Water absorbent resin and method for production thereof

Abstract

According to the present invention, a balance between lowering of the fine powder generated during the production, and lowering of the amount of residual monomer in the resulting water absorbent resin can be achieved. More particularly, according to the present invention, a water absorbent resin which includes a small amount of residual monomer and has a favorable physical property can be obtained while effectively reutilizing the fine powder generated in the production step of the water absorbent resin, without increasing using amount of an additive (at least one selected from the group consisting of thermal initiators, oxidizing agents and reducing agents) for reducing the residual monomer. In other words, the method for production of a water absorbent resin of the present invention is a method in which the residual monomer can be efficiently reduced, and in addition, an effect in terms of the production cost, i.e., recycling of the fine powder, can be also achieved similarly to conventional recycling methods of fine powder through agglomeration.

1 . A method for production of a water absorbent resin which comprises: 
 a step of obtaining a polymer gel having a water absorbing property,    a step of obtaining an agglomerated gel by adding an aqueous fluid containing at least one additive selected from the group consisting of thermal initiators, oxidizing agents and reducing agents to the fine powder which is obtained in the production of a water absorbent resin and has a weight average particle diameter falling within the range of 10 to 150 μm, and    a step of drying while allowing said agglomerated gel and said polymer gel to coexist.    
     
     
         2 . The method for production of a water absorbent resin according to  claim 1  wherein said aqueous fluid is previously heated before addition to the fine powder.  
     
     
         3 . The method for production of a water absorbent resin according to  claim 1  wherein said additive comprises a persulfate.  
     
     
         4 . The method for production of a water absorbent resin according to  claim 2  wherein said additive comprises a persulfate.  
     
     
         5 . The method for production of a water absorbent resin according to  claim 1  wherein ratio (A/B) of the solid content rate A (%) of the agglomerated gel to the solid content rate B (%) of the polymer gel is not less than ⅓ but not greater than 3, in said step of drying while allowing said agglomerated gel and said polymer gel to coexist.  
     
     
         6 . The method for production of a water absorbent resin according to  claim 2  wherein ratio (A/B) of the solid content rate A (%) of the agglomerated gel to the solid content rate B (%) of the polymer gel is not less than ⅓ but not greater than 3, in said step of drying while allowing said agglomerated gel and said polymer gel to coexist.  
     
     
         7 . The method for production of a water absorbent resin according to  claim 3  wherein ratio (A/B) of the solid content rate A (%) of the agglomerated gel to the solid content rate B (%) of the polymer gel is not less than ⅓ but not greater than 3, in said step of drying while allowing said agglomerated gel and said polymer gel to coexist.  
     
     
         8 . The method for production of a water absorbent resin according to  claim 4  wherein ratio (A/B) of the solid content rate A (%) of the agglomerated gel to the solid content rate B (%) of the polymer gel is not less than ⅓ but not greater than 3, in said step of drying while allowing said agglomerated gel and said polymer gel to coexist.  
     
     
         9 . The method for production of a water absorbent resin according to  claim 1  wherein quantity of solid content in the agglomerated gel becomes not greater than 40% by weight of the quantity of solid content in the polymer gel, in said step of drying while allowing said agglomerated gel and said polymer gel to coexist.  
     
     
         10 . The method for production of a water absorbent resin according to  claim 2  wherein quantity of solid content in the agglomerated gel becomes not greater than 40% by weight of the quantity of solid content in the polymer gel, in said step of drying while allowing said agglomerated gel and said polymer gel to coexist.  
     
     
         11 . The method for production of a water absorbent resin according to  claim 3  wherein quantity of solid content in the agglomerated gel becomes not greater than 40% by weight of the quantity of solid content in the polymer gel, in said step of drying while allowing said agglomerated gel and said polymer gel to coexist.  
     
     
         12 . The method for production of a water absorbent resin according to  claim 4  wherein quantity of solid content in the agglomerated gel becomes not greater than 40% by weight of the quantity of solid content in the polymer gel, in said step of drying while allowing said agglomerated gel and said polymer gel to coexist.  
     
     
         13 . The method for production of a water absorbent resin according to  claim 5  wherein quantity of solid content in the agglomerated gel becomes not greater than 40% by weight of the quantity of solid content in the polymer gel, in said step of drying while allowing said agglomerated gel and said polymer gel to coexist.  
     
     
         14 . The method for production of a water absorbent resin according to  claim 6  wherein quantity of solid content in the agglomerated gel becomes not greater than 40% by weight of the quantity of solid content in the polymer gel, in said step of drying while allowing said agglomerated gel and said polymer gel to coexist.  
     
     
         15 . The method for production of a water absorbent resin according to  claim 7  wherein quantity of solid content in the agglomerated gel becomes not greater than 40% by weight of the quantity of solid content in the polymer gel, in said step of drying while allowing said agglomerated gel and said polymer gel to coexist.  
     
     
         16 . The method for production of a water absorbent resin according to  claim 8  wherein quantity of solid content in the agglomerated gel becomes not greater than 40% by weight of the quantity of solid content in the polymer gel, in said step of drying while allowing said agglomerated gel and said polymer gel to coexist.  
     
     
         17 . A water absorbent resin which comprises a polymer gel having a water absorbing property and an agglomerated gel, wherein said agglomerated gel consists of fine powders having a weight average particle diameter falling within the range of from 10 to 150 μm; ratio (A/B) of the solid content rate A (%) of said agglomerated gel to the solid content rate B (%) of the polymer gel is not less than ⅓ but not greater than 3, and the amount of residual monomer is not lower than 0 but not higher than 500 ppm.