Method for producing polyacrylic acid (salt)-based water-absorbent resin
A method for producing polyacrylic acid (salt)-based water-absorbing resin includes a step of polymerizing an acrylic acid (salt)-based aqueous monomer solution, the polymerization step involving use of a polymerization reaction apparatus including a polymerization reaction unit covered with a casing, the polymerization step involving a polymerization reaction under internal pressure of the polymerization reaction unit, the internal pressure being slightly reduced and having a pressure reduction level of more than 0 kPa and not more than 10 kPa relative to the ambient pressure at the periphery of the polymerization reaction unit.
1. A method for producing polyacrylic acid (salt)-based water-absorbing resin comprising a step of polymerizing an acrylic acid (salt)-based aqueous monomer solution,
wherein a polymerization reaction apparatus used in said polymerization step has a polymerization reaction unit covered with a casing, and
wherein, in said polymerization step, a polymerization reaction is carried out under conditions that internal pressure of said polymerization reaction unit is reduced by a range of more than 0 kPa and not more than 10 kPa relative to ambient pressure at the periphery of said polymerization reaction unit.
2. The method according to claim 1 , wherein the internal pressure of said polymerization reaction unit is measured inside discharge piping connected to a discharge outlet installed at said polymerization reaction unit.
3. The method according to claim 2 , wherein a plurality of said discharge outlets are installed.
4. The method according to claim 1 , wherein a gas is supplied from outside to inside of said polymerization reaction unit.
5. The method according to claim 4 , wherein said gas is air or a mixed gas containing air as a main component.
6. The method according to claim 4 , wherein a temperature of said gas is lower than 50° C.
7. The method according to claim 4 , wherein a range of temperature change for said gas is 5° C. to 40° C. by the hour, by the day or by the season.
8. The method according to claim 1 , wherein, in said polymerization step, a temperature of an aqueous monomer solution that is supplied to said polymerization reaction unit is not lower than 40° C. and lower than 100° C.
9. The method according to claim 1 , wherein, in said polymerization step, a water content of an aqueous monomer solution that is supplied to said polymerization reaction unit is 30 weight % to 70 weight %.
10. The method according to claim 1 , wherein, in said polymerization step, a polymerization initiator contained in an aqueous monomer solution that is supplied to said polymerization reaction unit is a thermal decomposition initiator.
11. The method according to claim 1 , wherein, in said polymerization step, time for polymerization is not longer than 60 minutes.
12. The method according to claim 1 , wherein, in said polymerization step, a highest temperature point is not lower than 100° C.
13. The method according to claim 1 , wherein said polymerization reaction unit includes a continuous kneader polymerization device or a continuous belt polymerization device.
14. The method according to claim 1 , wherein a rate of volatilization defined as a proportion of an entire weight of a hydrogel obtained through said polymerization step relative to an entire weight of an aqueous monomer solution that is supplied in said polymerization step is not less than 1 weight %.
15. The method according to claim 1 , wherein a volatile component that is volatilized in said polymerization step is collected after discharged to outside said polymerization reaction unit.
16. The method according to claim 15 , wherein said volatile component is collected by cooling or by water outside said polymerization reaction apparatus.
17. The method according to claim 15 , wherein said collected volatile component is reused during said polymerization step.