Water-absorbent cross-linked polymers in foam form

Details Water-absorbent cross-linked polymers in foam form Water-absorbent cross-linked polymers in foam form

1998-08-26

2001-01-16

Foelak, Morton (Department: 1711)

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

Synthetic resins

Cellular products or processes of preparing a cellular...

C521S149000, C522S086000

Reexamination Certificate

active

06174929

ABSTRACT:

The invention relates to water-absorbent foamed crosslinked polymers, processes for their preparation and their use in sanitary articles which are used for absorbing body fluids and in dressing materials for covering wounds.
Water-absorbent crosslinked polymers are termed superabsorbent polymers or superabsorbents, because they are able to absorb many times their own weight of aqueous liquids, forming hydrogels. Superabsorbents are used in practice in diapers, for example, for absorbing urine. The superabsorbents have the property of retaining the absorbed liquid even under mechanical loading.
For variation in the performance properties of superabsorbents, two different types of foams are known: (1) mixtures which contain superabsorbents in a foamed matrix, and (2) foams which consist of a superabsorbent material.
A category (1) foam is prepared, for example, from a mixture consisting on the one hand of components for forming a polyurethane foam and on the other hand of polymerizable monomers, a crosslinker and a polymerization initiator for preparing a superabsorbent. From a mixture of this type, in a polycondensation reaction from the polyurethane components, the foam is formed which contains the superabsorbent, resulting from polymerization of the monomers, in the form of an interpenetrating network, cf. U.S. Pat. No. 4,725,628, U.S. Pat. No. 4,725,629 and U.S. Pat. No. 4,731,391.
U.S. Pat. No. 4,985,467 discloses a polyurethane foam which contains a chemically bound superabsorbent. Furthermore, combinations of latex foams are known into which superabsorbent finely particulate materials are incorporated after the foaming process, cf. EP-A-427 219 and U.S. Pat. No. 4,990,541.
Category (2) foams include, for example, products which are obtained by mixing a previously prepared superabsorbent with a polyhydroxy compound and a blowing agent in an extruder at elevated temperature. The foam forms as the mixture is forced out of the extruder. Processes of this type are described, for example, in U.S. Pat. No. 4,394,930, U.S. Pat. No. 4,415,388 and GB-A-2 136 813.
U.S. Pat. No. 4,529,739 and U.S. Pat. No. 4,649,154 disclose processes for preparing foams, in which a water-swellable, carboxyl-bearing material is foamed with a blowing agent which releases the blowing gas in a reaction neutralizing the polymer carboxyls.
According to the information in WO-A-94/22502, superabsorbent foams based on crosslinked partially neutralized polycazboxylates are prepared by foaming a monomer mixture with a water-insoluble blowing agent which has a boiling point below 50° C., and polymerizing the foam to completion virtually simultaneously with foaming.
EP-A-04 21 264 discloses the preparation of foamlike superabsorbents, in which an aqueous monomer mixture containing an emulsified oil phase is polymerized, with the oil occupying the space of the later foam pores and being removed by evaporation, after polymerization is completed, when the foamed material is dried.
WO-A-88/09801 discloses that hydrophilic polymers, eg. poly(sodium acrylate), in the presence of crosslinkers such as polyepoxides and blowing agents, can be processed by heating to give a foamlike superabsorbent.
A procedure for preparing foamlike superabsorbents is also known in which carbonates, hydrogencarbonates or carbon dioxide are added as blowing agent to a mixture of carboxyl-bearing monomers, crosslinking agent and polymerization initiator, polymerization of the monomers being initiated at the same time as the blowing agent is added or shortly thereafter. The superabsorbent is given a foam structure by the carbon dioxide formed in the neutralization reaction, cf. EP-A-2 954 438 and U.S. Pat. No. 4,808,637. According to the process disclosed by WO-A-95/02002, a foamed superabsorbent, following its preparation, is admixed with one or more compounds capable of reacting to form subsequent surface crosslinking and heated to from 100 to 300° C.
In the above-described processes for preparing superabsorbent foams, foam formation and polymerization proceed either synchronously or at insignificantly staggered times. The as yet incompletely polymerized foams have only a short life, generally only a few minutes. A disadvantage of the above-specified processes is, for example, the use of relatively large amounts of blowing agent, in particular the use of CFCs in the case of WO-A-94/22502.
It is an object of the present invention to provide foamed superabsorbents. It is a further object of the present invention to indicate an improved process for preparing foamed superabsorbents.
We have found that these objects are achieved according to the invention with water-absorbing foamlike crosslinked polymers which are obtainable by
(I) foaming a polymerizable aqueous mixture which comprises
(a) monoethylenically unsaturated monomers containing acid groups, which are at least 50 mol % neutralized,
(b) with or without other monoethylenically unsaturated monomers,
(c) crosslinkers,
(d) initiators,
(e) from 0.1 to 20% by weight of at least one surfactant,
(f) with or without one or more solubilizers and
(g) with or without thickeners, foam stabilizers, polymerization regulators, fillers and/or cell nucleating agents,
the foaming being performed by dispersing fine bubbles of a gas inert to free radicals, and
(II) polymerizing the foamed mixture with formation of a foamed hydrogel, with or without adjustment of the water content of the polymer to from 1 to 60% by weight.
The invention also relates to a process for preparing water-absorbing foamlike crosslinked polymers, which comprises foaming a polymerizable aqueous mixture of
(a) monoethylenically unsaturated monomers containing acid groups, which are at least 50 mol % neutralized,
(b) with or without other monoethylenically unsaturated monomers,
(c) crosslinkers,
(d) initiators,
(e) from 0.1 to 20% by weight of at least one surfactant,
(f) with or without one or more solubilizers and
(g) with or without thickeners, foam stabilizers, polymerization regulators, fillers and/or cell nucleating agents
in a first process stage by dispersing fine bubbles of a gas inert to free radicals and polymerizing the resulting foam in a second process stage with formation of a foamlike hydrogel, with or without adjustment of the water content of the foamlike hydrogel to from 1 to 60% by weight.
According to the invention, a polymerizable aqueous mixture is processed to form a foam which is stable to processing and can be shaped as desired. The polymerizable aqueous mixture includes as component (a) monoethylenically unsaturated monomers which contain acid groups, which are at least 50 mol % neutralized. Examples of such monomers are monoethylenically unsaturated C
3
-C
25
-carboxylic acids or anhydrides, for example acrylic, methacrylic, ethacrylic, &agr;-chloroacrylic, crotonic and maleic acids, maleic anhydride, itaconic, citraconic, mesaconic, glutaconic, aconitic and fumaric acids.
In addition, monoethylenically unsaturated sulfonic acids can be used as monomers of the group (a), for example vinylsulfonic acid, allylsulfonic acid, sulfoethyl acrylate, sulfoethyl methacrylate, sulfopropyl acrylate, sulfopropyl methacrylate, 2-hydroxy-3-acryloxypropylsulfonic acid, 2-hydroxy-3-methacryloxypropylsulfonic acid, vinylphosphoric acid, allylphosphonic acid and 2-acrylamido-2-methylpropanesulfonic acid. The monomers can be used alone or in a mixture with each other in the preparation of the superabsorbent foams. Preferred monomers of the group (a) used are acrylic, methacrylic, vinylsulfonic and acrylamidopropanesulfonic acids or mixtures of these, eg. mixtures of acrylic and methacrylic acids, mixtures of acrylic and acrylamidopropanesulfonic acids or mixtures of acrylic and vinylsulfonic acids.
The monomers are at least 50 mol % neutralized with, for example, alkali metal bases or ammonia or amines. Preferably, sodium hydroxide solution or potassium hydroxide solution is used for neutralization. However, the neutralization can also be carried out using sodium carbonate, sodium hydrogencarbonate, potassium carbonate or p

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