Water-absorbing agent having water-absorbent resin as a main component and production method of the water-absorbing agent

A water-absorbing agent, comprising as a main component a water-absorbent resin having a crosslinked structure obtained by polymerizing an unsaturated monomer having a carboxyl group, wherein

• an internal crosslinking agent comprising a non-high molecular weight compound having 4 to 10 functional groups, each capable of forming a covalent bond with a carboxyl group, wherein the functional groups are one or more functional groups selected from the group consisting of a hydroxyl group, amino group, oxetane group, oxazoline, cyclocarbonate, oxazolidinone, and cyclic urea, 0.5 to 4 times as many carbon atoms as the number of said functional groups, a molecular weight of not less than 90 but not more than 5000, and a functional group equivalent weight of not less than 20 g/mol but not more than 200 g/mol is introduced into an inside of the water-absorbent resin, and the inside of the water-absorbent resin is crosslinked with at least a part of the non-high molecular weight compound introduced into the inside of the water-absorbent resin and where the inside of the water-absorbent resin is further crosslinked with an internal crosslinking agent other than the non-high molecular weight compound, which internal crosslinking agent has two or more polymerizable ethylenically unsaturated double bonds per molecule, wherein the water-absorbent resin is further subjected to surface crosslinking with an organic surface crosslinking agent and/or an inorganic surface crosslinking agent, and the water-absorbing agent including 100 ppm but not more than 10,000 ppm of the non-high molecular weight compound in which any of the functional groups capable of forming a covalent bond with a carboxyl group does not form a bond with the carboxyl group of the water-absorbent resin, and where an amount of usage of the non-high molecular weight compound is not less than 0.02 mol % but less than 24 mol % relative to the unsaturated monomer having the carboxyl group, and an amount of usage of the internal crosslinking agent having the two or more polymerizable ethylenically unsaturated double bonds per molecule is not less than 0.005 mol % but less than 0.1 mol % relative to the unsaturated monomer having the carboxyl group.

A method for producing a water-absorbing agent which includes as a main component a water-absorbent resin having a crosslinked structure obtained by polymerizing an unsaturated monomer having a carboxyl group,

• said method comprising the steps of:
• (i) polymerizing the unsaturated monomer having a carboxyl group and included in an aqueous monomer solution in the presence of an internal crosslinking agent comprising a non-high molecular weight compound having 4 to 10 functional groups, each capable of forming a covalent bond with a carboxyl group, wherein the functional groups are one or more functional groups selected from the group consisting of hydroxyl group, amino group, oxetane group, oxazoline, cyclocarbonate, oxazolidinone, and cyclic urea, 0.5 to 4 times as many carbon atoms as the number of said functional groups, a molecular weight of not less than 90 but not more than 5000, and a functional group equivalent weight of not less than 20 g/mol but not more than 200 g/mol, so as to obtain a hydropolymer, the aqueous monomer solution including an internal crosslinking agent other than the non-high molecular weight compound, which internal crosslinking agent has two or more polymerizable ethylenically unsaturated double bonds per molecule;
• (ii) drying the hydropolymer, obtained in the step (i), so as to obtain the water-absorbent resin, and
• (iii) crosslinking a surface of the water-absorbent resin with an organic surface crosslinking agent and/or an inorganic surface crosslinking agent, wherein
• relative to the unsaturated monomer having a carboxyl group, an amount of usage (Y) (mol %) of the non-high molecular weight compound is in the range expressed by the following equation (1):
• Y<0.06/{2−(1.35X/100)} (1)
• where X represents a neutralization ratio (mol %) of the carboxyl group of the water-absorbent resin, X being not less than 45 and not more than 85, and where the amount of usage of the internal crosslinking agent having the two or more polymerizable ethylenically unsaturated double bonds per molecule is not less than 0.005 mol % but less than 0.1 mol % relative to the unsaturated monomer having the carboxyl group.