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
1998-01-30
2002-09-10
Le, Hoa Van (Department: 1752)
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...
C524S320000
Reexamination Certificate
active
06448320
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a superabsorbent resin composition which has high water absorption and undergoes neither degradation nor deterioration even in a water-containing state after it absorbs aqueous liquids or body fluids, such as urine, blood and perspiration; and to a method for producing the same.
2. Description of Related Art
Superabsorbent resins have been widely used as a water-absorbing material in absorbent articles in the sanitary field, such as disposable diapers for babies, adults or those suffering from incontinence, and sanitary napkins; as water-retaining materials in the agricultural and horticultural fields; and as sludge coagulants, moisture condensation preventive agents, and water stopping agents in the field of civil engineering.
It is known that water-soluble polymers (crosslinked polymers) constituting such superabsorbent resins undergo molecular weight reduction (degradation) and deterioration with time in the presence of radical generating species, such as hydrogen peroxide or L-ascorbic acid or a salt thereof. Because L-ascorbic acid or a salt thereof is present in body fluids, such as urine, blood, and perspiration, superabsorbent resin degradation and deterioration in disposable diapers or sanitary napkins has been a serious problem.
The degradation reaction of a water-soluble polymer due to such radical generating species is conspicuous after the polymer has absorbed an aqueous liquid or a body fluid, such as urine, blood or perspiration (hereinafter referred to as a water-containing condition), especially in the co-presence of transition metal ions capable of having more than one oxidation number, such as iron ions or copper ions, in the air.
This is because traces of transition metal ions, such as iron or copper, serve as a catalyst to markedly accelerate decomposition and radical generation of hydrogen peroxide or L-ascorbic acid or a salt thereof, as described, e.g., in
J. Am. Chem. Soc
., Vol. 89, p. 4176 (1967) and
Carbohydrate Research
, Vol. 4, p. 63 (1967).
While such transition metal ions are sometimes added intentionally or as a third component to a system containing a superabsorbent resin and a radical generating species, e.g., hydrogen peroxide or L-ascorbic acid or a salt thereof, metallic ions are present in water or raw materials as impurities in a very trace amounts, and it is known that such extremely small amounts of transition metal ions can have sufficient catalyzing activity for severing the water-soluble high-molecular weight chains of the superabsorbent resin over time (see, for example,
Free Radical Research Communications
, Vol. 1, p. 349 (1986)).
Known approaches for inhibiting the above-described degradation and deterioration of superabsorbent resins include (1) a method comprising sealing the superabsorbent resin under reduced pressure, or in a nitrogen atmosphere so as to avoid contact with air (especially oxygen), (2) a method comprising using highly purified water or raw materials so as to prevent metallic ions from entering the superabsorbent resin, (3) a method comprising adding an antioxidant or a reducing agent to the superabsorbent resin, (4) a method of adding proteins or enzymes to the superabsorbent resin, and (5) a method of adding to the superabsorbent resin, metal chelating agents, such as citric acid, (poly)phosphoric acid or a salt thereof, and ethylenediaminetetraacetic acid (EDTA) or a salt thereof.
However, the methods (1) and (2) are in many cases practically impossible to apply depending on the use of the superabsorbent resin. Methods (3), (4), and (5) that rely on conventional additives achieve some effect in suppressing degradation and deterioration of superabsorbent resins but are not always sufficient in effect. In many cases, an additive must be added in a large quantity or an additive exerting a very strong action must be used before the desired effect can be manifested. Under such circumstances, the essential physical properties or functions of the superabsorbent resin tend to be seriously ruined, and some of the additives can bring about unfavorable results in terms of the hygiene of the working environment, i.e., some emit an offensive odor, and some have poor stability.
Mixing or dispersing a metal chelating agent in a superabsorbent resin is disclosed in Japanese Patent Laid-Open Nos. 89838/81, 230046/84, and 275661/89. For example, it has been revealed that the water-absorbing performance is not reduced even in the presence of water containing salts or ions by using EDTA, sodium tripolyphosphate, etc.
According to the inventors' study, however, it has turned out that the use of EDTA or sodium tripolyphosphate is not so effective in stabilizing a superabsorbent resin in the presence of an aqueous solution or water containing a radical generating species, e.g., hydrogen peroxide or L-ascorbic acid or a salt thereof.
In addition to the above-mentioned performance properties of superabsorbent resins, i.e., stability over time in a water-containing state (gel stability with time), the water absorption capacity (the amount of water absorbed), the rate of water absorption, the gel strength after swelling, liquid permeability, and the like are important requirements for superabsorbent resins. However, many of these properties conflict with each other, and it is very difficult to meet all of these requirements, which has been one of the problems in developing superabsorbent resins. For example, an attempt to increase water absorption is generally accompanied by reductions in gel strength after swelling and liquid permeability.
Various methods have recently been proposed in order to solve these problems. For example, Japanese Patent Publication Nos. 18690/85 and 48521/86 propose forming a highly crosslinked layer as a surface layer of a superabsorbent resin. Japanese Patent Publication No. 19563/93, and Japanese Patent Laid-Open Nos. 211305/86, 264006/86, and 36411/87 propose graft-treating a carboxyl- and/or carboxylate-containing superabsorbent resin with a silane coupling agent. Japanese Patent Laid-Open No. 306118/94 proposes treating a superabsorbent resin with a titanium alkoxide. Additionally, a method comprising spraying an aqueous solution of a compound capable of reacting easily with a functional group of a superabsorbent resin (e.g., a carboxylate group), such as an aqueous solution of a polyvalent metal salt, a polyglycidyl ether, or a polyisocyanate, onto a superabsorbent resin, followed by heating to form a highly crosslinked layer on the surface of the superabsorbent resin is known. Nevertheless, these methods are still insufficient for satisfying both superabsorbent performance (e.g., a water absorption capacity) and gel stability with time after swelling.
Japanese Patent Laid-Open No. 145326/95 discloses the addition of a sulfate of a polyvalent metal selected from titanium, zirconium and vanadium to a superabsorbent polymer as one method for simultaneously improving gel strength, stability, and stickiness after water absorption.
According to the inventors' study, however, the gel stability over time achieved by this method is insufficient particularly for polymers having high water absorption. Besides, addition of a polyvalent metal sulfate tends to reduce the initial rate of water absorption of the superabsorbent polymer or tends to make the polymer particles before water absorption ready to agglomerate due to the hygroscopicity of the polyvalent metal sulfate.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a superabsorbent resin composition which exhibits high water absorption and is yet stable against degradation and/or deterioration even in the presence of an aqueous solution or water containing a radical generating species, such as L-ascorbic acid or a salt thereof, or a transition metal ion, such as an iron ion or a copper ion.
In particular, the object of the present invention is to provide a superabsorbent resin composition which has a high water absorption and undergo
Furugaki Hisakazu
Hanada Yoko
Igarashi Tadashi
Clarke Yvette M.
Kao Corporation
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