METHOD FOR PRODUCING HYDROPHILIC POLYMER RESIN

Abstract: A hydrophilic polymer is denatured with a gaseous denaturant or a powdery denaturant substance made from a liquid denaturant substance. The liquid denaturant substance is made into powders by, for example, a method of mixing the liquid denaturant substance with a water-insoluble compound, a method of cooling the liquid denaturant substance to or below its melting point to turn the same into a solid state. Consequently, the hydrophilic polymer can be denatured uniformly. In case that the hydrophilic polymer is denatured with a gaseous denaturant, the hydrophilic polymer and denaturant react with each other efficiently in a safe manner regardless of the size or shape of the hydrophilic polymer. In case that the hydrophilic polymer is denatured with the powdery denaturant substance, not only the liquid denaturant substance which is substantially in the solid state can be mixed with the hydrophilic polymer, but also the denaturation timing can be readily controlled.

Description: TECHNICAL FIELD The present invention relates to a production method for a hydrophilic resin, and more particularly, to a production method for a hydrophilic resin having excellent physical properties, such as an absorbing property, by uniformly denaturing a hydrophilic polymer, for example, by applying a crosslinking treatment to an absorbent resin. TECHNICAL BACKGROUND Recently, an absorbent resin is used extensively in sanitary goods, such as paper diapers, sanitary napkins, and incontinence pads, to absorb body fluids. Besides the sanitary goods, the absorbent resin is also used extensively in water-retaining materials for soil to keep water in soil or drip absorbing materials to absorb drip from foods or the like. Examples of such absorbent resins include: a partially neutralized crosslinked polymer of polyacrylic acid, a hydrolyzed graft polymer of starch-acrylonitrile, a neutralized graft polymer of starch-acrylic acid, a saponified copolymer of vinyl acetate-acrylic ester, a hydrolyzed copolymer of acrylonitrile or acrylamide, or a crosslinked product thereof, and a crosslinked polymer of a cationic monomer. Notable properties of the absorbent resin include absorbency and absorbing rate when brought into contact with an aqueous liquid like a body fluid, liquid permeability, gel strength of swelled gel, and absorbing power of absorbing water from a base material containing an aqueous liquid, etc. However, each property is not necessarily correlated positively to one another. For example, the higher the absorbency, the lower the absorbing rate, liquid permeability, and gel strength. To solve the above problem, a crosslinking treatment (secondary crosslinking treatment) using a crosslinking agent (denaturant) is conventionally applied to the absorbent resin to well balance (improve) these properties and enhance the absorbing property. A secondary crosslinking structure is introduced into the absorbent resin in the following manner: a swelling liquid in which the absorbent resin is swelled with a solvent, or a dispersing liquid in which the absorbent resin is dispersed in a dispersing medium is prepared, and a crosslinking agent or a solution thereof is added to the above prepared liquid and mixed with each other, so that the reaction of the absorbent resin and crosslinking agent takes place in a so-called solid-liquid system. The solvent or dispersing medium referred herein is, for example, a hydrophilic compound, such as water and alcohol. Examples of the above method of introducing the secondary crosslinking structure into the absorbency resin using a crosslinking agent include: a method of using polyhydric alcohol (Japanese Laid-open Patent Application No. 180233/1983 ( Tokukaisho No. 58-180233 ) and Japanese Laid-open Patent Application No. 16903/1986 ( Tokukaisho No. 61-16903 )), a method of using a polyglycidyl compound, a polyaziridine compound, a polyamine, or polyisocyanate (Japanese Laid-open Patent Application No. 189103/1984 ( Tokukaisho No. 59-189103 )), a method of using glyoxal (Japanese Laid-open Patent Application No. 117393/1977 ( Tokukaisho No. 52-117393 )), a method of using polyvalent metal compound (Japanese Laid-open Patent Application No. 136588/1976 ( Tokukaisho No. 51-136588 ), Japanese Laid-open Patent Application No. 257235/1986 ( Tokukaisho No. 61-257235 ), and Japanese Laid-open Patent Application No. 7745/1987 ( Tokukaisho No. 62-7745 )), a method of using a silane coupling agent (Japanese Laid-open Patent Application No. 211305/1986 ( Tokukaisho No. 61-211305 ), Japanese Laid-open Patent Application No. 252212/1986 ( Tokukaisho No. 61-252212 ), and Japanese Laid-open Patent Application No. 264006/1986 ( Tokukaisho No. 61-264006 )), a method of using an epoxy compound and a hydroxy compound (Japanese Laid-open Patent Application No. 132103/1990 ( Tokukaihei No. 2-132103 )), a method of using alkylene carbonate (German Patent No. 4,020,780), etc. Also, a variety of methods are proposed to distribute the crosslinking agent over the surface of the absorbent resin more evenly to crosslink near the surface of the absorbent resin uniformly. Examples of such methods are: a method of adding the crosslinking agent to the absorbent resin in the presence of inactive inorganic powders (Japanese Laid-open Patent Application No. 163956/1985 ( Tokukaisho No. 60-163956 ) and (Japanese Laid-open Patent Application No. 255814/1985 (Tokukaisho No. 60-255814 )), a method of adding the crosslinking agent to the absorbent resin in the presence of dihydric alcohol (Japanese Laid-open Patent Application No. 292004/1989 ( Tokukaihei No. 1-292004 )), a method of adding the crosslinking agent to the absorbent resin in the presence of an ether compound (Japanese Laid-open Patent Application No. 153903/1990 ( Tokukaihei No. 2-153903 )), a method of adding the crosslinking agent to the absorbent resin in the presence of an alkylene oxide adduct of monohydric alcohol, an organic acid salt, or lactam (European Patent No. 555,692). However, the above conventional methods have following problems. That is, because the conventional methods use the solvent or dispersing medium to react the absorbent resin with the crosslinking agent, these methods must include a post-treatment process, such as a removing step for removing the solvent or dispersing medium and a drying step. This is the reason why the entire procedure of introducing the secondary crosslinking structure into the absorbent resin is complicated. In addition, the crosslinking agent and the solvent or dispersing medium may reside in the post-reaction absorbent resin, thereby possibly making the resulting absorbent resin unsafe. Further, it is quite difficult to remove or collect an excessive crosslinking agent when the reaction ends. Moreover, when microscopic powders of the absorbent resin is used, for example, a mixture of the absorbent resin and the solvent or dispersing medium produces agglomerate, thereby making it impossible to swell or disperse the absorbent resin uniformly in a satisfactory manner. Thus, the secondary crosslinking structure can not be introduced into all kinds of absorbent resin uniformly because of their shape or size. In addition, the swelling liquid or dispersing liquid must be stirred relatively hard to let the absorbent resin swell or disperse the same uniformly in a satisfactory manner. Thus, the absorbent resin is susceptible to a physical damage, and for example, the surface of the absorbent resin is often damaged when the reaction ends. Note that the agglomerate referred herein means masses of agglomerated particles. Also, to carry out the crosslinking treatment at a relatively high degree, for example, to attain high crosslinking density and depth, a relatively large amount of solvent or dispersing medium must be used, which makes the reaction of the absorbent resin and crosslinking agent inefficient. In addition, when a relatively large quantity of the solvent or dispersing medium is used, not only the agglomerate is readily produced, but also the energy cost of the post-treatment process increases undesirably. Further, the conventional methods can change (improve) the balance the notable properties of the absorbent resin to a certain extent, but not beyond the extent of practical use. For example, the recent sanitary goods tend to use more amount of absorbent resin while reducing its thickness. However, if the desired properties for the absorbent resin used in the materials of the sanitary goods, that is, an absorbent material, the conventional methods can not balance the properties in a practical manner. Therefore, there has been an increasing need for a method of balancing the properties in a practical manner, in other words, a method of further improving the quality of the absorbent resin. More specifically, in case of an absorbent material containing a large amount of absorbent resin, namely, having a high concentration of absorbent resin, the desired properties are the absorbing property under pressure, such as absorbency and water retaining ability under pressure, bonding and shape-keeping properties when water is absorbed into spaces among the particles of the absorbent resin which are disclosed in Japanese Laid-open Patent Application No. 96159/1993 ( Tokukaihei No. 5-96159 ), and bonding and shape-keeping properties of an absorbent material made of the absorbent resin, cellulose fiber, etc. when the absorbent material has absorbed water. However, the conventional methods are not effective enough to further improve the absorbing property under pressure. In addition, when the method disclosed in above Japanese Laid-open Patent Application No. 96159/1993 ( Tokukaihei No. 5-96159 ) is adopted, although it becomes possible to control the inconveniences, for example, the absorbent resin is released from the absorbent material or migrates within the absorbent material while water is being absorbed, the absorbency under pressure can be hardly improved in some kinds of absorbent resin. Further, once the absorbent resin is produced, its bonding property deteriorates over time before the absorbent resin is actually used. Therefore, the conventional methods cause a number of problems specified as above in a reaction of the absorbent resin and crosslinking agent, namely, in a reaction of a hydrophilic polymer and a denaturant. DISCLOSURE OF THE INVENTION The present invention is devised to solve the above problems, and therefore, has an object to provide a new production method for a hydrophilic resin which does not cause the above specified problems. The present invention has a further object to provide a production method for a hydrophilic resin which has an excellent absorbing property under pressure, such as absorbency and water retaining ability under pressure, and can show excellent performance (absorbing property) even when used in the sanitary goods or the like having a high percent by weight of a hydrophilic polymer (high resin concentration). The present invention has still another object to provide a production method for a hydrophilic resin which has an excellent absorbing property under pressure, and when used in an absorbent material, a hydrophilic polymer contained therein is hardly released from the absorbent material while water is being absorbed; moreover, the hydrophilic resin maintains its bonding property over time after the production until being put into practical use. According to the invention a production method of the hydrophilic resin of the present invention is characterized in that the hydrophilic polymer is mixed with a powdery denaturant substance made from a liquid denaturant substance, for example, by mixing the liquid denaturant substance with a water-insoluble compound, or by cooling the liquid denaturant substance to or below the melting point. In the conventional methods, to denature the hydrophilic polymer, more specifically, to apply a surface crosslinking treatment to an absorbent resin using a crosslinking agent, for example, the absorbent resin is mixed with a the crosslinking agent alone or an aqueous solution of the same. Generally, the crosslinking agent is in a liquid state at room temperature and has high affinity with the absorbent resin. For this reason, the absorbent resin starts to absorb the crosslinking agent, or the reaction of the absorbent resin and crosslinking agent takes place as soon as the absorbent resin and crosslinking agent are mixed with each other. Thus, the crosslinking agent can not be distributed evenly on the surface of the absorbent resin, and hence, the surface crosslinking treatment can not be applied uniformly to the absorbent resin. In contrast, according to the production method of the hydrophilic resin of the present invention, a liquid denaturant substance is made into powders first, and thence mixed with the hydrophilic polymer. Thus, the hydrophilic polymer can be mixed with the liquid denaturant substance which is substantially turned into a solid state. Consequently, compared with a case where the hydrophilic polymer is mixed with the liquid denaturant substance directly, the hydrophilic polymer can be mixed homogeneously with the liquid denaturant substance which has been made into powders in effect. Also, according to the producing method of the hydrophilic resin of the present invention, the liquid denaturant substance is made into powders. Thus, when the hydrophilic polymer and powdery denaturant substance are mixed with each other, the hydrophilic polymer does not start to absorb the liquid denaturant substance contained in the powdery denaturant substance, nor the reaction of the hydrophilic polymer and the denaturant contained in the liquid denaturant substance does not take place immediately. The hydrophilic polymer is denatured when a mixture of the hydrophilic polymer and powdery denaturant substance has absorbed an aqueous medium or when the mixture is heated. Therefore, according to the producing method of the hydrophilic resin of the present invention, since the hydrophilic polymer and powdery denaturant substance can be mixed with each other homogeneously, not only the hydrophilic polymer can be denatured uniformly, but also the denaturation timing can be controlled. Thus, according to the above method, not only the hydrophilic polymer can be denatured uniformly, but also the hydrophilic resin with a good balance of the properties can be obtained. In the following, the present invention will be described in detail. In the present invention, a powdery denaturant substance made from a liquid denaturant substance is used to denature the hydrophilic polymer uniformly. Note that, in the present invention, "denaturation" means to change the structure or physical properties of the hydrophilic polymer by bringing the hydrophilic polymer into contact with the denaturant or reacting the hydrophilic polymer with the denaturant. To be more specific, "denaturation" means, for example, a crosslinking treatment (secondary crosslinking treatment). The reaction of the hydrophilic polymer and denaturant includes a crosslinking reaction, an addition reaction, a substitute reaction, and an esterification reaction. The hydrophilic polymer subject to denaturation (hereinafter, referred to as treatment) by the denaturing method and producing method of the present invention is not especially limited, and a solid or gel of any hydrophilic polymer having a reaction group (functional group) is applicable. A carboxyl group is particularly preferred as the above reaction group. Also, the hydrophilic polymer having the crosslinking structure inside is preferred. More specifically, a good example of the hydrophilic polymer is partially neutralized poly(meta)acrylic acid having the crosslinking structure inside, such as an absorbent resin. The hydrophilic polymer can be obtained by, for example, polymerizing a monomeric composition mainly composed of acrylic acid and a salt thereof (hereinafter, referred to as acrylic acid/salt). Examples of the. hydrophilic polymer are known absorbent resins including: a partially neutralized crosslinked polymer of polyacrylic acid (U.S. Patent Nos. 4,625,001, 4,654,039, 5,250,640, and 5,275,773, and European Patent No. 456,136), a hydrolyzed graft polymer of starch-acrylonitrile, a neutralized or partially neutralized crosslinked graft polymer of starch-acrylic acid (U.S. Patent No. 4,076,663), a saponified copolymer of vinyl acetate-acrylic acid (U.S. Patent No. 4,124,748), a saponified copolymer of vinyl acetate-acrylic ester, a hydrolyzed (co)polymer of acrylonitrile (U.S. Patent No. 3,935,099) or a crosslinked product thereof, a hydrolyzed copolymer of acrylamide (U.S. Patent No. 3,959,569) or a crosslinked product thereof, a crosslinked carboxymethyl cellulose, a crosslinked cationic monomer, a crosslinked copolymer of isobutylene-maleic anhydride, a crosslinked copolymer of 2-acrylamide-2-methylpropane sulfonic acid and acrylic acid, a crosslinked polyethylene oxide, and a crosslinked copolymer of methoxypolyethylene glycol and acrylic acid. Of all the above example hydrophilic polymers, crosslinked polyacrylic acid/salt is preferred. It is preferable that 50mol% - 90mol% of acidic groups in the crosslinked polyacrylic acid/salt are neutralized. Also, alkali metal salt, alkali earth metal salt, ammonium salt, hydroxyammonium salt, amine salt, and alkylamine salt are preferred as the salt. Besides acrylic acid/salt, the monomeric composition may optionally contain a hydrophilic monomer copolymerizable with acrylic acid/salt. Examples of the hydrophilic monomer include: anionic unsaturated monomers, such as methacrylic acid, crotonic acid, maleic acid, maleic anhydride, fumaric acid, itaconic acid, cinnamic acid, sorbic acid, β-acryloyloxypropionic acid, 2-(meta)acryloylethane sulfonic acid, 2-(meta)acryloylpropane sulfonic acid, 2-(meta)acrylamide-2-methylpropane sulfonic acid, vinyl sulfonic acid, styrene sulfonic acid, allyl sulfonic acid, vinyl phosponic acid, and 2-(meta)acryloyloxyethyl phosphoric acid, or salts of these acids (for example, alkali metal salt, alkali earth metal salt, ammonium salt, and alkylamine salt); nonionic unsaturated monomers, such as acrylamide, methacrylamide, N-ethyl(meta)acrylamide, N-n-propyl (meta) acrylamide, N-isopropyl(meta)acrylamide, N,N-dimethyl(meta)acrylamide, 2-hydroxyethyl(meta)acrylate, 2-hydroxypropyl(meta)acrylate, methoxypolyethylene glycol (meta)acrylate, polyethylene glycol mono(meta)acrylate, vinylpyridine, N-vinylpyrrolidone, N-acryloylpiperidine, and N-acryloylpyrrolidine; cationic unsaturated monomers, such as N,N-dimethylaminoethyl(meta)acrylate, N,N-diethylaminoethyl(meta)acrylate, N,N-dimethylaminopropyl(meta)acrylate, and N,N-dimethylaminopropyl(meta)acrylamide. or quaternary compounds thereof (for example, a reaction product with alkyl halide, or a reaction product with dialkyl sulfric acid). One member or a mixture of more than one member selected from these example hydrophilic monomers can be used effectively. In case that the monomeric composition is mainly composed of acrylic acid/salt, an amount of the phydrophlic monomers other than acrylic acid/salt is preferably below 50mol%, more preferably 30mol% or below, and most preferably 10mol% or below based on the combined weight of the monomeric compositions. It is preferable that the hydrophilic polymer produced by (co)polymerizing the above monomeric composition(s) has(have) the crosslinking structure (primary crosslinking structure) inside. The above crosslinking structure can be readily introduced into the hydrophilic polymer using an internal crosslinking agent when the monomeric composition is copolymerized, so that the monomeric composition copolymerizes or reacts with the internal crosslinking agent. The hydrophilic polymer may be of a self-crosslinking type that does not need the internal crosslinking agent. Examples of the internal crosslinking agent include: a compound having a plurality of vinyl groups (polymeric unsaturated groups) within a molecule; a compound having at least one vinyl group and at least one functional group reactive with a reaction group contained in the monomeric composition within a molecule; and a compound having a plurality of functional groups reactive with the above reaction group within a molecule. One member or a mixture of more than one member selected from these example internal crosslinking agents can be used effectively. Examples of the compound having a plurality of vinyl groups within a molecule include N,N'-methylenebis(meta)acrylamide, (poly)ethylene glycol di(meta)acrylate, (poly)propylene glycol di(meta)acrylate, trimethylolpropane tri(meta)acrylate, trimethylolpropane di(meta)acrylate, glycerin tri(meta)acrylate, glycerin acrylate methacrylate, ethyleneoxide denatured trimethylolpropane tri(meta)acrylate, pentaerythritol tetra (meta) acrylate, dipentaerythritol hexa(meta)acrylate, N,N-diallylacrylamide, triallyl cyanurate, triallyl isocyanurate, triallyl phosphate, triallyl amine, diallyloxy acetic acid, N-methyl-N-vinylacrylamide, bis(N-vinyl carboxylic amide), poly (meta) allyloxyalkanes, such as tetraallyloxy ethane. The compound having at least one vinyl group and at least one functional group reactive with the reaction group within a molecule means an ethylenic unsaturated compound having at least one of a hydroxyl group, an epoxy group, or a cationic group, and the examples of which include: glycidyl(meta)acrylate, N-methylol acrylamide, and (meta)acrylic acid dimethylamino ethyl. The compound having a plurality of functional groups reactive with the reaction group within a molecule means, for example, a compound having at least two groups selected from a hydroxyl group, an epoxy group, a cationic group, or an isocyanate group. Examples of such a compound include: (poly)ethylene glycol diglycidyl ether, glycerol diglycidyl ether, ethylene glycol, polyethylene glycol, propylene glycol, glycerin, pentaerythritol, ethylene diamine, ethylene carbonate, polyethyleneimine, and aluminium sulfate. Of all these example internal crosslinking agents, the compound having a plurality of vinyl groups within a molecule is preferred, because using such a compound can further improve the properties of the resulting hydrophilic polymer, for example, the absorbing property in case that the hydrophilic polymer is an absorbent resin. Although an amount of the internal crosslinking agent with respect to an amount of the hydrophilic monomer varies with a combination of the hydrophilic monomer and internal crosslinking agent, a preferable range is between 0.005mol% and 3mol%, and a more preferable range is between 0.01mol% and 1.5mol%. If less than 0.005mol% or more than 3mol% of the internal crosslinking agent is used when the hydrophilic polymer is the absorbent resin, for example, the resulting absorbent resin may not have the desired absorbing property. When the monomeric composition is (co)polymerized, hydrophilic high polymers, such as starch or derivative of starch, cellulose or derivative of cellulose, polyvinylalcohol, polyacrylic acid/salt and a crosslinked product of the same, or chain transfer agents, such as hypophosphorous acid and hypophosphite, may be added to the reactant system. The polymerization method for (co)polymerizing the monomeric composition is not especially limited, and known methods, such as the aqueous solution polymerization, reverse phase suspension polymerization, bulk polymerization, and precipitation polymerization, can be adopted. Of these polymerization methods, the methods in which an aqueous solution of the monomeric composition is polymerized, that is, the aqueous solution polymerization and reverse phase suspension polymerization are preferable because the polymerization reaction can be readily controlled and the properties of the resulting hydrophilic polymer can be further improved. The reaction conditions, such as reaction temperature and time, are not especially limited, and can be set arbitrary depending on the kinds of the hydrophilic monomer. The aqueous solution polymerization and reverse phase suspension polymerization can be carried out by known polymerization methods disclosed in, for example, U.S. Patent Nos. 4,625,001, 4,769,427, 4,873,299, 4,093,776, 4,367,323, 4,446,261, 4,683,274, 4,690,996, 4,721,647, 4,738,867, and 4,748,076. When the monomeric composition is (co)polymerized, a radical polymerization initiator or activation energy rays, such as UV rays and electron beams, can be used. Examples of the radical polymerization initiator include peroxides, such as potassium persulfate, sodium persulfate, ammonium persulfate, t-butylhydro peroxide, hydrogens peroxide, 2,2'-azobis(2-amidinopropane)dihydrochloride, 2,2'-azobisisobutylonitrile, benzoil peroxide, cumene hydroperoxide, and di-t-butylperoxide. One member or a mixture of more than one member selected from these radical polymerization initiators can be used effectively. An oxidizing radical polymerization initiator may is used as a redox initiator when combined with a reducing agent, such as sulfites including sodium sulfite and sodium hydrogen sulfite, bisulfite, thiosulfate, formamidine sulfenic acid, ferrous sulfate, and L-ascorbic acid. An amount of the polymerization initiator with respect to an amount of the hydrophilic monomer varies with a combination of the hydrophilic monomer and polymerization initiator, but a preferable range is between 0.001mol% and 2mol%, and a more preferable range is between 0.01mol% and 0.5mol%. The shape of the hydrophilic polymer obtained as the result of the above (co)polymerization is not especially limited, and the hydrophilic polymer can be made into spherical, substantially spherical, granular, leaflet, or flat particles; a fiber; a bar, a sheet, a film, a plate, a block, or have an undefined shape (pulverized undefinedly). The hydrophilic polymer may be a porous product or a sponge having sequential bubbles therein. The size of the hydrophilic polymer is not especially limited either, and the hydrophilic polymer may be microscopic powders. In other words, the hydrophilic polymer does not have to be of specific shape and size when treated by the denaturant, and can be of any suitable shape and size for the intended use. In short, the denaturing method of the present invention is applicable to any hydrophilic polymer regardless of its shape and size. The particle size of the hydrophilic polymer may be adjusted through sieving or the like when necessary. In case that the hydrophilic polymer is the absorbent resin, a preferable particle size is in a range between 200µm and 600µm. In this case, it is further preferable that the particles having a particle size of smaller than 150µm is 10wt% or less, and more preferably 5wt% or less, based on the weight of the hydrophilic polymer. When an average particle size of the hydrophilic polymer is outside of the above range, it becomes difficult to obtain the hydrophilic polymer with an excellent absorbing property. The producing method of the hydrophilic resin of the present invention is the method of mixing the hydrophilic polymer with a powdery denaturant substance made from a liquid denaturant substance. In the present invention, the liquid denaturant substance means a denaturant substance that remains in the liquid state when being added to the hydrophilic polymer. The liquid denaturant substance can be a pure liquid of the denaturant or a solution or dispersing liquid prepared by dissolving or dispersing the denaturant into an adequate solvent. In other words, the liquid denaturant substance is not especially limited as long as it contains the denaturant and remains in the liquid state when being added to the hydrophilic polymer. The denaturant is not especially limited either, and can be any compound reactive with a reaction group contained in the hydrophilic polymer. In this case, the denaturant is a compound having a number of functional groups reactive with a reaction group contained in the hydrophilic polymer within a molecule (that is, multifunctional compound), or a compound having a single functional group reactive with a reaction group contained in the hydrophilic polymer within a molecule (unifunctional compound). An example of the compound having a number of functional groups reactive with a reaction group contained in the hydrophilic polymer within a molecule and serving as the denaturant is a crosslinking agent (surface crosslinking agent). In case that the reaction group contained in the hydrophilic polymer is a carboxyl group, examples of preferred crosslinking agent include, but are not limited to the known surface crosslinking agents as follow: polyhydric alcohols, such as ethylene glycol, propylene glycol, diethylene glycol, dipropylene glycol, triethylene glycol, tripropylene glycol, tetraethylene glycol, tetrapropylene glycol, polyethylene glycol, polypropylene glycol, 1,3-propanediol, 2,2,4-trimethyl-1,3-pentanediol, glycerin, polyglycerin, 2-butene-1,4-diol, 1,3-butanediol, 1,4-butanediol, 1,5-pentanediol, 2,4-pentanediol, 1,6-hexanediol, 2,5-hexanediol, 1,2-cyclohexane dimethanol, 1,2-cyclohexanol, trimethylolpropane, diethanolamine, triethanolamine, polyoxypropylene, a block copolymer of oxyethylene-oxypropylene, pentaerythritol, and sorbitol; polyepoxides, such as ethylene glycol diglycidyl ether, polyethylene glycol diglycidyl ether, glycerol polyglycidyl ether, diglycerol polyglycidyl ether, polyglycerol polyglycidyl ether, propylene glycol diglycidyl ether, polypropylene glycol diglycidyl ether, and glycidol; polyamines, such as ethylene diamine, diethylene triamine, triethylene tetramine, tetraethylene pentamine, pentaethylene hexamine, polyallylamine, and polyethyleneimine; alkylene carbonates, such as 1,3-dioxolane-2-one, 4-methyl-1,3-dioxolane-2-one, 4,5-dimethyl-1,3-dioxolane-2-one, 4,4-dimethyl-1,3-dioxolane-2-one, 4-ethyl-1,3-dioxolane-2-one, 4-hydroxymethyl-1,3-dioxolane-2-one, 1,3-dioxane-2-one, 4-methyl-1,3-dioxane-2-one, and 4,6-dimethyl-1,3-dioxane-2-one; polyisocyanates, such as 2,4-tolylene diisocyanate and hexamethylene diisocyanate; polyoxazoline compounds, such as 1,2-ethylenebis oxazoline; haloepoxides, such as epichlorohydrin, epibromohydrin, α-methylepichlorohydrin; polyvalent metal compounds, namely, a hydroxide and a chloride of polyvalent metals, such as zinc, calcium, magnesium, aluminum, iron, and zirconium. One member or a mixture of more than one member selected from these crosslinking agents can be used effectively. Of all these examples, a crosslinking agent containing at least one kind of compound selected from a group consisting of polyhydric alcohol, polyepoxide, polyamine, and alkylene carbonate is preferred. Of all the polyamines, a polyamine (hereinafter, referred to as high molecular weight polyamine) having a weight average molecular weight (Mw) of 2,000 or greater is preferable, and a high molecular weight polyamine having a weight average molecular weight ranging from 10,000 to 10,000,000 is particularly preferable. If a high molecular weight polyamine having a weight average molecular weight less than 2,000 is used, the resulting hydrophilic resin may not have satisfactory bonding property or shape-keeping property when water is being absorbed into spaces among particles of the hydrophilic polymer, or an absorbent material made of the hydrophilic resin and cellulose fiber (for example, paper and comminuted pulp) may not have satisfactory bonding property or shape-keeping property when water is being absorbed. In case that the crosslinking agent includes the high molecular weight polyamine, the hydrophilic polymer and high molecular weight polyamine contact and start to react with each other when the hydrophilic resin, namely, the hydrophilic polymer contained in the hydrophilic resin, has absorbed an aqueous liquid, whereupon the bonding force among the particles of the hydrophilic polymer or shape-keeping ability of the absorbent product is developed. Therefore, according to the present invention, the reaction timing of the hydrophilic polymer and high molecular weight polyamine can be controlled, and the deterioration of the bonding property over time can be prevented more effectively. In case that the high molecular weight polyamine is used as the crosslinking agent, the surface crosslinking treatment may be applied to the surface portion of the hydrophilic polymer in advance by any known surface crosslinking method or the method of the present invention. Examples of the high molecular weight polyamine include: (1) a monopolymer of monoallylamine derivative and a monopolymer of a diallylamine derivative; (2) a copolymer of more than one kind of monoallylamine derivatives, a copolymer of more than one kind of diallylamine derivatives, a copolymer of a monoallylamine derivative and a diallylamine, derivative; (3) a copolymer of a monoallylamine derivative and/or diallylamine derivative, and a dialkyldiallylammonium salt; (4) a homopolymer of an unsaturated carboxylic acid derivative containing a tertiary amino group (hereinafter referred to as the unsaturated carboxylic acid derivative a); (5) a copolymer of more than one kind of the unsaturated carboxylic acid derivatives a; (6) a copolymer of the unsaturated carboxylic acid derivative a, and a protonic and/or alkylated tertiary amino group of the unsaturated carboxylic acid derivative a (hereinafter simply referred to as quaternary ammonium salt), and/or a dialkyldiallylammonium salt; (7) a ternary copolymer of the unsaturated carboxylic acid derivatives a, quaternary ammonium salt and/or a dialkyldiallylammonium salt, and a vinyl monomer copolymerizable with the above monomers; (8) a polymer produced by copolymerizing an unsaturated carboxylic acid and an unsaturated monomer copolymerizable with the unsaturated carboxylic acid first, and thence reacting alkyleneimine with a carboxyl group contained in the resulting copolymer; (9) polyalkyleneimine; (10) polyalkyleneimine·epihalohydrin resin; (11) polyalkylenepolyamine; (12) a polymer of (2-methacryloyloxyethyl) ethyleneimine, and a copolymer of (2-methacryloyloxyethyl)ethyleneimine and an unsaturated monomer copolymerizable with (2-methacryloyloxyethyl) ethyleneimine; (13) polyamidepolyamine; (14) polyamideamine·epihalohydrin resin; (15) denatured polyacrylamide as a result of Mannich reaction and denatured polymethacrylamide as a result of Mannich reaction; (16) polyvinylamine, and a copolymer of vinylamine and an unsaturated monomer copolymerizable with vinylamine; (17) a condensation polymer of dicyandiamide·diethylenetriamine. To be more specific, examples of the high molecular weight polyamine include: polyallylamine, polydiallylamine, poly(N-alkylallylamine), poly(alkyldiallylamine), a copolymer of monoallylamine-diallylamine, a copolymer of N-alkylallylamine-monoallylamine, a copolymer of monoallylamine-dialkyldiallylammonium salt, a copolymer of diallylamine-dialkyldiallylammonium salt, polydimethylaminoethyl acrylate, polydiethylaminoethyl acrylate, polydimethylaminoethyl acrylamide, straight-chain polyethyleneimine, branched-chain polyethyleneimine, polyethylenepolyamine, polypropylenepolyamine, polyamidepolyamine, polyetherpolyamine, polyvinylamine, polyamidepolyamine·epichlorohydrin resin, and polyamidine. The examples also include amino denatured products produced by reacting formaldehyde and diethylamine with polyacrylamide or polymethacrylamide. Also, the high molecular weight polyamine may be neutralized by an acidic compound either completely or partially. Examples of the compound having a single functional group reactive with a reaction group contained in the hydrophilic polymer within a molecule and serving as the denaturant include, but are not limited to: compounds having a hydroxyl group, such as pentanol, hexanol, heptanol, octanol, decanol, alkoxy polyethylene glycol, lactic acid, and ethyl lactate; compounds having an epoxy group, such as 2-ethylhexylglycidyl ether, phenylglycidyl ether, butylglycidyl ether, 2-methyloctylglycidyl ether, allylglycidyl ether, and glycidyl(meta)acrylate; compounds having an amino group, such as methylamine, ethylamine, diethylamine, triethylamine, n-propylamine, isopropylamine, diisopropylamine, 3-methoxy propylamine, 3-ethoxy propylamine, 3-(2-ethylhexyloxy)propylamine, 3-(dibutylamino)propylamine, n-butylamine, t-butylamine, sec-butylamine, diisobutylamine, 2-ethylhexylamine, and di-2-ethylhexylamine, tri-n-octylamine, and salts of these compounds; cationic surfactants having an amino group and salts of such surfactants. One member or a mixture of more than one member selected from these compounds can be used effectively. Further, a mixture of the compound having a single functional group within a molecule and the compound having a number of functional groups within a molecule may be used as the denaturant of the present invention. Although an amount of the denaturant with respect to an amount of the hydrophilic polymer varies with the kinds or combination of the hydrophilic polymer and denaturant or the use of the resulting hydrophilic resin, a preferable amount is in a range between 0.001 part by weight and 10 parts by weight with respect to 100 parts by weight of the hydrophilic polymer in solid, and a more preferable amount is in a range between 0.01 part by weight and 5 parts by weight. Limiting an amount of the denaturant within the above range makes it possible to obtain the hydrophilic resin serving as an absorbing agent with an excellent absorbing property under pressure and bonding and shape-keeping properties. If more than 10 parts by weight of the denaturant is used, a part of the denaturant is wasted uneconomically. Also, an excessive denaturant may prevent to realize a desired denaturing (improving) effect. More specifically, in case that the hydrophilic polymer is an absorbent resin and the denaturant is a crosslinking agent, if the crosslinking agent is used excessively, the crosslinking density becomes too high to maintain good absorbency of the resulting absorbing agent, namely, hydrophilic resin. On the other hand, in case of less than 0.001 part by weight of the denaturant is used, the desired denaturing effect can be hardly attained. The method of producing a powdery denaturant substance by turning the liquid denaturant substance into powders is not especially limited in the present invention. For example, a method of mixing the liquid denaturant substance with powders of a water-insoluble compound, or a method of cooling the liquid denaturant substance to or below the melting point, are applicable. The powders of the water-insoluble compound is not especially limited as long as it remains inactive in response to the reaction of the hydrophilic polymer and denaturant and does not affect the properties of the resulting hydrophilic resin. Examples of the water-insoluble compound include, but are not limited to: inorganic powders, such as silicon dioxide, titanium dioxide, aluminum oxide, magnesium oxide, zinc oxide, talc, calcium phosphate, barium phosphate, silicic acid, silicate, clay, diatomaceous earth, zeolite, bentonite, kaolin, hydrotalcite, perlite, isolite, activated clay, silica sand, quartzite, strontium ore, fluorite, and bauxite; organic powders, such as polyethylene, polypropylene, polyvinyl chloride, polystyrene, polyamide, melamine resin, polymethyl methacrylate, denatured starch, powders of cellulose, ethyl cellulose, sawdust, activated carbon, and tea-leaves; microscopic powders of absorbent resins. One member or a mixture of more than one member selected from these water-insoluble compounds can be used effectively. Of all these example water-insoluble compounds, microscopic inorganic powders are preferable, and microscopic powders of clay are more preferable. Also, a preferable particle size of the water-insoluble compound is 1,000µm or less, more preferably 500µm or less, and most preferably 100µm or less. Although a ratio of the liquid denaturant substance and water-insoluble compound in the powdery denaturant substance varies with the kinds and combination of the denaturant and water-insoluble compound or the use of the resulting hydrophilic resin, a ratio of the liquid denaturant substance (that is, denaturant or a solution or dispersing liquid of the same) with respect to 100 parts by weight of the water-insoluble compound is preferably in a range between 1 part by weight and 1,000 parts by weight, and more preferably in a range between 10 parts by weight and 500 parts by weight. When more than 1,000 parts by weight of the liquid denaturant substance is used, the resulting powdery denaturant substance readily turn into slurry, and when this happens, the hydrophilic polymer starts to absorb or starts to react with the denaturant contained in the powdery denaturant substance as soon as the hydrophilic polymer and powdery denaturant substance are mixed with each other. Consequently, it has become difficult to distribute the denaturant uniformly over the surface of the hydrophilic polymer, thereby making it impossible to attain desired denaturating effect. For example, in case that the hydrophilic polymer and denaturant are an absorbent resin and a crosslinking agent, respectively, an absorbing agent (hydrophilic resin) having an excellent absorbing property under pressure and bonding and shape-keeping properties can not be obtained. Likewise, when less than 1 part by weight of liquid denaturant substance is used, the desired denaturing effect can not be attained, either. The method of mixing the liquid denaturant substance and water-insoluble compound is not especially limited, and a method of simply mixing the denaturant and water-insoluble compound, or a method of mixing a solution or dispersing liquid of the denaturant and the water-insoluble compound, are applicable. In case that a solution or dispersing liquid of the denaturant and the water-insoluble compound are mixed, the solution or dispersing liquid in the denaturant substance is generally sprayed or dropped on the water-insoluble compound first, and thence the solution or dispersing liquid and water-insoluble compound are mixed with each other. Examples of a mixing apparatus used for mixing the liquid denaturant substance and water-insoluble compound include: a cylindrical mixer, a screw type mixer, a screw type extruder, a turbulizer, a nauter type mixer, a V-shaped mixer, a ribbon type mixer, a two-arm type kneader, a fluidized mixer, a flash type mixer, a rotary disk mixer, a roll mixer, and a rotary mixer. The mixing speed can be either high or low. In case that the liquid denaturant substance is a solution or dispersing liquid of the denaturant, water and/or hydrophilic organic solvent are removed when the mixing with the water-insoluble compound ends. As has been explained, the powdery denaturant substance of the present invention is obtained by mixing the liquid denaturant substance and water-insoluble compound. It is preferable that the powdery denaturant substance is in the form of particles. Generally, a particle size of the powdery denaturant substance is 1/2 or less, and preferably 1/5 or less of the average particle size of the hydrophilic polymer. When the powdery denaturant substance is produced, the kinds or particle size of the water-insoluble compound and mixing conditions of the water-insoluble compound and liquid denaturant substance are selected arbitrary depending on, for example, the intended use of the resulting hydrophilic resin. According to the above arrangement, it has become possible to obtain the powdery denaturant substance which not only denatures the hydrophilic polymer more uniformly, but also further facilitates the control of the denaturation timing. In addition, in case that the denaturant remains in the liquid state at room temperature, the powdery denaturant substance can be obtained by cooling the liquid denaturant substance to or below a melting point to turn the same into a solid. The method of mixing the hydrophilic polymer and powdery denaturant substance is not especially limited in the present invention. A mixing apparatus used for mixing the hydrophilic polymer and powdery denaturant substance can be any of the aforementioned example mixing apparatuses. The mixing speed can be either high or low. According to the present invention, the hydrophilic polymer and denaturant can be mixed homogeneously by the step of mixing the hydrophilic polymer and powdery denaturant substance. Consequently, it has become possible to readily obtain the hydrophilic resin with an excellent balance of properties. In addition, if the heat treatment (heating step) is carried out optionally subsequent to the mixing step to heat the resulting mixture, the hydrophilic polymer and denaturant can react with each other efficiently. Further, if a liquid or gas of an aqueous liquid is added to the resulting mixture before the optional heat treatment and after the mixing step, the hydrophilic polymer and denaturant can also react with each other efficiently. In other words, to denature the hydrophilic polymer with the powdery denaturant substance, the hydrophilic polymer and powdery denaturant substance are mixed with each other, and the hydrophilic polymer and the denaturant contained in the powdery denaturant substance are brought into contact with each other by an adequate means, such as adding an aqueous medium and/or heating. The aqueous medium may be in a liquid or gaseous state, for example, water or vapor. In case that a covalent binding is formed as a result of the reaction of the denaturant and hydrophilic polymer, and for example, when the denaturant is a polyhydric alcohol, polyepoxide, or alkylene carbonate, an absorbing agent having a more excellent absorbing property under pressure can be obtained as the hydrophilic resin by adding the aqueous liquid to the mixture of the hydrophilic polymer and powdery denaturant substance. In this case, an amount of water contained in the aqueous liquid varies with a chemical make-up or average particle size of the hydrophilic polymer, a chemical make-up of the powdery denaturant substance, or the intended use of the resulting hydrophilic resin, but a preferable amount with respect to 100 parts by weight of the hydrophilic polymer in solid is 10 parts by weight or less, and a more preferable amount is in a range between 1 part by weight and 5 parts by weight. The method of adding the aqueous liquid is not especially limited. Although a treatment temperature in the heating treatment varies with the kinds of the denaturant or the like, the treatment temperature is preferably 80°C or higher, more preferably in a range between 100°C and 230°C, and most preferably in a range between 160°C and 220°C. The surface of the hydrophilic polymer is denatured by the heat treatment. When the treatment temperature is below 80°C, it becomes difficult to denature the hydrophilic polymer uniformly, and therefore, if the resulting hydrophilic resin is used as an absorbing agent, the absorbing property under pressure of the absorbing agent is hardly improved. In addition, since the heat treatment takes a long time, the productivity of the hydrophilic resin is reduced. The heat treatment is carried out by a typical dryer or furnace. Examples of the dryer include: a channel mixing dryer, a rotary dryer, a disk dryer, a fluidized-bed dryer, a flash type dryer, and an infra-red dryer. The heat treatment can be carried out while the hydrophilic polymer and powdery denaturant are being mixed with each other. In short, the mixing step and heat treatment can be carried out in parallel. As has been explained, the producing method of the hydrophilic resin of the present invention is the method including the mixing step of mixing the hydrophilic polymer and the powdery denaturant substance made from a liquid denaturant substance, and optionally the heating step of heating the mixture obtained in the mixing step. The hydrophilic polymer and liquid denaturant substance are, for example, an absorbent resin containing an acidic group and a crosslinking agent, respectively. According to the above method, the liquid denaturant substance absorbed in the powdery denaturant substance resides on the surface or inside the water-insoluble compound. For this reason, the liquid denaturant substance can be mixed with the hydrophilic polymer while remaining substantially in the solid state. Thus, according the above method, the hydrophilic polymer and the liquid denaturant substance, in effect, the powdery denaturant substance, can be mixed with each other more homogeneously compared with the method of mixing the liquid denaturant substance directly with the hydrophilic polymer. The hydrophilic polymer and liquid denaturant substance reside separately in the mixture of the hydrophilic polymer and powdery denaturant substance. In other words, in case that the liquid denaturant substance is made into a powdery denaturant substance, the hydrophilic polymer neither starts to absorb the liquid denaturant substance contained in the powdery denaturant substance nor starts to react with the denaturant contained in the liquid denaturant substance as soon as the hydrophilic polymer and powdery denaturant substance are mixed with each other. The hydrophilic polymer and the denaturant contained in the liquid denaturant substance are brought into contact with each other when the hydrophilic resin has absorbed the aqueous liquid or is heated, thereby making it possible to denature the hydrophilic polymer uniformly. In case that the powdery denaturant substance are produced by cooling the liquid denaturant substance to or below the melting point, the liquid denaturant substance can be mixed with the hydrophilic polymer while remaining substantially in the solid state. Since the denaturant is contained in the powdery denaturant substance, the hydrophilic polymer and denaturant reside in the hydrophilic resin separately until it is practically used, thereby remaining inactive with each other. When the powdery denaturant substance are fused, the hydrophilic polymer and denaturant are brought into contact and react with each other, whereupon the hydrophilic polymer is denatured (for example, in case that a crosslinking agent is used as the liquid denaturant substance, the surface of the hydrophilic polymer is crosslinked). A method of fusing the powdery denaturant substance is not especially limited, and heat treatment or the like can be adopted. As has been explained, according to the above producing method, not only the hydrophilic polymer can be denaturated uniformly, but also the denaturation timing can be controlled. For example, according to the above producing method, when the liquid denaturant substance is polyhydric alcohol, polyepoxide, or alkylene carbonate, ― better properties, such as the absorbing property under pressure, can be developed by the denaturation. Particularly, when a crosslinking agent containing a polyamine or the like is used as the denaturant, the bonding ability among the particles of the hydrophilic polymer and the aforementioned shape-keeping ability develop when the hydrophilic resin has absorbed the aqueous liquid during the practical use. Further, in this case, the timing of interaction of the particles of the hydrophilic polymer triggered by the polyamine or the like can be controlled. Consequently, the deterioration of the properties, such as gel breaking strength, over time can be prevented more efficiently. Thus, according to the above arrangement, it has become possible to provide a hydrophilic resin which has an excellent absorbing property under pressure, such as absorbency or water retaining ability under pressure, and shows excellent performance (absorbing property) even when used in the sanitary goods or the like including a high percent by weight of the hydrophilic polymer (resin concentration). In the hydrophilic resin, a liquid introduction space, through which the aqueous liquid migrates to the inside of the hydrophilic polymer, is secured under pressure. Thus, even when 50wt% or more of the hydrophilic polymer is used, the liquid introduction space can be secured under pressure as well. Also, according to the above arrangement, it has become possible to provide a hydrophilic resin which has an excellent absorbing property under pressure, prevents the hydrophilic polymer contained therein from releasing from the absorbent material, and maintains its bonding property over time after the production until being put into actual use. The hydrophilic resin contains the hydrophilic polymer and liquid denaturant substance, and when the liquid denaturant substance is the powdery denaturant substance produced by making a crosslinking agent having polyamine or the like into powders, the above-described effect is more significant. When an absorbing agent is obtained as the hydrophilic resin of the present invention, the absorbing agent is used after combined with, for example, cellulose fibers, such as paper and comminuted pulp. The absorbing agent has an excellent dispersing ability for the aqueous liquid and does not get wet much. Moreover, since the absorbing agent has an excellent absorbing property, the absorbing agent can be extensively used for sanitary goods, such as paper diapers, sanitary napkins, assisting material for incontienet patients, such as incontinence pads, wound protectors, and wound healing materials, to absorb body fluids; drip absorbing materials to absorb drip from foods or the like, or freshness preserving materials; water-retaining materials for soil to keep water in soil; or water stopping materials. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic cross section of a measuring device for measuring an amount of absorbed water under pressure, which is one of the abilities of an absorbing agent, namely, a hydrophilic polymer denatured by the denaturing method of the present invention; Figure 2 is a schematic cross section of a measuring device for measuring absorbency under pressure, which is one of the abilities of an absorbing agent, namely, a hydrophilic polymer denatured by the denaturing method of the present invention. THE BEST MODE OF EMBODIMENT OF THE PRESENT INVENTION The present invention will be described in detail in the following by ways of Examples and Comparative Examples; however, the present invention is not limited to the following disclosure. Also, six kinds of absorbent resins serving as the hydrophilic polymer is produced in the following manner, and the abilities (physical properties) of the resulting hydrophilic polymers and denatured hydrophilic resins (which are collectively referred to as an absorbing agent in some occasions) are measured in the manner described below. Note that the "part(s)" and "%" respectively mean "part(s) by weight" and "percent by weight" unless otherwise specified. (1) Producing Method of Hydrophilic Polymer Six kinds of absorbent resins are produced as the hydrophilic polymer by the following producing methods (a) through (f), respectively. (a) Here, 5,000 parts of 30wt% sodium acrylate aqueous solution with a neutralization ratio of 70mol% is placed in a reactor applicable to a so-called nitrogen gas sealing and cooling/heating. More specifically, an aqueous solution made of 371 parts of acrylic acid and 1,129 parts of sodium acrylate serving as the hydrophilic monomer, and 3,500 parts of water is placed in the reactor. Then, 6.76 parts of polyethylene glycol diacrylate serving as the internal crosslinking agent is dissolved into the above aqueous solution, and a nitrogen gas is blown into the resulting aqueous solution (bubbling) for 30 minutes to eliminate oxygen dissolved in'the aqueous solution. Then, 1.8 part of potassium persulfate serving as the radical polymerization initiator and 0.09 part of L-ascorbic acid serving as the reducing agent are added to the aqueous solution with stirring. Then, the aqueous solution is allowed to polymerization at 30°C in a nitrogen gas flow. The temperature inside the reactant system rises to 70°C after 5 minutes or so since the polymerization took place. Then, the reactant solution is stirred for 2 hours while the reactant system is heated to keep the inside temperature at approximately 70°C to complete the polymerization reaction. Consequently, hydrous gel of crosslinked polymer made of sodium polyacrylate is obtained. The hydrous gel of crosslinked polymer is taken out and divided minutely with a meat chopper. The divided product is spread across a 60-mesh metal gauze and dried with a hot air at 150°C for 135 minutes using a hot air dryer. Then, the dried product is pulverized with a coffee mill, and sieved through a 20-mesh (the size of each mesh: 840µm) metal gauze. Consequently, particles of a crosslinked polymer having an average particle size of approximately 400µm, that is, an absorbent resin (hereinafter, referred to as absorbent resin (a)), is obtained as the hydrophilic polymer. The absorbent resin (a) is neither moist nor viscous, but dry and smooth. (b) Here, an aqueous solution is prepared by dissolving 1,000 parts of acrylic acid serving as the hydrophilic monomer and 5 parts of tetraallyloxyethane serving as the internal crosslinking agent into 3,500 parts of water and mixed with each other in a reactor applicable to a so-called nitrogen gas sealing and cooling/heating. Then, a nitrogen gas is blown into the resulting aqueous solution (bubbling) for 30 minutes to eliminate oxygen dissolved in the aqueous solution. Then, 6 parts of hydrogen peroxide and 1 part of 2,2'-azobis(2-amidinopropane)dihydrochloride both serving as the radical polymerization initiator, and 0.3 part of L-ascorbic acid are added to the aqueous solution with stirring. Then, the aqueous solution is allowed to polymerization at 25°C in a nitrogen gas flow. The reactant solution is stirred for approximately 2 hours while the temperature inside the reactant system is kept at a constant level to complete the polymerization reaction. Consequently, hydrous gel of crosslinked polymer made of polyacrylic acid is obtained. Then, after the resulting hydrous gel of crosslinked polymer is taken out and divided minutely, 868 parts of 48wt% sodium hydroxide aqueous solution is added to the divided product and kneaded homogeneously to neutralize approximately 75mol% of carboxyl groups contained in the resulting polymer. In other words, approximately 75mol% of carboxyl groups contained in the resulting polymer is made into a sodium salt. The partially neutralized hydrous gel of crosslinked polymer is taken out, and dried at 150°C with a drum dryer. Then, the dried product is pulverized with a coffee mill and sieved through a 20-mesh metal gauze. Consequently, particles of a crosslinked product having an average particle size of approximately 410µm, namely, an absorbent resin (hereinafter referred to as absorbent resin (b)), is obtained as the hydrophilic polymer. The absorbent resin (b) is neither moist nor viscous, but dry and smooth. (c) Here, 5,000 parts of 35wt% sodium acrylate aqueous solution with a neutralization ratio of 80mol% is placed in a reactor applicable to a so-called nitrogen gas sealing and cooling/heating. More specifically, an aqueous solution made of 281 parts of acrylic acid and 1,469 parts of sodium acrylate serving as the hydrophilic monomer, and 3,250 parts of water is placed in the reactor. Then, 2.2 parts of N,N'-methylenebisacrylamide serving as the internal crosslinking agent is dissolved into the aqueous solution, and a nitrogen gas is blown into the resulting aqueous solution (bubbling) for 30 minutes to eliminate oxygen dissolved in the aqueous solution. Then, 4 parts of 2,2'-azobis(2-amidinopropane)dihydrochloride is added to the aqueous solution with stirring, and the aqueous solution is stirred further for 5 minutes. Then, the aqueous' solution turns to white, and the production of white microscopic particles of 2,2'-azobis(2-amidinopropane)diacrylate is acknowledged in the reactant system. Immediately, 1.8 part of potassium persulfate and 0.09 part of L-ascorbic acid are added to the aqueous solution with stirring. Then, the aqueous solution is allowed to polymerization at 25°C in a nitrogen gas flow. The temperature inside the reactant system rises to 80°C after 5 minutes or so since the polymerization took place. Then, the reactant solution is stirred for 2 hours while the reactant system is heated to keep the inside temperature at approximately 75°C to complete the polymerization reaction. Consequently, hydrous gel of crosslinked polymer made of sodium polyacrylate is obtained. The hydrous gel of crosslinked polymer is taken out and divided minutely with a meat chopper. The divided product is spread across a 60-mesh metal gauze and dried with a hot air at 150°C for 125 minutes using a hot air dryer. Then, the dried product is pulverized with a coffee mill, and sieved through a 20-mesh metal gauze. Consequently, particles of a crosslinked polymer having an average particle size of approximately 400µm, that is, an absorbent resin (hereinafter, referred to as absorbent resin (c)), is obtained as the hydrophilic polymer. Since 2,2'-azobis(2-amidinopropane)diacrylate can also serve as a foaming agent, the absorbent resin (c) is a porous resin having pores with an average diameter of 50µm. In addition, the absorbent resin (c) is neither moist nor viscous, but dry and smooth. (d) The absorbent resin (b) obtained in the producing method (b) above is further sieved through a 100-mesh (the size of each mesh: 150µm) metal gauze, and a substance having passed through a 93-mesh metal gauze is obtained as an absorbent resin (hereinafter, referred to as absorbent resin (d)). The absorbent resin (d) is neither moist nor viscous, but dry and smooth. (e) Here, 3.59g of trimethylolpropane triacrylate serving as the internal crosslinking agent is dissolved into 5,500g of 39wt% sodium acrylate (hydrophilic monomer) aqueous solution with a neutralization ratio of 75mol% to prepare a reactant solution. Then, the reactant solution is deaerated for 30 minutes under a nitrogen gas atmosphere. Subsequently, the resulting reactant solution is supplied to a rector, which is a stainless lidded two-arm type kneader of 10ℓ equipped with two sigma blades and a jacket. Then, the reactant system is displaced by introducing a nitrogen gas while keeping the reactant solution at 30°C. Then, 2.4g of ammonium persulfate and 0.12g of L-ascorbic acid both serving as the polymerization initiator are added to the reactant solution with stirring, and the resulting reactant solution starts to polymerize after 1 minute later. The polymerization proceeds at 30°C-80°C and completes 60 minutes later, after which the resulting hydrous gel of polymer is taken out. The resulting hydrous gel of polymer is divided minutely to have a particle size of approximately 5mm. The divided hydrous gel of polymer is spread across a 50-mesh metal gauze and heated and dried at 150°C for 90 minutes. The resulting dry product is pulverized with a vibrating mill, and sieved through a 20-mesh metal gauze. Consequently, an absorbent resin precursor having an average particle size of 400µm is obtained and 5wt% of which is the particles having a particle size smaller than 106µm. Next, a solution of a surface crosslinking agent made of 0.5 part of glycerin serving as the crosslinking agent, 3 parts of water, and 0.75 part of isopropyl alcohol serving as the hydrophilic organic solvent is mixed with 100 parts of the resulting absorbent resin precursor. The resulting mixture is heated at 200°C for 35 minutes, and consequently, an absorbent resin (hereinafter, referred to as absorbent resin (e)) is obtained as the hydrophilic polymer. The resulting absorbent resin (e) has an average particle size of 400µm and 3% of which is the particles having a particle size smaller than 106µm. (f) Here, 3.56g of polyethylene glycol diacrylate serving as the internal crosslinking agent is dissolved into 5,500g of 33wt% sodium acrylate (hydrophilic monomer) aqueous solution with a neutralization ratio of 75mol% to prepare a reactant solution. Then, the reactant solution is deaerated under a nitrogen gas atmosphere. Then, 2.4g of ammonium persulfate and 0.12g of L-ascorbic acid both serving as the polymerization initiator are added to the reactant solution with stirring to start the polymerization. When the polymerization ends, the resulting hydrous gel of polymer is divided minutely and dried at 150°C in the hot air dryer until an amount of water contained therein drops to 5% or less. Then, the dried product is pulverized with a roll granulator and sieved through a 20-mesh metal gauze. Consequently, particles of a crosslinked polymer having an average particle size of 400µm, namely, an absorbent resin (hereinafter, referred to as absorbent resin (f)), is obtained as the hydrophilic polymer, and 3wt% of which is the particles having a particle size smaller than 106µm. (2) Amount of Retained Water Here, 0.2g of the absorbing agent is uniformly placed into a tea-bag sack (60mmx60mm) made of non-woven fabric and the opening is heat sealed, after which the tea-bag sack is dipped in a 0.9wt% sodium chloride aqueous solution (physiological saline solution) at room temperature for 30 minutes. Then, the tea-bag sack is taken out, and subjected to hydro-extraction for 3 minutes at 250G using a centrifugal separator, after which the weight W 1 (g) of the tea-bag sack is measured. Further, the same processes are carried out with an empty tea-bag sack, and the weight W 0 (g) of the empty tea-bag sack is measured. The amount of retained water (g/g) is calculated using the weights W 1 and W 0 on the basis of the following equation: Amount of Retained Water(g/g) = Weight W 1 (g)-Weight W 0 (g) Weight of Absorbing Agent(g) . (3) Absorbency Here, 0.2g of the absorbing agent is uniformly placed into a tea-bag sack (60mm×60mm) made of non-woven fabric, and dipped in a 0.9wt% sodium chloride aqueous solution (physiological saline solution) at room temperature for 60 minutes. Then, the tea-bag sack is taken out, and subjected to hydro-extraction for 3 minutes at 250G using a centrifugal separator, after which the weight W 1 (g) of the tea-bag sack is measured. Further, the same processes are carried out with an empty tea-bag sack, and the weight W 0 (g) of the empty tea-bag sack is measured. The absorbency (g/g) is calculated using the weights W 1 and W 0 on the basis of the following equation: Absorbency ( g / g ) = Weight W 1 ( g )- Weight W 0 ( g ) Weight of Absrobing Agent ( g ) . (4) Absorbing Amount Under Pressure To begin with, a measuring apparatus used for measuring an absorbing amount, namely, absorbency, under pressure of the absorbing agent will be explained briefly with reference to Figure 1. As shown in the drawing, the measuring apparatus comprises a balance 1, a container 2 of a predetermined capacity placed on the balance 1, an air intaking pipe 3, and a conduit 4 made of silicone resin, a glass filter 6, and a measuring section 5 placed on the glass filter 6. The container 2 has an opening 2a at the top and another opening 2b at the side surface, and the air intaking pipe 3 is inserted through the opening 2a while the conduit 4 is fixed to the opening 2b. Further, a predetermined amount of physiological saline solution (0.9wt% sodium chloride aqueous solution) 12 is withheld in the container 2. The lower end portion of the air intaking pipe 3 is dipped into the physiological saline solution 12. The air intaking pipe 3 is provided to keep the inside of the container 2 substantially at normal pressure. The glass filter 6 has a diameter of 55mm. The container 2 and the glass filter 6 communicate with each other through the conduit 4. The glass filter 6 is fixed at predetermined position and height with respect to the container 2. The measuring section 5 is provided with a paper filter 7, a supporting cylinder 9, a metal gauze 10 affixed to the bottom of the supporting cylinder 9, and a weight 11. In the measuring section 5, the paper filter 7 and supporting cylinder 9 (in effect, the metal gauze 10) are sequentially placed on the glass filter 6 in this order, and the weight 11 is placed on the metal gauze 10 inside the supporting cylinder 9. The metal gauze 10 is made of stainless steel to have a 400-mesh (the size of each mesh: 38µm). A predetermined amount of an absorbing agent 15 of a certain particle size is uniformly scattered on the metal gauze 10. The upper surface of the metal gauze 10 where the absorbing agent 15 touches the metal gauze 10 is aligned with the lower surface 3a of the air intaking pipe 3. The weight 11 is adjusted in such a manner to apply a load of 50g/cm 2 evenly to the metal gauze 10, namely, the absorbing agent 15. The absorbing amount under pressure is measured using the above-arranged measuring apparatus in the manner described below. To begin with, preparatory operations are carried out. More specifically, a predetermined amount of physiological saline solution 12 is poured into the container 2, and the air intaking pipe 3 is inserted into the container 2. Then, the paper filter 7 is placed on the glass filter 6. At the same time, 0.9g of the hydrophilic resin serving as the absorbing agent 15 is uniformly scattered on the metal gauze 10 inside the supporting cylinder 9, and the weight 11 is placed on the absorbing agent 15. Subsequently, the metal gauze 10, in effect, the supporting cylinder 9 having the absorbing agent 15 and the weight 11 inside, is placed on the paper filter 7 in such a manner that the center of the supporting cylinder 9 is superimposed on the center of the glass filter 6. Then, the weight W 2 (g) of the physiological saline solution 12, which has been absorbed by the absorbing agent 15 for 60 minutes since the supporting cylinder 9 was placed on the paper filter 7, is measured by the balance 1. The same procedure is taken without using any absorbing agent 15, and the weight W 3 (g) of the physiological saline solution 12 absorbed by the components other than the absorbing agent 15, for example, the paper filter 7, is measured by the balance 1. Then, the absorbing amount (g/g) under pressure is calculated on the basis of the following equation: Absorbing Amount (g/g) under Pressure = Weight W 2 (g) - Weight W 3 (g) Weight of Absorbing Agent (g) . (5) Absorbency Under Pressure To begin with, a measuring apparatus used for measuring absorbency under pressure will be explained briefly with reference to Figure 2. As shown in the drawing, the measuring apparatus comprises a balance 1, and a container 22 of a predetermined capacity placed on the balance 1, an air intaking pipe 23, and a conduit 24, a glass filter 26, and a measuring section 25 placed on the glass filter 26. The container 22 has an opening 22a at the top and another opening 22b at the side surface, and the air intaking pipe 23 is inserted through the opening 22a while the conduit 24 is fixed to the opening 22b. Further, a predetermined amount of a synthetic urine 32 is withheld in the container 22. The synthetic urine 31 is an aqueous solution composed of 0.2% of sodium sulfate, 0.2% of potassium chloride, 0.05% of magnesium chloride hexahydrate, 0.025% of calcium chloride dihydrate, 0.085% of ammonium dihydrogenphosphate, and 0.015% of diammonium hydrogenphospate. The lower end portion of the air intaking pipe 23 is dipped into the synthetic urine 32. The glass filter 26 has a diameter of 70mm. The container 22 and the glass filter 26 communicate with each other through the conduit 24. The glass filter 26 is fixed to a predetermined position, so that the upper surface of the same is aligned with the lower end of the air intaking pipe 23. The measuring section 25 is provided with a paper filter 27, a supporting cylinder 28, a metal gauze 29 affixed to the bottom of the supporting cylinder 28, and a weight 30. In the measuring section 25, the paper filter 27 and supporting cylinder 28 (in effect, the metal gauze 29) are sequentially placed on the glass filter 26 in this order, and the weight 30 is placed on the metal gauze 29 inside the supporting cylinder 28. The supporting cylinder 28 is made to have a bore diameter of 60mm. The metal gauze 29 is made of stainless steel to have a 400-mesh according to JIS, Japanese Industrial Standard (the size of each mesh: 38µm). A predetermined amount of an absorbing agent 35 is uniformly scattered on the metal gauze 29. The weight 30 is adjusted in such a manner to apply a load of 50g/cm 2 evenly to the metal gauze 29, namely, the absorbing agent 35. The absorbency under pressure is measured using the above-arranged measuring apparatus in the manner described below. To begin with, preparatory operations are carried out. More specifically, a predetermined amount of the synthetic urine 31 is poured into the container 22, and the air intaking pipe 23 is inserted into the container 22. Then, the paper filter 27 is placed on the glass filter 26. At the same time, 0.9g of an absorbing agent serving as the absorbing agent 35 is uniformly scattered on the metal gauze 29 inside the supporting cylinder 28, and the weight 30 is placed on the absorbing agent 35. Subsequently, the metal gauze 29, in effect, the supporting cylinder 28 having the absorbing agent 35 and weight 30 inside, is placed on the paper filter 27. Then, the weight W 4 (g) of the synthetic urine 31, which has been absorbed by the absorbing agent 35 for 60 minutes since the supporting cylinder 28 was placed on the paper filter 27, is measured by the balance 1. Then, the absorbency (g/g) under pressure after 60 minutes since the absorption started is calculated on the basis of the following equation: Absorbency (g/g) under Pressure = Weight W 4 (g) Weight of Absorbing Agent(g) . (6) Gel Breaking Strength Here, 25.3g of a synthetic urine and 0.84g of an absorbing agent are placed in a supporting cylinder having a bore diameter of 50mm and a height of 13mm, so that the absorbing agent absorbs the synthetic urine and turns into gel. The synthetic urine has the same chemical make-up explained in the above (5) absorbency under pressure column. Then, the supporting cylinder is placed on a measuring table having a 20-mm-dia. opening at the center in such a manner that the center of the supporting cylinder is superimposed on the center of the measuring table, and breaking strength of the gel inside the supporting cylinder is measured. The breaking strength is measured by a neocard meter of Model No. M-302 of ITEC Corp. under the following conditions: diameter and lowering speed of a pressure sensitive axis is 8mm and 0.36cm/sec., respectively. The breaking strength referred herein means a minimum strength necessary to break an elastic body, namely, the gel inside the supporting cylinder, against the elastic limit. (7) Gel Breaking strength and Retention After Gel Breaking Strength Lowering Promotion Test (GBSLPT) Here, 0.84g of an absorbing agent is allowed to stand at 40°C and a relative humidity of 90% for 1 ○ 24 hours or 2 ○ 48 hours (gel breaking strength lowering promotion test, hereinafter, simply referred to as lowering promotion test). Then, the gel breaking strength is measured using the absorbing agent when the lowering promotion test ends. More specifically, 0.84g of the absorbing agent and 25.3g of the synthetic urine are placed in the supporting cylinder having a bore diameter of 50mm and a height of 13mm, so that the absorbing agent absorbs the synthetic urine and turns into gel. The breaking strength of the gel thus obtained is measured in the same manner as the above section (6). The retention (%) referred herein is a ratio of gel breaking strength (I) with respect to gel breaking strength (II) in percentage, where the gel breaking strength (I) is the gel breaking strength after the lowering promotion test by allowing the absorbing agent to stand for 48 hours at 40°C and a relative humidity of 90% and the gel breaking strength (II) is the gel breaking strength of the absorbing agent without the lowering promotion test. <Example 1> Herein, 3 parts of 50% polyethyleneimine aqueous solution having a weight average molecular weight of 70,000 (Epomine P-1050 of Nippon Shokubai Co., Ltd.) serving as the crosslinking agent (liquid denaturant substance) and 9 parts of aluminium oxide (average particle size: 80µm) serving as the water-insoluble compound are mixed with each other with stirring. Consequently, a powdery denaturant substance (1) having an average particle size of 80µm is obtained. Then, 12 parts of the powdery denaturant substance (1) is added to 100 parts of the absorbent resin (e) serving as the hydrophilic polymer and mixed with each other. Then, the resulting mixture is heated and dried at 90°C for 20 minutes. The resulting dry product is sieved through a metal gauze having 840µm openings, and the substance having passed through the metal gauze is obtained as the hydrophilic resin to be used as the absorbing agent. The absorbency, absorbency under pressure, gel breaking strength, gel breaking strength after the gel breaking strength lowering promotion test, and retention of the resulting absorbing agent are measured in the above-described manners, and the results of which are set forth in Table 1 below. <Example 2> Herein, perlite (particles of volcanic rocks, 80% of which is composed of SiO 2 and Al 2 O 3 ) as the water-insoluble compound is sieved through a metal gauze having 500µm openings, and the substance having passed through the metal gauze is collected. Then, 5 parts of the collected substance and 5 parts of 30% polyethyleneimine aqueous solution having a weight average molecular weight of 70,000 (Epomine P-1000 of Nippon Shokubai Co., Ltd.) are mixed with each other with stirring, and a powdery denaturant substance (2) having an average particle size 200µm is obtained. Then, 10 parts of the powdery denaturant substance (2) is added to 100 parts of the absorbent resin (e) serving as the hydrophilic polymer and mixed with each other. The resulting product is sieved through a metal gauze having 840µm openings, and the substance having passed through the metal gauze is obtained as the hydrophilic resin serving the absorbing agent. The absorbency, absorbency under pressure, gel breaking strength, gel breaking strength after the gel breaking strength lowering promotion test, and retention of the resulting absorbing agent are measured in the above-described manners, and the results of which are set forth in Table 1 below. <Example 3> To begin with, 5 parts of 30% polyethyleneimine aqueous solution having a weight average molecular weight of 70,000 (Epomine P-1000 of Nippon Shokubai Co., Ltd.) serving as the crosslinking agent (liquid denaturant substance) and 5 parts of powders of polyethylene (Floucene UF-1.5 of Sumitomo Seika Chemicals Co., Ltd. containing 99.9% or more of particles having a particle size of 75µm or less) are mixed with each other with stirring, and a powdery denaturant (3) is obtained. Then, 10 parts of the powdery denaturant (3) is added to 100 parts of the absorbent resin (e) serving as the hydrophilic polymer and mixed with each other. The resulting product is sieved through a metal gauze having 840µm openings, and the substance having passed through the metal gauze is obtained as the hydrophilic resin serving an absorbing agent. The absorbency, absorbency under pressure, gel breaking strength, gel breaking strength after the gel breaking strength lowering promotion test, and retention of the resulting absorbing agent are measured in the above-described manners, and the results of which are set forth in Table 1 below. <Comparative Example 1> Herein, 5 parts of 30% polyethyleneimine aqueous solution having a weight average molecular weight of 70,000 (Epomine P-1000 of Nippon Shokubai Co., Ltd.) serving as the crosslinking agent (liquid denaturant substance) is added to 100 parts of the absorbent resin (e) serving as the hydrophilic polymer and mixed with each other. The resulting mixture is heated and dried at 90°C for 20 minutes. The resulting dried product is sieved through a metal gauze having 840µm openings, and the substance having passed through the metal gauze is collected as the treated hydrophilic polymer to be used as a comparative absorbing agent. The absorbency, absorbency under pressure, gel breaking strength, gel breaking strength after the gel breaking strength lowering promotion test, and retention of the comparative absorbing agent are measured in the above-described manners, and the results of which are set forth in Table 1 below. EXAMPLE COMPARATIVE EXAMPLE 1 1 2 3 ABSORBENCY (g/g) 27 27 27 27 ABSORBENCY UNDER PRESSURE (g/g) 27 28 27 25 GEL BREAKING STRENGTH (Kdyne/cm 2 ) 195 216 206 210 BEFORE GBSLPT 164 185 145 117 AFTER 24 HOURS FM GBSLPT 132 168 115 80 AFTER 48 HOURS FM GBSLPT RETENTION (%) 68 78 56 38 The results set forth in Table 1 reveal that the hydrophilic resin of the present invention can maintain high gel breaking strength and retention even after the lowering promotion test, because the hydrophilic resin contains the hydrophilic polymer having a functional group, and the powdery denaturant substance containing both the denaturant reactive with the above functional group and particles of the water-insoluble compound. This means that the hydrophilic resin of the present invention can maintain its bonding property over time after the production until being put into actual use. The reason is assumed to be as follows. Since the denaturant is contained in the powdery denaturant substance, the hydrophilic polymer and denaturant exist separately in the hydrophilic resin and never react with each other until being put into actual use. The hydrophilic polymer and denaturant start to react with each other, for example, when the absorbing resin has absorbed the aqueous liquid, and as a consequence, the hydrophilic polymer is denatured. Thus, not only the hydrophilic polymer can be denatured uniformly, but also the denaturing timing can be controlled, thereby making it possible to prevent the deterioration of the properties over time caused by denaturation. As has been explained, the hydrophilic resin of the present invention can prevent the deterioration of the properties over time more efficiently. Since the hydrophilic resin of the present invention is denatured after the powdery denaturant substance and hydrophilic polymer are mixed with each other homogeneously, the hydrophilic resin has an excellent absorbing property under pressure. <Example 4> To begin with, 0.1 part of ethylene glycol diglycidyl ether serving as the crosslinking agent (denaturant), 2 parts of isopropyl alcohol serving as the hydrophilic organic solvent, and 1 part of hydrophilic microscopic particles of silicon dioxide (Aroesil 200 of Nippon Aroesil Co., Ltd.) serving as the water-insoluble compound are mixed with each other with stirring, and a powdery denaturant substance (4) having an average particle size of 10µm is obtained. Then, 3.1 parts of the powdery denaturant substance (4) is added to 100 parts of the absorbent resin (f) serving as the hydrophilic polymer and mixed each other, and after which 5 parts of water is added to the resulting mixture and mixed with each other. The resulting mixture is heated at 180°C for 25 minutes and sieved through a metal gauze having 840µm openings, and the substance having passed through the metal gauze is obtained as the crosslinked hydrophilic resin to be used as the absorbing agent. The absorbency and absorbency under pressure of the resulting absorbing agent are measured in the above-described manners, and the results of which are set forth in Table 2 below. <Example 5> Herein, 1 part of glycerin serving as the crosslinking agent (denaturant), 1 part of isopropyl alcohol serving as the hydrophilic organic solvent, and 1 part of hydrophilic microscopic particles of silicon dioxide (Carplex 22S of Shionogi & Co., Ltd.) serving as the water-insoluble compound are mixed with each other with stirring, and a powdery denaturant substance (5) having an average particle size of 20µm is obtained. Then, 3 parts of the powdery denaturant substance (5) is added to 100 parts of the absorbent resin (f) serving as the hydrophilic polymer and mixed with each other, and after which 3 parts of water is added to the resulting mixture and mixed with each other. The resulting mixture is heated at 200°C for 40 minutes and sieved through a metal gauze having 840µm openings, and the substance having passed through the metal gauze is obtained as the crosslinked hydrophilic resin serving an absorbing agent. The absorbency and absorbency under pressure of the resulting absorbing agent are measured in the above-described manners, and the results of which are set forth in Table 2 below. <Example 6> Herein, 0.1 part of ethylene glycol diglycidyl ether serving as the crosslinking agent (denaturant) and 7 parts of water are mixed with each other with stirring to obtain a liquid denaturant substance. The resulting liquid denaturant substance is cooled at -20°C for 12 hours to turn the liquid denaturant substance into a solid. Then, the solid denaturant substance is pulverized and sieved through a metal gauze having 2mm openings, and the substance having passed through the metal gauze is obtained as a powdery denaturant substance (6). Then, 7.1 parts of the powdery denaturant substance (6) is added to 100 parts of the absorbent resin (f) serving as the hydrophilic polymer and mixed with each other. The resulting mixture is heated at 140°C for 30 minutes and sieved through a metal gauze having 840µm openings, and the substance having passed through the metal gauze is obtained as the crosslinked hydrophilic resin serving an absorbing agent. The absorbency and absorbency under pressure of the resulting absorbing agent are measured in the above-described manners, and the results of which are set forth in Table 2 below. <Comparative Example 2> The absorbent resin (f) is used as a comparative absorbing agent and the absorbency and absorbency under pressure of the same are measured in the above manners, the results of which are set forth in Table 2 below. <Comparative Example 3> Here, 0.1 part of ethylene glycol diglycidyl ether serving as the crosslinking agent (denaturant) and 2 parts of isopropyl alcohol serving as the hydrophilic organic solvent, and 5 parts of water are added to 100 parts of the absorbent resin (f) serving as the hydrophilic polymer and mixed with each other. The resulting mixture is heated at 180°C for 25 minutes, and sieved through a metal gauze having 840µm openings, and the substance having passed through the metal gauze is obtained as the crosslinked hydrophilic resin to be used as a comparative absorbing agent. The absorbency and absorbency under pressure of the comparative absorbing agent are measured in the above-described manners, and the results of which are set forth in Table 2 below. <Comparative Example 4> Here, 1 part of glycerin serving as the crosslinking agent (denaturant) and 1 part of isopropyl alcohol serving as the hydrophilic organic solvent, and 3 parts of water are added to 100 parts of the absorbent resin (f) serving as the hydrophilic polymer and mixed with each other. The resulting mixture is heated at 200°C for 40 minutes, and sieved through a metal gauze having 840µm openings, and the substance having passed through the metal gauze is obtained as the crosslinked hydrophilic resin to be used a comparative absorbing agent. The absorbency and absorbency under pressure of the comparative absorbing agent are measured in the above-described manners, and the results of which are set forth in Table 2 below. <Comparative Example 5> A comparative absorbing agent is obtained in the same manner as Example 17 except that the liquid denaturant substance is not made into powders. In other words, 0.1 part of ethylene glycol diglycidyl ether serving as the crosslinking agent (denaturant) and 7 parts of water are mixed with each other with stirring to obtain a liquid denaturant substance. Then, 7.1 parts of the liquid denaturant substance is added to 100 parts of the absorbent resin (f) serving as the hydrophilic polymer and mixed with each other. The resulting mixture is heated at 140°C for 30 minutes, and sieved through a metal gauze having 840µm openings, and the substance having passed through the metal gauze is obtained as the crosslinked hydrophilic resin to be used as a comparative absorbing agent. The absorbency and absorbency under pressure of the comparative absorbing agent are measured in the above-described manners, and the results of which are set forth in Table 2 below. EXAMPLE COMPARATIVE EXAMPLE 4 5 6 2 3 4 5 ABSORBENCY (g/g) 40 38 36 54 44 36 44 ABSORBENCY UNDER PRESSURE (g/g) 28 27 28 8 24 25 10 The results set forth in Table 4 reveal that the hydrophilic resin of the present invention has an excellent absorbing property under pressure because the hydrophilic resin is denatured (surface crosslinked) after the powdery denaturant substance and the hydrophilic polymer are mixed homogeneously. POTENTIAL INDUSTRIAL APPLICATION According to the producing method of the hydrophilic-resin of the present invention, the hydrophilic polymer and a powdery denaturant substance made from a liquid denaturant substance are mixed with each other. Therefore, the liquid denaturant substance which is substantially in the solid state can be mixed with the hydrophilic polymer. Consequently, the hydrophilic polymer and the liquid denaturant substance, in effect, the powdery denaturant substance, can be mixed with each other homogeneously compared with a case where the liquid denaturant substance and hydrophilic polymer are mixed with each other. In addition, since the phydrophlic polymer and liquid denaturant substance exist separately, the hydrophilic polymer does not start to absorb the liquid denaturant substance, or the reaction of the hydrophilic polymer and denaturant contained in the liquid denaturant substance does not take place as soon as the hydrophilic polymer and the powdery denaturant substance are mixed with each other. The hydrophilic polymer can be denatured uniformly, for example, when the hydrophilic resin has absorbed an aqueous liquid or the hydrophilic resin is heated, thereby making it possible to control the denaturation timing. Consequently, according to the producing method of the hydrophilic resin of the present invention, the hydrophilic resin having a good balance of properties can be readily produced. In the hydrophilic resin obtained by the above producing method, the hydrophilic polymer is denatured when the hydrophilic polymer and denaturant are brought into contact and react with each other, for example, when the hydrophilic polymer has absorbed an aqueous liquid or the hydrophilic resin is heated. Therefore, not only the hydrophilic polymer can be denatured uniformly, but also the denaturation timing can be controlled. Thus, better properties, such as absorbency under pressure, can be obtained through the denaturation. In particular, in case that the liquid denaturant substance is a crosslinking agent containing a polyamine, the aforementioned bonding ability among the particles of the hydrophilic polymer and shape-keeping ability of the absorbent material are developed when the absorbent resin has absorbed the aqueous liquid. Further in this case, since it becomes possible to control the timing of interaction of the particles of the hydrophilic polymer triggered by polyamine or the like, deterioration caused by the change of the aforementioned properties, such as gel breaking strength, can be prevented more efficiently. The hydrophilic resin obtained by the above producing method has an excellent absorbing property under pressure, such as absorbency and water retaining ability under pressure, and for example, the hydrophilic resin can show excellent performance (absorbing property) even when used in sanitary goods or the like having a high percent by weight of the hydrophilic polymer (resin concentration). Further, the hydrophilic resin has an excellent absorbing property under pressure, and the hydrophilic polymer contained therein is hardly released from the absorbent material while water is being absorbed; moreover, the hydrophilic resin maintains its bonding property over time after the production until being put into actual use. For the reasons specified as above, the hydrophilic resin can be extensively and suitably used for sanitary goods, such as paper diapers, sanitary napkins, assisting material for incontienet patients, such as incontinence pads, wound protectors, and wound healing materials, to absorb body fluids; drip absorbing materials to absorb drip from foods or the like, or freshness preserving materials; water-retaining materials for soil to keep water in soil;or water stopping materials.

Claims: A production method for a hydrophilic resin characterized in that a hydrophilic polymer is mixed with a powdery denaturant substance made from a liquid denaturant substance. The production method of Claim 1, wherein said liquid denaturant substance is made into powders by being mixed with powders of a water-insoluble compound. The production method of Claim 1, wherein said liquid denaturant substance is made into a powder by being cooled to or below a melting point of said liquid denaturant substance and turned into a solid state. The production method of Claim 1, wherein a mixture of said hydrophilic polymer and said powdery denaturant substance is heated. The production method of Claim 1, wherein an aqueous medium in a liquid or gaseous state is added to a mixture of said hydrophilic polymer and said powdery denaturant substance first, and thence a resulting mixture is heated. The production method of Claim 1, wherein said hydrophilic polymer is an absorbent resin having an acidic group. The production method of Claim 1, wherein said liquid denaturant substance contains a crosslinking agent. The production method of Claim 7, wherein said crosslinking agent contains a polyamine whose weight average molecular weight is 2,000 or greater. The production method of Claim 1, wherein the denaturant changes the structure or physical properties of the hydrophilic polymer by reaction of the denaturant with a functional group of the hydrophilic polymer. The production method according to Claim 1 or Claim 9, wherein the denaturant is a high molecular weight polyamine, and the denaturant brings about a bonding force among the particles of the hydrophilic polymer. The production method of Claim 10, wherein said high molecular weight polyamine has a weight average molecular weight of 2,000 or greater.

Inventors: HARADA NOBUYUKI, MASUDA YOSHIHIKO, MOTONO YOSHIHIRO, NAGASUNA KINYA, NAKAMURA MASATOSHI, SAKAMOTO SHIGERU, SHIMOMURA TADAO, WADA KATSUYUKI, HARADA, NOBUYUKI, MASUDA, YOSHIHIKO, MOTONO, YOSHIHIRO, NAGASUNA, KINYA, NAKAMURA, MASATOSHI, SAKAMOTO, SHIGERU, SHIMOMURA, TADAO, WADA, KATSUYUKI
Applicants: NIPPON CATALYTIC CHEM IND, NIPPON SHOKUBAI CO., LTD.
Application year: 1997
Main classification: C08J3
IPC codes: C08J3