Powdery polymers capable of absorbing aqueous liquids, a process

Details Powdery polymers capable of absorbing aqueous liquids, a process Powdery polymers capable of absorbing aqueous liquids, a process

1996-03-26

1997-09-30

Foelak, Morton

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Cellular products or processes of preparing a cellular...

521 56, 521149, 521150, C08J 900

Patent

active

056726333

DESCRIPTION:

BRIEF SUMMARY
The present invention relates to powdery, cross-linked polymers absorbing water and aqueous liquids, referred to as superabsorbers, having improved swelling properties and an improved retention capacity for aqueous liquids. The present invention further relates to a process for the manufacture of these polymers and to their use in absorbent sanitary articles and in technical fields.
Superabsorbers are water-insoluble, cross-linked polymers which, under swelling and formation of hydrogels, are capable of absorbing large amounts of body fluids, such as urine or blood, or other aqueous liquids and of retaining the absorbed liquid amount under a certain pressure. Owing to said characteristic absorption properties the polymers are mainly used in sanitary articles, for example, diapers and sanitary napkins.
The superabsorbers commercially available today are cross-linked polyacrylic acids or cross-linked starch-acrylic-acid-graft-polymers partially neutralized with sodium hydroxide solution or potassium hydroxide solution. In principle, powdery superabsorbers are manufactured by two methods:
According to the first method, partially neutralized acrylic acid in aqueous solution in the presence of a multi-functional cross-linking agent is converted into a gel by radical polymerization, the gel is then crumbled, dried, ground, and screened out to the desired particle size. The polymerization in solution may either be carried out continuously or discontinuously. Patent literature gives a wide spectrum of alternatives with respect to ratios of concentration, temperatures, kind and amount of cross-linking agents and initiators. Typical methods are described, for example, in U.S. Pat. No. 4,286,082, DE 27 06 135 and U.S. Pat. No. 4,076,633.
The second method includes the inverse suspension and emulsion polymerization. In these processes, an aqueous, partially neutralized acrylic acid solution is dispersed in a hydrophobic organic solvent by means of protective colloids or emulsifiers, and the polymerization is started by radical initiators. After completion of the polymerization, the water is azeotropically removed from the reaction mixture and the polymeric product filtered off and dried. The cross-linking reaction may be effected by incorporating a polyfunctional cross-linking agent, which is dissolved in the monomer solution, by polymerization, and/or by reacting suitable cross-linking agents with functional groups of the polymer during one of the production steps. The principle is described, for example, in U.S. Pat. No. 4,340,706, DE 37 13 601 and DE 28 40 010.
Initially, only the very high swelling capacity on contact of the absorber with the liquid, also referred to as free swelling capacity, had been the main factor in the development of superabsorbers; later it turned out, however, that not only the amount of absorbed liquid is of importance but also the stability of the swollen gel. However, absorbency, also referred to as swellability or free swelling capacity, on the one hand, and gel strength on the other hand, represent contrary properties, as is known by U.S. Pat. No. 3,247,171 and U.S. Pat. No. Re. 32,649. This means that polymers having a particularly high absorbency exhibit a poor strength of the swollen gel so that the gel is deformable under an exerted pressure when loaded by the body weight and further liquid distribution and liquid absorption is prevented at the same time. According to U.S. Pat. No. Re. 32,649 a balanced relation of the properties of these superabsorbers in a diaper construction between liquid absorption, liquid transport and dryness of the diaper on the skin is to be ensured. In this connection, not only is the polymer's capability of retaining a liquid under subsequent pressure, after swelling freely first, of importance, but also the fact that liquids are absorbed even against a simultaneously acting pressure, i.e., during the liquid absorption. This is the case in practice when a person sits or lies on a sanitary article or when shear forces are acting, e.g., by movements of le

REFERENCES:
patent: 4295987 (1981-10-01), Parks
patent: 5264495 (1993-11-01), Irie et al.
patent: 5462972 (1995-10-01), Smith et al.

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