Acidic Superabsorbent Hydrogels

A process for preparing a hydrogel-forming polymer capable of absorbing aqueous fluids, wherein said hydrogel-forming polymer is an acrylic acid (co)polymer partially neutralized with an alkali metal, alkaline earth metal, ammonium, or amine salt and having a pH absorbency index pHAI of at least 90 and a pH of not more than 5.7, wherein pHAI is calculated as follows:

• pHAI=ΔpH (AUL0.7 psi+CRC),
• wherein ΔpH=7-pH of product, AUL0.7 psi is an absorbency under pressure at 0.7 psi, and CRC is a centrifuge retention capacity, comprising a step of surface postcrosslinking a base polymer having a pH of not more than 5.7 with a surface postcrosslinking agent consisting of a di- or polyglycidyl compound, a polyamine, a polyamidoamine, a polyamidoamine reaction product with epichlorohydrin, a polyamide reaction product with epichlorohydrin, or 2-oxazolidinone.

A process for preparing a hydrogel-forming polymer capable of absorbing aqueous fluids, wherein said hydrogel-forming polymer is an acrylic acid (co)polymer partially neutralized with an alkali metal, alkaline earth metal, ammonium, or amine salt and having a pH absorbency index pHAI of at least 90 and a pH of not more than 5.7, wherein pHAI is calculated as follows:

• pHAI=ΔpH (AUL0.7 psi+CRC),
• wherein ΔpH=7-pH of product, AUL0.7 psi is an absorbency under pressure at 0.7 psi, and CRC is a centrifuge retention capacity, comprising a step of surface postcrosslinking a base polymer having a pH of not more than 5.7 with a surface postcrosslinking agent consisting of a di- or polyglycidyl compound, a polyamine, a polyamidoamine, or a polyamidoamine or polyamine reaction product with epichlorohydrin or 2-oxazolidone, and a step of forming complexes on particle surfaces of the hydrogel-forming polymers by spraying the particle surfaces with a solution of a divalent or more highly valent metal salt.