Polyacrylic acid (salt)-type water absorbent resin and method for producing of same

• US 8,859,685 B2
• Filed: 08/27/2010
• Issued: 10/14/2014
• Est. Priority Date: 08/27/2009
• Status: Active Grant

• Current Assignee: Nippon Shokubai Company Limited
• Original Assignee: Nippon Shokubai Company Limited
• Inventors: Nogi, Kozo, Ishizaki, Kunihiko, Konishi, Kazumasa
• Primary Examiner(s): Rabago, Roberto
• Application Number:
  US13/392,214
• Publication Number:
  US 20120157650A1
• Time in Patent Office: 1,509 Days
• Field of Search: 525/329.7, 525/329.9, 525/330.2, 526/317.1
• US Class Current: 525/329.7
• CPC Class Codes: A61L 15/24 Macromolecular compounds obtained by reactions only involving carbon-to-carbon unsaturated bonds; Derivatives thereof A61L15/225 takes precedence
• CPC Class Codes: A61L 15/60 Liquid-swellable gel-forming materials, e.g. super-absorbents
• CPC Class Codes: C08F 120/06 Acrylic acid; Methacrylic acid; Metal salts or ammonium salts thereof
• CPC Class Codes: C08F 220/06 Acrylic acid; Methacrylic acid; Metal salts or ammonium salts thereof
• CPC Class Codes: C08F 222/102 of dialcohols, e.g. ethylene glycol di(meth)acrylate or 1,4-butanediol dimethacrylate
• CPC Class Codes: C08J 2300/14 Water soluble or water swellable polymers, e.g. aqueous gels
• CPC Class Codes: C08J 2333/02 Homopolymers or copolymers of acids; Metal or ammonium salts thereof
• CPC Class Codes: C08J 3/24 Crosslinking, e.g. vulcanising, of macromolecules mechanical aspects B29C35/00; crosslinking agents C08K
• CPC Class Codes: C08J 3/245 Differential crosslinking of one polymer with one crosslinking type, e.g. surface crosslinking
• CPC Class Codes: C08L 33/08 Homopolymers or copolymers of acrylic acid esters

Abstract

A process for producing a water absorbent resin is provided with which it is possible to efficiently and inexpensively obtain a surface-crosslinked water-absorbing resin having excellent material properties, while ensuring high productivity. In continuous production scaled up to a large scale (in particular, 1 t/hr or more), the physical properties are improved and stabilized (for example, a reduction in standard deviation of the physical properties) by a surface crosslinking treatment to further improve absorption against pressure (AAP) and saline flow conductivity (SFC). The process for producing a polyacrylic acid (salt)-type water absorbent resin includes a surface treatment step in which after addition of a surface-crosslinking agent, a crosslinking reaction is conducted in a transverse type continuous stirring apparatus having stirring means including a feeding inlet and a discharging outlet of a water absorbent resin and one or more rotary shafts having with a plurality of stirring discs, and heating means, at a stirring-power index of the device of 3-15 W·hr/kg.
(Stirring-power index)=((power consumption of apparatus at the time surface treatment)−(power consumption at the time of idling))×average retention time)/(treatment amount per unit time×average retention time).

Claims

1. A method for producing a polyacrylic acid (salt)-type water absorbent resin, comprising a step of preparing an aqueous monomer solution of an acrylic acid (salt), a step of continuously polymerizing the aqueous monomer solution, a step of finely shredding a hydrous gel-like crosslinked polymer during or after polymerization, a step of drying the obtained particulate hydrous gel-like crosslinked polymer, and a surface treatment step of adding and reacting a surface treatment agent to and with the dried water absorbent resin powder,

wherein crosslinking reaction is carried out at a stirring power index of 3 to 15 W·

hr/kg in a transverse type continuous stirring apparatus having stirring means including a feeding inlet and a discharging outlet of a water absorbent resin and one or more rotary shafts having with a plurality of stirring discs, and heating means after the addition of the surface-crosslinking agent in the surface treatment step;

wherein

(stirring power index)=((power consumption of apparatus at the time surface treatment)−

(power consumption at the time of idling))×

average retention time)/(treatment amount per unit time×

average retention time).

2. The production method according to claim 1 further comprising forcibly cooling the water absorbent resin powder by using a transverse type continuous stirring apparatus having a stirring power index of 5 to 13 (W·

hr/kg) after the heating treatment.

3. The production method according to claim 1, wherein the surface treatment step has two or more lines for one line in the polymerization step.

4. The production method according to claim 1, wherein a periodical shielding apparatus is installed in the inlet and/or the outlet of the transverse type continuous stirring apparatus to be used in the surface treatment step.

5. The production method according to claim 1, wherein the transverse type continuous stirring apparatus to be used in the surface treatment step has a downward inclined angle of 0.1 to 5)(°

).

6. The production method according to claim 1, wherein the transverse type continuous stirring apparatus to be used in the surface treatment step has a mass surface area ratio defined below of 100 (kg/m²/hr) or lower:

wherein,

(mass surface area ratio)=(mass flow rate of water absorbent resin per unit time)/(heat transfer area of apparatus).

7. The production method according to claim 1, wherein the transverse type continuous stirring apparatus to be used in the surface treatment step has an aspect ratio of 1.5 to 5.

8. The production method according to claim 1, wherein the transverse type continuous stirring apparatus to be used in the surface treatment step has scraping blades.

9. The production method according to claim 1, wherein the transverse type continuous stirring apparatus to be used in the surface treatment step has a surface roughness (Rz) in the inner face of 800 (nm) or lower.

10. The production method according to claim 1, wherein the bulk specific gravity of the polyacrylic acid (salt)-type water absorbent resin is 0.50 to 0.75 (g/cm³).

11. The production method according to claim 1, wherein a surfactant is added to the polyacrylic acid (salt)-type water absorbent resin.

12. The production method according to claim 1, wherein the inside of the transverse type continuous stirring apparatus is kept in slightly reduced pressure.

13. The production method according to claim 1, wherein a heat medium for the transverse type continuous stirring apparatus to be used in the surface treatment step is pressurized steam.

14. The production method according to claim 1, wherein the surface treatment step is carried out by adding a dehydration reactive crosslinking agent and heating treatment at 150 to 250°

C.

15. The production method according to claim 1, wherein the surface treatment step is carried out by adding an organic surface-crosslinking agent and/or a polyvalent metal salt.

16. The production method according to claim 1, wherein the polymerization step is carried out by continuous kneader polymerization or continuous belt polymerization.

17. The production method according to claim 1, wherein the polyacrylic acid (salt)-type water absorbent resin has 25 (×

10<sup>−

7</sup>·

cm³·

s·

g<sup>−

1</sup>) or higher of flow conductivity of physiological saline solution (SFC).