Olejnik, K. Effect of the Free Swelling of Refined Cellulose Fibres on the Mechanical Properties of Paper.FIBRES & TEXTILES in Eastern Europe 2012, 20, 1(90) 113-116.

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Effect of the Free Swelling of Refined Cellulose Fibres on the Mechanical Properties of Paper

Konrad Olejnik

Lodz Technical University, Institute of Papermaking and Printing,

ul. Wólczańska 223, 90-924, Łódź, PolandE-mail: konrad.olejnik@p.lodz.pl

AbstractThe purpose of this work was to determine the optimal duration of free swelling for refined unbleached kraft pine pulp. Free swelling is the effect of water sorption by cellulose fibres without their simultaneous mechanical treatment (e.g. refining). The second purpose was to determine the possibility of improving the mechanical properties of paper by testing various combinations of refining and free swelling processes. The pulp SR value and water reten-tion value (WRV) of the fibres were tested. As regards paper properties, the tensile index and tear index were tested. Based on the results obtained, it was found that for the pulp tested, refined to 35 °SR, the optimal time of free swelling was 70 min. It was also proved that the free swelling had a significant impact on both the WRV of fibres and the mechani-cal parameters of paper tested. For the scheme in which refining was followed by free swelling, a 7% increase in the tear index was obtained. However, an 11% increase in the tear index was attained for the scheme with three refining stages, followed by free swelling after each refining.

Key words: pulp, free swelling, WRV, refining, paper strength.

n IntroductionIt is well known that the unrefinedplant-basedcellulosefibresused in pa-per production have low papermakingpotential (the ability to create a desiredpaper structureofdesiredpaperproper-ties).Papermadefromthistypeoffibreischaracterisedbyhighporosityandlowstrength.Suchcharacteristicsresultfromthe fact that thestrengthpropertiesofapaper product dependon the surface offibrebonding.Unrefinedcellulosefibresare relatively stiff, therefore the surfaceoftheirmutualcontactinapaperproductwillbemarginal.Papermadefromsuchfibreswillhavealoosestructureandlowstrength(Figure 1.a).Basicefforts-aimedatimprovingpapermakingability-havetobefocusedonthemaximaldevelopment

todeterminetheprogressofinternalfibril-lationisthewaterretentionvalue(WRV),developedbyJayme[2,3].Thetests[7,10]showedthatanincreaseintheWRVoffibresisclearlyconnectedwithanincreaseinthemechanicalpropertiesofthepaper.It is important to remember thatwaterpenetration and swelling are processesofspecifickinetics[6,9].Acertainperiodoftimeisrequiredforwatertopenetrateinto all available spaces.Therefore fi-bre swellingoccurs not onlyduring therefiningoperationbut also (in a limited

oftheboundsurfaceinpaper.Thiseffectisobtainedbyincreasingfibreflexibility[7,8].

Intheformingandpressingstage,suffi-cientlyflexibleandsoftenedfibresbuildapaperwebofcompactstructure,wheretheareaoffibremutual contact is large[7,8].Inthedryingprocess,intheplacesoffibrecontact,thebondsinfluencingthehighstrengthofthefinalproductarefixed(Figure 1.b).

To increase the flexibility of cellulosefibres,itisnecessarytoweakenthebondsconnecting their structural elements. Inpractice,thisisdoneinthepulprefiningprocess.Itshouldbenotedthatthedesiredeffect of this operation is possible onlyinthepresenceofpolarliquid(onanin-dustrialscale–onlyinawatermedium).Mechanicalinteractionsbetweenknivesinrefiningequipmentdamagetheexternal,impermeable layers of cellulose fibresandmake internal layers available forwater [7].Watermolecules are stronglyattractedbyfree,polarcarboxyl,hydroxylorsulfogroups[11-13],whicharepresentin chemical compounds (e.g. cellulose,hemicellulose, lignin), constituting thebuildingmaterial of the plant fibre cellwallorarebroughtintothefibrestructureby their treatment (pulping, bleaching)[1,11-13].Theeffectofthisattractionresults in the replacement of hydrogenbondswithhydrousbridgesthatloosen,asaconsequence,eachlayerofthecellwall.Inpapertechnologythisphenomenoniscalled internalfibrillation.The swellingofcellulosefibresistheresultofinternalfibrillation.Oneofthefactorsthatallows

Figure 1. a) Structure of a paper sample made from unrefined pulp (stiff fibres), b) Structure of paper made from refined pulp (flexible fibres).

a)

b)

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range) during free contactwithwater(freeswelling).ThemostcomprehensiveresearchworksinthisfieldwerecarriedoutbyWultschandWeissmann[15]inthe1950s,inwhichtheyfoundthatthetimeafterwhich the pulp reachesmaximumhydrationdependsonseveralfactors,in-cludingthepulpgradeandrefiningrate.Theyalsodiscoveredthatafterfourhoursoffreeswellingitwaspossibletoobtainanincreaseinpaperstrengthof15%.Usinglimitedrefiningcombinedwithfreeswell-ing,theyobtainedthesamestrengthwithbetterpulpdrainability,whichhadagoodresultasithelpedtodewaterthepaperwebonthepapermachine’swire.Theswell-ingofpapermakingcellulosefibreswasthe subjectofMaloneyandPaulapuro’sresearchwork.Theresearchers,however,mainlyfocusedonmeasurementsofthefundamental relationships between thefibrestructureandwater.Theydeterminedthesizeofcapillariesinthefibrecellwallsandwaterdistributioninthefibrestructure[4,5].Therearealsoavailabletestresultsforfreeswellingitself,butonlyregardingfibres dedicated to textile purposes [14,16].The tests used solutions of variouscompounds,e.g.NaOH,KOH,LiOHordifferentalcohols.Forthisreasontheyarenot that important forpaper technology.Literatureanalysesshowthatfewauthorshaveconcentratedtheirworkonthepos-sibleimprovementofpapermakingabilitybycombiningrefiningwithfreeswelling.

sultofacompromisebetweendecreasing(alongwithrefining)thedrainabilityandincreasingthepapermakingpotential(de-finedbycertainproductqualityindices).Itwasdecidedthatthebasiclimitationoftherefiningprocesswouldbetheabilitytodrainthepulptested(measuredbySRvalue).TheSRvaluewasusedasthemostpopular(especiallyinindustrialpractice)andwasthesimplesttoaccomplish,beingthe indicator of drainage.Free swellingwascharacterisedbythefibers’WRV.Asregardspaperproperties,thetensileindexandtearindexweretested.

n Materials and methodsUnbleachedkraftpinepulpwasused intheexperiments.AllrefiningsweredoneinaPFImillaccordingtostandardmethodTAPPIT248.Thewaterretentionvalue(WRV)offibreswastestedaccordingtoSCAN-C102XE (8measurements persampleweremade).Itwasassumedthat,after centrifuging of the pulp sampletested, free swellingwas finished.Therefinedpulpwasexamined,andlabora-tory sheets of 70 g/m2were formed inRapid-Köthen apparatus according toStandard ISO5259-2:2001. Paper sam-pleswere conditioned according to theISO187:1990standard.Themechanicalpropertiesofpaperweretestedinaccord-ancewithadequateISOstandards:tensileindex - ISO1924-2:2008 (10measure-ments per sampleweremade), and tearindex-ISO1974:1990(4measurementspersampleweremade).Theschemesofrefining and subsequent swelling testedare shown inFigure 2. TR stands forthe optimal refining time andTS standsfor theoptimal freeswelling time (bothvalueswereexperimentallydetermined).Information about the typical accuracyofallmeasurementsisgiveninTable 1.

n ResultsOn the basis of an analysis of the SRvalue for similar pulp grades processedinindustrialconditions,itwasassumedthatuponrefining,theSRvalueofthetestpulp could not exceed 35 °SR.For thelimitdefinedinthisway,itwasassumedthat optimal pulp and paper propertieswere obtained for that refining degree.Figure 3 showstherelationshipbetweentherefiningtimeandSRvaluesobtained.TheSRvalueassumedwasachievedaftertimeTR=6min.Therefore,furthertestsoffreeswellingwereconductedforpulprefinedfordeterminedtimeTR.

Atpresent, thoroughknowledgeof thekineticsintheswellingprocessofpaper-makingfromcellulosefibresisbecomingparticularly important.Thegrowing ca-pacitiesoftechnologicallinesconnectedwithhigherpapermachinecapacitiesaremakingthetimeofspecificunitoperationsshorter. Batch refining systems,wherepapermaking pulp had enough time toswell,havebeenreplacedbycontinuoussystems,meaningthatrefinedpulpflowsthroughalltherefinerswithinarelativelyshortperiodof time, afterwhich- it isimmediatelyusedintheproductioncycle.Sucha system doesnotusually ensuretheminimaltimeneededforappropriatefibreswelling,theeffectofwhichmayin-volvethenon-optimaldevelopmentofthepapermakingpropertiesofagivenpulp.Asaresult,lowerpaperqualitycouldbeobtained.

Thelackofnewresearchresultsregardingfree swelling encouraged the author toundertakearesearchprojectinthisfield.Theaimoftheprojectwastodeterminethe optimal time of free swelling of aselectedcellulosepulp,withtheotheraimbeingtodeterminepossibleimprovementsinthestrengthpropertiesofpaperbytest-ingvariouscombinationsofrefiningandfreeswellingprocesses.

Onemustrememberthattherefiningproc-essinindustrialconditionsisalwaysare-

Figure 2. Schemes of refining and swelling sequences tested.

Table 1. Typical accuracy of all laboratory measurements accomplished (Student-Fischer method, confidence level = 95%)

Measurement Mean Square Error Standard DeviationSR value, °SR 0.239 0.76WRV, % 0.0732 0.17Tensile index, N×m/g 0.853 2.71Tear index, mN×m2/g 0.0645 0.21

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therefiningprocess,theimpactofforcesintherefiningzoneforcessomewatertobedrainedfromfibresonceagain,resultinginarepeateddecreaseintheswellingrate.Itisalsoworthmentioningthatusingsev-eralshorterrefiningsinsteadofonelongonedidnotimprovetheWRV.Therefore,inordertoobtaintheoptimaleffect,thebestsolutionistousefreeswellingafterrefining.

Theanalysisofthetensileindexoflabo-ratorypapersamplesmadeofpulpsfromthedifferentschemesofrefiningandfreeswelling(Figure 6, seepage116)showsthattfreeswellingconsiderablyimprovesthestrengthpropertiesofpaper.Whereverfree swellingwas used, higher valuesof the tensile indiceswere obtained forscheme1,whichonlyinvolvedrefining.On the basis of these results it can beconcludedthatthebesteffects(thehighesttensileindex)wereachievedforscheme2,whereonerefiningwasused,followedbyfreeswelling.Inthisinstance,almost

Fromapracticalpointofview,thetimeofthepulpstaying(andswelling)inthetechnologicallineisafunctionofthetotalcapacity of the stock system and papermachine. If the pulp has to stay longerthere,thecapacityofthetechnologicallinehastobeincreased(e.g.byconstructionofanadditionalchest).Toreducecosts,itisnecessarytodetermineanoptimalandpossiblyshortswellingtime.Inthesetests,theboundarytimeofthefreeswellingwasdefinedasthatafterwhichtheincreasesinswellingdegreeswerelessthan5%ofthetotalWRVincrease(obtainedingivenconditions for a given pulp).Figure 4 showschanges in thisvaluedeterminedfor different timesoffreeswelling.Onthebasisofthepreviouslygivendefinitionoftheswellingtimelimit,itwasfoundthat the time required for the test pulpwasTS=70min.Thereforeinallfurtherexperiments, free swellingwas carriedoutfortimeTS.Figure 4alsoshowsthatfreeswellingisnotalinearprocess.Freeswellingisfastestwithintheinitialmin-utes,thenthedynamicsofwatersorptionstart to drop, reaching a definedmaxi-mumboundaryvalueintheend.Itcanbesuspectedthatthiseffectcanbearesultof the fact thatwaterabsorptioncreatespressureon thefibrecellwall structure,andthereispressureinducedcreepduringthe looseningof the cellwall structure.Thewhole process of free swelling isrelativelyslow,lastingseveralminutes.Inthecasepresented,theWRVoffibresafterTS=70minutesoffreeswellingaddition-allyincreasedbyaround4%.Onthebasisoftherefiningandfreeswell-ingtimedetermined,6differentschemescombining refining and free swellingoperations in different sequenceswere

developed.The sequence of the opera-tionswasmarked in thefollowingway:R–refining,andS–freeswelling.Intheschemes,where several refiningswereused,thetotalrefiningtimehadtobethesameastheTRtime.Insuchcases,eachoftherefiningslastedforashortertime,proportionaltothenumberoftherefinings(e.g.with two refinings,eachof whichlasted0.5×TR).Nevertheless,ineachcase,freeswellingwasconductedforatimeTS.

Figure 5 shows the finalWRVvaluesobtained for each refining-free swell-ing scheme.The results indicate that,dependingonthesequenceusedandthenumberofthesetwounitoperations,theWRVmayhavevaluesdifferingevenby11%.ThehighestWRVof 146.6%wasobtained for theR-S sequence,whereasthe lowestwasfor theR-S-R(135.1%).Generally,itcanbeobservedthathigherWRVswere obtained for the sequencesthatwere finishedwith free swelling.Therefore,itcanbeconcludedthatduring

Figure 3. Changes in dewatering ability (measured in SR degrees) of unbleached kraft pine pulp refined in a laboratory PFI mill.

Figure 4. Determination of the optimum duration of free swelling for test pulp refined to 35 °SR.

Figure 5. Fibre WRV for different refining and free swelling schemes.

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a 7% increase in the tensile indexwasachieved.Anotmuchlowerincreasewasobtainedforscheme6(threerefiningsfol-lowedbyfreeswellingaftereach).Therelatively lowest increasewasobtainedforschemes3and4.Therefore,itcanbeconcludedthat,whenthetensileindexistheparameterdeterminingpaperquality,thebestsolutionwouldbetouseonefreeswellingafterrefining(scheme2).

Thesituationlooksdifferentforthetearindex.This isaparameterreflecting thedynamic strength properies of paper.These properties usually decrease afterrefining. Figure 7 showsvaluesofthetearindexobtainedforalltheschemestested.Itturnedoutthatfreeswellingalsohasapositiveimpactonthisindex.Inthecasediscussed,thebestresultswereobtainedforscheme6,wherethepulpwasrefinedthreetimes(alwaysforatimeof1/3×TR),andaftereachrefiningthepulpunderwentfreeswellingforatimeTS,resultinginan11%increaseinthetearindexforscheme1(refiningonly).Onthebasisoftheresultsobtained,ageneralrulecanbeformulatedthatforthetearindex,shortrepeatedrefin-ingswithnecessaryinterruptionsforfreeswellingareadvantageous.

n SummaryOnthebasisofthetestresults,itcanbeconcludedthatfreeswellingisalong-last-ingprocess.Forthepulptested,refinedtoavalueof35°SR,theoptimaltimeoffreeswellingwas70minutes.Thetestresultsshowthatfreeswellingdefinitelyhasa

positiveeffectonboththeWRVoffibresandthestrengthpropertiesofpapertested.Thebestresultswereachievedwhenfreeswellingwasusedattheveryend,afterthefinalrefining.Intheexperimentwithonefinalfreeswellingafterasinglerefiningstage,a7%increaseinthetensileindexwas obtained.The highest increase inthetearindex(11%)wasobtainedintheexperimentwhere 3 refining stages and3 stages of free swellingwere appliedalternately.

AcknowledgmentThis study was supported by the Polish Minis-try of Science and Higher Education (project NN 209 122 139)

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Received 23.02.2011 Reviewed 13.06.2011

Figure 6. Values of the tensile index of laboratory paper samples made from the test pulp for different schemes of fibre refining and free swelling.

Figure 7. Values of the tear index of laboratory paper samples made from the test pulp for various refining and free swelling schemes.