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How to combine a hydrophobic matrix and a hydrophilic filler without adding a compatibilizer – Co-grinding enhances use properties of renewable PLA-starch composites

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To link to this article: DOI: 10.1016/j.cep.2012.03.005

URL :

http://dx.doi.org/10.1016/j.cep.2012.03.005

This is an author-deposited version published in: http://oatao.univ-toulouse.fr/

Eprints ID: 5695

To cite this version:

Le Bolay, Nadine and Lamure, Alain and Gallego Leis, Nora and Subhani,

Arfan How to combine a hydrophobic matrix and a hydrophilic filler

without adding a compatibilizer – Co-grinding enhances use properties of

renewable PLA-starch composites. (2012) Chemical Engineering and

Processing: Process Intensification, vol. 56 . pp. 1-9. ISSN 0255-2701

O

pen

A

rchive

T

oulouse

A

rchive

O

uverte (

OATAO

)

OATAO is an open access repository that collects the work of Toulouse researchers

and makes it freely available over the web where possible.

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(2)

How

to

combine

a

hydrophobic

matrix

and

a

hydrophilic

filler

without

adding

a

compatibilizer

– Co-grinding

enhances

use

properties

of

Renewable

PLA–starch

composites

Nadine

Le

Bolay

a,∗

,

Alain

Lamure

b

,

Nora

Gallego

Leis

b

,

Arfan

Subhani

b aUniversitédeToulouse,LGC,UPS-INPT-CNRS,ENSIACET,4alléeEmileMonso,BP44362,31030ToulouseCedex4,France bUniversitédeToulouse,CIRIMAT,UPS-INPT-CNRS,ENSIACET,4,alléeEmileMonso,BP44362,31030ToulouseCedex4,France

Keywords: Poly(lacticacid) Starch Grinding Composites Properties

a

b

s

t

r

a

c

t

In ordertoavoid theuseofcompatibilizersorplasticizers,co-grindingwasperformedtoproduce PLA–starchcompositematerials.Fragmentationandagglomerationphenomenawereanalysedto pro-poseaproductionmechanism.Co-grindingenhancesdispersionofthefillerinthematrixandinteractions betweenthematerials.Consequentlywhileblendingthetwomaterialshasanegativeeffecton mechani-calproperties,co-grindingpermitstoimprovethemifoptimizedoperatingconditionsareapplied.Water uptakeanddiffusionarealsocontrolledbyco-grindingconditions.Thistreatmentallowstheproduction ofcompositematerialsofferinggoodusepropertieswithoutanyuseofacompatibilizeroraplasticizer.

1. Introduction

Eco-designofpolymersandcompositesisbasedonanimportant

issuethatfacedsocietyoverthelasttwodecades:whattodowith

tonsofnon-degradablewastesissued frompetrochemical

poly-merswhich have alreadycaused seriousenvironment damage?

Forseveralyears,theuseofbiodegradablepolymersisfavoured,

inparticularinshort-lifeapplicationsforwhichpetroleum

poly-mersareparticularlynotinteresting.Oneofthewaystopropose

biodegradablepolymersisthedevelopmentofsyntheticpolymers

usingmonomersfromnaturalresources.Apromisingpolymerin

thisregardispoly(lacticacid)(PLA),becauseitismadefrom

agricul-turalproductsandisbiodegradable.PLAiseasilyhydrolysedinthe

presenceofmoisture,anditshydrolysedproductsarenon-toxic.It

offersmanyexcellentpropertiessuchasgoodbiocompatibilityand

manufacturing. However,synthetic biodegradablepolymers are

oftenmoreexpensivethanpetrochemicalpolymersand

mechan-icalproperties(stressandelongation)mustbeadaptedtotheir

application(drugdelivery,scaffold,packaging,etc.).

Like manypolymersfrompetroleum,polymersfrom

renew-able resources are rarely used alone, and biodegradablefillers

are oftenadded. Amongthem, starchis widelyused since itis

∗ Correspondingauthor.Tel.:+33534323682;fax:+33534323700. E-mailaddress:nadine.lebolay@ensiacet.fr(N.LeBolay).

completely biodegradable, renewable, cheap (what permits to

reducethecompositecost)andenhancesthecomposite

biodegrad-abilityrate.

Unfortunately,PLAandstarcharethermodynamically

immisci-blesincePLAishydrophobicandstarchishydrophilic.Thisleads

topooradhesionbetweenthetwocomponents,andconsequently

poorand irreproducibleperformances. Zhangand Sun[1] have

determinedthetensilestrengthandtheelongationofPLAaloneand

ofaPLA–starchblendandtheyhaveobservedthatbothproperties

oftheblendaredividedbytwocomparedtothoseofPLA.

Sinceinterfacialadhesionplaysavitalroleinpropertiesof

poly-mericcomposites,various compatibilizerswereinvestigated by

differentauthorstoimprovetheinteractionsbetweenthe

com-ponentsconstitutingtheblends.Yuetal.[2],Acioli-MouraandSun

[3]andWangetal.[4]usedmethylenediphenyldiisocyanate,what

permittedtoimprovetheinterfacebetweenthematrixandthe

filler.PLA–starchmixtureshadmechanicalpropertiesclosetothose

ofPLA.However,methylenediphenyldiisocyanateisnota good

candidateforenvironmentandisunsuitableforfoodpackaging.

ZhangandSun[1]usedmaleicanhydrideandaddedaninitiator

toimprovecompatibilitybetweenPLA,starch andmaleic

anhy-dride.Mechanical propertieswereimprovedcomparedtovirgin

PLA/starchbuttheywerelowerthanthoseofPLAalone.However,

compatibilizersareundesirableforfull-biodegradation.

Other authors added plasticizers to improve the interface

betweenPLAandstarch.Theproductsoftenusedasplasticizers

(3)

aresmallmolecularagentssuchascitricacidester,glycerol,

for-mamide,polyethyleneglycolorwater[5–7].Theuseofplasticizers

leadstoadditionalproductioncost.

Co-grindingisanalternativetoimprovetheinterfacebetween

thematrixandthefillerwithoutaddinganyagent.Thisprocesswas

usedtoproducecompositesmadeofpetrochemicalpolymersfilled

withamineral[8],non-degradablepolymers[9]orstarch[10,11],

anditwasshownthattheusepropertiesofcompositematerial

couldbesignificantlyimprovedincomparisontoblendsdueto

betterinteractionsbetweenthecomponents.

Inthiswork,starch-filledPLAcompositematerialswere

pro-ducedbyco-grindinginatumblingballmill.Fragmentationand

agglomerationphenomenawerestudiedtounderstandhowthe

compositematerialsareformed.Inthisway,thesizedistribution

andmeansizeoftheparticlesweremeasuredandtheparticles

wereobservedbyscanningelectronmicroscopyinorderto

pro-poseamechanismbywhichthecompositematerialisproduced.

Thenastudyofthesurfacepropertiesofthematerialsgroundalone

andco-groundwasperformedtounderstandtheevolutionofthe

interactionsbetweenmatrixandfillerduringgrinding.These

dif-ferentpropertieshaveaninfluenceontheuseproperties;thatis

whyit isveryimportanttounderstandhowtheyevolveduring

co-grinding.Finally,theevolution ofuseproperties(mechanical

properties,water uptake and diffusion) duringco-grindingwas

characterized,theobjectivebeingtostudyifco-grindingpermits

toenhancethesepropertiescomparedtothoseofblendsandto

definethebestco-grindingconditions.

2. Materialsandmethods

l,d-Polylacticacid(PLA)PABRL-68(Galactic,Belgium)

contain-ingapproximately12%ofd-lacticacidwasusedasthematrix.Its

molecularweight,Mn,givenbythesupplier,isequalto68kDa,

whilethepolymolecularindexMw/Mnis2.78.SincePLAwas

sup-pliedintheformofgranulesofapproximately3mm,theseones

werepreviouslyground during3mininathermostatically

con-trolledlaboratoryknifemill.Thepowderwasthensievedandthe

particlesbelow400mmwerekeptfortheexperimentsperformed

inthepresentwork.ASEMpictureofpre-groundparticlesis

pre-sentedinFig.1a.ThePLAspecialgravityis1250kgm−3anditsglass

transitiontemperature,determinedexperimentallybydifferential

scanningcalorimetry(DSCNetzschPhenix204),is58◦C.A

simi-larvalueofTgwasproposedbyLuandChen[12].Themechanical

propertiesofthematrixweredeterminedexperimentally.Theyare

thefollowing:Youngmodulus=3.6GPa;stressatbreak=13.7MPa;

elongationatbreak=8.7%.Thesevaluesareintheorderof

magni-tudeofthedatagivenbythesupplier.

Thefillerwasawaxymaizestarch(Waxilys–Roquette,France).

Itismainlycomposedofamylopectine(99%). Theparticlessize

rangesfrom4to32mm.AsshowninFig.1b,theinitialsample

containsindividualparticlesandsomeagglomerates.Thefiller

spe-cialdensity is1330kgm−3 and itsglasstransitiontemperature,

determinedexperimentallybydifferentialscanningcalorimetry,is

90◦C.

Theproductsweredrygroundinalaboratoryceramictumbling

ballmill(Prolabo,France) havinga capacity of 5Land rotating

arounditshorizontalaxis.Themillchamber containedceramic

ballshavingthreediameters(5.5,9.3and17.5mm)witha

propor-tionof¼,¼and½respectively.Theinterestofusingdifferentball

sizesistohavealwaysaballsizeadaptedtothedecreasingparticle

diameter.Theballfillingratewasfixedat20%ofthechamber

vol-ume.Eachmaterialwasfirstgroundalonetounderstanditsown

behaviourduringthetreatmentandthenthetwoproductswere

groundtogethertoproducethecomposite.Duringeachexperiment

thepowderfillingrate,correspondingtothepercentageofthefree

Fig.1. SEMmicrographof(a)pregroundPLAparticlesand(b)starchparticles.

volumebetweentheballsoccupiedbythepowder,wasfixedat

10vol.%.Theballandthepowderrateswerelowcomparedto

val-uesusuallyusedinindustrialtumblingballmillstominimizethe

powderconsumption.Therotationspeedofthemillwasfixedat

100rpmwhichcorrespondsto75%ofthecriticalspeed.Thestarch

percentageinthemixtureswasequalto20wt.%,sinceitwasshown

inpreviousstudiesthatafillerrateof20–30%permitstohavethe

bestusepropertiesofco-groundcompositescontainingminerals

orstarchasfillers[13–15].

Powdersamplesweretakenatdifferentgrindingtimesin

var-iouszonesofthemilltobesurethattheywererepresentative

ofthewholeparticles.Thesamplequantitiesweresmallenough

nottosignificantlymodifythepowdervolumeinthechamber.For

someanalyses(granulometry,morphology)thepowderwasused

assampled,whileforothers(surfacecharacterization,mechanical

propertiesandwateruptake)pelletswereneeded.Inthislastcase,

thepowderwasintroducedinacylindricalmouldandcompactedat

60◦CinaCarverpress.Apressureof150barswasappliedduring

15min.Theseconditionspermittohavethebestreproducibility

oftheanalyses.Thepelletsdiameterwas12mmandtheirmass

wasaround500mgforwateruptakeand200mgforsurfaceand

mechanicalproperties.

Thesizedistributionsofthepowders,expressingthevolume

percentageofparticlesindifferentsizeclasses,weremeasuredby

meansofadrylaserdiffractiongranulometerMalvernMastersizer

2000.ThedataweretreatedaccordingtotheMietheorywhich

permitstolimitartefactsatsmallsizesofthesizedistributions.The

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Table1

Propertiesoftheliquidsat20◦ C. (kgm−3) (Pas) L(mJm−2) d(mJm−2) p(mJm−2) Diiodomethane 3320 2.8×10−3 50.8 50.8 0 Formamide 1130 4.55×10−3 58 39 19 Water 1000 1×10−3 72.8 21.8 51

wascalculated.Thereproductioninthesizemeasurementswas

verifiedandthedifferencebetweenthemeansizeswaslessthan

1%.Selectedpowdersamplesandpelletssurfaceswereobserved

usingascanningelectronmicroscopeLeo435VP.

Matrix–fillerinteractionswerecharacterizedbyanalysing

sur-faceproperties.Inthatway,asessiledropmethodwasadopted,

usingaDigidropContactAngleMeterfromGBXScientific

Instru-ments.Drops (3mL) of different liquidswere deposited onthe

pelletssurfaceandthestaticcontactangle()wasmeasuredby

meansofahighresolutioncameraandsoftware,calculatingthe

slopeofthetangenttothedropattheliquid–solidinterface.Five

measurementsweredonewitheachliquidondifferentlocations

ofthepelletsandthedifferencebetweentheangleswaslessthan

1◦.Twopolarliquids(waterandformamide)andanon-polar

liq-uid(diiodomethane)wereused,whosepropertiesarepresented

inTable1.Theenergycomponentsofthetwoproductsandofthe

compositesweredeterminedfromtheanglesandtheinfluenceof

grindingandco-grindingontheseparameterswasstudied.

Mechanicalpropertiesofpelletsweredeterminedatroom

tem-perature(21◦C)usingdiametralcompressiontestscalledBrazilian

disctests[16].Pelletswereplacedverticallybetweentwoplates

andanincreasingcompressionforce(F)wasapplieduntilthe

rup-tureofthepellets.Theappliedforceandthespacebetweenthe

platesweremeasured.Therelatedtensilestress()canbe

calcu-latedfromtheforceaccordingtothefollowingequation:

= 2F

.Dp.H

(1)

whereDpandHarerespectivelytheinitialdiameterandthe

thick-nessofthepellet.

Thestrain(ε)istheratiobetweenthediametervariationofthe

pelletanditsinitialdiameter,expressedinpercentage.Thestress

andstrainatbreakandtheYoungmoduluswerecalculatedfrom

themeasureddata.Threeexperimentswererealizedforeach

sam-pleanditwascheckedthatthereproducibilityerrorwaslessthan

5%.

Tostudythebehaviourofthegroundandco-ground

materi-alsinwater,pelletswereintroducedinflaskscontainingdistilled

waterandregularlyremovedfromwater,driedonapapersheet

andweighted.Theexperimentswereperformedduring16months

andthewateruptake,WU,wascalculatedwiththeequation:

WU=mt

mi

mi

×100 (2)

wheremiistheinitialpelletmassandmtthepelletmassat

immer-siontimetimm.

3. Experimentalresultsanddiscussions

3.1. Evolutionofthesizeandmorphologyoftheparticles

Thetwoproductswerefirstgroundseparatelytostudytheir

fragmentation and agglomeration mechanisms. Concerning PLA

grinding, the evolution of the size distribution is presented in

Fig.2.Initially,thesizedistributionisspread.Rapidlyaftergrinding

begins,adisplacementofthesizedistributiontowardstherightis

observed,whichmeansthatthesmallestPLAparticlesarenomore

presentinafreestateinthemillchamber.Thisisduetoan

agglom-erationphenomenonasitcanbenoticedontheSEMmicrographof

Fig.2.VariationofthesizedistributionsofgroundPLA.

Fig.3takenafter15minofgrinding.SmallPLAparticlesarestuck

onbigones.Thisphenomenonhasbeennoticedduringthefirst3h

oftheexperiment.After8h,thesizedistributionshiftstowardsthe

left,i.e.towardsthesmallsizesbecausePLAparticlesbegintobreak.

Alltheseobservationsleadtothevariationofthemeansize

plot-tedinFig.4.ThePLAmeansize,initiallyequalto200mm,increases

rapidlyduringthefirst3htoreach400mm,andthenlevelsofffor

5h,tofinallydecreaseslowly.

Asforstarch(Fig.4),itsinitialmeansizeis13mm,i.e.lowerthan

theminimumsize(severaltensofmm)thatcanbereachedwhen

grindingmacromolecules[17].Consequently,itisnotreduced

dur-ingthetreatment.Itisconstantduringthefirst2handincreases

then.ItwascontrolledbySEMthatduringthefirstperioda

dis-aggregationof thestarch agglomerateshappens. Then, thesize

increase is the result of agglomeration of individual particles.

Finallythesizeoscillatesduetoacompetitionbetween

agglom-erationofindividualparticlesandfragmentationofagglomerates.

Inasecondtime,PLA–starchmixtureswereco-ground.The

vari-ationwithco-grindingtimeofthesizedistributionispresentedin

Fig.5.Theinitialsizedistributionisbimodal(PLApeakatlargesizes

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9

Fig.4.InfluenceofthegrindingtimeonPLAandstarchmeansizes.

andstarchpeakatsmallsizes).Undertheeffectofco-grinding,the

starchpeakdisappearsduringthefirst3hduetoagglomerationof

itssmallparticlesonPLAbigones(Fig.6a).OnthisSEMmicrograph,

onecanseethatstarchparticlesareindividual,whatmeansthatthe

agglomeratesobservedinFig.1bhavebeendissociated.Themean

sizeincreasesfrom176mmto250mmduringthisperiod.

After-wardstheremainingpeak(athighsizes)shiftstowardstheright.

ThismaybeattributedtothefactthatsmallPLAparticlesstickon

bigones,asitcanbeseenontheSEMmicrographofFig.6btaken

after8h.Thusthemeansizeincreasesdrasticallytoreach392mm

atthattime.Finallythepeakatlargesizesshiftstowardstheleft,

i.e.towardssmallersizesandthemeansizedecreases,stronglyin

afirsttimetoreach52mmafter45handmoreslowlythenupto

25mmafter76h.TheSEMmicrographofFig.6cpermitstoexplain

thisevolutionbyabreakageoftheagglomerates.Composite

parti-clesareproduced,inwhichstarchiswelldispersedinPLAandwell

stuckonthematrix.Moreover,thepresenceofstarchpermitsto

limitagglomerationbetweenPLAparticlesthemselvesandfavours

thefragmentationphenomenonsinceafter30hthemeansizeis

equalto209mmforPLAaloneandto151mmforthemixture.

3.2. Characterizationofthesurfacepropertiesoftheproducts

groundaloneandco-ground

The characterization ofthesurface properties ofthe matrix,

thefillerandthemixturepermitsabetterunderstandingofthe

intermolecularinteractionsbetweenthematerialsconstitutingthe

compositeandoftheeffectofco-grindingontheseinteractions.

Theseonesinfluencetheusepropertiesofthecomposite.Whileit

ispossibletodeterminedirectlythesurfaceenergyofliquids,inthe

caseofsolidsthelackofmoleculesandatomsmobilityimposesto

Fig.5.Variationofthesizedistributionsoftheco-groundmixtures.

Fig.6.SEMmicrographsofmixtureparticlesataco-grindingtimeof(a)1h;(b)9h; and(c)30h.

useindirectmethodssuchasthestudyoftheinteractionsbetween

thesolidandaliquid.

Whena liquid dropis deposited ona solid surface,

equilib-riumbetweenthesolid,theliquidandthevapourisestablished,

leadingtoacontactangle,,definingthewettabilitybetweenthe

liquidandthesolid.Thisanglecorrespondstotheminimumenergy

betweenthethreephasesatequilibrium. Differentmodelshave

beenproposedintheliteraturetoestablisharelationshipbetween

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them,twomodelsareoftenusedintheliteratureandwerealready

appliedtocompositesproducedbyco-grindingandmadeof

syn-theticpolymericmatrix(poly(vinylacetate)andpolystyrene)anda

fillerconsistingofamineral(calciumcarbonateandsilica)orstarch

[8,10,11].Sincetheyhavebeendevelopedindetailsinthese

refer-ences,onlyashortdescriptionwillbegiven,necessaryforthestudy

presentedhere.

Intheirmodel,OwensandWendt[18]consideredthatatotal

surfacetensioncan beexpressedasthesumof dispersive( d)

andnon-dispersiveorpolar( p)components,thelastone

result-ingfromhydrogenbonding anddipole–dipole interactions.The

relationpermittingtodeterminetheenergycomponentsfromthe

contactangleis:

1+cos 2 L=

q

d S Ld+

q

Sp Lp (3)

whereindexesSandLcorrespondtosolidandliquidrespectively.

Van Oss et al. [19] proposed the so-called Lifshitz–van der

Waalsapproachinwhich thetotalsurfacetensionisdividedin

Lifshitz–vanderWaals( LW)andacid–base( AB)components.The

lastoneisdecomposedinacid( +)andbasic( )components.The

expressionusedinthismodelisthus:

1+cos 2 L=

q

LW S LLW+

p

+ S − L +

p

− S + L (4)

In previousstudies [8,10,11],it was shown that the

disper-sivecomponentforthefirstmodelandtheLifshitz–vanderWaals

componentforthesecondonedonotevolvesignificantlyduring

grindingorco-grinding.TheacidcomponentoftheLifshitz–van

derWaalsmodelwasverylowinallcases,indicatingthatthe

poly-mersandthecompositesareelectrondonors.Moreoverthepolar

componentofthefirstmodelandthebasiccomponentofthe

sec-ondoneevolveinthesamewayduringgrindingorco-grinding.

Finally,consideringtheLifshitz–vanderWaalsmodel,thestudy

oftheinfluenceofgrindingorco-grindingonsurfaceproperties

shouldnotbebasedontheanalysisoftheglobalacid–base

com-ponentbecauseitisacombinationoftwophenomena(acidand

basic)thatcanevolveinanoppositemanner.

Duetoalltheseconsiderations,itwasdecidedtoapplyonly

themodelofOwensandWendttotheresultsofthisstudy.The

evolutionofthecontactanglesmeasuredwiththetwomaterials

groundseparatelyisnotpresentedhere.Onlythedataonthewater

anglesarediscussed.Indeed,initiallytheyareequalto16◦forstarch

and72◦ forPLA,indicatingthatstarchisveryhydrophilicwhile

PLAishydrophobic.Asgrindingproceedstheanglebetweenwater

andPLApelletsdoesnotevolvesignificantly.Onthecontrary,as

detailedinapreviousstudy[11],forstarchitincreasesduringthe

first6hofgrindingduetothefactthatthematerialbecomes

pro-gressivelymorehydrophobic.Thecontactanglelevelsoffthensince

thermodynamicequilibriumisreached.

Theenergyvalues (dispersive,polarand total)oftheOwens

and Wendt modelare gathered in Fig.7 for thetwo materials

ground alone.The dispersive componentsdo not evolve

signif-icantly.Concerning thepolarenergy, the initialvalueof starch

ishigh(33mJ/m2)duetothepresenceofnumeroushydrophilic

hydroxyl( OH−)groupsinglucoseunitswhilethatofPLAislower,

resultingfromabalancebetweenpolarcarboxyl( C O)groups

andnon-polarmethyl( CH3)groups.Duringgrinding,thestarch

polarenergydecreases,probablyduetoconformation

modifica-tionsofstarchmolecules,whilePLApolarenergyremainsquite

constant.Thetotalenergiesevolveasthepolarones.

Thesamekindofstudywasdonewhileco-grindingthe

mix-ture(Fig.8).Asalreadyindicatedpreviouslyforthetwomaterials

groundalone,thevariationofthedispersiveenergyisnot

signifi-cantwhenco-grindingthem.Theinitialpolarenergyofthesimple

mixtureisequalto25mJ/m2(i.e.betweenthevaluesofbothinitial

Fig.7. InfluenceofgrindingonthesurfaceenergiesofPLAandstarch.

materials),indicating theirsimultaneouspresence atthepellets

surface (seeFig. 9a where starchcan beseen amongPLA).

Co-grindingleadstoadrasticdecreaseofthisparameterduringthefirst

8h.Thiscanbeexplainedbytheobservationsmadeonsize

distribu-tions.IndeedsmallPLAparticlesstickprogressivelyonPLA–starch

agglomerates,thuscoveringstarchparticles(seeFig.9bwherethe

pelletsurfaceismorehomogeneousthanthesurfaceobservedin

Fig.9a).Consequently,thepelletssurfacebecomesmore

consti-tutedofPLAwhosepolarenergyislower.Forlongertimes,since

agglomeratesarebroken,starchmaybemorepresentonthepellet

surface.This,combinedtointermolecularinteractionsbetweenthe

twomaterialsgeneratesaprogressiveincreaseofthepolarenergy.

Finally,itremainsconstantatavaluecomprisedbetweenthoseof

thetwoseparatelygroundmaterials.

3.3. Characterizationofthemechanicalpropertiesofthe

materials

Twoseriesofexperimentswerecarriedout:onewithPLAalone

and onewiththecomposite mixture.Examplesofstress–strain

curves(ofPLAandmixturesgroundorco-groundduring1hand

30h)arepresentedinFig.10.ItcanbeseenthatthePLAbehaviouris

brittlewhilethecompositebehaviourvariesfrombrittletoductile

whentheco-grindingtimeisincreased.Moreover,afteratreatment

of1h,thelimitstressandstrainarelowerforthecomposite

mix-turethanforPLA,whileitistheoppositeafter30h.Fromthesefirst

data,onecanalreadysaythatadaptedco-grindinghasapositive

effectonthemechanicalpropertiesofthemixture.

Toenrichthisfirstanalysis,theevolutionoftheYoung

modu-lusversusthetreatmenttimeisplottedinFig.11aswellasthe

stressandstrainatbreakinFig.12.Thethreeparametersdonot

evolvesignificantlyforPLA.Onthecontrary,addingstarchtoPLA

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Fig.9.SEMphotoofapelletsurface((a)co-grindingtime=0and(b)co-grinding time=6h).

andmixingthem(withoutgrinding)hasaverynegativeeffecton

theparameterssinceadhesionofthefilleronthematrixisbad

andstarchhaslowermechanicalpropertiesthanPLA.Thiskindof

reductionofthemechanicalpropertieswhenaddingstarchtoPLA

wasalreadyobservedbyZhangandSun[1]whoaddeda

compati-bilizertoenhancethemechanicalproperties.

Thethreeparametersincreasewhenco-grindingisproceeded.

Thevaluesafter30hofco-grindingare2.2times(forε),3.5times

(for)and4.9times(forE)higherthanwithasimplemixingand

Fig.10. Examplesofstress-straincurvesforPLAandmixtures.

Fig.11.VariationoftheYoungmodulusofPLAandmixtures.

evenhigherthanthevaluesmeasuredforPLAalone.Thefilleris

welldispersedinthematrixandinteractionsbetweenthe

materi-alsarefavouredbyco-grinding.Theenhancementofthedifferent

parametersismoreimportantthatwhatwasobservedinaprevious

studyonco-groundcompositesmadeupofpolystyreneandstarch

(mechanicalparametersmultipliedbyafactorof1.5–2)[20]since

PLApossessescarboxylgroupswhoseinteractionswithhydroxyl

groupsofstarch,favouredbyco-grinding,certainlyhaveamore

positiveeffectonthepropertiescomparedtopolystyrene–starch

interactions.However,a toolong treatmentleadstoadecrease

ofthethreeparameters,probablybecauseofadegradationofthe

molecularchains.Co-grindingthemixtureduringanoptimumtime

permitstohavebettermechanicalpropertiesthanforthematrix

alone,withoutusingachemicalagent,whatisinterestingfor

envi-ronment.

3.4. Studyofthematerialsbehaviourinwater

Usingdegradablematerialsforpackagingpresentsagreat

inter-estforenvironment,butitisimportantthatthematerialisnottoo

rapidlydegraded.Itwasshowninapreviousstudy[11]thatadding

starchtopolystyrenehasanegativeeffectontheresistancetowater

ofblendswhentheyareonlymixedandaminimumco-grinding

time,thevalueofwhichincreaseswiththefillerrate,is

neces-sarytoenhancetheresistanceofthecompositestowaterwithout

anycompatibilizer.Inthiswork,itisshownhowtheincorporation

ofstarchinPLAandtheimplementationofco-grindingmayhave

aninfluenceonthewateruptakeanddiffusioninthematerials

usedinapelletform.Resultsonthematerialsgroundalonewillbe

presentedbeforethoseobtainedwiththemixtures.

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Fig.13.Influenceofgrindingonthewateruptakeof(a)PLA;(bandc)mixtures.

Concerningstarch,dataobtainedfor5monthsweredescribed

andcommentedindetailsinapreviousarticle[10].Insummary,

itwasshownthataminimumgrindingtimeof2hwasneededto

avoidquasi-instantaneousdisaggregationofthepelletsinwater.

Thereasonofthisistheverygoodaffinityofstarchwithwater,as

alreadyindicatedinthesectiononthesurfaceproperties,

result-ingofthepresenceofthehydroxylgroupsofglucose.Forgrinding

timesbetween2and5h,thepelletsresistancetowaterwasgood,

probablyduetoamodificationofthemolecularchainsofstarch,

andconsequentlya progressiveincreaseofthepelletsmass,i.e.

ofthewateruptake,wasnotedduringimmersioninwater.The

wateruptakeraisedupto60%after1000minofimmersion.After

onemonthofimmersion,thepelletconsistencychangedduetoa

progressivedegradationofstarch.Finally,atoolonggrindingtime,

generatinganexcessivedegradationofthemolecularchainsbythe

millballs,inducedarapiddisaggregationofthepellets.

ResultsobtainedwithPLAinthisstudyarepresentedinFig.13a.

Theimmersiontimeisinalogarithmicformtobeabletoanalyse

easilytheperiodofWUincreaseduringthefirstdaysofimmersion.

Threepartscanbeobservedinthecurves.Inthefirstone,before

60minofimmersion,WUdoesnotevolvesignificantly.Inthe

sec-ondpart(before30,000minofimmersion,i.e.about21days),WU

increasesduetoprogressivewaterabsorption.However,sincethe

wateruptakeneverexceeds2%,onecansaythatPLAhaslittle

affin-itywithwater.Thiscorroboratesthehighcontactanglesbetween

waterandPLAmeasuredwhenstudyingthesurfacepropertiesof

thematrix.DuetothelowvaluesofWUobserved,itisdifficultto

analyseanyinfluenceofthegrindingtime.Inthethirdpartofthe

curves(after30,000min),phasesofWUdecreaseandphasesofWU

increaseareobserved.Thisisduetotheprogressivedegradationof

PLA.Thewateruptakeisevennegative,whenthepelletsmassis

lowerthantheinitialmass,butthevaluesarelessnegativewhen

thegrindingtimeislong(6.6%fort=0and2.2%for1800min).

Finally,themixturesdataareshowninFig.13bandc.AsforPLA

alone,oneobservesthreezonesinthecurves:thefirstonebefore

60minofimmersionwhereWUdoesnotincreaseexceptwhen

thetwomaterialsarejustmixed,thesecondonebetween60and

10,000minofimmersion(around7days)wherethewateruptake

increases,andthethirdoneafter10,000minofimmersionwhere

thewateruptakefluctuatesstrongly.Theimmersiontimeatwhich

thefirstwateruptakefluctuationappearsislowerforthemixture

thanforPLAalone.

Asindicatedabove,thewateruptakedoesnotincreaseduring

the first 60min of immersion when thetwo materialsare

co-ground.Thismeansthatwaterdoesnotdiffuseinthepelletsas

soonastheyareintroducedinwater.Whenthetwomaterialsare

justmixed(co-grindingtime=0),thestarchagglomeratesobserved

ontheSEMphotoofFig.9afavourimmediateabsorptionofwater.

Inthesecondzone,thewateruptakeincreasesprogressively

withthe immersion time, and since starch is very hydrophilic,

adding 20% of filler permitstoincrease significantly thewater

uptake compared to the hydrophobic matrix alone. The water

uptakekineticsdependsontheco-grindingtime.Indeed,whenthe

twomaterialsarejustmixed(co-grindingtime=0),WUisthe

high-estwhenthepelletisimmersedlessthan1000min,whatisnotthe

caseafter1000min.Thismaybeattributedtothebadhomogeneity

ofthemixturealreadynoticedpreviouslyontheSEMmicrograph

ofFig.9a,butalsotobadinteractionsbetweenthematrixandthe

filleralreadyevoked,thatmayfavourabsorptionofwaterbystarch

inthefirstimmersiontimes.Whenco-grindingthetwomaterials

15min,thefilleriswelldispersedinthepelletanditsproportion

onthepelletsurfaceislessthanfortheungroundmixtureandis

certainlymorerepresentativeoftheproportioninthewhole

sam-ple.Thewater uptakeislowerthanforco-grindingtime=0.An

increaseoftheco-grindingtime(between15and180min)hasa

positiveeffectonthewateruptake.Thismaybeduetothe

disag-glomerationofstarchagglomeratesrelatedpreviouslythatleads

tothedispersionofsmallerindividualstarchparticleswithinthe

pelletsvolumeandfavourswaterabsorption.After360minof

co-grinding,therateofwateruptakedecreasesstronglyfirstandthen

more slowly(for co-grindingtimes higherthan1000min). This

decreaseoftheratemaybeattributedtoagglomerationofsmall

PLAparticlesonPLA–starchagglomeratesthatlimitsthepresence

ofthefilleronthepelletssurfaceandcreatesabarriertowater,

butalsotoanincreaseoftheinteractionsbetweenthematrixand

thefillerandadegradationofthemolecularchainsundertheeffect

ofalongactionofballs.After10,000minofimmersion,diffusion

equilibriumisreached.ThemaximumvalueofWU(atequilibrium)

isreachedforashorterimmersiontimethanwithPLAalone.This

canbeattributedtothepresenceofstarchthatisprogressivelyand

partiallyreleasedinwaterasalreadysuggestedbyAngellieretal.

[21].Thisreleaseoccursbeforethedegradationofthepelletsand

leadstoamassreduction,i.e.tothefirstWUdecrease.Thepellet

massmeasuredatequilibriumwillbenamedm∞.

Thethirdzonecorrespondstodegradationphenomenaasfor

PLAalone.Fortheveryfirstco-grindingtimes,WUismore

nega-tiveforthemixture(−8.3%)thanforPLA(−6.6%).Thismeansthat

starchcontributestodegradability.Then,mixturevaluesbecome

(9)

Fig.14.Determinationofthewaterdiffusionmechanismforthecomposites.

(WU=−10.6%).Itcanbeobservedthatthehigherthewateruptake

duringthewaterabsorptionphase(secondpartofthecurves),the

lowerthenegativevalueofWU.

For high co-grinding times (i.e. when the maximum water

uptakeislowinpart2ofthecurves),WUislessnegativetoreach

−2%.Thismaybearesultofstronginteractionsbetweenmatrix

andfiller,butalsoprobablyadegradationofthemoleculesunder

alongeffectofthemillballs.

To studythewater diffusionobserved inthesecond partof

thewateruptakecurves,Frisch[22]hasexpressedthediffusion

mechanismaccordingtothefollowingequation:

mt−mi

m∞−mi

=k·timmn (5)

In thisequation,nisthediffusionalexponentwhich

charac-terizesthediffusionmechanism.Indeed,whenwaterdiffusesina

polymer,thisoneswellsandtheswellingratedependsonthe

poly-merchainsrelaxation.Ifthewaterdiffusionrateislowerthanthe

relaxationrate,thediffusionisfickianandnisequalto0.5.When

theoppositeoccurs,themechanismiscontrolledbychain

relax-ationandnisequalto1.Thismodelhasalreadybeenusedtostudy

waterdiffusioninstarch[23,24]andinstarchfilledco-ground

com-posites[11,25].Moreover,inEq.(5),kisaconstantwhichcanbe

expressedasfollowswhenthediffusionisfickian[26]:

k= 4 H

r

D 

!

(6)

whereDisthediffusioncoefficientandHthepelletthickness.

Itwastriedtoapplythemodeltotheresultsofthisstudy.Since

PLAishydrophobic,waterdiffusionisverylowandnodiffusional

exponentanddiffusioncoefficientcanbedetermined.Asforstarch,

Seynietal.[11]haveshownthat,whenthegrindingtimepermits

tohaveagoodresistanceofthepelletstowater,thediffusionis

fickianandadiffusioncoefficientequalto1.1×10−10m2s−1was

determined.Russoetal.[23]havefoundasimilarvalue.

Concerning the mixtures, the variation of

ln[(mt−mi)/(m∞−mi)] has been plotted versus the logarithm

of theimmersion time (expressed in min) in Fig. 14 for some

representative co-grinding times. When no co-grinding is

applied, the points are gathered around a straight line for

valuesofln[(mt−mi)/(m∞−mi)]lowerthan−0.5,i.e.forvaluesof

(mt−mi)/(m∞−mi)lowerthan0.6whichisthevaliditylimitof

Eq.(5)[22].Forco-groundcomposites,whenlntimmislowerthan

4(immersiontime lowerthan1h)themodelisnotconvenient

since,asalready indicated, nowater uptake happens.Thenthe

pointsare distributed arounda line, the slope of which is the

diffusionalexponent.Thelinesequationshavebeendetermined

bylinearregression,andvaluesofnaregatheredinTable2.Since

thevaluesarecloseto0.5,onecansaythatthediffusionisfickian.

Table2

Valuesofthediffusionalexponents.

Co-grindingtime(min)

0 15 30 60 180 360 480 1260 1800 4570 n 0.54 0.51 0.46 0.51 0.55 0.55 0.54 0.52 0.54 0.55

Fig.15.Influenceoftheco-grindingtimeonthediffusioncoefficientsofthe com-posites.

Thediffusioncoefficients werethencalculated for thedifferent

co-grindingtimesusingEq.(6).TheyarepresentedinFig.15.The

mixturediffusioncoefficientsarelowerthanthatofstarchsince

thematrixismorehydrophobicthanthefiller.ThevariationofD

withtheco-grindingtimelogicallyfollowsthewateruptakerate.

Disthehighestatt=0sincethepresenceofstarchagglomerates

favourswaterdiffusion.Whenco-grindingisimplemented,

diffu-sioncoefficientsarelowerand,asforthewateruptakerateand

forthesamereasonsthatwillnotberepeatedhere,Dincreases

untiltheco-grindingtimeisequalto180minanddecreasesthen

toleveloffatlongtreatmenttimes.

4. Conclusions

PLA–starchcompositematerialswereproducedbyco-grinding.

Thetwoproductswerefirstgroundaloneandthenco-ground,and

sizeaswellasSEManalyseshavepermittedtoproposea

mecha-nismofcompositeproduction.

Thecharacterizationofthesurfacepropertiesoftheproducts

hasallowedshowingthatPLAisahydrophobicmatrixandstarch

isahydrophilicfiller,duetothepresenceofnumeroushydroxyl

groupsinglucose,butgrindingreducedthishydrophilicbehaviour

andthestarchpolarenergycomponent.Co-grindingthemixture

enhancesinteractionsbetweenthetwomaterialsthathaveagreat

effectontheusepropertiesofthecomposite.Indeed,asimpleblend

ofhydrophobicPLA andhydrophilicstarch leadstoa reduction

ofthemechanicalpropertiesofthemixturebyafactor2.5

com-paredtothoseofthesinglematrixduetophaseseparationandbad

adhesion.Onthecontrary,applyingco-grindingpermitstoincrease

themechanicalproperties,whichareeven higherthanthoseof

thesinglePLAmatrixforoptimizedoperatingconditions.Finally

interactionsbetweenmatrixandfillerinfluencealsothe

compos-itebehaviourinwater.Thewaterdiffusionrateandthemaximum

wateruptakeofthecompositesvary,dependingontheco-grinding

time.

Co-grindingpermitstoproducecomposite materialsoffering

goodusepropertieswithoutaddingacompatibilizerora

(10)

Acknowledgments

TheauthorswouldliketothanktheInstitutNational

Polytech-niqueofToulouseforthefinancialsupportofthisinterlaboratory

work.

StarchwasgraciouslyprovidedbyDr.S.Molina-Boisseaufrom

CERMAVGrenoble.

AppendixA. Nomenclature

d50 particlemeansize(mm)

D diffusioncoefficient(m2s−1)

Dp pelletdiameter(mm)

E Youngmodulus(GPa)

F compressionforce(N)

H pelletthickness(mm)

mi initialpelletmass(kg)

mt pelletmassatimmersiontimetimm(kg)

n diffusionalexponent

t grindingtime(min)

timm immersiontime(min)

WU wateruptake(%)

ε strain(%)

surfaceenergy(mJm−2)

 tensilestress(MPa)

 contactanglebetweenaliquiddropandthepelletsurface

(◦)

References

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Figure

Fig. 1. SEM micrograph of (a) preground PLA particles and (b) starch particles.
Fig. 3 taken after 15 min of grinding. Small PLA particles are stuck on big ones. This phenomenon has been noticed during the first 3 h of the experiment
Fig. 5. Variation of the size distributions of the co-ground mixtures.
Fig. 7. Influence of grinding on the surface energies of PLA and starch.
+4

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