test method for exterior tire rubbercompounds.docx

test method for exterior tire rubbercompounds.docx

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testmethodforexteriortirerubbercompounds

PolymerDegradationandStabilityVolume74,Issue2,2001,Pages353-362

Developmentofaservice-simulating,acceleratedagingtestmethodforexteriortirerubber

compoundsI.Cyclicaging

DengHuanga,BartJ.LaCounta,JoseM.Castro

aandFrederickIgnatz-Hooverb

aDepartmentofIndustrial,Welding,andSystemsEngineering,TheOhioStateUniversity,Columbus,OH43210,USA

bFlexsysAmerica,L.P.,260SpringsideDrive,POBox5444,Akron,OH44334-0444,USA

Received2May2001; revised2July2001; accepted4July2001. Availableonline25October2001.

Abstract

Agingoftire

rubber

compoundsinrealserviceisaffectedbymultiplefactors.Thisprocesscannotbeaccuratelyrepresentedbyconventionalsingle-factortests.Anovelcyclicagingtestisproposedasamultiplefactor,servicesimulating,acceleratedagingtestmethod.Itincludesfoursub-testsofoxygenbomb,dynamicozone,ultravioletlight,andwatersolutionattack.Theproposedcyclictestiscomparedagainstthermal,dynamicozone,anddynamicoutdooragingbycharacterizingmaterialpropertydegradationasafunctionofagingtime.Fourformulationsofexteriortire

rubber

compoundsareusedasexperimentalmaterials.Theexperimentalresultsshowthattheproposedcyclicagingtestisclosertodynamicoutdooragingthantheconventionaldynamicozoneandthermalagingtests.Thestaticmodulusincreaseswiththermalaging,anddecreaseswithdynamicozoneagingtime.Fordynamicoutdoorandcyclicaging,themodulusinitiallyincreases,butthenlaterdecreases,showingtracesofbothofthepreviouslymentionedresults.Itisalsofoundthatthesizeofthespecimenhasasignificantaffectonaging.Sucheffectcanbeutilizedtoacceleratetheartificialagingwithoutelevatingthetemperature.

AuthorKeywords:

Rubber

aging;Acceleratedaging;Dynamicozone;Thermaldegradation;Dynamicoutdooraging

1.Introduction

Duringstorageandinservice,rubbermaterialsaresubjecttophysicalandchemicalpropertychangesduetoaging.Forgoodlong-termperformance,sustainingthesepropertiesisnolessimportantthanobtainingtheinitialdesiredproperties[1].Inthetirerubberindustry,itisnecessarytoevaluatetheagingresistanceofdifferentcompounds.Unfortunately,thistaskisdifficulttoaccomplishbyroadtestingbecauseitusuallytakesyearstoobserveacompletedagingprocessinrealservice,moreover,theserviceconditionscanbetooarbitrarytobeusedforanyscientificstudy.

Usinganoutdooragingtestcanavoidthecomplicatedserviceconditions,butconsiderablevariationinconditionsfromsitetositeandseasonaldifferencesmakeitverydifficulttomeasurenaturalweatheringinabsoluteterms.Onlyresultsproducedatthesamesiteandduringthesametimeperiodarecomparativelymeaningful[2].Thus,insteadofusingarealserviceoranoutdooragingtest,itisnecessarytoestablishanartificial,service-simulating,acceleratedagingtestmethod.

Currently,therearemanystandardrubberagingtestmethods,butmostoftheminvolveonlyoneoratmosttwoinfluencingfactors.Thus,thesemethodsareusefulinevaluatingperformancewhenfailureisdeterminedbyasinglemodeofdegradation,orinpredictingrelativeperformanceforvariouscompoundswhereinasinglemodeoffailureisclearlydominant.Inreallife,however,exteriortirerubbercompounds,areexposedtoacombinationoffactorsincludingoxygen,ozone,heat,ultravioletlight,variousliquids,anddynamicflexing[3and4].Agingisacompositeresultofallthesefactors.Thus,itisdifficulttopredicttheagingresistanceofexteriortirecompoundsinrealservicebasedoncurrentlyavailablestandardtestmethods.

Theultimategoalofthisresearchprojectistodevelopamultiplefactor,artificialagingtestmethodtosimulatetheserviceagingofexteriortirerubbermaterials.Thistestmethodshouldachievetheindustrialrequirementsof

（1）simulatingcapability,

（2）repeatability,and（3）acceleration.Asthefirstparttowardsouroverallgoal,amultiple-factortest-cyclicagingisproposed.Itconsistsofseveralsingle-factorsub-tests,involvingoxygenbomb,dynamicozone,ultravioletlight,andaqueousexposure.Itisdesignedtosimulatethereal-serviceagingbyartificialmeans.Adynamicoutdooragingtest,andtwoconventionalagingtests—thermalanddynamicozonearecarriedoutandcomparedtotheproposedcyclicagingtest.Fourformulationsofexteriortirerubbercompoundsareusedasexperimentalmaterials.Thedegradationofmaterialpropertiesasafunctionofagingarestudiedinordertoevaluatetheagingmethods.

2.Experimental

2.1.Rubbermaterials

2.1.1.Compoundformulations

Fourformulationsoftirerubberwereusedasexperimentalmaterials,whichincludescrosscombinationsoftwobasecompounds:

NR（tread）andNR/BR（sidewall）;andtwoantidegradants:

6QDI[N-（1,3-dimethylbutyl）-N′-phenyl-p-quinonediimine]and6PPD[N-（1,3-dimethylbutyl）-N′-phenyl-p-phenylenediamine].DetailsoftheformulationsaregiveninTable 1.

Table1.Rubbercompoundformulations

Whileadifferentantidegradantwasusedforeachbasecompound,thesameantioxidant,TMQ（2,2,4-Trimethyl-1,2-dihydroquinoline）,wasusedthroughout.Whenusedincombinationwithanantidegradantthatprovidesresistanceagainstozoneandflexfatigue,suchas6QDIor6PPD,averyhighperformanceprotectoriscreated[5].TMQisa"chain-stopping"antioxidantthatinterruptsthepropagationcyclebydonatingahydrogenatomtoreactwithfreeradicals,andthereforeremovingitfromthemedium.

Bothantidegradantsarep-phenylenediaminebasedandprovideprotectionagainstbothoxidativeandozonedegradation.Whenusedattheconcentrationlevelsgiven,theantioxidantprotectionislow,whiletheozoneandflexfatigueresistanceisverygood.Thep-phenylenediaminetypeantiozonantspreventcrackingbyformingaprotectivelayeronthesurfaceoftherubber.Sinceozonereactsveryquicklywiththedoublebondsintherubbertoformanozonide,itdoesnotdiffuseintotherubber,leavingonlyasurfacephenomenon[5].Theozonidesformedaretheneasilydecomposedcausingscissionofthechains,andthuscracksunderstrain.Thesurfacelayerthattheantidegradantsformisduetothereactionoftheseozonideswiththeantidegradant.Theystopfurtherreactionoftheozoneandcarbondoublebondsandalsopreventtheformationofcracksbyincreasingthecriticalstrainenergyrequiredforcrackgrowthtooccur[4].

Theantidegradantusedincompounds1and3,6QDI,hasadualfunction.Duringcuring,themajorityofthe6QDIconvertsinto6PPD.Theremainingbindstothemainrubberchain.Thus,theboundportionisresistanttoleaching,butispreventedfrommigratingtothesurfaceandprotectingagainstozoneattack.Thisboundportioncanprovidealonglastingprotectionagainstoxidationwhiletherestcanfightozonedegradation.

2.1.2.Processing

Foreachbasecompound,amasterbatchincludingallingredientsbuttheantidegradants,accelerators,andvulcanizingagentswasfirstproducedusingaBanburyinternalmixer.Therestoftheadditiveswerethenincorporatedintothemasterbatchonarollmill.Therollmillingtemperatureswere70 °CforNRand60 °CforNR/BR.Conventionalprocessingprocedureswereappliedtoproduceahomogenousmix.

ThecurecharacteristicsweremeasuredwithaMovingDieRheometer（MDR）,whichrecordstheoscillatingtorqueasafunctionoftimeatthecuretemperature.Acuretemperatureof150Cwasappliedforallmaterials.Thetimesof90%oftorquebuild-upplusoneminutewereusedforactualcuretimes,specifically,16,13,18,and11.6minforcompounds1,2,3and4,respectively.

Thecuredrubbersheetsweredie-cutintospecimenswithdimensionsasshowninFig. 1.Thesespecimensareconventionallyusedforthedynamicozonetest.

Fig.1.Specimendimensionsinmm（samplethickness:

1.8mm）.

2.2.Agingtestmethods

2.2.1.Thermalaging

ThethermalagingtestusedwastheASTMD865testtubemethod.Cross-contaminationbetweencompoundsisavoidedbyplacingnomorethanthreesamplesofthesamecompoundintesttubes,whicharethenheatedinanaluminumblock.Convectiveaircirculationispermittedthroughinsertsinthetube

stoppers.

Theagingtemperaturewas70 °C.Atthistemperature,themechanismofagingisusuallyconsideredsimilartothatatambienttemperature[6].

Roadtemperaturesmaysometimesexceed100 °Cundercertainconditions,however,exposureisgenerallytimeweightedtolowertemperatures.Theimportantfactorinthiscaseisthattheagingmechanismisafunctionoftemperature.Whenoneobservestheeffectonmodulusofoxygenabsorptionatvarioustemperatures,thisbecomesclearer.ForNRrubbers,lowertemperaturesleadtoincreasesinmodulus,whileagingathighertemperaturesleadstoadecreaseinmodulus[3].Thustheneteffectatlowtemperaturesistoincreasecross-linkdensity,andathightemperaturesdecreasecross-linkdensity.Theoxidativechainscissionscausedbydecompositionofhydroperoxidesbecomemorecritical.Thusthechoiceof70 °Cissimplyastartingpointtomimicalongertimeaverageconditionwithoutbiastowardchangingtheoverallmechanismofdegradation.

2.2.2.Dynamicozoneaging

Thedynamicozonetestwasanotherconventionalagingtestmethod.Thesampleswereattachedtoadynamicstretchingdevice,andplacedinanozonechamber.Theozonesuppliedbythegeneratorflowsthroughthechambertokeepitataconstantconcentrationlevel.

Forthisstudy,thesampleswerestrainedfrom0to25%elongationatafrequencyof90cyclesperminute.Noadjustmentsweremad