Method for producing polyacrylic acid (salt)-based water absorbent resin powder
From a view pint of decreasing an out of spec product after surface crosslinking, particularly, when an alkylene carbonate compound is used as a surface crosslinking agent, influence due to an air temperature is great, and it is necessary to reduce ethylene glycol which is produced as a byproduct. There is provided a process for producing a polyacrylic acid (salt)-based water absorbent resin powder, comprising storing a non-polymerized organic compound being a raw material (modified raw material) of a water absorbent resin powder in a heated state, thereafter, mixing the non-polymerized organic compound with other compound at a predetermined ratio and, further, mixing the mixture into a water absorbent resin powder.
1. A method for producing a polyacrylic acid (salt)-based water absorbent resin powder, comprising melt-heating a non-polymerized organic compound, where said non-polymerized organic compound is a surface crosslinking agent, mixing the non-polymerized organic compound in a heated-melted state with one or more of other compounds at a predetermined ratio to obtain a mixture, and mixing the resulting mixture with a polyacrylic acid (salt)-based water absorbent resin particle, wherein the polyacrylic acid (salt)-based water absorbent resin particle has a ratio of internal gas bubbles of 0.5 to 8.0%.
2. The method for producing a polyacrylic acid (salt)-based water absorbent resin powder according to claim 1 , further comprising mixing a mixture of said non-polymerized organic compound and said one or more other compounds, the mixture being controlled at a predetermined ratio with a mass flowmeter, with a polyacrylic acid (salt)-based water absorbent resin particle.
3. The method according to claim 1 , wherein a mass flowmeter is used for controlling a mixing ratio of the non-polymerized organic compound and the one or more other compounds, or, for controlling a mixing ratio of a mixture of the non-polymerized organic compound and the one or more other compounds, with a water absorbent resin.
4. The method according to claim 3 , wherein the mass flowmeter is a Coriolis-type mass flowmeter.
5. The method according to claim 2 , wherein the mass flowmeter is a Coriolis-type mass flowmeter.
6. The method according to claim 1 , wherein the mixture obtained by mixing at a predetermined ratio is an aqueous solution.
7. The method according to claim 1 , wherein the non-polymerized organic compound has a melting point of 10 to 100° C., and the heated-melted state is a state to be heated to a range exceeding a melting point by 100° C. or less and 30 to 90° C.
8. The method according to claim 1 , wherein a temperature of the one or more other compounds is 30° C. to 100° C., or a temperature of the one or more other compounds is controlled so that a temperature of the mixture of the non-polymerized organic compound and the one or more other compounds before mixing into the water absorbent resin is 30° C. to 100° C.
9. The method according to claim 1 , wherein an amount of water of the non-polymerized organic compound is 1.0% by weight or less.
10. The method according to claim 1 , wherein the non-polymerized organic compound is one or a plurality of dehydration reactive surface crosslinking agent of the water absorbent resin powder.
11. The method according to claim 1 , further comprising a step of mixing a polyvalent metal compound into a water absorbent resin simultaneously with, or separately from the mixture, and/or, a step of mixing at least one selected from a surfactant and a hydroxycarboxylic acid compound, into a water absorbent resin simultaneously, or separately from the mixture.
12. The method according to claim 1 , wherein the water absorbent resin particle to be mixed with the mixture has been heated to 35° C. or higher in advance.
13. The method according to claim 1 , wherein β-hydroxypropionic acid (salt) in the water absorbent resin particle before mixed with the mixture is 1000 ppm or less.
14. The method according to claim 1 , wherein the water absorbent resin powder is continuously produced at 1 ton or more per 1 hour.
15. The method according to claim 1 , wherein a strainer is used in a step of transporting the non-polymerized organic compound.
16. The method according to claim 1 , wherein an atmospheric dew point is 20° C. or lower in storage of the non-polymerized organic compound.
17. The method according to claim 1 , wherein a temperature of the mixture is controlled at 50° C. or lower and, the mixture is mixed into the water absorbent resin particle within 10 days from the time of mixing.
18. The method according to claim 1 , wherein the mixture consists of 3 or more components.
19. The method of claim 1 , wherein said surface crosslinking agent is an alkylene carbonate.
20. The method of claim 19 , wherein said alkylene carbonate is ethylene carbonate.
21. The method of claim 1 , wherein the resulting mixture of said surface crosslinking agent and said one or more other compounds is an aqueous solution including 10 to 30% by weight of said surface crosslinking agent.
22. The method of claim 1 , wherein said one or more other compounds is selected from the group consisting of water, a polyhydric alcohol, and a polyvalent metal salt.