Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Cellular products or processes of preparing a cellular...
Reexamination Certificate
2001-09-04
2004-06-15
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...
C521S065000
Reexamination Certificate
active
06750262
ABSTRACT:
The invention relates to water-absorbing, expanded, crosslinked polymers having an improved distributing action, 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 super absorbents 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. They can be in the form of, for example, granules or powders or in expanded form. 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. 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 bonded superabsorbent, while WO94/07935, WO95/32860, WO96/16099, WO96/31555 and WO98/14508 describe polyurethane foams containing particles of superabsorbent polymers (SAP) without chemical attachment. 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. U.S Pat. No. 5,506,277 describes a closed-cell starch foam containing 20-40% superabsorbent.
Foams of this type have the fundamental disadvantage that the superabsorbent present in the foam matrix fills up the pore volume during absorption of aqueous liquids and thus increasingly blocks the pore structure of the foam. These materials therefore show a distinct fall in the absorption speed and an increasing deterioration in the distributing action as absorption increases. This behavior is very disadvantageous for use in hygiene articles.
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. No. 4,394,930, U.S. Pat. No. 4,415,388 and U.S. Pat. No. 4,410,571.
U.S. Pat. No. 4,529,739 and U.S. Pat. No. 4,649,164 disclose processes for producing foams in which a water-swellable polymer 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.
WO-A-88/09801 discloses that hydrophilic polymers, for example sodium polyacrylate, can be processed in the presence of crosslinkers such as polyepoxides and blowing agents by heating to give an expanded superabsorbent.
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. The disadvantage of this process is, one the one hand, the use of large amounts of blowing agent, in particular the use of CFC, and, on the other hand, the elaborate monitoring of the process.
EP-A-0421264 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.
Another procedure known for producing expanded superabsorbents is to add carbonates, bicarbonates or carbon dioxide as blowing agents to a mixture of carboxyl-containing monomers, 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. DE-A 3831261, U.S. Pat. No. 5,118,719, EP-A-538983, U.S. Pat. No. 4,808,637 and U.S. Pat. No. 5,750,585. In the processes disclosed in WO-A 95/02002 and EP-A 644207, an expanded and ground superabsorbent is mixed after production with one or more reactive compounds for subsequent surface crosslinking, and is heated to from 100 to 300° C.
The procedure where the foaming process and the polymerization take place substantially synchronously involves a number of disadvantages. Thus, the resulting foams often have inhomogeneous foam structures containing in some cases considerable amounts of closed-cell structures. In addition, there is a great tendency to skinning, and the resulting materials are hard and brittle. The consequence of these problems is that these foams are used not as whole foams but only in ground form as is stated in the abovementioned publications. The grinding process eliminates the inhomogeneity described. In addition, this breaks up the closed-cell structures and the skin layers on the foamed material.
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, their use properties are entirely inadequate. Thus, their water absorption speed is much too low for use in hygiene articles, and they moreover show no distributing action and have a pronounced tackiness which greatly restricts handling.
DE-A-19607551 discloses water-absorbing, expanded, crosslinked polymers obtainable by
(I) foaming a polymerizable aqueous mixture which comprises
(a) monoethylenically unsaturated monomers which contain acidic groups at least 50% mol are 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, 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.
A considerable problem in the production of superabsorbing foams from monomer mixtures is the control of the temperature. As explained in detail in WO-A-94/22502 and U.S. Pat. No. 5,750,585, control of the temperature during the foaming and polymerization processes is of crucial importance. Obtaining the required open-cell foam structures is ensured only when the change in temperature is strictly controlled. Control of the temperature during the mechanical blowing in the process of DE-A-19607551 is also of considerable importance because it crucially influences the structure of the monomer foam produced, and thus of the polymer foam.
A strict control of the temperature is always a problem and complicated to implement on the industrial scale when products like foams, which are good insulators, are involved and when large amounts of heat are released at the same time in the process. Large amounts of heat are produced on the one hand by the enormous heat of polymerization of the monomers employed,
Anstock Thomas
Hähnle Hans-Joachim
Heider Wolfgang
Schornick Gunnar
Schröder Ulrich
BASF - Aktiengesellschaft
Foelak Morton
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