Water-absorbent polymer and process for producing the same

A method for producing a water-absorbing resin, comprising:

• polymerizing a monomer solution to form a polymer by first adding a crosslinking agent (D) polymerizable with unsaturated monomers and then adding a condensation crosslinking agent (C) for a carboxyl group,
• crushing the resulting polymer,
• drying the crushed polymer,
• milling the dried polymer, and then
• heat-treating the milled polymer;
• or
• polymerizing a monomer solution to form a polymer by first adding a crosslinking agent (D) polymerizable with unsaturated monomers and then adding a condensation crosslinking agent (C) for a carboxyl group,
• crushing the resulting polymer,
• milling the crushed polymer,
• drying the milled polymer, and then
• heat-treating the dried polymer,
• wherein heat treating is performed at a temperature from 100 to 250° C.,
• wherein the monomer solution contains a carboxyl group-containing monomer and satisfies the following conditions (1) and (2):
• (1) before adding the crosslinking agent (D) and the condensation crosslinking agent (C), the monomer solution contains unsaturated carboxylate monomers comprising an ammonium unsaturated carboxylate (A) and an unsaturated carboxylic acid alkali metal salt (B);
• and
• (2) the percentage of the ammonium unsaturated carboxylate (A) is more than 50 mol % and 100 mol % or less, and the percentage of the unsaturated carboxylic acid alkali metal salt (B) is 0 mol % or more and less than 50 mol %, based on the total amount of unsaturated carboxylate monomers in the monomer solution.

A water-absorbing resin comprising a polymer molecular chain in which 50 mol % or more of repeating units in the chain are carboxyl group-containing units, wherein:

• a carboxyl group neutralization ratio in the central part of the resin is 60 mol % or more and a carboxyl group neutralization ratio on the external surface of the resin is 50 mol % or less,
• a condensation crosslinking agent (C) for a carboxyl group is distributed throughout the water-absorbing resin,
• the polymer has a skin layer having a thickness of 13 μm or more extending inward from the external surface of the polymer,
• the skin layer has a carboxyl group neutralization ratio less than that in the central part of the polymer, and
• the water-absorbing resin is produced by a process comprising polymerizing a monomer solution to form a polymer by first adding a crosslinking agent (D) polymerizable with unsaturated monomers and then adding the condensation crosslinking agent (C) for a carboxyl group.