Filler–elastomer interactio

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JournalofColloidandInterfaceScience267(2003)

86–91

http://wendang.chazidian.com/locate/jcis

Filler–elastomerinteractions:in?uenceofsilanecouplingagentoncrosslinkdensityandthermalstabilityofsilica/rubbercomposites

Soo-JinPark?andKi-SookCho

AdvancedMaterialsDivision,KoreaResearchInstituteofChemicalTechnology,P.O.Box107,Yusong,Taejon305-600,SouthKorea

Received29October2002;accepted31January2003

Abstract

Inthiswork,thecrosslinkdensityandthermalstabilityofthesilica/rubbercompositestreatedbysilanecouplingagents,i.e.,γ-aminopropyltriethoxysilane(APS),γ-chloropropyltrimethoxysilane(CPS),andγ-methacryloxypropyltrimethoxysilane(MPS),wereinvestigated.Thechemicalstructuresofmodi?edsilicaswerestudiedintermofsolid-state29SiNMRspectroscopy.Thecrosslinkdensityofthecompositeswasdeterminedbyswellingmeasurement.Thedevelopmentoforganicfunctionalgroupsonsilicasurfacestreatedbycouplingagentsledtoanincreaseinthecrosslinkdensityofthecomposites,resultinginincreasing?nalthermalstabilityofthecomposites.ThecompositestreatedbyMPSshowedthesuperiorcrosslinkdensityandthermalstabilityinthesesystems.Theresultscouldbeexplainedbythefactthattheorganicfunctionalgroupsofsilicasurfacesbysilanesurfacetreatmentsledtoanincreaseoftheadhesionatinterfacesbetweensilicasandtherubbermatrix.?2003ElsevierInc.Allrightsreserved.

Keywords:Silica;Silanecouplingagent;Crosslinkdensity;Thermalstability

1.Introduction

Commercialapplicationsofelastomersoftenrequiretheuseofparticulate?llerstoobtainthedesiredreinforcement.Intherubberindustry,besidescarbonblacks,silicasaretheotherreinforcing?llerusedtoimpartspeci?cpropertiestorubbercompounds[1].Itiswellknownthatcarbon-black-?lledrubbercompositeshavemultiphasesystemsdependingonthemobilityofrubbermolecules,whichin?uencethereinforcementofthecomposites.Extensiveworkhasalsobeencarriedoutonstructuraldevelopmentinsilica/rubbercomposites[2,3].

Thesurfacefunctionalenvironmentofsilicaparticlesisquitedifferentfromthatofcarbonblacksduetotheexis-tenceofsilanolgroupsintheparticles.Thus,theprimarydiscussiononthestructuraldevelopmentinthesilica/rubbersystemsisfocusedontheinteractionsbetweensilicaparti-clesandrubbermolecules[4,5].WolffandWang[6]studiedtheeffectsofsurfaceenergiesof?llersonrubberreinforce-mentandreportedthatthesurfaceenergeticsofsilicasarecharacterizedbyalowLondondispersivecomponentanda

*Correspondingauthor.

E-mailaddress:psjin@krict.re.kr(S.-J.Park).

0021-9797/$–seefrontmatter?2003ElsevierInc.Allrightsreserved.doi:10.1016/S0021-9797(03)00132-2

highpolarcomponent.Thehighpolarcomponentleadstostronginteractionsamongsilicaparticles;ontheotherhand,thelowLondondispersivecomponentcausesweak?ller–rubberinteractions,leadingtothelowmoduliofthevulcan-izateatahighelongation.Therefore,surfacetreatmentsareneededtoimprovethereinforcementofthecompositesbyincreasingtheLondondispersivecomponentofsurfaceen-ergetics[7–9].

Variousmethodsusedtomodifythesurfacepropertiesofthesilicasarelargelyintroducedintermsofthermal,chemical,electrochemical,andcouplingagenttreatments.Amongthem,silanecouplingagentshavebeenusedintherubberindustrytoimprovetheperformanceofsilicasandothermineral?llersinrubbercompounds.Asilanecouplingagentcontainsfunctionalgroupsthatcanreactwiththerubberandthesilicas.Inthisway,therubber–silicaadhesionisincreasedandconsequentlythereinforcingeffectofthesilicasisenhanced[10,11].

Ingeneral,crosslinkdensityofrubbercompositesisoneoftheimportantpropertiesofthermosetsandgenerallyrestrictsthedegreeofswellinginpolymer.Ahighdegreeofcrosslinkdensityindicatesthattherubberisnotsuitableforuseinthatenvironment.Socorrelationsofmechanical

S.-J.Park,K.-S.Cho/JournalofColloidandInterfaceScience267(2003)86–9187

behaviorandthermalstabilitywithcrosslinkdensityhavebeeninvestigated[12–15].

Inthelightofthesestudies,weinvestigatethechangesincrosslinkdensityofsilica/rubbercompositesfromdifferentsilanecouplingagenttreatments.Mechanicalinterfacialpropertiesandthermalstabilityofthecompositesarealsostudiedasfunctionsofcrosslinkdensity.

2.Experimental

2.1.Materialsandsamplepreparation

Virginalsilicas,denotedasVS(productname:VN3),weresuppliedbyDegussaCo.ThesamplesdenotedbyAPS,CPS,andMPSwerepreparedinVStreatedwithsilanecouplingagents,respectively,γ-aminopropyltriethoxysi-lane(APS),γ-chloropropyltrimethoxysilane(CPS),andγ-methacryloxypropyltrimethoxysilane(MPS).Silanecou-plingagentsweresuppliedbyShinetsuCo.

Forthepresentinvestigation,allofthesilanecouplingagentsstudiedwerepreparedunderconstantconditionsinordertotreatsilicasurfaces.Inthesesolutions,acosolventofmethanol(95wt%intotalsolvent)anddistilledwater(5wt%intotalsolvent)wasused,andthesilaneconcen-trationwas?xedat0.4wt%[3].Afterthesilanecouplingagentswerehydrolyzedfor1hwithaceticacidsolution,vir-ginalsilicasweredippedinthehydrolyzedsilanesolutionfor1handdriedat120?Cfor3h.Rubber(productnameSBR1500S;styrenecontent23.5%)obtainedfromKoreaKumhoPetrochemicalCo.wasusedinthisstudy.Disper-siveagent(productnameEF44;compositionblendoffattyacidderivatives;zinecontent8.5%;density1070g/cm3),wassuppliedbyStruktolCo.Thecompoundingformula-tionswerelistedinTable1.Forthemeasurementofmechan-icalpropertiesof?lledvulcanizedsamples,thecompoundswerecuredat1.5MPaand160?Cfor60min.2.2.Solid-stateNMRstudiesofsilicas

Aftersilanesurfacetreatmentsonsilicasamples,thesil-icasurfaceswerecon?rmedbysolid-state29SiNMRspec-troscopy(BrukerDSX-300solidFT-NMRspectrometer).

Table1

CompoundingformulationsIngredients

Loading[phr]

Rubber(styrenebutadienerubber)100Silica(VN3)40Zincoxide5Stearicacid

2Dispersiveagent(EF44)

3Accelerator(N-oxydiethylene-2-benzothiazolesulfenamide)1Sulfur

2

2.3.Swellingmeasurementsofsilica/rubbercompositesThedegreeofswellingwasmeasuredaccordingtoASTMD366-82andcalculatedusingtherelation[13,14]Q(%)=

m?m0

m0

×100,(1)

wherem0andmwerethemassesofthesamplebeforeandafterswelling5(measuredusinganelectricbalanceofsensitivity10?g),respectively.Thesolventusedinthisworkwastoluene(molarvolume107cm3/mol;cohesiveenergydensity37.2(J/cm3)0.5).

2.4.Mechanicalinterfacialpropertiesofsilica/rubbercomposites

Thetearingenergy(GIIIC),whichwasoneofthecriticalstrainenergyreleaserates(GC),wascharacterizedbytrouserbeamtestsforthemechanicalinterfacialbehaviorofrubbercompounds.Rectangularspecimensabout70mmlong,50mmwide,and2mmthickwerecutfromasheetthatwasmanufacturedbyatwo-rollmilltechnique.Alltestswereconductedatacrossheadspeedof2mm/min.

Thetensilestrengthofthesilica/rubbercompositeswasmeasuredaccordingtoASTMD412usingaUTM(Univer-salTestingMachine,Instron1125).Alltestswereconductedatacrossheaddisplacementrateof500mm/min.2.5.Thermalstabilityofsilica/rubbercompositesToinvestigatethethermalstabilityofthesilica/rubbercomposites,thermogravimetricanalyseswereperformedinnitrogenusingaTGA951DuPontthermalanalyzerataheatingrateof10?C/minfromroomtemperatureto800?C.

3.Resultsanddiscussion

3.1.Solid-stateNMRstudiesofsilicasurfaces

Figure1showsthe29Si-NMRspectroscopyofsilicasmodi?edbysilanecouplingagents.VSshowsthreepeaks,assignedtothreepossibletypesofsiliconenvironments,toallowtheunambiguousassignmentofthemeasuredres-onancesat?90(a),?100(b),and?110ppm(c)to(HO)2Si(OSi≡)2,(HO)Si(OSi≡)3,andSi(OSi≡)onthesil-icasurfaces,respectively[16].Aftertreatmentwithsilanecouplingagents,separate29Sisignalsfromboththesur-faceandtheattachedsilaneofAPS,CPS,andMPSareobservedtomonitorhydrolysisreactionscausedbyaco-solventofmethanol(95wt%intotalsolvent)anddistilledwater(5wt%intotalsolvent).Thesearesuf?cienttodistin-guishthetwosignalsandshowmainpeaksat?49(d)and?57ppm(e)duetoSi(OH)2RandSi(OSi)(OH)Ronthesurfaces[17],respectively.Also,silanesurfacetreatmentsincreasetheintensityofSi(OSi≡)groupsanddecreasethe

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86–91

Fig.1.Solid-state29SiNMRspectroscopyofsilicasmodi?edbysilanecouplingagents.

intensityof(HO)2Si(OSi≡)2and(HO)Si(OSi≡)3groups

comparedtoVS.MPS-treatedsilicashavethemaximumin-

tensityofSi(OSi≡),Si(OH)2R,andSi(OSi)(OH)Rgroups.

Theresultsindicatethatsilanesurfacetreatmentsleadtoa

decreaseofthehydroxygroupsonsilicasurfacesthrough

siloxaneorhydrogenbondingattheinterfacesbetweensili-

casandsilanecouplingagents.

3.2.Crosslinkdensityofsilica/rubbercomposites

Thedegreesofswellingofthesilica/rubbercomposites

arelistedinTable2.Theswellingbehaviorofthecomposites

oftensilanesurfacetreatmentissigni?cantlydecreasedcom-

paredtothatofuntreatedones.Figure2showstheweight

swelling(g)forthefourdifferentcompositesintoluenewith

dippingtime.Theswellingcurvesofmodi?edsilica/rubber

compositesaresimilartothoseofvirginalsilica/rubbercom-

posites.Theweightswellingofthecompositesisincreased

Table2

Degreeofswellingofthesilica/rubbercomposites

VS

Q(%)676APS646CPS623

MPS492Fig.2.Weightofswellingasafunctionofsquarerootoftimeforsilica/rubbercomposites.rapidlyuntildippingtime13h;after13hitreachesanequilibriumweight.Buttheweightswellingatequilibriumofthecompositesbysilanesurfacetreatmentisdecreased

comparedtothatofVS-?lledrubbercomposites.There-

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89

Table3

Crosslinkdensityofthesilica/rubbercomposites

VS

APSCPSMPSVe×1029(m?3)

0.413

0.464

0.497

0.762

sultscouldbeexplainedbysilanecouplingagentsformingmorecompactcrosslinkingstructuresinsilane-treatedsil-ica/rubbercompositesthaninuntreatedones.

Thecrosslinkdensity,Veperunitvolumeinaperfectnetwork,isgivenbytheequation[13,14]Ve=

ρpNA

M,C

(2)

whereρpisthepolymerdensity,NAisAvogadro’snumber,andMCistheaveragemolecularweightofthepolymerbetweencrosslinks.

EquilibriumswellingiswidelyusedtodetermineMC.AccordingtothetheoryofFlory,foraperfectnetwork[13,14],

(φp1/3

?φ

pMC=?V1ρp2)

[ln(1?φp)+φp+χ1φ2(3)

p

],whereMCistheaveragemolecularweightofthepolymerbetweencrosslinks,V1isthemolarvolumeofthesolvent,ρpthepolymerdensity,φpthevolumefractionofpolymerintheswollengel,andχ1theFlory–Hugginsinteractionparameterbetweensolventandpolymer.

UsingEq.(2),thecrosslinkdensity,Ve,iscalculatedforthesilica/rubbercomposites,asshowninTable3.Thecrosslinkdensityofthecompositesaftersilanesurfacetreatmentsisincreasedcomparedtothatofuntreatedsil-ica/rubbercomposites.Ingeneral,whensilanecouplingagentsareintroducedontosilicasurfacesinthecompos-ites,twointerfacesexistbetweenthesilicasandtherub-ber:theinterfacesbetweensilicasandsilanecouplingagentsandbetweensilanecouplingagentsandrubber.Therefore,thecompositeswithoutsilanecouplingagentsshowlesscrosslink-densitycomparedwiththatoftreatedonesduetothelackofcureaccelerationbyadsorptionofacceleratorontothesilicasurfaces.

FromtheresultsofNMRstudies,MPS-treatedsilicasareobservedinthehigherintensityofSi(OSi≡),Si(OH)2R,andSi(OSi)(OH)RpeaksthanforVS,APS,andCPS.ThecompositestreatedbyMPSshowthesuperiorcrosslinkdensityinthesesystems.ItisseenthatMPShasorganicfunctionalgroups,whichcanreactwiththedoublebondofvinylester.

3.3.Mechanicalpropertiesofsilica/rubbercompositesAccordingtoKraus[18],thedegreeofadhesionbetween?llersurfacesandrubbercanbeassessedfromtheswellingbehaviorofthesampleinasolvent.Therefore,theswellingratioofthesilica/rubbercomposites,whichismainlydue

to

Fig.3.Tearingenergy(GIIIC)ofthesilica/rubbercomposites.

thecrosslinkdensityofthecomposites,in?uencestheme-chanicalinterfacialproperties.Inotherwords,themechan-icalinterfacialpropertiesofthe?nalproductaredependent

oncrosslinkdensity.Sothetearingenergycanbeconsideredtobeaninterfacialcharacteristicoftheconstitutiveelementsofamaterial.Thetearingenergies(GIIIC)aremeasuredbyatrouserbeamtestandarecalculatedusingequation[19]GIIIC=

2×F

t

,(4)whereFistheappliedforceandtthewidthofthetearpathaftertearingiscompleted.

AsshowninFig.3,thetearingenergies(GIIIC)ofthecompositesmadefromsilanetreatmentsarelargelyin-creasedcomparedtothoseofVSandincreasedasafunc-tionofthecrosslinkdensity,asseeninTable3.Thesilanesurfacetreatmentsremovesilanolgroupsandintroducenewfunctionalgroupsonsilicasurfaces,whichcanreactwiththerubber.Thesurfacecharacteristicsofsilicasaftersilanetreatmentsleadtoanincreaseofthecrosslinkdensityofthesilica/rubbercompositescomparedtothatofuntreatedsil-ica/rubbercomposites,resultinginincreasedthetearingen-ergyofthecomposites.Theincreasingtearingenergyofthecompositesthenleadstoanincreaseofthemechanicalprop-erties,suchasstressandstrain,asshowninFig.4.Thecom-positestreatedbyMPShavehighercrosslinkdensitythanthatofthecompositesmadefromAPSorCPS,observedinhighertearingenergy(GIIIC).Therefore,itisrecognizedthattheincreasedofcrosslinkdensityofthecompositesim-provesmechanicalproperties,mainlyduetoincreasedadhe-sionatinterfacesbetweensilicasandrubbermatrix,asseeninFigs.3and4[20].3.4.Thermalstabilityofsilica/rubbercomposites

Thethermalstability,measuredintermsoftheonsettemperatureofdegradation[21],isenhancedasthecrosslinkdensityincreases[21,22].

Figure5showstheTGAthermogramofeachofthecomposites.Ascanbeseenfromtheresults,thereisa

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Fig.4.Stress–straincurvesofthesilica/rubber

composites.

Fig.5.TGAthermogramsofthesilica/rubber

composites.

Fig.6.Plotsofln[ln(1?α)?1]versusθfordeterminingthedecomposition

activationenergy.Table4Thermalstabilityparametersofthesilica/rubbercompositesIDT[?C]Tmax[?C]IPDT[?C]Et[kJ/mol]VS364460711108APS364463764113CPS370464787113MPS372466803118littleincreaseintheinitialtemperatureofdegradationofthecomposites.Theresidualweightsofthecompositesbysilanesurfacetreatmentareincreasedcomparedtothoseofuntreatedsilica/rubbercomposites.FromtheTGAcurvesofthecomposites,thermalstabilityisalsogivenbyinitialdecompositiontemperature(IDT),temperatureofmaximumrateofweightloss(Tmax),thermalstabilityconstant,integralproceduraldecompositiontemperature(IPDT),andactivationenergyfordecomposition,Et[22,23],aslistedinTable4.Asaresult,thermalconstantsandIPDTofthecompositestreatedbycouplingtreatmentsaresigni?cantlyincreasedascomparedtothoseofVS.Itisfoundthatthesilanesurfacetreatmentsleadtoalowerdegradationofthecrosslinkedcompositesathighertemperaturethanuntreatedones.Activationenergyfordecomposition,Et,oftherubbercompositescanbecalculatedfromTGAcurvesbytheintegralmethodofHorowitzandMetzger,accordingtotheequation[24]ln??ln(1?α)?1??=Etθ/RTmax2,(5)whereαisthedecomposedfraction,Ettheactivationenergyfordecomposition,Tmaxthetemperatureatmaximumrateofweightloss,andRthegasconstant.Fromtheplotsofln[ln(1?α)?1]vsθ,whichareshowninFig.6,theactivationenergyfordecompositioncanbecalculatedfromtheslopeofthestraightlineinEq.(5).Asaresult,Etofthesilanetreatedsilica/rubbercompos-itesisincreasedcomparedwiththatofVS.ThecompositestreatedbyMPShavesigni?cantlyincreasedEtinthesesys-tems.Thisresultcouldbeexplainedbythesilanesurfacetreatmentsleadingtoanincreaseofcrosslinkdensityofthecomposites,resultinginincreasedthethermalstability,asseeninTable4.Theresultismainlyduetotheformationofacompactedcrosslinkstructureofthecomposites[25].Figure7showsthedependencesofthetearingenergy(GIIIC)andtheactivationenergyfordecomposition(Et)ofthecompositesonthecrosslinkdensity(Ve)ofthecomposites.Asaresult,itisfoundthatthecrosslinkdensityofthecompositesislargelycorrelatedwiththeGIIIC(regressioncoef?cient,R=0.94)andtheEt(regressioncoef?cient,R=0.91)oftherubbercomposites.ThusitisrecognizedthatincreasingVeplaysamajorroleinimprovingthemechanicalinterfacialpropertiesandthermalstabilitiesoftheorganicrubbermatrixofthecomposites.