METHOD FOR INCREASING THE GEL STRENGTH OF A WATER-ABSORBENT COMPOSITION

Abstract: Method for increasing the gel strength of a water-absorbent compositon comprising a water-absorbent amount of a crosslinked water-swellable hydrophilic polymer and an inorganic powder in an amount sufficient to increase the gel strength of the water-absorbent composition. The method is characterized by physically blending the inorganic polymer with the polymer after the polymer has been polymerized and crosslinked. The compositions produced by the method of this invention can be incorporated into applications useful in removing water from numerous systems.

Description: A METHOD FOR INCREASING THE GEL STRENGTH OF A WATER-ABSORBENT COMPOSITION The present invention relates to polymeric agents which are useful as water absorbents, and in particular, to polymeric agents which are useful in removing water from numerous systems. Water-swellable polymers have found various uses as absorbents for water. Examples of such polymers are disclosed in U.S. Patent Nos. 3,926,891,- 4,190,562 and 4,293,609. Unfortunately, such known water-swellable polymers have limited gel strength. The low gel strengths of such polymers introduce integrity problems to water- -absorbent systems resulting in poor handling problems as well as poor product efficiency (i.e. poor utilization of water absorbent capacity). Thus, although water- -absorbent polymers have high water-absorbent capabilities, the utilization of such polymers has been severely limited. Gel strengths can be increased by increasing the amount of crosslinking agent in the polymer. However, such polymers are very rigid gels which are difficult to produce and difficult to handle. U.S. Patent No. 3,900,378 is directed to particulate, free-flowing, insoluble swellable polymers blended with inert fillers. The mixtures are prepared by blending a water-soluble hydrophilic polymer and the inert filler in the dry state, adding water while the blending is continued, and then exposing the mixture to ionizing radiation for a period of time to crosslink the polymer. The amount of water added to the blended mixture is such that the filler is maintained in a free-flowing state. In view of the prior art, it would be highly desirable to prepare a water-absorbent composition having high water-absorbent capacity and increased gel strength without the use of ionizing radiation to crosslink the polymer. The present invention is directed to a method for increasing the gel strength of a water-absorbent composition comprising a water-absorbent amount of a crosslinked, water-swellable hydrophilic polymer " and an inorganic powder on an amount sufficient to increase the gel strength of the water-absorbent composition characterized by physically blending the inorganic powder with the polymer after the polymer has been polymerized and crosslinked. The present invention is directed to a method for increasing the gel strength of a water-absorbent composition comprising a water absorbent amount of a crosslinked water-swellable hydrophilic polymer and an inorganic powder in an amount sufficient to increase the gel strength of the water-absorbent composition, characterized by physically blending the inorganic powder with the polymer after the polymer has been polymerized and crosslinked. The method of this invention provides the skilled artisan with a composition for effectively absorbing water in the form of an aqueous medium or moisture wherein the increased gel strength of the water-absorbent composition is desirable. The composi¬ tion produced by the method of this invention can be employed in a wide variety of applications in which water and moisture absorbent materials have been employed. The water-swellable hydrophilic polymers useful in the method of this invention can be any of the known hydrophilic polymers that can be constructed in a composite of polymeric absorbent and inorganic filler. Preferred hydrophilic polymers are lightly crosslinked. Such hydrophilic polymers include, for example, acrylic copolymers, starch/graft copolymers, water-insoluable alkali salt of saponified, and gelatinized starch/polyacrylonitrile graft polymers. Examples of suitable polymers are disclosed in U.S. Patent Nos. 3,997,484; 3,926,891; 3,395,099; 4,090,013; 4,190,562; 4,405,387. Especially preferred polymers are disclosed in U.S. Patent Nos. 4,117,184; 4,176,677; 4,293,609; and 4,424,247. The preferred hydrophilic polymers useful in this invention are polyelectrolytes exemplary of which are ammonium or alkali metal salts of homopolymers of acrylic or methacrylic acid, and copolymers thereof with one or more ethylenically unsaturated comonomers. Examples of suitable monomers useful in preparing polymers; polymers and copolymers; crosslinking agents; etc. are disclosed in U.S. Patent No. 4,424,247. Also useful of this invention are polymers comprising polymerized sulfonate monomers which can be used in place of or in addition to the acrylate monomers. Examples of suitable water-soluble, ethylenically unsaturated sulfonate monomers include N-sulfoalkyl, ,β-ethylenically unsaturated amide salts such as the alkali metal salts of 2-acrylamido-2-methylpropane sulfonic acid, 2-acrylamido propane sulfonic acid, 2-acrylamido ethane sulfonic acid as well as other such monomers listed in U.S. Patent No. 3,692,673, alkali metal salts of sulfoalkyl esters of unsaturated carboxylic acids such as 2-sulfoethyl methacrylate and other such sulfoalkyl esters as listed in U.S. Patent No. 4,075,134; salts of sulfoarylalkenes such as vinylbenzyl sulfonic acid and the various salts of vinylbenzyl sulfonate, p-styrene sulfonic acid, salts of sulfoalkenes such as vinyl sulfonic acid. Of the foregoing sulfonate monomers, the sulfoalkyl derivatives of acrylamide and methacrylamide are preferred with those of acrylamide being especially preferred, particularly the sodium and potassium salts of 2-acrylamido-2-methylpropane sulfonic acid (AMPS), and 2-acrylamido-2-propane sulfonic acid. In the most preferred embodiments, the sulfo group is in the form of an alkali metal sulfonate such as sodium sulfonate. For the purpose of this invention, a moisture absorbent or water-swellable polyelectrolyte or polymer is one that absorbs several times its weight of an aqueous liquid, preferably more than 15 times its weight in water. The amount of water which a particular polymer will absorb (i.e., the water-absorbent capacity) is dependent upon the pH of the aqueous medium which is being absorbed, the molecular weight of the polymer, the amount of ionic character of the polymer, and the amount of crosslinking in the polymer. By the term "powder" is meant a very finely divided water insoluble or sparingly water-soluble grouping or agregate of solid particles, usually smaller than 1,000 μm. Inorganic powders especially preferred herein include a wide variety of clays. Specific examples include sodium bentonite (montmorillonite clay), kaolinite and attapulgite. Other inorganic powders include, for example, white carbon, synthetic silicate, basic magnesium carbonate, ultrafine magnesium silicate, light or heavy calcium carbonate, soft or hard clays, talc, vermiculite, pearlite, barium sulfate, and mica. In the method of this invention the desired inorganic powder is blended with the desired" polymer. Although the materials can be physically blended using a wide variety of means, dry blending is preferred. That is, essentially dry polymer is physically mixed with the desired essentially dry inorganic powder. Thorough mixing insures good gel strength of the composition. Generally the order of addition of components is not particularly critical. The amount of polymer employed in this inven- tion can range from 1 to 99, preferably from 20 to 90, most preferably from 40 to 75, especially preferably from 50 to 60, weight percent of polymer based on the total weight of the polymer and filler. Conversely, the amount of inorganic powder useful herein ranges from 1 to 99, preferably from 10 to 80, most preferably from 25 to 60, especially preferably from 40 to 50, weight percent of said inorganic powder based on the total weight of the polymer and filler. The compositions produced by the method of this invention exhibit increased gel strengths over those compositions containing a polymer and not containing the inorganic powder. The high gel strengths of such compositions allow for products exhibiting good utilization of water absorbent capacity as well as good integrity. In this regard, the compositions of this invention are easy to handle and can be employed in a wider variety of applications as have water absorbent materials been previously employed. The compositions of this invention can be incorporated into film laminates and other such materials as are described, for example, in U.S. Patent Nos. 4,117,184; 4,176,677; 4,293,609 and 4,424,247. For example, thoroughly mixed powder/polymer compositions can be incorporated in wicking substrates and treated as described in U.S. Patent No. 4,293,609. Such materials exhibit high water-absorbent capabilities, good integrity and resistance to separation of polymer and inorganic powder. Uses of the increased gel strength composi¬ tion produced by the method of the present invention include applications such as shut-off filters for fuels such as gasoline, oil, and hydraulic fluids. Other applications include the incorporation of such composi¬ tions, as is necessary, in disposable diapers, and other personal hygiene products and applications. The following examples are presented to further illustrate but not limit the scope of this invention. Examples 1 through 6 and Comparative Run A A crosslinked polyacrylamide polymer having a molecular weight of 4 million which is crosslinked with 500 ppm methylene bisacrylamide is dry blended with sodium bentonite in a Waring Blendor. The weight ratio of polymer to clay is 50/50. One (1) gram (g) of this composite is dispersed in 50 g of deionized water. After 20 minutes the composite gel/water mixture is filtered through a 115 mesh nylon screen (125 μm) . The gel is allowed to drip dry on the screen for 20 minutes. The gel is scraped from the wire mesh with a spatula and placed onto a 1.5 inch (38.1 mm) diameter cylinder which is equiped with a 20 mesh wire screen (850 μm) at the bottom end. Enough of the gelled composition is added to provide a 1 inch (25.4 mm) height of gel in the 1.5 inch (38.1 mm) diameter cylinder. A spatula is then used to make a small hole in the gel that runs from the top of the gel to the bottom of the screen in order to allow trapped air to escape. A weighed piston is then placed onto the cylinder which presses the gel onto the screen. An empty quart (9.46 x 10 -4m3) jar is placed on top of the piston and enough water is added until a small quantity of gel is extruded through the wire screen. The force of the jar plus water and that of the piston is used in order to determine the gel strength. Data concerning the gel strength of the composition is presented in Table I. TABLE I Examples and Percent Percent Gel 1 Compartive Inorganic Water-Swellable Strength, psi Run Powder Polymer (kPa) 1 90 10 <0.1 (<0.7) 2 75 25 0.9 (6.2) 3 50 50 1.6 (11.0) 4 40 60 2.1 (14.5) 5 25 75 1.5 (10.3) 6 10 90 0.4 (2.8) A 0 100 <0.1 (<0.7) ^ • The gel strength in pounds per square inch (psi) is determined by dividing the total force on the gel by the area of the piston. The data in Table I indicates that good gel strengths are observed for those compositions containing a blend of inorganic and water-swellable polymer.

Claims: 1. A method for increasing the gel strength of a water-absorbent composition comprising a water absorbent amount of a crosslinked water-swellable hydrophilic polymer and an inorganic powder in an amount sufficient to increase the gel strength of the water- bsorbent composition, characterized by physical blending the inorganic powder with the polymer after the polymer has been polymerized and crosslinked. 2. The method of Claim 1 characterized in that the inorganic powder is a clay. 3. The method of Claim 1 characterized in that the amount of said polymer ranges from 1 to 99 weight percent, and the amount of said inorganic powder ranges from 1 to 99 weight percent, based on the total weight of the polymer and inorganic powder. 4. The method of Claim 3 characterized in that the amount of said polymer ranges from 20 to 90 weight percent, and the amount of said inorganic powder ranges from 10 to 80 weight percent, based on the total weight of the polymer and inorganic powder. 5. The method of Claim 4 characterized in that the amount of said polymer ranges from 40 to 75 weight percent, and the amount of said inorganic powder ranges from 25 to 60 weight percent, based on the total weight of the polymer and inorganic powder. 6. The method of Claim 5 characterized in that the amount of said polymer ranges from 50 to 60 weight percent, and the amount of said inorganic powder ranges from 40 to 50 weight percent, based on the total weight of the polymer and inorganic powder. 7. The method of Claim 1 characterized in that essentially dry polymer is physically blended with essentially dry inorganic powder. 8. The method of Claim 2 characterized in that the clay is sodium bentonite or attapulgite. 9. The method of Claim 1 characterized in that the powder is a grouping or aggregate of solid particles smaller than 1000 μm. 10. The method of Claim 1 characterized in that the water-swellable hydrophilic polymer contains polymerized monomers of N-sulfoalkyl, α,β-ethylenically unsaturated salts.

Inventors: MCKINLEY MARK J, SHERIDAN DAN P, MCKINLEY, MARK, J., SHERIDAN, DAN, P.
Applicants: DOW CHEMICAL CO, THE DOW CHEMICAL COMPANY
Application year: 1985
Main classification: C08K3
IPC codes: A61L15, B01J20, C08J3, C08K3, C08L33, C08L101, G07C5