J.AdhesionSci.Technol.,Vol.17,No.12,pp.1713–1726(2003Chemicalmodicationofstyrene–butadiene–styrene(SBSrubberbyreactivegraftingwithmaleicanhydrideV.L.LASALLE1,M.D.FAILLA1,ENRIQUEM.VALLéS1,CARMENM.CEPEDA-JIMéNEZ2,ROSATORREGROSA-MACIá2andJOSéMIGUELMARTíN-MARTíNEZ2;¤1PLAPIQUI,PlantaPilotodeIngenieríaQuímica,UNS/CONICET,C.C717-8000BahíaBlanca,Argentina2AdhesionandAdhesivesLaboratory,DepartmentofInorganicChemistry,UniversityofAlicante,03080Alicante,SpainReceivedinnalform14June2003Abstract—Aproceduretoincreasetheadhesionofblockstyrene–butadiene–styrene(SBSrubberconsistingofthereactivegraftingwithmaleicanhydride(MAinthepresenceofanorganicperoxideradicalinitiatorisproposed.TheinuenceofthereactivegraftingonthesurfacepropertiesofSBShasbeenstudiedwithspecialemphasisontheimprovementoftheadhesiontopolyurethaneadhesive.ThegraftingofMAontoSBSwascarriedoutinthepresenceofdifferentconcentrationsof2,5-dimethyl-2,5-di(tertbutylperoxyhexane(DBPHasinitiatortogenerateoxygenradicalsbythermaldecomposition,whichinducethegraftingreaction.ThemodicationprocesswasperformedinthemoltenstateusingaBrabendermixertopremixthereactantsandahotpresstoinitiatethefunctionalizingreaction.ATR-IRandXPSspectroscopieswereemployedtoverifythegraftingofMAonSBS.ThechangesinwettabilityonthemodiedSBSrubberweredeterminedbycontactanglemeasurements.AdhesionpropertieswereevaluatedfromT-peeltestsofSBSrubber/polyurethaneadhesivejoints.ReasonableextentsofMAgraftingonSBSwereobtained(evidencedbythepresenceofaweakcarbonylvibrationat1700cm?1intheATR-IRspectraandbythecarbon–oxygenbandatabindingenergyof287.0eVintheXPSspectra.ThehighertheDBPHamount,thehighertheMAamountgraftedontotheSBSsurface.Themaximumgraftinglevelwasobtainedusing2wt%MA.GraftedspeciesseemedtobemainlyconcentratedonthesurfaceoftheSBS-moldedsheets.Thewettabilityofthemodiedrubberincreasedwithrespecttotheoriginalpolymer,newcarbon–oxygenmoietieswerecreatedandtheC/Oratioincreased.Anoticeableenhancementinpeelstrengthvalueswasobserved,whichwasascribedtothecreationofinterfacialinteractionsbetweenthepolyurethaneandtheSBSrubbersurfaces.Keywords:Grafting;maleicanhydride;rubber;surfacecharacterization;XPS;ATR-IRspectroscopy.¤Towhomcorrespondenceshouldbeaddressed.Phone:(34-96590-3977.Fax:(34-96590-3454.E-mail:jm.martin@ua.es1714V.L.Lasalleetal.1.INTRODUCTIONThechemicalmodicationofcommercialpolymershasbecomeacommonap-proachtoproducenovelmaterialswithimprovedproperties[1].Oneofthemostusefulmethodsofmodicationisthegraftingofpolargroupsontopolymerchains.Severalreactantscontainingdifferentpolargroupshavebeenusedforthispurpose.Short-chainunsaturatedpolycarboxylicacid,nitriles,amides,anhydridesandestersaresomeofthereactantsmorefrequentlyusedforthefunctionalizationofpoly-olens.Thegraftingoffunctionalgroupsontothepolymerchainsispromotedbyadditionoforganicperoxides.ThethermaldecompositionoftheROOR0bondinducestheformationofRO2freeradicalsthatattackthemoleculesofthepolymer,subtractinghydrogenatomsandproducingreactiveradicals,which,inturn,reactwiththeaddedfunctionalizingmonomers.Styrene–butadiene–styrene(SBSisawell-knownthermoplasticwithablockco-polymerstructurewidelyemployedintherubberindustry.Asaresultofitslowsurfaceenergy,theadhesionofSBStoothermaterialsusingstandardadhesivesrequireschemicalormechanicaltreatmenttoimprovebonding.Inordertoreachadequateadhesion,itsapplicationsintheshoeindustryrequirephysicaland/orchemicalmodicationoftheSBSsurfacesinordertobeglued.Inthiscontext,theaimofthisstudywastolookforsimplerprocedurestochemicallymodifySBSpolymericchainswithmonomerscontainingpolargroups.Thismaybeausefulproceduretoavoidtheapplicationofsurfacetreatmenttoimprovetheadhesionofrubberwithpolyurethaneadhesive.Thereisnotenoughinformationintheliteratureaboutgraftingmaleicanhy-dride(MAontoSBS[2].Gergenetal.[3]usedatitrationtechniquetoquantifythedegreeofMAgraftinginducedbyorganicperoxides.Saitoetal.[4]graftedMAontopolymericchainswithoutusingchemicalinitiators.Theyshowedthatgraftingreactioncouldbecarriedoutbythermaleffectfollowingtheso-called‘n-reaction’mechanism.SteinkampandGrailreportedsimilarndings[5].TheyalsoclaimedanincreaseinadhesionandmechanicalpropertiesofSBS.Inre-centstudies,SanchezSolísetal.[6]investigatedthegraftingofMAontoSBStoinducecompatibilityandimproveimpactresistanceofSBS–PET(poly(ethyleneterephthalate-blendsusingbenzoylperoxideasradicalinitiator.Theyalsofoundthattheadditionofadhesionpromotershadagreatinuenceonthephasedisper-sionandmechanicalpropertiesoftheseblends.Furthermore,differentresearchers[7–10]showedthatMAwasgraftedontoSBSonlybyremovalofanallylichydro-genatom.Inthispaper,somepreliminaryresultsarepresentedtodemonstratethefeasibilityofreactivegraftingofSBSwithMAtoimprovetheadhesionofSBSrubbertopolyurethaneadhesives.ChemicalmodicationofSBSrubberbyreactivegraftingwithMA17152.EXPERIMENTAL2.1.MaterialsRadialblockco-polymerstyrene–butadiene–styrene(Solprene416,RepsolQuímica,Santander,Spainwasusedinthisstudy.Ithasadensityof0.94g/cm3(ASTMD-297,anintrinsicviscosityof2.2Pas(ASTMD-2857,andastyreneblockcontentof30%.2,5-dimethyl2,5-di(tertbutylperoxyhexane(DBPHfromAzkoNobelChemicalswasusedasradicalinitiator.MaleicanhydridewasprovidedbyAnedra(BuenosAires,Argentina.2.2.GraftingprocedureBatchesof35geachwerepreparedbypremixingallthereactants(MA,DBPHandSBSinaBrabenderPlasticorderMixerat40rpmand120±Cfor5min.Aftermixing,thegraftingreactionwascarriedinanhydraulicpress.Thetemperaturewasxedat190±Candthereactiontimewas20min.Withthisprocedure,moldedsheetsofgraftedSBS,150￡150￡3mminsize,wereobtained.Toassurethatthenon-graftedMAontotheSBSrubberwasremoved,1gsamplesweredissolvedin100mltolueneatboilingtemperatureandthenprecipitatedin1000mlcoldacetone.Theresultingmaterialwasdriedatroomtemperatureundervacuumfor36h.CastsheetsofgraftedSBSwerewipedwithisopropanolbeforecarryingouttheexperimentalmeasurements.2.3.CharacterisationofthegraftedSBS2.3.1.ATR-IRspectroscopy.About51mdepthofthegraftedSBSsheetswereexaminedbyFT-IRinaNicolet205spectrometeremployingtheattenuatedtotalreectance(ATRtechnique.AKRS-5(thalliumbromoiodidecrystalwasused.ATR-IRspectrawererecordedfrom4000to400cm?1withanincidenceangleof45±.200scanswereaveragedforeachspectrumwith4cm?1resolution.Similarstudieswerecarriedoutonbulk-modiedSBSafteritssurfacelayerhadbeenremoved.2.3.2.X-rayphotoelectronspectroscopy(XPS.TheoriginalandmodiedSBSsurfaceswereanalyzedwithaVGScienticMicrotechMultilabspectrometer,withanMgK?X-raysource(1253.6eVoperatingat15keVand300W.TheangleofincidenceoftheX-raysonthesamplewassetto45±.Priortoanalysis,sampleswereoutgassedinavacuumchambertoapressurelowerthan5￡10?8torr.Theanalysiswasperformedon5￡2mmrubbersurfaces.Foreachsample,asurveyscanencompassingtheregion0–1200eVwasrstobtained.Moredetailedscanningsofallobservedphotopeaksinthesurveyscanwerecarriedoutinthe20eVrange.BindingenergiesofallphotopeakswerereferencedtotheC1sphotopeakpositionforCCandCH(hydrocarbonsspeciesat285.0eV.AtomicconcentrationcalculationswerecarriedoutusingaVGX900-Wsystem.1716V.L.Lasalleetal.2.3.3.Contactanglemeasurements.ThedifferencesinthesurfacepropertiesofgraftedSBSwithrespecttooriginal(non-graftedrubberwereevaluatedbythistechnique.Adropofthetestliquid(CH2I2,approx.41l,wasplacedontheSBSrubbersurfacewithasyringe.TheSBSsamplewasmaintainedinanenvironmentalchambersaturatedwiththevapourofthetestliquid.Thecontactanglemeasure-mentsweremadeafter15minofdropdepositiontoensureequilibriumconditions.Thereportedmeasurementswereanaverageofsixdeterminationstakenindifferentareasofthesurface.2.3.4.T-peeltests.T-peeltestswereperformedonadhesivejointsobtainedusingpolyurethaneadhesiveandspecimensoftheoriginalandmodiedSBSrubber.Thepolyurethaneadhesivewaspreparedbydissolving15wt%ofcommercialpolyurethanepellets(Desmocoll540providedbyBayer,Leverkusen,GermanyinMEK(methylethylketone.ACowlesmechanicalstirreroperatingat1000rpmwasusedforthispurpose.Thedissolutionofthepolyurethanewascompletedafterstirringfor2h.ThisadhesivesolutionwasemployedtojointwosimilarlymodiedrectangularstripsofSBSrubber3￡10￡150mminsize.About1mlofadhesivesolutionwasappliedoneachSBSstrip.A30-minperiodunderopenairwasallowedtofacilitatesolventevaporationandensureuniformrubberwetting.Then,thesolidpolyurethanelmsontheSBSsurfaceswerereactivatedusinganinfraredlampat90–100±Cfor24s,andthemoltenpolyurethanelmswerebondedfor11satapressureof0.8MPa.Theadhesivejointsweremaintainedatroomtemperature(23±Cunderarelativehumidityof50%forperiodsof15minand72h.Attheendoftheseperiods,peeltestswerecarriedoutusinganInstron4411machinewithapullrateof100mm/min.ThepeelstrengthvaluesreportedinthisworkaretheaverageofthreemeasurementswitheachSBS-modiedsample.3.RESULTSANDDISCUSSIONFigure1comparestheATR-IRspectraintheregionbetween1500–2000cm?1ofavirginSBSsample,agraftedunpuriedsample(containingalltheaddedMA,thesurfaceofwhichwasonlywashedwithacetonetoremoveorganiccontaminantsduringprocessingandmanipulation,andapuriedgraftedSBSsampleobtainedbytreatingthegraftedunpuriedSBSwithhottoluene(thuscontainingonlythegraftedMAbecausetheresidualnon-graftedMAwasremoved.TheATR-IRspectrashowthatthereactionofSBSpolymerchainswith2wt%MAand0.05wt%DBPHgraftssuccessfullyareasonablesmallamountofMAontotheSBS.ThepreciseamountofMAgraftedontoSBSwasnotdeterminedinthisstudy.AlthoughseveralprocedurestoestablishthegraftingdegreeofMAinpolymershavebeenproposed[9,10],noneofthemprovidesadequateresults.Infact,theATR-IRspectrumofthevirginSBSdoesnotshowanyvibration,whereastheunpuriedgraftedSBSshowsasharpbandduetocarboxylicacid(1710cm?1whichindicatesthepresenceofbothnon-graftedandgraftedMAontotheSBSChemicalmodicationofSBSrubberbyreactivegraftingwithMA1717Figure1.ATR-IRspectraoforiginal,andunpuried(containingallMAandpuried(containingMAgraftedonlySBSrubber.0.050wt%DBPHwasaddedasradicalinitiator.chains.Thebandlocation,slightlyshiftedfromtheabsorptionbandsofthetypicalcarbonylabsorptionoftheanhydride(1730–1790cm?1,indicatesthatprobablytheMAhasbeenhydrolysedduringthemixingorreactionprocess.ThesendingsareinagreementwithrecentresultsbyAiminandChao[10].Asresultofremovalofthenon-graftedMAontheSBSwithhottoluene,thebandintensityat1710cm?1diminisheswithrespecttothatoftheunpuriedrubberbecausetheunreactedMAisremovednotonlyfromthesurfacebutalsofromthebulkpolymer,andonlythegraftedMAremains.VerysimilarresultswereobtainedwiththeSBSsamplesmodiedwithdifferentconcentrationsofDBPHandMA.Figure2comparesthesurface(Fig.2aandbulk(Fig.2bATR-IRspectraoftheunpuriedSBSrubbermodiedwithdifferentamountsofMA.TheATR-IRspectraofthebulkwereobtainedbyremovingtheexternalsurfaceoftheSBSrubberusingasurgicalblade.InthebulkATR-IRspectrathebandsassociatedwiththebondarenotpresentindetectableamounts.ThisisanindicationthattheMAmoietiesintheSBSrubbertendtoconcentrateonitssurface.XPStechniqueprovidesfurtherevidenceofreactivegraftingofMAontheSBS.Tables1and2showtheelementalcompositionofthemodiedSBSsurfacesbeforeandafterpuricationwithhottoluene,respectively.Thepresenceofsiliconreveals1718V.L.Lasalleetal.(aFigure2.ATR-IRspectraofmodiedSBSrubberwithdifferentamountsofMAand0.050wt%DBPH.(aSurface;(bafterremovaloftheuppermostsurfacelayer.Table1.XPSelementalcomposition(at%ofunpuriedSBSrubbercontainingdifferentamountsofMAand0.050wt%DBPHElement0wt%MA1wt%MA2wt%MA4wt%MAC87.089.684.087.4O9.28.513.310.5N0.2Si1.9O/C0.110.090.160.12ChemicalmodicationofSBSrubberbyreactivegraftingwithMA1719(bFigure2.(Continued.thatsomesilicacontaminationofthesamplesduringprocessingwaspresent.Thiscomplicatestheinterpretationofthedatabecausesilicaalsocontainsoxygenatoms(twooxygenatomsforeachsiliconatom.Ifacorrectionfortheoxygenpresentintheseimpuritiesisperformed(Table3,alltherubbersamplescontainingMAshowhigheroxygencontentthantheuntreatedSBS.Afterpuricationwithhottoluene,thecarboncontentincreasesandtheoxygencontentdecreases(i.e.theO/CratiodecreasesindicatingtheremovalofunreactedMA.AstheamountofMAaddedtotherubberisincreasedfrom1to4wt%thedecreaseinoxygencontentobservedafterpuricationbecomesmoresignicant.WhereastheunpuriedSBS1720V.L.Lasalleetal.Table2.XPSelementalcomposition(at%ofpuriedSBSrubbercontainingdifferentamountsofMAand0.050wt%DBPHElement0wt%MA1wt%MA2wt%MA4wt%MAC87.090.684.489.7O9.27.511.46.8N0.3Si3.1O/C0.110.080.140.08Table3.Corrected¤XPSelementalcomposition(at%ofpuriedSBSrubbercontainingdifferentamountsofMAand0.050wt%DBPHElement0wt%MA1wt%MA2wt%MA4wt%MAC97.094.095.496.0O3.6N0.4O/C0.030.060.050.04¤Percentagesofelementswerecorrectedbysubstractingsiliconandthecorrespondingamountofoxygen(astoichiometryofSiO2wasassumend.Table4.CH2I2contactangles(degreesonSBSrubbergraftedwithdifferentamountsofMAand0.050wt%DBPHMA(wt%μ(CH2I2050133239440containing1wt%MAhasanoxygencontentof8.5at%anditdecreasesto7.5at%afterpurication,fortheSBScontaining4wt%MAtheoxygencontentdecreasesfrom10.5at%to6.8at%.ThisisanindicationthattheamountofMAgraftedontherubberchainsislimitedtoacertainvalueandhigheramountsofMAdonotnecessarilyproducehigherdegreeofgraftingoftherubber.BecausethegraftingofMAseemstobemainlyrestrictedtotheSBSsurface,thewettabilityofthemodiedSBSrubberswasdeterminedusingcontactanglemeasurements.EquilibriumcontactanglesonvirginandfunctionalizedrubberswithMAareshowninTable4.TheCH2I2contactanglevaluesongraftedSBSdecreases,indicatinganimprovedwettabilityofSBSrubberaftergrafting.ChemicalmodicationofSBSrubberbyreactivegraftingwithMA1721ThedataprovidedbyATR-IRspectroscopycorrespondtoabout51mdepthontheSBSsurface.Therefore,XPSwasusedtoobtaininformationonthechemistryintheoutermostSBSsurface(about5nm.Table2showsthechemicalcompositionofthepuriedSBSrubbercontainingdifferentamountsofMAand0.050wt%DBPH.GraftingwithMAproducesanincreaseinO/Cratio,whichisanindicationofthereactionofMAwiththerubberchains.ThehighestoxygencontentisseenfortheSBSrubbergraftedwith2wt%MA.Siliconisalsopresentonthegraftedrubbersurfacesasacontaminantandatinyamountofnitrogenalsoappears(likelyduetoimpuritiesduringprocessingofSBSrubber.TheamountofDBPHalsodeterminesthegraftingdegreeofMAontotherubberchains.Tables2and5showthattheincreasingamountofDBPHfavoursthegraftingof2wt%MA(i.e.higheramountsofoxygenandloweramountsofcarbonareobtained,andthiseffectislessmarkedfortheSBSrubbergraftedwith4wt%MA.Figure3showstypicalexamplesofcurvettingofC1sphotopeaksforunpuriedandpuriedSBSrubberscontaining2wt%MA.Table5showsthepercentagesofcarbonspeciesforallunpuriedandpuriedSBSrubbers.ThecurvettingofunpuriedgraftedSBSshowstheexistenceofcarboxylgroups(bindingenergyD289.4eVduetothehydrolysisoftheMAdutingprocessing.BecausethemaleicacidisnotgraftedontotheSBSrubber,thesegroupsdisappearafterpuricationinhottoluene(Fig.3.Furthermore,anewbandduetonewcarbon–oxygengroupsatabindingenergyof287.0eVappearsinbothunpuriedandpuriedgraftedSBS.Typicalcarbonylbandappearsat288.0eVbutthepresenceofachemicalenvironmentlesselectronegativeproducesdisplacementofthebindingenergytolowervalues[11].ThegraftingofMAontothepolymerchainsallowstheformationofcarbonyl-typegroupswhichseemstobedisplacedtolowerbindingenergy.Infact,puricationofgraftedSBSmaintainsthebandat287.0eV,andthepercentagesarerelativelysimilarinbothunpuriedandpuriedsamples(Table5.Ontheotherhand,Table5showshigherpercentagesofcarbon–oxygenmoietiesbyincreasingtheamountofMAupto2wt%,althoughforthe4wt%MAgraftedSBS,areductionincarbon–oxygengroupsisobtained,probablyduetothelowerdegreeofgrafting.Finally,theloweramountofDBPH(i.e.0.025wt%produceslessdegreeofgrafting.ThemodicationsinbothsurfacechemistryandwettabilityintheSBScontainingMAaffectthepeelstrengthvaluesofSBS/polyurethaneadhesivejoints.Figure4showsthattheinitialpeelstrengthvalue(obtained15minafterjointformationofvirginSBS/polyurethaneadhesivejointisoftheorderof300N/m,thisvalueincreasestoabout500N/mafter72h(nalpeelstrengthduetopolyurethanecrystallization.Thesepeelstrengthvaluesareconsideredtobelow,whichmeansthatvirginSBSexhibitspooradhesionpropertiestopolyurethaneadhesives.TheimprovementinadhesionpropertiesbyreactivegraftingofSBSrubberwithMAissignicant.ThepeelstrengthvaluesincreaseinthejointsproducedwithSBScontainingMAismoremarkedbyincreasingtheMAcontentupto2wt%.ThisisinagreementwiththevariationsinsurfacechemistryinthegraftedSBSrubbers1722V.L.Lasalleetal.Figure3.XPSC1scurvettingofunpuried(containingallMAandpuried(containinggraftedMAonlySBSrubbercontaining2wt%MAand0.050wt%DBPH.0.025wt%DBPH.ChemicalmodicationofSBSrubberbyreactivegraftingwithMATable5.XPSat%ofspeciesobtainedfromC1scurvettingofunpuriedandpuriedSBSrubbercontainingdifferentamountsofMAandDBPHMA(wt%DBPH(wt%Unpuried285.0eV012424—0.0500.0500.0500.0250.02596.597.191.796.897.498.3287.0eV289.4eV———Puried285.0eV96.597.395.097.897.597.9287.0eV1723Figure4.T-peelstrengthvaluesofSBSrubber/polyurethaneadhesivejointsasafunctionoftheMAcontentinthemodiedSBS(0.050wt%DBPH.Alwaysanadhesionfailurewasobtained(visualinspection.(Fig.2aandTables2and5.AstheconcentrationofMAincreases,theinitialpeelstrengthincreasesthree-foldwithrespecttothejointproducedwiththevirginrubber,andthenalpeelstrengthincreasesbyafactoroftwo.Allpeeltestsshowbyvisualinspectionanadhesionfailure,i.e.thefailureisproducedbetweentheadhesiveandtheSBSrubbersurface,indicatingthattheadhesionofSBStopolyurethaneadhesiveincreasesbyreactivegraftingwithMA.TheimprovedadhesionbetweenMA-graftedSBSrubberandthepolyurethanecouldbeduetoimprovedinterfacialinteraction.ATR-IRspectroscopywasusedto1724V.L.Lasalleetal.(aFigure5.ATR-IRspectraofthe(apolyurethaneadhesivelmsand(bMA-modiedSBSrubbers(4wt%MAand0.050wt%DBPHwiththinpolyurethanelmsontheirsurface.Numbersindicatetheconcentrationofthepolyurethaneinthesolutionusedtospin-coatthemodiedSBSrubbersamples.determinethechemicalcompositionofthinpolyurethanelmsdepositedbyspincoatingofdilutedadhesivesolutions.ThepolyurethanelmswereobtainedusingsolutionsoftheadhesiveinMEKatdifferentconcentrationsrangingfrom0.5to18wt%,followedbydryingatroomtemperatureandreactivationat90–100±Cfor24s(thesameexperimentalconditionsasusedduringjointsformation.Figure5showstheATR-IRspectraofthepolyurethanelmandoftheSBSrubbermodiedwith4wt%MAwithdepositedthinpolyurethanelms.TheATR-IRspectrumofthepolyurethanelm(Fig.5ashowstypicalbandscorrespondingtotheurethane:NH(1538,3347cm?1,CO(1741cm?1andCOC(1068,1225cm?1groups.ThesebandsaredifferentfromthosefortheSBSrubber.TheATR-IRspectracorrespondingto0.5and1.0wt%polyurethaneadhesivesolutions(Fig.5bshowthedisappearanceoftheCOCgroupsat1216and1063cm?1andcarbonylgroupsat1700cm?1,whichisindicativeofsomekindofinteractionbetweenthesefunctionalgroupsandtheadhesive.Furthermore,thebandsofthepolyurethanecannotbedistinguished,probablybecausesomechainsinterdiffusionbetweenthepolyurethaneandtheSBSrubbersurfacestakesplace.IntheATR-IRspectracorrespondingtohigherconcentrationsoftheadhesive,theregionofthecarbonylgroupsisoccupiedbythecharacteristicbandsofthepolyurethaneandthedisap