Chlorine free extraction of cellulose from rice husk and whisker isolation

Chlorinefree extraction of cellulose from rice husk and whisker isolation Chlorinefree extraction of cellulose from rice husk and whisker isolationChlorinefree extraction of cellulose from rice husk and whisker isolationChlorinefree extraction of cellulose from rice husk and whisker isolationChlorinefree extraction of cellulose from rice husk and whisker isolationChlorinefree extraction of cellulose from rice husk and whisker isolationChlorinefree extraction of cellulose from rice husk and whisker isolationChlorinefree extraction of cellulose from rice husk and whisker isolation

j our na l h o me p ag e : w w w e l s e v i e r c o m / l o c a t e / c a r b p o l

Simone M.L Rosa, Noor Rehman, Maria Inez G de Miranda, Sônia M.B Nachtigall, Clara I.D Bica∗

Chemistry Institute, Federal University of Rio Grande do Sul, PO Box 15003, ZIP 91501-970, Porto Alegre, RS, Brazil

a r t i c l e i n f o

Article history:

Received 13 May 2011

Received in revised form 1 August 2011

Accepted 24 August 2011

Available online 31 August 2011

Keywords:

Cellulose whiskers

Rice husk

Biomaterials

Microscopy

Peroxide bleaching

Thermal analysis

a b s t r a c t

Thisworkreportstheisolationofcellulosewhiskersfromricehusk(RH)bymeansofan environ-mentalfriendlyprocessforcelluloseextractionandbleaching.Themultistepprocessbeginswiththe removalofpectin,cutin,waxesandotherextractivesfromricehusk,thenanalkalinetreatmentfor theremovalofhemicellulosesandlignin, andatwo-stepbleachingwithhydrogen peroxide/tetra-acetylethylenediamine(TAED),followedbyamixtureofaceticandnitricacids,forfurtherdelignification

ofthecellulosepulp.Thetechniquesofinfraredabsorptionspectroscopy(ATR-FTIR),scanning elec-tronmicroscopy(SEM),thermogravimetricanalysis(TGA),modulateddifferentialscanningcalorimetry (MDSC)andX-raydiffraction(XRD)showedthattheoverallprocessisadequatetoobtaincellulosewith highpurityandcrystallinity.Thiscellulosewassubmittedtosulfuricacidhydrolysiswiththeaimto iso-latethewhiskers.Theyshowedthetypicalelongatedrod-likeaspectasrevealedbytransmissionelectron microscopy(TEM)andatomicforcemicroscopy(AFM)

© 2011 Elsevier Ltd All rights reserved

1 Introduction

Ricehusk (RH)isoneofthemajoragriculturalresidues

gen-eratedasa byproductduringtherice millingprocess.TheFood

andAgricultureOrganizationoftheUnitedNations(FAO)forecasts

thattheglobalriceproductionstandsataround466milliontonnes

in2010/2011(FAO,2010).About23%ofthis amountconsistsof

RH(Chandrasekhar,Satyanarayana,Pramada,Raghavan,&Gupta,

2003).TheBrazilianriceproductionhasbeenintheorderof12

mil-liontonne/yearandRioGrandedoSul(thesouthernmoststateof

Brazil)isresponsiblefor60%ofthisproduction(IBGE,2010).Most

oftheRHproducediseitherusedasabeddingmaterialforanimals

anddiscardedinlandfillingsorsimplyburnedinthefields

lead-ingtoairandsoilpollution.Theexpressivecontentofabout20%

silicainRHand,afterburning,morethan90%silicainRHashhave

stimulatedextensiveresearchwhichsuggestedthepotentialuse

ofRHanditsashassourcesofinorganicchemicals(Chandrasekhar

etal.,2003).InthepresentworkweproposetheuseofRHasanew

sourceforobtainingcellulosewhiskersandweemployatotally

chlorine-freetechnique(TCF)toextractandbleachcellulosefrom

RH.Theisolationofhighlypurecellulosefromwheatstraw(Sun,

Sun,Su,&Sun,2004)andbarleystraw(Sun,Xu,Sun,Xiao,&Sun,

2005)usingtotallychlorine-freetechnologieshasbeenaddressed

inthescientificliteraturebutnotyetfromricehusk

Itiswellknownthatthemaincomponentsofplantfibersare

cellulose,hemicellulosesandlignin.Cellulose,whichawardsthe

∗ Corresponding author Tel.: +55 51 3308 7236; fax: +55 51 3308 7304.

E-mail address: claraism@iq.ufrgs.br (C.I.D Bica).

mechanicalpropertiesofthesematerials,isorderedin microfib-rils enclosed by theother two components,hemicellulose and lignin(Morán,Alvarez,Cyras,&Vazquez,2008).Celluloseisthe most ubiquitous and abundant natural polymer onthe planet, given its presence in plants and its widespread usefor ropes, sails,paper,timberforhousingandmanyotherapplications.By far,the most commerciallyexploited natural resource contain-ingcelluloseiswood(Eichhornetal.,2010)butcellulose isthe main component of severalotherwell employednatural fibers suchascotton,flax,hemp,juteandsisal(Moránetal.,2008).It

isexpectedthatthesupplyofwoodatareasonablepricewillbe insufficientinthefutureand,apartfromthenaturalfibers men-tionedabove,agriculturalbyproductswillbecomemoreattractive

assourcesofcellulose(Leitner,Hinterstoisser,Wastyn,Keckes,& Gindl,2007)

In recent years therehasbeen a remarkableinterestin cel-lulose fibers of nanometricdimensions Cellulose whiskersand microfibrilsareexamplesofnanocelluloseandresultfrom differ-entisolationmethodsleadingtodiversedimensions andaspect ratios(Siró&Plackett,2010).Cellulosewhiskersareelongated crys-tallinerod-likenanoparticlesbeinggenerallyisolatedbymeansof acidhydrolysiswhichremovestheamorphousdomainsexisting

incellulosefibers.Cellulosemicrofibrilsinturnareobtainedfrom mechanicaltreatmentbeinglongandflexiblenanoparticleswhich consistofalternatingcrystallineandamorphousstrings(Siqueira, Bras,&Dufresne,2009).Cellulosewhiskershavebeenisolatedfrom differentvegetablesourcessuchascottonandeucalyptus(Berg, Capadona,&Weder,2007;Dong,Revol,&Gray,1998;Hafraouiet al.,2008)andfromanimalsourcessuchastunicates(Bergetal., 2007;Hafraouietal.,2008).Consideringvegetableorigin,there

0144-8617/$ – see front matter © 2011 Elsevier Ltd All rights reserved.

areonly afew papers which describetheisolation ofwhiskers

fromagriculturalbyproducts,asforexamplewheatstraw(Herbert,

Cavaillé,&Dufresne,1996),peahullfiber(Chen,Liu,Chang,Cao,

&Anderson,2009),branch-barksofmulberry(Lietal.,2009)and

coconuthusks(Rosaetal.,2010)

Asfaras we know, theisolation of cellulose whiskersfrom

ricehusksourceshasnotbeenyetdescribedintheliteraturebut

onlytheisolationofsiliconcarbidewhiskersfromRH(Sujiroti&

Leangsuwan,2003)andofcellulosewhiskersfromricestraws(Orts

etal.,2005).So,thefractionationoflignocellulosicmaterialsofrice

husksintoitsconstitutivecomponentsbyenvironmentalfriendly

techniqueshasbeenthesubjectofourworkwiththeobjective

ofcellulosewhiskerisolation.RHandintermediateRHproducts

ofthemultistepextractionprocedurewerecharacterizedthrough

scanning electron microscopy (SEM), thermogravimetry (TGA)

andattenuatedtotalreflectance-infraredabsorptionspectroscopy

(ATR-FTIR) The purified cellulose was characterizedby

modu-lateddifferentialscanningcalorimetry(MDSC),wide-angleX-ray

diffraction(WAXD)aswellasTGAand ATR-FTIR.Thewhiskers

werecharacterizedbytransmissionelectronmicroscopy(TEM)and

atomicforcemicroscopy(AFM).Thepropertiesofpurified

cellu-loseisolatedfromricehuskwerecompared tothepropertiesof

commercialmicrocrystallinecellulose(MCC)

2 Experimental

2.1 Materials

Rice husk was supplied by Engenho Meirebe (Eldorado

do Sul/RS, Brazil) Hexane (Fmaia, Brazil), ethanol (Fmaia,

Brazil), sodium hydroxide (Labsynth, Brazil), hydrogen

perox-ide(CAQ Química,Brazil),nitricacid(Fmaia,Brazil),aceticacid

(CAQQuímica,Brazil),tetra-acetylethylenediamine(TAED)(Acros

Organics,NewJersey,USA)wereusedasreceived.Allsolventsand

reagentswereofanalyticalgrade.Microcrystallinecellulose(MCC)

wassuppliedbyQuimsul

2.2 Procedures

2.2.1 Isolationofcellulose

Ricehuskswerepreviouslyground.ThedriedRHwas

sequen-tiallydewaxedwithhexane/ethanol/waterinaSoxhletapparatus

Theextractivecontentwasfoundtobe6.8%.Delignificationwas

doneat121◦C,inautoclave(Stermax20EHD),usinga5%aqueous

NaOHwitha1:30strawtoliquorratio(g/mL)for30minbeing

this stepbased ona proceduredescribed by Uesu,Pineda, and

Hechenleitner(2000),adaptedtoricehusk.Thedispersionswere

treatedwithultrasoundfor30min.Inordertoremovethe

remain-ing hemicelluloses and lignin, theresulting pulp was bleached

followingaproceduredescribedbySun,Sun,Su,etal.(2004):the

pulpwastreated with2% H2O2 and0.2% TAED solution,atpH

11.8,for12h,at48◦C.Theliquortopulpratiowas25:1(mL/g)

Topurifythecellulosepulp,5.0mLof 80%(v/v)aceticacidand

0.5mLofconcentratednitricacid(70%,v/v)wereaddedto150mg

ofpulp,themixturewasthenplacedintoapreheatedoilbathat

120◦C,for15minor30min.Oncecooled,thesupernatantwasthen

carefullydecantedandthecellulosewaswashedsequentiallywith

95%ethanol(20mL),distilledwater(20mL),andagain95%ethanol

(20mL)toremoveextractionbreakdownproductsandtracesof

nitricacid.Finally,thepurifiedcellulosewasdriedinanovenat

60◦Cuntilconstantmass.Departingfromrawricehusks(∼9wt%

water),thetotalyieldofextractedcellulosewas28wt%

2.2.2 Isolationofthecellulosewhiskers

Thepurifiedcellulosewasmixedwithsulfuricacid64%(w/w)

ataratioof1:8.75(g/mL)asdescribedbyDongetal.(1998),at

temperatureof 25◦C The hydrolysis time wasfixedat 60min Thereactionswerestoppedbypouringthemixtureintoa large amountofcoldwater.Theexcessofsulfuricacidwasremovedby centrifugation(3000rpm,30min),usinganALCcentrifugePK120, followedbyaprolongeddialysis(regeneratedcellulosemembrane Fisher,cut-off10,000–14,000Da)againstpurewater.This proce-dureensuredthatallionicmaterialswereremovedexcepttheH3O+

counterionsassociatedwiththesulfategroupsonthesurfaceofthe whiskers(Dongetal.,1998).Thewhiskerswerefurtherdispersed

byanultrasonictreatment(UltrasonicequipmentThornton,Model USC-1400)

Althoughthestrongnitricandsulfuricacidswereusedinthe overallprocedure,theeffluentsturnedtobediluteandwereeasily neutralized

2.3 Characterization 2.3.1 Ricehuskandcellulose ScanningelectronmicrographsofdriedRH,extractivefreeand alkalinetreatedRHwereobtainedusingaJEOL®microscopeJSM

6060operatingat20kV.Thetestspecimenswereattachedtoan aluminumstubandsputteredwithgoldtoeliminatetheelectron chargingeffects

WAXD experimentswereperformed usinga Siemens D-500 diffractometer.PurifiedRHcellulose(after30minbleachingand alsocalledasRHcellulose)andMCCwerescannedinthereflection modeusinganincidentX-rayofCuK␣withwavelengthof1.54 ´˚A

atastepwidthof0.05◦min−1from2=0to40◦.TheSegalmethod wasusedtocalculatethecrystallinityofthesamples(Thygesen, Oddershede,Lilholt,Thomsen,&Stahl,2005).Eq.(1)wasusedto calculatethesamplecrystallinity(XCR)

XCR=I200−IAM

whereI200istheheightofthe200peak,which representsboth crystallineandamorphousmaterialandIAMisthelowestheight betweenthe 200 and 110peaks, which representsamorphous materialonly

Inourstudyweperformedapreliminaryexperimentinamuffle furnaceunderairatmospheretodeterminetheashcontentofrice husk

TGA scans werecarried out from35 to700◦C at a heating rateof10◦Cmin−1andunderinertatmosphereofN2inafluxof

50mLmin−1(TAInstrumentsmodelTGAQ5000IR).Sampleweight wastypicallykeptat17mg.TheTGAmicrobalancehasaprecision

of±0.1␮g

MDSCwas performed using a DSC Q2000 differential scan-ningcalorimeterfromTAInstruments.Sampleweightwaskeptat

∼7mgusinghermeticallysealedpanswithapinholeinthelid.Two proceduresweremadeusingpurifiedRHcellulose(after30min bleaching)inMDSC.Inthefirstone,thesampleswereanalysedas obtainedafterbleachingtreatment,equilibratedat35◦Cfor5min andheatedupto395◦Catheatingrateof5◦Cmin−1.Inaddition,a secondprocedurewasmadeapplyingarampof30◦Cmin−1from roomtemperatureto150◦Candequilibratingatthistemperature for5mintoremoveadsorbedwater,assuggestedintheliterature (Cabrales&Abidi,2010;Picker&Hoag,2002).Afterthisisothermal condition,sampleswerecooleduntil35◦Candasecondscanwas performedat5◦Cmin−1upto395◦C.TheMDSCanalyseswere car-riedonunderinertatmosphereofN2inafluxof50mLmin−1using

anamplitudeof temperaturemodulationof±1◦C anda period

modulationof60s

StructuralchangesbetweenMCC, RH,RHintermediate prod-uctsand purifiedRHcellulosewererevealedbyusingATR-FTIR with 64 scans and a resolution of 2cm−1, in a Nicolet 6700 spectrophotometer

Fig 1. ATR-FTIR spectra for RH, RH extractive-free, alkaline treated RH (RH after

15 min autoclave), RH cellulose (after 30 min bleaching) and commercial cellulose

(MCC) in the range from 2000 to 800 cm −1

2.3.2 Cellulosewhiskers

FortheTEMimages,dropsofRHwhiskeraqueoussuspensions

weredepositedonglow-dischargedcarboncoatedTEMgridsand

theexcessofwaterwaslettoevaporate.Thespecimenswere

neg-ativelystainedwith2%uranylacetateandobservedusingaJEOL

JEM1200FxIIelectronmicroscopeoperatingat80kV.Thewhisker

dimensionsweredeterminedwiththeaidoftheImageTools

soft-ware

AFMobservations werecarried outusing a Molecular

Imag-ingPicoPlusmicroscopeoperatinginairandintermittentcontact

modewithaMicromashNC36tip.Dropsofdiluteaqueous

suspen-sionsofRHcellulosewhiskersweredepositedontofreshlycleaved

mica.After30min,theexcessliquidwasremovedandthe

remain-ingfilmallowedtodry

3 Results and discussion

3.1 Characterizationofthericehuskandricehuskcellulose

3.1.1 Spectroscopiccharacterization

FTIR spectroscopy has been extensively used in cellulose

research,sinceitpresentsarelativelyeasymethodofobtaining

directinformationonchemicalchangesthatoccurduringvarious

chemicaltreatments(Sun,Sun,Zhao,&Sun,2004).Byidentifying

thefunctionalgroupspresent,FTIRallowstoknowaboutthe

chem-icalstructureofeachcompound.InthisworkFTIRwasemployed

withtheaimofverifyingifligninandhemicelluloseswereremoved

fromtheextractedcellulose

InthisworkFTIRspectraofRH,RHfreeofextractives,

com-mercialcellulose(MCC),andpurifiedRHcellulosewereobtained

Allsamplespresentedtwomainabsorbanceregions.Thefirstone

athighwavenumberscorrespondstotherange2700–3500cm−1

(Fig.1S,Supplementarymaterial),and thesecondone atlower

wavenumbers,totherange800–1800cm−1approximately.The

lat-tercanbeseeninFig.1.Thebroadabsorptionbandwithpeaks,

dependingonthesample,locatedfrom3330to3360cm−1isdue

tostretchingof–OHgroupsandthatonenear2900cm−1isrelated

totheC–Hstretchingvibrations

Thebandat1640cm−1couldbeassignedtotheC Cstretching

ofaromaticringsofligninbutitisalsopresentinthespectrum

ofcommercialcellulose.Accordingtovariousauthors(Moránet

al., 2008; Sun, Sun, Su, et al.,2004; Zuluaga, Putaux, Restrepo, Mondragon,&Ga ˜nán,2007),thisbandrelatestothebendingmode

ofadsorbedwater.Allsampleswerecarefullydriedbeforethe ATR-FTIRspectraweretaken, but,as reportedintheliterature,it is difficulttocompletelydrycelluloseduetoitsstronginteraction withwater(Moránetal.,2008;Szczesniak,Rachocki,&Tritt-Goc,

2008).Allmaterialsanalysedpresentedthisabsorptionbandbut specificabsorptionscanalsobeseeninthespectra.The absorp-tionbandat1176cm−1correspondstoC–O–Casymmetricalbridge stretching.AspointedoutbySun,Sun,Su,etal.(2004),astrong peakat1049cm−1arisesfromC–O–Cpyranoseringskeletal vibra-tion.InFig.1itcanbeseenthatthispeakchangesitsforminthe

RHcelluloseasfarasitappearsasadoublet.Incomparisontothe spectrumofcommercialcellulose,itcanbeconcludedthat hemi-celluloseswereextensivelyremoved.Thesharppeakat910cm−1

ischaracteristicof␤-glycosidiclinkagesbetweenthesugarunits (Sun,Sun,Su,etal.,2004).ThespectraofRHandRHfreeof extrac-tivesshowtwoabsorptionscharacteristicoflignin:aweakbandat

1510cm−1(alsoC Cstretchingofaromaticring)andabroad shoul-derat1244cm−1 (C–Ostretchingoftheetherlinkage)whichare absentinthespectrumofRHcelluloseaswellasthatof commer-cialcellulose.AccordingtoVieraetal.(2007),theabsenceofthese bandsindicatesthatmostoftheligninwasremoved.SoinRH cel-lulosetheextractionproceduresremovedmostofligninpolymers becauseofthedisappearanceofthelignin-associatedabsorbances

at1510cm−1and1244cm−1.InthespectrumofRHcelluloseitcan alsobeidentifiedapeakat1725cm−1(C Oofketone)which proba-blyarisesfrompartialacetylationofRHcelluloseduringthesecond bleachingstepwhereaceticacidisemployedasalsomentioned

byotherauthors(Moránetal.,2008;Sun, Sun,Su,etal.,2004; Zuluagaetal.,2007).InthespectraofRHandRHfreeof extrac-tivesapeakat1734cm−1canbeseenwhichcanalsobeassigned

toC Oofketonebutduetohemicelluloses.Fig.1alsoshowsan ATR-FTIRspectrumofextractive-freecellulosepulpobtainedafter

15minofalkalinetreatment,inautoclave,andbeforethebleaching steps.Inthisspectrumthereisnotanyabsorbanceinthecarbonyl region

3.1.2 Scanningelectronmicroscopy(SEM)

BySEMitwaspossibletodetectdifferenteffectsontheRH sur-faceaccordingtothestagesofpre-extractionandpulping,asshown

inFig.2.Thechangesintheouterepidermisshowthechemical attacksufferedbythematerialatdifferentstages

Incomparisontoextractive-freericehusk(Fig.2a),after15min

ofalkalinetreatmentinautoclave,itcanbeseenthatthericehusk particleschangedfromflattorolledshape(Fig.2b).Fig.2cshows thesurfaceofextractive-freericehuskwiththepresenceof sil-icaparticles.AsimilaraspectofRHsurfacewasalsoreportedby Chandrasekharetal.(2003).Fig.2 showsthattheseparticleswere removedafter15mininautoclave.InFig.2 filamentscanbeseen

ontheouterepidermisintheregionswhereprotuberanceswere removedafter30mininautoclave.Fig.2fshowsthatafter1hin autoclavethesurface didnot changesignificantly.So30minin autoclavewaschosenastheoptimumtime.BycomparingFig.2 (15mininautoclave)andFig.2h(30mininautoclave)itcanbe noticedthattheinnerepidermisisalsomodifiedwhenthe auto-clavetreatmentisincreasedto30min.Thisalkalinetreatmentin autoclavealsocausesareductionoftheaveragesizeofRHparticles (Fig.2S,Supplementarymaterial)

3.1.3 Thermogravimetricanalysis(TGA) Fig.3showsthethermaldegradationpatternofthe commer-cialcellulose(MCC),crudeRH,andRHcellulose(after15minand

30min bleaching) All samples showed a thermal event below

150◦Ccorrespondingtodehydration.Themasslossofwaterinthis stepwasdeterminedfrom45◦Cto150◦C.Itwasabout3.8wt%for

Fig 2.SEM micrographs of RH after various stages of chemical attack: a, b) outer epidermis of RH extractive-free (bars: 200 ␮m and 10 ␮m, respectively); c, d) RH outer epidermis after 15 min in autoclave (100 ␮m and 10 ␮m, respectively); e) RH outer epidermis after 30 min in autoclave (bar: 10 ␮m, both); f) RH outer epidermis after 60 min

in autoclave (bar: 10 ␮m); g) RH inner epidermis after 15 min in autoclave (bar: 10 ␮m); h) RH inner epidermis after 30 min in autoclave (bar: 10 ␮m).

MCC,9.0wt%forcrudeRH,5.8wt%forRHcelluloseafter15min

bleachingand5.7wt%forRHcelluloseafter30minbleaching.The

effectivethermaldegradationoftheRHconstituentsbeginsabove

200◦Candreferstobondcleavageofhemicellulose,celluloseand

lignin

ItispossibletoverifythatRHcelluloseshowedhigher

ther-malstabilitythan theprecursorRH,since inthesesamplesthe

componentsthatstarttodegradeatlowertemperaturehadbeen

removed.ThecrudeRHmaindecompositionpeakisconsiderably

widerthanthoseoftheothersamplesduetothedecompositionof

hemicellulosesandlignin.CommercialcelluloseandRHcellulose

decomposedinasinglestep.Thisbehaviorsuggeststheabsence

ofhemicelluloseandligninintheRHcelluloseobtained.TheDTG

curveofRHcellulosedoesnotshowtheshoulderclosetothe

cellu-losepeakthatreferstothehemicellulose.Thisisinaccordancewith

theFTIRresultspreviouslyshown.Themaximumrateof

decompo-sitionofRHcelluloseoccurredat345◦C.Thistemperatureagrees

wellwiththevalueof348◦CfoundbyMoránetal.(2008)forthe

Fig 3. DTGA curves for commercial cellulose (MCC), RH, RH cellulose after 15 min

decompositionpeakofcommercialcelluloseand355◦Cfoundby Yang,Yan,Chen,Lee,&Zheng(2007),determinedatsame heat-ingrate.ThecommercialcelluloseshowedhigherTmaxthanthe

RHcelluloseisolatedinthisstep.Accordingtotheliterature,the higherthedecompositiontemperatureobtainedby thermogravi-metricanalysisthegreaterthecrystallinityofcellulose(Alemdar& Sain,2008;Chenetal.,2011;Uesu,Pineda,&Hechenleitner,2000) However,thediscussionshavebeenrecentlyimprovedconsidering othereffectsthatcaninfluencethetemperaturepeakof degrada-tion:presenceofsubstancesbondedtomicrofibrilsurfaces(Vila, Barneto,Fillat,Vidal,&Ariza,2011),crystalsizeofcellulose(Kim, Eom,&Wada,2010)andtheatmosphereenvironmentused (usu-allynitrogenorair)(Mamleev,Bourbigot,&Yvon,2007;Vilaetal.,

2011)

RHpresenteda highresidualmassat theend ofthe experi-ment(700◦C),around26%.Theashcontentofricehuskdetermined underairatmosphereinthisworkwas16wt%at1000◦C.Theresult agreesperfectlywellwithZhaoetal.(2009).Evenconsideringthat theanalysiswasperformedundernitrogenatmosphere,thiswas

anespeciallyhighvalueanditwasrelatedtothehighsilica con-tentofRH(Rosa,Nachtigall,&Ferreira,2009).At700◦Cresiduesof about8%forcommercialcellulose,15%forRHcelluloseafter15min bleachingand11%forRHcelluloseafter30minbleachingcanbe determinedfromtheTGAcurves(Fig.3S,Supplementarymaterial)

AsindeedevidencedbytheX-raydiffractionstudy,thecrystallinity indexofRHcelluloseislowerthanthatofthecommercialcellulose AnotherexplanationmayberelatedtothepartialacetylationofRH celluloseevidencedbythepresenceofanabsorptionat1725cm−1

intheATR-FTIRspectrum

3.1.4 Modulateddifferentialscanningcalorimetry(MDSC) Modulated differentialscanningcalorimetry(MDSC) permits theseparationofthetotalheatflowsignalintoitsreverseheatflow andnon-reverseheatflowcomponents.Theseparationisbasednot onlyonthermodynamicreversibilitybutalsoonchangesoccurring whenasinusoidalmodulationisoverlaidonaconventionallinear heatingrateduringanexperiment.Inthissense,MDSCarisesasan excitingwaytoincreasetheunderstandingofricehuskcellulose thermalproperties.Theeffectsoftemperatureonamorphousand crystallineregionsofricehuskcellulosewerestudiedbyMDSC

Fig 4. MDSC curves of total (1), reverse (2) and non-reverse (3) heat flow at 5 ◦ C min −1 : (a) RH cellulose (30 min bleaching), under first procedure; (b) RH cellulose (30 min bleaching), under second procedure; (c) MCC, under first procedure; (d) MCC, under second procedure.

Considering literature using DSC technique, as reported by

Morán et al (2008) and Yang et al (2007), the fusion of the

crystalline fractionof some types of cellulose shows a narrow

endothermicpeakcloseto330◦C.Thistransitioncanmovetolower

temperatures depending on factors such as molecular weight,

amountofamorphouscontent,crystallitesizes,etc Sometimes,

anexothermicpeakisfoundinthesameregion,whichhasbeen

relatedtoadegradationprocess(Moránetal.,2008).Accordingto

Mamleevetal.’sstudies(2007),adepolymerizationby

transglyco-sylationoccursat310◦Cduringcellulosepyrolysis.Asbothevents

canbesuperimposed,theycannotbeeasilydistinguishedinmany

cases

Fig.4ashowstheheatflowcurvesofRHcelluloseanalysedby

MDSC.Consideringtotaland non-reverseheatflow curves,they

showtwomainevents.Thefirstendothermicpeakobservedbelow

150◦Cisduetolossofwater.Thesecondendothermictransition

startsaround270◦Cwitha peakat320◦Candisrelatedto

cel-lulosemelting.Asmooth exothermictransitioncanbedetected

near340◦C.Thiseventhasitsonsetoverlappedwiththeendofthe

endothermicregionandcanberelatedtothedepolymerizationof

celluloseassupportedbytheequivalentpeakinthenon-reverse

heatingcurve.Such conclusionisalsocorroboratedbytheTGA

studywhichshowsamaximumofweightlossforRHcelluloseinthe

sametemperatureregion.Ontheotherhand,animportantchange

intheheatcapacityofthemediumcanalsobeseenbetween300◦C

and330◦Cinthereverseheatflowcurve.Thisindicatesachange

inchemicalcompositionasaresultofthedepolymerization

reac-tion.Theabsenceofanendotherminreversingsignalindicatesthat

thisthermaleventisakinetictransformation.Byvisualinspection

ofthepan,veryfewsolidresidueswerefoundatthisstageand charringprocesswasevident

Fig.4 showstheMDSCcurvesofRHcellulosesubmittedtothe secondproceduredescribedintheexperimentalsection,withan isothermalsteptoeliminatewater.Asexpected,itwasnotfound anypeakduetowaterrelease.Awell-definedendothermic tran-sitionispresentbeyond300◦CwhichissimilartothatofFig.4a beingrelatedtomeltingandvolatilizationaswell.Howeverthe smallexothermicpeakfollowing this transitionwasnotclearly seeninthetotalandnon-reversecurves.Thethermogramsprofiles

ofRHcelluloseareverysimilartothoseofMCCwhichareshown

inFig.4candd.IncomparisontoMCC,thepeakmaximumofthe endothermictransitiondetectedbeyond300◦Coccurredatlower temperatureforRHcellulose(asassignedbyarrowsTRH=321◦C,

inFig.4aandb,andTMCC=344◦CinFig.4candd)independentlyof waterpresenceasitwasobservedintotalandnon-reverseheating curves

Inthis study,itwasobservedthatallsamplesshowed well-defined endothermic peaks corresponding to the fusion of its crystallinepart,asshown inFig.4a–d.However,cellulose sam-pleswithwateradsorbed(Fig.4aandc)showedmoreclearlythe exothermicpeaksfollowingmelting.Thissuggeststhatthe degra-dationmechanismresponsiblefortheexothermicpeakisaffected

bythepresenceofwater

3.1.5 WideangleX-raydiffraction(WAXD)

It can be observed in Fig.5 that the major crystallinepeak foreachsampleoccurredataround2=22◦whichrepresentsthe cellulosecrystallographicplane(200).Thecrystallinityindexof

Fig 5.X-ray diffraction patterns of MCC and RH cellulose (30 min bleaching).

RHcellulose(calculatedbySegalformula)wasapproximately67%

whilethatofMCCwasestimatedas79%.Forcomparison,the

crys-tallinityindexofothersamples,asreportedintheliterature,was

foundtobearound66%forpotatotubercellulose,68%forricestraw

celluloseand71%forwoodcellulose(Abe&Yano,2009).Itcan

beconcludedthattheprocedureemployedinthisstudyfor

cel-luloseextractionfromricehuskisadequateforobtainingsamples

withhighcrystallinity.Itwasreportedthathighlycrystallinefibers

andfibrilaggregatescouldbemoreeffectiveinachievinghigher

reinforcementforcompositematerials(Cheng,Wang,Rials,&Lee,

2007).InadditionitcanbenoticedinFig.5thatRHcellulosecan

beclassifiedascelluloseI,sincethereisnodoubletintheintensity

ofthepeakatca.2=22◦.AsimilarfindingwasreportedbyMorán

etal.(2008)forsisalcelluloseextractedbyotherprocedures

3.2 Characterizationofcellulosewhiskers

Basically, microscopy has been the preferred technique for

themorphologicalcharacterizationinstudiesinvolvingcellulose

whiskers.Inthisstudy,AFMandTEMwereusedtoinvestigatethe

morphologyandsizeofthedispersedstructures

TheatomicforcemicrographinFig.6showsthesampleobtained

after60minofhydrolysis.Itwaspossibletoseetheisolated

cellu-losefibrilsfreefromtheothercomponentsofricehusks.Mostof

Fig 6.AFM image of RH cellulose whiskers obtained after 60 min of acid hydrolysis.

thehydrogenbondsthatkeptthewhiskersassociatedwere prob-ablydisruptedafterthisprocedure.However,someaggregatesare stillpresent

ByTEM(Fig.7aandb),structuresintheformofneedles rang-ingfrom100to400nminlengthand6to14nminwidthwere observed.TheaveragelengthvaluewasL=(143±64)nmwhilethe averagethicknesswasd=(8±2)nm.Suchdimensionsare compara-bletothoseofwhiskersoriginatingfromcotton(Beck-Candanedo, Roman, & Gray, 2005; Bica, Borsali, Rochas, & Geissler, 2006; Hafraouietal.,2008),wood(Beck-Candanedoetal.,2005),peahull fiber(Chenetal.,2009)andcoconuthusks(Rosaetal.,2010).The

RHwhiskersshowlengthsshorterthaninthecaseofbranch-barks

ofmulberry(Lietal.,2009)butRHwhiskersaremuch thinner Theaspectratioshowsanaveragevaluenear18.InFig.6someRH whiskersappearedmoreaggregatedintheformofbundlesasalso observedbyHeux,Chauve,andBonini(2000)inthecaseofcotton whiskers.AccordingtoHafraouietal.(2008),suchnanostructures canbecomposedofavaryingnumberofparallelsubunitsof cellu-losechains.Thehighaspectratioofthecellulosewhiskersobtained fromrice huskindicatesthatthesestructuresexhibitpromising

ofthisworldwideproducedagriculturalwaste

4 Conclusions

Residuesfromplants areinterestingalternativesascellulose

sourcesforseveralapplications.Inthisworkachlorine-free

pro-cedurefortheisolationofcellulosefromricehuskwasshownto

beveryefficient.Theoverallprocessdoesnotproduceanytoxic

effluents.Onthebasisofthewholecellulosecontentexpectedfor

ricehusk,thismethodresultedinayieldaround74%.TGA

anal-ysisperformedundernitrogenshowedhighresidualmassforRH

at700◦C.Thiscanbepartiallyattributedtothehighsilica

con-tentofthematerial.Inourstudy,theashcontentofRHat1000◦C

wasdeterminedtobe16wt%.FTIR,TGAandMDSCanalysesagreed

wellwithrespecttotheeliminationofhemicelluloseandlignin

fromricehuskafterthepurificationprocedureusedtoisolate

cel-lulose.WAXDexperimentsindicated thatthecrystallinityofRH

cellulose (67%)waslower than thatof MCC(79%) Lower

crys-tallinityhasbeenpointedoutasafactor,amongothers,thatcan

lowerthethermaldegradationtemperature.Thedecomposition

temperatureofRHcellulosewasfoundtobelowerthan

commer-cialmicrocrystallinecellulose.Besideswaterelimination,theMDSC

analysesshowedonemainendothermiceventforcellulosesamples

(RHcelluloseandMCC),whichwasrelatedtothemeltingof

cellu-losecrystals.TheTGAandMDSCresultsagreewellwithrespectto

thethermalstabilityofricehuskcelluloseandhelpedtoimprove

theknowledgeonthecomplexbehaviorofcellulosedegradation

Cellulosewhiskersweresuccessfullyobtainedbysulfuricacid

hydrolysisofthericehuskcellulose.AccordingtoTEMandAFM

images,itwaspossibletoisolateneedle-likestructuresof

cellu-losewhiskers,withsizesvaryingfrom6to14nminwidthand

100–400nminlength.Theaveragevaluesoflengthandthicknessof

thesewhiskersgiveanaspectratioaround18.Suchavalueofaspect

ratioisadequateforapplicationofRHwhiskersasnanofillersin

polymermatrices.Inthiswaytheuseofricehuskasanovelmaterial

sourceallowstoobtainnewparticleswithnanometricdimensions

wideningthesupplyofnanostructuredmaterialsusablefor

poly-mernanocomposites

Acknowledgements

TheauthorswouldliketothankConselhoNacionalde

Desen-volvimentoCientíficoeTecnológico(CNPq)forgrant

474278/2007-7andfellowshipTWAS/CNPq;Coordenac¸ãodeAperfeic¸oamento

dePessoaldeEnsinoSuperior(CAPES)andFundac¸ãodeAmparo

àPesquisadoEstadodoRioGrandedoSul(FAPERGS)for

fellow-ships(alsoCAPES/REUNI);CentrodeMicroscopiaEletrônicaofthe

FederalUniversityofRioGrandedoSul(CME/UFRGS)andMs.M

Queirozfor technicalassistanceduring theTEMandSEM

anal-yses;Mr.O.Machado(InstitutodeFísica/UFRGS)forperforming

theWAXDmeasurements,Dr.J.Vaghetti(IQ/UFRGS)fortechnical

assistanceduringTGAandMDSCanalysesandMs.N.Reisforhelp

inthefirstexperimentsofthisproject

Appendix A Supplementary data

Supplementarydataassociatedwiththisarticlecanbefound,in

theonlineversion,atdoI:10.1016/j.carbpol.2011.08.084

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