Water-absorbing resin

A water-absorbent resin that is a crosslinked polymer of a water-soluble ethylenically unsaturated monomer, the water-absorbent resin having a dry-up index of 2.2 or more, the dry-up index being represented by the following equation (1):

• dry-up index=overall absorption capacity term α× water absorption rate term β (1),
• wherein the overall absorption capacity term α and the water absorption rate term β are determined according to the following equations (2) and (3):
• overall absorption capacity term α =
• overall
• ⁢
• ⁢
• absorption
• ⁢
• ⁢
• capacity
• ⁢
• ⁢
• term
• ⁢
• ⁢
• α
• =
• (
• physiological
• ⁢
• ⁢
• saline
• ⁢
• ⁢
• absorption
• ⁢
• capacity
• ⁢
• ⁢
• under
• ⁢
• ⁢
• a
• ⁢
• ⁢
• load
• ⁢
• ⁢
• of
• ⁢
• ⁢
• 2.07
• ⁢
• ⁢
• kPa
• -
• physiological
• ⁢
• ⁢
• saline
• ⁢
• ⁢
• absorption
• ⁢
• capacity
• ⁢
• ⁢
• under
• ⁢
• ⁢
• a
• ⁢
• ⁢
• load
• ⁢
• ⁢
• of
• ⁢
• ⁢
• 4.82
• ⁢
• ⁢
• kPa
• )
• (
• physiological
• ⁢
• ⁢
• saline
• ⁢
• ⁢
• absorption
• ⁢
• ⁢
• capacity
• -
• physiological
• ⁢
• ⁢
• saline
• ⁢
• ⁢
• retention
• ⁢
• ⁢
• capacity
• )
• ;
• (
• 2
• )
• water absorption rate term β =dynamic water absorption rate/static water absorption rate (3),
• wherein the degree of neutralization of the water-soluble ethylenically unsaturated monomer is 77 to 80 mol % of all acid groups in the water-soluble ethylenically unsaturated monomer,
• the water-soluble ethylenically unsaturated monomer is (meth)acrylic acid,
• the overall absorption capacity term α is 0.95 or more,
• the water absorption rate term β is 1.56 or more, and
• the water-absorbent resin is prepared by a polymerization method satisfying all of conditions (i), (ii), and (iii):
• (i) the molar ratio of a water-soluble ethylenically unsaturated monomer and an internal crosslinking agent used in polymerization in the first stage (water-soluble ethylenically unsaturated monomer/internal crosslinking agent) is 10×103 to 15×103;
• (ii) the molar ratio of a water-soluble ethylenically unsaturated monomer and an internal crosslinking agent used in polymerization in the second stage (water-soluble ethylenically unsaturated monomer/internal crosslinking agent) is 15×103 to 25×103;
• (iii) the molar ratio of the total amount of water-soluble ethylenically unsaturated monomer used in polymerization in which a resin used in a post-crosslinking reaction is prepared and a post-crosslinking agent (water-soluble ethylenically unsaturated monomer/post-crosslinking agent) is 2.5×103 to 4.5×103.