Patent ID: 8859700

Claim:
A method for producing a water-absorbent resin by a multi-stage reversed-phase suspension polymerization to polymerize a water-soluble ethylenically unsaturated monomer, wherein the first stage polymerization comprising at least the following steps: (A) performing a primary dispersion in the absence of surfactants by stirring to mix an aqueous solution of a water-soluble ethylenically unsaturated monomer containing a water-soluble radical polymerization initiator in a petroleum hydrocarbon dispersion medium in which a hydrophobic polymeric dispersion agent is dispersed or dissolved; (B) performing a secondary dispersion by adding a surfactant to the resultant dispersion liquid; and (C) performing a radical polymerization to obtain water-absorbent resin particles in a hydrous gel state which disperse in the petroleum hydrocarbon dispersion medium; and the second stage polymerization comprising at least the following steps: (D) precipitating at least a part of the surfactant; (E) stirring to mix the aqueous solution of the water-soluble ethylenically unsaturated monomer of the second stage polymerization containing a water-soluble radical polymerization initiator therein to be absorbed and aggregated in the polymerized gel at the first stage; and (F) performing a radical polymerization again; wherein the aqueous solution of the water-soluble ethylenically unsaturated monomer is added in step (A) to the petroleum hydrocarbon dispersion medium at an addition rate V of 0.30 [min −1 ] or less defined by the following Equation (I): V=F×A/T wherein V: Addition rate [min −1 ], F: Average linear flow rate from nozzle [m/min], A: Cross-sectional area of nozzle [m 2 ], and T: Total amount [m 3 ] of aqueous monomer solution added to a polymerization reaction tank.