WO2009048157A1

WO2009048157A1 - Surface treatment method for water-absorbent resin

Info

Publication number: WO2009048157A1
Application number: PCT/JP2008/068688
Authority: WO
Inventors: Yoshiro Mitsukami; Taku Iwamura
Original assignee: Nippon Shokubai Co., Ltd.
Priority date: 2007-10-09
Filing date: 2008-10-08
Publication date: 2009-04-16
Also published as: JP2011500872A; JP5390509B2

Abstract

Provided is a method that is capable of producing a surface treated water-absorbent resin excellent in water-absorbing characteristics (in particular, absorption capacity against pressure or fluid permeability), at low temperature and in a short period of time, at low cost and by a safe method. In a surface treatment method for a water-absorbent resin having a surface cross-linking step for subjecting a water-absorbent resin to surface cross-linking in the presence of a radical polymerization initiator and water, is provided. In the surface cross-linking step, activated energy beam having a wavelength of equal to or shorter than ultraviolet ray is not irradiated to a reaction system of surface cross-linking, and the surface cross-linking step is carried out so that water content of the water-absorbent resin after the surface cross-linking step is larger than that before the surface cross-linking step.

Description

DESCRIPTION

SURFACE TREATMENT METHOD FOR WATER-ABSORBENT RESIN

TECHNICAL FIELD The present invention relates to a surface treatment method for a water-absorbent resin, and more specifically, the present invention relates to technology for improving cost or safety in production, while maintaining water-absorbing characteristics of the water-absorbent resin.

BACKGROUND ART

The water-absorbing agent containing a copolymer (a water-absorbent resin) as a main component has been hitherto used as one constituent material for hygienic materials such as sanitary cotton, disposable diaper or other absorbents for body fluid. As an example of such a water-absorbent resin composing the water-absorbing agent, there is included, for example, a hydrolyzate of a starch-acrylonitrile graft polymer, a neutralized starch-acrylic acid graft polymer, a saponified vinyl acetate-acrylic acid ester copolymer, a hydrolyzate of an acrylonitrile copolymer or acrylamide copolymer, and a cross-linked substance thereof, partially neutralized cross-linked polyacrylic acid or the like. Any of these water-absorbent resins possess an internal crosslink structure and exhibit no solubility in water.

Characteristic properties, which these water-absorbent resins are expected to possess, include high absorption capacity, excellent absorption speed, high gel strength, and excellent suction force necessary for sucking water from a substrate, fluid permeability and the like.

As technology aiming at improving various

-j water-absorbing characteristics of the water-absorbent resin, there has been proposed conventionally a method for enhancing cross-link density at the surface of the water-absorbent resin, by treatment of the surface of the water-absorbent resin using a cross-linking agent . For example, inU . S . Patent No.4783510, there has been disclosed a technology for subj ecting an aqueous solution containing a peroxide-type radical initiator to bringing into contact with the surface part of the water-absorbent resin, and heating. In addition, in JP-A-1-113406, there has been disclosed a technology for cross-linking the water-absorbent resin, by subjecting water-absorbent resin particles to mixing with a cross-linking agent having two or more functional groups, and bringing into contact with steam. Still more, in JP-A-1-297430, there has been disclosed a technology for introducing a cross-link structure at the surface of the water-absorbent resin, by subjecting the water-absorbent resin particles to mixing with a surface cross-linking agent having two or more functional groups, and an organic solvent, and heating under atmosphere of specific humidity. In addition, in U.S. Serial No.2007-0078231, there has been disclosed a technology for heating the water-absorbent resin particles in the presence of a peroxide, an organic cross-linking agent (a polyvalent alcohol) and water. Any of these technologies is the technology for introducing a cross-link structure at the surface of the water-absorbent resin, by subjecting a specific surface cross-linking treatment agent to bringing into contact with the water-absorbent resin, and heating. On the other hand, there has also been proposed a technology for surface cross-linking the water-absorbent resin, by carrying out irradiation treatment with activated energy beam such as ultraviolet ray, instead of heat treatment. For example, in WO 2006/62258 Pamphlet, there has been disclosed a technology for mixing the water-absorbent resin and a water-soluble radical polymerization initiator, and irradiating activated energy beam to the mixture obtained thereby. In addition, in WO 2006/62253 Pamphlet, there has been disclosed a technology for subj ecting the water-absorbent resin to mixing with predetermined amount of a radical polymerization initiator and a radically polymerizable compound, and irradiating activated energy beam to the mixture obtained thereby.

According to disclosures in the above Patent Literature, it is said that surface cross-link density of the water-absorbent resin is increased by cross-linking or the like of reactive functional groups themselves derived from a monomer present at the surface of the water-absorbent resin, by use of a surface cross-linking agent, resulting in enhancement of various water-absorbing characteristics.

DISCLOSURE OF INVENTION

However, surface treatment of the water-absorbent resin by using technology described in the above Patent Literature, had a problem of raising production cost due to requiring heating at high temperature for a long period of time, or providing inferior safety due to high reactivity of a cross-linking agent, in the case where a cross-linking agent that is capable of reacting at low temperature was adopted. In addition, water-absorbing characteristics of the obtained surface cross-linkedwater-absorbent resin is not necessarily sufficient, therefore, still more enhancement of, in particular, water-absorbing characteristics under pressure

(absorption capacity against pressure) or fluid permeability

Q has been required.

Under these circumstances, it is an object of the present invention to provide a method that is capable of producing a surface treated water-absorbent resin with excellent water-absorbing characteristics (in particular, absorption capacity against pressure or fluid permeability) at low temperature and in a short period of time, at low cost and by a safe method.

In view of the above problem, the present inventors have intensively studied, as a result, have found that water-absorbing characteristics (in particular, absorption capacity against pressure or fluid permeability) of the obtained surface cross-linked water-absorbent resin can be enhanced, by using a low-cost and safe method, by carrying out a surface cross-linking step so that water content of the water-absorbent resin after the surface cross-linking step is larger than that before the surface cross-linking step, in the surface cross-linking of the water-absorbent resin, in the presence of a radical polymerization initiator and water, and have thus completed the present invention.

That is, the surface treatment method for the water-absorbent resin relevant to the present invention is a surface treatment method for the water-absorbent resin having a surface cross-linking step for subjecting a water-absorbent resin to surface cross-linking in the presence of a radical polymerization initiator and water, wherein in the surface cross-linking step activated energy beamhaving a wavelength of equal to or shorter than ultraviolet ray, is not irradiated to a reaction system of surface cross-linking, and the surface cross-linking step is carried out so that water content of the water-absorbent resin after the surface cross-linking step is larger than that before the surface cross-linking step.

According to the surface treatment method of the present invention, the surface treated water-absorbent resin excellent in water-absorbing characteristics (in particular, absorption capacity against pressure or fluid permeability) can be produced at low temperature and in a short period of time, by using a low-cost and safe method .

Still more other objects, features and advantages of the present invention will become apparent with reference to the preferable embodiments exemplified in the following explanation.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a schematic drawing of a measurement apparatus used in measurement of saline flow conductivity (SFC)

BEST MODE FOR CARRYING OUT THE INVENTION

It should be noted that, in the present invention, "weight" and "mass" are treated as synonyms, and "% by weight" and "% by mass" are treated as synonyms. In addition, unless otherwise specified, "physiological saline" means an aqueous 0.9% by weight sodium chloride solution, and "water content" means water content of the water-absorbent resin, specified by reduced weight at 180⁰C for 3 hours in examples to be described later.

Hereinafter, although explanation is given about the surface treatment method of water-absorbent resin relevant to the present invention, the technical scope of the present invention should not be restricted by these explanation, and beyond the following exemplification, the proper variation can be carried out within not to impair the spirit of the present invention. The present invention relates to a surface treatment method for a water-absorbent resin having a surface cross-linking step for subjecting a water-absorbent resin to surface cross-linking in the presence of a radical polymerization initiator and water, wherein in the surface cross-linking step activated energy beam having a wavelength of equal to or shorter than ultraviolet ray, is not irradiated to a reaction system of surface cross-linking, and the surface cross-linking step is carried out so that water content of the water-absorbent resin after the surface cross-linking step is larger than that before the surface cross-linking step. (1) A water-absorbent resin (the base polymer)

A water-absorbent resin (the base polymer) is a water-swelling, water-insoluble polymer formed by introducing a cross-link structure to a aqueous gel-like polymer. In the present invention, the term "water-swelling" refers to having a "Centrifuge Retention Capacity (CRC)" in physiological saline of equal to or higher than 2 g/g, preferably 5 to 100 g/g and more preferably 10 to 60 g/g. It should be noted that as value of CRC, value measured by a method described in Examples is adopted. In addition, the term "water-insoluble" refers to the uncross-linked water-extractable parts (hereafter may be referred to as "extractable parts") in the water-absorbent resin, with a content of 0 to 50% by weight, preferably 0 to 25% by weight, more preferably 0 to 15% by weight, and still more preferably 0 to 10% by weight. It should be noted that as value of extractable parts, value measured by a method described in Examples is adopted.

[A measurement method for elution soluble parts]

In a plastic container with lid (6 cm in diameter * 9 cm in height) with a volume of 250 ml, 184.3 g of physiological saline was weighed, and 1.00 g of the water-absorbent resin was added thereto, and then they were stirred together by the use of a magnetic stirrer with 8 mm in diameter and 25 mm in length, at a rotation number of 500 rpm for 16 hours to extract the soluble part in the resin. The extracted solution was filtered with one filter paper (0.26 mm in thickness and 5 μm in retained particle diameter; manufactured by Advantec Toyo K. K. and sold under the product name of "JIS P 3801 No.2") and 50.0 g of the resultant filtrate was weighed and used as a measurement solution.

First, physiological saline alone was titrated with an aqueous 0. IN NaOH solution till pH 10, and subsequently titrated with an aqueous IN HCl solution till pH 2.7 to obtain blank titers ( [bNaOH] ml and [bHCl] ml, respectively). By performing the similar titrating operation on the measurement solution, the titers ( [NaOH] ml and [HCl] ml, respectively) were obtained.

For example, in the case of the water-absorbent resin which is composed of known amounts of acrylic acid and sodium salt thereof, the extractable parts of this water-absorbent resin were calculated in accordance with the following mathematical expression, based on average molecular weight of the monomer and the titer which is obtained from the aforementioned operation. When the amounts were unknown, the average molecular weight of the monomer was calculated by using the neutralization ratio determined by the titration in accordance with the following mathematical expression. [Expression 1] Extractable parts (% by weight) = 0.1 * (average molecular weight of monomer) x 184.3 * 100 x ([HCl] - [bHCl]) / 1000 / 1.0 / 50.0 Neutralization ratio (%bymol) = [1 - ([NaOH] - [b (NaOH)]) / ([HCl] - [bHCl])] x 100

(2) A production method for the water-absorbent resin (the base polymer) (2-1) A polymerization step for polymerizing a monomer component

In the present invention, the water-absorbent resin can be obtained, for example, by a polymerization step for polymerizing a monomer component. As the polymerization method, aqueous solution polymerization, reversed phase suspension polymerization, bulk polymerization, precipitation polymerization, or the like may be adopted. In consideration of performance aspect or control easiness of polymerization, it is preferable to carry out aqueous solution polymerization or reversed phase suspension polymerization, by using a monomer component as an aqueous solution, and it is particularly preferable to carry out aqueous solution polymerization.

Concentration of the aqueous solution of the monomer component is preferably 20% by weight to saturated concentration, more preferably 30 to 70% by weight, and still more preferably 35 to 60% by weight, as the monomer. The concentration less than 20% by weight is disadvantageous in that larger heat quantity and time are required, since the water content of the obtained polymer (hydrogel) becomes large .

Such polymerization methods are described, for example, in U.S. Patent No. 4625001, U.S. Patent No. 4769427, U.S. Patent No. 4873299, U.S. Patent No. 4093776, U.S. Patent No. 4367323, U.S. Patent No. 4446261, U.S. Patent No. 4683274, U.S. Patent No. 4690996, U.S. Patent No. 4721647, U.S. Patent No. 4738867, U.S. Patent No. 4748076, U.S. Patent No. 6906159 and the like.

Kind of the monomer composing the water-absorbent resin is not especially limited, and an unsaturated monomer such as an ethylenic unsaturatedmonpmermaybe used . The ethylenic unsaturated monomer is not especially limited, and is preferably a monomer having an unsaturated double bond at the terminal thereof, and there is included, for example, an anionic monomer such as (meth) acrylic acid, 2- (meth) acryloyl ethane sulfonic acid, 2- (meth) acryloyl propane sulfonic acid, 2- (meth) acrylamide-2-methyl propane sulfonic acid, vinyl sulfonic acid, or styrene sulfonic acid, or a salt thereof; a nonionic hydrophilic group-containing monomer such as (meth) acrylamide, N-substituted

(meth) acrylamide, 2-hydroxyethyl (meth) acrylate, or 2-hydroxypropyl (meth) acrylate; an amino group-containing unsaturated monomer such as N,N-dimethylaminoethyl

(meth) acrylate, N,N-diethylaminoethyl (meth) acrylate,

N,N-diethylaminopropyl (meth) acrylate, or

N,N-dimethylaminopropyl (meth) acrylamide, or a quaternized compound thereof; or the like. These monomers may be used either alone as one kind, or in combination of two or more kinds. Among these, (meth) acrylic acid, 2- (meth) acryloyl ethane sulfonic acid, 2- (meth) acrylamide-2-methylpropane sulfonic acid, and/or a salt thereof; N, N-dimethylaminoethyl (meth) acrylate; a quaternized compound of N, N-dimethylaminoethyl (meth) acrylate; and (meth) acrylamide are preferably used, and acrylic acid and/or a salt thereof are particularly preferably used. Amount of an acrylic acid (salt) is preferably in range of from 50 to 100% by mol, more preferably from 70 to 100% by mol, still more preferable from 90 to 100% by mol, relative to 100% by mol of total amount of the monomer components. When an acrylate salt is used as the monomer, a monovalent salt of acrylic acid selected from an alkali metal salt, an ammonium salt , and an amine salt of acrylic acid, is preferable in view of water-absorbing performance of the water-absorbent resin. It is more preferable to be the alkali metal salt of acrylic acid, and particularly preferably the acrylic acid salt selected from a sodium salt, a lithium salt, and a potassium salt of acrylic acid.

In producing the water-absorbent resin, monomer components other than the above monomers may be used without impairing the effect of the present invention. For example, there may be exemplified a hydrophobic monomer such as an aromatic ethylenic unsaturated monomer having carbon atoms of 8 to 30, an aliphatic ethylenic unsaturated monomer having carbon atoms of 2 to 20, an alicyclic ethylenic unsaturated monomer having carbon atoms of 5 to 15, an alkyl ester of

(meth) acrylic acid containing an alkyl group having carbon atoms of 4 to 50 or the like. Ratio of such a hydrophobic monomer is generally in a range of 0 to 20 parts by weight, relative to 100 parts by weight of the above ethylenic unsaturated monomer. The hydrophobic monomer over 20 parts by weight may deteriorate water-absorbing performance of the obtained water-absorbent resin in certain cases.

Explanation was given above on preferable embodiments of the present invention with reference to the case where the ethylenic unsaturated monomer was used as the monomer component, however, technical scope of the present invention should not be limited to these embodiments. For example, the water-absorbent resin of the present invention may be a cross-linked body of polyamino acid, cross-linked bodies of polysaccharides, a cross-linked body of polyester, a cross-linked body of polyacetal, and a composite thereof. In a polymerization step, first, by polymerization of theinonomer component in the presence of a cross-linking agent, a aqueous gel-like cross-linked polymer is obtained. The aqueous gel-like cross-linked polymer may be a self cross-linking-type one without using a cross-linking agent, however, it is preferably one obtained by copolymerization or reaction of a cross-linking agent having two or more polymerizable unsaturated groups or two or more reactive functional groups in one molecule. By having an internal cross-link in this way, the water-absorbent resin becomes water insoluble.

As a specific example of a cross-linking agent used in the polymerization step (an internal cross-linking agent) , there is included, for example, N, N' -methylene bis (meth) acrylamide, (poly) ethylene glycol di (meth) acrylate, (poly) propylene glycol di (meth) acrylate, (ethylene oxide modified) trimethylolpropane tri (meth) acrylate, trimethylolpropane di (meth) acrylate, glycerin di (meth) acrylate, glycerin tri (meth) acrylate, (ethylene oxide modified) glycerin acrylate methacrylate, pentaerythritol tri (meth) acrylate, pentaerythritol tetra (meth) acrylate, dipentaerythritol hexa (meth) acrylate, N,N-diallyl acrylamide, triallyl cyanurate, triallyl isocyanurate, triallyl phosphate, triallyl amine, diallyloxyacetic acid, bis (N-vinylcarboxylic amide),

(ethylene oxide modified) tetraallyloxyethane, poly (meth) allyloxyalkane, (poly) ethylene glycol diglycidyl ether, glycerin diglycidyl ether, ethylene glycol, polyethylene glycol, propylene glycol, glycerin, pentaerythritol, ethylene diamine, polyethylene imine, glycidyl (meth) acrylate or the like. These internal cross-linking agents may be used alone as one kind or two or more kinds may be used in combination. In particular, in consideration of water-absorbing characteristics or the like of the obtained water-absorbent resin, it is preferable that a compound having two ormore polymerizable unsaturated groups is used as the internal cross-linking agent. Specifically,

(poly) ethylene glycol di (meth) acrylate, (ethylene oxide modified) trimethylolpropane tri (meth) acrylate, glycerin di (meth) acrylate, (ethylene oxide modified) glycerin acrylate methacrylate, or pentaerythritol tri (meth) acrylate is preferably used, and (poly) ethylene glycol di (meth) acrylate or (ethylene oxide modified) trimethylolpropane tri (meth) acrylate is more preferably used. Used amount of the internal cross-linking agent in the polymerization step is preferably 0.0001 to 1% by mol, more preferably 0.001 to 0.5% by mol, and still more preferably 0.005 to 0.2% by mol, relative to total amount of the monomer components. The used amount of the internal cross-linking agent of equal to or more than 0.0001% by mol provides introduction of internal cross-linking, while the used amount of the internal cross-linking agent of equal to or less than

1% by mol leads to prevent excessive increase in gel strength of the obtained water-absorbent resin as well as the corresponding reduction of water-absorbing characteristics .

In polymerization, a hydrophilic polymer such as a starch-cellulose, a derivative of the starch-cellulose, polyvinyl alcohol, polyacrylic acid (salt), a cross-linked body of polyacrylic acid (salt) orthelike, or a chain transfer agent such as hypophosphorous acid (salt) or the like may be added into a reaction system. Used amount of the hydrophilic polymer in the polymerization step is preferably 0 to 50% by weight, more preferably 0 to 30% by weight, and still more preferably 0 to 10% by weight, relative to total amount of the monomer components. In addition, used amount of the chain transfer agent in the polymerization step is preferably 0.001 to 1% by mol, more preferably 0.005 to 0.5% by mol, and still more preferably 0.01 to 0.3% by weight, relative to total amount of the monomer components.

In polymerization initiation of the above polymerization reaction, for example, a polymerization initiator or activated energy beam such as radiation ray, electron beam, ultraviolet ray, or electromagnetic beam may be used. The polymerization initiator is not especially limited, and a thermal polymerization initiator, a photo-polymerization initiator or the like may be used.

As a thermal polymerization initiator, there is included a persulfate such as sodium persulfate, potassium persulfate, ammonium persulfate or the like; a peroxide such as hydrogen peroxide, t-butyl peroxide, methyl ethyl ketone peroxide or the like; an azo compound such as an azonitrile compound, an azoamidine compound, a cyclic azoamidine compound, an azoamide compound, an alkylazo compound, 2, 2' -azobis (2-amidinopropane) dihydrochloride,

2,2' -azobis [2- (2-imidazolin-2-yl) propane] dihydrochloride or the like. In addition, as a photo-polymerization initiator, there is included a benzoin derivative, a benzil derivative, an acetophenone derivative, a benzophenone derivative, an azo compound or the like. These polymerization initiators may be used alone as one kind or in combination of two or more kinds. In addition, in the case where peroxide such as persulfate is used as the polymerization initiator, for example, oxidation-reduction (redox) polymerization may be carried out using in combination of a reducing agent such as a sulfite salt, a bisulfate salt, L- ascorbic acid or the like. Used amount of the polymerization initiator in the polymerization step is preferably 0.001 to 2% by mol, and more preferably 0.01 to 0.5% by mol , relative to total amount of the monomer components. Temperature inpolymerization initiation depends on kind of the polymerization initiator to be used, however, it is preferably 15 to 130°C, and more preferably 20 to 120⁰C. The temperature in polymerization initiation outside the above range could increase residual monomers of the obtained water-absorbent resin, or lower water-absorbing performance of the water-absorbent resin, due to progress of an excessive self cross-linking reaction. The polymerization time may be set as appropriate, and is generally from 0.1 to 60 minutes.

It should be noted that, as for the unsaturated monomer component, a partially neutralized substance of acrylic acid or the like may be polymerized, or after polymerization of an acid group-containing monomer such as acrylic acid, the obtained polymer may be neutralized with an alkali compound such as sodium hydroxide or sodium carbonate. The water-absorbent resin used in the present invention is preferably one containing an acid group and having predetermined neutralization ratio (% by mol of neutralized acid groups relative to total acid groups) . As an acid group, there is included a carboxyl group, a sulfonic group, a phosphate group, orthe like . Inthis case, the neutralization ratio of the obtained water-absorbent resin is preferably 25 to 100% by mol, more preferably 50 to 90% by mol, still more preferably 50 to 75% by mol, and particularly preferably 60 to 70% by mol. By the polymerization step as above, an aqueous gel-like cross-linked polymer is obtained. Solid content (water content) of such an aqueous gel-like cross-linked polymer is determined as appropriate, based on concentration of the aqueous solution of the monomer or water vaporization in polymerization.

(2-2) Α drying/crushing/classification step In the present invention, the water-absorbent resin is obtained by a drying step where the aqueous gel-like cross-linked polymer obtained in the polymerization step is dried. In addition, particle-like one obtained by being subjected to a further crushing step or a classification step after the drying step is also included in concept of the water-absorbent resin.

The drying step is a step for drying the aqueous gel-like cross-linked polymer obtained in the polymerization step. It should be noted that "drying" in the present invention means to increase solid content by equal to or higher than 10%, or to decrease water content to equal to or lower than 25%.

Drying means is not especially limited, and there may be suitably used a conventionally known dryingmeans for using, one kind or two or more kinds of, for example, a band drier, a stirring drier, a fluidized-bed drier or the like. Water content (it is specified by reduced weight at 18O⁰C for 3 hours) of the water-absorbent resin after drying is preferably 0 to 25% by weight, more preferably 1 to 15% by weight, and still more preferably 2 to 10% by weight.

Drying temperature or drying time in the drying step is not especially limited, and dryingmaybe carried out usually at 70 to 250°C, preferably at 150 to 230°C and more preferably at 160 to 180°C. It should be noted that "drying temperature" means temperature of a heating medium in the drying step, and in the case where temperature of the heating medium can not be determined, as in the case of drying by using microwave, it is determined by temperature of the water-absorbent resin as a drying target. The drying temperature of equal to or higher than 70°C prevents the drying time from becoming longer than necessary. In addition, the drying temperature of equal to or lower than 250⁰C prevents deterioration of the water-absorbent resin in drying. The drying time may be set as appropriate, and is usually 1 minute to 5 hours, and preferably 10 minute to 2 hours.

Aiming at converting a block-like aqueous gel-like cross-linked polymer obtained in the polymerization step into a particle form, a fine pulverization step of gel for crushing the polymer may be carried out before the above drying step. The particle-formed aqueous gel is capable of progressing the above drying step smoothly, due to having enlarged surface area of the gel . Crushingmaybe carried out by various cutting means, such as, but not especially limited to, a roller-type cutter, a guillotine cutter, aslicer, a roll cutter, a shredder, scissors or the like, alone or in combination, as appropriate.

In addition, in a preferable embodiment, the drying step may be further followed by the crushing step and/or the classification step. The crushing step is a step for converting a dried substance obtained in the drying step into a particle-like form by crushing with a crusher. As a crusher to be used in the crushing step, in the type name of the crushers classified in Table 1.10 of Particle Technology Handbook

(first edition, edited by Particle Technology Association) , the crusher that is classified to shear rough crushers, impact shredders, and high speed rotary crushers, and possesses at least one of crushing mechanisms such as cutting, shearing, striking, and rubbing is preferably used. Among the crushers corresponding to these types, those having cutting and shearing as main mechanisms can be used particularly preferably. The preferable crusher includes, for example, a roll mill, a knife mill, a hammer mill, a pin mill, a jet mill, or the like, and is preferably provided with a unit for heating the inner wall surface of the crusher itself. The classification step is a step for continuously classifying particles of the dried substance obtained in the crushing step. The classification step is not especially limited, however, it is preferably carried out by sieve classification (a metal sieve made of stainless steel) . In addition, it is preferable that, in order to attain target property and particle size, a plurality of sieves are used at the same time in the classification step, and in addition, the classification step is carried out preferably once before a surface cross-linking step to be described later, more preferably twice or more times before and after the surface cross-linking step. The continuous sieve classification step is preferably carried out under heating the sieve or keeping the sieve warm.

The water-absorbent resin used in the present invention is preferably in a particle form. It is more preferable to be the powder-like water-absorbent resin containing the particles having a particle diameter in a range of 150 to 850 μm (it is specified by sieve classification) , in an amount of 90 to 100% by weight, and particularly preferably 95 to 100% by weight. The water-absorbent resin larger than 850 μm may sometimes provide poor skin touch feeling, or break a top sheet of disposable diaper, when the obtained surface cross-linked water-absorbent resin is used as a disposable diaper or the like. On the other hand, the water-absorbent resin where amount of particles smaller than 150 μm is over 10% by weight, may results in scattering of fine powder, generation of clogging in use, and lowering of water-absorbing performance of the surface cross-linkedwater-absorbent resin. Weight average particle diameter is preferably 150 to 850 μm, more preferably 200 to 600 μm, and still more preferably 300 to 500 μm. The weight average particle diameter of equal to or larger than 150 μm is preferable in view of safety and hygiene. On the other hand, the weight average particle diameter of equal to or smaller than 850 μm makes possible suitable use as a disposable diaper or the like. In addition, logarithmic standard deviation of particle size distribution (σζ) is preferably 0.23 to 0.45, and more preferably 0.25 to 0.40. It should be noted that as value of weight average particle diameter or logarithmic standard deviation of particle size distribution (σζ) , value measured by a method described below in Examples shall be adopted. It should be noted that the water-absorbent resin used in the surface treatment method for the water-absorbent resin of the present invention is not limited to one produced by the above method, and may be one prepared by other methods. In addition, the water-absorbent resin obtained by the above method is usually the non-surface cross-linked water-absorbent resin, however, the water-absorbent resin used in the present invention may be one surface cross-linked in advance, by using a polyvalent alcohol, a polyvalent epoxy compound, an alkylene carbonate, an oxazolydone compound or the like.

(3) A surface cross-linking step

The surface cross-linking step is a step for surface cross-linking the water-absorbent resin. "Surface cross-linking" means to increase cross-link density at the surface of the water-absorbent resin. Surface cross-linking can be confirmed by decrease in CRC (value after correction of water content) from the water-absorbent resin (the base polymer) before the surface cross-linking step. In this surface cross-linking step, the water-absorbent resin is surface cross-linked in the presence of the radical polymerization initiator and water. And, the present invention is characterized in that the surface cross-linking step is carried out so that water content of the water-absorbent resin after the surface cross-linking step is larger than that before the surface cross-linking step. In this way, by carrying out surface cross-linking of the water-absorbent resin, it is possible to enhance water-absorbing characteristics (in particular, absorption capacity against pressure or fluid permeability) of the obtained surface cross-linked water-absorbent resin, at low cost and by a safe method. (3-1) In the presence of the radical polymerization initiator and water

A cross-link structure at the surface of the water-absorbent resin may be introduced, for example by heating the water-absorbent resin in the presence of the radical polymerization initiator and water. It should be noted that the surface cross-linking step in the surface treatment method of the present invention is carried out without irradiation of activated energy beam having a wavelength of equal to or shorter than ultraviolet ray to a reaction system of surface cross-linking. It is because irradiation of such activated energy beam could induce deterioration of a resin. Here, "irradiation of activated energy beam having a wavelength of equal to or shorter than ultraviolet ray" means, for example, positive irradiation by using an irradiation apparatus such as a metal halide lamp, and does not include inevitable irradiation of activated energy beam contained in natural light, or light from illumination in production equipment.

In the present invention, usually, the water-absorbent resin obtained in the polymerization step is mixed with an aqueous solution containing the radical polymerization initiator andwater (hereaftermaybe referred to as "treatment solution"), to obtain a water-absorbent resin composition. Then, this water-absorbent resin composition is subjected to heat treatment. In this way, a cross-link structure is introduced at the surface of the water-absorbent resin. It should be noted that technical scope of the present invention should not be limited only to such an embodiment; and order of the addition of each component present in a reaction system, or timing of between addition of each component and heat treatment is not especially limited. For example, the radical polymerization initiator or water may be added separately to the water-absorbent resin, or they may be added to the water-absorbent resin under heat treatment.

In the case where heat treatment is carried out after mixing the water-absorbent resin with the treatment solution, "water content of the water-absorbent resin before the surface cross-linking step" means water content after mixing of the treatment solution and before the heat treatment, and "water content after the surface cross-linking step" means water content after the heat treatment. In addition, in the case where themixing of the water-absorbent resin and the treatment solution is carried out under the heat treatment, "water content of the water-absorbent resin before the surface cross-linking step" means water content before the heat treatment, and "water content after the surface cross-linking step" means water content after the heat treatment . Therefore, in industrial production equipment, water content of the water-absorbent resin entering reaction equipment for carrying out a surface cross-linking reaction corresponds to "water content before the surface cross-linking step", and water content of the water-absorbent resin coming out from reaction equipment for carrying out the surface cross-linking reaction corresponds to "water content after the surface cross-linking step".

Conventionally it has been general that surface cross-linking of the water-absorbent resin is carried out by formulating a surface cross-linking agent. Formulation of the surface cross-linking agent provides firm chemical bonds between functional groups present at the surface of the water-absorbent resin and the surface cross-linking agent, and thereby a stable surface cross-link structure can be introduced at the resin surface. In addition, by selecting a chain length of the surface cross-linking agent as appropriate, adjustment of distance between surface cross-links becomes easy, andby adjustment of the formulation amount, cross-link density can be controlled. However, in the present invention, by treatment of the water-absorbent resin in the presence of the radical polymerization initiator and water, even without formulation of such a surface cross-linking agent, a cross-link structure can be introduced at the surface of the water-absorbent resin.

In the present invention, the surface cross-linking step of the water-absorbent resin is carried out in the presence of the radical polymerization initiator. The radical polymerization initiator used in the surface cross-linking step of the water-absorbent resin is not especially limited, and a conventional knowledge canbe referred to as appropriate . As an example of the radical polymerization initiator, there is included, for example, a water-soluble radical polymerization initiator and a thermal decomposition-type radical polymerization initiator, however, it is not limited thereto . It shouldbe noted that hereafter "the water-soluble radical polymerization initiator and the thermal decomposition-type radical polymerization initiator" may also be meant collectively and simply as "the radical polymerization initiator". In addition, the water-soluble radical polymerization initiator and the thermally decomposable radical polymerization initiator are duplicated partially. In the surface cross-linking step, presence of "the water-soluble radical polymerization initiator" along with the water-absorbent resin, is capable of uniformly dispersing the polymerization initiator at the surface of the water-absorbent resin having excellent hydrophilic nature and water-absorbing property. In this way, the water-absorbent resin excellent in water-absorbing characteristics can be produced.

"The water-soluble radical polymerization initiator" means one soluble into water (25°C) in an amount of equal to or more than 1% by weight, preferably equal to or more than 5% by weight, and more preferably equal to or more than 10% by weight. Specifically, there is included a persulfate such as ammonium persulfate, sodium persulfate, potassium persulfate or the like; hydrogen peroxide; an azo compound such as 2, 2 ' -azobis (2-amidinopropane) dihydrochloride, 2,2' -azobis [2-2 (-imidazolin-2-yl) propane] dihydrochloride or the like. Among these, in particular, use of the persulfate or the soluble azo compound is preferable in respect of providing enhancement capability in any of the water-absorbing characteristics such as absorption capacity against pressure (herein, it may also be referred to simply as "absorption capacity against pressure") to physiological saline, fluid permeability of the surface treated water-absorbent resin.

On the other hand, the thermal decomposition-type radical polymerization initiator is referred to as a compound which generates a radical by heating. In a preferred embodiment, the thermal decomposition-type radical polymerization initiator has 10 hour half-life decomposition temperature of 0 to 120 ⁰C, more preferably f 20 to 100 ⁰C particularly preferably 40 to 80 ⁰C. The 10 hour half-life decomposition temperature of equal to or higher than 0 °C is capable of providing stable storage, and the 10 hour half-life decomposition temperature of equal to or lower than 120 °C is capable of securing sufficient reactivity.

In the surface cross-linking step, when surface cross-linking of the water-absorbent resin is carried out in a state of presence of "the thermal decomposition-type radical polymerization initiator" along with the water-absorbent resin, surface cross-link can be introduced at low temperature and in a short period of time, resulting in attainment of the water-absorbent resin having high gel strength and excellent water-absorbing characteristics. It should be noted that the thermal decomposition-type radical polymerization initiator may be any of an oil-soluble type or a water-soluble type. The oil-soluble type radical polymerization initiator has features that decomposition rate is less labile to pH or ionic strength as compared with the water-soluble type radical polymerization initiator. However, because the water-absorbent resin is hydrophilic, in consideration of permeability to the water-absorbent resin, it is more preferable to use the water-soluble type photo-polymerization initiator.

The thermal decomposition-type radical polymerization initiator is relatively inexpensive, and production process and production equipment can be simplified, because the strict light-shielding is not necessarily required, as compared with a compound commercially available as a photo-polymerization initiator. As typical examples of the thermal decomposition-type radical polymerization initiator, there is included a persulfate salt such as sodium persulfate, ammonium persulfate, potassium persulfate; a percarbonate salt such as sodium percabonate or the like; peracetatic acid; a peracetate salt such as sodium peracetate or the like; hydrogen peroxide; and an azo compound such as 2,2' -azobis (2-amidinopropane) dihydrochloride, 2, 2 ' -azobis [2- (2-imidazolin-2-yl) propane] dihydrochloride, 2, 2 ' -azobis (2-methylpropionitrile) or the like. Among these, use of a persulfate salt such as sodium persulfate, ammonium persulfate, potassium persulfate; and an azo compound such as 2, 2 ' -azobis (2-amidinopropane) dihydrochloride, 2, 2 ' -azobis [2- (2-imidazolin-2-yl) propane] dihydrochloride, 2, 2 ' -azobis (2-methylpropionitrile) , which have a 10 hour half-life decomposition temperature of 40 to 80°C, are preferable. Amongthese, inparticular, use of the persulfate salt is preferable, in respect of being capable of providing excellent absorption capacity against pressure and fluid permeability of the obtained surface treated water-absorbent resin. It shouldbe noted that, in the present invention, "the radical polymerization initiator" is not limited to the above embodiments .

Amount of the radical polymerization initiator, contained in the water-absorbent resin in the surface cross-linking step, or mixed with the water-absorbent resin before the surface cross-linking step, is preferably 0.01 to 20% by weight, more preferably 0.05 to 10% by weight, still more preferably 0.1 to 10% by weight, and particularly preferably 0.2 to 5% by weight, relative to amount (100% by weight as converted to solid content) of the water-absorbent resin. The amount of the radical polymerization initiator present of equal to or more than 0.01% by weight is capable of introducing a cross-link structure effectively at the surface of the water-absorbent resin; on the other hand, the amount of the radical polymerization initiator present of equal to or lower than 20% by weight is capable of suppressing reduction of water-absorbing characteristics of the obtained surface treated water-absorbent resin.

As an embodiment of presence of the radical polymerization initiator alongwith the water-absorbent resin in the surface cross-linking step, there may be an embodiment of mixing the radical polymerization initiator directly with the water-absorbent resin, however, there may be preferably adopted an embodiment of mixing the radical polymerization initiator in an aqueous solution state dissolved in water with the water-absorbent resin. Because the water-absorbent resin has water-absorbing characteristics, by mixing the radical polymerization initiator in a form of an aqueous solution, it is possible to uniformly disperse the radical polymerization initiator at the surface of the water-absorbent resin, and uniformly mix with the water-absorbent resin. It should be noted that the aqueous solution used for adding the radical polymerization initiator may contain, in addition of water, other solvents or other components (for example, a radically polymerizable compound or a mixed co-agent or the like to be described later) in a range which does not impair solubility of the radical polymerization initiator. Then, in the surface cross-linking step of the present invention, presence of water along with the water-absorbent resin in a reaction system is essential. According to the above embodiment where the radical polymerization initiator is mixed with the water-absorbent resin in a form of the aqueous solution, not only the radical polymerization initiator but also water can be added at the same time to the reaction system, therefore, operation required in surface cross-linking can be simplified.

The water content of the water-absorbent resin obtained before the surface cross-linking treatment in the surface cross-linking step (in the case where heat treatment is carried out after mixing the water-absorbent resin and the treatment solution, it means water content after mixing the treatment solution and before heat treatment) is preferably 1 to 60% by weight, more preferably 5 to 50% by weight, still more preferably 7 to 45% by weight, and particularly preferably 10 to 40% by weight, relative to amount (100% by weight as converted to solid content) of the water-absorbent resin. The water content of the water-absorbent resin of equal to or higher than 1% by weight is capable of introducing a cross-link structure efficiently at the surface of the water-absorbent resin, and is capable of shortening time of heat treatment to be required in order to enhance absorption capacity against pressure or fluidpermeability up to a desired level. On the other hand, the water content of the water-absorbent resin of equal to or lower than 60% by weight is capable of preventing excessive increase in energy to be required in the drying step after the surface cross-linking step. As described above, in the case where the radical polymerization initiator is mixed in a form of the aqueous solution with the water-absorbent resin, concentration of the aqueous solution may be enough to be adjusted so that water content of the water-absorbent resin after the addition is value within the above range. The amount of water added here is about 1 to 50% by weight, preferably 2 to 30% by weight, more preferably 3 to 20% by weight and still more preferably 5 to 15% by weight, relative to amount of the water-absorbent resin (100% by weight as converted to solid content) . It should be note that a mixing pattern of water with the water-absorbent resin is not necessarily limited to the case where it is mixed in a form of the aqueous solution containing the radical polymerization initiator. Water may be added separately after mixing the radical polymerization initiator with the water-absorbent resin. In addition, the addition of water may be substituted by drying the aqueous gel-like cross-linked polymer obtained in the polymerization step as appropriate and adjusting water content in the polymer to about 10 to 60% by weight.

As described above, in the surface cross-linking step of the present invention, use of the surface cross-linking agent, which has been essential conventionally, is not essential. However, depending on a case, the surface cross-linking of the water-absorbent resin may be carried out using the surface cross-linking agent. In other words, the surface cross-linking treatment (heat treatment) of the water-absorbent resin may be carried out in the presence of the surface cross-linking agent. According to such an embodiment, there is an advantage that a cross-link structure can be introduced efficiently and in a short period of time, at the surface of the water-absorbent resin.

The surface cross-linking agent to be used is not especially limited, and a conventionally known knowledge can be referred to as appropriate. As an example of the surface cross-linking agent, there is included, for example, a polyhydric alcoholic compound, an epoxy compound, a polyvalent amine compound, or a condensate thereof with a halo-epoxy compound, an oxazolin compound, a mono-, di-, or polyoxazolidinone compound, a multivalent metal salt, an alkylene carbonate compound or the like . Specifically, these compounds are exemplified in ϋ. S. Patent No. 6,228,930, U.S. Patent No.6, 071, 976, U . S . Patent No.6,254,990. For example, there is included a polyhydric alcoholic compound such as mono-, di-, tri-, tetra-, or polyethylene glycol, 1, 2-propylene glycol, 1, 3-propanediol, dipropylene glycol, 2, 3, 4-trimethyl-l, 3-pentanediol, polypropyrene glycol, glycerin, polyglycerin, 2-butene-l, 4-diol, 1, 4-butanediol, 1, 3-butanediol, 1, 5-pentanediol, 1, 6-hexanediol, or 1 , 2-cyclohexanedimethanol ; an epoxy compound such as ethylene glycol diglycidyl ether, or glycidol; a polyvalent amine compound such as ethylene diamine, diethylene triamine, triethylene tetramine, tetraethylene pentamine, pentaethylene hexamine, polyethylene imine, or polyamide polyamine; a halo-epoxy compound such as epichlorohydrin, epibromohydrin, or α-methyl epichlorohydrin; a condensate of the above polyvalent amine compound with the above halo-epoxy compound; an oxazolidinone compound such as 2-oxazolidinone; an oxetane compound; a cyclic urea compound; a alkylene carbonate compound such as ethylene carbonate. Among these, it is preferable to use at least one kind selected from the oxetane compound, the cyclic urea compound or the polyhydric alcohol, and more preferably at least one kind selected from an oxetane compound or a polyhydric alcohol having carbon atoms of 2 to 10 is used, and still more preferably a polyhydric alcohol having carbon atoms of 3 to 8 is used.

Used amount of the surface cross-linking agent depends on compounds used or combination thereof, however, it is preferably 0.001 to 10% by weight, and more preferably 0.01 to 5% by weight, relative to amount of the water-absorbent resin (100% by weight as converted to solid content) . (3-2) A radically polymerizable compound In the above surface cross-linking step, surface cross-linking of the water-absorbent resin is carried out preferably in the presence of the radical polymerization initiator. According to such an embodiment, a cross-linked structure can be introduced still more effectively at the surface of the water-absorbent resin without essentially using a cross-linking agent.

"The radically polymerizable compound" means a compound polymerizable by radical polymerization, and specifically there may be preferably used, for example, an ethylenic unsaturated monomer (a mono-functional radically polymerizable compound) used in production of the water-absorbent resin in the column of the above ^λλwater-absorbent resin", or a compound included as an example of the internal cross-linking agent (a multi-functional radically polymerizable compound) . Therefore, detailed explanation is omitted here. It should be noted that as the radically polymerizable compound, for example, only either of the mono-functional radically polymerizable compound or the multi-functional radically polymerizable compound may be used, or both thereof may be used in combination. In addition, as the radically polymerizable compound, one kind maybe used alone or two ormore kinds maybe used in combination. In a preferable embodiment, in view of simple production, a compound used as the ethylenic unsaturated monomer and the internal cross-linking agent in producing the water-absorbent resin, is used as the radically polymerizable compound in the present step. It should be noted that used amount molar ratio between the mono-functional radically polymerizable compound and the multi-functional radically polymerizable compound used in such an embodiment may be the same as or different from molar ratio between the ethylenic unsaturated monomer and the internal cross-linking agent in producing the water-absorbent resin. It is preferable to use a little larger amount of themulti-functional radicallypolymerizable compound as compared with the composition in producing the water-absorbent resin. Used amount of the multi-functional radically polymerizable compound is preferably 0.001 to 100% by mol, more preferably 0.01 to 50% by mol, still more preferably 0.05 to 30% by mol, particularly preferably 0.1 to 20% by mol, and most preferably 0.5 to 10% by mol, relative to total amount of the mono-functional radically polymerizable compound. Such an embodiment is advantageous in view of an efficient increase of cross-linked density at the surface of the water-absorbent resin. In addition, in the present step, preferably, the radically polymerizable compound contains a compound having two or more polymerizable unsaturated groups in one molecule (for example, a compound having two or more polymerizable unsaturated groups, among the above internal cross-linking agents) . According to such an embodiment, it is possible to still more enhance absorption capacity against pressure or fluid permeability. Preferably, the mono-functional radically polymerizable compound has an acid group and predetermined neutralization ratio (% by mol of neutralized acid groups relative to total acid groups) . The mono-functional radically polymerizable compound having an acid group attains excellent water-absorbing characteristics. As an acid group, there is included a carboxyl group, a sulfonic group, a phosphate group, or the like. In a preferred embodiment, the acid group-containing mono-functional radically polymerizable compound is the acid group-containing monomer among the monomer included above as an example of the ethylenic unsaturated monomer. Specifically, there is included (meth) acrylic acid, 2- (meth) acryloyl ethane sulfonic acid, 2- (meth) acryloyl propane sulfonic acid, 2- (meth) acrylamide-2-methyl propane sulfonic acid, vinyl sulfonic acid, or styrene sulfonic acid, or a salt thereof. Among them, in view of water-absorbing performance of the water-absorbing, (meth) acrylic acid or 2- (meth) acrylamide-2-methylpropane sulfonic acid is more preferably used, and acrylic acid is still more preferably These acid group-containing mono-functional radically polymerizable compounds may be used either alone as one kind, or in combination of two or more kinds.

In the case where the acid group-containing mono-functional radically polymerizable compound is neutralized (in a salt form) , the acid group-containing mono-functional radically polymerizable compound is preferably a monovalent salt selected from an alkali metal salt, an ammonium salt, and an amine salt. Among them, the alkali metal salt is more preferable, and a salt selected from a sodium salt, a lithium salt, and a potassium salt is particularly preferable. Additionally, in the case where the ethylenic unsaturated monomer is used as the radically polymerizable compound, neutralization ratio of the monomer may be the same as or different from neutralization ratio of the water-absorbent resin as the base polymer. It is preferable that the neutralization ratio of the ethylenic unsaturated monomer as the radically polymerizable compound is relatively small. According to such an embodiment, surface treatment can be progressed in a short period of time. Specifically, the neutralization ratio of the ethylenic unsaturated monomer as the radically polymerizable compound is preferably 0 to 90% by mol, more preferably 0 to 70% by mol, still more preferably 0 to 60% by mol, particularly preferably 0 to 50% by mol, most preferably 0 to 20% by mol, and most particularly preferably 0 to 10% by mol. Such a neutralization ratio is advantageous in view of enhancement in surface treatment rate . In the case where two or more kinds of acid group-containing radically polymerizable compounds are used, "total acid groups of acid group-containing radically polymerizable compound" means total acid groups present in all the acid group-containing radically polymerizable compounds, and "neutralized acid group" means total neutralized acid groups present in all the acid group-containing radically polymerizable compounds. For example, when as the acid group-containing radically polymerizable compound, an acrylic acid and a sodium acrylate are used at a molar ratio of the acrylic acid to the sodium acrylate of 1:1, the neutralization ratio of the acid group-containing radically polymerizable compound is 50% by mol.

In the case where surface cross-linking of the water-absorbent resin is carried out in the presence of the radically polymerizable compound, amount of the radically polymerizable compound present in a reaction system of the surface cross-linking is preferably 0.5 to 50% by weight, more preferably 1 to 30% by weight, further preferably 1.5 to 20% by weight, further more preferably 2 to 15% by weight, particularly preferably 2.5 to 10% by weight, and most preferably 3 to 7% by weight, relative to amount (100% by weight as converted to solid content) of the water-absorbent resin. The amount of the radically polymerizable compound of equal to or higher than 0.5% by weight is capable of maintaining absorption capacity against pressure of the water-absorbent resin at sufficient value . On the other hand, the amount of the radically polymerizable compound of equal to or lower than 50% by weight is capable of suppressing reduction of absorption capacity of the surface treated water-absorbent resin. (3-3) A mixed co-agent

As described above, in the surface cross-linking step of the present invention, the radical polymerization initiator and water (along with, if necessary, the surface cross-linking agent or the radically polymerizable compound) are present with the water-absorbent resin, however, in a reaction system of the surface cross-linking, it is preferable that a mixed co-agent is further present in the reaction system in order to enhance mixingproperty of these substances present in the reaction system. "The mixed co-agent" is a water-soluble or water-dispersible compound other than the radical polymerization initiator and the radically polymerizable compound, and it is not especially limited, as long as it is capable of suppressing agglomeration of the water-absorbent resin with water and enhancing mixing property of the aqueous solution with the water-absorbent resin. By the addition of the mixed co-agent, agglomeration of the water-absorbent resin with water can be suppressed and the aqueous solution and the water-absorbent resin can be mixed uniformly. As a result, surface cross-linking treatment such as heat treatment to be described later, is uniformly carried out to the water-absorbent resin, and it becomes possible to carry out uniform surface cross-linking throughout the water-absorbent resin.

As a specific example of the mixed co-agent, there may be included a surfactant, a water-soluble polymer, a hydrophilic organic solvent, a water-soluble inorganic compound, an inorganic acid (salt) , and an organic acid (salt) .

As a surfactant, there is included at least one kind or two or more kinds of a surfactant selected from the group consisting of a nonionic surfactant and an anionic surfactant having an HLB of equal to or higher than 7. For example, there may be exemplified a sorbitan aliphatic ester, a polyoxyethylene sorbitan aliphatic ester, a polyglycerin aliphatic ester, a polyoxyethylene alkyl ether, a polyoxyethylene alkylphenol ether, a polyoxyethylene acyl ester, a sucrose aliphatic ester, a higher alcohol sulfate ester salt, an alkyl naphthalene sulfonate salt, an alkylpolyoxyethylene sulfate salt, a dialkyl sulfosuccinate salt or the like. Among these surfactants, polyoxyethylene alkyl ether may be preferably used. Number average molecular weight of the polyoxyethylene alkyl ether is preferably 200 to 100,000, and more preferably 500 to 10,000. When polyoxyethylene alkyl ether to be used as the mixed co-agent, has the number average molecular weight of equal to or higher than 200, an effect as the mixed co-agent can be obtained effectively. On the other hand, the number average molecular weight of equal to or lower than 100, 000 is capable of ensuring sufficient solubility to water and suppressing increase in viscosity of a solution as a reaction system, resulting in ensuring sufficient mixing property of the reaction system including the water-absorbent resin.

As the water-soluble polymer, there is included, for example, polyvinyl alcohol, polyethylne oxide, polyethylene glycol, polypropylene glycol, polyacrylamide, polyacrylic acid, sodium polyacrylate, polyethylene imine, methyl cellulose, carboxymethyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, dextrin, sodium alginate, starch or the like. Among these polymers, polyethylene glycol is preferable. Number average molecular weight of these water-soluble polymers, is preferably 200 to 100,000, and more preferably 500 to 10,000.

As the hydrophilic organic solvent, there is included alcohols such as methyl alcohol, ethyl alcohol, propyl alcohol, isopropyl alcohol, butyl alcohol, isobutyl alcohol, or t-butyl alcohol; ketones such as acetone, or methylethyl ketone/ ethers such as dioxane, alkoxy (poly) ethylene glycol, or tetrahydrofuran; amides such as e-caprolactam, or N,N-dimethyl formamide; sulfoxides such as dimethyl sulfoxide; polyhydric alcohols such as ethylene glycol, diethylene glycol, propylene glycol, triethylene glycol, tetraethylene glycol, 1, 3-propane diol, dipropylene glycol, 2, 2, 4-trimethyl-l, 3-pentane diol, glycerin, 2-butene-l, 4-diol, 1,3-butane diol, 1,4-butane diol, 1, 5-pentane diol, 1,6-hexane diol, 1, 2-cyclohexane dimethanol, 1, 2-cyclohexanol, trimethylol propane, diethanol amine, triethanol amine, polyoxypropylene, pentaerythritol, or sorbitol; or the like.

As the water-soluble inorganic compound, there is included an alkali metal salt such as sodium chloride, sodium hydrogen sulfate, or sodium sulfate; an ammonium salt such as ammonium chloride, ammonium hydrogen sulfate, or ammonium sulfate; an alkali metal hydroxide such as sodium hydroxide, or potassium hydroxide; a polyvalent metal salt such as aluminum chloride, polyaluminium chloride, aluminum sulfate, potassium alum, calcium chloride, alkoxy titanium, zirconium ammonium carbonate, or zirconium acetate; a non-reducible alkali metal salt pH buffer agent such as hydrogencarbonate, dihydrogen phosphate, ormonohydrogen phosphate; or the like. As the inorganic acid (salt) , there is included hydrochloric acid, sulfuric acid, phosphoric acid, carbonic acid, boric acid, and a salt thereof (for example, an alkali metal salt, or an alkali earth metal salt) . In addition, as the organic acid (salt), there is exemplified acetic acid, propionic acid, lactic acid, citric acid, succinic acid, malic acid, tartaric acid, and a salt thereof (for example, an alkali metal salt, and an alkali earth metal salt) .

Among the above exemplifications, a polyoxyethylene alkyl ether, polyethylene glycol, a water-soluble polyvalent meta salt, sodium chloride, ammonium hydrogen sulfate, ammonium sulfate, sulfuric acid, and hydrochloric acid is preferably used as the mixed co-agent.

These mixed co-agents may be used alone as only one kind or in a mixture form of two or more kinds . In addition, amount of the mixed co-agent present in the reaction system of surface cross-linking is not especially limited, as long as it suppresses aggregation of the water-absorbent resin with water, and enhances mixing property of the aqueous solution with the water-absorbent resin, however, for example, it is preferably 0.0001 to 40% by weight, more preferably 0.001 to 10% byweight, particularlypreferably 0.005 to 5% byweight, and most preferably 0.01 to 1% by weight, relative to amount

(100% by weight as converted to solid content) of the water-absorbent resin.

Order of the addition of the mixed co-agent, when it is used, is not also especially limited, and there may be used a method for adding the mixed co-agent to the water-absorbent resin in advance and then adding and mixing thereto water or the radical polymerization initiator (the aqueous solution containing these, depending on the case) ; and a method for dissolving the mixed co-agent in the aqueous solution, and simultaneously mixing the obtained solution with the water-absorbent resin. (3-4) Mixing

Mixing condition, in the case where the above each component (the water-absorbent resin, radical polymerization initiator, water, and if necessary, the surface cross-linking agent, the radically polymerizable compound, the mixed co-agent) is mixed in advance, is not especially limited. For example, mixing temperature is preferably 0 to 150 °C, more preferably 10 to 120⁰C, still more preferably 20 to 100°C, particularly preferably 30 to 90°C, and most preferably 40 to 70⁰C. Mixing time is usually 0.1 to 60 minutes, but is not especially limited thereto. In addition, mixing means is also not especially limited, and there may be used, an ordinary mixer, for example, a V-shape mixer, a ribbon-type mixer, a screw-type mixer, a rotation circular plate-type mixer, an air-flow-type mixer, a batch-type kneader, a continuous-type kneader, a paddle-type mixer, a plow-type mixer or the like, as the mixing means. (3-5) Heat treatment

Subsequently, the water-absorbent resin is subjected to surface cross-linking. This surface cross-linking step is carried out so that water content of the water-absorbent resin after the surface cross-linking step is larger than that before the surface cross-linking step. By carrying out surface cross-linking of the water-absorbent resin in this way, it becomes possible to produce the surface treated water-absorbent resin with excellent water-absorbing characteristics (in particular, absorption capacity against pressure or fluid permeability) at low temperature and in a short period of time, at low cost and by a safe method.

To introduce a cross-link structure at the surface of the water-absorbent resin, for example, it is enough to heat the reaction system of the surface cross-linking containing the above each component . Explanation will be given in detail below on the surface cross-linking treatment, however, the present invention should not be limited to these specific embodiments .

In order to surface cross-link the water-absorbent resin by heat treatment, it is enough to heat the reaction system containing the above each component, and preferably to heat under relatively high humidity atmosphere. Specific condition of such atmosphere or specific condition of the heat treatment is not especially limited, and it may be determined as appropriated, so that water content of the water-absorbent resin after the surface cross-linking step is larger than that before the surface cross-linking step, in other words, water content of the water-absorbent resin increases as compared with before the surface cross-linking treatment.

As an example of atmosphere condition in carrying out the heat treatment, temperature of the atmosphere is preferably 50 to 150⁰C, more preferably 70 to 130°C, still more preferably 90 to 110⁰C, and particularly preferably 95 to 100⁰C. In addition, pressure of the atmosphere may be any of reduced pressure, normal pressure, and under pressure, and not especially limited, however, it is preferably 1013 to 4906 hPa, more preferably 1013 to 2758 hPa, and still more preferably 1013 to 1450 hPa. Further, relative humidity of the atmosphere is preferably 50 to 100% RH, more preferably 70 to 100% RH, still more preferably 90 to 100% RH, particularly preferably 95 to 100%RH, andmost preferably 100% RH (saturated steam) . By using saturated steam as the atmosphere in heat treatment, it becomes possible to surely increase water content of the water-absorbent resin obtained by the surface cross-linking step, as compared with before the surface cross-linking, In addition, oxygen concentration in the atmosphere is preferably 0 to 25% by volume, more preferably 0 to 15% by volume, still more preferably 0 to 10% by volume, still further preferably 0 to 5% by volume, particularly preferably 0 to 1% by volume, and most preferably 0 to 0.5% by volume. When the oxygen concentration in the atmosphere is adjusted to relatively low concentration in this way, it is capable of preventing oxidative degradation of the water-absorbent resin in heating, and thus preferable.

Heating time in carrying out the heat treatment is also not especially limited, however, it is preferably 1 to 90 minutes, more preferably 2 to βOminutes, still more preferably 3 to 30 minutes, and still further preferably 5 to 15 minutes. The heating time of equal to or longer than 1 minute is capable of introducing a cross-link structure at the surface of the water-absorbent resin; on the other hand, the heating time of equal to or shorter than 90 minute is capable of preventing degradation of the water-absorbent resin caused by heating.

In the case where the water-absorbent resin is surface cross-linked by heat treatment, apparatus used in heat treatment, is not especially limited, and a known drier may be used, however, one equipped with an air supply apparatus for supplying air containing steam is preferably used. For example, there is preferably used a dryer, equipped with the air supply apparatus, of a heat conduction type, a radiation heat conduction type, ahot air conduction type, or a dielectric heating type . Specifically, there is preferably used a dryer of a belt type, a thin-type stirring type, a fluidized-bed type, an air flow type, a rotation type, a mixing type, an infrared ray type, or an electron beam type. (3-6) A fluid permeability enhancing agent

In addition, a fluid permeability enhancing agent may be added into the surface cross-linked water-absorbent resin after the above surface cross-linking treatment. As an example of such a fluid permeability enhancing agent, there is included minerals such as talc, kaolin, fuller's earth, bentonite, activated clay, cawk, natural asphaltum, strontium ore, ilmenite, pearlite; aluminum compounds such as aluminum sulfates 14 - 18 hydrates (or anhydrides) , potassium aluminum sulfates 12 hydrate, sodium aluminum sulfate 12 hydrate, aluminum chloride, polyaluminum chloride, aluminum oxide, and zirconium compounds such as zirconium sulfate, zirconium nitrate, zirconium acetate, zirconium carbonate, ammonium zirconium acetate, ammonium zirconium carbonate, and an aqueous solution thereof; other polyvalent metal salts; hydrophilic amorphous silicas (for example, the product of the dry method made by Tokuyama K. K. and sold under the trademark designation of " Reolosil QS-20" and the products of the precipitation method made by DEGUSSA Corp. and sold under the trademark designation of "Sipernat 22S and Sipernat 2200"); oxide composites such as a silicon oxide'aluminum oxide*magnesium oxide composite (for example, the product of ENGELHARD Corp. sold under the trademark designation of "Attagel #50) , a silicon oxide"aluminum oxide composite, a silicon oxide-magnesium oxide composite; or the like. Such a fluid permeability enhancing agent is mixed in an amount of preferably 0 to 20% by weight, more preferably 0.01 to 10% by weight, and particularly preferably 0.1 to 5% by weight, relative to amount (100% by weight as converted to solid content) of the surface cross-linked water-absorbent resin. The fluid permeability enhancing agent may be added in a form of an aqueous solution when it is soluble in water or in a form of powder or slurry when it is insoluble in water. Other additives such as an antibacterial agent, a deodorant, or a chelating agent may be added, as appropriate, in an amount within the above range. (4) The surface treated water-absorbent resin

In the present invention, by applying the above treatment to the water-absorbent resin, the surface treated water-absorbent resin can be produced. The obtained water-absorbent resin obtained in this way has excellent water-absorbing characteristics such as absorption capacity against pressure or fluid permeability.

In the present invention, water content of the water-absorbent resin after the surface cross-linking step becomes larger than that before the surface cross-linking step, however, the water content of the surface treated water-absorbent resin obtained after the surface cross-linking step is preferably 1 to 50% by weight, more preferably 2 to 45% by weight, still more preferably 3 to 40% by weight, and particularly preferably 5 to 30% by weight . In addition, amount of increase in the water content after the surface cross-linking step compared to that before the surface cross-linking step is not especially limited, however, as absolute value of water content value (= "the water content after the surface cross-linking step" - "the water content before the surface cross-linking step") , it increases preferably by 0.1 to 40% by weight, more preferably by 0.5 to 30% by weight, still more preferably by 1 to 20% by weight, and particularly preferably by 2 to 15% by weight. When the water content or amount of the increase in the water content of the surface treated water-absorbent resin is equal to or higher than the lower limit value of the above range, effect of the present invention is sufficiently fulfilled, and the surface treated water-absorbent resin excellent in water-absorbing performance such as absorption capacity against pressure or fluid permeability can be obtained. On the other hand, when this value is equal to or lower than the upper limit value of the above range, reduction of water-absorbing characteristics with the increase in water content can be prevented.

It has been hitherto known that formation of surface cross-link results in slightly lowering of centrifuge retention capacity (CRC) but enhances capability to retain absorbed fluid even in a pressurized state, namely enhances absorption capacity against pressure. According to a method of the present invention, absorption capacity against pressure (hereafter may be also referred to as simply "AAP") under 4.83 kPa of the water-absorbent resin is increased by equal to or more than 1 g/g, without using a surface cross-linking agent as an essential component. This shows that according to a method of the present invention, a cross-link structure is introduced at the surface of the water-absorbent resin. In a preferable embodiment, absorption capacity against pressure of the water-absorbent resin after the surface cross-linking step is increased compared to that before the surface cross-linking step, by preferably equal to or more than 8 g/g, more preferably equal to or more than 10 g/g, and still more preferably equal to or more than 12 g/g. In addition, AAP value of the obtained surface treated water-absorbent resin is preferably 15 to 30 g/g, more preferably 18 to 25 g/g, and still more preferably 20 to 24 g/g. It should be noted that as AAP value, value (value after correction of water content) measured by a method described in examples to be described later is adopted.

In addition, centrifuge retention capacity (CRC) of the surface treated water-absorbent resin is preferably equal to or higher than 8 g/g, more preferably equal to or higher than 15 g/g, still more preferably equal to or higher than 20 g/g, and particularly preferably equal to or higher than 25 g/g. The upper limit is not especially limited, however, it is preferably equal to or lower than 50 g/g, more preferably equal to or lower than 40 g/g, and still more preferably equal to or lower than 35 g/g. The CRC of equal to or higher than 8 g/g provides the water-absorbent resin sufficiently applicable to hygienic materials such as a disposable diaper. On the other hand, the CRC of equal to or lower than 50 g/g provides the water-absorbent resin ensuring sufficient gel strength and excellent in fluid permeability. It should be noted that as CRC value, value (value after correction of water content) measured by a method described in examples to be described later is adopted.

Furthermore, saline flow conductivity (SFC) , which is an index of fluid permeability in the obtained surface treated water-absorbent resin, is preferably equal to or higher than 10 (unit: 10^"7XCm³XSXg^"1), more preferably equal to or higher than 15 (unit: 10^"7XCm³XSXg^"1) , still more preferably equal to or higher than 20 (unit: 10^"7XCm³XSXg^"1), and particularly preferably equal to or higher than 50 (unit: 10^"7XCm³XSXg^"1) . The upper limit is not especially limited, however, it is preferably equal to or lower than 300 (unit: 10^"7XCm³XSXg^"1) . The case of the saline flow conductivity (SFC) of equal to or higher than 10 (unit: 10^"7XCm³XSXg^"1) results in sufficient fluid permeability and can permeate fluid sufficiently into an absorbing body, and surely absorb excretion fluid such as urine or the like in usage. It should be noted that as value of SFC, value measured by a method described in examples to be described later is adopted. A shape of the surface treated water-absorbent resin obtained by the present invention may be adjusted, as appropriate, depending on treatment condition such as a shape of the water-absorbent resin before treatment, granulation/formation after treatment, or the like, however, it is in general a powder form. Such powders have a weight average particle diameter (specifiedby sieve classification) of 150 to 850 μm, preferably 200 to 600 μm, and more preferably 300 to 500 μm; and content of particles with the particle diameter of 150 to 850 μm is preferably 90 to 100% by weight, and still more preferably 95 to 100% by weight, relative to total amount of the water-absorbent resin. In addition, logarithmic standard deviation (σζ) of particle size distribution is preferably 0.23 to 0.45, and more preferably 0.25 to 0.35.

The surface treated water-absorbent resin obtained by the method of the present invention is formed with a surface cross-link having higher crosslink density at the vicinity of the surface as compared with the inside, and preferably uniform and high crosslink density throughout the whole surface of the water-absorbent resin, and thus is capable of improving characteristics to be desired in the water-absorbent resin, for example, properties such as absorption capacity, absorption speed, gel strength, suction force, and fluid permeability to extremely high level.

The surface treated water-absorbent resin obtained by the method of the present invention, because of having excellent characteristics as described above, starting with hygienic materials field such as sanitary goods, a disposable diaper, or an incontinence pad, is suitably used in various fields including a medical field such as medical supplies; an agriculture field such as a soil water retention agent; foods field such as freshness retention; an industrial field such as a due condensation prevention material and cold insulation material, and depending on object or function, other additives such as silica, zeolite, an antioxidant, a surfactant, silicone oil, a chelating agent, a deodorant, perfume, a drug, a plant growth co-agent, a bacteriacide, a fungicide, a foaming agent, a pigment, a dye, a fibrous substance (hydrophilic staple fiber, pulp, synthetic fibers or the like) , fertilizer, or the like may be added. The addition amount of these other additives is preferably about 0.001 to 10% by weight, relative to total weight of a product in various applications.

EXAMPLES Explanation will be given below specifically on the present invention with reference to Examples and Comparative Examples, however, technical scope of the present invention is not limited thereto. Hereinafter, the ^Xλ% by weight" may be expressed simply as "%" and the "liters" simply as "L", for the sake of simplicity. In addition, unless otherwise described, the work was carried out under condition of room temperature (23±1°C) and a humidity of 30% RH. Measurement methods and evaluation the methods indicated in the Examples and Comparative Examples will be shown below. (1) Particle size distribution: weight average particle diameter (D50) and logarithmic standard deviation (σζ) of particle distribution

A 10 g sample of water-absorbent resin was charged in JIS standard sieves having mesh openings of 850 μm, 710 μm, 600 μm, 500 μm, 425 μm, 300 μm, 212 μm, 150 μm and 45 μm (THE IIDA TESTING SIEVE: diameter of 8 cm) and classified by a vibration classifier (IIDA SIEVE SHAKER, type: ES-65, SER. No.0501) for 5 minutes. Residual % R was plotted in logarithm probability paper to read particle diameter corresponding to R=50% by weight, as weight average particle diameter (D50) . In addition, logarithmic standard deviation (σζ) is represented by the following expression, when particle diameters for R=84.1% by weight and R=15.9% by weight are expressed as Xl and X2, respectively. That is, smaller value of σζ means narrower particle size distribution. [Expression 2] σζ=0.5^χln(X2/Xl)

(2) Water content

In an aluminum cup having 4 cm in bottom diameter and 2 cm in height, a 1.00 g sample of water-absorbent resin was uniformly spread on the bottom surface of the aluminum cup to measure weight Wl (g) of the aluminum cup containing this water-absorbent resin. The sample in the cup was left standing for three hours in a drier (EYELA, constant temperature thermostat drier (Natural oven) NDO-450, manufactured by Tokyo Rikakiki Co. , Ltd.) adjusted in advance to 180⁰C. Then weight W2 (g) of the aluminum cup containing this water-absorbent resin was measured just after taking out (at least within 1 minute) from the hot air dryer. Water content

(% by weight) was calculated according to the following expression, based on these Wl and W2. [Expression 3]

Water content (% by weight) =[ (Wl (g)-W2 (g) )/ (weight of the water-absorbent resin (g) ) ] χlOO

(3) Centrifuge Retention Capacity (CRC)

A 0.200 g sample of water-absorbent resin was uniformly placed in a pouch of non-woven fabric (size: 85 mm x 60 mm; manufactured by Nangoku Pulp Kogyo K. K. and sold under the trademark of "Heatlon Paper, Model GSP-22) . Then the pouch with the sample was heat-sealed and immersed in a large excess

(about 500 ITiL) of physiological saline at room temperature.

After 30 minutes, the pouch was pulled up and drained at centrifugal force (250 G) described in "edena ARSORBENCY II 441.1-99", for 3 minutes, by the use of a centrifugal separator

(manufactured by Kokusan Co., Ltd, model: H-122) to measure weight W3 (g) of the pouch. The similar procedure was repeated without using the water-absorbent resin and the weight W4

(g) of the pouch was measured. CRC (g/g) was calculated in accordance with the following expression using W3 and W4. [Expression 4]

CRC (g/g)= [W3 (g)-W4 (g)- (weight (g) of the water-absorbent resin (g) ) ] / (weight of the water-absorbent resin (g) ) (4) Absorption capacity against pressure (AAP)

A 400-mesh wire mesh-screen of stainless steel (38 μm inmesh opening) was fused to the bottom of a plastic supporting cylinder with an inner diameter of 60 mm. Under the conditions of room temperature (25 ±2⁰C) and a humidity of 50 RH%, 0.900 g of water-absorbent resin was uniformly scattered on the wire mesh-screen, and a piston and a load with an outer diameter of slightly smaller than 60 mm, which were adjusted so as to add uniformly a load of 4.83 kPa to the water-absorbent resin, not to generate a gap among the inner wall surface of the supporting cylinder, and not to prevent a vertical motion, weremountedthereon sequentially in this order . Then, the whole weight W5 (g) of the resultant measuring apparatus was determined.

A glass filter with a diameter of 90 mm (pore diameters: from 100 to 120 μm: manufactured by Sogo Rikagaku Glass ManufactoryK. K. ) was placed inside a Petri dishwith a diameter of 150 mm, and an aqueous 0.9 % by weight sodium chloride solution (physiological saline) (20 to 25°C) was added so as to become the same level as the upper surface of the glass filter. One filter paper with a diameter 90 mm (0.26 mm in thickness and 5 μm in retained particle diameter; manufactured by Advantec Toyo K. K. and sold under the product name of "JIS

P 3801, No. 2") was mounted thereon so as to have the surface thereof thoroughly wetted, and the excess liquid was removed.

The measuring apparatus was wholly mounted on the wetted filter paper and the water-absorbent resin was allowed to absorb the solution under the load, for a predetermined time. This absorption time was set at one hour with calculating from the start of the measurement. Specifically, the whole measuring apparatus was lifted after 1 hour and the weight thereof Wβ (g) was measured. This mass measurement must be carried out as quickly as possible without giving anyvibration to the apparatus . Absorption capacity against pressure (AAP) (g/g) was calculated in accordance with the following expression using W5 and W6. [Expression 5] AAP (g/g) = [W5 (g) - W6 (g) ] / (weight of the water-absorbent resin (g) ) (5) Saline flow conductivity (SFC)

The saline flow conductivity (SFC) is a value indicating fluid permeability in a swelling state of the water-absorbent resin. The higher SFC value shows having the higher fluid permeability. It should be noted that unit of SFC value is

.

Measurement of SFC was carried out in accordance with the test for saline flow conductivity (SFC) described in U . S . Patent No.5849405.

Specifically, first, an apparatus shown in Fig. 1 was prepared. Explanation will be given on the apparatus with reference to Fig. 1: Tank 31 had a glass tube 32 inserted therein and the lower end of the glass tube 32 was set up so that an aqueous 0.69 % by weight saline 33 was maintained to a height of 5 cm from the bottom of the swollen gel 44 in a cell 41. The aqueous 0.69 % by weight saline solution in the tank 31 was supplied to the cell 41 via an L-letter tube 34. An L-letter tube 34 was fitted with a cock 35. Under the cell 41, a collection container 48 for collecting the passed fluid is set up, and this collecting container 48 was set up on an even balance 49. The cell 41 had an inner diameter of 6 cm. A 400-mesh wire mesh-screen (with a mesh opening of 38 μm) 42 made of stainless steel was placed on the bottom surface in the lower part of the cell . A piston 46 was provided in the lower part thereof with holes 47 sufficient for passing fluid and was fitted at the bottom part thereof with a glass filter 45 having good permeability capable of preventing the particles of the water-absorbent resin or the swollen gel thereof from entering the hole 47. The cell 41 was laid on a stand for mounting the cell. The surface of the stand contacting with the cell was placed on a wire mesh-screen 43 made of stainless steel which does not inhibit fluid permeation.

By using this apparatus, the water-absorbent resin (0.900 g) were uniformly placed in a container 40 and were left swelling in artificial urine under a pressure of 0.3 psi (2.07 kPa) for 60 minutes, and then height of a layer of gel 44 was recorded. It should be noted that the artificial urine mentioned above was one prepared by the addition of 0.25 g of dihydrate of calcium chloride, 2.0 g of potassium chloride, 0.50 g of hexahydrate of magnesium chloride, 2.0 g of sodium sulfate, 0.85 g of ammonium dihydrogen phosphate, 0.15 g of diammonium hydrogen phosphate and 994.25 g of purified water. Then, under a pressure of 0.3 psi (2.07 kPa) , 0.69% by weight saline 33 was passed fromthe tank 31 under constant hydrostatic pressure through a swollen gel layer. This measurement for SFC was carried out at room temperature (20to25°C). By means of a computer and a balance, amounts of fluid passing the gel layer were recorded at intervals of 20 seconds for 10 minutes, as a function of time. Flow speed Fs (T) passing through the swollen gel 44 (mainly between particles) was determined in unit of [g/s] by dividing the increased weight (g) with the increased time (s) . The time, in which the constant hydrostatic pressure and the stable flow speed were attained, was denoted by Ts. Only the data obtained between Ts and 10 minutes was used for calculation of the flow speed. The value Fs (T = 0) , namely, the initial flow speed passing through the gel layer was calculated by using the flow speed obtained between Ts and 10 minutes. The Fs (T = 0) was calculated by extrapolating, the result of the least-squares method of the Fs (T) against time, to T = 0. [Expression 6] Saline flow conductivity (SFC) = (Fs (t = 0) x LO) /(p x A xΔP) = (Fs (t = 0) x LO) /139506 wherein Fs (t = 0) : flow speed expressed in unit of g/s; LO: height of the gel layer expressed in unit of cm; p: density of the NaCl solution (1.003 g/ cm³); A: upper side area of the gel layer in the cell 41 (28.27 cm²) ; ΔP: hydrostatic pressure added on the gel layer (4920 dyne/cm²) .

(Production Example 1)

In a reactor formed by a 10 L inner volume of stainless kneader provided with two sigma-type blades, a jacket and a cover, 5433 g (24.2 mols) of an aqueous solution of sodium acrylate (monomer concentration: 39% by weight) was charged, and 12.83 g (0.0246 mols) of polyethylene glycol diacrylate (number of average ethylene oxide units: n = 9) as an internal cross-linking agent was dissolved in the aqueous solution to prepare a reaction solution. Then, this reaction solution was purged under nitrogen gas atmosphere. Subsequently, 29.43 g of an aqueous solution of 10% by weight of sodium persulfate as a polymerization initiator, and 24.53 g of an aqueous solution of 0.1% by weight of L-ascorbic acid were added to the reaction solution under stirring. As a result, polymerization was initiated after about 1 minute. Then under crushing generated gel, polymerization was carried out at 20 to 95°C, and at 30 minutes after initiation of polymerization, a aqueous gel-like cross-linked polymer was taken out. The obtained aqueous gel-like cross-linked polymer had a particle diameter of equal to or smaller than 5 mm. This crushed aqueous gel-like cross-linked polymer was spread on a 50 mesh (mesh opening of 300 μm) metal mesh screen and dried in a hot air drier set at 175⁰C for 50 minutes. In this way, irregular, easily crushable, powder-like agglomerate was obtained.

The obtained powder-like agglomerate was crushed by using a roll mill, and further classified with a JIS standard sieve having a mesh opening of 710 μm. Then, by classifying particles, which had passed through the sieve having a mesh opening of 710 μm in the above operation, by use of a JIS standard sieve having a mesh opening of 150 μm, particles passing through the sieve having a mesh opening of 150 μm were removed. In this way, a water-absorbent resin (A) was obtained. (Production Example 2)

A water-absorbent resin (B) was obtained similarly in Production Example 1, except that used amount of polyethylene glycol diacrylate (number of average ethylene oxide units: n = 9) was changed to 11.55 g (0.0221 mols) , and the mesh opening for classification was changed from 710 and 150 μm to 850 and 212 μm.

Particle size distributions of the obtained water-absorbent resins (A) and (B) are shown in Table 1 below, and various evaluation results thereof are shown in Table 2 below.

Table 1

(Example 1-1)

500 g of the water-absorbent resin (A) as a base polymer was added to 5-L Loedige mixer (Type M5R, manufactured by Loedige Co . , Ltd. ) , and a treating solutionwhere 1.Og (0.00192 mols) of polyethylene glycol diacrylate (number of average ethylene oxide units : n = 9), 27.4 g (0.381 mols) of acrylic acid, 45.0 g of water and 5.0 g of ammonium persulfate were mixed in advance, was sprayed under stirring at 300 rpm, . After continuing the mixing under stirring at room temperature for 3 min, and making the added water permeate and diffuse into the inner part of particles, stirring was terminated once, and a sample charging port of the Loedige mixer was taken off.

10 g of the water-absorbent resin (water content: 13.0% by weight) discharged from the Loedige mixer was uniformly spread on a stainless steel circular Petri dish with a diameter of 9 cm and a depth of 1.5 cm. Then, the water-absorbent resin was charged with the Petri dish in a water oven (trade name: Healthio, type: AX-HC3, manufactured by Sharp Co. , Ltd. ) , and was heated for 1 minute in steamed food (strong) mode. By the treatment in this mode, a substance to be treated is heated with saturated steam, therefore heating temperature reaches 100 ⁰C. In addition, pressure inside the water oven during the heating was normal pressure (1013 hPa) , and oxygen concentration in the water oven was equal to or lower than 0.5% by volume. It should be noted that the water oven was preheated by operation for 15 minutes in steamed food (strong) mode, in advance, just before the above operation.

The water-absorbent resin obtained by the above treatment was pulverized till it passed through a JIS standard sieve with a mesh opening of 850 μm to obtain a surface treated water-absorbent resin (1-1) . Various evaluation results of the obtained surface treated water-absorbent resin (1-1) are shown in Table 2 below. It should be noted that amount of each component in the column of "Treatment solution" in Table 2 indicates % by weight (wt %) relative to amount (500 g) of the water-absorbent resin (A) as a base polymer. In addition, "CRC after correction of water content" and "AAP after correction of water content" shown in Table 2 below were calculated in accordance with calculation equations shown below. Here, in the following expressions, "CRC before correction of water content" indicates centrifuge retention capacity (CRC) of the water-absorbent resin before measurement of water content of the above (2) , and also "AAP before correction of water content" indicates absorption capacity against pressure (AAP) of the water-absorbent resin before measurement of water content of the above (2) . [Expression 7] CRC after correction of water content (g/g)= [((CRC before correction of water content (g/g) ) +1) / (100- (water content of the water-absorbent resin) ) ] ^χlOO-1 [Expression 8]

AAP after correction of water content (g/g)= (AAP before correction of water content (g/g))/ [100- (water content of the water-absorbent resin) ]^χlOO (Example 1-2)

A surface treated water-absorbent resin (1-2) was obtained by a similar method to in the above Example 1-1, except that heating time of the water-absorbent resin in the water oven was changed to 3 minutes . Various evaluation results of the obtained surface treated water-absorbent resin (1-2) are shown in Table 2 below. (Example 1-3) A surface treated water-absorbent resin (1-3) was obtained by a similar method to in the above Example 1-1, except that heating time of the water-absorbent resin in the water oven was changed to 5 minutes . Various evaluation results of the obtained surface treated water-absorbent resin (1-3) are shown in Table 2 below. (Example 1-4) A surface treated water-absorbent resin (1-4) was obtained by a similar method to in the above Example 1-1, except that heating time of the water-absorbent resin in the water oven was changed to 10 minutes. Various evaluation results of the obtained surface treated water-absorbent resin (1-4) are shown in Table 2 below. (Example 1-5)

A surface treated water-absorbent resin (1-5) was obtained by a similar method to in the above Example 1-1, except that heating time of the water-absorbent resin in the water oven was changed to 15 minutes. Various evaluation results of the obtained surface treated water-absorbent resin (1-5) are shown in Table 2 below. (Example 2-1)

A surface treated water-absorbent resin (2-1) was obtained by a similar method to in the above Example 1-2, except that a solution where 1.0 g (0.00192 mols) of polyethylene glycol diacrylate (number of average ethylene oxide units : n= 9), 4.Og (0.056 mols) of acrylic acid, 10.0 g (0.106 mols) of sodium acrylate, 45.0 g of water and 5.0 g of ammonium persulfate were mixed in advance, was used as a treatment solution. It should be noted that water content of the water-absorbent resin discharged from the Loedige mixer, after mixing of the base polymer and the treatment solution under stirring, was 13.3% by weight. Various evaluation results of the obtained surface treated water-absorbent resin (2-1) are shown in Table 2 below. (Example 2-2) A surface treated water-absorbent resin (2-2) was obtained by a similar method to in the above Example 2-1, except that heating time of the water-absorbent resin in the water oven was changed to 5 minutes . Various evaluation results of the obtained surface treated water-absorbent resin (2-2) are shown in Table 2 below. (Example 2-3)

A surface treated water-absorbent resin (2-3) was obtained by a similar method to in the above Example 2-1, except that heating time of the water-absorbent resin in the water oven was changed to 10 minutes. Various evaluation results of the obtained surface treated water-absorbent resin (2-3) are shown in Table 2 below. (Example 2-4) A surface treated water-absorbent resin (2-4) was obtained by a similar method to in the above Example 2-1, except that heating time of the water-absorbent resin in the water oven was changed to 15 minutes. Various evaluation results of the obtained surface treated water-absorbent resin (2-4) are shown in Table 2 below. (Example 3-1)

A surface treated water-absorbent resin (3-1) was obtained by a similar method to in the above Example 1-3, except that a solution where 2.5 g of polyethylene glycol monomethyl ether (number average molecular weight: about 2000), 40.0 g of water and 5.0 g of ammonium persulfate were mixed in advance, was used as a treatment solution. It should be noted that water content of the water-absorbent resin discharged from the Loedige mixer, after mixing of the base polymer and the treatment solution under stirring, was 12.8% by weight . Various evaluation results of the obtained surface treatedwater-absorbent resin (3-1) are shown in Table 2 below. (Example 3-2)

A surface treated water-absorbent resin (3-2) was obtained by a similar method to in the above Example 3-1, except that heating time of the water-absorbent resin in the water oven was changed to 10 minutes. Various evaluation results of the obtained surface treated water-absorbent resin (3-2) are shown in Table 2 below. (Example 3-3)

A surface treated water-absorbent resin (3-3) was obtained by a similar method to in the above Example 3-1, except that heating time of the water-absorbent resin in the water oven was changed to 30 minutes. Various evaluation results of the obtained surface treated water-absorbent resin

(3-3) are shown in Table 2 below. (Example 4-1)

A surface treated water-absorbent resin (4-1) was obtained by a similar method to in the above Example 1-2, except that 500 g of the water-absorbent resin (B) as a base polymer was used, a solution where 0.5 g (0.00234 mols) of NK ester 701A (glycerine acrylate methacrylate, manufactured by Shin-Nakamura Chemical Co., Ltd.), 20.O g (0.278 mols) of acrylic acid, 35.0 g of water, 0.05 g of polyethylene glycol monomethyl ether (number average molecular weight: about 2000), and 0.5 g of VA-044 (2, 2' -azobis (2- (2-imidazoline -2-yl) propane) dihydro-chloride) , manufactured by Wako Pure Chemical Industries, Ltd.) were mixed in advance, was used as a treatment solution . It should be noted that water content of the water-absorbent resin discharged fromthe Loedige mixer, after mixing of the base polymer and the treatment solution under stirring, was 10.5% by weight. Various evaluation results of the obtained surface treated water-absorbent resin (4-1) are shown in Table 2 below. (Example 4-2)

A surface treated water-absorbent resin (4-2) was obtained by a similar method to in the above Example 4-1, except that heating time of the water-absorbent resin in the water ovenwas changed to 5 minutes . Various evaluation results of the obtained surface treated water-absorbent resin (4-2) are shown in Table 2 below. (Example 4-3)

A surface treated water-absorbent resin (4-3) was obtained by a similar method to in the above Example 4-1, except that heating time of the water-absorbent resin in the water oven was changed to 10 minutes. Various evaluation results of the obtained surface treated water-absorbent resin

(4-3) are shown in Table 2 below. (Comparative Example 1)

A comparative water-absorbent resin (1) was obtained by a similar method to in the above Example 3-3, except that a solution where 2.5 g of polyethylene glycol monomethyl ether

(number average molecular weight: about 2000), and 40.0 g of water were mixed in advance, was used as a treatment solution. It should be noted that water content of the water-absorbent resin discharged from the Loedige mixer, after mixing of the base polymer and the treatment solution under stirring, was 12.9% by weight . Various evaluation results of the comparative water-absorbent resin (1) are shown in Table 2 below. (Comparative Example 2-1)

A comparative water-absorbent resin (2-1) was obtained by a similar method to in the above Example 3-3, except that mode of the water oven was changed to water oven cake (set temperature: 120°C) and heating time was changed to 3 minutes . In the treatment by this mode, heating is carried out with superheated steam obtained by further heating after vaporization. In addition, pressure inside the water oven in heating was normal pressure (1013 hPa) , and oxygen concentration in the water oven was equal to or lower than 0.5% by volume. It should be noted that the water oven was preheated by operation for 15 minutes in water oven cake (set temperature: 120°C) mode, in advance, just before the above operation. Various evaluation results of the comparative water-absorbent resin (2-1) are shown in Table 2 below. (Comparative Example 2-2) A comparative water-absorbent resin (2-2) was obtained by a similar method to in the above Comparative Example 2-1, except that heating time of the water-absorbent resin in the water oven was changed to 5 minutes. Various evaluation results of the comparative water-absorbent resin (2-2) are shown in Table 2 below. (Comparative Example 3)

A comparative water-absorbent resin (3) was obtained by a similar method to in the above Comparative Example 2-1, except that, as a means for heating the water-absorbent resin treated with the treatment solution, a hot air dryer (HISPEC HT320, manufactured by ETHAC Div. , Kusumoto chemicals, Ltd. ) was used instead of the water oven, to carry out heat treatment at 120⁰C for 30 minutes. Various evaluation results of the obtained comparative water-absorbent resin (3) are shown in Table 2 below.

(Comparative Example 4)

A comparative water-absorbent resin (4) was obtained by a similar method to in the above Comparative Example 3, except that heating temperature was changed to 160⁰C. Various evaluation results of the comparative water-absorbent resin (4) are shown in Table 2 below. (Comparative Example 5) A comparative water-absorbent resin (5) was obtained by a similar method to in Comparative Example 4, except that a solution where 1.0 g (0.00192 mols) of polyethylene glycol diacrylate (number of average ethylene oxide units: n = 9) , 4.0 g (0.056 mols) of acrylic acid, 10.0 g (0.106 mols) of sodium acrylate, 45.0 g of water and 0.05 g of ammonium persulfate weremixed in advance, was useda treatment solution. It should be noted that water content of the water-absorbent resin discharged from the Loedige mixer, after mixing of the base polymer and the treatment solution under stirring, was 12.5% by weight. Various evaluation results of the obtained comparative water-absorbent resin (5) are shown in Table 2 below. (Comparative Example 6) A comparative water-absorbent resin (6) was obtained by a similar method to in the above Comparative Example 5, except that heating temperature was changed to 210⁰C. Various evaluation results of the comparative water-absorbent resin

(6) are shown in Table 2 below. (Comparative Example 7)

A comparative water-absorbent resin (7) was obtained by a similar method to in the above Comparative Example 5, except that heating temperature was changed to 80⁰C, and amount of ammonium persulfate in the treatment solution was changed to 25.0 g. Various evaluation results of the comparative water-absorbent resin (7) are shown in Table 2 below. Table- 2

(Ti

P: Polyethylene glycol dxacrylate (number of average ethylene oxide units, n = 9) ftA: Acrylic acid

SA: Sodium acrylate

W: Pure watei

APS: Ammonium persulfate

PEGOMe: Polyethylene glycol monomethyl ether (number average molecular weight; about 2,000)

701A: Glycerine acrylate methacrylate (NK ester 701A, manufactured by Shm-Nakamura Chemical Co., Ltd.)

VA-044: 2, 2' -azobis (2- (2-imidazolme-2-yl)propane) dihydrochloride, manufactured by Wako Pure Chemical Industries, Ltd.)

WAR: water-absorbent resin

SWAR: Surface treated water-absorbent resin

SCWAR: Surface treated comparative water-absorbent resin

From the results shown in Table 2, it is understood that water-absorbing characteristics (in particular, absorption capacity against pressure (AAP) or fluid permeability (SFC)) of the obtained surface cross-linked water-absorbent resin, is enhanced, by surface cross-linking of the water-absorbent resin in the presence of a radical polymerization initiator and water and so that water content of the water-absorbent resin increases (for example by heating in saturated steam) . In addition, it is understood that, by carrying out the above surface cross-linking under condition that a surface cross-linking agent and a radically polymerizable compound such as acrylic acid (salt) are further present, water-absorbing characteristics of the surface cross-linked water-absorbent resin can be still more enhanced.

Claims

CLAIMS :

1. A surface treatment method for a water-absorbent resin comprising: a surface cross-linking step for subjecting a water-absorbent resin to surface cross-linking in the presence of a radical polymerization initiator and water, wherein in said surface cross-linking step, an activated energy beam having a wavelength of equal to or shorter than ultraviolet ray is not irradiated to a reaction system of surface cross-linking; and said surface cross-linking step is carried out so that a water content of said water-absorbent resin after said surface cross-linking step is larger than a water content of said water-absorbent resin before said surface cross-linking step.

2. The surface treatment method according to claim 1, wherein said water-absorbent resin is heated, in said surface cross-linking step.

3. The surface treatment method according to claim 2, wherein the heating is carried out in saturated steam.

4. The surface treatment method according to any one of claims 1 to 3 , wherein an amount of said radical polymerization initiator present in said reaction system of the surface cross-linking is 0.01 to 20% by weight relative to an amount of said water-absorbent resin.

5. The surface treatment method according to any one of claims 1 to 4, wherein said surface cross-linking step is carried out in the presence of a radically polymerizable compound.

6. The surface treatment method according to claim 5, wherein an amount of said radically polymerizable compound present in the reaction system of the surface cross-linking is 0.5 to 50% by weight relative to an amount of said water-absorbent resin.

7. The surface treatment method according to claims 5 or 6, wherein said radically polymerizable compound contains an acid group-containing radically polymerizable compound; and a neutralization ratio of said acid group-containing radically polymerizable compound is 0 to 60% by mol.

8. The surface treatment method according to any one of claims 1 to 7, wherein said water-absorbent resin is a resin obtained by polymerization of an unsaturated monomer containing acrylic acid (salt) as a main component.