Water-absorbent resin particles, absorber, and absorbent article

Abstract

A water-absorbing resin particle according to the present invention has, in terms of iron atoms, 0.03-10 ppm of a water-insoluble iron-containing substance.

Description

Technical Field

[0001] The present invention relates to water-absorbing resin particles, an absorber, and a disposable absorbent article.

Background Art

[0002] Water-absorbing resin particles are used in sanitary materials such as disposable diapers and sanitary products, water retention materials, agricultural and horticultural materials such as soil improvers, water stop materials for cables, and industrial materials such as dew condensation prevention materials. It is known that water-absorbing resin particles deteriorate over time when they absorb liquid and become swollen gels. In order to compensate for this, studies have been made to improve the stability of the gel state of water-absorbing resin particles. Patent Document 1 discloses a technique for improving the stability of a swollen gel over time by adding a metal chelating agent.

[0003] On the other hand, from the viewpoint of environmental protection, recycling of pulp from used disposable diapers has been studied. Patent Document 2 discloses that, in order to reuse used disposable diapers, a salt of a strong acid and a nitrogen-containing basic compound is added to treat aggregates of water-absorbing resin powder after water absorption.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0005] Although water-absorbing resin particles need to form a stable swollen gel by water absorption, it is required that the swollen gel is easily decomposed under acidic conditions used in the recycling process.

[0006] The object of the present invention is to provide water-absorbing resin particles that can form a stable swollen gel upon water absorption, but the swollen gel is easily decomposed under acidic conditions.

Means for Solving the Problems

[0007] The water-absorbing resin particles of the present invention contain a water-insoluble iron-containing substance in an amount of 0.03 ppm or more and 10 ppm or less in terms of iron atoms.

[0008] Preferably, the iron-containing substance contains at least one selected from the group consisting of iron, iron compounds, and iron alloys. The iron alloy may be stainless steel. Preferably, the iron-containing substance is attached to the surface of the water-absorbing resin particles.

[0009] The present invention also provides an absorber containing the water-absorbing resin particles. The present invention further provides an absorbent article including the absorber. The absorbent article may be a paper diaper.

Effects of the Invention

[0010] The present invention provides water-absorbing resin particles that can form a stable swollen gel upon water absorption, but the swollen gel is easily decomposed under acidic conditions, an absorber using the water-absorbing resin particles, and an absorbent article.

Brief Description of the Drawings

[0011]

Figure 1

Modes for Carrying Out the Invention

[0012] Hereinafter, embodiments of the present invention will be described in detail. However, the present invention is not limited to the following embodiments, and various modifications can be made within the scope of the gist thereof.

[0013] In this specification, "acrylic" and "methacrylic" are collectively referred to as "(meth)acrylic". Similarly, "acrylate" and "methacrylate" are referred to as "(meth)acrylate". In the numerical ranges described step by step in this specification, the upper limit or lower limit of a numerical range at a certain step can be arbitrarily combined with the upper limit or lower limit of a numerical range at another step. In the numerical ranges described in this specification, the upper limit or lower limit of the numerical range may be replaced with the values shown in the examples. "Water-soluble" means showing a solubility of 5% by mass or more in water at 25°C. The materials exemplified in this specification may be used alone or in combination of two or more. The content of each component in the composition means the total amount of the plurality of substances present in the composition when there are a plurality of substances corresponding to each component in the composition, unless otherwise specified. "Physiological saline" means an aqueous solution of 0.9% by mass sodium chloride. "Room temperature" means 25°C ± 2°C.

[0014] [Water-absorbing resin particles] The water-absorbing resin particles according to this embodiment have a water-insoluble iron-containing substance in an amount of 0.03 ppm or more and 10 ppm or less in terms of iron atoms. By having such a configuration, the water-absorbing resin particles according to this embodiment can form a stable swollen gel upon water absorption, but the swollen gel is easily decomposed under acidic conditions.

[0015] The water-absorbing resin particles are not particularly limited as long as they are composed of a resin having water-absorbing properties. The water-absorbing resin particles may, for example, contain a crosslinked polymer formed by polymerization of a monomer containing an ethylenically unsaturated monomer. The crosslinked polymer can have monomer units derived from the ethylenically unsaturated monomer. The water-absorbing resin particles can be produced, for example, by a method including a step of polymerizing a monomer containing an ethylenically unsaturated monomer. Examples of the polymerization method include inverse phase suspension polymerization method, aqueous solution polymerization method, bulk polymerization method, precipitation polymerization method, etc. Among these, from the viewpoint of ensuring good water absorption characteristics of the obtained water-absorbing resin particles and easy control of the polymerization reaction, the inverse phase suspension polymerization method or the aqueous solution polymerization method is preferable. In the following, as a method of polymerizing an ethylenically unsaturated monomer, the inverse phase suspension polymerization method will be described as an example.

[0016] The ethylenically unsaturated monomer may be water-soluble. Examples of the water-soluble ethylenically unsaturated monomer include (meth)acrylic acid and its salts, 2-(meth)acrylamido-2-methylpropanesulfonic acid and its salts, (meth)acrylamide, N,N-dimethyl(meth)acrylamide, 2-hydroxyethyl (meth)acrylate, N-methylol(meth)acrylamide, polyethylene glycol mono(meth)acrylate, N,N-diethylaminoethyl (meth)acrylate, N,N-diethylaminopropyl (meth)acrylate, diethylaminopropyl (meth)acrylamide, etc. When the ethylenically unsaturated monomer has an amino group, the amino group may be quaternized. The ethylenically unsaturated monomer may be used alone or in combination of two or more. Functional groups such as carboxyl groups and amino groups of the above-mentioned monomers can function as functional groups capable of crosslinking in the surface crosslinking step described later.

[0017] From the viewpoint of industrial availability, the ethylenically unsaturated monomer may contain at least one compound selected from the group consisting of (meth)acrylic acid and its salts, acrylamide, methacrylamide, and N,N-dimethylacrylamide. From the viewpoint of further enhancing the water absorption properties, it is more preferable that the ethylenically unsaturated monomer contains at least one compound selected from the group consisting of (meth)acrylic acid and its salts. That is, it is preferable that the water-absorbing resin particles have a structural unit derived from at least one selected from the group consisting of (meth)acrylic acid and its salts.

[0018] As the monomer for obtaining the water-absorbing resin particles, monomers other than the above-described ethylenically unsaturated monomer may be used. Such monomers can be used, for example, by mixing them with an aqueous solution containing the above-described ethylenically unsaturated monomer. The amount of the ethylenically unsaturated monomer used may be 70 to 100 mol% based on the total amount of the monomers. The proportion of (meth)acrylic acid and its salts may be 70 to 100 mol% based on the total amount of the monomers.

[0019] The ethylenically unsaturated monomer is usually preferably used as an aqueous solution. The concentration of the ethylenically unsaturated monomer in the aqueous solution containing the ethylenically unsaturated monomer (hereinafter simply referred to as "monomer aqueous solution") is preferably 20 mass% or more and the saturation concentration or less, more preferably 25 to 70 mass%, and still more preferably 30 to 55 mass%. Examples of the water used in the aqueous solution include tap water, distilled water, ion-exchanged water, and the like.

[0020] When the ethylenically unsaturated monomer has an acid group, the acid group may be neutralized with an alkaline neutralizing agent and then used in the polymerization reaction. The degree of neutralization with the alkaline neutralizing agent in the ethylenically unsaturated monomer may be, for example, 10 to 100 mol%, 50 to 90 mol%, or 60 to 80 mol% of the acidic groups in the ethylenically unsaturated monomer.

[0021] Examples of the alkaline neutralizing agent include alkali metal salts such as sodium hydroxide, sodium carbonate, sodium bicarbonate, potassium hydroxide, and potassium carbonate; and ammonia. The alkaline neutralizing agent may be used alone or in combination of two or more. The alkaline neutralizing agent may be used in an aqueous solution state to simplify the neutralization operation. Neutralization of the acid group of the ethylenically unsaturated monomer can be carried out, for example, by dropping an aqueous solution of sodium hydroxide, potassium hydroxide, etc. into the above-mentioned monomer aqueous solution and mixing them.

[0022] In the inverse suspension polymerization method, in the presence of a surfactant, an aqueous monomer solution can be dispersed in a hydrocarbon dispersion medium, and polymerization of an ethylenically unsaturated monomer can be carried out using a radical polymerization initiator or the like.

[0023] Examples of surfactants include nonionic surfactants, anionic surfactants, etc. Examples of nonionic surfactants include sorbitan fatty acid esters, (poly)glycerin fatty acid esters (where “(poly)” means both cases with and without the prefix “poly”; the same applies hereinafter), sucrose 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 alkyl phenyl ethers, polyoxyethylene castor oil, polyoxyethylene hydrogenated castor oil, alkyl allyl formaldehyde condensed polyoxyethylene ether, polyoxyethylene polyoxypropylene block copolymers, polyoxyethylene polyoxypropyl alkyl ethers, and polyethylene glycol fatty acid esters. Examples of anionic surfactants include fatty acid salts, alkylbenzene sulfonates, alkyl methyl taurates, polyoxyethylene alkyl phenyl ether sulfate esters, polyoxyethylene alkyl ether sulfonates, phosphoric acid esters of polyoxyethylene alkyl ethers, and phosphoric acid esters of polyoxyethylene alkyl allyl ethers. The surfactant may be used alone or in combination of two or more.

[0024] From the viewpoints that the state of the W / O type inverse suspension is good, it is easy to obtain water-absorbing resin particles having a suitable particle size, and it is industrially easy to obtain, the surfactant preferably contains at least one compound selected from the group consisting of sorbitan fatty acid esters, polyglycerin fatty acid esters, and sucrose fatty acid esters. From the viewpoints that it is easy to obtain an appropriate particle size distribution of the water-absorbing resin particles, and the water absorption characteristics of the water-absorbing resin particles and the performance of the absorber and absorbent article using the same are likely to be improved, the surfactant preferably contains sucrose fatty acid ester, and more preferably contains sucrose stearate.

[0025] From the viewpoints of obtaining sufficient effects with respect to the amount used and being economical, the amount of the surfactant used is preferably 0.05 to 10 parts by mass, more preferably 0.08 to 5 parts by mass, and still more preferably 0.1 to 3 parts by mass with respect to 100 parts by mass of the aqueous monomer solution.

[0026] In inverse suspension polymerization, a polymer dispersant may be used together with the above-described surfactant. Examples of the polymer dispersant 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 ethyl hydroxyethyl cellulose. The polymer dispersant may be used alone or in combination of two or more. From the viewpoint of excellent dispersion stability of the monomer, at least one selected from the group consisting of 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 is preferable.

[0027] From the viewpoints of obtaining sufficient effects with respect to the amount used and being economical, the amount of the polymer dispersant used is preferably 0.05 to 10 parts by mass, more preferably 0.08 to 5 parts by mass, and still more preferably 0.1 to 3 parts by mass with respect to 100 parts by mass of the aqueous monomer solution.

[0028] The hydrocarbon dispersion medium may contain at least one compound selected from the group consisting of linear aliphatic hydrocarbons having 6 to 8 carbon atoms and alicyclic hydrocarbons having 6 to 8 carbon atoms. Examples of the hydrocarbon dispersion medium include linear aliphatic hydrocarbons such as n - hexane, n - heptane, 2 - methylhexane, 3 - methylhexane, 2,3 - dimethylpentane, 3 - ethylpentane, n - octane; alicyclic hydrocarbons such as cyclohexane, methylcyclohexane, cyclopentane, methylcyclopentane, trans - 1,2 - dimethylcyclopentane, cis - 1,3 - dimethylcyclopentane, trans - 1,3 - dimethylcyclopentane; and aromatic hydrocarbons such as benzene, toluene, and xylene. The hydrocarbon dispersion medium may be used alone or in combination of two or more kinds.

[0029] From the viewpoint of easy industrial availability and stable quality, the hydrocarbon dispersion medium may contain at least one selected from the group consisting of n - heptane and cyclohexane. Also, from the same viewpoint, as the mixture of the above - mentioned hydrocarbon dispersion media, for example, commercially available Exxon Heptane (manufactured by ExxonMobil: containing 75 - 85% of n - heptane and isomeric hydrocarbons) may be used.

[0030] From the viewpoint of moderately removing the heat of polymerization and easily controlling the polymerization temperature, the amount of the hydrocarbon dispersion medium used is preferably 30 to 1000 parts by mass, more preferably 40 to 500 parts by mass, and still more preferably 50 to 400 parts by mass with respect to 100 parts by mass of the aqueous monomer solution. When the amount of the hydrocarbon dispersion medium used is 30 parts by mass or more, the polymerization temperature tends to be easily controlled. When the amount of the hydrocarbon dispersion medium used is 1000 parts by mass or less, the productivity of the polymerization tends to be improved and it is economical.

[0031] The radical polymerization initiator is preferably water-soluble. For example, persulfates such as potassium persulfate, ammonium persulfate, and sodium persulfate; peroxides such as methyl ethyl ketone peroxide, methyl isobutyl ketone peroxide, di-t-butyl peroxide, t-butyl cumyl peroxide, t-butyl peroxyacetate, t-butyl peroxyisobutyrate, t-butyl peroxypivalate, and hydrogen peroxide; azo compounds such as 2,2'-azobis(2-amidinopropane) dihydrochloride, 2,2'-azobis[2-(N-phenylamidinopropane)] dihydrochloride, 2,2'-azobis[2-(N-allylamidinopropane)] dihydrochloride, 2,2'-azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride, 2,2'-azobis{2-[1-(2-hydroxyethyl)-2-imidazolin-2-yl]propane} dihydrochloride, 2,2'-azobis{2-methyl-N-[1,1-bis(hydroxymethyl)-2-hydroxyethyl]propionamide}, 2,2'-azobis[2-methyl-N-(2-hydroxyethyl)-propionamide], and 4,4'-azobis(4-cyanovaleric acid) can be mentioned. The radical polymerization initiator may be used alone or in combination of two or more. As the radical polymerization initiator, at least one selected from the group consisting of potassium persulfate, ammonium persulfate, sodium persulfate, 2,2'-azobis(2-amidinopropane) dihydrochloride, 2,2'-azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride, and 2,2'-azobis{2-[1-(2-hydroxyethyl)-2-imidazolin-2-yl]propane} dihydrochloride is preferable, and sodium persulfate is more preferable.

[0032] The amount of the radical polymerization initiator used may be 0.05 to 10 millimoles per mole of the ethylenically unsaturated monomer. When the amount of the radical polymerization initiator used is 0.05 millimoles or more, the polymerization reaction does not require a long time and is efficient. When the amount of the radical polymerization initiator used is 10 millimoles or less, it is easy to suppress the occurrence of a rapid polymerization reaction.

[0033] The above radical polymerization initiator can also be used as a redox polymerization initiator in combination with a reducing agent such as L-ascorbic acid.

[0034] During the polymerization reaction, the aqueous monomer solution used for polymerization may contain a chain transfer agent. Examples of the chain transfer agent include hypophosphites, thiols, thiolic acids, secondary alcohols, and amines.

[0035] In order to control the particle size of the water-absorbing resin particles, the aqueous monomer solution used for polymerization may contain a thickening agent. Examples of the thickening agent include hydroxyethyl cellulose, hydroxypropyl cellulose, methyl cellulose, carboxymethyl cellulose, polyethylene glycol, polyacrylamide, polyethyleneimine, dextrin, sodium alginate, polyvinyl alcohol, polyvinyl pyrrolidone, and polyethylene oxide. Note that if the stirring speed during polymerization is the same, the higher the viscosity of the aqueous monomer solution, the greater the tendency for the median particle size of the resulting particles to be larger.

[0036] Crosslinking occurs by self-crosslinking during polymerization, but crosslinking may be promoted by using an internal crosslinking agent. When an internal crosslinking agent is used, it is easy to control the water absorption characteristics (such as water retention amount) of the water-absorbing resin particles. The internal crosslinking agent is usually added to the reaction solution during the polymerization reaction.

[0037] Examples of the internal crosslinking agent include: diesters or triesters of (meth)acrylic acid with polyols such as ethylene glycol, propylene glycol, trimethylolpropane, glycerin, polyoxyethylene glycol, polyoxypropylene glycol, and polyglycerin; unsaturated polyesters obtained by reacting the above polyols with unsaturated acids (such as maleic acid and fumaric acid); bis(meth)acrylamides such as N,N'-methylenebis(meth)acrylamide; diesters or triesters of (meth)acrylic acid obtained by reacting polyepoxides with (meth)acrylic acid; dicarbamyl esters of (meth)acrylic acid obtained by reacting polyisocyanates (such as tolylene diisocyanate and hexamethylene diisocyanate) with hydroxyethyl (meth)acrylate; compounds having two or more polymerizable unsaturated groups such as allylated starch, allylated cellulose, diallyl phthalate, N,N',N''-triallyl isocyanurate, and divinylbenzene; polyglycidyl compounds such as (poly)ethylene glycol diglycidyl ether, (poly)propylene glycol diglycidyl ether, (poly)glycerin diglycidyl ether, (poly)glycerin triglycidyl ether, (poly)propylene glycol polyglycidyl ether, and polyglycerol polyglycidyl ether; haloepoxy compounds such as epichlorohydrin, epibromohydrin, and α-methylepichlorohydrin; and compounds having two or more reactive functional groups such as isocyanate compounds (such as 2,4-tolylene diisocyanate and hexamethylene diisocyanate). The internal crosslinking agent may be used alone or in combination of two or more. As the internal crosslinking agent, polyglycidyl compounds are preferred, diglycidyl ether compounds are more preferred, and at least one selected from the group consisting of (poly)ethylene glycol diglycidyl ether, (poly)propylene glycol diglycidyl ether, and (poly)glycerin diglycidyl ether is even more preferred.

[0038] The amount of the internal crosslinking agent used is preferably 30 mmol or less, more preferably 0.01 to 10 mmol, still more preferably 0.012 to 5 mmol, particularly preferably 0.015 to 1 mmol, extremely preferably 0.02 to 0.1 mmol, and very preferably 0.025 to 0.08 mmol per mole of the ethylenically unsaturated monomer, from the viewpoint that the resulting polymer is moderately crosslinked to suppress its water-soluble property and it is easy to obtain a sufficient water absorption amount.

[0039] In a state where an aqueous phase containing an ethylenically unsaturated monomer, a radical polymerization initiator, and an internal crosslinking agent as required, and an oil phase containing a hydrocarbon dispersion medium, a surfactant, and a polymer dispersant as required are mixed, heating is performed with stirring, and inverse suspension polymerization can be carried out in a water-in-oil system.

[0040] When carrying out inverse suspension polymerization, an aqueous monomer solution containing an ethylenically unsaturated monomer is dispersed in a hydrocarbon dispersion medium in the presence of a surfactant (and a polymer dispersant as required). At this time, before starting the polymerization reaction, the addition time of the surfactant, the polymer dispersant, etc. may be either before or after the addition of the aqueous monomer solution.

[0041] Among them, from the viewpoint of easily reducing the amount of the hydrocarbon dispersion medium remaining in the obtained water-absorbing resin, it is preferable to disperse the surfactant after dispersing the aqueous monomer solution in the hydrocarbon dispersion medium in which the polymer dispersant is dispersed and then carry out the polymerization.

[0042] Inverse suspension polymerization can be carried out in one stage or in multiple stages of two or more stages. From the viewpoint of enhancing productivity, inverse suspension polymerization is preferably carried out in 2 to 3 stages.

[0043] When performing inverse suspension polymerization in multiple stages of two or more stages, after performing the first-stage inverse suspension polymerization, an ethylenically unsaturated monomer is added to and mixed with the reaction mixture obtained in the first-stage polymerization reaction, and the second-stage and subsequent inverse suspension polymerizations may be carried out in the same manner as in the first stage. In the inverse suspension polymerization in each stage after the second stage, in addition to the ethylenically unsaturated monomer, the above-mentioned radical polymerization initiator and / or internal cross-linking agent are added within the range of the molar ratio of each component to the above-mentioned ethylenically unsaturated monomer based on the amount of the ethylenically unsaturated monomer added during the inverse suspension polymerization in each stage after the second stage, and it is preferable to carry out inverse suspension polymerization. In addition, in the inverse suspension polymerization in each stage after the second stage, an internal cross-linking agent may be used as necessary. When using an internal cross-linking agent, it is preferable to carry out inverse suspension polymerization by adding it within the range of the molar ratio of each component to the above-mentioned ethylenically unsaturated monomer based on the amount of the ethylenically unsaturated monomer used in each stage.

[0044] The polymerization reaction can be carried out using various stirrers having stirring blades. As the stirring blades, flat blades, lattice blades, paddle blades, propeller blades, anchor blades, turbine blades, Faudler blades, ribbon blades, full zone blades, max blend blades, etc. can be used.

[0045] The temperature of the polymerization reaction varies depending on the radical polymerization initiator used, but from the viewpoints of promoting the polymerization rapidly, shortening the polymerization time to enhance the economy, and easily removing the polymerization heat to carry out the reaction smoothly, 20 to 150 °C is preferable, and 40 to 120 °C is more preferable. The reaction time is usually 0.5 to 4 hours. The completion of the polymerization reaction can be confirmed, for example, by the cessation of the temperature rise in the reaction system. Thereby, the polymer of the ethylenically unsaturated monomer is usually obtained in the state of a water-containing gel.

[0046] Polymer particles (e.g., polymer particles having structural units derived from ethylenically unsaturated monomers) are obtained by drying the resulting water-containing gel polymer to remove water. Examples of the drying method include (a) a method in which azeotropic distillation is carried out by heating from the outside in a state where the water-containing gel polymer is dispersed in a hydrocarbon dispersion medium, and the hydrocarbon dispersion medium is refluxed to remove water; (b) a method in which the water-containing gel polymer is taken out by decantation and dried under reduced pressure; (c) a method in which the water-containing gel polymer is filtered by a filter and dried under reduced pressure, etc. Among them, from the simplicity in the manufacturing process, it is preferable to use the method (a).

[0047] In the production of the water-absorbing resin particles, surface crosslinking of the surface portion (surface and near the surface) of the water-containing gel polymer may be carried out using a crosslinking agent in the drying step (water removal step) or a step subsequent thereto. By performing surface crosslinking, it is easy to control the water absorption characteristics and the like of the water-absorbing resin particles. The surface crosslinking is preferably carried out at a timing when the water-containing gel polymer has a specific water content rate. The timing of the surface crosslinking is preferably when the water content rate of the water-containing gel polymer is 5 to 35% by mass, more preferably when it is 10 to 35% by mass, and even more preferably when it is 15 to 30% by mass.

[0048] The water content rate (% by mass) of the water-containing gel polymer is calculated by the following formula. Water content rate = [Ww / (Ww + Ws)] × 100 Ww: The amount of water in the water-containing gel polymer obtained by adding the amount of water used as necessary when mixing an inorganic reducing agent, a surface crosslinking agent, etc. to the amount obtained by subtracting the amount of water discharged to the outside of the system by the drying step from the amount of water contained in the monomer aqueous solution before polymerization in the entire polymerization step. Ws: The solid content calculated from the charged amounts of materials such as ethylenically unsaturated monomers, crosslinking agents, initiators, etc. constituting the water-containing gel polymer.

[0049] Examples of the crosslinking agent (surface crosslinking agent) for performing surface crosslinking include compounds having two or more reactive functional groups. Examples of the surface crosslinking agent include polyols such as ethylene glycol, propylene glycol, 1,4-butanediol, trimethylolpropane, glycerin, polyoxyethylene glycol, polyoxypropylene glycol, and polyglycerin; polyglycidyl compounds such as (poly)ethylene glycol diglycidyl ether, (poly)glycerin diglycidyl ether, (poly)glycerin triglycidyl ether, trimethylolpropane triglycidyl ether, (poly)propylene glycol polyglycidyl ether, and (poly)glycerol polyglycidyl ether; haloepoxy compounds such as epichlorohydrin, epibromohydrin, and α-methylepichlorohydrin; isocyanate compounds such as 2,4-tolylene diisocyanate and hexamethylene diisocyanate; oxetane compounds such as 3-methyl-3-oxetanemethanol, 3-ethyl-3-oxetanemethanol, 3-butyl-3-oxetanemethanol, 3-methyl-3-oxetaneethanol, 3-ethyl-3-oxetaneethanol, and 3-butyl-3-oxetaneethanol; oxazoline compounds such as 1,2-ethylenebisoxazoline; carbonate compounds such as ethylene carbonate; and hydroxyalkylamide compounds such as bis[N,N-di(β-hydroxyethyl)]adipamide. The surface crosslinking agent may be used alone or in combination of two or more. As the surface crosslinking agent, polyglycidyl compounds are preferred, and at least one selected from the group consisting of (poly)ethylene glycol diglycidyl ether, (poly)glycerin diglycidyl ether, (poly)glycerin triglycidyl ether, (poly)propylene glycol polyglycidyl ether, and polyglycerol polyglycidyl ether is more preferred.

[0050] From the viewpoint of easily obtaining suitable water absorption characteristics, the amount of the surface crosslinking agent used is preferably 0.01 to 20 mmol, more preferably 0.05 to 10 mmol, still more preferably 0.1 to 5 mmol, particularly preferably 0.15 to 1 mmol, and extremely preferably 0.2 to 0.5 mmol, per 1 mol of the ethylenically unsaturated monomer used in the polymerization.

[0051] After surface crosslinking, polymer particles that are surface-crosslinked dry products can be obtained by distilling off water and a hydrocarbon dispersion medium by a known method, drying under heating and reduced pressure, and the like.

[0052] The water-absorbing resin particles may further contain various additional components selected from, for example, a gel stabilizer, a metal chelating agent (ethylenediaminetetraacetic acid and its salts, diethylenetriaminepentaacetic acid and its salts, such as sodium diethylenetriaminepentaacetate, etc.), a fluidity improver (lubricant), and the like. The additional components can be arranged inside the polymer particles, on the surface of the polymer particles, or both. When the water-absorbing resin particles contain a metal chelating agent, the gel stability tends to be further enhanced. Since trace transition metal ions contained in urine and the like are considered to promote the deterioration of the swollen gel, it is considered that the deterioration of the swollen gel can be suppressed and gel stability can be obtained by capturing the transition metal ions with a metal chelating agent. The additional component may be a fluidity improver (lubricant), and the fluidity improver may contain inorganic particles. Examples of the inorganic particles include silica particles such as amorphous silica.

[0053] The shape of the water-absorbing resin particles is not particularly limited, and may be, for example, substantially spherical, crushed, or granular, or may be a shape in which primary particles having these shapes are aggregated.

[0054] The median particle diameter of the water-absorbing resin particles may be 100 to 800 μm, 150 to 700 μm, 200 to 600 μm, or 250 to 500 μm. The median particle diameter can be measured by the following method. Combine JIS standard sieves from the top in the order of a sieve with an aperture of 600 μm, a sieve with an aperture of 500 μm, a sieve with an aperture of 425 μm, a sieve with an aperture of 300 μm, a sieve with an aperture of 250 μm, a sieve with an aperture of 180 μm, a sieve with an aperture of 150 μm, and a receiving tray. Put 50 g of the water-absorbing resin particles into the topmost combined sieve and classify them according to JIS Z 8815 (1994) using a rotary tap shaker (manufactured by Iida Seisakusho Co., Ltd.). After classification, calculate the mass percentage of the particles remaining on each sieve with respect to the total amount to obtain the particle size distribution. Regarding this particle size distribution, plot the relationship between the aperture of the sieve and the integrated value of the mass percentage of the particles remaining on the sieve on logarithmic probability paper by integrating the sieves from the larger particle diameter side in order. By connecting the plots on the probability paper with a straight line, obtain the particle diameter corresponding to an integrated mass percentage of 50 mass% as the median particle diameter.

[0055] From the viewpoint of obtaining excellent gel strength, the water absorption of the water-absorbing resin particles in physiological saline may be 10 g / g or more, 20 g / g or more, 30 g / g or more, 40 g / g or more, 50 g / g or more, or 60 g / g or more, and may be 80 g / g or less, 70 g / g or less, or 60 g / g or less. From these viewpoints, the water absorption of the water-absorbing resin in physiological saline may be 10 to 80 g / g. The water absorption of the water-absorbing resin particles in physiological saline can be measured by the following procedure.

[0056] Mix 500 g of physiological saline and 2.0 g of the water-absorbing resin particles and stir at room temperature (25 °C) for 60 minutes. Filter the above mixture using a JIS Z 8801-1 standard sieve with an aperture of 75 μm having a mass Wa (g). Leave the filtrate on the sieve for 30 minutes with the sieve tilted at an inclination angle of about 30 degrees with respect to the horizontal. Measure the total mass Wb (g) of the water-absorbed water-absorbing resin particles and the sieve, and obtain the water absorption by the following formula. Water absorption of physiological saline = (Wb - Wa) / 2.0

[0057] (Iron-containing substance) The water-absorbing resin particles according to this embodiment have a water-insoluble iron-containing substance in an amount of 0.03 ppm or more and 10 ppm or less in terms of iron atoms. The water-insoluble iron-containing substance dissolves under acidic conditions and promotes the decomposition of the swollen gel, facilitating the regeneration treatment of the swollen gel. The content of the water-insoluble iron-containing substance in the water-absorbing resin particles may be 0.05 ppm or more, 0.1 ppm or more, 0.3 ppm or more, or 0.5 ppm or more in terms of iron atoms, and may also be 8 ppm or less, 5 ppm or less, 3 ppm or less, 2 ppm or less, or 1 ppm or less. The amount of the iron-containing substance in the water-absorbing resin particles can be analyzed by methods such as flame atomic absorption spectrometry and inductively coupled plasma optical emission spectrometry.

[0058] When the amount of the iron-containing substance exceeds 10 ppm in terms of iron atoms, the color of the water-absorbing resin particles changes from white, which is not preferable. For example, when the water-absorbing resin particles have 20 ppm of iron oxide in terms of iron atoms, they appear faintly pink. Generally, when the water-absorbing resin particles in a disposable diaper are colored, the colored water-absorbing resin particles look like spots from the inner surface of the pure white disposable diaper, so such disposable diapers tend to be shunned by general consumers in terms of appearance.

[0059] The solubility of the water-insoluble iron-containing substance in 100 g of water may be less than 1 g at 25°C. The iron-containing substance may contain at least one selected from the group consisting of iron, iron compounds, and iron alloys. Examples of the iron compounds include iron(II) oxide, iron(III) oxide, iron(II) oxalate, iron(II) hydroxide, iron(III) hydroxide, iron(II) carbonate, and iron(III) fluoride. Examples of the iron alloys include stainless steel, chromium steel, chromium molybdenum steel, nickel chromium steel, nickel chromium molybdenum steel, and manganese molybdenum steel.

[0060] Since the iron-containing substance adheres to the surface of the water-absorbing resin particles rather than inside them, the effects of the present invention can be more effectively exhibited. The iron-containing substance does not necessarily have to be chemically bonded to the water-absorbing resin particles. The water-insoluble iron-containing substance may be derived from a production apparatus or the like used in the production process of the water-absorbing resin particles, and the amount of the water-insoluble iron-containing substance may be adjusted in the step of producing the water-absorbing resin particles. Further, the water-absorbing resin particles according to the present embodiment may be prepared by mixing the produced water-absorbing resin particles and a predetermined amount of the water-insoluble iron-containing substance. The size of the iron-containing substance may be, for example, 50 μm or less, 48 μm or less, or 46 μm or less, and may also be 0.5 μm or more, 1 μm or more, or 2 μm or more.

[0061] [Absorbent] The water-absorbing resin particles according to the present embodiment can be suitably used for an absorbent. The absorbent contains water-absorbing resin particles having 0.03 ppm or more and 10 ppm or less of a water-insoluble iron-containing substance in terms of iron atoms. The absorbent according to the present embodiment can contain a fibrous material, and for example, is a mixture containing the water-absorbing resin particles and the fibrous material according to the present embodiment. As the configuration of the absorbent, for example, a configuration in which the water-absorbing resin particles and the fibrous material are uniformly mixed may be used, or a configuration in which the water-absorbing resin particles are sandwiched between fibrous materials formed in a sheet shape or a layer shape may be used, or other configurations may be used.

[0062] Examples of the fibrous material include finely pulverized wood pulp; cotton; cotton linter; rayon; cellulose-based fibers such as cellulose acetate; synthetic fibers such as polyamide, polyester, and polyolefin; and mixtures of these fibers. The fibrous material may be used alone or in combination of two or more. As the fibrous material, hydrophilic fibers can be used. From the viewpoint of a material with less environmental impact as a disposable diaper material, it is preferable that the fibrous material used in the absorbent is wood pulp derived from natural products.

[0063] The mass ratio of the water-absorbing resin particles in the absorber may be 40% by mass or more, 45% by mass or more, 50% by mass or more, 55% by mass or more, 60% by mass or more, 65% by mass or more, or 70% by mass based on the total of the water-absorbing resin particles and the fibrous material. Further, the mass ratio of the water-absorbing resin particles in the absorber may be 100% by mass or less, 95% by mass or less, 90% by mass or less, 85% by mass or less, or 80% by mass based on the total of the water-absorbing resin particles and the fibrous material. The mass ratio of the water-absorbing resin particles in the absorber may be 40 to 100% by mass, 50 to 95% by mass, or 60 to 90% by mass based on the total of the water-absorbing resin particles and the fibrous material.

[0064] In order to enhance the shape retention property of the absorber before and during use, the fibers may be adhered to each other by adding an adhesive binder to the fibrous material. Examples of the adhesive binder include heat-fusible synthetic fibers, hot melt adhesives, and adhesive emulsions. The adhesive binder may be used alone or in combination of two or more kinds.

[0065] Examples of the heat-fusible synthetic fibers include all-melting type binders such as polyethylene, polypropylene, and ethylene-propylene copolymers; non-all-melting type binders having a side-by-side or core-sheath structure of polypropylene and polyethylene, etc. In the above-mentioned non-all-melting type binder, only the polyethylene portion can be heat-fused.

[0066] Examples of the hot melt adhesive include mixtures of base polymers such as ethylene-vinyl acetate copolymer, styrene-isoprene-styrene block copolymer, styrene-butadiene-styrene block copolymer, styrene-ethylene-butylene-styrene block copolymer, styrene-ethylene-propylene-styrene block copolymer, and amorphous polypropylene, and tackifiers, plasticizers, antioxidants, etc.

[0067] Examples of the cohesive emulsion include polymers of at least one monomer selected from the group consisting of methyl methacrylate, styrene, acrylonitrile, 2-ethylhexyl acrylate, butyl acrylate, butadiene, ethylene, and vinyl acetate.

[0068] The absorber according to this embodiment may contain a deodorant, an antibacterial agent, a fragrance, etc.

[0069] The absorber according to this embodiment may further contain additives such as inorganic powder (e.g., amorphous silica), deodorant, pigment, dye, antibacterial agent, fragrance, adhesive, etc. Various functions can be imparted to the absorber by these additives. When the water-absorbing resin particles contain inorganic particles, the absorber may contain inorganic powder separately from the inorganic particles in the water-absorbing resin particles. Examples of the inorganic powder include silicon dioxide, zeolite, kaolin, clay, etc.

[0070] The shape of the absorber according to this embodiment is not particularly limited, and may be, for example, in the form of a sheet. The thickness of the absorber (e.g., the thickness of the sheet-shaped absorber) may be, for example, 0.1 to 20 mm, or 0.3 to 15 mm.

[0071] [Absorbent article] The absorbent article according to this embodiment includes the absorber according to this embodiment. The absorbent article according to this embodiment may include, in addition to the absorber, for example, a core wrap, a liquid-permeable topsheet, and a liquid-impermeable backsheet. The core wrap is for maintaining the shape of the absorber. The liquid-permeable topsheet is disposed on the outermost side where the liquid to be absorbed enters. The liquid-impermeable backsheet is disposed on the outermost side opposite to the side where the liquid to be absorbed enters.

[0072] Examples of absorbent articles include diapers (e.g., paper diapers), training pants, incontinence pads, sanitary products (sanitary napkins, tampons, etc.), sweat pads, pet sheets, simple toilet members, animal excrement treatment materials, etc.

[0073] According to this embodiment, by using water-insoluble iron-containing substances in an amount of 0.03 ppm or more and 10 ppm or less in terms of iron atoms in the water-absorbing resin particles, used water-absorbing articles can be easily decomposed and treated with an acidic aqueous solution. The water-absorbing resin particles do not deteriorate under normal use conditions and are decomposed when in an acidic atmosphere under the condition of pulp recycling. By using the water-absorbing resin particles according to this embodiment in disposable diapers, when performing pulp regeneration from used disposable diapers, the gel of the swollen water-absorbing resin can be solubilized and easily separated, so that the cost of the pulp regeneration process is reduced, which is industrially useful.

Examples

[0074] Hereinafter, the content of the present invention will be described in more detail using examples and comparative examples, but the present invention is not limited to the following examples.

[0075] [Production Example 1] A round-bottom cylindrical separable flask with an inner diameter of 11 cm and an internal volume of 2 L equipped with a reflux condenser, a dropping funnel, a nitrogen gas inlet tube, and a stirrer (a stirring blade having two stages of 4 inclined paddle blades with a blade diameter of 5 cm) was prepared. 293 g of n-heptane was added to this separable flask as a hydrocarbon dispersion medium, and 0.736 g of maleic anhydride-modified ethylene-propylene copolymer (manufactured by Mitsui Chemicals, Inc., Hiwax 1105A) was added as a polymer-based dispersant to obtain a mixture. While stirring this mixture with a stirrer, the temperature was raised to 80°C to dissolve the dispersant in n-heptane, and then the mixture was cooled to 50°C.

[0076] Next, 92.0 g (1.03 mol of acrylic acid) of an aqueous acrylic acid solution containing 80.5% by mass of a water-soluble ethylenically unsaturated monomer was added to a beaker with an inner volume of 300 mL. Subsequently, while cooling from the outside, 147.7 g of a 20.9% by mass aqueous sodium hydroxide solution was dropped into the beaker to achieve 75 mol% neutralization. Thereafter, 0.092 g of hydroxyethyl cellulose (manufactured by Sumitomo Seika Chemicals Co., Ltd., HEC AW-15F) as a thickener, 0.0736 g (0.272 mmol) of potassium persulfate as a water-soluble radical polymerization initiator, and 0.010 g (0.057 mmol) of ethylene glycol diglycidyl ether as an internal crosslinking agent were added and dissolved to prepare the first-stage aqueous liquid.

[0077] Then, while stirring at a rotation speed of 550 rpm of the stirrer, the above-mentioned first-stage aqueous liquid was added to the above-mentioned separable flask and stirred for 10 minutes. Thereafter, a surfactant solution obtained by heating and dissolving 0.736 g of sucrose stearate (surfactant, manufactured by Mitsubishi Chemical Foods Co., Ltd., Ryoto Sugar Ester S-370, HLB value: 3) in 6.62 g of n-heptane was added to the above-mentioned separable flask. Then, while stirring at a rotation speed of 425 rpm of the stirrer, the system was sufficiently purged with nitrogen. Thereafter, the flask was immersed in a water bath at 70 °C and heated, and polymerization was carried out for 60 minutes to obtain the first-stage polymerization slurry liquid.

[0078] Next, 128.8 g (1.43 mol of acrylic acid) of an aqueous acrylic acid solution containing 80.5% by mass of a water-soluble ethylenically unsaturated monomer was added to another beaker with an inner volume of 500 mL. Subsequently, while cooling from the outside, 159.0 g of a 27% by mass aqueous sodium hydroxide solution was dropped into the beaker to achieve 75 mol% neutralization. Thereafter, 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 then dissolved to prepare the second-stage aqueous liquid.

[0079] Next, while stirring at a rotation speed of 1000 rpm of the stirrer, after cooling the inside of the above-mentioned separable flask to 25°C, the total amount of the above-mentioned second-stage aqueous liquid was added to the above-mentioned first-stage polymerization slurry liquid. Subsequently, after replacing the inside of the system with nitrogen for 30 minutes, the flask was immersed again in a water bath at 70°C and heated, and a polymerization reaction was carried out for 60 minutes to obtain a second-stage water-containing gel-like polymer.

[0080] To 0.589 g of an aqueous solution of 45 mass% of sodium diethylenetriaminepentaacetate was added to the above-mentioned second-stage water-containing gel-like polymer with stirring. Thereafter, the temperature of the reaction solution was raised in an oil bath at 125°C, and 248.4 g of water was withdrawn from the system while refluxing n-heptane by azeotropic distillation of n-heptane and water. Then, 4.42 g of an aqueous solution of 2 mass% of ethylene glycol diglycidyl ether (ethylene glycol diglycidyl ether: 0.507 mmol 0.507 mmol) as a surface cross-linking agent was added to the flask, and then held at 83°C for 2 hours.

[0081] Thereafter, the polymer particles (dry product) were obtained by evaporating and drying n-heptane at 125°C. After passing these polymer particles through a sieve with an opening of 850 μm, 0.1 mass% of amorphous silica (manufactured by Oriental Silica Corporation, Tokusil NP-S) was mixed with the polymer particles based on the total mass of the polymer particles to obtain 228.0 g of water-absorbing resin particles A containing amorphous silica. The median particle diameter of the water-absorbing resin particles A was 342 μm, and the water absorption amount of physiological saline was 60 g / g.

[0082] [Example 1] Into a sealed container with a volume of 1 L, 100 g of water-absorbing resin particles A and 0.0010 g of iron(III) oxide (particle size: 5 μm or less) were placed and mixed with a cross rotary mixer (Meiwafosis Co., Ltd., CM-3 type) for 30 minutes to obtain water-absorbing resin particles B. In the particle size distribution of the water-absorbing resin particles, almost no change was observed due to the addition of iron(III) oxide.

[0083] [Example 2] In a sealed container with a capacity of 1 L, 90 g of water-absorbing resin particles A and 10 g of water-absorbing resin particles B were placed and mixed for 30 minutes with a cross rotary mixer to obtain water-absorbing resin particles C.

[0084] [Example 3] In a sealed container with a capacity of 1 L, 99 g of water-absorbing resin particles A and 1 g of water-absorbing resin particles B were placed and mixed for 30 minutes with a cross rotary mixer to obtain water-absorbing resin particles D.

[0085] [Example 4] In a sealed container with a capacity of 1 L, 100 g of water-absorbing resin particles A and 0.0010 g of iron powder (particle size: 45 μm or less) were placed and mixed for 30 minutes with a cross rotary mixer to obtain water-absorbing resin particles E. Almost no change was observed in the particle size distribution of the water-absorbing resin particles due to the addition of the iron powder.

[0086] [Example 5] In a sealed container with a capacity of 1 L, 90 g of water-absorbing resin particles A and 10 g of water-absorbing resin particles E were placed and mixed for 30 minutes with a cross rotary mixer to obtain water-absorbing resin particles F.

[0087] [Example 6] In a sealed container with a capacity of 1 L, 99 g of water-absorbing resin particles A and 1 g of water-absorbing resin particles E were placed and mixed for 30 minutes with a cross rotary mixer to obtain water-absorbing resin particles G.

[0088] [Comparative Example 1] In a sealed container with a capacity of 1 L, 99 g of water-absorbing resin particles A and 1 g of water-absorbing resin particles F were placed and mixed for 30 minutes with a cross rotary mixer to obtain water-absorbing resin particles H.

[0089] [Comparative Example 2] In a sealed container with a capacity of 1 L, 100 g of water-absorbing resin particles A and 0.0010 g of iron(II) sulfate were placed and mixed for 30 minutes with a cross rotary mixer to obtain water-absorbing resin particles I. Almost no change was observed in the particle size distribution of the water-absorbing resin particles due to the addition of the iron(II) sulfate.

[0090] [Reference Example] Evaluation was carried out according to various test methods using the water-absorbing resin particles A.

[0091] [Gel Stability Evaluation] The stability of the swollen gels of the water-absorbing resin particles A to I was evaluated. The results are shown in Table 1.

[0092] (Preparation of Gel) Weighed 49.0 g of physiological saline into a beaker with an inner volume of 100 mL, added a magnetic stirrer bar (8 mm φ × 30 mm without ring), and placed it on a magnetic stirrer (manufactured by iuchi: HS-30D). Subsequently, the magnetic stirrer bar was adjusted to rotate at 600 revolutions per minute. Next, 1.0 g of the water-absorbing resin particles was added into the stirring beaker, and stirring was continued until the swirling disappeared and the liquid surface became horizontal to prepare a swollen gel. Immediately after preparing the swollen gel, the beaker containing the swollen gel was covered with wrap (manufactured by Mitsubishi Chemical Corporation, Dialwrap).

[0093] (Measurement of Gel Strength) The gel strength of the swollen gel was measured using an apparatus having the measurement principle shown in Fig. 1. The apparatus shown in Fig. 1 is composed of a support part 50, a movable platen 60, a drive part 70 for driving the movable platen 60, and a measurement part 80. In the support part 50, a gantry 53 is fixed to the upper part of a support column 52 erected on a support base 51. A movable platen 60 is attached to the support column 52 so as to move up and down. The movable platen 60 can mount a measurement sample (swollen gel) 61. A pulse motor 71 is mounted on the gantry 53, and by rotating a pulley 72, the movable platen 60 is moved up and down via a wire 73.

[0094] In the measurement part 80, a pressure-sensitive shaft 84 is attached to a load cell 81 for measuring the strain generated by deformation via a precision spring 82 and a continuous shaft 83. The pressure-sensitive shaft 84 has a disk at its tip. The diameter of the disk can be changed according to the measurement conditions. A weight 90 can be mounted on the upper part of the pressure-sensitive shaft 84.

[0095] The operating principle of the apparatus for measuring gel strength is as follows. A precision spring 82 is fixed to the upper load cell 81 (stress detector), and a pressure-sensitive shaft 84 with a disk is connected to the lower part and vertically suspended with a predetermined weight 90 placed thereon. The movable platen 60 on which the measurement sample 61 is placed rises at a constant speed by the rotation of the pulse motor 71. A constant-speed load is applied to the measurement sample 61 via the precision spring 82, the strain generated by the deformation is measured by the load cell 81, and the hardness is measured and calculated.

[0096] The gel strength was measured using a card meter (manufactured by ITECNO ENGINEERING CO., LTD.: Card Meter Mini ME-600) with a disk of the pressure-sensitive shaft of 16 mmφ, a load of 400 g, a speed of 7 seconds / inch, and in the viscous mode.

[0097] After leaving the swollen gel at room temperature of 25 ± 2°C for 30 minutes from the preparation, the gel strength (initial gel strength value) was measured by the above method. Also, in order to evaluate the stability of the swollen gel, the beaker containing the swollen gel was placed in a thermo-hygrostat at 40°C and 60% RH and left standing for 2 days, and then the gel strength (gel strength value after 2 days) of the swollen gel returned to 25°C was measured. Each gel strength was measured 3 times, and the average value was used.

[0098] [Evaluation of Gel Deterioration by Acidic Aqueous Solution] For the water-absorbing resin particles A to I, the deterioration of the swollen gel by the acidic aqueous solution was evaluated. The results are shown in Table 1.

[0099] (Preparation of Artificial Urine) Each component was added to ion-exchanged water so that urea was 2.0% by mass, sodium chloride was 0.8% by mass, calcium chloride dihydrate was 0.03% by mass, magnesium sulfate heptahydrate was 0.08% by mass, and L(+)-ascorbic acid was 0.02% by mass to prepare artificial urine.

[0100] (Preparation of Filtrate) 1.0 g of the water-absorbing resin particles were put into a beaker containing 29.0 g of artificial urine and stirred at 400 rpm for 15 seconds to prepare a swollen gel. Next, the swollen gel was added to a beaker containing 200.0 g of ion-exchanged water and 20.0 g of 1N-HCl and stirred at 600 rpm for 1 hour. After stirring, the total amount of the liquid containing the gel was transferred to a heat-resistant bottle and sealed. Then, the heat-resistant bottle was placed in a hot air dryer at 90 °C and left standing for 1 week. After 1 week, the heat-resistant bottle was cooled to 25 °C, the gel was separated with a 75-μm sieve, and the filtrate was collected.

[0101] (Calculation of the rate of increase in deteriorated elution) Using burettes (manufactured by Miyahara Measuring Instrument Co., Ltd., capacity 25 mL, 1 scale 0.1 mL) filled with 0.1N-HCl aqueous solution and 0.1N-NaOH aqueous solution respectively, the filtrate was titrated to evaluate the deterioration of the swollen gel. The deterioration of the swollen gel was relatively compared with the titration value of the filtrate of the water-absorbing resin particles in the example or comparative example, taking the titration value of the filtrate of the reference example using water-absorbing resin particles A as a blank. The titration was carried out according to the following procedure.

[0102] (Titration) Using a pH meter (HORIBA pH / 10N METER F-24) and a pH electrode (HORIBA pH composite electrode, model 6261), pH calibration was performed at three points with pH standard solutions (7, 9, 4). Blank titration was carried out while stirring 50 g of the filtrate. 0.1N-NaOH aqueous solution was dropped until the pH reached 10, the dropped amount was read from the scale of the burette, and the dropped amount was calculated from the difference from the scale before dropping. Next, 0.1N-HCl aqueous solution was dropped until the pH reached 2.7, the dropped amount was read from the scale of the burette, and the dropped amount was calculated from the difference from the scale before dropping.

[0103] From the dropped amount of 0.1N-NaOH aqueous solution until the pH of the filtrate reached 10, the increased amount (mass of the increased acrylic acid unit) of the acrylic acid unit eluted into the filtrate due to the decomposition of the swollen gel was calculated from the following formula. Molar amount (mmol) of increased acrylic acid units = [Drop amount (mL) of NaOH aqueous solution in the example or comparative example - Drop amount (mL) of NaOH aqueous solution in the reference example] × NaOH normality (0.1 N) Mass (mg) of increased acrylic acid units = Molar amount (mmol) of increased acrylic acid units × Molecular weight of acrylic acid (72 g / mol) × 250 / 50

[0104] From the drop amount of 0.1 N - HCl aqueous solution until the pH of the filtrate reached 2.7, the increase amount (mass of increased sodium acrylate units) of sodium acrylate units eluted into the filtrate due to the decomposition of the swollen gel was calculated from the following formula. Molar amount (mmol) of increased sodium acrylate units = [Drop amount (mL) of HCl aqueous solution in the example or comparative example - Drop amount (mL) of HCl aqueous solution in the reference example] × HCl normality (0.1 N) - Molar amount (mmol) of increased acrylic acid units Mass (mg) of increased sodium acrylate units = Molar amount (mmol) of increased sodium acrylate units × Molecular weight of sodium acrylate (94 g / mol) × 250 / 50

[0105] The deterioration elution increase rate per 1 g of water-absorbing resin particles was calculated from the following formula. Deterioration elution increase rate (%) = [Mass (mg) of increased acrylic acid units + Mass (mg) of increased sodium acrylate units] / 1000 / Mass of water-absorbing resin particles (1.0 g) × 100

[0106]

Table 1

[0107] In the water-absorbing resin particles of the example, a swollen gel having a stable gel strength was formed even after 2 days from water absorption, and the swollen gel could be easily decomposed by an acidic aqueous solution.

Explanation of symbols

[0108] 50... Support section, 51... Support stand, 52... Support column, 53... Mounting base, 60... Movable platen, 61... Measurement sample, 70... Driving section, 71... Pulse motor, 72... Pulley, 73... Wire, 80... Measuring section, 81... Load cell, 82... Precision spring, 83... Coupling shaft, 84... Pressure-sensitive shaft, 90... Weight.

Claims

1. A water-absorbing resin particle having an iron content of 0.3 ppm or more and 8 ppm or less in terms of iron atoms, and The iron-containing substance is adhered to the surface of the water-absorbing resin particle.

2. The water-absorbing resin particle according to claim 1, wherein the iron-containing substance contains at least one selected from the group consisting of iron, iron compounds, and iron alloys.

3. The water-absorbing resin particle according to claim 2, wherein the iron alloy is stainless steel.

4. The water-absorbing resin particle according to claim 2, wherein the iron compound is at least one selected from the group consisting of iron(II) oxide, iron(III) oxide, iron(II) oxalate, iron(II) hydroxide, iron(III) hydroxide, iron(II) carbonate, and iron(III) fluoride.

5. An absorber containing the water-absorbing resin particle according to any one of claims 1 to 4.

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

7. The absorbent article according to claim 6, which is a paper diaper.

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