Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Cellular products or processes of preparing a cellular...
Reexamination Certificate
2000-01-10
2001-11-27
Cooney, Jr., John M. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Cellular products or processes of preparing a cellular...
C521S061000, C521S063000, C521S064000, C521S123000, C526S317100, C526S318100, C526S318200
Reexamination Certificate
active
06323252
ABSTRACT:
In the gel polymerization of water-swellable polymers, monomers are polymerized in aqueous solution. Certain additives, such as cross-linking agents, may be incorporated into the monomer mixture. The product of the polymerization process is typically dried and subjected to mechanical means of particle size reduction and classification including chopping, grinding, and sieving. If desired, the dried superabsorbent polymer particles may be further surface modified and/or heat treated. Such post-heat-treatment leads to an almost entirely dry product with a strong tendency to build up static electricity during handling, which can, in turn, cause processing difficulties. Such static electricity build-up is extremely undesirable since it negatively influences the accuracy of superabsorbent polymer dosing and its distribution in personal care articles, such as diapers or sanitary napkins. Certain treatments are then required to minimize the static electricity build-up during the handling of the superabsorbent polymer (SAP) thereby increasing the electric conductivity of the surface of the SAP particles. Such treatments include, for example, addition of water, or polyethylene glycols.
It is known that more plasticized surfaces are less friable. Friability can cause property degradation and dust production in the polymer. It is also known that water can be a plasticizer for superabsorbent polymers. However, one major problem associated with the addition of water is the tendency of the SAP particles to swell on the surface and become sticky. As increasing amounts of water are added, the superabsorbent polymer becomes more cohesive and less processable.
In the past, the production of SAP did not comprise a post-heating treatment, thus, the addition of water to the polymerized and dried SAP was not required since the SAP thus prepared still contained moisture. However, it has been known to add aqueous solutions to the superabsorbent polymers in order to enhance certain properties thereof. The addition of such aqueous solutions, for example, to add surface post-cross-linkers or other additives or processing agents, is typically carried out in the presence of organic solvents, or anti-agglomerating agents, in order to prevent or minimize agglomeration caused by the water, which otherwise would occur.
European Patent Application 605,215 discloses a method for treating an absorbent resin by adding to the absorbent resin powder a substance capable of reacting with the residual monomer in the absorbent resin powder, thereby forming an absorbent resin composition having a water content in the range of 10 to 70 percent by weight and heat-treating the absorbent resin composition at a temperature in the range of 100° C. to 200° C. for not less than 10 minutes while retaining the water content of the absorbent resin within 20 percent. An alternative method comprises the addition to the absorbent resin powder of a substance capable of reacting with the residual monomer in the absorbent resin powder, thereby forming an absorbent resin composition having a water content in the range of 25 to 55 percent by weight and heat-treatment of the absorbent resin composition at a temperature in the range of 120° C. to 200° C. for a period of not less than 10 minutes while retaining the water content of the absorbent resin in the range mentioned above, and then drying the absorbent resin composition at a temperature not exceeding 120° C.
European Patent Application 309,187 discloses absorbent materials, such as cross-linked, water-soluble and water-swellable particulate polymers, which are immobilized and safely handled by adding an aqueous liquid, such as water or saline in amounts sufficient to form hydrates in which the water comprises from 20 percent to 80 percent by weight of the total hydrate.
U.S. Pat. No. 4,043,952 discloses a process for surface treating a water-absorbent composition to improve its aqueous dispersibility by a) forming a dispersion comprising a water-absorbent composition based on anionic polyelectrolyte, polyvalent cations of at least one metal, and a dispersing medium in which said composition is substantially insoluble; b) maintaining said dispersion at a temperature of −40° C. to +150° C. for a period of time sufficient for said cations to ionically complex the outer surface of said composition exposed to said dispersing medium; and c) removing said dispersing medium.
However, the surface treatment techniques of the state of the art, especially those not employing any anti-agglomerating agent, still present a major drawback of agglomerating the polymer particles, which agglomeration is substantially irreversible. Thus, it would be highly desirable to find means of preventing or minimizing undesirable and irreversible agglomeration without the need of using organic solvents, or water-insoluble, non-swellable powders as anti-agglomerating agents. Therefore, industry would find great advantage in improved aqueous fluid absorbent polymers which possesses reduced tendencies to build up static electricity and to generate dust, sufficient fine dust binding capability without causing undesirable agglomeration of the polymer particles in the surface treatment process. Industry would further find advantage in a process for distributing more homogeneously additives on the surface of superabsorbent polymers.
Accordingly, the present invention provides a composition comprising aqueous fluid absorbent polymer particles which have been heat-treated at temperatures greater than 170° C. for more than 10 minutes, wherein the composition has been remoisturized, after the heat-treatment, with an aqueous additive solution, in the absence of an organic solvent or water-insoluble, non-swellable powders, and comprises up to 10 percent by weight, based on the total weight of the composition, water and wherein the composition is characterized by the ability to absorb at least 20 grams of a 0.9 weight percent aqueous saline solution under a pressure of 0.3 psi (21,000 dynes/cm
2
), that is, a 60 minute 0.3 psi (21,000 dynes/cm
2
) AUL greater than 20 grams/gram as measured in accordance with the Absorption Under Load Test set forth in U.S. Pat. No. 5,147,343.
The present invention further provides a process comprising:
(a) preparing a water-swellable hydrogel by a gel polymerization process;
(b) drying and sizing the hydrogel to form a composition comprising dried and sized particles, the composition comprising particles having a particle size distribution of 50 to 1500 microns;
(c) heat-treatment; and
(d) contacting the composition with an aqueous additive solution in the absence of an organic solvent or water-insoluble inorganic powders, wherein the composition is characterized by a 60 minute 0.3 psi (21,000 dynes/cm
2
) AUL greater than 20 grams/gram.
Yet another objective of the present invention is a process comprising:
(a) preparing a water-swellable hydrogel by a gel polymerization process;
(b) drying and sizing the hydrogel to form a composition comprising dried and sized particles, the composition comprising particles having a particle size distribution of 50 to 1500 microns;
(c) contacting the composition with an aqueous additive solution in the absence of an organic solvent or water-insoluble inorganic powders, wherein the composition is characterized by a 60 minute 0.3 psi (21,000 dynes/cm
2
) AUL greater than 20 grams/gram;
(d) drying an/or heat-treating the composition; and, optionally,
(e) remoisturization of the heat-treated SAP so that the resultant SAP contains up to 10 percent of water.
Surprisingly, it has been found that the remoisturized polymer particles of the present invention are more homogeneously moisturized to the desired level, with substantially reduced tendency for irreversible agglomeration during the process of remoisturization without sacrificing other physical and/or chemical properties. In view of the reduced tendency for irreversible agglomeration, the remoisturized polymer particles of the present invention exhibit reduced static electricity build-up. Furthermo
Fialkowski Michael A.
Gartner Herbert A.
Staples Thomas L.
Cooney Jr. John M.
The Dow Chemical Company
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