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
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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
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,FranceKeywords: 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
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
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
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
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
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.
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
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
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
[1]J.F.Zhang,X.Sun,Mechanicalpropertiesofpoly(lacticacid)/starchcomposites compatibilizedbymaleicanhydride,Biomacromolecules5(2004)1446–1451. [2] L.Yu,K.Dean,Q.Yuan,L.Chen,X.Zhang,Effectofcompatibilizerdistribution ontheblendsofstarch/biodegradablepolyesters,J.Appl.Polym.Sci.103(2007) 812–818.
[3] R.Acioli-Moura,X.S.Sun,Thermaldegradationandphysicalagingofpoly(lactic acid)anditsblendswithstarch,Polym.Eng.Sci.48(2008)829–836. [4]H.Wang,X.Sun,P.Seib,Mechanicalpropertiesofpoly(lacticacid)andwheat
starchblendswithmethylenediphenyldiisocyanate,J.Appl.Polym.Sci.84 (2002)1257–1262.
[5] L.V.Labrecque,R.A.Kumar,V.Dave,R.A.Gross,S.P.McCarthy,Citrateestersas plasticizersforpoly(lacticacid),J.Appl.Polym.Sci.66(1997)1607–1613. [6]N.Wang,J.Yu,P.R.Chang,X.Ma,Influenceofformamideandwateronthe
propertiesofthermoplasticstarch/poly(lacticacid)blends,Carbohydr.Polym. 71(2008)118–119.
[7]O. Martin, L. Averous, Poly(lactic acid): plasticization and properties of biodegradablemultiphasesystems,Polymer42(2001)6209–6219. [8]C.Zapata-Massot,N.LeBolay,Effectofthemineralfilleronthesurface
prop-ertiesofco-groundpolymericcomposites,Part.Part.Syst.Charact.24(2007) 339–344.
[9]J. Pan,W.J.D.Shaw, Effectofprocessing parameters onmaterial proper-ties ofmechanicallyprocessedpolyamide, J.Appl. Polym.Sci. 56(1995) 557–566.
[10]A.Seyni,N.LeBolay,S.Molina-Boisseau,Ontheinterestofusing degrad-ablefillersinco-groundcompositematerials,PowderTechnol.190(2009) 176–184.
[11] A.Seyni,N.LeBolay,A.Lamure,Matrix–fillerinteractionsinaco-ground ecocomposite-surfacepropertiesandbehaviourinwater,PowderTechnol.208 (2011)390–398.
[12]Y.Lu,S.C.Chen,Microandnano-fabricationofbiodegradablepolymersfordrug delivery,Adv.DrugDeliv.Rev.56(2004)1621–1633.
[13]C.Zapata,N.LeBolay,C.Frances,S.Molina-Boisseau,Productionofsmall compositeparticlesbyco-grindinginamediamill– characterizationofthe granulometricandthemechanicalproperties,Trans.IChemEPartA:Chem. Eng.Res.Des.82(A5)(2004)631–636.
[14] A.Seyni,N.LeBolay,S.Molina-Boisseau,Mechanicalpropertiesand degrada-tionofstarch-filledpolystyreneco-groundcomposites,in:CHISA2008,Prague, CzechRepublic,2008.
[15]N.LeBolay,V.Santran,G.Dechambre,C.Combes,C.Drouet,A.Lamure,C.Rey, Production,byco-grindinginamediamill,ofporousbiodegradablepolylactic acid–apatitecompositematerialsforbonetissueengineering,PowderTechnol. 190(2009)89–94.
[16]F.F.L.Carneiro,A.Barcellos,Tensilestrengthofconcrete,RILEMBull.18(1953) 99–107.
[17]K.Schönert,Sizereduction(fundamentals)–Chapter1Ullmann’sEncyclopedia ofIndustrialChemistry.UnitOperationsI,vol.B2,VCHVerlagsgesellschaft, Weinheim,1988,pp.5.1–5.14.
[18]D.K.Owens,R.C.Wendt,Estimationofthesurfacefreeenergyofpolymers,J. Appl.Polym.Sci.13(1969)1741–1747.
[19]C.J.VanOss,M.K.Chaudhury,R.J.Good,InterfacialLifshitz–VanderWaals and polar interactions in macroscopic systems, Chem. Rev. 88 (1988) 927–941.
[20]A.Seyni,Propriétésphysico-chimiquesetd’usagedematériauxcompositesà chargedégradableproduitsparco-broyage,ThesisofInstitutNational Poly-techniqueofToulouse(2008).
[21]H.Angellier,S.Molina-Boisseau,L.Lebrun,A.Dufresne,Processingand struc-turalpropertiesofwaxymaizestarchnanocrystalsreinforcednaturalrubber, Macromolecules38(2005)3783–3792.
[22] H.L.Frisch,Sorptionandtransportinglassypolymers– areview,Polym.Eng. Sci.20(1980)2–13.
[23]M.A.L.Russo,E.Strounina,M.Waret,T.Nicholson,R.Truss,P.J.Halley,Astudyof waterdiffusionintoahigh-amylosestarchblend:theeffectofmoisturecontent andtemperature,Biomacromolecules8(2007)296–301.
[24] X.Yu,A.R.Schmidt,L.A.B.Perez,J.S.Schmidt,Determinationofthebulk mois-turediffusioncoefficientforcornstarchusinganautomatedwatersorption instrument,J.Agric.FoodChem.56(2008)50–58.
[25] N.LeBolay,A.Seyni,M.Hemati,Wateruptakeanddiffusioninco-ground starch-filledpoly(vinylacetate)composites,in:8thWorldCongressof Chem-icalEngineering–GreenPolymersSymposium,Montreal,Canada,2009. [26]J.Crank,MathematicsofDiffusion,2nded.,OxfordUniversityPress,NewYork,