Water-absorbing agent and its production process

1. A production process for a water-absorbing agent containing water-absorbent resin particles having an internal cross-linked structure obtained by polymerizing a water-soluble unsaturated monomer, the production process comprising the steps of:

(A) forming non-surface-treated water-absorbent resin particles having an internal cross-linked structure obtained by polymerizing the water-soluble unsaturated monomer in the presence of a covalent bonding cross-linking agent;

(B) obtaining surface treated water-absorbent resin fine particles by reacting resin fine particles in the presence of a surface-cross-linking agent; and

(C) causing the surface-treated water-absorbent resin fine particles to be contained in the non-surface-treated water-absorbent resin particles.

2. The production process according to claim 1 , wherein;

a mass ratio of the non-surface-treated water-absorbent resin particles and the surface-treated water-absorbent resin fine particles ranges from 99:1 to 50:50.

3. A water-absorbing agent obtained in the production process for a water-absorbing agent according to claim 2 .

4. The production process according to claim 1 , further comprising the step of:

(D) heating non-surface-treated water-absorbent resin particles having the surface-treated water-absorbent resin fine particles contained therein, or mixing water-absorbent resin particles having the surface-treated water-absorbent resin fine particles contained therein with a second surface cross-linking agent and then heating a resultant mixture, so as to cross-link vicinities of surfaces of the water-absorbent resin particles.

5. The production process according to claim 4 , wherein:

the step (D) is the following step (d):

(d) heating water-absorbent resin particles having a first water-soluble multivalent metal salt contained therein, or mixing the water-absorbent resin particles having the first water-soluble multivalent metal salt contained therein with the second surface cross-linking agent and then heating a resultant mixture, so as to cross-link vicinities of surfaces of the water-absorbent resin particles.

6. The production process according to claim 5 , further comprising the step of:

(e) adding a second water-soluble multivalent metal salt to surfaces of the water-absorbent resin particles obtained in the step (d).

7. The production process according to claim 6 , wherein:

in the step (c), a content of the first water-soluble multivalent metal salt in the water-absorbent resin particles ranges from 0.001 to 1 mass %, with respect to 100 parts by mass of the water-absorbent resin particles;

in the step (e), the amount of the second water-soluble multivalent metal salt added ranges from 0.001 to 4 mass %, with respect to 100 parts by mass of the water-absorbent resin particles; and

the content of the first water-soluble multivalent metal salt is less than the amount of the second water-soluble multivalent metal salt added.

8. The production process according to claim 6 , wherein:

the first water-soluble multivalent metal salt and the second water-soluble multivalent metal salt each include aluminum.

9. The production process according to claim 5 , wherein:

the surface cross-linking agent used in the step (d) includes polyhydric alcohol.

10. The production process according to claim 1 , wherein:

the steps (A) and (C) are respectively the following steps (a) and (c):

(a) step of forming water-absorbent resin particles not containing first water-soluble multivalent metal salt, the water-absorbent resin particles having an internal cross-linked structure and being obtained by polymerizing a water-soluble unsaturated monomer in the presence of a covalent bonding cross-linking agent; and

(c) step of causing water-absorbent resin fine particles containing the first water-soluble multivalent metal salt to be contained in the water-absorbent resin particles not containing the first water-soluble multivalent metal salt.

11. The production process according to claim 10 , wherein:

the water-absorbent resin fine particles including the first water-soluble multivalent metal salt are so small in size as to pass through a sieve having a mesh size of 300 μm.

12. The production process according to claim 10 , wherein:

the step (c) is realized by agglomeration of the water-absorbent resin particles formed in the step (a) and the water-absorbent resin fine particles containing the first water-soluble multivalent metal salt.

13. The production process according to claim 12 , wherein:

the agglomeration is performed in the presence of water.

14. The production process according to claim 10 , wherein:

the step (a) and step (c) are a step of, by polymerizing a water-soluble unsaturated monomer in the presence of a covalent bonding cross-linking agent before and/or during the addition of the water-absorbent resin fine particles containing the first water-soluble multivalent metal salt, forming water-absorbent resin particles not containing the first water-soluble multivalent metal salt and having an internal cross-linked structure.

15. The production process according to claim 10 , wherein:

the step (c) is realized by mixing a hydrogel water-absorbent resin having a moisture content of not less than 20 mass % and the water-absorbent resin fine particles containing the first water-soluble multivalent metal salt.

16. A water-absorbing agent obtained in the production process for a water-absorbing agent according to claim 1 .