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The Performance of MultifunctionalAcrylates as Anti-ReversionAgents in Tire Applications

By Steven K. Henning

Cray Valley USA, LLCExton, Pennsylvania

USA

Cray Valley USA, LLC • Oaklands Corporate Center • 468 Thomas Jones Way, Suite 100 • Exton, PA 19341Tel: 877-871-2729 • Web: www.crayvalley.com

5066 01/10

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ABSTRACTMultifunctional acrylates have found utility as effectiveanti-reversion agents in sulfur cured applications whereovercure or high temperature cure conditions exist. Inthis paper, a new multifunctional acrylate product isevaluated in both conventional and semi-efficientvulcanization systems. Reversion resistance and curedphysical properties of the resulting natural rubber/carbon black vulcanizates are compared to othercommercial products and efficient vulcanizationsystems.

Multifunctional acrylates (Cray Valley® SR534 andSR534D) provide excellent reversion resistance whileproviding a minimal impact on optimally curedproperties.

The object of the present study is to further explorethe benefits of antireversion agents. Torque rheometryis used to predict the effect of these agents onpreventing reversion in overcure or high-temperaturecure conditions. Physical properties of optimally curedand overcured vulcanizates are provided,demonstrating the relative efficacy of anti-reversionagents. Further, flexural fatigue testing at elevatedtemperatures is used to differentiate the compoundswith and without anti-reversion agents in simulated in-service conditions.

It will be shown that, with the addition of anti-reversionagents, conventional and semi-efficient vulcanizationsystems can provide the reversion resistance propertiesof efficient vulcanization while maintaining theadvantage in dynamic properties associated with thesecure types. Finally, the mechanism of reversionprevention using SR534 is explored throughfundamental swelling experiments and a model reactionsystem, with results differentiating it from an imide-based product.

INTRODUCTIONAs tires and other engineered rubber articles aredesigned to meet increasing process and servicecondition demands, inherent drawbacks of the sulfur-derived network must be addressed. The bondstrength of sulfur linkages determine the uniqueperformance attributes of the system, but also accountfor service limitations. A network derived frompolysulfidic linkages is preferred for componentssubject to dynamic strain where improved flexuralfatigue and tear properties are desired. Having lowerbond dissociation energies,1,2 polysulfidic linkagesbreak more readily under strain, but due to theirchemical nature also possess the ability to reform andalleviate stresses.3,4 The major disadvantage ofpolysulfidic networks is poor thermal stability.Thermal stress on the vulcanizate can lead to adegradation process known as reversion.

In sulfur cured vulcanizates, reversion can be definedby a reduction in physical properties associated with aloss of network integrity. The reversion process isthermally initiated and primarily associated withovercure or high temperature service conditions.Reversion involves reactions that lead to thedesulfuration of polysulfidic linkages and main-chainmodification which results in weaker networkstructures.5,6 The result is a decline in physicalproperties and a decrease in the performance of therubber article.7

Reversion can take place both during (in process) andafter (in service) the initial vulcanization event. A desirefor improved manufacturing efficiency may lead toincreased curing temperatures which can cause higherrates of reversion. A move toward high performanceand ultra high performance tire design places increaseddemands on tire components, including loads anddeflections which can lead to higher runningtemperatures. Many run-flat designs utilize inserts whichbear the load of the vehicle under zero-inflationconditions. Heat build-up in these scenarios can leadto in-service reversion and a loss in physical propertiesof the critical internal components. Durability of therubber article can be negatively affected.

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Anti-reversion agents are often used to mediate theeffects of the reversion process. The most effectiveapproach is to compensate for the destruction ofcrosslinks by adding chemical additives that react toform new crosslinks when reversion processes areinitiated. Ideally, the processes have similar rates suchthat crosslink density is maintained. Bothmultifunctional acrylate esters and imide-basedantireversion agents have been shown to effectivelycombat reversion induced in laboratory conditions.8,9

Previous work compared a proprietary multifunctionalacrylate ester product (Cray Valley SR534) to 1,3-bis(citraconimidomethyl)benzene (CIMB).10 Bothproducts effectively prevented reversion in a modelcarbon black filled cis-poly(isoprene) compound. Inaddition, SR534 minimized the effects on optimallycured properties (t90) and provided protection underconditions which induce reversion. Tensile, shear, anddynamic properties were compared. The dataprovided suggests that SR534 is capable of formingadditional, heat-stable crosslinks preferentially duringovercure conditions which would typically result inreversion. The object of the present study is to furtherexplore the benefits of antireversion agents. SR534and CIMB are added to conventional and semi-efficientvulcanization systems and compared to an efficient curesystem. Torque rheometry is used to predict the effectof these agents on preventing reversion in overcure orhigh-temperature cure conditions. Physical propertiesof optimally cured and overcured vulcanizates areprovided, demonstrating the relative efficacy of anti-reversion agents. Further, flexural fatigue testing atelevated temperatures is used to differentiate thecompounds with and without anti-reversion agents insimulated in-service conditions. It will be shown that,with the addition of anti-reversion agents, conventionaland semi-efficient vulcanization systems can providethe reversion resistance properties of efficientvulcanization while maintaining the advantage indynamic properties associated with these cure types.Finally, the mechanism of reversion prevention usingSR534 is explored through fundamental swellingexperiments and a model reaction system, with resultsdifferentiating it from the imide-based product.

EXPERIMENTAL

Rubber CompoundingA model compound formulation was designed for itspredilection for reversion.10 The formulation, given inTable I, is based on cis-poly(isoprene) (naturalrubber), as it has the highest allylic hydrogenconcentration of the diene-based elastomers. Thecurative loadings for conventional (CV), semi-efficient(S-EV), and efficient (EV) vulcanizations are provided.These are model formulations that have not beenoptimized for a specific application.

Table I. Model formulations.

Ingredient, phr CV S-EV EV

Non-Productive Natural Rubber (CV-60) 100.0 100.0 100.0Carbon Black (N 330) 50.0 50.0 50.0Zinc Oxide 5.0 5.0 5.0Stearic Acid 1.5 1.5 1.5

Productive Antioxidanta 1.0 1.0 1.0Anti-Reversion Agent 0, 2.0, 5.0 0, 2.0, 5.0 0, 2.0, 5.0Sulfur 2.5 1.2 0.2TBBSb 0.6 1.6 2.0TMTDc 2.0

a2,2,4-trimethyl-1,2-hydroquinolinebN-t-butylbenzothiazole-2-sulfenamidectetramethylthiuram disulfide

Using an internal mixer, a non-productive masterbatchwas prepared containing all ingredients except anti-reversion agents and curatives. The productive stagewas mixed using a two-roll mill. Milling times wereapproximately 7 minutes. Productive stock was agedovernight prior to testing.

Physical TestingThe determination of vulcanization behavior wasperformed on a Tech Pro MDPT moving die rheometer(MDR) according to ASTM D 5289 at both 160ºCand 180ºC. The individual calculated t90 values (timeto 90% of maximum torque, optimal cure) were usedfor subsequent test sample preparation (compressionmolded). In addition, reverted samples were preparedby curing to 60 minutes (overcure). Stress-strain andtear data was acquired on a Thwing-Albert MaterialsTester following ASTM D 412 and D 624-C. DeMattiaflex fatigue was determined according to ASTM D

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813 under conditions starting at room temperature andalso at an elevated constant temperature of 100ºC.

Swelling ExperimentsSamples were mixed in a Brabender Prep Mixer usingthe same formulation in Table I with the carbon blackomitted from the masterbatch. The unfilled compoundswere mixed for 4 minutes at 60 rpm and 60ºC. TheMDR was used to determine optimal cure times fromwhich vulcanizates were prepared. Swellingexperiments were conducted on cylindrical samplesof known weight and dimensions. Crosslink densitywas determined using the experimental methodologyoutlined by Flory and Rhener11 and modified withequivalent terms derived from theories of rubberelasticity. The c parameter used for the toluene/cis-poly(isoprene) system was 0.37.

RESULTS and DISCUSSION

Torque RheometryOriginating from weaker polysulfidic crosslinks,reversion can be minimized using efficient vulcanizationsystems which promote the formation of stable mono-and disulfidic linkages. However, this strategy has adisadvantage in that tear properties and dynamicperformance (flexural fatigue) can be adverselyaffected. Strong crosslinks possess high dissociationenergies and resist rupture to a limiting point, then failcatastrophically. Having lower dissociation energies,polysulfidic bonds break more readily under strain, butdue to their chemical nature also possess the ability toreform and alleviate stresses.1,2 The labile nature ofthe polysulfidic crosslinks contribute to both improveddynamic properties,3 but also causes reversion.

The rheometer cure profiles for the control formulationsare provided in Figure 1 (no anti-reversion agentadded) measured at 160ºC. It is clear that as the sulfur-to-accelerator ratio decreases reversion was reduced.In the EV case, sulfur donors are utilized and noreversion was evident.

Figure 1. Cure profiles of conventional (CV), semi-efficient (SE-V) and efficient (EV) vulcanizations;160ºC.

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The performance of the anti-reversion agents in thereversion-prone CV and S-EV formulations is sum-marized in Figures 2 and 3, respectively, again at

160ºC. SR534 and CIMB are added at 2 phr in bothcompounds, with the EV compound (no reversionagent) included for comparison.

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Figure 2. Activity of anti-reversion agents in CV formulation; 2 phr, 160ºC.

CV Formulation

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Both SR534 and CIMB effectively reduce reversionin the CV formulation at this loading. After an apparentinduction period, CIMB tends to produce a marchingmodulus while SR534 plateaus to a constant torque

value. In the S-EV formulation, both anti-reversionagents closely follow the rheometer profile of the EVcure and more effectively compensate for desulfuration.

Figure 3. Activity of anti-reversion agents in S-EV formulation; 2 phr, 160ºC.

S-EV Formulation

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These agents were also evaluated in the S-EVformulation at an elevated cure temperature of 180ºCin an attempt to simulate accelerated high temperatureservice conditions or model the results of a hightemperature cure cycle. Figure 4 shows that whileboth agents reduced reversion, SR534 more closely

matches the eventual torque value (60 minutes) of theEV system, while at this loading CIMB suffers frommarching modulus. Again, an induction period is seenbefore apparent crosslink density is increasedsubsequent to the initial maximum torque value (MH).It should be noted that loading of the anti-reversionagents has not been optimized.

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Figure 4. Activity of anti-reversion agents in S-EV formulation; 2 phr, 180ºC.

S-EV Formulation

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Physical PropertiesAnti-reversion agents are often used in applicationswhere cured rubber components are subjected toextreme in-service conditions. Elevated temperatures,whether generated internally by hysteretic heating orby high ambient conditions, accompanied by largedeformations may induce reversion. Testing thephysical properties of vulcanizates cured both to thet90 of the compound and after a substantial overcure(60 minutes) may provide insight to the utility of anti-reversion additives.

Tensile and tear properties. The physical propertiesof the model S-EV compounds employing 2 phr ofanti-reversion agents were compared to the controlS-EV compound after being cured to both t90(approximately 4 minutes in each case) and 60 minutesof cure time (160ºC). The properties of the EVcompound have also been included for reference.Table II provides the data.

Table II. Physical properties of vulcanizates cured to optimal times and overcured to 60 minutes(reverted).

Anti-reversion Agent (2 phr) none SR534 CIMB none none SR534 CIMB none Cure System S-EV S-EV S-EV EV S-EV S-EV S-EV EV Cure Time (minutes) 4 4 4 4 60 60 60 60 Tensile Strength (MPa) 23.92 21.79 22.90 17.90 19.37 20.77 21.23 12.79 Elongation (%) 550 523 552 481 505 510 475 368 100% Modulus (MPa) 2.59 2.60 2.54 2.36 2.28 2.80 3.03 2.44 300% Modulus (MPa) 11.39 11.18 10.74 9.10 9.71 10.67 11.84 13.33 Tear Strength (Die C) (kN/m) 109.6 83.0 79.6 50.8 42.4 61.3 58.6 43.4

original reverted

The S-EV control displayed the largest net change inphysical properties. The addition of anti-reversionagents compensated for this loss and more effectivelymaintained properties. Note that the original tearstrength of the S-EV compounds was significantlybetter than the EV compound. The anti-reversion

compounds, when subject to overcure and an overallreduction in tear strength, maintained values at leastequivalent to the original EV compound tear. The EVcompound produced the least change in tear withovercure.

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Figure 5 presents a comparison of the normalizeddeviation in properties and provides a visual measureof the overall change in physicals with reversion.Equation 1 was used to calculate the deviation inphysical properties as a function of reversion.

Deviation = [property(60 min.) – property(t90)]

Equation 1. Deviation calculation.The results for each physical property tested werenormalized to the deviation in the control (set at -100,as each property of the control was reduced with

overcure). Any departure from zero net change wouldbe considered detrimental upon compound ageing, butit can be seen that the compound containing SR534demonstrates the least amount of deviation fromoptimally cured physicals after the overcure event. Theperformance of SR534 is partly driven by maintainingcrosslink density at this loading, rather than a loss incrosslink density (control) or marching modulus(CIMB) (Figure 3). Similar improvements inperformance when using multifunctional acrylates wasreported by other investigators, including dynamicshear measurements.8

Figure 5. Normalized comparison of the deviation in physical properties as a function of overcure(reversion); S-EV formulation, 2 phr anti-reversion agent.

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Flexural fatigue properties. DeMattia flexural fatiguetesting was performed under both conditions startingat room temperature and at an elevated constanttemperature of 100ºC. The latter conditions wereinvestigated as an attempt to test the efficacy of anti-reversion agents under severe in-service conditions.The most notable difference is that the DeMattia testingwas performed under aerobic conditions which maynot be the case for in-service reversion of internal tirecomponents, for example. However, the testing doesoffer a direct comparison between the agents and thecontrol formulations.

The benefits of polysulfidic crosslinks in the vulcanizateare illustrated in Figure 6 which compares the controls

(no anti-reversion agent) of the various vulcanizationsystems and provides results when 2 phr anti-reversionagents are included. The 100% modulus values weresimilar (approximately 2.0 MPa). As reported in earlierstudies,14 improvements in flexural fatigue propertiescan be correlated to the polysulfidic crosslink contentof the vulcanized network. The flex fatigue resistanceof the controls improves EV < S-EV < CV. Theaddition of anti-reversion agents in the formulation doesnot reduce the flex fatigue. In the CV formulation,SR534 improves the crack growth resistance. Theability of anti-reversion agents to improve upon heat-build up and durability under high temperature testingconditions when tested on a compression flexometer(Goodrich) has previously been demonstrated.15

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Figure 6. DeMattia flex fatigue results performed under ambient (room temperature) conditions.

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Equilibrium swelling experiments. Equilibriumswelling experiments are utilized to determine thecrosslink density (Vx) of vulcanizates. An unfilled (nocarbon black) version of the CV model formulationwas used to produced compounds including eitherSR534 or CIMB additives (5 phr). A controlcompound was also included. Samples cured to botht90 (4 minutes) and 60 minutes were prepared.Crosslink densities were then calculated for eachvulcanizate based on the swelling experiment. The

results are provided in Figure 7. The control losesconsiderable network integrity with reversion. SR534provides crosslink density values most similar to thecontrol at optimal cure, and maintains crosslink densitywhen subjected to overcure conditions. CIMBappeared to partially react during vulcanization andincreased crosslink density prematurely. CIMB alsolost density as a function of mold residence time.Swelling experiments may most clearly characterizethe potential advantage of incorporating SR534 into areversion-prone formulation.

Figure 7. Crosslink density (Vx) of unfilled vulcanizates prepared at optimal cure times (t90) and 60minutes.

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These results, in conjunction with the rheometer, tear,and flexural fatigue data in the previous sections,demonstrate that the inherent compromise betweenimproved dynamic physical properties and reversionin CV and S-EV cure systems may be correctedthrough the use of anti-reversion agents. This is possiblewith minimal reformulating due to the documentedcrosslink-compensating activity of SR534.

Mechanistic StudyThe anti-reversion agents compared in this study arebased on different reactive groups. It is thereforepossible that the reaction mechanisms through whicheach form new crosslinks may also be different.Crosslinking through Diels-Alder reactions involvingthe methyl-substituted imide groups of CIMB has beendocumented.9 The mechanism is initiated by 1,4-elimination reactions during desulfuration whichproduces conjugated unsaturation in the polymer chain.Crosslinks are formed through Diels-Alder reactions

between the conjugation and imide structures, resultingin load-bearing crosslinks formed through thedifunctional CIMB. However, the ability of CIMB andSR534 to participate in other reactions, including “ene”and radical addition has also been postulated.8

A basic study was devised in an attempt to elucidatethe relative activity of SR534 and CIMB in the modelcompound system. To the non-productive masterbatchof the formulation provided in Table II, 5 phr SR534or CIMB was added; no curatives (sulfur, accelerators)were included. The MDR was employed to measureany evolution of torque as a function of anti-reversionagent loading and temperature. Compounds weretested between 100ºC and 200ºC for 60 minutes. Theincrease in torque is taken as a measure of crosslinkdensity. The control (no anti-reversion agents)produced a torque rise of 0.21 points when tested at160ºC. Figures 8 and 9 provide cure as a function oftemperature for CIMB and SR534, respectively.

Figure 8. MDR torque profiles for CIMB as a function of testing temperature; no curatives, 5 phrCIMB.

CIMB, 5 phr

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As expected, CIMB demonstrated little evidence ofreaction, consistent with the previously reportedmechanism. The absence of initial polysulfidic

crosslinks precludes the formation of conjugatedunsaturation, rendering the Diels-Alder reactionpathway unavailable.

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Figure 9. MDR torque profiles for SR534 as a function of testing temperature; no curatives, 5 phrSR534.

SR534, 5 phr

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Only SR534 built appreciable torque at elevatedtemperatures, and showed some baseline activity attemperatures below 160ºC. This data does noteliminate a Diels-Alder reaction pathway but suggeststhat another mechanism may be available to SR534.As both compounds are susceptible to radical additionreactions, the differentiation in activity would suggestan alternate pathway. The ene reaction, requiring onlya reactive enophile (acrylate ester) and allylic hydrogen,could be a possibility. It is possible that the sterichindrance involving the methyl substituent of the imideeffectively excludes an ene reaction for CIMB at lowertemperatures. The ene reaction has also beendocumented having a higher activation energy thanDiels-Alder reactions.1 So it would be conceivable thatSR534, capable of reaction through both mechanisms,can offer crosslink-compensating activity at lowertemperatures or under conditions where conjugatedunsaturation is not available. While the results of thisstudy are not definitive, they do suggest a difference inthe potential activity of the two anti-reversion agentsas a direct result of their structure. It is possible, througha potential ene reaction pathway, that SR534 may offerexpanded application utility.

SUMMARY and CONCLUSIONSSulfur cure systems can be differentiated by the relativeamounts of polysulfidic crosslinks they form. Networksof high polysulfidic linkage concentration improve tearand dynamic properties and extend flexural fatigue life.Heat sensitivity, which is manifested as networkreversion, is an issue when employing CV or S-EVsystems. The EV system eliminates reversion in thecured compound, but displays poorer physicalproperties.

Antireversion agents which compensate for the loss ofpolysulfidic crosslink density by forming new, stablecrosslinks can better maintain physical properties ofcompounds subject to overcure or high temperaturein-service conditions. With the inclusion of SR534 in aCV or S-EV formulation, initial tensile and dynamicproperties are similar for compounds cured underoptimal conditions and largely maintained as a functionof reversion. Tear strength and flexural fatigueperformance are improved versus EV curedcompounds after subject to high temperature orovercure conditions. SR534 therefore provides afavorable alternative to using EV cure systems as ananti-reversion strategy.

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REFERENCES1 L. H. Palys and P. A. Callais, Rubber World 229(3), 35(2003).2 P. R. Dluzneski, Rubber Chem. Technol. 74, 451 (2001).3 A. G. Thomas, J. Polym. Sci. 31, 467 (1958).4 L. Bateman, ed., “The Chemistry and Physics of Rubber-Like Substances,” MacLaren, London, 1963.5 M. Mori, Rubber Chem. Technol. 76, 1259 (2003).6 C. H. Chen, J. L. Koenig, J. R. Shelton, and E. A Collins,Rubber Chem. Technol. 54, 734 (1981).7 U. Shankar, Rubber Chem. Technol. 25, 241 (1952).8 E. J. Blok, M. L. Kralevich and J. E. Varner, Rubber Chem.Technol. 73, 114 (2000).

9 A. H. M. Schotman, P. J. C. van Haeren, A. J. 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).10 S. Henning and S. Shapot, Paper 13, Fall TechnicalMeeting, Rubber Division, ACS, Pittsburgh, PA,November 1-3, 2005.11 P. J. Flory and J. J. Rhener, J. Chem. Phys. 11, 521 (1943).12 W. Cooper, J. Polym. Sci. 28, 195 (1958).13 L. Bateman, ed., “The Chemistry and Physics of Rubber-Like Substances,” MacLaren, London, 1963.14 W. L. Cox and C. R. Parks, Rubber Chem. Technol. 39,785 (1966).15 R. N. Datta and M. S. Ivany, Rubber World 212(5), 24(1995).16 B. B. Snider, Acc. Chem. Res. 13, 426 (1980).

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