Method for producing water absorbent polyacrylic acid (salt) resin powder, and water absorbent polyacrylic acid (salt) resin powder

1. A method for producing water absorbent polyacrylic acid (salt) resin powder, comprising the steps of:

(i) polymerizing an acrylic acid (salt) monomer aqueous solution;

(ii) during or after the step of (i), performing gel grinding on a hydrogel crosslinked polymer which is obtained by the polymerization and which has a neutralization ratio of 45 mol % to 90 mol %, the step of (ii) being carried out on the condition that steps (1) and (2) are met:

(1) the gel grinding being carried out with a gel grinding energy (GGE) of 18[J/g] to 60[J/g];

(2) a weight average molecular weight of a water soluble content of the hydrogel crosslinked polymer being increased by 10,000[Da] to 500,000[Da]; and optionally

(3) the particulate hydrogel crosslinked polymer obtained by this step (ii) having a weight average particle diameter (D50) of 350 μm to 2000 μm, and a logarithmic standard deviation (σζ) of a particle size distribution of 0.2 to 1.0, wherein in a case of the hydrogel crosslinked polymer being subjected to the gel grinding with the above condition (3) being satisfied, the particulate hydrogel crosslinked polymer to be supplied into a through-flow belt drier has a resin solid content of 10 wt % to 80 wt %, and the through-flow belt drier sends to the particulate hydrogel crosslinked polymer hot air of 150° C. to 250° C. at a velocity of 0.8[m/s] to 2.5[m/s] in a vertical direction (up-and-down direction); and wherein the hydrogel crosslinked polymer has a resin solid content of 10 wt % to 80 wt %; and

(iii) drying a particulate hydrogel crosslinked polymer obtained by the gel grinding, wherein the drying is performed at 150° C. to 250° C. by use of a drier; and

(iv) carrying out a surface treatment to the dried particulate hydrogel crosslinked polymer.

2. The method as set forth in claim 1 , wherein the particulate hydrogel crosslinked polymer obtained by the step of (ii) has resin solid content of 10 wt % to 80 wt %.

3. The method as set forth in claim 1 ,

in the case where the gel pulverization of the step (ii) has been carried out by any one of (1) to (3),

a drier used in the step of (iii) is a through-flow belt drier configured to send hot air at a velocity of 0.8[m/s] to 2.5[m/s] in a direction vertical to the particulate hydrogel crosslinked polymer.

4. The method as set forth in claim 1 , wherein the step of (i) performs kneader polymerization or belt polymerization.

5. The method as set forth in claim 1 , wherein the step of (ii) is carried out with respect to:

(a) a hydrogel crosslinked polymer having gel CRC of 10[g/g] to 35[g/g];

(b) a hydrogel crosslinked polymer having gel Ext of 0.1 wt % to 10 wt %; or

(c) a hydrogel crosslinked polymer having gel CRC of 10[g/g] to 35[g/g] and gel Ext of 0.1 wt % to 10 wt %.

6. The method as set forth in claim 1 , wherein the step of (ii) is carried out with respect to a hydrogel crosslinked polymer having resin solid content of 40 wt % to 80 wt %.

7. The method as set forth in claim 1 , wherein the step of (ii) is carried out with respect to:

(d) a hydrogel crosslinked polymer having a monomer polymerization ratio of not less than 90 mol %; or

(e) a hydrogel crosslinked polymer having a monomer polymerization ratio of not less than 90 mol % and a neutralization ratio of 45 mol % to 90 mol %.

8. The method as set forth in claim 1 , wherein the step of (ii) is carried out with respect to a hydrogel crosslinked polymer having a gel temperature of 40° C. to 120° C.

9. The method as set forth in claim 1 , wherein the step of (ii) employs a screw extruder including a casing having an edge where a porous die is provided.

10. The method as set forth in claim 1 , wherein the step of (ii) is carried out so that gel Ext of the hydrogel crosslinked polymer is increased by not more than 5 wt %.

11. The method as set forth in claim 1 , wherein in the step of (ii), 0 to 4 part(s) by weight of water is added to 100 parts by weight of the hydrogel crosslinked polymer.

12. The method as set forth in claim 1 , wherein the particulate hydrogel crosslinked polymer thus obtained in the step of (ii) has at least one property of:

(g) gel Ext of 0.1 wt % to 10 wt %;

(h) gel CRC of 10[g/g] to 35[g/g]; and

(i) resin solid content of 10 wt % to 80 wt %.

13. The method as set forth in claim 1 , wherein the particulate hydrogel crosslinked polymer to be supplied into a through-flow belt drier in the step of (iii) has a temperature of 60° C. to 110° C.

14. The method as set forth in claim 1 , further comprising a classifying step,

the classifying step providing classified water absorbent resin particles having a weight average particle diameter (D50) of 250 μm to 500 μm and logarithmic standard deviation (σζ) of particle size distribution of 0.25 to 0.50.

15. The method as set forth in claim 1 , wherein the surface treatment includes a surface crosslinking step of performing surface crosslinking to the particulate hydrogel crosslinked polymer thus dried, and a step of adding, to the particulate hydrogel crosslinked polymer thus dried, at least one of (i) polyvalent metal salt, (ii) a cationic polymer and (iii) inorganic fine particles, the step of adding being carried out concurrently or non-concurrently with the surface crosslinking step.