THE STUDY ON MECHANISM OF BINDER MIGRATION DURING COATING PROCESS

JOURNAL OF2004 ApiTIANJIN UNIVERSITY OF SCIENCE AND TECHNOL0GYVol.19 SuppTHE STUDY ON MECHANISMbinder migration-migration from an ideal state tononuniform distribution.OFBINDERMIGRATIONMuch of the carly work on binder migrationDURING COATING PROCESSconcluded it has great effect on the quality of coatedpaper[2]-[4] and the mechanisms of binder migrationYun Liangalso have been discussed.Capillary suctionmechanism and convection flows mechanism haveCollege of Material Science & Eng.been proposed successively [5]， and beingSouth China Univ. ofTech, Guangzhou, Chinasupplemented by the following research [6]. GroundE-mail: ppliangyun@yahoo.comon the explanations of these two theories, bindermigration to coating surface takes place duringKefu Chendrying. But a new theory based on the rheologicalphenomenon of wall effect was proposed by RangerThe State Key Lab. Of Pulp & Paper Eng.in the mid of 1990's[7]. According to this new theorySouth China Univ, of Tech., Guangzhou, China.surface binder enrichment occurs during coatingapplication. It is obvious, a conflict exists amongABSTRACTthese theories, but unfortunately, there has noexperiment report to solve this problem till now.The present work has sought to determine the exactBinder migration during coating process and thmechanism of binder migration were studied in thisphase that binders migrate to coating surface andpaper. After the latex was tagged by osmium, thediscusses the possible mechanism of migration.degree of binder migration was measured byStyrene-butadiene binder (SBR), one of the bindersenergy-dispersive X-ray spectrometer. For the wetbeing widely used currently, is selected to study.sample just after coating application,the realinformation of binder distribution was kept byEXPERIMENTALquenching the sample in liquid nitrogen followed byfreeze-drying.The results showed: under thCoating applicationcondition of this research, binder migration occurredThe coating color were based on 50% clay and 50%both in the process of coating application and drying.calcium carbonate，with 20 parts SBR latex as theBut the amount of binder migration occurred duringsole binder and 0.35 parts CMC as the thickener. Thecoating application was much little than that occurredsolids content was 60% . The coating suspensionsduring drying. The mechanism of binder migrationwere applied on paperboard at laboratory rod coater.during the process of coating application was studiedThe real informnation of binder distribution in wetby force analyses. And one viewpoint was proposedcoating layer was obtained by quenching the wethat was binder migration was caused by Magnuscoated paper in liquid nitrogen,followed byforce and Saffiman force.freeze-drying. As a compare, 160°C hot air dryingwas used.KEYWORDS: coating process, binder migration,Tagging and measuringThe binder distribution in coated papers wasmechanismdetermined by tagging the binder with osmiumArticle ID:1672- 6510 (2004) supp- 0345- 04followed by energy-dispersive X-ray analysis. Thetagging procedure was carried out in a fume hoodINTRODUCTIONand a small glass desiccator worked as a stainingchamber.4% OsO4 solution was placed in theBinders are the second most abundant component inbottom of the desiccator below the sample holder,and then the samples were placed on the samplethe coating color after the pigment.Binders areholder. The desiccator was sealed and left in the hoodadded into coating colors to fulill the followingfor 36hours.The samples were then removed andpurposes, binding of pigment particles to each other,allowed to degas in a vacuum desiccator for 1 hour.binding of pigment particles to base paper, adjustingAfter that the samples were ready to be measured byof ink absorption, protecting of colloid, affectingenergy-dispersive X-ray analysis. The amount ofstability and flowability of coating color [1]. Unlikebinder was revealed bv the counters of characteristicuniform distribution in well prepared coating colorsuspension, binder distribution is nonuniform along.mea中国煤化工Cros5-section was二ge binder contentthe cross section of coating layer. It is referred to ason CYH ;CNMHGecrosssecin,5.345-Jourrual of 7ianjininiversity of Science und TechnologyVol.19 Supppoints were selected to measure (represent the binderamounts of binder migration take place during dryingcontent at the cxact point).The degree of binderare much more than that occur during coatingmigration (DBM) was express by the counters at eachapplication.point departure fom the average one.Base on the mechanism of wall effect, binderenrichment exists at the out coating surface from theRESULTS AND DISCUSSIONvery beginning of its existence, so the DBM shouldreach the max. just after coating application. But theBinder migration during coating applicationresults of this study show, the DBM during drying isFig. 1 shows the change of DBM during coatingmuch higher than that just after application. So newprocess. The abscissa is the depth in microns fromtheory is needed to explain the cause of binderthe outer coating surface and the ordinate is themigration during coating application.DBM. It can be seen every curve is departure fromMechanism of binder migration during coatingthe zero datum line. It confirms binder migrationapplicationexist during coating process.The 0 sec.curveThe previous part testifies binder migration doesrepresent the sample been prepared by fast freeze andoccurs during coating application, but it couldn't befreeze-drying. It reveals the effect of coatingexplained by the currently views. Following theapplication on binder migration. At the outer coatingpossible theory will be discussed through the forcesurface (point of 0 micron), the DBM is positive, thatanalysis.means the binder has a tendency to migrate to theFig.2 shows the schematic diagram of coatingouter coating surface. The curve ends at point aboutapplication. The applicator rod moves relatively to50 micron, the interface of coating and substrate. Thethe paper web and the coating flm is laid down onDBM is also positive at this point and this can bethe paper. During this process, the main forcesexplained by the penetration of coating color into theloading on coating color are shear stress and pressuresubstrate. At the middle area of the curve, degree ofderiving from the relative motion of rod and paperbinder migration is negative. It is mainly due to theweb. And the forces loading on binder can be dividedbinder migrate both toward the outer coating surfaceinto two kinds. One is depends on the liquid-binderand the substrate.relative motion and the other not. Inertia force,gravity, colloid force and Brownian force belong to10;the first kind. Pressure gradicnt force，resistance,◆-0second一个- 4 scondadded mass force，Basset force，Magnus force and..--8second - ◆.20 secondSaffiman force pertain to the sceond kind.Thephysical meaning of these forces will be explained asfollowing and the calculable force will be expressedwith the round shape binder.cothng dirction of rodappskr abon rodcoaig color-10Fig.2 Schematic diagramn of coating application20一3C0I.grawitydepth of catinglayer μmG=古πd°ρL8(1)Fig. 1 DBW vs. depth of coating layer during coatingprocess2.inertia forceF=q πd°ρl(2)The other three curves in figure1 present the sampleprepared under different time of hot air drying, These3.colloid forcethree curves have the same tendency as the curve 0Colloid force derives fom the interaction betweensec. i.e. the DBM is positive at the out coatingparticles. It may cause the particles attract each othersurface and the interface of substrate, and at theor repulse each other.Colloid force couldn't bemiddle area of curves the DBM is negative. However_ledge of colloidcomparing with the cure 0 sce.，the fluctuation of cher中国煤化工~these three curse are more serious. That signifes the4.BrYHCNMHG●346-Vol.19 SuppJourndl of Tianjin Unirersity of Science and TechnolagyBrownian action is one of the characters of colloidshear lift force also would formed. It is namedparticles. Under the action of Brownian force, binderSaffiman force and could be express as follow[Il]:particles are under the state of moving ceaselessly.Brownian force is also noncalculable.i_=1.62d2V ρsμ/ (4-u)V5.pressiure gradient forceThere are many forces act on binder particles duringDuring the applicating，the rod moves relative tocoating application，but they don't have the samepaper web, so pressure gradient force forms withinimportant impact on binder migration. The forcesthe coating layer. For the binder particles, that has thecause binder migrate to outer coating surface shouldspecifical volume，the pressure gradient is various atperpendicular to this surface (z direction), so binderdifferent point on the binder particle surface，so themigration could be explained by analyses of the forcebinder particle has to bear the pressure gradientthat perpendicular to the coating surface. Theseforce.forces include gravity, Magnus force, Saffman forceF=士πnAP(3)and the components of colloid force and Brownianforce. When comparing with hydromechanical force ,6.slocksresistancethe components of colloid force and Brownian forceCoating is viscous fluid, the resistance act on binderare ignorable. As the direction of Magnus force andparticle could be expressed by formula stocks [8]:Saffman fore is opposite to the direction of gravity,F;=6πCrμ A(u.-uy)so if binder could migrate to the outer coatingT.added mass forcesurface, the driving force should be Magnus forceDifferent from the vacuum condition, when binderand Saffman force. Under the limit of theory aboutparticles moving in the fluid, the fluid around binderhigh solid content liquid-solid flow, some parametersparticles also will move with binder particles. Thisof Magnus force and Saffman force couldn't beeffect equal to a certain mass was added to the bindercalculated, but Magnus and Saffman force alsoparticles. For a round particle, the added mass forcecapable of explaining the binder migration to outerequals to half the mass of fluid that being taken bycoating surface during coating application.the binder particles [9].F=方πd2ρ(5)CONCLUSIONSthe relative acceleration was defined as follow:Binder migration during coating process was study inξ= du.. dy(6)this paper, it was clear that binder migration occuredduring the whole coating process i.e. not only at the8.Basset forceWhen the speed of binder particles changes, the fluiddrying phase but also at the coating applicationphase, though the amount of binder migration occursfield around the particles couldlot achieveduring drying was much more than that occuredstabilization immediately. So a force derived fromduring coating application.Since the binderthis ffect will act on the particles. It not onlymigration during coating application couldn't be welldepends on the speed and acceleration of particles,explained by the exist theories, a new mechanismbut also depends on the history of aceleration. Thisbase on two-phase flow was proposed, Magnus andforce is named Basset force. It equals to theSaffinan force were consider to be the main cause ofdifference of transient value between stress changebinder migration during coating application.and stabilization achieve [10].F。=号4'pVπD1' ξ凹) -du'(7)NOMENCLATURE: .!0 Vt-t'9.Magnus forceCJ is the corrction cofficient of binder particlesDuring the coating application, binder particles weredf is the diameter of binder particles, munder the nonuniform speed field, and due to theFg is the Basset force, Ndifferent speed at top and bottom of particle, a spinF, is the ineria force,Nlift force made binder particles revolving formed. It isFM is the Magnus force, Nnamed Magnus force and could be express by theF, is the pressure gradient force, Nfollowing formula [{10]:is the Stockes resistance, NErg πdi p/w(uy-u)(8)is the Saffman force, N。ic the addad maco frarra N10.Saffiman force中国煤化工Besides the spin lift force, because the speed gradientexists at the different point of the binder particles, the,IYHCNMH GS,347-Journal of Tiaujin Unitersity of Science and TechnologyVol.19 Supprn is the radius of binder particles, mof binder migration and latex flm formation during coatingis the time ,sconsolidation. Tappi J.1993,76<5):79-84u is the speed of binder particles, m/s4. Engst?m.,G.,Rigdahl,M.Binder migration-effect onprintablility and print quality. Nord.Pulp Pap.Res.J. 1992, 7uy is the speed of fuid, m/s(2):55-74ρ1 is the density of binder particles, kg/m25. Vanderhof J, Bradford E,The simulation for binderμs is the viscosity of fluid, Pa.smigration in latex clay coating using a modeling systcm.w is the angle velocity, rad/sTAPPI CoatingConference Proceedings , Atlanta,USA,72, I: oatngis the surface tension, N/m13-176. Hagen, K G,A fundamental asessment of the eftect ofis the relative acceleration, m/s2drying on coating quality, TAPPI Coating ConferenceProceedings, Atlanta ,USA, 1985, 131-140.REFERENCES7. Ranger A E,FInstp C, Binder migration during drying ofpigment coatings.Paper Technology, 1994 ,35(10):40- 46.I. Zhang, Y.H Technology of converted paper, China Light8. Bo,S.Y, Two phases flow, National Defence Publication,Industry Publications, BeiJing, PRC, 1991PRC.19852. Zang, Y.H, Aspler, JS. The efet of surface binder content9. Guo,YH and Lu,SJ，Conspectus of hydrodynamics,on print density and ink receptivity of coated paper. TappiScience Publications, PRC, 1981Adv .Coat.Fundam.Symp.Proc. Atlanta,Ga, USA. 1997,10. Bawwet,A.B.. Hydrodynamics. Vol.2. Dover. New York.165-1751961:285-2923. K.Yamazaki, T. Nishioka, YHatori,et al. Print mottle effect 11. Saffmnan, P. G. The lift on a small spbere in a slow shearflow. J. Fluid of Mech.1965, 22: 385-400中国煤化工MYHCNMHG-348-