Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymers from only ethylenic monomers or processes of...
Reexamination Certificate
2003-03-04
2004-12-14
Wu, David W. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Polymers from only ethylenic monomers or processes of...
C526S317100, C525S055000, C525S057000, C525S061000, C525S244000, C525S246000, C525S249000, C525S253000
Reexamination Certificate
active
06831142
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to pulverulent, crosslinked polymers which absorb water, aqueous liquids and blood (superabsorbers) and have improved properties, in particular an improved retention and an improved retention capacity for liquids under pressure and an improved capacity for transportation of liquids, their preparation and their use as absorbents in hygiene articles and in industrial fields.
BACKGROUND OF THE INVENTION
Superabsorbers are water-insoluble, crosslinked polymers which are capable of absorbing large amounts of aqueous liquids and body fluids, such as e.g. urine or blood, with swelling and the formation of hydrogels, and of retaining them under a certain pressure. As a result of these characteristic properties, these polymers are chiefly used for incorporation into sanitary articles, such as e.g. babies' nappies, incontinence products or sanitary towels.
The superabsorbers which are currently commercially available are substantially crosslinked polyacrylic acids or crosslinked starch-acrylic acid graft polymers, in which some of the carboxyl groups are neutralized with sodium hydroxide solution or potassium hydroxide solution.
For aesthetic reasons and from environmental aspects, there is an increasing trend to make sanitary articles, such as babies' nappies, incontinence products and sanitary towels, ever smaller and thinner. To ensure a constant total retention capacity of the sanitary articles, this requirement can be met only by reducing the content of large-volume fluff. As a result of this, further tasks fall to the superabsorbers in respect of transportation and distribution of liquid, which can be summarized as permeability properties.
In the case of superabsorber materials, permeability is understood as meaning the capacity for transportation and three-dimensional distribution of added liquids in the swollen state. This process proceeds in the swollen superabsorber gel by a capillary transportation through intermediate spaces between the gel particles. Transportation of liquid through swollen superabsorber particles itself follows the laws of diffusion and is a very slow process which plays no role in the distribution of the liquid in the use situation of the sanitary article. In superabsorber materials which cannot effect capillary transportation because of a lack of gel stability, separation of the particles from one another, avoiding the gel blocking phenomenon, has been ensured by embedding these materials into a fiber matrix. In nappy constructions of the new generation, there is only little fiber material, or none at all, in the absorber layer to assist in transportation of the liquid. The superabsorbers used here must accordingly have a sufficiently high stability in the swollen state so that the swollen gel still has a sufficient amount of capillary spaces through which liquid can be transported.
To obtain superabsorber materials with a high gel strength, on the one hand the degree of crosslinking of the polymer can be increased, which necessarily results in a reduction in the swellability and the retention capacity. An optimized combination of various crosslinking agents and comonomers, as described in the patent specification DE 196 46 484, is indeed capable of improving the permeability properties, but not to a level which allows, for example, incorporation of a layer which optionally comprises only superabsorbers in a nappy construction.
Methods for surface after-crosslinking of the polymer particles can furthermore be used. In so-called after-crosslinking, the carboxyl groups of the polymer molecules on the surface of the superabsorber particles are reacted with various after-crosslinking agents which can react with at least two of the carboxyl groups close to the surface. In addition to increasing the gel strength, the ability to absorb liquid under pressure is greatly improved in particular, since the known phenomenon of gel blocking, in which swollen polymer particles stick together and as a result a further absorption of liquid is prevented, is suppressed.
The surface treatment of liquid-absorbing resins is already known. To improve dispersibility, ionic complexing of the carboxyl groups close to the surface with polyvalent metal cations is proposed in U.S. Pat. No. 4,043,952. The treatment is carried out with salts of polyvalent metals dispersed in organic, optionally water-containing solvents (alcohols and other organic solvents).
An after-treatment of superabsorber polymers with reactive surface-crosslinking compounds (alkylene carbonates) to increase the absorptive ability for liquids under pressure is described in DE-A40 20 780. A surface after-crosslinking of superabsorbent polymers with polyfunctional crosslinking agents, such as polyvalent metal compounds, in the presence of an inert, inorganic powder, such as SiO
2
, to improve the absorption properties and to produce a non-tacky gel of the polymer particles is described in DE-A-35 03 458.
According to EP-A-0 574 260, superabsorbent polymers with a low residual monomer content which does not change decisively even by surface crosslinking are obtained if certain conditions are maintained during the polymerization and the after-crosslinking is carried out with conventional polyfunctional crosslinking agents, such as polyols, alkylene carbonates and polyvalent metal salts, under conventional conditions.
The after-crosslinked polymers show a good absorption without the use of pressure. According to EP-A-0 889 063, superabsorbent polymers which are already surface-crosslinked can be treated against free-radical degradation by body fluids, in particular L-ascorbic acid, by after-treatment with a compound of titanium or zirconium and a compound which chelates these metal compounds.
EP 0 233 067 describes water-absorbing resins which are crosslinked on the surface and are obtained by reaction of a superabsorbent polymer powder with 1-40 wt. %, based on the polymer powder, of an aluminium compound. A mixture of water and diols, which is said to make the use of lower alcohols as solvents superfluous, is used as the treatment solution. 100 parts by wt. of crosslinking agent solution are applied to 100 to 300 parts by weight of absorber. The diols (e.g. polyethylene glycol 400 and 2000, 1 ,3-butanediol or 1,5-pentanediol) added to the reaction medium of water also serve to prevent lumping together of the superabsorber during treatment with the large amounts of aqueous treatment solution used here. The solvent is removed in a subsequent drying step at 100° C. The polymers treated in this way have an inadequate level of properties, and an improvement in the absorptive ability under pressure is not achieved. Furthermore, treatment with large amounts of treatment solution cannot be carried out economically in modem, continuously operating processes.
WO 96/05234 describes a process for the treatment of superabsorbent polymers, according to which the surface of the absorber particles, which contain at least 10 wt. % water, was treated with a crosslinked layer obtained by a reaction of a reactive, hydrophilic polymer or a reactive organometallic compound with an at least bifunctional crosslinking agent at temperatures below 100° C. Metal salts are not mentioned. The metal compounds employed must be able to react with the functional groups of the crosslinking agent. Organometallic compounds are therefore recommended as the metal compounds, and should be present in a weight ratio of 0.1 to 30 to the crosslinking compound. The polymers obtained are said to have a balanced ratio of absorption, gel strength and permeability, the measurement values stated being determined under less critical conditions. Thus, for example, the absorption and the permeability are determined without any pressure loading. A disadvantage of this known process is the use of solvents and toxically unacceptable crosslinking reagents, such as e.g. the polyimines, alkoxylated silane or titanium compounds and epoxides mentioned as preferred.
According to WO 95/22356 and WO 97/12575, an improvement
Jörg Harren
Mertens Richard
Sastri Satya
Smith Moore LLP
Stockhausen GmbH & Co. KG
Wu David W.
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