WO2006014031A1

WO2006014031A1 - Particulate water-absorbing agent with water-absorbing resin as main component, method for production of the same, and absorbing article

Info

Publication number: WO2006014031A1
Application number: PCT/JP2005/014809
Authority: WO
Inventors: Katsuyuki Wada; Hiroko Ueda; Teruyuki Kanto; Kunihiko Ishizaki; Motohiro Imura
Original assignee: Nippon Shokubai Co., Ltd.
Priority date: 2004-08-06
Filing date: 2005-08-05
Publication date: 2006-02-09
Also published as: CN103272571B; CN1993176A; RU2007108309A; JP2014014818A; CN103272571A; JP5922623B2; JP2006068731A; SG187519A1; EP1796831A1; EP1796831A4; IN2007CN00955A; US20090036855A1; EP1796831B1; SG155191A1

Abstract

A particulate water-absorbing agent including a water-absorbing resin having superior property, and being suited for practical applications at high concentrations in absorbing cores such as diapers and the like without generation of any odor of the organic solvent can be obtained because no organic solvent is included.

Description

Particulate water-absorbing agent with water-absorbing resin as main component, method for production of the same, and absorbing article

Technical Field

The present invention relates to a particulate water-absorbing agent including a water-absorbing resin as a main component. In particular, it relates to a particulate water-absorbing agent fulfilling more superior absorption ability without generating odor in practical applications as absorbing articles such as a disposable diaper (hereinafter referred to as a diaper) .

Background Art

At present, as component materials in sanitary goods such as a disposable diaper, a sanitary napkin, an incontinence pad, and the like, a water-absorbing resin (water-absorbing agent) to absorb body fluid and hydrophilic fibers such as pulp are widely used. As the water-absorbing resin, for example, partially neutralized and crosslinked polyacrylic acid, hydrolysates of starch-acrylic acid graft polymer, saponified vinyl acetate-acrylic acid ester copolymers, hydrolysates of acrylonitrile copolymers or acrylamide copolymers, or crosslinked polymers thereof, crosslinked polymers of cationicmonomers, and the like are used as main rawmaterials. Water-absorption properties suggested to be desired for the water-absorbing resin have conventionally included high absorption capacity, excellent absorption speed, liquid permeability and gel strength of swollen gel in contact with aqueous liquid such as bodily fluid, as well as absorbing amount for sucking water from a substrate containing aqueous liquid. Accordingly, water-absorbing agents adjusted to have specific property have been proposed.

Such a water-absorbing resin has been required to have superior property such as liquid absorption amount, water absorption speed, gel strength and gel permeability in contact with aqueous liquid such as body fluid, along with superior water suction force to suck water from a substrate containing aqueous liquid. Further, as recent trends, water-absorbing resin powder has been expected to exhibit very narrow particle size distribution, with high absorption capacity and low water-extractables . In addition, high absorbency against pressure or liquid permeability under pressure has essentially been required.

For example, there are many patent applications on many parameters specifying various properties of these water-absorbing resins, or on measurement methods thereof (US Reissued 32649, UK2267094B, US5051259, US5419956, US6087002, EP0629441, EP0707603, EP0712659, EP1029886, US5462972, US5453323, US5797893, US6127454, US6184433, US6297335, US Reissued 37021, US5140076, US6414214B1, US5994440, US6444744, US6194531, EP0940148, EP1153656, EP0605215, US5147343, 5 US5149335, EP0532002, US5601452, US5562646, US5669894,

US6150582, WO02/053198, EP0937739, JP60158861, JP11241030, US4732968, WO03/095510 and US6388000) .

Water-absorbing resins superior in gel strength, extractables andabsorption capacityareproposedinUS Reissued

10 32649. A water-absorbing resin superior in liquidpermeability under no pressure, water absorption speed and absorption capacity is proposed in UK2267094B. Technology specifying specific particle size distribution is proposed in US5051259, US5419956, US6087002 and EP0629441. Further, a

15. water-absorbing resin superior in absorbency against pressure under various loads, or many measurement methods therefore are also proposed, and water-absorbing resins with superior absorbency against pressure alone or in combination with other property are proposed in EP0707603, EP0712659, EP1029886,

20 US5462972, US5453323, US5797893, US6127454, US6184433, US6297335, US Reissued 37021 and the like.

Moreover/ water-absorbing resins with superior resistance to impact that decreases the property are proposed inUS5140076, US6414214B1andthelike. Awater-absorbingresin

25 with specific powdery dust amount is proposed in US5994440 and the like, and a water-absorbing resin with less coloring is proposed in US6444744 and the like. Water-absorbing resins superioringeldurabilityinanaqueousL-ascorbicacidsolution as index of urine resistance or superior in water absorption ability are proposed in US6194531 and EP0940148. A water-absorbing resin with superior air permeability is proposed in EP1153656. A water-absorbing resin with lower amount of residual monomers is proposed in EP0605215.

Further, inUS5147343, US5149335, EP0532002, US5601452, US5562646,US5669894,US6150582,WO02/053198 and the like, water-absorbing resins with specific property are proposed as suitable to absorbing articles such as a diaper having specific property, composition or polymer concentration. Furthermore, hygiene materials such as a disposable diaper have changed into thinner type, in other words, the water-absorbing resin concentration tends to be increased in recent years, and thus, demands for improvement in connection with odors generated from a water-absorbing resin have increased. Withrespecttoodors, inattemptstoimpartdeodorization performance to a water-absorbing resin, combinations of a water-absorbing resin and various types of deodorants or antimicrobials have been hitherto proposed. For example, a water-absorbingresincompositioncomprisingawater-absorbing resin and an extract fromleaves of Theaceae plant (for example, see, JP60158861), and a water-absorbing resin composition comprising an extract fromconiferplant and awater-absorbing resin having a specific performance (for example, see, JP11241030) and the like have been known. A technique of agglomerating a water-absorbing resin in a dispersion solvent for particle size control has been known (US4732968) . Also, in recent years, a technique focused on acetic acid and propionic acid which are> impurities in acrylic acid as the odor of water-absorbing resins was proposed (WO03/095510) . Additionally, in connection with residual monomer, a technique focused on β-hydroxy propionic acid which is an impurity in acrylic acid salt was also proposed (US6388000) .

Many techniques have been proposed as described above, however, usingamountofthewater-absorbingresinhas increased in absorbing cores of disposable diapers and the like in recent years, therefore, the absorbing cores are trended to have high water-absorbing resin concentrations (weight ratio of water-absorbing resin being high) . Accordingly, there was a problem that conventional water-absorbing resins have not exhibited sufficient performance for use at high concentrations, and that also deodorization performance is not sufficient at high concentrations. As using amount of water-absorbing resin has increased, reduction of the amount of residual monomer has been further required. An object of the present invention is to provide a particulate water-absorbing agent comprising a water-absorbing resin which is suited for practical applications at high concentrations in absorbing cores such as a diaper to supply superior absorbing articles, and a method for production of the same. More specifically, as means for solving problems (supplying superior absorbing articles) ,the present invention provides a water-absorbing agent which has an additional function and the water-absorbing agent being superior in deodorization performance with no odor generated after swelling, and being suited for practical applications, and a method for production of the same .

Disclosure of the Invention

Conventionally, attentions have been focused on an odor of urine in investigations of deodorization performances of a water-absorbing resin. The present inventors found in the process of elaborate investigations of deodorization performances of a water-absorbing resin as an additional function, that water-absorbing resins, particularly water-absorbing resins obtained by reversed phase suspension polymerization, themselves have a specific odor, and that the odor of the water-absorbing resin itself deteriorates the deodorization performance in practical applications. In addition, as an important factor for practical applications in a diaper, it was found that specific narrow particle size distribution and specific absorption capacity of a water-absorbingresinandtheparticle shapeareof importance. It was conventionally believed that water-absorbing resins produced by reversed phase suspension polymerization include less amount of residual monomer, however, as a result of elaborate investigations on cause for a specific odor of the water-absorbing resin itself produced by reversed phase suspension polymerization, it was found that the residual monomer is generated and the amount thereof is increased in fact. In addition, as a result of exploration of cause for the odor, increase of the amount of residual monomer, volatile organic solvent used in the reversed phase suspension polymerization, and generation of an odor derived from other impurities in monomer in reversed phase suspension polymerization were found.

After elaborate investigations to solve the problems, it was found that because polymerization, surface crosslinking, and drying and agglomeration were carried out in an organic solvent having low boiling points (for example, cyclohexane having a boiling point of 80.7⁰C) for the water-absorbing agents producedby conventional reversedphase suspension polymerization, a slight amount of the organic solvent remains according to conventional methods for production and in conventional products, and this organic solvent accounts for the odor in practical applications of the diaper. According to an aspect of the present invention, the problems are resolved for reducing the odor,by controlling minor components (acetic acid, propionic acid, acrylic acid dimerandthe like) inthemonomer, particularlyinacrylic acid, without substantially using an organic solvent in surface crosslinking after reversed phase suspension polymerization, and by heating at high temperature (preferably not lower than 150°Candnothigherthan250⁰C) alsowithout substantiallyusing an organic solvent in agglomeration. Therefore, awater-absorbingagentaccordingtoanaspect of thepresent invention is aparticulate water-absorbing agent comprising water-absorbing resin particles as amain component which are obtained by polymerization of an unsaturatedmonomer containing an acid group and/or a salt thereof and which are at least ones of nearly spherical shapes, aggregates thereof and aggregates derived from nearly spherical shapes, and said water-absorbing agent satisfies the following (a), (b) , (c.) and (d) ;

(a) centrifuge retention capacity (CRC) in a physiological saline solution being not lower than 32 g/g;

(b) mass median particle size (D50) being not smaller than 200 μm and not larger than 400 μm;

(c) ratio of particles having diameter of smaller than 150 μm (defined by sieve classification ) being not lower than 0 % by weight and not higher than 5% by weight; and (d) content ofvolatile organic compounds as atmospheric concentration measured by a gas detector tube being not lower than 0 ppm and not higher than 100 ppm.

The nearly spherical shape referred to herein means a spherical or elliptical particule (includingperfect spherical particulate shape)without havinganyangulate corner andhaving a ratio of the average major axis and average minor axis of the particle being not lower than 1.0 and not higher than 3.0, with the upper limit being preferably 2.0, and more preferably 1.5. The aggregate of the nearly spherical shape referred to hereinmeansparticles formedbyaggregationofnearlyspherical shapes which are primary particles to give a shape like bunch of grapes. Specific examples are illustrated in the electron micrograph shown in Fig. 1 described later. The aggregate derived from nearly spherical shapes referred to herein means rock-shapedparticles, rice grain-shapedparticles andthe like formed by aggregation, fusion and integration of the nearly spherical shapes which are primary particles . Specific examples are illustrated in electron micrograph shown in Fig. 2 and Fig. 3 described later. In these cases, presence of the nearly spherical shapes which are primary particles can not be identified in appearance. As particle shape of a water-absorbing resin obtained by reversed phase suspension polymerization, a shape of particles with wrinkles is illustrated in Fig.l~Fig.4 of EP516925B1 (WO92/16565,US5744564) . A shape of particles with perfect spherical body is illustrated in Fig.5 of EP516925B1. A sausage shape of particles is illustrated in Fig.2 of US4973632. The water-absorbing resin particles of the present invention are at least one kind or not less than 2 kinds of nearly spherical shapes, aggregates thereof and aggregates derived from nearly spherical shapes, and may be selected from them.

The particulate water-absorbing agent referred to herein means an absorbing and solidifying of an aqueous liquid, which comprises a water-absorbing resin as a main component, and contains a small amount of additive or water as needed. Content of the water-absorbing resin is not lower than 70% by weight and not higher than 100% by weight of the entire water-absorbing agent, and the lower limit thereof is preferably 80% by weight, and more preferably 90% by weight. In the minor component, water is generally a main component or essential, and in addition thereto, additives described later may be used. The aqueous liquid is not limited to water, but may be any one as long as it contains water such as e.g., urine, blood, excrement, waste liquid, humidity or steam, ice, a mixture of water and organic solvents or inorganic solvents, rain water and underground water and the like but not particularly limited thereto. Preferably, the absorbing and solidifying may be used for urine, and particularly human urine.

Also, measurement method of the content of volatile organic compounds as atmospheric concentration measured by a gas detector tube in the above item (d) may be according to the measurement method (8) described in Examples below.

A water-absorbing agent (or a water-absorbing resin) of6.0Og is added to the sniffing bag, and odorless air of 1.2 L is fed in the sniffing bag. Subsequently, 30 ml of a 0.90% by weight aqueous solution of sodium chloride is poured to the water-absorbing agent in the sniffing bag. The water-absorbing agent is left to permit swelling. The sniffing bag is left to standinanincubatorat 37°Cfor 60minutes, andsubsequently, left to stand at room temperature for 10 minutes. Then, atmospheric concentration of volatile organic compounds is measured using a gas detector tube. In the present invention, the content of volatile organic compounds is not a content of volatile organic compounds in a water-absorbing agent. The content of volatile organic compounds is an amount of gas in theatmospherewhichis specifiedbyameasuringmethodmentioned later. The inventors foundthat the content of volatile organic compounds specified inthepresent invention correlates to odor by a water-absorbing articles in practical applications (i.e. as a diaper, especially as a high concentration diaper) , and completed the present invention.

A second water-absorbing agent of the present invention is a particulate water-absorbing agent comprising water-absorbing resin particles as a main component which are obtainedbypolymerizationofanunsaturatedmonomercontaining an acid group and/or a salt thereof and which are at least ones of nearly spherical shapes, aggregates thereof and aggregates derived fromnearly spherical shapes, and said water-absorbing agent satisfies the following (a) , (b) , (c) and (e) ; (a) centrifuge retention capacity (CRC) in a physiological saline solution being not lower than 32 g/g

(b) mass median particle size (D50) being not smaller than 200 μm and not larger than 400 μm

(c) ratio of particles having diameter of smaller than 150 μm(defined by sieve classification ) being not lower than

0 % by weight and not higher than 5% by weight

(e) content of residual monomer after heating at 180°C for 3 hours being not lower than 0 ppm and not higher than 500 ppm. A third water-absorbing agent of the present invention is a particulate water-absorbing agent comprising agglomerated water-absorbing resin particles as a main component which are obtained by reversed phase suspension polymerization of an unsaturated monomer containing an acid group and/or a salt thereof, and said water-absorbing agent satisfies the following (a) , (b) , (c) and (d) ;

(c) ratio of particles having diameter of smaller than 150 μm(defined by sieve classification ) being not lower than 0 % by weight and not higher than 5% by weight; and

(d) content of volatile organic compounds as atmospheric concentration measured by a gas detector tube being not lower than 0 ppm and not higher than 100 ppm.

A fourth water-absorbing agent of the present invention is a particulate water-absorbing agent comprising agglomerated water-absorbing resin particles as a main component which are obtained by reversed phase suspension polymerization of an unsaturated monomer containing an acid group and/or a salt thereof, and said water-absorbing agent satisfies the following (a) , (b) , (c) and (e) :

(a) centrifuge retention capacity (CRC) in a physiological saline solution being not lower than 32 g/g

(c) ratio of particles having diameter of smaller than 150 μm(defined by sieve classification ) being not lower than 0 % by weight and not higher than 5% by weight (e) content of residual monomer after heating at 180°C for 3 hours being not lower than 0 ppm and not higher than 500 ppm.

A method for production of the water-absorbing agent of the present invention comprises:

(1) a step of providing acrylic acid having a content of acetic acid and propionic acid of not higher than 500 ppm, and a content of acrylic acid dimer of not higher than 1000 ppm; (2) a step of reversedphase suspensionpolymerization of an unsaturated monomer containing the acrylic acid and/or asalt thereofasamaincomponentofthemonomerinahydrophobic organic solvent;

(3) a step of adding an aqueous solution of a surface crosslinking agent without including any organic solvent to cross-linked polymer particles which are obtained by the polymerization and which satisfy the following (a), (b) and (c) :

(b) mass median particle size (D50) being not smaller than 200 μm and not larger than 400 μm; and

(c) ratio of particles having diameter of smaller than 150 μm being not lower than 0 % by weight and not higher than 5% by weight; and (4) a step of surface crosslinking by heating at a temperature of not lower than 150°C and not higher than 250°C. According to the water-absorbing agent of the present invention, unprecedentedlly comfortable use is enabled without generation of odor after swelling, and excellent absorption ability is achieved upon practical applications in absorbing articles such as a diaper.

Brief Description of the Drawings Fig. 1 illustrates an electron micrograph showing a water-absorbing resin forming particles with a shape like bunch of grapes through aggregation of nearly spherical shapes which are primary particles.

Fig. 2 illustrates an electron micrograph showing a water-absorbing resin forming rock-shaped particles through aggregation and fusion of nearly spherical shapes which are primary particles.

Fig. 3 illustrates an electron micrograph showing a water-absorbing resin forming rice grain-shaped particles through aggregation and fusion of nearly spherical shapes which are primary particles.

Best Mode for Carrying Out the Invention

Raw materials used for the water-absorbing resin and the water-absorbing agent of the present invention and reaction conditions will be explained below. (1) A water-absorbing resin

A water-absorbing resin of the present invention means a cross-linked polymer which can form hydrogel and is water swelling and non-dissolving in water. For example, water swelling indicates one absorbing large quantity of water in ion exchanged water, such as essentially 5 times or more own weight and preferably 50 to 1000 times, while non-dissolving in water means one with extractables of not higher than 50% by weight (lower limit: 0% by weight ) and still more, in the range described later. Measurement methods thereof are specified in Examples.

As the water-absorbing resin in the present invention, to attainobjectives ofthepresent invention, awater-absorbing resinobtainedbycrosslinkingpolymerization of anunsaturated monomer containing an acid group and/or salts thereof (may be a water-absorbing resin having a cross-linked and polymerized structure, and may also be a water-absorbing resin obtained by a crosslinking reaction with a crosslinking agent following polymerization of an unsaturated monomer containing an acid group and/or a salt thereof) is essentially used. Preferably, partially neutralized polymer of polyacrylic acid obtained by polymerizing and crosslinking of an unsaturatedmonomer mainly composed of acrylic acid and/or salts (neutralized product) thereof is used. (2) An unsaturated monomer

As an unsaturatedmonomer composing the water-absorbing resin (hereinafter, abbreviated simply as a monomer) , acrylic acidand/or a saltthereof ispreferablyusedas amain component. Further, in order to obtain the water-absorbing agent of the present invention, it is preferredthat acrylic acid is provided having a content of acetic acid and propionic acid in acrylic acid being not'higher than 500 ppm (lower limit being 0 ppm) , and acrylic acid dimer content being not higher than 1000 ppm (lower limit being 0 ppm) . The content of acetic acid and propionic acid in acrylic acid, and the acrylic acid dimer content are preferably not higher than 500 ppm, more preferably not higher than 300 ppm, and particularly preferably not higher than 100 ppm, respectively. The content of acetic acid and propionic acid being higher than 500 ppm in acrylic acid, and the acrylic acid dimer content being higher than 1000 ppm may result in generation of. the odor or the residual monomer,, therefore, the water-absorbing agent of the present invention with less amount of volatile organic compounds can not be obtained.

In a method for obtaining the acrylic acid described above, oxidizing conditions for preparing acrylic acid may be controlled, or conditions for distillation and crystallization may be strictly controlled to fall within the above range. Degree ofrectificationmaybe respectivelyelevatedbyincrease in number of theoretical plates (for example, increase 6 to

20 plates more than before) , increase in reflux ratio and the like when a distillation method is employed as a purification method, or alternatively, by increase in number of times of crystallization (for example, increase 3 to 10 times) when a crystallization method is employed as a purification method.

The distillation method and the crystallization method may be also employed in combination. Moreover, during storage of the acrylic acid following purification, the temperature may be controlled at a low temperature (preferablylO to 25°C,more preferablyl4 to 23⁰C) .

In addition, other monomer except for acrylic acid may be used in combination, and the water-absorbing resin may be obtained from the other monomer alone. Examples of the monomer which may be additionally used include aqueous or hydrophobic unsaturated monomers such as methacrylic acid, maleic anhydride, maleic acid, fumaric acid, crotonic acid, itaconic acid, vinylsulfonic acid, 2- (meth) acrylamide-2-methylpropane sulfonic acid, (meth) acryloxyalkane sulfonic acid and its alkali metal salts, ammonium salts, N-vinyl-2-pyrrolidone, N-vinylacetamide, (meth) acrylamide, N-isopropyl (meth) acrylamide, N,N-dimethyl (meth) acrylamide, 2-hydroxyethyl (meth) acrylate, methoxypolyethyleneglycol (meth) acrylate, polyethyleneglycol (meth) acrylate, isobutylene, lauryl (meth)acrylate, andthe like. These canbeacopolymer!zation component.

When monomers other than acrylic acid (salts) are used in combination, to attain objectives of the present invention, use ratio of the monomer (excluding the following a crosslinkable monomer ) other than acrylic acid (salt thereof) is preferably 0 to 30% by mole based on total amount with acrylic acid and salts thereof used as a main component, more preferably 0 to 10% by mole and most preferably 0 to 5% by mole.

When an unsaturated monomer containing an acid group is used as a monomer, examples of the salt thereof include alkali metal salts, alkaline earthmetal salts and ammonium salts. They may be used in combination. In view of performance, industrial availability and safety of the water-absorbing resin obtained, sodium salts or potassium salts are preferable.

Also, an unsaturatedmonomer containing an acid group such as acrylic acid is preferably neutralized at the acid group in view of property and pH. Neutralization ratio of the acid group is usually 20 to 100% by mole, preferably

30 to 95% by mole, and more preferably 40 to 80% by mole.

Neutralization of the acid groupmaybe conductedwith amonomer or with a polymer, either of which may be used in combination. (3) A crosslinkable monomer (an internal crosslinking agent)

The water-absorbing resin according to the present invention is a cross-linked polymer and the crosslinkage may^¬ be a self-crosslinked type without using a crosslinkable monomer. However, in view of property, it is preferably obtained by copolymerization or reaction of th the above-mentioned monomer or polymer ,with a crosslinkable monomer (may be also referred to as an internal crosslinking agent of the water-absorbing resin) having not less than 2 polymerizable unsaturated groups or not less than 2 reactable groups in a molecule. Being a cross-linked polymer may be also specified by being non-dissolving in water as described above.

Specific examples of these internal crosslinking agent include, for example, N,N' -methylenebis (meth) acrylamide, (poly) ethyleneglycol di (meth) acrylate,

(poly)propyleneglycol di (meth)acrylate, trimethylolpropane tri (meth) acrylate, glycerine tri (meth) acrylate, glycerine acrylate methacrylate, ethylene oxide modified trimethylolpropane tri (meth) acrylate, pentaerythritol tetra (meth) acrylate, triallyl cyanurate, triallyl isocyanurate, triallyl phosphate, triallylamine, poly(meth) allyloxyalkane, (poly) ethyleneglycol diglycidyl ether, glycerol diglycidyl ether, ethyleneglycol, polyethyleneglycol, propyleneglycol, glycerine, pentaerythritol, ethylenediamine, ethylene carbonate, propylene carbonate, polyethyleneimine, glycidyl (meth)acrylate, and the like.

These internal crosslinking agents may be used alone or in amixture of two or more kinds, as appropriate. These internal crosslinking agents may be added as a whole into a reaction system or in portion wise. When at least one kind or not less than 2 kinds of internal crosslinking agents are used, in consideration of absorption properties of the water-absorbing resin or the water-absorbing agent finally obtained, it is preferable to use essentially a compoundhavingnot less thantwopolymerizable unsaturated groups, in polymerization.

Using amount of these internal crosslinking agents is preferably in the range of 0.001 to 2% by mole based on the aforementioned monomer (excluding the internal crosslinking agents), more preferably 0.005 to 0.5% by mole, further preferably 0.01 to 0.2% by mole, and particularly preferably 0.03 to 0.15% by mole. The using amounts of the internal crosslinking agents less than 0.001%bymole and over 2% bymolemaynot provide sufficient absorption properties.

Whencrosslinkedstructure is introducedinside apolymer by using the internal crosslinking agent, the internal crosslinking agent may be added to a reaction system before, during or after polymerization of the monomer, or after neutralization.

(4) A polymerization solvent

In the present invention,in view of property, it is preferable that the monomer is dissolved in polymerization solvent (especially water) ,and the monomer in the solution is polymerized by reversed phase suspension polymerization. Monomer concentration in the aqueous solution(hereinafter referred to as a monomer aqueous solution) ,when an aqueous solution of the monomer is prepared,is determined by temperature of the aqueous solution or by the types of the monomer and not especially limited,however,preferably 10 to 70% by weight,and further preferably 20 to 60% by weight.When aqueous solutionpolymerizationiscarriedout, asolventother than water may also be used,if necessary, a solvent type used in combination is not especially limited.

(5) A polymerization initiator

An initiator used in polymerization of the monomer to obtain the water-absorbing resin used in the present invention includes a radical polymerization initiator such as potassium persulfate, ammoniumpersulfate, sodiumpersulfate, potassium peracetate, sodiumperacetate, potassiumpercarbonate, sodium percarbonate, tert-butyl hydroperoxide, hydrogen peroxide or 2,2'-azobis(2-amidinopropane) dihydrochloride; or a photopolymerization initiator such as 2-hydroxy-2-methyl -1-phenylpropane-l-one. They may be used for polymerization initiation by redox or ultraviolet rays. Using amount of the polymerization initiator is, in view of property, 0.001 to 2% by mole, preferably 0.01 to 0.1% by mole (based on total monomers) . When the using amount of the polymerization initiator is less than 0.001% by mole, unreacted residual monomers increase, while, when the amount of the polymerization initiator is over 2% by mole, polymerization control becomes difficult and thus not preferable. (6) A polymerization method

When the monomer is polymerized in order to obtain the water-absorbing agent of the present invention, reversed phase suspension polymerization can be employed in which an aqueous solution of this monomer is dispersed in a hydrophobic organic solvent that is inert to polymerization in the presence of a dispersant to allow for polymerization. This polymerization method is described in, . for example, US4093776, US4367323., US4446261, US4683274, US5185413, US5244735, US5998553, US6573330 and the like. Monomers, crosslinking agents, solvents , initiators and the like exemplified in these polymerization methods may be also employed in the present invention.

The reversed phase suspension polymerization is a polymerization method in which an aqueous solution of a polymerizable monomer is suspended or emulsified in a hydrophobic organic solvent to allow the monomer to be dispersed. Illustrative examples of the surfactant (US6458896) or dispersant for allowing the monomer to be dispersed include e.g., anionic surfactants, nonionic surfactants (US2003-153887) , cationic surfactants, amphoteric surfactants and the like. Usable surfactant is exemplified in , for examle,US6107358.

Specific examples of the anionic surfactant which may be used include fatty acid sodium such as mixed fatty acid sodium soap and sodium stearate; higher alcohol sodium sulfate, sodium alkylsulfate, alkyl benzene sulfonate and the like. Examples of the nonionic surfactant include polyoxyethylene alkyl ether such as polyoxyethylene higher alcohol ether, sorbitan fatty acid esters, glycerol fatty acid esters and the like. Examples of the cationic surfactant and amphoteric surfactant include alkylamines, alkylbetaine and the like. Additionally,, examples of other dispersant include ethyl cellulose, hydroxyethyl cellulose and the like.

Using amount of these surfactants or dispersants may be selected as appropriate depending on kind of the polymerization. In general, the using amount is preferably 0.1 to 30 parts by weight, and more preferably 0.3 to 5 parts by weight based on 100 parts by weight. of entire monomer components including the polymerizable monomer and the crosslinkablemonomer. Also, usingamount ofthesedispersants or surfactants may be 0.001 to 10% by weight, and preferably 0.001 to 1% by weight based on the organic solvent described later.

The organic solvent (dispersion solvent) for use in the reversed phase suspension polymerization may be any one as long as it is fundamentally hardly soluble in water, and is inert to polymerization. Examples of the organic solvent include aliphatic hydrocarbons such as n-pentane, n-hexane, n-heptane and n-octane, alicyclic hydrocarbons such as cyclohexane andmethyl cyclohexane, aromatic hydrocarbons such as benzene, toluene and xylene, and the like. Among them, preferable examples of the solvent in view of stability of industrial obtention , quality and the like include n-hexane, n-heptane and cyclohexane. Using amount of these hydrophobic solvents may be 0.5 to 10 parts by weight, and preferably 0.6 to 5 parts by weight based on 1 part by weight of the aqueous solution containing the polymerizable monomer. (7) Addition of a chain transfer agent In the present invention, a chain transfer agent may be used in polymerization. By polymerization in the presence of an aqueous chain transfer agent in addition to the unsaturated monomer, internal crosslinking agent and polymerization initiator, and when the water-absorbing resin thus obtained is used as the water-absorbing agent of the present invention, an absorbing core with high absorbing ability and superior stability to urine can be obtained.

The aqueous chaintransferagentusedforpolymerization in the present invention is not especially limited as long as it dissolves in water or an aqueous ethylenic unsaturated monomer, and examples of the agent include thiols, thiolates, secondary alcohols, amines, hypophosphites and the like. Specific examples include mercaptoethanol, mercaptopropanol, dodecylmercaptan, thioglycols, thiomalic acid, 3-mercaptopropionic acid, isopropanol, sodium hypophosphite, formic acid and salts thereof. Although one kind or not less than 2 kinds selected from the group can be used, in view of the effect, a hypophosphite such as sodium hypophosphite is preferably used. Using amount of the aqueous chain transfer agent depends on kind of the aqueous chain transfer agent and concentration of a monomer aqueous solution, however, is 0.001 to 1% by mole, and preferably 0.005 to 0.3% by mole based on total monomers.

(8) Drying After conducting reversed phase suspension polymerizationaccordingtothemethodofthepresent invention, depending on the water content following the polymerization, the resulting hydrated gel-like polymer is further dried to put into use as the water-absorbing resin. The drying method is not particularly limited but any known drying method can be used, for example, a variety of method such as a method by azeotropic dehydration in a hydrophobic organic solvent used in polymerization, or common forced draft furnace, dryer under reduced pressure, microwave dryer, high humidity drying using high temperature steam or the like may be employed after filtration of the hydrated gel-like polymer so that intended water content is attained. Preferably the method by azeotropic dehydration is used.Water content of the water-absorbing resin used in the present invention (defined by amount of water included in the water-absorbing resin/measured based on loss on drying at 180°C for 3 hours) is not particularly limited, however, it is selected so as to provide powder exhibiting fluidity even at room temperature in view of property of the resulting water-absorbing agent, and powder state with water content of more preferably 0.2 to 30% by weight, further preferably 0.3 to 15% by weight, and particularly preferably 0.5 to 10% by weight.

(9) Washing with lower-boiling point organic solvent

When awater-absorbingresin is producedbyreversedphase suspension polymerization, polymerization was conducted using an organic solvent such as n-hexane, n-heptane or cyclohexane, followed by surface crosslinking, drying and agglomeration as described above. Therefore, there exists a problem involving presence of a slight amount of the organic solvent in conventional methods for production and commercially available products. Theparticulate water-absorbing agent canbe obtained by means of the second production method(i.e. washing with lower-boiling point organic solvent) .

In order to reduce the amount of residual organic solvent, amethodinwhichthehydratedgel-likepolymerobtained as described above is washed with an organic solvent or an inorganic solvent having a lower-boiling point may be used. This lower-boiling point organic solvent has a boiling point of, for example, preferably not lower than 0°C and not higher than 70°C, andmore preferably not lower than 30°C and not higher than 5O⁰C. Among them, acetone, dimethyl ether, methylene chloride and the like are preferred. Specifically, after filtrating the hydrated gel-like polymer, it may be washed with the lower-boiling point organic solvent described above, and dried by the hot air drying or the like. This washing may be carriedout afterthedryingtreatment. Alternatively, without or in combination with washing in this manner, the organic solvent may be removed in surface crosslinking treatment by heating at high temperature described later (the first productionmethod:heattreatment) . Usingamountofthe solvent for washing may be usually 0.5 to 100 parts by weight, and preferably 1 to 10 parts by weight ,more preferably 2 to 5 parts by weight based on 1 part by weight the water-absorbing resin. The temperature may be not lower than the.room temperature and not higher than the boilng point. (10) Classification and particle size control, and absorption capacity control

A water-absorbing resin (cross-linked polymer) of the present invention is adjusted preferably to have specific particle size as powder to obtain the water-absorbing agent of the present invention. When the particle size is out of range described below, performance as a diaper can be hardly achieved. Particle size may be adjusted, when crosslinked polymer is produced by reversed phase suspension polymerization, by dispersion polymerization in particulate state and distribution drying, however, it may be adjusted to have a specific particle size by agglomerating.Also, it may be adjusted to have a specific particle size by pulverizing and classification after drying.

Particle diameter of the water-absorbing resin (cross-linked polymer) particle of the present invention is usually controlled to being not smaller than 180 μm and not larger than 420μm as mass median particle size (D50), with the lower limit being preferably 200 μm, more preferably 225μ m, and particularly preferably 250 μm, while the upper limit beingpreferably400μm, morepreferably380 μm, andparticularly preferably 350 μm; and ratio of particles with diameter of smaller than 150 μm and/or particles with diameter of larger than 850 μm is controlled to be 0 to 5% by weight, preferably 0 to 2% by weight, and more preferably 0 to 1% by weight in order to obtain the water-absorbing agent of the present invention.

Bulk density (specified by JIS K-3362 (1998) ) of the water-absorbing resin of the present invention is adjusted to be in the range of preferably 0.40 to 0.90 g/ml, and more preferably 0.50 to 0.80 g/ml to obtain the water-absorbing agent of the present invention. Ratio of particles with diameter of 150 to 600 /J!tι is preferably 90 to 100% by weight, more preferably 95 to 100% by weight, and further more preferably 98 to 100% by weight in whole particles. Logarithmic standard deviation (σζ) of particle size distribution is controlled to be preferably 0.20 to 0.50, more preferably 0.20 to 0.45, and particularly preferably 0.20 to 0.40. When the particle size is out of range, the ability of watertight is not achieved upon practical applications in absorbing articles such as a diaper. Although the water-absorbing resin obtained in the present invention as described above is adjusted to fall within the aforementioned particle size, centrifuge retention capacity in a physiological saline solution before the surface crosslinking is preferably controlled to fall within the range of not lower than 32 g/g, with the lower limit being more preferably 35 g/g, more preferably 40 g/g, and particularly preferably 45 g/g, while the upper limit being more preferably 70 g/g, more preferably 65 g/g, and particularly preferably 60 g/g. When centrifuge retention capacity is out of range, improving of he property is not achieved after surface crosslinking.Consequently ,a diaper with superior property may not be obtained. Control of the absorption capacity may be perfectedby controlling the polymerization conditions such as the internal crosslinking agent, and drying conditions. (11) Surface crosslinking treatment (may be merely referred to as surface crosslinking)

Awater-absorbing resinused in the water-absorbing agent of the present inventionmay be one obtained by the crosslinking polymerization and drying (or partial drying) , and pulverizationandclassificationas needed, followedbyfurther subjecting the surface to a crosslinking (secondary crosslinking) treatment.

Although there exist a variety of crosslinking agents which may be used for subjecting the surface to crosslinking, in view of the property, a polyhydric alcohol compound, an epoxy compound, a polyvalent amine compound or a condensate thereof with a haloepoxy compound, an oxazoline compound, a mono-, di~, or polyoxazolidinone compound, a polyvalent metal salt, an alkylene carbonate compound or the like has been generallyused. The surface crosslinking agents whichmay be used in the present invention are exemplified specifically, in US6228930, US6071976, US6254990 and the like. Examples thereof include e.g., polyhydric alcohol compounds such as mono-, di- or polyethyleneglycol, propyleneglycol, 1, 3-propanediol, dipropyleneglycol, 2, 3, 4-trimethyl-l, 3-pentanediol, glycerin, 2-butene-l, 4-diol, 1, 4-butanediol, 1, 3-butanediol, 1, 5-pentanediol, 1, β-hexanediol, 1, 2-cyclohexanedimethanol and the like; epoxy compounds such as mono-, di- or polyethyleneglycol diglycidyl ether, glycidol and the like; oxazolidinone compounds such as 2-oxazolidinone; alkylene carbonate compounds such as ethylene carbonate, and the like, but not particularly limited thereto. To maximize effects of the present invention, it is preferable to essentially use a polyhydric alcohol among these surface crosslinking agents. A polyhydric alcohol having carbon atoms of 2 to 10, and preferably carbon atoms of 3 to 8 may be used.

Using amount of the surface crosslinking agent is generally 0.001 to 10 parts by weight, and preferably 0.01 to 5 parts by weight based on 100 parts by weight of the water-absorbing resin. In the present invention, it is preferred that water as a main component, particularly water alone is used as a solvent in the surface crosslinking. Amount of water used as the solvent may fall within the range of 0.5 to 20 parts by weight, and preferably 0.5 to 10 parts by weight based on 100 parts by weight of the water-absorbing resin. Although an organic solvent canbe used in combination, residual solvent must be^' noted. Using amount of the solvent may be not higher than 0.5 part by weight, and preferably not higher than 0.1 part by weight, more preferably not higher than 0 part by weight based on 1 part by weight the water. In the present invention, among a variety of mixing methods, preferable method is premixing of a surface crosslinking agent with water as needed, followed by spraying or drop-wise addition of the aqueous solution to the water-absorbing resin, and the spraying method is more preferable. Average particle diameter of the droplet to be sprayed is preferably 1 to 300 μm, and more preferably 2 to 200 μm.

The water-absorbing resin after mixing with the surface crosslinking agent is preferably subjected to a heat treatment. Conditions for carrying out the heat treatment may be: temperature of the water-absorbing resin or heat medium temperature being usually not lower than 60°C and not higher than 280°C, with the lower limit being preferably 100°C, and more preferably 150°C, and the upper limit being preferably 25O⁰C, and more preferably 240°C; and heating time being preferably 1minute or longer and2 hours or less,morepreferably 10 minutes or longer and 1 hour or less, further preferably 15 minutes or longer and 45 minutes or less. In order to obtain the water-absorbing agent of the present invention, suitably, a large amount of dispersion solvent is not used as in conventional reversed phase, but a heat treatment at high temperature without substantially using a dispersion solvent is preferred upon the surface crosslinking treatment according to the present invention. In other words, because water-absorbing resins according to conventional reversed phase polymerization were obtained by dispersion in an organic solvent followed by polymerization and surface crosslinking, it was found that dispersion solvent was easy to remain.

According to the present invention, a water-absorbing agent (first water-absorbing agent) in which a volatile organic material is not substantially present is obtained through heating the water-absorbing resin at high temperature (for example, not lower than 150°C andnot higher than 250°C) , without using a dispersion solvent for the water-absorbing resin or an organic solvent for the surface crosslinking agent. Moreover, content of the acrylic acid dimer in the raw material is extremely slight as described above, and this dimer is decomposed into monomers during heating. Therefore, a water-absorbing agent (second water-absorbing agent) enabling suppression of increase in generation of the residual monomer through the following heating can be obtained. Heating at high temperature according to the present invention is preferred because it leads to removal of volatile organic compounds, and reduction of the amount of residual monomer which may be increased due to heat.

(12) Agglomeration

Inordertoobtainthewater-absorbingagent ofthepresent invention, agglomeration is carried out during or after polymerization. Agglomeration results in production of a water-absorbing agent which is further superior in water absorption speed and particle size, and is suited for practical applications e.g., as a diaper. Method of agglomeration is not particularly limited in its procedure as long as multiple polymerized particles are bound to form a large particle. Agglomerationby reversedphase suspension, more specifically, aggregation during reversed phase polymerization (EP0695762,US4732968) or two step polymerization (EP0807646) , or reversedphase agglomeration by adding an inactive inorganic substance following polymerization (US4732968) may be carried out. Owing to the agglomeration, a water-absorbing agent that is more suited for practical applications in a diaper can be obtained. The agglomeration method by reversed phase suspension as described above can be also employed in the present invention. However, because a problem of the odor due to the residual dispersion solvent was found in agglomeration by reversed phase suspension, an agglomeration method in which any dispersion solvent is not used in the present invention. In other words, an agglomeration method which has not been conventionally employed so often for reversed phase suspension polymerization, more specifically, a method in which water-absorbing resin particles dried and filtrated following polymerization are agglomerated using water is suitably employed in the present invention.

Accordingly, to obtain a particulate water-absorbing agent of the present invention, more preferably, the agglomeration step comprises heating with keeping the water content of not lower than 1% by weight and not higher than 10% by weight; and crush as needed, after subjecting the water-absorbing resin to the surface crosslinking treatment. Thus, thewater-absorbingagent canbe adjustedtohave specific particle size as powder. Water to be added to the water-absorbing resin may contain other additives such as chelating agents described later,components made from plants, antimicrobials, aqueous polymers, inorganic salts,etc. Content of them falls within the range of 0.001 to 50% by weight as a concentration in an aqueous solution.

In order to obtain the particulate water-absorbing agent of the present invention, particle diameter is usually controlled to fall within narrow range of not smaller than 200 μm and not larger than 400 μm, with the lower limit being preferably 225 μm, and more preferably 250 μm, while the upper limit being preferably 380 μm, and more preferably 350 μm, as mass median particle size (D50) of the particulate water-absorbing agent. In addition, ratio of particles with diameteroflowerthan150μm (definedbysieveclassification) is controlled to be not lower than 0 % by weight and not higher than 5% by weight, with the upper limit being preferably 2% by weight, and more preferably 1% by weight.

Mass median particle size of the particulate water-absorbing agent obtained in agglomeration step of the present invention is controlled to be larger by preferably 5 to 30%, more preferably 7 to 25%, and still more preferably 9 to 20% than mass median particle size of the water-absorbing resin (cross-linked polymer) particles of the present invention. In the particulate water-absorbing agent of the present invention, agglomerated particles account for 10% by weight or more and 100% by weight or less, with the lower limit being preferably 30% by weight, and more preferably 50% by weight in entire water-absorbing agent. Additionally, in order to obtain the particulate water-absorbingagent ofthepresent invention, itsbulkdensity (specified by JIS K-3362 (1998) ) is adjusted to be in the range of preferably 0.40 to 0.90 g/ml, and more preferably 0.50 to 0.80 g/ml. Further, the particulate water-absorbing agent of the present invention is produced to have ratio of particles with diameter of 150 to 600μm is preferably 90 to 100% byweight, morepreferably 95 to 100% byweight, andfurthermorepreferably 98 to 100% by weight in whole particles. . Logarithmic standard deviation (σζ) of particle size distribution is controlled to be preferably 0.20 to 0.50, more preferably 0.20 to 0.45, and particularly preferably 0.20 to 0.40.

In the present invention, agglomerating is preferably performed by a method for spraying or drop-wise addition of water or an aqueous solution dissolved with other additives components to the water-absorbing resin, and a spraying method is more preferable. Droplet size to be sprayed is, as volume average particle diameter, preferably in the range of 0.1 to

300 μm, and more preferably in the range of 0.1 to 200 μm.

As agglomerating equipment used in agglomeration, those having strong mixing force are preferable. Suitable examples of the agglomerating equipment include e.g., a cylinder type mixer, a double wall conical mixer, a high speed agitation type mixer, a v-shapedmixer, a ribbon typemixer, a screw type mixer, a double-arm kneader, a crashing type kneader, a rotation type mixer, an air-flow type mixer, a turbulizer, a batch type Lδdige mixer, a continuous type Lδdige mixer and the like.

For promoting the agglomeration and reducing the amount of residual organic materials, the water-absorbing resin after mixing with water or the aqueous solution is preferably subjected to a heat treatment.-. Heat treatment is performed, in view of agglomerating ratio or agglomerating strength, while maintaining water content (specified by weight loss in drying at 180⁰C for 3 hours) to be 1 to 10% by weight, more preferably 2 to 8% by weight and further preferably 2.5 to 6% by weight. Heating medium such as hot air can be used in heating, and heating temperature (temperature of heating medium or temperature of material) is preferably in the range of 40 to 120°C, more preferably in the range of 50 to 100°C.

Heating time is preferably in the range of 1 minute to 2 hours. A suitable combination example of heating temperature and heating time is 60°C for 0.1 to 1.5 hour, and 100°C for 0.1 to 1 hour. Heating and the addition of water may be carried out by the same equipment or by separate equipment. Also, heating maybe carriedout with stirringorwhile standing still (without stirring) , as long as temperature or water content can be controlled, but hardening (to give looseblock shape) byheating while standing still (without stirring) is preferable. Preferably, in heating, curing may be performed by heating the water-absorbing resin added with water after heaping up of approximately 1 to 100 cm, more preferably 5 to 80 cm, and particularlypreferably10to70cm. Ahardenedwater-absorbing resin is then pulverized., and preferably classified to obtain intended agglomerate of the present invention.

In the agglomeration according to the present invention, because curing by heating is carried out after providing water alone or water as a main component (60 to 100%) for use as a binder, an absorbing core can be obtained which is not only safe but excellent because the agglomerate can be appropriately destroyed in practical applications, and mass median particle size can be controlled to fall within the range according to the present invention, even though the size is reduced by impact.

(13) Addition of chelating agent

To the water-absorbing agent of the present invention, a chelating agent, in particular, a polyvalent carboxylic acid and salts thereof can be formulated.

The chelating agent which may be used in the water-absorbing agent of the present invention is preferably a chelating agent with high ion blocking ability or chelating ability for Fe or Cu, specifically, a chelating agent with stability constant for Fe ion of not lower than 10, preferably not lower than 20. The chelating agent is further preferably an aminopolyvalent carboxylic acid and salts thereof, and particularly preferably an aminocarboxylic acid with not less than 3 carboxyl groups, preferably not less than 4 carboxyl groups,more preferably not less than 5 carboxyl groups,while the upper limit being polymer, usually not more than 100 carboxyl groups ,preferably not more than 20 carboxyl groups, and salts thereof.

Specific examples of these polyvalent carboxylic acids include ^'diethylenetriamine pentaacetic acid, triethylenetetramine hexaacetic acid, cyclohexane-1, 2-diamine tetraacetic acid, N-hydroxyethyl ethylenediamine triacetic acid, ethyleneglycol diethylether diamine tetraacetic acid, ethylenediamine tetra propionic acetic acid, N-alkyl-N' -carboxymethyl aspartic acid,

N-alkenyl-N' -carboxymethyl aspartic acid and alkaline metal salts thereof;alkaline earth metal salts ammonium salts or amine salts thereof. Among these, diethylenetriamine pentaacetic acid, triethylenetetramine hexaacetic acid, N-hydroxyethyl ethylenediamine triacetic acid and salts thereof are most preferable.

Using amount of the chelating agent, in particular, the aminopolyvalentcarboxylicacidisassmallasgenerally0.00001 to 10 parts by weight based on 100 parts by weight of the water-absorbing resin, a main component, preferably 0.0001 to 1 part by weight,andmore preferably 0.01 to 0.5 part byweight.

The usingamount over 10 parts byweight is not onlyuneconomical due to failure to achieve enough effect relative to using amount but also poses a problem of reducing absorption capacity and the like. On the other hand, the using amount less than 0.00001 part by weight does not provide sufficient addition effect.

(14) Other additives

In the present invention, the following (A) components made from plants, (B) polyvalent metal salts of organic acids,

(C) inorganic fine particles (including (D) composite hydrated oxides) may be also added as minor components in addition to thechelatingagent, bywhichvarious functionscanbe imparted to the water-absorbing resin of the present invention. ' Although using amount of these additives (A) to (D) and (E) mayvarydependingontheobjectives andfunctiontobe imparted, exemplary amount of adding one of them is in the range of 0 to 10 parts by weight, preferably 0.001 to 5 parts by weight, and more preferably 0.002 to 3 parts by weight based on 100 parts by weight of the water-absorbing resin. In general, the amount lessthan0.001partbyweightdoesnotprovide sufficient effect or additional function, while the amount over 10 parts by weight may not achieve effect to meet with the added amount, or may deteriorate absorbing performance, (ft) Components made from plants

A water-absorbing agent of the present invention can be addedwith a component made fromaplant in the amount described above to fulfill deodorizationeffect. Thecomponentmade from a plant to be used in the present invention is preferably at least one compound selected from polyphenol, flavone, derivatives thereof and caffeine. It is further preferable thatthecomponentmade fromplantisatleastonekindofcompound selected from tannin, tannic acid, stachyurus praecox, gall or gallic acid.

Plants containing the component made from a plant to be used in the present invention include, for example, Theaceae plant such as camellia, Hikasaki plant and Sprague; Graiαineae plant such as rice plant, sasa-bamboo, bamboo, corn, wheat and the like; and Rubiaceae plant such as coffee.

Form of the components made,from a plant to be used in the present invention includes plant extract (essential oil), plant itself (plant-ground powder), plant residue or extract residue as by-products generated in production processes from plant processing industry or foods processing industry, however, not particularly limited thereto. The above-mentioned components made froma plant are exemplified in EP1352927,US2004-48955 and the like. (B) A polyvalent metal salt

To a water-absorbing agent of the present invention may be added a polyvalent metal salt, in particular, a polyvalent metal salt of organic acids in the amount described above to improve powder fluidity, and to prevent blocking in moisture absorption.

A polyvalent metal salt of an organic acid used and a method for mixing the polyvalent metal salt are exemplified in WO2004-69936, and examples of the polyvalent metal salt of organicacids usedinthepresent invention, havingcarbonatoms of not less than 7 in a molecule include metal salts other than alkali metal salts of fatty acid, petroleum acid or polymeric acid, .

Illustrative examples of the organic acid composing the polyvalent metal salt of an organic acid include long chain orbranched fattyacids such as caproicacid, octylic acid, octynoic acid, decanoic acid, lauric acid, myristic acid, palmitic acid, oleic acid, stearic acid and the like; petroleum acids such as benzoic acid, myristicinic acid, naphthenic acid, naphthoic acid, naphthoxyacetic acid and the like; polymeric acids such as poly(meth) acrylic acid, polysulfonic acid, and the like. The organic acid preferably has a carboxyl group in a molecule. More preferably, the organic acid may be a fatty acid such as caproic acid, octylic acid, octynoic acid, decanoic acid, lauric acid, myristic acid, palmitic acid, oleic acid, stearic acid, tallow acid or hydrogenated fatty acid of castor oil or the like. Still more preferably the organic acid may be a fatty acid without an unsaturated bond in a molecule, for example, caproic acid, octylic acid, decanoic acid, lauric acid, myristic acid, palmitic acid or stearic acid. Most, preferably, the organic acid may be a long chain type fatty acid having not less than 12 carbon atoms in a molecule, without an unsaturated bond in a molecule such as laulic acid, myristic acid, palmitic acid or stearic acid.

(C) An ^'inorganic fine particle

To a water-absorbing agent of the .present invention may be addedaninorganic fineparticle, inparticular, aninorganic fine particle of non-dissolving in water to prevent blocking inmoisture absorption. Specific examples of inorganicpowder to be used in the present invention include e.g., metal oxides such as silicon dioxide, titanium oxide and the like; silica acid (silicate salt) such as natural zeolite, synthetic zeolite andthelike; kaolin, talc, clay, bentonite, andthe like. Among these, silicon dioxide and silica acid (silicate salt) aremore preferable and silicon dioxide and silica acid (silicate salt) with average particle diameter, measured by a coulter counter method, of 0.001 to 200 μm are further preferable. The deodorization ability can be achieved by above-mentioned zeolite.Amass ratioofthe SiO₂andAl₂O₃inzeoliteispreferably l:l~100,more preferablyl:2~10.

(D) A composite hydrated oxide

To a water-absorbing agent of the present invention may be added a composite hydrated oxide containing zinc and silicon orzincandaluminum (forexample, exemplifiedinJapanesePatent Application No. 2003-280373) to add superior blocking ratio after moisture absorption (powder fluidity after the water-absorbing resin or the water-absorbing agent absorbs moisture) and further superior deodorization performance.

(E) A reducing agent

In a water-absorbing agent of the present invention may be preferably used a reducing agent, still more an inorganic reducing agent, and particularly a reducing agent containing sulfur or containing oxygen to prevent odor or coloring. Examples of the reducing agent to be used include sulfurous acid (sulfite) or sulfurous acid (hydrogen sulfite) and the like exemplified in US4863989.

(F) Others Other additives such as an antimicrobial, an aqueous polymer, a water insoluble polymer, water, an organic fine particle, and the like may be added arbitrarily, as long as the water-absorbing agent of the present invention can be particularly obtained. (15) A particulate water-absorbing agent of the present invention

The particulate water-absorbing agent can be obtained by means of the first production method (i.e. heat treatment) , the second production method (i.e. washing with lower-boiling point solvent), and so on. By the new water-absorbing agent, the problems of the present invention are solved. With the conventional water-absorbing resin and absorbing articles (especially diapers) , there was a problem in practical applications as a diaper (especially as a diaper with high water-absorbing resin concentration) that rewet amount in a diaper was large, thoughmanyparameters of the water-absorbing resin were controlled. Moreover, it was a problem in certain instances that deodorization effect and reduction of the amount of residual monomers were insufficient, consequently a diaper with superior property may not be obtained. To solve the above-mentioned problems (i.e. to provide water-absorbing articles which were suited for practical applications) in the present invention, the present inventors gave attention to content of the volatile organic compounds and/or the amount of residual monomer after heating in a water-absorbing agent (water-absorbing resin) , which have conventionally not been focused, as means for solving the problems. The present inventorshavealsofoundthatanewwater-absorbingagentsolves the above-mentioned problems by controlling particle size, absorption capacity, moreover content of the volatile organic compounds and/or the amount of residual monomer after heating in the water-absorbing agent.

That is, the water-absorbing agent of the present invention is: a particulate water-absorbing agent comprising water-absorbing resin particles as a main component which are obtainedbypolymerizationofanunsaturatedmonomercontaining an acid group and/or a salt thereof and which are at least ones of nearly spherical shapes, aggregates thereof and aggregates derived fromnearly spherical shapes, and saidwater-absorbing agent satisfies the following (a) , (b) , (c) and (d) ;

(b) mass median particle size (D50) being not smaller than 200 μm and not larger than 400 μm; (c) ratio of particles having diameter of smaller than 150 μm being not lower than 0 % by weight and not higher than 5% by weight; and

The water-absorbing agent of the present invention is: a particulate water-absorbing agent comprising a water-absorbing resin particles as a main component which are obtainedbypolymerizationofanunsaturatedmonomercontaining an acid group and/or a salt thereof and which are at least ones of nealy spherical shapes, aggregates thereof and aggregates derived from nearly spherical shapes, and the water-absorbing agent satisfies the following (a) , (b) , (c) and (e) ; (a) centrifuge retention capacity (CRC) in a physiological saline solution being not lower than 32 g/g

(c) ratio of particles having diameter of smaller than 150 μm being not lower than 0 % by weight and not higher than

5% by weight

(e) content of residual monomer after heating at 180°C for 3 hours being not lower than 0 ppm and not higher than 500 ppm. The water-absorbing agent of the present invention is: a particulate water-absorbing agent comprising agglomerated water-absorbing resin particles as a main component which are obtained by reversed phase suspension polymerization of an unsaturated monomer containing an acid group and/or a salt thereof, and the water-absorbing agent satisfies the following (a) , (b) , (c) and (d) ;

(c) ratio of particles having diameter of smaller than 150 μm being not lower than 0 % by weight and not higher than 5% by weight; and

The water-absorbing agent of the present invention is: a particulate water-absorbing agent comprising agglomerated water-absorbing resin particles as a main component which are obtained by reversed phase suspension polymerization of an unsaturated monomer containing an acid group and/or a salt thereof, and the water-absorbing agent satisfies the following (a) , (b) , (c) and (e) :

(a) centrifuge retention capacity (CRC) in a physiological saline solution being not lower than 32 g/g (b) mass median particle size (D50) being not smaller than 200 μm and not larger than 400 μm

(c) ratio of particles having diameter of smaller than 150 μm being not lower than 0 % by weight and not higher than 5% by weight

(e) content of residual monomer after heating at 180°C for 3 hours being not lower than 0 ppm and not higher than 500 ppm.

A water-absorbing agent of the present invention is controlled to have (b) mass median particle size (D50) of usually being not smaller than 200 μm and not larger than 400 μm, with the lower limit being preferably 225μm, and particularly preferably 250 μm, while the upper limit being preferably 380 μm, and particularly preferably 350 μm. In addition, (c) ratio of particles with diameter of smaller than 150 μm is controlled to be not lower than 0 % by weight and not higher than 5% by weight, with the upper limit being preferably 3% by weight, more preferably 2% by weight, and particularly preferably 1% by weight. Particle size adjustment is preferably carried out before surface crosslinking, however, it may be adjusted by pulverizing , classification and agglomerating after surface crosslinking to give specific particle size. When mass median particle size is especially less than 200 μm and ratio of particles with diameter of smaller than 150 μm is larger than 5% by weight, a problem in connection with handling characteristics, particularly dust maybe raised. Also, when it is used in absorbing articles such as a diaper, leakage from the top sheet could possibly occur. When mass medianparticle size is especiallymore than 400 μm, excellent absorption speed can not be achieved, thus, high property can not be obtained upon practical applications in absorbing articles such as a diaper.

The (a) centrifuge retention capacity (CRC) of the present invention may be not smaller than 32 g/g, with the lower limit being more preferably 35 g/g, still more preferably 40 g/g, and particularly preferably 45 g/g, while the upper limit being more preferably 70 g/g, still more preferably 65 g/g, and particularly preferably 60 g/g- When the absorption capacity is smaller than 32 g/g, high property can not be obtained in a diaper application.

The (d) content of volatile organic compounds as atmospheric concentration measured by a gas detector tube of the present invention is 0 ppm to 100 ppm, with the upper limit being preferably 50 ppm, more preferably 20 ppm, and still more preferably 5 ppm. This content means a content of volatile organic compounds in sealed air (atmospheric concentration) as determined by a measurement method of content of volatile organic compounds described later in Example (8) . When the content of volatile organic compounds is higher than 100 ppm, odor of the organic compounds is generated in a diaper application, and thus, not only sense of discomfort is caused, but deodorization performance is deteriorated. Accordingly, high property can not be obtained. The volatile organic compounds means an organic solvent having boiling point at ordinary pressure of not higher than 150°C, preferably boiling point of 65 to 145°C, morepreferably75to 130⁰C, andparticularly preferably 80 to 125°C. Ingeneral, itmaybe anyorganic solvent used in manufacture, and examples thereof include e.g., the dispersion solvents essentially used in reversed phase polymerization, organic solvents may be used in the surface crosslinking as needed, as well as acetic acid, propionic acid, acrylic acid dimer and the like which are impurities of the monomer. It was found that remaining of these substances in a slight amount account for odor of the water-absorbing agent or inhibition of deodorization effect. Thus, these problems were dissolved by the present invention.

According to the present invention, (e) content of the residual monomer after heating the water-absorbing agent at 180⁰C for additional 3 hours is not lower than 0 ppm and not higher than 500 ppm, with the upper limit being preferably 300 ppm, and more preferably 100 ppm. The content of the residual monomer after heating for 3 hours of being higher than 500 ppm is not preferred. Because decomposition and the like of the acrylic acid dimer may be caused during practical applications (especially during practical applications under high temperature) , or with additional reforming or treatment, leading to generation and increase of residual monomer, and to generation of an odor derived from the residual monomer, even though initial content of residual monomer is low.

In conventional reversed phase suspension polymerization of water-absorbing agents, as described above, polymerization ^■ and surface crosslinking, drying and agglomeration have been carried out in lower-boiling point organic solvent (for example, cyclohexane having boiling point of 80.7°C) . Thus, it was found that a slight amount of organic solvent remains according to conventional methods for production and in commercially available products and the like, and that the residual organic solvent causes the odor in practical applications as a diaper. According to the present invention, such problems can be resolved by not substantially using organic solvent in surface crosslinkingandagglomeration after reversed phase polymerization, and by heating at high temperature (preferably not lower than 15O⁰C and not higher than 250⁰C) , while controlling minor components in the monomer, particularly in acrylic acid (acetic acid, propionic acid, acrylic acid dimer and the like) .

(16) Other properties of a particulate water-absorbing agent of the present invention (f) Particles with diameter of 150 to 600 μm; (g) Logarithmic standard deviation

Bulk density (specified by JIS K-3362 (1998) ) of a water-absorbing agent of the present invention is adjusted to bepreferablyintherangeof0.40to0.90g/itιlandraorepreferably 0.50 to 0.80 g/ml. Ratio of (f) particles with diameterbetween 150 and 600 μm is preferably 90 to 100% by weight based on total particles, more preferably 95 to 100% by weight, and further preferably98 to-100%byweight. Logarithmicstandarddeviation

(σζ) of particle size distribution (g) is in the range of preferably 0.20 to 0.50, more preferably 0.20 to 0.45, and particularly preferably 0.20 to 0.40. When particles without this range are used in a diaper, superior property may not be obtained. Because the water-absorbing agent of the present invention has narrow particle size distribution, handling characteristics (in particular, feeding characteristics) are favorable, and superior water absorption speed is provided. Therefore, when it is used as an absorbing article, superior liquid diffusibility is also achieved, (h) Absorbency against pressure A water-absorbing agent of the present invention has absorbency against pressure at 1.9 kPa (under load) in a 0.90% by weight aqueous sodium chloride solution of preferably not lower than 20 g/g, more preferably not lower than 25 g/g, still more preferably not lower than 30 g/g, and particularly preferably not lower than 35 g/g. When the absorbency against pressure is lower than 20 g/g, effect of the present invention may not be exerted. The upper limit is not particularly limited, however, approximately 60 g/g will be enough as the case may be, in view of elevation of the cost due to difficulty in production.

(i) Blocking ratio after moisture absorption

A water-absorbing agent of the present invention is superior in powder handling characteristics due to having low blocking ratio after moisture absorption described later in Example. Blocking ratio aftermoisture absorption is preferably

0 to 30% by weight, more preferably 0 to 20% by weight, further preferably 0 to 10% by weight and particularly preferably 0 to 5% by weight. Blocking ratio after moisture absorption over

30% byweightprovides aproblemsuch as difficulty inproduction of a diaper and the like, for example, owing to poor powder fluidity. Theblockingratioaftermoisture absorptionfalling within the above range can be attained by using the additives.

(j) Increased extractables by deterioration

Increased extractables by deterioration of the present invention is usually 0 to 15% by weight, more preferably 0 to

10% by weight, and still more preferably 0 to 5% by weight.

When increased extractables by deterioration is over 15% by weight, stability of the water-absorbing agent against urine becomes insufficient and enough absorption ability cannot be fulfilled in longperiodpractical application of the absorbing core .

(k) Increased ratio of extractables by deterioration Increased ratio of extractables by deterioration of the present invention is usually 1 to 4 times, preferably 1 to 2 times, more preferably 1 to 1.5 times, and particularly preferably 1 to 1.3 times. When increasedratio of extractables by deterioration is over 3 times, stability of the water-absorbing agent against urine becomes insufficient and enough absorption ability cannot be fulfilled in long period practical application of the absorbing core.

(1) Absorption speed with vortex method Absorption speed of the water-absorbing agent of the present invention is shorter than 60 seconds, preferably 1 to 55 seconds and more preferably 2 to 50 seconds. A water-absorbing agent with absorption speed over 60 seconds may not fulfill sufficient absorption ability when it used in absorbing cores such as a diaper.

(17) Shape of the particulate water-absorbing agent of the present invention In general, particle shape of the water-absorbing resin

(particulate water-absorbing agent) of the present invention obtainedbyreversedphase suspensionpolymerization exhibits nearly spherical shape, aggregated shape thereof, or aggregated shape derived from nearly spherical shapes, depending on the polymerization mechanism. This particle shape can be readily distinguished from irregular shape (pulverized shape or crushedshape) ofthewater-absorbingresinobtainedbygeneral aqueous polymerization. Fig. 1 illustrates an electron micrograph showing a water-absorbing resin forming particles with a shape like bunch of grapes through aggregation of nearly spherical shapes which are primaryparticles. Others,particles with wrinkles, particles with a perfect spherical body and particles with a sausage shape are exemplified. Fig. 2 illustrates an electron micrograph showing a water-absorbing resin forming rock-shaped particles through aggregation and fusion of nearly spherical shapes which are primary particles . Fig. 3 illustrates an electron micrograph showing a water-absorbing resin forming rice grain-shaped particles through aggregation and fusion of nearly spherical shapes which are primaryparticles. The water-absorbing resin obtainedby reversed phase suspension polymerization has high bulk density of 0.9 g/mL in comparison with water-absorbing resins of irregularshapeowingtotheparticle shapethereof. Therefore, it is advantageous in regard to transportation of the water-absorbing resin and in reduction in thickness of the_^ absorbing core. Moreover, because of large surface area of the aggregates of nearly spherical shapes or aggregates derived from nearly spherical shapes, comparatively high liquid absorptionspeedcanbeattained. Therefore, it is advantageous in prevention of liquid leakage and skin irritation when used in absorbing cores such as a diaper.

(18) An absorbing article

Applications of the particulate water-absorbing agent of the present invention are not especially limited, however, the water-absorbing agent is used preferably in an absorbing core and an absorbing article.

The absorbing core of the present invention is obtained using the particulate water-absorbing agent. The absorbing core in the present invention means an absorbing material formed using the particulate water-absorbing agent and hydrophilic fibers as main components. Content of the water-absorbing agent (core concentration) in the absorbing core of the present invention, based on total weight of the water-absorbing agent and hydrophilic fibers is preferably not lower than 20 and not higher than 100% by weight, with the lower limit being more preferably 30% by weight, and particularly preferably 40% by weight.

Further, an absorbing article of the present invention is equippedwith the absorbing core of the present invention, a surface sheet with liquid permeability and a back sheet with liquid non-permeability.

Absorbing articles of the present invention, in particular, a disposable diaper for a child, a disposable diaper for an adult or a sanitary napkin can be fabricated by, for example, preparing the absorbing core (absorbing substrate) through blending or sandwiching fiber substrates and the water-absorbing agent, followed by sandwiching the absorbing core between the substrate with liquid permeability (the surface sheet) and the substrate with liquid non-permeability (the back sheet) andmounting of elastic parts, diffusion layers, pressure sensitive adhesivetapes, andthelike, if necessary. Such absorbing core is subjected to compression molding to have density of 0.06 to 0.50 g/cc and basis weight in the range of 0.01 to 0.20g/cκι². Illustrative examples of the fiber substrate to be used include hydrophilic fibers, such as e.g., ground wood pulp, or cotton linters, crosslinked cellulosic fibers, rayon fibers, cotton fibers, wool fibers, acetate fibers, vinylon fibers, and the like. Preferably they are used as airlied.

A water-absorbing agent of the present invention is one exhibiting superior absorption properties. Specific examples of such absorbing article include sanitary goods starting froma disposable diaper for anadult, whose growth is significant recently, a diaper for a child, a sanitary napkin, so to speak a pad for incontinence, and the like, but not limited thereto. Because a water-absorbing agent present in an absorbing article of the present invention provides less rewet amount and significant dry feeling. loads of person wearing such goods and nursing staffs can be significantly reduced.

Examples Thepresent inventionwillbe elucidatedspecifically withthe followingExamples andComparative Examples below, without being limited to the following Examples. Various performances 'of a water-absorbing resin, a particulate water-absorbing agent (hereinafter, referred to as water-absorbing agent) and an absorbing article were measured by the following methods. In the following description of the measurement method, explanation will be made for cases in which a property of a water-absorbing agent is measured. Electrical equipment was always used under conditions of 100 V and 60 Hz in Examples. In addition, the water-absorbing resin, the water-absorbing agent and the absorbing article were used under conditions of 25°C ± 2^DC and 50% RH (relative humidity), unless particularly specified. An aqueous solution of 0.90% by weight of sodium chloride was used as a physiological saline solution.

When moisture is absorbed by the water-absorbing agent (water-absorbing resin) , measurement is carried out after drying under reduced pressure (for example, for about 16 hrs at 60 to 80°C) , as appropriate, to equilibrium moisture content (2 to 8% by weight, about 5% by weight) of the water-absorbing agent (water-absorbing resin) . When a water-absorbing agent (water-absorbing resin) , a diaper, or a water-absorbing agent (a water-absorbing resin) taken out of a diaper on the market which may absorb moisture on distribution is used in a comparison test, comparison may be made after drying under reduced pressure (for example, for about 16 hrs at 60 to 80°C) as appropriate to equilibrium moisture content (2 to 8% by weight, about 5% by weight) of the water-absorbing agent (water-absorbing resin) .

(1) Centrifuge Retention Capacity (CRC) for a physiological saline solution (an aqueous solution of 0.90% by weight of sodium chloride)

A water-absorbing agent of 0.20 g was uniformly put in a bag (60 mm x 85 mm) made of unwoven fabric and immersed in a physiological saline solution controlled at 25 ± 2°C. The bag containing the water-absorbing agent was taken out of the saline solution after 30 minutes and subjected to dewatering for 3 minutes at 250 G (250 x 9.81 m/sec²) using a centrifuge (Model H-122 small size centrifugemade byKokusan Corporation) and then weighed to determine weight W₂ (g) . Weight Wi (g) of the bag then was measured after similar operation without using any water-absorbing agent. Centrifuge Retention Capacity (g/g) was calculated from the weights Wi and W₂ according to the following formula. Centrifuge Retention Capacity (g/g) = { (weight W₂ (g) - weight Wi (g) }/weight of water-absorbing agent (g) - 1

(2) Absorbency Against Pressure at 1.9 kPa (AAP 1.9 kPa) in a physiological saline solution A water-absorbing agent of 0.900 g was uniformly scattered on a 400-Mesh wire mesh made of stainless steel

(mesh size: 38 μm) welded to the bottom end face of a plastic support cylinder with inner diameter of 60 mm. A piston (cover plate), which has outer diameter a little smaller than 60 mm, forms no gap against the inner surface of the support cylinder and can move up and down smoothly, was mounted on the water-absorbing agent. Total weight W₃ (g) of the support cylinder, the water-absorbing agent and the piston was measured. A load adjusted weight to press the water-absorbing agent uniformly at 1.9 kPa including the weight of the piston was mounted on the piston, thereby completing a set of measuring apparatus. A glass filter (mesh size: 40~50μm ) with, diameter of 90 mm and thickness of 5 mm was placed in a Petri dish with diameter of 150 mm and a physiological saline solution controlled at 25 ± 2⁰C was poured up to the same level as the upper surface of the glass filter. A sheet of filter paper with diameter of 9 cm (No.2 from Toyo Roshi Kaisha -Ltd. ) was placed on the surface of the glass filter to be entirely wetted, and then excess solution over the wetted filter paper was removed.

The set of the measuring apparatus was placed on the wetted filter paper and the liquid was absorbed with the water-absorbing agent under load. The liquid level was kept constant by adding the liquid when the liquid surface became lower than the upper surface of the glass filter. The set of the measuring apparatus was lifted up after an hour and weight W^ (g) (the total weight of the support cylinder, the swollen water-absorbing agent and the piston) excluding the load was measured again. The absorbency against pressure Cg/g) was calculated from the weights W₃ and W₄ according to the following formula.

AbsorbencyAgainst Pressure (g/g) = {weightW₄ (g) -weight W₃ {q) }/weight of water-absorbing agent (g) (3) Mass median particle size (D50) , logarithmic standard deviation (σζ) and percentage by weight of particles less than 150 urn in diameter

A water-absorbing agent was subjected to sieve classification using JIS standard sieves (JIS Z-8801-l(2000) ) having mesh opening size of 850 um, 710 μm, 600 μm, 500 μm, 425μm, 300 μm, 212μm, 150 μm, 106 μm and 45 μm, andpercentage byweight ofparticles less than 150 μmin diameterwasmeasured, while oversize percentages R at each particle size were plotted on a logarithmic probability paper. Particle size corresponding to R = 50% by weight was thus determined as mass median particle size (D50) . Logarithmic standard deviation (σζ) is represented by the following formula, wherein smaller value of σζ means narrower particle-size distribution.

(wherein Xi and X₂ are particle diameters for R = 84.1% by weight and R = 15.9% by weight, respectively)

For sieve classification, a water-absorbing agent of 10.00 g is 'charged into each of the JIS standard mesh sieves (The IIDA TESTING SIEVE: inner diameter of 80 mm) and sieved for classification for 5 minutes using a Ro-tap type sieve shaker (Model ES-65 sieve shaker from Iida Seisakusho Co., Ltd.) .

Mass median particle size (D50) means particle diameter for a standard sieve, corresponding to 50% by weight basedon the whole particles, when sieving is carried out by standard sieves with particular meshes as described in US5051259 and the like.

(4) Extractable polymer

First of all, preparation for measurement of extractable polymer will be described. A pH electrode was calibrated with buffer solutions of pH 4.0, pH 7.0 andpH 10.0. Aphysiological saline solution^'of 50 ml, prepared beforehand for a blank test, was poured into a 100 ml glass beaker and added in dropwise with a 0.1 mol/L aqueous solution of sodium hydroxide until pH 10, while stirring with a stirrer chip with length of 30 mm, to determine titration amount V_ab (ml) of the aqueous solution of sodium hydroxide for a blank test. The saline solution was further added in dropwise with a 0.1 mol/L aqueous solution of hydrochloric acid until pH 2.7, while stirring, to determine titration amount V_bb (ml) of hydrochloric acid for a blank test.

A physiological saline solution of 200 ml, prepared beforehand, was poured into a 250 ml polypropylene cup withalidandaddedwith l.Og (= m(g) ) ofawater-absorbing agent obtained in Examples or Comparative Examples to be describedlater. The saline solutionwas thenstirredwith a stirrer with length of 30 mm and diameter of 8 mm at 500 ± 50 rpm for one hour or 16 hours to extract extractable polymer. After stirring for one hour or 16 hours, the saline solution was filtrated with a filter paper (No.2, made by Toyo Roshi Kaisha Ltd., retaining particle size of 5 μm according toJISP3801(1995)) to obtain a filtrate.

Thus obtained filtrate of 20 ml (recorded as F (ml) ) was poured into a 100 ml glass beaker and added with a physiological saline solution to obtain 50 ml of a filtrate for titration. When thus obtained filtrate is less than 20 ml, the total amount of thus obtained filtrate was recorded as F (ml) and added with a 0.90% by weight aqueous solution of sodium chloride to obtain 50 ml of a filtrate for titration. Subsequently, the filtrate for titration was added in dropwise with a 0.1 mol/L aqueous solution of sodium hydroxide, while stirring with a cylinder-type stirrer with length of 30 mm and diameter of 8 mm until pH 10, to determine titration amount V_a (ml) of the aqueous solution of sodium hydroxide. The filtrate was further added in dropwise with a 0.1 mol/L aqueous solution of hydrochloric -acid, while stirring, until pH 2.7, to determine titration amount V_b (ml) of the aqueous solution of hydrochloric acid. Extractable polymer content (%) is calculated by the following formula.

Extractable polymer content (%) = { (W_a + W_b) /m} x 100 Wherein W_a (g) is relative weight of units having an acid group in extractable polymer included in a water-absorbing agent, and W_b (g) is relative weight of units having a carboxylate group neutralized by an alkali metal in extractable polymer included in the water-absorbing agent. Each of them is calculated by the following formulae. W_a (g) = N_a x 72 x 200/F Wb (g) = N_b x 94 x 200/F

Wherein the value 72 is weight per 1 mole of a repetition unit in polyacrylic acid, and when a monomer with an acidgroup other than acrylic acid is copolymerized, the value is altered to average weight of a repetition unit including such a monomer. The value 94 is weight per 1 mole of a repetition unit in polysodium acrylate and it is altered, as appropriate, when a monomer having an acid group other than acrylic acid is copolymerized, or when potassium, lithium or the like is used instead of sodium as an alkali metal salt.

N_a (mol) is number ofmoles of an acid group of extractable polymer in a filtrate and Nb (mol) is number of moles of a carboxylate group, neutralized by an alkali metal, of extractablepolymer inthefiltrate. Eachofthemis calculated by the following formulae.

N_a (mol) = { (V_a - V_ab)/1000} x 0.1 N_b (mol) = Ni - N_a

Wherein Nx (mol) is number of total moles of extractable polymer in a filtrate to be measured and is calculated by the following formula.

N₁ (mol) = { (V_b - V_M,) /1000} x 0.1

The contents of extractable polymer determined by the above formulae were differentiated as extractable polymer content for one hour (%) on a filtrate obtained by stirring a physiological saline solution for one hour, and extractable polymer content for 16 hours (%) on a filtrate obtained by stirring a physiological saline solution for 16 hours.

(5) Evaluation of resistance to urine A physiological saline solution prepared beforehand was added with L-ascorbic acid to give 0.05% by weight so as to obtain mimicked artificial urine for deterioration test. Specifically, 0.50 g of L-ascorbic acid was dissolved in 999.5 g of a physiological saline solution to prepare mimicked artificial urine for deterioration test.

Mimicked artificial urine for deterioration test of 25 ml was poured into a 250 ml polypropylene cup with a lid and addedwith 1.0 gofawater-absorbingagent obtained in Examples or Comparative Examples to be described later to formswollen gel. After the cup was sealed with the lid, the swollen gel was left for standing in atmosphere of 37°C for 16 hours.

After 16hours, 175ml ofthephysiological saline solution was added and then extractable polymer after deterioration was extracted from the hydrated gel by stirring for one hour similarly as in the previous item (4) with a cylinder-type stirrer with length of 30 mm and diameter of 8 mm.

After extraction by stirring for one hour, the saline solution was filtrated and pH-titrated by the same method as intheprevious item (4) todetermineextractablepolymercontent after deterioration Exi(% by weight) using the same formulae as in the previous item (4) . To compare absolute amount of increased extractable polymer content by deterioration, for evaluation of resistance to urine, increased extractables by deterioration (% by weight) was calculated by the following formula. Increased extractables by deterioration (.% by weight) = Exi - Ex₂ wherein Ex₂ is extractable polymer content for one hour (% by weight) . To compare extractable polymer content formed by deterioration with extractable polymer content before deterioration, for evaluating resistance to urine, increased ratio of extractables by deterioration (quotient) was calculated by the following formula. Increased ratio of extractables by deterioration = Exi /Ex₂

(6) Evaluation of absorption speed (Vortex method) To a 0.90% by weight aqueous solution of sodium chloride (physiological saline solution) prepared previously of 1000 parts by weight was added 0.02 part by weight of a brilliant blue-FCF, a food additive, and kept at 30⁰C. The physiological saline solution of 50 ml was poured into a 100 ml beaker and addedwith2.0 gofawater-absorbingagent obtained in Examples or Comparative Examples to be described later, while stirring at 600 rpm with a cylinder-type stirrer with length of 40 mm and diameter of 8 mm, to measure absorption speed (second) . Absorption speed (second) was measured in terms of time required for the test liquidto completelycover the stirrer chip as the water-absorbing agent absorbs the physiological saline solution, which was measured with end point determined accordingtothe standarddescribedinJISK7224 (1996) "Testing method for water absorption speed of super absorbent resins - Description".

(7) Blocking ratio (Fluidity) after moisture absorption (% by weight)

Aparticulatewater-absorbingagent of2 gdescribedlater was uniformly scattered on the bottom of an aluminum cup with diameter of 52- mm and height of 22 mm and quickly put in a humidity-controllable incubator (PLATIOOUS LUCIFER PL-2G from ESPEC Corp.) controlled beforehand at 25⁰C and 90% relative humidity, and left for standing for 60 minutes. The moisture-absorbed water-absorbing agent was then transferred to a JIS standard sieve with diameter of 7.5 cm and 2000 μm ofmeshopen. Whenthemoisture-absorbedwater-absorbingagent was adhered to aluminum cup too rigidly to do so, the moisture-absorbed water-absorbing agent accompanied by blocking should be detached from the cup and transferred to the sieve very carefully not to destroy the block. The transferred water-absorbing agent to the sieve was immediately sieved for 8 seconds using a sieve shaker (IIDA SIEVE SHAKER, TYPE: ES-65, SER. No. 0501) . Weight W₅ (g) of the oversize water-absorbing agent left on the sieve and weight We (g) of the undersize water-absorbing agent passed through the sieve were measured. Blocking ratio after moisture absorption (% by weight) was calculated by the following formula. The lower blocking ratio after moisture absorption means the water-absorbing agent that is more superior in fluidity after moisture absorption, with improved powder handling nature. Blocking ratio after moisture absorption (% by weight) = {weight W₅ (g) / (weight W₅ (g) + weight W₆ (g) ) } x 100 (8) Content of volatile organic compounds (ppm) A water-absorbing resin of 6.0Og was uniformly spread in a glass Petri dish (Code: 305-07, external diameter x height =120mmx25mm, describedinGENERALCATALOGUEA-1000 (published in 2003) published by SOGO LABORATORY GLASS WORKS CO., LTD) . Then, the water-absorbing agent was covered by one piece of breathable and liquid permeable heatlon paper (Nangoku Pulp Technical Corporation, type: GSP-22) cut into circular (diameter: 116mm) . Whenheatlonpaper cannotbeused, nonwoven fabric may be substituted. Circumference of the heatlon paper (or nonwoven fabric) was fixed on inner wall of the glass Petri dish with a tape (10 mm x 10 mm) at three sites. One edge of a 3 L sniffing bag (manufactured by OMI ODORAIR SERVICE Corporation) was opened, and after placing the glass Petri dish includingthe spreadwater-absorbing resintherein, openedpart of the sniffing bag was closed with an adhesive tape not to leave any space. After depressurizing inside of the sniffing bag once froma glass tube attached to the sniffing bag, odorless air of 1.2 L was fed therein, and subsequently, 30 ml of a 0.90% by weight aqueous solution of sodium chloride (physiological saline solution) adjusted to a temperature of 25 ± 2°C was pouredat once to the dish inthe sniffingbag, whilepreventing contamination of the ambient air, using a glass funnel connected to a Teflon (registered trade name) tube. Then, the water-absorbing agent was uniformly swollen, and the bag was sealed hermetically with a silicon rubber stopper. The bag was left to stand in an incubator at 37°C to permit swelling.

After leaving to stand in an incubator at 37°C for 60 minutes, the bag was taken out, and subsequently was left to stand at room temperature ( 20 to 30°C) . After leaving to stand at room temperature for 10 minutes, the silicon rubber stopper was removed, and atmospheric concentration was measured using a gas sampler (manufactured by Gastec Corporation, GV-IOOS) and a gas detector tube (manufactured by Gastec Corporation, No. 102L) while preventing contamination of the ambient air.

Distance between tip of the gas detector tube and the swollen water-absorbing agent was kept to be 5 to 10 cm during the time period from initiation to termination of the measurement. Measurement using the gas sampler and gas detector tube was carried out according to the method specified for the gas detector tube. Thus determined atmospheric concentration was defined as volatile organic compounds content (ppm) . In this measurement method, as the case may be, the gas detector tube may execute detection to result in change of color even though similar operation is conducted using a physiological saline solutionalonewithout usingawater-absorbingagent. In such a case, correction was performed by subtracting a blank value detected in the indicator range yielded when a physiological saline solution alone was used (detection limit: 10 ppm) . (9) Evaluation of an absorbing core performance (Rewet amounts after 10 minutes)

An absorbing core was prepared using a water-absorbing agent obtained in Examples or Comparative Examples described later and subjected to a rewet test in order to evaluate performance as an absorbing core.

To begin with, a method for preparation of an absorbing core for evaluation is shown below.

A water-absorbing agent to be described later of 1 part byweight andcrushedwoodpulpof2partsbyweightwere subjected to dry mixing using a mixer. Thus obtained mixture was spread on a wire screen of 400-Mesh (mesh size: 38 μm) to form a web with diameter of 90 mm. The web was pressed under pressure of 196.14 kPa (2 kgf/cm²) for 1 minute to obtain an absorbing core for evaluation with basis weight of 0.05 g/cm². Subsequently, a method for evaluation of rewet amount after 10 minutes is shown below. The absorbing core for evaluation was placed on the bottom of a Petri dish with inner diameter of 90 mm made of stainless steel (SUS) , and nonwoven fabric with diameter of 90 mm was placed thereon. Then, 30 ml of themimicked artificial urine for deteriorationtest which had been prepared previously, used in the above item (5); evaluation of resistance to urine, was then poured over the nonwoven fabric and subjectedto absorption for 10 minutes under conditions of no load. Thereafter, 30 sheets of filter paper with diameter of 90 mm (No. 2 from Toyo Roshi Kaisha Ltd.), whosetotalweight (W₇ (g) ) wasmeasuredbeforehand, wereplaced, and a piston and a load (total weight of the piston and load being 20 kg) with diameter of 90 mm were placed on the filter paper so as to press uniformly the absorbing core, the nonwoven and the filter paper. The filter papers were made to absorb rewet liquid, while pressing for 5 minutes. The 30 sheets of the filter paper were then weighed (Wa (g) ) to calculate rewet amount after 10 minutes.

Rewet amount after 10 minutes (g) = Ws (g) - W₇ (g) (10) Amount of residual monomer

A water-absorbing agent of 1.000 g was dispersed in 200 ml of a physiological saline solution, which was stirred with amagnetic stirrer (500 rpm) for 1hour, andthe resulting swollen gel was filtrated through a filter (0.45 μm) . The amount of residual monomer was quantitatively determinedby analyzing thus obtained filtrate on liquid chromatography (HPLC) . Measurement was carried out by a specific measurement method accordingtoMeasurementmethodoftheamountofresidualmonomer of superabsorbent materials (edana RESIDUALMONOMERS 410.2-02) recommended by EUROPEAN DISPOSABLES AND NONWOVENS ASSOCIATION (EDANA) .

[Production Example of acrylic acid 1]

Commercially available acrylic acid (containing 2000 ppm acrylic acid dimer, 500 ppm acetic acid and 500 ppm propionic acid) was supplied into column the bottom of a high boiling point impurities-separating column having 50 dual-flow perforated plates,and then distilled in a reflux ratio of 1 to remove maleic acid and a dimmer of acrylic acid (acrylic acid dimer) and crystalized. Accordingly, acrylic acid (containing 20 ppm acrylic acid dimer, 50 ppm acetic acid and 50 ppm propionic acid) was obtained. [Production Example of acrylic acid 2]

Commercially available acrylic acid (containing 2000 ppm acrylic acid dimer, 500 ppm acetic acid and 500 ppm propionic acid) was supplied into column the bottom of a high boiling point impurities-separating column having 20 dual-flow perforated plates, and then distilled in a reflux ratio of- 1 to remove maleic acid and a dimmer of acrylic acid (acrylic acid dimer) and crystalized. Accordingly, acrylic acid (containing 20 ppm acrylic acid dimmer, 500 ppm acetic acid and 500 ppm propionic acid) was obtained. [Production of aqueous solution of sodium acrylate]

Acrylic acid in an amount of 1390 g was neutralized with

48% sodium hydroxide according to Example 9 in US5210298 at 20 to 4O⁰C to obtain 100 % by mol neutralized sodium acrylate at the concentration of 37 % by weight. [Example 1]

Into a 2-L four-neck separable flask equipped with a stirrer, a reflux condenser, a thermometer, an inlet tube for nitrogen gas and a dropping funnel was charged 1.0 L of cyclohexane, and thereto was added 3.8 g of sucrose fatty acid ester (manufactured by DAI-ICHI KOGYO SEIYAKU CO.,LTD., DK-ester F-50, HLB = 6) as a dispersant which was dissolved therein. Dissolved oxygen was degassed by bubbling nitrogen gas. In another flask, 84.6 g of sodium acrylate that is a neutralized product of acrylic acid of Production Example 1, 21.6 g of acrylic acid of Production Example 1 and 0.016 g of N,N' -methylenebisacrylamide were dissolved in 197 g of ion exchangedwater, andthereinwas dissolved 0.4 g of hydroxyethyl cellulose (manufactured by Daicel Chemical Industries, Ltd., HEC-Daicel EP-850) to prepare an aqueous solution of monomer havingmonomer concentration of 35% by weight. To this aqueous solution of monomer was added 0.15 g of potassium persulfate which was dissolved therein. Thereafter, dissolved oxygen was degassed by bubbling nitrogen gas. Then the aqueous solution of monomer in this flask was added to the separable flask to allow for dispersion by stirring. Thereafter, polymerization reaction was initiated by raising the bath temperature to 60⁰C, followed by keeping this temperature for 2 hours. After completing the polymerization, water included in the hydrated gel was removed by azeotropic dehydration with cyclohexane, followed by filtration, drying under reduced pressure at 80⁰C to obtain spherical polymer powder. Thus resulting polymer powder had water content of 5.6%. With 100 parts by weight of the polymer was mixed 3.53 parts by weight of a surface crosslinking agent consisting of 0.5 part by weight of propyleneglycol, 0.03 part by weight of ethyleneglycol diglycidyl ether, 0.3 part by weight of 1, 4-butanediol and 2.7 parts by weight of water. The mixture was subjected to a heat treatment at 210°C for 45 min. After surface crosslinking, 3 parts by weight of water was further added to the mixture, which was heated hermetically at 60°C for 30 minutes. Agglomerated particulate water-absorbing agent (1) was obtainedby classification for 850 μm. Centrifuge retention capacity, absorbency against pressure at 1.9 kPa, particle size distribution, mass median particle size (D50) , logarithmic standard deviation (σζ) and percentage by weight ofparticles having diameter of smaller than 150 μm, extractable polymer, evaluation of resistance to urine, absorption speed, blocking ratio after moisture absorption, content of volatile organic compound, and the amount of residual monomer after heating at 180°C for 3 hours of thus obtained particulate water-absorbing agent (1) are shown in Table 1 and Table 2. [Comparative Example 1] Into a 2-L four-neck separable flask equipped with a stirrer, a reflux condenser, a thermometer, an inlet tube nitrogen gas and a dropping funnel was charged 1.0 L of cyclohexane, and thereto was added 3.8 g of sucrose fatty acid ester {manufactured by DAI-ICHI KOGYO SEIYAKϋ CO.,LTD., DK-ester F-50, HLB = 6) as a dispersant which was dissolved therein. Dissolved oxygen was degassed by bubbling nitrogen gas. Separately in another flask, 84,6 g of sodium acrylate that is aneutralizedproduct of commerciallyavailable acrylic acid (containing 2000 ppm acrylic acid dimer, 500 ppm acetic acid and 500 ppm propionic acid), 21.6 g of commercially available acrylic acid and 0.016 g of

N,N'-methylenebisacrylamide were dissolved in 197 g of ion exchangedwater, andthereinwas dissolved 0.4 gofhydroxyethyl cellulose (manufactured by Daiσel Chemical Industries, Ltd., HEC-ϋaicel EP-850) to prepare an aqueous solution of monomer havingmonomer concentration of 35% byweight. To this aqueous solution of monomer was added 0.15 g of potassium persulfate whichwas dissolved therein. Thereafter, dissolved oxygen was degassed by bubbling nitrogen gas. Then the aqueous solution of monomer in this flask was added to the saparable flask to allow for dispersion by stirring- Thereafter, polymerisation reactionwas initiated by raising the bath temperature to 6O⁰C, followed by keeping this temperature for 2 hours. After completing the polymerization, water was removed from the hydrated gel-like polymer by aseotropic dehydration. After adding 85.0mg of ethyleneglycol diglycidyl ether as a surfacecrosslinkingagenttodehydratedhydratedgel-like polymer, surface crosslinking treatment was carried out by keeping bath at 80°C for 2 hours. After agglomeration by reversed phase suspension (organic solvent: cyclohexane) , the mixturewas filtrated, followedbydryingunder reducedpressure at 80°C to obtain a comparative particulate water-absorbing agent (1) • Results of evaluation of thus obtained comparative particulate water-absorbing agent (1) in a similar manner to Example 1 are showninTable 1 andTable 2. The amount ofresidual monomer before heating in Comparative Example 1 was 50 ppm.

[Example 2]

To 100 parts by weight of the particulate water-absorbing agent (1) obtained in Example 1 was added andmixed (dryblended) 0.3 part by weight of fine particulate silicon dioxide

(manufactured by NIPPON AEROSIL CO. , LTD. , AEROSIL ® 200 (mean particle size of primary particles: 12 ran)) to obtain a particulate water-absorbing agent (2) . Results of evaluation of thus obtained particulate water-absorbing agent (2) in a similar manner to Example 1 are shown in Table 1 and Table 2. [Example 3]

With 100 parts by weight of the particulate water-absorbing agent (1) obtained in Example 1 was mixed by spraying 2 parts by weight of an aqueous solution comprising diethylenetriamine pentaacetatic acid aqueous solution and a 15% by weight aqueous solution of extract from plant leaves of Theaceae plant (Trade Name: FS-80MO, Supplier: Shiraimatu Shinyaku Co., Ltd., Address: 37-1 Ugawa, Minakuchi-Cho, Kouka-Gun, Shiga-Ken, Japan) (amount of each component being adjusted so that diethylenetriamine pentaacetatic acid and the 15% by weight aqueous solution of extract from plant leaves of Theaceae plant became 50 ppm by weight and 0.1% by weight by weight relative to the water-absorbing agent (1) , respectively) . The resultant mixture was cured at 60°C for 1 hour, and thereto was then added and mixed (dry blended) 0.3 part by weight of fine particulate calcium stearate (manufactured by Kanto Chemical Co., Inc.) to obtain a particulate water-absorbing agent (3) . Results of evaluation of thus obtained particulate water-absorbing agent (3) in a similar manner to Example 1 are shown in Table 1 and Table 2. [Example 4]

To evaluate an absorbing core performance of the water-absorbing agent (1) obtained in Example 1, an absorbing core for evaluation (1) was prepared according to the method for evaluation of an absorbing core performance in the above item (9), to measure rewet amount after 10 min. Results are shown in Table 3.

[Examples 5 and 6]

The same procedures as in Example 4 were repeated except for using the particulate water-absorbing agents (2) and (3) obtained in Examples 2 and 3 instead of the particulate water-absorbing agent (1) used in Example 4, to obtain absorbingcores forevaluation (2) and (3) , respectively. Rewet amounts of thus obtained absorbing cores for evaluation (2) and (3) were measured. The results are shown in Table 3. [Comparative Example 2]

The same procedures as in Example 4 were repeated except for using the comparative particulate water-absorbing agent (1) obtained Comparative Example 1 instead of the particulate water-absorbing agent (1) used in Example 4, to obtain a comparative absorbing core for evaluation (1) . Rewet amount of thus obtained comparative absorbing core for evaluation (1) was measured. The results are shown in Table 3.

[Comparative Example 3] The same procedures as in Example 1 were repeated except for using the acrylic acid obtained in Production Example 2 instead of the acrylic acid obtained in Production Example 1, neutralizing the acrylic acid to obtain sodium acrylate, followed by polymerization similarly to Example 1. Results from thus obtained comparative particulate water-absorbing agent (3) are shown in Table 1 and Table 2. [Comparative Example 4]

The same procedures as in Example 1 were repeated except for using the commercially available acrylic acid directly, which was neutralized to obtain sodium acrylate, followed by polymerization similarly to Example 1. Results from thus obtained comparative particulate water-absorbing agent (4) are shown in Table 1 and Table 2.

Table 1

Note) Amount of residual monomer before heating in Comparative Example 1 is 50 ppm. o

Table 2

OO

Table 3

OO

A particulate water-absorbing agent of the present inventiondoes not containavolatile organiccompoundas shown in Tables 1 to 3, therefore, no odor is generated from the gel in swollen state, and the agent is superior in absorption capacity, absorbency against pressure and absorption speed. Mass median particle size and logarithmic standard deviation are also falling within a range to achieve favorable handling characteristics of the water-absorbing agent and to attain high property of the water-absorbing agent in use as an absorbing article. Use of the particulate water-absorbing agent of the present invention results in an absorbing article (a diaper in Table 3) with less rewet amount and high performance. Such a particulate water-absorbing agent of the present invention exhibits still largerblocking ratio aftermoisture absorption, urine resistance and deodorization performance by adding inorganic fine particles, a deodorant and the like as needed.

Industrial Applicability

As described hereinabove, the present invention relates to a particulate water-absorbing agent including a water-absorbing resin as a main component. More specifically, it relates to a particulate water-absorbing agent fulfilling more superior absorption ability without generating odor in practical applications as absorbing articles such as a diaper.

Claims

Claims :

1. A particulate water-absorbing agent comprising water-absorbing resin particles as a main component which are obtainedbypolymerizationofanunsaturatedmonomercontaining an acid group and/or a salt thereof and which are at least ones of nearly spherical shapes, aggregates thereof and aggregates derived fromnearly spherical shapes, and saidwater-absorbing agent satisfies the following (a) , (b) , (c) and (d) ; (a) centrifuge retention capacity (CRC) in a physiological saline solution being not lower than 32 g/g;

5% by weight; and

2. A particulate water-absorbing agent comprising a water-absorbing resin particles as a main component which are obtainedbypolymerizationofanunsaturatedmonomercontaining an acid group and/or a salt thereof and which are at least ones of nearly spherical shapes, aggregates thereof and aggregates derived fromnearly spherical shapes, and saidwater-absorbing agent satisfies the following (a) , (b) , (c) and (e) ;

(e) content of residual monomer after heating at 18O⁰C for 3 hours being not lower than 0 ppm and not higher than 500 ppm.

3. A particulate water-absorbing agent comprising water-absorbing resin particles as a main component which are obtained by reversed phase suspension polymerization of an unsaturated monomer containing an acid group and/or a salt thereof, andsaidwater-absorbingagent satisfiesthefollowing (a), (b), (C) and (d)';

(a) centrifuge retention capacity (CRC) in a physiological saline solution being not lower than 32 g/g; (b) mass median particle size (D50) being not smaller than 200 μm and not larger than 400 μm;

(c) ratio of particles having diameter of smaller than 150 μm being not lower than 0 % by weight and not higher than 5% by weight; and (d) content of volatile organic compounds as atmospheric concentration measured by a gas detector tube being not lower than 0 ppm and not higher than 100 ppm.

4. A particulate water-absorbing agent comprising water-absorbing resin particles as a main component which are obtained by reversed phase suspension polymerization of an unsaturated monomer containing an acid group and/or a salt thereof, andsaidwater-absorbingagent satisfies the following (a), (b), (C) and (e) :

5. A particulate water-absorbing agent according to anyoneofclaims 1to4whereinatleastaparttheagentcomprises agglomerated particles.

6. A particulate water-absorbing agent according to any one of claims 1 to 5 which is obtained by further comprising water insoluble inorganic fine particles.

7. A particulate water-absorbing agent according to anyone of claims 1 to 6which is obtainedby further comprising a reducing agent.

8. A particulate water-absorbing agent according to anyoneofclaims 1to7whereinthewater-absorbingagent further satisfies the following (f) :

(f) absorbency against pressure at 1.9 kPa in a physiological saline solution being not lower than 20 g/g.

9. A method for production of the particulate water-absorbing agent according to any one of claims 1 to 8, which comprises: a step of providing acrylic acid having a content of acetic acid and propionic acid of not higher than 500 ppm, and a content of acrylic acid dimer of not higher than 1000 ppm; a reaction step of reversed phase suspension polymerizationofanunsaturatedmonomercontainingtheacrylic acid and/or a salt thereof as a main component of the monomer in a hydrophobic organic solvent; a step of adding an aqueous solution of a surface crosslinking agent without including any organic solvent to cross-linked polymer particles which are obtained by the polymerization and which satisfy the following (a) , (b) and (C) :

(a) centrifuge retention capacity (CRC) in a physiological saline solution being not lower than 32 g/g; (b) mass median particle size (D50) being not smaller than 200 μm and not larger than 400 μm; and

(c) ratio of particles having diameter of smaller than 150 μm being not lower than 0 % by weight and not higher than 5% by weight; and a step of surface crosslinkingbyheating at a temperature of not lower than 150°C and not higher than 250°C.

10. A method for production according to claim 9 which further comprises a step of agglomeration.

11. A method for production according to claim 10 wherein the agglomeration is carried out in the reaction step of reversed phase suspension polymerization.

12. Amethod for production according to claim 10 which further comprises a step of agglomeration comprising: adding water, heating in the state of keeping the water content of not lower than 1 % by weight and not higher than 10% by weight and crushing of thus resulting aggregates.

13. An absorbing article for excrement, urine or blood, which is molded by comprising a particulate water-absorbing agent according to any one of claims 1 to 8 and hydrophilic fibers.

14. An absorbing article according to claim 13 wherein content of the particulate water-absorbing agent based on total weight of the particulate water-absorbing agent and the hydrophilic fibers is not lower than 30% byweight andnot grater than 100% by weight.