jute ﬁbers with treated incorporated chemically pulverizednano/micro Modeling of the and creep analysis behavior of jute/green epoxycomposites Industrial Crops and Products

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IndustrialCropsandProducts84(2016)230–240

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Industrial Crops and Products

j o u r n a l h o m e p a g e :w w w . e l s e v i e r . c o m / l o c a t e / i n d c r o p

Modeling and analysis of the creep behavior of jute/green epoxy composites incorporated with chemically treated pulverized nano/micro jute ﬁbers

Abdul Jabbar

^a

, Jiˇrí Militk ´y

^a,∗

, Bandu Madhukar Kale

^a

, Samson Rwawiire

^a

, Yasir Nawab

^b

, Vijay Baheti

^a

aDepartmentofMaterialEngineering,TechnicalUniversityofLiberec,Studentská2,46117Liberec,CzechRepublic

bDepartmentofFabricManufacturing,NationalTextileUniversity,Faisalabad37610Pakistan

a r t i c l e i n f o

Articlehistory:

Received7November2015

Receivedinrevisedform7December2015 Accepted19December2015

Keywords:

Juteﬁber

Polymercomposites Creep

Greenepoxy Compressionmolding Pulverization

a b s t r a c t

Thispaperreportsthecreepbehaviorofalkalitreatedjute/greenepoxycompositesincorporatedwith variousloadings(1,5and10wt%)ofchemicallytreatedpulverizedjuteﬁbers(PJF)atdifferentenvi- ronmenttemperatures.Compositeswerepreparedbyhandlayupmethodandcompressionmolding technique.Thecreepanddynamicmechanicaltestswereperformedinthree-pointbendingmodeby dynamicmechanicalanalyzer(DMA).TheincorporationofPJFisfoundtosigniﬁcantlyimprovethecreep resistanceandstrainrateofcomposites.Threecreepmodelsi.e.Burger’smodel,Findley’spowerlaw modelandasimplertwo-parameterpowerlawmodelwereusedtomodelthecreepbehaviorinthis study.Thetimetemperaturesuperpositionprinciple(TTSP)wasappliedtopredictthelong-termcreep performance.TheFindley’spowerlawmodelwasfoundtobesatisfactoryinpredictingthelong-term creepbehavior.Dynamicmechanicalthermalanalysis(DMTA)resultsrevealedtheincreaseinstorage modulus,glasstransitiontemperatureandreductioninthetangentdeltapeakheightofcompositeswith higherloadingofPJF.

1. Introduction

Naturalfiberpolymercomposites(NFPC)areincreasinglyused nowadaysinindustrialapplicationsasasubstituteofpolymercom- positesmadewithmostlyusedsyntheticfiberssuchasglassand carbonetc.duetotheirenvironmentalandeconomicbenefits.Nat- uralfibersarerenewable,biodegradable,costeffective,safetouse, availableinhugequantities,lowfossil-fuelenergyrequirements andthemostimportantlytheirhighspecificstrength toweight ratio(JohnandAnandjiwala,2009;Mishraetal.,2003).Thisisof distinctiveimportanceespeciallyininteriortransportationappli- cationsasitleadstoreductionofvehicleweightforhigherfuel efficiencyandenergy saving.Polymercompositesusedinengi- neeringapplicationsareoftensubjectedtostressforalongtime andathightemperatures.Creep(aprogressivedeformationofa materialatconstantstress)isveryimportantend-usepropertyfor materialapplicationsrequiringlongtermdurabilityandreliabil- ity(KremplandKhan,2003).Considerablestudiescanbefoundin

∗ Correspondingauthor.

E-mailaddress:jiri.militky@tul.cz(J.Militk ´y).

literatureonthecreepbehaviorofnaturalﬁberpolymercompos- ites.Differentmathematicalmodelingtechniqueshavealsobeen appliedtoanalyzethecreepbehaviorofcompositematerials(Acha etal.,2007;BledzkiandFaruk,2004;Haoetal.,2014;Jiaetal., 2011;MarcovichandVillar,2003;Xuetal.,2010).Researchershave alsotriedtostudythecreepbehaviorofpolymercompositesby additionofdifferentkinds ofﬁllersinmatrices(Jiaetal.,2011;

Shenetal.,2004;Siengchin,2013;SiengchinandKarger-Kocsis, 2009;Yangetal.,2006a,b).Additionofnano/microfillerstopoly- mershasshownimprovementsinthestrengthandstiffnessofthe resultingcomposites,however,researchshowsthatthesefillers tendtoplasticizethecomposites.Totheauthor’sbestknowledge, thereisnostudyavailableinopenliteratureonthecreepbehavior ofalkalitreatedjutereinforcedgreenepoxycompositesincorpo- ratedwithchemicallytreatedpulverizedjutefibersasreinforcing fillers.Amongallthenaturalreinforcingmaterials,juteappearsto beapromisingfiberduetoitshightoughnessandaspectratioin comparisonwithothernaturalreinforcements(Achaetal.,2007) andoccupiesthesecondplaceintermsofworldproductionlevels ofcellulosicfibers(Caietal.,2000).Theobjectiveofthepresent studyistoinvestigatetheincorporationofpulverizednano/micro jutefiberspreparedfromwastejuteonthecreepbehaviorunder

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theconditionsofdifferenttemperaturesanddynamic mechani- calbehaviorofalkalitreatedwovenjute/greenepoxycomposites.

Themodelingofexperimentalcreepdataissatisfactorilyconducted usingdifferentcreepmodels.Furthermore,thetime-temperature superpositionprinciple(TTSP)isemployedinordertopredictthe long-termcreepbehavior ofcompositesbased onexperimental data.

2. Materialsandmethods

2.1. Materials

Jutewovenfabricproduced fromtossa jute(Corchorusolito- rius)ﬁbershavinganarealdensityof600gm⁻²with5-endsatin weavedesignwasproducedonashuttleloom.Warpandweftden- sitiesofthefabricwere6.3threadspercmand7.9threadspercm respectively.Wastejuteﬁbers,sourcedfromajutemill,wereused forpulverization.GreenepoxyresinCHS-EpoxyG520andhard- enerTELALIT0600weresuppliedbySpolchemie,CzechRepublic.

Sodiumhydroxide(NaOH)wassuppliedbyLach-Ner,CzechRepub- lic.Sodiumsulfate(Na₂SO₄)andsodiumhypochlorite(NaOCl)were suppliedbySigma–Aldrich,CzechRepublic.

2.2. Chemicalpretreatment

Theuntreated fabric wasfirst washed with2wt% non-ionic detergentsolutionat70^◦Cfor1.0hpriortosurfacetreatmentsto removeanydirtandimpuritiesstickingtothesurface.Thejutefab- ricandwastejutefiberswerethenimmersedseparatelyin2%NaOH solutionfor1hat80^◦Cmaintainingaliquorratioof15:1.Alkali treatedwastejutefiberswerefurthertreatedwith7g/lNaOClsolu- tionatroomtemperaturefor2hunderpH10–11andsubsequently antichloredwith0.1%Na₂SO₄at50^◦Cfor20min.Bothfabricand wastefibers,afterchemicalpretreatment,werethenwashedwith freshwaterseveraltimesuntilthefinalpHwasmaintainedat7.0 andthenallowedtodryatroomtemperaturefor48handat100^◦C inanovenfor2h.

2.3. Pulverizationofjuteﬁbers

Pulverizationofchemicallytreatedwastejuteﬁberswascarried outusingahigh-energyplanetaryballmillofFritschpulverisette7.

Pulverizationprocessreliesontheprincipleofenergyreleaseatthe pointofimpactbetweenballsaswellasonthehighgrindingaction createdbyfrictionofballsonthewall(BahetiandMilitky,2013).

Thesinteredcorundumcontainerof80mlcapacityandzirconium ballsof10mmdiameterwerechosenfor1hofpulverization.The balltomaterialratio(BMR)waskeptat10:1andthespeedwas keptat850rpm.

2.4. Preparationofcomposites

Thecompositeswerepreparedbyhandlayupmethod.Theresin andhardenerweremixedinaratioof100:32(byweight)according tomanufacturerrecommendationsandthenweighedamountsof pulverizedjutefibers(PJF)under1,5and10wt%weremechanically mixedwithepoxyresinatroomtemperatureuntilahomogeneous mixturewasobtained.Thepreparedresin/PJFmixturewaspoured onfabriclayersandspreadoutbyahandroller.Thegentlerolling actionofhandrollerconfirmedthewettingofjute fabrics.Each compositelaminatecomprised3layersofjutefabricwithorienta- tionofeachlayerinthesamedirection.Thecompositelayupalong withTeflonsheetsweresandwichedbetweenapairofsteelplates andcuredat120^◦Cfor1.0hinamechanicalconvectionovenwith predeterminedweightonittomaintainuniformpressureofabout 50kPa(Mishraetal.,2014).Thefibervolumefraction(V_f)ofall

compositeswasintherangeof0.25–0.27.Thepreparedcomposite samplesweredesignatedasU(untreated),A-0%(alkalitreatedjute fabricwith0wt%ofPJF),1%,5%and10%(alkalitreatedjutefabric with1,5,10wt%ofPJF)respectively.

2.5. Characterizationandtesting

2.5.1. Characterizationofpulverizedjuteﬁbers

Theuntreatedandtreatedjutefabricandfiberswereanalyzed byFTIRspectroscopy.AThermoFisherFTIRspectrometer,model NicoletiN10,wasusedinthisstudy.Thespectrometerwasusedin theabsorptionmodewitharesolutionof4cm⁻¹.Particlesizedistri- butionofpulverizedjuteparticleswasstudiedonMalvernZetasizer nanoseriesbasedondynamiclightscatteringprincipleofBrown- ianmotionofparticles.Deionizedwaterwasusedasdispersion mediumanditwasultrasonicatedfor5minwithBANDELINultra- sonicprobeSONOPLUSbeforecharacterization.Refractiveindexof 1.52wasusedtocalculateparticlesizeofpulverizedjute.Inaddi- tion,morphologiesofpulverizedjutefiberswereobservedwith Vega-TescanTS5130ScanningElectronMicroscopeat30KVaccel- eratingvoltage.ThesurfaceoffiberswasgoldcoatedpriortoSEM inspection.

2.5.2. Characterizationofcomposites

Short-termcreeptestswereperformedinthreepointbending modeattemperatures40^◦C,70^◦Cand100^◦CusingQ800Dynamic mechanicalthermalanalysis(DMTA)instrumentofTAinstruments (NewCastleDL,USA)for30min.Thestaticstressof2.0MPawas appliedat thecenter point of long sideof the samplethrough thesamplethicknessfor30minafterequilibratingatthedesired temperatureandcreepstrainwasmeasuredasafunctionoftime.

Thestaticstresswasselectedafterperformingastrainsweeptest, wherethelinearviscoelasticregionwasdeﬁnedforeach ofthe compositesensuringthatthecreeptestswereconductedinthe linearviscoelasticregion.TheTTSP wasselectedfor short-term creeptestsperformedatvarioustemperaturesforjute/greenepoxy compositesincorporatedwithvariouscontentsofPJF. Thetem- peraturerange was40–100^◦C,in5^◦Csteps,andtheisothermal testswererunonthesamespecimeninthetemperaturerange.

The2.0MPastresswasappliedfor10minateachtemperature.In everymeasurement,thespecimenwasequilibratedfor5minat eachtemperature,inordertoevenlyadjustforthecorrecttem- peratureofthespecimen.Thedynamicmechanicalpropertiesof composites weremeasuredin 3-pointbendingmodeusing the abovesameinstrument.Thetestingconditionswerecontrolledin thetemperaturerangeof35–200^◦C,withaheatingrateof3^◦C/min, ﬁxedfrequencyof1Hz,preloadof0.1N,amplitudeof20␮m,and forcetrackof125%.Thesampleshavingathicknessof4.5–5mm, widthof12mmandspanlengthof50mmwereusedforbothcreep andDMTAtesting.Tworeplicatesamplesweretestedforeachtest conditionandaveragevaluesarereported.

2.6. Creepmodeling

Thenon-linearcurvefitfunctionoftheOriginPro9.0software wasusedformodelingthecreepcurvesandfittingthemodelsto theexperimentaldata.Theminimumsumofsquareddeviationof experimentaldatafromthecreepmodelsandcorrelationcoeffi- cient(R²)wereselectedascriterion(MelounandMilitky,2011).The correlationcoefficientvalueR²isdefinedasmodelsumofsquares dividedbytotalsumofsquares.Abettergoodness-to-fitisobtained whenR²iscloserto1.

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232 A.Jabbaretal./IndustrialCropsandProducts84(2016)230–240

Fig.1. FTIRspectraofjuteﬁbers.

3. Resultsanddiscussion

3.1. Characterizationofjuteﬁbers

FTIRanalysiswascarriedouttoconﬁrmsomeremovalofnon- cellulosiccontents(e.g.hemicellulosesandlignin)fromthesurface ofjuteﬁbersafterpretreatments.TheFTIRspectraofuntreatedand treatedjute areshownin Fig.1.Themajordifferenceobserved betweenthespectrais thedisappearance/reductionof peaksat

∼1730cm⁻¹ and∼1240cm⁻¹.Thepeakat∼1730cm⁻¹ isdueto stretchvibrationofC Obondsincarboxylicacidandestercom- ponentsofcelluloseandhemicellulose(Morshedetal.,2010)and alsoachemicalgroupoflignin(Haqueetal.,2009;Tserkietal., 2005). The peak at ∼1240cm⁻¹ is due to C O C asymmetric stretchingoftheacetylgroupoflignin(Liuetal.,2004).Thereduc- tion/disappearanceofthesepeaksconﬁrmthepartialremovalof hemicelluloseandopeningupoftheligninstructureinthejute ﬁbersafterpretreatments.

Thejuteﬁberswerepulverizedtoparticleswithanaveragesize of1480nminwiderparticlesizedistributionasshowninFig.2a.

Fig.2bshowstheshapeandsurfacemorphologyofpulverizedjute ﬁbers.ThePJFcanbeseeninirregularshapeandsizewithcertain aspectratioaswell.

3.2. Creepbehavior

3.2.1. Creepmodels

Fourparameters(ortheBurger’s)modelisoneofthemostly usedphysicalmodelstogivetherelationshipbetweenthemor- phologyofpolymercompositesandtheircreepbehavior(Findley et al.,1989; Ward and Sweeney, 2012). It is based ona series combinationofaMaxwellelementwithaKelvin–Voigtelement.

Thetotalcreepstrainisdividedintothreeseparateparts:ε_Mthe instantaneouselasticdeformation(Maxwellspring),εKviscoelas- ticdeformation(Kelvinunit)andε_∞viscousdeformation(Maxwell dash-pot).Thus,totalstrainasafunctionoftimecanberepresented bythefollowingequations:

ε (t)=εM+εK+ε_∞ (1)

ε (t)= ₀ E_M+₀

E_K(1−e⁻EKt

⁄

K)+ ₀

_Mt (2)

Whereε(t)isthecreepstrain,0isthestress,tisthetime,EMand E_KaretheelasticmoduliofMaxwellandKelvinsprings,and_M andKaretheviscositiesofMaxwellandKelvindashpots.K/EK

isusuallydenotedas,theretardationtimerequiredtogenerate 63.2%deformationintheKelvinunit(Yangetal.,2006b).ε_Misa constantvalueanddoesnotchangewithtime.ε_Krepresentsthe earlieststageofcreepandattainsasaturationvalueinshorttime andε_∞representsthetrendinthecreepstrainatsufﬁcientlylong time,andappearssimilartothedeformationofaviscousliquid obeyingNewton’slawofviscosity.ThefourparametersEM,EK,M,

_KcanbeobtainedbyﬁttingtheEq.(2)totheexperimentaldata andcanbeusedtodescribethecreepbehaviorofcomposites.The creeprateofviscoelasticmaterialscanbeobtainedbytakingthe derivativeofEq.(2).

dε (t) dt =0

E_K(e⁻E_Kt

⁄

_K)+ 0

_M (3)

TheFindley’spowerlawmodelisanempiricalmathematical modelusedtosimulatethecreepbehaviorofpolymercomposites.

ThemodelcanberepresentedbythefollowingEq.(4)(Findleyetal., 1989);

ε (t)=at^b+ε₀ (4)

Where,aandbarethematerialconstantsandε₀istheinstanta- neousstrain.TheabilityofFindley’spowerlawmodeltosimulate thecreepdatahasbeenfoundtobesatisfactoryinseveralstud- ies(Jiaetal.,2011;PlaseiedandFatemi,2008;Yangetal.,2006b).

However,thismodelisnotabletoexplainthecreepmechanism ofmaterial.Atwoparameterempiricalpowerlawmodelhasalso beenusedinsomestudies(Tajvidietal.,2005;Xuetal.,2010)to simulatethecreepdate.Ithastheform;

ε (t)=at^b (5)

Where,aandbarethematerialconstantsandε0istheinstanta- neousstrain.

Thelongtermcreepisanimportantparametertoevaluatethe end-useperformanceofnaturalﬁberpolymercompositesbutit

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Fig.2. (a)Particlesizedistributionand(b)SEMimageofjuteﬁbersafterpulverization.

isoftenimpracticaltoperformacreeptestforanextremelylong period oftime. Time-temperaturesuperposition (TTS)isone of thecommonestimationtechniquestopredictthelongtermcreep behaviorbyshiftingthecurves fromtestsatdifferenttempera- tureshorizontallyalongthelogarithmictimeaxistogeneratea singlecurveknownasmastercurve(WardandSweeney,2012).

Theshiftingdistanceiscalledshiftfactor.

3.2.2. Shorttermcreep

Fig.3showsthecreepstrainsforjutecompositesasafunction oftimewith0,1,5,10wt%ofPJFcontentatthreedifferenttem- peratureconditions.Itisvisiblyapparentthatthecompositeshave lowinstantaneousdeformationε_Mandcreepstrainat40^◦Cdue tohigherstiffnessofcompositesbutthisdeformationincreasesat highertemperaturesduetodecreaseincompositesstiffness.The creepstrainofallcompositesalsoincreasedathighertempera- turesbuttheuntreatedjutecompositeswasaffectedmorethan

theothers. The creepstrain of alkalitreated with0% PJF com- positeislessthanuntreatedone.Thismaybeexplaineddue to increaseinsurfaceroughnessofjutefabricafteralkalitreatment and decrease infrictional slippageof matrixpolymer chainsat theﬁber/matrixinterfaceresultinginlesscreepdeformationthan untreatedcomposite.Theleastcreepstrainisshownbycompos- iteincorporatedwith10%PJFatalltemperaturesfollowedby5%

and 1%PJF incorporatedcomposites. At100^◦C,5% and 10%PJF compositeshavealmostsameinstantaneouselasticdeformation but10%composite haslessviscousdeformationovertime. This maybeattributedtogreaterinhibitionofslippageandreorienta- tionofpolymerchainwithincreasingcontentsofPJF.TheBurger’s modelcurvesshowasatisfactoryagreementwiththeexperimen- taldata(Fig.3).ThefourparametersEM,EK,M,K of Burger’s model,usedtoﬁttheEq.(2)totheexperimentaldata,aresum- marizedinTable1.Allfourparameterswerefoundtodecreasefor allcompositesastemperatureincreased(Table1).EMcorresponds

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Fig.3.CreepcurvesofcompositesincorporatedwithdifferentloadingsofPJFatdifferenttemperatures.

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Fig.3.Continued.

Table1

SimulatedfourparametersinBurger’smodelforshorttermcreepofthecomposites.

Temperature Parameters Compositetypes

Untreated Alkali-0% 1% 5% 10%

40^◦C EM(MPa) 2477.24±78.3 3259.41±138.0 3492.53±133.8 3810.53±133.6 3774.75±149.1 EK(MPa) 23876.27±10854.5 38244.62±19726.9 42496.38±21110.6 44220.12±21281.1 40549.80±19016.6

M(Pas) 2.72E7±1.33E7 3.54E7±1.22E7 4.77E7±2.04E7 4.54E7±1.97E7 5.11E7±2.47E7

K(Pas) 2.11E6±2.38E6 1.37E6±2.09E6 1.73E6±2.5E6 2.53E6±3.49E6 1.90E6±2.62E6

SS^* 2.65033E-9 1.29037E-9 1.04302E-9 9.92292E-10 1.08146E-9

Adj.R² 0.97524 0.97061 0.96424 0.96929 0.96304

70^◦C EM(MPa) 1985.89±91.9 2403.61±102.7 2921.19±118.5 3116.67±118.6 3323.10±131.9 EK(MPa) 7790.84±2752.5 11972.90±4497.3 14228.47±5360.3 16946.57±6037.4 15615.45±5726.5

M(Pas) 9.48E6±3.81E6 1.32E7±5.14E6 1.98E7±9.71E6 2.33E7±1.08E7 2.27E7±1.13E7

K(Pas) 956179.43±6.90E5 1.32E6±1.09E6 1.71E6±1.34E6 1.81E6±1.45E6 1.94E6±1.44E6

SS^* 1.08937E-8 5.93037E-9 3.81543E-9 2.74811E-9 2.87718E-9

Adj.R² 0.98881 0.98689 0.9851 0.9847 0.98596

100^◦C EM(MPa) 1380.05±294.3 1743.43±306.1 2417.83±218.3 2653.56±183.6 2616.45±123.7 EK(MPa) 665.35±117.7 865.63±121.6 3543.54±958.0 6087.72±1783.7 8457.41±2683.5

M(Pas) −4.13E20±0.0 −6.00E32±0.0 7.44E6±3.92E6 1.08E7±5.26E6 1.31E7±5.97E6

K(Pas) 219089.75±1.03E5 255733.51±9.91E4 457813.67±2.43E5 705184.01±4.39E5 1.11E6±6.84E5

SS^* 8.30033E-7 3.37586E-7 2.87859E-8 1.31777E-8 6.79893E-9

Adj.R² 0.97565 0.98358 0.99023 0.98826 0.98971

SS^*:Sumofsquareddeviations.

totheelasticityofthecrystallizedzonesinasemicrystallizedpoly- mer.Comparedtotheamorphousregions,thecrystallizedzones aresubjectedtoimmediatestressduetotheirhigherstiffness.The instantaneouselasticmodulusisrecoveredimmediatelyoncethe stressis removed.E_Kisalsocoupledwiththestiffnessofmate- rial.ThedecreaseinparametersE_M,E_Kresultedfromtheincrease intheinstantaneousandtheviscoelasticdeformationsastemper- atureincreased.TheviscosityMcorrespondstodamageinthe crystallizedzonesandirreversibledeformationintheamorphous regionsandtheviscosityKisassociatedwiththeviscosityofthe amorphousregionsinthesemicrystallizedpolymer(Militk ´yand Jabbar,2015).The decreaseinviscosityparameters_M,_Kpro- poseanimprovementinthemobilityofmolecularchainsathigher

temperature.Theparametersforuntreatedandalkalitreatedwith 0%PJFcompositeshaveundergonealargestdecrease,resultingin highercreepstrain.ThecompositesincorporatedwithPJF,espe- cially5and10%,havecomparativelybettervaluesofparameters particularly_Mwhichisrelatedtothelongtermcreepstrainand validateslesstemperaturedependenceofthesecomposites(Fig.3).

TheviscosityMincreaseswiththeincreaseinPJF%andpermanent deformationdecreases.Fig.4aandbcomparesthecreepstrainand strainrateofuntreatedand10%PJFcompositesatvarioustem- peratures.Comparatively,temperaturehadmoreinﬂuenceonthe creepdeformationofuntreatedjutecompositethanthatof10%PJF composite.

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Fig.4.Creepstrain(a)andstrainrate(b)ofuntreatedand10%PJFcompositesatdifferenttemperatures.

3.2.3. Time-temperaturesuperposition(TTS)

Thecreepcurvescorrespondingtodifferenttemperaturelev- els were shifted along the logarithmic time axis according to time–temperaturesuperpositionprinciple usingTAInstruments ThermalAdvantage^TMsoftwaretogenerateamastercurveataref- erencetemperatureof40^◦C.Theshiftingprocedureofthiscurve

obeystheWilliams–Landel–Ferry(WLF)equation.TheWLFequa- tionisgivenbyEq.(6);

log˛_T= −C1(T−T0)

C2+ (T−T0) (6)

Where␣T isthehorizontal(ortime)shiftfactor,C₁ and C₂ are constants,T0 isthe referencetemperature(K) andT is thetest

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Fig.5.TTSmastercurvesforcreepofthecompositesincorporatedwithdifferentloadingsofPJFatareferencetemperatureof40^◦C.

temperature(K).Themastercurves,whichgiveanindicationof long-termcreepperformanceofcomposites,areplottedinlog–log scaleandpresentedinFig.5.Themastercurvesshowbettercreep resistanceofcompositeswithincreasingcontentofPJF.Itisobvi- ousthatthebestlongtermperformanceisshownbycomposites incorporatedwith5and10%PJFindicatingtheirgoodreinforcing effectiveness.Itisalsointerestingtonotethatabovelog-time4.0s, thecreepdeformationofuntreated,0and1%compositesshowa fastertendencyofincreasecompared to5and10%PJFcompos- ites.Theseﬁndingsshowthatunderthesmallstress,thematerials enteredintoaviscoelasticstateoveranextremelylongperiodof time,andinviscoelasticstatetheroleof1%incorporationofPJFin thereinforcementeffectivenessislessthanthatof5and10%PJF.

Fig.6showsthatthepredictionabilityoftheBurger’smodel, Findley’smodelandtwo parameterpowerlawmodel(Eqs. (2), (4)and(5))forthelog-termcreepbehaviorofcompositesatthe referencetemperatureof40^◦C.TheincorporationofPJFincom- positesdecreasedthelongtermcreepstraincompareduntreated and0%ﬁlledalkalitreatedcomposites.Itcanbeclearlyseenthat Findley’smodel is good to predictthe long-termcreep performance. However, this model provides the adequate prediction abilitywithinthesteady statecreepandgiven time intervalas revealed by some researchers (Siengchin, 2013; Siengchin and Karger-Kocsis,2009)whileforlongertimedurationthecalculated datamayshowconsiderabledeviationfromtheexperimentaldata.

ThepredictionoftheBurger’smodelshowssomedeviationand thatoftwoparameterpowerlawmodel,showsalittlelargedevi- ationfromtheexperimentaldata.Similarﬁndingswerereported byotherresearchers(Haoetal.,2014;Jiaetal.,2011).Thesim- ulatedparametersofBurger,Findleyandtwoparameterspower

lawmodelsaresummarizedinTable2.TheparametersofBurger model,resultedfromﬁttingcreepcurves,areverydifferentfrom thoseoftheshorttermcreeptests.It canbeseenfromTable2 thatalltheBurgermodelparametersincreasewiththeincrease inloadingofPJF%.The_M,whichdetermineslong-termcreep,is lowestforuntreatedcompositeandhighestfor10%PJFcomposite.

Therefore,theuntreatedcompositeshowsthehighestand10%PJF compositeshowsthelowestcreepdeformation.Itisalsoobvious forFindleymodelthattheparametera(reflectingshort-termcreep) increasedandparametersε₀ (reflectingtheinstantaneousinitial creepstrain)andb(reflectinglog-termcreep)decreasedwiththe increasingcontentofPJFwhichindicatesanenhancedlong-term creepperformancewithPJF loading.Similarly, fortwo parameterpowerlawmodel,parametera(reflectingshort-termcreep) increasedandparameterb(reflectinglog-termcreep)decreased withtheincreasingcontentofPJF.

3.3. Dynamicmechanicalthermalproperties

Dynamicmechanicalthermalanalysiscancharacterizethevis- coelasticpropertiesofthematerialsanddeterminetheinformation ofstoragemodulus,lossmodulus(theenergydissipationassoci- atedwiththemotionofpolymerchains)andlossfactor(tandelta) ofpolymer compositeswithinthemeasuredtemperaturerange (Wangetal.,2015).Thevariationofstoragemodulus(E)ofcom- positesincorporatedwithdifferentcontentofPJFasafunctionof temperatureatfrequencyof1HzisshowninFig.7.Itcanbeseen fromFig.7athatthereisagradualfallinthestoragemoduliwith temperature,whichshouldberelatedwithanenergydissipation phenomenoninvolvingcooperativemotionsofthepolymerchains

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Fig.6. Long-termcreeppredictionofcompositesby(a)Burger’smodel,(b)Findleymodeland(c)twoparameterpowerlawmodelat40^◦C.

Table2

SimulatedparametersofBurger’smodel,Findley’spowerlawmodelandtwoparameterspowerlawmodelforlongtermcreeppredictionofthecompositesat40^◦C.

Temperature Parameters Compositetypes

Untreated Alkali-0% 1% 5% 10%

Burger’s model

EM(MPa) 2713.76±193.6 3002.45±184.0 2904.39±179.0 3478.01±191.0 3517.07±125.5 EK(MPa) 4781.98±2005.8 5470.29±2194.5 6271.53±2337.0 15949.06±7113.4 31490.67±16174.2

M(Pas) 2.64E9±1.29E9 2.94E9±1.29E9 4.01E9±2.19E9 9.44E9±6.93E9 2.10E10±2.04E10

K(Pas) 1.0227E8±8.73E7 1.35E8±1.05E8 9.24E7±7.83E7 6.76E7±9.00E7 9.49E7±1.53E8

SS^* 4.86566E-7 3.00571E-7 2.96188E-7 1.2249E-7 4.56987E-8

Adj.R² 0.9696 0.97551 0.9658 0.92279 0.88442

Findley’s model

a 1.05E-5±4.83E-06 6.49E-6±3.69E-06 1.79E-5±1.14E-05 3.49E-5±3.45E-05 3.35E-5±5.92E-04 b 0.34341±3.39E-02 0.3699±4.22E-02 0.28033±4.63E-02 0.17143±6.61E-02 0.1284±2.27E+00 ε0 6.70E-4±2.68E-05 6.17E-4±2.58E-05 6.13E-4±3.82E-05 5.00E-4±5.74E-05 5.11E-4±9.03E-03

SS^* 3.83097E-8 4.27255E-8 4.37949E-8 1.91548E-8 1.30874E-8

Adj.R² 0.99761 0.99653 0.99496 0.98797 0.96701

Two parameter powerlaw model

a 4.63E-4±1.41E-04 4.24E-4±1.39E-04 4.82E-4±1.00E-04 4.85E-4±3.61E-05 5.22E-4±2.08E-05 b 0.08443±3.37E-02 0.08145±3.64E-02 0.06828±2.37E-02 0.03723±9.03E-03 0.01951±5.01E-03

SS^* 2.4937E-6 2.31209E-6 1.0253E-6 9.22041E-8 2.50976E-8

Adj.R² 0.8452 0.8128 0.88238 0.94226 0.93694

SS^*:sumofsquareddeviations.

withtemperature(Fengetal.,2011).Theincreaseinstoragemodu- lusoverthewholetemperaturerangewasobservedforcomposites incorporatedwithdifferentloadingsofPJF,forexample,additionof 1,5and10%PJFcausesasigniﬁcantincreaseof∼18%,22%and43%

inthestoragemodulusrespectivelyat35^◦C.Moreover,thestorage moduluscurvesofcompositeshavebeenshiftedtohighertemper- aturesafteradditionofthePJF,particularly5and10%loading.This signiﬁcantimprovementinstoragemodulusisduetobetterrein- forcingeffectofPJFleadingtoincreasedstiffnessandthemobility restrictionofpolymerchains(Jiaetal.,2011).Thechangeinloss

factor(tanı,theratiooflossmodulustocorrespondingstorage modulus)ofcompositeswithdifferentloadingofPJFasafunction oftemperatureisshowninFig.7c.Untreatedcompositedisplayed ahighertanıpeakvaluethanothers.Thismaybeattributedto moreenergydissipationduetofrictionaldampingattheweaker untreatedﬁber/matrixinterface.Thetemperatureatwhichtan␦ attainsamaximumvaluecanbereferredtoastheglasstransition temperature(Tg)(ShanmugamandThiruchitrambalam,2013).A positiveshiftinTgcanbeobservedforallcompositesincorporated withPJFcomparedtountreatedcompositeThelowertan␦peak

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Fig.7.DynamicmechanicalpropertiescompositesincorporatedwithdifferentloadingsofPJF;(a)storagemodulus,(b)lossmodulus,(c)tandelta.

Table3

TgvaluesobtainedfromEcurves.

Composites TgfromEmaxcurve(^◦C)

Untreated 110.10

Alkali-0% 110.60

1% 114

5% 123

10% 137

heightisshownbycompositeincorporatedwith10%PJFfollowed by5%and1%PJFcomposites,exhibitingastrongﬁber/matrixinter- facialinteractionswhichcanrestrictthesegmentalmovementof thepolymerchainsleadingtotheincreasedTg.

IthasbeenreportedthatTgvaluesobtainedfromlossmodulus (E)curvepeakaremorerealisticascomparedtothoseobtained fromlossfactor(tanı)(Akay,1993).ApositiveshiftinTgtohigher temperatureforallcompositesincorporatedwithPJFisobserved, i.e.Tgincreasedfrom110.1^◦Cforuntreatedto∼110.6,114,123and 137^◦Cforcompositesincorporatedwith0,1,5and10%PJFrespec- tivelyaspresentedinTable3andFig.7b.Thismaybeduetoreduced mobilityofmatrixpolymerchainsandbetterreinforcementeffect ofPJF.Ithasbeenreportedthatsystemscontainingmorerestric- tionsandahigherdegreeofreinforcementtendtoexhibithigher Tg(Almeidaetal.,2012).

4. Conclusion

Creepbehaviorofalkalitreatedwovenjute/greenepoxycom- positesincorporatedwithdifferentloadingsofchemicallytreated pulverizedjuteﬁbers(PJF)waspresentedatvariousenvironment temperatures.Thecreepdeformationwasfoundtoincreasewith

temperature. The creep resistanceof composites was found to improvesigniﬁcantlywiththeincorporationofPJF.Themodeling ofcreepdatawassatisfactorilyconductedbyusingBurger’smodel, Findley’spowerlawmodelandasimplertwo-parameterpowerlaw model.TheBurger’smodelﬁttedwelltheshort-termcreepdata.

However,theFindley’spowerlawmodelwassatisfactoryforpre- dictingthelong-termcreepperformanceofcompositescompared toBurgerandtwoparametermodel.Themastercurves,generated byTTSprinciple,indicatedthepredictionofthelong-termperfor- manceofcomposites.Dynamicmechanicaltest resultsrevealed theincreaseinstoragemodulus,glasstransitiontemperatureand reduction in the tangent delta peak heightof PJF incorporated composites.Basedontheanalysisofresults,theimprovedcreep resistanceofthecompositeswerelikely attributedtotheinhib- itedmobilityofpolymermatrixmolecularchainsinitiatedbylarge interfacialcontactareaofPJFaswellastheirinterfacialinteraction withthepolymermatrix.

Acknowledgements

Thisworkwassupportedunderthestudentgrantscheme(SGS- 21085)byTechnicalUniversityofLiberec,CzechRepublic.Oneof theauthorsisthankful toDr.Ing.Antonín Potˇeˇsiland Ing.Petr HornikforprovidingDMAtestingfacilities.

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