1. Field of the Invention
The present invention relates to the production of superabsorbent acrylic polymer powders capable of absorbing large amounts of water or aqueous fluids and which are well suited for a variety of hygienic applications.
2. Description of the prior Art
It is known to this art to produce polymer particulates having a high capacity for water absorption by inverse suspension polymerization of ethylenically unsaturated monomers, more particularly of acrylic monomers. The powders which are thus obtained swell greatly in the presence of water, providing gels of high mechanical strength. These properties are useful, inter alia, for the manufacture of sanitary appliances, e.g., sanitary napkins, for absorption and retention of body fluids.
One significant improvement in the production of such absorbent powders is described in EP-0,441,507, comprising polymerization of the acrylic monomer in at least two separate stages. In a first discrete stage, an inverse suspension polymerization is carried out in conventional manner, such polymerization resulting in the formation of a gel. In a second stage, a fresh monomer charge is absorbed into this gel and polymerization thereof is initiated within the actual gel formed previously. If appropriate, this absorption/polymerization sequence can be repeated. In this fashion, polymeric resins are prepared having a particle size which is appreciably larger than the resins obtained via single inverse suspension polymerization. Their degree or extent of swelling in the presence of water, elastic modulus, plasticity and resistance to collapse under pressure of the gel, are also appreciably improved.
The absorbent properties of the final polymer depend on the degree of neutralization of the monomeric acrylic acid; it is now well known to this art that, for hygienic applications, the optimum degree of neutralization must be close to 75%, in particular taking account of the required compatibility of the superabsorbent with human skin. Thus, in the multistage process, a first inverse suspension polymerization of a first aqueous charge of 75% neutralized acrylic acid is conducted, and subsequent charges of aqueous acrylic acid, also 75% neutralized, are absorbed by the gel thus formed.