Rubber Compounding_Brendan Rodger

11Vulcanization

Frederick Ignatz-Hoover and Brendan H. ToFlexsys America LP, Akron, Ohio, U.S.A.

I. INTRODUCTION—TERMINOLOGY***

The following is a short list of terminology commonly used within rubberindustry discussions of vulcanization of general-purpose elastomers. Whereindicated, reference is made to specific test methodologies.

Vulcanization is the process of treating an elastomer with a chemical todecrease its plasticity, tackiness, and sensitivity to heat and cold andto give it useful properties such as elasticity, strength, and stability.Ultimately, this process chemically converts thermoplastic elasto-mers into three-dimensional elastic networks. This process convertsa viscous entanglement of long-chain molecules into a three-dimensional elastic network by chemically joining (cross-linking)these molecules at various points along the chain. The process ofvulcanization is depicted graphically in Figure 1. In this diagram,the polymer chains are represented by the lines and the cross-linksby the black circles.

Scorch refers to the initial formation of an extensive three-dimensionalnetwork rendering the compound elastic. The compound is thus nolonger plastic or deformable and cannot be shaped or further pro-cessed. Scorch safety is the length of time for which the compound

* Although based on ASTM D-1566-80b, these definitions have been modified to fit this

discussion.

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MD: RODGERS, JOB: 03286, PAGE: 505

Copyright © 2004 by Taylor & Francis

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can be maintained at an elevated temperature and still remain plas-tic. This time marks the point at which the plastic material beginsthe chemical conversion to the elastic network. Thus if the com-pound scorches before it is formed into the desirable shape or com-posite structure it can no longer be used. Time to scorch is thusimportant because it indicates the amount of time (heat history) thecompound may be exposed to heat during shaping and formingoperations before it becomes an intractable mass.

Rate of cure or cure rate describes the rate at which cross-links form.After the point of scorch, the chemical cross-linking continues pro-viding more cross-links and thus greater elasticity or stiffness (mod-ulus). The rate of cure determines how long a compound must becured in order to reach ‘‘optimum’’ properties.

Cure time is the time required to reach a desired state of cure. Mostcommon lab studies use the t90 cure time, which is the time requiredto reach 90% of the maximum cure.

State of cure refers to the degree of cross-linking (or cross-link density)of the compound. State of cure is commonly expressed as a percent-age of the maximum attainable cure (or cross-link density) for a givencure system. The elastic force of retraction, elasticity, is directly pro-portional to the cross-link density or number of cross-links formed inthe network.

Reversion refers to the loss of cross-link density as a result of non-oxidative thermal aging. Reversion occurs in isoprene-containingpolymers to the extent that the network contains polysulfidic cross-links. Reversion converts a polysulfidic network into a network richin monosulfidic and disulfidic cross-links and, most important, onethat has a lower cross-link density than the original network. Re-

Figure 1 In vulcanization the randomly oriented chains of raw rubber becomecross-linked as indicated diagrammatically at the right.

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version does not occur or hardly occurs in isoprene polymers curedwith vulcanization systems designed to produce networks rich inmonosulfidic and disulfidic cross-links. Reversion is commonlycharacterized by the time required for a defined drop in torque inthe rheometer as measured from the maximum observed torque.

‘‘Network maturation’’ is a term used to describe chemical changes tothe network imparted by the action of the curatives through con-tinued heating beyond the cure time required to provide for optimalproperties. In isoprene polymers the effect is commonly referred toas reversion. However, in butadiene-containing polymers the effectis to reduce polysulfidic networks to networks rich in monosulfidicand disulfidic cross-links and having greater cross-link density thanthe original network. This slow increase in modulus with time isoften called a ‘‘marching modulus.’’

Vulcanizing agents are chemicals that will react with active sites in thepolymer to form connections or cross-links between chains.

An accelerator is a chemical used in small amounts with a vulcanizingagent to reduce the time of (accelerate) the vulcanization process. Insulfur vulcanization today, accelerators are used to control the on-set, speed, and extent of reaction between sulfur and elastomer.

Activators are materials added to an accelerated vulcanization systemto improve acceleration and to permit the system to realize its fullpotential of cross-links.

Retarders are chemicals used to reduce the tendency of a rubber com-pound to vulcanize prematurely by increasing scorch delay (timefrom beginning of the heat cycle to the onset of vulcanization).Ideally, a retarder would have no effect on the rate of vulcanization.Such an ideal retarder has been called a prevulcanization inhibitor,or PVI.

The kinetics of vulcanization are studied using curemeters or rheom-eters that measure the development of torque as a function of time at a giventemperature. An idealized cure curve is given in Figure 2. Several importantvalues derived from the rheometer characterize the rate and extent ofvulcanization of a compound. Critical values include the following.

MI or Rmin. The minimum torque in the rheometer. This parameteroften correlates well with the Mooney viscosity of a compound(Fig. 2).

Mh or Rmax. The maximum torque achieved during the cure time.ts2. The time required for the state of cure to increase to two torque

units above the minimum at the given cure temperature. This param-eter often correlates well with the Mooney scorch time.

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t25. The time required for the state of cure to reach 25% of the fullcure defined as (Mh � Ml). Generally a state of cure of about 25–35% is necessary to prevent the development of porosity when alarge rubber article is removed from a curing press. This level ofcure also provides enough strength to prevent the article from tearingas it is removed from a curing mold.

t90. The time required to reach 90% of full cure defined as Mh � Ml.t90 is generally the state of cure at which the most physical pro-perties reach optimal results.

II. VULCANIZING AGENTS

Sulfur is the oldest and most widely used vulcanizing or cross-linking agentand will be the vulcanizing agent of interest in most of this discussion. Themajority of cure systems in use today involve the generation of sulfur-containing cross-links, usually with elemental sulfur in combination with anorganic accelerator. In recent years, the proportion of sulfur has tended tofall and the levels of accelerator and the use of sulfur donors have increasedto give great improvements in the thermal and oxidative stability of thevulcanizate. Other vulcanization systems that do not use sulfur or sulfurdonors are less commonly used and include various resins such as resorcinol-formaldehyde resins, urethanes, or peroxides. Metal oxides or sulfur-acti-vated metal oxides can be used for halogenated elastomers.

Figure 2 Rheometer curve.

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About 150 years ago, Goodyear (1) in the United States and Hancock(2) in England discovered that India rubber could be changed by heating itwith sulfur so that it was not greatly affected by heat, cold, and solvents. Thisprocess was termed ‘‘vulcanization’’ deriving from the association of heat andsulfur with the Vulcan of mythology.

Since that time, many other chemicals have been examined as possiblevulcanizing agents with some degree of success. Sulfur vulcanizates providean outstanding balance of cost and performance, exhibiting excellent strengthand durability for very low cost. No other cure system has, on its own,successfully competed with sulfur as a general-purpose vulcanizing agent.One limitation imposed upon the use of sulfur as a vulcanizing agent is thatthe elastomer must contain some chemical unsaturation. In saturated elas-tomers, other chemicals, particularly organic peroxides, have been foundquite useful.

We will therefore consider elemental sulfur and sulfur-bearing chem-icals (sulfur donors) as one class of vulcanizing agents and non-sulfur vul-canizing agents as a second class.

A. Sulfur and Sulfur Donors

Sulfur vulcanization occurs by the formation of sulfur linkages or cross-linksbetween rubber molecules, as shown in Figure 3. In conventional sulfurvulcanization (generally formulated as a high sulfur-to-accelerator ratio) theresultant network is rich in polysulfidic sulfur linkages. Sulfur chain linkagescan contain six or more sulfur atoms. Lower sulfur-to-accelerator ratiosproduce networks that are characterized by a greater number of sulfurlinkages containing fewer sulfur atoms. Thus, the so-called efficient vulcan-ization systems produce higher cross-link densities for the same loading ofsulfur. At very low sulfur-to-accelerator ratios, networks can be producedthat are composed predominantly of monosulfidic and disulfidic cross-links.

Figure 4 depicts the general changes in vulcanizate physical propertiesas the vulcanization state of the rubber changes. As the cross-link density ofthe vulcanizate increases (or the molecular weight between cross-linksdecreases), elastic properties such as tensile and dynamic modulus, tear and

Figure 3 Sulfur vulcanization.

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tensile strength, resilience, and hardness increase whereas viscous loss prop-erties such as hysteresis decrease. Further increases in cross-link density willproduce vulcanizates that tend toward brittle behavior (see Fig. 4). Thus athigher cross-link densities such properties as hardness and tear and tensilestrength plateau or begin to decrease. As a consequence, proper compoundingmust be done to provide the best balance in properties for the specifiedapplication.

Unaccelerated sulfur vulcanization is a slow, inefficient process. For thisreason, over a century of research efforts have been directed toward the de-velopment of materials to improve the efficiency of this process. The activa-tors, accelerators, and retarders to be discussed in later sections have resultedfrom these endeavors.

Another class of chemicals, known as sulfur donors, have been devel-oped to improve the efficiency of sulfur vulcanization. These materials areused to replace part or all of the elemental sulfur normally used in order toproduce vulcanized products containing fewer sulfur atoms per cross-link. Inother words, these materials make more efficient use of the available sulfur.The two most common sulfur donors are the disulfides tetramethylthiuram(TMTD*) (1) and dithiodimorpholine (DTDM) (2).

* A complete list of the abbreviations used in this chapter is given in Table 1.

Figure 4 Effects of vulcanization on physical properties. 1, Tear strength; 2, dynamicmodulus; 3, hardness; 4, hysteresis, permanent set; 5, static modulus; 6, tensile strength.

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Table 1 Recognized Industry Abbreviations for Accelerators

Abbreviation Chemical name Flexsys trade name

CBS N-Cyclohexyl-2-benzothiazolesulfenamide Santocure CBSCTP N-(Cyclohexylthio)phthalimide Santogard PVI

DBTU N,NV-DibutylthioureaDCBS N,N-Dicyclohexyl-2-benzothiazolesulfenamide Santocure DCBSDETU N,NV-Diethythiourea

DOTG Di-o-tolylguanidineDPG Diphenylguanidine Perkacit DPGDPTH Dipentamethylenethiuram hexasulfide

DTDM Dithiodimorpholine Sulfasan DTDMETU EthylenethioureaMBS 2-(Morpholinothio)benzothiazolesulfenamide Santocure MBSMBT 2-Mercaptobenzothiazole Perkacit MBT

MBTS Benzothiazyl disulfide Perkacit MBTSNDPA N-NitrosodiphenylaminePEG Polyethylene glycol

TBBS N-t-Butyl-2-benzothiazolesulfenamide Santocure TBBSTDEDC Tellurium diethyldithiocarbamate Perkacit TDECTETD Tetraethylthiuram disulfide Perkacit TETD

TMQ Polymerized 2,2,4-trimethyl-1,2-dihydroquinoline Flectol TMQTMTD Tetramethylthiuram disulfide Perkacit TMTDTMTM Tetramethylthiuram monosulfide Perkacit TMTM

TMTU TrimethylthioureaZBDC Zinc dibutyldithiocarbamate Perkacit ZDBCZBPD Zinc o-di-n-butylphosphorodithioate Vocol ZBPDZDEC Zinc diethyldithiocarbamate Perkacit ZDEC

ZDMC Zinc dimethyldithiocarbamate Perkacit ZDMCZMBT Zinc salt of 2-mercaptobenzothiazole Perkacit ZMBT6PPD N-1,3-Dimethylbutyl-N-phenyl-p-phenylenediamine Santoflex 6PPD

ETPT Bis(diethyl thiophosphoryl) trisulfideBDITD Bis(diisopropylthiophosphoryl) disulfide

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Tetramethylthiuram acts as an accelerator as well as a sulfur donor. As aconsequence, compounds containing TMTD tend to be cure rate activated;that is, they are more scorchy and have faster cure rates. These materials areusually used with the objective of improving thermal and oxidative agingresistance. Use of sulfur donors increases the level of mono- and disulfidiccross-links, which are reversion-resistant and more stable toward oxidativedegradation. However, sulfur donors can also be used to reduce the possibilityof sulfur bloom (by reducing the level of free sulfur in a formulation) and tomodify curing and processing characteristics.

B. Non-Sulfur Cross-Links

The vast majority of rubber products are cross-linked by using sulfur. Thereare, however, special cases or special elastomers for which non-sulfur cross-links are necessary or desirable.

1. Peroxide Vulcanization

In peroxide vulcanization, direct carbon cross-links are formed between elas-tomer molecules as shown in Figure 5 (i.e., no molecular bridges as there arein sulfur cures.)

The peroxides decompose under vulcanization conditions, forming freeradicals on the polymer chains, which leads to the direct formation of cross-links. Peroxides can be used to cross-link a wide variety of both saturated andunsaturated elastomers, whereas sulfur vulcanization will occur only in un-saturated species.

In general, carbon–carbon bonds from peroxide-initiated cross-linksare more stable than the carbon–sulfur–carbon bonds from sulfur vulcan-ization. Thus, peroxide-initiated cures often give superior aging propertiesto the rubber products. However, peroxide-initiated cures generally representhigher cost to the processor and require greater care in storage and processing.

A wide variety of organic peroxides are available, including productssuch as benzoyl peroxide and dicumyl peroxide. Proper choice of peroxideclass must take into account its stability, activity, intended cure temperature,and effect on processing properties.

Figure 5 Peroxide-initiated vulcanization.

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Carbon–carbon cross-links can also be initiated by gamma or X-radia-tion; these presently find limited commercial application.

2. Resin Vulcanization

Certain difunctional compounds form cross-links with elastomers by reactingwith two polymer molecules to form a bridge. Epoxy resins are used withnitrile, quinone dioximes, and phenolic resins with butyl rubber and dithiolsor diamines with fluorocarbons. The most important of these is the use ofphenolic resins to cure butyl rubber. This cure system is widely used for thebladders used in curing new tires and the curing bags used in the retreadindustry. The low levels of unsaturation of butyl rubber does require resincure activation by halogen-containing materials such as SnC12.

3. Metal Oxide Vulcanization

The polychloroprene rubber (CR or neoprene) and chlorosulfonated poly-ethylene (CSM or HypalonR) are vulcanized with metal oxides. The reactioninvolves active chlorine atoms, but not much is known about the nature of theresultant cross-links.

4. Urethane Vulcanization

Workers at the Malaysian Rubber Producers Association (MRPRA) haveproposed urethanes as an alternative form of cross-linking to that based onsulfur bridges (3), and vulcanizing chemicals based on such products arecommercially available. The vulcanizing agent in these systems is derivedfrom p-benzoquinone monoxime ( p-nitrosophenol) and a di- or polyisocya-nate. Unlike sulfur vulcanization, accelerators are not necessary, but theefficiency of the process is improved by the presence of free diisocyanate andby ZDMC. The latter catalyzed the reaction between the nitrosophenol andthe polymer chain to form pendant groups.

The principal advantage of these systems lies in the high stability of thecross-links, which give very little modulus reversion even on extreme over-cure. Problems can occur with their lower scorch, rate of cure, and modulus.However, modulus and fatigue life retention on aging are very good. Work ina number of laboratories is aimed at seeking cross-link systems that will bethermally labile at high temperatures but perform elastically at operatingtemperatures, thus bringing rubber molding closer to plastics technology.

One such patent (4) uses an elastomer obtained by reacting a metal saltwith a coordinating basic group present in an elastomer containing anelectron-donating atom. Co polymers of butadiene rubber, styrene butadienerubber, and vinylpyridine may be used with zinc, nickel, and cobalt chlorides.

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III. ACTIVATORS

Realization of the full potential of most organic accelerators and cure systemsrequires the use of inorganic and organic activators. Zinc oxide is the mostimportant inorganic activator, but other metallic oxides (particularly mag-nesium oxide and lead oxide) are also used. Although zinc has long beentermed an activator, zinc or another divalent metal ion should be consideredto be an integral and required part of the cure system. As shown below, zinchas a profound effect on the extent of cure achievable in accelerated sulfurvulcanization and thus should be expected to be inherently active at the sul-furation step. The most important organic activators are fatty acids, althoughweak amines, guanidines, ureas, thioureas, amides, polyalcohols, and aminoalcohols are also used.

The large preponderance of rubber compounds today use a combina-tion of zinc oxide and stearic acid as the activating system. Several studies(5–9) have been published on the effects of variations in the concentrationsof these activators. In general the use of the activators zinc oxide and stearicacid improves the rate and efficiency of accelerated sulfur vulcanization.Rheographs obtained on stocks containing various combinations of curesystem components are shown in Figure 6.

In the absence of an accelerator, the activators zinc oxide and stearicacid are ineffective in increasing the number of cross-links produced (Fig. 6,

Figure 6 Effect of activators on cure rate (100 NR).

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compound 2). The use of an unactivated sulfenamide accelerator with sulfurproduces a significant increase in torque (cross-links) in a reasonable period oftime (Fig. 6, compound 3). This stock, however, would not be considered to bevery well cured by today’s standards.

The addition of zinc oxide to the accelerated stock as the only activatorproduces a dramatic effect and a well-cured stock. This demonstrates thecritical role of zinc in accelerated sulfur vulcanization. The boost in efficiencysuggests that zinc should be considered an integral component of theintermediate responsible for the attachment of sulfur to the rubber in thecross-link reactions. In order for zinc to be used effectively, it must be presentin a form that can react with the accelerator system. This means that the zincmust be in a soluble form, or a very fine particle size zinc oxide must be used(so that it can be readily solubilized). Most natural rubbers and somesynthetics contain enough fatty acids to form soluble zinc salts (from addedzinc oxide) that interact with the accelerators. Sulfenamide-accelerated cureswill release free amine, which produces a soluble zinc amine complex from thezinc oxide. To ensure that sufficient acids are available to solubilize zinc, it iscommon to add 1–4 phr of stearic acid or a similar fatty acid. In addition tosolubilizing zinc, the fatty acid serves as a plasticizer and/or lubricant toreduce the viscosity of the stock. The use of fatty acid soaps permits fulldevelopment of cross-links by the organic accelerator as shown for compound9 in Figure 6.

Other methods are also used to provide a soluble form of zinc ions. Basiczinc carbonates are more soluble in rubber than fine-particle zinc oxide andcan therefore be used in higher concentrations. Soluble fatty acid zinc salts areused to provide both better dispersion and solubility of zinc ions. Commonsalts are zinc stearate and zinc 2-ethylhexoate.

IV. ACCELERATORS

Although many people consider that the development of accelerators began inthe early 1900s, the first vulcanization patent issued in the United States (1)described the ‘‘combination of said gum with sulfur and white lead to form atriple compound.’’ Whatever the course of Goodyear’s experimentation in1839, his first patent covered an accelerated vulcanization with sulfur. Sincethat time, many people have studied the use of inorganic and organiccompounds as accelerators for sulfur vulcanization.

In the nineteenth century, a number of inorganic compounds, particu-larly oxides and carbonates, were used as accelerators. These materials didgive shorter curing times but gave little improvement in physical properties. In

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the early 1900s, the accelerating effect of basic organic compounds was dis-covered. In 1906, Oenslager (10) found that aniline and other amines ac-celerated sulfur vulcanization. Since that time, emphasis has been placed onnitrogen- and sulfur-containing organic compounds. Important milestonesalong the way have been the discovery of dithiocarbamates in 1918, of 2-mercaptobenzothiazole (MBT) in 1921, and of benzothiazole sulfenamides in1937.

Today, the rubber compounder has available more than 100 singleproducts of known composition and 37 blends and unspecified materials(11–13). Accelerators and accelerator systems are chosen on the basis of theirability to control the following processing/performance properties of rubbercompounds:

1. Time delay before vulcanization begins (scorch safety)2. Speed of the vulcanization reaction after it is initiated (cure rate)3. Extent of the vulcanization after the vulcanization reaction is com-

plete (state of cure)4. Other factors such as green stock storage stability, fiber or steel

adhesion, and bloom tendency

The job of the compounder, therefore, becomes one of selecting andevaluating individual accelerators and/or combinations of accelerators. Theproliferation of accelerator types should be viewed as an opportunity, becauseit often gives compounders a chance to custom fit curing systems to theirprocessing and/or performance needs. This section attempts to categorize andpredict performance within and between generic classes of accelerators. Likemany reviews, it draws generalizations that may often be violated. The ex-perienced compounder will find numerous instances where performanceorders are reversed or otherwise out of order in compounds he has developed.Rather than a definitive list of exact properties, the following reflects anexpectation of what an accelerator response might be if there are no other dataavailable from which to draw conclusions.

A. Accelerator Classes

Accelerators can beclassified chemicallyand functionally. The principal chem-ical classes of accelerators in commercial use today are listed in Table 2.Functionally, these compounds are typically classified as primary or sec-ondary accelerators (including ultra-accelerators, or ‘‘ultras’’). Compoundsclassified as primary accelerators usually provide considerable scorch delay,medium-to-fast cure rates, and good modulus development. Compoundsclassified as secondary accelerators or ultras usually produce scorchy, veryfast curing stocks.

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Generally accepted functional classifications of the accelerators are asshown in Figure 7.

By proper selection of these accelerators and their combinations, it ispossible to vulcanize rubber at almost any desired time and temperature. Ofcourse, the speed of vulcanization is not the same for all polymers. Elastomersthat contain 100% unsaturation (i.e., NR, BR) will cure faster with a givenvulcanization system than will polymers that contain fewer double bonds suchas SBR (85 mol% unsaturation) and NBR (50–75 mol% unsaturation). Inthese polymers, it is common to use higher accelerator levels and less sulfur.However, the relative relationships between accelerators are similar in all ofthese elastomers, and the comparison between accelerator classes shown inFigure 8 is typical.

The development of an activated sulfenamide cure system to meetspecific requirements of processing and physical properties requires both aselection and a refining process. The initial selection of the primary and pos-sibly secondary accelerators to be used is based primarily upon the needs ofcure rate, time, and processability balanced by cost. After this decision hasbeen made, a systematic study is required to fit these accelerators to the spe-cific process conditions to be encountered.

To assist in this process, we first look at a comparison of primary andsecondary accelerators. Then, the effects of primary-to-secondary ratiosand total concentrations will be examined. In each case, the comparison willbe based upon Mooney scorch, rheometer cure characteristics, and tensilemodulus.

1. The Mechanism of Zinc-Mediated Accelerated SulfurVulcanization

Historical and General Aspects Related to the Mechanism of Sulfur Vul-canization. Much is known about accelerated sulfur vulcanization of the

Table 2 Accelerator Classes

Class Response speed Acronyms

Aldehyde-amine Slow —

Guanidines Medium DPG, DOTGThiazoles Semi-fast MBT, MBTSSulfenamides Fast, delayed action CBS, TBBS, MBS, DCBS

Dithiophosphates Fast ZBPDThiurams Very fast TMTD, TMTM, TETDDithiocarbamates Very fast ZDMC, ZDBC

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various diene elastomers. various elastomers. Each elastomer shows differ-ences in various aspects of its vulcanization chemistry. These differences arerelated to the physical and chemical nature of the elastomer under consid-eration and to the cure systems employed. Several reviews discuss in detailthe early work that led to the prevailing theories on vulcanization: Chapmanand Porter (12) rigorously summarize the chemistry of sulfur vulcanizationin natural rubber, and Kresja and Koenig (13) cover sulfur vulcanizationin various other elastomers. Most recently, quantitative structure–activityrelationship studies (QSAR) have shed more insight into the nature of theactive sulfur–accelerator–zinc complex involved in the vulcanization reac-tion (41).

There are many classes of compounds that can serve as accelerators insulfur vulcanization as shown in Table 2. A feature common to vulcanizationaccelerators is some form of a tautomerizable double bond. In fact, the mostactive contain the UNjCUSUH functionality. This is the common struc-

Figure 7 Primary and secondary accelerators.

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tural unit found in all of the 2-mercapto-substituted nitrogen heterocyclicaccelerators known today. Note that the delayed action precursors, 2-mecaptobenzothiazole disulfide, sulfenamides, and sulfenimides of 2-mercap-tobenzothiazole decompose to form 2-mercaptobenzothiazole, a structurethat contains this NjCUSU functionality.

By comparing vulcanization activity in accelerators derived from 4-mercaptopyridine and 2-mercaptopyridine, Rostek et al. (14) showed that theposition of sulfur ortho to the heteroatom (which in this case is nitrogen) is astructural requirement for activity as an accelerator for sulfur vulcanization.It has been suggested that the function of the nitrogen atom is to act as ahydrogen acceptor during the sulfuration and cross-linking reactions (15,16).This empirically derived mechanism has been used to explain the allylicsubstitution (17) and concomitant formation of MBT during sulfuration andcross-linking (18).

A typical rubber vulcanizate will contain various components in addi-tion to the sulfur and accelerator. An example of a natural rubber vulcanizateprepared using a conventional cure system is given in Table 3. As discussedin the preceding section, the rates of vulcanization and states of cure dependnot only on the type of accelerator used but also on the amount and type(s)of activator(s) (e.g., stearic acid, zinc oxide, and/or secondary acceleratorssuch as DPG or TMTD). The time to the onset of cure varies with the class

Figure 8 A comparison of common classes of accelerators.

Vulcanization 519

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of accelerator used. Some accelerators provide only a relatively short delaybefore network formation begins. The sulfenamides and sulfenimides arespecial classes of accelerators that provide for a long delay period before theonset of network formation. Each component of a cure system plays an im-portant role in determining the rate and nature of the vulcanization reaction.

Major commercial interest lies in the sulfenamide and sulfenimideclasses of accelerators. These classes are important in the preparation oflarge rubber articles such as tires. Large items require a great deal of shapingand forming to prepare the final form. Once they are in the final form,vulcanization should commence rapidly to allow for high productivity. Themechanical shaping and forming processes involve mixing, calendering, andextrusion. Each activity produces considerable heat due to the viscous natureof the rubber compound. The delayed action provided by the sulfenamide andsulfenimide accelerators allows a period of time for processing before theonset of vulcanization.

The mechanism of vulcanization long remained unclear because of theinherent nature of the problem. During vulcanization, a very small percentageof material reacts with the polymer, transforming it into a network of in-tractable material that is difficult to analyze by traditional methodology.Much of the understanding of the process has been developed through modelcompound studies, studies of vulcanization reaction kinetics, and tracing thefate of the accelerator and sulfur chemicals through extraction and HPLCanalysis. Recently, NMR spectroscopic methods have helped to elucidate thenature of the sulfur attachment to the rubber. Most recently, insight has beendeveloped through the use of QSAR studies.

Generally speaking, it is the role of accelerators and cure activators toactivate the elemental sulfur and/or the rubber for the cross-linking reaction.

Table 3 Composition of a TypicalRubber Vulcanizate

Ingredient phra

Natural rubber 100N-330 Black 50Oil 3.0

Stearic acid 2.0Zinc oxide 5.0Antidegradant 2.0Sulfur 2.4

Accelerator (e.g., TBBS) 0.6

a phr = parts per hundred parts rubber.

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Sulfur may be activated by reaction of the amine with the sulfur molecules,which generates ammonium polysulfide anions or polysulfidic radical anions.These combine or react to form amine polysulfides or alkylammoniumpolysulfides, which have been proposed as intermediates in vulcanization(19–25). McCleverty suggests that it is the role of zinc to liberate the aminefrom the accelerator in order for the amine to react with the sulfur. Accordingto McCleverty, this sulfur–amine reaction product subsequently reacts withthe rubber.

Various zinc accelerator complexes have long been postulated as theactive sulfurating agents in zinc-containing cure system (5,25–27). These zincaccelerator complexes have the general structures shown in Figure 9. Suchcomplexes are modified through the action of ligands derived from acceler-ators (amines from sulfenamides), activators (stearic acid or zinc stearate), orsecondary accelerators such as amines, amides, ureas, and guanidines. Thecomplex species of polysulfidic analogs of such structures have been proposedto be involved in the reactions by which sulfur is attached to the rubber andcross-links are formed (28–33).

The zinc accelerator complexes may incorporate additional atoms ofsulfur to form zinc accelerator perthiolate type complexes as in B in Figure 9(25). Sulfur has also been shown to insert into zinc complexes of dithioacids(34,35). The sulfur atoms in the perthiolato zinc complexes are labile andthus readily exchange sulfur atoms. These complexes of labile sulfur havebeen shown to be effective accelerators (36). In fact, it was proposed (34,35)

Figure 9 Generalized structures of sulfurating intermediates.

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that this type of sulfur insertion reaction may be general in zinc-mediatedaccelerated sulfur vulcanization. Ultimately, these sulfur exchange andinsertion reactions form the bulk of the prereactions that occur duringdelayed action sulfenamide- or sulfenimide-catalyzed sulfur vulcanization.

Many different mechanisms for sulfur vulcanization have been sug-gested. Proposed pathways often involve several competitive and/or consec-utive reactions and can involve numerous intermediates. Sometimes as manyas 15 different chemical intermediates have been proposed (12). With the largenumber of competitive reactions and the large number of intermediates,identifying one structure as a critical intermediate appears to be an insur-mountable and unrealistic task. In fact, the large numbers of species foundthrough experimentation indicate that a complex competitive pathway mayprovide the best explanation for vulcanization chemistry. Although severalintermediates are probably capable of and likely to cause sulfuration andcross-linking of the rubber, it is likely that one mechanism with a character-istic intermediate dominates the process.

Reaction mechanisms can sometimes be elucidated through the identi-fication of critical chemical intermediates followed by comparison to knownreactions. More often, information regarding structure–activity relationshipsis instrumental in understanding the steps or mechanism of a chemicalreaction. These relationships have traditionally correlated empirically de-rived structural parameters to chemical activity and are referred to as QSARstudies.

Historical QSAR Studies. Quantitative structure–activity relation-ships (QSARs) were born in the first part of this century. In 1935, Hammettformulated his famous equation in an effort to mathematically relate struc-tural changes to chemical reactivity (37). Three basic sets of parameters wereinitially developed. Each set of ‘‘j constants’’ quantifies the effects of asubstituent on a reaction such as the dissociation equilibrium of benzoic acids(j) or substituted phenols (j�) or the rate of solvolysis of cumyl chlorides[XC6H4CCl(CH3)2] (j+). Since the early days of the Hammett equation,numerous reactivity scales have been generated and large numbers of reac-tivity constants have been accumulated. Chief among these are the Taft–Hammett j and the Taft steric parameters Es.

The Hammett relations quantify differences between ground-stateenergies of reactants and transition state energies of active intermediatesand are often referred to as linear free energy relationships. Understandinghow substituents (or a homologous series of chemical reactants) alter thekinetics of reaction provides direct evidence for identification of the chemicalnature of transition state complexes and ultimately the mechanism of thechemical reaction under consideration. Thus, defining the ‘‘electronic’’ effects

4871-9_Rodgers_Ch11_R2_052404



of various compounds or substituents on the kinetics of reaction or under-standing influential factors that alter the activation energy of reaction servesto explicitly define the nature of the studied reaction.

Whereas the Hammett sigma constants account for ‘‘resonance’’ and‘‘inductive’’ effects in aromatic systems, Taft developed the first generallysuccessful method for numerically explaining the steric effects and inductivepolar effects in organic chemistry (38,39).

Early QSAR Studies of Sulfur Vulcanization. Morita (40) correlatedthe inductive effects of a series of sulfanamides and bis-thioformanalidesto vulcanization activity. Steric effects were considered negligible (or at leastuniform) in this series of substituted phenylthioaniline- and substitutedaniline–based mercaptobenzothiazole sulfenamides. Morita showed that pKavalues and vulcanization parameters correlated reasonably well to the j*constant even though these parameters were developed for conventionalorganic chemistry (not chemistry involving sulfur and nitrogen).

Although the correlations are reasonable for the mercaptobenzothia-zole sulfenamides based on the substituted aniline series used in this example,they are not consistent with the narrow subset of aliphatic amines included inMorita’s study. Morita shows, in plots of cure properties vs. j*, disconti-nuities that separate aliphatic amines from the substituted phenylamines.Morita observed two linear relationships with slopes of opposite sign for N-substituted phenyl-sulfenamides and N-alkyl-sulfenamides. Longer scorchdelays were observed for electron-withdrawing substituted phenyl com-pounds and the sterically hindered alkyl substituents. Morita concluded thatthe more basic amino derivatives generally gave faster acceleration rates andhigher cross-link efficiencies and longer scorch delays.

The discontinuity shown in Morita’s data suggests that steric factors orelectronic (inductive) effects are significantly different in the two amine classesof sulfenamides. On the other hand, Morita shows that the 13C NMR plot ofthe C-2 carbon in the parent sulfenamide vs. j* are continuous across bothclasses of amine sulfenamides. Thus, the factors affecting chemical shifts inthe 13C spectra of the parent sulfenamide are different from the factors af-fecting vulcanization characteristics.

The parameters used in Morita’s study have been derived for organicreactions that at most involve only oxygen at the reactive centers or transitionstates. In the case of sulfur vulcanization, the reactions clearly involve sulfurand carbon and possibly zinc and nitrogen as well. Hence, the relationsderived by Morita are surprisingly good considering the differences inchemistry involved. Morita thus showed that the electronic and steric effectsof the amine moiety of the derived sulfenamide provide a critical influence incontrolling the rate of sulfur vulcanization. No insight could be provided into

4871-9_Rodgers_Ch11_R2_052404



the role and influence of the heterocyclic portion of the accelerator, mercap-tobenzothiazole.

Recent QSAR Studies. The previous studies of Morita were based onHammett constants that had been developed for carbon- and oxygen-centered organic reactions. Although sulfur is isoelectronic with oxygen,chemically it is somewhat different—softer, more polarizable, and less elec-tronegative. Thus, studies using parameters based in sulfur and nitrogenchemistry would be more beneficial in understanding the nature of sulfurvulcanization.

Recently, a detailed QSAR study provided significant insight into themechanism of sulfur vulcanization (41). It was based on semiempiricalquantum-mechanical calculations describing ‘‘proposed’’ zinc complexesderived from a series of 24 sulfenimides and sulfenamides derived fromvarious amines and sulfur-substituted nitrogen heterocycles. Thus, this studyused parameters calculated to characterize sulfur- and nitrogen-containingstructures pertinent to sulfur vulcanization, thereby overcoming the previousshortcomings.

Sulfenamides and sulfenimides were modeled in generalized zinc com-plex structures that basically took two factors into account. First, thestoichiometry of the accelerator fragments should be preserved in the zinccomplex. Second, the zinc complex would be modeled as a tetrafunctionalcomplex. In the case of sulfenimides, a fatty acid carboxylate would providethe fourth ligand. Further interaction of the zinc complex with additionalsulfur or the unsaturation on the polymer chain would then be assumed toproceed by zinc assuming coordinate states expanded from the tetravalentstate (Figure 10).

The result of this study clearly showed the effects of both the aminemoiety and the heterocyclic thiol on the rate of vulcanization. A modeldescribing the rate of vulcanization was derived that employed four termsthat accounted for more than 96% of the variance in the rates of reaction(R2=0.9667). The four parameters were (Figure 11).

1. Electron density in the Zn–S bond (electron–electron repulsion)2. Electron density in the CjN bond (electron–electron repulsion)

Figure 10 Sulfenamide and sulfenimide zinc complex.

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3. Interaction parameter for an NUH bond (measure of the qualityof interaction of the amine ligand with zinc)

4. Molecular surface area

Generalized conclusions can thus be drawn from these results. The datasupport the idea that heterocyclic thiols forming strong S–Zn complexes tendto make for slower accelerators. Increasing the electron density in the CjNbond tends to increase the rate of reaction. Improving the quality of theinteraction of the amine ligand with the zinc increases the rate of vulcaniza-tion. A structure that favors the general flow of electrons away from the ZnUSbond and into the CjN bond will tend to be a faster accelerator (as depictedin Fig. 13). And finally, because the reaction involves diffusion of metalcomplexes through a viscous liquid, the rate of reaction is diffusion-controlledand thus depends upon the surface area of the complex. Thus, largeaccelerator complexes provide for slower reaction kinetics.

This model rationalizes the differences between primary and secondaryamine–based sulfenamides. A more quantitative discussion is given below,but the effects are readily understood in qualitative terms. Primary amine-based sulfenamides are typically faster accelerators than those based on sec-ondary amines. In terms of traditional logic, stronger bases would providefor faster reaction kinetics. Thus, neglecting steric effects, secondary aminesmight be expected to provide for faster vulcanization rates. This discrepancycan now be readily understood because the greater steric nature of the sec-ondary amines reduces the effectiveness of the interaction of the nitrogenwith zinc.

The complex as modeled is significant in understanding the possiblestructure of a sulfurating intermediate (Fig. 12). In historically proposed zinccomplexes, the heterocyclic thiol was attached to the zinc atom by a chain ofsulfur atoms. In the structures above, accelerator thiolate ions are attacheddirectly to the zinc atom. In the historically proposed structure, it is unlikelythat electronic effects derived from the nature of a heterocyclic thiol joined tozinc through a polysulfidic chain (as in Fig. 9) would significantly influence

Figure 11 Arrows indicate the directional ‘‘characteristic flow’’ of electrons

favoring faster rates of vulcanization.

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the kinetics of sulfur vulcanization. Any number of sulfur atoms in a chainattaching the thiol to zinc should significantly modulate electronic effects ofvarious heterocycles. Thus for polysulfidic linkages between zinc and theaccelerator, the electronic influence on the complex is nonexistent.

In this model, sulfur (to be added to the polymer chain) is directlyattached to zinc, and during reaction zinc would be found in an expandedligand site (i.e., 4-coordinate Zn going to 5-coordinate Zn, where the fifthcoordination site is occupied by the sulfur). This 5-coordinate structure theninteracts with the double bond in the polymer, and reaction takes place,inserting sulfur in the allylic position (Fig. 13).

Figure 12 Proposed structure for the sulfurating intermediate that leads to cross-link formation.

Figure 13 Cross-link formation.

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Polysulfidic zinc structures such as 3 have been shown to be chemicallypoised for the sulfuration reaction. Zinc hexasulfide complexes have beenshown to serve as polysulfidic sulfur donors (42). These complexes are soinherently reactive that when heated to vulcanization temperatures, com-pounds having an accelerator (such as a sulfenamide) undergo rapid vulcan-ization with exceptionally short scorch delay. The resulting network is rich inpolysulfidic sulfur cross-links. Rapid vulcanization is normally achievedthrough the use of combinations of secondary accelerators with sulfenamidesbut normally results in networks having short sulfur linkages (primarilymono- and disulfidic networks).

2. Molecular Explanations of Various Accelerator Activities

The reactivity of heterocyclic thiol-based sulfenamides or sulfenimides andthe influence of the corresponding amines can now be understood in a morequantitative fashion. Table 4 compares accelerators that have various degreesof activity. For each accelerator the relative contribution to the maximum rateof vulcanization for the critical structural features is provided along with

Table 4 Accelerator Type and Rate of Vulcanizationa

Electron

density,

Electron

density,

Cmpdb InterceptZnUSbond

CUNbond

Exchangeenergy,NUH

Molecularsurface

area Pred. Obsv

TBBS �178.98 �1.87 47.22 141.23 �2.46 5.14 5.6

TBSI �178.98 �3.05 46.63 140.09 �2.53 2.16 2.57CDMPS �178.98 �3.26 44.44 141.87 �2.44 1.63 1.1DCBS �178.98 �0.60 47.07 138.49 �3.44 2.54 2.6CDMPSI �178.98 �3.09 43.93 141.41 �2.50 0.76 1.1

a Contributions to the maximum rate of vulcanization from structural features of corresponding zinc

complexes.b For structures, see Figure 14.

Max rate of

vulcanization

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the overall observed and the predicted maximum rates of vulcanization. Ingeneral, as can be seen from the table, sulfenamides are faster than sulfeni-mides and primary amine sulfenamides are faster than secondary amine–based sulfenamides.

The accelerators whose structure are shown in Figure 14 can now becompared using TBBS as a reference point. The steric nature of the dicyclo-hexylamine is so great that a number of interactions are altered includingthe ZnUS bond and the N–Zn bond. As a result, the complex behaves as asomewhat electron starved system, and the resulting rate is slower than that ofthe TBBS system. In addition, the surface area of the complex is so large thatthis effect alone accounts for a nearly 20% reduction in reactivity of the DCBSaccelerator system compared to the TBBS.

Because it is sulfenimide, TBSI is modeled as a complex with one amineand one acid moiety. The electronic effect of substituting the acid for theamine is to withdraw electrons from the heterocyclic amine CjN bond andincrease electron density in the ZnUS bond. The increase in electron densityin the ZnUS bond is a result of reduced steric hindrance allowing for betterinteraction between the Zn and S atoms and also a result of the inductiveeffect of the oxygen (oxygen being more electronegative than nitrogen). Theinductive effect of the oxygen also reduces the N–Zn interaction, as can beseen in the N–H exchange energy. The total result is an accelerator with sig-nificantly slower kinetics than TBBS.

Figure 14 Structures of accelerators listed in Table 4.

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Finally, although it has the same amine moiety as TBBS (t-butylamine),CDMPS has a different heterocyclic thiol (4,6-dimethyl-2-mercaptopyrimi-dine). In this complex, the N–Zn interaction is similar to that observed in theTBBS complex and the molecular surface area is nearly the same. Thedifference in reactivity is attributed to the electronic character of the pyrim-idine ring system. The electron density the CjN bond is significantly lower,and the electron density in the ZnUS bond is considerably higher than thoseobserved for TBBS. This balance in electrons is consistent with a tendency tofavor the thiol tautomer in the tautomeric equilibrium. Accelerators favoringthe thione form tend to be faster accelerators.

The ability of the pyrimidine thiol to form strong bonds may also playa role in the maturation or reversion chemistry. Lin (44) has shown thatCDMPS produces vulcanizates that exhibit better heat aging characteristicsthan TBBS.

3. Molecular Effects on the Activation Energy for Vulcanization

The vulcanization characteristics (including Arrhenius activation energy) forseven 2-mercaptobenzothiazole-based sulfenamides were measured and re-lated to the effects of the amine in the zinc complex as modeled above (43). Inthat report, the maximum rate of vulcanization was correlated to the NUZnbond length in the zinc complex (R2 = 0.987, df = 13). Other likely amineconstants of characterization such as Taft steric constants, pKa or Hammettj* constants gave poor coorelations.

The Arrhenius activation energy also correlated well with the NUZnbond length (R2 = 0.9040, df = 5.) Recent calculations produced a singleparameter model correlating Ea with maximum net atomic charge on Nhaving R2 = 0.9554, df = 6. The maximum charge on the nitrogen atom isfound on the heterocyclic ring nitrogen. The fact that the coefficient for the Ncharge parameter is negative supports the expectation that the heterocyclicring nitrogen serves as the hydrogen acceptor in the sulfuration step (F.Ignatz-Hoover, unpublished results).

All of these results provide strong support for the idea that a complexsimilar to those shown in figures is likely to play a strong role in the sulfu-rization step. These complexes then can be characterized as having a

ð1Þ

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heterocyclic thiol directly bonded to the zinc atom and sulfur attachedseparately to the zinc as shown in the zinc hexasulfide complexes. Clearly,kinetic effects will be altered in practice as various compounding ingredientscan influence the equilibrium and, in fact, the nature of the zinc complex.Practical compounding examples are provided in the next section.

B. Practical Comparison of Primary Accelerators

The response of an elastomer to a specific accelerator varies with the numberand activity of the double bonds present. Natural rubber and styrenebutadiene rubber are typical of the highly unsaturated polymers in use andwill be used as examples in this presentation.

1. Natural Rubber

Typical responses of PerkacitR MBTS and the common sulfenamides arecompared in NR in Table 5 and Figure 15. Compared to Perkacit MBTS,the sulfenamides provide longer scorch delay, faster cure rates, and highermodulus values.

2. Styrene Butadiene Rubber

Typical responses in SBR are shown in Table 6 and Figure 16. The com-parison of the thiazole accelerator, Perkacit MBTS, with the sulfenamides issimilar to that found in NR. The differences between sulfenamides are, how-ever, more pronounced than those found in NR.

3. Performance Comparison

At equal concentrations, the sulfenamides can generally be ranked as follows:Scorch Delay

Perkacit MBTS < Santocure CBS c Santocure TBBS

< Santocure MBS < Santocure DCBSð2Þ

Cure Rate

Santocure CBS c Santocure TBBS > Santocure MBS

> Santocure DCBSð3Þ

Modulus Development

Santocure TBBS > Santocure MBS c Santocure CBS

> Santocure DCBSð4Þ

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The observed differences in scorch delay are larger and more important thanthe differences in cure rate or modulus. These differences are a function of theamine from which the sulfenamide is derived. Generally, the more basicamines produce sulfenamides that are scorchier and faster curing. Addition-ally, steric hindrance will produce more slowly curing accelerators as in thecase of Santocure DCBS.

C. Comparison of Secondary Accelerators

There are a large number of secondary accelerators that could be used witheach of the sulfenamides, thereby providing a wide range of flexibility. Tosimplify matters, this presentation will examine only the more commonsecondary accelerators and their effect on Santocure TBBS as the primary

Table 5 Comparison of Primary Accelerators in Vulcanization of NR

SMR-5CV 100.0

N-330 Black 50.0Sundex 790 3.0Zinc oxide 5.0Stearic acid 2.0

Flectol TMQ 1.0Sulfur 2.4

Perkacit MBTS 0.6 — — —

Santocure CBS — 0.6 — —Santocure TBBS — — 0.6 —Santocure MBS — — — 0.6

Mooney scorch at 121jCMinimum viscosity 50.0 45.9 45.2 46.9t5, min 6.7 12.0 12.5 12.0

Rheometer at 144jCMin. torque, in.-lb 8.4 7.7 7.5 8.1Max. torque, in.-lb 30.9 37.8 39.1 37.3t2, min 4.9 8.2 9.5 8.0

t90, min 26.3 20.6 23.0 22.8t90 � t2 21.4 12.4 13.5 14.8

Stress–strain (t90 cure)

Shore A hardness 64.0 67.0 69.0 66.0100% modulus, psi 290.0 375.0 435.0 365.0300% modulus, psi 1440.0 1930.0 2160.0 1890.0

Ult. tensile, psi 3570.0 4080.0 4220.0 4120.0Ult. elongation, % 570.0 550.0 590.0 555.0

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Figure 15 Primary accelerators in vulcanization of NR at 144jC. (For data, seeTable 5.)

Table 6 Comparison of Primary Accelerators in SBR

SBR 1606 162.0

Zinc oxide 5.0Stearic acid 1.0Flectol TMQ 2.0

Sulfur 1.8

Perkacit MBTS 1.2 — — —SantocureRCBS — 1.2 — —

Santocure TBBS — — 1.2 —Santocure MBS — — — 1.2Mooney scorch at 135jC

Min viscosity 43.2 42.0 42.0 41.3t5, min 24.5 34.0 35.7 54.1

Rheometer at 160jCMin torque, in.-lb 6.7 6.7 6.2 6.8

Max torque, in.-lb 27.4 31.4 32.8 32.4t2, min 5.2 8.0 7.7 10.6t90, min 37.5 16.7 17.5 21.5

t90 � t2 32.3 8.7 9.8 10.9Stress–strain (t90 cure)

Shore A Hardness 67.0 67.0 68.0 68.0

100% modulus, psi 265.0 285.0 325.0 305.0300% modulus, psi 1095.0 1375.0 1500.0 1445.0Ult. tensile, psi 3000.0 3130.0 3345.0 3125.0Ult. elongation, % 645.0 570.0 565.0 550.0

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accelerator. The effects of these materials on the other sulfenamides aresimilar.

These comparisons have been made in NR, SBR, and NBR using aPerkacit MBTS/DPG system as a control in each case. Within a givenpolymer, the sulfur is held at a single concentration. Initial comparisons aremade at the same concentration and in the same ratio of primary to secondaryaccelerator. Variations in concentration and in the ratio of primary tosecondary accelerator will be discussed in Section VI.

1. Natural Rubber

Seven secondary accelerators were evaluated with Santocure TBBS and anNR compound and compared with a Perkacit MBTS/DPG control. Theformulations used are shown in Table 7.

As shown in Figure 17, all of the activated sulfenamide stocks providemore scorch delay than does the activated thiazole stock. Of the secondaryaccelerators tested, Perkacit ZDMC is the scorchiest, and Perkacit TETDprovides the longest scorch delay. Conversely, those stocks containing adithiocarbamate or thiuram show cure times (see Fig. 18) at least as short asthat of the activated thiazole control, even though they exhibit much longerscorch delays. Only the use of DOTG as a secondary accelerator gives a longercure time than the control. Therefore, one can obtain significant improve-ments in scorch protection with no increase in cure time through the use of anactivated sulfenamide.

Figure 16 Primary accelerators in vulcanization of SBR at 160jC.

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Figure 17 Comparison of secondary accelerators (100 NR/2.5 sulfur).

Table 7 Comparison of Secondary Accelerators in NRa

Sulfur 2.5

Perkacit MBTS 1.2 — — — — — — —Perkacit DPG 0.4 — — — — — — —Santocure TBBS — 0.6Perkacit TMTD — 0.4 — — — — — —

Perkacit TMTM — — 0.4 — — — — —Perkacit TETD — — — 0.4 — — — —Perkacit ZDMC — — — — 0.4 — — —

Perkacit ZDEC — — — — — 0.4 — —Perkacit ZDBC — — — — — — 0.4 —DOTG — — — — — — — 0.4

Mooney scorch at 121jCt5, min 7.2 16.8 21.5 23.5 13.7 16.7 20.2 21.7

Rheometer at 143jC

t90, min 9.2 7.5 9.5 9.8 7.0 7.8 9.3 16.5t90 � t2 6.2 2.0 2.2 2.5 2.0 2.0 2.6 10.0

Stress–Strain (t90 cure)100% modulus,

psi

380.0 490.0 510.0 440.0 455.0 415.0 410.0 410.0

a Formula (phr): #1RSS, 100; FEF Black, 40; Circolite RPO, 10; zinc oxide, 5.0; stearic acid, 1.5; Santoflex

6PPD, 2.0.

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At the level of accelerator used in this study, all of the activated sulfen-amides produced a higher modulus than the activated thiazole (see Fig. 19).

Of course, concentration adjustments can be made to equalize modulusif desired, and such adjustments will be discussed in Section VI. The thiuramsare known sulfur donors and therefore generally require more adjustment toequalize modulus.

2. Styrene Butadiene Rubber

The same chemicals were also evaluated as secondary accelerators in SBR,as shown in Table 8. The responses obtained in SBR are summarized inFigure 20 (scorch delay), Figure 21 (cure time), and Figure 22 (modulus).Again, all of the activated sulfenamide stocks exhibit greater scorch pro-tection than does the Perkacit MBTS/DPG stock. In this polymer, VocolZBPD provides the longest scorch delay, followed by Perkacit TMTM.

Although Vocol produces a long scorch delay, as can be seen in Figure20, it also produces a very slow cure and lower modulus, as shown in Figures21 and Figure 22, respectively. For these reasons, the use of Vocol is not rec-ommended in SBR.

The comparisons shown in Figures 21–23 indicate that Perkacit TMTMprovides the better combination of scorch delay, cure rate, and modulusdevelopment in the SBR compound. Again, in SBR, it is feasible to obtain

Figure 18 Comparison of rheometer readings with secondary accelerators (100NR/2.5 sulfur).

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Table 8 Comparison of Secondary Accelerators in SBRa

Sulfur 1.8Perkacit MBTS 1.2 — — — — — —Perkacit DPG 0.4 — — — — — —

Santocure TBBS — 0.5Perkacit TMTD — 0.3 — — — — —Perkacit TMTM — — 0.3 — — — —

Perkacit TETD — — — 0.3 — — —Perkacit ZDMC — — — — 0.3 — —Perkacit ZDBC — — — — — 0.3 —

Vocol ZBPD — — — — — — 0.3Mooney scorch at 135jCt5, min 10.4 12.3 22.0 14.5 13.2 18.7 24.4

Rheometer at 160jC

t90, min 9.2 7.4 9.3 8.6 8.7 12.0 21.3t90 � t2 5.9 3.6 3.6 4.2 4.5 6.5 14.5

Stress–Strain (t90 cure)

100% modulus, psi 290.0 310.0 300.0 285.0 275.0 270.0 230.0

a Formula (phr): SBR 1500, 100; N-330, 50; Circosol 4240, 10; zinc oxide, 4.0; stearic acid, 2.0; Santoflex

6PPD, 2.0.

Figure 19 Comparison of moduli obtained with secondary accelerators (100 NR/

2.5 sulfur).

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improved scorch delay with no increase in cure time or loss of physicalproperties.

3. Nitrile Rubber

Typical responses of the same secondary accelerators, used with SantocureTBBS in a black-filled nitrile compound, are shown in Table 9. Again, the

Figure 21 Comparison of cure times of secondary accelerators (100 SBR/1.8 sulfur).

Figure 20 Comparison of scorch delay with secondary accelerators (100 SBR/1.8sulfur).

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Figure 23 Comparison of cure times at 135jC with secondary accelerators (100NBR/1.5 sulfur).

Figure 22 Comparison of modulus achieved with secondary accelerators (100 SBR/1.8 sulfur).

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responses are compared with the Perkacit MBTS/DPG control cure systemin Figures 24–26. It should be noted that magnesium carbonate–treated sul-fur was used to obtain adequate sulfur dispersion.

Figure 23 shows the effect of changing from the activated thiazole curesystem to an activated sulfenamide. It produces greater processing safety withall secondaries tested in this nitrile stock. Increases in processing safety,depending on the secondary accelerator used, range between 20% and 140%.As shown in Figure 24, the activated sulfenamide stocks exhibit much shortercure times than the activated thiazole stock in this polymer. The improvementin cure time is much greater than that realized in NR or SBR. Even so, therelative relationships between secondary accelerators are similar to thosefound in NR. Again, the modulus values realized (see Fig. 25) are similar tothose obtained with the MBTS/DPG system.

D. Effect of Fillers

The preceding results show that the responses realized with the varioussecondary accelerators vary significantly from elastomer to elastomer. Alogical extension, therefore, is to examine these responses with various fillers.For this reason, the accelerator combinations just discussed were also

Table 9 Comparison of Secondary Accelerators in NBRa

MC sulfur 1.5

Perkacit MBTS 1.2 — — — — — —Perkacit DPG 0.4 — — — — — —Santocure TBBS — 0.5Perkacit TMTD — 0.3 — — — — —

Perkacit TMTM — — 0.3 — — — —Perkacit TETD — — — 0.3 — — —Perkacit ZDMC — — — — 0.3 — —

Perkacit ZDBC — — — — — 0.3 —Vocol ZBPD — — — — — — 0.3Mooney scorch at 135jC

t5, min 2.4 4.7 5.2 5.8 2.9 3.4 3.0Rheometer at 160jCt90, min 9.7 3.7 4.5 3.7 2.8 3.5 4.7

t90 � t2 8.2 1.7 2.2 1.3 1.3 1.5 3.0Stress–Strain (t90 cure)

100% modulus, psi 605.0 705.0 760.0 640.0 655.0 595.0 550.0

a Formula (phr): Krynac 34.50, 100; N-550 Black, 45; N-770 Black, 40; DOP, 15; zinc oxide, 5.0; stearic

acid, 1.5; Santoflex 6PPD, 2.0.

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Figure 25 Comparison of moduli obtained with secondary accelerators (100 NBR/1.5 sulfur).

Figure 24 Comparison of cure times at 160jC with secondary accelerators (100NBR/1.5 sulfur).

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Figure 26 Secondary accelerators with different fillers (100 NR/0.5 Santocure TBBS/0.3 secondary).

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_R2_052404

MD:RODGERS,JOB:03286,PAGE:541


Table 10 Effects of Variation of Secondary Accelerator and Fillera in NR Vulcanizates

NR 100.0Sundex 790 10.0

Zinc oxide 5.0Stearic acid 1.5Santoflex 6PPD 2.0

Sulfur 2.5

Perkacit MBTS 1.2 — — — — — —Perkacit DPG 0.4 — — — — — —Santocure TBBS — 0.5Perkacit TMTD — 0.3 — — — — —

Perkacit TMTM — — 0.3 — — — —Perkacit TETD — — — 0.3 — — —Perkacit ZDMC — — — — 0.3 — —

Perkacit ZDBC — — — — — 0.3 —Vocol ZBPD — — — — — — 0.3

1. SMR-5CV, 100 phr; FEF Black, 40 phrMooney scorch at 121jC

Min. viscosity 33.5 31.8 30.0 29.0 29.4 25.7 25.4

t5, min 7.3 19.5 26.0 26.8 15.3 22.8 21.2Rheometer at 143jt2, min 3.0 5.8 7.3 7.3 5.0 6.7 6.5t90, min 9.7 8.5 10.0 10.0 7.8 10.2 14.8

t90 � t2 6.7 2.7 2.7 2.7 2.8 3.5 8.3

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Stress–Strain/t90 cure at 143jC

100% modulus, psi 385 435 430 410 440 450 310

2. Pale crepe, 100 phr; hard clay, 80 phr; PEG, 2.0 phr

Mooney scorch at 121jCMin. viscosity 24.7 23.2 23.3 22.5 24.0 22.0 21.8t5, min 9.0 12.2 11.0 18.5 9.7 18.0 12.5

Rhoemeter at 143jt2, min 3.7 4.0 3.8 5.8 3.8 6.2 4.3t90, min 9.8 6.3 6.2 8.3 6.5 9.3 14.8t90 � t2 6.1 2.3 2.4 2.5 2.7 3.1 10.5

Stress–Strain/t90 cure at 143jC100% modulus, psi 425.0 460.0 435.0 455.0 440.0 415.0 360.0

3. Pale crepe, 100 phr; Hisil 233, 40 phr; PEG, 2.0 phrMooney scorch at 121jC

Min. viscosity 36.8 40.4 39.4 40.5 40.7 39.4 36.8

t5, min 12.8 14.7 15.8 22.0 12.2 22.7 14.7Rheometer at 143jt2, min 4.7 4.7 4.3 6.2 4.2 6.7 5.0t90, min 12.7 6.7 6.7 9.0 6.9 10.0 16.0

t90 � t2 8.0 2.0 2.4 2.8 2.5 3.3 11.0Stress–strain/t90 cure at 143jC

100% modulus, psi 230.0 225.0 220.0 250.0 210.0 225.0 180.0

aFillers: FEF Black, hard clay, Hisil 233.

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evaluated in natural rubber stocks filled with FEF black, hard clay, andhydrated silica. Concentrations of the vulcanizing agents were held constant,but 2 phr PEG was added to the mineral-filled stocks. The formulationsstudied are summarized in Table 10.

Figure 26 provides an overall view of the data obtained. Relative curetime rankings of the secondary accelerators are similar for the three fillers, asare the actual cure times. Modulus rankings are also quite similar. However,the actual modulus values produced in this silica-filled stock are lower thanthose obtained with the other fillers.

The only major difference noted with the change in fillers is that whichoccurs with Perkacit TMTM. In the black-filled stock, Perkacit TMTM pro-duces an excellent combination of long scorch delay, fast cure, and highmodulus. With mineral fillers, Perkacit TMTM does not exhibit an advantagein scorch delay.

E. Variation in Ratio and Concentration of Accelerators

To this point, we have discussed the effects of changing primary and sec-ondary accelerator types in three different polymers.

After a basic cure system is selected, several adjustments are usuallynecessary before requirements are satisfied. The most common adjustmentsfall into one of the following categories:

Change cure time.Change induction time.Reduce cure time but do not change induction time.Increase induction time but do not change cure time.Increase or decrease modulus.

In order to make adjustments in activated cure systems, the general effects ofprimary-to-secondary accelerator ratio and total accelerator concentrationsneed to be known.

1. Systematic Studies

Determination of the effects of concentrations and ratios of accelerators forall possible combinations of primary and secondary accelerators would bevery time consuming, to say the least. Therefore, to provide guidelines forrefining cure systems, we illustrate the responses to changes in concentrationsand ratios in an NR/SBR blend (Table 11).

For purposes of illustration, let us assume that it is desired to match thephysical properties obtained with the following MBTS/DPG system and toincrease the scorch time to 28–30 min with no increase in cure time.

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We will attempt to obtain these properties with Santocure TBBS asthe primary accelerator and Perkacit TMTD as the secondary accelerator.Let us now look at an efficient and systematic way to study these changes. Astatistical procedure known as response surface experimentation providesgood estimations of the properties obtainable through a range of concentra-tion changes. Therefore, that combination can be chosen which produces themost desirable compromise of properties.

Basically, response surface experimentation requires the evaluation ofa comparatively small number of batches in a regular manner, followed by amathematical analysis to produce contour plots. Though it sounds compli-cated, the procedure is, in fact, quite simple.

The philosophy of response surface experimentation can best be ex-plained by using the simple case of one independent variable (i.e., the concen-tration of one ingredient in a rubber compound) and one dependent variable(i.e., a measured property such as scorch time or modulus).

Figure 27 shows a case in which it is desired to evaluate the effect of someindependent variable X upon some dependent variable Y for values of Xranging from 1 to 3. At five evenly spaced levels of X, the value of Y is deter-mined and plotted on the graph (dots); the best line through these data isthen determined so that we can predict the value of Y for any value of Xbetween 1 and 3.

Table 11 Base Stock

SBR 1712 68.75

SMR-5 50.00N-550 (FEF) 50.00Zinc oxide 3.00Stearic acid 2.00

Flectol TMQ 1.50Sulfur 2.25Perkacit MBTS 1.20

Perkacit DPG 0.50Mooney scorch, t5 at 250jF 19.60Rheometer, t90 at 307jF 8.80

Stress–Straina

Shore A hardness 60.0300% modulus, psi 2000.0

Ult. tensile, psi 2880.0Ult. elongation, % 405.0

a After curing for 10 min at 307jF.

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The best line through these data will have the mathematical form

Y ¼ b0 ¼ b1Xþ b2X2 ð3Þ

In this case, with one X, we frequently draw the line by eye and do almost aswell as we would if we calculated the equations. Calculation of the equationsby regression techniques does, however, provide us with confidence limits forany prediction we might make.

Response surface experimentation is nothing more than the applicationof this technique to more than one independent variable. In this illustration, itwas decided to perform an experiment in which the independent variableswere the concentrations of Santocure TBBS and Perkacit TMTD. Theexperimental design chosen is depicted graphically in Figure 28.

Again, our purpose is to perform trials or evaluate batches over a rangeof accelerator concentrations within which we are fairly sure the bestcombination is to be found. Then, by mathematical interpolation (i.e.,regression analysis) we can predict the combination that will best meet thespecifications and then confirm our result by further trials.

The equation calculated for two independent variables (in SantocureTBBS and Perkacit TMTD) will take the form

Yi þ B0 þ B1X1 þ B2X2 þ B11X21 þ B22X

22 þ B12X1X2 ð5Þ

where Yi represents the measured properties (e.g., Y1 = scorch, Y2 =rheometer cure time, etc.) Today, we usually calculate such equations on acomputer, but they are not particularly difficult to perform on a desk cal-

Figure 27 Relationship between dependent and independent variables.

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Figure 28 Experimental design.

Figure 29 Contours of equal scorch delay (t5 at 250jF).

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culator. The details of these calculations and the evaluation of their utility canbe found in any statistics test. Today, we are more concerned with the graphsthat can be calculated from these equations. These graphs are called contourplots and have the general form shown in Figure 29, which shows contours ofequal scorch delay for changes in Santocure TBBS and Perkacit TMTD.

These plots are read as follows: Any combination of Santocure TBBSconcentrations that coincides with the line labeled 30 will produce scorchtimes of approximately 30 mins, while any combination that coincides withthe line labeled 50 will produce scorch times of approximately 50 min. Thecontours show that as Perkacit TMTD ratio is increased, Mooney scorch isdecreased, indicating that the ratio of accelerators is the predominant factorcontrolling Mooney scorch time. Similar equations and contour plots wereobtained for rheometer and stress–strain data and are shown in Figures 30and 31.

The stated objective here was to develop a compound with 28–30 minscorch time and no increase in cure times over the approximately 9 minrealized with the Perkacit/DPG system. Therefore, we superimpose the 30

Figure 30 Contours of cure time (t90 at 307jF). Dashed line is 30 min scorch at

250jF.

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min scorch contour over the contour plot for cure time in Figure 30. Thesecontours predict that the desired scorch and cure properties can be obtainedat point A, i.e., 0.85 phr Santocure TBBS and 0.25 phr Perkacit TMTD. Curetime response to a change in Perkacit TMTD ratio is similar to Mooneyscorch: i.e., increased Perkacit TMTD decreases cure time.

Now let us superimpose the 30 min scorch contour and the 8 and 9 mincure contours over the contour plot for 300% tensile modulus (Fig. 31). Thecomposite contours confirm that point A is the concentration and ratio ofaccelerators that can produce the desired properties. Additionally, themodulus contours show that modulus response depends on total acceleration,indicating that total accelerator level, not ratio, is the predominant factorcontrolling modulus development.

Also, we find that we can meet the desired scorch with a shorter curetime at point B, i.e., 1.1 phr Santocure TBBS and 0.375 phr Perkacit TMTD.The concentration of accelerators at point A would result in a less expensivecuring system. The concentration of accelerators at point B would be moreexpensive but might result in a cheaper product through increased produc-tivity. Therefore, it was decided to evaluate both points.

Figure 31 Contours of 300% modulus (psi). (- - - -) 30 min Scorch at 250jF. (- - -)Cure time at 307jF.

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2. Confirmatory Examples

The predicted and observed values obtained with these concentrations ofaccelerators are shown in Table 12.

V. RETARDERS

Santogard PVI (N-cyclohexylthiophthalimide) was the first rubber chemicalable to delay the onset of sulfur vulcanization in a predictable manner.Santogard PVI is almost the ‘‘ideal’’ retarder, because small additions (0.1–0.5 phr) produce large increases in processing safety (see Fig. 32).

Figure 33 shows that increases in processing safety are obtained,without affecting the rate of cure or final cured modulus, at the normal levelsused (0.1–0.3 phr). With conventional retarders, reductions in cure rate thatresult in increases in cure times have frequently been confused with trueretardation of scorch where cure rate is unaffected. The predictable influenceof Santogard PVI on processing safety with respect to level of addition andtemperature is shown in Figures 34 and 35.

Santogard PVI is highly effective with sulfenamide accelerators asshown in Figure 35, where the response of processing safety to increases inthe level of Santogard PVI is illustrated for the most commonly usedsulfenamide accelerator Santocure TBBS. This linear response enables the

Table 12 Confirmatory Experiments

Santocure TBBS (phr) 0.85 1.1

Perkacit TMTD (phr) 0.25 0.375 1.2 PerkacitMBTS, 0.5

Predicted Actual Predicted Actual Perkacit DPG

Mooney scorch at 250jF

t5, min 32.9 33.5 30.1 30.0 19.6Rheometer at 307jFt2, min 5.1 5.1 4.5 4.6 3.1

t90, min 9.4 9.2 7.8 8.2 8.8Max. torque, in.-lb 36.6 36.2 38.0 37.0 37.8

Stress–Strain (t90 cure at 307jF)Shore A hardness 62.0 63.0 63.0 64.0 60.0

300% modulus, psi 2065.0 1950.0 2275.0 2130.0 2000.0Ult. tensile, psi 2600.0 2850.0 2630.0 2530.0 2880.0Ult. elongation, % 365.0 400.0 355.0 355.0 405.0

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exact amount required for a given increase in processing safety to be quicklydetermined and enables the ‘‘calibration’’ of a compound in processing terms.

Predictability with respect to temperature is demonstrated in Figure 35.Unlike other retarders (e.g.,N-nitrosodiphenylamine), Santogard PVI will notdecompose over the normal range of processing and curing temperatures. Thegraph in Figure 35 shows two lines representing the relationship between

Figure 32 Effect on Mooney scorch (100 NR/2.5 sulfur/0.6 Santocure TBBS).

Figure 33 Effect on curing characteristics (100 NR/2.5 sulfur/0.6 Santocure TBBS).

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processing safety (rheometer T2) and processing temperature for a SantocureTBBS–accelerated SBR compound. The dashed line demonstrates the effect ofadding 0.25 phr Santogard PVI. This addition produces a parallel shift to theleft.Thus,movingfrompointAatatemperatureof140jCinaverticaldirectionby the addition of 0.25 phr Santogard PVI will permit a 10jC increase inprocessing temperature while maintaining the same processing safety. Thistemperature predictability extends the applications of Santogard PVI from asimple retarder to that of a much more versatile additive with which heat inputcan be considered a controllable factor in the same way as processing safety.

The linear relationship between Santogard PVI level and processingsafety shown in Figure 35 occurs with a wide range of polymers, accelerators,sulfur levels, filler types and level, and other compounding ingredients. Inpractically all cases, a straight-line relationship is obtained, the slope andposition of which depends on the particular formulation. Although thehighest response occurs with sulfenamides, Santogard PVI is active withnearly all accelerators for sulfur-curable elastomers but normally ineffectivewith peroxide, resin, or metal oxide curing systems. It is not normally used inlatex formulations. Santogard PVI is most effective with the fastest curingpolymers, and an approximate order of response is

NR > NBR > SBR > EPDM > IIR > CR ð6Þ

Figure 34 Santogard PVI concentration vs. Mooney scorch (100 NR).

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The response is determined not only by the elastomer but also by theaccelerator system. The specialty elastomers show lower response to Santo-gard PVI due to the slower curing rate of the polymer and also due to the factthat they are generally cured with accelerators showing a low response toSantogard PVI (e.g., thiurams, dithiocarbamates). An exception to this is inbutyl formulations, where Santogard PVI shows the best response with curingsystems based on dithiocarbamates. Santogard PVI has also been usedeffectively in NBR/PVC, polyacrylic rubber, and sulfur-curable polyure-thanes. Sulfur level is also very important; the best response is found withconventional levels (1.5–3.0 phr), with a tendency to poorer response as sulfurlevel increases (ebonites). The response in systems with low sulfur levels islargely dependent on the accelerator.

The addition of Santogard PVI produces no deterioration in aging,fatigue, or ozone resistance in compounds cured to optimum. Within normalusage levels (0.1–0.4 phr), it has no effect on modulus, resilience, creep, per-manent set, heat buildup, abrasion resistance, oil swelling resistance, etc.

Figure 35 Effect of Santogard PVI over a range of processing temperatures (100SBR).

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Santogard PVI is also not known to have any detrimental effects on theadhesion of cured rubber to textiles (rayon, nylon, polyester, aramid) or steel(brass- or zinc-coated wire or chemically treated surfaces). It is widely usedin skim stocks to maintain high levels of adhesion in steel-belted radial tires.

When levels over 0.4 phr are required, attention must be paid to thefinal cured modulus, because it may be reduced slightly with possible effectson compression set, heat buildup, resilience, and creep. If such high levelsare required, it is usual to readjust the modulus with a small increase in thesulfur level (up to 40% of the level of Santogard PVI) or accelerator (up to20% of the Santogard PVI level). A surface bloom may also occur in somecases. Santogard PVI will not cause contact or migration staining to paintedsurfaces but may impart a slight discoloration to white or light-coloredstocks.

VI. CURE SYSTEMS FOR SPECIALTY ELASTOMERS

A. EPDM

Properly compounded EPDM exhibits many desirable vulcanizate propertiesincluding resistance to ozone, heat, ultraviolet radiation, weathering, andchemicals. Because of the attractive combinations of properties, EPDM hasgained acceptance in a wide variety of applications. However, the relativelylow unsaturation of EPDM requires complex cure systems to achieve thedesired properties.

Nearly every conceivable combination of curing ingredients has beenevaluated in the various EPDM polymers, and over the years certain of thesehave shown particular merit. A brief description of four of these cure systemsused in practice follows.

This is one of the earliest cure systems developed for EPDM. It exhibits amedium cure rate and develops satisfactory vulcanizate properties. Theprimary advantage of this system is its low cost, but a major shortcoming isits severe tendency to bloom.

Cure Package 1. Low Cost

Sulfur 1.5

PerkacitRTMTD 1.5Perkacit MBT 0.5

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This common, nonblooming cure package has been labeled the ‘‘TripleEight’’ system for obvious reasons. It provides excellent physical propertiesand very fast cures but tends to be scorchy and is relatively expensive.

Excellent compression set and good heat aging properties characterize curepackage 3. Its drawbacks are a tendency to bloom and very high cost.

This general-purpose, nonblooming system offers good performance and isincluded as another example of a widely used EPDM cure package.

Cure Package 3. Low Set

Sulfur 0.5Perkacit ZDBD 3.0

Perkacit ZDMC 3.0Sulfasan DTDM 2.0Perkacit TMTD 3.0

Cure Package 4. General Purpose

Sulfur 2.0Perkacit MBTS 1.5

Perkacit ZDBD 2.5Perkacit TMTD 0.8

Cure Package 5. 2121 System

Vocol ZBPD 2.0Thiurad 1.0Santocure TBBS 2.0

Sulfur 1.0

Cure Package 2. Triple Eight

Sulfur 2.0Perkacit MBT 1.5

Perkacit TDED 0.8Perkacit DPTT 0.8Perkacit TMTD 0.8

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An attractive balance of fast cure, good physical properties, and goodresistance to compression set and heat aging are features of the 2121 curesystem, which was derived from a complex, statistically designed experimentto optimize the level of each ingredient. (Full details of the development of thissystem are given in the Flexsys publication ‘‘Systematic Development of anEPDM Accelerator System,’’ issued September 1977.)

Tables 13–16 summarize the properties obtained with these cure pack-ages when evaluated in three EPDM polymers varying in type and amount ofunsaturation. These data confirm the features of each cure system describedabove. The polymers used are listed in Table 13.

The advent of the faster curing, more unsaturated EPDMs made itpossible to use simpler accelerator systems such as the activated thiazoles andactivated sulfenamides used in NR, SBR, and the other highly unsaturatedpolymers. The use of these systems in EPDM is illustrated in Table 17.

These data compare one of the faster curing packages (the Triple8 system) with the simpler systems. The simple systems offer low-cost,bloom-free stocks and provide good scorch delay and satisfactory physicalproperties, but they are slower curing. The addition of a second activatingaccelerator, such as zinc dialyldithiocarbamate, speeds up the cure with noreal change in physical properties.

A common problem with the widely known EPDM curing packages isthe fact that the systems that produce low compression set also exhibit severebloom. This adverse combination of properties has recently been overcomewith the development of the ‘‘2828’’ system (2.0 Sulfasan DTDM/0.8 PerkacitTMTD/2.0 Perkacit ZDBC/0.8 Perkacit DPTT) illustrated in Table 18. Thiscure system provides compression set comparable to that of the low-setpackage discussed earlier; however, no bloom has been observed on stockscured with this system.

B. Nitrile Rubber

Nitrile rubber is a general term describing a family of elastomers obtainedby the copolymerization of acrylonitrile and butadiene. Although each

Table 13 Polymer Description

Polymera Third monomer type % Unsaturation

Nordel 1070 1,4-Hexadiene 2.5

Vistalon 5600 ENB 4.5Vistalon 6505 ENB 9.5

a Nordel is a registered trademark of E.I. duPont de Nemours and Company;

Vistalon is a registered trademark of Exxon Chemical Company

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Table 14 Low Saturation EPDM

Masterbatch

Nordel 1070 100.0N-550 Black 100.0N-774 Black 100.0Paraffinic oil 110.0

Flectol TMQ 2.0Zinc oxide 5.0Stearic acid 2.0

Cure system 1 2 3 4 5Low-cost X‘‘Triple 8’’ X

Low-set XGeneral-purpose X‘‘2121’’ X

Mooney scorch at 135jCMin viscosity 41.0 49.0 43.0 46.0 41.0t5, min 11.4 6.0 17.5 9.5 15.2t35, min 14.4 8.3 24.8 12.4 19.7

Rheometer at 160jC; 1j arcMax. torque, in.-lb 23.5 29.6 24.5 27.5 22.5t2, min 3.5 2.5 4.8 3.0 5.8

t90, min 17.5 17.3 14.5 15.5 18.0Stress–Strain Cure t90 at 160jC

Shore A hardness 67.0 71.0 69.0 71.0 66.0

100% modulus, psi 480.0 705.0 520.0 600.0 385.0Ult. tensile, psi 1690 1860 1600 1715 1615Ult. elongation, % 320.0 280.0 325.0 295.0 430.0

Stress–strain After 70 hr at 121jC

Shore A hardness 72.0 77.0 73.0 77.0 73.0100% modulus, psi 850.0 1370.0 805.0 1330.0 705.0Ult. tensile, psi 1950.0 2015.0 1675.0 1900.0 1770.0

Ult. elongation, % 235.0 160.0 225.0 155.0 280.0Retained TE factor,a % 84.0 62.0 72.0 58.0 71.0

Compression set, % (after 22 hr at 122jC,

t90 cure + 5 min at 160jC)

68.0 67.0 40.0 67.0 68.0

a Retained TE factor =aged ult: tensile � aged ult: elong

Ult: tensile � ult: elongation� 100

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Table 15 Medium Saturation EPDM

Masterbatch

Vistalon 5600 100.0N-774 Black 100.0N-550 Black 100.0Paraffinic oil 110.0


Cure system 1 2 3 4 5Low-cost X‘‘Triple 8’’ X


Mooney scorch at 135jCMinimum viscosity 41.0 46.0 38.0 39.0 38.0t5, min 7.3 4.2 11.0 7.0 10.5t35, min 9.8 6.2 17.8 10.0 14.5

Rheometer at 160jC; 1j arcMax. torque, in.-lb 28.0 31.0 25.0 29.0 28.0t2, min 3.2 1.5 3.4 2.5 4.2

t90, min 12.8 9.3 8.0 13.8 12.0Stress–Strain (t90 cure at 160jC)

Shore A hardness 74.0 76.0 74.0 76.0 74.0

100% modulus, psi 610.0 620.0 445.0 585.0 600.0Ult. tensile, psi 1515 1600 1405 1605 1645Ult. elongation, % 305.0 275.0 375.0 310.0 400.0

Stress–Strain (after 70 hr at 121jC)

Shore A hardness 78.0 80.0 77.0 79.0 78.0100% modulus, psi 955 1045 770.0 1000 750Ult. tensile, psi 1835 1790 1605 1655 1870

Ult. elongation, % 207.0 175.0 235.0 175.0 280.0Retained TEa factor,a % 82.0 71.0 72.0 58.0 80.0

Compression set, % (after 22 hr at 122jC,


67.0 67.0 50.0 65.0 63.0



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Table 16 High Saturation EPDM

Masterbatch

Vistalon 6500 100.0N-774 Black 100.0N-550 Black 100.0Paraffinic oil 110.0


Cure System 1 2 3 4 5Low-cost X‘‘Triple 8’’ X


Mooney scorch at 135jCMinimum viscosity 41.0 48.0 44.0 37.0 37.0t5, min 9.1 5.3 14.0 12.8 14.5t35, min 13.5 7.7 29.5 12.8 14.5

Rhoemeter at 160jC; 1j arcMax. torque, in.-lb 30.0 35.0 29.0 33.0 28.0t2, min 2.8 1.5 3.4 2.5 3.4

t90, min 11.1 8.0 9.0 11.2 9.5Stress–Strain (t90 cure at 160jC)

Shore A hardness 75.0 77.0 75.0 76.0 76.0

100% modulus, psi 970.0 1170.0 890.0 1100.0 815.0Ult. tensile, psi 1490.0 1550.0 1390.0 1630.0 1390.0Ult. elongation, % 160.0 135.0 165.0 155.0 175.0

Stress–Strain (after 70 hr at 121jC)

Shore A hardness 79.0 83.0 78.0 81.0 80.0100% modulus, psi 1500.0 — 1270.0 — 1130.0Ult. tensile, psi 1680.0 1715.0 1480.0 1690.0 1540.0

Ult. elongation, % 115.0 85.0 120.0 85.0 140.0Retained TEa factor, % 81.0 70.0 77.0 57.0 89.0

Compression set, %

(after 22 hr at 122jC,t90 cure + 5 min at 160jC)

66.0 62.0 43.0 65.0 67.0



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polymer’s specific properties depend primarily upon its acrylonitrile content,they all exhibit excellent abrasion resistance, heat resistance, low compressionset, and high tensile properties when properly compounded. Probably thepredominant feature dictating their use is their excellent resistance to petro-leum oils.

Cure systems for nitrile rubber are somewhat analogous to those used inSBR except that magnesium carbonate–treated sulfur is usually used to aid inits dispersion into the polymer. Typical cure systems employ approximately1.5 phr of the treated sulfur with appropriate accelerators to obtain thedesired rate and state of cure. Common accelerator combinations include thethiazole/thiuram or sulfenamide/thiuram types. Examples of these sulfur-based cure systems are shown in Table 19.

As operating requirements for nitrile rubber become more stringent,improved aging and set resistance become important. These improvementsare realized by reducing the amount of sulfur and by using a sulfur donorsuch as Sulfasan DTDM or Perkacit TMTD. Examples of these sulfur donor

Table 17 Thiazole and Sulfenamide Curing Systems in Vistalon 5600a

Santocure CBS 1.2 — 1.2 — ‘‘Triple 8’’

Perkacit MBTS — 1.5 — 1.5 ‘‘Triple 8’’Perkacit TMTD 0.7 0.8 0.7 0.8 ‘‘Triple 8’’Perkacit ZDEC — — 0.7 0.8 ‘‘Triple 8’’Sulfur 1.5 1.5 1.5 1.5 ‘‘Triple 8’’

Mooney scorch at 135jCt5, min 9.2 7.4 7.4 6.3 2.4

Rheometer at 160jC

t2, min 3.0 2.7 3.1 2.3 1.1t90, min 17.7 22.0 13.6 17.0 10.0Max. torque, in.-lb 60.0 60.0 60.0 60.0 70.0

Physical properties,optimum cure at 160jCUTS, psi 1465.0 1565.0 1520.0 1580.0 1620.0

100% modulus, psi 825.0 855.0 865.0 925.0 1010.0Elongation at break, % 220.0 220.0 210.0 200.0 150.0

Compression set,Opt. cure, ASTM-B

22 hr at 100jC, % 54.0 51.0 43.0 48.0 47.0Compression set, % (after

overcure 1 hr at 160jC,

22 hr at 100jC)

27.0 24.0 23.0 22.0 24.0

a General Formula (phr): Polymer, 100; FEF Black, 200; oil, 120; zinc oxide, 5; stearic acid, 1.

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Table 18 Curing Systems for ‘‘Low Set’’

Vistalon 3708 100.0

N-550 Black 50.0N-762 Black 150.0Circosol 4240 120.0Stearic acid 1.0

Santoflex 6PPD 2.0Zinc oxide 5.0

1 2 3 4

Sulfur 0.5 0.5 — 0.5Sulfasan DTDM 2.0 1.7 2.0 —Perkacit TMTD 3.0 2.5 0.8 1.0

Perkacit ZDMC 3.0 2.5 — —Perkacit ZDBC 3.0 2.5 2.0 —Perkacit DPTT — — 0.8 —

Santocure CBS — — — 2.0Vocol ZBPD — — — 3.2Accelerator cost, $/phr 17.57 14.74 11.93 8.63Mooney viscometer at 135jC

Minimum viscosity 20.0 21.4 21.2 20.5t5, min 14.2 13.7 16.4 14.2

Rheometer at 160jC

Min. torque, in.-lb 3.6 3.3 2.9 3.4Max. torque, in.-lb 23.0 22.1 18.5 15.8t2, min 5.0 4.8 6.7 5.2

t90, min 11.2 11.2 15.2 11.5Stress–Strain (t90 cure at 160jC)

Shore A hardness 70.0 69.0 70.0 66.0100% modulus, psi 430.0 390.0 330.0 275.0

300% modulus, psi 995.0 930.0 835.0 690.0Ult. tensile, psi 1280.0 1210.0 1160.0 940.0Ult. tensile, psi 550.0 545.0 670.0 640.0

Ult. elongation, %Aged 70 hr at 121jC

Shore A hardness 72.0 73.0 70.0 71.0

100% modulus, psi 500.0 460.0 370.0 405.0300% modulus, psi 1200.0 1130.0 940.0 970.0Ult. tensile, psi 1330.0 1290.0 1150.0 1160.0

Ult. elongation, % 435.0 425.0 560.0 450.0Retained TE factor, % 82.0 83.0 83.0 87.0

Compression set, % (after70 hr at 121jC,


60.0 58.0 57.0 76.0

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cure systems are shown in Tables 20 and 21. The advantage of these systemsis that they have better set resistance and aging while maintaining adequatescorch safety and fast cures. Note also in Table 21 that when using equallevels of Sulfasan DTDM, Santocure TBBS, and Perkacit TMTD andadjusting only total accelerator concentration, a wide modulus range isachieved while adequate scorch safety and a fast cure rate are maintained.Therefore, we have a viable method to control cross-link density in sulfurdonor systems.

C. Neoprene

Ethylene thiourea (ETU) has traditionally been the accelerator of choice forattaining maximum physical properties in Neoprene W compounds. How-ever, ETU is now available only in predispersed forms, and the rubber in-dustry is actively looking for a viable replacement.

We have found A-1k thiocarbanilide accelerator to be effective in neo-prene, particularly the Neoprene W types, which require additional acceler-ation beyond that provided by metal oxides alone.

Table 19 High-Sulfur Nitrile Rubber Systems

MC Treated sulfura 1.5 1.5 1.5

Perkacit TMTM 0.4 — —Perkacit MBTS — 1.5 —Santocure TBBS — — 1.2Perkacit TMTD — — 0.1

Processing and curing propertiesMooney scorch at 121jCt5, min 6.8 8.1 5.7

Rheometer cure time at 160jCt90, min 8.7 15.2 4.7

Physical properties on t90 cure

Hardness 73.0 71.0 75.0100% modulus, psi 610.0 520.0 730.0Ult. tensile, psi 2370.0 2355.0 2510.0

Ult. elongation, % 380.0 475.0 355.0Heat aging resistance (70 hr at 100jC)

Shore A hardness 80.0 78.0 82.0Retained TE factor, % 85.0 68.0 57.0

Compression set, % (after 22 hr at 100jC) 31.0 50.0 55.0

a Magnesium carbonate–treated sulfur; used to improve dispersion.

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Table 20 High Sulfur vs. Low Sulfur in Nitrile Rubber

MC-treated sulfura 1.5 0.3

Perkacit MBTS 1.5 —Santocure TBBS — 1.0Perkacit TMTD — 1.0Mooney scorch at 121jC

t5, min 8.1 8.1Rheometer cure time at 160jCt90, min 15.2 10.5

Physical properties on t90 cureShore A hardness 71.0 69.0100% modulus, psi 520.0 450.0

Ult. tensile, psi 2355.0 2190.0Ult. elongation, % 475.0 485.0

Heat aging resistance (after 70 hr at 100jC)

Shore A hardness 78.0 74.0Retained TE factor, % 68.0 89.0

Compression set, % (after 22 hr at 100jC) 50.0 24.0

a Magnesium carbonate–treated sulfur; used to improve dispersion.

Table 21 Sulfurless Cure Systems for Nitrile Rubber

MC-treated sulfur 1.5 — — —

Perkacit MBTS 1.5 — — —Santocure TBBS — 1.0 1.0 1.0Perkacit TMTD — 1.0 1.0 1.0

Sulfasan DTDM — 1.0 1.0 2.0Mooney scorch at 135jCt5, min 8.1 10.7 7.0 7.9

Rheometer cure time at 160jC

t90, min 15.2 14.7 12.5 13.3Physical properties on t90 cure at 160jC

Shore A hardness 71.0 68.0 71.0 73.0

100% modulus, psi 520.0 445.0 620.0 830.0Ult. tensile, psi 2355.0 2235.0 2370.0 2430.0Ult. elongation, % 475.0 485.0 360.0 290.0

Heat aging resistance (70 hr at 100jC)Shore A hardness 78.0 73.0 75.0 78.0Retained TE factor, % 68.0 87.0 89.0 83.0

Compression set, % (after 22 hr at 100jC) 50.0 22.0 13.0 12.0

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The A-1 accelerated compounds exhibit good processing safety; fast,level cures with excellent tensile properties; and compression set resistance.The advantages of A-1 described above are demonstrated for Neoprene W inTable 22 and Figure 36. Of particular interest is the very flat plateau obtainedwith the A-1 compared with the marching modulus of the other systems.

An unexpected advantage of the A-1 cure is its dramatic response toSantogard PVI for providing longer scorch delay. However, a sacrifice incompression set and modulus is observed as shown in Table 23. Also included

Table 22 Neoprene W Curing systems

MasterbatchNeoprene W 100.0N-990 Black 20.0

N-774 Black 40.0Aromatic oil 15.0Flectol TMQ 1.0

Stearic acid 4.0Stan Maga beads 1.0Zinc oxideb 5.0

Additive 1 2 3

NA-22 0.5 — —Perkacit TMTM — 1.0 —DOTG — 1.0 —

Sulfur — 0.5 —A-1TM — — 0.7

Mooney scorch at 135jC

Minimum viscosity 32.8 28.9 30.9t5, min 7.7 34.5 9.5

Rheometer at 160jC (MPC dies, F1j arc)

Min torque, in.-lb 5.0 4.0 4.1Max. torque, in.-lb 31.2 29.3 25.6t2, min 2.2 5.2 2.3t90, min 20.8 25.0 5.8

Stress–Strain (t90 cure at 160jC)Shore A hardness 61.0 58.0 57.0100% modulus, psi 345.0 280.0 270.0

300% modulus, psi 1715.0 1375.0 1320.0Ult. tensile, psi 2655.0 2440.0 2520.0Ult. elongation, % 470.0 550.0 540.0

Compression set, % (after 22 hr at 100jC) 12.0 23.0 10.0

a Stan Mag is a trademark of Harwick Chemical Corp.b Zinc oxide added to the mill.

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Table 23 Variations of A-1 Cure in Neoprene W

System Control Faster cure Longer scorch safety

StockA-1k 0.7 0.7 0.7

Vocol ZBPD-pdr — 0.5 —Santogard PVI — — 0.2Mooney scorch at 121jC

t5, min 18.0 7.7 30.0Rheometer at 160jCt90, min 7.4 4.8 10.7

Physical properties (t90 cure)Shore A hardness 62.0 63.0 60.0300% modulus, psi 1950.0 1705.0 1240.0Ult. tensile, psi 2905.0 2900.0 2700.0

Ult. elongation, % 435.0 475.0 530.0Compression set, %

(after 22 hr at 100jC)17.0 19.0 24.0

Figure 36 Comparison of neoprene cure systems.

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in this table is an A-1/Vocol ZBPD-pdr combination that offers very fast,scorchy compounds with excellent compression set resistance. This curesystem should be compatible with applications employing continuous vulcan-ization processes, wherein rapid onset of cure is mandatory.

D. Butyl Rubber

Because of its low unsaturation, butyl rubber possesses excellent resistanceto weathering, heat, and ozone as well as exhibiting excellent fatigue re-sistance. Of course, its predominant attribute is low gas permeability, whichmakes it the preferred elastomer for interliners, innertubes, bladders, andother air containment parts. The requirements for butyl tubes, for bothtruck and passenger tires, include good heat resistance and low set uponstretching (or maintaining dimensions after inflation), which can possiblyalso be related to compression set. Another major problem in most tubes isthe weakness of the splice, which results in premature failure due to sepa-ration. This appears to be particularly acute in tubes used with steel-beltedradial truck tires.

Table 24 Butyl Rubber Cure Systemsa

Semi-E.V. Conventional

Sulfur 0.5 2.0Perkacit TMTD 1.0 1.0

Perkacit MBT — 0.5Sulfasan DTDM 1.2 —Santocure TBBS 0.5 —

Mooney scorch at 121jCt5, min 36.2 18.5

Rheometer at 160jCt90, min 21.0 21.8

Physical propertiesShore A hardness 68.0 68.0300% modulus, psi 800.0 1030.0

Ult. tensile, psi 1590.0 1640.0Ult. elongation, % 600.0 510.0

Compression set, % (70 hr at 121jC) 56.0 81.0

Heat aging resistance (70 hr at 121jC)Retained tensile, % 76.0 57.0

E.V.= efficient vulcanization.a Masterbatch (phr) Butyl 218, 100; GPF Black, 70; paraffinic oil, 25; zinc oxide, 5.

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One method to improve splicing behavior is to develop longer scorchtimes, which permit better flow and thus better knitting at the splice prior tocure. This would have to be accomplished with no loss in other key properties.It is our objective to compare properties of a semi-efficient vulcanization curesystem to those of a conventional sulfur cure system recommended by thepolymer manufacturer. Table 24 summarizes these formulations and theproperties. As observed earlier in the case of nitrile rubber, the SulfasanDTDM–based cure systems offer significant improvement in heat andcompression set resistance as well as improved processing safety, all qualitiesthat contribute to improved product performance.

REFERENCES

1. Goodyear C. US Patent 3633, 1844.2. Hancock T. Br Patent 9952, 1843.3. Baker CSL, Barnard D, Porter M. Rubber Chem Technol 1970; 43:501.

4. Aquitaine Total Organico. Br Patent 1,339,653, 1973.5. Barton BC, Hart EJ. Ind Eng Chem 1952; 44:2444–2448.6. Adams HE, Johnson BL. Ind Eng Chem 1953; 45:1539–1546.

7. Trivette CD Jr, Morita E, Young EJ. Rubber Chem Technol 1962; 35(5):1360–1426.

8. Hall WE, Jones HC. Presented at a Meeting of ACS Rubber Division, Chicago,IL, October 1970.

9. Hall WE, Fox DB. Presented at a Meeting of ACS Rubber Division, SanFrancisco, CA, Oct 5–8, 1976.

10. Oenslager G. Ind Eng Chem 1933; 25:232.

11. Van Alpen J. Rubber Chemicals. Boston: Reidel, 1973.12. Chapman AV, Porter M. In: Roberts AD, ed. Natural Rubber Science and

Technology. Oxford: Oxford Univ Press, 1988:511–620.

13. Kresja MR, Koenig JL. Rubber Chem Technol 1993; 66:376–410.14. Rostek CJ, Lin H-J, Sikora DJ, Katritzky AR, Kuzmierkiewicz W, Shobana N.

Rubber Chem Technol 1996; 69(2):180–202.

15. Wolfe JR Jr. Rubber Chem Technol 1968; 41:1339.16. Coran AY. In: Eirich FR, ed. Science and Technology of Rubber. New York:

Academic Press, 1978:301.17. Skinner TD. Rubber Chem Technol 1972; 45:182.

18. Campbell RH, Wise RW. Rubber Chem Technol 1964; 37:635.19. Kratz GD, Flower AH, Coolidge C. J Indian Eng Chem 1920; 12:317.20. Bedford CW, Scott W. J Indian Eng Chem 1920; 12:31.

21. Moore CG, Saville RW. J Chem Soc 1954, 2082.22. Krebs H. Rubber Chem Technol 1957; 30:962.23. Milligan B. Rubber Chem Technol 1966; 39:1115.

4871-9_Rodgers_Ch11_R2_052404



24. Chivers T. Nature 1974; 252:32.25. McCleverty JA. In: Muller A, Krebs B, eds. Sulfur. Its Significance for Chem-

istry, for the Geo-, Bio-, and Cosmosphere Technol 1984; 5:311–329.

26. Coran AY. Rubber Chem Technol 1964; 37:679, 685, 1965; 38:1.27. Milligan B. J Chem Soc 1966; 1:34.28. Higgins GMC, Saville B. J Chem Soc, 1963, 2812.

29. Coates E, Rigg B, Saville B, Skelton D. J Chem Soc 1965, 5613.30. Spacu G, Macarovici CGh. Bull Sect Sci Acad Roumaine 1938–1939; 21:173.31. Lichty JG. US Patent 2,129,621, 1938.

32. Tsurugi J, Nakabayashi T. J Soc Rubber Ind Jp 1952; 25:267.33. Gupta SK, Srivastava TS. J Inorg Nucl Chem 1970; 32:1611.34. Fackler JP Jr, Coucouvanis D, Fetchin JA, Seidel WC. J Am Chem Soc 1968;

90:2784.35. Coucouvanis D, Fackler JP Jr. J Am Chem Soc 1967; 89:1346.36. Goda K, Tsurgi J, Yamamoto R, Ohara M, Sakuramoto Y. Nippon Gomu

Kyokaishi 1973; 46:63.

37. Hammett LP. Chem Rev 1935; 17:125.38. Taft RW. J Am Chem Soc 1952; 74:3120.39. Taft RW. In: Newman MS, ed. Steric Effects in Organic Chemistry. New York:

Wiley, 1956:556.40. Morita E. Rubber Chem Technol 1984; 57:744.41. Ignatz-Hoover F, Katritzky AR, Lobanov VS, Karelson M. Rubber Chem

Technol 1999; 72(2):318–333.42. Rauchfuss TB, Maender OW, Ignatz-Hoover F. US Patent 6,114,469, 2000.43. Ignatz-Hoover F, Kuhls G. Delayed action accelerated sulfur vulcanization of

high diene elastomers; the effects of the amine on vulcanization characteristicsof sulfenamide accelerated cure systems. ACS Rubber Division, Spring Meet-ing, Chicago, IL, 1990, Paper 3.

44. Lin H-J, Datta RN, Meander OW. U.S. Pat. 6,646,029, Nov. 11, 2003.

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