Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Reexamination Certificate
1998-04-06
2001-02-13
Wu, David W. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C526S088000, C526S240000
Reexamination Certificate
active
06187872
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a water-absorbent agent and a manufacturing method of the same, and more particularly to a water-absorbent agent suitable for sanitary articles such as paper diapers (disposable diapers) and sanitary napkins, with a significantly improved water absorbing ability, and also relates to a manufacturing method of the same.
BACKGROUND OF THE INVENTION
In recent years, for absorption of large quantities of water, a water-absorbent agent such as water-absorbent resin has been widely adopted as one of the materials constituting sanitary articles such as paper diapers, sanitary napkins, and adult incontinence pads. Also, for absorbing and holding of water, such a water-absorbent agent (water-absorbent resin, etc.) has also been widely adopted as soil conditioners and drip sheets for food.
Examples of a conventionally known water-absorbent agent are as follows: (1) a partially neutralized and crosslinked polyacrylic acid, (2) hydrolyzed copolymer of starch-acrylonitrile, (3) neutralized graft polymer of starch-acrylic acid, (4) saponified copolymer of vinyl acetate-acrylic ester, (5) a crosslinked polymer of a hydrolyzed copolymer of acrylonitrile or a crosslinked polymer of a hydrolyzed copolymer of acrylamide, and (6) a crosslinked cation monomer, etc.
The properties which the water-absorbent resin is desired to possess, in order to be adopted as sanitary articles, include, for example, (a) high absorbency and high absorbing rate to be manifested upon contact with aqueous liquids such as body fluids, (b) liquid permeability, (c) high strength exhibited by a gel swollen with liquid, and (d) an ability to aspirate water from a substrate impregnated with aqueous liquid.
However, these properties are not necessarily proportionally related to one another such that, for example, as absorbency of water-absorbent resin increases, such properties as the liquid permeability, the gel strength, and the absorbing rate decrease. In order to improve a balance of the various water-absorbent properties of the water-absorbent resin, various techniques of crosslinking the surface region of the water-absorbent resin have been suggested.
As such techniques, the following methods in which specific compounds are adopted as a crosslinking agent are known. (a) A method using a polyhydric alcohol as a crosslinking agent (Japanese Unexamined Patent Application No. 180233/1983 (Tokukaisho 58-180233), and Japanese Unexamined Patent Application No. 16903/1986 (Tokukaisho 61-16903)), (b) a method using a polyglycidyl compound, a polyaziridine compound, a polyamine compound, and a polyisocyanate compound as a crosslinking agent (Japanese Unexamined Patent Application No. 189103/1984 (Tokukaisho 59-189103)), (c) a method using a glyoxal as a crosslinking agent (Japanese Unexamined Patent Application No. 117393/1977 (Tokukaisho 52-117393)), (d) a method using a polyvalent metal as a crosslinking agent (Japanese Unexamined Patent Application No. 136588/1976 (Tokukaisho 51-136588), Japanese Unexamined Patent Application No. 257235/1986 (Tokukaisho 61-257235) and Japanese Unexamined Patent Application No. 7745/1987 (Tokukaisho 62-7745)), (e) a method using a silane coupling agent as a crosslinking agent (Japanese Unexamined Patent Application No. 211305/1986 (Tokukaisho 61-211305), Japanese Unexamined Patent Application No. 252212/1986 (Tokukaisho 61-252212) and Japanese Unexamined Patent Application No. 264006/1986 (Tokukaisho 61-264006)), (f) a method using an epoxy compound and a hydroxy compound as crosslinking agents (Japanese Unexamined Patent Application No. 132103/1990 (Tokukaihei 2-132103)), and (g) a method using an alkylene carbonate as a crosslinking agent (DE-4020780, and U.S. Pat. No. 5,409,771).
Also, as a technique of uniformly carrying out surface crosslinkage in which a crosslinking agent is uniformly spread over the surface of the water-absorbent resin during a crosslinkage reaction, for example, the following methods, applied when adding the crosslinking agent, are known. (h) A method using an inorganic inactive powder (Japanese Unexamined Patent Application No. 163956/1985 (Tokukaisho 60-163956) and Japanese Unexamined Patent Application No. 255814/1985 (Tokukaisho 60-255814)), (i) a method using a dihydric alcohol (Japanese Unexamined Patent Application No. 292004/1989 (Tokukaihei 1-292004)), (j) a method using an ether compound (Japanese Unexamined Patent Application No. 153903/1990 (Tokukaihei 2-153903)), and (k) a method using alkylene oxide additive of a monchydric alcohol, an organic acid salt, lactam, etc. (Japanese Unexamined Patent Application No. 200046/1994 (Tokukaihei 6-200046), and EP-0555692).
With the described surface processing methods, some improvements in the balance of the various water-absorbent properties of the water-absorbent resin are attained, yet further improvements are needed to reach a desirable level. That is to say, in light of recent trend of an absorbent of thin sanitary articles containing a large amount of water-absorbent resin, considering the properties required for such water-absorbent resin, the water-absorbent resin made by the described methods has not reached a satisfactory level. Therefore, a further improvement in the quality of the water-absorbent resin is demanded.
The properties required for water-absorbent resin contained in high concentration in the absorbent are (1) high absorbency under no applied pressure and (2) water-absorbing properties, such as absorbency and liquid diffusivity under heavy load and high pressure, which are superior than that of conventional water-absorbent resin. Also, in the case where the absorbent containing in high concentration the water-absorbent resin is used for an extended period of time, a water soluble component (mainly water soluble polymer component) of the water-absorbent resin is gradually removed. This may result in lowering of diffusivity of aqueous liquid such as body fluids, and an increase in the amount of backlash of the aqueous liquid. For this reason, it is demanded to further reduce the amount of the water soluble component in the water-absorbent resin.
Incidentally, as a common manufacturing method of the water-absorbent resin, the following methods are known. (I) A method in which a monomer of acrylic acid or other compounds as a main component, which have been neutralized, are polymerized, and (II) a method (so-called a post-neutralization polymerization method) in which after polymerizing a monomer of acrylic acid or other compounds as a main component, which have not been neutralized or have been neutralized at a relatively low neutralization ratio within a predetermined range, resulting hydrogel polymer is neutralized as required. For example, the method (I) is adopted as the manufacturing method of water-absorbent resin disclosed in Japanese Unexamined Patent Application No. 126310/1989 (Tokukaihei 1-126310), and U.S. Pat. No. 4,985,518. However, in method (I), because a relatively large amount of water soluble component is present, when the water-absorbent agent is adopted in sanitary articles, there is a chance that the diffusivity of aqueous liquid such as body fluids is lowered and the amount of backlash of the aqueous liquid is increased. For this reason, the method (II) is considered to be more desirable as a manufacturing method of water-absorbent resin, and is suitably adopted as a method of reducing the water soluble component while maintaining high absorbency of the water-absorbent resin under no applied pressure.
Specifically, the following methods are known as an example of method (II). (1) A method in which after polymerizing the acrylic acid in the presence of a vinyl crosslinking agent, the acrylic acid thus polymerized is neutralized with alkali metals, and resulting water containing neutralized gel is further crosslinked by divalent metal ions (U.S. Pat. No. 4,295,987), (m) a method in which an alkali metal containing compound is added to a hydrogel polymer which has been prepared by polymerizing monomers containing a free ac
Fujimaru Hirotama
Fujino Shin-ichi
Harada Nobuyuki
Ishizaki Kunihiko
Kimura Kazuki
Nippon Shokubai Co. , Ltd.
Nixon & Vanderhye
Wu David W.
Zalukaeva Tanya
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