Twin-screw extrusion impact on natural fibre morphology and material properties in poly(lactic acid) based biocomposites

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Twin-screw extrusion impact on natural fibre

morphology and material properties in poly(lactic acid) based biocomposites

Guillaume Gamon, Philippe Evon, Luc Rigal

To cite this version:

Guillaume Gamon, Philippe Evon, Luc Rigal. Twin-screw extrusion impact on natural fibre morphol-

ogy and material properties in poly(lactic acid) based biocomposites. Industrial Crops and Products,

Elsevier, 2013, vol. 46, pp. 173-185. �10.1016/j.indcrop.2013.01.026�. �hal-00790824�

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To link to this article : DOI: 10. 1016/j.indcrop.2013.01.026 URL : http://dx.doi.org/10.1016/j.indcrop.2013.01.026

To cite this version : Gamon, guillaume and Evon, Philippe and Rigal, Luc Twin-screw extrusion impact on natural fibre morphology and material properties in poly(lactic acid) based biocomposites. (2013) Industrial Crops and Products, vol. 46 . pp.

173-185. ISSN 0926-6690

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Twin-screw extrusion impact on natural fibre morphology and material properties in poly(lactic acid) based biocomposites

G. Gamon

, Ph. Evon

, L. Rigal

aUniversitédeToulouse,INP,LaboratoiredeChimieAgro-industrielle,ENSIACET,4AlléeEmileMonso,BP44362,31030ToulouseCedex4,France

bINRA,LaboratoiredeChimieAgro-industrielle,31030ToulouseCedex4,France

Keywords:

Twin-screwextrusion Biocomposite Poly(lacticacid) Naturalfibre

Mechanicalandthermalproperties

a b s t r a c t

Naturalfibresfrommiscanthusandbamboowereaddedtopoly(lacticacid)bytwin-screwextrusion.

Theinfluenceofextruderscrewspeedandoftotalfeedingratewasstudiedfirstonfibremorphologyand thenonmechanicalandthermalpropertiesofinjectedbiocomposites.Increasingthescrewspeedfrom 100to300rpmsuchasincreasingthefeedingrateinthesametimeupto40kg/hhelpedtopreservefibre length.Indeed,ifshearratewasincreasedwithhigherscrewspeeds,residencetimeintheextruderand blendviscositywerereduced.However,suchconditionsdoubledelectricalenergyspentbyproduced matterweightwithoutsignificanteffectonmaterialproperties.

Thecomparisonoffourbamboogradeswithvariousfibresizesenlightenedthatfibrebreakageswere moreconsequentwhenlongerfibreswereaddedintheextruder. Longerfibreswerebeneficialfor materialmechanicalpropertiesbyincreasingflexuralstrength,whileshortfibresrestrainedmaterial deformationunderheatbypromotingcrystallinityandhinderingmorechainmobility.

1. Introduction

Environmentalconcernshaveledoverthepastyearstogrowing researchovernewsolutionsforplastics.Workover“greenplastics”

havebeenmotivatedbytwospecificgoals:reducingdependenceof plasticproductiononpetroleumsupplies,thatwilldecreaseinthe future,anddevelopingsolutionstoplasticwasteaccumulation.The developmentofbiobasedandbiodegradablepolymersisa‘cradle tograve’approachaimingtouserenewableresourcesandtolimit waste.

Thermoplastic starch (TPS), polyhydroxyalkanoates (PHAs), polylactidesandtheirblendsarepromising candidatesfor such replacement and are subject to many researches. Poly(lactic acid)(PLA)hasbeenintensivelyinvestigated inpastyears.This biodegradablepolyester,whichcanbeusedinmanyapplications from packaging to biocompatible materials, has a thermoplas- ticbehaviourcombinedwithhighmechanicalperformance,good appearanceandlowtoxicity(Jamshidianetal.,2010).

Blending polyolefins and polyesters withnatural fibres is a knownwaytoreduceproductioncostswhilesavingorincreas- ingthematrixproperties(Mohantyetal.,2000;Faruketal.,2012).

Naturalfibresarerenewable,andtheyhavetheadvantagesover

∗Correspondingauthorat:Agromat,sitedel’ENIT,47Avenued’Azereix,BP1629, 65016TarbesCedex,France.Tel.:+33562446084;fax:+33562446082.

E-mailaddresses:guillaume.gamon@ensiacet.fr(G.Gamon), philippe.evon@ensiacet.fr(Ph.Evon),luc.rigal@ensiacet.fr(L.Rigal).

glassorcarbonfibrestobeabundantandcheaper.Moreover,they haveahightoughness–densityratioandgoodthermalproperties ontheinsulationviewpoint.KymäläinenandSjöberg(2008)refer- encedthethermalconductivityofdifferentflaxandhempfibre mats(from33to94mW/mK)andshowedthatitwascomparable tothermalconductivitiesofglasswool(50mW/mKand below) orstonewool(from35to71mW/mK).Forcomparison,Nature- WorksLLCannouncedthermalconductivityof160mW/mKforits PLAgrades.Despitetheseadvantages,fibrehydrophilicbehaviour canbeasourceofincompatibilitywiththematrixandcanincrease biocompositemoisture-sensitivity.PLA-basedbiocompositeshave beenwidelystudiedandseveraltypesofvegetalfibreshavebeen incorporated(Faruketal.,2012).Fibrescomingfromwood(Huda etal.,2006;Sykaceketal.,2010),flax(Oksmanetal.,2003),hemp (Masireketal.,2007),kenaf(Ogbomoetal.,2009)andjute(Plackett, 2004)have been tested among others. Lezak et al. (2008) and Nyambo et al. (2010)also studiedtheincorporation of various agriculturalresidues.Fromthesestudies,itwasrevealedthatthe fibretypehasarealimportanceoncompositeperformance.They enlightenedalackofinteractionsbetweenthePLAandthetested fibresresultinginaweakinterfaceunabletotransferefficiently stressfromthematrixtothereinforcingfibreduringmechanical solicitation.Thisresultedinmechanicalstrengthreductionwith- outanychemicalorphysicalcompatibilisationmadetoenhance fibre–matrixinterface.

Miscanthus(Miscanthusgiganteus)andbamboo(Thyrsostachys oliverii)aretwoperennialcropscharacterizedbyhighyields.Mis- canthusisarhizomatousgrassrelatedtosugarcanethatoriginated

fromSoutheastAsia,andwasoriginallyintroducedintoEuropeas anornamentalgardengrass(Johnsonetal.,2005).Bamboogrows upto40mofheightinmonsoon climates.Generally,it isused inconstruction,carpentry,weavingandplaiting,etc.(Faruketal., 2012).Bothmiscanthusandbambooattractedattentionforfuel production(Hongetal.,2011)butalsoforbiocompositesreinforce- ment.BourmaudandPimbert(2008)tested,bynanoindentation, modulus and hardness of miscanthus fibre being 9.49GPa and 0.34GPa,respectively,showingittohavemodulusbetweensisal (8.52GPa)andhemp(12.14GPa).Theyworkedtoincorporatemis- canthusinPLAandpolypropylene(PP)too,showingcomparable performance toother fibres. Johnsonet al. (2005) alsoworked toblend miscanthuswithMater-Bi^® toimproveitsimpactper- formance. Mater-Bi^® had a 0.8J puncture energy, tested by an instrumentedfallingdartimpacttester,thatwasincreasedupto 1.9Jwithmiscanthusfibres.Okuboetal.(2004)comparedbamboo fibrepropertiestothoseofjutefibres.Testedbamboofibreshada 441MPatensilestrengthanda35.9GPamodulus,whatwashigher tojutefibresstrengthandmodulus(370MPaand22.7GPa,respec- tively).BamboowasinvestigatedwithPLAbyTokoroetal.(2008).

Theyprovedthatdependingontheextractionmethodofbamboo fibres,theseonescanreinforceornotmaterialbendingstrength.

Shortbundles(215mminlengthand39mmindiameter)wereless efficientthanlongerfibresfromsteamexplodedpulp(1740mmin lengthand24mmindiameter),andtheirrespectivestrengthswere around80MPaand115MPa.Theyalsoshowedthat3mmbamboo fibrebundlescouldimprovePLAIzodimpactstrengthfrom1.5to morethan5kJ/m²,while0.2mmbundlesdecreaseditto1kJ/m².

Theprocessusedforbiocompositeproductionhasahighimpor- tancetoo.Agoodfibredispersionisneededtoaimgoodmaterial performance.Fibreorientationwillplaya rolesincefibresrein- forcemorethematerialintheirlongitudinaldirection(Josephetal., 1999).Inaddition,manymodelsoncompositeshaveenlightened theimportanceoffibreaspectratioinmechanicalproperties,which isdefinedbytheratiobetweenitslength(L)anditsdiameter(d).

Accordingtothem,keepingahighaspectratiobringsmorestiffness tothebiocomposite.Shearappliedduringthecompoundingand mouldingprocesseswillcausefibrebreakages.However,natural fibres,oftenlinkedtogetherbypecticsubstancesintobundles,have theabilitytogetseparatedundershear,whatreducestheirfinal diameter.Josephetal.(1999)showedthatincreasingrotorspeed duringPP-sisalmelt-mixingcaused more fibrebreakage witha largeincreaseofsmalllengthfractioninfibresizedistribution.Also, lessfibrebreakagewasobservedwhentemperaturewashigher andsoreducedblendviscosity.LeDucetal.(2011)investigated flaxfibrebehaviourundershearinarheo-opticalsystem,observ- ingseveralfibrebendingsbeforerupture.Theyalsoobserved a highlossinlengthaftercompoundinginaninternalmixer.Flax fibreswitha10mminitiallengthwerereducedtoa96mmaver- agelengthinnumber.Twin-screwextrusionisahighshearprocess thatcanhelptomatchagoodfibredispersion.Bledzkietal.(2005) observedbettermechanical propertiesbycompounding PP and woodinatwin-screwextrudercomparedtohighspeedmixerand two-rollmill.Nevertheless,extrusionleadstoseveralfibrebreak- ages.Tokoroetal.(2008)haveseenlengthreductionfrom215to 86.3mmanddiameterreductionfrom39.2to21.3mmforshort bamboofibrebundles,afterextrusionandinjection.Wollerdorfer andBader(1998)observedthis drasticshorteningoffibrestoo, inthecaseofflax-basedbiodegradablepolymercomposites.They sawdifferencesin fibrelengthdistributions, due tothechosen matrixanditsrheology.Indeed,distributions,afterfibreaddition inBionolle^®withlowmeltviscosity,werewiderwithlongfibres thandistributionsforTPSorBiocell^®biocomposites.Byincreasing thefibrecontentinTPSfrom10to20%,theyshowedconsiderable increaseofthefibrepercentageinthelowerlengthfractionunder 200mm.BeaugrandandBerzin(2012)correlatedhempfibrelength

reductiontotheincreasingspecificmechanicalenergy(SME)spent duringcompoundingbytwin-screwextrusioninpolycaprolactone (PCL)matrix.The couplebarreltemperatureand fibremoisture contentwasalsofoundtohaveanimportance.Fibrelengthand aspectratioweremorepreservedwitha100^◦Cbarreltempera- tureanda22.5wt%moistureinfibre,thanwith140^◦Cand9.8wt%

moisture.Twostudiesobservedthatfibrebreakageandseparation frombundletoelementaryfibrewouldbemoreorlessimportant dependingonthefibreorigin.First,Oksmanetal.(2009)compared sisal,flax,bananaandjute,andfoundthatflaxfibresobtainedby enzymaticrettingprocessandhavinglowlignincontent,arebet- terseparatedthantheothersinnaturalfibrereinforcedPP.InLe Moigneetal.(2011)study,flaxfibreswerealsoseparatedinele- mentaryfibreswhilesisalfibresremainedpartlyinbundlesand wheatstrawprovidedbundlesandlargeamountsofsmallparticles.

Thisstudyaimedtoinvestigatetheinfluenceofthecompound- ingparametersonnaturalfibremorphology,mechanicalproperties andthermalpropertiesinpoly(lacticacid)basedbiocomposites.

Forthis,miscanthusfibreswerecompoundedtoaPLAcommer- cialgradeinatwin-screwextruderatdifferentfeedingratesand differentscrewspeedstocontrolshear inthemachine,andthe miscanthus/poly(lacticacid)blendsweretheninjected.Thefibre sizepreservationdependingonitsinitialsizewasalsostudied.For that,differentcalibratedgradesofbamboofibreswereusedand comparedtomiscanthus.

2. Materialsandmethods 2.1. Materials

Fibresareavailablecommercialgrades.Miscanthusfibres(MIS) wereprovidedbyMiscanthusGreenPower(France),andbamboo fibreswereprovidedbyBambooFibersTechnology(France).Four differentgradesofbamboofibres,namedB1toB4fromthelonger totheshorterone,wereusedforthisstudy.Thechemicalcomposi- tionsandinitialmorphologiesofthefivefibrestestedarereported inTable1.SEMimagesofthedifferentfibresbeforecompounding, takenwithaJEOLJSM-700F(Japan)scanningelectronmicroscope, usinga5kVacceleratingvoltage,areshowninFig.1.Fibreswere vacuum-coatedtwotimeswithpalladiumforobservationtoavoid chargingundertheelectronbeam.Itcanbeseenthatfibreswere heldtogetherinbundles.Moreover,itwasnoticedthatcontraryto miscanthusbundles,bamboobundlesexhibitedfibreseparationat theirend.TheshorterbamboogradeB4wascomposedofsmaller bundlesonwhichthisfibreseparationwaslessvisible.

Poly(lacticacid)wasaNatureworksLLC(USA)Ingeo^TMgrade, anditwassuppliedintheformofgranules.

2.2. Fibrechemicalcomposition

Characterizationof thedifferentfibrechemicalcompositions focused onlignocellulose, lipids, ashand hot water extractible compounds.Amountsofcellulose,hemicellulosesandligninswere determinedaccordingtotheADF-NDFtechnique(VanSoestand Wine,1967,1968).Ligninswereoxidizedbyapotassiumperman- ganatesolution.Resultsobtainedformiscanthus(Table1)were closetothoseobtainedbyVanHulleetal.(2010)withthesame technique. Lipids weredetermined by Soxhlet extractionusing cyclohexaneasextractingsolvent(FrenchstandardNFV03-908).

Sampleswereburntoffat550^◦Cduring5hfortheashcontent determination(FrenchstandardNFV03-322).Forthehotwater extractscontentdetermination,extractionwasmadebyrefluxing distilledwater for1h. Thehotwater extractscancontaininor- ganiccompounds,tannins,gumsorsugars.Alldeterminationswere carriedoutinduplicate.

Table1

Chemicalcompositionandinitialmorphologyofthedifferentfibrestested.

Miscanthus(MIS) Bamboo(B1) Bamboo(B2) Bamboo(B3) Bamboo(B4)

Chemicalcomposition(wt%ofthedrymatter)

Cellulose 61.1(0.4) 65.5(1.4) 66.1(1.3) 60.2(1.8) 43.6(1.2)

Hemicelluloses 23.3(1.0) 12.3(2.0) 11.0(1.4) 13.3(1.5) 17.4(0.8)

Lignins 6.8(0.6) 14.5(0.4) 14.3(0.6) 17.6(0.9) 21.0(0.5)

Lipids 0.8(0.0) 0.2(0.1) 0.2(0.1) 0.1(0.0) 0.3(0.1)

Ash 1.7(0.1) 1.9(0.1) 1.2(0.0) 2.1(0.0) 3.4(0.0)

Hotwaterextracts 5.3(0.3) 5.8(0.8) 6.8(1.5) 6.1(0.3) 12.8(0.5)

Fibremorphology

Meanlength(mm) 1.69(0.85) 3.24(1.68) 2.12(0.83) 1.29(0.76) 0.49(0.26)

Meandiameter(mm) 0.23(0.14) 0.26(0.20) 0.20(0.10) 0.16(0.10) 0.08(0.04)

Meanaspectratio 8.6(5.0) 17.2(11.1) 13.0(7.3) 9.6(6.6) 8.6(7.2)

Numbersinparenthesescorrespondtothestandarddeviations.

2.3. Compounding

Priortoextrusion,fibres havebeendriedovernightat80^◦C.

After drying, 3–4wt% remaining moisture in the fibres was measuredaccordingtotheFrenchstandardNFENISO665.Com- poundingwascarriedoutinaco-rotatingtwin-screwextruderwith a44L/dratioanda 28.3mmscrewdiameter.Itconsistedin 11 successivemodules.PLAwasfedinmodule1oftheextruderand thefibreintroductionwasmadethroughasidefeederinmodule 6afterPLA melting.Threedistinctzonesmadeofkneadingele- mentswerelocatedinmodules7–9todispersefibresinthemelted PLA.Temperaturewassetat190^◦Cinthemeltingzone, andat 165^◦Cinthekneadingzone.Fourdifferentscrewspeeds(n)were testedduringthestudy:100,150,225and300rpm,with20kg/h intotalfeedingrate(Q),i.e.PLAplusnaturalfibres.Q/nratioswere 0.20,0.13,0.09and0.07kg/h/rpm,respectively.Inaddition,com- poundingwascarriedoutatthesefourdifferentscrewspeedsfora 0.13kg/h/rpmQ/nratio,correspondingtofeedingratesof13,20,30 and40kg/h,respectively.Matterpressure(P_mat,bars)andtempera- tureinthedieaswellastesttorque(T,%)oftheextrudermotorhave beenmeasuredwithspecificdetectors,andrecordedeveryminute duringproduction.Theresultingspecificmechanicalenergy(SME, (Wh)/kg)wascalculatedwiththefollowingequation:

SME= PM×(T/Tmax)×(n/nmax)

Q .

P_M(41W)isthemotor’selectricpower,Tmax(100%)isthemax- imumtorqueoftheextrudermotor,andnmax(1200rpm)isthe maximumspeedoftherotatingscrews.TheSMEcorrespondsto theelectricalenergyconsumedbythemotorperweightunitof mattertoensurethecompounding.

Compoundrodswerecooledwithwaterandair,andthenwere pelletized.Pelletlengthwasaround3.61±0.26mm(meanvalue obtainedafterthelengthmeasurementof20pelletsusinganelec- tronicdigitalslidingcalliperhavinga0.01mmresolution).

2.4. Injectionmoulding

Aninjectionpresswith150tonnesclampingforcewasused tomakestandarddumbbellshapedsamplesformechanicalprop- ertymeasurements.Pelletsweredriedat 60^◦C during4hprior to moulding. Temperature profile along the plasticating screw was30–155–160–165–165^◦Candthedietemperaturewas170^◦C.

Screwspeedformeltingwassetat150rpmandinjectionspeedwas setat50mm/s.Themouldwaskeptat18^◦Cwithwatercirculation.

2.5. Rheologicalmeasurements

Relative viscosity measurements have been carried out on extrudedpelletsinaThermoHaake(Germany)MiniLabmicrocom- pounder. It consistsin a co-rotating twin-screwsconfiguration.

TheMiniLabisequippedwithabackflowchanneldesignedasa slitcapillary.Pressureismeasuredatthecapillaryentranceand exit.Shearstressisdeducedfromthepressuredropintheback flowchannelduringmeltpolymerrecirculation.Differentrelative shearrateswerestudiedbychangingscrewspeed.Measurements havebeenmadeat170^◦Cwithscrewspeedsfrom50to250rpm correspondingtorelative shearrates between177and889s⁻¹. Measuredviscositiesandshearratesarecalled“relative”inthecase ofMiniLabmeasurements,asvolumicflowisnotsetbutestimated thankstothescrewspeed.Slipeffectsalongthedevicewallcould alsocausedeviationfromabsoluteviscosityvalues,asstatedon thedevice’stechnicaldocumentation.Toavoidanyinterferenceof residencetimeinourcomparisons,atimegapof1.5minbetween eachmeasurementwasset.Alldeterminationswerecarriedoutin duplicate.

2.6. Fibreextractionandfibresizemeasurements

Fibres wereextracted frombiocompositesby dissolvingPLA inchloroformusingaSoxhletextractionapparatus.Fibrepictures weretakenwithaNachet(France)Rubisbinocularmagnifierwitha 5.5×–10×observingmagnificationdependingonthefibresize.An imagewastakenforeachanalysedsampleusingtheArchimed4.0 (France)software.Thepictureresolutionwas14and7mm/pixel, respectively,whatwasagoodresolutiontohaveasufficientnum- beroffibrestocharacterizeonthepictureandtoobservevarious bundle sizes. However,this method waslimited for measuring elementary fibres. Due to picture resolution, measurements of specimenunder30mmweredifficult.Lengthanddiameterwere manuallymeasuredwiththeImageJ(USA)software.200fibresby sampleweremeasuredinordertosetlength,diameterandaspect ratiodistributions.Carehasbeentakentolabelfibresmeasured toavoidduplicates.Inordertocomparethesamples,sizedistri- butionswerebuiltandsuperposed.Inthispaper,meanvaluesof length,diameterandaspectratioarepresented,toclearlyrepresent observationsmadeonthesizedistributions.Theywerecalculated innumber.

2.7. Mechanicaltesting

TensileandflexuralpropertiesweremeasuredusingaTinius Olsen(USA)universaltestingmachinefittedwitha5kNloadcell accordingtotheFrenchstandardsNFENISO527and178,respec- tively.Thecrossheadspeedfortensiletestingwas5mm/minand tensilemoduluswasdeterminedwithanextensometeratthespeed of1mm/min.Thecrossheadspeedforthethree-pointbendingflex- uraltest was2mm/minfor a 64mmgap.Sampleswerestored fortwo weeksin aclimaticchambersetat 25^◦C and60%rela- tivehumiditybeforetesting.Sixsamplespertestingmodewere analysed.

Fig.1.SEMimagesofmiscanthusfibres(MIS)andbamboofibregrades(B1toB4)beforecompounding.

2.8. Differentialscanningcalorimetry(DSC)

PLAtransitiontemperaturesweredeterminedusingaMettler Toledo(Switzerland)DSC821^e calorimeter undernitrogen flow.

SamplesforDSCanalysiswereobtainedfrominjectedcomposites afteratwo-weekstorageinaclimaticchambersetat25^◦Cand60%

relativehumidity.Afirstheatingrampat10^◦C/minuntil200^◦Cwas performedtoerasesamplethermalhistory.Afteracoolingramp until25^◦Cat15^◦C/minand10minisothermat25^◦Ctoendthe cooling,asecondheatinginthesameconditionswascarriedout.

Twoanalysesweremadebysample.Theglasstransition(Tg),cold

crystallization(T_cc),and melting(T_m)temperaturesweredeter- minedfromthelastheatingramp.Tgwastakenasthemidpoint oftheDSCcurvedeflectionfrombaseline.Enthalpyvalueswere determinedusingSTAR^eSW9.30softwarefromMettler-Toledoby integratingtheareaofthecoldcrystallizationandmeltingpeaks anddoingtheratiobetweenthemeasuredareaandtherealPLA massinthebiocomposite.Crystallinityrate()aftercoolingstep wascalculatedasfollowing:

=1Hf−1Hcc

1Hfth ,

where1Hfisthesamplemeltingenthalpy(J/g),1Hccisthesample coldcrystallizationenthalpy(J/g)and1Hfththetheoreticalmelt- ingenthalpyfor100%crystallinePLAtakentobe93.6J/g(Fischer etal.,1973;Tuominenetal.,2002).Variationsinusedvaluesfor thistheoreticalmeltingenthalpycouldbefoundrangingfrom93 to93.7J/gintwoofthepreviouslyreportedworks(Tokoroetal., 2008;Nyamboetal.,2010).

2.9. Dynamicmechanicalanalysis(DMA)

Dynamicmechanicalproperties,i.e.storagemodulus,lossmod- ulusandlossfactor(tanı),definedastheratioofthelossmodulus tothestorageone,weredeterminedusinga TritonTechnology (UK)TTDMAdevice.Testswereperformedusingsinglecantilever geometryoversampleswith25mmlength,10mmwidthand4mm thickness.Distance betweenclampswas 10mm. Strain of1Hz frequency and 20mmamplitude was used.Heating ramp from ambienttemperatureto160^◦Cwascarriedoutatarateof2^◦C/min.

Analyseswereperformedoninjectedsamplesstoredatleasttwo weeksincontrolledconditions(25^◦C;60%RH)andwereduplicated toconfirmtherepeatability.

2.10. Heatdeformation

DeformationofPLAinjectedobjectscouldlimittheirusagein thermallyprocessedpackagesforinstance(Jamshidianetal.,2010).

Thenon-normalizedtestproposedhereisacomparativestudyof heatstabilitiesforPLAandPLAbiocomposites.Dimensionalstabil- ityofinjectedpieceswasdeterminedonstandarddumbbellshaped samplesafterstorageinanovenat80^◦Cduring1h.Suchtempera- turewaschosentobehigherthanPLAglasstransitiontemperature, andlowerthancoldcrystallizationandmeltingones.Pieceswere holdbyagripononeside,theothersidebeingletfreeintheair.

Aftertheheattreatment,thesamplebendbyitsfreesideandangle variationbetweenthetwosidescouldbethendetermined(Fig.2).

Thisvariationwasmeasuredonasideviewpictureofthesam- plewiththeImageJsoftware.Twosampleswereheat-treatedby lot,afterastorageofatleasttwoweeksincontrolledconditions (25^◦C;60%RH).TheheatdeformationofPLAbiocomposites(%) wasthendefinedastheratiobetweentheanglevariationandthe initialhorizontalangle(180^◦).Inthisnon-normalizedtest,noforce wasappliedtothesample,exceptitsownweightandpossiblyair flowintheoven.Consequently,theresultspresentedinthisstudy canbeconsideredasafirstclueofshapedeformationimprovement butdonotrepresentheatstabilityunderloadasthetestdetermin- ingtheheatdeflectiontemperature(HDT)cando(standardASTM D648).

Fig.2. Representationofsamplebendinganddeformationangle,blackcirclesrep- resentingthegripholdingthesampleononeside.

2.11. Size-exclusionchromatography(SEC)

A Dionex(France) size exclusion chromatography equipped withaIota2refractiveindex(RI)detectorwasusedtodetermine PLAmolecularweightdistributionintheinjectedmaterial.Three PLgel columnswere associated in seriesof 10³,500 and 100 ˚A alongwithaprecolumn.Columnswerekeptata30^◦Ctemperature.

PLAseparatedfromfibres,duringpreviousSoxhletextractionin chloroform,wasusedforcharacterization.PLAwasremovedfrom extractingchloroformbyevaporation.Itwasthendissolvedagain in clear chloroform at an approximate 5mg/mL concentration.

Chloroformwasalsousedaseluentfortheanalyses.Polystyrene standardswereused forthecalibration. Soxhletextractionwas comparedtoPLAextractionbysimpledissolutioninchloroform atambienttemperaturefollowedbyBuchnerfiltrationtoremove fibresandsolventevaporation.NodifferenceswereobservedinPLA molecularweightdistributions.

3. Resultsanddiscussion 3.1. Compositesprocessing

Duringextrusion,torqueevolutionwasmeasuredinorderto comparetheextruderforceneededtoconveyandtomixthetwo rawmaterials,i.e.PLAandnaturalfibres.Fromthetorquevalue, theSMEvaluewasdeduced,representingthespecificmechanical energy(perweightunitofmatter)spenttocompoundthefibres withPLA.Passingthroughthediewasalsoanimportantstepduring extrusionprocess.Asaresponseofcompoundstateintothedie,the matterpressurewascontrolled.Thisparametercouldbeinfluenced bymanyvariablessuchasthediegeometry,itsfillingdegree,the temperatureorthecompoundviscosity.

Table2

Extrusionparametersforvariousfibretypesandloadingswitha150rpmscrewspeed.

Fibretype Fibreloading(wt%) Q(kg/h) T(%) Pmat(bars) SME((Wh)/kg)

Withoutfibre 0 20 46(0.0) 18(0.1) 118

MIS

10 20 51(0.6) 27(0.6) 130

20 20 57(0.7) 34(0.6) 145

40 20 67(0.9) 56(1.9) 170

B1 20 20 51(0.5) 28(0.5) 131

40 15 41(1.0) 44(1.2) 140

B2 20 20 53(0.7) 29(0.5) 136

40 12 43(0.8) 35(1.3) 184

B3 20 20 52(0.7) 31(0.5) 133

40 20 65(0.9) 55(1.7) 167

B4 20 20 53(0.6) 34(0.7) 136

40 20 68(4.0) 73(5.3) 174

Numbersinparenthesescorrespondtothestandarddeviations.

Fig.3.MiniLabrelativerheologicalmeasurements(A)forPLA-miscanthuscompositeswithincreasingfibrecontentsfrom0to40wt%and(B)for60/40PLA–bamboogrades.

ResultsforPLA-miscanthus blends,in Table2,showeda lin- ear torqueincrease withfibreconcentration in the compound.

The more fibreadded, the more torque needed and electricity consumedforthecompoundpreparation.BlendingPLAwithmis- canthusfibresalsoledtoapressureriseintothedie(from18to 56bars).Asdiegeometryandtemperaturewerekeptconstant,P_mat responsemustbelinkedtotwodifferentparameters,i.e.diefill- ingdegreeandthecompoundviscosity.First,iffeedingratewas keptconstant,itwasrelatedtothematterweight(20kg/h).Nev- ertheless,fibresandPLAhavedifferentdensities,whatcouldcause volumevariationsinthediedependingonthefibre/polymerratio.

NaturalfibredensityisusuallyhigherthanPLAone(Nyamboetal., 2010)soaddingmorefibreshouldreducemattervolume.Butarod expansionwasobservedatthedieexitandwasaclueoftrappedair intotheblendwhilemixingwasdone.Thiscausedvolumeincrease indieandpossiblyhigherpressure.Anotherparameterwasthe blendviscosityinthedie.Therefore,therheologicalbehaviourof pelletsproducedwasanalysedtocompletetheobservationmade duringcompounding.Thesemeasurementsshowedanincreasing relativemeltedviscosityfrom0to40wt%fibreloading(Fig.3A).

ThecompositesrheologicalcurveswerefittedwiththeOswald–De Waelepower-law:

=K˙ⁿ⁻¹,

whereistheviscosity(Pas),˙ istheshearrate(s⁻¹),Kisthe consistency(Pasⁿ)andnisthepower-lawindex.Thecoefficient ofdeterminationR²wasaround0.99forallthecurves.TheKval- uesforthecompoundswith0,10,20and40wt%were550,1620, 5742and86676Pa,respectively.Thenpower-lawindexvalues were0.57,0.39,0.21 and−0.24,respectively.In thelatter case, anegativeindexwasquitesurprisingbutcouldbeattributedto slipeffectsalongtheMiniLabwalls(Fraihaetal.,2011).However, theriseofconsistencyvalueasthedecreaseofpower-lawindex showedthedifficultiesfortheblendstoflowathighfibreconcen- trations.Indeed,fibres,thatremainedsolid,weredispersedinto moltenpolymer,hinderingitsflowandcausingviscosityincrease, especiallyinthelowshearrateregion(177–435s⁻¹).Theseobser- vationswerecoherentwithpreviousworksdoneinfilledpolymer systems(Kalaprasadetal.,2003;Guoetal.,2005).Theorientationof thefibres,theirinteractionsbetweeneachothersuchastheirinter- actionswiththematrixwerepossibleviscosityincreasecauses.At

lowshear,fibresweredisorientedandpolymerchainentangled.

Shearratewasinsufficienttoensurethemobilityofthesystem.

Perturbationsinnormalflowresultedinviscosityincrease.Fibre- to-fibrecollisionsandfrictionsinsuchdisorientedsystemwere moreimportant.Kalaprasadetal.(2003)showedalsothatthemore theaffinitybetweenthematrixandthepolymer,thehigherthe blendviscosity.Inthepresentcase,theaffinitybetweenPLAandthe fibreswaslowbecausenocouplingwasconsidered.Therefore,the influenceofpolymer–fibreinteractionswaslikelylowerthanthe onesoffibredisorientationandfibre-to-fibrecollisions.Athigher shear,fibresgotorientedintheflowdirection,thefibre-to-fibre collisionswerediminished,andtheirimpactonviscositybecame lower.Thus,viscosityincreasewithfibreconcentrationmostlikely explainedmatterpressureandtorqueincreasesobservedduring compounding.

Whenthefeedingratewasincreasedproportionallytoscrew speedataset20wt%fibreloading,torqueandmatterpressurerose followingthesametrend:from47.3to63.9%andfrom30.3to37.5 bars,respectively(Fig.4).BykeepingQ/nratioconstant,globalfill- ingratio(F_R)alongthescrewswaskeptconstanttoo,astheywere

Fig.4.Evolutionoftesttorque(T)andmatterpressureinthedie(Pmat)withthe screwspeed(n)atsetfeedingrate(Q=20kg/h)andatsetQ/nratio(0.13kg/h/rpm) for20wt%miscanthusfilledcompounds.

Fig.5.Evolutionofspecificmechanicalenergy(SME)withthescrewspeed(n)atset feedingrate(Q=20kg/h)andatsetQ/nratio(0.13kg/h/rpm)for20wt%miscanthus filledcompounds.

linkedbytherelationFR=A×(Q/n)(Vergnesetal.,1998),whereA isaconstantvaluedependingontheextrudergeometryandthe screwprofilethatwerekeptthesameforallthestudy.Actually,if theamountofmatterintroducedinthemachinebecamebigger,it wascompensatedbylowerresidencetimealongthescrewsdueto afasterscrewrotation.Despitekeepingaconstantscrewfilling,a torqueincreasewasobservedasthescrewspeedincreased.Thedie fillingmustbethecause.Indeed,eveniffillingwaskeptconstant alongthescrews,morematterwaspassingthroughthedieinthe sameperiod.Astheflowrateofmatterthroughthedieincreased, thepressureappliedbythearrivingmatteronthematteralready inthediebecamehigher.Torquevariationscouldbearepercussion ofmatteraccumulationinthedie,hinderingscrewrotation.

Onthecontrary,whenthescrewspeedvariedatthesamefeed- ingrate,Q/nratiowaseitherincreasedorreduced,andthistended tofillortoemptythescrews,respectively.ItcanbeseeninFig.4,for asetfeedingrate,thatbothtorqueandmatterpressuredecreased whenthescrewspeedincreased:from64.8to43.2%andfrom36.9 to26.9bars,respectively.Additionallytoglobalfillingreduction byusinghigherscrewspeed,sheartransferredtothemattergot highertoo.Thus,thematterintheextruderwasfluidized,dueto thePLAthermoplasticbehaviour,andthiscouldalsoexplainthe torqueandmatterpressuredecreases.

Inthepointofviewofelectricalconsumption,itcanbeseen inFig.5thatincreasingthescrewspeedresultedinmoreenergy spent.Thetorquereductionobservedata20kg/hfeedingratewas notsufficienttocompensatescrewspeedincreaseinthefinalSME calculation,andsotheSMEincreasedfrom111to221(Wh)/kg.

WhenQ/nratiowaskeptconstant,theSMElogicallyroseinthe sameproportionthanthetorque(from122to164(Wh)/kg),this onebeingtheonlyparameterofSMEcalculationvarying.

ForthefourbamboogradesreportedinTable2,nosignificant differencewas observed onelectrical consumption when com- poundedat20wt%loading(between131and136(Wh)/kgforthe SMEvalue).Moreover,theenergyneededforcompoundingbam- boofibreswithPLAwaslowerthanformiscanthus(145(Wh)/kg).

Inthecaseofmiscanthus,matterpressureinthediewasalsohigher thanfor bamboo gradeswhile nodifferenceswereobserved in rheologicalmeasurementsbetweenthefibresat20wt%.Thisobser- vationcouldbelinkedtotheself-heatingofthematterinthedie.

Temperatureinthediewassetat165^◦C.However,realmattertem- peraturemeasuredinthatzonewasaround168^◦Cformiscanthus filledcompoundandaround170^◦Cforbamboofilledcompounds.

Thedifferenceinself-heating,thatwasalittlehigherwithbamboo fibres,couldcomefromdifferenceinfibreabrasiveness,depending

onitsoriginandchemicalcomposition.Meltviscosityinthediewas surelylowerforthebamboobasedblendswiththehighestmea- suredtemperaturethanwithmiscanthusbasedblends,whatcould explainthelowermatterpressureandtorque,andsothelower SMEvalue,observedwithbamboocomparetomiscanthus.Aslight increaseinmatterpressure(from28to34bars)wasobservedwith decreasingfibresizeattheentrance.Ata40wt%loading,thefeeder usedinthisstudywaslimitedtointroduceenoughbamboofibres fromB1andB2gradestoreacha 20kg/htotalfeedingrate.For B1,thereductionoffeedingrateto15kg/hcausedadecreasein torque(from51%at20wt%loadingand20kg/hfeedingrateto41%

at40wt%loadingand15kg/hfeedingrate).Theenergyspentforthe compoundingwaslowerforB1thanforthethreeothergradespre- paredatthesame40wt%loading:140(Wh)/kginsteadofatleast 167(Wh)/kg.Onthecontrary,forB2grade,wheretotalfeedingrate wasevenmorereduced(12kg/h),thetorquereductionwasinthe samerangethanforB1grade(from53%at20wt%loadingto43%

at40wt%loading).Consequently,thefinalenergyspentwasmuch higher(184(Wh)/kg).B3andB4bamboogradesandmiscanthus werecompoundedinthesameconditionsforthetwotestedfibre loadings,i.e.ata20kg/htotalfeedingrate.Productionofthe40wt%

B4filledcompoundappearedtobeinstablewithhighvariations oftorqueanddiematterpressure.Thesevariationsdidnotcome fromthegravimetricfeederused,asanalertmessageisshownon thesupervisionassoonasthemeasuredfeedingratedriftedof1%

fromthesetvalue.Nofibreaccumulationwasneitherobservedat thesidefeederbasis.Thevariationswerepossiblyduetothevery smallfibresize,implyinghigherspecificsurfacetowetbyPLAand moredifficultiestodispersethefibreinthatcase.Thesedifficul- tiesresultedinaninhomogeneousmixing.Meltpolymerflowwas probablyperturbedbythepresenceoffibreagglomeratesorpar- tiallywettedfibres,whichhaddifficultiestogetorientedintheflow direction.Indeed,moststudiesshowedthatathighfibreconcentra- tions,interactionsbetweenfibresaremorenumerous.Interstitial spacesbetweentheagglomeratedfillers,containingimmobilized polymer,canchangesystembehaviourasifthefillerconcentra- tionwasactuallyhigherthanwhathadbeenadded(Utrackiand Fisa,1982).Fig.3BshowedthatB4gradeactuallyexhibitedhigher viscositycomparetotheothergrades.Theviscosityincreaseprob- ablyexplainedtheincreasedpressuresinthedieobservedduring extrusionwithB4.

3.2. Fibresize

Processimpactonthefibresizewasevaluatedbycomparing miscanthusbundlelength,diameterandaspectratiodistributions afterthedifferentstepsoftheprocess,i.e.extrusion,pelletizing andinjection-moulding.AsseeninTable3,themainsizereduction occurredintheextrusionpart.Indeed,a37%lossinfibrelength anda 13%lossinfibrediameterwereobserved in20wt%filled compoundrods.Thisresultedinanaspectratiodecrease(from8.6 to5.7).Pelletizingstepcauseda smalldecreaseinlengthwith- outaffectingthediameter.Thiswasduetofibreorientationinrod, mainlyfollowingtheflow,andresultedinanadditionalaspectratio loss(from5.7to5.3)oflowsignificance comparetothereduc- tionafterextrusion.Injection-mouldingisahigh-shearprocessbut withlowresidencetimecomparetoextrusion.Italsoaffectedfibre size.Bothlengthanddiameterweresubjectedtoreductioninthe sameproportion:from1.01to0.84mmandfrom0.21to0.17mm, respectively.Attheend,aspectratioremainedthesamethaninthe pellets(5.2).Consequently,itappearedthattwin-screwextrusion compoundingwastheprocessstepcausingthemostfibrebreakage, butalsothemostfibreseparationfrombundles.

Increasingthescrewspeedduringcompoundingcausedmore sheartothematterinthescrewrestrictedareas.Consequently,it wasreasonabletothinkthatthemoreshearprovided,themore

Table3

Miscanthusfibresizeinextrudedrods,pelletsandinjectedmaterials.

80/20PLA/MIScomposites 60/40PLA/MIScomposites

Length(mm) Diameter(mm) Aspectratio Length(mm) Diameter(mm) Aspectratio

Extrusion 1.06(0.59) 0.20(0.12) 5.7(2.3) 0.88(0.43) 0.19(0.10) 5.0(2.0)

Pelletizing 1.01(0.46) 0.21(0.10) 5.3(2.3) 0.83(0.44) 0.17(0.10) 5.1(2.1)

Injection-moulding 0.84(0.40) 0.17(0.09) 5.2(1.8) 0.78(0.35) 0.17(0.08) 4.8(1.8)

Numbersinparenthesescorrespondtothestandarddeviations.

fibrebreakageobtained.Infact,resultsofmeanlengthandmean diameterinthefinalmaterialrevealedfewvariationsinsizeinthe caseof20wt%miscanthusfilledcompoundsobtainedatsetfeeding rateand withscrewspeed from100to300rpm(Fig.6).Distri- butionswereveryclosetoeachother,andmaximumfibrelength anddiameter(1.00mmand0.21mm,respectively)wereobtained witha225rpmscrewspeed.Meanaspectratioswere4.9,5.2,5.1 and5.4forscrewspeedsof100,150, 225and300rpm,respec- tively.Whenbothfeedingrateandscrewspeedincreased,i.e.Q/n ratiokeptconstant,thesametendencywasstillobservedinsaving fibrelength.Atthesametime,fibrediameterincreasedprogres- sivelybutslightlywiththescrewspeed(from0.17to0.20mm).

Meanaspectratioswere5.9,5.2,6.3and5.6,forfeedingratesof 13,20,30and40kg/h,respectively.Forinvestigatedcompound- ingconditions,screwspeedappearedtobeinsufficientneitherto breakfibresnortoseparatebundles.Byincreasingscrewspeed, residencetimeintotheextruderdecreased,leadingtomoreshear butonashorterperiod.Inaddition,meltedPLAwasfluidizeddueto itsshear-thinningbehaviour,andsoithadfewerdifficultiestobe conveyedthroughrestrictedareasalongthetwin-screwextruder.

WollerdorferandBader(1998)haveshownthatthematrixrheo- logyhadanimpactonthefinalsizedistributionsofflaxfibres.When feedingratewaskeptat20kg/h,byincreasingthescrewspeed,the fillinginmixingzoneregionswasreduced,whatcouldalsoreduce theirefficiencytotransfershear.Inthatcase,a0.09kg/h/rpmQ/n ratiocorrespondingtoa225rpmscrewspeed couldbeconsid- eredasanoptimalconditiontopreservefibresize.Despitethese differencesinlengthanddiameter,inallcases,thefibreaspect ratiodistributionsinthesecompoundswerefoundtobecloseto eachother.TheseresultswentinthesamewaythanBeaugrand and Berzin (2012) observations in PCL/hemp compounding by twin-screwextrusion,whichenlightenedalsoasmallscrewspeed influenceonfibresize.

Lengthanddiameterforbamboofibresinthefinalmaterialare presentedinTable4.For80/20PLA/bamboocomposites,theB1

Fig.6. Miscanthusfibremeanlength(L)andmeandiameter(d)ininjectedmaterials from20wt%filledcompoundsextrudedatdifferentscrewspeeds(n),atsetfeeding rate(Q=20kg/h)andatsetQ/nratio(0.13kg/h/rpm).

gradethatcontainedinitiallythelongerandwiderfibresalsokept higherlengthanddiameteraftertheglobalprocess,i.e.extrusion, pelletizingandinjection-moulding,thanthethreeothergrades(B2 toB4).However,fibresfromB1gradeweresubjectedtothegreatest reductioninsizeaslengthlosswas72%insteadofaround62%for B2andB3grades,andonly32%forB4grade.Reductionindiameter waslessimportant,between25and35%forB1toB3gradesand only12%forB4grade.ThemoreimportantdiameterlossfortheB1 toB3gradescomparetoB4andmiscanthus(−13%)wasexplained bySEMimagespresentedinFig.1.Fibreswerepartlyseparated atthebundleendsinthesethreegrades,whathavesurelyeased fibreseparationduringprocess.Aspectratiosdrasticallydecreased forthefourbamboogrades,andparticularlyforB1grade(−69%).

Attheend,gapbetweengradeswasdramaticallyreducedanddif- ferencesinaspectratioswerefoundtobeminor(from4.8to5.6) comparedtowhattheywerebeforeprocess(from8.6to17.2).Few differencesinsize,between80/20and60/40PLA/bamboocompos- ites,wereobservedforB3andB4grades.ForB2gradeandmore particularlyforB1grade,anextrasizereductionwasnoticedin theinjectedmaterial,especiallyforfibrelength,asitwasalready observedformiscanthus(Table3).B2gradeprovidedthelonger fibres(0.74mm),whatcouldbeexplainedbytheuseofalower Q/nratio(0.08kg/h/rpminsteadof0.10kg/h/rpmforB1gradeand 0.13kg/h/rpmforB3andB4grades).Thus,asformiscanthuswhere amaximuminlengthwasobtainedfora0.09kg/h/rpmQ/nratio, theQ/nratiothatgavethelongerfibresforB2bamboogradewas approximatelythesame(0.08kg/h/rpm).Aspectratiodistributions arerangingfrom4.5to5.6(Table4),whatwasclosetothedistribu- tionsobtainedforthevariousPLA/miscanthuscomposites(ranging from4.9to6.3).Analysesofmechanicalandthermalproperties willenlightentheinfluenceofsuchsizerangesonthecomposite properties.

3.3. Mechanicalproperties

Mechanical properties of the fourbamboo composites were comparedinFig.7tomiscanthuscompositesandPLAonesatthe twodifferentfibreloadings tested(20and40wt%).Forthefive fibretypes,importantflexuralandtensilemodulusincreaseswere observed by adding 20 and especially 40wt% of natural fibres.

Moreover,itappearedthatB2gradeprovidedbetterincreaseat thetwofibreconcentrationsforbothflexuralandtensilemodu- lus.Asanexample,at40wt%B2gradeloading,flexuralandtensile modulusincreaseswere+164%and+197%,respectively,compare toneatPLA.Ontheotherpart,tensilestrengthwasreducedfor thefivefibretypes(upto−18%for80/20compositesandupto

−21%for60/40composites)asnospecifictreatmenttoimprove fibre/polymerinterfacewasmadeduringthisstudy.Nevertheless, lesstensilestrengthreductionwasobservedwithB2grade:only

−7%at both 20wt%and 40wt% fibreloading.For thefive fibre types,thesame reductionwasobservedforflexuralstrengthat 20wt%fibreloading.Onthecontrary,inthecaseof60/40com- posites,reductioninflexuralstrengthwasobservedonlywithB3 andB4grades,meaningthatcompoundsfilledwithmiscanthusor withB1andB2gradesrevealedflexuralstrengthshigherthanPLA one.Bestflexuralstrengthat40wt%fibreloading(80MPa)was

Table4

BamboofibresizeininjectedmaterialsfromB1toB4bamboogrades.

80/20PLA/bamboocomposites 60/40PLA/bamboocomposites

Length(mm) Diameter(mm) Aspectratio Length(mm) Diameter(mm) Aspectratio

B1 0.90(0.45) 0.18(0.11) 5.3(2.1) 0.69(0.42) 0.15(0.08) 4.9(1.7)

B2 0.80(0.41) 0.15(0.07) 5.6(1.8) 0.74(0.47) 0.15(0.09) 5.0(1.7)

B3 0.46(0.25) 0.10(0.05) 5.1(2.2) 0.46(0.23) 0.10(0.05) 4.5(1.6)

B4 0.33(0.15) 0.07(0.03) 4.8(2.4) 0.31(0.17) 0.06(0.03) 5.4(2.3)

Numbersinparenthesescorrespondtothestandarddeviations.

Fig.7.FlexuralandtensilepropertiesforPLA(crossedsquare),80/20composites(filledsymbol)and60/40composites(opensymbol)filledwithMIS(downtriangle),B1 (circle),B2(uptriangle),B3(diamond)andB4(righttriangle).

stillobtainedwithB2grade,andit was8%more thanPLAone.

If20wt%loadedPLA/B2gradecompoundperformanceinflexural andtensilestrengthscanbeattributedtoahigheraspectratio(5.6 insteadof4.8–5.3forthefourotherfibres),itwasnotthecasefor 40wt%loadedone.Forthisconcentration,B4gradehadthehigher medianaspectratio(5.4insteadof5.0forB2grade).Differences betweenaspectratiosforbamboogrades(Table4)wereinthesame rangeofthoseobservedbetweenmiscanthuscomposites,extruded withvariousscrewspeedsandvariousfeedingrates(from4.9to 6.3).Consequently,theresultsofmechanicalpropertiesforthese compoundswerecomparedtoestimatesignificanceofaspectratio influenceonmechanicalproperties.Tensilepropertiesresultswere chosenforsuchcomparison.

Fig.8 shows thetensile property variationsfor 20wt%mis- canthusfilledcompoundsproducedatdifferentscrewspeeds.No significantdifference wasobserved between thesecompounds, regarding tothestandard deviations. Assaid previously,screw speed impact on fibresize was of low significance and varia- tionsobserved at 225rpm (Fig. 6) werenot enough important toimprove tensileproperties.Modifyingresidencetime ofboth fibreandPLAorprovidingmoresheardidnothelpimprovingthe mixingbetweenboth components,astensilestrengthremained quitethesameforallthecompoundstested.BeaugrandandBerzin (2012)alsoobservedasmallinfluenceofscrewspeedonPCL/hemp compositemechanicalproperties.Contrarilytotheobservationsin Fig.5,feedingratewasfoundtohaveamoreimportantroleintheir studyonfibresizeandcompositemechanicalproperties.However, theyusedverylowfeedingrates(0.85and1.5kg/h)comparetothe rangeoffeedingrate(from13to40kg/h)usedhere.Itwasbelieved thatthequitehigherresidencetimeintheircasecausedahigher dependenceoffibresizeandcompositepropertiestothefeeding

rate.Ifscrewspeedwasfoundtohavealittleimpactonproperties, thecompoundspreparedata100rpmscrewspeedexhibitedvery largestandarddeviationsfortensilestrengthandtensilemodulus comparedtotheothers.Thiscouldbeanevidenceofaninhomo- geneousmixing.Atthisscrewspeed,indeed,shearratemaynotbe sufficienttofluidizemeltPLAenoughforagoodfibredispersion orwettability.Fromtheseresults,fibreaspectratiomaynotbethe parameterinfluencingthestiffnessdifferencesobservedbetween thefourbamboogrades.

Fig.8.Evolutionoftensilestrengthandtensilemoduluswiththescrewspeedvalue (n)atsetfeedingrate(Q=20kg/h)andatsetQ/nratio(0.13kg/h/rpm)for20wt%

miscanthusfilledcompounds.

Ifaspectratiovariationsbetweenbamboofibreswerefoundto benegligible,variationsinlengthanddiameterweremoreimpor- tant(Table4).Thelongestfibres,i.e.B1andB2grades,reinforced themorePLAinflexuralstrength,evenleadingtoitsincreaseat a40wt%fibreloadingasabovementioned.Diameterandparticle shapemostlikelyaffectedtensileandflexuralstrengths,andlarger bundlesobservedforB1andMISfibrescouldexplainworseper- formancesforthemthanforB2gradeonthematerialstrengthening (Fig.7).Otherparameterscouldhaveplayedarolesuchasthefibre dispersion,thefibresurfaceoritsmorphology.Chemicalcomposi- tionisalsowellknowntoinfluenceintrinsicfibrepropertiesand so,itsreinforcement.Amountofcelluloseisoneimportantparam- eter.Themorethecellulosein thefibre, thehigher itsintrinsic modulusandstrength(Mohantyetal.,2000).Thebestmechani- calpropertieswereobtainedfromB1,B2andMISfibres(Fig.7) whichwerericherincellulosethanotherfibres(Table1).Onthe otherhand,B4grade,whichimprovedthelessmodulusandcaused themorestrengthreductionforbothmechanicaltests,wasalso thebamboogradecontainingthelesscellulose.Concentrationin hotwaterextractiblecomponentswasalsoclearlyhigherinthat grade(13wt%ofthedrymatterinsteadof5–7wt%forthefourother fibres).Apreviousstudy(AshoriandNourbakhsh,2010)reported bettermechanicalpropertiesinPP/woodcompositesbyremoving theseextractiblecomponentsinoakandpinebeforecompounding, confirmingtheirnegativeeffectonmechanical propertieswhen presentinthecompound.

3.4. Thermalandthermo-mechanicalproperties

DSC thermograms revealed that incorporating miscanthus fibres in PLA using different fibre loadings and compounding conditionsslightlyincreaseditsglasstransition(Tg)andcoldcrys- tallizationtemperatures(Tcc):from55.1^◦C and97.1^◦C for neat PLA,respectively,to56.3–58.0^◦Cand 97.8–101.8^◦Cforcompos- ites(Table5).Onthecontrary,meltingtemperature(Tm)remained unchangedbythefibrepresence.Changesincoldcrystallization temperatureuptoalmost5^◦Csurelytraducedmoredifficultiesfor thePLAchainstorearrangethemselvesinpresenceofmiscanthus fibres.Indeed,ahighertemperaturemeantthatmoreenergywas neededtoinitiatethisrearrangement.Changesinchainmobility andinitialPLAcrystallinestructurecouldbeanexplanationtothese observations.Crystallinityrateaftercoolingwasindeedfoundto beslightlyincreasedinmostcases(upto10.9%insteadof9.1%for neatPLA)bytheadditionoffibres.However,inthecaseof80/20 PLA/miscanthuscomposites,itwasnoticedthatcrystallinityrate wasslightlyreducedto8.6–8.9%whenalowscrewspeed(100rpm) wasusedduringtwin-screwextrusioncompounding.Thisispossi- blyduetoaninhomogeneousfibremixinganddispersion.Mathew etal.(2006)observedthatfibrescouldpromotecrystallinityinPLA, crystallitegrowthbeinginitiatedatthefibresurface.Aninhomoge- nousfibredispersionandclusterformationreduced thespecific

Fig.9.Storagemodulusandlossfactor(tanı)forneatPLAandPLA/miscanthus compositesproducedata20kg/hfeedingrateanda150rpmscrewspeed.

fibresurfaceincontactwithPLA,andsotheareaswherecrystallites canbeformed.Regardingtochainmobility,sampleswereanalysed byDMAandthermogramsrevealedaheightreductionoflossfac- torpeak(Fig.9).Thisphenomenonwasimputedinpreviousworks toareducedchainmobility(Nyamboetal.,2010;Sreekumaretal., 2010)andresultedfromthesterichindrancecausedbyfibredis- persiononpolymerchainmotion,aswellastheslightcrystallinity increasementionedabove.Indeed,mobilityofpolymerchainorga- nizedincrystalliteswasevenmorereduced.Consequently,cold Table5

DSCandheatdeformationdataforPLA/miscanthuscomposites.

Fibreloading Q(kg/h)–n(rpm) Tg(^◦C) Tcc(^◦C) Tm(^◦C) (%) Heatdeformation(%)

NeatPLA 20–150 55.1 97.1 166.2 9.1 57.7(3.1)

10wt%MIS 20–150 56.3 97.8 166.4 10.2 51.0(1.8)

20wt%MIS 20–100 56.5 99.2 166.4 8.6 28.6(2.2)

20wt%MIS 20–150 56.9 99.6 165.9 10.4 34.2(2.2)

20wt%MIS 20–225 56.8 100.3 164.9 9.7 38.1(2.9)

20wt%MIS 20–300 56.9 101.8 166.4 9.4 37.5(2.4)

20wt%MIS 13–100 57.9 101.6 166.7 8.9 33.1(2.8)

20wt%MIS 30–225 57.0 101.1 166.3 10.9 35.4(5.2)

20wt%MIS 40–300 56.5 99.9 166.2 9.9 34.6(0.7)

40wt%MIS 20–150 58.0 99.3 166.7 10.2 7.8(0.7)

Numbersinparenthesesforheatdeformationdatacorrespondtothestandarddeviations.

Table6

DSCandheatdeformationdataforPLA/bamboocompositesproducedata150rpmscrewspeed.

Fibreloading^a Tg(^◦C) Tcc(^◦C) Tm(^◦C) (%) Heatdeformation(%)

20wt%B1 57.5 103.6 166.8 9.7 37.7(3.2)

40wt%B1 57.8 101.7 166.4 12.4 11.0(1.2)

20wt%B2 57.1 102.5 166.3 10.0 39.2(5.8)

40wt%B2 57.7 100.7 166.9 9.4 6.6(0.4)

20wt%B3 57.1 91.7 165.8 14.9 27.9(1.2)

40wt%B3 57.1 95.9 166.3 12.9 4.9(1.2)

20wt%B4 51.8 87.9 162.3 15.0 27.6(3.2)

40wt%B4 57.6 96.9 167.4 12.8 4.8(0.2)

Numbersinparenthesesforheatdeformationdatacorrespondtothestandarddeviations.

aFeedingratewas12kg/hforthe40wt%B2formulation,15kg/hforthe40wt%B1formulation,and20kg/hforalltheothers.

crystallizationoccurredathighertemperature.Forglasstransition, itwasalsoshown infilledsystemthatpolymermodificationat thematrix–fillerinterfacecausedfreevolumeandchainmobility decreaseresultinginglasstransitiontemperatureincrease(Droste andDibenedetto,1969).Itwasseenonmechanicalpropertiesthat interfaceinteractionsbetweenthePLAandthenaturalfibreswere weak.However,theseweakinteractions,thechainmobilityreduc- tionduetothefibrepresencesuchasthecrystallinityformationon thefibresurfacecouldexplaintheobservedTgincrease.

Anothereffectoftheloweringofpolymerchainmobilitywasthe reductionofsampleheatdeformation:58%withoutfibreaddition, 51%at10wt%fibreloading,29–38%at20wt%fibreloading,andonly 8%at40wt%fibreloading(Table5).Themorefibreadded,themore sterichindrancetheycaused,decreasingchainmobilityandmate- rialdeformation.StoragemoduliinDMA(Fig.9)wereveryclose beforePLAglasstransitionforallcomposites,itsincreaseoccur- ringonlyat40wt%offibrescomparedtoneatPLA.However,after glasstransition,thestoragemoduluswasfoundtoremainhigher along withthe fibreconcentration.Thus, it wasconcludedthat 40wt%filledmaterialeffectivelyenhancedPLAthermalstabilityby restrainingpolymerchainmobility.Lessdeformationwasobserved at40wt%fibreloading,andcorrespondingcompositekeptsome stiffnessafterglasstransition.

DSCresultsforbamboogradesarementionedinTable6.Nodif- ferenceswereobservedontransitiontemperaturedependingon thegrade,exceptwith20wt%B4filledcomposite.FortheB4grade, glasstransitionandmeltingtemperatureswerereducedfrom40 to20wt%fibreloading,whatcouldbetheclueofpolymerchain reduction. Size-exclusion chromatography (SEC) measurements gavePLA weightaveragemolarmasses(M_w)of 18,800,19,000, 18,800and18,400g/molfor20wt%ofB1,B2,B3andB4grades, respectively.Thecorrespondingpolydispersityindexes(Ip),repre- sentingthemolecularweightdistributionwidth,were1.8,2.4,2.4 and1.8,respectively.Thus,itwasfoundnosignificantdifferences inPLAmolarmassdistributionsininjectedmaterialsdepending ontheincorporatedbamboo fibres.Reductionof PLAtransition temperaturesinthecaseof20wt%ofB4didnotcomefromPLA molecularweightdistribution,butmightcomefromdifferences inthecrystallizationbehaviourcomparetotheothercomposites.

Indeed,theshortestfibres,fromB3andB4grades,promotedPLA crystallization.Crystallinityratewasatleast12.8%forthesetwo smallestgradesinsteadof9.4–12.4%forB1andB2grades.Tokoro etal.(2008)alsoobservedthisnucleatingactionofshortbamboo fibrebundlesinPLA.Defects duetofibresurfaceroughnesscan initiatecrystalgrowth.Mathewetal.(2006)observedthesedif- ferencesofcrystallinitypromotiondependingonthefibresurface topography.ForB3andB4grades,crystallinityratewashigherat 20wt%thanat40wt%andreached14.9and15.0%forthe80/20 PLA/B3gradeandPLA/B4gradecomposites,respectively.Atthis amount,coldcrystallizationtemperaturewaslowerprovingthat PLAchainrearrangementwaseasedbynucleatingeffectofB3and B4aslessenergywasneededtoinitiatethisrearrangement.With

40wt%,itwasthoughtthatthetoolargeamountoffibrescurbed crystallitegrowth.Atthesameweight,smallerfibresrepresented morenumerousparticlesthanthelongerones,andsomorespecific surfaceincontactwiththematrix,whatcausedmoresterichin- drance.Therewasacompetitionbetweenfibrenucleatingeffect andhindranceofchainmobility,limitingthecrystallitegrowth.

Thiscompetitionbetweenthefillernucleatingeffectanditsimped- ingeffectoncrystallizationathighloadingwasalsoobservedinPLA filledwithclaybyWuetal.(2007).

Fig.10.Storagemodulusandlossfactor(tanı)for60/40PLA/bamboocomposites producedata150rpmscrewspeed(totalfeedingratewas12kg/hforB2grade, 15kg/hforB1grade,and20kg/hforB3andB4grades).