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

Abstract

The first embodiment of the method for producing water-absorbent resin particles of the present application includes a polymerization step of obtaining a polymer by polymerizing a monomer in at least a part of the surface of a surface-crosslinked polymer particle. The second embodiment of the method for producing water-absorbent resin particles includes a polymerization step of obtaining a polymer by polymerizing a monomer in the presence of a crosslinking agent in at least a part of the surface of a non-surface-crosslinked polymer particle.

Description

TECHNICAL FIELD

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

[0002] Water-absorbent resin particles are widely used in various fields such as sanitary materials such as paper diapers, sanitary products, and simple toilets; horticultural and farm materials such as water-retaining agents and soil conditioners; industrial materials such as water-stopping agents and anti-fogging agents. In the production of sanitary materials, water-absorbent resin particles can be damaged by collision of water-absorbent resin particles with each other, friction with equipment, and the like, and thus can lose the original water-absorbing ability. In view of such a problem, a technique for improving the impact resistance and the like by controlling the internal bubble rate of water-absorbent resin particles is known (for example, see Patent Literature 1).

[0003] Prior Art Documents

[0004] Patent Literature

[0005] Patent Literature 1: Japanese Patent Application Laid-Open No. 2014-098172 SUMMARY

[0006] Technical Problem to be Solved by the Invention

[0007] According to the present inventor's findings, in the related art relating to water-absorbent resin particles, it is difficult to improve the impact resistance while achieving excellent water-absorbing performance under load.

[0008] An object of one aspect of the present application is to provide a method for producing water-absorbent resin particles capable of improving the impact resistance while achieving excellent water-absorbing performance (water-absorbing performance of the same level or higher) under load in comparison with a method for producing water-absorbent resin particles using the same content of raw materials. Another object of one aspect of the present application is to provide water-absorbent resin particles capable of improving the impact resistance while achieving excellent water-absorbing performance under load in comparison with water-absorbent resin particles obtained using the same content of raw materials.

[0009] Means for Solving the Technical Problem

[0010] The present inventors have found the following insight. That is, as a technique for improving water absorption performance under load, surface crosslinking of polymer particles can be considered. However, when surface crosslinking is performed alone, not only is it the case that water absorption performance under load cannot be sufficiently improved, but also it is the case that water-absorbent resin particles are easily broken (have low impact resistance). In contrast, by performing a process of polymerizing a monomer on the surface of polymer particles after surface crosslinking of the polymer particles, or a process of polymerizing a monomer in the presence of a crosslinking agent on the surface of polymer particles that have not been surface crosslinked, in comparison with a manufacturing method for water-absorbent resin particles using the same content of raw materials, it is possible to achieve excellent water absorption performance under load (water absorption performance under load) while improving impact resistance.

[0011] A first embodiment of the manufacturing method for 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 at least a portion of the surface of a surface-crosslinked polymer particle.

[0012] A second embodiment of the manufacturing method for 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 in at least a portion of the surface of a polymer particle that has not been surface crosslinked.

[0013] According to these manufacturing methods for water-absorbent resin particles, in comparison with a manufacturing method for water-absorbent resin particles using the same content of raw materials, it is possible to obtain water-absorbent resin particles that achieve excellent water absorption performance under load while improving impact resistance.

[0014] 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 portion of the surface of the polymer particle.

[0015] According to this water-absorbent resin particle, in comparison with water-absorbent resin particles obtained using the same content of raw materials, it is possible to achieve excellent water absorption performance under load while improving impact resistance.

[0016] Effects of the Invention

[0017] According to one aspect of the present invention, it is possible to provide a manufacturing method for water-absorbent resin particles that achieves excellent water absorption performance under load while improving impact resistance, in comparison with a manufacturing method for water-absorbent resin particles using the same content of raw materials. According to another aspect of the present invention, it is possible to provide a water-absorbent resin particle that achieves excellent water absorption performance under load while improving impact resistance, in comparison with water-absorbent resin particles obtained using the same content of raw materials. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of a device for measuring the water absorption of water-absorbing resin particles under load. Detailed Implementation

[0019] The embodiments of the present invention will now be described in detail. However, the present invention is not limited to the following embodiments, and various modifications and implementations can be made within its spirit and scope.

[0020] In this specification, "acrylic acid" and "methacrylic acid" are both referred to as "(meth)acrylic acid". "acrylate" and "methacrylate" are also referred to as "(meth)acrylate". "(poly)" refers to both the case with and without the prefix "poly". The upper or lower limit of a numerical range described in this specification can be arbitrarily combined with the upper or lower limits of other numerical ranges. The upper or lower limit of the numerical range described in this specification can also be replaced with the values ​​shown in the examples. Room temperature refers to 25°C ± 2°C. The materials exemplified in this specification can be used alone or in combination of two or more. When multiple substances equivalent to each component are present in the composition, unless otherwise specified, the content of each component in the composition refers to the total amount of the multiple substances present in the composition.

[0021] The method for manufacturing water-absorbing resin particles according to the first embodiment includes a polymerization step in which a polymer is obtained by polymerizing a monomer in at least a portion of the surface of a surface-crosslinked polymer particle. The method for manufacturing water-absorbing resin particles according to the second embodiment includes a polymerization step in which a polymer is obtained by polymerizing a monomer in 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, the polymer, as the monomer, can be obtained as a polymer disposed on at least a portion of the surface of the polymer particle.

[0022] According to the production method of the water-absorbent resin particles according to the present embodiment (including the first embodiment and the second embodiment), by performing the process of polymerizing the monomer on the surface of the polymer particles after the surface crosslinking of the polymer particles or the process of polymerizing the monomer in the presence of the crosslinking agent on the surface of the polymer particles which is not surface-crosslinked, among the production methods of the water-absorbent resin particles using the same content (same kind, same amount) of raw materials, it is possible to obtain the water-absorbent resin particles which can improve the impact resistance while achieving excellent water-absorbing properties under load, as compared with the case where these processes are not performed. According to the production method of the water-absorbent resin particles according to the present embodiment, by adjusting the order of the polymerization of the monomer and the use of the crosslinking agent, it is possible to improve the impact resistance while achieving excellent water-absorbing properties under load.

[0023] The water-absorbent resin particles according to the present embodiment have the polymer particles which are surface-crosslinked, and the polymer which is disposed on at least a part of the surface of the polymer particles. Such water-absorbent resin particles can be obtained by the production method of the water-absorbent resin particles according to the first embodiment. According to the water-absorbent resin particles according to the present embodiment, among the water-absorbent resin particles obtained using the same content of raw materials, it is possible to improve the impact resistance while achieving excellent water-absorbing properties under load.

[0024] According to the water-absorbent resin particles and the production method thereof according to the present embodiment, by improving the impact resistance, it is possible to suppress the generation of small-diameter particles when a pressure is applied to the water-absorbent resin particles.

[0025] The present inventors and others have speculated that the following mechanism is one reason for the improvement of the impact resistance while achieving excellent water-absorbing properties under load. That is, when the surface crosslinking is performed on the polymer particles, a solidified layer is obtained as the outermost layer due to the increase in the crosslinking density on the surface thereof. In this case, even if excellent water-absorbing properties under load are obtained, the solidified outermost layer is easily damaged (low impact resistance) due to the collision of the particles with each other.

[0026] On the other hand, according to the production method of the water-absorbent resin particles according to the first embodiment, by polymerizing the monomer on the surface of the polymer particles after the surface crosslinking of the polymer particles, it is possible to suppress the exposure of the solidified layer obtained by the surface crosslinking as the outermost layer while maintaining the state where the polymer particles are surface-crosslinked. Thereby, it is possible to improve the impact resistance while achieving excellent water-absorbing properties under load.

[0027] Further, according to the method for producing water-absorbent resin particles according to the second embodiment, by polymerizing the monomer in the presence of the crosslinking agent on the surface of the polymer particles that are not surface-crosslinked, the crosslinking of the outermost layer can be promoted while suppressing the crosslinking density of the outermost layer from becoming excessively high, and thus the impact resistance can be improved while achieving excellent water-absorption performance under load.

[0028] However, the mechanism of the appearance effect is not limited to these.

[0029] The median particle diameter of the water-absorbent resin particles according to the present embodiment can be in the following range. The median particle diameter of the water-absorbent resin particles can 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 can 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 can be 100 to 800 μm.

[0030] In the polymerization step of the method for producing water-absorbent resin particles according to the first embodiment, the monomer is polymerized in at least a part of the surface of the polymer particles that are surface-crosslinked. The "polymer particles that are surface-crosslinked" refer to polymer particles in which the crosslinking density of the surface is higher than that of the inside.

[0031] In the polymerization step of the method for producing water-absorbent resin particles according to the first embodiment, the monomer is brought into contact with the surface of the polymer particles. In the polymerization step, the monomer can be added to a liquid containing the polymer particles, or a liquid containing the polymer particles and a liquid containing the monomer can be mixed. 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. In addition, the "absence of a crosslinking agent" has the same meaning as the absence of the addition of a crosslinking agent in the polymerization step. When an internal crosslinking agent is used in the formation of the polymer particles, in the polymerization step, a trace amount of the internal crosslinking agent remaining in the polymer particles can possibly leak to the outside of the polymer particles, but even in this case, as long as no crosslinking agent is newly added in the polymerization step, it is included in the "absence of a crosslinking agent".

[0032] In the polymerization step of the method for producing the water-absorbent resin particles according to the second embodiment, the monomer is polymerized in the presence of the crosslinking agent in at least a part of the surface of the polymer particles that are not surface-crosslinked. The "polymer particles that are not surface-crosslinked" refer to polymer particles in which the crosslinking density inside the particles is substantially equal to the crosslinking density on the surface.

[0033] In the polymerization step of the method for producing the water-absorbent resin particles according to the second embodiment, the monomer and the crosslinking agent are brought into contact with the surface of the polymer particles. The monomer and the crosslinking agent can be added to a liquid containing the polymer particles, or a liquid containing the monomer and the crosslinking agent can be mixed with a liquid containing the polymer particles, or a liquid containing the monomer, a liquid containing the crosslinking agent, and a liquid containing the polymer particles can be mixed.

[0034] The shape of the polymer particles is not particularly limited, and for example, can be substantially spherical, irregular, particulate, or the like, or can be a shape resulting from agglomeration of primary particles having such shapes. The polymer particles of irregular shape can be obtained, for example, by crushing a block of polymer using a crusher.

[0035] The polymer particles can have water-absorbing properties. The water-absorbing amount of ion-exchanged water at 25°C (water-absorbing amount under normal pressure) in the polymer particles can be, for example, 10 g / g or more.

[0036] The polymer particles can contain a gel stabilizer, a metal chelating agent, a flowability improver (lubricant), or the like. These components can be disposed inside the polymer particles, on the surface of the polymer particles, or both.

[0037] The polymer obtained in the polymerization step can be water-soluble or non-water-soluble (or poorly water-soluble). In the case where the polymer is water-soluble, the solubility of the polymer can be, for example, 1 g or more (for example, 1 to 150 g) per 100 g of ion-exchanged water at 25°C. In the case where the polymer is poorly water-soluble, the solubility of the polymer can be, for example, less than 1 g per 100 g of ion-exchanged water at 25°C.

[0038] The polymer obtained in the polymerization step can constitute a coating portion that coats at least a part of the surface of the polymer particles (coated body). The coating portion can coat at least a part of the surface of the polymer particles, or a part or all of the surface of the polymer particles. In the water-absorbent resin particles according to the present embodiment, the crosslinking density of the coating portion can be lower than the crosslinking density of the surface of the polymer particles.

[0039] The reaction temperature in the polymerization process can be, for example, 15–200°C. The polymerization reaction in the polymerization process can be a chain polymerization reaction, a successive polymerization reaction, etc. Examples of constituent materials of the polymer obtained in the polymerization process include chain polymerization products such as poly(meth)acrylic acid, poly(meth)acrylamide, polyvinyl alcohol, polyepoxide, and polyalkylene glycol; and successive polymerization products such as polyurethane (urethane resin), phenolic resin (e.g., condensates of phenolic compounds and aldehydes), polyester, polyamide, and polycarbonate. The polymer can be a cross-linked polymer.

[0040] From the viewpoint of easily achieving excellent water absorption under load while simultaneously improving impact resistance, the polymer obtained in the polymerization process preferably comprises a polymer having structural units derived from olefinic unsaturated monomers (polymers having olefinic unsaturated monomers as monomer units). From the viewpoint of easily achieving excellent water absorption under load while simultaneously improving impact resistance, the polymer obtained in the polymerization process preferably comprises polyurethane.

[0041] Examples of olefinically unsaturated monomers include (meth)acrylic acid and its salts, (meth)acrylates (methyl (meth)acrylate, ethyl (meth)acrylate, 2-hydroxyethyl (meth)acrylate, 2-(diethylamino)ethyl (meth)acrylate, 2-(diethylamino)propyl (meth)acrylate, etc.), (meth)acrylamide monomers ((meth)acrylamide, N-isopropyl (meth)acrylamide, 2-(meth)acrylamide-2-methylpropanesulfonic acid and its salts, N,N-dimethyl (meth)acrylamide, N-hydroxymethyl (meth)acrylamide, diethylaminopropyl (meth)acrylamide, etc.), and polyethylene glycol mono(meth)acrylates. From the viewpoint of easily achieving excellent water absorption under load while simultaneously improving impact resistance, the olefinically unsaturated monomer preferably includes at least one selected from the group consisting of (meth)acrylic acid and its salts. From the viewpoint of easily achieving excellent water absorption under load while simultaneously improving impact resistance, the olefinically unsaturated monomer preferably includes a (meth)acrylamide monomer.

[0042] In the polymerization process, when multiple substances react with each other to obtain a polymer, examples of combinations of multiple substances include polyols and polyisocyanates; aldehydes and phenolic compounds; polyols and polycarboxylic acids; polyamines and polycarboxylic acids; phenolic compounds and carbonates; phenolic compounds and chlorine carbonates, etc.

[0043] Polyols can be compounds with two or more hydroxyl groups, and can include diols, triols, etc. Examples of polyols include polyether polyols, polyester polyols, polycarbonate polyols, polysiloxane polyols, polyisoprene polyols, and polyolefin polyols.

[0044] Polyisocyanates can be compounds having two or more isocyanate groups, and diisocyanates, triisocyanates, etc., can be used. Examples of polyisocyanates include aromatic isocyanates such as diphenylmethane diisocyanate, dimethyl diphenylmethane diisocyanate, toluene diisocyanate (e.g., toluene-2,4-diisocyanate), benzene-xylene diisocyanate, and terephthalene diisocyanate; alicyclic isocyanates such as dicyclohexylmethane diisocyanate and isophorone diisocyanate; and aliphatic isocyanates such as hexamethylene diisocyanate.

[0045] Examples of aldehydes include aliphatic aldehydes such as formaldehyde, acetaldehyde, and propionaldehyde; and aromatic aldehydes such as benzaldehyde.

[0046] Examples of phenolic compounds include phenol, cresol, catechol, naphthol, and hydroquinone.

[0047] From the viewpoint of easily obtaining excellent impact resistance, and from the viewpoint of easily improving water absorption and / or impact resistance under load in comparison with manufacturing methods of water-absorbing resin particles using mutually identical raw materials, the amount of monomer in the polymerization step of the water-absorbing resin particle manufacturing method according to this embodiment is preferably within the following range relative to 100 moles of monomer used to obtain polymer particles in the particle production step described later (in the case of multi-stage polymerization, the total amount of monomer in each stage). The amount of 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 monomer is preferably less than 100 mol, less than 100 mol, less than 80 mol, less than 60 mol, less than 50 mol, less than 40 mol, less than 30 mol, less than 25 mol, less than 20 mol, less than 15 mol, less than 14 mol, less than 13 mol, less than 12 mol, less than 11 mol, less than 10 mol, less than 5 mol, less than 2 mol, less than 1 mol, less than 0.5 mol, or less than 0.1 mol. From these views, the amount of monomer is preferably from 0.01 to 100 mol.

[0048] Examples of crosslinking agents used in the polymerization step of the method for manufacturing water-absorbing resin particles according to the second embodiment include polyglycidyl compounds such as (poly)ethylene glycol diglycidyl ether, (poly)propylene glycol diglycidyl ether, (poly)glycerol diglycidyl ether, (poly)glycerol triglycidyl ether, (poly)propylene glycol polyglycidyl ether, and polyglycerol polyglycidyl ether.

[0049] From the viewpoint of easily achieving excellent water absorption performance under load while improving impact resistance, the amount of crosslinking agent in the polymerization step of the method for manufacturing water-absorbing resin particles according to the second embodiment is preferably within the following range relative to 100 moles of monomer in the polymerization step. The amount of crosslinking agent is preferably 0.001 moles or more, more preferably 0.003 moles or more, further preferably 0.005 moles or more, particularly preferably 0.008 moles or more, extremely preferably 0.01 moles or more, and very preferably 0.02 moles or more. The amount of crosslinking agent is preferably 1 mole or less, more preferably 0.5 moles or less, further preferably 0.1 moles or less, particularly preferably 0.05 moles or less, and extremely preferably 0.03 moles or less. From these viewpoints, the amount of crosslinking agent is preferably 0.001 to 1 mole.

[0050] The method for manufacturing water-absorbing resin particles according to the second embodiment does not include a surface crosslinking step for surface crosslinking of the polymer particles before the polymerization step. The method for manufacturing water-absorbing resin particles according to the first embodiment includes a surface crosslinking step for surface crosslinking of the polymer particles before the polymerization step. In the surface crosslinking step, the polymer particles are surface crosslinked by mixing the polymer particles and a surface crosslinking agent. In the surface crosslinking step, polymerization of the monomers may not be present.

[0051] Examples of surface crosslinking agents include poly(poly)ethylene glycol diglycidyl ether, poly(poly)propylene glycol diglycidyl ether, poly(poly)glycerol diglycidyl ether, poly(poly)glycerol triglycidyl ether, poly(poly)propylene glycol polyglycidyl ether, polyglycerol polyglycidyl ether, and other polyglycidyl-based compounds.

[0052] From the viewpoint of easily achieving excellent water absorption under load while improving impact resistance, the amount of surface crosslinking agent in the surface crosslinking process is preferably within the following range relative to 100 moles of monomer used to obtain polymer particles (in the case of multi-stage polymerization, the total amount of monomers in each stage). The amount of surface crosslinking 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 surface crosslinking agent is preferably 0.5 moles or less, more preferably 0.1 moles or less, and even more preferably 0.05 moles or less.

[0053] The method for manufacturing water-absorbing resin particles according to this embodiment includes a particle-making step that polymerizes monomers to obtain polymer particles before the surface crosslinking step and the polymerization step. In the particle-making step, the monomers can be polymerized once or multiple times.

[0054] Polymer particles can be obtained, for example, by polymerizing monomers containing olefinic unsaturated monomers. That is, polymer particles can have structural units derived from olefinic unsaturated monomers (having olefinic unsaturated monomers as monomeric units). Examples of polymerization methods for olefinic unsaturated monomers include reverse suspension polymerization, aqueous solution polymerization, bulk polymerization, and precipitation polymerization.

[0055] The olefinic unsaturated monomer can be a water-soluble olefinic unsaturated monomer (for example, an olefinic unsaturated monomer with a solubility of 1 g or more relative to 100 g of deionized water at 25°C). Examples of olefinic unsaturated monomers include (meth)acrylic acid and its salts, (meth)acrylates ((meth)acrylate, (meth)acrylate, (meth)acrylate 2-hydroxyethyl meth)acrylate, (meth)acrylate 2-(diethylamino)ethyl meth)acrylate, (meth)acrylate 2-(diethylamino)propyl meth)acrylate, etc.), (meth)acrylamide monomers ((meth)acrylamide, N-isopropyl(meth)acrylamide, 2-(meth)acrylamide-2-methylpropanesulfonic acid and its salts, N,N-dimethyl(meth)acrylamide, N-hydroxymethyl(meth)acrylamide, diethylaminopropyl(meth)acrylamide, etc.), polyethylene glycol mono(meth)acrylate, etc. From the viewpoint of easily suppressing particle aggregation while obtaining water-absorbing resin particles, the olefinic unsaturated monomer may contain at least one selected from the group including (meth)acrylic acid and its salts. From the viewpoint of obtaining water-absorbing resin particles while easily suppressing particle aggregation, polymer particles preferably have structural units derived from at least one selected from the group consisting of (meth)acrylic acid and its salts.

[0056] When an olefinic unsaturated monomer has an acidic group, it can be neutralized before being used in a polymerization reaction. The degree of neutralization in the olefinic unsaturated monomer can be 10–100 mol%, 50–90 mol%, or 60–80 mol% of the acidic group in the olefinic unsaturated monomer.

[0057] As monomers for obtaining polymer particles, monomers other than the olefinically unsaturated monomers described above can be used. Such monomers can be used, for example, by mixing them in an aqueous solution containing the aforementioned olefinically unsaturated monomers. The amount of olefinically unsaturated monomer used relative to the total monomer amount (the total amount of monomers used to obtain polymer particles, for example, the total amount of monomers imparting structural units to the crosslinked polymer; the same applies hereinafter) is preferably 70 to 100 mol%. The proportion of (meth)acrylic acid and its salts relative to the total monomer amount is more preferably 70 to 100 mol%. "The proportion of (meth)acrylic acid and its salts" refers to the total stoichiometric proportion of (meth)acrylic acid and its salts.

[0058] To obtain polymer particles, an internal crosslinking agent can be used. When an internal crosslinking agent is used during the polymerization of the monomer, it is easy to increase the crosslinking density approximately uniformly throughout the polymer particles. Examples of internal crosslinking agents include polyglycidyl compounds such as (poly)ethylene glycol diglycidyl ether, (poly)propylene glycol diglycidyl ether, (poly)glycerol diglycidyl ether, (poly)glycerol triglycidyl ether, (poly)propylene glycol polyglycidyl ether, and polyglycerol polyglycidyl ether; diethylene compounds; diol compounds; and diacrylate compounds.

[0059] The method for manufacturing water-absorbing resin particles according to this embodiment includes a step of classifying the water-absorbing resin particles by sieving after the polymerization step. This allows for adjustment of the particle size distribution.

[0060] According to this embodiment, a liquid absorption method using the absorbent resin particles described in this embodiment can be provided. The liquid absorption method of this embodiment includes a step of contacting the liquid to be absorbed with the absorbent resin particles described in this embodiment.

[0061] Example

[0062] The present invention will be further described below using examples and comparative examples, but the present invention is not limited to the following examples.

[0063] (Example 1)

[0064] A round-bottomed cylindrical separating 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 inlet tube, and a stirrer (with a two-stage, 5 cm diameter, four-bladed inclined impeller). 293 g of n-heptane (hydrocarbon dispersion medium) and 0.736 g of maleic anhydride-modified ethylene-propylene copolymer (polymeric dispersant, Mitsui Chemicals, Inc., Hi-WAX 1105A) were added to the separating flask to obtain a mixture. The dispersant was dissolved by heating the mixture to 80 °C while stirring at 300 rpm, and then the mixture was cooled to 55 °C.

[0065] Next, 92.0 g of an 80.5% by mass aqueous solution of acrylic acid (acrylic acid: 1.03 mol) was added to a 500 mL Erlenmeyer flask. Then, while cooling from the outside, 102.2 g of a 30% by mass aqueous solution of sodium hydroxide was added dropwise to neutralize 75 mol% of the acrylic acid. Subsequently, 0.092 g of hydroxyethyl cellulose (thickener, SUMITOMO SEIKA CHEMICALSCO.,LTD., HEC AW-15F), 0.0736 g (0.272 mmol) of potassium persulfate (water-soluble free radical polymerization initiator), 0.0101 g (0.0581 mmol) of ethylene glycol diglycidyl ether (internal crosslinking agent), and 32.85 g of deionized water were added and dissolved to prepare the first-stage monomer aqueous solution.

[0066] Then, the first-stage monomer aqueous solution was added to the separating flask and stirred for 10 minutes. Next, 7.356 g of a surfactant solution (obtained by dissolving 0.736 g of sucrose stearate (surfactant, manufactured by Mitsubishi Chemical Corporation, RYOTO Sugar Ester S-370, HLB:3) in 6.62 g of n-heptane by heating) was added to the separating flask to obtain the reaction mixture. The system was then thoroughly purged with nitrogen while stirring the reaction mixture at 550 rpm. The separating flask was then immersed in a 70°C water bath to raise the temperature of the reaction mixture, and the first-stage polymerization was carried out for 10 minutes to obtain the first-stage reaction mixture.

[0067] Next, 128.8 g of an 80.5% by mass aqueous solution of acrylic acid (acrylic acid: 1.44 mol) was added to another 500 mL Erlenmeyer flask. Then, while cooling from the outside, 143.1 g of a 30% by mass aqueous solution of sodium hydroxide was added dropwise to neutralize 75 mol% of the 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 deionized water were added and dissolved to prepare the second-stage monomer aqueous solution.

[0068] Then, the first-stage reaction mixture was stirred at 1000 rpm and cooled to 25°C. The total amount of the second-stage monomer aqueous solution was then added to the first-stage reaction mixture to obtain a reaction solution. The system was then thoroughly purged with nitrogen while stirring the reaction solution. The separating flask was then immersed in a 70°C water bath to heat the reaction solution, and the second-stage polymerization was carried out for 5 minutes to obtain the second-stage reaction mixture (polymer particles before surface crosslinking).

[0069] Following the second-stage polymerization, the reaction mixture was heated in an oil bath at 125°C, and 267 g of water was discharged from the system while refluxing the heptane through azeotropic distillation of n-heptane and water. Next, 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, thereby obtaining a dispersion of surface-crosslinked polymer particles.

[0070] Next, 111.4 g of an 80.5% by mass aqueous solution of acrylic acid (acrylic acid: 1.25 mol) was added to another 500 mL Erlenmeyer flask. Then, while cooling externally, 125.8 g of a 30% by mass aqueous solution of sodium hydroxide was added dropwise to neutralize 75 mol% of the acrylic acid. Subsequently, 0.0891 g (0.3296 mmol) of potassium persulfate and 0.84 g of deionized water were added to dissolve the potassium persulfate, thus preparing the third-stage monomer aqueous solution.

[0071] Then, the dispersion of the surface-crosslinked polymer particles was maintained at 83°C for 2 hours and then naturally cooled to 50°C. Next, the total amount of the third-stage monomer aqueous solution was added to the dispersion of the surface-crosslinked polymer particles to obtain a reaction solution. Then, while stirring the reaction solution, the system was thoroughly purged with nitrogen and maintained at 45°C for 30 minutes. Furthermore, the separating flask was immersed in a water bath at 75°C and heated to allow the third-stage polymerization to proceed for 15 minutes, thereby obtaining a third-stage reaction mixture.

[0072] Following the third-stage polymerization, the reaction mixture was heated in an oil bath at 125°C, and subjected to azeotropic distillation of n-heptane and water. While refluxing the n-heptane, water was drained from the system until the temperature inside the flask reached 90°C. The n-heptane was then evaporated and dried to obtain the polymer. Passing this polymer through an 850 μm sieve yielded 282.62 g of superabsorbent resin particles in the form of aggregated spherical particles. The median particle size of the superabsorbent resin particles was 436 μm.

[0073] (Comparative Example 1)

[0074] In Comparative Example 1, surface crosslinking was not performed before the third-stage polymerization. Instead, the third-stage polymerization and surface crosslinking were performed sequentially after obtaining the second-stage reaction mixture (polymer particles before surface crosslinking). Otherwise, water-absorbing resin particles were produced in the same manner as in Example 1.

[0075] First, the polymerization is carried out in the same manner as in Example 1 up to the second level.

[0076] After the second-stage polymerization, the reaction mixture of the second stage was heated in an oil bath at 125°C, and 245g of water was discharged from the system while the n-heptane was refluxed through azeotropic distillation of n-heptane and water.

[0077] Next, 111.4 g of an 80.5% by mass aqueous solution of acrylic acid (acrylic acid: 1.25 mol) was added to a 500 mL Erlenmeyer flask. Then, while cooling externally, 125.8 g of a 30% by mass aqueous solution of sodium hydroxide was added dropwise to neutralize 75 mol% of the acrylic acid. Subsequently, 0.0891 g (0.3296 mmol) of potassium persulfate and 0.84 g of deionized water were added to dissolve the potassium persulfate, thus preparing the third-stage monomer aqueous solution.

[0078] Then, the reaction mixture of the second stage (after removing water) was kept at 83°C for 2 hours and then naturally cooled to 50°C. Next, the total amount of the monomer aqueous solution of the third stage was added to the reaction mixture of the second stage to obtain a reaction solution. Then, while stirring the reaction solution, the system was thoroughly purged with nitrogen and kept at 45°C for 30 minutes. Furthermore, the separating flask was immersed in a water bath at 75°C and heated to allow the third-stage polymerization to proceed for 15 minutes, thereby obtaining the reaction mixture of the third stage.

[0079] After the third-stage polymerization, the reaction mixture of the third stage was heated in an oil bath at 125°C, and 204g of water was discharged from the system while the n-heptane was refluxed through azeotropic distillation of n-heptane and water.

[0080] Subsequently, 0.0884 g (0.5075 mmol) of ethylene glycol diglycidyl ether was added as a surface crosslinking agent, and the mixture was kept at 83 °C for 2 hours.

[0081] Then, the heptane was evaporated and dried by heating in an oil bath at 125°C to obtain the polymer. By passing the polymer through a sieve with a pore size of 850 μm, 283.36 g of water-absorbing resin particles in the form of aggregated spherical particles were obtained. The median particle size of the water-absorbing resin particles was 448 μm.

[0082] (Example 2)

[0083] In Example 2, the amount of water discharged after the second-stage polymerization was changed from 267g to 241g, and the contents of the third-stage monomer aqueous solution were also changed. Otherwise, 217.07g of water-absorbing resin particles (aggregated into spherical particles) were obtained in the same manner as in Example 1. The third-stage monomer aqueous solution was prepared as follows: 44.6g of an 80.5% by mass acrylic acid aqueous solution (acrylic acid: 0.50 mol) was added to a 500mL Erlenmeyer flask. While cooling from the outside, 50.3g of a 30% by mass sodium hydroxide aqueous solution was added dropwise to neutralize 75 mol% acrylic acid. Then, 0.0357g (0.1319 mmol) of potassium persulfate and 0.5g of deionized water were added to dissolve the potassium persulfate. The median particle size of the water-absorbing resin particles was 370μm.

[0084] (Comparative Example 2)

[0085] In Comparative Example 2, the amount of water discharged after the second-stage polymerization was changed from 245g to 241g, the contents of the third-stage monomer aqueous solution were changed, and the amount of water discharged after the third-stage polymerization was changed from 204g to 42g. Otherwise, 216.32g of water-absorbing resin particles were obtained in the same manner as in Comparative Example 1. Regarding the third-stage monomer aqueous solution, it was prepared as follows: 44.6g of an 80.5% by mass acrylic acid aqueous solution (acrylic acid: 0.50 mol) was added to a 500mL Erlenmeyer flask. While cooling from the outside, 50.3g of a 30% by mass sodium hydroxide aqueous solution was added dropwise to neutralize 75 mol% acrylic acid. Then, 0.0357g (0.1319 mmol) of potassium persulfate and 0.5g of deionized water were added to dissolve the potassium persulfate. The median particle size of the water-absorbing resin particles was 386μm.

[0086] (Example 3)

[0087] In Example 3, the contents of the third-stage monomer aqueous solution were changed, and a third-stage monomer solution and a crosslinking agent were added simultaneously. Otherwise, in the same manner as in Example 1, 209.29 g of water-absorbing resin particles (with the morphology of aggregated spherical particles) were obtained.

[0088] First, the polymerization is carried out in the same manner as in Example 1 up to the second level.

[0089] Next, 33.4 g of an 80.5% by mass aqueous solution of acrylic acid (acrylic acid: 0.37 mol) was added to a 500 mL Erlenmeyer flask. Then, 75 mol% of the acrylic acid was neutralized by adding 37.7 g of a 30% by mass aqueous solution of sodium hydroxide dropwise while cooling from the outside. Subsequently, 0.0267 g (0.0989 mmol) of potassium persulfate and 0.38 g of deionized water were added to dissolve the potassium persulfate, thus preparing the third-stage monomer aqueous solution.

[0090] Then, the second-stage reaction mixture (polymer particles before crosslinking) was heated in an oil bath at 125°C, and 267g of water was discharged from the system while refluxing the heptane through azeotropic distillation of n-heptane and water. Next, 0.0884g (0.5075 mmol) of ethylene glycol diglycidyl ether was added as a crosslinking agent, and the above-mentioned third-stage monomer aqueous solution was added. The mixture was then maintained at 83°C for 2 hours, thereby obtaining a dispersion of crosslinked polymer particles.

[0091] The dispersion of cross-linked polymer particles was then heated in an oil bath at 125°C, and subjected to azeotropic distillation of n-heptane and water. While refluxing the n-heptane, water was drained from the system until the temperature inside the flask reached 90°C. The n-heptane was then evaporated and dried to obtain the polymer. Passing this polymer through a sieve with a pore size of 850 μm yielded 209.29 g of water-absorbing resin particles in the form of aggregated spherical particles. The median particle size of the water-absorbing resin particles was 372 μm.

[0092] (Comparative Example 3)

[0093] In Comparative Example 3, the amount of water discharged after the second-stage polymerization was changed from 245g to 241g, the contents of the third-stage monomer aqueous solution were changed, and the amount of water discharged after the third-stage polymerization was changed from 204g to 32g. Otherwise, 210.70g of water-absorbing resin particles were obtained in the same manner as in Comparative Example 1. Regarding the third-stage monomer aqueous solution, it was prepared as follows: 33.4g of an 80.5% by mass acrylic acid aqueous solution (acrylic acid: 0.37 mol) was added to a 500mL Erlenmeyer flask. While cooling from the outside, 37.7g of a 30% by mass sodium hydroxide aqueous solution was added dropwise to neutralize 75 mol% acrylic acid. Then, 0.0267g (0.0989 mmol) of potassium persulfate and 0.38g of deionized water were added to dissolve the potassium persulfate. The median particle size of the water-absorbing resin particles was 378μm.

[0094] (Example 4)

[0095] In Example 4, the contents of the third-stage monomer aqueous solution were changed. Otherwise, 201.46 g of absorbent resin particles (aggregated into spherical particles) were obtained in the same manner as in Example 1. The third-stage monomer aqueous solution was prepared as follows: 22.3 g of an 80.5% by mass acrylic acid aqueous solution (acrylic acid: 0.25 mol) was added to a 500 mL Erlenmeyer flask. While cooling from the outside, 25.1 g of a 30% by mass sodium hydroxide aqueous solution was added dropwise to neutralize 75 mol% acrylic acid. Then, 0.0178 g (0.0658 mmol) of potassium persulfate and 0.30 g of deionized water were added to dissolve the potassium persulfate. The median particle size of the absorbent resin particles was 387 μm.

[0096] (Comparative Example 4)

[0097] In Comparative Example 4, the contents of the third-stage monomer aqueous solution and the amount of water discharged after the third-stage polymerization were changed from 204 g to 22 g. Otherwise, 200.02 g of water-absorbing resin particles were obtained in the same manner as in Comparative Example 1. The third-stage monomer aqueous solution was prepared as follows: 22.3 g of an 80.5% by mass acrylic acid aqueous solution (acrylic acid: 0.25 mol) was added to a 500 mL Erlenmeyer flask. While cooling from the outside, 25.1 g of a 30% by mass sodium hydroxide aqueous solution was added dropwise to neutralize 75 mol% acrylic acid. Then, 0.0178 g (0.0658 mmol) of potassium persulfate and 0.30 g of deionized water were added to dissolve the potassium persulfate. The median particle size of the water-absorbing resin particles was 381 μm.

[0098] (Example 5)

[0099] In Example 5, the amount of water discharged after the second-stage polymerization was changed from 267g to 245g, and the contents of the third-stage monomer aqueous solution were also changed. Otherwise, 173.68g of water-absorbing resin particles (aggregated into spherical particles) were obtained in the same manner as in Example 1. The third-stage monomer aqueous solution was prepared as follows: 0.2g of an 80.5% by mass acrylic acid aqueous solution (acrylic acid: 0.002 mol) was added to a 500mL Erlenmeyer flask. While cooling from the outside, 0.3g of a 30% by mass sodium hydroxide aqueous solution was added dropwise to neutralize 75 mol% acrylic acid. Then, 0.000178g (0.00066 mmol) of potassium persulfate and 0.84g of deionized water were added to dissolve the potassium persulfate. The median particle size of the water-absorbing resin particles was 356μm.

[0100] (Comparative Example 5)

[0101] In Comparative Example 5, the contents of the third-stage monomer aqueous solution and the amount of water discharged after the third-stage polymerization were changed from 204 g to 0.2 g. Otherwise, 172.62 g of water-absorbing resin particles were obtained in the same manner as in Comparative Example 1. The third-stage monomer aqueous solution was prepared as follows: 0.2 g of an 80.5% by mass acrylic acid aqueous solution (acrylic acid: 0.002 mol) was added to a 500 mL Erlenmeyer flask. While cooling from the outside, 0.3 g of a 30% by mass sodium hydroxide aqueous solution was added dropwise to neutralize 75 mol% acrylic acid. Then, 0.000178 g (0.00066 mmol) of potassium persulfate and 0.84 g of deionized water were added to dissolve the potassium persulfate. The median particle size of the water-absorbing resin particles was 366 μm.

[0102] (Example 6)

[0103] In Example 6, the contents of the third-stage monomer aqueous solution were changed. Otherwise, 193.20 g of superabsorbent resin particles (aggregated into spherical particles) were obtained in the same manner as in Example 3. The third-stage monomer aqueous 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 deionized water. The median particle size of the superabsorbent resin particles was 385 μm.

[0104] (Comparative Example 6)

[0105] In Comparative Example 6, the amount of water discharged after the second-stage polymerization was changed from 245g to 237g, the contents of the monomer aqueous solution in the third stage were changed, and the amount of water discharged after the third-stage polymerization was changed from 204g to 21g. Otherwise, 198.28g of superabsorbent resin particles (aggregated into spherical particles) were obtained in the same manner as in Comparative Example 1. The monomer aqueous solution in the third stage was obtained by mixing 22.1g (0.31 mol) of acrylamide, 0.0177g (0.0655 mmol) of potassium persulfate, and 27.22g of deionized water. The median particle size of the superabsorbent resin particles was 413 μm.

[0106] (Example 7)

[0107] In Example 7, the amount of water discharged after the second-stage polymerization was changed from 267g to 269g, and the contents of the monomer aqueous solution in the third stage were changed. Otherwise, 227.32g of water-absorbing resin particles (in the form of aggregated spherical particles) were obtained in the same manner as in Example 1.

[0108] First, the polymerization is carried out in the same manner as in Example 1 up to the second level.

[0109] Following the second-stage polymerization, the reaction mixture (polymer particles before surface crosslinking) was heated in an oil bath at 125°C, and 269 g of water was discharged from the system while refluxing the heptane through azeotropic distillation of n-heptane and water. Next, 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, thereby obtaining a dispersion of surface-crosslinked polymer particles.

[0110] Next, as the third-stage monomer solution, 4.4 g of polyether polyol (DKS Co. Ltd., DK polyol 3817) and 83.6 g of distilled water were mixed to prepare 88 g of mixture A (polyol aqueous solution), and 31.14 g of mixture B (isocyanate acetone solution) was prepared by mixing 3.12 g of toluene-2,4-diisocyanate and 28.02 g of acetone.

[0111] Then, the dispersion of the surface-crosslinked polymer particles was maintained at 83°C for 2 hours. Afterward, mixture A was added to the dispersion of the surface-crosslinked polymer particles, and the mixture was stirred at 80°C for 30 minutes. Next, mixture B was added, and the mixture was stirred at 80°C for 60 minutes, and a successive polymerization reaction (tertiary polymerization) was carried out on the surface of the polymer particles to polymerize the polyurethane, thereby obtaining a third-stage reaction mixture.

[0112] Following the third-stage polymerization, the reaction mixture was heated in an oil bath at 125°C, and azeotropic distillation of n-heptane and water was performed, evaporating n-heptane while simultaneously removing water and acetone from the system. This continued until the temperature inside the flask reached 90°C. The n-heptane was then evaporated and dried to obtain the polymer. Passing this polymer through an 850 μm sieve yielded 227.32 g of superabsorbent resin particles in the form of aggregated spherical particles. The median particle size of the superabsorbent resin particles was 373 μm.

[0113] (Comparative Example 7)

[0114] In Comparative Example 7, surface crosslinking was not performed before the third-stage polymerization. Instead, the third-stage polymerization and surface crosslinking were performed sequentially after obtaining the second-stage reaction mixture (polymer particles before surface crosslinking). Otherwise, water-absorbing resin particles were produced in the same manner as in Example 7.

[0115] First, the polymerization is carried out in the same manner as in Example 1 up to the second level.

[0116] After the second stage of polymerization, the reaction mixture (polymer particles before surface crosslinking) of the second stage is heated in an oil bath at 125°C, and 245g of water is discharged from the system while the heptane is refluxed through azeotropic distillation of n-heptane and water.

[0117] Next, as the third-stage monomer solution, 4.4 g of polyether polyol (DKS Co. Ltd., DK polyol 3817) and 83.6 g of distilled water were mixed to prepare 88 g of mixture A (polyol aqueous solution), and 31.14 g of mixture B (isocyanate acetone solution) was prepared by mixing 3.12 g of toluene-2,4-diisocyanate and 28.02 g of acetone.

[0118] Next, the above-mentioned mixture A was added to the second-stage reaction mixture (the reaction mixture after water removal), and stirred at 80°C for 30 minutes. Then, after adding the above-mentioned mixture B, the mixture was stirred at 80°C for 60 minutes, and polyurethane was polymerized by a successive polymerization reaction (third-stage polymerization) on the surface of the polymer particles, thereby obtaining the third-stage reaction mixture.

[0119] After the third-stage polymerization, the reaction mixture of the third stage was heated in an oil bath at 125°C, and 47g of water and acetone were discharged from the system by azeotropic distillation of n-heptane and water while n-heptane was refluxed.

[0120] Subsequently, 0.0884 g (0.5075 mmol) of ethylene glycol diglycidyl ether was added as a surface crosslinking agent, and the mixture was kept at 83 °C for 2 hours.

[0121] Then, the heptane was evaporated and dried by heating in an oil bath at 125°C to obtain the polymer. By passing the polymer through a sieve with a pore size of 850 μm, 230.16 g of water-absorbing resin particles in the form of aggregated spherical particles were obtained. The median particle size of the water-absorbing resin particles was 450 μm.

[0122] (Example 8)

[0123] In Example 8, after polymer particles were prepared by aqueous solution polymerization, water-absorbing resin particles were produced.

[0124] A round-bottomed cylindrical separating 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 inlet tube, and a stirrer (with a two-stage, 5 cm diameter, four-bladed inclined impeller). 509.71 g (7.07 mol) of 100% acrylic acid was added to the separating flask. While stirring the acrylic acid, 436.47 g of deionized water was added to the separating flask. Then, 444.68 g of 48% sodium hydroxide was added dropwise under ice bath (1 °C), thus preparing 1390.86 g of partially neutralized acrylic acid solution with a monomer concentration of 45.08% by mass (neutralization rate: 75.44 mol%). This operation was repeated, preparing a total of 2781.72 g of partially neutralized acrylic acid solution.

[0125] 406.89 g of deionized 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 above-mentioned acrylic acid partially neutralized solution to obtain a reaction solution (monomer aqueous solution). The reaction solution was then purged with nitrogen for 30 minutes under a nitrogen atmosphere. Next, the reaction solution was supplied to a stainless steel double-arm kneader equipped with a thermometer, nitrogen inlet pipe, openable and closable cap, two sigma-type blades, and a sleeve, and the system was purged with nitrogen while maintaining the reaction solution at 30°C. Then, while stirring the reaction solution, 92.63 g (7.780 mmol) of a 2.0% by mass sodium persulfate aqueous solution and 15.85 g of a 0.5% by mass L-ascorbic acid aqueous solution were added. After approximately 1 minute, the temperature began to rise and polymerization began. The highest temperature during polymerization was 93°C, observed after 6 minutes. Then, while maintaining the sleeve temperature at 60°C and continuing stirring, the hydrogel, which served as the first-stage polymerization reactant, was extracted 60 minutes after polymerization began. The obtained hydrogel was then sequentially fed into a Kiren Royal Co., Ltd. 12VR-750SDX meat grinder for further subdivision. The diameter of the holes in the plate at the tip of the meat grinder was 6.4 mm.

[0126] The finely subdivided hydrogel was spread on a metal mesh with a pore size of 0.8 cm × 0.8 cm and then dried in hot air at 160 °C for 60 minutes to obtain the dried product.

[0127] Next, the dried material was pulverized using a centrifugal pulverizer (Retsch ZM200, 1mm sieve diameter, 12000rpm) to obtain irregularly broken resin powder A. Furthermore, this resin powder A was graded using metal meshes with 850μm, 250μm, and 180μm pore sizes, thereby obtaining graded resin powder B, which passes through the 850μm pore size metal mesh but not the 250μm pore size metal mesh.

[0128] A round-bottomed cylindrical separating 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 inlet pipe, and a stirrer (with a two-stage stirring blade with four inclined blades and a 5 cm diameter). After adding 100 g of the above-mentioned resin powder B to the separating flask, 560 g of n-heptane was added as a hydrocarbon dispersion medium.

[0129] Subsequently, the separation flask was heated to 83°C in an oil bath at 125°C, and 0.040 g (0.230 mmol) of ethylene glycol diglycidyl ether was added as a surface crosslinking agent. The mixture was then kept at 83°C for 2 hours, thereby obtaining a dispersion of surface-crosslinked polymer particles.

[0130] Next, 10.1 g of an 80.5% by mass aqueous solution of acrylic acid (acrylic acid: 0.11 mol) was added to a 500 mL Erlenmeyer flask. Then, while cooling externally, 11.4 g of a 30% by mass aqueous solution of sodium hydroxide was added dropwise to neutralize 75 mol% of the acrylic acid. Afterward, 0.00810 g (0.0300 mmol) of potassium persulfate was added to dissolve the potassium persulfate, thereby preparing the second-stage monomer aqueous solution.

[0131] Then, the dispersion of the surface-crosslinked polymer particles was maintained at 83°C for 2 hours and then naturally cooled to 50°C. Next, the total amount of the second-stage monomer aqueous solution was added to the dispersion of the surface-crosslinked polymer particles to obtain a reaction solution. Then, while stirring the reaction solution, the system was thoroughly purged with nitrogen and maintained at 45°C for 30 minutes. Furthermore, the separating flask was immersed in a 75°C water bath for heating and second-stage polymerization was carried out for 15 minutes to obtain a second-stage reaction mixture.

[0132] Following the second-stage polymerization, the reaction mixture was heated in an oil bath at 125°C, and subjected to azeotropic distillation of n-heptane and water. While refluxing the n-heptane, water was drained from the system until the temperature inside the flask reached 90°C. The n-heptane was then evaporated and dried to obtain the polymer. Passing this polymer through an 850 μm sieve yielded 109.19 g of superabsorbent resin particles. The median particle size of the superabsorbent resin particles was 460 μm.

[0133] (Comparative Example 8)

[0134] In Comparative Example 8, surface crosslinking was not performed before the second-stage polymerization. Instead, the second-stage polymerization and surface crosslinking were performed sequentially after obtaining resin powder B. Otherwise, water-absorbing resin particles were produced in the same manner as in Example 8.

[0135] First, resin powder B was obtained in the same manner as in Example 8, and then 100g of resin powder B and 560g of n-heptane (hydrocarbon dispersion medium) were mixed in a separating flask.

[0136] Subsequently, the separation flask was heated to 83°C in an oil bath at 125°C and then naturally cooled to 50°C to obtain a dispersion A of polymer particles before surface crosslinking.

[0137] Next, a reaction solution was obtained by adding the same total amount of the second-stage monomer aqueous solution as in Example 8 to the dispersion A of polymer particles before surface crosslinking. Then, after thoroughly purging the system with nitrogen while stirring the reaction solution, the mixture was maintained at 45°C for 30 minutes. Furthermore, the separating flask was immersed in a water bath at 75°C for heating and second-stage polymerization was carried out for 15 minutes, thereby obtaining a second-stage reaction mixture.

[0138] After the second-stage polymerization, the reaction mixture of the second stage was heated in an oil bath at 125°C, and 9g of water was discharged from the system while the heptane was refluxed through azeotropic distillation of n-heptane and water, thereby obtaining a dispersion B of polymer particles before surface crosslinking.

[0139] Then, 0.040 g (0.230 mmol) of ethylene glycol diglycidyl ether was added to the dispersion B of polymer particles before surface crosslinking as a surface crosslinking agent, and the mixture was kept at 83°C for 2 hours to obtain the dispersion of polymer particles after surface crosslinking.

[0140] Subsequently, the dispersion of the surface-crosslinked polymer particles was heated in an oil bath at 125°C to evaporate and dry the n-heptane, thereby obtaining the polymer. This polymer was then passed through a sieve with a pore size of 850 μm, yielding 230.16 g of water-absorbing resin particles in the form of aggregated spherical particles. The median particle size of the water-absorbing resin particles was 454 μm.

[0141] <Median Particle Size>

[0142] The median particle size of the superabsorbent polymer (SAP) particles was determined in the following order. The particle size distribution of 5 g of SAP particles was determined using a continuous fully automated ultrasonic vibrating sieve analyzer (ROBOT SHIFTER RPS-205, manufactured by SEISHIN ENTERPRISE Co., Ltd.), JIS standard sieves with apertures of 850 μm, 710 μm, 600 μm, 500 μm, 400 μm, 300 μm, 250 μm, and 150 μm, and a tray. Regarding the particle size distribution, the cumulative percentage of particles remaining on the sieve, arranged in descending order of particle size, was plotted on logarithmic probability paper. By connecting the plotted particles on the logarithmic probability paper with straight lines, the particle size corresponding to 50% of the cumulative mass was obtained as the median particle size.

[0143] <Water absorption under load>

[0144] use Figure 1 The apparatus Y shown measures the water absorption of physiological saline under load (pressurization) of absorbent resin particles (room temperature, 25℃±2℃). The apparatus Y comprises a burette section 61, a conduit 62, a measuring stage 63, and a measuring part 64 placed on the measuring stage 63. The burette section 61 includes: a burette 61a extending vertically; a rubber stopper 61b disposed at the upper end of the burette 61a; a stopcock 61c disposed at the lower end of the burette 61a; an air inlet tube 61d extending into the burette 61a at one end near the stopcock 61c; and a stopcock 61e disposed at the other end of the air inlet tube 61d. The conduit 62 is installed between the burette section 61 and the measuring stage 63. The inner diameter of the conduit 62 is 6 mm. A 2 mm diameter hole is provided in the center of the measuring stage 63, and the conduit 62 is connected to it. The measuring unit 64 includes: a cylinder 64a (made of acrylic resin (plexiglass)); a nylon mesh 64b bonded to the bottom of the cylinder 64a; and a counterweight 64c. The inner diameter of the cylinder 64a is 20 mm. The pore size of the nylon mesh 64b is 57 μm (255 mesh). During measurement, the absorbent resin particles 65 of the test object are uniformly distributed on the nylon mesh 64b. The counterweight 64c has a diameter of 19 mm and a mass of 120 g. The counterweight 64c is placed on the absorbent resin particles 65, and can apply a load of 4.14 kPa relative to the absorbent resin particles 65.

[0145] After adding 0.100 g of absorbent resin particles 65 to the cylinder 64a of the measuring apparatus Y, a counterweight 64c was placed and the measurement began. Air of the same volume as the physiological saline absorbed by the absorbent resin particles 65 was rapidly and smoothly supplied to the burette 61a through an air inlet tube. Therefore, the decrease in the physiological saline level inside the burette 61a represents the amount of physiological saline absorbed by the absorbent resin particles 65. The burette 61a was marked with 0 mL to 0.5 mL graduations from top to bottom to represent the physiological saline level. The burette 61a was read at the graduation Va before absorption began and at the graduation Vb 60 minutes after absorption began. The absorption volume and increase rate under load were calculated using the following formula. The results are shown in Table 1.

[0146] Water absorption under load [mL / g] = (Vb - Va) / 0.1

[0147] Increase in water absorption under load [%) = {(Water absorption of the example - Water absorption of the comparative example) / (Water absorption of the comparative example)} × 100

[0148] <Damage Rate>

[0149] Fifteen (approximately 100g) spherical aluminum balls (HD-15, manufactured by NIKKATO CORPORATION) with a diameter of 15mm and 10g of absorbent resin particles were added to a 400mL aluminum ball mill container. The absorbent resin particles were then pulverized at 140rpm for 15 minutes. Afterward, the pulverized material was manually passed through a 150μm JIS standard sieve for 2 minutes. The total mass W of the particles passing through the sieve was calculated. A The total mass W of particles that did not pass through the sieve B The breakage rate and improvement rate were calculated using the following formula. The results are shown in Table 1.

[0150] Breakage rate [mass %] = {W A / (W A +W B )}×100

[0151] The rate of increase in breakage rate [%] = {|Breakage rate of the example - Breakage rate of the comparative example| / (Breakage rate of the comparative example)} × 100

[0152] [Table 1]

[0153]

[0154] Symbol Explanation

[0155] 61-burette section, 61a-burette, 61b-rubber stopper, 61c, 61e-stopcock, 61d-air inlet tube, 62-conduit, 63-measuring stage, 64-measuring section, 64a-cylinder, 64b-nylon mesh, 64c-counterweight, 65-absorbent resin particles, Y-measuring device.

Claims

1. A method for manufacturing water-absorbing resin particles, comprising: The polymerization process involves obtaining a polymer by polymerizing monomers in at least a portion of the surface of surface-crosslinked polymer particles. in, The polymer particles are hygroscopic. The amount of the monomer is 5 moles or more relative to 100 moles of the monomer used to obtain the polymer particles. The polymer obtained by polymerizing the monomer is a chain polymerization product or a successive polymerization product. The chain polymerization reactant is a polymer having structural units derived from at least one selected from the group consisting of acrylic acid and its salts. The successive polymerization reactants comprise polyurethane.

2. A method for manufacturing water-absorbing resin particles, comprising: The polymerization process involves polymerizing monomers in the presence of a crosslinking agent in at least a portion of the surface of uncrosslinked polymer particles to obtain a water-insoluble polymer. in, The polymer particles are hygroscopic. The amount of the monomer is less than 25 moles relative to 100 moles of the monomer used to obtain the polymer particles. The polymer obtained by polymerizing the monomer is a chain polymerization product or a successive polymerization product. The chain polymerization product is a polymer having structural units derived from (meth)acrylamide monomers. The (meth)acrylamide monomer comprises at least one selected from the group consisting of (meth)acrylamide, N-isopropyl (meth)acrylamide, 2-(meth)acrylamide-2-methylpropanesulfonic acid and its salts, N,N-dimethyl (meth)acrylamide, N-hydroxymethyl (meth)acrylamide, and diethylaminopropyl (meth)acrylamide. The successive polymerization reactants comprise polyurethane.

3. The method for manufacturing water-absorbing resin particles according to claim 1, wherein, The polymer obtained by polymerizing the monomer is poorly water-soluble.

4. A water-absorbing resin particle, comprising: Surface-crosslinked polymer particles; and The polymer disposed on at least a portion of the surface of the polymer particle, in, The polymer particles are hygroscopic. The amount of monomer of the polymer used to obtain at least a portion of the surface of the polymer particles is 5 moles or more relative to 100 moles of monomer used to obtain the polymer particles. The polymer disposed on at least a portion of the surface of the polymer particles is a chain polymerization product or a successive polymerization product. The chain polymerization reactant is a polymer having structural units derived from at least one selected from the group consisting of acrylic acid and its salts. The successive polymerization reactants comprise polyurethane.

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