Method for producing polyacrylic acid (salt)-based water absorbent resin

1. A method for producing a polyacrylic acid (salt)-based water absorbent resin, the method comprising a polymerization step of supplying as a base material an aqueous solution containing an acrylic acid and/or an acrylic acid salt as a monomer component and polymerizing the monomer in the presence of a polymerization initiator,

wherein in the polymerization step, there is used a polymerization apparatus which comprises a polymerization part covered with a case, said polymerization part comprising at least a supply line for supplying the aqueous solution, an external gas supply port, and a gas discharge port, and has a structure that a liquid contact part in contact with the aqueous solution and a gas supplied from the outside of the polymerization apparatus are brought into contact with the aqueous solution during a polymerization, and

wherein a controlled temperature of the liquid contact part is set as TS, a temperature of the gas is set as TG, and TT is (TS+TG)/2, the polymerization is carried out under temperature conditions satisfying the following Equations 1 to 3

35° C.≦TS≦85° C.  Equation 1

40° C.≦TG≦90° C.  Equation 2

47° C.≦TT≦73° C.  Equation 3

2. The method according to claim 1 , wherein in the polymerization step, the temperature, TW, of the aqueous solution to be supplied to the polymerization part is not less than 40° C. and lower than 100° C.

3. The method according to claim 2 , wherein in the polymerization step, the temperature, TW, of the aqueous solution is not lower than 40° C. over a period from the preparation of the aqueous solution to the start of polymerization.

4. The method according to claim 1 , wherein a weight of the aqueous solution per unit area at a site of the polymerization part in contact with the aqueous solution is 6.2 to 23.4 kg/m 2 .

5. The method according to claim 1 , wherein a water content, PW, of the aqueous solution is 70 wt % or less.

6. The method according to claim 5 , wherein a difference (PW−AW) between the PW and the AW is 5 wt % or more, wherein “AW” is a water content of a polymerization product obtained at the time when the polymerization step is completed.

7. The method according to claim 1 , wherein the polymerization apparatus is a continuous kneader type apparatus or a continuous belt type apparatus.

8. The method according to claim 1 , the method further comprising a drying step, a pulverization step; and a surface cross-linking step.

9. The method according to claim 1 , the method further comprising a fine powder recycling step.

10. The method according to claim 1 , wherein a polymerization peak temperature is higher than 100° C.

11. The method according to claim 1 , wherein a polymerization time is 10 minutes or shorter.

12. The method according to claim 1 , wherein the gas contains oxygen.

13. The method according to claim 1 , wherein the gas contains air as a main component of the gas.

14. The method according to claim 1 , wherein a temperature of the gas supplied from the outside is 40 to 90° C.

15. The method according to claim 1 , wherein supply of the gas from the outside is carried out by gas supply and/or suction.

16. The method according to claim 1 , wherein an amount (m 3 /min) of the gas supplied is in the range of 0.01 to 1000 m 3 /min.

17. The method according to claim 1 , wherein the polymerization initiator is a heat decomposition type polymerization initiator.