Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – At least one aryl ring which is part of a fused or bridged...
Patent
1995-06-30
1997-03-11
Reddick, Judy M.
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
At least one aryl ring which is part of a fused or bridged...
524114, 524108, 524280, 524503, 524547, 524556, 5253298, 525340, 5253282, 5253285, 442417, C08K 332
Patent
active
056102208
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. The present invention further relates 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 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 into 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, the gel is then crumbled, dried, ground, and screened out to the desired particle size. This polymerization in solution may either be carried out continuously or discontinuously. 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 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 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 having 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 further liquid distribution and absorption is prevented. According to U.S. Pat. No. Re 32,649 a balanced proportion of such superabsorbers in a diaper construction ensures liquid absorption, liquid transport, and dryness of the diaper and the skin. 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; this is the case in practice when a baby or person sits or lies on a sanitary article or when shear forces are acting, e.g. by movements of legs. In EP No. 03 39 461 A 1 this special absorption property is referred to as absorpt
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Brehm Helmut
Klimmek Helmut
Chemische Fabrik Stockhausen GmbH
Reddick Judy M.
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