The chemical modification of lignins with succinic anhydride in aqueous systems

www.elsevier.nl/locate/polydegstab

Thechemicalmodi®cationofligninswithsuccinicanhydridein

aqueoussystems

B.Xiaoa,X.F.Suna,RunCangSunb,*

aCentreforStrawUtilization,TheNorth-WesternUniversityofAgriculturalandForestSciencesandTechnology,Yangling,People'sRepublicofChinabSchoolofPulpandPaperSciencesandEnvironmentalEngineering,SouthChinaUniversityofTechnology,Guangzhou,People'sRepublicofChina

Received22June2000;accepted14July2000

Abstract

Chemicalmodi®cationofoilpalmtrunk®brelignin,poplarlignin,maizestemlignin,andbarley,wheat,andryestrawligninswasachievedbyesteri®cationwithsuccinicanhydrideinaqueoussolutions.FT±IRspectroscopyclearlyrevealedthedi󰂀erencesinthestructureofthemodi®edligninsasaresultofthissuccinoylation.Thesechangeswerere¯ectedbythermalanalysessuchasthermogravimetricanalysisanddi󰂀erentialscanningcalorimetry.Allthesuccinylatedligninpreparationsshowedanincreasedthermalstabilitycomparedtothecorrespondingunmodi®edlignins.#2001ElsevierScienceLtd.Allrightsreserved.

Keywords:Lignin;Succinoylation;Aqueoussolution;Thermalstability

1.Introduction

Ligninaccountsfor24±33%ofthedrymatterofsoftwoodand16±24%ofthehardwoodusuallyusedinpulpingprocesses[1].Agriculturalresiduessuchasstrawcontain12±18%lignin[2].Duringthechemicalpulping(inthekraft,soda,organosolvorsulphitepro-cesses),thepartiallydegradedligninisdissolvedintheblackliquor,fromwhichitcanbelaterisolatedbyvar-iousmethods.Currentannualglobalproductionoflig-nininpulpmillsamountstosome50milliontons,whichapproachesthewholeproductionoftheplasticsindus-try.Theseligninsinprinciplecanbemodi®edeitherbyfurtherdegradationandpolymerizationoftheresultingoligomers,orbydirectchemicalmodi®cationofthepolymerfragmentsusingsmallmoleculesorgraftontootherlargemolecules[1].

Alargenumberofmodi®cationreactionsforligninshasbeenreportedintheliterature[3±7].Somespecialreactionsonthermalstabilityandadhesiveproperties,engineeringplastics,sulphurlignin,andelectricallyconductinglignins,havebeenstudiedintensivelyinthelasttwodecades[1,8,9].Oneveryattractivefeatureofligninsistheirchemicalcomposition,withalarge

*Correspondingauthoratpresentaddress:TheBioCompositesCentre,UniversityofWales,BangorLL572UW,UK.Tel.:+44-01248-370588;fax:+44-01248-370594.

E-mailaddress:bcs00a@bangor.ac.uk(R.Sun).amountofrelativelyeasilyaccessiblehydroxylunitsthatcanbeusedfortheattachmentofavarietyoffunctionalgroups[10].Itisevidentthatmostofthereactionstypicalforsubstitutedphenolsandphenolicethersarepossibleinthecaseoflignins.Ofalltheche-micalmodi®cationreactions,oneofthekeypropertiesrequiredformaterialsintendedtobeusedinhygienicproductsistheirabilitytoabsorbliquids.Carboxylicacidsareknowntoimprovewaterabsorptioninthematerials[11±13].

Modifyingwoodwithcyclicanhydridessuchassuc-cinic,maleic,orphthalicanhydridehasbeenpointedasawayofintroducingnewmaterialproperties[10,14],inwhichthecarboxylategroups(ÀCOOÀ)canlinkligninorwood®brethroughesteri®cationorhydrogenbond-ing.Furthermore,themodi®cationresultsinapendantcarboxylicmoietyattachedtotheligninorwoodviaacovalentesterbond,providingasiteuponwhichfurtherreactivechemistryispossible.Forexample,reactionofwoodwithsuccinic,maleic,orphthalicanhydride,andsubsequentreactionwithavarietyofepoxidehasbeenreportedinanextensiveseriesofpublications[7,15±17].Thisisaninterestingwaytoachieveligninorwoodthermoplasticizationbyesteri®cationwithdicarboxylicanhydridesfollowedbyoligoesteri®cationwithepoxides.Particularly,maleicanhydrideisana,b-unsaturatedcarbonylcompound,containingonecarbon±carbondoublebond(C󰂈C).Thisconjugatedstructuregreatlyincreasesthegraftreactivityofthecarbon±carbondouble

0141-3910/01/$-seefrontmatter#2001ElsevierScienceLtd.Allrightsreserved.PII:S0141-3910(00)00133-6

224B.Xiaoetal./PolymerDegradationandStability71(2001)223±231

bondthroughonresultingthethefaceincrosslinkingconjugateheterocyclicadditionringwiththepolymermatrixorstrongunderadhesionaradicalattheinitiator,inter-GlasserAs[8,18].

pioneersontionalitytheligninandco-workersinthechemicalmodi®cationofligninderivatives[3,19,20]withuniformmadeanterminalinitialstudyfunc-ofandsuperiorhandlingcharacteristicsintionsolubility.Hydroxyalkylationandcarboxymethyla-termsfunctionalityoflignin[21].thathaveenhancesbeenshownsurfacetoactiveproducehydrophilicpulpAlkenylsivelyandtheseaspaperpapersuccinicanhydridesarewellcharacteristicsknowninthesizingindustry,agentswhere[3].theyHowever,areusedexten-modifyingreagentssuchassuccinicanhydride(SA)theasuseligninofforewithofinterestagentstohasstudynotbeenreported.Itwasthere-modi®edtheaimofexploitingthethee󰂀ectreactiveofsuchchemistrymodi®cationsites.alkalineThisligninspaperforuseinnovellignin-basedcompo-ofSAperoxideligninfromdealsoilwithpalmthechemicaltrunk®bremodi®cationofwheatdrideandligninsryefrompoplartree,maizestems,andalkalinebarley,conditionsinaqueousstrawssystems.byreactionThein¯uencewithofsuccinicanhy-stabilityoftheonmodi®edthechemicallignincompositionaredescribed.andthereactionthermal2.Experimental2.1.Materials

ResearchOilpalmlengthInstitutetrunk®breofMalaysia.waskindlyItwassuppliedcut2±3byForestcminstems,priorweretheobtainedbarley,topulping.wheatfromtheandOtherexperimentalryematerialsstraws,includingmaizefarmandorpoplarforesttreeofeststrawsSciencesNorth-WesternandTechnologyUniversity(Yangling,ofAgriculturalPRandFor-pieces.weredriedinsunlightandthencutChina).intosmallTheoutercutThematerialsandTheinnerharvestedwerebarksgroundandpoplartreewaspeeledo󰂀thetothenpasschippeda0.8anddried.Thelatinggroundsampleswerefurtherdriedmminansizeair-circu-screen.drideovenat60󰀎Cfor16hbeforeuse.Succinicanhy-(Xian)wasandpurchaseditwasusedfromasreceived.

SigmaChemicalCompany2.2.degradedPulpinglignin

ofoilpalmtrunk®breandisolationofthewithTheoilpalmtrunk®brewaspulpedina8ldigesterchargeacookingthewascomposedliquorofto®breratioof12:1.Chemicalcookingrawwasmaterial.180󰀎C.TheAtthemaximum18%KOHbeginningtemperatureand0.1%AQofthepulping,intheofthe

timewastotemperatureof180󰀎Cwas1h.ThecookingtimefromThe2h.

procedureforwasHCl.®rsttheacidi®edblackliquorisolationtopHwasasfollows:ofthethedegradedblackliquorligninweretureseparatedThenthe6.0bydropwiseadditionof6Mbypolysaccharideprecipitationofdegradationtheneutralizedproductsethanol,with3volofethanol.Afterevaporationofmix-pHtheligninswereobtainedbyreprecipitationtheatsolution.2.0adjustedby6MHClfromthesupernatantwithat5󰀎CanuntilaqueousTheisolatedligninswerepuri®edbywashinganalysissolution(Fig.of1).pH2.0,air-dried,andkept2.3.Alkalineperoxidetreatments

wheat,Thedriedtoluene±ethanolbarley,powderandsampleofpoplar,maizestem,andrespectively.(2:1,ryev/v)strawswas®rstextractedwith1.8%atHThedewaxedinsampleaSoxhletwasextractorthensoakedfor8inh,a2Otion,45󰀎Cfor2solution15hatpHwith11.6,a1:18sampletoliquorratioandhemicellulosesthenthe®ltrateconcentrated.wasneutralizedrespectively.Thealkalinewith6MHClAftertopH®ltra-6.0mentionedandligninwereisolatedperoxideasthemethodsoluble®breabovefortheisolationofoilpalmtrunkprocedureligninisfromillustratedtheblackinFig.liquor.2.Thedetailedisolation2.4.Succinoylationoflignin

systemsTheschemeinportsa150ismlillustratedforsuccinoylationinFig.3.Theofreactionligninwasinaqueouscarriedmowell,capableglassofandpHofmeteraccommodatingreactorequippedwithvariousaccessprobe.Inaadditionfunnel,ther-withlignin60mlsampledistilledwaswater.chargedTheintotypicalreaction,3.0gsolutionthepHreactorwastogetheradjusted

Fig.palm1.trunkScheme®breforpulping.

isolationofligninsfromtheblackliquorofoilB.Xiaoetal./PolymerDegradationandStability71(2001)223±231225

to8.5±9.0with2%NaOHsolution.Thecorrespondingamountofsuccinicanhydride(seeTable2)wasaddedslowlytothereactionmixtureoveraperiodof0.5h,withstirringthereactionmixtureat28󰀎C.Theresultingreactionmixturewascontinuedstirringforatotalperiodof1,2,4,and12hat28󰀎C,respectively.BothpHandtemperatureweremonitoredduringthereaction.pHwasstabilizedatbetween8.5and9.0bytheinter-mittentadditionof2%NaOHsolutiontothereactionmixture.Thetemperatureofthereactionwaskeptat28Æ0.5󰀎C.TheproductwasseparatedbyprecipitationofthereactionmixtureatpH3.0adjustedby6MHCl.Thesolidwasallowedtosettlefor1dayandthesuper-natantwasdiscarded.Theproductwaswashedthree

timeswithacidi®edwater(pH2±3)bydecantationandthenfreeze-dried.2.5.Characterization

Thequalitativeidenti®cationandquantitativedeter-minationofphenolicacidsandaldehydesreleasedfromthealkalinenitrobenzeneoxidation(170󰀎C,3h)oftheisolatedparentligninswerecarriedoutonaHichromH5ODSHPLCcolumnofdimensions250Â4.6mm(purchasedfromPhenomenexCo.,Beijing).Thesepara-tionwasobtainedusingalineargradientoftwosolventsystems:solventA(water±methanol±aceticacid,84:15:1)andsolventB(methanol±water±aceticacid,90:9:1).Alineargradientwasrunover30minfrom0%to40%Bata¯owrateof1ml/min.Thecompoundsweredetectedat280nmbycomputercomparisonoftheretentiontimesandpeakareaswiththeauthenticphenolics.

ThechemicalstructureoftheligninderivativeswasevaluatedbyFouriertransforminfrared(FT±IR).ANicolet510spectrophotometerwasusedtorecordFT±IRspectrausingaKBrdisccontaining1%®nelygroundsamples.Thermalstabilityofligninderivativeswasperformedusingthermogravimetricanalysis(TGA)anddi󰂀erentialscanningcalorimetry(DSC)onasimultaneousthermalanalyser(NETZSCHSTA-409).Thesampleweighedbetween10and15mg.Thescanswererunfromroomtemperatureto600󰀎Catarateof10󰀎C/minunderadryairatmosphere.3.Resultsanddiscussion

3.1.Phenoliccompositionoftheparentlignins

Fig.2.Schemeforextractionandisolationofligninfrompoplartreeandvariousstraws.

Alkalinenitrobenzeneoxidationisoneofthemostfrequentlyusedmethodsforanalysingligninsbychemical

Table1

Theyield(%ligninsample,w/w)ofphenolicacidsandaldehydesfromalkalinenitrobenzeneoxidationofparentligninsamples

Unmodi®edligninsample

Phenolicacidandaldehydep-Hydroxybenzoicacidp-HydroxybenzaldehydeVanillicacidSyringicacidVanillin

SyringaldehydeAcetovanilloneAcetosyringonep-CoumaricacidFerulicacidTotal

abOilpalmtrunk®bre0.180.430.322.623.3114.86NaN0.15Tb21.87

Poplartree0.560.420.782.4512.2416.800.680.690.390.1135.12

Barleystraw0.961.760.803.8214.2111.320.680.250.801.4836.08

Maizestems0.785.020.492.457.209.810.280.121.261.1228.53

Wheatstraw0.461.420.822.1611.859.870.500.161.021.1228.56

Ryestraw0.691.871.063.6216.0512.081.060.621.581.90.59

N=notdetectable.T=trace.

226

Table2

Theyieldaofsuccinylatedlignins

B.Xiaoetal./PolymerDegradationandStability71(2001)223±231

Succinoylationcondition

LigninsampleOilpalmtrunkOilpalmtrunkOilpalmtrunkOilpalmtrunkOilpalmtrunkOilpalmtrunkOilpalmtrunkOilpalmtrunkBarleystrawPoplartreeMaizestemWheatstrawRyestraw

abcSuccinylatedlignin

Time(h)11111241211111

pHc8.5±9.08.5±9.08.5±9.08.5±9.08.5±9.08.5±9.08.5±9.08.5±9.08.5±9.08.5±9.08.5±9.08.5±9.08.5±9.0

PreparationNo.123456710111213

Yield(%)94.798.5108.6106.3105.6106.3108.1102.4.292.598.096.6.9

Molarratiob0.5/11/11.5/12/13/11/11/11/11/11/11/11/11/1

Temperature28󰀎C28󰀎C28󰀎C28󰀎C28󰀎C28󰀎C28󰀎C28󰀎C28󰀎C28󰀎C28󰀎C28󰀎C28󰀎C

Basedonthepercentageofdriedparentlignin(w/w).

Molarratiorepresentsthemolofsuccinicanhydride/molofC9unitoflignin.pHrepresentsthereactionpH.

degradationmethods,whichresultsintheformationofwell-de®nedlowmolecularweightproducts.Thequantitiesandrelativedistributionofsuchdegradationproductscanbeusedtoderiveinformationaboutcom-positionoftheoriginallignin.Inthiscase,thethreeconstitutivemonomericligninunitsp-hydroxyphenyl,guaiacyl,andsyringylareoxidizedintocorrespondingp-hydroxybenzaldehyde,vanillin,andsyringaldehyde[22].Toverifythestructuralfeaturesoftheligninpoly-mers,nitrobenzeneoxidationofthesixparentligninpreparationswasperformedat170󰀎Cfor3h,andthephenoliccompositionineachpreparationisgiveninTable1.Obviously,thepredominantoxidationproductoftheoilpalmtrunk®breligninwassyringaldehyde,whichaccountedfor68%ofthetotalnitrobenzeneoxi-dationproducts.Vanillinwasmajorbutlessabundantdegradationproduct.Thisrevealedthattheparentlig-ninpreparationfromoilpalmtrunk®brecontainedalargeamountofthenoncondensedsyringylunits.Ontheotherhand,bothvanillinandsyringaldehydeweredominatedintheoxidationproductsofparentligninpreparationsofpoplartree,barley,wheat,andryestraws,andmaizestems,indicatingthatnoncondensedguaiacylandsyringylunitswerethemajorligninunitsinthese®veparentligninfractions.Anoticeablehigheramountofsyringaldehydethanvanillinwasidenti®edintheoxidationproductsofpoplarandmaizestemparentligninfractions,whereasareversetrendwasfoundintheoxidationcompoundsoftheparentligninprepara-tionsfrombarley,wheat,andryestraws.Thepresenceofasmallamountofp-hydroxybenzaldehydeintheoxidationproductsofallthesixparentligninprepara-tionswasgenerallyconsideredtobeindicativeofp-hydroxyphenylunitswithinthelignins.Thehigheryieldsfromnitrobenzeneoxidationofpoplar,barley

straw,andryestrawligninsimpliedalesscondensedligninstructureinthesethreeparentligninpreparationsthantheparentligninfractionsobtainedfromoilpalmtrunk®bre,maizestems,andwheatstraw.3.2.Modi®cationandyield

Asmentionedabove,ligninsareproducedbypoly-merizationofthreemonomericprecursors,p-hydro-xycinnamylalcohol,coniferylalcohol,andsinapylalcohol.ThesecompoundsallhaveaterminalaliphatichydroxylgroupatC-gonthesidechaininadditiontoaphenolichydroxylgroupatC-4ofthearomaticring[23].Milledwoodligninshavetotalaliphatichydroxylcontentsofmorethan1.1mol/C9unitincontrasttoatotalphenolichydroxylcontentof0.28mol/C9unit[24].Furthermore,thetotalphenolichydroxylcontentofligninsinwoodisusuallylessthan0.1mol/C9unit[25].Thereasonforanaliphatichydroxylcontentofmorethan1mol/C9unitissigni®cantnotonlyinthechemicalstructureandbiogenesisoflignins,butalsowithregardtochangesoccurringinthestructureofligninsfromacid-catalyzedhydrolysisorhomolyticcleavageofa-aryletherbondsduringtheisolationprocess.Ontheotherhand,thetechnicallignins,whichdonothaveaC9unitstructure,containlessthan1molofaliphatichydroxylgroupinaC9unit,butslightlyhighphenolichydroxylcontent.Forexample,pinekraftlignincon-tain0.77molaliphatichydroxylgroupand0.58phe-nolichydroxylgroupinaC9unit[24].Inthisstudy,modi®cationofligninswithsuccinicanhydrideunderalkalineconditionallowsforthepossibilityoftheadductbeingbondedtobothaliphaticandphenolichydroxylgroupsviaamonoesterlink(allowingapen-dantcarboxylicgroup)orviaadiesterlinkages.Inthe

B.Xiaoetal./PolymerDegradationandStability71(2001)223±231227

latterwithincasechemicalthedridemodi®cationlignin.adiesterBasedlinkagemayleadtocrosslinkingofonScotsanpineextensivewithsuccinicstudyonanhy-thereportedoroctenylreactionthatthesuccinicdiesteranhydride,contentHillandMallon[7]observedtemperatureof100󰀎C.isnegligibleSimilarresultbelowwastheanhydrideforathebroadabsorptionpeakat28the󰀎centredCmodi®cationinbycarbonylatthisofligninwithsuccinic1737study.bondscmÀThis1wascon®rmedbyin,whichestersresultedat1740cmfromÀ1andstronglycarboxylicacidsat1712cmÀ1.Thelatterwastraofmodi®edoverlappedligninswiththepreviousone,inthespec-eousThe9.0).solutionchemicalundermodi®cation(Fig.4).

wascarriedoutinaaqu-reactionInthiscase,ligninweakwasalkalinesu󰂁cientlyconditionsoluble(pHfor8.5±thedecantingisolationtoproceedprocedureinhomogeneousincludedacidi®cation,phase.However,thesettling,drying.succinylatedSigni®cantor®ltration,losseswashing,wereoftenandrecoveryencounteredbyfreezemediumsinceligninsmallwasamountsprecipitatedofesteri®edfromtheligninreactionwhenwith

Fig.3.Schemeforsuccinoylationoflignin.

Fig.a)and4.succinylatedFT±IRspectraligninofunmodi®edpreparationoil4(spectrumpalmtrunkb).

lignin(spectrumlowsolution.molecularweightToeliminateweightwerepossiblestilllossessolubleofintheacidi®edperformedsuccinylatedreactioninwhichthelignin,ligninscontrolwereexposedexperimentslowmoleculartowereanhydride.conditions,beThedeterminedalthoughweightinthethesamelossesabsenceofsuccinicdegreehigh,centrationof8.6±9.8%succinoylationofthewerefoundtoofinitialparentlignins.Theparentgain,occurringandligninofligninsasfunctionofcon-thetypesuccinicanhydride,reactiontime,andresultswasaredeterminedgiveninTablebymeasuring2.ThereactionweightanhydridethatcarboxyliconeattachedisbetweenschematicallyligninshowncomponentsinFig.and3,assumingsucciniccinylatedacid.AssuccinicshownanhydrideinTable2,willtheresultyieldinoneto(mol108.6%lignininpreparationoccurredfrom3.94.7%inpreparationofsuc-1fromofsuccinicanhydride/molAnincreaseofCofmolarratio9unitinlignin)from0.5/1ment94.7%toto1.5/1108.6%.resultedTheinanincreaseintheyieldconcentrationofsuccinoylationabilitywasprobablybyincreasingreasonforthisenhance-duetosuccinicanhydrideofesterifyingtheofligninsuccinicmoleculesanhydridemoleculestheingreatertheproximityavail-impliedabilitythatagent.thereactionsInotherathigherwords,concentrationthisobservationofthenitymolarofofthesuccinicanhydridemainlymoleculesreliedoninthetheavail-vici-parationratiohydroxylfromgroups.1.5/1inHowever,preparationfurther3toincrease3/1inin108.6%morecesseslossesto5105.6%.ledtoaslightdecreaseintheyieldfrompre-oftheproductsThiswouldduringbeduethetoisolationtheslightlypro-increasein(preparationinpreparationtheyieldsamplesofsuccinoylation4and5.Furthermore,anprolongingdirectthe2)reactionandduration108.1%from(preparationbetween98.5%1h7)bydi󰂀usionconsequenceofthefavourablee󰂀ecttoof4timehwasonaesterifyingandadsorptionofthereactantsbetweenthetheagentsandtheligninmolecules.Incontrast,ofsumed102.8%.reactionThisfor12lowerhinpreparationyield8gaveonlytheyieldpolymericdueoligophenolicsligninstotheofsuccinoylationwaspre-intodegradationofsomeamountsofnoylation®breslightligninforsuccinylated12duringh.Inthemuchcomparisonprolonginglowermolecularperiodofweightsucci-withmolarwithratiooilofpalm1/1fortrunk1observedlowerconditioninyieldstheligninofsuccinoylationpreparations(92.5±98.0%)h,underthewerestems,duewheatgivenstraw,fromandbarleyryestraw.straw,Thispolartree,maizesamexylicto®veknownalkalinegroupstherelativelyinperoxideoilpalmhigheramountsofphenolicwasprobablyhydro-solubletrunk®breligninsligninsincethanitinisotherwellmildthatasolutionof1.8%H2O2atpH11.6isani®cantlydelignifyinga󰂀ecttheagentoverallandstructurethetreatmentofpolymericdidnotsig-lig-

228B.Xiaoetal./PolymerDegradationandStability71(2001)223±231

nins.wereThement,mostlypredominantsurvivedbduring-O-4interunitlinkagesofligninchemicalwhereashigherpulpingtheyprocessaresubstantiallyalkalineperoxidetreat-[2].cleavedduringtheparentreactiveligninamountsestingly,thanfromofphenolicThishydroxylicresultedgroupsinrelativelyinthethealiphaticoilpalmhydroxylictrunk®bregroupswhich[3].areInter-morebleachedbasedonchemicalmodi®cationofhighpulpnholmwithkraftpulpandchemicalthermomechanicalxylic[10]revealedsuccinicthatanhydride,theamountGellerstedofreactiveandhydro-Gate-comparativegroupswallstudywaslowthereactivityinlignin.ofInisolatedcontrast,withahavecomponentsthatreachacontradictwithaceticconclusionanhydride,Rowellwoodetal.cell[6]lignin,theachievedandmostreactiveisolatedcellwallinwhichcomponenttheystatedwasacetylwithinan15acetylmincontentofreactionofalmostwitha10%wasthecinicreactivitycontentofofabout18%after4h.Thedi󰂀erencesmaximuminre¯ectanhydrideshydroxylicgroupswithaceticandsuc-lignin,thebetweenthetwoligninsamplesmayandin¯uencetheconditionoffactorsofesteri®cation,suchasanhydride,etc.parent3.3.FT±IRspectra

supportThemodi®edsionfortheligninabovepreparationssuccinoylationswereusingexaminedtooilligninpalmFT±IR.trunkThus,®brelignintheFT±IR(spectrumspectraforunmodi®edtransmis-4.ofTheypreparationshowedthe4(spectrumb)area)illustratedandsuccinylatedinFig.di󰂀erencesligninbroughtdi󰂀erencesinthechemicalstructure1000broadcmÀ1.wereaboutbythesuccinoylation.TheseInthemostlyspectrumre¯ectedofintheregion1800±Cbandat1705cmÀ1orasaparentshoulderlignin,theweakring.󰂈OareAromaticstretchingskeletoninnon-conjugationisassignedtovibrationswiththearomatic1460givenat1610,1513,and1425cminÀthe1.parentligninvibrations.cmÀ1isAbsorptionatbreathingAattributedtothearomaticmethylgroupbandwithweakCOstretching)peakat1328andcmÀ1a(syringylringsyringylat1118lignintype)cmÀ1strongabsorptionareoriginated(aromaticCHin-planedeformation,breathingmolecules,withCOwhilestretching),thebandfromandatthe10331218syringylcm(guaiacylunitsÀ1ringinCHguaiacylin-planedeformation,guaiacyltype)arise(aromaticinAsshownunitsininFig.lignin4,themoleculesfromtheesteri®cation[26].

1712thethecmintensi®cationÀ1,ofthepeakofbetweenligninresulted1740±tionligninasduecarboxyltotheornewester.carbonylIngroupslinkedto1745-1740bycarbonylthiscase,theabsorp-cmÀ1bondsinestersgivespeaksbetweeninstronglycarboxylicoverlappedacids,andexhibitstheabsorptionwithathebandbycarbonylbondspreviousat1712onecmÀ1isand,whichthere-

Fig.a),25.(spectrumFT±IRspectrab),3(spectrumofsuccinylatedc),andlignins5(spectrumpreparationsd).

1(spectrumforeabsenceresultedofbandsintheat1790peakandcentred1860atcm1737À1cmÀ1.Theofunreactedsuccinicanhydride,provedthat,characteristicpeakesters,at1143anhydridecmÀ1intheproduct.thereInwasaddition,nomorethe1118whichwaspartiallyrepresentsoverlappedtheCÀOwithstretchingthebandintheattheintensitiesparentcmÀ1ligninforaromatic(syringylCHtype).in-planeAsadeformationfrombeofthepeaksat1737cmÀ1andresult,1143thecmrelativeÀ1underusedtoexaminethereactivityofthecanninFig.di󰂀erent5showsconditionssuccinoylationthegiven.

trumpreparations1(spectrumsuccinylateda),oil2(spectrumpalmtrunkb),®bre3(spec-lig-ofdistinctivethec),typicaland5peakssuccinylated(spectrumd).at1737cmligninTheÀ1andspectraspectra1138aswerecmshownindicativeÀ1bytheTheforesters.betweenrathertheothermodi®edthesimilarfourabsorptionpatternandintensitylignins.spectrarevealedasimilarstructureofstraw,parenttheandryeligninsfromSimilarly,poplartree,theestermaizereactionsstem,wheatof1735appearancesstrawwerealsomonitoredbyobservingcmÀ1andanofCanÀOesterabsorptioncarbonylatabsorbance1138cmÀ1atin

Fig.trum6.a),FT±IR11(spectrumspectrab),of12succinylated(spectrumc),ligninsand13preparations(spectrumd).

10(spec-B.Xiaoetal./PolymerDegradationandStability71(2001)223±231229

Fig.7.Thermogramsofsuccinylatedligninpreparations1(a),7(b),and13(c).

230B.Xiaoetal./PolymerDegradationandStability71(2001)223±231

esterspeakat(Fig.17356).cmInspectionÀ1ofFig.6,arelativelyweakerpoplari®edtreelignin(spectrumforestersina)spectrumthaninspectraofsuccinylatedof(spectrummaizecatedc),stemandligninrye(spectrumstrawb),wheatstrawligninmod-astions.comparedalowertodegreeofsuccinoylationlignin(spectrumofpoplard)ligninindi-succinylatedThisobservationthoseofthecorrespondedlatterthreetolignintheprepara-yieldof96.9±98.0%ligninobtained,92.5%frompoplarandstraw.

frommaizestem,wheatstraw,andrye3.4.Thermalstability

beFiveoilgenerallydistinctobservedstagesininthethethermaldissociationcouldandpalmtypicalitssuccinylatedtrunk®breligninTGA(thermogramscurvesfornottheshown)parentligninwere20±100approximatelysamplesthermogramslignin1(a)andofmodi®edpreparations.oilpalmExamplestrunk®breof󰀎situated7(b)inaretheshowntemperatureinFig.7.They600󰀎C,101±207󰀎C,208±300󰀎C,301±405󰀎C,andranges406±larityC.couldinThesethetypicalgeneralthermogramspatterns.However,exempli®edthethesimi-turetheandbeweightsdi󰂀erentiatedlosses.byThesetheirdi󰂀erentcharacteristicsamplesthermogramstempera-ofbility.thevariousalterationsmodi®edinchemicalligninpreparationsstructureandwerethermalindicativeofwaterThethefrom®rstthestagesamples.wasassignedThistotheevaporationsta-of11%modi®edtemperaturewaterwasligninpreparationwas7signi®cantly(b)inwhichtrueaboutintheincreasedevaporatedfrom20fromto100the󰀎sampleC.Consequently,whenthestarted®rsttionatthermal101󰀎C.degradationInweightstepwasconsideredtobefortemperatureat10%weightlosslossterms,occurredthedecomposi-at236󰀎Cligninunmodi®edsamplepreparationoilpalm1,andlignin,258󰀎265󰀎Cforsuccinylatedtemperature7.WhentheweightlossaccountedCformodi®edfor50%,ligninthenin,for508󰀎Cforraisedto502󰀎Cfortheparentoilpalmlig-datathesuccinylatedtheesteri®edoilpalmligninligninsamplepreparation1,and532󰀎Csuccinylatedimpliedoiloilinpalmfactaligninhigherthermalstabilityfor7.Thethemodi®edpalmligninandasatisfactorycomparedthermaltothestabilityunmodi®edofthe®edthreeandligninsuccinylatedupto210󰀎ryeC.strawInaddition,ligninbothunmodi-indistinctstages(20±100󰀎C,101±190󰀎exhibitedC,191±600only󰀎C)positiontheirTGAsampletemperaturecurvesinforanitsairparentatmosphere.andmodi®edThedecom-cinylatedstartedat186󰀎Cand190󰀎C,respectively.Theligninsuc-that58%itsmaximumryestrawrateligninofweightpreparationlossbetween(Fig.7c)10%showedwassampleslightlyoccurredattemperaturefrom240󰀎Cto530󰀎C,whichand(230±512higher󰀎C,thermogramthanthatofnotitsshown).unmodi®edTheresults

ligninrevealedhadlignin.

aslightlythatthehighermodi®edthermalryestrawstabilityligninthanpreparationitsparentpalmFig.7alsogivestheDSCpeak,ligninpreparations1(a)curvesand7oftheesteri®edoilwasobservedwhichrepresentsatamaximumheatreleased(b).temperaturefromTheoftheexothermic468product,󰀎bothlatedunmodi®edoilpalmligninsampleandsucciny-Cforexhibitedligninpreparations.SuccinoylatedryestrawligninitsasymmetricDSCacurve.maximumNotably,temperaturetheexothermicat483󰀎Caspeakshownwasinfromduetooverlappingendothermicresponse[27].tionsThebothtively,1andmeltingunreacted7waspointlignindetectedfortheandthereactionproductatmodi®ed234ligninprepara-increasingasshownbythebroadendothermicand241󰀎C,respec-growthsampleoftrenddegreeinofmeltingsuccinoylationpointcorrespondedpeaks.fromsampletoThis1thetoaqueousInconclusion,7(Table2).

inexpensive,alkalinethesolutionsuccinoylationwassimple,ofe󰂀ective,variousseeminglyligninsindegreenylatedofsubstitutionandenvironmentalwaslow.Thefriendly,althoughthestepproductprecipitation,ligninpreparationswascollectivelyisolationeasyofbysucci-one-nottionberecovered(8.6±9.8%indueofwhichtothetheinitialnoticeablesolubleparentamountsoftheinanlignins)aqueouscouldanhydride/CofpH3.Anincreaseinmolarratioofsuccinicsolu-growthSimilarof9unitofligninfrom0.5/1to1.5/1ledtoextensionincreasingthesuccinylatedstabilityofreactiontrendtimewasligninfromalsofrom94.7%to108.6%.1observedto4h.Theduringthecinoylation.oftheproductwasincreasedbychemicalthermalsuc-Acknowledgements

ScholarsThefundingdationProgramme,ofthisNationalprojectNaturalbytheCheungKongUSAisofgratefullyChina,acknowledged.

andChinaBridgeInternationalScienceFoun-inReferences

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