Influence of silaned nano SiO2 on impact properties of carbon fiber.docx

Influence of silaned nano SiO2 on impact properties of carbon fiber.docx

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InfluenceofsilanednanoSiO2onimpactpropertiesofcarbonfiber

InfluenceofsilanednanoSiO2onimpactpropertiesofcarbonfiberreinforcedpolyethylenecomposites

RongLiu

SchoolofMechanical&ElectronicEngineering,HainanUniversity,Hainan878805,P.R.China

Abstract

SilanednanoSiO2wasusedtoimprovetheadhesionpropertiesofcarbonfiber/polyethylene（CF/PE）composites.ThenanoSiO2weretreatedbysilaneunderdifferentdischargetime.Andthechangesonthesurfacepropertiesofthetreatedanduntreatedcompositeswerestudiedbyimpact,threepointbendingtestsandscanningelectronmicroscope（SEM）analysis.ThemeasurementshowedthatCFincreasesfracturetoughnesswiththeincreaseofCFcontent.WhiletoomuchhighcontentofCFdidnotfurthercausetheincreaseofthetoughnessofCF/PEcomposite.Theimpactstrengthofsilanedspecimensisstillhigherthanthoseoftheunsilanedones.Themodifiedcompositewiththegoodmatrix/fiberadhesionpossessed20%higherinterlaminarshearstrengthscomparedtothecompositehavingweakinterface.

Keywords:

SiO2;CF/PE;toughness

1.Introduction

Duringthelastdecade,considerableattentionwaspaidtoinorganic–organichybridmaterialsbecausetheirsolidstatepropertiescouldbetailoredinrelationtothenatureandrelativecontentoftheirconstitutivecomponents.Low-volumeadditions（1–5wt.%）ofhighlyanisotropicnanoparticles,suchaslayeredsilicates,providepropertyenhancementwithrespecttotheneatpolymerthatarecomparabletothoseachievedbyconventionalloadings（15–40wt.%）oftraditionalfillers.Besides,uniquevalueaddedpropertiesnotnormallypossiblewithtraditionalfillersarealsoobserved,suchasenhancedstrength,electricalconductivity,electrostaticdischarge,remote-actuatedshaperecoveryandablationresistance[1-3].

Inotherpublications,fracturetoughnesswasimprovedbyaddingsphericalnanoparticlesinwhichqualitynano-dispersioniseasiertoachieve[4,5].Filler/matrixinteractionandachievingqualitydispersionseemtobethekeyfactorsforfracturetoughnessenhancementfornano-scalefillers.

Oneofthemajorconcernsindesigningcompositestructuresistheirsusceptibilitytoimpactloading.Fiber-reinforcedpolymer-matrixcompositesareknowntobehighlysusceptibletointernaldamagecausedbytransverseloads,evenunderlow-velocityimpacts[6,7].Thecompositescanbedamagedonthesurfaceaswellasbeneaththesurfacewithrelativelylightimpactscausingbarely-visibleimpactdamage,whilethesurfacemayappeartobeundamagedtovisualinspection.Fortheeffectiveuseoffiber-reinforcedpolymer-matrixcompositesforhigh-performanceapplications,understandingthecausesfortheformationofsuchdamageunderlow-velocityimpactandimprovingthedamage-resistancecharacteristicsofthecompositesareimportantconsiderationswhichhavebeenthetopicofextensiveresearchforthelastfewyears.Reviewarticlesontheimpactbehaviourofpolymer-matrixcompositescoveringcon-tactlaws,impactdynamics,stressanalysis,damagemechanics,post-impactresidualpropertycharacterizationanddamage-resistanceimprovementsareavailableintheliterature[8,9].Manyresearchpublicationsareavailableontheimpactbehaviourofpolymer-matrixcompositescoveringspecificaspects[10,11].

Matrixdeformationandmicro-cracking,interfacialdebonding,laminasplitting,delamination,fiberbreak-ageandfiberpull-outarethepossiblemodesoffailureincompositessubjectedtoimpactloading.Eventhoughfiberbreakageistheultimatefailuremode,thedamagewouldinitiateintheformofmatrixcracking/laminasplittingandwouldleadtodelamination.Damage-freecompositesarenecessaryfortheireffectiveuse[12].

Inthiswork,SiO2istreatedbysilaneinordertoimprovethesurfacepropertiesofcarbonfiber.ThepurposeofthisworkistostudytheimpactpropertiesofthePEcompositesfilledwithsurfacemodifiedCF.SomeinsightsintotheimpactfracturemechanismsofthePEcompositearealsogiven.

2.Experimental

2.1Materials

Highdensitypolyethylene（HDPE）[Tg:

85℃,Tm:

130℃,p:

0.95gcm-3andMw:

125,000gmol-1],wereallMerck（Germany）productsandusedaspurchased.

Forthepresentinvestigation,thereinforcementmaterialswerecontinuouspolyacrylonitrile（PAN）-basedcarbonfibersmanufacturedbyShanghaisxcarbonTechnologyCo.Ltd.

Thesilane（KH550）wasdissolvedinethanolbeforeuse.Theparticlesofnano-SiO2weredispersedinethanolandsubjectedtoultrasonicagitationfor15min,thesilanesolutionwasthenintroducedandtheultrasonictreatmentcontinuedfor1h.

2.2Compositepreparation

PEsampleswerefirstdissolvedin40mLofxyleneinathreeneckedflask.RequiredamountofCFwasaddedintotheabovesolution.Indolewasalsodissolvedin10mLofxyleneandaddedintothesameflask.Thereactionmixturewaswashedwithdistilledhotwaterseveraltimes,anddriedinavacuumovenat70℃for24h.

2.3Impactandthreepointbending（TPB）tests

ACharpyunnotchedimpactstrengthtestwascarriedoutonapendulumimpacttester（ModelPH125,Amsler&Co.,Schaffhouse,Switzerland）withacapacityof0.98J.Thetestspanwas20mm,measuredbetweenthetwospecimensupports.

Thespecimenswereimpactedonthenarrow1mmsurface,withthelineofimpactmidwaybetweenthesupports,andthedirectionofblownormaltotheplaneofreinforcement.

AnINSTRON4206electromechanicalmachinewasusedtoimplementthethreepointbendingtest.

3.Resultsanddiscussion

3.1impactstrength

Fig.1TheimpactstrengthofCF/PEcompositewithCFcontent

TheresultsoffracturetoughnessmeasurementsareshowninFig1.AsseeninthisFig,CFincreasesfracturetoughnesswiththeincreaseofCFcontent.WhiletoomuchhighcontentofCFdidnotfurthercausetheincreaseofthetoughnessofCF/PEcomposite.BasedontheresultsshowninFig1,itcanbeexpectedthatcompositesmadeusingCFshowimprovedinterlaminarfracturetoughness.

Fig.2TheimpactstrengthofCF/PEcompositefilledwithSiO2withandwithouttreatment

TostudytheeffectofsilanednanoSiO2modificationontoughnessimprovementoffinalcompositeagainstthroughthicknesscracks,theresultsofimpactstrengthversusSiO2contentfornotchedtestspecimensarereportedinFig2.Itisinterestingthatallsamplesfracturedthroughthicknessandincreaseinmodifiercontentledtoincreasedimpactstrength.WhilesimilartypeofimpacttestwasusedtoevaluateimpactstrengthofCFmodifiedPEcomposites,revealednosignificantimprovementinimpactstrengthofthecomposite（Fig1）.ThiscanbeattributedtothefactthatthePEusedinthatparticularinvestigationwasnotductileenoughtobetoughenedbyCFmodification.Thisiswhynoobviousimprovementinimpactstrengthofthecompositewasobserved.ThecontributionoffibersinabsorbingtheimpactenergyisnotinfluencedeffectivelybyCFmodification.Inaddition,Fig2showsthatimpactstrengthofsilanedspecimensarestillhigherthanthoseoftheunsilanedones.Themodifiedcompositewiththegoodmatrix/fiberadhesionpossessed20%higherinterlaminarshearstrengthscomparedtothecompositehavingweakinterface.ThereforeonemayconcludethatsilanedSiO2modificationimprovescrackinitiationenergyofthecomposites.

Themechanicalbenefitofthesenanoscalematerialsappearstoincreaseasstressintensitydecreases.Theyarenotcapableofhinderingcrackpropagation（fracturetoughness）onceacrackbeginstoadvance,buttheirnano-scaledimensionsenablethemtobetterinteractwiththematrixbeforeacriticalcrackisinitiated.Inaddition,SiO2maintaintheirhighaspectratiopost-processing.

Thisallowsforadelayincrazeformationandcoalescencepriortocrackinitiation.CFsdonotpresentanaspectratiowhichfavoursastronginteractionwiththematrix,reducingcreepimprovement.Ontheotherhand,thelargersizeoftheCFsallowsthemtoslowthegrowthofthecrackbydeflection,pull-outandcrackbridgingmechanisms.WhenanadvancingcrackispresentedwithaCF,itisdeflectedfollowingtheCF/matrixinterface,eventuallyleadingtothepulloutofthefilament.ThesurfacemodificationwassaidtohelpinexfoliatingagglomerationsandstrengtheningSiO2–matrixinteractions.

TheeffectofSiO2additiononthefailurebehaviorofCF/PEisillustratedbytheSEMmicrographshowninFig.3.Fiberpull-outisstillevidentonthefractureplaneandnoPEmatrixappearstoadheretothefibersurface.TheductilefailureofthePEmatrixchangestoabrittleonethatisassociatedwithsomelimitedcrazing.Thecrazeremnantsonthefracturesurfaceareclearlyvisible.Thus,itcanbeexpectedthat,duringthefractureprocess,theresistancetocrackpropagationwillbereducedmarkedlyandthesamplesfailinamorebrittlemanner.

（a）

（b）

（c）

Fig.3TheimpactfracturemorphologyofCF/SiO2/PEcomposite

Asseen,plasticdeformationsattheplasticzoneofmodifiedresinsareaccompaniedbystresswhitening.ThesizeofstresswhiteningincreaseswithincreasingtheSiO2content;suchthatin5vol%specimen,itexpandstothewholesurfaceinfrontofthecracktip（Fig.3a）.ThedifferenceinthedamagezonesizesseeninFig.3isinagreementwithfracturetoughnessdatareportedinFig2.

TheflatandalmostfeaturelessimageseeninFig.3aindicatesthetypicalbrittlefractureofneatPE.Ontheotherhand,theroughfracturesurfaceof3vol%specimenseeninFig.3bshowsthesignificantamountofplasticdeformationoccurredinthismaterialpriortofracture.Itisfoundthatcavitationisaprerequisiteformassivesheardeformationofthematrix.Thelatterisknownasthemainsourceofenergyabs