US 20040138362A1 (19) United States (12) Patent Application Publication (10) Pub. No.: US 2004/0138362 A1 Kim (43) Pub. Date: Jul. 15, 2004 (54) WATER-ABSORBENT (86) PCT No.: PCT/US02/20573 CARBOXYL-CONTAINING POLYMERS WITH LOW MONOMER CONTENT (30) Foreign Application Priority Data (76) Inventor: Young-Sam Kim, Midland, MI (US) Jun. 29, 2001 (US) - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 60302330 Correspondence Address: Publication Classification THE DOW CHEMICAL COMPANY 7 INTELLECTUAL PROPERTY SECTION (51) Int. Cl." ....................................................... C08K 3/10 P. O. BOX 1967 (52) U.S. Cl. ............................................ 524/403; 524/556 MIDLAND, MI 48641-1967 (US) (57) ABSTRACT (21) Appl. No.: 10/469,664 A water absorbent polymer with reduced residual monomer (22) PCT Filed: Jun. 26, 2002 content is prepared using Silver ions and/or colloidal Silver. US 2004/O138362 A1 Jul. 15, 2004 WATER-ABSORBENT CARBOXYL-CONTAINING and bisulfites. The surfactant and the vinylic addition com POLYMERS WITH LOW MONOMER CONTENT pound may be used in admixture with oxidizing anions, Such as perOXOdisulfate and peroxide. An acqueous Solution of the 0001. This invention relates to water-absorbent carboxyl additives is mixed with the water-absorbent polymer in the containing polymers having a low residual monomer content form of Swollengels or beads or dry polymer. The presence and a process for their preparation. of Surfactants is believed to negatively affect the liquid 0002 Water-absorbent polymers, also referred to as distribution when the polymer is wetted. In addition, vinyl Superabsorbent polymers or aqueous fluid absorbent poly addition compounds like Sulfites and bisulfites generally mers, are primarily used in personal care products which cause offensive odor problems during processing. absorb body fluids, for example, baby diapers, adult incon 0006 U.S. Pat. No. 5,629,377 discloses a process for tinence products and feminine hygiene products. In Such preparing water-absorbent polymer particles comprising applications, Superabsorbent polymer particles are incorpo polymerizing unsaturated carboxyl-containing monomers in rated into absorbent Structures that contain Synthetic and/or the presence of a chlorine- or bromine-containing oxidizing natural fiber or paper based, woven and nonwoven Struc agent to form a hydrogel which is then heated at a tempera tures, or toughened masses of fibers, Such as fluff pads. The ture of from 170° C. to 250° C., preferably of from 210°C. materials used in Such structures can quickly absorb aqueous to 235 C. Alternatively, the chlorine- or bromine-containing fluids and distribute them throughout the whole absorbent oxidizing agent may be added to the polymerized hydrogel. Structure. The Structures, in the absence of Superabsorbent The method is effective for improving absorbency, for polymers, have limited absorption capacity, are bulky due to example, centrifuge capacity and absorbency under load the large amount of material needed to provide acceptable (AUL), while keeping the amount of residual monomers at absorption capacity, and do not retain fluid under pressure. an acceptable level. However, the high heat treat tempera A means for improving the absorbency and fluid retention ture needed to activate the chlorine- or bromine-containing characteristics of Such absorbent Structures is to incorporate oxidizing agent is detrimental for various reasons, including Superabsorbent polymer particles that imbibe fluids to form high energy usage and loSS of moisture. a Swollen hydrogel material. 0007 U.S. Pat. No. 4,659,793 teaches that very small 0003. The Superabsorbent polymer particles quickly amounts of certain metal ions (Zn", Co", Co", Cui", absorb fluids and retain Such fluids to prevent leakage and Mo, Fe", Fe, Cr", Ni, Ce" and Cet) promote the give the absorbent structure a “dry feel” even when wetted. copolymerization of ethylenically unsaturated dicarboxylic See U.S. Pat. No. 4,610,678 for examples of such polymers. acid monomers, especially maleic acid, and C.f3-ethyleni See also U.S. Pat. No. 4,654,039 and Re. No. 32,649, which cally unsaturated monomers having carboxyl or Sulfonic disclose a process for the preparation of Superabsorbent acid groups, such as (meth)acrylic acid or 2-acrylamido-2- polymers and the use of known crosslinking agents for Such methyl propane Sulfonic acid, whereby the amount of unre polymers, and also U.S. Pat. Nos. 4,295,987 and 4,303,771. acted dicarboxylic acid monomer is significantly reduced. A variation of the basic process is taught in GB Patent The metal ions are added to the monomer mixture. The 2,119,384, which discloses a post polymerization Surface resulting copolymer is not described as a Superabsorbent crosslinking proceSS in which the previously polymerized polymer but is useful in antiscalants, dispersants, detergent absorbent polymer powder is mixed with crosslinkers, pref additives, deflocculants etc. erably polyalcohols, a Solvent and water, to coat the polymer Surface and is heated to temperatures in the range of 90 to 0008 Numerous other methods are known to reduce the 300° C. to crosslink the Surface. U.S. Pat. No. 5,506,324 amount of residual monomers in Superabsorbent materials, discloses Superabsorbent polymer particles comprising poly for example, the use of Sulfites, bisulfites, ammonia, amines, mers containing carboxyl moieties which are crosslinked amino acids like cystein and lysine, Sulfurous acid, phos using Co polyhydric hydrocarbons which are ethoxylated phorous acid, pyrophosphorous acid, hypophosphorous with from 2 to 8 ethylene oxide units per hydroxyl moiety acid, thiosulfuric acid, hydroxylamine or a Salt thereof, and of the polyhydric hydrocarbon wherein the hydroxyl moiety ascorbic acid (see U.S. Pat. Nos. 5.229,488, 5.866,678, at the end of each ethylene oxide chain is esterified with a 4,766,173, and 4,929,717). Co unsaturated carboxylic acid or ester thereof. In a 0009. The various methods taught in the prior art use preferred embodiment, the Superabsorbent polymer particles additional non-reactive additives in addition to the reactive are Subjected to a heat-treatment process after drying and agents mentioned above and/or additional mixing measures sizing the particles. to improve the distribution of the reactive agents. The non-reactive additives may be Substances that can modify 0004. A basic problem with commercially available hydrogel particles, and may be not only Surfactants but also, water-absorbent polymer has been the presence of residual for example, organic Solvents, mineral oils and very fine monomers. A water-absorbent polymer product with inorganic or organic particles like Silicon dioxide, aluminum reduced residual monomer content is highly desired. Various dioxide, Zeolite or poly(methyl methacrylate) particles. The methods to lower the residual monomer content are known use of Such additives complicates the manufacturing pro in the art. ceSS, which then is more time-consuming and therefore leSS 0005 European Patent Publication 505 163 relates to a economical. In addition, the use of additives Such as Sur method for reducing residual (meth)acrylic acid present in factants and very fine powderS may adversely affect absor poly(acrylic acid) water-absorbent gel polymers which com bence and liquid distribution. prises treating these polymers with a combination of a 0010. It would be highly desirable to provide a novel Surfactant having a certain HLB and a vinylic addition method for the preparation of water-absorbent polymers compound that can react with a vinylic double bond. having a low residual monomer level, which method would Examples of the vinylic addition compound include Sulfites eliminate the above-mentioned disadvantages. US 2004/O138362 A1 Jul. 15, 2004 0.011 This invention relates to a process for the prepara 0025 The silver ions may be added as one or more dry tion of a water-absorbent polymer, which comprises Silver Salts or as a Solution of one or more Silver Salts. The Solution may be an aqueous Solution, a non-aqueous organic 0012 (I) polymerizing a polymerization mixture com Solution or a Solution in a mixture of water and organic prising: Solvent, with an aqueous Solution being preferred. An acque 0013 (a) one or more ethylenically unsaturated car ous Solution may also be prepared by complexation of boxyl-containing monomers, water-insoluble silver salts. 0026. The silver salts are applied to the Superabsorbent 0014 (b) one or more crosslinking agents, polymer either in powdered Salt form or as a Solution or 0015 (c) optionally one or more comonomers copo Suspension. The Solution can be acqueous, organic, or a lymerizable with the carboxyl-containing monomer, mixture of these. Water-soluble silver salts, which are and termed “Soluble Silver Salts' in the present application, are the preferred source of silver ions. The solubility of various 0016 (d) a polymerization medium to form a Silver Salts generally can be improved by acidifying them, crosslinked hydrogel, dissolving them in alkalis, dissolving them in organic Sol vent, dissolving them at elevated temperatures, and/or inten 0017 (II) comminuting the hydrogel to create particles Sive mixing during the dissolution proceSS. The degree of and solubility of the silver salt is not particularly critical. The 0018) (III) drying the hydrogel; wherein silver ions or soluble silver salts preferably have a solubility in pH neutral colloidal silver are added in at least