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

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OULOUSE

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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

Any correspondence concerning this service should be sent to the repository

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Twin-screw

extrusion

impact

on

natural

fibre

morphology

and

material

properties

in

poly(lactic

acid)

based

biocomposites

G. Gamon

a,b,∗

_,

_Ph.

_Evon

a,b

_,

_L.

_Rigal

a,b

a_Université_de_Toulouse,_INP,_Laboratoire_de_Chimie_{Agro-industrielle,}_ENSIACET,₄_Allée_Emile_Monso,_BP_44362,₃₁₀₃₀_Toulouse_Cedex_4,_France b_INRA,_Laboratoire_de_Chimie_{Agro-industrielle,}₃₁₀₃₀_Toulouse_Cedex_4,_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 knownwaytoreduceproductioncostswhilesavingor increas-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.Thisresultedinmechanicalstrengthreduction with-outanychemicalorphysicalcompatibilisationmadetoenhance fibre–matrixinterface.

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

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fromSoutheastAsia,andwasoriginallyintroducedintoEuropeas anornamentalgardengrass(Johnsonetal.,2005).Bamboogrows upto40mofheightinmonsoon climates.Generally,it isused inconstruction,carpentry,weavingandplaiting,etc.(Faruketal., 2012).Bothmiscanthusandbambooattractedattentionforfuel production(Hongetal.,2011)butalsoforbiocomposites reinforce-ment.BourmaudandPimbert(2008)tested,bynanoindentation, modulus and hardness of miscanthus fibre being 9.49GPa and 0.34GPa,respectively,showingittohavemodulusbetweensisal (8.52GPa)andhemp(12.14GPa).Theyworkedtoincorporate mis-canthusinPLAandpolypropylene(PP)too,showingcomparable performance toother fibres. Johnsonet al. (2005) alsoworked toblend miscanthuswithMater-Bi® _to_improve_its_impact

per-formance. Mater-Bi® _had _a _0.8_J _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_,_while_0.2_mm_bundles_decreased_it_to₁_kJ/m2_.

Theprocessusedforbiocompositeproductionhasahigh impor-tancetoo.Agoodfibredispersionisneededtoaimgoodmaterial performance.Fibreorientationwillplaya rolesincefibres rein-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 fibreswitha10mminitiallengthwerereducedtoa96mm aver-agelengthinnumber.Twin-screwextrusionisahighshearprocess thatcanhelptomatchagoodfibredispersion.Bledzkietal.(2005)

observedbettermechanical propertiesbycompounding PP and woodinatwin-screwextrudercomparedtohighspeedmixerand two-rollmill.Nevertheless,extrusionleadstoseveralfibre break-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®_with_low_melt_viscosity,_were_wider_with_long_fibres

thandistributionsforTPSorBiocell®_{biocomposites.}_By_increasing

thefibrecontentinTPSfrom10to20%,theyshowedconsiderable increaseofthefibrepercentageinthelowerlengthfractionunder 200mm.BeaugrandandBerzin(2012)correlatedhempfibrelength

reductiontotheincreasingspecificmechanicalenergy(SME)spent duringcompoundingbytwin-screwextrusioninpolycaprolactone (PCL)matrix.The couplebarreltemperatureand fibremoisture contentwasalsofoundtohaveanimportance.Fibrelengthand aspectratioweremorepreservedwitha100◦_C_barrel

tempera-tureanda22.5wt%moistureinfibre,thanwith140◦_C_and_9.8_wt%

moisture.Twostudiesobservedthatfibrebreakageandseparation frombundletoelementaryfibrewouldbemoreorlessimportant dependingonthefibreorigin.First,Oksmanetal.(2009)compared sisal,flax,bananaandjute,andfoundthatflaxfibresobtainedby enzymaticrettingprocessandhavinglowlignincontent,are bet-terseparatedthantheothersinnaturalfibrereinforcedPP.InLe Moigneetal.(2011)study,flaxfibreswerealsoseparatedin ele-mentaryfibreswhilesisalfibresremainedpartlyinbundlesand wheatstrawprovidedbundlesandlargeamountsofsmallparticles. Thisstudyaimedtoinvestigatetheinfluenceofthe compound-ingparametersonnaturalfibremorphology,mechanicalproperties andthermalpropertiesinpoly(lacticacid)basedbiocomposites. Forthis,miscanthusfibreswerecompoundedtoaPLA commer-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.Thechemical composi-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)IngeoTM_grade,

anditwassuppliedintheformofgranules.

2.2. Fibrechemicalcomposition

Characterizationof thedifferentfibrechemicalcompositions focused onlignocellulose, lipids, ashand hot water extractible compounds.Amountsofcellulose,hemicellulosesandligninswere determinedaccordingtotheADF-NDFtechnique(VanSoestand Wine,1967,1968).Ligninswereoxidizedbyapotassium perman-ganatesolution.Resultsobtainedformiscanthus(Table1)were closetothoseobtainedbyVanHulleetal.(2010)withthesame technique. Lipids weredetermined by Soxhlet extractionusing cyclohexaneasextractingsolvent(FrenchstandardNFV03-908). Sampleswereburntoffat550◦_C_during₅_h_for_the_ash_content

determination(FrenchstandardNFV03-322).Forthehotwater extractscontentdetermination,extractionwasmadebyrefluxing distilledwater for1h. Thehotwater extractscancontain inor-ganiccompounds,tannins,gumsorsugars.Alldeterminationswere carriedoutinduplicate.

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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.Threedistinctzonesmadeofkneading ele-mentswerelocatedinmodules7–9todispersefibresinthemelted PLA.Temperaturewassetat190◦_C_in_the_melting_zone, _and_at

165◦_C_in_the_kneading_zone._Four_different_screw_speeds_(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(Pmat,bars)and

tempera-tureinthedieaswellastesttorque(T,%)oftheextrudermotorhave beenmeasuredwithspecificdetectors,andrecordedeveryminute duringproduction.Theresultingspecificmechanicalenergy(SME, (Wh)/kg)wascalculatedwiththefollowingequation:

SME= PM×(T/Tmax)×(n/nmax)

Q .

PM(41W)isthemotor’selectricpower,Tmax(100%)isthe

max-imumtorqueoftheextrudermotor,andnmax(1200rpm)isthe

maximumspeedoftherotatingscrews.TheSMEcorrespondsto theelectricalenergyconsumedbythemotorperweightunitof mattertoensurethecompounding.

Compoundrodswerecooledwithwaterandair,andthenwere pelletized.Pelletlengthwasaround3.61±0.26mm(meanvalue obtainedafterthelengthmeasurementof20pelletsusingan elec-tronicdigitalslidingcalliperhavinga0.01mmresolution).

2.4. Injectionmoulding

Aninjectionpresswith150tonnesclampingforcewasused tomakestandarddumbbellshapedsamplesformechanical prop-ertymeasurements.Pelletsweredriedat 60◦_C _during₄_h_prior

to moulding. Temperature profile along the plasticating screw was30–155–160–165–165◦_C_and_the_die_temperature_was₁₇₀◦_C.

Screwspeedformeltingwassetat150rpmandinjectionspeedwas setat50mm/s.Themouldwaskeptat18◦_C_with_water_circulation.

2.5. Rheologicalmeasurements

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

TheMiniLabisequippedwithabackflowchanneldesignedasa slitcapillary.Pressureismeasuredatthecapillaryentranceand exit.Shearstressisdeducedfromthepressuredropintheback flowchannelduringmeltpolymerrecirculation.Differentrelative shearrateswerestudiedbychangingscrewspeed.Measurements havebeenmadeat170◦_C_with_screw_speeds_from₅₀_to₂₅₀_rpm

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,whatwasagoodresolutiontohaveasufficient num-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,size distri-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-pointbending flex-uraltest was2mm/minfor a 64mmgap.Sampleswerestored fortwo weeksin aclimaticchambersetat 25◦_C _and_60%

rela-tivehumiditybeforetesting.Sixsamplespertestingmodewere analysed.

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

2.8. Differentialscanningcalorimetry(DSC)

PLAtransitiontemperaturesweredeterminedusingaMettler Toledo(Switzerland)DSC821e _calorimeter _under_nitrogen _flow.

SamplesforDSCanalysiswereobtainedfrominjectedcomposites afteratwo-weekstorageinaclimaticchambersetat25◦_C_and_60%

relativehumidity.Afirstheatingrampat10◦_C/min_until₂₀₀◦_C_was

performedtoerasesamplethermalhistory.Afteracoolingramp until25◦_C_at₁₅◦_C/min_and₁₀_min_isotherm_at₂₅◦_C_to_end_the

cooling,asecondheatinginthesameconditionswascarriedout. Twoanalysesweremadebysample.Theglasstransition(Tg),cold

crystallization(Tcc),and melting(Tm)temperatureswere

deter-minedfromthelastheatingramp.Tgwastakenasthemidpoint

oftheDSCcurvedeflectionfrombaseline.Enthalpyvalueswere determinedusingSTARe_SW_9.30_software_from_{Mettler-Toledo}_by

integratingtheareaofthecoldcrystallizationandmeltingpeaks anddoingtheratiobetweenthemeasuredareaandtherealPLA massinthebiocomposite.Crystallinityrate()aftercoolingstep wascalculatedasfollowing:

=1Hf−1Hcc 1Hfth ,

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where1Hfisthesamplemeltingenthalpy(J/g),1Hccisthesample

coldcrystallizationenthalpy(J/g)and1Hfththetheoretical

melt-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,loss mod-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◦_C_was_carried_out_at_a_rate_of₂◦_C/min.

Analyseswereperformedoninjectedsamplesstoredatleasttwo weeksincontrolledconditions(25◦_C;_60%_RH)_and_were_duplicated

toconfirmtherepeatability.

2.10. Heatdeformation

DeformationofPLAinjectedobjectscouldlimittheirusagein thermallyprocessedpackagesforinstance(Jamshidianetal.,2010). Thenon-normalizedtestproposedhereisacomparativestudyof heatstabilitiesforPLAandPLAbiocomposites.Dimensional stabil-ityofinjectedpieceswasdeterminedonstandarddumbbellshaped samplesafterstorageinanovenat80◦_C_during₁_h._Such

tempera-turewaschosentobehigherthanPLAglasstransitiontemperature, andlowerthancoldcrystallizationandmeltingones.Pieceswere holdbyagripononeside,theothersidebeingletfreeintheair. Aftertheheattreatment,thesamplebendbyitsfreesideandangle variationbetweenthetwosidescouldbethendetermined(Fig.2). Thisvariationwasmeasuredonasideviewpictureofthe sam-plewiththeImageJsoftware.Twosampleswereheat-treatedby lot,afterastorageofatleasttwoweeksincontrolledconditions (25◦_C;_60%_RH)._The_heat_deformation_of_PLA_{biocomposites}_(%)

wasthendefinedastheratiobetweentheanglevariationandthe initialhorizontalangle(180◦_)._In_this_{non-normalized}_test,_no_force

wasappliedtothesample,exceptitsownweightandpossiblyair flowintheoven.Consequently,theresultspresentedinthisstudy canbeconsideredasafirstclueofshapedeformationimprovement butdonotrepresentheatstabilityunderloadasthetest determin-ingtheheatdeflectiontemperature(HDT)cando(standardASTM D648).

Fig.2. Representationofsamplebendinganddeformationangle,blackcircles rep-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◦_C_temperature.

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 20₄₀ 20₁₅ 51₄₁(0.5)_(1.0) 28₄₄(0.5)_(1.2) 131₁₄₀ 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 20₄₀ 20₂₀ 53₆₈(0.6)_(4.0) 34₇₃(0.7)_(5.3) 136₁₇₄

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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.BlendingPLAwith mis-canthusfibresalsoledtoapressureriseintothedie(from18to 56bars).Asdiegeometryandtemperaturewerekeptconstant,Pmat

responsemustbelinkedtotwodifferentparameters,i.e.die fill-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_),_K_is_the

consistency(Pasn₎_and_n_is_the_power-law_index._The_coefficient

ofdeterminationR2_was_around_0.99_for_all_the_curves._The_K

val-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 showedthedifficultiesfortheblendstoflowathighfibre concen-trations.Indeed,fibres,thatremainedsolid,weredispersedinto moltenpolymer,hinderingitsflowandcausingviscosityincrease, especiallyinthelowshearrateregion(177–435s−1_)._These

obser-vationswerecoherentwithpreviousworksdoneinfilledpolymer systems(Kalaprasadetal.,2003;Guoetal.,2005).Theorientationof thefibres,theirinteractionsbetweeneachothersuchastheir inter-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,global fill-ingratio(FR)alongthescrewswaskeptconstanttoo,astheywere

Fig.4.Evolutionoftesttorque(T)andmatterpressureinthedie(Pmat)withthe

screwspeed(n)atsetfeedingrate(Q=20kg/h)andatsetQ/nratio(0.13kg/h/rpm) for20wt%miscanthusfilledcompounds.

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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,whenthescrewspeedvariedatthesame feed-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,theenergyneededforcompounding bam-boofibreswithPLAwaslowerthanformiscanthus(145(Wh)/kg). Inthecaseofmiscanthus,matterpressureinthediewasalsohigher thanfor bamboo gradeswhile nodifferenceswereobserved in rheologicalmeasurementsbetweenthefibresat20wt%.This obser-vationcouldbelinkedtotheself-heatingofthematterinthedie. Temperatureinthediewassetat165◦_C._However,_real_matter

tem-peraturemeasuredinthatzonewasaround168◦_C_for_miscanthus

filledcompoundandaround170◦_C_for_bamboo_filled_compounds.

Thedifferenceinself-heating,thatwasalittlehigherwithbamboo fibres,couldcomefromdifferenceinfibreabrasiveness,depending

onitsoriginandchemicalcomposition.Meltviscosityinthediewas surelylowerforthebamboobasedblendswiththehighest mea-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 compoundingwaslowerforB1thanforthethreeothergrades pre-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.These difficul-tiesresultedinaninhomogeneousmixing.Meltpolymerflowwas probablyperturbedbythepresenceoffibreagglomeratesor par-tiallywettedfibres,whichhaddifficultiestogetorientedintheflow direction.Indeed,moststudiesshowedthatathighfibre concentra-tions,interactionsbetweenfibresaremorenumerous.Interstitial spacesbetweentheagglomeratedfillers,containingimmobilized polymer,canchangesystembehaviourasifthefiller concentra-tionwasactuallyhigherthanwhathadbeenadded(Utrackiand Fisa,1982).Fig.3BshowedthatB4gradeactuallyexhibitedhigher viscositycomparetotheothergrades.Theviscosityincrease prob-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 smalldecreaseinlength with-outaffectingthediameter.Thiswasduetofibreorientationinrod, mainlyfollowingtheflow,andresultedinanadditionalaspectratio loss(from5.7to5.3)oflowsignificance comparetothe reduc-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

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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,fibrediameterincreased progres-sivelybutslightlywiththescrewspeed(from0.17to0.20mm). Meanaspectratioswere5.9,5.2,6.3and5.6,forfeedingratesof 13,20,30and40kg/h,respectively.Forinvestigated compound-ingconditions,screwspeedappearedtobeinsufficientneitherto breakfibresnortoseparatebundles.Byincreasingscrewspeed, residencetimeintotheextruderdecreased,leadingtomoreshear butonashorterperiod.Inaddition,meltedPLAwasfluidizeddueto itsshear-thinningbehaviour,andsoithadfewerdifficultiestobe conveyedthroughrestrictedareasalongthetwin-screwextruder.

WollerdorferandBader(1998)haveshownthatthematrix rheo-logyhadanimpactonthefinalsizedistributionsofflaxfibres.When feedingratewaskeptat20kg/h,byincreasingthescrewspeed,the fillinginmixingzoneregionswasreduced,whatcouldalsoreduce theirefficiencytotransfershear.Inthatcase,a0.09kg/h/rpmQ/n ratiocorrespondingtoa225rpmscrewspeed couldbe consid-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,gapbetweengradeswasdramaticallyreducedand dif-ferencesinaspectratioswerefoundtobeminor(from4.8to5.6) comparedtowhattheywerebeforeprocess(from8.6to17.2).Few differencesinsize,between80/20and60/40PLA/bamboo compos-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),whatwasclosetothe distribu-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 thetwofibreconcentrationsforbothflexuralandtensile modu-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,inthecaseof60/40 com-posites,reductioninflexuralstrengthwasobservedonlywithB3 andB4grades,meaningthatcompoundsfilledwithmiscanthusor withB1andB2gradesrevealedflexuralstrengthshigherthanPLA one.Bestflexuralstrengthat40wt%fibreloading(80MPa)was

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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.Thiscouldbeanevidenceofan inhomo-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.

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Ifaspectratiovariationsbetweenbamboofibreswerefoundto benegligible,variationsinlengthanddiameterweremore impor-tant(Table4).Thelongestfibres,i.e.B1andB2grades,reinforced themorePLAinflexuralstrength,evenleadingtoitsincreaseat a40wt%fibreloadingasabovementioned.Diameterandparticle shapemostlikelyaffectedtensileandflexuralstrengths,andlarger bundlesobservedforB1andMISfibrescouldexplainworse per-formancesforthemthanforB2gradeonthematerialstrengthening (Fig.7).Otherparameterscouldhaveplayedarolesuchasthefibre dispersion,thefibresurfaceoritsmorphology.Chemical composi-tionisalsowellknowntoinfluenceintrinsicfibrepropertiesand so,itsreinforcement.Amountofcelluloseisoneimportant param-eter.Themorethecellulosein thefibre, thehigher itsintrinsic modulusandstrength(Mohantyetal.,2000).Thebest mechani-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)andcold

crys-tallizationtemperatures(Tcc):from55.1◦C and97.1◦C for neat

PLA,respectively,to56.3–58.0◦_C_and _97.8–101.8◦_C_for

compos-ites(Table5).Onthecontrary,meltingtemperature(Tm)remained

unchangedbythefibrepresence.Changesincoldcrystallization temperatureuptoalmost5◦_C_surely_traduced_more_difficulties_for

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.Thisis possi-blyduetoaninhomogeneousfibremixinganddispersion.Mathew etal.(2006)observedthatfibrescouldpromotecrystallinityinPLA, crystallitegrowthbeinginitiatedatthefibresurface.An inhomoge-nousfibredispersionandclusterformationreduced thespecific

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

fibresurfaceincontactwithPLA,andsotheareaswherecrystallites canbeformed.Regardingtochainmobility,sampleswereanalysed byDMAandthermogramsrevealedaheightreductionofloss fac-torpeak(Fig.9).Thisphenomenonwasimputedinpreviousworks toareducedchainmobility(Nyamboetal.,2010;Sreekumaretal., 2010)andresultedfromthesterichindrancecausedbyfibre dis-persiononpolymerchainmotion,aswellastheslightcrystallinity increasementionedabove.Indeed,mobilityofpolymerchain orga-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)

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Table6

DSCandheatdeformationdataforPLA/bamboocompositesproducedata150rpmscrewspeed.

Fibreloadinga _T g(◦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.

a_Feeding_rate_was₁₂_kg/h_for_the₄₀_wt%_B2_formulation,₁₅_kg/h_for_the₄₀_wt%_B1_formulation,_and₂₀_kg/h_for_all_the_others.

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

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

DSCresultsforbamboogradesarementionedinTable6.No dif-ferenceswereobservedontransitiontemperaturedependingon thegrade,exceptwith20wt%B4filledcomposite.FortheB4grade, glasstransitionandmeltingtemperatureswerereducedfrom40 to20wt%fibreloading,whatcouldbetheclueofpolymerchain reduction. Size-exclusion chromatography (SEC) measurements gavePLA weightaveragemolarmasses(Mw)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)observedthese dif-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,whatcausedmoresteric hin-drance.Therewasacompetitionbetweenfibrenucleatingeffect andhindranceofchainmobility,limitingthecrystallitegrowth. Thiscompetitionbetweenthefillernucleatingeffectandits imped-ingeffectoncrystallizationathighloadingwasalsoobservedinPLA filledwithclaybyWuetal.(2007).

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

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Decreaseinpolymerchainmobilitywiththeshortestbamboo fibres,duetothepromotionofPLAcrystallinityandthesteric hin-dranceofmorenumerousparticles,wasconfirmedbyDMAresults (Fig.10).Forthesegrades,theheightoflossfactorpeakwasthe low-est.Consequently,fibresizeinfinalmaterial,reportedinTable4, playedaroleoncompositeheatdeformation(Table6)by reduc-ingpolymerchainmobility.Indeed,bestthermalstabilitieswere obtainedfromB3andB4grades,especiallywitha40wt%fibre load-ing:4.9and4.8%forheatdeformation,respectively.DMAprofiles of60/40PLA/bamboocompositesreportedinFig.10alsoshowed anadvantageofB3andB4shortergradesonthestorage modu-lusafterglasstransition.Theseobservationsareconfirmedinthe heatdeformationresultsof80/20PLA/miscanthuscompounds pro-ducedata20kg/hfeedingrate(Table5).Heatdeformationwas foundtobehigherforcompoundsextrudedathighscrewspeeds (225and300rpm,respectively),i.e.withlowerQ/nratios(0.09and 0.07kg/h/rpm,respectively).Thesecompoundscontainedlonger andlargerfibresinthefinalmaterial,especiallytheoneproduced ata225rpmscrewspeed(Fig.6).

Itcanbeconcludedfromtheseresultsthatmaterialthermal stabilityisimprovedbyreducingthepolymerchainmobility.This effectwasobtainedwithhighfibreconcentrationsandwiththe useofsmallfibres,whichbothrestrictedpolymerchainmotionby stericimpedingandcrystallinitypromotion.

4. Conclusion

MiscanthusfibreandbamboofibrereinforcedPLAcomposites werepreparedbytwin-screwextrusion.Influenceofcompounding parameterssuchasscrewspeedandfeedingrateonfibre morphol-ogyandfinalmaterialspropertieswereevaluated.Increasingthe screwspeed,andsotheshearrateintotheblend,asincreasingthe feedingratehelpedtosavefibrebundlelength,whatwasattributed toalowerresidencetimeinthemachineandareducedblend vis-cosity.However,thedifferencesin fibremorphologyinside the injectedmaterialswerenotenoughimportanttoseedifferences intheirmechanicalproperties.Compositeheatstabilitywas bet-terforcompoundsextrudedatlowerscrewspeedforwhichless mechanicalenergywasspent.Using150rpmscrewspeedwasthe bestchoiceforenergysavingas100rpmwasinsufficienttohave homogenousproperties.

Thebamboogradestudywithfourdistinctinitialsize distribu-tionsshowedthatthelongerthefibreswerebeforetheprocess,the moretheywerebroken.Onthemechanicalpropertypointofview, longerfibreswouldbepreferredastheyreinforcedthematerial themost,especiallyinflexuralsolicitation.Dispersingnumerous shortfibresimprovedthermo-mechanicalpropertiesandreduced thematerialheatdeformation,asthesefibrespromotedPLA crys-tallinityandrestrainedpolymerchainmobility.

Acknowledgements

WewouldliketothanktheFrenchEnvironmentandEnergy Management Agency (ADEME) and the “Conseil Général des Hautes-Pyrénées”whichhaveparticipatedtothisworkvia finan-cialsupport,aswellasNathalieAubazacandJoëlAlexis(LGP)for theirhelpforSEMimages.

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