Method for producing water-absorbing resin particles, water-absorbing resin particles, absorber, and absorbent article

Abstract

Provided is a novel method for producing water-absorbing resin particles having gel stability with respect to iron-containing artificial urine, long-term gel stability against artificial urine, and yellowing resistance.　This method for producing water-absorbing resin particles comprises: a step for polymerizing water-soluble ethylenically unsaturated monomers to obtain polymer particles; and a surface crosslinking step for subjecting the polymer particles to surface crosslinking. Before and / or after the surface crosslinking step, a phosphonic acid-based chelating agent is added to the polymer particles having a water content of 5-100 mass%. The amount of the phosphonic acid-based chelating agent added is 0.055-0.6 parts by mass with respect to 100 parts by mass of the water-soluble ethylenically unsaturated monomer.

Description

Method for producing water-absorbent resin particles, water-absorbent resin particles, absorbent body, and absorbent article

[0001] The present invention relates to a method for producing water-absorbent resin particles, water-absorbent resin particles, an absorbent body, and an absorbent article, and more particularly to a method for producing water-absorbent resin particles constituting an absorbent body suitably used in hygiene materials such as disposable diapers, sanitary napkins, and incontinence pads, water-absorbent resin particles, an absorbent body using the water-absorbent resin particles, and an absorbent article.

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

[0003] As such water-absorbent resin particles, a crosslinked product of a polymer of a water-soluble ethylenically unsaturated monomer, more specifically a crosslinked product of a polymer of a partially neutralized polyacrylic 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 preferable water-absorbent resin particle (see, for example, Patent Document 1).

[0004] Absorbent articles such as disposable diapers, sanitary napkins, and incontinence pads are mainly composed of an absorbent core located in the center for absorbing and retaining body fluids such as urine and menstrual blood excreted from the body, a liquid-permeable surface sheet (top sheet) located on the side that comes into contact with the body, and a liquid-impermeable back sheet (back sheet) located on the opposite side that comes into contact with the body. The absorbent core is usually composed of hydrophilic fibers such as pulp and water-absorbent resin particles.

[0005] Japanese Patent Application Publication No. 3-227301

[0006] In such absorbent articles, the water-absorbent resin particles contained in the absorbent body are affected by vitamin C in urine and iron contained in urine and the water-absorbent resin particles, and a phenomenon occurs in which the gel deteriorates.

[0007] As a method for suppressing gel deterioration of the water absorbent resin particles as described above, a method of adding a metal chelating agent in the manufacturing process of the water absorbent resin particles is known. For example, an aminocarboxylic acid chelating agent coordinates with iron or the like, and exerts an effect of suppressing deterioration of the water absorbent resin particles caused by vitamin C and iron.

[0008] Furthermore, the aminocarboxylic acid chelating agent not only inhibits gel deterioration of the water-absorbent resin particles but also exerts the effect of inhibiting yellowing of the water-absorbent resin particles.

[0009] However, the inventors of the present invention have found that when the iron concentration in urine is high, the effect of the aminocarboxylic acid chelating agent in inhibiting gel deterioration is not sufficient.

[0010]

[0010] Under these circumstances, a main object of the present invention is to provide a novel method for producing water-absorbent resin particles that have excellent gel stability in artificial urine containing iron, long-term gel stability in artificial urine, and yellowing resistance. Another object of the present invention is to provide water-absorbent resin particles that are excellent in high absorption performance (particularly, saline water retention capacity and saline water absorption capacity under a load of 4.14 kPa), and that have excellent gel stability in artificial urine containing iron, long-term gel stability in artificial urine, and yellowing resistance, and an absorbent body and absorbent article that utilize the water-absorbent resin particles.

[0011] The present inventors have conducted extensive research to solve the above-mentioned problems. As a result, they have found that in a method for producing water-absorbent resin particles, comprising a step of polymerizing a water-soluble ethylenically unsaturated monomer to obtain polymer particles and a surface-crosslinking step of surface-crosslinking the polymer particles, by adding a predetermined amount of a phosphonic acid chelating agent to polymer particles adjusted to a predetermined water content at least either before or after the surface-crosslinking step, water-absorbent resin particles having gel stability in artificial urine containing iron, long-term gel stability in artificial urine, and yellowing resistance can be obtained. The present invention has been completed based on this finding and through further extensive research.

[0012] That is, the present invention provides the following configurations. Item 1. A method for producing water-absorbent resin particles, comprising: a step of polymerizing a water-soluble ethylenically unsaturated monomer to obtain polymer particles; and a surface cross-linking step of surface-crosslinking the polymer particles, wherein a phosphonic acid chelating agent is added to the polymer particles having a water content of 5% by mass or more and 100% by mass or less before and / or after the surface cross-linking step, and the amount of the phosphonic acid chelating agent added is 0.055 parts by mass or more and 0.6 parts by mass or less relative to 100 parts by mass of the water-soluble ethylenically unsaturated monomer. Item 2. A method for producing water-absorbent resin particles according to Item 1, wherein the water content of the polymer particles is 20% by mass or more and 80% by mass or less. Item 3. A method for producing water-absorbent resin particles according to Item 1 or 2, wherein the water-absorbent resin particles have the following properties (A) to (E): (A) A physiological saline water retention capacity is 20 g / g or more and 70 g / g or less. (B) The amount of physiological saline solution absorbed under a load of 4.14 kPa is 5 mL / g or more and 40 mL / g or less. (C) The yellowness index after being left for 7 days in an environment of 70°C and 90% relative humidity is less than 25. (D) The gel swelled 40 times with artificial urine has a gel strength of 6000 N / m after being left for 14 hours in an environment of 37°C and 60% relative humidity. 2 (E) A gel swollen 40 times with iron-containing artificial urine having an iron concentration of 10 ppm has a gel strength of 4000 N / m after being left for 2.5 hours in an environment of 25°C and 50% relative humidity. 2 or more. Item 4. Water-absorbent resin particles which are crosslinked polymers of water-soluble ethylenically unsaturated monomers, the water-absorbent resin particles having the following properties (A) to (E): (A) A saline water retention capacity of 20 g / g or more and 70 g / g or less. (B) A saline water absorption capacity under a load of 4.14 kPa of 5 mL / g or more and 40 mL / g or less. (C) A yellowness index after being left for 7 days in an environment of 70°C and 90% relative humidity is less than 25. (D) A gel swelled 40 times with artificial urine has a gel strength of 6000 N / m or more after being left for 14 hours in an environment of 37°C and 60% relative humidity. 2(E) A gel swollen 40 times with iron-containing artificial urine having an iron concentration of 10 ppm has a gel strength of 4000 N / m after being left for 2.5 hours in an environment of 25°C and 50% relative humidity. 2 Item 5. An absorbent body comprising the water-absorbent resin particles according to Item 4. Item 6. An absorbent article comprising the absorbent body according to Item 5.

[0013] According to the present invention, it is possible to provide a novel method for producing water-absorbent resin particles that combine gel stability with artificial urine containing iron, long-term gel stability with artificial urine, and yellowing resistance. Furthermore, according to the present invention, it is also possible to provide water-absorbent resin particles that are excellent in high absorption performance (particularly, saline water retention capacity and saline water absorption capacity under a load of 4.14 kPa), and that combine gel stability with artificial urine containing iron, long-term gel stability with artificial urine, and yellowing resistance, as well as absorbent bodies and absorbent articles that use the water-absorbent resin particles.

[0014] 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.

[0015] As used herein, the term "comprising" includes "consisting essentially of" and "consisting of." Furthermore, as used herein, "(meth)acrylic" means "acrylic or methacrylic," "(meth)acrylate" means "acrylate or methacrylate," and "(poly)" refers to the presence or absence of the prefix "poly." Furthermore, as used herein, "water-soluble" means exhibiting a solubility of 5% by mass or more in water at 25°C.

[0016] 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 "~".

[0017] 1. Method for Producing Water-Absorbent Resin Particles The method for producing water-absorbent resin particles of the present invention includes, in this order, a step of polymerizing a water-soluble ethylenically unsaturated monomer to obtain polymer particles, and a surface cross-linking step of surface-cross-linking the polymer particles.

[0018] In the method for producing water-absorbent resin particles of the present invention, a phosphonic acid chelating agent is added to polymer particles having a water content of 5% by mass or more and 100% by mass or less at least either before or after a surface cross-linking step, and the amount of the phosphonic acid chelating agent added is in the range of 0.055 parts by mass or more and 0.6 parts by mass or less relative to 100 parts by mass of a water-soluble ethylenically unsaturated monomer.

[0019]

[0033] By virtue of these features, the method for producing water-absorbent resin particles of the present invention can suitably produce water-absorbent resin particles that have gel stability against iron-containing artificial urine, long-term gel stability against artificial urine, and yellowing resistance. Hereinafter, the method for producing water-absorbent resin particles of the present invention will be described in detail.

[0020] <Polymerization Step> The polymerization step is a step of polymerizing a water-soluble ethylenically unsaturated monomer to obtain polymer particles. Typical polymerization methods for polymerizing a water-soluble ethylenically unsaturated monomer include aqueous solution polymerization, spray-droplet polymerization, emulsion polymerization, and reverse-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 needed. In reverse-phase suspension polymerization, polymerization is carried out by heating the water-soluble ethylenically unsaturated monomer in a hydrocarbon dispersion medium while stirring. Among these, reverse-phase suspension polymerization is preferred from the viewpoint of improving the general water absorption performance of water-absorbent resin particles (water retention capacity of physiological saline solution and water absorption capacity of physiological saline solution under a load of 4.14 kPa). In the polymerization step, an internal crosslinking agent may be added to the water-soluble ethylenically unsaturated monomer as needed to form crosslinked polymer particles (a hydrogel-like substance) having an internal crosslinked structure. An example of the polymerization step is described below.

[0021] [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.

[0022] Among these, acrylic acid and its salts are widely used as raw materials for water-absorbent resin particles, and these acrylic acid and / or salts thereof may be copolymerized with the other water-soluble ethylenically unsaturated monomers described above for use. 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.

[0023] 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.

[0024] 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.

[0025] The degree of neutralization of the water-soluble ethylenically unsaturated monomer with the alkaline neutralizing agent is preferably 40 to 100 mol %, more preferably 50 to 90 mol %, even more preferably 60 to 85 mol %, and still more preferably 70 to 80 mol %, as the degree of neutralization with respect to all acid groups possessed by the water-soluble ethylenically unsaturated monomer.

[0026] [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.

[0027] 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.

[0028] [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 such compounds include compounds having two or more polymerizable unsaturated groups such as benzene; diglycidyl compounds such as (poly)ethylene glycol diglycidyl ether, (poly)propylene glycol diglycidyl ether, and (poly)glycerin diglycidyl ether, and polyglycidyl compounds such as triglycidyl compounds; epihalohydrin compounds such as epichlorohydrin, epibromohydrin, and α-methylepichlorohydrin; compounds having two or more reactive functional groups such as isocyanate compounds such as 2,4-tolylene diisocyanate and hexamethylene diisocyanate; and oxetane compounds such as 3-methyl-3-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.

[0029] 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.

[0030] [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.

[0031] 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).

[0032] [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.

[0033] 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.

[0034] 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.

[0035] (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.

[0036] 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.

[0037] 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.

[0038] [Other Components] In the method for producing water-absorbent resin particles, if desired, other components may be added to an aqueous solution containing a water-soluble ethylenically unsaturated monomer to carry out reverse phase suspension polymerization. As the other components, various additives such as a thickener and a chain transfer agent can be added.

[0039] 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.

[0040] 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.

[0041]

[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.

[0042] 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.

[0043] Such reversed-phase suspension polymerization can be carried out in one stage or in two or more stages, and is preferably carried out in two or three stages from the viewpoint of increasing productivity.

[0044] 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.

[0045] 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.

[0046] <Dehydration Step> After the above-described reversed-phase suspension polymerization, a dehydration step may be included in which water, a hydrocarbon dispersion medium, and the like are removed by distillation by external application of energy such as heat. When dehydrating the hydrous gel-like material after reversed-phase suspension polymerization, the system in which the hydrous gel-like material is dispersed in the hydrocarbon dispersion medium is heated, and the water and the hydrocarbon dispersion medium are temporarily removed from the system by azeotropic distillation. In this case, if only the evaporated hydrocarbon dispersion medium is returned to the system, continuous azeotropic distillation is possible. 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. By controlling the treatment conditions of this dehydration step after polymerization to adjust the amount of dehydration (i.e., adjusting the water content of the polymer particles), it is possible to control the various properties of the resulting water-absorbent resin particles. As described above, in the present invention, it is necessary to control the water content when adding the phosphonic acid chelating agent to the polymer particles. Furthermore, from the viewpoint of more suitably exhibiting the effects of the present invention, it is also preferable to adjust the water content of the polymer particles to be subjected to the surface crosslinking step.

[0047] In the dehydration step, dehydration treatment by distillation may be carried out under normal pressure. When dehydration treatment is carried out under normal pressure, the dehydration temperature is preferably 70 to 250°C, more preferably 80 to 180°C, even more preferably 80 to 140°C, and even more preferably 90 to 130°C.

[0048] <Surface cross-linking step> The surface cross-linking step is a step of subjecting the polymer particles obtained in the polymerization step to surface cross-linking. When the polymer particles are cross-linked polymer particles (hydrogel-like material), the surface cross-linking step is a step of adding a surface cross-linking agent to a hydrogel-like material having an internal cross-linked structure obtained by polymerizing a water-soluble ethylenically unsaturated monomer to perform cross-linking (surface cross-linking reaction). This surface cross-linking reaction is preferably carried out in the presence of a surface cross-linking agent after the polymerization of the water-soluble ethylenically unsaturated monomer. In this way, by subjecting a hydrogel-like material having an internal cross-linked structure to a surface cross-linking reaction after the polymerization, the cross-linking density near the surface of the water-absorbent resin particles can be increased, and water-absorbent resin particles having improved performance such as water absorption capacity under load can be obtained.

[0049] Examples of the surface cross-linking agent include compounds having two or more reactive functional groups. For example, polyols such as ethylene glycol, propylene glycol, 1,4-butanediol, 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 and hydroxyalkylamide compounds such as bis[N,N-di(β-hydroxyethyl)]adipamide. Among these surface cross-linking agents, polyglycidyl compounds such as (poly)ethylene glycol diglycidyl ether, (poly)glycerin diglycidyl ether, (poly)glycerin triglycidyl ether, trimethylolpropane triglycidyl ether, (poly)propylene glycol polyglycidyl ether, and (poly)glycerol polyglycidyl ether are preferred.These surface cross-linking agents may be used alone or in combination of two or more.

[0050] 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.

[0051] 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.

[0052] The timing of addition of the surface crosslinking agent may be after the polymerization reaction of the water-soluble ethylenically unsaturated monomer has almost completely finished, and the surface crosslinking agent is added in the presence of water in a range of preferably 1 to 400 parts by mass, more preferably 5 to 200 parts by mass, still more preferably 10 to 100 parts by mass, and still more preferably 20 to 60 parts by mass, relative to 100 parts by mass of the water-soluble ethylenically unsaturated monomer. Note that the amount of water means the total amount of water contained in the reaction system and water used as necessary when adding the surface crosslinking agent.

[0053] From the viewpoint of more suitably exerting the effects of the present invention, the water content of the polymer particles when the surface crosslinking agent is added is preferably 1% by mass or more, more preferably 10% by mass or more, still more preferably 20% by mass or more, and is preferably 60% by mass or less, more preferably 40% by mass or less, still more preferably 35% by mass or less. Preferred ranges include 1 to 60% by mass, 1 to 40% by mass, 1 to 35% by mass, 10 to 60% by mass, 10 to 40% by mass, 10 to 35% by mass, 20 to 60% by mass, 20 to 40% by mass, and 20 to 35% by mass.

[0054] 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.

[0055] <Addition of Phosphonic Acid Chelating Agent> In the method for producing water-absorbent resin particles of the present invention, a phosphonic acid chelating agent is added to the polymer particles at least either before the surface cross-linking step or after the surface cross-linking step. The water content of the polymer particles at the time of addition is 5% by mass or more and 100% by mass or less. Furthermore, the amount of the phosphonic acid chelating agent added at this time is 0.055 parts by mass or more and 0.6 parts by mass or less relative to 100 parts by mass of the water-soluble ethylenically unsaturated monomer.

[0056] The phosphonic acid chelating agent is not particularly limited as long as it achieves the effects of the present invention, and preferred examples include ethylenediaminetetramethylenephosphonic acid (EDTMP), diethylenetriaminepentamethylenephosphonic acid (DTPMP), and salts thereof. From the viewpoint of more suitably achieving the effects of the present invention, among these, it is preferable to include pentasodium ethylenediaminetetramethylenephosphonic acid (EDTMP.5Na) and hexasodium diethylenetriaminepentamethylenephosphonic acid (DTPMP.7Na), and it is even more preferable to include pentasodium ethylenediaminetetramethylenephosphonic acid (EDTMP.5Na). The phosphonic acid chelating agent added to the polymer particles may be one type only, or two or more types.

[0057] The water content of the polymer particles when the phosphonic acid chelating agent is added may be in the range of 5% by mass to 100% by mass, and from the viewpoint of more suitably exhibiting the effects of the present invention, it is preferably 15% by mass or more, more preferably 20% by mass or more, even more preferably 25% by mass or more, and is preferably 90% by mass or less, more preferably 80% by mass or less, even more preferably 70% by mass or less, and preferred ranges include 5 to 90% by mass, 5 to 80% by mass, 5 to 70% by mass, 15 to 90% by mass, 15 to 80% by mass, 15 to 70% by mass, 20 to 90% by mass, 20 to 80% by mass, 20 to 70% by mass, 25 to 90% by mass, 25 to 80% by mass, 25 to 70% by mass, etc. The water content of the polymer particles when the phosphonic acid chelating agent is added can be adjusted by the drying step (including dehydration) described below, the addition of water to the polymer particles, or the like.

[0058] The amount of the phosphonic acid chelating agent added may be in the range of 0.055 parts by mass or more and 0.6 parts by mass or less, relative to 100 parts by mass of the water-soluble ethylenically unsaturated monomer. From the viewpoint of more suitably exerting the effects of the present invention, the amount is preferably 0.055 parts by mass or more, more preferably 0.056 parts by mass or more, even more preferably 0.057 parts by mass or more, and is preferably 0.6 parts by mass or less, more preferably 0.4 parts by mass or less, even more preferably 0.3 parts by mass or less. Preferred ranges include 0.055 to 0.6 parts by mass, 0.055 to 0.4 parts by mass, 0.055 to 0.3 parts by mass, 0.056 to 0.6 parts by mass, 0.056 to 0.4 parts by mass, 0.056 to 0.3 parts by mass, 0.057 to 0.6 parts by mass, 0.057 to 0.4 parts by mass, and 0.057 to 0.3 parts by mass.

[0059] In the present invention, the addition of phosphonic acid chelating agent to polymer particles can be after polymerization process and before surface cross-linking process, or after surface cross-linking process, or can be after polymerization process and before surface cross-linking process and both after surface cross-linking process.From the viewpoint of more suitably exhibiting the effect of the present invention, it is more preferable to add phosphonic acid chelating agent to polymer particles at least after polymerization process and before surface cross-linking process.It should be noted that when adding phosphonic acid chelating agent to polymer particles both before and after surface cross-linking process, at least one addition time, the water content of polymer particles should be in the range of 5 mass% or more and 100 mass% or less, and the addition amount of phosphonic acid chelating agent should be in the range of 0.055 mass parts or more and 0.6 mass parts or less relative to 100 mass parts of water-soluble ethylenically unsaturated monomer, but it is also preferable that both addition times satisfy these ranges.

[0060] The method for adding the phosphonic acid chelating agent to the polymer particles is not particularly limited, and examples thereof include a method of mixing the phosphonic acid chelating agent in a solid state, such as a powder, with the polymer particles, a method of dripping an aqueous solution of the phosphonic acid chelating agent onto polymer particles dispersed in an organic solvent, and a method of spraying the aqueous solution of the phosphonic acid chelating agent onto the surface of the polymer particles. Methods for spraying the aqueous solution of the phosphonic acid chelating agent include a method using a spray nozzle device, an ultrasonic device using an ultrasonic vibrator, or a rotary atomization centrifugal spray device. In the present invention, a method using a spray nozzle device is preferred. The spray nozzle device is not particularly limited as long as it is capable of spraying an aqueous solution, but is preferably a one-fluid or two-fluid spray with a spray pattern such as a flat spray, hollow cone, or full cone.

[0061] <Drying step> After the above-mentioned surface cross-linking, a drying step may be included in which water, a hydrocarbon dispersion medium, and the like are removed by distillation by applying energy such as heat from the outside. The polymer particles after surface cross-linking are dried, and the water and the hydrocarbon dispersion medium are distilled off, thereby obtaining water-absorbent resin particles. The treatment conditions of the drying step after this surface cross-linking are controlled to adjust the amount of dehydration (i.e., to adjust the water content of the polymer particles), thereby making it possible to control the water content when adding a phosphonic acid chelating agent to the polymer particles.

[0062] 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.

[0063] 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.

[0064] 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, reducing agents, metal chelating agents (metal chelating agents different from phosphonic acid chelating agents), radical chain inhibitors, antioxidants, antibacterial agents, etc. 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 inorganic powder relative to 100 parts by mass of the water-absorbent resin particles. The additives are preferably hydrophilic or water-soluble.

[0065] 2. Water-absorbent resin particles By employing the above-described method for producing water-absorbent resin particles of the present invention, it is possible to produce water-absorbent resin particles that have gel stability in artificial urine containing iron, long-term gel stability in artificial urine, and yellowing resistance. Furthermore, by employing the above-described method for producing water-absorbent resin particles of the present invention, it is also possible to improve the saline water retention capacity and saline water absorption capacity under a load of 4.14 kPa, in addition to these properties. More specifically, by employing the method for producing water-absorbent resin particles of the present invention, it is possible to suitably produce water-absorbent resin particles having, for example, the following properties (A) to (E):

[0066] (A) The saline water retention capacity is 20 g / g or more and 70 g / g or less. (B) The saline water absorption capacity under a load of 4.14 kPa is 5 mL / g or more and 40 mL / g or less. (C) The yellowness index after being left for 7 days in an environment of 70°C and 90% relative humidity is less than 25. (D) The gel swelled 40 times with artificial urine has a gel strength of 6000 N / m or more after being left for 14 hours in an environment of 37°C and 60% relative humidity. 2 (E) A gel swollen 40 times with iron-containing artificial urine having an iron concentration of 10 ppm has a gel strength of 4000 N / m after being left for 2.5 hours in an environment of 25°C and 50% relative humidity. 2 That's all.

[0067] The (A) physiological saline water retention capacity of the water-absorbent resin particles of the present invention is preferably 20 g / g or more, more preferably 30 g / g or more, and is also preferably 70 g / g or less, more preferably 50 g / g or less, further preferably 45 g / g or less, and further preferably 43 g / g or less, and preferred ranges include 20 to 70 g / g, 20 to 50 g / g, 20 to 45 g / g, 20 to 43 g / g, 30 to 70 g / g, 30 to 50 g / g, 30 to 45 g / g, 30 to 43 g / g, etc.

[0068]

[0113] Furthermore, (B) the water absorption amount of physiological saline solution under a load of 4.14 kPa of the water absorbent resin particle of the present invention is preferably 10 mL / g or more, more preferably 15 mL / g or more, still more preferably 18 mL / g or more, and is preferably 30 mL / g or less, and preferred ranges include 10 to 30 mL / g, 15 to 30 mL / g, 18 to 30 mL / g, etc.

[0069] Furthermore, the water-absorbent resin particles of the present invention (C) have a yellowness index after being left for 7 days in an environment of 70°C and 90% relative humidity, of preferably less than 25, more preferably less than 20, and even more preferably less than 15. The lower limit of the yellowness index is, for example, 0.

[0070] The initial yellowness index of the water-absorbent resin particles of the present invention is preferably less than 20, more preferably less than 15, and even more preferably less than 12. The lower limit of the yellowness index is, for example, 0.

[0071] Furthermore, the gel strength of the water-absorbent resin particles of the present invention after being swollen 40 times with (D) artificial urine and left for 14 hours in an environment of 37°C and 60% relative humidity is preferably 6000 N / m 2 More preferably, 6500 N / m 2 More preferably, 7000 N / m 2 or more, and preferably 30,000 N / m 2 or less, more preferably 20,000 N / m 2 or less, more preferably 15,000 N / m 2 or less, more preferably 10,000 N / m 2More preferably, the range is 6000 to 30000 N / m 2 , 6000~20000N / m 2 , 6000~15000N / m 2 , 6000~10000N / m 2 , 6500~30000N / m 2 , 6500~20000N / m 2 , 6500~15000N / m 2 , 6500~10000N / m 2 , 7000~30000N / m 2 , 7000~20000N / m 2 , 7000~15000N / m 2 , 7000~10000N / m 2 The upper limit of the initial value of the gel strength is, for example, 20,000 N / m 2 is.

[0072] Furthermore, the gel obtained by swelling the water-absorbent resin particles of the present invention 40 times with (E) iron-containing artificial urine having an iron concentration of 10 ppm is preferably left for 2.5 hours in an environment of 25°C and 50% relative humidity, and the gel strength is preferably 4000 N / m 2 More preferably, 4300 N / m 2 or more, and preferably 15,000 N / m 2 or less, more preferably 10,000 N / m 2 More preferably, 8000 N / m or less 2 More preferably, the range is 4000 to 15000 N / m 2 etc.

[0073] The properties (A) to (E) of the water-absorbent resin particles are measured by the measurement methods described in Examples. The initial value of the yellowness index of the water-absorbent resin particles of the present invention is also measured by the measurement method described in Examples.

[0074] The water-absorbent resin particles of the present invention are constituted by crosslinking a polymer of a water-soluble ethylenically unsaturated monomer, that is, by a crosslinked polymer having structural units derived from a water-soluble ethylenically unsaturated monomer.

[0075] The water-absorbent resin particles of the present invention are in the form (secondary particles) of aggregates of fine particles (primary particles). Examples of the shape of the primary particles include substantially spherical, irregularly crushed, and plate-like shapes. The water-absorbent resin particles of the present invention, which are secondary particles, may have various shapes. Examples of the shape of the water-absorbent resin particles include granular, substantially spherical, irregularly crushed, plate-like, fibrous, flake-like shapes, and shapes obtained by aggregating these resins. The water-absorbent resin particles are preferably granular, substantially spherical, irregularly crushed, fibrous, or shapes obtained by aggregating these resins.

[0076] The median particle diameter of the water-absorbent resin particles is preferably 200 μm or more, 250 μm or more, 280 μm or more, 300 μm or more, or 320 μm or more. From the same viewpoint, the median particle diameter is preferably 700 μm or less, 600 μm or less, 550 μm or less, 500 μm or less, 450 μm or less, or 400 μm or less. That is, the median particle diameter is preferably 200 to 700 μm, preferably 200 to 600 μm, more preferably 250 to 500 μm, even more preferably 300 to 450 μm, and even more preferably 320 to 400 μm.

[0077] The median particle size of the water-absorbent resin particles can be measured using a JIS standard sieve, and specifically, it is a value measured by the method described in the examples.

[0078] The physiological saline water absorption speed of the water-absorbent resin particles is preferably 20 seconds or more, more preferably 25 seconds or more, even more preferably 30 seconds or more, and is preferably 65 seconds or less, more preferably 60 seconds or less, even more preferably 55 seconds or less, and more preferably in the range of 20 to 65 seconds, 25 to 60 seconds, etc.

[0079] The physiological saline water absorption rate of the water-absorbent resin particles is a value measured by the method described in the examples.

[0080] 3. Absorbent Material, 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.

[0081] The absorbent of the present invention contains the water-absorbent resin particles of the present invention. The absorbent may further contain hydrophilic fibers. Examples of the absorbent's configuration 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. The absorbent may also 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.

[0082] The basis weight of the water-absorbent resin particles in the absorbent body of the present invention is 50 g / m 2 More than 400g / m 2 The basis weight is preferably 100 g / m or less. 2 More preferably, 120 g / m 2 More preferably, 140 g / m 2 or more, and preferably 300 g / m 2 or less, more preferably 250 g / m 2 More preferably 200 g / m or less 2 The following is the result.

[0083] The hydrophilic fiber may be at least one selected from the group consisting of finely ground wood pulp, cotton, cotton linter, rayon, cellulose acetate, polyamide, polyester, and polyolefin. Examples 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 fiber is usually 0.1 to 10 mm, or may be 0.5 to 5 mm.

[0084] The basis weight of the hydrophilic fiber in the absorbent body of the present invention is 0 g / m 2 800g / m or more 2 The basis weight is preferably 100 g / m or less. 2More preferably, 120 g / m 2 More preferably, 140 g / m 2 and preferably 700 g / m 2 or less, more preferably 600 g / m 2 More preferably 500 g / m or less 2 The following is the result.

[0085] 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.

[0086] 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.

[0087] Examples of liquid-permeable sheets include air-through, spunbond, chemical-bond, and needle-punched nonwoven fabrics 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. The liquid-permeable sheet is preferably at least one selected from the group consisting of thermal-bonded nonwoven fabrics, air-through nonwoven fabrics, spunbonded nonwoven fabrics, and spunbonded / meltblown / spunbonded nonwoven fabrics.

[0088] The basis weight of the liquid permeable sheet is 5 g / m 2 More than 100g / m 2 Preferably, it is 10 g / m or less. 2 60g / m or more 2It is more preferable that the liquid-permeable sheet has a surface embossed or perforated to improve the liquid diffusibility. The embossing or perforation can be carried out by a known method.

[0089] Examples of liquid-impermeable sheets include sheets made of synthetic resins such as polyethylene, polypropylene, and polyvinyl chloride; sheets made of nonwoven fabrics such as spunbond / meltblown / spunbond (SMS) nonwoven fabrics in which a water-resistant meltblown nonwoven fabric is sandwiched between high-strength spunbond nonwoven fabrics; and sheets made of composite materials of these synthetic resins and nonwoven fabrics (for example, spunbond nonwoven fabrics, spunlace nonwoven fabrics). As the liquid-impermeable sheet, a sheet made of a synthetic resin mainly composed of low-density polyethylene (LDPE) resin can also be used. The liquid-impermeable sheet has, for example, a basis weight of 10 to 50 g / m. 2 The sheet may be made of a synthetic resin.

[0090] The absorbent article preferably comprises a laminate having an absorbent body containing water-absorbent resin particles and a core wrap sandwiching the absorbent body from above and below, a liquid-permeable sheet disposed on the upper surface of the laminate, and a liquid-impermeable sheet disposed on the surface of the laminate opposite to the liquid-permeable sheet side.

[0091] 4. Additional Notes This specification includes at least the inventions shown in (1) to (9) below. (1) A method for producing water-absorbent resin particles, comprising: a step of polymerizing a water-soluble ethylenically unsaturated monomer to obtain polymer particles; and a surface cross-linking step of surface-crosslinking the polymer particles, wherein a phosphonic acid chelating agent is added to the polymer particles having a water content of 5% by mass or more and 100% by mass or less before and / or after the surface cross-linking step, and the amount of the phosphonic acid chelating agent added is 0.055 parts by mass or more and 0.6 parts by mass or less relative to 100 parts by mass of the water-soluble ethylenically unsaturated monomer. (2) The method for producing water-absorbent resin particles according to (1) above, wherein the water content of the polymer particles is 20% by mass or more and 80% by mass or less. (3) The method for producing water-absorbent resin particles according to (1) or (2), wherein the amount of the phosphonic acid chelating agent added is 0.055 to 0.4 parts by mass, 0.055 to 0.3 parts by mass, 0.057 to 0.4 parts by mass, or 0.057 to 0.3 parts by mass, relative to 100 parts by mass of the water-soluble ethylenically unsaturated monomer. (4) The method for producing water-absorbent resin particles according to any one of (1) to (3), wherein the water content of the polymer particles when adding a surface crosslinking agent is 10 to 60% by mass, 10 to 40% by mass, or 10 to 35% by mass. (5) Water-absorbent resin particles that are crosslinked products of polymers of water-soluble ethylenically unsaturated monomers, and the water-absorbent resin particles have the following properties (A) to (E): (A) A water retention capacity of physiological saline is 20 g / g or more and 70 g / g or less. (B) The amount of physiological saline solution absorbed under a load of 4.14 kPa is 5 mL / g or more and 40 mL / g or less. (C) The yellowness index after being left for 7 days in an environment of 70°C and 90% relative humidity is less than 25. (D) The gel swelled 40 times with artificial urine has a gel strength of 6000 N / m after being left for 14 hours in an environment of 37°C and 60% relative humidity. 2 (E) A gel swollen 40 times with iron-containing artificial urine having an iron concentration of 10 ppm has a gel strength of 4000 N / m after being left for 2.5 hours in an environment of 25°C and 50% relative humidity. 2(6) The water-absorbent resin particles according to (5) above, which have a saline water-retention capacity of 20 to 50 g / g, 30 to 45 g / g, or 30 to 43 g / g. (7) The water-absorbent resin particles according to (5) above, which have a saline water-absorption capacity under a load of 4.14 kPa of 10 to 30 mL / g, 15 to 30 mL / g, or 18 to 30 mL / g. (8) The water-absorbent resin particles according to (5) or (6) above, which have a saline water-absorption capacity under a load of 4.14 kPa of 10 to 30 mL / g, 15 to 30 mL / g, or 18 to 30 mL / g. (9) The water-absorbent resin particles according to (6) above, which have a gel strength of 6,000 to 20,000 N / m after being swelled 40 times with artificial urine and left for 14 hours in an environment of 37°C and 60% relative humidity. 2 , 6500~15000N / m 2 , 7000~10000N / m 2 (9) A water-absorbent resin particle according to any one of (5) to (7) above, wherein the gel is swollen 40 times with iron-containing artificial urine having an iron concentration of 10 ppm, and the gel strength after being left for 2.5 hours in an environment of 25°C and 50% relative humidity is 4000 to 15000 N / m 2 The water-absorbing resin particles according to any one of (5) to (8) above.

[0092] The present invention will be described in detail below with reference to examples and comparative examples, but the present invention is not limited to these examples.

[0093] The polymer particles (crosslinked polymer 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%.

[0094] Example 1 Polymerization Process 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 this flask as a hydrocarbon dispersion medium, and 0.736 g of maleic anhydride-modified ethylene-propylene copolymer (Mitsui Chemicals, Inc., Hiwax 1105A) 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.

[0095] 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.

[0096] 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 crosslinked polymer particles.

[0097] [Dehydration Step] After obtaining the crosslinked polymer particles, the flask was immersed in an oil bath set at 125°C, and 183.8 g of water was extracted from the system by azeotropic distillation of n-heptane and water while refluxing n-heptane. Then, 4.37 g of a 3.0 mass% aqueous solution of pentasodium ethylenediaminetetramethylenephosphonic acid was added with stirring. At this time, the water content of the crosslinked polymer particles when the aqueous solution of pentasodium ethylenediaminetetramethylenephosphonic acid was added was 57 mass%. Then, 67.6 g of water was extracted from the system by azeotropic distillation of n-heptane and water again while refluxing n-heptane.

[0098] [Surface Cross-Linking Step] Thereafter, 4.42 g (0.507 mmol) of a 2 mass % aqueous solution of ethylene glycol diglycidyl ether was added as a surface cross-linking agent to the flask, and the mixture was maintained for 2 hours at 83° C. At this time, the water content of the cross-linked polymer particles when the aqueous solution of ethylene glycol diglycidyl ether was added was 28 mass %.

[0099] [Drying step] Thereafter, n-heptane was evaporated at 125°C to dry the mixture, and the mixture was further passed through a sieve with an opening of 850 μm to obtain 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 water-absorbent resin particles to obtain 220.3 g of water-absorbent resin particles (1). The water-absorbent resin particles (1) had a saline water retention capacity of 41 g / g, a saline water absorption capacity under a load of 4.14 kPa of 20 mL / g, a saline water absorption rate of 35 seconds, and a median particle diameter of 338 μm.

[0100] Example 2 [Polymerization step] 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 this flask as a hydrocarbon dispersion medium, and 0.736 g of maleic anhydride-modified ethylene-propylene copolymer (Mitsui Chemicals, Inc., Hiwax 1105A) 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. Thereafter, 0.0184 g (0.068 mmol) of potassium persulfate and 0.092 g (0.339 mmol) of 2,2′-azobis(2-amidinopropane) dihydrochloride as water-soluble radical polymerization agents, and 0.0046 g (0.026 mmol) of ethylene glycol diglycidyl ether as an internal crosslinking agent were added and dissolved 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.

[0101] 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 0.0258 g (0.095 mmol) of potassium persulfate and 0.129 g (0.475 mmol) of 2,2′-azobis(2-amidinopropane) dihydrochloride 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.

[0102] 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 crosslinked polymer particles.

[0103] [Dehydration Step] After obtaining the crosslinked polymer particles, the flask was immersed in an oil bath set at 125°C, and 183.7 g of water was extracted from the system by azeotropic distillation of n-heptane and water while refluxing n-heptane. Then, 4.37 g of a 3.0 mass% aqueous solution of pentasodium ethylenediaminetetramethylenephosphonic acid was added with stirring. At this time, the water content of the crosslinked polymer particles when the aqueous solution of pentasodium ethylenediaminetetramethylenephosphonic acid was added was 57 mass%. Then, 47.9 g of water was extracted from the system by azeotropic distillation of n-heptane and water again while refluxing n-heptane.

[0104] [Surface Cross-Linking Step] Thereafter, 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 for 2 hours at 83° C. At this time, the water content of the cross-linked polymer particles when the aqueous solution of ethylene glycol diglycidyl ether was added was 37 mass %.

[0105] [Drying step] Thereafter, n-heptane was evaporated at 125°C to dry the mixture, and the mixture was further passed through a sieve with an opening of 850 μm to obtain 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 water-absorbent resin particles to obtain 219.7 g of water-absorbent resin particles (2). The water-absorbent resin particles (2) had a saline water retention capacity of 46 g / g, a saline water absorption capacity under a load of 4.14 kPa of 17 mL / g, a saline water absorption rate of 48 seconds, and a median particle diameter of 346 μm.

[0106] Example 3 The same operation as in Example 1 was performed, except that the amount of water extracted from the system by azeotropic distillation of n-heptane and water was changed from 67.6 g to 61.0 g, and the water content of the crosslinked polymer particles when adding a 2 mass% aqueous solution of ethylene glycol diglycidyl ether as a surface crosslinking agent was changed from 28 mass% to 31 mass%, to obtain 221.3 g of water-absorbent resin particles (3). At this time, the water-absorbent resin particles (3) had a water retention capacity of saline solution of 34 g / g, a water absorption capacity of saline solution under a load of 4.14 kPa of 28 mL / g, a water absorption rate of saline solution of 38 seconds, and a median particle diameter of 340 μm.

[0107] Example 4 The same operation as in Example 1 was carried out, except that in the [dehydration step] of Example 1, 4.37 g of a 3.0 mass% aqueous solution of diethylenetriaminepentamethylenephosphonic acid hexasodium was used instead of 4.37 g of a 3.0 mass% aqueous solution of ethylenediaminetetramethylenephosphonic acid pentasodium. 220.2 g of water-absorbent resin particles (4) was obtained. The water content of the crosslinked polymer particles when a 2 mass% aqueous solution of ethylene glycol diglycidyl ether was added as a surface crosslinking agent was 28 mass%. The water-retention capacity of the water-absorbent resin particles (4) was 38 g / g, the amount of water absorption of saline solution under a load of 4.14 kPa was 21 mL / g, the water absorption rate of saline solution was 37 seconds, and the median particle diameter was 340 μm.

[0108] <Comparative Example 1> In the [dehydration step] of Example 1, the amount of water extracted from the system by azeotropic distillation of n-heptane and water was changed from 183.8 g to 199.1 g, and 4.86 g of a 4.5 mass% diethylenetriaminepentaacetic acid-pentasodium (DTPA-5Na) aqueous solution was used instead of 4.37 g of a 3.0 mass% ethylenediaminetetramethylenephosphonic acid-pentasodium aqueous solution, and the amount of water extracted from the system by azeotropic distillation of n-heptane and water was changed from 67.6 g to 52.7 g. Except for this, the same operation as in Example 1 was performed to obtain 222.0 g of water-absorbent resin particles (5). At this time, the water content of the crosslinked polymer particles when the diethylenetriaminepentaacetic acid-pentasodium aqueous solution was added was 50 mass%, and the water content of the crosslinked polymer particles when a 2 mass% ethylene glycol diglycidyl ether aqueous solution was added as a surface crosslinking agent was 28 mass%. The saline water retention capacity of the water-absorbent resin particles (5) was 41 g / g, the saline water absorption capacity under a load of 4.14 kPa was 19 mL / g, the saline water absorption speed was 35 seconds, and the median particle diameter was 363 μm.

[0109] <Production Example> [Polymerization Step] 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 stirring blade with two stages of four inclined paddle blades with a blade diameter of 5 cm. 293 g of n-heptane was added to this flask as a hydrocarbon dispersion medium, and 0.736 g of maleic anhydride-modified ethylene-propylene copolymer (Mitsui Chemicals, Inc., Hiwax 1105A) 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.

[0110] 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.

[0111] 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 crosslinked polymer particles.

[0112] [Dehydration Step] After obtaining the crosslinked polymer particles, the flask was immersed in an oil bath set at 125° C., and 247.17 g of water was extracted from the system by azeotropic distillation of n-heptane and water while refluxing n-heptane.

[0113] [Surface Cross-Linking Step] Thereafter, 4.42 g (0.507 mmol) of a 2 mass % aqueous solution of ethylene glycol diglycidyl ether was added as a surface cross-linking agent to the flask, and the mixture was maintained for 2 hours at 83° C. At this time, the water content of the cross-linked polymer particles when the aqueous solution of ethylene glycol diglycidyl ether was added was 28 mass %.

[0114] [Drying Step] Thereafter, n-heptane was evaporated at 125°C to dry the mixture, and 50.09 g of water was extracted from the system. The mixture was then passed through a sieve with an opening of 850 µm to obtain 220.1 g of crosslinked polymer particles (A). At this time, the water content of the crosslinked polymer particles (A) was 7% by mass.

[0115] Comparative Example 2 To 20 parts by mass of the crosslinked polymer particles (A) obtained in Production Example, 0.04 parts by mass of triethylenetetraminehexaacetic acid (TTHA) as a chelating agent was added in powder form, 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 mixture to obtain water-absorbent resin particles (6). The saline water retention capacity of the water-absorbent resin particles (6) was 40 g / g, the saline water absorption capacity under a load of 4.14 kPa was 21 mL / g, the saline water absorption rate was 36 seconds, and the median particle diameter was 344 μm.

[0116] Example 5 20 parts by mass of the crosslinked polymer particles (A) obtained in the Production Example were weighed into a round-bottomed cylindrical separable flask with an inner diameter of 11 cm and equipped with a fluororesin anchor-shaped stirring blade. Next, while stirring at 300 rpm, 0.40 parts by mass of a 3.0% by mass aqueous solution of pentasodium ethylenediaminetetramethylenephosphonate was taken with a 3 mL syringe, and a Fine Atomizer Oral (Yoshikawa Kasei Co., Ltd.) was attached to the tip of the syringe. The solution was sprayed into the separable flask over 1 second. After stirring for 10 minutes, the mixture was heated at 100°C for 30 minutes to obtain a mixture. 0.5 parts by mass of amorphous silica (Toxil NP-S, Oriental Silicas Corporation) was mixed with 100 parts by mass of the mixture to obtain water-absorbent resin particles (7). The water-absorbent resin particles (7) had a saline water retention capacity of 40 g / g, a saline water absorption capacity under a load of 4.14 kPa of 21 mL / g, a saline water absorption speed of 36 seconds, and a median particle diameter of 344 μm.

[0117] Example 6 The same procedure as in Example 1 was carried out, except that in the [dehydration step] of Example 1, the amount of water extracted from the system by azeotropic distillation of n-heptane and water was changed from 183.8 g to 242.8 g, the water content of the crosslinked polymer particles when adding the aqueous solution of pentasodium ethylenediaminetetramethylenephosphonic acid was changed from 57 mass% to 30 mass%, and the amount of water extracted from the system by azeotropic distillation of n-heptane and water was changed from 67.6 g to 8.6 g, to obtain 221.5 g of water-absorbent resin particles (8). The water content of the crosslinked polymer particles when adding a 2 mass% aqueous solution of ethylene glycol diglycidyl ether as a surface crosslinking agent was 28 mass%. The physiological saline water retention capacity of the water-absorbent resin particles (8) was 38 g / g, the physiological saline water absorption capacity under a load of 4.14 kPa was 21 mL / g, the physiological saline water absorption rate was 35 seconds, and the median particle diameter was 358 μm.

[0118] Example 7 The same procedure as in Example 1 was carried out, except that in the [dehydration step] of Example 1, the amount of water extracted from the system by azeotropic distillation of n-heptane and water was changed from 183.8 g to 133.6 g, the water content of the crosslinked polymer particles when adding the pentasodium ethylenediaminetetramethylenephosphonic acid aqueous solution was changed from 57 mass% to 80 mass%, and the amount of water extracted from the system by azeotropic distillation of n-heptane and water was changed from 67.6 g to 117.8 g, to obtain 224.5 g of water-absorbent resin particles (9). The water content of the crosslinked polymer particles when adding a 2 mass% ethylene glycol diglycidyl ether aqueous solution as a surface crosslinking agent was 28 mass%. The physiological saline water retention capacity of the water-absorbent resin particles (9) was 38 g / g, the physiological saline water absorption capacity under a load of 4.14 kPa was 21 mL / g, the physiological saline water absorption rate was 35 seconds, and the median particle diameter was 356 μm.

[0119] <Comparative Example 3> In the [dehydration step] of Example 1, 4.37 g of a 3.0 mass% aqueous solution of pentasodium ethylenediaminetetramethylenephosphonic acid was added to the obtained crosslinked polymer particles without extracting water from the system by azeotropic dehydration, and the water content of the crosslinked polymer particles when adding the pentasodium ethylenediaminetetramethylenephosphonic acid aqueous solution was changed from 57 mass% to 141 mass%, and the amount of water extracted from the system by azeotropic distillation of n-heptane and water was changed from 67.6 g to 251.4 g, except that the same operation as in Example 1 was performed to obtain 222.0 g of water-absorbent resin particles (10). The water content of the crosslinked polymer particles when adding a 2 mass% aqueous solution of ethylene glycol diglycidyl ether as a surface crosslinking agent was 28 mass%. The water-absorbent resin particles (10) had a saline water retention capacity of 40 g / g, a saline water absorption capacity under a load of 4.14 kPa of 19 mL / g, a saline water absorption speed of 36 seconds, and a median particle diameter of 400 μm.

[0120] Example 8 The same procedure as in Example 2 was carried out, except that in the [dehydration step] of Example 2, 4.37 g of 5.0 mass % ethylenediaminetetramethylenephosphonic acid pentasodium solution was used instead of 4.37 g of 3.0 mass % ethylenediaminetetramethylenephosphonic acid pentasodium aqueous solution, and the amount of water extracted from the system by azeotropic distillation of n-heptane and water was changed from 47.9 g to 47.8 g, to obtain 219.5 g of water-absorbent resin particles (11). The water content of the crosslinked polymer particles when a 2 mass % ethylene glycol diglycidyl ether aqueous solution was added as a surface crosslinking agent was 37 mass %. The physiological saline water retention capacity of the water-absorbent resin particles (11) was 45 g / g, the physiological saline water absorption capacity under a load of 4.14 kPa was 20 mL / g, the physiological saline water absorption rate was 45 seconds, and the median particle diameter was 346 μm.

[0121] Example 9 Water-absorbent resin particles (12) were obtained by the same operation as in Example 5, except that 0.28 parts by mass of a 32.0% by mass aqueous solution of pentasodium ethylenediaminetetramethylenephosphonic acid was used instead of 0.40 parts by mass of the 3.0% by mass aqueous solution of pentasodium ethylenediaminetetramethylenephosphonic acid in Example 5. The water-absorbent resin particles (12) had a water retention capacity of physiological saline of 40 g / g, a water absorption capacity of physiological saline under a load of 4.14 kPa of 18 mL / g, a water absorption rate of physiological saline of 38 seconds, and a median particle diameter of 344 μm.

[0122] Comparative Example 4 The same procedure as in Example 1 was carried out, except that in the [dehydration step] of Example 1, 1.31 g of 0.5 mass% pentasodium ethylenediaminetetramethylenephosphonic acid was used instead of 4.37 g of a 3.0 mass% aqueous solution of pentasodium ethylenediaminetetramethylenephosphonic acid, and the amount of water extracted from the system by azeotropic distillation of n-heptane and water was changed from 67.6 g to 64.7 g, to obtain 219.0 g of water-absorbent resin particles (13). The water content of the crosslinked polymer particles when a 2 mass% aqueous solution of ethylene glycol diglycidyl ether was added as a surface crosslinking agent was 28 mass%. The physiological saline water retention capacity of the water-absorbent resin particles (13) was 42 g / g, the physiological saline water absorption capacity under a load of 4.14 kPa was 18 mL / g, the physiological saline water absorption rate was 37 seconds, and the median particle diameter was 366 μm.

[0123] <Water Content of Polymer Particles> The water content of the polymer particles was calculated as follows. The water content of the polymer particles was calculated according to the following formula using the amount of water W1 (g) contained in the aqueous liquid used in the production of polymer particles in the [polymerization step], the amount of water W2 (g) contained in the aqueous additive solution added to the system in the [dehydration step], the amount of water W3 (g) contained in the aqueous surface crosslinking agent solution added to the system in the [surface crosslinking step], the amount of water W4 (g) extracted to the outside of the system by azeotropic distillation of n-heptane and water in the [dehydration step], the amount of water W5 (g) extracted to the outside of the system by azeotropic distillation of n-heptane and water in the [surface crosslinking step] and the [drying step], and the amount M1 (g) of water-soluble ethylenically unsaturated monomer contained in the aqueous liquid used in the production of polymer particles in the [polymerization step]. Water content of polymer particles (% by mass) = (W1 + W2 + W3 - W4 - W5) × 100 / M1

[0124] <Median particle size (particle size distribution)> 10 g of water-absorbent resin particles were sieved using an ultrasonic vibration sieving measuring instrument (Robot Sifter RPS-01, manufactured by Seishin Enterprise Co., Ltd.), sieves with JIS standard openings of 850 μm, 600 μm, 500 μm, 425 μm, 300 μm, 250 μm, and 180 μm, and a tray. The mass of the particles remaining on each sieve was calculated as a mass percentage with respect to the total amount. The mass percentages of the particles remaining on each sieve were integrated in order from the largest particle size, and the relationship between the sieve opening and the integrated value of the mass percentage of the particles remaining on the sieve was plotted on logarithmic probability paper. By connecting the plots on the probability paper with a straight line, the particle size corresponding to an integrated mass percentage of 50 mass% was determined, and this was taken as the median particle size.

[0125] <Water absorption rate of physiological saline> 50±0.1 g of physiological saline placed in a 100 ml beaker was adjusted to a temperature of 25±0.2°C in a thermostatic water bath, and then the solution was stirred at 600 rpm with a magnetic stir bar (8 mmφ×30 mm, without ring) to generate a vortex. 2.0±0.002 g of water-absorbent resin particles were added to the physiological saline at once, and the time (seconds) from the addition of the water-absorbent resin particles until the vortex disappeared and the liquid surface became flat was measured, and this time was defined as the water absorption rate of the physiological saline by the water-absorbent resin particles.

[0126] <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

[0127] <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 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 has a burette tube 21 with a scale, a rubber stopper 23 that seals the opening at the top of the burette tube 21, a cock 22 connected to the tip of the bottom of the burette tube 21, and an air introduction tube 25 and a cock 24 connected to the bottom of the burette tube 21. The burette unit 1 is fixed with a clamp 3. The flat measurement table 13 has a through-hole 13a with a diameter of 2 mm formed in its center, and is supported by a height-adjustable stand 11. The through hole 13a of the 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.

[0128] 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.

[0129] 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).

[0130] 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 amount of saline absorbed by the water-absorbent resin particles 10a under a load of 4.14 kPa was calculated by the following formula: Amount of saline absorbed under a load of 4.14 kPa (mL / g) = Wc (mL) / Mass of water-absorbent resin particles (g)

[0131] <Evaluation of Stability of Gel Absorbing Iron-Containing Artificial Urine> Using iron-containing artificial urine (iron concentration 10 ppm) having the following composition, the stability of a gel (swollen gel) in which water-absorbent resin particles have absorbed water was evaluated.

[0132] (Preparation of Iron-Containing Artificial Urine) Iron-containing artificial urine was prepared with the following composition: 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 Ferrous sulfate II heptahydrate: 0.05 g Ion-exchanged water: 970.9 g

[0133] (Preparation of swollen gel) 39.0 g of the above iron-containing artificial urine was weighed into a 100 mL beaker, a magnetic stirrer bar (8 mmφ x 30 mm, without ring) was added, and the magnetic stirrer 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 was 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). The beaker containing the swollen gel covered with plastic wrap was left in a room at a temperature of 25 ± 2 ° C. and a relative humidity of 50 ± 10% for 2.5 hours, after which the plastic wrap was removed from the beaker containing the swollen gel, and the gel strength was measured by the following method.

[0134] (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 of the pressure-sensitive shaft 84 with a disk, a load of 400 g, a speed of 7 seconds / inch, and viscous mode settings.

[0135] <Evaluation of Long-Term Stability of Gel Absorbing Artificial Urine> Using artificial urine having the following composition, evaluation of the long-term stability of a gel (swollen gel) in which water-absorbent resin particles have absorbed water was carried out.

[0136] (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

[0137] (Preparation of Swollen Gel) A swollen gel was prepared in the same manner as in <Evaluation of Stability of Gel Absorbed with Iron-Containing Artificial Urine>, except that in <Test of Gel Absorbed with Iron-Containing Artificial Urine>, the above artificial urine was used instead of the iron-containing artificial urine, and instead of leaving the gel for 2.5 hours in a room at a temperature of 25±2°C and a relative humidity of 50±10%, the gel was left for 14 hours in a thermo-hygrostat (LHU-113, manufactured by Espec Corporation) at a temperature of 37±2°C and a relative humidity of 60±10%.

[0138] (Measurement of Gel Strength) The temperature of the swollen gel was adjusted to 25±2° C., and the gel strength was measured in the same manner as in <Evaluation of Stability of Gel That Absorbed Iron-Containing Artificial Urine>.

[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 colorimeter, 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 using the following formula, and was taken as an initial value. Yellowness index=100(1.28X−1.06Z) / Y

[0140] 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 7 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 above formula.

[0141]

[0142] 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 15 nylon mesh sheet 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: a step of polymerizing a water-soluble ethylenically unsaturated monomer to obtain polymer particles; and a surface cross-linking step of subjecting the polymer particles to surface cross-linking, wherein a phosphonic acid chelating agent is added to the polymer particles, having a water content of 5% by mass or more and 100% by mass or less, before and / or after the surface cross-linking step, and the amount of the phosphonic acid chelating agent added is 0.055 parts by mass or more and 0.6 parts by mass or less, relative to 100 parts by mass of the water-soluble ethylenically unsaturated monomer.

2. The method for producing water-absorbent resin particles according to claim 1, wherein the water content of the polymer particles is 20% by mass or more and 80% by mass or less.

3. The method for producing water-absorbent resin particles according to claim 1 or 2, wherein the water-absorbent resin particles have the following characteristics (A) to (E): (A) the physiological saline water retention capacity is 20 g / g or more and 70 g / g or less; (B) the physiological saline water absorption capacity under a load of 4.14 kPa is 5 mL / g or more and 40 mL / g or less; (C) the yellowness index after being left for 7 days in an environment of 70°C and 90% relative humidity is less than 25; (D) the gel swelled 40 times with artificial urine has a gel strength of 6000 N / m or more after being left for 14 hours in an environment of 37°C and 60% relative humidity. 2 (E) The gel is swollen 40 times with artificial urine containing iron with an iron concentration of 10 ppm, and after being left for 2.5 hours in an environment of 25° C. and 50% relative humidity, the gel strength is 4000 N / m 2 That's all.

4. A water-absorbent resin particle which is a crosslinked polymer of a water-soluble ethylenically unsaturated monomer, the water-absorbent resin particle having the following characteristics (A) to (E): (A) a physiological saline water retention capacity of 20 g / g or more and 70 g / g or less; (B) a physiological saline water absorption capacity under a load of 4.14 kPa of 5 mL / g or more and 40 mL / g or less; (C) a yellowness index of less than 25 after being left for 7 days in an environment of 70°C and 90% relative humidity; (D) a gel strength of 6000 N / m after a gel swelled 40 times with artificial urine is left for 14 hours in an environment of 37°C and 60% relative humidity. 2 (E) The gel is swollen 40 times with artificial urine containing iron with an iron concentration of 10 ppm, and after being left for 2.5 hours in an environment of 25° C. and 50% relative humidity, the gel strength is 4000 N / m 2 That's all.

5. An absorbent comprising the water-absorbent resin particles according to claim 4.

6. An absorbent article comprising the absorbent body according to claim 5.