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Linear Dynamic Viscoelastic Response of Melts for Crosslinkable Polyethylene Insulation Cable Granules During Cross-linking

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Linear Dynamic Viscoelastic Response of Melts for Crosslinkable Polyethylene Insulation Cable Granules During Cross-linking

    Linear Dynamic Viscoelastic Response of

    Melts for Crosslinkable Polyethylene

    Insulation Cable Granules During

    Cross-linking

    CHEM.RES.CHINESEUNIVERSITIES2010.26(1).142l47

    LinearDynamicViscoelasticResponseofMeltsforCrosslinkable

    PolyethyleneInsulationCableGranulesDuringCross?linking

    YANGHongmei',YANGYong.zhu,HEXiao,ZHENGQiangandZHOUHou.qiang.

    j.DepartmentofPolymerScienceandEngineering,2.KeyLaboratoryofMacromolecularSynthesisandFunctionalization,

    MinistryofEducation,ZhejiangUniversity,Hangzhou310027,PR.China; 3.NingboDongfangCableLtd.,Ningbo315040,PRChina

    AbstractTwocommercialcrosslinkablepolyethyleneinsulationcablegranules.designedforl10and35kVvoltage

    insulation,withsimilarcrosslinkingtemperaturebutdifferentmeltingtemperature,werechosenasexperimental

    samplesforexaminingtheirlineardynamicresponsesduringCROSS

    linking.Ithasbeenfoundthatthegelcontentsof

    cablecompoundsforl10and35kVinsulationarealmostthesameaftertheyhavebeencross

    linkedatthesame

    temperature,pressureandtime.Andthesamplemeltsshowthesimilardependenceofthedynamicstoragemodulus,

    Gonstrain.Ontheotherhand.thedynamictemperatureramptestandthedynamictimesweeptestindicatethatthe

    samplesexhibitdifferentdynamicviscoelasticresponsesduringtheircrosslinking.Anexpressionfordescribingtheir

crosslinkingprocesswasproposedviaprobingrheokineticsofcrosslinkingforthetwosampl

    es.

    KeywordsXLPE;Insulationcable;Crosslinking;Rheologicalbehavior

    ArticleID1005-9040(20l0,-O1-14206

    1lntroduction

    Crosslinkinghasbeenemployedasaneffective methodtoenhancethemechanicalpropertiesofthe linearpolymers.Lowdensitypolyethylene(LDPE)isa non??polarpolymer,andiscommonlyusedasanelec-- tricalinsulatorofhighvoltagecablesduetoitslow cost,excellentelectricproperties,goodchemicalrc- sistanceandeasyprocessing'一川.Overthepasttwo

    decades,crosslinkedpolyethylene(XLPE)haspro

    gressivelyreplacedlowdensitypolyethylene(LDPE) inextrudedinsulationcablegranulesbecauseXLPE hasbe~ermechanicalpropertiesandchemicalresis

    tanceatahightemperaturethanLDPE.

    ItisknownthatthepurityofLDPEresinisvery importantfortheinsulationpropertyofinsulationca- ble,especiallyfortransmittingelectricityapplica- tionwithhighvoltages.Inaddition,thecrosslinking processalsoplaysanimportantroleintheperfor

    manceofinsulationcable.However,itoftenincludes partiallysomeimpuritiesintotheinsulationcable,and resultsinthechangesofmicrostructureofthecom

    pounds[3,51.

    Itisworthnotingthatasanimportanttesting method,rheologicalmeasurementcangweusalotof valuableinformationaboutthepolymermorphologies andstructures,suchasmolecularweight,molecular

    weightdistribution,viscoelasticpropertiesofpoly- mersandtheircorrespondingstructuralevolution duringtheprocess[一羽.

    Fenouilloteta1.9investigated

    thetheologicalbehaviorofthermoset/thermoplastic blendsduringisothermalcuring,andproposedarheo

    kineticmodeltopredicttheviscoelasticbehaviorof thesvstem.CiardelliPt1.1.1monitoredthechemical

    crosslinkingreactionofpropylenepolymerthrough rheologicaltesting.Zhou1Jexaminedtherheo.

    kineticsofmeltpoly(ethylened?octene1elastomer

    withdicumylperoxide.

    Inthiswork.twokindsofcommercialcrosslin- kablepolyethyleneinsulationcablecompounds,de

    signedfor110and35kVvoltageinsulation,werese- lectedasthemodelmaterialsstudied.Inordertode- scribesystematicallytheviscoelasticparametersof thetwosamplesduringcrosslinkingprocess,wefo

    cusedoura~entiononthelineardynamicresponsesof meltsforcrosslinkablepolyethyleneinsulationcable compoundsduringtheCROSSlinkingthroughrheolo?-

    gicalmeasurement.

    Correspondingauthor.E-mail:yanghongmei@zju.edu.cn ReceivedApril2,2009;acceptedMay7,2009. SupportedbytheNationalKeyTechnologiesResearchandDevelopmentProgramofChina

    Duringthe1lthFive'YearPlan

    Period(No.2007BAE19B01)andtheIndustrialResearchProjectsoftheScienceandTechno

    logyPlanofNingboCity,China

    (No.2007B10033).

    No.1YANGHong.meieta1.143

2Experimental

    2.1Materials

    Bothl10kV(HFOB.4201sc)and35kV(YJ.35) insulationcablegranuleswerekindlysuppliedby NingboDongfangCableLtdChina.110kVsample

    wasacommercia1productfromBorealis.Denmark andFinland,and35kVsamplewasmanufacturedby ZhejiangWanmaGroupChina.

    2.2SamplePreparation

    Disksof1.5mminthicknessand25inl/1india- meterweremadebycompressionmouldingfromthe pelletsoftheinsulationcablecompoundsforrheolo

    gicaltests.Thepelletswerepressedat125.Cand15 M_Pafromapatternwithl6holesof1.5mminthick. hessand25mmindiameterinahot.press(XLB25.D,. Afterwards.thesampleswerecooledinairunderl6 ?【f)aandatroomtemperature.Inordertoavoideros. slinking,thewholetimeforpreparationwas10minat 125.C.

    2.3Measurements

    2.31DsCMeasurement

    Thermalpropertiesofthejnsulationcablegra

    nuleswereexaminedwithaDSCQ100TAInstrument. Inordertodetectthecrosslinkingtemperatureofthe twoinsulationcablecompounds.510mgofthe

    specimenscutoutfromthepelletsofLDPEinsulation cablecompoundswasanalyzedbyheatingfrom40.C to220.Catthreerampratesof2.5and10.C/min.In ordertocalculatethecrystallinityofthecompounds

beforecrosslinking.5l0mgofthespecimenswas

    meltedat130.Cforl0mintoeliminatethethermal history,thencooledtolO.CatthreerampratesOf2. 5andl0.C/min.andfinallyheatedfroml0.CtO 130.Cat10.C/min.Antheexperimentswerecarried outundernitrogenatmospheretoavoidtherlTlalde. gradation.ThecrystallinityOfb0thinsulationcable compoundswascalculatedviathefusionenthalpy duringthesecondheatingthroughuniversalanaly

    sissoftware.Thefusionenthalpyofl00%crystalliza

    tionpolyethylenewas287.3J/g

    2.3.2RheologicatMeasurement

    Therheologicaltestswereperformedwithan advancedrheometricexpansionsystem(ARES. RheometricsSci.Ltd.,USA)inairatmosphereusing parallelplatesmode1andthesystemwasoperated underthefollowingconditions:(1)frequency.10 rad/sindynamicstrainsweep;(2)strain1%indy- namicfrequencysweep;(3)10rad/s,1%,at differenttemperaturesindynamictimesweep;(4) 09=10rad/s,y=1%indynamictemperatureramp. 2.33GelFractionMeasurement

    Thedegreeofcrosslinkingwasobtainedbygel fractionmeasurement.Gelfractionwasdeterminedby meansofaSoxhletextractorwithxyleneassolvent. Sampleswereexposedtorefluxingxyleneforatleast 48hat200.Cuntilthesampleweightapproacheda constant.Theextractionresiduewastakenasthegel content[13

    .

andthege1contentwascalculatedviathe

    followingequation

    Gelcontent(%)=(m1/mo)×100%(1)

    wherem0andmlareoriginalmassandmassofextrac- tionresidue,respectively.

    3ResultsandDiscussion

    Fig.1showstheDSCtracesforthetwoinsulation cablegranules.Itcanbefoundthatthemeltingtem

    perature(about109.C,ofll0kVinsulationcable granulesisslightlyhigherthanthat(about107.C1of 35kVinsulationcablegranules.MoreoveLthecrys

    tallinityofb0ththeinsulationcablegranulescanbe calculatedthefusionenthalpyduringheating.The resultsrevealthatecrystallinityOfll0kVsampleis 52.2%.higherthan48.1%Of35kVsamplecalculated 306O90l2O15Ol8O2l0240

    t{?

    efc

    Fig.1DSCtracesfor110kV(A)and35kV(B)insulation cablegranulesatdifferentmeltingrates Meltingrate/(.C-min):a.2;b.5;c.10.

    144CHEM.RES.CHINESEUNIVERSITIESVO1.26 with287.3J/gasthefusionenthalpyof100%crystal

    lizationpolyethylene[12.Furthermore.moreobvious

    crosslinkingreactioncouldbeobservedwhentempe- ratureisabout150.C.Forthisreason,allthesamples werepreparedat125.Cinordertoavoidcrosslinking. Foreachsample,thetemperatureatwhichtheheat flowisuptothetopvalueincreaseswiththeincrease oframprate.

    Ingeneral,polymermeltsdemonstratefllinear viscoelasticbehaviorinflsmallstrain(orstress)re. gion,inwhichthedynamicstoragemodulusGand IOSSmodulusGareindependentOfthestrain.andthe structureofsamplecouldhardlybedestroyedduring thetesting.Fig.2showsy-dependentGthroughstrain sweepexperimentsat=10rad/sand130.Cfortwo samples.TheplotofGvs.displaysaplateaucor

    respondingtothelinearviscoelasticbehaviorwhen isbelowthecorrespondingcriticalstrain.However themeltsdemonstrateadecreaseinGandthosetwo sampleshavealmostthesamedependentGwhen

    thevalueishiigherthantheYcvalue.Theseresults implythatthesamplemeltsexhibitsimilarlinearvis

    coelasticregionsbeforecrosslinking,and',=1%is preferredtochooseforallthedynamicrheological measurementsinordertoensurethatthet, estiskeptin

    thelinearviscoelasticregion.

    

    y)

    Fig.2DependenceofstoragemodulusGonstrain f0r110kVr?1and35kV(o)insulationcable granulesat130.Candafrequencyco=10rad/s Thedynamicviscoelasticityofapolymermeltat alow?isconsiderablysensitivetostructurechanges ortotheformationofflnetworkstructure.Thedy. namiclinearviscoelasticityforthesampleswas examinedatagivenstrainof1%toensurethetestto becarriedoutinthelinearregionfirstly.Itcanbe

    foundfromthefrequencysweeptestingaspresented inFigs.3and4,respectivelythatthevaluesofGand G"forthesamplemeltsaresimilarinthewholetested ?regionatlowertestingtemperatures(120and 125.C1.andthedependenceofmodulusoncoforthe samplesissimilar,exhibitingtheexistence0ftheli. quid-likeviscoelasticbehavior.Butwiththeincrease ofthetestingtemperature,thevaluesofGandG" increaseatlower09region,duetothecrosslinking. Expressly,itseemsthatI10kVsamplecancrosslink morequicklyat160.C,andasaresult,extraordinary viscoelasticresponsescanbeobserved,i.e.,Gshows lessdependenceon?withinthetestedregion.

    Ingeneral,polymermeltsexhibitelasticres

    ponsesinahigh(orshorttime)regionandviscous responsesinalow(orlongtime)region.Theelas

    tic?-viscousresponsetransformationpointcorrespon?- dingtothelosstangent(tariff)canbegivenby: tan::1(22

    G

    ForuncrosslinkedLDPE,thetransformation pointchangeslittlewiththeincreasingtemperature, andappearsat?1rad/sforthetwosamplesfrom

    120.Cto150.CasshowninFigs.3and4.Howeve~ crosslinkingresultsintheappearanceofasecond transformationpointatthelowerforthesamples, 1O

    10

    

    10

l0

    l0

    l0

    1O:

    l00

    ~o/(rad?s.) Fig.3DependenceofstoragemodulusG(A),loss

    modulusGr(B)andlosstangenttand(C)on

    frequencyfor110kVinsulationcablegra nulesatdifferenttemperatures

    m

    t)

    No.1YANGHongmeieta1.145

    l0

    10

    

    104

    l0

    1O0

    l0

    1O

    10

    lO

    lO0

    10

    l00

    (A)

    

    

20?

    25?

    30?

    40?

    5O?

    60?

    co/(rad?s)

    (C)

     20?

    25?

    30?

    40?

    50?

    60?

    :2

    lOI2l01l00

    o~/(rad?s

    10:

    Fig.4DependenceofstoragemodulusG(A),Joss modulusG,(B)andlosstangenttan~(C)on frequency?for35kVinsulationcablegranule

    atdifferenttemperatures

    andthesecondtransformationpointmovestohigher regionwiththeincreaseoftemperaturetestedfor thetwosamplesasshowninFig.3(c)andFig.4(C), implyingthatthehigherelasticityappearsearlierfor thesamplesathighertemperaturesduetocrosslinking Inparticular,tan8islessthan1withinthewhole measuredcoregionforthesamplestestedat160.C. Hence,forthesamplestestedat130,140and150.C.

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