Method for producing polymer dispersion and method for producing water-absorbing resin particle

Abstract

This method for producing a polymer dispersion includes: a step for preparing a polymer solution containing a polymer in which a crosslinked structure of a water-absorbing resin has been cleaved; and a step for bringing the polymer solution into contact with a poor solvent for the polymer to deposit a solid polymer and thereby obtaining a polymer dispersion in which the solid polymer is dispersed. The transmittance of the polymer dispersion over a 1 cm optical path of light, having wavelength of 425 nm, is 0.85% or less.

Description

Method for producing polymer dispersion and method for producing water-absorbent resin particles

[0001] The present invention relates to a method for producing a polymer dispersion and a method for producing water-absorbent resin particles.

[0002]

[0003] Generally, disposable sanitary products are constructed by enclosing pulp fibers and a water-absorbent resin between a water-impermeable cover sheet and a water-permeable nonwoven fabric, and the water-absorbent resin absorbs water and swells to absorb bodily waste. After use, such sanitary products are disposed of by incineration or landfilling, but recently, research has been conducted into the recovery and reuse of components from used sanitary products. For example, Patent Document 1 discloses a water-absorbent resin decomposition technology that focuses on cleavage of crosslinks within the water-absorbent resin, and a technology for recovering the water-soluble recycled polymer obtained by decomposing the water-absorbent resin as an aggregate by crosslinking with multivalent metal ions.

[0003] Japanese Patent Application Laid-Open No. 2020-49398

[0004] When a polymer solution containing a polymer whose crosslinked structure has been cleaved is re-crosslinked in a reversed-phase suspension system to regenerate a water-absorbent resin, if the polymer exhibits an ideal shape (spherical) in the dispersion medium, the water-absorbent resin regenerated from the polymer also tends to become spherical. According to the findings of the present inventors, when an attempt is made to regenerate a water-absorbent resin by subjecting a polymer solution as it is to re-crosslinking in a reversed-phase suspension system, the polymer is unlikely to become spherical in the dispersion medium, and it is therefore difficult to regenerate a spherical water-absorbent resin.

[0005] Therefore, one aspect of the present invention relates to a method for producing a polymer dispersion in which the polymer is likely to become spherical in a dispersion medium in a reversed-phase suspension system.

[0006] One aspect of the present disclosure includes, for example, the following [1] to [5]. [1] A method for producing a polymer dispersion, comprising: preparing a polymer solution containing a polymer in which a crosslinked structure of a water-absorbent resin has been cleaved; and contacting the polymer solution with a poor solvent for the polymer to precipitate a solid polymer, thereby obtaining a polymer dispersion in which the solid polymer is dispersed, wherein the polymer dispersion has a transmittance of 0.85% or less for light with a wavelength of 425 nm at an optical path length of 1 cm. [2] The production method according to [1], wherein the viscosity of the polymer dispersion at 25°C is 5000 mPa·s or less. [3] The production method according to [1] or [2], wherein the poor solvent contains at least one selected from the group consisting of alcohols, phenols, and ketones having 1 to 10 carbon atoms. [4] The production method according to any one of [1] to [3], wherein the degree of neutralization of the polymer is 40 mol% or more. [5] A method for producing water-absorbent resin particles, comprising a step of crosslinking a solid polymer in the polymer dispersion obtained by the production method according to any one of [1] to [4] to obtain a water-absorbent resin.

[0007] According to one aspect of the present invention, there is provided a method for producing a polymer dispersion in which the polymer is likely to become spherical in a dispersion medium in a reversed-phase suspension system.

[0008] Fig. 1 is a cross-sectional view showing an example of an absorbent article having an absorbent core; Fig. 2 is an SEM image of the polymer powder obtained in Example 1; Fig. 3 is an SEM image of the polymer powder obtained in Comparative Example 1; Fig. 4 is an SEM image of the polymer powder obtained in Comparative Example 2; Fig. 5 is an SEM image of the polymer powder obtained in Comparative Example 3.

[0009] The present invention is not limited to the following examples.

[0010] In this specification, "(meth)acrylic" refers to both acrylic and methacrylic. "Acrylate" and "methacrylate" are also written as "(meth)acrylate." The same applies to other similar terms. "(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. The materials exemplified in this specification may be used alone or in combination of two or more. "Standard sieve" refers to a test sieve (metal mesh sieve) specified in JIS Z 8801-1:2019. Unless otherwise specified, the examples and comparative examples were performed in an environment of 1 atmosphere, room temperature, and normal humidity, and the various parameters disclosed in this specification were also measured in the same environment, with the temperature of the various samples being room temperature. "1 atmosphere" is 101,325 Pa, "normal temperature" is 25° C., and "normal humidity" is 50% RH.

[0011] An example of a method for producing a spherical polymer includes a step of preparing a polymer solution containing a polymer in which the crosslinked structure of a water-absorbent resin has been cleaved (preparation step), and a step of bringing a poor solvent for the polymer into contact with the polymer solution to precipitate the polymer, thereby obtaining a polymer dispersion in which a solid polymer is dispersed (dispersion step).

[0012] <Water-absorbent resin> The water-absorbent resin may be a recycled water-absorbent resin. The recycled water-absorbent resin may be a used water-absorbent resin (i.e., a water-absorbent resin that has become gelatinous due to absorption of liquid), an unused water-absorbent resin (e.g., waste water-absorbent resin generated during the manufacturing process of a water-absorbent resin), or a mixture of both.

[0013] Examples of absorbent articles containing recycled water-absorbent resins include diapers (e.g., disposable diapers), toilet training pants, incontinence pads, sanitary materials (sanitary napkins, tampons, etc.), sweat pads, pet sheets, portable toilet components, and animal waste treatment materials. The absorbent articles may be used. The recycled water-absorbent resin recovered from used absorbent articles may form a gel due to absorption of liquid during use.

[0014] The recycled water-absorbing resin contains water and a water-absorbing resin. The recycled water-absorbing resin may contain fibrous materials such as cellulosic fibers and synthetic fibers.

[0015] The water-absorbent resin contains, for example, a polymer (crosslinked polymer) of an ethylenically unsaturated monomer. Examples of the ethylenically unsaturated monomer include (meth)acrylic acid and its salts, 2-(meth)acrylamido-2-methylpropanesulfonic acid and its salts, (meth)acrylamide, N,N-dimethyl(meth)acrylamide, 2-hydroxyethyl(meth)acrylate, N-methylol(meth)acrylamide, polyethylene glycol mono(meth)acrylate, N,N-diethylaminoethyl(meth)acrylate, N,N-diethylaminopropyl(meth)acrylate, and diethylaminopropyl(meth)acrylamide. The ethylenically unsaturated monomer may contain at least one compound selected from the group consisting of acrylic acid and its salts, methacrylic acid and its salts, acrylamide, methacrylamide, and N,N-dimethylacrylamide.

[0016] The crosslinked polymer may contain monomer units other than ethylenically unsaturated monomer units. The proportion of ethylenically unsaturated monomer units (particularly, (meth)acrylic acid and salts thereof) in the crosslinked polymer may be 70 mol % to 100 mol % relative to the total amount of monomer units. The proportion of (meth)acrylic acid and salts thereof in the ethylenically unsaturated monomers may be 70 mol % to 100 mol %.

[0017] The water-absorbing resin contains, for example, a crosslinked polymer having a poly(meth)acrylic acid structure.As such crosslinked polymer, for example, the polymer obtained by polymerizing a monomer composition containing (meth)acrylic acid and a crosslinking agent capable of reacting with the carboxyl group of (meth)acrylic acid to form a covalent bond; the polymer obtained by polymerizing a monomer composition containing (meth)acrylic acid to obtain a polymer, and then treating the surface of the polymer with a crosslinking agent capable of reacting with the carboxyl group of the polymer to form a covalent bond; the polymer obtained by polymerizing a monomer composition containing (meth)acrylic acid and a crosslinking agent capable of reacting with the carboxyl group of (meth)acrylic acid to form a covalent bond, and then treating the surface of the polymer with a crosslinking agent capable of reacting with the carboxyl group of the polymer to form a covalent bond.

[0018] The shape of the recycled water absorbent resin is not particularly limited, and if it is a used gel, it may be in the form of an irregular lump, and if it is an unused one (generally a dry powder), it may be in the form of an irregular crushed shape, flakes, granules, etc.

[0019] <Preparation Step> In the preparation step, a polymer solution containing a polymer in which the crosslinked structure of the water-absorbent resin has been cleaved is prepared. In the preparation step, for example, the water-absorbent resin is brought into contact with a base component to obtain a polymer solution containing a polymer in which the crosslinked structure of the water-absorbent resin has been cleaved. The polymer solution may be a mixture of a polymer (a crosslinked polymer in which the crosslinked structure has been cleaved) and a base component. The polymer solution may or may not contain a water-absorbent resin (a crosslinked polymer in which the crosslinked structure has not been cleaved). When the crosslinked polymer contained in the water-absorbent resin is a crosslinked polymer having a poly(meth)acrylic acid structure, the polymer may be a polymer having a poly(meth)acrylic acid structure, and the polymer solution may be a mixture of a polymer having a poly(meth)acrylic acid structure and a base component.

[0020] The base component may be an inorganic base component, for example, at least one inorganic base component selected from the group consisting of sodium hydroxide, ammonia, potassium hydroxide, and calcium hydroxide, or may be a monovalent inorganic base component from the viewpoint of suppressing aggregation of the polymer after cleavage of the crosslinked structure, for example, at least one monovalent inorganic base component selected from the group consisting of sodium hydroxide, ammonia, and potassium hydroxide.

[0021] The degree of neutralization of the polymer solution may be adjusted by contacting the polymer solution with an acid component or the like. The degree of neutralization of the polymer refers to the ratio of neutralized acid groups (acid groups that have formed salts) to the total amount of acid groups in the polymer (including neutralized acid groups). For example, when the polymer is a partially neutralized product of polyacrylic acid, the degree of neutralization of the polymer refers to the ratio of carboxylate groups to the total amount of carboxyl groups and carboxylate groups contained in the partially neutralized product of polyacrylic acid.

[0022] The degree of neutralization of the polymer contained in the polymer solution tends to decrease affinity for the poor solvent if the degree of neutralization of the polymer is high, and from the viewpoint of easily suppressing the amount of poor solvent used to precipitate the solid polymer, it may be 40 mol% or more, 50 mol% or more, 60 mol% or more, or 70 mol% or more, and from the viewpoint of improving the skin compatibility of the regenerated water-absorbent resin, it may be 100 mol% or less, 90 mol% or less, 80 mol% or less, or 75 mol% or less. From these viewpoints, the degree of neutralization of the polymer contained in the polymer solution may be 40 mol% to 100 mol%, 50 mol% to 100 mol%, 60 mol% to 100 mol%, or 70 mol% to 100 mol%.

[0023] The concentration of the polymer in the polymer solution may be, for example, 0.1% by mass or more, 0.5% by mass or more, or 1% by mass or more, and 10% by mass or less, 7% by mass or less, or 5% by mass or less. The concentration of the polymer in the polymer solution may be 0.1% by mass to 10% by mass, 0.5% by mass to 7% by mass, or 1% by mass to 5% by mass.

[0024] <Dispersion Step> In the dispersion step, a poor solvent for the polymer is brought into contact with the polymer solution to precipitate a solid polymer, thereby obtaining a polymer dispersion in which the solid polymer is dispersed. At this time, the transmittance of the polymer dispersion at a wavelength of 425 nm over an optical path length of 1 cm (hereinafter sometimes simply referred to as transmittance) is 0.85% or less. The poor solvent refers to a solvent in which the solubility of the polymer in 100 g of solvent at 25°C is less than 0.1 g. The poor solvent may be a water-soluble organic solvent.

[0025] When a poor solvent is brought into contact with a polymer solution, a solid polymer precipitates, thereby reducing the transmittance of the polymer dispersion. According to the findings of the present inventors, by adjusting the type and / or amount of the poor solvent to be contacted so that the transmittance of the final polymer dispersion becomes 0.85% or less, a polymer dispersion can be obtained in which the polymer easily becomes spherical in the dispersion medium in a reversed-phase suspension system.

[0026] Examples of methods for contacting a polymer solution with a poor solvent for the polymer include adding the poor solvent to the polymer solution, or adding the polymer solution to the poor solvent. When adding a poor solvent to a polymer solution from the viewpoint of facilitating adjustment of the transmittance of light with a wavelength of 425 nm at an optical path length of 1 cm, the poor solvent may be added to the polymer solution over time. From a similar viewpoint, when adding a polymer solution to a poor solvent, the polymer solution may be added to the poor solvent over time. Note that "adding over time" means adding the poor solvent (or polymer solution) to the polymer solution (or poor solvent) continuously or intermittently over a predetermined time.

[0027] Examples of poor solvents include alcohols having 1 to 10 carbon atoms, phenols, ketones, ethers, nitriles, etc. From the viewpoint of facilitating the production of spherical polymers with higher purity, the poor solvent may be at least one selected from the group consisting of alcohols having 1 to 10 carbon atoms, phenols, and ketones.

[0028] Examples of alcohols having 1 to 10 carbon atoms include methanol, ethanol, propanol, butanol, pentanol, ethylene glycol, and glycerin. Examples of phenols include phenol, cresol, dibutylhydroxytoluene, and eugenol. Examples of ketones include acetone, methyl ethyl ketone, and diethyl ketone. Examples of ethers include dimethyl ether, ethyl methyl ether, diethyl ether, furan, and tetrahydrofuran. Examples of nitriles include acrylonitrile. The poor solvent may be at least one selected from the group consisting of methanol, ethanol, and acetone.

[0029] The concentration of the poor solvent in the finally obtained polymer dispersion may be 60% by mass or less, 55% by mass or less, or 50% by mass or less, from the viewpoint of preventing an increase in the transmittance of the polymer dispersion due to the generation of aggregated precipitates caused by excessive precipitation of the polymer. The concentration of the poor solvent in the polymer dispersion may be 10% by mass or more or 20% by mass or more, from the viewpoint of precipitating a sufficient amount of polymer. The concentration of the poor solvent in the polymer dispersion may be 10% by mass to 60% by mass or 20% by mass to 55% by mass.

[0030] The transmittance of the polymer dispersion at a 1 cm optical path length for light with a wavelength of 425 nm is 0.85% or less, and from the viewpoint of sufficiently precipitating a solid polymer that easily forms spherical particles in a dispersion medium in a reversed-phase suspension system, the transmittance may be 0.80% or less, 0.70% or less, or less than 0.70%. From the viewpoint of suppressing aggregation of the precipitated solid polymer and thereby facilitating the formation of spherical particles in a dispersion medium in a reversed-phase suspension system, the transmittance of the polymer dispersion at a 1 cm optical path length for light with a wavelength of 425 nm may be 0.01% or more, 0.10% or more, 0.20% or more, or 0.30% or more. From these viewpoints, the transmittance of the polymer dispersion may be 0.01% to 0.80%, 0.01% to 0.70%, or 0.01% or more but less than 0.70%. The transmittance of the polymer dispersion at a 1 cm optical path length for light with a wavelength of 425 nm is measured by the method described in the Examples below.

[0031] The viscosity of the polymer dispersion at 25°C may be 5000 mPa·s or less, 3000 mPa·s or less, or 2000 mPa·s or less, from the viewpoint of facilitating dispersion in the dispersion medium of a reversed-phase suspension system, thereby suppressing aggregation of the solid polymer in the dispersion medium, and facilitating the formation of spherical particles. The viscosity of the polymer dispersion at 25°C may be 10 mPa·s or more, 50 mPa·s or more, 100 mPa·s or more, 500 mPa·s or more, 1000 mPa·s or more, 1500 mPa·s or more, 3000 mPa·s or more, 5000 mPa·s or more, or 10000 mPa·s or more. From these viewpoints, the viscosity of the polymer dispersion at 25°C may be 50 mPa·s to 5000 mPa·s, 50 mPa·s to 3000 mPa·s, or 50 mPa·s to 2000 mPa·s, 100 mPa·s to 5000 mPa·s, 100 mPa·s to 3000 mPa·s, or 100 mPa·s to 2000 mPa·s. The viscosity of the polymer dispersion at 25°C is measured by the method described in the examples below.

[0032] The median particle size of the dried solid polymer after adding the polymer dispersion to the dispersion medium of the reversed-phase suspension system may be 100 μm or more, 150 μm or more, or 200 μm or more, or may be less than 300 μm or less than 290 μm, or may be 100 μm or more and less than 300 μm. The median particle size of the solid polymer is measured by the method described in the Examples below (method for measuring the median particle size of polymer powder).

[0033] <Crosslinking step> The polymer dispersion is likely to become spherical in the dispersion medium in a reversed-phase suspension system, and therefore can be suitably used for reversed-phase suspension polymerization. Therefore, after obtaining the polymer dispersion, the solid polymer in the obtained polymer dispersion may be crosslinked to obtain a water-absorbent resin. That is, another embodiment of the present invention is a method for producing water-absorbent resin particles, comprising a step (crosslinking step) of crosslinking the solid polymer in the polymer dispersion obtained by the above-mentioned production method to obtain a water-absorbent resin.

[0034] After obtaining the solid polymer and before carrying out the crosslinking step, in order to reduce the viscosity of the polymer dispersion, the polymer dispersion may be diluted, heated, or a salt (a salt containing a monovalent cation) may be added to the polymer dispersion. Furthermore, after obtaining the solid polymer and before crosslinking the solid polymer, the solid polymer may be subjected to a sterilization treatment.

[0035] In the crosslinking step, for example, a solution containing a crosslinking agent capable of forming a covalent bond with the functional group of the solid polymer is prepared, and the solution containing the crosslinking agent is added to the polymer dispersion to crosslink the solid polymer via the covalent bond to obtain a crosslinked polymer. In the crosslinking step, the polymer dispersion may be gelled. Alternatively, the polymer dispersion may be mixed with a dispersion medium containing a poor solvent other than the poor solvent used to obtain the polymer dispersion, and then mixed with a solution containing a crosslinking agent to crosslink the spherical polymer to obtain a crosslinked polymer. Alternatively, a crosslinking agent may be added to a dispersion medium containing a poor solvent other than the poor solvent used to obtain the polymer dispersion, and then mixed with the polymer dispersion to crosslink the spherical polymer to obtain a crosslinked polymer. By crosslinking the solid polymer, the entire polymer dispersion loses fluidity, and a gel containing the crosslinked polymer and water is formed. When crosslinking progresses to the extent that a gel is formed, water-absorbent resin particles having high water absorption capacity and low soluble content are particularly likely to be obtained.

[0036] Examples of functional groups possessed by the solid polymer include carboxyl groups. When the solid polymer has carboxyl groups, the solid polymer is crosslinked by a reaction between the carboxyl groups and a crosslinking agent and / or by a reaction between the carboxyl groups themselves. The covalent bond may be at least one selected from the group consisting of an ester bond, a thioester bond, an amide bond, an ether bond, and a carbon-carbon bond. The spherical polymer may be crosslinked, for example, via at least one group selected from the group consisting of a carboxylic acid ester group, a thioester group, an amide group, an acid anhydride group, an oxyalkylene group, and an oxyarylene group.

[0037] Examples of the crosslinking agent include aliphatic polyhydric alcohols such as (poly)ethylene glycol, (poly)propylene glycol, (poly)glycerin, and pentaerythritol; glycidyl compounds such as (poly)ethylene glycol diglycidyl ether, (poly)propylene glycol diglycidyl ether, and (poly)glycerin diglycidyl ether; bisacrylamide compounds such as N,N'-methylenebis(meth)acrylamide; allylated starch; diallyl phthalate; N,N',N"-triallyl isocyanurate; divinylbenzene; ethylenediamine, polyethyleneimine, and glycidyl (meth)acrylate.

[0038] The amount of the crosslinking agent may be 0.0001 parts by weight or more, 0.001 parts by weight or more, or 0.005 parts by weight or more, and may be 10 parts by weight or less, 10 parts by weight or less, or 5 parts by weight or less, per 100 parts by weight of the solid polymer. The amount of the crosslinking agent may be 0.0001 parts by weight to 15 parts by weight, 0.001 parts by weight to 10 parts by weight, or 0.005 parts by weight to 5 parts by weight, per 100 parts by weight of the solid polymer.

[0039] The reaction temperature when crosslinking the solid polymer is appropriately set depending on the type, amount, etc. of the crosslinking agent used, but may be 50° C. or higher, 80° C. or higher, or 100° C. or higher, and may be 220° C. or lower, 200° C. or lower, or 180° C. or lower. The reaction temperature when crosslinking the solid polymer may be 50° C. to 220° C., 80° C. to 200° C., or 100° C. to 180° C.

[0040] The reaction time for crosslinking the solid polymer is appropriately set depending on the type and amount of the crosslinking agent used, the reaction temperature, etc., but may be 1 to 200 minutes, or 5 to 150 minutes.

[0041] After obtaining a crosslinked polymer through the crosslinking step, a surface crosslinking step of surface crosslinking the crosslinked polymer may be carried out. The surface crosslinking can be carried out, for example, by adding a crosslinking agent for surface crosslinking (surface crosslinking agent) to the crosslinked polymer and allowing it to react.

[0042] The surface cross-linking agent may be a compound having two or more reactive functional groups that are reactive with the cross-linked polymer. The surface cross-linking agent may be the same as or different from the cross-linking agent in the cross-linking step.

[0043] The reactive functional groups of the surface cross-linking agent may be carbonate groups, alcoholic hydroxyl groups, epoxy groups, halogeno groups in haloepoxy compounds, isocyanate groups, oxetanyl groups, oxazoline groups, or combinations thereof. Carbonate groups are considered two reactive functional groups because they can react with two other molecules.

[0044] Examples of surface cross-linking agents having a carbonate group include alkylene carbonates (ethylene carbonate, etc.). Examples of surface cross-linking agents having an alcoholic hydroxyl group include polyol compounds such as ethylene glycol, propylene glycol, 1,4-butanediol, trimethylolpropane, glycerin, polyoxyethylene glycol, polyoxypropylene glycol, and polyglycerin, and hydroxyalkylamide compounds (bis[N,N-di(β-hydroxyethyl)]adipamide, etc.). Examples of surface cross-linking agents having two or more epoxy groups include (poly)ethylene glycol diglycidyl ether, (poly)glycerin diglycidyl ether, (poly)glycerin triglycidyl ether, trimethylolpropane triglycidyl ether, (poly)propylene glycol polyglycidyl ether, and (poly)glycerol polyglycidyl ether. Examples of haloepoxy compounds having an epoxy group and a halogeno group include epichlorohydrin, epibromohydrin, and α-methylepichlorohydrin. Examples of surface crosslinking agents having an isocyanate group include 2,4-tolylene diisocyanate and hexamethylene diisocyanate. Examples of surface crosslinking agents having an oxetanyl group include 3-methyl-3-oxetanemethanol, 3-ethyl-3-oxetanemethanol, 3-butyl-3-oxetanemethanol, 3-methyl-3-oxetaneethanol, 3-ethyl-3-oxetaneethanol, and 3-butyl-3-oxetaneethanol.

[0045] The amount of the surface crosslinking agent may be 0.001 parts by mass or more, 0.005 parts by mass or more, or 0.01 parts by mass or more, and may be 5 parts by mass or less, 3 parts by mass or less, or 1 part by mass or less, relative to 100 parts by mass of the crosslinked polymer. The amount of the crosslinking agent may be 0.001 parts by mass to 5 parts by mass, 0.005 parts by mass to 3 parts by mass, or 0.01 parts by mass to 1 part by mass, relative to 100 parts by mass of the crosslinked polymer.

[0046] When the polymer dispersion is mixed with a dispersion medium, the dispersion medium used is a liquid composed mainly of a hydrophobic substance, for example, a hydrocarbon-based dispersion medium. Examples of hydrocarbon-based dispersion media include chain aliphatic hydrocarbons such as n-hexane, n-heptane, 2-methylhexane, 3-methylhexane, 2,3-dimethylpentane, 3-ethylpentane, and n-octane; alicyclic hydrocarbons such as cyclohexane, methylcyclohexane, cyclopentane, methylcyclopentane, trans-1,2-dimethylcyclopentane, cis-1,3-dimethylcyclopentane, and trans-1,3-dimethylcyclopentane; and aromatic hydrocarbons such as benzene, toluene, and xylene. The hydrocarbon-based dispersion media may be used alone or in combination of two or more.

[0047] The dispersion medium may contain a polymer dispersant, a surfactant, etc., in order to improve the dispersibility of the solid polymer.

[0048] Examples of polymer dispersants include hydrophilic polymer dispersants such as polyvinyl alcohol (PVA), polyvinylpyrrolidone (PVP), polyethylene glycol (PEG), polypropylene glycol, polyethylene glycol-polypropylene glycol block copolymers, polyglycerin, polyoxyethylene glycerin, polyoxypropylene glycerin, polyoxyethylene-polyoxypropylene glycerin copolymers, and polyoxyethylene sorbitan fatty acid esters; and hydrophobic polymer dispersants such as maleic anhydride-modified polyethylene, maleic anhydride-modified polypropylene, maleic anhydride-modified ethylene-propylene copolymers, maleic anhydride-modified EPDM (ethylene-propylene-diene terpolymer), maleic anhydride-modified polybutadiene, maleic anhydride-ethylene copolymers, maleic anhydride-propylene copolymers, maleic anhydride-ethylene-propylene terpolymers, maleic anhydride-butadiene copolymers, polyethylene, polypropylene, ethylene-propylene copolymers, oxidized polyethylene, oxidized polypropylene, oxidized ethylene-propylene copolymers, ethylene-acrylic acid copolymers, ethyl cellulose, and ethylhydroxyethyl cellulose.

[0049] Examples of surfactants include nonionic surfactants such as sorbitan fatty acid esters, (poly)glycerin fatty acid esters, sucrose fatty acid esters, polyoxyethylene sorbitan fatty acid esters, polyoxyethylene glycerin fatty acid esters, sorbitol fatty acid esters, polyoxyethylene sorbitol fatty acid esters, polyoxyethylene alkyl ethers, polyoxyethylene alkylphenyl ethers, polyoxyethylene castor oil, polyoxyethylene hydrogenated castor oil, alkylallyl formaldehyde condensed polyoxyethylene ethers, polyoxyethylene polyoxypropylene block copolymers, polyoxyethylene polyoxypropyl alkyl ethers, and polyethylene glycol fatty acid esters; and anionic surfactants such as fatty acid salts, alkylbenzenesulfonates, alkylmethyltaurates, polyoxyethylene alkylphenyl ether sulfates, polyoxyethylene alkyl ether sulfonates, polyoxyethylene alkyl ether phosphates, and polyoxyethylene alkylallyl ether phosphates.

[0050] When a polymer dispersion is mixed with a dispersion medium in a reversed-phase suspension system, the volume ratio of the polymer dispersion to the dispersion medium (polymer dispersion / dispersion medium) may be 0.1 or more, 0.15 or more, 0.3 or more, or 0.4 or more, from the viewpoint that the median particle size of the solid polymer in the dispersion medium is likely to increase (i.e., the median particle size of the resulting crosslinked polymer can be expected to increase). The volume ratio of the polymer dispersion to the dispersion medium (polymer dispersion / dispersion medium) may be 0.7 or less, 0.6 or less, or 0.55 or less, from the viewpoint that crosslinking is likely to occur in the reversed-phase suspension system. The volume ratio of the polymer dispersion to the dispersion medium (polymer dispersion / dispersion medium) may be 0.1 to 0.7, 0.15 to 0.6, or 0.15 to 0.55.

[0051] A regenerated water-absorbent resin containing a crosslinked polymer is formed by a method including removing water from a crosslinked polymer obtained through an optional surface crosslinking step. When the reaction liquid to be subjected to the crosslinking step is an aqueous solution, the water-absorbent resin particles are formed by a method including a drying step of removing water from a block of crosslinked polymer formed by gelling the reaction liquid itself, drying the crosslinked polymer to form a dried product, and a pulverizing step of pulverizing the dried product.

[0052] The method for drying the crosslinked polymer may be a common method such as a squeezing method such as centrifugation, dehydration using an organic solvent, natural drying, heat drying, air drying, freeze drying, or a combination thereof. The heating temperature for drying may be 80°C to 220°C, 90°C to 200°C, or 100°C to 180°C from the viewpoint of efficiently removing water.

[0053] The moisture content of the dried product may be, for example, 20% by mass or less, 10% by mass or less, or 5% by mass or less. The moisture content of the dried product means the proportion of moisture in the dried product based on the total amount of the dried product. When a crosslinked polymer containing water is heated at 200°C for 2 hours, the difference in mass of the crosslinked polymer before and after heating can be considered to be the moisture content of the dried product.

[0054] Before drying the crosslinked polymer, the crosslinked polymer may be crushed to form a crushed product containing structures of a certain size. By forming a crushed product, water can be efficiently removed. The structures constituting the crushed product can be, for example, elongated structures, granular structures (particles), or a combination thereof. The crushed product may include a plurality of structures having a shape that can pass through a circular hole with a diameter of 10 mm or 7 mm. The elongated structures may be curved, and as long as their maximum width is 10 mm or less, they can be said to have a shape that can pass through a circular hole with a diameter of 10 mm. The granular structures (particles) may be irregular in shape and may have a shape that can pass through a circular hole with a diameter of 10 mm while changing direction. Examples of crushing devices for crushing the crosslinked polymer include kneaders (e.g., pressure kneaders, double-arm kneaders), meat choppers, cutter mills, and farm mills.

[0055] The dried product is pulverized to form water-absorbent resin particles. The pulverization method is not particularly limited. For example, the dried product can be pulverized using a pulverizer such as a centrifugal pulverizer, a roller mill, a stamp mill, a jet mill, a high-speed rotary pulverizer, or a container-driven mill.

[0056] The water-absorbent resin particles obtained by pulverization may be classified. Classification refers to an operation of dividing a particle group (powder) into two or more particle groups having different particle size distributions. A part of the water-absorbent resin particles after classification may be pulverized and classified again.

[0057] The classification method is not particularly limited, and may be, for example, screen classification or air classification. Screen classification is a method of classifying particles on a screen into particles that pass through the meshes of the screen and particles that do not pass through by vibrating the screen. Screen classification can be performed using, for example, a vibrating sieve, a rotary sifter, a cylindrical stirring sieve, a blower sifter, or a rotary shaker. Air classification is a method of classifying particles by utilizing an air flow.

[0058] The median particle size of the crosslinked polymer powder obtained through pulverization and, if necessary, classification may be, for example, 200 μm to 500 μm, or 200 μm to 500 μm. The particle size distribution may be adjusted by mixing two or more powders with different median particle sizes obtained by classification. The median particle size of the crosslinked polymer powder can be adjusted by the mixing ratio of the polymer dispersion and the dispersion medium, the addition of a surfactant, etc.

[0059] <Absorbent article> Fig. 1 is a cross-sectional view showing an example of an absorbent article having an absorbent body containing water-absorbent resin particles. The absorbent article 100 shown in Fig. 1 comprises a water-absorbent sheet 50 having a film-like absorbent body 10, a liquid-permeable sheet 30, and a liquid-impermeable sheet 40.

[0060] The water-absorbent sheet 50 includes an absorbent body 10 containing a powder of water-absorbent resin particles 1, and two core wrap sheets 20a and 20b. The absorbent body 10 is disposed inside the core wrap sheets 20a and 20b. The absorbent body 10 maintains its shape by being sandwiched between the two core wrap sheets 20a and 20b. The core wrap sheets 20a and 20b may consist of two sheets, a single folded sheet, or a single bag. A sheet member that does not have any other components on the outside of the core wrap sheets 20a and 20b that wrap the absorbent body 10 is sometimes referred to as a water-absorbent sheet.

[0061] The absorbent body 10 is a component that mainly contains a powder of water-absorbent resin particles 1 and is retained to have a certain shape. The absorbent body 10 may contain fibrous material 3 in addition to the powder of water-absorbent resin particles 1, or may not contain fibrous material 3. The content of the water-absorbent resin particles 1 in the absorbent body 10 may be 50% by mass or more and 100% by mass or less, 60% by mass or more and 100% by mass or less, 70% by mass or more and 100% by mass or less, 80% by mass or more and 100% by mass or less, or 90% by mass or more and 100% by mass or less, based on the mass of the absorbent body 10.

[0062] The thickness of the absorbent body 10 may be, for example, 20 mm or less, 15 mm or less, 10 mm or less, 5 mm or less, 4 mm or less, or 3 mm or less, or may be 0.1 mm or more, or 0.3 mm or more. The thickness of the absorbent body 10 may be 0.1 mm or more and 20 mm or less. The mass per unit area of ​​the absorbent body 10 is 1000 g / m ２ Below, 800g / m ２ or less than 600 g / m ２ or less, and ２ It may be more than that.

[0063] The fibrous material 3 can be, for example, a cellulosic fiber, a synthetic fiber, or a combination thereof. Examples of cellulosic fibers include comminuted wood pulp, cotton, cotton linters, rayon, and cellulose acetate. Examples of synthetic fibers include polyamide fibers, polyester fibers, and polyolefin fibers. The fibrous material may also be a hydrophilic fiber (e.g., pulp).

[0064] The absorbent 10 may further contain inorganic powder (e.g., amorphous silica), a deodorant, an antibacterial agent, a fragrance, etc. When the water-absorbent resin particles 1 contain inorganic particles, the absorbent 10 may contain inorganic powder in addition to the inorganic particles in the water-absorbent resin particles 1.

[0065] The water-absorbent sheet 50 may further have an adhesive 21 interposed between the core wrap sheet 20a and the absorbent body 10. An adhesive layer may be interposed between the core wrap sheets 20a, 20b on both sides and the absorbent body 10. The adhesive 21 is not particularly limited, and may be, for example, a hot-melt adhesive.

[0066] The core wrap sheets 20a, 20b may be, for example, nonwoven fabrics. The two core wrap sheets 20a, 20b may be the same or different nonwoven fabrics. The nonwoven fabric may be a nonwoven fabric made of short fibers (i.e., staple) (short fiber nonwoven fabric) or a nonwoven fabric made of long fibers (i.e., filaments) (long fiber nonwoven fabric). The staple may have a fiber length of, but is not limited to, typically several hundred millimeters or less.

[0067] The core wrap sheets 20a, 20b may be a thermal bonded nonwoven fabric, an air-through nonwoven fabric, a resin bonded nonwoven fabric, a spunbonded nonwoven fabric, a meltblown nonwoven fabric, an airlaid nonwoven fabric, a spunlace nonwoven fabric, a point bonded nonwoven fabric, or a laminate containing two or more types of nonwoven fabric selected from these.

[0068] The nonwoven fabric used as the core wrap sheets 20a, 20b can be made of synthetic fibers, natural fibers, or a combination thereof. Examples of synthetic fibers include fibers containing synthetic resins selected from polyolefins such as polyethylene (PE) and polypropylene (PP), polyesters such as polyethylene terephthalate (PET), polytrimethylene terephthalate (PTT) and polyethylene naphthalate (PEN), polyamides such as nylon, and rayon. Examples of natural fibers include fibers containing cotton, silk, hemp, or pulp (cellulose). The fibers forming the nonwoven fabric may be polyolefin fibers, polyester fibers, or a combination thereof. The core wrap sheets 20a, 20b may also be tissue paper.

[0069] The water-absorbent sheet 50 may be used to manufacture various other absorbent articles. Examples of absorbent articles include diapers (e.g., disposable diapers), toilet training pants, incontinence pads, sanitary materials (sanitary napkins, tampons, etc.), sweat pads, pet sheets, portable toilet components, and animal waste disposal materials. The absorbent bodies that make up these absorbent articles often move or deform due to the movements of the user of the absorbent article, etc.

[0070] The liquid-permeable sheet 30 is positioned as the outermost layer on the side into which the liquid to be absorbed penetrates. The liquid-permeable sheet 30 is positioned outside the core wrap sheet 20b while in contact with the core wrap sheet 20b. The liquid-impermeable sheet 40 is positioned as the outermost layer on the opposite side of the absorbent article 100 from the liquid-permeable sheet 30. The liquid-impermeable sheet 40 is positioned outside the core wrap sheet 20a while in contact with the core wrap sheet 20a. The liquid-permeable sheet 30 and the liquid-impermeable sheet 40 have main surfaces that are wider than the main surface of the water-absorbent sheet 50, and the outer edges of the liquid-permeable sheet 30 and the liquid-impermeable sheet 40 extend around the absorbent body 10 and the core wrap sheets 20a, 20b. However, the size relationships among the absorbent body 10, the core wrap sheets 20a, 20b, the liquid-permeable sheet 30, and the liquid-impermeable sheet 40 are not particularly limited and may be appropriately adjusted depending on the intended use of the absorbent article, etc.

[0071] The liquid-permeable sheet 30 may be a nonwoven fabric. The nonwoven fabric used as the liquid-permeable sheet 30 may have appropriate hydrophilicity from the viewpoint of the liquid absorption performance of the absorbent article. From that viewpoint, the liquid-permeable sheet 30 may be a nonwoven fabric having a hydrophilicity of 5 to 200 as measured in accordance with the Pulp and Paper Testing Method No. 68 (2000) of the Paper and Pulp Technology Association. The hydrophilicity of the nonwoven fabric may also be 10 to 150. For details of Pulp and Paper Testing Method No. 68, see, for example, WO2011 / 086843.

[0072] Hydrophilic nonwoven fabrics may be made from fibers with moderate hydrophilicity, such as rayon, or from fibers obtained by hydrophilizing hydrophobic chemical fibers, such as polyolefin and polyester fibers. Methods for obtaining nonwoven fabrics containing hydrophilized hydrophobic chemical fibers include spunbonding a mixture of hydrophobic chemical fibers and a hydrophilizing agent, adding a hydrophilizing agent to the hydrophobic chemical fibers, and impregnating a spunbond nonwoven fabric obtained from hydrophobic chemical fibers with a hydrophilizing agent. Examples of hydrophilizing agents include anionic surfactants such as aliphatic sulfonates and higher alcohol sulfates, cationic surfactants such as quaternary ammonium salts, nonionic surfactants such as polyethylene glycol fatty acid esters, polyglycerin fatty acid esters, and sorbitan fatty acid esters, silicone surfactants such as polyoxyalkylene-modified silicones, and stain release agents made from polyester, polyamide, acrylic, or urethane resins.

[0073] The basis weight (mass per unit area) of the nonwoven fabric used as the liquid-permeable sheet 30 is set to 5 g / m from the viewpoint of imparting good liquid permeability, flexibility, strength, and cushioning properties to the absorbent article, and from the viewpoint of increasing the liquid permeation rate of the absorbent article. ２ ~200g / m ２ , 8 g / m ２ ~150g / m ２ , or 10 g / m ２~100g / m ２ The thickness of the liquid-permeable sheet 30 may be 20 μm to 1400 μm, 50 μm to 1200 μm, or 80 μm to 1000 μm.

[0074] The liquid-impermeable sheet 40 prevents liquid absorbed by the absorbent body 10 from leaking out through the liquid-impermeable sheet 40. The liquid-impermeable sheet 40 may be a resin sheet or a nonwoven fabric. The resin sheet may be a sheet made of a synthetic resin such as polyethylene, polypropylene, or polyvinyl chloride. The nonwoven fabric may be a spunbond / meltblown / spunbond (SMS) nonwoven fabric in which a water-resistant meltblown nonwoven fabric is sandwiched between high-strength spunbond nonwoven fabrics. The liquid-impermeable sheet 40 may be a composite sheet of a resin sheet and a nonwoven fabric (e.g., a spunbond nonwoven fabric or a spunlace nonwoven fabric). The liquid-impermeable sheet 40 may be breathable to reduce stuffiness during wear and to alleviate discomfort to the wearer. For example, a low-density polyethylene (LDPE) resin sheet can be used as the breathable liquid-impermeable sheet 40.

[0075] In order to ensure flexibility and not impair the wearing comfort of the absorbent article, the basis weight (mass per unit area) of the liquid impermeable sheet 40 is 10 g / m ２ ~50g / m ２ may be.

[0076] The present invention will be described in more detail below with reference to examples.

[0077] <Evaluation Method> The transmittance, viscosity, shape, and median particle size were measured according to the following evaluation methods. The measurement results are shown in Table 1.

[0078] [Transmittance] A spectrophotometer (model number: UV-1850) manufactured by Shimadzu Corporation was used to measure the transmittance. The polymer dispersion was placed in a UV measurement cell (optical path length 1 cm) and degassed by operating a tabletop centrifuge (model number: H-36) manufactured by Kokusan at 2000 revolutions per minute for 1 minute. If degassing was not complete, the same operation was performed using the tabletop centrifuge to degas the sample. Next, the cell was set in the spectrophotometer, and the transmittance was measured at a measurement wavelength of 425 nm. Note that the transmittance measurement was calibrated so that the transmittance of distilled water was 100%.

[0079] [Viscosity] The polymer dispersion was placed in a 100 ml centrifuge tube and degassed using a centrifuge (Kokusan Corporation, tabletop centrifuge H-36). Using a Brookfield viscometer (Brookfield, model number: DV1MRVTJ0), the viscosity was measured at 25°C after 1 minute at a rotation speed of 20 revolutions per minute. The rotors used for the measurements were rotor No. 4 when the viscosity was less than 8000 mPa·s, rotor No. 6 when the viscosity was 10000 mPa·s to 40000 mPa·s, and rotor No. 7 when the viscosity was 40000 mPa·s or higher.

[0080] [Shape] The shape of the polymer powder was observed using a scanning electron microscope (SEM). SEM images of the polymer powder obtained in Example 1 are shown in Figure 2, those of the polymer powder obtained in Comparative Example 1 in Figure 3, those of the polymer powder obtained in Comparative Example 2 in Figure 4, and those of the polymer powder obtained in Comparative Example 3 in Figure 5.

[0081] [Median particle size (Robot sifter method)] The median particle size of the polymer powder was measured using a sonic sieving automatic particle size distribution measuring instrument (Robot sifter, RPS-01, manufactured by Seishin Enterprise Co., Ltd.). The measurement conditions were sonic intensity: 50 Lv, frequency: 51 Hz, classification time: 5 min, sweep time: 0.1 min, and pulse interval: 1 second. The openings of the standard sieves used in the measurement were 710, 400, 150, 106, 75, 63, 45, and 38 μm.

[0082] Example 1 94 g of ion-exchanged water was charged into a 500 ml capacity polybeaker equipped with a stirrer having four inclined paddle blades with a diameter of 7.5 cm. While stirring the solution at 400 rpm at room temperature, 6 g of partially neutralized uncrosslinked polyacrylic acid (Acupara AP-70, manufactured by Sumitomo Seika Chemicals Co., Ltd.) was added. When the solution thickened and began to wrap around the stirring shaft, the stirring speed was increased to 100 rpm and the solution was stirred for 6 hours to dissolve the solution, preparing a 6 mass % aqueous sodium polyacrylate solution (neutralization degree 70 mol%) as a polymer solution. While stirring the resulting polymer solution (100 g) at 1000 rpm using the stirrer, 100 g of a 70 mass % aqueous ethanol solution as a poor solvent was added dropwise at 10 ml / min using a liquid feed pump, preparing a polymer dispersion in which a solid polymer precipitated. 300 g of normal heptane was weighed out as a hydrocarbon dispersion medium into a 2000 ml separable flask equipped with a stirrer with four 5.0 cm diameter inclined paddle blades, a reflux condenser, and a nitrogen tube. 0.1 g of maleic anhydride-modified ethylene-propylene copolymer (trade name: Hiwax 1105A, manufactured by Mitsui Chemicals, Inc.) as a polymer dispersant, and 0.1 g of sucrose stearate ester (trade name: Ryoto Sugar Ester S-370, HLB value 3, manufactured by Mitsubishi Chemical Foods Corporation) as a surfactant were added, and the mixture was heated and dissolved while stirring at 500 rpm, and then allowed to cool to 70 ° C. The stirring speed was then increased to 1000 rpm, and the polymer dispersion (200 g) was added using a funnel. The volume ratio of the polymer dispersion to the dispersion medium (polymer dispersion / dispersion medium) was 0.50. After holding at 70 ° C for 30 minutes, the polymer dispersion was heated using a 125 ° C oil bath to remove the dispersion medium and solvent, yielding a powder. The obtained powder was classified using a standard sieve with an opening of 300 μm to obtain a polymer powder (crosslinked polymer).

[0083] Example 2 A polymer powder (crosslinked polymer) was obtained by the same operation as in Example 1, except that a 98% by mass aqueous methanol solution was used as the poor solvent and that the temperature after adding the polymer dispersion to the liquid containing the hydrocarbon dispersion medium was set to 60°C.

[0084] Example 3 A polymer powder (crosslinked polymer) was obtained in the same manner as in Example 1, except that a 70% by mass aqueous solution of isopropyl alcohol was used as the poor solvent.

[0085] Example 4 A polymer powder (crosslinked polymer) was obtained by the same operation as in Example 1, except that an 80 mass % acetone aqueous solution was used as the poor solvent, the holding temperature after adding the polymer dispersion to the liquid containing the hydrocarbon dispersion medium was set to 60°C, and the standard sieve used to classify the obtained powder had an opening of 850 μm.

[0086] (Example 5) A polymer powder (crosslinked polymer) was obtained by the same operation as in Example 1, except that 96 g of ion-exchanged water was used, 4 g of partially neutralized uncrosslinked polyacrylic acid was used, and an 80 mass % aqueous ethanol solution was used as the poor solvent.

[0087] Example 6 A polymer powder (crosslinked polymer) was obtained by the same operation as in Example 1, except that the partially neutralized uncrosslinked polyacrylic acid was changed to Akupana AP-40F manufactured by Sumitomo Seika Chemicals Co., Ltd., a 6 mass % aqueous solution of sodium polyacrylate (neutralization degree 40 mol %) was prepared as the polymer solution, an 80 mass % aqueous solution of ethanol was used as the poor solvent, and the standard sieve used to classify the obtained powder had an opening of 850 μm.

[0088] Example 7 A polymer powder (crosslinked polymer) was obtained by the same operation as in Example 1, except that the amount of ion-exchanged water was 90.67 g, 3.33 g of a 30 mass % aqueous sodium hydroxide solution was added to the partially neutralized uncrosslinked polyacrylic acid to prepare a polymer solution of an aqueous sodium polyacrylate solution having a neutralization degree of 100 mol %, 42 mass % aqueous ethanol solution was used as the poor solvent, and the standard sieve used to classify the obtained powder had an opening of 850 μm.

[0089] Example 8 A polymer powder (crosslinked polymer) was obtained in the same manner as in Example 1, except that the volume ratio of the polymer dispersion to the dispersion medium (polymer dispersion / dispersion medium) was changed to 0.15.

[0090] Example 9 A polymer powder (crosslinked polymer) was obtained in the same manner as in Example 1, except that the volume ratio of the polymer dispersion to the dispersion medium (polymer dispersion / dispersion medium) was changed to 0.30.

[0091] Comparative Example 1 The same operation as in Example 1 was carried out, except that the amount of ion-exchanged water was changed to 194 g, the aqueous sodium polyacrylate solution (polymer solution) was mixed with normal heptane without adding a poor solvent, and the obtained polymer could not be classified because it was too large.

[0092] Comparative Example 2 Polymer aggregates were obtained by the same procedure as in Example 1, except that a 60% by mass aqueous ethanol solution was used as the poor solvent. The obtained aggregates were amorphous.

[0093] Comparative Example 3 A 10% by mass aqueous solution of sodium polyacrylate (degree of neutralization: 70 mol%) was prepared as a polymer solution by the same procedure as in Example 1, except that the amount of ion-exchanged water was 171 g and the amount of partially neutralized uncrosslinked polyacrylic acid was 19 g. Next, while the obtained polymer solution was stirred at 1,000 rpm using the stirrer, 10 g of a 100% by mass aqueous solution of ethanol as a poor solvent was added dropwise at 10 mL / min using a liquid feed pump. However, no solid polymer precipitated, and a mixed solution containing the polymer solution and the poor solvent was obtained. Next, 300 g of normal heptane was weighed out as a hydrocarbon dispersion medium into a 2000 ml separable flask equipped with a stirrer having four inclined paddle blades with a diameter of 5.0 cm, a reflux condenser, and a nitrogen tube. 0.3 g of maleic anhydride-modified ethylene-propylene copolymer (trade name: Hiwax 1105A, manufactured by Mitsui Chemicals, Inc.) as a polymer dispersant, and 0.3 g of sucrose stearate ester (trade name: Ryoto Sugar Ester S-370, HLB value 3, manufactured by Mitsubishi Chemical Foods Corporation) as a surfactant were added, and the mixture was heated and dissolved while stirring at 500 rpm, and then allowed to cool to 70 ° C. Then, the stirring speed was increased to 1000 rpm, and the mixture (140 g) was added using a pressure filter. After holding at 70 ° C. for 30 minutes, the solvent was distilled off by heating the mixture using a 125 ° C. oil bath to obtain polymer aggregates. The resulting aggregates were amorphous and too large to be classified.

[0094]

[0095] In Examples 1 to 9, a poor solvent was brought into contact with the polymer solution to precipitate a solid polymer so that the transmittance was 0.85% or less, and thus a polymer powder that was spherical or nearly spherical in the dispersion medium of the reversed-phase suspension system could be obtained. On the other hand, in Comparative Example 1, a poor solvent was not mixed, so no solid polymer was precipitated, and a spherical polymer powder could not be obtained in the dispersion medium of the reversed-phase suspension system. In Comparative Examples 2 and 3, mixing the polymer solution with the poor solvent resulted in almost no polymer precipitation, and the transmittance of the polymer dispersion exceeded 0.85%, so a spherical or nearly spherical polymer powder could not be obtained in the dispersion medium of the reversed-phase suspension system.

[0096] 1...water-absorbent resin particles, 10...absorbent body, 20a, 20b...core wrap sheets, 30...liquid-permeable sheet, 40...liquid-impermeable sheet, 50...water-absorbent sheet, 100...absorbent article.

Claims

1. A method for producing a polymer dispersion, comprising: a step of preparing a polymer solution containing a polymer in which the crosslinked structure of a water-absorbent resin has been cleaved; and a step of bringing the polymer solution into contact with a poor solvent for the polymer to precipitate a solid polymer, thereby obtaining a polymer dispersion in which the solid polymer is dispersed; wherein the polymer dispersion has a transmittance of 0.85% or less for light with a wavelength of 425 nm at an optical path length of 1 cm.

2. The method according to claim 1, wherein the viscosity of the polymer dispersion at 25°C is 5000 mPa·s or less.

3. The production method according to claim 1, wherein the poor solvent comprises at least one selected from the group consisting of alcohols, phenols, and ketones having 1 to 10 carbon atoms.

4. The method according to claim 1, wherein the degree of neutralization of the polymer is 40 mol % or more.

5. A method for producing water-absorbent resin particles, comprising a step of crosslinking the solid polymer in the polymer dispersion obtained by the method according to any one of claims 1 to 4 to obtain a water-absorbent resin.

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