Mechanical Properties of Biodegradable Compositesfrom Poly
Lactic Acid (PLA) and MicrocrystallineCellulose (MCC)Aji P.
Mathew,1Kristiina Oksman,1Mohini Sain21Department of Engineering
Design and Materials, Norwegian University of Science and
Technology,Trondheim, Norway2Earth Sciences Centre, Faculty of
Forestry and Chemical Engineering, University of Toronto, Toronto,
CanadaReceived 22 March 2004; accepted 27 October 2004DOI
10.1002/app.21779Published online in Wiley InterScience
(www.interscience.wiley.com).ABSTRACT: Biodegradable composites
were preparedusing microcrystalline cellulose (MCC) as the
reinforcementandpolylacticacid(PLA)asamatrix.
PLAispolyesteroflactic acid and MCC is cellulose derived from high
qualitywoodpulpbyacidhydrolysistoremovetheamorphousregions. The
composites were prepared with different MCCcontents, upto25wt%,
andwoodour(WF)andwoodpulp (WP) were used as reference materials.
Generally, theMCC/PLA composites showed lower mechanical
propertiescompared to the reference materials. The dynamic
mechan-ical thermal analysis (DMTA) showed that the storage
mod-uluswasincreasedwiththeadditionof MCC. TheX-raydiffraction
(XRD) studies on the materials showed that thecomposites were less
crystalline than the pure components.However, the scanning electron
microscopy (SEM) study ofmaterialsshowedthat
theMCCwasremainingasaggre-gates of crystalline cellulose brils,
which explains the poormechanical properties. Furthermore, the
fracture surfaces ofMCC composites were indicative of poor adhesion
betweenMCC and the PLA matrix. Biodegradation studies in
com-postsoil
at58CshowedthatWFcompositeshavebetterbiodegradabilitycomparedtoWPandMCCcomposites.Thecompositeperformancesareexpectedtoimprovebyseparationof
thecelluloseaggregates tomicrobrils andwith improved adhesion. 2005
Wiley Periodicals, Inc. J ApplPolym Sci 97: 20142025, 2005Key
words: bio-composites; microcrystalline cellulose; mor-phology;
dynamic mechanical thermal properties; mechani-cal
propertiesINTRODUCTIONCurrently, natural
berreinforcedpolymercompos-itestechnologyisfocusedoncreatinglowcost,highperformance,
and lightweight materials to
replacepurepolymersorglassbercomposites.Therehavebeen intensive
research and product development ofcomposite materials from
petroleum based polymerslike polypropylene and polyethylene
reinforced
withnaturalbers.15Thesecompositematerialsareusedextensivelyinautomotiveapplications,
buildingma-terials, and household products.68The advantages ofusing
lignocellulosic bers as reinforcements in differ-ent polymers are
reduced weight, relatively good stiff-nessandstrength, aswell
aslowcost andeaseofdisposal. Composites preparedusingjute, ax,
ba-nana, sisal, pineapple, coir, oil palm,
etchavebeenstudiedbyseveral scientists.911Thepotential prop-erty
improvement of any composite material dependson the degree of
dispersion and the degree of interac-tion/adhesion between the
matrix and reinforcingphase.12,13The effect of processing
techniques on theproperties of composites was investigated by
Mattosoandcoworkers, andtheyconcludedthattwinscrewextrusion
resulted in better ber dispersion with
bersbeingdissociatedtotheformof individual ultima-tum.14,15The use
of reinforcements that provide largesurface area is considered as a
method for
obtainingbetterinteractionbetweenthematrixandreinforce-ment,
leadingtobetter mechanical properties, heatresistance, dimensional
stability, etc.16,17However, of late, inspired by the growing
environ-mental awareness by all and new standards includingEndof
LifeVehicle(ELV) regulations intheEUautomotive sector, there is a
deliberate interest to lookfor systems that are even more
environmental friendlyand biodegradable. Therefore, materials based
on rawmaterialsderivedfromnaturalresourcesofplantoranimal origin
and synthetic polymers with biodegrad-ablebackbonesarebeingstudied.
Biopolymerslikesoy-oil based epoxy, starch based polymers,
polycap-rolactone(PCL), polyhydroxybutyrate(PHB), poly-lactic acid
(PLA), and polyester amide have been in-vestigated by scientists as
a potential matrix for biode-gradableandenvironmental
friendlycomposites.1822ThestudiesonPLAandespeciallyPLAbasedcom-Correspondence
to: K. Oksman ([email protected]).Journal of Applied Polymer
Science, Vol. 97, 20142025 (2005) 2005 Wiley Periodicals,
Inc.posites are very few, and works on natural ber com-posites lled
PLA is yet to be explored.2326Polylacticacid is a versatile polymer
made from renewable ag-ricultural rawmaterials andis
fullybiodegradable.Another feature that makes this polymer
interesting isthe fact that it can be processed similarly to
polyole-ns; furthermore, PLA owns good stiffness andstrength.
Polylacticacid(PLA) productsaremainlyusedinapplications as plastic
bags for householdwastes, barriers for sanitaryproducts
anddiapers,planting, anddisposablecups andplates, productswithnot
veryhighperformances. Themaindraw-backs withPLAproperties are
lowtoughness andthermal stability. Therefore, it is interesting to
study iftheincorporationofreinforcementscanimprovethetoughness and
thermal stability. In the present study,PLAbasedbiodegradable
composites are preparedwith microcrystalline cellulose as the
reinforcingphasebytwin-screwextrusionfollowedbyinjectionmolding.
Our previous study of PLA composites hasshown that there is no
degradation of PLA during theextrusion process.26Microcrystalline
cellulose (MCC), where the amor-phous regions are removed by acid
hydrolysis, can beaverypromisingcellulosic reinforcement for
poly-mers. It is basically crystalline cellulose derived fromhigh
quality wood pulp, and it is expected to disinte-grate into
cellulose whiskers after a complete hydro-lysis. Native cellulose
is one of the strongest and stiff-est natural bers available; the
theoretical modulus isestimated at 167.5 GPa,27and it has a high
potential toact as reinforcing agent in biopolymers. This
cellulosebril can be about 510 nm in diameter, and the lengthvaries
from 100 nm to several micrometers.28MCC hasthe advantage of high
specic surface area comparedto other conventional cellulose
bers.Thisworkwasconductedasapreliminarystudytowards PLA based high
performance nano compos-ites using MCC, and we wanted to explore
the possi-bilities of getting MCC dispersed in the PLA matrix
ascrystallinenanoreinforcements duringtheextrusionprocess. Wood our
(WF) and wood pulp (WP) com-posites were usedfor comparison. The
mechanicalpropertiesof thecompositeswerestudied, andthemorphology
of composite fracture surfaces was
exam-inedusingscanningelectronmicroscopy(SEM).Theviscoelasticproperties
andcrystallinityof materialswere studied. Further, the
biodegradability of the
sys-temwaslookedintobydegradationstudiesincom-post
soil.EXPERIMENTSMethodsMatrix. Poly-l-Lactic Acid, POLLAIT, was
suppliedby Fortum Oil and Gas Oy, Porvoo, Finland. The MFIfor the
PLA is between 1 and 2 g/10 min (190C, 2.16kg). The molecular
weight (Mw) of the PLAwas97,000g/mol. The synthesis and chemical
structure ofPLA from glucose is shown in Figure 1.Reinforcements.
The microcrystalline cellulose used asthe reinforcement is a powder
with a particle size of1015m.
ItwassuppliedbyBorregaardChemcell,Sarpsborg, Norway, and contained
93% of microc-rystalline cellulose. Wood our (WF) and wood pulp(WP)
were used as reference materials. WF (Pine 42)was supplied by
Scandinavian Wood Fiber AB, Orsa,Sweden, and WP (TerracelTM) was
obtained from Ray-onier, USA. The physical and chemical
characteristicsof all
threereinforcementsandthePLAmatrixaresummarized in Table
I.ProcessingThecompositematerialswerecompoundedusingatwin-screw
extruder (Coperion Werner and PeidererZSK 25 WLE) with a side
feeder and gravimetric feed-ing systems for both main and side
feeders. The pro-cessing parameters for extrusion are already
describedFigure 1 Synthesis and chemical structure of
PLA.BIODEGRADABLECOMPOSITESFROMPLA/MCC 2015in our earlier work on
PLA composites.26Compositeswere pelletizedfor further
injectionmolding.
TestsampleswereinjectionmoldedaccordingtotheISO294standardfor
thermoplastics usingaCincinnatiACT Milacron 50 injection molding
machine. The in-jectionmoldingwascarriedoutat200Cwithave-locity of
60mm/s. The mold temperature was kept at50C, and the pack pressure
was 400 bars. A coolingtime of 15 s was givenbefore demoldingthe
testsamples. No process aids or other additives wereused.
Thecompositionsof thecompositespreparedare given in Table
II.Scanning Electron MicroscopyThe morphology of used bers, MCC,
WF, and WP, aswell as composite microstructure was studied using
ascanning electron microscope (SEM), Cambridge 360.The sample
surfaces were sputter coated with Au toavoid charging.Mechanical
testingThemechanical propertiesofcompositesweremea-sured using an
Instron universal testing machine(model 8800) with a crosshead
speed of 2mm/min anda load cell of 10 kN. The tensile testing was
performedaccording to the ISO 527 standard for tensile testing
oncomposites and plastics. At least six specimens
weretestedforeachcomposition, andtheresultsarepre-sented as an
average for tested samples.Dynamic mechanical thermal
analysisDynamic Mechanical Thermal Analysis (DMTA) wasconducted
using a Rheometrics Scientic V, to
studythetantemperatureanddynamicmodulusofdif-ferentcompositesystems.
DMTAwasruninadualcantilever bending mode with typical sample
dimen-sions: thickness 4mm, length 30mm, and width 10mm.Heating
rate was 1.5C/min, strain rate was 1mm/s,and frequency was 1
Hz.Crystallinity studies (WAXD)Wide angled X-ray diffraction (with
a Siemens Diffrac-tometer D5000) was used to study the
crystallinity ofthe pure components and MCC composites. The
sam-ples were exposed for a period of 1.5 s for each
angleofincidence()usingaCuKX-raysourcewithawavelength() of 1.541.
Theangleof
incidenceisvariedfrom4to50bystepsof0.02s.Theperiodicaldistances(d)
of themainpeakswerecalculatedac-cording to Braggs equation ( 2 days
sin).Biodegradation studiesBiodegradability of pure PLA and the
composites wasstudiedat58CaccordingtotheASTMD5338stan-dard. The
water content of the soil was around 60% byweight.
ThecompostusedwasofgardenwasteandsuppliedbyTrondheimcommune.
Further, thesoilwasusedasobtainedandhadnospecial microbialactivity.
The test specimens were compression moldedto2mmthickness,
andthesampledimensionswereapproximately 6 6 cm2. The samples were
recoveredfromthesoil at different stagesof degradationandwashed
with distilled water, dried in the oven at 50C,andweighed.
Photographsofthesampleswerealsotaken for visual
comparison.RESULTSANDDISCUSSIONMorphologyTheoverviewanddetailedappearanceof
theusedMCCisshowninFigure2. Itcanbeseenfromthegurethat
theMCCisinparticulateformandtheTABLEICharacteristics of the Used
Raw MaterialsPLA MCC WP WFDensity (g/cm3) 1.26 0.130.25 0.30.4
0.5pH 5.07.0 5.58.0 5.08.0Appearance Transparent pale, pellets
White, free owing powder White, brous pellets Pale, coarse
powderParticle size 1015 m 2030 m 150750 mToxicity No No No
NoTABLEIICompositions of the Studied CompositesMaterials Matrix (wt
%) Filler (wt %)PLA 100 0PLA/MCC10 90 10PLA/MCC15 85 15PLA/MCC20 80
20PLA/MCC25 75 25PLA/WP25 75 25PLA/WF25 75 252016 MATHEW, OKSMAN,
ANDSAINparticledimensions areintherangeof 1015
m,havinganaspectratio(1/d)around1[(Fig. 2(b)]. Itcan also be seen
that the MCC exists as aggregates ofcrystallinecelluloseentities.
Itisalsopossibletoseesome nano brils on the MCC particle surfaces,
whichmight be evidence that the MCC particles are agglom-erates of
hundreds of individual cellulose nano brils.Figure3shows
theappearanceof woodpulpandwood our. Figure 3(a) shows wood pulp
bers (WP),whichareintheformofsinglebershavingasizearound 20 m in
diameter and 3500 m in length. Thedetailedviewof WPis asingleber
withhighestaspect ratio (1/d 175). Figure 3(b) shows WF, whichis in
the form of wood ber bundles with dimensionsrangingbetween200and400
mindiameter and10001500m in length, giving a 1/d around
10.Thefracturesurfacesof
thecompositespecimenswerestudiedtounderstandthefailuremechanismsand
also study possible interaction between differentcomponents. The
fracture surfaces of PLA/MCC com-positesaregiveninFigure4.
Figure4(a)isanover-viewof PLA/MCCcomposites
showingauniformdispersion of MCC in the PLA matrix. Another
obser-vation is that the MCC still remains as aggregates
ofcrystallinebrilsandnoseparationhadtakenplaceduring the extrusion
process. Further in Figure 4(a),
alargenumberofholesinthePLAmatrixarevisiblewhereMCChavebeenlocatedbeforethefracture.Figure4(b),
amoredetailedmicrograph, showsthatthere are voids aroundthe
MCCaggregates. Bothobservations, voids around the MCC and many
holesinthematrix, indicatethatthereisnoadhesionbe-tween the PLA and
MCC.Figure 5 shows the fracture surface of PLA/WP
andPLA/WFcomposites. Figure5(a) showswoodpulpcomposite. It is
possible to see that wood pulp remainsas single woodbers
withexcellent dispersibility,whichis not usuallythe case
whenwoodpulpisblendedwithpolyolens.29This couldbeduetoaspecic
interaction between the polyester matrix andcellulose. Gatenholm
and coworkers have made sim-ilar conclusions in an earlier study of
PHB-based cel-lulose composites.30Further observations inFigure5(a)
are that the single wood ber surfaces are cleanandit is
alsopossible tosee ber pullouts. Theseobservations indicate that
there is no chemical adhe-sion with the PLA matrix, but if compared
to Figure 4,PLA/MCCcomposites, theinteractionisbetter. Fig-ure 5(b)
shows a wood our composite. The rst ob-servationis that thewoodber
bundles arelargerthan the MCC and WP. The second observation,
whichwasmadeduringtheSEM,isthatitwasdifculttoidentify the wood bers
from the PLA matrix; this isFigure2 Structure and appearance of
MCC: a) overview;and b) detailed view.Figure 3 Morphology of used
a) wood pulp, and b) woodour.BIODEGRADABLECOMPOSITESFROMPLA/MCC
2017usually the case when there is good adhesion
betweenthematrixandbers becausethebers arecoatedwith the polymer.
In this case, it was not possible toobtaincoatedwoodbers but we
believe that themechanical interlockingis better inthis
systembe-causeof theroughnessof thewoodour andthatmight contribute
to improved adhesion.Mechanical
propertiesTableIIIsummarizesthemeanandstandarddevia-tionsofthemechanical
propertiesofPLAandPLAcomposites. Theresults
arealsopresentedinmoredetail in separate gures.Figure 6 shows
typical tensile curves for pure PLAand PLA composites with
different MCC content. Thegure clearly shows that both tensile
stress and elon-gation to break are lower for the composites
comparedtopurePLAandthat thecompositeshaveslightlybetter stiffness
compared to pure PLA. Figure 7 showsthe modulus and tensile
strength as a function of MCCcontent. It is clearly shown that
increased MCC con-tent has a negative effect on composites strength
com-paredtopure PLA, while the modulus is
slightlyincreasedwithincreasedMCCcontent. Asthestan-dard deviations
of strength values are within the lim-its of differences,
therefore, we can conclude that thestrength remains almost constant
between 10 to 25%MCC
content.Figure8showstypicaltensilecurvesofpurePLAandcompositeswith25%ofWF,
WP, andMCCre-spectively. The tensile strengthis againhigher forpure
PLA compared to the composites, but thestrengthis better for
bothWFandWPcompositesthanfor MCCcomposite. The
elongationtobreakdecreasesagaininthecompositescomparedtothepure
PLA, being in the order WP WF MCC.Figure 9 compares the tensile
strength and the mod-ulus of composites withMCC25, WF25,
andWP25with pure PLA. All the studied composites have ten-sile
modulus higher than pure PLA, and the WF
sys-temhashighestmodulus,followedbyWPandthenMCC. However, inthis
case, WFandWPcanbeconsideredtohave the same modulus
statistically,taking the standard deviations into consideration.
Theconclusionofthemechanicaltestingisthatallcom-posites have higher
modulus than pure PLA but theincorporation of both WF and WP
increases the mod-ulus more than MCC does.PLAis a brittle polymer,
andit seems that
thebrittlenessevenincreaseswiththeadditionofcellu-losereinforcements.
Theelongationtobreak, forallFigure 4 Microstructure of PLA/MCC
composites: a) over-view of PLA/MCC10, and b) detailed view of
PLA/MCC25.Figure5 Microstructureof a) PLA/WP25andb) PLA/WF25
composites.2018 MATHEW, OKSMAN, ANDSAINcomposites, is lower than
that for pure PLA. The low-ering of elongation to break with the
addition of bersto polymers is a common trend observed in
thermo-plastic composites.31,32When the three cellulose sam-ples
are compared, the elongation is the highest for
thePLA/WPsystemfollowedbythePLA/WFsystemand is lowest for the
PLA/MCC system. As the SEMstudyof compositemicrostructureshowed,
theWPwas acting as single bers with higher l/d ratio
thanWForMCCandmanyberpulloutscouldbeob-served. Another observation
made on WP compositeswas that wood pulp bers seem to be soft
(twisted andcurled). Soft and longer bers and ber pullouts
mightresultinbettertoughnessand, therefore,
alsobetterelongationtobreak. Theresultsmight
indicatethatbothWPandWFact betterasreinforcementscom-pared to
MCC.Themechanical performanceof compositesisex-pected to depend on
the following factors: 1) adhesionbetween the PLAmatrix and
cellulosic reinforce-ments, stress transfer efciencyof the
interface; 2)volumefractionof thebers; 3) aspect ratioof
thereinforcements; 4) ber orientation; and 5) the degreeof
crystallinity of the matrix.33Thedecreaseinstrengthisanindicationof
poorstress transfer across the interphase, which means thatthere is
practically no interfacial bonding between thereinforcing ber and
the polymer matrix.34The
pooradhesionbetweenthematrixandberinitiatesnu-merousvoidsat theber
matrixinterface, andthestress transfer to the bers, which are the
load bearingentities, becomesinefcient
leadingtolowstrengthvalues.35The higher tensile strength,
elongation, and modu-lus of WP and WF systems can be explained
based ontheaspectratioofthethreereinforcements.TheWPand WF have
higher aspect ratio than MCC and
thatcouldenhancethemechanicalperformanceseveninthe case when the
adhesion is poor. This enhancementwould occur in the case if the
ber length were equaltoor higher thanthecritical ber length, Lc. It
ispossible to see from the fracture surfaces in Figures 4and 5 that
there are debonding and ber pullouts in allcomposite systems, which
means that the ber lengthislowerthanthecritical berlength.
InthecaseofMCC, which is a particulate reinforcement, the LLcand,
therefore, it
ispossibletoobtainmanydebondedsitesonthefracturesurface.Thelowef-ciency
of stress transfer by particulate reinforcementsin composites has
been reported earlier.36,37The lowaspect ratio and particulate
nature leads to low elon-Figure6 Typical
tensilecurvesforPLAandPLA/MCCcomposites with different MCC
contents.Figure 7 Stiffness and strength as a function of MCC
con-tent.TABLEIIIMechanical Properties PLA and Its
CompositesMaterialsTensile strength(MPa)Elongation atbreak
(%)E-Modulus(GPa)PLA 49.6 1 2.4 0.1 3.6 0.2PLA/MCC10 38.2 0.5 1.8 0
4.1 0.7PLA/MCC15 37.8 0.8 1.9 0.2 4.4 0.2PLA/MCC20 38.1 0.7 1.8 0.1
4.7 0.3PLA/MCC25 36.2 0.9 1.7 0.2 5.0 0.2PLA/WP25 45.2 1.3 1.9 0.3
6.0 0.7PLA/WF25 45.2 1.3 1.7 0.1 6.3
0.2BIODEGRADABLECOMPOSITESFROMPLA/MCC 2019gationat
breakandbrittleness. Thehighinterfacialarea is the only favorable
aspect in the case of MCCand, therefore, the MCC system has
mechanical prop-erties comparable withwoodpulpber (WP) andwoodber
(WF) systems inspiteof its particulatenature.The PLAis a
semicrystalline polymer, andit ispredicted that the crystallinity
of the PLA will increasewiththeadditionofcellulosicreinforcements.
Thesecrystalline regions then could act as physicalcrosslinks or
ller particles, which could subsequentlyincrease the modulus
substantially. The higher mod-ulus of the composites compared to
pure PLA mightbeanindicationthat thecrystallinityis higher
forcomposites. However, duringthefabricationof thecomposites,
thecoolingrateisfast, it isconsideredthat the matrix is mostly
amorphous, and the crystal-linity may be primarily due to
transcrystallinity of thePLAonthebersurface. Inaddition,
MCChasthehighest surface area, whichmeans that we wouldexpect the
highest transcrystallinity in the PLA/MCCsystem. However,
thecrystallinityinthesystemre-quires more detailed study, and the
effect of crystal-linity on composite properties will be reported
in ourforthcoming article in detail.Dynamic mechanical thermal
analysisThe DMTA of PLA/MCC composites was
performedtoinvestigateiftheadditionoftheMCCwouldim-provethethermalproperties,
suchasmaximumusetemperature, for PLA. Five materials, PLA,
PLA/MCC10, PLA/MCC15, PLA/MCC20, and PLA/MCC25, were tested to nd
the maximum use temper-ature and also to see the possible
interaction betweenthe PLA matrix and MCC. Figure 10 shows how
theaddition of different contents of MCC inuenced thetan delta and
dynamic modulus of PLA. Figure 10(a)shows the effect of MCC content
on the tan delta peak.It is possible to see that the tan delta peak
(-transi-tion) is slightlyshiftedtohigher temperaturewithincreased
MCC content. The shift to higher
tempera-tureusuallyindicatesrestrictedmoleculemovementbecause of
improved interaction in lled polymers.38The -relaxation involves
the movement of amor-phous chains, and the presence of
reinforcementsand/or crystalline regions can act as
physicalcrosslinks, decreasing the mobility of the amorphousregions
and increasing the modulus.35,33The tan-deltapeakof
PLAwasmeasuredtobeapproximatelyat67Candthatwasincreasedto69CforMCCcom-posites.
However, in this case, the shift is not signi-cant enough to
indicate a strong interaction
betweenthePLAandtheMCC,whichwasalsoindicatedbySEMstudyof this
compositesystem. Thedynamicmoduluscurvesforcompositeswithdifferent
MCCcontent are giveninFigure 10(b). The modulus isalmost the same
for all the systems until the softeningtemperature. It
couldbenotedthat theadditionofFigure 8 Tensile curves of pure PLA,
and composites with25% of wood our, wood pulp, and MCC,
respectively.Figure 9 Comparison of the mechanical properties of
PLAand its composites: a) tensile modulus, and b)
tensilestrength.2020 MATHEW, OKSMAN, ANDSAINMCCin PLAincreasedthe
softening temperaturefrom57to60C. Further,
itcanbeseenthataround8090C, the modulus increases again. This could
beattributedtothecoldcrystallizationofthesemicrys-tallinePLAmatrix,
whichisinconformitywiththework carried out earlier.26The values of
glassy and rubbery modulus obtainedfrom DMTA are given in Table IV.
The rubbery mod-ulus is obtained from the modulus values in the
tem-perature range of 7080C, which is after the polymerrelaxation
and before the onset of cold crystallization.Above 80C, the cold
crystallization takes over and themodulus is governed by the
crystallinity of the matrix.The rubbery modulus of MCC composites
(at 75C) ishigherthanthatofPLAanditincreaseswithMCCcontent. The
rubbery modulus is known to depend onthe crystallinity of the
system, aspect ratio, and inter-action between the phases. In the
case of MCC com-posites, the adhesion and interaction between
thephases was found to be poor and the aspect ratio is
toolowtohaveanysignicant effect onmodulus. Weexpect that the MCC
would act as a nucleating agentin PLA, and during cooling a
transcrystalline layer ofPLAontheMCCsurfacewouldbedeveloped.
Thisphenomenonmight becorrelatedtothehigher
dy-namicmodulusobservedinMCCcomposites.How-ever, detailedstudyof
thenucleatingcapabilityofMCC and the transcrystallization of PLA is
required toconrm this assumption and will be discussed in de-tail
in our next article.Figure 11 shows the DMTA curves for PLA,
PLA/Figure 10 DMTA curves for PLA/MCC composites: a) tandelta, and
b) dynamic modulus.TABLEIVThe Effect of Cellulose Based
Reinforcements on theDynamic ModulusDynamic modulusMaterialsGlassy
stage(108Pa)Rubbery stage(106Pa)PLA 6.4 2.15PLA/MCC10 10.1
3.35PLA/MCC15 10.9 3.53PLA/MCC20 10.9 5.71PLA/MCC25 10.4
8.45PLA/WP25 11.9 46.7PLA/WF25 13.1 24.5Figure 11 DMTAcurves for
PLA, PLA/MCC25, PLA/WF25, andPLA/WP25 composites: a) tandelta,
andb)dynamic modulus.BIODEGRADABLECOMPOSITESFROMPLA/MCC 202125MCC,
PLA/25WF, andPLA/25WP. Figure 11(a)shows the tan-delta peaks of
PLAandcompositeswith25%ofMCC, WP, andWF. TheDMTAresultsindicatethat
thecompositeshaveahigher thermalstability compared to pure PLA. It
was found that the-transitionfor pure PLAis around67Candin-creases
to 70C for WF and 69.1C in the case of WPand MCC. This can be
explained based on the retar-dation in the relaxation of amorphous
regions due tothe physical interaction with the reinforcing phase
andthe crystalline regions of the matrix. Figure 11(b)shows
thestoragemodulus curveof PLAandthecomposites.
Thecompositesexhibitanimprovementin dynamic storage modulus and
heat distortion tem-perature compared to pure PLA. The storage
modulusvaluesforglassyandrubberyaregiveninTableIV.The rubbery
modulus (between 70 and 80C) is higherfor the composites than for
pure PLA and is highestfor WP bers followed by WF and then MCC.
Whenthethreecellulosesystemsarecompared, it canbeseen that the
increase in rubbery modulus is in directcorrelation with the aspect
ratio. Therefore, the aspectratio can be considered as the
governing factor in therelaxed modulus of the composites. The
curves showadecreaseonmodulusataround60C, whichindi-cates that
themodulus increases again, whichis
atypicaleffectofcoldcrystallization.
Astheadhesionbetweenthephasesispoor inall thesystems, thiseffect
depends on aspect ratio of the reinforcement aswell as
crystallinity of the
matrix.TheseDMTAresultsleadtotheconclusionthatasmall improvement in
thermal stability is obtained
bytheadditionofcellulosicreinforcementstothePLAmatrix. The modulus
also showed some improvementbut not signicant enough to indicate
good interactionoradhesionbetweenthephases. Therefore,
theper-formance can be improved by enhancing the interac-tion
between the matrix and reinforcement by addingcompatibilizers,
functionalizingthereinforcementoradding nano-sized
reinforcements.Wide angled X-ray scatteringThe pure polymer and ber
components and
compos-iteswerecharacterizedbyX-raydiffractiontostudythe effect of
the type of reinforcement and MCC con-tent on the crystallinity of
PLA. The X-ray diffractionpatterns for PLA, WP, and MCC are shown
in Figure12. In the gure, the WAXS pattern of wood bers isnot shown
due to the difculty in testing coarse woodour.Except for a narrow
peak at 2 16.4, PLA exhibitsan amorphous nature and it can be
considered as in asemicrystalline phase. The WP and MCC show
peaksat 2 22.5 and 34.5. In the case of MCC,
additionalpeaksareseenat215.4and16.2whereasinthecaseofwoodpulpbers,
thesepeaksarenotsharpand a shoulder is observed around the region 2
14to17. ThepeaksaremoreprominentandsharpforMCC, showing the
crystalline nature of this reinforce-ment.
Thedvaluesassociatedwith215.4, 16.2,22.5, and 34.5 are 6.14, 5.46,
3.95, and 2.59 , which iscorresponding to the cellulose I polymorph
structure.Other scientists havereportedthesameresults
forMCCfromX-rayanalysisearlier.37,38Youngandco-workers have
concluded that high crystalline MCC isrequired for high performance
composites with MCC.The wood pulp also shows peaks corresponding to
theCelluloseI structure, but theyareweaker andlessprominent,
indicating lower crystallinity. It is possiblethat wood pulp
contains some residual lignin and
thatcontributestotheslightlylowercrystallinityof thisber compared
to that of MCC.In Figure 13, the effect of MCC content on
crystal-linity of the system is shown. The peaks at 2 16.5and 22.6
are the most prominent and is indicative
ofPLAandcellulosecrystallinity, respectively. AstheMCC content
increases to 25%, a steady increase in
theintensityofthepeaksat222.6isobserved. Thepeaks at 2 15.2 and
16.3 are not prominent at low Figure 12 WAXD curves for raw
materials used.Figure 13 Diffraction pattern for PLA/MCC
composites.2022 MATHEW, OKSMAN, ANDSAINreinforcement content,
andtheyare replacedbyabroadshoulder around216, whichindicates amore
predominant amorphous material. For MCCcomposites,
theXRDdataisindicativeof verylowPLA crystallinity and the peaks due
to PLA and cel-lulose-I are weak. The low intensity of the PLA peak
ispossibly explained by a low level of matrix crystallin-ity
developed due to fast cooling rates during extru-sion and injection
molding.InFigure14diffractogramsofPLAandcompos-ites with 25% of WF,
WP, and MCC are given. It canbe seen that all the composites have
weak crystallinebandsandaremainlyamorphousinnature, whichcanbe due
tothe fast coolingduringcompositepreparation. However, further
study is necessary toconrmthis effect. In the WF composite, a
lowintensity peak is observed at 2 16.7 correspond-ing to PLA and a
shoulder like hump is observed at2 22.4, which is indicative of low
crystallinity ofWF. In the case of the WP system, the peak
isobservedat216.7, correspondingtoPLA. Thepeak at 2 22.7 from the
cellulose-I is also presentinthecomposite. However, thepeakat
215.4and16.2 is not identiable, andthe systemas
awholeappearstohaveverylowcrystallinity.
TheMCCsystemhasalowintensitypeakat22.7fromcellulose.
Inadditiontothis, ashoulderisvisiblearound215.8and16.6,
whichoriginatesfromcellulose and PLA crystallinity. The MCC
compositeshows low PLA crystallinity compared to wood
andpulpbercomposites, whichisinagreementwiththe DMTA results.
Therefore, it can be inferred
thatthePLA/WFandPLA/WPsystemshaveahigherlevel of matrix
crystallinity than the PLA/MCC sys-tem.
ThisexplainsthelowermodulusoftheMCCsystem compared to the WF and WP
systems.In all cases, it can be seen that the magnitude of thepeaks
indicating crystallinity of the composite is lowerthanthat of
PLAandthereinforcement used. Thisprobably results from the
intermixing with PLA andalso the random orientation of the
reinforcement. Thereinforcementishavingarandomarrangement,
andtheresultantcrystallinityisexpectedtobelessthanthe case when the
reinforcement is oriented in a plane.Thistypeof
observationwasreportedbyDufresneandcoworkerswhileworkingwithtunicinwhiskerslled
composites.39BiodegradabilityFigure 15 shows the picture of samples
recovered fromcompost soil at different stages of degradation.
Whenbiodegradableapplicationsareconsidered,
compost-ingisthemostpreferreddisposalroute. Therateofhydrolytic
degradation of PLA is dependent on tem-perature and the humidity
level. In an environment ofhigh humidity and a temperature of
around 58 2C,PLApolymers were foundtodegrade
rapidly.40,41Therefore, for the present study, temperature
wasmaintained at 58C and moisture content of the soilwas60%.
InPLAdegradation,
moisturesuscepti-bilityistheprimarydrivingforcetowardsdegrada-tionandinvolvesfoursteps,
namely,. waterabsorp-tion, ester cleavage forming oligomers,
solubilizationof oligomer fractions, anddiffusionof soluble
oli-gomersbybacteria.42ItwasfoundthattheonsetofPLAdegradationoccurredasearlyas2weeksandwas
marked by the embrittlement of the samples. Theslower degradation
rate in the composites is due to theresistance in water uptake and
diffusion through thecomposite compared to pure PLA, which readily
takesupwater. However, the weight loss was not verysignicant for
PLA or the composites until 8 weeks incompost soil. InFigure16,
theresidual weight per-centage (Rw %) of the composites as a
function of time(indays)incompostsoil isshown. Itwasseenthatafter
60 days in the compost soil, the composites
alsoshowedmarkeddegradation. After 75days, all
thesamplesshowarapidincreaseindegradation,espe-ciallytheWFcomposites.
ThegureshowsthattheWF composites have a higher rate of degradation
thanthe WP and MCC composites. This leads to the con-clusion that
PLA/WF composites are more susceptibleto water and thereby more
biodegradable, comparedto WP and MC composites.CONCLUSIONSThis
study was carried out as an initial step towardstheuseof
microcrystallinecellulose(MCC) asrein-forcement in the PLA matrix.
The morphology
studiesofMCCparticlesshowedthatMCCexistsasaggre-gatesofnano-bers/whiskersofcellulose.However,the
morphology studies of the composites revealed theFigure 14
Comparison of crystallinity of PLA/MCC25,PLA/WF25, and PLA/WP25
composites using WAXD.BIODEGRADABLECOMPOSITESFROMPLA/MCC 2023fact
that the MCC did not separate into nano-whiskersduring the
extrusion process and remained as micro-particles in the
composite.Further, the evaluation of the mechanical propertiesof
the composites demonstrated that the tensile mod-ulus was improved
with increased MCC content, buttensile strength and elongation to
break was de-creased. ItwasalsoobservedthatbothWFandWPresulted in
composites with better mechanical proper-ties compared to MCC
composites, which may be duetopoor interfacial
adhesionbetweenthephases asobserved from SEM. Further, the
morphology studiesshowedagooddispersionofall
cellulosereinforce-mentsinPLA, whichdependsonthenatureof
thepolyester matrix.DMTAstudies showedthat
thestoragemodulusandthermal
stabilityincreasedmarginallywiththeaddition of cellulosic
reinforcement. The storage mod-ulus of the composites changed in
the order WP WFMCCPLA. Thischangeinstoragemodulusispredicted to be
governed by the aspect ratio of cellu-lose reinforcements.XRD
studies showed that both MCC and wood pulphave cellulose-I
structure but the crystallinity is higherfor MCC than for wood
pulp. The results also showedthat PLA exhibited a single sharp peak
and is semic-rystalline in nature. All composites
showedlowercrystallinitythanthe pure component phases. Thehigher
crystallinity of PLA in WP and WF
compositescomparedtoMCCcompositescanbeconsideredasone of the
possible reasons for the better mechanicalperformance of WF and WP
composites compared toMCC composites.Figure 15 Photographs showing
different stages of biodegradation of PLA and its composites in
compost soil.Figure16 Residual w/oofPLAanditscompositesasafunction
of time in the compost soil.2024 MATHEW, OKSMAN,
ANDSAINBiodegradationstudiesonthecompositesshowedthatPLAstarteddegradingby34weekswhilethecomposites
started degrading rapidly during 48weeks. The degradation rate was
found to be higherfor WF composites than for MCC and WP
composites.We believe that the incorporation of MCC in PLA
afterseparation into nano-whiskers will result in high per-formance
biodegradable composites.The authors thank Fortum Oil and Gas Oy,
Finland, Borre-gaardChemcell, Norway, ScandinavianWoodFiber
AB,Sweden, and Rayonier, USA, for supplying the
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