Grinding characteristics of a highly water-absorbent polymer

Paper Grinding characteristicsof a highlywater-absorbent polymer ENDOH andKAZUO YOSHIKAZU SHIGEHISA TAKEUCHI KUGA, The Institute andChemical Hirosawa Saitama Wako-shi, 351-01, Research, Japan ofPhysical Received forAPT 23July 26October 1990 1990, accepted Abstract-A water-absorbent alcohol/sodium was ina gelofvinyl acrylate copolymer ground highly and inaninert fine We obtained with amean size of3.5 vacuum toobtain drygas particles. products µm inavacuum after for24h.Products andinnitrogen were smaller than those slightly grinding ground in i n e ffect of added to the feed on of and helium. We the the size ground argon investigated attapulgite theground andfound thatitwas effective forupto10hforproducing fine The products particles. relation between size and water ofground with andwithout theattapulgite absorbency products particle was was also Forparticle sizes than thewater ofground absorbency products investigated. greater 5 µm, with than orthesame asthat ofthefeed. The water decreased size for slightly higher absorbency particle smaller than 5µm,especially inthecase ofgrinding with particles attapulgite. " NOMENCLATURE A absorbance ofUVlightinblue-dextrin solution[-] solution[-] ofUVlightinresidue Ao absorbance ofblue-dextrin [wt%] CBDconcentration undersize distribution D(x)cumulative (-] ofwater absorbed[kg] Mg weight . ofVA/SA[kg] Mp weight ' t time [h] grinding ofblue-dextrin solution[m3] Yo volume W water defined byEq.(1) [kg/kg] absorbency x particle size [jum] diameter ofthecumulative undersize distribution [pm] x5o median ofwater[kg/m;] p density 1.INTRODUCTION water-absorbent ofcross-linked can Highly polymer gelcomposed polyelectrolytes absorbwaterupto several hundred timesits ownweight [1].A gelof vinyl VA/SA isa typical alcohol/sodium acrylate (termed hereafter) copolymer highly water-absorbent Thegelparticle hasan inhomogeneous in structure, polymer. which sodium ina polyvinyl alcohol aredispersed phases phase [2,3]. polyacrylate Theabsorbed watercanberetained inthedispersed sodium mainly polyacrylate phases [4]. Inthepolymerization it isdifficult to control theparticle sizeofthe reaction, In particular, smaller than200 pm cannot beproduced particles by polymers.

152 ofthismaterial sizes polymerization [5].Nevertheless, manyapplications require smaller thanthislimit. Forexample, theparticle sizemustbesmaller than10tlmto beusedasthehydrophobic inpaintandinotherbuilding materials for component Atthissize,theparticles retain a smooth surface without protection against dewing. absorbs a largeamount cracks evenwhen thepolymer ofwater. insize,areobtained available ofVA/SA, Commercially particles 10-30pm byjet ina flowofcoldnitrogen arenotsufficiently small grinding gas[5].These particles fortheapplications mentioned above. Inthisstudy, weground VA/SA inseveral kindsofatmosphere smaller than (vacuum, N2,HeandAr)to obtainparticles Wealsotestedtheeffectof attapulgite 10,um. (MggAl2S'8020(OH2) 8H20)on Asattapulgite haspositive it adsorbs onthesurface of grinding. charges, tightly and VA/SA ittofillthespaces between theVA/SA particles [6].Thus,weexpect soretard thecoagulation ofthefineparticles. Thegrinding characteristics of VA/SAwithandwithout were attapulgite to obtain information onsizereduction. Therelation between water investigated andparticle sizeoftheground werealsoinvestigated. absorbency products 2.EXPERIMENT 2.1.Grinding andmethod apparatus Ahighly suchasVA/SA beground water-absorbent cannot wellinroom polymer itabsorbs water fromtheairandcoagulates. Wedesigned a millpotin air,because inavacuum which wecould control thegrinding either orinadryinert atmosphere, 1shows themillpotconsisting ofa vacuum linejointforthegasinlet gas.Figure andoutlet, a ballvalve, a stainless steelfilter(125 filterandstainless mesh), paper steelballswitha diameter of5mmusedasthegrinding media.

1.Grinding mill Figure pot.

153 weobtained available VA/SA Asthefeedinourexperiments, commercially [5] thathadbeenground toyield byjetmillina flowofcoldnitrogen gasandsieved of8gofVA/SA wasplaced inthemillpot a mean sizeof11 lum.Acharge particle with960gofstainless steel balls. Forsome runsattapulgite wt%)wasalsoput (5-20 inthemillpot.Thenvacuum wasperformed at 60-70°C untilthetotal drying inaninertgasatmosphere 10-2 fellbelow Torr.Inthecaseofgrinding pressure (N2, HeorAr),themillwasfilled andflashed several times toavoid contamination by airandmoisture andfinally setata pressure of1atm. Thegrinding wasperformed ballmill.Thecondition wasfixed at bya vibration a frequency ofvibration of994r.p.m.andanamplitude of10mm.Thegrinding timewasalsofixedat 1,2.5,5,10and24h. 2.2.Measurement andwater absorbency ofsizedistribution Thesizedistributions of feedandproducts weremeasured bylaserrefraction. theparticles werewelldispersed in water-free ethanol Before measurement, by vibration: 5minwitha 150 Wvibrator then2minwitha 50Wvibrator ultrasonic thatwasattached tothemeasurement apparatus. Thewater wasmeasured thetea-bag method andUV bytwomethods: absorbency Inthefirstmethod aknown ofVA/SA, in weight absorptiometry. Mp,wasplaced a fibermembrane of37,um stainless steelmesh. Thebagwassuspended bagmade inwater timeandthenremoved, drained andweighed toobtain the forafixed again ofwater wasdefined asfollows: absorbed, weight absorbency Mg.Thewater A Thebasisofthesecond method isthatblue-dextrin isnotabsorbed byVA/SA. volume of0.03wt%blue-dextrin solution sodium chloride (0.9wt%) J.-ó containing ofUVlight,Ao,at 333nmwasmeasured wasprepared anditsabsorbance bya a weight wasplaced ina volume ofthe Next, spectrophotometer. J.-ó MpofVA/SA 0.03% blue-dextrin solution fora fixedtime.Theabsorbance ofUV,A,inthe residue solution wasmeasured. Weconfirmed thattheabsorbance, experimentally asshown inFig.2. totheconcentration ofblue-dextrin, A,wasproportional CBD,

at333 nmasafunction oftheconcentration ofblue-dextrin. 2.Absorbance ofUV light Figure

154 theweight ofthewater absorbed Therefore, byVA/SA, bythe Mg,isdetermined following equation: where ofwater. p isthedensity AND DISCUSSION 3.RESULTS ' characteristics 3.1.Grinding 3shows thecumulative undersize ofproducts for distribution, Figure D(x), ground 5h ina vacuum, inAr,inHeorinN2at 1atm.InFig.3, xstands fortheparticle size.Thesizedistribution ofproducts ina vacuum wasapproximately the ground same asthatofproducts inN2.Theparticle sizes ofproducts forotherruns, ground were thanthose ofproducts inavacuum andinN2 however, slightly larger ground allsubsequent ina vacuum. wasperformed Therefore, grinding 4shows thesizedistributions ofproducts for1,2.5,5,10and24h Figure ground withgrinding time.Themeansizeof in a vacuum. Theparticle sizesdecreased for24h wasapproximately Therefore, thegrinding of products ground VA/SA ina vacuum ballmillwaseffective forobtaining bythevibration particles than10pm. smaller Toinvestigate theeffect ofattapulgite ontheproduct sizedistribution, VA/SA % 5shows wasground for5h with5,10and20 wt attapulgite. thatthesize Figure distributions foralltheproducts were thesame. Astheproduct size approximately inourexperimental distribution isinsensitive totheamount ofadded attapulgite wefixed theadded amount to 10%insubsequent runs. conditions, Themedian diameter ofthecumulative undersize distribution, x5o,ofground inFig.6asa function withandwithout areshown ofgrinding products attapulgite time.Thex5ovalues oftheproducts ina vacuum for24h wereapproxiground inbothgrindings. There a difference inxsobetween mately 3.5 pm was,however, thecases withandwithout ofproduct attapulgite upto10h.From3to10h,thex5o withattapulgite wassmaller thanthexsoofproduct without the ground ground aidinthisperiod. isonlyeffective asa grinding Therefore, attapulgite. attapulgite

3.Size inN2. inHe,andinAr. distributions ofproducts for5hinavacuum, ground Figure

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4.Size distributions ofproducts for1, 2.5, 5,10and24hinavacuum. ground Figure

5.Size distributions ofproducts for5hwith 5,10and20wt%attapulgite. Figure ground

6.xso forproducts with asafunction ofgrinding time. (0)andwithoutattapulgite Figure ground

156 Ananomaly wasobserved thexsoofproducts for1h withthe initially: ground wasslightly for1h without attapulgite largerthanthexsoof products ground Thisresult arises fromthetightadsorption oftheattapulgite attapulgite. probably onthesurface oftheVA/SA tothepositive oftheattapulgite owing charge [6] 3.2.Water-absorbent characteristics Therelationship between thewater W,andx5oforproducts absorbency, ground without isshown inFig.7.These results were measured attapulgite bythetea-bag andtheerrorbarshows thestandard deviation ofthemeasurements. Water method, absorbencies measured weresimilar. AsthemeasurebytheUVabsorptiometry mentsbyUVabsorptiometry weremorecomplicated, wemeasured thewater method thereafter. AscanbeseenfromFig.7, the absorbency bythetea-bag ofproducts ofxsofrom11to 6 yriwassimilar toor absorbency (0)intherange thantheabsorbency ofthefeed(8).However, theabsorbency oftheparticles higher forwhich decreased witha decrease ofparticle size. xsowaslessthan5 pm 8shows therelationship between thewaterabsorbency, Figure W,andxsofor theproducts withattapulgite. Forxsosmaller than thedecrease of ground thewater ofproducts withattapulgite wasgreater thanthatof absorbency ground theproduct without asshown inFig7. ground attapulgite 9shows thewater oftheproducts withandwithout Figure absorbency ground asafunction ofgrinding time.Theabsorbency ofproducts more attapulgite ground than5h decreased withgrinding when withattapulgite. time,especially ground 10shows thescanning electron ofthefeedandFig.11shows Figure micrographs for1-24h without products ground (a-e)andwithattapulgite (f-j).After5h,the toshowcoagulated particle shapes (c,d,e,h, i,j) changed structures, especially inthecaseofproducts withattapulgite correlated ground (h-j).Thiscoagulation wellwiththedecrease ofwater withgrinding timeafter5h asshown in absorbency thatthedecrease ofwaterabsorbency andtheformation of Fig.9. Wesuspect structure oftheground arecasued ofcrosscoagulated products bythebreakage linked inVA/SA duetotheheatproduced andaccumulated inthe polyelectrolytes thelonggrinding time. system during

7.Correlation ofwater with forproducts without Figure absorbency xso ground attapulgite.

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8.Correlation ofwater with forproducts with Figure absorbency ground xso attapulgite.

9.Water ofproducts with andwithout asafunction Figure of absorbency ground (0)attapulgite time. grinding -

10.Scanning electron ofthefeed. Figure micrograph

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11.Scanning electron ofproducts after (a)Ihwithout micrographs grinding: attapulgite. Figare

11.(b)2.5hwithout attapulgite. Figure

11.(c)5hwithout attapulgite. Figure

159

11.(d)10hwithout Figure attapulgite.

11.(e)24hwithout attapulgite. Figure

11.(f)1hwith attapulgite. Figure

160

11.(g)2.5hwith Figure attapulgite.

11.(h)5hwith Figure attapulgite.

11.(i)10hwith Figure attapulgite.

161

' 11.(j)24hwith attapulgite. Figure 4.CONCLUSIONS Wehavedemonstrated amethod forgrinding thehighly water-absorbent polymer, toobtain smaller than10 pm fordeveloping suitable VA/SA, particles applications. Fromourexperiments wederive thefollowing conclusions. ofproducts ina vacuum andinnitrogen were (1)Thesizedistributions ground smaller thanthoseofproducts inargonandinhelium. slightly ground diameter of cumulative undersize distribution of theproduct (2)Themedian decreased from11 to 3.5 tlm byourmethod. added tothegrinding waseffective fine (3)Attapulgite upto10h forobtaining particles. sizes than5 thewater ofground (4)Forparticle greater absorbency products thanorthesame asthatofthefeed. wasslightly higher ofground smaller than5tlminsizedecreased (5)Thewater absorbency products withparticle when withattapulgite. size,especially ground REFERENCES 1.K.Nukushina, absorbent. Yuki Gosei 1980. 6,546-554, Kagaku Super 2.T.Motohashi, Kako 1984. Kobunshi 33,452-458, Kokyusuisei zyushi. 3.M.Ogura, Mikuro hidoro tooyou. 1984. Sobunrigata Kagaku Kogyo 2,137-142, gelnotokusei and ofsodium 4.S.Sakohara, F.Muramoto and M.Asaeda, Swelling shrinking processes polyacrylatesolutions. J.Chem. 1990. 23,119-124, type super-absorbent gelinelectrolyte Eng. Japan 5.Catalog ofSumika-gel, Sumitomo Chemicals Co.Ltd. 6.F.Masuda, T.Itoand E.Sodeyama, Kokai Koho Kyusuizai oyobi kyusyusei buppin, Japan Tokkyo 1986. (Patent) JP61/58657,