Method for producing water absorbent resin particles

Abstract

Disclosed is a method for producing water absorbent resin particles, the method comprising: forming a hydrous gel-like polymer that contains a polymer of a water-soluble ethylenically unsaturated monomer and water by reversed phase suspension polymerization in a reaction liquid that contains the water-soluble ethylenically unsaturated monomer, water, and a dispersion medium; removing water from the reaction liquid by heating the reaction liquid while stirring the reaction liquid with a stirring blade, which is fixed to a stirring shaft rotating in a concentration chamber, so as to form a concentrate which is a concentrated reaction liquid; and recovering polymer particles that contain the polymer from the concentrate. When the stirring shaft is inserted into the reaction liquid, the required stirring power calculated from a torque value for rotating the stirring shaft is 0.20 kW / m3 to 3.00 kW / m3 on average from a time point at which stirring of the reaction liquid is started to a time point at which the amount of water contained in the concentrate reaches 120 mass%.

Description

Method for producing superabsorbent polymer particles

[0001] This disclosure relates to a method for producing water-absorbent polymer particles.

[0002] Superabsorbent polymer particles are manufactured using various polymerization methods, such as reverse-phase suspension polymerization and aqueous solution polymerization. Products made from superabsorbent polymer particles are widely used in various fields, including sanitary materials such as disposable diapers and sanitary napkins, horticultural materials such as water-retaining agents and soil conditioners, water-stopping materials, and industrial materials such as condensation inhibitors. In these applications, a low proportion of fine powder in the superabsorbent polymer particles is sometimes required (for example, Patent Document 1).

[0003] International Publication No. 2007 / 123188

[0004] This disclosure relates to a method for producing superabsorbent resin particles by reverse-phase suspension polymerization, specifically to reducing the proportion of fine powder in the resulting superabsorbent resin particles.

[0005] This disclosure includes: [1] forming a water-containing gel polymer containing a polymer of the water-soluble ethylenically unsaturated monomer and water by reverse-phase suspension polymerization in a reaction solution containing a water-soluble ethylenically unsaturated monomer, water, and a dispersion medium; removing water from the reaction solution by heating the reaction solution containing the water-containing gel polymer in a concentration chamber while stirring with a stirring blade fixed to a rotating stirring shaft, thereby forming a concentrate which is the concentrated reaction solution; and extracting polymer particles containing the polymer from the concentrate, wherein one or more stirring shafts are inserted into the reaction solution, and the required stirring power calculated from the torque values ​​for rotating each of the one or more stirring shafts is 0.20 kW / m² on average from the time stirring of the reaction solution by the stirring blades begins until the amount of water contained in the concentrate reaches 120% by mass based on the mass of the polymer. ３ 3.00kW / m or more ３The following is a method for producing superabsorbent polymer particles: [2] The average power required for stirring from the time stirring of the reaction solution by the stirring blade until the time when the amount of water contained in the concentrate becomes 120% by mass based on the mass of the polymer, and from the time stirring of the reaction solution by the stirring blade until the time when the amount of water contained in the concentrate becomes 100% by mass based on the mass of the polymer, is 0.20 kW / m ３ 3.00kW / m or more ３ The method according to [1], which is as follows: [3] The method according to [1] or [2], wherein the number of stirring shafts inserted into the reaction solution is one. [4] The method according to any one of [1] to [3], wherein when the maximum width of the concentration chamber is D and the depth of the reaction solution in the concentration chamber before heating is h, the ratio h / D is 0.80 or more and 1.60 or less. [5] The method according to any one of [1] to [4], wherein when the maximum width of the concentration chamber is D and the maximum width of the stirring blade is d, the ratio d / D is 0.20 or more and 0.80 or less. [6] The method according to any one of [1] to [5], wherein the stirring blade is a paddle blade. [7] The method according to any one of [1] to [6], wherein the water-soluble ethylenically unsaturated monomer comprises (meth)acrylic acid and its alkali metal salt.

[0006] In a method for producing superabsorbent resin particles by reverse-phase suspension polymerization, the proportion of fine powder in the resulting superabsorbent resin particles can be reduced.

[0007] This is a schematic diagram showing an example of a concentration apparatus for concentrating the reaction solution.

[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 forming a water-containing gel-like polymer containing a polymer of a water-soluble ethylenically unsaturated monomer and water by inverse phase suspension polymerization in a reaction solution containing the water-soluble ethylenically unsaturated monomer, water, and a dispersion medium; heating the reaction solution containing the water-containing gel-like polymer in a concentration chamber while stirring with a stirring blade fixed to a rotating stirring shaft to remove water from the reaction solution, thereby forming a concentrate which is the concentrated reaction solution; and taking out polymer particles containing the polymer from the concentrate.

[0010] The reaction solution for inverse phase suspension polymerization is formed in an arbitrary reaction chamber. The reaction solution can be a suspension containing an oily liquid containing a dispersion medium, and a particulate aqueous liquid containing water and a water-soluble ethylenically unsaturated monomer and dispersed in the oily liquid. During the polymerization reaction, the reaction solution is usually stirred. In addition, ordinary conditions regarding the production of water-absorbent resin particles by inverse phase suspension polymerization can be applied without particular limitation.

[0011] 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 salts of (meth)acrylic acid and 2-(meth)acrylamido-2-methylpropanesulfonic acid may be, for example, alkali metal salts. The alkali metal salts of (meth)acrylic acid and 2-(meth)acrylamido-2-methylpropanesulfonic acid may be, for example, sodium salts.

[0012] The water-soluble ethylenically unsaturated monomer may contain (meth)acrylic acid and an alkali metal salt of (meth)acrylic acid. In that 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, and may be substantially 100 mol%.

[0013] The aqueous liquid may contain an internal crosslinking agent that crosslinks a polymer of the water-soluble ethylenically unsaturated monomer formed by a polymerization reaction. The internal crosslinking agent may be a compound having two or more reactive functional groups having reactivity with the water-soluble ethylenically unsaturated monomer. The reactive functional group can 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.

[0014] Examples of the internal crosslinking agent having two or more (meth)acryloyl groups include (meth)acrylic acid ester 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)acrylic acid ester compounds formed from polyepoxide compounds and (meth)acrylic acid, and (meth)acrylic acid carbamyl ester compounds formed from polyisocyanate compounds (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.

[0015] The vinyl group as a reactive functional group may be part of the 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.

[0016] Examples of internal crosslinking agents 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.

[0017] Examples of internal crosslinking agents having two or more isocyanate groups include 2,4-tolylene diisocyanate and hexamethylene diisocyanate.

[0018] The amount of the internal crosslinking agent may be, for example, 0 mmol to 0.5 mmol, 0 mmol to 0.2 mmol, 0 mmol to 0.1 mmol, or 0 mmol to 0.05 mmol per mole of water-soluble ethylenically unsaturated monomer.

[0019] The aqueous solution may contain a radical polymerization initiator. The radical polymerization initiator may include, for example, an azo compound, a peroxide, or a combination thereof.

[0020] The amount of radical polymerization initiator may be, for example, 0.0001 moles or more and 1 mole or less, 0.001 moles or more and 0.1 mole or less, 0.005 moles or more and 0.08 moles or less, or 0.01 moles or more and 0.05 moles or less per 100 moles of water-soluble ethylenically unsaturated monomer.

[0021] The aqueous solution may also contain other components such as thickeners, hydrophilic polymer dispersants, chain transfer agents, foaming agents, dispersion stabilizers (e.g., surfactants, hydrophobic polymer dispersants), or combinations thereof.

[0022] The dispersion medium may be a hydrocarbon. Examples of hydrocarbons that can be used as a dispersion medium include linear 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 may be a single type or a combination of two or more types.

[0023] The amount of dispersion medium may be 30 to 1000 parts by mass, 50 to 650 parts by mass, 70 to 550 parts by mass, or 100 to 450 parts by mass per 100 parts by mass of water-soluble ethylenically unsaturated monomer.

[0024] The reaction solution may be heated for the polymerization reaction. The heating temperature for the polymerization reaction may be, for example, 40°C to 90°C. The time for the polymerization reaction may be, for example, 30 minutes to 240 minutes.

[0025] As reverse-phase suspension polymerization progresses, particulate, hydrated gel-like polymers are typically formed in the reaction solution. After the formation of the hydrated gel-like polymers, the reaction solution is concentrated by removing some of the water in the reaction solution in a concentration chamber. The reaction solution after reverse-phase suspension polymerization may remain in the reaction chamber and be used as a concentration chamber. Alternatively, the reaction solution after reverse-phase suspension polymerization may be transferred to a concentration chamber separate from the reaction chamber. The concentration chamber can be the containment section of any container. The concentration chamber may have a portion formed by the cylindrical inner surface of the container.

[0026] By heating the reaction solution containing the water-containing gel polymer in a concentration chamber while stirring to a temperature above the boiling point of water, or above the azeotropic point of the azeotropic mixture containing water and the dispersion medium, water can be efficiently removed from the reaction solution. This allows for efficient concentration of the reaction solution. Heating causes the water in the reaction solution to vaporize, and the vaporized water can be discharged outside the concentration chamber. Removal of water results in the formation of a concentrate, which is the concentrated reaction solution, within the concentration chamber. The heating temperature for concentration may be, for example, 70°C to 150°C, 80°C to 140°C, or 100°C to 130°C.

[0027] The concentrate formed by the concentration of the reaction solution typically contains particulate water-containing gel polymers, including the polymer and water. In other words, the concentrate contains polymer particles, including the polymer and water. Water is removed until the amount of water in the concentrate is any amount, such as 120% by mass or less or 100% by mass or less, based on the mass of the polymer. That is, the amount of water in the concentrate at the end of concentration may be 120% by mass or less or 100% by mass or less, based on the mass of the polymer. For example, the amount of water in the concentrate at the end of concentration may be 10% by mass or more and 120% by mass or less, 12% by mass or more and 100% by mass or less, or 15% by mass or more and 65% by mass or less, based on the mass of the polymer. The amount of water in the concentrate at the end of concentration may be 10% by mass or more, 12% by mass or more, or 15% by mass or more, based on the mass of the polymer, or 120% by mass or less, 100% by mass or less, or 65% by mass or less, based on the mass of the polymer. The amount of water in the reaction solution before concentration may be 125% by mass or more and 180% by mass or less, based on the mass of the polymer.

[0028] One or more stirring shafts, each with a fixed stirring blade, are inserted into the reaction solution, and the reaction solution is stirred by the rotation of the stirring shafts and the stirring blades. According to the inventor's findings, by setting the stirring conditions by focusing on the power required for stirring, the proportion of fine powder in the water-absorbent resin particles can be reduced. Specifically, the stirring conditions are set so that the power required for stirring during concentration falls within a predetermined range. The power required for stirring is an index for quantitatively evaluating the power required to stir a fluid. In this disclosure, the power required for stirring is a value calculated from the following formula (I): Power required for stirring (unit: kW / m) ３ ) = [{(2π / 1000) × 60}] × rotational speed (unit: rpm) × torque value (unit: N・m) / fluid volume (unit: m ３ ) ... (I) The rotational speed is the number of rotations per minute of the stirring shaft and impellers. The torque value is the force required to rotate a single stirring shaft to which the impellers are fixed. The torque value can be measured, for example, by a torque sensor. The fluid volume is the volume of the reaction liquid or concentrate being stirred.

[0029] The required stirring power, calculated from the torque values ​​for rotating one or more stirring shafts, averages 0.20 kW / m² from the time stirring of the reaction solution by the stirring blades begins until the amount of water in the concentrate reaches 120% by mass relative to the mass of the polymer. ３ 3.00kW / m or more ３It can be as follows. The amount of water contained in the concentrate may be calculated as the difference between the total amount of water contained in the reaction solution before concentration and the amount of water removed outside the system by concentration. When the average stirring power required from the time when stirring of the reaction solution is started by the stirring blade for concentration until the amount of water contained in the concentrate reaches 120% by mass based on the mass of the polymer is within this range, the proportion of fine powder in the powder of the water-absorbing resin particles can be effectively reduced. When the stirring power required is moderately large, it is difficult for the water-containing gel polymer to deposit in the reaction solution, so the decomposition of the water-containing gel polymer due to rubbing between the deposited water-containing gel polymers is suppressed, and as a result, the proportion of fine powder in the water-absorbing resin particles is considered to decrease. When the stirring power required is moderately small, the decomposition of the water-containing gel polymer due to the collision of the water-containing gel polymers dispersed in the reaction solution is suppressed, and as a result, the proportion of fine powder in the water-absorbing resin particles is considered to decrease. When the stirring power required is 0.20 kW / m ３ or more, the formation of coarse powder due to the deposition of the water-containing gel polymer can also be suppressed.

[0030] From the same viewpoint as described above, the average stirring power required from the time when stirring of the reaction solution is started by the stirring blade for concentration until the amount of water contained in the concentrate reaches 120% by mass based on the mass of the polymer is 0.20 kW / m ３ or more and 2.80 kW / m ３ or less, 2.60 kW / m ３ or less, 2.40 kW / m ３ or less, 2.20 kW / m ３ or less, 2.00 kW / m ３ or less, 1.80 kW / m ３ or less, 1.60 kW / m ３ or less, 1.40 kW / m ３ or less, 1.20 kW / m ３ or less, 1.10 kW / m ３ or less, or 1.00 kW / m ３The following is also acceptable: When the reaction solution is concentrated until the amount of water in the concentrate is 100% by mass or less based on the mass of the polymer, the average power required for stirring from the time stirring of the reaction solution with a stirring blade for concentration is started until the time when the amount of water in the concentrate reaches 100% by mass based on the mass of the polymer is 0.20 kW / m ３ The above, and 2.80 kW / m². ３ Below, 2.60kW / m ３ Below, 2.40kW / m ３ Below, 2.20kW / m ３ Below, 2.00kW / m ３ Below, 1.80kW / m ３ Below, 1.60kW / m ３ Below, 1.40kW / m ３ Below, 1.20kW / m ３ Below, 1.10kW / m ３ The following, or 1.00 kW / m² ３ The following may also be used: The power required for stirring can be adjusted, for example, by the rotational speed of the stirring shaft and stirring blades, the shape of the stirring blades, etc.

[0031] If the concentration continues after the amount of water in the concentrate reaches 120% by mass or 100% by mass based on the mass of the polymer, the average power required for stirring from the point when the amount of water in the concentrate reaches 120% by mass or 100% by mass based on the mass of the polymer until stirring of the concentrate (concentrated reaction solution) by the impeller is stopped should be 0.20 kW / m ３ 3.00kW / m or more ３ The following or other ranges described above may also apply: When surface crosslinking water-containing polymer particles (water-containing gel polymer) in a mixture containing a concentrate and a surface crosslinking agent, the average stirring power required from the time the amount of water in the concentrate becomes 120% by mass or 100% by mass based on the mass of the polymer until the concentrate and the surface crosslinking agent (or its solution) are mixed is 0.20 kW / m². ３ 3.00kW / m or more ３ The following or other options within the scope described above may also be used.

[0032] The average stirring power required from the time stirring of the reaction solution with the impeller for concentration is started until the time when the amount of water in the concentrate reaches 120% by mass and 100% by mass based on the mass of the polymer can be determined, for example, by determining the stirring power required multiple times at predetermined time intervals from the torque value, the rotation speed of the stirring shaft, and the volume of the reaction solution or concentrate, and then calculating the average of the determined stirring powers. If the rotation speed of the stirring shaft is constant and the change in the volume of the reaction solution or concentrate is small, the torque value alone can be measured at predetermined time intervals, and the value calculated from equation (I) using the average of the measured torque values, the initial volume of the reaction solution, and the set rotation speed of the stirring shaft can be considered as substantially the average stirring power required. The time interval for measuring the torque value, etc., may be, for example, 5 minutes.

[0033] When multiple stirring shafts are inserted into the reaction solution, the required stirring power can be determined for each stirring shaft. The average required stirring power for all of the multiple stirring shafts while the reaction solution is being stirred by the impellers for concentration may be within the above numerical range. The number of stirring shafts inserted into the reaction solution in the concentration chamber may be only one.

[0034] The concentration of the reaction solution by removing water may be carried out in two or more stages, with the heating of the reaction solution temporarily stopped between each stage. In this case, the average power required for stirring may be within the above-mentioned range for all two or more concentration stages.

[0035] Figure 1 is a schematic diagram showing an example of a concentration apparatus for concentrating a reaction solution. The concentration apparatus 100 shown in Figure 1 comprises a concentration tank 105, a stirrer 170, a stirring shaft 180, a stirring blade 120, and a thermometer 160. The inner surface of the concentration tank 105 forms a concentration chamber 105A, and the reaction solution 110 is contained within the concentration chamber 105A. The stirring shaft 180 is attached to the stirrer 170, and the stirring blade 120 is fixed to the tip of the stirring shaft 180. The tip of the stirring shaft 180, the stirring blade 120, and the thermometer 160 are inserted into the reaction solution 110. During concentration, the concentration tank 105 is heated by any heating means such as a heater or an oil bath. By heating the reaction solution 110 while stirring it with the rotating stirring blade 120, the water in the reaction solution 110 vaporizes, and the water is discharged outside the concentration tank 105. In this case, water may be discharged outside the concentration tank 105 using a reflux condenser.

[0036] The concentration tank 105 has a cylindrical portion. In the cylindrical portion of the concentration tank 105, the concentration chamber 105A has a maximum width D. The maximum width D is the maximum width of the concentration chamber 105A in the horizontal direction.

[0037] The stirring shaft 180 is rotationally driven by the agitator 170. The stirring blade 120 is a paddle blade composed of multiple plate-like sections extending along a direction perpendicular to the longitudinal direction of the stirring shaft 180. The maximum width d of the stirring blade 120 is the maximum total width of the stirring blade 120 (multiple plate-like sections) when viewed from a direction perpendicular to the longitudinal direction of the stirring shaft 180, and is sometimes called the blade diameter. In the example in Figure 1, one stage of stirring blade 120 is fixed to one stirring shaft 180. Two or more stages of stirring blades may be fixed at different positions along the length of one stirring shaft. The number of stirring blades fixed to one stirring shaft may be one to three stages. The type of stirring blade can be arbitrarily selected. For example, the stirring blade may be a paddle blade, a flat blade, a grid blade, a propeller blade, an anchor blade, a turbine blade, a Faudler blade, a ribbon blade, a full-zone blade, or a Maxblend blade.

[0038] The ratio h / D of the depth of the reaction solution to the maximum width of the concentration chamber 105A may be 0.80 or more and 1.60 or less, from the viewpoint of reducing the proportion of fine powder in the water-absorbing resin particles. From a similar viewpoint, the ratio h / D may be 0.85 or more and 1.40 or less, or 0.90 or more and 1.30 or less. From a similar viewpoint, the ratio h / D may be 0.85 or more or 0.90 or more, or 1.40 or less or 1.30 or less.

[0039] The ratio d / D of the maximum width of the stirring blade 120 to the maximum width of the concentration chamber 105A may be 0.20 or more and 0.80 or less, from the viewpoint of reducing the proportion of fine powder in the water-absorbing resin particles. From a similar viewpoint, the ratio d / D may be 0.30 or more and 0.70 or less, or 0.35 or more and 0.50 or less. From a similar viewpoint, the ratio d / D may be 0.30 or more or 0.35 or more, or 0.70 or less or 0.50 or less.

[0040] By removing water and the dispersion medium from the concentrate, a dry polymer particle powder can be obtained. The concentrate may be heated for this purpose.

[0041] Before obtaining dried polymer particles from the concentrate, the polymer particles containing water (hydrated gel-like polymer) may be surface-crosslinked in a mixture containing the concentrate and a surface crosslinking agent.

[0042] The surface crosslinking agent may be a compound having two or more reactive functional groups, and examples 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, epibromhydrin This includes halo epoxy compounds such as α-methylepichlorohydrin; isocyanate compounds such as 2,4-tolylene diisocyanate and hexamethylene diisocyanate; oxetane compounds such as 3-methyl-3-oxetane methanol, 3-ethyl-3-oxetane methanol, 3-butyl-3-oxetane methanol, 3-methyl-3-oxetane ethanol, 3-ethyl-3-oxetane ethanol, and 3-butyl-3-oxetane ethanol; oxazoline compounds such as 1,2-ethylenebisoxazoline; carbonate compounds such as ethylene carbonate; and hydroxyalkylamide compounds such as bis[N,N-di(β-hydroxyethyl)]adipamide. The surface crosslinking agent may include polyglycidyl compounds such as (poly)ethylene glycol diglycidyl ether, (poly)glycerin diglycidyl ether, (poly)glycerin triglycidyl ether, (poly)propylene glycol polyglycidyl ether, and polyglycerol polyglycidyl ether. These surface crosslinking agents may be used individually or in combination of two or more.

[0043] The amount of surface crosslinking agent may be 0.01 mmol to 10 mmol, 0.03 mmol to 3 mmol, or 0.05 mmol to 1 mmol per mole of monomer units constituting the polymer in the polymer particles.

[0044] The mixture may be heated for surface crosslinking. The heating temperature may be, for example, 60°C to 200°C, or 80°C to 150°C. The reaction time for the surface crosslinking reaction may be, for example, 1 minute to 300 minutes, or 5 minutes to 200 minutes.

[0045] Various additives may 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, superabsorbent resin particles mean particles containing polymer particles. Superabsorbent resin particles may contain polymer particles and additives. The amount of additive (e.g., lubricant) may be, for example, 0.001 parts by mass to 10 parts by mass, 0.01 parts by mass to 5 parts by mass, or 0.1 parts by mass to 2 parts by mass per 100 parts by mass of polymer particles.

[0046] The median particle size of the water-absorbent resin particles may be, for example, 50 μm or more and 850 μm or less. The median particle size 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, or 400 μm or less. The median particle size of the water-absorbent resin particles may be 50 μm or more, 100 μm or more, or 200 μm or more and 850 μm or less. The median particle size of the water-absorbent resin particles can be measured using a JIS standard sieve as described in the examples below.

[0047] The present invention is not limited to the following embodiments.

[0048] In the following example of the production of superabsorbent polymer particles, a Three One Motor (BLh1200R, manufactured by Shinto Kagaku Co., Ltd.) was used as the agitator. This agitator has a function to measure the torque value required to rotate the attached stirring shaft.

[0049] 1. Production of superabsorbent polymer particles (Example 1) Polymerization process <First stage polymerization reaction> A round-bottom cylindrical separable flask (internal diameter (maximum width) 11 cm, internal volume 2 L) equipped with a reflux condenser, dropping funnel, and nitrogen gas inlet tube was prepared. A stirring shaft with two-stage stirring blades (four inclined paddle blades with a blade diameter of 5 cm) fixed to it was attached to a stirrer so that the stirring blades were positioned inside the separable flask. 294 g of n-heptane as a dispersion medium and 0.644 g of maleic anhydride-modified ethylene-propylene copolymer (Mitsui Chemicals, Inc., High Wax 1105A) as a hydrophobic polymer dispersant were placed in the separable flask. The mixture in the separable flask was heated to 80°C while being stirred by the rotation of the stirring blades to form an n-heptane solution containing maleic anhydride-modified ethylene-propylene copolymer. After stopping the stirring, the n-heptane solution was cooled to 40°C.

[0050] 92.0 g of an 80.5% by mass acrylic acid aqueous solution (acrylic acid: 1.03 mol) was placed in a 300 mL beaker. 103.24 g of a 30% by mass sodium hydroxide aqueous solution was added dropwise while cooling from the outside to neutralize 75 mol% of the acrylic acid. To the partially neutralized acrylic acid aqueous solution formed by neutralization, 0.0736 g (0.272 mmol) of potassium persulfate was added as a radical polymerization initiator, 0.0046 g (0.0264 mmol) of ethylene glycol diglycidyl ether was added as an internal crosslinking agent, and 44.41 g of deionized water was added to prepare the first aqueous solution.

[0051] The first aqueous solution was added to the n-heptane solution in the separable flask described above, and the resulting reaction mixture was stirred with a stirring blade for 10 minutes. Next, a surfactant solution containing 5.80 g of n-heptane and 0.644 g of sucrose stearate (surfactant, HLB: 3, Mitsubishi Chemical Foods Corporation, Ryoto Sugar Ester S-370) was further added to the reaction mixture. The reaction mixture was stirred with a stirring blade at 500 rpm while the system was thoroughly purged with nitrogen. Subsequently, the separable flask was immersed in a 70°C water bath and held in that state for 60 minutes to allow the first stage of polymerization to proceed.

[0052] <Second Stage Polymerization Reaction> 128.8 g of 80.5% by mass acrylic acid aqueous solution (acrylic acid: 1.44 mol) was placed in a 300 mL beaker. 144.53 g of 30% by mass sodium hydroxide aqueous solution was added dropwise while cooling from the outside to neutralize 75 mol% of the acrylic acid. To the partially neutralized acrylic acid aqueous solution formed by neutralization, 0.0902 g (0.334 mmol) of potassium persulfate was added as a radical polymerization initiator, 0.0116 g (0.0665 mmol) of ethylene glycol diglycidyl ether was added as an internal crosslinking agent, and 16.42 g of deionized water was added to prepare the second stage aqueous solution.

[0053] The reaction solution in the separable flask after the first stage of polymerization was cooled to 26°C while being stirred with a stirring blade at 1000 rpm. Next, the entire volume of the second stage aqueous solution was added to the reaction solution, and the system was purged with nitrogen for 60 minutes. The separable flask was again immersed in a 70°C water bath to raise the temperature and start the second stage of polymerization. By allowing the second stage of polymerization to proceed for 60 minutes while maintaining stirring, a reaction solution (polymerization slurry) containing particulate water-containing gel polymer was formed.

[0054] The separable flask containing the concentration reaction solution was immersed in an oil bath at 125°C. While stirring with a stirring blade, the temperature of the reaction solution was raised to above the azeotropic point of the azeotropic mixture containing n-heptane and water, and the removal of water contained in the second stage polymerization slurry was initiated by azeotropic distillation of n-heptane and water. The stirring power required when stirring of the reaction solution with the stirring blade was initiated was 0.23 kW / m ３ (Rotation speed: 100 rpm, Torque value: 0.025 N·m, Volume of reaction solution: 0.0011622 m³) ３ The reaction solution in the separable flask was 131 mm deep. An intermediate concentrate (concentrated reaction solution) was formed by concentrating the solution by removing water while maintaining the rotation speed of the stirring blade at 100 rpm. The torque value of the stirring shaft during stirring was measured every 5 minutes and saved to a data logger. After removing 55 g of water from the reaction solution, the stirring power required when the amount of water in the concentrate reached 120% by mass (263 g) based on the mass of the polymer was 0.22 kW / m ３(Rotation speed: 100 rpm, Torque value: 0.023 N·m, Volume of concentrate (concentrated reaction solution): 0.0011066 m³) ３ The average power required for stirring from the time stirring of the reaction solution by the impeller was started until the amount of water in the concentrate reached 120% by mass based on the mass of the polymer was 0.22 kW / m². ３ Furthermore, after removing 44 g of water from the reaction solution and the amount of water in the concentrate reached 100% by mass (219 g) of the polymer mass, the required stirring power was 0.21 kW / m². ３ (Rotation speed: 100 rpm, Torque value: 0.021 N·m, Volume of concentrate (concentrated reaction solution): 0.0010616 m³) ３ The average power required for stirring from the time stirring of the reaction solution by the impeller was started until the amount of water in the concentrate reached 100% by mass based on the mass of the polymer was 0.23 kW / m². ３ The amount of water in the concentrate reached 100% by mass based on the mass of the polymer, at which point 78 g of water was removed from the reaction solution.

[0055] To the intermediate concentrate, 2.58 g of a 3.0% by mass aqueous solution of sodium sulfite and 4.91 g of a 4.5% by mass aqueous solution of sodium diethylenetriaminepentaacetate were added and stirred to mix. While maintaining the rotation speed of the stirring blade at 100 rpm, the intermediate concentrate was further concentrated by removing the water contained in the intermediate concentrate by azeotropic distillation of n-heptane and water. The concentration process was terminated when 83 g of water was removed from the intermediate concentrate. At this point, the amount of water in the concentrate was 26% by mass based on the mass of the polymer.

[0056] To the concentrate after surface crosslinking concentration, 5.52 g of a 2.0% by mass aqueous solution of ethylene glycol diglycidyl ether (ethylene glycol diglycidyl ether (surface crosslinking agent): 0.634 mmol) was mixed. Surface crosslinking of the water-containing gel polymer (polymer particles) was promoted in the mixture by maintaining the rotation speed of the stirring blade at 100 rpm and keeping the internal temperature at 83°C for 2 hours.

[0057] The mixture after surface crosslinking was heated in an oil bath at 125°C, and water and n-heptane were evaporated until almost no more evaporates were distilled from the system, thereby obtaining 238.96 g of dried polymer particles (absorbent resin particles).

[0058] (Example 2) Under the same conditions as in Example 1, except that the rotation speed of the stirring blade in the concentration process was changed to 350 rpm, 236.32 g of superabsorbent resin particles were obtained. In the concentration process, the required stirring power at the time stirring of the reaction solution by the stirring blade was started was 0.73 kW / m ３ (Torque value: 0.023 N·m, volume of reaction solution: 0.0011622 m³) ３ ) and the required stirring power when the amount of water in the concentrate reaches 120% by mass based on the mass of the polymer is 0.79 kW / m ３ (Torque value: 0.024 N·m, Volume of concentrate (concentrated reaction solution): 0.0011066 m³) ３ The average power required for stirring from the time stirring of the reaction solution by the impeller is started until the time when the amount of water in the concentrate reaches 120% by mass based on the mass of the polymer is 0.78 kW / m². ３ The required stirring power when the amount of water in the concentrate reached 100% by mass based on the mass of the polymer was 1.04 kW / m². ３ (Torque value: 0.030 N·m, Volume of concentrate (concentrated reaction solution): 0.0010616 m³) ３ The average power required for stirring from the time stirring of the reaction solution by the impeller is started until the amount of water in the concentrate reaches 100% by mass based on the mass of the polymer is 0.79 kW / m². ３ That was the case.

[0059] (Comparative Example 1) Under the same conditions as in Example 1, except that the rotation speed of the stirring blade in the concentration process was changed to 50 rpm, 237.90 g of superabsorbent resin particles were obtained. In the concentration process, the stirring power required at the time stirring of the reaction solution by the stirring blade was started was 0.10 kW / m ３ (Torque value: 0.022 N·m, volume of reaction solution: 0.0011622 m³) ３) and the required stirring power when the amount of water in the concentrate reaches 120% by mass based on the mass of the polymer is 0.11 kW / m ３ (Torque value: 0.023 N·m, Volume of concentrate (concentrated reaction solution): 0.0011066 m³) ３ The average power required for stirring from the time stirring of the reaction solution by the impeller is started until the amount of water in the concentrate reaches 120% by mass based on the mass of the polymer is 0.11 kW / m². ３ The required stirring power when the amount of water in the concentrate reached 100% by mass based on the mass of the polymer was 0.12 kW / m². ３ (Torque value: 0.025 N·m, Volume of concentrate (concentrated reaction solution): 0.0010616 m³) ３ The average power required for stirring from the time stirring of the reaction solution by the impeller is started until the amount of water in the concentrate reaches 100% by mass based on the mass of the polymer is 0.11 kW / m². ３ That was the case.

[0060] (Comparative Example 2) Under the same conditions as in Example 1, except that the rotation speed of the stirring blade in the concentration process was changed to 1000 rpm, 238.65 g of superabsorbent resin particles were obtained. In the concentration process, the required stirring power at the time stirring of the reaction solution by the stirring blade was started was 4.41 kW / m ３ (Torque value: 0.049 N·m, volume of reaction solution: 0.0011622 m³) ３ ) and the required stirring power when the amount of water in the concentrate reaches 120% by mass based on the mass of the polymer is 5.02 kW / m ３ (Torque value: 0.053 N·m, Volume of concentrate (concentrated reaction solution): 0.0011066 m³) ３ The average power required for stirring from the time stirring of the reaction solution by the impeller is started until the amount of water in the concentrate reaches 120% by mass based on the mass of the polymer is 4.89 kW / m². ３ The required stirring power when the amount of water in the concentrate reached 100% by mass based on the mass of the polymer was 5.33 kW / m². ３ (Torque value: 0.054 N·m, Volume of concentrate (concentrated reaction solution): 0.0010616 m³)３ The average power required for stirring from the time stirring of the reaction solution by the impeller is started until the amount of water in the concentrate reaches 100% by mass based on the mass of the polymer is 4.99 kW / m². ３ That was the case.

[0061] 2. Evaluation of water-absorbing resin particles (1) Medium particle size (D50) Water-absorbing resin particles were passed through a JIS standard sieve with a mesh size of 150 μm. If the amount passing through the sieve was 5.0 mass% or more of the total amount, the medium particle size was measured using the sieve combination (A) below, and if it was less than 5.0 mass%, the sieve combination (B) below was used. (A) The JIS standard sieves were combined in the following order from top to bottom: sieve with a mesh size of 710 μm, sieve with a mesh size of 600 μm, sieve with a mesh size of 500 μm, sieve with a mesh size of 425 μm, sieve with a mesh size of 300 μm, sieve with a mesh size of 250 μm, sieve with a mesh size of 150 μm, sieve with a mesh size of 75 μm, and a receiving tray. (B) The JIS standard sieves were arranged from top to bottom in the following order: a sieve with a mesh size of 850 μm, a sieve with a mesh size of 710 μm, a sieve with a mesh size of 600 μm, a sieve with a mesh size of 500 μm, a sieve with a mesh size of 425 μm, a sieve with a mesh size of 300 μm, a sieve with a mesh size of 250 μm, a sieve with a mesh size of 150 μm, and a receiving tray.

[0062] Five grams of superabsorbent polymer particles were placed in the top sieve, and the particles were classified using a continuous fully automatic ultrasonic vibration sieving analyzer (Robot Shifter RPS-205, manufactured by Seishin Corporation). After classification, the percentage (mass percentage) of the total amount of superabsorbent polymer particles remaining on each sieve was calculated. The relationship between the sieve opening and the cumulative percentage of superabsorbent polymer particles remaining on the sieve was plotted on logarithmic probability paper by accumulating the percentages starting from the fraction with the largest particle size. A graph showing the relationship between the cumulative mass percentage and particle size was created by connecting the plots on the probability paper with straight lines. From the obtained graph, the particle size (unit: μm) corresponding to a cumulative mass percentage of 50% was determined, and this value was defined as the median particle size (D50).

[0063] (2) 5% sieve particle size (D5) From the graph showing the relationship between cumulative mass percentage and particle size, which was created in the above-mentioned measurement of the medium particle size, the particle size (unit: μm) corresponding to a cumulative mass percentage of 95.0% was determined, and this value was defined as the 5% sieve particle size (D5).

[0064] (3) 90% sieve diameter (D90) From the graph showing the relationship between cumulative mass percentage and particle diameter, which was created in the above-mentioned measurement of the medium particle diameter, the particle diameter (unit: μm) corresponding to a cumulative mass percentage of 10.0 mass% was determined, and this value was defined as the 90% sieve diameter (D90).

[0065] (4) Fine powder content ratio The fine powder content ratio was calculated using the following formula. When the proportion of fine powder in the water-absorbing resin powder is small, the fine powder content ratio approaches 1. Fine powder content ratio = D5 / D50

[0066] (5) Coarse powder ratio The coarse powder ratio was calculated using the following formula. When the proportion of coarse powder in the water-absorbing resin powder is small, the coarse powder ratio will be closer to 1. Coarse powder ratio = D90 / D50

[0067] The evaluation results are shown in Table 1. Regarding the average stirring power required in Table 1, the values ​​for "Start to 120% by mass of water content" and "Start to 100% by mass of water content" are the average stirring power required from the time stirring of the reaction solution by the impeller is started until the time when the amount of water in the concentrate reaches 120% by mass and 100% by mass, respectively, based on the mass of the polymer.

[0068]

[0069] As shown in Table 1, the average power required for stirring from the time stirring of the reaction solution by the impeller is started until the time when the amount of water in the concentrate reaches 120% by mass based on the mass of the polymer was 0.20 kW / m². ３ 3.00kW / m or more ３ It was confirmed that the proportion of fine particles in the water-absorbent resin powder decreases when the particles are within the following range.

[0070] 100...Concentration device, 105...Concentration tank, 105A...Concentration chamber, 110...Reaction solution, 120...Agitator blade, 160...Thermometer, 170...Agitator, 180...Agitator shaft.

Claims

1. Forming a water-containing gel polymer containing a polymer of the water-soluble ethylenically unsaturated monomer and water by reverse-phase suspension polymerization in a reaction solution containing a water-soluble ethylenically unsaturated monomer, water, and a dispersion medium; removing water from the reaction solution by heating the reaction solution containing the water-containing gel polymer in a concentration chamber while stirring with a stirring blade fixed to a rotating stirring shaft, thereby forming a concentrate which is the concentrated reaction solution; and extracting polymer particles containing the polymer from the concentrate, wherein one or more stirring shafts are inserted into the reaction solution, and the required stirring power, calculated from the torque values ​​for rotating one or more of the stirring shafts, is an average of 0.20 kW / m² from the time stirring of the reaction solution by the stirring blades begins until the amount of water contained in the concentrate reaches 120% by mass based on the mass of the polymer. ３ 3.00kW / m or more ３ The following is a method for producing superabsorbent polymer particles.

2. The average power required for stirring from the time stirring of the reaction solution by the impeller is started until the time when the amount of water contained in the concentrate reaches 120% by mass based on the mass of the polymer, and from the time stirring of the reaction solution by the impeller is started until the time when the amount of water contained in the concentrate reaches 100% by mass based on the mass of the polymer, is 0.20 kW / m ３ 3.00kW / m or more ３ The method according to claim 1, which is as follows.

3. The method according to claim 1, wherein the number of stirring shafts inserted into the reaction solution is one.

4. The method according to claim 1, wherein when the maximum width of the concentration chamber is D and the depth of the reaction solution in the concentration chamber before heating is h, the ratio h / D is 0.80 or more and 1.60 or less.

5. The method according to claim 1, wherein when the maximum width of the concentration chamber is D and the maximum width of the stirring blade is d, the ratio d / D is 0.20 or more and 0.80 or less.

6. The method according to claim 1, wherein the stirring blade is a paddle blade.

7. The method according to any one of claims 1 to 6, wherein the water-soluble ethylenically unsaturated monomer comprises (meth)acrylic acid and its alkali metal salt.

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