatic solvent such as 1,2,4-trichlorobenzene, nitrobenzene or m-dinitrobenzene in the presence of a catalyst, usually ammonium molybdate. The solvent alsoacts as a heat-transfer medium. On heating to 120 °C an exothermic reaction begins andthis temperature is maintained for about an hour. The temperature is then raised to 160–180 °C and kept constant for 6–12 hours. During this time ammonia and carbon dioxide areevolved, together with some solvent; the reaction is complete when ammonia evolutionceases. The remaining solvent is then removed by either steam or vacuum distillation. Theyield is 90–95%. For many years the solvent process was in almost exclusive use.

Dry bake method

As the name implies, no solvents are used in this process and the expensive solvent recoverystage is not needed. Instead of phthalic anhydride, the more expensive phthalonitrile is usedas reactant. Urea is not necessary as a source of nitrogen but seems to have been used bysome manufacturers. The copper needed comes from copper(I) chloride. Continuousprocessing on moving heated copper belts has been attempted, but there were mechanicaldifficulties in handling the reactants and products after hard solid masses had formed. Thisproblem was only partly solved by adding neutral inorganic diluents such as sodium sulphateor common salt to improve the fluidity. Interest in the dry bake process was renewed whenpolychlorobiphenyls (PCBs) were recognised as carcinogens and detected in crude bluesproduced by the solvent process.

Carcinogenic by-products

The mechanism of formation of PCBs from the trichlorobenzene solvent involves hydrogenabstraction by the urea present, leading to the formation of trichlorophenyl radicals. Twosuch radicals can then combine (Scheme 2.2) to form a PCB molecule (2.36). This reactioncan be suppressed by adding antioxidants such as hydroquinone or sodium phosphite to yieldhydrogen (H•) radicals, which convert (Scheme 2.3) the trichlorophenyl radicals back tothe parent substance (2.37).

ClCl

Cl Cl

ClCl

Cl

Cl Cl Cl Cl

Cl

+

2.36Scheme 2.2

ClCl

Cl Cl

Cl Cl

H

2.37

+

Scheme 2.3

chpt2(1).pmd 15/11/02, 14:4268

69

Other high-boiling solvents such as nitrobenzene and 2-nitrotoluene can also be used inthe solvent process, but 4-nitrobiphenyl, a listed carcinogen, is produced when nitrobenzeneis employed. One way to avoid the formation of substituted biphenyls is to employ apolysubstituted toluene as solvent, the substituents being chlorine or alkyl groups, mainlypropyl. These are difficult to prepare but they have been patented by Toyo Inks.

Another approach is to use high-boiling aliphatic solvents with minimal aromatic content;boiling points are in the range 150–250 °C. Much higher ratios of solvent to reactants areneeded than when aromatic solvents are used, owing to the high viscosity of the system.

Yet another method of avoiding PCB formation in the solvent process is to dispense withthe need for urea as a reactant by using the more expensive phthalonitrile instead ofphthalic anhydride.

Isolation of crude blue

No method of synthesis produces crude blue in pigmentary form. The solvent (if any) andthe impurities must first be removed. Vacuum or steam distillation is used to remove thesolvent, the former being generally cheaper. Impurities can then be removed by thoroughwashing for about two hours at 90–95 °C, first with 10% aqueous sodium hydroxide solutionand then with 10% aqueous hydrochloric acid, followed by filtration, pressing, water washingand drying. From the purified crude blue, the copper phthalocyanine blue pigment is isolatedin either the α- or β-crystal form. By introducing halogeno substituents into the molecule aseries of green pigments can be derived (section 2.4.2).

Conversion of crude blue into copper phthalocyanine blue pigments

Purified crude blue exists in the most stable crystal form of copper phthalocyanine, the β-form,but the particles are much too large to make a good pigment. The β-form was that prepared byRobertson [20], from which he proved the molecular formula as mentioned earlier. Threeother crystal forms of copper phthalocyanine have been reported but only one of them, the α-form, is of interest in the pigment industry. Mixtures of α- and β-forms have caused varioustechnical problems because the former is reddish blue and the latter is greenish blue. The α-form has a tendency to revert to the more stable β-form in the presence of the organic solventsthat are constituents of many paint formulations, with a consequent shade change andsometimes a loss of colour strength. Hence there is a need to prepare solvent-stable α-formpigments. Since β-form crystals also tend to grow and lose strength in the presence of organicsolvents, there is a need to render them solvent-stable too.

Methods for making both forms solvent-soluble were the subject of many patents andclosely guarded industrial secrets, but much of the mystery was cleared up in two papers byGerstner [23] and Smith and Easton [24] published in 1966, by which time X-raydiffraction, electron microscopy and disc centrifuge particle sizing methods had beenbrought to bear on the problem.

Converting the β-form to the α-form

The β-form of crude blue can be converted to the α-form by complete disintegration of thecrystals. This can be readily achieved by dissolving them in concentrated sulphuric acid,

PHTHALOCYANINE PIGMENTS

chpt2(1).pmd 15/11/02, 14:4269


thus forming a solution of single molecules, and then re-precipitating as α-form crystals byrunning the solution into a large volume of water. Careful control of the flow rate, coupledwith stirring and temperature control (since heat is evolved in the process) and perhaps thepresence of surfactants, can give tinctorially strong α-form pigments. Other methods,termed ‘acid slurrying’ or ‘permutoid swelling’ involve the use of insufficient sulphuric acidto bring about complete solution. Copper phthalocyanine sulphate crystals are formed; theseare then drowned in water as before, forming α-form crystals. Yet a third method ofconversion is by prolonged salt grinding in a ball mill in the absence of any solvent. The α-form so produced, however, is not solvent-stable.

Solvent-stable forms

Smith and Easton [24] give the scientific basis of many of the patent claims. Salt grinding ofeither α- or β-form and treatment with solvent either during or after grinding gives the β-form. In the absence of solvent the product is the α-form, irrespective of the startingmaterial. However, mixtures of α- and β-forms can result from salt grinding in the presenceof solvent, depending on the grinding efficiency and the time of grinding in the mill. Twostages, first grinding and then solvent treatment, are often easier to control. For example, ifthe grinding process in the absence of solvent is stopped when about one-third of thematerial remains as the β-form, exposure to solvent will then cause a rapid reversion of theα-form back to the β-form, giving solvent-stable β-form. Solvent-stable α-form pigment canbe made by incorporating some copper monochlorophthalocyanine in the pigment or bycarrying out the salt grinding in the presence of compounds that can be adsorbed on thesurface of the growing α-form crystals, hindering further growth and stabilising them againstreversion to the β-form.

2.4.2 Other phthalocyanine derivatives

The discovery of the phthalocyanine chromogen gave rise to much research, particularly inview of its outstanding fastness properties. Many other metallic derivatives have beenprepared and many substituents introduced into the phthalocyanine nucleus. Sulphonationhas given some soluble dyes, which have even been converted back into pigments by makingthem insoluble again as barium salts. The only widely produced colorants are a range ofgreen pigments made by halogenating copper phthalocyanine using chlorine either alone ormixed with bromine.

There are 16 hydrogen atoms on the benzenoid rings of the phthalocyanine molecule(2.35) that can in theory be replaced by halogens. Those in the eight 3,6-positions can bereplaced easily whereas more severe conditions are needed for substitution in the 4,5-positions. The products of different manufacturers vary in the chlorine content, 14 or 15atoms per molecule being usual in pigments under the generic name CI Pigment Green 7.As the number of chlorine atoms increases, the green pigments become increasingly yellowin hue. This trend can be carried still further by using mixtures of chlorine and brominecontaining increasing amounts of bromine. The yellowest pigments have twelve bromineand two chlorine atoms per molecule and carry the generic name CI Pigment Green 36.

Direct chlorination of copper phthalocyanine can be carried out by passing the gas into amolten eutectic of aluminium chloride and sodium chloride at a temperature of 180–200 °C,

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the copper phthalocyanine content being about 20% of the eutectic by mass. Iron(III)chloride is present as a catalyst [25]. Other methods include:(a) chlorination in trichlorobenzene at 200 °C using 10% antimony trisulphide as a

catalyst;(b) chlorination under a pressure of 5–10 atmospheres at a lower temperature of 160–

170 °C, a process in which the release of hydrogen chloride gas is used to control thepressure; and

(c) dry chlorination in a fluidised bed.

C

HN

NH

C

O

O

2.38


QUINACRIDONE PIGMENTS

Yet other processes have used sulphuryl chloride (SO2Cl2), thionyl chloride (SOCl2) orsulphur dichloride (SCl2) as chlorinating agents.

The crude polychlorinated copper phthalocyanine requires a finishing process to developthe full potential colour strength, but the problems are less than those experienced withcopper phthalocyanine itself, as no problems of polymorphism occur. Finishing is carried outby acid pasting of the crude green in oleum or chlorosulphonic acid. The crystallinity, asshown by the sharpness of the X-ray diffraction pattern, can be improved by solventtreatment as there is no tendency for this to cause crystal growth. The light fastness rating(ISO 8) and solvent resistance (generally 5) are both as high as those of phthalocyanineblues, making them exceptionally high-quality pigments.

A great deal of research has been carried out to try to find organic pigments of red colourto match phthalocyanines in these two fastness requirements. The many red azo pigmentsshow much inferior light fastness in reductions with titanium dioxide. Pigments made fromvat dyes, mentioned earlier, have met the need to some extent, but there is still a gap in theorganic colour pigment range for brilliant reds of the highest fastness properties.

2.5 QUINACRIDONE PIGMENTS

The synthesis of linear trans-quinacridone (2.38) was reported in 1935 by Liebermann [26]and was cursorily looked at as a red vat colorant but not developed commercially. It wasmore than twenty years later that the DuPont company introduced these compounds aspigments under the trade name of Cinquasia. Their chemical structures are based on CIPigment Violet 19 (2.38). As with the phthalocyanines, this compound can exist in severalpolymorphic forms; in this case there are three, termed α-, β- and γ- forms; only the last twobeing useful as pigments. The first three pigments were called Cinquasia Red B (γ-form, sizeabove 1000 nm), Cinquasia Red Y (γ-form, size below 1000 nm) and Cinquasia Violet R (β-form).

Quite apart from the problems of chemical synthesis, it is clear that the application ofphysico-chemical methods to pigment manufacture, first needed in the development of the

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phthalocyanines, was essential to the successful introduction of these new pigments. X-raydiffraction methods are needed to recognise the crystal forms. The β-form was made fromthe crude product as synthesised by milling with a large amount of salt in the presence ofxylene, whilst preparation of the γ-form required wet milling with dimethylformamide,followed by removal of the solvent and the salt with aqueous acid. Since the Red B and RedY pigments differed only in their particle size, methods capable of measuring particles lessthan 1000 nm in diameter were obviously necessary. Such methods were not yet available in1958 when DuPont first marketed these products.

The chemical synthesis of linear trans-quinacridones and their substituted derivativesthat have been marketed subsequently is a complicated multi-stage sequence, making suchpigments very expensive and sustainable only where high-fastness red pigments are essential,as in the car industry. There are four routes of synthesis, details of which have been given byPollack [27].

The DuPont company used the succinic acid ester process, in which diethyl succinate isfirst cyclised to diethyl succinylsuccinate (2.39) in a sodium/alcohol mixture. One mole ofthe product is next condensed with two moles of aniline under oxidising conditions, formingdiethyl 2,5-dianilino-3,6-dihydroterephthalate (2.40). Ring closure at 250 °C givesdihydroquinacridone, from which quinacridone is obtained by oxidising away the two extrahydrogen atoms.

HC

H2CC

CH

CH2

C

OEtOOC

COOEtO

2.39Diethyl succinylsuccinate

EtOOC

HN

COOEt

NHH H

HH

2.40

The Sandoz company used the dibromoterephthalic acid method. This acid was madefrom p-xylene by brominating it to form 2,5-dibromo-p-xylene and then oxidising this to 2,5-dibromoterephthalic acid. Reaction of one mole of this acid with two moles of an arylaminein the presence of copper(II) acetate gives 2,5-bis(arylamino)terephthalic acid, which can bering-closed to a linear quinacridone. Unsymmetrical substitution using two differentarylamines is possible.

The BASF route started from hydroquinone, which was converted to 2,5-dihydroterephthalic acid by a Kolbe-Schmitt reaction. One mole of this acid was treatedwith two moles of an arylamine, both components being in the form of a suspension inaqueous methanol. This was added to a small amount of a solution of vanadium(III)chloride and sodium chlorate. Gentle heating gave a 95% yield of 2,5-bis(arylamino)benzo-1,4-quinone-3,6-dicarboxylic acid. Ring closure to the trans-quinacridonequinone tookplace in the presence of concentrated sulphuric acid at 60–80 °C. This was then reduced tothe required crude pigment by zinc or aluminium powder in caustic soda under pressure,inan aluminium chloride/urea melt or by the use of a sulphuric acid/polyphosphoric acidmixture.

The fourth synthesis, developed by the Swiss Lonza company and known as the diketeneprocess, resembles the succinic acid ester process of DuPont in that the central ring of the

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quinacridone molecule is totally synthesised. Acetone vapour is passed over an electricallyheated wire to form first ketene, H2C=C=O, which then dimerises to form diketene(2.41). Chlorination in the presence of alcohol yields ethyl 4-chloroacetoacetate,ClCH2COCH2COOEt. This reaction proceeds at low temperature but is exothermic to a con-siderable extent and can only be successfully carried out on a large scale in special reactorshaving an extensive surface area. Diketene also reacts with aromatic amines to yield aceto-acetanilides, which are the coupling components needed to prepare Hansa Yellows (Table 2.4).

C OO

CH2CH2C

2.41

Diketene

In the manufacture of quinacridone pigments only the first and last of the four routesoutlined have been operated commercially. Synthesis is followed by the milling processesnecessary to give products with the crystal structure and particle size required for their useas pigments.

NH

Cl

Cl

Cl

Cl

O

N N

O

Cl

Cl

Cl

Cl

HN

2.42


2.7 DIOXAZINE PIGMENTS

Triphenodioxazine compounds have been used to make brilliant blue direct and reactivedyes for cellulosic fibres, but only one pigment in this chemical class, CI Pigment Violet 23(2.43), is widely used. It is prepared by condensing 3-amino-9-ethylcarbazole with chloranilin trichlorobenzene [29]. As with many other pigments, manufacturers offer it in manyphysical forms adapted to the intended end-use.

DIKETOPYRROLOPYRROLE PIGMENTS

2.6 ISOINDOLINONE PIGMENTS

Another new chemical class of pigments, called Irgazins, was produced in 1964 by the Geigycompany [28]. These products are made by condensing two moles of 4,5,6,7-tetrachloroiso-indolin-1-one with one mole of an aromatic diamine. Thus p-phenylenediamine gives theyellow pigment CI Pigment Yellow 110 (2.42), which has high light fastness and heat resist-ance. The analogous CI Pigment Yellow 109 derived from 2-methylphenylene-1,4-diamine isgreener in hue. Both of these pigments are used in plastics coloration, particularly ofpolyolefins.

2.8 DIKETOPYRROLOPYRROLE PIGMENTS

This novel brilliant red chromogen was discovered by Iqbal and Cassar of Ciba-Geigy in1983. It has yielded several useful derivatives with first-class application and fastness

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properties in this region of the colour gamut [30]. A typical example is the dichlorinatedcompound CI Pigment Red 254 (2.44).

2.9 FLUORESCENT PIGMENTS

A colorant of this type is not a pigment in the same sense that has been used so far in thischapter, that is, essentially a single chemical compound that can be used to colour media inwhich it is not soluble. Fluorescent pigments are based on fluorescent dyes which are solublein certain resins. A resin coloured in this way is ground to a powder of small particle size andthis ‘fluorescent pigment’ is dispersed into various media in the same way as other pigments.The molecular mechanism by which dissolved dye exhibits fluorescence is discussed insection 11.2. The overall result is that the paints, printing inks and plastics into whichfluorescent pigments have been incorporated have vivid bright colours which attract theeye. This property is very valuable for safety clothing worn by police, rescue workers andothers working in hazardous situations. Their vehicles also become much more visible whenparts of them are painted with special finishes containing these colorants. Other usesinclude advertising and plastic toys.

The resins used to make fluorescent pigments are usually toluenesulphonamide-melamine-formaldehyde matrices. The dyes used for this purpose include CI Disperse Yellow11, Rhodamine 6G (CI Basic Red 1) and Rhodamine B (CI Basic Violet 10). More details ofthe fluorescent dyes used have been given in a review by Christie [31].

Unfortunately, the fluorescent effect is not directly proportional to the concentration ofcolorant present, since there is considerable quenching if quite low concentrations areexceeded. The light fastness of the fluorescent pigments is also less than that of many otherorganic pigments now available, but improvement can be achieved using overlayerscontaining ultra-violet absorbers. This is an area in which further research will clearly beneeded.

N

N

O

O

N

N

H5C2

Cl

Cl

C2H5

2.43


ClC

HN

CNHO

Cl

O

2.44

CI Pigment Red 254

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2.10 INORGANIC PIGMENTS

Inorganic pigments have a special place in pigment chemistry for several reasons. Some arecultural and historical. The art galleries of the world are filled with oil paintings in which thecolours were, until the industrial revolution, produced entirely from mixtures of inorganicpigments obtained from natural sources, the so-called earth pigments. Later some of thesewere made industrially.

Another reason for the importance of inorganic pigments is that there are no whiteorganic pigments. White pigments are essential to provide opacity to the paints and printinginks used on metal, wood, paper, textile fabrics and plastic films. White inorganic pigmentsare also used to provide opacity to synthetic fibres and the vast numbers of articles madefrom plastics by moulding and extrusion processes. The term plastics covers a whole range ofdifferent polymers, including polyamides, polyesters, ABS (acrylonitrile-butadiene-styrene)copolymers, poly(vinyl chloride) and polyolefins. The uses to which these plastics are put,their different physical and chemical properties, and the technologies used in fabricatingthem sometimes including the use of quite high temperatures, impose stringent demands onany colorants used in them. Inorganic pigments, both white and coloured, can generallymeet these demands more often than organic pigments. There are no general rules whichapply and the only way to find out which individual pigments are suitable is by practicaltrials. In uses where high light and weathering fastness is required, such as in paint for cars,many organic pigments do not meet the required standards. Inorganic pigments are the onlyones capable of withstanding the very high temperatures used in the manufacture of glassand in the glazing of pottery after it has been decorated.

Carbon black in one of its many forms is the colorant most widely used in printing inks.Vast amounts are also used in formulating rubber for vehicle tyres, where the carbon blackalso plays an important part in the vulcanisation process when rubber is hardened byreacting it with sulphur during manufacture of the tyres. The primary criteria for carbonblack selection in fibre coloration are colour and undertone, but secondary properties mustmatch the final application. Carbon black performance is determined by prime particle sizeand aggregate structure. Inconsistent dispersion performance may arise by sub-optimalcompounding leading to poor dispersion of the carbon black agglomerates, or from low-quality carbon black containing undispersible contaminants. Recent quality improvementsinclude a new carbon black for increasing the lifetime of continuous filters and close controlof residual impurities improving carbon quality in black-pigmented polyester film used infood packaging [32].

A recent survey of the technical and environmental aspects of inorganic pigments bySchwarz and Endriss [33] has put the importance of these products into numericalperspective. Of a total worldwide production of some 5M tonnes of pigments, 96% areinorganic. Of this 4.8M tonnes, carbon black and various whites account for 84%, that isjust over 4M tonnes. This means that world production of coloured inorganic pigments isapproximately 0.8M tonnes.

2.10.1 White inorganic pigments

The primary requirement for a compound to be of a white appearance is that it should notabsorb any of the radiation in the visible part of the electromagnetic spectrum, that is, ofwavelengths between 400 and 650 nm; in practice this is seldom attainable and slightly less

INORGANIC PIGMENTS

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than 100% transmission can be tolerated, but there must be no marked absorption bands.The additional requirement for a substance to be usable as a white pigment is that it shouldhave as high a refractive index throughout the visible spectrum as possible. A third criterionis that it should be in the form of particles fairly uniform in size, with diameters of the orderof 1000 nm.

Because pigments are used as dispersions of particles in media of a refractive index ofabout 1.5, this is the lower limit for their practical use. There are several white inorganiccompounds with refractive indices between 1.5 and 1.63 which are called ‘transparentwhites’ or ‘extenders’. These include whiting (CaCO3; CI Pigment White 18), china clay(CI Pigment White 19), barytes (BaSO4; CI Pigment White 22), gypsum (CaSO4.2H2O; CIPigment White 25), talc and silica. These compounds confer very little opacity and aremainly used to thicken the paints into which they are incorporated.

White pigments, some of rather variable composition, that have been used in the pastinclude white lead (CI Pigment White 1), refractive index 1.9; zinc oxide (CI PigmentWhite 4), r.i. 1.9; lithopone (CI Pigment White 5), r.i. 1.84; zinc sulphide (CI PigmentWhite 7), r.i. 2.3; antimony oxide (CI Pigment White 11), r.i. 2.2. All have been eclipsed bytitanium dioxide (CI Pigment White 6).

Titanium dioxide

As a pigment this is used in two crystalline forms: rutile (r.i. 2.71) and anatase (r.i. 2.51). Itis these high refractive indices which, combined with modern manufacturing methods, haveensured its pre-eminence.

The ore used for making commercial titanium dioxide is the black ilmenite, nominallyFeO.TiO2 but containing small amounts of many impurities, depending on the source. Thedevelopment of manufacturing methods to prepare pure titanium dioxide in the form ofeither rutile or anatase took many years. Anatase pigments appeared in the mid 1930s andrutile pigments in the 1940s. One of the problems was to remove the last traces of iron andother metallic impurities which distorted the crystalline lattice, leading to inferior whiteness.

There are now two processes in widespread use for making titanium dioxide pigments. Inthe sulphate process, finely ground ilmenite is digested in sulphuric acid and the iron isreduced and separated as iron(II) sulphate. The titanium(IV) sulphate is hydrolysed bysteam to a hydrous oxide, which is thoroughly washed to remove soluble impurities andfinally calcined at a temperature of about 1000 °C to give the anatase form of titaniumdioxide.

In the chloride process, developed in about 1960, the titanium in the ore is converted totitanium(IV) chloride by heating it to 800 °C with chlorine in the presence of carbon, whichcombines with the released oxygen. The purified chloride is then oxidised to titaniumdioxide at 1000 °C and the chlorine formed is recycled. Technical problems arise becausethe oxidation of titanium(IV) chloride is not sufficiently exothermic to make the reactionself-sustaining but these can be overcome by pre-heating the reactants and by burningcarbon monoxide in the reactor to raise the temperature. By careful control of theconditions, it is possible to produce pure rutile particles of a mean size of 200 nm.

Titanium dioxide is non-toxic, in marked contrast to pigments such as white lead, and iswidely used as a pigment in utensils and on surfaces coming into contact with food inindustry, as well as in almost all other uses calling for white pigment. It is not the easiest

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pigment to disperse and, after a long struggle to produce it in pure form, most manufacturersnow produce a range of quite heavily coated forms, each prepared to facilitate and maintaindispersion in particular media.

Both crystal forms of titanium dioxide are photochemically active, anatase more thanrutile. Under ultraviolet light the pigment releases active oxygen, which can tender textilefibres or degrade paint films, causing ‘chalking’. In the dark, atmospheric oxygen regeneratesthe titanium dioxide and the cycle of degradation recurs. In applications where this is amajor problem, rutile-form pigments containing additives such as antimony(III) sulphide aretherefore used.

Much research had to be done during the development of large-scale processes to preparetitanium dioxide in two different crystal forms and in a high state of purity. The knowledgeso gained has been put to good use and led to the production of several new colouredinorganic pigments. These products are based on the discovery that the rutile lattice canabsorb certain metal oxides into its structure to give coloured pigments which retain thehigh fastness properties of pure titanium dioxide. These pigments are discussed in the nextsection.

INORGANIC PIGMENTS

2.10.2 Coloured inorganic pigments

During the first half of the 20th century, when linseed oil was the primary medium fromwhich paints were made and phenol-formaldehyde resin was the basis of most plasticmaterials, both paints and plastics were usually coloured using inorganic pigments. Thesecond half of the century saw the introduction of many new types of plastics and theannual world production of them rose above the l00M tonnes mark. Much of this had to becoloured for identification or decorative purposes. The simplest way to do this is to colourthe whole of the plastic before extruding or moulding it into the shape of the finishedarticle, rather than to apply a coloured surface coating later. Whilst some of the pigment wasno doubt wasted by being buried in the interior, this loss was completely outweighed by theeconomy of a single-step operation to the required finished article and uniformity of colourbetween separate batches.

The result was a big increase in the demand for pigments which met the necessaryfastness standards and withstood the heating involved. Many organic pigments could notmeet these requirements and some of them adversely affected the moulding process itself, orthe dimensional stability of the finished articles. Inorganic pigments, or mixed organic/inorganic products, overcame the problems in some instances, despite the less easydispersibility of the inorganics. The coloured inorganic pigments in current production aregenerally those that have particular advantages which organic pigments cannot match.

Ultramarine

This is the synthetic form of the blue mineral lapis lazuli, which was imported from Chinaand India in the Middle Ages. It is characterised by its attractive blue colour, high lightfastness (ISO 7–8), excellent resistance to alkalis and all organic solvents, easy dispersibilityand non-toxicity, but it has poor resistance to acids. It was first made by Guimet in France in1830 after having been noticed as an impurity in the glass furnaces at St Gobain. It is nowmade by calcining a mixture of the ingredients shown in Table 2.5, which are first finely

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ground and formed into bricks. The reaction takes 4–5 days in a sealed muffle furnace at800 °C, during which time the sodium carbonate reacts with the alumina and silica to forma zeolite, a hydrated silicate of calcium and aluminium. This reacts with the sodiumpolysulphides formed at the same time to give primary green ultramarine. As the furnacecools, air diffuses into it and the primary green oxidises to form raw blue ultramarine. Theoxidation process can be accelerated by blowing in sulphur dioxide. The product at thisstage contains 20–25% sodium sulphate, which is leached out with water. The remainder isthen ball-milled in the wet state until the size of the particles is in the range 500–6000 nm.The particles are next separated into size fractions by sedimentation or centrifugal methods.The coarsest fraction is used for laundry blues, the intermediate fraction in plasticscoloration and the finest in high-grade printing inks.

Table 2.5 Ingredients used in the manu-facture of ultramarine

Ingredient Amount (%)

China clay 30Sodium carbonate 32Sulphur 30Silica 4Rosin 4

The formula as deduced from chemical and X-ray structural analysis is Na6Al6Si6O24S4.There is an open framework of SiO4 tetrahedra with shared corners. Some silicon ions arereplaced by aluminium ions, however, and to preserve electrical neutrality sodium cationsare needed. These are in excess and the overall balance is maintained by the S2–, S2

2– andS3

2– ions present. Consequently, chemical formulae containing fractional proportions ofelements are possible.

The colour of inorganic compounds is associated with charge transfer within thestructure. This can be brought about by specific light absorption, so that the variability ofultramarine composition that can be achieved opens up many possibilities for colourchanges. Pure starting materials, particularly the absence of iron impurities, are necessary inmaking consistent pigments.

As well as the basic ultramarine with two sulphur atoms in its formula, which is CIPigment Blue 29, two other pigments based on it are commercially available. These productsare CI Pigment Violet 15 and CI Pigment Red 259, the latter having a pink colour. Bothpigments are made from ultramarine by oxidation and ion exchange but have a lower colourstrength.

The refractive index of these pigments is about 1.5, which means that they give atransparent blue when used in paints and clear plastics. Opacity can be increased by addingsmall amounts of titanium dioxide. The principal failing of ultramarine is its lack ofresistance to acid, which can even decompose the pigment if there is sufficient available.Coated grades are made with substantially improved acid resistance.

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Cadmium pigments

A range of mixtures containing cadmium sulphide and varying amounts of zinc sulphide orcadmium selenide can be made, the resulting colours being as shown in Table 2.6. Theoutstanding brilliance of shade of these pigments cannot be matched by any other inorganicpigment. Equally bright organic pigments have lower heat stability than have cadmiumpigments, which can withstand temperatures of 400 °C. Other advantages of the cadmiumpigments are better solvent fastness and high fastness to migration, particularly in plastics,including ABS, polyolefins and extended PVC.

Table 2.6 Colours obtainable with cadmium pigments

Colour Components CI Pigment

Primrose CdS, ZnS Yellow 35Yellow CdS Yellow 37Orange CdS, 0.2 CdSe Orange 20Red CdS, 0.4 CdSe Red 108Maroon CdS, 0.7 CdSe Red 108

The starting material for making these pigments is cadmium sulphate, which must be freefrom iron, nickel and copper impurities. Cadmium sulphide is precipitated from the sulphatesolution by adding an alkaline solution of pure sodium sulphide under controlled conditionsof pH, temperature and addition rate. The yellow product is in the cubic crystal form, whichis converted into the required hexagonal form by calcination at 500–600 °C in the absenceof air.

Primrose pigment containing zinc sulphide is made by adding zinc sulphate to thecadmium sulphate starting material and the zinc sulphide appears in solid solution in thefinal product. The pigments consisting of mixtures of cadmium sulphide and selenide aremade rather differently. An alkaline slurry of cadmium carbonate is made by mixingsolutions of cadmium sulphate and sodium carbonate. At the same time selenium is addedto a solution of sodium sulphide to form sodium sulphoselenide as a source of S-Sepolyanions. This must be kept alkaline to prevent the precipitation of selenium. Mixing of asolution of the correct composition with the cadmium carbonate slurry ensures theprecipitation of cadmium sulphoselenide that will yield the desired hue. After washing, theprecipitate is calcined to give the pigment.

Both cadmium and selenium are poisonous and there are regulations about the limits offree cadmium in the pigment (100 p.p.m.) and on the disposal of waste liquors frommanufacturing plants.

As well as the uses in plastics already mentioned, these pigments are used in car paintsand stoving enamels, and also in leather finishes and rubber.

Iron oxide pigments

Naturally occurring iron oxide pigments are widely distributed geographically and can befound in a wide range of colours from black to reds and yellows, depending on compositionand crystal structure. Ochres of many kinds and from different sources were often used in oil

INORGANIC PIGMENTS

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paintings by the Old Masters The use of magnetic iron oxides in recording tapes hasstimulated their investigation at advanced technological level, but in the field of pigmentstheir use has declined to that of cheap fillers for controlling paint flow properties and offacing colours for building bricks.

Iron(II) sulphate is a by-product in many industrial processes, such as the manufacture oftitanium dioxide, the pickling of steel sheet before galvanising and the reduction of aromaticnitro compounds to amines using iron catalysts. Conversion of waste iron(II) salts to usableiron oxide pigments, where the quality requirements are not too stringent, is therefore auseful proposition, since it uses up chemicals that would otherwise be regarded as wasteproducts.

Yellow iron oxides can be prepared by precipitating iron(II) hydroxide from a solution ofiron(II) sulphate by adding sodium hydroxide and then bubbling air through the heatedsuspension to oxidise it to α-iron(III) oxide hydrate (α-FeOOH or Fe2O3.H2O). Analternative method uses a single step of passing a mixture of air and ammonia into theiron(II) sulphate solution, followed by a heating stage. Seeding of the precipitated dispersionwith a previously prepared pigment of the required crystal form helps to control the qualityof the final product.

Red, brown and black iron oxides are prepared by first heating green iron(II) sulphatecrystals to remove six of the molecules of water of crystallisation (leaving FeSO4.H2O) andthen calcining the product to the desired form of iron(III) oxide with the evolution ofsulphur oxides.

Currently produced iron oxide pigments are listed in Table 2.7. Similar pigments occurnaturally in many parts of the world, but are of course mixed with other mineral substances.

The crystallography of iron oxides is complex because there are two oxides, FeO and Fe2O3,as well as α-FeOOH, which can occur in mixed crystals. Kittel [34] has given a full account ofthe chemistry and Feitknecht [35] has discussed the theory of the colours of these pigments.

Table 2.7 Synthetic iron oxide pigments

Colour Formula CI Pigment

Yellow FeOOH Yellow 42Red Fe2O3 Red 101Brown Fe2O3 & FeO.Fe2O3 mixtures Brown 6Black FeO.Fe2O3 Black 11

Lead Chromate pigments

This group of pigments includes chrome yellow, chrome orange and molybdate red (Table2.8). Their colours range from lemon yellow to bluish red. The chrome yellow pigments arepure lead chromates or mixed-phase pigments of lead chromate and lead sulphate. Partialreplacement of chromate by sulphate in the mixed-phase crystals causes a gradual reductionof tinting strength and hiding power, but allows production of chrome yellows with agreenish yellow hue [33]. Chrome orange pigments are basic lead chromates that vary incontent of PbO and PbSO4. Molybdate red pigments are mixed-phase pigments containinglead chromate, lead sulphate and lead molybdate. Their colour depends on the proportion ofmolybdate, crystal form and particle size.

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81

The presence of lead and hexavalent chromium in these products is of chronic hazardconcern and the EU has classified these pigments as harmful substances. Lead is soluble atstomach-acid concentrations and can accumulate in the organism. The results of a high leadintake include inactivation of enzymes and disturbances in the synthesis of haemoglobin.Hexavalent chromium compounds are considered to be carcinogenic. For these reasons theusage of lead chromate pigments has declined considerably in recent years.

Complex coloured inorganic pigments

There are two types of pigments in this category, rutile and spinel pigments. The first, asmentioned already in section 2.10.1, have been developed as a result of the research done toproduce titanium dioxide in pigmentary form. By calcining pure rutile with nickel andantimony oxides at a high temperature, the rutile lattice becomes able to absorb about 3% ofnickel(III) oxide and 10% of antimony(V) oxide. The first of these imparts a yellow colourand the second serves to maintain the overall valency of the cations at four. Thecharacteristic properties of nickel and antimony oxides are no longer apparent afterincorporation in the rutile lattice. The commercial product is designated CI Pigment Yellow53. A buff pigment of this type can be made using chromium(III) oxide instead of nickeloxide [36]. In either of these products niobium oxide can be used instead of antimony oxide,giving a pigment of higher colour strength. Brown pigments are produced if manganese(III)oxide is used instead of nickel or chromium oxide in the calcination, an example being CIPigment Brown 37.

Spinels are minerals which have the formula MO.R2O3, where M is a divalent metal suchas magnesium and R is a tervalent metal, for example aluminium. Blue pigments areobtained when the metals in the spinel MgO.A12O3 are partially or completely replaced bycobalt. Cobalt Blue is CI Pigment Blue 28 with the formula CoO.Al2O3. The main uses ofthese pigments are in paints and in plastics, where their fastness properties are excellent.Their high-temperature stability is particularly useful in plastics.

Metallic pigments

Attractive metallic effects can be obtained by incorporating very thin, small flakes ofaluminium, copper or copper/zinc alloys in otherwise transparent surface coatings. Themetal flakes act as tiny mirrors within these paints, which are particularly effective on thecurved surfaces of cars since the colour changes with the angle from which the surface isviewed. The orientation of the flakes within the paint film also changes the colour seen bythe eye, so that careful dispersion of the metal components of the paint is essential.

Table 2.8 Lead chromate pigments

Common name Formula CI Pigment

Chrome yellow Pb(Cr,S)O4 Yellow 34Chrome orange Pb(Cr,S)O4 Orange 21Molybdate red Pb(Cr,S,Mo)O4 Red 104

INORGANIC PIGMENTS

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Aluminium flakes are listed as CI Pigment Metal 1, whilst copper and copper/zinc alloys areCI Pigment Metal 2. The latter can give the appearance of gold in suitable formulations.The transparency of the surface coatings in which the metallic pigments give their besteffect is achieved in formulations which use solvent dyes, rather than coloured pigments, asthe principal colorants. However, adequate fastness of the entire coating must be theoveriding consideration.

2.11 HOW PIGMENTS ACT AS COLORANTS

This chapter has been concerned mainly with the chemistry of making pigments but has alsostressed the importance of preparing them in the correct physical form. In the case of thosepigments which can be made in more than one crystal form, such as the anatase and rutileforms of titanium dioxide, this may mean that all the pigment should be in one form only. Inothers, a mixture of polymorphic forms may be required, and it is then necessary to ensurethat the desired ratio of these forms is present.

With all pigments, the particle size distribution of the pigment as synthesised is the mostimportant single factor in ensuring its optimum potential performance in use, but thispotentia1 can only be achieved if the particles are properly dispersed. To understand whyparticle size is so important it is necessary to look in some detail at the optical behaviour oftiny pigment particles dispersed in a transparent organic medium. The exact nature of thismedium depends on whether the end-use is a paint, a printing ink or a plastic.

In the case of paints and printing inks, the initial preparations will be in the semi-solidstate because solvents are needed both in the process of dispersing the pigment in the paintor ink medium and for application purposes. These solvents dry out after the paint or ink isapplied. When making coloured plastic articles, both heating and solvents may be used toaid dispersion in the plastic medium as part of the moulding process. However, from theviewpoint of the optical properties in all of these pigment uses, what is most important isthat each of these media has a refractive index close to 1.5.

Refractive index is strictly defined as the ratio of the velocity at which light travels in avacuum to that at which it travels through the medium. For present purposes, it is asufficiently close approximation to use the velocity at which light travels in air, rather thanin a vacuum.

Figure 2.2 gives a simplified analysis, based only on the theory of geometric optics andignoring the wave properties of light, of what happens when a single particle of a light-absorbing (coloured) pigment, embedded in a transparent medium, is struck by a ray ofwhite light. The size of the particle is many times greater than the wavelength of visible light(400–650 nm) and its refractive index is greater than that of the medium, which is about1.5.

At point A in Figure 2.2, a ray of white light enters the medium and its velocitydiminishes, with the result that its path is bent towards the normal at this point. Snell’s lawof refraction states that the relation between the angles of incidence (i) and refraction (r) isgiven by Equation 2.1, where n is the ratio of refractive indices of the medium and air.

�sinsin

in

r (2.1)

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83

The ray then travels on in the medium and strikes the pigment particle at point B. Here it isagain refracted and its path bent towards the normal. The relation between the angles i′ andr′ at point B is again given by Equation 2.1, but this time n is the ratio of refractive indices ofthe particle and medium, that of the particle being the greater. As the ray passes through thepigment particle some wavelengths of the light are absorbed. Which wavelengths these areand how much of them are absorbed, is determined by the shape of the absorption curve ofthe particular pigment involved. When the ray reaches the point C it will certainly becoloured to some extent, depending on the distance BC. At point C, one of two things mayoccur. If the angle of incidence i″ is smaller than the critical angle, the coloured light ray willemerge from the pigment particle and re-enter the medium, following a path CD with theangle r″ greater than i″, as shown in Figure 2.2. If the angle i″ at point C is greater than thecritical angle, however, total internal reflection will occur at that point and the ray will notemerge from the particle. The direction of travel of the ray will be effectively reversed and itwill start on a path CE back through the particle and then possibly out into the medium andthe air and thus to the observer’s eye. Its exact path depends on the particle shape and itsorientation in the medium. Which of these two possible paths the ray follows at point Cdepends on the value of the critical angle, Φc. This is given by Equation 2.2, where np andnm are the refractive indices of the particle and medium, respectively. The important pointis that the greater the refractive index of the particle, the greater is the chance of the criticalangle being exceeded and the more light will start back towards the eye. A complete anddetailed analysis would be very complicated, if not impossible, since in a real case lots ofother particles, including those of different pigments, would be present.

Colouredlight

Pigmentparticle

Medium

Air

Whitelight

A

B

r ′

i ′

i ′′

r ′′

C

r

i

D

E

Figure 2.2 Optics of a single coloured pigment particle in a medium when struck by a ray of whitelight. The light becomes increasingly coloured as it travels from point B to point C

HOW PIGMENTS ACT AS COLORANTS

chpt2(1).pmd 15/11/02, 14:4283


pc

m

nsin

n� � (2.2)

If total internal reflection does occur at point C, the amount of absorption could be doubled.Too long a path within the particle, or a very high absorption coefficient, could mean that allthe light is absorbed and the colouring power of the particle will thus be lost. Thisdemonstrates one reason why the mean particle size of the coloured pigment must bearranged to minimise this possibility.

So far, the effect of light scattering has been ignored. Scattering occurs when light raysstrike particles of linear size considerably less than the wavelength of visible light. Whatoccurs is shown in Figure 2.3. The intensity of the scattered light is proportional to itsincident intensity and also to the 1/λ4 power of its wavelength. Short-wavelength violet orblue light is scattered ten times more than red light. Incidentally, this explains why a clearsky in daylight appears blue, the scattering agent in such a clear sky being the molecules thatform the atmosphere. This effect is, however, relatively unimportant in considering theoptics of pigment particles dispersed in a medium. The most important points that followfrom a consideration of the scattering phenomenon are that white pigments should(a) not significantly absorb visible light,(b) consist of particles of mean size well below the wavelength of visible light, and

Pigmentparticle

Medium

Air

Whitelight

Observer

Figure 2.3 Scattering of lightproduced by a single very smallpigment particle

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85

(c) have a refractive index greater than 1.5, this being that of the media in which whitepigments are generally used.

In practical applications of pigments there will usually be white pigment to confer opacityand one or more coloured pigments to provide the colour. The main effects in this case areindicated in Figure 2.4, which shows the combination of a coloured pigment particle of a sizeconsiderably greater than the wavelength of light and a white pigment particle considerablysmaller than that wavelength. If the coloured particle is also smaller than this, its colouringpower is lost and its effect virtually the same as that of the white pigment particle. Inpractice most pigment particles produce both scattering and absorption effects.

Small whitepigment particle

Colouredpigment particle

Medium

Air

White light Observer

Coloured light Coloured light

Figure 2.4 Combination of acoloured pigment particle and avery small white pigment particle

HOW PIGMENTS ACT AS COLORANTS

Even though this analysis takes account of both the predictions of geometric optics and ofthe wave theory of light, there are still further factors to consider. Every refraction isaccompanied by some degree of reflection. Coefficients of light absorption depend on theplane of polarisation of the light with respect to crystal planes in the structure and pigmentsshow both birefringence and dichroism, topics which are fully discussed in textbooks ofphysical optics.

In a concentrated dispersion of white and coloured pigments, each present in a range ofparticle sizes, a detailed analysis of ray directions becomes impossible. It is possible to deducesome general principles, however. These are that white pigments should be of a narrow sizedistribution, peaking in the region of 200 nm, and that coloured pigments should be of largersizes, peaking around 500 nm, depending on the value of their absorption coefficients.

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This analytical approach is difficult to apply to individual pigments because physical datarelating to refractive index, dispersion curves and the absorption curves in the solid state arenot available. A colligative approach, based on the Kubelka-Munk analysis whichcharacterises pigments by only two constants, an absorption and a scattering coefficient, hasbeen applied with considerable success to the computation of the proportions of pigments inmixtures needed to match a given colour. Much of the book Colour physics for industry isdevoted to this topic [37].

There are also problems arising from the electrical interactions between differentpigments and between pigments and the media in which they are dispersed, particularlywhen these are liquid paints and inks. This topic is discussed in the textbooks by Parfitt andApps (amongst others), given in the bibliography at the end of this chapter.

2.12 SOLVENT DYES

Textile dyes can be dissolved or solubilised in water to a greater or lesser extent, this beingessential for all conventional dyeing processes. Since solubility in water is generallyaccompanied by insolubility in nonpolar solvents, most textile dyes have this property.Solvent dyes, on the other hand, are soluble in organic solvents but insoluble in water.Solvent dyes are used as colour markers for the many different hydrocarbon fractionsproduced in oil refineries. Although the concentration of dye is low, the vast volumesinvolved makes the overall dye usage considerable.

Solvent dyes are also used for tinting transparent varnishes and lacquers in the furnitureand leather industries, where the surface texture of the finished articles needs to beprotected but not obscured. There is also a growing demand for coloured but transparentinks for printing on the ever-increasing amounts of plastic packaging of all kinds used for thefood sold in supermarkets.

Solvent dyes are used to colour the transparent paints used to exploit the specialproperties of metallic pigments, as mentioned earlier. Another use for solvent dyes is in thecolouring of many plastic articles to supplement the colouring power of pigments, if the useof pigments alone in a particular application is found to have undesirable effects on thephysical properties of the plastic.

The new Colour Index volume Pigments and Solvent Dyes lists some 350 solvent dyes andgives their chemical structures, unlike earlier editions which named 800 dyes but includedfew structures. This fall in numbers is not because of any decreased use but rather thegeneral contraction in numbers of all dyes used in the textile industry. Solvent dyes havebeen introduced not by attempts to synthesise new colorants but by selection and in somecases modification of known disperse dyes to meet the technical requirements. The majorityof solvent dyes are azo compounds but among the blue dyes there are anthraquinones. Theaqueous solubility of some of the parent sulphonated dyes has been reduced to acceptablelevels by formation of their salts with heavy metals or long-chain alkylamines.

2.13 CONCLUSION

The diversity of chemical structures involved in pigment chemistry is much less than that ofdyes, but extends from organic to inorganic compounds. The technology involved in the useof pigments in a wide range of different media is that of dispersing the pigment in the

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medium, a process requiring quite different properties from those needed for dyes. As aresult, even a compound that is chemically suitable for pigment use may not be acceptablein practice because it cannot be prepared in a physical form that allows its potentialcolouring properties to be exploited fully as a stable dispersion. This occurs quite frequentlywith the wide variety of plastic media now available and requiring to be coloured.Manufacturers of pigments meet these difficulties by selling the same pigment chemical in aseries of different forms with additives and coatings chosen to suit the intended end-use.

This development has also been necessary because of the increased automation in manycolour-using industries. For example, a computer colour match prediction recipe may call forthe use of exact amounts of particular pigments and these have to be metered by volumeinto the machine that shapes the finished article. This simply cannot be done using drypowders, which were at one time the principal products of pigment makers. Dispersions ofthe component pigments with precise and constant colour strength are essential if largenumbers of the finished articles are to be kept within acceptable colour tolerances.

The range of pigments commercially available is diminishing, though producers usuallymake several physical forms of the same pigment, each intended for specified applications.There are several reasons for the fall in numbers of different pigments. As well as the needsof the automated systems mentioned above, there has been a rationalisation of the range ofintermediates available from the chemical industry, especially if an intermediate is no longerrequired for dye making. Certain pigments have been discontinued because they cannotmeet the current demands from users for higher fastness standards, particularly to light.Low-volume production may be uneconomic when suitable substitutes are available.

The increasing use of instrumental colour match prediction, with the possibility, at leastin theory, of matching wide colour ranges using only three or four pigments, as alreadyachieved in colour printing, is yet another factor contributing to contraction of the pigmentrange. No doubt technical limitations such as dispersion stability will prevent such a drasticchange happening in practice.

REFERENCES 1. L M Valentine, J. Oil Col. Chem. Assoc., 67 (1984) 157. 2. R Kearton, J.S.D.C., 111 (1995) 368. 3. H Skelton, Rev. Prog. Coloration, 29 (1999) 43. 4. A G Perkin and A E Everest, The natural organic colouring matters, (London: Longmans, 1918). 5. A W C Harrison, The Manufacture of lakes and precipitated pigments, (London: Leonard Hill, 1930). 6. A Whitaker, J.S.D.C., 104 (1988) 294. 7. V C Vesce, Official digest, 28 (1956) 1; 31 (1959) 530. 8. H Gaertner, J. Oil Col. Chem. Assoc., 46 (1963) 13. 9. BIOS Report 1661, (London: HMSO, 1946) 49, 83, 98-100.10. D Patterson, Bunsengesell. Phys. Chem., 71 (1967) 271.11. J D Hildreth and D Patterson, J.S.D.C., 79 (1962) 519.12. S Wang and C Zhou, Dyes and Pigments, 37 (1998) 327; 38 (1998) 185.13. A Zitscher, J.S.D.C., 70 (1954) 530.14. J C Harris, J.Amer. Chem. Soc., 70 (1948) 3671.15. F G Riddell et al, Dyes and Pigments, 35 (1997) 191.16. H B Weiser and E E Porter, J. Phys. Chem., 31 (1927) 1704.17. N H Haddock, J.S.D.C., 61 (1945) 68.18. W Braun and O Tcherniac, Ber. , 40 (1907) 2709.19. H de Diesbach and E van der Weid, Helv. Chim. Acta, 10 (1927) 886.20. J Robertson, J.C.S., (1935) 615.21. BIOS Report 960, (London: HMSO, 1946) 47.

REFERENCES

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22. FIAT Report 1313, Vol 3, 273.23. W Gerstner, J. Oil Col. Chem. Assoc., 49 (1966) 954.24. F M Smith and J D Easton, J. Oil Col. Chem. Assoc., 49 (1966) 614.25. FIAT Report 1313, Vol 3, 285.26. H Liebermann, Ann., 518 (1935) 245.27. P Pollack, Chimia, 30 (1976) 357.28. Geigy, US Pat. 2 973 358 (1961).29. B L Kaul, Rev. Prog. Coloration, 23 (1993) 19.30. A Iqbal et al, J. Coatings Technol., 60 (1988) 37.31. R M Christie, Rev. Prog. Coloration, 23 (1993) 1.32. M Potter, Chemical Fibers Internat., 48 (Mar 1999) 62.33. S Schwarz and H Endriss, Rev. Prog. Coloration, 25 (1995) 6.34. H Kittel, Pigments, (Stuttgart: Wissenschaft Verlag, 1960).35. W Feitknecht in Pigments, an introduction to their physical chemistry, (London: Elsevier, 1967).36. D Huguenin and T Chopin, Dyes and Pigments, 37 (1998) 129.37. Colour physics for industry, 2nd Edn, Ed R McDonald (Bradford: SDC, 1997).

BIBLIOGRAPHYThe most extensive compilation of information about pigments is in the volume Pigments and Solvent Dyes (1997), whichis part of the Colour Index. For various reasons, there is now less technical information about the chemistry of thesynthesis and manufacturing methods for pigments than appeared in the 1982 volume, although much of this was inreferences to patents.

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89

CHAPTER 3

Dye structure and application properties

John Shore

3.1 DYE CHARACTERISTICS AND CHEMICAL STRUCTURE

Virtually all commercial textile dyeing and printing processes take place by the application ofa solution or a dispersion of the dyes to the textile material followed by some type of fixationprocess. The dye solution or dispersion is almost always in an aqueous medium. A majorobjective of the fixation step is normally to ensure that the coloured textile exhibitssatisfactory fastness to subsequent treatment in aqueous wash liquors. In view of theoverriding importance of water as a transfer medium in dyeing and printing it seemsreasonable to begin with a discussion of the properties of dyes in solution and in dispersion.

3.1.1 The solubility of dyes in water

In a pure ice crystal each oxygen atom is surrounded by four hydrogen atoms located in atetrahedral configuration. Two of them are covalently bonded to the oxygen atom,conveniently forming an angle of 120 degrees. Hydrogen atoms from two other moleculesare thus positioned equidistantly on opposite sides of the plane of this H–O–H structure. Insteam, on the other hand, each molecule is freely and rapidly in motion, frequently collidingwith other molecules but not capable of forming any type of bond with them at suchelevated temperatures. The association between water molecules in the liquid staterepresents an intermediate condition. Molecular motion is slower than in steam and water ismuch denser. The formation, motion, collision and re-formation of clusters of two or moremolecules is a characteristic feature of the liquid state.

A hydrogen atom normally consists of a positively charged nucleus (a proton) and a muchsmaller orbiting electron. The proton is thus virtually unscreened, permitting the twoelectronegative oxygen atoms in neighbouring water molecules to approach one anothermore closely than they could in the absence of the intervening hydrogen atom. Thisreduction in the force of repulsion between the two electronegative atoms is known as ahydrogen bond [1].

This idealised two-dimensional representation of a seven-molecule cluster (3.1)illustrates that some molecules are more firmly held than others. In reality, of course, eachcluster is three-dimensional in shape and the individual molecules adopt a more randomorientation than indicated here. As the temperature of the liquid increases and the clusterbecomes more mobile, the loose network of hydrogen bonds (indicated by dashed lines)holding the molecules together is weakened. In a collision the cluster is more likely to loseone or more fringe members (a) depending on a single hydrogen bond than those held inplace by two (b), three (c) or four (d) links. The average cluster size thus decreases as thetemperature rises. A higher proportion of free molecules is formed as the boiling point isapproached, heralding the incipient evaporation as steam.

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90 DYE STRUCTURE AND APPLICATION PROPERTIES

The dissolution of an inert nonpolar solute molecule in water has been described in termsof the ‘iceberg’ concept, by which transient local clusters of water molecules are stabilised bycohesive association around the nonpolar molecule. The enveloping cluster surrounding thelatter contains a higher proportion of multilinked members (b–d) than are present in thefreely mobile clusters typical of pure water. Hydrophobic interaction between nonpolarsolute molecules arises directly from this ‘iceberg’ effect. Pairs of such stabilised cohesiveclusters tend to merge and redistribute, forming a larger and more stable sheath around twoor more solute molecules. These molecules then behave as if a binding force exists betweenthem, described as hydrophobic interaction [2]. An analogous process within a clusteringaqueous sheath favours preferential interaction between the hydrophobic segments ofdissolved solute ions in concentrated solutions of dyes or surfactants. A dissolved ionicsolute always has a more stable cluster of water molecules around the hydrophobic portionof the ion. This asymmetrical distribution favours coalescence of solute molecules inconcentrated solution to form a dye aggregate or surfactant micelle. Once formed, however,such an associated moiety is further stabilised by the inner sheath of charged ionic groupingsoriented outwards towards the aqueous clusters.

Many anionic dyes (section 1.6) depend on their sulphonic acid groups for their solubilityin water. Dye sulphonic acids have pK values within the range of pH 1–2 and are fullyionised under dyeing conditions as either the free acid or the sodium salt. The mutualelectrostatic repulsion between dye sulphonate anions ensures their uniform separation anddistribution in dilute aqueous solution. At higher concentrations, however, this repulsion iscounterbalanced by mutually attractive forces of various kinds operating at shorter range[3]:(1) hydrogen bonding(2) van der Waals forces(3) hydrophobic or nonpolar bonding.

The most important types of hydrogen bond are the hydroxy-ether, hydroxy-amine andimino-amine linkages (Figure 3.1). Thus one of the most significant forces of attractionbetween dye molecules containing hydroxy groups is identical with that governing mutualinteraction in clusters of water molecules. Many water-soluble dyes (section 1.6) containprimary, secondary or tertiary amine groupings, all of which are capable of participating viahydrogen bonding in dye–dye association or dye–water solubilisation mechanisms. The vander Waals forces include dipole–dipole interaction between atoms of unlike polarity in

3.1

H

Oa

H

H

Od

H

Ob

H

Oa

H

H

Ob

H H

H

Oc

H

H

Oa

H

H

chpt3(1).pmd 15/11/02, 14:4290

91

neighbouring molecules and less specific p-bonding or dispersion forces between unsaturatedgroupings and aryl nuclei. Hydrophobic bonding between nonpolar segments of dye ions isparticularly significant for acid dyes of high wet fastness solubilised by a single sulphonategroup (section 1.6.7) and for basic dyes, which normally contain only one quaternarynitrogen atom (section 1.6.6).

O H O NO H >> . . .. . . NN H . . .

Figure 3.1 Types of hydrogen bond in order of relative strength (from the left: hydroxy-ether, hydroxy-amine and imino-amine)

The bonding forces involved in hydrophobic interaction can be quite specific to thestructure of the dye ions involved. A series of isomeric derivatives of 2-phenylazophenol-4-sulphonic acid, each containing a trifluoromethyl substituent, was synthesised recently. Theaqueous solubility of these monosulphonated acid dyes was found to be dependent on thelocation of this specific grouping in the dye molecule [4].

The solubility of an ionic dye in water normally increases with temperature, since theenhanced mobility favours electrostatic repulsion between ions rather than closer approachto form aggregates by means of the short-range attractive forces. Addition of a simpleinorganic electrolyte, on the other hand, normally lowers the solubility limit at a giventemperature. Such additions enhance the ionic character of the aqueous phase and help tostabilise the structure of dye aggregates by forming an electrical double layer within thesheath of clustered water molecules around them.

In contrast, the addition of a water-miscible nonionic solute such as urea or an alkanol toan aqueous solution containing aggregates of dye ions has a disaggregating action at a giventemperature and electrolyte concentration. Urea, for example, has a strong tendency toinhibit clustering because of the competitive influence of urea–water hydrogen bonds.Consequently the stabilising ‘iceberg’ cluster surrounding each dye aggregate tends todisintegrate. Dye–urea hydrogen bonds or dipole–dipole attractive forces further disrupt thehydrophobic interaction between pairs of dye ions.

Nonionic solutes with a directional polar/nonpolar structural arrangement, such as long-chain alkyl polyoxyethylene adducts, destabilise the loose structure of dye aggregates by asomewhat different mechanism. These additives readily form micelles in which thehydrophobic alkyl groups form the core and the oxyethylene chains are hydrogen-bondedwithin an extensive sheath of water molecules clustered around them. The formation ofthese large clusters denudes the less firmly held envelopes surrounding the dye aggregates,which become thinner and less stable. The lower dielectric constant of the surfactantsolution enhances the electrostatic forces of repulsion between monomeric dye ions.Hydrogen bonding or dipolar interaction between the oxyethylene chains and individual dyeions further weakens the hydrophobic attraction between pairs of dye ions.

All aqueous solutions of anionic or cationic dyes have lower surface tension than purewater. Solutions of the free sulphonic acids give lower values than solutions of the sodiumsalts of anionic dyes. The decrease in surface tension depends more on the degree ofsulphonation of the dye than on its relative molecular mass, although alkyl substituents may

DYE CHARACTERISTICS AND CHEMICAL STRUCTURE

chpt3(1).pmd 15/11/02, 14:4291


exert a further lowering effect [5]. Thus a relatively hydrophilic tetrasulphonated disazo dyesuch as CI Direct Blue 1 (3.2) has comparatively little influence on surface tension, whereasa monosulphonated milling acid dye such as CI Acid Yellow 72 (3.3) can exhibit a milddetergency effect [6].

NN

N

HO

NH2

NaO3S

O

H2N

SO3Na

SO3NaNaO3S H3CO

3.2

H

OCH3

N

CI Direct Blue 1

N

N

H

N

N

O

CH3(CH2)11

Cl

SO3Na

Cl

H3C 3.3

CI Acid Yellow 72

Measurements of the surface tension of aqueous solutions of various sulphonated andunsulphonated phenylazonaphthol dyes showed that the degree of surface activity (that is,the lowering of surface tension) tended to increase progressively with the degree of alkylsubstitution in the series of dyes [7]. The surface-active behaviour of such alkylated dye ionsensures that they become more concentrated at the interface between the dyebath and thefibre surface, just as they do at the air–water interface of the dye solution. Foaming ofdyebaths can be a serious practical problem with relatively hydrophobic dye structuressolubilised by means of a single ionised group.

Increasing concern about respiratory sensitisation by dust from dyes in powder form,especially reactive dyes, has generated extensive development work on concentratedsolutions and low-dusting cold-dissolving granular forms of water-soluble dyes (section1.7.1). Difficulties arise in attempting to market dissolved brands at concentrations close tothe solubility limit, although these can be overcome by adding miscible solvents orsolubilising agents. Hydrolysis during storage is another problem with reactive dyes ofinherently high reactivity. Fluorotriazine dyes show maximum stability when buffered to pH7–8, whereas sulphatoethylsulphone dye solutions require pH 4–5 to minimise prematurehydrolysis. Dyes marketed as granules should show neither dusting nor hydrolytic instability,but the rate of dissolution may depend on average granule size and dust can be formed fromgranules during transportation [8].

Dyes marketed as aqueous solutions or dispersions should exhibit the followingcharacteristics [8,9]:(1) Constant flow properties under normal ambient conditions(2) Complete and rapid homogenisation on stirring after a freeze/thaw cycle

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(3) Ease of dispensing in automatic metering equipment(4) Complete miscibility with water and other liquid products used in dyebath or print-

paste formulations(5) No drying-out or deposition of dye or other component on the inside of the container(6) No crystallisation of solid particles from solution(7) No agglomeration or sedimentation of particles in dispersion(8) No microbial growth on storage.

Disperse liquid brands of sulphur, vat or disperse dyes, including mixed formulations ofdisperse and vat dyes in matching hues for the dyeing of polyester/cellulosic blends, havebeen commercially available for many years. The proportion of reactive dyes marketed asaqueous solutions is expected to increase markedly [10] because of:(1) Optimum safety for operatives(2) Versatility of formulation to suit specific customer needs(3) Rapid cleaning of dispensing equipment when changing the target shade(4) Ease of dispensing via automatic metering devices(5) Unrestricted application with special suitability for continuous dyeing and printing.

3.1.2 Aggregation of dyes in aqueous solution

Dyes often exist in aqueous solution as aggregates of several ions or molecules rather thanindividual moieties. It is unlikely that all component anions in a multisulphonated dyeaggregate are in the same state of ionisation. Unlike surfactants, dye molecules in general donot possess structures with directional polar and nonpolar segments. Thus they do notassociate as micelles of ordered structure with a well-defined hydrophobic core and acharged hydrophilic sheath. Dye aggregates should be envisaged as relatively amorphous incomposition with zones of more or less polar character distributed within them, althoughthere will be a tendency for individual molecules to become oriented with their ionisedgroupings towards the aqueous phase.

Within a structural series, aggregation is usually greater for dyes with a higher ratio ofrelative molecular mass (Mr) to ionic group content, that is, a higher value for theequivalent mass per sulphonic acid group in the case of sulphonated anionic dyes.Multisulphonated dye anions may be stabilised as dimers in which the two componentsadopt a coplanar arrangement with the sulpho groups located if possible at opposite ends ofthe dimer. More hydrophobic structures of higher equivalent mass, especiallymonosulphonates, can readily form aggregates of larger sizes. Dimers are formed first,growing further by accretion of more dye anions to form lamellar micelles, in which the dyemoieties are stacked like cards in a pack [3]. Planar chromogens, especially the sulphonatedphthalocyanine dyes, are particularly prone to such stacking.

Equilibrium constants for dimerisation (KD) have been determined [3] for several classicaldyes from different chemical classes (Table 3.1). Although differing considerably in Mr andstructural type, these values are all within the same order of magnitude (logKD = 3–4 at25 °C). It seems likely that with most dyes the forces governing aggregation are mainly of thevan der Waals type, including dipole–dipole and dispersion forces. Unlike hydrogen bonds,these are able to operate through successive layers of stacked molecules. An attempt to obtainequilibrium constants for the dimerisation of four p-substituted phenylazo-1-naphthol-6-


chpt3(1).pmd 15/11/02, 14:4293


sulphonic acids (3.4) failed because of the superimposed effects of acid-base, tautomeric(Scheme 3.1) and oligomeric equilibria [11].

Thermodynamic aspects of the aggregation behaviour of three monoazo acid dyes inaqueous solution have been studied by a potentiometric method. An anion-selectivemembrane electrode permitted the direct derivation of dye monomer concentration in thesystem. Graphs of e.m.f. against dye concentration were non-linear owing to dye–dyeassociation at the higher concentrations. These deviations enable the dye monomerconcentration to be determined at any total dye concentration. Adoption of an appropriatestepwise association model provided a framework for computing the successivemultimerisation constants and the respective concentrations of dimers and higher multimers[l2]. Another novel procedure to determine the aggregation constants of water-soluble dyesand the extinction coefficients of the monomeric dye ions has been proposed recently [13].

Most measurements of aggregation number (the average number of ions or molecules peraggregate) have been carried out on solutions of direct dyes, because these aggregate morereadily than most of the other classes of water-soluble dyes. Regrettably, aggregationnumbers determined for the same direct dyes under the same conditions using differentmethods of measurement are seldom consistent. Most results indicate, however, that suchsolutions contain a mixture of aggregates of various sizes in dynamic equilibrium, somewhatanalogous to the mixed clusters of molecules in pure water. When individual anions ordimers are absorbed by a cellulosic fibre during dyeing, larger aggregates break down tomaintain a similar overall distribution of aggregate sizes. The extent of aggregation of adirect dye increases with increasing concentration of dye or added electrolyte, but decreasesas the dyebath temperature is increased. Although many direct dyes are highly aggregated at

Table 3.1 Equilibrium constants for dimerisation of dyes [3]

CI Generic name Chemical class Mr Log KD

Basic Orange 14 Acridine 265 4.02Basic Red 6 Azine 422 3.78Basic Blue 9 Thiazine 284 3.77Basic Red 1 Xanthene 443 3.36Mordant Violet 5 Monoazo 344 3.29Acid Orange 7 Monoazo 328 3.12

N

N

H O

R SO3Na

N

N

O

R SO3Na

H

Ketohydrazone

Hydroxyazo

3.4

Scheme 3.1

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95

ambient temperature, the degree of aggregation is often negligible under exhaust dyeingconditions at the boil even in the presence of electrolyte.

Although quantitative agreement between the aggregation numbers obtained usingdifferent techniques of measurement is poor, meaningful conclusions can be drawn fromthem regarding the relative tendency of different direct dye structures to aggregate. Undergiven conditions of temperature and electrolyte concentration, CI Direct Orange 26 (3.5) ismuch more highly aggregated than CI Direct Yellow 12 (3.6). Both are disulphonated disazodyes, but presumably the two o-ketohydrazone groupings and especially the ureido residue inthe orange dye provide much greater scope for intermolecular hydrogen bonding than theetherified phenolic groups in the yellow dye. The sulpho groups on the central stilbenechromogen of Yellow 12 tend to inhibit aggregation. In a detailed study CI Direct Red 81(3.7; X = H), an important dye that has good migration properties but low affinity and poorfastness to washing, was compared with its p-benzoylamino analogue (3.7; X = NHCOPh).The higher affinity and wet fastness but inferior migration of the latter was found to beassociated with an increased tendency to aggregate, presumably due to the greater scope forhydrogen bonding via the amide groupings.

N

H H

NC O

NN

O

N

H

O

N

H

SO3NaNaO3S 3.5

CI Direct Orange 26

N

N

CH3CH2O

SO3Na

CH

CH

NaO3S

N

N OCH2CH3

3.6

CI Direct Yellow 12

NaO3S N

N N

N

H O

N

C

H

O

X

NaO3S3.7

Three disazo analogues derived from naphthionic acid as coupling component are allhighly aggregated in solution: CI Direct Red 28 (3.8; X = Y = H), CI Direct Red 2 (3.8; X= CH3, Y = H) and its m-tolidine isomer (3.8; X = H, Y = CH3). The two tolidinederivatives aggregate more readily than does Red 28, although the nonplanar m-tolidine dyeis not so highly aggregated as the coplanar o-tolidine structure in Red 2, which is moresensitive to flocculation by salt and several times more substantive to cellulose than its m-tolidine isomer. The tetrasulphonated dye CI Direct Blue 1 (3.2) is considerably lessaggregated than these disulphonated naphthionic acid derivatives.


chpt3(1).pmd 15/11/02, 14:4295


In a recent investigation the aggregation of CI Direct Red 1 (3.9) in aqueous solution wasstudied as a function of pH, temperature and dye concentration [14]. At a given temperatureand concentration, the high aggregation number at pH 4 confirmed that only the sulphonategroup in the γ acid residue at one end of this disazo structure was ionised. At higher pH valuesthe carboxyl group in the salicylic acid residue at the other end of the molecule becameionised, increasing the solubility and lowering the aggregation number. The degree ofaggregation decreased with increasing temperature as expected, but this effect of pH onionisation and disaggregation was evident at temperatures between 25 °C and 60 °C.

N N

N N

X Y Y X

NH2

SO3Na

NH2

SO3Na3.8

N N

N

N

SO3Na

H2N

HOHO

NaOOC

3.9

CI Direct Red 1

Although acid dyes in general are considerably less aggregated than direct dyes undersimilar conditions, they show broadly similar trends between structure and aggregationbehaviour. Thus an increase in the size of the coplanar aryl nuclei in monoazo acid dyes (forexample, replacement of a phenyl by a naphthyl residue) increases the tendency toaggregate. A recent spectrophotometric study of aggregation equilibria in a series of alkyl-substituted azo acid dyes revealed that only dimerisation took place with alkyl chain-lengthsup to hexyl. Formation of multimers, however, was observed with octyl or decyl groups. As ageneral rule the dimerisation equilibrium constant increased progressively with the length ofthe n-alkyl chain, but the presence of a sec-butyl substituent produced an anomalousdecrease in equilibrium constant attributable to steric interference with hydrophobicinteraction between dye anions [15].

The aggregation and tautomeric equilibria of CI Acid Red 138 (3.10) have beenexamined recently. In spite of the length of the dodecyl substituent in the diazo componentonly monomers and dimers were present [16], presumably because the H acid residuecontributes two sulpho groups to solubilise the molecule. Both monomer and dimer specieswere predominantly in the hydrazone form, as expected, and hydrophobic interactionbetween the terminal dodecyl chains played an important part in formation of the dimer.

An increase in the degree of sulphonation of a typical azo acid dye structure minimisesaggregation, especially if the sulpho groups are widely separated from one another and fromthe azo group. Light-scattering studies demonstrated freedom from aggregation with the

chpt3(1).pmd 15/11/02, 14:4296

97

important monoazo monosulphonate CI Acid Orange 7 (3.11) and many simple monoazodi- or trisulphonates, but typical disulphonated milling dyes such as CI Acid Red 111 (3.12)showed salt-sensitive aggregation. With the disazopyrazolone disulphonate CI Acid Orange63 (3.13) the aggregation appeared to be virtually independent of the concentrations of dyeand electrolyte at low temperatures. Aggregation greatly decreased above about 60 °C, ingood agreement with the anomalous dyeing properties of this dye. It may form stablemicelles analogous to those of surfactants and this effect is apparently highly sensitive totemperature in the 50–70 °C region.

NaO3S N

N

H O

3.11

CI Acid Orange 7

H3C SO2

O N

N

CH3

N

N

H O

NaO3S

SO3Na

H3C

3.12

CI Acid Red 111

H3C SO2

O N

N N

N

H

N

N

O

SO3Na

3.13 CH3

CI Acid Orange 63

CH3(CH2)11 N

N

H O

SO3Na

HN

C

O

CH3

NaO3S 3.10

CI Acid Red 138

The aggregation behaviour and tautomerism of three o,o′-dihydroxy and one o-hydroxy-o′-methoxy monoazo dyes have been studied by UV-visible spectroscopy [17]. Evidence ofmonomer-dimer equilibria was obtained for all four of these mordant dye structures.Intermolecular hydrogen bonding between the hydroxy groups and hydrophobic interactionbetween aryl nuclei contribute to the dimerisation effect.

The influence of metal ions on the aggregation of CI Reactive Red 2 (3.14) in aqueoussolution at various pH values has been examined in detail recently [18]. Sodium ions have aprofound effect on these disulphonated dye anions, so that the enhancement of substantivityduring the dyeing process is accompanied by aggregation attributable to hydrophobicinteraction between the phenyl and s-triazine ring systems. Aggregation is much greater,


chpt3(1).pmd 15/11/02, 14:4297


however, in the presence of divalent calcium or magnesium ions, which can formintermolecular electrostatic linkages between sulpho groups in different dye anions. Thisaccounts for the adverse influence of hard water on the level dyeing properties of reactivedyes.

N

N

H

SO3Na

O

HN

N

N

N

Cl

Cl

3.14

CI Reactive Red 2

NaO3S

NHN

CNHO

N

H O

NaO3S

SO3Na

HN

N

N

N NH

Cl SO3Na

3.15

The introduction of a cyclic amide system into a monoazo dye molecule considerablyenhances its substantivity for cellulose. This was demonstrated recently by evaluating aseries of aminochlorotriazine structures derived from 5-aminobenzimidazolone as diazocomponent, typically the bluish red H acid derivative (3.15). The urea residue in theimidazolone ring is a powerful hydrogen bonding moiety. These dyes exhibited excessiveaggregation when dyed at a temperature lower than 95 °C in the presence of a conventionalsalt concentration but showed markedly higher exhaustion at a relatively lower ionicstrength, where the degree of aggregation was similar to analogous dye structures derivedfrom aniline as diazo component. These results indicate that benzimidazolone-type reactivedyes are of potential interest as ‘low-salt’ products [19].

Owing to experimental difficulties, knowledge of aggregation effects in alkaline dithionitesolutions of leuco vat dyes is sporadic [20,21]. Investigations based on absorption spectra haveshown that, depending on concentration and temperature, planar polycyclic molecules such asthe violanthrone derivatives CI Vat Blues 19, 20 and 22 and the perylene tetracarboxydiimidederivatives CI Vat Reds 23 and 32 are mainly present as monomers or dimers in leuco vatsolutions. Violanthrones that do not have a coplanar structure because of the presence of

chpt3(1).pmd 15/11/02, 14:4298

99

alkoxy groups in the 16,17-positions, such as CI Vat Blue 16 and the widely used Greens 1, 2and 4, are always present as single molecules under vat dyeing conditions.

3.1.3 Decomposition of dyes under reducing conditions

The primary objective of an exhaust dyeing process is to transfer as much as possible of thedye in the system from the dyebath into the fibre, provided this can be achieved with thedye uniformly distributed throughout the dyed material at the end of the process. If dyeing isunduly prolonged, either to improve the levelness or to attain an exact colour match tostandard, decomposition of a minor proportion of the dye present may occur. The rate ofdecomposition is normally much more rapid in the dyebath phase than on the fibre and theonset of degradation can sometimes be observed as a change in colour of the exhaust liquor.This effect usually occurs as a hypsochromic shift, such as reddening of a blue or yellowingof a red solution.

Many direct dyes show instability of this nature if applied at a temperature above the boil,as in the dyeing of polyester/cellulosic blends, for example. A frequent cause of the problemis reduction of azo linkages. This may be accelerated in the presence of viscose, which has areducing action under alkaline dyeing conditions. Direct dyes with exposed azo groups freefrom o-substituents, like the two azo links in CI Direct Yellow 12 (3.6), are particularlyprone to reductive decomposition. However, some dyes with o-aminoazo groups, such as CIDirect Red 2 (3.8; X = CH3, Y = H) or Red 28 (3.8; X = Y = H), are also reduction-sensitive. In the relatively unstable trisazo structure of CI Direct Green 33 (3.16), the mostvulnerable of the three azo groups is the unprotected central linkage, so that the likely initialproducts of reduction (Scheme 3.2) are two monoazo dyes (3.17 and 3.18), resulting in ahypsochromic reddening and dulling of the blue-green solution.

N

N N

N N

N

H

SO3Na

HN

COCH3

OSO3Na

H3C

H3C

NaO3S

OCH2CH3

NaO3S

N

N

H3C

H3C

SO3NaNH2

OCH2CH3

NH2N

N

H

NaO3S

O

HN

COCH3

SO3Na

NaO3S

3.16

CI Direct Green 33

3.17

+

3.18

4H

Scheme 3.2


chpt3(1).pmd 15/11/02, 14:4299


Dyes containing azo links protected by o,o′-substituents, especially o-sulphonate groups,are generally relatively resistant to reductive breakdown during dyeing. Azothiazole dyessuch as CI Direct Yellow 29 (3.19) and especially copper-complex structures such as CIDirect Blue 98, in which both azo groups are fully protected as the metal complex (3.20),show excellent stability in high-temperature dyeing conditions [22]. Buffering withammonium sulphate confers a stabilising influence when dyes of inferior stability have to beused on viscose. Mild oxidants such as sodium m-nitrobenzenesulphonate are occasionallyuseful and more powerful inorganic oxidants, such as potassium chlorate or dichromate, canbe most effective if used with care. Excessive use of these additives, however, can result inoxidative decomposition of azo dyes.

SO3Na

H3C

N

S

N

S

N

N

S

N

S

N

SO3Na

CH3

3.19CI Direct Yellow 29

NH

O

N

SO3Na

N

O

OH2

O

N

N

OCuCu

H2O

NH

NaO3S

3.20CI Direct Blue 98

Direct dyes of the disazo diarylurea type, such as CI Direct Orange 26 (3.5) and CI DirectRed 79 (3.21), are particularly prone to decomposition in high-temperature dyeing becausemolecular breakdown can occur by hydrolysis as well as by the reductive mechanism underalkaline conditions. Thus the ureido linkage in this red dye may be broken by hydrolysis togive two monoazo dye fragments (3.22), or the azo groups can be reduced to yield adiaminodiphenylurea (3.23) and two molecules of H acid (Scheme 3.3).

The azo linkage in some acid dyes is sensitive to the mild reducing action of the cystineand cysteine residues in wool under hot alkaline conditions. CI Acid Blue 113 (3.24) is acommodity dye showing good all-round wet fastness on wool and nylon, but it tends to turnbrown during dyeing if exposed to alkaline reducing conditions at high temperatures. Thedegradation involves reduction to metanilic acid and a brown monoazo dye (3.25), the rateincreasing rapidly above pH 7 (Scheme 3.4).

Hydrolysis of ester groups can also occur in certain wool dyes. For example, CI Acid Red1 (3.26) loses its acetyl group (Scheme 3.5) on prolonged dyeing in an acid dyebath, the huebecoming bluer and duller and the dyeing much less fast to light.

An interesting study of the relative rates of reduction of N,N-bis(2-chloroethyl)-aminoazobenzene and various monosubstituted derivatives (3.27) was initiated because ofthe ability of such dyes to inhibit the growth of animal tumours [23], even though some

chpt3(1).pmd 15/11/02, 14:42100

101

Scheme 3.3

N

OH

N N

H H

N

NaO3S

NaO3S

OCH3 H3CO

N

N

SO3Na

SO3Na

H3C O CH3

HO

NH2N

H3CO

N

HO

CH3

SO3Na

SO3Na

N

H H

NH2N NH2

OH3C CH3

H3CO

SO3Na

H2N

HO

SO3Na

OCH3

3.21

2+

3.22

CI Direct Red 79

H acid

2

3.23

HCOONa

NaOH

8H

N

N N

N NH

SO3Na

NaO3S

NH2

NaO3S

N

N NH

SO3Na

H2N

3.24

CI Acid Blue 113

+

3.25

4H

Scheme 3.4


chpt3(1).pmd 15/11/02, 14:42101


dialkylamino analogues such as 4-(dimethylamino)azobenzene have long been known tocause liver cancer in rats. A selection from the results is presented in Table 3.2, expressed interms of the relative rate constant 100 k/k0 where k and k0 are the respective rate constantsfor the substituted and unsubstituted compounds. Electron-donating substituents (methyl,methoxy) in the 3- and 4-positions were found to retard reduction, whereas electron-withdrawing groups (bromo, chloro, acetyl, carboxy, sulpho) in these positions enhanced therate. Steric effects tended to predominate, however, when relatively bulky substituents(phenyl, bromo, iodo) were present in the 2-position.

N

H

N

HN

COCH3

O

SO3Na

NaO3S

N

H

N

H2N

O

SO3Na

NaO3S

H+

H2O

+ HOOC.CH3

3.26

CI Acid Red 1

Scheme 3.5

N

N N

CH2CH2Cl

CH2CH2ClZ

Y X

3.27

Table 3.2 Rates of reduction of monosubstituted monoazo dyesby tin(II) chloride [23]

Substituent in structure 3.27 Relative rate constant

X = nitro very highX = carboxy very highX = sulpho 2470Z = acetyl 2100Z = carboxy 1180Z = sulpho 374Z = bromo 254Z = chloro 206X = chloro 113Z = phenyl 101None 100Y = methyl 94X = bromo 80X = phenyl 68X = methyl 62X = iodo 39Z = methyl 36Z = methoxy 20

chpt3(1).pmd 15/11/02, 14:42102

103

The mechanism of the reduction reaction involves the initial protonation of the azonitrogen further from the N-chloroethyl groups (Scheme 3.6), formation of the quinonoidcationic species (3.28) being responsible for the bathochromic effect observed when dyes ofthis series are treated with acid. In the case of the 2-carboxy and 2-nitro derivatives of theparent dye, reduction of the azo linkage took place so rapidly that the rate could not bemeasured. The 2-carboxyphenylazo grouping readily chelates with metal ions and it seemslikely that chelation of the tin(II) ion exerts a powerful catalytic effect on the reduction rate.In the 2-nitrophenylazo dye both the nitro and azo groups are potential sites for reduction tooccur. In fact the azo linkage was reduced very rapidly but the yield of o-nitroaniline wasquantitative, indicating that the nitro group was quite stable under these circumstances [23].

N

N N

CH2CH2Cl

CH2CH2Cl

N

N N

CH2CH2Cl

CH2CH2Cl

N

N N

CH2CH2Cl

CH2CH2Cl

HH+

_

+

+3.28

Scheme 3.6

Mordant dyes are notoriously troublesome from the viewpoint of colour matching becausethe hue of the chromium complex usually differs greatly from that of the unmetallised parentdye (section 5.4.1). If other metal ions are present in the treatment bath or on the fibreduring chroming, the colour obtained is likely to differ from that of the pure chromiumcomplex. Certain important chrome dyes, including CI Mordant Black 11 (3.29) and Black17 (3.30), are particularly sensitive to traces of iron or copper. The hue of the black dyeingsobtained is redder in the presence of copper and browner with iron contamination. Thefastness to light and wet treatments may also prove inferior under these conditions. Evencertain 1:2 metal-complex acid dyes show similar effects in the presence of these impurities,

N

OH

NaO3S

N

H O

O2N 3.29

CI Mordant Black 11

NaO3S

OH

N

N

H O

3.30

CI Mordant Black 17


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indicating that the iron or copper ions are able to displace chromium from the desiredchromium complex. Sequestering agents of the sodium polyphosphate type (section 10.2)can help to minimise these problems.

With some leuco vat dyes, chemical changes may take place in alkaline reducing media athigh temperatures [24,25]. Many vat dyes contain benzoylamino substituents to conferhigher affinity for cellulosic fibres. Alkaline hydrolysis of such dyes may result fromprolonged treatment with loss of the benzoyl groups and a gradual lowering of substantivity,leading to unexpectedly low exhaustion of the dyebath. Thus CI Vat Yellow 3 (3.31) ishydrolysed under these conditions (Scheme 3.7), the hue becoming redder and dullerbecause of the formation of the red 1,5-diaminoanthraquinone, and the dyeing exhibitslower fastness to washing.

NC

O

OH

O NH C

O

COONa H2N

O

O

NH2

3.31

CI Vat Yellow 3

2 +

NaOH

Scheme 3.7

High-temperature vatting or dyeing conditions may cause dehalogenation of vat dyescontaining bromo or chloro substituents, resulting in a change of hue towards the parentchromogen. Thus if dichloro (CI Vat Violet 1) or monobromo (Violet 9) derivatives ofisoviolanthrone are vatted and dyed on cotton at 90 °C rather than normally at 60 °C, amarkedly bluer hue closer to the unsubstituted compound (Violet 10) is obtained. Undersimilar conditions, dyeings of dibromo (Green 2), dichloro (Blue 17) or trichloro (Blue 18)derivatives of violanthrone do become slightly redder and duller, but the contribution of theparent dye (Blue 20) formed is small because these halogenated structures are more stablethan their isomeric analogues. Chlorinated acridone derivatives such as Red 39 and Violet14 may also suffer dechlorination. Monochloro (Blue 14) or dichloro (Blue 6) derivatives ofindanthrone lose chlorine even when dyed above 50 °C and this involves a deterioration infastness to bleaching.

Indanthrone (3.32; CI Vat Blue 4) and its derivatives require the reduction of only two ofthe four keto groups to form the sparingly soluble disodium leuco compound (Scheme 3.8).Hydrogen bonding between the inner phenolic groups and the neighbouring azine nitrogenatoms stabilises the structure (3.33). Loss of oxygen atoms can occur, however, as a result ofover-reduction at temperatures above 60 °C. The tendency for this irreversible reaction toensue increases with time and temperature of reduction and with the concentrations of

chpt3(1).pmd 15/11/02, 14:42104

105

alkali and dithionite. Additions of glucose or sodium nitrite have a stabilising influence onthe disodium enolate solution. Over-reduction yields redder and duller dyeings because ofthe formation of a brown derivative that will not respond to the oxidising agents normallyused after vat dyeing.

Flavanthrone (3.34; CI Vat Yellow 1) behaves in a similar way (Scheme 3.9). Only twohydrogen atoms are required to yield the blue disodium leuco form (3.35), but this is morestable than the analogous disodium enolate of indanthrone. Nevertheless, over-reduction tothe brown flavanthraquinol (3.36) can take place under severe conditions of high alkalinityand temperature. In these circumstances the dyeings can no longer be reoxidised toflavanthrone even using acidified dichromate solution. These irreversible overreductionreactions are not known for any classes of anthraquinonoid vat dyes other than theindanthrone and flavanthrone chromogens.

N

N

HO

OH

O

O

N

N

O

O

ONa

ONa

H

H

3.32 3.33

Indanthrone

Scheme 3.8

N

N

O

N

N

N

N

ONa

ONa

H

H

ONa

3.34

Flavanthrone

3.35

3.36

2H

O ONa

Scheme 3.9


chpt3(1).pmd 15/11/02, 14:42105


3.1.4 Rates of reduction and redox potentials of vat dyes

The rate of reduction of a vat dye depends partly on the intrinsic chemical properties of thedye and partly on the size and physical form of the dispersed particles undergoing thisreaction. The physical factors are much less important than the chemical aspects [26]. Thevatting process entails conversion of the insoluble keto form into the soluble sodium enolate(section 1.6.1). The reaction takes place in two stages at ambient temperature. Extremelyrapid reduction to the hydroquinone is followed by slower dissolution in the alkalinesolution. At higher temperatures, however, the dissolution rate approximates more closely tothe rate of reduction. Temperature and dithionite concentration are the important variablesand the rate of reduction is much less dependent on dye or alkali concentration.

The ease of reduction of a vat dye depends on its chemical structure in relation to thereducing power of the agent selected. In practice this question is seldom critical because thereduction potential developed by a typical reducing agent is low enough to permit thereduction of all vat dyes of practical interest. Indeed, such dyes would never have beenexploited commercially if they had not responded effectively to reduction underconventional conditions of application.

The times of half-reduction of five typical vat dyes are listed in Table 3.3. Most of thecommercially important products give values within the range 25–500 seconds. Few dyes areas slow to reduce as CI Vat Red 1 and even fewer are reduced as quickly as flavanthrone. Infact, when these times were measured the average particle size of the sample of flavanthroneunder test was greater than that of the CI Vat Green 1 sample, which was reduced at leastten times more slowly [26].

Vat dye reduction is a reversible reaction leading to the formation of an equilibriummixture of the oxidised and reduced forms. The relative rates of reduction of vat dyes bearno obvious relation to their intrinsic reduction potentials, which determine the position ofthe equilibrium reached when the keto form of the vat dye is dissolved under a given set ofreducing conditions. The relative stabilities of the reduced and oxidised forms can beestimated from the reduction potential developed at a bright platinum electrode (Table 3.4).An unstable leuco compound tends to revert to the oxidised form by transfer of electrons tothe electrode, developing a high negative reduction potential. On the other hand, a dyewith a small negative reduction potential is easy to reduce because the leuco form isrelatively stable.

Few vat dyes are as difficult to reduce as CI Vat Yellow 3 (3.31) in which the amide NHgroups form hydrogen bonds with the keto groups, or as easy to reduce as tetrabrominated

Table 3.3 Times of half-reduction of typical vat dyes [26]

CI Vat Chemical name Time(s)

Yellow 1 Flavanthrone <5Blue 17 16,17-Dichloroviolanthrone 31Orange 9 Pyranthrone 36Green 1 16,17-Dimethoxyviolanthrone 50Red 1 6,6′-Dichloro-4,4′-dimethylthioindigo 2880

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107

indigo. Most dyes of commercial interest have reduction potentials within the range –200 to–300 mV. It is interesting to note from Tables 3.3 and 3.4 that CI Vat Yellow 1 is inherentlyless stable in the reduced form than is CI Vat Red 1, even though the yellow flavanthrone isreduced approximately a thousand times quicker than is the red thioindigo derivative.Although standard reduction potentials have to be measured in anhydrous solvent solutionfor practical reasons, the results appear to be in general agreement with the qualitativebehaviour of vat dyes in aqueous media. Thus indigoid dyes are more stable in the leucoform than are the anthraquinone derivatives in general, but they tend to be reduced moreslowly and need a higher temperature for complete reduction.

Cyclic voltammetry has been used to determine the standard heterogeneous rateconstants and standard Gibbs free energy for the oxidation-reduction of thioindigo (CI VatRed 41) on cellulose. The substrate plays an active role in these reactions of the dye whenthey take place during dyeing, including lowering of the activation energy of the redoxprocesses. This technique allows the vat dyeing mechanism to be modelled more closely[27].

The application of indirect electrolysis as a reduction technique in indigo dyeing has beenreported recently. The build-up, penetration and final depth attained were similar to thoseobserved in conventional dyeing with alkaline dithionite. Various reversible redox systemswere evaluated as potentially suitable for indigo dyeing, particularly an iron(II)-triethanolamine complex. The iron(III) form of this complex can be transformed into theiron(II) form by cathodic reduction, thus leading to a regenerable reducing agent. Thisapproach offers the prospect of improved process stability, because the reduction state of thedyebath can be monitored by measuring the reduction potential. By regenerating themediator system, significant reductions in consumption of chemicals and water can beachieved. The process engineering involved can readily be applied to the commercialproduction of indigo-dyed denim [28].

3.1.5 Substantivity and ionisation of indigo

Indigo was in short supply during the 1970s and development work at Dan River Inc. (USA)demonstrated that the consumption of indigo in the dyeing of cotton denim yarn could be

Table 3.4 Standard reduction potentials of vat dyes in pyridine [26]

CI Vat Chemical name Potential(mV)

Yellow 3 1,5-Dibenzoylaminoanthraquinone –490Orange 9 Pyranthrone –270Green 1 16,17-Dimethoxyviolanthrone –250Yellow 4 1,2,4,5-Dibenzopyrenequinone –240Yellow 1 Flavanthrone –230Red 6 6-Chloro-6′methoxy-4-methylthioindigo –220Blue 1 Indigo –220Blue 4 Indanthrone –220Blue 20 Violanthrone –220Blue 17 16,17-Dichloroviolanthrone –210Red 1 6,6′-Dichloro-4,4′-dimethylthioindigo –190Blue 5 5,5′,7,7′-Tetrabromoindigo –70


chpt3(1).pmd 15/11/02, 14:42107


markedly reduced using a sodium carbonate/hydroxide buffer rather than caustic soda aloneto generate lower dyebath pH values. Dyebaths produced in this way were less stable,however, resulting in a variable pH, more deposition of inadequately dissolved dye and agreater risk of unlevel dyeing. In recent years the effect of dyebath pH on the yield andpenetration of indigo dyeings has been studied in greater detail [29].

Measurements of apparent colour yield, expressed in terms of the reflectance absorptivitycoefficient of the indigo-dyed yarn, showed this to be a maximum at pH 10.5–11.5 and todecrease progressively with pH, until at pH 13.5 the colour yield was only about one-sixth ofthat at pH 11 (Figure 3.2). The degree of penetration of the dyeings was also highlydependent on dyebath pH, poorly penetrated ring dyeings being obtained at pH 11 andalmost fully penetrated ones at pH 13. This is attributable to a rapid rate of strike onto thefibre surface under conditions of much higher substantivity at the lower pH.

It is well-known that the reduction of indigo (3.37) proceeds according to Scheme 3.10,to give firstly the monoionised sodium hydrogen enolate (3.38) and then its conjugate basethe disodium enolate (3.39). The relative amounts of these species present is governed bythe dyebath pH. By extrapolation from the data for the pK values of the various sulphonatedderivatives of indigo, it was shown that approximate pK values of 8.0 and 12.7 respectivelycan be taken to represent the ionisation equilibria in Scheme 3.10. Using these statisticallyestimated values, the fractions of reduced indigo present as species 3.34 and 3.35 at a givenpH can be calculated [29].

Dyebath pH

10.5 11 11.5 12 12.5 13 13.5

50

100

150

200

Ref

lect

ance

abs

orpt

ivity

coe

ffici

ent

Figure 3.2 Effect of dyebath pH on apparent colour yield in a five-dip indigo dyeing process [29]

N

N

N

N

N

N

O

OH

H Na+O

H

H

OH

Na+O

H

H

O Na+

_ _

_

3.37 3.38 3.39

Indigo Mono-enolate Di-enolate

Scheme 3.10

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109

Sorption studies have revealed that both equilibrium sorption and the ratio of themonoionised to diionised forms of indigo are markedly higher at pH 10.5–11.5, where thehighest colour yield is achieved in indigo dyeing [30]. Above this region of pH, the fractionof indigo that exists as the monoionised species decreases rapidly with increasing pH. Aboveabout pH 12.7, a greater proportion of the leuco dye is present as the disodium enolate(3.39). Good linear correlation was obtained (Figure 3.3) between the colour yield dataconverted to a fractional scale and the fraction of indigo present in the dyebath as themonoionised form over the pH range 10.5–13.5. Improved formulation of buffered alkalishas enabled the degree of ring dyeing obtained in the commercial indigo dyeing of cottondenim yarn to be closely controlled [29].

Fraction of monoionised indigo present

0.2 0.4 0.6 0.8 1.0

0.2

0.4

0.6

1.0

0.8

Frac

tiona

l col

our

yiel

d

Figure 3.3 Relation between fractional colour yield and fraction of monoionised indigo [29]

3.1.6 Oxidative decomposition of dyes

Oxidising agents have always been important reagents for the dyer and printer, not only forthe reoxidation of vat or sulphur dyes but also in bleaching, stripping and dischargetreatments. In recent years, interest in the oxidative decomposition of dyes has greatlyincreased as a means of dealing with the problem of colour in waste-water (section 1.7.3).Catalytic oxidation is probably the simplest approach to decolorisation of residual dyes. Poly-p-phenylene-1,3,4-oxadiazole may be used to induce oxidative decomposition under UVirradiation [31]. Azo acid dyes derived from H acid or K acid can be degraded by photo-assisted catalysis in an aqueous suspension of titanium dioxide. The reaction followedapparent first-order kinetics, the effect of pH varying with the dye structure [32]. Hydroxylradicals play an essential role in the oxidative fission of the azo groups. The triplet statelifetimes of the dye species are almost independent of the presence of titanium dioxide,implying that the triplet state does not react with hydroxyl radicals or anionic superoxidefree radicals [33].

Ozone treatment has potential as a powerful means of decolorising exhaust dyebaths withat least 95% colour removal for good reproduction of bright shades after recycling [34]. Theeffect of ozone concentration on the decomposition of reactive dye hydrolysates (eightorange or red monoazo and two blue anthraquinone dyes) was investigated [35]. Theecological properties of ozone-treated N-containing hydrolysates and the influence of


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surfactants on the ozonolysis reaction were evaluated. GLC, HPLC and mass spectrometryare valuable techniques for following the reactions and identifying the products ofdecomposition [35,36]. Fundamental study of the ozonolysis mechanism has shown that theazostilbene CI Direct Yellow 12 (3.6) yields mainly 4-carboxy-4′-ethoxyazobenzene-3-sulphonate (3.40) and hydrogen peroxide, indicating that the stilbene group is moreoxidation-sensitive than the azo linkages in spite of the protection afforded by the two o-sulphonate groups [37]. The stability to ozone of 2-arylazo-1-naphthol-3,6-disulphonatedyes (3.41) was investigated. Electron-withdrawing nitro or bulky perfluoroalkyl substituentsin the diazo component conferred increased resistance to ozone attack [38].

N

N

SO3Na

COONa

CH3CH2O

3.40

N

N

H O

SO3Na

NaO3S 3.41

Commercially viable systems for the decolorisation of spent dyebaths can be based onhydrogen peroxide treatment initiated by UV radiation. A representative selection of sixdisulphonated monoazo acid dyes and two disazo disulphonated types was exposed forvarious times in a pilot-scale photochemical reactor of this kind. The controlling parameterswere dye structure, pH, peroxide dosage and UV light intensity [39]. In a wider survey ofthe response of various classes of dyes to the combination of UV radiation and hydrogenperoxide, viable candidates for further in-plant treatment trials were the water-solublereactive, direct, acid and basic classes. On the other hand, water-insoluble colorants such asdisperse and vat dyes did not appear to be viable [40].

The photo-oxidation of several water-soluble monoazo dyes was studied in hydrogenperoxide solution, irradiation being carried out using a low-pressure mercury vapour lamp.The reaction was found to be pseudo-first-order with respect to dye concentration. A labilehydrogen atom in the dye molecule was the primary target of attack and the predominanthydrazone form of the chromogen was more sensitive to oxidation than the azo tautomer[41]. An alternative hydrogen peroxide treatment at temperatures close to 100 °C in theabsence of UV irradiation was evaluated using three repreeentative anionic dyes ofcommercial importance: a monosulphonated anthraquinone milling dye (CI Acid Blue 25),a tetrasulphonated disazo dye (CI Direct Blue 25) and a sulphatoethylsulphonephthalocyanine dye (CI Reactive Blue 21). The effects of pH, temperature and Fe(II) orFe(III) ions as catalysts were examined and a reaction mechanism proposed [42].

The influence of substituents on the rates of degradation of arylazo reactive dyes based onH acid, caused by the action of hydrogen peroxide in aqueous solution and on cellulose, hasbeen investigated [43]. The results suggested that the oxidative mechanism involves attackof the dissociated form of the o-hydroxyazo grouping by the perhydroxyl radical ion [.OOH].The mechanism of oxidation of sulphonated amino- and hydroxyarylazo dyes in sodiumpercarbonate solution at pH 10.6 and various temperatures has also been examined. Theinitial rate and apparent activation energy of these reactions were determined. Theketohydrazone form of such dyes is more susceptible to attack than the hydroxyazo tautomer[44].

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The anthraquinone derivative CI Acid Blue 45 (3.42) is somewhat sensitive to oxidation(by sodium nitrite for example), especially in the presence of trace metals such as Fe(III) orCu(II) ions, forming the dull reddish blue tetrahydroxyanthraquinone disulphonate, whichexhibits inferior levelling properties. The formation of isatin (3.43) and anthranilic acid(3.44) as oxidative decomposition products from indigo (3.37) during the acid washing ofdenim with permanganate has been confirmed. Either of these compounds is capable ofcausing undesirable yellowing and dulling of the indigo dyeing if not removed in subsequenthot water rinsing [45].

HO

H2N

NH2

OH

O

O

NaO3S

SO3Na

3.42

CI Acid Blue 45

N

O

O

H3.43

Isatin

NH2

COOH

3.44

Anthranilic acid

The decolorisation of disodium 1-phenylazo-2-naphthol-6,8-disulphonate (CI AcidOrange 10) and CI Direct Red 2 (3.8; X = CH3, Y = H) by aqueous sodium hypochloritewas followed by UV spectroscopy. The rates of degradation of both dyes in the initial stageof the reactions conformed to a second-order rate equation [46]. The mechanism ofoxidation of a series of orange disodium 2-arylazo-1-naphthol-3,6-disulphonates (3.45) indilute sodium hypochlorite involved initial attack at the imino nitrogen of the dye in itsketohydrazone tautomeric form by the electrophilic chloronium ion (Cl+). The diazocomponent is regenerated and the naphthol residue is oxidised to the o-quinonoid analogue(Scheme 3.11). Sulphonic or carboxylic acid groups located ortho to the azo linkage areparticularly effective in retarding the rate of azo decomposition [47].

N

N

HO

NaO3S

SO3Na N

N

O

NaO3S

SO3Na

H

N

N

O

NaO3S

SO3Na

Cl

Cl+

N N Cl

O

O

O

NaO3S

SO3Na

_

+

3.45

+

Scheme 3.11


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The effect of ionic strength on the aggregation and degradation rates of the orange dyedisodium 1-(2′-methylphenylazo)-2-naphthol-3,6-disulphonate (3.46) in sodium hypo-chlorite solutions was studied by visible absorptiometry. The slowest rate of decompositionof the dye was observed under conditions in which the degree of aggregation was at amaximum. This confirmed that the dye molecules shielded inside aggregates are oxidisedsignificantly more slowly than the free monomer in solution [47]. When this oxidationreaction was followed in surfactant solutions, the rate of degradation was markedlydependent on the chemical structure of the surfactant molecule [48]. The influence ofsubstituents on the rates of degradation of red arylazo reactive dyes based on H acid, causedby the action of hypochlorite in solution and on cellulose, has been evaluated [49]. At pHvalues above 5 only the dissociated form of the o-hydroxyazo grouping undergoes attack bythe chloronium ion, so that the pKa value of this group becomes a decisive criterion for thefastness to hypochlorite treatment.

N

CH3

N

H O SO3Na

SO3Na3.46

N

NAr

N

SO3Na

H

X

3.47

O

N

SO3Na

NN

X

H

NAr

H

NaO3S

N

Ar

3.48

The influence on colour fastness to chlorinated water resulting from incorporating asulpho group into various positions of the arylazo-1-naphthol chromogen in typical orangereactive dyes was examined. Enhanced fastness could be attributed to steric protection ofthe imino site in the predominant ketohydrazone form of these dyes [50]. In a similar studyof orange reactive dyes based on the arylazoarylamine system, it was found particularlyeffective to incorporate the sulpho group into the 8-position of these 4-arylazo-1-naphthylamine dyes (3.47; Ar = aryl, X = reactive group), peri to the imino group bearingthe reactive system. In the case of blue disazo reactive dyes derived from twice-coupled Hacid (3.48), the two phenylazo groups located in the 2,7-positions make a majorcontribution to protecting the imino group in the 8-position. In all such imino-substitutedacid, direct or reactive azo dyes of these or related types, electrophilic attack on the iminogroup by the chloronium ion in sodium hypochlorite solution initiates the degradationprocess, drastically accelerating the subsequent cleavage of the azo linkage [51].

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3.1.7 Dispersion properties and aqueous solubility of disperse dyes

An aqueous dispersion of a disperse dye contains an equilibrium distribution of solid dyeparticles of various sizes. Dyeing takes place from a saturated solution, which is maintainedin this state by the presence of undissolved particles of dye. As dyeing proceeds, the smallestinsoluble particles dissolve at a rate appropriate to maintain this saturated solution. Only thesmallest moieties present, single molecules and dimers, are capable of becoming absorbed bycellulose acetate or polyester fibres. A recent study of three representative CI Disperse dyes,namely the nitrodiphenylamine Yellow 42 (3.49), the monoazo Red 118 (3.50) and theanthraquinone Violet 26 (3.51), demonstrated that aggregation of dye molecules dissolvedin aqueous surfactant solutions does not proceed beyond dimerisation. The proportionpresent as dimers reached a maximum at a surfactant:dye molar ratio of 2:5 for all threedyes, implying the formation of mixed dye-surfactant micelles [52].

NH SO2

N

HO2N

3.49

CI Disperse Yellow 42

N

N N

CH2CH2OH

CH2CH2OHO2N

Cl

Cl

Cl

3.50

CI Disperse Red 118O

O

O

O

NH2

NH2

3.51

CI Disperse Violet 26

Surfactants are invariably incorporated in the formulation of commercial disperse dyepowders, grains or liquid brands. Further additions of surfactants to the dyebath may bemade in batchwise or continuous application. The initial purpose of the agents included bythe manufacturer is to prevent the finely divided particles of dye from agglomeration duringmilling or spray drying. They also help to maintain concentrated dispersions in a stablecondition during transportation and handling, to minimise agglomeration and settling duringstorage of liquid brands.

In the dyeing process, where the dispersion is much less concentrated, the function of thesurfactant addition is to control a process of dynamic change rather than to preserve a staticequilibrium for as long as possible. Here the agents play an active part in accelerating thedisintegration and dissolution of dispersed particles and thus in increasing the concentrationof dissolved dye present in the dyebath. This has the secondary effect of increasing the rateat which dye can be adsorbed onto the fibre surface. During cooling of the dyebathsurrounding the dyed material, the presence of a surfactant helps to retard agglomeration,coalescence or crystallisation of dye particles from the saturated solution as it cools.


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Coalescence of dyes during cooling is characteristic of their existence in a viscous state atelevated temperatures [53]. Surfactants minimise the redeposition of insolubilised dye ontothe surface of the fibres from the cooling bath.

Numerous disperse dyes are marketed in a metastable crystalline form that givessignificantly higher uptake than the corresponding more stable modification. The molar freeenthalpy difference can be used as a criterion of the relative thermodynamic stabilities oftwo different modifications [53]. Certain dyes can be isolated in several differentmorphological forms. For example, an azopyrazole yellow disperse dye (3.52) was prepared infive different crystal forms and applied to cellulose acetate fibres. Each form exhibited adifferent saturation limit, the less stable modifications giving the higher values [54].

In a later investigation three different crystal modifications of each of three commerciallyimportant monoazo CI Disperse dyes, namely Yellow 3 (3.53), Yellow 42 (3.49) and Red 54(3.54), were characterised in terms of melting point, X-ray diffraction pattern and dyeingbehaviour on polyester fibres. Different fibre saturation values were found for the variousforms of Yellow 42 and Red 54, but the three modifications of Yellow 3 did not differ in thisrespect [55]. A dispersing agent may influence the aqueous solubility of crystal modificationsof the same dye to differing extents, so that the rate of change from a metastable to a morestable form on heating may be modified in the presence of an agent. On the other hand, ametastable form of Yellow 3 showed no tendency to revert to the stable form in the absenceof any dispersing agent [56].

N

NO2

NN

N

H2N

H3C

CH3CH2SO2

3.52

N

N

CH3CONH

HO

CH3

3.53


Cl

N

N N

CH2CH2OCOCH3

CH2CH2CNO2N

3.54

CI Disperse Red 54

The saturation solubility of a disperse dye in water at a given temperature may be definedas the maximum amount of pure dye that will give a single homogeneous phase whendissolved in a given volume of pure water. The qualification as ‘saturation solubility’ impliesthat any further addition of dye would remain undissolved as a separate solid phase.Solubilisation of a disperse dye refers to the marked increase in saturation solubility thatoccurs when a water-soluble solubilising agent is added. If a water-miscible co-solvent suchas acetone or ethanol is added to an aqueous suspension of a disperse dye, there is a gradualincrease in the amount of dissolved dye that is proportional to the quantity of co-solventadded.

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A disperse dye suspension responds somewhat differently when a surface-activesolubilising agent is added. At low concentrations of this type of additive the saturationsolubility of the dye remains relatively little changed, but when the critical micelleconcentration of the surfactant is reached a sudden marked increase in dye solubility isobserved [57]. When micelles of the surfactant begin to form in the solution, these providea more amenable environment into which the dye molecules or dimers can transfer. Abovethe critical micelle concentration the increase in solubility of the dye is directly proportionalto the concentration of surfactant present [1,56].

The saturation solubility of pure disperse dyes in water is often extremely low at ambienttemperature. An early survey of about forty typical dyes for cellulose acetate yielded valueswithin the range 0.1 to 32 mg/l [58]. Raising the temperature to 80 °C results in aconsiderable increase in saturation solubility, but the solubilising power of surfactants isessential for the satisfactory dyeing of cellulose acetate with the less soluble dyes. Dyes forpolyester tend to be more hydrophobic in structural balance. These require considerablesolubilisation by agents even at 130 °C in order to give satisfactory dyeing behaviour andfreedom from agglomeration or redeposition on cooling.

The hydrocarbon groupings in disperse dye molecules represent the most hydrophobicportions and these favour dimerisation and agglomeration in an aqueous environment.Substituents containing electronegative atoms (such as nitrogen, oxygen or sulphur),especially when bearing hydrogen atoms (as in =N–NH–, –NH2, –CONH–, –OH, –SHgroups) are able to interact strongly with water to form hydrogen bonds and thus tocontribute to solubility in water [1].

Much of the available published data on the aqueous solubility of disperse dyes wasaccumulated during the 1960s in the course of studies of the mechanism of dyeing celluloseacetate with disperse dyes. Most of the dyes examined were low-energy types for dyeingacetate (Table 3.5). Particular attention was given to monoazo dyes derived from aniline or

Table 3.5 Aqueous solubility of disperse dyes of related structure [59-63}

Solubility(mg/l)

Temp (°C): 60 80 80 90 100CI Disperse Structure Lit. ref.: 59 60 61 62 63

3.55; X = H, Y = H 1413.55; X = NH2, Y = H 74

Orange 3 3.55; X = NO2, Y = H 3 16.6Orange 1 3.55; X = NO2, Y = Ph 1.1

3.56; X = Ph 2143.56; X = 1-naphthyl 7.8

Red 19 3.57; X = Y = CH2CH2OH 26.4Red 1 3.57; X = CH2CH3, Y = CH2CH2OH 7.7

3.57; X = Y = CH2CH3 ca 0.2Blue 23 3.58; X = Y = NHCH2CH2OH 61Blue 3 3.58; X = NHCH3, Y = NHCH2CH2OH 30Blue 14 3.58; X = Y = NHCH3 2Violet 1 3.58; X = Y = NH2 21.7Red 15 3.58; X = OH, Y = NH2 11.7 38

3.58; X = Y = OH 17


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p-nitroaniline coupled to N,N-disubstituted anilines with ethyl or 2-hydroxyethylsubstituents and to 1,4-disubstituted anthraquinones containing potential hydrogen bondinggroups (–OH, –NH2, –NHCH3 or –NHCH2CH2OH).

In the azo series, unsubstituted aniline as a diazo component gave more soluble dyes thandid p-phenylenediamine or p-nitroaniline, in that order. Introduction of an additional arylring resulted in a marked decrease in solubility, as shown by comparing CI Disperse Orange1 with Orange 3, or phenylazo-N,N-bis(2-hydroxyethyl)aniline (3.56; X = Ph) with its 1-naphthylazo analogue. Hydroxyethyl groups are readily solvated in water by hydrogenbonding and therefore confer much higher solubility than alkyl substituents in analogousdyes. Examples are given in Table 3.5 from both the monoazo (3.57) and the anthraquinone(3.58) series. The solubility is approximately doubled if two hydroxyethyl groups areintroduced instead of one. Aminoanthraquinone dyes are markedly more soluble than theirhydroxy analogues.

In a recent study of twenty disperse and solvent dyes, data for water solubility, octanol/water partition coefficient, entropy of fusion and melting point were subjected to regressionanalysis. Complicating factors such as impurities, polymorphism, tautomerism, polarisationand hydrogen bonding precluded the development of reliable predictions of solubility andpartition coefficient. Anthraquinone dyes exhibited much lower entropy of fusion thanmany of the azo dyes [64,65].

3.2 DYEABILITY OF FIBRES IN RELATION TO DYE STRUCTURE

The chemical forces that operate between dye molecules and fibrous polymers are essentiallythe same as those governing the clustering of water molecules, dye solubilisation in aqueousmedia and the aggregation or agglomeration of dyes. In dyeing systems depending on anessentially ionic mechanism, such as the dyeing of acrylic fibres with basic dyes or theapplication of anionic dyes to nylon, silk or wool, a decisive role is played by electrostaticattraction between the charged dye ions and sites of opposite charge in the substrate.Shorter-range forces make a notable contribution to the affinity of the dye ion but theirfunction is essentially supportive.

In all other dyeing systems, however, the charge (if any) on the dye ion or molecule doesnot enhance affinity. A cellulosic fibre, for example, normally carries a negative charge likethat of an approaching dye anion and measures must be taken to minimise the forces of

X N

N N

Y

H

3.55

X N

N N

CH2CH2OH

CH2CH2OH

3.56

O2N N

N N

Y

X

3.57

O

O

X

Y

3.58

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repulsion arising from this similarity. Under these circumstances the less specific shorter-range forces (hydrogen bonding and van der Waals or dispersion forces) make a much moresubstantial contribution to dye-fibre bonding.

3.2.1 Dyeing of cellulosic fibres with anionic dyes

Within a structural series of direct dyes, the lower the equivalent mass (that is, the ratio ofrelative molecular mass to sulphonic acid group content) the higher will be the aqueoussolubility of the dye. As already noted, the converse of this general trend is that dyemolecules of higher equivalent mass tend to aggregate together more readily because of thegreater opportunity for hydrophobic interaction. In the presence of a cellulosic substrate,such dyes will tend to escape more readily from the aqueous dyebath by adsorption onto thefibre surface. It is reasonable to assume that the dye molecules will become adsorbed in sucha way that as far as possible the hydrophilic sulphonate groups are directed outwards towardsthe aqueous phase.

Just as addition of an electrolyte to a solution of a direct dye tends to lower theelectrostatic repulsion between the negatively charged dye ions and promote aggregation,the effect of the presence of salt when dyeing a cellulosic fibre is to overcome the long-rangeforces of repulsion between the dye anions and the negatively charged fibre surface. Thecloser approach then allows hydrogen bonding and other short-range attractive forces tooperate between the dye molecules and the glucoside units of cellulose. This favourableinfluence of electrolyte addition is as useful with anionic leuco forms of vat or sulphur dyesas it is with sulphonated direct or reactive dyes.

Hydrogen bonding between the hydroxy groups of cellulose and centres ofelectronegativity (nitrogen, oxygen or sulphur), especially those substituted with hydrogenatoms (as in =N–NH–, –NH2, –CONH–, –OH, –SH groups), is widely acknowledged tocontribute to adsorption and retention of the dye molecules. Stereochemical studies haveshown that hydrogen bonding is possible for almost all cellulose hydroxy groups in theamorphous regions of the fibre. Measurements of the heat of dyeing confirm that a typicaladsorption process entails the formation of at least two hydrogen bonds per molecule of dyeadsorbed [66,67]. It is obvious that retention of the adsorbed molecule will be favoured thehigher the number of potential dye–fibre bonding points and the more widely they arespaced along the length of the dye molecule. The more hydrogen bonds that a dye can formwith the glucosidic polymer, the more readily it can compete with and rupture the fibre–fibrebonds in order to penetrate more deeply into the amorphous structure of the fibre.

The recognition that the existence of conjugation in all organic colorants was essential toprovide intense colour (section 1.5) and that an increase in the length of the conjugatedchain produced a bathochromic shift of the main waveband in the absorption spectrum ledto the adoption of related ideas in the context of the theory of substantivity. Many directdyes with high affinity for cellulose are disazo or trisazo structures in which the azo groupsare located in para positions relative to one another, so that the longest conjugated chainthrough the azo-linked aryl nuclei tends to be as near linear as practicable. The presence ofan amide group that is conjugated via a 1,4-phenylene or 2,6-naphthylene residue to thenearest azo group, as in the commercially important CI Direct Red 81 (3.7; X = H),enhances the substantivity of the molecule by lengthening the conjugated chain, amodification that also causes a bathochromic shift.

DYEABILITY OF FIBRES IN RELATION TO DYE STRUCTURE

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A methine group oriented para to an azo group through an intervening aryl nucleusboosts substantivity even more than does an amide group in a similar position. This wasdemonstrated for a series of azostilbene and related dyes (3.59; Ar = Ph, X = CH=CH,CONH or CO) with their thiophene analogues (Table 3.6). Affinity increased in the orderof increasing electron mobility (CH=CH > CONH > CO) and was greater for theheterocyclic derivatives than for the corresponding phenyl-substituted dyes [68]. A series ofanalogues of the most substantive of these dyes (containing both the thiophene ring and themethine linkage) proved to be highly sensitive to the nature of the coupling component atthe opposite end of the conjugated chain, however (Table 3.7). The presence of an aminogroup favours substantivity (naphthionic acid > NW acid, H acid > chromotropic acid) butthe extra sulpho group in the three disulphonated couplers affects it adversely.

The significance of conjugation as a contributor to the substantivity of dyes for celluloseis not always easy to distinguish from the effect of the degree of linearity of the molecule.Almost all direct dye molecules possess flexible chains of aryl nuclei linked by azo or otherunsaturated groups. Such structures can readily adopt a near-linear spatial conformation, as

X N

N

ArNH2

SO3Na3.59

CH

CH N

N

HS

Y

O

SO3Na

O

H2N

SO3Na

NaO3S

O

HO

SO3Na

NaO3S

O SO3Na

SO3Na

3.60

Y1 = Y2 =

Y3 =

Y4 =

Table 3.6 Substantivity and structure of p-substitutedmonoazo naphthionic acid derivatives [68]

Substituents in structure 3.59

Ar X Affinity(kJ/mol)

Thienyl CH=CH –15.8Phenyl CH=CH –14.3Thienyl CONH –13.1Phenyl CONH –12.0Thienyl CO –8.8Phenyl CO –7.0

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far as the angular constraints of the participating bonds will allow. Azo groups attached tocentral naphthylene residues are frequently oriented in 1,4- or 2,6-positions relative to oneanother. Typical central diamines that give linear dyes on tetrazotisation include p-phenylenediamine, benzidine, p,p′-diaminostilbene, p,p′-diaminodiphenylamine and p,p′-diaminodiphenylurea. For similar reasons, 1,4-bis(arylamino)anthraquinone vat dyes aremarkedly more substantive than their 1,5-disubstituted analogues.

Table 3.7 Substantivity of thienyl monoazo dyes with various couplingcomponents [68]

Coupling component Structure Affinity(kJ/mol)

Naphthionic acid 3.59; Ar = thienyl, X = CH=CH –15.8NW acid 3.60; Y = Y1 –9.7H acid 3.60; Y = Y2 –9.4Chromotropic acid 3.60; Y = Y3 –7.1R acid 3.60; Y = Y4 –6.3

N

N

H

N

N

H

SO3Na

NaO3S NaO3S

O O

H2N

3.61

N

N

H

NaO3S

O

N N

H

O

SO3Na

SO3Na

H2N

3.62

N

N

H

NaO3S

O

HN

H2N

SO3Na

SO3Na

O N

3.63

The ability to adopt an extended configuration has been recognised for many years to bea desirable feature of substantive dyes. It helps to explain why J acid is such a popular choiceas a central component in unsymmetrical disazo dyes. For example, it is much easier foraniline→J acid→H acid (3.61) with the 2,6-naphthylene substitution pattern to adopt alinear conformation than for the similar disazo dyes aniline→γ acid→H acid (3.62) andaniline→H acid→J acid (3.63) with 2,7- and 2,8-disubstitution respectively.


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A further refinement of the concept that linear conjugated systems such as aryl–azo–aryl–azo–aryl represented the preferred design for highly substantive molecules was thestipulation that the aryl nuclei must be able to adopt a coplanar conformation [69]. Thisfavours the adsorption of one dye molecule onto another during the formation of laminaraggregates, as well as the multipoint adsorption of a dye molecule onto two or moreglucoside units of a cellulose chain via hydrogen bonding sites. There are unequivocalexamples that clearly demonstrate the validity of this interpretation. Thus CI Direct Red 2(3.8; X = CH3, Y = H) and other 3,3′-disubstituted analogues are markedly moresubstantive than their meta analogues with 2,2′-disubstitution (such as 3.8; X = H, Y =CH3). These substituents adjacent to the central bond in these symmetrical disazostructures inhibit free rotation of the phenylene nuclei and thus prevent the adoption of acoplanar conformation.

The scope for intramolecular hydrogen bonding between each azo group and couplingcomponent in symmetrical disazo structures of this kind also contributes significantly to themaintenance of coplanarity and hence to the affinity for cellulose. The number ofcommercial direct dyes in which coupling has been directed para to the amino or hydroxysubstituent in the coupling component is much smaller than those manufactured by orthocoupling. Within the series of symmetrical o-tolidine disazo dyes in which the naphthionicacid of CI Direct Red 2 (3.64; X = SO3Na, Y = H) is replaced in turn by Laurent’s acid,Cleve’s 6 and 7-acids, and finally Peri acid (Table 3.8), there is a marked lowering of affinityfor the 6-, 7- and 8-sulpho derivatives compared with the first two members of the series[70]. Naphthylamine-4- and –5-sulphonic acids can only couple ortho to the aminosubstituent and the derived dyes preferentially adopt an extended coplanar arrangement(3.64) by intramolecular hydrogen bonding of the amino group to the more distant nitrogenatom of the adjacent azo group. In the remaining members of the series, however, couplingreadily takes place in the para position (3.65) and the amino groups are no longer able toform intramolecular hydrogen bonds. Free rotation is possible on either side of the azolinkages and the preferred coplanar arrangement is no longer preferentially stabilised.

N

N

N

H

H

N

N

H

CH3H3C

N

H

X

YY

X 3.64

N

N

H2N

Z

Y X

N

CH3H3C

N NH2

Z

X Y3.65

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Vat dye molecules are relatively hydrophobic and are usually capable of assuming acoplanar configuration that favours aggregation in a laminar arrangement as well as theformation of multiple attachments to a segment of the cellulose chain. Hydrogen bondingcan be invoked to account for the favourable influence of arylamino or benzoylaminosubstituents on the affinity of the parent anthraquinonoid nuclei in vat dye structures.Purely carbocyclic quinones such as pyranthrone, violanthrone and dibenzopyrenequinonemust depend mainly on coplanarity, hydrophobic interaction and hydrogen bonding betweendye keto groups and cellulose hydroxy groups. In any closely related series of vat dyes theaffinity does seem to increase with molecular size, but the differences between members ofdifferent subclasses emphasise the specific influence of their characteristic structuralfeatures, especially nitrogen-containing groupings, on the forces of affinity.

A full interpretation of the relationships between direct or vat dye structure andsubstantivity for cellulose must take into account the contribution of multilayer adsorptionof dye molecules within the pore structure of the fibre [71]. The great difference insubstantivity between CI Direct Red 28 (3.66) and the monoazo acid dye (3.67) that is the‘half-size’ analogue of this symmetrical disazo dye may be interpreted in terms of theirrelative tendencies to form multilayers within the fibre pores as a result of dye–dyeaggregation. Saturation adsorption values of these two dyes on viscose fibres at pH 9 and50 °C corresponded to monolayer coverage areas of approximately 90 and 11 m2/g ofinternal surface respectively [72]. In view of the smaller molecular area and greater mobilityof the ‘half-size’ acid dye, higher uptake than the direct dye would be anticipated if therewere only a limited area of internal surface available for true monolayer adsorption.

Table 3.8 Substantivity of symmetrical disazo dyes derived fromnaphthylamine monosulphonic acids [70]

Coupling component Structure Affinity(kJ/mol)

Naphthionic acid 3.64; X = SO3Na, Y = H –26.8Laurent’s acid 3.64; X = H, Y = SO3Na –28.0Cleve’s 6-acid 3.65; X = SO3Na, Y = Z = H –22.6Cleve’s 7-acid 3.65; Y = SO3Na, X = Z = H –22.2Peri acid 3.65; Z = SO3Na, X = Y = H –23.4

NH2

N

N

SO3Na

N

N

NH2

SO3Na3.66

CI Direct Red 28

N

N

NH2

SO3Na3.67


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The much higher adsorption of the direct dye at saturation equilibrium can be explainedsatisfactorily on the basis that both dyes are adsorbed initially on essentially the sameinternal surface area, but the direct dye has a much greater tendency to aggregate and sobuild up readily into multilayers. The greater length of the direct dye molecule relative tothe acid dye of similar cross-sectional area favours penetration into the intermicellar spacesbetween the cellulose fibrils, followed by multipoint adsorption onto the glucoside units oranother dye molecule already adsorbed. Inorganic electrolytes assist powerfully in promotingthese interactive forces leading to multilayer adsorption of direct dyes, since they are presentin the intermicellar pore spaces as well as the external phase. Acid dyes aggregate much lessreadily in solution (section 3.1.2) and are thus less likely to form multilayered aggregates inthe fibre pores.

The pore structure of cellulosic fibres has a decisive influence on direct dyeing. It can bedefined using probe molecules of specific graduated sizes, measurement being by either thestationary non-solute water method or by chromatographic elution on a cellulose column.The distribution of pores can be determined from chromatographic values, revealing theaccessibility of the inner voids to dye molecules of different sizes [73]. Pore structure datahave been related to the adsorption and diffusion kinetics of the disazo dyes CI Direct Blue1 (3.2) and Red 81 (3.7; X = H). The specific pore surface can be decisive for equilibriumuptake, whereas the porosity or the amount of free water can be important wheninterpreting differences in diffusion rate [74].

3.2.2 Dyeing of amide fibres with anionic dyes

Wool, silk and nylon contain both basic and acidic groups, amongst which by far the mostimportant are amino and carboxy groups respectively. Just like the parent amino acids fromwhich protein fibres are derived, all three of these polymeric amides show zwitterioniccharacteristics at pH values close to the isoelectric point, the pH at which the fibre containsequal numbers of protonated basic and ionised acidic groups. As the pH decreases belowthis point, the carboxylate anions are progressively neutralised by the adsorption of protonsand the fibre acquires a net positive charge (Scheme 3.12). Conversely, as the pH risesabove the isoelectric point, the fibre becomes negatively charged as a result of deprotonationof the amino groups by adsorption of hydroxide ions or other simple anions (Scheme 3.13).

H3N [fibre] COO H3N [fibre] COOH_+ +

+ H+

Scheme 3.12

H3N [fibre] COO H2N [fibre] COO + H2O_+

+ OH__

Scheme 3.13

The capacity of nylon or the protein fibres to adsorb simple organic or inorganic acids isclosely equivalent to their respective contents of accessible amino groups. More complex dyeanions, however, differ in their affinity for these fibres owing to nonpolar bonding betweenthe hydrophobic portions of the dye molecule (alkyl substituents and unsubstituted aryl

chpt3(1).pmd 15/11/02, 14:42122

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nuclei) and nonpolar side-chain or main-chain segments in the fibrous polymer. Thecontributions to dye anion affinity of electrostatic attraction to protonated amino groups inthe fibre and hydrophobic interaction with nonpolar segments tend to mask any furtherinfluence of hydrogen bonding forces, although this form of bonding seems likely toreinforce the effects of the other factors. According to specific adsorption studies on wool,silk and nylon, the tendency of proton donor molecules to form hydrogen bonds with amidegroups in these polymers is in the order: ArOH > ArNHCOCH3 > ArNH2, ArNHR,AlkOH.

As a general rule, the lower the dyebath pH the more rapid is the rate of initialadsorption by the amide fibre and the subsequent approach towards equilibrium exhaustion.Consequently, the attainment of a safely controllable rate of dyeing becomes easier thehigher the pH at which dyeing begins. Eventually, however, a pH is reached at which thedyebath is no longer exhausted within a reasonable time.

Dyeing conditions are usually controlled to give a moderate initial rate of dyeing foroptimum levelling, followed by a gradual lowering of pH by acid release to achieve as muchexhaustion as possible at the end of the process. Selection of the preferred initial pH andrate of acidification is determined by the affinity of the dyes at neutral pH values. Neutral-dyeing affinity is dependent on the structural features and hydrophilic-lipophilic balance ofthe dye molecule.

The degree of sulphonation of an acid dye (section 1.6.7) has a major influence on theprofile of exhaustion values reached at various pH values on a specific substrate. At low pHin the absence of electrolyte all acid dyes show high exhaustion, irrespective of their sulphogroup content. In neutral solution, on the other hand, multisulphonated dyes generally showmuch lower exhaustion than dye monosulphonates. Above the isoelectric point the fibre isnegatively charged and the forces of electrostatic repulsion increase with the degree ofsulphonation. Hydrophobic interaction permits adsorption of a dye monosulphonate ontothe fibre surface in such a way that the charged group is directed outwards towards thedyebath phase.

The rate of dyeing of acid dyes of relatively small molecular size decreases with increasingdegree of sulphonation. Simple monoazo monosulphonate types are rapidly absorbed butreadily desorbed again. Thus they migrate easily but show extremely poor fastness to wettreatments. There are two main types of levelling acid dye:(1) Disulphonated dyes of relative molecular mass (Mr) about 400–600 that are somewhat

sensitive to dyeability differences in the substrate and show the lowest wet fastness ofall dye classes used on wool

(2) Monosulphonated dyes of lower Mr (300–500) and slightly higher wet fastness thatmigrate more readily and cover dyeing faults (such as carbonising damage) moreeffectively.

In an early study of the dyeing of blends of normal and chlorinated wool with a series ofnaphthionic acid→2-naphthol dyes [75], the trisulphonated CI Acid Red 18 (3.68; X = Y= SO3Na) showed the most marked contrast. Penetration of the hydrophobic epicuticle ofnormal wool by this hydrophilic dye was negligible in the initial stage of dyeing butchlorinated fibres were rapidly and deeply dyed. The monosulphonated Red 88 (3.68; X = Y= H), however, revealed relatively little contrast in dyeability between the two types ofwool, whereas Red 13 (3.68; X = SO3Na, Y = H) gave a degree of contrast intermediate


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between those of the other two dyes. Apparently the relatively hydrophobic 2-naphtholresidue in CI Acid Red 88 is almost as effective a means of attachment to the epicuticle ofnormal wool as the negatively charged naphthionic acid residue is towards the hydrophilicsurface of chlorinated wool.

NaO3S N

N

H

Y

X

O

3.68

There are three main subclasses of milling acid dyes:(1) Monosulphonated dyes of Mr 500–600 are somewhat more hydrophobic than the

monosulphonated levelling acid dyes. They also migrate and cover well but they are alittle inferior to disulphonated milling dyes in wet fastness and thus have sometimesbeen described as ‘half-milling’ dyes;

(2) Disulphonated dyes of Mr 600–900 diffuse much more slowly than typical levelling aciddyes. Thus they exhibit correspondingly higher wet fastness but their migration andcoverage properties are inferior;

(3) Certain disulphonated milling acid dyes contain higher alkyl groups (such as butyl,octyl, dodecyl) to confer higher neutral dyeing affinity, better coverage of dyeabilityirregularities and exceptionally good wet fastness. They are sometimes described as‘super-milling’ dyes to distinguish them from category (2), the largest of the threesubclasses.

Direct evidence has been provided to demonstrate the contribution of hydrophobicinteraction to the substantivity of milling dyes for wool [76]. In a series of monosulphonatedphenylazopyrazolone dyes (Table 3.9), substantivity for wool increased with chain length inthe alkyl series (B >> D > F) but the increased polarisability of alkenyl groups almostcompletely negated this effect (B >> E). Cyclisation of the saturated alkyl group producedmaximum substantivity (A > B) and formation of an unsaturated aryl ring had a similareffect (C >> E). Finally, a hydroxy substituent almost completely inhibited the hydrophobicinteraction attributable to the ethyl group (D > G). It was evident from these results thatcoulombic interaction and hydrogen bonding played insignificant roles in these equilibria.

Table 3.9 Exhaustion of p-substituted phenylazopy-razolone dyes on wool [76]

Dye Substituent (R) in 3.69 Exhaustion (%)

A Cyclohexyl 85B n-Hexyl 65C Phenyl 59D Ethyl 17E Hexatrienyl 10F Hydrogen 8G 2-Hydroxyethyl 3

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Mordant dyes have always been important for the dyeing of wool. Chromium has becomethe only significant mordant because it yields exceptionally high fastness to light and wettreatments. Aftertreatment with dichromate is the only chroming process of any practicalimportance on wool. Chromic salts must be used on silk, however, which lacks the reducingpower of wool to convert dichromate Cr(VI) anions to chromic Cr(III) cations. Despite theemergence of various ranges of metal-complex acid dyes and reactive dyes, chrome dyes stillretain a market share at about one-third of all dyes for wool (Table 3.10).

N

N

H

N

N

SO3Na

R

H3C

O

3.69

Table 3.10 Estimated proportions of wool dyed worldwide in1995 with different ranges of dyes [77]

Dye range Proportion (%)

Chrome mordant dyes 291:2 Metal-complex acid dyes 29Milling acid dyes 20Levelling acid dyes 91:1 Metal-complex acid dyes 7Reactive dyes for wool 6

The practical advantages of chrome dyes include economic build-up to full depths, goodlevel-dyeing properties (Mr only 300–600) and excellent fastness after chromium-complexformation. Unfortunately, the marked change in colour of the dyeing during aftertreatmentcomplicates the colour matching stage. Further drawbacks include the prolonged dyeingprocedure, oxidative damage to the wool and the environmental hazards associated withchromium-containing effluent. All dyes requiring the use of chromium compounds havecome under increasing pressure recently for ecological reasons. Hexavalent chromium in theform of dichromate salts is causing concern because of its toxic and mutagenic propertiesrequiring special safety measures in handling [77].

The 1:1 metal-complex acid dyes are almost all monosulphonates of Mr 400–500. This givesthem dyeing characteristics not unlike those of the monosulphonated levelling acid dyes, withgood migration and coverage of damaged wool. In fabric dyeing it is usual to apply the 1:1metal-complex dyes at pH 2, sacrificing optimum physical properties of the wool in favour ofbetter migration and coverage. The 1:2 metal-complex dyes that were originally developedfollowing the successful adoption of the 1:1 metal-complex types were symmetrical structuresfree from ionic solubilising groups and some were applied from aqueous dispersion. Most dyesof this type now used on wool or nylon are solubilised by one nonionised but polar group, suchas sulphonamide (–SO2NH2), methylsulphonamide (–SO2NHCH3) or methylsulphone(–SO2CH3) on each of the dye ligand groups, which are often dissimilar. These are moreexpensive to manufacture than the traditional symmetrical structures, however, and share


chpt3(1).pmd 15/11/02, 14:42125


most of their drawbacks. They show high neutral-dyeing affinity and very good fastness tolight, although they are significantly inferior to chrome dyes in fastness to the most severe wettreatments. Such hydrophobic structures favour coverage of dye-uptake variations in wool buttheir high affinity can cause rapid initial strike and they diffuse and migrate slowly (Mr 700–900).

Advances were made in developing dyeing methods and auxiliary products (sections 5.8.3and 12.2) to improve the level-dyeing and coverage properties of sulphonated 1:2 metal-complex dyes, leading to the widespread adoption of the more economical of themonosulphonates in uses where unsulphonated dyes were previously considered essential.Sulphonated 1:2 metal-complex dyes may be divided into two subclasses:(1) Unsymmetrical monosulphonated dyes of Mr 700–900. The presence of the ionic

solubilising group reduces manufacturing costs and minimises the staining of adjacentsin severe wet tests, but these dyes are more sensitive to dye-affinity differences than arethe unsulphonated 1:2 metal-complex types.

(2) Disulphonated dyes of Mr 800–1000, many of them symmetrical in structure. Althoughoften cheaper to manufacture than the unsymmetrical monosulphonates and theunsulphonated types, these slow-diffusing complexes have intrinsically poor levellingand migration properties.

Reactive dyes for wool are unmetallised acid dye structures that contain reactive groupscapable of forming covalent bonds with nucleophilic sites in wool. Several types of reactivesystem have proved effective on wool, the most important being α-bromoacrylamide, 5-chloro-2,4-difluoropyrimidine and the N-methyltaurine precursor of vinylsulphone. Specialamphoteric levelling agents were developed to minimise the inherent skittery dyeingproperties of these relatively hydrophilic dyes (section 12.7.2).

Inherently more expensive than traditional dyes for wool, reactive dyes also requireaftertreatment with ammonia to ensure optimum fixation and wet fastness. Consequently,their usage has been limited (Table 3.10) mainly to bright dyeings on machine-washablewool and other high-quality goods. Their moderate to good levelling and migration beforefixation (Mr 500–900, usually two or three sulpho groups), together with excellent wetfastness make them highly suitable for dyeing shrink-resist wool, usually in slubbing or yarnform.

If the wool cuticle is damaged by local chemical attack, abrasion or exposure to light,more rapid strike of dye occurs on the exposed cortical cells. At low dyeing temperatures allanionic dyes are taken up preferentially by damaged fibres and fibre tips [78], but attemperatures close to the boil the more polar the dye the stronger is the preferentialabsorption by damaged fibres [79]. Dyes of low Mr, particularly milling acid and sulphonated1:2 metal-complex dyes, cover such differences well by subsequent migration away from thetip towards the undamaged root and good coverage can be promoted using cationicauxiliaries. These interact with anionic dyes to form hydrophobic complexes that are lesssensitive to differences in fibre dyeability. Chrome dyes reveal a marked contrast betweenroot and tip, since both the mordant and the dye are preferentially absorbed by the damagedtips and the reducing action of the cystine breakdown products resulting from wool damageaccelerates the conversion of dichromate Cr(VI) to chromic Cr(III) ions. Most reactive dyesare highly sensitive to dye-affinity variations, especially on unchlorinated wool.

Normal nylon 6.6 fibres contain only about 35–45 milliequiv./kg of amine end groups and

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this limits the build-up attainable with the hydrophilic levelling acid dyes. Differential acid-dyeable nylon variant yarns can be made by modifying the amino-group content. Deep-dye(60–70 milliequiv./kg) and ultra-deep (>80 milliequiv./kg) variants are obtained by theinclusion of a primary aliphatic diamine containing a tertiary amino group, such as N,N-bis(2-aminoethyl)piperazine, in place of some of the hexamethylenediamine used inmanufacture of the normal polymer. Pale-dye nylon (15–20 milliequiv./kg) is made byreacting a proportion of the amine end groups in the normal polymer with a suitableblocking reagent such as γ-butyrolactone. Basic-dyeable nylon is produced by replacing someof the adipic acid used in the polymerisation by a suitable tribasic acid, such as 5-sulphoisophthalic acid. The use of acid, basic and disperse dyes to achieve multicolouredeffects on differential dyeing nylon has been described in detail elsewhere [80].

The distribution of an acid dye between the components of blends of acid-dyeable nylonvariants depends on the dyeing conditions and the molecular structure of the dye, especiallythe degree of sulphonation. At a given pH in the neutral region, dyebath exhaustion of aseries of dyes differing only in degree of sulphonation decreases as the number of sulphogroups in the molecule increases. These differences in substantivity become more marked asthe content of basic groups in the fibre increases, so that the deeper-dyeable variants aremore sensitive than the normal fibre to differences in degree of sulphonation within theseries of dyes. Consequently, more highly sulphonated dyes give better differentiationbetween the variant yarns at a given pH than the less-sulphonated dyes with higher neutral-dyeing affinity.


3.2.3 Dyeing of ester fibres with disperse dyes

Since disperse dyes and ester fibres (cellulose esters and polyester) do not contain any ionicgroups, dye-fibre attraction clearly depends on hydrogen bonding, dipole–dipole interactionand dispersion forces. Simple nonionised proton donors, such as phenol, are readily adsorbedby cellulose acetate and this has been taken as evidence of hydrogen bonding between thephenolic hydroxy groups and keto groups in the polymer. Most disperse dye structurescontain proton-donating groups such as hydroxy and amino, as well as proton acceptorssuch as keto, methoxy, azo and dimethylamino groups. Refractometric studies havedemonstrated that the α-keto hydrogen atoms in the acetate grouping –OCOCH3 aresufficiently polarisable to take part in hydrogen bonding with proton acceptors [81]. Thus allthe polar groups in a disperse dye molecule can contribute to its affinity for an ester fibreand not merely those with proton-donating character.

Monolayer experiments have indicated that face-to-face complexing occurs between azodisperse dye molecules and the hexa-acetylcellobiose units of cellulose triacetate, with themolecular segments lying parallel [82,83]. The relatively hydrophobic disperse dye moleculeleaves the aqueous dyebath and penetrates between the polymer chains, entering a nonpolarenvironment where water molecules are unable to follow. New sites for adsorption becomeaccessible as the dye molecule diffuses further into the amorphous regions of the fibre. Themore numerous the substituents on the dye molecule that can interact with the polymersegments, the more it is able to break the interchain bonds of the fibre structure. Thus thegreater the bonding capacity of a disperse dye molecule, the higher will be its saturationlimit in the fibre.

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It is possible to account for the absorption of any disperse dye by cellulose acetate,triacetate or polyester in terms of the combined action of powerful hydrogen bonds andweak dispersion forces. The latter forces (p-bonding) between aryl rings in the dye andterephthalate residues make a more important contribution on polyester, but onpolypropylene only weak aliphatic dispersion forces can operate [84]. Provided the dyemolecule is planar and can lie flat against the cyclic segments of the ester polymer, all polargroups on the fringes of the rings have the potential to contribute to the overall strength ofthe dye-fibre multiple bonding. This accounts for the much higher affinity of planar dispersedye molecules compared with nonplanar analogues (Table 3.11). Partition coefficient in thistable is defined as the ratio of the concentration (mol/kg) of dye in cellulose acetate to that(mol/l) in water.

N

O

O

R

3.70 (planar)

O

O

CH

CH

ArAr

N

R

Ar=

3.71 (non-planar)

Measurements of aqueous solubility and partition coefficient between cellulose acetateand water were compared for thirty disperse dyes and an approximate inverse relationshipwas postulated [60]. This can only be valid to a limited extent, however, because thepartition ratio also depends on the saturation solubility of the dye in cellulose acetate. Thisproperty varies from dye to dye and is not directly related to aqueous solubility. Thesolubilities of four dyes in a range of solvents were compared with their saturation values oncellulose acetate. Solubilities in benzene showed no significant correlation. With the othersolvents the degree of correlation increased in the order: ethanol < ethyl acetate < 20%aqueous diethylene glycol diacetate (CH3COOCH2CH2OCH2CH2OCOCH3). The last-named compound was suggested as a model with polar groups similar to those in celluloseacetate [86].

Table 3.11 Partition coefficient and saturation value of disperse dyes on secondarycellulose acetate [85]

Dye structure Substituent (R) Partition coeff. Satn. Value(mol/kg)

3.70 COOCH2CH3 998 0.2183.71 COOCH2CH3 39 0.03193.70 CONHCH2CH2OH 194 0.1353.71 CONHCH2CH2OH 2.6 0.0031

3.70 216 0.212

3.71 17 0.018

N OCO

N OCO

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129

Dyes containing hydrophilic N-2-hydroxyethyl groups interact with unsubstitutedhydroxy or keto groups in cellulose acetate by hydrogen bonding, whereas dyes with morehydrophobic N-alkyl substituents depend much more on nonpolar bonding. Hence dyes ofthe latter type have more affinity for cellulose triacetate or polyester than those of theformer type, which are more suitable for acetate dyeing. Unsymmetrically disubstituted dyesoften give higher absorption on secondary acetate than their symmetrical analogues (Table3.12), presumably because they can readily participate in both types of dye–fibre bondingmechanism.

Table 3.12 Solubility of disubstituted dyes in secondary celluloseacetate [86]

Dye structure X Y Solubility at 80 °C (g/kg)

3.72 H H ca 2.43.72 H OH 183.72 OH OH 143.73 H H ca 4.63.73 H CH2OH 223.73 CH2OH CH2OH 6.6

N

N N

CH2CH2

CH2CH2 X

Y

O2N

3.72

O

O

NHCH2

NHCH2

X

Y

3.73

In order to achieve efficient build-up to heavy depths when dyeing cellulose acetate at80 °C it is customary, particularly for navy blues, to use a mixture of two or morecomponents of similar hue. If these behave independently, each will give its saturationsolubility in the fibre. In practice, certain mixtures of dyes with closely related structures are20–50% less soluble in cellulose acetate than predicted from the sum of their individualsolubilities [87]. Dyes of this kind form mixed crystals in which the components are able toreplace one another in the crystal lattice. The melting point depends on composition,varying gradually between those of the components, and the mixed crystals exhibit lowersolubility than the sum of solubilities of the component dyes [88]. Dyes of dissimilarmolecular shape do not form mixed crystals, the melting point curve of the mixture shows aeutectic point and they behave additively in mixtures with respect to solubility in water andin the fibre.

Quite small variations in disperse dye structure can markedly modify substantivity forpolyester [89]. This is evident from Figure 3.4, where the two blue dye structures differ onlyin the 3-acylamino substituent of the diethylaniline coupling component. Replacingacetylamino by propionylamino in dye 3.74 increases the colour yield by at least 30% for a1.5% depth applied to polyester fibre for 45 minutes at 130 °C. An even more strikingexample is provided in Figure 3.5, illustrating two isomeric greenish blue dyes applied to


chpt3(1).pmd 15/11/02, 14:42129


polyester under the conditions already specified. In this case, replacement of the N-n-hexylgroup in dye 3.75 by N-1-methylpentyl increases the colour yield at 0.5% depth by about25%.

Dye applied/%

0.3 0.6 0.9 1.2 1.5

10

20

30

70

60

50

40

Col

our

yiel

d (In

teg

valu

e)

Dye 3.74: R = ethyl

Dye 3.74: R = methyl

Figure 3.4 Colour yield of acylamino-substituted blue monoazo disperse dyes [89]

Dye applied/%

0.1 0.2 0.3 0.4 0.5

10

20

30

40

Col

our

yiel

d (In

teg

valu

e)

Dye 3.75: R = n-hexyl

Dye 3.75: R = 1-methylpentyl

Figure 3.5 Colour yield of N-alkylated greenish blue monoazo disperse dyes [89]

O2N

CN

N

Br

N N

CH2CH3

CH2CH3

HN

CO R

3.74

O2N

CN

N

NO2

N NH

HN

COCH3

R

3.75

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131

One of the criteria that determine the substantivity of disperse dyes for ester fibres is thatthe solubility parameter of the dye must be within one unit (cal/cm3)½ of that of the fibre[90]. Thus the 3-acetylamino dye (3.76; R = CH3) with a solubility parameter(s) of 11.3 hasmuch higher substantivity for polyester (s = 10.8) than has the 3-propionylamino analogue(3.76; R = CH2CH3, s = 13.0). The effects of dyeing auxiliaries on the exhaust dyeingprocess can also be interpreted by reference to the solubility parameters of fibre, dye andauxiliary [91]. The solubilities of six azo disperse dyes in water and polyester weredetermined and the thermodynamic parameters influencing solubility were calculated.Water solubility is governed by the mixing enthalpy but this does not play a role in thepolyester environment and cannot be predicted satisfactorily from solubility parameters [92].

Cl

N

N

CH3SO2

HN

CO R

N

CH2CH3

CH2CH3

3.76

Derivatives of diaminoanthrarufin (3.77; X = Y = H) and its 1,8-dihydroxy-4,5-diaminoisomer (diaminochrysazin) have been among the most widely used anthraquinone dyes forester fibres. For example, methylation of diaminoanthrarufin gives CI Disperse Blue 26, amixture of several components. Study of the pure N-alkylated derivatives from the baseconfirmed that monosubstitution (3.77; X = H, Y = alkyl) gives mid-blue dyes withexcellent dyeing properties and acceptable fastness on polyester, but the bis-alkyl dyes (3.77;X = Y = alkyl) are greener and inferior in application properties. Mixtures of theunsubstituted base with alkylated components, as obtained industrially, were especiallyadvantageous for build-up to heavy depths, however [93].

O

O

NH

HO

X OH

HN Y3.77

The effects of disperse dye-substrate interaction on the tensile properties of colouredpoly(ethylene terephthalate) film was examined recently [94]. Incorporation of varioushydroxy derivatives of anthracene and anthraquinone into the polymer gave indications thataggregation of dye molecules within voids was favoured when their affinity for the substratewas inadequate. Lowering of the glass-transition temperature (Tg) of polymers wasinvestigated recently by incorporating various concentrations of a series of yellow or orangedisperse dyes of increasing Mr, namely p-nitroaniline, p-nitrophenylazoaniline (CI DisperseOrange 3), p-nitrophenylazodiphenylamine (CI Disperse Orange 1) and the azopyridone CIDisperse Yellow 231 (3.78). As expected, the Tg values were lowered with increasingconcentrations of dye in the polymer and on poly(vinyl butyral) the degree of lowering of Tg


chpt3(1).pmd 15/11/02, 14:42131


increased in the order of increasing Mr of the dye present. On a copolymer of vinyl chlorideand vinyl acetate, however, the degree of lowering of Tg with Yellow 231 was less than withOrange 1 or Orange 3, suggesting that Yellow 231 interacts in different ways with these twopolymers [95].

CH2CH2OOC N

N

H

N

O

O

CH

CN

CH2CH3

H3C

H3C3.78


3.2.4 Dyeing of acrylic fibres with basic dyes

Most acrylic copolymer fibres contain sulphate and sulphonate groups on the polymerchains, derived from the inorganic redox catalysts (potassium persulphate and sodiumbisulphite) that initiate the polymerisation reaction. The adsorption of basic dyes at low pHoccurs mainly on these strongly acidic groups [96–99]. Carboxylic groups are sometimespresent, however, if benzoyl peroxide has been used as initiator, resulting in dyeingbehaviour that varies considerably more with dyebath pH [100]. The concentration ofstrongly acidic sites for dye adsorption in an acrylic fibre, as determined by sulphur analysisor by non-aqueous titration with a suitable base, agrees extremely well with saturation valuesderived from Langmuir isotherms defining the uptake of basic dyes [97,101]. Thiscorrespondence provides excellent confirmation that the dyeing mechanism is essentially aprocess of ion exchange.

In commercial acrylic fibres the strongly acidic groups are normally stabilised in sodiumsalt form. If this is transformed into the acid form during preparation, anomalous colourchanges (halochromism) may take place in the subsequent dyeing stage. Thus when anacrylic fibre prepared in the free acid form was dyed at 80 °C with CI Basic Orange 33 (3.79;X = Y = H) the dyeing became increasingly much redder than normal, only reverting slowlyto the usual orange hue after prolonged dyeing for 1.5 hours or more. A control dyeing onthe same fibre prepared as the sodium salt showed no hue change, building up to the normalorange hue. Chloro-substitution in the diazo component inhibited the effect, since with CIBasic Red 18 (3.79; X = Cl, Y = H) and especially CI Basic Brown 30 (3.79; X = Y = Cl)the hue changes were much less marked [102]. This halochromic change is attributed totransient formation of the redder associated complex 3.80 on the acid-treated fibre, followedby reversion to the usual ion-exchange product 3.81 that is formed by conventional dyeing(Scheme 3.14). Addition of a neutral electrolyte such as sodium sulphate suppresses theprotonation of the strongly acidic groups in the fibre.

O2N

X

N

Y

N N

CH2CH2

CH2CH3

N

CH3

CH3

CH3

Cl

+

_3.79

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133

The rate of dyeing of basic dyes on acrylic fibres is highly sensitive to temperature. Thisoriginates in the response of the segmental mobility of the polymer chains to changes intemperature. At or near the glass-transition temperature (Tg), the rate of diffusion of dyeinto the polymer structure increases markedly owing to the onset of enhanced mobility ofthe chain segments. Water acts as a plasticising agent for this process, by lowering the Tg ofthe acrylic polymer by 30–35 °C [103]. This acceleration of the rate of dyeing as soon as Tgis exceeded, together with the virtual irreversibility of the electrostatic interaction betweenthe dye cation and the anionic dyeing site (3.81) in an otherwise nonpolar localenvironment, means that careful control of application conditions is essential if level dyeingis to be achieved.

Difficulties of incompatibility can arise with mixtures of basic dyes on acrylic fibresbecause of competition for the limited number of dyeing sites available and the differencesbetween dyes in terms of affinity and rate of diffusion. The rate of uptake of each dye whenapplied in admixture with another is invariably slower than when the dye is applied alone atthe same concentration. Competition effects of this kind can lead to serious practicalproblems unless the dyes are carefully designed and selected to have similar dyeingcharacteristics [97,98,104,105]. Dyes with exceptionally low affinity and rapid rates ofdiffusion have been developed, offering improved migration on acrylic fibres [106]. Thesedyes have migration properties not unlike those of monosulphonated acid dyes on nylon.

With the majority of basic dyes for acrylic fibres, however, levelling by migration isextremely limited because of their high affinity. A slow rate of increase of dyeingtemperature (0.5 °C/min or even less) over the range within which the diffusion rate isincreasing rapidly gives an improved degree of control. Colourless cationic retarding agentsprovide temporary competition for the available anionic sites in the early stage of adsorptionof the dye. The amount of retarder applied is adjusted according to the proportion ofunoccupied sites remaining when the dyeing equilibrium is attained, in order to minimisethe risk of undesirable restraining. Since dye uptake is related to the number of unoccupiedsites remaining, the temporary blocking action of the retarder effectively slows the rate ofdyeing [107].

[acrylic] SO3H [dyeN] Cl Cl[acrylic] SO3H

[acrylic] SO3H Cl Na2SO4 [acrylic] SO3

[acrylic] SO3 Cl [acrylic] SO3

+_ _

Acid form 3.80

+

_+

_

3.813.80

+ NaCl + NaHSO4

_Na+ +

_ _

3.81

+ NaCl

Sodium salt form

[dyeN]+

[dyeN]+

[dyeN]+

[dyeN]+

[dyeN]+

Scheme 3.14


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3.3 APPLICATION PROPERTIES AND CHEMICAL STRUCTURE

The choice of dyes for a required colour on a given substrate almost always involvescompromise between conflicting requirements. Similarly there is seldom an ‘ideal’ dyestructure of a desired hue that embodies all possible attractive features with regard toapplication and fastness properties. Many bright azo dyes have only moderate fastness tolight. The incorporation of a metal atom to form a complex of higher fastness often entails asignificant loss in brightness. The gain in aqueous solubility provided by an extra sulphogroup in a direct or reactive dye often has to be paid for by a decrease in affinity forcellulose. An increase in the hydrophobic character of an acid dye for wool or nylon mayimprove the neutral dyeing affinity at the expense of impaired level-dyeing properties.Likewise, the migration behaviour of a disperse dye is often affected adversely by an increasein molecular size intended to confer higher wet fastness on cellulose acetate or improvedsublimation fastness on polyester. Further examples of such conflicting requirements willemerge in the remainder of this chapter.

3.3.1 Dyeing kinetics and dye structure

The rates of uptake of direct dyes by cellulose vary extremely widely, as indicated by the datain Table 3.13. The time of ‘half-dyeing’ (the time to reach an exhaustion level half of thatattained at equilibrium) on viscose for the highly substantive disazo J acid derivative CIDirect Red 23 is approximately 400 times that for the rapidly diffusing azostilbene dye CIDirect Yellow 12 [108]. Coefficients of diffusion for five of these dyes have been measured incellophane film [109] and these values do account for some of the differences in dyeing rate,although the inverse relationship is only approximate. There is a tendency for smallermolecules to diffuse more quickly, in that structures 3.82 to 3.84 contain only four or fivearyl nuclei and one or two sulpho groups, whereas structures 3.88 to 3.90 have six to eightaryl rings and two to four solubilising groups.

A much more decisive factor in determining the rates of dyeing of many direct dyes fromaqueous salt solutions is their tendency to aggregate (section 3.1.2), since this greatly retardstheir diffusion into the water-swollen voids of the substrate. Ureido-linked J acid residues

Table 3.13 Times of half-dyeing on viscose [108] and diffusion co-efficients in cellophane film [109] for direct dyes at 90 °C

Diffusion coeff. in Time of half-dyeingcellophane film on viscose

CI Direct Structure × 1014 (m2/s) yarn (min)

Yellow 12 3.82 150 0.26Red 81 3.83 50 0.84Yellow 59 3.84 1.0Red 31 3.85 25.3 1.7Red 2 3.86 7.7 8.9Blue 1 3.87 13.3 15.9Red 26 3.88 43.8Black 22 3.89 43.8Red 23 3.90 100

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135

CH3CH2O N

N

SO3Na

CH

CH N

N

NaO3S

OCH2CH3

3.82

CI Direct Yellow 12

N

S

N

S

NH2

H3C

SO3Na

3.84

CI Direct Yellow 59

O

N

N

H

N

N

H

H2N SO3Na

SO3Na

OCH3H3CONaO3S

NaO3S

O

NH2

3.87

CI Direct Blue 1

NaO3S N

N N

N

H

N

C

H

O

NaO3S

O

3.83

CI Direct Red 81

NH2

SO3Na

N

N

H3C CH3

N

N

NH2

SO3Na

3.86

CI Direct Red 2

N

N

H

N

OO

N

H

NaO3S

NH

SO3Na

3.85

CI Direct Red 31

APPLICATION PROPERTIES AND CHEMICAL STRUCTURE

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(CI Direct Reds 23 and 26), especially with an acetanilide residue elsewhere in the molecule(as in Red 23), provide opportunities for hydrogen-bond stabilisation of the weaker p-bonding attraction between aryl rings in neighbouring component molecules of an aggregate.

The kinetic behaviour of anionic dyes on amide fibres tends to be much more closelyrelated to molecular size and hydrophilic-lipophilic balance. Thus within a related series ofdye structures it is possible to discern more specific relationships. For example, in the seriesof p-substituted aniline→R acid dyes (3.91; R = H, methyl, n-butyl or n-dodecyl) thelogarithm of the rate of dyeing on wool is inversely proportional to the molecular volume[110].

N

N

H

N

H H

NC O

N

N

HO

O

SO3Na

SO3NaNaO3S

OCH3

3.88

CI Direct Red 26H2N

NH2

N N

O

N

SO3Na

N

H

N

N

H

N

NaO3S

NaO3S

SO3Na N

H2N

NH2

3.89

CI Direct Black 22

O

N

N

H

N

H H

NC

N

N

O O

N

C

H

CH3

O

NaO3S

OH

SO3Na3.90

CI Direct Red 23

H

R

O SO3Na

SO3Na

N

3.91

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A somewhat similar inverse relationship between dyeing rate and molecular size wasinvoked to interpret the relative degree of penetration of three anthraquinone blue dyes intonylon fibres after dyeing under various conditions [111]. After only 5–10 minutes at 90 °C,CI Acid Blue 25 (3.92) had penetrated the nylon almost completely, with no evidence ofring dyeing. CI Acid Blue 138, a disulphonated analogue with an additional dodecyl groupon the aniline ring (3.93), showed a marked difference in degree of penetration after dyeingfor one hour (40% at 90 °C, compared with 100% at 100 °C). The extremely slow-diffusingdisulphonated dye CI Acid Blue 175 (3.94), with seven interlinked rings able to form acoplanar arrangement, attained only 40–50% penetration of the substrate even after dyeingfor two hours at 100 °C.

NH2

HN

SO3Na

O

O

3.92

CI Acid Blue 25

NH2

HN

(CH2)11CH3

NaO3S

SO3Na

O

O

3.93

CI Acid Blue 138

HN

HN

H2C

H2C

O

O

SO3Na

SO3Na

3.94

CI Acid Blue 175

The relative rates of diffusion of disperse dyes in hydrophobic fibres tend to exhibit anapproximately inverse relationship to molecular size, although here again the overall trend issomewhat obscured by specific forces of interaction between hydrogen-bonding substituentsin the dye and polymer chain segments. CI Disperse Orange 3 (Mr 242) diffuses about fivetimes more quickly than CI Disperse Blue 24 (Mr 328) in nylon and polyester but only about1.5 times in cellulose acetate [112]. These differences (Table 3.14) suggest that the orangeazo dye interacts more strongly than the blue anthraquinone dye with the acetate fibre,possibly by hydrogen bonding of the terminal primary amino group in Orange 3 with anacetyl group in cellulose acetate.

The relative rates of diffusion of CI Disperse Orange 3 on nylon, acetate and polyesterindicated in Table 3.14 are reasonably consistent with those measured independently in theearly days of polyester dyeing [113], as shown in Table 3.15. These figures, for three disperse


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dyes of almost equal Mr also emphasise a marked increase in diffusion rate on polyesterbetween 85 and 100 °C, that is much less evident on nylon or cellulose acetate. Dispersedyes tend to diffuse more rapidly in polypropylene than in other hydrophobic fibres but togive relatively low saturation values indicative of poor affinity for this fibre. Table 3.16illustrates these differences for 1-anilinoanthraquinone (3.96; X = NHPh, Y = H) oncellulose acetate and polypropylene [84].

Table 3.14 Diffusion coefficients of disperse dyes in hydrophobic fibres [112]

Diffusion coefficient at 100°C × 1015 (m2/s)

CI Disperse Structure Nylon Acetate Polyester

Orange 3 3.95; X = Y = H 23.1 15.1 1.3Blue 24 3.96; X = NHCH3, Y = NHPh 4.2 10.6 0.3

N

N N

Y

XO2N

3.95

O

O

X

Y

3.96

Table 3.15 Relative diffusion coefficients of disperse dyes [113]

Relative diffusion coefficient

Nylon Acetate Polyester

Rel. mol.CI Disperse Structure mass (Mr) 85°C 85°C 85°C 100°C

Orange 3 3.95; X = Y = H 242 680 460 1 48Red 15 3.96; X = NH2, Y = OH 239 1000 286 1 34Violet 1 3.96; X = Y = NH2 238 450 452 1 31

Table 3.16 Saturation values and diffusion coefficients of 1-anilinoanthraquinone [84]

Saturation Diffusion coeff.Dyed at 80°C value (g/kg) × 1014 (m2/s)

Polypropylene 3.2 6.74Cellulose acetate 6.0 1.12

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139

There is a roughly inverse relationship for a series of structurally related dyes between thetime of half-dyeing and the saturation solubility in an appropriate substrate, as illustrated forseveral 4-alkylamino derivatives of 1-anilinoanthraquinone on cellulose acetate (Table 3.17).It is interesting that methylamino and 2-hydroxyethylamino substituents confer goodsolubility in this substrate, but ethylamino groups are even less effective than isobutylaminogroups in this respect [114].

Table 3.17 Times of half-dyeing and solubilities of disperse dyes in cellu-lose acetate at 85°C [114]

Time ofSubstituent (R) in 3.97 half-dyeing (min) Solubility (g/kg)

Methyl 30 8.82-Hydroxyethyl 35 4.2Isobutyl 4.1Ethyl 37 1.5

The times of half-dyeing on cellulose acetate listed in Table 3.18 give some significantpointers to the relationship between disperse dye structure and dyeing rate on this fibre. Themost mobile molecular structure shown is the dinitrodiphenylamine dye CI Disperse Yellow1, closely followed by the unsubstituted p-nitrophenylazoaniline Orange 3. The introductionof one N-phenyl group into the latter structure (to form Orange 1) has a much morepronounced effect than two ethyl or 2-hydroxyethyl groups (Red 1 or Red 19). In the seriesof 1,4-diaminoanthraquinone (3.96) derivatives, the unsymmetrically substituted types (Red15, Violet 4 and Blue 3) are absorbed much more rapidly than is the symmetrical analoguewith two methylamino groups (Blue 14). This dye is greatly retarded in comparison with theunmethylated dye (Violet 1). In a similar way, Blue 26:1 containing mono- and di-methylated diaminoanthrarufin is absorbed several times more slowly than Blue 26, amixture of the monomethyl and unmethylated derivatives. Possibly the presence ofmethylamino substituents favours aggregation of these anthraquinone dyes.

Diffusion coefficients [115] and dyeing rate constants [116] for the same fifteen dyes onnylon are given in Table 3.19, confirming many of the trends already noted above. Yellow 1,Orange 3 and Violet 4 are again the three most rapidly absorbed dyes, in the same order.The diffusion coefficients place the first two dyes in reverse order, however, suggesting thatthe terminal phenolic group in Yellow 1 interacts more effectively with proton-acceptor sites

O

O

HN

HNR

3.97


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Table 3.18 Times of half-dyeing of disperse dyes on cellulose acetate at 85 °C [115]

Time ofCI Disperse Structure half-dyeing (min)

Yellow 1 3.98 0.75Orange 3 3.95; X = Y = H 0.8Violet 4 3.96; X = NH2, Y = NHCH3 1.6Red 15 3.96; X = NH2, Y = OH 2.4Blue 3 3.96; X = NHCH3, Y = NHCH2CH2OH 2.5Violet 1 3.96; X = Y = NH2 3.0Red 19 3.95; X = Y = CH2CH2OH 3.3Red 1 3.95; X = CH2CH3, Y = CH2CH2OH 4.0Red 11 3.99 4.8Blue 26 3.100; X = H 6.5Orange 1 3.95; X = H, Y = Ph 14.0Red 13 3.101 16.5Blue 14 3.96; X = Y = NHCH3 16.5Orange 13 3.102 19.0Blue 26:1 3.100; X = CH3 28.5

O2N OH

NO2

NH

3.98


O

O

NH2

NH2

OCH3

3.99

CI Disperse Red 11

HO

H2N

NHCH3

OH

HO

NH

HN

OH

O

O

O

OX

X

+

3.100

O2N

Cl

N

N N

CH2CH2OH

CH2CH3

3.101

CI Disperse Red 13N

N N

N OH

3.102

CI Disperse Orange 13

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141

in nylon than the primary amino terminal group in Orange 3. As before, an N-phenylsubstituent (Orange 1) exerts a markedly greater retarding influence than N-ethyl-N-2-hydroxyethyl (Red 1) or N,N-bis(2-hydroxyethyl) (Red 19) substitution of the parentstructure (Orange 3). Once again in the 1,4-disubstituted anthraquinone series, the rateconstants for the unsymmetrical structures (Red 15, Violet 4 and Blue 3) are much higherthan that for the symmetrical bis-methylamino analogue (Blue 14). The rates for the twomixed diaminoanthrarufin derivatives, however, are much closer on nylon than on celluloseacetate. The diffusion coefficients of these dyes do differ markedly, perhaps because theunmethylated component of Blue 26 interacts more strongly with proton-acceptor sites innylon than does the more fully methylated mixture Blue 26:1.

Table 3.19 Diffusion coefficients [115] and dyeing rate constants [116] for disperse dyeson nylon at 80 °C

Diffusion coeff. RateCI Disperse Structure × 1015 (m2/s) constant

Yellow 1 3.98 11 1820Orange 3 3.95; X = Y = H 23 940Violet 4 3.96; X = NH2,Y = NHCH3 840Blue 3 3.96; X = NHCH3,Y = NHCH2CH2OH 570Red 19 3.95; X = Y = CH2CH2OH 450Violet 1 3.96; X = Y = NH2 11 350Red 15 3.96; X = NH2,Y = OH 280Red 1 3.95; X = CH2CH3,Y = CH2CH2OH 3 200Orange 13 3.102 1 160Red 13 3.101 130Blue 14 3.96; X = Y = NHCH3 11 120Red 11 3.99 15 80Blue 26 3.100; X = H 5 80Blue 26:1 3.100; X = CH3 17 68Orange 1 3.95; X = H,Y = Ph 3 56

The study of the degree of penetration of blue acid dyes in nylon mentioned earlier alsoincluded a further series of measurements for two blue disperse dyes on polyester [111]. Afully penetrated dyeing with CI Disperse Blue 56 (3.103; X = Br) was achieved after 90minutes at 120 °C, but with the Blue 83 mixture (3.103; X = PhOCOCH3 or PhOCOPh)complete penetration within this time of dyeing could only be attained at 140 °C. Inpractice, the dyeing of polyester fibres is often restricted to 30–60 minutes at 125–130 °C inorder to minimise the release of oligomer into the dyebath or to preserve desirable fabricproperties. When applying high-energy dyes of the Blue 83 type these conditions canproduce dyeings that are only 30–40% penetrated.

H2N

HO

OH

NH2

O

O

3.103

X


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Diffusion coefficients for five disperse dyes on polyester are compared in Table 3.20. Fourof these structures were included in the corresponding data for the nylon and celluloseacetate systems. As before, the unsymmetrical 1,4-diaminoanthraquinone derivative (CIDisperse Blue 3) diffuses significantly more rapidly than the symmetrical bis-dimethylaminoanalogue (Blue 14). It is interesting that the disazo dye Orange 13 with a terminal p-hydroxygroup shows a higher diffusion coefficient on polyester than the two monoazo dyes with aterminal p-nitro group (Reds 7 and 13).

Table 3.20 Diffusion coefficients of disperse dyes on unset polyester at 100 °C [61]

Diffusion coeff.CI Disperse Structure × 1014 (m2/s)

Blue 3 3.96; X = NHCH3, Y = NHCH2CH2OH 1.6Orange 13 3.102 1.4Red 7 3.104 1.25Blue 14 3.96; X = Y = NHCH3 1.0Red 13 3.101 0.9

O2N N

N N

CH2CH2OH

CH2CH2OH

Cl

3.104

CI Disperse Red 7

As in the case of disperse dyes on polyester, the coefficients of diffusion of basic dyes inacrylic fibres tend to decrease as molecular size increases. Most basic dyes have a singlecationic centre surrounded by a relatively compact hydrophobic system containing three orfour aryl rings. Five typical structures are compared in Table 3.21 in terms of their relativediffusion coefficients in an acrylic fibre and in a basic-dyeable polyester variant in the presenceand absence of a diphenyl-type carrier [117]. Individual substituents exert only minor effectson the overall rate of diffusion. Thus the N-ethyl-N-2-hydroxyethyl substitution pattern in CIBasic Blue 41 produces only slightly lower diffusion coefficients than the dimethylaminoanalogue (Blue 54) and even the dimethyltriazole diazo component in Yellow 25 has a minorretarding effect relative to the monomethylthiazole group in Red 29.

Table 3.21 Relative diffusion coefficients of basic dyes on basic-dyeable polyester andacrylic fibres at 100 °C [117]

Basic-dyeable polyester

CI Basic Structure No carrier Carrier Acrylic fibre

Red 29 3.105 1.1 1.9 3.6Blue 54 3.106; X = CH3, Y = CH3 1.1 1.8 3.4Blue 41 3.106; X = CH2CH3, Y = CH2CH2OH 1.0 1.8 3.1Yellow 25 3.107 1.0 1.7 2.8Yellow 11 3.108 0.7 1.6 2.6

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143

3.3.2 Dye structure and affinity for the fibre

As already noted in relation to kinetic behaviour, minor changes in direct dye structure canhave marked effects on dyeing properties. Within the closely related series of J acid disazo dyesin Table 3.22, insertion of an acetylamino group in CI Direct Orange 26 to form Red 23 gives asignificant boost to the affinity as well as a bathochromic influence on the colour. Comparingthe other two members of this series, an additional aryl ring (Red 26) has a much greater effecton the overall affinity of the molecule than a simple methyl group (Red 24).

NNN

S

N+

CH3

Cl_

3.105

CI Basic Red 29

N+

S

N

N N

Y

X

CH3

H3CO

Cl_

3.106

NNN

NN+

N

CH3

CH3

Cl_

3.107

CI Basic Yellow 25

N+

CH3H3C

HC

CH3

CH NH

Cl

H3CO

OCH3

_

3.108

CI Basic Yellow 11

Table 3.22 Affinity of direct dyes for viscose at 60 °C [67]

CI Direct Substituents in structure 3.109 Affinity(kJ/mol)

Red 26 –23.8

Red 23 –21.3

Orange 26 –20.1

Red 24 –18.4

OCH3

X = Y = SO3Na

X = Y = NHCOCH3

X = Y =

X = Y =

OCH3 H3C

SO3Na


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The important contribution of hydrogen bonding to the substantivity of direct dyes wasexplored by the synthesis of red (3.110; X = CHO) and violet (3.110; X = PhNHCSNH2)monoazo derivatives of R acid. The existence of hydrogen bonding between the X group andhydroxy groups in cellulose was demonstrated. This research was followed up by a morecomprehensive survey of various disazo R acid dyes derived from a range of diamines (3.111; Y= O, NH, CH2, CO, SO2, CH=CH, CONH, NH.CO.NH, NH.CS.NH, CO.NH.NH.CO,NH.CO.CO.NH, NH.CO.CH2.CO.NH, NH.C=NH.NH and CO.NH.C=NH.NH.CO). Thesignificance of such central functional groups with scope for hydrogen bonding or dipolarinteraction with hydroxy groups in cellulose was clearly evident [118].

Numerous linear diamines, such as many of those in the survey mentioned above, havebeen evaluated as potential replacements for benzidine, an inexpensive and highly versatileintermediate that was banned in the 1970s because it posed a carcinogenic threat. Twotrisazo dyes have been synthesised recently using 4,4′-diaminodiphenyl thioether instead ofthe benzidine component of CI Direct Black 38 (3.112; X = Y = NH2) and CI DirectGreen 1 (3.112; X = OH, Y = H). These new dyes exhibited higher substantivity andfastness to washing than the two traditional products on cellulosic fibres [119].

N

NX

H

N

H H

NC

NaO3S

O

O

O

N

SO3Na

N

H

Y

3.109

N

N

H

SO3Na

O SO3Na

X

3.110

YN N

N

H

N

HNaO3S SO3Na

NaO3S SO3Na

O O

3.111

N N

N NX

SO3NaNaO3S

NH2 OY

N

NH

3.112

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145

Affinity values of the symmetrical disazo dyes CI Direct Yellow 12 (3.6), Red 2 (3.8; X =CH3, Y = H) and Blue 1 (3.2) on cellophane have been measured recently underhydrostatic pressures up to 600 Mpa [120]. The affinity of Yellow 12 increased slightly butvalues for the other two dyes decreased considerably with increasing hydrostatic pressure(Table 3.23). The sulpho groups on the central stilbene nuclei of the Yellow 12 moleculetend to inhibit aggregation, whereas Red 2 and Blue 1 aggregate much more readily. Thesmall increase in the affinity observed with Yellow 12 may indicate that isomerisation fromthe cis to the more stable trans form may occur as the hydrostatic pressure is increased.

Table 3.23 Affinity of direct dyes for cellophane at 55 °C [120]

Affinity (kJ/mol)

Hydrostaticpressure (MPa) CI Direct Yellow 12 CI Direct Red 2 CI Direct Blue 1

0.1 –18.2 –20.9 –31.0100 –18.2 –17.5 –28.3200 –18.8 –16.2 –25.3400 –19.3 –14.9 –22.7600 –19.8 –14.1 –21.6

The attraction between cellulose and the leuco form of a simple polycyclic vat dye isbelieved to be mainly attributable to dispersion forces. Not surprisingly therefore, affinitytends to increase with relative molecular mass in the series shown in Table 3.24. This is notthe only factor, however, because the eight-ring pyranthrone (3.113) has a higher affinitythan the nine-ring violanthrone isomers (3.114 and 3.115). The configuration of the ringsand the location of the keto groups are also significant, since violanthrone is moresubstantive than isoviolanthrone.

Table 3.24 Affinity of polycyclic vat dyes for cotton at 40 °C [115,121]

Polycyclic vat dye Structure Mr Affinity (kJ/mol)

Pyranthrone 3.113 406 –23.2Violanthrone 3.114 456 –22.8Isoviolanthrone 3.115 456 –19.9Dibenzopyrenequinone 3.116 332 –15.7Anthanthrone 3.117 306 –12.0Benzanthraquinone 3.118 258 –4.9

O

O

3.113

Pyranthrone

O O

3.114

Violanthrone


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Hydrogen bonding is believed to strongly influence the substantivity of benzoylamino-anthraquinone vat dyes, as detailed studies have shown (Table 3.25). Introduction of asecond benzoylamino group into 1-benzoylaminoanthraquinone approximately doubles theaffinity, 1-4-disubstitution having a greater effect than 1,5-disubstitution. The enhancedaffinity conferred by the p-R-substituted benzoylamino group increases with the dipolemoment of the C–R bond (Table 3.25). Such substituents are almost equally effective ineither m- or p-positions, but o-R-substituted analogues are much less substantive becausecoplanarity of the aryl rings with the plane of the anthraquinone nucleus is inhibited.

O

O

3.115

Isoviolanthrone

O

O3.116

Dibenzopyrenequinone

O

O3.117

Anthanthrone

O

O3.118

Benzanthraquinone

Table 3.25 Affinity of benzoylaminoanthraquinone vat dyes on cotton at 40 °C [115]

Affinity(kJ/mol)

Substituent (R) H CH3 CH3O ClDipole moment (C-R) 0.41 1.16 1.56

Structure 3.119

–15.3 –16.6 –16.0 –19.6

–12.0 –12.8 –14.5 –15.6

–6.9 –8.2 –9.1 –9.6

X = Y = R CONH, Z = H

X = Z = R CONH, Y = H

X = R CONH, Y = Z = H

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147

Early studies of the affinity of acid dyes for wool revealed noteworthy correlations withdye structure. For example, in four pairs of monoazo dyes differing only by replacement of abenzene by a naphthalene nucleus, the affinity increase in each pair was consistently withinthe range -4.6 to -6.3 kJ/mol. In three related pairs of dyes, an additional sulpho groupreduced the affinity by about -4 kJ/mol. In a series of alkylsulphuric acids (ethyl, octyl,dodecyl) and in two series of monoazo dyes containing alkyl chains of increasing length, theincrement per methylene group was consistently about -1.66 kJ/mol. A close correlationbetween affinity and Mr was also obtained for a series of substituted phenylazo-1-naphthol-4-sulphonic acid dyes [115].

Two further interesting series of systematic changes in structure have shed light on therelative effects of alkyl and aryl groupings in conferring enhanced affinity for wool. Thephenylazo member (3.120; X = Ph) of the series shown in Table 3.26 has an affinity of -16.7kJ/mol and the introduction of a 4-n-butyl substituent has almost as great an effect as theadditional aryl ring when the phenylazo is replaced by a naphthylazo grouping. Likewise, inthe three anthraquinone derivatives shown in Table 3.27, replacing the 4-methylamino byan n-butylamino group in structure 3.121 boosts the affinity by about 10%, but the effect ofan anilino residue represents almost a 15% increase.

X

Y Z

O

O

3.119

NaO3S

O

N

NX

H

3.120

NH2

N

SO3Na

H X

O

O

3.121

Table 3.26 Affinity of monoazo acid dyes for wool at 70 °C [122]

Substituent (X) in 3.120 Affinity (kJ/mol)

–22.6

–22.2

–16.7

CH3(CH2)3


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Careful measurements have been made of the affinity of many levelling and milling aciddyes for nylon using various experimental techniques. Radio tracer analysis was used tofollow the uptake of a series of 1-naphthylazo-2-naphthol dyes (3.122) on nylon 6.6 film.The highest neutral-dyeing affinity was shown by the monosulphonate CI Acid Red 88(3.122; X = Y = Z = H), which exhibited the greatest degree of overdyeing in excess of theamine end group content of the nylon. The disulphonate Red 13 (3.122; Y = SO3Na,X=Z=H) and the trisulphonate Red 18 (3.122; X=Y=SO3Na, Z=H) gave progressivelylower neutral-dyeing affinity and overdyeing, whilst the tetrasulphonate Red 41 (3.122; X =Y = Z = SO3Na) showed no tendency to overdye [124].

Table 3.27 Affinity of anthraquinone acid dyes for wool at 50° Cand pH 4.6 [123]

Substituent (X) in 3.121 Affinity (kJ/mol)

–25.1

–(CH2)3CH3 –24.2–CH3 –22.2

NaO3S N

N

H ZO

X

Y3.122

In a recent study of the series of acid dyes obtained by diazotisation of p-alkylanilines andcoupling with 1- or 2-naphtholsulphonic acids [125], the substantivity, wet fastness andwater repellency characteristics of these dyes on nylon were found to increase progressivelywith alkyl chain length, provided the dyes were dissolved completely in the dyebath.Aggregation to form multimers and ultimately micelle stabilisation complicates thebehaviour of alkylated dye monosulphonates of this kind (section 3.1.2). This research wasfollowed up more recently in a detailed evaluation of anthraquinone blues (3.123) preparedby condensation of bromamine acid with a series of p-n-alkylanilines (R = H, butyl, octyl,dodecyl or hexadecyl). Related series of monoazo dyes were synthesised using the same p-n-alkylanilines as diazo components with various couplers such as H acid (as in structure3.124), γ acid, M acid, NW acid or R acid [126]. In some cases (as in 3.124; X = H orCOCF2CF2CF3) the effects of inserting a perfluorobutyroylamino substituent in the couplingcomponent were also assessed [127]. Although the perfluoro group and the long-chain alkylgroups adversely affected substantivity for nylon 6, they did enhance wet fastness and waterrepellency as expected. The perfluorobutyroyl grouping was particularly effective inconferring water repellency [126].

Affinity values have been used to interpret the important practical problem of mutualblocking effects in mixture recipes on nylon [128]. Typical values for some commercially

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149

NH2

HN

SO3Na

O

O

3.123

R

R N

N

H O

3.124

HN

X

NaO3S

SO3Na

Table 3.28 Affinity of acid dyes for nylon 6.6 by chloride desorption at 75 °C [128]

CI Acid Structure Affinity (kJ/mol)

Red 88 3.125; X = NaO3S Y = H –32.0

Violet 5 3.126; X = CH3CONH Y = SO2 CH3 –27.7

Orange 20 3.127 –25.8

Orange 8 3.125; X = NaO3S

CH3

Y = H –25.2

Red 18 3.125; X = NaO3S Y = SO3Na –24.6

Orange 7 3.125; X = NaO3S Y = H –24.3

Violet 7 3.126; X = CH3CONH Y = COCH3 –22.4Yellow 17 3.128; X = CH3 Y = Cl –22.2Red 1 3.126; X = H Y = COCH3 –21.4Yellow 23 3.128; X = COOH Y = H –20.9

Orange 10 3.125; X = Y = SO3Na –20.9

important monoazo acid dyes are given in Table 3.28. CI Acid Orange 7 is the ortho isomerof Orange 20 (3.127). The latter has a significantly higher affinity, probably becausehydrogen bonding of the keto group with proton-donor sites in nylon is favoured. Orange 7and Orange 10 have the same 1-phenylazo-2-naphthol chromogenic system but theadditional sulpho group in Orange 10 lowers the affinity by about -3.4 kJ/mol. Similarly, thetwo extra sulpho groups in Red 18 reduce the affinity by about -7.4 kJ/mol compared withthe analogous Red 88, the most substantive of the dyes listed in Table 3.28. Orange 8 differsfrom Orange 7 only in having an additional methyl substituent ortho to the azo linkage. Thisonly raises the affinity by about -0.9 kJ/mol, but the bathochromic change from phenylazo tonaphthylazo in going from Orange 7 to Red 88 gives an increase of approximately -7.7 kJ/mol, virtually the same magnitude as the lowering effect of two sulpho groups.


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In the series of three N-acylated phenylazo H acid dyes (3.126), the additional p-acetylamino group in Violet 7 enhances the affinity above that of Red 1 by about –1.0 kJ/mol, as well as producing a marked bathochromic shift in hue. Replacing the N-acetylaminogroup in the H acid residue by N-p-tosylamino (Violet 5) has less effect on hue but raises theaffinity by about –5.3 kJ/mol. Somewhat surprisingly, replacing the carboxyl group in theazopyrazolone chromogen (3.128) of Yellow 23 by a methyl group and introducing twochloro substituents into the N-phenyl ring only increases the overall affinity by about –1.3kJ/mol.

The yield of disperse dyes on polyester is limited by their slow rate of diffusion into thefibre rather than by inherently low substantivity. If dyeing times are sufficiently long,saturation values on polyester approach those on cellulose acetate and often exceed thoseon nylon (Table 3.29; for structures of these dyes see Tables 3.14 and 3.18).

Some early measurements of the partition of various aminoanthraquinone derivativesbetween cellulose acetate and ethanol (Table 3.30) dramatically illustrate the value ofprimary amino groups in conferring affinity for cellulose acetate. Indeed, the introduction of

N

O

NX

H

Y

Y3.125

X N

N

H

SO3Na

HN

Y

O

NaO3S3.126

NaO3S N

N

H

O

3.127

CI Acid Orange 20NaO3S N

N

H

N

N

O

X

Y

Y

SO3Na

3.128

Table 3.29 Saturation values of disperse dyes on hydrophobic fibres at85 °C [113]

Saturation value (% o.w.f.)

CI Disperse Structure Acetate Nylon Polyester

Yellow 1 3.98 16.0 5.0 7.1Red 15 3.96; X = NH2, Y = OH 11.2 4.4 12.0Violet 1 3.96; X = Y = NH2 8.3 4.9 4.4Orange 3 3.95; X = Y = H 5.1 2.0 4.1

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a 4-hydroxy group into 1-aminoanthraquinone actually lowers the affinity for the fibreslightly, perhaps by favouring intermolecular hydrogen bonding and thus greater aggregationin the ethanolic phase. Methylation of the 1-amino group promotes affinity, butintroduction of a 2-methyl substituent into 1-aminoanthraquinone has a greater effect. Themost impressive increases, however, are caused by further amino substitution. The partitioncoefficients of the 1,4-, the 1,8- and the 1,5-isomers are respectively about twice, three timesand four times that of the monoamino analogue. Further amino substitution has a lessmarked effect, the 1,4,5,8-tetra-substituted derivative having a partition coefficient aboutfive times that of 1-aminoanthraquinone.

3.3.3 Effect of dye structure on migration and levelling

Early investigators of the skittery dyeing of wool concluded that the differences in dyeabilitybetween individual fibres are attributable to the variations in extent of degradation that theyhave undergone in the fleece and during preparation [130,131]. The responses of levellingand milling acid dyes to such differences were studied in detail [132]. With typical levellingacid dyes such as CI Acid Red 1 (3.129; R = H) the proportion of ring-dyed fibres presentgradually increases with dyeing temperature up to a maximum at about 70 °C. As dyeingcontinues above this temperature the content of ring-dyed fibres gradually falls again anduniformly penetrated fibres begin to predominate. Eventually the migration process ensuresthat a level and fully penetrated dyeing is achieved.

Table 3.30 Partition coefficients of aminoanthraquinone deriv-atives between cellulose acetate and ethanol at 60 °C [129]

Anthraquinone derivative Partition coefficient

1,4,5,8-Tetra-amino 8.771,5-Diamino 6.641,8-Diamino 5.731,4-Diamino 2.801-Amino-2-methyl 2.671-Methylamino 2.381-Amino 1.791-Amino-4-hydroxy 1.59

R N

N

H

SO3Na

NaO3S

HN

COCH3

O

3.129

Milling acid dyes behave in a more individual way. The disazo monosulphonate CI AcidRed 151 (3.130) is absorbed extremely rapidly at low temperature, all fibres becoming ring-dyed. Little further uptake takes place until the temperature approaches 90 °C. Diffusion


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into the fibre then accelerates markedly, the proportion of ring-dyed fibres rapidly falling tozero and soon all fibres present become deeply and uniformly dyed. The disazo disulphonateCI Acid Red 85 (3.131), however, shows virtually no uptake at all below 70 °C and ringdyeing is at a maximum in the range 80–90 °C. Only after boiling for about 30 minutes doesthe proportion of fully penetrated fibres exceed that of ring-dyed ones. A level dyeing ofsatisfactory fastness requires at least an hour at the boil. More hydrophobic ‘super-milling’dyes such as CI Acid Red 138 (3.129; R = n-dodecyl), the p-alkylated analogue of Red 1,give ring dyeings at 70 °C and above. Even after prolonged boiling, it is difficult to attainmore than a small proportion of fully penetrated fibres. Such dyes tend to be especiallysensitive to skitteriness and to root-tip differences of dyeability in weathered wool.

NaO3S N

N N

N

H O

3.130

CI Acid Red 151

H3C SO2

O N

N N

N

H

NaO3S

SO3Na

O

3.131

CI Acid Red 85

As already indicated in section 3.3.2, the introduction of a further sulpho group into anacid dye tends to lower the affinity by about –4 kJ/mol. The results in Table 3.31 for twoseries of phenylazo-l-naphthol dyes with varying degrees of sulphonation clearly demonstratethat the increase in hydrophilic character brought about in this way significantly impairstheir migration behaviour. Thus the monosulphonated levelling acid dye sulphanilicacid→1-naphthol has excellent migration properties, but the pentasulphonated analogueaniline-2,5-disulphonic acid→oxy-Koch acid migrates only with great difficulty.

Systematic studies of the relationship between disperse dye structure and levellingproperties on the ester fibres have shown that in general levelling tends to decrease asmolecular size increases. Thus an inverse linear correlation was found between the molarvolume of a series of disperse dyes and their barriness rating on polyester [90]. As molecularsize increased, migration became less effective in covering dye affinity variation in a texturedpolyester fabric prone to show barriness. Molecular size is not the only relevant factor,however, because disperse dyes with significant aqueous solubility show better migrationproperties than less soluble dyes of similar molecular size.

Disperse dyes containing n-alkyl substituents within the range butyl to octadecyl showedonly 20-40% exhaustion on polypropylene fabric. Exhaustion and washing fastness increasedbut levelling decreased with increasing alkyl chain length. Dyeing with 1,4-bis(octylamino)anthraquinone in the presence of an ethoxylated octadecanol surfactant,

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however, gave 70-90% exhaustion and markedly improved levelling. Exhaustion increasedprogressively with the degree of ethoxylation of the nonionic levelling agent [133].

An interesting comparison of the migration behaviour of a related series ofalkylaminothiazine basic dyes on an acrylic fibre is outlined in Table 3.32. Methylene blue(CI Basic Blue 9), the best known of these, is the tetra-N-methylated derivative with amigration rating of 2–3. Insertion of a nitro group meta to one of the dimethylamino groupsslightly impairs migration. As might be expected, the completely unmethylateddiaminothiazine structure shows excellent migration, whereas the tetra-N-ethyl analogue ofmethylene blue migrates only with great difficulty. Surprisingly, removal of one of the N-methyl groups from the methylene blue structure reduces migration slightly, but the isomerictrimethylated derivative with one of the methyl groups ortho to the unmethylated aminogroup has a migration rating almost as good as the parent unmethylated diaminothiazine.

Table 3.31 Migration properties of phenylazonaphthol acid dyes on wool [115]

Structure (3.132; unspecified X and Y = H) Basicity Migration

Y4 = SO3Na 1 5X4 = Y4 = SO3Na 2 4X2 = X5 = Y4 = SO3Na 3 3–4X4 = SO3Na 1 5X4 = Y4 = SO3Na 2 4X4 = Y6 = Y8 = SO3Na 3 3–4X4 = Y3 = Y6 = Y8 = SO3Na 4 3X2 = X5 = Y3 = Y6 = Y8 = SO3Na 5 2

X4 N

X5

X2

N

H

Y3 Y4

Y6

Y8

O

3.132

Table 3.32 Migration properties of thiazine basicdyes on acrylic fibre [134]

Structure (3.133) Migration

R = X = Y = Z = H 5R = Y = CH3, X = Z = H 4R = X1 = CH3, X2 = Y = Z = H 2R = X = CH3, Y = Z = H 2–3R = X = CH3, Y = H, Z = NO2 2R = X = CH2CH3, Y = Z = H 1

N

SNR

R

Z

Y

NX2

X1+

3.133


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3.3.4 Dye structure and light fastness

The fastness to light of a dyed fibre depends on many factors. These include the inherentstability of the dye chromogen when exposed to photochemical attack and the way in whichthis interaction is modified by the polymer substrate and the conditions of exposure. Generalreviews of the mechanisms of fading reactions and the influence of dye structure andaggregation behaviour on these processes are available [135–140]. Kinetic evidenceindicates that the light fastness of direct dyes on cotton is markedly dependent on theformation of aggregates within the fibre. This can sometimes lead to apparently anomalousdifferences in fastness rating for the same dye applied and aftertreated in various ways.Hence the light fastness of direct dyeings is determined more by their state of aggregationthan by structural features of the dye chromogen [141–145].

Interesting pointers to the reasons for the limited light fastness of monoazo red and disazoblue reactive dyes on cellulosic fibres, particularly those derived from H acid couplingcomponents, have emerged in recent years. Experimental evidence for free-radicalformation has been presented and the important role of radical reactions in the fadingmechanisms of reactive dyes has been defined [146]. The environmental effects ofatmospheric oxygen on the fading behaviour of aminochlorotriazine reactive dyes on drycotton fabric have been investigated. Most of the dyes were degraded in two stages, a rapidinitial fade followed by a slower and incomplete process. Oxygen accelerated the majorfading step for almost all dyes, but a phthalocyanine derivative was more sensitive in anitrogen atmosphere. In some instances the mechanism of fading in air was initiallyoxidative but became reductive in the second stage [147].

Accelerated fading in the wet state is characteristic of many azo reactive dyeings. Ifcellulose dyed with such dyes is exposed to light in aerated water the fading mechanism is anoxidative one. The rate of fading depends on dye structure, pH and oxygen concentrationpresent [148]. Photoreduction can take place on exposure in deaerated water, however,especially in the presence of sodium mandelate (3.134) under a nitrogen atmosphere.Catalytic wet fading of certain azopyrazolone yellow reactive dyes in mixture dyeings withphthalocyanine or triphenodioxazine blues has long been recognised as an importantproblem of reactive dye selection. The effect is very slight in the dry state and is almostcompletely suppressed in the absence of oxygen. Considerable protection of theazopyrazolone component is achieved by adding the singlet oxygen quencher 1,4-diazabicyclo[2,2,2]octane (3.135). Oxidative destruction of the azopyrazolone chromogendoes not rupture the ether bond formed in the dye-cellulose fixation process [148].

Complaints of accelerated or anomalous fading of reactive dyeings often implicate azodyes derived from H acid and may involve wetting of the affected textile by perspirationduring sunlight exposure. The mechanism of fading by light and perspiration simultaneouslyis not always the same as fading by light alone. In the dry state, dyes of the H acid typegenerally fade by a reductive mechanism but in the presence of aerated water the

CH COONa

OH3.134

Sodium mandelate

N CH2

CH2

CH2

DABCO

3.135

NCH2

CH2

CH2

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155

degradation reaction becomes oxidative. Dyes containing strongly electron-donatingsubstituents tend to fade oxidatively, whereas those with strongly electron-withdrawinggroups are more vulnerable to reductive attack. The reductive fading mechanism is greatlyenhanced when the exposed dyeing is wetted with acidic or alkaline perspiration [149].

In a recent study of the combined effects of perspiration and light, garments dyed withfive typical reactive dyes of various chromogenic types (Table 3.33) were worn by tennisplayers in bright sunlight. The fastness ratings showed good agreement with results from theJapanese ATTS test on these dyeings. The sensitivity to this problem shown by thecommercially predominant disazo CI Reactive Black 5 (3.136) derived from H acid wasconfirmed, compared with the much more stable anthraquinone derivative CI Reactive Blue19 (3.137).

Table 3.33 Wearer trial and fastness tests on dyeings representing variouschromogens [150]

Perspiration(acid)/light Light

CI Reactive Chromogen Wearer trial ATTS test Xenon arc

Blue 19 Anthraquinone 4 3–4 5Blue 221 Copper-formazan 3 3 5Blue 28 Copper-monoazo 1–2 1 5Blue 222 Azo 2–3 2–3 4–5Black 5 H acid disazo 2 1 4–5

N

N

H O

SO3Na

NH2N N

CH2CH2SO2NaO3SO

SO2CH2CH2 OSO3Na

NaO3S3.136

CI Reactive Black 5

NH2

HN

O

O

SO3Na

SO2CH2CH2 OSO3Na

3.137

CI Reactive Blue 19


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The tendency of Black 5 and other widely used azo reactive dyes of acceptable lightfastness to give rise to customer complaints when exposed to light in the presence ofperspiration was examined further. Dyeings of eight reactive dyes representing a variety ofazo chromogens, mostly with a sulphatoethylsulphone reactive system but also including onedichlorotriazine dye and one with a bifunctional system, were subjected to ATTS tests andxenon arc exposure (Table 3.34). Surprisingly, the J acid dye was more resistant toperspiration/light fading than to a conventional light fastness test. The four dyes derivedfrom H acid or K acid faded much more rapidly in the ATTS test, but the azopyrazolone,azonaphthalene and γ acid monoazo chromogens appeared to be unaffected by the presenceof perspiration during exposure.

Table 3.34 Perspiration and light fastness of various azo reactive dyes [150]

Perspiration(acid)/light Light

Reactive system Azo chromogen ATTS test Xenon arc

Sulphatoethylsulphone (SES) Azopyrazolone 4–5 5Bifunctional (SES/MCT) Azonaphthalene 5 5SES γ acid monoazo 4 4Dichlorotriazine (DCT) J acid monoazo 4–5 3–4SES H acid monoazo 2 4–5SES K acid monoazo 1 4–5SES H acid Cu-monoazo 1 4–5SES H acid disazo 1 4–5

These marked differences in response between different types of azo chromogen have beeninterpreted [150] in terms of tautomerism of their aminonaphthol structures (Scheme 3.15).When the imino nitrogen is peri to the hydroxy group, as in H acid (3.138) and K acid (3.139)dyes, the reduction-sensitive hydroxyazo form is stabilised by hydrogen bonding with the iminogrouping. In J acid (3.140) and γ acid (3.141) derivatives, however, hydrogen bond stabilisationof either tautomer by the imino group cannot take place. Independent studies of the fadingbehaviour of tautomeric azo dyes have shown that the hydrazone form is more resistant tophotoreduction than the azo form, but has a lower stability to photo-oxidation by singletoxygen [151]. In the monoazo red reactive dyes derived from H acid, the location and natureof the reactive system has a significant influence on fastness to the perspiration/light test(Table 3.35). When the reactive group is attached to the diazo component this leads to lowerfastness ratings than when it is linked via an imino group to the coupling component (3.138).

The fading behaviour of azoic dyeings on cotton is determined mainly by the chemicalcharacteristics of the diazo and coupling components. Although they show generally highfastness when exposed in full depths under dry conditions, this stability declines sharply withincreasing humidity or decreasing applied depth. Electron-withdrawing substituents in thephenylazo grouping, especially trifluoromethyl or nitro, enhance light fastness but electron-donating groups such as o-methyl or o-methoxy usually lower the ratings. Substituents in theanilide nucleus of Naphtol AS combinations, however, influence light fastness in theopposite way. For example, in a series of coupling components used with o-nitroaniline asdiazo component (3.142), the highest light fastness was found for R = p-methoxy and thelowest for R = m-nitro [152].

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157

NaO3S

N

O

NH

H

NArX SO3Na

Y

NaO3S

N

O

NH

NArX SO3Na

Y

H

3.138

H acid dyesScheme 3.15

N

NaO3S

SO3Na

N

O

H

N

H

ArR

R

3.139

K acid dyes

NaO3S

NH

N

O

NArR

H

R

3.140

J acid dyes

NaO3S

N

O

NArR

H

NH R

3.141

γ acid dyes

NO2

N

N

H CO N

H

O R

3.142

Table 3.35 Location of reactive system in H acid monoazo red reactive dyes [150]

Perspiration/light ATTS test

Reactive system Location (3.138) Acid Alkali

Dichlorotriazine Y 2 1–2Monochlorotriazine (MCT) Y 3 2–3Dichloroquinoxaline Y 2–3 2Bifunctional (SES/MCT) Y 3 2–3Sulphatoethylsulphone X 2 1–2Sulphatoethylsulphone X 1–2 1Bifunctional (SES/MCT) X 1–2 1


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The fading of azo acid dyes for amide fibres has attracted a great deal of interest.Photochemical oxidation of the commercially important H acid disazo CI Acid Black 1(3.143) in aqueous solution is accelerated by hydrogen peroxide but retarded by mannitoladdition. The reaction rate is increased by incorporating a triplet-sensitising dye, suggestingthat the azo groups, particularly the o-hydroxyazo in its hydrazone form, undergo attack byhydroxyl free radicals and triplet-sensitised degradation [153]. Measurements of the fadingof a series of monoazo dyes of the phenylazo R acid type on wool demonstrated that this wasa reductive process influenced particularly by the nature of ortho substitution in the diazocomponent;. Anionic groups (sulpho, carboxyl) conferred higher fastness to light butstrongly electron-withdrawing nonionised substituents (chloro, nitro) had adverse effects[154]. A similar study of p-substituted phenylazo γ acid dyes revealed that electron-donatinggroups, such as ethoxy, amino or anilino, markedly lowered the light fastness.

N

N

H O

H2N N

SO3Na

N

NO2

NaO3S 3.143

CI Acid Black 1

The kinetics of fading of the monoazo dyes CI Acid Orange 8 (3.144) and Red 1 (3.145)on exposure to sunlight in aqueous solutions at pH values in the range 2 to 4.5 and on silkfabric were investigated recently [155]. In the presence of silk these dyes were morephotoreactive, exerting a protective action against photodegradation of the silk fibroin. Aspart of a broad chemometric strategy to design novel acid dyes for silk [156], eightphenylazo J acid dyes with o,p-substitution (3.146) were synthesised and their light fastnessratings compared on this fibre (Table 3.36). The results indicated that electron-withdrawingsubstituents (nitro, benzoyl), or an additional phenyl group in the p-position, enhanced lightfastness slightly, whereas benzoate ester groups or an o-phenyl substituent lowered thefastness significantly. Disubstitution by propoxy did not modify the rating from that shownby the parent dye. All these dyes retained the 6-amino group in the coupling component,which tends to impair light fastness.

N

N

H O

3.144

CH3

NaO3S

CI Acid Orange 8

N

N

H O

HN

SO3Na

NaO3S 3.145

CI Acid Red 1

C

O

CH3

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159

Aggregates of dye molecules normally fade more slowly than do single moleculesdistributed uniformly through the substrate. These aspects were examined in the dyebathand on nylon for three monoazo dyes, CI Acid Orange 10, Red 18 and Red 88 (forstructures see Table 3.28). Aggregation was found to influence the fastness to light, washingand rubbing [157]. When alkyl groups are introduced into the structures of milling acid dyesto confer higher neutral-dyeing affinity and better wet fastness on wool, relatively shortchains (C4–C6) can have a favourable influence on light fastness by promoting aggregationin the fibre [136]. If the chain length is increased (C12–C16), however, the fastness to lightdecreases markedly, owing to the higher surface activity of such dyes tending to destabiliseaggregates in favour of monomolecular distribution [158]. For example, the light fastness onwool of the phenylazopyrazolone levelling acid dye (3.147; X = H) is 6 but that of the long-chain alkylated CI Acid Yellow 72 (3.147; X = n-dodecyl) is only 4–5.

Table 3.36 Light fastness of substituted phenylazo J aciddyes on silk [156]

Substituents in structure 3.146 Light fastness

X = Y = nitro 4X = H, Y = benzoyl 4X = phenyl, Y = nitro 4X = benzoyl, Y = H 4X = Y = H 3–4X = Y = propoxy 3–4X = nitro, Y = phenyl 2–3X = Y = benzoate 2

Y

N

N

H O

NaO3S

NH2X

3.146

N

N

H

N

N

H3C

O

Cl

SO3Na

Cl

X

3.147

The influence of various dyeing and heat setting parameters on the light fastness of threeacid dyes on nylon 66 has been examined recently. The capability of the dyes to quench thephotoactive luminescent species in the polyamide matrix was found to be related closely tolight fastness rating. Since acid dyes are linked to terminal amino groups throughelectrostatic bonds, they are able to protect their own chromogens and the polymer chainfrom photodecomposition through excitation energy transfer via the ionic bonds [159].


chpt3(1).pmd 15/11/02, 14:43159


The nature of the substrate has little effect on the light fastness of anthraquinone aciddyes on amide fibres. Most substituents hardly affect the slow rate of fading of thesestructures, but photodealkylation may occur when alkylaminoanthraquinones are exposed tolight. The fastness can be improved, however, by incorporating arylamino in place ofalkylamino groups [160,161]. The photodegradation in dimethylformamide solution and innylon 6.6 of the 1:2 chromium-complex CI Acid Orange 60 (3.148) and the importantanthraquinone CI Acid Green 25 (3.149) used on nylon automotive carpets was studied.Each of the dyes was found to fade by a reductive mechanism in both media.Dimethylformamide was found to be a suitable model for nylon in characterisingphotofading [162]. Consequently, the photodecomposition of several 1:2 metal-complex azodyes was investigated in DMF solutions exposed to daylight or ultraviolet radiation at 300 or350 nm. The coordinated metal atom and the wavelength of photolysis influenced thedegradation reaction significantly. The presence of a ketone sensitiser accelerated dyedecomposition, confirming that hydrogen abstraction is responsible for initiation of thefading reaction. Dyes containing cobalt(II) atoms were found to be effective quenchers ofsinglet oxygen [163].

N

N N

N

O

O

CH3

SO2NH2

Cr

N

NN

N

O

O

H3C

H2NO2S3.148

CI Acid Orange 60

NH

HN

NaO3S CH3

NaO3S CH3

O

O

3.149

CI Acid Green 25

The photochemistry of four triphenylmethane acid dyes was studied in poly(vinylalcohol), methylcellulose and gelatin films. These model systems were chosen with a view toelucidating the complex free-radical reactions taking place in the heterogeneous dyed wool/water/air system on exposure to UV radiation. The dye fading mechanism seems to involvean excited triplet state of the dye molecule [164]. The rate of fading is governed by:(1) the positions of the sulpho groups in the dye molecule(2) the ability of the substrate or residual solvent to donate electrons or hydrogen atoms to

the dye molecule(3) the degree of aggregation of the dye in the substrate(4) the internal physical and chemical structure of the substrate.

Catalytic fading of certain mixtures of acid dyes on nylon, wool and their blends is aparticularly significant problem of dye selection for carpets [165]. Xenotest ratings as low as3–4 (much bluer) were found for green shades on nylon containing an azopyrazolonecomponent such as CI Acid Yellow 19 (3.150), which fades much more rapidly in the presence

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161

of the widely used disulphonated 1,4-bis(arylamino)anthraquinones (3.151) such as CI AcidBlue 140, Green 25 (X = H, R = Me), Green 27 (X = H, R = n-Bu) or Green 28 (X = OH,R = n-Bu). Furthermore, commercially important unmetallised azopyrazolones such as Yellow19, 49 or 79 are catalytically faded by typical 1:2 chromium-complex azopyrazolone dyes fromthe yellow to violet sectors of the colour gamut, including CI Acid Yellow 59 (3.152; X = H),Brown 384 (3.152; X = SO3Na) and substituted 2-hydroxyphenylazopyrazolone Reds 225,226, 359 and 399.

SO3Na

N

N

H

N

N

O

H3C3.150

CI Acid Yellow 19

Cl

SO3Na

Cl

HN

HN

X

X

NaO3S

NaO3S R

R

O

O

3.151O

CrO

C

N

NN

N

O

OO

C

N

NN

OX

N

X

CH3

H3C3.152

Apart from the catalytic effects between these fairly well-defined subclasses of acid dyeson nylon, that do not occur on wool [165], certain 1:2 metal-complex dyes such as CI AcidYellow 59 (3.152; X = H) are able to catalyse the fading of various unmetallised milling aciddyes in mixture recipes on either wool or nylon [166]. For example, orange or scarlet shadesformulated with Yellow 59 and Red 114 (3.153) can give light fastness as low as 2 (muchyellower). Unacceptable off-tone fading can also arise in green mixtures of Yellow 59 withdisulphonated 1,4-bis(arylamino)anthraquinones (3.151) that also fade much yellower.Photosensitised degradation of dyed nylon fibres is another practical problem that ischaracteristic of Yellow 59 and similar 1:2 chromium-complex azopyrazolone dyes.

For the same disperse dyes on various hydrophobic fibres the highest light fastness is

H3C SO2

O N

N

CH3

N

N

H

NaO3S

SO3Na

O

H3C

3.153

CI Acid Red 114


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found on polyester, normally followed by cellulose acetate and triacetate, with nylon givingthe lowest ratings. Anthraquinone disperse dyes often contain one or two primary orsecondary amino groups in the 1,4-positions. Tetra-amino derivatives have lower fastnessthan mono- or diamino analogues. Light fastness on cellulose acetate or polyester may beenhanced by:(1) a hydroxy group para to an amino group(2) acetylation of a primary amino group(3) replacement of an alkylamino by an arylamino group.

Electron-donating groups (amino, methylamino, hydroxy, methoxy) in the 2-position, onthe other hand, are extremely undesirable because, unlike similar substituents in the 1,4-positions, they are unable to form intramolecular hydrogen bonds with the keto groups ofanthraquinone and hence are highly susceptible to photo-oxidation [167].

The photochemical decomposition of typical azo and anthraquinone disperse dyes wasfound to correlate closely with the wavelength of UV radiation at which the fibre substrateshowed maximum photodegradation (230 nm on cellulose acetate, 259 nm on triacetate and316 nm on polyester). When exposed to UV radiation in ethyl acetate solution, the fading ofCI Disperse Red 60 (3.154) occurred most rapidly in the 316 nm region [168]. Applicationof a UV absorber showing peak absorption in this region conferred some protection todyeings of CI Disperse Red 73 (3.155) on polyester against dye fading and fibrephotodegradation [169]. The primary mechanism of fading of the latter azo dye was areductive reaction, whereas decomposition of the anthraquinone derivative Red 60 wasessentially an oxidative process [168].

O

O

NH2

OH

O

3.154

CI Disperse Red 60

CN

O2N N

N N

CH2CH2CN

CH2CH2

3.155

CI Disperse Red 73

The fading rates of substituted 4-phenylazo-1-naphthylamine dyes on cellulose acetatewere consistent with a photo-oxidative mechanism [170], but studies of the fading ofphenylazo-2-naphthol dyes on polypropylene were indicative of photoreduction [171,172].In simple azobenzene derivatives on polyester, an electron-withdrawing 4-nitro or especially3-nitro group enhances light fastness (Table 3.37). An electron-donating 3′-methoxysubstituent in the opposite ring boosts these effects further, but the 2-nitro-3′-methoxycombination yields the same fastness as azobenzene itself. The 2′-methoxy-5′-methylsubstitution pattern also reinforces the favourable influence of the 3- or 4-nitro group butmarkedly lowers the rating of 2-nitroazobenzene. All three nitroazobenzenes are adverselyaffected by a 2′-hydroxy-5′-methyl arrangement [173].

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163

Photofading studies of the simple azobenzene derivatives CI Disperse Orange 1 (3.156; R= Ph) and Orange 3 (3.156; R = H) were carried out in solution and in the solid phaseunder a variety of conditions. Attempts were made to interpret light fastness ratings onnylon and polyester substrates in terms of behaviour of the dyes in solution in the presenceof model compounds representing functional groups in the polymers. Various mechanisms offading were postulated and potential photostabilisers evaluated [174].

Many important orange, red, brown and blue disperse dyes belong to the substituted p-phenylazoaniline subclass (Table 3.38). On nylon, light fastness in the p-nitrophenylazoseries (3.157; X = NO2) is marginally improved by electron donation (methyl, methoxy) butmarkedly lowered by electron withdrawal (chloro and especially cyano or nitro) in the orthoposition [175]. On polyester, electron-withdrawing groups oriented ortho (chloro, cyano) orpara (nitro, acetyl, methylsulphonyl or diethylaminosulphonyl) to the azo linkage improvethe fastness but electron-donating substituents (methyl, methoxy) lower the ratings. The2,4-dinitrophenylazo arrangement is also highly unfavourable. In the 2-chloro-4-nitrophenylazo series (3.158), the presence of N-cyanoethyl or N-acetoxyethyl groups in thecoupling component confers much higher light fastness than the N,N-diethyl analogue butan N-2-hydroxyethyl group is slightly unfavourable in this respect.

Only a limited range of nitro, azo and anthraquinone disperse dyes exhibit adequatefastness to dry heat, light and weathering for application on polyester automotive fabrics.The structure of CI Disperse Yellow 86 was modified to incorporate UV absorbers of thebenzophenone, benzotriazole or oxalanilide types into the dye molecule. The derived dyesshowed better fastness properties than the parent unsubstituted dye. Positioning of thephotostabilising moiety within the dye molecule had little influence on the light fastnessobtained, however. Built-in benzophenone residues were more effective than the other twotypes [177]. Nevertheless, several further monoazo and nitrodiphenylamine disperse dye

Table 3.37 Light fastness of substituted azobenzenes on polyester [173]

Azobenzene substituents Light fastness

3-Nitro-3′-methoxy 7–83-Nitro-2′-methoxy-5′-methyl 7–84-Nitro-2′-methoxy-5′-methyl 7–83-Nitro 74-Nitro-3′-methoxy 74-Nitro 6–73-Nitro-2′-hydroxy-5′-methyl 6–72-Nitro 6None 62-Nitro-3′-methoxy 64-Nitro-2′-hydroxy-5′-methyl 52-Nitro-2′-methoxy-5′-methyl 4–52-Nitro-2′-hydroxy-5′-methyl 3

O2N N

N N

R

H

3.156


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Table 3.38 Light fastness of substituted aminoazobenzene dyes on nylon [175] andpolyester [89,176]

Light fastness

CI Disperse Structure Nylon Polyester

Orange 45 3.158; X = Y = CN 7Red 54 3.158; X = CN, Y = OCOCH3 7Red 169 3.158; X = CN, Y = OCH2CH2CN 6–7Red 186 3.158; X = Y = OCOCH3 6–7Red 73 3.157; X = NO2, Y = CN, Z = H 1–2 6–7Red 50 3.158; X = H, Y = CN 3–4 6

3.158; X = H, Y = OCOCH3 6Orange 25 3.157; X = NO2, Y = Z = H 4 5

3.157; X = NO2, Y = H, Z = CH3 53.157; X = NO2, Y = CH3, Z = H 4–5 4–5

Red 56 3.158; X = OH, Y = CN 43.157; X = COCH3, Y = H, Z = CH3 43.157; X = SO2CH3, Y = H, Z = CH3 43.158; X = Y = H 3–43.157; X = NO2, Y = OCH3, Z = H 4 3–4

Red 13 3.158; X = H, Y = OH 33.157; X = SO2NEt2, Y = H, Z = CH3 33.157; X = Y = NO2, Z = H 1 23.157; X = Y = H, Z = CH3 23.157; X = OCH3, Y = H, Z = CH3 2

N

N

X

Y

Z

N

CH2CH2CN

CH2CH3

3.157

N

N

O2N

Cl

N

CH2CH2

CH2CH2

Y

X

3.158

structures containing an oxalanilide photostabilising grouping were synthesised andevaluated as potential dyes for automotive trims. The light fastness of CI Disperse Red 167(3.159) was enhanced by inclusion of an electron-donating o-ethyl group in the diazocomponent [178].

N

N

O2N

Cl

N

CH2CH2OCOCH3

CH2CH2OCOCH3

3.159

HN

C

O

CH2CH3

CI Disperse Red 167

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165

A similar approach has been adopted in modifying the anthraquinone system representedby CI Disperse Blue 77 (3.160), the aniline residue being replaced by a more elaboratearylamine with UV absorbing characteristics, The modifying intermediates selected includedhydroxybenzophenone and chlorobenzotriazole derivatives, as well as a hinderedphotostabiliser. In general, the improvement in light fastness from this approach is inferior tothat given by carefully formulated physical mixtures of the corresponding dye and stabilisingagent. The modified dye structures tend to exhaust more slowly and show poor levelling butdo offer higher fastness to sublimation [179].

HO O OH

HNOO2N

3.160

CI Disperse Blue 77

Two commercial disazo disperse dyes of relatively simple structure were selected for arecent study of photolytic mechanisms [180]. Both dyes were found to undergophotoisomerism in dimethyl phthalate solution and in films cast from a mixture of dye andcellulose acetate. Light-induced isomerisation did not occur in polyester film dyed with thetwo products, however. The prolonged irradiation of CI Disperse Yellow 23 (3.161; X = Y =H) either in solution or in the polymer matrix yielded azobenzene and variousmonosubstituted azobenzenes. Under similar conditions the important derivative Orange 29(3.161; X = NO2, Y = OCH3) was degraded to a mixture of p-nitroaniline and partiallyreduced disubstituted azobenzenes.

X N

N

Y

N

N OH

3.161

3.3.5 Dye structure and wet fastness

The fastness of a dye to wet treatments, such as washing or perspiration, is a function ofkinetic (diffusion) and thermodynamic (affinity) effects. Certain trends can be discerned,but conclusions derived from simple structural changes in closely related structures haveonly limited applicability. Relationships between wet fastness ratings and molecularstructure of direct dyes on cellulosic fibres are somewhat tenuous because of the majorobscuring influence of hydrogen bonding and other forces of association between dye anionsleading to aggregation within the substrate (sections 3.1.2 and 3.3.4). In general, the wetfastness of disperse dyes on hydrophobic fibres tends to increase with the molecular size ofthe dye and with the increasingly hydrophobic character of the substrate (acetate < nylon< triacetate < polyester).


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Acid dyes are normally applied to wool at acidic pH values but are washed under neutralor mildly alkaline conditions. Forces of affinity between dye and substrate tend to retarddesorption. The higher the degree of sulphonation in a series of naphthylazo-2-naphtholdyes (Table 3.39), the higher is the rate of desorption into sodium borate solution at 40 °C.With a series of monosulphonated acid dyes the desorption rate under these conditions wasinversely related to the affinity of the dye for wool at pH 3 and 60 °C (Table 3.40).

Table 3.39 Basicity and desorption rate of monoazo acid dyeson wool [181]

Desorption rateSubstituents in 3.162 Basicity (%/min)

R = X = Y = Z = SO3Na 4 16.0R = Y = Z = SO3Na, X = H 3 12.1R = X = H, Y = Z = SO3Na 2 5.2R = SO3Na, X = Y = Z = H 1 1.6

Table 3.40 Affinity and desorption rate of monosulphonated acid dyes on wool [181]

Desorption rate (%/min)

CI Acid Structure Affinity (kJ/mol) 40°C 60°C

Orange 7 3.163 –18.4 6.4 18.5Yellow 36 3.164 –21.7 5.4 16.6Violet 43 3.165 –27.3 3.0 9.1Blue 25 3.166 –29.7 2.0 6.0

R N

N

H

Z

O X

Y3.162

NaO3S N

N

H O

3.163

CI Acid Orange 7

NaO3S

N

N NH

3.164

CI Acid Yellow 36

HNO

O

NaO3S CH3

3.165

CI Acid Violet 43

NH2

HN

SO3Na

O

O

3.166

CI Acid Blue 25

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167

The increase in affinity for wool imparted to acid dye structures by the inclusion of long-chain alkyl groups has a noteworthy effect on wet fastness. Thus the incorporation of such agroup into the levelling acid dye CI Acid Red 1 (3.129; R = H) to form the ‘super-milling’acid dye Red 138 (3.129; R = n-dodecyl) raises the fastness to washing at 50 °C (effect onpattern) from 2–3 to 4–5. Similar effects for a series of alkylated phenylazo-2-naphthol-6-sulphonate dyes are recorded in Table 3.41.

Table 3.41 Structure and wash fastness of monoazodyes on wool [182]

Wash fastnessSubstituents in 3.167 (effect on pattern)

X = (CH2)11CH3, Y = H 4X = (CH2)3CH3, Y = H 3X = CH2CH3, Y = H 2X = H, Y = CH2CH3 2X = CH3, Y = H 1–2X = Y = H 1

N

N

H O

SO3Na

X

Y

3.167

The series of eight phenylazo J acid dyes with o,p-substitution in the diazo component(3.146) synthesised for evaluation on silk was subjected to various wet fastness tests (Table3.42). The results indicated that fastness to these agencies was lowered slightly by o,p-benzoate ester disubstitution or by a benzoyl group in the p-position. Increases in fastnessratings with dipropoxy, dinitro, o-nitro-p-phenyl or p-nitro-o-phenyl substitution were alsoslight, but an o-benzoyl grouping was surprisingly effective in this respect. Possibly the ketomoiety stabilises this dye in its hydrazone form (3.168) by hydrogen bonding, implying thatthe tautomeric hydroxyazo form of these dyes has lower affinity for silk than thecorresponding ketohydrazone.

Table 3.42 Wet fastness of substituted phenylazo J acid dyes on silk [156]

Washing Perspiration

Substituents in structure 3.146 40°C 50°C 60°C Acid Alkaline

X = benzoyl, Y = H 5 4 3 5 5X = Y = propoxy 4 2–3 1 5 5X = Y = nitro 3–4 3 1 5 5X = nitro, Y = phenyl 4 2 1 4–5 5X = phenyl, Y = nitro 4 2 1 4–5 5X = Y = H 3 2 1 5 5X = H, Y = benzoyl 3–4 3 1 4 4X = Y = benzoate 2–3 1–2 1 4 5


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In a further exploration of the relationship between dye structure and wet fastness onsilk, four novel monoazo J acid derivatives (3.169; X = X1 to X4), including 3.168 (X = X2)made from 2-aminobenzophenone, were synthesised. Silk was dyed at pH 4 and 85 °C andthe dyeings tested for fastness to washing, perspiration and dry cleaning. The highestallround fastness was shown by the 4-aminobenzophenone derivative (X = X4), a structurethat resembles the anti-parallel pleated sheet arrangement of polypeptide chains in silk[183].

C O

N

N

H

NaO3S

NH2

O

3.168

O2N

NO2

C O

C

O

N

N

O

NH2X

H

NaO3S= X1

= X3

= X2

= X4

3.169

Affinity values were determined for an interesting series of five anthraquinone acid dyes[184]. The first four were monosulphonated derivatives that differed only in the length ofthe n-alkyl R substituent (3.170). The fifth dye (3.171) was composed of twomonosulphonated molecules linked at the R position by means of a methylene group. Theaffinity for nylon increased progressively with relative molecular mass as the length of the n-alkyl chain was extended (Table 3.43). The average increment per methylene group up to n-butyl was -2.55 kJ/mol. The affinity of the disulphonated dye could be calculated to a goodapproximation from the sum of twice that of the non-alkylated control dye plus theincrement for the linking methylene group, as in Equation 3.1.

2( 23.8) ( 2.55) 50.15 kJ/mol� � � � � (3.1)

The wet fastness of these dyeings increased consistently with the Mr of the dye.

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169

3.3.6 Dye structure and heat fastness

This fastness requirement is of practical significance only for disperse dyes on hydrophobicfibres. Dyes of low fastness are sublimed from the surface of the heated fibre at a ratedependent on the temperature of treatment. The diffusion coefficient of the dye in the drypolymer controls the rate at which loss of dye from the surface by volatilisation isreplenished from the interior of the fibre. Dye decomposition during the heat fastness testcan be highly significant in determining the overall rating attainable with a given dyestructure [185].

Inverse relationships between the vapour pressure of disperse dyes and their molecularsize and polarity have been established [186]. In Table 3.44 the vapour pressure values at200 °C for three typical disperse dyes of low relative molecular mass are given. A decrease ofonly 5% in Mr between CI Disperse Red 11 (3.172; X = OCH3, Y = NH2) and Violet 4(3.172; X = H, Y = NHCH3) produces a tenfold increase in vapour pressure, although afurther decrease of 4% to give Orange 3 (3.173) has only a doubling effect. A more strikingexample appears in Table 3.45. The 12% decrease in Mr from Blue 14 (3.174; X = NHCH3)to Red 9 (3.174; X = H) has profound effects, raising the diffusion coefficient in polyesterabout sixfold and the vapour pressure almost 700-fold. Both factors have an adverse

O

O

NH2

SO3Na

HN R

3.170

NHHN

O

O

O

O

SO3Na NaO3S

NH2 H2N

CH2

3.171

Table 3.43 Affinity and relative molecular mass of anthra-quinone acid dyes on nylon [184]

Structure Mr Affinity (kJ/mol)

3.170; R = H 394 –23.83.170; R = methyl 408 –26.63.170; R = n-butyl 450 –34.03.170; R = n-dodecyl 562 –45.63.171 800 –52.8


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influence on heat fastness. The monoazo Yellow 3 (3.175) is almost identical with Blue 14 inMr but diffuses twice as rapidly and vaporises much more readily, possibly because it has onlytwo aryl nuclei and a linear conformation.

The requirements of disperse dyes for the transfer printing of polyester fabrics are almostexactly opposite to those of the dyer. Adequate sublimation during the transfer step cannormally be achieved only with dyes of low Mr (<350) that are substantially free from highlypolar substituents [189]. For steric reasons, however, there are some exceptions to thisgeneral rule. Thus 2,4,6-trisubstituted phenylazo groups, which often feature in useful brownand navy blue members of the aminoazobenzene class, can interfere with the coplanarity ofsuch structures. This may hinder dye-dye and dye-fibre bonding to such an extent that suchdyes of Mr exceeding 400 can sometimes be used for transfer printing.

The relationships between dye structure and heat fastness for two series ofphenylazopyrazolone dyes (Table 3.46) demonstrate the significance of changes in molecular

Table 3.44 Vapour pressure and relative molecular mass of disperse dyes [187]

Vapour pressureCI Disperse Structure Mr at 200°C × 103 (kPa)

Orange 3 3.173 242 18.4Violet 4 3.172; X = H, Y = NHCH3 252 9.67Red 11 3.172; X = OCH3, Y = NH2 268 1.03

Table 3.45 Vapour pressure, diffusion coefficient and structure of disperse dyes [188]

Vapour pressure Diffusion coefficientCI Disperse Structure Mr at 160°C × 105 (cm Hg) × 1013 (m2/s)

Red 9 3.174; X = H 237 692 6.87Yellow 3 3.175 269 13.5 2.39Blue 14 3.174; X = NHCH3 266 1.01 (1.2)

O

O

NH2

X

Y 3.172

N

N

O2N

NH2

3.173


O

O

NHCH3

X 3.174

N

H

CH3CO

N

N

HO

CH33.175


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171

size and polar substituents for this property [175]. In the 2-chloro-4-nitrophenylazo (3.176)series, the tetramethylenesulphone ring enhances heat fastness far more than does aconventional N-phenyl group, even though both changes in molecular size are relativelymodest. A 3-carbethoxy substituent in the pyrazolone ring raises heat fastness about onepoint higher than that of the 3-methyl analogue, a difference roughly equivalent to theinsertion of an N-phenyl ring into the 3-methylpyrazolone parent dye. In the 4-carbomethoxyphenylazo (3.177) structure, an improvement of this magnitude can beachieved merely by using the pyrazol-5-imine instead of the pyrazol-5-one as the couplingcomponent.

Variations in the dialkylamino terminal substitution pattern of typical alkylamino-azobenzene derivatives have a greater effect on heat fastness than have substituents in thearyl nuclei of the diazo and coupling components [138]. The fastness can be significantlyimproved by incorporating polar groups (acetoxy, acetylamino, cyano) capable of hydrogenbonding or dipole–dipole interaction with polymer segments. The effect on heat fastness ofreplacing an acetate ester by a benzoate ester group in the coupling component is clearlyillustrated by comparing two commercially important yellow brown disperse dyes of the 2,6-dichloro-4-nitrophenylazo series (Table 3.47).

Dyes of high Mr with numerous substituents or bulky groups almost invariably showhigher fastness to sublimation than simpler aminoazobenzene or aminoanthraquinoneanalogues [182]. An interesting and comprehensive survey of 2,4,6-trisubstituted phenylazodyes comparing the relative influence of all four possible halogeno substituents exploredmany such mutual steric and electron-withdrawing effects in the diazo components (Table3.48). In all three series of structures represented (3.179–3.181), heat fastness of thetrisubstituted derivatives increased consistently in the order: 2,6-dihalogeno-4-nitro < 2,4-dinitro-6-halogeno < 2-cyano-4-nitro-6-halogeno. As expected, heat fastness increasedmarkedly with the size of the halogen atom and in the halogenated phenylazo-N-

O2N N

Cl

H

N N

NY

X

O

3.176

CH3OOC N

H

N N

N

H3C

X

3.177


Table 3.46 Structure and heat fastness of phenylazopyrazolonedisperse dyes on polyester [175]

Structure Heat fastness

3.176; X = CH3, Y = 5

3.176; X = COOCH2CH3, Y = Ph 3–43.177; X = NH 3–43.176; X = CH3, Y = Ph 2–33.177; X = O 2–33.176; X = CH3, Y = H 1–2

SO2

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Table 3.47 Structure and heat fastness of esterified N-hydroxy-ethyl disperse dyes on polyester [89]

Substituent R Heat fastnessCI Disperse in structure 3.178 (30s at 210°C)

Orange 62 O.CO.Ph 4–5Orange 30 O.CO.Me 3–4

O2N

Cl

N

Cl

N N

CH2CH2

CH2CH2CN

R

3.178

Table 3.48 Structure and heat fastness of halogenated phenylazo disperse dyes onpolyester [190]

Sublimation for 2.5% depth (°C)

Structure Iodo Bromo Chloro Fluoro

3.179; X = nitro, Y = halogeno, Z = cyano 210 210 2103.181; X = nitro, Y = halogeno, Z = cyano 200 190 1803.180; X = nitro, Y = halogeno, Z = cyano 200 180 1803.181; X = Z = nitro, Y = halogeno 200 180 170 1703.179; X = Z = nitro, Y = halogeno 190 180 180 1703.180; X = Z = nitro, Y = halogeno 190 180 170 1703.180; X = nitro, Y = Z = halogeno 190 180 1703.181; X = nitro, Y = Z = halogeno 190 180 1703.179; X = nitro, Y = Z = halogeno 190 170 1703.179; X = nitro, Y = halogeno, Z = H 180 170 170 1603.179; X = Z = H, Y = halogeno 170 160 160 1503.179; X = halogeno, Y = Z = H 170 150 160 140

cyanoethyl-N-hydroxyethylanilines (3.179) heat fastness increased in the order: 4-halogeno< 2-halogeno < 2-halogeno-4-nitro. Heat fastness in Tables 3.48, 3.50 and 3.51 is definedas the dry heat treatment temperature at which a 2.5% o.w.f. dyeing on polyester gives asatisfactory rating [190].

Proton-donor groups (amino, hydroxy) in the 2-position of 1,4-disubstitutedanthraquinone dyes enhance the fastness to heat because they are readily able to formintermolecular hydrogen bonds with the substrate or other dye molecules. Similar groups inthe 1,4-positions normally form intramolecular hydrogen bonds with the keto groups ofanthraquinone. The main drawback of electron-donating groups in the β-positions ofanthraquinone dyes, however, is their usually adverse effect on light fastness (section 3.3.4).

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173

Table 3.49 Structure and heat fastness of substituted aminohydroxy-anthraquinone disperse dyes on polyester [175]

Structure Heat fastness

3.182; X = NHCOCH3 4–5

3.183; R = SO2NHCH2CH2CH2OCH2CH3 4–5

3.182; X = SO2 3–4

3.183; R = Cl 3–4

3.183; R = –CH2CH2CH2CH2CH2CH2OH 3–4

3.182; X = OCH3 2–3

3.182; X = –CH2CH2COOCH2CH3 2–3

3.183; R = SCH3 2–3

3.183; R = 2–3

3.182; X = 1–2

3.182; X = H 1–23.183; R = CH3 1–2

Table 3.50 Structure and heat fastness of N-substituted amino-dihydroxyanthraquinone disperse dyes on polyester [190]

Sublimation forSubstituent R in structure 3.184 2.5% depth (°C)

Benzothiazol-2-yl 1804-n-Dodecylphenyl 1803-Fluoromethylphenyl 180Morpholin-4-ylpropyl 160Pyrid-3-ylmethyl 160Phenyl 1602,3-Dihydroxypropyl 1602-Hydroxypropyl 160Morpholin-4-ylethyl 150Benzyl 150Cyclohexyl 150Methoxypropyl 150Hydroxypropyl 150Hydroxyethyl 150Pyran-2-ylmethyl 140Methoxyethyl 140Ethyl 140Methyl 140Hydrogen 140


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Table 3.51 Structure and heat fastness of p-substituted anilinoanthraquinone disperse dyeson polyester [192]

Sublimation for 2.5% depth (°C)

Anthrarufin derivatives Chrysazin derivatives

3.187 3.188 3.189 3.190

1-amino- 1-anilino- 1-amino- 1-anilino-Substituent R in 3.187–3.190 5-anilino 5-nitro 8-anilino 8-nitro

Benzoylamino 200 200 210 210Acetylamino 200 200 200 210Aminosulphonyl 200 200 200 200Benzoyl 190 200 200 210Carbethoxy 210 190 210 190Acetyl 180 190 200 190Hydroxyethyl 170 180 160 180Methoxy 170 160 160 180Ethoxy 160 170 160 170Methyl 160 170 160 170Hydrogen 160 160 160 170

N

N

X

N

CH2CH2CN

CH2CH2OH

Y

Z

3.179 N

N

X

N

CH2CH2CN

CH2CH2OH

Y

Z

HN

COCH3

OCH33.180N

N

X

Y

Z

NHCH2CH2OH

3.181NO

O

H X

3.182

OHNH2O

O

O R

OH

3.183

OHO

O

NH R

OH

3.184

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175

Trends indicating the capability of various polar and nonpolar substituents to enhance thelow heat fastness of 1-amino-4-hydroxyanthraquinone (3.182; X = H) and its 2-hydroxyderivative (3.183; R = H) are revealed in Table 3.49. In the series of secondary aminoderivatives (3.182) a phenyl group offers little or no improvement. An ethyl propionateresidue or a p-methoxyphenyl group produces a modest increase in fastness but more polarmoieties such as tetramethylenesulphone or p-acetanilide groupings are necessary to achievehigh ratings. Methylation of the 2-hydroxy group in structure 3.183 improves the lightfastness but not the fastness to sublimation. A phenyl or p-methylthiophenyl group raisesheat fastness somewhat, but more marked improvements follow with 6-hydroxy-n-hexyl, p-chlorophenyl and the complex alkoxyalkylsulphamoylphenyl grouping shown in Table 3.49.

In a related series of 1,2,4-trisubstituted anthraquinone compounds, the effectiveness ofvarious polar and nonpolar substituents to improve on the low heat fastness of 2-amino-1,4-dihydroxyanthraquinone (3.184; R = H) was examined (Table 3.50). Short-chain alkylgroups (methyl, ethyl) and even the pyranylmethyl ether are relatively ineffective buthydroxyalkyl, cyclohexyl, benzyl and morpholinylethyl groups show moderate increases.Further improvement is given by phenyl, pyridylmethyl and morpholinylpropyl.Outstandingly effective, however, are the benzothiazolyl, dodecylphenyl and fluoro-methylphenyl groupings.

The influence of the number and orientation of bromo substituents on heat fastness intwo systematic series of multibrominated derivatives of 1,5-diaminoanthrarufin (3.185) and

NH2O

O OHH2N

HO

3.185

NH2O

O OHHO

H2N

3.186

NH2O

O OHNH

HO

R 3.187

HNO

O OHO2N

HO

3.188

R

NH2O

O OH

NH

HO

R

3.189HNO

O OH

O2N

HO

R

3.190


chpt3(1).pmd 15/11/02, 14:43175


1,8-diaminochrysazin (3.186) was investigated in detail [191]. More recently, p-substitutionof the aryl ring in four series of aminoanilino and anilinonitro analogues has been studied(Table 3.51). Only moderate improvements in heat fastness were afforded by methyl,methoxy or ethoxy substituents. Groupings incorporating more polar moieties, includinghydroxyethyl, acetyl, carbethoxy and benzoyl, were much more effective, but the highestratings were shown by the amido groupings, namely aminosulphonyl, acetylamino andbenzoylamino.

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47. K Hamada, M Nishizawa and M Mitsuishi, Dyes and Pigments, 16, No 3 (1991) 165. 48. K Hamada et al., Dyes and Pigments, 36 (1998) 313. 49. K Bredereck and C Schumacher, Dyes and Pigments, 21, No 1 (1993) 23, 45. 50. T Omura, Y Kayane and Y Tezuka, Dyes and Pigments, 20, No 4 (1992) 227. 51. T Omura, Dyes and Pigments, 23 (1993) 179; 24 (1994) 125; 26 (1994) 33. 52. A K Mairal and S B Guha, J.S.D.C., 109 (1993) 120. 53. Zhang Zhi-bai et al., AATCC Internat. Conf.& Exhib. (Oct 1987) 227. 54. W Biedermann, J.S.D.C., 87 (1971) 105; 88 (1972) 329. 55. V A Shenai and M C Sadhu, J. Appl. Polymer Sci., 20 (1976) 3141. 56. A S Ghanekar and F Jones, J.S.D.C., 92 (1976) 445. 57. H Schott, J. Phys. Chem., 70 (1966) 2966. 58. C L Bird, J.S.D.C., 70 (1954) 68. 59. J R Aspland and C L Bird, J.S.D.C., 77 (1961) 9. 60. C L Bird and P Harris, J.S.D.C., 73 (1957) 199. 61. E Merian et al., J.S.D.C., 79 (1963) 505. 62. D Patterson and R P Sheldon, J.S.D.C., 76 (1960) 178. 63. W McDowell and R Weingarten, J.S.D.C., 85 (1969) 589. 64. G L Baughman and E J Weber, Dyes and Pigments, 16, No 4 (1991) 261. 65. M Hou, G L Baughman and T A Perenich, Dyes and Pigments, 16, No 4 (1991) 291. 66. H F Willis et al., Trans. Far. Soc., 41 (1945) 506. 67. W J Marshall and R H Peters, J.S.D.C., 63 (1947) 446. 68. S Fisichella, G Scarlata and M Torre, J.S.D.C., 94 (1978) 521. 69. H H Hodgson, J.S.D.C., 49 (1933) 213. 70. K Nishida, J.S.D.C., 82 (1966) 313. 71. H Bach et al., Angew. Chem., 75 (1963) 407. 72. C H Giles and A S A Hassan, J.S.D.C., 74 (1958) 846. 73. M Grunwald, E Burtscher and O Bobleter, Textilverediung, 27 (1992) 45. 74. K Bredereck, F Bader and U Schmitt, Textilveredlung, 24 (1989) 142. 75. F Townend, J.S.D.C., 61 (1945) 144. 76. J Meybeck and P Galafassi, Appl. Polymer Symposia, 18 (1971) 463. 77. K Hannemann and H Flensberg, Melliand Textilber., 78 (1997) 160. 78. J Hertig and H Scheidegger, Textilveredlung, 18 (1983) 325. 79. J A Bone, Proc. Internat. Wool Text. Res.Conf., Aachen, 10 (1984) 170. 80. J Shore, Blends Dyeing, (Bradford: SDC, 1998). 81. F M Arshid, C H Giles and S K Jain, J.C.S., (1956) 1272. 82. A Cameron, C H Giles and T H MacEwan, J.C.S., (1958) 1224. 83. C H Giles, V G Agnihotri and A S Trivedi, J.S.D.C., 86 (1970) 451. 84. C L Bird and A M Patel, J.S.D.C., 84 (1968) 560. 85. E H Daruwalla, S S Rao and B D Tilak, J.S.D.C., 76 (1960) 418, 680. 86. C L Bird, J.S.D.C., 74 (1958) 688. 87. C L Bird and P Rhyner, J.S.D.C., 77 (1961) 12. 88. K Hoffmann, W McDowell and R Weingarten, J.S.D.C., 84 (1968) 306. 89. J F Dawson, J.S.D.C., 107 (1991) 395. 90. H Gerber, J.S.D.C., 94 (1978) 298. 91. A Urbanik, AATCC Nat. Tech. Conf., (Oct 1981) 258. 92. W Biedermann and A Datyner, Text. Res. J., 61 (1991) 637. 93. A Fisher, C S Lee and A T Peters, J. Chem. Tech. Biotechnol., 32 (1982) 532. 94. Y Muraoka et al., Text. Res. J., 66 (1996) 104. 95. M Mitsuishi et al., J.S.D.C., 112 (1996) 333. 96. J F Laucius, R A Clarke and J A Brooks, Amer. Dyestuff Rep., 44 (1955) 362. 97. O Glenz and W Beckmann, Melliand Textilber., 38 (1957) 296. 98. W Beckmann, J.S.D.C., 77 (1961) 616. 99. S Rosenbaum, Text. Res. J., 33 (1963) 899.100. K Meldrum and J S Ward, J.S.D.C., 74 (1958) 140.101. D Balmforth, C A Bowers and T H Guion, J.S.D.C., 80 (1964) 577.102. M Hoten, J.S.D.C., 112 (1996) 123.103. S Rosenbaum, J. Polymer Sci., A3 (1965) 1949.104. U Mayer, W Ender and A Würz, Melliand Textilber., 47 (1966) 653, 772.105. U Mayer, Amer. Dyestuff Rep., 56 (1967) 869.106. J A Koller and M Motter, Textilveredlung, 11 (1976) 3.

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107. S Rosenbaum, Text. Res. J., 34 (1964) 159.108. J Boulton, J.S.D.C., 54 (1938) 268; 60 (1944) 5.109. C C Wilcock, J.S.D.C., 63 (1947) 136.110. C H Giles, A P Montgomery and A H Tolia, Text. Res. J., 32 (1962) 99.111. T L Dawson and J C Todd, J.S.D.C., 95 (1979) 417.112. C Renard, Teintex, 36 (1971) 845.113. E Waters, J.S.D.C., 66 (1950) 609.114. T Vickerstaff and E Waters, J.S.D.C., 58 (1942) 116.115. T Vickerstaff, The physical chemistry of dyeing, 2nd Edn (London: Oliver & Boyd, 1954).116. T Vickerstaff, J.S.D.C., 59 (1943) 92.117. E Siepmann, Melliand Textilber., 49 (1968) 577.118. E Riesz, Bull. Sci. Inst. Text. France, 11 (43) (1982) 61; Textilveredlung, 25 (1990) 19.119. Y C Chao and Y I Pan, Dyes and Pigments, 31 (1996) 253.120. H Maruyama et al., J.S.D.C., 112 (1996) 53.121. J A Fowler, A G H Michie and T Vickerstaff, Melliand Textilber., 32 (1951) 296.122. B G Ferrini, Y Kimwe and H Zollinger, Proc. Internat. Wool Text. Res. Conf., Paris, 3 (1965) 291.123. S R S Iyer, A S Ghanekar and G S Singh, Symp. on Dye-polymer Interactions, Bombay (1971) 13.124. D M Lewis, Rev. Prog. Coloration, 28 (1998) 12.125. Y C Chao and C W Yeh, Dyes and Pigments, 23 (1993) 285.126. Y C Chao, Colour Science ’98, Vol. 2, Ed. S M Burkinshaw (Leeds: Leeds Univ., 1998) 112.127. Y C Chao, M J Chang and C H Chung, Dyes and Pigments, 39 (1998) 183.128. E Atherton, D A Downey and R H Peters, Text. Res. J., 25 (1955) 977.129. V Kartaschoff and G Farine, Helv. Chim. Acta, 11 (1928) 813.130. T Barr, F M Rowe and J B Speakman, J.S.D.C., 58 (1942) 52.131. G L Royer, H E Millson and E I Stearns, Amer. Dyestuff Rep., 32 (1943) 288.132. W H Watkins, G L Royer and H E Millson, Amer. Dyestuff Rep., 33 (1944) 52.133. D Fiebig and S Frick, Melliand Textilber., 78 (1997) 604.134. J T Voltz, Text. Chem. Colorist, 9 (1977) 113.135. N A Evans and I W Stapleton, J.S.D.C., 89 (1973) 208.136. P Bentley, J F McKellar and G O Phillips, Rev. Prog. Coloration, 5 (1974) 33.137. G E Krichevsky et al., Text. Res. J., 45 (1975) 608.138. G Hallas, J.S.D.C., 95 (1979) 285.139. R Grecu, M Pieroni and R Carpignano, Dyes and Pigments, 2 (1981) 305.140. N S Allen, Rev. Prog. Coloration, 17 (1987) 61.141. C H Giles, J.S.D.C., 73 (1957) 127.142. G Baxter, C H Giles and W J Lewington, J.S.D.C., 73 (1957) 386.143. C H Giles et al., J.S.D.C., 88 (1972) 433.144. C H Giles, D J Walsh and R S Sinclair, J.S.D.C., 93 (1977) 348.145. C H Giles and E A Strevens, Text.Res. J., 49 (1979) 724.146. K J Sirbiladze et al., Dyes and Pigments, 19 No 4 (1992) 235.147. Y Okada, F Fukuoka and Z Morita, Dyes and Pigments, 37 (1998) 47.148. Y Okada et al., Dyes and Pigments, 12, No 3 (1990) 197; 19, No 1 (1992) 1; 19, No 3 (1992) 203; 20, No 2 (1992)

123; 24 (1994) 1.149. K Bredereck and C Schumacher, Dyes and Pigments, 23 (1993) 135.150. K Imada, N Harada and T Takagishi, J.S.D.C., 110 (1994) 231.151. J Griffiths and C Hawkins, J. Appl. Chem. Biotechnol., 27 (1977) 558; J. Chem. Soc. Perkin II, (1977) 747.152. J Monheim and R Löwenfeld, Melliand Textilber., 53 (1972) 1134.153. M T Ball et al., Dyes and Pigments, 19, No 1 (1992) 51.154. R H Kienle, E I Stearns and P A van der Meulan, J. Phys. Chem., 50 (1946) 363.155. M R Massafra et al., Dyes and Pigments, 40 (1998) 171.156. M R de Giorgi et al., J.S.D.C., 109 (1993) 405; Dyes and Pigments, 30 (1996) 79.157. B Küster, Chemiefasern und Textilind., 37/89 (1987) 1236; Textil Praxis, 43 (1988) 60.158. J Meybeck, P Ruckstuhl and J M Thumann, Teintex, 25 (1960) 241.159. N S Allen et al., Dyes and Pigments, 34 (1997) 169.160. C H Giles and R S Sinclair, J.S.D.C., 88 (1972) 109.161. D Wegerle, J.S.D.C., 89 (1973) 54.162. H S Freeman and J Sokolowska-Gajda, Text. Res. J., 60 (1990) 221.163. J Sokolowska-Gadja, Dyes and Pigments, 36 (1998) 149.164. D F Duxbury, Dyes and Pigments, 25 (1994) 131, 179.165. K Dunkerley, J.S.D.C., 108 (1992) 268.

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166. G Reinert, Melliand Textilber., 69 (1988) 58.167. N S Allen, P Bentley and J F McKellar, J.S.D.C., 91 (1975) 366.168. N Katsuda, T Omura and T Takagishi, Dyes and Pigments, 34 (1997) 147; 36 (1998) 231.169. N Katsuda et al., Dyes and Pigments, 31 (1996) 291; 36 (1998) 193.170. E Atherton and R H Peters, J.S.D.C., 68 (1952) 64.171. M Ahmed and V N Mallet, J.S.D.C., 84 (1968) 313.172. V N Mallet and B T Newbold, J.S.D.C., 90 (1974)4.173. S V Sunthankar and V Thanumoorthy, Indian J. Chem., 8 (1970) 598.174. J P Binkley, T M Evans and A R Horrocks, Adv. Col. Sci. Tech., 2 (1999) 114.175. C Muller, Amer. Dyestuff Rep., 59 (Mar 1970) 37.176. M F Sartori, J.S.D.C., 83 (1967) 144.177. M E Mason, J C Posey and H S Freeman, AATCC Internat. Conf.& Exhib. (Oct 1991) 114.178. H S Freeman, M E Mason and J Lye, Dyes and Pigments, 42 (1999) 53.179. D O Ukponmwan, Colourage, 45 (Apr 1998) 35.180. H S Freeman and S A McIntosh, Text. Research J., 64 (1994) 309.181. C H Nicholls, J.S.D.C., 72 (1956) 479.182. C H Giles, D G Duff and R S Sinclair, Rev. Prog. Coloration, 12 (1982) 58.183. M R de Giorgi, Melliand Textilber., 73 (1992) 578.184. K Grieder, J.S.D.C. 92 (1976) 8.185. W McDowell, J.S.D.C., 89 (1973) 177.186. H E Schroeder and S N Boyd, Text. Res. J., 27 (1957) 275.187. R B Chavan and A K Jain, J.S.D.C., 105 (1989) 73.188. F Jones and T S M Leung, J.S.D.C., 90 (1974) 286.189. J Griffiths and F Jones, J.S.D.C., 93 (1977) 176.190. A T Peters and N O Soboyejo, J.S.D.C., 104 (1988) 486; 105 (1989) 315.191. A T Peters and B A Tenny, J.S.D.C., 93 (1977) 378.192. A T Peters and Y C Chao, J.S.D.C., 104 (1988) 435.

REFERENCES

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180 CHEMISTRY OF AZO COLORANTS

180

CHAPTER 4

Chemistry of azo colorants

Geoffrey Hallas

4.1 INTRODUCTION

Shortly after Perkin had produced the first commercially successful dyestuff, a discovery wasmade which led to what is now the dominant chemical class of dyestuffs, the azo dyes. Thisdevelopment stemmed from the work of Peter Griess, who in 1858 passed ‘nitrous fumes’(which correspond to the formula N2O3) into a cold alcoholic solution of 2-amino-4,6-dinitrophenol (picramic acid) and isolated a cationic product, the properties of whichshowed it to be a member of a new class of compounds [1]. Griess extended hisinvestigations to other primary aromatic amines and showed his reaction to be generallyapplicable. He named the products diazo compounds and the reaction came to be known asthe diazotisation reaction. This reaction can be represented most simply by Scheme 4.1, inwhich HX stands for a strong monobasic acid and Ar is any aromatic or heteroaromaticnucleus.

Ar NH2 HX HNO2 Ar N N X+ ++ _

+ 2H2O

Scheme 4.1

Griess subsequently recognised that the orange compound formed by the reaction ofdiazobenzene with phenol in neutral solution contained two benzene nuclei joined togethervia an azo (–N=N–) linkage [2] and again the reaction was found to be general and notconfined to phenols. This second reaction became known as the diazo coupling reaction andthe products as azo compounds. The wide applicability of this reaction opened the way to avast multitude of compounds produced in generally high yields in simple plant and led to theazo dyes becoming the largest and most versatile class of dyes.

The high reactivity of diazonium salts enables them to enter into a great many reactionsother than the azo coupling reaction, but these fall outside the scope of the present chapter.Various other methods can be used for the preparation of azo compounds, although theseare of minor importance compared with the diazo coupling reaction and they will only betouched on briefly where appropriate.

4.2 MECHANISM OF DIAZOTISATION AND COUPLING

Studies of the mechanism of the diazotisation reaction date back to the end of thenineteenth century and have continued ever since, progressively incorporating the growingknowledge of theoretical chemistry [3,4]. The reaction, which involves nitrosation, has beenstudied under a wide range of acidities using a variety of amines and nitrosating agents.

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Numerous effects arising from variations in conditions have been described. In essence, theaccepted mechanism is represented by Scheme 4.2, in which X–NO represents one of themany varieties of nitrosating species such as nitrous acid (HO–NO), nitrosyl chloride (Cl–NO), dinitrogen trioxide (O2N–NO), the nitrosyl cation (NO+) or the nitrous acidium ion(H2NO2

+) that may be present in solutions containing nitrous acid. According to theconditions employed, either the formation of the nitrosating agent or its initial attack on theamine is the slowest reaction step and therefore determines the overall rate, the subsequentsteps in the reaction sequence being rapid. The concentrations of the various possiblenitrosating species can be influenced by other ions, such as chloride ions, or compounds,such as thiourea, present in the reaction mixture so that catalytic effects are observed.

Ar NH2 X NO Ar NH2 NO X Ar NH NO HX

Ar NH NO Ar N N OH Ar N N X H2OHX

+_+

+

Nitrosamine

_+

Diazo hydroxide

+

Scheme 4.2

The diazonium ion is a powerful electrophile that can readily attack compounds having anucleophilic centre, with the expulsion of a proton, or other entity, from the site of attack[5]. This mode of behaviour leads to the formation of azo compounds by the azo couplingreaction, the compound with the nucleophilic site being termed the coupling component.Suitable coupling components include arylamines, phenols, naphthols and keto–enolcompounds. The mechanism with an arylamine coupling component can be represented byScheme 4.3. In the coupling component illustrated both the o- and p-positions are activatedby the dialkylamino group. However, an approach of the diazonium ion to the o-positionwould cause the electron-donating group to be twisted out of plane and thus to lose itsactivating influence. For this reason, coupling occurs selectively at the p-position. Thesituation is somewhat different in non-aqueous media where the counter-ion of thediazonium salt can influence the o-/p-ratio. In dichloromethane, diazonium halides coupleto 1-naphthol almost exclusively in the 4-position whereas the corresponding haloacetatescouple mainly in the 2-position [6].

N(CH3)2

H

N

N(CH3)2

NAr

NNAr N(CH3)2

+

+

NNAr+

– H

Scheme 4.3

MECHANISM OF DIAZOTISATION AND COUPLING

chpt4(1).pmd 15/11/02, 15:23181


With a phenol, naphthol or keto–enol coupling component the mechanism is given byScheme 4.4, in which blocking of the p-position forces coupling at the o-position. In certaincases involving the use of feebly reactive diazonium salts, loss of the proton from thetransition state (4.1) in Scheme 4.4 may be slow but may often be facilitated by the additionof a tertiary base, such as pyridine, to the coupling mixture [7,8].

–O O

H

N

HO

NNAr

NAr

Ar N N+

4.1

+

Scheme 4.4

In these two coupling schemes the arylamine participates in the reaction as the free base,whereas the naphthol coupling component reacts in its ionised form. Thus, whilst raisingthe pH increases the rate of coupling in both cases, much higher pH values are required tobring about satisfactory coupling with hydroxy compounds than with arylamines. Thissituation is taken advantage of in the case of certain aminonaphthol coupling components,particularly 8-amino-1-naphthol-3,6-disulphonic acid (4.2; H acid), where one azo groupcan be selectively directed into the molecule by the amino group at low pH, followed by asecond azo group directed by the ionised hydroxy group at higher pH.

SO3H

OHH2N

HO3S

higher pH (7–10)low pH (3–5)

4.2

H acid

4.3 DIAZO COMPONENTS AND DIAZOTISATION METHODS

A vast array of arylamines can be converted into the corresponding diazonium salts; fromthese diazo components and the host of available coupling components an almost limitlessrange of azo dyes is accessible.

It has already been pointed out (section 1.7.4) that various arylamines are hazardous. InJuly 1994, the German government introduced, unilaterally, stringent regulations banningimports of various textile and clothing products which contain specific arylamine-based azodyes. Only those colorants which on cleavage of one or more azo groups could form any oneof the twenty listed arylamines were affected. These amines (Table 4.1) are classified asproven carcinogens. Two additional arylamines, 2-methoxyaniline and 4-aminoazobenzene,were added subsequently by the EU. The German ordinance restricts the use of only about5% of azo dyes (about 150 products). Similar regulations have been introduced in the

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Netherlands and in France. By mid 1997, most azo pigments, due to their very low solubility,had been exempted [9]. The legislative requirements have been discussed [10]. A voluntaryregistration scheme for harmless dyestuffs is available and makes use of the Internet [11].

In 1996 an official German analytical method was published for the detection of bannedamines in relation to cotton, viscose, wool and silk; a second method was introduced forleather. The official procedure uses sodium dithionite in a weakly acidic citrate-bufferedmedium; more aggressive methods are known to produce false positive results where theamine detected is an artefact of the test procedure, resulting from chemical reactions otherthan azo cleavage. Reactions not involving azo groups can sometimes lead to the formationof forbidden amines [12]. Methods of detection involving thin-layer chromatography,leading to the unequivocal identification of the specified amines, have been devised [13,14].2-Naphthylamine has been detected in dyes derived from 1-naphthylamine [15], due tocontamination of the intermediate. Benzidine can be formed from 4-nitroaniline afterdiazotisation by dediazoniation, followed by radical coupling to give 4,4′-dinitrobiphenyl andfinally reduction [15].

A variety of diazotisation techniques has been evolved to cope with the variation inphysical properties across the range of amines.

4.3.1 Direct method

The rudiments of the diazotisation technique consist of treating the amine with at least 2.25equivalents of a mineral acid (usually hydrochloric acid) and one equivalent of sodiumnitrite, in accordance with Scheme 4.5. The slight excess of acid specified is required toensure that the reaction medium remains acid throughout the diazotisation. In the directmethod sodium nitrite is added to a solution of the amine in dilute mineral acid. This

Table 4.1 List of specified arylamines

Name on list Alternative name

o-Aminoazotoluene 2-Amino-4,4′-dimethylazobenzene4-AminobiphenylBenzidine 4,4′-Diaminobiphenylp-Chloroaniline 4-Chloroaniline4-Chloro-o-toluidine 4-Chloro-2-methylaniline3,3′-Dichlorobenzidine3,3′-Dimethoxybenzidine o-Dianisidine3,3′-Dimethylbenzidine o-Tolidine4-Methoxy-m-phenylenediamine 2,4-Diaminoanisole6-Methoxy-m-toluidine 2-Methoxy-5-methylaniline4,4′-Methylene-bis(2-chloroaniline) 4,4′-Diamino-3,3′-dichlorodiphenylmethane4,4′-Methylenedianiline 4,4′-Diaminodiphenylmethane4,4′-Methylene-o-toluidine 4,4′-Diamino-3,3′-dimethyldiphenylmethane4-Methyl-m-phenylenediamine 2,4-Diaminotoluene2-Naphthylamine 2,Aminonaphthalene5-Nitro-o-toluidine 2,Methyl-5-nitroaniline4,4′-Oxydianiline 4,4′-Diaminodiphenyl ether4,4′-Thiodianiline 4,4′-Diaminodiphenyl thioethero-Toluidine 2-Methylaniline2,4,5-Trimethylaniline

DIAZO COMPONENTS AND DIAZOTISATION METHODS

chpt4(1).pmd 15/11/02, 15:23183


method is suitable for aniline and its simple derivatives with groups such as alkyl, alkoxy orchloro as substituents in the aromatic ring, simple naphthylamines and benzidinederivatives, many of which are known carcinogens. These compounds can be readilydiazotised or tetrazotised by dissolving the amine in dilute hydrochloric acid, cooling thesolution to 0–5 °C with ice and adding the requisite amount of sodium nitrite solution asrapidly as the nitrous acid formed from it is used in the reaction. Spotting the solution ontostarch–iodide paper is carried out to test for an excess of nitrous acid. On occasions when itis difficult to determine the end point with starch–iodide paper, a dilute acid solution of 4,4′-diaminodiphenylmethane-2,2′-sulphone (4.3), which gives a bright blue colour with nitrousacid, can be used [16]. The reaction is complete when a slight excess of nitrous acid haspersisted for about 15 minutes. The excess of nitrous acid is then conveniently and rapidlydestroyed by the addition of a little sulphamic acid, in accordance with Scheme 4.6,producing innocuous products. The diazonium salt solution is then ready for immediate usein dye synthesis.

Ar NH2 NaNO2 Ar N N X NaX+ 2HX +_

+ + 2H2O+

Scheme 4.5

HNO2 H2NSO3H H2SO4 N2 H2O+ + +

Sulphamic acidScheme 4.6

SH2N NH2O O

4.3

For the diazotisation reaction to succeed, it is necessary that the amine should becompletely converted into the hydrochloride before the addition of sodium nitrite, becauseany free amine can react with the diazonium salt to form a diazoamino compound (Scheme4.7). This complication needs to be avoided in the case of amines, such as the dichloro-anilines, which do not dissolve easily in dilute hydrochloric acid [17]. With such compoundsit is convenient first to dissolve them in a hot acid solution, which is then cooled to 0–5 °C.This procedure ensures the absence of free amine, and even if the hydrochloride precipitateson cooling it readily redissolves as the diazotisation proceeds. Similar precautions arerequired with the nitroanilines and here an increased amount of mineral acid isadvantageous [18].

Ar NH2 Ar N N X Ar NH N N Ar HX+_+

+

Scheme 4.7

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185

Low temperatures are generally necessary for successful diazotisation, except in a fewcases where the diazonium salt is exceptionally stable. Temperatures of 50–60 °C are usedfor the diazotisation of 3-aminodibenzofuran (4.4), for example [l9].

O NH2

4.4

3-Aminodibenzofuran

4.3.2 Inverse method

In this method the order of addition of the reactants is changed. The most generalapplication is to the diazotisation of aniline- and naphthylamine-sulphonic acids, many ofwhich are sparingly soluble in acidic media and form sparingly soluble diazonium salts.Typically, the arylaminesulphonic acid is dissolved as its sodium salt in dilute alkali andsodium nitrite is then added to the solution. The resulting solution is run slowly into amixture of hydrochloric acid and ice, the diazonium salt being formed almostinstantaneously.

In a variation of this procedure [19], a small amount of acid is added to a suspension of asulphonated amine in sodium nitrite solution; the sulphonic acid group on the aminegenerates nitrous acid and diazotisation proceeds to completion.

4.3.3 Use of nitrosylsulphuric acid

Very feebly basic amines cannot usually be diazotised in dilute acid media and in theseinstances the reaction has to be carried out in a concentrated acid, normally sulphuric acid.The usual technique is first to dissolve dry sodium nitrite in the concentrated acid, whenreaction occurs in two stages (Scheme 4.8), resulting in the formation of nitrosylsulphuricacid (4.5). The nitrosyl ion – nitrous acid equilibrium has been evaluated spectroscopically.In 96% sulphuric acid the 15N-n.m.r. signal is characteristic of the free nitrosyl ion [4].Reaction (2) of Scheme 4.8 is slow at room temperature and it is desirable to heat themixture to 70 °C in order to attain equilibrium within a reasonable time. After cooling, theamine is added gradually and after a short time the reaction mixture is poured onto ice,giving an aqueous solution of the diazonium salt [20].

This technique is used to diazotise anilines carrying two or more electron-withdrawingsubstituents, such as 2,4-dinitro- and 2-cyano-4-nitro-aniline, as well as aminoanthra-quinones and heteroaromatic amines [4]. Some diazonium salts prepared by this technique,such as those from 6-halogeno-2,4-dinitroanilines, are unstable in water and have to beadded directly to the coupling component solution.

NO HSO4 H2O

H2SO4

H2SO4

NaNO2

HNO2

NaHSO4 HNO2

4.5

+

++

+

(2)

(1)

_+

Nitrosylsulphuric acidScheme 4.8

DIAZO COMPONENTS AND DIAZOTISATION METHODS

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4.3.4 Other methods

Although not used in dye preparation, three other methods of diazotisation deserve passingmention, one being chiefly of historical interest and the other two are sometimes of use inpreparative work. The oldest is the method originally used by Griess (section 4.1), whogenerated the first recognised diazonium salt by passing ‘nitrous gases’ into a cold ethanolicsolution of the amine. This reagent is produced by reducing nitric acid with arsenic(III)oxide or starch; it behaves as if it consisted of dinitrogen trioxide. The method can be usefulfor the preparation of solid diazonium salts, but a better procedure is that due toKnoevenagel [21]. In this second method an alkyl nitrite is added to a solution of the amineand sulphuric acid in ethanol or other non-aqueous solvent. This procedure allows greatercontrol of the reaction than is possible in the Griess method.

Finally, in a method originated by Witt [22], which is occasionally useful for thediazotisation of stable amines of high relative molecular mass that pose solubility problems,the amine is dissolved in nitric acid before the addition of sodium metabisulphite. Nitrousacid is produced in the medium according to Scheme 4.9, enabling diazotisation to proceed.

Na2S2O5 + 2HNO3 Na2S2O7 + 2HNO2

Scheme 4.9

Diazonium salts vary greatly in electrophilicity and therefore in reactivity therebyaffecting the ease with which they couple. The presence in the aromatic nucleus of electron-attracting groups, such as nitro and cyano groups, causes difficulties in the diazotisation stepbut helps the resulting diazonium salt to couple extremely readily. Electron-donatingsubstituents such as methoxy and acetylamino groups, which facilitate diazotisation,diminish the coupling power. An extreme case is posed by the diazonium salts derived fromo- and p-aminophenols and aminonaphthols, which are very feeble couplers, requiringhighly alkaline conditions and often the use of a coupling assistant such as pyridine to bringabout a satisfactory reaction. This very low level of reactivity is due to the hydroxysubstituent interacting structurally with the diazonium group, leading to a mesomeric systemin which the limiting forms are represented by Scheme 4.10 [23].

O

N

O–

N+ +

N– NScheme 4.10

4.4 PREPARATION AND USE OF COUPLING COMPONENTS

As well as being presented with an array of primary amines that can be used for thepreparation of azo dyes, the dye chemist has available a multiplicity of coupling components.The coupling components useful in the synthesis of azo dyes cover a range of chemical typeswhich can all be seen to contain a keto–enol system, a hydroxy or an amino group. Since theuse of these compounds is largely restricted to the azo dye field, their preparation is in manycases not covered in the common organic chemistry books. In the discussion of theseclasses, therefore, a brief mention of methods of preparation is given where appropriate.

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4.4.1 Acetoacetarylamides

Coupling components of this class have the general structure 4.6 (Scheme 4.11), in whichAr represents an aromatic, usually benzenoid, nucleus that may carry substituents. Initiallysuch compounds were prepared by the reaction of an arylamine with ethyl 3-oxobutanoate(ethyl acetoacetate) but now are more conveniently made by treatment of an arylamine withdiketene (Scheme 4.11). The electron-attracting effect of the two carbonyl groups flankingthe methylene group in structure 4.6 gives the protons on this latter group an acidiccharacter permitting solubility in alkaline solution, the negative charge produced by theionisation being spread over the molecule as shown in Scheme 4.12. This resonanceprovides an electron-rich site on the carbon atom of the methylene group, at which couplingoccurs. Coupling of simple diazotised arylamines with acetoacetarylamides produces greenishyellow pigments and dyes of particular value in the direct and azoic dye ranges, under whichheadings specific structures are mentioned. Related to the acetoacetarylamides is barbituricacid (4.7), which has found similar use as a coupling component in synthetic dyes.

Ar NH2 H5C2OOCCH2COCH3 Ar NH2 O C CH2

O C CH2

ArNHCOCH2COCH3

++

4.6

Ethyl acetoacetate

Diketene

Scheme 4.11

ArNHCO CH C CH3

O

ArNHCO CH C CH3

O_

_

Scheme 4.12

NH

NH

O

O

OH

H

4.7

Barbituric acid

4.4.2 Pyridones

The pyridone coupling components (4.8), which came into use in the 1960s chiefly for thepreparation of greenish yellow disperse and reactive dyes, are made by the condensation ofan alkylamine with ethyl acetoacetate and ethyl cyanoacetate. Coupling occurs at theposition indicated by the arrow in Scheme 4.13.

PREPARATION AND USE OF COUPLING COMPONENTS

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4.4.3 Pyrazolones

The 1-arylpyrazol-5-ones (4.9), prepared by the two-step condensation of an arylhydrazinewith ethyl acetoacetate, are the most commonly used coupling components for the synthesisof greenish yellow azo dyes. Coupling occurs at the 4-position of the pyrazolone ring which,as in the case of the acetoacetarylamides discussed above, is activated towards electrophilicattack by the two flanking unsaturated carbon atoms (Scheme 4.14).

H2C

COC2H5

CO

O

CH3

H2C

CO

CN

H5C2O

NH2

R

N

H

H

O

CN

R

CH3

O

+ +

Ethyl acetoacetate Ethyl cyanoacetate

4.8Scheme 4.13

H2C

COC2H5

COCH3

O

N

Ar

NH2H

NN

H

H

CH3

O

Ar

+

4.9Scheme 4.14

4.4.4 Aminopyrazoles

Closely related to the pyrazolones, but less commonly used, are the 5-aminopyrazoles (4.10).Again, these compounds give greenish yellow dyes when coupled at the activated 4-positionindicated by the arrow in Scheme 4.15. The aminopyrazoles are prepared by condensation ofan arylhydrazine with 3-aminobut-2-enenitrile (diacetonitrile), which is itself produced bydimerisation of ethanenitrile (acetonitrile) over a nickel catalyst.

NN

H

H

CH3

HN

Ar

NCCH C

NH2

CH3 +2CH3CN Ni ArNHNH2

4.10

Scheme 4.15

4.4.5 Phenols

When phenol is treated with one equivalent of a diazonium salt at pH 7–8 coupling occursmainly at the 4-position of the benzene ring, but small amounts of the 2-azo and 2,4-disazocompounds are also formed [24]. With an excess of diazonium salt under neutral conditionsthe 2,4-disazo derivative is formed [25] and under strongly alkaline conditions the 2,4,6-

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trisazo derivative is produced. These factors can lead to impure products so that phenolitself is not often used as a coupling component. 4-Arylazophenols have the disadvantage ofbeing pH-sensitive, ionisation of the hydroxy group under alkaline conditions resulting in amarked change of hue. With 2-arylazophenols, hydrogen bonding between the hydroxygroup and the azo linkage (4.11) prevents such ionisation. Because of this effect the mostcommonly employed simple phenols are compounds such as 4-methylphenol (p-cresol) inwhich blocking of the 4-position ensures that coupling takes place at the 2-position. Withsimple diazonium salts p-substituted phenols yield yellow azo compounds.

N

N

OH

4.11

Benzene-1,3-diol (resorcinol), in which the two hydroxy groups reinforce one another, isof greater technical importance than phenol, not only because it couples much morestrongly but also because the presence of two activating hydroxy groups allows more thanone azo group to be introduced into the molecule. The preferred first coupling under neutralconditions takes place at the 4-position [24] and when a further equivalent of diazoniumsalt is added in the presence of sodium acetate or carbonate a second azo group isintroduced into the 2-position. In the presence of sodium hydroxide the second group entersthe 6-position. At a sufficiently high pH 2,4,6-trisazo compounds are produced [26–28]. Theability to introduce two different azo groups into resorcinol is exploited in the production ofbrown dyes mainly for use on leather.

The other two dihydric phenols, benzene-1,2-diol (catechol) and benzene-1,4-diol(hydroquinone) are of little interest in this context since they reduce diazonium salts onattempted coupling, being themselves oxidised to the corresponding quinones.

4.4.6 Aniline derivatives

Aniline itself couples inefficiently with normal diazonium salts [29], the reaction product ofa diazotised amine and aniline often being the diazoamino compound. However, thepresence of electron-donating substituents, such as methyl or methoxy groups, in thebenzene nucleus makes coupling much easier and allows the introduction of an azo groupinto the 4-position of compounds such as 3-methylaniline (m-toluidine), 2,5-dimethylaniline(p-xylidine), 3-amino-4-methoxytoluene (cresidine) and 2,5-dimethoxyaniline. In the eventof the 4-position being blocked, as in 2,5-dimethoxy-4-methylaniline, the azo group can beintroduced into the unoccupied o-position [28]. Like the 4-arylazophenols, these 4-arylazoanilines display pH sensitivity, becoming protonated under acid conditions; they aretherefore mainly of use as intermediates in the synthesis of disazo dyes.

N-Alkylation of primary aromatic amines increases their nucleophilic character, makingthem couple much more readily, the introduction of the azo group occurring in the 4-position. Thus, in contrast to aniline, N-methylaniline couples readily and N,N-dimethyl-aniline very readily with simple diazonium salts. Diphenylamine also couples in the 4-position, but less readily than N-methylaniline.


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The increase in coupling power gained on N-alkylation can be exploited to bring aboutthe coupling of primary arylamines by warming the base with an aqueous solution of oneequivalent of sodium bisulphite and one equivalent of formaldehyde. The complex thusformed reacts with the arylamine, giving a clear solution of the methyl-ω-sulphonatederivative (4.12) which couples readily. The methyl-ω-sulphonate group is labile and can beconveniently removed by acid hydrolysis, affording a facile preparation of aminoazobenzenesnot accessible by direct coupling (Scheme 4.16).

ArNH2 + HOCH2SO3Na ArNHCH2SO3Na + H2O

4.12Scheme 4.16

The N,N-dialkylanilines are of paramount importance in the synthesis of disperse dyes,the whole gamut of shades from yellow to greenish blue being obtainable from them bysuitable choice of diazo component. If the 4-position in an N,N-dialkylaniline is blocked, asfor example in N,N-dimethyl-4-methylaniline, steric hindrance prevents the diazonium ionapproaching the o-position and coupling does not take place [30].

As with resorcinol, the 1,3-diamines of the benzene series are useful in the preparation ofdisazo dyes that can form the basis of brown shades. Generally the 2,4-disazo compound isthe product of coupling with two equivalents of diazonium salt, but at higher pH valuesformation of the 4,6-isomer can be significant [31]. Again, the 1,2- and 1,4-diamines reducediazonium salts on attempted coupling. 1,3,5-Triaminobenzene has been found to couplethree times [32].

4.4.7 Aminophenols

3-Aminophenol is a useful dyestuff intermediate, capable of coupling in either the 2- or the4-position. It is often used in preference to resorcinol or benzene-1,3-diamine (m-phenylenediamine) as a final coupling component in the preparation of complex dyescontaining more than one azo group, typically trisazo direct dyes. Other aminophenols are oflittle interest in this context.

4.4.8 Naphthols

In this discussion of coupling components it is convenient to separate naphthalenecompounds from their benzenoid analogues since important differences in behaviour existbetween the two classes. Naphthalene, although formally containing two fused benzenerings, only possesses ten π-electrons, has much less than twice the resonance stabilisationenergy of benzene and therefore has a lower level of aromaticity. The reason for thissituation is related to the fact that in only one of the three limiting canonical forms (4.13a–c) are both rings ‘benzenoid’, the other two forms each containing one ‘benzenoid’ and one‘quinonoid’ ring. A consequence is that the contribution of 4.13b to the overall structure ofnaphthalene is greater than that of the other two canonical forms; so that the bondsbetween adjacent α- and β-positions show a degree of double-bond character. This type ofbond stabilisation has an effect on the coupling behaviour of naphthols and naphthylamines,as will be shown later.

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The two isomeric naphthols and some of their sulphonic acids are very widely used ascoupling components in dye manufacture, generally giving orange hues when coupled withsimple diazonium salts. 1-Naphthol (4.14) couples in both the 2- and 4-positions and, aswith phenol, it is difficult to make the coupling selective. Highly reactive diazonium saltscouple chiefly in the 4-position whilst weakly reactive ones, especially those derived fromaminophenols and aminonaphthols, couple mainly in the 2-position. The position ofcoupling in 1-naphthols is, of course, also influenced by substituents; for example, asulphonic acid group in the 3-, 4-, or 5-position inhibits 4-coupling and directs thediazonium ion to attack at the 2-position.

B Q

4.13a

B Bβ

α

4.13b

Q B

4.13c

OH

4.14

With 2-naphthol (4.15) the special characteristics of the structure of naphthalenementioned above come into play and coupling is directed exclusively into the 1-position.When this position is blocked, as with 1-methyl-2-naphthol, no coupling occurs. The reasonfor this behaviour lies in the nature of the relevant resonance canonicals (4.15a–c) of thenaphtholate ion (Scheme 4.17). In structure 4.15a the unsubstituted ring retains itsbenzenoid character and is therefore vastly preferred over structure 4.15c, in which theunsubstituted ring is forced to assume a quinonoid configuration. This difference localisesthe lone pair of electrons and the negative charge on the 1-position and leads to selectiveattack of the diazonium ion at this site.

OH

4.15

O O O––

–

4.15a 4.15b 4.15c

Scheme 4.17


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4.4.9 Naphthylamines

The aminonaphthalenes couple much more easily than do the amines of the benzene series.This increased reactivity allows azo compounds to be prepared without the need to reinforcethe effect of the amino group with additional electron-donating substituents. The resultingdyes have hues similar to those obtained from naphthol coupling components and the parentamines display directive influences similar to those shown by the naphthols in the couplingreaction. Thus with one equivalent of a diazonium salt 1-naphthylamine couples largely in the4-position [33]. With two equivalents of diazonium salt the 2,4-disazo compound can beobtained [34,35]. 2-Naphthylamine, like 2-naphthol, couples exclusively in the 1-position [31]and derivatives with this position blocked by non-labile substituents fail to couple. Whentreated with powerful diazonium salts, 2-naphthylamines with labile substituents in the 1-position, such as 2-naphthylamine-1-sulphonic acid, couple by introducing the azo group intothe 1-position with concomitant expulsion of the labile substituent.

In the distant past, 2-naphthylamine was an important intermediate for the synthesis ofdyestuffs, but its recognition as a potent carcinogen resulted in its total withdrawal byreputable dye manufacturers. Stringent precautions also need to be taken when using 1-naphthylamine to ensure that the material is not contaminated with the 2-isomer. Thenaphthylaminesulphonic acids, however, remain amongst the most important dyestuffintermediates, finding wide-ranging applications as both diazo and coupling components.4-Substituted 3-aminoacetanilides have been examined as replacements for 1-naphthylamine in monoazo dyes [36]. 3-Amino-4-methoxyacetanilide can be used as aneffective substitute for 1-naphthylamine in the synthesis of disazo dyes [37].

4.4.10 Aminonaphthols

There are 14 isomers carrying both an amino group and a hydroxy group attached to anaphthalene nucleus, but only those in which the two groups are located in different ringsare of use as coupling components. In these cases, each group exerts the major directiveinfluence within its own ring, so that at pH 7 or below coupling occurs in the amino-substituted ring, whereas at alkaline pH coupling in the hydroxylated ring becomesimportant. Often there is still a choice of possible coupling sites, leading to mixtures ofproducts. Very few of the aminonaphthol isomers have found use in dye manufacture, theirvalue being determined by ease of manufacture, toxicological considerations and theproperties of the derived dyes.

Of far greater importance are the sulphonated aminonaphthols, in which the versatilityconferred by the presence of both an amino and a hydroxy group makes them among the mostimportant group of azo intermediates. Three are outstanding, namely, 6-amino-1-naphthol-3-sulphonic acid (4.16; J acid), 7-amino-1-naphthol-3-sulphonic acid (4.17; γ acid) and H acid(4.2), which couple under the relevant conditions of pH at the arrowed positions.

OH

SO3HH2N

alk

4.16

J acid

acid

OH

SO3H

H2N

acid

alk

4.17

γ acid

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Particularly important dyes are produced when the N-acylated derivatives are coupled,bright orange hues resulting from the use of N-acyl J acid and bright reds from N-acyl γ acidand N-acyl H acid derivatives. Coupling of γ acid under acid conditions leads to redmonoazo dyes that cannot be further coupled and have exceptional fastness to light. Thesecharacteristics result from hydrogen bonding between both the amino and the hydroxygroups and the azo linkage (4.18), thus preventing ionisation of the hydroxy group which isessential for coupling to occur and also protecting the azo linkage by involving both lonepairs of electrons in coordination.

H

N

NN

OH

SO3H

H

Ar

4.18

Other important dye classes are formed by coupling aminonaphtholsulphonic acids withtwo equivalents of diazonium salt, giving a disazo dyestuff. In the case of H acid it isessential that the acid coupling stage is carried out first, the products being navy blue incolour. With J acid, which again gives dark blue disazo dyes, the initial coupling shouldpreferably be carried out under alkaline conditions.

4.5 STRUCTURE OF AZO DYES

4.5.1 Stereoisomerism

The simplest azo compound, azobenzene, exists as a mixture (Scheme 4.18) of a stable trans(4.19) and an unstable cis (4.20) form [38,39]. Formation of the cis isomer is induced byexposure to light, the quantum yield of the process depending upon the wavelength of thelight employed [40]. The proportion of cis isomer can be appreciable in an equilibriummixture. Thus a concentration of 24% of this unstable form builds up within a few hourswhen an acetic acid solution of azobenzene is exposed to sunlight in shallow white trays.Reversion to the trans form occurs readily on heating and is catalysed by a variety ofsubstances that can function as electron donors or acceptors [41].

The two isomers display different properties and can be separated by exploitingdifferences in their distribution between immiscible solvents or by chromatography onalumina. The molecule of the trans form is very nearly planar, whereas in the cis isomer the

N N N N

4.19 4.20

trans cis

Scheme 4.18

STRUCTURE OF AZO DYES

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benzene rings are inclined at an angle of about 56 degrees to one another [42,43]. The transisomer forms monoclinic crystals, has no dipole moment and melts at 68 °C whereas the cisisomer forms orthorhombic bipyramidal crystals, has a dipole moment of 3.0 debye and meltsat 71.4 °C. The two isomers also differ in hue. Some substituted azobenzenes display similarstereoisomerism but these are mainly simple structures, increasing molecular complexitycausing steric factors to control the configuration exclusively in the trans form. Some simpleazobenzene derivatives serve as disperse dyes, but in these the difference in hue between thecis and trans forms gives rise to the phenomenon of photochromism, manifested as a changein hue on exposure to light; this property is an undesirable feature of such dyes. The thermalcis–trans isomerisation of 4-diethylamino-3′-nitroazobenzene has been studied in varioussolvents. The conversion appears to involve an inversion mechanism passing through an sp-hybridised linear transition state [44].

4.5.2 Tautomerism

Whilst azo compounds prepared from diazonium salts and phenolic or keto–enol couplingcomponents are often depicted in the hydroxyazo form (4.11), an alternative tautomericstructure can be drawn for such compounds (Scheme 4.19). This ketohydrazone tautomer(4.21) can, in cases where the azo and hydroxy groups are located on adjacent carbon atoms,exhibit hydrogen bonding between the two groups as shown. Similar pairs of structures, butwithout hydrogen bonding, can be drawn for p-hydroxyazo compounds.

N

N

OH

N

N

OH

4.11 4.21

The actual structure of any particular azo compound will depend on the relative energylevels of the two tautomers which can be drawn for that compound. Where one form has amuch lower energy level than the other, then the lower-energy form will predominate; atautomeric mixture will result when the energy levels of the two forms are similar. In thecase of azo compounds derived from keto–enol coupling components (acetoacetarylamides,pyridones, pyrazolones and aminopyrazoles) the ketohydrazone form has a much lowerenergy level than the hydroxyazo alternative and represents the true structure of thesecompounds. Physical measurements support this conclusion. For example, spectroscopicevidence shows that azo compounds derived from acetoacetarylamides have structure 4.22[45]. This finding is confirmed by n.m.r. evidence, which indicates that pyrazolone-basedazo compounds have the structure 4.23 [46,47]. Substituted phenylazopyridones favour theketohydrazone form in solution [48]. The most appropriate representation of tautomericcolorants has been discussed [49].

The converse of this situation is presented by diazotised arylamines coupled to phenols,where assumption of a ketohydrazone structure (4.21) would necessitate loss of the aromaticcharacter of the phenolic ring. Consequently, these compounds exist solely in thehydroxyazo form.

Scheme 4.19

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The phenylazonaphthols present a particularly interesting series. Despite the partial lossof aromaticity in the hydrazone structures that can be drawn for 1-phenylazo-2-naphthol, 2-phenylazo-1-naphthol and 4-phenylazo-1-naphthol, these structures are not much higher inenergy than the azo forms [50], so that there should be no significant preference for oneform over the other. When the visible absorption curves of 4-phenylazo-1-naphthol aremeasured in a variety of solvents, they all pass through a common point (isosbestic point) asin Figure 4.1 [51,52]. 1-Phenylazo-2-naphthol shows similar behaviour. This finding is clearevidence that in solutions of these compounds both tautomers are present.

The solvent effect on the azo–hydrazone equilibrium of 4-phenylazo-1-naphthol has beenmodelled using ab initio quantum-chemical calculations. The hydrazone form is more stablein water and in methylene chloride, whereas methanol and iso-octane stabilise the azo form,The calculated results were in good agreement with the experimental data in these solvents.Similar studies of 1-phenylazo-2-naphthol and 2-phenylazo-1-naphthol providedconfirmation. Substituent effects in the phenyl ring were rationalised in terms of theHOMO–LUMO orbital diagrams of both tautomeric forms [53].

Tautomerism both in the solid state and in solution is confirmed by infrared spectroscopicmeasurements on all three compounds [54,55]. The highest content of ketohydrazone formwithin the three isomeric phenylazonaphthols occurs, in solution, with 2-phenylazo-1-naphthol [56]. A 15N-n.m.r. study of some azo dyes derived from H acid and relatedintermediates has confirmed the dominance of the ketohydrazone tautomer [57]. Similarfindings have been obtained using high-field 1H- and 13C-n.m.r. spectroscopy [58].

N

N

OH

O

H

N

H3C

Ar

4.22

N

N

OH

N

N

R

Ar

4.23

350 400 450 500 550

1

2

Wavelength/nm

Mol

ar e

xtin

ctio

n co

effic

ient

/l (m

ol c

m)–

1 ×

10–4

PyridineMethanolAcetic acid

Figure 4.1 Effect of solvent on 4-phenylazo-1-naphthol spectrum

STRUCTURE OF AZO DYES

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A totally different picture is presented by 3-phenylazo-2-naphthol. This unusual isomercannot be prepared by a normal coupling procedure but has been obtained by reaction of 3-amino-2-naphthol with thionyl chloride to give the N-sulphinylamine (4.24), condensationof which with N-phenylhydroxylamine yields the desired product [59]. Here, assumption ofa ketohydrazone form would entail loss of aromatic character in both rings of thenaphthalene nucleus and the energetic unfavourability of this situation ensures that thecompound exists solely in the hydroxyazo form.

In the case of phenylazonaphthylamines, the aminoazo tautomer is very much favouredrelative to the iminohydrazone form [57,60].

OH

N S O

4.24

4.6 PREPARATION AND IMPORTANCE OF NAPHTHALENEINTERMEDIATES

4.6.1 Importance of naphthalene

When purely benzenoid intermediates are employed in the preparation of azo dyes, theproducts exist solely in the hydroxyazo form since the energy level of this tautomer issignificantly lower than that of the ketohydrazone form. The position changes when anaphthalene intermediate is used, for here the two tautomers are of comparable energy and socoexist. The ketohydrazone form generally possesses a higher extinction coefficient than doesthe hydroxyazo alternative. The practical consequence is that the former has a greatertinctorial strength – in crude terms, more colour value per unit mass. The participation of theketohydrazone form in naphthalene-based dyes allows them to gain this strength advantagewhich, together with the ready availability of naphthalene and its convenient processing toyield ideally substituted derivatives, has made this hydrocarbon of great importance as aprimary starting material in the manufacture of intermediates for the dyestuffs industry.

Naphthalene intermediates [61] are always built up by substitution reactions starting fromthe cheap and plentiful hydrocarbon using, in the main, only seven basic reactions. Most ofthese reactions are generally familiar from benzene chemistry but with some modification,since naphthalene has two different possible positions of substitution. These positions are oftendesignated α and β, the four α-positions being ortho and the four β-positions meta to thenearest carbon atom of the central bond. A further modifying influence is the lower level ofaromaticity of naphthalene compared with benzene, leading to increased reactivity.

The main reactions used in the manufacture of dyestuff intermediates from naphthaleneare described in sections 4.6.2 to 4.6.8.

4.6.2 Sulphonation

Treatment of naphthalene with concentrated sulphuric acid [62] results in substitution atone of the α-positions (4.25) if the temperature is below about 60 °C, but the reaction is

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reversible (Scheme 4.20) and at higher temperatures rearrangement occurs, involvingdesulphonation and resulphonation, yielding the thermodynamically stable β-isomer (4.26).

Under more forcing conditions, such as the use of higher operating temperatures or ofoleum (sulphur trioxide dissolved in anhydrous sulphuric acid), more than one sulphonicacid group can be introduced. This approach can lead to mixtures of isomers and, inpractice, the reaction conditions must be carefully controlled if the desired isomer is to beproduced in maximum yield.

SO3H

SO3H< 60 °C > 60 °C

4.25 4.26Scheme 4.20

4.6.3 Nitration

Nitration [63], unlike sulphonation, is not reversible and results very largely in α-substitution, yielding 1-nitronaphthalene (4.27).

NO2

4.27

4.6.4 Reduction

Reduction of a nitro compound to the corresponding naphthylamine proceeds exactly as inthe benzene series. A batch process using iron and hydrochloric acid is traditional but hasbeen somewhat superseded by catalytic hydrogenation.

4.6.5 Replacement of a sulphonic acid group by a hydroxy group

This displacement is accomplished by heating the sulphonated derivative at a hightemperature with sodium or potassium hydroxide [64]. Typical is the preparation of 2-naphthol (4.15) from naphthalene-2-sulphonic acid (Scheme 4.21). Displacement of thesulphonic acid group occurs more readily when it is located at the α- rather then at the β-position, the former requiring a fusion temperature of about 200 °C and the latter one ofabout 250 °C. This difference in reactivity can be exploited to prepare naphtholsulphonicacids by fusion of suitable naphthalenedisulphonic acids.

SO3H OH

4.26 4.15

NaOH/250 °C

Scheme 4.21

PREPARATION AND IMPORTANCE OF NAPHTHALENE INTERMEDIATES

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4.6.6 Bucherer reaction

This reaction [65–67] is only rarely encountered in the benzene series but is extremelyuseful for appropriate derivatives of naphthalene, where the mechanism of the reaction hasbeen investigated extensively. The reaction allows a hydroxy group to be exchanged for anamino group or vice versa. When a hydroxy group is to be converted into an amino group,the naphthol is heated under pressure with ammonium bisulphite (often produced in situ byintroduction of ammonia liquor and sulphur dioxide into a sealed autoclave) at atemperature of 100–150 °C; the naphthol is thereby converted into the correspondingnaphthylamine. The mechanism of the reaction is outlined in Scheme 4.22.

NH2HO

SO3Na

OO

SO3NaSO3Na

SO3NH4 SO3NH4

NH

SO3NaSO3NH4

NH

SO3NaSO3Na

NH2

NH4HSO3

NH3

4.28 4.29 4.30

– H2O

4.31

H+

4.324.33Imino tautomer

Keto tautomer

Scheme 4.22

In the initial step the naphthol reacts in its keto tautomeric form (4.28), addingammonium bisulphite across the isolated double bond in the sulphonated ring. Nucleophilicattack by ammonia on the tetralonesulphonic acid (4.29) is followed by a proton shift fromnitrogen to oxygen, producing an intermediate (4.30) which then loses water to give theimine (4.31). After cooling and discharging the autoclave, the reaction mixture is acidifiedand boiled. The added bisulphite is thereby eliminated from the molecule andrearrangement of the imine tautomer (4.32) so produced completes the conversion into thenaphthylamine (4.33).

The reverse of this reaction, which allows a naphthylamine to be converted into thecorresponding naphthol, is similarly carried out by heating the amine with sodium bisulphiteand finally acidifying the mixture; the mechanism is outlined in Scheme 4.23.

The Bucherer reaction can also be used in the preparation of substituted naphthylamines.Carrying out the reaction on a naphthol but employing an alkylamine in place of ammoniaresults in the production of the corresponding N-alkylnaphthylamine. Alternatively, a

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naphthylamine can be used as starting material, whereupon the excess of alkylamineemployed results in the amino group being displaced by the alkylamino group. Of greaterimportance is the preparation of N-ary1naphthylamines by heating a naphthol (or anaphthylamine) with sodium bisulphite and an arylamine, whereupon the hydroxy (oramino) group in the starting material is exchanged for the arylamino group.

In the above reaction mechanisms it is noteworthy that the sulphonic acid groupintroduced has been shown to enter at the 3-position and not the 4-position as previouslypostulated. A consequence of this situation is that an attempted Bucherer reaction on anaphthol (or a naphthylamine) carrying a sulphonic acid group located meta to the hydroxy(or amino) group would require a second sulphonic acid group to be introduced at thisposition. Since it is impossible to locate two sulphonic acid groups on the same carbon atom,these compounds cannot undergo the transformation.

NH2

SO3Na

NH

SO3Na

O

SO3NaSO3Na SO3Na

OH

SO3Na

NaHSO3 H2O

H+4.33

Scheme 4.23

4.6.7 Arylation

In some cases where it is difficult to carry out the Bucherer reaction successfully, it is easierto prepare N-arylnaphthylamines by heating together a naphthylamine and an arylamine. Inparticular, this reaction is useful in the preparation (Scheme 4.24) of 1-phenylaminonaphthalene-8-sulphonic acid (4.34; N-Phenyl Peri acid) and its N-4-methylphenyl analogue (Tolyl Peri acid), both of which intermediates are valuablecomponents for the production of navy blue dyes.

NHO3S NH2 HO3SNH2

200°C

4.34

H

Scheme 4.24


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4.6.8 Kolbe-Schmitt reaction

This carboxylation reaction is of importance mainly for the manufacture from 2-naphthol ofβ-oxynaphthoic acid (BON) acid, the parent acid of sodium 3-hydroxy-2-naphthoate(4.35). The carboxylation is achieved by heating the sodium salt of 2-naphthol underpressure in an atmosphere of carbon dioxide (Scheme 4.25). Under these conditions anequilibrium mixture of the naphthol and its carboxylated derivative is established, themixture containing about 30% of the carboxylic acid. The desired product is extracted fromthe reaction mixture and the unchanged 2-naphthol is recycled.

ONa OH

COONa

4.35

CO2

Scheme 4.25

4.6.9 Preparation of specific intermediates

The above reactions can be used to prepare a variety of multi-purpose intermediates that areof great value in dyestuffs manufacture. Many of these intermediates were prepared andfound to be of value in dye manufacture before the orientation of their substituents wasestablished. This early lack of basic knowledge resulted in these intermediates being giventrivial names that have persisted in the dyestuffs industry.

In the preparation of naphthalene intermediates the reactions must be employed in thecorrect sequence to achieve a desired orientation in the final product. A further crucialconsideration can be the need to avoid steps that would result in the formation ofcarcinogenic materials. These points are illustrated in the following examples.

Naphtholsulphonic acids and γ acid

The preparation of 2-naphthol by high-temperature sulphonation of naphthalene followedby alkali fusion of the resulting naphthalene-2-sulphonic acid has been mentionedpreviously. Further sulphonation of 2-naphthol yields several useful naphtholsulphonic acidsand conditions can be chosen to make one or other of these compounds the main product.The initial product is the unstable 2-naphthol-1-sulphonic acid, which readily rearranges to2-naphthol-6-sulphonic acid (4.36; Schaeffer’s acid). Further sulphonation leads to 2-naphthol-6,8-disulphonic acid (4.37; G acid) at low temperature and 2-naphthol-3,6-disulphonic acid (4.38; R acid) at higher temperature.

HO3S

OH

4.36

Schaeffer’s acid

SO3H

OH

HO3S4.37

G acid

HO3S

OH

SO3H4.38

R acid

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Fusion of 2-naphthol-6,8-disulphonic acid with sodium hydroxide yields naphthalene-1,7-diol-3-sulphonic acid (4.39), the more readily displaced α-sulphonic acid group beingreplaced. When this compound is subjected to a Bucherer reaction only the 7-hydroxy groupis exchanged for an amino group, yielding γ acid (4.40), as in Scheme 4.26. This selectivityarises because the 1-hydroxy group is unable to form the intermediate addition productnecessary for the Bucherer reaction to proceed, the sulphonic acid group being located inthe meta position.

OH

HO3S

OH

HO3S

NH2

4.39 4.40

γ acid

OH

Scheme 4.26

H acid

H acid (4.2) is possibly the most important single naphthalene-based intermediate. Thepreparation of this intermediate starts with a high-temperature sulphonation of naphthaleneusing 65% oleum (anhydrous sulphuric acid in which 65% by mass of sulphur trioxide hasbeen dissolved) to give mainly naphthalene-1,3,6-trisulphonic acid, the nitration productfrom which is purified by selective isolation. Reduction of the nitro group followed byhydrolysis of the 1-sulphonic acid substituent by heating with sodium hydroxide solution at180 °C completes the process (Scheme 4.27).

Use of a higher temperature (280 °C) at the final stage results in more extensivehydrolysis, with the amino group also being replaced by a hydroxy group, leading to theformation of naphthalene-1,8-diol-3,6-disulphonic acid (chromotropic acid).

HO3S

HO3S SO3H

HO3S NO2

HO3S SO3H

HO3S NH2

HO3S SO3H

OH NH2

HO3S SO3H

H2SO4/SO3 HNO3

H2/Pd

Koch’s acid

NaOH

4.2H acid

Scheme 4.27


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J acid

Of particular interest is the chain of reactions leading to J acid (4.43), since several of thecompounds that feature at intermediate stages in the chain are themselves useful as dyestuffintermediates. The starting point for this chain is 2-naphthol which, in early syntheses, wasconverted by means of a Bucherer reaction into 2-naphthylamine, this then beingsulphonated to yield 2-naphthylamine-5,7-disulphonic acid (4.42; Amido J acid). Therecognition that 2-naphthylamine is a potent carcinogen caused this route to be abandoned.In the method of preparation now in use (Scheme 4.28), a sulphonic acid group isintroduced into the 1-position of the naphthalene nucleus and is carried through the earlystages, so that 2-naphthylamine-1-sulphonic acid (4.41; Tobias acid) rather than theunsulphonated amine figures in the preparation.

OH

SO3H

OH

SO3H

NH2

HO3S

SO3H

NH2

SO3H

HO3S NH2

SO3HOH

NH2HO3S

4.41

Tobias acid

4.42

Amido J acid

4.43

J acid

Scheme 4.28

After sulphonation to 2-naphthylamine-1,5,7-trisulphonic acid, the labile 1-sulphonicacid substituent, which has now served its purpose, is eliminated by diluting thesulphonation mixture and heating. Fusion of the resulting disulphonic acid (4.42) withsodium hydroxide replaces the more labile 5-sulphonic acid group by a hydroxy group,forming J acid.

2-Naphthylamine-5,7-disulphonic acid and 2-naphthylamine-1-sulphonic acid, which areintermediate products in Scheme 4.28, as well as 2-naphthylamine-1,5-disulphonic acid(obtained by careful low-temperature sulphonation of Tobias acid), are all used in thesynthesis of azo dyes.

1-Naphthylaminesulphonic acids

Nitration of naphthalene-1-sulphonic acid produces two isomeric nitronaphthalenes thathave very similar solubilities. It is convenient to reduce the mixture without separation,giving a mixture of 1-naphthylamine-8-sulphonic acid (4.44; Peri acid) and 1-naphthylamine-5-sulphonic acid (4.45; Laurent’s acid), as in Scheme 4.29. These two

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compounds can be readily separated since the former has a much lower solubility. Theisomers also differ markedly in their value as intermediates, Peri acid being a very useful dyecomponent whilst Laurent’s acid finds little use.

In all the above sequences, single isomers are produced by careful control of the reactionconditions combined with purification by selective isolation at various points in the synthesis.Occasionally two isomers are produced which give dyestuffs that have very similar properties;in these cases it is often quite acceptable and economically beneficial not to separate theindividual components but to use the total mixture in dye preparation. An example is themixture of 1-naphthylamine-6- and 7-sulphonic acids (4.46; mixed Cleve’s acids), which arisesby nitration and reduction of naphthalene-2-sulphonic acid (Scheme 4.30).

Scheme 4.29

SO3H

SO3H

SO3HNO2

NO2

SO3H

SO3HNH2

NH2

4.44

Peri acid

4.45

Laurent’s acid

+ H2HNO3+

NO2

NO2

NH2

NH2

SO3H

SO3H SO3H

SO3H

SO3H

4.46

Cleve’s acids

+ +

H2HNO3

Scheme 4.30


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4.7 SCHEMATIC REPRESENTATION OF COUPLING

The simple process of diazotising an amine and joining the resulting diazonium salt with acoupling component produces a monoazo dye, that is, a dye containing a single azo group.Such dyes are often represented as A→E dyes, A standing for the amine, E the end couplingcomponent and the arrow symbol meaning ‘diazotised and coupled with’. The certainty withwhich diazotisation and coupling proceed, together with the wide variety of componentsavailable, allows more complex dye structures to be built up by combining two or morediazotisation and coupling sequences. A convenient shorthand way of classifying these morecomplex products has gained common acceptance. This shorthand, known as Winthersymbols, uses arrows combined with the following capital letters to indicate the nature of thecomponents:– A: an amine that is diazotised– E: an end coupling component– D: a diamine that is tetrazotised (both amino groups are diazotised)– M: an amine that is first coupled with a diazotised amine and then is itself diazotised– Z: a coupling component that can couple twice.

Using this nomenclature, disazo dyes (dyes containing two azo groups) fall into one or otherof the three classes represented thus:

E1←D→E2 A1→Z←A2 A→M→E

4.8 SULPHONATED AZO DYES

In the majority of dyeing processes the dye is transferred from aqueous solution onto thefibre, so that adequate aqueous solubility is a desirable property in a dye. The sulphonic acidgroup provides the cheapest way of solubilising a dyestuff and does not generally cause anydiminution in the light fastness of the chromogen to which it is attached. Consequently,dyes containing sulphonic acid groups find use in the dyeing of cellulosic fibres (section3.2.1) and amide fibres (section 3.2.2). In the ranges of dyes available for the dyeing of thesefibres, sulphonated monoazo structures provide many of the dyes in the yellow to red sectoras well as a few of the more bathochromic hues. As indicated previously, the couplingcomponent is of prime importance in determining the hue, with the diazo componentmerely modifying it, although when the diazo component contains powerful electron-withdrawing or -donating substituents this modification can be profound. Pyridone andpyrazolone coupling components are generally used in yellow dyes and naphthol,naphthylamine, aminonaphthol or acylaminonaphthol coupling components to cover theorange to red region. Disazo dyes span the spectrum from yellow to greenish blue and finduse mainly on cellulosic fibres but also on wool and silk. Trisazo and polyazo dyes arerestricted to duller hues on cellulosic fibres.

The actual structures chosen vary according to the particular fibre as well as the end useto which the dyed material will be put. It is appropriate to divide the discussion of thestructures of these dyes according to the fibres on which they are used.

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4.8.1 Acid dyes for wool

Simple monoazo dyes containing one or two sulphonic acid groups are used to dye wool, thedye being retained on the fibre by electrostatic bonding. Pyrazolones, naphthols,naphthylamines and acylaminonaphthols are commonly used coupling components. These‘levelling acid’ dyes are typified by tartrazine (4.47; CI Acid Yellow 23) and CI Acid Orange20 (4.48). These simple dyes are markedly hydrophilic in character and this necessitatesthem being applied to the substrate from a strongly acidic dyebath. Under these conditionsthe dyes readily penetrate the wool fibres and distribute themselves evenly, but their highlyhydrophilic character makes them easily desorbed and thus limits their fastness to washing.A higher level of wash fastness can be achieved by using disazo dyes of larger molecular size,usually carrying two sulphonic acid groups. These ‘milling acid’ dyes are typified by CI AcidYellow 42 (4.49), CI Acid Red 151 (4.50) and CI Acid Blue 116 (4.51). 4-Amino-2′-nitrodiphenylamine has been used as the A component in disazo dyes based on structure4.51 [68].

N

N

HO3S

OH

4.48

CI Acid Orange 20

N

N N

N

OHHO3S

4.50

CI Acid Red 151

N

N N

N

SO3H

N

H

SO3H4.51

CI Acid Blue 116

N

N

OH

N

NHO3S

HOOC

SO3H

4.47

CI Acid Yellow 23

O H

N

NN

N N

N

OH

N

N

CH3

H3C

HO3S

SO3H

4.49

CI Acid Yellow 42

SULPHONATED AZO DYES

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Much superior wet fastness can be achieved by incorporating a highly hydrophobicweighting group into the dye molecule [69]. Non-polar bonding between this group andhydrophobic side chains in wool imparts neutral-dyeing affinity to these ‘super-milling’ dyes(section 3.2.2). A typical example is CI Acid Red 138 (4.52). Confirmation of theketohydrazone structure has been obtained by spectroscopic and PPP-MO techniques [70].

OH

N

N

NH COCH3

HO3S SO3H

CH3(CH2)11

4.52

CI Acid Red 138

Of particular interest are the light-fast red dyes obtained by coupling onto γ acid underacid conditions. In this latter type, an example of which is CI Acid Red 32 (4.53), both theamino group and the hydroxy group are hydrogen bonded to the azo linkage [71]. Thisdouble hydrogen bonding prevents the tautomerism that is normally a feature ofnaphthalene-based azo dyes by locking the dye in the hydroxyazo form, leading both to thered hue and to the high level of light fastness.

Strongly electron-withdrawing or -donating substituents in the diazo component canhave a profound effect on the hue, producing a large bathochromic shift (section 1.4). Theuse of diazo components of the naphthalene series produces a similar but less marked effect.These effects are exploited in CI Acid Blue 92 (4.54), prepared by coupling diazotisedO-tolylsulphonyl H acid with N-Phenyl Peri acid and hydrolysis of the product. Here theproximity of the peri-sulphonic acid group to the secondary amino group prevents anymarked pH sensitivity, removing the need to stabilise the hue by acylation. Again thehydroxy group on the diazo component is hydrogen bonded onto the azo linkage, resulting inexcellent light fastness.

NH

O

N

HN

NHCOCH3

SO2NCH2CH3

H

HO3S4.53

CI Acid Red 32

N

H

O

N N

SO3H

HO3S

HO3S

4.54

CI Acid Blue 92

H

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4.8.2 Acid dyes for nylon

The synthetic polyamide fibre nylon can be dyed with either acid dyes or disperse dyes, theformer giving better fastness and the latter better levelling properties. The types of acid dyeused are broadly similar to those for wool, that is, monosulphonated levelling acid dyes withrelative molecular mass (Mr) in the 300–500 region and disulphonated milling acid dyeswith Mr of 600–900. Dyes of small molecular size are often deficient in wet fastness andthose of Mr higher than about 800 tend to give unlevel dyeings. Within these crudeguidelines certain structural features exert specific effects; for instance, hydrogen bondingsubstituents such as hydroxy and acylamino groups boost fastness to washing but may impairlevel dyeing behaviour.

Monoazo dyes containing a single sulphonic acid group conveniently fall into thelevelling acid group. A typical structure is CI Acid Red 266 (4.55).

Monosulphonated levelling acid dyes are mainly yellow or red in colour, whilst the largermolecular size that can be tolerated with a disulphonated milling acid dye allows the use ofdisazo compounds, such as CI Acid Blue 118 (4.56), to cover more bathochromic hues suchas navy blue.

NH

O

N

HN

Cl

CF3

HO3S

H

4.55

CI Acid Red 266

N

N N

N

SO3H

SO3H4.56

CI Acid Blue 118

N

H

CH3

4.8.3 Direct dyes

Certain water-soluble dyes are directly adsorbed onto cotton that has not been pretreatedwith a mordant (section 3.2.1). The first dye in which this phenomenon was observed wasCongo red (4.57; CI Direct Red 28), discovered in 1884 by Böttiger. The extreme pHsensitivity of this dye now restricts its use to that of an indicator, but it deserves mention asthe forerunner of the direct dye class.

N

N N

N

H2N

SO3H

HO3S

NH2 4.57

CI Direct Red 28


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If the solubility of dyes of this type is kept to an acceptable minimum, adequate washfastness can be achieved. This low solubility requirement is met by the use of the minimumnumber of solubilising groups consistent with the need to prepare an aqueous dyebath.Hydrogen bond formation with the fibre depends upon having numerous groups containingelectron-rich atoms, such as oxygen and nitrogen, positioned at intervals along an extendedplanar molecule. Suitable features conferring substantivity include amino and acylaminogroupings, hetero atoms in aromatic rings and the azo group itself.

The simplest monoazo dyes fail to meet these requirements, but by choosingintermediates known to confer substantivity and by building up the molecule to provide thenecessary length and coplanarity (section 3.2.1), direct dyes can be produced from this class.Thus the highly substantive character of the benzothiazole nucleus is exploited in CI DirectYellow 8 (4.58), as is the alignment of the azo, ureido and acylamino groups in thesubstituted J acid coupling component of CI Direct Red 65 (4.59).

N

S

N

H O

N

N

H

OH3C

SO3HH3C

4.58

CI Direct Yellow 8CH3

N

N

OH

N N

ONHCOCH3

HH

HO3S

4.59

CI Direct Red 65

N H

O H

N

N

HO3S

HO3S

OCH3

N

H

H3CO

N

OH

SO3H

SO3H

NH

H

4.60

CI Direct Blue 1

The required long planar shape is more readily supplied by simple disazo structures suchas CI Direct Blue 1 (4.60). Copper complexes of such disazo compounds are important andare dealt with elsewhere (section 5.5.3). A widely used method of producing A→Z←A typedisazo dyes relies on treating J acid with phosgene (COCl2) to give the bis-couplingcomponent carbonyl J acid (4.61).

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Disazo dyes can also be prepared by phosgenating suitable amino monoazo dyes. Thephosgenation technique for building up direct dye molecules by joining together sub-unitsgenerally produces symmetrical structures. However, the use of cyanuric chloride (4.62)allows different sub-units to be joined together and this intermediate has been employed tolink non-substantive anthraquinone structures to substantive azo moieties in green directdyes, such as CI Direct Green 28 (4.63).

N N

O

OH

HO3S

H H

OH

SO3H

4.61

Carbonyl J acid

N

N N

NN

N

N

N N

H

O NH2

SO3H

OH

H HHO3S

4.63

CI Direct Green 28

OH

COOH

N

N

N

Cl

Cl

Cl4.62

Cyanuric chloride

It is in the direct dye class that the more complex polyazo dyes come into their own, andtrisazo structures such as CI Direct Blue 78 (4.64), CI Direct Brown 222 (4.65) and CIDirect Black 38 (4.66) are classic examples of this type of dye. The last-named dye is nowknown to be carcinogenic [72]. Generally, the A→M1→M2→E type (such as 4.64) affordreasonably bright blue shades whilst dyes prepared according to other sequences, such as theE←D→Z←A type (examples 4.65 and 4.66) yield drab shades.

The last of this trio highlights an area of concern in direct dye chemistry. Many dyes ofthis type relied on the extremely useful intermediate 4,4′-diaminobiphenyl (benzidine). Inthe tetrazonium salt prepared from this diamine, the two diazonium groups are conjugatedand strongly activate one another. This activation makes the first coupling proceed veryreadily, but after completion the second diazonium group is no longer subject to the


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powerful activating influence of a conjugated diazonium group and it couples far moresluggishly. The two diazonium groups can thus be coupled selectively in a stepwise mannerto produce a host of useful products. In the preparation of CI Direct Black 38, for example,one diazonium grouping in tetrazotised benzidine undergoes coupling at low pH with H acidand diazotised aniline is then added. On raising the pH, the benzenediazonium chloridecouples next to the hydroxy group on the H acid unit. Finally, m-phenylenediamine is addedand the second benzidinediazonium grouping couples slowly to produce the third azolinkage. The withdrawal of the notorious carcinogen benzidine by major dyestuffmanufacturers in the 1970s led to an extensive search for replacements for these dyes. Oneproduct that has emerged from this effort is CI Direct Black 166 (4.67), in which 4,4′-diaminobenzanilide is used as a benzidine replacement. Sulphonated diaminobenzanilideshave also been examined [73]. Other alternative diamines have been reported [74,75].Interesting examples include 5,5′-diamino-2,2′-bipyridyl [76] and 3,8-diaminophen-anthridone [77], a benzidine analogue in which the biphenyl rings are linked by an amidegroup. The genotoxicity of azo dyes has been reviewed [78].

SO3H

N

N N

N N

N

OH

N

HO3S

SO3H

HO3S

4.64

CI Direct Blue 78

H

N

N N

N

N N

HO

HOOC

H3C

H2N

CH3H2N

4.65

CI Direct Brown 222

SO3H

CH3

N

N N

N

H2N

N N

H2N

HO

SO3H

HO3S

NH2

4.66

CI Direct Black 38

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Tetrakisazo dyes (dyes containing four azo groups) are less common but some do find use,these products often arising from the phosgenation of an amino disazo dye.

Direct dyes have only modest fastness to washing, which may be improved by after-treatments such as metal-complex formation (section 5.5.3) or by diazotisation of the dye onthe fibre and further coupling of the diazonium salt with an insoluble coupling component(section 1.6.14). In addition to their use on cotton and viscose, direct dyes are important inthe dyeing of leather. The cheapest members of this class are also used in the coloration ofpaper, since for this purpose fastness properties are largely irrelevant and price is all-important.

4.9 UNSULPHONATED MONOAZO DYES

4.9.1 Solvent-soluble dyes

These dyes, already described in section 2.12, require adequate solubility for the colorationof various organic solvents and must be cheap [79]. The simplest and least polar dyes of thisclass are used for the coloration of petrol and ball-pen inks, with more polar types being usedin lacquers, stains and varnishes. Some products of lower solubility are used in masscoloration.

These unexacting requirements make the simplest unsulphonated azo structures, oftenmonoazo types, quite acceptable [80]. Typical of the least polar members of this class are CISolvent Yellow 2 (4.68), CI Solvent Orange 1 (4.69) and CI Solvent Red 17 (4.70). Simpleazo structures carrying sulphonamide, sulphone or carboxylate ester groups are used where asomewhat more polar, less soluble dye is needed. Simple disazo compounds (4-amino-azobenzene→2-naphthol, for example) are used as red solvent dyes. Probably the onlystructural feature worthy of note in this class is the occasional adoption of structurescarrying long alkyl chains to enhance solubility, as in the case of the disazo dye CI SolventYellow 107 (4.71).

H2N

NH2

N

N N

H

O

N

N

N N

HO3S

H2N

HO

SO3H

4.67

CI Direct Black 166

N

N N(CH3)2

4.68

CI Solvent Yellow 2

N

N OH

HO 4.69

CI Solvent Orange 1

UNSULPHONATED MONOAZO DYES

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4.9.2 Disperse dyes

The introduction of the man-made fibre cellulose acetate in the early 1920s posed a newproblem for the dyestuff chemist, an initial solution to which was supplied by Green andSaunders in 1923 with the launch of the Ionamine range [81,82]. These dyes carried, as solesolubilising group, an ω-methanesulphonate grouping, which was incorporated into themolecule by coupling onto an arylamine ω-methanesulphonate. During the dyeing processthis grouping was hydrolysed, leaving a fine dispersion of a water-insoluble dye as thedyebath. This type of dye, known as a disperse dye [83,84], is now used for the dyeing ofcellulose acetate, triacetate and polyester fibres as well as offering an alternative to acid dyesfor the dyeing of nylon.

In order to facilitate satisfactory dye uptake, the molecular size of a disperse dye must bekept small; monoazo structures are therefore exceptionally important, particularly in thecoloration of polyester and cellulose triacetate. In the yellow shade area, molecular sizegenerally poses no problem and the various available coupling components can all be usedwithout making the molecule too large. A very simple example of the type of structureemployed using a phenolic coupling component is CI Disperse Yellow 3 (4.72). This dye isknown to cause skin sensitisation when on nylon [85] and can also provoke an allergicreaction [86].

H3C

OCH3

N

H

N

O

4.70

CI Solvent Red 17

H19C9

OH

N

N

H3C

CH3

N

N

HO

C9H19

4.71

CI Solvent Yellow 107

N

N

CH3CONH

HO

CH3

4.72


Heterocyclic coupling components are widely used in the disperse dye field for theproduction of yellow dyes. Numerous conventional dyes are based on simple pyrazolones,often combined with an o-nitroaniline diazo component, the o-nitro group being particularlyfavourable in ensuring good light fastness. CI Disperse Yellow 8 (4.73), which uses a verysimple pyrazolone coupling component, is an example.

A very noticeable feature has been the use of a wider range of heterocyclic couplingcomponents than in water-soluble azo dyes. An early example of this trend was CI DisperseYellow 5 (4.74), which used 4-hydroxy-1-methylcarbostyril as coupling component. It was in

H3C N

N

OH

N

NH

NO2

H3C4.73


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213

this field that the pyridones made their first impact. A typical example is CI Disperse Yellow211 (4.75). These compounds are now the most important yellow-producing couplingcomponents, giving exceptionally bright greenish yellow hues. Their success has stimulatedthe investigation of other compounds, with the particular aim of extending the range of huesavailable from such simple coupling components. Thus, derivatives of 2,6-diaminopyridinewith simple diazo components yield orange dyes, such as 4.76.

N

N

OH

O2N N

CH3O4.74


Cl

NO2

N

HO

N

N

CH3

CN

O

C2H54.75


N

N

NO2N

H2N CN

NHC2H5

H2N

Cl 4.76

With the more bathochromic hues, the restriction posed by the requirement for smallmolecular size can only be overcome by using ω-substituted N,N-dialkylaniline couplingcomponents and allying the powerful dialkylamino auxochrome with electron-attractinggroups in the diazo component. This arrangement, with electrons being donated from oneend of the molecule and attracted to the other, produces highly polar structures, with huesvarying from yellow to red to blue as the polarity increases. Examples are CI Disperse Red 72(4.77) and CI Disperse Blue 183 (4.78). In this latter dye the polarity is further increased byincorporating an amide group in the coupling component to reinforce the electron-donatingeffect of the dialkylamino group.

N

N

O2N

N

CN

CH2CH2CN

CH2CH2COOCH3

4.77

CI Disperse Red 72

N

N

O2N

N(C2H5)2

Br

NH

COC2H5

CN

4.78

CI Disperse Blue 183


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The coupling component in CI Disperse Red 72 is a complex tertiary amine, carryingcyano and methoxycarbonyl groups on the alkyl chains. Such a coupling component isprepared by reaction of the primary amine with acrylonitrile (H2C=CHCN) and methylacrylate (H2C=CHCOOCH3), the amino group adding across the activated double bond.The reason for incorporating these substituents is to enhance the fastness properties of thedye. Thus, the use of a cyanoethyl substituent in preference to a simple ethyl group in thetertiary amine coupling component generally improves fastness to light and sublimation.

In addition to benzenoid diazo components, diazotised heterocyclic amines in which theamino group is attached to a nitrogen- or sulphur-containing ring figure prominently in thepreparation of disperse dyes [87,88], since these can produce marked bathochromic shifts.The most commonly used of these are the 6-substituted 2-aminobenzothiazoles, prepared bythe reaction of a suitable arylamine with bromine and potassium thiocyanate (Scheme 4.31).Intermediates of this type, such as the 6-nitro derivative (4.79), are the source of red dyes,as in CI Disperse Red 145 (4.80). It has been found that dichloroacetic acid is an effectivesolvent for the diazotisation of 2-amino-6-nitrobenzothiazole [89]. Subsequent couplingreactions can be carried out in the same solvent system. Monoazo disperse dyes have alsobeen synthesised from other isomeric nitro derivatives of 2-aminobenzothiazole [90].Various dichloronitro derivatives of this amine can be used to generate reddish blue dyes forpolyester [91].

O2N

NH2

O2N

NH2

SCN O2N S

N

NH2KSCN/Br2

4.79Scheme 4.31

S

N

N

N N

C2H5

CH2CH2CN

O2N4.80

CI Disperse Red 145

The use of heterocyclic diazo components is particularly important in the preparation ofblue disperse dyes. It is often difficult to prepare, diazotise and/or couple the highlyelectronegatively substituted diazo components of the benzene series needed to achieve bluehues, and feebly basic aminoheterocycles offer a convenient alternative. The smallmolecular size of these heterocyclic amines, compared with that of heavily substitutedanilines, is another factor favouring their use.

One of the earliest diazo components of this type to be introduced was 2-amino-5-nitrothiazole (4.81), prepared by condensation of thiourea with chloroacetaldehyde andnitration of the resultant 2-aminothiazole (Scheme 4.32). This component yields brightdischargeable blues, such as CI Disperse Blue 82 (4.82), which have outstanding build-up,very high extinction coefficients and good fastness to burnt gas fumes. Use of diazocomponent 4.81 with coupling component 4.83 yields a greenish blue dye.

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Other examples include 3-aminobenzisothiazoles, especially the 5-nitro-derivative(4.84)[92] (synthesised as shown in Scheme 4.33) and electronegatively substituted 2-aminothiophenes (Scheme 4.34). Both of these amines, when diazotised, are capable ofgiving blues with arylamine coupling components. The non-benzenoid intermediate 4.84 hasa strong bathochromic effect, as in CI Disperse Blue 148 (4.85). Diazotised 5-acetyl-2-amino-3-nitrothiophene (4.86) gives a greenish blue dye when coupled with compound 4.83[93]. The corresponding dye from the diacetyl derivative of 4.83 and 2-amino-3,5-dinitrothiophene is green on polyester. An extensive range of thiophene-based monoazodisperse dyes has been examined [94].

CHO

H2C Cl

NH2

C NH2S

S

N

NH2

S

N

NH2

O2N

+

Chloroacetaldehyde Thiourea 4.81

Scheme 4.32

S

N

N

N

O2N H3C

N

C2H5

CH2CHCH2OH

4.82

CI Disperse Blue 82

OH

OCH3

N(CH2CH2OH)2

N

COCH3

H4.83

S

N

O2N

NH2

CN O2N

NH2

C O2NNH2

S

NH2

H2S H2O2

4.84Scheme 4.33

N

C2H5

CH2CH2COOCH3

4.85


N

NNS

NO2


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Whilst monoazo dyes of small molecular size dominate the disperse dye field, the use ofhigh-temperature dyeing and printing methods for the coloration of polyester has permittedsome relaxation in the necessity for limiting molecular size. This flexibility has allowed theuse of monoazo dyes such as CI Disperse Red 220 (4.87), employing a typical azoic couplingcomponent, particularly as disperse dyes in processes for the dyeing of polyester/cottonblends. A novel development in the dyeing of such blends has been the introduction ofdisperse dyes such as CI Disperse Red 167 (4.88), in which the tertiary amine couplingcomponent contains ester groupings. These groups can be hydrolysed in an alkaline clearingbath to facilitate removal of loose dye at the end of the dyeing process, thus minimisingstaining of the cellulosic portion of the blend.

N

H

N

N

O

Cl

O

H OC2H5

CH3

4.87

CI Disperse Red 220

S SS

S

Cl CH3CO Cl CH3CO

NO2

Cl

CH3CO

NO2

NH2

CH3COCl

AlCl3

HNO3

NH4OH

4.86Scheme 4.34

N

N

O2N

N

COC2H5

H

Cl

N(CH2CH2OCOCH3)2

4.88

CI Disperse Red 167

The widespread adoption of high-temperature dyeing methods has also allowed the use ofsimple disazo structures, such as CI Disperse Orange 13 (4.89) and CI Disperse Orange 29(4.90), as economic dyes giving chiefly yellow and orange hues. The latter dye is known toexist in the syn conformation in the crystal [95]; the unsubstituted parent dye prefers theanti conformation. A few monoazo and disazo disperse dyes have absorption bands in thenear infrared [96].

Considerable research effort has been devoted to developing a new generation of dispersedyes designed to optimise fastness to washing and minimise cross-staining of the cellulosiccomponent of polyester/cellulosic blends [97,98]. Diester-containing monoazo disperse dyestructures (4.91) that yield a dicarboxylic acid on hydrolysis and certain thienylazo blues

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(4.92) that are capable of being rendered soluble by a mild alkaline aftertreatment offerconsiderable benefits when dyeing polyester/cellulosic blends, including:(1) minimal cross-staining of the cellulosic fibre(2) minimal processing time because the alkaline fixation for reactive dyes clears the

disperse dye stain(3) avoidance of a reduction clear with dithionite(4) good washing fastness performance after heat setting during finishing.

N

N N

NO2N OH

4.90


OCH3

O2N N

N

HN

C

O

CH3

N

CH2CH2COOCH3

CH2CH2COOCH3

4.91

N N

CH2CH3

CH2CH3

HN

C

O

CH3

NS

O2N

NO2

4.92

N

N N

N OH

4.89


An investigation of more than twenty blue monoazo structures, mostly derived from 2-amino-3-carboxymethyl-5-nitrothiophene (4.93; X = COOCH3) or its 2-amino-3,5-dinitroanalogue (4.93; X = NO2) and some containing hydrolysable ester groupings in the couplingcomponent, was reported recently [99]. Depending on the dye structure, applied depth andconditions of heat setting, alkali clearing of these dyeings was as effective as a reductionclear in most instances. Comparisons between dyes containing either acetoxy (OCOCH3) orcarboxymethyl (COOCH3) ester groups demonstrated that the lability of the thienyl ringsystem contributes more to alkali clearing efficiency than the nature of these hydrolysableesters.

SO2N NH2

X

4.93


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A different approach to the incorporation of a hydrolysable grouping is to synthesisemonoazo derivatives of 4-aminophthalimide (4.94; R = Me or Et). This diazo componentundergoes ring opening under relatively mild alkaline conditions (Scheme 4.35) to give awater-soluble structure containing a 3,4-dicarboxyphenyl grouping (4.95) without affectingthe azo group present [100]. Therefore these dyes display good alkali-clearing properties,thus avoiding the need for a reduction clearing step that can result in the liberation ofcarcinogenic amines.

N

N N

R

RC

HNC

O

O

C

NaO

O

N

N N

R

R

C

O

NaO

4.94

Na2CO3

H2O

CO2 + NH3 +

4.95Scheme 4.35

4.10 BASIC AZO DYES

This class of dyes [101] includes some old-established products such as chrysoidine (4.96;CI Basic Orange 2) that have been of importance in the past due to their bright, strong,economical colours. Nowadays the main area of application of basic azo dyes is on acrylicfibres. The more advanced dyes developed for this purpose differ from the older ones in thatthe positive charge is supplied by a quaternary ammonium group rather than merely by aprotonated amine. In the dyeing of acrylic fibres problems arise because the levellingproperties of the adsorbed dye are poor, since the electrostatic attraction of a basic dye tothe acidic centres in the fibre provides extremely strong binding, resulting in excellentfastness properties.

N

N

H2N

NH3

+Cl–

4.96

CI Basic Orange 2

The two main classes of basic dyes vary in the way in which the basic centre is built intothe molecule. The use of pendant quaternary ammonium groups, where the charge isinsulated from the chromogen, allows disperse dyes to be given a cationic character.Alternatively, the cationic charge is delocalised within the chromogen.

Typically, CI Basic Red 24 (4.97) has a structure reminiscent of a disperse dye, exceptthat a quaternary ammonium group is carried on the pendant alkyl chain in the couplingcomponent. This coupling component is prepared by reaction of N-ethylaniline withethylene oxide followed by conversion of the resulting β-hydroxyethyl derivative into the β-

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chloroethyl compound, which is then treated with trimethylamine. By contrast, in CI BasicOrange 24 (4.98) the basic site is located in the diazo component.

Basic dyes in which the positive charge is supplied by a quaternised heterocyclic ring thatforms part of the chromogenic system often have particularly bright, strong hues. Theintermediates used here are again typical disperse dye components, quaternisation beingcarried out after coupling by treating the dye with an alkylating agent such as dimethylsulphate. Using typical tertiary amine coupling components, bright blue dyes, such as CIBasic Blue 41 (4.99), are readily obtained.

N

N N

C2H5

CH2CH2N(CH3)3

CN

O2N

CH3OSO3

+

–4.97

CI Basic Red 24

(CH3)3NCH2C N

N N

CH3

CH2CH2CN

OCl

–4.98

+

CI Basic Orange 24

N

S

N

N N

C2H5

CH2CH2OH

CH3

H3CO

+4.99

CH3OSO3–

CI Basic Blue 41

Considerable difficulties are often experienced in the preparation of basic dyes fromquaternised heterocyclic diazo components. An alternative technique is to use an oxidativecoupling procedure, in which a mixture of a hydrazone and a coupling component is treatedwith a mild oxidising agent, such as a hexacyanoferrate(III) [102]; an azo compound is thenproduced as shown in Scheme 4.36 for the synthesis of CI Basic Red 30 (4.100) [103].Quaternisation of heterocyclic derivatives can lead to the formation of mixtures of isomers,as in the case of CI Basic Red 22 (4.101). As well as the 2,4-dimethyl derivative shown, theproduct contains about 15% of the 1,4-dimethyl isomer [104].

N

CH3

N NH2

N

N

N NH2NH2

CH3

+

+

–4e

–3H+

CI Basic Red 30

4.100

X –

Scheme 4.36

BASIC AZO DYES

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It is convenient to include in this class certain tautomeric structures that can exist eitherin the azo form or in the alternative hydrazone form. These dyes are thediazatrimethinecyanines, which can be viewed as being derived from the trimethinecyanines(R–CH=CH–CH=R) by replacement of two of the CH units by nitrogen atoms (R–N=N–CH=R). Such dyes are important in achieving yellow and red shades and they are oftenmost conveniently prepared by an oxidative coupling procedure using coupling componentspeculiar to basic dyes. CI Basic Red 29 (4.102) and CI Basic Yellow 24 (4.103) are typical ofthis group.

NN

N

CH3

CH3

N

N N(C2H5)2

CH3OSO3–

+ 4.101

CI Basic Red 22

N

S

CH3

N

NN

H3C

CH3

+

Cl–4.102

CI Basic Red 29

N

S

N

N CH

N

NH3C

CH3

CH3CH3OSO3

+–

4.103

CI Basic Yellow 24

4.11 AZOIC DIAZO AND COUPLING COMPONENTS

In 1880 a process for the coloration of cotton by initial impregnation with an alkaline solutionof 2-naphthol and subsequent treatment with a solution of a diazonium salt was patented byThomas and Robert Holliday. In this process a water-insoluble monoazo dye of good fastnessto washing was formed within the fibre pores. The dyer’s need to diazotise the arylaminecomponent at low temperature led to these dyes becoming known as ‘ice colours’ and theproduct obtained using diazotised 4-nitroaniline became known as Para Red. Over the nextthirty years minor improvements in the process were introduced, but a major step forward wasmade in 1912 when Griesheim-Elektron introduced a new type of coupling component,3-hydroxy-N-phenylnaphthalene-2-carboxamide (4.104; 3-hydroxy-2-naphthoic anilide),under the trade name Naphtol AS (CI Azoic Coupling Component 2). The guiding principlebehind this advance was that the new intermediate was substantive towards cellulose andtherefore was better held in place on the fibre prior to and during the treatment with adiazonium salt. Furthermore, the increased substantivity and diminished solubility of theresulting azo combination gave a marked improvement in the level of wash fastness obtained.The term ‘azoic colours’ was introduced in the late 1920s [105].

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Naphtol AS is best prepared by heating together β-oxynaphthoic (BON) acid, anilineand phosphorus trichloride in an inert solvent, the initially formed intermediate (4.105)reacting further with the naphthoic acid to give the desired anilide. A range of similarproducts followed, with a variety of arylamines being used in place of aniline. A selection ofthese compounds is given in Table 4.2. An attempt has been made recently to correlate thechemical structure of azoic coupling components with their substantivity for cottoncellulose. Theoretically computed equations were derived to relate substantivity values tothe van der Waals’ surface and to molecular volume and hydrophobicity of the sorbedmolecule. The results provide support for the hypothesis of microcrystalline multilayeredmicelles available for sorption of these compounds [106].

O

HO

NH

4.104

CI Azoic CouplingComponent 2

N

P N

H

4.105

Table 4.2 Azoic coupling components

Naphtol A.C.C. Chemical name

AS–D 18 3-Hydroxy-2-naphthoic 2′-methylanilideAS–ITR 12 3-Hydroxy-2-naphthoic 5′-chloro-2′,4′-dimethoxyanilideAS–OL 20 3-Hydroxy-2-naphthoic 2′-methoxyanilideAS–PH 14 3-Hydroxy-2-naphthoic 2′-ethoxyanilideAS–TR 8 3-Hydroxy-2-naphthoic 4′-chloro-2′-methylanilide

A.C.C. CI Azoic Coupling Component

Ranges of diazotisable amines were supplied, designed to go with these couplingcomponents and known as Fast Bases, despite being marketed as the phenylammoniumchlorides (hydrochloride salts). Whilst the hues obtainable at first were limited to theorange to red region of the colour gamut, the later introduction of amines heavilysubstituted with electron-donating groups allowed extension into the violet to blue region.Examples of Fast Bases are given in Table 4.3; the last two entries in this table have beenclassified by ETAD as toxic compounds [107].

AZOIC DIAZO AND COUPLING COMPONENTS

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Extension into other hues was not possible using anilides of BON acid, and, to meet thisrequirement, coupling components of differing structure but similar levels of substantivitywere introduced (4.106 to 4.109; the arrows indicate coupling positions). Yellow dyeingswere obtained by using the bis-acetoacetarylamide derived from 3,3′-dimethylbenzidine,marketed as Naphtol AS-G (4.106; CI Azoic Coupling Component 5). Browns weresupplied by using the 2′-methylanilide, Naphtol AS-BR (4.107; X = CH3, Y = H; CI AzoicCoupling Component 3) and the 4′-chloroanilide, Naphtol AS-LB (4.107; X = H, Y = Cl;CI Azoic Coupling Component 15) of 2-hydroxycarbazole-1-carboxylic acid. Blacks resultedfrom the 4″-methoxy-2″-methylanilide of 3′-hydroxy-1,2-benzocarbazole-2′-carboxylic acid,Naphtol AS-SR (4.108; CI Azoic Coupling Component 25). Duller greens stemmed fromthe 2′-methylanilide of 3-hydroxyanthracene-2-carboxylic acid, Naphtol AS-GR (4.109; CIAzoic Coupling Component 36), whilst bright greens were eventually provided by NaphtolAS-FGGR (CI Azoic Coupling Component 108), the reaction product of copper or nickelphthalocyanine with an aminoarylpyrazolone, the constitution of which has not beendisclosed [108].

Table 4.3 Fast bases

Fast base A.D.C. Chemical name

Blue BB 20 4-Benzoylamino-2,5-diethoxyaniline hydrochlorideOrange GC 2 3-Chloroaniline hydrochlorideOrange RD 49 2-Chloro-5-trifluoromethylaniline hydrochlorideRed ITR 42 2-Methoxyaniline-5-sulphondiethylamide hydrochlorideRed KB 32 5-Chloro-2-methylaniline hydrochlorideRed RC 10 5-Chloro-2-methoxyaniline hydrochlorideRed RL 34 2-Methyl-4-nitroaniline hydrochlorideRed TR 11 4-Chloro-2-methylaniline hydrochlorideScarlet GG 3 2,5-Dichloroaniline hydrochlorideViolet B 41 4-Benzoylamino-2-methoxy-5-methylaniline hydrochloride

A.D.C. CI Azoic Diazo Component

N

H

O

N

O

H

X

Y

H

4.107

CI Azoic CouplingComponent 3

N

H

4.108

O

O

H

N

H

OCH3

H3C

CI Azoic Coupling Component 25

H3C

N N

CH3

H3C

H

O O

CH3

H

O O

4.106


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O

O

NH

CH3

H

4.109


NCH2CH2N

C

C

C

C

O

CH3

O

CH3

O

CH3

O

CH3

4.110

Azoic coupling can be achieved on silk where free amino groups have been introducedinto the fibroin structure by condensation with 4-aminobenzaldehyde. The diazotisedmaterial can then be treated with an azoic coupling component [109].

As recommended laundering temperatures have tended to fall in recent years, a bleachconsisting of sodium perborate activated by addition of tetra-acetylethylenediamine (4.110;TAED) has become an important component of household detergent formulations. Thissystem is effective at temperatures as low as 40–50 °C. A recent study of the effects ofTAED-activated peroxy bleaching on the colour fastness of azoic dyeings has demonstratedthat the sensitivity of these products can be related to their chemical structure. Electron-donating substituents in the diazo component enhance resistance to oxidative attack underthese conditions, as do the size and complexity of substituents present in the couplingcomponent [110].

4.12 STABILISED DIAZONIUM SALTS AND AZOIC COMPOSITIONS

4.12.1 Fast Salts

As the range of components available for use in the azoic dyeing process expanded, researchwas simultaneously targeted on improvements designed to make the process more attractiveto the commercial dyer. The necessity for the dyer to diazotise the Fast Base was removedwith the introduction of stabilised diazonium salts [111], known as Fast Salts. Stabilisationwas achieved by a judicious selection of the counter-ion to the diazonium cation; variousanions have found use in commercial Fast Salts and some examples are listed in Table 4.4.Particularly effective is the diazonium tetrachlorozincate, which can be readily prepared byadding an excess of zinc chloride solution to a solution of the diazonium salt. Theprecipitated complex diazonium salt is usually admixed with an inert diluent, whichenhances its stability, and in use the dyer only needs to dissolve the powder in water toprepare the necessary diazonium salt solution.

4.12.2 Rapid Fast colours

In addition to these ‘active’ stabilised diazonium salts that give a diazonium salt solutionimmediately on dissolving in water, there are certain derivatives of diazonium compoundsfrom which the diazonium salt can be readily regenerated. One group of such derivatives isthe class of compounds known as anti-diazotates [112]. Diazotates are formed when the pH

STABILISED DIAZONIUM SALTS AND AZOIC COMPOSITIONS

chpt4(1).pmd 15/11/02, 15:23223


of a diazonium salt solution is raised to between 9 and 12 with an alkali metal hydroxide.The syn-diazotates initially formed are somewhat unstable and isomerise into the stable anti-diazotate form. The evidence overwhelmingly points to the two forms being stereoisomers(4.111 and 4.112, respectively).

Stable mixtures of anti-diazotates and Naphtols were marketed as Rapid Fast colours forprinting onto fabric with development of the azoic dye by steaming. The anti-diazosulphonates (4.113) [113], which were prepared by treatment of a diazonium salt withsodium sulphite and which regenerate the diazonium ion on treatment with an oxidisingagent, found similar use. Both ranges are now of only historical interest.

Table 4.4 Stabilised diazonium salts

Fast salt A.D.C. Stabilising anion and parent base

TetrachlorozincateBlack K 38 4-Amino-2,5-dimethoxy-4′-nitroazobenzeneBlue B 48 3,3′-dimethoxybenzidine (o-dianisidine)Blue BB 20 4-Benzoylamino-2,5-diethoxyanilineBordeaux GP 1 4-Methoxy-2-nitroanilineOrange GR 6 2-NitroanilineRed AL 36 1-AminoanthraquinoneRed 3GL 9 4-Chloro-2-nitroanilineRed ITR 42 2-Methoxyaniline-5-sulphondiethylamideScarlet GG 3 2,5-DichloroanilineScarlet R 13 2-Methoxy-5-nitroaniline

TetrafluoroborateOrange GC 2 3-ChloroanilineRed GG 37 4-Nitroaniline

Naphthalene-1,5-disulphonateRed B 5 2-Methoxy-4-nitroanilineRed TR 11 4-Chloro-2-Methylaniline

ChlorideVariamine Blue B 35 4-Amino-4′-methoxydiphenylamine

A.D.C. CI Azoic Diazo Component

4.12.3 Rapidogens and Neutrogenes

Superior ‘passive’ stabilised diazo compounds are afforded by the diazoamino compounds(triazenes) that arise by reaction of diazonium salts with a variety of secondary amines [114].Typically, sarcosine (CH3NHCH2COOH), which gives products based on structure 4.114,as well as N-methyltaurine (CH3NHCH2CH2SO3H) and N-methylaniline-4-sulphonic acid,

N N

Ar O–

4.111

syn-diazotate

N N

Ar

O–

4.112

anti-diazotate

N N

Ar

O SO3Na

4.113

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have been used for this purpose. Again these were marketed in admixture with a coupler forapplication to cotton and colour development by acid steaming, under the tradenameRapidogen in the 1930s [114]. A disadvantage was the corrosive effect of the acid (usuallyacetic or formic) on the equipment, but this problem was eventually overcome with theNeutrogene range [115], which used an N-substituted anthranilic acid, such as 2-carboxyphenylglycine (4.115), as the secondary amine required for triazene formation.

These products, which have been copied by other manufacturers under varioustradenames, are designed specifically for printing applications. Azoic dyeing applicationswere catered for in 1967 when Hoechst introduced the Azanil salts, a limited range ofmixtures of substantive triazenes and Naphtols that can be absorbed onto the fabric and thecolour developed by acidification of the dyebath.

Ar N

N N

CH3

CH2COOH

4.114

COOH

NHCH2COOH

4.115

4.13 AZO PIGMENTS PRODUCED BY FINAL COUPLING

Insoluble azo pigments produced by a final coupling stage are in the main made fromconventional azoic dye intermediates and find wide use as general-purpose pigments wherethe highest fastness is not essential. For yellow pigments acetoacetarylamide couplingcomponents are the most commonly employed, but the monoazo combinations derived fromthese, such as CI Pigment Yellow 1 (4.116), are generally of poor fastness to heat andsolvents. Superior properties are shown by disazo pigments of the E1←D→E2 type, whichare often based on the use of 3,3′-dichlorobenzidine as tetrazo component. A typical disazopigment is CI Pigment Yellow 170 (4.117). Colour and constitution relationships inazoacetoacetanilide pigments have been examined [116]. Pigments of this type can beidentified and analysed by spectroscopic and chemical methods [117].

H3CO

N

H

O

O

N

N

H

N

N

H

N

O

O

H

OCH3

H3C

Cl

CH3

Cl4.117


H3C N

NO2

N

H

N

H

O

O

H3C

4.116

CI Pigment Yellow 1

AZO PIGMENTS PRODUCED BY FINAL COUPLING

chpt4(1).pmd 15/11/02, 15:23225


For orange colours, simple 2-naphthol derivatives are the most commonly used couplingcomponents as, for instance, in 2,4-dinitroaniline→2-naphthol (4.118; CI Pigment Orange5). As in the yellow series, superior disazo pigments can be prepared using 3,3′-dichlorobenzidine as tetrazo component with derivatives of 1-phenyl-3-methylpyrazol-5-oneas couplers.

O2N

NO2

N

H

N

O

4.118

CI Pigment Orange 5

In the red to violet sector Naphtol AS and its derivatives are widely used. Simplemonoazo combinations such as 2,5-dichloroaniline and Naphtol AS serve where high heatand solvent fastness are not essential, but for better properties polar features, such as amidelinkages, are incorporated. CI Pigment Red 245 (4.119), CI Pigment Red 188 (4.120) andCI Pigment Violet 50 (4.121) are typical of this group.

N

H2NOC

H

N

O O

NH

OCH3

4.119

CI Pigment Red 245

Cl N

H

O

N

O

H3CO

H

N

O O

NH

H3CO

Cl

4.120

CI Pigment Red 188

N

H

O

N

H

N

O O

NH

H3C

OCH3

4.121


In the blue region, conventional azoic combinations are used as pigments and theadoption of o-dianisidine as tetrazo component allows the disazo type to be produced, suchas CI Pigment Blue 25 (4.122).

chpt4(1).pmd 15/11/02, 15:23226

227

N

H

O

O

N

N

H

H3CO

N

OCH3

N

H O

O

N

H

4.122

CI Pigment Blue 25

The objective in preparing a pigment is to produce a highly insoluble organic compoundand in order to approach this ideal it is necessary to resort to intermediates that themselveshave only very limited solubility. These inevitable difficulties in preparation limit thefastness achieved from a final coupling process and have led to the development ofalternative procedures for the production of high-grade azo pigments; these alternatives arediscussed elsewhere (section 2.3).

The wholly organic azo compounds discussed above have the soft texture and hightinctorial strength that are desired pigmentary properties. Somewhat inferior in theseproperties are the ‘lakes’ of sulphonated azo compounds, which are also widely used aspigments. These ‘lakes’ are insoluble salts, usually calcium or barium salts of sulphonatedarylamines linked to cheap coupling components, which rely on the polar character of theionic bond to provide their low solubility. Examples are CI Pigment Red 57:1, the calciumlake of compound 4.123, and CI Pigment Red 53:1, the barium lake of compound 4.124.

Organic pigments have been reviewed [118]. X-ray powder diffraction data of manyorganic colorants have been collected and discussed [119].

CH3

NN

O

H

COONa

SO3Na

4.123

CI Pigment Red 57

Cl

NN

O

H

NaO3S

CH3

4.124

CI Pigment Red 53

4.14 IMPLICATIONS OF NEW TECHNOLOGY IN DIAZOTISATION ANDCOUPLING

The manufacture of azo dyestuffs has been for many years a batch operation, individualbatches of dye being made in multipurpose plant, isolated by filtration techniques, stove-dried and standardised. The traditional processes [120] are now steadily being replaced[121,122] by more efficient production methods that offer greater cost-effectivenes and

IMPLICATIONS OF NEW TECHNOLOGY IN DIAZOTISATION AND COUPLING

chpt4(1).pmd 15/11/02, 15:23227


improved working conditions. Although most azo dye production is still carried out by batchoperation in vastly improved plants, more dedicated continuous units for the manufacture ofhigh-volume products are being introduced. In these plants the diazotisation and couplingsteps are carried out continuously by computer-controlled metering of the intermediates andreactants into a tube, flow rates being chosen to allow the reaction time to be less than thedwell time of the reaction mixture in the tube. The product is still usually isolated byfiltration but, even here, automatic discharging has removed much of the heavy physicaleffort and dirty conditions previously endured. Spray drying of pre-standardised pastes withcareful attention to particle form provides much improved products for the user. The supplyof dyes as concentrated solutions allows the automatic metering of dyestuffs by the user,making automation easier and allowing greater control over dyeing conditions. Theseimprovements in manufacturing technique, together with the availability of a much widervariety of analytical methods, serve to ensure that the user is supplied with a high-qualityproduct at an acceptable price.

REFERENCES 1. P Griess, Ann., 106 (1858) 123; 113 (1860) 201; Compt. Rend., 49 (1859) 77. 2. P Griess, Ann., 137 (1866) 85; Ber., 9 (1876) 627. 3. H Zollinger, Color chemistry, 2nd Edn (Weinheim: VCH, 1991) Chapter 7.2. 4. P Rys in Physico-chemical principles of color chemistry, Eds A T Peters and H S Freeman (London: Blackie, 1996)

Chapter 1.2. 5. W Bradley, J.S.D.C., 75 (1959) 289. 6. K Bredereck, B Gülec, B Helfrich and S.Karaca, Dyes and Pigments, 9 (1988) 153. 7. H Zollinger, Color chemistry, 2nd Edn (Weinheim: VCH, 1991) Chapter 7.3. 8. V Chmatal and Z J Allan, Coll. Czech. Chem. Commun., 30 (1965) 1205. 9. ETAD Information Notice No 6 (July 1997). 10. A Geisberger, J.S.D.C., 113 (1997) 197. 11. Y S Szeto and G Taylor, J.S.D.C.,113 (1997) 103. 12. U Sewekov and A Westerkamp, Melliand Textilber., 78 (1997) 56. 13. B Küster and U Wahl, Textilveredlung, 32 (1997) 121. 14. K Hübner, E Schmele and V Rossbach, Melliand Textilber., 78 (1997) 720. 15. S W Oh, M N Kang, C W Cho and M W Lee, Dyes and Pigments, 33 (1997) 119. 16. O Stein, Ber., 27 (1894) 2806. 17. E Noelting and E Kopp, Ber., 38 (1905) 3506. 18. C Schwalbe, Z.Farb.Text.Ind., 4 (1905) 433. 19. K H Saunders and R L M Allen, Aromatic diazo compounds, 3rd Edn (London: Edward Arnold, 1985) Chapter 1.2. 20. P Rys in Physico-chemical principles of color chemistry, Eds A T Peters and H S Freeman (London: Blackie, 1996)

Chapter 1.3. 21. E Knoevenagel, Ber., 23 (1890) 2994; 28 (1895) 2048. 22. O N Witt, Ber., 42 (1909) 2953. 23. V V Ershov, G A Nikiforov and C R H I de Jonge, Quinone diazides, (Amsterdam: Elsevier, 1981). 24. T S Gore and K Venkataraman, Proc. Indian Acad. Sci., 34A (1951) 368. 25. H H Hodgson and D E Nicholson, J.C.S., (1943) 379. 26. W R Orndorff and B J Ray, Ber., 40 (1907) 3211. 27. F Muhlert, Z.Angew.Chem., 21 (1908) 2611. 28. K H Saunders and R L M Allen, Aromatic diazo compounds, 3rd Edn (London: Edward Arnold, 1985) Chapter 6.2. 29. H V Kidd, J. Org. Chem., 2 (1937) 206. 30. W Schwarin and O Kaljanov, Ber., 41 (1908) 2056. 31. F Muzik and Z J Allan, Chem. Listy, 49 (1955) 212. 32. F Muzik and Z J Allan, Chem. Listy, 46 (1952) 774. 33. E R Ward, D B Pearson and P R Wells, J.S.D.C., 75 (1959) 484. 34. C Krohn, Ber., 21 (1888) 3241. 35. V Chmatal and Z J Allan, Coll. Czech. Chem. Commun., 27 (1962) 1835. 36. N Sekar, J.S.D.C., 111 (1995) 390; 112 (1996) 204.

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229

37. N Sekar, J.S.D.C., 112 (1996) 242. 38. G S Hartley, Nature, 140 (1937) 281. 39. G S Hartley, J.C.S., (1938) 633. 40. G Zimmerman, L Y Chow and U J Paik, J. Amer. Chem. Soc., 80 (1958) 3528. 41. D Schulte-Frohlinde, Ann., 612 (1957) 131. 42. D J W Bullock, C W N Cumper and A I Vogel, J.C.S., (1965) 5316. 43. D L Beveridge and H H Jaffe, J. Amer. Chem. Soc., 88 (1966) 1948. 44. B Marcandalli, P L Beltrame, E Dubini-Paglia and A Seves, Dyes and Pigments, 11 (1989) 179. 45. Y Yagi, Bull. Soc. Chim. Japan, 36 (1963) 487. 46. R Jones, A J Ryan, S Sternhill and S E Wright, Tetrahedron, 19 (1963) 1497. 47. S Toda, Nippon Kagaku Zasshi, 80 (1959) 402. 48. Q Peng, M Li, K Gao and L Cheng, Dyes and Pigments, 18 (1992) 271. 49. G Hallas and A H M Renfrew, J.S.D.C., 112 (1996) 207. 50. E Fischer and Y F Frei, J.C.S., (1959) 3159. 51. P F Gordon and P Gregory, Organic chemistry in colour, (Berlin: Springer-Verlag, 1983) 100. 52. A Burawoy, A G Salem and A R Thompson, J.C.S., (1952) 4793. 53. L Antonov et al., Dyes and Pigments, 38 (1998) 157; 40 (1998) 163. 54. K J Morgan, J.C.S., (1961) 2151. 55. D Hadzi, J.C.S., (1956) 2143. 56. L Antonov and S Stoyanov, Dyes and Pigments, 28 (1995) 31. 57. K Yamamoto, K Nakai and T Kawaguchi, Dyes and Pigments, 11 (1989) 173. 58. C Szantay, Z Csepregi, P Aranyosi, I Rusznak, L Toke and A Vig, Magn. Res. Chem., 35 (1997) 306. 59. H E Fierz-David, L Blangey and E Merian, Helv. Chim. Acta, 34 (1951) 34, 846. 60. J Kelemen, Dyes and Pigments, 2 (1981) 73; J Kelemen, G Kormany and G Rihs, Dyes and Pigments, 3 (1982) 249; J

Kelemen, S Moss, H Sauter and T Winkler, Dyes and Pigments, 3 (1982) 27. 61. N Donaldson, The chemistry and technology of naphthalene compounds, (London: Edward Arnold, 1958). 62. N N Vorozhtsov, The chemistry of synthetic dyes, Vol 3, Ed K Venkataraman (New York: Academic Press, 1970) 93. 63. N N Vorozhtsov, The chemistry of synthetic dyes, Vol 3,Ed K Venkataraman (New York: Academic Press, 1970) 110. 64. N N Vorozhtsov, The chemistry of synthetic dyes, Vol 3, Ed K Venkataraman (New York: Academic Press, 1970) 146. 65. H T Bucherer, J.Prakt.Chim., (2), 69 (1904) 49. 66. N L Drake, Organic Reactions, 1 (1942) 105. 67. H Seeboth, Angew. Chem. Internat., 6 (1967) 307. 68. J Kraska and K Blus, Dyes and Pigments, 31 (1996) 97. 69. J Meybeck and P Galafassi, Appl. Polymer Symp., 18 (1971) 463. 70. S Stoyanov, T Iijima, T Stoyanova and L Antonov, Dyes and Pigments, 27 (1995) 237. 71. D L Ross and E Reissner, J. Org. Chem., 31 (1966) 2571. 72. C T Helmes, Amer. Dyestuff Rep., 83 (Aug 1994) 40. 73. W Czajkowski, Dyes and Pigments, 17 (1991) 297. 74. J Shore, Rev. Prog. Coloration, 21 (1991) 23. 75. Y C Chao and S S Yang, Dyes and Pigments, 29 (1995) 131. 76. F Calogero, H S Freeman, J F Esaney, W M Whaley and B J Dabney, Dyes and Pigments, 8 (1987) 431. 77. J Szadowski, Dyes and Pigments, 14 (1990) 217. 78. H S Freeman, D Hinks and J F Esaney in Physico-chemical principles of color chemistry, Eds A T Peters and H S

Freeman (London: Blackie, 1996) Chapter 7. 79. Chi-Kang Dien, The chemistry of synthetic dyes, Vol 8, Ed K Venkataraman (New York: Academic Press, 1978) 81. 80. Colour Index International, Pigments and Solvent Dyes (Bradford: SDC, 1997) 198. 81. A G Green and K H Saunders, J.S.D.C., 39 (1923) 10. 82. A G Green and K H Saunders, J.S.D.C., 40 (1924) 138. 83. J F Dawson, J.S.D.C., 107 (1991) 395. 84. S Abeta and K Imada, Rev. Prog. Coloration, 20 (1990) 19. 85. K D Wozniak, A Keil and D Müller, Textil Praxis, 45 (1990) 965. 86. P Elsner, Textilveredlung, 29 (1994) 98. 87. N Sekar, Colourage, 45 (Dec 1998) 37. 88. A D Towns, Dyes and Pigments, 42 (1999) 3. 89. J Sokolowska-Gajda and H S Freeman, Dyes and Pigments, 20 (1992) 137. 90. A T Peters, S S Yang and E Chisowa, Dyes and Pigments, 28 (1995) 151. 91. A T Peters and S S Yang, Dyes and Pigments, 30 (1996) 291. 92. H G Wippel, Melliand Textilber., 50 (1969) 1090. 93. G Hallas in Developments in the chemistry and technology of organic dyes, Ed J Griffiths (Oxford: Blackwell Scientific

Publications, 1984) 31.

REFERENCES

chpt4(1).pmd 15/11/02, 15:23229


94. G Hallas and A D Towns, Dyes and Pigments, 31 (1996) 273; 32 (1996) 135; 33 (1997) 205; 33 (1997) 215; 33(1997) 319; 34 (1997) 133; 35 (1997) 45; 35 (1997) 219.

95. H S Freeman, S A McIntosh and P Singh, Dyes and Pigments, 35 (1997) 11. 96. M Matsuoka in Analytical chemistry of synthetic colorants, Eds A T Peters and H S Freeman (London: Blackie, 1995)

Chapter 3. 97. P W Leadbetter and A T Leaver, Rev. Prog. Coloration, 19 (1989) 33. 98. P W Leadbetter and S Dervan, J.S.D.C., 108 (1992) 369. 99. J H Choi, S H Hong and A D Towns, J.S.D.C., 115 (1999) 32.100. J S Koh and J P Kim, J.S.D.C., 114 (1998) 121, Dyes and Pigments, 37 (1998) 265.101. R Raue, Rev. Prog. Coloration, 14 (1984) 187.102. S Hünig, Angew. Chem. Internat., 7 (1968) 335.103. P F Gordon and P Gregory, Organic chemistry in colour (Berlin: Springer-Verlag, 1983) 87.104. D Brierley, P Gregory and B Parton, J. Chem. Research, 5 (1980) 174.105. E B Higgins, J.S.D.C., 43 (1927) 213.106. C Daescu and D Hadaruga, Dyes and Pigments, 40 (1998) 235.107. R Anliker, G Dürig, D Steinle and E J Moriconi, J.S.D.C., 104 (1988) 223.108. F Gund, J.S.D.C., 76 (1970) 151.109. S Ifrim, Amer. Dyestuff Rep., 84 (Mar 1995) 38.110. J Wang, AATCC Internat. Conf. & Exhib. (Oct 1998) 144.111. K H Saunders and R L M Allen, Aromatic diazo compounds, 3rd Edn (London: Edward Arnold, 1985) Chapter 3.5.112. K H Saunders and R L M Allen, Aromatic diazo compounds, 3rd Edn (London: Edward Arnold, 1985) Chapter 5.1.113. K H Saunders and R L M Allen, Aromatic diazo compounds, 3rd Edn (London: Edward Arnold, 1985) Chapter 5.3.114. K H Saunders and R L M Allen, Aromatic diazo compounds, 3rd Edn (London: Edward Arnold, 1985) Chapter 7.1.115. B Jomain, J.S.D.C., 69 (1953) 661.116. R M Christie and P N Standring in The design and synthesis of organic dyes and pigments, Eds A T Peters and H S

Freeman (London: Elsevier Applied Science, 1991) Chapter 11.117. C Nicolaou and M Da Rocha in Analytical chemistry of synthetic colorants, Eds A T Peters and H S Freeman

(London: Blackie, 1995) Chapter 8.118. W S Czajkowski in Modern colorants: Synthesis and structure, Eds A T Peters and H S Freeman (London: Blackie,

1995) Chapter 3.119. A Whitaker in Analytical chemistry of synthetic colorants, Eds A T Peters and H S Freeman (London: Blackie, 1995)

Chapter 1.120. H E Fierz-David and L Blangey, Fundamental processes of dye chemistry, (New York: Interscience, 1949).121. G Booth, Rev. Prog. Coloration, 8 (1977) 1; 14 (1984) 114.122. G Booth, The manufacture of organic colorants and intermediates, (Bradford: SDC, 1988).

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231

CHAPTER 5

Chemistry and properties of metal-complex andmordant dyes

John Shore

5.1 INTRODUCTION

The complexing of chelatable organic compounds with transition-metal atoms is widelyexploited in catalytic reactions, biological systems, metal recovery and sequestration of tracemetals. Such complexes are important in many non-textile applications, including solventdyes of high fastness, charge-control agents in photocopying, dye developers in instantcolour photography and specific reagents in colorimetric analytical chemistry. This chapteris mainly concerned with the interaction between organic chromogenic systems andchromium or copper (or occasionally iron, cobalt or nickel) to give acid dyes for amide fibresand direct or reactive dyes for cellulosic fibres [1–3].

It has been recognised for centuries that certain natural dyes, including alizarin, kermes,cochineal and fustic, now known to contain o-dihydroxy phenolic or anthraquinonoidresidues in their structures, can be fixed on natural fibres using oxides or salts of transitionmetals as mordants. Although mordanted wool dyed with alizarin showed excellent fastness,reproducibility of shade was difficult to achieve because of the variable composition of theraw materials available. The famous Turkey red, in which alizarin was applied to aluminium-mordanted wool in the presence of calcium salts, formed a metallised complex the nature ofwhich remains in considerable doubt.

The research of Werner at Zürich in the early l900s on the theory of coordination valency[4] established the concept of a metal-complex dye and was the key to understanding of themordanting process. He concluded that a metal ion was characterised by two types ofvalency, which he called primary and secondary. The primary valency satisfied the chemicalequivalence (the number of positive charges on the metal cation) and the secondary valencytook part in coordination bonding. We now refer to the primary valency as the oxidationnumber or state of the central metal atom and the secondary valency as the number ofassociated ligand atoms required to complete the coordination number. It must be borne inmind that the concept of bond angles, such as the tetrahedral configuration of the C-Hbonds in methane, was then unknown and knowledge of the mode of attachment of a ligandto the central metal atom was extremely vague.

Synthetic alizarin (5.1), CI Mordant Red 5 (5.2) and CI Mordant Orange 1 (5.3), the firstazo dye capable of forming a metal complex, in this case via the salicylic acid residue, areexamples of the simple mordant dyes in widespread use at the time when Wernerpropounded his theory. The first metal-complex dyes to be prepared in substance, ratherthan within the fibre, were discovered by Bohn of BASF in 1912 by treatinghydroxyanthraquinonesulphonic acids with a warm solution of a chromium(III) salt. In the

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232 CHEMISTRY AND PROPERTIES OF METAL-COMPLEX AND MORDANT DYES

same year chromium complexes of mordant azo dyes were also patented. However, it was notuntil Ciba in 1915 proposed applying these sulphonated 1:1 chromium-dye complexes towool from a sulphuric acid solution at low pH that economic exhaustion values could beattained. CI Acid Yellow 104 is a typical example of these mono- or disulphonated azo dyecomplexes still in current use.

O

O

OH

OH

5.1

Alizarin

NN

OO

SO2

H3CN

H

C

H3CH2CO

O

Cr

NN

OO

N

C

H

OCH2CH3

O

O2S

CH3

5.4

CI Acid Black 58

NN

OOCr

NN

OO

NO2

O2N

NaO3S

SO3Na

5.5

CI Acid Green 12

NN OH

5.2

CI Mordant Red 5

OHHO

NaO3S

NN OH

COOH

O2N

5.3

CI Mordant Orange 1

The first water-soluble symmetrical 1:2 metal-complex dye, which contained no sulphogroups and was solubilised by a polar methylsulphonyl group on each organic ligand, wasdescribed by Schetty of Geigy for application to wool under neutral dyeing conditions [5].This dye was CI Acid Black 58 (5.4), still an important basis for grey shades in wool dyeing.If sulphonated intermediates were used to form symmetrical disulphonated 1:2 metal-

chpt5(1).pmd 15/11/02, 15:28232

233

complex analogues, such as CI Acid Green 12 (5.5), unlevel dyeing remained a problembecause of the low pH necessary to achieve satisfactory exhaustion.

This difficulty was gradually overcome by the use of nonionic or weakly cationic levellingagents, but the most important factor was the introduction of a range of polar but nonionisedsolubilising groups [l,6]. These included alkylsulphony1 (RSO2–), as in structure 5.4, mono-and dialkylsulphonamides (R2NSO2–) and cyclic sulphones. Typical sulphonamido-substituteddyes include CI Acid Orange 60 (5.6; R = H) and 87 (5.6; R = CH3). Insoluble 1:2 metal-complex dyes devoid of polar solubilising groups are also available, such as CI Acid Black 63(5.7). They are mainly of interest for the dyeing of nylon from aqueous dispersion.

NN

OOCr

NN

OO

N

N

N

N

CH3

H3C

R

R

O2S

NH

R

SO2

HN

R

5.6

NN

OOCr

NN

OO

NO2

O2N

5.7

CI Acid Black 63

5.2 FUNDAMENTAL CONCEPTS

Metal-complex formation entails the interaction of one or more organic ligands with amultivalent metal cation. This brings about certain fundamental changes in thecharacteristics of the components of the complex.

5.2.1 Ligand systems

A ligand system is any charged or uncharged polar entity or atomic component of asubstituent group containing an excess electron or electron pair. The nature of the bondbetween the ligand and the metal atom depends on the ligand acting as an electron-pairdonor and the metal as an electron-pair acceptor. The size of the atom and the electrondistribution in the outermost shell of a transition element of the first series are especiallyconducive to the formation of metal complexes. The donation of an electron pair fromligand atom to metal gives rise to a covalent or sigma bond, in molecular orbitalterminology. It is often convenient to distinguish between conventional covalent bondsarising from the sharing of electrons, denoted by a straight line, and coordinated sigmabonds arising by electron-pair donation from a ligand system, denoted by an arrow. However,there is no real difference between them once the bonds are formed.

FUNDAMENTAL CONCEPTS

chpt5(1).pmd 15/11/02, 15:28233


In the synthesis of metal-complex dyes the organic ligand may form part of either a basicor an acidic functional group. A basic ligand carries a lone pair of electrons that mayinteract with the metal ion. Examples include –NH2 (amino), =NH (imino), =N– (in azoor azomethine), =O (in carbonyl) or –S–(in thioether). An acidic ligand, such as –OH(from carboxylic, phenolic or enolic groups), –SH (from thiophenolic or thioenolic groups)or –NH– (from amino or imino groups), loses a proton during metallisation to give a formalnegative charge.

In the case of a ligand molecule such as ammonia or water, only one pair of electrons isutilised in forming a coordinated sigma bond with the metal. Ligand systems of this type aresaid to be unidentate and are found in many metal-complex dye structures, satisfying sitesaround the central metal atom not already sterically occupied by coordinated dye ligands.Bi- or tridentate ligand systems form respectively two or three coordinated or covalentbonds with the central atom. Dye molecules used as intermediates in synthesising metalcomplexes have at least two ligand donor groups arranged so that it is geometrically possibleto coordinate at two or more positions in the coordination shell of a single metal ion.Examples found throughout this chapter contain from two to six ligand groups bound to theone transition-metal ion. Where two groups originating in the same bidentate moleculecombine with the same metal ion, a heteroatomic ring is formed.

Ring formation between the metal M and a salicylic acid residue (in a dye structure suchas 5.3) containing two donor groups is shown in Scheme 5.1. This process was first describedby Morgan and Drew [7] as chelation, which profoundly affects the chemical and physicalproperties of both the ligand system and the metal atom. Although the formation of a singleheterocyclic ring imparts some stability to the complex, this is not generally sufficient to bedirectly useful in dyeing. Scheme 5.1 is a reversible reaction, so that demetallisation can takeplace to an extent that depends on the severity of the conditions. The metal chelate may bethermodynamically stable yet kinetically labile. The chromogenic ligand system may bereplaced by other electron-pair donors, such as water molecules. This sensitivity tohydrolysis is shown by certain complexes of o-nitrosonaphthols, such as the 1:3 iron(II)complex (5.8) of 1-nitroso-2-naphthol-6-sulphonic acid, although this dye is still of someinterest for the dyeing of wool. On the other hand, multidentate ligands form numerouscomplexes with copper, chromium or cobalt that contain two condensed heteroatomic ringsystems. These are much more stable, both thermodynamically and kinetically, undergoingligand exchange reactions extremely slowly.

5.2.2 Coordination number

The number of ligand systems associated with a metal atom in a complex is known as thecoordination number, often abbreviated to CN. For transition metals in their lower oxidationstates (1+, 2+ or 3+) the CN is usually 4 or 6. The metals used in complex formation with

NN

Ar

COH

O

OH

NN

Ar

CO

O

OM

M2++ + 2H+

Scheme 5.1

chpt5(1).pmd 15/11/02, 15:28234

235

dye molecules fall into this category. In less common complexes the CN can range from 2 to10. This value depends on the size and geometry of the ligand system relative to the size ofthe metal ion. The larger the central cation, the greater the number and variety of ligandsystems that can be accommodated. The charge on the cation also influences the CN.Metals in a higher oxidation state (such as 6+ or 7+) generally have low CN values, sincethe removal of six or seven electrons reduces the ionic size and so the accommodation ofmany ligand systems simultaneously around the ion becomes sterically impossible.

5.3 ELECTRONIC STRUCTURE OF TRANSITION-METAL IONS

The structures of metal-complex dyes, which must exhibit a high degree of stability duringsynthesis and application, is limited to certain elements in the first transition series, notablycopper, chromium, iron, cobalt and nickel. The remaining members of the transition seriesform relatively unstable chelated complexes. The following description of the influence ofelectronic structure, however, is applicable to all members of the series.

The principal characteristic of the transition elements is an incomplete electronic subshellthat confers specific properties on the metal concerned. Ligand systems may participate incoordination not only by electron donation to the 3d levels in the first transition series but alsoby donation to incomplete outer 4s and 4p shells. Figure 5.1 shows that the differences inorbital energy levels between the 4s, 4p and 3d orbitals are much smaller than, for example,the difference between the inner 2s and 2p levels. Consequently, transitions between the 4s,4p and 3d levels can easily take place and coordination is readily achieved. The manner inwhich ligand groups are oriented in surrounding the central metal atom is determined by thenumber and energy levels of the electrons in the incomplete subshells.

SO3Na

N

O

O

SO3Na

N

O O

NaO3S

N

O

O

FeNa+

5.8

CI Acid Green 1

ELECTRONIC STRUCTURE OF TRANSITION-METAL IONS

chpt5(1).pmd 15/11/02, 15:28235


The first transition series of elements with atomic number from 21 to 29 (scandium,titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper) is built up by theaddition of electrons to the outer 3d shell, which can accommodate a total of ten electronsin five subshells of equal energy, each capable of containing two electrons. The influence ofthe electrical fields originating from the ligand atoms as they approach the electrons of theseoutermost 3d, 4s and 4p shells is described by ligand field theory. On approach, hybridisationbetween available ligand electrons and metal electrons takes place as bonds are formed. It isthis and the strength of the ligand field that determines the final arrangement of the ligandsystems around the central metal atom.

For trivalent elements of CN6, such as chromium(III) or cobalt(III), the most stablespatial arrangement of six identical ligand groups is that of an octahedron with four of thegroups situated in equatorial quadrants and the other two directly above and below thecentral metal atom, as shown in Figure 5.2(a). It is possible to obtain a distorted octahedralor tetragonal configuration by an increase in the distance of separation of the ligand groupsin the pole positions of the octahedron. In the extreme, when the two pole positions areligand-free, this leads to a square planar arrangement. For divalent elements of CN4, such ascopper(II), there are two possibilities: a square planar distribution on the equatorial plane, asin Figure 5.2(b), or the tetrahedral configuration shown in Figure 5.2(c). In early objectionsto the electrostatic theory of bonding in metal complexes it was argued that the symmetricalarrangement of four ligands about a spherical central atom was best explained in terms of atetrahedral structure. It was not appreciated that in fact the d shell electrons of the metalatom are symmetrical, so that stable square planar arrangements are possible.

s p dLevels

Incr

easi

ng e

nerg

y

4p

4s

3s

2s

1s

3d

3p

2p

Figure 5.1 Relative energy levels up to atomic number 36

chpt5(1).pmd 15/11/02, 15:28236

237

5.3.1 Octahedral arrangement

As described above, in each transition-metal atom there are five d orbitals, each of whichcan accommodate two electrons. These orbitals are arranged spatially in two groups, oneacting between the main x-, y- and z-axes of the metal ion and the other acting along theseaxes. In an octahedral weak ligand field environment, the influence of the sixelectronegative ligand atoms is to cause the ligand electrons to avoid those regions wherethe charge density associated with the d electrons is greatest. This charge densitydistribution is shown in Figure 5.3. Each line of closest approach is therefore along a line ofminimum charge repulsion. For the dxy, dxz and dyz orbitals, this is along the main x-, y- andz-axes. The dx2–y2 and dz2 orbitals, on the other hand, have maximum charge densities alongthese main axes, Hence they conflict with the line of approach of the ligand atoms and aredestabilised. The five orbitals are thus split into two groups of different energy: an upperdoublet referred to as eg and a lower triplet level known as t2g.

The distribution of the d electrons between these various orbitals is determined by thedifferences in energy level between them. The energy difference is directly proportional tothe electric field strength of the ligand systems taking part in complex formation. Since thetransition-metal ions do not exist in isolation, they must be evaluated in association withcounter-ions or solvents such as water or other ligand systems present.

In isolation the electron distribution in the trivalent chromium(III) ion consists of threeunpaired electrons in the d shell, as indicated in line (a) of Table 5.1. In line (b) the sixelectron pairs donated to the central chromium atom by oxygen atoms of water moleculesgive rise to sp3d2 hybridisation. This is characteristic of an octahedral structure. A similarsituation arises with the trivalent cobalt(III) complex in line (e), where each of the three t2glevels is doubly occupied by an electron pair from each cyano ligand.

With the divalent chromium(II) ion there are two distinct possibilities, shown in lines (c)and (d). With such an isolated ion having four electrons in the d shell the fourth electronmay occupy either an eg level (c) or a t2g level (d), depending on whether the energyrequired to cause spin pairing is less or greater than the difference between these energy

(a)(c)

(b)

Figure 5.2 Ligand arrangements about a central nucleus


chpt5(1).pmd 15/11/02, 15:28237


x

y

z

dxy

x

y

z

dxz

x

y

z

dyz

x

y

z

dz2

x

y

z

dx2–y2

Figure 5.3 Atomic d orbitals in transition elements

levels. These alternatives result in a high-spin (c) or low-spin (d) complex respectively, thefield strength of the ligand systems determining which is actually formed. The unpairedelectrons in the chromium(II) ion are unstable, so divalent chromium cannot be utilised inpreparing dye complexes. Invariably a d2 electron is lost and oxidation to chromium(III)ensues, with concurrent reduction of the ligand chromogenic system [8]. The oxidationstate and electron distribution in the 3d shell at least partly accounts for the fact that mostmetal-complex dyes of value for wool or nylon dyeing are based on chromium(III) orcobalt(III) ions.

The presence of unpaired electrons in a complex can be detected by the study of itsmagnetic properties. A single spinning electron has an associated magnetic moment. Whenan organometallic complex is weighed in a magnetic field any unpaired electrons contributeto an apparent increase in mass compared with that observed in the absence of the field.The magnitude of attraction of the complex to a magnet is a measure of the content ofunpaired electrons. Such a complex is described as paramagnetic. Where there is nodetectable difference in mass, the complex contains no unpaired electrons and is said to bediamagnetic. Thus these results are of value in determining the electronic distributionaround the central metal atom.

5.3.2 Tetrahedral arrangement

Depending on the ligand field strength and the number of ligand systems that can beaccommodated, hybridisations other than octahedral are possible. The tetracyanocuprateion in line (f) of Table 5.1 has a tetrahedral configuration arising from sp3 hybridisation, as

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239

in the case of bonding to a saturated carbon atom. The copper(I) ion has a completely filledd shell and electron-pair donation from the four cyano ligands occurs in the 4s and 4p levels.In a tetrahedral arrangement, the eg orbitals are more stable than the t2g orbitals becausethey are situated at the maximum distance from the influence of the four ligand systems.Conversely, the t2g orbitals become destabilised. The ligands are best visualised as occupyingalternate corners of a cube, the main axes of which represent those of the metal ion, asshown in Figure 5.2(c). The energy difference between eg and t2g orbitals in a tetrahedralstructure is approximately half that for the octahedron. This effect favours formation of anoctahedral rather than a tetrahedral configuration.

5.3.3 Square planar arrangement

The square planar configuration illustrated in Figure 5.2(b) can be considered as anoctahedral structure in which any opposing pair of ligands on a common axis are completelyremoved from the sphere of influence of the central metal atom. If two such ligands aremoved outwards only slightly, the result is a distorted octagonal or tetragonal structure.

Table 5.1 Electron distribution in some transition-metal ions (1s, 2s, 2p, 3s and 3p shells are completein all cases)

Ion 3d 4s 4p

(a) Cr3+

(b) Cr(H2O)63+ sp3d2

(c) Cr2+ high spin

(d) Cr2+ low spin

(e) Co(CN)63– sp3d2

(f) Cu(CN)43– sp3

(g) Cu(NH3)2+ sp2d

denotes electron-pair donation to metal ion


chpt5(1).pmd 15/11/02, 15:28239


5.4 STRUCTURAL CHARACTERISTICS NECESSARY FOR COMPLEXFORMATION

5.4.1 Bidentate ligand systems

In a bidentate ligand system, three molecules of a dye containing either a terminal salicylicacid unit (as in 5.2) or an o-nitrosonaphthol residue are able to chelate simultaneously witha trivalent metal ion of CN6, such as chromium(III) or iron(III), to form a 1:3 metal–dyecomplex (as in 5.8). Historically, the most important bidentate ligand system was alizarin(5.1). It has been suggested that both hydroxy groups and the keto group in the peri positionare all involved with the metal atom in the chelation mechanism.

Most metal-complex dyes of commercial significance belong to the azo class. In practice,tridentate ligand systems incorporating the azo group and both o,o′-substituents are necessaryto achieve adequate stability and fastness. Each nitrogen atom in an azo (–N=N–) orazomethine (–CH=N–) grouping carries a lone electron pair that can participate incoordination. There is no evidence, however, that the azo group in unsubstituted azobenzene,even the cis isomer, can yield a stable complex with any first-series transition metal. On theother hand, an o-aminoazo or o-hydroxyazo grouping can act as a bidentate ligand system withthe azo group participating in coordination. Such 1:2 complexes containing divalent cobalt,nickel or copper were synthesised from o-substituted azobenzenes (5.9; M = Co, Ni or Cu, X= O or NH) and the analogous azomethine derivatives (5.10), demonstrating that the β-nitrogen of the azo linkage normally acts as the donor atom [9].

NN

X

NN

XM

5.9

CHN

X

CHN

XM

5.10

In general, metal complexes formed from bidentate azo chromogens are little used as dyesbut do find important applications as pigments (section 2.3.2). Rare exceptions exist,however, such as the nickel(II) complex of p-nitroaniline→BON acid (5.11). This has beenused for bordeaux prints of high light fastness on cotton fabrics. Two possible modes ofbidentate attachment to the nickel atom can be envisaged (Scheme 5.2).

Metallisation of terminal coupling components

Salicylic acid has been found useful as a terminal coupling component in direct or mordantdyes to provide a bidentate site for metallisation. The free acid is also capable of complexingand ammonium chromisalicylate (5.12) is used as an intermediate in the synthesis of metal–dye complexes. CI Mordant Orange 1 (5.3) and Yellow 5 (5.13) are examples of dyes

chpt5(1).pmd 15/11/02, 15:28240

241

NN

O2N

HO COOH

NN

O2N

O COOH

NN

O2N

O COOH

NN

O2N

O C

H

Ni

H2OOH2

O

ONi

H2O OH2

Ni (II) Ni (II)

5.11

H

Scheme 5.2

O

C O

O

O

CO

O

Cr

NH3

NH3

NH4.3H2O

5.12

+

_

Ammonium chromisalicylate

NN

HOOC

HOOH

COOH5.13

CI Mordant Yellow 5

NN

HO

SO3Na

H2NSO2

HO

HOOC

NaO3S

5.14

CI Mordant Red 26

STRUCTURAL CHARACTERISTICS NECESSARY FOR COMPLEX FORMATION

chpt5(1).pmd 15/11/02, 15:28241


containing terminal salicylic acid residues. Since both ligands in these residues areelectronically associated with the azo chromogen, changes in hue on metallisation can bedramatic. Difficulties in shade matching are particularly evident when afterchroming suchdyeings. This effect may be minimised by introducing an insulating group, such asmethylene, sulphonyl or sulphonamido, between the salicylic acid residue and thechromogen. CI Mordant Red 26 (5.14) is a typical example.

Another terminal bidentate ligand that has been exploited occasionally in bright disazodirect dyes is the sulphated 8-hydroxyquinoline residue (5.15). On aftercoppering, fastness tolight and wet tests is enhanced by hydrolysis catalysed by the copper(II) ion and formation of abidentate 1:2 complex (Scheme 5.3). Apparently, electron withdrawal by sulphur facilitatesremoval of the sulphite grouping and approach of the copper(II) cation [10].

The o-nitrosonaphthol ligand system also belongs to the terminal bidentate category (asin structure 5.8). If the sulpho group in each ligand is replaced by a nonionised polarsolubilising group, such as methylsulphonyl (as in 5.4) or sulphonamido (as in 5.6), theoverall negative charge on the 1:3 metal–dye complex is reduced from -4 to -1, so thatdyeing can take place from neutral or weakly acidic dyebaths. Similarly, green to olive huescan be obtained on wool or nylon using 1:3 iron(II) complexes of 6-hydroxy-7-nitroso-indazoles. The amphoteric nature of the three heterocyclic rings in such complexes accountsfor their solubility. Protonation of the N atom in each indazole ring (Scheme 5.4) dependson the pH, so that the charge on the complex can vary from -1 for the unprotonated form(5.16) to +2 for the fully protonated form (5.17). The use of iron(III) salts in their synthesisinitiates partial oxidation of the hydroxynitroso-indazole with consequent reduction toiron(II), which then interacts to form the 1:3 metal–dye complex [11].

5.4.2 Tridentate ligand systems

Many of the premetallised 1:1 chromium-dye complexes introduced from 1915 onwards forthe one-bath dyeing of wool at low pH were o,o′-dihydroxyazo compounds containingchelated chromium(III) ions. Typical examples still in use today include CI Acid Orange 74(5.18) and other azopyrazolones ranging in hue from yellow to bluish red, as well as

N

OSO3Na N

NaO3SO

N

O N

O

Cu

+

Cu (II)

2 Na2SO3 +

5.155.15

2Na+

Scheme 5.3

chpt5(1).pmd 15/11/02, 15:28242

243

azoacetoacetanilides such as CI Acid Yellow 99 (5.19). For some time after theirintroduction these dyes were formulated as bidentate systems, utilising only one of thehydroxy groups in conjunction with the azo group. This misconception may have arisen byanalogy with true bidentate ligands such as salicylate complexes [7] and the o-substitutedazo and azomethine analogues [9]. Only in 1939 were these 1:1 chromium complexes ofo,o′-dihydroxyazo dyes unequivocally confirmed to be tridentate structures, in which bothhydroxy groups and the azo linkage are coordinated with the metal atom. The remainingthree sites are taken up by unidentate ligands such as water molecules [12].

The tridentate azo systems have proved to be the most important of all possiblemultidentate chromogenic ligand systems available for metal-complex formation. Thosecapable of coordination with metals of CN6 (trivalent chromium or cobalt) or CN4(divalent copper) are mainly limited to o-hydroxy-o′-substituted diaryl azo or azomethinederivatives (5.20; X = OH, NH2, or COOH, Y = N or CH). The more important dyes forwool or nylon are chromium complexes in which X is usually a hydroxy group but may be anamino, carboxyl or other ligand group from which a proton is lost during complex formation.Coordination of these systems leads to a bicyclic ring system with the N→M bond shared. Ifthe o-substituent X is hydroxy or amino that ring is five-membered (5.21), but if it iscarboxyl both annelated rings are six-membered (5.22). This difference has an importantbearing on the physical properties of these complexes (section 5.6.1). Annelated-ringformation enhances the stability of the resultant metal–dye complex to an extent that is fullyadequate for conventional dyeing conditions. Single-ring bidentate systems, on the otherhand, seldom exhibit this degree of stability.

NHN

O

N O

Fe

NH

N

O

N

O

HN

N

O

N

O

NHHN

O

N O

Fe

NH

NH

O

N

O

HN

HN

O

N

O

5.17

3H

3HO

+_

– 2+

5.16

Scheme 5.4

NN

N

N

H3CO2N

NaO3S OO

Cr

OH2H2O

OH2

5.18

CI Acid Orange 74

NN C

C

C

OO2N

NaO3S OO

Cr

OH2H2O

OH2

CH3

NH

5.19

CI Acid Yellow 99


chpt5(1).pmd 15/11/02, 15:28243


YN

HOX

5.20

NN

OHN M

5.21

5

6 NN

OC

O

O M

5.22

6

6

1:1 Metal–dye complexes

As already described, these are tridentate structures in which the three remaining sites on thechromium(III) ion are occupied by colourless monodentate ligands (as in 5.18). These arequite stable at low pH values, whereas similar cobalt complexes are generally unstable.However, 1:1 cobalt-dye complexes have been obtained by heating a tridentate dye with acobalt(II) salt in the presence of excess ammonia, the monodentate ligand groups being threeammonia molecules. The reaction involved oxidation of the cobalt(II) ion to cobalt(III) priorto complex formation [13]. Even so, the tesulting complexes were insufficiently stable for useas dyes. Stable 1:1 cobalt-dye complexes suitable for dyeing can be prepared, however, byreplacing the colourless monodentate ligands by a tridentate molecule such asdiethylenetriamine (as in 5.23). By varying the number of sulpho groups in the chromogenicligand from zero to three, the net charge on the complex varies from +1 to –2.

NN

SO3O2N

O CoO

NH2

H2N NH

H2C CH2

CH2

CH2

O3S

K

5.23

_

+

The presence of sulphonic acid groups in 1:1 chromium or cobalt complexes enhancessolubility in water and permits electrostatic bonding to protonated amino groups in wool(section 3.2.2). Strongly acidic conditions are necessary to achieve high substantivity andsome degradation of the wool is unavoidable. The relatively low Mr (400–500) of these 1:1premetallised dyes ensures good level dyeing properties under these conditions. Thepresence of ionised sulpho groups in these complexes can give rise to anomalous conclusionswhen trying to elucidate structure from elemental analysis data, since simple metal salts mayalso be formed between transition-metal cations and these anionic sulpho groups.

1:2 Metal–dye complexes

The unfilled sites in 1:1 metal–dye complexes of chromium(III) or cobalt(III) ions can be

chpt5(1).pmd 15/11/02, 15:28244

245

occupied by colourless ligands (three mono-, mono-/bi- or one tridentate). Such a triad ofsites can generally be replaced by a second tridentate chromogen, as shown by Drew andFairbairn [12]. This results in a more stable configuration consisting of two bicyclic ringsystems as in the general formula 5.24 (M = Co or Cr, X = N or CH). Colour, solubility,dyeing and fastness properties can be controlled by selection from a wide range ofsulphonated or unsulphonated azo or azomethine tridentate ligand systems.

XN

OOM

NX

OO

56

56

5.24

NN

C

C

OOCo

NN C

C

OO

H3C

C

O

N

H

X

X

CH3

C

O

N

H

5.25

NN

OOCo

NN

OO

SO2

O2S

N

N

CH3H2N

NH2N

C

H

O

H3C

5.26

CI Acid Brown 240

Symmetrical 1:2 cobalt complexes of phenylazoacetoacetanilides, such as CI Acid Yellow119 (5.25; X = NO2) or Yellow 151 (5.25; X = SO2NH2), are important in the yellow toorange sector. Many orange and red dyes of this class are symmetrical 1:2 chromiumcomplexes of phenylazopyrazolones, like CI Acid Orange 60 (5.6; R = H). Browns are oftenunsymmetrical chromium or cobalt types with azopyrazolone and azonaphthol ligands, CIAcid Brown 240 (5.26) being a good example. Dyes in the violet, blue, green and blacksectors may be symmetrical or unsymmetrical but often contain two arylazonaphthol ligands,as in C.I.Acid Black 58 (5.4).

The reaction of two moles of a tridentate chromogen (A) with one equivalent of themetal M leads exclusively to the symmetrical AMA structure. Reaction with two differentchromogenic ligands A and B simultaneously yields a mixture of structures: AMA, AMBand BMB. Such mixtures are commercially important, as they are less expensive to producethan the specific unsymmetrical product AMB, which can be obtained in a pure form onlyby the reaction of a pure 1:1 complex (AM) with the other ligand (B). Monosulphonatedunsymmetrical chromium complexes with two negative charges per molecule are particularlyimportant because they have good neutral-dyeing affinity for wool or nylon.


chpt5(1).pmd 15/11/02, 15:28245


NNAr NNAr CH

NO2

N N ArCH3NO2Cl2 ++ _

Scheme 5.5

N

N N

NC

R

ArAr

N

NN

NC

R

Ar ArM

5.27

N

NC

N

N

O

Cu

N

5.28

CN

N N

N

O

CN

NN

N

O M

5.29

Tridentate formazans

Formazans may be synthesised by the selective coupling under alkaline conditions of twoidentical or different diazonium cations with an activated methylene group in a suitablecoupling component such as acetone or nitromethane, as shown in Scheme 5.5. Bycoordination with both of the two azo groupings, divalent metals such as copper, nickel orcobalt will give monocyclic 1:2 metal complexes (5.27) of relatively low stability [14].Formazans containing an o-hydroxy or o-carboxy substituent in one of the phenylazo groupingscan be used to give intensely coloured neutral bicyclic 1:1 metal–dye complexes [15]. Thus thered tridentate ligand shown becomes violet on metallisation with copper(II) ions. Oncompleting the CN4 by coordination with an aryl monodentate ligand such as pyridine (5.28),a further bathochromic shift to reddish blue is observed. With trivalent metals of CN6,tridentate formazans give 1:2 complexes with two bicyclic ring systems (5.29).

5.4.3 Quadridentate ligand systems

Quadridentate formazansIf both of the phenylazo groupings in the formazan chromogen contain an o-hydroxy or o-carboxy substituent, coordination with a divalent metal atom such as copper yields a highlystable annelated tricyclic ring system (5.30). Only a 1:1 metal:dye ratio is possible in thiscase. For trivalent metals of CN6 the coordination sphere is completed using a colourlessligand molecule such as pyridine, ammonia or water. Solubility and dyeing properties may bevaried by the introduction of a sulpho group or an uncharged solubilising group such assulphonamido. In a recent evaluation of a series of quadridentate formazan complexes with

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247

copper(II) of the 5.30 type, the colours of the products obtained from a given formazan werefound to vary according to the method of synthesis [16].

Similar blue formazan–copper complexes that are sufficiently solubilised by theintroduction of sulphonic acid substituents in the phenyl rings have become established asimportant members of most ranges of reactive dyes. In general, they are redder than theturquoise blue copper phthalocyanines, less costly than the anthraquinone blues and exhibithigher fastness to light than the twice-coupled H acid blues. Many of them have the generalstructure represented by 5.31. Those that react with cellulose by nucleophilic additionmostly have the substituents X = vinylaulphone or precursor, Y = sulpho, whereas thosereacting by a nucleophilic substitution mechanism have X = sulpho, Y = NH–Z, where Z isthe haloheterocyclic reactive system.

CN

N

M

N

N

O CO

O

6

5 6

5.30

CN

N

Cu

N

N

O CO

OY

X

SO3Na

5.31

Phthalocyanine metal-complex colorants

Copper phthalocyanine is another important representative of a quadridentate ligandsystem. It is predominant as a turquoise blue pigment of outstanding light fastness and itshalogenated derivatives are important green pigments (section 2.4). The copperphthalocyanine chromogen is a tetracyclic annelated ring system of exceptionally highkinetic and thermal stability. It can be sublimed without decomposition in vacuo attemperatures as high as 550 °C, although the solid form may undergo polymorphic change at250 °C under atmospheric conditions. Many other transition metals and rare-earth elementsare able to form 1:1 complexes with phthalocyanine but the brilliant colour and outstandingstability of the copper complex ensure its predominance. A series of 3- and 4-hydroxylatedphthalocyanine complexes of tri- or tetravalent metals (A1, Cr, Ge, In and Sn) has beenprepared recently. The stabilities of these complexes to thermal oxidation in air have beencompared and the effects of the position of the hydroxy group on the electronic spectra invarious solvents were studied [17].

Copper phthalocyanine derivatives are well established as turquoise blue direct andreactive dyes for cellulosic fibres. Chlorosulphonation at the 3-position, followed byhydrolysis, yields sulphonated direct dyes such as CI Direct Blue 86 (5.32; X = H) and Blue87 (5.32; X = SO3Na). Solubility and dyeing properties can be varied by introducing fourchlorosulphonyl groups, some of which are hydrolysed and some converted to sulphonamideby reaction with ammonia or alkylamines. This approach is also the main route to reactivedyes of the copper phthalocyanine type. The reactive system Z is linked to a 3-sulphonyl site


chpt5(1).pmd 15/11/02, 15:28247


by means of an aryldiamine, as in structure 5.33 where Y = H or SO3Na, and the dyeingcharacteristics varied by the nature of X, which may be NH2 or ONa. Nickel phthalocyanineis the basis of some brilliant green reactive dyes, into which a similar range of 4-sulphonylsubstituents can be introduced.

N

NN

NN

N

N

N

Cu

NaO3S

X

SO3Na

5.32

N

NN

NN

N

N

N

Cu

SO2

O2S

O2S

X

X

X

SO2

HN Y

HN Z

5.33

In an interesting study, phthalocyanine complexes containing four anthraquinone nuclei(5.34) were synthesised and evaluated as potential vat dyes and pigments [18].Anthraquinone-1,2-dicarbonitrile or the corresponding dicarboxylic anhydride was reactedwith a transition-metal salt, namely vanadium, chromium, iron, cobalt, nickel, copper, tin,platinum or lead (Scheme 5.6). Substituted analogues were also prepared from amino,chloro or nitro derivatives of anthraquinone-1,2-dicarboxylic anhydride.

5.5 PREPARATION OF METAL-COMPLEX COLORANTS

There are numerous ways in which a metal atom can be incorporated into a dye molecule toform a metal complex. Most attention has been given to complexes of chromium, cobalt andcopper. There are essential differences in the conditions of preparation of these compounds.

5.5.1 Chromium complexes

Table 5.1 in section 5.3.1 reveals that the trivalent hexa-aquo chromium complex cationpossesses three low-energy t2g orbitals, each singly occupied. Consequently, the ligand fieldstabilisation energy is considerable. Since this energy has to be overcome, replacement ofthe coordinated water molecules by other ligand groups is a slow process. The mostsuccessful practical methods are designed to take this into account.

The replacement reactions may be represented generally by a two-stage process.Introduction of the first tridentate monoazo dye ligand D2– is the rate-determining step(Scheme 5.7). Where a typical o,o′-dihydroxyazo dye ligand is used in excess with a hydratedchromium(III) salt under alkaline conditions (pH >9) favouring 1:2 metal–dye complexformation, none of the 1:1 complex remains. This indicates that the presence of one

chpt5(1).pmd 15/11/02, 15:28248

249

chelated dye ligand favours attachment of the other and the forward reaction in Scheme 5.8is more rapid than that in Scheme 5.7. In order to obtain a 1:1 complex in high yield,therefore, it is necessary first to synthesise the 1:2 complex in alkaline medium and then totreat this product under strongly acidic conditions. The reverse reaction in Scheme 5.8 thenyields the desired 1:1 complex. The chromium salt used can be the acetate, chloride orsulphate, the reactions being carried out in aqueous media at the boil.

Various chromium-complex dyes were prepared recently by reacting ammoniumchromium sulphate with a series of chelatable o,o′-dihydroxyazopyridone structures (Scheme5.9). Elemental analyses corresponded to a 1:2 metal–dye ligand ratio (5.35). The

N

NN

NN

N

N

N

M

O

O

O

O

O

O

O

O

O

O

O

O

CN

CN C

OCO

O

+ Metal (M) saltor4 4

5.34Scheme 5.6

D2– [Cr(H2O)6]3++slow

[D.Cr(H2O)3]+ + 3H2O

Scheme 5.7

HClD2–+[D.Cr(H2O)3]+ + 3H2O[D.Cr.D]–

rapid

Scheme 5.8

PREPARATION OF METAL-COMPLEX COLORANTS

chpt5(1).pmd 15/11/02, 15:28249


NN

N

O

C

O

NH2

RO

H3C

OH

H

NNN

OOCr

NN

N

OO

O

R

C

O

H2N CH3

H3C C

O

NH2

O

R

2+ [Cr(NH3)6]2 [SO4]3

5.35Scheme 5.9

substituted azopyridone chromogens are more stable in the ketohydrazone form. Thedyeability and fastness properties of these unmetallised yellow mordant dyes and theirderived chromium complexes were evaluated for the dyeing of wool and polyester/woolblends [l9].

If the metallisable dye is insoluble in water, a miscible solvent such as ethanol or ethyleneglycol may be added. Polar solvents such as formamide or molten urea have sometimes beenpreferred. It is likely that such solvents will preferentially displace water molecules andcoordinate with the chromium(III) ion as the first step in the reaction. If colourless organicchelates of chromium, such as those derived from oxalic or tartaric acid, are used instead ofor in addition to hydrated chromium(III) salts, the difficulty of replacing the stronglycoordinated water molecules in the first stage of the reaction is eliminated. In this way theinitial reaction can be carried out at high pH without contamination by the precipitation ofchromium hydroxide. Use of the complex ammonium chromisalicylate (5.12) in thisconnection should also be noted (section 5.4.1).

5.5.2 Cobalt complexes

Like trivalent chromium the hexa-aquo cobalt(III) cation contains three low-energy t2gorbitals, this time each doubly occupied. In general, trivalent cobalt complexes are easier toprepare than their chromium analogues and often exhibit significantly higher fastness tolight. Chromium complexes normally absorb at longer wavelengths than the correspondingcobalt compounds, however. This bathochromic shift is evident from comparisons between,for example, either CI Acid Violet 128 (5.36; M = Co, X = NO2) and Blue 335 (5.36; M =Cr, X = H), or Brown 403 (5.37; M = Co) and Black 172 (5.37; M = Cr).

Direct use of cobalt(III) salts in the synthesis of metal–dye complexes is generally avoidedbecause the cobalt(III) ion is strongly oxidising and slow to react with dye ligands. Most 1:2cobalt-dye complexes are prepared by the reaction between the chelatable dye and acobalt(II) salt at a relatively high pH. The dominant product is invariably the diamagneticcobalt(III) complex, oxidation of the cobalt(II) ion taking place by reduction of some of theazo dye molecules present. This can be avoided, however, by adding a less stable oxidant.

Unlike the corresponding 1:1 chromium complexes, the stability of 1:1 cobalt-dyecomplexes is inadequate for them to be obtained in acceptable yield at low pH. However,advantage can be taken of the strong affinity for cobalt ions of ammonia or amines as

chpt5(1).pmd 15/11/02, 15:28250

251

electron donors. Reaction of the cobalt(II) ion with a tridentate dye molecule in an excess ofaqueous ammonia yields a 1:1 complex in which the coordination sphere is completed bythree ammonia molecules. This again is diamagnetic, indicating the absence of unpairedelectrons necessary to achieve an sp3d2 octahedral structure. Although 1:1 cobalt-dyecomplexes have little significance as potential premetallised acid dyes for wool or nylon, theyare widely used as intermediates in the synthesis of unsymmetrical 1:2 cobalt-dye complexes,especially when the asymmetry arises from the presence of a sulphonic acid group in one ofthe chelatable dye ligands.

The dyeing and fastness properties of chromium and cobalt symmetrical 1:2 complexes(5.38) of the mordant dye 2-amino-4-methylphenol→2-naphthol were compared recentlyon nylon 6 and wool fabrics [20]. The dyebath exhaustion and fastness performance wereconsistently higher on nylon than on wool. The chromium-complex dyeings were slightlymore bathochromic, especially on wool, than those of the cobalt complex but the lattershowed marginally higher fastness to washing on wool. Fastness to wet rubbing on woolfavoured the chromium complex, however (Table 5.2).

NN

OOM

NN

OO

NaO3S

Cl

X

5.36

NN

OOM

NN

OO

NaO3S

O2N

NO2

SO3Na

5.37

NN

OOM

NN

OO

CH3

H3C

5.38


chpt5(1).pmd 15/11/02, 15:28251


Table 5.2 Dyeing and fastness properties of 1:2 metal-complex dyes on nylon 6 and wool [20]

Fastness properties

Exhaustion (%)

Dye λmax (nm)Washing Rubbing

Complex Nylon 6 Wool on Light5.38 (80 °C) (90 °C) Fibre (C arc) E C W Dry Wet

M = Cr 93 569 >5 5 5 5 5 5M = Co 93 565 >5 5 5 5 5 5M = Cr 88 572 5 3–4 4 4 4–5 4M = Co 88 551 5 4 5 4–5 4–5 3

E Effect on patternC Cotton stainW Wool stain

Chromium and cobalt complexes of azoic dyes

Complexes of chromium(III) and cobalt(III) ions with insoluble azoic dyes, such as that (5.39)prepared by coupling Naphtol AS (CI Azoic Coupling Component 2) with diazotised picramicacid (Scheme 5.10), were synthesised recently for use as negative charge control agents inphotocopying [21–23]. The aggregation behaviour and tautomerism of structure 5.39 as thefree acid (X = H) and the ionised phenolate (X = Na) were studied by UV/visiblespectrophotometry in polar solvents. Monomer–dimer equilibria were detected indimethylformamide and in dimethylsulphoxide. Analysis of the deconvoluted absorption bandsattributed the spectral changes to a distribution in favour of dimerised aggregates in theketohydrazone form, with the monomeric species being more stable in the azo form (5.39).

The coupling of Naphtol AS or its phenyl-substituted derivatives with diazonium saltsfrom variously substituted anilines in aqueous alkaline solution (section 4.11) gaveincomplete reactions and impure products in some instances, probably because thesecoupling components have inadequate solubility in aqueous media. Pure dyes in ca. 90%yields were obtained by reaction in dimethylformamide in the presence of sodium acetate.Metallisation of these o,o′-dihydroxyazo ligands with sodium chromium salicylate or acobalt(II) salt gave metal-complex dyes in 80–100% yields [22]. Specific structural isomersof these complexes were identified by i.r., n.m.r., Raman and UV/visible spectroscopy [23].

N N

OO2N

O2N

X HO C

O

N

H

NN

O2N O X

O2N

HO C

O

N

HCl +

5.39

_+

Scheme 5.10

chpt5(1).pmd 15/11/02, 15:28252

253

Mixed chromium/cobalt complexes in reactive dyes

An important subclass of black reactive dyes for the dyeing of cellulosic fibres consists ofsymmetrical 1:2 metal–dye complexes in which the metal content is a deliberate mixture ofchromium(III) and cobalt(III) ions, usually rich in the chromium component. The two mostimportant structural types are the nitrophenylazo H acid complex 5.40 and thenitronaphthylazo J acid complex 5.41. In both cases, each reactive system Z is linked to theNH site in each coupling component.

NN

OOM

NN

OO

NO2

O2N

SO3Na

SO3Na

NaO3S

NaO3S

NHZ

HNZ

5.40

NN

OOM

NN

OO

SO3Na

NH

NaO3S

NH

NO2

SO3Na

Z

Z

O2N

NaO3S

5.41

5.5.3 Copper complexes

The only copper complexes of tridentate azo compounds are 1:1 structures, since copper(II)has a CN of 4. They can be prepared by the reaction of the azo compound with a copper(II)salt in an aqueous medium at 60 °C. The major application for copper-complex azo dyes isas direct or reactive dyes for the dyeing of cellulosic fibres. They are seldom developed foruse on wool or nylon, although various orange and red 1:1 copper-complex azopyrazolones(5.42) were synthesised recently and evaluated on these fibres by application from a weaklyacidic dyebath [24].

Copper-complex azo direct dyes

Almost all of these depend on the availability of two o,o′-dihydroxyazo ligands in theunmetallised direct dye molecule, although these may be present as the o-methoxy-o′-hydroxyazo precursor system. In the manufacture of these dyes it is often easier to synthesise


chpt5(1).pmd 15/11/02, 15:28253


NN

N

N

H3C

NSO2

H

OO Cu

OH2

5.42

the latter grouping because the o-methoxyarylamines are diazotised and coupled moreefficiently than the corresponding o-hydroxyarylamines. Metallisation of the chromogen canbe achieved using cuprammonium sulphate in the presence of an alkanolamine such asdiethanolamine. During this reaction the methoxy groups present become demethylated. Itis probable that coordination occurs between the copper ion and the electron-rich oxygen ofthe methoxy group. Simultaneously or subsequently the methyl group is lost as a carboniumion that reacts immediately with hydroxide ion to form methanol, as in Scheme 5.11.Removal of the methyl group is probably assisted by coordination of the methoxy oxygen.

NN

OO Cu

H3COH2

NN

OO Cu

OH2

HOCH3OH+

_

Scheme 5.11

Many of the premetallised direct dyes are symmetrical structures in the form of bis-1:1complexes with two copper(II) ions per disazo dye molecule. Scheme 5.12 illustratesconversion of the important unmetallised royal blue CI Direct Blue 15 (5.43), derived fromtetrazotised dianisidine coupled with two moles of H acid, to its much greener copper-complex Blue 218 (5.44) with demethylation of the methoxy groups as described above.Important symmetrical red disazo structures of high light fastness, such as CI Direct Red 83(5.45), contain two J acid residues linked via their imino groups. Unsymmetrical disazo bluesderived from dianisidine often contain a J acid residue as one ligand and a different coupleras the other, such as Oxy Koch acid in CI Direct Blue 77 (5.46), for example.

Copper-complex azo reactive dyes

In contrast to direct dyes, metal-complex azo reactive dyes are almost always monoazochromogens coordinated to one copper(II) ion per molecule. The important structural typesinclude phenylazo J acid reds (5.47), phenylazo H acid violets (5.48) and naphthylazo H acidblues (5.49), where Z represents the reactive system attached through the imino group inthe coupling component. Less often the reactive system is located on the diazo component,as in CI Reactive Violet 5 (5.50) and analogous red to blue members of various ranges.

chpt5(1).pmd 15/11/02, 15:28254

255

NN

NN

OO HH

H2NNH2 O

H3C

O

CH3

NaO3S SO3Na

NaO3SSO3Na

NN

NN

OO

H2NNH2 OO

NaO3S SO3Na

NaO3SSO3Na

Cu Cu

H2O OH2

5.43

2 Cu(II) + 2 HO

5.44

2 CH3OH

+

_

CI Direct Blue 15

CI Direct Blue 218

Scheme 5.12

NN

N

NaO3S

OO

NaO3S

NN

N

SO3Na

OO

SO3Na

C

O

HH

Cu Cu

OH2 H2O

5.45

CI Direct Red 83

NN

NN

OO

SO2 OO

NaO3S NH

NaO3SSO3Na

Cu Cu

H2O OH2NaO

5.46

CI Direct Blue 77


chpt5(1).pmd 15/11/02, 15:28255


NN

NaO3S

NH Z

OO Cu

OH2

NaO3S

5.47

NN

NaO3S

SO3Na

OO Cu

OH2

NaO3S

HN

Z

5.48

NN

NaO3S

SO3Na

OO Cu

OH2

NaO3S

HN

ZSO2

NaO

5.49 NN

NaO3S

SO3Na

OO Cu

OH2

HN

COCH3

NaO3SOCH2CH2SO2

5.50

CI Reactive Violet 5

Oxidative coppering of monohydroxyazo ligands

This is an alternative method of introducing copper into an o-hydroxyazo dye structure. Theazo compound is treated with a copper(II) salt and an oxidant in an aqueous medium at 40–70 °C and pH 4.5–7.0. Sodium peroxide, sodium perborate, hydrogen peroxide or other saltsof peroxy acids may be used as oxidants, the function of which is to introduce a secondhydroxy group in the o′-position [25]. This process is reminiscent of earlier work on CI AcidRed 14 (5.51; X = H), an o-hydroxyazo dye that will not react with a chromium(III) salt toform a 1:1 complex but will do so by oxidation with an acidified dichromate solution. Thisoxidation product was later found to be identical with that obtained by conventionalreaction of CI Mordant Black 3 (5.51; X = OH) with a chromium(III) salt [7].

Replacement of labile o-halogeno substituents

The halogen atom in an o-halogeno-o′-hydroxyazo compound may be replaced by a hydroxygroup under mildly alkaline conditions, provided that the halogeno substituent is activatedby the presence of electron-withdrawing groups (acetyl, cyano, nitro) in the o- and/or p-positions [26]. The mechanism is believed to involve formation of an intermediate complex(5.52; R = electron-withdrawing substituent) of low stability in which chlorine iscoordinated with the copper atom [27]. This facilitates attack by hydroxide ion at the

chpt5(1).pmd 15/11/02, 15:28256

257

carbon atom to which the chlorine is attached, leading to the formation of a conventionalbicyclic 1:1 complex, as in Scheme 5.13.

5.5.4 Iron complexes

With the exception of the 1:3 iron complexes of bidentate ligand systems, for example the o-nitrosonaphthols (such as 5.8 in section 5.2.1), there has been little interest shown iniron(III) complexes of azo dye ligands as textile dyes until quite recently. Difficulties arisingfrom the presence of chromium residues in effluents from factories involved in themanufacture or use of premetallised dyes have stimulated research on complexes of othertransition metals, notably iron(III) which is unlikely to give rise to significant effluentproblems because of a much higher permitted level in effluent than that for chromium.Certain black iron-complex dyes are already claimed to be strong candidates for use on woolor nylon in carpets, furnishings and automotive fabrics where high fastness to light isessential [28].

The same naphthylazo-2-naphthol ligand grouping is present in the 1:1 complex CI AcidBlack 52 (5.53; M = Cr) and its symmetrical 1:2 analogue Black 172 (5.54; M = Cr), whichare both widely used in the dyeing of wool. The corresponding 1:1 and 1:2 complexes oftrivalent iron (M = Fe) have been synthesised and their properties compared with the

NN

HOClR

R

NN

OClR

R

Cu

OH2

NN

OOR

R

Cu

OH2HO

Cu(II)

5.52

_

NN

O

SO3Na

NaO3S

X

H

5.51

Scheme 5.13


chpt5(1).pmd 15/11/02, 15:28257


existing chromium blacks. Excellent light fastness was found but shade differences wereobserved that would prevent direct replacement in existing recipe formulations [29]. In afurther evaluation of this kind, the symmetrical 1:2 iron complexes (5.55; M = Fe)analogous to CI Acid Violet 83 (5.55; M = Cr, X = H) and Black 99 (5.55; M = Cr, X =NHCOCH3) were synthesised by reacting the metal-free ligands with iron(III) sulphate.Black dyeings on wool and nylon based on these iron complexes exhibited ratings of fastnessto light and washing that were similar to those of control dyeings using the conventionalchromium-complex dyes [30].

This work has been extended to the synthesis and evaluation [31,32] of various 1:2iron(III) complexes of tridentate formazan ligands containing two bicyclic ring systems (as5.29 in section 5.4.2). In an interesting study of the photodegradation of dyes of this kind,their light fastness ratings on nylon and their rates of photofading in dimethylformamidesolution were determined. The light fastness of the three unmetallised formazans (X = H,Cl or NO2) was improved by metallisation, the 1:2 cobalt(III) complexes (5.56; M = Co)

NN

O M

OH2H2O

OH2O

O2N

NaO3S

5.53

NN

OOM

NN

OO

X

X

O2S

NH2

SO2

H2N

5.55

CN

N N

N

O

CN

NN

N

O M

X

X SO2NH2

H2NO2S

5.56

NN

OOM

NN

OO

SO3Na

NaO3S

O2N

NO2

5.54

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259

giving consistently higher ratings than the 1:2 iron(III) analogues (5.56; M = Fe). Thesecobalt formazans, unlike their iron counterparts, were found to be effective quenchers ofsinglet oxygen. The presence of an electron-withdrawing substituent in the o-hydroxy-phenylazo grouping also enhanced photostability (H < Cl < NO2), the highest light fastnessbeing shown by the nitro-substituted cobalt complex [33].

More recently, attention has turned to the aftertreatment of commercially availablemordant dyes on wool with iron(II) and iron(III) salts as a potential source reductionapproach to eliminating chromium ions from dyebath effluent [34]. The anticipatedimprovements in fastness performance were achieved. The structures of the conventional1:2 iron-dye complexes formed on the wool fibres were characterised by negative-ion fast-atom bombardment spectroscopy and HPLC analysis [35].

Symmetrical premetallised 1:2 metal–dye complexes of unsulphonated monoazostructures with aluminium (5.57) or trivalent iron (5.58) have been patented recently foruse as solvent dyes [36]. They contain a polar methoxypropylaminosulphone grouping ineach diazo component and are marketed as alkylamine salts. It remains to be seen, however,whether a full colour gamut of bright aluminium and iron complex dyes can be discoveredwith light fastness performance equivalent to that of currently available chromium andcobalt complex dyes.

NN C

C CH3

NHC

O

O

O

(OCH3)nCH3OC3H6NHSO2

NNC

CH3C

C

O

O

O Al

SO2NHC3H6OCH3

(H3CO)n

C

H2C

CH

CH2

C

NH2

OH

CH3

H3C

H3C

CH3

SH

5.57

+

_

NN

OO

NN

OO Fe

CH3OC3H6NHSO2

SO2NHC3H6OCH3

H3NCH2 CH

CH2CH2CH2CH3

CH2CH3

5.58

+

_


chpt5(1).pmd 15/11/02, 15:28259


5.6 ISOMERISM IN METAL-COMPLEX DYES

5.6.1 Stereoisomerism

The three-dimensional character of metal-complex structures allows the possibility that theycan form stereoisomers. Thus all structures represented in this chapter should be regarded asa convenient simplification. As already noted, metals of CN4 can participate in eithertetrahedral or planar arrangements (section 5.3). Two symmetrical and identical bidentateligands surrounding a metal atom in a tetrahedral configuration is an entirely unambiguousstructure. A pair of unsymmetrical bidentate systems, however, can exist as enantiomerspossessing non-superimposable mirror images and exhibiting optical isomerism. Likewise,equatorial planar arrangements of unsymmetrical bidentate entities, such as copper 8-hydroxyquinolate (5.59), exist in cis and trans forms (Scheme 5.14). Copper salicylate 1:2complexes (5.60) exhibit only the trans planar configuration. Apparently the cis isomer is toounstable to be isolated in this instance.

N

O

Cu N

O

N

O

Cu N

O

cis trans

5.59

Copper 8-hydroxyquinolateScheme 5.14

C

O

O

O

C

O

O

O

Cu

5.60

Copper salicylate

In those 1:2 complexes of major commercial importance as premetallised dyes containingtwo unsymmetrical tridentate ligands coordinated to a trivalent metal of CN6 such aschromium(III) or cobalt(III), the possible number of stereoisomers becomes much greater.The requirement that the tridentate dye molecules retain a high degree of coplanarity informing the complex, however, does restrict this number. As early as 1939 it was concluded[12] that in symmetrical 1:2 complexes of o,o′-dihydroxyazo dyes with chromium(III) themajor planes of the two dye molecules are arranged about the central atom in a mutuallyperpendicular fashion. This spatial structure later became known as the Drew–Pfitznermodel or a meridial complex [6]. Such an arrangement is unambiguous and these complexesdo not exhibit isomerism.

In 1941 two different isomers of the 1:2 chromium complex of an o-carboxy-o′-hydroxyazo dye were isolated. It was concluded that the three donor ligand atoms of each

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261

dye molecule were situated at the apices of an equilateral triangle forming the face of aregular octahedron [37]. During the early 1960s the existence of such isomers was elegantlyconfirmed in an extensive series of papers [38,39]. In meridial complexes derived from o,o′-dihydroxyazo dyes the angle subtended by the three coordinating groups is a right angle andis best fitted by the Drew–Pfitzner model. On metallisation such dye ligands form a 5,6′-membered bicyclic ring system (5.24 in section 5.4.2). In contrast, o-carboxy-o′-hydroxyazoligands form a 6,6′-membered ring system (5.22), with an angle of 60 degrees between thethree coordinating groups. Such a configuration can be accommodated more fittingly at thecorners of the equilateral faces of the octahedron. This so-called Pfeiffer–Schetty ‘sandwich’model or facial complex exhibits a minimum of three isomeric forms, each existing as anenantiomeric mirror-image pair.

Further studies were extended to o-amino-o′-hydroxyazo ligands and their azomethineanalogues, as well as o-carboxy- and o-hydroxyformazan complexes [40], fully confirmingthat all diarylazo chromogens yielding 5,6′-membered annelated rings adopt the meridialconfiguration, whereas those forming a 6,6′-membered ring system give isomers of the facialtype. An elegant confirmation of the latter arrangement has been obtained by X-raycrystallography [41]. Facial stereoisomers may differ in solubility or absorption spectra andcan be separated by chromatographic techniques. The activation energy of conversion fromone form to another is low, however, so that a solution of one isomer rapidly transforms intoan equilibrium mixture containing several components. Attempts to demonstrate differencesin dyeing properties between facial isomers are difficult to assess. Nevertheless, facialchromium isomers of o-carboxyazopyrazolone dyes have been claimed to show significantlyinferior wet fastness but marginally higher light fastness on wool than analogous meridialstructures [42].

5.6.2 Nα/Nβ isomerism

A different type of isomerism in metal-complex azo dyes, originally defined by Zollinger [43],has become known as Nα/Nβ isomerism. It is found in 1:1 complexes of copper(II) ornickel(II), as well as 1:2 complexes of chromium(III) and cobalt(III) ions, provided that theo,o′-dihydroxyazo ligands are unsymmetrical. There are two isomers (α and β) of thebicyclic 1:1 complexes but the doubly bicyclic 1:2 complex structures can form three,namely the α,α (5.61), α,β (5.62) and β,β (5.63) configurations. If group R is alkyl,differences between the proton magnetic resonance signals associated with the alkyl groupprotons are dependent on the relative positions of the R groups and the N atoms thatcoordinate with the central metal atom M. From these data and corresponding results forazomethine analogues in which the coordinating N atoms can be identified unambiguously,the proportions of the three isomers present in a mixture can be calculated [44].

STABILITY OF METAL-COMPLEX DYES

5.7 STABILITY OF METAL-COMPLEX DYES

5.7.1 Factors governing the stability of metal complexes

The formation of any metal complex is a reversible reaction and at equilibrium the complexis always partially dissociated into its ligand (L) and metal ion (M) components (Scheme5.15). The thermodynamic stability constant (K) is a measure of the extent of this

chpt5(1).pmd 15/11/02, 15:28261


dissociation. The simplest general definition of K is as the ratio of the factor of theconcentrations of the end-products to that of the starting materials, as in Equation 5.1where the square brackets denote concentration terms such as mol/l. In dilute solutiondissolved solutes show an approximation to ideal behaviour, but at higher concentrationsactivity coefficients should also be included. If the complex L–M is thermodynamicallystable, only traces of the components L and M remain at equilibrium and K1:1 has a highvalue.

11:1

1

[L M]K

[L ][M]�

� (5.1)

NN

R

OO

M

NN

OO

R

5.61

NN

OO

M

R

NNR

OO

5.62

NN

OO

M

NN

OO

R

R

5.63

Scheme 5.15ML1

K1L1 + M

Formation of a 1:1 dye–ligand complex, as in Scheme 5.15, is the simplest case. Furtherreaction to give a 1:2 complex L–M–L (Scheme 5.16) involves a second equilibriumconstant K2 related to the concentrations of the 1:1 and 1:2 complexes present atequilibrium (Equation 5.2). If the two ligands are different, the stability constant of the 1:2complex (K1:2) is the product of K1 and K2 (Equation 5.3). In the special but widely

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263

prevalent case of a symmetrical 1:2 complex, the K1:2 value is related to the product of themetal ion concentration and the square of the ligand concentration (Equation 5.4). Inpractice, premetallised 1:2 dyes have K values of 1015 or greater.

1 22

1 2

[L M L ]K

[L M][L ]� �

�� (5.2)

Scheme 5.16


ML1 L1 M L2

K2+ L2

1 21:2 1 2

1 2

[L M L ]K K K

[L ][L ] [M]� �

� � (5.3)

1:2 2[L M L]

symm. K[L] [M]

� �� (5.4)

As already discussed (section 5.4.2), factors influencing the stability of metal-complexstructures include the size and number of annelated ring systems formed on metallisation.Chelates with five- and six-membered rings are more stable than complexes formed fromterminal salicylic acid residues that contain only a single coordinated ring. Tridentate ligandsyielding 6,7-annelated ring systems, such as those formed by o,o′-dicarboxyazo groupings,show stabilities closer to those of bidentate ligand systems.

Metal-complex stability is also related to the basic strength of the ligand entity. For aseries of 1:2 complexes of the bidentate naphthylazophenol ligand (5.64) with copper(II)ion, the acidic dissociation constants (pKa) are linearly related to the stability constants (logK1:2), the more acidic groups forming the less stable complexes. Thus where X = NO2 instructure 5.64 then pKa = 8.1 and log K1:2 = 17.2, and where X = OCH3 then pKa = 8.5

NN

O

Cu

NN

O

SO3Na

NaO3S

X

X

5.64

chpt5(1).pmd 15/11/02, 15:28263


and log K1:2 = 19.2. This relationship is only valid, however, for a series of strictly relatedligand groups and is dependent on the nature of the solvent [45].

In a similar investigation of the tautomeric tridentate ligand 2′-hydroxyphenylazo-2-naphthol (5.65 in Scheme 5.17), the first and second acidic dissociation constants (pKa)related to the two hydroxy groups in the parent structure (X = H) were found to be 11.0and 13.75 respectively. On introduction of an electron-withdrawing substituent (X) the firstdissociation constant decreased from 11.0 to 10.55 (X = Cl) or 7.67 (X = NO2). Thestability constants (log K1:1) of the derived 1:1 complexes were dependent on the metal ionintroduced [46], being particularly high for nickel(II) at 19.6 and copper(II) at 23.3.

NN

HOOH

X

NN

OOH

X

H

5.65

Hydroxyazo Ketohydrazone

Scheme 5.17

The nature of the donor atom in a coordinating group plays a significant part in theresultant stability of the metal complex. As already mentioned (section 5.5.2), stable 1:1cobalt(III) complexes can be obtained when the coordination sphere is completed by threenitrogen atoms. Iron(III) atoms tend to form more stable complexes with oxygen donoratoms, as in the 1:3 o-nitrosonaphthol structures. Chromium(III) complexes, on the otherhand, show equal stability with either nitrogen or oxygen donors, which helps to explaintheir ubiquitous versatility. A series of o-substituted-o′-hydroxyazo ligands was derived fromthe same pyrazolone coupling component, where the o-substituent was methoxy, methylthioor dimethylamino. It was shown that even though the acidic dissociation constants of themethoxy- and methylthio-substituted ligands were practically identical, those metalcomplexes containing sulphur donors exhibited greater stability constants than thosecontaining oxygen donors [47].

5.7.2 Instability of metal-complex dyes during textile processing

The sensitivity of premetallised acid dyes to the presence of traces of other metal ions duringdyeing and finishing is attributable to demetallisation of the complex, leading to markedchanges in colour, dyeing properties and fastness to light or other agencies. The chromium-complex CI Acid Brown 360, of interest specifically for the dyeing of leather, is sensitive tothe presence of copper(II) and nickel(II) ions even at ambient temperature [48].Ultrafiltration can be applied to distinguish between the chelated metal colorants and freecopper, chromium or nickel ions in textile effluents. Chromium transfer through themembrane is the least efficient [49]. Proposed ecological criteria for the EU ecolabel withrespect to metal-complex dye effluents are ≤50 mg/kg Cr and ≤75 mg/kg Cu or Ni [50].

It is well known that transition-metal ions such as copper(II) or iron(III) will catalyse the

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265

decomposition of hydrogen peroxide in solution to form excessive concentrations ofperhydroxide (HOO–) anions. If colour-woven cotton fabrics containing undyed yarns aswell as yarns dyed with copper-complex reactive dyes, for example, are subjected to a post-bleaching treatment with alkaline peroxide, copper ions desorbed from the dyed materialmay catalyse oxidative degradation of the cotton. Addition of salts of ethylenediaminetetra-acetic acid (EDTA) to the bleach liquor is an effective means of sequestering these releasedions (5.66) and hence suppressing the damage and discoloration of the substrate [51].

O

NCH2CH2N

O

O

O

O

O

O

O

Cu

H2O

H2O

5.66

Cu

OH2

OH2

An appropriate ion-specific electrode was found to provide a convenient, precise andrelatively inexpensive method for potentiometry of copper(II) ion in copper-complex azo orformazan dyes. Copper(II) ion in copper phthalocyanine dyes can be quantified after anionexchange. Twelve commercial premetallised dyes evaluated using this technique containedcopper(II) ion concentrations in the range 0.007 to 0.2%. Thus many copper-complex director reactive dyes are likely to contribute low but possibly significant amounts of ionic copperto textile dyeing effluents [52].

The presence of residual unbound transition-metal ions on a dyed substrate is a potentialhealth hazard. Various eco standards quote maximum permissible residual metal levels.These values are a measure of the amount of free metal ions extracted by a perspirationsolution [53]. Histidine (5.67) is an essential amino acid that is naturally present as acomponent of perspiration. It is recognised to play a part in the desorption of metal-complexdyes in perspiration fastness problems and in the fading of such chromogens by thecombined effects of perspiration and sunlight. The absorption of histidine by cellophane filmfrom aqueous solution was measured as a function of time of immersion at various pHvalues. On addition of histidine to an aqueous solution of a copper-complex azo reactivedye, copper-histidine coordination bonds were formed and the stability constants of thespecies present were determined [54]. Variations of absorption spectra with pH thataccompanied coordination of histidine with copper-complex azo dyes in solution wereattributable to replacement of the dihydroxyazo dye molecule by the histidine ligand [55].

On immersion of cellophane films dyed with four copper-complex azo reactive dyes inaqueous histidine, the absorption spectra of the dyeings changed as a result of abstraction of


N NH

CH2CHCOOH

NH2

5.67

Histidine

chpt5(1).pmd 15/11/02, 15:28265


copper ions by the amino acid. The coordination of histidine to a copper-complex dyeappears to be promoted by hydrogen bonding of the amino acid to cellulose hydroxy groups.Shade variations and lowering of light fastness were observed following the immersion ofcotton fabrics dyed with various premetallised reactive dyes in a histidine solution [56,57].

5.7.3 Decolorisation of waste liquors containing metal-complex dyes

The problem of residual colorants in waste-waters is a subject of increasing concern (section1.7.3) and various techniques are exploited to treat coloured effluents. Destructivedecolorisation using powerful oxidising agents such as ozone is one important approach. Thereactivities with this reagent of disodium 1-(2′-hydroxyphenylazo)-2-naphthol-3,6-disulphonate and its 1:1 complex (5.68) with copper(II) were compared with those ofseveral classical dye structures: Alizarin (CI Mordant Red 11), Chrysophenine (CI DirectYellow 12), Indigo carmine (CI Acid Blue 74), Malachite green (CI Basic Green 4) andOrange II (CI Acid Orange 7). The relative ease of oxidation increased in the order:Malachite green < Alizarin < Cu-complex 5.68 < Orange II < Chrysophenine <Unmetallised 5.68 < Indigo carmine. Ozonation of the unsulphonated analogue of thecopper-complex 5.68 yielded phenol, 2-naphthol and phthalic anhydride [58].

An alternative approach to decolorising waste dyebaths of particular relevance to anionicmetal-complex dyes is by means of ion-pair extraction using long-chain amines. The metal-complex dyes can be recovered from the organic phase by extraction with caustic soda andthen reused in dyeing. The amines that result after dye recovery are practically colourlessand may be recycled. Waste water from a cotton dyeing containing the hydrolysed form ofthe copper phthalocyanine CI Reactive Blue 41 was shaken at pH 3 with a secondaryalkylamine and a mixture of aliphatic hydrocarbons. The dye was almost totally removedand the copper content of the liquor reduced from 30 mg/l to 0.1 mg/l.

In similarexperiments [59] on wool dyeing effluents (a) the concentration of CI Acid Red 214 (5.69)was lowered from 204 to 10 mg/l and the chromium content from 5.0 to 0.1 mg/l, and (b) asolution of 25 mg/l CI Acid Red 362 (5.70) became practically colourless and the AOX fromthis halogenated dye fell from 1.6 to 0.5 mg/l.

An evaluation of the macrocyclic ligand cucurbituril in powder form as a precipitant fordirect dyes varying in molecular size, including CI Direct Red 79 (disazo), Blue 71 (trisazo)and Red 80 (tetrakisazo), established the influence of pH and ligand concentration onadsorption [60]. The cucurbituril molecule contains six acetylenediurein units linked in

NN

SO3Na

SO3Na

OO Cu

OH2

5.68

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267

NN

N

OO Cr

OH2

NaO3S

O2N

H2O

OH2

HO5.69

CI Acid Red 214

NN

OOCr

NN

OO

N

N

N

N

H3C

CH3

Cl

Cl

Cl

NaO3S

Cl

SO3Na

5.70

CI Acid Red 362

annular form via methylene groups between pairs of nitrogen atoms in adjacent units. Mostof the copper in effluents from direct dyeing is in the form of copper-complex dyes. Thusselective removal of the dye from the waste liquor by treatment with cucurbiturilsimultaneously reduces the heavy metal content present. An investigation of thiscomplexing agent to remove the copper phthalocyanines CI Reactive Blues 41 and 116, thecopper-complex disazo Blue 120 and the chromium-complexes CI Acid Yellow 120, Reds214 (5.69), 296 and 362 (5.70), Violet 58 and Black 194 from dyehouse effluents gave highlypromising results [61].

Quite recently, the removal of the symmetrical 1:2 chromium complex of an azomethineligand (5.71) from waste-water using a chromium-tolerant bacterial culture wasinvestigated. At low dye concentration initial biosorption was insignificant but about 50% ofdye was subsequently decolorised by bacterial degradative activity. At high dyeconcentration, however, an initial biosorption of about 20% led to only 27% decolorisationoverall. The concentration of chromium in the solution followed a similar pattern [62]. Theproperties of the corresponding manganese(II) complex for dyeing wool and nylon 6.6 havebeen assessed [63]. Interest has also been shown in the analogous azomethine-2-naphtholligand (5.72) and its complexes with copper(II), nickel(II) and cobalt(III) ions, which havebeen evaluated for the dyeing of wool under controlled conditions [64].


NCH

N

CHN N

OCrO ClO4

5.71

+

_

NCH2

N

O

5.72

chpt5(1).pmd 15/11/02, 15:28267


5.8 CHROMIUM-RELATED PROBLEMS IN THE MORDANT DYEING OFWOOL

There are problems of definition with the term ‘mordant dye’ (section 1.6.8) and it is oftenmore precise to refer to those chelatable dyes, mostly o,o′-dihydroxyazo ligands, that areapplied to wool at low pH and fixed by dichromate aftertreatment as chrome dyes. Neverthe-less, mordant dyeing is a convenient way to describe this two-stage process that has becomethe focus of substantial development work in recent years because of increasing concern aboutthe environmental hazards associated with residual chromium in dyehouse effluent.

Virtually all of the chrome dyes that remain of major commercial importance are simplemonoazo structures, such as CI Mordant Violet 5 (5.65; X = SO3Na). These products areeasy to manufacture from low-cost intermediates, readily soluble and build up well to heavydepths. Owing to their relatively small molecular size (Mr 300–600), they show good level-dyeing properties when applied to wool at pH 4 and the boil. Glauber’s salt is an effectivelevelling agent for chrome dyes but it impairs the efficiency of the afterchroming stage. Theunique combination of level dyeing behaviour and outstanding wet fastness offered bychrome dyes made them increasingly important in the 1970s, when shrink-resist woolknitwear suitable for treatment in household washing machines was launched [65].

Levelling agents of the weakly cationic type, often based on ethoxylated alkylamines, aresatisfactory for use with chrome dyes. The effect of cationic surfactants with and withoutphenyl rings on the spectral properties of chrome dyes of the o,o′-dihydroxyazo series hasbeen investigated recently. There was evidence of dye–agent interaction by means of acombination of electrostatic forces and hydrophobic bonding. The specific orientation of theassociated molecules was strongly influenced by opportunities for stacking of the planar arylrings in dye and agent structures. Stacking interactions were promoted by low electrondensity in the aryl rings [66].

The growing reliance of the dyeing industry on rapid instrumental colour matching hastended to be a handicap for users of chrome dyes. A marked change in hue normallyaccompanies conversion of the mordant dye to its chromium complex during aftertreatment,although matching difficulties can be overcome by careful control of conditions [67]. Moreserious problems include the prolonged dyeing procedure, oxidative damage to the wool bydichromate treatment, as well as growing awareness of the environmental hazards associatedwith chromium compounds, especially the hexavalent chromium form [68]. Although 10 mgper day of chromium(III) in food is normal for good health, it is important to concede thatchromium(VI) is highly toxic to mankind and aquatic life [69]. Typical limiting values in theUK regarding permissible amounts of chromium for discharge to effluent are 0.2–0.5 mg/l asthe more potent chromium(VI) dichromate anion and 2–4 mg/l as the chromium(III)cation. Proposed ecological criteria for the EU ecolabel with respect to chroming baths are0.5 mg/l for chromium(VI) and 5 mg/l for chromium(III) ions [50]. Legislation covering therelease of chromium-containing effluents is becoming increasingly strict, especially inGermany, the UK and the USA [70]. Draft regulations indicate that no more than 0.1 mg/ltotal chromium will be tolerated in future [69].

Dichromate anions are readily absorbed under acidic conditions by wool that has beendyed with chrome dyes. The chromium(VI) on the fibre is then gradually reduced by thecystine residues in wool keratin to chromium(III) cations, which react with the dye ligandsto form a stable complex. In this way the cystine disulphide bonds are destroyed, resulting inoxidative degradation of the wool fibres [71].

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It has been demonstrated recently that the dichromate oxidation of wool cystine proceedsas shown in Scheme 5.18. The cystine crosslinks (5.73) are cleaved to give dehydroalanine(5.74) and perthiocysteine (5.75) residues, the latter being readily oxidised by thedichromate ions to yield cysteine-S-sulphonate (Bunte salt) residues (5.76).

CO

CH

NH

CH2 S S CH2 CH

CO

NH

CO

C CH2

NH

H S S CH2 CH

CO

NH

HO S S CH2 CH

CO

NH

O

O

+

Cr2O72

5.73

5.74

5.75

5.76

_

Cystine

Dehydroalanine

Perthiocysteine

Cysteine-S-sulphonateScheme 5.18

5.8.1 Controlling factors in the afterchroming process

By far the most widely used chroming agent is sodium dichromate, although the potassiumsalt has occasionally been preferred. The dichromate may be applied before the dye(mordant method), simultaneously with the dyeing process (metachrome method) or as anaftertreatment (afterchrome method), but only the afterchrome process remains of practicalsignificance. The theoretical aspects of chroming have been reviewed [72,73] and themechanism of the reactions may be represented as follows:(1) Dichromate anions are absorbed and interact with protonated amino groups in wool

(Scheme 5.19), the sorption proceeding most rapidly below pH 3.5.(2) The absorbed chromium(VI) is gradually reduced to chromium(III) as a result of

participation in the oxidative decomposition of cystine crosslinks as represented byScheme 5.18.

(3) The chromium(III) cations then combine with the carboxylate groups in the wool fibre(Scheme 5.20).

(4) The dye ligand interacts with free or complexed chromium(III) to form 1:1 and 1:2chromium-dye complexes (Scheme 5.21), mainly the more stable 1:2 complex. Thesecoloured complexes are bound to the wool primarily through van der Waals andelectrostatic forces. Any excess chromium(III) will remain linked to carboxylate sites inthe wool.

Originally the amounts of dichromate used in the traditional afterchrome process variedbetween about 25 and 50% of the total amount of chrome dyes present, with the lower andupper limits set at 0.25% and 2.5% of the mass of wool. These quantities were well in excess

CHROMIUM-RELATED PROBLEMS IN THE MORDANT DYEING OF WOOL

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2H3N [wool] COOH

HOOC [wool] NH3 H3N [wool] COOH

Cr2O72–+

Cr2O72–

+

+ +

Scheme 5.19

H3N [wool] C

OH

O

H3N [wool] C

O

O

Cr2+ + H+ Cr3+ ++ +

Scheme 5.20

H3N [wool] C

OH

O

H3N [wool] C

O

O

Cr2+

NN

OH

OH

NN

OO Cr

+

+ H

+

+ +

+

+

Scheme 5.21

of the stoichiometric amount required even for formation of the 1:1 complex. This approachresulted in excess dichromate remaining in the aftertreatment bath for discharge to effluent,as well as on the wool fibre where it contributed to further oxidative degradation. Users ofchrome dyes are increasingly concerned with dyeing under mild conditions at pH 4–5 and85 °C, followed by chroming at pH 3.5–3.8 and 90 °C to minimise wool damage [73,74].Chroming in a fresh bath tends to give lower residual chromium content but does increasethe processing costs. Hercosett-treated wool generally requires more dichromate than doesuntreated wool, since some of the dichromate is absorbed by the cationic polymer layer [75].

Every effort should be made to exhaust the dyebath as much as possible, because anyresidual mordant dye will complex with chromium(III) ions in the dye liquor. Apart fromcomplicating effluent treatment, this raises the possibility of lower fastness resulting fromdeposition on the surface of the wool. Methods that give lower residual chromium tend toproduce unlevel chroming, but this can be effectively countered using appropriate levellingagents. The amphoteric agents originally developed for the reactive dyeing of wool areparticularly effective but nonionic alkylarylethoxylates can also be used. Water free from

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traces of iron or copper is essential, since sequestering agents such as EDTA (5.66) orpolyphosphates must be avoided. An aftertreatment in ammonia solution (pH 8.5) for 20minutes at 80 °C, followed by acidification to pH 5, helps the development of optimumfastness, especially on shrink-resist wool.

Concern regarding exposure to chromium is not just related to effluent discharge. It isobvious that residual unbound chromium present on the fibre is also a potential hazard. TheOeko-Tex ecolabel specifies 1 ppm total chromium or cobalt on babywear and 2 ppmchromium or 4 ppm cobalt on other garments [76]. These figures represent the amount offree metal extracted by a standard perspiration solution. In general, typical 1:2 metal-complex dyeings will satisfy these requirements in full depths, chrome dyeings only tomedium depths and premetallised 1:1 complexes only in pale-depth dyeings [65].


5.8.2 Improved mordant dyeing methods to minimise residual chromium

In one low-chrome method the wool is initially dyed at the boil to facilitate maximumpenetration and levelling. The temperature is then lowered to 75 °C, since maximumexhaustion of chrome dyes takes place below the boil. After adjusting to pH 3.5–3.8 byaddition of formic acid, the near-stoichiometric quantity of dichromate as recommended bythe dye supplier is added and the temperature again raised to the boil. About 7.5% o.w.f.sodium sulphate is added after chroming for 10–15 minutes and boiling is continued for afurther 30 minutes. Sodium sulphate gradually displaces chromium from the woolcarboxylate sites, making the chromium(III) cations more readily available for interactionwith the dye ligands so that less dichromate is needed [73].

An alternative method relies on a constant temperature of 92 °C for dyeing and chroming,thus minimising damage to the wool and dispensing with the need to lower the dyebath temp-erature before commencing the chroming stage [68]. After dyeing at 92 °C and exhausting thedyebath with a small amount of formic acid, an addition of dichromate is made according toEquation 5.5, where C% is the amount of dichromate required for a dyeing at D% depth.

C 0.2 0.15D� � (5.5)

Chroming is allowed to continue at a relatively mildly acidic pH for about 10 minutes toimprove the uniformity of chroming with this reduced quantity of dichromate. The pH isthen lowered to 3.5–3.8 to ensure maximum utilisation of the chromium and treatment iscontinued at 92 °C for 45 minutes.

There are several redox methods of chroming that depend on the addition of a reducingagent to the chroming bath after about 10 minutes at or near the boil. The precise mechanismis not fully understood, but clearly the increased rate and extent of reduction of hexavalent totrivalent chromium plays a crucial part. The first technique of this type used 1–3% o.w.f. lacticacid, which was found to be the most effective of the α-hydroxymonocarboxylic acidsevaluated to assist the rapid conversion of chromium(VI) anions to chromium(III) cations[77]. The method should be used in conjunction with reduced amounts of dichromate if theadvantages to be gained from a lack of residual chromium fixed in the fibre are to be attained.Lactic acid is only effective at high concentrations, resulting in higher processing costs and anincreased total oxygen demand in the effluent.

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Sodium thiosulphate was later proposed [78] instead of lactic acid. This enabled manychrome dyes to be effectively aftertreated at a temperature as low as 80 °C with consequentadvantages in respect of improved levelling and decreased fibre damage. Some dyes areinadequately chromed at 80 °C, however, so a minimum temperature of 90 °C was laterrecommended [75]. When sodium thiosulphate is present in the chroming bath, theconcentration of Bunte salt residues (5.76 in Scheme 5.18) is considerably increased. Thethiosulphate nucleophile (5.77) will readily add to the activated double bond indehydroalanine (5.74), as shown in Scheme 5.22. The protective effect of thiosulphate ionsin preventing some of the wool damage is attributable to their reducing properties, so thatthey compete with perthiocysteine (5.75) in the overall reduction of chromium(VI) tochromium(III) ions [72]. In a modification of this redox approach, a glucose-basedproprietary product Lyocol CR (Clariant) is recommended instead of sodium thiosulphate.This agent also accelerates the conversion of Cr(VI) to Cr(III) and forms a complex withthe chromium(III) cations that is then absorbed by the wool [79].

Sulphamic acid has been evaluated as a replacement for formic acid in controlling theoptimum pH of chroming. If the process is initiated at a pH lower than 3.5 before thedichromate is added and then allowed to gradually become less acidic when reduction ofmost of the dichromate has taken place, the optimum result for both Cr(VI) and Cr(III) inthe effluent should be achieved. Sulphamic acid slowly hydrolyses under these conditions(Scheme 5.23) to give ammonium hydrogen sulphate. It is also able to react [80] withprimary amino and hydroxy groups in wool keratin, releasing ammonia (Scheme 5.24). Allthese reactions will contribute to an increase in pH of the chroming bath. There are nosignificant differences in wet fastness of chrome dyeings or in the damage caused to the woolby the sulphamic acid process compared with the conventional afterchrome method usingformic acid [81].

CO

C CH2

NH

Na S S ONa

O

OH2O

+

5.74 5.765.77

+ NaOH

Dehydroalanine Sodium thiosulphate Cysteine-S-sulphonate

CO

CH

NH

CH2 S S

O

O

ONa

Scheme 5.22

NH4HSO4H2NSO3H + H2OScheme 5.23

HO [wool] NH2 HO3SO [wool] NHSO3H+ 2 H2NSO3H + 2 NH3

Scheme 5.24

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Rare earth cations form complexes of low stability with mordant dyes and can be used tocontrol the chroming process. Mixtures of rare earth chlorides are available at low prices(ca. US$1 per kg in China) and are abundant in some countries. Neodymium and ceriumions have an outer electron distribution similar to that of trivalent chromium. If mordant-dyed wool is treated with such a mixture 20 minutes before the dichromate addition in aconventional chroming sequence, coordination complexes of the rare earth elements aretemporarily formed. Ion-exchange reactions with chromium(III) cations allow time for thelatter to migrate throughout the wool substrate and level chroming is achieved in spite ofthe low level of dichromate addition. Although the rare earth complexes are significantlydifferent in colour, the ultimate hue of the chromium-complex dyeing is normal. The colourfastness properties are not adversely affected and wool damage is slightly reduced. Theenvironmental impact of the ca. 60 mg/l rare earth cations in the effluent has yet to beevaluated [82].

An interesting new approach has been evaluated to avoid the environmental hazards andoxidative degradation of wool associated with these conventional and modified dichromateaftertreatments. Chromium(III) salts such as chromium fluoride have been used withmordant dyes in steam fixation processes of printing or continuous dyeing but seldom for theaftertreatment of exhaust dyeings. It has now been shown that the chromium lactate 1:2complex (5.78) formed by reacting chromium(III) chloride with lactic acid (Scheme 5.25) issubstantive to wool at pH 3–4. It is capable of adequately chroming a wide variety of chromedyes to give satisfactory wet fastness, less fibre damage and effluents of low toxicity [83].

More recently, 1:2 complexes of the chromium(III) ion with 5-sulphosalicylic [84,85],salicylic and citric acids have been compared with the lactate complex [85]. Like the latter,the citrate (5.79) and 5-sulphosalicylate (5.80; X = SO3H) complexes are readily water-soluble but the unsubstituted salicylate (5.80; X = H) is not. If formic acid is added,however, the mixed formate–salicylate complex (5.81) shows moderate solubility. The pHvalue for maximum uptake of the complexes varied slightly but was always within the rangeof pH 2–4. Compared with conventional dichromate treatment, the lactate complexsometimes gave inferior wet fastness. Fastness ratings equal to conventional aftertreatmentwere given by the two salicylate mordant complexes (5.80) without the drawbackscharacteristic of the dichromate process [85].

5.8.3 Improved products and processes to compete with chrome dyes

In spite of their long-recognised disadvantages, or as a result of the substantial efforts toovercome them, chrome dyes still represent about 30% of total dye consumption in wooldyeing (Table 3.10 in section 3.2.2). Premetallised acid dyes and reactive dyes for woolaccount for a further 40%, so it is not surprising that much attention has been given to


CH3 CH

OH

COOHC

CO O

H3C OC

CCH3O

OO

Cr

OH2

OH2

CrCl3.6H2O + 2

5.78

_

Scheme 5.25

chpt5(1).pmd 15/11/02, 15:28273


improving the performance of these ranges, which have their own characteristic limitations.There has been considerable activity in the selection of compatible members from differentbut related classes of wool dyes, such novel selections often being given a distinctive brandname [67].

Carbonised wool can be dyed uniformly with 1:1 metal-complex dyes in 8% sulphuric acidsolution without prior neutralisation. These dyes show excellent penetration of tightlywoven fabrics and better wet fastness than levelling acid dyes in full depths [86]. Theirmajor disadvantage is the need to apply them at pH 2, causing damage to the wool andrequiring a subsequent neutralisation step. The Neolan Plus system was introduced in 1988,consisting of eight Neolan P (Ciba) modified 1:1 metal-complex dyes containingfluorosilicate anions. They are applied with formic acid at pH 3.5–4 using Albegal Plus(Ciba), a synergistic mixture of quaternary and esterified alkylamine ethoxylates andfluorosilicates [87]. High dyebath exhaustion ensures shade reproducibility not attainablehitherto, as well as low residual chromium in the dyebath. Wool quality is preserved by theabove-normal dyebath pH and post-neutralisation is not necessary. An improved BASFprocess for conventional 1:1 metal-complex dyes utilises sulphamic acid, commencing at pH2–2.5 and gradually increasing to pH 3–3.5 (Schemes 5.23 and 5.24). Together with a novellevelling agent, these conditions promote good exhaustion and dye migration, improvedwool quality and low values for residual chromium [70].

The Lanaset (Ciba) system introduced in 1983 is based on a compatible range of fifteenmilling acid and 1:2 metal-complex dyes. For minimum damage to the wool, they are appliedin the isoelectric region (pH 4.5–5) in the presence of Albegal SET (Ciba), an amphotericlevelling agent [88]. The required pH is maintained with formic acid for loose stock andtops. A dyebath accelerant, Miralan T (Ciba), is mainly of interest on these goods. Fullexhaustion is attained in 10–20 minutes at the boil, resulting in increased reproducibilityand improved wool quality. Dye liquors can be reused where a full-volume reserve tank isavailable, giving energy savings and lower volumes of effluent [74]. Glauber’s salt is

C

CO O

OC

CO

OO

Cr

OH2

OH2

HOOCH2C

HOOCH2C

CH2COOH

CH2COOH

5.79

_

O

C O

O

O

CO

O

Cr

OH2

OH2

X

X

5.80

_

O

C O

O

CO

CO

Cr

OH2

OH2O

H

O

H

5.81

_

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recommended for yarn and piece dyeing, with an acetate/acetic acid buffer system forimproved control of pH. Although adopted primarily for the dyeing of loose stock, slubbingand yarn, the Lanaset system can be applied with care to the winch dyeing of woollens andthe jet or overflow dyeing of worsteds.

The versatility of this system is reflected in its suitability for blends of wool with otherfibres [89]. The ideal conditions for the dyeing of wool/acrylic blends are at pH 4-5 andAlbegal SET inhibits the risk of co-precipitation between Lanaset anionic dyes and basicdyes. Lanaset dyes are quite stable when dyeing polyester/wool at pH 4-5 and 115-120°Cusing Irgasol HTW (Ciba) as wool protectant. These dyes are also suitable for dyeing woolin its blends with silk, nylon or cellulosic fibres.

The new Neolan A (Ciba) range of eight optimised combinations of metal-free acid dyeshas been developed specially for the dyeing of wool and wool/nylon blends. Five members ofthe range can be further combined in trichromatic recipes, enabling wool dyers to achieve80-90% of fashion shades [90]. Brilliant hues and excellent migration properties areobtained in hank and piece dyeing. Selected products can be used in dyeing wool yarns forcarpets. Neolan A dyes are applied at pH 5–6 to give exhaustion values exceeding 95% anda uniform migration rate of about 30%. Albegal NA (Ciba) is a levelling agent developedspecially for use with these dyes [91].

Although reactive dyes account for only about 5% of total dye usage on wool, no otherclass can achieve such brilliant hues of high fastness to light and wet treatments. Chromedyes are mainly used for navy blue and black dyeings, where reactive dyes in general areparticularly costly by comparison. Mill trials have been carried out to compare the fastness ofa reactive black dyeing based on CI Reactive Black 5 (5.82) with a dyeing of the chromeblack CI Mordant Black 9 (5.83) that gives the highest wet fastness. In the most demandingroute of dyeing loose wool and assembling into knitted garments that are given an oxidativeshrink-resist treatment followed by resin application, the reactive dyeing met all processingand end-use fastness requirements [65]. Chrome-dyed wool is brittle and has acharacteristically harsh handle. This is attributed to oxidative damage and the excessivecrosslinking imparted during afterchroming. Reactive dyes, however, offer fibre-protectiveeffects during dyeing by:(1) inhibiting thiol-disulphide interchanging and thus restricting the degree of permanent

set during dyeing(2) minimising the hydrolytic peptide bond breakdown that is responsible for impairing the

dry tensile strength of wool fibres.

In 1996 Ciba introduced a new reactive black, Lanasol Black PV, that is an attractivealternative to established chrome blacks, even as regards price. It has been designed to givetinctorial strength, shade metamerism and light fastness that are close to those of theafterchromed complex of Eriochrome Black PV (5.83). Only in the case of fastness toextreme processes such as potting and cross-dyeing with acetic acid is the target standardnot entirely achieved. To attain optimum exhaustion and good level dyeing behaviour, 2%o.w.f. Albegal B (Ciba) must be used and aftertreatment with ammonia is necessary toensure the highest wet fastness ratings [92].

The new range of Lanasol CE (Ciba) metal-free reactive dyes for wool has beendeveloped to offer the option of replacing chrome dyes in full depths by cost-effectivealternatives. These five dyes (yellow, red, blue, navy and black) are based on the α-


chpt5(1).pmd 15/11/02, 15:28275


NN

OH

CH2CH2SO2

NaO3S

NaO3SO

H2N

SO3Na

N N

SO2CH2CH2 OSO3Na

5.82

CI Reactive Black 5

NN

O

OH

H

OH

NaO3S5.83

CI Mordant Black 9

bromoacrylamide reactive system and can be shaded with existing Lanasol brands using thestandard application methods for these dyes [93]. Lanasol Navy CE is particularly versatile,being suitable for the exhaust dyeing of untreated, chlorinated or machine-washable wool,as well as continuous dyeing, vigoreux printing and coloured discharge styles and in silkdyeing [90]. Excellent fastness to light and wet treatments with outstanding fixation,levelling and reproducibility are claimed for Lanasol CE dyes, which fulfil the requirementsof the Oeko-Tex 100 ecolabel [76].

The influence of transition-metal salts, mordant dyes and premetallised acid dyes on thephotodegradation of wool has been investigated [94]. Certain yellow dyes that absorb UVradiation in the 320–400 nm region will inhibit phototendering but most other dyesaccelerate the process. The uranyl(II) ion (UO2) also absorbs in this region and was foundto be an effective UV screen. Treatment with aluminium or iron(III) hydroxide broughtabout a light-induced strengthening of wool fabric, but a harsher handle developed onprolonged exposure. Treatment of wool with alizarin-3-sulphonate (5.84; CI Mordant Red 3)resulted in some light-induced increase of breaking strength but on aluminium-treated woolit was considerably more effective and durable for at least 6000 h irradiation. Woolpretreated with a uranyl salt followed by alizarin-3-sulphonate dyeing was phototenderedextremely rapidly, in contrast to the protective effects when either component was appliedseparately. The mordant dye 1-nitroso-2-naphthol-6-sulphonate (5.85) on aluminium-mordanted wool imparted protection from strength loss for 3000 h but acceleratedphotodegradation on untreated wool. The 1:2 chromium-complex dyes CI Acid Yellow 59(5.86) and Black 58 (5.4) did not significantly modify the rate of phototendering of wool,with or without an aluminium mordant.

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REFERENCES 1. G Schetty, J.S.D.C., 71 (1955) 705. 2. R Price in Chemistry of synthetic dyes, Ed K Venkataraman, Vol 3 (New York: Academic Press, 1970) 303. 3. F Beffa and G Back, Rev. Prog. Coloration, 14 (1984) 33. 4. A Werner, Ber., 41 (1908) 1062. 5. G Schetty, Textilrundschau, 5 (1950) 399. 6. H Pfitzner, Melliand Textilber., 35 (1954) 649. 7. G T Morgan and H D K Drew, J.C.S., 117 (1920) 1456. 8. R Price, Chimia, 28 (1974) 221. 9. H D K Drew and J K Landquist, J.C.S., 140 (1939) 292.10. R W Hay and J A G Edmonds, Chem. Commun. (1967) 969.11. R F M Sureau, Chimia, 19 (1965) 254.12. H D K Drew and R E Fairbairn, J.C.S., 141 (1939) 823.13. P A Mack and R Price, Ind. Chim. Belge Suppl. (1967) 31.14. L Hunter and L B Roberts, J.C.S., 143 (1941) 820.15. R Wizinger, Chimia (suppl.), 22 (1968) 82.16. Z Yu-zhen and L Dong-zhi, Dyes and Pigments, 29 (1995) 57.17. V E Maizlish et al., Zhur. Obshch. khim., 67 (1997) 846.18. S Hussamy, AATCC Internat. Conf. & Exhib. (Oct 1991) 102.19. I J Wang, Y J Hsu and J H Tian, Dyes and Pigments, 16, No 2 (1991) 83.20. H Kocaokutgen, E Erdem and I E Gümrükcüoglu, J.S.D.C., 114 (1998) 93.21. B R Hsieh, D Desilets and P M Kazmaier, Dyes and Pigments, 14, No 3 (1990) 165.22. B R Hsieh, Dyes and Pigments, 14, No 4 (1990) 287.23. B R Hsieh, R K Crandall and B A Weinstein, Dyes and Pigments, 17, No 2 (1991) 141.24. K Blus, Dyes and Pigments, 25 (1994) 15.25. H Pfitzner and H Baumann, Angew. Chem., 70 (1958) 232.26. B I Stepanov in Recent progress in the chemistry of natural and synthetic colouring matters’, Eds T S Gore et al. (New

York: Academic Press, 1962) 451.27. R Price in Chemistry of synthetic dyes, Ed K Venkataraman, Vol 3 (New York: Academic Press, 1970) 348.

O

O

OH

OH

SO3Na

5.84

CI Mordant Red 3

HO

SO3Na

NO

5.85N

N

OCCr

NN

CO

N

N

CH3

H3C

O

O

N

N

5.86

CI Acid Yellow 59

OO

REFERENCES

chpt5(1).pmd 15/11/02, 15:28277


28. J Sokolowska-Gajda, H S Freeman and A Reife, Text. Res. J., 64 (1994) 388.29. M Ryder, Wool Record, 151 (Nov 1992) 5.30. H S Freeman, L D Claxton and V S Houk, Text. Chem. Colorist, 27 (Feb 1995) 13.31. H S Freeman et al., AATCC Internat. Conf. & Exhib., (Oct 1995) 397.32. J Sokolowska-Gajda, H S Freeman and A Reife, Dyes and Pigments, 30 (1996) 1.33. J Sokolowska-Gajda, J.S.D.C., 112 (1996) 364.34. H A Bardole, H S Freeman and A Reife, Text. Research J., 68 (1998) 141.35. W Czajkowski and M Szymczyk, Dyes and Pigments, 37 (1998) 197.36. H S Freeman and J Sokolowska-Gajda, Rev. Prog Coloration, 29 (1999) 8.37. P Pfeiffer and S Saure, Ber., 74 (1941) 935.38. G Schetty and W Kuster, Helv. Chim. Acta, 44 (1961) 2193.39. G Schetty, Helv. Chim. Acta, 45 (1962) 1095, 1473; 46 (1963) 1132; 47 (1964) 921.40. L Beffa et al., Helv. Chim. Acta, 46 (1963) 1369.41. H Jaggi, Helv. Chim. Acta, 51 (1967) 580.42. G Schetty, Textilveredlung, 1 (1961) 3.43. H Zollinger, Chemie der azofarbstoffe, (Basel: Verlag Birkhäuser, 1958) 238.44. G Schetty and E Steiner, Helv. Chim. Acta, 57 (1974) 2149.45. A Johnson et al., de Tex, 21 (1962) 453.46. F A Snavely, W C Fernelius and B E Douglas, J.S.D.C., 73( 1957) 491.47. F A Snavely, B D Krecker and C G Clark, J. Amer. Chem. Soc., 81 (1959) 2337.48. W R Dyson and M A Knight, J. Amer. Leather Chem. Assoc., 86 (1991) 14.49. G L Baughman, H A Boyter and W G O’Neal, AATCC Internat. Conf. & Exhib. (Oct 1997) 314.50. L Benisek, Wool Record, 158 (Apr 1999) 42.51. Y Imabayashi, J. Soc. Fibre Sci. Technol. Japan (1995) 39.52. J H Kim and G L Baughman, AATCC Internat. Conf. & Exhib. (Oct 1998) 582.53. K Parton, J.S.D.C., 114 (1998) 8.54. Y Okada et al., Dyes and Pigments, 24 (1994) 99.55. Y Okada, T Kawanishi and Z Morita, Dyes and Pigments, 27 (1995) 271.56. Y Okada, M Asano and Z Morita, Dyes and Pigments, 31 (1996) 53; IFATCC Cong., Vienna (Jun 1996) 360.57. Y Okada et al., Dyes and Pigments, 33 (1997) 239; 38 (1998) 19.58. M Matsui et al., Bull. Chem. Soc. Japan, 64, No 19 (1991) 2961.59. I Steenken-Richter and W D Kermer, J.S.D.C., 108 (1992) 182.60. H J Buschmann, D Raderer and E Schollmeyer, Textilveredlung, 26 (1991) 157.61. H J Buschmann and E Schollmeyer, Textilveredlung, 28 (1993) 182.62. H Matanic et al., J.S.D.C., 112 (1996) 158.63. Z Grabaric et al., J.S.D.C., 109 (1993) 199.64. N Koprivanac et al., Dyes and Pigments, 22, No 1 (1993) 1.65. K Parton, Colour Science ’98, Vol. 2, Ed. S M Burkinshaw (Leeds: Leeds Univ., 1999) 120.66. H R Sagaster and G Robisch, Dyes and Pigments, 13, No 3 (1990) 187.67. J A Bone, J Shore and J Park, J.S.D.C., 104 (1988) 12.68. K Schaffner and W Mosimann, Textilveredlung, 14 (1979) 12.69. D M Lewis, J.S.D.C., 113 (1997) 193.70. P A Duffield, R R D Holt and J R Smith, Melliand Textilber., 72 (1991) 938.71. F R Hartley, J.S.D.C., 85 (1969) 66.72. D M Lewis and G Yan, J.S.D.C., 109 (1993) 193.73. G Meier, J.S.D.C., 95 (1979) 252.74. A Hoyes, Wool Record, 151 (Apr 1992) 49.75. P A Duffield and K H Hoppen, Melliand Textilber., 68 (1987) 195.76. Oeko-Tex, Zürich Edn. (Feb 1997) 15.77. L Benisek, J.S.D.C., 94 (1978) 101.78. P Spinacci and N C Gaccio, IFATCC Cong., (Budapest: June, 1981).79. A C Welham, J.S.D.C., 102 (1986) 126.80. B A Cameron and M T Pailthorpe, Text. Research J., 57 (1987) 619.81. J Xing and M T Pailthorpe, J.S.D.C., 108 (1992) 17.82. J Xing and M T Pailthorpe, J.S.D.C., 108 (1992) 265.83. D M Lewis and G Yan, J.S.D.C., 109 (1993) 13; 110 (1994) 281.84. J Xing and M T Pailthorpe, Text. Research J., 65 (1995) 70.85. D M Lewis and G Yan, J.S.D.C., 111 (1995) 316.86. L C De Meulemeester, I Hammers and W Mosimann, Melliand Textilber., 71 (1990) 898.87. W Mosimann, Amer. Dyestuff Rep., 80 (Mar 1991) 26.

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88. H Flensberg, W Mosimann and H Salathe, Melliand Textilber., 65 (1984) 472.89. H Flensberg and K Hannemann, Wool Record, 156 (Oct 1997) 42.90. P Runser, Wool Record, 157 (Oct 1998) 25.91. K Hannemann and P Runser, Melliand Textilber., 80 (1999) 278.92. K Hannemann and H Flensberg, Melliand Textilber., 78 (1997) 160.93. K Hannemann and P Runser, Wool Record, 157 (1998) 53.94. I J Miller and G J Smith, J.S.D.C., 111 (1995) 103.

REFERENCES

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280 CHEMISTRY OF ANTHRAQUINONOID, POLYCYCLIC AND MISCELLANEOUS COLORANTS

280

CHAPTER 6

Chemistry of anthraquinonoid, polycyclic andmiscellaneous colorants

Geoffrey Hallas

6.1 ANTHRAQUINONE ACID, DISPERSE, BASIC AND REACTIVE DYES

It was pointed out in Chapter 1 that, after the azo class, anthraquinone derivatives form thenext most important group of organic colorants listed in the Colour Index. The majorapplication groups are vat dyes, disperse dyes and acid dyes (Table 1.1).

Of the various anthracenedione isomers, only the 9,10-compound is used for thesynthesis of dyes; it is usually referred to simply as anthraquinone (6.1). The parentcompound is pale yellow in colour, having a weak absorption band in the visible region(n→π* transition). The presence of one or more electron-donating substituents leads tosignificant bathochromic effects so that relatively simple derivatives are of commercialimportance as dyes. The colour of such compounds, which usually contain amino or hydroxygroups, can be attributed to the existence of a charge-transfer absorption band [1].

From a historical viewpoint, the advent of synthetic anthraquinone dyes can be traced tothe elucidation of the structure of the important naturally occurring compound 1,2-dihydroxyanthraquinone (6.2; alizarin) by Graebe and Liebermann in 1868 [2]. Naturalalizarin is the colouring matter of madder and was the source of the then valuable mordantdye Turkey red (CI Mordant Red 11), an aluminium–calcium complex. In the following yearCaro and Perkin independently discovered a commercially viable route to alizarin fromanthraquinone via the 2-sulphonic acid (6.3) Treatment of the sodium salt ofanthraquinone-2-sulphonic acid (called ‘silver salt’, from its appearance) with aqueoussodium hydroxide under pressure at an elevated temperature in the presence of an oxidisingagent gives the disodium salt of alizarin as the main product (Scheme 6.1). Perkin soondevised a more efficient and cheaper route that involved chlorination of anthracene to give9,10-dichloroanthracene, which was then sulphonated, oxidised and fused with sodiumhydroxide to yield alizarin [3].

6.1.1 Methods of synthesis

Anthraquinone itself is traditionally available from the anthracene of coal tar by oxidation,often with chromic acid or nitric acid; a more modern alternative method is that of airoxidation using vanadium(V) oxide as catalyst. Anthraquinone is also produced in thereaction of benzene with benzene-1,2-dicarboxylic anhydride (6.4; phthalic anhydride) usinga Lewis acid catalyst, typically aluminium chloride. This Friedel–Crafts acylation gives o-benzoylbenzoic acid (6.5) which undergoes cyclodehydration when heated in concentratedsulphuric acid (Scheme 6.2). Phthalic anhydride is readily available from naphthalene orfrom 1,2-dimethylbenzene (o-xylene) by catalytic air oxidation.

chpt6(1).pmd 15/11/02, 15:29280

281

Although anthraquinone is the starting point for the preparation of many derivatives,involving substitution and replacement reactions, certain compounds are obtained directlyby varying the components in the above synthesis. Thus, for example, replacement ofbenzene with methylbenzene (toluene) leads to the formation of 2-methylanthraquinone. Aparticularly important variation on the phthalic anhydride route is the synthesis of 1,4-dihydroxyanthraquinone (6.6; quinizarin) using 4-chlorophenol with sulphuric acid andboric acid as catalyst (Scheme 6.3). The absence of aluminium chloride permits hydrolysis ofthe chloro substituent to take place.

O

O

O

O

SO3H

O

O

OH

OH

NaOH

1α

2β

3

45

6

7

8

6.1 6.3

6.2

Anthraquinone

Alizarin

H2SO4

Scheme 6.1

O

O

O

O

COOHAlCl3

H2SO46.1

6.4 6.5

Phthalic anhydride

Anthraquinone

Scheme 6.2

O

O

O

OH

Cl

O

O

OH

OH

H2SO4

H3BO3

6.4

+

6.6

QuinazarinScheme 6.3

ANTHRAQUINONE ACID, DISPERSE, BASIC AND REACTIVE DYES

chpt6(1).pmd 15/11/02, 15:29281


Electrophilic substitution at the anthraquinone ring system is difficult due to deactivation(electron withdrawal) by the carbonyl groups. Although the 1-position in anthraquinone israther more susceptible to electrophilic attack than is the 2-position, as indicated by π-electron localisation energies [4], direct sulphonation with oleum produces the 2-sulphonicacid (6.3). The severity of the reaction conditions ensures that the thermodynamicallyfavoured 2-isomer, which is not subject to steric hindrance from an adjacent carbonyl group,is formed. However, the more synthetically useful 1-isomer (6.7) can be obtained bysulphonation of anthraquinone in the presence of a mercury(II) salt (Scheme 6.4). Itappears that mercuration first takes place at the 1-position followed by displacement. Somedisulphonation occurs, leading to the formation of the 2,6- and 2,7- or the 1,5- and 1,8-disulphonic acids, respectively. Separation of the various compounds can be achievedwithout too much difficulty. Sulphonation of anthraquinone derivatives is also of someimportance.

O

O

O

O

SO3H

H2SO4

Hg(II)

6.1 6.7

Scheme 6.4

Anthraquinone-1-sulphonic acid is the traditional precursor of 1-aminoanthraquinone(6.8), the most important anthraquinone intermediate. Since it is expensive to eliminatemercury(II) ions from waste water, an alternative route via 1-nitroanthraquinone has beeninvestigated. Nitration of anthraquinone gives, as well as the desired 1-nitro derivative,significant amounts of the 2-isomer together with 1,5- and 1,8-dinitroanthraquinones.Nevertheless, chemists at Sumitomo in Japan have optimised the nitration procedure withrespect to both yield and purity of the 1-nitro compound. In particular, nitration is stoppedwhen 80% of the anthraquinone has been substituted [5]. Nitration of anthraquinonederivatives is also of some significance.

Direct halogenation is normally carried out only on derivatives of anthraquinone. Theimportance of chloro or bromo derivatives lies mainly in the displacement of the halogen bynucleophiles, especially amines. Simple derivatives, such as 1-chloroanthraquinone (6.9),are available from the appropriate sulphonic acid by replacement (Scheme 6.5). 1-Amino-4-bromoanthraquinone-2-sulphonic acid (6.10; bromamine acid) is a very useful intermediatein the synthesis of dyes derived from 1,4-diaminoanthraquinone. Bromamine acid isobtained from 1-aminoanthraquinone (6.8) by sulphonation to give the 2-sulphonic acid(6.11), which is then brominated (Scheme 6.6). Direct bromination of 1-aminoanthraquinone affords the 2,4-dibromo derivative. Hydroxyanthraquinones are alsosusceptible to halogenation. In the case of alizarin (6.2), for example, the 1-hydroxy group isinvolved in intramolecular hydrogen bonding with the adjacent carbonyl group so thatbromination takes place at the 3-position (6.12), which is both ortho to the 2-hydroxy groupand meta to the other carbonyl group (Scheme 6.7).

chpt6(1).pmd 15/11/02, 15:29282

283

Although some hydroxyanthraquinones, such as quinizarin (6.6), are available by directsynthesis, most hydroxy derivatives are produced by nucleophilic displacement of sulphonicacid groups or halogen atoms. Thus 1,5-dihydroxyanthraquinone (6.13; anthrarufin) and the1,8-isomer (6.14; chrysazin) are obtained by alkaline hydrolysis of the correspondingdisulphonic acids using calcium hydroxide or dilute sodium hydroxide. Under such relativelymild conditions additional hydroxy groups are not introduced. In the case of anthraquinone-2-sulphonic acid (6.3), as pointed out earlier, hydroxylation as well as displacement can takeplace to produce alizarin (6.2). From a mechanistic point of view it seems likely that thereaction first involves hydroxylation to produce a hydroquinone (6.15), which is thenoxidised to 1-hydroxyanthraquinone-2-sulphonic acid (6.16) before nucleophilic displace-ment of the sulphonic acid group takes place (Scheme 6.8).

O

O

SO3H

O

O

Cl

O

O

NH2

NH3

HCl

NaClO3

6.8 6.7

6.9Scheme 6.5

O

O

NH2

SO3H

O

O

NH2

Br

SO3HBr2

6.11 6.10Bromamine acidScheme 6.6

O

O

O

OH

O

O

O

OH

H H

Br2

6.2 6.12

Br

Scheme 6.7


chpt6(1).pmd 15/11/02, 15:29283


Polyhydroxyanthraquinone derivatives can be obtained by means of the Bohn–Schmidtreaction, in which hydroxyanthraquinones containing at least one α-hydroxy group reactwith fuming sulphuric acid; thus, for example, alizarin can be converted into 1,2,5-trihydroxyanthraquinone (6.17). The presence of boric acid leads to the formation of mixedanhydrides with sulphuric acid and to cyclic intermediates. The substitution mechanisms arecomplex but it has been established [6] that sulphur trioxide can interact with the 1-hydroxygroup and the adjacent carbonyl group to form esters containing six-membered rings,exemplified by structure 6.18. The sulphuric acid esters are readily hydrolysed to hydroxycompounds during work-up. Sulphonation and subsequent desulphonation can also takeplace. The Bohn–Schmidt procedure is also applicable to aminoanthraquinones. Thus 1,4-diaminoanthraquinone (6.19) can be converted into 1,4-diamino-5,8-dihydroxyanthra-quinone (6.20).

Nucleophilic displacement of halogen by amines is an important method of introducingamino groups into the anthraquinone ring system. In the Ullmann reaction the displacementis catalysed by metallic copper or by copper ions so that relatively mild conditions can beused. Mechanistic studies suggest that copper(I) ions exert a catalytic effect via complexformation. Derivatives of 1,4-diaminoanthraquinone are of considerable industrialsignificance. Many compounds are prepared from the reduced form of quinizarin (6.6).

O

O

OH

OH

6.13

Anthrarufin

O

O

OHOH

6.14

Chrysazin

O

O

SO3Na

O

O

OHHSO3Na

O

O

OH

SO3Na

O

O

OH

SO3Na

OH

oxidation6.2

6.166.15

_ _

_

_Alizarin

Scheme 6.8

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285

Leucoquinizarin can be represented by various tautomers such as structures 6.21 and 6.22;however, 1H- and 13C-n.m.r. have shown the dominant species to be the 2,3-dihydrotautomer (6.22) [7]. Alkylamines react with leucoquinizarin in a stepwise manner to givethe corresponding 1,4-dialkylamino derivative (6.23) after air oxidation (Scheme 6.9). Theless nucleophilic arylamines do not react unless boric acid is present as a catalyst. Thecomplex thus formed (6.24) is more susceptible to nucleophilic attack at the 1- and 4-positions than is 1eucoquinizarin. Unsymmetrical 1,4-diaminoanthraquinones can besynthesised by using two different amines; the corresponding symmetrical dyes are alsoformed to a certain extent.

6.1.2 Acid, disperse, basic and reactive dyes

Anthraquinone dyes tend to predominate in the violet, blue and green hue sectors.Although they have the advantages of brightness and chemical stability, they are moreexpensive to manufacture than are azo dyes, which are also tinctorially stronger.

In general, 1-substituted derivatives of anthraquinone are more bathochromic than thecorresponding 2-substituted isomers, in accordance with PPP-MO calculations [1].Intramolecular hydrogen bonding is not possible between the carbonyl group and a 2-

O

O

OH

OH

OH

6.17

O

O

OSO3H

O

OS

O O

H SO3H

BHO3SO OSO3H

6.18

O

O

NH2

NH2

6.19


O

O

NH2

NH2

OH

OH

6.20

OH

OH

OH

OH

OH

OH

O

O6.21 6.22

Leucoquinizarin


chpt6(1).pmd 15/11/02, 15:29285


substituent and, in the 2-substituted series, the bathochromic shift increases with electron-donor power in the sequence: OH < OCH3 < NHCOCH3 < NH2 < NHCH3 < N(CH3)2.An enhanced bathochromic effect is observed when a 1-substituent, such as hydroxy ormethylamino (6.25), is able to hydrogen-bond with the adjacent carbonyl group and therebyassist the conjugation of the donor lone pair of electrons with the anthraquinone ringsystem; for example, a methylamino group is a more effective donor than is a dimethylaminogroup in the 1-substituted series. Moreover, the latter group is unable to conjugate fully dueto steric hindrance between the bulky substituent and the adjacent carbonyl group, which

OH

OH

O

O

OH

OH

NHR

O

OH

OH

NHR

NHR

O

O

NHR

NHR

O

O

NHR

NHR

RNH2

RNH2

– H2O

– H2O

air

6.23

6.22

Scheme 6.9

O

O

O

O

B

B

HO OH

HO OH

6.24

O

O

NH CH3

6.25

chpt6(1).pmd 15/11/02, 15:29286

287

decreases the bathochromic effect and reduces the tinctorial strength (intensity). Stericinteraction is minimised by rotation of the dimethylamino group out of the plane of theanthraquinone ring system (6.26). Some illustrative spectral data are given in Table 6.1 [8].Significant crowding effects can also arise from the presence of ortho groups, as in 1,2-disubstituted anthraquinones.

Table 6.1 Spectral data for some monosubstituted anthra-quinones in methanol [8]

1-position 2-position

Substituent λmax (nm) εmax λmax (nm) εmax

OH 402 5500 368 3900OCH3 378 5200 363 3950NHCOCH3 400 5600 367 4200NH2 475 6300 440 4500NHCH3 503 7100 462 5700NHC6H5 500 7250 467 7100N(CH3)2 503 4900 472 5900

Table 6.2 Spectral data for some disubstituted anthra-quinones in methanol [8]

Substituents λmax (nm) εmax

1,4-diOH 470 17 0001,5-diOH 425 10 0001,8-diOH 430 10 9601,4-diNH2 550, 590 15 850, 15 8501,5-diNH2 487 12 6001,8-diNH2 507 10 000

Anthraquinone itself can be regarded approximately as consisting of two ortho-interlinkedbenzoyl groups [9]. Thus, the introduction of a second group into the unsubstituted ring of amonosubstituted anthraquinone effectively doubles the extinction coefficient (compare εmaxvalues for α-hydroxy and α-amino substitution in Tables 6.1 and 6.2). However, thepresence of two donor groups in the 1,4-positions gives rise to a pronounced increase inintensity, together with a significant bathochromic shift (Table 6.2) [8]. These enhancedeffects have been widely exploited in commercial dyes. The order of bathochromicity foramino-substituted anthraquinones is: 2 < 1 < 1,5 < 1,8 < 1,2 < 1,4 < 1,4,5,8. Thissequence can be explained by VB theory [9] as well as by PPP-MO calculations [1], but onlythe latter approach can account for variations in tinctorial strength. The twin absorptionpeaks of 1,4-diaminoanthraquinones are thought to be caused by vibrational fine structureassociated with one electronic transition in the visible region. 1,4-Bis(alkylamino)-anthraquinones are generally blue. The introduction of arylamino substituents into the 1,4-


chpt6(1).pmd 15/11/02, 15:29287


positions gives rise to an additional absorption band in the 400 nm region that imparts ayellow component to the dominant blue colour. A good example is CI Acid Green 25 (6.27),which in aqueous solution absorbs at 410, 608 and 646 nm [10].

O

O

NH

NH

CH3

HO3S

HO3S

CH3

6.27

CI Acid Green 25

Acid dyes

In 1871 Graebe and Liebermann discovered that alizarin (6.2) could be applied to wool bymordant dyeing after sulphonation to produce the 3-sulphonic acid (6.28). This dye is stilllisted in the latest revision of the Colour Index as a commercial product [11]. Althoughmany sulphonated polyhydroxyanthraquinones have been examined, few remain in currentuse. Another, and more important, classic dye that continues in commercial use as an aciddye is CI Acid Blue 45 (6.29). This dye was discovered in 1897 by Schmidt and can be madefrom anthrarufin (6.13) by disulphonation, subsequent dinitration and reduction. The dyegives an attractive blue on wool with good all-round fastness properties.

O

O

OH

OH

SO3H

6.28

CI Mordant Red 3

O

O

NH2

SO3H

OH

OH

NH2

HO3S

6.29

CI Acid Blue 45

Many blue acid dyes have been derived from bromamine acid (6.10) by nucleophilicdisplacement of the halogen atom using arylamines of varying complexity in the presence of acopper catalyst. Simple examples include CI Acid Blue 25 (6.30; R = H) and the analogouscyclohexyl derivative CI Acid Blue 62 (6.31). Reaction with p-butylaniline gives CI Acid Blue230 (6.30; R = butyl), which possesses good washing fastness owing to hydrophobicinteraction with the substrate. Somewhat more complex structures are illustrated by CI AcidBlue 40 (6.32) and CI Acid Blue 129 (6.33). CI Acid Blue 40 is greener than the parent dye(6.30; R = H) due to the electronic effect of the acetylamino substituent.

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289

The progressive introduction of methyl groups into the pendant phenyl ring of CI AcidBlue 25 (6.30; R = H) leads to an increase in dye uptake and to improved wet fastnessproperties on wool [12]. Steric crowding in the case of CI Acid Blue 129 (6.33) reduces theconjugation of the 4-substituent with the remainder of the system and results in a reddishblue hue. The aliphatic cyclohexyl ring in CI Acid Blue 62 (6.31) has a similar effect on thehue.

Use of the intermediates 1-amino-2,4-dibromoanthraquinone and 1-amino-4-bromo-2-methylanthraquinone leads to similar dyes, exemplified by CI Acid Blue 78 (6.34; X = Br)and CI Acid Blue 47 (6.34; X = CH3) respectively; in the production of these two dyes,sulphonation is the final step.

O

O

NH2

SO3H

NH R

6.30

O

O

NH2

SO3H

NH

6.31

CI Acid Blue 62

O

O

NH2

SO3H

NH NHCOCH3

6.32

CI Acid Blue 40

O

O

NH2

SO3H

6.33

HN

CH3

CH3H3CCI Acid Blue 129

O

O

NH2

X

NH

HO3S

CH3

6.34

Milling dyes with very good wet fastness are obtained by the reaction of one mole of adiamine with two moles of bromamine acid, as in the case of CI Acid Blue 127 (6.35). Thisdye is suitable for dyeing wool bright blue from a neutral or weakly acid bath.


chpt6(1).pmd 15/11/02, 15:29289


Condensation of one mole of leucoquinizarin (6.22) with two moles of an arylamine inthe presence of boric acid, followed by oxidation and sulphonation, is the route to somesymmetrically substituted acid dyes; these are blue to green according to the structure of theamine. The classical, but still important, dye CI Acid Green 25 (6.27) has already beenmentioned. A related dye of commercial significance is CI Acid Blue 80 (6.36), which canbe obtained either from leucoquinizarin or from 1,4-dichloroanthraquinone. Unsym-metrically substituted 1,4-diaminoanthraquinone derivatives bearing only one sulphonicacid group are better for the level dyeing of nylon than are their disulphonated analogues.

O

O

NH2

SO3H

NH

O

O

NH2

HO3S

NHC

CH3

CH3

6.35

CI Acid Blue 127

O

O

6.36

CI Acid Blue 60

HN

HN

SO3H

CH3H3C

CH3

CH3

CH3H3C

SO3H

A suitable example is CI Acid Blue 27 (6.37; X = NHCH3), which is made from 4-bromo-1-methylaminoanthraquinone by nucleophilic displacement followed bysulphonation. Some commercial acid dyes based on 1,4-diaminoanthraquinone containsubstituents in both the 2- and the 3-positions. Thus, for example, CI Acid Violet 41 (6.38)is produced by the condensation of 1,4-diamino-2,3-dichloroanthraquinone with phenol inthe presence of sodium sulphite and manganese dioxide.

Aminohydroxyanthraquinone derivatives are available from leucoquinizarin by heatingwith an appropriate arylamine and boric acid in aqueous ethanol, followed by oxidation. Theuse of p-toluidine and subsequent sulphonation gives CI Acid Violet 43 (6.37; X = OH).

The photodegradation of CI Acid Green 25 (6.27) on nylon proceeds by hydrogen atomabstraction to produce the leuco compound [13].

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Disperse dyes

Anthraquinone dyes are second only to azo dyes in importance as disperse dyes and arepredominant in the red, violet, blue and blue-green sectors [14]. Because anthraquinonedisperse dyes are relatively expensive to manufacture, successful attempts were made toreplace some of them with technically equivalent and more economical products [15]. Thereplacement process has been most successful in the red region using, for example,heterocyclic azo dyes and novel chromogens. The brilliance of the anthraquinones withtheir narrow spectral absorption bands is difficult to attain with other structures, however, asis their high light fastness and chemical stability. The development of anthraquinonedisperse dyes is included in a review by Dawson [16].

Dyes for cellulose acetate are relatively simple molecules, typified by CI Disperse Red 15(6.39; X = OH), CI Disperse Violet 4 (6.39; X = NHCH3) and CI Disperse Blue 3 (6.40),the last-named being manufactured from leucoquinizarin and the appropriate amines. Theunsymmetrically substituted product inevitably contains significant amounts of the relatedsymmetrical compounds. The widely used CI Disperse Blue 3 is known to cause skinsensitisation when on nylon [17] and can also provoke an allergic reaction [18]. Bright red2-alkoxy-1-amino-4-hydroxyanthraquinones, such as CI Disperse Red 4 (6.41), can beobtained from 1-amino-2,4-dibromoanthraquinone by hydrolysis to give 1-amino-2-bromo-4-hydroxyanthraquinone (CI Disperse Violet 17), which is then condensed with theappropriate alcohol.

O

O

X

NH CH3

HO3S

6.37

O

O

NH2

NH2

SO3H

O

6.38

CI Acid Violet 41

O

O

NH2

X

6.39

O

O

NHCH3

NHCH2CH2OH

6.40

CI Disperse Blue 3

O

O

NH2

OH

OCH3

6.41

CI Disperse Red 4

Suitable disperse dyes for polyester require good sublimation fastness and generallycontain additional or more hydrophobic substituents compared with acetate dyes. Thus, forexample, CI Disperse Red 60 (6.42) is important for the dyeing of polyester fabrics but hasonly moderate sublimation fastness. It is, however, the most important red dye for transfer


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printing. Improved sublimation fastness can be achieved if substituents are introduced intothe phenoxy ring. Exceptional fastness is achieved by chlorosulphonation of CI Disperse Red60 followed by condensation with 3-methoxypropylamine to give the brilliant red dye 6.43[16].

O

O

NH2

OH

O

6.42

CI Disperse Red 60

O

O

NH2

OH

O SO2NH(CH2)3OCH3

6.43

Monoarylation of 1-amino-4-hydroxyanthraquinone (6.39; X = OH) results in violetdyes such as CI Disperse Violet 27 (6.44; R = H) and the bluish violet CI Disperse Blue 72(6.44; R = CH3); the latter dye is also important as CI Solvent Violet 13. Chlorination of1,4-diaminoanthraquinone with sulphuryl chloride gives the 2,3-dichloro derivative (CIDisperse Violet 28), which on condensation with phenol yields CI Disperse Violet 26 (6.45).Monoaryl or dialkyl derivatives of 1,4-diaminoanthraquinone (6.19; CI Disperse Violet 1)are blue. Typical examples include CI Disperse Blue 19 (6.46) and CI Disperse Blue 23(6.47).

O

O

NH

OH

R

6.44

O

O

NH2

NH2

O

O

6.45


O

O

NH

NH2

6.46

CI Disperse Blue 19

O

O

NHCH2CH2OH

NHCH2CH2OH

6.47

CI Disperse Blue 23

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Many commercially important blue dyes are derived from tetra-α-substitutedanthraquinones, especially the diamino derivatives of anthrarufin (6.13) and chrysazin(6.14). Examples include CI Disperse Blue 26 (6.48) and CI Disperse Blue 56 (6.49).Progressive methylation of 1,8-diamino-4,5-dihydroxy- and 1,5-diamino-4,8-dihydroxy-anthraquinone leads to a series of reddish-blue to greenish-blue dyes [19]. The presence ofN,N-dimethylamino substituents results in marked hypsochromic shifts in the visibleabsorption maxima due to steric hindrance (compare 6.26). The presence of amino groups isoften associated with inferior fastness to burnt gas fumes. This drawback can be overcomeby the use of dyes such as CI Disperse Blue 27 (6.50), which, although relatively expensive,has very good all-round fastness properties on both cellulose acetate and polyester. This dyecan be obtained from chrysazin (6.14) by nitration and subsequent condensation with p-(2-hydroxyethyl)aniline. Bright blue dyes for polyester can be manufactured from the well-known acid dye CI Acid Blue 45 (6.29) by the C-arylation of its boric ester with, forexample, a mixture of phenol and anisole, followed by removal of the sulphonic acid groups.This procedure gives CI Disperse Blue 73 (6.51; R = H or CH3). 1,4,5,8-Tetra-aminoanthraquinone (CI Disperse Blue 1) is a cellulose acetate dye of long standing, as isthe product of partial methylation, CI Disperse Blue 31. The parent dye is now known to becarcinogenic [20].

O

O

NHCH3

OH

OH

H3CHN

6.48

CI Disperse Blue 26

O

O

NH2

OH

OH

NH2

Br

6.49

CI Disperse Blue 56

O

O

NH

OH

NO2

OH

CH2CH2OH

6.50

CI Disperse Blue 27

O

O

NH2

OH

OH

NH2

OR

6.51

Few heterocyclic anthraquinone analogues are of established importance as disperse dyes.An interesting exception is the system 6.52, which provides bright turquoise blue dyes ofvery good light fastness on polyester [21]. A route to CI Disperse Blue 60 (6.52; R =CH2CH2CH2OCH3) involves hydrolysis of 1,4-diamino-2,3-dicyanoanthraquinone followedby alkylation of the resulting imide; several closely related products can be obtained byvarying the alkyl group.


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Not many green anthraquinone disperse dyes are available commercially. One example isCI Disperse Green 6:1 (6.53), which has wider use as CI Solvent Green 3 and is theprecursor of CI Acid Green 25 (6.27).

O

O

NH2

NH2

N

O

O

R

6.52

O

O

NH

NH

CH3

CH3

6.53

CI Disperse Green 6:1

Basic dyes

Some anthraquinone dyes with pendant cationic groups are used commercially on acrylicand modacrylic fibres [22]. Only two disclosed structures are included in the latest revisionof the Colour Index, the dyes being the reddish blue CI Basic Blue 47 (6.54) and the greenishblue CI Basic Blue 22 (6.55).

O

O

NH2

NH CH2NH(CH3)2 X

6.54

+ _

CI Basic Blue 47

O

O

NHCH3

NHCH2CH2CH2N(CH3)3 X

6.55

+ _

CI Basic Blue 22

Reactive dyes

The chemistry of reactive dyes is discussed in detail in Chapter 7. However, it is appropriateto point out in this section that the anthraquinone system provides several commerciallyvaluable bright blue reactive dyes. Bromamine acid (6.10) is the key intermediate for thesynthesis of the vinylsulphone dye CI Reactive Blue 19 (6.56), which finds application indyeing and printing. The dichlorotriazinyl dye CI Reactive Blue 4 (6.57) is derived from thesame intermediate and is used in protein purification [23]. CI Reactive Blue 6 (6.58),obtained from the reaction of 1,4-diaminoanthraquinone with epichlorohydrin, was the onlyblue dye in the Procinyl (ICI) range for nylon. Anthraquinone blues have been replaced tosome extent by derivatives of the intrinsically bright triphenodioxazine chromogen [24].

6.2 POLYCYCLIC VAT DYES

Vat dyes are insoluble in water and are applied to cellulosic fibres, usually with sodiumdithionite under alkaline conditions, by a vatting process involving reduction to produce a

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water-soluble leuco form which possesses affinity for the substrate. Air or chemical oxidationthen results in regeneration of the insoluble form of the colorant trapped within the fibrestructure.

Vat dyes based on anthraquinone and related polycyclic systems generally exhibitoutstanding fastness properties. Most vat dyes contain two carbonyl groups linked by aconjugated (quinonoid) system (Scheme 6.10). They can be applied as the water-solubledisulphuric esters of the leuco forms; these have low affinity for cellulose and can beobtained by esterifying the leuco compound with chlorosulphonic acid. Reduction andesterification may be carried out without isolating the leuco compound using pyridine andiron together with chlorosulphonic acid [25].

O

O

NH2

NH NH

SO3H

N

N

N

Cl

Cl

SO3H6.57

CI Reactive Blue 4

O

O

NH2

NH SO2CH2CH2OSO3H

SO3H

6.56

CI Reactive Blue 19

O

O

NHCH2CHCH2Cl

NHCH2CHCH2Cl

OH

OH6.58

CI Reactive Blue 6

O

O ONa

ONa

OH

OH

OSO3H

OSO3H

Na2S2O4 / NaOH

air

2H+

2ClSO3H

Scheme 6.10

POLYCYCLIC VAT DYES

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Anthraquinonoid systems are the most important vat dyes, providing hues covering thewhole of the visible spectrum. Nevertheless, the main commercial products are found withinthe blue, green, brown and black sectors. Some of the relatively simple yellow and orangedyes cause phototendering of the dyed substrate, bringing about adverse structural changeson exposure to light.

Most of the currently available vat dyes have been known for many years and very fewnew dyes have been marketed during the last thirty years [26]. Recent research efforts haveconcentrated on improved methods of manufacture and more economical finishing oftraditional vat dyes.

The general stability of the polycyclic vat dyes permits many of them to be used aspigments [27]. Several types of polycyclic vat colorants provide structures suitable for use oncellulosic fibres for infrared camouflage [28].

6.2.1 Anthraquinones

In 1909 Deinet observed that the product of N-benzoylation of 1-aminoanthraquinonecould be applied as a vat dye. Although the leuco form of 1-aminoanthraquinone has noaffinity for cotton, the corresponding benzoyl derivative does have a moderate affinity.

Acylaminoanthraquinones are structurally the simplest of the vat dyes, and are used foryellow, orange, red and violet hues. Numerous acylaminoanthraquinones have beenexamined for use as vat dyes but only a few have survived competition from more cost-effective types. One example is CI Vat Yellow 26 (6.59), which is made from 1-aminoanthraquinone and isophthaloyl chloride. Another yellow dye of commercialimportance is CI Vat Yellow 33 (6.60), which is made by reducing 4′-nitrobiphenyl-4-carboxylic acid with glucose and sodium carbonate. The resulting azo compound is acylatedwith thionyl chloride before condensing with 1-aminoanthraquinone. It is of significancethat the azo bridge remains intact during the vatting process; in this dye the azo group isunable to tautomerise into a hydrazone and consequently resists reduction [29].

O

O

C CHN

O O

O

O

NH

6.59

CI Vat Yellow 26

O

O

HNC

O

N NO

O

NHC

O

CI Vat Yellow 33

6.60

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Derivatives of 1,4- and 1,5-diaminoanthraquinone provide some important examples inthis class. Relatively simple dyes include CI Vat Yellow 3 (6.61; X = H) and CI Vat Violet 15(6.61; X = OH).

O

O X

CHN

O

X

C NH

O

6.61

It is generally found that dyes based on the 1,4-diaminoanthraquinone system are morebathochromic than isomers derived from 1,5-diamino-anthraquinone. This difference can berelated to the relative stabilities of the dipolar excited states of the dyes. Qualitatively, the1,4-disubstituted derivatives are aromatic (6.62) and therefore at a lower energy, whereasthe 1,5-disubstituted isomers (6.63) are not. Consequently, the energy difference betweenthe ground state of the molecule and its first excited state is smaller for 1,4-diaminoanthraquinones than for the 1,5-isomers. For example, the 1,4-isomer of CI VatYellow 3 (6.61; X = H) is CI Vat Red 42, although this dye is no longer in commercial use.A more complex example is CI Vat Red 21 (6.64), which contains amino groups in the acylring systems.

O

O

NH2

NH2

6.62

+_

_+

O

O

NH2

H2N

6.63

+

+

_

_

O

O

NH2

CONH

O

O

NH2

NHCO

OO

6.64

CI Vat Red 21

POLYCYCLIC VAT DYES

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6.2.2 Indanthrones

In 1901 Bohn discovered the first vat dyes based on anthraquinone, a landmark in thehistory of synthetic dyes. In an attempt to prepare an anthracene analogue of indigo (6.113),Bohn applied the Heumann synthesis to 2-aminoanthraquinone by fusing the condensationproduct of this amine and chloroacetic acid with potassium hydroxide. The brilliant blueproduct was named indanthrene, a name coined from those of indigo and anthracene. Bohnshowed that the same product resulted from the alkali fusion of 2-aminoanthraquinone, sothe dye could not be an analogue of indigo. The new vat dye was marketed by BASF asIndanthren Blue R (CI Vat Blue 4) and was subsequently given the chemical nameindanthrone (6.65). The structure was proposed on the basis of elemental analysis(C28H14O4N2), suggesting the loss of four atoms of hydrogen from two molecules of 2-aminoanthraquinone), the absence of azo and of primary amino groups and the exceptionalstability. This was confirmed by synthesis: for example, Bohn obtained indanthrone byheating 1-amino-2-bromoanthraquinone in nitrobenzene solution under Ullmannconditions.

O

O

N

N

O

O

H

H

1

2

3

4

7

81011

1213

16

17

6.65

CI Vat Blue 4

Indanthrone has low fastness to bleaching because the corresponding yellowish greenazine (6.66) is formed. Its importance at the time of its discovery lay in the dearth of fastdyes for cotton. Indanthrone is also used as a pigment (CI Pigment Blue 60).

Very careful control of the fusion conditions is necessary in order to minimise theformation of by-products. Alkali fusion at approximately 220 °C gives mainly the blue alkalisalt of leuco indanthrone; air oxidation is then required to produce the dye. Fusion at lowertemperatures results in the formation of alizarin (6.2), whereas reaction at a highertemperature (above 300 °C) affords significant amounts of a yellow dye as well as the blueindanthrone. The yellow dye was first marketed by BASF as Flavanthren (CI Vat Yellow 1)and was subsequently given the chemical name flavanthrone (6.67). Today flavanthrone isused mainly as a pigment (CI Pigment Yellow 24). Flavanthrone is readily reduced to a blueleuco form; hydrogen abstraction to generate the leuco form can also take place oncellulose, leading to fibre damage. The leuco compound is believed, on the basis ofspectroscopic evidence, to have the structure 6.68 [30]. The yellow to blue colour change ismade use of in testing for the presence of a reducing agent under alkaline conditions. Thestructure of flavanthrone was confirmed by an Ullmann synthesis using the Schiff base(6.69) obtained from 2-amino-1-chloroanthraquinone, which gave a bianthraquinonylprecursor (6.70), as in Scheme 6.11. Hydrolysis of compound 6.70 gives the corresponding

O

O

N

N

O

O

6.66

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299

diamine, which subsequently undergoes cyclodehydration to flavanthrone. In a relatedindustrial synthesis, the diphthalimide of 2-amino-1-chloroanthraquinone is converted intothe corresponding bianthraquinonyl, which undergoes hydrolysis and ring closure in dilutesodium hydroxide.

O

N

N

O

6.67

CI Vat Yellow 1 OH

N

N

OH

6.68

The formation of indanthrone and flavanthrone, as well as alizarin, during the alkalifusion of 2-aminoanthraquinone can be explained mechanistically on the basis of the initialloss of a proton. The resulting anionic species can be represented as a resonance hybrid andis also tautomeric (Scheme 6.12). Primary 1-hydroxylation of 2-aminoanthraquinone isprobably the first step in the formation of the alizarin by-product (compare Scheme 6.8).Such an attack may initiate the formation of flavanthrone [31]. It is also possible to envisagethe formation of all three species by a radical mechanism [32].

The normal blue vat dye obtained from indanthrone corresponds to adihydroanthraquinone (compare flavanthrone) and yields a sparingly soluble disodium salt.It seems likely that strong hydrogen bonding between the inner hydroxy groups and theazine nitrogen atoms accounts for the ready formation of the planar molecule 6.71. Theinfrared spectrum of the disodium salt indicates the absence of carbonyl groups. Treatmentof the salt with dimethyl sulphate produces the corresponding dimethyl diether.

Halogenated indanthrones provide improved resistance to hypochlorite treatments andmany derivatives have been examined. Direct halogenation of indanthrone leads tosubstitution at the 7-, 8-, 16- and 17-positions. The commercially important 7,16-dichloroindanthrone (6.72; CI Vat Blue 6) can be synthesised by a rather lengthy route fromphthalic anhydride and chlorobenzene (Scheme 6.13). Polycyclic vat dyes of this type alsofind use as electrophotographic photoreceptors.

O

O

Cl

N CH

O

O

N CH

O

O

NCH

6.69

2

6.70Scheme 6.11

POLYCYCLIC VAT DYES

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O

O

NH2

O

O

NH

O

O

NH

HO

O

NH2

O

O

NO

O

NH2

H

H

O

OO

O

NH2

HH2N

O

O

NH

O

ONH2

O

O

H2N

O

O

NH2

O

O

H2N

O

O

NH2

O

O

NH

O

ONH2

6.65

Flavanthrone Indanthrone

– H2O

+ OH

tautomerism

2-amino AQ300 °C

2-amino AQ220 °C

+ OH– H2O

+ OH– H2O

oxidation oxidation

cyclodehydration repeat sequence

_

_

_ _

__

_ _

_

_

_

_

resonance

6.67

Scheme 6.12

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301

O

ONa

N

N

ONa

O

H

H

6.71

O

O

N

N

O

O

H

H

Cl

Cl

6.72

CI Vat Blue 6

O

O

O

Cl

O

ClCO2H

O

ClCO2H

NO2

O

ClCO2H

NH2

O

O

Cl

NH2NO2

O

O

Cl

NH2

Br

AlCl3

H2SO4

Br2

CI Vat Blue 6

+

HNO3 / H2SO4

Fe / HCl

Ullmann reaction

Phthalic anhydride

6.72Scheme 6.13

6.2.3 Benzanthrone derivatives

Benzanthrone (6.73) is the source of various commercially important violet, blue and greenvat dyes. This tetracyclic system can be prepared from a mixture of anthraquinone andpropane-1,2,3-triol (glycerol) by heating with iron powder in concentrated sulphuric acid.The reaction involves reduction of anthraquinone to anthrone (6.74) followed bycondensation (Scheme 6.14) with propenal (acrolein), the latter compound being generated

POLYCYCLIC VAT DYES

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from glycerol by dehydration. The structure of benzanthrone was confirmed by Scholl whoprepared it by cyclising 1-benzoylnaphthalene with aluminium chloride.

The benzanthrone system is susceptible to both electrophilic and nucleophilic attack.The most reactive sites towards electrophiles are the 3- and 9-positions, which can becompared with the 4,4′-positions in biphenyl. The 9-position is somewhat deactivated by thecarbonyl group, however. Thus, for example, monobromination takes place at the 3-positionand further substitution gives 3,9-dibromobenzanthrone. Nitration and benzoylationsimilarly give rise to the 3-substituted product. The 3-position is in fact peri-hindered(compare naphthalene) so that sulphonation yields the 9-sulphonic acid. Electron with-drawal by the carbonyl group activates the 4- and 6-positions towards nucleophilic attack:for example, hydroxylation occurs at these sites.

In 1904 Bally obtained a bluish violet solid by alkali fusion of benzanthrone at approxi-mately 220 °C. Two isomeric compounds were isolated by vatting the reaction mixture andfiltering off a sparingly soluble sodium salt. Oxidation of the filtrate gave a blue vat dye,violanthrone (6.75; CI Vat Blue 20), as the main component. The less soluble residuesimilarly afforded a violet product, isoviolanthrone (6.76; CI Vat Violet 10). The formationof isoviolanthrone can be suppressed by carrying out the fusion in a solvent such asnaphthalene or a polyethylene glycol in the presence of sodium acetate and sodium nitrite.Dyes of this type are often referred to as dibenzanthrones.

O

HH

CHCHOCH2

O

1

2

3

4

5

689

10

11

6.74 6.73

Anthrone Benzanthrone

Scheme 6.14

O

O

1

2

3

4

6

78

911

12

13

14

1516

17

18

6.75

CI Vat Blue 20

O

O1

2

34

67

8

5

11

12

1314

1516

17

10

6.76

CI Vat Violet 10

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303

It was established in 1929 by Lüttringhaus and Neresheimer that 4,4′-bibenzanthronyl(6.77) is an intermediate in the formation of violanthrone. Thus, compound 6.77 resultswhen an Ullmann reaction is carried out on 4-chlorobenzanthrone; the same product isobtained when benzanthrone reacts under relatively mild conditions (approximately 110 °C)with a mixture of potassium hydroxide and potassium acetate in 2-methylpropan-1-ol(isobutanol). Alkali fusion at a higher temperature then converts 4,4′-bibenzanthronyl intoviolanthrone. Use of aluminium chloride also leads to ring closure (Scholl reaction).

Mechanistically it is likely that the resonance-stabilised anion resulting from deprotonationat the 4-position of benzanthrone couples with non-ionised benzanthrone. Linkage across the3- and 3′-positions under more strongly alkaline conditions may also involve loss of a proton,coupling and oxidation (Scheme 6.15). Chlorination of benzanthrone followed by an Ullmannreaction produces 3,3′-bibenzanthronyl (6.78), which readily undergoes 4,4′-coupling undermild conditions. 3,3′-Bibenzanthronyl is available from benzanthrone directly by treatmentwith manganese dioxide in sulphuric acid.

Isoviolanthrone (6.76) was first made by heating 3-chlorobenzanthrone with ethanolicpotassium hydroxide at 150 °C (Scheme 6.15). The reaction probably involvesdeprotonation at the 4-position to form a carbanion which then displaces the chlorosubstituent in another molecule of 3-chlorobenzanthrone. Repeating the sequence in theintermediate 3,4′-bibenzanthronyl derivative then gives isoviolanthrone. In an improved,but mechanistically obscure, method of synthesis 3-chlorobenzanthrone is treated withsodium sulphide to produce 3,3′-bibenzanthronyl sulphide; this compound is converted intoisoviolanthrone on refluxing with potassium hydroxide in isobutanol. Isoviolanthrone itselfis no longer used as a vat dye but some halogenated derivatives are commercially available.For example, dichlorination of isoviolanthrone in nitrobenzene gives CI Vat Violet 1, whichis also useful as a pigment (CI Pigment Violet 31). The greenish blue 6,15-dimethoxyderivative (CI Vat Blue 26) is also of some importance.

Nitration of violanthrone gives CI Vat Green 9, the main component of which is the 16-nitro derivative. Vatting gives the corresponding amino compound, which gives a bluishblack hue when oxidised on the fibre. Several halogenated derivatives, such as the trichloroderivative CI Vat Blue 18, are of industrial significance.

Of special importance is the outstanding bright green vat dye originally marketed byScottish Dyes as Caledon Jade Green (6.79; CI Vat Green 1), discovered in 1920 by Davies,Fraser-Thomson and Thomas. Oxidation of violanthrone with manganese dioxide andsulphuric acid gives 16,17-dihydroxyviolanthrone. Methylation of this diol yields thedimethoxy derivative (6.79). This dye has excellent fastness to light, alkali and chlorine. Itsconstitution was confirmed by a lengthy synthesis from 2-methoxybenzanthrone. Analternative method of manufacture involves the oxidation of 4,4′-bibenzanthronyl (6.77) withmanganese dioxide and sulphuric acid to give violanthrone-16,17-dione (6.80), which onreduction with sodium bisulphite gives an excellent yield of the 16,17-diol. Treatment of CIVat Green 1 with bromine in oleum gives the 3,12-dibromo derivative (CI Vat Green 2). Thesubstitution pattern was confirmed by the n.m.r. spectrum of the dimethyl diether of the leucocompound. Many other alkyl derivatives of 16,17-dihydroxyviolanthrone have been examinedbut few remain of commercial significance. Of some interest is the navy blue vat dye CI VatBlue 16, in which the 16- and 17-positions are connected by an ethylenedioxy bridge (6.81).

Violanthrone and isoviolanthrone are effectively cis and trans isomers. The important ciscompound absorbs more intensely and at a longer wavelength than the trans isomer (Table

POLYCYCLIC VAT DYES

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O

H

O

OO

H

OO

OO

OO O

Cl

+ OH

– H2O

+

6.73

6.73 KOH / KOAc110 °C

oxidation

6.77

(1) KOH / KOAc or AlCl3 220 °C(2) oxidation

6.75

KOH / KOAc110 °C

6.78

KOH / EtOH150 °C

6.76

Ullmann

reaction

_

_

_

_

Benzanthrone

resonance

4,4′-Bibenzanthronyl

ViolanthroneIsoviolanthrone

3,3′-Bibenzanthronyl

Scheme 6.15

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305

6.3). These differences are found to be general for this type of vat dye and can be accountedfor theoretically by the PPP-MO approach [33]. It is of interest to compare the absorptionmaxima of violanthrone and some of its derivatives in Table 6.3. Steric crowding at anessential single bond [34] normally results in a hypsochromic shift of the absorption band asshown by the 16,17-dimethyl derivative and, to a lesser extent, by the 16-methoxy compound.In contrast, however, the blue 16,17-bridged violanthrone (6.81) exhibits a bathochromiceffect that is somewhat greater in the green dimethoxy analogue. Since the steric requirementsof the two methoxy groups are not significantly less than those of the equivalent methylgroups, it must be concluded that this bathochromic shift is electronic in origin. A possibleexplanation is the formation of a stabilised excited state (6.82) in the case of the dimethoxydye in which some attraction exists between the adjacent oxygen atoms; any steric clash canbe minimised by rotation of the methoxy groups away from the plane of the ring system. Apronounced bathochromic effect is also exhibited by the 7,8-dimethoxy isomer, in which thesubstituents are similarly situated, but not by the 3,12-dimethoxy compound [32]. It is likelythat the constraint imposed by the puckered eight-membered ring in the 16,17-bridged dyereduces the interaction between the oxygen lone-pair orbitals and the π-orbitals of thepolycyclic system.

OO

O O

H3C CH3

6.79

312

CI Vat Green 1

OO

O O

6.80

Violanthrone-16,17-dione

OO

O O

H2C CH2

6.81

CI Vat Blue 16

Table 6.3 Spectral data for some dibenzanthrones in dimethylformamide

CI Vat Compound λmax (nm) εmax

Violet 10 Isoviolanthrone (6.76) 588 41 700Blue 20 Violanthrone (6.75) 600 60 260

16-Methoxyviolanthrone 598 38 900Green 1 16,17-Dimethoxyviolanthrone 636 41 700Blue 16 16,17-Bridged violanthrone 618

16,17-Dimethylviolanthrone 575

POLYCYCLIC VAT DYES

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6.2.4 Carbazole derivatives

Diphthaloylcarbazole derivatives constitute an important group of vat dyes, possessing goodall-round fastness properties and providing hues ranging from yellow to black.

The precursors of the carbazoles are the anthrimides, in particular the 1,1′-dianthraquinonylamines (1,1′-dianthrimides). These intermediates were originally used asvat dyes themselves, but very few give bright colours and the vatting process also bringsabout reduction to 1-aminoanthraquinone. Only one dianthrimide is currently active in theColour Index [11]; this grey vat dye is CI Vat Black 28 (6.83). 1,1′-Dianthrimides are easilyprepared by heating a 1-aminoanthraquinone with a 1-chloroanthraquinone in nitrobenzeneor another solvent in the presence of sodium carbonate and a copper catalyst.

OO

O O

CH3H3C

6.82

+

_

O

O

HN

O

O

NH

NHC

O

C

O

6.83

CI Vat Black 28

In 1910 Mieg found that 1,1′-dianthrimides undergo carbazolisation (ring closure) whenheated with potassium hydroxide at approximately 220 °C or with aluminium chloride at asomewhat lower temperature (Scholl reaction). The simplest dye of this type (6.84; CI VatYellow 28) is no longer manufactured. Cyclisation under strongly basic conditions is probablyinitiated by deprotonation (Scheme 6.16); an oxidation step is necessary after ring closure.The characteristic structure of this type of vat dye, which is stabilised by intramolecularhydrogen bonding (6.84), was established by an alternative synthesis based on the Graebe

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307

route to carbazole and involving an Ullmann reaction with 2-chloro-1-nitroanthraquinone,followed by reduction and cyclisation.

The parent dye (6.84), which has a phototendering action on cellulose, is now obsolete butseveral relatively simple benzoylamino derivatives are of commercial importance. Thus CI VatOrange 15 (6.85; R1 = R4 = NHCOPh, R2 = R3 = H) is a 1,5-diaminoanthraquinone typewhereas the more bathochromic olive vat dye CI Vat Black 27 (6.85; R1 = R4 = H, R2 = R3

= NHCOPh) is a 1,4-diaminoanthraquinone derivative. The unsymmetrically substitutedisomer (6.85; R1 = R3 = H, R2 = R4 = NHCOPh) is a useful brown vat dye (CI VatBrown 3).

O

O

O

O

N

H

O

O

O

O

N

H

O

O

O

O

N

H

H

O

O

O

O

N

H

+ OH

– H2O

oxidation

6.84

_

_

_

CI Vat Yellow 28

Scheme 6.16

O

O

O

O

N

H

R2 R3

R4R1

6.85

The presence of two benzoylamino groups in the dianthrimide system permits carbazole ringclosure to take place relatively easily in concentrated sulphuric acid at approximately 30 °C. Itis likely that the benzoylamino groups assist protonation at the adjacent carbonyl group andthereby aid the subsequent cyclisation process. The final oxidation step can take place with

POLYCYCLIC VAT DYES

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sulphuric acid alone. It is possible that the cyclisation occurs by a radical mechanism [32].Various more complex dyes of industrial significance contain more than one carbazole

system. For example, CI Vat Orange 11 (6.86) contains two carbazole components and acentral unit derived from 1,5-diaminoanthraquinone. This dye is prepared by carbazolisation ofthe trianthrimide produced when two moles of 1-chloroanthraquinone react with one mole of1,5-diaminoanthraquinone. The equivalent isomeric dye obtained from 1,4-diamino-anthraquinone is reddish brown (6.87; CI Vat Brown 1). The interesting, symmetrically-substituted tetracarbazole dye CI Vat Green 8 (6.88) was first synthesised in 1911 by Heppfrom 1,4,5,8-tetrachloroanthraquinone. Not surprisingly, the product (C70H28N4Ol0; relativemolecular mass 1084) is of very low solubility. The structure was confirmed in 1957 byJayaraman, who found no evidence of uncyclised anthrimides in the UV spectrum of the dyesolution in concentrated sulphuric acid [32].

O

O

O

O

N

HO

O

N

H

6.86

CI Vat Orange 11

O

O

O

O

N

H

NH

O

O

6.87

CI Vat Brown 1

O

O

OO

N

H

N

H

O

O

O

O

N

H

O

O

N

H

6.88

CI Vat Green 8

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6.2.5 Acridone derivatives

The first anthraquinone vat dye containing an acridone ring system was synthesised in 1909by Ullmann. The parent compound (6.89) can be made by condensation of 1-chloroanthraquinone with anthranilic acid in the presence of copper, followed bycyclodehydration in concentrated sulphuric acid (Scheme 6.17).

O

O

Cl NH2

COOH

O

O

NH

COOH

O

O

NH

O

O

O

N

OH

Cu

H2SO4

+

6.89

1

2

3

4 6

7

9

10

11

12

Scheme 6.17

Alternatively, the synthesis may begin by condensing aniline with the 1-chloro-2-carboxyintermediate. Acridone vat dyes of this type have excellent light fastness but only moderateresistance to alkali due to the keto-enol equilibrium. It is interesting that this pentacyclicdye is approximately 30 nm more bathochromic than the closely related tetracyclic 1-amino-2-benzoylanthraquinone.

The parent acridone is not useful as a vat dye, although some halogenated derivatives arecommercially important. Chlorination of dye 6.89 with sulphuryl chloride gives the reddishviolet CI Vat Violet 14, which consists mainly of the 6,10,12-trichloro compound. Thecolour of acridone derivatives is very sensitive to the number and position of substituents;for example, the 6-chloro derivative is violet, whereas the 9,12-dichloro compound is redand its 9,11-isomer is orange. The presence of a substituted amino group at the 6-positionleads to blue dyes such as CI Vat Blue 21 (6.90). This dye is obtained from bromamine acid(6.10) and 4-trifluoromethylanthranilic acid, followed by ring closure, during whichdesulphonation takes place, and finally benzoylation. The relatively simple bisacridone CIVat Violet 13 (6.91) is the original dye made in 1909 by Ullmann from 1,5-dichloroanthraquinone; this product is still of industrial importance.

POLYCYCLIC VAT DYES

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Various complex carbazole-acridone dyes have been exploited to obtain brown, khaki,grey and olive hues. The fused carbazole-acridone CI Vat Brown 55 (6.92), which possessesexcellent all-round fastness properties, is currently available. This compound can beobtained from the 6,10,12-trichloro derivative (CI Vat Violet 14) by condensation with twoequivalents of 1-amino-5-benzoylaminoanthraquinone, followed by carbazolisation.

The important olive green dye CI Vat Green 3 (6.93; R = H), which has outstanding lightfastness, can be regarded as an acridone analogue. This dye is readily available by condensing3-bromobenzanthrone with 1-aminoanthraquinone under Ullmann conditions followed by ringclosure with potassium hydroxide. A useful derivative (6.93; R = 1-anthraquinonylamino),prepared in a similar manner from 3,9-dibromobenzanthrone, is a brownish grey dye (CI VatBlack 25).

O

ON

N

H

O

O

H

6.91

CI Vat Violet 13

N

O

R

O

OH

6.93

NH

O

O

NO

O

H

O

Cl

NH C

O

NH

O

O

NH

CO

6.92

CI Vat Brown 55

O

O

N

CF3

O

H

NH

O

6.90

CI Vat Blue 21

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311

6.2.6 Miscellaneous polycyclic quinones

The carbocyclic analogue of flavanthrone was first prepared in 1905 by Scholl. Chlorinationof 2-methylanthraquinone with sulphuryl chloride yields the 1-chloro derivative, whichunder Ullmann conditions gives 2,2′-dimethyl-1,1′-bianthraquinonyl (6.94). Treatment ofthe latter compound with ethanolic potassium hydroxide produces pyranthrone (6.95; CIVat Orange 9). This cyclisation reaction (Scheme 6.18) was the first example of its kind andprobably proceeds via carbanion formation. As well as being an anthrone derivative,pyranthrone is a fused benzanthrone and also contains a central pyrene ring system.Pyranthrone, a powerful phototenderer, gives rise to brilliant orange hues of good fastnessproperties. Bromination of pyranthrone in chlorosulphonic acid gives mainly the reddishorange 4,12-dibromo derivative, CI Vat Orange 2.

An alternative route to pyranthrone, involving baking 1,6-dibenzoylpyrene (6.96) withaluminium chloride, was also devised by Scholl (Scheme 6.18). The Scholl reaction is a keystep in the synthesis of several polycyclic quinones; the cyclisation of 1-benzoylnaphthaleneto give benzanthrone (6.73) has already been mentioned. The mechanism of thiscyclodehydrogenation reaction may involve an initial protonation step, if traces of water arepresent, or complexation with aluminium chloride. Electrophilic substitution is thereby

O

O

CH3

O

O

H3C

O

O

O

O

KOH AlCl3

1

6

1

2

3

4

5

679

10

11

12

13

14

15

6.94

6.96

6.95

CI Vat Orange 9Scheme 6.18

POLYCYCLIC VAT DYES

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promoted and is then followed by an oxidation step (Scheme 6.19). It has been suggestedthat radical cations are formed as intermediates [35].

The anthanthrone system (6.97) was discovered in 1912 by Kalb. In the latter stages thesynthesis of anthanthrone involves hydrolysis of naphthostyril (6.98), diazotisation andbiaryl formation under Gattermann conditions to give 1,1′-binaphthyl-8,8′-dicarboxylic acidbefore cyclodehydration (Scheme 6.20). The leuco derivative of anthanthrone itself haslittle affinity for cellulose and the oxidation product has low tinctorial power. However,bromination of anthanthrone or of the intermediate dicarboxylic acid leads to the brightreddish orange 4,10-dibromo derivative CI Vat Orange 3, which possesses good general

Scheme 6.19

HOOH

HH

H

OH

H

OHO

– H

oxidation

– 2H

6.73

+

+

+

+

Benzanthrone

HN CO COOHNH2 COOHN2

CO OH

COHO

O

O

KOH HNO2

Cu

2 2 2

AlCl3 or

H2SO4

6.97

1

2

3

4

5

8

9

10

11

7

6.98Naphthostyril

AnthanthroneScheme 6.20

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313

fastness properties and finds use as a pigment, CI Pigment Red 168. This derivative is usedas a charge generation material in commercial photocopiers and laser printers [36].Treatment of one mole of 4,10-dibromoanthanthrone with two moles of 1-amino-4-benzoylaminoanthraquinone gives the bluish grey dye CI Vat Black 29.

O

O

1

2

34

5

68

9

10

1112

13

6.99

CI Vat Yellow 4

O

Cl

O

Cl

6.100

CI Vat Brown 45

There are several isomeric dibenzopyrenequinones, of which only CI Vat Yellow 4 (6.99)is of commercial importance. This compound is synthesised from 1,5-dibenzoylnaphthaleneusing the Scholl cyclisation procedure; the same product can be obtained in a similarmanner from 3-benzoylbenzanthrone. Its dibromo derivative CI Vat Orange 1, prepared bydirect bromination, possesses improved fastness to light, washing and bleaching. The parentdye is now known to be carcinogenic [20].

The reddish brown dye CI Vat Brown 45 (6.100) is a dichloro derivative of the unusualacedianthrone system. This dye is prepared by condensing two moles of 2-chloroanthronewith one mole of glyoxal followed by oxidative cyclisation, which probably takes place in astepwise manner [31].

The 1,9-heterocyclic derivative flavanthrone (6.67) has already been described. Severalrelated heterocyclic compounds are of importance as vat dyes. The 1,9-pyrimidanthronederivative 6.101 is of commercial significance both as a vat dye (CI Vat Yellow 20) and alsoas a pigment (CI Pigment Yellow 108). The pyrimidine ring system, which preventsphototendering on cellulose, can be obtained from the appropriately substituted 1-aminoanthraquinone with dimethylformamide and either thionyl chloride (SOCl2) orphosgene (COCl2). 1,9-Pyrazolanthrones, like the anthrapyrimidines, can be regarded asheterocyclic analogues of benzanthrone. The parent tautomeric pyrazolanthrone system(6.102) is available from 1-aminoanthraquinone by diazotisation, reduction to the hydrazinewith sodium bisulphite and ring closure in sulphuric acid. As with benzanthrone,dimerisation takes place on fusion with alkali. Improved resistance to alkali is achieved byalkylation. Thus the useful bluish red dye CI Vat Red 13 (6.103) is obtained by ethylation ofthe parent dimer. Several other valuable dyes also contain a pyrazolanthrone system, such asthe navy blue dye CI Vat Blue 25 (6.104), which is prepared by condensing 3-bromobenzanthrone with pyrazolanthrone followed by treatment with alkali, and the greydye CI Vat Black 8 (6.105), which is made in a similar manner from 3,9-dibromobenzanthrone, pyrazolanthrone and 1-aminoanthraquinone.

POLYCYCLIC VAT DYES

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Heterocyclic ring systems are also used to connect two anthraquinone groups. Typicalexamples include CI Vat Red 10 (6.106), which is an oxazole derivative obtained from 2-amino-3-hydroxyanthraquinone and the appropriate acyl chloride, the similar thiazolederivative CI Vat Blue 31 (6.107) and the oxadiazole derivative CI Vat Blue 64 (6.108).

O

NN

CONH

O O

6.101

CI Vat Yellow 20

N

O

N

H5C2

N

O

NC2H5

6.103

CI Vat Red 13

N

O

N

O

6.104

CI Vat Blue 25

N

O

N

O

N

O

O

H

6.105

CI Vat Black 8

N

O

NH

N

O

NH

6.102

Pyrazolanthrone

O

O

O

NO

O

NH2

6.106

CI Vat Red 10

O

O

S

NO

O

NH2

NHC

O

6.107

CI Vat Blue 31

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315

Although not anthraquinonoid, some related vat dyes derived from perylene-3,4,9,10-tetracarboxylic acid and from naphthalene-1,4,5,8-tetracarboxylic acid are worthy ofmention. Thus, for example, when N-methylnaphthalimide is fused with alkali the resultingdimer is CI Vat Red 23 (6.109). This product is also useful as a pigment (CI Pigment Red179). Dyes of this type also find an application in electrophotographic recording processes[36]. Water-soluble dyes of this class (6.110) have been patented recently by Clariant forink-jet printing [37]. Treatment of naphthalene-1,4,5,8-tetracarboxylic anhydride with o-phenylenediamine gives rise to two isomeric diimidazoles, each of which is useful as a vatdye and as a pigment. The products are CI Pigment Orange 43 (6.111) and CI Pigment Red194 (6.112). The two isomers can be seperated chromatographically or by treatment withethanolic potassium hydroxide. Dichroic pigments of this type are also useful in liquid crystaldisplay devices.

O

O

NH2

N

N

O

N O

O

NH2

N

6.108

CI Vat Blue 64

H HOO

N N

O

O

O

O

CH3H3C

6.109

CI Vat Red 23

NH

N

N

N

R

NaOOCCH2HN

N

NaO3S O

O

N

O

O

NH

N

N

N

R

NHCH2COONaSO3Na

6.110

N N

N

NO

O

6.111

CI Vat Orange 7

N N

N

OO

N

6.112

CI Vat Red 15

POLYCYCLIC VAT DYES

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6.3 INDIGOID AND THIOINDIGOID DYES

The natural blue dye indigo (6.113; CI Vat Blue 1) has been known since antiquity; thereare several plants from which indigo can be isolated. Of particular importance is the genusIndigofera, which can be grown in India, China and other tropical or subtropical regions[38]. Aqueous extraction of the indigo plant yields indican, the colourless glucoside of 3-hydroxyindole (6.114; indoxyl). Removal of the sugar residue in a fermentation process andair oxidation of the resulting indoxyl gives indigo. As well as the essential colouring matter,indigotin (6.113), natural indigo contains varying amounts of the isomeric by-productindirubin (6.115) and other compounds. In Europe the woad plant (Isatis tinctoria) wascultivated to provide indigo of lower purity in the guise of woad.

N

N

O H

OH

1

23

4

5

6

7

1′

2′3′

4′5′

6′7′

6.113

Indigotin

N

OH

H

N

O

H6.114

Indoxyl

N N

O

OH

H

6.115

Indirubin

The constitution of indigotin was elucidated by Adolf von Baeyer between 1865 and1883, on the basis of degradation and synthesis. It was already known that distillation ofindigo gave aniline (anil is the Portuguese word for indigo). Among other conversions, vonBaeyer showed that indigo could be reduced to indole and oxidised to isatin (6.116). The cisisomer (Z configuration) was proposed by von Baeyer for indigo. The correct E configuration(trans isomer) was eventually established in 1926 by Posner, using X-ray crystallography [39].The indigo molecule is almost planar in the solid state and forms a hydrogen-bonded latticein which each molecule is linked to four others. The observed bond lengths suggest somecontribution to the ground state of the molecule from charge-separated structures such asthe dipolar species 6.117. Not surprisingly, indigo has a high melting point (390–392 °C)and low solubility. Infrared studies have shown the existence of intermolecular hydrogenbonding in the parent compound and in many derivatives, but intramolecular hydrogenbonding (6.113) cannot be discounted. Unlike thioindigo, the trans isomer cannot readily beconverted into the cis form although derivatives of the cis isomer are known, such ascompound 6.118.

The colour of indigo depends dramatically upon its physical state and environment; forexample, the vapour is red but the colour on the fibre is blue. The marked solvatochromismof indigo (Table 6.4) is attributable mainly to hydrogen bonding. A progressivebathochromic shift of the visible absorption band is observed as the solvent polarity

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317

N

O

O

H

6.116

Isatin

N

N

O

H O

H

6.117

+

_

N N

O O

OO

6.118

Table 6.4 Solvatochromism of indigo

Solvent λmax (nm)

Vapour 540Carbon tetrachloride 588Xylene 591Ethanol 606Dimethyl sulphoxide 642Solid 660

N

N

O

H O

H

6.119

N

NH3C

H3C

H

O

CH3

CH3

O

H

6.120

(dielectric constant) increases. N,N′-Dimethylindigo and thioindigo, in which hydrogenbonding is absent, exhibit only small solvatochromic effects.

The isolated chromogen in indigo vapour, like that of thioindigo (λmax 543 nm in DMF),is red. Klessinger and Lüttke have shown that the structural unit responsible for the redcolour is the cross-conjugated H-shaped chromophore 6.119. The secondary role of thebenzene rings in indigo has been established by the synthesis of simple analogues such ascompound 6.120 [40]. PPP-MO calculations support the idea of an H chromogen [41].Despite the relatively minor role of the benzene rings in the indigo molecule, the type andposition of substituents in the chromogen are important for determining the colour of aderivative. In general, electron-donating groups at the 5,5′- and 7,7′-positions bring aboutbathochromic shifts, whereas electron-withdrawing substituents at these sites causehypsochromic shifts. Qualitatively, a donor substituent that is para or ortho to the donor NHgroup will stabilise the first excited state whereas an acceptor group has a destabilisingeffect. The reverse situation obtains at 4,4′- and 6,6′-positions that are ortho or para to theacceptor carbonyl group. The VB explanation is supported by PPP-MO calculations [39].

Indigo is still of considerable commercial importance, especially in warp dyeing for wovencotton denim, and finds use as a pigment, CI Pigment Blue 66. Light fastness of indigodyeings is only moderate but it fades on tone to a pale blue. Photofading of indigo probably

INDIGID AND THIOINDIGOID DYES

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involves attack by singlet oxygen at the central double bond [42]. Indigoid vat dyes havegradually declined in importance relative to anthraquinone derivatives.


The synthesis of indigo was much more difficult than that of alizarin (6.2) [43]. In 1865 vonBaeyer first attempted to obtain indigo by reductive dimerisation of isatin (6.116); he finallyachieved a seven-step synthesis from phenylglycine via isatin in 1878. Many syntheses havebeen developed subsequently for indigo, but very few of these have achieved industrialimportance.

In 1890 Heumann obtained a low yield of indoxyl (6.114) by fusing phenylglycine withpotassium hydroxide at 300 °C. Indoxyl, once formed, is readily oxidised by atmosphericoxygen to give indigo. An improved yield was obtained at a lower fusion temperature (200 °C)from 2-carboxyphenylglycine, starting from anthranilic acid (Scheme 6.21). The initialcyclisation is like a Dieckmann condensation [44]. This route was adopted by BASF whensynthetic indigo was first marketed in 1897. The advent of synthetic indigo soon led to a rapiddecline in demand for the relatively impure natural product. At the time the route toanthranilic acid was difficult and relatively expensive [45]. In 1901 Pfleger found that it ispossible to cyclise phenylglycine efficiently by adding sodamide at 190 °C to a low-meltingeutectic mixture of sodium and potassium hydroxides. This discovery enabled aniline to beused as a cheaper starting material. Pfleger’s route is still used today, except that the requiredphenylglycine is produced via the N-cyanomethyl derivative obtained by the condensation ofaniline with sodium formaldehyde–bisulphite and sodium cyanide; alkaline hydrolysis thengives sodium phenylglycinate.

Derivatives of indigo can be made by synthesis or by direct substitution. Sulphonation ofindigo using oleum gives mainly the 5,5′-disulphonic acid (6.121) together with the 5,7′-isomer. This colorant is of interest as a food dye (CI Food Blue 1) and as an acid dye (CIAcid Blue 74); the aluminium salt is a pigment (CI Pigment Blue 63). Several halogenateddyes have been marketed, exemplified by CI Vat Blue 5 (6.122), which is obtained fromindigo by bromination in an organic solvent such as nitrobenzene. The natural dye Tyrianpurple (CI Natural Violet 1) is 6,6′-dibromoindigo [46].

COOH

NH2

COONa

NHCH2COONa

N

ONa

COONa

H

NaOH

ClCH2COONa

Na2CO3

air

–CO26.113

Indigo

Scheme 6.21

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319

The sulphur analogue of indigo, thioindigo (6.123), was first synthesised in 1905 byFriedländer from anthranilic acid, using a reaction sequence similar to the earlier Heumannsynthesis of indigo (Scheme 6.21); the amino group was first converted into a thiol bydiazotisation, followed by treatment with sodium disulphide and reduction of theintermediate diphenyl disulphide. Reaction of the thiol with chloroacetic acid andsubsequent cyclisation gives thioindoxyl, which is then oxidised, usually with sulphur. Thefinal oxidation step, as in the case of indigo, may proceed by a hydride transfer mechanismor by a free-radical coupling reaction [44]. Thioindigo (CI Vat Red 41) is a bright red dyethat is still available commercially. Unlike indigo, it is photochromic; on exposure to lightthe trans stereoisomer (λmax 546 nm in chloroform) is converted into the morehypsochromic cis compound (λmax 490 nm in chloroform) giving a marked change of colourfrom violet to yellowish red.

N

NHO3SO

H

H

OSO3H

6.121

CI Food Blue 1

N

NBrO

H

H

OBr

Br

Br

6.122

CI Vat Blue 5

S

S

O

O

6.123

Thioindigo

Substituted thioindigoid dyes are usually obtained via the appropriate benzenethiol in aHeumann-type synthesis. The final cyclisation of the phenylthioglycolic acid derivative canoften be achieved in concentrated sulphuric acid or by using chlorosulphonic acid. Severalroutes make use of the Herz reaction (Scheme 6.22), in which a substituted aniline isconverted into the corresponding o-aminothiophenol by reaction with sulphurmonochloride followed by hydrolysis of the intermediate dithiazolium salt [47]. Afterreaction between the thiol and chloroacetic acid, the amino group is converted into a nitrilegroup by a Sandmeyer reaction. Hydrolysis of the nitrile leads to the formation of therequired thioindoxyl derivative.

NH2

R S

S

N NH2

SHR

ClH2OS2Cl2 +

_

R

Scheme 6.22

INDIGID AND THIOINDIGOID DYES

chpt6(1).pmd 15/11/02, 15:29319


The colour of the thioindigo chromogen is more sensitive to the effects of substituentsthan that of indigo. Several derivatives of thioindigo are of commercial significance. Thepresence of donor substituents para to the carbonyl groups leads to a pronouncedhypsochromic shift, as in the case of CI Vat Orange 5 (6.124).

S

O

H5C2O

S

O

OC2H5

6.124

CI Vat Orange 5

Colorants containing chloro and methyl substituents are of importance, particularly aspigments [27]. Thus, CI Vat Red 1 (6.125) also finds use as a pigment, CI Pigment Red 181.Tetrachlorothioindigo (6.126) is widely used as a reddish violet pigment (CI Pigment Red88). A naphthalene analogue of thioindigo (6.127; CI Vat Brown 5) can be prepared fromnaphthalene-2-thiol and is used for textile printing. A few unsymmetrically-substitutedthioindigo dyes are also produced; CI Vat Red 2 (6.128) is an example.

S

O

Cl

S

O

Cl

CH3

CH36.125

CI Vat Red 1

S

O

S

O

Cl

ClCl

Cl

6.126

CI Pigment Red 88

S

O

S

O6.127

CI Vat Brown 5

S

O

S

O

CH3

CH3

Cl

Cl Cl

6.128

CI Vat Red 2

Dull tertiary hues are available from mixed indigoid-thioindigoid dyes, such as CI VatBrown 42 (6.129) and CI Vat Black 1 (6.130). Hybrid dyes of this kind are relatively difficultto prepare. In one method of synthesis a suitable indoxyl or thioindoxyl reacts with 4-nitroso-N,N-dimethylaniline to form an anil at the 2-position (keto tautomer). Furthercondensation between the anil and a different thioindoxyl or indoxyl gives theunsymmetrical dye.

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321

6.3.2 Application

Vat dyes in general have the disadvantage that reduction to a water-soluble form isnecessary before application to cellulosic textiles. Traditionally sodium dithionite in alkalinesolution serves as the reducing agent. An alternative sometimes used in vat printing isthiourea dioxide, which is converted into formamidine sulphinate in alkaline solution(section 12.9.1).

Indigo and its derivatives are readily reduced under weakly alkaline conditions to thewater-soluble leuco form. The disodium salt Indigo white (6.131) is classified in the ColourIndex as CI Reduced Vat Blue 1 and is the form in which it is absorbed by cellulosic fibres.Acidification of Indigo white gives the more stable acid leuco form. The related leucodisulphuric ester (6.132; CI Solubilised Vat Blue 1) was mainly used for wool dyeing. Asmentioned in section 6.2, reduction and esterification can be carried out without isolatingthe leuco compound by using pyridine and iron together with chlorosulphonic acid. Indirectelectrolysis can also be used as a reduction technique in indigo dyeing [48].

N

O

S

O

Cl

Cl H

6.129

CI Vat Brown 42

N

O

S

O

Br

H

Cl

6.130

CI Vat Black 1

N

ONa

N

ONaH

H

6.131

Indigo white

N

HO3SO

N

OSO3HH

H

6.132

CI Solubilised Vat Blue 1

Nearly all the commercially produced indigoid and thioindigoid vat dyes have beenmarketed in their solubilised forms. Thus, for example, CI Vat Blue 5 is also available as CISolubilised Vat Blue 5. In addition, several vat dyes have been sold only as their solubilisedderivatives. Usage of these expensive derivatives has declined in recent years.

6.4 SULPHUR AND THIAZOLE DYES

Although many types of dye contain sulphur other than in sulphonic acid groups, sulphurdyes are usually considered to be those dyes that are best applied from a sodium sulphidedyebath. Like vat dyes, sulphur dyes are water-insoluble before and after application tocellulosic fibres. Disulphide linkages in the dye molecules are readily reduced by sodium

SULPHUR AND THIAZOLE DYES

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sulphide to give alkali-soluble thiol groups. The more environmentally friendly thioureadioxide can be used as a substitute for sodium sulphide [49]. The resulting leuco dye hassubstantivity for cellulose. Subsequent oxidation within the fibre by exposure to air andoxidising agents such as hydrogen peroxide leads to the regeneration of di- and poly-sulphide linkages between aromatic ring systems. The dyeings generally possess good wetfastness and satisfactory light fastness. The fastness to bleaching of sulphur dyes is poor andthe dyes are often sensitive to acid attack with the evolution of hydrogen sulphide.Nevertheless they are relatively cheap to produce and offer a full range of hues from yellowto black, albeit rather dull colours. The blues and blacks are of greatest importance,especially CI Sulphur Black 1, which is manufactured on a very large scale [50].Environmental legislation imposes increasing constraints on the use of dyes. Sulphur dyeeffluents contain, in particular, harmful sulphides which cannot be discharged into seweragetreatment systems directly. The preferred method of removing sulphides is by air oxidationusing efficient aerators, known as helixors, to produce thiosulphates [51].

The first commercial sulphur dye was discovered accidentally in 1873 by Croissant andBretonnière who heated lignin-containing organic waste, such as sawdust, with sodiumpolysulphide at about 300 °C; the product was sold under the name Cachou de Laval [52].Even today an equivalent dye (CI Sulphur Brown 1) is derived from lignin sulphonate,which is readily available from waste liquors from wood pulp manufacture. The real pioneerof sulphur dyes was Vidal, the first chemist to obtain dyes of this type from specific organiccompounds. In particular, Sulphur Black T (CI Sulphur Black 1) was made from 2,4-dinitrophenol in 1899. At the turn of the century many of the intermediates available weresubjected to sulphurisation (thionation), that is, treatment with sulphur, sodium sulphide orsodium polysulphide to introduce sulphur linkages.

6.4.1 Methods of formation

Two basic methods are used for the manufacture of sulphur dyes. In one, the startingmaterials are baked either with sulphur alone or with sulphur and sodium sulphide at atemperature between 180 and 350 °C. Alternatively the intermediates are heated underreflux in aqueous or alcoholic sodium polysulphide; this process may also be carried outunder pressure at temperatures up to about 130 °C. Following sulphurisation the dye isprecipitated by means of air or chemical oxidation, acidification or a combination of thesemethods. The sulphurisation process results in the evolution of hydrogen sulphide, which isusually absorbed in aqueous sodium hydroxide for use elsewhere – in the reduction of nitrocompounds, for example.

The commercially available sulphur dyes can be divided into three categories. Theconventional CI Sulphur dyes are water-insoluble, polymeric products, which can also beproduced in a finely dispersed form [50]. CI Leuco Sulphur dyes are obtained by treatingsulphur dyes with, typically, an alkaline mixture of sodium sulphide and sodiumhydrosulphide. These dyes are suitable for direct application and are available inconcentrated solutions (liquid dyes). Liquid dyes are also produced directly from thethionation mixture without isolation; powder or granular products can be made by drying aslurry of the dye together with a reducing agent. CI Solubilised Sulphur dyes, which arethiosulphonic acid derivatives of sulphur dyes, are obtained by the action of sodium sulphiteor bisulphite on the parent dye. These products are highly water-soluble but possess only low

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substantivity for cotton until they have been converted into the leuco form with sodiumsulphide. After application an oxidative treatment converts the thiol groups into disulphidebonds, leading to dimeric or polymeric dyes [53]. The thiosulphate dyes (ArSSO3Na) belongto a group of compounds known as Bunte salts [54].

In the Colour Index both conventional sulphur dyes and their leuco counterparts areallocated the same CI constitution number; a different number is given to the relatedsolubilised version. Thus, for example, CI Sulphur Black 1 and CI Leuco Sulphur Black 1have the reference CI 53185 whereas CI Solubilised Sulphur Black 1 appears under CI53186. Because of the complexity of the final products, sulphur dyes are classified accordingto the chemical structure of the organic starting material that predominates in themanufacturing process. Typical intermediates include aromatic amines, with or without nitroand phenolic groups, and diphenylamine derivatives.

6.4.2 Structural features

The dry baking procedure generally results in yellow, orange or brown sulphur dyes, many ofwhich are known to contain benzothiazole groups. For example, CI Sulphur Yellow 4 can beobtained from a melt of 2-(4′-aminophenyl)-6-methylbenzothiazole (6.133; dehydrothio-p-toluidine) 4,4′-diaminobiphenyl (6.134; benzidine) and sulphur. The thiazole intermediate isobtained when p-toluidine is heated with sulphur. Further sulphurisation gives the dithiazolederivative primuline base, sulphonation of which led to the discovery of primuline (CIDirect Yellow 59) by Green in 1887. Primuline base is now known to contain three relatedcomponents having one, two or three benzothiazole units, respectively [55]. Benzidine (acarcinogen) is no longer used by European manufacturers but sulphur dyes derived from thisintermediate are still available commercially in other parts of the world. The structure of CISulphur Yellow 4 was investigated in 1948 by Zerweck et al. [56]. These workers degradedthe dye in a potassium hydroxide melt and obtained various o-aminothiophenols, whichwere condensed with chloroacetic acid to produce identifiable lactams. From the ratio oflactams obtained, it was deduced that CI Sulphur Yellow 4 was a mixture of four maincomponents, exemplified by structure 6.135; these were all synthesised and gave, onadmixture, a product that was effectively identical with the commercial dye.

S

N

NH2

H3C

6.133

NH2H2N

6.134

Benzidine

N

S

H2N

S N

S

NH2

S6.135


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A similar study of the dye CI Sulphur Orange 1, obtained by heating 2,4-diaminotoluenewith sulphur, led to the isolation of compound 6.136 after reaction of the alkali melt of the dyewith chloroacetic acid. It was concluded that the commercial dye is a polybenzothiazole. Thefinal hue of the product depends on the temperature and the duration of the sulphurisation;the use of a smaller proportion of sulphur produces CI Sulphur Brown 10. A similar product(CI Sulphur Brown 8) is obtained when 2,4-dinitrotoluene is baked with sodium polysulphide.A brown sulphur dye of high fastness to light (CI Sulphur Brown 52) results when thepolycyclic hydrocarbon decacyclene (6.137) is baked with sulphur at 350 °C [57].

S

N O

H

H2N

H3C

6.136

6.137

Decacyclene

The use of intermediates capable of forming quinonimine ring systems leads to thesynthesis of red, blue, green and black sulphur dyes on treatment with aqueous or alcoholicsodium polysulphide. Thus, for example, condensation of N,N-dimethyl-p-phenylene-diamine with phenol gives a diphenylamine intermediate (6.138), which on sulphurisation inalcoholic sodium polysulphide first yields an indoaniline oxidation product (6.139). Thisquinonimine in turn undergoes thionation to form a phenothiazone chromogen (6.140).Further reaction leads to the synthesis of CI Sulphur Blue 9. A similar sequence can beenvisaged using indophenol derivatives. For example, the simple indophenol 6.141, which ismade by oxidising a mixture of 4-aminophenol and phenol with sodium hypochlorite, givesCI Sulphur Blue 14 on sulphurisation with aqueous sodium polysulphide. Similar blue dyesare available from related intermediates. For example, condensation of diphenylamine with4-nitrosophenol in sulphuric acid gives compound 6.142; this intermediate reacts withaqueous sodium polysulphide to form CI Sulphur Blue 13. The incorporation of anaphthalene ring system enables green dyes to be produced. The important dye CI SulphurGreen 3, for example, is made by heating compound 6.143 with aqueous sodium poly-sulphide under reflux in the presence of copper(II) sulphate. CI Sulphur Green 14 is basedon copper phthalocyanine [50].

(CH3)2N

N

H

OH

6.138

(CH3)2N

N

O

6.139

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Sulphurisation of 4-hydroxydiphenylamine in a glycol solvent gives CI Sulphur Red 10. Auseful red dye (CI Sulphur Red 6) is prepared by heating 3-amino-6-hydroxy-2-methylphenazine (6.144) with aqueous sodium polysulphide. The phenazine derivative canbe made by oxidising the condensation product of 2,4-diaminotoluene and 4-aminophenol.The substituted phenothiazone 6.145 gives CI Sulphur Red 5 on treatment with aqueoussodium polysulphide in the presence of glycerol. Zerweck et al. [56] showed that thechlorine atoms in compound 6.145 could be displaced stepwise by mercapto groups. Theresulting polymeric dye was found to be essentially the same as that generated bysulphurisation of the parent 4-hydroxy-4′-methyldiphenylamine. It can therefore be deducedthat sulphur dyes of this type contain structural units similar to that shown in structure6.146, which possesses a central thianthrene ring system isosteric with indanthrone (6.65).

Only one violet sulphur dye is commercially available: CI Sulphur Violet 1, obtained fromthe substituted indoaniline 6.147. Several black sulphur dyes are currently in use, especially

S

N

O(CH3)2N

6.140

N

OHO

6.141

N

ON

H

6.142

N

H

N

H

HO3S

OH

6.143

N

NHO

CH3

NH2

6.144

N

SH3C

Cl

O

Cl

6.145

N

SH3C O

S

S N

SO

S

S

CH3

6.146


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CI Sulphur Black 1, which is produced from the relatively simple intermediate 2,4-dinitrophenol and aqueous sodium polysulphide. A similar product (CI Sulphur Black 2) isobtained from a mixture of 2,4-dinitrophenol and either picric acid (6.148; X = NO2) orpicramic acid (6.148; X = NH2). A black dye possessing superior fastness to chlorine whenon the fibre (CI Sulphur Black 11) can be made from the naphthalene intermediate 6.149 byheating it in a solution of sodium polysulphide in butanol. An equivalent reaction using thecarbazole intermediate 6.150 gives rise to the reddish blue CI Vat Blue 43 (Hydron blue).This important compound, which also possesses superior fastness properties, is classified as asulphurised vat dye because it is normally applied from an alkaline sodium dithionite bath.Interestingly, inclusion of copper(II) sulphate in the sulphurisation of intermediate 6.150leads to the formation of the bluish black CI Sulphur Black 4.

The many sulphur dyes synthesised via quinonimine intermediates are polymeric productscontaining numerous disulphide crosslinkages that can be broken by reduction in aqueoussodium sulphide; thioether groups survive the reduction process. The smaller thiolate-containing molecules formed are substantive to cellulose. Although the actual structures ofsuch dyes are complex, their essential features can be illustrated in an idealised model(Scheme 6.23), in which X = S, NH or O, and R indicates substituents or annelated rings.

N

NH2

H2N

NHCOCH3

O

6.147

OH

XO2N

NO26.148

N

H

OH

6.149

N

H

OHN

H6.150

X

N S

S N

X

R

O

S

O

S

R

X

N S

S N

X

R

OH

SNa

HO

SNa

R

H

H

reduction

oxidation

Scheme 6.23

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327

6.4.3 Sulphurised vat dyes and thiazole derivatives

CI Vat Blue 43 and the equivalent dye derived from N-ethylcarbazole (CI Vat Blue 42) aresometimes referred to as sulphurised vat dyes. This term is also applied to someanthraquinonoid thiazole derivatives, such as CI Vat Yellow 2 (6.151). This compound canbe made by the reaction of 2,6-diamino-1,5-dichloroanthraquinone with benzaldehyde andsulphur. Another anthraquinonoid vat dye containing a thiazole ring system (6.107) hasalready been mentioned.

The thiazole ring system is found in many types of dye. Thiazole-containing sulphur dyesand primuline were considered in section 6.4.2. Quaternised dehydrothio-p-toluidine 6.133is available as CI Basic Yellow 1 (6.152). Other derivatives of this intermediate are used asdirect dyes, such as CI Direct Yellow 8 (4.58). The benzothiazole ring appears in various azodisperse dyes [14], quaternisation of which gives useful cationic dyes, an important examplebeing CI Basic Blue 41 (4.99). Another example containing a quaternised thiazole ring is CIBasic Red 29 (4.102).

O

O

N

S

N

S

6.151

CI Vat Yellow 2

N

S

N(CH3)2

CH3

H3C

Cl

6.152

+ _

CI Basic Yellow 1

6.5 DIARYLMETHANE AND TRIARYLMETHANE DYES

Derivatives of triphenylmethane were among the earliest synthetic colorants, and are still indemand where bright, intense colours are needed without the necessity for outstandingfastness to light and chemical reagents. Basic dyes of this type, as well as other cationic dyes,are suitable for dyeing conventional acrylic fibres, on which they show better fastnessproperties than on natural fibres. The photodegradation of triphenylmethane dyes has beenreviewed [42].

Traditionally dyes in this category are referred to as triarylmethane dyes; carbocyclic ringsystems other than benzene, particularly naphthalene, and various heterocyclic systems arealso encountered. Few diarylmethane dyes are of commercial significance, although someheterocyclic derivatives and analogues are of interest. Di- and tri-arylmethane dyes aremesomeric cations (charge-resonance systems) in which the positive charge is localisedmainly on the terminal nitrogen atoms, as in the case of Michler’s Hydrol Blue (Scheme6.24). One of the resonance canonical forms (6.153) is a carbonium ion, so that dyes of thiskind are sometimes referred to as di- or tri-arylcarbonium ions [58]. Since the centralcarbon atom in these systems is sp2 hybridised, Zollinger suggests that the terms di- and tri-arylmethine dyes are more appropriate [59]; this class of dyes is structurally similar to thepolymethine dyes.

DIARYLMETHANE AND TRIARYLMETHANE DYES

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Michler’s Hydrol Blue absorbs at 607.5 nm (ε = 147 500) in 98% acetic acid [60]. Thisbright blue dye and related diphenylmethane derivatives have poor fastness properties andare not of commercial importance. Attachment of an amino group to the central carbonatom of Michler’s Hydrol Blue gives the yellow dye Auramine (6.154; CI Basic Yellow 2),which absorbs at 434 nm in ethanol and is used in the dyeing of paper and in otherapplications, although the dye is a known carcinogen. The electron-donating amino groupproduces a very marked hypsochromic shift. Auramine is somewhat more stable thanMichler’s Hydrol Blue but, being a ketimine, it is hydrolysed in boiling water to formMichler’s ketone (6.155) and ammonia. The dye was first obtained in 1883 by Kern andCaro who heated Michler’s ketone with ammonium chloride and zinc chloride. Thesynthesis first requires the formation of the important ketone intermediate by a Friedel–Crafts acylation (Scheme 6.25). An improved route was devised in 1889 by Sandmeyer,

Scheme 6.24

H

(CH3)2N N(CH3)2

H

N(CH3)2(CH3)2N

H

(CH3)2N N(CH3)2

6.153

+ +

+

Michler’s Hydrol Blue

(CH3)2N (CH3)2N

COCl

N(CH3)2

O

N(CH3)2(CH3)2NN(CH3)2

+NH2Cl

(CH3)2N

COCl2

ZnCl2

NH4Cl

ZnCl2

6.154

_

CI Basic Yellow 26.155

Michler’s ketone

Scheme 6.25

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329

involving the corresponding thioketone (6.157). Condensation of N,N-dimethylaniline withformaldehyde gives 4,4′-bis(dimethylamino)diphenylmethane (6.156), which is then heatedwith sulphur and ammonium chloride, with sodium chloride as diluent, in an atmosphere ofammonia (Scheme 6.26). The resulting Auramine base (6.158) gives Auramine whentreated with hydrochloric acid. The ethyl analogue of Auramine is CI Basic Yellow 37.

Scheme 6.26

(CH3)2N

H

H

N(CH3)2(CH3)2N

(CH3)2N N(CH3)2

SNH

N(CH3)2(CH3)2N

H

NH3

HCl

S

2 + HCHO

6.154

6.1586.157

6.156

2

3

56

2′

3′

5′

6′

+

CI Basic Yellow 2

Reduction of Michler’s ketone gives Michler’s hydrol (6.159), which forms Michler’sHydrol Blue in the presence of acid (Scheme 6.27). Michler’s hydrol is produced industriallyby the oxidation of the diphenylmethane precursor (6.156); further oxidation to giveMichler’s ketone takes place readily.

OH

H

N(CH3)2(CH3)2N

OH2

H

N(CH3)2(CH3)2N

H

6.153

+

+

Michler’s Hydrol Blue

6.159

Michler’s hydrol

Scheme 6.27


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The more important triphenylmethane dyes probably date from 1834, when Rungeobtained red compounds of unknown constitution from crude phenol and from crudeaniline. Shortly after Perkin’s discovery of mauveine (section 6.6.1) in 1856 Natansonobtained the triphenylmethane dye magenta from crude aniline by oxidation. Althoughmany related dyes were made empirically over the following two decades, it was not until1880 that Fischer was able to establish structural relationships within this group of dyes. Theessential triphenylmethane ring system was identified by its conversion (Scheme 6.28) into4,4′,4″-triaminotriphenylmethane (6.160), the source of pararosaniline (6.161; CI BasicRed 9).

H

NH2H2N

NH2

OH

NH2H2N

NH2

NH2ClH2N

NH2

HClHO

oxidation

reduction

6.160

6.161

+ _

_

Dye (colour salt)

Leuco base

Dye (carbinol) base

Scheme 6.28

Nearly all commercial triarylmethane dyes contain two or three electron-donorsubstituents at para sites. In general, dyes containing two p-amino groups (the malachitegreen series) are green or greenish blue, whereas those with three p-amino groups (thecrystal violet series) are reddish to bluish violet. Of lesser importance are monoaminostructures containing alkoxy groups, although these orange or red dyes have very good lightfastness on acrylic and modacrylic fibres [22].


In the synthesis of the triarylmethane skeleton the central carbon atom is provided either bya simple compound, such as formaldehyde or phosgene, or as part of an aromatic

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intermediate. Depending on the oxidation state of the component used, the resulting dye isobtained directly or as the leuco base. The various reactions involved are essentiallyelectrophilic aromatic substitutions in which, usually, the para position is activated by apowerful electron-donor substituent such as a dialkylamino group.

The leuco base of the dye is produced by condensation of one mole of an aromaticaldehyde with two moles of an aromatic amine having an unsubstituted para position (thealdehyde synthesis) in the presence of a mineral acid or a Friedel–Crafts catalyst. Schiff basesare obtained with primary aromatic amines devoid of crowding substituents adjacent to theamino group. The aldehyde method is illustrated by the classical synthesis of malachitegreen (6.162; CI Basic Green 4), first carried out in 1877 by Fischer (Scheme 6.29). Theresulting leuco base is readily oxidised to the dye base and thence to the colour salt. Formany years lead(IV) oxide has been used as the oxidising agent but more recently, forenvironmental reasons, lead-free processes have been used. Of particular interest isoxidation by chloranil (tetrachloro-p-benzoquinone) or by air in the presence of catalysts.Mechanistically it is likely that the conversion involves hydride abstraction in the case ofchloranil, so that the dye is formed directly from the leuco base [61]. The chemistry of leucotriarylmethanes has been reviewed [62].

Condensation of a diphenylmethanol derivative, such as Michler’s hydrol (6.159), with areactive aryl component under acid conditions (the hydrol synthesis) also provides a leucobase. The dye 6.163 (CI Acid Green 50) is made by reacting Michler’s hydrol with R acid(2-naphthol-3,6-disulphonic acid) and oxidising the resulting leuco compound (Scheme6.30).

(CH3)2N

CHO

H

N(CH3)2(CH3)2N

OH

N(CH3)2(CH3)2NN(CH3)2(CH3)2N

H

HCl

Cl

2 +

oxidation

6.162

+

+

_

Colour salt

Leuco base

Dye base

Scheme 6.29


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Michler’s ketone (6.155) and related carbonyl compounds can be used to obtain coloursalts directly by reaction with a reactive intermediate such as an aromatic amine (the ketonesynthesis), as in the case of crystal violet (6.164; CI Basic Violet 3) shown in Scheme 6.31.

A few triphenylmethane dyes are still obtained by empirical methods first used in theearly processes of dye manufacture. For example, in the production of magenta (CI BasicViolet 14), a mixture of aniline, toluidines and nitrotoluenes is heated with zinc and iron(II)

OH

(CH3)2N N(CH3)2

H

(CH3)2N N(CH3)2

H

OH

SO3HHO3S

OH

SO3HHO3S

(CH3)2N N(CH3)2

HO

SO3HO3S

+

oxidation

6.163

+

_

R acid

CI Acid Green 50Scheme 6.30

O

(CH3)2N N(CH3)2

N(CH3)2

(CH3)2N N(CH3)2

N(CH3)2

Cl

+ZnCl2 or

POCl3

6.155

6.164

+

_

CI Basic Violet 3Scheme 6.31

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333

chlorides for several hours. It is likely that the reaction involves initially the oxidation of amethyl group to an aldehyde group. Condensation to form triphenylmethane derivatives, notsurprisingly, leads to mixtures. It is now known that magenta is carcinogenic. The maincomponents of magenta are rosaniline (6.165) and pararosaniline (6.161). The yellowishbrown by-product chrysaniline (6.166; CI Basic Orange 15), an acridine dye, is present inthe mother liquors of magenta manufacture. In the manufacture of methyl violet (CI BasicViolet 1), air oxidation of N,N-dimethylaniline in the presence of a copper(II) salt gives amixture of the more highly methylated pararosanilines.

H2N NH2

CH3

NH2

Cl

6.165

_

+

Rosaniline

N

H

NH2

NH2

X

6.166

_

+

CI Basic Orange 15

In the laboratory pure dye bases can be prepared by the reaction of diaryl ketones, such asMichler’s ketone (6.155), with appropriate Grignard reagents or organolithium compounds.These tertiary alcohols are also available from related reactions between aryl esters andorganometallic reagents. A new method of synthesis has been reported which involvesdisplacement of benzotriazole in diarylbenzothiazolylmethanes by aryl Grignard reagents[63].

The basic dyes are usually marketed as chlorides, oxalates or zinc chloride double salts; inthe case of malachite green (6.162), the last-named derivative has the formula3[Dye]Cl.2ZnCl2.2H2O. Acid dyes of this type are often isolated as the sodium salt.

6.5.2 Acid, basic and mordant dyes

Alkylation of aminotriphenylmethane dyes has a bathochromic effect, which is even morepronounced on arylation, as illustrated in Table 6.5.

Table 6.5 Spectral data for some triphenylmethane dyes in 98% aceticacid

Trivial name CI Basic Structure λmax (nm)

6.167; R = H 538Crystal violet Violet 3 6.167; R = CH3 589Ethyl violet Violet 4 6.167; R = C2H5 592.5

6.168; R = H 570Malachite green Green 4 6.168; R = CH3 621Brilliant green Green 1 6.168; R = C2H5 629.5


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Unlike Michler’s Hydrol Blue (6.153) and crystal violet (6.167; R = CH3), malachite green(6.168; R = CH3) shows two absorption bands in the visible region of the electromagneticspectrum (Figure 6.1). In 98% acetic acid the long-wavelength absorption band (x-band) of

NR2

NR2R2N

X

6.167

_

+

NR2R2N

X

6.168

x

y_

+

N(CH3)2(CH3)2N

Cl

Cl

6.169

_

+

CI Basic Blue 1

N(CH3)2(CH3)2N

Cl

6.170

2

3

4

5

6

2′

3′

5′

6′

2′′

3′′

5′′

6′′ _

+

CI Basic Green 4p

o

m

400 500 600 700

2

4

6

8

10

Wavelength/nm

Mol

ar e

xtin

ctio

n co

effic

ient

/l (m

ol c

m)–

1 ×

10–4 x-band

y-band

Figure 6.1 Absorption spectrum of malachite green

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335

malachite green, which is associated with polarisation along the x-axis, appears at 621 nm(ε = 104 000) whereas the less significant y-band, which is polarised along the y-axis, is foundat 427.5 nm (ε = 20 000) [64]; the green colour of the dye arises from these two components.Consequently, the presence of an ortho substituent in the phenyl ring of malachite green givesrise to a significant crowding effect, which decreases the importance of the y-band by partialdeconjugation; at the same time the x-band is enhanced. Dyes of this type are therefore blue;for example, the 2-chloro derivative (6.169; CI Basic Blue 1) of malachite green (6.170)displays absorption bands at 635 nm (ε = 121 000) and 415.5 nm (ε = 13 000) [64].

As shown by X-ray crystallography, triphenylmethane dyes are unable to adopt a planarconformation and thus the three aromatic rings are twisted out of the molecular plane byapproximately 30 degrees. For example, the molecule of pararosaniline perchlorate is shapedlike a three-bladed propeller [65]. Steric effects are therefore significant for o-substitutedanalogues of malachite green but not for those with only m- or p-substitution (6.170). Inaccordance with theory [66], crowding substituents adjacent to the central carbon atom intriarylmethane dyes give rise to bathochromic shifts of the long-wavelength absorption band.

The x-band in malachite green arises from an NBMO→π* transition, so that 3- and 4-substituents affect the energy of the excited state only and bring about spectral shifts of thefirst absorption band which vary linearly with the appropriate Hammett substituentconstants. Thus, electron-withdrawing groups cause bathochromic shifts of the x-bandwhereas donor substituents cause hypsochromic shifts (Table 6.6) [64,67]. The y-band arisesfrom a π→π* transition [68] so that substituent effects are less predictable. As the donorstrength of the 4-substituent increases, however, the y-band moves bathochromically andeventually coalesces with the x-band – at 589 nm in the case of crystal violet (6.164), whichpossesses two NBMOs that are necessarily degenerate [69].

Table 6.6 Spectral data for some derivatives of malachite green

x-band y-band

Substituent λmax (nm) εmax λmax (nm) εmax

None 621 104 000 427.5 20 0003-Cl 630 103 000 426 17 0004-Cl 627.5 104 000 433 22 0003-CF3 634 106 000 424 16 0004-CF3 637 104 000 424 15 0003-CN 637 90 000 426 15 0004-CN 643 88 000 429 16 0003-NO2 637.5 87 000 425 14 0004-NO2 645 83 000 425 17 0003-CH3 618.5 106 000 433 22 0004-CH3 616.5 106 000 437.5 25 0003-OCH3 622.5 107 000 435 18 0004-OCH3 608 106 000 465 34 0004-C6H5 626 100 000 457 30 0004-N(CH3)2 589 117 000 589 117 000


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Acid dyes

The presence of at least two sulphonic acid groups in the triarylmethane ring system permitsthe derivatives to be applied as acid dyes. Although sulphonic acid groups are often presentin the intermediates used, acid dyes can also be obtained by direct sulphonation of the basicdye itself or at the leuco stage.

Typically, the aldehyde synthesis is used to make the 2,4-disulphonic acid (6.171; CI AcidBlue 1) of Brilliant Green (6.168; R = C2H5) from N,N-diethylaniline and benzaldehyde-2,4-disulphonic acid. The closely related dye CI Acid Blue 3 (6.172) is obtained bysulphonating the 3-hydroxy derivative of Brilliant Green (CI Basic Green 1) at the leucobase stage. After oxidation the dye is isolated as the calcium salt, which also finds use as afood colorant (CI Food Blue 5). Direct sulphonation of the colour salt is used to prepare CIAcid Blue 93 from diphenylamine blue (6.173; CI Solvent Blue 23). The latter dye is madeby phenylation of pararosaniline (6.161) with an excess of aniline in the presence of benzoicacid at about 180 °C. The main product of sulphonation is the trisulphonic acid. CI AcidGreen 50 (6.163) has already been mentioned as an example of the hydrol synthesis. Thevery similar CI Acid Green 16 (6.174) is obtained by condensing Michler’s hydrol withnaphthalene-2,7-disulphonic acid followed by oxidation.

N(C2H5)2(C2H5)2N

SO3

SO3Na 6.171

+

_

CI Acid Blue 1

N(C2H5)2(C2H5)2N

SO3

SO3

HO

6.172_

+

_

CI Acid Blue 3

1Ca2+

2

NHNH

HN

Cl

6.173

+

_

CI Solvent Blue 23

(CH3)2N N(CH3)2

SO3HO3S

6.174

+

_

CI Acid Green 16

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Several acid dyes are derived from N-sulphobenzyl-substituted amines. Thus in theproduction of CI Acid Violet 17 (6.175), N-ethyl-N-(3-sulphobenzyl)aniline is condensedwith formaldehyde to produce the substituted diphenylmethane, which is then oxidised tothe hydrol and condensed with N,N-diethylaniline. Further oxidation of the leuco base givesthe dye. The presence of an o-methyl group in the diethylaminophenyl ring gives a brightblue dye (CI Acid Blue 15). Condensation of benzaldehyde-2-sulphonic acid with the sameintermediate followed by oxidation of the leuco base gives CI Acid Blue 9, which is used indrop-on-demand ink-jet printing [36]. This dye is also a food colorant (CI Food Blue 2) andthe barium salt is used as a pigment (CI Pigment Blue 24). The corresponding 2-chloroderivative is CI Acid Green 9.

The nucleophilic displacement of suitable para substituents in analogues of malachitegreen is used as a means of synthesis of some important acid dyes. For example, CI AcidBlue 83 (6.176; R = H) is made by the aldehyde method, using 4-chlorobenzaldehyde andN-ethyl-N-(3-sulphobenzyl)aniline. The resulting leuco base is oxidised to the colour salt,

N NH5C2

CH2

C2H5

H2C

N(C2H5)2

SO3HO3S

6.175

+

_

CI Acid Violet 17

N NH5C2

CH2

C2H5

H2C

NH

SO3HO3S

R R

OC2H5

6.176

+

_


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which is then condensed with p-phenetidine to give the required product. The stericallyhindered dimethyl derivative of this dye, CI Acid Blue 90 (6.176; R = CH3) is also ofinterest. It is synthesised by first condensing benzaidehyde with N-benzyl-N-ethyl-m-toluidine to give a leuco base, which is then trisulphonated; after oxidation, the 4-sulphonicacid group is displaced by reaction with p-phenetidine.

The incorporation of an indole ring system often leads to an improvement in the lightfastness. A suitable example is CI Acid Blue 123 (6.177), which is derived from 4,4′-dichlorobenzophenone. Condensation with 1-methyl-2-phenylindole in the presence ofphosphorus oxychloride produces the triarylmethane ring system. Replacement of thechlorine atoms with p-phenetidine, followed by sulphonation, gives the dye.

NH NH

N

H5C2O

HO3S

OC2H5

SO3

CH3

6.177

_

+

CI Acid Blue 123

A study of some triphenylmethane acid dyes on model polymer systems has revealed theoperation of a complex fading mechanism which probably involves excited triplet-state dyemolecules [70].

Basic dyes

Several examples of typical triarylmethane dyes have already been mentioned, in particular,pararosaniline (6.161), malachite green (6.162) its o-chloro derivative (6.169), crystal violet(6.164), rosaniline (6.165) and diphenylamine blue (6.173). CI Basic Green 1 (6.168; R =C2H5), the ethyl analogue of malachite green, is prepared by the aldehyde route and isisolated as the sulphate. The ethyl analogue of crystal violet is CI Basic Violet 4 (6.167; R =C2H5) and is obtained by the ketone route.

In general, the presence of substituents adjacent to terminal dialkylamino groups leads topartial deconjugation and to complex spectral shifts [71]. Steric effects are minimised,however, in the case of primary and secondary p-amino groups. Thus rosaniline (6.165)contains one o-toluidine residue and three such groupings are present in new magenta(6.178; CI Basic Violet 2), which is effectively obtained by the hydrol route, involving thereaction of two moles of o-toluidine with one of formaldehyde, oxidation to the hydrol,condensation of this intermediate with o-toluidine and oxidation of the leuco base. Methylgroups are also sited in two rings of CI Basic Blue 5 (6.179), made by the aldehyde routeusing 2-chlorobenzaldehyde and N-ethyl-o-toluidine. The leuco base of the stericallyhindered 2′,2″-dimethyl derivative of malachite green (6.170) is used as a charge transportmaterial in electrophotography [36].

Various derivatives of naphthyldiphenylmethane are of value as blue dyes; the naphthylsubstituent brings about bathochromic shifts by extending the conjugation and also by a

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steric effect [72]. The Victoria Blues have been used commercially for many years. Typicalexamples include CI Basic Blue 26 (6.180), which was discovered in 1883 by Caro and Kern.The dye can be prepared by the ketone route using Michler’s ketone (6.155) and 1-phenylaminonaphthalene. A similar condensation between the ethyl analogue of Michler’sketone and 1-ethylaminonaphthalene gives CI Basic Blue 7 (6.181), now classified as toxicby the Ecological and Toxicological Association of the Dyestuffs Manufacturing Industry(ETAD) [73].

H2N NH2Cl

NH2

CH3

CH3H3C

6.178

+ _

CI Basic Violet 2

H5C2NH NHC2H5Cl

CH3H3C

Cl

6.179

+ _

CI Basic Blue 5

The phosphotungstomolybdic acid salts (PTMA lakes) of many basic dyes are used asviolet, blue and green pigments [27]. Examples include methyl violet (CI Pigment Violet 3),Victoria Pure Blue BO (CI Pigment Blue 1) and Brilliant Green (CI Pigment Green 1). Thecopper hexacyanoferrate(II) complex of methyl violet is CI Pigment Violet 27. The free dyebases of various triarylmethane derivatives are used as solvent dyes. Illustrative examplesinclude methyl violet (CI Solvent Violet 8), Victoria Blue B (CI Solvent Blue 4) andmalachite green (CI Solvent Green 1).

Certain derivatives of basic triarylmethane dyes are used in pressure-sensitive papers.Carbonless copy papers that make use of the so-called crystal violet lactone (6.182) are ofparticular importance. The reverse side of the top sheet is coated with microcapsulescontaining a solution of the colourless lactone (colour former) in a non-polar solvent. Thebottom sheet is coated with an acidic clay or an acidic phenolic resin. Application ofpressure ruptures the minute capsules and interaction between the released lactone and thelower acidic surface leads to instant dye formation (structure 6.183) at the point of contact(Scheme 6.32). The resulting colour gradually fades; slower-acting colour formers that giverise to longer-lasting images, such as benzoylated leuco methylene blue, are therefore usually

(CH3)2N N(CH3)2

HN

Cl

6.180

+

_

CI Basic Blue 26

(C2H5)2N N(C2H5)2

NHC2H5

Cl

6.181

+

_

CI Basic Blue 7


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incorporated into the capsules. The synthesis and properties of phthalide-type colourformers have been reviewed [74].

Mordant dyes

Hydroxytriphenylmethane derivatives give rise to anionic charge-resonance systems that areisoconjugate with the corresponding amino-substituted dyes. Thus, for example, the oxonolanalogue of malachite green is benzaurine (Scheme 6.33). The quinonoid neutral form(6.184), which is pale yellow, produces a violet anion (6.185) on the addition of alkali; in

C

O

O

(CH3)2N N(CH3)2

(CH3)2N

(CH3)2N N(CH3)2

COO

N(CH3)2

H

6.182 6.183

+

+

_

Crystal violet lactone

Scheme 6.32

OHHO

H

HO O

O OHO OH

H

OH

oxidation

6.184

6.1856.186

+

+

_

_

Leuco baseNeutral form

BenzaurineScheme 6.33

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341

HO O

COOH COOH

CH3H3C

ClCl

6.187

CI Mordant Blue 1

HO O

COOH COOH

CH3H3C

N(C2H5)2

6.188

CI Mordant Violet 1

O

O

OHHO

O

O

OO

O O

COO

OH

OO

COO

OH

OH

6.189

6.1906.191

__

_

_ _ _

_

_

Phenolphthalein

Scheme 6.34

strongly acidic solution the neutral form is protonated to give a cationic charge-resonancesystem (6.186) which is also violet [75]. The analogous trihydroxytriphenylmethane systemis aurine.

Technically important dyes are salicylic acid derivatives that function as chrome mordantdyes for wool. Thus CI Mordant Blue 1 (6.187) is made by the aldehyde synthesis from 2,6-dichlorobenzaldehyde and 2-hydroxy-3-methylbenzoic (o-cresotinic) acid in concentratedsulphuric acid. Oxidation of the leuco base is achieved by the addition of sodium nitrite. Onwool the product, which is isolated as the sodium salt, is a dull maroon colour, changing to abright blue on treatment with a chromium salt. Some dyes of this type, such as CI MordantViolet 1 (6.188), also contain a basic group. This compound is also prepared by the aldehyderoute.

The acid–base indicator phenolphthalein is a derivative of benzaurine (Scheme 6.34).Condensation of phthalic anhydride with phenol generates the colourless lactone form


chpt6(1).pmd 15/11/02, 15:29341


(6.189), which ionises in alkaline solution and undergoes ring opening to produce the reddianion (6.190) within the pH range 8.3–10.0. The charge-resonance system is destroyed inan excess of alkali by the formation of the colourless triphenylmethanol derivative (6.191).Several derivatives of phenolphthalein are also used as indicators. The relatedphenolsulphonephthaleins are particularly versatile as acid-base indicators, covering a widerange of pH. Typical examples include bromophenol blue (6.192) (pH range 3.0–4.6) andbromocresol purple (6.193) (pH range 5.2–6.8).

6.5.3 Xanthene dyes

The most important heterocyclic analogues of the di- and triarylmethane dyes are xanthenederivatives containing an oxygen bridge. These dyes produce red, pink and greenish yellowhues that are strongly fluorescent [76]. The oxygen bridge confers greater rigidity on thesystem, leading to fluorescence and, in accordance with PMO theory [77], the electrondonation results in hypsochromic shifts of the long-wavelength absorption band relative tothat of the parent dyes.

The xanthene analogue of Michler’s Hydrol Blue (6.153) is Pyronine G (6.194), which isused as a red biological stain. Condensation of 3-dimethylaminophenol with formaldehydefollowed by dehydration in concentrated sulphuric acid gives the leuco base. Dyes ofcommercial significance contain three aromatic rings. The bright reddish violet dyeRhodamine B (6.195; CI Basic Violet 10), discovered in 1887 by Cérésole, is a goodexample. Condensation of 3-diethylaminophenol with phthalic anhydride yields the dye.The free dye base is used as a solvent dye (CI Solvent Red 49) and thephosphotungstomolybdic acid lake finds use as a pigment (CI Pigment Violet 1); the ethylester of Rhodamine B is CI Basic Violet 11. A similar reaction between 3-ethylamino-p-cresol and phthalic anhydride gives a redder product on esterification (6.196; CI Basic Red1). The PTMA lake is a valuable pink pigment (CI Pigment Red 81), as is the copperhexacyanoferrate(II) complex (CI Pigment Red 169).

Several xanthene derivatives are applied as acid dyes. For example, condensation of 3-diethylaminophenol with benzaldehyde-2,4-disulphonic acid, followed by cyclisation andoxidation, gives Acid Rhodamine B (6.197; CI Acid Red 52). This dye is used in drop-on-demand ink-jet printing [36].

Fluorescein (6.198; X = H; CI Acid Yellow 73) is the xanthene analogue ofphenolphthalein (6.190) and gives rise to an anionic charge-resonance system. This was the

SO2

O

OHHO

Br

Br Br

Br

6.192

Bromophenol blue

SO2

O

OHHO

Br

CH3 CH3

Br

6.193

Bromocresol purple

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343

(CH3)2N N(CH3)2O

Cl

6.194

+

_

Pyronine G

(C2H5)2N N(C2H5)2O

COOH

Cl

6.195

+

_

CI Basic Violet 10

H5C2NH NHC2H5ClO

COOC2H5

CH3H3C

6.196

+ _

CI Basic Red 1

(C2H5)2N N(C2H5)2O

SO3

SO3Na

6.197

+

_

CI Acid Red 52

first xanthene dye, originally made in 1871 by von Baeyer from phthalic anhydride andresorcinol. This dye has a strong green fluorescence in solution even at very great dilutionand is widely used as a marker. Various halogenated derivatives are of interest, especially thebright red tetrabromo compound eosine (6.198; X = Br; CI Acid Red 87), and theequivalent tetraiodo compound erythrosine (6.198; X = I; CI Acid Red 51). Eosine is widelyused for colouring paper, inks and crayons; CI Pigment Red 90:1 is the aluminium salt.Purified erythrosine is a permitted food colorant (CI Food Red 14); the aluminium salt isalso used for this purpose (CI Food Red 14:1) and as a pigment (CI Pigment Red 172).Condensation of tetrachlorophthalic anhydride with resorcinol, followed bytetrabromination of the product, gives the bright pink Phloxine B (6.199; CI Acid Red 92);the free acid is used as a solvent dye (CI Solvent Red 48) and the aluminium salt is apigment (CI Pigment Red 174).

NaO OO

COONa

X X

XX

6.198

NaO OO

COONa

Br Br

BrBr

Cl

Cl

Cl

Cl

6.199

CI Acid Red 192


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O

O

O CH3

NH

(C2H5)2N

6.200

O NN

SO3

SO3H

6.201

+

_

Rhodamine 101

(CH3)2N N(CH3)2N

H

Cl

6.202

+

_

CI Basic Orange 14

H2N NH2N

CH3

Cl

6.203

+ _

Acriflavine

Xanthene dyes are used as colour formers. These so-called fluorans usually contain aminogroups sited para and meta to the central carbon atom. Such a substitution pattern gives riseto broad absorption bands and leads to almost black colour production; the lactone 6.200 isa typical example. This xanthene derivative finds use in direct thermal printing [36]. Thechemistry of fluoran leuco dyes has been reviewed [78].

Various xanthene derivatives, including fluorescein, are also used as laser dyes to coverthe spectral region from 500 to 700 nm. A modern example is the julolidine-based dyeRhodamine 101 (6.201), which absorbs at 576 nm and lases at 648 nm [79].

6.5.4 Acridine dyes

The presence of a nitrogen bridge in di- and tri-arylmethane dyes gives rise to the lesssignificant acridine dyes; the parent heterocyclic ring system is so-called because of itsirritant action in the nose and throat. The acridine analogue of Michler’s Hydrol Blue isacridine orange (6.202; CI Basic Orange 14). This dye is made by nitration of 4,4′-bis(dimethylamino)diphenylmethane (6.156), reduction of the resulting dinitro derivativeand oxidation of the cyclised diamino intermediate.

The antiseptic acriflavine (6.203) is obtained by condensation of m-phenylenediaminewith glycerol and oxalic acid, followed by methylation of the product.

The triphenylmethane acridine analogue chrysaniline (6.166) has already beenmentioned as a by-product in the synthesis of magenta.

6.6 MISCELLANEOUS COLORANTS

6.6.1 Azines, oxazines and thiazines

Heterocyclic analogues of diarylmethane dyes in which the central carbon atom has been

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345

replaced by nitrogen are of some importance. Such a replacement leads to significantbathochromic shifts of the long-wavelength absorption band in accordance with PMOtheory [77]. Thus Bindschedler’s Green (6.204), the aza analogue of Michler’s Hydrol Blue,absorbs at 725 nm. This indamine dye is only of historical interest but a related indoanilinederivative is used as a solvent dye (6.205; CI Solvent Blue 22). Of more importance are theazines, oxazines and thiazines which contain a heteroatom bridge.

N

(CH3)2N N(CH3)2

Cl

6.204

+

_

Bindschedler’s Green

N

(CH3)2N O

6.205

CI Solvent Blue 22

Azine dyes

The first synthetic dye, mauveine, belongs to this group. The previously accepted structurefor Perkin’s dye (CI 50245) has been shown to be incorrect [80]. Chromatography of anauthentic sample of mauveine yielded two major components (6.206 and 6.207) which weresubjected to spectroscopic analysis. Few dyes of this type remain of industrial significance, anexception being Safranine T (6.208; CI Basic Red 2), discovered in 1859 by GrevilleWilliams. The oxidative sequence leading to the formation of Safranine T (Scheme 6.35)was only elucidated in recent times [81]. The 1:1 mixture of o-toluidine and 2-methyl-1,4-diaminobenzene required for the first oxidative step can be obtained by reductive cleavage ofthe monoazo dye formed by coupling diazotised o-toluidine with o-toluidine. Furtheroxidation of the intermediate diphenylamine derivative gives an electrophilicquinonediimine (indamine), which then reacts with aniline. Oxidative cyclisation producesa dihydroazine that yields the dye in a final oxidative step.

Aniline black (CI Oxidation Base 1) is a complex polymeric phenazine that can beproduced on cotton fabric by impregnation with aniline hydrochloride and suitable inorganicoxidants, such as sodium chlorate, ammonium vanadate and copper hexacyanoferrate(II).Aniline black is also made directly for use as a pigment (CI Pigment Black 1).

N

H2N N

CH3

NH

H3C

X

6.206

_

+

MISCELLANEOUS COLORANTS

chpt6(1).pmd 15/11/02, 15:29345


N

H2N NH2ClN

H3C CH3

6.208

+ _

NH2

CH3

NH2

CH3

NH2

NH

H2N NH2

CH3H3C

N

NH2H2N

CH3H3C

NH2

NH

NH2

CH3H3C

H2N NH

N

H2N NH2NH

CH3H3C N

N

H

NH2

CH3H3C

H2N

+oxidation

oxidation

oxidation

oxidative

cyclisation

oxidation

6.208

+

+

CI Basic Red 2

Scheme 6.35

N

H2N N

CH3

NH

H3C

CH3

X

6.207

_

+

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347

Oxazine dyes

Bindschedler’s Green (6.204) can be made by condensing p-nitroso-N,N-dimethylanilinewith N,N-dimethylaniline. A similar condensation with 2-naphthol gives Meldola’s Blue(6.209; CI Basic Blue 6), the first oxazine dye, discovered in 1879. The symmetrical CI BasicBlue 3 (6.210) is of more commercial significance. It is synthesised by nitrosation of N,N-diethyl-m-anisidine followed by condensation with N,N-diethyl-m-aminophenol, and is usedfor dyeing acrylic fibres. This dye is now classified by ETAD as toxic [73].

The dioxazine ring system is the source of some valuable violet pigments, such as CIPigment Violet 23 (6.211). This colorant is obtained by condensing 3-amino-9-ethylcarbazole with chloranil. Sulphonation of the pigment gives the dye CI Direct Blue108. Triphenodioxazines have recently been the source of some blue reactive dyes [24].Examples are known of symmetrical bifunctional structures (6.212; NHRNH =alkylenediamine, Z = haloheterocyclic system) and unsymmetrical monofunctional typessuch as 6.213 [37].

N

O N(CH3)2

Cl

6.209

_

+

CI Basic Blue 6

N

O N(C2H5)2(C2H5)2N

Cl

6.210

+

_

CI Basic Blue 3

N

O

O

NN

C2H5

N

C2H5Cl

Cl

6.211


O

N

N

O

SO3Na

ZHNRHN

Cl

Cl

SO3Na

NHRNHZ

6.212

O

N

N

O

H2N

Cl

Cl

NHCO SO2CH

6.213

CH2


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Thiazine dyes

The most important thiazine dye is methylene blue (6.214; CI Basic Blue 9), which is alsowidely used as a biological stain and as a redox indicator. Its synthesis (Scheme 6.36)involves the oxidative thiosulphonation of p-amino-N,N-dimethylaniline. Condensation ofthe product with N,N-dimethylaniline then gives an intermediate indamine thatsubsequently undergoes oxidative ring closure. Nitration of methylene blue producesmethylene green (6.215; CI Basic Green 5), which shows better light fastness on acrylicfibres. The N-benzoyl derivative of leuco methylene blue is used as a colour former incarbonless copy papers (section 6.5.2).

N

S N(CH3)2(CH3)2N

Cl

6.214

+

_

CI Basic Blue 9

N

S N(CH3)2(CH3)2N

NO2

Cl

6.215

+

_

CI Basic Green 5

N(CH3)2

NH2

N(CH3)2

SSO3H

NH2

N(CH3)2

S

N

N(CH3)2

SO3

(CH3)2N

+ Na2S2O3

oxidation

Na2Cr2O7

oxidative

cyclisation6.214

+

_

CI Basic Blue 9

Scheme 6.36

6.6.2 Polymethine dyes

Cyanine dyes fall within the more general category of polymethine dyes, in which a chain ofmethine groups is terminated with a donor group and an acceptor group respectively [82].

The first cyanine dye was made in 1856 by Greville Williams. Thus the blue charge-resonance system 6.216 was produced when oxidative coupling took place between N-

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349

pentylquinolinium iodide and the corresponding 4-methyl derivative. Dyes of this kind werelater found to have colour-sensitising properties in photographic films and hundreds ofrelated compounds have been synthesised, making use of a great variety of heterocyclicterminal groups [83]. Long-chain cyanines absorb beyond the visible region and are useful asinfrared sensitisers in photography. Few cyanine colorants have found use as textile dyes,however, owing to inferior fastness properties. Astra Phloxine (6.217; CI Basic Red 12) is anexample of a carbocyanine (trimethine) dye derived from 1,3,3-trimethylindoline. Thisproduct is now classified by ETAD as toxic [73]. Structurally unsymmetrical dyes of this typeare exemplified by CI Basic Orange 21 (6.218), which is obtained by condensing Fischer’saldehyde (6.219) with 2-methylindole. The use of polymethine cyanine dyes in photographyand dye lasers has been reviewed [84].

N CH N C5H11H11C5

6.216

+

I–N

CH CH CH

N

CH3

H3CCH3

H3CCH3

H3C Cl

6.217

+

_

CI Basic Red 12

N

CH3

H3CCH3

CH CH

NHH3C

Cl

6.218

_+

CI Basic Orange 21

N

CH3

H3CCH3

CH CHO6.219

Fischer’s aldehyde

Of some importance as textile dyes are aza analogues of polymethine (cyanine) dyes.Azacarbocyanines result when Fischer’s aldehyde is heated with primary aromatic amines.Thus CI Basic Yellow 11 (6.220) is obtained when Fischer’s aldehyde is condensed with 2,4-dimethoxyaniline. The equivalent reaction with 2-methylindoline gives CI Basic Yellow 21(6.221), which has superior light fastness but has been classified by ETAD as toxic [73]. Thetinctorially strong golden yellow diazacarbocyanine dye CI Basic Yellow 28 (6.222) isprepared by coupling diazotised p-anisidine with Fischer’s base (6.223), followed byquaternisation with dimethyl sulphate. Some triazacarbocyanine dyes are also usedcommercially.

N

CH3

H3CCH3

CH CH NH OCH3

H3COCl

6.220

_+

CI Basic Yellow 11

N

CH3

H3CCH3

CH CH N

CH3

Cl

6.221

_+

CI Basic Yellow 21


chpt6(1).pmd 15/11/02, 15:29349


Hemicyanine (styryl) dyes are readily obtained by heating Fischer’s base with anappropriate aldehyde. Typical products include CI Basic Red 14 (6.224) and CI Basic Violet16 (6.225). The latter dye has been classified by ETAD as toxic [73].

N

CH3

H3CCH3

CH N N

CH3

OCH3

CH3OSO3

6.222

_+

CI Basic Yellow 28

N

CH3

H3CCH3

CH2

6.223

Fischer’s base

N

CH3

H3CCH3

CH CH N

CH3

CH2CH2CN

Cl

6.224

_+

CI Basic Red 14

N

CH3

H3CCH3

CH CH N(C2H5)2

Cl

6.225

_+

CI Basic Violet 16

Cationic monoazo dyes can be classified as diazahemicyanines; examples of such dyes areconsidered in section 4.10.

Uncharged styryl (methine) disperse dyes were originally introduced to provide greenishyellow colours on cellulose acetate fibres. One such dye still in use is CI Disperse Yellow 31(6.226), which is made by condensing 4-(N-butyl-N-chloroethylamino)benzaldehyde withethyl cyanoacetate. Suitable compounds for polyester usually contain the electron-acceptingdicyanovinyl group, introduced with the aid of malononitrile. An increased molecular sizeleads to improved fastness to sublimation, as in the case of CI Disperse Yellow 99 (6.227). Anovel polymethine-type structure of great interest is present in CI Disperse Blue 354(6.228), which is claimed to be the most brilliant blue disperse dye currently available [85].

CHN

CH3CH2CH2CH2

ClCH2CH2

C

COOC2H5

CN

6.226


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351

Quinophthalone (6.229) and its derivatives [86] also fall into the methine category,although they appear in the Colour Index under quinoline colouring matters. The parentcompound was discovered in 1882 by Jacobsen, who condensed 2-methylquinoline(quinaldine) with phthalic anhydride. The product, quinoline yellow, is used as a solventdye (CI Solvent Yellow 33). The light fastness is improved by the presence of a hydroxygroup in the quinoline ring system. Derivatives of this type provide greenish yellow dispersedyes for polyester. The moderate sublimation fastness of CI Disperse Yellow 54 (6.230; R =H) is improved by the introduction of an adjacent bromine atom in CI Disperse Yellow 64(6.230; R = Br).

The chromium complexes of some azomethine derivatives are used as solvent dyes. CISolvent Yellow 32 (6.231) is an example, obtained by condensing one mole of salicyl-aldehyde with the appropriate amine, followed by treatment with one equivalent ofchromium.

CH N

C2H5

CH2CH2OCO(CH2)4COOCH2CH2

H3C

C

NC

NC

CH

H5C2

CH3

C

CN

CN

N

6.227


(H13C6)2N CH

O2S

CCN

NC

6.228


N

H

O

O

6.229

Quinophthalone

N

H

O

O

R OH

6.230

CH

OH

N

NO2

SO3HHO

6.231

CI Solvent Yellow 32

O

O

CH3(CH2)2O

O

O

6.232

CI Disperse Red 356


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A new cross-conjugated methine-type chromogen was introduced in 1984. The dye CIDisperse Red 356 (6.232) exemplifies this system, which contains two α,ω-donor-acceptordienone segments. The development of such benzodifuranone disperse dyes has beendescribed [87].

6.6.3 Nitro and nitroso dyes

Yellow to brown compounds are obtained when one or more nitro groups are conjugatedwith electron-donor substituents such as hydroxy or, of more importance, amino groups.Such dyes are relatively cheap. Picric acid, 2,4,6-trinitrophenol, can be regarded as theoldest synthetic dye; it was produced in 1771 by Woulfe by treating indigo with nitric acid.

The hydroxy-nitro dye Naphthol Yellow S (6.233; CI Acid Yellow 1) was discovered in1879 by Caro and is still manufactured. It is produced by sulphonation of 1-naphthol to give1-naphthol-2,4,7-trisulphonic acid, followed by replacement of the 2- and 4-sulpho groupsin nitric acid medium. Nucleophilic substitution of 1-chloro-2,4-dinitrobenzene with 4-aminodiphenylamine-2-sulphonic acid gives CI Acid Orange 3 (6.234).

HO3S

OH

NO2

NO26.233

CI Acid Yellow 1

O2N NH

NO2

SO3H

NH

6.234

CI Acid Orange 3

Of particular importance are disperse dyes based on o-nitrodiphenylamine. The lightfastness of such compounds is better than that of their p-substituted isomers, owing tointramolecular hydrogen bonding. The o-nitro derivatives are more bathochromic than thep-nitro analogues but the latter possess greater tinctorial strength. Dyes such as CI DisperseYellow 1 (6.235; X = OH) and CI Disperse Yellow 9 (6.235; X = NH2) are examples oftypical products used on cellulose acetate. Such dyes generally have lower substantivity forpolyester, but this can be improved by minor structural changes. Thus CI Disperse Yellow 42(6.236; X = C6H5) has higher substantivity for polyester than has the parent dye CIDisperse Yellow 33 (6.236; X = H) [21]. The inclusion of a photostabiliser grouping in theterminal phenyl ring of CI Disperse Yellow 42 (6.236; X = C6H5) leads to improved lightfastness [88]. The incorporation of an azo group into the diphenylamine system improvessublimation fastness and colour strength; for example, CI Disperse Yellow 9 (6.235; X =NH2) is the source of CI Disperse Yellow 70 (6.237).

O2N NH

NO2

X

6.235

NH SO2NH

O2N

X

6.236

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353

The introduction of sulphonic acid groups into dyes of this kind provides acid dyessuitable for nylon, such as CI Acid Yellow 199 (6.238).

The so-called nitroso dyes are metal complexes of 1-nitroso-2-naphthol. In the parentsystem a tautomeric equilibrium (Scheme 6.37) exists between hydroxynitroso (6.239) andquinoneoxime (6.240) forms. Only the green iron(II) complexes, which have good lightfastness, have been exploited commercially. The unsubstituted iron(II) complex is CIPigment Green 8. The corresponding 6-sulphonic acid complex, Naphthol Green B (CIAcid Green 1), was discovered in 1885 by Hoffmann.

NH NO2N

NO2

N OH

6.237


NH NHO3S

NO2

N OH

H3C

6.238

CI Acid Yellow 199

O

HNO

O

NO

H

6.239 6.240

Scheme 6.37

REFERENCES 1. Y Kogo, H Kikuchi, M Matsuoka and T Kitao, J.S.D.C., 96 (1980) 475. 2. C Graebe and C Liebermann, Ber., 1 (1868) 49, 104. 3. M R Fox, Dye-makers of Great Britain 1856–1976, (Manchester: ICI, 1987) 97. 4. M Titz and M Nepras, Coll. Czech. Chem. Commun., 37 (1972) 2674. 5. S Abeta and K Imada, Rev. Prog. Coloration, 20 (1990) 19. 6. J Winkler and W Jenny, Helv. Chim. Acta, 48 (1965) 119. 7. M Kikuchi, T Yamagishi and M Hida, Dyes and Pigments, 2 (1981) 143. 8. M Nepras, J Fabian, M Titz and B Gas, Coll. Czech. Chem. Commun., 37 (1982) 2569. 9. P F Gordon and P Gregory, Organic chemistry in colour, (Berlin: Springer-Verlag, 1983) Chapter 4.6.

REFERENCES

chpt6(1).pmd 15/11/02, 15:29353


10. C F H Allen, C V Wilson and G F Frame, J. Org. Chem., 7 (1942) 169.11. Colour Index International, 3rd Edn (4th revision) Vol 9 (Bradford: SDC, 1992).12. Y Zhang and W Hou, Dyes and Pigments, 30 (1996) 283.13. H S Freeman and J Sokolowska-Gajda, Text. Res. J., 60 (1990) 221.14. G Hallas in Developments in the chemistry and technology of organic dyes, Ed. J Griffiths (Oxford: Blackwell Scientific

Publications, 1984) 31.15. O Annen, R Egli, R Hasler, B Henzi, H Jakob and P Matzinger, Rev. Prog. Coloration, 17 (1987) 72.16. J F Dawson, Rev. Prog. Coloration, 14 (1984) 90.17. K D Wozniak, A Keil and D Müller, Textil Praxis, 45 (1990) 965.18. P Elsner, Textilveredlung, 29 (1994) 98.19. A T Peters and X Ma, J.S.D.C., 108 (1992) 244.20. C T Helmes, Amer. Dyestuff Rep., 83 (Aug 1994) 40.21. J F Dawson, J.S.D.C., 99 (1983) 183.22. R Raue, Rev. Prog. Coloration, 14 (1984) 187.23. S B McLoughlin and C R Lowe, Rev. Prog. Coloration, 18 (1988) 17.24. A H M Renfrew, Rev. Prog. Coloration, 15 (1985) 15.25. H Zollinger, Color chemistry, 2nd Edn, (Weinheim: VCH, 1991) Chapter 8.11.26. U Baumgarte, Rev. Prog. Coloration, 17 (1987) 29.27. W Herbst and K Hunger, Industrial organic pigments, 2nd Edn (Weinheim: Wiley-VCH, 1997).28. S M Burkinshaw, G Hallas and A D Towns, Rev. Prog. Coloration, 26 (1996) 47.29. M Schellenberg and R Steinmetz, Helv. Chim. Acta, 52 (1969) 431.30. J Aoki, Bull. Chem. Soc. Japan, 41 (1968) 1017.31. H Zollinger, Color chemistry, 2nd Edn (Weinheim: VCH, 1991) Chapter 8.9.32. K Venkataraman and V N Iyer in The chemistry of synthetic dyes, Vol 5, Ed. K Venkataraman ( New York: Academic

Press, 1971) Chapter 3.33. P F Gordon and P Gregory, Organic chemistry in colour (Berlin: Springer-Verlag, 1983) Chapter 5.2.34. J Griffiths, Colour and constitution of organic molecules (New York: Academic Press, 1976) Chapter 4.6.35. G A Clowes, J.C.S., C (1968) 2519.36. P Gregory, Rev. Prog. Coloration, 23 (1993) 1.37. H S Freeman and J Sokolowska-Gajda, Rev. Prog. Coloration, 29 (1999) 8.38. K Venkataraman, The chemistry of synthetic dyes, Vol 2 (New York: Academic Press, 1952) 1003.39. P F Gordon and P Gregory, Organic chemistry in colour (Berlin: Springer-Verlag, 1983) Chapter 5.3.40. H Zollinger, Color chemistry, 2nd Edn (Weinheim: VCH, 1991) Chapter 2.5.41. M Klessinger, Dyes and Pigments, 3 (1982) 235.42. N Kuramoto in Physico-chemical principles of colour chemistry, Eds. A T Peters and H S Freeman (London: Blackie,

1996) Chapter 6.4.43. J Shore, Textilveredlung, 12 (1986) 207.44. H Zollinger, Color chemistry, 2nd Edn (Weinheim: VCH, 1991) Chapter 8.3.45. P F Gordon and P Gregory, Organic chemistry in colour (Berlin: Springer-Verlag, 1983) Chapter 1.3.46. K McLaren, The colour science of dyes and pigments, 2nd Edn (Bristol: Adam Hilger, 1986) Chapter 1.2.47. S Geyer and R Meyer, Chem. Abs., 95 (1981) 203782r.48. T Bechtold, E Burtscher, G Kühnel and O Bobleter, J.S.D.C., 113 (1997) 135.49. W Czajkowski and J Misztal, Dyes and Pigments, 26 (1994) 77.50. R A Guest and W E Wood, Rev. Prog. Coloration, 19 (1989) 63.51. J Robinson Ltd, J.S.D.C., 111 (1995) 172.52. C Heid, K Holoubek and R Klein, Melliand Textilber., 54 (1973) 1314.53. H Zollinger, Color chemistry, 2nd Edn (Weinheim: VCH, 1991) Chapter 9.1.54. C D Weston in The chemistry of synthetic dyes, Vol 7, Ed. K Venkataraman (New York: Academic Press, 1974)

Chapter 2.55. E Riesz, Textilveredlung, 26 (1991) 403.56. W Zerweck, H Ritter and M Schubert, Angew. Chem., 60 (1948) 141.57. D G Orton in The chemistry of synthetic dyes, Vol 7, Ed. K Venkataraman (New York: Academic Press, 1974)

Chapter 1.58. P Rys and H Zollinger, Fundamentals of the chemistry and application of dyes (New York: Wiley-Interscience, 1972)

Chapter 8.59. H Zollinger, Color chemistry, 2nd Edn (Weinheim: VCH, 1991) Chapter 4.1.60. C C Barker, G Hallas and A Stamp, J.C.S. (1960) 3790.61. N Nishimura and T Motoyama, Bull. Chem. Soc. Japan, 57 (1984) 1.62. R Muthyala,and X Lan in Chemistry and application of leuco dyes, Ed. R Muthyala (New York: Plenum, 1997)

Chapter 5.

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63. R Muthyala, A R Katritzky and X Lan, Dyes and Pigments, 25 (1994) 303.64. C C Barker, M H Bride, G Hallas and A Stamp, J.C.S. (1961) 1285.65. L L Koh and K Erika, Acta Cryst., B, 27 (1971) 1405.66. M J S Dewar in Steric effects in conjugated systems, Ed. G W Gray (London: Butterworths, 1958) 46.67. A S Ferguson and G Hallas, J.S.D.C., 87 (1971) 187.68. A C Hopkinson and P A H Wyatt, J.C.S., B (1970) 530.69. J Griffiths, Colour and constitution of organic molecules (New York: Academic Press, 1976) Chapter 9.3.70. D F Duxbury, Dyes and Pigments, 25 (1994) 179.71. G Hallas, J.S.D.C., 83 (1967) 368.72. G Hallas and D R Waring, J.C.S., B (1970) 979.73. R Anliker, G Dürig, D Steinle and E J Moriconi, J.S.D.C., 104 (1988) 223.74. R Muthyala and X Lan in Chemistry and application of leuco dyes, Ed. R Muthyala (New York: Plenum, 1997)

Chapter 4.75. J Griffiths, Colour and constitution of organic molecules (New York: Academic Press, 1976) Chapter 9.4.76. R M Christie, Rev. Prog. Coloration, 23 (1993) 1.77. M J S Dewar in Recent advances in the chemistry of colouring matters, (London: Chemical Society, 1956) 79.78. R Muthyala and X Lan in Chemistry and application of leuco dyes, Ed. R Muthyala (New York: Plenum, 1997)

Chapter 6.79. P F Gordon and P Gregory in Developments in the chemistry and technology of organic dyes, Ed. J Griffiths (Oxford:

Blackwell Scientific Publications, 1984) 66.80. O Meth-Cohn and M Smith, J.C.S., Perkin I, (1994) 5.81. J F Corbett, J.S.D.C., 88 (1972) 438.82. H Zollinger, Colour chemistry, 2nd Edn (Weinheim: VCH, 1991) Chapter 3.1.83. L G S Brooker in The theory of the photographic process, Ed. T H James (New York: Macmillan, 1971) 205.84. Z Zhu, Dyes and Pigments, 27 (1995) 77.85. J Hu, P Skrabal and H Zollinger, Dyes and Pigments, 8 (1987) 189.86. F Kehrer, P Niklaus and B K Manukian, Helv. Chim. Acta, 50 (1967) 2200.87. C W Greenhalgh, J L Carey, N Hall and D F Newton, J.S.D.C., 110, (1994) 178.88. H S Freeman and J C Posey, Dyes and Pigments, 20 (1992) 171.

REFERENCES

chpt6(1).pmd 15/11/02, 15:29355

356 CHEMISTRY OF REACTIVE DYES

356

CHAPTER 7

Chemistry of reactive dyes

John Shore

7.1 INTRODUCTION

The concept of attaching a coloured molecule to cellulose by means of a chemical bond is atleast a century old, but until the 1950s a commercially viable technique for achievingdyeings of high wet fastness in this way remained elusive. The viscose process devised forthe manufacture of regenerated cellulosic fibres involved the conversion of alkali-treatedcotton to the soluble sodium cellulose xanthate by exposure to carbon disulphide vapour.While working on this process in the 1890s, Cross and Bevan described the synthesis of acoloured polymer by benzoylation of alkali-treated cellulose, nitration of the benzoate ester,reduction to the aminobenzoate and finally diazotisation and coupling [1].

During the following half-century, much fundamental work on the structure of cellulosicfibres was undertaken and numerous esters and ethers of cellulose were prepared,occasionally involving the attachment of coloured sidechain substituents. Although suchnecessarily complex and esoteric reactions confirmed the formation of covalent bondsbetween typical chromogenic systems and the hydroxy groups in cellulose, they remainedessentially of academic interest only [2,3]. Surprisingly, few attempts were made to adaptreaction conditions or to develop appropriate reagents that would allow such derivatives tobe formed under typical dyehouse conditions. Drawbacks of the treatments applied in thisperiod included their multi-stage complexity and the use of costly and hazardous solventmedia. Degradative attack of the cellulosic fibres by the vigorous reagents or reactionconditions necessary, or sensitivity of the colorant–fibre linkage to hydrolytic attack duringsubsequent handling or storage of the coloured product, were further problems thatprevented exploitation in practical dyeing systems.

The first commercially available dye capable of covalent reaction with a textile fibre isbelieved to be Supramine Orange R (CI Acid Orange 30). This was introduced by I GFarbenindustrie in the 1930s for the dyeing of wool. It contained a chloroacetylaminosubstituent from which the labile chlorine atom can be readily displaced under conventionalweakly acidic dyeing conditions at the boil to form a dye–fibre bond (Scheme 7.1). It doesnot seem to have been realised at the time that the high fastness to washing shown by thisacid dye was partly attributable to reaction with nucleophilic groups in wool.

In 1952 Hoechst marketed two Remalan vinylsulphone dyes that were capable of reactingwith wool. These were applied under near-neutral conditions and functioned by nucleophilicaddition across the activated double bond of the vinylsulphone group. The chemistry thathad been elucidated in the development of these novel dyes provided a springboard forHoechst to respond quickly with the first range of Remazol dyes when the possibility of dye–fibre reaction was finally achieved on cellulosic fibres.

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357

Stephen and Rattee at ICI were also evaluating speculative reactive dyes for wool in theearly 1950s. These products included a series containing the dichloro-s-triazine reactivesystem [4]. Recognising that compounds with activated chloro substituents can be inducedto react with cellulose under alkaline conditions, Rattee decided to investigate this byimmersing alkali-treated cotton in solutions of these new dyes [5]. Various refinements ofthis process were necessary including the addition of salt to enhance substantivity andlowering of the pH to minimise hydrolysis of the highly reactive dichlorotriazine system,before a trichromatic set of three Procion (ICI) dyes could be marketed in 1956.

Exploitation of the dichlorotriazine dyes soon led to parallel development of the muchless reactive aminochloro-s-triazine derivatives, which ultimately became the mostsuccessful of all reactive dye systems. Aminochlorotriazine dyes (such as 7.2) are readilyprepared by a substitution reaction (Scheme 7.2) at 30–40 °C between an arylamine and thedichlorotriazine precursor (7.1 in this instance). Dyes of the aminochlorotriazine type [6]were launched simultaneously as Cibacron (Ciba) and Procion H (ICI) brands shortly afterthe dichlorotriazine dyes had been introduced. More stable pad liquors could be formulated

H2N(CH2)4 CH

NH

CO

NHCOCH2Cl[dye] [dye] NHCOCH2NH(CH2)4 CH

NH

CO

+

Chloroacetyl dye Lysine in wool Dye-fibre bond

Scheme 7.1

NN

O

HN

N

N

N

SO3Na

H

NaO3S

Cl

Cl

NN

O

HN

N

N

N

SO3Na

H

NaO3S

NH

Claniline

7.1

7.2

CI Reactive Red 1

CI Reactive Red 3

SO3Na

SO3Na

Scheme 7.2

INTRODUCTION

chpt7(1).pmd 15/11/02, 15:32357


using these less reactive dyes. The range of reactivities offered by the two classes of dyes incombination with various alkalis greatly extended the scope of novel dyeing methods forthem [7].

The growth of reactive dyes over the intervening years has been steady rather thanspectacular [8]. It was originally thought that they would replace most of the other classes ofdyes for cellulosic fibres, except for vat dyes in the most demanding sectors, and eventuallydominate the field. This did not in fact occur and the traditional demands for the moreeconomical direct and sulphur dyes on woven cotton goods remained largely unchanged.One reason for this was the need for complete removal of unfixed or hydrolysed dyes inorder to achieve satisfactory wet fastness. As much as 50% of the total cost of a reactivedyeing process must be attributed to washing-off and treatment of the resulting effluent, alimitation that has prevented reactive dyes from attaining the level of success originallypredicted for them [2].

7.2 REACTIVE SYSTEMS

The characteristic features of a typical reactive dye molecule include:(1) the chromogen, contributing the colour and much of the substantivity for the fibre;(2) the reactive system, enabling the dye to form a covalent bond with the fibre and often

also contributing some substantivity;(3) a bridging group that links the reactive system to the chromogen and often exerts

important influences on reactivity, stability and substantivity;(4) one or more solubilising groups, usually sulphonic acid substituents on the aryl rings of

the chromogen;(5) in some instances, as with the important aminochlorotriazine system, a colourless

arylamino or other residue attached to the reactive grouping that also modifiessolubility and substantivity.

All the important chemical classes of chromogen have been included in reactive dyestructures. The sulphatoethylsulphone precursor of the vinylsulphone reactive groupcontributes significantly to the aqueous solubility of dyes of this type. The nature of thebridging group, especially in dyes of the haloheterocyclic type, greatly influences thereactivity and other dyeing characteristics of such dyes [9]. The structure of the reactivegrouping and substituents attached to it is decisive with regard to the chemical stability ofthe dye–fibre bond that is formed.

Numerous factors have to be taken into account in designing reactive dyes of commercialinterest [10]. Some of the most important are:(1) Economy – any reactive system selected for a range of dyes must enable them to be

produced at acceptable cost.(2) Availability – the system selected must be free from patent restrictions, health hazards

or other limitations to exploitation.(3) Versatility – it must be possible to attach the reactive system to a variety of dye

chromogenic groupings in manufacture.(4) Storage stability – dyes containing the reactive system must be stable to storage under

ambient conditions worldwide.

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(5) Efficiency – the manufacturing yield must be economically viable and the dye fixationmust be high under conventional conditions of application.

(6) Bond stability – the dye–fibre bonds must be reasonably stable to a range of relativelysevere fastness tests.

Only a relatively few reactive systems (Table 7.1) have met these requirements sufficientlywell to become commercially established in a significant segment of the market for reactivedyes. In addition to these important types, several others have been marketed [3,11] asalternative ranges that have failed to maintain a foothold in the marketplace, or asindividual members of established ranges where they show reactivity characteristics similarto one of the more important systems. Many of these systems of relatively minor significanceare listed in Table 7.2.

Interesting intermediates and reactions were involved in synthesis of some of the systemsincluded in Table 7.2. A carbonamido bridging group was often used to attach the reactivesystem to the chromogen. This could be a weak point, in that it provided another site atwhich hydrolytic attack could rupture the dye–fibre bond. The carbonyl group in the highlyreactive dichloropyrimidine-5-carbonamide system served to further activate the chlorosubstituents in the heterocyclic ring as well as providing the means of attachment to thechromogen. The route to this system started from urea and formylenemalonic ester, whichwere condensed to give 2,4-dihydroxypyrimidine-5-carboxylic acid. This was reacted withphosphorus oxychloride to yield 2,4-dichloropyrimidine-5-carbonyl chloride, which was thencondensed with a suitable amino group in the dye chromogen.

Isomeric with the dichloropyrimidine was the 3,6-dichloropyridazine system, also linkedto the chromogen through a carbonamido bridge. Analogous to the dichloropyridazine wasthe 1,4-dichlorophthalazine-6-carbonamide system. This was synthesised from trimelliticacid (benzene-1,2,4-tricarboxylic acid) by condensation with hydrazine and treatment of theresulting 1,4-dihydroxyphthalazine-6-carboxylic acid with phosphorus oxychloride andphosphorus pentachloride. Both sulphur and nitrogen were used as activating atoms in the2-chlorobenzothiazole reactive system [12]. The intermediate 2-chlorobenzothiazole waschlorosulphonated in the 6-position and the derived 6-sulphonyl chloride was condensedwith the amino group of the chromogen.

REACTIVE SYSTEMS

Table 7.1 Important reactive systems for cellulosic dyeing

Monofunctional Homobifunctional

Dichlorotriazine

Aminochlorotriazine

Aminofluorotriazine

Trichloropyrimidine

Chlorodifluoropyrimidine

Chlorofluoromethylpyrimidine

Dichloroquinoxaline

Sulphatoethylsulphone

Sulphatoethylsulphonamide

Bis(aminochlorotriazine)Bis(aminofluorotriazine)Bis(aminonicotinotriazine)Bis(sulphatoethylsulphone)

Heterobifunctional

Aminochlorotriazine-sulphatoethylsulphoneAminofluorortriazine-sulphatoethylsulphoneDifluoropyrimidine-sulphatoethylsulphone

chpt7(1).pmd 15/11/02, 15:32359


A different type of activating arrangement was evident in the dichloropyridazone system.Here the reactive 4-chloro substituent is not activated directly by the two nitrogen atoms inthis quinonoid heterocyclic ring but by the C=C–C=O grouping, which is a vinylogue of acarboxylic acid chloride [13]. The starting materials for this system were dichloromaleicacid, made by chlorination of 2-butyne-1,4-diol, and hydrazinopropionitrile fromacrylonitrile and hydrazine. These were cyclised to give dichloropyridazonyl propionitrile,which was then hydrolysed and converted to the acid chloride using thionyl chloride.

The acrylamide group and its precursor the 3-sulphatopropionamide system (Scheme 7.3)both show only feeble reactivity with cellulose under conventional alkaline dyeingconditions [14] and this has greatly limited their usefulness in spite of their economicattractiveness. Similar considerations apply to the 2-chloroethyl- and 2-sulphatoethyl-sulphamoyl systems, which are both precursors of the three-membered aziridine (cyclicethyleneimine) ring system (Scheme 7.4). This unsaturated group undergoes a nucleophilicaddition reaction [15] with alkali-treated cellulose (Scheme 7.5). The importantvinylsulphone group and its various precursor systems are much more reactive and theseprovided the only really successful ranges of dyes that react by the nucleophilic additionmechanism.

Table 7.2 Reactive systems of minor or historical significance only

N

N

Cl

[dye] – HNOC

Cl

2,4-Dichloropyrimidine-5-carbonamide

N

N

Cl

[dye] – HNOC

Cl

3,6-Dichloropyridazine-4-carbonamide

[dye] – HNOC

N

N

Cl

Cl

1,4-Dichlorophthalazine-6-carbonamide

N

S

Cl

[dye] – HNO2S

2-Chlorobenzothiazole-6-sulphonamide

N

N

O Cl

Cl[dye] – HNOCH2CH2C

3′-(4,5-Dichloropyridaz-3-on-2-yl)propionamide

[dye]–NHCOCH=CH2 Acrylamide[dye]–NHCOCH2CH2–OSO3Na 3-Sulphatopropionamide[dye]–SO2CH=CHCl 2-Chlorovinylsulphone[dye]–SO2NHCH2CH2Cl 2-Chloroethylsulphamoyl[dye]–SO2NHCH2CH2–OSO3Na 2-Sulphatoethylsulphamoyl

chpt7(1).pmd 15/11/02, 15:32360

361

[dye] NHCOCH2CH2 OSO3Na [dye] NHCOCH CH2

NaOHNa2SO4 H2O+ +

3-Sulphatopropionamide Acrylamide

Scheme 7.3

[dye] SO2NHCH2CH2Cl

[dye] SO2NHCH2CH2 OSO3Na

[dye] SO2 NCH2

CH2

NaOH

NaOH

2-Chloroethylsulphamoyl Aziridine

2-SulphatoethylsulphamoylScheme 7.4

[dye] SO2 NCH2

CH2[cellulose] OH [dye] SO2NHCH2CH2 O [cellulose]+

Aziridine Dye-fibre bond

Scheme 7.5

7.3 MONOFUNCTIONAL SYSTEMS

The monofunctional reactive systems of outstanding importance contain only one possiblereactive centre, such as the halogeno substituent in the aminohalotriazine dyes, or theactivated terminal carbon atom in the vinylsulphone system. In others there are twoequivalent replaceable halogeno substituents, as in the dichloroquinoxaline or dichloro-triazine heterocyclic ring systems. When one of these halogen atoms is displaced by reactionor hydrolysis, the reactivity of the remaining halogeno substituent is greatly inhibited by thepresence of the new hydroxy or cellulosyl substituent.

The influence of the reactive system itself on the substantivity of the dye containing itwas recognised in the early years of the development of reactive dyes [16]. Thedichlorotriazine grouping and even more so the aminohalotriazine or dichloroquinoxalinesystems considerably enhance the overall substantivity of the dye molecule. The pyrimidine-based structures, with one less heterocyclic nitrogen atom than the corresponding triazinederivatives, contribute much less to the total substantivity of the dye. The non-aromatic,reactive systems, notably the 2-sulphatoethylsulphone group, modify the substantive effectof the chromogen only slightly or not at all. This can offer a significant practical advantage,since a low overall substantivity that remains unaffected by the reactive system greatlyfacilitates removal of the unfixed dye on washing-off, a property of especial value for printingor continuous dyeing.

The s-triazine ring is unique amongst the six-membered nitrogen heterocycles inpossessing three electronegative atoms ideally placed to provide the necessary activation ofthe halogen atoms attached to the adjacent carbon atoms. Calculations of charge

MONOFUNCTIONAL SYSTEMS

chpt7(1).pmd 15/11/02, 15:32361


distribution over these various heterocyclic rings [17] show that the largest possible chargeattained in the series is found on the carbon atoms of the s-triazine ring (Figure 7.1). As aresult, none of the dyes derived from the alternative chloro-substituted heterocyclic systemsare capable of showing reactivity as high as that of the dichloro-s-triazine derivatives.

NN

N

N

N N

N N

N

N

–0.007+0.071

–0.083 –0.109

+0.064

+0.042

–0.031

+0.202

–0.178

+0.101

–0.144

+0.133–0.072

+0.23

–0.20

Pyridazine Pyrazine Pyridine Pyrimidine s-Triazine

Figure 7.1 Levels of reactivity of various heterocyclic rings

Not all reactive systems are sufficiently versatile to be used successfully with arepresentative selection of chromogens covering the entire colour gamut. In some cases,attempts to achieve a complete range in this way based on the use of only one type ofreactive group have resulted in lack of compatibility because of the dominant influences ofdifferent chromogens on reactivity and substantivity. An alternative approach designed toovercome such difficulties has been to decide beforehand on a target profile of moderatereactivity and high substantivity. The range is then built up accordingly, using variouscombinations of reactive system and chromogen that will yield the desired profile. Whenfocusing in this way on gaps in a range of reactive dyes, this approach to researchencourages the development of new chromogens as well as novel reactive systems [18].

7.3.1 Dichloro-s-triazine dyes

The key intermediate for these dyes is cyanuric chloride (7.3), in which all three chlorosubstituents are attached to equivalent carbon atoms with fractional positive chargesinduced by the neighbouring negative nitrogen atoms. Polarisation of the carbon–chlorinebonds makes the chloro substituents labile and readily susceptible to stepwise nucleophilicsubstitution. If the attacking nucleophile is a water-soluble amine, the reaction can beconveniently carried out with the cyanuric chloride suspended in an agitated aqueoussolution of the amine at a temperature close to 5 °C. Maintaining the pH close to neutralityby addition of alkali as required allows the first of the chlorine atoms to be smoothlydisplaced (Scheme 7.6), yielding the aminodichlorotriazine product (7.4). The optimumconditions for preparing various 6-alkyl-, 6-aryl- and 6-heteroaryl-substituted derivatives of2,4-dichloro-s-triazine have been defined recently [19].

Selecting a sulphonated dye molecule containing an amino group as the nucleophile leadsdirectly to a dichlorotriazine dye. In certain cases a suitable intermediate may be condensedwith cyanuric chloride and then the chromogenic grouping is synthesised from this reactionproduct. Both of these routes are illustrated in a simple way for CI Reactive Red 1 (7.1) inScheme 7.7. In these dyes the electronic effects responsible for the lability of the chlorosubstituents are muted by feedback of electrons from the electron-donating imino bridging

chpt7(1).pmd 15/11/02, 15:32362

363

R NH2

N

N

N

Cl

Cl

Cl

N

N

N

Cl

Cl

Cl

R NH2

N

N

N

Cl

NH

Cl

R

NaOH

+

+ NaCl + H2O

7.4

7.3Cyanuric chloride Transient species

Scheme 7.6

HO

NaO3S

H2N

SO3Na

N

N

N

Cl

Cl

Cl

HO

NaO3S

HN

SO3Na

N

N

N Cl

Cl

SO3

N N

N

N

N Cl

Cl

NN

HN

SO3Na

NaO3S

OH

SO3NaNN

O

NaO3S

SO3NaH

SO3NaN

N

N

Cl

Cl

Cl

NaOH

NaOH

H2N

+

+

7.1

_

+

H acid Cyanuric chloride

Coupling component

Diazocomponent

CI Reactive Red 1

Cyanuric chlorideDye base

Scheme 7.7

link, making the dye quite stable in neutral solution. This highly reactive system issusceptible to attack by hydroxide ions on the alkaline side and subject to autocatalytichydrolysis on the acid side. To guard against decomposition in these ways a buffer is added tothe dye solution to ensure stability during isolation and further buffer is incorporated intothe isolated paste before drying.


chpt7(1).pmd 15/11/02, 15:32363


The dichlorotriazine dyes are so reactive that they can be readily fixed to cellulosicmaterials by pad-batch dyeing at ambient temperature or by exhaust methods at 30–40 °C.This means that relatively small chromogens are preferred to ensure adequate mobility ofdye on the fibre during the exhaustion stage. This requirement makes these dyes eminentlysuitable for bright dyeings but less satisfactory for deep tertiary hues, since the larger-sizechromogens used for this purpose often fail to give acceptable performance by low-temperature application. A weakness with certain dichlorotriazine dyes, particularly redmonoazo derivatives of H acid such as CI Reactive Red 1 (7.1), is that under conditions oflow pH the dye–fibre bond is susceptible to acid-catalysed hydrolysis leading to deficienciesin fastness to washing or acid perspiration.

Ionisation of the hydroxy groups in cellulose is essential for the nucleophilic substitutionreaction to take place. At neutral pH virtually no nucleophilic ionised groups are presentand dye–fibre reaction does not occur. When satisfactory exhaustion of the reactive dye hastaken place, alkali is added to raise the pH to 10–11, causing adequate ionisation of thecellulose hydroxy groups. The attacking nucleophile (:X–) can be either a cellulosate anionor a hydroxide ion (Scheme 7.8), the former resulting in fixation to the fibre and the latterin hydrolysis of the reactive dye. The fact that the cellulosic substrate competes effectivelywith water for the reactive dye can be attributed to three features of the reactive dye/cellulosic fibre system:(1) the concentration of dye in the fibre phase is greater than that in the solution phase(2) the greater nucleophilicity of the cellulosate anion compared with hydroxide(3) the high selectivity of the electrophilic reactive group

Because the pKa of the cellulosic substrate is lower than that of water, reaction with thefibre according to Scheme 7.8 is favoured [20].

When partial hydrolysis occurs as in Scheme 7.8 to form the 2-chloro-4-hydroxy species(7.5), the dye does not have a further chance to achieve fixation via the remaining chlorineatom. Under the alkaline conditions of fixation, ionisation of the acidic 4-hydroxysubstituent leads to a massive feedback of electronegativity into the triazine ring, causingtotal deactivation of the remaining 2-chloro substituent. The remaining chlorine atom in

N

N

N

Cl

NH

Cl

[dye]

N

N

N

Cl

NH[dye]

Cl

X

N

N

N

Cl

NH

X

[dye]

X+

+ Cl

_

_

_

Dichlorotriazine dye Transient species

Scheme 7.8

chpt7(1).pmd 15/11/02, 15:32364

365

the still active but fixed species (7.6), on the other hand, may be subsequently hydrolysed(Scheme 7.9) to give an inactive fixed species (7.7). Under more extreme conditions of pHor temperature, a crosslinked species (7.8) can be formed linking two neighbouring cellulosechains in an amorphous region of the fibre [21,22].

N

N

N

Cl

NH

OH

[dye]

7.5

N

N

N

OH

NH

O

[dye]

[cellulose]

N

N

N

Cl

NH

O

[dye]

[cellulose]

N

N

N

O

NH

O

[dye]

[cellulose]

[cellulose]

7.6

7.87.7

Active fixed dye

Crosslinked dye-fibre bondHydrolysed fixed dyeScheme 7.9

7.3.2 Monochloro-s-triazine dyes

Controlled reaction of dichlorotriazine dyes with either amines or alcohols at 30–40 °C leadsto two further classes of monofunctional dyes, the 2-amino-4-chloro- and 2-alkoxy-4-chloro-s-triazines respectively. The latter are more reactive than the former but less reactivethan the parent dichlorotriazine types. They are now essentially of historical interest, the 2-isopropoxy-4-chloro system forming the basis of the Cibacron Pront (Ciba) range forprinting. The bulky isopropoxy group was chosen in order to disrupt the planarity of thesubstituted triazine system and thus favour removal of unfixed dye at the washing-off stage.The mechanism of reaction of methyl-α-D-glucoside (7.9), as a soluble model for cellulose,with a model Cibacron Pront dye in homogeneous solution was examined recently. It wasdemonstrated that reaction occurs only with the hydroxy groups attached at the C4 and C6positions, the ratio of reactivities being 12:1 in favour of the primary alcoholic site [23].

In another recent study, the relative rates of hydrolysis of a series of orange N-methyl Jacid dyes of the 2-alkoxy-4-chloro-s-triazine type (7.10; R = methyl, ethyl or isopropyl)


chpt7(1).pmd 15/11/02, 15:32365


were compared. Surprisingly, the hydroxide ion displaced the methoxy group about 1.5 timesmore rapidly than the chloro substituent (Scheme 7.10; kOR > kCl). Increase in the size andelectron-donating capacity of the alkoxy group resulted in a decreasing propensity forsubstitution, so that displacement of methoxide ion proceeded about twelve times quickerthan that of isopropoxide ion [24].

More energetic conditions of fixation, typically 80 °C and pH 11 for exhaust application,are necessary to achieve efficient fixation of 2-amino-4-chloro-s-triazine dyes on cellulosicfibres. Early studies of the relationships between structure and substantivity ofaminochlorotriazine dyes revealed that the NH bridging groups linking the chromogen andthe uncoloured 2-arylamino substituent to the heterocyclic ring exert marked effects on thesolubility and dyeing properties of these dyes [2]. Replacement of such an NH imino link byan N-methylimino group lowers the substantivity by inhibiting hydrogen bonding tocellulose hydroxy groups. The use of a sulphonated arylamine to form the uncolouredsubstituent in the 2-position of a monochlorotriazine system is helpful in enhancing dyesolubility and migration behaviour. The relationship between the distortion angle caused bysteric hindrance and the reactivity of aminochlorotriazine dyes has been demonstrated in akinetic study of the hydrolysis of model compounds. Bulky substituents in the ortho positionwith respect to the imino bridge in 2-anilino derivatives cause sectional distortion ofcoplanarity between the phenyl and triazine rings, leading to an increase in reactivity owingto partial impediment of p-π conjugation [25].

7.3.3 Monofluoro-s-triazine dyes

Fluorine and chlorine are the only important choices for the labile substituents on aheterocyclic reactive system but numerous other leaving groups have been patented, mostly assubstituent on an s-triazine ring. These include sulpho, cyano, thiocyanato, azido andtrichloromethyl [26], as well as more elaborate groupings (Figure 7.2). Such derivatives are

CH

CH

CH CH

CH

O

OCH3HO

CH2OH

OH OH

1

23

4

5

6

7.9

Methyl-α-D-glucoside

NN

O

NaO3S

NaO3S

SO3Na

N

NN

O

ClN

R

CH3H

7.10

N

N

N

OH

NH

Cl

[dye]

N

N

N

O

NH

Cl

[dye]

R

N

N

N

O

NH

OH

[dye]kOR kCl

R

Scheme 7.10

chpt7(1).pmd 15/11/02, 15:32366

367

almost always made by replacing a chlorine atom in a chloro-s-triazine dye by reaction withthe sodium salt of the appropriate nucleophile. Thus they are invariably more costly than theparent chloro compound and offer no obvious benefits, so they have not been exploitedcommercially.

A fluorine atom is used as the leaving group in the Cibacron F (Ciba) range of 2-amino-4-fluoro-s-triazine dyes, as exemplified by the red monoazo H acid dye 7.11. The essentialhighly reactive intermediate cyanuric fluoride required for their manufacture is made fromcyanuric chloride by an exchange reaction with potassium fluoride (Scheme 7.11). Thegreater electronegativity of fluorine compared with chlorine results in a markedly higher

O2N

O SO3Na

NaO3S

O SO3Na

O2N

S NO2

S

N

S S C

S

N

CH2CH3

CH2CH3

CH

COOCH2CH3

COOCH2CH3

P

OCH3

OCH3

O

Figure 7.2 Alternative leaving groups for heterocyclic systems

NN

O

NaO3S

NaO3S

SO3Na

HN

N

N

N NH

F

H

7.11

SO3Na

N

N

N

Cl

Cl

Cl

N

N

N

F

F

F

+ 3 KF + 3 KCl

Cyanuric chloride Cyanuric fluoride

Scheme 7.11


chpt7(1).pmd 15/11/02, 15:32367


level of reactivity for these dyes than for their 2-amino-4-chloro analogues. The compactcharacter of the fluorine atom, resulting from the close binding of the nine-electron spherearound the atomic nucleus, favours reaction with dye bases carrying aliphatic amino groups.This need has prompted the synthesis and exploitation of novel dye intermediates bearingsuch substituents, such as 5-aminomethyl-l-naphthylamine-2-sulphonic acid (7.12). This isprepared (Scheme 7.12) by condensing 1-acetylaminonaphthalene-2-sulphonic acid with N-methylolphthalimide (Tscherniac–Einhorn reaction), followed by hydrolysis of the resulting5-substituted derivative.

N

O

O

CH2OH

SO3H

CH3COHN

N

O

O

CH2

SO3H

CH3COHN

NH2

SO3H

CH2H2N

COOH

COOH

+

CH3COOH +

+

7.12Scheme 7.12

Considerable work was carried out in the 1980s by Mitsubishi on the preparation andevaluation of disperse dyes containing the highly reactive 2-alkoxy-4-fluoro-s-triazine system,an example being the blue dicyanonitrophenylazo structure 7.13 [27]. These products weredesigned specifically for the dyeing of both component fibres in polyester/cellulosic blends.Although marketed commercially, these novel dyes have not gained widespread acceptance[8]. The absence of water-solubilising groups allowed these molecules to enter the polyesterfibre but the high relative molecular mass (490 in the case of 7.13) implies slow diffusion andpoor migration properties. The combination of high reactivity and lack of water solubilityseems likely to favour fixation near the cellulosic fibre surface with poor penetration into theinterior. In the case of structures like 7.13, there may be a possibility of some self-deactivation

chpt7(1).pmd 15/11/02, 15:32368

369

of the reactive system by temporary quaternisation between the tertiary amino group in onemolecule and the fluorotriazine centre in another (Scheme 7.13).

CN

O2N

CN

N N N

CH2CH3

CH2CH3

HN

N

N

N OCH3

F

7.13

N

CH2CH3

CH2CH3 N

N

N

OCH3

F

N

N

N

OCH3

N CH2CH3

CH2CH3

N

CH2CH3

CH2CH3N

N

N

OCH3

OH H2O

+

+ + HF

F–

Scheme 7.13


7.3.4 Trichloropyrimidine dyes

The 1,3-diazine arrangement in the pyrimidine ring provides less activation of chlorosubstituents than the s-triazine system (Figure 7.1). Fixation to a cellulosic fibre by exhaustdyeing methods requires treatment at the boil rather than the 80 °C found most suitable foraminochlorotriazine dyes, but the dye–fibre linkage through a diazine ring is more stablethan that containing a triazine nucleus [16]. The intermediate required for these dyes,tetrachloropyrimidine (7.15), is prepared by chlorination of barbituric acid (7.14) andtreatment of the resulting 5-chloro derivative with phosphorus oxychloride and dimethyl-aniline as catalyst (Scheme 7.14). Two isomeric aminotrichloropyrimidine structures arepossible with the amino substituent in either the 2- or the 4-position. A study of the initialreaction between tetrachloropyrimidine and various arylamines demonstrated thatnucleophilic substitution occurs mainly by displacement of the 4-chloro substituent [28], asillustrated for the red monoazo H acid dye CI Reactive Red 17 (7.16).

Preferential substitution at the 4-position is much less pronounced in the reaction of2,4,6-trichloropyrimidine with arylamines, so that the dichloropyrimidine dyes formed in this

chpt7(1).pmd 15/11/02, 15:32369


way contain a mixture of 2,6- and 4,6-dichloro isomers. These dyes are even less reactivethan the trichloropyrimidine dyes but are correspondingly more resistant to acidic oralkaline hydrolysis.

The 5-chloro substituent in the trichloropyrimidine reactive system is much less activatedby the nitrogen atoms in the heterocyclic ring and is thus not normally capable of hydrolysisor reaction with the fibre. Dichloropyrimidine dyes containing a 5-methyl substituent areless reactive but more stable than their trichloropyrimidine analogues [16]. Dyes preparedwith an electron-withdrawing group in the 5-position, such as 5-cyano or 5-nitro, showenhanced reactivity of the 2,6-dichloro substituents but somewhat lower stability of the dye–fibre bond. An example of a 5-cyano-2,4-dichloropyrimidine dye is CI Reactive Red 219,which is compatible in reactivity and dyeing behaviour with dichlorotriazine dyes. Theintermediate required for dyes of this type is 5-cyano-2,4,6-trichloropyrimidine (7.17),prepared from barbituric acid (7.14). Reaction of this with sodium cyanate and hydrochloricacid yields the 5-carbonamido derivative and this is treated with phosphorus oxychlorideand dimethylaniline as catalyst, resulting in simultaneous dehydration of the carbonamidogroup and replacement of the hydroxy groups to give the desired product (Scheme 7.15).

NN

H

NaO3S

OSO3Na

SO3Na

HN

N

N

Cl

Cl

Cl

7.16

CI Reactive Red 17

N

N

CH

O

OH

O

H

N

N

CH

OH

OH

HO

N

N

OH

OH

HO Cl

N

N

Cl

Cl

Cl Cl

7.14

7.15

Barbituric acid

Tetrachloropyrimidine

Scheme 7.14

chpt7(1).pmd 15/11/02, 15:32370

371

N

N

CH

O

OH

O

H

N

N

CH

OH

OH

HO

N

N

OH

OH

HO CONH2

N

N

Cl

Cl

Cl CN

7.14

7.17

Barbituric acid

Scheme 7.15

NN

H

NaO3S

OSO3Na

NH

N

N

F

FCl

7.18

CH3O

N

N

Cl

Cl

Cl

N

N

F

F

F

Cl Cl+ 3 KF + 3 KCl

7.15

Scheme 7.16


7.3.5 Chlorodifluoropyrimidine dyes

Another important route to more reactive halopyrimidine dyes is to use fluorine rather thanchlorine in the reactive centres. As with the fluoro-s-triazine dyes, this results in a markedlyhigher level of reactivity compared with the corresponding chloro-substituted analogues.Exhaust dyeing temperature for optimal fixation of a 5-chloro-2,4-difluoropyrimidinederivative, such as the red monoazo J acid dye 7.18, is 40–50 °C. The dye–fibre bond formedwith cellulose by fixation of these highly reactive products is more stable to acid conditionsthan that formed by the competing dichlorotriazine dyes but it does tend to undergo oxidativecleavage more readily under the influence of light exposure in the presence of peroxycompounds. The exchange reaction of tetrachloropyrimidine (7.15) with potassium fluorideyields 5-chloro-2,4,6-trifluoropyrimidine (Scheme 7.16). When this is reacted with the dye

chpt7(1).pmd 15/11/02, 15:32371


base, however, a mixture of two isomers is formed (Scheme 7.17), the symmetrical 5-chloro-4,6-difluoro arrangement (7.19) and the asymmetrical 5-chloro-2,4-difluoro group (7.20).

Recent work at DyStar has led to the development of 4,5-difluoropyrimidine dyes such asthe bluish red 7.21 for the dyeing of cellulosic and polyamide fibres [29]. This reactivesystem is reported to be superior to the conventional 5-chloro-2,4-difluoro arrangement instructure 7.18, for example. Another novel and interesting system is the 2,4,6-trifluoropyrimidine grouping (7.22) included by Clariant in dyes designed for cellulosicdyeing [20]. This is environmentally more attractive than the 5-chloro-2,4-difluoro systemcontaining the relatively inert 5-chloro substituent that will contribute to AOX values. Dyesbased on structure 7.22 should also be stable to perborate attack during laundering.

NN

SO3Na

H O

SO3Na

N

N

HN

F

F

NaO3S 7.21

NH

N

N

F

F

F

[dye]

7.22

N

N

F

F

F

Cl

N

N

F

NH

F

Cl

[dye] NH2

[dye]

N

N

F

NH

F

[dye]

Cl

2 + 2

+

7.197.20Scheme 7.17

7.3.6 Chloromethylpyrimidine dyes

In the Levafix P (DyStar) range of rapid-fixing reactive dyes for printing, the leaving groupwas a methylsulphonyl substituent [30]. Preparation of the required reactive intermediate(7.23) involved condensation of S-methylthiourea with ethyl acetoacetate, chlorination ofthe product, oxidation of the methylthio group and conversion of the hydroxy group tochloro by the action of phosphorus oxychloride (Scheme 7.18). The 4-chloro substituent instructure 7.23 was preferentially eliminated by neutral condensation with the dye base

chpt7(1).pmd 15/11/02, 15:32372

373

(Scheme 7.19), leading to dyes with a 5-chloro-4-methyl-2-methylsulphonylpyrimidinereactive system (7.24). During reaction with the cellulosic fibre under alkaline conditions,the methylsulphonyl moiety is a particularly effective leaving group and rapid fixation takesplace when the reactive system approaches the cellulosate anion.

The Levafix P dyes have been replaced by the Levafix PN (DyStar) range, which is basedon the 5-chloro-2-fluoro-4-methylpyrimidine system. These two systems are essentiallysimilar, but the Levafix PN dyes have a fluorine atom as the labile entity rather than themethylsulphonyl grouping. The necessary intermediate 5-chloro-2,6-difluoro-4-methyl-pyrimidine (7.25) can be made from 4-methyl-2,5,6-trichloropyrimidine by an exchangereaction with potassium fluoride (Scheme 7.20). Condensation with the dye base (Scheme7.21) will give a mixture of isomers, namely, the 5-chloro-6-fluoro-4-methyl (7.26) and 5-chloro-2-fluoro-4-methyl (7.27) derivatives.

CH3CH2O C

H2C

O

C

CH3

O

H2N C

NH2

S CH3

N

N

CH3

HO

S CH3

N

N

CH3

HO

SO2CH3

Cl

N

N

CH3

Cl

SO2CH3

Cl

+

7.23

Scheme 7.18

[dye] NH2

N

N

CH3

Cl

SO2CH3

Cl

N

N

CH3

NH

SO2CH3

Cl

[dye]

+

7.23

H2O + NaCl +

7.24

NaOH

Scheme 7.19


chpt7(1).pmd 15/11/02, 15:32373


7.3.7 Dichloroquinoxaline dyes

The reactivity of this system is much higher than that of analogous dichloropyrimidine dyesand comparable with aminofluorotriazine or difluoropyrimidine dyes, optimal fixation beingachieved by exhaust dyeing at about 50 °C. Thus these three reactive systems are mutuallycompatible with one another and all three have been used in members of the Levafix(DyStar) range for long-liquor dyeing. The structure of a typical red monoazo H acid dyecontaining the dichloroquinoxaline system is illustrated (7.28).

N

N

CH3

Cl

Cl

Cl

N

N

CH3

F

F

Cl+ 2 KF + 2 KCl

7.25

Scheme 7.20

[dye] NH2

N

N

CH3

F

F

Cl

N

N

CH3

NH

F

Cl[dye]

N

N

CH3

NH

F

Cl

[dye]

2 + 2

7.25

7.26

+

7.27

Scheme 7.21

NN

H

NaO3S

O N

N Cl

Cl

SO3Na

C

HN

O

SO3Na

7.28

chpt7(1).pmd 15/11/02, 15:32374

375

The key intermediate (7.29) required for reaction with the dye base is manufactured from3,4-diaminobenzoic acid and oxalic acid [15]. These are condensed to yield 2,3-dihydroxyquinoxaline-6-carboxylic acid and the chloro substituents are introduced usingphosphorus oxychloride and phosphorus pentachloride (Scheme 7.22). Unlike all otherimportant haloheterocyclic reactive systems, the bridging link between the chromogen andthe quinoxaline nucleus is amidic and thus expected to be readily hydrolysed under acidicconditions. The 1,4-diazine ring in the dye–fibre linkage formed by these dyes, like the 1,3-diazine ring present after fixation of the pyrimidine systems, tends to undergo oxidativecleavage when exposed to light or heat under peroxidic conditions. In spite of suchpotentially severe defects, the commercial success of these dyes over a long period indicatesthat they do not give rise to serious practical problems under normal circumstances [2].

S CH2CH2[dye]

O

O

OSO3Na S CH[dye]

O

O

CH2

NaOH

7.30

+ Na2SO4 + H2O

7.31

Scheme 7.23

7.3.8 Sulphatoethylsulphone and -sulphonamide dyes

Reactive dyes of the vinylsulphone type (7.31) are normally marketed in the form of the 2-sulphatoethylsulphonyl precursor (7.30) in order to enhance the aqueous solubility andstorage stability of the dye. In the presence of alkali the precursor group is converted intothe active vinylsulphone form (Scheme 7.23), this being necessary for the reaction with thecellulosic fibre [31–33]. The aqueous solubility of commercial sulphatoethylsulphone dyesgenerally varies according to the number of sulphonate and sulphate ester groups present.Further reactions at 90–100 °C and pH values above 11.5 in the presence of hydroxide orcarbonate anions (Scheme 7.24) result in generation of the 2-hydroxyethylsulphone (7.32)and eventually the diethyl ether form (7.33), which can amount to about 10% of the totaldye present [34].


NH2

NH2

HOOCC

C

O OH

O OH N

N OH

OH

N

N Cl

Cl

HOOC

OCCl

+

7.29Scheme 7.22

chpt7(1).pmd 15/11/02, 15:32375


NN

NaO3S

SO3Na

HN

COCH3

OO

Cu

NaO3SO CH2CH2SO2

7.34

CI Reactive Violet 5

In a recent investigation of the effect of low-frequency ultrasonic waves on the stability ofa 1:1 copper-complex phenylazo H acid dye (7.34), reaction rates were determined for the1,2-elimination of the sulphato group to form the vinylsulphone (Scheme 7.23) and for thehydrolysis step that leads to the 2-hydroxyethylsulphone form [35]. Elimination of thesulphato moiety is strongly accelerated by the presence of the electron-attracting sulphonegroup situated in the β-position relative to the leaving group and is the rate-determiningstep [36]. This permits control of the kinetics of the fixation process and has importantimplications for dye application and level dyeing.

S CH[dye]

O

O

CH2 S CH2CH2[dye]

O

O

OH

S CH[dye]

O

O

CH2

O

[dye]HO CH2CH2 S

O

S[dye]

O

CH2CH2 O CH2CH2 S [dye]

O

O O

7.31

+ H2O

7.32

7.31

+

7.32

7.33

Scheme 7.24

In contrast to the various haloheterocyclic reactive systems already discussed, thevinylsulphone group reacts by a nucleophilic addition mechanism rather than bysubstitution. The carbon–carbon double bond forming the vinyl moiety is polarised by thepresence of the strongly electron-attracting sulphone group. This polarisation confers apartial positive charge on the terminal carbon atom, favouring nucleophilic addition(Scheme 7.25). The attacking nucleophile (:O–X) can be either a cellulosate anion or ahydroxide ion, the former resulting in fixation to the fibre and the latter in hydrolysis to the2-hydroxyethylsulphone form.

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377

[dye] S CH

O

CH2

O

O X [dye] S CH

O

CH2

O

O X

[dye] S CH2CH2

O

O

O X

H

+_ _

+Transient species

Scheme 7.25

Table 7.3 Relationship between substantivity and nature of thereactive group for monofunctional dyes [37]

Reactive group X Primary exhaustion (%)

–SO2CH2 CH2OSO3Na 20–SO2CH =CH2 80–SO2CH2 CH2OH 38

X

N

OCH3

N

H O

SO3Na

7.35

CI Reactive Red 22


The substantivity of the reactive vinylsulphone is much higher than that of thesulphatoethylsulphone precursor or the hydroxyethylsulphone hydrolysis product. Typicalvalues for primary exhaustion of these three forms of CI Reactive Red 22 (7.35) onunmercerised cotton in the presence of 50 g/l sodium sulphate at neutral pH are given inTable 7.3. This dye system offers potentially high fixation through the highly substantivereactive form combined with excellent wash-off potential owing to the low substantivity ofthe hydroxyethylsulphone. The disadvantage of the system is the major difference insubstantivity between the precursor form and the vinylsulphone because the rate ofsecondary exhaustion after alkali addition is more difficult to control [37].

The kinetics of homogeneous reaction of several reactive dyes of the vinylsulphone typewith methyl-α-D-glucoside (7.9), selected as a soluble model for cellulose, were studied inaqueous dioxan solution. The relative reactivities of the various hydroxy groups in the modelcompound were compared by n.m.r. spectroscopy and the reaction products were separatedby a t.l.c. double-scanning method [38]. The only sites of reaction with the vinylsulphonesystem were the hydroxy groups located at the C4 and C6 positions [39,40].

chpt7(1).pmd 15/11/02, 15:32377


O

O

NH2

SO3Na

SO2CH2CH2

HN

OSO3Na7.37CI Reactive Blue 19

Sulphatoethylsulphone dyes are intermediate in reactivity between the high-reactivityheterocyclic systems, such as dichlorotriazine or difluoropyrimidine, and the low-reactivityranges, such as aminochlorotriazine or trichloropyrimidine. Exhaust dyeing temperaturesbetween 40 and 60 °C may be chosen, depending on pH, since caustic soda is often selectedto bring about alkaline hydrolysis of the precursor sulphate ester. The substantivity of manyof these dyes is markedly lower than that of typical haloheterocyclic dyes. Not only has thevinylsulphone group, unlike the heterocyclic ring systems, little if any inherent affinity forcellulose, but the terminal sulphato group enhances the aqueous solubility of the precursorform before 1,2-elimination to the vinylsulphone. In contrast to the haloheterocyclicsystems, the dye–fibre bonds formed by the vinylsulphone dyes are at their weakest underalkaline conditions [41].

Two of the earliest Remazol dyes to be discovered by Hoechst turned out to be amongstthe most successful of all reactive dyes. The four solubilising groups in the precursor form ofCI Reactive Black 5 (7.36) confer high solubility but unusually low substantivity. This dye isalmost symmetrical in structure and when the sulphate ester groups are lost by 1,2-elimination, the substantivity for cellulose is enhanced and the bis(vinylsulphone) structureformed shows highly efficient fixation under alkaline conditions. After fixation theinherently low substantivity of the unfixed bis(hydroxyethylsulphone) dye facilitateswashing-off in a region of the colour gamut where this is often notoriously difficult.

The extremely attractive bright blue hue combined with excellent light fastness of CIReactive Blue 19 (7.37) remained unchallenged by competing blue reactive dyes for manyyears. The aqueous solubility of this structure is inherently low, depending only on the 2-sulphonate group after 1,2-elimination of the sulphate ester has taken place. This has led to

NN

H

NaO3S

O

H2N

SO3Na

N

NaO3SO CH2CH2SO2

7.36

CI Reactive Black 5

N

SO2CH2CH2 OSO3Na

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379

CH3CONH CH3CONH SO2Cl

CH3CONH SO2NaCH3CONH SO2CH2CH2OH

H2N SO2CH2CH2OH H2N SO2CH2CH2 OSO3Na

ClCH2CH2OH

ClSO3H

Na2SO3

dil.

conc.

H2SO4

7.38

HCl

Scheme 7.26

poor reproducibility and levelling problems, but nevertheless this dye has remained secondonly to Black 5 in terms of market share amongst reactive dyes.

In the various ranges of haloheterocyclic reactive dyes, the reactive system is usuallyattached to the chromogen via a simple NH imino or methylimino linkage. Preparation of dyesin the vinylsulphone series, however, often requires the synthesis of special arylamineintermediates that contain the reactive system. These are mostly anilines or naphthylaminescontaining a β-sulphatoethylsulphonyl substituent. The intermediate 4-(β-sulphatoethylsul-phonyl)aniline (7.38), used as the diazo component for Black 5 (7.36), is synthesised by thechlorosulphonation of acetanilide, reduction with sodium sulphite and condensation of thesulphinate with 2-chloroethanol to give the 2-hydroxyethyisulphone. Hydrolysis of theacetylamino group and esterification of the hydroxy group with concentrated sulphuric acidyields the desired product (Scheme 7.26). Synthesis of the isomeric 3-(β-sulphatoethyl-sulphonyl)aniline (7.39), the key intermediate for Blue 19 (7.37), proceeds by chlorosul-phonation of nitrobenzene, reduction to the sulphinate, condensation with 2-chloroethanol,reduction of the nitro group and finally formation of the sulphate ester (Scheme 7.27).

A level of reactivity similar to the vinylsulphone dyes is offered by the vinylsulphonamidesystem. The essential intermediate in this instance is carbyl sulphate (7.40), the cyclic an-hydride of ethionic acid, which is readily available from ethylene and sulphur trioxide.Reaction with the amino group of an intermediate or the dye base yields the 2-sulphato-ethylsulphonamide (7.41) precursor system directly (Scheme 7.28). As with the sulphatoethyl-sulphone analogues (Schemes 7.23–7.25), in the presence of alkali these dyes generate theactive vinylsulphonamide form and this undergoes nucleophilic addition with a cellulosateanion during fixation treatment. Attack by a hydroxide ion produces the deactivated 2-hydroxyethylsulphonamide and this can react further with the vinylsulphonamide to give thediethyl ether form. Tertiary sulphonamides (–NRSO2–, where R≠H) are used because theirsecondary analogues are deactivated under alkaline conditions [36]:

NHSO2 NSO2


chpt7(1).pmd 15/11/02, 15:32379


CH2

CH2

O2S

OSO2

CH2

CH2

O

NHCH3[dye]

N[dye]

SO2CH2CH2

CH3

OSO3H

2 SO3 +150°C

7.41

7.40

Scheme 7.28

H2N

SO2CH2CH2 OSO3Na

H2N

SO2CH2CH2OH

O2N

SO2CH2CH2OH

O2N

SO2Na

O2N

SO2Cl

O2NClSO3H Na2SO3

ClCH2CH2OH

H2SO4

reduction

7.39

conc.

Scheme 7.27

7.3.9 Acid-fixing reactive dyes

The realisation that the reaction of phosphonic acid derivatives with alcohols to givephosphonate monoesters could be exploited in a reactive dyeing system originated at theStanford Research Institute in 1973. Four years later the Procion T (ICI) range of dyescontaining such groups was launched commercially. They were intended for the continuousdyeing and printing of cellulosic fabrics, especially polyester/cellulosic blends. The unusualconditions of application under mildly acidic conditions (pH 5–6) followed bythermofixation at 200–220 °C were quite different from those of conventional reactive dyes,making them much more compatible with the thermofixation of disperse dyes on thepolyester component of the blend.

The entire range of Procion T dyes was based on one versatile intermediate, 3-aminophenylphosphonic acid (7.42), readily manufactured by nitration of phenylphosphonicacid followed by reduction (Scheme 7.29) In most instances this intermediate became the

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381

P

O

OH

OH

P

O

OH

OH

O2N

P

O

OH

OH

H2N

7.42Scheme 7.29


NN

O

HN

COCH3

SO3 NH4

H4N O3S

P

OO

O

H4N

H4N H

7.43

CI Reactive Red 177

H2N CN

7.44

Cyanamide

H2N C NH

NH

CN

7.45Dicyandiamide

diazo component as in structure 7.43, but it could also be attached to a naphthylaminecoupler or to a non-azo chromogen through the imino link [8]. These dyes were marketed asaqueous solutions of their ammonium salts, which generated the free phosphonic acid formunder thermofixation conditions.

An activating agent of the carbodiimide type, such as cyanamide (7.44) or dicyandiamide(7.45), is necessary to bring about fixation to cellulose and this plays a decisive part in thereactions involved. At least two possible mechanisms of fixation have been proposed and itseems likely that either or both may be operative [8]. In the early work with dyes containingthe phosphonic acid group [42], formation of the phosphonic anhydride (7.46) was believedto precede the esterification step, with one dye phosphonate moiety being released forfurther reaction with another molecule of the carbodiimide activator (Scheme 7.30). It hadalready been shown [43], that monoesters of arylphosphonic acids could be prepared in highyield by reaction with alcohols in the presence of dicyclohexyl carbodiimide, theesterification proceeding via the arylphosphonic anhydride.

An alternative mechanism [8] entails reaction of cyanamide (or dicyandiamide) with thedye phosphonate to give an O-acylisourea derivative (7.47). This is able to react directlywith cellulose to form dye-fibre bonds, urea being released as the anticipated by-product(Scheme 7.31). In support of this mechanism, it is known that O-acylisourea derivatives ofarylcarboxylic acids react readily with alcohols and this constitutes an efficient route for thepreparation of carboxylic esters [44].

More recently, the fixation efficiency on cotton of CI Reactive Red 177 (7.43) and its 4-carboxyphenylazo analogue in the presence of various carbodiimides (including 7.44 and7.45) was investigated, as well as homogeneous reactions of selected carboxylic acids withalcohols (including acetylcellulose in acetone). The carboxylated dye reacted moreeffectively with cotton cellulose in the presence of cyanamide rather than dicyandiamide,

chpt7(1).pmd 15/11/02, 15:32381


whereas either type of carbodiimide was fully effective with the dye phosphonate. Loss ofcarbodiimide by hydrolysis to urea, followed by thermal decomposition to ammonia andcarbon dioxide is the most important factor limiting the optimum level of dye fixation [45].

Further work with the same dye (7.43) and carbodiimides (7.44 and 7.45) concentratedon this problem of limited efficiency. Cotton fabric padded with the dye phosphonatesolution was aftertreated with the carbodiimide dissolved in various alcoholic solutions toavoid hydrolytic decomposition. Under these conditions cyanamide was much moreeffective than dicyandiamide. With conventional reactive dyes the efficiency of the dye–fibre reaction is limited by competing hydrolysis of the active dye. Although phosphonatedor carboxylated reactive dyes do not hydrolyse, their level of fixation is limited by competinghydrolysis of the carbodiimide activator [46].

P[dye] ONH4

ONH4

O

H2N CN

[dye] P

O

O

OH

P

O

[dye]

OH

H2N CONH2

[cellulose] OH

[cellulose] O P

O

OH

[dye] [dye] P

O

OH

OH

+

7.46

++2 NH3

+

Cyanamide

Dye anhydride

Urea

Scheme 7.30

P[dye] ONH4

ONH4

O

H2N CN P[dye] O

OH

O

C NH

NH2

[cellulose] OH

PO [dye]

OH

O

[cellulose] H2N CONH2

+ + 2 NH3

+

7.47Cyanamide

UreaScheme 7.31

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383

Although offering significant benefits, the Procion T (ICI) dyes suffered from certaintechnical drawbacks. These included dye migration during drying, especially on heavyfabrics such as corduroy, and strength losses of cellulosic fabrics during thermofixationunder the acidic conditions required. The main reasons for their withdrawal in 1987,however, were economic rather than technical [8].

Interest in acid-fixing reactive dyes has remained active because of their environmentallyattractive features (section 1.7). The freedom from competing hydrolytic reactionspotentially offers exceptionally high fixation, extreme stability of the dye–fibre bonds andcomplete suitability of the unfixed dyes for recycling. In contrast to conventional reactivedyes, sensitisation problems arising from reaction with skin proteins are not anticipated.Unlike the haloheterocyclic reactive dyes, there is no risk of release of AOX compounds towaste waters. Heavy metals are not involved in the application of acid-fixing reactive dyes,nor are the electrolytes or alkalis that normally contaminate effluents from conventionalreactive dyeing.

A homobifunctional dye (7.48; X = NHCH2CH2PO3H2) containing two alkyl-phosphonate groups was synthesised by condensing two moles of aminoethylphosphonic acidwith the commercially available bis-aminochlorotriazine dye CI Reactive Red 120 (7.48; X= Cl). For comparison, a model dye (7.49) of the Procion T type was prepared by diazotising3-aminophenylphosphonic acid (7.42) and coupling with R salt (disodium 2-naphthol-3,6-disulphonate). After isolation as the free acids, these dyes were converted to theirammonium salts. When applied to cotton by a pad-batch-bake method in the presence ofcyanamide (7.44) and ammonium dihydrogen phosphate, the bis-phosphonoethyl dye gavemore than 90% fixation, compared with only 54% for the mono-phosphonophenyl modeldye [47].

NH NH

N

N

NN

N

N

X X

HN NH

SO3Na NaO3S

O O

N NN

H

N

H

NaO3SSO3Na

NaO3S SO3Na7.48

O SO3Na

SO3Na

N

N

H

P

O

HO

HO

7.49

HO P

O

HO

H2C

N CH2CH2

HO P

HO

O

H2C

OHP

O

OH

CH2

N

OHP

OH

O

CH27.50

EDTMP


chpt7(1).pmd 15/11/02, 15:32383


(CH3)2N N

N

C

O

OH

7.53

+ C OCH2CH3

O

Cl

+ N(CH2CH3)3

(CH3)2N N

N

C

O

7.54

C OCH2CH3

O

O

+ Cl– NH(CH2CH3)3

DMF –5 °C

Scheme 7.32

An interesting variation on this type of pad-bake fixation system is to use the commercialsequestering agent ethylenediaminetetramethylphosphonic acid (7.50) as a crosslinkingagent to react with cotton cellulose and with a dye containing nucleophilic hydroxyethylgroups. A suitable bis-hydroxyethyl dye structure (7.48; X = NHCH2CH2OH) of this kindwas derived by condensing two moles of ethanolamine with CI Reactive Red 120 (7.48; X =Cl). Dye fixation values exceeding 80% were achieved in this way in the presence ofcyanamide as dehydrating catalyst [47].

A similar concept evaluated recently depends on pad-bake application of the commercialcrosslinking agent butane-1,2,3,4-tetracarboxylic acid (7.51) capable of esterificationreactions with a hydroxyethyl group in the dye molecule and with cellulose OH groups. Thehydroxyethyl derivative (7.52) was made by condensing ethanolamine with thechlorotriazine group in the commercial dye CI Reactive Red 3 (7.2). Thermofixationtreatment at 170 °C in the presence of sodium hypophosphite (NaH2PO2) as catalyst gavedyeings of high fastness to washing but no fixation values were reported [48].

A tetracarboxylated derivative was prepared recently by reaction of a commercial reactivedye with two molar equivalents of aspartic acid. This novel derivative was evaluated by pad–dry–bake and pad-batch–bake methods under slightly acidic conditions in the presence ofcyanamide as activator [49]. An interesting disperse dye containing a novel reactiveanhydride system (7.54) was prepared from the parent dye carboxylate (7.53) by reactionwith ethyl chloroformate in the presence of a tertiary base (Scheme 7.32). Such dyes will

CH2 COOH

CH COOH

CH COOH

CH2 COOH

7.51

BTCA

SO3Na

N

N

H

HN

O

NaO3S

N

N

N

HOCH2CH2HN

NH

SO3Na

7.52

chpt7(1).pmd 15/11/02, 15:32384

385

react with wool at the boil and pH 6–7, liberating ethanol as a by-product of the reaction.Unlike conventional reactive dyes for wool, no aftertreatment with ammonia is necessary toattain optimum fastness [50].

N

N

N

Cl

Cl

NH [dye] NH

N

N

N

Cl

Cl7.55

BIFUNCTIONAL SYSTEMS

7.4 BIFUNCTIONAL SYSTEMS

Evidence soon emerged in the 1960s that monofunctional reactive dyes containing tworeactive centres, such as the dichlorotriazines, were capable of forming crosslinks betweenadjacent cellulose chains (7.8). This resulted in cellulosic fibres that had been dyed in thisway showing anomalous behaviour in cuprammonium solubility tests [51]. Individualmembers of the early ranges of reactive dyes, including CI Reactive Black 5 (7.36),contained more than one reactive system but it was not until around 1970 that ICIintroduced the Procion Supra (printing) and Procion H-E (dyeing) ranges of high-fixationdyes containing two aminochlorotriazine groups per molecule.

If other factors are equal, the use of a reactive dye containing two reactive groups ratherthan its analogue with only one reactive group per molecule increases the fixation from atypical 60% to approximately 80% on average in exhaust dyeing. In pad–batch processes thecorresponding fixation efficiency levels are about 75% and 95% respectively [52].Bifunctional systems containing two different kinds of reactive group are popular in exhaustdyeing and gaining ground, especially on account of their relative insensitivity of fixation tofluctuations in dyeing temperature [53].

A detailed study of representative bis(aminochlorotriazine) dyes, as well as otherpotentially crosslinking reactive systems (dichlorotriazine, chloromethoxytriazine,trichloropyrimidine, chlorodifluoropyrimidine and dichloroquinoxaline) provided convincingevidence of the extent of crosslinking that could take place. Crosslinking was non-existentor relatively insignificant for typical pad–batch dyeings at ambient temperature, but thermalfixation by the pad–dry–steam method resulted in much more prevalent crosslinking by dyemolecules [54].

7.4.1 Bis(aminochlorotriazine) dyes

Several different approaches have been employed in order to build the bifunctionalityconcept into known dye chromogens. In the case of bis(dichlorotriazine) dyes, which werenot exploited commercially, only the arrangement 7.55 is feasible. The difficulty ofintroducing sufficient extra sulphonic acid groups into the chromogen to compensate for thepresence of these two reactive systems and provide adequate solubility and mobility at thelow dyeing temperature necessary for such highly reactive molecules proved insuperable.

chpt7(1).pmd 15/11/02, 15:32385


The versatility of the bis(aminochlorotriazine) concept is much greater, with threedifferent arrangements being possible (7.56–7.58; RNH2 = arylamine, –NH–X–NH– =linking diamine). In the Procion Supra (ICI) dyes marketed for textile printing, therelatively more costly 7.56 pattern was preferred for technical reasons. Arrangements 7.57and 7.58 have been used to design low-reactivity dyes of high substantivity intendedprimarily for exhaust dyeing. Structures of the 7.57 type are symmetrical about the centrallinking diamine and most of them contain a simple yellow, orange or red monoazochromogen at each end of the molecule, CI Reactive Red 120 (7.48; X = Cl) being a typicalexample. Blue and green bis(aminochlorotriazine) dyes, including those derived from non-azo chromogens as well as the twice-coupled H acid disazo blues (7.59), usually fit thegeneral structure 7.58.

NH [dye] NH

N

N

NN

N

N

Cl

NH RHN

Cl

7.58

R

[dye] NH

N

N

N

Cl

NH X NH

N

N

N

Cl

NH R7.56

[dye] NH

N

N

N

Cl

NH X NH

N

N

N

Cl

NH [dye]

7.57

N

N

H

NaO3S

H2N

SO3Na

O

7.59

SO3Na

NH

N N

SO3Na

NH

Unsymmetrical analogues of type 7.57 can be readily prepared using phenylene-1,3-diamine-4-sulphonic acid as the central linking unit, because the bulky sulpho groupingprotects the 3-amino group by steric hindrance whilst monoacylation takes place at the 1-amino position. This approach, however, introduces a generally undesirable extra sulphonicacid group into the structure. Recent research has demonstrated, however, that it is possibleto monoacylate phenylene-1,3-diamine itself using one dichlorotriazine dye and to react the

chpt7(1).pmd 15/11/02, 15:32386

387

product with a different dichlorotriazine dye to yield novel unsymmetrical structures [55].Thus CI Reactive Red 1 (7.1) was condensed with one mole of the diamine to give theintermediate 7.60. In the second acylation step the analogous dye CI Reactive Red 11 (7.61)was used, resulting in the formation of the unsymmetrical bis(aminochlorotriazine) dye 7.62.


N

N

H

NaO3S

SO3Na HN

SO3Na

O

N

N

N NH

Cl

NH2

7.60

N

N

H

NaO3S

SO3Na HN

SO3Na

O

N

N

N Cl

Cl

7.61

N

N

H

NaO3S

SO3Na HN

SO3Na

O

N

N

N NH

Cl

NH

7.62

N

N

N

Cl

NH

O

NaO3S

SO3Na

N

N

H

NaO3S

These bifunctional dye molecules are approximately twice the size of analogousmonofunctional structures (compare 7.2 with 7.48). Their high substantivity ensuresexcellent exhaustion at the preferred dyeing temperature of 80 °C, leading to fixation valuesof about 70–80%, although in full depths typical members of the range may require saltconcentrations as high as 100 g/l for optimal yield. High-temperature application conditionsensure good levelling and the high fixation leads to better utilisation of the dyes applied withless hydrolysis and less coloration of the effluent. Unfortunately, the rate of removal ofunfixed dyes at the washing stage is slow owing to the high intrinsic substantivity of thesedyes.

A range of bis(aminofluorotriazine) dyes has been marketed by Ciba for long-liquordyeing under the brand name Cibacron LS, which require only a low salt (LS) concentrationin the dyebath. The patent literature indicates that the most likely structures (7.63) to meetthis requirement contain two high-substantivity chromogens linked through the central unitcarrying the fluorotriazine reactive groups [56]. This system offers an environmental

chpt7(1).pmd 15/11/02, 15:32387


advantage over the chlorotriazine analogues; organofluorine compounds do not fall into theAOX classification because the fluoride ion liberated as soluble silver fluoride according tothe test protocol is not detected [57].

Conferring high substantivity to the dye molecule in these various ways can lead toproblems when dyeing in full depths, especially when it comes to the soaping-off stage. Itwould therefore be interesting to develop labile central linking units between thechromogenic groupings that could be cleaved during fixation or soaping. Such dyes wouldprovide intrinsically high substantivity in the primary exhaustion stage to reduce the saltrequirement but in the later alkaline fixation stage their molecular size would be halved andthe unfixed single chromogens of much lower substantivity would show good soaping-offbehaviour [56].

7.4.2 Bis(aminonicotinotriazine) dyes

An aminochlorotriazine dye will react with a tertiary amine possessing a sterically accessiblenitrogen atom to form a quaternary ammonium derivative (Scheme 7.33). The positivecharge carried by the quaternary nitrogen atom increases the polarisation of the C–N bondthat links it to the triazine ring, so making such a compound much more reactive than theparent dye. Within a few years of the introduction of reactive dyes, the utility of tertiaryamines as catalysts to assist the fixation of aminochlorotriazine dyes to cellulose wasevaluated [58]. Suitable tertiary amines include trimethylamine (7.64), N,N-dimethylhydrazine (7.65), 1,4-diazabicyclo[2,2,2]octane (DABCO, 7.66), pyridine (7.67; R= H) and substituted pyridines such as nicotinamide (7.67; R = CONH2 ) or nicotinic acid(7.67; R = COOH).

[dye] NH

N

N

N

Cl

NH Ar

[dye] NH

N

N

N

N(CH3)3

NH Ar

N(CH3)3

Cl

+

7.64

_+

Trimethylamine

Scheme 7.33

H2N N(CH3)2

7.65Dimethylhydrazine

N CH2CH2 N

CH2CH2

CH2CH2

7.66DABCO

N

R7.67

N

N

N

NH

F

NH

[dye]

X

N

N

N

HN

NH

F

[dye]

7.63

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389

In the case of quaternary derivatives made from the non-planar aliphatic amines 7.64,7.65 and 7.66, steric strains further destabilise the C–N+ bond so that reaction withcellulose occurs under alkaline conditions at 30 °C, whereas temperatures of about 40–50 °Care required for the pyridinium derivatives 7.67. The quaternisation approach appeared tooffer the opportunity to prepare dyes yielding reactivity levels intermediate between those ofaminochloro- and dichlorotriazine dyes without loss of the desirable stability of the dye–fibrebond to acidic conditions that is characteristic of aminohalotriazine dyes. Unfortunately,this ideal was not attainable because of the objectionable odours of the tertiary aminesliberated by the fixation reaction and the sensitivity of the reactivity behaviour of thequaternised derivatives to the nature of the chromogen attached to the triazine ring, makingit difficult to select compatible combinations of dyes.

A further difference in behaviour between certain substituted pyridinium dyes and otheranalogues was revealed when the relative rates of hydrolysis of a series of quaternaryderivatives of a monoazo N-methyl J acid dye (7.68) were compared. It was found that wherethe leaving group X was nicotinamide (7.67; R = CONH2) or nicotinic acid (7.67; R =COOH) the dye was rapidly and unexpectedly converted to the aminotriazine, whereas withpyridine (7.67; R = H), isonicotinic acid (4-carboxypyridine) or other leaving groups theexpected hydroxytriazine was produced, although much more slowly [59]. The mechanism ofaminotriazine formation apparently involves attack by hydroxide ion at the 2-position,deprotonation by the 3-carboxylate ion, ring-opening by cleavage of the 1,2-bond and finallyhydrolysis of the 1,6- bond (Scheme 7.34).

The kinetics of reaction of DABCO (7.66) and nicotinic acid (7.67; R = COOH) with theaminochlorotriazine dye CI Reactive Red 3 (7.2) were studied under neutral conditions attemperatures in the range 100–130 °C. Quaternisation by DABCO was much more rapid thanby nicotinic acid under these conditions. Neutral exhaust dyeing tests at 130 °C using thebis(aminochlorotriazine) analogue CI Reactive Red 120 (7.48; X = Cl) with the two catalystsconfirmed these trends, in that the degree of fixation was greatly increased by DABCO butnicotinic acid showed no appreciable catalytic effect [60]. This difference may be attributableto steric strain of the C–N+ bond in the quaternised triazine structure by the non-planarDABCO substituent.

Nicotinic acid (7.67; R = COOH) is a component of the Vitamin B complex that isessential to the mammalian diet, a deficiency causing pellagra. As a dye intermediate itoffers relatively low cost and environmental acceptability. When used as a leaving entity itreadily dissolves in the dyebath, a significant practical advantage over the tertiary amineswith lower solubility and unpleasant odours. This was recognised at ICI in 1979 when thebis(aminonicotinotriazinyl)-substituted triphenodioxazine dye Procion Blue H-EG (CIReactive Blue 187) was introduced for exhaust dyeing at 80 °C with the other

N

O

NaO3S

N

CH3

N

N

N

N

CH3

X

SO3Na

NaO3S

7.68

N

H


chpt7(1).pmd 15/11/02, 15:32389


bis(aminochlorotriazine) members of the range. Although this product enjoyed somecommercial success it was more reactive than typical Procion H-E dyes, rendering itsomewhat incompatible in terms of dyeing behaviour. Indeed, it was evident that fixation tocellulose commenced prior to the addition of alkali to the dyebath [8]. Procion Blue H-EGwas eventually superseded by the more conventional bis(aminochlorotriazinyl)-substitutedanalogue Procion Blue H-EGN (CI Reactive Blue 198).

The observation that alkali was not essential for the fixation of nicotinotriazines wasexploited by Nippon Kayaku in 1983. A full range of bis(aminonicotinotriazine) dyes wasintroduced under the Kayacelon React (KYK) brandname, an example being CI Reactive Red221 (7.48; X = nicotino) [60]. All three possible arrangements of the two reactive systems arerepresented in the range, these being described as the two-step, one-arm (7.56), linkage (7.57)and two-arm (7.58) types [61]. Most of the commonly encountered chromogens have beenpatented in this way, including monoazo, disazo, metal-complex azo, copper formazan andcopper phthalocyanine [8]. Exhaust dyeing from a neutral bath at 130 °C is recommended,making these dyes particularly suitable for the one-bath dyeing of polyester/cellulosic blends inconjunction with disperse dyes. By operating under these conditions, the diffusion problemsanticipated with such large molecules are minimised.

In spite of the anomalous ring-opening decomposition of nicotinotriazine compoundsunder conditions of alkaline hydrolysis (Scheme 7.34), the product of reaction of abis(aminonicotinotriazine) dye with cellulose is the same as that from the analogousbis(aminochlorotriazine) dye in terms of hue, colour fastness and stability of the dye–fibrebond. If desired, these bis(aminonicotinotriazine) dyes can be applied satisfactorily at 80 °Cand pH 11, as was evident for CI Reactive Blue 187. They have slightly higher reactivity

N

COO

N

N

N

NCH

COO

N

N

N

OHN

CH

COOH

N

N

N

O

NHC

COOH

N

N

N

ON

CH

COO

N

N

N

OH

N

N

N

NH2

HC

HC O

CH

CH OH

COO

OH

H2O+

_

+

_

_

_

_

Scheme 7.34

chpt7(1).pmd 15/11/02, 15:32390

391

than vinylsulphone or chlorodifluoropyrimidine dyes, but are less reactive thandichlorotriazine or dichloroquinoxaline systems [62].

Six bis-quaternary derivatives of C.I.Reactive Red 120 (7.48; X = Cl) were synthesisedusing trimethylamine (7.64), DABCO (7.66), pyridine (7.67; R = H), nicotinamide (7.67; R= CONH2), nicotinic acid (7.67; R = COOH) and isonicotinic acid (4-carboxypyridine) asthe tertiary amines, the nicotinic acid derivative (7.48; X = nicotino) representing thecontrol dye CI Reactive Red 221. The conditions required to achieve at least 90%conversion to the bis-quaternary species were 2 hours at 70 °C and pH 6.5 for DABCO andtrimethylamine. The less reactive pyridine and its analogues required 2 or more hours at90 °C for completion of this reaction. In exhaust dyeing tests on cotton, all six dyes showedsimilar levels of fixation at neutral pH, irrespective of dyeing temperature. At alkaline pH,however, fixation of the nicotinamide derivative declined from about 70% at pH 7 to 40% atpH 9. Values of exhaustion for this dye also decreased steadily as the dyebath temperaturerose from 100 to 130 °C. These poor results were attributed to ring-opening decompositionof the pyridine-3-carbonamide system (as in Scheme 7.34) with formation of theaminotriazine under these adverse conditions [63].

CH2CH2SO2

OCH3

H3CO

N

N

H O

NaO3S

H2N N

SO3Na

N

SO2CH2CH2

NaO3SO

OSO3Na

7.69


7.4.3 Bis(sulphatoethylsulphone) dyes

The most commercially successful reactive dye of all, CI Reactive Black 5 (7.36) containstwo sulphatoethylsulphone precursor groups that contribute markedly to its initial solubility.When these are hydrolysed in alkali to release the reactive bis(vinylsulphone) form, theconsiderable increase in substantivity (Table 7.3) leads to highly efficient fixation. Furtherhydrolysis of the vinylsulphone groups to give the inactive bis(hydroxyethylsulphone)derivative, however, lowers the substantivity and hence contributes to favourable wash-offperformance.

A dimethoxy analogue of Black 5 has also been marketed but this (7.69) has not been sosuccessful as it is more expensive to make, greener in hue and thus less suitable as a basis forblack. Structures of the bis(sulphatoethylsulphone) type have been commercialised in othersectors of the colour gamut, including the bluish red 1:2 metal-complex CI Reactive Red 23(7.70), anthrquinone blues such as the symmetrical structure 7.71 and phthalocyanine bluessimilar to 7.72.

chpt7(1).pmd 15/11/02, 15:32391


NN

NN N

NN N

Cu

O2SNH

SO2CH2CH2 OSO3Na

SO3Na

NaO3S

SO2

HNCH2CH2O2SNaO3SO

7.72

NaO3S

NaO3S

N

O

N

SO2CH2CH2

O

SO3Na

SO3Na

N

O

N

CH2CH2O2S

OCu

OSO3Na

NaO3SO

7.70CI Reactive Red 23

O HN

HNO

CH3

CH3

SO2CH2CH2 OSO3Na

SO2CH2CH2 OSO3Na

7.71

A highly substantive bluish red bis(sulphatoethylsulphone) structure (7.73) has beenpatented recently [29]. In a kinetic study using the t.l.c. double-scanning method, thecondensation reactions between the bis(aminochlorotriazine) dye CI Reactive Red 120(7.48; X = Cl) and the two isomeric sulphatoethylsulphone anilines 7.38 and 7.39 to yieldthe two corresponding bis(sulphatoethylsulphone) isomeric dyes were compared. The rateconstant of the reaction between Red 120 and the meta isomer 7.39 was about ten times aslarge as that for the para isomer 7.38 [64].

chpt7(1).pmd 15/11/02, 15:32392

393

CH

2CH

2SO

2N

NHO

NaO

3S

H2N

N

SO

3Na

N

SO

2

NaO

3SO

SO

2CH

2CH

2N

NHO S

O3N

a

NH

2N

NaO

3SN

HN

OS

O3N

a

7.73

N

NHO

NaO

3S

HN

SO

3Na

7.74

SO

3Na

NaO

3S

NN

NCl

N

CH

2CH

2SO

2CH

2CH

2Cl

CH

2CH

2SO

2CH

2CH

2Cl

CH

2CH

2SO

2NH

H3C

N

NH

NN

O

H3C

HN

SO

2CH

2CH

2O

SO

3Na

NaO

3SO

7.75


chpt7(1).pmd 15/11/02, 15:32393


Not all homobifunctional reactive dyes that react with cellulose by the nucleophilicaddition mechanism are marketed as sulphatoethylsulphones. Thus the bluish red structure7.74 contains two chloroethylsulphone precursor groups attached via a diethylamine residueand an activated chlorotriazine grouping to the H acid coupling component. Theazopyrazolone yellow structure 7.75 depends for its reactivity on sulphatoethylsulphonamideprecursor groups located separately at the diazo and coupler extremities of the molecule.

7.4.4 Aminochlorotriazine-sulphatoethylsulphone dyes

Reaction of a dichloro-s-triazine dye with an anilino intermediate containing a 2-sulphatoethylsulphone substituent, such as 7.38 or the more nucleophilic 7.39, is the preferredroute to heterobifunctional reactive dyes of the Sumifix Supra (NSK) class introduced bySumitomo in 1980. Rate constants for the reactions between two vinylsulphonylanilineisomers and a model dichlorotriazine dye have been determined in DMF by the t.l.c. scanningmethod. As expected, the meta isomer reacted more quickly than para-vinylsulphonylaniline[65]. The Sumifix Supra dyes are capable of reacting with cellulose via either themonochlorotriazine moiety or the vinylsulphone group released by the precursor sulphate ester.A typical structure is that of the monoazo H acid dye shown (7.76) [66]. Notable features arethe high substantivity contributed by the triazine bridging nucleus and the capability this givesto link the two reactive centres to a wide variety of chromogens.

The marked differences in substantivity between the various forms of monofunctionalvinylsulphone dyes (section 7.3.8) recur to a moderated extent in the Sumifix Supra dyesbecause of the influence of the substantive triazine ring. The scarlet chromogen (7.77) linkedvia a chlorotriazine unit to the three variants of the vinylsulphone grouping showed similartrends (Table 7.4) to those already seen for those of CI Reactive Red 22 tested under the sameconditions (Table 7.3). The rate of secondary exhaustion will be easier to control in thisinstance because of the lower difference in substantivity between the precursor and thevinylsulphone form [37].

Rate constants and the products formed in the hydrolysis of CI Reactive Red 194 (7.76)at 50 °C and pH values in the 10–12 region were determined by high-pressure liquidchromatography. In addition to the normal hydrolysis of the two reactive systems, the iminolink between the triazine and benzene nuclei was also hydrolysed [67]. Theheterobifunctional copper formazan dye CI Reactive Blue 221 and two blue anthraquinonemonofunctional reactive dyes of the bromamine acid type, namely the aminochlorotriazineBlue 5 and the sulphatoethylsulphone Blue 19, were compared in terms of their sensitivity to

NN

SO3Na

H O

NaO3S

SO3Na

HN

N

N

N

Cl

NH

SO2CH2CH2 OSO3Na

7.76CI Reactive Red 194

chpt7(1).pmd 15/11/02, 15:32394

395

hydrolysis over a wide range of pH (1–12). Three main products of hydrolysis were isolatedfrom Blue 221 by dialysis and thin-layer chromatography. It was confirmed that most of thefixation of Blue 221 to cotton cellulose in exhaust dyeing at 60 °C occurs via thevinylsulphone group [68].

The presence of two reactive groups that differ in reactivity gives dyes that are lesssensitive to exhaust dyeing temperature than any of the typical monofunctional reactivesystems. They can be applied over a wider range of temperatures (50–80 °C) andreproducibility of hue in mixture recipes is improved. Moreover, they show minimalsensitivity to electrolyte concentration and are less affected by changes in liquor ratio [69].Low dyeing temperatures favour reaction via the vinylsulphone group, whereas at highertemperatures the contribution of the chlorotriazine system to fixation becomes moreimportant [70]. This was demonstrated by controlled enzymatic degradation of amechanically milled cotton fabric that had been exhaust dyed at 60 °C, generating thefollowing conclusions after analysis of the products isolated [71]:(1) About 80% of the vinylsulphone groups had reacted with hydroxy groups in cellulose(2) About 50% of the chlorotriazine groups had not reacted and only about half of these

had been hydrolysed to the hydroxytriazine(3) A considerable proportion of the dye molecules had formed crosslinks by reacting via

both reactive systems.

For further studies of this kind a scarlet monoazo dye of this type was synthesisedcontaining three 13C-labelling atoms in the triazine ring. Enzymatic digestion of cotton dyedby four different exhaust methods yielded various dye-sugar derivatives that were estimatedquantitatively using 13C-NMR liquid spectroscopy. As well as crosslinking through bothgroups, monofixation can occur via either group with the other either hydrolysing orremaining intact. Distribution patterns between these five modes of fixation to cellulosewere elucidated for all four dyeing methods. In isothermal dyeing at 50, 60 or 80 °C, theproportion of fixation via the chlorotriazine group increased at the expense of vinylsulphone

Table 7.4 Relationship between substantivity and nature ofthe reactive group for heterobifunctional dyes [37]

Reactive group X Primary exhaustion (%)

–SO2CH2 CH2OSO3Na 43–SO2CH =CH2 82–SO2CH2 CH2OH 63

H3CO

SO3Na

N

N

H O

NaO3S

HN

N

N

N HN

Cl

X7.77


chpt7(1).pmd 15/11/02, 15:32395


fixation and crosslinking. Crosslinking could be optimised by neutral exhaustion at 40 °Cfollowed by two-step fixation at 60 °C and 80 °C [72].

The levelling properties of CI Reactive Red 194 (7.76), a twice-coupled H acid blue dyecontaining the same heterobifunctional system, the bis(sulphatoethylsulphone) dye CIReactive Black 5 (7.36) and six monofunctional sulphatoethylsulphone dyes were comparedrecently in considerable detail. The results confirmed that those anthraquinone blue dyes ofthe bromamine acid type that rely only on the 2-sulphonate group and the precursorsulphatoethylsulphone substituent for aqueous solubility, such as CI Reactive Blue 19(7.37), are especially prone to unlevel dyeing. These relatively hydrophobic characteristicspromote aggregation in salt solution at relatively low dyeing temperatures [73]. Prematureloss of the sulphate ester group should be avoided and such dyes should be applied at thelowest salt concentration and highest temperature that are consistent with the attainment ofacceptable exhaustion and fixation performance.

The incorporation of two different reactive systems and the high substantivity ofheterobifunctional dyes favour the achievement of unusually high fixation. Although thisleads to better utilisation of the dyes applied, with less of the hydrolysed by-products tocolour the effluent, removal of these unfixed dyes at the washing-off stage may presentdifficulties because of their high intrinsic substantivity [8]. The formation of two differenttypes of dye–fibre bond has beneficial consequences for fastness performance.Heterobifunctional dyes show superior fastness to acid storage compared withdichlorotriazine or dichloroquinoxaline systems and better fastness to peroxide washing thandifluoropyrimidine or dichloroquinoxaline dyes [53].

In a detailed investigation, the dye–fibre bond stabilities of exhaust dyeings on cotton ofthe orthodox heterobifunctional dye CI Reactive Red 194 (7.76) and two monofunctionalcontrol dyes were compared. Both were analogues of Red 194, one having a hydroxytriazinegroup instead of the normal chlorotriazine and the other a hydroxyethylsulphone groupinstead of the normal sulphate ester. As expected, in acidic buffer solutions the orthodoxdye 7.76 showed higher stability than the hydroxytriazine-vinyisulphone analogue, whichwas itself more stable than the chlorotriazine-hydroxyethylsulphone dye. Under alkalineconditions the hydroxytriazine-vinylsulphone dye was much less stable than the two dyeswith an active chlorotriazine group [74].

Two unorthodox heteromultifunctional dyes were also included in this investigation,neither being of commercial interest. The anthraquinone blue dye 7.78 containing the sameheterobifunctional reactive system as Red 194 exhibited surprisingly low fixation. This wasattributed to the non-planar conformation of bromamine acid derivatives of this kind [74].All three imino groups linking together the phenyl-triazine-phenyl-anthraquinone series ofnuclei in this structure have a twisting effect on the aryl systems on both sides of each NHlink. Thus the triazine ring is twisted through about 90 degrees relative to the plane of theanthraquinone chromogen. Another multifunctional dye that gave disappointingly lowfixation was the symmetrical structure 7.79, even though there were no less than fivereactive systems linked together with two red monoazo H acid chromogens. Apparently thisexceptionally large dye molecule is absorbed so gradually and diffuses so slowly thatadequate fixation cannot be attained within an acceptable dyeing time [74].

Another comprehensive evaluation of cotton dyeing parameters has been carried out foreighteen isomeric monoazo H acid red dyes containing aminochlorotriazine and

chpt7(1).pmd 15/11/02, 15:32396

397

O

O

NH2

SO3Na

HN

SO3Na

N

N

N

NH NH

Cl

SO2CH2CH2 OSO3Na

7.78

N N

N

Cl

N

N

N

Cl

HN NH

N

SO3Na

N O

NaO3S

HO2S

CH2

CH2

OSO3Na

HN

N

N

N

Cl

NHNH

N

NaO3S

NO

SO3Na

H SO2

CH2

CH2

OSO3Na

NH

SO3Na NaO3S

7.79


sulphatoethylsulphone (Z) reactive groups [66]. Representative results for the affinityparameter, rate of hydrolysis and fixation yield are presented in Table 7.5. Threecommercially available dyes are represented in the series (CI Reactive Reds 194, 198 and227). Substitution in the o-position usually lowers the affinity parameter by stericallyhindering the coplanarity of the aryl nuclei. Thus the two dyes with ortho substituents inboth rings have the lowest affinity values. Surprisingly, however, the two dyes (Reds 194 and227) with an o-sulpho group in ring A and the reactive system in the m- or p-position of ringB show the highest affinity values.

Dyes with the sulphatoethylsulphone group in the m-position of ring B are the mostreactive, especially Red 194, which hydrolyses about ten times more quickly than those dyeswith an ortho reactive group in this ring. Two of the commercial products (Reds 194 and227) achieved the highest fixation values. The combination of a m- or p-sulpho group inring A and the reactive group in the o-position of ring B leads to poor affinity and the lowestreactivity, resulting in fixation yields less than half those of Reds 194 and 227. In general,dyes like these four with both reactive systems attached via the imino link in the couplingcomponent show more extremes of dyeing behaviour than those in which thesulphatoethylsulphone group is in the diazo component (ring A).

chpt7(1).pmd 15/11/02, 15:32397


7.4.5 Aminofluorotriazine-sulphatoethylsulphone dyes

In 1988 Ciba launched the Cibacron C range of bifunctional reactive dyes. They contain anew aliphatic vinylsulphone system and either a monofluorotriazine bridging group or anarylvinylsulphone function [75]. Owing to the small size of the fluorine atom, thedifluorotriazine precursor reacts more smoothly with an alkylamine carrying the sulphato-

Table 7.5 Affinity parameter, rate of hydrolysis and fixation yield [66]

Substituent in

Rate ofRing A Ring B Affinity hydrolysis FixationSO3Na Z CI Reactive parameter (10–2/min) yield (%)

o o 0.29 1.9 28.4o m Red 194 0.62 15.0 41.8o p Red 227 0.64 4.8 44.8m o 0.33 1.1 18.7m m 0.50 7.4 37.1m p 0.50 2.1 37.8p o 0.35 1.3 20.6p m 0.52 8.3 34.7p p 0.51 2.5 39.1

Substituent in

Rate ofRing A Ring B Affinity hydrolysis FixationZ SO3Na CI Reactive parameter (10–2/min) yield (%)

o o 0.27m o 0.36 4.9 28.3p o 0.36 4.6 32.4o m 0.46m m 0.58 5.4 34.5p m Red 198 0.54 4.3 37.9o p 0.46m p 0.54 3.4 36.3p p 0.51 3.0 38.9

Z Sulphatoethylsulphone

p

o

N

N

H O

NaO3S7.80

m

SO3Na

HN

N

N

N HN

p

m

o

Cl

Ring A

Ring B

chpt7(1).pmd 15/11/02, 15:32398

399

H2NCH2CH2SO2CH2CH2OSO3Na

7.81

ethylsulphone system such as 7.81 than with an arylamine intermediate such as 7.38 or 7.39.The Cibacron C dyes are designed mainly for pad applications and are characterised by lowto moderate affinity, good build-up, ease of washing-off and high fixation (often >90%).Their good stability under padding conditions, high solubility, efficiency of reaction andoutstanding fixation make them especially suitable for the pad–batch process [76]. Thepresence of the vinylsulphonylalkyl group assists solubility and most of these dyes form stableliquids without the addition of urea [36].


The stability of the dye–fibre bonds in these dyeings is high to both acid and alkali,compared with monofunctional halotriazine analogues, because of the major contribution ofthe vinylsulphone function to the fixation mechanism. The fluorotriazine group confersmuch higher stability to alkali than is shown by monofunctional sulphatoethylsulphone dyes[77]. A characteristic feature of the Sumifix Supra (NSK) heterobifunctional system is themajor difference in reactivity between the aminochlorotriazine moiety and the much morereactive vinylsulphone group. There are some practical conditions, notably in pad–batchapplication, that do not allow full advantage to be taken of both types of reactive grouppresent. The combination of aminofluorotriazine and sulphatoethylsulphone in the CibacronC (Ciba) synchronised bifunctional system, both groups offering effective fixation undervirtually the same conditions, exploits the concept of bifunctionality more effectively.

These factors have been demonstrated elegantly in a detailed evaluation by pad–batchdyeing of cotton with three commercially important copper formazan reactive dyes:(1) Cibacron Blue F-R (Ciba; CI Reactive Blue 182) with a monofunctional aminofluoro-

triazine group.(2) Sumifix Supra Blue BRF (NSK; CI Reactive Blue 221) with a heterobifunctional

aminochlorotriazine-sulphatoethylsulphone system.(3) Cibacron Blue C-R (Ciba) with a synchronised bifunctional aminofluorotriazine-

sulphatoethylsulphone system.

Under the relatively mild conditions of pad–batch fixation within 6 hours batching time, theheterobifunctional dye (Blue 221) behaved as if it were a monofunctional sulphato-ethylsulphone dye and only the synchronised Cibacron C system showed truly bifunctionalperformance [78].

Interfibrillar crosslinking induced by a colourless cellulose reactant early in the wetprocessing sequence can prevent further fibrillation of lyocell fibres. Certain bifunctionalreactive dyes exert a similar effect but specific molecular characteristics must be present.These include steric orientation and separation of the reactive groups, degree of reactivity,size of chromogen, molecular flexibility and diffusion properties. Cibacron LSbis(aminofluorotriazine) dyes applied by exhaust dyeing and Cibacron C amino-fluorotriazine-sulpatoethylsulphone dyes in the cold pad-batch process largely fulfil theserequirements and thus provide control of undesirable post-fibrillation of lyocell duringfinishing and laundering [79].

chpt7(1).pmd 15/11/02, 15:32399


7.5 CHROMOGENS IN REACTIVE DYES

Providing there are enough sulpho groups to ensure adequate solubility in water, the onlyessential feature of a chromogen needed to build it into a reactive dye molecule of thehaloheterocyclic type is a primary or secondary amino group to which the heterocyclicsystem can be attached. This also applies to the various ranges of bifunctional dyes describedin section 7.4, since they all contain this same type of amino-s-triazine grouping attached tothe chromogen. Only in the case of monofunctional sulphatoethylsulphone dyes is theselection of chromogens more limited by the various ways in which conventionalintermediates for these dyes, such as 7.38 and 7.39, can form an integral part of thechromogenic grouping. These intermediates have wide applicability in azo dye structures,since they are conveniently used as diazo components with a variety of orthodox couplers.The para isomer (7.38) is less nucleophilic than the meta-substituted one (7.39), which istherefore more versatile.

Most ranges of reactive dyes contain examples of monoazo, disazo, metal-complex azo,anthraquinone and phthalocyanine chromogens, with copper formazan or triphenodioxazineblues sometimes also present. Within each sector of the colour gamut it is often possible tofind the same specific chromogen being used in various ranges, with individualmonofunctional dyes differing only in the nature of the reactive system. These trends areoften dictated by the need to consider the ready accessibility of key intermediates. Inresearch centred around the design of novel dye structures, computer-based techniques ofmolecular modelling and statistical experimental design to reveal structure–propertyrelationships are now common practice [80].

NN

H

N

H3C

OSO3Na

NHZCl

Cl

SO3Na

N

7.82

NN

N

OH

SO3Na

NHZ

O

CONH2H3C

CH2CH3

7.83

7.5.1 Greenish yellow chromogens

These dyes are invariably monoazo compounds with the reactive system attached to thediazo component, owing to the ready availability of monosulphonated phenylenediamineintermediates. Pyrazolone couplers are most commonly used, as in structure 7.82 (where Z isthe reactive grouping), and this is particularly the case for greenish yellow vinylsulphonedyes. Catalytic wet fading by phthalocyanine or triphenodioxazine blues is a characteristicweakness of azopyrazolone yellows (section 3.3.4). Pyridones (7.83), barbituric acid (7.84)and acetoacetarylide (7.85; Ar = aryl) coupling components are also represented in thissector, with the same type of diazo component to carry the reactive function.

chpt7(1).pmd 15/11/02, 15:32400

401

NN

N

N

OH

SO3Na

OH

HO

NHZ

7.84N

N C

C NH

CH3CH O

O

Ar SO3Na

SO3Na

NHZ

7.85

SO3H

HO3S SO3H

N N

SO3Na

NaO3S SO3Na

OH

NH2

HN

CONH2

SO3Na

NaO3S SO3Na

N N

HN

CONH2

NH2

Cl

7.87

SO3Na

NaO3S SO3Na

N N

HN

CONH2

NH

N

N

N

Cl

NH2

7.88

CI Reactive Orange 12

kd

4 NaOH

+ N2 + NaCl + 3 H2O

+

kc 4 NaOH

_

+ NaCl + 4 H2O

Cyanuricchloride,

thenammonia

7.86

Scheme 7.35

CHROMOGENS IN REACTIVE DYES

7.5.2 Reddish yellow chromogens

Azo structures covering this sector of the colour gamut are prepared from di- ortrisulphonated naphthylamine diazo components and p-coupling anilines, such as 3-aminophenylurea as in structure 7.88, 3-aminoacetanilide or cresidine (3-amino-4-methoxytoluene), with the reactive system attached to the terminal amino group. Thekinetics of the azo coupling reaction (kc) to form the Orange 12 chromogen from diazotised2-naphthylamine-3,6,8-trisulphonic acid (7.86) and 3-aminophenylurea (7.87), as well asthe rate of decomposition (kd) of the diazonium salt (Scheme 7.35), were measured

chpt7(1).pmd 15/11/02, 15:32401


potentiometrically [81]. Stepwise condensations with cyanuric chloride and then withammonia yield the aminochlorotriazine dye (7.88).

A well-recognised practical problem with arylazoaniline golden yellow dyes of this kind isphotochromism. This slight change in hue is attributed to a reversible, photochemicallyinduced transformation from the more stable trans to the less stable, less linear cis isomer(Scheme 7.36). It occurs very quickly on exposure to light but the thermal reversion from cisto trans in the dark is a much slower process. Greenish yellow chromogens (7.82–7.85) andorange reactive dyes of the J acid type (e.g. 7.89) do not exhibit this phenomenon. Thesestructures are all derived from o-hydroxyazo coupling components that exist predominantlyin the ketohydrazone form, so that rotation around the N–N axis is possible and cis–transisomerism cannot occur [82].

7.5.3 Orange chromogens

Dye bases for these dyes are obtained from a sulphonated arylamine as the diazo componentand J acid as the coupler. Yellowish orange dyes are given by mono- or disulphonatedanilines, as in structure 7.89, whereas reddish orange hues result from di- or trisulphonatednaphthylamines, structure 7.92 being a typical example. If N-methyl J acid is used, as in thisinstance, the high substantivity that is characteristic of J acid dyes becomes somewhat lower,making it easier to wash-off the unfixed dyes after fixation. In a recent investigation [83],the kinetics of the coupling reaction between diazotised 2-naphthylamine-1,5-disulphonicacid (7.90) and the dichlorotriazinyl derivative of N-methyl J acid (7.91) that yields theOrange 4 chromogen (Scheme 7.37) were examined in detail.

In orange dyes of the haloheterocyclic type, the reactive system is invariably attached viathe nitrogen of the J acid coupler. In vinylsulphone dyes, on the other hand, it is normallymore convenient to use as diazo component an intermediate such as 7.38 or 7.39 bearingthe precursor grouping together with an N-acetylated derivative of J acid or γ acid ascoupler, structure 7.93 being typical.

NN

SO3Na

HN

COCH3

NH Z

NaO3S

SO3NaSO3Na

SO3Na

NaO3S

NN

HN

NH Z

COCH3

trans

cisScheme 7.36

NN

SO3Na

H O

NH Z

NaO3S

7.89

chpt7(1).pmd 15/11/02, 15:32402

403

SO3H

HO3S

N N

N

CH3

N

NN

NaO3S

OH Cl

Cl

SO3Na

NaO3S

NN

H

N

N

N

N

O

NaO3S

Cl

Cl

Cl

+

7.91

3 NaOH

+ NaCl + 3 H2O7.92

+ _


7.90

CH3

Scheme 7.37

NN

OH

NaO3S

CH2CH2O2SNaO3SO NHCOCH3

7.93


NN

OH

NaO3S

CH3O

SO3Na

NH Z

7.94

NN

OH

CH2CH2O2SNaO3SO

OCH3SO3Na

7.95

CI Reactive Red 22


7.5.4 Scarlet chromogens

Dyes in this hue sector are also derived from J acid, N-methyl J acid or γ acid but the diazocomponent is usually a sulphonated 2- or 4-anisidine, as a methoxy substituent has abathochromic influence. The highly substantive structure 7.94 is found in varioushaloheterocyclic (Z) dyes. As in the orange region, vinylsulphone dyes have the precursorgrouping located on the diazo arylamine, as exemplified by structure 7.95.

chpt7(1).pmd 15/11/02, 15:32403


7.5.5 Red chromogens

Bluish red reactive dyes are almost totally dominated by H acid as the indispensablecoupling component. Various mono- or disulphonated anilines or naphthylamines aresuitable diazo components, but the outstandingly important one is orthanilic acid (7.96). Aswith orange and scarlet dyes, haloheterocyclic (Z) reactive systems are linked via the iminogroup of the H acid residue but sulphatoethylsulphone substituents are found in the diazocomponent with N-acetyl H acid as the typical coupler. A characteristic problem associatedwith reactive dyes derived from H acid is their accelerated fading under the simultaneousinfluence of perspiration and light (section 3.3.4).

NN

OO

Cu

NaO3S

NaO3S

NH Z

7.97

NN

OH

SO3Na

NaO3S

HN

Z

SO3Na

7.96

7.5.6 Rubine chromogens

Bordeaux and rubine hues are given by 1:1 copper-complex monoazo structures made from adiazotised aminophenolsulphonic acid and J acid or γ acid, with the haloheterocyclic group(Z) on the coupling component (7.97). As usual in the vinylsulphone series it is moreconvenient to use a precursor-substituted aminophenol with N-acetyl J acid as coupler. Inthe red–violet–blue zone of the colour gamut the higher light fastness shown by coppercomplexes compared with their unmetallised analogues is particularly desirable in spite ofthe sacrifice of some brightness.

7.5.7 Violet chromogens

These very much resemble the corresponding rubine dyes but have H acid rather than J acidas the coupling component. This does mean that there is some risk of accelerated fading as aresult of the combined effects of perspiration and light. The histidine component ofperspiration is able to abstract the copper from some metal-complex azo dyes (section 5.7.2)and the demetallised H acid structure will then be vulnerable to this problem (section3.3.4). Nevertheless, these copper-complex violet dyes are much less sensitive than theunmetallised bright bluish reds in this respect. Examples of typical structures include thosewith a haloheterocyclic (Z) group on the H acid residue (7.98) and those such as CIReactive Violet 5 (7.34) with a sulphatoethylsulphone group in the diazo component.

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405

7.5.8 Dull blue chromogens

These are reddish blue 1:1 copper-complex monoazo dyes derived from a 2-naphthylamine-or 2-aminonaphtholsulphonate as diazo component and another aminonaphtholsulphonateas coupler. Often such dyes are more easily prepared using a 2-naphthylaminesulphonateand oxidatively coppering the resulting monoazo dye (section 5.5.3). In orthodox structuresthe imino link of H acid carries the reactive system (7.99), but in other instances thenaphthylamine diazo component provides the site of attachment of a haloheterocyclic(7.100) or sulphatoethylsulphone (7.101) grouping.

NN

OO

Cu

NaO3S

SO3Na

NaO3S

HN

Z

7.98

NN

OO

Cu

NaO3S

SO3Na

NaO3S

HN

ZSO3Na

7.99 NN

OO

Cu

NaO3S

NHZ

H2N SO3Na

SO3Na

7.100

NN

OO

Cu

NaO3S

SO2

CH2CH2NaO3SO

NaO3S

SO3Na

HN

COCH3

7.101


7.5.9 Bright blue chromogens

Two different chemical classes contribute to this sector. Initially it was entirely dominated byanthraquinone dyes typified by structure 7.102. The dye bases for attachment ofhaloheterocyclic (Z) systems are prepared by condensing bromamine acid (7.103) withvarious phenylenediamines. The outstandingly successful CI Reactive Blue 19 (7.37) is the

chpt7(1).pmd 15/11/02, 15:32405


condensation product of bromamine acid with the precursor arylamine 7.39. More recently,bright blue symmetrical structures of the triphenodioxazine class (7.104; –NH–R–NH– =alkylenediamine, Z = haloheterocyclic system) have been developed. These are tinctoriallymuch more intense than anthraquinone chromogens and thus offer economic advantages.The marketing of a bis(aminonicotinotriazine) dye of this class (CI Reactive Blue 187) hadan important influence on the subsequent development and success of the Kayacelon React(KYK) range of dyes containing this bis-quaternary system (section 7.4.2).

O

N

N

O

NaO3S

HNRHNZ

Cl

Cl SO3Na

NH R NH Z

7.104

7.5.10 Turquoise chromogens

This colour makes an important contribution to the appeal of reactive dyeings and prints. Itis totally dominated by dyes derived from copper phthalocyanine (section 5.4.3). A notableearly example is structure 7.105, made from copper phthalocyanine by chlorosulphonation ofall four 3-positions, partial reaction with phenylene-1,3-diamine-4-sulphonic acid andammonia, hydrolysis of the remaining chlorosulphonyl groups, then finally introduction ofthe reactive system using cyanuric chloride and ammonia. Such products areinhomogeneous mixtures of slightly different components, the overall balance ofsubstituents totalling four per molecule as shown in formula 7.105. Solubility and dyeingproperties of these dyes can be modified by varying the proportion of 3-sulphonamide(polar) to 3-sulphonic acid (hydrophilic) groups, as well as selection from a variety ofphenylenediamines. An important dye of this class is CI Reactive Blue 21, in which thereactive function is provided by a 3-SO2NHPhSO2CH2CH2OSO3Na grouping.

O

O

NH2

SO3Na

NH

HN

Z

SO3Na

7.102

O

O

NH2

Br

SO3Na

7.103

Bromamine acid

chpt7(1).pmd 15/11/02, 15:32406

407

7.5.11 Green chromogens

Only relatively few green reactive dyes have been marketed and most of them have beendesigned by linking up separate blue and yellow chromogens. The most important approachhas been to attach a yellow monoazo (Y = N) or stilbene (Y = CH) chromogen to abromamine acid residue, either directly (7.106) or via a phenylenediamine linking group andan aminohalotriazine system (7.107; X = Cl or F). Another possibility is to incorporate ayellow chromogen together with the reactive grouping of a copper or nickel phthalocyanine,but such dyes have unattractive dyeing properties because of the length of this extendedsubstituent. In a few instances, dull bluish greens can be achieved from twice-coupled Hacid structures if the two diazo components are correctly chosen, as in thebis(aminochlorotriazine) dye CI Reactive Green 19.

N

NN

N

N

NN

N

Cu

NaO3S

NaO3S

SO2NH2

SO2NH

NaO3S

NH

N

N

N

Cl

NH2

7.105

CI Reactive Blue 7

O

O

NH2

SO3Na

HN

Y Y

SO3Na

NH Z

SO3Na

7.106

O

O

NH2

HN

HN

SO3Na

N

N

N

HN Y Y NO2

NaO3S

SO3NaNaO3S

X

7.107


7.5.12 Brown chromogens

Unmetallised disazo dyes of the type A→M→E in Winther symbols (section 4.7) dominatethis sector. The A component is usually a disulphonated aniline or naphthylamine.Orthanilic acid or p-xylidine (2,5-dimethylaniline) for yellow browns, or a variety ofmonosulphonated 1-naphthylamines for redder browns, are selected as the M and Ecomponents. The terminal amino group provides the site for attachment of the reactive

chpt7(1).pmd 15/11/02, 15:32407


system (Z). Structure 7.108 illustrates a typical reddish brown. Monoazo J acid, γ acid orpyrazolone ligands have been used in unsymmetrical 1:2 cobalt or chromium complexes witha reactive group in each ligand to give brown dyes of high fastness to light and wettreatments.

N

SO3Na

NaO3S

N NN

H3C

OO

Cu

NaO3S

N

CH3

Z

7.109

SO3Na

NaO3S

N

N N

NaO3S

N NH

SO3Na

Z

7.108

7.5.13 Navy blue chromogens

Three major approaches have been followed to provide reactive dyes in this importantsector. One category is closely related to the reddish blue monoazo 1:1 copper complexesalready described (section 7.5.8). To provide the higher substantivity and deeper intensityfor build-up to navy blue shades, a second unmetallised azo grouping is introduced. As withthe brown dyes, the A→M→E pattern is adopted for their synthesis. Component A isnormally a sulphonated aniline, M an aminophenol or aminocresol and E a sulphonatednaphthol or aminonaphthol. The reactive system (Z) is usually, but not invariably, locatedon the E component and the copper atom always coordinates with an o,o′-dihydroxyazogrouping provided by the M and E components (7.109).

This hue sector is dominated, however, by the more economical unmetallised twice-coupled H acid derivatives. In these dyes the amino group in the H acid molecule is notused to attach the reactive system because that would prevent conjugation between the twoazo groups. As in structure 7.110, the diazo components are almost always aniline-2,5-disulphonic acid and a sulphonated phenylenediamine. Usually the former is attached byacid coupling ortho to the amino group and the latter, which provides the site for thehaloheterocyclic (Z) group, by alkaline coupling ortho to the hydroxy group. Inmonofunctional vinylsulphone dyes of this type, a precursor-bearing intermediate such as7.38 or 7.39 is introduced by alkaline coupling to the hydroxy side of the H acid residue.

In the outstandingly successful CI Reactive Black 5, two such precursor-bearing units areused in the synthesis of this near-symmetrical bifunctional structure (7.36). Following thisprecedent, competing bifunctional dyes of analogous structure were designed with twophenylene-1,3-diamine-4-sulphonate groupings to accommodate the reactive systems

chpt7(1).pmd 15/11/02, 15:32408

409

(7.111). Navy blue reactive dyeings that meet exceptional levels of fastness to light and wettreatments can be achieved (at a price) by turning to copper formazan complexes (7.112). Inthose dyes with a haloheterocyclic reactive system, this is normally attached through animino link at position Y and X is a sulpho group. Conversely, in vinylsulphone analogues Y issulpho and X is the sulphatoethylsulphone precursor.

7.5.14 Black chromogens

This is almost a contradiction in terms for reactive dyes. As in the green sector, it is difficultto design a homogeneous reactive dye that will yield deep black dyeings without shading.The economically attractive twice-coupled H acid dyes build up well but even thosedescribed as blacks, such as CI Reactive Black 5, are really dark navy blues. Dyes of thiskind are readily shaded with low-cost browns of the A→M→E type (section 7.5.12), or withsmaller amounts of yellow and red shading components, to give general-purpose blackmixtures of considerable versatility. Copper-complex navy blues can be used in a similar waywith shading components of high light fastness for more demanding outlets.

Mixed cobalt/chromium complexes of the symmetrical 1:2 type in which each ligandcontains a low-reactivity system linked to the coupling component (section 5.5.2) have beenwidely used for continuous dyeing and printing. They offer high light fastness and the

N

N

SO3Na

NHZ

H O

NaO3S

SO3Na

H2N N N

NaO3S SO3Na

7.110

N

N

SO3Na

NHZ

H O

NaO3S

SO3Na

H2N N N

NaO3S NH

7.111

Z

N

NC

N

N

O

Cu

O C

OY

X

SO3Na

7.112


chpt7(1).pmd 15/11/02, 15:32409


unfixed dye is more readily washed-off because such structures tend to be less substantive tocellulose than the unmetallised disazo dyes designed principally for exhaust dyeing.

Homogeneous black chromogens are well represented among direct dyes, where highlysubstantive trisazo or tetrakisazo structures are often encountered. Quite often thesecontain primary amino groups as auxochromes and hydrogen bonding sites positioned ateach end of the molecule. It would not be difficult to convert the typical trisazo dye CIDirect Black 80, for example, into its bis(aminochlorotriazine) derivative (7.113). Thishypothetical structure, however, would possess such a high substantivity and such a slowrate of diffusion that undesirable characteristics of poor levelling, inadequate penetrationand inefficient fixation would be likely, together with great difficulty at the washing-offstage. This performance profile is reminiscent of that shown by the pentafunctional red dye(7.79) described in section 7.4.4 [74].

7.6 STABILITY OF DYE–FIBRE BONDS

The chromogens used to synthesise reactive dyes normally exhibit poor wet fastness in theunfixed state. The high wet fastness of reactive dyeings depends almost entirely on theresistance of the dye–fibre bonds to the agencies characteristic of wet fastness tests,including pH, temperature, surfactants and oxidants. When the dye–fibre reaction productis formed during the fixation process (Schemes 7.8 and 7.25), many of the factorsresponsible for the susceptibility of the reactive system to hydrolysis remain operative andmay play a decisive part in determining the wet fastness attainable [10]. Thus when adichlorotriazine dye reacts with a cellulosate anion, one of the chloro substituents isreplaced by an electron-releasing cellulosyl grouping that is less electronegative. The otherelectronegative chlorine and three nitrogen atoms remain, however, activating thesubstituents on the heterocyclic ring to hydrolytic attack (Scheme 7.38). This can result ineither stabilisation or rupture of the dye–fibre bond.

A somewhat different balance exists with the less reactive aminochlorotriazine dyes.Since an arylamino group is more strongly electron-releasing than the cellulosyl grouping,the electronegative influence of the three nitrogen atoms in the ring predominates and onlythe activated cellulosyl ether bond is subject to hydrolytic attack (Scheme 7.39). For thisreason, therefore, such dye–fibre linkages will be marginally less stable under alkalineconditions than those formed by the reaction of cellulose with a dichlorotriazine dye. Themarginal differences in dye-fibre bond stability between aminochlorotriazine, bis(amino-chlorotriazine) and bis(aminofluorotriazine) dyes were examined by treating cotton dyeings

NH

N

N

N

Cl

H2N

SO3Na

N N NN

OH

NaO3S

NaO3S

O

HN

N

N

N

Cl

NH2N

N

H

7.113

chpt7(1).pmd 15/11/02, 15:32410

411

[dye] NH

N

N

N

NH [aryl]

O [cellulose]

[dye] NH

N

N

N

NH [aryl]

OH HO [cellulose]

H2O

NaOH

+

Dye-fibre bond Hydrolysed dye

Scheme 7.39

STABILITY OF DYE–FIBRE BONDS

[dye] NH

N

N

N

OH

Cl

HO [cellulose]

[dye] NH

N

N

N

O [cellulose]

Cl

[dye] NH

N

N

N

O [cellulose]

OH

H2O

NaOH

NaOH

+

+ NaCl

Hydrolysed dyeActivateddye-fibre bond

Stabiliseddye-fibre bondScheme 7.38

in buffer solutions (pH 10 or 12) at various temperatures (60, 85 or 98 °C). The results wereexpressed in terms of the percentage of hydrolysed dye released [84].

If a pyrimidine ring forms the basis of a chloroheterocyclic reactive system, the effects aresimilar to those for chlorotriazine analogues but the activation by only two ring nitrogenatoms is much less marked (Figure 7.1). Thus pyrimidine dye–fibre linkages are more stableto alkali than either of the corresponding chlorotriazine systems. The rate-determining stepfor acidic hydrolysis of haloheterocyclic systems is nucleophilic attack of the protonatedheterocyclic ring by water molecules.

The relative fixation given by various haloheterocyclic dyes on cotton and the stability oftheir dye–fibre bonds under acidic and alkaline conditions have been compared. Among thetriazines, the highest relative fixation (70%) was shown by 2-arylamino and 2-heterylaminoderivatives. Among the pyrimidines, the 5-cyano-2,4-dichloro derivatives gave 73% but the5-chloro-2,4-difluoro derivatives achieved 84%. The latter system also yielded the moststable dye-fibre bonds in acidic as well as alkaline media [85]. A series of quantummechanical calculations for cellulosic dyeings prepared with analogous dyes representingtwelve different reactive systems containing either a pyrimidine or an s-triazine ringprovided rate constants for hydrolysis under acidic and alkaline conditions [86].

When fixation to cellulose is achieved by a nucleophilic addition mechanism (Scheme

chpt7(1).pmd 15/11/02, 15:32411


7.25), an important factor is the reversibility of formation of the dye–fibre bond. Severealkaline treatments are capable of rupturing this linkage to regenerate the unsaturatedreactive system, which is able to react again either with cellulose or with water (Scheme7.40). The presence of a less electronegative activating group than sulphone, as in thevinylsulphonamide dyes (7.41), increases the stability of the dye–fibre bond but reduces thereactivity of the dye.

S[dye] CH2CH2OH

O

O

[dye] S CH

O

O

CH2

[dye] S CH2CH2

O

O

O [cellulose]

H2O

NaOH

Hydrolysed dye Active dye

Dye-fibre bondScheme 7.40

The kinetics of alkaline hydrolysis of a series of eleven vinylsulphone reactive dyes fixedon cellulose have been investigated at 50 °C and pH 11. Bimodal hydrolytic behaviour wasobserved under these conditions, the reaction rates being rapid at first but becoming sloweras the concentration of fixed dye remaining gradually decreased. These results wereattributed to differences in the degree of accessibility of the sites of reaction of the dyeswithin the fibre structure [87].

In an interesting comparison of dye–fibre bond stabilities over the pH range 3.5–10,dyeings of an aminochlorotriazine-sulphatoethylsulphone bifunctional dye were comparedwith those of two monofunctional analogues of almost identical structure. Under acidicconditions the bifunctional dyeing showed higher stability than the vinylsulphone dyeing,which in turn was more stable than the monofunctional aminochlorotriazine analogue. Atalkaline pH, on the other hand, the aminochlorotriazine analogue and the bifunctionaldyeing were virtually identical in stability, both being markedly more stable than themonofunctional vinylsulphone [74]. For similar reasons, bifunctional reactive dyeings of theaminofluorotriazine-sulphatoethylsulphone type show better allround stability to acidic andalkaline conditions than analogous monofunctional dyeings based on dichlorotriazine,aminochlorotriazine, aminofluorotriazine or vinylsulphone systems [77].

Reactive tendering refers to the strength loss of reactive-dyed cellulosic garments thatcan occur during commercial laundering. The effect is attributed to the inductive electron-withdrawing influence of the dye structure on the dye–fibre bond, which accelerates acidhydrolysis of β-1,4-glycosidic linkages in the cellulose chain [88]. Conversion of sodiumsulphonate groups in the dye molecule to the free acid form by prolonged rinsing can alsocontribute to acid tendering.

The hydroxide ion is not the most active nucleophile with which the dye–fibre bonds inreactive dyeings have to contend. Many commercial detergent formulations contain sodium

chpt7(1).pmd 15/11/02, 15:32412

413

N N

Cl

Cl

X

NH[dye]

N N

OH

O

X

NH[dye] [cellulose]

N N

OH

O

OOH

NH[dye]

[cellulose]HO

[cellulose]

HO

NaOH

HOONaOH

+

_

_Trichloropyrimidine (X = Cl) orCyanodichloropyrimidine (X = CN)

Scheme 7.41

perborate or percarbonate that may release peroxy species in washing treatments. Theperhydroxide anion (HOO–) is an exceptionally powerful nucleophile capable of attackingcertain types of haloheterocyclic reactive system to give unstable products. Studies of thereaction of alkaline peroxide solutions with dissolved reactive dyes have shown that reactivechloro substituents are readily displaced. Heterocyclic dyes containing other leaving groups,such as fluoro- or nicotinotriazines, and systems that fix by nucleophilic addition, e.g.vinylsulphone dyes, generally do not show perhydroxide formation.

Surprisingly, however, dyes of the chlorodifluoropyrimidine type readily form aperhydroxide derivative that leads to cumulative damaging effects on dye–fibre bondstability. A comparison between seven different haloheterocyclic systems each attached tothe same chromogen (phenylazo H acid) demonstrated several important conclusions [89]:(1) Only dye–fibre linkages that carry on the heterocyclic ring an electronegative

substituent that is ortho (or para) to the dye–fibre bond show both peroxidation(Schemes 7.41 to 7.43) and bond breakage (Scheme 7.44).

(2) Dye–fibre linkages that carry on the heterocyclic ring an electronegative substituentthat is meta to the dye–fibre bond may show peroxidation (Scheme 7.45) but only slightbreakage of bonds.

(3) Dye–fibre linkages with no electronegative substituents on the heterocyclic ring showneither peroxidation nor bond breakage, e.g. aminochlorotriazine, aminofluorotriazine,bis(aminochlorotriazine) and bis(aminonicotinotriazine) dyes.

An amine aftertreatment has been developed recently to protect haloheterocyclic dyes withsubstituents vulnerable to attack by perborates or other peroxy compounds in detergents.This product displaces such substituents, deactivating the dye–fibre bond system andrendering it resistant to peroxidic attack [90].

The relationship between dye–fibre bonding and light fastness was examined for tensulphatoethylsulphone reactive dyes on cellulose and it was shown that the stronger thebonding between dye and substrate, the more stable was the dyeing when exposed to light

STABILITY OF DYE–FIBRE BONDS

chpt7(1).pmd 15/11/02, 15:32413


N N

F

F

Cl

NH[dye]

N N

OH

O

Cl

NH[dye] [cellulose]

N N

OH

O

OOH

NH[dye]

[cellulose]HO

[cellulose]

HOO–

HO–

NaOH

NaOH

+

Chlorodifluoropyrimidine

Scheme 7.42

N

N Cl

ClCO[dye]

[cellulose]HO

N

N Cl

OCO[dye] [cellulose]

N

N OOH

OCO[dye] [cellulose]

NaOH

HOONaOH

+

_Dichloroquinoxaline

HO

Scheme 7.43

N N

OH

O [cellulose]NH[dye]

OOH

N N

O

ONH[dye]

OH

[cellulose]HOfree-radical

mechanism+

Scheme 7.44

[91]. Light-fading studies on cotton dyeings of five trichloropyrimidine reactive dyesrevealed that the rate of photodegradation of the cellulose was influenced by the presence ofthe dye. Compared with an undyed control, the blue dyeing was degraded more rapidly butthe yellow, orange, red and green dyes tested exerted a protective effect [92]. Repeated

chpt7(1).pmd 15/11/02, 15:32414

415

perborate oxidation of dyed cotton and viscose yarns demonstrated increased tendering ofdyeings containing metal-complex reactive dyes. Viscose yarns were more significantlyaffected than cotton, possibly owing to the lower crystallinity, greater accessibility andhigher carboxy content of viscose fibres [93].

7.7 REACTIVE DYES ON WOOL

Wool keratin contains several different types of nucleophilic site for reaction with dyes [94-96]. By far the most important ones in intact wool are the amino groups of the N-terminalaminoacid residues and the sidechain groups of lysine and histidine (Figure 7.3). These arethe same groups that provide positively charged sites under acidic conditions for absorptionand electrostatic bonding with reactive or unreactive anionic dyes. A reactive dye with alabile halogen atom (X) will undergo a nucleophilic substitution reaction with one of theseamino sites in the uncharged state (Scheme 7.46). The thiol groups of cysteine residuesformed by the hydrolysis of cystine disulphide groups in wool provide further sites forreaction with reactive dyes (Scheme 7.47).

The hydrolytic breakdown of the disulphide bonds of cystine liberates some hydrogensulphide and this can gradually deactivate a vinylsulphone dye by formation of the

Scheme 7.45

NH2

CH2

CO

NH

CH

CO

CH2CH2CH2CH2NH2

NH

CH

CO

CH2N

HN NH

CH

CO

CH2 SH

N-terminalglycine

Lysine Histidine Cysteine

Figure 7.3 Nucleophilic sites in wool keratin for reaction with dyes

REACTIVE DYES ON WOOL

Y

N

N

Cl

ClNH[dye]

[cellulose]HO

Y

N

N

Cl

ONH[dye] [cellulose]

Y

N

N

OOH

ONH[dye] [cellulose]

NaOH

HOONaOH

+

_

Dichloropyrimidine (Y = CH) orDichlorotriazine (Y = N)

HO

chpt7(1).pmd 15/11/02, 15:32415


unreactive thioether derivative (Scheme 7.48) [97]. In a similar way, hydrogen sulphide willdisplace hydrogen bromide from an α-bromoacrylamide reactive system (Scheme 7.49). Thethiirane intermediate initially formed is unstable, decomposing to the less reactiveunsubstituted acrylamide with the precipitation of sulphur [98]. It is likely thathaloheterocyclic reactive dyes are also readily deactivated by hydrogen sulphide, the halosubstituent being replaced by a thiol group. The ability of wool reactive dyes to scavengehydrogen sulphide explains their fibre protective effect when dyeing wool in medium to fulldepths; damage is related to the extent of the thiol-disulphide interchange or ‘setting’reaction, which is promoted by cystine degradation in hot aqueous media [99].

[dye]X SO3 NH3

CH

CO

R

NH2

CH

CO

R

X [dye] SO3NaNH

CH

CO

R

[dye] SO3Na

NH3

H

HO ++ NH4X

+

__

N-terminal amino acid

Scheme 7.46

NH

CH

CO

CH2 SH X [dye] SO3Na

NH

CH

CO

CH2 S [dye] SO3NaNH3

+

+ NH4X

Scheme 7.47

CH CH2SO2[dye] + H2S SO2CH2CH2SH[dye]

Scheme 7.48

[dye] NHCO C

Br

CH2 + H2S [dye] NHCO CH CH2 + HBr

S

[dye] NHCO CH CH2Scheme 7.49

Traditional wool dyeing methods have often involved a rapid unlevel initial strike at lowtemperature, followed by a prolonged migration treatment at the boil to attain optimumlevelness. To fit in with these requirements, ranges of reactive dyes developed for woolneeded to react slowly with the fibre and this implied reactive systems with low intrinsicreactivity. One such group that was found to react too slowly for exploitation on cellulosic

chpt7(1).pmd 15/11/02, 15:32416

417

fibres was selected for the Procilan (ICI) dyes, the first full range of reactive dyes to bedesigned for application to wool.

These were unsymmetrical 1:2 chromium complexes of Mr 800–900, derived fromdifferent monoazo ligands with one sulpho substituent and one acrylamido reactive groupper dye molecule. They were intended for application to wool at the boil, at least an hourbeing necessary for slow addition of the nucleophilic groups across the activated doublebonds of the acrylamide dyes (Scheme 7.50). Like their unreactive analogues (section 3.2.2)these chromium-complex dyes exhibited high neutral-dyeing affinity, but this was a problemfrom the viewpoint of removal of the unfixed hydroxypropionamide derivative at thewashing-off stage. Dullness of hue was another limitation and the range was later extendedby adding brighter unmetallised monoazo chromogens of similar Mr with two sulpho groupsand an aryloxychlorotriazine reactive system. The technical performance achieved did notjustify the relatively high cost of this hybrid range, which was eventually withdrawn.

An important contributory factor was the emergence in 1966 of the most successful rangeof reactive dyes designed for wool [100,101]. These are the Lanasol (Ciba) α-bromoacrylamide dyes (7.115), usually marketed in the form of their α,β-dibromopropionamide precursors (7.114). In dilute alkaline solution this group readilyreleases hydrogen bromide to yield the active form (Scheme 7.51). The introduction ofacrylamide, α-bromoacrylamide or α,β-dibromopropionamide reactive groups into dyemolecules is readily achieved by direct acylation of the dye base [dye]–NH2 with theappropriate acid chloride Cl–CO–R, where R is –CH=CH2, –CBr=CH2 or –CHBr–CH2Brrespectively [36].

A bromine atom attached to a vinyl group is deactivated towards nucleophilicsubstitution. The sp2 hybridisation of the carbon causes the C–Br bond to shorten andbecome stronger, whilst the delocalisation of the electrons by resonance causes theactivation energy for displacement of the bromine to increase [96]. It is now widely acceptedthat initial reaction with wool involves addition at the double bond [102]. Ring closure toform a three-membered aziridino ring then occurs by intramolecular nucleophilicsubstitution and this group finally hydrolyses to a β-substituted α-hydroxypropionamidedye–fibre bond (Scheme 7.52). Recent laser Raman spectroscopic studies have providedsupport for this mechanism and suggest that cysteine thiol groups are particularly important

NaO3S [dye] NHCOCH CH2

N-terminal

amino acidNaO3S [dye] NHCOCH2CH2 NH

CH

CO

[wool]

R

NaO3S [dye] NHCOCH2CH2OH

H2O

Scheme 7.50

[dye] NHCO CH CH2Br

Br

[dye] NHCO C CH2

BrNaOH

+ NaBr + H2O

7.1157.114

Scheme 7.51


chpt7(1).pmd 15/11/02, 15:32417


sites for reaction, especially with damaged wool [103]. Further detailed studies using modelcompounds have confirmed that the reaction with primary amino groups resulting in theformation of aziridine rings is important. Reactions of the α-bromoacrylamide grouping withthe imidazole nitrogen of N-acetylhistidine and the thiol group of N-acetylcysteine were alsoobserved [104].

Although reactive dyes account for only about 5% of total dye usage on wool, no otherclass can offer such brilliant hues of high fastness to light and wet treatments. Chrome dyesare mainly used for economical navy blue and black dyeings, where reactive dyes in generalare particularly costly by comparison. In 1992 Ciba supplemented the Lanasol range withtwo new dyes, Navy B and Black R, that occupy the shade areas corresponding to chromenavy and black brands. More recently, Lanasol Black PV has been introduced as strongcompetition for established chrome blacks, even as regards price. It has been designed togive tinctorial strength, shade, metamerism and light fastness close to those of theafterchromed complex from Eriochrome Black PV (CI Mordant Black 9). An amphotericlevelling agent and ammonia aftertreatment are necessary and the fastness to severe wettests such as potting and cross-dyeing is not quite as high as chrome dyeings [105].

Few of the haloheterocyclic reactive systems that became established for the dyeing ofcellulosic fibres have transferred successfully to wool dyeing conditions. The one exceptionis the 5-chloro-2,4-difluoropyrimidine system (section 7.3.5). This has been utilised in theDrimalan (Clariant) and Verofix (DyStar) ranges of reactive dyes for wool [106]. These arebased on conventional reactive dye chromogens (section 7.5) with two or three sulphogroups per molecule and an Mr of 600–800. The dye base usually has an alkylaminosubstituent (section 7.3.3) for reaction with chlorotrifluoropyrimidine to yield the reactivedye. This is a mixture of two isomers (7.19 and 7.20) and thus gives rise to a variety ofdifferent dye–fibre bond structures when fixation to the various nucleophilic sites in woolkeratin (Figure 7.3) takes place. The high fixation ratio shown by these dyes has been

[dye] NHCO C CH2

Br

R CH CO [wool]

NH2

[dye] NHCO CH CH2

Br NH

CHR CO [wool]

[dye] NHCO CH CH2

NH

CHR CO [wool]

[dye] NHCO CH CH2

N

CHR CO [wool]

[dye] NHCO CH CH2

NH

CH

OH

R CO [wool]

NH3

H2O

Br

+

+ NH4Br

_ +

N-terminal amino acid

7.115

Scheme 7.52

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419

attributed to the continuing activity of partially hydrolysed dye, since the reactivity of thesecond fluorine atom is only slightly decreased by reaction of the first with an amino groupin wool [107]. A novel range of heterobifunctional dyes has been introduced recently underthe brand name Realan (DyStar). These products contain a 5-chloro-2,4-difluoropyrimidinesystem and a sulphatoethylsulphone grouping in the same molecule. They are recommendedfor the dyeing of wool at a mildly acidic pH and of silk under mildly alkaline conditions[108].

Although the first two reactive dyes intended for fixation on wool contained thevinylsulphone reactive system and were marketed by Hoechst as long ago as 1952, it proveddifficult to develop a fully compatible coherent range along these lines. Some of the earlysulphatoethylsulphone dyes, including the bifunctional Hostalan Black SB (7.36) and theattractive Remalan Brilliant Blue R (7.37), had become highly successful as Remazolequivalents for cellulosic dyeing (section 7.3.8). Various secondary alkylamines, such asdiethylamine or N-methyltaurine, were exploited as leaving groups in other Hostalan dyes(Scheme 7.53). Under mildly acidic conditions at the boil gradual elimination of the aminetook place, allowing the dye to level more effectively before the vinylsulphone form couldreact with the nucleophilic sites in wool [109]. This assortment of precursor types and theusual need to vary the chromogen considerably (section 7.5) gave rise to marked differencesin Mr (600–900) even though most members of the range were disulphonated. This ratherincompatible range of products was eventually discontinued in the 1980s.

[dye] SO2CH2CH2 N

CH2CH3

CH2CH3

[dye] SO2CH2CH2 N

CH2CH2SO3Na

CH3

[dye] SO2 CH CH2

CH3CH2NHCH2CH3

CH3NHCH2CH2SO3Na

+

+

Vinylsulphone form

N-methyltaurine

Diethylamine

Precursor forms

Scheme 7.53


It is widely accepted that reactive dyes protect wool from hydrolytic damage during hotaqueous dyeing processes at pH 3–7. In the dyeing of wool/cotton blends, the woolcomponent is significantly protected if it is first dyed with appropriate wool-reactive dyesbefore exposure to the strongly alkaline conditions necessary for satisfactory fixation ofreactive dyes on cotton [110]. The successful dyeing of severely damaged carbonised woolwith the bifunctional sulphatoethylsulphone dye CI Reactive Black 5 (7.36) was attributedto the ability of this product to crosslink the wool keratin chains and thus to exert aprotective effect [111].

In a comparison between various reactive dyes for wool and several commerciallyavailable colourless fibre-protecting agents, it was shown that reactive dyes in medium andfull depths are significantly more effective. Reaction readily occurs between typical reactivedyes and the cysteine thiol groups (Scheme 7.47) released when the cystine disulphidebonds are hydrolysed under dyeing conditions. Such reactions inhibit thiol-disulphide

chpt7(1).pmd 15/11/02, 15:32419


interchange reactions and thus significantly interfere with the level of set produced in aboiling dyebath. Reactive dyes also react preferentially with non-keratinous proteins in theintercellular material and the endocuticle, thus minimising their tendency to hydrolyse andpartially dissolve in the hot aqueous dyebath [112].

The presence of sulphur-containing aminoacids in keratinous fibres seems to have specialsignificance for the clothes moth, which attacks wool and other animal hairs but not silk orother fibre types. An effective method of mothproofing is to reduce the disulphide bonds tothiol groups using thioglycolic acid at pH 7 and 50 °C, followed by an alkylation reactionunder similar conditions with an alkylene dihalide (Scheme 7.54). More recently, thepossibility of using a bifunctional reactive dye instead of the alkylene dihalide has beenexplored [113]. The resistance of wool flannel to damage by clothes moth larvae wasmarkedly improved, but the treatment caused adverse effects on physical properties similarto those found with the alkylene bis-thioether crosslinks in wool.

NH

CH

CO

CH2 S S CH2 CH

NH

CO

NH

CH CH2 SH

CO

HS CH2 CH

NH

CO

X [dye] X

NH

CH CH2

CO

S [dye] S CH2 CH

NH

CO

NH

CH

CO

CH2 S S CH2 CH

NH

CO

HSCH2COOH

BrCH2CH2Br

+

Cystine

Thioglycolicacid

Cysteine

CH2CH2

Scheme 7.54

7.8 REACTIVE DYES ON SILK

Silk fibroin contains no cystine and the content of lysine and histidine is also low (about 1%in total), but it does contain tyrosine phenolic (13%) and serine alcoholic (16%) sidechains.Since glycine accounts for 44% of the total aminoacid content, an N-terminal glycineresidue is reasonably representative of most of the primary amino dyeing sites in silk fibres.Amino acid analysis of hydrolysed reactive-dyed silk indicates that the reaction betweenfibroin and reactive dyes takes place mainly at the ε-amino group of lysine, the imino groupof histidine and the N-terminal amino group of the peptide chain. In an alkaline medium,the hydroxy groups of tyrosine and serine also react [114].

Commercial ranges of reactive dyes have not been marketed specifically for silk dyeing, sothe more important types of reactive system developed successfully for wool or cellulosicfibres have been evaluated on silk. Apparel made from silk traditionally required dry

chpt7(1).pmd 15/11/02, 15:32420

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cleaning to avoid colour loss, so the high wet fastness offered by reactive dyes has beenadvantageous. Vinylsulphone dyes in particular provide good dischargeable grounds forprinting styles and show excellent fastness to perspiration [115].

Theoretically, all reactive dyes can be used for silk dyeing. However, to achieve the bestquality of dyed silk, reactive dyes have to satisfy the following requirements [114]:(1) Brilliance of hue: this is especially important on mulberry silk. Many dyeings on tussah

silk are much duller and the dyed silk shows a lower colour yield because of inferiorexhaustion.

(2) High reactivity: silk is damaged in an alkaline medium at high temperature, so reactivedyeing should be carried out in an acidic or neutral dyebath.

(3) Good storage stability: the consumption of dyes for the batchwise dyeing of silk is small,so the dyes should be highly stable to storage.

Silk can be readily dyed with conventional high-reactivity dyes of the dichlorotriazine,dichloroquinoxaline or difluoropyrimidine classes. Exhaust dyeing at 60–70 °C and pH 5–6gives satisfactory results, especially if a mildly alkaline aftertreatment is given to enhancefixation. Dichlorotriazine dyes can also be applied by pad–batch dyeing with bicarbonate anda batching time of 4–6 hours. The relatively low reactivity of aminochlorotriazine dyes,however, results in moderate to poor build-up on silk. Tertiary amine catalysts such asDABCO (7.66) can be used to accelerate the dye–fibre reaction and increase the fixationsubstantially [116], but it is difficult to achieve satisfactory compatibility in mixture dyeingsby this method (section 7.4.2).

In contrast to cellulosic dyeing with reactive dyes, the fibroin–dye bonds are remarkablystable in aqueous media of pH 4 to 10 [117]. Since there exists only a negligible amount ofbond hydrolysis even at high temperature and in a medium of pH 2, the cleavage of thefibroin–dye bond is not a problem in reactive-dyed silk. The stability of these bonds whendyeing with difluoropyrimidine dyes is the highest in both acidic and basic media [118].

Chloroacetyl reactive dyes have occasionally been used for the dyeing of wool but theynormally require alkaline fixation for acceptable colour yields. Silk can be damaged byalkaline conditions but research has indicated that certain haloacetyl dyes will givesatisfactory performance on silk under mild conditions. Table 7.6 presents selected resultsfrom a comparison of mono- and bifunctional dyes containing bromo- or chloroacetylreactive groups [119]. The exhaustion tended to decrease as the dyebath pH or temperaturewas increased but the fixation values showed trends in the opposite direction. The twosymmetrical bifunctional structures yielded consistently higher exhaustion and fixation thantheir unsymmetrical monofunctional analogues. An additional bromine atom boosted theexhaustion by about 16%. The most striking difference between the bromoacetyl andchloroacetyl reactive groups was the outstandingly higher reactivity of the bromoacetylsystem under mild conditions (pH 7 and 75 °C).

Sulphatoethylsulphone dyes have proved highly suitable for silk, yielding brilliant hues ofhigh wet fastness by application at 80 °C and pH 7–8 in the presence of Glauber’s salt,usually followed by an alkaline fixation treatment to ensure optimum fixation [115,120,121].A new approach to this dyeing system has been explored recently [122], in which thesulphatoethylsulphone dyes were first activated fully by a pretreatment at pH 8 for 5 minutesat the boil (Scheme 7.23). The free vinylsulphone dyes were then applied to silk at 80 °Cand pH 4–4.5 in the absence of salt. Under these conditions this highly reactive system

REACTIVE DYES ON SILK

chpt7(1).pmd 15/11/02, 15:32421


readily fixes to amino groups in silk by a nucleophilic addition mechanism (Scheme 7.55).The nonionised vinylsulphone groups ensure much higher exhaustion than thesulphatoethylsulphone precursor form, especially in the case of the bifunctional CI ReactiveBlack 5 (7.36). Avoidance of electrolyte additions is highly beneficial from the environ-mental viewpoint [122].

The Lanasol (Ciba) dyes marketed for the dyeing of wool, containing α-bromoacrylamideor α,β-dibromopropionamide groups, can also be used for dyeing silk to achieve highfixation in an alkaline medium [118,123]. The percentage fixation of the homobifunctionalbis(dibromopropionamide) dye 7.117 is exceptionally high. This dye is almost completelyfixed on the silk fibre, whereas the monofunctional analogue 7.118 shows much lowerfixation [118]. At pH 9, the dibromopropionamide groups in dye 7.117 rapidly eliminatehydrogen bromide to yield the reactive α-bromoacrylamide form.

The high fixation levels achieved on silk by various homobifunctional reactive dyescontaining two identical reactive systems per molecule, as well as the increasing success of

Table 7.6 Exhaustion and fixation of haloacetyl-substituted disazo dyes on silk [119]

Substituents inExhaustion (%) Fixation (%)

structure 7.116

75 °C 90 °C 90 °C 75 °C 90 °C 90 °CX Y pH 7 pH 7 pH 11 pH 7 pH 7 pH 11

Br Br 94 93 86 94 94 98Cl Cl 86 86 94 39 89 98Br H 78 76 70 79 76 91Cl H 83 80 74 15 38 90

NH

NaO3 S

SO3Na

O

N O N

H

N

NaO3S

SO3Na

N

O

CH2 Y

7.116

H2CX

N

H

C

O

C

O

[dye]NaO3S SO2CH CH2

NaO3S [dye] SO2CH2CH2OH

NaO3S [dye] SO2CH2CH2

CO

[silk]

R

H2O

N-terminal

amino acid

NH

CH

Scheme 7.55

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423

heterobifunctional dyes on cellulosic fibres, have prompted a detailed study of CI ReactiveRed 194 (7.76) on silk. This monoazo dye contains a sulphatoethylsulphone precursorgrouping linked to the H acid coupler through a chlorotriazine bridging group of lowerreactivity.

Maximum exhaustion and fixation of this bifunctional red dye on silk was achieved bydyeing at 90 °C and a neutral pH in the presence of 60 g/l electrolyte. The isoelectric pointof silk fibroin is found at pH 3–4 and the salt addition is necessary to suppress the negativecharge on the surface of the fibre. For optimum fixation, however, the pH must be highenough for the nucleophilic amino groups in the fibre to be present almost entirely in theirunprotonated form and for the sulphatoethylsulphone precursor groups to yield theirvinylsulphone active form. Dyeing under alkaline conditions results in lower exhaustion andfixation, not only because of the high concentration of ionised carboxylate groups in thefibre but also because of increased alkaline hydrolysis of the reactive groups in the dye [124].

All reactive dyes inhibit the dissolution of silk fibroin in appropriate solvents but theeffect is much more pronounced with bifunctional dyes because these are capable of formingcrosslinks between the polymer chains. Solubility tests are particularly sensitive to theformation of crosslinks and can give an estimate of the degree of crosslinking [125–127].The solvent selected should be effective in disrupting hydrogen bonding between polymersegments without bringing about scission of covalent bonds. The solubility of silk in asolution of calcium chloride in aqueous ethanol (molar ratio 1:2:8) was determined fordyeings of one monofunctional and three bifunctional reactive dyes at various applieddepths. A few typical results are given in Table 7.7. Even at a concentration of 0.05 mol/kg

REACTIVE DYES ON SILK

O

O

NH2

SO3Na

HN

NaO3S NH

C

ONHCO CH CH2Br

Br

NHCO CH CH2Br

Br7.117

O

O

NH2

SO3Na

HN

NaO3S NH

C

O

7.118

C

Br

CH2

chpt7(1).pmd 15/11/02, 15:32423


of CI Reactive Orange 16, the solubility of the dyed silk was still 83%, indicating how muchgreater is the effect of bifunctional crosslinking on this property.

The formation of crosslinks in silk fibroin increases the tenacity and resistance todeformation of the fibres, as reflected in the initial modulus and the yield point. Thisprotective effect conferred by fixation of the bifunctional dye CI Reactive Red 194 was notshown by the monofunctional Orange 16, which is unable to form crosslinks. The loss intenacity of undyed silk that is observed on treatment at 90 °C and pH 7 for 2 hours isattributable to lowering of the degree of polymerisation (DP) by hydrolysis of peptide bonds.The crosslinking action of bifunctional dyes tends to compensate for this loss in DP andprovides an intermolecular network that helps to maintain the physical integrity of the fibrestructure [124].

7.9 REACTIVE DYES ON NYLON

In spite of their unique status as the only commercial range of reactive dyes designedspecifically for the dyeing of nylon, the Procinyl (ICI) disperse reactive dyes were developedand maintained on a shoestring budget compared with conventional reactive dyes thatachieved greater success on other fibres. The range never contained more than six members,yet three different reactive systems were represented. The only blue was a 1,4-diaminoanthraquinone containing two 3-chloro-2-hydroxypropyl groups, precursors of theglycidyl reactive system that reacted only slowly and inefficiently with N-terminal aminogroups in nylon (Scheme 7.56). The only yellow chromogen was a 4-aminoazobenzenederivative bearing an aminochlorotriazine grouping of only low to moderate reactivity. Theremaining dyes of moderate light fastness covering the orange to rubine sector were eachbased on a simple monoazo chromogen with a 2-chloroethylsulphamoyl group, precursor ofthe relatively more active aziridinyl reactive system (Scheme 7.57).

The degree of fixation of these dyes under conventional dyeing conditions at 80–90 °Cwas rather low. Migration and coverage of dye-affinity variations was good but the wet

Table 7.7 Solubility of silk dyed with various reactive dyes [124]

Applied depth Dye on fibre SolubilityCI Reactive Structure (% o.w.f.) (mol/kg) (%)

Orange 16 7.119 1 0.010 99Black 5 7.36 2 0.012 60Red 194 7.76 2 0.013 51Red 120 7.48; X = Cl 6 0.010 41

CH2CH2SO2

NaO3S7.119

C.I. Reactive Orange 16

NHCOCH3

O

N

N

H

NaO3SO

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425

fastness was not much better than that of high-energy disperse dyes. Only by alkalineaftertreatment or high-temperature dyeing at 120 °C could fixation be improved, but withsome risk of unlevel dyeing if a high pH or temperature was reached too quickly. The dyeingof nylon/cellulosic blends with Procinyl dyes and anionic reactive dyes was more successful,because the alkaline fixation stage necessary for the latter on the cellulosic component alsoensured good fixation of the Procinyl dyes on nylon. In general, however, the technicalperformance of this small range of dyes seldom justified their cost and they were eventuallywithdrawn.

Anionic reactive dyes designed for cellulosic fibres or wool are able to give bright shadesof high wet fastness on nylon but their drawbacks are clearly evident. The degree of fixationis limited by the number and accessibility of primary amine end groups in nylon. Below thislimit (roughly 1% o.w.f. on normal nylon, higher on deep-dye variants) such dyeings showexcellent wet fastness but at greater depths the fastness deteriorates as the proportion ofunfixed dye present increases. Most anionic reactive dyes contain two or more sulpho groupsper molecule and this can result in blocking effects in mixture recipes containing dyes thatvary markedly in affinity for nylon. When high-reactivity dyes are applied in depths up tothe limit of available end groups, these dyes show very poor migration properties. Catalyticfading can be a problem in certain combination shades.

It has been pointed out [99] that, in theory at least, a monofunctional vinylsulphone dyeis capable of reacting twice with a primary amino N-terminal group in nylon (Scheme 7.58).In practice, however, fixation limits for these dyes are not significantly higher than for high-reactivity dyes of the haloheterocyclic types. When one multisulphonated reactive dyemolecule has reacted, steric hindrance and strong mutual electrostatic repulsion effectsinhibit access of a second reactive molecule to the imino group in the dye-fibre linking unit.

REACTIVE DYES ON NYLON

[dye] NHCH2CHCH2Cl

OH

[dye] NHCH2CH CH2

O

[dye] NHCH2CHCH2NH

OH

[nylon]

NaOH

N-terminal

Chlorohydroxypropylamino Glycidylamino

Dye-fibre bond

amino group

Scheme 7.56

[dye] SO2NHCH2CH2Cl [dye] SO2 NCH2

CH2

[dye] SO2NHCH2CH2NH [nylon]

NaOH

N-terminal

Chloroethylsulphamoyl Aziridinylsulphone

Dye-fibre bond

amino group

Scheme 7.57

chpt7(1).pmd 15/11/02, 15:32425


This problem has been confirmed recently in a study of the mechanism of covalentreaction between nylon 6.6 and the sulphatoethylsulphone dye CI Reactive Blue 19 (7.37).Acid hydrolysis of the dyed fibre and HPLC analysis of the hydrolysate yielded the 6-aminohexylaminoethylsulphonyl derivative of Blue 19. Even when the dyeing procedure wasoptimised to achieve maximal exhaustion and fixation to the fibre [128], only about 30% ofthe N-terminal amino groups in the nylon 6.6 were accessible because of mutual blockingeffects between these bulky anionic dye molecules.

[dye] SO2 CH CH2 + H2N [nylon]

[dye] SO2 CH2CH2 NH [nylon]

[dye] SO2CH2CH2

N

[dye] SO2CH2CH2

[nylon]

[dye] SO2 CH CH2

Scheme 7.58

7.10 NOVEL REACTIVE DYEING PROCESSES

Precedents show that attempts to forecast the probability of attainment of truly originaltechniques of coloration are fraught with difficulty. No dyeing expert of the early 1950scould have predicted the impact that reactive dyes would make within that decade. With anew millennium offering continuing challenges, however, the acknowledged world experts inthis field are not deterred by this from speculating how this game will go next [129,130].What is beyond doubt is that there are still important research targets remaining unfulfilledand that reactive systems still offer sufficient versatility to play a major part in thesedevelopments.

There has been a noteworthy revival of interest recently in adapting the concept ofcrosslinking reactants, so overwhelmingly successful in chemical finishing processes, to themore demanding task of attaching coloured molecules to cellulose or other fibres containingnucleophilic groups. Early attempts to achieve this technology transfer failed because theyrelied too closely on the chemistry of the existing finishing processes. Thus in the ProcionResin process of the early 1960s, a reactive dye of the chlorotriazine type was condensedwith an amino sugar, such as N-methy1glucosamine. The carbohydrate residue on theresulting dye molecule participated in crosslinking of the cellulosic fibre using a suitable N-methylol reactant [131].

The introduction of Calcobond dyes a few years later by American Cyanamid exploited asimilar principle but incorporated the N-methylol groups into the dye molecule itself [132].The labile chloro substituents in dichlorotriazine dyes were converted to amino groups bysubstitution with ammonia and the resulting melamine residue made cellulose-reactiveagain by reaction with formaldehyde (Scheme 7.59). A typical member of this range was CIReactive Red 92 (7.120). A characteristic problem of the Procion Resin process and of the

chpt7(1).pmd 15/11/02, 15:32426

427

Calcobond dyes was that the colour fastness to light was adversely affected. The fastness toacid treatment was no better than that of the resin finish, i.e. the dyeings could be strippedby boiling at a pH below 5.

[dye] NH

N

N

N

Cl

Cl

[dye] NH

N

N

N

NH2

NH2

[dye] NH

N

N

N

NHCH2OH

NHCH2OH

2 NH3

2 HCHO

Scheme 7.59

NN

SO3Na

H O

NaO3S

SO3Na

HN

N

N

N NHCH2OH

HOCH2NH

7.120

CI Reactive Red 92

NOVEL REACTIVE DYEING PROCESSES

The most interesting departure from conventional reactive dyeing methods of the 1960swas the Basazol (BASF) approach [133]. Selected unmetallised chromogens and 1:2chromium complexes containing nucleophilic groups such as primary or secondary amino,arylsulphonamide or unsubstituted pyrazolone residues were used (7.121 and 7.122 aretypical examples). These were applied to cellulosic fabrics by padding or printing, togetherwith the non-substantive crosslinking agent 1,3,5-tris(acryloyl)hexahydro-s-triazine (7.123).Above pH 10.5 the sulphonamide group is sufficiently nucleophilic to undergo Michaeladdition with the acryloyl groups of Fixing Agent P. This trifunctional product is sufficientlyreactive and versatile to form dye–fibre links, as well as dye dimers or trimers and crosslinksbetween cellulose molecules [134]. Although Basazol dyes were not hydrolysable,dimerisation and cellulose crosslinking do represent competitive side reactions (Scheme7.60). Some acryloyl groups will be hydrolysed (Scheme 7.61). The system was not adaptableto exhaust dyeing and the Basazol products were later withdrawn from the market.

chpt7(1).pmd 15/11/02, 15:32427


[dye] SO2NH2 HO [cellulose]N

N

NC C

C CHO

CH2

HC

O

H2C CH

O

CH2

COCHN

N

N

SO2NHCH2CH2

CH2

[dye]

CO N

N

N

COCH2CH2

COCH2CH2

HCH2C

O

O

[cellulose]

[cellulose]

N N

N

COCH

COCH2CH2SO2NHCH2CH2CO

CH2

O [cellulose][dye]

NaOH

CO

SO2NHCH2CH2[dye] CO

+ +

+ +

NaOH

Dye dimer Fibre crosslink

Dye-fibre bond

7.123

NN

N

NH O

HO

OH

OCH3

H2NO2S

7.121

NN

N

HN

OO

CH3

NN

N

NH

OO

H3C

Cr

SO3Na

NaO3S

7.122

Scheme 7.60

CN

O

CH CH2 CN

O

CH2CH2OHH2O

NaOH

Scheme 7.61

CH3CH2N

CH2

CH2

CH3CH2

CH OH

7.124

Cl–

NaO3S

N N

NHN Cl

Cl7.125

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429

A relatively uncontrolled attempt to enhance the colour yield and fastness to washing ofselected direct dyes involved addition to the dyebath of a colourless reactant that wasintended to form covalent attachments between these dyes and hydroxy groups in cellulose[135]. Most of the dyes selected contained at least two primary amino groups inaminonaphthol residues. The reactants evaluated were cyanuric chloride (7.3), N,N-diethyl-3-hydroxyazetidinium chloride (7.124) and 2,4-dichloro-6-(4′-sulphoanilino)-s-triazine(7.125), all of which will indeed react readily with primary arylamines. Some of the dyesselected, however, contained only diphenylurea or salicylic acid residues and these are mostunlikely to react efficiently under dyeing conditions.

In a later development [136,137], cyanuric chloride was proposed as an aftertreatment forcotton already dyed with direct dyes containing amino groups. This approach appears evenless likely to succeed than in situ addition to the dyebath. Serious hazards are associated withthe handling of cyanuric chloride under these conditions. This highly reactive compound isa primary skin irritant and is known to cause severe allergic reactions in certain individuals.Dye–agent reaction will be inefficient because of hydrolytic deactivation. Uptake of cyanuricchloride (or its hydrolysis products) by the dyed cotton will be poor.

A much more controlled approach to the concept involves incorporating the reactivefunction into the substrate and carrying out the dyeing with a non-hydrolysable dyecontaining a nucleophilic group [138]. Various nucleophilic groups come into consideration,but the aliphatic amino group gives the best results in practice as it is more reactive thanarylamino, phenolic or aliphatic hydroxy groups. In terms of nucleophilicity, the aliphaticthiol group is even more reactive, but thiol compounds are often malodorous and tend tooxidise to their disulphide derivatives [52]. An aminoalkyl derivative of the monoazo H aciddye CI Reactive Red 58 was prepared by reaction with ethylenediamine (Scheme 7.62).

Acrylamidomethylated cellulose prepared by the pad–bake condensation of N-methylolacrylamide with cotton (Scheme 7.63) reacts readily with nucleophilic dyes of thiskind. High fixation is achieved either by exhaustion at pH 10.5 in 80 g/l salt solution or bypad–batch treatment at the same pH for 24 hours (Scheme 7.64). These aminoalkyl dyesshow zwitterionic characteristics below pH 8 and this lowers the nucleophilicity of theprimary amino group (Scheme 7.65). Practically no dye is removed on alkaline soaping ofthese dyeings at the boil, compared with ca. 30% desorption of unfixed dye after applicationof the dichlorotriazine analogue CI Reactive Red 58 to untreated cotton. The dyeing ofactivated cotton with the aminoalkylated derivative thus offers significant savings, sinceonly a minimal rinse would be adequate and soaping at the boil could be omitted [52].

A simple yet rather drastic method of introducing reactive sites into the cellulosemacromolecule is to use periodate as a relatively specific oxidant. This ring-opening reactionconverts the vic-diol grouping in the 2,3-positions of some of the glycoside rings into adialdehyde (Scheme 7.66). The aldehyde groups can provide sites for fixation of dyemolecules containing nucleophilic aminoalkyl groups, such as a symmetrical bis(aminoalkyl)derivative of a bis(aminochlorotriazine) triphenodioxazine blue reactive dye [139]. Theazomethine groups formed are sensitive to hydrolysis unless reduced to the secondary amine.

The highly reactive compound disodium 4,4′-bis(dichlorotriazinylamino) stilbene-2,2′-disulphonate (7.126; known as DAST or T-DAS), an important intermediate in themanufacture of stilbene-type fluorescent brightening agents (section 11.6.1), has beenevaluated recently as a crosslinking agent for the fixation of dyes with nucleophilic groups.Compound 7.126 was applied to cotton simultaneously with the hydrolysed form of CI


chpt7(1).pmd 15/11/02, 15:32429


Reactive Black 5 (7.127) by an exhaust process and optimal conditions for fixation weredetermined [140]. The substantivity of this agent for cellulose is not ideal, as it requires ahigh salt concentration to ensure a fixation level of 80%. To achieve 85% fixation of dye7.127 by this method, an addition of 50 g/l sodium sulphate was necessary [29].

The novel cationic reactant 2,4-dichloro-6-(2′-pyridinoethylamino)-s-triazine (7.128)was evaluated as a means of activating cellulose by pad–batch application at pH 8.5 for 24hours. This agent contains two reactive chlorine atoms but under these mild conditions only

N N

N

NH Ar

ClHN[dye]

N N

N

NH Ar

NHCH2CH2NH2

NaOH

+ H2NCH2CH2NH2

+ NaCl + H2O

NaO3S

NaO3S

NaO3S

HN[dye]

NaO3S

NaO3S

NaO3S

Scheme 7.62

[cellulose] OH [cellulose] O CH2NHCOCH CH2HOCH2NHCOCH CH2

ZnCl2

150°C+

Scheme 7.63

N N

N

NH Ar

NHCH2CH2NH2 H2C HC CONHCH2 O [cellulose]

N N

N

NH Ar

NHCH2CH2NHCH2CH2CONHCH2 O [cellulose]

+HN[dye]

NaO3S

NaO3S

NaO3S

HN[dye]

NaO3S

NaO3S

NaO3S

Scheme 7.64

chpt7(1).pmd 15/11/02, 15:32430

431

one of them reacts with a hydroxy group in cellulose (Scheme 7.67). The electron-donatingcellulosyl substituent deactivates the triazine ring, ensuring that the second chlorine atomremains unreacted. Aliphatic amino groups are more powerful nucleophiles than cellulosehydroxy groups, so that an efficient substitution reaction is still possible during subsequentdyeing with an aminoalkylated dye (Scheme 7.68). After a cold water wash, dyeings werecarried out on the modified cotton fabric without salt at pH 9, using the monofunctionalaminoalkylated derivative of CI Reactive Red 58 and a bifunctional analogue (7.129)synthesised by reacting one mole of CI Reactive Red 120 (7.48; X = Cl) with two moles ofethylenediamine. Good substantivity and fixation were achieved on cotton treated with5.4% o.w.f. of the cationic reactant, particularly using the bifunctional nucleophilic dye[141].

Nucleophilic aminoalkyl dyes of the types described above can be fixed on cotton that has

Scheme 7.65

HC

HC CH

CH

OHC

HO OH

CH2OH

periodateHC

HC CH

CH

OHC

O O

CH2OH

HC

H2C CH2

CH

OHC

NH

CH2OH

4HHC

HC CH

CH

OHC

CH2OH

HN

[dye]H2CH2C CH2CH2[dye]

NN

[dye]H2CH2C CH2CH2[dye]

2 H2NCH2CH2[dye]

Scheme 7.66


N N

N

NH Ar

NHCH2CH2NH3X

N N

N

NH Ar

NHCH2CH2NH2 + HX

_+HN[dye]

NaO3S

NaO3S

NaO3S

HN[dye]

NaO3S

NaO3S

NaO3S

chpt7(1).pmd 15/11/02, 15:32431


N

N

N

Cl

Cl

NH

SO3Na

HC

CH

NaO3S

NH

N

N

N

Cl

Cl7.126

HOCH2CH2SO2 N

N

H O

NaO3S

H2N N

SO3Na

N

SO2CH2CH2OH

7.127

N N

N Cl

Cl

CH2CH2HNN

N N

N O

Cl

CH2CH2HNN [cellulose]

Cl

Cl

pH 8.5 cellulose

+

+

_

_

7.128

Scheme 7.67

been pretreated with the trifunctional anionic reactant 2-chloro-4,6-bis(4′-sulphato-ethylsulphonylanilino)-s-triazine (7.130), which has been given the codename XLC [142].After conversion to the active bis-vinylsulphone form (XLC–VS), crosslinking of thecellulose is possible under conditions that do not excessively hydrolyse the chlorotriazinefunction. As in the case of the cationic pretreating agent 7.128, this provides a suitable sitefor attachment of the aminoalkyl dye (Scheme 7.69).

chpt7(1).pmd 15/11/02, 15:32432

433

If the size of this extended bridge grouping containing five ring systems (including Ar) iscompared with that needed by the original vinylsulphone dyes (SO2CH2CH2O in Scheme7.25) to link the dye chromogen to cellulose, one must ask whether the gain in fixation isjustifiable. Some of the active vinylsulphone and chloro substituents in reactant XLC–VSwill be lost by hydrolysis (as in Scheme 7.70).

This versatile water-soluble reactant has been evaluated in wool and nylon dyeing. Thenucleophilic aminoalkyl derivatives of orthodox aminochlorotriazine dyes behave liketraditional acid dyes on wool owing to their zwitterionic character under neutral-dyeingconditions (Scheme 7.65). Improved wet fastness can be achieved using the reactant XLC

NN

HN

NH HN

NH

NN

SO3Na

H O

NaO3S

H2NCH2CH2HN NHCH2CH2NH2

SO3Na NaO3S

SO3Na

O H

NaO3S

7.129

N N

N O

Cl

CH2CH2HNN [cellulose]

N N

N

NH Ar

NHCH2CH2NH2

N N

N

NH Ar

NHCH2CH2HN

N N

N OCH2CH2HNN [cellulose]

+

+

+

HN[dye]

O3S

O3S

O3S

HN[dye]

O3S

O3S

O3S

Scheme 7.68


chpt7(1).pmd 15/11/02, 15:32433


[143]. The effect of this additive on the conventional dyeing of wool with theanthraquinone sulphatoethylsulphone dye CI Reactive Blue 19 (7.37) was investigated.

N N

NHN NH

Cl

CH2CH2SO2 SO2CH2CH2 OSO3NaNaO3SO

N N

NHN NH

Cl

SO2 SO2 CH CH2CHCH2

[cellulose] OH HO [cellulose]

N N

NHN NH

Cl

CH2CH2SO2 SO2CH2CH2 O [cellulose]O[cellulose]

N N

N

NH Ar

NHCH2CH2NH2

N N

NHN NHCH2CH2SO2 SO2CH2CH2 O [cellulose]O[cellulose]

N N

N

NH Ar

NHCH2CH2HN

NaOH NaOH

HN[dye]

NaO3S

NaO3S

NaO3S

HN[dye]

NaO3S

NaO3S

NaO3S

7.130

+

XLC-VS

Dye-fibre bond

XLC

Scheme 7.69

H2C HC SO2 [XLC-VS]

Cl

SO2 CH CH2

[XLC-VS]

OH

HOCH2CH2SO2 SO2CH2CH2OH

NaOH3 H2O

Scheme 7.70

chpt7(1).pmd 15/11/02, 15:32434

435

Aminoacid analysis and solubility tests for crosslinking revealed that XLC participated invarious reactions leading to crosslinked wool [144].

In further work on nylon [145], this trifunctional reactant was applied simultaneouslywith various nucleophilic dyes of the aminoalkyl type (Scheme 7.71). As in the case of theBasazol system on cellulose (Scheme 7.60), the intended formation of covalent dye–fibrelinkages has to compete with side reactions, such as partial hydrolysis (Scheme 7.70), di- ortrimerisation that may lead to less than optimum fastness, or substrate crosslinking that mayadversely influence desirable fibre characteristics.

NHCH2CH2NH2 H2N [nylon]

N N

NNH HN

Cl

CH2CH2SO2 SO2CH2CH2 OSO3NaNaO3SO

Cl

N

N

N

NH

NHNHCH2CH2NHCH2CH2SO2

NHCH2CH2NHCH2CH2SO2

Cl

N

N

N

NH

NH SO2CH2CH2NH

SO2CH2CH2NH

[nylon]

[nylon]

N N

NHN NH

Cl

NHCH2CH2NHCH2CH2SO2 SO2CH2CH2NH [nylon]

NaOH NaOH

[dye]

NaO3S

NaO3S

NaO3S

[dye]

NaO3S

NaO3S

NaO3S

[dye]

NaO3S

NaO3S

NaO3S

[dye]

NaO3S

NaO3S

NaO3S

+ +

+

+

7.130

Dye dimer

Fibre crosslink

Dye-fibre bond

XLC

Scheme 7.71


More controlled and efficient fixation is possible when the reactant is applied as apretreating agent [146]. If nylon given such a pretreatment is subsequently dyed with theconventional chlorotriazine dye CI Reactive Red 3 (7.2), the substantivity and fixation ofthe latter are markedly lowered because the anionic XLC residues have reacted with N-terminal amino groups in the fibre. Treatment of the modified nylon with ammonia,however, restores some degree of dyeability. Opposite effects are observed if CI ReactiveRed 3 is reacted with ethylenediamine to form an aminoalkyl derivative (7.131). Thisnucleophilic dye exhibits a high degree of fixation only on the modified nylon that has beenpretreated with XLC.

chpt7(1).pmd 15/11/02, 15:32435


In more recent work by the same authors [147], a monofunctional aminoalkyl dyeshowed high exhaustion on nylon under mildly acidic conditions. Aftertreatment of thisdyeing with XLC–VS, the active bis-vinylsulphone form of the precursor 7.130, resulted in asignificant degree of covalent bonding of the dye to nylon. This approach of using acrosslinking reactant to fix a nucleophilic dye showing orthodox dyeing behaviour on nylonwas also applied to a specially prepared cationic dye. This contained a vinylsulphonyl groupand was converted to a nucleophilic aminoethylsulphonyl derivative by reaction withammonia (Scheme 7.72). This was readily absorbed on nylon C-terminal sites at pH valuesabove 10. The anionic crosslinking agent XLC was subsequently applied to provide bridginglinks between these dye molecules and N-terminal amino groups in nylon. The weak link inthis system (7.132) is the electrostatic bond between the cationic dye and the C-terminalcarboxylate group. Any dye di- or trimers formed with the same trifunctional XLC moleculewill be held to the nylon only by virtue of this electrostatic attraction and will be deficient inwet fastness. Partial premature hydrolysis (Scheme 7.70) of the crosslinking agent thatreacts with single dye molecules may detract from the colour yield or wet fastness properties.

Interest in Fixing Agent P (7.123) has been revived in the context of nucleophilicaminoalkyl dyes of the 7.129 and 7.131 types. Nylon pretreated with this symmetricaltrifunctional reactant contains residual acryloyl sites that will undergo an addition reactionwith nucleophilic dye molecules [145]. Although in theory each N-terminal amino group inthe fibre can give rise to two acryloyl sites (Scheme 7.73), crosslinking between two N-

NN

SO3Na

H O

NaO3S

SO3Na

HN

N

N

N

H2NCH2CH2NH

NH

7.131

R3N [dye] SO2CH CH2 R3N [dye] SO2CH2CH2NH2+ NH3

+ +

Scheme 7.72

N N

NHN NH

Cl

SO2CH2CH2NHCH2CH2SO2 SO2CH2CH2NH

[nylon]

[dye]R3NCOO

[nylon]

_ +

C-terminalgroup Dye-fibre bond

N-terminalgroup

7.132

chpt7(1).pmd 15/11/02, 15:32436

437

terminal groups, with or without subsequent dye fixation, will increase the proportion oftrifunctional reactant required. Partial premature hydrolysis (Scheme 7.61) will have asimilar effect on consumption of the agent.

On wool, dyeings of the bis(aminoethyliminotriazine) dye 7.129 and of the correspondingaminoalkyl derivative of CI Reactive Red 58 were carried out at the boil and pH 5 for 60minutes. A dispersion of the symmetrical reactant 7.123 was then added and dyeingcontinued for 30 minutes before adjusting to pH 7 to complete the fixation stage in a further30 minutes. Compared with a control dyeing of the bis(aminochlorotriazine) analogue CIReactive Red 120 (7.48; X = Cl) for the same total time at the boil and pH 5, the newapproach offers deferred fixation and hence improved levelness. The wet fastnessperformance was significantly enhanced but light fastness was adversely affected [148].

Reactant 7.123 can be prepared by reacting acrylonitrile with s-trioxane (Scheme 7.74)using an acid catalyst in carbon tetrachloride as solvent [147]. Two commercially availablebifunctional reactants, N,N′-bis(acryloyl)methylenediamine (7.133) and its water-solublebis-quaternary precursor (7.134), have been evaluated to achieve dyeings of similar qualityat lower cost. The precursor is readily converted to the active reactant at about pH 8 during

CON

N

N

C

C

CH CH2

O

HC

O

HC

H2C

H2C

COCH2CH2NHN

N

N

C

C

O

HC

O

HC

H2C

H2C

COCH2CH2NHN

N

N

C

C

O

NHCH2CH2NHCH2CH2

O

NHCH2CH2NHCH2CH2

[nylon]H2N

[nylon]

NHCH2CH2NH2

[nylon]

+

7.123

2

+

[dye]

NaO3S

NaO3S

NaO3S

[dye]

NaO3S

NaO3S

NaO3S

[dye]

NaO3S

NaO3S

NaO3S

Scheme 7.73


chpt7(1).pmd 15/11/02, 15:32437


the dyeing process (Scheme 7.75). Monofunctional (7.131) and bifunctional (7.129)nucleophilic dyes were applied to wool by variants of the process already established usingreactant 7.123. The reactivity of agent 7.133 was lower (Scheme 7.76), however, so thatfixation was higher when it was applied at the start of the process. The bis-quaternaryprecursor cannot be used in this way because of its cationic character. Partial hydrolysis ofthe active reactant (Scheme 7.61) is unavoidable. The light fastness of dyeings using the bis-quaternary precursor was marginally lower and the aminoalkyl derivatives of cellulose-substantive chlorotriazine dyes tended to stain adjacent cotton in tests of fastness towashing and perspiration [149].

The alicyclic tertiary base hexamethylenetetramine (7.135) hydrolyses at the boil torelease the simple crosslinking agent formaldehyde (Scheme 7.77). This has been evaluatedin wool dyeing by applying the aminoethyl analogue of CI Reactive Red 3 (7.131) andhexamethylenetetramine simultaneously at the boil and pH 5. Covalent dye–fibre fixationdid not commence until sufficient formaldehyde had been released at the boil, so the dyewas able to migrate during the heating-up phase and good level dyeing was achieved [150].

Recent trends in unorthodox reactive dyeing systems, including Schemes 7.63–7.76generally employing nucleophilic aminoethyliminotriazine dyes prepared as outlined in

O

H2C

O

CH2

O

CH2

CH CNH2C H2C CH C

O

N CH2

N

N

NC C

C

CH

O

HC

O

CHO

CH2

CH2

H2C

CCl4

+ 3acid

3

7.123

s-Trioxane

Acrylonitrile

Scheme 7.74

R3N CH2CH2CONHCH2NHCOCH2CH2 NR3

H2C HC CONHCH2NHCO CH CH2

X X

pH8 2 NaOH

7.133

+ 2 NR3 + 2 NaX + 2 H2O

+ +

_ _

7.134

Scheme 7.75

chpt7(1).pmd 15/11/02, 15:32438

439

Scheme 7.62 together with a variety of reactive intermediates or crosslinking agentsintended to link together the dye and the substrate, raise some important questions. Thisresearch has encompassed variants of such systems on cotton, wool and nylon, mainly byexhaust application. Conventional reactive dyes have so far accounted for only about 20% ofall dyes used worldwide on cellulosic fibres. Reactive dyes amount to only about 5% of alldyes used on wool and their current usage on nylon is negligible.

The special nucleophilic dyes designed for these novel systems are derived from cellulose-substantive chromogens and below pH 8 their aminoalkyl groups impart a zwitterioniccharacter (Scheme 7.65), especially to the bifunctional type (7.129). Like direct dyes withboth amino and sulpho substituents (section 3.1.2), aggregation problems can be foreseen.Although these novel dyes are free from problems of hydrolytic deactivation, the risk ofpremature hydrolysis leading to impaired fixation has been transferred to the activated fibre(Scheme 7.67) or the crosslinking reactant (7.130) essential for their application.Pretreatment of the fibre with a colourless reactant must be exceptionally reproducible, toensure that the subsequent exhaustion, levelling and fixation of the dyes remains acceptablyconsistent. If penetration of the pretreating agent or crosslinking reactant into the fibre ispoor, ring dyeing will occur and the colour yield and fastness performance will be adverselyaffected.

NHCH2CH2NH2 H2C HC CONHCH2NHCO CH CH2 H2N [wool]

HS [wool]

NHCH2CH2NHCH2CH2CONH

NHCH2CH2NHCH2CH2CONH

CH2 H2C

HN

HN COCH2CH2

COCH2CH2

NH [wool]

S [wool]

NHCH2CH2NHCH2CH2CONHCH2NHCOCH2CH2 NH [wool]

[dye]

NaO3S

NaO3S

NaO3S

[dye]

NaO3S

NaO3S

NaO3S

[dye]

NaO3S

NaO3S

NaO3S

[dye]

NaO3S

NaO3S

NaO3S

+

++

Dye dimer Fibre crosslink

Dye-fibre bondScheme 7.76

H2C

N

CH2

N

CH2

N

CH2

N

CH2

H2C

+ 6 H2O 6 HCHO + 4 NH3

7.135Scheme 7.77


chpt7(1).pmd 15/11/02, 15:32439


The high substantivity for cellulose of these nucleophilic dyes inevitably ensures thatstaining of adjacent cotton is unacceptable in wet fastness tests when the novel systems areevaluated on wool or nylon [149]. For the control dyeing representing target behaviour onwool a conventional α-bromoacrylamide dye of similar hue, such as CI Reactive Red 66,should be used. The monochlorotriazine dye from which the aminoethyliminotriazinederivative is prepared is not a valid control [148,149], because such dyes have never givensatisfactory results on wool. Recent developments with conventional reactive dyes for woolhave highlighted the design of low-cost, dull chromogens to compete more effectively withchrome dyes (section 7.7). It is not easy to see how such targets can be reconciled withthese relatively elaborate novel systems.

The problems of dyeing nylon with reactive dyes are rather different (section 7.9),although the demand to compete with low-cost, non-reactive alternatives is even greater.Normal nylon contains only a limited number (ca. 40 milliequiv./kg) of accessible primaryamine end groups available for covalent fixation of reactive dyes or crosslinking reactants. Itis not normally difficult to ensure that virtually all of these react with a conventionalmonofunctional reactive dye. What is much more challenging is to design a compatiblerange of dyes to cover the entire colour gamut without unacceptable blocking effects inmixture recipes. In full depths the demand for accessible sites of reaction in normal nylongreatly exceeds their supply. The search for a viable reactive dyeing system in this area willprobably have to involve an ultra-deep dyeing variant (>80 milliequiv./kg) as the preferredsubstrate, although this would be more costly than normal nylon and would exhibit less thanideal physical properties.

It will be interesting to see whether such novel reactive dyeing systems will becomecompetitive alongside the long-established orthodox monofunctional ranges (section 7.3)and still-proliferating bifunctional structures (section 7.4) that have made gradual if notspectacular inroads into the textile sectors traditionally dyed with various ranges of non-reactive dyes. Safe handling of any necessary crosslinking reactants in substance or as anactive component of the textile substrate will have to be given careful consideration. Eachunusual intermediate required in the synthesis of a novel colorant or co-reactant molecule,as well as every possible side reaction or degradation pathway leading from the dyed textileand its associated waste waters, can no longer be introduced without posing searchingquestions of health and safety (section 1.7).

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REFERENCES

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Colorants and Auxiliaries Vol 1