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

Abstract

A first embodiment of a method for producing water-absorbent resin particles is provided with a polymerization step for obtaining a polymer by polymerizing monomer on at least a portion of the surface of surface-crosslinked polymer particles. A second embodiment of a method for producing water-absorbent resin particles is provided with a polymerization step for obtaining a polymer by polymerizing monomer, in the presence of a crosslinking agent, on at least a portion of the surface of polymer particles that are not surface crosslinked.

Description

[Technical Field]

[0001] The present invention relates to a method for producing water-absorbent resin particles, water-absorbent resin particles, and the like. [Background technology]

[0002] Water-absorbent resin particles are widely used in various fields, such as sanitary materials such as disposable diapers, sanitary products, and portable toilets; agricultural and horticultural materials such as water retention agents and soil conditioners; and industrial materials such as waterproofing agents and anti-condensation agents. During the production of sanitary materials, water-absorbent resin particles may be damaged by collisions between water-absorbent resin particles or friction with machinery, resulting in the loss of their original water absorption performance. To address this problem, a technique is known for improving impact resistance and the like by controlling the internal air bubble ratio of water-absorbent resin particles (see, for example, Patent Document 1 below). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2014-098172 Summary of the Invention [Problem to be solved by the invention]

[0004] According to the findings of the present inventors, it is difficult to improve impact resistance while achieving excellent water absorption performance under load in the conventional techniques relating to water-absorbent resin particles.

[0005] An object of one aspect of the present invention is to provide a method for producing water-absorbent resin particles that can improve impact resistance while achieving excellent water absorption performance under load (equal to or greater than water absorption performance) compared to methods for producing water-absorbent resin particles using the same contents of raw materials.An object of another aspect of the present invention is to provide water-absorbent resin particles that can improve impact resistance while achieving excellent water absorption performance under load compared to water-absorbent resin particles obtained using the same contents of raw materials. [Means for solving the problem]

[0006] The present inventors have found the following. That is, as a technique for improving water absorption performance under load, surface cross-linking of polymer particles is considered. However, surface cross-linking alone may not only not sufficiently improve water absorption performance under load, but may also make the water-absorbent resin particles prone to breakage (low impact resistance). In contrast, by performing a treatment in which a monomer is polymerized on the surface of the polymer particles after surface cross-linking the polymer particles, or a treatment in which a monomer is polymerized on the surface of polymer particles that have not been surface-cross-linked in the presence of a cross-linking agent, it is possible to achieve excellent water absorption performance under load (water absorption performance under load) and improve impact resistance compared to a case in which these treatments are not performed, in comparison with a method for producing water-absorbent resin particles using the same raw materials.

[0007] A first embodiment of the method for producing water-absorbent resin particles according to one aspect of the present invention includes a polymerization step of obtaining a polymer by polymerizing a monomer on at least a part of the surface of the surface-crosslinked polymer particles.

[0008] A second embodiment of the method for producing water-absorbent resin particles according to one aspect of the present invention includes a polymerization step of obtaining a polymer by polymerizing a monomer in the presence of a crosslinking agent on at least a part of the surface of a polymer particle that is not surface-crosslinked.

[0009] According to these methods for producing water-absorbent resin particles, in comparison with methods for producing water-absorbent resin particles using the same raw materials, it is possible to obtain water-absorbent resin particles that can achieve excellent water-absorbing performance under load while improving impact resistance, as compared with a case in which these polymerization steps are not performed.

[0010] A water-absorbent resin particle according to another aspect of the present invention includes a surface-crosslinked polymer particle and a polymer disposed on at least a part of the surface of the polymer particle.

[0011] Such water-absorbent resin particles can achieve excellent water absorption performance under load and improve impact resistance, compared to water-absorbent resin particles obtained using the same raw materials. [Effects of the Invention]

[0012] According to one aspect of the present invention, it is possible to provide a method for producing water-absorbent resin particles that can achieve excellent water-absorbing performance under load while improving impact resistance, as compared with methods for producing water-absorbent resin particles using the same contents of raw materials. According to another aspect of the present invention, it is possible to provide water-absorbent resin particles that can achieve excellent water-absorbent performance under load while improving impact resistance, as compared with water-absorbent resin particles obtained using the same contents of raw materials. [Brief explanation of the drawings]

[0013] [Figure 1] FIG. 1 is a schematic diagram showing a device for measuring the water absorption under load of water-absorbent resin particles. DETAILED DESCRIPTION OF THE INVENTION

[0014] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail below with reference to the preferred embodiments. However, the present invention is not limited to the following embodiments and can be practiced in various modified forms within the scope of the present invention.

[0015] In this specification, "acrylic" and "methacrylic" are collectively referred to as "(meth)acrylic." "Acrylate" and "methacrylate" are similarly referred to as "(meth)acrylate." "(Poly)" refers to both cases with and without the prefix "poly." In the numerical ranges described in stages in this specification, the upper or lower limit of a certain numerical range can be arbitrarily combined with the upper or lower limit of another numerical range. In the numerical ranges described in this specification, the upper or lower limit of that numerical range may be replaced with a value shown in the Examples. Room temperature refers to 25°C ± 2°C. The materials exemplified in this specification may be used alone or in combination of two or more. When multiple substances corresponding to each component are present in the composition, the content of each component in the composition refers to the total amount of those multiple substances present in the composition, unless otherwise specified.

[0016] The method for producing water-absorbent resin particles according to the first embodiment includes a polymerization step of obtaining a polymer by polymerizing a monomer on at least a portion of the surface of a surface-crosslinked polymer particle. The method for producing water-absorbent resin particles according to the second embodiment includes a polymerization step of obtaining a polymer by polymerizing a monomer on at least a portion of the surface of a non-surface-crosslinked polymer particle in the presence of a crosslinking agent. In the polymerization step, a polymer of the monomer can be obtained that is arranged on at least a portion of the surface of the polymer particle.

[0017] According to the method for producing water-absorbent resin particles of this embodiment (including the first and second embodiments), by carrying out a process of polymerizing a monomer on the surface of the polymer particles after surface-crosslinking the polymer particles, or a process of polymerizing a monomer on the surface of a polymer particle that has not been surface-crosslinked in the presence of a crosslinking agent, it is possible to obtain water-absorbent resin particles that can achieve excellent water-absorbent performance under load (under pressure) while improving impact resistance, compared to a process for producing water-absorbent resin particles that uses the same raw materials (same type, same amount, etc.). According to the method for producing water-absorbent resin particles of this embodiment, by adjusting the order of polymerization of the monomer and use of the crosslinking agent, it is possible to achieve excellent water-absorbent performance under load while improving impact resistance.

[0018] The water-absorbent resin particles according to this embodiment include surface-crosslinked polymer particles and a polymer disposed on at least a portion of the surface of the polymer particles. Such water-absorbent resin particles can be obtained by the method for producing water-absorbent resin particles according to the first embodiment. The water-absorbent resin particles according to this embodiment can achieve excellent water absorption performance under load and improved impact resistance, compared to water-absorbent resin particles obtained using the same raw materials.

[0019] According to the water-absorbent resin particles and the method for producing the same according to the present embodiment, the impact resistance is improved, and therefore it is possible to suppress the generation of particles with small particle diameters when pressure is applied to the water-absorbent resin particles.

[0020] The present inventors speculate that the following mechanism is one of the reasons why excellent water absorption performance under load is achieved while impact resistance is improved. That is, when polymer particles are surface-crosslinked, the crosslink density on the surface increases, resulting in a hard layer as the outermost layer. In this case, even if excellent water absorption performance under load is achieved, the hard outermost layer is easily damaged by collisions between particles (low impact resistance). On the other hand, according to the method for producing water-absorbent resin particles according to the first embodiment, by polymerizing a monomer on the surface of the polymer particles after surface-crosslinking the polymer particles, it is possible to prevent the hard layer obtained by surface-crosslinking from being exposed as the outermost layer while maintaining the surface-crosslinked state of the polymer particles, thereby achieving excellent water absorption performance under load and improving impact resistance. Furthermore, according to the method for producing a water-absorbent resin particle of the second embodiment, by polymerizing a monomer in the presence of a crosslinking agent on the surface of a polymer particle that is not surface-crosslinked, crosslinking of the outermost layer is promoted while suppressing an excessive increase in the crosslink density of the outermost layer, and therefore it is possible to achieve excellent water absorption performance under load while improving impact resistance. However, the mechanism by which the effect is exerted is not limited to these.

[0021] The median particle diameter of the water-absorbent resin particles according to this embodiment may be in the following ranges. The median particle diameter of the water-absorbent resin particles may be 100 μm or more, 150 μm or more, 200 μm or more, 250 μm or more, 300 μm or more, 350 μm or more, 360 μm or more, 370 μm or more, 380 μm or more, 400 μm or more, 420 μm or more, or 450 μm or more. The median particle diameter of the water-absorbent resin particles may be 800 μm or less, 700 μm or less, 600 μm or less, 500 μm or less, 450 μm or less, 420 μm or less, 400 μm or less, 380 μm or less, 370 μm or less, or 360 μm or less. From these viewpoints, the median particle diameter of the water-absorbent resin particles may be 100 to 800 μm.

[0022] In the polymerization step of the method for producing water-absorbent resin particles according to the first embodiment, a monomer is polymerized on at least a part of the surface of the surface-crosslinked polymer particles. The "surface-crosslinked polymer particles" are polymer particles having a higher crosslink density on the surface than inside the particles.

[0023] In the polymerization step of the method for producing water-absorbent resin particles according to the first embodiment, a monomer is brought into contact with the surface of the polymer particles. In the polymerization step, a monomer may be added to a liquid containing the polymer particles, or the liquid containing the polymer particles may be mixed with a liquid containing the monomer. In the polymerization step, the monomer is polymerized in the absence of a crosslinking agent. The liquid containing the monomer does not contain a crosslinking agent. Note that the "state in the absence of a crosslinking agent" is synonymous with no crosslinking agent being added in the polymerization step. When an internal crosslinking agent is used to form the polymer particles, there is a possibility that a trace amount of the internal crosslinking agent remaining in the polymer particles may leak out of the polymer particles during the polymerization step. However, even in such a case, as long as no new crosslinking agent is added in the polymerization step, this is included in the "state in the absence of a crosslinking agent."

[0024] In the polymerization step of the method for producing water-absorbent resin particles according to the second embodiment, a monomer is polymerized in the presence of a crosslinking agent on at least a part of the surface of a polymer particle that is not surface-crosslinked. The "polymer particle that is not surface-crosslinked" is a polymer particle in which the crosslink density inside the particle is approximately equal to the crosslink density on the surface.

[0025] In the polymerization step of the method for producing water-absorbent resin particles according to the second embodiment, a monomer and a crosslinking agent are brought into contact with the surfaces of the polymer particles. The monomer and the crosslinking agent may be added to a liquid containing the polymer particles, the liquid containing the polymer particles may be mixed with a liquid containing the monomer and the crosslinking agent, or the liquid containing the polymer particles may be mixed with a liquid containing the monomer and a liquid containing the crosslinking agent.

[0026] The shape of the polymer particles is not particularly limited, and may be, for example, substantially spherical, irregular, granular, etc., or may be an aggregate of primary particles having these shapes. Irregularly shaped polymer particles can be obtained, for example, by crushing polymer lumps with a crusher.

[0027] The polymer particles may have water absorption properties. The polymer particles may have a water absorption capacity of ion-exchanged water at 25° C. (water absorption capacity under normal pressure) of, for example, 10 g / g or more.

[0028] The polymer particles may also contain gel stabilizers, metal chelating agents, flow improvers (lubricants), etc. These components may be located inside the polymer particles, on the surface of the polymer particles, or both.

[0029] The polymer obtained in the polymerization step may be water-soluble or not water-soluble (may be poorly water-soluble). When the polymer is water-soluble, the solubility of the polymer may be, for example, 1 g or more (e.g., 1 to 150 g) per 100 g of ion-exchanged water at 25° C. When the polymer is poorly water-soluble, the solubility of the polymer may be, for example, less than 1 g per 100 g of ion-exchanged water at 25° C.

[0030] The polymer obtained in the polymerization step may constitute a coating portion that coats at least a part of the surface of the polymer particle (subject to be coated). The coating portion may coat at least a part of the surface of the polymer particle, and may coat a part or the whole of the surface of the polymer particle. In the water-absorbent resin particle according to this embodiment, the crosslink density of the coating portion may be lower than the crosslink density of the surface of the polymer particle.

[0031] The reaction temperature in the polymerization step may be, for example, 15 to 200°C. The polymerization reaction in the polymerization step may be a chain polymerization reaction, a step-growth polymerization reaction, or the like. Constituent materials for the polymer obtained in the polymerization step include chain polymerization products such as poly(meth)acrylic acid, poly(meth)acrylamide, polyvinyl alcohol, polyalkylene oxide, and polyalkylene glycol; and step-growth polymerization products such as polyurethane (urethane resin), phenolic resin (for example, a condensation product of a phenol compound and an aldehyde), polyester, polyamide, and polycarbonate. The polymer may be a crosslinked polymer.

[0032] The polymer obtained in the polymerization step preferably contains a polymer having structural units derived from an ethylenically unsaturated monomer (a polymer having an ethylenically unsaturated monomer as a monomer unit), from the viewpoint of easily improving impact resistance while achieving excellent water absorption performance under load.The polymer obtained in the polymerization step preferably contains polyurethane, from the viewpoint of easily improving impact resistance while achieving excellent water absorption performance under load.

[0033] Examples of the ethylenically unsaturated monomer include (meth)acrylic acid and its salts, (meth)acrylic acid esters (methyl (meth)acrylate, ethyl (meth)acrylate, 2-hydroxyethyl (meth)acrylate, 2-(diethylamino)ethyl (meth)acrylate, 2-(diethylamino)propyl (meth)acrylate, etc.), (meth)acrylamide-based monomers ((meth)acrylamide, N-isopropyl(meth)acrylamide, 2-(meth)acrylamido-2-methylpropanesulfonic acid and its salts, N,N-dimethyl(meth)acrylamide, N-methylol(meth)acrylamide, diethylaminopropyl(meth)acrylamide, etc.), and polyethylene glycol mono(meth)acrylate. The ethylenically unsaturated monomer preferably includes at least one selected from the group consisting of (meth)acrylic acid and its salts, from the viewpoint of achieving excellent water absorption performance under load while easily improving impact resistance. The ethylenically unsaturated monomer preferably contains a (meth)acrylamide-based monomer, from the viewpoint of easily improving impact resistance while achieving excellent water absorption performance under load.

[0034] In the polymerization step, when a polymer is obtained by reacting a plurality of substances with each other, combinations of the plurality of substances include polyol and polyisocyanate; aldehyde and phenol compound; polyol and polycarboxylic acid; polyamine and polycarboxylic acid; phenol compound and carbonate ester; phenol compound and carbonic acid chloride, etc.

[0035] The polyol may be any compound having two or more hydroxyl groups, and may include diols, triols, etc. Examples of the polyol include polyether polyols, polyester polyols, polycarbonate polyols, polysiloxane polyols, polyisoprene polyols, and polyolefin polyols.

[0036] The polyisocyanate may be any compound having two or more isocyanate groups, and diisocyanates, triisocyanates, etc. Examples of polyisocyanates include aromatic isocyanates such as diphenylmethane diisocyanate, dimethyldiphenylmethane diisocyanate, tolylene diisocyanate (e.g., tolylene-2,4-diisocyanate), xylylene diisocyanate, and p-phenylene diisocyanate; alicyclic isocyanates such as dicyclohexylmethane diisocyanate and isophorone diisocyanate; and aliphatic isocyanates such as hexamethylene diisocyanate.

[0037] Examples of the aldehyde include aliphatic aldehydes such as formaldehyde, acetaldehyde, and propionaldehyde; and aromatic aldehydes such as benzaldehyde.

[0038] Examples of the phenolic compounds include phenol, cresol, catechol, naphthol, and hydroquinone.

[0039] The amount of the monomer in the polymerization step of the method for producing water-absorbent resin particles according to the present embodiment is preferably within the following ranges relative to 100 moles of monomer (in the case of multi-stage polymerization, the total amount of monomer in each stage) used to obtain polymer particles in the particle production step described below, from the viewpoint of easily obtaining excellent impact resistance and easily improving the water absorption amount and / or impact resistance under load in comparison with methods for producing water-absorbent resin particles using the same raw materials. The amount of the monomer is preferably 0.01 moles or more, 0.05 moles or more, 0.1 moles or more, 0.5 moles or more, 1 mole or more, 2 moles or more, 5 moles or more, 10 moles or more, 11 moles or more, 12 moles or more, 13 moles or more, 14 moles or more, 15 moles or more, 20 moles or more, 25 moles or more, 30 moles or more, 40 moles or more, or 50 moles or more. The amount of the monomer is preferably 100 mols or less, less than 100 mols, 80 mols or less, 60 mols or less, 50 mols or less, 40 mols or less, 30 mols or less, 25 mols or less, 20 mols or less, 15 mols or less, 14 mols or less, 13 mols or less, 12 mols or less, 11 mols or less, 10 mols or less, 5 mols or less, 2 mols or less, 1 mol or less, 0.5 mols or less, or 0.1 mol or less. From these viewpoints, the amount of the monomer is preferably 0.01 to 100 mols.

[0040] Examples of the crosslinking agent in the polymerization step of the method for producing water-absorbent resin particles according to the second embodiment include polyglycidyl compounds such as (poly)ethylene glycol diglycidyl ether, (poly)propylene glycol diglycidyl ether, (poly)glycerin diglycidyl ether, (poly)glycerin triglycidyl ether, (poly)propylene glycol polyglycidyl ether, and polyglycerol polyglycidyl ether.

[0041] The amount of crosslinking agent in the polymerization step of the method for producing water-absorbent resin particles according to the second embodiment is preferably in the following range relative to 100 moles of monomer in the polymerization step, from the viewpoint of easily improving impact resistance while achieving excellent water absorption performance under load. The amount of crosslinking agent is preferably 0.001 mole or more, more preferably 0.003 mole or more, even more preferably 0.005 mole or more, particularly preferably 0.008 mole or more, extremely preferably 0.01 mole or more, and very preferably 0.02 mole or more. The amount of crosslinking agent is preferably 1 mole or less, more preferably 0.5 mole or less, even more preferably 0.1 mole or less, particularly preferably 0.05 mole or less, and extremely preferably 0.03 mole or less. From these viewpoints, the amount of crosslinking agent is preferably 0.001 to 1 mole.

[0042] The method for producing water-absorbent resin particles according to the second embodiment does not include a surface cross-linking step of surface-cross-linking polymer particles before the polymerization step. The method for producing water-absorbent resin particles according to the first embodiment may include a surface cross-linking step of surface-cross-linking polymer particles before the polymerization step. In the surface cross-linking step, the polymer particles are surface-cross-linked by mixing the polymer particles with a surface cross-linking agent. The surface cross-linking step does not need to involve polymerization of monomers.

[0043] Examples of the surface cross-linking agent include polyglycidyl compounds such as (poly)ethylene glycol diglycidyl ether, (poly)propylene glycol diglycidyl ether, (poly)glycerin diglycidyl ether, (poly)glycerin triglycidyl ether, (poly)propylene glycol polyglycidyl ether, and polyglycerol polyglycidyl ether.

[0044] The amount of the surface cross-linking agent in the surface cross-linking step is preferably within the following range relative to 100 moles of monomer used to obtain the polymer particles (in the case of multi-stage polymerization, the total amount of monomers in each stage), from the viewpoint of easily improving impact resistance while achieving excellent water absorption performance under load. The amount of the surface cross-linking agent is preferably 0.0005 moles or more, more preferably 0.001 moles or more, and even more preferably 0.002 moles or more. The amount of the surface cross-linking agent is preferably 0.5 moles or less, more preferably 0.1 moles or less, and even more preferably 0.05 moles or less.

[0045] The method for producing water-absorbent resin particles according to the present embodiment may include a particle preparation step of polymerizing a monomer to obtain polymer particles, prior to the surface cross-linking step and the polymerization step. In the particle preparation step, the monomer can be polymerized once or multiple times.

[0046] The polymer particles can be obtained, for example, by polymerizing a monomer containing an ethylenically unsaturated monomer. That is, the polymer particles can have structural units derived from the ethylenically unsaturated monomer (having the ethylenically unsaturated monomer as a monomer unit). Examples of polymerization methods for the ethylenically unsaturated monomer include reverse phase suspension polymerization, aqueous solution polymerization, bulk polymerization, and precipitation polymerization.

[0047] The ethylenically unsaturated monomer may be a water-soluble ethylenically unsaturated monomer (for example, an ethylenically unsaturated monomer having a solubility of 1 g or more in 100 g of ion-exchanged water at 25° C.). Examples of the ethylenically unsaturated monomer include (meth)acrylic acid and its salts, (meth)acrylic acid esters (methyl (meth)acrylate, ethyl (meth)acrylate, 2-hydroxyethyl (meth)acrylate, 2-(diethylamino)ethyl (meth)acrylate, 2-(diethylamino)propyl (meth)acrylate, etc.), (meth)acrylamide-based monomers ((meth)acrylamide, N-isopropyl(meth)acrylamide, 2-(meth)acrylamido-2-methylpropanesulfonic acid and its salts, N,N-dimethyl(meth)acrylamide, N-methylol(meth)acrylamide, diethylaminopropyl(meth)acrylamide, etc.), and polyethylene glycol mono(meth)acrylate. The ethylenically unsaturated monomer may contain at least one selected from the group consisting of (meth)acrylic acid and salts thereof, from the viewpoint of easily obtaining water-absorbent resin particles while suppressing particle aggregation. The polymer particles preferably have a structural unit derived from at least one selected from the group consisting of (meth)acrylic acid and salts thereof, from the viewpoint of easily obtaining water-absorbent resin particles while suppressing particle aggregation.

[0048] When the ethylenically unsaturated monomer has an acidic group, the acidic group may be neutralized before use in the polymerization reaction. The degree of neutralization in the ethylenically unsaturated monomer may be 10 to 100 mol %, 50 to 90 mol %, or 60 to 80 mol % of the acidic group in the ethylenically unsaturated monomer.

[0049] Monomers other than the above-mentioned ethylenically unsaturated monomers may be used as monomers for obtaining polymer particles. Such monomers can be used, for example, by mixing them with an aqueous solution containing the above-mentioned ethylenically unsaturated monomer. The amount of the ethylenically unsaturated monomer used is preferably 70 to 100 mol% relative to the total amount of monomers (total amount of monomers for obtaining polymer particles, for example, total amount of monomers that provide structural units of a crosslinked polymer; the same applies hereinafter). In particular, it is more preferable that the proportion of (meth)acrylic acid and salts thereof is 70 to 100 mol% relative to the total amount of monomers. The "proportion of (meth)acrylic acid and salts thereof" means the proportion of the total amount of (meth)acrylic acid and salts thereof.

[0050] An internal cross-linking agent may be used to obtain polymer particles. When an internal cross-linking agent is used during polymerization of the monomer, it is easy to increase the cross-link density substantially uniformly throughout the polymer particles. Examples of the internal cross-linking agent include polyglycidyl compounds such as (poly)ethylene glycol diglycidyl ether, (poly)propylene glycol diglycidyl ether, (poly)glycerin diglycidyl ether, (poly)glycerin triglycidyl ether, (poly)propylene glycol polyglycidyl ether, and polyglycerol polyglycidyl ether; divinyl compounds; dialcohol compounds; and diacrylate compounds.

[0051] The method for producing water-absorbent resin particles according to this embodiment may include a step of classifying the water-absorbent resin particles using a sieve after the polymerization step, thereby adjusting the particle size distribution.

[0052] According to the present embodiment, it is possible to provide a liquid absorbing method using the water-absorbent resin particles according to the present embodiment. The liquid absorbing method according to the present embodiment includes a step of bringing the water-absorbent resin particles according to the present embodiment into contact with a liquid to be absorbed. [Example]

[0053] The present invention will be further explained below using examples and comparative examples, but the present invention is not limited to the following examples.

[0054] Example 1 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, and a stirrer (a stirrer with two stages of four inclined paddle blades with a blade diameter of 5 cm). 293 g of n-heptane (hydrocarbon dispersion medium) and 0.736 g of maleic anhydride-modified ethylene-propylene copolymer (polymeric dispersant, Mitsui Chemicals, Inc., Hiwax 1105A) were added to the separable flask to obtain a mixture. The mixture was heated to 80 °C while stirring at 300 rpm to dissolve the dispersant, and then cooled to 55 °C.

[0055] Next, 92.0 g of an 80.5% by weight acrylic acid aqueous solution (acrylic acid: 1.03 mol) was placed in a 500 mL Erlenmeyer flask. Subsequently, while cooling externally, 102.2 g of a 30% by weight sodium hydroxide aqueous solution was added dropwise to neutralize the 75 mol% acrylic acid. Subsequently, 0.092 g of hydroxyethyl cellulose (thickener, Sumitomo Seika Chemicals Co., Ltd., HEC AW-15F), 0.0736 g (0.272 mmol) of potassium persulfate (water-soluble radical polymerization initiator), 0.0101 g (0.0581 mmol) of ethylene glycol diglycidyl ether (internal crosslinking agent), and 32.85 g of ion-exchanged water were added and dissolved to prepare a first-stage monomer aqueous solution.

[0056] The first-stage monomer aqueous solution was then added to the separable flask and stirred for 10 minutes. Subsequently, 7.356 g of a surfactant solution obtained by dissolving 0.736 g of sucrose stearate (surfactant, manufactured by Mitsubishi Chemical Foods Corporation, Ryoto Sugar Ester S-370, HLB: 3) in 6.62 g of n-heptane under heat was added to the separable flask to obtain a reaction solution. The reaction solution was stirred at 550 rpm while the system was thoroughly purged with nitrogen. The separable flask was then immersed in a 70°C water bath to heat the reaction solution, and the first-stage polymerization was carried out for 10 minutes to obtain a first-stage reaction mixture.

[0057] 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, 143.1 g of a 30% by mass sodium hydroxide aqueous solution was added dropwise to neutralize the 75 mol% acrylic acid. Subsequently, 0.1030 g (0.3812 mmol) of potassium persulfate, 0.0116 g (0.0655 mmol) of ethylene glycol diglycidyl ether (internal crosslinking agent), and 0.63 g of ion-exchanged water were added and dissolved to prepare a second-stage monomer aqueous solution.

[0058] The first-stage reaction mixture was cooled to 25°C while stirring at 1000 rpm, and the entire second-stage aqueous monomer solution was added to the first-stage reaction mixture to obtain a reaction solution. The reaction mixture was then thoroughly purged with nitrogen while stirring. The separable flask was then immersed in a 70°C water bath to heat the reaction mixture, and second-stage polymerization was carried out for 5 minutes to obtain a second-stage reaction mixture (polymer particles before surface crosslinking).

[0059] After the second-stage polymerization, the reaction mixture was heated in a 125°C oil bath, and 267g of water was extracted from the system by azeotropic distillation of n-heptane and water while refluxing n-heptane. Subsequently, 0.0884g (0.5075mmol) of ethylene glycol diglycidyl ether was added as a surface cross-linking agent, and the mixture was maintained at 83°C for 2 hours to obtain a dispersion of surface-cross-linked polymer particles.

[0060] Next, 111.4 g of an 80.5% by mass acrylic acid aqueous solution (1.25 mol of acrylic acid) was placed in another 500 mL Erlenmeyer flask. Subsequently, while cooling externally, 125.8 g of a 30% by mass sodium hydroxide aqueous solution was added dropwise to neutralize the 75 mol% acrylic acid. After that, 0.0891 g (0.3296 mmol) of potassium persulfate and 0.84 g of ion-exchanged water were added, and the potassium persulfate was dissolved to prepare a third-stage monomer aqueous solution.

[0061] The dispersion of the surface-crosslinked polymer particles was then maintained at 83°C for 2 hours and then allowed to cool to 50°C. Subsequently, the entire amount of the third-stage aqueous monomer solution was added to the dispersion of the surface-crosslinked polymer particles to obtain a reaction solution. The reaction solution was then thoroughly purged with nitrogen while stirring, and then maintained at 45°C for 30 minutes. The separable flask was then immersed in a 75°C water bath to increase the temperature, and the third-stage polymerization was carried out for 15 minutes to obtain a third-stage reaction mixture.

[0062] After the third-stage polymerization, the temperature of the third-stage reaction mixture was raised in an oil bath at 125°C, and water was extracted from the system by azeotropic distillation of n-heptane and water while refluxing n-heptane until the temperature in the flask reached 90°C. The n-heptane was then evaporated and dried to obtain a polymer. The polymer was passed through a sieve with an opening of 850 μm to obtain 282.62 g of water-absorbent resin particles in the form of agglomerated spherical particles. The median particle diameter of the water-absorbent resin particles was 436 μm.

[0063] (Comparative Example 1) In Comparative Example 1, water-absorbent resin particles were produced in the same manner as in Example 1, except that surface crosslinking was not carried out before the third-stage polymerization, and after obtaining a second-stage reaction mixture (polymer particles before surface crosslinking), third-stage polymerization and surface crosslinking were carried out in this order. First, the same procedures as in Example 1 were carried out up to the second stage polymerization. After the second-stage polymerization, the reaction mixture was heated in a 125°C oil bath, and 245 g of water was extracted from the system by azeotropic distillation of n-heptane and water while refluxing n-heptane. Next, 111.4 g of an 80.5% by mass acrylic acid aqueous solution (1.25 mol of acrylic acid) was placed in a 500 mL Erlenmeyer flask. Subsequently, while cooling from the outside, 125.8 g of a 30% by mass sodium hydroxide aqueous solution was added dropwise to neutralize the 75 mol% acrylic acid. After that, 0.0891 g (0.3296 mmol) of potassium persulfate and 0.84 g of ion-exchanged water were added, and the potassium persulfate was dissolved to prepare a third-stage monomer aqueous solution. The second-stage reaction mixture (the reaction mixture after water removal) was maintained at 83°C for 2 hours and then allowed to cool to 50°C. The entire third-stage aqueous monomer solution was then added to the second-stage reaction mixture to obtain a reaction mixture. The reaction mixture was then thoroughly purged with nitrogen while stirring, and then maintained at 45°C for 30 minutes. The separable flask was then immersed in a 75°C water bath to raise the temperature, and third-stage polymerization was carried out for 15 minutes to obtain a third-stage reaction mixture. After the third-stage polymerization, the reaction mixture was heated in a 125°C oil bath, and 204 g of water was extracted from the system by azeotropic distillation of n-heptane and water while refluxing n-heptane. Thereafter, 0.0884 g (0.5075 mmol) of ethylene glycol diglycidyl ether was added as a surface cross-linking agent, and the mixture was maintained at 83° C. for 2 hours. Then, the temperature was raised in an oil bath of 125°C to evaporate n-heptane and dry the mixture, thereby obtaining a polymer. The polymer was passed through a sieve with an opening of 850µm, thereby obtaining 283.36g of water-absorbent resin particles in the form of agglomerated spherical particles. The median particle diameter of the water-absorbent resin particles was 448µm.

[0064] Example 2 In Example 2, 217.07 g of water-absorbent resin particles (in the form of agglomerated spherical particles) was obtained in the same manner as in Example 1, except that the amount of water withdrawn after the second-stage polymerization was changed from 267 g to 241 g and the contents of the third-stage aqueous monomer solution were changed. The third-stage aqueous monomer solution was prepared by placing 44.6 g of an 80.5 mass % acrylic acid aqueous solution (acrylic acid: 0.50 mol) in a 500 mL Erlenmeyer flask, and then neutralizing 75 mol % of the acrylic acid by adding 50.3 g of a 30 mass % aqueous sodium hydroxide solution dropwise while cooling from the outside. Furthermore, 0.0357 g (0.1319 mmol) of potassium persulfate and 0.5 g of ion-exchanged water were added, and then dissolving the potassium persulfate. The median particle diameter of the water-absorbent resin particles was 370 μm.

[0065] (Comparative Example 2) In Comparative Example 2, 216.32 g of water-absorbent resin particles were obtained in the same manner as in Comparative Example 1, except that the amount of water withdrawn after the second-stage polymerization was changed from 245 g to 241 g, the contents of the third-stage aqueous monomer solution were changed, and the amount of water withdrawn after the third-stage polymerization was changed from 204 g to 42 g. The third-stage aqueous monomer solution was prepared by placing 44.6 g of an 80.5 mass% acrylic acid aqueous solution (acrylic acid: 0.50 mol) in a 500 mL Erlenmeyer flask, and then neutralizing 75 mol% of the acrylic acid by adding 50.3 g of a 30 mass% aqueous sodium hydroxide solution dropwise while cooling from the outside, and then adding 0.0357 g (0.1319 mmol) of potassium persulfate and 0.5 g of ion-exchanged water, followed by dissolving the potassium persulfate. The median particle diameter of the water-absorbent resin particles was 386 μm.

[0066] Example 3 In Example 3, 209.29 g of water absorbent resin particles (in the form of agglomerated spherical particles) was obtained in the same manner as in Example 1, except that the content of the third-stage aqueous monomer solution was changed, and that the third-stage monomer solution and the crosslinking agent were added simultaneously. First, the same procedures as in Example 1 were carried out up to the second stage polymerization. Next, 33.4 g of an 80.5% by mass acrylic acid aqueous solution (acrylic acid: 0.37 mol) was placed in a 500 mL Erlenmeyer flask. Subsequently, while cooling from the outside, 37.7 g of a 30% by mass sodium hydroxide aqueous solution was added dropwise to neutralize the 75 mol% acrylic acid. After that, 0.0267 g (0.0989 mmol) of potassium persulfate and 0.38 g of ion-exchanged water were added, and the potassium persulfate was dissolved to prepare a third-stage monomer aqueous solution. The second-stage reaction mixture (polymer particles before crosslinking) was heated in an oil bath at 125°C, and 267 g of water was extracted from the system by azeotropic distillation of n-heptane and water while refluxing n-heptane. Subsequently, 0.0884 g (0.5075 mmol) of ethylene glycol diglycidyl ether was added as a crosslinking agent, and the aqueous monomer solution from the third stage was added. The mixture was then maintained at 83°C for 2 hours to obtain a dispersion of crosslinked polymer particles. Thereafter, the temperature of the dispersion of crosslinked polymer particles was raised in an oil bath at 125°C, and water was extracted from the system by azeotropic distillation of n-heptane and water while refluxing n-heptane until the temperature in the flask reached 90°C, and then the n-heptane was evaporated and dried to obtain a polymer. This polymer was passed through a sieve with an opening of 850 μm to obtain 209.29 g of water-absorbent resin particles in the form of agglomerated spherical particles. The median particle diameter of the water-absorbent resin particles was 372 μm.

[0067] (Comparative Example 3) In Comparative Example 3, 210.70 g of water-absorbent resin particles were obtained in the same manner as in Comparative Example 1, except that the amount of water withdrawn after the second-stage polymerization was changed from 245 g to 241 g, the contents of the third-stage aqueous monomer solution were changed, and the amount of water withdrawn after the third-stage polymerization was changed from 204 g to 32 g. The third-stage aqueous monomer solution was prepared by placing 33.4 g of an 80.5 mass% acrylic acid aqueous solution (acrylic acid: 0.37 mol) in a 500 mL Erlenmeyer flask, and then neutralizing 75 mol% of the acrylic acid by adding 37.7 g of a 30 mass% aqueous sodium hydroxide solution dropwise while cooling from the outside, and then adding 0.0267 g (0.0989 mmol) of potassium persulfate and 0.38 g of ion-exchanged water, followed by dissolving the potassium persulfate. The median particle diameter of the water-absorbent resin particles was 378 μm.

[0068] Example 4 In Example 4, 201.46 g of water-absorbent resin particles (in the form of agglomerated spherical particles) was obtained in the same manner as in Example 1, except that the contents of the third-stage aqueous monomer solution were changed. The third-stage aqueous monomer solution was prepared by placing 22.3 g of an 80.5 mass % acrylic acid aqueous solution (acrylic acid: 0.25 mol) in a 500 mL Erlenmeyer flask, then neutralizing 75 mol % of the acrylic acid by adding 25.1 g of a 30 mass % aqueous sodium hydroxide solution dropwise while cooling from the outside, and further adding 0.0178 g (0.0658 mmol) of potassium persulfate and 0.30 g of ion-exchanged water, followed by dissolving the potassium persulfate. The median particle diameter of the water-absorbent resin particles was 387 μm.

[0069] Comparative Example 4 In Comparative Example 4, 200.02 g of water-absorbent resin particles were obtained in the same manner as in Comparative Example 1, except that the contents of the third-stage aqueous monomer solution were changed and the amount of water withdrawn after the third-stage polymerization was changed from 204 g to 22 g. The third-stage aqueous monomer solution was prepared by placing 22.3 g of an 80.5 mass % acrylic acid aqueous solution (acrylic acid: 0.25 mol) in a 500 mL Erlenmeyer flask, then neutralizing 75 mol % of the acrylic acid by adding 25.1 g of a 30 mass % aqueous sodium hydroxide solution dropwise while cooling from the outside, and further adding 0.0178 g (0.0658 mmol) of potassium persulfate and 0.30 g of ion-exchanged water, followed by dissolving the potassium persulfate. The median particle diameter of the water-absorbent resin particles was 381 μm.

[0070] Example 5 In Example 5, 173.68 g of water-absorbent resin particles (in the form of agglomerated spherical particles) was obtained in the same manner as in Example 1, except that the amount of water withdrawn after the second-stage polymerization was changed from 267 g to 245 g and the contents of the third-stage aqueous monomer solution were changed. The third-stage aqueous monomer solution was prepared by placing 0.2 g of an 80.5 mass% aqueous acrylic acid solution (0.002 mol of acrylic acid) in a 500 mL Erlenmeyer flask, then neutralizing 75 mol% of the acrylic acid by adding 0.3 g of a 30 mass% aqueous sodium hydroxide solution dropwise while cooling from the outside, and then adding 0.000178 g (0.00066 mmol) of potassium persulfate and 0.84 g of ion-exchanged water, followed by dissolving the potassium persulfate. The median particle diameter of the water-absorbent resin particles was 356 μm.

[0071] (Comparative Example 5) In Comparative Example 5, 172.62 g of water-absorbent resin particles were obtained in the same manner as in Comparative Example 1, except that the contents of the third-stage aqueous monomer solution were changed and the amount of water withdrawn after the third-stage polymerization was changed from 204 g to 0.2 g. The third-stage aqueous monomer solution was prepared by placing 0.2 g of an 80.5 mass % acrylic acid aqueous solution (acrylic acid: 0.002 mol) in a 500 mL Erlenmeyer flask, then neutralizing 75 mol % of the acrylic acid by adding 0.3 g of a 30 mass % aqueous sodium hydroxide solution dropwise while cooling from the outside, and further adding 0.000178 g (0.00066 mmol) of potassium persulfate and 0.84 g of ion-exchanged water, followed by dissolving the potassium persulfate. The median particle diameter of the water-absorbent resin particles was 366 μm.

[0072] Example 6 In Example 6, 193.20 g of water-absorbent resin particles (in the form of agglomerated spherical particles) was obtained in the same manner as in Example 3, except that the contents of the third-stage aqueous monomer solution were changed. The third-stage aqueous monomer solution was obtained by mixing 22.1 g (0.31 mol) of acrylamide, 0.0177 g (0.0655 mmol) of potassium persulfate, 0.0884 g (0.5075 mmol) of ethylene glycol diglycidyl ether (crosslinking agent), and 22.80 g of ion-exchanged water. The median particle diameter of the water-absorbent resin particles was 385 μm.

[0073] (Comparative Example 6) In Comparative Example 6, 198.28 g of water-absorbent resin particles (in the form of agglomerated spherical particles) was obtained in the same manner as in Comparative Example 1, except that the amount of water withdrawn after the second-stage polymerization was changed from 245 g to 237 g, the contents of the third-stage aqueous monomer solution were changed, and the amount of water withdrawn after the third-stage polymerization was changed from 204 g to 21 g. The third-stage aqueous monomer solution was obtained by mixing 22.1 g (0.31 mol) of acrylamide, 0.0177 g (0.0655 mmol) of potassium persulfate, and 27.22 g of ion-exchanged water. The median particle diameter of the water-absorbent resin particles was 413 μm.

[0074] Example 7 In Example 7, 227.32 g of water absorbent resin particles (in the form of agglomerated spherical particles) was obtained in the same manner as in Example 1, except that the amount of water extracted after the second-stage polymerization was changed from 267 g to 269 g, and the content of the aqueous monomer solution in the third stage was changed. First, the same procedures as in Example 1 were carried out up to the second stage polymerization. After the second-stage polymerization, the second-stage reaction mixture (polymer particles before surface crosslinking) was heated in an oil bath at 125°C, and 269 g of water was extracted from the system by azeotropic distillation of n-heptane and water while refluxing n-heptane. Subsequently, 0.0884 g (0.5075 mmol) of ethylene glycol diglycidyl ether was added as a surface crosslinking agent, and the mixture was maintained at 83°C for 2 hours to obtain a dispersion of surface-crosslinked polymer particles. Next, as the third-stage monomer solution, 4.4 g of a polyol (Dai-ichi Kogyo Seiyaku Co., Ltd., DK Polyol 3817) and 83.6 g of distilled water were mixed to prepare 88 g of mixed solution A (aqueous polyol solution), and 3.12 g of tolylene-2,4-diisocyanate and 28.02 g of acetone were mixed to prepare 31.14 g of mixed solution B (isocyanate acetone solution). The dispersion of the surface-crosslinked polymer particles was then maintained at 83°C for 2 hours. The mixed solution A was then added to the dispersion of the surface-crosslinked polymer particles, followed by stirring at 80°C for 30 minutes. The mixed solution B was then added, followed by stirring at 80°C for 60 minutes, allowing a sequential polymerization reaction (third-stage polymerization) to proceed on the surfaces of the polymer particles, resulting in the polymerization of polyurethane, thereby obtaining a third-stage reaction mixture. After the third-stage polymerization, the temperature of the third-stage reaction mixture was raised in an oil bath at 125°C, and water and acetone were removed from the system while evaporating n-heptane by azeotropic distillation of n-heptane and water until the temperature in the flask reached 90°C. The n-heptane was then evaporated and dried to obtain a polymer. The polymer was passed through a sieve with an opening of 850 μm to obtain 227.32 g of water-absorbent resin particles in the form of agglomerated spherical particles. The median particle diameter of the water-absorbent resin particles was 373 μm.

[0075] (Comparative Example 7) In Comparative Example 7, water absorbent resin particles were produced in the same manner as in Example 7, except that surface crosslinking was not performed before the third-stage polymerization, and after obtaining a second-stage reaction mixture (polymer particles before surface crosslinking), third-stage polymerization and surface crosslinking were performed in this order. First, the same procedures as in Example 1 were carried out up to the second stage polymerization. After the second-stage polymerization, the second-stage reaction mixture (polymer particles before surface crosslinking) was heated in an oil bath at 125°C, and 245 g of water was extracted from the system by azeotropic distillation of n-heptane and water while refluxing n-heptane. Next, as the third-stage monomer solution, 4.4 g of a polyol (Dai-ichi Kogyo Seiyaku Co., Ltd., DK Polyol 3817) and 83.6 g of distilled water were mixed to prepare 88 g of mixed solution A (aqueous polyol solution), and 3.12 g of tolylene-2,4-diisocyanate and 28.02 g of acetone were mixed to prepare 31.14 g of mixed solution B (isocyanate acetone solution). Thereafter, the above-mentioned mixed solution A was added to the second-stage reaction mixture (the reaction mixture after water removal), and the mixture was stirred for 30 minutes at 80° C. Subsequently, the above-mentioned mixed solution B was added, and the mixture was stirred for 60 minutes at 80° C., and a sequential polymerization reaction (third-stage polymerization) was carried out on the surface of the polymer particles to polymerize polyurethane, thereby obtaining a third-stage reaction mixture. After the third-stage polymerization, the reaction mixture for the third stage was heated in an oil bath at 125°C, and 47 g of water and acetone were extracted from the system by azeotropic distillation of n-heptane and water while refluxing n-heptane. Thereafter, 0.0884 g (0.5075 mmol) of ethylene glycol diglycidyl ether was added as a surface cross-linking agent, and the mixture was maintained at 83° C. for 2 hours. Then, the temperature was raised in an oil bath of 125°C to evaporate n-heptane and dry the mixture, thereby obtaining a polymer. The polymer was passed through a sieve with an opening of 850µm, thereby obtaining 230.16g of water-absorbent resin particles in the form of agglomerated spherical particles. The median particle diameter of the water-absorbent resin particles was 450µm.

[0076] Example 8 In Example 8, polymer particles were prepared by aqueous solution polymerization, and then water-absorbent resin particles were prepared.

[0077] A round-bottomed, cylindrical, separable flask with an inner diameter of 11 cm and a capacity 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). 509.71 g (7.07 mol) of 100% acrylic acid was placed in the separable flask. While stirring the acrylic acid, 436.47 g of ion-exchanged water was added to the separable flask. Then, 444.68 g of 48% by weight sodium hydroxide was added dropwise in an ice bath (1°C) to prepare 1390.86 g of a partially neutralized acrylic acid solution with a monomer concentration of 45.08% by weight (neutralization rate: 75.44 mol%). This procedure was repeated to prepare a total of 2781.72 g of a partially neutralized acrylic acid solution.

[0078] 406.89 g of ion-exchanged water and 2.90 g (5.576 mmol) of polyethylene glycol diacrylate (internal crosslinking agent, n = 9) were added to 2781.72 g of the partially neutralized acrylic acid solution described above to obtain a reaction solution (monomer aqueous solution). This reaction solution was then purged with nitrogen gas for 30 minutes under a nitrogen atmosphere. The reaction solution was then fed into a stainless steel double-arm kneader equipped with a thermometer, a nitrogen inlet tube, a retractable lid, two sigma-type blades, and a jacket. The system was then purged with nitrogen gas while maintaining the reaction solution at 30°C. Next, 92.63 g (7.780 mmol) of a 2.0 wt% aqueous solution of sodium persulfate and 15.85 g of a 0.5 wt% aqueous solution of L-ascorbic acid were added while stirring the reaction solution. After approximately 1 minute, the temperature began to rise, and polymerization began. After 6 minutes, the maximum temperature during polymerization reached 93°C. After that, stirring was continued while maintaining the jacket temperature at 60°C, and 60 minutes after the start of polymerization, the hydrogel, the product of the first polymerization reaction, was removed. The obtained hydrogel was sequentially placed into a meat chopper 12VR-750SDX manufactured by Kiryu Royal Co., Ltd. and chopped into small pieces. The diameter of the hole in the plate located at the tip of the meat chopper was 6.4 mm.

[0079] This finely divided particulate hydrogel was spread on a wire mesh with openings of 0.8 cm x 0.8 cm, and then dried with hot air at 160°C for 60 minutes to obtain a dried product.

[0080] The dried product was then pulverized using a centrifugal pulverizer (Retsch, ZM200, screen diameter 1 mm, 12,000 rpm) to obtain irregularly pulverized resin powder A. This resin powder A was then classified using a wire mesh with an opening of 850 μm, a wire mesh with an opening of 250 μm, and a wire mesh with an opening of 180 μm to obtain resin powder B, which was a fraction that passed through the wire mesh with an opening of 850 μm but did not pass through the wire mesh with an opening of 250 μm.

[0081] A round-bottomed cylindrical separable flask with an inner diameter of 11 cm and a capacity 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, each 5 cm in diameter). 100 g of the above-mentioned resin powder B was placed in this separable flask, and then 560 g of n-heptane was added as a hydrocarbon dispersion medium.

[0082] Thereafter, the separable flask was heated to 83°C in a 125°C oil bath, and 0.040 g (0.230 mmol) of ethylene glycol diglycidyl ether was added as a surface cross-linking agent. The mixture was then maintained at 83°C for 2 hours to obtain a dispersion of surface-cross-linked polymer particles.

[0083] Next, 10.1 g of an 80.5% by mass acrylic acid aqueous solution (acrylic acid: 0.11 mol) was placed in a 500 mL Erlenmeyer flask. Subsequently, while cooling from the outside, 11.4 g of a 30% by mass sodium hydroxide aqueous solution was added dropwise to neutralize the 75 mol% acrylic acid. After that, 0.00810 g (0.0300 mmol) of potassium persulfate was added, and the potassium persulfate was dissolved to prepare a second-stage monomer aqueous solution.

[0084] The dispersion of the surface-crosslinked polymer particles was then maintained at 83°C for 2 hours and then allowed to cool to 50°C. Subsequently, the entire amount of the second-stage aqueous monomer solution was added to the dispersion of the surface-crosslinked polymer particles to obtain a reaction solution. The reaction solution was then thoroughly purged with nitrogen while stirring, and then maintained at 45°C for 30 minutes. The separable flask was then immersed in a 75°C water bath to increase the temperature, and second-stage polymerization was carried out for 15 minutes to obtain a second-stage reaction mixture.

[0085] After the second-stage polymerization, the temperature of the second-stage reaction mixture was raised in an oil bath at 125°C, and water was extracted from the system by azeotropic distillation of n-heptane and water while refluxing n-heptane until the temperature in the flask reached 90°C. The n-heptane was then evaporated and dried to obtain a polymer. The polymer was passed through a sieve with an opening of 850 μm to obtain 109.19 g of water-absorbent resin particles. The median particle size of the water-absorbent resin particles was 460 μm.

[0086] (Comparative Example 8) In Comparative Example 8, a water-absorbent resin particle was produced in the same manner as in Example 8, except that surface crosslinking was not performed before the second-stage polymerization, and after obtaining resin powder B, the second-stage polymerization and surface crosslinking were performed in this order. First, similarly to Example 8, resin powder B was obtained, and then 100 g of resin powder B was mixed with 560 g of n-heptane (hydrocarbon dispersion medium) in a separable flask. Thereafter, the separable flask was heated to 83°C in an oil bath at 125°C and then allowed to cool to 50°C, thereby obtaining a dispersion A of polymer particles before surface crosslinking. Subsequently, the entire amount of the second-stage monomer aqueous solution similar to that in Example 8 was added to the dispersion A of polymer particles before surface crosslinking to obtain a reaction solution. Then, the reaction solution was stirred while the inside of the system was thoroughly purged with nitrogen, and then the system was maintained at 45°C for 30 minutes. Furthermore, the separable flask was immersed in a water bath at 75°C to raise the temperature, and second-stage polymerization was carried out for 15 minutes to obtain a second-stage reaction mixture. After the second-stage polymerization, the temperature of the second-stage reaction mixture was raised in an oil bath at 125°C, and 9 g of water was extracted from the system by azeotropic distillation of n-heptane and water while refluxing n-heptane, thereby obtaining a dispersion B of polymer particles before surface crosslinking. Then, 0.040 g (0.230 mmol) of ethylene glycol diglycidyl ether was added as a surface cross-linking agent to the dispersion B of polymer particles before surface cross-linking, and the mixture was kept at 83°C for 2 hours to obtain a dispersion of polymer particles after surface cross-linking. Thereafter, the dispersion of the surface-crosslinked polymer particles was heated in an oil bath at 125°C to evaporate n-heptane and dry, thereby obtaining a polymer. This polymer was passed through a sieve with an opening of 850µm, thereby obtaining 230.16g of water-absorbent resin particles in the form of agglomerated spherical particles. The median particle diameter of the water-absorbent resin particles was 454µm.

[0087] <Median particle size> The above-mentioned median particle diameter of the water-absorbent resin particles was measured by the following procedure. Using a continuous, fully automatic ultrasonic vibration sieving measuring instrument (Robot Sifter RPS-205, manufactured by Seishin Enterprise Co., Ltd.), JIS standard sieves with mesh sizes of 850 μm, 710 μm, 600 μm, 500 μm, 400 μm, 300 μm, 250 μm, and 150 μm, and a tray, the particle size distribution of 5 g of water-absorbent resin particles was measured. Regarding this particle size distribution, the masses of particles remaining on the sieves were integrated in descending order of particle size, and the relationship between the sieve mesh size and the integrated value of the mass percentage of the particles remaining on the sieves was plotted on logarithmic probability paper. The particle diameter corresponding to an integrated mass percentage of 50% by mass was obtained as the median particle diameter by connecting the plots on the logarithmic probability paper with a straight line.

[0088] <Water absorption under load> The water absorption amount (room temperature, 25°C ± 2°C) of physiological saline solution under load (pressure) of water-absorbent resin particles was measured using a measuring device Y shown in Fig. 1. The measuring device Y is composed of a burette unit 61, a conduit 62, a measurement table 63, and a measurement unit 64 placed on the measurement table 63. The burette unit 61 has a burette 61a extending vertically, a rubber stopper 61b arranged at the upper end of the burette 61a, a cock 61c arranged at the lower end of the burette 61a, an air introduction tube 61d having one end extending into the burette 61a near the cock 61c, and a cock 61e arranged on the other end of the air introduction tube 61d. The conduit 62 is attached between the burette unit 61 and the measurement table 63. The inner diameter of the conduit 62 is 6 mm. A hole with a diameter of 2 mm is drilled in the center of the measurement table 63, and the conduit 62 is connected to it. The measuring unit 64 has a cylinder 64a (made of acrylic resin (Plexiglas)), a nylon mesh 64b adhered to the bottom of the cylinder 64a, and a weight 64c. The inner diameter of the cylinder 64a is 20 mm. The openings of the nylon mesh 64b are 57 μm (255 mesh). During measurement, the water-absorbent resin particles 65 to be measured are uniformly scattered on the nylon mesh 64b. The diameter of the weight 64c is 19 mm, and the mass of the weight 64c is 120 g. The weight 64c is placed on the water-absorbent resin particles 65, and can apply a load of 4.14 kPa to the water-absorbent resin particles 65.

[0089] After placing 0.100 g of water-absorbent resin particles 65 in cylinder 64a of measuring device Y, weight 64c was placed on top and measurement was started. Air of the same volume as the physiological saline solution absorbed by water-absorbent resin particles 65 was quickly and smoothly supplied from the air inlet tube into the inside of burette 61a, so that the decrease in the water level of the physiological saline solution inside burette 61a corresponds to the amount of physiological saline solution absorbed by water-absorbent resin particles 65. The scale of burette 61a is engraved from top to bottom in increments of 0.5 mL from 0 mL. The scale Va of burette 61a before the start of water absorption and the scale Vb of burette 61a 60 minutes after the start of water absorption were read as the water level of the physiological saline solution, and the water absorption amount under load and the improvement rate were calculated using the following formulas. The results are shown in Table 1. Water absorption under load [mL / g] = (Vb-Va) / 0.1 Improvement rate of water absorption under load [%] = {(water absorption of Example - water absorption of Comparative Example) / (water absorption of Comparative Example)} × 100

[0090] <Damage rate> Fifteen spherical alumina balls (Alumina Ball HD-15, manufactured by Nikkato Corporation) with a diameter of 15 mm (approximately 100 g) and 10 g of water-absorbent resin particles were placed in a 400 mL alumina pot for a ball mill, and the water-absorbent resin particles were pulverized for 15 minutes at a rotation speed of 140 rpm. Thereafter, the pulverized material was manually passed through a JIS standard sieve with a mesh size of 150 μm for 2 minutes. The total mass W of the particles that passed through the sieve was A , and the total mass of particles that did not pass through the sieve W B Based on this, the breakage rate and improvement rate were calculated using the following formula. The results are shown in Table 1. Damage rate [mass%] = {W A / (W A +W B )}×100 Improvement rate of breakage rate [%] = {|Breakage rate of Example - Breakage rate of Comparative Example| / (Breakage rate of Comparative Example)} × 100

[0091] [Table 1] [Explanation of symbols]

[0092] 61...burette part, 61a...burette, 61b...rubber stopper, 61c, 61e...cock, 61d...air introduction tube, 62...conduit, 63...measurement table, 64...measurement part, 64a...cylinder, 64b...nylon mesh, 64c...weight, 65...water-absorbent resin particles, Y...measuring device.

Claims

1. a polymerization step of polymerizing a monomer on at least a part of the surface of the surface-crosslinked polymer particles to obtain a polymer, the amount of the monomer in the polymerization step is 25 to 100 moles relative to 100 moles of the monomer used to obtain the polymer particles; the polymer obtained in the polymerization step includes a polymer having a structural unit derived from an ethylenically unsaturated monomer, The method for producing water-absorbent resin particles, wherein the ethylenically unsaturated monomer comprises at least one selected from the group consisting of (meth)acrylic acid and its salts, and (meth)acrylamide.

2. 2. The method for producing water-absorbent resin particles according to claim 1, wherein the ethylenically unsaturated monomer comprises at least one selected from the group consisting of (meth)acrylic acid and salts thereof.

3. The method for producing water-absorbent resin particles according to claim 1, wherein the ethylenically unsaturated monomer comprises (meth)acrylamide.

4. a polymerization step of polymerizing a monomer in the presence of a crosslinking agent on at least a portion of the surface of a polymer particle that is not surface-crosslinked, thereby obtaining a polymer; the amount of the monomer in the polymerization step is 25 to 100 moles relative to 100 moles of the monomer used to obtain the polymer particles; the amount of the crosslinking agent in the polymerization step is 0.001 to 0.5 moles per 100 moles of the monomer in the polymerization step; the polymer obtained in the polymerization step includes a polymer having a structural unit derived from an ethylenically unsaturated monomer, The method for producing water-absorbent resin particles, wherein the ethylenically unsaturated monomer contains at least one selected from the group consisting of (meth)acrylic acid and salts thereof.

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