Method for producing water absorbent resin particles, and method for controlling particle diameters of water absorbent resin particles

Abstract

Disclosed is a method for producing water absorbent resin particles, the method including: preparing an aqueous liquid that contains a monomer containing a (meth)acrylic acid compound of at least one of (meth)acrylic acid and a salt thereof, an alcohol compound having 1-10 carbon atoms, and water; forming a particulate hydrous gel polymer that contains a polymer of the monomer and water by reversed phase suspension polymerization in a reaction liquid that contains the aqueous liquid and a dispersion medium; and taking out water absorbent resin particles that contain the polymer from the reaction liquid. The aqueous liquid is prepared to contain the alcohol compound in an amount of 800 ppm by mass or less with respect to the amount of the (meth)acrylic acid compound.

Description

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

[0001] The present disclosure relates to a method for producing water-absorbent resin particles and a method for controlling the particle size of water-absorbent resin particles.

[0002] Water-absorbent resins are produced in the form of particles, and are widely used in various fields, such as hygiene materials such as disposable diapers and sanitary products, gardening materials such as water retention agents and soil conditioners, and industrial materials such as water-stopping materials and anti-condensation agents.

[0003] In such water-absorbent resin particles, it is required that the proportion of particles having an excessively large particle size is small (for example, Patent Document 1).

[0004] International Publication No. 2012 / 081355

[0005] When water-absorbent resin particles are obtained by reversed-phase suspension polymerization, depending on the polymerization conditions, a trace amount of impurities contained in an aqueous liquid may cause the formation of coarse particles in which a large number of primary particles are linked in a chain shape. As a result, the proportion of coarse particles having an excessively large particle size may increase.

[0006] The present disclosure relates to suppressing the formation of chain-like coarse particles formed by linking primary particles in the production of water-absorbent resin particles by reversed-phase suspension polymerization.

[0007] The present disclosure includes the following: [1] A method for producing water-absorbent resin particles, comprising: preparing an aqueous liquid containing a monomer containing a (meth)acrylic acid compound of at least one of (meth)acrylic acid and a salt thereof, an alcohol compound having 1 to 10 carbon atoms, and water, forming a particulate hydrogel polymer containing a polymer of the monomer and water by reverse-phase suspension polymerization in a reaction liquid containing the aqueous liquid and a dispersion medium, and extracting water-absorbent resin particles containing the polymer from the reaction liquid, wherein the aqueous liquid contains the alcohol compound in an amount of 800 ppm by mass or less relative to the amount of the (meth)acrylic acid compound, and when the aqueous liquid contains a salt of (meth)acrylic acid, the amount of the (meth)acrylic acid compound is the sum of the amount of the (meth)acrylic acid and an amount of (meth)acrylic acid corresponding to the substance amount of the salt. [2] The method according to [1], further comprising preparing a (meth)acrylic acid raw material containing (meth)acrylic acid and an alcohol compound having from 1 to 10 carbon atoms, the (meth)acrylic acid raw material being transported so that the amount of the alcohol compound is 800 mass ppm or less relative to the amount of the (meth)acrylic acid, and the aqueous liquid is prepared from the (meth)acrylic acid raw material. [3] The method according to [1] or [2], wherein the water-absorbing resin particles comprise spherical primary particles. [4] The method according to any one of [1] to [3], wherein the alcohol compound comprises at least one selected from isononyl alcohol, 1-butanol, and isopropyl alcohol.

[0008] According to the present disclosure, in the production of water-absorbent resin particles by reversed-phase suspension polymerization, it is possible to suppress the formation of chain-like coarse particles in which primary particles are linked together.

[0009] 1 is a scanning electron microscope (SEM) image showing an example of a water-absorbent resin particle; 2 is an SEM image showing an example of a water-absorbent resin particle containing chain-like coarse particles; 3 is an SEM image showing an example of a water-absorbent resin particle containing chain-like coarse particles; 4 is an SEM image showing an example of a water-absorbent resin particle containing chain-like coarse particles.

[0010] The present invention is not limited to the following examples: In this specification, (meth)acrylic acid means acrylic acid, methacrylic acid, and combinations thereof.

[0011] An example of a method for producing water-absorbent resin particles includes preparing an aqueous liquid containing a monomer containing a (meth)acrylic acid compound which is at least one of (meth)acrylic acid and a salt thereof, an alcohol compound having 1 to 10 carbon atoms, and water; forming a particulate hydrogel polymer containing a polymer of the monomer and water by reverse phase suspension polymerization in a reaction liquid containing the aqueous liquid and a dispersion medium; and extracting water-absorbent resin particles containing the polymer from the reaction liquid.

[0012] The reaction liquid for reversed-phase suspension polymerization can be formed by stirring a mixed liquid containing an aqueous liquid and a dispersion medium. According to the findings of the present inventors, the amount of the alcohol compound having 1 to 10 carbon atoms contained in the aqueous liquid is related to the degree of formation of chain-like coarse particles in which primary particles are linked together in the water-absorbent resin particles extracted from the reaction liquid. Therefore, in the aqueous liquid before forming the reaction liquid, the amount of the alcohol compound having 1 to 10 carbon atoms is adjusted to 800 mass ppm or less relative to the amount of the (meth)acrylic acid compound. When the amount of the alcohol compound having 1 to 10 carbon atoms is 800 mass ppm or less, water-absorbent resin particles with a low proportion of chain-like coarse particles can be stably produced. From a similar viewpoint, the amount of the alcohol compound having 1 to 10 carbon atoms in the prepared aqueous liquid may be 750 ppm by mass or less, 700 ppm by mass or less, 650 ppm by mass or less, 600 ppm by mass or less, 550 ppm by mass or less, 500 ppm by mass or less, 450 ppm by mass or less, 400 ppm by mass or less, 360 ppm by mass or less, 320 ppm by mass or less, 280 ppm by mass or less, 240 ppm by mass or less, 200 ppm by mass or less, 180 ppm by mass or less, 160 ppm by mass or less, 140 ppm by mass or less, 120 ppm by mass or less, or 100 ppm by mass or less, relative to the amount of the (meth)acrylic acid compound. The amount of the alcohol compound having 1 to 10 carbon atoms in the prepared aqueous liquid may be more than 0 ppm by mass, or may be 1 ppm by mass or more, 5 ppm by mass or more, or 10 ppm by mass or more relative to the amount of the (meth)acrylic acid compound.

[0013] The amount of the alcohol compound having 1 to 10 carbon atoms in the prepared aqueous liquid may be more than 0 ppm by mass and not more than 800 ppm by mass, 750 ppm by mass, 700 ppm by mass, 650 ppm by mass, 600 ppm by mass, 550 ppm by mass, 500 ppm by mass, 450 ppm by mass, 400 ppm by mass, 360 ppm by mass, 320 ppm by mass, 280 ppm by mass, 240 ppm by mass, 200 ppm by mass, 180 ppm by mass, 160 ppm by mass, 140 ppm by mass, 120 ppm by mass, or 100 ppm by mass. The amount of the alcohol compound having 1 to 10 carbon atoms in the prepared aqueous liquid may be 1 ppm by mass or more and 800 ppm by mass or less, 750 ppm by mass or less, 700 ppm by mass or less, 650 ppm by mass or less, 600 ppm by mass or less, 550 ppm by mass or less, 500 ppm by mass or less, 450 ppm by mass or less, 400 ppm by mass or less, 360 ppm by mass or less, 320 ppm by mass or less, 280 ppm by mass or less, 240 ppm by mass or less, 200 ppm by mass or less, 180 ppm by mass or less, 160 ppm by mass or less, 140 ppm by mass or less, 120 ppm by mass or less, or 100 ppm by mass or less, relative to the amount of the (meth)acrylic acid compound. The amount of the alcohol compound having 1 to 10 carbon atoms in the prepared aqueous liquid may be 5 ppm by mass or more and 800 ppm by mass or less, 750 ppm by mass or less, 700 ppm by mass or less, 650 ppm by mass or less, 600 ppm by mass or less, 550 ppm by mass or less, 500 ppm by mass or less, 450 ppm by mass or less, 400 ppm by mass or less, 360 ppm by mass or less, 320 ppm by mass or less, 280 ppm by mass or less, 240 ppm by mass or less, 200 ppm by mass or less, 180 ppm by mass or less, 160 ppm by mass or less, 140 ppm by mass or less, 120 ppm by mass or less, or 100 ppm by mass or less, relative to the amount of the (meth)acrylic acid compound.The amount of the alcohol compound having 1 to 10 carbon atoms in the prepared aqueous liquid may be 10 ppm by mass or more and 800 ppm by mass or less, 750 ppm by mass or less, 700 ppm by mass or less, 650 ppm by mass or less, 600 ppm by mass or less, 550 ppm by mass or less, 500 ppm by mass or less, 450 ppm by mass or less, 400 ppm by mass or less, 360 ppm by mass or less, 320 ppm by mass or less, 280 ppm by mass or less, 240 ppm by mass or less, 200 ppm by mass or less, 180 ppm by mass or less, 160 ppm by mass or less, 140 ppm by mass or less, 120 ppm by mass or less, or 100 ppm by mass or less, relative to the amount of the (meth)acrylic acid compound.

[0014] When the aqueous liquid contains a salt of (meth)acrylic acid, the amount of the (meth)acrylic acid compound relative to the amount of the alcohol compound having from 1 to 10 carbon atoms is the sum of the amount of (meth)acrylic acid and the amount of (meth)acrylic acid equivalent to the amount by substance of the salt of (meth)acrylic acid. The amount of (meth)acrylic acid equivalent to the amount by substance of the salt of (meth)acrylic acid compound means the amount of (meth)acrylic acid that is the same amount by substance as the salt of (meth)acrylic acid. When the aqueous liquid contains two or more alcohol compounds having from 1 to 10 carbon atoms, the aqueous liquid is prepared so that their total amount is within the above-mentioned range.

[0015] Examples of alcohol compounds having 1 to 10 carbon atoms that can be contained in the aqueous liquid include isononyl alcohol, 1-butanol, isopropyl alcohol, 1,3-butanediol, 1,4-butanediol, propylene glycol, cyclohexanol, methyl t-butyl alcohol, 1-octanol, and isobutyl alcohol. The alcohol compounds having 1 to 10 carbon atoms may be used alone or in combination of two or more.

[0016] The aqueous liquid may contain (meth)acrylic acid and a salt of (meth)acrylic acid as the (meth)acrylic acid compound. Based on the total amount of the (meth)acrylic acid compounds, the proportion of the salt of (meth)acrylic acid may be, for example, 50 mol % or more and 100 mol % or less. The salt of (meth)acrylic acid may be, for example, an alkali metal salt of (meth)acrylic acid. The alkali metal salt of (meth)acrylic acid may be a sodium salt.

[0017] The aqueous liquid can be prepared, for example, by a method including: preparing a (meth)acrylic acid raw material containing (meth)acrylic acid and an alcohol compound having from 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 a basic compound. The basic compound may be an alkali metal hydroxide (e.g., sodium hydroxide).

[0018] The (meth)acrylic acid raw material may be transported such that the amount of alcohol compounds having from 1 to 10 carbon atoms is 800 ppm by mass or less relative to the amount of (meth)acrylic acid. When a transport container (e.g., an ISO tank container) in which the (meth)acrylic acid raw material is stored for transportation has been used to transport alcohol compounds having from 1 to 10 carbon atoms before the (meth)acrylic acid raw material is stored, even after the interior of the container has been appropriately cleaned, a trace amount of alcohol compound remaining in the transport container may be contained in the (meth)acrylic acid raw material. By managing the transport container so that a large amount of alcohol compound does not remain in the transport container for transporting the (meth)acrylic acid raw material, the amount of alcohol compound contained in the (meth)acrylic acid raw material can be controlled to a range of 800 ppm by mass or less. The amount of alcohol compounds having from 1 to 10 carbon atoms in the (meth)acrylic acid raw material removed from the transport container may be within the same range as the amount of alcohol compound relative to the amount of (meth)acrylic acid compound in the aqueous liquid.

[0019] The monomers in the aqueous liquid may further include a water-soluble ethylenically unsaturated monomer other than a (meth)acrylic acid compound. The proportion of the (meth)acrylic acid compound 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, or may be substantially 100 mol%. The water-soluble ethylenically unsaturated monomer other than a (meth)acrylic acid compound may include, for example, at least one selected from the group consisting of 2-(meth)acrylamido-2-methylpropanesulfonic acid and its alkali 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.

[0020] 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 (particularly, a (meth)acrylic acid compound). The reactive functional groups may be, for example, a (meth)acryloyl group, a vinyl group, an epoxy group, a halogeno group in a haloepoxy compound, an isocyanate group, or a combination thereof.

[0021] Examples of internal crosslinking agents having two or more (meth)acryloyl groups include (meth)acrylic acid ester compounds formed from a polyol compound and (meth)acrylic acid, unsaturated polyesters formed from a polyol compound and an unsaturated carboxylic acid (maleic acid, fumaric acid, etc.), bis(meth)acrylamide compounds (N,N'-methylenebis(meth)acrylamide, etc.), (meth)acrylic acid ester compounds formed from a polyepoxide compound and (meth)acrylic acid, and (meth)acrylic acid carbamyl ester compounds formed from a polyisocyanate compound (tolylene diisocyanate, hexamethylene diisocyanate, etc.) and hydroxyethyl (meth)acrylate. The polyol compound for forming the (meth)acrylic acid ester compound or the unsaturated polyester may be, for example, ethylene glycol, propylene glycol, trimethylolpropane, glycerin, polyoxyethylene glycol, polyoxypropylene glycol, polyglycerin, or a combination thereof.

[0022] The vinyl group as a reactive functional group may be a part of an allyl group. Examples of internal crosslinking agents having two or more vinyl groups (or allyl groups) include allylated starch, allylated cellulose, diallyl phthalate, N,N',N''-triallyl isocyanurate, and divinylbenzene.

[0023] Examples of the internal crosslinking agent having two or more epoxy groups include (poly)ethylene glycol diglycidyl ether, (poly)propylene glycol diglycidyl ether, (poly)glycerin diglycidyl ether, (poly)glycerin triglycidyl ether, (poly)propylene glycol polyglycidyl ether, and polyglycerol polyglycidyl ether.

[0024] Examples of internal crosslinking agents having two or more isocyanate groups include 2,4-tolylene diisocyanate and hexamethylene diisocyanate.

[0025] The amount of the internal crosslinking agent may be 0 mmol or more and 0.5 mmol or less, 0 mmol or more and 0.2 mmol or less, 0 mmol or more and 0.1 mmol or less, 0 mmol or more and 0.05 mmol or less, or 0 mmol or more and 0.02 mmol or less, or may be substantially 0 mmol, relative to 1 mol of the monomer.

[0026] The aqueous liquid may contain, as other components, a thickener, a hydrophilic polymer dispersant, a radical polymerization initiator, a chain transfer agent, a foaming agent, or a combination thereof.

[0027] Examples of thickeners include hydroxyalkyl celluloses such as hydroxyethyl cellulose (HEC) and hydroxypropyl cellulose (HPC), hydroxyalkyl alkyl 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. The thickeners may be used alone or in combination of two or more.

[0028] The amount of the thickener may be 0.05 parts by mass or more and 20 parts by mass or less, 0.2 parts by mass or more and 10 parts by mass or less, or 0.4 parts by mass or more and 5 parts by mass or less, relative to 100 parts by mass of the monomer.

[0029] Examples of hydrophilic polymer dispersants include polyvinyl alcohol (PVA), polyvinylpyrrolidone (PVP), polyethylene glycol (PEG), polypropylene glycol, polyethylene glycol-polypropylene glycol block copolymer, polyglycerin, polyoxyethylene glycerin, polyoxypropylene glycerin, polyoxyethylene-polyoxypropylene glycerin copolymer, and polyoxyethylene sorbitan fatty acid ester. The hydrophilic polymer dispersants may be used alone or in combination of two or more.

[0030] The amount of the hydrophilic polymer dispersant may be 0.001 parts by mass or more and 10 parts by mass or less, 0.005 parts by mass or more and 5 parts by mass or less, 0.01 parts by mass or more and 3 parts by mass or less, or 0.01 parts by mass or more and 1.5 parts by mass or less, relative to 100 parts by mass of the monomer.

[0031] The radical polymerization initiator may include, for example, an azo-based compound, a peroxide, or a combination thereof.

[0032] Examples of the azo compounds include 2,2'-azobis[2-(N-phenylamidino)propane]dihydrochloride, 2,2'-azobis{2-[N-(4-chlorophenyl)amidino]propane}dihydrochloride, 2,2'-azobis{2-[N-(4-hydroxyphenyl)amidino]propane}dihydrochloride, 2,2'-azobis[2-(N-benzylamidino)propane]dihydrochloride, 2,2' -Azobis[2-(N-allylamidino)propane]dihydrochloride, 2,2'-azobis(2-amidinopropane)dihydrochloride, 2,2'-azobis{2-[N-(2-hydroxyethyl)amidino]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-diazepin-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-imidazolin-2-yl]propane] dihydrochloride 2,2'-azobis[N-(2-carboxyethyl)-2-methylpropionamidine]tetrahydrate, and 2,2'-azobis[2-methyl-N-(2-hydroxyethyl)propionamide].

[0033] 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-t-butyl peroxide, t-butyl cumyl peroxide, t-butyl peroxyacetate, t-butyl peroxyisobutyrate, and t-butyl peroxypivalate; and hydrogen peroxide.

[0034] The amount of the radical polymerization initiator may be, for example, 0.0001 mol or more and 1 mol or less, 0.0003 mol or more and 0.05 mol or less, 0.0005 mol or more and 0.03 mol or less, or 0.001 mol or more and 0.015 mol or less, relative to 100 mol of the monomer.

[0035] Examples of chain transfer agents include hypophosphites, thiols, thiolic acids, secondary alcohols, and amines.

[0036] Examples of the blowing agent include inorganic blowing agents such as ammonium carbonate, sodium bicarbonate, and ammonium bicarbonate; nitroso compounds such as dinitrosopentamethylenetetramine; azo compounds such as azodicarbonamide and azobisisobutyronitrile; and organic blowing agents such as sulfonylhydrazide compounds such as 4,4'-oxybisbenzenesulfonylhydrazide and p-toluenesulfonylhydrazide.

[0037] A reaction solution for reversed-phase suspension polymerization can be prepared by stirring a mixture containing the prepared aqueous solution and an oily solution, which is a hydrophobic liquid primarily composed of a hydrophobic dispersion medium. The reaction solution contains particulate aqueous solution dispersed in the oily solution. The polymerization reaction in the reaction solution forms a particulate hydrogel polymer containing a monomer polymer and water.

[0038] The reaction solution may contain a surfactant. The surfactant may be contained in the oily liquid. The surfactant may be a nonionic surfactant or an anionic surfactant. Examples of nonionic surfactants include sorbitan fatty acid esters, polyglycerin 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, alkylaryl formaldehyde condensed polyoxyethylene ethers, polyoxyethylene polyoxypropylene block copolymers, polyoxyethylene polyoxypropyl alkyl ethers, and polyethylene glycol fatty acid esters. Examples of anionic surfactants include fatty acid salts, alkylbenzenesulfonates, alkylmethyltaurates, polyoxyethylene alkylphenyl ether sulfates, polyoxyethylene alkyl ether sulfonates, polyoxyethylene alkyl ether phosphates, and polyoxyethylene alkylallyl ether phosphates. The surfactant may be used alone or in combination of two or more.

[0039] The oily liquid in the reaction mixture is a hydrophobic liquid mainly composed of a hydrophobic dispersion medium. The dispersion medium may be a hydrocarbon dispersion medium. The oily liquid may further contain optional additives such as a hydrophobic polymer dispersant.

[0040] 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 media may be used alone or in combination of two or more.

[0041] The amount of the dispersion medium contained in the oily liquid may be 30 parts by mass or more and 1,000 parts by mass or less, 50 parts by mass or more and 650 parts by mass or less, 70 parts by mass or more and 550 parts by mass or less, or 100 parts by mass or more and 450 parts by mass or less, relative to 100 parts by mass of the monomer.

[0042] Examples of hydrophobic polymer dispersants include maleic anhydride-modified polyethylene, maleic anhydride-modified polypropylene, maleic anhydride-modified ethylene-propylene copolymer, maleic anhydride-modified EPDM (ethylene-propylene-diene terpolymer), maleic anhydride-modified polybutadiene, maleic anhydride-ethylene copolymer, maleic anhydride-propylene copolymer, maleic anhydride-ethylene-propylene copolymer, maleic anhydride-butadiene copolymer, polyethylene, polypropylene, ethylene-propylene copolymer, oxidized polyethylene, oxidized polypropylene, oxidized ethylene-propylene copolymer, ethylene-acrylic acid copolymer, ethyl cellulose, and ethylhydroxyethyl cellulose. The hydrophobic polymer dispersants may be used alone or in combination of two or more.

[0043] The amount of the hydrophobic polymer dispersant may be 0.05 parts by mass or more and 10 parts by mass or less, 0.08 parts by mass or more and 5 parts by mass or less, or 0.1 parts by mass or more and 3 parts by mass or less, relative to 100 parts by mass of the monomer.

[0044] For the polymerization reaction, the reaction solution may be heated. The heating temperature for the polymerization reaction may be, for example, 40° C. or higher and 90° C. or lower. The polymerization reaction time may be, for example, 30 minutes or higher and 240 minutes or lower.

[0045] During the polymerization reaction, the reaction solution is usually stirred. The rotation speed of the stirring during the polymerization reaction may be, for example, 200 rpm or more and 1000 rpm or less.

[0046] Water-absorbent resin particles containing a polymer are extracted from a reaction liquid (slurry) containing a particulate hydrogel polymer. Extracting the water-absorbent resin particles from the reaction liquid includes, for example, forming a concentrate by extracting a portion of the water from the reaction liquid by azeotropic distillation of the dispersion medium and water, and evaporating the alcohol compound contained in the dispersion medium, water, and aqueous liquid from the concentrate. After removing the dispersion medium and the like by evaporation, a powder of water-absorbent resin particles containing dried polymer particles can be obtained. A certain amount of water may remain in the dried polymer particles (water-absorbent resin particles).

[0047] After the water is removed, the polymer particles may be surface-crosslinked in a mixture containing a concentrate containing polymer particles (water-absorbent resin particles) containing a polymer and a surface-crosslinking agent.

[0048] The surface cross-linking agent may be a compound having two or more reactive functional groups, examples of which include polyols such as ethylene glycol, propylene glycol, 1,4-butanediol, trimethylolpropane, glycerin, polyoxyethylene glycol, polyoxypropylene glycol, and polyglycerin; polyglycidyl compounds such as (poly)ethylene glycol diglycidyl ether, (poly)glycerin diglycidyl ether, (poly)glycerin triglycidyl ether, trimethylolpropane triglycidyl ether, (poly)propylene glycol polyglycidyl ether, and (poly)glycerol polyglycidyl ether; epichlorohydrin, epibromohydrin and α-methylepichlorohydrin; isocyanate compounds such as 2,4-tolylene diisocyanate and hexamethylene diisocyanate; oxetane compounds such as 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; oxazoline compounds such as 1,2-ethylenebisoxazoline; carbonate compounds such as ethylene carbonate; and hydroxyalkylamide compounds such as bis[N,N-di(β-hydroxyethyl)]adipamide. The surface cross-linking agent may include a polyglycidyl compound such as (poly)ethylene glycol diglycidyl ether, (poly)glycerin diglycidyl ether, (poly)glycerin triglycidyl ether, (poly)propylene glycol polyglycidyl ether, polyglycerol polyglycidyl ether, etc. These surface cross-linking agents may be used alone or in combination of two or more.

[0049] The amount of the surface cross-linking agent may be 0.01 mmol or more and 10 mmol or less, 0.03 mmol or more and 3 mmol or less, or 0.05 mmol or more and 1 mmol or less, per mole of the monomer unit constituting the polymer in the polymer particles.

[0050] For the surface cross-linking, the mixture may be heated. The heating temperature may be, for example, 60° C. or higher and 200° C. or lower, or 80° C. or higher and 150° C. or lower. The reaction time for the surface cross-linking reaction may be, for example, 1 minute or higher and 300 minutes or lower, or 5 minutes or higher and 200 minutes or lower.

[0051] The dispersion medium can be removed from the concentrate to obtain dried polymer particles (water-absorbent resin particles), and for this purpose the concentrate may be heated.

[0052] Various additives may be further added to the dried polymer particles, examples of which include lubricants, metal chelating agents, surface modifiers, heat stabilizers, antioxidants, and antibacterial agents.

[0053] The lubricant may be, for example, amorphous silica particles. Examples of metal chelating agents include ethylenediaminetetraacetic acid and its salts (such as disodium ethylenediaminetetraacetic acid) and diethylenetriaminepentaacetic acid and its salts (such as pentasodium diethylenetriaminepentaacetic acid). Examples of surface modifiers include polyvalent metal compounds such as aluminum sulfate, potassium alum, ammonium alum, sodium alum, (poly)aluminum chloride, and hydrates thereof; and polycation compounds such as polyethyleneimine, polyvinylamine, and polyallylamine.

[0054] The additive may be attached to the surface of the polymer particles or may penetrate into the interior of the polymer particles. The additive (e.g., a lubricant) can be added to the polymer particles after drying to attach the additive to the surface of the polymer particles. The additive may be added to the liquid for polymerization or to the concentrate after water extraction. In this specification, water-absorbent resin particles mean particles containing polymer particles. The water-absorbent resin particles may contain polymer particles and an additive.

[0055] The amount of the additive (e.g., lubricant) may be, for example, 0.001 to 10 parts by mass, 0.01 to 5 parts by mass, or 0.1 to 2 parts by mass relative to 100 parts by mass of the polymer particles.

[0056] The water-absorbent resin particles may include spherical primary particles. Primary particles obtained by reverse-phase suspension polymerization may form chain-like coarse particles due to trace amounts of impurities contained in the aqueous liquid. However, by controlling the amount of alcohol compound in the aqueous liquid within an appropriate range, the formation of chain-like coarse particles from the spherical primary particles can be effectively suppressed. The spherical primary particles may be true spherical or nearly true spherical. The aspect ratio (major axis diameter / minor axis diameter) of the spherical primary particles may be, for example, 1.0 or more and 1.5 or less.

[0057] The present invention is not limited to the following examples. 1. Production of Water-Absorbent Resin Particles Example 1 A round-bottomed, cylindrical, separable 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 gas inlet tube, and a stirrer. A stirring blade with four inclined paddle blades with a blade diameter of 5 cm was attached to the stirrer. 283 g of n-heptane as a hydrocarbon dispersion medium and 0.736 g of maleic anhydride-modified ethylene-propylene copolymer (Hiwax 1105A, Mitsui Chemicals, Inc.) as a hydrophobic polymer-based dispersant were placed in the prepared separable flask. The mixture in the separable flask was heated to 80°C while stirring to form an n-heptane solution containing the maleic anhydride-modified ethylene-propylene copolymer. After stirring was completed, the n-heptane solution was cooled to 56°C.

[0058] 74.06 g (1.03 mol) of acrylic acid and 0.0074 g of isononyl alcohol were placed in a 300 mL beaker. While cooling the beaker with ice water, 102.8 g of a 30% by weight aqueous sodium hydroxide solution was added dropwise to the beaker to prepare a 75 mol% neutralized acrylic acid. Next, 69.0 g of water, 0.0920 g of hydroxyethyl cellulose (Sumitomo Seika Chemicals, AW-15F) as a thickener, 0.0368 g (0.136 mmol) of potassium persulfate as a radical polymerization initiator, and 0.0101 g (0.058 mmol) of ethylene glycol diglycidyl ether as an internal crosslinking agent were added and dissolved in the beaker to prepare an aqueous solution containing acrylic acid and sodium acrylate.

[0059] The prepared aqueous solution was added to the n-heptane solution containing the maleic anhydride-modified ethylene-propylene copolymer in a separable flask. The mixture in the separable flask was stirred for 10 minutes. Next, a surfactant solution containing 0.736 g of sucrose stearate (Ryoto Sugar Ester S-370, HLB: 3, Mitsubishi Chemical Foods Corporation) and 6.62 g of n-heptane was added to the mixture. The reaction solution formed in the separable flask was stirred with a stirrer at 300 rpm while the atmosphere in the separable flask was thoroughly purged with nitrogen. After the nitrogen purge, the separable flask was immersed in a 70°C water bath to raise the temperature of the reaction solution and initiate the polymerization reaction. The polymerization reaction was allowed to proceed for 60 minutes while maintaining stirring, resulting in the formation of a slurry containing a hydrogel polymer.

[0060] The separable flask containing the slurry was immersed in an oil bath at 125°C. While refluxing the n-heptane contained in the slurry in the separable flask, 127.5 g of water contained in the slurry was extracted to the outside of the separable flask by azeotropic distillation of n-heptane and water. Next, the separable flask was heated in an oil bath at 125°C to evaporate the n-heptane and the like, thereby obtaining 96.2 g of dried water absorbent resin particles.

[0061] Examples 2 and 3 and Comparative Examples 1 to 3 Water-absorbent resin particles were obtained in the same procedure as in Example 1, except that, instead of 0.0074 g of isononyl alcohol, alcohol compounds (isononyl alcohol, 1-butanol and isopropyl alcohol) in the amounts shown in Table 1 were added to adjust the aqueous liquid. The amount of the alcohol compound shown in Table 1 is a ratio (ppm by mass) based on the mass of acrylic acid before neutralization.

[0062] 2. Evaluation of Water-Absorbent Resin Particles The powder of water-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 chain-like coarse particles formed by linking of primary particles. FIG. 1 is an SEM image of the water-absorbent resin particles of Example 2. FIGS. 2, 3, and 4 are SEM images of the water-absorbent resin particles of Comparative Examples 1, 2, and 3, respectively. The powder of water-absorbent resin particles of the Examples contained spherical primary particles, and no formation of chain-like coarse particles due to aggregation or linking of primary particles was observed. From these results, it was confirmed that the formation of chain-like coarse particles could be suppressed by adjusting the amount of the alcohol compound relative to the amount of the (meth)acrylic acid compound.

[0063]

Claims

1. A method for producing water-absorbent resin particles, comprising: preparing an aqueous liquid containing a monomer containing at least one (meth)acrylic acid compound of (meth)acrylic acid or a salt thereof, an alcohol compound having 1 to 10 carbon atoms, and water; forming a particulate hydrogel polymer containing a polymer of the monomer and water by reverse phase suspension polymerization in a reaction liquid containing the aqueous liquid and a dispersion medium; and extracting water-absorbent resin particles containing the polymer from the reaction liquid, wherein the aqueous liquid containing the alcohol compound in an amount of 800 ppm by mass or less relative to the amount of the (meth)acrylic acid compound is prepared, and when the aqueous liquid contains a 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 the (meth)acrylic acid equivalent to the substance amount of the salt.

2. The method according to claim 1, further comprising preparing a (meth)acrylic acid raw material that contains (meth)acrylic acid and an alcohol compound having 1 to 10 carbon atoms, the amount of the alcohol compound being 800 mass ppm or less relative to the amount of the (meth)acrylic acid, and the aqueous liquid is prepared from the (meth)acrylic acid raw material.

3. The method of claim 1, wherein said water-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 the group consisting of isononyl alcohol, 1-butanol, and isopropyl alcohol.

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