Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Cellular products or processes of preparing a cellular...
Reexamination Certificate
2000-09-05
2002-09-24
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...
C521S064000
Reexamination Certificate
active
06455600
ABSTRACT:
The invention relates to water-absorbing, expanded, crosslinked polymers, to a process for the production thereof and to the use thereof in hygiene articles employed to absorb body fluids and in dressing material for covering wounds.
Water-absorbing, crosslinked polymers are referred to as superabsorbents or superabsorbing polymers because they are able to absorb a multiple of their own weight of aqueous liquids to form hydrogels. Superabsorbents are used in practice, for example, in diapers for absorbing urine. The superabsorbents have the property of retaining the absorbed liquid even under mechanical stress.
In order to alter the use 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 superabsorbing material.
A foam belonging to category (1) is produced, for example, from a mixture which comprises, on the one hand, components for forming a polyurethane foam and, on the other hand, polymerizable monomers, a crosslinker and a polymerization initiator to produce a superabsorbent. The foam is formed from the polyurethane components in a mixture of this type in a polycondensation reaction and contains the superabsorbent which has been produced by polymerization of the monomers in the form of an interpenetrating network, cf. U.S. Pat. Nos. 4,725,628, 4,725,629 and 4,731,391.
U.S. Pat. No. 4,985,467 discloses a polyurethane foam which contains a chemically bonded superabsorbent. Also known are combinations of latex foams into which superabsorbing, fine-particle materials are incorporated after the foaming process, cf. EP-A-427 219 and U.S. Pat. No. 4,990,541.
Products belonging to category (2) of foams are those, for example, which are obtained by mixing a prefabricated superabsorbent in an extruder with a polyhydroxy compound and a blowing agent at elevated temperature. The foam is formed when the mixture is expelled from the extruder. Processes of this type are described, for example, in U.S. Pat. Nos. 4,394,930, 4,415,388 and GB-A-2 136 813.
U.S. Pat. Nos. 4,529,739 and 4,649,154 disclose processes for producing foams in which a water-swellable material having COOH groups is foamed with a blowing agent which liberates the blowing gas in a neutralization reaction with the COOH groups of the polymer.
According to the statements in WO-A 94/22502, superabsorbing foams based on crosslinked, partially neutralized polycarboxylates are produced by foaming a monomer mixture with a blowing agent which is insoluble in water and has a boiling point below 50° C., and completing polymerization of the foam at virtually the same time as the foaming.
EP-A-04 21 264 discloses the production of foam-like superabsorbents by polymerizing an aqueous monomer mixture which contains an emulsified oil phase. The action of the oil in this case is to occupy the space for the later pores in the foam and it is removed by evaporation, after the polymerization is complete, on drying the expanded material.
WO-A 88/09801 discloses that it is possible to process hydrophilic polymers, e.g. sodium polyacrylate, in the presence of crosslinkers such as polyepoxides and blowing agents, by heating, to an expanded superabsorbent.
Another procedure known for producing expanded superabsorbents is to add carbonates, bicarbonates or carbon dioxide as blowing agents to a mixture of monomers which contain carboxyl groups, crosslinker and polymerization initiator, with the polymerization of the monomers being started at the same time as the addition of the blowing agent or shortly thereafter. The superabsorbent acquires a foam structure due to the carbon dioxide formed in the neutralization reaction, cf. EP-A-2 954 438 and U.S. Pat. No. 4,808,637. In the process disclosed in WO-A 95/02002, an expanded superabsorbent is mixed after production with one or more reactive compounds for subsequent surface crosslinking, and is heated to from 100 to 300° C.
In the processes described above for producing superabsorbing foams, the foam formation and the polymerization take place either synchronously or at negligibly different times. The foams which have not yet completely polymerized have only a short pot life, usually only a few minutes. It is a disadvantage in the processes indicated above that, for example, relatively large amounts of blowing agent are used, especially the CFCs used in the case of WO-A 94/22502.
DE-A-19607551 discloses water-absorbing, expanded, crosslinked polymers which are obtainable by
(I) foaming a polymerizable aqueous mixture which comprises
(a) monoethylenically unsaturated monomers which contain acidic groups and are at least 50 mol % neutralized,
(b) optionally other monoethylenically unsaturated monomers,
(c) crosslinkers,
(d) initiators,
(e) 0.1-20% by weight of at least one surfactant,
(f) optionally at least one solubilizer and
(g) optionally thickeners, foam stabilizers, polymerization regulators, fillers and/or cell nucleating agents
where the foaming takes place by dispersing fine bubbles of a gas which is inert to free radicals in the polymerizable aqueous mixture, and
(II) polymerizing the foamed mixture to form an expanded hydrogel and, where appropriate, adjusting the water content of the polymer to 1-60% by weight.
According to the disclosure of DE-A-19540951, the water content of the expanded hydrogel is adjusted to 1-45% by weight. The expanded polymers obtainable in this way are used in hygiene articles, for example for absorbing body fluids.
JP-A-08073507 describes soft and flexible superabsorbent films which are produced by polymerizing an aqueous acrylate solution which is partly neutralized by an alkanolamine in the presence of a crosslinker. Although the resulting films are described as soft and flexible, no information is given on the climatic conditions under which the flexibility is retained. In particular, no information is available for elevated or reduced temperatures. In addition, these products have several serious disadvantages. Thus, their absorption speed is completely inadequate for use in hygiene articles, and their absorption capacity is also in need of improvement. Furthermore, the films show pronounced tackiness which greatly impairs handling.
Superabsorbent foams based on crosslinked polymers with acidic groups, which are produced, for example, as described in the abovementioned references DE-A-19540951, DE-A-19607551 or WO-A-94/22502 can be obtained in soft and flexible form by adjusting a defined moisture content of about 25%. Even if additional flexibilization measures as described in the above patents are taken, nevertheless a moisture content of at least 20% is usually necessary. Flexibilization by such a high water content entails the disadvantage that it is not stable at low humidities. At a relative humidity below 50%, the foam starts to dry out and thus increasingly loses its flexibility. Likewise, the flexibility of the foam decreases rapidly at temperatures below 5° C.
Although the loss of flexibility is reversible, i.e. when the temperature rises or the humidity increases the foams again become soft and flexible, nevertheless this represents a serious limitation on use, for example if a hygiene article is stored in very dry air at home. Furthermore, the resulting foam layers feel moist (moist “handle”) which is likewise regarded as disadvantageous for use in hygiene articles.
It is an object of the present invention to provide a superabsorbing foam which remains flexible and, at the same time, overcomes the other disadvantages described under conditions relevant to use, for example at temperatures between −15° C. and +50° C. and a relative humidity of 20 to 95%.
We have found that this object is achieved by water-absorbing, expanded, crosslinked polymers obtainable by
(I) foaming a polymerizable aqueous mixture which comprises
(a) monoethylenically unsaturated monomers which contain acidic groups and are optionally neutralized,
(b) optionally other monoethylenically unsaturated monomers,
(c) crosslinkers,
(d) initiators,
(e) 0.
Anstock Thomas
Beck Martin
Hähnle Hans-Joachim
Heider Wolfgang
Schornick Gunnar
BASF - Aktiengesellschaft
Foelak Morton
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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