Enhancing absorption rates of superabsorbents by incorporating a blowing agent

Abstract: An improved superabsorbent polymer having increased rate of water absorption is obtained by the addition, preferably prior to polymerization, of a carbonate blowing agent to a monomer solution of the monomers used to form the superabsorbent polymer. Preferred monomers include (meth)acrylic acids, preferably partially neutralized prior to polymerization and appropriate cross-linking agents. The carbonate cross-linking agents are any carbonate salt soluble or dispersible in the monomer solution, prior to polymerization. The multi-valent cationic salts of carbonate are preferred, especially the complex carbonates, for example, of magnesium.

Description: This invention relates to methods of forming superabsorbent polymer compositions. Superabsorbent polymer compositions are normally formed by polymerization in solution to form a hydrogel polymer in a gel form, by polymerization in water-in-oil latex to form hydrogel dispersed superabsorbent polymer dispersed in an oil continuous phase, or polymerization using any other technique that will derive and then essentially dry a superabsorbent, substantially water-insoluble, slightly cross-linked and partially neutralized hydrogel-forming polymer composition. These hydrogel-forming superabsorbent polymeric materials are useful as absorbents for water and/or for aqueous body fluids when the polymers are incorporated in absorbent structures, absorbent articles and the like, such as disposable diapers, adult incontinence pads and sanitary napkins. The water insoluble hydrogel forming polymers are materials capable of absorbing large quantities of aqueous fluids, such as water, brines, and body fluids and wastes. Superabsorbents are capable of retaining such absorbed fluids under moderate pressures. These materials are especially useful in absorbent articles, such as disposable diapers. These hydrogel forming absorbent materials are normally composed of polymers of polymerizable unsaturated carboxylic acids, or derivatives and salts thereof. Suitable monomeric acids are e.g. acrylic acid and/or acrylic acid salts, methacrylic acid and/or salts thereof, alkylacrylates and acrylamides. The polymers are rendered water insoluble by cross-linking using any number of cross-linking agents. The carboxylic acid monomers, other monomers useful in these superabsorbent polymers, and the useful cross-linking agents have been described in the art, particularly by US-A-4,654,039, Brandt, et al. Other U.S. patents representative of superabsorbent polymer formation are listed below: US-A-3,514,510, Hoffman, Jr. US-A-3,670,731, Harmon US-A-3,935,363, Burkholder, et al. US-A-4,062,817, Westerman US-A-4,064,071, Gilmour, et al. US-A-4,123,397, Jones US-A-4,286,082, Tsubakimoto, et al US-A-4,340,706, Obayashi, et al. US-A-4,342,858, Herman, et al. These patents do not, however, contain any reference to increasing the rate of water absorption by incorporating in the solution, prior to polymerization, a carbonate based blowing agent. There is some art discussing foamed materials, which, with strong hydrolysis of oil soluble polyesters, become superabsorbent, i.e. US-A-4,529,739. Also, a reference to the use of nitrogen blowing agents; AT-B-391,321, September 25, 1990, (AT-A-88-2108, August 29, 1988) has been abstracted. These nitrogen blowing agents use ammonium carbamate, and accumulate excess ammonia. In EP-A-295438 a fully neutralized composition contains large quantities of carbonate as neutralizing agent. Any foaming effect appears to be a by-product. DE-A-3831261 discloses a method of forming a foamed polyacrylate with adjustable swell rate by using blowing agents. The polymer reaction mixture is prepared, is partially or completely neutralized, and is polymerized. Before or after the polymerization, but before heating, a nitrogen-containing blowing agent is added, to over-neutralize the mixture. During heating the blowing agent is decomposed to foam the already-polymerized material. It is then comminuted. Once such blowing agent disclosed is ammonium carbonate. In Examples 1 to 15 where a blowing agent is present before polymerization, the mixture is flushed with nitrogen for one hour before the start of polymerization. The present invention provides a method of forming a dry superabsorbent polymer composition, which method comprises: (i) forming an aqueous monomer solution containing 50 to 80 mol%-neutralised olefinically-unsaturated carboxylic acid monomer, and crosslinking agent in an amount of from 0.005 to 2.0 wt% based on said aqueous monomer solution, the total monomer content of the solution being at least 20 wt%; (ii) dispersing or dissolving a carbonate blowing agent throughout the monomer solution to constitute a carbonated monomer composition, the amount of carbonate blowing agent being from 0.05 to 2.5 wt% based on the total weight of the carbonated monomer composition; (iii) between 5 minutes preceding and 15 minutes following procedure (ii), adding free radical initiator to said carbonated composition to initiate free radical polymerisation therein, with crosslinking by the crosslinking agent, to form a microcellular, water-insoluble aqueous hydrogel polymer; (iv) chopping the hydrogel polymer into pieces having a diameter in the range from 0.1 mm to 5.0 cm; (v) drying the pieces at temperature in the range from 85°C to 210°C, and grinding the dried pieces to a size in the range from 0.05 mm to 5.0 mm. The Monomers Our method is preferably practised using monomers in an aqueous solution, or monomers dissolved in water and then dispersed in a water-in-oil emulsion. The monomers are most preferably selected from acrylic acid, the water-soluble salts of acrylic acid, methacrylic acid, the water-soluble salts thereof, and admixtures thereof. However, any olefinically unsaturated carboxylic acid and carboxylic acid anhydride monomers can be used, including the acrylic acids and their anhydrides, typified by acrylic acid itself, methacrylic acid, ethacrylic acid, alpha-chloroacrylic acid, alpha-cyano acrylic acid and beta-methyl acrylic acid. Sometimes the term "(meth)acrylic acid" is used, and represents the presence of acrylic acid alone, methacrylic acid alone, any admixture of these acids, and any water-soluble salt of these acids, either alone or in any admixture. The unsaturated carboxylic acids can also include itaconic acid, citraconic acid, maleic acid, fumaric acid and maleic anhydride. Other olefinic unsaturated monomers may also be used, such as the sulfonic acid monomers. These monomers can be chosen from, but are not necessarily limited to, vinyl sulfonic acids, allyl sulfonic acids, styrene sulfonic acid, the sulfo-acrylic and methacrylic acid esters, including sulfoethylacrylate, sulfoethylmethacrylate, and sulfopropylacrylate, sulfopropylmethacrylate. Also, the sulfo(meth)acrylamide materials, such as acrylamido N-methylene sulfonic acid, acrylamido-N-ethylene sulfonic acid, 2-acrylamido-2-methylpropane sulfonic acid, and the like are useful. Other olefinically unsaturated monomers, such as acrylamide and methacrylamide may also be useful in forming copolymers which can be cross-linked by the described cross-linkers to form polymeric superabsorbents. The Cross-Linking Agents The cross-linking agents can include, but are not necessarily limited to, compounds having at least two polymerizable double bonds, compounds having at least one polymerizable double bond and at least one functional group reactive with the acid containing monomer material, compounds having at least two functional groups reactive with the acid containing monomer material, and polyvalent metal compounds whose metallic cations can form ionic cross-linkages. The cross-linking agents are particularly exemplified by materials containing at least two polymerizable double bonds including, but not limited to, di, tri, or polyvinyl compounds, such as divinyl benzene, divinyl toluene, or di, tri, or polyesters of unsaturated mono or poly carboxylic acids with polyols including, for example, di or tri acrylic acid esters of polyol, such as ethylene glycol, trimethylpropane, glycerine, or polyoxyethylene glycols. Other cross-linking agents are alkylene bis-acrylamides, such as N,N-methylene-bis-acrylamide, and carbamyl esters obtained by reacting polyisocyanates with hydroxyl group containing monomers; di, tri, or poly allyl ethers of polyols; di, tri, or poly allyl esters of polycarboxylic acids, such as diallyl phthalate and diallyl adipate. Other cross-linkers are the esters of unsaturated mono or poly-carboxylic acids with mono-allyl esters of polyols, such as the acrylic acid ester of polyethylene glycol monoallyl ether, di or triallyl amine, and the alkylene glycol diglycidyl ethers. The cross-linking agent is present at from 0.005 weight percent of the total formulation to 2.0 weight percent of the total formulation. Preferably, the cross-linking agent is present at from about 0.10 wt% to about 10 wt% percent based on the total monomer formulation. The most preferred cross-linking agents are selected from N,N'-methylene bis-acrylamide, trimethylol propane triacrylate, diallylamine, triallylamine, ethylene glycol diglycidyl ether and mixtures thereof. The Superabsorbent Polymer The polymer is formed in solution, or in a water-in-oil emulsion. The superabsorbent polymer is preferably formed from a thin layer of the carbonated monomer solution which has been deaerated (purged of oxygen) to which is added the free radical initiator of choice. As explained previously the catalyst, or mixture of catalysts, is added to the monomer solution within no more than five minutes before or fifteen minutes after the addition of the carbonate blowing agent. Simultaneous addition of the catalyst and the blowing agent, or addition of the catalyst after the addition of blowing agent are preferred. Most preferred is the addition of catalyst following the formation of the carbonated blowing agent. Initial temperatures of the carbonated monomer solution are from 0°-20°C. The thin layer solution is preferably deaerated and protected from air before polymerization, and after initiating polymerization by the addition of free radical initiators. The polymerization forms an aqueous hydrogel of the cross-linked, water insoluble polymer. The polymerization is exothermic, causing the gel temperature to increase from an initial temperature of about 0° to 20°C to temperatures of about 80°-110°C. The aqueous gel, in the presence of the carbonate blowing agent, as heating and polymerization occurs (polymerization is exothermic) develops as a microcellular hydrogel, since the carbonate blowing agent is releasing carbon dioxide gases so that the hydrogel has dispersed in it gaseous bubbles of carbon dioxide. The microcellular structure may appear cloudy, demonstrating relatively small dispersed gas bubbles, may appear opaque, normally representing somewhat larger gas bubbles, (or higher quantities of CO 2 ), or may appear quite foamy, with volume increases in excess of ten times the initial volume of the carbonated monomer solution. The microcellular gel volume increase can range from about 1.01 up to at least 10.0 times the volume of the original carbonated monomer solution, depending upon, primarily, the concentration of the carbonate blowing agent contained in the treated monomer solution. The preferable hydrogel-forming polymer materials are synthesized from aqueous solutions containing at least 20, and most preferably at least 30-40 weight percent solution of acrylic acid, methacrylic acid, mixtures thereof and/or their water soluble salts. These monomers or mixtures of these monomers are preferably dissolved in an aqueous solution, which solution also contains the cross-linking agent in appropriate amount. The aqueous solution, herein referred to as the monomer solution, has at least 20 percent by weight total monomer content, preferably at least 25 percent by weight total monomer content, and most preferably at least 30-40 percent by weight total monomer content. This monomer solution also contains the amounts of the cross-linking agents mentioned above. These are selected most preferably from the group consisting of the bis-acrylamides, such as N-N'-methylene bis-acrylamide, the di, tri, or polyesters of unsaturated mono or polycarboxylic acid polyols, such as trimethylpropane triacrylate, or the di, or tri glycidyl ethers of polyols, such as ethylene glycol diglycidyl ether, or the multi-substituted allyl amines, such as diallyl amine and triallyl amine, or any combination thereof. The polymerization is initiated with free radical initiators known to the artisan. These initiators may include, but are not limited to, peroxide or persulfate catalysts, azo catalysts, the so-called redox catalysts, ionizing radical-forming radiation, or any combination of these free radical initiators. The hydrogel material is made from partially neutralized monomers. Any suitable basic salt-forming cation including the alkaline metals, ammonia, or ammonium salts, and amines may be used for the purpose of the partial neutralization. It is preferred to have a degree of neutralization of the starting carboxylic acid monomers of from 60 to 80 mole percent. When appropriate, the degree of neutralization within the ranges above may be partially accomplished by the addition of the carbonate based blowing agents to be described below. The Blowing Agents The carbonate blowing agents may be any carbonate or bicarbonate containing salt, or mixed salt, and may include carbon dioxide as a gas or a solid, sodium carbonate, potassium carbonate, ammonium carbonate, magnesium carbonate, or magnesium (hydroxic) carbonates, calcium carbonate, barium carbonate, bicarbonates and hydrates of these, or other cations, as well as naturally occurring carbonates, such as dolomite, or mixtures thereof. These blowing agents release carbon dioxide when heated while dissolved or dispersed in the carbonated monomer solutions. A most preferred carbonate blowing agent is MgCO 3 , which may also be represented by the formula (MgCO 3 ) 4 .Mg(OH) 2 .5H 2 0. Another preferred agent is (NH 4 ) 2 CO 3 . The MgCO 3 and (NH 4 ) 2 CO 3 may also be used in mixtures. The amount of blowing agent is from 0.05 to 2.5 weight percent based on total carbonated monomer solution weight. It is most preferred to add from 0.2 weight percent to 2.5 weight percent blowing agent. The blowing agent must be added prior to, or immediately after polymerization is initiated. The blowing agents are not effective if added after the polymer gel (hydrogel) is formed, nor are they effective added after chopping or drying the gelled polymer, as will be demonstrated in the later examples. The preferred blowing agents are carbonate salts of multi-valent cations, such as Mg, Ca, Zn, and the like. Although certain of the multi-valent transition metal cations may be used, some of them, such as ferric cation, can cause colour staining and may be subject to reduction-oxidation reactions or hydrolysis equilibria in water. This may lead to difficulties in quality control of the final polymeric product. Also, other multi-valent cations, such as Ni, Ba, Cd, Hg would be unacceptable because of potential toxic or skin sensitizing effects. Our method is primarily related to improving the rate of absorption by superabsorbent polymers of synthetic urine, aqueous brines, or aqueous solutions, including biological fluids, such as urine, sweat, blood, and the like. The improved superabsorbents are preferably (meth)acrylic acid based superabsorbent polymers and our method of manufacturing these materials comprises the steps of dissolving at least one acrylic acid monomer, and/or partially neutralized salts thereof (preferably the sodium salts) in an aqueous solution, which solution also contains the cross-linking agent, to yield the 50 to 80 mole percent neutralized solution. The monomer-crosslinker solution (referred to herein as the monomer solution) is then completed by adding to it an effective microcellular forming amount of the carbonate blowing agent, in the concentration of from 0.05 to 2.5 weight percent of the total solution weight, thereby forming the carbonated monomer composition. The carbonate blowing agent may contribute to the partial neutralization mentioned above. The free radical catalyst is added immediately before, coincident with or immediately following the addition of the blowing agent, which catalyst can be any free radical catalyst or combination of catalysts known in the art. This initiated carbonated monomer composition is then heated appropriately, often by the exothermic polymerization reaction, to complete polymerization. The temperatures of polymerization range from an initial temperature of about 0°C-20°C and are exothermic to temperature of about 80°C-110°C. Polymerizing this carbonated monomer solution forms a hydrogel, which has a microcellular structure from dispersion of carbon dioxide formed in the gel by the decomposition of the carbonate blowing agent. This microcellular gel is then masticated by chopping, grinding, or otherwise forming gel pieces of said microcellular hydrogel, which microcellular gel pieces have particle diameter sizes ranging from about 0.1 millimeters to about 5.0 centimeters, preferably from about 10 mm to about 2.0 cm. These masticated gel pieces are then dried at temperatures of from 85°C to 210°C to form a dry superabsorbent polymer which is then ground to a particle size having a diameter of from about 0.05 mm to about 5.0 mm. The ground superabsorbent has improved rates of absorption of the aqueous body fluids and synthetic solutions above. The improved dry superabsorbent polymer essentially retains the gel strength and water (or brine or synthetic urine) capacity of base formulations made without the carbonate blowing agents. To exemplify our invention, the following examples are presented: An aqueous formulation containing about 30-40 weight percent of a combination of acrylic acid and sodium acrylate (about 40/60 - 20/80) and containing from about 0.1 to 0.5 weight percent of an amine based cross-linking agent is formed. To this monomer solution is added the following carbonate blowing agents. (See Table I.) In separate tests, polymerization was initiated and completed, the resultant gel (absent blowing agent) or microcellular gel (in the presence of blowing agent) was ground and dried, and the superabsorbent characteristics measured, using standard tests for gel strength, 0.9% saline capacity, and swell rate. The results are presented in Table I: The results in Table I demonstrate that, compared to a blank superabsorbent (ave. of 5 samples) made from the same neutralized acrylic acid and cross-linking agent, the polymers containing the carbonate blowing agents provide for greatly increased swell rates, using a synthetic urine. Also, gel strength seemed to be improved, essentially with minimal effects on capacity. The number of seconds required to absorb a preset amount of synthetic urine is a measure of the rate of water absorption. Decreasing time is a good result. Also, the final improved superabsorbent also maintains and improves gel strength, and saline capacity of the superabsorbents formed is essentially still acceptable. To demonstrate the ineffectiveness of adding the carbonate blowing agents after polymer gel is formed, the following tests and results are provided in Table II: As can be seen, addition of the carbonate blowing agents before polymerizing is necessary, since the addition after gel formation is ineffective in obtaining improved superabsorbent polymer. The synthetic urine tested contains the following cations and anions or elements dissolved in water: Elements Concentrations Na + 600 - 700 ppm Ca ++ 65 - 75 ppm Mg ++ 55 - 65 ppm K + 1100 - 1200 ppm Phosphorus 240 - 280 ppm Sulfur 450 - 500 ppm Chloride 1100 - 1300 ppm Sulfate 1300 - 1400 ppm

Claims: A method of forming a dry superabsorbent polymer composition, which method comprises: (i) forming an aqueous monomer solution containing 50 to 80 mol%-neutralised olefinically-unsaturated carboxylic acid monomer, and crosslinking agent in an amount of from 0.005 to 2.0 wt% based on said aqueous monomer solution, the total monomer content of the solution being at least 20 wt%; (ii) dispersing or dissolving a carbonate blowing agent throughout the monomer solution to constitute a carbonated monomer composition, the amount of carbonate blowing agent being from 0.05 to 2.5 wt% based on the total weight of the carbonated monomer composition; (iii) between 5 minutes preceding and 15 minutes following procedure (ii), adding free radical initiator to said carbonated composition to initiate free radical polymerisation therein, with crosslinking by the crosslinking agent, to form a microcellular, water-insoluble aqueous hydrogel polymer; (iv) chopping the hydrogel polymer into pieces having a diameter in the range from 0.1 mm to 5.0 cm; (v) drying the pieces at temperature in the range from 85°C to 210°C, and grinding the dried pieces to a size in the range from 0.05 mm to 5.0 mm. The method of claim 1 wherein the carbonate blowing agent is present in a quantity of 0.2 weight percent to 2.5 weight percent of the carbonated monomer composition. The method of claim 1 or claim 2 wherein the 50-80 mol% neutralised carboxylic acid monomer contained in the monomer solution is a mixture of acrylic acid and the sodium salt of acrylic acid; the cross-linking agent is divinyl benzene, divinyl toluene, a di or triacrylic acid ester of a polyol, bis-acrylamide, a di, tri, or poly allyl ester of a polycarboxylic acid; di or tri allyl amine, or admixtures thereof; and the carbonate blowing agent is one or more of carbon dioxide, Na 2 CO 3 , K 2 CO 3 , (NH 4 ) 2 CO 3 , MgCO 3 , CaCO 3 , NaHCO 3 , KHCO 3 , NH 4 HCO 3 , Mg(HCO 3 ) 2 , Ca(HCO 3 ) 2 , ZnCO 3 , (MgCO 3 ) 4 .Mg(OH) 2 .5H 2 O, BaCO 3 , or a bicarbonate or hydrate thereof. The method of claim 1 or claim 2 wherein (a) the aqueous carbonated monomer composition contains at least 30 weight percent of the combination of acrylic acid and sodium acrylate; (b) the aqueous carbonated monomer composition contains at least 0.05 weight percent of the cross-linking agent, that is methylene bis-acrylamide, trimethylol propane triacrylate, di-allyl amine, triallyl amine, ethylene glycol diglycidyl ether, or mixture thereof; (c) the aqueous carbonated monomer composition contains at least 0.1 weight percent of the carbonate blowing agent, that is Na 2 CO 3 , (MgCO 3 ) 4 .Mg(OH) 2 .5H 2 O, MgCO 3 , CaCO 3 , (NH 4 ) 2 CO 3 , or mixture thereof. The method of claim 1 or claim 2 wherein in the carbonated monomer composition said monomer consists essentially of from 20-40 weight percent acrylic acid and from 60 to 80 weight percent sodium acrylate; the cross-linking agent is methylene-bis-acrylamide, ethylene glycol diglycidyl ether, diallyl amine, triallyl amine, or mixture thereof, said cross-linking agent is present in the carbonated monomer composition at from 0.05 to 0.5 weight percent; the carbonate blowing agent is (NH 4 ) 2 CO 3 , MgCO 3 , (MgCO 3 ) 4 . Mg(OH) 2 . 5H 2 O, or mixture thereof, and the free radical initiator is one or more of hydrogen peroxide, sodium persulfate, azo catalysts, organic peroxides, sodium bisulfite, or peracetate catalysts. The method of any preceding claim wherein the polymerization step is carried out at between 0°C and 110°C. The method of any preceding claim including the further step of incorporating the dry superabsorbent polymer in an absorbent structure on a diaper, an incontinence brief, a sanitary napkin, or a bandage.

Inventors: LIND ERIC J, LIND, ERIC J.
Applicants: STOCKHAUSEN LOUISIANA LIMITED
Application year: 1992
Main classification: C08J9
IPC codes: A61L15, C08J9, B01J20