wa of compostes

8/2/2019 Wa of Compostes

1/17

1

WaterAbsorptionandDurabilityofWoodPlasticCompositesMarekGnatowski,Ph.D.,ResearchDirector,PolymerEngineeringCompanyLtd.

Burnaby,BC,Canada

AbstractWaterabsorptioninwoodplasticcomposites(WPC)isstillacontroversialbut

importantissueassociatedwithcompositedurability.

Inthispaper,waterabsorptionbyWPCboardsexposedtoexteriorconditionsin

Vancouver,BCforaperiodofupto6yearsisexamined. Theboardsweremade

usingdifferentmaterialformulationsthatmatchedthewaterabsorption

characteristicsofavarietyofcommercialproductsavailableinNorthAmericain

20012002.

Thetotalwaterabsorptionandwaterdistributionwithintheboardswasevaluated

withthefocuson:(1)periodofexposure(2)woodcontentincomposite(3)exposure

location(sun

or

shadow)

(4)

exposure

geographical

location

(5)

wood

species

(6)

presenceofUVstabilizers(7)presenceofzincborate. Theeffectofwaterabsorption

onbiologicalactivitywithintheWPCboardsaswellasmechanicalpropertieswillbe

discussed.

ItwasfoundthatWPCmayabsorbasignificantquantityofwaterduringprolonged

exposuretoexteriorconditions,andthewatercontentinthematerialseemsto

increaseduringtheexposureperiod. Thismaybeassociatedwiththelossofsome

mechanicalpropertiesandthepresenceofbiologicalactivity,includingdecayfungi.

IntroductionWater

seems

to

be

an

environmental

factor

that

affects

practically

all

materials

exposed

toexteriorconditions. Waterispresenteverywhere,andevenondrysunnydayswe

mayseelongperiodsofheavydewformationduringthenight,fromearlyeveningto

latemorning. Itisexpectedthatwatermayalsoaffectperformanceandpropertiesof

WoodPlasticComposites(WPC)includingmechanicalproperties,dimensionalstability,

warping,andintensificationofbiologicalactivity.(117)

Forthisreasonwaterabsorption

byWPCshasbecomeourprimeinterestinresearchondurabilityofthesenew

materials. Itiswidelyknownthatthereissomecontroversysurroundingwater

absorptionbywoodplasticcomposites. Fromonesidethereisaperceptioninindustry

thatwoodparticlesareeffectivelyencapsulatedinwaterimperviousplastic. While

woodmay

absorb

as

much

as

about

25%

water

to

fiber

saturation

point,

polyolefins,

includingpolyethylenewhichcreatesacontinuousphaseofthecomposite,mayabsorb

only0.01%moistureafterimmersion. Therearehowever,signsthatcontradictthese

optimisticexpectations. Thereispublisheddatathatindicatesexpansion,warpage,

decay,andeventhepresenceoffruitingbodiesofdecayfungiontheWPCsurfacein

thefield.(4,10,12,13,14)

Thissuggestsaratherhighwaterpresenceintheevaluated

materialsexposedtoweatherelements.


2/17

2

Toshedsomelightonthiscontroversy,in2000PolymerEngineeringCompanyLtd.

undertookresearchonwaterabsorptionbyWPCexposedtoexteriorconditionsto

gatherscientificdatainthisrespect. Theexperimentalsamplesusedinthiswork

simulatedtheextremeperformanceofcommercialcomposites. Thesamplesdiscussed

inthispaperwereexposedinthemild,temperateclimateofVancouver,BCforupto6

years,andalsoanevaluationwasdoneforboardsexposedlaterintropicalHilo,Hawaii.

Theobjectiveofthispaperistosharewithyousomeourresults. Certainresultshave

beendiscussedinpreviousWPCconferencesinMadison(15)

andinToronto(17)

andtheymayberepeatedhereforcomparisonpurposes.

SamplePreparationandExposureSamplesusedforevaluationweremadeintwogroupsrepresentedbyformulations#5

and#12,containingnominalwoodcontentof50%and65%.(table1). Woodcontentof

50%and65%areshownforsimplicity. Further,samples#5and#12wereoptionally

modifiedbytheadditionofaUVstabilizerpackagecontainingorganicUVabsorbers,

freeradicalsscavengersandinorganicpigments,and/orzincboratebiocide. Also,for

simplicity,thequantityofHDPEisgivenas45%and30%respectively. Acorrectionto

accommodateadditives

was

done

accordingly

without

displaying

the

relatively

small

changesrequiredinwoodandresincontents.

Formulations#5and#12wereestablishedsothatwaterabsorptioninequilibriumand

kineticsresembledsomecommercialWPCwithextremeperformancewithrespectto

waterabsorptioncollectedinNorthAmericaandtestedforthispurposeduringthe

periodfrom2000to2002.(figures1and2)

0%

5%

10%

15%

20%

25%

30%

35%

40%

45%

W

aterAbsorption

(%

)

A B C E F G H I J K L M N O 5 6 12 13

Commercial Experimental

Figure1. Waterabsorptioninequilibriumforcommercial(AO)

andexperimental(5,6,12,13)WPC


3/17

3

0%

5%

10%

15%

20%

25%

30%

35%

40%

Start 6 hours 1day 7 day 10 day 11day 14 day 17 day 21 day

Length of Immersion

W

aterAbsorption

(%

)

G

L

6

5

0%

5%

10%

15%

20%

25%

30%

35%

40%

Star t 6 hours 1 day 7 day 10 day 11 day 14 day 17 day 21 day

Length of Immersion

WaterAbsorption(%)

N

B

13

12

Figure2. Kineticsofwaterabsorptionbyselectedsamplesof

commercialandexperimentalWPC

WPCboardsusedfortestingwereextrudedatWashingtonStateUniversity,WoodMaterials

andEngineeringLaboratoryunderwellcontrolledconditionsusinga55mmconicalcounter

rotatingMillicrontwinscrewextruder.

Sampleswerecollectedinthefieldbycuttingtheendoftheexposedboardtobecross

sectionedin

the

laboratory,

as

shown

in

figure

3.

Care

was

taken

to

avoid

loss

of

moisture

duringmaterialhandling.

Figure3. Specimencuttingpatternandpresentationofmoisturecontent(MC)

andwaterabsorption(WA)intestedsamples

bc


4/17

4

BydryingsectionB,theaveragewaterabsorption(WA)forthetestedboardwas

established. Withtheknowledgeoftheexactwoodcontentinthesamples,moisture

content(MC)inwoodforeachspecimenwasthencalculated. Thismoisturecontentin

wood,asanimportantfactorincompositeperformance,isshowninbargraphs(figure

3)inthispaper. Furthermore,bywaferingsectionCofthesample,wewereableto

findthemoisturecontentdistributionwithintheexposedboardsasafunctionofthe

boardthicknessasshowninfigure3. Allgraphsclearlyindicatethemoisturecontentrequiredtoinitiatedecay,whichisabout25%.

Thetestedsampleswereexposedinsunandshadowintwogeographicallocations;

Vancouver,BCandHilo,Hawaii. Thelocationswereselectedinsuchawaythatthey

representedamoderate,temperateclimateaswellasatropicalclimatethatcouldbe

expectedtobemoreaggressive. Thisaggressivenesswithrespecttobiologicalactivity

couldberepresentedbytheSchefferIndex: Hilo331,andVancouver(Seattle)49.

ResultsanddiscussionThe

evaluation

of

the

exposed

samples

allowed

for

the

collection

of

information

about

waterabsorption,takingintoconsiderationthefollowingfactors:

Periodofexposure

Woodcontentincomposite

Exposurelocation(sunandshadow)

Exposuregeographicallocation

Woodspecies

PresenceofUVstabilizers

Presenceofbiocide(zincborate)


5/17

5

Ascanbeseeninfigure4,duringthefirst4yearsmoisturecontentinthetested

samplesgrewrapidly,andlateron,betweenthe4th

and6th

years,aplateauseemedto

beachieved. However,lookingatwaterdistributionwithintheboardswecouldseea

constantincreaseinmoisturecontentinthecenterofthesamples,withadecreasein

MCnearthesurface. Thiscouldbeduetoclimaticvariabilitypriortosampleharvesting.

Aportionofthespecimensexceededthemoisturecontentrequiredforinitiationofdecay.

Thiscanalsobeseenonthemajorityofthegraphs.

Figure4. MoisturecontentanditsdistributioninsamplesofWPCexposedin

Vancouver,BC(sunnylocation)for70months


6/17

6

Woodcontent,asexpected,seemstobeanimportantfactorinwaterabsorptionby

WPC. Justanincreaseinpineflourcontentfromabout50to65%,causedanincreasein

moisturecontentofabout100%regardlessoftheexposureperiod(figure5). Therewas

alsoasignificantdifferenceinmoisturedistributionwithintheboard,particularlywith

increaseddistancefromthesurface.

Figure5. MoisturecontentanditsdistributioninsamplesofWPCcontaining50and65%wood

exposedin

Vancouver,

BC

(sunny

location)

for

70

months


7/17

7

Surprisingly,therewererelativelysmalldifferencesinmoisturecontentforsamples

exposedinsunandshadow. Samplesexposedinsunseemedtohaveevenhigher

moisturecontent,particularlyinthecaseofcompositeswithlargerwoodcontentas

showninfigure6.

Figure6. MoisturecontentanditsdistributioninsamplesofWPCexposedinVancouver,BC

insunandshadowfor70months


8/17

8

Evenmoresurprising,wastheperformanceofoursampleswhenexposureina

moderatevs.tropicalclimatewascompared. SamplesexposedinVancouvershowed

significantlyhighermoisturecontentthanthoseexposedinhotandverywetHilo,

Hawaii(figure7). Thismaybeexplainedbythehigherevaporationrateintropical

Hawaii,andthefactthatsampleswerealwaysharvestedinVancouverattheendofthe

wetterwintermonthsofFebruaryorMarch.

Figure7. MoisturecontentanditsdistributioninsamplesofWPCexposedfor46months

inVancouver,BCand51monthsinHilo,HI


9/17

9

Woodspeciesalsoseemstoinfluencethewaterabsorptionofcomposites. Samples

containingthesamequantityoftheoakflouralwaysshowedlowerwaterabsorptionin

comparisontopinewhenexposedatthesamelocation(figure8).

Figure8. MoisturecontentanditsdistributioninsamplesofWPCmadefromdifferentwoodspecies(pine

oroak)exposedinVancouver,BC(sunnylocation)for70months


10/17

10

AdditivesusedinourUVstabilizerpackageseemedtoincreasethewaterabsorptionof

thesamplesascanbeseeninfigure9. Thisappliedtobothformulationswithlowerand

higherwoodcontent. Alargeincreaseinmoisturecontentoccurredallacrossthe

boards,includingtheareaneartheboardsurfaceandalsointhecentre.

Figure9. MoisturecontentanditsdistributioninsamplesofWPC,withandwithoutUV

stabilizerpackage,exposedinVancouver,BC(sunnylocation)for70months


11/17

11

Interestingandunexpectedwastheadditionofzincborate,knowntobeeffectiveasa

fungicide. Thepresenceofzincborateseemedtodecreasewaterabsorptionin

practicallyallofthetestedformulationsasshowninfigure10.

Figure10. MoisturecontentanditsdistributioninsamplesofWPC,withandwithout

zincborate,exposedinVancouver,BC(sunnylocation)for70months


12/17

12

WaterabsorptionaffectedthedurabilityoftheWPCboards. Duringtestingofthe

flexuralpropertiesofboardsexposedtoexteriorconditionsinVancouverforalmostsix

years,wefoundthattheMOEwasreducedbyabout30%,regardlessofsunorshadow

exposure(figure11). BoardsexposedinshadowseemedtohaveaslightlyhigherMOE

buttherewasonlyarelativelysmallvariationinMOEbetweenthedifferentexposure

conditions. Itmaybepointedoutthatthepresenceofzincborateonceagainaffected

theboardperformance. WhileunexposedreferencesamplescontainingzincborateshowedlowerMOE,alloftheexposedsampleswithzincboratehadstatistically

significanthigherMOEasshownbycapitallettersonthegraph.

Figure11. ChangeinMOEofWPC,withUVstabilizerpackage,andwithandwithout

zincborate,exposedinsunandshadowinVancouver,BCfor70months


13/17

13

Moisturecontentwasalsomeasuredinthetestedsamplesdirectlyafterremovalfrom

thefield. Inallsamplestested,theaverageMCwaswellbelowthedecaypoint. The

averageMCwasinitiallyabout15to20%anddecreasedduring6dayconditioning(75F,

50%RH)onlytoabout10to15%. Moisturecontentinthetestedsamplesisshownin

figure12. Thereferencesamplesused,whichwerestoredforoversixyearsina

shelteredlocationthatwasheatedduringthewinter,hadonlyabout3%MC,evenafter

conditioning.Moisture

was

typically

significantly

higher

near

the

sample

surface,

which

likelyaffectedthetestingresults.

Figure12. MoisturecontentanditsdistributionforsamplesofWPCexposedinVancouver,BCfor70monthsandtestedformechanicalproperties(figure11)


14/17

14

Anotherconsequenceoftheobservedhighmoisturecontent,whichoftenexceeded

25%inmanysamples,seemedtobedecay. Decaywasfoundafterathorough

inspectionofthesamplesafteronly28monthsexposureinHilo. Atthispoint,Iwould

liketorepeatwhatwasmentionedearlier,thatHiloisaveryaggressiveplacewith

respecttodecayandhasaSchefferindexofabout330,versusonlyabout50expected

inVancouver. Aninitialbrieflookatthesampledidnotshowanythingunusual,onlyafurthercarefulinspectionusingamagnifyingglassshowedsomestrangesurface

topographyinthecentre,andsomedarkerthanusualsegmentsofwood. Further

examinationofthesample,withopticalandSEMmicroscopes,revealedwhatseemed

tobeadvanceddecaywithevidenceoffungalmyceliainsomeplaces(figure13). Ithas

tobementionedthatthedecayregionwashardtothetouchandwaslimitedtothe

internalportionofthesample. Theexternalpartoftheboardwasundamaged.

Samplescollectedmorerecently,inHiloandVancouver,arestillunderevaluationinthis

respect.


15/17

15

Figure13. MicroscopicinspectionofinteriorofWPC(sample#8)exposedinHilo,HI

(sunnylocation)

for

28

months

(A)

digital

photomicrography

(B

and

C)

optical

microscopy (DandE)SEMmicroscopy. Pleasenotethedecayedwoodintheboard

centre,andfungimyceliumfillingthecavitywithremainsofthewood. FiguresCandD

showtheidenticalsampleareausingopticalmicroscopyandSEMrespectively.

A

B

C

D

E


16/17

16

Conclusions WoodPlasticComposites(WPC)boardsprogressivelyabsorbasignificant

quantityofwaterduringexteriorexposure. Moisturecontentdistributioninthe

boardcrosssectionshaveacharacteristicUshape,frequentlyexceedingthe

concentrationrequiredfordecayinitiation.

Waterabsorptionisalongprocess,andevenafter6yearsexposureinamoderateclimate,equilibriumhaslikelynotbeenreached

AmajorfactorinwaterabsorptionbyWPCwastheratioofwoodtoplastic

binder;withtheincreaseofwoodcontent,moisturecontentprogressesvery

quickly

AnotherfactorinwaterabsorptionisthematerialcompositionofWPC. Certain

additivesmaysignificantlyincreaseordecreasewaterabsorption(forexample

zincboratedecreasedwaterabsorptioninthetestedformulations)

Climateandsamplelocation(sunorshadow)maynotbeamajorfactorinwater

absorption

Itwas

shown

that

water

absorption

in

exterior

exposure

most

likely

influenced

MOEofselectedsamples(decrease~30%). Itwasobservedthatsome

experimentalsampleswithasignificantamountofwaterabsorptionunderwent

decayinexteriorexposure.

References:

1. Lopez,J,Sain,M.,andP.A.Copper,2004,DurabilityofNaturalFibrePlasticComposites

forOutdoorApplications,ProgressinWoodfibrePlasticCompositesConference

Proceedings,Toronto,Canada,May1011,2004

2. Ibach,R.E.,

and

C.M.

Clemons,

2004,

Field

Evaluations

of

Extruded

Woodfiber

Plastic

Composites,ProgressinWoodfibrePlasticCompositesConferenceProceedings,

Toronto,Canada,May1011,2004

3. Ibach,R.E. ClemonsC.M.,andN.M.Stark,2003,CombinedUVandWaterExposureas

aPreconditioningMethodinLaboratoryFungalDurabilityTesting,7thAnnual

ConferenceonWoodfiberPlasticComposites,Madison(WN)May1920,2003

4. Morris,P.I.andP.A.Cooper,1997,Recycledplastic/woodcompositelumberattacked

byfungi,ForestProductsJournal48(1):8688

5. Klyosov,A.,2004,NaturalandWoodfiberCompositesintheRealWorld,Progressin

WoodfibrePlasticCompositesConferenceProceedings,Toronto,Canada,May1011,

2004

6. Rowell,R.A.,Lange,S.E.,andR.E.Jacobson,2002,EffectsofMoistureonAspen

Fiber/PolypropyleneComposites,ProgressinWoodfibrePlasticComposites

ConferenceProceedings,Toronto,Canada,May2324,2002

7. Verhey,S.A.,andP.E.Laks,2002,StrengthLossFollowingFungalAttackonWood

Fiber/ThermoplasticComposites,ProgressinWoodfibrePlasticCompositesConference

Proceedings,Toronto,Canada,May2324,2002

8. Sigworth,B.,2002,AdditivesforWoodfilledPolyolefinsCouplingAgents,Progressin

WoodfibrePlasticCompositesConferenceProceedings,Toronto,Canada,May2324,

2002


17/17

17

9. Gnatowski,M.andC.Mah,May2003,TestingofWoodPlasticComposites,7th

Annual

ConferenceonWoodfiberPlasticComposites,Madison(WN)May1920,2003

10.Manning,M.,2004,CreatingValueinWPCProductswithAntiMicrobialsandStain

ResistantAdditives,WPCConference,Baltimore,MD,October1112,2004

11.Zabel,R.A.andJ.J.Morrell.1992,WoodMicrobiology,DecayandItsPrevention

12.Morris,P.I.andP.A.Cooper,1997,ObservationsonPlasticLumberasaSubstitutefor

PreservativeTreatedWood,18thAnnualMeetingoftheCWPA,pg117128,Vancouver,

BC,November

34,

1997

13.Manning,M.,2006,FieldTestingofWPCsaCriticalComponentofDurability

Evaluation,ProgressinWoodandBiofibrePlasticCompositesConferenceProceedings,

Toronto,Canada,May12,2006

14.Manning,M.,Ascherl,F.,2007,WoodPlasticCompositeDurabilityandtheCompelling

CaseforFieldTesting9thInternationalConferenceonWood&BiofiberPlastic

Composites,Madison,WN,May2123,2007

15. Gnatowski,M.,2005,WaterAbsorptionbyWoodPlasticCompositesinExterior

Exposure,8thInternationalConferenceonWoodfiberPlasticComposites,Madison,

WI,May2325,2005

16.Gnatowski,M.,2007,RadiationInducedDegradationinWPCintheFieldandin

LaboratoryConditions,9thInternationalConferenceonWood&BiofiberPlastic

Composites,Madison,WN,May2123,200717.Gnatowski,M.,2008,WaterAbsorptionbyWoodPlasticCompositesFieldand

LaboratoryChallenges,10thInternationalConferenceonProgressinBiofibrePlastic

Composites,Toronto,Ontario,May1213,2008

AcknowledgementsTheauthorofthispaperwouldliketothankthefollowingPolymerEngineeringstaffinvolvedinthisresearch: CeciliaStevens,Ph.D.,MathewLeung,B.Sc.,KateMao,B.Eng.,DavidLesewick,ChristineMah,B.Sc.,andBeverlyStart.IwouldalsoliketothankWashingtonStateUniversity(WoodMaterialsandEngineeringLab)fortheirsupport

in

sample

preparation.

wa of compostes

Documents