rubber chemicals

Vol. 11 RUBBER CHEMICALS 577

RUBBER CHEMICALS

Introduction

Rubber processing chemicals are used extensively to impart performance and pro-cessability to rubber and the products made thereof. These chemicals are typicallyorganic compounds but several inorganic materials are also included under theumbrella of rubber chemicals. The rubber compound is described as a mixture ofone or more rubber polymers (elastomers) with a combination of one or more offillers, oils, and rubber chemicals (see RUBBER COMPOUNDING). The compound, oncevulcanized, provides technologically useful properties, such as may be applied totires, hoses, belts, tracks, and a variety of mechanical goods.

Vulcanization systems and antidegradants are the two dominant classes ofrubber chemicals, given that the largest class of rubbers used in industry (the so-called general-purpose rubbers) require cross-linking by sulfur or other curativeto provide meaningful performance, and require protection from oxygen and ozoneto provide meaningful service lifetimes (see ANTIOXIDANTS).

The present article reviews several recent developments in the field of rubberchemicals of particular interest to practitioners. These topics include

(1) non-nitrosamine curatives as a response to health concerns and legislation(2) reversion-resistance curatives to enhance product durability and support

higher temperature cures(3) silica-to-rubber coupling agents to support expanded use of silica filler(4) nonstaining and persistent antidegradants for enhanced performance

Non-Nitrosamine Curatives

General-purpose rubbers are those made from the monomers isoprene, 1,4-butadiene, and styrene. Each isoprene and butadiene unit contributes one

Encyclopedia of Polymer Science and Technology. Copyright John Wiley & Sons, Inc. All rights reserved.

578 RUBBER CHEMICALS Vol. 11

carbon–carbon double bond to the final structure, plus a number of allylic hy-drogen sites, which are the reactive centers for sulfur vulcanization. In sulfurvulcanization, polymer chains are connected by covalent bonds (cross-linked) ofsulfur atoms or chains of sulfur atoms attached to the allylic positions of indi-vidual chains. The chemical components necessary for this process include thepolymer(s) with allylic hydrogens, elemental sulfur or a source of sulfur, pluschemical promoters called accelerators and activators. In current practice, thecurative promoter package typically employs a primary accelerator, one or moresecondary accelerators, and zinc oxide and fatty acid (or a zinc fatty acid salt)activator. Several excellent reviews have been written detailing the chemistry in-volved between these ingredients and the influence they have on the developmentof final vulcanizate properties (1–9).

The benefits of an optimized sulfur vulcanization system are several: eco-nomical, good kinetics, and good performance. Economical in that sulfur is veryinexpensive, and typical accelerators and activators are also low cost commod-ity chemicals. Good kinetics in that rapid cross-linking is achievable in a mat-ter of a few minutes at usual curing temperatures of 130–180◦C but that therubber compound can withstand many minutes at processing temperatures of-ten in the range of curing temperatures (ie, 100–140◦C) before the onset ofcross-linking.

The undesirable premature cross-linking of polymer chains in the processingstep is called scorch; the ability of the vulcanization system to withstand the heat ofprocessing without premature cross-linking is called scorch safety. This attributeof delayed action (scorch safety) with fast cure is unique to accelerated sulfurvulcanization, and allows for safe and rapid mixing and fabrication of rubbercompounds and parts coupled with rapid cure.

Good performance derives from the structure of the cross-links as a blendof sulfur chain lengths, from one sulfur atom (monosulfidic) to two (disulfidic) tothree or more, up to eight and more (polysulfidic). The differing chain lengthsimpart differing physical characteristics: shorter chain impart heat stability andresistance to permanent set, longer chain lengths impart tear resistance. The ratioof sulfur to primary accelerator controls cross-link length distribution, and thuscan be used to tailor physical properties, even as the concentration of curativescontrols cross-link density, itself a dominant contributor to compound propertiessuch as modulus, strength, hardness, and abrasion resistance.

Figure 1 illustrates the curing process as measured by a rheometer. Therheometer follows the development of cure state as a function of time. Vulcan-ization characteristics such as minimum torque ML (a measure of compoundgreen viscosity), onset of cure (T1 pt rise), MH (a measure of maximum cure statereached), and T90% cure (time required to reach 90% cure state) are valuableproperties defining cure behavior. Likewise the rheometer cure profile can be bro-ken into three regions of interest, an estimate of scorch safety, an idea of whatthe cure state development looks like during the cross-linking phase, and how thecross-linked vulcanizate network matures with additional heat.

Primary Accelerators. The primary accelerator of choice is a mercapto-benzothiazole sulfenamide of general chemical structure (1). Specific examples ofthis class are listed in Tables 1 and 2.


Fig. 1. Example of rheometer cure curve illustrating major cure attributes. To convertdNm to in lb, multiply by 0.885.

The sulfenamide S N bond strength controls the scorch safety and cure rate.The more labile this bond, the more readily the active cure promoter mercapto-benzothiazole is released at the onset of the cross-linking process. A family ofsulfenamides has been developed to span a range of cure rates. Common histor-ical members of this class are shown in Table 3, arranged from fast curing to

Table 1. Nitrosamine-Generating Sulfenamides

Chemical name Abbreviation Chemical structure

4-Morpholinyl-2-benzothiazoledisulfide

MBSS

N-Oxydiethylenebenzothiazole-2-sulfenamide

MBS

N,N-Diisopropylbenzothiazole-2-sulfenamide

DIBS


Table 2. Non-Nitrosamine-Generating Sulfenamides and Sulfenimides


N-Cyclohexyl benzothiazole-2-sulfenamide CBS

N-tert-Butyl-2-benzothiazole sulfenamide TBBS

N-Dicyclohexyl-2-benzothiazole sulfenamide DCBS

N-tert-Butyl-2-benzothiazole sulfenimide TBSI

slow, based on primary amines of weakening base strength (ie, a to b to c) and/orincreasing steric hindrance (ie, c to d to e) (10).

German legislation targeting elimination of stable and volatile nitrosaminesas by-products of vulcanization became effective in September 1988 as the “Tech-nical Rules for Hazardous Materials TRGS 552” (11,12). It recognized 11 ni-trosamines as carcinogenic and set industry limits to exposure. Nitrosaminesare potent carcinogens in laboratory animal testing, and are suspect human car-cinogens. The amines released from sulfenamide accelerators in the necessaryearly step of vulcanization can combine with nitrogen oxides (NOx) in the at-mosphere to produce nitrosamines. Primary amines form unstable nitrosaminesthat immediately decompose and do not constitute a health risk, whereas sec-ondary amines produce stable nitrosamines by this process, (R)2 N N O). The

Table 3. Sulfenamide Structure Influence or Cure Rate and Scorch Safety

a b c d e

CBS TBBS MBS DIBS DCBS

CyclohexlyamineBase strengthpKa 10.7

t-ButylamineBasestrengthpKa 10.5

MorpholineBasestrengthpKa 8.3

DiisopropylamineBase strengthpKa 11.0

DicyclohexylamineBase strengthpKa 11.3

Steric hindrancepredominates

Steric hindrancepredominates


Table 4. Comparison of sulfenamide cure propertiesnormalized to MBSa,b

Cure time (T90%) Scorch time

Nitrosamine-generatingMBS 100 100MBSS 27% shorter 45% shorterDIBS 7% longer 10% longer

Non-nitrosamine alternativesCBS 27% shorter 29% shorterTBBS 15% shorter 22% shorterDCBS 31% longer 30% longer

aBase compound: SBR, 50; polyisoprene, 50; N299 carbon black, 50;naphthenic oil, 28; stearic acid, 1; ZnO, 3; antioxidant, 1; sulfur, 1.75;sulfenamides at an equal molar level (4.74 mmol).bSamples tested at 150◦C.

immediate effect of the legislation was thus to target for elimination from vulcan-ization systems of all sulfenamides made from secondary amines able to form 1of the 11 listed nitrosamines. The accelerators given in Table 1, MBS and MBSS(derived from morpholine) and DIBS (derived from diisopropyl amine), had to bereplaced at least for European markets.

Compromise cure systems based on the faster and slower curing “non-nitrosamine” sulfenamides listed in Table 2 have been recommended as alterna-tives (12–14). The sulfenimide variant N-tert-butyl-2-benzothiazole sulfenimide(TBSI) was also produced to satisfy this need (15). As with DCBS the large sterichindrance associated with the sulfenimide slowed the reaction kinetics, resultingin slower cures and extended scorch safety.

Table 4 (16) makes a comparison of cure properties for the sulfenamides listedin Tables 1 and 2 relative to MBS. No exact replacements are available for MBS,MBSS, and DIBS, but DCBS can serve as an alternative for MBS and DIBS andCBS and TBBS can serve as alternatives for MBSS. Proper adjustments in sulfurand sulfenamide levels can provide similar cure performance. CBS and TBBS alsosatisfy the need in providing rapid curing kinetics where required. Additionalscorch protection, if needed, can be obtained by application of the widely usedscorch inhibitor N-(cyclohexylthio)phthalimide (17).

Secondary Accelerators. Even bigger changes have come to the sec-ondary curative segment. Three classes of chemicals served historically as sec-ondary accelerators, useful in low levels to speed up sulfenamide cures: the thiu-ram (2) and dithiocarbamate (3) types, and the guanidine type (4). Specific exam-ples for these classes are listed in Tables 5 and 6.


Table 5. Nitrosamine-Generating Thiurams


Tetramethylthiuram disulfide TMTD

Tetramethylthiuram monosulfide TMTM

Two workhorse members of the thiuram class, particularly active in cure rateas well as economical, are TMTD and TMTM (R= methyl, x = 2 and 1, respectively,in 2). Both TMTD and TMTM are stable nitrosamine generators, as they liberatethe secondary amine, dimethyl amine, during vulcanization (12,13). Replacementsfor these rubber chemicals has been a target of considerable technical effort in therubber industry. Several candidates have emerged, each with its own attributes.None is a direct substitute for either TMTD or TMTM but several have proven

Table 6. Low or Non-Nitrosamine-Generating Thiurams, Dithiocarbamates, andGuanidines


N,N,N′,N′-Tetraisobutylthiuramdisulfide

IBT

N,N,N′,N′-Tetraisobutylthiurammonosulfide

IBM

Tetrabenzylthiuramdisulfide

TBzTD

Zinc dibenzyldithiocar-bamate

ZBEC

Diphenylguanidine DPG

Di-o-tolylguanidine DOPG


Table 7. Comparison of Cure Properties Relative to TMTDa ofNon-Nitrosamine Secondary Accelerator Alternativesb,c

Cure time (T90%) Scorch time

TBzTD (eq molar, 0.67 phr) 18% longer 33% longerIBT (eq molar, 0.47 phr) 18% longer 33% longerZBzDC (eq molar, 0.76 phr) 6% longer EqualaAt an equal molar level (0.30 phr).bBase compound: SBR, 50; Polyisoprene, 50; N299 carbon black, 50; naph-thenic oil, 28; stearic acid, 1; ZnO, 3; antioxidant, 1; sulfur, 1.75; sulfe-namides at an equal molar level (4.74 mmol).cSamples tested at 150◦C.

useful in non-nitrosamine cure packages. Cure properties of likely replacementsare compared to TMTD in Table 7 (16).

As secondary curatives, both TBzTD and IBT provide slightly longer curetimes but have the advantage of prolonging scorch safety. A much closer matchto TMTD is ZBzDC in both cure time and scorch time. When used at equal molarconcentrations, significantly more weight of the non-nitrosamine replacementsare required versus TMTD.

TBzTD and ZBzTD do, in fact, generate stable nitrosamines from the sec-ondary amine dibenzyl amine; however, this nitrosamine is nonvolatile and isnot readily released from the vulcanized product into the atmosphere. It is alsoconsidered noncarcinogenic (18). Therefore, these materials are considered non-nitrosamine in usage. IBT likewise generates a stable nitrosamine but its levelhas been claimed to be 100 times less than TMTD (19).

Sulfur Donors. A fourth class of sulfur vulcanization chemicals has beenaffected by the move to non-nitrosamine generators: the sulfur donors. These areorganic chemicals which have a relatively high level by weight of sulfur atomsthat are able to be donated in the vulcanization process to create sulfur cross-links. Elemental sulfur is the material of choice as a source of the sulfur atomswhich create these cross-links, but some articles cannot tolerate free sulfur forrequirements of color, solubility, or performance.

With regard to solubility, compounds which require a very high cure statemay require more sulfur than can be supplied as elemental sulfur. Sulfur hasa solubility limit of about 1 part per hundred rubber at room temperature (20)and at higher levels will “bloom,” that is, crystallize out of solution onto the rub-ber compound surface, where it can be detrimental to processing, particularly to“tack,” the ability of rubber to stick to itself or other rubber component in thebuilding of a composite product. So-called insoluble sulfur has been developed asan alternative form of elemental sulfur: it has a polymerized long-chain structure( Sx ) as opposed to crystalline sulfur (S8 rings) (21,22). Insoluble sulfur can besubstituted for crystalline sulfur and can be used at levels well above the solu-bility limits without bloom, in theory. In practice, however, insoluble sulfur willrevert to soluble sulfur at the temperatures often attained in mixing and shapingrubber components, and thus can result in bloom in processing (23). Suffice it tosay that some alternative forms for sulfur are sometimes needed for processing.


Table 8. Nitrosamine-Generating Sulfur Donors


4,4′-Dithiodimorpholine DTDM

4-Morpholinyl-2-benzothiazoledisulfide

MBSS

N-Oxydiethylenethiocarbamoyl-N′-oxydiethylenesulfenamide

OTOS

Tetramethylthiuram disulfide TMTD

Likewise, for performance, sometimes a sulfur donor is needed as well. Ele-mental sulfur gives rise to variable cross-link lengths, regulated to some extent bysulfur-to-accelerator ratio. In some applications, particularly those which requirevery high temperature and or oxidative stability, one may need a preponderance ofcross-links of very low rank, mostly monosulfidic. For this purpose, organic sulfurdonors may be useful (24).

Four conventional sulfur donor curatives employed historically are all ni-trosamine generators, derived from either morpholine or dimethylamine, as shownin Table 8. The industry has developed non-nitrosamine substitutes, as shown inTable 9. As discussed earlier, TBzTD is used primarily as a secondary acceleratorto further accelerate cure while offering good scorch protection. Similarly, IBTperforms like TBzTD. CLD appears to be as active as DTDM in donating sulfurevidenced by its ability to generate the same cure state as DTDM when used atequal available sulfur levels. CLD however can introduce more scorch. These at-tributes are illustrated in Table 10 (16). TESPT has found value as a sulfur donorin what has been described as an equilibrium cure, or EC for short (25). The con-cept relies on the slow release of sulfur from TESPT during the curing processto control the reversion process illustrated in the rheometer chart of Figure 1.Rhenocure® SDT/S is a thiophosphite and thus not amine-based. It can be usedwhere high levels of short cross-links of the mono- and di-type are desirable forlong-term heat age stability (26). The Vultac® class of Table 9 has found uses indiene rubbers as partial or total replacement for sulfur (27). Benefits claimed in-clude improvements in aging properties and promoting covulcanization of blends.Table 10 illustrates the sulfur-donating ability of the Vultac® class by showingVultac® 7 provides a similar cure state as sulfur when used as a total replacement.One drawback is the introduction of scorch.

The rubber chemist has reacted vigorously to the challenge of eliminat-ing volatile nitrosamine-generating chemicals in the workplace, even thoughmany had been fixtures of the sulfur vulcanization process. Non-nitrosamine pri-mary and secondary accelerators and sulfur donors are now available and usedextensively.

Table 9. Non-Nitrosamine Sulfur Donors


Tetrabenzylthiuram disulfide TBzTD

N,N,N′,N′-Tetraisobutylthiuram disulfide IBT

Caprolactam disulfide CLD

Bis-(3-triethoxysilylpropyl)tetrasulfane TESPT

Bis-(O,O-di-2-ethylhexyl-thiophosphoryl)polysulfide Rhenocure® SDT/S

Alkylphenol disulfide oligmer Vultac®

585


Table 10. Activity of Sulfur Donors Versus Free Sulfur Compared Using aRheometer Cure Tracea,b

Sulfur donor Sulfur DTDM CLD Vultac® 7

Sulfur donor, phr 1.0 3.7 4.5 3.3Available sulfur, phr 1.0 1.0 1.0 1.0Delta Tq , dNm (cure state) 19.2 26.4 26.6 19.6T90%, min 21.7 36.3 19.7 21.5T1 pt rise, min 9.0 12.3 5.7 4.0aBase compound: SBR, 50; polyisoprene, 50; N299 carbon black, 50; naphthenic oil, 28; stearicacid, 1; ZnO, 3; antioxidant, 1; CBS, 1.25.bSamples tested at 150◦C.

Reversion-Resistance Curatives

Concomitant with the several benefits of sulfur vulcanization described aboveare certain deficiencies owing to the complexity of the formation of, and in-stability of, the sulfur cross-link system. Reversion is defined as the anaerobic(nonoxidative) degradation of sulfur-cured vulcanizates which results in a lossof compound performance. This phenomenon is generally associated with loss incross-link density via thermal degradation of sulfur cross-links caused by heatand/or extraneous accelerator fragments (28,29). It occurs during overcure con-ditions, elevated cure temperatures (30), and severe service conditions (31). Re-version is most pronounced for natural rubber, which tend to lose cross-links andsoften with heat and aging, whereas synthetics (polybutadiene, styrene-butadienecopolymers (qv)) tend to add cross-links and harden with heat and aging (seeBUTADIENE POLYMERS).

Overcure, elevated cure temperatures, and severe service are all growingtrends industrially, particularly for large tires as are used in over-the-highwaytrucks, off-the -road equipment, and aircraft tires, all of which also use a lot ofnatural rubber. Manufacturing efficiencies promote higher temperature cure con-ditions for better throughput. Tires are cured from heat applied at the outside(mold) and inside (bladder) surfaces. For tires with thick rubber cross sections,and given the poor thermal conductivity of rubber, a temperature gradient is pro-duced from surfaces to interior. The surfaces may see significantly more total heathistory than the interior. This may be so even allowing for slow cooling of the in-terior and continued cure of the inside after the tire is removed from the mold.Cure systems are tailored for individual components to try to balance the cureprofile across the cross section, but a given thick tread may still be subjected toa broad range of heat inputs, so that only a portion of that one component willbe optimally cured while a portion will be overcured. (No portion can be under-cured without generating a useable product. Indeed, cure conditions are set so asto require all portions of the tire to achieve a minimum acceptable level of cure.)Tires, like other rubber goods, perform to ever higher levels: mileage, service life,and service conditions. Still, high operating temperatures and loads may impartreversion. Thus, there has been a growing interest in chemicals which inhibit thetendency to reversion of sulfur-cured natural rubber.


Fig. 2. Reversion during cross-link formation.

Reversion can occur during cross-link formation (high temperature cure) andafter cross-link formation (overcure or in service). These are shown pictorially inFigures 2 and 3.

For processes which contribute to reversion seen during cross-link forma-tion, several pathways can be envisioned contributing to loss in ultimate curestate development. As Figure 4 shows, these pathways most likely involve desul-furation or decomposition of intermediate polythio-sulfurating species or thepolythio-benzothiazole cross-link precursor formed by the heat of the curing pro-cess. This decomposition pathway effectively reduces the concentration of activecross-linking agents resulting in lower cross-link density. Also at very high temper-atures as cross-links are forming, they may, in a similar manner, be immediatelydesulfurated and decomposed.

In the development of maximum cure state, the initial cross-link structure,which may be high in polysulfides, goes through a rearrangement process in whichthe Zn-accelerator complexes, which promote curing, extrude sulfur from the ini-tial cross-links and reutilize the extruded sulfur to form additional cross-links (29).After the maximum cure state is established with a desired proportion of polysul-fide, disulfide, and monosulfide cross-links for compound performance, the net-work cross-link structures will continue to evolve with additional heat introduced

Fig. 3. Reversion after cross-link formation.


Fig. 4. Reversion pathways during cross-link Formation.

during an overcure situation (30) or during the service life of the compound (31).Figure 5 shows that this reversion process involves loss of polysulfide cross-linkswith the formation of more mono and disulfide types and finally decompositionof the mono and disulfidic types yielding modifications in the polymer backbone.Modifications would include carbon–carbon double bond rearrangement, growthof diene and triene structures, and isomerization of existing double bonds. For

Fig. 5. Cross-link network maturation and reversion with heat from curing or high tem-perature service.


Fig. 6. Effect of temperature on cure rate and cure state. Base compound: NR, 100; N347carbon black, 50; naphthenic oil, 8; stearic acid, 1; ZnO, 3; antioxidant, 0.75; sulfur, 3; TBBS,0.75. To convert dNm to in lb, multiply by 0.885.

natural rubber, this results in creation of some trans double bonds in the other-wise all-cis structure. Additionally, unproductive (nonload bearing) cyclic sulfidesincrease in number in reversion: sulfur attached by both of its valences to neigh-boring segments of the same polymer chain. Zinc sulfide is produced as a stablebut inactive by-product of both vulcanization and reversion. All these changes arefollowed by a drop in cross-link density (32–35).

The effect of cure temperature on the cure kinetics and cure state profile of asimple carbon black filled, sulfenamide/sulfur cured natural rubber compound isshown in Figure 6 (16). As shown in the growth of the cure state, an article whichwould require 110 min to cure to its maximum cure state at 125◦C can be curedat 135◦C in 50 min, at 150◦C in 18 min, at 165◦C in about 7 min, and at 182◦Cin about 3 min. However, the maximum state of cure achieved keeps dropping ascure temperature increases.

The same process continues beyond “optimum” or maximum cure state if cureis allowed to continue (“overcure”). Again, as seen in Figure 6, the cure state nowdrops from the maximum to attain a lower plateau. This decline is faster and morepronounced as cure temperature increases. The window of time for near-optimumcure narrows, and the regime of overcure grows.

Physical manifestations of reversion, beyond loss in cure state, include

(1) loss in resilience(2) greater heat buildup(3) loss in abrasion resistanec(4) loss in wire adhesion(5) initial increase then large loss in tear and fatigue resistance(6) loss in modulus and hardness(7) loss in tensile strength


How much of the decay in properties is due to cis–trans isomerization, cyclicsulfides, growth of dienes, reduction in sulfur rank, and reduction in total cross-link density is a subject of ongoing research, but all processes likely contribute.

Because of the large demands put on rubber compounds to address issues ofincreased productivity via high cure temperatures or increased service tempera-tures brought about by overloads and high speeds, reversion-resistance chemicalsare now becoming part of the compounder’s arsenal to combat reversion issues.

Reversion-resistance chemicals operate by one of two mechanisms:

(1) Relink polymer chains to maintain cross-link density. These may be viasulfur or non-sulfur covalent bonding.

(2) Form heat stable cross-links that are more resistant to scission than con-ventional sulfur cross-links. These may or may not contain sulfur.

Several rubber chemicals have been developed to inhibit or compensate forsulfur cross-link reversion. Several of these are shown in Table 11.

MPBM operates under the mechanism of forming heat-stable cross-links.It has been reported that the network structures formed with MPBM are notprone to revert (36,37) and offer heat resistance during high temperature curingand overcure. Detailed mechanistic studies have found the major reactions occur-ring during vulcanization in sulfur-containing systems to be 1,2-cross-linking ofthe imide double bond across polymer chains generating a heat-stable carbon–carbon cross-link. Additional reactions included homopolymerization, copoly-merization giving multiple cross-links, and Michael addition reactions with 2-mercaptobenzothiazole and amines derived from sulfenamide curatives as well asother extraneous sulfur species formed during cure (38).

BCI-MX also provides its heat-resistant performance by the formation ofcarbon–carbon cross-links but does it by the mechanism of relinking polymerchains to maintain cross-link density (38,39). The relinking mechanism takesplace only during reversion when sulfur cross-links are broken and in their placecyclic sulfides, dienes, and trienes are formed along the polymer backbone (16,35).The structural differences of BCI-MX from MPBM prevent it from undergoing thereactions discussed about MPBM. Instead, the citraconimide group undergoes aDiels–Alder addition reaction with these chain modifications to relink polymerchains and compensate for the lost, reverted, sulfur cross-links (39–41). For BCI-MX to be effective, some initial reversion must take place. This is nicely illustratedin Figure 7 (16) which shows the reversion characteristics of BCI-MX versus thesame compound with no BCI-MX added.

As discussed earlier, TESPT functions as a reversion-resistant chemical bythe slow release of sulfur to compensate for cross-links lost (25). It has also beenfound that TESPT leads to higher levels of monosulfidic cross-links (42), whichyields improved reversion (thermal aging) resistance (43,44).

Both HTS and BDTCH can be classified as proceeding through the mech-anism, which forms heat-stable cross-links that are more resistant to scission.The cross-link has been referred to as a hybrid cross-link in which a hexylmethy-lene dithio group Sn [(CH2)6 Sx]k is the bridging group between two poly-mer chains (45–49). In this scheme, k is presumed to be small. At high sulfur

Table 11. Anti-Reversion Agents


N-N′-m-phenylenedimaleimide MPBM

1,3-Bis(citraconimidomethyl) benzene BCI-MX

Bis(3-triethoxysilylpropyl)tetrasulfane TESPT

Hexamethylene bis-thiosulfate disodium salt dihydrate HTS

Mixture of zinc salts of aliphatic and aromatic carboxylic acids Activator® 73

R = aliphatic–aromatic mix1,6-Bis(N,N-dibenzylthiocarbamyldithio)hexane BDTCH

Trifunctional acrylate coagent Sartomer SR534

591


Fig. 7. Example of reversion-resistant agent effect on reversion (cure temperature 165◦C).Base compound: NR, 100; N347 carbon black, 50; naphthenic oil, 8; stearic acid, 1; ZnO, 3;antioxidant, 0.75; sulfur, 3; TBBS, 0.75. To convert dNm to in lb, multiply by 0.885.

levels both n and x would be greater than 1. As reversion proceeds, the numberof n and x sulfur atoms would be reduced to one resulting in a hybrid cross-linkwith monosulfide linkages for thermal stability and the hexylmethylene bridge toadd flexibility to the cross-link for improved tear and fatigue properties. Figure 7illustrates the reversion-resistant capabilities the hybrid cross-link can offer.

Since Activator 73 A is a mixture of zinc salts of linear aliphatic and aromaticcarboxylic acids (50), it would be expected that the mechanism of action would besimilar to zinc stearate, which can extract sulfur from polysulfidic cross-links andreuse that sulfur to generate more cross-links (51). Ultimately this would resultin a network structure high in monosulfidic content (42).

Trifunctional acrylates, such as Sartomer SR534, have been found to providereversion-resistant protection (52). Test results of the three trifunctional acry-late and methacrylate monomers, pentaerythritol triacrylate, trimethylolpropanetrimethacrylate, and trimethylolpropane triacrylate, versus BCI-MX prove inter-esting (53). Rheometer traces of cure development and reversion characteristicswith these trifunctional monomers duplicated BCI-MX behavior even to the pointof having initial reversion chemistry occurring before they became active. Thiswould suggest these materials operate by the same mechanism as BCI-MX.

To conclude this section, the rubber chemist has vigorously pursued materi-als and means to inhibit and counteract the tendency to reversion, so as to permithigher temperature cures, buttress rubber goods against overcure, and permitincreasingly greater operating conditions and service lifetimes.

Silica-to-Rubber Coupling Agents

Rubber must be cross-linked to be technologically useful, hence the emphasis his-torically and in the two sections above, on chemicals to facilitate and optimize thisprocess. However, even cured rubber is practically of little value unless the rubber


is reinforced by a particulate filler (see FILLERS). Cured, unfilled (“gum”) rubberhas very low tensile strength, and poor tear and abrasion resistance. Contrast apencil eraser or a rubber band to a tire tread or transmission belt, for example.The early tires and rubber goods were “filled” with inert fillers such as zinc ox-ide. The discovery about a century ago that carbon black (qv) greatly enhancedrubber performance began the evolution of tough durable goods made from rub-ber. It turns out that very few other substances reinforce rubber as does carbonblack. None do at the same cost. Hence, even today, carbon black is the dominantreinforcing filler for rubber, even as talc, calcium carbonate, kaolin clay, and otherlower cost, nonreinforcing fillers retain uses in selected nontire applications.

The characteristics shared by reinforcing fillers are few but vital:

(1) Small hard particles, with diameters upon ultimate dispersion below micronsize

(2) Ability of said particles to disperse in the rubber medium(3) Ability of said particles to interact with themselves, so as to produce an

independent filler network, which interpenetrates with and synergisticallysupports the polymer network. This attribute is called “filler/filler interac-tion.”

(4) Ability of said particles to interact with the polymer matrix. This attributeis called “polymer/filler interaction.”

Carbon Black. Carbon black (qv) has all of these characteristics. Carbonblack is amorphous elemental carbon with a graphitic-like microcrystalline sub-structure (54). The furnace process rubber grades come in a range of particle andaggregate sizes and shapes, with reinforcing grades in the 10–100 nm ultimateparticle diameters. Carbon black disperses very well in general-purpose rubbers,with which it shares an organic chemical nature. It has a strong self-affinity; oncea sufficient loading is achieved in a rubber matrix, it imparts electrical conductiv-ity to the composite via the filler network, for example. This filler network is alsoresponsible for the enhanced low strain dynamic stiffness of filled rubber, and theloss (recoverable upon relaxation) of this stiffness with strain. This phenomenonis called the Payne effect (55). Finally, and most importantly, carbon black has astrong affinity to general-purpose rubbers, as a function of attributes built intothe carbon black in its manufacture (“surface activity”) and the unsaturated or-ganic structures of the polymers, such as carbon–carbon double bonds, and styreneunits.

Much has been written on the nature of the carbon black/rubber interaction,which is still not fully understood (selected references include References 56–59).One thing is clear: carbon black has a strong affinity to general-purpose rubbers. Itis manifest in the simple experiment called “bound rubber” or “insoluble polymer”measurement. A sample of uncured rubber, which may or may not be devoid ofrubber chemical additives, is admixed with an amount of reinforcing carbon black.The composite is then immersed in a solvent in which the polymer is soluble.Whereas all, or all save any gelled portion, of the unfilled rubber dissolves in thesolvent, the same polymer, as a carbon black filled composition, produces only aportion of soluble polymer. For a typical recipe containing 30–50 parts by weight


of carbon black per hundred parts rubber, a typical value for the nondissolvedor bound polymer might be 20–40%. Empirically, the quantity of bound rubbercorrelates with abrasion resistance, or tread wear for a (subsequently cured) tiretread compound (60). Bound rubber cannot be measured on cured rubber, of course;all but a fraction of a percent will be bound in that case, regardless of the type oramount of filler. But the phenomenon of polymer/filler interaction, measured inthe uncured state, carries forward to the cured compound and product, in manytoughness and resilience related properties.

The search for alternative reinforcing fillers to carbon black has continuedfor decades, and has recently produced one material which offers in selected ap-plications, even greater performance: precipitated silica.

Silica. Silica (a mineral with a repeat unit of SiO2) is, in its crystallineform, as quartz and sand/sandstone, the most abundant compound on the earth’scrust. Synthetic silica can be produced by a fumed process via silicon tetrachloride,SiCl4, or aqueous process, via sodium silicate, itself made from natural silica.The product of the aqueous process is precipitated in an amorphous form, whichprovides two of the attributes of a reinforcing filler: small particle size and highdegree of self-affinity or networking. The self-affinity is due to hydrogen bondingbetween surface silanol groups (Si OH), which are present in large amount (5–7per nm2) on the surface. Fumed silica has much fewer surface silanol groups. Arepresentation of the silica surface is given in Figure 8.

In the early 1990s, a new generation of precipitated silicas was developed:the so-called highly dispersable silicas (HDS) (61–65). This class provides anothervital attribute of a reinforcing filler for rubber: easy dispersability at high loadingsto submicron sizes.

The remaining attribute, polymer/filler interaction, is inherently missingfrom silica but can be provided by the use of a specific class of rubber chemicals,the silica-to-rubber coupling agents. Silica-to-rubber coupling agents have beenknown since the 1960s, and had permitted limited use of silica filler, much of thisin nonblack mechanical goods and shoe soles. Use in tires was restricted to lowlevels in skim compounds for wire adhesion and treads for cut resistance. A goodreview of the state of the technology in the mid-1970s is given by Wagner (66).

With the advent of HDS, the ability to disperse silica to levels equal to thatof carbon black is now possible for the creation of highly silica-filled rubber com-pounds. Thus, there has been a revolution in growing use of silica in place of car-bon black in tires since the early 1990s, requiring growing use of specific couplingagents, whose number and chemistries have also expanded in this time period. Adescription of this class of rubber chemicals follows.

Fig. 8. Silica surface chemistry.


The predominant chemical class of silica-to-rubber coupling agents are thefunctionalized silane esters of general structure 5 (where typically R = Me, Et,n = 3; R′′ = propylene [ (CH2)3 ]; and X is a variable function). Many of thesewere developed in the mid-1970s (67).

The most commonly used silane coupling agents (qv) for rubber are shownin Table 12.

All Silane Coupling Agents (qv) work by a twofold bonding mechanism(68–70):

(1) The silane ester moiety hydrolyzes because of heat and moisture presenton the surface of the silica (RO Si → HO Si) and the then-formed silanolcondenses with a silanol group on the silica surface to produce a siloxanebond linking coupling agent to silica ( Si O Si ).

(2) The functional group (X above) on the other end of the coupling agent reactsdirectly either with the polymer chain or with the vulcanization system soas to provide a covalent bond between the coupling agent (already attachedto silica) and the polymer.

Step 1, reaction with silica, occurs in the mixing (with heat) of the silica,rubber, and coupling agent. The polymer is as yet uncured and not bonded tothe filler. This step is called hydrophobation: the hydrophilic silanol containingsurface is converted to a hydrophobic organic modified surface. The silica losesmuch of its self-affinity by this process, rendering it more compatible with rubber.The Payne effect is reduced and may be nearly eliminated if hydrophobation isthorough.

Step 2, reaction with polymer, occurs in the vulcanization step (with heat,after curatives have been added in a lower temperature stage addition). Figure 9shows the two-step coupling mechanism in the general case.

A subclass of silane rubber chemicals exists in which the “X” function ofstructure 5 is replaced by an inert alkyl group, creating a silica-reactive but non-coupling process aid, of the general formula 6 (where R′ = C1 C18 n-alkyl). Thesematerials can be very effective in reducing compound viscosity and the Payneeffect (71) but do not themselves provide cured compound reinforcement. A se-lected series of alkyl silanes is shown in Table 13. Alkyl silanes may be usedin conjunction with coupling agent silanes to balance processability and curedperformance.

The nature of function “X” is dependent on the nature of the cure system.Historically, the methacrylate function was used for peroxide-cured rubber andthe mercapto function was first used for sulfur-cured rubber (see a and b inTable 12). Definitive studies of the role of coupling bonds on physical properties

Table 12. Selected Organofunctional Silane Coupling Agents for Silica-Filled Rubber

Flash Supplier PolymerChemical name Formula CAS no. point, ◦C (trade name) application(s)

Chloro Polychloroprene3-Chloropropyltriethoxysilane Cl(CH2)3Si(OCH2CH3)3 [5089-70-3] N/A Degussa (Si

230)Amino Polyamide,

epoxy, acrylicγ -Aminopropyltriethoxysilane H2N(CH2)3Si(OCH2CH3)3 [919-30-2] 96 GE Silicones

(A-1100)Degussa (VP

Si 251)γ -Aminopropyltrimethoxysilane H2N(CH2)3Si(OCH3)3 [13822-56-5] 82 GE Silicones

(A-1110)N-β-(Aminoethyl)-γ -

aminopropylmethyldimethyoxysilaneH2N(CH2)2NH(CH2)3SiCH3(OCH3)3 [3069-29-2] >93 GE Silicones

(A-2120)Vinyl Polyolefin,

SBR, BR,NR (peroxidecure)

Vinyltriethoxysilane CH2=CHSi(OCH2CH3)3 [78-08-0] 44 GE Silicones(A-151)

Degussa (VPSi 225)

Vinyltrimethoxysilane CH2=CHSi(OCH3)3 [2768-02-7] 28 GE Silicones(A-171)

Epoxy Acrylic, nitrile,polyurethane,SBR

γ -Glycidoxypropyltrimethoxysilane CH2CH(O)CH2O(CH2)3Si(OCH3)3 [2530-83-8] 110 GE Silicones(A-187)

Methacryloxy Butyl,polyolefin,SBR, BR, NR(peroxide)

596

γ -Methacryloxypropyltrimethoxysilanea CH2=C(CH3)CO2(CH2)3Si(OCH3)3 [2530-85-0] 108 GE Silicones(A-174)

Degussa (VPSi 123)

Sulfur Nitrile, SBR,BR, NR(sulfur cure)

γ -Mercaptopropyltrimethoxysilaneb HS(CH3)3Si(OCH3)3 [4420-74-0] 88 GE Silicones(A-189)

Degussa (VPSi 163)

Bis-(triethoxysilylpropyl)tetrasulfidec [(CH3CH2O)3Si(CH2)3]2S4 [40372-72-3] 104 GE Silicones(A-1289)

Degussa (Si69)

Bis-(triethoxysilylpropyl)disulfided [(CH3CH2O)3Si(CH2)3]2S2 [56706-10-6] 75 GE Silicones(A-1589)

Degussa (Si266, Si 75)

3-Octanoylthio-1-propyltriethoxysilanee CH3(CH2)6(C=O)S(CH2)3Si(OCH2CH3)3 N/A N/A GE Silicones(NXT)

Thiocyanato SBR, BR, NR(sulfur cure)

3-Thiocyanatopropyltriethoxysilane N=C=S(CH2)3Si(OCH2CH3)3 [34708-08-2] N/A Degussa(Si 264)

Miscellaneous Polyurethane,phenolic,urea-formaldehyde

γ -Ureidopropyltrimethoxysilane H2N(C=O)NH(CH2)3Si(OCH3)3 [23843-64-3] 99 GE Silicones(A-1524)

γ -Isocyanatopropyltriethoxysilane O=C=N(CH2)3Si(OCH2CH3)3 [24801-88-5] 77 GE Silicones(A-1310)

aMAS.bMPTMS.cTESPT.dTESPD.eNXT.

597


Fig. 9. Silane coupling chemistry.

and tire performance of silica-filled tire rubber compounds was done by Wagnerusing these two coupling agents (72).

Several other silanes were originally used to adhere rubber to glass sub-strates as well as applied to various rubbers with mineral fillers such as clays.Some of these are given in Table 12 as well.

Despite the performance benefits of mercaptosilane, serious drawbacks lim-ited its use in tires: a strong and bad odor, plus a large reduction in scorch safety,resulting in inability to process the rubber. The mercapto group was simply tooreactive to the rubber, when combined with sulfur and curatives. Cross-linkingwould initiate in mixing or extrusion.

Table 13. Selected Alkyl Silane (Silane Ester) Hydrophobating Agents for Silica-FilledRubber

Flash Supplierpoint, (trade

Chemical name Formula CAS no. ◦C name)

Methyltriethoxysilane CH3Si(OCH2CH3)3 [2031-67-6] 29 GE Silicones(A-162)

Propyltriethoxysilane CH3(CH2)2Si(OCH2CH3)3 [2550-02-9] N/A Degussa (VPSi 203)

Octyltriethoxysilane CH3(CH2)7Si(OCH2CH3)3 [2943-75-1] 41 GE Silicones(A-137)

Degussa (VPSi 208)

Methyltrimethoxysilane CH3Si(OCH3)3 [1185-55-3] 12 GE Silicones(A-1630)

Hexadecyltriethoxysilane CH3(CH2)15Si(OCH2CH3)3 [16415-13-7] N/A Degussa (VPSi 216)


In the 1970s, the first step change improvement in coupling agents for sulfur-cured rubber, as used in tires, came with the development of the tetrasulfide silane,TESPT (see c in Table 12) (67,68). Now, the mercaptan function was protected inthe form of a polysulfide. To generate the active mercaptan, a sulfur–sulfur bondcleavage was needed. By analogy to the generation with scorch safety of mercapto-benzothiazole (MBT) by the cleavage of the disulfide MBTS, the tetrasulfide silaneimparted scorch safety yet retained polymer reactivity of the mercaptosilane.

TESPT remained the industry standard coupling agent for sulfur cured rub-ber into the 1990s. With the development of HDS came expanded applications forsilica-based compounds, particularly in high performance passenger tire treadswhich needed a combination of tread wear, low rolling resistance, and good wettraction (73). This combination of performance attributes can be simultaneouslyoptimized to unique levels with silica. Recipes with 50–100 phr silica were nowpermissible and desirable.

As silica technology expanded to meet market performance targets, certainprocessing deficiencies grew in importance, and remedies through materials de-velopment emerged and continue to do so presently.

First, it became apparent that highly silica-filled compounds, even usingHDS, required longer mixing times and processed worse than their carbon blackcounterparts (extrusion, tire building). It became clear that TESPT itself did nothave sufficient scorch safety for all operating conditions; the middle sulfur–sulfurbonds in the polysulfide bridge are relatively weak and can be broken, liberatingelemental sulfur to impart scorch. TESPT is a known sulfur donor curative, afterall. So, the market for TESPD expanded (see c and d in Table 12). The strongersulfur–sulfur bonds in TESPD provided extra processability in production whileretaining necessary polymer reactivity.

The newest sulfur silane coupling agent, NXT (see e in Table 12) (74,75) car-ries this approach even further. A substituted carbonyl group is used to protecta mercaptosilane during processing and reaction at the silanol end with the sil-ica (7), deprotection occurs during cure, catalyzed by curatives, to release activemercaptan function (8), which, through reactions with the vulcanization system,bonds to polymer (9), as shown below.

The rubber chemist has recognized and reacted to the growing trend to em-ploy silica as reinforcing filler for rubber with development of multiple silaneester coupling agents and alkyl silane process aids for improved processabilityand performance.


Nonstaining and Persistent Antidegradants

General-purpose rubbers are susceptible to degradation by atmospheric oxygen(O2) and ozone (O3).

Chemical antioxidants (qv) act to preserve rubber from degradation bymolecular oxygen. This degradation is manifest in hardening (cross-linking) forpolybutadiene and SBR and initial softening (chain scission) than hardeningfor polyisoprene rubber (natural or synthetic). All rubbers lose tensile strengthand cut and tear resistance due to oxidation as well. Antioxidants inhibit thefree-radical autoxidation cyclic chain reaction process via several mechanisticpathways. The primary antioxidants scavenge polymer carbon and hydroper-oxy radicals by donation of an active hydrogen. Secondary antioxidants decom-pose hydroperoxides. A review is available, with attendant references to thehistorical development of antidegradants and their mechanisms of action in rub-ber (76). Autoxidation and antioxidant mechanisms have also been reviewed(77,78).

The hydrogen-bonded intermediate complex of a hindered phenolic antioxi-dant with a stable aminoxyl radical (TEMPO), used as a model for a hydrocarbonoxidant, has been isolated and its structure determined, as confirmation of theradical scavenging mechanism (79).

The principal classes of primary antioxidants for rubber remain the pheno-lics (nonblack articles) and amines (black-filled articles). Among the amine typesare the substituted diphenylamines, para-phenylenediamines (PPD, (10), whereR,R′ = alkyl or aryl), and polymeric trimethyl-dihydroquinoline (TMQ). Repre-sentative phenolic antioxidants are given in Table 14, and selected amine an-tidegradants are given in Table 15. The majority of tire and mechanical goodsmade from general-purpose rubbers are protected by the phenolic and amine an-tioxidants.

Macroscopically, ozonation leads to cracking of the rubber surface and ulti-mate product failure. A polar mechanism was proposed by Criegee for ozonationof unsaturated hydrocarbon rubbers, with addition of ozone to the carbon–carbondouble bond followed by decomposition of the intermediate ozonide leading tocleavage of the polymer chain (80). Polymers without carbon bond unsaturationin the backbone do not crack in the presence of ozone. Zhang recently proposeda free-radical mechanism and attributed PPD antiozonant activity to its low ion-ization potential and ability to quench polymer radicals (81).

The PPD class of antidegradants provides by far the best antiozonant pro-tection (for a review of antiozonants, see Reference 82). PPDs inhibit ozone degra-dation of rubber by multiple mechanisms (76,83–85):

(1) Migration to the surface of the rubber article(2) Competitive reaction with and scavenging of ozone at the surface

Table 14. Representative Phenolic Antioxidants Used in Rubber

Chemical name Structure CAS no. Supplier (trade name)

a. Styrenated phenol [61788-44-1] Akcros Chemical (Akcrostab C53-P)Flexsys (Montaclere SPH)Great Lakes Chemical (Anox G2)Goodyear Chemical (Wingstay S)Harwick Standard (Antioxidant SP)PMS Specialties (Prodox 120)RT Vanderbilt (Wingstay S)Sovereign Chemical (AO 47)Sumitomo Chemical (Sumilizer S)Crompton Uniroyal (Naugard SP)

b. Alkylated hindered phenol Akrochem (Antioxidant 32, 33, 43)CP Hall (Stabiwhite Powder 49-454)Goodyear Chemical (Wingstay C, T)Great Lakes Chemical (Anox T)Inspec Fine Chemicals (Ionol K65, J65)RT Vanderbilt (Wingstay C, T)

c. Polybutylated bisphenol A [68610-51-5] BF Goodrich (Goodrite Superlite)RT Vanderbilt (Agerite Superlite)Sumitomo Chemical (Sumilizer NW)

601

Table 14. (Continued)

Chemical name Structure CAS no. Supplier (trade name)

d. 2,2′-Methylene-bis-(4-methyl-6-t-butylphenol)

[119-47-1] Akrochem (Antioxidant 235)Ashland Chemical (Ashland AO 46)Bayer Corp. (Vulkanox BKF)CP Hall (Cyanox 2246)Chemetall (Naftonox 2246)Cytec Industries (Cyanox 2246)Great Lakes Chemical (Lowinox 22M46)Harwick Standard (Stangard PC)PMC Specialties (CAO-5)RT Vanderbilt (Vanox MBPC)Raschig (Ralox 46)Sumitomo Chemical (Sumilizer MDP-S)

e. Butylated reactionproduct of p-cresol anddicyclopentadiene

[68610-51-5] Akrochem (Antioxidant 12)Ashland Chemical (Ashland AO CL)Goodyear Chemical (Wingstay L)Great Lakes Chemical (Lowinox CPL)PMC Specialties (Ralox LC LC)Raschig (Ralox LC)RT Vanderbilt (Wingstay L)Sovereign Chemical (AOL 600)

f. Mixed methylenic bridgedadducts of alkylatedphenol anddicyclopentadiene

Goodyear Chemical (Wingstay K)RT Vanderbilt (Wingstay K)

602

g. Tetrakis methylene (3,5-di-t-butyl-4-hydroxyhydro

[6683-19-8] Akrochem (Antioxidant 1010)Ashland Chemical (Ashland AO 610)Akcros Chemicals (Lankromark LE373)Ciba Specialty Chemicals (Irganox 1010)Dover Chemical (Dovernox 10)Great Lakes Chemical (Anox 20)Mayzo, Inc. (BNX 1010)PMC Specialties (Prodox 1010)Raschig (Ralox 630)Sumitomo Chemical (Sumilizer BP-101)

h. 1,3,5-Trimethyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxybenzyl)benzene

[1708-70-2] Albemarle Corp. (Ethanox 330)Ciba Specialty Chemicals (Irganox 1330)

603

Table 15. Representative Amine Antidegradants Used in Rubber

Chemical name(abbreviation) Structure CAS no. Supplier (trade name)

a. para-Styrenateddiphenylamine

[68442-68-2] Akrochem (AntioxidantPOSDA)

Goodyear Chemical(Wingstay 29)

RT Vanderbilt(Wingstay 29)

b. Octylateddiphenylamine(ODPA)

[68411-46-1] Akrochem (AntioxidantS)

BF Goodrich (GoodriteStalite S)

Bayer (VulkanoxOCD/SG)

Crompton Uniroyal(Octamine)

Flexsys (PermanoxODPA)

Harwick Standard(Stangard ODP)

RT Vanderbilt (Vanox12)

604

c. 2,2,4-Trimethyl-1,2-dihydroquinoline,polymerized(TMQ)

26780-96-1 Ashland Chemical(Ashland AO TQ-T)

Bayer (VulkanoxHS/LG)

BF Goodrich (GoodriteMA, Resin D)

Chemetall (NaftonoxTMQ)

Crompton Uniroyal(Naugard Q)

Flexsys (Flectol TMQ)Great Lakes Chemical

(Anox HB, HPG)PMC Specialties (Ralox

TMQ)RT Vanderbilt (Agerite

Resin D)Raschig (Ralox TMQ)Sovereign Chemical

(Pilnox TDQ)Sumitomo Chemical

(Antigene RD-G)Struktol (Struktol

TMQ)

d. Mixed diaryl-p-phenylenediamine(DPPD)

[68953-84-4] Akrochem (AntiozonantMPD-100)

Bayer (Vulkanox 3100)Goodyear Chemical

(Wingstay 100, 200)Crompton Uniroyal

(Novazone AS)RT Vanderbilt

(Wingstay 100)

605

Table 15. (Continued)

Chemical name(abbreviation) Structure CAS no. Supplier (trade name)

e. N,N′-Bis(1,4-dimethylpentyl)-p-phenylendiamine(77PD)

[3081-14-9] CP Hall (UOP 788)Crompton Uniroyal

(Flexzone 4L)Flexsys (Santoflex

77PD)UOP (UOP 788)

f. N-(1,3-Dimethylbutyl)-N′-phenyl-p-phenylenediamine(6-PPD)

[61931-82-6][793-24-8]

Akrochem (AntiozonantPD-2)

CP Hall (UOP 588)Chemetall (Naftonox

6PPD)Crompton Uniroyal

(Flexzone 7L, 7P)Duslo Sala (Dusantox

6PPD)Bayer (Vulkanox 4020)Flexsys (Santoflex

6PPD)RT Vanderbilt (Antozite

67P)Sovereign Chemical

(Dusantox 6PPD)Struktol (Struktol

6PPD)Sumitomo Chemical

(Antigene 6C)UOP (UOP 588)

606

g. Benzenamine,N-[4-(1,3-dimethyl(butyl)imino]-2,5-cyclohexadien-1-ylidene(6-QDI)

[52870-46-9] Flexsys (Q-Flex QDI)

h. 2,4,6-Tris(N-1,4-dimethyl-pentyl-p-phenylenediamino)-1,3,5-triazine(TAPDT)

Crompton Uniroyal(Durazone 37)607


(3) Formation, through its reaction products with ozone, of a protective film atthe rubber surface

(4) Increase in the critical stress of the rubber article. The critical stress is theapplied static stress at which ozone cracking appears on the surface of arubber article exposed to ozone.

N-(1,3-dimethylbutyl)-N′-phenyl-p-phenylenediamine; abbreviated 6-PPD,is a particularly versatile chemical offering a blend of antioxidant and antiozo-nant behavior combined with good rubber solubility and modest volatility, al-though other PPDs have applications as well. The diaryl PPDs have limited rub-ber solubility but lower volatility than 6-PPD. They provide excellent antioxidantprotection and decent persistence with diminished antiozonant protection.

All PPDs are highly staining and discoloring. This behavior is not necessarilya problem in carbon black filled articles, but restricts usage in colored goods—oreven tires with colored components.

Two long-standing deficiencies with the PPD antidegradants, lack of per-sistence and discoloration/staining, have been addressed, respectively, in recentadditions to the field: 6-QDI, a dehydrogenated PPD, and TAPDT, a trimeric, highMW form of PPD.

6-QDI (N-(1,3-dimethylbutyl)-N′-phenyl-p-quinonediimine, Table 15) offersimproved persistence because of its partial immobilization and binding to thepolymer matrix during mixing (86,87). As an efficient radical trap, 6-QDI acts asa chemical peptizer to reduce natural rubber chain length and viscosity in mix-ing by capping cleaved polymer chain end radicals. 6-QDI still provides chemicalantidegradant protection similarly to its hydrogenated parent, 6-PPD, and is con-sidered to be produced in situ from 6-PPD as part of the latter’s mechanism ofaction in the cured rubber compound, as shown in equation 1 (88,89). 6-QDI isalso reported to preserve sulfur network rank stability better than 6-PPD andimpart improved abrasion resistance to a natural rubber tread compound relativeto PPD and TMQ antidegradants (90).

(1)

TAPDT [2,4,6-tris-(N-1,4-dimethylpentyl-p-phenylenediamino)-1,3,5-triazine, Table 15 provides reduced discoloration as a result of its increasedmolecular size which imparts reduced mobility in the rubber matrix (91).Discoloration results from bloom of ozonation by-products of the antidegradantto the rubber surface. PPD containing compounds will stain adjacent compoundsin a composite structure because of migration of the PPD. An example is a blacksidewall protected by PPD staining the white sidewall of a tire. TAPDT does not


migrate like the monomeric PPDs. TAPDT gives excellent static ozone protection,flex resistance, as well as antioxidant behavior, but does not provide the samelevel of dynamic ozone protection (protection to product under stress, such as incyclic flexing) as the monomeric PPDs (91). Blends of TAPDT and 6-PPD havebeen shown to give excellent overall protection and durability (91).

A new partially graftable, aminic PPD of unspecified structure was recentlypresented as another means to nonstaining antidegradant protection for rubber(92).

In summary, the field of rubber chemicals is both mature and ever evolving.Recent advances and changes have been prompted by environmental concerns,the move to higher efficiency cures and processing, the pursuit of more durablerubber products, and the growth of silica as a reinforcing filler.

BIBLIOGRAPHY

“Rubber Chemicals” in EPST 1st ed., Vol. 12, pp. 256–280, by F. W. Shaver, TheB. F. Goodrich Co.; “Rubber Chemicals” in EPSE 2nd ed., Vol. 14, pp. 716–762, by J. A.Kuczkowski, The Goodyear Tire & Rubber Co.

1. P. Ghosh, S. Katare, P. Patkar, J. M. Caruthers, V. Venkatasubramanian, and K. A.Walker, Rubber Chem. Technol. 76, 592 (2003).

2. M. R. Krejsa and J. L. Koenig, Rubber Chem. Technol. 66, 376 (1993).3. M. Akiba and A. S. Hashim, Prog. Polym. Sci. 22, 475 (1997).4. M. A. Faith, Rubber World 208(5), 15 (1993).5. M. A. Faith, Rubber World 209(1), 22 (1993).6. M. A. Faith, Rubber World 209(3), 17 (1993).7. M. A. Faith, Rubber World 209(5), 18 (1994).8. M. A. Faith, Rubber World 210(1), 24 (1994).9. A. V. Chapman and M. Porter, in A. D. Roberts, ed., Natural Rubber Science and Tech-

nology, Oxford University Press, New York, 1988, pp. 512–620.10. J. O. Harris and C. D. Trivette Jr., in G. Alliger and I. J. Sjothum, eds., Vulcan-

ization of Elastomers, Robert E. Krieger Publishing Co., New York, 1978, pp. 174–178.

11. R. H Schuster, Hannover, Sowie F. Nabholz and M. Gmunder, Kautschuk Gummi Kun-ststoffe 43(2), 95 (1990).

12. W. Hoffmann, GAK 43, 562 (1990).13. W. Hofmann and J. Diederichsen, Int. Polym. Sci. Technol. 21(8), T/23 (1994).14. K. M. Davies, D. G. Lloyd, and A. Orband, Kautschuk Gummi Kunststoffe 42(2), 120

(1989).15. C. J. Hann, A. B. Sullivan, B. C. Host, and G. H. Kuhls Jr., Rubber Chem. Technol.

67(1), 76 (1994).16. R. M. D’Sidocky, The Goodyear Tire & Rubber Co., unpublished results, 2004.17. R. I. Keib, A. B. Sullivan, and C. D. Trivette, Rubber Chem. Technol. 43, 1188

(1970).18. D. B. Seeberger, Rubber Division, ACS, Detroit, Mich., Paper No. 70, Oct. 1989.19. D. W. Chasar, Rubber World 22(5), 26 (2002).20. B. H. To and A. D. Hoog, Rubber Plast. News Sept. 23, 14 (2002).21. A. A. Kuznetsov and O. A. Kulikova, Prog. Rubber Plast. Technol. 16, 255

(2000).


22. F. Cataldo, Die Angewandte Makromolekulare Chemie 249, 137 (1997).23. Y. B. Aziz and C. Hepburn, J. Rubber Res. Inst., Malaysia 27(2), 57 (1979).24. R. N. Datta, Rapra Rev. Rep. 12(12), 1 (2002).25. S. Wolff, Rubber Division, ACS, Las Vegas, Nev., Paper No. 30, May 1980.26. A. Johansson and T. Frueh, Rubber Division, ACS, Cleveland, Ohio, Paper No. 106,

Oct. 1997.27. Thio & Fine Chemicals, Vultac Rubber Chemicals. Available at http://www.

Atofinachemicals.com/Products.28. P. M. Lewis, in G. Scott, ed., Developments in Polymer Stabilization, Elsevier Applied

Science Publishers, New York, 1987, pp. 105–191.29. P. J. Nieuwenhuizen, J. Reedijk, M. Van Duin, and W. J. McGill, Rubber Chem. Technol.

70, 368 (1997).30. C. T. Loo, Polymer 15, 357 (1974).31. J. I. Cunneen and R. M. Russell, J. Rubber Res. Inst., Malaysia 22, 300 (1969).32. M. Mori, Rubber Division, ACS, San Francisco, Calif., Paper No. 4, Apr. 2003.33. C. H. Chen, J. L. Koenig, J. R. Shelton, and E. A Collins, Rubber Chem. Technol. 54,

734 (1981).34. E. J. Blackman and E. B. McCall, Rubber Chem. Technol. 43, 651 (1970).35. N. J. Morrison and M. Porter, Rubber Chem. Technol. 57(1), 63 (1984).36. L. M. Volchenok, A. D. Leiken, and E. A. Dzyura, Int. Polym. Sci. Technol. 21(7), T/44

(1994).37. U.S. Pat. 5,616, 279 (Apr. 1, 1997), R.M. D’Sidocky and N.A. Maly (to The Goodyear

Tire & Rubber Co.).38. R. N. Datta, A. G. Talma, and A. H. M. Schotman, Rubber Chem. Technol. 71, 1073

(1998).39. A. G. Talma, R. N. Datta, A. H. M. Schotman, and W. H. Helt, Rubber Division, ACS,

Cleveland, Ohio, Paper No. 64, Oct 1995.40. R. N. Datta and N. M. Huntink, Kautschuk Gummi Kunststoffe 55, 350

(2002).41. A. H. M. Schotman, P. J. C. Van Haeren, H. I. M. Weber, F. G. H. Van Wijk, J. W.

Hofstraat, A. G. Talma, A. Steenbergen, and R. N. Datta, Rubber Chem. Technol. 69,727 (1996).

42. R. N. Datta, Plast. Recycling Technol. 19(3), 143 (2003).43. P. M. Lewis, N. R. Technol. 17(4), 57 (1986).44. D. S. Campbell and A. V. Chapman, J. Natl. Rubber Res. 5, 246 (1990).45. G. Anthoine, E. R. Lynch, D. E. Mauer, and P. G. Moniotte, Rubber Division, ACS,

Indianaipolis, Ind., Paper No. 51, Spring 1984.46. D. G. Lloyd, Int. Polym. Sci. Technol. 15(9), T/1 (1988).47. F. Ignatz-Hoover, Rubber World 214(5), 19 (1996).48. J. Hahn, P. Palloch, E. Walter, and N. Thelen, Rubber Division, ACS, Providence, R.I.,

Paper No. 35, Apr. 2001.49. T. Kleiner, J. Neilsen, H. J. Weidenhaupt, W. Jeske, and H. Buding, Rubber Plast. News

14, 14 (Jan. 2002).50. Rubber Handbook, Struktol, Ohio, p. 58. Available at http://www.struktol.com/

rubber.asp.51. A. V. Chapman, Phosphorus, Sulfur, and Silicon, Vol. 59, Gordon and Breach Science

Publishers, United Kingdom, 1991, pp. 271–274.52. New Sartomer Products Bull. 3(14) (Nov. 2003).53. E. J. Blok, M. L. Kralevich, and J. E. Varner, Rubber Chem. Technol.

73(1), 114 (2000).54. D. T. Norman, in R. Ohm, ed., The Vanderbilt Rubber Handbook, 13th ed., 1990,

p. 397.


55. A. R. Payne, Rubber Plast. Age 963, (Aug. 1961).56. A. I. Medalia, Rubber Chem. Technol. 51, 437–523 (1978), and references cited

therein.57. E. Papirer, A. Voet, and P. H. Given, Rubber Chem. Technol. 42, 1200

(1969).58. J. LeBras and E. Papirer, Rubber Chem. Technol. 52, 43 (1979).59. J. A. Belmont and J. M. Funt, Kautsch Gummi Kunststoffe 44, 1135 (1991).60. G. R. Cotten and E. M. Dannenberg, Tire Sci. Technol. 2, 211 (1974).61. U.S. Pat. 5,089,554 (Feb. 18, 1992), F. Bomo, Y. Chevallier, P. Lamy, and J.-C. Morawski,

(to Rhone-Poulenc Industries).62. P. Cochet, P. Barruel, L. Barriquand, J. Grobert, E. Prat, and Y. Bomal, Rubber World

210(3), 20 (1994).63. P. Cochet, L. Barriquand, Y. Bomal, and S. Touzet, Rubber Division, ACS, Cleveland,

Ohio, Paper No. 74, Oct. 1995.64. S. Wolff, U. Gorl, M.-J. Wang, and W. Wolff, presentation at Tyretech ’93, Basel, Oct.

1993.65. U. Gorl, R. Bausch, H. Esch, and R. Kuhlman, in DIK German Rubber Conference 1994,

Stuttgart, June 1994.66. M. P. Wagner, Rubber Chem. Technol. 49, 703 (1976).67. F. Thurn and S. Wolff, Kautsch Gummi Kunststoffe 28, 733 (1975).68. S. Wolff, Rubber Chem. Technol. 55, 967 (1982).69. S. Wolff, Tire Sci. Technol. 15, 276 (1987).70. U. Gorl, A. Hunsche, A. Mueller, and H. G. Koban, Rubber Chem. Technol. 70, 608

(1997).71. L. Ladouce-Stelandre, Y. Bomal, P. Cochet, and J. Ramier, Tire Technol. Int. (2003).

Annual Review 36.72. M. P. Wagner, Rubber Chem. Technol. 47, 697 (1974).73. B. Schwaiger and A. Blume, Rubber World 222(1), 32 (Apr. 2000), and references cited

therein.74. P. Joshi, R. Cruse, R. Pickwell, K. Weller, M. Hofstetter, E. Pohl, M. Stout, and

F. Osterholtz, Rubber Plast. News 30 (Sept. 9, 2002).75. P. Joshi, R. Cruse, R. Pickwell, K. Weller, W. Sloan, M. Hofstetter, M. Stout, and

F. Osterholtz, Tire Technol. Int. (2003). Annual Review 44.76. J. Kuczkowski, Rubber Division, ACS, Cleveland, Ohio, Paper No. 83, Oct. 2003.77. J. R. Shelton, Rubber Chem. Technol. 45, 359 (1972).78. J. A. Howard, Rubber Chem. Technol. 47, 976 (1974).79. B. Ahrens, M. Davidson, V. Forsyth, M. Mahon, A. Johnson, S. Mason, R. Price, and P.

Raithby, J. Am. Chem. Soc. 123, 9164 (2001).80. R. Criegee, A. Kerckow, and H. Zinke, Berichte 88, 1878 (1955).81. J.-M. Zhang, Rubber Division, ACS, Cleveland, Ohio, Paper No. 99, Oct. 2003.82. R. Ohm, Rubber World 208(5), 18 (Aug. 1993).83. W. L. Cox, Rubber Chem. Technol. 32, 364 (1959).84. E. R. Erickson, R. A. Bernstein, E. L. Hill, and P. Kosy, Rubber Chem. Technol. 32, 1062

(1959).85. R. P. Lattimer, E. R. Hooser, R. W. Layer, and C. K. Rhee, Rubber Chem. Technol. 56,

431 (1983).86. F. Ignatz-Hoover, Rubber Division, ACS, Pittsburgh, Pa., Paper No. 141, Oct.

2002.87. F. Ignatz-Hoover and B To, Rubber Division, ACS, Cleveland, Ohio, Paper No. 98, Oct.

2003.88. M. E. Cain, I. R. Gelling, G. T. Knight, and P. M. Lewis, Rubber Ind. 216

(1975).


89. I. R. Gelling and G. T. Knight, Plast. Rubber Process. 83 (Sept. 1977).90. F. Ignatz-Hoover and B. To, Rubber Division, ACS, Cleveland, Ohio, Paper No. 119,

Oct. 2003.91. S. W. Hong and C.-Y. Lin, Rubber World 222(5), 36 (Aug. 2000).92. C. Fagouri and J. Fay, Rubber Division, ACS, Cleveland, Ohio, Paper No. 101, Oct.

2003.

MARTIN P. COHEN

RICHARD M. D’SIDOCKY

The Goodyear Tire & Rubber Company

rubber chemicals

Documents