Catalyst – solid sorbent – or support therefor: product or process – Solid sorbent – Organic
Reexamination Certificate
1999-10-20
2001-10-09
Hendrickson, Stuart L. (Department: 1754)
Catalyst, solid sorbent, or support therefor: product or process
Solid sorbent
Organic
Reexamination Certificate
active
06300275
ABSTRACT:
This invention relates to superabsorbent compositions which contain multivalent metal salts and a process for preparing said compositions.
Superabsorbent polymers are well-known materials which commonly are used in personal care articles such as diapers. These polymers are known to absorb several times their weight of, for example, water, saline solution, urine, blood, and serous bodily fluids. However, these polymers suffer from a phenomenon called gel blocking. Gel blocking refers to decreased rate of absorbency which results from rapid swelling of particle surfaces followed by clumping of polymer gel particles. This clumping tends to shield or block part of the absorbent polymer from the fluid to be absorbed, resulting in decreased absorption per unit weight of polymer.
Because superabsorbent polymers are subject to gel blocking, it is common practice in the manufacture of diapers to mix the superabsorbent polymer with a fibrous material such as cellulose fluff used in diapers. One purpose of the fluff is to separate the particles of superabsorbent polymer from one another in order to decrease the degree of gel blocking. Unfortunately, fluff prices recently have risen making it uneconomical to use as much fluff as was commonly used in the past. As a result, diaper manufacturers would like to replace some of the fluff with additional superabsorbent polymer. However, replacing fluff with superabsorbent is not an ideal solution since known superabsorbents tend to gel block more in diapers having less fluff. Therefore, it would be desirable to have a superabsorbent polymer having reduced gel blocking characteristics.
U.S. Pat. No. 5,115,011 addresses the gel blocking problem by contacting a water absorbent polymer with an aqueous solution of two water soluble salts, the first being a halogen, sulfate, acetate or nitrate of aluminum, calcium or magnesium, and the second being a monovalent metal salt or ammonium salt of at least one kind of an oxyacid selected from sulfurous acid and thiosulfuric acid. A dry blend of 0.6 g aluminum sulfate and 30 g polymer is prepared in Example for Comparison 3 of the patent, and is shown to have a blocking of 70 percent or more after 5 minutes.
U.S. Pat. No. 5,578,318 discloses the preparation of superabsorbent “hydrophobic coated particles” by dry blending materials, such as non-crosslinked polyacrylate salts, with a source of multivalent ions and, optionally, then adding an alcohol, certain wetting agents, and polysiloxane derivatives. The wetted material is dried prior to use. Example XXIII of this patent discloses a blend of 2.61 weight percent AQUALON A-250, 0.21 weight percent aluminum acetate, and 97.18 weight percent water. After drying, the resulting material of this example exhibited relatively poor performance as a superabsorbent.
U.S. Pat. No. 4,090,013 discloses materials prepared from a water-soluble anionic polyelectrolyte and a polyvalent metal cation source. However, the products are s characterized in U.S. Pat. No. 5,578,318 as exhibiting gel blocking.
U.S. Pat. No. 4,693,713 discloses an absorbent for blood and serous bodily fluids, the absorbent comprising a physical mixture of certain polymers and certain compounds. The compounds are described as water soluble, present in the form of a pourable powder at ambient temperature, and not harmful to health. The patent teaches that the compound may be added to the polymer by dissolving it in the monomer solution, or that the compound can be added to the polymer preparation process at any time in dry or dissolved form. Dry blends of polymer and compound are prepared in the examples of the patent.
J0172457-A discloses a composition prepared by dry blending a superabsorbent resin and polyaluminum chloride. This publication teaches that water absorption performance is greatly decreased when a salt other than polyaluminum chloride is used.
It would be desirable to have a superabsorbent polymer with improved resistance to gel blocking and which would not require the use of oxyacids in its preparation. It would be further desirable to have a superabsorbent polymer with improved absorption, absorption under load and decreased gel blocking. Furthermore, it would be desirable to have a simpler and more efficient process to prepare materials with such properties.
The superabsorbent polymer of the invention exhibits improved gel bed resiliency. Specifically, the superabsorbent polymer composition of the invention exhibits a gel bed resiliency of at least 5 millimeters. Gel bed resiliency is a property which describes the ability of a superabsorbent polymer gel mass to rebound following compression. While not wishing to be bound by any theory, it is believed that this improved resiliency reduces gel blocking by maintaining the porosity of the gel bed. It is further believed that improved gel bed resiliency makes the superabsorbent polymer of the invention especially suitable for use in absorbent articles, such as diapers, which have high loadings of superabsorbent polymer.
The invention includes a process for preparing the improved superabsorbent polymer. The invention also includes a process for preparing an improved superabsorbent polymer, the process comprising mixing the polymer with a multivalent metal salt, then intimately contacting the mixture from 0.1 to 10 percent binder, based on the weight of the polymer and multivalent metal salt mixture, with the proviso that the contacting is conducted in the substantial absence of volatile alcohols. The invention further includes absorbent articles comprising the composition of the invention.
The improved superabsorbent polymer of the invention is prepared by mixing, preferably by dry blending, a superabsorbent polymer with a multivalent metal salt and then contacting the mixture with a binder.
In the present invention, a multivalent metal salt suitably is employed in an amount sufficient to improve the gel bed resiliency property of the superabsorbent polymer. Preferably, at least 0.5 percent of multivalent metal salt, based the weight of the mixture, is employed. More preferably, at least about 1 percent of multivalent metal salt, and most preferably at least about 2 percent of multivalent metal salt are employed. Preferably, no more than about 10 percent of multivalent metal salt are employed based on the weight of the mixture of multivalent metal salt and polymer. More preferably, no more than about 8 weight percent multivalent metal salt is employed, and most preferably, no more than 6 weight percent multivalent metal salt is employed. Preferably, the amount of multivalent metal salt employed is from 0.5 to 10 weight percent based on the weight of multivalent metal salt and polymer. More preferably, from 1 to 8 percent of multivalent metal salt are employed, and most preferably from 2 to 6 weight percent are employed.
The multivalent metal salt is preferably water soluble. Examples of preferred metal cations include the cations of Al, Fe, Zr, Mg and Zn. Preferably, the metal cation has a valence of at least +3, with Al being most preferred. Examples of preferred anions in the multivalent metal salt include halides, chlorohydrates, sulfates, nitrates and acetates, with chlorides, sulfates, chlorohydrates and acetates being preferred, chlorohydrates and sulfates being more preferred, and sulfates being most preferred. Inorganic salts are more preferred. Aluminum sulfate is the most preferred multivalent metal salt and is readily commercially available. The preferred form of aluminum sulfate is hydrated aluminum sulfate, more preferably aluminum sulfate having from 12 to 14 waters of hydration. Mixtures of multivalent metal salts can be employed. Preferably, the contacting of the polymer and multivalent metal salt is conducted in the substantial absence of a monovalent metal salt or ammonium salt of at least one kind of an oxyacid selected from sulfurous acid and thiosulfuric acid. In a preferred embodiment, the contacting is conducted in the substantial absence of divalent metal salts.
The polymer and multivalent metal salt suitably ar
Hendrickson Stuart L.
The Dow Chemical Company
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