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

• US 8,907,021 B2
• Filed: 08/27/2010
• Issued: 12/09/2014
• Est. Priority Date: 08/27/2009
• Status: Active Grant

• Current Assignee: Nippon Shokubai Company Limited
• Original Assignee: Nippon Shokubai Company Limited
• Inventors: Kitano, Takahiro, Nogi, Kozo, Ishizaki, Kunihiko
• Primary Examiner(s): LEE, DORIS L
• Application Number:
  US13/392,639
• Publication Number:
  US 20120157625A1
• Time in Patent Office: 1,565 Days
• Field of Search: 525/329.7
• 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

Disclosed is a method for producing a water absorbent resin, by which a surface-crosslinked water absorbent resin having excellent physical properties can be efficiently obtained at low cost, while assuring high productivity. When the production scale is increased to a continuous production at 1 t/hr or more, the physical properties are improved and stabilized (for example, standard deviation of the physical properties is reduced) by a surface-crosslinking treatment, and the absorption against pressure (AAP) and liquid permeability (SFC) are further improved. Specifically disclosed is a method for producing a water absorbent resin, which is characterized in that the stirring shaft of the continuous mixing apparatus for the surface-crosslinking agent is heated during the mixing step of the surface-crosslinking agent or that the continuous mixing apparatus for the surface-crosslinking agent is operated at a pressure that is reduced relative to the ambient pressure and a gas flow is passed through the mixing apparatus during the mixing of the surface-crosslinking agent so that the gas flow in the mixing apparatus is 40° C. or more (when defined with respect to the gas temperature at the exit).

Claims

1. A method for producing a polyacrylic acid (salt)-type water absorbent resin comprising steps of:

preparing an aqueous monomer solution of an acrylic acid (salt),

continuously polymerizing the aqueous monomer solution into a hydrous gel-like crosslinked polymer,

finely shredding the hydrous gel-like crosslinked polymer during or after polymerization,

drying the obtained particulate hydrous gel-like crosslinked polymer to obtain a dried water absorbent resin powder,

adding a surface-crosslinking agent to the dried water absorbent resin powder with a continuous mixing apparatus, wherein the pressure inside of the continuous mixing apparatus is a reduced pressure greater than 0 kPa and is 10 kPa or lower than atmospheric pressure, and

carrying out reaction of the mixture, wherein

a stifling shaft of the continuous mixing apparatus for the surface-crosslinking agent is heated in the step of mixing the surface-crosslinking agent.

3. The production method according to claim 1 or 2, wherein the stirring shaft of the continuous mixing apparatus for the surface-crosslinking agent is so heated as to be at a higher temperature than the inner wall temperature of the continuous mixing apparatus.

4. The production method according to claim 1 or 2, wherein the pressure of the inside of the continuous mixing apparatus for surface-crosslinking agent is reduced.

5. The production method according to claim 1 or 2, wherein 50 to 100% in the inner surface area of the continuous mixing apparatus is of a metal.

6. The production method according to claim 1 or 2, wherein the inner face of the inside of the continuous mixing apparatus for the surface-crosslinking agent is of a stainless steel with a surface roughness (Rz) of 800 nm or lower.

7. The production method according to claim 1 or 2, wherein the reaction step for the mixture is carried out at 150°

C. to 250°

C.

8. The production method according to claim 1 or 2, wherein the water absorbent resin powder is pneumatically transported by using compressed air with a dew point of −

5°

C. to −

100°

C.

9. The production method according to claim 1 or 2, wherein the mixing apparatus is a hoe type blender.

10. The production method according to claim 1 or 2, wherein the temperature of the mixture is 50 to 95°

C. until the mixture flows to the reaction step after the surface-crosslinking agent is mixed.

11. The production method according to claim 1 or 2, wherein the reaction step of the mixture has two or more lines in the polymerization step.

12. The production method according to claim 1 or 2, wherein the average residence time of the water absorbent resin in the continuous mixing apparatus is within 1 minute.

13. The production method according to claim 1 or 2, wherein the temperature of the mixture is increased by 0.5°

C. or more by continuously mixing the crosslinking agent to the dried water absorbent resin powder.

14. The production method according to claim 1 or 2, wherein the water absorbent resin has 20 g/g or higher of water absorption against pressure (AAP) for an aqueous 0.9 wt. % sodium chloride solution at a pressure of 4.8 kPa, 1 (×

10<sup>−

7</sup>·

cm³·

s·

g<sup>−

1</sup>) or higher of saline flow conductivity of 0.69 wt. % physiological saline solution (SFC), and 20 g/g or higher of water absorption under no pressure (CRC).

2. A method for producing a polyacrylic acid (salt)-type water absorbent resin comprising steps of:

preparing an aqueous monomer solution of an acrylic acid (salt),

continuously polymerizing the aqueous monomer solution into a hydrous gel-like crosslinked polymer,

finely shredding the hydrous gel-like crosslinked polymer during or after polymerization,

drying the obtained particulate hydrous gel-like crosslinked polymer to obtain a dried water absorbent resin powder,

adding a surface-crosslinking agent to the dried water absorbent resin powder with a continuous mixing apparatus, and

carrying out reaction of the mixture, wherein

the continuous mixing apparatus for the surface-crosslinking agent is operated at below ambient pressure and at a gas current of at least 40°

C. to adjust the outlet gas temperature of the gas current in the mixing apparatus during mixing the surface-crosslinking agent, wherein the pressure inside of the continuous mixing apparatus is a reduced pressure greater than 0 kPa and is 10 kPa or lower than atmospheric pressure.