Method for producing water-absorbent resin particles and method for controlling particle size of water-absorbent resin particles

Abstract

Disclosed is a method for producing water-absorbent resin particles, which comprises: a step for preparing an aqueous liquid that contains a monomer of a (meth) acrylic acid compound that contains (meth) acrylic acid or / and a salt thereof, an alcohol compound having 1-10 carbon atoms, and water; a step for forming a particulate water-containing gel polymer containing a polymer of a monomer and water by reversed-phase suspension polymerization in a reaction liquid containing an aqueous liquid and a dispersion medium; and a step for extracting the water-absorbent resin particles containing the polymer from the reaction solution. The aqueous liquid containing an alcohol compound in an amount of 800 ppm by mass or less relative to the amount of the (meth) acrylic acid compound is prepared.

Description

Technical Field

[0001] This disclosure relates to a method for manufacturing water-absorbing resin particles and a method for controlling the particle size of water-absorbing resin particles. Background Technology

[0002] Absorbent polymers are manufactured in particle form and are widely used in various fields such as hygiene materials (diapers, sanitary products, etc.), water-retaining agents, horticultural materials (soil conditioners, etc.), waterproofing materials, and industrial materials (anti-fogging agents, etc.).

[0003] In such absorbent resin particles, the proportion of particles with excessively large particle sizes is required to be small (e.g., Patent Document 1).

[0004] Existing technical documents Patent documents Patent Document 1: International Publication No. 2012 / 081355 Summary of the Invention

[0005] The technical problem to be solved by the invention In the case of obtaining water-absorbing resin particles through reverse suspension polymerization, sometimes, depending on the polymerization conditions, trace impurities in the aqueous solution become the main cause, resulting in the formation of coarse particles composed of a large number of primary particles linked in a chain. As a result, the proportion of coarse particles with excessively large particle sizes sometimes increases.

[0006] This disclosure relates to a method for suppressing the formation of coarse, chain-like particles composed of primary particles in the manufacture of absorbent resin particles by reverse suspension polymerization.

[0007] means for solving technical problems This disclosure includes the following. [1] A method for manufacturing absorbent resin particles, comprising: The process of preparing an aqueous solution, wherein the aqueous solution contains a monomer of a (meth)acrylic acid compound comprising at least one of (meth)acrylic acid or a salt thereof, an alcohol compound having 1 or more and 10 or fewer carbon atoms, and water; The process of forming a particulate hydrogel polymer containing the monomer and water through reverse suspension polymerization in a reaction solution containing the aqueous liquid and dispersion medium; and The process of removing the water-absorbing resin particles containing the polymer from the reaction solution. Prepare the aqueous solution containing an amount of alcohol compound of less than 800 ppm by mass relative to the amount of the (meth)acrylic acid compound. When the aqueous solution contains the salt of the (meth)acrylic acid, the amount of the (meth)acrylic acid compound is the sum of the amount of the (meth)acrylic acid and the amount of (meth)acrylic acid equivalent to the amount of the salt. [2] According to the method described in [1], wherein, The method further includes a step of preparing a (meth)acrylic acid raw material, said (meth)acrylic acid raw material containing (meth)acrylic acid and an alcohol compound having 1 or more and 10 or fewer carbon atoms, and being conveyed in such a manner that the amount of said alcohol compound relative to the amount of said (meth)acrylic acid is 800 ppm by mass or less. The aqueous solution is prepared from the (meth)acrylic acid raw material. [3] According to the method described in [1] or [2], wherein, The absorbent resin particles comprise spherical primary particles. [4] According to any one of [1] to [3], wherein, The alcohol compound comprises at least one selected from isononol, 1-butanol and isopropanol.

[0012] Invention Effects According to this disclosure, it is possible to suppress the formation of coarse, chain-like particles formed by linking primary particles in the manufacture of absorbent resin particles by reverse suspension polymerization. Attached Figure Description

[0013] Figure 1 This is a scanning electron microscope (SEM) image showing an example of absorbent resin particles.

[0014] Figure 2 This is an SEM image showing an example of a hydrophobic resin particle containing chain-like coarse particles.

[0015] Figure 3 This is an SEM image showing an example of a hydrophobic resin particle containing chain-like coarse particles.

[0016] Figure 4 This is an SEM image showing an example of a hydrophobic resin particle containing chain-like coarse particles. Detailed Implementation

[0017] This invention is not limited to the following examples. In this specification, (meth)acrylic acid means acrylic acid, methacrylic acid, and combinations thereof.

[0018] An example of a method for manufacturing absorbent resin particles includes: a step of preparing an aqueous liquid containing a monomer of a (meth)acrylic acid compound comprising at least one of (meth)acrylic acid or a salt thereof, an alcohol compound having 1 or more and 10 or fewer carbon atoms, and water; a step of forming a particulate hydrogel polymer containing a monomer and water by reverse suspension polymerization in a reaction liquid containing the aqueous liquid and a dispersion medium; and a step of removing absorbent resin particles containing the polymer from the reaction liquid.

[0019] The reaction solution for reverse suspension polymerization can be formed by stirring a mixture containing an aqueous solution and a dispersion medium. According to the inventors, the amount of alcohol compounds with 1 to 10 carbon atoms in the aqueous solution is related to the degree of formation of chain-like coarse particles, which are formed from primary particles, in the absorbent resin particles taken from the reaction solution. Therefore, the amount of alcohol compounds with 1 to 10 carbon atoms in the aqueous solution before forming the reaction solution is adjusted to be 800 ppm by mass or less relative to the amount of (meth)acrylic acid compound. If the amount of alcohol compounds with 1 to 10 carbon atoms is 800 ppm by mass or less, absorbent resin particles with a low proportion of chain-like coarse particles can be stably produced. From the same perspective, the amount of alcohol compounds with 1 to 10 carbon atoms in the prepared aqueous solution relative to the amount of (meth)acrylic acid compound can be less than 750 ppm by mass, less than 700 ppm by mass, less than 650 ppm by mass, less than 600 ppm by mass, less than 550 ppm by mass, less than 500 ppm by mass, less than 450 ppm by mass, less than 400 ppm by mass, less than 360 ppm by mass, less than 320 ppm by mass, less than 280 ppm by mass, less than 240 ppm by mass, less than 200 ppm by mass, less than 180 ppm by mass, less than 160 ppm by mass, less than 140 ppm by mass, less than 120 ppm by mass, or less than 100 ppm by mass. The amount of alcohol compounds with 1 to 10 carbon atoms in the prepared aqueous solution relative to the amount of (meth)acrylic acid compound can exceed 0 ppm by mass, or can be more than 1 ppm by mass, more than 5 ppm by mass, or more than 10 ppm by mass.

[0020] The amount of alcohol compounds with 1 or more carbon atoms and less than 10 in the prepared aqueous solution may exceed 0 ppm by mass and be less than 800 ppm by mass, less than 750 ppm by mass, less than 700 ppm by mass, less than 650 ppm by mass, less than 600 ppm by mass, less than 550 ppm by mass, less than 500 ppm by mass, less than 450 ppm by mass, less than 400 ppm by mass, less than 360 ppm by mass, less than 320 ppm by mass, less than 280 ppm by mass, less than 240 ppm by mass, less than 200 ppm by mass, less than 180 ppm by mass, less than 160 ppm by mass, less than 140 ppm by mass, less than 120 ppm by mass, or less than 100 ppm by mass relative to the amount of (meth)acrylic acid compound.

[0021] The amount of alcohol compounds with 1 or more carbon atoms and less than 10 in the prepared aqueous solution relative to the amount of (meth)acrylic acid compounds can be more than 1 ppm by mass and less than 800 ppm by mass, less than 750 ppm by mass, less than 700 ppm by mass, less than 650 ppm by mass, less than 600 ppm by mass, less than 550 ppm by mass, less than 500 ppm by mass, less than 450 ppm by mass, less than 400 ppm by mass, less than 360 ppm by mass, less than 320 ppm by mass, less than 280 ppm by mass, less than 240 ppm by mass, less than 200 ppm by mass, less than 180 ppm by mass, less than 160 ppm by mass, less than 140 ppm by mass, less than 120 ppm by mass, or less than 100 ppm by mass.

[0022] The amount of alcohol compounds with 1 or more carbon atoms and less than 10 in the prepared aqueous solution relative to the amount of (meth)acrylic acid compounds can be more than 5 ppm by mass and less than 800 ppm by mass, less than 750 ppm by mass, less than 700 ppm by mass, less than 650 ppm by mass, less than 600 ppm by mass, less than 550 ppm by mass, less than 500 ppm by mass, less than 450 ppm by mass, less than 400 ppm by mass, less than 360 ppm by mass, less than 320 ppm by mass, less than 280 ppm by mass, less than 240 ppm by mass, less than 200 ppm by mass, less than 180 ppm by mass, less than 160 ppm by mass, less than 140 ppm by mass, less than 120 ppm by mass, or less than 100 ppm by mass.

[0023] The amount of alcohol compounds with 1 or more carbon atoms and less than 10 in the prepared aqueous solution relative to the amount of (meth)acrylic acid compounds can be more than 10 ppm by mass and less than 800 ppm by mass, less than 750 ppm by mass, less than 700 ppm by mass, less than 650 ppm by mass, less than 600 ppm by mass, less than 550 ppm by mass, less than 500 ppm by mass, less than 450 ppm by mass, less than 400 ppm by mass, less than 360 ppm by mass, less than 320 ppm by mass, less than 280 ppm by mass, less than 240 ppm by mass, less than 200 ppm by mass, less than 180 ppm by mass, less than 160 ppm by mass, less than 140 ppm by mass, less than 120 ppm by mass, or less than 100 ppm by mass.

[0024] When the aqueous solution contains a salt of (meth)acrylic acid, the amount of (meth)acrylic acid compound used as a basis for the amount of alcohol compounds with 1 or more but less than 10 carbon atoms is the sum of the amount of (meth)acrylic acid and the amount of (meth)acrylic acid equivalent to the amount of the salt of (meth)acrylic acid. The amount of (meth)acrylic acid equivalent to the amount of the salt of (meth)acrylic acid compound refers to the amount of (meth)acrylic acid that is the same as the amount of the salt of (meth)acrylic acid. When the aqueous solution contains two or more alcohol compounds with 1 or more but less than 10 carbon atoms, the aqueous solution is prepared in such a way that their sum is within the above-mentioned range.

[0025] Examples of alcohols containing 1 to 10 carbon atoms that may be present in aqueous solutions include isononanol, 1-butanol, isopropanol, 1,3-butanediol, 1,4-butanediol, propylene glycol, cyclohexanol, methyl tert-butanol, 1-octanol, and isobutanol. Alcohols containing 1 to 10 carbon atoms may be a single compound or a combination of two or more compounds.

[0026] The aqueous solution may contain (meth)acrylic acid and its salts as (meth)acrylic acid compounds. Based on the total amount of the (meth)acrylic acid compounds, the proportion of the (meth)acrylic acid salts may, for example, be 50 mol% or more and 100 mol% or less. The (meth)acrylic acid salts may, for example, be alkali metal salts of (meth)acrylic acid. The alkali metal salts of (meth)acrylic acid may be sodium salts.

[0027] Aqueous solutions can be prepared, for example, by a method comprising the following steps: preparing a (meth)acrylic acid raw material containing (meth)acrylic acid and an alcohol compound having 1 to 10 carbon atoms; and neutralizing a portion of the (meth)acrylic acid in the (meth)acrylic acid raw material with an alkaline aqueous solution containing an alkaline compound. The alkaline compound may be an alkali metal hydroxide (e.g., sodium hydroxide).

[0028] (Meth)acrylic acid raw materials can be transported in such a manner that the amount of alcohol compounds with 1 to 10 carbon atoms is 800 ppm by mass or less relative to the amount of (meth)acrylic acid. If the transport container (e.g., an ISO tank container) used to hold the (meth)acrylic acid raw material was previously used to transport alcohol compounds with 1 to 10 carbon atoms, trace amounts of alcohol compounds may remain in the transport container and sometimes be included in the (meth)acrylic acid raw material, even after proper cleaning of the container interior. By managing the transport container to prevent large amounts of alcohol compounds from remaining in the transport container used to transport the (meth)acrylic acid raw material, the amount of alcohol compounds contained in the (meth)acrylic acid raw material can be controlled within a range such as 800 ppm by mass or less. The amount of alcohol compounds with 1 to 10 carbon atoms in the (meth)acrylic acid raw material removed from the transport container can be within the same range as the amount of alcohol compounds in the aqueous liquid relative to the amount of (meth)acrylic acid.

[0029] The monomers in the aqueous solution may also contain water-soluble olefinic unsaturated monomers other than (meth)acrylic acid compounds. The proportion of (meth)acrylic acid compounds relative to the total amount of monomers may be 60 mol% or more, 70 mol% or more, 80 mol% or more, 90 mol% or more, or 95 mol% or more, and may substantially be 100 mol%. The water-soluble olefinic unsaturated monomers other than (meth)acrylic acid compounds may include, for example, at least one selected from the group consisting of 2-(meth)acrylamide-2-methylpropanesulfonic acid and its base salt, (meth)acrylamide, N,N-dimethyl(meth)acrylamide, 2-hydroxyethyl(meth)acrylate, N-hydroxymethyl(meth)acrylamide, polyethylene glycol mono(meth)acrylate, N,N-diethylaminoethyl(meth)acrylate, N,N-diethylaminopropyl(meth)acrylate, and diethylaminopropyl(meth)acrylamide.

[0030] The aqueous liquid may contain an internal crosslinking agent that crosslinks the polymer of the monomer formed by the polymerization reaction. The internal crosslinking agent may be a compound having two or more reactive functional groups that are reactive with the monomer (especially (meth)acrylic acid compounds). The reactive functional groups may be, for example, (meth)acryloyl, vinyl, epoxy, halogenated groups in halogenated epoxy compounds, isocyanate groups, or combinations thereof.

[0031] Examples of internal crosslinking agents having two or more (meth)acryloyl groups include (meth)acrylate compounds formed from polyol compounds and (meth)acrylic acid, unsaturated polyesters formed from polyol compounds and unsaturated carboxylic acids (maleic acid, fumaric acid, etc.), bis(meth)acrylamide compounds (N,N'-methylenebis(meth)acrylamide, etc.), (meth)acrylate compounds formed from polyepoxide compounds and (meth)acrylic acid, and (meth)acrylate carbamoyl ester compounds formed from polyisocyanate compounds (toluene diisocyanate, hexamethylene diisocyanate, etc.) and hydroxyethyl (meth)acrylate. The polyol compounds used to form (meth)acrylate compounds or unsaturated polyesters may, for example, be ethylene glycol, propylene glycol, trimethylolpropane, glycerol, polyoxyethylene glycol, polyoxypropylene glycol, polyglycerol, or combinations thereof.

[0032] Vinyl groups, as reactive functional groups, can be part of allyl groups. Examples of internal crosslinking agents having two or more vinyl (or allyl) groups include allylated starch, allylated cellulose, diallyl phthalate, N,N',N''-triallyl isocyanurate, and divinylbenzene.

[0033] Examples of internal crosslinking agents having two or more epoxy groups include (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.

[0034] Examples of internal crosslinking agents having two or more isocyanate groups include 2,4-toluene diisocyanate and hexamethylene diisocyanate.

[0035] The amount of the internal crosslinking agent relative to 1 mole of monomer can be more than 0 mmol and less than 0.5 mmol, more than 0 mmol and less than 0.2 mmol, more than 0 mmol and less than 0.1 mmol, more than 0 mmol and less than 0.05 mmol, or more than 0 mmol and less than 0.02 mmol, and can also be practically 0 mmol.

[0036] Aqueous liquids may contain thickeners, hydrophilic polymeric dispersants, free radical polymerization initiators, chain transfer agents, foaming agents, or combinations thereof as other components.

[0037] Examples of thickeners include hydroxyalkyl celluloses such as hydroxyethyl cellulose (HEC) and hydroxypropyl cellulose (HPC); hydroxyalkyl celluloses such as hydroxyethyl methyl cellulose, hydroxypropyl methyl cellulose, and hydroxyethyl ethyl cellulose; carboxyalkyl celluloses such as carboxymethyl cellulose; and carboxyalkyl hydroxyalkyl celluloses such as carboxymethyl hydroxyethyl cellulose. Thickeners can be a single agent or a combination of two or more agents.

[0038] The amount of thickener relative to 100 parts by mass of monomer can be more than 0.05 parts by mass and less than 20 parts by mass, more than 0.2 parts by mass and less than 10 parts by mass, or more than 0.4 parts by mass and less than 5 parts by mass.

[0039] Examples of hydrophilic polymeric dispersants include polyvinyl alcohol (PVA), polyvinylpyrrolidone (PVP), polyethylene glycol (PEG), polypropylene glycol, polyethylene glycol-polypropylene glycol block copolymers, glycerol, polyoxyethylene glycerol, polyoxypropylene glycerol, polyoxyethylene-polyoxypropylene glycerol copolymers, and polyoxyethylene dehydrated sorbitol fatty acid esters. Hydrophilic polymeric dispersants can be a single type or a combination of two or more.

[0040] The amount of hydrophilic polymeric dispersant relative to 100 parts by mass can be more than 0.001 parts by mass and less than 10 parts by mass, more than 0.005 parts by mass and less than 5 parts by mass, more than 0.01 parts by mass and less than 3 parts by mass, or more than 0.01 parts by mass and less than 1.5 parts by mass.

[0041] Free radical polymerization initiators may include, for example, azo compounds, peroxides, or combinations thereof.

[0042] Examples of azo compounds include 2,2'-azobis[2-(N-phenylamidinyl)propane] dihydrochloride, 2,2'-azobis{2-[N-(4-chlorophenyl)amidinyl]propane} dihydrochloride, 2,2'-azobis{2-[N-(4-hydroxyphenyl)amidinyl]propane} dihydrochloride, and 2,2'-azobis[2-(N-benzylamidinyl)propane] dihydrochloride. 2,2'-Azobis[2-(N-allylamidinyl)propane] dihydrochloride, 2,2'-Azobis(2-amidinylpropane) dihydrochloride, 2,2'-Azobis{2-[N-(2-hydroxyethyl)amidinyl]propane} dihydrochloride, 2,2'-Azobis[2-(5-methyl-2-imidazolin-2-yl)propane] dihydrochloride, 2,2'-Azobis[2-(2- [Imidazolin-2-yl)propane] dihydrochloride, 2,2'-azobis[2-(4,5,6,7-tetrahydro-1H-1,3-diazaphen-2-yl)propane] dihydrochloride, 2,2'-azobis[2-(5-hydroxy-3,4,5,6-tetrahydropyrimidin-2-yl)propane] dihydrochloride, 2,2'-azobis{2-[1-(2-hydroxyethyl)-2-imidazolinyl] [2,2'-azoline-2-yl]propane] dihydrochloride, 2,2'-azobis(2-methylpropamid) dihydrochloride, 2,2'-azobis[2-(2-imidazoline-2-yl)propane] disulfate dihydrate, 2,2'-azobis[N-(2-carboxyethyl)-2-methylpropamidine] tetrahydrate and 2,2'-azobis[2-methyl-N-(2-hydroxyethyl)propamid].

[0043] Examples of peroxides include persulfates such as potassium persulfate, ammonium persulfate, and sodium persulfate; organic peroxides such as methyl ethyl ketone peroxide, methyl isobutyl ketone peroxide, di-tert-butyl peroxide, tert-butyl cumene peroxide, tert-butyl peracetate, tert-butyl perisobutyrate, and tert-butyl perpentyl peroxide; and hydrogen peroxide.

[0044] The amount of free radical polymerization initiator relative to 100 moles of monomer can be, for example, more than 0.0001 moles and less than 1 mole, more than 0.0003 moles and less than 0.05 moles, more than 0.0005 moles and less than 0.03 moles, or more than 0.001 moles and less than 0.015 moles.

[0045] Examples of chain transfer agents include hypophosphites, thiols, thiolic acids, secondary alcohols, and amines.

[0046] Examples of foaming agents include inorganic foaming agents such as ammonium carbonate, sodium bicarbonate, and ammonium bicarbonate; nitroso compounds such as dinitrospentamethylenetetramine; azo compounds such as azodicarbonamide and azobisisobutyronitrile; and organic foaming agents such as sulfonyl hydrazides such as 4,4'-oxobisbenzenesulfonyl hydrazide and p-toluenesulfonyl hydrazide.

[0047] By stirring a mixture containing a prepared aqueous liquid and an oily liquid, which is mainly composed of a hydrophobic dispersion medium, a reaction solution for reverse suspension polymerization can be formed. The reaction solution contains particulate aqueous liquid dispersed in the oily liquid. Through polymerization in the reaction solution, a particulate hydrogel polymer containing monomers and water is formed.

[0048] The reaction solution may contain surfactants. Surfactants may be contained in oily liquids. Surfactants may be nonionic or anionic. Examples of nonionic surfactants include sorbitan fatty acid esters, polyglycerol fatty acid esters, sucrose fatty acid esters, polyoxyethylene sorbitan fatty acid esters, polyoxyethylene glycerol fatty acid esters, sorbitan fatty acid esters, polyoxyethylene sorbitan fatty acid esters, polyoxyethylene alkyl ethers, polyoxyethylene alkylphenyl ethers, polyoxyethylene castor oil, polyoxyethylene cured castor oil, alkylallyl formaldehyde condensed polyoxyethylene ethers, polyoxyethylene polyoxypropylene block copolymers, polyoxyethylene polyoxypropylene alkyl ethers, and polyethylene glycol fatty acid esters. Examples of anionic surfactants include fatty acid salts, alkylbenzene sulfonates, alkyl methyl taurate, polyoxyethylene alkylphenyl ether sulfates, polyoxyethylene alkyl ether sulfonates, polyoxyethylene alkyl ether phosphates, and polyoxyethylene alkylallyl ether phosphates. Surfactants may be a single type or a combination of two or more types.

[0049] The oily liquid in the reaction solution is a hydrophobic liquid mainly composed of a hydrophobic dispersion medium. The dispersion medium can be a hydrocarbon dispersion medium. The oily liquid may also contain any additives such as hydrophobic polymeric dispersants.

[0050] Examples of hydrocarbon 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 dispersion medium can be a single type or a combination of two or more types.

[0051] The amount of dispersion medium contained in the oily liquid relative to 100 parts by mass can be more than 30 parts by mass and less than 1000 parts by mass, more than 50 parts by mass and less than 650 parts by mass, more than 70 parts by mass and less than 550 parts by mass, or more than 100 parts by mass and less than 450 parts by mass.

[0052] Examples of hydrophobic polymeric dispersants include maleic anhydride-modified polyethylene, maleic anhydride-modified polypropylene, maleic anhydride-modified ethylene-propylene copolymer, maleic anhydride-modified EPDM (ethylene-propylene-diene-terpolymer), maleic anhydride-modified polybutadiene, maleic anhydride-ethylene copolymer, maleic anhydride-propylene copolymer, maleic anhydride-ethylene-propylene copolymer, maleic anhydride-butadiene copolymer, polyethylene, polypropylene, ethylene-propylene copolymer, oxidized polyethylene, oxidized polypropylene, oxidized ethylene-propylene copolymer, ethylene-acrylic acid copolymer, ethyl cellulose, and ethyl hydroxyethyl cellulose. Hydrophobic polymeric dispersants can be a single type or a combination of two or more types.

[0053] The amount of hydrophobic polymeric dispersant relative to 100 parts by mass of monomer can be more than 0.05 parts by mass and less than 10 parts by mass, more than 0.08 parts by mass and less than 5 parts by mass, or more than 0.1 parts by mass and less than 3 parts by mass.

[0054] For the polymerization reaction to proceed, the reaction solution can be heated. The heating temperature for the polymerization reaction can be, for example, above 40°C and below 90°C. The polymerization reaction time can be, for example, above 30 minutes and below 240 minutes.

[0055] During the polymerization reaction, the reaction liquid is typically stirred. The stirring speed during the polymerization reaction can be, for example, above 200 rpm and below 1000 rpm.

[0056] The process involves removing water-absorbing resin particles containing a polymer from a reaction solution (slurry) containing particulate hydrogel polymers. The process includes, for example, extracting a portion of the water from the reaction solution through azeotropic distillation of the dispersion medium and water to form a concentrate; and evaporating the dispersion medium, water, and alcohol compounds contained in the aqueous solution from the concentrate. After removing the dispersion medium and the like through evaporation, a powder containing dried polymer particles and water-absorbing resin particles is obtained. A certain amount of water may remain in the dried polymer particles (water-absorbing resin particles).

[0057] After water is extracted, polymer particles can be surface crosslinked in a mixture containing a concentrate and a surface crosslinking agent, wherein the concentrate contains polymer particles (hygroscopic resin particles).

[0058] Surface crosslinking agents can be compounds with two or more reactive functional groups, examples of which include polyols such as ethylene glycol, propylene glycol, 1,4-butanediol, trimethylolpropane, glycerol, polyoxyethylene glycol, polyoxypropylene glycol, and polyglycerol; polyglycidyl compounds such as (poly)ethylene glycol diglycidyl ether, (poly)glycerol diglycidyl ether, (poly)glycerol triglycidyl ether, trimethylolpropane triglycidyl ether, (poly)propylene glycol polyglycidyl ether, and (poly)glycerol polyglycidyl ether; halogenated epoxy compounds such as epichlorohydrin, epibromohydrin, and α-methylepicochlorohydrin; 2, Isocyanate compounds such as 4-toluene diisocyanate and hexamethylene diisocyanate; oxetane compounds such as 3-methyl-3-oxetane methanol, 3-ethyl-3-oxetane methanol, 3-butyl-3-oxetane methanol, 3-methyl-3-oxetane ethanol, 3-ethyl-3-oxetane ethanol, and 3-butyl-3-oxetane ethanol; oxazoline compounds such as 1,2-ethylene bisoxazoline; carbonate compounds such as ethylene carbonate; and hydroxyalkylamide compounds such as bis[N,N-di(β-hydroxyethyl)]hexamethylenediamide. Surface crosslinking agents may include polyglycidyl compounds such as (poly)ethylene glycol diglycidyl ether, (poly)glycerol diglycidyl ether, (poly)glycerol triglycidyl ether, (poly)propylene glycol polyglycidyl ether, and polyglycerol polyglycidyl ether. These surface crosslinking agents can be used alone or in combination of two or more.

[0059] The amount of surface crosslinking agent may be more than 0.01 mmol and less than 10 mmol, more than 0.03 mmol and less than 3 mmol, or more than 0.05 mmol and less than 1 mmol, relative to each mole of the monomer unit constituting the polymer in the polymer particles.

[0060] To achieve surface crosslinking, the mixture can be heated. The heating temperature can be, for example, above 60°C and below 200°C, or above 80°C and below 150°C. The reaction time for the surface crosslinking reaction can be, for example, above 1 minute and below 300 minutes, or above 5 minutes and below 200 minutes.

[0061] The dispersion medium can be removed from the concentrate to obtain dried polymer particles (hygroscopic resin particles). Therefore, the concentrate can be heated.

[0062] Various additives can be further added to the dried polymer particles. Examples of additives include lubricants, metal chelating agents, surface modifiers, heat stabilizers, antioxidants, and antibacterial agents.

[0063] Lubricants can be, for example, amorphous silica particles. Examples of metal chelating agents include ethylenediaminetetraacetic acid (EDTA) and its salts (such as disodium EDTA), and diethylenetriaminepentaacetic acid (DTA) and its salts (such as pentasodium DTA). Examples of surface modifiers include polyvalent metal compounds such as aluminum sulfate, potassium sulfate, ammonium sulfate, sodium sulfate, (poly)aluminum chloride, and their hydrates; and polycationic compounds such as polyethyleneimine, polyethyleneamine, and polyallylamine.

[0064] Additives can adhere to the surface of polymer particles or penetrate into the interior of polymer particles. Additives (e.g., lubricants) can be added to the surface of the polymer particles after they have been dried. Additives can also be added to the liquid used for polymerization or to the concentrate after water extraction. In this specification, water-absorbing resin particles refer to particles containing polymer particles. Water-absorbing resin particles may contain both polymer particles and additives.

[0065] The amount of additives (e.g., lubricants) relative to 100 parts by mass of polymer particles can be, for example, more than 0.001 parts by mass and less than 10 parts by mass, more than 0.01 parts by mass and less than 5 parts by mass, or more than 0.1 parts by mass and less than 2 parts by mass.

[0066] Superabsorbent resin particles can contain spherical primary particles. Primary particles obtained through reverse suspension polymerization may sometimes form chain-like coarse particles due to trace impurities in the aqueous solution. However, by controlling the amount of alcohol compounds in the aqueous solution within an appropriate range, the formation of chain-like coarse particles based on spherical primary particles can be effectively suppressed. The spherical primary particles can be spherical or approximately spherical. The aspect ratio (major axis diameter / minor axis diameter) of the spherical primary particles can, for example, be 1.0 or higher and 1.5 or lower.

[0067] Example The present invention is not limited to the following embodiments.

[0068] 1. Manufacturing of water-absorbing resin particles Example 1 A round-bottomed cylindrical flask with an inner diameter of 11 cm and an internal volume of 2 L was prepared, equipped with a reflux condenser, a dropping funnel, a nitrogen inlet tube, and a stirrer. The stirrer was fitted with four inclined blades, each with a single blade diameter of 5 cm. 283 g of n-heptane was added to the flask as a hydrocarbon dispersion medium, and 0.736 g of maleic anhydride-modified ethylene-propylene copolymer (Mitsui Chemicals, Inc., HIWAX 1105A) was added as a hydrophobic polymeric dispersant. The mixture in the flask was stirred while the temperature was raised to 80 °C, forming a n-heptane solution containing the maleic anhydride-modified ethylene-propylene copolymer. After stirring, the n-heptane solution was cooled to 56 °C.

[0069] 74.06 g (1.03 mol) of acrylic acid and 0.0074 g of isononol were added to a 300 mL beaker. While cooling the beaker with ice water, 102.8 g of a 30% (w / w) sodium hydroxide aqueous solution was added dropwise to prepare a 75 mol% neutralized acrylic acid. Next, 69.0 g of water, 0.0920 g of hydroxyethyl cellulose (Sumitomo Seika Chemicals Company, Limited, AW-15F) as a thickener, 0.0368 g (0.136 mmol) of potassium persulfate as a free radical polymerization initiator, and 0.0101 g (0.058 mmol) of ethylene glycol diglycidyl ether as an internal crosslinking agent were added to the beaker and dissolved to prepare an aqueous solution containing acrylic acid and sodium acrylate.

[0070] The prepared aqueous solution was added to a heptane solution containing maleic anhydride-modified ethylene-propylene copolymer in a removable flask. The mixture in the removable flask was stirred for 10 minutes. Next, a surfactant solution containing 0.736 g of sucrose stearate (Mitsubishi Chemical Foods Corporation, RYOTO SUGARESTER S-370, HLB: 3) and 6.62 g of heptane was added to the mixture. The reaction mixture formed in the removable flask was stirred with a stirrer at 300 rpm, and the removable flask was fully purged with nitrogen. After nitrogen purging, the removable flask was immersed in a 70°C water bath to raise the temperature of the reaction mixture, thereby initiating the polymerization reaction. The polymerization reaction was continued with stirring for 60 minutes, thereby forming a slurry containing a hydrogel polymer.

[0071] A detachable flask containing the slurry was immersed in an oil bath at 125°C. The n-heptane contained in the slurry was refluxed, and 127.5 g of water was extracted from the slurry and transferred to the outside of the detachable flask via azeotropic distillation of n-heptane and water. Next, the detachable flask was heated in an oil bath at 125°C to evaporate the n-heptane, thereby obtaining 96.2 g of dried water-absorbing resin particles.

[0072] Examples 2 and 3 and Comparative Examples 1-3 To adjust the aqueous solution, alcohol compounds (isononol, 1-butanol, and isopropanol) were added in amounts shown in Table 1 instead of 0.0074 g of isononol. Otherwise, water-absorbing resin particles were obtained in the same order as in Example 1. The amounts of alcohol compounds shown in Table 1 are proportions (ppm by mass) based on the mass of acrylic acid before neutralization.

[0073] 2. Evaluation of water-absorbing resin particles The powder of the absorbent resin particles obtained in the examples or comparative examples was observed using a scanning electron microscope (SEM) to confirm the presence or absence of coarse, chain-like particles formed by the linkage of primary particles. Figure 1 This is an SEM image of the water-absorbing resin particles from Example 2. Figure 2 , Figure 3 and Figure 4 The images shown are SEM images of the absorbent resin particles from Comparative Examples 1, 2, and 3. The absorbent resin particles of the examples contain spherical primary particles, and no chain-like coarse particles formed due to aggregation or agglomeration of the primary particles were observed. This result confirms that the formation of chain-like coarse particles can be suppressed by adjusting the amount of alcohol compound relative to the amount of (meth)acrylic acid compound.

[0074] [Table 1]

Claims

1. A method for manufacturing water-absorbing resin particles, comprising: The process of preparing an aqueous solution, wherein the aqueous solution contains a monomer of a (meth)acrylic acid compound comprising at least one of (meth)acrylic acid or a salt thereof, an alcohol compound having 1 or more and 10 or fewer carbon atoms, and water; The process of forming a particulate hydrogel polymer containing the monomer and water through reverse suspension polymerization in a reaction solution containing the aqueous liquid and dispersion medium; and The process of removing the water-absorbing resin particles containing the polymer from the reaction solution. Prepare the aqueous solution containing an amount of alcohol compound of less than 800 ppm by mass relative to the amount of the (meth)acrylic acid compound. When the aqueous solution contains the salt of the (meth)acrylic acid, the amount of the (meth)acrylic acid compound is the sum of the amount of the (meth)acrylic acid and the amount of (meth)acrylic acid equivalent to the amount of the salt.

2. The method according to claim 1, wherein, The method further includes a step of preparing a (meth)acrylic acid raw material, said (meth)acrylic acid raw material containing (meth)acrylic acid and an alcohol compound having 1 or more and 10 or fewer carbon atoms, and being conveyed in such a manner that the amount of said alcohol compound relative to the amount of said (meth)acrylic acid is 800 ppm by mass or less. The aqueous solution is prepared from the (meth)acrylic acid raw material.

3. The method according to claim 1, wherein, The absorbent resin particles comprise spherical primary particles.

4. The method according to any one of claims 1 to 3, wherein, The alcohol compound comprises at least one selected from isononol, 1-butanol and isopropanol.

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