Thermal performance and optical properties of wood–polymer composites

Article

Thermalperformance

andopticalproperties

ofwood–polymer

composites

SvetlanaButylina,OssiMartikkaand

¨rkiTimoKaJournalofThermoplasticCompositeMaterials26(1)60–73!TheAuthor(s)2011Reprintsandpermissions:sagepub.co.uk/journalsPermissions.navDOI:10.1177/0892705711419694

Abstract

Thethermalperformanceandopticalpropertiesofwood–polypropylenecompositesmadefromuntreatedwoodmaterialwithandwithoutmetal-containingpigments,andcompositesmadefromheat-treatedwoodmaterialwerestudied.Thethermalheatbuildupandlinearshrinkageweredetermined.Theimpactoftheopticalpropertiesofthesurfaceofthecompositesontheheatbuildupwasanalysed.Thewood–polypro-pylenecompositesmadefromuntreatedwoodwithoutpigmentwerefoundtohavethelowestheatbuildup;asimilarcompositecontaininggreypigmenthadthehighestheatbuildup.Thelinearshrinkageofthestudiedwood–polypropylenecompositeswasintherange0.05–0.16%.

Keywords

Wood–polypropylenecomposites,heatbuildup,opticalproperties

Introduction

Compositesmadefromablendofthermoplasticandnatural bershavebeenthesubjectofmuchresearchandbecomeattractivetomanufacturersbecauseoftheirmanyadvantages.Theseadvantagesincludeimprovedenvironmentalperformance,mainlyduetotheuseofbiodegradablematerialsandareductionintheuseofnonrenewable(oil-based)resourcesthroughoutthewholelifecycleofthecom-posite;lowcostofwood ourandnatural-organic llersingeneral;thelowerdensityofthese llersincomparisontotraditional llers,suchasglass bers;LappeenrantaUniversityofTechnology,Lappeenranta,Finland

Correspondingauthor:

SvetlanaButylina,LappeenrantaUniversityofTechnology,P.O.Box20,Lappeenranta,FIN-53851,Finland.Email:butylina@lut.fi

Butylinaetal.61improvementsinthesafetyofproductionemployees(reducedhazardinthecaseofaccidentalinhalation);specialaestheticpropertiesofthecomposites,whichcanbeeasilyprocessedandre ned,obtainingwood-likelookingproducts;andfullrecy-clabilityofthecomposites.1Therearemanypotentialindoorandoutdoorappli-cationswherecompositescanbeused.Forexample,wood–polymercomposites(WPCs),mostlymanufacturedthroughextrusionandinjectionmoldingprocesses,canbeusedintheautomotive(dashboardsorscreendoorsofvehicles)andcon-structionindustries(interior oorcoverings,pro ledpartsfordoorsandwindows,ornamentalpanels,externalshutters,pavements,garageorentrancedoors,etc).2Exteriornonstructuralorsemistructuralcompositebuildingproductssuchasdeck-ing,fencing,siding,androoftilesarebeingintroducedintothemarket.3InEurope,WPCdevelopmenthasstartedwithdecking.4

Theoutdoorapplicationofthesematerialshasraisedconcernsabouttheirdurability,includingfungalresistance,ultravioletresistance,moistureresistance,anddimensionalstability.5Additives,suchaspigments,ultravioletabsorbers,andhinderedaminelightstabilizersareusedtopreventtheWPCsfromcolorfading.AccordingtoastudyreportedbyKiguchietal.,6theadditionofdarkerpigmentsimprovesthecolorstabilityofwood–polypropylenecompositesbetterthantheadditionoflightcolorpigments.Deckingsaremostlystainedwithgreyorbrownasdominantcolors.

Itisknownthatasurfaceexposedtosolarradiationexhibitsbuildupoftem-perature.Theabsorbedsolarenergyisareasonforheatbuildup.TheSunisahighlyenergeticsystemconverting4milliontonsofhydrogenintoheliumandinverselyatthetemperatureofhundredsofmillionsdegreesCelsiuseverysecond,resultinginabout3.86Â1023kWgeneratedperyear.Despitethefactthatonly1.78Â1012kW/yearreachesthesurfaceoftheEarth,theamountofsolarenergyisstill20,000timesmorethanthetotalannualworldenergyproduc-tion.7TheamountofenergyreachingtheEarthgreatlyvarieswithitsverticalangle.ThelargestamountisreceivedatnoonwhentheSun’sraysfallattherightangle,andthesmallestwhentheSunrisesandsets.Thetotalamountofmonthlyradiationkeepsincreasingfromspringtosummer,anddecreasesgrad-uallytowardwintertime.8Theintensityofsolarradiationmayvarysigni cantly.Forexample,inindustrialareasitmayreach600–700W/m2,800–900W/m2inurbanareas,andaround1000–1100W/m2inhighmountainregions.

ThewavelengthrangeoftheSun’sradiationisbetween0.2and3.0mm,andtheenergyvarieswiththewavelength.Thesolarradiationspectrumconsistsoftheultravioletrange0.12–0.45mm(15.89%ofenergyiscollectedinthisregion),thevisiblerange0.45–0.75mm(35.80%),andtheinfraredrange0.75–1.00mm(48.30%).8Thethermalbuildupofasurfaceexposedtosolarradiationhasacor-relationwiththecolorofthesurface,thatis,white-coloredsurfacesstaycoolerthanblack-coloredsurfaces.Thecolorofasurfacecanbeexplainedintermsofthere ectanceforeachcomponentofthevisiblespectrum.Itisveryimportanttoknowtheabsorptance/re ectanceintheinfraredregioninordertopredictthethermalpropertiesofthematerial,becausealmosthalfofthesolarenergyis

62JournalofThermoplasticCompositeMaterials26(1)concentratedinthisregion.Somepigments,socalledcoolpigments,suchastita-niumdioxidein uencetheopticalandnear-infraredpropertiesofcoatings.9

Variousstudieshavebeenperformedtounderstandbetterthethermalandopticalperformanceofconstructionmaterialsandtheirimpactonthecityclimate.9–11Lowersurfacetemperaturescontributetodecreasingthetemperatureoftheambientair,astheheatconvectionintensityfromacoolersurfaceislower.Suchtemperaturereductionscanhavesigni cantimpactsoncoolingenergyconsumptioninurbanareas,afactofparticularimportanceforcitiesinahotclimate.Alaboratorytestwasintroducedtopredicttheincreaseintemperaturesaboveambientairtemperatureduetosolarenergyabsorption,usinganinfraredre ectiveheatlamptoimitatesolarradiation.

Inthepresentstudy,heatbuildupwasdeterminedforwood–polypropylenecompositesmadefromheat-treatedwoodandforcompositesmadefromuntreatedwoodwithandwithoutaddedmetal-containingpigment.Theopticalpropertiesofthesurfaceofthecompositeswerestudiedbyusingvisible(VIS)andFouriertransforminfrared(FT-IR)spectrophotometers.Linearshrinkageofextrudedpro leswasalsodetermined.

Experimental

Materials

Thecompositionofthestudiedwood–polypropylenecompositesareshowninTable1.ThesoftwoodpelletsusedintheexperimentswerepurchasedfromVAPO(Jyvaskyla,Finland).AccordingtothetechnicaldatasheetprovidedbyVAPO,thesizeofthepelletswas6–8mmindiameter,with10–30mmaveragelength.Inthiswork,birchtreatedaccordingtotheFinnishThermoWoodtech-nologywasusedtoprepareheat-treatedwood bers(HTW).

Aneatpolypropylenehomopolymer,EltexHY001P,suppliedbyINEOSOle ns&PolymersEurope(Brussels,Belgium),wasusedinthepreparationTable1.Formulationofwood–polypropylenecomposites.

Wood

No.

1

1a

2

2a

3

4

5TypePelletsPelletsHTWHTWPelletsPelletsPellets(wt%)70757075707069PolypropyleneTypeNeat/RecycledNeatNeatNeatNeatNeatRecycled(wt%)26222722262625MAPP(wt%)3333333PigmentType----GreenGreyGrey(wt%)----111Lubricant(wt%)1-----2MAPP:maleatedpolypropylene.

Butylinaetal.63ofthecomposites.RecycledpolypropylenehomopolymersuppliedbyEkiplastOy(Hauho,Finland)wasalsousedtocompoundthecomposites.Theneatpolypro-pylenehomopolymerhadthedensityof0.91g/cm3andmeltmass- owrateof45g/10min(230 C/2.16kg),andtherecycledpolypropylenehomopolymerhadthemeltmass- owrateof3g/10min(190 C/2.16kg).

Thecouplingagentwasmaleatedpolypropylene(MAPP),OREVACÕCA100(Ato na,France).TheOrevacCA100polymerhaslowfunctionality(1%)andahighmolarmass(25kg/mol).AccordingtoSainetal.,12theoptimumconcentra-tionofacouplingagentisaround3–4%byweightofthecomposite,thus3%MAPPwasaddedineachcase.

Twodi erentpigmentswereusedinthestudy:greenpigment(purchasedfromHollandColoursNV,TheNetherlands);andgreypigment(purchasedfromClariant(Finland)Oy,Vantaa,Finland).Theelementalanalysisofthepigmentswasperformedwithscanningelectronmicroscopy(SEM)coupledtoanenergy-dispersiveX-rayspectrometer(EDS).TheSEM–EDSresultsindicatedthepresenceofcobalt(Co)andchromium(Cr)asmajorcomponentsofthegreenpigment.Theanalysisofthegreypigmentrevealedthattitanium(Ti)wasitsmajorcomponent,althoughotherelements,suchassilicon(Si),sodium(Na),aluminium(Al)andsulfur(S)werealsodetected.

Fortheformulationofthewood–polypropylenecompositesmadefromrecycledpolymer,alubricantwasaddedinordertoimprovethe owabilityofthehotmelt.StructolÕTPW113(Ohio,UnitedStates),whichisablendofcomplex,modi edfattyacidesters,wasusedasthelubricant.

Processing

Thewoodmaterial,plastic,andadditiveswerecompoundedusingaWeberCE7.2conicaltwin-screwextruder(HansWeberMaschinenfabrikGmbH,Kronach,Germany).Thegravimetricfeedingsystemincludedamainfeederconnectedwithsidefeedersforeachindividualcomponent.Allcomponentswerefedintotheextruderthroughthemainfeeder.

Thescrewhadthelength-to-diameter(L/D)ratioof17,andthescrewspeedwas12rpm.Thebarreltemperaturesoftheextruderwere170–200 C,andthemelttem-peratureatthediewas180 C.Thepressureatthedievariedbetween4and7MPa,dependingonthematerialblend,andthematerialoutputwas25kg/h.Thesampleswereextrudedthrougharectangulardie;thehollowpro leisshowninFigure1.Heatbuilduptestinginlaboratoryapparatus

TheheatbuildupintheWPCswastestedaccordingtoTS15534(AnnexF).ThesetupisshowninFigure2.Sampleswithdimensions75mmÂ75mmÂ5mmweretested.Atotalof fteenspecimenspereachtypeofcompositeweremeasured.Awhiteinfraredheatlamphavingthenominalpowerof250W(purchasedfromGeneralElectric,Hungary)wasused.Thedistancebetweenthelowestpartof

64JournalofThermoplasticCompositeMaterials26(1)Figure1.Hollowprofileofwood–polypropylenecomposites.

thedownward-orientedlampandthebottomoftheboxwas400mm.Thetemper-atureofthecompositemeasuredatitsbottom,andtheincreaseofthetemperatureofthecompositecomparedtoambientair(ÁTexp)wererecordedwith1mininter-valsbyadigitalthermometerequippedwithadatalogger.

Opticalcharacteristics

There ectancecurvesofthecompositesinthevisiblerangeweremeasuredwithaMinoltaCM-2500dspectrophotometer(KonikaMinoltaSensingInc.,Japan).ThemeasurementsweremadeusingaD65illuminantanda2degreestandardobserver.ThetristimulusX,Y,andZvaluesofallspecimenswereobtainedfromthespectro-photometer.TheCIELABcolorsystemwasusedtocomputethesurfacecolorinL*,a*,b*coordinates.TheL*representsthelightnesscoordinate,anditvariesfrom100(white)to0(grey);a*representsthered(+a*)togreen(–a*)coordinate;andb*representstheyellow(+b*)toblue(–b*)coordinate.

TheglossvaluesofthecompositesweremeasuredusingaNovo-glossTRIOglossmeter(RhopointInstrumentsLtd,EastSussex,UK).Themeasurementsweremadeattheangleof60degrees,whichisrecommendedforWPCs.

Di use-re ectanceinfraredFouriertransformspectroscopy(DRIFTS)wasusedforpowderedsamplesofcompositeusingaPerkin-ElmerSystem2000FT-IRspectrophotometerequippedwithPerkin-Elmerdi usere ectanceaccessory.Powderofthetoplayerofthecompositewaspreparedbysandingwith

sandpaper

Butylinaetal.65Figure2.Setupforheatbuildupmeasurement.

(gritdesignationP240).Samplesofabout10mgwereanalysed.Potassiumbromide(KBr;Aldrich,FT-IRgrade)wasusedasreference.Re ectancespectrawereobtainedintherange10,000–2700/cm(1.0–3.7mm)using50scansand4/cmresolution.Heatreversion(linearshrinkage)testing

HeatreversionwasdeterminedaccordingtotheEN479standard.Thistestestab-lishesthepercentageoflinearshrinkageofapro leatanelevatedtemperature.Thehollowpro lewiththelengthof250mmwasplacedinanovenat100 Cfor60min.Amarkedlengthofthistestsamplewasmeasuredunderidenticalconditions(23Æ2 C),beforeandafterheatingintheoven.

TheheatreversionRwascalculatedasapercentageusingthefollowingEquation(1):

R¼ðLoÀL1ÞÂ100,Loð1

Þ

66JournalofThermoplasticCompositeMaterials26(1)whereLoandL1arethedistancesbetweenthemarksbeforeandafterheatingintheoven(mm),respectively.

Resultsanddiscussion

Heatbuilduptesting

TheresultspresentedinTable2showthemaximumtemperatures(Tmax)ofthecompositesandtheincreaseofpaneltemperatureabovethetemperatureofambi-entair(ÁTexp).Allcompositesreachedasteadystatetemperaturewithin1h.Forallmeasurements,theambienttemperatureofairwas22.3(Æ0.3 C).Thelowesttemperatures(TmaxandÁTexp)werefoundforthewood–polypropylenecompos-ites(N1andN1a)thatweremadefromuntreatedpelletizedwoodanddidnotcontainanypigments.Thehighesttemperatureswerefoundforthecompositescontaininggreypigmentmadeeitherfromneatorrecycledpolypropylene(N4andN5).Thetemperaturesmeasuredforthecompositesmadefromheat-treatedwood(N2andN2a),andforthecompositesmadefrompelletizedwoodandcontaininggreenpigment(N3)laybetweenthetwoabovementionedgroups.Thedi erencebetweenthecompositeshavingthehighestandlowesttemperaturewasestimatedas8.8degrees.Forcomparison,similarsamplesofuntreatedsoftwoodwoodandheat-treatedwoodweretested.Themaximumtemperaturereachedbyuntreatedwoodwas47.7 C,whileforheat-treatedwooditwas49.3 C.Becausetheheatabsorptionpropertyofamaterialdependsonitsopticalproperties,thelatterweremeasured,andtheresultsarepresentedbelow.

Opticalcharacteristicsofwood–polypropylenecomposites

TheresultsofthespectrophotometricmeasurementsinthevisiblerangeareshowninFigure3.There ectioncurveinthevisibleregionrepresentsmostaccuratelytheTable2.Maximumtemperaturereachedbywood–polypropylenecomposites(Tmax),andthetemperaturedifferencebetweenthemaximaltemperatureofthesampleandthetemperatureofambientair(ÁTexp)(theresultsaretheaverageoffifteenmeasurements).

No.

1

1a

2

2a

3

4

5Tmax( C)55.055.058.057.056.363.863.3ÁTexp32.932.735.834.634.541.240.7

Butylinaetal.67

Figure3.Reflectanceofwood–plasticcompositesinthevisiblewavelengthrange.

colorofamaterial.Wood–polypropylenecomposites(N1andN1a)showverystrongabsorptioninthe0.40–0.50micrometersband,followedbyhighre positesmadefromheat-treatedwood(N2andN2a)haveverystrongabsorptioninpartofthevisiblespectrum.Bothcompositescon-taininggreypigment(N4andN5)exhibitlowre ectance(about7%)inthewholevisiblerange.Thecompositecontaininggreenpigment(N3)hasasmallre ectancepeakaround0.50micrometers,andthenshowsstrongabsorptionintherestofthevisiblespectrum.Nocorrelationbetweenthemaximumtemperatureandre ectanceinthevisibleregionwasfoundforthestudiedwood–polypropylenecomposites.

Colorcoordinateswerecalculatedusingthevisiblespectrum,andtheresultsarepresentedinTable3,togetherwithglossmeasurementsforthecomposites.AscanbeseeninTable3,thecolorcoordinatesforthecompositesmadefromthesamerawmaterial(e.g.,N1andN1a,andN2andN2a)areslightlydi erent.Torecall,compositesN1andN2contained70%woodmaterial,whilecompositesN1aandN2ahad75%ofwood.Thecompositemadefromrecycledpolypropylene(N5)wascharacterizedbyhigherlightnessandspecularglosscomparedtothecompos-itemadefromneatpolypropylene(N4).Generally,allthestudiedcompositeshadlowspecularglossvalues(3.5–5.7).

There ectanceofthecompositesinthenear-infraredregion(1.0–2.5mm)wasmeasuredwithaninfrared(IR)spectrophotometer.AscanbeseeninFigure4,

68JournalofThermoplasticCompositeMaterials26(1)Table3.Colorcoordinatesandglossmeasurementsofwood–polypropylenecomposites(theresultsaretheaverageoftenmeasurements).

No.

1

1a

2

2a

3

4

5L*67.6866.6116.6416.0544.5330.9937.04a*7.038.1014.8515.19–39.43–0.76–0.86b*30.2936.3226.2925.7512.863.71–0.14Glossatangle60 5.395.564.905.533.643.535.07

Figure4.Reflectanceofwood–polypropylenecompositesinnear-infraredregion:TheyellowlinerepresentsN1,thebrownlineN2,thegreenlineN3,andthegreylinesN4andN5.

re ectanceinthenear-infrared(NIR)regionforcompositesN1andN2madefromuntreatedandheat-treatedwood,respectively,issimilar,eventhoughtheirre ec-tanceinthevisibleregionwasdi erent.Thecompositesmadewithoutpigmentshavehigherre ectancethantheonesmadewithpigments.There ectanceofthecompositecontaininggreenpigment(N3)ishigherthanthere ectanceof

the

Butylinaetal.69

Figure5.Maximumtemperaturesofwood–polypropylenecompositesasafunctionofreflec-tanceinthenear-infraredregion.

compositecontaininggreypigment(N4).Greycoloringisoftenusedfordecking,butascanbeseeninFigure4,thewood–polypropylenecompositescontaininggreypigmenthavethelowestre ectanceinthenear-infraredregion.

There ectanceoftherawwoodsampleswasnotmeasuredinthisstudy.Woodisknowntobeanexcellentmaterialtore ectlight.13Accordingtotheliteraturedata,moderatelydarkbarewoodtypicallyhasavisiblere ectanceof0.20,andtheNIRre ectanceofabout0.70,whichresultsinthesolarre ectanceofabout0.45.9Theouterlayerofthestudiedwood–polypropylenecompositepro leswascon-sistedofpolymer.Thisouterpolypropylenelayera ectsthere ection/absorptionoflight.Inorganicpigments,whichwereaddedinwood–polypropylenecompositesbelongtothetypicalpigmentsusedwidelytocolorplastics.

Intheirresearchonheatbuildupofpaintedsteelpanels,MoerkandReck14foundthatre ectanceat2.4mmhadagoodcorrelationwiththeactualexteriorheatbuildupofpaintedsteelpanelswithasimilarglosslevel.Inthepresentstudy,thecorrelationbetweenthemaximumtemperaturesobtainedatheatbuilduptest-ingandnear-infraredre ectancemeasuredatthemiddlepointofthespectrum(1.0–2.5mm)wasexamined.Figure5showsthatthecorrelationbetweenthemax-imumtemperatureandinfraredre ectancewasrelativelygood(R2=0.87).Applicationofintegralsofre ectancespectrabetween1.0and2.5mminstead

of

70JournalofThermoplasticCompositeMaterials26(1)singlepointre ectanceat1.75mmresultedinasimilarcorrelationbetweenthere ectanceparameterandthemaximumtemperatureofthecompositemeasuredinheatbuildupexperiment.Anexclusionofcompositemadefromheat-treatedwoodresultedinabettercorrelationbetweenthenear-infraredre ectanceparam-eterandmaximumtemperature(R2=0.97).Bettercorrelationbetweenthenear-infraredre ectanceandmaximumtemperatureofcompositematerialintheabsenceofheat-treatedwood–polypropylenecompositewasconsideredasaproofthatopticalpropertiesofouterpolymerlayeraredeterminingparameterinthiscase.

Ithastobekeptinmindthatsurfacere ectancedataforthewood–polypro-pylenecompositesobtainedbyaspectrophotometercangivearoughideaofthethermalbuildupofcompositesexposedtosolarradiation.Convectionheatloss,forcedconvectivewindcooling,andthevarietyoflocationsontheEarth’ssurfaceshouldbetakenintoaccountforpredictingtheheatbuildupofcompositesduetoasolarradiationundernaturalconditions.

Heatreversion(linearshrinkage)

Theproblemsofresidualthermalstressesin berreinforcedcompositeshavebeenextensivelystudied.15–17Residualstressesinthermoplasticsarepresentinthecom-positestructureimmediatelyaftertheprocessingandsubsequentcoolingtotheservicetemperature.Thesestressesin uencethepropertiesofthecompositestruc-turessigni cantly.15Themagnitudeofresidualstressesinthecompositestructuresdependsonfourparametersinthecasewhenthelong-termandenvironmentalparametersareignored:thetemperaturedi erence,thecoe cientsofthermalexpansion/shrinkageuponthecoolingofthecompositeconstituents,theelasticcoe cientsoftheseconstituents,andthe bervolumefraction.16

Inthiswork,thelinearshrinkageofahollowpro lewasdeterminedaccordingtotheEN479standard.Hollowcompositesaremoresusceptibletoresidualpostmanufactureshrinkagecomparedtosolidboards.Inhollowboards,asmuchas15%oftheoverallshrinkageisstill‘stored,’waitingforthetemperaturetogoup,suchasonadeckunderdirectsunlight.18Table4showsthevolumefractionofwood berandheatreversion(shrinkage)forthehollowpro lesofthestudiedwood–positesN1andN5,madefrompelletizedwoodmaterial,andcompositeN2a,madefromheat-treatedwoodmaterial,showlowerheatreversionscomparedtootherstudiedcomposites.

Themicromechanicalmodelsdeveloped19toestimatethethermalexpansion/shrinkageofacompositerequiresknowledgeofthepropertiesoftheconstituents(e.g.,coe cientofthermalexpansion,Young’smodulus,andvolumefraction)andmicrostructures(e.g., berorientation).Ingeneral,thecoe cientofthermalexpansionforthereinforcing bersismuchlowerthanforthethermoplasticmatri-ces;15thelinearcoe cientofthermalexpansion/shrinkage(LCTE)forpolypro-pylenehomopolymerisequalto8–10Â10–51/ C,andforwoodspecies(hardwoodsandsoftwoods)equal(alongthegrain)to0.31–0.45Â10–51/ C.18

Butylinaetal.71Table4.Fibervolumefraction(Vf)andheatreversion(linearshrinkage)ofwood–polypropyl-enecomposites.

No.

1

1a

2

2a

3

4

5Vf0.610.670.620.680.620.620.61Heatreversion,R(%)0.050.110.130.080.140.160.08

Theincreaseofthevolumefractionofreinforcing bers,havingLCTEvalueslowerthanthematrix,decreasesthelinearexpansion/shrinkageofthecomposite.Inourstudy,nocleartrendwasfoundbetweenthewoodvolume berfractionandheatreversion(linearshrinkage).

Inconnectionwiththequestionofthecompositemicrostructure( berorienta-tionanddistributioninthecompositestructure),itshouldbenotedthatourpre-viousstudyonmechanicalpropertiesofsimilarwood–polypropylenecompositesshowedthatthe bershadrandomorientationandsizedistribution.20Also,duetothehighwood berloadinginthestudiedcomposites,thepresenceofdefectssuchaswood beraggregatesandvoidswasdetected.Thepresenceof beraggregateswerefoundtobemorepronouncedforcompositesmadefrompelletizedwoodmaterial.Togetherwithalackofknowledgeonpropertiesofindividualconstitu-ents(e.g.,Young’smodulusforthe ber,whichisnormallyobtainedinasingle bertensiletesting),theabovementioneddefectsofstructurewerethemainrea-sonswhymodelingisabsentinourwork.Morethoroughresearchworkshouldtobedonetoidentifythepropertiesoftheconstituentsinordertousethemicro-mechanicalapproachtoexplainthebehaviorofthestudiedcomposites.

Asalastnote,aweightlosswasobservedafterthecompositeshadbeenkeptintheovenat100 Cfor1hinthecourseofthetestingprocedure(EN479standard).Themoisturecontentsoftheoriginalcompositeswereintherange1.5–2.6%.Thecompositeswithahighervolumefractionofwood berwerecharacterizedbyhighermoisturecontent,exceptforthecompositesmadefromheat-treatedwood,forwhichthedi erenceinthemoisturecontentwasverysmall.Thus,itisconsideredthatthelinearshrinkageofcompositesshowninTable4canincludetheshrinkageduetothedryingofwood.

Conclusions

Thethermalheatbuildup,opticalpropertiesandlinearshrinkageofwood–polypropylenecompositeswerestudied.Thefollowingconclusionsweredrawnonthebasisoftheexperimentalresults:

72JournalofThermoplasticCompositeMaterials26(1)

1.Thethermalheatbuildupwasfoundtobewellcorrelatedwiththenear-infraredre ectanceofthecomposites.Thecompositescontaininggreypigmenthad6–9degreeshigherheatbuildupvaluesthantheothercomposites,andtheywerecharacterizedbythelowestinfraredre ection.Becausegrey-coloredcompositesareoftenappliedfordecking,itwouldbedesirabletousere ectivepigmentsinordertodecreaseheatbuildup.

2.Generally,theheatbuildupforbothuntreatedandheat-treatedwoodensampleswaslowercomparedtothewood–polypropylenecomposites.Theincreaseinwoodcontent(from70to75%),ortheuseofrecycledpolypropyleneinsteadofneatonewerefoundtohavenosigni cante ectontheheatbuildupofthecomposites.

3.Thelinearshrinkageofthestudiedwood–polypropylenecompositeswasfoundtobeintherange0.05–0.16%.Therewasnocleartrendbetweenthe bervolumefractionandthelinearshrinkageofthecomposite.Thepresenceofmoistureinthecompositeshadane ectonthemeasuredvaluesoflinearshrinkage.

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