Superabsorbants with controlled absorption speed

Details Superabsorbants with controlled absorption speed Superabsorbants with controlled absorption speed

1999-10-21

2002-05-28

Lipman, Bernard (Department: 1713)

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

Synthetic resins

Mixing of two or more solid polymers; mixing of solid...

C525S929000, C525S329900, C525S342000

Reexamination Certificate

active

06395830

ABSTRACT:

The invention relates to hydrophilic, highly swellable polymers for aqueous liquids, so-called superabsorbers, which are coated with reactive, water-insoluble, film-forming, hydrophobic polymers and an additional reactive component capable of reacting with carboxyl groups or carboxylate anions to form additional crosslinking sites on the particle surface, the production of such coated superabsorbers and their use in hygienic articles used for absorbing body fluids.
Highly swellable polymers which may be obtained by polymerization of unsaturated acids such as acrylic acid, methacrylic acid, acrylamidopropanesulfonic acid, etc., or partially neutralized in the form of the alkali or ammonium salts thereof, in the presence of small amounts of polyunsaturated compounds are already known.
Likewise, superabsorbent crosslinked polymers which may be obtained by graft copolymerization of unsaturated acids onto various matrices, such as polysaccharides, polyvinyl alcohols, polyalkylene oxides and derivatives thereof, are well-known. The highly swellable polymers mentioned are remarkable for their ability to absorb large quantities of aqueous liquids such as blood or urine with swelling and formation of hydrogels and retain the absorbed quantity of liquid even under pressure.
As a result of their characteristic absorbing properties, these polymers are preferably used as absorbents in hygienic articles.
According to the state of the art, the properties of these hydrogels can be modified by a surface treatment using specific substances. To this end, conventional hydrogels which have been dried, milled and optionally screened, are reacted in the form of powders with reactive compounds, i.e., compounds containing groups capable of reacting with the carboxyl groups of the hydrogels to give covalent crosslinking on the surface of the granulate particles. These reactive compounds may be di- or polyols, bisepoxides, higher epoxides, or cyclic carbonic esters as well.
Such surface crosslinking has been described in DE 40 20 780 C1, for example, where alkylene carbonates are employed as surface crosslinkers.
The surface treatment of hydrophilic, highly swellable polymers using reactive substances for the purpose of surface crosslinking has been described in numerous other publications.
EP-A 349,240 (to achieve balance between absorptive capacity and absorption rate as well as gel stability and absorbency) describes an aftertreatment of polymers using crosslinking agents including two or more functional groups, which may undergo reaction with the carboxyl or carboxylate groups or other groups contained in the polymer. Conventionally, the hydrophilic, highly swellable powdered polymer is directly mixed with one or more reactive hydrophilic components, optionally using water and organic solvents.
Inter alia, EP-B 317,106 describes di- or polyols, di- or polyglycidyl ethers, haloepoxy compounds, polyisocyanates, polyamines, polyfunctional aziridine compounds, or alkyl di(tri)halides as reactive, surface-crosslinking components.
Surface subsequent crosslinking is effected by performing a thermal treatment of the products subsequent to coating the superabsorbent polymer powders.
The above-described procedures have in common that a significant improvement of the absorptive capacity under pressure for aqueous liquids such as urine, blood or other body fluids is achieved through the thermal treatment of the highly swellable, powdered polymers in the presence of the reactive components mentioned.
The surface treatment of granulated, hydrophilic, highly swellable polymers using additives for dedusting the granulated polymers, e.g., using polyols or polyethylene glycols in PCT/US93/02872, is also well-known. The laid-open document DE 44 26 008 A1 refers to the use of non-reactive, water-insoluble, film-forming polymers of, inter alia, homo- and copolymerizates of acrylic and methacrylic esters, vinyl esters, polyamides, polyesters for dedusting and improving the abrasion resistance. For the same purpose of improving the abrasion resistance, but also in order to reduce the caking tendency, the DE 195 24 724 A1 describes a surface treatment using non-reactive, water-soluble waxes. DE 44 14 117 A1 describes the use of non-reactive polysiloxanes, e.g., polydimethylsiloxanes and polymethylphenylsiloxanes for the surface treatment of hydrophilic, highly swellable polymers for use in dedusting a superabsorbent polymer.
EP-A 705,643 A1 refers to water-absorbing polymers having improved properties, which are obtained by treating crosslinked polymers based on acrylic acid or the salts thereof with a modified silicone oil having functional groups capable of reacting with carboxyl groups and/or carboxylate groups, with amino and epoxy-functional silicones being mentioned as examples. The silicones may be applied as substance, as a solution in an organic solvent, or as an emulsion. If necessary, the polyacrylate/silicone blend may be postcured at temperatures of 60-200° C.
The products obtained using the described functional silicone oils and the described process have improved anti-caking performance and a reduced dusting tendency. In particular, the absorption under pressure is markedly improved by the treatment described. The products have good initial absorption under pressure and absorptive capacity. The Examples 1 through 4 used in EP-A 705,643 show that the absorption of the products including functional silicone oil is higher both prior to and after postcure of the materials after 10 minutes and thus, the absorption rate is higher compared to the material according to Comparative Examples 1 through 8, which has not been surface-treated.
Therefore, the absorbent polymers from EP-A-705,643 also belong to those superabsorbers that are conventional in this respect, tending to rapid absorption of an aqueous liquid immediately upon contact with such a liquid, giving rise to rapid swelling near the entry site and so-called gel blocking.
Likewise, methods and additives for improving the absorption rate of superabsorbent hydrophilic polymers have been known for a long time.
DE 44 18 818 A1 describes the addition of, inter alia, CO
2
, alkaline, alkaline earth and ammonium carbonates and bicarbonates to the monomer solution as expanding agent, in order to enhance the absorption rate of highly swellable polymers.
The U.S. Pat. No. 4,548,847 describes hydrogels reversibly crosslinked by at least bivalent metal cations, such as Ca
2+
or Ba
2+
. By co-using a substance capable of removing the metal cations, it is possible to achieve delayed swelling. Water-soluble chelating agents such as Na
2
HPO
4
, sodium hexametaphosphate and the disodium salt of ethylenediamine tetraacetate are preferably used. These substances cause the reversible crosslinking sites formed by said at least bivalent metal cations to be degraded by complexing. The dropping crosslinking density permits stronger swelling of the products. The patent also describes absorbent articles such as wound dressings and tampons which contain the absorbent polymers having delayed swelling characteristics. Among other things, an absorbent article (wound dressing) made up of layers is described, wherein the polymer of the invention is contained at certain points in each layer. Combinations of the polymer according to the invention with other polymers absorbing aqueous liquids are not mentioned.
The patent applications GB 2,280,115 A and WO 95/00183 describe an absorbent article containing covered superabsorbent particles in that area into which body fluid is released. The cover of the superabsorbent particles prevents swelling thereof until the cover has dissolved in the test liquid or body fluid or is penetrated by same. Thus, they are superabsorbent particles having an activation period until swelling begins, which period can be varied by means of the cover material and the thickness thereof. Non-reactive polysaccharides such as gelatin, micro-crystalline cellulose and cellulose derivatives are mentioned as cover materials. The activation period until swelling

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