Powder-form cross-linked polymers capable of absorbing aqueous l

Details Powder-form cross-linked polymers capable of absorbing aqueous l Powder-form cross-linked polymers capable of absorbing aqueous l

1996-01-11

1998-01-27

Yoon, Tae

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

Synthetic resins

Cellular products or processes of preparing a cellular...

521142, 524729, 523105, 523132, 523173, 5253297, 5253302, C08J 908, C08F 2002, C08F 800

Patent

active

057123160

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 absorption velocity, swelling and retention capacity for aqueous liquids under high load. The present invention further relates to a process for the manufacture of said polymers and to their use 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 fluids, such as urine or blood, and of retaining the absorbed liquid amount under a certain pressure or load. Owing to said characteristic absorption properties the polymers are mainly used for incorporating them in sanitary articles, for example, diapers and sanitary napkins.
Initially, only the very 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 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 of a cross-linked polymer, on the other hand, represent contrary properties, as is known from U.S. Re No. 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. Re 32,649 a balanced relation between absorption capacity (gel volume) and gel strength is to be aimed at so as to ensure liquid absorption, liquid transport, dryness of the diaper and the skin when such superabsorbers are used in a diaper structure. In this connection, not only the polymer's capability 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. 0 339 461 this particular absorption property is referred to as absorption under load.
The only way to meet the increasing trend of reducing the size and thickness of sanitary articles for esthetic and environmental reasons (reduction of waste in the land fill) is to reduce the large-volume fluff pulp portion in diapers and to increase the portion of superabsorber at the same time. For that reason the superabsorber has to take over additional functions with respect to liquid absorption and transport thereof, which were previously performed by the fluff pulp and which cannot be accomplished by the known superabsorbers to a satisfactory extent.
This particularly applies to cases where the proportion of superabsorber in the absorbent zone of a sanitary article is increased to 40 to 60%-wt. and more. Owing to the poor absorption rate of the superabsorber in case of several mictions, so-called "leakage" may result caused by a blockade in the liquid transport and liquid distribution, in particular under pressure, i.e., the liquid is no longer absorbed by the sanitary article, thus resulting in a wet diaper surface and, consequently, wet skin.
To provide superabsorbent polymers having the characteristic properties, such as high absorption capacity, high gel strength and high absorbency under load, it is necessary to subject the polymer resins to a subsequent surface treatment.
For example, U.S. Pat. No. 4,043,952 recommends polyvalent metal compounds to improve the dispersibility in water, and U.S. Pat. No. 4,051,086 proposes the use of glyoxal to improve the absorption rate. DE-OS 27 40 169 describes the production of absorbents based on potassium

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