Method for producing water-absorbent resin

1. A method for producing a water-absorbent resin by a reversed-phase suspension polymerization method, the method comprising:

step 1 of dispersing a first aqueous solution containing a partially neutralized product (A) of a water-soluble ethylenically unsaturated monomer having acid groups in the molecule in a petroleum-type hydrocarbon dispersion medium in the presence of a dispersing agent, and then polymerizing the resulting dispersion to obtain a slurry containing primary particles of a polymer, and

step 2 of adding a second aqueous solution containing a partially neutralized product (B) of a water-soluble ethylenically unsaturated monomer having acid groups in the molecule to the slurry obtained in step 1, and then polymerizing the resulting mixture to obtain a slurry in which the primary particles are agglomerated, and

a drying step of removing the petroleum-type hydrocarbon dispersion medium and moisture from the slurry obtained in step 2,

wherein the dispersing agent is at least one member selected from the group consisting of a surfactant and a polymer-type dispersing agent, and

a molar neutralization degree X of the partially neutralized product (A) is set larger than a molar neutralization degree Y of the partially neutralized product (B), and a difference between the molar neutralization degree X and the molar neutralization degree Y is set to 5% or more.

2. The method for producing a water-absorbent resin according to claim 1 , wherein the addition and polymerization of the second aqueous solution are repeated in step 2.

3. The method for producing a water-absorbent resin according to claim 1 , wherein the molar neutralization degree X is set to 65 to 94%, and the molar neutralization degree Y is set to 56 to 89%.

4. The method for producing a water-absorbent resin according to claim 1 , wherein the water-soluble ethylenically unsaturated monomer having acid groups in the molecule is at least one member among those of acrylic acid and methacrylic acid.

5. The method for producing a water-absorbent resin according to claim 1 , wherein the surfactant is at least one kind selected from the group consisting of a polyglyceryl fatty acid ester, a sucrose fatty acid ester and a sorbitan fatty acid ester.

6. The method for producing a water-absorbent resin according to claim 1 , wherein the polymer-type dispersing agent is at least one kind selected from the group consisting of a maleic anhydride-modified polyethylene, a maleic anhydride-modified polypropylene, a maleic anhydride-modified ethylene-propylene copolymer, a maleic anhydride-ethylene copolymer, a maleic anhydride-propylene copolymer, a maleic anhydride-ethylene-propylene copolymer, a polyethylene, a polypropylene, an ethylene-propylene copolymer, an oxidized polyethylene, an oxidized polypropylene and an oxidized ethylene-propylene copolymer.

7. The method for producing a water-absorbent resin according to claim 1 , wherein the first aqueous solution is dispersed in the petroleum-type hydrocarbon solvent containing the polymer-type dispersing agent, and the surfactant is further added to the obtained dispersion, and then the resulting mixture is polymerized in step 1.

8. The method for producing a water-absorbent resin according to claim 1 , further comprising step 3 of post-crosslinking the water-absorbent resin obtained in step 2 using a post-crosslinking agent.

9. The method for producing a water-absorbent resin according to claim 8 , wherein the post-crosslinking agent is at least one kind selected from the group consisting of ethylene glycol diglycidyl ether, polyethylene glycol diglycidyl ether, ethylene glycol triglycidyl ether, polyethylene glycol triglycidyl ether, glycerol diglycidyl ether, polyglycerol diglycidyl ether, glycerol triglycidyl ether, polyglycerol triglycidyl ether, propylene glycol polyglycidyl ether, polypropylene glycol polyglycidyl ether, glycerol polyglycidyl ether and polyglycerol polyglycidyl ether.