Aramid Fibers From Fiber Chemistry 3rd

13 Aramid Fibers

� 2006 by Taylor & Francis Group

Vlodek Gabara, Jon D. Hartzler, Kiu-Seung Lee,David J. Rodini, and H.H. Yang

CONTENTS

13.1 Introduction .............................................................................................................976

13.1.1 Historical Perspective .................................................................................. 976

13.1.2 Aramid—Definition .................................................................................... 977

13.1.3 Examples of Compositions.......................................................................... 977

13.2 Aramid Products: Forms and Properties ................................................................. 977

13.3 Producers of Aramid Products.................................................................................978

13.4 Structure–Property Relationship..............................................................................979

13.4.1 Fine Structure..............................................................................................979

13.4.2 Thermal Properties ......................................................................................980

13.4.3 Solubility and Chemical Properties .............................................................981

13.4.4 Fiber Mechanical Properties .......................................................................981

13.4.5 Films and Papers......................................................................................... 984

13.5 Polymerization of Aromatic Polyamides..................................................................985

13.5.1 Introduction ................................................................................................985

13.5.2 Synthesis of Ingredients............................................................................... 986

13.5.2.1 m-Phenylene Diamine ..................................................................986

13.5.2.2 p-Phenylene Diamine ................................................................... 987

13.5.2.3 3,4’-Diaminodiphenyl Ether ......................................................... 988

13.5.2.4 Diacid Chlorides .......................................................................... 988

13.5.3 Polymerization Fundamentals..................................................................... 989

13.5.3.1 Reaction Mechanism ................................................................... 990

13.5.3.2 Reaction Energetics ..................................................................... 991

13.5.4 Direct Polymerization by Catalysis. ............................................................ 991

13.5.5 Polymerization Methods ............................................................................. 993

13.5.5.1 Interfacial Polymerization............................................................ 993

13.5.5.2 Solution Polymerization...............................................................995

13.5.5.3 Vapor-Phase Polymerization........................................................999

13.5.5.4 Plasticized Melt Polymerization................................................. 1000

13.6 Aramid Solutions ................................................................................................... 1001

13.6.1 Isotropic Solutions. ................................................................................... 1001

13.6.1.1 m-Aramid Solutions................................................................... 1001

13.6.1.2 p-Aramid Solutions. ................................................................... 1001

13.6.2 Anisotropic Solutions................................................................................ 1002

13.6.2.1 Phase Behavior........................................................................... 1002

13.6.2.2 Rheological Properties ............................................................... 1003

, LLC.

13.7 Preparation of Aramid Products............................................................................ 1003

13.7.1 Fibers......................................................................................................... 1003

13.7.1.1 Dry Spinning.............................................................................. 1003

13.7.1.2 Wet Spinning ............................................................................. 1005

13.7.1.3 Dry-Jet Wet-Spinning ................................................................ 1006

13.7.2 Film ........................................................................................................... 1009

13.7.3 Fibrids ....................................................................................................... 1010

13.7.4 Pulp ........................................................................................................... 1011

13.8 Applications ........................................................................................................... 1012

13.8.1 m-Aramid Fiber......................................................................................... 1013

13.8.1.1 Protective Apparel ..................................................................... 1013

13.8.1.2 Thermal and Flame-Resistant Barriers ...................................... 1014

13.8.1.3 Elastomer Reinforcement........................................................... 1015

13.8.1.4 Filtration and Felts .................................................................... 1015

13.8.2 m-Aramid Paper ........................................................................................ 1015

13.8.2.1 Electrical .................................................................................... 1015

13.8.2.2 Core Structures .......................................................................... 1016

13.8.2.3 Miscellaneous............................................................................. 1017

13.8.3 p-Aramid Fiber.......................................................................................... 1017

13.8.3.1 Armor ........................................................................................ 1017

13.8.3.2 Protective Apparel ..................................................................... 1018

13.8.3.3 Tires and Mechanical Rubber Goods ........................................ 1018

13.8.3.4 Composites................................................................................. 1019

13.8.3.5 Optical and Electromechanical Cables....................................... 1019

13.8.3.6 Ropes and Cables ...................................................................... 1020

13.8.3.7 Reinforced Thermoplastic Pipe.................................................. 1020

13.8.3.8 Civil Engineering........................................................................ 1021

13.8.4 p-Aramid Paper ......................................................................................... 1021

13.8.4.1 Core Structures .......................................................................... 1021

13.8.4.2 Printed Wiring Boards ............................................................... 1022

13.8.4.3 p-Aramid Pulp ........................................................................... 1022

13.9 Conclusions and Direction..................................................................................... 1024

References ........................................................................................................................ 1025

13.1 INTRODUCTION

13.1.1 HISTORICAL PERSPECTIVE

Development of aromatic polyamides had a very unique beginning. Its origin in an industrial

corporation (DuPont) led to a combination of fundamental science, engineering, and ap-

plications research from the very early stages of the development. In 1948, with the

commercialization of nylon fiber and the near-development of a polyester fiber, the manage-

ment of the DuPont Technical Division launched very broad, long-range research programs

with goals, among others, of developing very high-strength fibers and high-temperature-

resistant fibers.

The first phase covered a decade from the early 1950s to the early 1960s. Clearly, materials

with unusual properties are not easy to process and they would not have been possible without

the development of low-temperature solution polymerization techniques by Paul Morgan’s

� 2006 by Taylor & Francis Group, LLC.

group at DuPont [1]. The next critical step was to understand factors governing solubility of

these difficult to dissolve polymers. Beste and Stephens [2] elucidated the role of certain salts

that help in obtaining good solutions of these polymers. This work culminated in the

commercialization of Nomex, the first high-temperature-resistant, m-aramid fiber [3,4].

Starting in the early 1960s the work focused on new fibers with a performance superior

to Nomex, and p-aramids became a logical choice. Stephanie Kwolek focused her initial

work on the more tractable poly(1,4-benzamide) polymer and produced, in the mid-1960s, a

fiber with a spectacular modulus of 400 gpd. After additional work, a yarn with 7.0 gpd

tenacity and an unheard of modulus of 900 gpd was prepared. This fiber was known as fiber

B. Subsequent work shifted to poly( p-phenylene terephthalamide) (PPTA). After signifi-

cant effort by many in both polymerization and spinning areas, Herb Blades made a

processing breakthrough by focusing on the air-gap spinning of concentrated solutions of

high-molecular-weight PPTA polymer. The first PPTA fibers were produced by this process

in early 1970, and by 1972 Kevlara was introduced to the market place. This was clearly a

significant achievement considering the novelty and complexity of the technology involved

and the speed at which it was accomplished. In addition to the impressive blend of science and

engineering required to commercialize Kevlar, this was also the first demonstration of fiber

mechanical properties predicted by theoretical considerations developed as early as the mid-

1930s. This provided a fundamental basis as well as an impetus to study and commercialize

other materials with comparable properties.

13.1.2 ARAMID—DEFINITION

As alluded to in the introduction, properties of aromatic polyamides differ significantly from

those of their aliphatic counterparts. This led the U.S. Federal Trade Commission to adopt

the term ‘‘aramid’’ to designate fibers of the aromatic polyamide type in which at least 85% of

the amide linkages are attached directly to two aromatic rings.

13.1.3 EXAMPLES OF COMPOSITIONS

The superior properties of these materials were the reason why significant research effort

has been devoted to this chemistry. Yang [5] showed at least 100 different compositions

in this area and that number has doubled during the past 15 years since Yang’s book was

published.

The early work by Sweeny, Kwolek, and others demonstrated that progress in this

area of technology was the result of a constant trade-off between properties and processa-

bility. This is very likely the reason why after half-a-century of research only four compo-

sitions have reached commercial stage: poly(m-phenylene isophthalamide) (MPDI), PPTA,

copoly(p-phenylene=3,4’-diphenyl ether terephthalamide) (ODA=PPTA), poly[5-amino-

2-(p-aminophenyl)benzimidazole terephthalamide] (SVM), and its copolymers.

13.2 ARAMID PRODUCTS: FORMS AND PROPERTIES

The outstanding thermal and mechanical properties that can be derived from these

compositions led to the exploration, as well as commercial realization, of various product

forms. Currently these product forms include fibers, fibrids and pulps, films, papers,

and particles.

aKevlar—a registered trademark of E.I. DuPont de Nemours & Co., Inc., Wilmington, Delaware, USA.


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poly (p-phenylene terephthalamide) PPTA

copoly(p-phenylene/3,4'-diphenyl ether terephthalamide) ODA/PPTA

poly (m-phenylene isophthalamide) MPDI

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poly [5-amino-2-(p -aminophenyl)benzimidazole terephthalamide]–SVM

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The largest commercial volume of these materials is in the form of fibers. Continuous filament

yarns are preferred where very high mechanical properties are required and staple fiber is used

for textile applications. The significant volumes involved in these applications led to the

development of special spinning processes designed to produce these forms.

The excellent thermal properties of these materials led to high volume applications where

these materials were used as binders or as short fiber reinforcing agents. This required the

development of both fibrids and pulps. This chapter discusses both the processes of formation

as well as the principles of applications of these forms.

Various nonwoven structures have been developed as well. The least important among

sheet structures are films. There are two film products (see Section 13.3) based on p-aramids

and none on m-aramids. The significant cost differential is the most likely reason for this

situation. On the other hand a very large market has been developed for papers based on both

p-aramids and m-aramids. In general, these papers are based on short fibers (floc) and a

binder (fibrids), but other components have been explored as well. A very small market exists

for particles other than fibrids and pulps.

13.3 PRODUCERS OF ARAMID PRODUCTS

The basic development and the first commercial introduction of these materials were done by

DuPont, which continues to be the largest producer. m-Aramid fiber products (staple,


continuous filament yarn, and floc) with the trademark Nomexb are produced by DuPont in

the United States as well as Spain. The paper products come from the U.S. plant as well as

from a facility in Japan. The only other major m-aramid producer is Teijin, with its fiber

product Teijinconexc produced in Japan.

The situation is very similar on the para side of chemistry. The first and the largest

producer—DuPont—has three facilities throughout the world. The largest one in the United

States produces essentially all product forms except films. Fiber is also produced in Ireland

and Japan. The other producer of p-aramids is Teijin Co., which produces two basic fibers:

Twarond based on PPTA and Technorac based on a copolymer. Twaron is produced in the

Netherlands while Technora is manufactured in Japan.

A small amount of p-aramid fiber (Armos and Rusar) is produced in Russia. Both are

copolymers based on diaminophenylbenzimidazole—a unique but expensive monomer.

There are two producers of p-aramid film. The first one was Toray with its Mictrone

film based on a copolymer and Asahi with a product (Aramicaf) based on PPTA homo-

polymer.

13.4 STRUCTURE–PROPERTY RELATIONSHIP

13.4.1 FINE STRUCTURE

In general aramid homopolymers crystallize with relative ease. PPTA is a highly crystalline

material. Two structures have been identified for this polymer: the first was proposed by

Northolt [6] and the second by Haraguchi [7]. Haraguchi [7] and Roche [8] proposed

mechanisms for their formation. In both cases they proposed an interaction between the

solution and the coagulation process. Roche proposed that to form the Haraguchi struc-

ture, PPTA solution has to crystallize into a crystal solvate [9] prior to the removal of

sulfuric acid. After acid removal and drying the Haraguchi polymorph is formed. This is

the less stable form and at an elevated temperature rearranges into the Northolt form.

Coagulation of PPTA solution leads to the Northolt structure, according to Roche, and

that is why all commercial fibers exhibit essentially the Northolt structure. Northolt [6] and

later Tashiro [10] reported their estimates of the size of the orthorhombic unit cell. The

values are listed in Table 13.1. Commercial fibers based on PPTA are highly crystalline.

Estimates of the degree of crystallinity of Kevlar 29 are 68 to 85% and as high as 95% for

Kevlar 49 [11,12].

In addition to crystallinity, PPTA fibers exhibit a larger scale organization. It has been

proposed that PPTA fibers have an unusual radial orientation of pleated hydrogen-bonded

sheets [13]. This unique morphology has a significant impact on the mechanical properties of

the fibers.

MPDI has a triclinic unit cell and is significantly less crystalline than PPTA (Table 13.1).

Savinov [14] proposed that crystallinity depends on the conditions of polymer precipitation

from solution. Precipitation of polymer in water leads to a noncrystalline material while

precipitation in water containing some solvent leads to a crystalline form. Krasnov [15]

showed that increased fiber orientation leads to higher crystallinity. SVM, the Russian

bNomex—a registered trademark of E.I. DuPont de Nemours & Co., Inc., Wilmington, Delaware, USA.cTeijinconex, Technora—registered trademarks of Teijin, Ltd., Japan.dTwaron—a registered trademark of Akzo Nobel, The Netherlands.eMictron—a registered trademark of Toray Co., Japan.fAramica—a registered trademark of Asahi Co., Japan.


TABLE 13.1Crystallinity of Homopolymers

PPTA MPDI

Crystal system Orthorhombic Triclinic

Lattice constant

a (A) 7.80 5.27

b (A) 5.19 5.25

c (A) 12.9 11.3

a (degree) 111.5

b (degree) 111.4

g (degree) 90 88.0

Number of chains in a unit cell 2 1

Density (g=cm3)

Calculated 1.50 1.45

Observed 1.43–1.45 1.38

Source: From Northolt, M.G.; Eur. Polym. J., 10, 799, 1974; Haraguchi, K.,

Kajiyama, T., and Takayanagi, M.J., J. Appl. Polym. Sci., 23, 915, 1979;

Roche, E.J., Allen, S.R., Gabara, V., and Cox, B., Polymer, 30, 1776, 1989;

Gardner, K.H., Matheson, R.R., Avakin, P., Chia, Y.T., and Gierke, T.D.,

Polym. Prepr. (Am. Chem. Soc. Div. Polym. Chem.), 24(2), 469, 1983;Tashiro,

K., Kobayashi, M., and Tadokoro, H., Macromolecules, 10(2), 413, 1977.

product based on poly[5-amino-2-(p-aminophenyl) benzimidazoleterephthalamide] is the only

other commercial product based on a homopolymer. This material is noncrystalline, as might

be expected, based on the structural irregularities that can arise from the orientation of repeat

units in the polymer chain (cis–trans, head–tail).

Copolymers are noncrystalline materials. Blackwell has studied the fine structure of

Technora fiber [16].

13.4.2 THERMAL PROPERTIES

The search for materials with very good thermal properties was the original reason for

research into aromatic polyamides. Bond dissociation energies of C��C and C��N bonds

in aromatic polyamides are ~20% higher than those in aliphatic polyamides. This is the reason

why the decomposition temperature of MPDI exceeds 4508C [17]. Conjugation between

the amide group and the aromatic ring in PPTA increases chain rigidity as well as the

decomposition temperature, which exceeds 5508C [17,18].

Obviously, hydrogen bonding and chain rigidity of these polymers translates to very high

glass transition temperatures. Using low-molecular-weight polymers, Aharoni [19] measured

glass transition temperatures of 2728C for MPDI and over 2958C for PPTA (which in this

case had low crystallinity). Others have reported values as high as 4928C [20]. In most cases

the measurement of Tg is difficult because PPTA is essentially 100% crystalline. As one would

expect, these values are not strongly dependent on the molecular weight of the polymer above

a DP of ~10 [21].

We have discussed above the crystalline nature of most of the fibers based on homo-

polymers. While information on melting of the crystalline phase of these polymers differs, all

quoted melt temperatures are very high. For MPDI most values are similar to 4358C as


determined by Takatsuka [17]. On the other hand, most authors report the decomposition

temperature of PPTA to be lower than its melting point [17]. Chaudhuri [18] reported a value

of 530 8C. Table 13.2 summarizes some of the thermal properties of commercial aramid fibers

[22,140–146].

The almost perfect orientation of p-aramid fibers is reflected in the anisotropic behavior of

its thermal expansion coefficient. The linear expansion coefficient for these materials is

negative (Table 13.2). Because the volumetric thermal expansion coefficient is not affected

by orientation, the radial coefficient must increase as fiber orientation increases. The negative

expansion coefficient of these materials has opened a whole field of applications in electronics

(see section 13.8.4.2).

13.4.3 SOLUBILITY AND C HEMICAL P ROPERTIES

The same structural characteristics that are responsible for the excellent thermal properties of

these materials are responsible for their limited solubility as well as good chemical resistance.

PPTA is soluble only in strong acids like H2SO4, HF, and methanesulfonic acid. Preparation

of this polymer via solution polymerization in amide solvents is accompanied by polymer

precipitation. As expected, based on its structure, MPDI is easier to solubilize then PPTA. It

is soluble in neat amide solvents like N-methyl-2-pyrrolidone (NMP) and dimethylacetamide

(DMAc), but adding salts like CaCl2 or LiCl significantly enhances its solubility.

The significant rigidity of the PPTA chain (as discussed above) leads to the formation of

anisotropic solutions when the solvent is good enough to reach a critical minimum solids

concentration. The implications of this are discussed in greater detail later in this chapter.

It is well known that chemical properties differ significantly between crystalline

and noncrystalline materials of the same composition. In general, aramids have very good

chemical resistance as shown in Table 13.3. Obviously, the amide bond is subject to a

hydrolytic attack by acids and bases. Exposure to very strong oxidizing agents results in

a significant strength loss of these fibers. In addition to crystallinity, structure consolidation

affects the rate of degradation of these materials.

The hydrophilicity of the amide group leads to a significant absorption of water by all

aramids. While the chemistry is the driving factor, fiber structure also plays a very important

role; for example, Kevlar 29 absorbs ~7% water, Kevlar 49 ~4%, and Kevlar 149 only 1%.

Fukuda explored the relationship between fiber crystallinity and equilibrium moisture in

great detail [23].

Because of their aromatic character, aramids absorb UV light, which results in an

oxidative color change. Substantial exposure can lead to the loss of yarn tensile properties

[24]. UV absorption by p-aramids is more pronounced than with m-aramids. In this case a

self-screening phenomenon is observed, which makes thin structures more susceptible to

degradation than thick ones. Very frequently p-aramids are covered with another material

in the final application to protect them.

The high degree of aromaticity of these materials also provides significant flame resist-

ance. All commercial aramids have a limited oxygen index in the range of 28–32%, which

compares with ~20% for aliphatic polyamides (Table 13.2). The utilization of these properties

is discussed in greater detail in the Applications section of this chapter.

13.4.4 FIBER MECHANICAL P ROPERTIES

Typical properties of commercial aramid fibers are given in Table 13.4. While yarns of m-

aramids have tensile properties that are no greater than those of aliphatic polyamides, they do

retain useful mechanical properties at significantly higher temperatures. The high glass


TABLE 13.2Thermal Properties of Aramid Fibers

Trade name Nomex Teijinconex Kevlar Twaron

polymer MPDI MPDI PPTA PPTA

Fiber type 430 Std HT K-29 K-49 Std HM

Technora

ODA=PPTA

Property

Specific heat (J=kg-K) 72 60 60 81 81 81 81 96

Thermal

conductivity (W=m-K) 0.25 0.11 0.11 2.5 2.5 — — 0.5

Coefficient of thermal

expansion (cm=cm-8C) 1.8�10�5 1.5�10�5 1.5�10�5 �4.0�10�6 �4.9�10�6 �3.5�10�6 �3.5�10�6 �6�10�6

Heat of

combustion (J=kg) 28�106 — — 35�106 35�106 — — —

Flammability

LOI (%) 28 29—32 29—32 29 29 29 29 —

Decomposition (in N2)

Temperature (8C) 400–420 400–430 400–430 520–540 520–540 520–540 520–540 500

Source: From DuPont Technical Guide for Kevlar Aramid Fiber, H-77848, 4=00; DuPont Technical Guide for Nomex Brand Aramid Fiber, H-52720, 7=01; Teijin Ltd.,

Teijinconex Heat Resistant Aramids Fiber 02.05; Teijin Ltd., High Tenacity Aramids Fibre: Technora TIE-05=87.5; Akzo Nobel, Twaron—Product Information:Yarns, Fibers

and Pulp.

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TABLE 13.3Chemical Resistance of Aramid Fibers

Trade name Nomex Kevlar Technora

Polymer MPDI PPTA ODA=PPTA

Chemical Time (h)=temp. (8C) Percent Strength Retention

40% H2SO4 100=95 90

10% H2SO4 100=21 90–100 90–100

10% H2SO4 1000=21 95 35*

10% HCl 1000=21 20–60 35 10*

10% HNO3 100=21 60–80 90–100 20–60

10% NaOH 100=95 75

10% NaOH 1000=21 90–100 90 46

40% NaOH 1000=21 80–90 76

28% NH4OH 1000=21 90–100 90–100 65*

0.01% NaClO 1000=21 90–100 16

10% NaClO 100=95 55

0.4% H2O2 1000=21 90–100 56–75

10% NaCl 1000=21 90–100 100*

100% Acetic acid 1000=21 90–100 90*

90% Formic acid 100=21 90–100 90–100 90–100

90% Formic acid 100=99 60–80 90–100 0–20

100% Acetone 1000=21 90–100 90–100

100% Acetone 100=56 80–90 90–100

100% Benzene 1000=21 90–100 90–100 100

100% Ethyl alcohol 1000=21 90–100 90–100 100

100% Ethyl alcohol 100=77 90–100 90–100

50% Ethylene glycol 1000=99 80–90 90–100 60–80

100% Gasoline 1000=21 90–100 90–100 90–100

100% Methyl alcohol 1000=21 90–100 90–100 90–100

100% Perchloroethylene 10=99 90–100 90–100

100% Tetrahydrofuran 1000=21 90–100

*Measurements made after 3 months (2200 h) exposure at room temperature.

Source: From DuPont Technical Guide for Kevlar Aramid Fiber, H-77848, 4=00; DuPont Technical Guide for

Nomex Brand Aramid Fiber, H-52720, 7=01; Teijin Ltd., High Tenacity Aramids Fibre: Technora TIE-05=87.5.

TABLE 13.4Properties of Aramid Fibers

Trade name Nomex Teijinconex Kevlar Twaron

Polymer MPDI MPDI PPTA PPTA

Fiber type 430 std HT K-29 K-49 std HM

Technora

ODA=PPTA

Density (g=cm3) 1.38 1.38 1.38 1.44 1.44 1.44 1.45 1.39

Strength (Gpa) 0.59 0.61–0.68 0.73–0.86 2.9 3.0 2.9 2.9 3.4

Elongation (%) 31 35–45 20–30 3.6 2.4 3.6 2.5 4.6

Modulus (Gpa) 11.5 7.9–9.8 11.6–12.1 71 112 70 110 72


transition temperature leads to low (less than 1%) shrinkage at temperatures below 2508C. In

general, mechanical properties of m-aramid fibers are developed on drawing (see below). This

process produces fibers with a high degree of morphological homogeneity, which leads to very

good fatigue properties.

The mechanical properties of p-aramid fibers have been the subject of much study. This is

because these fibers were the first examples of organic materials with a very high level of both

strength and stiffness. These materials are practical confirmation that nearly perfect orienta-

tion and full chain extension are required to achieve mechanical properties approaching those

predicted for chemical bonds. In general, the mechanical properties reflect a significant

anisotropy of these fibers—covalent bonds in the direction of the fiber axis with hydrogen

bonding and van der Waals forces in the lateral direction.

Termonia has proposed a kinetic model for fiber strength [25–27]. His calculations suggest

that molecular mass, its distribution, and intermolecular forces control fiber strength. Allen’s

work linked the failure mode of these fibers with their morphology very closely [16, 28–30].

He was able to show that fiber pleating is responsible for the fact that one needs to consider

the asymptotic modulus (modulus close to the fiber breaking point) of these fibers rather than

the initial modulus to explain mechanical properties. This interpretation confirmed a clear

dependence of fiber strength on both local orientation (as measured by the asymptotic

modulus) and secondary interactions (as measured by shear properties).

The use of p-aramids in composites has focused much research effort on the compressive

properties of these fibers. Excellent tensile properties, approaching 80% of the theoretical

modulus, and 30% of the theoretical strength are not matched by their compressive proper-

ties. PPTA fiber yields under compression at ~0.5% of strain. This is caused by a buckling

phenomenon that is attributed to the relatively weak lateral properties of these highly

anisotropic fibers. However, aramids with their hydrogen bonding have significantly better

compressive strength than UHMWPE, which has extremely weak lateral properties. Allen

[31] measured compressive strength by a recoil test and obtained 258N=tex for Kevlar 49

compared to 7.5 N=tex for UHMWPE. Aramids also compare well with PBO, which has a

compressive strength of 0.133 N=tex. All high strength organic fibers yield under compressive

stress with formation of kink bands. However this, significant dislocation does not lead to

major tensile strength loss. At a strain of 3% the loss is only ~10%.

This high degree of anisotropy of the p-aramids is reflected in fatigue properties. Tension–

tension fatigue is very good. Wilfong [32] reported no failure after 107 cycles with loads at

60% of breaking strength. Compressive fatigue is not as good—especially at higher strains. At

a strain of 0.5% no strength loss is observed even after 106 cycles but at a strain of 1% the

strength loss begins at about 103 cycles [33].

Creep (long-term failure of fibers at loads below their breaking strength) is the final

mechanical property for review. The kinetic model of fiber failure was applied by Termonia

[25] to estimate creep behavior. His calculations suggest that the activation energy of covalent

bond breaking controls the lifetime of materials. That is why UHMWPE fails after 2.5 min

when strained to 50% of its breaking strain (measured at 1 sec). PPTA under the same

conditions fails after 100 years. Lafitte [34] measured creep strain for Kevlar 29 at a load of

50% of its breaking strength and found a strain of 0.3% after 107 sec.

13.4.5 FILMS AND PAPERS

Although the primary focus of this chapter is on fibers, we have included some illustrations of

sheet products based on this chemistry.

There are two examples of commercial p-aramid films. Toray produces a terpolymer film

under the trade name Mictron, while Asahi introduced a PPTA homopolymer film called


TABLE 13.5Properties of Aramid Films

Mictron Aramica

Producer toray asahi

Thickness (mm) 25 25

Density (g=cm3) 1.5 1.4

Mechanical properties:

Direction machine cross machine cross

Tensile strength GPa 0.5 0.5 0.3

Elongation % 60 15 25

Tensile modulus GPa 13 9 19 10

Initial tear strength Kg — 25

Long-term heat resistance 8C 180 ~200

Thermal expansion (1=8C � 10�5) 0.1 0.2

Moisture absorption

At 75% RH and

At room temp. % 1.5 2.8

Electrical properties:

Dielectric constant at 1KHz — 4

Dissipation factor at 1 KHz — 0.02

Volume resistivity V=cm 5 � 1017 1 � 1016

Surface resistivity V=cm — 1 � 1016

Dielectric strength KV=mm 300 230

Source: From Yasufuku, S., IEEE Elec. Insu. Mag., 11(6), 27, 1995; Teijin Ltd., High Tenacity Aramids Fibre:

Technora TIE-05=87.5; Asahi Chemical Industry America, Inc., Technical Brochure, Aramica Film, 1991; Akzo

Nobel, Twron Product Information: Yarns, Fibers and Pulp.

Aramica. In both cases the product goal was a high strength, thin film for mass storage

devices. Film properties are shown in Table 13.5.

Aramids papers are found in a much broader range of applications than films (see

Applications section). Most papers are comprised of a composite structure of short fibers

and a binder. Paper properties can be tailored by changing the composition and the process-

ing conditions. Selected properties are illustrated in Table 13.6.

13.5 POLYMERIZATION OF AROMATIC POLYAMIDES

13.5.1 INTRODUCTION

We began this discussion with a description of the high melting point and difficult solubility

of aromatic polyamides. Very clearly these properties present a significant challenge in their

synthesis and fabrication.

First, the infusible nature of many of these polymers precludes the use of conventional

bulk polymerization and melt processing techniques. Second, aromatic diamines are signifi-

cantly less reactive than aliphatic diamines toward polyamidation. This requires the use of

more reactive dicarboxylic acid intermediates or some activation mechanism to complete the

polycondensation in a reasonable period of time. Some technological breakthroughs were

necessary to make progress in the synthesis of aromatic polyamides. These came in the late


TABLE 13.6Properties of Aramids Paper

Nomex Nomex Nomex

Polymer MPDI MPDI PPTA

Producer DuPont DuPont DuPont

Type 410 410 N710

Thickness mm 127 127 97

Density g=cm3 0.87 0.87 0.64

Mechanical properties:

Direction machine cross machine

Tensile strength GPa 0.1 0.05 0.2

Elongation % 16 13 1.5

Tensile modulus GPa — — 5.4

Initial tear strength Kg 3.3 1.6 —

Long-term heat resistance 8C ~200 ~200

Thermal expansion (1=8C � 10�5) — 0.7

Moisture absorption

At 55% RH and

At room temp. % — 1.6

Electrical properties:

Dielectric constant at 1 KHz 2.4 3.9a

Dissipation factor at 1 KHz 0.006 0.02a

Volume resistivity V=cm 5 � 1016 —

Surface resistivity V=cm 1 � 1016 —

Dielectric strength KV=mm 25 82a

aMeasurements made after three months (2200 hrs) exposure at room temperature.

Source: From E.I. DuPont de Nemours & Co., Inc, NOMEX Aramid Paper Type 410—Typical Properties, H-22368,

8=98; Magellan International; Hendren, G.L., Kirayoglu, B., Powell, D.J., and Weinhold, M., Adv. Mater., 10(15),

1233, 1998; Yasufuku, S., IEEE Elec. Insn. Mag., 11(6), 27, 1995.

1950s and the early 1960s when it was demonstrated that high molecular weight wholly

aromatic polyamides could be prepared by low-temperature interfacial [35] and solution

[36,37] polycondensation processes.

13.5.2 S YNTHESIS OF I NGREDIENTS

It was also imperative to develop synthetic routes to high purity ingredients for these

polymerizations to be successful. The syntheses of commercially important ingredients will

be described here. Only one of several alternative routes will be illustrated. It must also be

noted that the chemistry is constantly being modified to achieve less costly, more efficient and

environmentally friendly processes.

13.5. 2.1 m -Phenylen e Dia mine

The first step in m-phenylene diamine (MPD) synthesis is the nitration of benzene in 20%

oleum (Equation 13.1). The nitration is a two-stage continuous process [38] replacing two

protons on the benzene ring with two nitro groups by the catalytic action of sulfuric acid. The

m-isomer is the dominant product.


�

NO2

NO2

NO2

NO2

NO2

NO2

� �

MinorMinorMajor

H2SO4

�2 HNO3 2 H2O ð13:1Þ

The resulting isomer mixture is washed with water and ammonia, centrifuged to remove acid

and phenolic by-products and then catalytically hydrogenated [39]. MPD is isolated from the

crude diamine mixture and purified by selective distillation (Equation 13.2)

NO2

NO2 � 6 H2 NH2

NH2

Catalyst

(Pt, Pd, Fe)� 4 H2O

MPD crudeIsomeric mixtureof dinitrobenzene

ð13:2Þ

13.5.2.2 p-Phenylene Diamine

The synthesis of p-phenylene diamine (PPD) starts with air oxidation of ammonia to form

N2O3 (in equilibrium with NO and NO2) (Equation 13.3)

4 NH3 þ 6O2 ��!Pt=Ru Catalyst

1000�C2N2O3 þ 6H2O ð13:3Þ

This mixture is then reacted with four moles of aniline to produce diphenyltriazine as follows:

N2O3 � 4 NH2N N

H

� 3 H2O2 N

ð13:4Þ

Diphenyltriazine is rearranged to form a mixture of p- and o- aminoazobenzene using nitric

acid as a catalyst

N NN

H

HNO3

Rearrangement

N N

NH2

NN

H2N

�

ð13:5Þ


2

Finally, the aminoazobenzenes are hydrogenated to the corresponding phenylene diamines

[40–42]. A mole of aniline is regenerated for every mole of phenylene diamine and is recycled.

The phenylene diamine isomers are then separated, and the o-isomer is sold as an ingredient

for the production of various fungicides.

N N

NH2

N N

H2N

2H2

1 2H2

NH2 H2N

H2N

H2N

1

NH

(13.6a)

(13.6b)

NH2

1 1

13.5.2.3 3,4’-Diaminodiphenyl Ether

The synthesis of 3,4’-diaminodiphenyl ether (3,4’-POP) is more complex than that of simple

aromatic diamines such as MPD and PPD and hence this monomer is more expensive.

Condensing 1,3-dinitrobenzene with 4-aminophenol using potassium carbonate in dimethyl-

formamide (DMF) or DMAc produces 3,4’-POP. The resulting 3-nitro-4’-aminodiphenyl

ether is then hydrogenated [42].

3-nitro-4'-aminodiphenyl ether

DMF/DMAc

K2CO3

O2N

O NH2� NH2HONO2

O2N

ð13:7Þ

3,4'-diaminodiphenyl ether

H2N

O NH2H2,DMF,110�C

Pd/C

O2N

O NH2

3-nitro-4'-aminodiphenyl ether

ð13:8Þ

A mixture of 4-aminophenol, 1-3-dinitrobenzene and K2CO3 in DMF was treated at 1508Cfor 4 h to give 96.3% 3-nitro-4’-aminodiphenyl ether. This was treated with Pd on C in DMF

at 1108C and H2(3 atm) for 5 h to give 98.0% 3,4’-diaminodiphenyl ether.

13.5.2.4 Diacid Chlorides

Terephthaloyl chloride (TCl) and isophthaloyl chloride (ICl) are produced by reacting the

corresponding dicarboxylic acid with phosgene [43].


OH � 2ClDMF

Cl � 2HCl � 2CO2CCl

O

C

O

C

O

ClCHO

O

C

O

ð13:9Þ

The reaction involves formation of a catalyst complex between DMF and phosgene, which

then reacts with terephthalic acid.

"Complex"

CO2+

Cl−

NCH3

CH3

C

Cl

HCl C

O

Cl+NCH3

CH3

C

O

H ð13:10Þ

2 NCH3

CH3

C

Cl

H

"Complex"

−2HCl

2 NCH3

CH3

C

O

H+CCl

O

C

O

Cl+CHO

O

C

O

OH

Cl−

ð13:11Þ

The reaction is carried out in a slurry of TPA, DMF, and TCl with countercurrent injection of

phosgene. The product, TCl, is degassed, heated to destroy the catalyst complex, and then

distilled to remove impurities.

13.5.3 POLYMERIZATION FUNDAMENTALS

The usual methods for preparing aliphatic polyamides are not suitable for preparing high-

molecular-weight aromatic polyamides because of the reduced reactivity of aromatic dia-

mines and the high melting point of the resulting polymers. Polymerization of wholly

aromatic polyamides is usually carried out in solution, instead of in bulk, using highly reactive

diacid chlorides vs. diacids. The reaction is fast and takes place at a much lower temperature

than conventional melt polymerizations. The synthesis is based on the familiar Schotten–

Baumann reaction [44–49].

H2ONaCl ++R

O

N

R1

R2

NaOHH

R1

R2

+RC

O

C Cl N C ð13:12Þ

Condensation polymers are formed if the complementary reagents are difunctional.

N

H

R

H

C

O

R' C

O

+ 2nH2O2nNaOH

nnClnH2N + + 2nNaClC

O

R' C

O

ClNH2R N

ð13:13Þ

A large amount of salt is generated in this reaction following neutralization of the by-product

hydrochloric acid (HCl). The high salt concentration in the process stream requires the


use of expensive corrosion resistant materials—one of the key contributors to the high cost of

aramid fibers.

An alternative route to aromatic polyamides is referred to as a hydrogen transfer reaction

[50]. This reaction between a diacid and diisocyanate is run at a low temperature to form an

intermediate polymer that loses carbon dioxide on subsequent heating to form the aromatic

polyamide (Equation 13.14).

H-transfer

HeatCO22n+

nN

H

R1 N

H

C

O

R2 C

O

C

O

N

H

R1 N

OH

O

O

R2 C

O

O

n

nHOnO

n

C

O

N

H

R1 N

OH

O

O

R2 C

O

OC

O

R2 C

O

OHNC R1 N O +C C C

C C

ð13 :14 Þ

This reaction is not widely used because of the higher cost of diisocyanates and the difficulty

in eliminating all the carbon dioxide.

13.5. 3.1 Reactio n Mech anism

The first step in the condensation reaction is the attack of the amine nitrogen at the carbonyl

carbon of the dicarboxylic acid. The local electron density at the aromatic amine nitrogen is

greatly reduced by participation of the lone pair electrons with the aromatic p-cloud, whereas

the local electron density of the aliphatic counterpart is enhanced by the inductive effect of

aliphatic hydrocarbon. This leads to a significant difference in the polycondensation reaction

rate between aromatic polyamides and aliphatic polyamides.

+ Aromatic amidation

HO C

O

CH2+

Aliphatic amidation

N

H

H

CH2

N

H

H

CX

O

pi-cloud overlap

inductive effect

To compensate for reduced electron density at the amine nitrogen, the dicarboxylic acid is

activated by increasing the partial positive charge at the carbonyl carbon. Halogen atoms (X)

have proven to be effective because of their high electronegativity. An amide linkage is

formed from the transition complex (Equation 13.15) by eliminating HX (Equation 13.16).

Because the eliminated acid, HX, will react with the opposing amine to form a quater-

nary ammonium salt, it must be removed for the polymerization to continue. An organic

amine, such as pyridine, is often used as an acid acceptor to regenerate the amine end


from the quaternary salt (Equation 13.17). Polymerization solvents such as N, N-dimethyl

acetamide (DMAc) and N-methylpyrrolidone (NMP) are sufficiently basic to function as acid

acceptors as well.

Transition complex

C

O

X

C X

O

NH2N

H

H

N

O

X

H

H

H2N

O

XC C

ð13 :15 Þ

N C

OH

H2N C

O

X

X

H

N

O

X

H

H

H2N

O

X

Transition complex

C C

ð13 :16 Þ

+

X N

H

Amine regeneration

N C

OH

H2N C

O

XN

N

OH

H2N

O

X

X

H

C C

ð13 :17 Þ

Factors that can limit the extent of the polymerization reaction include deactivation of chain-

ends, stoichiometric imbalance of reagents, monofunctional impurities, and insufficient mo-

bility of growing chain-ends. Some of these factors are used to control polymer molecular

weight.

13.5.3.2 Reaction Energetics

As shown in Table 13.7, the free energy of reaction of aramid polymerizations is reported to

be negative even with aromatic acid, ester, and diamine monomers. In spite of this driving

force, the rate of reaction is extremely slow because of the high activation energy of the

polymerization reaction [51].

13.5.4 DIRECT POLYMERIZATION BY CATALYSIS

Several different classes of catalysts, so-called condensing agents, have been reported in the

literature [52–55] for the polycondensation reaction of aromatic diamines with aromatic

diacids. This polycondensation is called ‘‘direct polymerization’’ because unmodified mono-

mers can be used in the reaction. The condensing agents, which are generally derived

from phosphorus or sulfur compounds, activate the dicarboxylic acid in situ during the


TABLE 13.7Energetics of Aromatic Polycondensation

DGr(T) (KJ=mole) DGr(T) (KJ=mole)

Diamine T8K IPA DMI DPI ICl TPA DMT DPT TCl

MPD 298 �8.5 32.5 �79.5 �158.0 8.5 �17.0 �63.5 �145.0

400 �23.0 — — �179.5 �4.0 �47.5 — �168.5

PPD 298 �35.5 �59.5 �106.5 �186.0 �21.5 �47.0 �94.5 �175.0

400 �50.0 — — �207.5 �49.5 �92.0 — �214.0

MPD: m-Phenylenediamine

DPI: Diphenylisophthalate

DMI: Dimethyl isophthalate

TPA: Terephthalic acid

DPT: Diphenylterephthalate

DGr(T): Free energy of reaction

PPD: p-phenylenediamine

IPA: Isophthalic acid

ICl: Isophthaloyl chloride

DMT: Dimethylterephthalate

TCl: Terephthaloyl chloride

Source: From Hand, D.R., Hartert, R., and Bottger, C., Stab resistant and Anti-ballistic material. Method of making

the same, U.S. Patent Application Publication U.S. 2004=0023580 A1, February 5, 2004; Karyakin, N.V. and

Rabinovich, I.B., Dokl. Akad. Nank. SSSR, 271(6), 1429, 1983.

polymerization. The best-known route involves an N-P type intermediate as the activated

complex. As an example, triphenyl phosphite is reacted with a carboxylic acid in the presence

of a tertiary amine (e.g., pyridine) to form the N-phosphonium salt 5, which gives the

corresponding amide on aminolysis (Equation 13.18).

5

OPh

+ + OHP OPhH

O

OPh

C

O

N

H

NH2NH

N

O C

O

PhO OPh+ CHO

O

P

OPh

OPhPhO P

ð13:18Þ

The reaction mechanism involves protonation of the triphenyl phosphite by a carboxylic acid

to form 2, which is transformed by pyridine into transition states 3 and 4. The N-phospho-

nium salt 4 reacts with the carboxylate anion to give 5.


543

2

H

N O

O

PhO

OPhCHO

O

OPh

N

POPhPhO

H

N

POPhPhO

H

NP

OPh

OPh

OPh

HH+P

OPh

OPhPhO

OPhOPh

CP

ð13:19Þ

In other words, the aromatic carboxylic acid is activated by the pyridinyl triphosphonate

cation so that the weakly basic aromatic amine can effectively attack the carbonyl center. The

reaction has not been utilized commercially because the costs of recovering and regenerating

triphenylphosphite far outweigh the cost advantage of using unmodified diacids.

Similar activation mechanisms of the P-O-P type [56], C-O-P type [57], and N-S=C-O-S

type [58], and reactions activated by silicon tetrachloride [59] and aromatic halo compounds

such as picryl chloride have also been reported in the literature [60].

13.5.5 POLYMERIZATION METHODS

The two principal methods used for the synthesis of aromatic polyamides are interfacial

polymerization and solution polymerization. Vapor-phase polymerization and plasticized

melt techniques have also been demonstrated but have not been adopted for practical use.

13.5.5.1 Interfacial Polymerization

In the interfacial method, the two fast-reacting intermediates are dissolved in a pair of

immiscible liquids, one of which is preferably water. The water phase contains the diamine

and any added alkali. The second phase consists of the diacid halide in an organic liquid such

as carbon tetrachloride, dichloromethane, xylene, or hexane, etc. The two solutions are

brought together with vigorous agitation and the reaction takes place at or near the interface

of the two phases; hence, the name interfacial polymerization.

13.5.5.1.1 Reaction at the InterfaceIn interfacial polycondensation, the polymerization reaction occurs very close to the interface

between the aqueous and organic layers generally just within the organic solvent layer that

contains the diacid chloride [60,61]. The adjacent aqueous phase generally contains, in

addition to the diamine, a basic reagent capable of neutralizing hydrogen chloride liberated

in the reaction. The reaction rate is so fast that the polymerization reaction becomes ‘‘diffu-

sion-controlled.’’ As the polymerization proceeds, the diffusion of additional monomers

through the formed polymer layer becomes increasingly difficult. As a result, the number of

growing chains is limited. For this reason, polymers with much higher molecular weights are


formed than are obtained in a normal step-growth polymerization reaction and these high

molecular weights are achieved at less than quantitative conversion. Furthermore, because

the polymerization reaction is diffusion controlled, it is not mandatory to start with an exact

balance of the two monomers in the respective phases.

There is no evidence that the interface has any special orienting or aligning effect on the

reactants, but it does provide, through solubility differences, a controlled introduction of the

diamine in the aqueous phase into an excess of diacid halide in the adjacent organic phase.

When the two phases are brought into contact, both reactants and solvents tend to

become partitioned with the opposing phase. The diamine nearly always has an appreciable

partition toward the organic phase, whereas the acid chloride has very little solubility in

water. Measured equilibrium partition coefficients for diamines in useful solvent systems have

varied from 400 to less than 1(CH2O=Csolvent). The values have been used to estimate the

relative tendency of diamines to transfer to the organic phase under polymerization condi-

tions. Partition equilibria are never achieved during polymerization because mass transfer of

diamine is the rate-controlling step at all concentrations and acylation takes place in the

organic phase as rapidly as diamine is transferred.

13.5.5.1.2 Amine AcylationAt the onset of the polycondensation reaction, diamine monomer sees excess acid chloride

and is presumably acylated at both ends. Ensuing diamine encounters a layer of acid chloride-

terminated oligomer and some acid chloride. The reaction proceeds by an irreversible coup-

ling of the oligomers by the diamine. The concentration and size of oligomers increase until a

layer of high polymer is obtained. Thus, high polymer forms because of the high reaction rate

and the increasing probability that the diamine will react with an acid chloride-terminated

oligomer rather than with a free acid chloride monomer.

13.5.5.1.3 Acid EliminationHydrogen chloride, the product of the fast reaction between amine and acid chloride, diffuses

to the aqueous phase. Any amine hydrochloride that might be formed is usually very

insoluble in the organic phase but is soluble in the aqueous phase. Both hydrogen chloride

and amine hydrochloride have to be neutralized in the aqueous phase with inorganic bases.

13.5.5.1.4 Major VariablesVariables affecting the polymerization include temperature, monomer ratio and concentra-

tion, impurities, additives, acid acceptor, and mode of addition. The polymerization of MPDI

is used as a model for interfacial polymerization in the following discussion.

13.5.5.1.4.1 Temperature

Most interfacial polycondensation are initiated at ambient temperature. Because the reactions

are rapid there is no need for heating and, in fact, cooling is frequently employed to

control the temperature rise, especially on a larger scale [62–64]. Raising the temperature

will change the solubility of both polymer and intermediates and will accelerate side reactions

as well as the desired polymerization reaction.

13.5.5.1.4.2 Reactant Equivalence

The molecular weight of polymers made by interfacial polycondensation is far less sensitive to

nonequivalence of reactants than that of polymers prepared by melt or solution methods for

reasons already discussed—high reaction rate, diffusion control of monomers, and the none-

quilibrium nature of the polymerization. The molecular weight of polymers precipitating as a

coherent film from an unstirred interface is completely insensitive to the contents of the

system as a whole, whereas the molecular weight of polymers from a stirred interface is

generally more sensitive to reactant nonequivalence.


13.5.5.1.4.3 Impurities and Additives

Interfacial polymerization will tolerate the presence of impurities in the reactants that simply

dilute the material and thereby produce nonequivalence of reactants. These diluents might be

water or inert contaminants in the acid chloride. Reactive monofunctional species are harmful

in either phase. To maximize molecular weight, it is essential to use high purity monomers.

Molecular weight control can be achieved, if desired, with appropriate use of monofunctional

reagents. Examples of impurities interfering with the interfacial polyamidation of MPDI

are half hydrolyzed acid chloride, monoamide, partially oxidized amines, and reactive

surfactants.

13.5.5.1.4.4 Acid Acceptors

Salts of basic diamines and strong acids are not sufficiently dissociated to permit the amine to

react further.At least twomoles of acid acceptor permole of diamineare needed tomaximize the

yield of high polymer [65]. Of water soluble inorganic acid acceptors used in MPDI polymeriza-

tions in a water–DMeTMS solvent system, sodium carbonate appeared to be the most prom-

ising. Use of two equivalents of sodium carbonate gave white polymer with inherent viscosity of

2.48 in 100% yield. With 1.1 equivalents of sodium carbonate, white polymer with an inherent

viscosity of 2.70 was obtained in 100% yield, while further reduction to one equivalent gave a

polymer with an inherent viscosity of 1.97. Polymer with an inherent viscosity of 1.83 (98.5%

yield) was obtained using two equivalents of sodium bicarbonate. Calcium hydroxide, potas-

sium carbonate, and sodium hydroxide all gave polymers with lower inherent viscosity.

13.5.5.1.4.5 Reactant Addition

The mode of addition of reactants will also influence the reaction. Perhaps the best procedure

would be to use a high-speed, low-volume mixer into which both solutions are charged

simultaneously. In a typical batch polymerization process, rapid addition of the diacid

chloride solution to a vigorously stirred diamine solution has given the best results. Rapid

initial stirring appears to be an essential requirement for obtaining high-molecular-

weight MPDI in water–DMeTMS. In two experiments employing rapid and slow stirring,

respectively, in a Waring blender, polymers with inherent viscosity of 2.48 and 0.66

were obtained. In another experiment polymer obtained with initial low speed stirring for

one minute followed by high speed stirring for an additional four minutes had a viscosity of

only 0.41 [66].

These and other factors affecting the interfacial polycondensation reaction are discussed

in more detail in P.W. Morgan’s book entitled, ‘‘Condensation Polymers,’’ published by

Interscience Publishers, John Wiley & Sons, 1965 [66].

13.5.5.2 Solution Polymerization

Solution polycondensation is carried out in an inert organic solvent.

Tertiary amines typically serve as the acid acceptor. The procedure generally starts with all

the ingredients in solution but this is not always an essential requirement. The polymer may

remain in solution or precipitate at any time.

13.5.5.2.1 Interfering FactorsBoth physical and chemical factors can limit the polymerization reaction. Several effects that

are classified as physical, even though they are physicochemical interactions, are the quality

of stirring, precipitation of diamine salts, and precipitation of the polymer. Chemical factors

include reactions with impurities and acid acceptors.

13.5.5.2.1.1 Impurities

The fast, low-temperature solution polymerization reactions are surprisingly tolerant of

impurities but this tolerance varies considerably. The purity of the reactants and solvents


must exceed the level required by the interfacial method. This is because all of the materials

are in intimate proximity in a single-phase system.

Nonreactive impurities in the solvent are of minor significance except as they might

depress the solubility of the polymer. Nonreactive impurities in the intermediates lead to an

imbalance in the reactants thereby limiting molecular weight.

Reactive impurities are substances that can react with the monomers, the growing chain-

ends, or the acid acceptor to terminate the polymerization prematurely. They can be

introduced with the solvent or with the intermediates. The acid chloride may contain

impurities originating in its synthesis or storage such as hydrogen chloride, thionyl chloride,

phosphorus halides, or monoacid halides. The diamine may contain monoamines, water, or

carbonates. It may degrade oxidatively in air or absorb moisture and carbon dioxide. The

degree of interference caused by these impurities depends on both the quantity of

the impurities as well the relative reaction rates of the desired polymerization vs. those

of the impurities.

13.5.5.2.1.2 Solvent Reactivity

The solvent should not react with either the amine or the acid halide during the course of

the polymerization. Solvent interference can be limited by minimizing the contact time

between the monomer and the solvent; for example, the intermediates can be dissolved and

allowed to react simultaneously. Alternatively, a small amount of nonreactive solvent can be

used to dissolve one or both intermediates prior to polymerizing them in a more reactive

medium.

13.5.5.2.1.3 Side Reactions with Acid Acceptors

Secondary amine acid acceptors can terminate chain growth by reacting with the diacid halide

unless amine reactivity is minimized by steric effects. Reactions between a tertiary amine acid

acceptor and the acid halide or certain solvents must also be avoided. An acid chloride and a

tertiary amine can react to form a monoamide and an alkyl halide (Equation 13.20). This

reaction is known to occur in fair yield at high temperatures and probably takes place to some

extent at room temperature [67–69]. In the usual preparative method wherein diacid halide is

added to a solution of diamine and a strongly basic acid acceptor, no difficulty is experienced

if the polycondensation reaction is rapid. As the polycondensation reaction rate decreases, the

potential for interference by side reactions increases. In a polymerization system, this would

be a chain terminating reaction.

R Cl+C

O

N

R

RC

O

N

R

R

RCl

N

R

R

R

+C

O

Cl

ð13:20Þ

A reaction that can occur between an acid chloride and a tertiary amine in the presence of

moisture is the formation of an acid anhydride (Equation 13.21).

NH

R

R

Cl+ 2RC

O

N

R

R

RCl

C

O

O

O

2 C

ð13:21Þ


An anhydride group in the polymer chain is a hydrolytically weak link and would likely be

subject to cleavage on isolation of the polymer in water.

13.5.5.2.1.4 Diacylation

Diacylation of an amine by the acid halide leads to branched and network polymers. This side

reaction has also been observed in interfacial polycondensation reactions [70].

13.5.5.2.2 Reaction RatesSolution polycondensation employs the same reactions as used in interfacial polycondensa-

tion and similar reaction rates are involved. This means that the fastest reactions have rates

on the order of 102–106 l=mole-sec. Polycondensations involving such reactions may be

completed in a few minutes at room temperature.

13.5.5.2.3 Physical and Mechanical Effects

13.5.5.2.3.1 Temperature

Solution polycondensation reactions between diamines and diacid halides produce polymers

with maximum molecular weight when carried out at room temperature or below.

While reaction rates and polymer solubility would be expected to increase with increasing

temperature, the rates of competitive side reactions will also increase.

13.5.5.2.3.2 Concentration

Solution polycondensation reactions have not shown any marked sensitivity to reactant

concentration except as the concentration affects stirrability or temperature control. Lower

concentrations are uneconomical and introduce relatively larger amounts of solvent impur-

ities. Higher concentrations may yield unstirrable masses when the polymer or by-product

salt precipitates, and the heat of the reaction is more difficult to control when reactants are

mixed rapidly at high concentration.

13.5.5.2.3.3 Equivalence of Reactants and Mixing

Although both interfacial and solution polycondensation reactions show unusual insensitivity

to nonequivalence of reactants, solution polycondensations are appreciably more sensitive to

reactant balance.

Features common to both polymerization methods include: (1) use of fast reacting

intermediates; (2) reaction irreversibility; (3) the reaction takes place essentially as fast as

the contact of complementary reactants occurs; and (4) the growing polymer is in solution or

highly swollen during the polymerization process. Unlike the interfacial process, the solution

process has no interface to provide for the flow of one reactant into a higher concentration of

the complementary reactant. It is this liquid–liquid interface that plays a significant role in

attaining reactant balance in the interfacial process. The success of the solution process shows

that an interfacial boundary, while helpful as a regulating device, is not essential for the

formation of a high-molecular-weight polymer.

A key rationale for the insensitivity to nonequivalence of reactants in a single-phase

system is that the rate of polymerization is often faster than the rate of mixing even in

the absence of an interfacial boundary. It is presumed that in a solution polymerization

system there are temporary interfaces or zones within which polymerization is proceeding

independently of any potential effect of the ratio of the two reactants in the system as a whole.

Thus, even a single drop of acid chloride solution in a large volume of diamine solution reacts

rapidly with the local, or immediately surrounding diamine, before the droplet is dispersed.

This leads to oligomers and polymer with higher molecular weight than would be obtained

from a random reaction at the known reactant ratio. Further dropwise addition of one

reactant continues this effect because each successive drop goes into a large system that


consists in part of an active polymer with a higher than random degree of polymerization.

Eventually as the system approaches equivalence and the concentration of reactive groups is

reduced, there is a greater chance of a wider distribution of the increment of added reactant

and the occurrence of random reaction [68]. Theoretical treatments of the effects of monomer

ratio as well as side reactions have been described by Flory [71]. Kilkson has analyzed the

problem of irreversible polymerization in both batch and steady-state reactors [72].

13.5.5.2.4 Acid AcceptorsPolycondensation reactions between diamines and diacid chlorides require the removal of the

by-product hydrogen chloride. The acid acceptor need not be a basic substance but must retain

the by-product acid in some way while the reaction proceeds. A variety of amines and some

sterically hindered secondary amines have been used as acid acceptors in the solution prepar-

ation of polyamides. From an empirical point of view, the base strength of the acid acceptor

should be about equal to or greater than the base strength of the terminal amine group at the end

of an oligomer or polymer chain. The pKa scale in water is used for base strength. A different

measure,E1=2, isused toquantify thebase strengthofamines inorganic solvents.Hall [73,74]has

defined the E1=2 of an amine as a potential (in millivolts) of solution at the half-titration point

with perchloric acid andhas shown thatE1=2 is parallel to the pKa scale inwater. Table 13.8 lists

these values for some acid acceptors frequently used for solution polyamidation.

13.5.5.2.5 SolventThe solvent has many roles. It dissolves the monomers and provides for their mixing and

contact; it dissolves or swells the growing polymer so that the reaction is maintained; it carries

the acid acceptor and facilitates the disposition of by-product salts; it influences the reaction

rate by polarity or solvation effects; and it absorbs the heat of reaction.

The solvent should be inert and should ideally be able to dissolve the intermediates before

the polymerization is started. A primary requisite for high polymer formation in all solution

polycondensation reactions is that the solvent must be able to dissolve or swell the poly-

mer sufficiently to permit the completion of the polymerization [75–77]. The solution

polycondensation process requires a stronger polymer–solvent interaction than does the

TABLE 13.8Basicity of Amine Acid Acceptors

E1=2 (mV)a

Acid acceptor pKa Ethyl acetate Acetonitrile

tert-Butylamine 10.45 130 —

Diisobutylamine 10.59 207 —

Triethylamine 10.74 197 66

Tri-n-propylamine 10.70 228 —

Tri-n-butylamine 10.89 210 —

N-Ethylpiperidine 10.45 190 84

N-Ethylmorpholine 7.70 290 221

N,N-Diethyl-m-toluidine 7.24 — —

N,N-Diethylaniline 6.56 467 425

Pyridine 5.26 — —

aE1=2 is the millivolt reading at the half-titration point at 258C with perchloric acid as the titrant

from the work of Hall.

Source: From Hall, H.K., J. Am. Chem. Soc., 79, 5439, 1957; Hall, H.K., J. Phys. Chem., 60, 63,

1956.


interfacial polycondensation method. The combination of solvent, diamine, and acid acceptor

must be such that the diamine does not precipitate as a salt with limited solubility.

Although little is known about the effects of solvent polarity, viscosity, and specific gravity on

these reactions, the reaction rate tends to increase with an increase in solvent polarity [78,79].

13.5.5.2.6 Solubilizing AidsOccasionally, solubilizing aids or auxiliary solvents are added to boost the solvating power of

the primary solvent. The polymerization of PPTA requires the presence of a solubilizing aid

to obtain a high-molecular-weight polymer. Alkaline or alkaline earth metal halides such as

CaCl2 and LiCl are known to be effective solubilizing aids in substituted amide solvents such

as NMP and DMAc. Solubilizing aids apparently increase the polarity of the solvent by

complexing with the carbonyl group (Equation 13.22).

CH3 C

O

N

CH3

CH3

+ LiCl

Li

CH3 C

O

N

CH3

CH3

Cl

Polarization ð13:22Þ

More recently, quaternary ammonium halides such as methyl tri-n-butyl ammonium chloride

were used in the polymerization of PPTA in NMP [80]. Effective shielding of the ammonium

cation by bulky alkyl groups stabilizes the ionized species in an organic medium so that it can

facilitate the polarization of NMP (Equation 13.23).

Cl

+

N

CH3

ON

CH3

O

NH3C

CH2CH2CH2CH3

CH2CH2CH2CH3

CH2CH2CH2CH3

Cl

NH3C

CH2CH2CH2CH3

CH2CH2CH2CH3

CH2CH2CH2CH3

ð13:23Þ

13.5.5.2.7 Reactivity of Precipitated PolymerIn the solution polymerization of PPTA in NMP–CaCl2 solvent, significant chain growth takes

place after the polymer precipitates. At the beginning of the reaction, the polymerization

proceeds in solution. As the molecular weight of the polymer increases, the viscosity of the

solution increases rapidly to a gel point and eventually the polymer precipitates. At this stage, the

molecular weight of the polymer is still very low (inherent viscosity ~2), but the polymerization

continues in the precipitated state to an inherent viscosity of >6, in the absence of interfering

contaminants such as water. This is a clear evidence that the chain-ends of the polymer are not

deactivated on precipitation but retain enough mobility to react with the neighboring active

groups. However, the rate of reaction becomes very slow after the polymer precipitates.

13.5.5.3 Vapor-Phase Polymerization

Vapor-phase polymerization has been described in the patent literature as an alternative route

to aromatic polyamides from aromatic diamines and aromatic diacid chlorides [81]. The

reaction is carried out in the gas phase by mixing vapors of the two monomers in the presence

of an inert gas. The temperature at the reaction zone has to be higher than the glass transition

temperature of the polymer to achieve segmental mobility of the growing polymer chain.



l

Polymer decomposition is minimal because the reaction time is very short. The polymer is

deposited on removable inorganic or organic substrates maintained in the reaction zone.

Monomer Avapor

Monomer Bvapor

Inert gas Mixer

Rea

ctor

Que

nch Sep

arat

or

Inert gas

Scrubber

Inert gasrecycle

Schematic of the Vapor-Phase Polymerization Process

Vapors of two different monomers (A and B) together with a hot inert gas are fed to a

mixer (such as a jet mixer, a simple short tube, or a combination of both) and then to the

reactor inlet. Additional inert gas can be introduced as needed. The reactor effluent stream

consisting of some polymer, possible oligomers, and by-product acid, is conducted through a

quench chamber where the stream is cooled by a flow of relatively cold inert gas. The cooled

stream is then led through a separator such as combination of a cyclone separator and filters

to remove solid material. The filtered stream is then passed through a water scrubber to

remove hydrogen halide and vented to the atmosphere or recycled.

Vapor-phase polycondensation has the distinct advantage of not having to use solvent and

it makes possible the elimination of by-product HCl in the gas phase. However, the resulting

polymers are usually highly branched due to the high reaction temperature required to

maintain chain mobility. In addition, the stoichiometric balance of reagents is much more

difficult to maintain than in the case of a condensed phase reaction.

13.5.5.4 Plasticized Melt Polymerization

Most aromatic polyamides cannot be made by a melt polymerization process because the

polymer melt temperature exceeds the decomposition temperature. Singh developed a unique

procedure for preparing certain aromatic polyamides by a melt process using an interna

plasticizer generated in-situ during the polymerization [82]. The following reaction scheme

was used to prepare aromatic polyamides in the absence of a solvent (Equation 13.24).

N

O

H

+

N

OC

O

C

O

N

O

x4C

O

CH2 N

H

+1−x

C

O

NC

H

O

N

H

+

NH2H2N

ð13:24Þ

The melt polycondensation of isophthaloyl-N,N-bis (valerolactam) with m-phenylene dia-

mine yielded the aromatic polyamide MPDI plasticized by liberated valerolactam. A small

amount of valerolactam is polymerized to poly(valerolactam) during the polymerization,

which the author claims can be minimized by adjusting the reaction parameters. It is proposed

that the plasticizer can be removed by water extraction after the shaping process thereby

recovering the infusible aromatic polyamide.

13.6 ARAMID SOLUTIONS

Aramid polymers have high melting points or melt with decomposition that makes fiber

processing by melt spinning impractical [1]g. Fibers are therefore spun from polymer solu-

tions. These polymers not only do not melt but also are not easy to dissolve. Highly polar

solvents, with or without the aid of inorganic salts such as lithium chloride or calcium

chloride, or acids like concentrated sulfuric acid have to be used [88].

13.6.1 ISOTROPIC S OLUTIONS

Some aramids are processed from isotropic solutions. Flexible chain homo-polymers like MPDI

can be dissolved in solvents like NMP and DMAc [88] to form such solutions but the degree of

solubility can be further enhanced by copolymerization [83]. Isotropic solutions can be also

obtained with p-aramids but in this case copolymerization is required to enhance solubility.

13.6. 1.1 m-Aram id So lutions

As previously mentioned, DuPont and Teijin are the two major manufacturers of m-aramid

fibers. Russian scientists also developed a commercial process for the manufacture of MPDI

polymer and fiber under the trade name of Fenilon [84]. However, at this point Fenilon

production has been suspended.

DuPont’s m-aramid polymer, MPDI, is polymerized using essentially a 1:1 molar ratio of

m-phenylenediamine and isophthaloyl chloride [85]. Patent literature indicates that the fiber,

Nomex, is spun directly from the polymerization solution in DMAc, which contains calcium

chloride. MPDI polymer solutions containing >3% by weight calcium chloride are quite

stable [2].

Teijin’s product, trademarked Teijinconex, is a 100 =97 =3 copolymer of MPD =ICl =TCl

[83]. The polymer is prepared by interfacial polymerization, isolated and dissolved in NMP to

form spin dopes of approximately 20% solids concentration [86]. The resulting isotropic

solutions are stable at 1008C and are suitable for wet spinning. The solution has two solubility

limits that include reversible and irreversible regions, as shown in Figure 13.1 [87]. If the

irreversible limit is exceeded, the polymer becomes soluble only in sulfuric acid.

The Russian Fenilon process utilizes low-salt content MPDI solutions [89]. Most of the

hydrochloric acid generated during the polymerization process is removed by treatment with

ammonia. The resulting insoluble ammonium chloride is filtered from the polymerization

solution. Residual HCl is likely neutralized with an organic base. The neutralized solution is

suitable for wet spinning of fibers.

13.6.1.2 p-Aramid Solutions

p-Aramids are soluble in strong acids and in highly polar solvents in the presence of in-

organic salts. They form isotropic solutions only at low polymer concentrations. Among

commercial products, copolyamides from the SVM family as well as copoly(p-phenylene=3,4’-diaminodiphenylether terephthalamide) (Teijin’s Technora base polymer) remain soluble in

their polymerization mixture [90] and can be spun directly from that solution.

gException Teijinconex mono-filament process.


10

20

30

00 100

Pol

ymer

con

c, %

Reversible

limit

Solution

region

Irreversible

limit

Temperature, 8C

FIGURE 13.1 Stability of Teijinconex spin solution. (From Fujie, H., Nikkyo Geppo, 40, 8, 1987. With

permission.)

13.6.2 A NISOTROPIC SOLUTIONS

13.6. 2.1 Phase Behavior

A distinctive feature of semirigid polymers such as p-aramids is that their solutions develop

molecular orientation under shear or extension with great ease. This results in a unique

difference in properties in the direction of shear or extension vs. those perpendicular to the

shear direction. There are two classes of materials that have this characteristic: lyotropic,

which form anisotropic solutions; and thermotropic, which form anisotropic melts. As

aramids do not melt we will focus here on lyotropic systems. Anisotropic solutions differ

from isotropic solutions in many physical characteristics including light depolarization,

rheological properties, phase behavior, and molecular orientation.

Observed structures of a lyotropic material are classified into three categories: nematic,

smectic, and cholesteric. Nematic and cholesteric mesophases can be readily identified by

microscopic examination. The existence of a smectic mesophase is not well defined and is only

suggested in some cases. Solvent, solution concentration, polymer molecular weight, and

temperature all affect the phase behavior of lyotropic polymer solutions. In general, the phase

transition temperature of a lyotropic solution increases with increasing polymer molecular

weight and concentration. It is often difficult to determine the critical concentration or

transition temperature of a lyotropic polymer solution precisely. Some polymers even degrade

below the nematic–isotropic transition temperature so that it is impossible to determine the

transition temperatures. Phase behavior is also affected by the polymer molecular conform-

ation and intermolecular interactions.

A good example of a lyotropic solution is that of PPTA in sulfuric acid. Figure 13.2

shows the viscosity–concentration relationship of a solution of PPTA of moderate molecular

weight [91]. At low polymer concentrations, the solution viscosity increases with increasing

concentration just like an isotropic solution of a flexible chain polymer. However, above a

critical concentration of ~12%, the solution viscosity decreases abruptly with increasing

concentration. This behavior is caused by the close packing of the rigid chain polymer

molecules to form ordered domains. The solution viscosity reaches a minimum point at

about 20% solids and then abruptly increases with additional solids. A solid phase will

eventually appear when the solution becomes supersaturated. The anisotropic PPTA–

H2SO4 solution exhibits liquid crystal behavior. It has the flow properties of a liquid and is

crystal-like with the ability to depolarize cross-polarized light. When the solution is subjected


0

10

20

30

40

50

5 10 15 20 25 30

Bro

okfie

ld v

isco

met

er r

eadi

ng

Concentration, wt %

FIGURE 13.2 Bulk viscosity vs. concentration of PPTA–H2SO4 solution. (From Bair, T.I. and Morgan,

P.W., U.S. Patent 3,673,143, 1972; U.S. Patent 3,817,941, 1974. With permission.)

to shear or elongational flow, the liquid crystal domains become aligned in the direction of

flow to achieve a high degree of molecular orientation.

For fiber preparation, a lyotropic solution is best processed at a solids concentration near

the minimum solution viscosity and at a temperature close to its anisotropic transition

temperature (Figure 13.2). These conditions maximize solution ordering prior to spinning.

13.6. 2.2 Rheologi cal Propert ies

Lyotropic solutions generally exhibit viscoelastic behavior. They are pseudoplastic and

exhibit shear thinning with increasing shear rate. For polymers of near-linear chain conform-

ation, their lyotropic solutions are known to give less die swell and are less tractable than

isotropic solutions. The PPTA–H2S04 solution was the first to be used commercially and has

been studied most extensively.

The rheological properties of PPTA–H2S04 solutions have been studied by several inves-

tigators [92–97]. Figure 13.3 and Figure 13.4 show the relationship between shear viscosity, �hh,

and shear rate, g, for Kevlar–H2SO4 solutions of various concentrations at 25 and 60 8C,

respectively. Figure 13.5 is a plot of shear viscosity vs. shear stress for PPTA solutions at 25 8C[97]. The change in the slope of these curves between 8 and 10% solutions shows the effect of

the isotropic–anisotropic phase transition. The viscosity–shear stress curves for 10 and 12%

solutions tend to infinity, indicating the presence of a yield stress [94].

13.7 PREPARATION OF ARAMID PRODUCTS

13.7.1 FIBERS

13.7. 1.1 Dry Spinnin g

Solutions of m-aramid polymers are currently produced using dry-or-wet spinning processes.

Processing steps after spinning can include drawing, drying, and heat treatment.

In the dry-spinning process, a solution of polymer is extruded through a spinneret that is

mounted at the top of a heated column. As the solution is extruded in the presence of hot inert


10�2 10�1

0.5%

6%

8%

12%

10%

100100

102

103

104

105

101

γ, sec�1

h, p

oise

.

FIGURE 13.3 Shear viscosity vs. shear rate for re-dissolved Kevlar–H2SO4 solution at 258C. (From

Aoki, H., White, J.L., and Fellers, J.F., J. Appl. Polym. Sci., 23, 2293, 1979. With permission.)

gas (or air), solvent evaporates from the incipient fiber. The temperature of the heated gases

in the column is above the boiling point of the solvent. The solidified fiber is collected at the

bottom of the column. The polymer solvent must be inert, stable at its boiling point, and a

good solvent for the polymer. The heat of vaporization of the solvent must not be too high, it

must have sufficient thermal resistance, low toxicity, a very low tendency to produce static

charges, low risk of explosion, and be relatively easy to recover [98]. The dry-spinning

process was initially developed for spinning acrylic fibers and was modified for spinning m-

aramid polymer. DuPont developed processes for dry spinning Nomex from DMF and

DMAc solutions [99]. The m-aramid polymer solution is disordered in the solution state.

Some orientation is imparted during the extrusion of the solution through the spinneret

capillary. The extent of fiber orientation tends to increase as the shear rate through the

spinneret capillary is increased. Radial structural inhomogeneities are generally introduced

during the solvent diffusion and evaporation stages of the dry-spinning process [10]. A skin

10�3 10�2 10�1

γ, sec−1

100 101101

102

10310%

12%

8%

6%

104

102

h, p

oise

FIGURE 13.4 Shear viscosity vs. shear rate for re-dissolved Kevlar–H2SO4 solution at 608C. (From



101 102

8%6%

10%

12%

102

103

104

105

σ12, dynes/cm2

h, p

oise

103 104

FIGURE 13.5 Shear viscosity vs. shear stress for re-dissolved Kevlar–H2SO4 solution at 258C. (From


core structure forms because the outer skin of the fiber loses solvent faster than the inner core.

As diffusion progresses, the loss of the solvent from the core through the solidified sheath

reduces the mass of the core. This results in the sheath collapsing inward. Since the evapor-

ation rate of the solvent in the sheath of the fiber is faster than the diffusion rate of solvent

from the core of the fiber the cross section shape of the fiber can change from round to dog-

bone. m-Aramid fibers are spun at a spin stretch ratio of 1–20x, which is far lower than

fibers processed from the melt, but this has little impact on fiber properties since there is

very little orientation produced during this part of the process. The resulting m-aramid fibers

at the bottom of the spin cell retain considerably more solvent ( >20%) than dry spun acrylic

fibers (<5%).

The as-spun fiber is then drawn to develop physical properties. Fiber drawing is generally

done in a dilute water solution of solvent. The solvent partially plasticizes the fiber and

facilitates drawing (3–5x). After the drawing step, the fibers are washed with water, dried, and

crystallized by heating at a temperature above the polymer Tg (~275 8C) [100,101]. Typical

fiber properties are in the order of 0.6 GPa with an elongation to break of 30%. A schematic

of the Nomex dry-spinning process is shown in Figure 13.6.

13.7. 1.2 Wet Spinni ng

In the wet-spinning process polymer solution is extruded through a spinneret that is sub-

merged in a coagulating medium consisting of solvent and nonsolvent. On coagulation, the

spinning solution undergoes spinodal decomposition into polymer-rich and polymer-poor

regions and ultimately into a solid phase. It is this polymer solvent–nonsolvent interaction

that has the greatest impact on the structure of the fiber and the ultimate properties that can

be achieved. The relative rates of solvent to nonsolvent diffusion control the process of phase

separation [102]. Important variables controlling this process are polymer solids, solution

composition and temperature, coagulating solution composition and temperature, the extru-

sion rate, and the residence time in the coagulating bath. Control of the size and character of


Spin solution frompolymerization

Dry spinning

Drawing

Washing

Drying and heattreatment

FIGURE 13.6 m-Aramid dry-spinning process.

the voids formed in such a process is key to achieving fibers with excellent mechanical

properties [103].

A schematic of the Teijinconex wet-spinning process is shown in Figure 13.7 [87]. The

process schematic for producing Fenilon is similar to that shown in Figure 13.7 with the

exception that the polymer solution is spun directly from the polymerization process [104].

While the above processes require little or no inorganic salt content in the spinning solution,

the process described by Tai et al. allows the use of salt-containing solutions [105].

13.7. 1.3 Dry-Je t Wet- Spinning

Kwolek [106] demonstrated in her early work at DuPont that p-aramid fibers could be spun

from amide and salt solutions using a conventional wet-spinning process. These solutions

were typically of low concentration. The resulting fibers had low strength but high modulus

after heat treatment. In later development, p-aramid fibers were spun from more concentrated

solutions using dry-jet wet-spinning processes [107]. These solutions contained aramid poly-

mer above a critical solids concentration and were anisotropic.

In 1970, Blades [108] discovered that high-strength, high-modulus fibers could be

spun from anisotropic solutions of aramid polymers by dry-jet wet spinning (Figure 13.8).

His process is shown schematically in Figure 13.8. The key feature of this process is that an

anisotropic solution is extruded through an air gap between the spinneret and the coagula-

tion bath. The coagulated filaments are washed, neutralized, and dried. This process


Dissolver

Wet spinning

Quenching

Washing

Wet draw

Dry polymer from interfacialpolymerization

Amide solvent

Drying, hot draw andheat treatment

FIGURE 13.7 Teijinconex wet-spinning process. (From Fujie, H., Nikkyo Geppo, 40, 8, 1987.)

produces a fiber with tenacity and initial modulus 2–4 times that of a fiber prepared by a

conventional wet-spinning process.

The mechanistic model of polymer molecular orientation in a dry-jet wet-spinning process

is shown in Figure 13.9 [109]. Shear at the capillary wall causes the liquid crystalline domains

to orient along the direction of flow when an anisotropic solution is extruded through a

spinneret capillary. At the capillary exit, some deorientation of liquid crystalline domains

occurs because of solution viscoelasticity. However, this deorientation is quickly overcome by

threadline tension on the attenuating filament in the air gap. The attenuated filaments retain

this highly oriented molecular structure on coagulation giving rise to highly crystalline, highly

oriented fibers.


=

Spin dope

Spinneret

Transfer line

Air gap

Coagulating liquid

Spin tube

GuidePump

Tube

p

Filaments

Container

Rotating bobbing

Container

Spinning block

p

FIGURE 13.8 Dry-jet wet-spinning process. (From Blades, H., U.S. Patent 3,767,756, 1973.)

Orientation

Partial deorientation

Reorientation

Air gap

Spinneret

Quenchwaterbath

FIGURE 13.9 Molecular orientation during dry-jet wet spinning. (From Yang, H.H., Aramid fibers, in

Fibre Reinforcement for Composite Materials, Bunsell, A.R., Ed., Elsevier, Amsterdam, 1988. With

permission.)


The operating conditions for dry-jet wet spinning are proprietary for fiber producers and

are therefore not revealed in detail. A review of the literature shows that the general

conditions are as follows [110]:

Polymer molecular weight 5,000–35,000

Polymer inherent viscosity 3–20 dL=g

Spinning speed >55 y=min (>50 m=min)

Number of filaments l0–l500

Spinneret hole diameter 0.002–0.004 in. (0.05l–0.l02 mm)

Filament size 1–6 denier=filament

The as-spun fiber from dry-jet wet spinning can be heat treated at high temperatures and

high tension to increase its crystallinity and degree of crystalline orientation [111]. The heat

treatment conditions are generally in the following ranges:

Temperature 250–5508CTime <10 min

Tension 5–50% of breaking strength

As discussed above, isotropic solutions are typically converted to fibers by a wet-spinning

process. Ozawa [90] disclosed that the polymerization mixture of copoly(p-phenylene=3,4’-diaminodiphenylether terephthalamide) remained isotropic. He deviated from traditional

spinning techniques and spun fiber from this solution using dry-jet wet spinning. Although

as-spun fiber tensile properties were modest, high strength fiber was achieved with subsequent

drawing. This fiber product was later commercialized as Technora aramid fiber by Teijin Ltd.

The use of dry-jet wet spinning to prepare fibers from isotropic solutions has since been

widely practiced.

The dry-jet wet-spinning process is unique in that the temperature of the spinning nozzle is

different than that of the spin bath. In comparison, the spinning nozzle in a conventional wet-

spinning process is immersed in the coagulation liquid and is therefore at the same temp-

erature. This gives rise to several inherent limitations with the wet-spinning process. First,

the coagulant temperature must exceed the freeze point of the spinning solution. Second, the

spinning solution is exposed to the coagulant as soon as it exits the spinneret holes. This can

limit attenuation of the incipient filament. The dry-jet wet-spinning method allows the use of

a low temperature coagulant without concern for freezing the spin solution. The air gap

permits the extruded solution to be more fully attenuated and to develop a higher degree of

molecular orientation.

Dry-jet wet spinning is, however, a much more mechanically complicated process and

requires careful control of both the air gap and the flow dynamics of the coagulant fluid.

13.7.2 FILM

Aramid films have been in development since the late 1990s by several Japanese com-

panies including Toray, Teijin, and Asahi. As with fibers, aramid solutions can be extruded

through flat dies to form films. The conventional wet process can be employed to produce

unidirectional and bi-oriented films from isotropic aramid solutions. Production of films

from anisotropic solutions requires unique processes as shown by the example of PPTA film.

Forming films from anisotropic solution is extremely difficult because of the ease with

which these solutions orient. Obviously once the films orient in the machine direction they are


Dry polymer H2SO4

Dissolver

Drum/Beltcasting

Steamtreatment

Coagulation

Acid andwater

Washing

Wetstretching

Drying

FIGURE 13.10 PPTA film process. (From Imanishi, T. and Muraoka S., U.S. Patent 4,752,643,

June 21, 1988.)

very weak in the cross direction and, as a result, tend to fibrillate. Asahi has developed a

process leading to a coherent film [112]. A schematic of this process is shown in Figure 13.10.

An anisotropic PPTA solution in sulfuric acid is extruded through a die onto a drum or a

belt where it is initially exposed to warm, humid air. Under these conditions, the solution

reverts to an isotropic state as moisture is absorbed to reduce the effective acid concentra-

tion and raise the temperature. This is the critical step as it leads to the formation of an

isotropic film. The structure is fixed on coagulation after which the film is washed with

water to remove the remaining sulfuric acid. The wet film is biaxially stretched to develop

mechanical properties in both directions and then dried. Finally, the film can be heat-treated

to further improve properties.

13.7.3 F IBRIDS

Fibrids are film-like particles that are formed when—aramid solutions are precipitated in a

nonsolvent under high shear [113,116]. The dimensions of as-formed fibrids are around 100

mm � 700 mm � 0.01 mm [113,114]. Fibrids have a high surface area, around 200–300 m2=g,

and can function as a thixotrope or a reinforcing agent in composite, sealing, coating, and

elastomer applications [114,115]. Fibrids are used primarily in aramid papers. Aramid papers

are composed of a mixture of fibrids and short Nomex fibers referred to as floc (Figure 13.11).

Fibrids serve as a binder for the short fibers and also improve the dielectric properties of high

temperature, heat-resistant aramid papers (Figure 13.12) [115,116]. A process for making m-

aramid papers is shown in Figure 13.13 [113].


Fibrid Floc

FIGURE 13.11 Photomicrographs of aramid fibrid and floc.

13.7.4 PULP

p-Aramid pulp is a highly fibrillated material that retains the key chemical and physical

properties, low creep performance, and high temperature and wear resistance of the precursor

p-aramid fiber. These characteristics make p-aramid pulp an excellent candidate to replace

asbestos in friction products such as brake linings and clutch facings, gaskets, and industrial

papers. The highly fibrillated structure of Kevlar pulp is characterized by a combination of

high fibril aspect ratio (>100) and high specific surface area [116,117]. The fibrils can be

attached to, or detached from, the core fiber.

Pulp is produced by passing a dilute slurry of short cut length, p-aramid fiber through one

or more high shear refiners. The highly oriented, crystalline fiber is cut and readily split into

fibrils of smaller diameter because of the relatively low compressive strength of the fiber. The

refining process is controlled to produce a certain balance between the final fiber length and

the degree of fibrillation or the degree of new surface generation. The optimum relation-

ship between these two parameters is dictated by the process or product performance

requirements of the specific end-use application. Water is removed from the resulting pulp

slurry to produce a wet product or, with additional drying, a dry product. Wet pulp contains

50–70% moisture depending on the producer. Dry pulp contains 4–8% moisture. Handling of

the pulp becomes difficult at lower moisture levels because of static problems.

FIGURE 13.12 Photomicrograph of a cross section of Nomex Type 411 paper.


http://www.crcnetbase.com/action/showImage?doi=10.1201/9781420015270.ch13&iName=master.img-002.jpg&w=178&h=132

http://www.crcnetbase.com/action/showImage?doi=10.1201/9781420015270.ch13&iName=master.img-003.jpg&w=178&h=131

Staple supply

Fibrid supply Stock tankBeater Head box

Product reel Dryer rolls Wet press Fourdinermachine

Broke

Slusher

Mixer

Calender

FIGURE 13.13 Process for making m-aramid papers.

Pulp is characterized in terms of fiber length, length distribution, and the degree of

fibrillation. Absolute fiber (or fibril) length typically ranges from less than a millimeter to

about 6 millimeters. The fiber length distribution is measured using a device such as a Kajaani

200 instrument and is reported in terms of a length-weighted average length (P

ni li2=P

ni li) or

weight-weighted average length ( P

ni li3=P

ni li2). The length-weighted average length of typ-

ical commercial pulps is in the range of 0.6–1.1 mm. The degree of fibrillation is related to the

specific surface area of the pulp or to the drainage rate of an aqueous pulp slurry determined

by the Canadian Standard Freeness or Schopper–Riegler methods. There is a fiber–fibril

diameter or width distribution in pulp just as there is a length distribution. The diameter will

range from 12 to 15 mm, the diameter of the precursor fiber, to less than 1 mm for the smallest

fibrils. Pulp specific surface area ranges from about 7 to 15 m2=g reflecting the breakdown of

the initial fiber, with a surface area of about 0.2 m2=g, into a broad distribution of smaller

diameter fibrils. Canadian Standard Freeness values range from about 100 ml for ‘‘high’’

surface area pulps to about 600 ml for less highly refined pulp merges. The highly fibrillated

morphology characteristic of p-aramid pulp is shown in Figure 13.14.

13.8 APPLICATIONS

The broad range of properties of aramids is the main reason for their utility in diverse

applications. Here we will attempt to illustrate how previously described properties of these

fibers are exploited in their applications.


EP6381TR3 Pulp KE IF538 Detecteur = SE1 Nom Utilisateur = PIERDOC

Date :7 Juil 2003WD = 12mmGrand = 1.00 KX

(Merge 1f538)

10μm

FIGURE 13.14 Scanning electron micrograph of Kevlar brand pulp.

13.8.1 m-ARAMID FIBER

Many of the applications of m-aramid fibers are due to their unique combination of flame

resistance with thermal and textile properties. Some applications also benefit from the fact

that m-aramid fibers are available in colored form. In general, these fibers are very difficult to

dye and thus most producers offer producer colored (pigmented) fiber. While pigments offer

in general better UV stability, this approach limits the number of colors available. At this time

only DuPont offers piece dyeable products. In general, dyeing of Nomex fibers requires the

use of carriers, and dyeing technology is kept as proprietary information by dye houses.

In general, flammability as well as thermal properties are bulk properties of the material.

When these properties are critical, compositions comprising 100% aramid fibers are used.

Blends with nonaramid materials do come into play when other fiber properties or charac-

teristics are desired.

13.8.1.1 Protective Apparel

Fabrics of m-aramids are widely used in thermal protective apparel because of their unique

combination of thermal and textile properties. The fibers from which these fabrics are made

are inherently flame resistant and do not melt or drip. A measure of the fiber’s flammability is

its limiting oxygen index (LOI), which is the concentration of oxygen in air that is required to

support combustion once the material is ignited. Materials with an LOI> 21 are considered

nonflammable. The inherent flame resistance of m- and p-aramids is essentially the same with

LOI values of ~28–29. For apparel applications, m-aramids are generally preferred over p-

aramids because the fabrics have a more comfortable, textile-like hand as a consequence of

lower fiber modulus and higher elongation. Even though m-aramids fibers exhibit high glass

transition temperature and high crystalline phase melting points (2758C and 4258C respect-

ively) both glass transition temperature and melting temperature of the crystalline phase are


high (2758C and 4258C respectively) in flame 100% m-aramid garments exhibit some shrink-

age, which in turn can lead to fabric ‘‘break opening’’ and loss of protective barrier. Blends

with p-aramids are often utilized to stabilize the protective garment against shrinkage and to

reduce fabric ‘‘break-open’’ during flame exposure. At higher exposure to flame MPDI

carbonizes and forms a tough char at a temperature of ~8008F (4278C). The intumescent

nature of the char provides additional protection. Decomposition products on combustion

will vary depending on the heating rate and the amount of oxygen present. In general,

combustion by-products are similar to those obtained on burning wood, wool, cotton,

polyester, and acrylic [118,121].

Both continuous filament and staple yarns are used in protective apparel fabrics. Typical

filament deniers range from 0.85 to 2. Staple fiber length is 1.5–2 in. for processing on the

cotton system. Yarns are available in dyeable and producer colored forms. Fabric forms

include woven, knit, and nonwoven. The mechanical toughness of the fiber results in higher

fabric strength than FRT cotton fabrics of even greater weight. Higher resistance to tear and

abrasion also provides greater durability and longer useful garment life. Ultimately fabric

selection will depend on the application and the end-use performance requirements such as

the degree of protection required, flammability, durability, comfort, cost, style, etc.

m-Aramid fabrics are widely used in industrial, military, fire fighting, and auto racing

applications. Chemical, petrochemical, and utility workers wear flame-resistant protective

clothing where flash fire or electrical arc hazards exist. Military applications include flight

suits and coveralls for combat vehicle and shipboard engineering crews. In firefighting

apparel, m-aramids and blends with p-aramids find use in turnout gear, station uniforms,

hood, gloves, and boots. The turnout is a three-component system (an outer shell, a moisture

barrier, and a thermal barrier) designed to provide basic thermal protection in hot environ-

ments and in flashover conditions in addition to maximizing comfort and minimizing the

potential for heat stress. Race car drivers and their crews wear clothing to protect themselves

from flash fires resulting from crashes and pit accidents. The protective gear includes suits,

underwear, socks, and gloves.

13.8.1.2 Thermal and Flame-Resistant Barriers

The same fiber properties that make m-aramids suitable for protective apparel applications

find utility in thermal and flame-resistant barrier fabrics found in transportation (aircraft,

train, and automobile) end-uses and in contract furnishings for hotels, offices, auditoriums,

hospitals, and day care centers. Fabrics involved in aircraft and railroad car interior appli-

cations include upholstery, floor coverings, bulkheads, wall coverings, and blankets.

Fire-blocking materials increase the probability of safe egress of passengers from the cabin

in a fire emergency. A fire-blocking fabric or thermal liner in aircraft seating provides a

barrier between the flame source and, for example, a high fuel content polyurethane seat

cushion. A typical construction would be a layer of a spunlaced fabric quilted to a woven m-

aramid fabric to provide both durability and lightweight. The fire-block is designed to retard

or delay ignition of the cushion once the flame has penetrated the outer upholstery fabric.

Because the fibers are inherently flame-resistant, there are no topical treatments that can

wear off or be removed during routine laundering. The abrasion resistance and toughness of

the fiber allows for easy maintenance of fabrics without concern for fading, cracking, or

degradation.

Yarns can be dyed or are producer colored. This allows for the design of attractive

interiors and at the same time, provides the safety of a flame-resistant material. The filament

denier for these applications is higher than that of yarns for apparel fabrics and is generally in

the range of 3–10.


13.8.1.3 Elastomer Reinforcement

There are a few elastomer reinforcement applications where m-aramid yarns are superior to

p-aramid yarns. Continuous filament m-aramid yarn is used in a loose knit construction

to reinforce automotive heater hose. Yarn on yarn abrasion resistance, and not strength, is

key to performance in this application where the hose is exposed to significant thermal,

impulse, and vibrational stresses. A second growing use is in the reinforcement of silicone

elastomer hose for automobile turbochargers where m-aramid provides high thermal stability.

13.8.1.4 Filtration and Felts

Filter bags of m-aramid fiber felts are the material of choice in the bag houses of the hot mix

asphalt (HMA) industry as well as in a variety of other applications. Bag houses are the

preferred air cleaning system because they provide compliance with pollution codes and

provide economic advantages over scrubbers. Bags can be manufactured from a variety of

materials including Teflon1h, fiberglass, polyester, and polyphenylene sulfide, but m-aramids

are the most suitable for HMA plants. Key factors determining this include filtration

performance, chemical resistance, tensile strength, durability, cost, temperature resistance,

and combustibility [119,122].

Bags of Nomex fiber can withstand a continuous operating temperature of 4008 F

(2048C). Additionally the fiber remains dimensionally stable at this temperature—neither

growing nor shrinking more than 1%. The common felt in the industry is a 14 oz=yd2 felt

made of 2 dpf fibers.

m-Aramid felts and fabrics are ideal for heavy-duty laundry textile covers used on

calendars and ironing presses. These materials can meet the thermal stability requirements

of calendars and presses operating at temperatures of up to 2008C. For equipment operating

at lower temperatures (150–1608C), m-aramid fabrics provide greater reliability than lower

cost polyester press covers whose use is still permissible at this temperature range. While heat

resistance is the key criterion for covers, m-aramids also have the advantages of abrasion

resistance, dimensional stability, and very good resistance to hydrolysis.

13.8.2 m-ARAMID PAPER

As we have mentioned earlier, m-aramid papers are produced exclusively by DuPont and thus

most of the application data are based on Nomex papers.

13.8.2.1 Electrical

In the form of paper or pressboards, m-aramids provide an optimum balance of properties for

use as electrical insulation in transformers, motor, generators, and other electrical equipment.

Properly used, these materials can extend the life of an electrical equipment, reduce the

frequency of premature failures, and protect against random electrical stress situations.

Papers and pressboards are made from two m-aramid forms—floc and fibrids. Floc is

yarn cut to a short length. Floc retains the intrinsic properties of the yarn and gives the paper

mechanical strength. Fibrids are microscopic film-like particles that provide dielectric

strength and bind the floc particles together to give the sheet integrity.

Key properties are inherent dielectric strength, mechanical toughness, thermal stability,

chemical compatibility, cryogenic capability, moisture insensitivity, and radiation resistance.

hTeflon1—a registered trademark of E.I. DuPont de Nemours & Co., Inc., Wilmington, Delaware, USA.


Depending on product type and thickness, densified products can withstand high short-term

electrical stresses without further treatment with varnishes or resins. Densified products have

good resistance to tear and abrasion and, in thin grades, are flexible. Electrical and mechan-

ical properties are unaffected at temperatures up to 2008C. Useful properties are maintained

for at least 10 years of continuous exposure at 2208C. Like m-aramid yarns, papers do not

melt and do not support combustion. Products are compatible with all classes of varnishes

and adhesives, transformer fluids, lubricating oils, and refrigerants. At the boiling point of

nitrogen (778K), selected types of Nomex paper and pressboards have tensile strengths

exceeding values at room temperature. In equilibrium at 95% relative humidity, densified

products retain 90% of their bone-dry dielectric strength. Products are unaffected by 800

megarads of ionizing radiation and retain useful electrical and mechanical properties after

eight times this exposure [120,123].

Papers are available in many forms varying in thickness, degree of densification, and

composition (additive type or floc to fibrid ratio). Pressboards, which differ from paper in

thickness and rigidity, are likewise available in several thicknesses and degrees of densifica-

tion. The product of choice will depend on many factors including end-use thermal and

mechanical performance requirements, formability or ease of fabrication, and the desired

degree of saturability.

Applications in transformers include conductor wrap, layer and barrier insulation, coil end

filler, core tubes, section or phase insulation, lead and tap insulation, case insulation,

and spacers. In motors and generators, the superior thermal properties of m-aramid pro-

ducts can enhance both performance and reliability. Their strength and resilience can also

help extend the life of rotating equipment in severe operation conditions. Insulating parts

where m-aramids are used in rotating equipment include conductor wrap, coil wrap, slot

liners, wedges, phase insulation, end-laminations, pole pieces and coil supports, commutator

V-rings, bushings, and lead insulation.

13.8.2.2 Core Structures

Core structures are more commonly referred to as honeycomb structures or cores. Cores of

m-aramid honeycombs with carbon-fiber skins were first used in flooring panels of the British

Aerospace VC-10 BOAC in the late 1960s. In 1970, Boeing’s new generation aircraft, the 747,

flew with a number of interior and exterior components fabricated with aramid core. Since

then, aramid honeycomb cores have become a standard design material for flooring panels,

fairings, radomes, rudders, elevators, cowlings, and thrust reversers. The primary purpose of

core structures is to minimize weight while [121] maximizing stiffness. Lower weight translates

to increased payloads and reduced fuel costs [124].

Aramid cores are made from paper (typically 1.5–4 mil in thickness) comprising m-aramid

floc and fibrids, similar to the papers used in electrical applications discussed in the previous

section. Adhesive node lines are printed on paper sheets that are then stacked, pressed, and

heated to cure the adhesive. The resulting block is expanded. The adhesive-free areas form the

hexagonal cells of the honeycomb configuration. The core is dipped several times in an epoxy

or phenolic resin solution until the desired density and mechanical property levels are

reached. The core is then cut into slices of the desired thickness. Face sheets are glued to

each side of the core. The most common face sheet today is a composite of carbon fiber and

epoxy resin.

Aramid cores have many attributes. m-Aramids have high thermal tolerance and are

compatible with resins with cure temperatures to 4008F. Cores can be fabricated in a wide

range of densities from 1.5 to 10 lb=ft3. They have higher specific shear strength than foam

cores and higher toughness, at equal density, than aluminum, glass, or foam cores. They have


high wet strength and exhibit excellent creep and fatigue performance. Aramid cores do not

corrode and do not promote galvanic action in contact with metals. They are easy to fabricate

and the self-extinguishing character of m-aramids allows the structures to meet stringent

flammability, smoke generation, and toxicity standards.

13.8.2.3 Miscellaneous

Tags and labels of m-aramid paper for in-process bar coding are used where high temperature

stability and chemical resistance are required. In loudspeakers, m-aramid sheets are used

for voice coil insulation and for the speaker cone itself. Bus bars in lithium ion batteries for

portable telephones and computers are insulated with m-aramid paper. Photocopiers and laser

printers that operate at high temperatures use cleaning rollers and webs made from m-aramid

paper.

13.8.3 P-ARAMID FIBER

As m-aramid fibers are best known for their flame resistance, p-aramid fibers are universally

recognized as the material of choice for ballistic protection. While p-aramids do play a critical

role in this application we will attempt to show that their unusual properties are also suitable

for a wide variety of other end-uses.

13.8.3.1 Armor

Aramid-based armor systems are designed to protect individuals and equipment against a

variety of threats in both civilian and military environments. Handgun bullets and knives are

the primary threats encountered in civilian law enforcement work. Military threats are more

wide ranging and generally deal with higher velocity projectiles including rifle bullets, flech-

ettes, and fragments from mortars, grenades, and mines. The design of the optimum protect-

ive system must take into consideration the nature of the threat and therefore civilian and

military systems will necessarily differ. Armor systems can be roughly divided into soft and

hard categories. Soft armor systems are assemblies of woven fabrics that are used to make

bullet-resistant vests, flak jackets, and soft structures such as blankets, curtains, and liners.

Hard or composite armor systems are used in helmets and in structures designed to protect

vehicles, vessels, or shelters. These systems are made of multiple fabric layers impregnated

with a vinyl ester or phenolic–polyvinylbutyral resin binder. Spall liners that are fitted inside

armored military vehicles and protect against fragments resulting from hits by high velocity

shells are a classical example of hard armor.

Beginning in the 1970s high strength fibers—particularly p-aramids—generally displaced

glass and nylon as the preferred fibers for ballistic protection in soft armor. The evolution of

vest design continues today with ever-increasing demands for greater ballistic protection,

less weight, and greater comfort. Initial aramid-based vests of the 1970s had a weight of

1.26 lb=ft2 compared to 1.3 for the incumbent nylon reinforced vests of the 1950s. Today’s

vest weighs even less, about 0.95 lb=ft2, while providing greater ballistic protection. These

advances have been made possible through the use of higher strength yarns with a broader

range of deniers, achieved through spinning process modifications, and by optimizing the

weave pattern of the reinforcing fabrics.

Vests providing ballistic protection do not necessarily provide adequate protection

against threats from sharp implements such as knives. For civilian use, particularly in penal

institutions, vests incorporating p-aramids have been designed that provide protection against

penetration by knife, ice pick, and awl [122,123,125,126,127,128]. Designs that offer both

ballistic and stab protection have also been claimed [124–130].


13.8.3.2 Protective Apparel

p-Aramid yarns are used in protective apparel where cut resistance, thermal resistance, or

abrasion resistance is critical. Applications include gloves and sleeves for automotive, glass,

steel and metal workers, chainsaw chaps and trousers for lumberjacks, and other apparel such

as aprons and jackets. p-Aramid yarn does not support combustion and does not melt

in contrast to competitive products made from nylon, polyester, and polyethylene. Gloves

of p-aramids offer exceptional cut resistance and can substantially reduce the risk of hand and

finger injuries in glass and metal handling operations.

Gloves are made primarily from spun yarns, although some are made from textured

continuous filament yarns for applications where the tendency to form lint must be minim-

ized. Yarn denier per filament can vary from 0.85 to 4.2 dpf with 2.25 dpf the predominant

product. Generally, cut resistance increases as the denier is increased but dexterity is

sacrificed. Gloves are made from 100% aramid yarns or from blends with other fibers,

such as nylon or polyester, to reduce cost or to improve comfort or abrasion resistance.

Yarns can also be spun with steel fibers to provide superior cut resistance. Most gloves are

made of a knit construction although some are cut and sewn from woven fabric. Some p-

aramid gloves are coated or ‘‘dotted’’ with elastomers to enhance grip; others have leather

sewn over the palms and fingers to provide puncture resistance or to increase abrasion

resistance.

p-Aramid gloves can be cleaned using conventional laundering or dry-cleaning processes

with minimal impact on cut resistance. Unlike cotton, these gloves do not shrink when

exposed to hot water or hot air. Overall cost per use can be reduced with cleaning and

reuse, rather than disposal, of soiled items.

13.8.3.3 Tires and Mechanical Rubber Goods

p-Aramids are particularly well suited as reinforcing agents for belts of radial tires and for a

variety of mechanical rubber goods because of their high strength and modulus, excellent

dimensional stability, high temperature durability, and favorable strength to weight ratio. In

spite of these attributes, lower cost steel wire continues to be the reinforcement of choice for

passenger car tires. Nevertheless, aramid cords have slowly made inroads into tire applica-

tions since their introduction in the mid-1970s, particularly in the high performance arena

where the performance to weight ratio is critical. Key performance criteria are speed capabil-

ity, handling, and comfort. Additional factors that favor increasing aramid usage in automo-

bile and truck tires are the ongoing efforts to reduce vehicle weight and to reduce rolling

resistance to reduce energy consumption. Aramids also find use in aircraft, motorcycle, and

bicycle tires where the performance attributes often outweigh cost. Typical yarn deniers for

tire applications are 1000–3000 with a 1.5–2.25 dpf fiber. Product variants include so-called

‘‘adhesion activated’’ yarns that have a surface treatment that facilitates adhesion to the

elastomer and can simplify subsequent tire cord and fabric processing steps by eliminating a

dip-coating step [128,131].

Mechanical rubber goods include hoses, power transmission (PT) belts, and conveyor

belts. Aramids compete with nylon, polyester, glass, and steel in these applications. Steel

dominates the rubber hydraulic hose market and polyester is the reinforcement of choice

in lower pressure thermoplastic hoses. Advantages of aramid vs. other textiles in hose

applications include higher strength, which can lead to constructions with fewer plies and

less weight, and better thermal stability, dimensional stability, and chemical resistance. When

compared with steel, aramid will not corrode and can be fabricated into lower weight, more

flexible hoses.


PT belts can be divided into two categories—v-belts and synchronous belts. Strength,

dimensional stability, fatigue resistance, and adhesion are key reinforcement criteria. Poly-

ester is the primary reinforcing fiber in v-belts where cost considerations are most important.

Aramids can replace polyester in those applications where strength, shock loading, and

dimensional stability requirements outweigh cost. Glass has been the primary reinforc-

ing fiber in timing belts. However, aramid yarn is beginning to replace glass where higher

fatigue performance is required to meet increasing demands for more durable, longer-

lived belts.

In conveyor belts, as in hoses and PT belts, the superior performance potential of

aramid reinforcement must be weighed against the higher material cost. Compared to steel,

equivalent belt strength is achieved at one fifth the weight resulting in ease of handling, lower

energy costs, and lower installation costs. Maintenance and repair costs are reduced because

the fiber does not corrode. Personnel safety is enhanced by the absence of sparking potential.

Aramid reinforced belts have higher strength and modulus than nylon or polyester belts and

can be made thinner or constructed with fewer plies to lower belt weight, simplify handling, or

increase section length by reducing the number of splices.

Yarns are available in high tenacity, high modulus, or high elongation versions to meet the

performance requirements of specific end-uses.

13.8.3.4 Composites

p-Aramids are widely used in composite materials as the sole matrix-reinforcing agent or as a

hybrid in combination with carbon or glass. Composite property balance will differ from

application to application but the key requirement is cost-effective performance at reduced

weight. Glass has lower strength and modulus and higher density than aramid or carbon but

is the most widely used reinforcing fiber because of its low cost. Carbon fibers have the

highest strength and modulus but the lowest elongation. Aramid fibers have a combination of

high strength and modulus (although lower than carbon) with low density and high elonga-

tion that results in improved impact resistance. Composite structures are found in a host of

applications including aerospace components, automobile parts, boats, sporting goods, pro-

truded articles, and pressure vessels. In aircrafts, aramids are used in storage bins, air ducts,

and a variety of core (honeycomb) structures. In general, aramid composites have demon-

strated satisfactory performance in secondary aircraft structures. Aramid’s high tensile

strength lends itself well to the manufacture of canoes where weight can be reduced signifi-

cantly while providing greater tear strength and puncture resistance than fiberglass compos-

ites. Hockey shafts, golf club shafts, fishing rods, skis, and tennis rackets have incorporated

aramid composites. Fishing rods with unidirectional carbon fibers to provide longitudinal

stiffness and aramid fibers woven to provide lateral stiffness yield a high performance rod that

is both light weight and stable. In skis, aramid fibers dampen vibration for smoother, more

comfortable skiing.

13.8.3.5 Optical and Electromechanical Cables

The primary function of p-aramid yarns in fiber optic and electromechanical cables is to

protect the optic glass fiber and ductile power conductors from excessive loading or axial

strain. p-Aramids are well suited to this task because of their high strength and modulus, low

density, and resistance to creep. Yarn is used in two forms. Untwisted yarn is laid along the

length of the cable to provide maximum modulus to resist stretching. Twisted yarn is inserted

as a ripcord to provide maximum strength for tearing the protective sheathing when installing

or repairing cable.


Initial usage as a reinforcing agent in ground cables has largely been replaced by less costly

glass fiber that can provide the necessary strength and modulus where cable weight is not a

critical factor. Aramid yarn is widely used in ADSS (all dielectric self-supporting) aerial

cables where glass is unsuited because of its weight. Higher modulus aramid merges are

used in this application to minimize cable sag and to prevent the cable from coming into close

contact with neighboring electrical lines. Typical yarn deniers are 2840 and multiples thereof.

More recent applications are in so-called premise cables that are used to connect devices

within buildings. These cables provide more bandwidth, have lower power requirements, and

are less costly to maintain than copper lines. Cable diameter is important in this application

and therefore lower yarn deniers are used. These range from 380 to 1420. In addition to the

attributes cited above, the aramid yarn is nonflammable, which allows the cable to pass

mandated burn tests.

For electromechanical cables that are subject to fluctuating loads in use, tension–tension

fatigue performance is key. For this application, aramids are superior to galvanized improved

plow steel wire in fatigue resistance [129,132]. The high strength-per-unit weight of aramids

also allows the cable designer to maximize payload or working length while retaining the ease

of handling of a smaller and lighter system.

13.8.3.6 Ropes and Cables

Like fiber optic and electromechanical cables, p-aramids provide high strength and modulus

and permit the design of cordage with high load carrying capability with smaller, lighter

systems. Yarns are used in a variety of rope and cordage designs such as eight-strand plaited,

single and double braids, parallel strands, and wire-lay construction. The choice of construc-

tion will depend on the balance of properties required for a specific application. Applications

include mooring cables for ship, towlines, elevator cables, and deep-sea cables. Compared to

heavy cables of steel wire, p-aramid cables provide equivalent strength at one fifth the weight

and have a creep rate that approaches that of steel. Lower cable weight can be a significant

factor in enhancing worker safety by reducing the potential for back injuries related to

handling mooring lines. Unlike steel, aramid ropes will not corrode in an aqueous environ-

ment. Aramid ropes must be designed and handled in a way that minimizes the potential for

severe internal or external abrasion and subsequent strength loss. This includes consider-

ations of both rope construction and the appropriate sheave size for a given rope diameter.

A recent innovative machine-room-less traction elevator (ISIS) from ThyssenKrupp takes

full advantage of the properties of p-aramid in the design of the hoist cable and associated

traction sheaves [130–133]. The cable has three times the life of a steel rope, is smaller in size,

and weighs 90% less than a steel rope at a comparable strength rating. The smaller size

permits the use of smaller sheaves thereby decreasing torque requirements and operating

costs. No lubrication is required because the inner strands are Teflon coated. Finally, the

cable transmits less noise and provides a smoother, quieter ride.

Yarns are available in a variety of deniers and merge types that vary in the balance of

tensile properties. Special finishes can be applied to increase lubricity, improve fatigue in wet

applications, or provide better UV resistance. Ropes using Kevlar or Twaron are particularly

useful for static applications or where maximum modulus is required. Technora-based ropes

are suited for dynamic applications where resistance to fatigue is important.

13.8.3.7 Reinforced Thermoplastic Pipe

Reinforced thermoplastic pipe (RTP) is a relatively new composite product. At present there

are four suppliers with products ranging in diameter from 4 to 10 in. and with pressure ratings


up to 100 bars. The pipes are made in continuous lengths of polyethylene with p-aramid

reinforcement [131,134]. Like the ISIS elevator example above, RTP takes full advantage of

the intrinsic attributes of p-aramid fibers in the design of this new fluid transport system.

The oil industry is a major user of pipelines to transport oil and gas. In the oil field, flow

lines connect individual wells to trunk lines that carry the crude to loading docks or to

processing plants. Steel piping has traditionally been used for this application but the pipe

is subject to corrosion from within or without over its lifetime. Leakage caused by corrosion is

inevitable. Prior to the development of RTP, no suitable alternative to steel piping had

been found. The pipeline operator has value for a system that can reduce installation and

lifetime maintenance costs per unit length of pipe while meeting temperature and pressure

requirements. RTP designs incorporating aramid reinforcement appear to have the necessary

characteristics to replace steel piping in the flow line application.

Pipes are constructed with twisted cords to ensure the flexibility required to reel long

lengths of pipe of relatively small diameter. The pipes are lightweight for ease of transporta-

tion and installation. Long lengths simplify installation and maintenance by reducing the

number of couplings. Pipes are corrosion resistant, damage tolerant, and able to withstand

high temperatures and pressures. Advantages of aramids over other reinforcement materials

such as carbon or glass fiber include flexibility, ease of assembly, and damage tolerance

during assembly.

13.8. 3.8 Civil En gineering

Use of composite materials for concrete infrastructure repair that was initiated in the mid-

1980s finally began to proliferate in the mid-1990s. Carbon and glass fiber reinforced epoxy

resin composites have received the most interest. Aramid-based reinforcement has been

viewed as a more specialty product for applications requiring high modulus and where the

potential for electrical conductivity would preclude the use of carbon; for example, in Japan,

aramid sheet is used for all tunnel repair. Product forms include dry fabrics or unidirectional

sheets as well as pre-cured strips or bars. Fabrics or sheets are applied to a concrete surface

that has been smoothed (by grinding or blasting) and wetted with a resin (usually epoxy).

After air pockets are removed using rollers or flat, flexible squeegees, a second resin coat

might be applied. The process is repeated for additional plies [132,135].

Reinforcement of concrete structures is important in earthquake prone areas such as Japan,

Turkey, and Taiwan. Although steel plate is the primary material used to reinforce and repair

concrete structures, higher priced fiber-based sheet structures offer advantages for small sites

where ease of handling and corrosion resistance are important. The high strength, modulus, and

damage tolerance of aramid-reinforced sheets makes the fiber especially suitable for protecting

structures prone to seismic activity. The use of aramid sheet also simplifies the application

process. Sheets are light inweight and canbe easilyhandledwithout heavymachinery and canbe

applied in confined working spaces. Sheets are also flexible, so surface smoothing and corner

rounding of columns are less critical than for carbon fiber sheets [133,136].

13.8.4 P-ARAMID PAPER

13.8.4.1 Core Structures

p-Aramid core structures are analogous to core structures based on m-aramids (Section

13.8.2.2) but the base paper uses stronger and stiffer p-aramid floc instead of m-aramid

floc. In addition the component ratio of floc to fibrid is increased. This results in a more

porous sheet structure that allows better penetration of the matrix resin in the dipping step. In

addition to retaining all the attributes of m-aramid based cores, p-aramid cores have higher


shear strength, higher modulus, and greater fatigue resistance at similar cell size and density.

They also have higher hot–wet shear and compression properties than the m-analogues.

p-Aramid cores also bring process advantages because of the lower thermal expansion

coefficient and lower moisture-regain of the component fibers. This translates to improved

dimensional stability and the ability to retain shape and dimensions throughout the fabrica-

tion and part consolidation process.

Because of their superior compression, shear, and fatigue properties, structures based on

p-aramid cores allow even greater weight reduction than incumbent m-aramid cores. Recent

commercial adoptions include flooring panels in weight critical programs such as the

extended Airbus A-340 and the double deck Airbus A-380. p-Aramid cores have also replaced

m-aramid cores in the elevators and rudders of these aircrafts [121,124], because of their

superior hot–wet characteristics.

13.8.4.2 Printed Wiring Boards

Printed wiring boards (PWDs) made of p-aramid papers take advantage of the low axial

coefficient of thermal expansion (CTE) of the fiber to restrain in-plane expansion of the

impregnated resin when heat is applied to the composite laminate. Low CTE boards reduce

the strain on solder joints of leadless ceramic chip carriers used in traditional avionics and

military applications. In addition, low CTE laminates provide a reliable base for mounting

new high-density chip packages where solder joint failure due to thermal cycling is a concern.

These include the thin small outline package (TSOP) used for memory chips, the solder grid

array (SGA) microprocessor package, and the high lead count ball grid array (BGA).

Nonwoven aramid reinforcement is prepegged with epoxy resin on the same vertical path

treaters that are used to process fine weave E-glass. At a resin loading of 45–55% by weight,

the finished PWB has an in-plane CTE of 9–11 ppm=8C. [134,139].

13.8.4.3 p-Aramid Pulp

13.8.4.3.1 Brake Linings or Pads and Clutch FacingsAsbestos was the primary reinforcing agent used in friction materials before it was banned by

Congressional legislation in 1978 for health reasons. Two classes of formulations were

developed to replace asbestos: semimetallic and nonasbestos organic. Each has its own

specific limitations and attributes. p-Aramid in the form of pulp is one of the few organic

materials suited to the thermal demands of friction applications. Acrylic fiber in the form of

pulp has also been used where temperature requirements are less severe. Pulp retains the

strength, stiffness, and thermal properties of the precursor fiber and, in addition, provides

surface area in the order of 7–15 m2=g. This high surface area serves as a processing aid in

certain manufacturing steps and also as a retention aid for multicomponent brake formula-

tions. High fiber strength can lead to higher pad shear strength and increased resistance to

cracking. Fiber thermal stability can influence the nature of the critical transfer layer that

forms between the pad and the rotor. Brake formulations are optimized for a variety of

performance characteristics such as wear, frictional behavior, and noise. Aramid pulp, at

volume percentage levels of <1 to ~10, will influence each of these properties but overall

performance is highly dependent on the combined performance of all of the components in

the formulation.

Clutch facings are made from wet pulp and staple yarn. Friction papers for automatic

transmissions are made from wet pulp that is formed into a sheet on a paper making machine

and then impregnated with phenolic resin. Pulp provides strength in the initial paper making

process and tensile strength in the final composite structure. The fibrillar pulp also influences

sheet porosity. Sheet porosity is essential in this application to ensure adequate permeation of


the transmission fluid to dissipate heat generated in service. The combined attributes of

strength, heat and wear resistance, and durability that pulp brings to friction papers have

become increasingly important as designers continue to reduce the number and size of plates

in the transmission and, at the same time, auto manufacturers extend the warranty period for

the transmission.

Manual transmissions use a clutch facing made from a resin impregnated wound structure

composed of staple yarns. p-Aramid in yarn form provides more strength and durability than

a pulp-based paper sheet in this more demanding application. Although aramid reinforced

facings have sufficient thermal stability for this application, compositions based on glass and

metal fibers dominate this market.

13.8.4.3.2 GasketsLike friction materials, asbestos was widely used in high temperature, high performance

gaskets prior to the legislation in 1978. Asbestos was highly effective, very cheap, and

comprised 80–85% of the weight of the gasket. Aramid pulp brought high strength and

thermal stability to this end-use but the fiber cost was an order of magnitude higher than

that of asbestos. To reduce this cost penalty, formulations with only 5–20% aramid and 60–

80% inert fillers have been developed that provide goal performance in both compressed and

beater-add type gaskets.

Compressed gaskets are made on a two-roll calendar from a mix of pulp, elastomer, fillers,

curing agents, and toluene. Final gasket properties are very dependent on both the processing

conditions and the specific gasket formulation. Tensile strength depends primarily on the

amount and type (length and surface area) of pulp selected. Stress retention, compression and

recovery, and sealability depend on a combination of factors including the relative amounts

of fiber and elastomer, the type and particle size of the filler materials, as well as the mixing

and calendaring conditions.

Beater-add gaskets are made in an aqueous paper making type process. Ingredients such

as pulp, elastomer, fillers, curing agent, precipitation regulator, and precipitant are com-

bined in water. The resultant slurry is laid down on a screen to drain the water and form a

sheet that is then calendared and press cured. As with compressed gaskets, properties

will depend on both process conditions and the relative amount and type of ingredients.

Beater-add gaskets have been used primarily in cylinder head and other engine gaskets.

Today, many auto manufacturers are replacing these beater-add gaskets with gaskets of

multilayered steel.

13.8.4.3.3 Elastomer and Resin Reinforcementp-Aramid in pulp and short fiber (1.5–6 mm length) forms is an effective reinforcing agent in

both elastomer and thermoplastic resin matrices. Compared to traditional particulate reinfor-

cing agents in elastomers such as carbon black and silica, aramid pulp provides superior

reinforcement at much lower loadings. Advantages of pulp-reinforced elastomers include

high low-strain modulus, property anisotropy, greater cut and abrasion resistance, improved

wear performance and, in tire stocks, lower rolling resistance. These attributes are achieved,

however, only when the pulp is fully dispersed in the rubber stock. Because high surface area

pulp is rather difficult to open and wet out using standard rubber compounding processes,

concentrated pulp masterbatches (Kevlar engineered elastomer and Rhenogran) have been

formulated that allow compounders to more easily achieve adequate dispersion using stand-

ard mixing techniques. These masterbatches are available in a variety of elastomers including

SBR, NBR, natural rubber, polychloroprene rubber, and EPDM [135,136,138,139].

Applications utilizing pulp and short fiber reinforcement include PT belts, tires, roll

covers, hoses, and footwear. In v-belts, both wear resistance and durability increase. In


synchronous timing belts, pulp placed in the tooth area increases modulus and reduces the

propensity of tooth chunking or chipping. Pulp is used in high performance bicycle and

motorcycle tires to improve handling characteristics and to increase puncture resistance. Use

in several components of automobile tires continues to be investigated. In roll covers,

improvements in tear and abrasion resistance are achieved without affecting compound

hardness or processibility.

Use of p-aramids in molded or extruded thermoplastic parts offers performance advan-

tages over neat resins or glass-reinforced resins. Aramid reinforced parts exhibit improved

mechanical and thermal properties and superior wear resistance with no abrasion to the

counter surface. Because the fiber is not abrasive, there is less damage to processing equipment

and machining of parts is simplified.

13.8.4.3.4 Sealants and AdhesivesDry pulp is used as a thixotrope in sealants and adhesives to provide viscosity control at low

cost. Viscosity is presumably built through the formation of physical networks of entangled

fibrils of the high surface area pulp. Sag or run of applied sealants, adhesives, or coatings is

thereby minimized. With shear, the viscosity of these fluids decreases as the networks break

down, which facilitates application by spraying, brushing, or other means.

Compared to a common thixotrope such as fumed silica, pulp provides equivalent

viscosity at less than one tenth the weight in a typical epoxy resin. In addition, fluid viscosity

is unaffected by further processing (agitation) or aging—in contrast to fumed silica modified

resin where viscosity drops and is not fully recovered under similar conditions. Pulp can also

provide reinforcement in an adhesive matrix as shown by the significant increase in tensile

strength, modulus, and tear strength of both a PVC plastisol adhesive and a silicone sealant

on the addition of pulp [137,140].

13.9 CONCLUSIONS AND DIRECTION

This brief review of aramid fibers has summarized the very broad range of unusual function-

alities that these products bring. While the chemistry plays an important role in defining the

scope of applications for which these materials are suited, it is equally important that the final

parts are designed to maximize the value of the inherent properties of these materials.

TABLE 13.9Properties of High Performance Fibers

Fiber Twaron HM Carbon HS PBO M5 experimental M5 target

Tenacity GPa 3.2 3.5 5.5 5.3 9.5

Elongation % 2.9 1.4 2.5 1.4 >2

E modulus GPa 115 230 280 350 400–450

Compressivea strength GPa 0.48 2.1 0.42 1.6 2

Compressivea strain % 0.42 0.9 0.15 0.5 0.5

Density g=cm3 1.45 1.8 1.56 1.70 1.7

Onset of thermal degradation 8C 450 800 550 530 530

LOI % 29 N=A 68 >50 >50

aIn epoxy resin—3-point bending test.

Source: From Magellan International; Teijin Ltd., Teijinconex Heat Resistant Aramids Fiber 02.05.


It is very clear that these unusual properties are derived from structures that are quite

different from those of incumbent materials; for example, to obtain very high strength and

stiffness the polymer molecules must be perfectly oriented and fully extended, which leads to

the highly anisotropic nature of the fibers. That is one of the major reasons why associated

applications research efforts have gained such importance. The ultimate products have to be

designed to take this anisotropy into account.

We hope that we were able to clearly exemplify the constant trade-off between function-

ality and processability that is an ongoing challenge with these advanced materials. The

functionality that allows these materials to perform under extreme conditions has to be

balanced against processability that allows them to be economically shaped into useful

forms. This requirement is responsible for the fact that from hundreds of compositions

evaluated in the laboratory only a handful are commercially viable.

The fundamental science of structure–property relationship developed as a result of work

on aramids is being extended to other chemistries and offers the potential to develop materials

with even more impressive properties (Table 13.9).

N

O

N

O

PBO

OHN

OH

N

NH

N

NH

M5

N

NH

N

NH

PBI

The structures shown above illustrate the movement to a higher level of aromatic ‘‘content’’

to obtain even better thermal and flame performance. In the case of PBO and M5, the

structures are even more rigid than those of p-aramids and offer the potential for even greater

properties. This is achieved at the expense of ease of processability and at a significantly

higher cost. It is very clear that these compositions will not replace p-aramids but will likely be

an important supplement to our ‘‘tool box’’ of solutions to problems that we face.

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Aramid Fibers From Fiber Chemistry 3rd