Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Including a second component containing structurally defined...
Patent
1992-12-29
1995-04-25
Reddick, Judy M.
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
Including a second component containing structurally defined...
428407, 524 52, 524108, 524280, 524729, 5253297, 5253301, 525383, B32B 516
Patent
active
054097711
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to powdery, cross-linked polymers absorbing aqueous liquids and blood (superabsorbers) and having improved properties with regard to swelling and retention capacity for aqueous liquids under load; to a process for the manufacture of said polymers as well as to the use thereof in absorbent sanitary articles, such as diapers, in the adult incontinence, feminine hygiene, and for wound dressing.
Superabsorbers are water-insoluble, cross-linked polymers which, under swelling and formation of hydrogels, are capable of absorbing large amounts of aqueous liquids and body liquids, such as urine or blood, and of retaining the absorbed amount of liquid under a certain pressure/load. Owing to said characteristic absorption properties the polymers are mainly used for incorporating them in sanitary articles, for example, diapers and sanitary napkins.
The superabsorbers which are commercially available today are cross-linked polyacrylic acids or cross-linked starch-acrylic-acid-graft-polymers the carboxyl groups of which are partially neutralized with sodium hydroxide solution or caustic potash.
In principle, the 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, which 1s then comminuted, dried, ground, and screened out to the desired particle size. This solvent polymerization may either be carried out continuously or discontinuously. The patent literature discloses a wide spectrum of variations with respect to concentration ratios, temperature, kind and amount of cross-linking agents and initiators. Typical methods are described, for example, in U.S. Pat. Nos. 4,286,082 and 4,076,663 and German patent No. 27 06 135.
The second method is the inverse suspension or emulsion polymerization. In this process, 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 process is described, for example, in U.S. Pat. No. 4,340,706 and German patent Nos. 37 13 601 and 28 40 010.
Initially, only the high swelling capacity on contact with liquids, also referred to as free swelling capacity, had been the main factor in the development of superabsorbers; later it was found, however, that it is not only the amount of absorbed liquid that is important but also the stability of the swollen gel.
However, the absorbency, also referred to as swellability or free swelling capacity, on the one hand, and the gel strength of a cross-linked polymer, on the other hand, represent contrary properties: this has been known from U.S. Pat. No. 3,247,171 and U.S. Pat. No. Re. 32,649. This means that polymers with a particularly high absorbency exhibit a poor strength of the swollen gel so that the gel is deformable under pressure (e.g., the load of a body) and prevents further liquid distribution and absorption. According to U.S. Pat. No. Re 32,649 a balanced relation between absorptivity (gel volume) and gel strength should be aimed at so as to ensure liquid absorption, liquid transport, and dryness of the diaper and the skin when using such superabsorbers in a diaper construction. In this connection, not only the polymer's capacity of retaining a liquid under subsequent pressure, after swelling freely first, is of importance but also the fact that liquids are absorbed even against a simultaneously acting pressure, i.e., during the liquid absorption
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Dahmen Kurt
Mertens Richard
Chemische Fabrik Stockhausen GmbH
Reddick Judy M.
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