Water-absorbent agent, method for production thereof, and water- absorbent composition

Abstract

A method for the production of a water-absorbent agent comprises mixing an absorbent resin containing a carboxyl group with an additive soluble in the aqueous solution of at least one member selected from the group consisting of inorganic acids, organic acids, and polyamino acids and a cross-linking agent capable of reacting with the carboxyl group. A water-absorbent agent obtained by adding from 0.005 to 8 parts by weight of an epoxy compound to 100 parts by weight of a water-absorbent resin, exhibiting an absorption capacity without load of at least 45 (g/g), an absorption capacity under load of 20 g/cm 2 of not less than 30 (ml/g) and having a residue amount of an epoxy compound of not more than 2 ppm. A water-absorbent composition comprises a polyamino acid (salt) and a water-absorbent resin containing a carboxyl group.

What is claimed is:  
     
         1 . A method for the production of a water-absorbent agent which comprises mixing a water-absorbent resin containing a carboxyl group with an additive soluble in the aqueous solution of at least one member selected from the group consisting of inorganic acids, organic acids, and polyamino acids and a cross-linking agent capable of reacting with said carboxyl group.  
     
     
         2 . A method according to  claim 1 , which further comprises mixing the water-absorbent resin with said additive and said cross-linking agent and subsequently subjecting the resultant mixture to a heat treatment at a temperature in the range of from 100 to 230° C.  
     
     
         3 . A method according to  claim 1 , which further comprises preparatorily mixing said additive and said cross-linking agent and subsequently adding the resultant mixture to the absorbent resin.  
     
     
         4 . A method according to  claim 1 , wherein said additive is at least one member selected from the group consisting of saturated organic carboxylic acids, saturated inorganic acids of elements of Group 3 in the periodic table of elements, and poly(monoaminodicarboxylic acids).  
     
     
         5 . A method according to  claim 1 , wherein the water-absorbent resin containing a carboxyl group is obtained by polymerizing a hydrophilic monomer having acrylic acid and/or a salt thereof as a main component thereof.  
     
     
         6 . A method according to  claim 1 , wherein the cross-linking agent capable of reacting with the carboxylic group is an epoxy compound.  
     
     
         7 . A method according to  claim 1 , wherein the acid dissociation index (pKa value) of said additive is in the range of from 2.0 to 4.0.  
     
     
         8 . A method according to  claim 4 , wherein the additive is a saturated organic carboxylic acid.  
     
     
         9 . A method according to  claim 8 , wherein said saturated organic carboxylic acid is at least one compound selected from the group consisting of citric acid, succinic acid, and lactic acid.  
     
     
         10 . A method according to  claim 1 , wherein the mixture is effected by the use of from 0.01 to 5 parts by weight of water based on 100 parts by weight of said water-absorbent resin.  
     
     
         11 . A method according to  claim 1 , wherein the water-absorbent resin comprises irregular broken particles of an average particle diameter of from 200 to 600 μm and includes not more than 10% by weight of a fraction of less than 150 μm in diameter.  
     
     
         12 . A method according to  claim 1 , wherein said additive is used at a ratio in the range of from 0.01 to 5 parts by weight to 100 parts by weight of said water-absorbent resin.  
     
     
         13 . A method according to  claim 1 , wherein said water-absorbent resin has a water content in the range of from 1 to 10%.  
     
     
         14 . A water-absorbent agent obtained by adding from 0.005 to 8 parts by weight of an epoxy compound to 100 parts by weight of a water-absorbent resin containing carboxyl group, exhibiting an absorption capacity without load of not less than 45 (g/g) and an absorption capacity under a load of 20 g/cm 2  of not less than 30 (ml/g) and having a residue amount of an epoxy compound content of not more than 2 ppm.  
     
     
         15 . A water-absorbent agent according to  claim 14 , wherein a hydroxyl group-containing saturated organic acid is contained at a concentration in the range of from 0.01 to 5% by weight (based on said absorbent resin).  
     
     
         16 . A water-absorbent agent according to  claim 14 , wherein the absorption ratio under a load of 20 g/cm 2  is not less than 35 (ml/g) and the epoxy compound is no longer detected.  
     
     
         17 . A water-absorbent composition comprising a polyamino acid (salt) and a water-absorbent resin containing a carboxyl group.  
     
     
         18 . A composition according to  claim 17 , wherein said water-absorbent resin is a cross-linked polymer of acrylic acid.  
     
     
         19 . A composition according to  claim 17 , wherein the ratio of said polyamino acid (salt) is from 0.01 to 30 parts by weight to 100 parts by weight of said water-absorbent resin.  
     
     
         20 . A composition according to  claim 19 , wherein said polyamino acid (salt) is at least one member selected from the group consisting of polyaspartic acid (salt), polyglutamic acid (salt), and polylysine (salt).  
     
     
         21 . A water-absorbent composition comprising an absorbent resin exhibiting an absorption capacity without load of at least 25 g/g to physiological saline solution and a suction power of not less than 14 g/g to physiological saline solution and a suction power of at least 7 g/g to bovine blood.  
     
     
         22 . A water-absorbent structure comprising from 10 to 95 parts by weight of a water-absorbent composition containing a polyamino acid(salt) and a water-absorbent resin having a carboxyl group and from 90 to 5 parts by weight of hydrophilic fibers.  
     
     
         23 . A water-absorbent structure according to  claim 22 , which further comprises up to 40 parts by weight of thermoplastic fibers based on 100 parts by weight of said hydrophilic fibers.  
     
     
         24 . A water-absorbent structure according to  claim 22 , wherein the density is in the range of from 0.06 to 0.5 g/cc.  
     
     
         25 . A water-absorbent structure comprising 50 to 95 parts by weight of a water-absorbent composition formed of a water-absorbent resin exhibiting an absorption capacity without load of at least 25 g/g to physiological saline solution and a suction power of not less than 14 g/g to physiological saline solution and suction power of at least 7 g/g to bovine blood and 50 to 5 parts by weight of a hydrophilic fibers.  
     
     
         26 . A water-absorbent structure according to  claim 25 , which further comprises up to 40 parts by weight of thermoplastic fibers based on 100 parts by weight of said hydrophilic fibers.  
     
     
         27 . A water-absorbent structure according to  claim 25 , wherein the density is in the range of from 0.06 to 0.5 g/cc.  
     
     
         28 . A water-absorbent article furnished with the water-absorbent structure set forth in any of claims  22  through  27  and/or the water-absorbent composition set forth in any of  claims 17  to  21 .