CA2116035A1 - Superabsorbent polymers and process for producing - Google Patents

Abstract

Superabsorbent polymers of .alpha.,.beta.-ethylenically unsaturated monomers are made by polymerizing said monomers with certain crosslinking agents, drying the polymer and heating the dried polymer at, e.g., about 165 ·C to about 230 ·C, for a time sufficient to produce a heat-treated polymer having a sixty minute absorbency under a 0.3 psi load of at least about 25 grams/gram.
Particularly preferred superabsorbent polymers incorporate a crosslinking agent methylenebisacrylamide, bis(acrylamido)acetic acid and its salts, allyl acrylate, allylmethacrylate and difunctional monovinyl and monoallyl esters and amides, and when heat-treated provide a polymer having a sixty minute absorbency under a 0.3 psi load of at least about 30 grams/gram.

Description

WO 93/05û80 2 1 1 6 0 3 `~j PCr/US9~/07611 ~ ~

SUPERABSORBENT POLYMERS AND PROCESS FOR PRODUCiNG

BACK6ROUN~ OF THE INVENTION
Water-swellable polymers are used as constituents in sanitary materials which 5 absorb body fluids such as sanitary napkins, incontinent devices and disposable baby diapers Ex3mples of such polymers are disclosed in U.S. Patent Nos. 3,926,891; 4,190,562; and ~.
4,2~3,60g. .,`~
Various processesare known for producing absorbent polymers. For ~xample, U.S. Patent No. 4,fi33,222 teaches a process for preparing an absorbent po,ymer from a 10 neutrali2ed monomer witllout a surface-active agent. U.S. Paten~ No. 4,808,637 teaches~the uniform reaction o~ acrylic acid, an ?Ikali metal salt o~ carbonic acid, aluminum acetate, sodium suifate and water, preferably using microwave radiation as a heat source to initiate polymerizatlon Another process comprises the steps of preparing a reaction mlxture consisting 15 essentiaily of partlcular amounts of polymeri~able acid grou~containlng monomers, crossl~nkinQ agent and optionally free radic~l initiator in an aqueous medium and thereafter polymerizing and neutralizing at least a portion of the acid functional groups.
Absorbent polymers used in perional care devices are ~harac~eri~ed by certain performance properties, including centrifuge capacity, ahsor~nce under load, shear modulus, 20 ~nd percent extractables.
The centrifuge capacity of a polymer is quotient of the weight of the 0.9 percent saline solution retained by a swollen polymer after centrifugation to remove ;nterstitial water diYided bythe non-swollen weight. Unless otherwi;e indicated, celltrifuge capacity values disclosed herein are determined as described i n U S. Patent 4,286,082, the relevant portlons of 25 which are incorporated herein by reference, except that an absorptlon tlme of 30 minutes rather than 3 to S minutes is employed. Preferred centrifuge capacities are those greater than a~out 26 gramslgram. Especially preferred centrlfuge capacities are those greater ~han about 30 grarTls/gram.

WO 93/050X0 21 16 0 3 ~ PCl`/US92/07611 Sixty minute 0 3 psi absorbance under load (AUL) refers to the ability of the polymer to swell against a pressure of 0.3 psi. Sixty minute 0.3 psi AUL reflects the ability of the polymer to absorb, e.g., when the user of a personal care device is seated. Unless otherwise indicated, sixty minute 0.3 psi AUL values disclosed herein are determined as described in European Patent Applicaticln EP 339,461- A1, the relevant portions of which are incorporated hereinbyreference. PreferredsixtyminuteO.3psiAULvaluesarethosegreaterthanabout25Especially preferred sixty minute 0.3 psi AUL values are those greater than about 30.
Shear modulus refers to the ratio of applied stress to induced strain as the polymer is deformed. Unless othen~vise indicated, modulus values disclosed herein are determined as described in U.S. Reissue Patent 32,649. Preferred shear moduli are those greater than about 40,000 dynes/cm2.
In general, capacity is inversely proportional to sixty minute û 3 psi AUL and shear modulus. One of the main goals of polymer design has been to increase the sixty minute 0.3 psi AUL of the polymer without decreasing the shear modulus or centrifuge capacity.
Further, absorbent polymers used in personal care devices should have rninimal 16 hour extractables levels, i.e., soluble, uncrosslinked polymer levels. 6enerally, higher levels of extractables retard the transfer of liquid through an absorptive product. Unless otherwise indicated, 16 hour extractables values disclosed herein are determined as described in U.S.
Reissue Patent 32,649. Preterred absorbent polymer will have a 16 hour extractables level less 20 than about 10 percent, more preferably less than about 5 percent.
U. S. Patent No. 4,666,983 disclosed an absorbent article obtained by mixing 100parts by weight of an absorbent resin powder having a carboxyl group with 0.001 to 10 parts by weight of a crosslinking agent, and reacting the absorbent resin powder with the crosslinking agent, typically by heating at a temperature between 90C and 300C to form surface 25 crosslinked polymers. The surface crosslinked polymers are said to have improved ratios of absorption, as compared ~o non-surface crosslinked polymers. This process suffers the disadvantage of requiring a crosslinking agent, in addition to that present in the monomer mixture, to effect the improvement in absorptive properties Accordingly, those in industry would find great advantage in a process for 30 improving the sixty minute 0.3 psi AUL of a water-swellable polymer while simultaneously maintaining a low 16 hour extractables level, and while not significantly decreasing (if not increasing) the centrifuge capacity and shear modulus. Such a ,orocess should not require ~he use of a secondary surface crosslinking agent during heat treatment.
SUMMARY OF THE INVENTION
The present invention provides a process for producing a superabsorbent polymer comprising polymerizing an ~,~ethylenically unsaturated monomer in an aqueous phase in the presence of a suitable initiator and a crosslinking agent and thereafter drying the polymer at a temperature between about 1 00C and aboul ' 80C to form a dried polymer, characterized in WO 93/05080 21 16 0 3 5 PCI`/US92/07611 that the process fu rther comprises heati ng the dried polymer in the absence of a surface crossl inki ng agent to a temperature between about 165C and about 230C for a ti me suff icient to produce a heat-treated polymer having a sixty minute 0.3 psi AUL of at least about 25 gramslgram.
In a particularly preferred embodiment, the aqueous phase will further comprise a C~ to C6 alkyl acrylate, whereupon the heat-treated polymer has a sixty minute 0.3 psi AUL of at least about 28 grams/gram.
In an especially preferred embodiment, the crosslinking agent will be selected from the group consisting of methylenebisacrylamide, bis(acrylamido)acetic acid and its salts, 10 allyl acrylate, allylmethacrylate and esters or amides having both a vinyl and an allyl functionality, whereupon the heat-treated polymer has a sixty minute 0.3 psi AU L of at least about 30 grams/gram.
The present invention further provides a superabsorbent polymer comprising polymeri~ed mo;eties of at least one a,~ethyleni~aliy unsaturated monomer and at least one 15 crosslinking agent, said polymer being characteri~ed by a 60 minute 0.3 psi AUL of at least about 30 granWgram, a centrifuge capacity of at least about 32 gramslgram, and a 16 hour extractables level less than about 4 percent.
The prffent invention further provides a method of using such a superabsorbent polymer, comprising retaining sa;d polymer in conjunction with hydrophilic fibers in an 20 absorbentstructure. Such absorben~structure may in turn be retained within a disposable diaper, a sanitary napkin, or an incontinence device.
The superabsorbent polymers made by the process of this invention enioy an increased AUL, a high centrifuge capacity and modulus, and a low 16 hour extractables level.
~ DETAILED DESCRIPTION OF THE INVENTION
- ~ - 25 Suitablewater-soluble~,~ethylenicallyunsaturatedmonomersthatareuseful tn the present invention include those monomers which are capable of conversion by a4ueous solution polymerization into a water-swellable andlor lightly crosslinked hydrophilic gel polymer. The crosslinked s;tructure may be obtained by the copc lymerization of a water-soluble monomer and a crosslinking agent possessing at least two polymeri~able double bonds 30 in the molecular uni$.
The water-swellable or lightly crosslinked hydrophilic polymers that are prepared ! ' by thé method of the present invention can be any of the known hydrophilic polymers which are capable of absorbing large quantities of flui~s. Examples of such polymers and processes for preparing them are disclosed in U.S. Patent Nos.3,997,484; 3,926,891; 3,935,099; 4,090,013;
4,093,776; 4,340,706; 4,446,261; 4,683,274; 4,459,396; 4,708,997; 4,076,663, and 4,190,562 herein incorporated by reference. Such hydrophilic polymers are prepared from water-soluble - a,~ethylenically unsaturated monomers such as rnonocarboxylic acids, polycarboxylic acids, acrylamide and their derivatives :
. ' , .
~:

WO 93/05080 PCl /US92/07611 tlab~e S~1 a"~ethylenically unsaturated monomers include, for example, acrylic acid, methacrylic acid, crotonic acid, isocrotonic acid and alkali metal salts and ammonium salts thereof; male;c acid, fumaric acid, itaconic acid, acrylamide, methacrylamide and 2-acrylamido-2-methyl-1-propanesulfonicacid and itssalts. The preferred monomers include acrylic acid and S methacrylic acid and their respective salt forms such as alkali metal or ammonium salts The water-soluble monomers useful in the present invention may be used in amounts ranging from about 10 percent to about 80 percent by weight based on the total weight of the aqueous monomer solution. Preferably, the amount ranges from about 20 percent to about 60 percent based on the total weight of the aqueous monomer solution.
Optionally, the m;nor amounts of other water-soluble, unsaturated monomers, such as alkyl esters of the acid monomers, e.g., methyl acrylate or methyl methacrylate may be present. In addition, certain grafting polymers, such as, for example, polyvinyl alcohol, starch and water soluble or swellable cellulose ethers may be employed to prepare products having superior properties. Such grafting polymers, when employed, are used in amounts up to about 15 10 weight percent based on the ~,~ethylenically unsaturated monomer. Further, it may be advantageous to include a chelating agent to remove trace metals from solution, e.g., when a metal reaction vessel is employed. One such chelating agent is VERSENEX 80 (an aqueoùs solution of the peneasodium salt of diethylenetriamine pentacetic acid) (Trademark of The Dow Chemical Company) . Such chelating agents, when employed, are generally used i~
20 amounts between about 100 and about 2000 ppm based on the a,~ethylenically unsat ~i~a~ed monomer.
It is dffirable to obtain a level of conversion of monomer to polymer of at least about 95 percent conversion. The polymerization may be carried out using acid monomers that are not neutralized or that have been neutralized or partially neutralized prior to the 25 polymerization. Neutralization is conveniently achieved by contacting the aqueous monomer with an amount of basic material sufficient to neutralize benNeen about 20 and about 95 percent of the acid groups present in the acid monomers. Preferably, the amount of basic material will be sufficient to neutralize between about 40 percent and 85 percent, and most preferably between about 55 percent and about 75 percent of the acid groups present in the 30 acid monomers. When pr~neutralizing the monomer solution, it is important to control the neutralizationconditionssothattheheatof neutralizationdoesnotcausethepremature polymerization of the monomer mixture. The neutralization is advantageously carried out at temperatures below about 40C, preferably at temperatures below about 35C.
Compounds which are useful to neutralize the acid groups of the monomer are 35 typically those which will sufficiently neutralize the acid groups without having a detrimental effect on the polymerization process. Such compounds include alkali metal hydroxides, alkali meta! carbonates and bicarbonates. Preferably, the material used to neutralize the monomer is sodium or potassium hydroxide or sodium carbonate or potassium carbonate. In determining WO 93/05080 2 116 0 3 ~ PCI /US92/07611 the desired degree of neutralization, care must be taken to ensure that the pH of the resulting crosslinked absorbent polymer, which will be contacted with or dispersed in an aqueous fluid to be absorbed, is main~ined in a range appropriate for the applications for which the polymer is intended. Alternatively, the polyrnerization may be carried out employing 5 unneutralized monomers and thereafter neutralizing, as is known in the art.
Conveniently, a conventional vinyl addition polymerization initiator is used in the polymerization of the water-soluble monomers and the crosslinking agent A free radical polymeri~ation initiator which is sufficiently soluble in the monomer solution to initiate polymerization is preferred. For example, water soluble peroxides such as potassium 10 persulfate, ammonium persulfate, sodium persulfate, and other alkali-metal persulfates, hydrogen peroxide and water soluble azo-compounds such as 2,2'-azobis (2-amidinopropane-HCL) may be used. Some of these initiators, such as hydrogen peroxide can be combined with reducing substances such as sulfites or amines to form known redox type initiators. The amount of initiator used may range from about 0.01 to about 1.0 weight percent, preferably 15 about 0.01 to about O.S weight percent, based on the total weight of a,~ethylenically unsaturated monomer reactants.
The crosslinking agent is present in an amount effective to crosslink the water-soluble polymer. The preferred amount of crosslinking agent is determined by the desired degree of absorption capacity and the dffired strength to retain the absorbed fluid. Typically, 20 the crosslinking agent is used in amounts ranging from about O.OOûS to about 5 parts by weight per 100 parts byweight of ~,~ethylenically unsaturated monomer used. More pre~erably, the amount ranges from about 0.1 to about 1 part by weight per 100 parts by weight of the ~
ethylenically unsaturated monomer. If an amount over about 5 parts by weight of crosslinking agentper100partsisused,theresultingpolymerhastoohighacrosslinkingdensityand 25 exhibits a reduced absorption capacity and increased strength to retain the absorbed fluid. If the crosslinking agent is used in an amount lessthan about 0.0005 part by weigh~ per 100 parts, the polymer hastoo low a crosslinking density, and when contacted with the fluid to be absorbed becomes sticky and exhibits a lower initial absorption rate.
While the crosslinking agent will typically be soluble in the aqueous solution of 30 the a,~ethylenically unsaturated monomer, the crosslinking agent may be merely dispersible in such a solution, without negative implications. The use of such dispersing agents is disclosed in U.S. Patent No. 4,833,222, the relevant portions of which are incorporated herei n by reference.
Suitable dispersing agents include carboxymethyl cellulose suspending aids, methyl cellulose, hydroxypropyl cellulose, and polyvinyl alcohol. Such dispersing agents are typically provided at 35 a concentration between about 0.005 and about 0 1 weight percent, based on the total weight of ~,~ethyîenical!y unsaturated monomer reactants.
Suitable crosslinking agents are those which, when used, increase the sixty minute 0.3 psi AUL of the polymer, without significantly decreasing (if not increasing) the centrifuge WO 93/05080 ~03~ PC~/US92/07611 capacity. Preferred crosslinking agents include trimethylolpropanetriacrylate, butyleneglycoldiacrylate, ethyleneglycoldimethacrylate, ethylenebisacrylamide and diallylcarbonate, methylenebisacrylamide, bis(acrylamido)acetic acid and its salts, allyl acrylate, allylmethacrylate and difunctional monovinyl and monoallyl esters and amides. Especially 5 preferred crosslinking agents include methylenebisacrylamide, bis(acrylamido)acetic acid and its salts, allyl acrylate, allylmethacrylate and, esters or amides having both a vinyl and an allyl functionality.
In a preferred embodiment for making polymers useful in the practice of this invention, an aqueous solution of the a,~ethyleni~cally unsaturated monomer in the partially 10 neutralized form, the crosslinking agent, the initiator and a grafting polymer substrate, if desired, is prepared.
The polymeri~ation of the mixture may be initiated by elevating the temperature of the mixture containing the initiator or by using a redox-type initiator as dffcribed above.
Generaliy, the tcmpcrature at whieh polymerization will begin ranges from about 20C to 15 about 45C. The temperature at which the polymerization is carried out is highly dependent on the type of monomers uscd and the specific initiator system employed. Preferably, the maximum temperatu-e of polymerization ranges from about 50C to about 100C, most pffferably from about 60C to about 90C. The method by which the temperature of the , ~ ~ polymerization is co*trolled is not critical so long as sufficient cooling is prffent to remove the 20 heat~which is generated d:uring~the polymerization.
After polymerization, the gel is dried in, e.g., a circulating air oven, to a moisture content less than about 10 percent, preferably less then about 5 percent, most preferably about 2 to about 5 percent, expressed on a dried polymer basis, to form a ~dried polymer" .
It has advantageously been discovered that when the dricd polymer is further 25 ~ heated at elevated temperatures, the sixty minute 0.3 psi AUL increases, the ccntrifuge capacity andthemodulusdonotsignificantlydecrease(iftheydonotincrease),andthe 16hour extractables levels remain low ~if they do not decrease). Such heat treatment is preferably carried out at temperatures between about 165C and about 230C, more preferably bet~veen about 1 90C and 21 0C.
30 . The time for heattreatment can best be determined empirically, with such factors as humidity within the heat treatment apparatus, water content of the dried polymer, rate of temperature elevation, and type of crosslinking agent employed affecting the time. However, it has been found that when trimethylolpropanetriacrylate, butyleneglycoldiacrylate, ethyleneglycoldimethacrylate, ethylenebisacrylamide or diallylcarbonate are used as 35 crosslinking agents, absorptive properties improve with additional heating to a point, after which further heating c?uses a decrease in absorptive properties. In contrast, it has been ~ound i that when methylenebisacrylamide, bis(acrylamido)acetic acid or one of its salts, ally! acrylate, allylmethacrylate, or an ester or amide havlng both a vinyl and an allyl functionality are used as - ", -i,. ~ , , WO~3/05080 2~ 3 5 PCI/US92/07611 crosslinking agents employed, absorptive properties do not appear to degrade upon heating beyond the point at which a maximum increase in absorptive properties has occurred. .
The method of heattreating is not critical; for example, forced air ovens, fluidi~ed bed heaters, heated screw conveyors and the like may be successfully employed. If S desired, the heat treated polymer may be remoisturized for ease in handling.
Increases in shear modulus, sixty minute AUL, free swell capacity, and centrifuge capacity can simultaneously be obtained at lower temperatures (and at higher temperatures for shorter heating times) by incorporating pendant ester groups into the polymer. While not wishing to be bound by theory, it is believed that the incorporation of pendant ester groups 10 intothepolymerincreasesthemodulusandAULofthepolymerbyfacilitatingtheformationof inter- or intramolecular anydride crosslinks, i.e., crosslinks wherein ester and acid functionalities combine with the release of an alcohol. While such crosslinks may be achieved by mere heating (with the release of water, rather than an alcohol), it is believed that such crosslinks are formed more easily by heating an ester with an acid group, since an alkoxy group 15 is a better leaving group than a hydrogen group.
Pendant ffter groups may be incorporated into the polymer by copolymeri~ing acrylic acid and/or sodium acrylate with an C1 to C6 acrylate or a methacrylate. Preferred acrylates and methacrylates are monotunctional crosslinking agents, and include ethyl, propyl, isopropyl, butyl, and isobutyl acrylates or methacrylatff. The acrylate or methacrylate wiU
20 preferably be soluble in the aqueous solution of the a,~ethylenically unsaturated monomer.
In the alternative, the acrylate or methacrylate will be merely dispersible in such a solution.
Suitable dispersing agents and concentrations are as set forth above with respect to agents suitable to disperse the c-osslinking agent. The use of dispersing agents will enable the use of larger molecular weight acrylates and methacrylates, e.g., pentyl and hexyl acrylates and 25 mhacrylates.
- To achieve the benefits such pendant es~er groups afford, the monofunctional acrylate andlor methacrylate will preferably be provided in the range of between about 0.5 and about 10 percent, and more preferably between about 1 and about 5 percent, based on the weight of the monomer reactan~s.
The following examples are provided for the purpose of explanation rather than limitation. Unless otherwise indicated, sixty minute 0.3 psi AUL was measured as described in European Patent Application EP 339,461- A1; centrifuge capacity was determir,ed as described inU.S.Patent4,286,082exceptthatanabsorptiontimeof30minutesratherthan3to5 minuteswasemployed; andtheextractablesand modulusweredeterrnined asdescribed in U.S. Reissue Patent 32,649, with the relevant portions of each of the cited references bei ng incorporated herein by reference.

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WO93/05080 ?,~ 6035 PCl`/US92/07611 Example l: General Heat Treatment for the Reduction of Extractables in 55 Percent Neutralized Polvacrvlic Acid Two point one (2.1) grams of trimethylolpropane triacrylate, 0.2 grams of VERSENEX 80 chelating agent (available from the Dow Chemical Company), and 1.5 grams of a S S percent aqueous solution of polyvinyl alco~ol such asAlRVOL V-205, were added to 300 grams of acrylic acid. The resultant solution was neutralized to 55 percent by the addition of 121.8 grams of sodium carbonate in 792.5 grams of water and diluted to a total weight of 1168 grams with water. The neutrali2ed solution was purged for 1 hour with nitrogen. he polymerization was initiated by adding 0.75 grams of sodium persulfate, 0.15 grams of WAKO
10 V-50 (2,2' azobis (2-amidino-propane) dihydrochloride), 0.13 grams of a 70% aqueous solution of t-butyl hydroperoxide, and 4.5 ML of a 15% aqueous solution of sodium thiosulfate. After the reaction temperature peaked, the temperature was decreased to 60CC, which temperature was maintained an additional three hours. The gel was dried at 1 00C for 6 hours and then ground through a knife grinder. Portions of the material were heated in a forced air oven at 15 various temperatures The analysis is set forth in the following Tables I and ll.
TABLE I
16 Hour Extractables Levels (percent) after Heating at 200C
for Indicated Number of Minutes 8.7 7.6 ~ 6.5 - 6.1 ~ 3 ~ 10 2 ~ ~ ~ 7 1 TABLE II
16 Hour Extractables Levels (percent after Heating at 150C
for Indicated Number of Minutes Sample L 1 20 ~ _40 ~ 60 . 1 1 8-7 1 9-0 1 9-0 l 8.8 : -8-WO 93/05080 2 1 ~l 6 0 ~ t.~ PCT/US92/076~1 Example 2 The Effect of Heat Treatment at Various Deqrees of Neutralizatlon on Percent Extractables A gel was prepared in accordance with Example 1. Various degr~es of neutralization were achieved by neutralizing the monomer solution with varying amounts of S sodium carbonate solution. The gel was dried and heat treated i n accordance with the procedure set forth in Example 1. The degree of neutralization, and the 16 hour extractables levels of the polymer upon heating at 200C for one hour are set forth in the following Table lll.

TABLE III: 16 Hour Extractables Levels (percent) Sa ple ~eu~r-lizod InL~Ial Tr-ated 1 0 7.7 6.2 . _ _ 2 40 4 . 3 2 . 7 3 50 5.8 2.8 Example 3: The Effect of Heat Treatment on AbsorPtive Properties Fivesamples of trimethylolpropanetriacrylate crosslinked Dow DRYTECH
20 superabsorbent polymer were obtai ned from the Dow DRYTECH superabsorbent polymer plant in Midland, Michigan. The properties of such samples, prior to heat treatment, are set forth in Table IV. Such sampies were heated at 200C in a forced air oven for various times.
Yarious absorptive properties, such as modulus, free swell capacity, centrifuge capaci~y, S
minute 0.3 psi AUL, 60 minute 0.3 psi AUL, and 16 hour extractables levels, were measured.
25 Free swell capacity is determined by allowing 1.0 gram of the polymer to absorb its limit of 300 grams of a 0.9 percent saline solution in 20 minutes. The excess salt solution which is not absor~ed is weighed and subtracted from the original 300 grams ~o give the free swell capacity.
Five minute 0.3 psi AUL is measured following the pr~edure set referenced for the measurement of si~y minute 0.3 psi AUL, excepting that~he amount of fiuid pickup is 30 measured after five minutes, rather than after sixty minutes.
The data are further set forth in the following Table IV.

WO 93/05080 ~ PCl`/US92/0761 1 G~3 ~ ~ TA13LE IV
_ ., Time Shear FreeCentrifuge S min 60 16 hour Sample (20aOItn~ (dynes/cm2) swell Capac~ty AUL (AM9lu9) Exa(tbOr/ae)st-~ ~ ., 1 0 40,400 35.5 32.7 3.1 13.8 tO.6 8 32.4 7.4 26.1 16 49,800 34.0 31.6 14.1 28.0 32 4g.800 34.9 30.4 15.5 26.8 64 52,300 36.3 28.0 13.9 26.3 10 _ __ . .
2 0 60,400 33.0 28.3 5.6 26.4 7.7 8 28.6 12.6 27 7 16 66,400 30.0 26.5 25.0 27.2 32 72,400 30.3 25.4 18.1 26.5 64 78,700 32.4 24.7 21.6 26.5 5.9 75,700 ~8 0 _ _ 3 0 43.6 2.6 7.5 8 35.8 4.4 12.2 16 32.9 5.5 16 32 30.2 7.6 21.2 ,, , . ~ ~.

4 16 52,100 32.6 29.8 . 27.5 32 69,700 28.1 24.8 24.7 64 77,300 25.5 22.9 22.6 5.1 _ _ _ S 0 27,700 37.1 37.7 .. -12.2 6.9 8 35,000 36.5 37.2 16.1 16 39,300 35.3 34.4 26.5 32 41,400 33.4 29.4 26.4 64 50,600 31 26.4 24.5 5.64 _ 6 0 32.8 8.9 19.4 8 31.2 8.9 26.0 16 28.9 8.7 26.4 . 32 27 84 257 Thisdatasuggeststhat heattreatment increases a low 60 minute 0.3 psi AUL, e.g., a 60 minute 0.3 psi P~UL less than 2û, to at least 25. Such heat treatments increase the modulus, and do not significantly adversely affect the free swell capacity, the centrifuge capacity, or the 5 minute 0.3 35 psi AUL of the polymer. This suggests that heat treatment would enable the modification of a batch of polym~ - which has a non-preferred low sixty minute 0.3 psi AUL, rendering it highly usefulandsaleab,le. Thedatafurthershowsthetendencyof heattreatmenttomaintainlow 16 hour extractables levels, if not decrease them.

WO 93/05080 21~ PCI/US92/07611 Example 4; Comparisan of Different Heat Treatment TemPeratures and Times on AbsorPtive ProPerties A sample of DRYTECHTM 533 trimethyolpropanetriacrylate crosslinked superabsorbent polymer was obtained from the Dow Chemical Company (Midland, Ml). The S sample was heat treated at an indicated temperature for an indicated period of time. The temperatures and times of heat treatment, and the resultant properties of the heat-treated polymer, are set forth in Table V. The method utilized to determine one hour extractable levels is substantially as described in U.S. Patent Reissue No.32,649, the relevant portions of which are incorporated herein by reference.
TABLE V
:~ _r~

r~one ~none~ ~21.1 l 32.6 8.5 145 1623.1 33.2 3222.7 33.2 8.1 ::
; ~ 167 825.3 33.2 ` ~ ~ 1623.8 32.5 3224.4 29.4 8.0 ~(: .
: 175 823.7 33.2 1626.0 31.7 - 3225.7 3û.9 7.7 198 26;5 29.7 ~ ~
1627.3 29.2 7.7 ~:, }his data suggests that for trimethylolpropane triacrylate crosslinked polymers, ~ .
heat treatment temperatures greater than about 165C are preferred, with heat treatment temperatures greater than about 175C being especially preferred.
Example S: The Effect of Heat Treatment on Co~olYmers of Neutralized Acrvlic Acid and Methvl Acrvlate To 300 grams of acrylic acid were added 2.1 grams of trime~hylolpropanetriacrylate,0.2 grams of VERSENEX 80 chelating agent (available from the Dow Chemical Company),1.5 grams of a 5 percent aqueous solution of AIRVOL V-205 polyvinyl alcohol (available from Air Products Corporation ), and 15 grams of methyl acrylate. The 35 sol ution was neutral ized by the addit;on of a sol ution of 121.8 grams of sodi um carbonate i n 792.5~ grams of water. The neutrali~ed solution wasdiluted to a total weight of 1168 grams with watér. The diluted solution was purged for one hour with nitrogen. To initiate polymeri2aoon~o~7sgramsofsodium~persulfate~o.1sgramsofwAKov-soazo-initiator ,~ " ~

WO93/05080 ~ 603 PCl/US92/0761I

(available from Pure Chemicals Inc.),0.09 gram~ of a 70% aqueous solution of t-butylhydroperoxide, and a reducing agent (such as sodium thiosulfate) were added to the purged solutlon. After the reaction temperature peaked, the polymer was heated an additional 3 hours at 60C to form a gel . The gel was dried at 100C for 6 hours, and was 5 ground through a knife grinder. Portions of the ground material were heated in a forced air oven at various temperatures for various periods of time. The resultant data is set forth in the following Tables Vl and Vll.
TABLE V I

¦ # MeehYI lSO'C ModulUs ~ C~p cjey C p ciey ~ psi ~ (%) 0~ 46,800 30.0 29.2 26.6 3.76 853,600 31.3 29.9 26.7 1653,700 31.0 30.1 27.4 3257,200 31.7 30.0 27.5 U53,900 3I.9 29.7 27 6 3.93 046,000 29.6 29.4 27.0 3.37 849,700 31.0 30.4 27.7 1650,900 31.5 30.4 27.8 .
~: 3252,200 31.1 30.6 28.6 6455;600 30.0 30 5 28 3 3.2 044,100 32.0 31.2 26.8 : 3.93 846,700 32.8 31.5 26.5 ~-- ~ 1648,500 33.0 31.7 28.4 3249,600 33.5 31.7 28.6 : _ 6453,500 33 3 31 5 29 6 3.76 , ' 1 ~ ' i I

~, ~ ' ' , , .

WO 93/05080 ~ 3 3 5 PCI`/US92/07611 .TABLE V I I

Mrey%tlhayte ¦ T~me at ¦ Modulus ¦ CaSpa ¦ cafpuagce~ty ¦ M ~ ExtractaUe~

O O 46,800 30.0 29.2 26.6 3.76 8 55,900 32 0 29.3 27.6 16 64,000 29.2 27.2 26.1 32 76,400 25.7 23.4 23.8 64 87,000 23.3 20.6 21 3 2.36 1 O 46,000 29.6 29.4 27.0 3.37 8 56,800 ~1.5 30.0 28.3 16 62,400 29.0 27.5 26.7 32 79,100 27.3 23.5 24.1 64 84,800 23.9 ~1.2 22.5 2.36 O 44,100 32.0 31.2 26.8 3.93 ; ~ 8 5 ,800 33.0 30.7 29.2 16 60,000 31.0 28.6 27.7 32 75,100 28.0 25.1 25.0 _ 64 81,600 26 5 22 2 23 3 2.64 The~data of Table Vl~suggests that heat treating a polymer containing methyl acrylate at 150C
and~200 C Improves the absorptive properties of the polymer, providing po!ymers hav;ng a slxty minute 0.3~psi AUL of at least about 28.
Examdle 6:~ Heat Treatment of Polvmers PreDared with Preferred Crosslinkina Aaents 25 ~ The reactor consisted of a 21iter glass resin reaction kettle bottom, a stainless steèl agitatof assembly and a high torque, elenric stirring-motor with gear-reducers. It had a - ~ ~ glass jxketto allow for either heating or cooling of the react~r contents utili~ing a separate, water-circulating bath. An O-ring, which fit into grooves in the glass joint of the b~ttom and the steel top, sealed the reanor to prevent gas leaks.
Monomer mix was prepared by adding the following reagents to a beaker in the follow~ng order: 300.00 g acrylic acid, the amount of the desired crosslinker indicated in Table Vlll,~O 75 9 VERSENEX 80 (Trademark of The Dow Chemical Company) chelating agent, 1.50 g of a 5 pe~rc~ent solution of VINO~ 205 (Trademark of Air Products Company) polyvinyl alcohol in water ~if necessary to help disperse a crosslinking agent not fully soluble in the monomer m~
and300mLwater Totheresultlngsolutionwasaddedasolutionofl~4.0gsodiumcarbonate dissolved in 515 mL of water Monome- mix was loaded to the reactor through the loading tube thal had a smali ball valve fo- flow ~ont-ol Aher no~omer~loading was complete, nitrogen gas was bubbl~d~lnto the rnonomer so~ut~on ~to rer~ove dissoi ~ea oxygen After the aes~ red sparg~r~g 2~5o~
WO 93/05080 PCl /lJS92/0761 1 time the free radical initiator solutions were added through the dip-tube opening in the reanortop~ bysyringe: 4.8 mL of a 10 percent solution of sodium persulfate in water, followed by1.2mLofa10percentsolutionofsodiumerythorbateinwater. Averysli~3htnitrogen pressure was maintained in the reactor during initiator addition to prevent air from entering 5 the reactor through the s)pening.
Typical Monomer Mix Recipe value 300.00 9 acrylic acid 24.8 wt.%
crosslinking agent varies 0.75 g VERSENEX 80 chelating agent 2500 ppm BOM*
1.50 g of a S~ solution of Vinol 205 polyvinylalcohol in water 250 ppm BOM
300 mL water 144.0 9 sodium carbonate dissolved in 65.2 mol~
515 mL of water (21.9%) neut.
4.8 mL of a 10% solution of sodium persulfate in water 1600 ppm BOM
1.2 mL of a 10% solution of sodium erythorbate 400 ppm BOM
*Based on monomer Within a few minutes, an exothermic polymerization began. The temperiture o~
20 the reaction mixture rose to 80C over a 40 minute period, and was then decreased to 65C and maintained there for three hours by means of the circulating bath. The crumbly, rubber-like gei product was removed from the reactor, spread onto a nylon screen and dried in a 1 00C
oven for 16 hours. The dried polymer was cooled and then pulverized in a food blender. The amountofextractables,centrifugedswellingcapacity,swollenmodulus,andtheAULwere 25 rneasured on these sampies.
The polymer powder thus made was placed in a flat-bottomed, glass crystallizing dish. The dish was put into a forced air oven that was maintained at 200C to achieve a heat-treatment separate from the mild drying step. Samples were withdrawn at various times and their properties were determined. The mass-loss upon heating for many of the polymers was 30 also measured. In these instances, individual dishes were prepared for treatment. The change in mass was measured as a function of heating time and the percentage loss was calculated.
The mass loss at 200C averaged abaut 6 percent for samples initially dried at 1 00C for 16 hours. ~Thermal analysis (thermogravimetry coupled with gas chromatography and mass spectrometry) showed this mass was water. Traces of carbon dioxide were detected in the 35 thermal analysis at higher temperatures.
The differences between products made with different crosslinking agents became apparent after the heat-treatment of the dried polymer. In Table Vlll below are given WO 93/05080 2 1 1 6 Q 3 ~ PCr/US92/07~1 1 .
data on the physical properties of the polymers before and after heating at 200C for 1~0 minutes.

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_ ~ ~ ~ ~, W . " ~ V ~ ~U V ,... 0 , WO 93/05080 21 1 6 0 3 ~ PCI/US92/07611 Example 7: Comc~arison of Different Heat Treatment TemPeratures and Times on AbsorPtive Pro~erties when a Preferred Crosslinkina Aqent is EmPloved A mixture of 15 grams VINOL V-205 (available from Air Products Corp.) in 250 5 gramsdeionized waterwas heated to form a solution. In a separate flask, 300 grams acrylic acid, 2.1 grams allylmethacrylate, 0.75 grams VERSENEX 80 chelating agent (available from the Dow Chemical Company) and 70 grams deionized water were mixed to form a clear monomer mixture. The clea- monomer mixture was neutralized with a solution of 144 grams sodium carbonate in 450 gramsdeionized water to form a neutralized monomer mixture. The10 neutralized monomer mixture was charged into a 2 liter reactor, which was then purged with nitrogen for 45 minutes. The polymerization was initiated with a mixture of 4.8 mL of a 10%
aqueoussolution of sodium persulfate, 0.6 mL of a 10% aqueous solution of sodiumerythorbate, and 0.5 mL of a 30% aqueous solution of hydrogen peroxide. Within a few minutes, an exothermic polymerization began. The temperature of the reaction mixture rose 15 to80~overa40minuteperiod,andwasthendecreasedto65Candmaintainedtherefor three hours by means of a circulating bath. The crumbly, rubber-like gel product was removed from the reactor, spread onto a nylon screen and dried in a 100C oven for 16 hours. The dried - ~ - polymer was cooled and then pulverized in a food blender. The centrifuge capacity and the sixty minute 0.3 psi AUL were measured on these samples. ~ ~
The polymer powder thus made was placed in a flat-bottomed, glass crystallizing dish. The dish was put into a forced air own that was maintained at the indicated temperature ~or the indicated time to achieve a heat-treatment separate from the mild drying step. Samples were withd~awn at various timff and their properties were determined. ~he data are set forth in the following Table IX.
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WOg3/05080 PCT/US92/07611 6~3~
TA~LE IX

Temperature Time for C t f Sixty of Heat Heat en rl uge minute 0.3 Treatment Treatment apaci y psi AUL
(C) (minutes) (g~g) (g/g) none none 32.3 17.3 170 5 33.1 19.8 32.2 24.8 31.8 29.4 180 5 34.8 20.3 34.2 28.4 34.1 29.4 190 5 35.2 22.1 35~4 28.4 34.2 29.4 ~, ~
200 5 32.0 18.9 32.4 28.6 Ç0 31 9 29.8 215 _ _ 30 _ 33.3 32.5 .
230 ~ 30 32.5 ~ ~ 32~.0 , ExamDle8: Graftina To illustrate the effect obtained when grafting polymers are.included in the allyl -~ ~: ,:: . : . . .
methacrylate cro#linked polymerization recipe, the followlng gel polymer~atlons were carrled - - 25 out in a 30gallon reactor using the process set forth in Example 6:
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W093/05080 2116 0 3 ~J~ PCT/US92/07611 Recipe Grafted Grafted Acrylic acid, pounds 46 50 Allyl methacrylate, grams 108 182 Poly~vinyl alcohol), pounds 0 2.5 VERSENEX 80 chelating agent, grams 52 30 Soda ash, pounds 22 23 Sodium Persulfate, grams 33.4 38.5 30~ Hydrogen Peroxide, grams34.8 40 Sodium erythorbate, grams 4.2 9.0 Water, total pounds added 107 - 125.5 Heat-treating air temperature 170 200 - (30 minutes), C
RESULTS
Centrifuge capacity, g/g 30.6 32.0 Swollen modulus, dynes/cm245,600 45,600 ~ extractable polyacrylate 4.8 ND
AUL, g/g at 0.3 psi for 60 min.30.7 29.6 The above results show that the presence of poly(vinyl alcohol) as a graft substrate resulted in improved centrifuge capacity with only a slight decrease in absorbency under load and equivalent modulus to the product obtained using a non-grafted polymer.
Whilethesubject invention has been described largely in terms of super bsorbent polymers prepared by a gel polymerization process, it is expected that superabsorbent polymers prepared by well-known suspension polymerization polymerizations could benefit equally from the disclosed heat treatments.
Various modifications may be made in the present invention without departing 25 from the spirit or scope thereof aswill be readily apparent to those skilled in the art.

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Claims (23)

WHAT IS CLAIMED IS:

1. A process for producing a superabsorbent polymer comprising polymerizing at least one .alpha.,.beta.-ethylenically unsaturated monomer in an aqueous phase in the presence of a suitable initiator and a crosslinking agent and thereafter drying the polymer at a temperature between about 100°C and about 180°C to form a dried polymer, characterized in that the process further comprises heating the dried polymer in the absence of a surface crosslinking agent to a temperature between about 165°C and about 230°C for a time sufficient to produce a heat-treated polymer having a sixty minute 0.3 psi AUL of at least about 25 grams/gram.

2. The process of Claim 1, wherein the .alpha.,.beta.-ethylenically unsaturated monomer is selected from the group consisting of acrylic acid, methacrylic acid, and alkali metal salts of said acids.

3. The process of Claim 1, wherein said crosslinking agent is selected from the group consisting of trimethylolpropanetriacrylate, butyleneglycoldiacrylate, ethyleneglycoldimethacrylate, ethylenebisacrylamide, diallylcarbonate, methylenebisacrylamide, bis(acrylamido)acetic acid and its salts, allylacrylate, allylmethacrylate and esters or amides having both a vinyl and an allyl functionality.

4. The process of any of Claims 1, 2, or 3, wherein said crosslinking agent is employed in an amount of from about 0.0005 to about 5 parts by weight based on 100 parts by weight of said .alpha.,.beta.-ethylenically unsaturated monomer.

5. The process of any of Claims 1, 2, or 3, wherein said crosslinking agent is employed in an amount of from about 0.1 to about 1 part by weight based on 100 parts by weight of said .alpha.,.beta.-ethylenically unsaturated monomer.

6. The process of any of Claims 1, 2, or 3, wherein said heat-treated polymer isfurther characterized by a centrifuge capacity of at least about 26 grams/gram.

7. The process of any of Claims 1, 2, or 3, wherein said heat-treated polymer isfurther characterized by a 16 hour extractable level less than about 8 percent.

8. The process of any of Claims 1, 2, or 3, wherein said dried polymer is characterized by a centrifuge capacity greater than about 30 grams/gram and a sixty minute 0.3 psi AUL less than about 25 grams/gram, and wherein said heating increases the sixty minute 0.3 psi AUL to at least about 25 grams/gram, without decreasing the centrifuge capacity to less than about 29 grams/gram.

9. The process of any of Claims 1, 2, or 3, wherein said .alpha.,.beta.-ethylenically unsaturated monomer is polymerized with between 1 and 5 percent by weight of a C1 to C6 acrylate or methacrylate based on the weight of the .alpha.,.beta.-ethylenically unsaturated monomer, wherein said dried polymer is characterized by a sixty minute 0.3 psi AUL of at least about 26, and wherein said heat-treated polymer is characterized by a sixty minute AUL of at least about 28.

10. The process of Claim 9, wherein said C1 to C6 acrylate or methacrylate is methyl acrylate or methyl methacrylate.

11. The process of Claim 9, wherein said heat-treated polymer is further characterized by a 16 hour extractable level less than about 4 percent.

12. The process of Claim 1, wherein said crosslinking agent is selected from thegroup consisting of methylenebisacrylamide, bis(acrylamido)acetic acid and its salts, allyl acrylate, allylmethacrylate and difunctional monovinyl and monoallyl esters and amides.

13. The process of Claim 12, wherein said crosslinking agent is allyl methacrylate.

14. The process of Claim 12, wherein said crosslinking agent is methylenebisacrylamide.

15. The process of Claim 12, wherein said crosslinking agent is bis(acrylamido)acetic acid or its salts.

16. The process of Claim 12, wherein said heat-treated polymer is characterized by a 60 minute 0.3 psi AUL of at least about 30 grams/gram.

17. The process of Claim 12, wherein said heat-treated polymer is characterized by a 60 minute 0.3 psi AUL of at least about 30 grams/gram, a centrifuge capacity of at least about 30 grams/gram, and a 16 hour extractables level less than about 5 percent.

18. The process of Claim 12, wherein said heat-treated polymer is characterized by a 60 minute 0.3 psi AUL of at least about 30 grams/gram, a centrifuge capacity of at least about 32 grams/gram, and a 16 hour extractables level less than about 4 percent.

19. The process of any of Claims 12, 16, 17, or 18, wherein said dried polymer is characterized by a 60 minute 0.3 psi AUL of at least about 25 grams/gram.

20. A superabsorbent polymer comprising polymerized moieties of at least one .alpha.,.beta.-ethylenically unsaturated monomer and at least one crosslinking agent, said polymer being characterized by a 60 minute 0.3 psi AUL of at least about 30 grams/gram, a centrifuge capacity of at least about 32 grams/gram, and a 16 hour extractables level less than about 4 percent

21. A superabsorbent polymer comprising polymerized moieties of at least one .alpha.,.beta.-ethylenically unsaturated monomer and at least one crosslinking agent, wherein said crosslinking agent is selected from the group consisting of methylenebisacrylamide, bis(acrylamido)-acetic acid and its salt, allyl acrylate, allylmethacrylate and difunctional monovinyl and monoallyl esters and amides.

22. A method of using the superabsorbent polymer of either Claim 20 or 21 comprising retaining said polymer in conjunction with hydrophilic fibers in an absorbent structure.

23. The method of claim 24, wherein said absorbent structure is further retained within a disposable diaper, a sanitary napkin, or an incontinence device.