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
1999-03-23
2002-04-16
Niland, Patrick D. (Department: 1714)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C525S050000, C525S055000, C525S242000, C525S243000, C525S244000, C525S298000, C525S330600
Reexamination Certificate
active
06372852
ABSTRACT:
BACKGROUND OF THE INVENTION
A. Technical Field
The present invention relates to a production process for a water-absorbing agent which is favorably used for sanitary materials such as paper-made diapers (disposable diapers), sanitary napkins and so-called incontinent pads.
B. Background Art
In recent years, water-absorbent resins that are hydrophilic resins are widely used as constituent materials of sanitary materials, such as disposable diapers, sanitary napkins, and so-called incontinent pads, for the purpose of causing the water-absorbent resins to absorb body fluids.
Known examples of the above water-absorbent resins are as follows: crosslinked products of partially neutralized polyacrylic acids; hydrolyzed products of starch-acrylic acid graft polymers; saponified products of vinyl acetate-acrylic acid ester copolymers; hydrolyzed products of acrylonitrile-or acrylamide copolymers, and their crosslinked products; and crosslinked polymers of cationic monomers.
Examples of the properties which the above water-absorbent resins should have are as follows: upon contact with aqueous liquids such as body fluids, excellent water absorption amount or speed, the liquid permeability, the gel strength of the swollen gel, the suction power to suck up water from a base material containing aqueous liquids. However, relations between these properties do not necessarily display positive correlations. For example, as the absorption capacity under no load increases, the absorption properties under a load deteriorate.
As to a method for improving such water-absorption properties of the water-absorbent resin in good balance, there is a known art in which the neighborhood of the surface of the water-absorbent resin is crosslinked, and various methods have been proposed as such.
For example, there are known methods in which the following materials are used as the crosslinking agents: polyhydric alcohols (JP-A-58-180233 and JP-A-61-016903); polyglycidyl compounds, polyaziridine compounds, polyamine compounds, or polyisocyanate compounds (JP-A-59-189103); glyoxal (JP-A-52-117393); polyvalent metals (JP-A-51-136588, JP-A-61-257235 and JP-A-62-007745); silane coupling agents (JP-A-61-211305, JP-A-61-252212, and JP-A-61-264006); alkylene carbonates (DE 4020780). In addition, there are also known methods in which the following materials are allowed to be present as third substances for the purpose of improving the dispersibility of the crosslinking agent when the crosslinking agent is mixed or when the crosslinking reaction is carried out: inert inorganic powders (JP-A-60-163956 and JP-A-60-255814); dihydric alcohols (JP-A-01-292004); water along with ether compounds (JP-A-02-153903); alkylene oxide adducts of monohydric alcohols, organic acid salts, lactams, and so on (EP 555692); and phosphoric acid (Publication of Internal Patent Application as entered the national phase in Japan (Kohyo) No. 08-508517).
However, there are cases where conventional arts of crosslinking the neighborhood of the surface of the water-absorbent resin cannot sufficiently meet the abilities as recently demanded to the water-absorbent resin to a high degree. For example, in recent years, the sanitary materials have a tendency to be designed to be thinned, so the water-absorbent resin concentration in the absorbent structure becomes high. Thus, excellent absorption capacities under no load and under a heavy load with good balance are exemplified as the properties that are desired to the water-absorbent resin in the absorbent structure which contains the water-absorbent resin in a large quantity, namely, high concentration, but the above conventional methods are still insufficient to further raise each of the above values.
Furthermore, there is a problem of the safety of the crosslinking agent as used. Generally, when the crosslinking agent has high reactive groups such as epoxy group, the crosslinking agent itself has property to stimulate skin. Thus, not only considering problems on environment of working, but also considering the application to sanitary materials, it is necessary to strictly control factors such as the amount of the crosslinking agent remaining in the resin, and further, complicated operations in the process are necessary for also decreasing the amount of the residual crosslinking agent. Therefore, in fact, the art (for obtaining water-absorbent resins as preferably used for sanitary materials) which is satisfactory with regard to the performance, the safety, and the process is still insufficient.
SUMMARY OF THE INVENTION
A. Object of the Invention
The present invention was made considering the above problems on the prior arts, and it is an object of the present invention to provide a new production process for a water-absorbing agent which is excellent in the absorption capacities under no load and under a load with good balance and can display excellent absorption properties even if the weight percentage of the water-absorbent resin (resin concentration) is high when the water-absorbent resin is used for materials such as sanitary materials.
B. Disclosure of the Invention
The present inventors studied and studied about a production process of a water-absorbing agent with encouragement to themselves and great efforts to achieve the above objects. As a result, the inventors completed the present invention by finding that the aimed water-absorbing agent (which is excellent in the absorption capacities under no load and under a load with good balance and can display excellent absorption properties even if the resin concentration is high when the water-absorbent resin is used for materials such as sanitary materials) can be produced by adding a vinyl compound to a water-absorbent resin and heating the resultant mixture.
Therefore, a production process for a water-absorbing agent, according to the present invention, is characterized by comprising the steps of adding a vinyl compound or both the vinyl compound and a polyhydric alcohol to a water-absorbent resin and heating the resultant mixture.
These and other objects and the advantages of the present invention will be more fully apparent from the following detailed disclosure.
DETAILED DESCRIPTION OF THE INVENTION
The water-absorbent resin, which is used as the hydrophilic resin to produce the water-absorbing agent of the present invention, is a conventionally known resin that absorbs as large a quantity of water as 50~1,000 times the original in ion-exchange water to thereby form a hydrogel. Particularly, those which have a carboxyl or hydroxyl group are preferable, and those which have a carboxyl group are more preferable. The water-absorbent resin is typically obtainable by polymerizing and crosslinking hydrophilic monomers of which the main component is either one or both of acrylic acid and a salt (neutralized product) thereof. In addition, as to the above water-absorbent resin, those which have an uncrosslinked water-soluble content of 25 weight % or below, preferably 15 weight % or below, more preferably 10 weight % or below, are used.
Examples of the above salt of acrylic acid include: alkaline metal salts, ammonium salts, and amine salts of acrylic acid. It is preferable that the constituent units of the above water-absorbent resin comprise acrylic acid of 10~40 mol % and its salt of 90~60 mol % (wherein the total of both is 100 mol %). The monomers, as used to produce the water-absorbent resin by polymerizing hydrophilic monomers (of which the main component is either one or both of acrylic acid and a salt thereof), may, if necessary, comprise not only acrylic acid or a salt thereof, but also monomers other than acrylic acid.
The monomers other than acrylic acid are not especially limited, but specified examples of them include: anionic unsaturated monomers, such as methacrylic acid, maleic acid, vinylsulfonic acid, styrenesulfonic acid, 2-(meth)acrylamido-2-methylpropanesulfonic acid, 2-(meth)acryloylethanesulfonic acid, and 2-(meth)acryloylpropanesulfonic acid, and their salts; nonionic unsaturated monomers containing a hydrophilic group, such a
Harada Nobuyuki
Hitomi Kazuhisa
Kirimura Shigenori
Yamamoto Yuji
Niland Patrick D.
Nippon Shokubai Co. Ltd
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