Water absorbing resin with improved internal structure and manufacturing method therefor
According to the present invention, the manufacturing method for the water absorbing resin involves the step of polymerizing a water-soluble unsaturated monomer, 0.06 of 5 mol % of which is composed of an internal crosslinking agent; and the step of drying a water-containing gel which has a thermally decomposing radical initiator content index of 40 to 100 at 100 to 250° C. The water absorbing resin of the present invention contains a water-soluble unsaturated monomer as a repeat unit for a major chain, 90 mol % of the monomer being composed of an acrylic acid and/or salt thereof, the resin having an internal crosslinking structure and exhibiting a weight-average molecular weight Mw of 360,000 to 1,000,000 daltons and an intrinsic viscosity IV of 2.1 to 6.0 dL/g where the weight-average molecular weight Mw and the intrinsic viscosity IV are measured after treatment under set 2 of hydrolysis conditions.
1. A water absorbing resin, comprising a water-soluble unsaturated monomer as a repeat unit for a major chain, 90 mol % or more, based on the total monomers making up the resin, of an acrylic acid and/or salt thereof, the resin having an internal crosslinking structure and exhibiting a weight-average molecular weight Mw of 360,000 to 1,000,000 daltons and an intrinsic viscosity IV of 2.1 to 6.0 dL/g where the weight-average molecular weight Mw and the intrinsic viscosity IV are measured after treatment under set 2 of hydrolysis conditions, in which treatment 20 mg of the water absorbing resin is left in 10 grams of a 0.1 mol/L aqueous solution of sodium hydroxide at 80° C. for 3 weeks, and the water absorbing resin regulates a thermally decomposing radical initiator content index for a water-containing gel to 40 to 100.
2. The water absorbing resin of claim 1 , wherein the resin shows a molecular weight distribution Mw/Mn after the treatment of 2.0 to 3.0.
3. The water absorbing resin of claim 1 , wherein the resin contains extractable polymer content which has a weight-average molecular weight Mw of 150,000 to 500,000 daltons.
4. The water absorbing resin of claim 1 , wherein the resin contains extractable polymer content which has an intrinsic viscosity IV of 1.0 to 2.0 dL/g.
5. The water absorbing resin of claim 1 , wherein the resin contains extractable polymer content which has a molecular weight distribution Mw/Mn of 2.0 to 3.0.
6. The water absorbing resin of claim 1 , wherein: the water absorbing resin has a mass-average particle diameter D50 of 200 to 600 μm; the water absorbing resin contains 0 to 5 mass % particles of such sizes that the particles can pass through a sieve having a mesh opening size of 150 μm; and the water absorbing resin has such a particle size distribution with a logarithm standard deviation σζ of 0.20 to 0.50.
7. The water absorbing resin of claim 1 , further containing an α-hydroxy carboxylic acid salt.