Kasetsart J. (Nat. Sci.) 48 : 908 - 915 (2014)

1 Kasetsart Agricultural and Agro-Industrial Product Improvement Institute (KAPI), Kasetsart University, Bangkok 10900, Thanland.

2 Clermont Université, Université Blaise Pascal, Institut de Chimie de Clermont-Ferrand (ICCF), CNRS UMR 6296, BP 10448, F-63000, France.

3 Clermont Université, ENSCCF, BP10448, F-63000, Clermont-Ferrand, France.* Corresponding author, e-mail: aaprmc@ku.ac.th

Received date : 05/11/13 Accepted date : 01/09/14

IntroductIon

Cellulosefiber based composites havebeen applied commercially in the constructionand automotive industries (Mohanty et al., 2000).Durability to outdoor environments forthese cellulose fiber-based compositeswas ofconcern, especially to solar light. Attention has mainly focused on the photo-degradation ofwood-polyethylene(PE)submittedtonaturaland

Effect of Palm Fiber on Photo and thermo-degradation of Polyethylene composites

Wirasak Smitthipong1, rungsima chollakup1,2,*, Wuttinan Kongtad1 and Florence delor-Jestin2,3

AbStrAct

Polyethylene/palmfibercomposites(PEP)werepreparedbyblendinglowdensitypolyethylene(PE)andpalmfibers(diameter<1mm)inatwin-screwextruder.ThinfilmsofPEPat0–30%fiberweightwereeitherexposedtoacceleratedartificialphoto-ageing(λ>300nm,60°C)ortothermo-oxidationat100°Cinanaeratedoven.Infrared(IR)spectraafterphoto-oxidationshowedthattheincorporationoffibersinPEincreaseditsdegradationratewiththeformationofcarbonylandvinylgroups.AdecreasewasobservedinthearomaticC=CabsorptionIRpeaksoftheligninstructureinthePEPcomposites.Thisindicatedthattheligninstructureinthefiberswasphoto-oxidizedandproducedradicalspeciesforoxidation.CrystallinitychangeofthePEPcomposites,analyzedusingdifferentialscanningcalorimetry,tendedtoincreaseuponphoto-andthermo-ageing.Thiseffectcouldbeexplainedbychainscissionsinvolvedintheoxidationprocess,astheshorterchainsofPEwouldhaveahighermobility,allowingthereorganizationofthePEmatrix.Comparedtothermo-degradation,theamountofvinylgroupsremainedconstantindicatingnoreactionoftheketonesinaNorrishtypeIIprocess.Thehigherthermo-oxidationrateinPEPcompositesmaybearesultofradicalspeciesofcelluloseandhemicellulosedegradationinpalmfibers.Keywords:polyethylene(PE),palmfiber,photo-degradation

artificialweathering (Stark andMatuana, 2004;Stark,2006;Ndiayeet al.,2008;Taibet al., 2010). These studies onwoodflour or cellulose-fiber-reinforcedPEhavefocusedeitheronchangesinthesurfacechemistrywithIRspectroscopyoronmechanical properties.Most studies found thatthefiberinclusionacceleratedtheoxidationrateuponultraviolet (UV) to visible light exposure(StarkandMatuana,2004;Stark,2006;Ndiayeet al., 2008). Stark andMutuana (2004) found

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that the carbonyl index of thewoodflour/highdensitypolyethylene(HDPE)washigherthanthatoftheHDPE.ThiswasexplainedbythefactthatlignininwoodflourisverysensitivetoUVlightdegradation (George et al., 2005). Inthecurrentstudy,bleachedpalmfiberfromwastes generated from oil production inThailandwerechosentoreinforcethePEmatrix.Theobjectiveofthisworkwastoinvestigatetheeffectofsmallfiberparticleinclusion(1mmsize)onthechemicalstructureandcrystallinityofthesecellulose fiber-reinforced PE composites afterexposuretoacceleratedphoto-andthermo-ageing.Photo-ageingconditioningwasperformedunderUV-lightirradiationat60°C.Thermo-oxidationused an aerated oven at 100 °C.The chemicalmodificationsofthecompositeswereinvestigatedusinginfraredspectroscopy.Differentialscanningcalorimetry(DSC)wasusedtoanalyzethechangein the crystallinity during photo- and thermo-oxidation.

MAtErIALS And MEtHodS

Materials composite preparation Palmfruitbunchesfromanoilproductionfactorywereusedasthesourceofthepalmfibers.Palm fiberswere extracted from agriculturalwastes according toChollakup et al., (2013). Briefly,palmfruitbunchesweretreatedwith25%NaOHonaweightbasis(aliquorratioofNaOHtofiberof15:1)at100°Cfor3hr.Then,thealkalinetreatedcoirfiberswerebleachedat100°Cfor2hrusing30%hydrogenperoxideataliquorratioofsolutiontofiberof30:1. Fiberswere ground into powder andpassed through a 20-mesh sieve using a rotor mill (Pulverisette14;Fritsch;Idar-Oberstein,Germany)ataspeedof16,000revolutionsperminute(rpm).Polyethylene (DowLDPE 320E, PE),with adensityof0.925g.cm-3andameltflowindexof1.0gper10min,wassuppliedbytheDowChemicalCompany (Edegem,Belgium).The palmfibers

at5-20%byfibervolumewereblendedwithPEgranulesinatwin-screwextruder(HaakeMinilab;ThermoScientific;Karlsruhe,Germany) at 170°Cwitharollerspeedof90rpmfor10min.Thentheextrudateswerecutintopalettesandpressedat150°Cfor1minunderapressureof20MPa.Compositefilmswereobtainedwithathicknessof90-130μm. composite ageing IrradiationwascarriedoutinaSEPAP12/24unit.This toolhadbeendesigned for thestudyofpolymerphoto-degradation inartificialageing conditions corresponding to medium acceleratedconditions(Lemaireet al., 1981). For theseexperiments,thetemperatureatthesurfaceofthesampleswasfixedat60°C.Lowtemperaturethermo-oxidationexperimentswerecarriedoutinaventilatedovenat100°C.Analytical method Infrared and ultraviolet spectroscopy Infrared spectra were recorded intransmissionmodewith aNicolet 760-FTIRspectrophotometer (Thermo Fisher ScientificCo.;Waltham,MA,USA)withOMNICsoftware(version 8.3; Thermo Fisher Scientific Co.;Waltham,MA,USA). Spectrawere obtainedusing 32 scans and a 4 cm-1 resolution. To avoid differencesduetofilmthicknessandPEcontents,the normalized absorbance (∆Inormalised) wasreportedaccordingtoEquation1(Morlat-Theriaset al., 2008): ∆

∆I

IInormalized

ref= υ (1)

where∆Iνcorrespondstotheintensitydifferenceofthebandatanywavenumberfromaninitialtimeand Irefcorrespondstotheintensityofthereferencebandat720cm-1,attributedtoC-Hrocking(Pagèset al., 1996). In addition, UV spectroscopy using aUV-2101PC spectrophotometer (Shimadzu;Kyoto,Japan)wasusedtostudythechangeinUVabsorptionafterphotoandthermo-degradation. differential scanning calorimetry Themelting temperature and heat of

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fusionofthecompositesweredeterminedusingDSC (DSC822;Mettler; Toledo, OH,USA).Sampleswerefirstrunfrom25to200°Cat10°Cper min to remove any thermal history. Then, the sampleswerecooledtoroomtemperatureat10°Cpermintorecordthecrystallizationbehaviorbefore reheating to100°Cat20°Cpermin totracethemeltingbehavior.ThecrystallinityofthecompositeswascalculatedaccordingtoEquation2: χ(%) = ×100 0

∆

∆

H

Hf

f (2)

where∆H0f and ∆Hf

are the heat of fusion ofthe 100% crystalline polymer and composites,respectively.∆H0

f is 290 J.g-1forPE.

rESuLtS And dIScuSSIon

Figure 1 shows the IR spectra of thePEcompositesatdifferentfibercontentsbeforeageing.The IR spectra of the PE compositescontainadditionalabsorptionbandsat3000–3500cm-1 attributed to hydroxyl linkages (hydrogenbonds of cellulose, alcohols, hydroperoxides,andofwateramongothers),at1600–1800cm-1 attributedtoconjugatedC=Obondingofcelluloseand ligno-cellulose, and at 900–1300 cm-1 attributed toC-Ovibration (Bajeret al., 2007)comparedtothePEspectra.Theintensityoftheseadditionalbandsincreasednaturallywiththefiber

content(beforeageing).Figure2showstheUV-visiblespectraofthePEPfilmsbeforeageing.Thisfigure shows that cellulosefibers absorb in theUV/visibleregionat280nmandtheUVintensityincreaseswiththefibercontent.LigninisthemaincomponentofthechemicalcompositionoffiberwhichabsorbsrelativelystronglyintheUV/visibleregion(George,2005).Asshownintheresults,thelignincontentofthepalmfiberafterbleachingwas10.9%. As reported for PE (Lacoste andCarlsson, 1992) photo-ageing results in the formationofhydroxylgroups(broadabsorptioncentered at 3430 cm-1), of a carbonyl envelopecentered at 1712 cm-1 and a vinyl group (908 cm-1).Figure3showstheevolutioninthecarbonyldomain for the purePE andPEP composite at30%byweight. It is noteworthy that the sameevolutionswere analogous forPEP compositesand look like the one observed in a PE film(LacosteandCarlsson,1992).Forthesametimeofirradiation,photoproductquantitiesarealwayshigherforPEPthanforPE(Ndiayeet al., 2008). Howeverthecurrentstudyfoundonlyadecreasein the aromaticC=C absorption band at 1595and 1506 cm-1 of PEP composites, indicatinglignin structures as seen in Figure 3. Decreases in these peaks depended on the fiber contentin the composites.For ahigherfiber content, ahigher ratewasobtained (datanot shown).The

Figure 1 Infraredspectraofpolyethylene/palmfibercompositesat0–30%byweightoffibercontentsbeforeageing.

Figure 2 UV (ultraviolet)-visible spectraof polyethylene/palm fiber (PEP)composites at 0–30% byweight offibercontentbeforeageing(PE=Lowdensity polyethylene).

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disappearance of this peakwhichwas seen bythe negative absorbance values proved that theligninstructureinfiberwasdegradedandfurtheroxidizedtoformparaquinonechromospheresasdescribedbyMuasherandSain(2006).DecreasesintheIRspectraofcellulosefibersat1595and1506 cm-1 afterphoto-oxidationwasobservedonPE/coirfibercomposite(Chollakupet al., 2012) Anincreaseincarbonylgroupsformedduring photo-ageing in the PE and in PEP compositesisshowninFigure4whichindicatesthat the oxidation rate of the PEP compositesincreasedwiththefibercontent.Incontrast,theformation of vinyl groups resulting from thedecompositionofketonesaccordingtotheNorrishIIprocessisthesameforPEandPEPcomposites(datanotshown).Theformationofvinylgroupsindicated that photo-degradation occurred via

Norrish II reactions, resulting in chain scission (Kaci et al., 2000). Figure 5 shows that theabsorbanceintheUVdomain(lessthan300nm)ofthePEPcompositesincreaseswithageingtime.IncreasingUVabsorbanceresultedfromoxidationproducts upon photo-degradation. Thermo-degradation occurred over 200 hr due to the tolerance conditionswithinthepolymerbulkat100°C.Figure6showsthechanges in the carbonyl spectra of thePE andPEP composite during thermo-oxidation.Theoxidation rate after thermo-ageingwas higherthanthatafterphoto-ageing.Therewasnochange

Figure 3 Changes in the infrared spectra of:(a) Low density polyethylene; (b)Polyethylene/palmfibercompositesat30%byweightoffibercontentduringphoto-oxidation in theSEPAPdeviceatλ>300nmand60°C.

Figure 4 Absorbancechangesat1712cm-1 during photo-oxidationofpolyethylene/palmfiber(PEP)compositesasafunctionofageingtime.(OD=theopticaldensity;Verticalerrorbarsindicate±SD;PE=Lowdensitypolyethylene.)

Figure 5 UV (ultraviolet)-visible spectraof polyethylene/palm fiber (PEP)compositesat16%byweightoffibercontentduringphoto-oxidation.

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inthevinylgroupformation(at908cm-1)whichwasanindicationofmainscissionaccordingtoonly aNorrish I reaction (data not shown). Ifthe degradation proceeds according to a Norrish II reaction, carbonyl groups and terminal vinylgroupswould be produced aswas found uponphoto-ageing.IncreasingthefibercontentinthePEP composites produced a distinct increase in carbonylgroupformationasshowninFigure7.However,therewasnodecreaseinthearomaticC=Cabsorptionbandat1506cm-1whichindicatedthatthearomaticlignininthefibercomponentisthermallystable.Ahigheroxidationrateduringthethermo-oxidationofthePEcompositesmayhaveledtothedecreaseinthedegreeofpolymerizationofcelluloseorhemicelluloseat100°C(LeVan,1989). Figure8 shows thechange in theUV-

visiblespectraofthePEPcompositesat16%byweightduringthermo-oxidation.Theabsorbancein the UV domain (less than 300 nm) increased withageingtimeandespeciallywithabsorbanceat280nmwhichincreasedatahigherratethanthatofphoto-oxidation.ThisUVresultsupportedtheIRresultsofthePEcompositesduringthermo-ageing. In photo-ageing, the crystallinity degree ofallPEandPEPcompositeincreasedasshowninFigure 9a.An increasewas observed in thecrystallinity degreewhich tended to a plateauafter200hrexposure.Withprogressinthephoto-

Figure 6 Changesintheinfraredspectraat1712cm-1of:(a)Lowdensitypolyethylene;(b)Polyethylene/palmfibercompositesat 30% byweight of fiber contentduringthermo-oxidationintheovenat100°C.

Figure 7 Absorbancechangesat1712cm-1 during thermo-oxidation of polyethylene/palmfiber (PEP) composites and asa functionofageing time. (OD= theoptical density;Vertical error barsindicate ± SD; PE = Low densitypolyethylene.)

Figure 8 UV (ultraviolet)-visible spectraof polyethylene/palm fiber (PEP)compositesat16%byweightoffibercontentduringthermo-oxidation.

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oxidationprocess,thenumberofchainscissionsincreasedwithareductioninthemolecularsize,whichallowedtherearrangementoftheseshortermolecular chains according to thewell knownchemi-crystallizationprocess(Wunderlich,1976).This resulted in an increase in the crystallinity degreeforthePEPcomposites. With thermo-ageing, no change incrystallinity ofPEwas found (Figure 9b).Thecrystallinity degree of the PEP compositesproduced similar results to photo-ageing. The longer the ageing time, the higher the crystallinity degree obtained for the PEP composites.Anincreasedcrystallinitydegreewasobservedonlyintheinitialageingtime(100–200hr).Thelongerthe ageing time, the more constant the crystallinity degree of the PE composites became. It washypothesized as described in the photo-ageingabove that themolecule segmentswith bulkyand randomly distributed chemical groups cannotfit into thecrystal lattice.This results inaninterruptionofthechemi-crystallization(StarkandMatuana, 2004). Furthermore, that rearrangement of the shorter PEmolecule should induce theformationofthechemi-crystallinityasexplainedinthe photo-degradation part and result in an increase inthecrystallinityofthepurePE.However,thecrystallinitychangewasnotfoundinthepurePE

due to this thermo-ageing condition accelerating further the degradationmechanism in the PEpolymer.The chemi-crystallinity of the shortermoleculesinthePEPcompositeswasstillbeingformed.Inthermalageing,thermalannealingcanalsoinfluencetheevolutionofcrystallinitywhichwasnotpossibleduringphoto-ageing.Theeffectofthepresenceoffiberwasmainlysignificantfortheevolutionofcrystallinityduringeachtypeofageing.TheinfluenceofsmallchainsinthePEPcompositeswasunderlinedwithspectralanalysisandcouldbeoneexplanationforthecrystallinityincrease. Figure 10a presents only Young’s modulusofthePEandPEPcompositeat5%byweightafterphoto-oxidation.ItcanbeobservedthattheYoung’smodulusofthepurePEincreasedsubstantiallywhen ageing time increased.TheYoung’smodulus of thePEPcomposite tendedtoincrease.Theformationofcarbonylandvinylgroups in the PE indicated main-chain scission, as the shorter chains from the photo-oxidationproducts of the PE have a higher mobilitywith rearrangement tomake the PEmolecularchains more crystalline (Stark and Matuana, 2004).Therefore, theYoung’smodulus of thepurePE increased.However, fiber inclusion inthePEmatrix preventedUVabsorptionofPE,

Figure 9 Crystallinitydegreeofthelowdensitypolyethylene(PE)andpolyethylene/palmfiber(PEP)compositesat8%byweightafter:(a)Photo-ageing;(b)Thermo-ageing.(Verticalerrorbarsindicate±SD.)

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Figure 10 Young’smodulusofthelowdensitypolyethylene(PE)andpolyethylene/palmfiber(PEP)compositesat8%byweightAfter:(a)Photo-ageing;(b)Thermo-ageing.(Verticalerrorbarsindicate±SD.)

consequently, shorter chains of PEwere lesscommoninthePEPcompositewhichmadeitlesscrystalline compared to the PE. Thus, increasing Young’smodulus of the PEP composite afterradiationwaslessinfluentialcomparedtothePE.After thermo-oxidation, the values ofYoung’smodulusofthepurePEandthePEPcomposite(Figure 10b) tended to increasewith increasedageing time.Thiswould describe the previousresults after photo-degradation.However, therewasnochangeinYoung’smodulusforanyofthePEP composites.

concLuSIon

Thepalmfibercontent incorporatedinthePEmatrixresultedinchangesinthefunctionalgroups and crystallinity of thePEP compositesafter photo- and thermo-ageing. Lignin in thefiberswas sensible toUV absorbancewhichproduced radical species and initiated the chain scissionofpolymersuponphoto-oxidation.Uponthermo-oxidation, cellulose and hemicellulosewere themain components producing radicalspeciestoincreasethepolymeroxidation.Chemi-crystallizationalsoproducedhighercrystallizationafterageing.ThisresultsofthisstudysuggesttheadditionofUV stabilizer inPEPcomposites toprevent photo- and thermo-degradation.

AcKnoWLEdgEMEntS

This workwas supported byKAPI,Kasetsart University, Bangkok, Thailand and Institut de Chimie de Clermont-Ferrand (ICCF), LeCentre national de la recherche scientifique(CNRS). Universite Blaise Pascal, Clermont-Ferrand, France.

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