ACMtokyohotn0744

ORIGINALARTICLEJWoodSci(2006)TheJapanWoodResearchSociety2006DOI10.
1007/s10086-005-0744-5N.
Okuda(*)·K.
Hori·M.
SatoDepartmentofGlobalAgriculturalSciences,GraduateSchoolofAgriculturalandLifeSciences,TheUniversityofTokyo,1-1-1Yayoi,Bunkyo-ku,Tokyo113-8657,JapanTel.
+81-3-5841-7507;Fax+81-3-5841-7507e-mail:aa47125@mail.
ecc.
u-tokyo.
ac.
jpNobuhisaOkuda·KekoHori·MasatoshiSatoChemicalchangesofkenafcorebinderlessboardsduringhotpressing(II):effectsonthebinderlessboardpropertiesReceived:September3,2004/Accepted:July1,2005AbstractToprovidebasicinformationonself-bondinginkenafcorebinderlessboards,aseriesofchemicalanalyseswasconductedonbinderlessboardsandtheirchemicalchangesduringhotpressingwereexaminedinourpreviousstudy.
Inthisstudy,binderlessboardsweremanufacturedunderconditionsthatmayacceleratethesupposedchemi-calchangestoinvestigatetheireffectontheboardproper-ties.
First,toinvestigatetheinuenceofthechemicalbondsformedbycarbonylcompoundsonself-bonding,theinu-enceofaceticacidadditionpriortoboardmanufacturingwasstudiedandtheeffectofmethanolextractives(contain-ingthecarbonylcompounds)wasalsoexamined.
Second,theinuenceofthecondensationreactioninligninwasdiscussedfromtheviewpointofboarddensity.
Last,toexaminetheinuenceofthermalsofteningoflignin,theinuencesoftemperatureconditionandmoisturecontent,aswellasthoseofmicrowavepretreatment,wereinvesti-gated.
Asaresult,theestimatedchemicalchangesweresuggestedtoinuencethebinderlessboardproperties.
KeywordsChemicalchange·Self-bondingmechanism·Binderlessboard·Thermalsofteningoflignin·Condensa-tionreactionIntroductionClaricationoftheself-bondingmechanismisconsideredtobeanimportantissueforthefurtherimprovementofbinderlessboardperformance,andforthepossibilityofexpandingitsapplicationtootherlignocellulosicmaterials.
Sofar,however,littleinformationhasemergedonthedetailoftheself-bondingmechanism.
Toprovidesomebasicinformationforthefurtherdiscussiononself-bonding,inourpreviousstudies,1,2aseriesofchemicalanalyseswasconductedandthechemicalchangesofbinderlessboardsduringthehot-pressingprocesswerediscussed.
Asaresult,severalchemicalchangeswereobserved:decompositionofpartoftheligninandhemicellulose,theprogressofconden-sationreactioninlignin,andtheformationofchemicalbondsbylowmolecularweightcarbonylcompoundsinmethanolextractives.
Thermalsofteningoflignin,althoughitmayinvolveslightchemicalchanges,wasalsoconsideredtobeanimportantfactorfortheexpressionofbinderlessboardperformance.
Inthisstudy,therefore,binderlessboardsweremanufacturedtoinvestigatetheinuencesofthesechemicalchangesonthebinderlessboardproperties.
First,theinuenceofmethanolextractiveswasinve-stigated.
Itwassuggestedthatlowmolecularweightcarbo-nylcompoundsintheextractivesformedchemicalbondsduringhotpressing.
2Thecompoundsmaycontainaromaticcarbonylgroups,however,inthisstudy,aceticacidwasusedasarstapproximationandwasaddedtokenafcorepowdertomanufacturebinderlessboards.
Itwasreportedthatfattyacidswerecontainedinlightpetroleumextractofkenafcoreandaceticacidwasconsideredtobeoneofthecomponentscontainedinthemethanolextractives.
3Then,accordingtothepreviousstudy,1,2itwassuggestedthatthemethanolextractivescontainedcompoundsderivedfromligninaswellasthecarbonylcompoundsofinterest,andhencetheeffectofmethanolextractiveswasinvesti-gatedbyremovingoraddingtheextractivespriortoboardmanufacture.
Next,thecontributionofligninwasstudied.
First,theinuenceofligninwasreviewedbymanufacturingbinderlessboardsfromdeligniedorlignin-addedrawmaterials.
Second,tostudytheinuenceoftheprogressofcondensationreactionsoflignin,binderlessboardswithdifferentboarddensitiesweremanufacturedtochangethedegreeofcontactamongthepowderelements,andtheinuencewasdiscussed.
Last,theinuenceofligninsoften-ingwasexamined.
Althoughitwasconsideredthatligninsofteninginvolvedlittlechemicalchange,itwassuggestedthatthephenomenonplayedanimportantroleinboard2performance,and,thus,itisworthfurtherinvestigation.
Itwasreportedthatthermalsofteningofligninisassociatedwiththetemperatureandthewateruptake.
4Fromtheview-pointofboardmanufacturing,thismeansthatthepressingtemperatureandthemoisturecontent(MC)oftherawmaterialsareconsideredtobethemostimportantcondi-tions.
Wealsoconsideredthatthesofteningcouldbeaccel-eratedbytheapplicationofmicrowavestoheatthematerialinsidetheboardbyactivatingwatermolecules.
Therefore,combinationsofthepressingtemperatureandtheMCoftherawmaterialwerestudied,andtheapplicationofmicro-wavesasapretreatmentwasinvestigated.
MaterialsandmethodsInuenceofmethanolextractivesInuenceofaceticacidadditionKenafcorewascrushedintopowder(averagepowdersize53mmandMC8%–9%)usingaourmill(ModelACM-10,HosokawaMicron,Japan).
Aceticacid,asoneofthetypicalcarbonylcompounds,wasaddedtothekenafcorepowderinconcentrationsof0%,2.
1%,and4.
7%tomanufacturebinderlessboards(A0,A2.
1,andA4.
7,respectively)bythesametechniqueusedinourpreviousstudy.
5Theboardmanufacturingconditionswereasfollows:pressingtem-perature180°C,pressingpressure5.
3MPa,pressingtime10min,boardthickness5mm,boardsize300300mm,andtargetdensity0.
8g/cm3.
Toevaluatethephysicaleffectofaceticacidaddition,modulusofrupture(MOR)anditsretention,modulusofelasticity(MOE),internalbondingstrength(IB)anditsretention,thicknessswelling(TS),andwaterabsorption(WA)weredeterminedaccordingtoJISA5905–1994(berboard).
MORretentionratiowasdeter-minedbymeasuringtheretainedMORaftersoakinginhotwater(70°C)for2h,followedbysoakinginwaterat20°Cwaterfor1h.
IBretentionwasdeterminedaftersoak-inginwaterat20°Cfor24handdryingat20°C.
Aftertestingtheirmechanicalproperties,binderlessboardsweregroundinaWileymill(WT-150,MIKISeisakusho,Japan)topassthrougha1-mmsieve.
ThepowderwassubsequentlyextractedwithmethanolusingaSoxhletextractorformorethan8h.
Fouriertransferinfra-red(FTIR)spectraoftheresultingresidueswererecordedonaJascoFT/IR-615FTIRspectrometerasKBrtablets,andtheeffectofaceticacidadditionwasdiscussedfromachemicalviewpoint.
InuenceofextractivesadditionToinvestigatetheinuenceofmethanolextractives,rst,thekenafcorepowder(estimatedaveragegrainsize0.
6mmandMC8%–9%)wasextractedwithmethanolandtheresultingresiduepowder(freefrommethanolextractives)andmetha-nolextractiveswereobtainedaftermethanolevaporation.
Then,binderlessboardswerepreparedfromthreetypesofrawmaterials:theresiduepowder(freefrommethanolex-tractives)(R-type),theoriginalpowdermixedwithanaddi-tional5%ofmethanolextractivesbasedontheoven-dryweight(E-type),andtheoriginalkenafcorepowder(origi-nal).
Themanufacturingconditionswerethesameasaboveexceptfortheboardsizeof200200mm.
Afterthat,toevaluatethebondingproperties,IBanditsretention(bythesamemethodasabove),TS,andWAweredetermined.
InuenceofligninTostudytheinuenceoflignin,rst,holocelluloseandperiodateligninwerepreparedunderthefollowingprocedures.
HolocellulosepreparationHolocellulosewaspreparedfromkenafcorepowder(esti-matedaveragegrainsize0.
6mmandMC8%–9%)inthesameprocedureusedinourpreviousstudy2exceptforthesamplequantity(around100gpertreatment),thequan-tityofthedilutedaceticacidaqueoussolution(around4l),thequantityofNaClO2(20g),andthetreatmentrepetitiontimes(sixtimes).
Theyieldofholocellulosewas82%.
PeriodateligninpreparationKenafcorepowder(asdescribedabove)(around100gperprocedure)waspretreatedwith1%sodiumhydroxidesolu-tion(NaOH)(3.
5l)for12h,neutralized,andsubsequentlyoxidizedwithsodiumperiodate(NaIO4)(125g)byaddingaceticacid(16.
7ml)for1weekinthedark.
Aftercentrifug-ing,theresultingresiduewasreducedwithsodiumborohy-dride(50g)in0.
1NNaOH(2l)for24h.
Aftercentrifuging,theresultingresiduewashydrolyzedwith0.
5Nhydrochlo-ricacid(HCl)(2l)for24h.
Aftercentrifuging,theresultingresiduewasdeterminedasperiodatelignin(yield22%).
Inthisprocedure,allthetreatmentswereconductedatroomtemperature.
Boardswerepreparedfromthreetypesofrawmaterials:thekenafcorepowdersubstitutedby20%periodateligninbasedonoven-dryweight(L-type),holocellulose(C-type),andtheoriginalkenafcorepowder(original)(kenafcorebinderlessboard).
Theboardmanufacturingconditionswerethesameasthosedescribedintheprevioussection.
Finally,IBanditsretention(bythesamemethodasabove),TS,andWAweredeterminedtoinvestigatetheinuenceofthelignincontent.
Inuenceoflignin:progressofcondensationreactionInuenceofboarddensityToexaminetheeffectoftheprogressofcondensationreac-tioninthechemicalstructureoflignin,theinuenceofthecontactdegreeamongthepowderelementswasstudiedbymanufacturingbinderlessboardsfromkenafcorepowder(averagepowdersize53mmandMC8%–9%)withdifferent3boarddensities:0.
5and1.
0g/cm3(D-0.
5,andD-1.
0,respec-tively).
Theboardmanufacturingconditionswerethesameasthosedescribedintheprevioussection.
Aftertestingthemechanicalproperties,thebinderlessboardswerereducedtopowderinaWileymill,subsequentlyextractedwithmethanol,andthentheresultingresidueswerecharacter-izedbythealkalinenitrobenzeneoxidationprocedure.
2Thesyringyl/vanillyl(S/V)ratiowasusedtodiscussthechangesintheligninstructure.
2Inuenceoflignin:thermalsofteningofligninInuenceofpressingtemperatureandmoisturecontentBinderlessboardswerepreparedfromkenafcorepowder(estimatedaveragegrainsize0.
6mmandinitialMC8%–9%)bychangingthepressingtemperatureandtheMCoftherawmaterial.
ThepressingtemperatureandtheMCinvestigatedinthisstudywereasfollows:MCof0%,5%,10%,and20%forapressingtemperatureof120°C,MCof0%,5%,10%,and15%for150°C,andMCof0%,2%,4%,5.
3%,8%,10%,12%,and14%for180°C.
Theothermanu-facturingconditionswerethesameasthosedescribedintheprevioussectionexceptforthepressingschedule:5MPafor4min,followedby3MPafor3min,and1MPafor3min(three-step-down).
ThetargetMCshigherthantheinitialvaluewereobtainedbysprayingwaterdirectlyandthoselowerthantheinitialvaluewereobtainedbysprayingwaterafterdryingwhichwascarriedoutinavacuumovenat70°Cfor48h.
Thesprayingwasperformedinasealedplasticbagandsufcientlymixedbeforetheboardpreparation.
Afterboardpreparation,MOR,MOE,IB,TS,andWAweredeterminedandtheeffectofthepressingtemperatureandtheMCwasexamined.
InuenceofpreheatingKenafcorechips(averagegrainsize6mmandMC8%)werepreheatedinamicrowaveoven(600W)for0,1,and2min(inaglasscontainersealedbyplasticlm)andimmediatelymadeintobinderlessboardunderthesamemanufacturingconditionsmentionedaboveexceptforthepressingtemperatureof170°C.
Afterthat,themechanicalpropertiesofthesebinderlessboardswereinvestigatedandtheinuenceofpreheatingexamined.
ResultsanddiscussionEffectofmethanolextractivesEffectofaceticacidadditionFigure1showstherelationshipsbetweentheaceticacidadditionratioandthemechanicalpropertiesofthebinderlessboardsprepared.
MORanditsretentionratio,IBanditsretentionratio,TSandWA,andMOEimprovedwithincreasingaceticacidadditionratio,indicatingthatthebondingpropertywasimprovedbytheaceticacidaddition.
Figure2showstheFTIRspectraofthemethanol-extractedresiduesofbinderlessboardsmanufacturedwiththeaddi-tionofaceticacid(A0,A2.
1,andA4.
7).
Thepeakaround1500cm-1(derivedfromaromaticmoieties)wasusedasaconstantstandardtoderiverelativeabsorbanceofallthesamplesatdifferentwavenumbers.
2AccordingtoFig.
2,thepeakintensityat1635cm-1(derivedfromC=Ostretchingvibrationofaromaticcarbonylcompounds),consideredtobeassociatedwithself-bonding,1,2increasedwithincreasingaceticacidadditionratio.
ItseemsreasonabletosupposeFig.
1a–d.
Modulusofrupture(MOR)indryandwetconditionsandtheretention(a),internalbondstrength(IB)anditsretentionafteragingtreatment(b),thicknessswelling(TS)andwaterabsorption(WA)(c),andmodulusofelasticity(MOE)(d)ofbinderlessboardspre-paredatdifferentaceticacidadditionratios4thattheaceticacid,asalowmolecularweightcarbonylcompound,experiencedchemicalchangeduringthehot-pressingprocessandremainedintheresiduesafterthemethanolextraction,indicatingthecontributiontoself-bonding.
Itwassuggestedthataceticacidadditioncouldbeaneffectivewaytoacceleratethechemicalchangesandtoimprovetheboardproperties.
EffectofextractivesadditionFigure3showstheIBanditsretention,andtheTSandWAfordifferentboardtypes.
AsshowninFig.
3a,theIBvalueswerealmostidenticaltoeachother,whereastheeffective-nessofthemethanolextractiveswasrecognizedintheIBaftertheagingtreatmentandtheIBretentionratio.
TheIBretentionafteragingtreatmentincreasedwiththemethanolextractiveaddition(E-type)anddecreasedwiththere-movalofmethanolextractives(R-type)whencomparedwiththeoriginalstate(original).
ThesametendencywasobservedforTSandWA(Fig.
3b),suggestingthatthemethanolextractivescontainingthecompoundsderivedfromligninandthecarbonylcompounds2contributedtosomeimprovementofthebondingproperties.
EffectofligninFigure4showstheIBanditsretention,andTSandWAfordifferentbinderlessboardtypes.
AsshowninFig.
4,themechanicalpropertiesoftheL-typeboardweresuperiortothoseoftheoriginal,whereasthepropertiesoftheoriginalwerefoundtobedecreasedtosomeextentbyligninre-moval(C-type).
Thisclearlyshowedthatligninplayedanimportantroleintheself-bondingmechanismandcontrib-utedtoimprovementsintheboardproperties,ashasbeensuggested.
6,7Fromourpreviousstudy,2theeffectsofligninwereconsideredtooccurthroughcondensationreactionsandthemalsoftening,andthesearediscussedbelow.
Effectoflignin:progressofcondensationreactionEffectofboarddensityTable1showstheinuenceoftheboarddensityontheprogressofcondensationreactioninthechemicalstructureoflignin.
Ifwecomparetheinitialstate(kenafcorepowder)andD-0.
5orD-1.
0,theS/Vvalueincreasedandtheyieldofthebenzaldehydederivativesdecreasedbythehot-pressingprocess,indicatingthatthecondensationreactionpro-gressedaffectedthechemicalstructureofligninduringboardmanufacturing.
Itisnotappropriatetoconcludethatthereactioncontributedtoself-bondingbecauseofthepossibilityofthereactionbeingprogressedonlywithineachparticle.
InTable1,smalldifferencesareobservedbetweenD-0.
5andD-1.
0:theD-0.
5sampleshowedalowerS/VratioandahigheryieldthantheD-1.
0sample.
Thismeansthattheextentoftheprogressofthecondensationreactionwasdependentontheboarddensity,or,inotherwords,thedegreeofthecontactamongpowderelements.
Itwassuggestedthatthecondensationreactionoccurrednotonlyinsidetheparticles,butalsobetweenparticlestosomeextent.
Thus,theprogressofthecondensationreactioninligninmightplayanimportantroleintheself-bondingmechanismandtheaccelerationofthereactionisthoughttobeeffectivefortheimprovementoftheboardproperties.
Fig.
2.
Fouriertransforminfraredspectraoftheextractedresiduesofbinderlessboardsmanufacturedwiththeadditionofdifferentamountsofaceticacid.
A0,0%aceticacid;A2.
1,2.
1%aceticacid;A4.
7,4.
7%aceticacidFig.
3a,b.
IBanditsretentionaftertheagingtreatment(a)andTSandWA(b)ofthreetypesofbinderlessboards.
R-type,theresiduepowder(freefrommethanolextrac-tives);E-type,theoriginalpowdermixedwith5%additionalmethanolextractivesbasedontheoven-dryweight;original,theoriginalkenafcorepowder5Effectoflignin:thermalsofteningofligninEffectofpressingtemperatureandmoisturecontentFigure5showsthemechanicalpropertiesofbinderlessboardsatdifferentMCandpressingtemperature.
Thebinderlessboardsmanufacturedat120°CattheMCof20%weredelaminatedbecauseofsteamexplosion,whichlow-eredtheMORandMOE(Fig.
5a,b).
Forthe120°Cand150°Csamples,MOR,MOE,andIBincreasedandTSandWAdecreasedwithincreasingMC.
However,inthe180°Csamples,noobvioustendencieswereobservedinMOR,MOE,andIB,whichsuggeststhethermalsofteningoflignin.
4Goring4examinedthesofteningtemperatureoftentypesofligninandreportedthatitwasdependentonthekindsoftheligninandwassubstantiallydecreasedbywateruptake.
Accordingtotheresults,4eventhoughkenafligninwasnotinvestigated,itcanbepresumedthatsoften-ingofthekenafcorelignindidnotoccuratthepressingtemperaturesof120°or150°CwithMCof0%,whichmadeTable1.
TheeffectoftargetboarddensityontheprogressofthecondensationreactioninligninSamplesTargetboarddensity(g/cm3)Lignincontenta(wt%)S/VratioYieldb(%)Kenafcorepowder–25.
31.
641.
7±0.
2D-0.
50.
525.
41.
740.
9±0.
1D-1.
0125.
71.
838.
9±0.
1S/Vratio,syringyl/vanillylratioaThesumofKlasonligninandacid-solubleligninbYieldoftotalbenzaldehydederivativesbasedonlignincontent,determinedastheaveragevalueofthreesamplesFig.
4a,b.
IBanditsretentionaftertheagingtreatment(a),andTSandWA(b)ofthreetypesofbinderlessboards.
L-type,thekenafcorepowdersubstitutedwith20%periodateligninbasedonoven-dryweight;C-type,holocellulose(includingasmallamountoflignin);original,theoriginalkenafcorepowderFig.
5a–e.
MOR(a),MOE(b),IB(c),TS(d),andWA(e)ofthebinderlessboardsmanufacturedunderdifferentcombinationsofmoisturecontentandpressingtemperatureconditions6Fig.
6a–c.
MORandMOE(a),IB(b),andTSandWA(c)ofthebinderlessboardsmanufacturedbychangingthemicrowavepreheatingtimetheIBvaluesalmostzero(Fig.
5c).
TheincreaseinMCenhancedligninsoftening,andinconnectionwiththis,themechanicalpropertiesshowedMCdependence(Fig.
5a–e).
Onthecontrary,forthe180°Csamples,itcouldbeinter-pretedthattheligninsoftenedat180°CeventhoughtheMCwas0%,and,therefore,themechanicalpropertieswereindependentoftheMCcondition.
Consequently,itisrea-sonabletosupposethatligninsofteningplaysanimportantroleintheexpressionofbinderlessboardproperties.
EffectofpreheatingFigure6showstheeffectsofpreheatingontheMOR,MOE,IB,TS,andWAvalues.
TheIBvalues(Fig.
6b)andtheotherboardproperties(Fig.
6a,c)improvedslightlywithapreheatingtimeof1min,whereasnoobviouseffectwasobservedwithapreheatingtimeof2min.
Theresultscouldbeexplainedonthebasisofligninsoftening.
Theimprove-mentswithapreheatingtimeof1minwereduetothemicrowavepreheating,whichcouldacceleratethethermalsofteningoflignin.
Ontheotherhand,withapreheatingtimeof2min,theeffectofthepreheatingtreatmentmaypartlybecompensatedbytheinuenceofexcessivedryingduringpreheating,consideringthefactthatpartoftherawmaterialwasburnedblackandtheresultingMCwaslessthan3%after2minofpreheating.
Thisinterpretationisconsistentwiththeideathatthethermalsofteningofligninistosomeextentresponsibleforboardperformance.
Here,weshouldpointoutthatthemicrowavepreheatingcouldbeusedasasimplepretreatmenttoimprovebinderlessboardproperties,althoughfurtherstudyofthepreheatingcondi-tionswouldberequired.
ConclusionsBinderlessboardsweremanufacturedunderconditionsthatmayacceleratethechangesobservedinourpreviousstudy2andtheinuenceofthechemicalchangesonthebinderlessboardpropertieswasinvestigated.
Theexperimentalresultsaresummarizedasfollows:1.
Theadditionofaceticacidwasconsideredtoacceleratethechemicalchangesduringhotpressingandwasfoundtobeeffectivefortheimprovementofboardproperties.
Itwasconsideredthattheaceticacidformednewchemi-calstructuresduringhotpressingandbecameinsolubletomethanol,althoughfurtherstudiesarerequiredfortheclaricationofthedetailedmechanism.
Thiseffectofthecarbonylcompoundswasfoundtobeinuencedbytheamountofmethanolextractives.
2.
Theprogressofthecondensationreactioninligninwasfoundtobedependentonthedegreeofthecontactamongparticlesaffectedbytheboarddensity,indicatingthatthereactionpartlyoccurredamongpowders.
Theaccelerationofthereactionwassupposedtobeeffectivefortheimprovementoftheboardproperties.
3.
Thethermalsofteningoflignin,althoughitmayinvolvelittlechemicalchange,wasindicatedbythefactthatthemechanicalpropertiesofbinderlessboardswereassoci-atedwiththetemperatureconditionandthemoisturecontent.
AcknowledgmentsTheauthorsexpresstheirdeepgratitudetoAraco,Corp.
,Japan,forsupportinsupplyingkenafmaterials.
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