Method for manufacturing water-absorbing resin particles

Abstract

This invention discloses a method for manufacturing water-absorbing resin particles. The method includes the following steps: preparing an aqueous liquid containing a water-soluble olefinic unsaturated monomer, water, and a free radical polymerization initiator; forming a reaction solution 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 olefinic unsaturated monomer through reverse suspension polymerization in the reaction solution; and obtaining polymer particles containing the polymer from the reaction solution. The temperature of the reaction solution at the moment the inert gas is introduced into the polymerization tank is above 47°C and below 70°C.

Description

Technical Field

[0001] This disclosure relates to a method for manufacturing water-absorbing resin particles. Background Technology

[0002] Superabsorbent polymer (SAP) particles are manufactured using various polymerization methods, including reverse suspension polymerization and aqueous solution polymerization. SAP products are widely used in various fields, such as hygiene materials (diapers, sanitary products), water-retaining agents, horticultural materials (soil conditioners), waterproofing materials, and industrial materials (anti-condensation agents). In these applications, a low proportion of coarse particles in the SAP particles is typically required (e.g., Patent Document 1).

[0003] Previous technical documents Patent documents Patent Document 1: International Publication No. 2012 / 081355 Summary of the Invention

[0004] The technical problem to be solved by the invention This disclosure relates to a step in a method for manufacturing superabsorbent polymer particles based on reverse suspension polymerization to reduce the proportion of coarse particles in the obtained superabsorbent polymer particles.

[0005] means for solving technical problems This disclosure includes the following. [1] A method for manufacturing water-absorbing resin particles, the method comprising the following steps: Prepare an aqueous solution comprising a water-soluble olefinic unsaturated monomer, water, and a free radical polymerization initiator; A reaction solution containing the aqueous liquid, dispersion medium, and dispersion stabilizer is formed in the polymerization tank; Inert gas is introduced into the polymerization tank; The polymer of the water-soluble olefinic unsaturated monomer is formed by reverse-phase suspension polymerization in the reaction solution; and Polymer particles containing the polymer are obtained from the reaction solution. The temperature of the reaction solution at the moment when the inert gas is introduced into the polymerization tank is above 47°C and below 70°C. [2] According to the method described in [1], wherein, The free radical polymerization initiator has a 10-hour half-life temperature above 40°C and below 80°C.

[0008] Invention Effects In the method for manufacturing superabsorbent resin particles based on reverse suspension polymerization, the proportion of coarse particles in the obtained superabsorbent resin particles can be reduced. Attached Figure Description

[0009] Figure 1 This is a schematic diagram illustrating an example of a reaction apparatus used for reverse suspension polymerization. Detailed Implementation

[0010] This 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 manufacturing water-absorbing resin particles includes the following steps: preparing an aqueous liquid comprising a water-soluble olefinic unsaturated monomer, water, and a free radical polymerization initiator; forming a reaction solution comprising 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 olefinic unsaturated monomer by reverse suspension polymerization in the reaction solution; and obtaining polymer particles comprising the polymer from the reaction solution.

[0012] The reaction solution can be formed, for example, by stirring a mixture of an oily liquid and an aqueous liquid containing a dispersion medium in a polymerization tank. Before introducing an inert gas into the polymerization tank, a suspension of particulate aqueous liquid, i.e., the reaction solution, is typically formed in the oily liquid containing the dispersion medium. The reaction solution can be formed in the polymerization tank under atmospheric conditions. The polymerization tank is a container with a reaction chamber capable of containing the reaction solution and allowing the introduction of an inert gas; its shape and size can be appropriately selected.

[0013] The temperature of the reaction solution at the moment the inert gas is introduced into the polymerization tank is above 47°C and below 70°C. It is believed that if the temperature of the reaction solution at the moment the inert gas is introduced into the polymerization tank is within this range, reverse suspension polymerization can be easily carried out under a stable dispersion state of the aqueous liquid in the reaction solution, thus enabling the stable production of water-absorbing resin particles with a low proportion of coarse particles. For example, it is possible to reduce the proportion of coarse particles with a particle size of 850 μm or more. From the same perspective, the temperature of the reaction liquid at the moment when the inert gas is introduced can be above 48°C and below 70°C or above 49°C and below 70°C, or above 47°C and below 68°C, or above 48°C and below 68°C or above 49°C and below 68°C, or above 47°C and below 66°C, or above 48°C and below 66°C or above 49°C and below 66°C, or above 47°C and below 65°C, or above 48°C and below 65°C or above 49°C and below 65°C, or above 47°C and below 64°C, or above 48°C and below 64°C or above 49°C and below 64°C.

[0014] By introducing an inert gas into the polymerization tank or its reaction chamber, at least a portion of the dissolved oxygen in the reaction liquid is replaced by the inert gas, thereby creating a state conducive to reverse-phase suspension polymerization. To introduce the inert gas into the polymerization tank, it can be introduced towards the gas phase portion of the polymerization tank or towards the reaction liquid. In this case, "the moment when the inert gas is introduced into the polymerization tank" refers to the moment when the inert gas is introduced towards the gas phase portion of the polymerization tank or towards the reaction liquid. From the viewpoint of effective replacement of dissolved oxygen in the reaction liquid, the inert gas can be introduced towards the reaction liquid. The reaction liquid can be stirred while introducing the inert gas. Examples of methods for introducing the inert gas into the reaction liquid include: blowing the inert gas into the reaction liquid through a gas inlet pipe inserted into the reaction liquid; blowing the inert gas into the reaction liquid through a hole provided in the polymerization tank; and introducing the inert gas into the reaction liquid after degassing the polymerization tank containing the reaction liquid. Examples of methods for introducing inert gas into the gas phase portion of the polymerization tank include: a method of blowing inert gas into the gas phase portion from a gas inlet pipe inserted into the gas phase portion of the polymerization tank; and a method of introducing inert gas into the gas phase portion after degassing the polymerization tank containing the reaction liquid.

[0015] The temperature of the reaction liquid before the introduction of inert gas can be lowered from above 70°C to above 47°C and below 70°C. The temperature of the reaction liquid before the introduction of inert gas can be raised from below 47°C to above 47°C and below 70°C. To adjust the temperature of the reaction liquid, the polymerization tank containing the reaction liquid can be heated or cooled using a water bath or a jacket installed around the polymerization tank. During the polymerization reaction of water-soluble olefinic unsaturated monomers based on reverse-phase suspension polymerization, the heating or cooling of the polymerization tank containing the reaction liquid can be maintained. During the polymerization reaction of water-soluble olefinic unsaturated monomers based on reverse-phase suspension polymerization, the temperature of the reaction liquid can be in the range of above 47°C and below 90°C.

[0016] Figure 1 This is a schematic diagram illustrating an example of a reaction apparatus used for reverse suspension polymerization. Figure 1The shown reaction apparatus 100 includes a polymerization tank 105 (a round-bottomed, cylindrical, separable flask), a stirrer 170, a stirring shaft 180, stirring blades 120 (four inclined blades), a gas inlet pipe 130, a reflux condenser 140, and a thermometer 160. A reaction liquid 110 is formed within the polymerization tank 105. The stirring shaft 180 is mounted on the stirrer 170, and stirring blades 120 are positioned at the front end of the stirring shaft 180. The gas inlet pipe 130, stirring blades 120, and thermometer 160 are inserted into the reaction liquid 110. The reflux condenser 140, having a gas exhaust pipe 150, is installed in the polymerization tank 105. By blowing inert gas into the reaction liquid 110 from the gas inlet pipe 130, at least a portion of the dissolved oxygen in the reaction liquid 110 is replaced by the inert gas.

[0017] Inert gases may include, for example, one or more selected from rare gases, nitrogen, and carbon dioxide. Examples of rare gases include helium, neon, argon, and krypton. From the viewpoint of industrial availability and economy, inert gases may include nitrogen.

[0018] During the polymerization reaction, the reaction liquid is typically stirred. The stirring speed during the polymerization reaction can be, for example, 10 rpm or more and 1000 rpm or 200 rpm or more and 1000 rpm. The polymerization reaction time can be, for example, 10 minutes or more and 240 minutes or less.

[0019] The free radical polymerization initiators contained in the aqueous solution may include, for example, azo compounds, peroxides, or combinations thereof.

[0020] 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-methylpropamidinyl) 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].

[0021] 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.

[0022] The 10-hour half-life temperature of the free radical polymerization initiator contained in the aqueous solution can be above 40°C and below 80°C. If the 10-hour half-life temperature of the free radical polymerization initiator is within this range, the polymerization reaction tends to proceed effectively when an inert gas is introduced at a temperature above 47°C and below 70°C. From the same perspective, the 10-hour half-life temperature of the free radical polymerization initiator can be above 45°C and below 80°C or above 50°C and below 80°C, or it can be above 40°C and below 75°C, above 45°C and below 75°C, or above 50°C and below 75°C, or it can be above 40°C and below 70°C, above 45°C and below 70°C, or above 50°C and below 70°C.

[0023] Free radical polymerization initiators with 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-methylpropanediamine) dihydrochloride, 2,2'-azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride, potassium persulfate, ammonium persulfate, and sodium persulfate.

[0024] The amount of free radical polymerization initiator relative to 100 moles of water-soluble olefinic unsaturated monomer can be, for example, more than 0.0001 moles and less than 1 mole, more than 0.001 moles and less than 0.1 moles, more than 0.005 moles and less than 0.08 moles, or more than 0.01 moles and less than 0.05 moles.

[0025] The water-soluble olefinic unsaturated monomer may, for example, comprise at least one selected from the group consisting of (meth)acrylic acid and its salts, 2-(meth)acrylamide-2-methylpropanesulfonic acid and its salts, (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. The salts of (meth)acrylic acid and 2-(meth)acrylamide-2-methylpropanesulfonic acid may, for example, be alkali metal salts. The alkali metal salts of (meth)acrylic acid and 2-(meth)acrylamide-2-methylpropanesulfonic acid may, for example, be sodium salts.

[0026] The water-soluble olefinic unsaturated monomer may include (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 olefinic 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, and may also be substantially 100 mol%.

[0027] The aqueous solution may contain an internal crosslinking agent that crosslinks a polymer of a water-soluble olefinic unsaturated monomer formed by a polymerization reaction. The internal crosslinking agent may be a compound having two or more reactive functional groups that are reactive with the water-soluble olefinic unsaturated monomer. Reactive functional groups may, for example, be (meth)acryloyl, vinyl, epoxy, halogenated groups in halogenated epoxy compounds, isocyanate groups, or combinations thereof.

[0028] 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)urethane compounds formed from polyisocyanate compounds (toluene diisocyanate, hexamethylene diisocyanate, etc.) and (meth)acrylate. The polyol compound used to form the (meth)acrylate compound or unsaturated polyester may, for example, be ethylene glycol, propylene glycol, trimethylolpropane, glycerol, polyoxyethylene glycol, polyoxypropylene glycol, polyglycerol, or combinations thereof.

[0029] 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.

[0030] 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.

[0031] Examples of internal crosslinking agents having two or more isocyanate groups include 2,4-toluene diisocyanate and hexamethylene diisocyanate.

[0032] The amount of the internal crosslinking agent relative to 1 mole of the water-soluble olefinic unsaturated 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.

[0033] Aqueous liquids may contain thickeners, hydrophilic polymeric dispersants, chain transfer agents, foaming agents, or combinations thereof as other components.

[0034] 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.

[0035] The amount of thickener relative to 100 parts by mass of water-soluble olefinic unsaturated 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.

[0036] Examples of hydrophilic polymeric dispersants include polyvinyl alcohol (PVA), polyvinylpyrrolidone (PVP), polyethylene glycol (PEG), polypropylene glycol, polyethylene glycol-polypropylene glycol block copolymers, polyglycerol, 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 types.

[0037] The amount of hydrophilic polymeric dispersant relative to 100 parts by mass of water-soluble olefinic unsaturated monomer 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.

[0038] Examples of chain transfer agents include hypophosphites, thiols, thiolic acids, secondary alcohols, and amines.

[0039] 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.

[0040] The dispersant stabilizer in the reaction solution is a component used to stabilize the dispersion of the aqueous liquid in the reversed-phase suspension, and is usually contained in the oily liquid. The dispersant stabilizer can be dissolved in the dispersion medium of the oily liquid. The dispersant stabilizer may contain surfactants, hydrophobic polymeric dispersants, or combinations thereof.

[0041] Surfactants can be nonionic surfactants, anionic surfactants, or combinations thereof. The HLB value of a surfactant can be 1 or higher and less than 16, 2 or higher and less than 12, or 3 or higher and less than 10.5. A surfactant can be a single type or a combination of two or more types.

[0042] Examples of nonionic surfactants include sorbitol fatty acid esters, polyglycerol fatty acid esters, sucrose fatty acid esters, polyoxyethylene sorbitol fatty acid esters, polyoxyethylene glycerol fatty acid esters, sorbitol fatty acid esters, polyoxyethylene sorbitol 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, alkylmethyl taurate, polyoxyethylene alkylphenyl ether sulfate salts, polyoxyethylene alkyl ether sulfonates, phosphate esters of polyoxyethylene alkyl ethers, and phosphate esters of polyoxyethylene alkylallyl ethers.

[0043] The surfactant may contain one or more selected from sorbitol fatty acid esters, polyglycerol fatty acid esters, and sucrose fatty acid esters. These surfactants can further reduce the proportion of coarse particles in the absorbent resin particles.

[0044] The amount of surfactant relative to 100 parts by mass of water-soluble olefinic unsaturated 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.

[0045] The hydrophobic polymeric dispersant introduced into the reaction solution can dissolve in the dispersion medium of the oily liquid. 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. The hydrophobic polymeric dispersant can be a single type or a combination of two or more types.

[0046] The amount of hydrophobic polymeric dispersant relative to 100 parts by mass of water-soluble olefinic unsaturated 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.

[0047] The amount of dispersant stabilizer or the total amount of surfactant and hydrophobic polymeric dispersant in the reaction solution can be more than 0.1 parts by mass and less than 30 parts by mass relative to 100 parts by mass of water-soluble olefinic unsaturated monomer.

[0048] The dispersion medium can be a hydrocarbon. Examples of hydrocarbons that can be used as a 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 hydrocarbon can be a single species or a combination of two or more species.

[0049] The amount of dispersion medium relative to 100 parts by mass of water-soluble olefinic unsaturated monomer 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.

[0050] As the polymerization reaction proceeds, a particulate hydrogel polymer containing the polymer is typically formed in the reaction solution. Polymer particles containing the polymer can be obtained from the reaction solution containing the hydrogel polymer as water-absorbing resin particles. The steps for obtaining polymer particles from the reaction solution include, for example, the following: extracting a portion of the water from the reaction solution through azeotropic distillation with water in a dispersion medium to form a concentrate; and evaporating the dispersion medium and water from the concentrate. After the evaporation-based removal of the dispersion medium and water, a powder of dried polymer particles can be obtained. A certain amount of water may remain in the dried polymer particles.

[0051] After a portion of the water is extracted from the reaction solution via azeotropic distillation with water through a dispersion medium, the polymer particles are surface-crosslinked in a mixture containing a concentrate of polymer particles containing the polymer and a surface crosslinking agent. Various additives can be further added to the dried polymer particles. Examples of additives include lubricants (e.g., silica particles), metal chelating agents, surface modifiers, heat stabilizers, antioxidants, and antibacterial agents. In this specification, water-absorbing resin particles refer to particles containing polymer particles. Water-absorbing resin particles may contain both polymer particles and additives. The amount of additives (e.g., lubricants) relative to 100 parts by mass of polymer particles may be, for example, 0.001 parts by mass or more and 10 parts by mass or less, 0.01 parts by mass or more and 5 parts by mass or less, or 0.1 parts by mass or more and 2 parts by mass or less.

[0052] In dried polymer particle (hygroscopic resin particle) powder, the proportion of particles with a diameter of 850 μm or larger relative to the total amount of hygroscopic resin particles can be less than 4% by mass, less than 3% by mass, less than 2% by mass, or less than 1% by mass. This proportion refers to the proportion before the powder is classified by sieving or other methods after drying. The method for determining the proportion of particles with a diameter of 850 μm or larger includes the following steps: sieving a powder with a mass W0 g of hygroscopic resin particles using a sieve with an 850 μm mesh; measuring the mass W1 g of particles remaining on the sieve; and calculating the proportion of particles with a diameter of 850 μm or larger using the formula: Proportion of particles with a diameter of 850 μm or larger = (W1 / W0) × 100. Particles with a diameter of 850 μm or larger are sometimes excluded from the product through classification. From the viewpoint of effectively utilizing manufacturing raw materials such as (meth)acrylic acid, a low proportion of particles with a diameter of 850 μm or larger in the powder before classification after polymerization is advantageous.

[0053] The median particle size of the superabsorbent resin particles can be, for example, 50 μm or more and 850 μm or less. The median particle size can also 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. As described in the examples below, the median particle size of the superabsorbent resin particles can be determined using JIS standard sieves.

[0054] Example The present invention is not limited to the following embodiments.

[0055] 1. Manufacturing of water-absorbing 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. The stirrer was fitted with four inclined blades, each with a two-stage blade diameter of 5 cm. 283 g of n-heptane as a hydrocarbon dispersion medium and 0.736 g of maleic anhydride-modified ethylene-propylene copolymer (Mitsui Chemicals, Inc., Hi-Wax 1105A) as a hydrophobic polymeric dispersant were placed in the prepared separable flask. While stirring the mixture in the separable flask, 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.

[0056] 74.06 g (1.03 mol) of acrylic acid was placed in 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 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-methylpropanediamine) dihydrochloride as a free 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.

[0057] The prepared aqueous solution was added to a 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 SugarEster S-370, HLB:3) and 6.62 g of heptane was added to the mixture. The separable flask was immersed in a 50°C water bath, and the reaction mixture formed in the separable flask was stirred with a stirrer at 300 rpm. After the temperature of the reaction mixture stabilized at 49°C, stirring was maintained and the reaction was carried out as described above. Figure 1 Nitrogen gas was then introduced into the reaction solution as an inert gas through a gas inlet tube inserted into the reaction solution, thus initiating the nitrogen introduction. As the removable flask was fully purged with nitrogen through the introduction, the polymerization reaction of acrylic acid and sodium acrylate began. Stirring was maintained from the start of the nitrogen introduction, and the polymerization reaction proceeded for 90 minutes, thereby forming a slurry containing a hydrogel polymer in the form of particles.

[0058] The separable flask containing the slurry was removed from a 50°C water bath and immersed in a 125°C oil bath. The n-heptane contained in the slurry was refluxed, and 127.4 g of water was extracted from the slurry to the outside of the separable flask through azeotropic distillation of n-heptane and water. Next, the separable flask was heated in a 125°C oil bath to evaporate the n-heptane, thereby obtaining 92.5 g of dried polymer particles as absorbent resin particles.

[0059] Example 2 The separable flask containing the reaction solution was immersed in a water bath at 55°C. Nitrogen gas was introduced when the temperature of the reaction solution stabilized at 54°C. Otherwise, under the same conditions as in Example 1, the water-absorbing resin particles of Example 2 were obtained.

[0060] Example 3 The separable flask containing the reaction solution was immersed in a water bath at 60°C. Nitrogen gas was introduced when the temperature of the reaction solution stabilized at 58°C. Otherwise, under the same conditions as in Example 1, the water-absorbing resin particles of Example 3 were obtained.

[0061] Example 4 The separable flask containing the reaction solution was immersed in a water bath at 65°C. Nitrogen gas was introduced when the temperature of the reaction solution stabilized at 63°C. Otherwise, under the same conditions as in Example 1, the water-absorbing resin particles of Example 4 were obtained.

[0062] Example 5 The amount of 2,2'-azobis(2-methylpropanediamine) 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. Nitrogen gas was introduced when the temperature of the reaction solution stabilized at 59°C. Otherwise, under the same conditions as in Example 1, the water-absorbing resin particles of Example 5 were obtained.

[0063] Example 6 The free radical polymerization initiator was changed to 0.0368 g (0.136 mmol) of potassium persulfate, and the separable flask containing the reaction solution was immersed in a water bath at 65°C. Nitrogen gas was introduced when the temperature of the reaction solution was stabilized at 64°C. Otherwise, under the same conditions as in Example 1, the water-absorbing resin particles of Example 6 were obtained.

[0064] Example 7 The amount of potassium persulfate was changed to 0.0736 g (0.272 mmol), and the water-absorbing resin particles of Example 7 were obtained under the same conditions as in Example 6.

[0065] Comparative Example 1 The separable flask containing the reaction solution was immersed in a water bath at 45°C. Nitrogen gas was introduced when the temperature of the reaction solution stabilized at 44°C. Otherwise, under the same conditions as in Example 1, the water-absorbing resin particles of Comparative Example 1 were obtained.

[0066] 2. Evaluation of water-absorbing resin particles (1) The proportion of particles with a diameter of 850 μm or larger The proportion of particles with a diameter of 850 μm or larger was determined at room temperature (25 ± 2 °C) and humidity (50 ± 10%) using the following steps. First, the powder of absorbent resin particles obtained in the examples or comparative examples was sieved using a sieve with a mesh size of 850 μm. The mass of absorbent resin particles with a diameter of 850 μm or larger remaining on the sieve was measured, and the proportion (mass %) of particles with a diameter of 850 μm or larger relative to the total amount of absorbent resin particles was calculated.

[0067] (2) Median particle size The median particle size was determined at room temperature (25±2℃) and humidity (50±10%) using the following steps. First, a series of JIS standard sieves were assembled from top to bottom: 500μm mesh, 425μm mesh, 250μm mesh, 180μm mesh, 150μm mesh, 106μm mesh, and 75μm mesh, along with a receiving tray. 5g of absorbent resin particles that had passed through an 850μm mesh were placed in the topmost sieve, and the particles were graded using a continuous fully automatic ultrasonic vibrating sieve separator (Robot Shifter RPS-02, manufactured by SEISHIN ENTERPRISE CO.,LTD.) for 5 minutes. After grading, the percentage (mass percentage) of absorbent resin particles remaining on each sieve relative to the total amount was calculated. By accumulating the proportions of the components with the largest particle sizes sequentially, the relationship between the sieve aperture and the cumulative proportion of absorbent resin particles remaining on the sieve is plotted on logarithmic probability paper. By connecting the plotted points on the probability paper with straight lines, the particle size equivalent to 50% of the cumulative mass is determined, and this value is set as the median particle size.

[0068] The evaluation results are shown in Table 1. The amounts of free radical polymerization initiators shown in Table 1 are percentages (mol%) relative to the amount of monomers. The types of free radical polymerization initiators shown in Table 1 are as follows.

[0069] (i) 2,2'-Azobis(2-methylpropanediamine) dihydrochloride (ii) Potassium persulfate [Table 1]

[0070] As shown in Table 1, it was confirmed that if the temperature of the reaction liquid at the moment the inert gas is introduced is above 47°C and below 70°C, the proportion of particles with a diameter of 850 μm or more in the obtained water-absorbing resin particles decreases.

[0071] Explanation of reference numerals in the attached figures 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 manufacturing water-absorbing resin particles, the method comprising the following steps: Prepare an aqueous solution comprising a water-soluble olefinic unsaturated monomer, water, and a free radical polymerization initiator; A reaction solution containing the aqueous liquid, dispersion medium, and dispersion stabilizer is formed in the polymerization tank; Inert gas is introduced into the polymerization tank; The polymer of the water-soluble olefinic unsaturated monomer is formed by reverse-phase suspension polymerization in the reaction solution; and Polymer particles containing the polymer are obtained from the reaction solution. The temperature of the reaction solution at the moment when the inert gas is introduced into the polymerization tank is above 47°C and below 70°C.

2. The method according to claim 1, wherein, The free radical polymerization initiator has a 10-hour half-life temperature above 40°C and below 80°C.

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