A superabsorbent polymer having improved absorbency properties

Abstract: Super-absorbent polymers having superior dryness properties when incorporated into absorbent articles are made from acrylic acid and crosslinking agent polymerized under controlled conditions.

Description: Background of the Invention The field of art to which this invention is directed is hydrogel-forming polymer compositions made from crosslinked polyacrylic acid. Water-insoluble hydrogel-forming polymers which are capable of absorbing large quantities of water and aqueous fluid are well know compositions. Such polymers, which are known as superabsorbent polymers, are lightly crosslinked acid functional polymers which swell in water of aqueous fluids but do not dissolve in the fluids. Superabsorbent polymers have been found to be particularly useful in diapers, feminine hygiene articles and surgical dressings. Descriptions of superabsorbent polymers and their uses are found in U.S. Patent Nos. 3,669,103 and 3,670,731. U.S. Patent No. 4,654,039 (Reissue 32,649) is directed to hydrogel forming polymer compositions which are described as substantially water-insoluble, slightly cross-linked, partially neutralized polymers, derived from unsaturated polymerizable, acid group-containing monomers and cross-liking agents. Such polymers are made by polymerizing the acid monomer and the crosslinking monomer in water using a redox initiator system, followed by partially neutralizing the acid groups with sodium hydroxide, then drying the polymer and pulverizing it to a powder. British Patent No. 2,119,384 discloses superabsorbent polymers made by polymerizing the water acrylic acid in admixture with sodium acrylate and a crosslinking monomer using a persulfate initiator, followed by drying and then heating with a crosslinking agent having at least two functional groups capable of reacting with carboxyl groups. In U.S. Patent No. 4,497,930, superabsorbent polymers are made by polymerizing acrylic acid in an inverse emulsion process followed by crosslinking the polymer with a diepoxide compound. According to U.S. Patent No. 4,295,987, superabsorbent polymers are made by polymerizing acrylic acid and a multi-functional acrylate monomer in water using a persulfate initiator followed by neutralizing the acid groups with caustic and then blending in a divalent cation salt, e.g., zinc acetate, for additional crosslinking. Numerous other patents disclose superabsorbent polymers and their uses, such as U.S. 4,076,663, 4,552,938, 4,507,438, 4,535,098, 4,820,773 and European Patent Application 189,163. Further prior art is DE-A-3 801 633, DE-A-3 617 311, US-A-4 826 917 and US-A-4 351 922. Many improvements have been made in the performance and properties of superabsorbent polymers over the years, such as in gel strength and absorbing capacity. However, such superabsorbent polymers do not have a balance of properties. Typically polymers with high elasticity modulus exhibit reduced absorbing capacity, which results, for example, in reduced diaper dryness. Polymers with increased absorbing capacity exhibit low absorbency under pressure and a reduced elasticity modulus which also reduces diaper dryness. European Patent Application 339,461 describes absorbent products made with superabsorbent polymers having the ability to swell against pressure. There is a need for a superabsorbent polymer having a balance of properties which, when the polymer is used in a diaper, results in improved diaper dryness and leakage. Summary of Invention This invention is directed to superabsorbent polymers, i.e., hydrogel forming polymers. In one aspect, this invention pertains to an improved superabsorbent polymer comprising a copolymer of acrylic acid, a crosslinking monomer and a water-soluble hydroxyl containing polymer wherein the crosslinking monomer contains at least 2 ethylenically unsaturated groups per molecule and is present in the amount of 0.005 to 1.0 mole percent based on the moles of acrylic acid and the water-soluble hydroxyl containing polymer is present in the amount of 0 to 10 weight percent based on the weight of acrylic acid and water soluble polymer, wherein said copolymer is neutralized with an aqueous base in the amount of 50 to 100 percent of the carboxylic acid groups and is post-crosslinked with a crosslinking compound in the amount of up to 1.8 mole percent based on the moles of acrylic acid, wherein said superabsorbent polymer has an Absorbency under Pressure of 30 g/g minimum, a Reabsorbing Capacity of 40 g/g minimum, an Elasticity modulus 0.9 N/cm 2 (of 9.0 x 10 4 dynes/cm 2 ) minimum, and a Recovery Ratio of 85 percent minimum. In another aspect, this invention pertains to articles made with superabsorbent polymers. One embodiment is an absorbent article having dispersed therein the polymer of the invention. In a preferred embodiment the absorbent polymer is present from 5-100 % by weight based on fiber. The superabsorbent polymer of this invention is made by forming an agueous solution of acrylic acid, a crosslinking monomer which is a polyethylenically unsaturated polymerizable monomer and, optionally, a hydroxy containing polymer, adding to the aqueous solution a redox initiator system and, optionally, a thermal free radical initiator, allowing the temperature to rise to a peak temperature under adiabatic conditions, holding at the peak temperature to reduce the free monomer content to less than 1000 ppm, neutralizing 50 to 100 percent of the acid groups of the resulting polymer with a base, and optionally adding a multi-functional compound having at least two groups capable of forming covalent bonds with carboxylic acid groups, drying the polymer to a moisture content below about 10 weight percent, and pulverizing the dried polymer to a powder. The superabsorbent polymer of this invention exhibits: absorbency under pressure of 30 g/g minimum; reabsorbing capacity of 40 g/g minimum; elasticity modulus of 0,9 N/cm 2 (9.0 x 10 4 ) dynes/cm 2 minimum, recovery ratio of 85 percent minimum. Articles made with the superabsorbent polymer of this invention exhibits dryness values of at least 40. Description of the Invention The primary monomer used to make the superabsorbent polymer of this invention is acrylic acid. The crosslinking monomer copolymerized with acrylic acid is a polyethylenically unsaturated polymerizable monomer having at least two polymerizable groups per molecule and which is soluble in water or in an acrylic acid-water solution. Examples of polymerizable groups are acrylic groups, methacrylic groups, allyl groups and vinyl groups. Crosslinking monomers include polyacrylic esters of polyols, polymethacrylic esters of polyols, polyallyl amines, polyallyl ethers, polyacrylamido compounds, polymethacrylamido compounds and divinyl-compounds. Specific examples of crosslinking monomers are tetrallyloxyethane, N,N'-methylene bisacrylamide, N,N'-methylene bismethacrylamide, triallylamine, trimethylol propane triacrylate, glycerol propoxy triacrylate and divinyl benzene. Preferred crosslinking monomers are those which contain at least two allyl groups, most preferably four allyl groups. A most preferred monomer is tetraallyloxyethane. Optional components used in making the superabsorbent polymers of this invention are water soluble hydroxy containing polymers, such as polysaccharides and vinyl or acrylic polymers. Examples of water soluble polysaccharides are starches, water soluble celluloses and polygalactommans. Suitable starches include the natural starches, such as sweet potato starch, potato starch, wheat starch, corn starch, rice starch and tapioca starch. Processed or modified starches, such as dialdehyde starch, alkyl-etherified starch, allyl-etherified starch, oxyalkylated starch, aminoethyl-etherified starch, and cyanoethyl-etherified starch are also suitable. The water-soluble celluloses useful in this invention are those obtained from such sources as wood, stems, bast and seed fluffs which are then deriviatized to form hydroxyalkyl cellulose, carboxymethyl cellulose and methyl cellulose. Suitable polyglactomannans are guar gum and locust bean gums as well as the hydroxyalkyl, carboxyalkyl, and aminoalkyl derivatives. Water soluble vinyl and acrylic polymers include polyvinyl alcohol and poly(hydroxyethyl acrylate). The preferred polysaccharide for use in this invention is natural starch, such as wheat starch, corn starch and alpha starches. In preparing the superabsorbent polymer of this invention, the acrylic acid and the water soluble polymer are reacted in the amount of 90 to 100 weight percent acrylic acid and 0 to 10 weight percent water soluble polymer, said weight percents being based on the weights of acrylic acid and water soluble polymer. The amount of polyethylenically unsaturated crosslinking monomer will vary from .005 to 1.0 mole percent based on the moles of acrylic acid, and preferably 0.01 to 0.3 mole percent. The polymerization initiators used in this invention are redox initiators and optionally thermal types. The redox initiators are used to initiate and to substantially complete the polymerization reaction. The thermal initiators if used are to ensure that the free monomer content of the product is reduced below 1000 ppm by weight. Referring to the redox initiators, any of the well known water soluble reducing agents and oxidizing agents can be used in this invention. Examples of reducing agents include such compounds as ascorbic acid, alkali metal sulfites, alkali metal bisulfites, ammonium sulfite, ammonium bisulfite, alkali metal hydrogen sulfite, ammonium hydrogen sulfite, ferrous metal salts, e.g., ferrous sulfates, sugars, aldehydes, primary and secondary alcohols. Oxidizing agents include such compounds as hydrogen peroxide, alkali metal persulfate, ammonium persulfate, alkylhydroperoxides, peresters, diacryl peroxides and silver salts. A particularly preferred redox initiator pair is ascorbic acid and hydrogen peroxide. In order to obtain superabsorbent polymers having the superior properties as claimed in this invention, the reducing agent must be used in the amounts of 2 x 10 -5 to 2.0 x 10 -2 mole percent based on moles of acrylic acid. The amount of oxidizing agent used will vary from 2.0 x 10 -3 to 1.1 mole percent, based on moles of acrylic acid. In order to ensure complete polymerization of the acrylic acid monomer and the crosslinking monomer, a thermal initiator can be included in the polymerization process. Useful thermal initiators are the "azo" initiators, i.e., compounds which contain the -N=N-structure. Any of the azo compounds which have some solubility in water or in an acrylic acid-water mixture and which have a 10 hour half life at 30°C or above can be used. Example of useful azo initiators are 2,2'-azobis(amidino propane) dihydrochloride, 4,4'-azobis(cyanovaleric acid), 4,4'-butylazo-cyanovaleric acid and 2,2'-azobis(isobutyronitrile) . A preferred azo initiator for use in this invention is 2,2'-azobis(amidinopropane) dihydrochloride. The thermal initiators are used in the amount of 0 to 1 mole percent based on the weight of acrylic acid. The polymerization process for preparing the compositions of this invention is conducted in water at a solids content, i.e., acrylic acid and water soluble polymer, of 20 to 30 weight percent, wherein the weight percent is based on the total weight of water, acrylic acid and water soluble polymer. The process used to prepare the compositions of this invention is an adiabatic reaction which is initiated at a temperature of 5°C to 20°C and which causes a rise in temperature which does not exceed a peak temperature of 90°C. Generally, the peak temperature will be 60°C to 75°C. The time required to reach peak temperature will vary depending upon the concentration of monomers, the amount of initiator and the specific initiator used as well as the size of the reaction batch and whether or not the reactor is insulated. Generally, this time will be 1 to 2 hours. After the peak temperature is reached, the temperature is held within 10°C of the peak temperature, and preferably within 5°C for 1 to 12 hours to ensure that the polymerization is complete and that the residual monomer content is below 1000 ppm. The carboxylic acid groups of the composition of this invention are then neutralized with an aqueous base in the amount of 50 to about 100 mole percent, preferably 65 to 75 mole percent. The preferred bases are the alkali metal hydroxides with the most preferred base being sodium hydroxide. Other bases, such as alkaline earth metal hydroxides, ammonium hydroxide, alkali metal, alkaline earth metal and ammonium carbonates, bicarbonates, and alcoholates and amines can also be used. The post-crosslinking compound, which optionally is added to the reactants after the neutralization step, is any compound which has at least two reactive moieties which can react with or form a bond with carboxylic acid or carboxylic acid salt groups and which is somewhat water soluble. Organic compounds which contain epoxy groups, hydroxyl groups, amine groups, phenolic groups and halohydrin groups are suitable for such use. Examples of useful post-crosslinking compounds are ethylene glycol diglycidyl ether, epichlorohydrin, glycerol, ethylene diamine and bisphenol A. A preferred compound is a polyglycidyl ether of polyol. The preferred post-crosslinking compound is ethylene glycol diglycidyl ether. The post-crosslinking compound is added to the polymer in the amount of 0 to 1.8 mole percent based on the moles of acrylic acid and preferably, when the crosslinking compound is a diglycidyl ether, in the amount of 0.02 to 0.15 mole percent, As stated hereinbefore, the polymerization reaction is an adiabatic reaction conducted without the application of external heat. The monomers, i.e., acrylic acid, the crosslinking monomer and the polysaccharide if used are dissolved in water in the reactor. Dissolved oxygen is removed from the solution by a sparge of inert gas, e.g., nitrogen, and the temperature is lowered to 5°C to 20°C. The polymerization initiators, i.e., the thermal initiator if used, the reducing agent and the oxidizing agent are added to the reactor with thorough mixing. After a short induction period, polymerization begins as indicated by a rise in temperature. Peak temperatures are reached in 1 to 3 hours. Generally, the peak temperature will be 60°C to 75°C. After the peak temperature is reached, the polymer is kept in a heat insulated container (which can be the reactor) for a time sufficient for the polymerization reaction to be completed as evidenced by the monomer content being reduced below 1000 ppm. Generally this time will be 2 to 12 hours. When the polymerization reaction is complete, the polymer gel is removed and is chopped into small particles. An aqueous base is then added to neutralize some or all of the acid groups. The gel is again chopped to ensure uniform mixing of the base with the polymer. An aqueous solution of a post-crosslinking agent can then be added and the gel is again chopped to ensure uniform mixing. The gel is then heated at a temperature of 20°C to 200°C to effect reaction of the post-crosslinking agent with the carboxylic acid groups and dry the polymer to a moisture content of less than 10 weight percent. The dried polymer is then ground and sieved to a particle size of 0,85 to 0,038 mm (20 to 400 mesh U.S. Standard Sieve), with a preferred range of 0,85 to 0,075 mm (20 - 200 mesh), and most preferably 95 percent of said particles are between 0,85 and 0,106 mm (20 and 140 mesh). In order to evaluate the polymer properties of the superabsorbent polymer, a model diaper is used. The diaper is constructed by placing a layer of pulp (14 x 37 cm) having a basis weight of 200 g/m 2 and a nonwoven backsheet. The superabsorbent polymer, 5 g, is then spread on the pulp layer as uniformly as possible. The polymer is then covered with a layer of pulp (14 x 37 cm) having a basis weight of 100 g/m 2 and a nonwoven top sheet. The testing of the model diaper is conducted as follows: 1) 50 ml of a 0.9 weight percent aqueous saline solution are poured onto the center of the diaper at 5 minute intervals until a total of 150 ml of solution have been applied; 2) at 5 minutes and at 2 hours after the final addition of saline solution to the diaper, the dryness of the diaper is evaluated by 10 trained people who touch the diaper and each rate it from 1 to 5 at each time interval. The rating description is: 5 - completely dry 4 - slightly damp 3 - damp 2 - slightly wet 1 - completely wet 3) The ratings by each person are added together for each time interval. The lowest possible rating using the above described test is 10 and the highest is 50. The dryness rating for diapers using the superabsorbent polymers of this invention is at least 40. The superabsorbent polymer of this invention is also evaluated by the following tests: 1) Absorbency Under Pressure This test determines the ability of a superabsorbent polymer to absorb under a pressure of 20 g/cm 2 (i.e., child sitting down). 2) Reabsorbing Capacity This test determines the ability of a superabsorbent polymer to absorb after it is partially hydrated (10 g/g saline concentration) and sheared a total of 50 times under a pressure of 22 g/cm 2 (i.e., slightly wet diaper being subjected to the stress of lying of sitting). 3) Elasticity Modulus This test determines the ability of the superabsorbent polymer to retain its integrity when it is totally saturated to prevent structure collapse and loss of fluid (i.e., child sitting on wet diaper). 4) Recovery Ratio This test determines the ability of the superabsorbent polymer to recover its shape after repeated applications of pressure. Absorbency under Pressure is measured using an Automatic Absorbency Tester, Model KM 350 (Kyowa Seiko Co., Ltd.) and a plastic tube having an inner diameter of 25 mm and a length of 50 mm with a wire net 0,15 mm (100 mesh) at the bottom of the tube. Samples having a mesh size of 32 - 100 = 0,5 - 0,15 mm are used in the test. A test sample, 0.100 ± .01 g, is placed in the plastic tube and is spread evenly over the wire net. A 100 g weight is placed on the sample. The plastic tube is placed in the center of the porous plate of the Tester under which is a reservoir containing saline solution (0.90 wt/vol. % NaCl aqueous solution). After 1 hour of absorption, the volume of absorbed saline solution is determined (a ml). A blank is run using the same procedure without the superabsorbent polymer (b ml). Absorbency under Pressure is equal to (a-b) x 10. Reabsorbing Capacity is determined as follows: a test sample, 1.00 g, is placed in a beaker containing 10.0 g of saline solution (0.9 wt/vol. % NaCl aqueous solution) and is left for 1 hour in order to obtain a uniform gel. The gel is then placed in a polyethylene bag which is sealed after removing the inside air. The bag containing the gel is placed on a press roller and is sheared under the following conditions: Wt. of roller: 1 kg Shearing speed by roller: 1 min./round Rolling frequency: 50 times Shear loading pressure: 22 g/cm 2 The Reabsorbing Capacity of the gel is then determined on 1.10 g of the sheared gel placed in a plastic tube using the procedure described under Absorbency under Pressure, with exception that the 100 g weight is not used. Elasticity Modulus is determined as follows: a test sample, 0.50 g, is placed in a beaker with 25.0 g of synthetic urine (aqueous solution of 0.8 weight percent NaCl, 2.0 weight percent urea, 0.08 weight percent Mg SO 4 · 7H 2 O, 0.03 weight percent CaCl 2 , said weight percents being based on weight of solution) and is left for 1 to 3 hours in order to obtain a uniform gel. A portion of the gel, 0.2 g ± 0.01 g, is then placed on a Creep Meter, Model RE 3305 (Yamaden Co., Ltd.) and the resistance to deformation is measured under a constant load of 15 g/cm 2 . The Elasticity Modulus in N/cm 2 (dynes/cm 2 ) is calculated from the ratio of the unit stress to the unit deformation. The Recovery Ratio is determined using the Creep Meter and synthetic urine described for the Elasticity Modulus test. For the Recovery Ratio test, 0.3 g of the superabsorbent polymer are added to 15 g of synthetic urine in a plastic bottle. The bottle is capped, shaken and left for 1 hours so as to obtain a uniform gel. A portion of the gelled sample, 0.1 g, is placed on the Creep Meter and the elasticity of the gel is measured during 20 strikes of the table and sample with the transducer. The Recovery Ratio is calculated as the final height of polymer/initial height of polymer x 100. The following example describes the invention in detail. Part and percentages, unless otherwise designated, are parts and percentages by weight. Example To an unheated/insulated reactor were added 800 parts of acrylic acid, 4.0 parts of tetraallyloxyethane, 300 parts of 8 percent oxidized starch in water and 2899.5 parts of water. Nitrogen was bubbled through the solution and the temperature was lowered to 10°C. When the dissolved oxygen was reduced below 1 ppm, the following initiators were added in the listed order: 0.8 parts of 2,2-azobisamidino propane dihydrochloride in 10 parts of water; 0.008 parts of ascorbic acid; 0.23 parts of 35 percent hydrogen peroxide. After an induction period of 20 minutes, polymerization began and a peak temperature of 60°C was reached in two hours. The product gel was kept in the insulated reactor for an additional 2 hours, wherein the residual monomer was reduced below 1000 ppm. To the polymer gel after being chopped in a meat grinder were added 644.38 parts of a 50 percent solution of sodium hydroxide in water. The temperature of the gel was about 60°C before the caustic addition and the temperature of the caustic solution was 38°C. The gel was again chopped to mix in the basic solution for uniform neutralization. To the gel, which had exothermed to a temperature of 88°C - 93°C, were then added 0.8 part of ethylene glycol diglycidyl ether, the temperature of the solution being 24°C. The gel was again chopped three times to obtain uniform distribution of the post crosslinking agent. The polymer was then dried to a moisture content of 10 percent on a rotary type drum dryer heated with 6895 · 10 5 Pa (100 psi) steam. The resulting flake polymer was then ground and sieved to a particle size of 0,85 - 0,038 mm [20 - 400 mesh (U.S. Standard Sieve)]. The polymer exhibited the following properties: Absorbency under pressure (AUP) - 30 g/g Reabsorbing capacity (Reab. Cap.) - 42 g/g Elasticity modulus (Elast. Mod.) - 0,9 N/cm 2 (9.3 x 10 4 dynes/cm 2 ) Recovery ratio - 86% Model diaper dryness - 5 minutes - 43 2 hours - 43 Comparative Example Polymers used in commercially available diapers were tested and compared with the polymer of this invention. The results of these tests are shown as follows: Polymer Dryness AUP Reab. Cap. Elast. Mod. 5 min. 2 hrs. g/g g/g 10 4 dyn/cm 2 N/cm 2 Luvs Deluxe 36 38 27 20 (11,8) 1,18 Ultra Pampers Plus 37 37 26 26 (13,1) 1,31 Ultra Pampers 34 36 23 28 (9,9) 0,99 Snuggums Ultra 34 38 24 29 (8,4) 0,84 Regular Pampers 31 33 25 23 (12,7) 1,27 Huggies Supertrim 31 35 21 31 (7,8) 0,78 Thick Pampers Plus 29 33 25 27 (8,6) 0,86 SANWET® IM-1500 30 38 12 38 (7,4) 0,74 SANWET® IM-1000 22 35 3 10 (4,7) 0,47 Invention 43 43 30 42 (9,3) 0,93 The superabsorbent polymers of this invention are useful in the manufacture of moisture absorbent articles, such as disposable diapers, sanitary napkins, incontinence garments, bandages, and the like. The superabsorbent compositions of this invention, due to the balance of properties described hereinabove, are particularly useful in the manufacture of thin and ultra thin disposable diapers which have excellent moisture absorbance capacity and reduced leakage. In making absorbent articles with the compositions of this invention, the superabsorbent composition is mixed with or dispersed in a porous matrix of fibers. Such matrices are made from wood pulp or fluff, cotton liners, melt blown synthetic fibers or a mixture of the meltblown fibers and the wood fluff. The synthetic fibers can be polyethylene, polypropylene, polyesters, copolymers of polyesters and polyamides. Absorbent articles, such as disposable diapers, are made with a liquid-impermeable backing material, a liquid-permeable bodyside facing material and the liquid-absorbing composite sandwiched between the backing material and the facing material. The liquid-impermeable backing material can be made from commercially available polyolefin film and the liquid-permeable facing material can be made from a commercially available nonwoven material, such as spunbonded or corded fibrous web which is wettable and capable of passing urine. The superabsorbent polymers of this invention can be used in the manufacture of absorbent articles such as those described in U.S. Patent Nos. 3,669,103; 3,670,731; 4,654,039; 4,699,823; 4,430,086; 4,973,325; 4,892,598; 4,798,603; 4,500,315; 4,596,567; 4,676,784; 4,938,756; 4,537,590; 4,935,022; 4,673,402; and European Patent Nos. 397,110; 378,247 and 339,461, The capacity of articles made with fluff and superabsorbent polymers to absorb repeated doses of liquid is determined as follows: 1) An absorbent pad comprised of about 2 g absorbent polymer and about 12 g wood pulp is formed and compressed to a density of about 0.2 g/cm 3 and a diameter of about six inches. 2) Said absorbent pad is then dosed with 30 ml of 0.9 wt. % saline solution. After 20 minutes, 25 g. of 11 cm filter paper are placed on the pad with enough weight to have a force of 6895 Pa (1 psi) applied to the pad. The filter paper is then weighed. The pad is again dosed with 30 ml of the saline solution and after 25 minutes, fresh filter paper and weight are applied as described above, and the filter paper is weighed. The test is then repeated for the third time. The rewet response is calculated as follows: wet wt. of filter paper - dry wt of filter paper = rewet response The rewet response of the superabsorbent polymer of this invention, Polymer A, and one having somewhat lower properties, Polymer B, as well as the properties of Polymers A and B are shown in the following tables. Polymer AUP Reab.Cat. Elast. Mod. Rec. Ratio A 30 42 9.3 x 10 4 86% B 29 39 8.8 x 10 4 83% Rewet Response of Polymer A Wt. Fluff Wt. Polymer 1st Dose 2nd Dose 3rd Dose 9 0.8 0 11.3 22.5 6 1.6 0 5.2 16.9 12 1.6 0 4.6 11.1 9 2.4 0 3.3 8.0 6 3.2 0 1.9 3.6 12 3.2 0 3.6 4.8 9 4.0 0 2.8 4.6 Rewet Response of Polymer B Wt. Fluff Wt. Polymer 1st Dose 2nd Dose 3rd Dose 9 0.8 0 12.5 22.4 6 1.6 0 7.3 18.4 12 1.6 0 5.8 12.4 9 2.4 0 3.8 8.8 6 3.2 0 1.8 4.8 12 3.2 0 4.5 5.9 9 4.0 0 3.0 5.6 As can be seen, both superabsorbent polymers had 0 rewet response after one dose. After the second and third doses, however, the overall rewet response for articles made with the superabsorbent polymer of the invention was superior, i.e., lower, than that of articles made with the other polymer. The principles, preferred embodiments and modes of operation of the present invention have been described in the foregoing specification.

Claims: Claims for the following Contracting States : AT, BE, CH, LI, DE, DK, FR, GB, GR, IT, LU, NL, PT, SE A superabsorbent polymer comprising a copolymer of acrylic acid, a crosslinking monomer and a water-soluble hydroxyl containing polymer wherein the crosslinking monomer contains at least 2 ethylenically unsaturated groups per molecule and is present in the amount of 0.005 to 1.0 mole percent based on the moles of acrylic acid and the water-soluble hydroxyl containing polymer is present in the amount of 0 to 10 weight percent based on the weight of acrylic acid and water soluble polymer, wherein said copolymer is neutralized with an aqueous base in the amount of 50 to 100 percent of the carboxylic acid groups and is post-crosslinked with a crosslinking compound in the amount of up to 1.8 mole percent based on the moles of acrylic acid, wherein said superabsorbent polymer has an Absorbency under Pressure of 30 g/g minimum, a Reabsorbing Capacity of 40 g/g minimum, an Elasticity modulus 0.9 N/cm 2 (of 9.0 x 10 4 dynes/cm 2 ) minimum, and a Recovery Ratio of 85 percent minimum. The polymer of claim 1 wherein the ethylenically unsaturated groups of the crosslinking monomer are allyl groups. The polymer of claim 2 wherein the crosslinking monomer is tetraallyloxyethane. The polymer of claim 3 wherein the tetraallyloxyethane is present in the amount of 0.01 to 0.3 mole percent. The polymer of claim 1 wherein said crosslinking monomer is selected from: a) N,N'-methylene bisacrylamide b) trimethylol propane triacrylate c) glycerol propoxy triacrylate and d) divinyl benzene. The polmyer of claim 1 wherein the water soluble hyroxyl containing polymer is a polysaccharide. The polymer of claim 6 wherein the polysaccharide is natural starch. The polymer of claim 1 wherein said polymer has a particle size wherein 95 % by weight of said particles are between 0.85 mm - 0.106 mm [20 - 140 (U.S. Standard mesh)]. The polymer of claim 1 wherein the post-crosslinking crosslinking compound is a polyglycidyl ether of polyol. The polymer of claim 9 wherein the polyglycidyl ether is the diglycidyl ether of diethylene glycol. An absorbent article comprising a porous matrix of fiber having dispersed therein the polymer of claim 1. An absorbent article according to claim 11 wherein said porous matrix is disposed between a liquid impermeable material and a liquid permeable material. An absorbent article according to claim 11 wherein the absorbent polymer is present from 5 - 100 % by weight based on said fiber. Process for preparing superabsorbent polymers of claims 1-10 which comprises forming an aqueous solution of 90 to 100 weight percent acrylic acid, a crosslinking monomer which is a polyethylenically unsaturated polymerizable monomer in an amount of 0.005 to 1.0 mole percent based on the moles of acrylic acid, and, optionally, a water soluble hydroxy containing polymer in an amount of 0 to 10 weight percent based on the weight of acrylic acid and water soluble polymer, adding to the aqueous solution a redox initiator system comprising reducing agents in amounts of 2 x 10 -5 to 2.0 x 10 -2 mole percent based on moles of acrylic acid and oxidizing agents in amounts of 2.0 x 10 -3 to 1.1 mole percent based on moles of acrylic acid, and, optionally, a thermal free radical initiator in an amount of 0 to 1 mole percent based on the weight of acrylic acid, allowing the temperature to rise to a peak temperature under adiabatic conditions, holding at the peak temperature to reduce the free monomer content to less than 1000 ppm, neutralizing 50 to 100 percent of the acid groups of the resulting polymer with a base, and optionally adding a crosslinking compound having at least two groups capable of forming covalent bonds with carboxylic acid groups, and drying the polymer to a moisture content below 10 weight percent. Claims for the following Contracting State : ES A process for preparing a superabsorbent polymer comprising a copolymer of acrylic acid, a crosslinking monomer and a water-soluble hydroxyl containing polymer wherein the crosslinking monomer contains at least 2 ethylenically unsaturated groups per molecule and is present in the amount of 0.005 to 1.0 mole percent based on the moles of acrylic acid and the water-soluble hydroxyl containing polymer is present in the amount of 0 to 10 weight percent based on the weight of acrylic acid and water soluble polymer, wherein said copolymer is neutralized with an aqueous base in the amount of 50 to 100 percent of the carboxylic acid groups and is post-crosslinked with a crosslinking compound in the amount of up to 1.8 mole percent based on the moles of acrylic acid, wherein said superabsorbent polymer has an Absorbency under Pressure of 30 g/g minimum, a Reabsorbing Capacity of 40 g/g minimum, an Elasticity modulus 0.9 N/cm 2 (of 9.0 x 10 4 dynes/cm 2 ) minimum, and a Recovery Ratio of 85 percent minimum, which comprises forming an aqueous solution of 90 to 100 weight percent acrylic acid, a crosslinking monomer which is a polyethylenically unsaturated polymerizable monomer in an amount of 0.005 to 1.0 mole percent based on the moles of acrylic acid, and, optionally, a water soluble hydroxy containing polymer in an amount of 0 to 10 weight percent based on the weight of acrylic acid and water soluble polymer, adding to the aqueous solution a redox initiator system comprising reducing agents in amounts of 2 x 10 -5 to 2.0 x 10 -2 mole percent based on moles of acrylic acid and oxidizing agents in amounts of 2.0 x 10 -3 to 1.1 mole percent based on moles of acrylic acid, and, optionally, a thermal free radical initiator in an amount of 0 to 1 mole percent based on the weight of acrylic acid, allowing the temperature to rise to a peak temperature under adiabatic conditions, holding at the peak temperature to reduce the free monomer content to less than 1000 ppm, neutralizing 50 to 100 percent of the acid groups of the resulting polymer with a base, and optionally adding a crosslinking compound having at least two groups capable of forming covalent bonds with carboxylic acid groups, and drying the polymer to a moisture content below 10 weight percent. The process of claim 1 wherein the ethylenically unsaturated groups of the crosslinking monomer are allyl groups. The process of claim 2 wherein the crosslinking monomer is tetraallyloxyethane. The process of claim 3 wherein the tetraallyloxyethane is present in the amount of 0.01 to 0.3 mole percent. The process of claim 1 wherein said crosslinking monomer is selected from: a) N,N'-methylene bisacrylamide b) trimethylol propane triacrylate c) glycerol propoxy triacrylate and d) divinyl benzene. The process of claim 1 wherein the water soluble hyroxyl containing polymer is a polysaccharide. The process of claim 6 wherein the polysaccharide is natural starch. The process of claim 1 wherein said polymer has a particle size wherein 95 % by weight of said particles are between 0.85 mm - 0.106 mm [20 - 140 (U.S. Standard mesh)]. The process of claim 1 wherein the post-crosslinking crosslinking compound is a polyglycidyl ether of polyol. The process of claim 9 wherein the polyglycidyl ether is the diglycidyl ether of diethylene glycol. An absorbent article comprising a porous matrix of fiber having dispersed therein the polymer of claim 1. An absorbent article according to claim 11 wherein said porous matrix is disposed between a liquid impermeable material and a liquid permeable material. An absorbent article according to claim 11 wherein the absorbent polymer is present from 5 - 100 % by weight based on said fiber.

Inventors: CHAMBERS DOUGLAS RONALD, FOWLER HUBERT HENRY JR, FUJIURA YOJI, MASUDA FUSAYOSHI, CHAMBERS, DOUGLAS RONALD, FOWLER, HUBERT HENRY, JR., FUJIURA, YOJI, MASUDA, FUSAYOSHI
Applicants: HOECHST CELANESE CORP, HOECHST CELANESE CORPORATION
Application year: 1992
Main classification: C08F220
IPC codes: C08F251, C08F220, A61L15