Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – At least one aryl ring which is part of a fused or bridged...
Reexamination Certificate
2000-03-10
2002-03-19
Jagannathan, Vasu (Department: 1714)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
At least one aryl ring which is part of a fused or bridged...
C524S556000, C524S555000, C524S548000, C604S358000, C604S372000
Reexamination Certificate
active
06359049
ABSTRACT:
FIELD OF THE INVENTION
The present invention generally relates to superabsorbent polymer (SAP) compositions having long-term color stability. The color-stable SAP compositions can be incorporated into articles, such as bandages, diapers, sanitary napkins, and other disposable paper products, wherein the SAP composition retains a clean, white color during extended storage periods, even under high temperature and humidity conditions.
BACKGROUND OF THE INVENTION
Water-absorbing resins are widely used in sanitary and hygienic goods, wiping cloths, water-retaining agents, dehydrating agents, sludge coagulants, disposable towels and bath mats, disposable door mats, thickening agents, disposable litter mats for pets, condensation-preventing agents, and release control agents for various chemicals. Water-absorbing resins are available in a variety of chemical forms, including substituted and unsubstituted natural and synthetic polymers, such as hydrolysis products of starch acrylonitrile graft polymers, carboxymethylcellulose, crosslinked polyacrylates, sulfonated polystyrenes, hydrolyzed polyacrylamides, polyvinyl alcohols, polyethylene oxides, polyvinylpyrrolidones, and polyacrylonitriles.
Such water-absorbing resins are termed “superabsorbent polymers,” or SAPs, and typically are lightly crosslinked hydrophilic polymers. SAPs are discussed generally in Goldman et al. U.S. Pat. Nos. 5,669,894 and 5,559,335, the disclosures of which are incorporated herein by reference. SAPs can differ in their chemical identity, but all SAPs are capable of absorbing and retaining amounts of aqueous fluids equivalent to many times their own weight, even under moderate pressure. For example, SAPs can absorb one hundred times their own weight, or more, of distilled water. The ability to absorb aqueous fluids under a confining pressure is an important requirement for an SAP used in a hygienic article, such as a diaper.
As used herein, the term “SAP particles” refers to superabsorbent polymer particles in the dry state, more specifically, particles containing from no water up to an amount of water less than the weight of the particles. The terms “SAP gel,” “SAP hydrogel,” or “hydrogel” refer to a superabsorbent polymer in the hydrated state, more specifically, particles that have absorbed at least their weight in water, and typically several times their weight in water.
SAPs have a tendency to degrade in color after long periods of storage. The tendency of an SAP to undergo a color transition from a clean, crisp, white color to a honey brown color accelerates as storage time, temperature, and humidity increases. In temperate climates, such as the United States and Europe, the rate at which an SAP undergoes color degradation is sufficiently slow such that the SAP, or article containing the SAP, typically is consumed before a color change is observable to the naked eye.
However, in tropical and subtropical climates, SAP color degradation is sufficiently rapid such that a color change often occurs before the SAP, or article containing the SAP, is consumed. In areas like southeast Asia, an SAP can change color from white to honey brown in about 4 to 6 weeks. This problem is exacerbated because the SAPs are produced far from the tropical climate, thereby increasing the time span of from SAP production to use. Furthermore, consumption of articles containing an SAP in such climates is relatively low, therefore further increasing the time period between SAP production and use.
The change in color of the SAP does not affect SAP performance, but adversely affects consumer acceptance of articles containing the color-degraded SAPs. In particular, consumers observing a color-degraded SAP in a diaper form an opinion that the diaper contains a contaminant, is somehow soiled or faulty, or is of low quality. The diaper typically is returned for a refund, and the consumer is less likely to repurchase that brand of diaper.
Problems also arise at the manufacturing level because manufacturers of diapers and other articles containing an SAP refuse to incorporate a discolored SAP into their products, and return the discolored SAP to the SAP manufacturer. A color-degraded SAP, therefore, ultimately adversely affects the manufacturer of articles and the manufacturer of the SAP, who must absorb the cost of the returned goods.
It would be desirable to provide an SAP that exhibits exceptional color stability properties, such that the SAP retains its crisp, white color throughout the useful life of the SAP, or the article containing the SAP, even when stored under high temperature and humidity conditions. Furthermore, it would be desirable to provide an SAP having a long-term color stability, without adversely affecting the absorbent properties of the SAP, such as absorbing a large amount of liquids quickly, having a good fluid permeability into and through the SAP, and having a high gel strength, such that a hydrogel formed from the SAP does not deform or flow under an applied stress or pressure.
SUMMARY OF THE INVENTION
The present invention is directed to an superabsorbent polymer (SAP) composition having long-term color stability, and to methods of manufacturing a color-stable SAP composition. More particularly, the present invention is directed to a color-stable SAP composition that retains its crisp, clean white color over an extended storage period at a high temperature and humidity, i.e., at least six (6) months when stored at 60° C. and 75% relative humidity.
One aspect of the present invention, therefore, is to provide a color-stable SAP composition comprising an SAP, about 0.05 to about 5 mole % of an inorganic reducing agent, and 0% to about 5 mole % of a metal salt, each mole % based on the total amount of monomers in the composition. When a metal salt is present, the metal salt and inorganic reducing agent are present in a mole ratio of about 3 to 1 to about 1 to 3, respectively.
Another aspect of the present invention is to provide a method of manufacturing a color-stable SAP composition, including the steps of polymerizing a monomer that provides an SAP, like an &agr;,&bgr;-unsaturated carboxylic acid, such as acrylic acid, either neutralized, unneutralized, or partially neutralized, to form a polymeric hydrogel, admixing an inorganic reducing agent and an optional metal salt into the hydrogel, and then drying the resulting admixture to form a color-stable SAP composition. The resulting color-stable SAP composition has the color-stabilizing compounds homogeneously dispersed throughout the SAP.
Another aspect of the invention is to provide a method of manufacturing a color-stable SAP composition including the steps of polymerizing a monomer that provides an SAP, for example, an &agr;,&bgr;-unsaturated carboxylic acid (either neutralized, unneutralized, or partially neutralized), to form a polymeric hydrogel, drying the hydrogel to form SAP particles, and then adding the inorganic reducing agent and optional metal salt to the SAP particles to provide a color-stable SAP composition. The color-stabilizing compounds can be added to the SAP particles by applying one or more solutions of the color-stabilizing compounds to the SAP particles. The color-stabilizing compounds also can be added to the SAP particles by admixing SAP particles with solid particles of the color-stabilizing compounds. The color-stabilizing compounds can be added to the SAP particles individually, or together, or in conjunction with a surface treatment that provides surface crosslinks on the SAP particle.
In another aspect of the present invention, the color-stabilizing compounds can be added to the SAP during manufacture of the SAP particles to prevent discoloration. The color-stabilizing compounds also can be added to previously prepared SAP particles to prevent any additional discoloration of the SAP particles.
In accordance with another aspect of the present invention, a color-stable SAP composition can be incorporated into articles used to absorb liquids, like diapers and catamenial devices. The articles resist color degradation over the expecte
Carrico Peter W.
Eckert David
BASF - Aktiengesellschaft
Jagannathan Vasu
Marshall Gerstein & Borun
Shosho Callie E
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