Method for producing superabsorbent polymer particles

Abstract

The present invention aims to provide a method for producing water-absorbent resin particles with suppressed coloration. [Solution] A method for producing water-absorbent resin particles, comprising a polymerization step of polymerizing a water-soluble ethylenically unsaturated monomer to obtain a polymer, wherein the polymer is maintained at a temperature of 125°C or lower from the time of the polymerization step until the end of the drying step.

Description

Technical Field

[0001] The present invention relates to a method for producing water-absorbing resin particles, and more particularly to a method for producing water-absorbing resin particles constituting an absorber suitably used for sanitary materials such as paper diapers, sanitary napkins, and incontinence pads.

Background Art

[0002] Water-absorbing resins have recently been widely used in the field of sanitary materials such as paper diapers, sanitary napkins, and incontinence pads.

[0003] Conventional water-absorbing resins have a problem of being colored yellow immediately after production depending on the production method. In the field of sanitary materials such as the above-mentioned paper diapers, when the water-absorbing resin in the absorbent article is colored, the commercial value of the absorbent article is significantly reduced.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] An object of the present invention is to provide a method for producing water-absorbing resin particles with suppressed coloring.

Means for Solving the Problems

[0006] To solve the above problems, the present invention includes the following. [1] A method for producing water-absorbing resin particles including a polymerization step of polymerizing a water-soluble ethylenically unsaturated monomer to obtain a polymer, A method for producing water-absorbing resin particles, wherein the polymer is maintained at a temperature of 125°C or lower from after the polymerization step until the end of the drying step. [2] A method for producing water-absorbent resin particles according to claim 1, wherein reverse-phase suspension polymerization is carried out in the polymerization step. [3] A method for producing water-absorbing resin particles according to claim 1, wherein the polymer is maintained at a temperature of 120°C or lower from the time of the polymerization step until the end of the drying step. [Effects of the Invention]

[0007] The manufacturing method according to the present invention provides water-absorbing resin particles with suppressed discoloration. [Modes for carrying out the invention]

[0008] Embodiments of the present invention will be described in detail below. However, the present invention is not limited to the following embodiments.

[0009] 1. Method for producing water-absorbent resin particles The present invention provides a method for producing water-absorbent resin particles, which includes a drying step in which a polymer obtained by polymerization of water-soluble ethylenically unsaturated monomers is dried at a temperature of 125°C or lower. By incorporating these components, the present invention can produce water-absorbent resin particles with suppressed discoloration. The method for producing water-absorbent resin particles according to the present invention will be described in detail below.

[0010] <Polymerization process> Typical polymerization methods for water-soluble ethylenically unsaturated monomers include aqueous solution polymerization, emulsion polymerization, and reverse-phase suspension polymerization. In aqueous solution polymerization, polymerization is carried out by heating an aqueous solution of water-soluble ethylenically unsaturated monomer while stirring as needed. In reverse-phase suspension polymerization, polymerization is carried out by heating the water-soluble ethylenically unsaturated monomer in a hydrocarbon dispersion medium while stirring. In the present invention, reverse-phase suspension polymerization is preferred from the viewpoint of being able to precisely control the drying temperature.

[0011] An example of a method for producing the water-absorbing resin particles according to the present invention is described below.

[0012] In reverse-phase suspension polymerization, an aqueous monomer solution containing a water-soluble ethylenically unsaturated monomer is dispersed in a hydrocarbon dispersion medium in the presence of a dispersion stabilizer. Conventionally, when obtaining water-absorbent resin particles by reverse-phase suspension polymerization of a water-soluble ethylenically unsaturated monomer, the reverse-phase suspension polymerization can be carried out in a single polymerization step, or it can be carried out in a multi-step polymerization step of two or more stages.

[0013] [Water-soluble ethylenically unsaturated monomers] Examples of water-soluble ethylenically unsaturated monomers include (meth)acrylic acid and its salts; 2-(meth)acrylamide-2-methylpropanesulfonic acid and its salts; nonionic monomers such as (meth)acrylamide, N,N-dimethyl(meth)acrylamide, 2-hydroxyethyl(meth)acrylate, N-methylol(meth)acrylamide, and polyethylene glycol mono(meth)acrylate; and amino group-containing unsaturated monomers such as N,N-diethylaminoethyl(meth)acrylate, N,N-diethylaminopropyl(meth)acrylate, and diethylaminopropyl(meth)acrylamide, and their quaternary derivatives. Among these water-soluble ethylenically unsaturated monomers, (meth)acrylic acid or its salts, (meth)acrylamide, and N,N-dimethylacrylamide are preferred, and (meth)acrylic acid and its salts are more preferred, from the viewpoint of being readily available industrially. These water-soluble ethylenically unsaturated monomers may be used individually or in combination of two or more types.

[0014] Among these, acrylic acid and its salts are widely used as raw materials for superabsorbent resin particles, and in some cases, these acrylic acids and / or their salts are copolymerized with the other water-soluble ethylenically unsaturated monomers mentioned above. In the present invention, the water-soluble ethylenically unsaturated monomer has a content of acrylic acid and its salts of 70 to 100 mol%. That is, the proportion of acrylic acid and its salts in the total water-soluble ethylenically unsaturated monomer is 70 to 100 mol%.

[0015] Water-soluble ethylenically unsaturated monomers are dispersed in a hydrocarbon dispersion medium in aqueous solution and subjected to reverse-phase suspension polymerization. By using an aqueous solution, the dispersion efficiency of water-soluble ethylenically unsaturated monomers in the hydrocarbon dispersion medium can be increased. The concentration of the water-soluble ethylenically unsaturated monomer in this aqueous solution is preferably in the range of 20% by mass to the saturation concentration or less.

[0016] Water-soluble ethylenically unsaturated monomers may be used in which their acidic groups (the acidic groups of acrylic acid) have been neutralized beforehand with an alkaline neutralizing agent, if necessary. Examples of such alkaline neutralizing agents include alkali metal salts such as sodium hydroxide, sodium carbonate, sodium bicarbonate, potassium hydroxide, and potassium carbonate; and ammonia. These alkaline neutralizing agents may also be used in aqueous solution form to simplify the neutralization process. The above-mentioned alkaline neutralizing agents may be used individually or in combination of two or more types.

[0017] The degree of neutralization of a water-soluble ethylenically unsaturated monomer by an alkaline neutralizing agent is preferably 10 to 100 mol%, more preferably 30 to 90 mol%, even more preferably 50 to 85 mol%, and even more preferably 70 to 80 mol%, from the viewpoint of facilitating particle size control, as the degree of neutralization relative to all acid groups of the water-soluble ethylenically unsaturated monomer.

[0018] [Polymerization initiator] The radical polymerization initiator is preferably water-soluble. For example, persulfates such as potassium persulfate, ammonium persulfate, and sodium persulfate; 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, t-butyl peroxy pivalate, and hydrogen peroxide; azo compounds such as 2,2'-azobis(2-amidinopropane) dihydrochloride, 2,2'-azobis[2-(N-phenylamidinopropane)] dihydrochloride, 2,2'-azobis[2-(N-allylamidinopropane)] dihydrochloride, 2,2'-azobis{2-[1-(2-hydroxyethyl)-2-imidazolin-2-yl]propane} dihydrochloride, 2,2'-azobis{2-methyl-N-[1,1-bis(hydroxymethyl)-2-hydroxyethyl]propionamide}, 2,2'-azobis[2-methyl-N-(2-hydroxyethyl)-propionamide], and 4,4'-azobis(4-cyanovaleric acid) can be mentioned. The radical polymerization initiator may be used alone or in combination of two or more. As the radical polymerization initiator, one selected from the group consisting of potassium persulfate, ammonium persulfate, sodium persulfate, 2,2'-azobis(2-amidinopropane) dihydrochloride, and 2,2'-azobis{2-[1-(2-hydroxyethyl)-2-imidazolin-2-yl]propane} dihydrochloride is preferred.

[0019] The amount of the radical polymerization initiator used may be 0.05 to 10 mmol per 1 mol of the water-soluble ethylenically unsaturated monomer. When the amount of the radical polymerization initiator used is 0.05 mmol or more, the polymerization reaction does not require a long time and is efficient. When the amount of the radical polymerization initiator used is 10 mmol or less, it is easy to suppress the occurrence of a rapid polymerization reaction.

[0020] In addition, the radical polymerization initiator can also be used as a redox polymerization initiator in combination with a reducing agent such as sodium sulfite, sodium bisulfite, ferrous sulfate, and L-ascorbic acid.

[0021] During the polymerization reaction, the aqueous monomer solution used for polymerization may contain a chain transfer agent. Examples of the chain transfer agent include hypophosphites, thiols, thiolic acids, secondary alcohols, and amines.

[0022] [Polymerization reaction temperature] From the viewpoint of accelerating the polymerization, shortening the polymerization time to enhance the economy, and easily removing the heat of polymerization to smoothly conduct the reaction, the reaction temperature of the polymerization reaction is preferably 20 to 110°C, and more preferably 40 to 90°C.

[0023] [Internal crosslinking agent] Examples of internal crosslinking agents include those capable of crosslinking the polymer of the water-soluble ethylenically unsaturated monomer used, such as unsaturated polyesters obtained by reacting polyols such as diols and triols, including (poly)ethylene glycol, (poly)propylene glycol, 1,4-butanediol, trimethylolpropane, and (poly)glycerin, with unsaturated acids such as (meth)acrylic acid, maleic acid, and fumaric acid; bisacrylamides such as N,N-methylenebisacrylamide; di(meth)acrylic acid esters or tri(meth)acrylic acid esters obtained by reacting polyepoxides with (meth)acrylic acid; di(meth)acrylic acid carbamyl esters obtained by reacting polyisocyanates such as tolylene diisocyanate and hexamethylene diisocyanate with hydroxyethyl (meth)acrylate; allylated starch, allylated cellulose, diallyl phthalate, N,N' Examples include compounds having two or more polymerizable unsaturated groups such as N''-trialyl isocyanurate and divinylbenzene; polyglycidyl compounds such as diglycidyl compounds and triglycidyl compounds such as (poly)ethylene glycol diglycidyl ether, (poly)propylene glycol diglycidyl ether, and (poly)glycerin diglycidyl ether; epihalohydrin compounds such as epichlorohydrin, epibromuhydrin, and α-methylepichlorohydrin; compounds having two or more reactive functional groups such as isocyanate compounds such as 2,4-tolylene diisocyanate and hexamethylene diisocyanate; and 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. Among these internal crosslinking agents, it is preferable to use unsaturated polyesters or polyglycidyl compounds, more preferably diglycidyl ether compounds, and especially (poly)ethylene glycol diglycidyl ether, (poly)propylene glycol diglycidyl ether, and (poly)glycerin diglycidyl ether. These internal crosslinking agents may be used individually or in combination of two or more types.

[0024] The amount of internal crosslinking agent used is preferably 30 mmol or less per mole of water-soluble ethylenically unsaturated monomer, more preferably 0 to 20 mmol, even more preferably 0 to 10 mmol, and still more preferably 0 to 5 mmol.

[0025] [Hydroxide dispersion medium] Examples of hydrocarbon dispersion media include aliphatic hydrocarbons with 6 to 8 carbon atoms 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. Among these hydrocarbon dispersion media, n-hexane, n-heptane, and cyclohexane are particularly suitable due to their easy industrial availability, stable quality, and low cost. These hydrocarbon dispersion media may be used individually or in combination of two or more types. As an example of a hydrocarbon dispersion media mixture, commercially available products such as Exsolheptane (manufactured by ExxonMobil: containing 75-85% by mass of heptane and its isomers) can also be used to obtain satisfactory results.

[0026] As for the amount of hydrocarbon dispersion medium used, from the viewpoint of uniformly dispersing the water-soluble ethylenically unsaturated monomer and facilitating control of the polymerization temperature, it is preferably 100 to 1500 parts by mass, more preferably 150 to 1000 parts by mass, and even more preferably 200 to 500 parts by mass, per 100 parts by mass of the first-stage water-soluble ethylenically unsaturated monomer.

[0027] [Dispersion stabilizer] (Surfactants) In reverse-phase suspension polymerization, dispersion stabilizers are used to improve the dispersion stability of water-soluble ethylenically unsaturated monomers in hydrocarbon dispersion media. Surfactants can be used as these dispersion stabilizers.

[0028] Examples of surfactants that can be used include sucrose fatty acid esters, polyglycerin fatty acid esters, sorbitan 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, alkylallylformaldehyde condensed polyoxyethylene ethers, polyoxyethylene polyoxypropylene block copolymers, polyoxyethylene polyoxypropyl alkyl ethers, polyethylene glycol fatty acid esters, alkyl glucosides, N-alkylgluconamides, polyoxyethylene fatty acid amides, polyoxyethylene alkylamines, phosphate esters of polyoxyethylene alkyl ethers, and phosphate esters of polyoxyethylene alkylallyl ethers. Among these surfactants, sorbitan fatty acid esters, polyglycerin fatty acid esters, and sucrose fatty acid esters are particularly preferred in terms of monomer dispersion stability. These surfactants may be used individually or in combination of two or more types.

[0029] The amount of surfactant used is preferably 0.1 to 30 parts by mass, and more preferably 0.3 to 20 parts by mass, per 100 parts by mass of the first-stage water-soluble ethylenically unsaturated monomer.

[0030] (Polymer-based dispersant) Furthermore, as a dispersion stabilizer used in reverse-phase suspension polymerization, polymer-based dispersants may be used in combination with the surfactants mentioned above.

[0031] Examples of 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, ethylcellulose, and ethyl hydroxyethylcellulose. Among these polymeric dispersants, it is particularly preferable to use maleic anhydride-modified polyethylene, maleic anhydride-modified polypropylene, maleic anhydride-modified ethylene-propylene copolymer, maleic anhydride-ethylene copolymer, maleic anhydride-propylene copolymer, maleic anhydride-ethylene-propylene copolymer, polyethylene, polypropylene, ethylene-propylene copolymer, oxidized polyethylene, oxidized polypropylene, and oxidized ethylene-propylene copolymer, from the viewpoint of monomer dispersion stability. These polymeric dispersants may be used individually or in combination of two or more types.

[0032] The amount of polymeric dispersant used is preferably 0.1 to 30 parts by mass, and more preferably 0.3 to 20 parts by mass, per 100 parts by mass of the first-stage water-soluble ethylenically unsaturated monomer.

[0033] [Other ingredients] In the method for producing water-absorbent polymer particles, other components may be added to an aqueous solution containing a water-soluble ethylenically unsaturated monomer to carry out reverse-phase suspension polymerization, if desired. Other components may include various additives such as thickeners and chain transfer agents.

[0034] <Dehydration process> The dehydration step includes a dehydration treatment in which, after the polymerization described above, energy such as heat is applied from the outside to remove water, hydrocarbon dispersion medium, etc., from the polymer by distillation, and may be performed as needed. The dehydration treatment by distillation may be performed under atmospheric pressure or under reduced pressure. Furthermore, from the viewpoint of increasing efficiency, it may be performed under a stream of gas such as nitrogen. When performing the dehydration treatment, the dehydration temperature is preferably 70°C to 125°C, 75°C to 125°C, 80°C to 120°C, 70°C to 120°C, 75°C to 120°C, 80°C to 110°C, 70°C to 118°C, 75°C to 118°C, or 80°C to 118°C.

[0035] <Surface crosslinking process> The surface crosslinking step is a step in which the surface of the polymer after the polymerization step described above is crosslinked, and may be performed as needed. It may also be obtained by adding a surface crosslinking agent to the polymer after the dehydration step to crosslink the polymer. This surface crosslinking reaction can be carried out in the presence of the surface crosslinking agent.

[0036] The amount of surface crosslinking agent added may be in the range of 0.001 to 1000 parts by mass, or in the range of 0.001 to 100 parts by mass, per 100 parts by mass of water-soluble ethylenically unsaturated monomer.

[0037] Examples of surface crosslinking agents include compounds having two or more reactive functional groups. For example, polyols such as ethylene glycol, propylene glycol, 1,4-butanediol, trimethylolpropane, glycerin, polyoxyethylene glycol, polyoxypropylene glycol, and polyglycerin; polyglycidyl compounds such as (poly)ethylene glycol diglycidyl ether, (poly)glycerin diglycidyl ether, (poly)glycerin triglycidyl ether, trimethylolpropane triglycidyl ether, (poly)propylene glycol polyglycidyl ether, and (poly)glycerol polyglycidyl ether; epichlorohydrin, epibromohydrin, and α-methylepichlorohydrin. Examples include halo epoxy 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-oxetaneethanol, 3-ethyl-3-oxetaneethanol, and 3-butyl-3-oxetaneethanol; oxazoline compounds such as 1,2-ethylenebisoxazoline; carbonate compounds such as ethylene carbonate; and hydroxyalkylamide compounds such as bis[N,N-di(β-hydroxyethyl)]adipamide. Among these surface crosslinking agents, 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 are preferred. These surface crosslinking agents may be used individually or in combination of two or more types.

[0038] Regarding the method of adding the surface crosslinking agent, it may be added as is, as an aqueous solution, or, if necessary, as a solution using a hydrophilic organic solvent. Examples of hydrophilic organic solvents include lower alcohols such as methyl alcohol, ethyl alcohol, n-propyl alcohol, and isopropyl alcohol; ketones such as acetone and methyl ethyl ketone; ethers such as diethyl ether, dioxane, and tetrahydrofuran; amides such as N,N-dimethylformamide; and sulfoxides such as dimethyl sulfoxide. These hydrophilic organic solvents may be used individually, in combination of two or more, or as a mixed solvent with water.

[0039] The reaction temperature for the surface crosslinking reaction is preferably between 70°C and 125°C. Furthermore, the reaction time for the surface crosslinking reaction is preferably between 1 and 300 minutes, and more preferably between 5 and 200 minutes.

[0040] Furthermore, when adding the surface crosslinking agent to the water-absorbent resin particles, the residual water content is preferably 5-60%, more preferably 5-50%, even more preferably 5-40%, and particularly preferably 10-30%.

[0041] The remaining water level is calculated using the following formula (1). Equation (1) Residual water content [%] = (Mass of water remaining in the polymer / Mass of water-soluble ethylenically unsaturated monomers used in the polymerization process) × 100

[0042] <Drying process> After the polymerization process described above, the polymer is dried by applying external energy such as heat to remove water and other substances from the polymer. This allows for obtaining a dried polymer product. Alternatively, hydrocarbon dispersion media may be removed during the drying process.

[0043] In the drying process, the drying treatment by distillation may be carried out under atmospheric pressure or under reduced pressure. Furthermore, from the viewpoint of improving drying efficiency, it may be carried out under a stream of gas such as nitrogen. From the viewpoint of improving drying efficiency, the lower limit of the drying temperature is preferably 70°C or higher, more preferably 75°C or higher, and even more preferably 80°C or higher. From the viewpoint of suppressing discoloration of the water-absorbing resin particles, the upper limit of the drying temperature is preferably 125°C or lower, more preferably 120°C or lower, and even more preferably 118°C or lower. From the viewpoint of improving drying efficiency and suppressing discoloration of the water-absorbing resin particles, the drying temperature is preferably 70°C to 125°C, 75°C to 125°C, 80°C to 120°C, 70°C to 120°C, 75°C to 120°C, 80°C to 110°C, 70°C to 118°C, 75°C to 118°C, and 80°C to 118°C.

[0044] The drying methods used in the above drying process include heating drying, hot air drying, reduced pressure drying, fluidized bed drying, infrared drying, microwave drying, drum dryer drying, drying by azeotropic dehydration with hydrophobic organic solvents, and high-humidity drying using high-temperature steam. Among these, heating drying, reduced pressure drying, and drying by azeotropic dehydration with hydrophobic organic solvents are preferred from the viewpoint of suppressing discoloration of the water-absorbing resin particles. As for the drying equipment used in the above drying process, commonly used dryers such as band dryers, groove dryers, rotary dryers, and kneaders equipped with drying devices can usually be used. The drying time varies depending on the drying temperature, but is usually about 0.3 to 5 hours.

[0045] The water-absorbing resin particles obtained by the manufacturing method of the present invention may consist only of polymer particles, but may further contain various additional components selected from, for example, inorganic powders, surfactants, oxidizing agents, reducing agents, metal chelating agents, antioxidants, and antibacterial agents. The additional components may be arranged inside the polymer particles, on the surface of the polymer particles, or both. The additional components may be fluidity improvers (lubricants). The fluidity improvers may be inorganic particles. Examples of inorganic particles include silica particles such as amorphous silica. [Examples]

[0046] The present invention will be described in detail below with reference to examples. However, the present invention is not limited to these examples. The water-absorbing resin particles obtained in the following examples and comparative examples were evaluated by the various test methods described later. Unless otherwise specified, evaluations were carried out in an environment with a temperature of 25±2℃ and a humidity of 50±10%.

[0047] <Manufacturing of water-absorbing resin particles> (Example 1) [First stage polymerization process] A reflux condenser, a dropping funnel, a nitrogen gas inlet tube, and a round-bottom cylindrical separable flask with an inner diameter of 11 cm and a capacity of 2 L were prepared. The flask was equipped with a stirring blade consisting of two stages of four inclined paddle blades with a blade diameter of 5 cm. 300 g of n-heptane was taken as the hydrocarbon dispersion medium into this separable flask, and 0.736 g of maleic anhydride-modified ethylene-propylene copolymer (Mitsui Chemicals, Inc., High Wax 1105A), a polymeric dispersant, and 0.736 g of sucrose stearate ester of HLB3 (Mitsubishi Chemical Foods Corporation, Ryoto Sugar Ester S-370), a surfactant, were added as dispersants. The mixture was heated to 80°C while stirring to dissolve the dispersants, and then cooled to 50°C.

[0048] Meanwhile, in a 300 mL beaker, 92.0 g (1.03 mol) of an 80.5% by mass acrylic acid aqueous solution was taken as a water-soluble ethylenically unsaturated monomer. While cooling from the outside, 102.8 g of a 30% by mass sodium hydroxide aqueous solution was added dropwise to neutralize it to 75 mol%, and then 0.092 g of hydroxyethylcellulose (Sumitomo Seika Co., Ltd., HEC AW-15F) as a thickening agent, 0.0736 g (0.272 mmol) of potassium persulfate as a water-soluble radical polymerization agent, 0.0101 g (0.058 mmol) of ethylene glycol diglycidyl ether as an internal crosslinking agent, and 44.63 g of ion-exchanged water were added and dissolved to prepare the first stage monomer aqueous solution. Then, the monomer aqueous solution prepared above was added to a separable flask, and while stirring at a stirrer speed of 550 rpm, the system was thoroughly purged with nitrogen. After that, the separable flask was immersed in a 70°C water bath and the temperature was raised, and polymerization was carried out for 60 minutes to obtain the first stage polymerization slurry.

[0049] [Second stage polymerization process] Meanwhile, in a separate 500 mL beaker, 128.8 g (1.44 mol) of an 80.5% by mass acrylic acid aqueous solution was taken as a water-soluble ethylenically unsaturated monomer. While cooling from the outside, 143.8 g of a 30% by mass sodium hydroxide aqueous solution was added dropwise to neutralize it to 75 mol%, and then 0.1030 g (0.381 mmol) of potassium persulfate as a water-soluble radical polymerization initiator, 0.0116 g (0.067 mmol) of ethylene glycol diglycidyl ether as an internal crosslinking agent, and 16.64 g of deionized water were added and dissolved to prepare the second stage monomer aqueous solution.

[0050] While stirring with a stirrer at 1000 rpm, the contents of the separable flask system were cooled to 27°C. Then, the entire amount of the second-stage monomer aqueous solution was added to the first-stage polymerization slurry, the system was purged with nitrogen for 30 minutes, and the separable flask was again immersed in a 70°C water bath to raise the temperature and carry out the polymerization reaction for 60 minutes to obtain the polymer after the second stage of polymerization.

[0051] [Dehydration process] Next, the separable flask was immersed in an oil bath set at 125°C, and 178g of water was removed from the system by azeotropic distillation of n-heptane and water while refluxing the n-heptane. 2.58g (0.62 mmol) of a 3% by mass sodium sulfite aqueous solution and 4.91g of a 4.5% by mass sodium diethylenetriaminepentaacetate aqueous solution were added to the separable flask. Then, the separable flask was immersed in an oil bath set at 125°C, and 83g of water was removed from the system by azeotropic distillation of n-heptane and water while refluxing the n-heptane.

[0052] [Surface crosslinking process] Subsequently, 5.52 g (0.633 mmol) of a 2% by mass aqueous solution of ethylene glycol diglycidyl ether was added to the separable flask as a surface crosslinking agent, and the flask was held at 83°C for 2 hours.

[0053] [Drying process] Subsequently, the separable flask was heated in an oil bath at 125°C. The temperature inside the separable flask system was 101°C when n-heptane and water were distilled off from the system (start of drying). The temperature inside the separable flask system was 100°C when evaporation continued until no more n-heptane and water were distilled off from the system (end of drying). The dried polymer was classified using a sieve with a mesh size of 850 μm to obtain 210 g of superabsorbent polymer particles.

[0054] (Example 2) [First stage polymerization process] A reflux condenser, a dropping funnel, a nitrogen gas inlet tube, and a round-bottom cylindrical separable flask with an inner diameter of 11 cm and a capacity of 2 L were prepared. The flask was equipped with a stirring blade consisting of two stages of four inclined paddle blades with a blade diameter of 5 cm. 300 g of n-heptane was taken as the hydrocarbon dispersion medium into this separable flask, and 0.736 g of maleic anhydride-modified ethylene-propylene copolymer (Mitsui Chemicals, Inc., High Wax 1105A), a polymeric dispersant, and 0.736 g of sucrose stearate ester of HLB3 (Mitsubishi Chemical Foods Corporation, Ryoto Sugar Ester S-370), a surfactant, were added as dispersants. The mixture was heated to 80°C while stirring to dissolve the dispersants, and then cooled to 50°C.

[0055] Meanwhile, in a 300 mL beaker, 92.0 g (1.03 mol) of an 80.5% by mass acrylic acid aqueous solution was taken as a water-soluble ethylenically unsaturated monomer. While cooling from the outside, 102.8 g of a 30% by mass sodium hydroxide aqueous solution was added dropwise to neutralize it to 75 mol%, and then 0.092 g of hydroxyethylcellulose (Sumitomo Seika Co., Ltd., HEC AW-15F) as a thickening agent, 0.0184 g (0.068 mmol) of potassium persulfate and 0.0920 g (0.339 mmol) of 2,2'-azobis(2-amidinopropane) dihydrochloride as water-soluble radical polymerization agents, 0.0046 g (0.026 mmol) of ethylene glycol diglycidyl ether as an internal crosslinking agent, and 44.60 g of ion-exchanged water were added and dissolved to prepare the first stage monomer aqueous solution. Then, the monomer aqueous solution prepared above was added to a separable flask, and while stirring at a stirrer speed of 550 rpm, the system was thoroughly purged with nitrogen. After that, the separable flask was immersed in a 70°C water bath and the temperature was raised, and polymerization was carried out for 60 minutes to obtain the first stage polymerization slurry.

[0056] [Second stage polymerization process] Meanwhile, in a separate 500 mL beaker, 128.8 g (1.44 mol) of an 80.5% by mass acrylic acid aqueous solution was taken as a water-soluble ethylenically unsaturated monomer. While cooling from the outside, 143.8 g of a 30% by mass sodium hydroxide aqueous solution was added dropwise to neutralize it to 75 mol%, and then 0.0258 g (0.095 mmol) of potassium persulfate, 0.1288 g (0.475 mmol) of 2,2'-azobis(2-amidinopropane) dihydrochloride, 0.0116 g (0.067 mmol) of ethylene glycol diglycidyl ether, and 16.59 g of deionized water were added and dissolved to prepare the second stage monomer aqueous solution.

[0057] While stirring with a stirrer at 1000 rpm, the contents of the separable flask system were cooled to 27°C. Then, the entire amount of the second-stage monomer aqueous solution was added to the first-stage polymerization slurry, the system was purged with nitrogen for 30 minutes, and the separable flask was again immersed in a 70°C water bath to raise the temperature and carry out the polymerization reaction for 60 minutes to obtain the polymer after the second stage of polymerization.

[0058] [Dehydration process] A separable flask was immersed in an oil bath set at 125°C, and n-heptane and water were extracted from the system by distillation while maintaining a pressure of 40 kPa. The amount of water extracted was 177 g. The pressure inside the separable flask was returned to atmospheric pressure, and 2.58 g (0.62 mmol) of a 3% by mass sodium sulfite aqueous solution and 4.91 g of a 4.5% by mass sodium diethylenetriaminepentaacetate aqueous solution were added to the separable flask. Next, the separable flask was immersed in an oil bath set at 125°C, and n-heptane and water were extracted from the system by distillation while maintaining a pressure of 40 kPa. The amount of water extracted from the system was 65 g.

[0059] [Surface crosslinking process] The pressure inside the separable flask was returned to atmospheric pressure, and 5.52 g (0.633 mmol) of a 2% by mass aqueous solution of ethylene glycol diglycidyl ether was sprayed into the separable flask as a surface crosslinking agent. The flask was then maintained at 83°C for 2 hours.

[0060] [Drying process] Subsequently, the pressure inside the separable flask system was maintained at 40 kPa, and the system was heated in an oil bath at 125°C. The temperature inside the separable flask system was 98°C when n-heptane and water were distilled off from the system (start of drying). The temperature inside the separable flask system was 99°C when evaporation continued until no more n-heptane and water were distilled off from the system (end of drying). The dried polymer was classified using a sieve with a mesh size of 850 μm to obtain 207 g of superabsorbent polymer particles.

[0061] (Example 3) [Polymerization process] A reflux condenser, a dropping funnel, a nitrogen gas inlet tube, and a round-bottom cylindrical separable flask with an inner diameter of 11 cm and a capacity of 2 L were prepared, along with a stirring blade consisting of two stages of four inclined paddle blades with a blade diameter of 5 cm (coated on the surface with fluororesin). 479 g of n-heptane was added to this separable flask as a hydrocarbon dispersion medium, and 1.10 g of HLB 8.6 sorbitan monolaurate (manufactured by NOF Corporation, trade name Nonion LP-20R) was added as a surfactant, and the mixture was heated to 46°C.

[0062] Meanwhile, 92.0 g (1.03 mol) of an 80.5% by mass aqueous solution of acrylic acid as a water-soluble ethylenically unsaturated monomer was taken into a 300 mL beaker, and while cooling with ice water, 102.8 g of a 30% by mass aqueous solution of sodium hydroxide was added dropwise to prepare a 75 mol% neutralized acrylic acid product. Furthermore, 44.6 g of water and 0.1012 g (0.374 mmol) of potassium persulfate as a water-soluble radical polymerization agent were added and dissolved to prepare an aqueous monomer solution. This aqueous monomer solution was added to a separable flask, and while stirring at a stirrer speed of 700 rpm, the inside of the separable flask was replaced with nitrogen for 30 minutes. After this, the separable flask was immersed in a 70°C water bath and the temperature was raised, and polymerization was carried out for 60 minutes to obtain a polymer.

[0063] After polymerization, 0.41 g (0.047 mmol) of a 2% by mass aqueous solution of ethylene glycol diglycidyl ether was added to the separable flask containing the polymerization slurry.

[0064] [Dehydration process] Next, the separable flask was immersed in an oil bath set at 125°C, and 98.7g of water was removed from the system by azeotropic distillation of n-heptane and water while refluxing the n-heptane.

[0065] [Surface crosslinking process] Subsequently, 2.76 g (0.317 mmol) of a 2% by mass aqueous solution of ethylene glycol diglycidyl ether was added to the separable flask as a post-crosslinking agent, and the mixture was held at 83°C for 2 hours.

[0066] [Drying process] Subsequently, the separable flask was heated in an oil bath at 125°C. The temperature inside the separable flask system was 86°C when n-heptane and water were distilled off from the system (start of drying). The temperature inside the separable flask system was 118°C when evaporation continued until no more n-heptane and water were distilled off from the system (end of drying). The dried polymer was classified using a sieve with a mesh size of 850 μm to obtain 69 g of superabsorbent polymer particles.

[0067] <Evaluation of water-absorbing resin particles> <Measurement of yellowness> 2.0 g of superabsorbent polymer particles were placed in a glass measuring container with an inner diameter of 3 cm. The yellowness of the superabsorbent polymer particles was measured using a colorimeter (Color Meter ZE6000, manufactured by Nippon Denshoku Industries Ltd.) with the tristimulus values ​​X, Y, and Z of the colorimeter corrected on a standard whiteboard. The yellowness was then calculated from the obtained X, Y, and Z (tristimulus values) of the superabsorbent polymer particles using the following formula.

[0068] Yellowness=100(1.28X-1.06Z) / Y

[0069] [Table 1]

Claims

1. A method for producing water-absorbent resin particles, comprising a polymerization step of polymerizing a water-soluble ethylenically unsaturated monomer to obtain a polymer, A method for producing water-absorbing resin particles, wherein the polymer is maintained at a temperature of 125°C or lower from the time of the polymerization step until the end of the drying step.

2. A method for producing water-absorbent resin particles according to claim 1, wherein reverse-phase suspension polymerization is carried out in the polymerization step.

3. A method for producing water-absorbing resin particles according to claim 1, wherein the polymer is maintained at a temperature of 120°C or lower from the time of the polymerization step until the end of the drying step.

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