Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Reexamination Certificate
2001-08-29
2003-08-12
Seidleck, James J. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C525S221000, C526S317100, C526S320000, C524S557000, C524S558000
Reexamination Certificate
active
06605673
ABSTRACT:
The invention relates to powdered, crosslinked polymers (superabsorbers) which absorb water, aqueous liquids, as well as blood, and have improved properties, particularly improved retention and improved liquid retention capability under pressure and an improved capability of conveying liquids, and to their production and their use as absorbents in hygiene articles and in technical fields.
Superabsorbers are water-insoluble, crosslinked polymers capable of absorbing large amounts of aqueous liquids and body fluids such as urine or blood with swelling and formation of hydrogels, and retaining them under a specific pressure. As a result of these characteristic properties, these polymers are predominantly used for incorporation in sanitary articles, e.g., in diapers for babies, incontinence articles, or in liners.
Essentially, the superabsorbers commercially available at present are crosslinked polyacrylic acids or crosslinked starch/acrylic acid graft polymers wherein the carboxyl groups are partially neutralized with sodium hydroxide or potassium hydroxide solution.
For aesthetic reasons and from environmental aspects, there is an increasing tendency of designing sanitary articles such as diapers for babies, incontinence articles and liners increasingly smaller and thinner. In order to ensure a constant retention capability of the sanitary articles, the above requirement can only be realized by reducing the percentage of large-volume fluff. As a result, the superabsorber also has to assume functions with respect to conveyance and distribution of liquid, which may be summarized as permeability properties.
Permeability in the case of a superabsorber material is understood to be the ability of conveying added liquids and distributing them in a three-dimensional fashion in its swollen state. In a swollen superabsorber gel, this process takes place via capillary conveyance through interstices between the gel particles. The actual conveyance of liquid through swollen superabsorber particles complies with the laws of diffusion and is an exceedingly slow process which, in the service condition of the sanitary article, does not play any role in the distribution of liquid. In superabsorber materials incapable of accomplishing capillary conveyance due to lacking gel stability, separation of the particles from each other has been ensured by embedding these materials in a fiber matrix, thereby avoiding the gel blocking phenomenon. In new generation diaper constructions, the absorber layer has only minor amounts of fiber material to support the conveyance of liquid, or none at all. Accordingly, the superabsorbers used therein must have sufficiently high stability in their swollen state, so that the swollen gel still has a sufficient quantity of capillary space, through which conveyance of liquid is possible.
In one aspect, in order to obtain superabsorber materials having high gel strength, the polymer crosslinking level could be increased, which would inevitably result in a loss of swelling capacity and retention capability. Indeed, an optimized combination of various crosslinkers and comonomers as described in the patent specification DE 196 46 484 is able to improve the permeability properties, but not to such a level that incorporation in a diaper construction of a layer optionally consisting of superabsorbers only would be possible.
Furthermore, methods of surface secondary crosslinking of the polymer particles may be used. During the so-called secondary crosslinking, the carboxyl groups of the polymer molecules at the surface of the superabsorber particles are reacted with various secondary crosslinking agents capable of reacting with at least two of the carboxyl groups near the surface. In addition to increasing the gel strength, the capability of absorbing liquids under pressure is highly improved in particular, because the well-known phenomenon of gel blocking is suppressed, where slightly swollen polymer particles adhere to each other, thereby preventing further absorption of liquid.
The surface treatment of liquid-absorbing resins is already well-known. To improve the dispersibility, ionic complexing of the carboxyl groups near the surface using polyvalent metal cations has been suggested in U.S. Pat. No. 4,043,952. This treatment is effected using salts of multivalent metals dispersed in organic solvents (alcohols and other organic solvents) optionally containing water.
A secondary treatment of superabsorber polymers using reactive, surface-crosslinked compounds (alkylene carbonates) to increase the liquid absorption capability under pressure has been described in DE-A-40 20 780.
The EP 0,233,067 describes water-absorbing resins crosslinked at their surface, obtained by reacting a superabsorbent polymer powder with an aluminum compound. A mixture of water and diols is used as treatment solution, which is intended to render the use of lower alcohols as solvents unnecessary. Preferably, 100 parts of crosslinker solution is applied on 100 to 300 parts of absorber. According to the examples, the reaction with the aluminum component takes place at room temperature. The diols (e.g., polyethylene glycol 400 and 2000, 1,3-butanediol or 1,5-pentanediol) added to the water reaction medium serve to prevent aggregation of the superabsorber during the treatment with such large amounts of aqueous treatment solution used therein. The solvent is removed in a subsequent drying operation at 100° C. The polymers thus treated have an insufficient level of properties, with improvement of the absorption capability under pressure not being achieved. Furthermore, a treatment using large amounts of treatment solution is not economically feasible in modern, continuously operating processes.
WO 96/05234 describes a process for the production of superabsorbing polymers, according to which a crosslinked layer is formed at the surface of the absorber particles containing at least 10 wt.-% of water by reacting a reactive, hydrophilic polymer or a reactive organometallic compound with an at least bifunctional crosslinker below 100° C. The polymer products are said to have a well-balanced correlation of absorption, gel strength and permeability, the measured values having been determined according to extremely poor criteria of evaluation. Thus, for example, the absorption and permeability have been determined without any pressure load. One drawback in this well-known process is the use of solvents and toxically critical crosslinking reagents such as polyimines, alkoxylated silicone or titanium compounds, and epoxides which are mentioned as being preferred.
An improvement in the properties of permeability and liquid conveyance is achieved in WO 95/22356 and WO 97/12575 by appropriately treating commercially available superabsorber products with aminopolymers in organic solvents. In addition to using toxicologically critical polyamines and polyimines, a serious drawback of the process described therein is the use of large amounts of organic solvents required in the treatment of the polymers. Industrial production is excluded by the safety aspect and cost associated therewith. In addition to the toxicological risk of these treatment agents, their tendency to decompose under the high temperatures of secondary crosslinking must also be taken into account which, among other things, can be seen in a yellow discoloration of the absorber particles.
The state of the art as described above does not provide any indication that a dramatic augmentation in the permeability properties is also possible in this secondary crosslinking stage, while retaining high retention capacity and capability of absorbing liquids under pressure.
It was therefore the object of the present invention to provide superabsorbing polymers which, as a combination of properties, not only have high absorbing capacity under pressure but also the normally contrary properties of high retention capability and good permeability in combination, i.e., a level of combined properties where, in addition to a retention value of at least ≧25 g/g, an SFC value of
Harren Jorg
Mertens Richard
Asinovsky Olga
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Seidleck James J.
Stockhausen GmbH & Co. KG
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