View
219
Download
0
Category
Preview:
Citation preview
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.
8/2/2019 Wa of Compostes
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
8/2/2019 Wa of Compostes
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
8/2/2019 Wa of Compostes
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)
8/2/2019 Wa of Compostes
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
8/2/2019 Wa of Compostes
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
8/2/2019 Wa of Compostes
7/17
7
Surprisingly,therewererelativelysmalldifferencesinmoisturecontentforsamples
exposedinsunandshadow. Samplesexposedinsunseemedtohaveevenhigher
moisturecontent,particularlyinthecaseofcompositeswithlargerwoodcontentas
showninfigure6.
Figure6. MoisturecontentanditsdistributioninsamplesofWPCexposedinVancouver,BC
insunandshadowfor70months
8/2/2019 Wa of Compostes
8/17
8
Evenmoresurprising,wastheperformanceofoursampleswhenexposureina
moderatevs.tropicalclimatewascompared. SamplesexposedinVancouvershowed
significantlyhighermoisturecontentthanthoseexposedinhotandverywetHilo,
Hawaii(figure7). Thismaybeexplainedbythehigherevaporationrateintropical
Hawaii,andthefactthatsampleswerealwaysharvestedinVancouverattheendofthe
wetterwintermonthsofFebruaryorMarch.
Figure7. MoisturecontentanditsdistributioninsamplesofWPCexposedfor46months
inVancouver,BCand51monthsinHilo,HI
8/2/2019 Wa of Compostes
9/17
9
Woodspeciesalsoseemstoinfluencethewaterabsorptionofcomposites. Samples
containingthesamequantityoftheoakflouralwaysshowedlowerwaterabsorptionin
comparisontopinewhenexposedatthesamelocation(figure8).
Figure8. MoisturecontentanditsdistributioninsamplesofWPCmadefromdifferentwoodspecies(pine
oroak)exposedinVancouver,BC(sunnylocation)for70months
8/2/2019 Wa of Compostes
10/17
10
AdditivesusedinourUVstabilizerpackageseemedtoincreasethewaterabsorptionof
thesamplesascanbeseeninfigure9. Thisappliedtobothformulationswithlowerand
higherwoodcontent. Alargeincreaseinmoisturecontentoccurredallacrossthe
boards,includingtheareaneartheboardsurfaceandalsointhecentre.
Figure9. MoisturecontentanditsdistributioninsamplesofWPC,withandwithoutUV
stabilizerpackage,exposedinVancouver,BC(sunnylocation)for70months
8/2/2019 Wa of Compostes
11/17
11
Interestingandunexpectedwastheadditionofzincborate,knowntobeeffectiveasa
fungicide. Thepresenceofzincborateseemedtodecreasewaterabsorptionin
practicallyallofthetestedformulationsasshowninfigure10.
Figure10. MoisturecontentanditsdistributioninsamplesofWPC,withandwithout
zincborate,exposedinVancouver,BC(sunnylocation)for70months
8/2/2019 Wa of Compostes
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
8/2/2019 Wa of Compostes
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)
8/2/2019 Wa of Compostes
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.
8/2/2019 Wa of Compostes
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
8/2/2019 Wa of Compostes
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
8/2/2019 Wa of Compostes
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.
Recommended