Particulate water absorbing agent and method for manufacturing same

1. A method for producing a particulate water absorbing agent, the particulate water absorbing agent being arranged such that particles having passed through a 150-μm mesh as defined through a standard-sieve classification are contained in an amount of 3 mass % or less, the method comprising the steps of:

(1) polymerizing a monomer aqueous solution including acrylic acid (salt) as a main component, the monomer aqueous solution having a monomer concentration of 30 mass % or more, the polymerization involving use of an internal crosslinking agent in an amount of 0.04 mol % or more and 0.07 mol % or less relative to the monomer and producing a hydrogel;

(2) drying the hydrogel, produced through the step (1), to produce a dried product having a swelling rate (CRC) of 35 g/g to 55 g/g;

(3) pulverizing the dried product into particles and optionally classifying the particles to produce a water absorbent resin powder including, at 80 mass % or more relative to the entire water absorbent resin powder, a particle having a size of 600 μm to 150 μm as defined through a standard-sieve classification;

(4) adding a surface-crosslinking agent, which includes a compound containing a hydroxyl group and/or a derivative group thereof, to the water absorbent resin powder, produced through the step (3), to decrease the swelling rate (CRC) by 3 g/g to 15 g/g to a swelling rate (CRC) of 32 g/g to 50 g/g to produce a surface-crosslinked water absorbent resin powder, and

(5) adding a liquid permeability improving agent to the water absorbent resin powder simultaneously with and/or after the step (4).

2. The production method according to claim 1 , wherein:

the step (4) involves two or more kinds of organic compounds as the surface-crosslinking agent.

3. The production method according to claim 1 , wherein:

the step (5) involves adding, as the liquid permeability improving agent, a water-insoluble inorganic fine particle in an amount of 0.05 part by mass to 5.0 parts by mass relative to 100 parts by mass of the water absorbent resin powder and/or surface-crosslinked water absorbent resin powder, wherein the water-insoluble inorganic fine particles each have a particle size of 1 nm or higher and 50 μm or less.

4. The production method according to claim 3 , wherein:

the water-insoluble inorganic fine particle is made of silicon dioxide, wherein the water-insoluble inorganic fine particles each have a particle size of 1 nm or higher and 50 μm or less.

5. The production method according to claim 1 ,

wherein:

the step (5) involves adding, as the liquid permeability improving agent, a water-soluble polyhydric metal cation in an amount of 0.01 part by mass to 5.0 parts by mass relative to 100 parts by mass of the water absorbent resin powder and/or surface-crosslinked water absorbent resin powder.

6. The production method according to claim 5 , wherein:

the water-soluble polyhydric metal cation is an aluminum cation.

7. The production method according to claim 1 , wherein:

the water absorbent resin powder to be subjected to the step (4) includes, at 5 mass % or less, a particle that passes through a 150-μm mesh as defined through a standard-sieve classification and/or includes, at 5 mass % or less, a particle that does not pass through a 710-μm mesh as defined through a standard-sieve classification.

8. The production method according to claim 1 , wherein:

a first classification step is performed before the step (4); and

a second classification step is performed after the step (4).