Stock material or miscellaneous articles – Coated or structually defined flake – particle – cell – strand,... – Particulate matter
Reexamination Certificate
2000-05-25
2002-05-21
Kiliman, Leszek (Department: 1773)
Stock material or miscellaneous articles
Coated or structually defined flake, particle, cell, strand,...
Particulate matter
C428S403000, C428S407000, C528S329100, C528S375000, C528S374000, C528S386000, C526S306000
Reexamination Certificate
active
06391451
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to surface-crosslinked superabsorbent polymer particles, and to methods of producing the surface-crosslinked superabsorbent particles. The present invention also relates to the use of the surface-crosslinked particles in articles, such as diapers, catamenial devices, and wound dressings. More particularly, the present invention relates to surface treating superabsorbent polymer (SAP) particles, such as a neutralized, crosslinked, homopolymer or copolymer of acrylic acid, with a hydroxyalkylamide to substantially improve the water absorption and water retention properties of the SAP particles.
BACKGROUND OF THE INVENTION
Water-absorbing resins are widely used in sanitary goods, 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 generally discussed 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 requirements for an SAP used in a hygienic article, such as a diaper.
As used here and hereafter, the term “SAP particles” refers to superabsorbent polymer particles in the dry state, i.e., particles containing from no water up to an amount of water less than the weight of the particles. The term “particles” refers to granules, fibers, flakes, spheres, powders, platelets, and other shapes and forms known to persons skilled in the art of superabsorbent polymers. The terms “SAP gel” and “SAP hydrogel” refer to a superabsorbent polymer in the hydrated state, i.e., particles that have absorbed at least their weight in water, and typically several times their weight in water. The terms “surface-treated SAP particle” and “surface-crosslinked SAP particle” refer to an SAP particle having its molecular chains present in the vicinity of the particle surface crosslinked by a compound applied to the surface of the particle. The term “surface crosslinking” means that the level of functional crosslinks in the SAP particle in the vicinity of the surface of the particle is generally higher than the level of functional crosslinks in the SAP particle in the interior of the particle.
SAP particles can differ in ease and cost of manufacture, chemical identity, physical properties, rate of water absorption, and degree of water absorption and retention, thus making the ideal water-absorbent resin a difficult composition to design. For example, the hydrolysis products of starch-acrylonitrile graft polymers have a comparatively high ability to absorb water, but require a cumbersome process for production and have the disadvantages of low heat resistance and decay or decomposition due to the presence of starch. Conversely, other water-absorbent polymers are easily and cheaply manufactured and are not subject to decomposition, but do not absorb liquids as well as the starch-acrylonitrile graft polymers.
Therefore, it would be extremely advantageous to provide a method of increasing the water absorption properties of a stable, easy to manufacture SAP particles to match the superior water absorption properties of a difficult to manufacture particle. Likewise, it would be advantageous to further increase the liquid absorption properties of already superior SAP particles.
In addition, conventional SAP particles all have a serious defect in that their rates of liquid absorption are lower than fluff pulp and paper. For example, when urine is excreted on a disposable diaper containing conventional SAP particles, the urine can remain in contact with the skin for a relatively long time and make the wearer uncomfortable. This is attributed to the low rate at which the diaper can absorb urine.
Attempts have been made to increase the liquid absorption rate by increasing the surface area of the SAP particle, i.e., by decreasing its particle size. However, when the particle size of the SAP particle is decreased, it generally forms a “fish eye” upon contact with urine, which retards the speed of liquid absorption. When the SAP particles are in the form of granules, each granule constitutes a “fish eye” and the speed of liquid absorption decreases. SAP particles in flake form exhibit a moderate increase in the speed of liquid absorption. But, SAP flakes are bulky and are difficult to transport and store.
Initially, the swelling capacity of an SAP particle on contact with liquids, also referred to as free swelling capacity, was the main factor in the design and development of SAP particles. Later, however, it was found that not only is the amount of absorbed liquid important, but the stability of the swollen gel, or gel strength, also important. The free swelling capacity, on one hand, and the gel strength, on the other hand, represent contrary properties. Accordingly, SAP particles having a particularly high absorbency typically exhibit a poor gel strength, such that the gel deforms under pressure (e.g., the load of a body), and prevents further liquid distribution and absorption.
A balanced relation between absorptivity (gel volume) and gel strength is desired to provide proper liquid absorption, liquid transport, and dryness of the diaper and the skin when using SAP particles in a diaper. In this regard, not only is the ability of the SAP particle to retain a liquid under subsequent pressure an important property, but absorption of a liquid against a simultaneously acting pressure, i.e., during liquid absorption also is important. This is the case in practice when a child or adult sits or lies on a sanitary article, or when shear forces are acting on the sanitary article, e.g., leg movements. This absorption property is referred to as absorption under load.
Currently, there is a trend to reduce the size and thickness of sanitary articles for esthetic and environmental reasons (e.g., reduction of waste in landfills). This is accomplished by reducing the large volume of fluff pulp and paper in diapers, and increasing the amount of SAP particles. The SAP particles, therefore, have to perform additional functions with respect to liquid absorption and transport which previously were performed by the fluff pulp and paper, and which could not be accomplished satisfactorily with conventional SAP particles.
Investigators have researched various methods of improving the amount of liquid absorbed and retained by SAP particles, especially under load, and the rate at which the liquid is absorbed. One preferred method of improving the absorption and retention properties of SAP particles is to surface treat the SAP particles.
The surface treatment of SAP particles is well known. For example, U.S. Pat. No. 4,043,952 discloses the use of polyvalent metal compounds as surface treating compounds. U.S. Pat. No. 4,051,086 discloses the use of glyoxal as a surface treatment to improve the absorption rate of SAP particles. The surface treatment of SAP particles with crosslinking agents having two or more functional groups capable of reacting with pendant carboxylate or other groups contained on the p
Beihoffer Thomas W.
Eckert David
Mitchell Michael A.
Kiliman Leszek
Marshall Gerstein & Borun
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