Method for producing water-absorbent resin particles and water-absorbent resin particles

Abstract

The present invention provides a method which is for producing water-absorbent resin particles, which comprises a step for obtaining polymer particles by subjecting a water-soluble ethylenically unsaturated monomer to reverse-phase suspension polymerization in a hydrocarbon dispersion medium, and by which water-absorbent resin particles having a narrow particle size distribution are obtained.　Provided is a method for producing water-absorbent resin particles, said method comprising a step for obtaining polymer particles by subjecting a water-soluble ethylenically unsaturated monomer to reverse-phase suspension polymerization in a hydrocarbon dispersion medium. The reverse-phase suspension polymerization is carried out in two or more stages. The method comprises a step for aggregating the polymer particles in the presence of a dispersion stabilizer. Two or more types of (poly)glycerol fatty acid esters which differ in precipitation temperature when a 0.46 mass% heptane solution is used as the dispersion stabilizer are used.

Description

Method for producing water-absorbent resin particles, and water-absorbent resin particles

[0001] The present invention relates to a method for producing water-absorbent resin particles, water-absorbent resin particles, an absorbent body, and an absorbent article, and more particularly to a method for producing water-absorbent resin particles constituting an absorbent body suitably used in hygiene materials such as disposable diapers, sanitary napkins, and incontinence pads, and water-absorbent resin particles.

[0002] BACKGROUND ART In recent years, water-absorbent resin particles have been widely used in the field of sanitary materials such as disposable diapers, sanitary napkins, and incontinence pads.

[0003] As such water-absorbent resin particles, a crosslinked product of a polymer of a water-soluble ethylenically unsaturated monomer, more specifically a crosslinked product of a polymer of a partially neutralized polyacrylic acid, has excellent water-absorbing ability, and since acrylic acid, which is a raw material thereof, is easily available industrially, it can be produced at low cost with constant quality, and is less susceptible to putrefaction and deterioration, and therefore, it is considered to be a preferable water-absorbent resin particle (see, for example, Patent Document 1).

[0004] Absorbent articles such as disposable diapers, sanitary napkins, and incontinence pads are mainly composed of an absorbent core located in the center for absorbing and retaining body fluids such as urine and menstrual blood excreted from the body, a liquid-permeable surface sheet (top sheet) located on the side that comes into contact with the body, and a liquid-impermeable back sheet (back sheet) located on the opposite side that comes into contact with the body. The absorbent core is usually composed of hydrophilic fibers such as pulp and water-absorbent resin particles.

[0005] Japanese Patent Application Publication No. 3-227301

[0006] The water-absorbent resin particles can be produced, for example, by subjecting a water-soluble ethylenically unsaturated monomer to reverse-phase suspension polymerization in a hydrocarbon dispersion medium to obtain polymer particles.

[0007] A main object of the present invention is to provide a method for producing water-absorbent resin particles, which method includes a step of obtaining polymer particles by reverse-phase suspension polymerization of a water-soluble ethylenically unsaturated monomer in a hydrocarbon dispersion medium, and which allows the production of water-absorbent resin particles having a narrow particle size distribution. Another object of the present invention is to provide a method for producing water-absorbent resin particles having a narrow particle size distribution.

[0008] The present inventors have conducted extensive studies to solve the above-mentioned problems. As a result, they have found that in a method for producing water-absorbent resin particles, which includes a step of obtaining polymer particles by reverse-phase suspension polymerization of a water-soluble ethylenically unsaturated monomer in a hydrocarbon dispersion medium, the method further includes a step of performing the reverse-phase suspension polymerization in two or more stages and aggregating the polymer particles in the presence of a dispersion stabilizer, and by using, as the dispersion stabilizer, two or more types of (poly)glycerin fatty acid esters that have different precipitation temperatures when used in a 0.46 mass % heptane solution, water-absorbent resin particles having a narrow particle size distribution can be obtained. The present invention was completed based on this finding and through further extensive studies.

[0009] That is, the present invention provides the following configurations: Item 1. A method for producing water-absorbent resin particles, comprising a step of obtaining polymer particles by reverse-phase suspension polymerization of a water-soluble ethylenically unsaturated monomer in a hydrocarbon dispersion medium, wherein the reverse-phase suspension polymerization is carried out in two or more stages, and the method includes a step of aggregating the polymer particles in the presence of a dispersion stabilizer, wherein two or more types of (poly)glycerin fatty acid esters having different precipitation temperatures when made into a 0.46 mass % heptane solution are used as the dispersion stabilizer. Item 2. A method for producing water-absorbent resin particles according to Item 1, wherein, among the two or more types of (poly)glycerin fatty acid esters, the difference between the precipitation temperature of the (poly)glycerin fatty acid ester having the highest precipitation temperature and the precipitation temperature of the (poly)glycerin fatty acid ester having the lowest precipitation temperature is 15°C or less. Item 3. Item 4. The method for producing water-absorbent resin particles according to Item 1 or 2, wherein at least one of the (poly)glycerin fatty acid esters is hexaglyceryl tristearate, and the amount of the hexaglyceryl tristearate used is in the range of 0.2 parts by mass or more and 4.0 parts by mass or less relative to 100 parts by mass of the water-soluble ethylenically unsaturated monomer used in the first-stage polymerization. Item 5. The method for producing water-absorbent resin particles according to Item 3, wherein the dispersion stabilizer comprises the hexaglyceryl tristearate and at least one of a (poly)glycerin fatty acid ester having a higher precipitation temperature than the hexaglyceryl tristearate and a (poly)glycerin fatty acid ester having a lower precipitation temperature, and the amount of the (poly)glycerin fatty acid ester having a higher precipitation temperature than the hexaglyceryl tristearate and the (poly)glycerin fatty acid ester having a lower precipitation temperature used is in the range of more than 0 part by mass and 3.0 parts by mass or less relative to 1 part by mass of the hexaglyceryl tristearate, respectively.Item 5. The method for producing water-absorbent resin particles according to Item 4, wherein the dispersion stabilizer comprises the hexaglyceryl tristearate, a (poly)glycerin fatty acid ester having a higher precipitation temperature than the hexaglyceryl tristearate, and a (poly)glycerin fatty acid ester having a lower precipitation temperature, and the amounts of the (poly)glycerin fatty acid ester having a higher precipitation temperature than the hexaglyceryl tristearate and the (poly)glycerin fatty acid ester having a lower precipitation temperature used are more than 0 part by mass and 2.0 parts by mass or less per part by mass of the hexaglyceryl tristearate. Item 6. Water-absorbent resin particles comprising polymer particles having a water-soluble ethylenically unsaturated monomer as a monomer unit, wherein the water-absorbent resin particles have a particle size distribution uniformity of 2.5 or less and contain the (poly)glycerin fatty acid ester in the vicinity of the surface.

[0010] According to the present invention, it is possible to provide a method for producing water-absorbent resin particles, which method includes a step of obtaining polymer particles by reverse-phase suspension polymerization of a water-soluble ethylenically unsaturated monomer in a hydrocarbon dispersion medium, and which can obtain water-absorbent resin particles having a narrow particle size distribution. Furthermore, according to the present invention, it is also possible to provide water-absorbent resin particles having a narrow particle size distribution.

[0011] As used herein, the term "comprising" includes "consisting essentially of" and "consisting of." Furthermore, as used herein, "(meth)acrylic" means "acrylic or methacrylic," "(meth)acrylate" means "acrylate or methacrylate," and "(poly)" refers to the presence or absence of the prefix "poly." Furthermore, as used herein, "water-soluble" means exhibiting a solubility of 5% by mass or more in water at 25°C.

[0012] In the present invention, the vicinity of the surface of the water-absorbent resin particle refers to the outermost surface of the water-absorbent resin particle and a region extending from the outermost surface to a depth of approximately 30 μm toward the center of the particle.

[0013] In this specification, a numerical value connected with "~" means a numerical range that includes the numerical values ​​before and after "~" as the lower limit and upper limit. When multiple lower limit values ​​and multiple upper limit values ​​are listed separately, any lower limit value and upper limit value can be selected and connected with "~".

[0014] 1. Method for Producing Water-Absorbent Resin Particles The method for producing water-absorbent resin particles of the present invention is a method for producing water-absorbent resin particles, which includes a step of obtaining polymer particles by reverse-phase suspension polymerization of a water-soluble ethylenically unsaturated monomer in a hydrocarbon dispersion medium (polymerization step).

[0015] In the method for producing water-absorbent resin particles of the present invention, reverse-phase suspension polymerization is carried out in two or more stages. The production method of the present invention also includes a step of aggregating polymer particles in the presence of a dispersion stabilizer. Furthermore, the production method of the present invention uses, as the dispersion stabilizer, two or more types of (poly)glycerin fatty acid esters that have different precipitation temperatures when dissolved in a 0.46% by mass heptane solution.

[0016] The method for producing water-absorbent resin particles of the present invention is provided with these features, and thereby can suitably produce water-absorbent resin particles having a narrow particle size distribution. The method for producing water-absorbent resin particles of the present invention will be described in detail below.

[0017] <Polymerization Step> The polymerization step is a step of polymerizing a water-soluble ethylenically unsaturated monomer by reversed-phase suspension polymerization to obtain polymer particles. In reversed-phase suspension polymerization, the water-soluble ethylenically unsaturated monomer is polymerized by heating under stirring in a hydrocarbon dispersion medium. In the polymerization step, an internal crosslinking agent may be added to the water-soluble ethylenically unsaturated monomer as needed to form crosslinked polymer particles having an internal crosslinked structure. An example of the polymerization step is described below.

[0018] [Water-soluble ethylenically unsaturated monomer] Examples of the water-soluble ethylenically unsaturated monomer include (meth)acrylic acid (in this specification, "acrylic" and "methacrylic" are collectively referred to as "(meth)acrylic", the same applies hereinafter) and salts thereof; 2-(meth)acrylamido-2-methylpropanesulfonic acid and salts thereof; nonionic monomers such as (meth)acrylamide, N,N-dimethyl(meth)acrylamide, 2-hydroxyethyl(meth)acrylate, N-methylol(meth)acrylamide, and polyethylene glycol mono(meth)acrylate; and amino group-containing unsaturated monomers such as N,N-diethylaminoethyl(meth)acrylate, N,N-diethylaminopropyl(meth)acrylate, and diethylaminopropyl(meth)acrylamide, and quaternized products thereof. Among these water-soluble ethylenically unsaturated monomers, (meth)acrylic acid or a salt thereof, (meth)acrylamide, and N,N-dimethylacrylamide are preferred, and (meth)acrylic acid and a salt thereof are more preferred, from the viewpoint of industrial ease of availability, etc. These water-soluble ethylenically unsaturated monomers may be used alone or in combination of two or more.

[0019] Among these, acrylic acid and its salts are widely used as raw materials for water-absorbent resin particles, and these acrylic acid and / or salts thereof may be copolymerized with the other water-soluble ethylenically unsaturated monomers described above for use. In this case, it is preferable that acrylic acid and / or its salts are used as the main water-soluble ethylenically unsaturated monomer in an amount of 70 to 100 mol % based on the total amount of water-soluble ethylenically unsaturated monomers.

[0020] The water-soluble ethylenically unsaturated monomer may be dispersed in a hydrocarbon dispersion medium in the form of an aqueous solution and subjected to reversed-phase suspension polymerization. By forming the water-soluble ethylenically unsaturated monomer into an aqueous solution, the dispersion efficiency in the hydrocarbon dispersion medium can be increased. The concentration of the water-soluble ethylenically unsaturated monomer in this aqueous solution is preferably in the range of 20% by mass to the saturated concentration or less. The concentration of the water-soluble ethylenically unsaturated monomer is more preferably 55% by mass or less, even more preferably 50% by mass or less, and even more preferably 45% by mass or less. Meanwhile, the concentration of the water-soluble ethylenically unsaturated monomer is more preferably 25% by mass or more, even more preferably 28% by mass or more, and even more preferably 30% by mass or more.

[0021] When the water-soluble ethylenically unsaturated monomer has an acid group, such as (meth)acrylic acid or 2-(meth)acrylamido-2-methylpropanesulfonic acid, the acid group may be neutralized in advance with an alkaline neutralizing agent, as necessary. Examples of such alkaline neutralizing agents include alkali metal salts such as sodium hydroxide, sodium carbonate, sodium bicarbonate, potassium hydroxide, and potassium carbonate; and ammonia. These alkaline neutralizing agents may be used in the form of an aqueous solution to simplify the neutralization operation. The alkaline neutralizing agents described above may be used alone or in combination of two or more.

[0022] The degree of neutralization of the water-soluble ethylenically unsaturated monomer with the alkaline neutralizing agent is preferably 40 to 100 mol %, more preferably 50 to 90 mol %, even more preferably 60 to 85 mol %, and still more preferably 70 to 80 mol %, as the degree of neutralization with respect to all acid groups possessed by the water-soluble ethylenically unsaturated monomer.

[0023] [Radical Polymerization Initiator] Examples of the radical polymerization initiator added to the polymerization step include persulfates such as potassium persulfate, ammonium persulfate, and sodium persulfate, peroxides such as methyl ethyl ketone peroxide, methyl isobutyl ketone peroxide, di-t-butyl peroxide, t-butylcumyl peroxide, t-butyl peroxyacetate, t-butylperoxyisobutyrate, t-butylperoxypivalate, and hydrogen peroxide, as well as 2,2'-azobis(2-amidinopropane) dihydrochloride and 2,2'-azobis[2-(N-phenyl)propane]. Examples of the radical polymerization initiator include azo compounds such as 2,2'-azobis[2-(N-allylamidino)propane] dihydrochloride, 2,2'-azobis{2-[1-(2-hydroxyethyl)-2-imidazolin-2-yl]propane} dihydrochloride, 2,2'-azobis{2-methyl-N-[1,1-bis(hydroxymethyl)-2-hydroxyethyl]propionamide}, 2,2'-azobis[2-methyl-N-(2-hydroxyethyl)-propionamide], and 4,4'-azobis(4-cyanovaleric acid). Among these radical polymerization initiators, potassium persulfate, ammonium persulfate, sodium persulfate, and 2,2'-azobis(2-amidinopropane) dihydrochloride are preferred from the viewpoints of ease of availability and handling. These radical polymerization initiators may be used alone or in combination of two or more. The radical polymerization initiator can also be used as a redox polymerization initiator in combination with a reducing agent such as sodium sulfite, sodium hydrogen sulfite, ferrous sulfate, or L-ascorbic acid.

[0024] The amount of radical polymerization initiator used is, for example, 0.00005 to 0.01 mole per mole of the water-soluble ethylenically unsaturated monomer. By using such an amount, it is possible to avoid a rapid polymerization reaction and complete the polymerization reaction within an appropriate time.

[0025] [Internal Crosslinking Agent] The internal crosslinking agent can be one that can crosslink the polymer of the water-soluble ethylenically unsaturated monomer used, such as (poly)ethylene glycol ("(poly)" refers to both the presence and absence of the prefix "poly"). the same applies hereinafter)], unsaturated polyesters obtained by reacting polyols such as diols and triols, such as (poly)propylene glycol, 1,4-butanediol, 1,6-hexanediol, trimethylolpropane, and (poly)glycerin, with unsaturated acids, such as (meth)acrylic acid, maleic acid, and fumaric acid; bisacrylamides such as N,N-methylenebisacrylamide; di(meth)acrylic acid esters or tri(meth)acrylic acid esters obtained by reacting polyepoxides with (meth)acrylic acid; di(meth)acrylic acid carbamyl esters obtained by reacting polyisocyanates, such as tolylene diisocyanate and hexamethylene diisocyanate, with hydroxyethyl (meth)acrylate; allylated starch, allylated cellulose, diallyl phthalate, N,N',N''-triallyl isocyanurate, divinyl Examples of the compound include a compound having two or more polymerizable unsaturated groups such as benzene; a diglycidyl compound such as (poly)ethylene glycol diglycidyl ether, (poly)propylene glycol diglycidyl ether, (poly)glycerin diglycidyl ether, and the like, and a polyglycidyl compound such as a triglycidyl compound; an epihalohydrin compound such as epichlorohydrin, epibromohydrin, and α-methylepichlorohydrin; a compound having two or more reactive functional groups such as an isocyanate compound such as 2,4-tolylene diisocyanate and hexamethylene diisocyanate; and an oxetane compound such as 3-methyl-3-oxetane methanol, 3-ethyl-3-oxetane methanol, 3-butyl-3-oxetane methanol, 3-methyl-3-oxetane ethanol, 3-ethyl-3-oxetane ethanol, and 3-butyl-3-oxetane ethanol. Among these internal crosslinking agents, it is preferable to use polyethylene glycol diacrylate, trimethylolpropane triacrylate, (poly)ethylene glycol diglycidyl ether, (poly)propylene glycol diglycidyl ether, and (poly)glycerin diglycidyl ether.These internal crosslinking agents may be used alone or in combination of two or more.

[0026] The amount of the internal crosslinking agent used is preferably 0.000001 to 0.02 mol, more preferably 0.00001 to 0.01 mol, even more preferably 0.00001 to 0.005 mol, and still more preferably 0.00005 to 0.002 mol, relative to 1 mol of the water-soluble ethylenically unsaturated monomer.

[0027] [Hydrocarbon Dispersion Medium] Examples of hydrocarbon dispersion media include aliphatic hydrocarbons having 6 to 8 carbon atoms, such as n-hexane, n-heptane, 2-methylhexane, 3-methylhexane, 2,3-dimethylpentane, 3-ethylpentane, and n-octane; alicyclic hydrocarbons, such as cyclohexane, methylcyclohexane, cyclopentane, methylcyclopentane, trans-1,2-dimethylcyclopentane, cis-1,3-dimethylcyclopentane, and trans-1,3-dimethylcyclopentane; and aromatic hydrocarbons, such as benzene, toluene, and xylene. Among these hydrocarbon dispersion media, n-hexane, n-heptane, and cyclohexane are particularly preferred because they are easily available industrially, have stable quality, and are inexpensive. These hydrocarbon dispersion media may be used alone or in combination of two or more. As an example of a mixture of hydrocarbon dispersion media, a commercially available product such as Exxol Heptane (manufactured by ExxonMobil Corporation; contains 75 to 85% by mass of hydrocarbons such as heptane and its isomers) can also be used to obtain favorable results.

[0028] The amount of hydrocarbon dispersion medium used is preferably 100 to 1500 parts by mass, and more preferably 200 to 1400 parts by mass, per 100 parts by mass of the water-soluble ethylenically unsaturated monomer in the first stage, from the viewpoints of uniformly dispersing the water-soluble ethylenically unsaturated monomer and facilitating control of the polymerization temperature. As will be described later, in the present invention, the reversed-phase suspension polymerization is carried out in two or more stages, and the above-mentioned first-stage polymerization refers to the first-stage polymerization reaction in the multistage polymerization (the same applies hereinafter).

[0029] [Dispersion stabilizer] The production method of the present invention includes a step of aggregating polymer particles in the presence of a dispersion stabilizer, and two or more types of (poly)glycerin fatty acid esters having different precipitation temperatures when dissolved in a 0.46% by mass heptane solution are used as the dispersion stabilizer.

[0030] The precipitation temperature of a 0.46% by mass heptane solution of the (poly)glycerin fatty acid ester (hereinafter sometimes simply referred to as "precipitation temperature") was measured by the method described in the Examples.

[0031] From the viewpoint of more suitably exerting the effects of the present invention, the difference between the precipitation temperature of the (poly)glycerin fatty acid ester having the highest precipitation temperature and the precipitation temperature of the (poly)glycerin fatty acid ester having the lowest precipitation temperature, among the two or more (poly)glycerin fatty acid esters, is preferably 15°C or less, more preferably 10°C or less, even more preferably 6°C or less, and is preferably 1°C or more, more preferably 2°C or more, even more preferably 3°C or more, and preferred ranges include 1 to 15°C, 1 to 10°C, 1 to 6°C, 2 to 15°C, 2 to 10°C, 2 to 6°C, 3 to 15°C, 3 to 10°C, and 3 to 6°C.

[0032] The precipitation temperature of the (poly)glycerin fatty acid ester is preferably 55°C or lower, more preferably 45°C or lower, even more preferably 40°C or lower, and even more preferably 35°C or lower, and is preferably 0°C or higher, more preferably 10°C or higher, even more preferably 15°C or higher, and even more preferably 20°C or higher. Preferred ranges include 0 to 55°C, 0 to 45°C, 0 to 40°C, 0 to 35°C, 10 to 55°C, 10 to 45°C, 10 to 40°C, 10 to 35°C, 15 to 55°C, 15 to 45°C, 15 to 40°C, 15 to 35°C, 20 to 55°C, 20 to 45°C, 20 to 40°C, and 20 to 35°C.

[0033] The (poly)glycerin fatty acid ester can reduce the coloring of the water absorbent resin particles due to heating. Specific examples of the (poly)glycerin fatty acid ester include monoglyceryl monostearate, tetraglyceryl monostearate, tetraglyceryl tristearate, tetrapolyglyceryl pentastearate, hexaglyceryl tristearate, decaglyceryl tristearate, decaglyceryl decastearate, decaglyceryl pentastearate, decaglyceryl dodecabehenate, decaglyceryl pentaisostearate, decaglyceryl pentaoleate, decaglyceryl pentapalmitate, decaglyceryl pentalaurate, decaglyceryl pentahydroxystearate, and hexaglyceryl condensed ricinoleate. In the present invention, it is preferable to use two or more types of (poly)glycerin fatty acid esters from among these (poly)glycerin fatty acid esters. It is more preferable that one of the two or more (poly)glycerin fatty acid esters is hexaglyceryl tristearate.

[0034] The total amount of two or more (poly)glycerin fatty acid esters used is preferably 0.2 parts by mass or more, more preferably 0.9 parts by mass or more, even more preferably 1.3 parts by mass or more, and still more preferably 1.4 parts by mass or more, relative to 100 parts by mass of the water-soluble ethylenically unsaturated monomer used in the first-stage polymerization of the reversed-phase suspension polymerization, and is preferably 12.0 parts by mass or less, more preferably 4.0 parts by mass or less, even more preferably 2.0 parts by mass or less, and still more preferably 1.7 parts by mass or less. parts or less, and preferred ranges include 0.2 to 12.0 parts by mass, 0.2 to 4.0 parts by mass, 0.2 to 2.0 parts by mass, 0.2 to 1.7 parts by mass, 0.9 to 12.0 parts by mass, 0.9 to 4.0 parts by mass, 0.9 to 2.0 parts by mass, 0.9 to 1.7 parts by mass, 1.3 to 12.0 parts by mass, 1.3 to 4.0 parts by mass, 1.3 to 2.0 parts by mass, 1.3 to 1.7 parts by mass, 1.4 to 12.0 parts by mass, 1.4 to 4.0 parts by mass, 1.4 to 2.0 parts by mass, and 1.4 to 1.7 parts by mass.

[0035] Furthermore, the amount of the (poly)glycerin fatty acid ester having the highest precipitation temperature among the two or more types of (poly)glycerin fatty acid esters used is preferably 0.1 parts by mass or more, more preferably 0.2 parts by mass or more, and even more preferably 0.3 parts by mass or more, relative to 100 parts by mass of the water-soluble ethylenically unsaturated monomer used in the first-stage polymerization of the reversed-phase suspension polymerization, and is preferably 12.0 parts by mass or less, more preferably 6.0 parts by mass or less, even more preferably 4.0 parts by mass or less, even more preferably 1.2 parts by mass or less, and even more preferably 1.0 part by mass or less. , and more preferably 0.5 parts by mass or less, and preferred ranges include 0.1 to 12.0 parts by mass, 0.1 to 6.0 parts by mass, 0.1 to 4.0 parts by mass, 0.1 to 1.2 parts by mass, 0.1 to 1.0 parts by mass, 0.1 to 0.5 parts by mass, 0.2 to 12.0 parts by mass, 0.2 to 6.0 parts by mass, 0.2 to 4.0 parts by mass, 0.2 to 1.2 parts by mass, 0.2 to 1.0 parts by mass, 0.2 to 0.5 parts by mass, 0.3 to 12.0 parts by mass, 0.3 to 6.0 parts by mass, 0.3 to 4.0 parts by mass, 0.3 to 1.2 parts by mass, 0.3 to 1.0 parts by mass, and 0.3 to 0.5 parts by mass.

[0036] Furthermore, the amount of the (poly)glycerin fatty acid ester having the lowest precipitation temperature, among the two or more types of (poly)glycerin fatty acid esters, used relative to 100 parts by mass of the water-soluble ethylenically unsaturated monomer used in the first-stage polymerization of the reversed-phase suspension polymerization, is preferably 0.2 parts by mass or more, more preferably 0.4 parts by mass or more, and is also preferably 12.0 parts by mass or less, more preferably 4.0 parts by mass or less, even more preferably 1.2 parts by mass or less, and still more preferably 0.8 parts by mass or less; preferred ranges include 0.2 to 12.0 parts by mass, 0.2 to 4.0 parts by mass, 0.2 to 1.2 parts by mass, 0.2 to 0.8 parts by mass, 0.4 to 12.0 parts by mass, 0.4 to 4.0 parts by mass, 0.4 to 1.2 parts by mass, and 0.4 to 0.8 parts by mass.

[0037] When at least one of the (poly)glycerin fatty acid esters is hexaglyceryl tristearate, the amount of hexaglyceryl tristearate used is preferably 0.2 parts by mass or more, more preferably 0.5 parts by mass or more, and preferably 9.0 parts by mass or less, more preferably 4.0 parts by mass or less, more preferably 0.9 parts by mass or less, and even more preferably 0.7 parts by mass or less, relative to 100 parts by mass of the water-soluble ethylenically unsaturated monomer used in the first-stage polymerization of the reversed-phase suspension polymerization. Preferred ranges include 0.2 to 9.0 parts by mass, 0.2 to 4.0 parts by mass, 0.2 to 0.9 parts by mass, 0.2 to 0.7 parts by mass, 0.5 to 9.0 parts by mass, 0.5 to 4.0 parts by mass, 0.5 to 0.9 parts by mass, and 0.5 to 0.7 parts by mass.

[0038] When at least one of the (poly)glycerin fatty acid esters is hexaglyceryl tristearate, and the composition contains hexaglyceryl tristearate and at least one of a (poly)glycerin fatty acid ester having a higher precipitation temperature than hexaglyceryl tristearate and a (poly)glycerin fatty acid ester having a lower precipitation temperature, one or more types of (poly)glycerin fatty acid esters having a higher precipitation temperature than hexaglyceryl tristearate may be used. Also, one or more types of (poly)glycerin fatty acid esters having a lower precipitation temperature than hexaglyceryl tristearate may be used.

[0039] In addition, when at least one of the (poly)glycerin fatty acid esters is hexaglyceryl tristearate, the composition contains hexaglyceryl tristearate and at least one of a (poly)glycerin fatty acid ester having a higher precipitation temperature than hexaglyceryl tristearate and a (poly)glycerin fatty acid ester having a lower precipitation temperature, and the amount of the (poly)glycerin fatty acid ester having a higher precipitation temperature than hexaglyceryl tristearate and the (poly)glycerin fatty acid ester having a lower precipitation temperature used is 1:10 ... With respect to parts by mass, it is preferably 0.0 part by mass or more, more preferably 0.3 part by mass or more, even more preferably 0.5 part by mass or more, and preferably 3 parts by mass or less, more preferably 2.0 parts by mass or less, even more preferably 1.3 parts by mass or less, and even more preferably 1.0 part by mass or less. Preferred ranges include 0.0 to 3.0 parts by mass, 0.0 to 2.0 parts by mass, 0.0 to 1.3 parts by mass, 0.3 to 3.0 parts by mass, 0.3 to 2.0 parts by mass, 0.3 to 1.3 parts by mass, 0.5 to 3.0 parts by mass, 0.5 to 2.0 parts by mass, and 0.5 to 1.3 parts by mass.

[0040] When at least one of the (poly)glycerin fatty acid esters is hexaglyceryl tristearate, and the composition contains hexaglyceryl tristearate, a (poly)glycerin fatty acid ester having a higher precipitation temperature than hexaglyceryl tristearate, and a (poly)glycerin fatty acid ester having a lower precipitation temperature, one or more types of (poly)glycerin fatty acid esters having a higher precipitation temperature than hexaglyceryl tristearate may be used. Also, one or more types of (poly)glycerin fatty acid esters having a lower precipitation temperature than hexaglyceryl tristearate may be used.

[0041] In addition, when at least one of the (poly)glycerin fatty acid esters is hexaglyceryl tristearate, the composition contains hexaglyceryl tristearate, and (poly)glycerin fatty acid esters having a higher precipitation temperature than hexaglyceryl tristearate and (poly)glycerin fatty acid esters having a lower precipitation temperature, and the amount of the (poly)glycerin fatty acid ester having a higher precipitation temperature than hexaglyceryl tristearate and the (poly)glycerin fatty acid ester having a lower precipitation temperature are used in an amount of 1 part by mass of hexaglyceryl tristearate, respectively. With respect to the total amount, it is preferably more than 0.0 part by mass, more preferably 0.3 part by mass or more, even more preferably 0.5 part by mass or more, and is preferably 2.0 parts by mass or less, more preferably 1.3 parts by mass or less, and even more preferably 1.0 part by mass or less. Preferred ranges include more than 0.0 part by mass and 2.0 parts by mass or less, more than 0.0 part by mass and 1.3 parts by mass or less, more than 0.0 part by mass and 2.0 parts by mass or less, 0.3 to 2.0 parts by mass, 0.3 to 1.3 parts by mass, 0.3 to 1.0 parts by mass, 0.5 to 2.0 parts by mass, 0.5 to 1.3 parts by mass, and 0.5 to 1.0 part by mass.

[0042] In addition, when at least one of the (poly)glycerin fatty acid esters is hexaglyceryl tristearate and the composition contains hexaglyceryl tristearate, a (poly)glycerin fatty acid ester having a higher deposition temperature than hexaglyceryl tristearate, and a (poly)glycerin fatty acid ester having a lower deposition temperature, the amount of the (poly)glycerin fatty acid ester having a higher deposition temperature than hexaglyceryl tristearate used is preferably 0.0 mass parts per 1 mass part of hexaglyceryl tristearate. The amount is preferably more than 0.0 parts by mass, more preferably 0.2 parts by mass or more, even more preferably 0.4 parts by mass or more, and is preferably 2.0 parts by mass or less, more preferably 1.0 part by mass or less, and even more preferably 0.8 parts by mass or less. Preferred ranges include more than 0.0 part by mass and 2.0 parts by mass or less, more than 0.0 part by mass and 1.0 part by mass or less, more than 0.0 part by mass and 0.8 parts by mass or less, 0.2 to 2.0 parts by mass, 0.2 to 1.0 parts by mass, 0.2 to 0.8 parts by mass, 0.4 to 2.0 parts by mass, 0.4 to 1.0 parts by mass, and 0.4 to 0.8 parts by mass.

[0043] Furthermore, when at least one of the (poly)glycerin fatty acid esters is hexaglyceryl tristearate and the composition contains hexaglyceryl tristearate, a (poly)glycerin fatty acid ester having a higher precipitation temperature than hexaglyceryl tristearate, and a (poly)glycerin fatty acid ester having a lower precipitation temperature than hexaglyceryl tristearate, the amount of the (poly)glycerin fatty acid ester having a lower precipitation temperature than hexaglyceryl tristearate used is preferably 0. It is more than 0 parts by mass, more preferably 0.2 parts by mass or more, even more preferably 0.5 parts by mass or more, and preferably 2.0 parts by mass or less, more preferably 1.5 parts by mass or less, and even more preferably 0.9 parts by mass or less. Preferred ranges include more than 0.0 parts by mass and 2.0 parts by mass or less, more than 0 parts by mass and 1.5 parts by mass or less, more than 0 parts by mass and 0.9 parts by mass or less, 0.2 to 2.0 parts by mass, 0.2 to 1.5 parts by mass, 0.2 to 0.9 parts by mass, 0.5 to 2.0 parts by mass, 0.5 to 1.5 parts by mass, and 0.5 to 0.9 parts by mass.

[0044] The two or more (poly)glycerin fatty acid esters having different precipitation temperatures in a 0.46 mass% heptane solution may be used in the present invention as long as they are two or more types, but preferably two to five types, more preferably two to four types, even more preferably two to three types, and particularly preferably three types.

[0045] In the aggregation step of the production method of the present invention, in addition to the two or more types of (poly)glycerin fatty acid esters described above, another dispersion stabilizer different from the (poly)glycerin fatty acid esters may also be present, and as the other dispersion stabilizer, a polymeric dispersant is preferred from the viewpoint of narrowing the particle size distribution. In the aggregation step, the total proportion of the two or more types of (poly)glycerin fatty acid esters in the total dispersion stabilizers is preferably 50% by mass or more, more preferably 60% by mass or more, even more preferably 80% by mass or more, and may be 90% by mass or more, 95% by mass or more, 99% by mass or more, etc.

[0046] As the dispersion stabilizer used in the step of aggregating polymer particles in the presence of a dispersion stabilizer, a surfactant or a polymeric dispersant can be used, and (poly)glycerin fatty acid ester is a surfactant. Examples of dispersion stabilizers other than (poly)glycerin fatty acid ester include the following surfactants and polymeric dispersants.

[0047] (Surfactant) In the reversed-phase suspension polymerization, a dispersion stabilizer can be used to improve the dispersion stability of the water-soluble ethylenically unsaturated monomer in the hydrocarbon dispersion medium. A surfactant can be used as the dispersion stabilizer.

[0048] Examples of surfactants that can be used include sucrose fatty acid esters, sorbitan fatty acid esters, polyoxyethylene sorbitan fatty acid esters, polyoxyethylene glycerin fatty acid esters, sorbitol fatty acid esters, polyoxyethylene sorbitol fatty acid esters, polyoxyethylene alkyl ethers, polyoxyethylene alkylphenyl ethers, polyoxyethylene castor oil, polyoxyethylene hydrogenated castor oil, alkylallyl formaldehyde condensed polyoxyethylene ethers, polyoxyethylene polyoxypropylene block copolymers, polyoxyethylene polyoxypropyl alkyl ethers, polyethylene glycol fatty acid esters, alkyl glucosides, N-alkyl gluconamides, polyoxyethylene fatty acid amides, polyoxyethylene alkylamines, phosphate esters of polyoxyethylene alkyl ethers, phosphate esters of polyoxyethylene alkyl allyl ethers, etc. These surfactants may be used alone or in combination of two or more.

[0049] (Polymer-Based Dispersant) As a dispersion stabilizer used in reversed-phase suspension polymerization, a polymer-based dispersant may be used in combination with the surfactant described above.

[0050] Examples of polymeric dispersants include maleic anhydride-modified polyethylene, maleic anhydride-modified polypropylene, maleic anhydride-modified ethylene-propylene copolymer, maleic anhydride-modified EPDM (ethylene-propylene-diene terpolymer), maleic anhydride-modified polybutadiene, maleic anhydride-ethylene copolymer, maleic anhydride-propylene copolymer, maleic anhydride-ethylene-propylene copolymer, maleic anhydride-butadiene copolymer, polyethylene, polypropylene, ethylene-propylene copolymer, oxidized polyethylene, oxidized polypropylene, oxidized ethylene-propylene copolymer, ethylene-acrylic acid copolymer, ethyl cellulose, and ethylhydroxyethyl cellulose. Among these polymeric dispersants, it is particularly preferable to use maleic anhydride-modified polyethylene, maleic anhydride-modified polypropylene, maleic anhydride-modified ethylene-propylene copolymer, maleic anhydride-ethylene copolymer, maleic anhydride-propylene copolymer, maleic anhydride-ethylene-propylene copolymer, polyethylene, polypropylene, ethylene-propylene copolymer, oxidized polyethylene, oxidized polypropylene, and oxidized ethylene-propylene copolymer from the viewpoint of dispersion stability of the monomer. These polymeric dispersants may be used alone or in combination of two or more.

[0051] [Other Components] In the method for producing water-absorbent resin particles, if desired, other components may be added to an aqueous solution containing a water-soluble ethylenically unsaturated monomer to carry out reverse phase suspension polymerization. As the other components, various additives such as a thickener and a chain transfer agent can be added.

[0052] For example, a thickener can be added to an aqueous solution containing a water-soluble ethylenically unsaturated monomer to carry out reversed-phase suspension polymerization. By adjusting the viscosity of the aqueous solution by adding a thickener in this way, it is possible to control the median particle size obtained in the reversed-phase suspension polymerization.

[0053] Examples of usable thickeners include hydroxyethyl cellulose, hydroxypropyl cellulose, methyl cellulose, carboxymethyl cellulose, polyacrylic acid, (partially) neutralized polyacrylic acid, polyethylene glycol, polyacrylamide, polyethyleneimine, dextrin, sodium alginate, polyvinyl alcohol, polyvinylpyrrolidone, polyethylene oxide, etc. If the stirring speed during polymerization is the same, the higher the viscosity of the water-soluble ethylenically unsaturated monomer aqueous solution, the larger the primary particles and / or secondary particles of the resulting particles tend to be.

[0054] [Reverse-phase suspension polymerization] In carrying out the reverse-phase suspension polymerization, for example, an aqueous monomer solution containing a water-soluble ethylenically unsaturated monomer is dispersed in a hydrocarbon dispersion medium in the presence of two or more types of (poly)glycerin fatty acid esters as dispersion stabilizers (and, if necessary, other dispersion stabilizers). In this case, the timing of adding the dispersion stabilizer may be either before or after the addition of the aqueous monomer solution, as long as it is before the start of the polymerization reaction.

[0055] Among these, from the viewpoint of easily reducing the amount of hydrocarbon dispersion medium remaining in the obtained water absorbent resin particles, it is preferable to disperse an aqueous monomer solution in a hydrocarbon dispersion medium having a polymeric dispersant dispersed therein, and then further disperse a surfactant therein, and then carry out polymerization.

[0056] Such reversed-phase suspension polymerization can be carried out in two or more stages, and is preferably carried out in two or three stages from the viewpoint of increasing productivity.

[0057] Inverse phase suspension polymerization carried out in two or more stages, after the first stage of reverse phase suspension polymerization, a water-soluble ethylenically unsaturated monomer is added to the reaction mixture obtained in the first stage of polymerization reaction and mixed, and the second and subsequent stages of reverse phase suspension polymerization are carried out in the same manner as the first stage.In the reverse phase suspension polymerization in each stage from the second stage onwards, in addition to the water-soluble ethylenically unsaturated monomer, it is preferable to add a radical polymerization initiator within the molar ratio of each component to the water-soluble ethylenically unsaturated monomer as described above, based on the amount of the water-soluble ethylenically unsaturated monomer added during the reverse phase suspension polymerization in each stage from the second stage onwards, to carry out reverse phase suspension polymerization.In addition, in the polymerization from the second stage onwards, an internal crosslinking agent may also be added to the water-soluble ethylenically unsaturated monomer as needed.

[0058] The reaction temperature of the polymerization reaction is preferably 20 to 110°C, more preferably 40 to 90°C, from the viewpoints of rapidly progressing the polymerization, shortening the polymerization time, thereby improving economic efficiency, and easily removing the heat of polymerization to smoothly carry out the reaction.

[0059] As long as the aggregation step of the polymer particles is carried out after the first-stage reversed-phase suspension polymerization, the aggregation step may be carried out between the first-stage reversed-phase suspension polymerization and the second-stage reversed-phase suspension polymerization, or may be carried out after the second-stage or later reversed-phase suspension polymerization. From the viewpoint of more suitably exhibiting the effects of the present invention, the aggregation step of the polymer particles is preferably carried out between the first-stage reversed-phase suspension polymerization and the second-stage reversed-phase suspension polymerization. In addition, in the aggregation step of the polymer particles, the polymerization reaction of the water-soluble ethylenically unsaturated monomer may or may not have progressed.

[0060] In the present invention, when aggregating the polymer particles obtained by the first-stage reversed-phase suspension polymerization in the aggregation step, the temperature of the slurry is adjusted to gradually precipitate two or more (poly)glycerin fatty acid esters having different precipitation temperatures, thereby controlling the degree of aggregation of the polymer particles. For example, before adding the aqueous solution of water-soluble ethylenically unsaturated monomer in the second-stage reversed-phase suspension polymerization, the temperature of the slurry is lowered to precipitate at least one of the two or more (poly)glycerin fatty acid esters. In this way, droplets of the water-soluble ethylenically unsaturated monomer added thereafter are not stabilized in the hydrocarbon dispersion medium and are absorbed by the polymer particles (gel-like primary particles), thereby promoting aggregation between the polymer particles. On the other hand, when the temperature of the slurry is increased, the (poly)glycerin fatty acid ester dissolves, and droplets of the water-soluble ethylenically unsaturated monomer are stabilized in the hydrocarbon dispersion medium, thereby suppressing aggregation between the polymer particles. In the present invention, by adjusting the promotion / suppression of aggregation between the polymer particles, water-absorbent resin particles with a narrow particle size distribution can be suitably produced.

[0061] The temperature range in the aggregation step is not particularly limited as long as it is a temperature range in which aggregation of polymer particles can be controlled, and examples thereof include a range of 5 to 50°C, preferably 10 to 40°C, and more preferably 15 to 30°C.

[0062] <Dehydration Step> After the above-described reversed-phase suspension polymerization, the process may include a dehydration treatment in which water, hydrocarbon dispersion medium, and the like are removed by distillation by applying energy such as heat from the outside. When dehydrating the hydrous gel-like material after reversed-phase suspension polymerization, the system in which the hydrous gel-like material is dispersed in the hydrocarbon dispersion medium is heated, and the water and hydrocarbon dispersion medium are temporarily distilled out of the system by azeotropic distillation. In this case, if only the evaporated hydrocarbon dispersion medium is returned to the system, continuous azeotropic distillation is possible. In this case, the temperature in the system during drying is maintained below the azeotropic temperature with the hydrocarbon dispersion medium, which is preferable from the viewpoint of preventing deterioration of the resin. By controlling the dehydration treatment conditions after this polymerization to adjust the amount of dehydration (i.e., adjusting the water content of the polymer particles), it is possible to control the various properties of the obtained water-absorbent resin particles.

[0063] In the dehydration step, dehydration treatment by distillation may be carried out under normal pressure. When dehydration treatment is carried out under normal pressure, the dehydration temperature is preferably 70 to 250°C, more preferably 80 to 180°C, even more preferably 80 to 140°C, and even more preferably 90 to 130°C.

[0064] <Surface cross-linking step> The surface cross-linking step is a step of subjecting the polymer particles obtained in the polymerization step to surface cross-linking. When the polymer particles are cross-linked polymer particles (hydrogel-like material), the surface cross-linking step is a step of adding a surface cross-linking agent to a hydrogel-like material having an internal cross-linked structure obtained by polymerizing a water-soluble ethylenically unsaturated monomer to perform cross-linking (surface cross-linking reaction). This surface cross-linking reaction is preferably carried out in the presence of a surface cross-linking agent after the polymerization of the water-soluble ethylenically unsaturated monomer. In this way, by subjecting a hydrogel-like material having an internal cross-linked structure to a surface cross-linking reaction after the polymerization, the cross-linking density near the surface of the water-absorbent resin particles can be increased, and water-absorbent resin particles having improved performance such as water absorption capacity under load can be obtained.

[0065] Examples of the surface cross-linking agent include compounds having two or more reactive functional groups. For example, polyols such as ethylene glycol, propylene glycol, 1,4-butanediol, diethylene glycol, triethylene glycol, trimethylolpropane, glycerin, polyoxyethylene glycol, polyoxypropylene glycol, and polyglycerin; polyglycidyl compounds such as (poly)ethylene glycol diglycidyl ether, (poly)glycerin diglycidyl ether, (poly)glycerin triglycidyl ether, trimethylolpropane triglycidyl ether, (poly)propylene glycol polyglycidyl ether, and (poly)glycerol polyglycidyl ether; haloepoxy compounds such as epichlorohydrin, epibromohydrin, and α-methylepichlorohydrin; isocyanate compounds such as 2,4-tolylene diisocyanate and hexamethylene diisocyanate; 3-methyl-3-oxetanemethanol and 3-ethyl-3-oxetane Oxetane compounds such as methanol, 3-butyl-3-oxetanemethanol, 3-methyl-3-oxetaneethanol, 3-ethyl-3-oxetaneethanol, and 3-butyl-3-oxetaneethanol; oxazoline compounds such as 1,2-ethylenebisoxazoline; ethylene carbonate, propylene carbonate, 4,5-dimethyl-1,3-dioxolan-2-one, 4,4-dimethyl-1,3-dioxolan-2-one, 4-ethyl and hydroxyalkylamide compounds such as bis[N,N-di(β-hydroxyethyl)]adipamide. Among these surface cross-linking agents, polyglycidyl compounds such as (poly)ethylene glycol diglycidyl ether, (poly)glycerin diglycidyl ether, (poly)glycerin triglycidyl ether, trimethylolpropane triglycidyl ether, (poly)propylene glycol polyglycidyl ether, and (poly)glycerol polyglycidyl ether are preferred.These surface cross-linking agents may be used alone or in combination of two or more.

[0066] The amount of the surface crosslinking agent used is preferably 0.00001 to 0.01 mol, more preferably 0.00005 to 0.005 mol, and further preferably 0.0001 to 0.002 mol, relative to 1 mol of the total amount of the water-soluble ethylenically unsaturated monomers used in the polymerization.

[0067] As a method for adding the surface crosslinking agent, the surface crosslinking agent may be added as it is or as an aqueous solution, or, if necessary, may be added as a solution using a hydrophilic organic solvent as a solvent. Examples of the hydrophilic organic solvent include lower alcohols such as methyl alcohol, ethyl alcohol, n-propyl alcohol, isopropyl alcohol, etc.; ketones such as acetone, methyl ethyl ketone, etc.; ethers such as diethyl ether, dioxane, tetrahydrofuran, etc.; amides such as N,N-dimethylformamide, etc.; sulfoxides such as dimethyl sulfoxide, etc. These hydrophilic organic solvents may be used alone, or two or more types may be used in combination, or as a mixed solvent with water.

[0068] The timing of addition of the surface crosslinking agent may be after the polymerization reaction of the water-soluble ethylenically unsaturated monomer has almost completely finished, and the surface crosslinking agent is added in the presence of water in a range of preferably 1 to 400 parts by mass, more preferably 5 to 200 parts by mass, still more preferably 10 to 100 parts by mass, and still more preferably 20 to 60 parts by mass, relative to 100 parts by mass of the water-soluble ethylenically unsaturated monomer. Note that the amount of water means the total amount of water contained in the reaction system and water used as necessary when adding the surface crosslinking agent.

[0069] From the viewpoint of more suitably exerting the effects of the present invention, the water content of the polymer particles when the surface crosslinking agent is added is preferably 1% by mass or more, more preferably 10% by mass or more, still more preferably 20% by mass or more, and is preferably 60% by mass or less, more preferably 40% by mass or less, still more preferably 35% by mass or less. Preferred ranges include 1 to 60% by mass, 1 to 40% by mass, 1 to 35% by mass, 10 to 60% by mass, 10 to 40% by mass, 10 to 35% by mass, 20 to 60% by mass, 20 to 40% by mass, and 20 to 35% by mass.

[0070] The reaction temperature in the surface crosslinking reaction is preferably 50 to 250° C., more preferably 60 to 180° C., further preferably 60 to 140° C., and even more preferably 70 to 120° C. The reaction time of the surface crosslinking reaction is preferably 1 to 300 minutes, and more preferably 5 to 200 minutes.

[0071] <Drying step> After the above-described surface cross-linking is performed, a drying treatment may be included in which water, a hydrocarbon dispersion medium, etc. are removed by distillation by applying energy such as heat from the outside. The polymer particles after surface cross-linking are dried and the water and the hydrocarbon dispersion medium are distilled off, thereby obtaining water-absorbent resin particles.

[0072] In the drying step, the drying treatment by distillation may be carried out under normal pressure or under reduced pressure. Moreover, from the viewpoint of increasing the drying efficiency, it may be carried out under a gas flow such as nitrogen. When the drying treatment is carried out under normal pressure, the drying temperature is preferably 70 to 250°C, more preferably 80 to 180°C, even more preferably 80 to 140°C, and even more preferably 90 to 130°C. Moreover, when the drying treatment is carried out under reduced pressure, the drying temperature is preferably 40 to 160°C, more preferably 50 to 110°C.

[0073] In addition, when the surface cross-linking step using a surface cross-linking agent is carried out after the polymerization of monomers by reverse phase suspension polymerization, the drying step by distillation described above is carried out after the surface cross-linking step is completed. Alternatively, the surface cross-linking step and the drying step may be carried out simultaneously.

[0074] The water-absorbent resin particles of the present invention may contain additives according to the purpose. Examples of such additives include inorganic powders, surfactants, oxidizing agents, reducing agents, metal chelating agents, radical chain inhibitors, antioxidants, antibacterial agents, etc. For example, the fluidity of the water-absorbent resin particles can be further improved by adding 0.05 to 5 parts by mass of amorphous silica as inorganic powder relative to 100 parts by mass of the water-absorbent resin particles. The additives are preferably hydrophilic or water-soluble.

[0075] 2. Water-absorbent resin particles By employing the method for producing water-absorbent resin particles of the present invention described above, it is possible to suitably produce water-absorbent resin particles having a narrow particle size distribution. More specifically, by employing the method for producing water-absorbent resin particles of the present invention, it is possible to suitably produce water-absorbent resin particles having, for example, a uniformity of particle size distribution of 2.5 or less and containing a (poly)glycerin fatty acid ester in the vicinity of the surface.

[0076] The uniformity of the particle size distribution of the water-absorbent resin particles of the present invention may be 2.5 or less, preferably 2.3 or less, more preferably 1.8 or less, and even more preferably 1.6 or less. The lower limit is 1, for example.

[0077] The uniformity of the particle size distribution of the water-absorbent resin particles is measured by the measurement method described in the Examples.

[0078] Furthermore, the water-absorbent resin particles of the present invention contain a (poly)glycerin fatty acid ester near the surface. In the method for producing water-absorbent resin particles of the present invention, a (poly)glycerin fatty acid ester is used as described above. Therefore, the (poly)glycerin fatty acid ester is contained near the surface of the water-absorbent resin particles. Specific examples of the (poly)glycerin fatty acid ester include the same as those exemplified in the method for producing water-absorbent resin particles of the present invention. It is preferable that two or more types of (poly)glycerin fatty acid esters are contained near the surface of the water-absorbent resin particles, and it is preferable that hexaglyceryl tristearate is contained.

[0079] The amount of the (poly)glycerin fatty acid ester contained in the vicinity of the surface of the water-absorbent resin particle of the present invention is preferably 1 part by mass or less, more preferably 0.65 parts by mass or less, and even more preferably 0.55 parts by mass or less, relative to 100 parts by mass of the water-absorbent resin particle. The lower limit is preferably 0.1 parts by mass or more, more preferably 0.2 parts by mass or more, and even more preferably 0.3 parts by mass or more.

[0080] Moreover, the water-absorbent resin particles of the present invention have a yellowness index after being heated at 200° C. for 2 hours of preferably 20 or less, more preferably 15 or less, and further preferably 12 or less. The lower limit of the yellowness index is, for example, 0.

[0081] The initial value of yellowness of the water absorbent resin particles of the present invention (yellowness before heating at 200°C for 2 hours) is preferably 15 or less, more preferably 12 or less, and further preferably 10 or less. The lower limit of the yellowness is, for example, 0.

[0082] The yellowness of the water-absorbent resin particles before and after heating at 200° C. for 2 hours is measured by the measurement method described in the Examples.

[0083] The water-absorbent resin particles of the present invention are preferably composed of a crosslinked polymer of a water-soluble ethylenically unsaturated monomer, i.e., a crosslinked polymer having structural units derived from a water-soluble ethylenically unsaturated monomer.

[0084] The water-absorbent resin particles of the present invention are in the form of aggregates (secondary particles) of fine particles (primary particles). Examples of the shape of the primary particles include substantially spherical, irregularly crushed, and plate-like shapes. The water-absorbent resin particles of the present invention, which are secondary particles, may have various shapes. Examples of the shape of the water-absorbent resin particles include granular, substantially spherical, irregularly crushed, plate-like, fibrous, flake-like shapes, and shapes obtained by aggregating these resins. The water-absorbent resin particles are preferably granular, substantially spherical, irregularly crushed, fibrous, or shapes obtained by aggregating these resins.

[0085] The median particle diameter of the water-absorbent resin particles is preferably 100 μm or more, 150 μm or more, 200 μm or more, 250 μm or more, 280 μm or more, 300 μm or more, or 320 μm or more. From the same viewpoint, the median particle diameter is preferably 700 μm or less, 600 μm or less, 550 μm or less, 500 μm or less, 450 μm or less, or 400 μm or less. That is, the median particle diameter is preferably 150 to 700 μm, preferably 200 to 600 μm, more preferably 250 to 500 μm, even more preferably 250 to 450 μm, and even more preferably 250 to 400 μm.

[0086] The median particle size of the water-absorbent resin particles can be measured using a JIS standard sieve, and specifically, it is a value measured by the method described in the examples.

[0087] The physiological saline water absorption speed of the water-absorbent resin particles is preferably 20 seconds or more, more preferably 25 seconds or more, even more preferably 30 seconds or more, and is preferably 65 seconds or less, more preferably 60 seconds or less, even more preferably 55 seconds or less, and more preferably in the range of 20 to 65 seconds, 25 to 60 seconds, etc.

[0088] The physiological saline water retention capacity of the water-absorbent resin particles of the present invention is preferably 20 g / g or more, more preferably 30 g / g or more, and is also preferably 80 g / g or less, more preferably 60 g / g or less, further preferably 55 g / g or less, further preferably 53 g / g or less, and preferred ranges include 20 to 80 g / g, 20 to 60 g / g, 20 to 55 g / g, 20 to 53 g / g, 30 to 80 g / g, 30 to 60 g / g, 30 to 55 g / g, 30 to 53 g / g, etc.

[0089] 3. Absorbent Material, Absorbent Articles The water-absorbing resin particles of the present invention constitute an absorbent material used in hygiene materials such as sanitary products and disposable diapers, and are suitably used in absorbent articles containing the absorbent material.

[0090] The absorbent of the present invention contains the water-absorbent resin particles of the present invention. The absorbent may further contain hydrophilic fibers. Examples of the absorbent's configuration include a sheet-like structure in which water-absorbent resin particles are fixed on a nonwoven fabric or between multiple nonwoven fabrics, a mixed dispersion obtained by mixing water-absorbent resin particles and hydrophilic fibers to form a uniform composition, a sandwich structure in which water-absorbent resin particles are sandwiched between layered hydrophilic fibers, and a structure in which water-absorbent resin particles and hydrophilic fibers are wrapped in tissue. The absorbent may also contain other components, such as adhesive binders such as heat-fusible synthetic fibers, hot-melt adhesives, and adhesive emulsions, to improve the shape retention of the absorbent.

[0091] The basis weight of the water-absorbent resin particles in the absorbent body of the present invention is 50 g / m 2 More than 400g / m 2 The basis weight is preferably 100 g / m or less. 2 More preferably, 120 g / m 2 More preferably, 140 g / m 2 and preferably 300 g / m 2 or less, more preferably 250 g / m 2 More preferably, 200 g / m or less 2 The following is the result.

[0092] The hydrophilic fiber may be at least one selected from the group consisting of finely ground wood pulp, cotton, cotton linter, rayon, cellulose acetate, polyamide, polyester, and polyolefin. Examples include cellulose fibers such as cotton-like pulp obtained from wood, mechanical pulp, chemical pulp, and semi-chemical pulp; artificial cellulose fibers such as rayon and acetate; and fibers made of synthetic resins such as hydrophilically treated polyamide, polyester, and polyolefin. The average fiber length of the hydrophilic fiber is usually 0.1 to 10 mm, or may be 0.5 to 5 mm.

[0093] The basis weight of the hydrophilic fiber in the absorbent body of the present invention is 50 g / m 2 800g / m or more 2 The basis weight is preferably 100 g / m or less. 2More preferably, 120 g / m 2 More preferably, 140 g / m 2 and preferably 700 g / m 2 or less, more preferably 600 g / m 2 More preferably 500 g / m or less 2 The following is the result.

[0094] The content of the water-absorbent resin particles in the absorbent body is preferably 5 to 100% by mass, more preferably 10 to 95% by mass, even more preferably 20 to 90% by mass, and even more preferably 30 to 80% by mass.

[0095] The absorbent article of the present invention can be produced by holding an absorbent body using the water-absorbent resin particles of the present invention between a liquid-permeable sheet (top sheet) through which liquid can pass and a liquid-impermeable sheet (back sheet) through which liquid cannot pass. The liquid-permeable sheet is arranged on the side that comes into contact with the body, and the liquid-impermeable sheet is arranged on the opposite side that comes into contact with the body.

[0096] Examples of liquid-permeable sheets include air-through, spunbond, chemical-bond, and needle-punched nonwoven fabrics made of fibers such as polyethylene, polypropylene, and polyester, as well as porous synthetic resin sheets. Examples of liquid-impermeable sheets include synthetic resin films made of resins such as polyethylene, polypropylene, and polyvinyl chloride. The liquid-permeable sheet is preferably at least one selected from the group consisting of thermal-bonded nonwoven fabrics, air-through nonwoven fabrics, spunbonded nonwoven fabrics, and spunbonded / meltblown / spunbonded nonwoven fabrics.

[0097] The basis weight of the liquid permeable sheet is 5 g / m 2 More than 100g / m 2 Preferably, it is 10 g / m or less. 2 60g / m or more 2It is more preferable that the liquid-permeable sheet has a surface embossed or perforated to improve the liquid diffusibility. The embossing or perforation can be carried out by a known method.

[0098] Examples of liquid-impermeable sheets include sheets made of synthetic resins such as polyethylene, polypropylene, and polyvinyl chloride; sheets made of nonwoven fabrics such as spunbond / meltblown / spunbond (SMS) nonwoven fabrics in which a water-resistant meltblown nonwoven fabric is sandwiched between high-strength spunbond nonwoven fabrics; and sheets made of composite materials of these synthetic resins and nonwoven fabrics (for example, spunbond nonwoven fabrics, spunlace nonwoven fabrics). As the liquid-impermeable sheet, a sheet made of a synthetic resin mainly composed of low-density polyethylene (LDPE) resin can also be used. The liquid-impermeable sheet has, for example, a basis weight of 10 to 50 g / m. 2 The sheet may be made of a synthetic resin.

[0099] The absorbent article preferably comprises a laminate having an absorbent body containing water-absorbent resin particles and a core wrap sandwiching the absorbent body from above and below, a liquid-permeable sheet disposed on the upper surface of the laminate, and a liquid-impermeable sheet disposed on the surface of the laminate opposite to the liquid-permeable sheet side.

[0100] The present invention will be described in detail below with reference to examples and comparative examples, but the present invention is not limited to these examples.

[0101] The water-absorbent resin particles obtained in the following Examples and Comparative Examples were evaluated by the following various tests. Unless otherwise specified, the measurements were carried out in an environment of a temperature of 25±2°C and a humidity of 50±10%.

[0102] [Production of Water-Absorbent Resin Particles] (Example 1) A round-bottomed cylindrical separable flask having an inner diameter of 11 cm and a volume of 2 L and equipped with a reflux condenser, a dropping funnel, a nitrogen gas inlet tube, and a stirrer (a stirring blade having two stages of four inclined paddle blades with a blade diameter of 5 cm) was prepared. 300 g of n-heptane (hydrocarbon dispersion medium) was placed in this separable flask, and 0.737 g of tetraglyceryl tristearate (dispersion stabilizer, manufactured by Sakamoto Pharmaceutical Industrial Co., Ltd., SY Glyster TS-3S) and 0.737 g of hexaglyceryl tristearate (dispersion stabilizer, manufactured by Sakamoto Pharmaceutical Industrial Co., Ltd., SY Glyster TS-5S) were added thereto, and the mixture was heated to 80°C with stirring to dissolve, and then cooled to 55°C.

[0103] Next, 92.0 g of an 80.5% by mass aqueous acrylic acid solution (acrylic acid: 1.03 mol) was placed in a 500 mL Erlenmeyer flask. Subsequently, while cooling from the outside, 102.78 g of a 30% by mass aqueous sodium hydroxide solution was added dropwise to perform 75 mol % neutralization, and then 0.0736 g (0.272 mmol) of potassium persulfate (a water-soluble radical polymerization initiator), 0.0101 g (0.0580 mmol) of ethylene glycol diglycidyl ether (an internal crosslinking agent), and 44.62 g of ion-exchanged water were added and dissolved to prepare a first-stage aqueous solution.

[0104] The first-stage aqueous solution was added to the separable flask, and the atmosphere in the separable flask was thoroughly purged with nitrogen while stirring at a rotation speed of 400 rpm. The separable flask was then immersed in a water bath at 70°C to raise the temperature of the reaction solution, and first-stage polymerization was carried out for 60 minutes to obtain a first-stage slurry.

[0105] Next, 128.8 g of an 80.5% by mass aqueous solution of acrylic acid (acrylic acid: 1.44 mol) was placed in another 500 mL Erlenmeyer flask. Subsequently, while cooling from the outside, 143.89 g of a 30% by mass aqueous solution of sodium hydroxide was added dropwise to perform 75 mol % neutralization, and then 0.1030 g (0.3810 mmol) of potassium persulfate, 0.0116 g (0.0666 mmol) of ethylene glycol diglycidyl ether (internal crosslinking agent), and 16.75 g of ion-exchanged water were added and dissolved to prepare a second-stage aqueous liquid.

[0106] While stirring the first-stage slurry at 1000 rpm, the separable flask system was cooled to 20°C, and then the entire amount of the second-stage aqueous liquid was added to the first-stage slurry to carry out an aggregation step. After the system was purged with nitrogen for 30 minutes, the separable flask was again immersed in a 70°C water bath to raise the temperature, and a polymerization reaction was carried out for 60 minutes to obtain a second-stage slurry.

[0107] After the second-stage polymerization, the temperature of the second-stage slurry liquid was raised in an oil bath at 125°C, and 248 g of water was extracted from the system by azeotropic distillation of n-heptane and water while refluxing n-heptane. Thereafter, the n-heptane was evaporated and dried to obtain a dried product. The dried product was passed through a sieve with an opening of 850 µm to obtain 203.8 g of water-absorbent resin particles in the form of aggregates of approximately spherical particles.

[0108] (Example 2) The same operation as in Example 1 was performed except that decaglyceryl pentastearate (dispersion stabilizer, manufactured by Nippon Surfactant Industrial Co., Ltd., NIKKOL Decaglyn 5-SV) was used instead of tetraglyceryl tristearate in Example 1, to obtain 200.3 g of water absorbent resin particles.

[0109] (Example 3) The same operation as in Example 1 was performed except that 0.737 g of tetraglyceryl tristearate and 0.737 g of hexaglyceryl tristearate in Example 1 were used, and 194.5 g of water absorbent resin particles were obtained.

[0110] (Example 4) The same operation as in Example 1 was performed except that 0.276 g of tetraglyceryl tristearate, 0.460 g of hexaglyceryl tristearate, and 0.368 g of decaglyceryl pentastearate were used instead of 0.737 g of tetraglyceryl tristearate and 0.737 g of hexaglyceryl tristearate in Example 1, to obtain 215.5 g of water absorbent resin particles.

[0111] Example 5 A round-bottomed cylindrical separable flask with an inner diameter of 11 cm and a volume of 2 L was prepared, equipped with a reflux condenser, a dropping funnel, a nitrogen gas inlet tube, and a stirrer (a stirring blade with two stages of four inclined paddle blades with a blade diameter of 5 cm). 293 g of n-heptane (hydrocarbon dispersion medium) was placed in this separable flask, and 0.276 g of maleic anhydride-modified ethylene-propylene copolymer (polymer dispersant, Mitsui Chemicals, Inc., Hiwax 1105A) was added. The mixture was dissolved by heating to 80 ° C. with stirring, and then cooled to 55 ° C.

[0112] Next, 92.0 g of an 80.5% by mass aqueous acrylic acid solution (acrylic acid: 1.03 mol) was placed in a 500 mL Erlenmeyer flask. Subsequently, while cooling from the outside, 102.78 g of a 30% by mass aqueous sodium hydroxide solution was added dropwise to perform 75 mol % neutralization, and then 0.0736 g (0.272 mmol) of potassium persulfate (a water-soluble radical polymerization initiator), 0.0558 g of tetra(ethylene glycol) diacrylate (an internal crosslinking agent), and 44.62 g of ion-exchanged water were added and dissolved to prepare a first-stage aqueous solution.

[0113] The first-stage aqueous solution was added to the separable flask and stirred for 10 minutes. Then, 0.276 g of tetraglyceryl tristearate, 0.460 g of hexaglyceryl tristearate, and 0.368 g of decaglyceryl pentastearate were dissolved in 6.62 g of n-heptane with heating to obtain a dispersion stabilizer solution. 7.724 g of the resulting dispersion stabilizer solution was added to the separable flask, and the atmosphere in the separable flask was thoroughly purged with nitrogen while stirring at a rotation speed of 400 rpm. The separable flask was then immersed in a 70°C water bath to raise the temperature of the reaction solution, and first-stage polymerization was carried out for 60 minutes to obtain a first-stage slurry.

[0114] Next, 128.8 g of an 80.5% by mass acrylic acid aqueous solution (1.44 mol of acrylic acid) was placed in another 500 mL Erlenmeyer flask. Subsequently, while cooling externally, 143.89 g of a 30% by mass aqueous sodium hydroxide solution was added dropwise to perform 75 mol% neutralization, and then 0.1030 g (0.3810 mmol) of potassium persulfate, 0.0447 g of tetra(ethylene glycol) diacrylate (internal crosslinking agent), and 16.75 g of ion-exchanged water were added and dissolved to prepare a second-stage aqueous solution. While stirring the first-stage slurry at a rotation speed of 1000 rpm, the separable flask system was cooled to 20 ° C., and the entire amount of the second-stage aqueous solution was added to the first-stage slurry to perform the aggregation process. After the system was purged with nitrogen for 30 minutes, the flask was again immersed in a water bath at 70° C. to raise the temperature, and the polymerization reaction was carried out for 60 minutes to obtain a second-stage slurry.

[0115] After the second-stage polymerization, the temperature of the second-stage slurry liquid was raised in an oil bath at 125°C, and 248 g of water was extracted from the system by azeotropic distillation of n-heptane and water while refluxing n-heptane. Thereafter, the n-heptane was evaporated and dried to obtain a dried product. The dried product was passed through a sieve with an opening of 850 µm to obtain 205.4 g of water-absorbent resin particles in the form of aggregates of approximately spherical particles.

[0116] Comparative Example 1 The same procedure as in Example 1 was carried out except that 1.289 g of tetraglyceryl tristearate was used instead of 0.737 g of tetraglyceryl tristearate and 0.737 g of hexaglyceryl tristearate in Example 1, to obtain 103.1 g of water-absorbent resin particles.

[0117] <Evaluation of Water-Absorbent Resin Particles> (Median Particle Diameter) A water-absorbent resin composition was prepared by mixing 100 g of water-absorbent resin particles with 0.5 g of amorphous silica (Toxil NP-S, manufactured by Oriental Silicas Corporation) as a lubricant. Seven types of JIS standard sieves (mesh openings: 600 μm, 500 μm, 425 μm, 300 μm, 250 μm, 150 μm, and 75 μm) were used in this measurement. The water-absorbent resin composition was placed on top of a sieve in which the selected JIS standard sieve and a tray were combined in order, and the mixture was shaken for 20 minutes using a Rotap shaker. Next, the mass of the water-absorbent resin composition remaining on each sieve was calculated as a mass percentage relative to the total amount, and the values ​​were integrated in order from the smallest particle diameter. The relationship between the sieve opening and the integrated value of the mass percentage remaining on the sieve was plotted on logarithmic probability paper. The plots on the probability paper were connected with a straight line to determine the particle size corresponding to a cumulative mass percentage of 50% by mass as the median particle size. The results are shown in Table 3.

[0118] (Uniformity) In the median particle size measurement, the particle size (A1) corresponding to an undersize cumulative mass percentage of 10 mass % and the particle size (A2) corresponding to 60 mass % were determined, and the uniformity was calculated using the following formula. The results are shown in Table 3. Uniformity = A2 / A1 In other words, when the particle size distribution is narrow, the uniformity approaches 1, and when the particle size distribution is broad, the uniformity becomes greater than 1.

[0119] (Precipitation Temperature of Dispersion Stabilizer) The precipitation temperature of the dispersion stabilizer was determined by cooling a mixed solution in which the dispersion stabilizer was dissolved in a solvent by heating, and measuring the turbidity of the mixed solution at each temperature. That is, 100 g of n-heptane (hydrocarbon dispersion medium) and 0.46 g of dispersion stabilizer were added to an eggplant-shaped flask. This solution was heated to 80 ° C. while stirring with a stirrer tip, thereby obtaining a mixed solution in which the dispersion stabilizer was dissolved. Thereafter, the mixed solution was cooled, and the turbidity was measured every 1 ° C. using an integrating sphere turbidimeter SEP-PT-706D (manufactured by Nitto Seiko Analytech Co., Ltd., optical path length 10 mm) according to a calibration curve created using a turbidity standard solution (kaolin 1000 ° C.). The precipitation temperature of the dispersion stabilizer was compared with the turbidity at a temperature 1 ° C. higher, and the temperature at which the turbidity rose by 10 ppm or more was taken as the precipitation temperature. The results are shown in Table 1.

[0120] <Yellowing Coloring Test (Yellowness Index Measurement) Before and After Heating at 200°C for 2 Hours> 2.0 g of water-absorbent resin particles were uniformly placed in a glass measuring container having an inner diameter of 3 cm, and the yellowness index of the water-absorbent resin particles was measured using a color difference meter (Color Meter ZE6000, manufactured by Nippon Denshoku Industries Co., Ltd.) in which X, Y, and Z, which are the tristimulus values ​​of the colorimeter, were corrected using a standard white board. The yellowness index was calculated from the X, Y, and Z (tristimulus values) of the obtained water-absorbent resin particles using the following formula, and was defined as the yellowness index before heating. The results are shown in Table 3. Yellowness index=100(1.28X−1.06Z) / Y

[0121] Furthermore, a test for the yellowness of the water-absorbent resin particles after heating was carried out as follows. That is, 2.0 g of water-absorbent resin particles were uniformly placed in a glass petri dish with an inner diameter of 3 cm and a depth of 1 cm. Nitrogen was passed through the petri dish so that the flow rate at the outlet was 400 mL / min. A vacuum dryer (AVO-310N, manufactured by AS ONE Corporation) preheated to 200±5°C was prepared, and a 3 cm thick glass wool heat insulating material was laid on the inner bottom. A stainless steel tray was placed on the heat insulating material inside the vacuum dryer, and the glass petri dish containing the water-absorbent resin particles and a surface thermometer were further placed on the stainless steel tray. From the point when the surface thermometer reached 200±5°C, heating was carried out for 2 hours. Thereafter, the glass petri dish containing the water-absorbent resin particles together with the stainless steel tray was removed from the vacuum dryer, and the glass petri dish was allowed to stand in a desiccator for 30 minutes to cool to room temperature. The entire amount of water-absorbent resin particles in the glass petri dish was placed in a glass measuring container having an inner diameter of 3 cm, and the yellowness of the water-absorbent resin particles was measured with a color difference meter (Color Meter ZE6000, manufactured by Nippon Denshoku Industries Co., Ltd.). The yellowness was calculated from X, Y, and Z (tristimulus values) of the obtained water-absorbent resin particles using the above formula, and was defined as the yellowness after heating. The results are shown in Table 3.

[0122]

[0123]

[0124] In Table 2, the amount of dispersion stabilizer added is the amount added based on 100 parts by mass of acrylic acid, the water-soluble ethylenically unsaturated monomer used in the first-stage polymerization.

[0125]

Claims

1. A method for producing water-absorbent resin particles, comprising a step of obtaining polymer particles by reverse-phase suspension polymerization of a water-soluble ethylenically unsaturated monomer in a hydrocarbon dispersion medium, wherein the reverse-phase suspension polymerization is carried out in two or more stages, and a step of aggregating the polymer particles in the presence of a dispersion stabilizer is included, and two or more types of (poly)glycerin fatty acid esters are used as the dispersion stabilizer, which have different precipitation temperatures when made into a 0.46 mass % heptane solution.

2. The method for producing water-absorbent resin particles according to claim 1, wherein, among the two or more types of (poly)glycerin fatty acid esters, the difference between the precipitation temperature of the (poly)glycerin fatty acid ester having the highest precipitation temperature and the precipitation temperature of the (poly)glycerin fatty acid ester having the lowest precipitation temperature is 15°C or less.

3. The method for producing water-absorbent resin particles according to claim 1 or 2, wherein at least one of the (poly)glycerin fatty acid esters is hexaglyceryl tristearate, and the amount of hexaglyceryl tristearate used is in the range of 0.2 parts by mass or more and 4.0 parts by mass or less per 100 parts by mass of the water-soluble ethylenically unsaturated monomer used in the first-stage polymerization.

4. The method for producing water-absorbent resin particles according to claim 3, wherein the dispersion stabilizer comprises the hexaglyceryl tristearate and at least one of a (poly)glycerin fatty acid ester having a higher precipitation temperature than the hexaglyceryl tristearate and a (poly)glycerin fatty acid ester having a lower precipitation temperature, and the amounts of the (poly)glycerin fatty acid ester having a higher precipitation temperature than the hexaglyceryl tristearate and the (poly)glycerin fatty acid ester having a lower precipitation temperature used are each in a range of more than 0 part by mass and not more than 3.0 parts by mass per part by mass of the hexaglyceryl tristearate.

5. The method for producing water-absorbent resin particles according to claim 4, wherein the dispersion stabilizer comprises the hexaglyceryl tristearate, a (poly)glycerin fatty acid ester having a higher precipitation temperature than the hexaglyceryl tristearate, and a (poly)glycerin fatty acid ester having a lower precipitation temperature than the hexaglyceryl tristearate, and the amounts of the (poly)glycerin fatty acid ester having a higher precipitation temperature than the hexaglyceryl tristearate and the (poly)glycerin fatty acid ester having a lower precipitation temperature used are each more than 0 parts by mass and 2.0 parts by mass or less per part by mass of the hexaglyceryl tristearate.

6. Water-absorbent resin particles containing polymer particles having a water-soluble ethylenically unsaturated monomer as a monomer unit, the particles having a particle size distribution uniformity of 2.5 or less, and containing a (poly)glycerin fatty acid ester near the surface.

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