WATER-ABSORBENT POLYMER COMPOSITION, METHOD OF FORMING IT AND ARTICLE CONTAINING IT

Abstract: A copolymer of recurring units of at least one α , β -unsaturated monomer and recurring units of at least one copolymerizable comonomer comprising, in its preferred embodiment from about 20 to about 80 percent pendant carboxylic acid units and from about 80 to about 20 percent pendant carboxylate salt units, is blended with at least one disaccharide or oligosaccharide to produce a water-absorbing composition upon curing. The composi­ tion is particularly suitable for fiber formation.

Description: This invention relates to water-absorbing compositions. In one of its more specific aspects, this invention relates to the incorporation of water-absorbing compositions into articles of manufacture for the purpose of improving the absorbent properties of the articles. Absorbent compositions are widely used in the manufacture of products which require high absorption capability. For example, water-absorbing compositions are used in the manufacture of surgical and dental sponges, tampons, sanitary napkins and pads, bandages, disposable diapers, meat trays, and household pet litter. Water-absorbing compositions are also used for the modification of soil to improve water retention and increase air capacity and for a host of other applications. As used herein, the term "water" when used in the phrases "water-absorbing" and "water-absorbent" is understood to mean not only water but also electrolyte solutions such as body fluids. A number of absorbent compositions have been developed which exhibit water absorption capacity. For example, U.S. Pat. Nos. 3,954,721 and 3,983,095 disclose preparations for derivatives of copolymers of maleic anhydride with at least one suitable vinyl monomer in fibrous form. The fibrous copolymers are rendered hydrophilic and water-swellable by reaction with ammonia or an alkali metal hydroxide. U.S. Pat. No. 3,810,468 discloses lightly cross-linked olefin-maleic anhydride copolymers prepared as substantially linear copolymers and then reacted with a diol or a diamine to introduce cross-linking. The resultant lightly cross-linked copolymers are treated with ammonia or an aqueous or alcohol solution of an alkali metal hydroxide. U.S. Pat. No. 3,989,586 describes the preparation of sorptive paper products by incorporating cross-linked copolymers of styrene or olefins with maleic anhydride in a paper web which is then treated to convert the copolymer to a water-swellable salt form. U.S. Pat. No. 3,980,663 describes water-swellable absorbent articles made from carboxylic polyelectrolytes via cross-linking with glycerine diglycidyl ether. U.S. Pat. Nos. 4,332,917 and 4,338,417 disclose blends of copolymers of styrene and maleic anhydride with polymers derived from a monomeric ester having vinyl unsaturation e.g., poly(vinyl acetate), cellulose triacetate, cellulose aceto-butyrate, poly(ethylacrylate) and poly(methylmethacrylate). U.S. Pat. No. 4,420,588 teaches a water-absorbing rubber composition comprising a 1,3-diene rubber and a water-absorbing resin dispersed in the rubber. And, U.S. Pat. No. 4,155,957 teaches a self-swelling, leakage-preventing rubber material which comprises (1) a copolymer of a lower olefin with maleic anhydride; (2) a polymer emulsion which is compatible with copolymer (1), e.g., an ethylene-vinyl acetate copolymer emulsion; and (3) a polyhydric or polyfunctional compound. The desirability of having water-absorbing compositions in fibrous forms is well known. For example, in sanitary products such as disposable diapers and tampons, fibers can be more easily confined within the product. In this respect, the prior art water-absorbing compositions are deficient; they do not facilitate fiber formation. Because of the speed of their crosslinking reactions, the prior art water-absorbing compositions possess no appreciable shelf life; fiber formation must be completed shortly after the compositions are prepared. This shelf life deficiency in the prior art composition is discussed in U.S. Pat. No. 3,983,095 which teaches that fiber formation should be completed within several hours and in some cases immediately, depending upon the reactivity of the cross-linking agent used. The water-absorbing compositions of this invention possess excellent shelf life and have been found suitable for forming into fibers several months after preparation. They facilitate fiber formation over a wide range of time and temperature. They also possess exellent integrity in the hydrogel or water-swollen state, exhibit excellent water and electrolyte solution absorption capacity, and are readily incorporated into conventional water-absorbing products using conventional methods. According to this invention there is provided a composition which is water-absorbent upon curing comprising: (a) a copolymer containing from 25 to 75 mole percent recurring units of at least one α,β-unsaturated monomer bearing at least one pendant group selected from carboxylic acid groups and carboxylate salt groups, from 20 to 80 percent of the total number of said pendant groups being carboxylic acid groups and from 80 to 20 percent being carboxylate salt groups, and from about 75 to about 25 mole percent recurring units of at least one copolymerizable copolymer, and (b) at least one carbohydrate compound selected from disaccharides and oligo-saccharides. According to this invention there is also provided a method of producing a water-absorbing composition comprising the steps of: (a) preparing a composition as defined above and (b) curing the resulting blend. According to this invention there is further provided a method of absorbing water and electrolyte solutions comprising the step of contacting the water or electrolyte solution to be absorbed with the cured water-absorbing composition. The invention further provides an article of manufacture comprising the cured water-absorbing composition and a means for supporting said composition to present said composition for absorption usage. According to this invention there is also provided a method of enhancing at least one water-absorbing characteristic of an article which method comprises the step of incorporating into the article the cured water-absorbing composition said composition being incorporated into the article in an effective amount to enhance at least one water-absorbing characteristic of the article as compared to the water-absorbing characteristics of the article in the absence of the composition. The copolymer of the water-absorbing composition of the invention may be derived from a copolymer containing 25 to 75 total mole percent recurring units of at least one α , β -unsaturated monomer and from 75 to 25 total mole percent recurring units of at least one copolymerizable comonomer. Preferably, the copolymer will contain from 35 to 65 total mole percent of recurring units of at least one α , β -unsaturated monomer and from 65 to 35 total mole percent of at least one copolymerizable comonomer. Most preferably, the copolymer will be an equimolar copolymer. Suitable α , β -unsaturated monomers are those bearing at least one pendant carboxylic acid unit or derivative of a carboxylic acid unit. Derivatives of carboxylic acid units include carboxylic acid salt groups, carboxylic acid amide groups, carboxylic acid imide groups, carboxylic acid anhydride groups and carboxylic acid ester groups. Suitable α , β -unsaturated monomers include maleic acid; crotonic acid; fumaric acid; mesaconic acid; the sodium salt of maleic acid; the sodium salt of 2-methyl, 2-butene dicarboxylic acid; the sodium salt of itaconic acid; maleamic acid; maleamide; N-phenyl maleimide; maleimide; maleic anhydride; fumaric anhydride; itaconic anhydride; citraconic anhydride; mesaconic anhydride; methyl itaconic anhydride; ethyl maleic anhydride; diethylmaleate; methylmaleate; and the like, and their mixtures. Any suitable copolymerizable comonomer can be employed. Suitable copolymerizable comonomers include ethylene, propylene, isobutylene, C₁ to C₄ alkyl methacrylates, vinyl acetate, methyl vinyl ether, isobutyl vinyl ether, and styrenic compounds having the formula: wherein R represents hydrogen or an alkyl group having from 1 to 6 carbon atoms and wherein the benzene ring may be substituted with low molecular weight alkyl or hydroxy groups. Suitable C₁ to C₄ alkyl acrylates include methyl acrylate, ethyl acrylate, isopropyl acrylate, n-propyl acrylate, n-butyl acrylate and their mixtures. Suitable C₁ to C₄ alkyl methacrylates include methyl methacrylate, ethyl methacrylate, isopropyl methacrylate, n-propylmethacrylate, n-butyl methacrylate and their mixtures. And, suitable styrenic compounds include styrene, α -methylstyrene, p-methylstyrene, t-butyl styrene, and the like, and their mixtures. The pendant units on the α , β -unsaturated monomer, will determine what, if any, additional reactions must be carried out to obtain a copolymer having the requisite pendant units necessary to produce the water-absorbing compositions of this invention i.e. 20 to 80 percent pendant carboxylic acid units and 80 to 20 percent pendant carboxylate salt units. Preferably, both units are present in an amount of from 30 to 70 percent. In general, if the α , β -unsaturated monomer bears only carboxylic acid amide, carboxylic acid imide, carboxylic acid anhydride, carboxylic acid ester groups, or mixtures thereof, it will be necessary to convert at least a portion of such carboxylic acid derivative groups to carboxylic acid groups by, for example, a hydrolysis reaction. If the α , β -unsaturated monomer bears only carboxylic acid salt groups, acidification to form carboxylic acid groups will be necessary. Similarly, the final copolymer must contain from 80 to 20 percent pendant carboxylate salt units. Accordingly, it may be necessary to carry out a neutralization reaction. Neutralization of carboxylic acid groups with a strong organic or inorganic base such as NaOH, KOH, ammonia, ammonia-in-water solution, or organic amines will result in the formation of carboxylate salt units, preferably carboxylate metal salt units. Moreover, the sequence and the number of reactions (hydrolysis, acidification, neutralization, etc.) carried out to obtain the desired functionality attached to copolymer backbone are not critical. Any number and sequence resulting in a final copolymer which possesses from 20 to 80 percent pendant carboxylic acid units and from 80 to 20 percent pendant carboxylate salt units is suitable. One copolymer particularly suitable for use is a copolymer of maleic anhydride and isobutylene. Others are copolymers of maleic anhydride and styrene, and of maleic anhydride and ethylene. Suitable copolymers will have peak molecular weights of from about 5,000 to about 500,000 or more. Suitable copolymers of maleic anhydride and isobutylene can be prepared using any suitable conventional method. Such copolymers are also commercially available from Kuraray Isoprene Chemical Company, Ltd., Osaka, Japan, under the trademark ISOBAM. ISOBAM copolymers are available in several grades which are differentiated by viscosity molecular weight: ISOBAM-10, 160,000 to 170,000; ISOBAM-06, 80,000 to 90,000; ISOBAM-04, 55,000 to 65,000; and ISOBAM-600, 6,000 to 10,000. To produce a water-absorbing composition of this invention, at least one copolymer as described above and at least one carbohydrate compound selected from the group consisting of disaccharides and oligosaccharides are blended such that the water-absorbing composition contains in weight percent, from 80 to 99 total copolymer and from 1 to 20 total carbohydrate compound. Preferably, the composition will contain from 85 to 95 weight percent total copolymer and from 5 to 15 weight percent total carbohydrate compound. Any suitable disaccharide or oligosaccharide can be employed in the practice of this invention. Disaccharides consist of two simple sugar units bound together by a glycosidic linkage. Disaccharides may, for example, be composed of two identical monosaccharides as in maltose, or may be composed of two dissimilar monosaccharides as in sucrose or in lactose (milk sugar), which is made up of a D-galactose and a D-glucose unit. Suitable disaccharides for use in the practice of this invention include: sucrose, maltose, lactose, and cellobiose. Oligosaccharides are water-soluble polymers consisting of 2-10 monosaccharide units. Oligosaccharides can be further classified as homopolymers containing only one type of monosaccharide, or heteropolymers containing several different kinds of monosaccharides. Examples of oligosaccharides suitable for use in the practice of this invention include: stachyose, a naturally occurring tetrasaccharide, maltopentaose, a five monosaccharide unit oligosaccharide, cyclomaltohexaose, a cyclic polysugar consisting of six monosaccharide units, maltoheptaose, and maltotriose. The water-absorbing composition of this invention can be prepared using any suitable blending method such as described in the Examples which follow. After the water-absorbing composition is prepared, but typically before it is cured but in some instances as it is curing, it is processed into any desired form using conventional methods of fabrication. For example, the water-absorbing composition can be subjected to casting; spray drying; air-assisted spray drying; air attenuation; wet, dry or flash spinning; and the like. The selection of the process is typically dictated by the shape or form needed for end use. Forms that the water-absorbing composition may be fabricated into include films or sheets, powders and granules, fibers and any form into which fibers can be processed such as for example milled fibers, chopped fibers, fluff or bulk fibers, strands, yarns, woven fabrics, non-woven mats and the like using a variety of methods, including twisting, beaming, slashing, warping, quilling, severing, texturizing, weaving, knitting, braiding etc. While not meaning to be limited to any theory, the disaccharide or oligosaccharide compound is believed to serve as a high temperature, slow-reacting, cross-linking type agent for the copolymer particles resulting in the formation of covalent cross-link type bonds upon curing. For example, it has been found that, if a partially neutralized styrene-maleic anhydride copolymer is blended with sucrose to form a water-absorbing composition according to this invention, a temperature about 150°C. or higher is typically required to achieve cure. Similarly, if a partially neutralized ethylene-maleic anhydride copolymer is employed, a temperature of 140°C. or higher is typically needed to achieve cure. And, if a partially neutralized isobutylene-maleic anhydride copolymer is employed, a temperature of 200°C. or higher is typically needed to achieve cure. Without meaning to limit the invention, the water-absorbing compositions of this invention are particularly well suited for being made into fibers because of the wide time and temperature ranges over which they can be shaped. More specifically, the water-absorbing compositions of this invention can be formulated to cure at temperatures within the range of from about 140°C to about 250°C. or higher and possess shelf lives in excess of two months. Hence, the water-absorbing compositions of this invention can be easily made into fibers using conventional fiber-forming methods and equipment. Moreover, no post-treatment (e.g., a salt-forming reaction as taught in U.S. Patent 3,983,095) of the cured fiber products is required. The water-absorbing compositions of this invention and articles of manufacture into which the compositions are incorporated are suitable for use in a wide range of absorptive functions. In general, the articles into which the water-absorbing compositions are incorporated serve the function of supporting the composition and presenting it in a form adapted for absorptive end use. Means to support and present the composition for absorptive use include, but are not meant to be limited to bandages, surgical and dental sponges, tampons, sanitary napkins and pads, disposable diapers, meat trays, pads for absorption of perspiration, and the like. In one embodiment, a water-absorbing composition of this invention is incorporated into a disposable diaper, using conventional fabrication methods to form a diaper composite having the following typical layers: (1) an outer layer (away from the body) of impermeable polyethylene film; (2) a first cellulosic pulp layer superimposed on the film; (3) a layer of (i) a cured water-absorbing composition of this invention in the form of, for example, fluff, a fibrous mass, a non-woven fiber mat or a woven fabric; or (ii) a layer comprising a blend of a cured water-absorbing composition of this invention and another fluff conventionally employed in diapers; (4) an optional, second cellulosic pulp layer; and (5) an inner permeable polyethylene film layer. Fibers made from the water-absorbing compositions of this invention are particularly suitable for absorbent applications. It is well known that a mass of fibers provides a large surface area for contact with the liquid material to be absorbed. Fibers as compared to powders can also be more easily confined within the article into which they are incorporated. The following examples serve to further demonstrate the invention. EXAMPLE 1 This example demonstrates the preparation of a water-absorbing composition of the invention using ISOBAM 10 isobutylene/maleic anhydride copolymer commercially available from Kuraray Isoprene Chemical Company, Limited. ISOBAM 10 has a molecular weight of 170,000 and a maleic anhydride content of about 59.3 weight percent (46.6 mole %) as determined by titration of maleic acid. About 1270 g of ISOBAM 10 isobutylene/maleic anhydride copolymer and about 2007 g of demineralized water were added to a mixing vessel with agitation and the vessel contents were heated to about 90°C. At a temperature of about 90°C about 658 g of a 50% sodium hydroxide solution prepared from 98.9% pure sodium hydroxide pellets were slowly added to the mixing vessel over a one hour period with agitation. After the addition of the sodium hydroxide solution, agitation was continued for about 12 hours at 90°C. to complete the reaction. The pH of the solution was found to be 6.5 and the neutralization reaction was calculated to have converted about 53.5% of the pendant carboxylic acid units on the 46.6 mole % anhydride units into carboxylate sodium salt units. The balance of the pendant units were converted to carboxylic acid units. About 311 g of the above aqueous solution of the partially neutralized isobutylene/maleic anhydride copolymer were mixed with about 7.0 g sucrose and stirred for about 30 minutes. After stirring, the solution was heated to drive off excess water and to provide a solution having approximately a 45% solids content suitable for fiber formation. The composition (I) contained 7 phr of sucrose based on the weight of the isobutylene/maleic anhydride copolymer. About 10 micron diameter fibers were prepared from the composition using a dry spinning process. The resultant fibers were divided into four portions and separately cured by heating in a hot air circulating oven at various cure conditions. Each sample of fibers was tested to determine Swell Index and Percent Solubility using the test procedures described below. The test results are shown in Table 1. Swell Index This test procedure is described in U.S. 4,454,055 the teachings of which are incorporated herein by reference thereto. The test procedure and equipment used herein were modified slightly as compared to the procedure and equipment described in U.S. 4,454,055. To determine the Swell Index at atmospheric (room) pressure, about 0.2 to 0.3 g of the water-absorbing composition to be tested is placed in an empty W-shaped tea bag. The tea bag containing the composition is immersed in brine (0.9 wt.% NaCl) for 10 minutes, removed and allowed to sit on a paper towel for 30 seconds to remove surface brine. The Swell Index of the composition, that is, the units of liquid absorbed per each unit of sample is calculated using the following formula: To determine Swell Index under pressure, the following modified procedure was used. After the tea bag containing the sample composition is immersed in brine and surface brine is removed, it is immediately placed in a 16 cm ID Buchner funnel fitted with a 2000 ml sidearm vacuum filter flask and connected to a manometer. Then, a piece of dental dam rubber sheeting is securely fixed over the mouth of the funnel such that the sheeting just rests on the tea bag. Next, a vacuum sufficient to create the desired pressure is drawn on the flask for a period of five minutes, and, the Swell Index under pressure is calculated using the above formula. Percent Solubility About 0.5 g of the water-absorbing composition sample to be tested is dispersed in about 150 g of brine (0.9 wt.% NaCl) at room temperature for 20 minutes with occasional gentle agitation. After 20 minutes, the mixture is filtered through a 150 micron polypropylene screen. Next, the filtrate is dried to a constant weight in an oven and the weight of soluble composition determined by subtracting the weight of the NaCl from the total weight of the dry filtrate. Percent solubility is then determined using the following formula: EXAMPLE 2 Using substantially the procedure of Example 1, about 314 g of the aqueous solution of the partially neutralized isobutylene/maleic anhydride copolymer produced in Example 1 were mixed with about 10.1 g of sucrose and concentrated to give a 45% solids composition (Composition II) containing 10 phr of sucrose based on the weight of the isobutylene/maleic anhydride copolymer. About 10 micron diameter fibers were prepared from the composition as described in Example 1. The absorbency of the fibers and the effect of cure conditions on samples of the fibers are shown in following Table 2. EXAMPLE 3 This example demonstrates the preparation of a copolymer of styrene/maleic anhydride suitable for use in this invention. A solution of about 240 g of maleic anhydride, about 255 g of styrene, and 2370 ml of methyl ethyl ketone was prepared and introduced into a one gallon stirred reactor at room temperature under a nitrogen atmosphere. Free radical polymerization was initiated by charging about 8.20 g of VAZO 65 polymerization initiator (asoisobutyronitrile, E. I. DuPont) into the reactor. The polymerization reaction was conducted for about 24 hours at a temperature of aboout 55°C., and then tetrahydrofuran was added to dilute the reactor contents. The resulting polymerization product, a copolymer of styrene and maleic anhydride, was recovered by precipitation into methanol with high speed stirring. The copolymer was dried overnight at 30°C. in a vacuum oven, and then at 90°C. for about one hour. About 499 g of copolymer were recovered. The dried styrene/maleic anhydride copolymer was analyzed by titration of maleic acid and found to contain 43 mole percent maleic anhydride (41.7 wt.%). The balance of the copolymer was styrene. The copolymer was tested and found to have a glass transition temperature of 234°C. by differential scanning calorimetry and a peak molecular weight of 108,000 by gel permeation chromatography using polystyrene standards. EXAMPLE 4 This example demonstrates the preparation of a water-absorbing composition of the invention using the styrene/maleic anhydride copolymer of Example 3. About 30 g of copolymer and about 300 g of demineralized water were added to a mixing vessel with agitation and the vessel contents were heated to about 90°C. At a temperature of about 90°C. about 10.2 g of a 50% sodium hydroxide solution prepared from 98.9% pure sodium hydroxide pellets were slowly added to the mixing vessel over a one hour period with agitation. After the addition was completed, agitation was continued for about one hour and the copolymer dissolved into solution. The neutralization reaction was calculated to have converted about 50% of the pendant carboxylic acid units on the 43 mole % anhydride units into carboxylate sodium salt units. The balance of the pendant units were converted to carboxylic acid units. About 3 g of sucrose (table sugar) were added to the solution with stirring at 90°C. for a few hours to drive off excess water and to provide solution of approximately a 45% solids content suitable for fiber formation. The composition (III) contained 10 phr of sucrose based on the weight of the styrene/maleic anhydride copolymer. About 1-5 micron diameter fibers were prepared from the composition using a dry spinning process. The resultant fibers were cured by heating at 160°C. for 30 minutes in a hot air circulating oven. The sample of fibers was tested to determine Swell Index and Percent Solubility. The test results are shown in Table 3. It will be evident from the foregoing that various modifications can be made to this invention. Such, however, are considered as being within the scope of the invention as claimed.

Claims: A composition which is water-absorbent upon curing comprising: (a) a copolymer containing from 25 to 75 mole percent recurring units of at least one α,β-unsaturated monomer bearing at least one pendant group selected from carboxylic acid groups and carboxylate salt groups, from 20 to 80 percent of the total number of said pendant groups being carboxylic acid groups and from 80 to 20 percent being carboxylate salt groups, and from about 75 to about 25 mole percent recurring units of at least one copolymerizable comonomer, and (b) at least one carbohydrate compound selected from disaccharides and oligo-saccharides. The composition of claim 1 in which said copolymer contains from 35 to 65 mole percent recurring units of said at least one α,β-unsaturated monomer and from 65 to 35 mole percent of said at least one copolymerizable comonomer. The composition of claim 1 in which said copolymer is an equimolar copolymer of said α,β-unsaturated monomer and said copolymerizable comonomer. The composition of any one of claims 1 to 3 comprising in weight percent from 80 to 99 of said copolymer and from 1 to 20 of said carbohydrate compound. A composition as claimed in any one of claims 1 to 3 comprising in weight percent, from 85 to 95 of said copolymer and from 5 to 15 of said carbohydrate compound. The composition of any one of claims 1 to 5 in which said copolymer is derived from a copolymer selected from copolymers of styrene and maleic anhydride, copolymers of ethylene and maleic anhydride and copolymers of isobutylene and maleic anhydride. The composition of any one of claims 1 to 6 in which said compound (b) is selected from sucrose, lactose, maltose, cellobiose and oligosaccharides consisting of 2 - 10 monosaccharide units. A method of producing a water-absorbing composition comprising the steps of: (a) preparing a blend comprising a composition as claimed in any one of claims 1 to 7, and (b) curing the resulting blend. The method of claim 8 comprising the step of forming the blend into a shaped article after the blend is prepared but before it is fully cured. The method of claim 7 in which the blend is formed into fibers, powder or film The method of claim 8, claim 9 or claim 10 in which said curing is induced by heating the blend. An article of manufacture comprising a cured water-absorbing composition and a means for supporting said composition for absorption usage, wherein said water-absorbing composition is as claimed in any one of claims 1 to 7. The article of manufacture of claim 12 in which said means for supporting said composition is selected from disposable diapers, tampons, sanitary napkins, surgical and dental sponges and bandages. The article of claim 12 or claim 13 in which said composition is incorporated into said means for supporting in a fibrous form, the form of powder and/or the form of film. A method of enhancing the water absorption characteristics of an article of manufacture which comprises the step of incorporation into said article a cured water absorbing composition wherein said composition is as claimed in any one of claims 1 to 7, said composition being incorporated into said article in an effective amount to enhance at least one water-absorbing characteristic of said article as compared to the water-absorbing characteristics of the article in the absence of the composition. The method of claim 15 in which said article of manufacture is selected from disposable diapers, tampons, sanitary napkins, surgical and dental sponges and bandages. The method of claim 16 or claim 17 in which said composition is incorporated into said means for supporting in a fibrous form, in the form of powder, and/or in the form of film. A method of absorbing water and electrolyte solutions comprising the step of contacting the water or electrolyte solution to be absorbed with a cured water-absorbing composition as claimed in any one of claims 1 to 7.

Inventors: LE-KHAC, BI
Applicants: ARCO CHEMICAL TECHNOLOGY, L.P.
Application year: 1987
Main classification: C08L35
IPC codes: A61F13, C08L33, C08F8, A61L15, B01J20, D06M15, D06M13, A61F5, C08K5, C08L35