Method for producing water absorbent resin particles

Abstract

The present invention provides a method for producing water absorbent resin particles which have good general water absorption performances (such as saline absorption) required for water absorbent resins, while additionally having high gel stability and being suppressed in yellowing under high temperature and high humidity conditions.　This method for producing water absorbent resin particles sequentially comprises in the following order: a step 1 in which a water-soluble ethylenically unsaturated monomer is polymerized, thereby obtaining hydrous gel particles; a step 2 in which a chelating agent, a sulfite compound and an organic antioxidant are added to the hydrous gel particles; and a step 3 in which the resulting hydrous gel particles are subjected to a surface crosslinking treatment. In the step 2, the moisture content of the hydrous gel particles when the sulfite compound is added thereto is 20% by mass to 75% by mass.

Description

Method for producing water-absorbent resin particles

[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 that constitute absorbents suitable for use in sanitary materials such as disposable diapers, sanitary napkins, and incontinence pads.

[0002] BACKGROUND ART In recent years, water-absorbent resins have been widely used in the field of sanitary materials such as disposable diapers, sanitary napkins, and incontinence pads.

[0003] As such a water-absorbent resin, a crosslinked polymer of a partially neutralized salt of acrylic acid has excellent water-absorbing ability, and since acrylic acid, which is a raw material thereof, is easily available industrially, it can be produced at low cost with constant quality, and is less susceptible to putrefaction and deterioration, and therefore, it is considered to be a preferred water-absorbent resin.

[0004] Japanese Patent Application Laid-Open No. 2005-29751

[0005] The gel formed by the absorption of body fluids such as human urine by water-absorbent resins is affected by various components in the body fluids (e.g., vitamin C, iron, etc. in urine), and the gel is subject to deterioration. For this reason, various additives (e.g., human urine stabilizers) have been added, and the manufacturing methods of water-absorbent resins have been improved.

[0006] For example, sulfite compounds, which are reducing agents, are expected to exhibit an effect as a stabilizer for human urine and to enhance the gel stability of water-absorbent resins. However, the addition of sulfite compounds to water-absorbent resins may cause problems such as deterioration of the general water-absorption performance of the water-absorbent resin (e.g., the amount of saline solution absorbed) and yellowing under high temperature and high humidity conditions.

[0007] Under such circumstances, a main object of the present invention is to provide a method for producing water-absorbent resin particles which have good general water-absorbing performance (for example, physiological saline water absorption capacity) required for a water-absorbent resin, and further which are inhibited from yellowing under high-temperature and high-humidity conditions and have high gel stability.

[0008] The present inventors have conducted extensive research to solve the above-mentioned problems. As a result, they have found that in a production method for water-absorbent resin particles, in which hydrogel particles obtained by polymerizing a water-soluble ethylenically unsaturated monomer are subjected to a surface-crosslinking treatment to obtain water-absorbent resin particles, a step of adding a chelating agent, a sulfite compound, and an organic antioxidant to the hydrogel particles before the surface-crosslinking treatment is carried out, and further, when the water content of the hydrogel particles when the sulfite compound is added in this step is within a predetermined range, water-absorbent resin particles having good water absorption performance generally required of a water-absorbent resin, suppressed yellowing under high-temperature and high-humidity conditions, and high gel stability can be suitably produced. The present invention was completed based on this finding and through further extensive research.

[0009] That is, the present invention provides the following configurations. Item 1. A method for producing water-absorbent resin particles, comprising: Step 1 of polymerizing a water-soluble ethylenically unsaturated monomer to obtain hydrous gel particles; Step 2 of adding a chelating agent, a sulfite compound, and an organic antioxidant to the hydrous gel particles; and Step 3 of subjecting the hydrous gel particles to a surface cross-linking treatment, in this order, wherein in Step 2, the water content of the hydrous gel particles when the sulfite compound is added is 20% by mass or more and 75% by mass or less. Item 2. The method for producing water-absorbent resin particles according to Item 1, wherein Step 2 further comprises a step of adjusting the water content of the hydrous gel particles. Item 3. The method for producing water-absorbent resin particles according to Item 1 or 2, wherein in Step 2, the water content of the hydrous gel particles when the chelating agent is added is 20% by mass or more. Item 4. Item 5. The method for producing water-absorbent resin particles according to any one of Items 1 to 3, wherein in Step 2, the water content of the hydrogel particles is 20% by mass or more when the organic antioxidant is added. Item 6. The method for producing water-absorbent resin particles according to any one of Items 1 to 5, wherein in Step 2, the amount of the sulfite compound added is 0.001 parts by mass or more and 3.0 parts by mass or less relative to 100 parts by mass of the water-soluble ethylenically unsaturated monomer. Item 7. The method for producing water-absorbent resin particles according to any one of Items 1 to 6, wherein in Step 2, the amount of the chelating agent added is 0.001 parts by mass or more and 2.0 parts by mass or less relative to 100 parts by mass of the water-soluble ethylenically unsaturated monomer. Item 8. The saline water absorption is 40-70 g / g, the yellowness index is less than 40 after being left in an environment of 70°C and 90% relative humidity for 14 days, and the gel strength is 5500 N / m after being left in an environment of 37°C and 60% relative humidity for 14 hours. 2 Item 9. An absorbent body comprising the water-absorbent resin particles according to Item 8. Item 10. An absorbent article comprising the absorbent body according to Item 9.

[0010] According to the present invention, it is possible to provide a method for producing water-absorbent resin particles that have good general water-absorbing performance required of a water-absorbent resin (for example, saline water absorption capacity), and further have suppressed yellowing under high temperature and high humidity conditions and high gel stability. Furthermore, according to the present invention, it is also possible to provide water-absorbent resin particles that have good saline water absorption capacity, suppressed yellowing under high temperature and high humidity conditions and high gel stability, an absorbent body containing the water-absorbent resin particles, and an absorbent article containing the absorbent body.

[0011] 1 is a schematic diagram of a measuring device for measuring the amount of saline water absorption under a load of 4.14 kPa. 2 is a schematic diagram of a measuring device used for measuring gel strength.

[0012] In this specification, the term "comprising" includes "consisting essentially of" and "consisting of". Furthermore, in this specification, "(meth)acrylic" means "acrylic or methacrylic", and "(meth)acrylate" means "acrylate or methacrylate". Furthermore, "water-soluble" means exhibiting a solubility of 5% by mass or more in water at 25°C.

[0013] In addition, in this specification, a numerical value connected with "~" means a numerical range that includes the numerical values ​​before and after "~" as the lower limit and upper limit. When multiple lower limit values ​​and multiple upper limit values ​​are listed separately, any lower limit value and upper limit value can be selected and connected with "~".

[0014] 1. Manufacturing Method of Water-Absorbent Resin Particles The manufacturing method of water-absorbent resin particles of the present invention comprises, in this order: Step 1: polymerizing a water-soluble ethylenically unsaturated monomer to obtain hydrogel particles; Step 2: adding a chelating agent, a sulfite compound, and an organic antioxidant to the hydrogel particles; and Step 3: subjecting the hydrogel particles to a surface cross-linking treatment. The method is characterized in that, in Step 2, the water content of the hydrogel particles when the sulfite compound is added is 20% by mass or more and 75% by mass or less. The water-absorbent resin particles manufactured by the manufacturing method of the present invention having such characteristics exhibit good water absorption performance (e.g., saline water absorption capacity) generally required of a water-absorbent resin, and further exhibit high gel stability by suppressing yellowing under high-temperature and high-humidity conditions. The manufacturing method of water-absorbent resin particles of the present invention will be described in detail below.

[0015] (Step 1) Step 1 is a step of polymerizing a water-soluble ethylenically unsaturated monomer to obtain hydrogel particles. Typical polymerization methods for polymerizing a water-soluble ethylenically unsaturated monomer include aqueous solution polymerization, emulsion polymerization, and reversed-phase suspension polymerization. In aqueous solution polymerization, polymerization is carried out by heating an aqueous solution of the water-soluble ethylenically unsaturated monomer while stirring as necessary. In reversed-phase suspension polymerization, polymerization is carried out by heating the water-soluble ethylenically unsaturated monomer in a hydrocarbon dispersion medium while stirring.

[0016] Step 1 may further include a step of pulverizing the hydrogel obtained by polymerizing the water-soluble ethylenically unsaturated monomer to obtain hydrogel particles. When reverse phase suspension polymerization is used as the polymerization method for the hydrogel particles, hydrogel particles are produced by polymerization, so pulverization of the hydrogel is usually not necessary. As an example of Step 1 for obtaining hydrogel particles, an example of reverse phase suspension polymerization will be described below.

[0017] In the method for obtaining hydrogel particles by reversed-phase suspension polymerization of a water-soluble ethylenically unsaturated monomer in a hydrocarbon dispersion medium, the polymerization is carried out in the presence of a radical polymerization initiator. As described later, if necessary, an internal crosslinking agent may be added to the water-soluble ethylenically unsaturated monomer to form hydrogel particles having an internal crosslinked structure.

[0018] <Polymerization Step> [Water-Soluble Ethylenically Unsaturated Monomer] Examples of the water-soluble ethylenically unsaturated monomer include (meth)acrylic acid (in this specification, "acrylic" and "methacrylic" are collectively referred to as "(meth)acrylic", the same applies hereinafter) and salts thereof; 2-(meth)acrylamido-2-methylpropanesulfonic acid and salts thereof; nonionic monomers such as (meth)acrylamide, N,N-dimethyl(meth)acrylamide, 2-hydroxyethyl(meth)acrylate, N-methylol(meth)acrylamide, and polyethylene glycol mono(meth)acrylate; and amino group-containing unsaturated monomers such as N,N-diethylaminoethyl(meth)acrylate, N,N-diethylaminopropyl(meth)acrylate, and diethylaminopropyl(meth)acrylamide, and quaternized products thereof. Among these water-soluble ethylenically unsaturated monomers, (meth)acrylic acid or a salt thereof, (meth)acrylamide, and N,N-dimethylacrylamide are preferred, and (meth)acrylic acid and a salt thereof are more preferred, from the viewpoint of industrial ease of availability, etc. 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 resins, and these acrylic acid and / or salts thereof may be copolymerized with the other water-soluble ethylenically unsaturated monomers described above. In this case, it is preferable that acrylic acid and / or its salts are used as the main water-soluble ethylenically unsaturated monomer in an amount of 70 to 100 mol % based on the total amount of water-soluble ethylenically unsaturated monomers.

[0020] The water-soluble ethylenically unsaturated monomer may be 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 or less. The concentration of the water-soluble ethylenically unsaturated monomer is more preferably 55% by mass or less, even more preferably 50% by mass or less, and even more preferably 45% by mass or less. Meanwhile, the concentration of the water-soluble ethylenically unsaturated monomer is more preferably 25% by mass or more, even more preferably 28% by mass or more, and even more preferably 30% by mass or more.

[0021] When the water-soluble ethylenically unsaturated monomer has an acid group, such as (meth)acrylic acid or 2-(meth)acrylamido-2-methylpropanesulfonic acid, the acid group may be 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.

[0022] 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 40 to 85 mol %, and still more preferably 50 to 80 mol %, as the degree of neutralization with respect to all acid groups possessed by the water-soluble ethylenically unsaturated monomer.

[0023] [Radical Polymerization Initiator] Examples of the radical polymerization initiator added to the polymerization step 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-butylperoxyisobutyrate, t-butylperoxypivalate, and hydrogen peroxide, as well as 2,2'-azobis(2-amidinopropane) dihydrochloride and 2,2'-azobis[2-(N-phenyl)propane]. 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-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). Among these radical polymerization initiators, potassium persulfate, ammonium persulfate, sodium persulfate, and 2,2'-azobis(2-amidinopropane) dihydrochloride are preferred from the viewpoints of ease of availability and handling. These radical polymerization initiators may be used alone or in combination of two or more. 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.

[0024] The amount of radical polymerization initiator used is, for example, 0.00005 to 0.01 mole per mole of the water-soluble ethylenically unsaturated monomer. By using such an amount, it is possible to avoid a rapid polymerization reaction and complete the polymerization reaction within an appropriate time.

[0025] [Internal Crosslinking Agent] The internal crosslinking agent can be one that can crosslink the polymer of the water-soluble ethylenically unsaturated monomer used, such as (poly)ethylene glycol ("(poly)" refers to both the presence and absence of the prefix "poly"). the same applies hereinafter)], unsaturated polyesters obtained by reacting polyols such as diols and triols, such as (poly)propylene glycol, 1,4-butanediol, 1,6-hexanediol, 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, divinyl Examples of the compound include a compound having two or more polymerizable unsaturated groups such as benzene; a diglycidyl compound such as (poly)ethylene glycol diglycidyl ether, (poly)propylene glycol diglycidyl ether, (poly)glycerin diglycidyl ether, and the like, and a polyglycidyl compound such as a triglycidyl compound; an epihalohydrin compound such as epichlorohydrin, epibromohydrin, and α-methylepichlorohydrin; a compound having two or more reactive functional groups such as an isocyanate compound such as 2,4-tolylene diisocyanate and hexamethylene diisocyanate; and an oxetane compound 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 cross-linking agents, it is preferable to use a polyglycidyl compound, it is more preferable to use a diglycidyl ether compound, 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.

[0026] The amount of the internal crosslinking agent used is preferably 0.000001 to 0.02 mol, more preferably 0.00001 to 0.01 mol, even more preferably 0.00001 to 0.005 mol, and still more preferably 0.00005 to 0.002 mol, relative to 1 mol of the water-soluble ethylenically unsaturated monomer.

[0027] [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 because they are easily available industrially, have stable quality, and are inexpensive. These hydrocarbon dispersion media may be used alone or in combination of two or more. As an example of a mixture of hydrocarbon dispersion media, a commercially available product such as Exxol Heptane (manufactured by ExxonMobil Corporation; contains 75 to 85% by mass of hydrocarbons such as heptane and its isomers) can also be used to obtain favorable results.

[0028] The amount of hydrocarbon dispersion medium used is preferably 100 to 1500 parts by mass, and more preferably 200 to 1400 parts by mass, per 100 parts by mass of the water-soluble ethylenically unsaturated monomer in the first stage, from the viewpoints of uniformly dispersing the water-soluble ethylenically unsaturated monomer and facilitating control of the polymerization temperature. As will be described later, the reversed-phase suspension polymerization is carried out in one stage (single stage) or in multiple stages of two or more stages, and the above-mentioned first stage polymerization refers to the polymerization reaction in the first stage of single-stage polymerization or multi-stage polymerization (the same applies hereinafter).

[0029] [Dispersion stabilizer] (Surfactant) In the reversed-phase suspension polymerization, a dispersion stabilizer can be 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.

[0030] 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, alkylallyl 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, phosphate esters of polyoxyethylene alkyl ethers, and phosphate esters of polyoxyethylene alkyl allyl ethers. Among these surfactants, sorbitan fatty acid esters, polyglycerin fatty acid esters, and sucrose fatty acid esters are particularly preferred from the standpoint of dispersion stability of the monomer. These surfactants may be used alone or in combination of two or more.

[0031] The amount of surfactant used is preferably 0.1 to 30 parts by mass, more preferably 0.3 to 20 parts by mass, per 100 parts by mass of the first stage water-soluble ethylenically unsaturated monomer.

[0032] (Polymer-Based Dispersant) As a dispersion stabilizer used in reversed-phase suspension polymerization, a polymer-based dispersant may be used in combination with the surfactant described above.

[0033] 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.

[0034] The amount of the polymeric dispersant used is preferably 0.1 to 30 parts by mass, more preferably 0.3 to 20 parts by mass, per 100 parts by mass of the first stage water-soluble ethylenically unsaturated monomer.

[0035] [Other Components] 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 other components, various additives such as a thickener and a chain transfer agent may be added.

[0036] For example, a thickener can be added to an aqueous solution containing a water-soluble ethylenically unsaturated monomer to carry out reversed-phase suspension polymerization. By adjusting the viscosity of the aqueous solution by adding a thickener in this way, it is possible to control the median particle size obtained in the reversed-phase suspension polymerization.

[0037] Examples of usable thickeners 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 of the resulting particles tend to be.

[0038]

[0033] In performing the reversed-phase suspension polymerization, for example, 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. In this case, the dispersion stabilizer (surfactant or polymeric dispersant) may be added either before or after the addition of the aqueous monomer solution, as long as it is before the start of the polymerization reaction.

[0039] Among these, from the viewpoint of easily reducing the amount of 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.

[0040] Such reversed-phase suspension polymerization can be carried out in one stage or in multiple stages of two or more stages, and is preferably carried out in two or three stages from the viewpoint of increasing productivity.

[0041] When carrying out reversed-phase suspension polymerization in two or more stages, after carrying out reversed-phase suspension polymerization in the first stage, a water-soluble ethylenically unsaturated monomer is added to the reaction mixture obtained in the polymerization reaction in the first stage and mixed, and reversed-phase suspension polymerization in the second and subsequent stages can be carried out in the same manner as in the first stage.In the reversed-phase suspension polymerization in each stage from the second stage onwards, it is preferable to carry out reversed-phase suspension polymerization by adding a radical polymerization initiator in addition to the water-soluble ethylenically unsaturated monomer within the molar ratio of each component to the water-soluble ethylenically unsaturated monomer as described above, based on the amount of the water-soluble ethylenically unsaturated monomer added during the reversed-phase suspension polymerization in each stage from the second stage onwards.In addition, in the polymerization in the second and subsequent stages, an internal crosslinking agent may be added to the water-soluble ethylenically unsaturated monomer as needed.

[0042] The reaction temperature of the polymerization reaction is preferably 20 to 110°C, more preferably 40 to 90°C, from the viewpoints of rapidly progressing the polymerization, shortening the polymerization time, thereby improving economic efficiency, and easily removing the heat of polymerization to smoothly carry out the reaction.

[0043] (Step 2) Step 2 is a step of adding a chelating agent, a sulfite compound, and an organic antioxidant to the hydrogel particles obtained in Step 1. In the present invention, the water content of the hydrogel particles when the sulfite compound is added in Step 2 is 20% by mass or more and 75% by mass or less.

[0044] From the viewpoint of more suitably exerting the effects of the present invention, the water content of the hydrogel particles when adding a sulfite compound to the hydrogel particles is preferably 20% by mass or more, more preferably 30% by mass or more, and even more preferably 40% by mass or more, and a preferred range is 20 to 75% by mass, 30 to 75% by mass, etc.

[0045] Furthermore, from the viewpoint of more suitably exerting the effects of the present invention, the water content of the hydrogel particles when the chelating agent is added to the hydrogel particles in step 2 is preferably 20% by mass or more, more preferably 30% by mass or more, even more preferably 40% by mass or more, and is preferably 150% by mass or less, more preferably 100% by mass or less, even more preferably 75% by mass or less, with preferred ranges being 20 to 150% by mass, 20 to 75% by mass, 30 to 75% by mass, etc.

[0046] From the viewpoint of more suitably exerting the effects of the present invention, the water content of the hydrogel particles when the organic antioxidant is added to the hydrogel particles in step 2 is preferably 20% by mass or more, more preferably 30% by mass or more, even more preferably 40% by mass or more, and is preferably 150% by mass or less, more preferably 100% by mass or less, even more preferably 75% by mass or less, and preferred ranges are 20 to 150% by mass, 20 to 75% by mass, 30 to 75% by mass, etc.

[0047] Step 2 preferably further includes a step of adjusting the water content of the hydrogel particles obtained in Step 1. For example, if the water content of the hydrogel particles produced in Step 1 is less than 20% by mass, water can be added to the hydrogel particles, and if the water content is more than 75% by mass, the hydrogel particles can be heated to evaporate the water, thereby adjusting the water content to within the range of 20% by mass to 75% by mass. For example, when hydrogel particles are produced by reverse phase suspension polymerization or the like, the water content of the resulting hydrogel particles usually exceeds 75% by mass (for example, the water content is often within the range of 100 to 250% by mass), so it is effective to provide a step of adjusting the water content of the hydrogel particles obtained in Step 1.

[0048] The order in which the chelating agent, sulfite compound, and organic antioxidant are added to the hydrogel particles is not particularly limited. For example, the chelating agent, sulfite compound, and organic antioxidant may be added in this order, or at least two of them may be added simultaneously.

[0049] From the viewpoint of more suitably exerting the effects of the present invention, the amount of the sulfite compound added in step 2 is preferably 0.001 part by mass or more, more preferably 0.01 part by mass or more, even more preferably 0.05 part by mass or more, and is preferably 3.0 parts by mass or less, more preferably 1.0 part by mass or less, even more preferably 0.50 part by mass or less, even more preferably 0.25 part by mass or less, relative to 100 parts by mass of the water-soluble ethylenically unsaturated monomer, and is preferably in the range of 0.001 to 3.0 parts by mass, 0.01 to 1.0 part by mass, etc.

[0050] Furthermore, from the viewpoint of more suitably exerting the effects of the present invention, the amount of the chelating agent added in step 2 is, relative to 100 parts by mass of the water-soluble ethylenically unsaturated monomer, preferably 0.001 part by mass or more, more preferably 0.002 part by mass or more, even more preferably 0.003 part by mass or more, and is preferably 2.0 parts by mass or less, more preferably 1.0 part by mass or less, more preferably 0.50 part by mass or less, even more preferably 0.10 part by mass or less, even more preferably 0.05 part by mass or less, and preferably in the range of 0.001 to 2.0 parts by mass, 0.002 to 1.0 part by mass, etc.

[0051] Furthermore, from the viewpoint of more suitably exerting the effects of the present invention, the amount of the organic antioxidant added in Step 2 is preferably 0.10 parts by mass or more, more preferably 0.30 parts by mass or more, and is preferably 20 parts by mass or less, more preferably 10 parts by mass or less, relative to 100 parts by mass of the sulfite compound, and preferably ranges from 0.10 to 20 parts by mass, 0.30 to 10 parts by mass, etc.

[0052] To more effectively achieve the effects of the present invention, the chelating agent, sulfite compound, and organic antioxidant are preferably added to the hydrogel particles in the form of an aqueous solution. The concentrations of the components in the aqueous solution may be adjusted to the respective amounts added as described above. For example, the concentration of the chelating agent aqueous solution added to the hydrogel particles is preferably 0.1 to 50% by mass, more preferably 0.5 to 40% by mass. The concentration of the sulfite compound aqueous solution added to the hydrogel particles is preferably 1 to 20% by mass, more preferably 5 to 20% by mass. The concentration of the organic antioxidant aqueous solution added to the hydrogel particles is preferably 0.01 to 10% by mass, more preferably 0.1 to 5% by mass.

[0053] From the viewpoint of more suitably exerting the effects of the present invention, preferred examples of the sulfite compound include sodium sulfite, potassium sulfite, calcium sulfite, sodium hydrogen sulfite, potassium hydrogen sulfite, ammonium hydrogen sulfite, sodium pyrosulfite, and potassium pyrosulfite. Only one type of sulfite compound may be used, or two or more types may be used.

[0054] From the viewpoint of more suitably exerting the effects of the present invention, preferred examples of the chelating agent include ethylenediamine-N,N'-disuccinic acid, diethylenetriaminepentaacetic acid, glycol ether diaminetetraacetic acid, ethylenediaminetetramethylenephosphonic acid, and diethylenetriaminepentamethylenephosphonic acid. The chelating agent may exist in the form of a salt. Examples of the salt include sodium salts and potassium salts. Only one type of chelating agent may be used, or two or more types may be used.

[0055] From the viewpoint of more suitably exhibiting the effects of the present invention, preferred examples of the organic antioxidant include ascorbic acids, erythorbic acids, gallic acids, protocatechuic acids, benzimidazoles, and alkylhydroxyanisoles. Only one type of organic antioxidant may be used, or two or more types may be used.

[0056] (Step 3) Step 3 is a step of subjecting the hydrogel particles to which the chelating agent, the sulfite compound, and the organic antioxidant have been added in Step 2 to surface cross-linking. The surfaces of the hydrogel particles are cross-linked by the surface cross-linking (surface cross-linking reaction). By subjecting the hydrogel particles having an internal cross-linked structure to a surface cross-linking reaction, the cross-linking density in the vicinity of the surface of the water-absorbent resin particles can be increased, and water-absorbent resin particles having improved performance such as the amount of saline water absorption under load can be obtained.

[0057] 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, diethylene glycol, triethylene glycol, 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; haloepoxy compounds such as epichlorohydrin, epibromohydrin, and α-methylepichlorohydrin; isocyanate compounds such as 2,4-tolylene diisocyanate and hexamethylene diisocyanate; 3-methyl-3-oxetanemethanol and 3-ethyl-3-oxetane Oxetane compounds such as methanol, 3-butyl-3-oxetanemethanol, 3-methyl-3-oxetaneethanol, 3-ethyl-3-oxetaneethanol, and 3-butyl-3-oxetaneethanol; oxazoline compounds such as 1,2-ethylenebisoxazoline; ethylene carbonate, propylene carbonate, 4,5-dimethyl-1,3-dioxolan-2-one, 4,4-dimethyl-1,3-dioxolan-2-one, 4-ethyl carbonate compounds (e.g., alkylene carbonates) such as 1,3-dioxolan-2-one, 4-hydroxymethyl-1,3-dioxolan-2-one, 1,3-dioxan-2-one, 4-methyl-1,3-dioxan-2-one, 4,6-dimethyl-1,3-dioxan-2-one, and 1,3-dioxopan-2-one; and hydroxyalkylamide compounds such as bis[N,N-di(β-hydroxyethyl)]adipamide.Among these surface cross-linking agents, polyols such as ethylene glycol, propylene glycol, 1,4-butanediol, diethylene glycol, triethylene glycol, trimethylolpropane, glycerin, polyoxyethylene glycol, polyoxypropylene glycol, and polyglycerin; (poly)ethylene glycol diglycidyl ether, (poly)glycerin diglycidyl ether, (poly)glycerin triglycidyl ether, trimethylolpropane triglycidyl ether, (poly)propylene glycol polyglycidyl ether, (poly)glycerin diglycidyl ether, trimethylolpropane triglycidyl ether, (poly)propylene glycol polyglycidyl ether, and (poly)glycerin diglycidyl ether; and carbonate compounds (e.g., alkylene carbonates) such as ethylene carbonate, propylene carbonate, 4,5-dimethyl-1,3-dioxolan-2-one, 4,4-dimethyl-1,3-dioxolan-2-one, 4-ethyl-1,3-dioxolan-2-one, 4-hydroxymethyl-1,3-dioxolan-2-one, 1,3-dioxan-2-one, 4-methyl-1,3-dioxan-2-one, 4,6-dimethyl-1,3-dioxan-2-one, and 1,3-dioxopan-2-one. These surface cross-linking agents may be used alone or in combination of two or more.

[0058] The amount of the surface crosslinking agent used is preferably 0.00001 to 0.01 mol, more preferably 0.00005 to 0.005 mol, and further preferably 0.0001 to 0.002 mol, relative to 1 mol of the total amount of the water-soluble ethylenically unsaturated monomers used in the polymerization.

[0059] As a method for adding the surface crosslinking agent, the surface crosslinking 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, isopropyl alcohol, etc.; ketones such as acetone, methyl ethyl ketone, etc.; ethers such as diethyl ether, dioxane, tetrahydrofuran, etc.; amides such as N,N-dimethylformamide, etc.; sulfoxides such as dimethyl sulfoxide, etc. These hydrophilic organic solvents may be used alone, or two or more types may be used in combination, or as a mixed solvent with water.

[0060] The water content of the hydrogel particles when the surface cross-linking agent is added to the hydrogel particles is preferably in the range of 1 to 75 mass%, more preferably in the range of 5 to 60 mass%, even more preferably in the range of 10 to 50 mass%, and still more preferably in the range of 15 to 40 mass%.

[0061] 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 of the surface crosslinking reaction is preferably 1 to 300 minutes, and more preferably 5 to 200 minutes.

[0062] Step 3 preferably further includes a drying step of the water-absorbent resin particles obtained by surface-treating the hydrous gel particles. Specifically, after the above-described reversed-phase suspension polymerization, a drying step may be included in which water, a hydrocarbon dispersion medium, and the like are removed by distillation by externally applying energy such as heat. When drying the hydrous gel particles after reversed-phase suspension polymerization, the system in which the hydrous gel particles are dispersed in the hydrocarbon dispersion medium is heated, and the water and the hydrocarbon dispersion medium are temporarily distilled out of the system by azeotropic distillation. Returning only the evaporated hydrocarbon dispersion medium to the system enables continuous azeotropic distillation. In this 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 preventing deterioration of the resin. Subsequently, the water and the hydrocarbon dispersion medium are distilled off to obtain dried 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 resulting water-absorbent resin particles.

[0063] In the drying step, the drying treatment by distillation may be carried out under normal pressure or under reduced pressure. Moreover, from the viewpoint of increasing the drying efficiency, it may be carried out under a gas flow 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. Moreover, when the drying treatment is carried out under reduced pressure, the drying temperature is preferably 40 to 160°C, more preferably 50 to 110°C.

[0064] The loss on drying of the water-absorbent resin particles obtained by the production method of the present invention is preferably 20% by mass or less, more preferably 15% by mass or less, and even more preferably 10% by mass or less, and a preferred range is 1 to 10% by mass, etc.

[0065] In addition, when the surface cross-linking step using a surface cross-linking agent is carried out after the polymerization of monomers by reverse phase suspension polymerization, the drying step by distillation described above is carried out after the surface cross-linking step is completed. Alternatively, the surface cross-linking step and the drying step may be carried out simultaneously.

[0066] The water-absorbent resin particles of the present invention may contain additives according to the purpose. Examples of such additives include inorganic powders, surfactants, oxidizing agents, radical chain inhibitors, and antibacterial agents. For example, the fluidity of the water-absorbent resin particles can be further improved by adding 0.05 to 5 parts by mass of amorphous silica as an inorganic powder relative to 100 parts by mass of the water-absorbent resin particles. The additives are preferably hydrophilic or water-soluble. The additives are preferably added to the water-absorbent resin particles obtained after the surface cross-linking step.

[0067] In the water-absorbent resin particles of the present invention, the content of the water-absorbent resin (excluding additives) including moisture (loss on drying) is preferably 70 mass % or more, more preferably 80 mass % or more, and further preferably 90 mass % or more.

[0068] The water-absorbent resin particles obtained by the production method of the present invention have good general water-absorbing performance (e.g., physiological saline water absorption capacity) required for a water-absorbent resin, and further, are inhibited from yellowing under high-temperature and high-humidity conditions and have high gel stability.

[0069] The water-absorbent resin particles of the present invention have, for example, a saline water absorption capacity of 40 to 70 g / g, a yellowness index of less than 40 after being left for 14 days in an environment of 70°C and 90% relative humidity, and a gel strength of 5500 N / m after being left for 14 hours in an environment of 37°C and 60% relative humidity. 2 The water-absorbent resin particles of the present invention are suitably produced by the production method of the present invention, but are not limited to water-absorbent resin particles produced by the production method, and may be any water-absorbent resin particles as long as they satisfy the above-mentioned physiological saline water absorption amount, yellowness index after being left to stand for 14 days in an environment of 70°C and 90% relative humidity, and gel strength after being left to stand for 14 hours in an environment of 37°C and 60% relative humidity.

[0070] The physiological saline water absorption amount of the water-absorbent resin particles of the present invention is preferably 40 g / g or more, more preferably 45 g / g or more, and further preferably 50 g / g or more, and from the viewpoint of suppressing sliminess of the gel after water absorption, the upper limit is preferably 70 g / g or less, and preferred ranges are 40 to 70 g / g, 45 to 70 g / g, etc.

[0071] The physiological saline water retention capacity of the water-absorbent resin particles of the present invention is preferably 25 g / g or more, more preferably 30 g / g or more, and is also preferably 60 g / g or less, more preferably 55 g / g or less, still more preferably 50 g / g or less, and preferred ranges are 25 to 60 g / g, 30 to 55 g / g, etc.

[0072]

[0113] Furthermore, the water absorption amount of physiological saline solution under a load of 4.14 kPa of the water absorbent resin particles obtained by the production method of the present invention is preferably 5 mL / g or more, more preferably 10 mL / g or more, and is also preferably 40 mL / g or less, more preferably 35 mL / g or less, still more preferably 30 mL / g or less, and preferred ranges are 5 to 40 mL / g, 10 to 30 mL / g, etc.

[0073] The physiological saline water absorption capacity, physiological saline water retention capacity and physiological saline water absorption capacity under a load of 4.14 kPa of the water-absorbent resin particles are values ​​measured by the methods described in the Examples.

[0074] Moreover, the water absorbent resin particles of the present invention have an "initial value of yellowness" evaluated by the method described in the Examples, which is preferably less than 20, more preferably less than 15, and even more preferably less than 10. The lower limit of the yellowness is, for example, 0.

[0075] Furthermore, the water absorbent resin particles of the present invention have a "yellowness index after being left to stand for 7 days in an environment of 70°C and 90% relative humidity," which is evaluated by the method described in the Examples, of preferably less than 40, more preferably less than 38, and even more preferably less than 30. The lower limit of the yellowness index is, for example, 5.

[0076] Furthermore, the water absorbent resin particles of the present invention have a "yellowness index after being left to stand for 14 days in an environment of 70°C and 90% relative humidity," which is evaluated by the method described in the Examples, of preferably less than 40, more preferably less than 38. The lower limit of the yellowness index is, for example, 10.

[0077] The water-absorbent resin particles of the present invention preferably have an "initial value of gel strength" of 5500 N / m 2 More preferably, 6200 N / m2 The upper limit of the initial value of the gel strength is, for example, 20,000 N / m 2 is.

[0078] Furthermore, the water-absorbent resin particles of the present invention preferably have a "gel strength after standing at 37°C for 14 hours" of 5500 N / m 2 More preferably, 6200 N / m 2 The upper limit of the gel strength is, for example, 20,000 N / m 2 is.

[0079] 2. Absorbent Material and Absorbent Articles The water-absorbing resin particles of the present invention constitute an absorbent material used in hygiene materials such as sanitary products and disposable diapers, and are suitably used in absorbent articles containing the absorbent material.

[0080] Here, the absorbent using the water-absorbent resin particles of the present invention contains the water-absorbent resin particles of the present invention. The absorbent may further contain hydrophilic fibers. Examples of the configuration of the absorbent include a sheet-like structure in which water-absorbent resin particles are fixed on a nonwoven fabric or between multiple nonwoven fabrics, a mixed dispersion obtained by mixing water-absorbent resin particles and hydrophilic fibers to form a uniform composition, a sandwich structure in which water-absorbent resin particles are sandwiched between layered hydrophilic fibers, and a structure in which water-absorbent resin particles and hydrophilic fibers are wrapped in tissue. Note that the absorbent may contain other components, such as adhesive binders such as heat-fusible synthetic fibers, hot-melt adhesives, and adhesive emulsions, to improve the shape retention of the absorbent.

[0081] The content of the water-absorbent resin particles in the absorbent body is preferably 5 to 100% by mass, more preferably 10 to 95% by mass, even more preferably 20 to 90% by mass, and even more preferably 30 to 80% by mass.

[0082] Examples of hydrophilic fibers include cellulose fibers such as cotton-like pulp obtained from wood, mechanical pulp, chemical pulp, and semi-chemical pulp, artificial cellulose fibers such as rayon and acetate, and fibers made of synthetic resins such as hydrophilically treated polyamide, polyester, and polyolefin. The average fiber length of the hydrophilic fibers is usually 0.1 to 10 mm, or may be 0.5 to 5 mm.

[0083] The absorbent article of the present invention can be produced by holding an absorbent body using the water-absorbent resin particles of the present invention between a liquid-permeable sheet (top sheet) through which liquid can pass and a liquid-impermeable sheet (back sheet) through which liquid cannot pass. The liquid-permeable sheet is arranged on the side that comes into contact with the body, and the liquid-impermeable sheet is arranged on the opposite side that comes into contact with the body.

[0084] Examples of liquid-permeable sheets include nonwoven fabrics such as air-through, spunbond, chemical-bond, and needle-punched types made of fibers such as polyethylene, polypropylene, and polyester, as well as porous synthetic resin sheets. Examples of liquid-impermeable sheets include synthetic resin films made of resins such as polyethylene, polypropylene, and polyvinyl chloride.

[0085] The present invention will be described in detail below with reference to examples and comparative examples, but the water-absorbent resin particles of the present invention and the method for producing the same are not limited to the examples.

[0086] The water-absorbent resin particles obtained in the following Production Examples and the water-absorbent resin particles obtained in the Examples and Comparative Examples were evaluated by the following various tests. Unless otherwise specified, the measurements were carried out in an environment of a temperature of 25±2°C and a relative humidity of 50±10%.

[0087] Example 1 [Process for Producing Hydrogel Particles (Process 1)] A round-bottomed, cylindrical, separable flask with an inner diameter of 11 cm and a capacity of 2 L was prepared, equipped with a reflux condenser, a dropping funnel, a nitrogen gas inlet tube, and a stirrer with a stirring blade having two stages of four inclined paddle blades with a blade diameter of 5 cm. 293 g of n-heptane was added to the flask as a hydrocarbon dispersion medium, and 0.736 g of 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 50°C. Separately, 92.0 g (1.03 mol) of an 80.5 mass% aqueous acrylic acid solution as a water-soluble ethylenically unsaturated monomer was placed in a 300 mL beaker, and while cooling with ice water, 147.7 g of a 20.9 mass% aqueous sodium hydroxide solution was added dropwise to carry out 75 mol% neutralization, followed by adding and dissolving 0.0736 g (0.272 mmol) of potassium persulfate as a water-soluble radical polymerization agent and 0.010 g (0.057 mmol) of ethylene glycol diglycidyl ether as an internal crosslinking agent, to prepare a first-stage aqueous liquid. The aqueous liquid prepared above was then added to a separable flask and stirred for 10 minutes. A surfactant solution prepared by heating and dissolving 0.736 g of a sucrose stearate ester with an HLB of 3 (Ryoto Sugar Ester S-370, Mitsubishi-Kagaku Foods Corporation) as a surfactant in 6.62 g of n-heptane in a 20 mL vial was then added. The system was thoroughly purged with nitrogen while stirring at a stirrer rotation speed of 500 rpm, and 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.

[0088] On the other hand, 128.8 g (1.44 mol) of an 80.5 mass% aqueous acrylic acid solution as a water-soluble ethylenically unsaturated monomer was placed in another 500 mL beaker, and while cooling with ice water, 160.0 g of a 27 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 and 0.0116 g (0.067 mmol) of ethylene glycol diglycidyl ether as an internal crosslinking agent were added and dissolved to prepare a second-stage aqueous liquid.

[0089] The contents of the separable flask system were cooled to 27°C while stirring at a stirrer speed of 1000 rpm, and then the entire amount of the second-stage aqueous liquid was added to the first-stage polymerization slurry liquid. The atmosphere in the system was replaced with nitrogen for 30 minutes, and the flask was again immersed in a water bath at 70°C to raise the temperature, and a polymerization reaction was carried out for 60 minutes to obtain hydrogel particles.

[0090] [Step 2 of Adding Additives to Hydrogel Particles] After obtaining the hydrogel particles, the flask was immersed in an oil bath set at 125°C, and 175.1 g of water was extracted from the system by azeotropic distillation of n-heptane and water while refluxing n-heptane (first water content adjustment step). Then, 4.42 g of a 0.5% by mass aqueous solution of trisodium ethylenediamine-N,N'-disuccinate was added with stirring. At this time, the water content of the hydrogel particles upon addition of the trisodium ethylenediamine-N,N'-disuccinate aqueous solution was 62% by mass. Next, 2.21 g of a 20% by mass aqueous solution of sodium sulfite was added with stirring. At this time, the water content of the hydrogel particles upon addition of the sodium sulfite aqueous solution was 64% by mass. Next, 2.21 g of a 0.2% by mass aqueous solution of L(+)-ascorbic acid was added with stirring. At this time, the water content of the hydrogel particles when the L(+)-ascorbic acid aqueous solution was added was 65% by mass.

[0091] [Surface cross-linking step (step 3)] Then, by azeotropic distillation of n-heptane and water again, while refluxing n-heptane, 84.7 g of water was extracted out of the system. Then, 4.42 g (0.507 mmol) of a 2 mass% aqueous solution of ethylene glycol diglycidyl ether was added to the flask as a surface cross-linking agent, and the mixture was maintained at 83 ° C. for 2 hours. Then, n-heptane was evaporated at 125 ° C. and dried, and further passed through a sieve with an opening of 850 μm to obtain 225.7 g of water-absorbent resin particles.

[0092] [Step of adding additives to water-absorbent resin particles] 0.5 parts by mass of amorphous silica (Toxil NP-S, Oriental Silicas Corporation) was mixed with 100 parts by mass of the obtained water-absorbent resin particles to obtain 226.8 g of water-absorbent resin particles (1). The physiological saline water absorption capacity of the water-absorbent resin particles (1) was 65 g / g, the physiological saline water retention capacity was 43 g / g, and the physiological saline water absorption capacity under a load of 4.14 kPa was 14 mL / g.

[0093] Comparative Example 1 The same operation as in Example 1 was carried out, except that 4.42 g of a 0.5 mass % aqueous solution of ethylenediamine-N,N'-disuccinic acid trisodium was not used, and the amount of water extracted from the system by azeotropic distillation of n-heptane and water in the [Surface cross-linking step] was changed from 84.7 g to 80.3 g, to obtain 224.0 g of water-absorbent resin particles (2). The water content of the hydrous gel particles when the aqueous sodium sulfite solution was added was 62 mass %, and the water content of the hydrous gel particles when the aqueous L(+)-ascorbic acid solution was added was 63 mass %. The physiological saline water absorption capacity of the water-absorbent resin particles (2) was 62 g / g, the physiological saline water retention capacity was 42 g / g, and the physiological saline water absorption capacity under a load of 4.14 kPa was 18 mL / g.

[0094] Comparative Example 2 The same procedure as in Example 1 was carried out, except that the 20% by mass aqueous sodium sulfite solution was not used, and the amount of water extracted from the system by azeotropic distillation of n-heptane and water in the [Surface Cross-linking Step] was changed from 84.7 g to 83.0 g, to obtain 225.5 g of water-absorbent resin particles (3). The water content of the hydrogel particles when the aqueous solution of trisodium ethylenediamine-N,N'-disuccinate was added was 62% by mass, and the water content of the hydrogel particles when the aqueous solution of L(+)-ascorbic acid was added was 64% by mass. The physiological saline water absorption capacity of the water-absorbent resin particles (3) was 61 g / g, the physiological saline water retention capacity was 37 g / g, and the physiological saline water absorption capacity under a load of 4.14 kPa was 18 mL / g.

[0095] Comparative Example 3 The same procedure as in Example 1 was carried out, except that the 0.1% by mass L(+)-ascorbic acid aqueous solution was not used, and the amount of water extracted from the system by azeotropic distillation of n-heptane and water in the [Surface Cross-linking Step] was changed from 84.7 g to 82.5 g, to obtain 224.8 g of water-absorbent resin particles (4). The water content of the hydrogel particles when the aqueous solution of trisodium ethylenediamine-N,N'-disuccinate was added was 62% by mass, and the water content of the hydrogel particles when the aqueous solution of sodium sulfite was added was 64% by mass. The physiological saline water absorption capacity of the water-absorbent resin particles (4) was 73 g / g, the physiological saline water retention capacity was 42 g / g, and the physiological saline water absorption capacity under a load of 4.14 kPa was 16 mL / g.

[0096] Example 2 The same operation as in Example 1 was performed except that the amount of water withdrawn from the system by azeotropic distillation of n-heptane and water in the first water content adjusting step was changed from 175.1 g to 253.8 g, and the amount of water withdrawn from the system by azeotropic distillation of n-heptane and water in the [surface cross-linking step] was changed from 84.7 g to 6.1 g, thereby obtaining 221.4 g of water-absorbent resin particles (5). The water content of the hydrous gel particles when the aqueous solution of ethylenediamine-N,N'-disuccinic acid trisodium was added was 26 mass%, the water content of the hydrous gel particles when the aqueous solution of sodium sulfite was added was 28 mass%, and the water content of the hydrous gel particles when the aqueous solution of L(+)-ascorbic acid was added was 29 mass%. The saline water absorption capacity of the water-absorbent resin particles (5) was 59 g / g, the saline water retention capacity was 36 g / g, and the saline water absorption capacity under a load of 4.14 kPa was 23 mL / g.

[0097] Example 3 The same operation as in Example 1 was performed except that the amount of water withdrawn from the system by azeotropic distillation of n-heptane and water in the first water content adjusting step was changed from 175.1 g to 223.2 g, and the amount of water withdrawn from the system by azeotropic distillation of n-heptane and water in the [surface cross-linking step] was changed from 84.7 g to 36.7 g, to obtain 223.8 g of water-absorbent resin particles (6). The water content of the hydrous gel particles when the aqueous solution of ethylenediamine-N,N'-disuccinic acid trisodium was added was 40 mass%, the water content of the hydrous gel particles when the aqueous solution of sodium sulfite was added was 42 mass%, and the water content of the hydrous gel particles when the aqueous solution of L(+)-ascorbic acid was added was 43 mass%. The water-absorbent resin particles (6) had a saline water absorption capacity of 66 g / g, a saline water retention capacity of 43 g / g, and a saline water absorption capacity under a load of 4.14 kPa of 12 mL / g.

[0098] Example 4 The same operations as in Example 1 were performed except that the amount of water withdrawn from the system by azeotropic distillation of n-heptane and water in the first water content adjusting step was changed from 175.1 g to 157.6 g, and the amount of water withdrawn from the system by azeotropic distillation of n-heptane and water in the [surface cross-linking step] was changed from 84.7 g to 102.2 g, to obtain 225.1 g of water-absorbent resin particles (7). The water content of the hydrous gel particles when the aqueous solution of ethylenediamine-N,N'-disuccinic acid trisodium was added was 70 mass%, the water content of the hydrous gel particles when the aqueous solution of sodium sulfite was added was 72 mass%, and the water content of the hydrous gel particles when the aqueous solution of L(+)-ascorbic acid was added was 73 mass%. The water-absorbent resin particles (7) had a saline water absorption capacity of 64 g / g, a saline water retention capacity of 42 g / g, and a saline water absorption capacity under a load of 4.14 kPa of 12 mL / g.

[0099] Comparative Example 4 The same operation as in Example 1 was performed except that the amount of water withdrawn from the system by azeotropic distillation of n-heptane and water in the first water content adjusting step was changed from 175.1 g to 135.8 g, and the amount of water withdrawn from the system by azeotropic distillation of n-heptane and water in the [surface cross-linking step] was changed from 84.7 g to 124.1 g, to obtain 223.8 g of water-absorbent resin particles (8). The water content of the hydrous gel particles when the aqueous solution of ethylenediamine-N,N'-disuccinic acid trisodium was added was 80 mass %, the water content of the hydrous gel particles when the aqueous solution of sodium sulfite was added was 82 mass %, and the water content of the hydrous gel particles when the aqueous solution of L(+)-ascorbic acid was added was 83 mass %. The saline water absorption capacity of the water-absorbent resin particles (8) was 67 g / g, the saline water retention capacity was 46 g / g, and the saline water absorption capacity under a load of 4.14 kPa was 12 mL / g.

[0100] Comparative Example 5 [Step 1 for Producing Hydrogel Particles] Hydrogel particles were obtained by carrying out the same operation as in the [Step 1 for Producing Hydrogel Particles] of Example 1.

[0101] [Step 2 of Adding Additives to Hydrogel Particles] After obtaining the hydrogel particles, the flask was immersed in an oil bath set at 125°C, and 135.8 g of water was extracted from the system by azeotropic distillation of n-heptane and water while refluxing n-heptane (first water content adjustment step). Then, 2.21 g of a 20% by mass aqueous solution of sodium sulfite was added with stirring. At this time, the water content of the hydrogel particles when the aqueous sodium sulfite solution was added was 80% by mass. Then, 89.2 g of water was extracted from the system by azeotropic distillation of n-heptane and water again while refluxing n-heptane (second water content adjustment step). Then, 4.42 g of a 0.5% by mass aqueous solution of ethylenediamine-N,N'-disuccinic acid trisodium was added with stirring. At this time, the water content of the hydrogel particles was 40% by mass when the aqueous solution of trisodium ethylenediamine-N,N'-disuccinate was added. Subsequently, 2.21 g of a 0.2% by mass aqueous solution of L(+)-ascorbic acid was added with stirring. At this time, the water content of the hydrogel particles was 42% by mass when the aqueous solution of L(+)-ascorbic acid was added.

[0102] [Surface cross-linking step (step 3)] Then, by azeotropic distillation of n-heptane and water again, while refluxing n-heptane, 34.9 g of water was extracted out of the system. Then, 4.42 g (0.507 mmol) of a 2 mass% aqueous solution of ethylene glycol diglycidyl ether was added to the flask as a surface cross-linking agent, and the mixture was maintained at 83 ° C. for 2 hours. Then, n-heptane was evaporated at 125 ° C. and dried, and further passed through a sieve with an opening of 850 μm to obtain 221.1 g of water-absorbent resin particles.

[0103] [Step of adding additives to water-absorbent resin particles] 222.2 g of water-absorbent resin particles (9) was obtained by performing the same operation as in [Step of adding additives to water-absorbent resin particles] of Example 1. The saline water absorption amount of the water-absorbent resin particles (9) was 65 g / g, the saline water retention amount was 43 g / g, and the saline water absorption amount under a load of 4.14 kPa was 14 mL / g.

[0104] Example 5 [Step 1 for Producing Hydrogel Particles] Hydrogel particles were obtained by carrying out the same operation as in the [Step 1 for Producing Hydrogel Particles] of Example 1.

[0105] [Step 2 of Adding Additives to Hydrogel Particles] After obtaining the hydrogel particles, the flask was immersed in an oil bath set at 125°C, and 135.8 g of water was extracted from the system by azeotropic distillation of n-heptane and water while refluxing n-heptane (first water content adjustment step). Then, 4.42 g of a 0.5 mass% aqueous solution of trisodium ethylenediamine-N,N'-disuccinic acid was added with stirring. At this time, the water content of the hydrogel particles when the aqueous solution of trisodium ethylenediamine-N,N'-disuccinic acid was added was 80 mass%. Then, 91.8 g of water was extracted from the system by azeotropic distillation of n-heptane and water again while refluxing n-heptane (second water content adjustment step). Then, 2.21 g of a 20 mass% aqueous solution of sodium sulfite was added with stirring. The water content of the hydrogel particles was 40% by mass when the sodium sulfite aqueous solution was added. Subsequently, 2.21 g of a 0.2% by mass L(+)-ascorbic acid aqueous solution was added with stirring. The water content of the hydrogel particles was 41% by mass when the L(+)-ascorbic acid aqueous solution was added.

[0106] [Surface cross-linking step (step 3)] Then, by azeotropic distillation of n-heptane and water again, while refluxing n-heptane, 32.3 g of water was extracted out of the system. Then, 4.42 g (0.507 mmol) of a 2 mass% aqueous solution of ethylene glycol diglycidyl ether was added to the flask as a surface cross-linking agent, and the mixture was maintained at 83 ° C. for 2 hours. Then, n-heptane was evaporated and dried at 125 ° C., and further passed through a sieve with an opening of 850 μm to obtain 222.7 g of water-absorbent resin particles.

[0107] [Step of adding additives to water-absorbent resin particles] 223.8 g of water-absorbent resin particles (10) was obtained by performing the same operation as in [Step of adding additives to water-absorbent resin particles] of Example 1. The saline water absorption capacity of the water-absorbent resin particles (10) was 65 g / g, the saline water retention capacity was 41 g / g, and the saline water absorption capacity under a load of 4.14 kPa was 20 mL / g.

[0108] Example 6 [Step 1 for Producing Hydrogel Particles] Hydrogel particles were obtained by carrying out the same operation as in the [Step 1 for Producing Hydrogel Particles] of Example 1.

[0109] [Step 2 of Adding Additives to Hydrogel Particles] After obtaining the hydrogel particles, the flask was immersed in an oil bath set at 125°C, and 135.8 g of water was extracted from the system by azeotropic distillation of n-heptane and water while refluxing n-heptane (first water content adjustment step). Then, 2.21 g of a 0.2% by mass L(+)-ascorbic acid aqueous solution was added with stirring. At this time, the water content of the hydrogel particles when the L(+)-ascorbic acid aqueous solution was added was 80% by mass. Then, 89.6 g of water was extracted from the system by azeotropic distillation of n-heptane and water again while refluxing n-heptane (second water content adjustment step). Then, 4.42 g of a 0.5% by mass aqueous solution of ethylenediamine-N,N'-disuccinic acid trisodium was added with stirring. At this time, the water content of the hydrogel particles was 40% by mass when the aqueous solution of trisodium ethylenediamine-N,N'-disuccinate was added. Subsequently, 2.21 g of a 20% by mass aqueous solution of sodium sulfite was added with stirring. At this time, the water content of the hydrogel particles was 42% by mass when the aqueous solution of sodium sulfite was added.

[0110] [Surface cross-linking step (step 3)] Then, by azeotropic distillation of n-heptane and water again, while refluxing n-heptane, 34.5 g of water was extracted out of the system. Then, 4.42 g (0.507 mmol) of a 2 mass% aqueous solution of ethylene glycol diglycidyl ether as a surface cross-linking agent was added to the flask, and the mixture was kept at 83 ° C. for 2 hours. Then, n-heptane was evaporated at 125 ° C. and dried, and further passed through a sieve with an opening of 850 μm to obtain 223.7 g of water-absorbent resin particles.

[0111] [Step of adding additives to water-absorbent resin particles] 224.8 g of water-absorbent resin particles (11) was obtained by performing the same operation as in [Step of adding additives to water-absorbent resin particles] of Example 1. The saline water absorption amount of the water-absorbent resin particles (11) was 59 g / g, the saline water retention amount was 39 g / g, and the saline water absorption amount under a load of 4.14 kPa was 17 mL / g.

[0112] Comparative Example 6 [Step 1 for Producing Hydrogel Particles] Hydrogel particles were obtained by carrying out the same operation as in the [Step 1 for Producing Hydrogel Particles] of Example 1.

[0113] [Surface cross-linking step (step 3)] After obtaining the hydrogel particles, the flask was immersed in an oil bath set at 125 ° C., and 251.5 g of water was extracted from the system by azeotropic distillation of n-heptane and water while refluxing n-heptane. Thereafter, 4.42 g (0.507 mmol) of a 2 mass% aqueous solution of ethylene glycol diglycidyl ether was added to the flask as a surface cross-linking agent, and the mixture was maintained at 83 ° C. for 2 hours. Thereafter, n-heptane was evaporated and dried at 125 ° C., and the mixture was further passed through a sieve with an opening of 850 μm to obtain 217.6 g of water-absorbent resin particles.

[0114] [Step of Adding Additives to Water-Absorbent Resin Particles] 0.7 parts by mass of diethylenetriaminepentaacetic acid (DTPA.5H), 2 parts by mass of sodium sulfite, and 0.02 parts by mass of L(+)-ascorbic acid were added as powder to 100 parts by mass of the water-absorbent resin particles obtained in the [Surface Cross-Linking Step], and the mixture was mixed for 30 minutes (conditions: revolution speed 50 rpm, rotation speed 50 rpm) using a cross rotary mixer manufactured by Meiwa Kogyo Co., Ltd. to obtain a mixture. Furthermore, 0.5 parts by mass of amorphous silica (Toxil NP-S, Oriental Silicas Corporation) was mixed with 100 parts by mass of the obtained mixture to obtain water-absorbent resin particles (12). The physiological saline water absorption capacity of the water-absorbent resin particles (12) was 60 g / g, the physiological saline water retention capacity was 42 g / g, and the physiological saline water absorption capacity under a load of 4.14 kPa was 14 mL / g.

[0115] Comparative Example 7 [Step 1 for Producing Hydrogel Particles] Hydrogel particles were obtained by carrying out the same operation as in the [Step 1 for Producing Hydrogel Particles] of Example 1.

[0116] [Surface cross-linking step (step 3)] After obtaining the hydrogel particles, the flask was immersed in an oil bath set at 125 ° C., and 251.5 g of water was extracted from the system by azeotropic distillation of n-heptane and water while refluxing n-heptane. Thereafter, 4.42 g (0.507 mmol) of a 2 mass% aqueous solution of ethylene glycol diglycidyl ether was added to the flask as a surface cross-linking agent, and the mixture was maintained at 83 ° C. for 2 hours. Thereafter, n-heptane was evaporated and dried at 125 ° C., and the mixture was further passed through a sieve with an opening of 850 μm to obtain 226.8 g of water-absorbent resin particles.

[0117] [Step of adding additives to water-absorbent resin particles] 20 g of the water-absorbent resin particles obtained in the [Surface cross-linking step] was weighed into a round-bottomed cylindrical separable flask with an inner diameter of 11 cm and equipped with an anchor-shaped stirring blade made of fluororesin. Next, while stirring at 300 rpm, 0.2 g of a 1.0 mass% aqueous solution of diethylenetriaminepentaacetic acid.pentasodium (DTPA.5Na) was added dropwise to the separable flask using a Pasteur pipette, and the mixture was stirred for 10 minutes to obtain a mixture. After heating this mixture at 100 ° C. for 30 minutes, 0.2 parts by mass of sodium sulfite and 0.002 parts by mass of L(+)-ascorbic acid were added as powder to 100 parts by mass of the mixture after heating, and the mixture was mixed for 30 minutes (conditions, revolution speed 50 rpm, rotation speed 50 rpm) using a cross rotary mixer manufactured by Meiwa Kogyo Co., Ltd. to obtain a mixture. Further, 0.5 parts by mass of amorphous silica (Toxil NP-S, Oriental Silicas Corporation) was mixed with 100 parts by mass of the obtained mixture to obtain water-absorbent resin particles. The saline water absorption capacity of the water-absorbent resin particles was 58 g / g, the saline water retention capacity was 40 g / g, and the saline water absorption capacity under a load of 4.14 kPa was 21 mL / g.

[0118] <Comparative Example 8> The same operation as in Comparative Example 7 was carried out except that 0.2 g of a 1.0 mass % aqueous solution of ethylenediamine-N,N'-disuccinic acid, trisodium salt (EDDS, 3Na) was used instead of 0.2 g of the 1.0 mass % aqueous solution of diethylenetriaminepentaacetic acid, pentasodium salt (DTPA, 5Na) in Comparative Example 7, to obtain 218.8 g of water-absorbent resin particles (14). The physiological saline water absorption capacity of the water-absorbent resin particles (14) was 56 g / g, the physiological saline water retention capacity was 39 g / g, and the physiological saline water absorption capacity under a load of 4.14 kPa was 23 mL / g.

[0119] Example 7 The same procedure as in Example 1 was carried out, except that 4.42 g of a 0.5 mass % aqueous solution of diethylenetriaminepentaacetic acid pentasodium (DTPA 5Na) was used instead of 4.42 g of a 0.5 mass % aqueous solution of ethylenediamine-N,N'-disuccinic acid tricodium in Example 1, to obtain 225.6 g of water-absorbent resin particles (15). The water content of the hydrogel particles when the aqueous solution of diethylenetriaminepentaacetic acid pentasodium was added was 62 mass %, the water content of the hydrogel particles when the aqueous solution of sodium sulfite was added was 64 mass %, and the water content of the hydrogel particles when the aqueous solution of L(+)-ascorbic acid was added was 65 mass %. The physiological saline water absorption capacity of the water-absorbent resin particles (15) was 58 g / g, the physiological saline water retention capacity was 39 g / g, and the physiological saline water absorption capacity under a load of 4.14 kPa was 14 mL / g.

[0120] Example 8 The same procedure as in Example 1 was carried out, except that 4.42 g of a 0.5 mass% aqueous solution of ethylenediaminetetramethylenephosphonic acid (EDTMP.8H) was used instead of 4.42 g of a 0.5 mass% aqueous solution of ethylenediamine-N,N'-disuccinic acid trisodium in Example 1, to obtain 224.6 g of water-absorbent resin particles (16). The water content of the hydrogel particles when the aqueous solution of ethylenediaminetetramethylenephosphonic acid was added was 62 mass%, the water content of the hydrogel particles when the aqueous solution of sodium sulfite was added was 64 mass%, and the water content of the hydrogel particles when the aqueous solution of L(+)-ascorbic acid was added was 65 mass%. The water absorption capacity of the water-absorbent resin particles (16) in physiological saline was 61 g / g, the water retention capacity of physiological saline was 37 g / g, and the water absorption capacity of physiological saline under a load of 4.14 kPa was 18 mL / g.

[0121] Example 9 [Step 1 for Producing Hydrogel Particles] Hydrogel particles were obtained by carrying out the same operation as in the [Step 1 for Producing Hydrogel Particles] of Example 1.

[0122] [Step 2 of Adding Additives to Hydrogel Particles] After obtaining the hydrogel particles, the flask was immersed in an oil bath set at 125°C, and 175.1 g of water was extracted from the system by azeotropic distillation of n-heptane and water while refluxing n-heptane (first water content adjustment step). Then, 4.42 g of a 0.5% by mass aqueous solution of trisodium ethylenediamine-N,N'-disuccinic acid was added with stirring. At this time, the water content of the hydrogel particles when the aqueous solution of trisodium ethylenediamine-N,N'-disuccinic acid was added was 62% by mass. Then, 15.3 g of water was extracted from the system by azeotropic distillation of n-heptane and water again while refluxing n-heptane (second water content adjustment step). Then, 2.21 g of a 20% by mass aqueous solution of sodium sulfite was added with stirring. At this time, the water content of the hydrous gel particles was 57% by mass when the sodium sulfite aqueous solution was added. Thereafter, 12.7 g of water was again extracted from the system by azeotropic distillation of n-heptane and water while refluxing n-heptane (third water content adjustment step). Subsequently, 2.21 g of a 0.2% by mass L(+)-ascorbic acid aqueous solution was added with stirring. At this time, the water content of the hydrous gel particles was 52% by mass when the L(+)-ascorbic acid aqueous solution was added.

[0123] [Surface cross-linking step (step 3)] Then, by azeotropic distillation of n-heptane and water again, while refluxing n-heptane, 56.7 g of water was extracted out of the system. Then, 4.42 g (0.507 mmol) of a 2 mass% aqueous solution of ethylene glycol diglycidyl ether as a surface cross-linking agent was added to the flask, and the mixture was maintained at 83 ° C. for 2 hours. Then, n-heptane was evaporated and dried at 125 ° C., and further passed through a sieve with an opening of 850 μm to obtain 219.7 g of water-absorbent resin particles.

[0124] [Step of adding additives to water-absorbent resin particles] 220.8 g of water-absorbent resin particles (17) was obtained by performing the same operation as in [Step of adding additives to water-absorbent resin particles] of Example 1. The saline water absorption amount of the water-absorbent resin particles (17) was 64 g / g, the saline water retention amount was 42 g / g, and the saline water absorption amount under a load of 4.14 kPa was 16 mL / g.

[0125] Example 10 The same operation as in Example 9 was carried out except that 8.83 g of a 0.5 mass % aqueous solution of ethylenediamine-N,N'-disuccinic acid trisodium was used instead of 4.42 g of the 0.5 mass % aqueous solution of ethylenediamine-N,N'-disuccinic acid trisodium in Example 9, and the amount of water extracted from the system by azeotropic distillation of n-heptane and water in the second water content adjustment step was changed from 15.3 g to 19.7 g, to obtain 225.7 g of water-absorbent resin particles (18). The water content of the hydrous gel particles when the aqueous solution of ethylenediamine-N,N'-disuccinic acid trisodium was added was 62 mass %, the water content of the hydrous gel particles when the aqueous solution of sodium sulfite was added was 57 mass %, and the water content of the hydrous gel particles when the aqueous solution of L(+)-ascorbic acid was added was 52 mass %. The saline water absorption capacity of the water-absorbent resin particles (18) was 64 g / g, the saline water retention capacity was 40 g / g, and the saline water absorption capacity under a load of 4.14 kPa was 11 mL / g.

[0126] Example 11 The same operation as in Example 9 was carried out, except that 2.21 g of a 0.5 mass % aqueous solution of ethylenediamine-N,N'-disuccinic acid trisodium was used instead of 4.42 g of the 0.5 mass % aqueous solution of ethylenediamine-N,N'-disuccinic acid trisodium, and the amount of water extracted from the system by azeotropic distillation of n-heptane and water in the second water content adjustment step was changed from 15.3 g to 13.1 g, to obtain 223.4 g of water-absorbent resin particles (19). The water content of the hydrous gel particles when the aqueous solution of ethylenediamine-N,N'-disuccinic acid trisodium was added was 62 mass %, the water content of the hydrous gel particles when the aqueous solution of sodium sulfite was added was 57 mass %, and the water content of the hydrous gel particles when the aqueous solution of L(+)-ascorbic acid was added was 52 mass %. The saline water absorption capacity of the water-absorbent resin particles (19) was 62 g / g, the saline water retention capacity was 46 g / g, and the saline water absorption capacity under a load of 4.14 kPa was 10 mL / g.

[0127] Example 12 The same operation as in Example 9 was carried out, except that 3.31 g of a 20 mass % aqueous sodium sulfite solution was used instead of 2.21 g of the 20 mass % aqueous sodium sulfite solution in Example 9, and the amount of water extracted from the system by azeotropic distillation of n-heptane and water in the third water content adjustment step was changed from 12.7 g to 13.6 g, to obtain 223.8 g of water-absorbent resin particles (20). The water content of the hydrous gel particles when the aqueous solution of ethylenediamine-N,N'-disuccinic acid trisodium was added was 62 mass %, the water content of the hydrous gel particles when the aqueous solution of sodium sulfite was added was 57 mass %, and the water content of the hydrous gel particles when the aqueous solution of L(+)-ascorbic acid was added was 52 mass %. The saline water absorption capacity of the water-absorbent resin particles (20) was 67 g / g, the saline water retention capacity was 50 g / g, and the saline water absorption capacity under a load of 4.14 kPa was 8 mL / g.

[0128] Example 13 The same operation as in Example 9 was carried out, except that 1.10 g of a 20 mass % aqueous sodium sulfite solution was used instead of 2.21 g of the 20 mass % aqueous sodium sulfite solution in Example 9, and the amount of water extracted from the system by azeotropic distillation of n-heptane and water in the third water content adjustment step was changed from 12.7 g to 11.8 g, to obtain 224.6 g of water-absorbent resin particles (21). The water content of the hydrous gel particles when the aqueous solution of ethylenediamine-N,N'-disuccinic acid trisodium was added was 62 mass %, the water content of the hydrous gel particles when the aqueous solution of sodium sulfite was added was 57 mass %, and the water content of the hydrous gel particles when the aqueous solution of L(+)-ascorbic acid was added was 52 mass %. The saline water absorption capacity of the water-absorbent resin particles (21) was 65 g / g, the saline water retention capacity was 45 g / g, and the saline water absorption capacity under a load of 4.14 kPa was 11 mL / g.

[0129] Example 14 The same operation as in Example 9 was performed except that 11.0 g of a 0.2 mass% L(+)-ascorbic acid aqueous solution was used instead of 2.21 g of a 0.2 mass% L(+)-ascorbic acid aqueous solution in Example 9, and the amount of water extracted from the system by azeotropic distillation of n-heptane and water in the [Surface cross-linking step] was changed from 56.7 g to 65.5 g, to obtain 226.8 g of water-absorbent resin particles (22). The water content of the hydrous gel particles when the aqueous solution of ethylenediamine-N,N'-disuccinic acid trisodium was added was 62 mass%, the water content of the hydrous gel particles when the aqueous solution of sodium sulfite was added was 57 mass%, and the water content of the hydrous gel particles when the aqueous solution of L(+)-ascorbic acid was added was 52 mass%. The saline water absorption capacity of the water-absorbent resin particles (22) was 62 g / g, the saline water retention capacity was 45 g / g, and the saline water absorption capacity under a load of 4.14 kPa was 22 mL / g.

[0130] Example 15 The same operation as in Example 9 was performed except that 1.10 g of a 0.2 mass% L(+)-ascorbic acid aqueous solution was used instead of 2.21 g of a 0.2 mass% L(+)-ascorbic acid aqueous solution in Example 9, and the amount of water extracted from the system by azeotropic distillation of n-heptane and water in the [Surface cross-linking step] was changed from 56.7 g to 55.6 g, to obtain 224.0 g of water-absorbent resin particles (23). The water content of the hydrous gel particles when the aqueous solution of ethylenediamine-N,N'-disuccinic acid trisodium was added was 62 mass%, the water content of the hydrous gel particles when the aqueous solution of sodium sulfite was added was 57 mass%, and the water content of the hydrous gel particles when the aqueous solution of L(+)-ascorbic acid was added was 52 mass%. The saline water absorption capacity of the water-absorbent resin particles (23) was 66 g / g, the saline water retention capacity was 46 g / g, and the saline water absorption capacity under a load of 4.14 kPa was 10 mL / g.

[0131] <Water Content of Hydrous Gel Particles> The water content of the hydrous gel particles was calculated as follows. Using the amount of water W1 (g) contained in the aqueous liquid used to produce the hydrous gel particles in the [hydrous gel particle production step (step 1)], the amount of water W2 (g) contained in the aqueous additive solution added to the system in the [additive addition step to hydrous gel particles (step 2)], the amount of water W3 (g) extracted from the system by azeotropic distillation of n-heptane and water in the [additive addition step to hydrous gel particles (step 2)], the amount of water W4 (g) extracted from the system by azeotropic distillation of n-heptane and water in the [surface cross-linking step (step 3)], and the amount of water-soluble ethylenically unsaturated monomer M1 (g) contained in the aqueous liquid used to produce the hydrous gel particles in the [hydrous gel particle production step (step 1)], the water content (% by mass) of the hydrous gel particles was calculated using the following formula: Water Content of Hydrous Gel Particles (% by mass) = (W1 + W2 - W3 - W4) × 100 / M1

[0132] <Saline Water Retention Capacity> 500 g of 0.9% by mass sodium chloride aqueous solution (saline) was weighed into a 500 mL beaker, and 2.0 g of water-absorbent resin particles were dispersed therein while stirring at 600 rpm with a 3 cm stir bar (without ring). The mixture was left to stand for 60 minutes while stirring, allowing the water-absorbent resin particles to fully swell. The mass Wa (g) of a standard sieve with a mesh size of 75 μm was then measured in advance, and the contents of the beaker were filtered using this. The sieve was tilted at an angle of approximately 30 degrees relative to the horizontal, and the sieve was left to stand for 30 minutes to filter out excess water. The mass Wb (g) of the sieve containing the swollen gel was measured, and the saline water absorption capacity was calculated using the following formula: Saline Water Absorption Capacity (g / g) = [Wb - Wa] / 2.0

[0133] <Saline Water Retention Capacity> A cotton bag (membrane broadcloth No. 60, width 100 mm x length 200 mm) containing 2.0 g of water-absorbent resin particles was placed in a 500 mL beaker. 500 g of a 0.9% by mass aqueous sodium chloride solution (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, and the mass Wd (g) of the cotton bag containing the swollen gel after dehydration was measured. The same operation was performed without adding water-absorbent resin particles, and the empty mass We (g) of the cotton bag when wet was measured, and the saline water retention capacity was calculated using the following formula: Saline Water Retention Capacity (g / g) = [Wd - We] / 2.0

[0134] <Water absorption amount of physiological saline solution under a load of 4.14 kPa> The water absorption amount of physiological saline solution under a load of 4.14 kPa (water absorption amount under load) was measured using a measuring device schematically shown in FIG. 1 . The measurement was performed twice for one type of water absorbent resin particle, and the average value was calculated. The measuring device 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 having 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 measuring 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.

[0135] The measurement unit 4 has a Plexiglas cylinder 31, 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 119.6 g, and can apply a load of 4.14 kPa (0.6 psi) to the water-absorbent resin particles 10a uniformly arranged on the polyamide mesh 32, as will be described later.

[0136] First, the stopcocks 22 and 24 of the burette part 1 were closed, and 0.9% by mass physiological 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 stopcocks 22 and 24 were opened. The inside of the conduit 5 was filled with 0.9% by mass saline solution 50 while preventing the inclusion of air bubbles. 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).

[0137] In the measurement unit 4, 0.10 g of water-absorbent resin particles 10a were uniformly arranged on the polyamide mesh 32 in the 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 reduced saline in the burette tube 21 (i.e., the amount of saline absorbed by the water-absorbent resin particles 10a) Wc (mL) 60 minutes after the water-absorbent resin particles 10a started to absorb saline from the conduit 5 was read, and the saline absorption capacity of the water-absorbent resin particles 10a under a load of 4.14 kPa was calculated by the following formula: saline absorption capacity under a load of 4.14 kPa (mL / g) = Wc (mL) / mass of water-absorbent resin particles (g)

[0138] <Loss on Drying> 2.0 g of water-absorbent resin particles were placed in an aluminum foil case (No. 8) previously adjusted to a constant weight (W1 (g)), the mouth of the aluminum foil case was loosely closed, and the total mass W2 (g) of the aluminum foil case containing the sample was precisely weighed. The aluminum foil case containing the sample described above was dried for 2 hours in a hot air dryer (manufactured by ADVANTEC, model: FV-320) with the internal temperature set to 105°C. After drying, the aluminum foil case containing the sample was allowed to cool to room temperature in a desiccator. The total mass W3 (g) of the aluminum foil case containing the sample after cooling was measured. The loss on drying of the sample was calculated using the following formula. Loss on Drying [mass %] = [{(W2 - W1) - (W3 - W1)} / (W2 - W1)] × 100

[0139] <Yellowing Coloring Test Under High Temperature and High Humidity Conditions (Measurement of Yellowness Index)> 2.0 g of water absorbent resin particles were placed in a glass measurement container having an inner diameter of 3 cm, and the yellowness index of the water absorbent resin particles was measured with a color difference meter (Color Meter ZE6000, manufactured by Nippon Denshoku Industries Co., Ltd.) in which X, Y, and Z, which are tristimulus values ​​of the colorimetric color difference meter, were corrected using a standard white board, and the yellowness index was calculated from the obtained X, Y, and Z (tristimulus values) of the water absorbent resin particles according to the following formula, and was set as an initial value.

[0140] Yellowness = 100 (1.28X-1.06Z) / Y

[0141] Further, a test for coloration of water-absorbent resin particles over time was carried out as follows. That is, 2.0 g of water-absorbent resin particles were uniformly placed in a glass petri dish having an inner diameter of 3 cm and a depth of 1 cm, and the container was stored for a predetermined number of days (7 days or 14 days) in a thermo-hygrostat (manufactured by Espec Corporation, LHU-113) set at a temperature of 70 ± 2 ° C. and a relative humidity of 90 ± 2%. Thereafter, the container was removed from the thermo-hygrostat and left to cool to room temperature for a while. The entire amount of water-absorbent resin particles in the container was placed in a glass measuring container having an inner diameter of 3 cm, and the yellowness of the water-absorbent resin particles was measured with a color difference meter (Color Meter ZE6000, manufactured by Nippon Denshoku Industries Co., Ltd.). The yellowness was calculated from the X, Y, and Z (tristimulus values) of the obtained water-absorbent resin particles according to the following formula.

[0142] Yellowness = 100 (1.28X-1.06Z) / Y

[0143] <Test on Gel Absorbing Artificial Urine> A test on gel (swollen gel) in which water-absorbent resin particles have absorbed water was carried out using artificial urine.

[0144] (Preparation of Artificial Urine) Artificial urine having the following composition was prepared: Urea: 20.0 g Sodium chloride: 8.0 g Calcium chloride dihydrate: 0.3 g Magnesium sulfate heptahydrate: 0.8 g L(+)-ascorbic acid: 0.2 g Ion-exchanged water: 970.9 g

[0145] (Preparation of swollen gel) 39.0 g of the above artificial urine was weighed into a 100 mL beaker, a magnetic stirrer bar (8 mmφ x 30 mm, without ring) was added, and the beaker was placed on a magnetic stirrer (manufactured by Iuchi Co., Ltd.: HS-30D), and the magnetic stirrer bar was rotated at 600 rpm. Next, 1.00 g of water-absorbent resin particles were added to the beaker during stirring, and stirring was continued until the rotating vortex disappeared and the liquid level became horizontal, to prepare a swollen gel to be used as a measurement sample. Immediately after preparing the swollen gel, the beaker containing the swollen gel was covered with plastic wrap (manufactured by Mitsubishi Chemical Corporation, DiaWrap).

[0146] (Measurement of Gel Strength) The gel strength at each temperature and after each rest period was measured using an apparatus having the measurement principle shown in FIG. 2. The apparatus shown in FIG. 2 is composed of a support unit 50a, a movable base 60, a drive unit 70 for driving the movable base 60, and a measurement unit 80. In the support unit 50a, a base 53 is fixed to the top of a support 52 erected on a support base 51. The movable base 60 is attached to the support 52 so that it can move up and down. A measurement sample (gel) 61 can be mounted on the movable base 60. A pulse motor 71 is mounted on the base 53, and by rotating a pulley 72, the movable base 60 is moved up and down via a wire 73. In the measurement unit 80, a disk-equipped pressure-sensitive shaft 84 is attached to a load cell 81 for measuring strain caused by deformation via a precision spring 82 and a connecting shaft 83. The disk-equipped pressure-sensitive shaft 84 has a disk at its tip. The diameter of the disk can be changed depending on the measurement conditions. A weight 90 can be mounted on top of the pressure-sensitive shaft 84 with a disk. The operating principle of the device for measuring gel strength is as follows: A precision spring 82 is fixed to a load cell 81 (stress detector) above, and the pressure-sensitive shaft 84 with a disk is connected to the bottom and suspended vertically with a predetermined weight 90 placed on it. The movable base 60 with the measurement sample 61 placed on it rises at a constant speed due to the rotation of the pulse motor 71. A constant load is applied to the measurement sample 61 via the precision spring 82, and the strain caused by deformation is measured by the load cell 81, and the hardness is measured and calculated. The gel strength value (N / m 2 ) was measured using a Curdmeter-MAX (manufactured by Asuka Kikai, product number: ME-500) with a disk of 16 mmφ on the pressure-sensitive shaft 84 with a disk, a load of 400 g, a speed of 7 seconds / inch, and viscous mode settings under the following temperature and standing time conditions: (initial value of gel strength) and (gel strength after standing at 37°C for 14 hours).

[0147] (Initial value of gel strength) The beaker containing the swollen gel covered with plastic wrap was left to stand for 60 minutes in an environment of a temperature of 25±2°C and a relative humidity of 50±10%, and then the plastic wrap was removed from the beaker containing the swollen gel, and the gel strength was measured by the method described above. The gel strength value at this time was defined as the gel strength (initial value).

[0148] (Gel strength after standing at 37°C for 14 hours) The beaker containing the swollen gel covered with plastic wrap was left to stand for 14 hours in a thermo-hygrostat (Espec Corporation, LHU-113) set at a temperature of 37±2°C and a relative humidity of 60±10%, and then the plastic wrap was removed from the beaker containing the swollen gel, and the gel strength was measured by the method described above. The gel strength value at this time was defined as the gel strength (14-hour value).

[0149]

[0150] *Chelating agent name: EDDS...Ethylenediamine N,N'-disuccinic acid, DTPA...Diethylenetriaminepentaacetic acid, EDTMP...Ethylenediaminetetramethylenephosphonic acid

[0151] REFERENCE SIGNS LIST 1 burette part 3 clamp 4 measuring part 5 conduit 10a water-absorbent resin particles 11 stand 13 measuring table 13a through-hole 21 burette tube 22 cock 23 rubber stopper 24 cock 25 air introduction tube 31 cylinder 32 polyamide mesh 33 weight 50 saline solution 50a support part 51 support base 52 support 53 stand 60 movable base plate 61 measurement sample 70 drive part 71 pulse motor 72 pulley 73 wire 80 measuring part 81 load cell 82 precision spring 83 connecting shaft 84 pressure-sensitive shaft with disk 90 weight

Claims

1. A method for producing water-absorbent resin particles, comprising the steps of: Step 1 of polymerizing a water-soluble ethylenically unsaturated monomer to obtain hydrous gel particles; Step 2 of adding a chelating agent, a sulfite compound, and an organic antioxidant to the hydrous gel particles; and Step 3 of subjecting the hydrous gel particles to a surface cross-linking treatment, in this order; wherein in Step 2, the water content of the hydrous gel particles when the sulfite compound is added is 20% by mass or more and 75% by mass or less.

2. The method for producing water-absorbent resin particles according to claim 1, wherein step 2 further comprises a step of adjusting the moisture content of the hydrogel particles.

3. The method for producing water-absorbent resin particles according to claim 1 or 2, wherein in step 2, the water content of the hydrogel particles when the chelating agent is added is 20 mass % or more.

4. The method for producing water-absorbent resin particles according to claim 1 or 2, wherein in step 2, the water content of the hydrogel particles when the organic antioxidant is added is 20 mass % or more.

5. A method for producing water-absorbent resin particles according to claim 1 or 2, wherein in step 2, the amount of the sulfite compound added is 0.001 parts by mass or more and 3.0 parts by mass or less per 100 parts by mass of the water-soluble ethylenically unsaturated monomer.

6. A method for producing water-absorbent resin particles according to claim 1 or 2, wherein in step 2, the amount of the chelating agent added is 0.001 parts by mass or more and 2.0 parts by mass or less per 100 parts by mass of the water-soluble ethylenically unsaturated monomer.

7. The method for producing water-absorbent resin particles according to claim 1 or 2, wherein in step 2, the amount of the organic antioxidant added is 0.1 parts by mass or more and 20 parts by mass or less per 100 parts by mass of the sulfite compound.

8. The saline water absorption is 40-70g / g, the yellowness index is less than 40 after being left in an environment of 70°C and 90% relative humidity for 14 days, and the gel strength is 5500N / m after being left in an environment of 37°C and 60% relative humidity for 14 hours. 2 The above is the water-absorbent resin particle.

9. An absorbent material comprising the water-absorbent resin particles according to claim 8.

10. An absorbent article comprising the absorbent body according to claim 9.