Method for producing a water-absorbent resin

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 [m2], and T:
• Total amount [m3] of aqueous monomer solution added to a polymerization reaction tank.