Process for producing water-absorbing resin
A process for preparing a water-absorbent resin made from an α,β-unsaturated carboxylic acid as an essential monomer characterized in that the process comprises allowing a metal chelating agent to be present at any step in the process in an amount of 0.001 to 6 parts by weight, based on 100 parts by weight of the α,β-unsaturated carboxylic acid; and adding a reducing agent or an oxidizing agent thereto in an amount of 0.001 to 6 parts by weight, based on 100 parts by weight of the α,β-unsaturated carboxylic acid before initiation of drying and/or during drying of a gelated product containing a water-absorbent resin obtained by polymerization.
1. A process for preparing a water-absorbent resin, wherein said process comprises:
a) polymerizing to completion an α,β-unsaturated carboxylic acid monomer to produce a polymerized water-containing gelated product;
b) adding a metal chelating agent at any step in the preparation of the water-absorbent resin, wherein said metal chelating agent is added in an amount of 0.001 to 6 parts by weight, based on 100 parts by weight of the α,β-unsaturated carboxylic acid;
c) adding a reducing agent or an oxidizing agent to the polymerized water-containing gelated product in an amount of 0.001 to 2 parts by weight, based on 100 parts by weight of the α,β-unsaturated carboxylic acid; and
d) drying the gelated product thereby yielding a polymerized water-absorbent resin having greater discoloration resistance than a polymerized water-absorbent resin having no reducing or oxidizing agent and no metal chelating agent added thereto.
2. The process for preparing a water-absorbent resin according to claim 1 , wherein the reducing agent is a sulfite, a hydrogensulfite, a dithionite or a pyrosulfite.
3. The process for preparing a water-absorbent resin according to claim 1 , wherein the oxidizing agent is hydrogen peroxide.
4. The process for preparing a water-absorbent resin according to claim 1 , wherein the metal chelating agent is at least one member selected from the group consisting of diethylenetriaminepentaacetic acid, triethylenetetraminehexaacetic acid, trans-1,2-diaminocyclohexanetetraacetic acid, ethylenediaminetetraacetic acid, tripolyphosphoric acid, and salts thereof.
5. A water-absorbent resin obtained by the process of any one of claims 1 to 4 , wherein the water-absorbent resin has Yellow Index of 12 or less, after allowing to stand at 50° C. and 90% relative humidity for 20 days.
6. An absorbent comprising a water-absorbent resin obtained by the process of any one of claims 1 to 4 , and a hydrophilic fiber.
7. An absorbent article comprising the absorbent of claim 6 , wherein the absorbent is kept between a liquid-permeable sheet and a liquid-impermeable sheet.
8. A process for preparing a water-absorbent resin, wherein said process comprises, in the following order:
a) polymerizing to completion an α,β-unsaturated carboxylic acid monomer to produce a polymerized water-containing gelated product;
b) adding a metal chelating agent at any step in the preparation of the water-absorbent resin, wherein said metal chelating agent is added in an amount of 0.001 to 6 parts by weight, based on 100 parts by weight of the α,β-unsaturated carboxylic acid;
c) adding an oxidizing agent to the polymerized water-containing gelated product in an amount of 0.001 to 2 parts by weight, based on 100 parts by weight of the α,β-unsaturated carboxylic acid; and
d) drying the gelated product thereby yielding a polymerized water-absorbent resin having greater discoloration resistance than a polymerized water-absorbent resin having no oxidizing agent and no metal chelating agent added thereto.
9. The process for preparing a water-absorbent resin according to claim 1 , wherein the amount of the reducing agent or oxidizing agent added in step c) is 0.01 to 2 parts by weight, based on 100 parts by weight of the α,β-unsaturated carboxylic acid.
10. The process for preparing a water-absorbent resin according to claim 8 , wherein the amount of the oxidizing agent added in step c) is 0.01 to 2 parts by weight, based on 100 parts by weight of the α,β-unsaturated carboxylic acid.