Method for producing water-absorbing resin particle

Abstract

To provide a method for producing water-absorbing resin particles containing a reductive inorganic salt with rapid water-absorption speed.SOLUTION: There is provided a method for producing absorbent resin particles, which includes a polymerization process in which water-soluble ethylene-type unsaturated monomers are subjected to inverse emulsion polymerization to obtain a reaction liquid containing a hydrous gel-like polymer, a dehydration process in which water is removed from the reaction liquid and a reducing inorganic salt is added, and a surface crosslinking process in which a surface crosslinking agent is added to the reaction liquid to surface crosslink the hydrous gel-like polymer, characterized in that the difference between the residual water ratio of the hydrous gel-like polymer when adding reducing inorganic salt in the dehydration process and the residual water ratio of the hydrous gel-like polymer when adding the surface crosslinking agent in the surface crosslinking process is 5-80%.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 containing a reducing inorganic salt. [Background technology]

[0002] Absorbent articles such as disposable diapers and sanitary napkins are generally formed by sandwiching an absorbent body made of hydrophilic fibers and a water-absorbent resin between a liquid-permeable sheet arranged on the side that comes into contact with the body and a liquid-impermeable sheet arranged on the opposite side. On the other hand, the gel formed when water-absorbent resin particles absorb body fluids such as human urine generally deteriorates over time, resulting in a decrease in its absorbency, and therefore, gel stability is required. Therefore, in order to improve the stability of the gel, water-absorbent resin particles containing a reducing inorganic salt have been proposed (for example, Patent Document 1).

[0003] In general, in the above-mentioned applications, the water-absorbent resin particles are required to have a high water absorption capacity and a high water absorption speed in order to prevent liquid leakage from the water-absorbent article. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 63-118375 Summary of the Invention [Problem to be solved by the invention]

[0005] An object of the present invention is to provide a method for producing water-absorbent resin particles containing a reducing inorganic salt, which have a high water absorption rate. [Means for solving the problem]

[0006] In order to solve the above problems, the present invention includes the following. [1] a polymerization step of subjecting a water-soluble ethylenically unsaturated monomer to reverse-phase suspension polymerization to obtain a reaction liquid containing a hydrous gel polymer; a dehydration step of removing water from the reaction solution and adding a reducing inorganic salt; a surface cross-linking step of adding a surface cross-linking agent to the reaction liquid and surface cross-linking the hydrous gel polymer, A method for producing water-absorbent resin particles, wherein a difference between a residual water rate of the hydrogel polymer when a reducing inorganic salt is added in the dehydration step and a residual water rate of the hydrogel polymer when a surface cross-linking agent is added in the surface cross-linking step is 5 to 80%. [2] 2. The method for producing water-absorbent resin particles according to claim 1, wherein the residual water content of the hydrous gel polymer when the reducing inorganic salt is added in the dehydration step is 15 to 100%. [Effects of the Invention]

[0007] The production method according to the present invention provides water-absorbent resin particles containing a reducing inorganic salt, which have a high water absorption rate. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 is a schematic diagram showing a method for measuring the amount of water absorption of a water-absorbent resin particle in physiological saline under a load of 2.07 kPa. DETAILED DESCRIPTION OF THE INVENTION

[0009] Hereinafter, embodiments of the present invention will be described in detail, but the present invention is not limited to the following embodiments.

[0010] 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 reverse-phase suspension polymerization of a water-soluble ethylenically unsaturated monomer. 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, a dehydration step of removing water from a reaction solution containing the hydrous gel polymer obtained in the polymerization step, and a surface crosslinking step of surface-crosslinking the hydrous gel polymer. Furthermore, a reducing inorganic salt is added to the hydrous gel polymer in the dehydration step. In the method for producing water-absorbent resin particles of the present invention, the reverse-phase suspension polymerization preferably includes two or more polymerization stages.

[0011] The present invention can produce water-absorbent resin particles by providing these features. The method for producing water-absorbent resin particles of the present invention will be described in detail below.

[0012] [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.

[0013] Conventionally, when water-soluble ethylenically unsaturated monomers are subjected to reverse phase suspension polymerization to obtain water-absorbent resin particles, the reverse phase suspension polymerization can be carried out in only one polymerization step, or in multiple steps of two or more. From the viewpoint of enhancing the productivity of water-absorbent resin particles while suitably exhibiting the effects of the present invention, the reverse phase suspension polymerization is preferably carried out in two to three polymerization steps.

[0014] In reversed-phase suspension polymerization, after the first polymerization step (first-stage reversed-phase suspension polymerization), a water-soluble ethylenically unsaturated monomer is added to the reaction mixture obtained in the first polymerization step, and the second and subsequent polymerization steps (second-stage reversed-phase suspension polymerization) can be carried out in the same manner as in the first step. In the reversed-phase suspension polymerization in each step from the second stage onwards, 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 step from the second stage onwards. In addition, in the second and subsequent polymerization steps, an internal crosslinking agent may be added to the water-soluble ethylenically unsaturated monomer, if necessary.

[0015] In 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, as long as it is added before the polymerization reaction begins.

[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, and the polymerization reaction of the water-soluble ethylenically unsaturated monomer is allowed 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 derivatives 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 above-mentioned other water-soluble ethylenically unsaturated monomers for use. In the present invention, the content of acrylic acid and its salts in the water-soluble ethylenically unsaturated monomers is 70 to 100 mol %. That is, the proportion of acrylic acid and its salts in 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 facilitating 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 facilitating particle size control.

[0022] The concentration of the water-soluble ethylenically unsaturated monomer during 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 facilitating 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 facilitating 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 bicarbonate, potassium hydroxide, and potassium carbonate; and ammonia. These alkaline neutralizing agents may be used in the form of an aqueous solution to simplify the neutralization operation. The alkaline neutralizing agents described above may be used alone or in combination of two or more.

[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-butylcumyl 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 Examples of the radical polymerization initiator include 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-[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. 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 radical polymerization initiator used may be 0.05 to 10 millimoles per mole of the water-soluble ethylenically unsaturated monomer. When the amount of 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 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] During the polymerization reaction, the aqueous monomer solution used for polymerization may contain a chain transfer agent, such as hypophosphites, thiols, thiolic acids, secondary alcohols, and amines.

[0030] [Polymerization reaction temperature] The reaction temperature of the polymerization reaction is preferably 20 to 110°C, and more preferably 40 to 90°C, from the viewpoints of improving economy by rapidly progressing the polymerization and shortening the polymerization time, and also of easily removing the heat of polymerization to smoothly carry out the reaction.

[0031] [Internal crosslinking agent] Examples of the internal crosslinking agent 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, 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; diglycidyl compounds such as (poly)ethylene glycol diglycidyl ether, (poly)propylene glycol diglycidyl ether, (poly)glycerin diglycidyl ether, and other polyglycidyl compounds, and triglycidyl compounds; 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-oxetanemethanol, 3-ethyl-3-oxetanemethanol, 3-butyl-3-oxetanemethanol, 3-methyl-3-oxetaneethanol, 3-ethyl-3-oxetaneethanol, and 3-butyl-3-oxetaneethanol. Among these internal cross-linking 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 cross-linking agents may be used alone or in combination of two or more.

[0032] 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 mole of the water-soluble ethylenically unsaturated monomer.

[0033] [Hydrocarbon dispersion medium] Examples of hydrocarbon dispersion media include aliphatic hydrocarbons having 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 preferred due to their industrial availability, stable quality, and low cost. These hydrocarbon dispersion media may be used alone or in combination of two or more. Suitable results can also be obtained using a mixture of hydrocarbon dispersion media, such as commercially available Exxol Heptane (manufactured by ExxonMobil Corporation; containing 75 to 85% by mass of heptane and its isomeric hydrocarbons).

[0034] The amount of the hydrocarbon dispersion medium used 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, 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.

[0035] [Dispersion stabilizer] (surfactant) In 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. A surfactant can be used as the dispersion stabilizer.

[0036] 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, alkylaryl formaldehyde condensed polyoxyethylene ethers, polyoxyethylene polyoxypropylene block copolymers, polyoxyethylene polyoxypropyl alkyl ethers, polyethylene glycol fatty acid esters, alkyl glucosides, N-alkyl gluconamides, polyoxyethylene fatty acid amides, polyoxyethylene alkylamines, polyoxyethylene alkyl ether phosphate esters, and polyoxyethylene alkyl allyl ether phosphate esters. Among these surfactants, sorbitan fatty acid esters, polyglycerin fatty acid esters, and sucrose fatty acid esters are particularly preferred in terms of dispersion stability of the monomer. These surfactants may be used alone or in combination of two or more.

[0037] The amount of surfactant used is preferably 0.1 to 30 parts by mass, more preferably 0.3 to 10 parts by mass, and even more preferably 0.5 to 2 parts by mass, per 100 parts by mass of the first-stage water-soluble ethylenically unsaturated monomer.

[0038] (polymer dispersant) As a dispersion stabilizer used in the reversed phase suspension polymerization, a polymeric dispersant may be used in combination with the surfactant described above.

[0039] 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 the monomer. These polymeric dispersants may be used alone or in combination of two or more.

[0040] The amount of polymeric dispersant used is preferably 0.1 to 30 parts by mass, more preferably 0.3 to 10 parts by mass, and even more preferably 0.5 to 2 parts by mass, per 100 parts by mass of the first-stage water-soluble ethylenically unsaturated monomer.

[0041] [Other ingredients] In the method for producing water-absorbent resin particles, if desired, other components may be added to the aqueous solution containing the water-soluble ethylenically unsaturated monomer 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.

[0042] <Dehydration process> This method includes a dehydration treatment in which, after the above-mentioned reversed-phase suspension polymerization, energy such as heat is applied from the outside to remove water, hydrocarbon dispersion medium, and the like by distillation from a reaction solution containing a hydrous gel polymer. The dehydration treatment by distillation may be carried out under normal pressure or under reduced pressure. Furthermore, from the viewpoint of improving efficiency, it may be carried out in a gas stream such as nitrogen. When the dehydration treatment is carried out under normal pressure, the dehydration temperature is preferably 70 to 250°C, more preferably 80 to 180°C, even more preferably 80 to 140°C, and even more preferably 90 to 130°C. When the dehydration treatment is carried out under reduced pressure, the dehydration temperature is preferably 40 to 160°C, and more preferably 50 to 110°C.

[0043] In the present invention, a reducing inorganic salt is added to the reaction solution in the dehydration step. The reducing inorganic salt is preferably added to the reaction solution in the form of an aqueous solution.

[0044] In the dehydration step, the residual water content of the hydrogel polymer when a reducing inorganic salt is added to the reaction liquid is preferably 15 to 100%, more preferably 20 to 90%, and even more preferably 30 to 80%.

[0045] Examples of reducing inorganic salts include sulfites such as sodium sulfite, potassium sulfite, calcium sulfite, zinc sulfite, and ammonium sulfite; bisulfites such as sodium bisulfite, potassium bisulfite, calcium bisulfite, and ammonium bisulfite; pyrosulfites such as sodium pyrosulfite, potassium pyrosulfite, and ammonium pyrosulfite; dithionites such as sodium dithionite, potassium dithionite, ammonium dithionite, calcium dithionite, and zinc dithionite; trithionates such as potassium trithionate and sodium trithionate; tetrathionates such as potassium tetrathionate and sodium tetrathionate; thiosulfates such as sodium thiosulfate, potassium thiosulfate, and ammonium thiosulfate; and nitrites such as sodium nitrite, potassium nitrite, calcium nitrite, and zinc nitrite. Among these, sulfites, bisulfites, pyrosulfites, dithionites, and nitrites are preferred, with sodium sulfite and potassium sulfite being more preferred, from the viewpoint of enhancing the stability of the gel.

[0046] From the viewpoint of enhancing the stability of the gel, the amount of the reducing inorganic salt added is preferably 0.005 to 5 parts by mass, more preferably 0.01 to 3 parts by mass, per 100 parts by mass of the water-soluble ethylenically unsaturated monomer used in the polymerization step.

[0047] <Surface crosslinking process> The surface cross-linking step is a step of cross-linking the surface of the hydrogel polymer after the dehydration step. The water-absorbent resin particles of the present invention are obtained by adding a surface cross-linking agent to a reaction liquid containing the hydrogel polymer after the dehydration step, and cross-linking the hydrogel polymer. This surface cross-linking reaction is carried out in the presence of the surface cross-linking agent after the dehydration step.

[0048] The amount of the surface cross-linking agent added is, relative to 100 parts by mass of the water-soluble ethylenically unsaturated monomer, The range is preferably 0.001 to 1 part by mass, more preferably 0.005 to 0.8 parts by mass, even more preferably 0.01 to 0.5 parts by mass, and even more preferably 0.02 to 0.2 parts by mass.

[0049] 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.

[0050] As a method for adding the surface cross-linking agent, the surface cross-linking agent may be added as it is or as an aqueous solution, or, 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, in combination of two or more, or as a mixed solvent with water.

[0051] The reaction temperature in the surface cross-linking reaction is preferably 50 to 250° C., more preferably 60 to 180° C., even more preferably 60 to 140° C., and even more preferably 70 to 120° C. The reaction time in the surface cross-linking reaction is preferably 1 to 300 minutes, and more preferably 5 to 200 minutes.

[0052] In the present invention, in the surface cross-linking step, the surface cross-linking agent is added when the difference between the residual water percentage of the hydrogel polymer when adding the reducing inorganic salt and the residual water percentage when adding the surface cross-linking agent is 5 to 80%. In this case, the water-absorbent resin particles obtained by the production method of the present invention have a high water absorption rate.

[0053] Further, from the viewpoint of increasing the water absorption rate of the water absorbent resin particles, the difference between the residual water rate of the hydrous gel polymer when a reducing inorganic salt is added and the residual water rate when a surface crosslinking agent is added is preferably 5 to 60%, more preferably 5 to 50%, still more preferably 5 to 40%, and particularly preferably 10 to 30%.

[0054] The residual water rate is calculated using the following formula (1). Equation (1) Residual water rate [%] = (mass of water remaining in the hydrogel polymer / mass of water-soluble ethylenically unsaturated monomer used in the polymerization step) × 100

[0055] <Drying process> After the above-mentioned surface cross-linking step, the surface-cross-linked hydrogel polymer may be dried by removing water, hydrocarbon dispersion medium, etc. from the reaction solution containing the hydrogel polymer by applying energy such as heat from the outside. This allows polymer particles, which are a dried product of the surface-cross-linked hydrogel polymer, to be obtained.

[0056] In the drying step, the drying treatment by distillation may be carried out under normal pressure or under reduced pressure. Furthermore, from the viewpoint of increasing the drying efficiency, the drying treatment may be carried out under a stream of gas such as nitrogen. When the drying treatment is carried out under normal pressure, the drying temperature is preferably 70 to 250°C, more preferably 80 to 180°C, even more preferably 80 to 140°C, and even more preferably 90 to 130°C. When the drying treatment is carried out under reduced pressure, the drying temperature is preferably 40 to 160°C, and more preferably 50 to 110°C.

[0057] The water-absorbent resin particles obtained by the production method of the present invention may be composed 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 disposed inside the polymer particles, on the surfaces of the polymer particles, or both. The additional component may be a flow improver (lubricant). The flow improver may be inorganic particles. Examples of inorganic particles include silica particles such as amorphous silica.

[0058] The water-absorbent resin particles may contain a plurality of inorganic particles arranged on the surface of the polymer particles. For example, the inorganic particles can be arranged on the surface of the polymer particles by mixing the polymer particles with the inorganic particles. The inorganic particles may be silica particles such as amorphous silica. When the water-absorbent resin particles contain inorganic particles arranged on the surface of the polymer particles, the ratio of the inorganic particles to the mass of the polymer particles may be 0.01% by mass or more, or 0.05% by mass or more, and may be 5.0% by mass or less, 1.0% by mass or less, or 0.5% by mass or less. The inorganic particles here are usually very small compared to the size of the polymer particles. For example, the average particle diameter of the inorganic particles may be 0.1 to 50 μm, 0.5 to 30 μm, or 1 to 20 μm. The average particle diameter here may be a value measured by dynamic light scattering or laser diffraction / scattering. [Example]

[0059] The present invention will be described in detail below with reference to examples. However, the present invention is not limited to the examples. The water-absorbent resin particles obtained in the following Examples and Comparative Examples were evaluated by various test methods described later. Unless otherwise specified, the evaluations were carried out in an environment of a temperature of 25±2°C and a humidity of 50±10%.

[0060] <Production of water-absorbent resin particles> Example 1 [First stage polymerization process] A 2-L round-bottom cylindrical separable flask (11 cm inner diameter) equipped with a reflux condenser, a dropping funnel, a nitrogen gas inlet, and a stirrer with two 5-cm-diameter four-paddle inclined blades was prepared. 322 g of n-heptane (hydrocarbon dispersion medium) was added to the flask. 0.92 g of a polymeric dispersant, maleic anhydride-modified ethylene-propylene copolymer (Hiwax 1105A, Mitsui Chemicals, Inc.), and 0.92 g of a surfactant, sucrose stearate ester with HLB3 (Ryoto Sugar Ester S-370, Mitsubishi Chemical Foods Corporation), were added. The mixture was heated to 80°C with stirring to dissolve the dispersant, and then cooled to 60°C.

[0061] 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. Thereafter, 0.11 g (0.41 mmol) of potassium persulfate, 0.0083 g (0.047 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 aqueous monomer solution.

[0062] Then, the inside of the system was thoroughly purged with nitrogen while stirring at a stirrer speed of 700 rpm, and then the flask was 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.

[0063] [Second-stage polymerization process] In a 500 mL beaker, 128.2 g (1.43 mol) of an 80.5 mass% aqueous acrylic acid solution was placed as a water-soluble ethylenically unsaturated monomer. While cooling from the outside, 143.0 g of a 30 mass% aqueous sodium hydroxide solution was added dropwise to neutralize the solution to 75 mol%, and then 0.154 g (0.57 mmol) of potassium persulfate as a water-soluble radical polymerization initiator, 0.0115 g (0.066 mmol) of ethylene glycol diglycidyl ether as an internal crosslinking agent, and 16.0 g of ion-exchanged water were added and dissolved to prepare a second-stage aqueous monomer solution.

[0064] The contents of the separable flask were cooled to 26°C while stirring at a stirrer speed of 1000 rpm, and then the entire amount of the second-stage aqueous solution was added to the first-stage polymerization slurry. The atmosphere in the system was replaced with nitrogen for 30 minutes, and the flask was again immersed in a 70°C water bath to raise the temperature, and the polymerization reaction was carried out for 60 minutes. By these operations, a hydrogel polymer was obtained after the second-stage polymerization.

[0065] [Dehydration process] The flask was immersed in an oil bath set to 125°C, and the temperature inside the flask was maintained at 90°C and the pressure at 40 kPa, and n-heptane and water were removed from the system by distillation. The amount of water removed from the system was 181.7 g (72% residual water content of the hydrous gel polymer when the reducing inorganic salt was added). The pressure inside the flask was returned to normal, and 7.34 g (1.75 mmol) of a 3% by mass aqueous solution of sodium sulfite was added to the flask. Next, the temperature inside the flask was kept at 90°C and the pressure at 40 kPa, and 120.2 g of water was extracted from the system by distillation (residual water rate of the hydrous gel polymer when the surface crosslinking agent was added: 8%).

[0066] [Surface crosslinking process] The pressure inside the flask was returned to normal pressure, and 8.14 g (0.935 mmol) of a 2% by mass aqueous solution of ethylene glycol diglycidyl ether as a surface cross-linking agent was sprayed into the flask, and the flask was maintained at 83° C. for 2 hours.

[0067] Thereafter, polymer particles were obtained by heating in an oil bath set at 125° C. The polymer particles were classified using a sieve with an opening of 850 μm, and 0.2 mass % of amorphous silica (Toxil NP-S, Oriental Silicas Corporation) was mixed with the polymer particles that had passed through the sieve with an opening of 850 μm, thereby obtaining 208 g of water-absorbent resin particles.

[0068] Example 2

[0113] 202 g of water absorbent resin particles were obtained in the same manner as in Example 1 except that the amount of water extracted out of the system when adding a reducing inorganic salt in a dehydration step was changed from 181.7 g to 222.5 g (residual water rate of hydrous gel polymer when adding a reducing inorganic salt: 49%), and the amount of water extracted out of the system when adding a surface crosslinking agent was changed from 120.2 g to 79.8 g (residual water rate of hydrous gel polymer when adding a surface crosslinking agent: 8%).

[0069] Example 3

[0113] 205 g of water absorbent resin particles were obtained in the same manner as in Example 1 except that the amount of water extracted out of the system when adding a reducing inorganic salt in a dehydration step was changed from 181.7 g to 263.3 g (residual water rate of hydrous gel polymer when adding a reducing inorganic salt: 26%), and the amount of water extracted out of the system when adding a surface crosslinking agent was changed from 120.2 g to 26.6 g (residual water rate of hydrous gel polymer when adding a surface crosslinking agent: 15%).

[0070] Example 4

[0113] Except for changing the amount of water extracted out of the system when adding a reducing inorganic salt in a dehydration step from 181.7 g to 270.4 g (residual water rate of hydrous gel polymer when adding a reducing inorganic salt: 22%), and changing the amount of water extracted out of the system when adding a surface crosslinking agent from 120.2 g to 33.7 g (residual water rate of hydrous gel polymer when adding a surface crosslinking agent: 7%), in the same manner as in Example 1, 204 g of water absorbent resin particles was obtained.

[0071] (Comparative Example 1)

[0111] The amount of water extracted to the outside of the system in the dehydration step was changed from 181.7 g to 293.4 g (the residual water rate of the hydrous gel polymer when a reducing inorganic salt is added, and the residual water rate of the hydrous gel polymer when a surface crosslinking agent is added: 9%), thereafter, the inside of the flask was returned to normal pressure, 7.34 g (1.75 mmol) of a 3 mass % aqueous solution of sodium sulfite was added into the flask, successively, 8.14 g (0.935 mmol) of a 2 mass % aqueous solution of ethylene glycol diglycidyl ether was added by spraying as a surface crosslinking agent, and the mixture was kept at 83°C for 2 hours, and 203 g of a water absorbent resin particle was obtained in the same manner as in Example 1 except that the amount of water extracted to the outside of the system in the dehydration step was changed from 181.7 g to 293.4 g (the residual water rate of the hydrous gel polymer when a reducing inorganic salt is added, and the residual water rate of the hydrous gel polymer when a surface crosslinking agent is added: 9%), thereafter, the inside of the flask was returned to normal pressure, 7.34 g (1.75 mmol) of a 3 mass % aqueous solution of sodium sulfite was added into the flask, and successively, 8.14 g (0.935 mmol) of a 2 mass % aqueous solution of ethylene glycol diglycidyl ether was added by spraying as a surface crosslinking agent, and the mixture was kept at 83°C for 2 hours.

[0072] <Evaluation of water-absorbent resin particles> [Water absorption rate] The water absorption rate of water-absorbent resin particles in physiological saline was measured using the Vortex method according to the following procedure. First, 50±0.1 g of physiological saline adjusted to a temperature of 25±0.2°C in a thermostatic water bath was weighed into a 100 mL beaker. Next, a vortex was generated by stirring at 600 rpm using a magnetic stir bar (8 mmφ×30 mm, without ring). 2.0±0.002 g of water-absorbent resin particles were added to the physiological saline all at once. The time [seconds] from the addition of the water-absorbent resin particles to the point at which the vortex on the liquid surface converged was measured, and this time was recorded as the water absorption rate of the water-absorbent resin particles.

[0073] [Saline water retention capacity] A cotton bag (membrane broadcloth No. 60, 100 mm wide x 200 mm long) containing 2.0 g of water-absorbent resin particles was placed in a 500 mL beaker. 500 g of 0.9% by mass sodium chloride aqueous solution (physiological saline) was poured into the cotton bag containing the water-absorbent resin particles all at once, taking care not to allow the bag to swell. The top of the cotton bag was tied with a rubber band and left to stand for 30 minutes to allow the water-absorbent resin particles to swell. After 30 minutes, the cotton bag was dehydrated for 1 minute using a dehydrator (manufactured by Kokusan Co., Ltd., product number H-122) set to a centrifugal force of 167 G. The mass (Wa) of the cotton bag containing the swollen gel after dehydration was measured. The same procedure was repeated without adding the water-absorbent resin particles. The empty mass (Wb) of the cotton bag when wet was measured, and the water retention capacity of the physiological saline solution was calculated using the following formula: Saline water retention capacity (g / g) = [Wa-Wb] / 2.0

[0074] [Water absorption under 2.07kPa load] The water absorption of water-absorbent resin particles under a load of 2.07 kPa (water absorption under a load of 2.07 kPa) in physiological saline was measured using an apparatus outlined in FIG. 1. The water absorption under load was measured twice for one type of water-absorbent resin particle, and the average of the measured values was calculated. The apparatus in FIG. 1 includes a burette unit 1, a clamp 3, a conduit 5, a stand 11, a measurement table 13, and a measurement unit 4 placed on the measurement table 13. The burette unit 1 includes a burette tube 21 with a scale, a rubber stopper 23 that seals the opening at the top of the burette tube 21, a cock 22 connected to the tip of the bottom of the burette tube 21, and an air introduction tube 25 and a cock 24 connected to the bottom of the burette tube 21. The burette unit 1 is fixed with a clamp 3. The flat measurement table 13 has a through-hole 13a with a diameter of 2 mm formed in its center, and is supported by a height-adjustable stand 11. The through hole 13a of the measurement table 13 and the cock 22 of the burette part 1 are connected by a conduit 5. The inner diameter of the conduit 5 is 6 mm.

[0075] The measurement unit 4 has a cylinder 31 made of acrylic resin, a polyamide mesh 32 adhered to one opening of the cylinder 31, and a weight 33 that is movable up and down within the cylinder 31. The cylinder 31 is placed on the measurement table 13 via the polyamide mesh 32. The inner diameter of the cylinder 31 is 20 mm. The opening of the polyamide mesh 32 is 75 μm (200 mesh). The weight 33 has a diameter of 19 mm and a mass of 59.8 g, and can apply a load of 2.07 kPa to the water-absorbent resin particles 10a uniformly arranged on the polyamide mesh 32, as will be described later.

[0076] First, the cocks 22 and 24 of the burette part 1 were closed, and 0.9% by mass saline solution adjusted to 25°C was poured into the burette tube 21 through the opening at the top of the burette tube 21. Next, the top opening of the burette tube 21 was sealed with a rubber stopper 23, and then the cocks 22 and 24 were opened. The inside of the conduit 5 was filled with 0.9% by mass saline solution 50 while taking care to prevent air bubbles from entering. The height of the measurement table 13 was adjusted so that the height of the water surface of the 0.9% by mass saline solution 50 that had reached the through-hole 13a was the same as the height of the upper surface of the measurement table 13. After the adjustment, the height of the water surface of the 0.9% by mass saline solution 50 in the burette tube 21 was read on the scale of the burette tube 21, and this position was designated as the zero point (the reading at 0 seconds).

[0077] In the measurement part 4, 0.10 g of water-absorbent resin particles 10a were uniformly arranged on a polyamide mesh 32 in a cylinder 31, a weight 33 was placed on the water-absorbent resin particles 10a, and the cylinder 31 was installed so that its center coincided with the conduit opening at the center of the measurement table 13. The amount of decrease Wc (mL) of the saline solution in the burette tube 21 (i.e., the amount of saline solution absorbed by the water-absorbent resin particles 10a) 60 minutes after the water-absorbent resin particles 10a started to absorb the saline solution from the conduit 5 was read, and the amount of saline solution absorbed by the water-absorbent resin particles 10a under a load of 2.07 kPa was calculated by the following formula. Water absorption of saline solution under a load of 2.07 kPa (mL / g) = Wc (mL) / 0.10 (g)

[0078] [Median particle size] 50 g of water-absorbent resin particles were used for measuring the median particle size. JIS standard sieves were arranged in the following order from top to bottom: a 710 μm mesh sieve, a 600 μm mesh sieve, a 500 μm mesh sieve, a 425 μm mesh sieve, a 300 μm mesh sieve, a 250 μm mesh sieve, a 150 μm mesh sieve, and a tray. The water-absorbent resin particles were placed on the top sieve and shaken for 10 minutes using a Rotap shaker for classification. After classification, the mass of the water-absorbent resin particles remaining on each sieve was calculated as a mass percentage relative to the total mass to determine the particle size distribution. The sieves on this particle size distribution were integrated in descending order of particle size, and the relationship between the sieve openings and the integrated value of the mass percentage of the water-absorbent resin particles remaining on the sieves was plotted on logarithmic probability paper. By connecting the plots on the probability paper with a straight line, the particle size corresponding to a cumulative mass percentage of 50 mass % was determined as the median particle size.

[0079] [Table 1]

[0080] As shown in Table 1, it was confirmed that the water-absorbing resin particles obtained by the manufacturing methods of the Examples had a faster water absorption rate than the Comparative Examples. [Explanation of symbols]

[0081] 1 Burette section 3 Clamp 4 Measuring part 5 Conduit 10a Water-absorbing resin particles 11 Mounting stand 13 Measuring table 13a Through hole 21 Burette 22 Cook 23 Rubber stopper 24 Cook 25 Air intake pipe 31 Cylinder 32 Polyamide mesh 33 Weight 50 0.9% by mass salt solution

Claims

1. a polymerization step of subjecting a water-soluble ethylenically unsaturated monomer to reverse-phase suspension polymerization to obtain a reaction liquid containing a hydrous gel polymer; a dehydration step of removing water from the reaction solution and adding a reducing inorganic salt; a surface cross-linking step of adding a surface cross-linking agent to the reaction liquid and surface cross-linking the hydrous gel polymer, a difference between a residual water rate of the hydrous gel polymer when a reducing inorganic salt is added in the dehydration step and a residual water rate of the hydrous gel polymer when a surface crosslinking agent is added in the surface crosslinking step is 5 to 80%.

2. 2. The method for producing water-absorbent resin particles according to claim 1, wherein the residual water content of the hydrous gel polymer when the reducing inorganic salt is added in the dehydration step is 15 to 100%.

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