Method for producing 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 water-soluble ethylenically unsaturated monomer, water, and a radical polymerization initiator; forming a reaction liquid that contains the aqueous liquid, a dispersion medium, and a dispersion stabilizer in a polymerization tank; introducing an inert gas into the polymerization tank; forming a polymer of the water-soluble ethylenically unsaturated monomer by means of reversed phase suspension polymerization in the reaction liquid; and obtaining polymer particles that contain the polymer from the reaction liquid. The temperature of the reaction liquid at the time when the introduction of the inert gas into the polymerization tank is started is 47°C to 70°C inclusive.

Description

Method for producing water-absorbent resin particles

[0001] The present disclosure relates to a method for producing water-absorbent resin particles.

[0002] Water-absorbent resin particles are produced by various polymerization methods such as reverse-phase suspension polymerization, aqueous solution polymerization, etc. Products of water-absorbent resin particles are widely used in various fields such as sanitary 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. In these applications, it is generally required that the proportion of coarse particles in the water-absorbent resin particles is small (for example, Patent Document 1).

[0003] International Publication No. 2012 / 081355

[0004] The present disclosure relates to a method for producing water-absorbent resin particles by reversed-phase suspension polymerization, among methods for producing water-absorbent resin particles, in which the proportion of coarse particles in the obtained water-absorbent resin particles is reduced.

[0005] The present disclosure includes the following: [1] A method for producing water-absorbent resin particles, comprising: preparing an aqueous liquid containing a water-soluble ethylenically unsaturated monomer, water, and a radical polymerization initiator; forming a reaction liquid containing the aqueous liquid, a dispersion medium, and a dispersion stabilizer in a polymerization tank; introducing an inert gas into the polymerization tank; forming a polymer of the water-soluble ethylenically unsaturated monomer by reverse-phase suspension polymerization in the reaction liquid; and obtaining polymer particles containing the polymer from the reaction liquid, wherein the temperature of the reaction liquid at the time when introduction of the inert gas into the polymerization tank is started is 47° C. or higher and 70° C. or lower. [2] The method according to [1], wherein the 10-hour half-life temperature of the radical polymerization initiator is 40° C. or higher and 80° C. or lower.

[0006] In the method for producing water-absorbent resin particles by reversed-phase suspension polymerization, the proportion of coarse particles in the obtained water-absorbent resin particles can be reduced.

[0007] FIG. 1 is a schematic diagram showing an example of a reaction apparatus for reversed-phase suspension polymerization.

[0008] The present invention is not limited to the following examples: In this specification, (meth)acrylic acid means acrylic acid, methacrylic acid, and combinations thereof.

[0009] An example of a method for producing water-absorbent resin particles includes preparing an aqueous liquid containing a water-soluble ethylenically unsaturated monomer, water, and a radical polymerization initiator, forming a reaction liquid containing the aqueous liquid, a dispersion medium, and a dispersion stabilizer in a polymerization tank, introducing an inert gas into the polymerization tank, forming a polymer of the water-soluble ethylenically unsaturated monomer by reverse phase suspension polymerization in the reaction liquid, and obtaining polymer particles containing the polymer from the reaction liquid.

[0010] The reaction liquid can be formed, for example, by a method including stirring a mixed liquid containing an oily liquid containing a dispersion medium and an aqueous liquid in a polymerization tank. Before the introduction of an inert gas into the polymerization tank is started, a reaction liquid is usually formed as a suspension in which a particulate aqueous liquid is dispersed in an oily liquid containing a dispersion medium. The reaction liquid may be formed in the polymerization tank under atmospheric conditions. The polymerization tank is a container having a reaction chamber that can accommodate the reaction liquid and that can introduce an inert gas into the polymerization tank, and its shape, size, etc. can be selected as appropriate.

[0011] The temperature of the reaction liquid at the time when the introduction of the inert gas into the polymerization tank is started is 47° C. or higher and 70° C. or lower. When the temperature of the reaction liquid at the time when the introduction of the inert gas into the polymerization tank is started is within this range, it is considered that the reversed-phase suspension polymerization easily proceeds in a state where the dispersion of the aqueous liquid in the reaction liquid is stable, and therefore water-absorbent resin particles with a small proportion of coarse particles can be stably produced. For example, the proportion of coarse particles having a particle diameter of 850 μm or higher can be reduced. From a similar viewpoint, the temperature of the reaction liquid at the time when the introduction of the inert gas is started may be 48°C or more and 70°C or less, or 49°C or more and 70°C or less, or 47°C or more and 68°C or less, or 48°C or more and 68°C or less, or 47°C or more and 66°C or less, or 48°C or more and 66°C or less, or 49°C or more and 66°C or less, or 47°C or more and 65°C or less, or 48°C or more and 65°C or less, or 47°C or more and 64°C or less, or 48°C or more and 64°C or less, or 49°C or more and 64°C or less.

[0012] By introducing an inert gas into a polymerization tank or its reaction chamber, at least a portion of the dissolved oxygen in the reaction solution is replaced with the inert gas, thereby facilitating the progress of reverse suspension polymerization. To introduce the inert gas into the polymerization tank, the inert gas may be introduced toward the gas phase in the polymerization tank or toward the reaction solution. In this case, the "point at which the introduction of the inert gas into the polymerization tank begins" refers to the point at which the inert gas begins to be introduced toward the gas phase in the polymerization tank or toward the reaction solution. From the perspective of efficient replacement of dissolved oxygen in the reaction solution, the inert gas may be introduced toward the reaction solution. The inert gas may be introduced while stirring the reaction solution. Examples of methods for introducing the inert gas into the reaction solution include a method of blowing the inert gas into the reaction solution through a gas inlet tube inserted into the reaction solution, a method of blowing the inert gas into the reaction solution through a hole provided in the polymerization tank, and a method of degassing the polymerization tank containing the reaction solution under reduced pressure and then introducing the inert gas into the reaction solution. Examples of the method for introducing the inert gas toward the gas phase portion in the polymerization tank include a method of blowing the inert gas toward the gas phase portion from a gas inlet pipe inserted into the gas phase portion in the polymerization tank, and a method of degassing the inside of the polymerization tank containing the reaction liquid under reduced pressure and then introducing the inert gas toward the gas phase portion.

[0013] The temperature of the reaction liquid before the inert gas is introduced may be lowered from a temperature higher than 70°C to 47°C or higher and 70°C or lower. The temperature of the reaction liquid before the inert gas is introduced may be raised from a temperature lower than 47°C to 47°C or higher and 70°C or lower. In order to adjust the temperature of the reaction liquid, the polymerization tank containing the reaction liquid may be heated or cooled using a water bath or a jacket provided around the polymerization tank. During the polymerization reaction of the water-soluble ethylenically unsaturated monomer by reversed-phase suspension polymerization, the polymerization tank containing the reaction liquid may be maintained heated or cooled. During the polymerization reaction of the water-soluble ethylenically unsaturated monomer by reversed-phase suspension polymerization, the temperature of the reaction liquid may be within the range of 47°C or higher and 90°C or lower.

[0014] FIG. 1 is a schematic diagram showing an example of a reaction apparatus for reversed-phase suspension polymerization. The reaction apparatus 100 shown in FIG. 1 includes a polymerization vessel 105 (a round-bottomed cylindrical separable flask), a stirrer 170, a stirring shaft 180, a stirring blade 120 (four-blade inclined paddle blade), a gas inlet pipe 130, a reflux condenser 140, and a thermometer 160. A reaction solution 110 is formed in the polymerization vessel 105. The stirring shaft 180 is attached to the stirrer 170, and the stirring blade 120 is attached to the tip of the stirring shaft 180. The gas inlet pipe 130, the stirring blade 120, and the thermometer 160 are inserted into the reaction solution 110. The reflux condenser 140 has a gas outlet pipe 150 and is attached to the polymerization vessel 105. By blowing an inert gas into the reaction solution 110 through the gas inlet pipe 130, at least a portion of the dissolved oxygen in the reaction solution 110 is replaced with the inert gas.

[0015] The inert gas may include, for example, one or more selected from a noble gas, nitrogen gas, and carbon dioxide gas. Examples of the noble gas include helium, neon, argon, and krypton. From the viewpoint of industrial availability and economy, the inert gas may include nitrogen gas.

[0016] During the polymerization reaction, the reaction solution is usually stirred. The stirring speed during the polymerization reaction may be, for example, 10 rpm or more and 1000 rpm or less, or 200 rpm or more and 1000 rpm or less. The polymerization reaction time may be, for example, 10 minutes or more and 240 minutes or less.

[0017] The radical polymerization initiator contained in the aqueous liquid may include, for example, an azo compound, a peroxide, or a combination thereof.

[0018] 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-methylpropionamidine)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-imidazoline-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, pan} dihydrochloride, 2,2'-azobis(2-methylpropionamide) dihydrochloride, 2,2'-azobis[2-(2-imidazolin-2-yl)propane] disulfate dihydrate, 2,2'-azobis[N-(2-carboxyethyl)-2-methylpropionamidine] tetrahydrate, and 2,2'-azobis[2-methyl-N-(2-hydroxyethyl)propionamide].

[0019] 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.

[0020] The 10-hour half-life temperature of the radical polymerization initiator contained in the aqueous liquid may be 40°C or higher and 80°C or lower. When the 10-hour half-life temperature of the radical polymerization initiator is within this range, the polymerization reaction tends to proceed efficiently when introduction of the inert gas is initiated at a temperature of 47°C or higher and 70°C or lower. From a similar viewpoint, the 10-hour half-life temperature of the radical polymerization initiator may be 45°C or higher and 80°C or lower, or 50°C or higher and 80°C or lower, or 40°C or higher and 75°C or lower, or 45°C or higher and 75°C or lower, or 50°C or higher and 75°C or lower, or 40°C or higher and 70°C or lower, or 45°C or higher and 70°C or lower, or 50°C or higher and 70°C or lower.

[0021] The radical polymerization initiator having a 10-hour half-life temperature of 40°C or higher and 80°C or lower may contain one or more compounds selected from 2,2'-azobis(2-methylpropionamidine) dihydrochloride, 2,2'-azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride, potassium persulfate, ammonium persulfate, and sodium persulfate.

[0022] The amount of the radical polymerization initiator may be, for example, 0.0001 mol or more and 1 mol or less, 0.001 mol or more and 0.1 mol or less, 0.005 mol or more and 0.08 mol or less, or 0.01 mol or more and 0.05 mol or less, relative to 100 mol of the water-soluble ethylenically unsaturated monomer.

[0023] The water-soluble ethylenically unsaturated monomer may include, for example, at least one selected from the group consisting of (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 salt of (meth)acrylic acid and 2-(meth)acrylamido-2-methylpropanesulfonic acid may be, for example, an alkali metal salt. The alkali metal salt of (meth)acrylic acid and 2-(meth)acrylamido-2-methylpropanesulfonic acid may be, for example, a sodium salt.

[0024] The water-soluble ethylenically unsaturated monomer may contain (meth)acrylic acid and an alkali metal salt of (meth)acrylic acid. In this case, the total proportion of (meth)acrylic acid and its alkali metal salt based on the total amount of the water-soluble ethylenically unsaturated monomer 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%.

[0025] The aqueous liquid may contain an internal crosslinking agent that crosslinks the polymer of the water-soluble ethylenically unsaturated 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 water-soluble ethylenically unsaturated monomer. 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.

[0026] 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.

[0027] 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.

[0028] 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.

[0029] Examples of internal crosslinking agents having two or more isocyanate groups include 2,4-tolylene diisocyanate and hexamethylene diisocyanate.

[0030] 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, relative to 1 mole of the water-soluble ethylenically unsaturated monomer.

[0031] The aqueous liquid may contain, as other components, a thickener, a hydrophilic polymer dispersant, a chain transfer agent, a foaming agent, or a combination thereof.

[0032] 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.

[0033] 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 water-soluble ethylenically unsaturated monomer.

[0034] 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.

[0035] 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 water-soluble ethylenically unsaturated monomer.

[0036] Examples of chain transfer agents include hypophosphites, thiols, thiolic acids, secondary alcohols, and amines.

[0037] 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.

[0038] The dispersion stabilizer in the reaction solution is a component for stabilizing the dispersion of the aqueous liquid in the reversed-phase suspension, and is usually contained in the oily liquid. The dispersion stabilizer may be dissolved in a dispersion medium in the oily liquid. The dispersion stabilizer may include a surfactant, a hydrophobic polymer dispersant, or a combination thereof.

[0039] The surfactant may be a nonionic surfactant, an anionic surfactant, or a combination thereof. The HLB of the surfactant may be 1 or more and 16 or less, 2 or more and 12 or less, or 3 or more and 10.5 or less. The surfactant may be used alone or in combination of two or more.

[0040] 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.

[0041] The surfactant may include one or more selected from a sorbitan fatty acid ester, a polyglycerin fatty acid ester, and a sucrose fatty acid ester. These surfactants can further reduce the proportion of coarse particles in the water-absorbent resin particles.

[0042] The amount of the surfactant 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 water-soluble ethylenically unsaturated monomer.

[0043] The hydrophobic polymer dispersant introduced into the reaction liquid may be dissolved in the dispersion medium in the oily liquid. 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 dispersant may be used alone or in combination of two or more.

[0044] 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 water-soluble ethylenically unsaturated monomer.

[0045] The amount of the dispersion stabilizer in the reaction liquid, or the total amount of the surfactant and the hydrophobic polymer dispersant, may be 0.1 parts by mass or more and 30 parts by mass or less per 100 parts by mass of the water-soluble ethylenically unsaturated monomer.

[0046] The dispersion medium may be a hydrocarbon. Examples of hydrocarbons that can be used as the dispersion medium 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 hydrocarbons may be used alone or in combination of two or more.

[0047] The amount of the dispersion medium 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 water-soluble ethylenically unsaturated monomer.

[0048] As the polymerization reaction progresses, a particulate hydrogel polymer containing a polymer is usually formed in the reaction liquid. Polymer particles containing a polymer can be obtained as water-absorbent resin particles from the reaction liquid containing the hydrogel polymer. Obtaining polymer 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 dispersion medium and water from the concentrate. After removing the dispersion medium and water by evaporation, a powder of dried polymer particles can be obtained. A certain amount of water may remain in the dried polymer particles.

[0049] After a portion of the water is extracted from the reaction solution by azeotropic distillation of the dispersion medium and water, the polymer particles may be surface-crosslinked in a mixture containing a concentrate containing polymer particles containing a polymer and a surface crosslinking agent. Various additives may be further added to the dried polymer particles. Examples of the additive include a lubricant (e.g., silica particles), a metal chelating agent, a surface modifier, a heat resistance stabilizer, an antioxidant, and an antibacterial agent. In this specification, water-absorbent resin particles refer to particles containing polymer particles. The water-absorbent resin particles may contain polymer particles and an additive. 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.

[0050] In the powder of dried polymer particles (water-absorbent resin particles), the ratio of particles having a particle diameter of 850 μm or more may be 4 mass % or less, 3 mass % or less, 2 mass % or less, or 1 mass % or less with respect to the total amount of water-absorbent resin particles. The ratio here means the ratio after drying and before the powder is classified by sieving or the like. The method for determining the ratio of particles having a particle diameter of 850 μm or more is to calculate the mass W of the water-absorbent resin particles. ０ g of powder is sieved through a sieve with an opening of 850 μm, and the mass W of the particles remaining on the sieve１ g and the ratio of particles having a particle diameter of 850 μm or more = (W １ / W ０ ) × 100 to calculate the proportion of particles having a particle diameter of 850 μm or more. Particles having a particle diameter of 850 μm or more may be excluded from the product by classification. A small proportion of particles having a particle diameter of 850 μm or more in the powder after polymerization and before classification is advantageous in terms of effective utilization of raw materials for producing (meth)acrylic acid, etc.

[0051] The median particle diameter of the water-absorbent resin particles may be, for example, 50 μm or more and 850 μm or less. The median particle diameter of the water-absorbent resin particles may be 50 μm or more and 800 μm or less, 600 μm or less, 500 μm or less, 400 μm or less, 350 μm, 300 μm or less, 250 μm or less, or 200 μm or less. The median particle diameter of the water-absorbent resin particles can be measured using a JIS standard sieve, as described in the examples described later.

[0052] The present invention is not limited to the following examples.

[0053] 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 gas inlet tube, and a stirrer. A stirring blade with two stages of 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 maleic anhydride-modified ethylene-propylene copolymer. After stirring was completed, the n-heptane solution was cooled to 56°C.

[0054] 74.06 g (1.03 mol) of acrylic acid was placed in a 300 mL beaker. While the beaker was cooled with ice water, 102.8 g of a 30% by mass aqueous solution of sodium hydroxide was added dropwise to the acrylic acid to prepare a 75 mol% neutralized product of acrylic acid. Next, 69.0 g of water, 0.0552 g (0.204 mmol) of 2,2'-azobis(2-methylpropionamidine) dihydrochloride as a radical polymerization initiator, and 0.0102 g (0.059 mmol) of ethylene glycol diglycidyl ether as an internal crosslinking agent were added to the beaker to prepare an aqueous solution containing acrylic acid and sodium acrylate.

[0055] The prepared aqueous solution was added to the n-heptane solution containing 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 (surfactant, Mitsubishi Chemical Foods Corporation, Ryoto Sugar Ester S-370, HLB: 3) and 6.62 g of n-heptane was added to the mixture. The separable flask was immersed in a 50°C water bath, and the reaction solution formed in the separable flask was stirred with a stirrer at 300 rpm. After the temperature of the reaction solution stabilized at 49°C, nitrogen gas introduction was initiated by blowing nitrogen gas as an inert gas into the reaction solution through a gas inlet tube inserted into the reaction solution while maintaining stirring, as shown in Figure 1. The introduction of nitrogen gas sufficiently replaced the atmosphere in the separable flask, initiating the polymerization reaction of acrylic acid and sodium acrylate. The polymerization reaction was allowed to proceed for 90 minutes from the start of nitrogen gas introduction while maintaining stirring, thereby forming a slurry containing particulate hydrogel polymer.

[0056] The separable flask containing the slurry was removed from the 50°C water bath and immersed in an oil bath at 125°C. While refluxing the n-heptane contained in the slurry in the separable flask, 127.4 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, thereby obtaining 92.5 g of dried polymer particles as water-absorbent resin particles.

[0057] Example 2

[0123] Water absorbent resin particles of Example 2 were obtained under the same conditions as in Example 1, except that the separable flask containing the reaction liquid was immersed in a water bath at 55°C, and the introduction of nitrogen gas was started when the temperature of the reaction liquid was stabilized at 54°C.

[0058] Example 3

[0123] Water absorbent resin particles of Example 3 were obtained under the same conditions as in Example 1, except that the separable flask containing the reaction liquid was immersed in a water bath at 60°C, and the introduction of nitrogen gas was started when the temperature of the reaction liquid was stable at 58°C.

[0059] Example 4

[0123] Water absorbent resin particles of Example 4 were obtained under the same conditions as in Example 1, except that the separable flask containing the reaction liquid was immersed in a water bath at 65°C, and the introduction of nitrogen gas was started when the temperature of the reaction liquid was stabilized at 63°C.

[0060] Example 5 Water absorbent resin particles of Example 5 were obtained under the same conditions as in Example 1, except that the amount of 2,2′-azobis(2-methylpropionamidine) dihydrochloride was changed to 0.1104 g (0.407 mmol), and the separable flask containing the reaction solution was immersed in a water bath at 60°C, and the introduction of nitrogen gas was started when the temperature of the reaction solution was stabilized at 59°C.

[0061] Example 6

[0123] Water absorbent resin particles of Example 6 were obtained under the same conditions as those of Example 1, except that the radical polymerization initiator was changed to 0.0368 g (0.136 mmol) of potassium persulfate, and the separable flask containing the reaction liquid was immersed in a water bath at 65°C, and the introduction of nitrogen gas was started in a state where the temperature of the reaction liquid was stabilized at 64°C.

[0062] Example 7 Water-absorbent resin particles of Example 7 were obtained under the same conditions as in Example 6, except that the amount of potassium persulfate was changed to 0.0736 g (0.272 mmol).

[0063] Comparative Example 1 Water absorbent resin particles of Comparative Example 1 were obtained under the same conditions as in Example 1, except that the separable flask containing the reaction liquid was immersed in a water bath at 45°C, and the introduction of nitrogen gas was started when the temperature of the reaction liquid was stabilized at 44°C.

[0064] 2. Evaluation of Water-Absorbent Resin Particles (1) Proportion of Particles with a Particle Diameter of 850 μm or More The proportion of particles with a particle diameter of 850 μm or more was measured in an environment of room temperature (25±2°C) and humidity of 50±10% by the following procedure. First, the powder of water-absorbent resin particles obtained in Examples or Comparative Examples was sieved with a sieve with a mesh size of 850 μm. The mass of the water-absorbent resin particles remaining on the sieve and having a particle diameter of 850 μm or more was measured, and the proportion (mass%) of particles having a particle diameter of 850 μm or more to the total amount of water-absorbent resin particles was calculated.

[0065] (2) Median Particle Diameter The median particle diameter was measured in an environment of room temperature (25±2°C) and humidity of 50±10% according to the following procedure. First, the JIS standard sieves were arranged in the following order from top to bottom: a 500 μm mesh sieve, a 425 μm mesh sieve, a 250 μm mesh sieve, a 180 μm mesh sieve, a 150 μm mesh sieve, a 106 μm mesh sieve, a 75 μm mesh sieve, and a tray. 5 g of water-absorbent resin particles that had passed through a 850 μm mesh sieve were placed on the top sieve, and the water-absorbent resin particles were classified by shaking for 5 minutes using a continuous fully automatic ultrasonic vibration sieving measuring instrument (Robot Sifter RPS-02, manufactured by Seishin Enterprise Co., Ltd.). After classification, the proportion (mass percentage) of the water-absorbent resin particles remaining on each sieve relative to the total amount of water-absorbent resin particles was calculated. By integrating the proportions of fractions in order from the largest particle diameter, the relationship between the sieve openings and the integrated values ​​of the proportions of water absorbent resin particles remaining on the sieve was plotted on a logarithmic probability paper. By connecting the plots on the probability paper with a straight line, a particle diameter corresponding to an integrated mass percentage of 50 mass% was determined, and this value was taken as the median particle diameter.

[0066] The evaluation results are shown in Table 1. The amount of radical polymerization initiator shown in Table 1 is the ratio (mol %) to the amount of monomer. The types of radical polymerization initiators shown in Table 1 are as follows: (i) 2,2'-azobis(2-methylpropionamidine) dihydrochloride (ii) potassium persulfate

[0067]

[0068] As shown in Table 1, it was confirmed that when the temperature of the reaction liquid at the time the introduction of the inert gas began was 47°C or higher and 70°C or lower, the proportion of particles having a particle diameter of 850 μm or more in the obtained water-absorbing resin particles was reduced.

[0069] 100: Reaction apparatus, 105: Polymerization tank (separable flask), 110: Reaction liquid, 120: Stirring blade, 130: Gas inlet pipe, 140: Reflux condenser, 150: Gas outlet pipe, 160: Thermometer, 170: Stirrer, 180: Stirring shaft

Claims

1. A method for producing water-absorbent resin particles, comprising: preparing an aqueous liquid containing a water-soluble ethylenically unsaturated monomer, water, and a radical polymerization initiator; forming a reaction liquid containing the aqueous liquid, a dispersion medium, and a dispersion stabilizer in a polymerization tank; introducing an inert gas into the polymerization tank; forming a polymer of the water-soluble ethylenically unsaturated monomer by reverse phase suspension polymerization in the reaction liquid; and obtaining polymer particles containing the polymer from the reaction liquid, wherein the temperature of the reaction liquid at the time when introduction of the inert gas into the polymerization tank is started is 47°C or more and 70°C or less.

2. The method according to claim 1, wherein the 10-hour half-life temperature of the radical polymerization initiator is 40°C or higher and 80°C or lower.

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