Method for producing water-absorbing resin particle

Abstract

To provide a method for producing water-absorbing resin particles having less coarse powder and fine powder.SOLUTION: A method for producing water-absorbing resin particles includes a step of subjecting a water-soluble ethylenic unsaturated monomer to reversed phase suspension polymerization in a hydrocarbon dispersion medium, in the presence of a dispersion stabilizer, wherein the hydrocarbon dispersion medium contains n-heptane, C6-8 aliphatic hydrocarbon excluding the n-heptane, and C6-8 alicyclic hydrocarbon, and the n-heptane is 20 pts.mass or more with respect to 100 pts.mass of the hydrocarbon dispersion medium.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a method for producing water-absorbent resin particles, and more particularly to a method for producing water-absorbent resin particles by inverse suspension polymerization of a water-soluble ethylenically unsaturated monomer. [Background technology]

[0002] Water-absorbent resin particles are used in the fields of sanitary materials such as diapers, sanitary products, and incontinence pads, and industrial materials such as waterproofing materials and dew condensation prevention agents. For example, water-absorbent resin particles are produced by a method of polymerizing ethylenically unsaturated monomers to obtain polymer particles (for example, Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] International Publication No. 2012 / 014748 Summary of the Invention [Problem to be solved by the invention]

[0004] An object of the present invention is to provide a method for producing water-absorbent resin particles containing less coarse powder and fine powder. [Means for solving the problem]

[0005] In order to solve the above problems, the present invention includes the following. [1] A method for producing water-absorbent resin particles, comprising a step of subjecting a water-soluble ethylenically unsaturated monomer to reverse phase suspension polymerization in a hydrocarbon dispersion medium in the presence of a dispersion stabilizer, the hydrocarbon dispersion medium contains n-heptane, an aliphatic hydrocarbon having 6 to 8 carbon atoms other than n-heptane, and an alicyclic hydrocarbon having 6 to 8 carbon atoms; The n-heptane is 20 parts by mass or more relative to 100 parts by mass of the hydrocarbon dispersion medium; A method for producing water-absorbent resin particles. [2] 2. The method for producing water-absorbent resin particles according to claim 1, wherein the alicyclic hydrocarbon having 6 to 8 carbon atoms is 60 parts by mass or less with respect to 100 parts by mass of the hydrocarbon dispersion medium. Effect of the Invention

[0006] The production method according to the present invention provides water-absorbent resin particles with little coarse powder and fine powder. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0007] Hereinafter, embodiments of the present invention will be described in detail, but the present invention is not limited to the following embodiments.

[0008] 1. Manufacturing method of water-absorbent resin particles The method for producing water-absorbent resin particles of the present invention is a method for producing water-absorbent resin particles by reversed-phase suspension polymerization of a water-soluble ethylenically unsaturated monomer in a hydrocarbon dispersion medium. The method for producing water-absorbent resin particles of the present invention includes a polymerization step of mixing an aqueous solution containing the water-soluble ethylenically unsaturated monomer, a hydrocarbon dispersion medium, a radical polymerization initiator, a dispersion stabilizer, and an internal crosslinking agent, and a surface crosslinking step of surface-crosslinking the polymer obtained in the polymerization step. In the method for producing water-absorbent resin particles of the present invention, the reversed-phase suspension polymerization includes two or more polymerization steps.

[0009] By providing these configurations, the present invention can produce water-absorbent resin particles. The method for producing water-absorbent resin particles of the present invention will be described in detail below.

[0010] [Reverse Phase Suspension Polymerization] In carrying out the reversed 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.

[0011] Conventionally, when a water-soluble ethylenically unsaturated monomer is subjected to reverse phase suspension polymerization to obtain water-absorbent resin particles, the reverse phase suspension polymerization can be performed in only one polymerization step, but the reverse phase suspension polymerization in the present invention is performed in two or more polymerization steps. From the viewpoint of enhancing the productivity of water-absorbent resin particles while favorably exerting the effects of the present invention, the reverse phase suspension polymerization is preferably performed in two to three polymerization steps.

[0012] In the reversed-phase suspension polymerization, after the first-stage polymerization step (first-stage reversed-phase suspension polymerization), a water-soluble ethylenically unsaturated monomer is added to the reaction mixture obtained in the first-stage polymerization reaction and mixed, and the second-stage and subsequent polymerization steps (second-stage reversed-phase suspension polymerization) can be carried out in the same manner as in the first stage. In the reversed-phase suspension polymerization in each stage after the second stage, it is preferable to carry out the reversed-phase suspension polymerization by adding a predetermined amount of a radical polymerization initiator in addition to the water-soluble ethylenically unsaturated monomer based on the amount of the water-soluble ethylenically unsaturated monomer added during the reversed-phase suspension polymerization in each stage after the second stage. In the second-stage and subsequent polymerizations, an internal crosslinking agent may also be added to the water-soluble ethylenically unsaturated monomer as necessary.

[0013] In the reversed-phase suspension polymerization, polymerization is carried out in the presence of a dispersion stabilizer (surfactant or polymeric dispersant). The dispersion stabilizer may be added either before or after the addition of the aqueous monomer solution, so long as it is added before the start of the polymerization reaction.

[0014] From the viewpoint of easily reducing the amount of the hydrocarbon dispersion medium remaining in the obtained water absorbent resin particles, it is preferable to disperse an aqueous monomer solution in a hydrocarbon dispersion medium having a polymeric dispersant dispersed therein, and then further disperse a surfactant therein, and then carry out polymerization.

[0015] The reaction temperature of the polymerization reaction is preferably 20 to 110°C, and more preferably 40 to 90°C, from the viewpoints of rapidly progressing the polymerization and shortening the polymerization time to thereby improve economic efficiency, and of easily removing the heat of polymerization to allow the reaction to proceed smoothly.

[0016] <Polymerization process> In the first and second and subsequent polymerization steps, the water-soluble ethylenically unsaturated monomer is polymerized by reversed-phase suspension polymerization using a radical polymerization initiator. Specifically, an aqueous solution containing the water-soluble ethylenically unsaturated monomer, a hydrocarbon dispersion medium, a radical polymerization initiator, and an internal crosslinking agent are mixed to allow the polymerization reaction of the water-soluble ethylenically unsaturated monomer to proceed.

[0017] The first and second and subsequent polymerization steps will be described in detail below.

[0018] [Water-soluble ethylenically unsaturated monomers] Examples of water-soluble ethylenically unsaturated monomers include (meth)acrylic acid and its salts; 2-(meth)acrylamido-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 and their quaternized products such as N,N-diethylaminoethyl(meth)acrylate, N,N-diethylaminopropyl(meth)acrylate, and diethylaminopropyl(meth)acrylamide. 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 industrial availability. These water-soluble ethylenically unsaturated monomers may be used alone or in combination of two or more.

[0019] Among these, acrylic acid and its salts are widely used as raw materials for water-absorbent resin particles, and these acrylic acid and / or its salts may be copolymerized with the other water-soluble ethylenically unsaturated monomers described above for use. In the present invention, the content of acrylic acid and its salts in the water-soluble ethylenically unsaturated monomer is 70 to 100 mol %. That is, the ratio of acrylic acid and its salts to the total water-soluble ethylenically unsaturated monomers is 70 to 100 mol %.

[0020] The water-soluble ethylenically unsaturated monomer is dispersed in a hydrocarbon dispersion medium in the form of an aqueous solution and subjected to reversed-phase suspension polymerization. By forming the water-soluble ethylenically unsaturated monomer into an aqueous solution, the dispersion efficiency 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 saturated concentration.

[0021] The concentration of the water-soluble ethylenically unsaturated monomer in the first-stage polymerization is preferably 55% by mass or less, more preferably 50% by mass or less, and even more preferably 48% by mass or less, from the viewpoint of easy particle size control. On the other hand, the concentration of the water-soluble ethylenically unsaturated monomer is preferably 25% by mass or more, more preferably 30% by mass or more, and even more preferably 35% by mass or more, from the viewpoint of easy particle size control.

[0022] The concentration of the water-soluble ethylenically unsaturated monomer in the second-stage polymerization is preferably 55% by mass or less, more preferably 50% by mass or less, and even more preferably 48% by mass or less, from the viewpoint of easy particle size control. On the other hand, the concentration of the water-soluble ethylenically unsaturated monomer is preferably 30% by mass or more, more preferably 35% by mass or more, and even more preferably 40% by mass or more, from the viewpoint of easy particle size control.

[0023] The ratio of the amount of the water-soluble ethylenically unsaturated monomer used in the second stage polymerization to the amount of the water-soluble ethylenically unsaturated monomer used in the first stage polymerization (amount used in the second stage / amount used in the first stage) is preferably 0.1 to 3.0, more preferably 0.5 to 2.5, even more preferably 1.0 to 2.0, and even more preferably 1.2 to 1.8, from the viewpoint of facilitating particle size control.

[0024] The water-soluble ethylenically unsaturated monomer may be used in which the acid group (acid group of acrylic acid) has been neutralized in advance with an alkaline neutralizing agent as necessary. Examples of such alkaline neutralizing agents include alkali metal salts such as sodium hydroxide, sodium carbonate, sodium hydrogen carbonate, potassium hydroxide, and potassium carbonate; ammonia, etc. In addition, these alkaline neutralizing agents may be used in the form of an aqueous solution in order to simplify the neutralization operation. The above-mentioned alkaline neutralizing agents may be used alone or in combination of two or more kinds.

[0025] From the viewpoint of facilitating particle size control, the degree of neutralization of the water-soluble ethylenically unsaturated monomer with the 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%, as the degree of neutralization with respect to all acid groups possessed by the water-soluble ethylenically unsaturated monomer.

[0026] [Polymerization initiator] The radical polymerization initiator is preferably water-soluble, and examples thereof include 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 peroxypivalate, and hydrogen peroxide; 2,2'-azobis(2-amidinopropane) dihydrochloride, 2,2'-azobis[2-(N-phenyl azo compounds such as 2,2'-azobis[2-(N-allylamidino)propane] 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). The radical polymerization initiator may be used alone or in combination of two or more kinds. The radical polymerization initiator is preferably 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.

[0027] The amount of the radical polymerization initiator used may be 0.05 to 10 millimoles per mole of the water-soluble ethylenically unsaturated monomer. When the amount of the radical polymerization initiator used is 0.05 millimoles or more, the polymerization reaction does not take a long time and is efficient. When the amount of the radical polymerization initiator used is 10 millimoles or less, it is easy to suppress the occurrence of a rapid polymerization reaction.

[0028] The radical polymerization initiator can also be used as a redox polymerization initiator in combination with a reducing agent such as sodium sulfite, sodium hydrogen sulfite, ferrous sulfate, or L-ascorbic acid.

[0029] In the polymerization reaction, the aqueous monomer solution used for the polymerization may contain a chain transfer agent, such as hypophosphites, thiols, thiolic acids, secondary alcohols, and amines.

[0030] [Internal crosslinking agent] Examples of the internal crosslinking agent include those capable of crosslinking the polymer of the water-soluble ethylenically unsaturated monomer used. For example, unsaturated polyesters obtained by reacting polyols such as diols and triols, such as (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' ,N''-triallyl isocyanurate, divinylbenzene, and other compounds having two or more polymerizable unsaturated groups; 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, epibromohydrin, 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, it is more preferable to use diglycidyl ether compounds, and it is preferable to use (poly)ethylene glycol diglycidyl ether, (poly)propylene glycol diglycidyl ether, or (poly)glycerin diglycidyl ether. These internal crosslinking agents may be used alone or in combination of two or more kinds.

[0031] The amount of the internal crosslinking agent used is preferably 30 mmol or less, more preferably 0.001 to 20 mmol, even more preferably 0.005 to 10 mmol, and even more preferably 0.01 to 5 mmol, per mol of the water-soluble ethylenically unsaturated monomer.

[0032] [Hydrocarbon dispersion medium] The hydrocarbon dispersion medium contains at least n-heptane as a hydrocarbon. The hydrocarbon dispersion medium preferably contains an aliphatic hydrocarbon having 6 to 8 carbon atoms other than n-heptane, and more preferably contains an alicyclic hydrocarbon having 6 to 8 carbon atoms. Examples of the aliphatic hydrocarbons having 6 to 8 carbon atoms excluding n-heptane include n-hexane, 2-methylhexane, 3-methylhexane, 2,3-dimethylpentane, 3,3-dimethylpentane, 3-ethylpentane, n-octane, 2,2,4-trimethylpentane, 2,2-dimethylhexane, 2,4-dimethylhexane, and 2,5-dimethylhexane. Examples of the alicyclic hydrocarbon having 6 to 8 carbon atoms include cyclohexane, methylcyclohexane, cyclopentane, methylcyclopentane, ethylcyclopentane, 1,1-dimethylcyclopentane, cis-1,2-dimethylcyclopentane, trans-1,2-dimethylcyclopentane, cis-1,3-dimethylcyclopentane, trans-1,3-dimethylcyclopentane, 1,1,2-trimethylcyclopentane, 1,1,3-trimethylcyclopentane, 1,2,3-trimethylcyclopentane, 2,2,4-trimethylcyclopentane, cis-1,trans-2,4-trimethylcyclopentane, trans-1,cis-2,3-trimethylcyclopentane, etc. In addition to the above-mentioned hydrocarbons, aromatic hydrocarbons such as benzene, toluene, and xylene may be included. Two or more types of hydrocarbons may be mixed and subjected to the step of reverse phase suspension polymerization, or a commercially available product that is a mixture of two or more types of hydrocarbons may be provided.

[0033] In the drying step described below, the hydrocarbon dispersion medium removed by distillation may be recovered and reused as the hydrocarbon dispersion medium for reversed-phase suspension polymerization. The recovered hydrocarbon dispersion medium may be purified by additional distillation or washing with water or the like before use.

[0034] The content of n-heptane in the hydrocarbon dispersion medium is 20 parts by mass or more relative to 100 parts by mass of the hydrocarbon dispersion medium. In this case, the water absorbent resin particles obtained by the production method of the present invention have less coarse powder and fine powder.

[0035] The content of n-heptane in the hydrocarbon dispersion medium is, from the viewpoint of reducing coarse powder and fine powder of the water absorbent resin particles, preferably 25 parts by mass or more, more preferably 35 parts by mass or more, even more preferably 50 parts by mass or more, still more preferably 70 parts by mass or more, and particularly preferably 95 parts by mass or more, relative to 100 parts by mass of the hydrocarbon dispersion medium.

[0036]

[0043] From the viewpoint of reducing coarse powder and fine powder of the water absorbent resin particles, the content of the alicyclic hydrocarbon having 6 to 8 carbon atoms in the hydrocarbon dispersion medium is preferably 60 parts by mass or less, more preferably 50 parts by mass or less, even more preferably 40 parts by mass or less, still more preferably 30 parts by mass or less, and particularly preferably 20 parts by mass or less, relative to 100 parts by mass of the hydrocarbon dispersion medium.

[0037]

[0043] From the viewpoint of reducing coarse powder and fine powder of the water absorbent resin particles, the content of the aliphatic hydrocarbon having 6 to 8 carbon atoms excluding n-heptane in the hydrocarbon dispersion medium is preferably 60 parts by mass or less, more preferably 50 parts by mass or less, even more preferably 40 parts by mass or less, still more preferably 30 parts by mass or less, and particularly preferably 20 parts by mass or less, relative to 100 parts by mass of the hydrocarbon dispersion medium.

[0038]

[0113] From the viewpoint of reducing coarse powder and fine powder of the water absorbent resin particles, the content of aromatic hydrocarbon in the hydrocarbon dispersion medium is preferably 0.2 parts by mass or less, more preferably 0.1 parts by mass or less, even more preferably 0.05 parts by mass or less, still more preferably 0.01 parts by mass or less, and particularly preferably 0.005 parts by mass or less, relative to 100 parts by mass of the hydrocarbon dispersion medium.

[0039] The amount of the hydrocarbon dispersion medium used is preferably 100 to 1,500 parts by mass, more preferably 150 to 1,000 parts by mass, and even more preferably 200 to 500 parts by mass, relative to 100 parts by mass of the first-stage water-soluble ethylenically unsaturated monomer, from the viewpoints of uniformly dispersing the water-soluble ethylenically unsaturated monomer and facilitating control of the polymerization temperature.

[0040] [Dispersion stabilizer] (Surfactant) In the reversed-phase suspension polymerization, a dispersion stabilizer is used to improve the dispersion stability of the water-soluble ethylenically unsaturated monomer in the hydrocarbon dispersion medium. As the dispersion stabilizer, a surfactant can be used.

[0041] As the surfactant, for example, sucrose fatty acid ester, polyglycerin fatty acid ester, sorbitan fatty acid ester, polyoxyethylene sorbitan fatty acid ester, polyoxyethylene glycerin fatty acid ester, sorbitol fatty acid ester, polyoxyethylene sorbitol fatty acid ester, polyoxyethylene alkyl ether, polyoxyethylene alkylphenyl ether, polyoxyethylene castor oil, polyoxyethylene hardened castor oil, alkylaryl formaldehyde condensed polyoxyethylene ether, polyoxyethylene polyoxypropylene block copolymer, polyoxyethylene polyoxypropyl alkyl ether, polyethylene glycol fatty acid ester, alkyl glucoside, N-alkyl gluconamide, polyoxyethylene fatty acid amide, polyoxyethylene alkylamine, polyoxyethylene alkyl ether phosphate, polyoxyethylene alkyl allyl ether phosphate, etc. can be used. Among these surfactants, it is particularly preferable to use sorbitan fatty acid ester, polyglycerin fatty acid ester, and sucrose fatty acid ester from the viewpoint of dispersion stability of monomer. These surfactants may be used alone or in combination of two or more.

[0042] The amount of the surfactant used is preferably 0.1 to 30 parts by mass, more preferably 0.3 to 10 parts by mass, even more preferably 0.5 to 1 part by mass, and even more preferably 0.6 to 0.9 parts by mass, relative to 100 parts by mass of the first-stage water-soluble ethylenically unsaturated monomer.

[0043] (Polymer Dispersant) As the dispersion stabilizer used in the reversed phase suspension polymerization, a polymeric dispersant may be used in combination with the above-mentioned surfactant.

[0044] 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, ethyl cellulose, and ethylhydroxyethyl cellulose. 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 dispersion stability of monomers. These polymeric dispersants may be used alone or in combination of two or more kinds.

[0045] From the viewpoint of reducing fine powder of the water-absorbent resin particles, the weight-average molecular weight (Mw) of the polymer dispersant is preferably 500 or more, more preferably 1000 or more, even more preferably 1500 or more, and even more preferably 2000 or more. From the viewpoint of reducing coarse powder of the water-absorbent resin particles, the weight-average molecular weight (Mw) is preferably 30000 or less, more preferably 25000 or less, even more preferably 20000 or less, and even more preferably 15000 or less.

[0046] The molecular weight distribution (Mw / number average molecular weight (Mn)) of the polymer dispersant is preferably 1 to 100, more preferably 2 to 70, even more preferably 3 to 50, and still more preferably 5 to 45, from the viewpoint of reducing coarse powder and fine powder of the water absorbent resin.

[0047] The amount of the polymeric dispersant used is preferably 0.1 to 30 parts by mass, more preferably 0.3 to 10 parts by mass, even more preferably 0.5 to 1 part by mass, and even more preferably 0.6 to 0.9 parts by mass, relative to 100 parts by mass of the first-stage water-soluble ethylenically unsaturated monomer.

[0048] [Other ingredients] In the method for producing water-absorbent resin particles, other components may be added to the aqueous solution containing the water-soluble ethylenically unsaturated monomer, if desired, to carry out reverse phase suspension polymerization. As the other components, various additives such as a thickener and a chain transfer agent can be added.

[0049] (Thickener) For example, a thickener may be added to an aqueous solution containing a water-soluble ethylenically unsaturated monomer before carrying out reverse suspension polymerization. By adjusting the viscosity of the aqueous solution by adding a thickener in this manner, it is possible to control the median particle size obtained in the reverse suspension polymerization.

[0050] Examples of the thickener that can be used include hydroxyethyl cellulose, hydroxypropyl cellulose, methyl cellulose, carboxymethyl cellulose, polyacrylic acid, partially neutralized polyacrylic acid, polyethylene glycol, polyacrylamide, polyethyleneimine, dextrin, sodium alginate, polyvinyl alcohol, polyvinylpyrrolidone, polyethylene oxide, etc. If the stirring speed during polymerization is the same, the higher the viscosity of the water-soluble ethylenically unsaturated monomer aqueous solution, the larger the primary particles and / or secondary particles obtained tend to be.

[0051] (Chain transfer agent) For example, in order to control the water absorbing performance of the water absorbent resin particles, the polymerization of the water-soluble ethylenically unsaturated monomer may be carried out in the presence of a chain transfer agent.

[0052] Examples of the chain transfer agent include thiols such as ethanethiol, propanethiol, and dodecanethiol; thiolic acids such as thioglycolic acid, thiomalic acid, dimethyldithiocarbamic acid, diethyldithiocarbamic acid, and salts thereof; secondary alcohols such as isopropanol; phosphorous acid compounds such as phosphorous acid, normal salts of phosphorous acid such as disodium phosphite, dipotassium phosphite, and diammonium phosphite, and acidic salts of phosphorous acid such as sodium hydrogen phosphite, potassium hydrogen phosphite, and ammonium hydrogen phosphite; phosphoric acid compounds such as normal salts of phosphoric acid such as phosphoric acid, sodium phosphate, potassium phosphate, and ammonium phosphate, and acidic salts of phosphoric acid such as sodium dihydrogen phosphate, potassium dihydrogen phosphate, ammonium dihydrogen phosphate, disodium hydrogen phosphate, dipotassium hydrogen phosphate, and diammonium hydrogen phosphate; hypophosphorous acid compounds such as hypophosphite, hypophosphites such as sodium hypophosphite, potassium hypophosphite, and ammonium hypophosphite; pyrophosphoric acid, tripolyphosphoric acid, polyphosphoric acid, and salts thereof; trimethyl phosphate, nitrilo trimethylene triphosphonic acid, and the like. These chain transfer agents may be used alone or in combination of two or more. The chain transfer agent may also be a hydrate thereof.

[0053] <Surface crosslinking process> The surface cross-linking step is a step of cross-linking the surface of the hydrogel polymer obtained by polymerizing the water-soluble ethylenically unsaturated monomer in the polymerization step after the reversed-phase suspension polymerization. The water-absorbent resin particles of the present invention are obtained by mixing the hydrogel polymer obtained in the polymerization step with a surface cross-linking agent and cross-linking the hydrogel polymer. This surface cross-linking reaction is preferably carried out in the presence of a surface cross-linking agent after the polymerization of the water-soluble ethylenically unsaturated monomer.

[0054] In the surface crosslinking step, the molar ratio of the surface crosslinking agent to the water-soluble ethylenically unsaturated monomer (surface crosslinking agent / water-soluble ethylenically unsaturated monomer) is preferably in the range of 0.00001 to 0.01, more preferably in the range of 0.00005 to 0.008, even more preferably in the range of 0.0001 to 0.005, and even more preferably in the range of 0.0002 to 0.002.

[0055] Examples of the surface cross-linking agent 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, α-methylepichlorohydrin, isocyanate compounds such as 2,4-tolylene diisocyanate and hexamethylene diisocyanate; oxetane compounds such as 3-methyl-3-oxetanemethanol, 3-ethyl-3-oxetanemethanol, 3-butyl-3-oxetanemethanol, 3-methyl-3-oxetaneethanol, 3-ethyl-3-oxetaneethanol, and 3-butyl-3-oxetaneethanol; oxazoline compounds such as 1,2-ethylenebisoxazoline; carbonate compounds such as ethylene carbonate; and hydroxyalkylamide compounds such as bis[N,N-di(β-hydroxyethyl)]adipamide. Among these surface cross-linking agents, preferred are 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, (poly)glycerol polyglycidyl ether, etc. These surface cross-linking agents may be used alone or in combination of two or more kinds.

[0056] As a method for adding the surface cross-linking agent, the surface cross-linking agent may be added as it is, or may be added as an aqueous solution, but if necessary, may be added as a solution using a hydrophilic organic solvent as a solvent. Examples of the hydrophilic organic solvent 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 alone, or two or more of them may be used in combination, or as a mixed solvent with water.

[0057] The timing of adding the surface cross-linking agent is preferably in the presence of water in the range of 1 to 400 parts by mass, more preferably in the presence of water in the range of 5 to 100 parts by mass, even more preferably in the presence of water in the range of 10 to 50 parts by mass, even more preferably in the presence of water in the range of 15 to 35 parts by mass, and particularly preferably in the presence of water in the range of 15 to 30 parts by mass, relative to 100 parts by mass of the water-soluble ethylenically unsaturated monomer. The amount of water means the total amount of water contained in the reaction system and water used as necessary when adding the surface cross-linking agent.

[0058] The reaction temperature in the surface crosslinking reaction is preferably 50 to 250° C., more preferably 60 to 180° C., further preferably 60 to 140° C., and even more preferably 70 to 120° C. The reaction time in the surface crosslinking reaction is preferably 1 to 300 minutes, and more preferably 5 to 200 minutes.

[0059] <Drying process> After the above-mentioned reversed-phase suspension polymerization, a drying step may be included in which water, the hydrocarbon dispersion medium, and the like are removed by distillation by adding energy such as heat from the outside. When dehydrating the hydrous gel polymer after the reversed-phase suspension polymerization, the system in which the hydrous gel polymer is dispersed in the hydrocarbon dispersion medium is heated, and the water and the hydrocarbon dispersion medium are once distilled out of the system by azeotropic distillation. At this time, if only the distilled hydrocarbon dispersion medium is returned to the system, continuous azeotropic distillation is possible. In that case, the temperature in the system during drying is maintained below the azeotropic temperature with the hydrocarbon dispersion medium, which is preferable from the viewpoint of the resin being less likely to deteriorate. Subsequently, the water and the hydrocarbon dispersion medium are distilled off to obtain water-absorbent resin particles. By controlling the processing conditions of the drying step after this polymerization to adjust the amount of dehydration, it is possible to control the various properties of the obtained water-absorbent resin particles.

[0060] In the drying step, the drying treatment by distillation may be performed under normal pressure or under reduced pressure. From the viewpoint of increasing the drying efficiency, the drying treatment may be performed under a stream of nitrogen or the like. When the drying treatment is performed under normal pressure, the drying temperature is preferably 70 to 250°C, more preferably 80 to 180°C, further preferably 80 to 140°C, and even more preferably 90 to 130°C. When the drying treatment is performed under reduced pressure, the drying temperature is preferably 40 to 160°C, and more preferably 50 to 110°C.

[0061] In addition, when a surface crosslinking step using a surface crosslinking agent is carried out after polymerization of monomers by reversed-phase suspension polymerization, the above-mentioned drying step by distillation is carried out after the surface crosslinking step is completed. Alternatively, the surface crosslinking step and the drying step may be carried out simultaneously.

[0062] The water-absorbent resin particles obtained by the production method of the present invention may contain additives according to the purpose. Examples of such additives include inorganic powders, surfactants, oxidizing agents, reducing agents, metal chelating agents, radical chain inhibitors, antioxidants, antibacterial agents, and the like. For example, inorganic powders may be used to further improve the fluidity of the water-absorbent resin particles. The amount of inorganic powder used is preferably in the range of 0.01 to 5 parts by mass, more preferably 0.1 to 1 part by mass, relative to 100 parts by mass of the water-absorbent resin particles.

[0063] 2. Water-absorbing resin particles The water-absorbent resin particles have a median particle size of preferably 100 to 650 μm, more preferably 150 to 600 μm, even more preferably 200 to 550 μm, even more preferably 250 to 500 μm, and particularly preferably 300 to 450 μm.

[0064] In the water-absorbent resin particles having a median particle diameter of 300 to 450 μm, the ratio of particles (coarse powder) having a particle diameter of 850 μm or more is preferably 15 mass % or less, and more preferably 10 mass % or less, based on the total amount of the water-absorbent resin particles.

[0065] In the water-absorbent resin particles having a median particle diameter of 300 to 450 μm, the ratio of particles (fine powder) having a particle diameter of less than 75 μm is preferably 3 mass % or less, and more preferably 1 mass % or less, based on the total amount of the water-absorbent resin particles. EXAMPLES

[0066] The present invention will be described in detail below with reference to examples, but the present invention is not limited to these examples.

[0067] 1. Hydrocarbon dispersion medium Hydrocarbon dispersion media (1) to (4) were prepared. Table 1 shows the contents of n-heptane, aliphatic hydrocarbons having 6 to 8 carbon atoms excluding n-heptane, and alicyclic hydrocarbons having 6 to 8 carbon atoms in each hydrocarbon dispersion medium. The contents shown in the table are parts by mass based on 100 parts by mass of the hydrocarbon dispersion medium.

[0068] [Table 1]

[0069] 2. Manufacturing of water-absorbent resin particles Example 1 [First stage polymerization process] A round-bottomed cylindrical separable flask with an inner diameter of 11 cm and a volume of 2 L was prepared, equipped with a reflux condenser, a dropping funnel, a nitrogen gas inlet tube, and a stirring blade with two stages of four inclined paddle blades with a blade diameter of 5 cm. The hydrocarbon dispersion medium used was the hydrocarbon dispersion medium (1) in Table 1. 293 g of the hydrocarbon dispersion medium was placed in the flask, and 0.736 g of a maleic anhydride-modified ethylene-propylene copolymer (Hiwax 1105A, Mitsui Chemicals, Inc.) was added as a polymeric dispersant. The mixture was heated to 80°C with stirring to dissolve the dispersant, and then cooled to 60°C.

[0070] In a 300 mL beaker, 92.0 g (1.03 mol) of an 80.5 mass% aqueous acrylic acid solution was placed as a water-soluble ethylenically unsaturated monomer, and while cooling from the outside, 102.8 g of a 30 mass% aqueous sodium hydroxide solution was added dropwise to carry out 75 mol% neutralization, and then 0.0736 g (0.272 mmol) of potassium persulfate, 0.0101 g (0.058 mmol) of ethylene glycol diglycidyl ether as an internal crosslinking agent, and 44.5 g of ion-exchanged water were added and dissolved to prepare a first-stage monomer aqueous solution.

[0071] The aqueous solution prepared above was added to a separable flask and stirred for 10 minutes, after which a surfactant solution prepared by heating and dissolving 0.736 g of sucrose stearate with HLB3 (Ryoto Sugar Ester S-370, manufactured by Mitsubishi Chemical Foods Corporation) as a surfactant in 6.62 g of a hydrocarbon dispersion medium was further added, and the system was thoroughly purged with nitrogen while stirring at a stirrer speed of 550 rpm. The flask was then immersed in a water bath at 70°C to raise the temperature, and polymerization was carried out for 60 minutes, thereby obtaining a first-stage polymerization slurry liquid containing primary particles.

[0072] [Second-stage polymerization process] In a 500 mL beaker, 128.8 g (1.44 mol) of an 80.5 mass% aqueous acrylic acid solution was placed as a water-soluble ethylenically unsaturated monomer, and while cooling from the outside, 143.9 g of a 30 mass% aqueous sodium hydroxide solution was added dropwise to carry out 75 mol% neutralization, and then 0.103 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 16.8 g of ion-exchanged water were added and dissolved to prepare a second-stage aqueous monomer solution.

[0073] The contents of the separable flask were cooled to 26°C while stirring at a stirrer speed of 1000 rpm, and the entire amount of the second-stage aqueous solution was added to the first-stage polymerization slurry, and the atmosphere in the system was replaced with nitrogen for 30 minutes. The flask was then immersed again in a 70°C water bath to raise the temperature and carry out the polymerization reaction for 60 minutes. By the above operations, a hydrous gel polymer was obtained after the second-stage polymerization.

[0074] [Surface crosslinking] Then, the flask was immersed in an oil bath set at 125°C, and 242.2 g of water was extracted from the system by azeotropic distillation of the hydrocarbon dispersion medium and water while refluxing n-heptane. Then, 4.42 g (0.507 mmol) of a 2 mass% aqueous solution of ethylene glycol diglycidyl ether was added as a surface crosslinking agent to the flask, and the flask was kept at 83°C for 2 hours.

[0075] Thereafter, the water and the hydrocarbon dispersion medium were evaporated by heating in an oil bath set at 125° C. to obtain a dried product. 0.2% by mass of amorphous silica (Toxil NP-S, Oriental Silicas Corporation) was mixed with this dried product to obtain 240 g of water-absorbent resin particles.

[0076] Example 2 239 g of water absorbent resin particles were obtained in the same manner as in Example 1, except that the hydrocarbon dispersion medium (2) in Table 1 was used as the hydrocarbon dispersion medium.

[0077] Example 3 243 g of water-absorbent resin particles were obtained in the same manner as in Example 1, except that the hydrocarbon dispersion medium (3) in Table 1 was used as the hydrocarbon dispersion medium.

[0078] Example 4 The same procedure as in Example 1 was repeated except that the hydrocarbon dispersion medium (4) in Table 1 was used as the hydrocarbon dispersion medium. Similarly, 242 g of water-absorbent resin particles were obtained.

[0079] 3. Evaluation of water-absorbent resin particles 50 g of water-absorbent resin particles obtained in the examples were used to measure the ratio of particles (coarse powder) having a particle diameter of 850 μm or more, the ratio of particles (fine powder) having a particle diameter of less than 75 μm, and the median particle diameter. JIS standard sieves were combined in the order of a sieve with an opening of 850 μm, a sieve with a opening of 600 μm, a sieve with a opening of 425 μm, a sieve with a opening of 300 μm, a sieve with a opening of 180 μm, a sieve with a opening of 125 μm, a sieve with a opening of 75 μm, and a tray. Water-absorbent resin particles were placed on the top sieve and classified according to JIS Z 8815 (1994) using a Rotap type shaker (manufactured by Iida Seisakusho Co., Ltd.). After classification, the mass of the particles remaining on each sieve was measured, and the mass of the particles remaining on the 850 μm sieve was W1, and the mass of the particles on the tray was W2. The proportion of particles having a particle size of 850 μm or more (coarse powder) and the proportion of particles having a particle size of less than 75 μm (fine powder) were calculated from the following formulas (1) and (2). Formula (1) Ratio of particles (coarse powder) having a particle diameter of 850 μm or more [mass%] = (W1 / 50) × 100 Equation (2) Percentage of particles (fine powder) having a particle size of less than 75 μm [mass%] = (W2 / 50) × 100 The mass of the particles remaining on each sieve was calculated as a mass percentage relative to the total mass to obtain the particle size distribution. The particles remaining on the sieves were integrated in order from the largest particle size to obtain a relationship between the sieve opening and the integrated value of the mass percentage of the particles remaining on the sieve on a logarithmic probability paper. The particle size corresponding to an integrated mass percentage of 50% by mass was obtained as the median particle size by connecting the plots on the probability paper with a straight line.

[0080] [Table 2]

[0081] As shown in Table 2, it was confirmed that the water-absorbent resin particles obtained by the production method of the Examples contained little coarse powder and fine powder.

Claims

1. A method for producing water-absorbent resin particles, comprising a step of subjecting a water-soluble ethylenically unsaturated monomer to reverse phase suspension polymerization in a hydrocarbon dispersion medium in the presence of a dispersion stabilizer, the hydrocarbon dispersion medium contains n-heptane, an aliphatic hydrocarbon having 6 to 8 carbon atoms other than n-heptane, and an alicyclic hydrocarbon having 6 to 8 carbon atoms; The n-heptane is 20 parts by mass or more relative to 100 parts by mass of the hydrocarbon dispersion medium; A method for producing water-absorbent resin particles.

2. 2. The method for producing water-absorbent resin particles according to claim 1, wherein the alicyclic hydrocarbon having 6 to 8 carbon atoms is 60 parts by mass or less with respect to 100 parts by mass of the hydrocarbon dispersion medium.

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