JP7733571B2 - Water-absorbing resin particles - Google Patents

Description

The present invention relates to water-absorbent resin particles.

Water-absorbent resins are used in the field of sanitary products, specifically as materials for absorbents contained in absorbent articles such as diapers (for example, Patent Document 1).

Japanese Patent Application Publication No. 5-184622

It is desirable for absorbent articles such as diapers to be able to absorb as much liquid as possible when worn and to prevent liquid leakage to the outside of the absorbent article.

The present invention aims to provide water-absorbent resin particles that provide absorbent articles that have a high liquid absorption capacity when worn by a user and that suppress liquid leakage.

The water-absorbent resin particles of the present invention have a gel settling rate of 45 seconds or more, measured by a method including the following (A), (B), and (C) in this order:
(A) 300 g of saline is stirred in a 300 ml beaker at 600 rpm using a magnetic stirrer.
(B) 0.5 g of water-absorbent aqueous resin particles is added to the physiological saline solution.
(C) The stirring bar is stopped 15 minutes after the addition of the physiological saline solution, and the time from the time the stirring bar is stopped until the gel formed by the swelling of the water-absorbent resin particles in the water is completely precipitated is defined as the gel settling rate.

It is preferable that the above-mentioned water-absorbent resin particles have a bulk density of 0.5 g/ml or more.

It is preferable that the above-mentioned water-absorbent resin particles have a saline water retention capacity of 20 g/g or more.

The present invention also provides an absorbent body containing the above-mentioned water-absorbent resin particles.

The present invention also provides an absorbent article comprising the above-mentioned absorbent body.

The absorbent article may be a diaper.

The present invention also provides a method for producing water-absorbent resin particles, comprising selecting water-absorbent resin particles having a gel sedimentation rate of 45 seconds or more, measured by a method comprising the following (A), (B), and (C) in this order:
(A) 300 g of saline is stirred in a 300 ml beaker at 600 rpm using a magnetic stirrer.
(B) 0.5 g of water-absorbent aqueous resin particles is added to the physiological saline solution.
(C) The stirring bar is stopped 15 minutes after the addition of the physiological saline solution, and the time from the time the stirring bar is stopped until the gel formed by the swelling of the water-absorbent resin particles in the water is completely precipitated is defined as the gel settling rate.

By using the water-absorbent resin particles obtained by the above manufacturing method, it is possible to obtain absorbent articles that have a high liquid absorption capacity and suppress the occurrence of liquid leakage.

The present invention also provides a method for suppressing liquid leakage from an absorbent article containing the water-absorbent resin particles, the method comprising adjusting a gel settling rate of the water-absorbent resin particles, measured by a method comprising the following (A), (B), and (C) in this order, to 45 seconds or more:
(A) 300 g of saline is stirred in a 300 ml beaker at 600 rpm using a magnetic stirrer.
(B) 0.5 g of water-absorbent aqueous resin particles is added to the physiological saline solution.
(C) The stirring bar is stopped 15 minutes after the addition of the physiological saline solution, and the time from the time the stirring bar is stopped until the gel formed by the swelling of the water-absorbent resin particles in the water is completely precipitated is defined as the gel settling rate.

The present invention provides water-absorbent resin particles that provide absorbent articles that have a high liquid absorption capacity when worn by a user and suppress liquid leakage.

FIG. 1 is a cross-sectional view showing an example of an absorbent article. FIG. 1 is a plan view showing the general shape of an agitating blade (a flat blade having a slit in the flat plate portion). FIG. 1 is a schematic diagram showing a method for measuring non-pressure DW.

The following describes in detail an embodiment of the present invention. However, the present invention is not limited to the following embodiment and can be implemented in various modifications within the scope of its essence.

In this specification, "acrylic" and "methacrylic" are collectively referred to as "(meth)acrylic." Similarly, "acrylate" and "methacrylate" are also referred to as "(meth)acrylate." "(Poly)" refers to both cases with and without the prefix "poly." In the numerical ranges described in this specification, the upper or lower limit of a certain range can be arbitrarily combined with the upper or lower limit of another range. In the numerical ranges described in this specification, the upper or lower limit of that range may be replaced with a value shown in the examples. "Water-soluble" refers to 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. When multiple substances corresponding to each component are present in the composition, the content of each component in the composition refers to the total amount of those substances present in the composition, unless otherwise specified. "Saline" refers to a 0.9% by mass aqueous solution of sodium chloride.

The water-absorbent resin particles according to this embodiment have a gel settling rate of 45 seconds or more, measured by a method including the following (A), (B), and (C) in this order:
(A) 300 g of saline is stirred in a 300 ml beaker at 600 rpm using a magnetic stirrer.
(B) 0.5 g of water-absorbent aqueous resin particles is added to the physiological saline solution.
(C) The stirring bar is stopped 15 minutes after the addition of the physiological saline solution, and the time from the time the stirring bar is stopped until the gel formed by the swelling of the water-absorbent resin particles in the water is completely precipitated is defined as the gel settling rate.

When water-absorbent resin particles having a gel settling rate of 45 seconds or more according to this embodiment are used in an absorbent article, the absorbent article's liquid absorption capacity is improved, making it possible to suppress liquid leakage to the outside of the absorbent article from the back, crotch, etc. Although the reason for this effect is unclear, the inventors speculate as follows. However, the present invention is not limited to the following mechanism. Water-absorbent resin particles tend to settle more slowly in a swollen state (gel) after absorbing water than in a dry powder state. Even within a single water-absorbent resin particle, if there are areas (dry powder-like areas) that are not sufficiently swollen due to uneven swelling, the settling rate of the particle is thought to be faster. Water-absorbent resin particles having a gel settling rate equal to or greater than a specific value are thought to have more uniform intra-particle crosslinking, thereby enabling more uniform swelling within the particle. By having a gel settling rate of 45 seconds or more, the water-absorbent resin particles according to this embodiment can absorb water more evenly within the particle when used in an absorbent article, improving the absorbent capacity of the absorbent article and suppressing liquid leakage from the absorbent article.

It is preferable to measure the gel settling rate five or more times and use the average value excluding the minimum and maximum values as the gel settling rate. The gel settling rate may be, for example, 100 seconds or less, 90 seconds or less, or 80 seconds or less.

The water-absorbent resin particles according to this embodiment may have a bulk density of, for example, 0.5 g/ml or more, 0.6 g/ml or more, 0.65 g/ml or more, 0.68 g/ml or more, or 0.7 g/ml or more. In this specification, the bulk density of the water-absorbent resin particles refers to the bulk density of the water-absorbent resin particles in a dry state. Even if the water-absorbent resin particles according to this embodiment have a high bulk density in a dry state, the settling rate of the gel after swelling tends to be sufficiently slow. The water-absorbent resin particles according to this embodiment may have a bulk density of, for example, 1.0 g/ml or less, 0.9 g/ml or less, or 0.85 g/ml or less. The bulk density is measured by the method described in the Examples below. The bulk density in a dry state refers to the bulk density of water-absorbent resin particles having a moisture content of 10% by mass or less.

From the viewpoint of easily increasing the water absorption capacity of the absorbent body, the saline water retention capacity of the water-absorbent resin particles according to this embodiment is preferably 20 g/g or more, 30 g/g or more, 33 g/g or more, 35 g/g or more, 38 g/g or more, 40 g/g or more, or 45 g/g or more. The saline water retention capacity of the water-absorbent resin particles may be 80 g/g or less, 75 g/g or less, 70 g/g or less, 65 g/g or less, 60 g/g or less, or 55 g/g or less. The saline water retention capacity of the water-absorbent resin particles is preferably 20 to 80 g/g, more preferably 30 to 55 g/g. The water retention capacity may be a value at 25°C. The saline water retention capacity of the water-absorbent resin particles can be measured by the method described in the Examples below.

The water-absorbent resin particles according to this embodiment may have a 10-minute unpressurized DW value of, for example, 30 to 80 ml/g. From the viewpoint of easily increasing the water absorption capacity of the absorbent body, the 10-minute unpressurized DW value is preferably 35 ml/g or more, more preferably 40 ml/g or more, and even more preferably 50 ml/g or more. The 10-minute unpressurized DW value may be, for example, 70 ml/g or less.

Examples of the shape of the water-absorbent resin particles according to this embodiment include substantially spherical, crushed, and granular. Furthermore, the water-absorbent resin particles according to this embodiment may each be composed of a single particle, or may be in the form of an aggregate of fine particles (primary particles) (secondary particles). The median particle diameter of the water-absorbent resin particles according to this embodiment (water-absorbent resin particles before water absorption) may be 130 to 800 μm, 200 to 850 μm, 250 to 700 μm, 300 to 600 μm, or 300 to 450 μm. The water-absorbent resin particles according to this embodiment may have a desired particle size distribution when obtained by the manufacturing method described below, but the particle size distribution may also be adjusted by performing an operation such as particle size adjustment using sieve classification.

The water-absorbent resin particles according to this embodiment may contain, for example, a crosslinked polymer (a crosslinked polymer having structural units derived from an ethylenically unsaturated monomer) obtained by polymerizing a monomer containing an ethylenically unsaturated monomer. That is, the water-absorbent resin particles according to this embodiment may contain a polymer having structural units derived from an ethylenically unsaturated monomer, and may contain polymer particles containing a crosslinked polymer having structural units derived from an ethylenically unsaturated monomer. A water-soluble ethylenically unsaturated monomer may be used as the ethylenically unsaturated monomer. Examples of polymerization methods include reverse-phase suspension polymerization, aqueous solution polymerization, bulk polymerization, and precipitation polymerization. Among these, reverse-phase suspension polymerization and aqueous solution polymerization are preferred from the viewpoints of ensuring good water absorption properties of the resulting water-absorbent resin particles and facilitating control of the polymerization reaction. Below, reverse-phase suspension polymerization is used as an example of a method for polymerizing an ethylenically unsaturated monomer.

The ethylenically unsaturated monomer is preferably water-soluble, and examples thereof 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, and diethylaminopropyl(meth)acrylamide. 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 types. The 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 process described below.

Among these, from the viewpoint of ease of industrial availability, the ethylenically unsaturated monomer preferably includes at least one compound selected from the group consisting of (meth)acrylic acid and its salts, acrylamide, methacrylamide, and N,N-dimethylacrylamide, and more preferably includes at least one compound selected from the group consisting of (meth)acrylic acid and its salts, and acrylamide. From the viewpoint of further enhancing water absorption properties, the ethylenically unsaturated monomer more preferably includes at least one compound selected from the group consisting of (meth)acrylic acid and its salts. In other words, the water-absorbent resin particles preferably have structural units derived from at least one compound selected from the group consisting of (meth)acrylic acid and its salts.

Monomers other than the above-described ethylenically unsaturated monomers may be used as monomers for obtaining water-absorbent resin particles. 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%, 80 to 100 mol%, 90 to 100 mol%, 95 to 100 mol%, or 100 mol% of the total amount of monomers (total amount of monomers for obtaining water-absorbent resin particles; for example, total amount of monomers that provide structural units of crosslinked polymers; the same applies below). In particular, the proportion of (meth)acrylic acid and its salts may be 70 to 100 mol%, 80 to 100 mol%, 90 to 100 mol%, 95 to 100 mol%, or 100 mol% of the total amount of monomers. The "proportion of (meth)acrylic acid and its salts" refers to the proportion of the total amount of (meth)acrylic acid and its salts.

According to this embodiment, as an example of water-absorbent resin particles, it is possible to provide water-absorbent resin particles containing a cross-linked polymer having structural units derived from an ethylenically unsaturated monomer, in which the ethylenically unsaturated monomer contains at least one compound selected from the group consisting of (meth)acrylic acid and salts thereof, and the proportion of (meth)acrylic acid and salts thereof is 70 to 100 mol % relative to the total amount of monomers used to obtain the water-absorbent resin particles.

The ethylenically unsaturated monomer is typically 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 the "aqueous monomer solution") is preferably 20% by mass or more and not more than the saturated concentration, more preferably 25 to 70% by mass, and even more preferably 30 to 55% by mass. Examples of water used in the aqueous solution include tap water, distilled water, and ion-exchanged water.

When the ethylenically unsaturated monomer contains an acid group, the acid group may be neutralized with an alkaline neutralizing agent before use. The degree of neutralization of the ethylenically unsaturated monomer with the alkaline neutralizing agent is preferably 10 to 100 mol %, more preferably 50 to 90 mol %, and even more preferably 60 to 80 mol % of the acid group in the ethylenically unsaturated monomer, from the viewpoint of increasing the osmotic pressure of the resulting water-absorbent resin particles and further improving the water absorption properties. Examples of alkaline neutralizing agents include alkali metal salts such as sodium hydroxide, sodium carbonate, sodium bicarbonate, potassium hydroxide, and potassium carbonate; ammonia, etc. The alkaline neutralizing agents may be used alone or in combination of two or more. The alkaline neutralizing agent may be used in the form of an aqueous solution to simplify the neutralization process. Neutralization of the acid group of the ethylenically unsaturated monomer can be performed, for example, by adding an aqueous solution of sodium hydroxide, potassium hydroxide, or the like dropwise to the above-mentioned aqueous monomer solution and mixing.

In reverse phase suspension polymerization, an aqueous monomer solution is dispersed in a hydrocarbon dispersion medium in the presence of a surfactant, and ethylenically unsaturated monomers can be polymerized using a radical polymerization initiator, etc.

Examples of surfactants include nonionic surfactants and anionic surfactants. Nonionic surfactants include sorbitan fatty acid esters, polyglycerin fatty acid esters, 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 alkylphenyl ethers, polyoxyethylene castor oil, polyoxyethylene hydrogenated castor oil, alkylaryl formaldehyde condensed polyoxyethylene ethers, polyoxyethylene polyoxypropylene block copolymers, polyoxyethylene polyoxypropyl alkyl ethers, and polyethylene glycol fatty acid esters. Anionic surfactants include fatty acid salts, alkylbenzene sulfonates, alkylmethyl taurates, polyoxyethylene alkylphenyl ether sulfates, polyoxyethylene alkyl ether sulfonates, polyoxyethylene alkyl ether phosphates, and polyoxyethylene alkyl allyl ether phosphates. Surfactants may be used alone or in combination of two or more.

From the viewpoints of achieving a favorable W/O type reverse phase suspension state, facilitating the production of water-absorbent resin particles having an appropriate particle size, and being readily available industrially, it is preferable that the surfactant 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 of facilitating the production of an appropriate particle size distribution of the water-absorbent resin particles and of facilitating the improvement of the water absorption properties of the water-absorbent resin particles and the performance of absorbents and absorbent articles using the same, it is preferable that the surfactant contains a sucrose fatty acid ester, and sucrose stearate ester is more preferable.

From the viewpoint of obtaining sufficient effect relative to the amount used and from the viewpoint of economy, the amount of surfactant used is preferably 0.05 to 10 parts by mass, more preferably 0.08 to 5 parts by mass, and even more preferably 0.1 to 3 parts by mass per 100 parts by mass of aqueous monomer solution.

In reversed-phase suspension polymerization, a polymeric dispersant may be used in combination with the surfactants described above. 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. Polymeric dispersants may be used alone or in combination of two or more. As the polymer dispersant, 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 preferred.

From the viewpoint of obtaining sufficient effect relative to the amount used and from the viewpoint of economy, the amount of polymeric dispersant used is preferably 0.05 to 10 parts by mass, more preferably 0.08 to 5 parts by mass, and even more preferably 0.1 to 3 parts by mass per 100 parts by mass of aqueous monomer solution.

The hydrocarbon dispersion medium may contain at least one compound selected from the group consisting of chain aliphatic hydrocarbons having 6 to 8 carbon atoms and alicyclic hydrocarbons having 6 to 8 carbon atoms. Examples of hydrocarbon dispersion mediums include chain aliphatic hydrocarbons 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. Hydrocarbon dispersion mediums may be used alone or in combination of two or more.

The hydrocarbon dispersion medium may contain at least one selected from the group consisting of n-heptane and cyclohexane, as these are easily available industrially and have stable quality. For the same reason, a mixture of the above-mentioned hydrocarbon dispersion mediums may also be used, such as commercially available Exxol Heptane (manufactured by ExxonMobil Corporation; contains 75-85% n-heptane and isomeric hydrocarbons).

From the viewpoint of adequately removing the heat of polymerization and facilitating control of the polymerization temperature, the amount of hydrocarbon dispersion medium used is preferably 30 to 1,000 parts by mass, more preferably 40 to 500 parts by mass, and even more preferably 50 to 400 parts by mass, per 100 parts by mass of aqueous monomer solution. Using an amount of hydrocarbon dispersion medium of 30 parts by mass or more tends to make it easier to control the polymerization temperature. Using an amount of hydrocarbon dispersion medium of 1,000 parts by mass or less tends to improve polymerization productivity and is more economical.

The radical polymerization initiator is preferably water-soluble, and examples thereof include persulfates such as potassium persulfate, ammonium persulfate, and sodium persulfate; peroxides such as methyl ethyl ketone peroxide, methyl isobutyl ketone peroxide, di-t-butyl peroxide, t-butylcumyl peroxide, t-butyl peroxyacetate, t-butylperoxyisobutyrate, t-butylperoxypivalate, and hydrogen peroxide; 2,2'-azobis(2-amidinopropane) dihydrochloride, 2,2'-azobis[2-(N-phenylamidino)propane] dihydrochloride, and 2,2' Examples of the radical polymerization initiator include azo compounds such as 2,2'-azobis[2-(N-allylamidino)propane]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). The radical polymerization initiator may be used alone or in combination of two or more. The radical polymerization initiator is preferably 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, more preferably potassium persulfate, ammonium persulfate, or sodium persulfate, and even more preferably sodium persulfate.

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

The above-mentioned radical polymerization initiators can also be used as redox polymerization initiators in combination with reducing agents such as sodium sulfite, sodium hydrogen sulfite, ferrous sulfate, and L-ascorbic acid.

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

To control the particle size of the water-absorbent resin particles, the aqueous monomer solution used in polymerization may contain a thickener. Examples of thickeners include hydroxyethyl cellulose, hydroxypropyl cellulose, methyl cellulose, carboxymethyl cellulose, polyethylene glycol, polyacrylamide, polyethyleneimine, dextrin, sodium alginate, polyvinyl alcohol, polyvinylpyrrolidone, and polyethylene oxide. Note that, given the same stirring speed during polymerization, the higher the viscosity of the aqueous monomer solution, the larger the median particle size of the resulting particles tends to be.

Internal crosslinking can occur during polymerization due to self-crosslinking, but crosslinking can also be achieved by using an internal crosslinking agent. Using an internal crosslinking agent makes it easier to control the water absorption properties of the water-absorbent resin particles. An internal crosslinking agent is usually added to the reaction liquid during the polymerization reaction. Examples of the internal crosslinking agent include di- or tri(meth)acrylic acid esters of polyols such as ethylene glycol, propylene glycol, trimethylolpropane, glycerin, polyoxyethylene glycol, polyoxypropylene glycol, and polyglycerin; unsaturated polyesters obtained by reacting the above-mentioned polyols with unsaturated acids (maleic acid, fumaric acid, and the like); bis(meth)acrylamides such as N,N'-methylenebis(meth)acrylamide; di- or tri(meth)acrylic acid esters obtained by reacting polyepoxides with (meth)acrylic acid; di(meth)acrylic acid carbamyl esters obtained by reacting polyisocyanates (tolylene diisocyanate, hexamethylene diisocyanate, and the like) 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; Examples of the internal crosslinking agent include polyglycidyl compounds such as 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 (2,4-tolylene diisocyanate, hexamethylene diisocyanate, etc.). The internal crosslinking agent may be used alone or in combination of two or more. As the internal crosslinking agent, a polyglycidyl compound is preferred, a diglycidyl ether compound is 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.

The amount of internal crosslinking agent used is preferably 30 millimoles or less, more preferably 0.01 to 10 millimoles, even more preferably 0.012 to 5 millimoles, particularly preferably 0.015 to 1 millimoles, extremely preferably 0.02 to 0.1 millimoles, and extremely preferably 0.025 to 0.08 millimoles per mole of ethylenically unsaturated monomer, from the viewpoint of easily achieving an excellent penetration rate in absorbent articles and of suppressing the water-soluble properties of the resulting polymer by being appropriately crosslinked, making it easier to achieve a sufficient amount of water absorption.

Reverse phase suspension polymerization can be carried out in a water-in-oil system by mixing an aqueous phase containing an ethylenically unsaturated monomer, a radical polymerization initiator, and optionally an internal crosslinking agent with an oil phase containing a hydrocarbon dispersion medium, a surfactant, and optionally a polymeric dispersant, and heating the mixture while stirring.

When performing reversed-phase 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 optionally a polymeric dispersant). The surfactant, polymeric dispersant, etc. may be added either before or after the addition of the aqueous monomer solution, as long as it is before the polymerization reaction begins.

Among these, from the viewpoint of easily reducing the amount of hydrocarbon dispersion medium remaining in the obtained water absorbent resin, it is preferable to disperse an aqueous monomer solution in a hydrocarbon dispersion medium in which a polymeric dispersant has been dispersed, and then further disperse a surfactant before carrying out polymerization.

Reverse-phase suspension polymerization can be carried out in one stage, or in two or more stages. From the perspective of increasing productivity, reverse-phase suspension polymerization is preferably carried out in two to three stages.

When performing reversed-phase suspension polymerization in two or more stages, after the first stage of reversed-phase suspension polymerization, an ethylenically unsaturated monomer is added to and mixed with the reaction mixture obtained in the first polymerization stage, and the second and subsequent stages of reversed-phase suspension polymerization are performed in the same manner as the first stage. In each stage of reversed-phase suspension polymerization from the second stage onward, it is preferable to perform reversed-phase suspension polymerization by adding, in addition to the ethylenically unsaturated monomer, the above-mentioned radical polymerization initiator and/or internal crosslinking agent within the molar ratio range of each component to the ethylenically unsaturated monomer, based on the amount of ethylenically unsaturated monomer added during the reversed-phase suspension polymerization in each stage from the second stage onward. Note that an internal crosslinking agent may also be used if necessary in the reversed-phase suspension polymerization from the second stage onward. When an internal crosslinking agent is used, it is preferable to perform reversed-phase suspension polymerization by adding the agent within the molar ratio range of each component to the ethylenically unsaturated monomer, based on the amount of ethylenically unsaturated monomer used in each stage.

The temperature of the polymerization reaction varies depending on the radical polymerization initiator used, but from the perspective of improving economy by rapidly progressing the polymerization and shortening the polymerization time, as well as easily removing the heat of polymerization and allowing the reaction to proceed smoothly, a temperature of 20 to 150°C is preferred, and 40 to 120°C is more preferred. 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. This results in a polymer of the ethylenically unsaturated monomer usually in the form of a hydrogel.

After polymerization, the resulting hydrogel polymer may be crosslinked by adding a post-polymerization crosslinking agent and heating. Crosslinking after polymerization increases the degree of crosslinking in the hydrogel polymer, further improving its water absorption properties.

Examples of crosslinking agents for post-polymerization crosslinking include polyols such as ethylene glycol, propylene glycol, 1,4-butanediol, trimethylolpropane, glycerin, polyoxyethylene glycol, polyoxypropylene glycol, and polyglycerin; compounds having two or more epoxy groups such as (poly)ethylene glycol diglycidyl ether, (poly)propylene glycol diglycidyl ether, and (poly)glycerin diglycidyl ether; haloepoxy compounds such as epichlorohydrin, epibromohydrin, and α-methylepichlorohydrin; compounds having two or more isocyanate groups such as 2,4-tolylene diisocyanate and hexamethylene diisocyanate; oxazoline compounds such as 1,2-ethylenebisoxazoline; carbonate compounds such as ethylene carbonate; and hydroxyalkylamide compounds such as bis[N,N-di(β-hydroxyethyl)]adipamide. Among these, polyglycidyl compounds such as (poly)ethylene glycol diglycidyl ether, (poly)glycerin diglycidyl ether, (poly)glycerin triglycidyl ether, (poly)propylene glycol polyglycidyl ether, polyglycerol polyglycidyl ether, etc. are preferred. The crosslinking agents may be used alone or in combination of two or more.

The amount of post-polymerization crosslinking agent may be 30 millimoles or less, 10 millimoles or less, or 0.01 to 5 millimoles per mole of ethylenically unsaturated monomer, from the viewpoint of easily obtaining favorable water absorption properties.

The post-polymerization crosslinking agent may be added after the polymerization of the ethylenically unsaturated monomer used in the polymerization. In the case of multi-stage polymerization, it is preferably added after the multi-stage polymerization. Taking into consideration heat generation during and after polymerization, retention due to process delays, opening of the system when the crosslinking agent is added, and fluctuations in moisture content due to the addition of water accompanying the addition of the crosslinking agent, the post-polymerization crosslinking agent is preferably added within the range of [moisture content immediately after polymerization ±3% by mass] from the perspective of moisture content (described below).

The resulting hydrogel polymer is then dried to remove water, yielding polymer particles (e.g., polymer particles having structural units derived from ethylenically unsaturated monomers). Drying methods include, for example, (a) a method in which the hydrogel polymer is dispersed in a hydrocarbon dispersion medium and then heated externally to perform azeotropic distillation, refluxing the hydrocarbon dispersion medium to remove water; (b) a method in which the hydrogel polymer is removed by decantation and then dried under reduced pressure; and (c) a method in which the hydrogel polymer is filtered through a filter and then dried under reduced pressure. Among these, method (a) is preferred due to its simplicity in the production process.

The particle size of the water-absorbent resin particles can be adjusted by adjusting the rotation speed of the agitator during the polymerization reaction, or by adding a flocculant to the system after the polymerization reaction or at the beginning of drying. Adding a flocculant increases the particle size of the resulting water-absorbent resin particles. Inorganic flocculants can be used as the flocculant. Examples of inorganic flocculants (e.g., powdered inorganic flocculants) include silica, zeolite, bentonite, aluminum oxide, talc, titanium dioxide, kaolin, clay, and hydrotalcite. From the perspective of superior flocculation effect, the flocculant is preferably at least one selected from the group consisting of silica, aluminum oxide, talc, and kaolin.

In reversed-phase suspension polymerization, the preferred method for adding a flocculant is to first disperse the flocculant in the same hydrocarbon dispersion medium as that used in the polymerization or in water, and then mix it with stirring into the hydrocarbon dispersion medium containing the hydrous gel polymer.

The amount of flocculant added is preferably 0.001 to 1 part by mass, more preferably 0.005 to 0.5 parts by mass, and even more preferably 0.01 to 0.2 parts by mass, per 100 parts by mass of the ethylenically unsaturated monomer used in the polymerization. By keeping the amount of flocculant added within the above range, it becomes easier to obtain water-absorbent resin particles having the desired particle size distribution.

In the production of water-absorbent resin particles, it is preferable to perform surface cross-linking on the surface portion (surface and near-surface) of the hydrogel polymer using a cross-linking agent in the drying process (moisture removal process) or a subsequent process. Surface cross-linking makes it easier to control the gel settling rate and water absorption characteristics of the water-absorbent resin particles. Surface cross-linking is preferably performed when the hydrogel polymer has a specific moisture content. The timing of surface cross-linking is preferably when the moisture content of the hydrogel polymer is 5 to 50% by mass, more preferably when it is 10 to 40% by mass, and even more preferably when it is 15 to 35% by mass.

The water content (mass%) of the hydrous gel polymer is calculated by the following formula.
Moisture content = [Ww/(Ww+Ws)]×100
Ww: the water content of the hydrogel polymer, obtained by subtracting the amount of water discharged to the outside of the system in the drying step from the amount of water contained in the aqueous monomer solution before polymerization in all polymerization steps, and adding the amount of water used as necessary when mixing a flocculant, a surface cross-linking agent, etc.
Ws: solid content calculated from the amounts of materials such as ethylenically unsaturated monomers, crosslinking agents, and initiators that constitute the hydrogel polymer.

Examples of crosslinking agents (surface crosslinking agents) used for surface crosslinking include compounds with two or more reactive functional groups. Examples of surface crosslinking agents 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; epichlorohydrin, epibromohydrin, and α-methylepichlorohydrin. haloepoxy compounds such as oxetane diisocyanate, 2,4-tolylene diisocyanate, and the like; isocyanate compounds such as 2,4-tolylene diisocyanate, hexamethylene diisocyanate, and the like; 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, and the like; oxazoline compounds such as 1,2-ethylenebisoxazoline, and the like; carbonate compounds such as ethylene carbonate, and the like; hydroxyalkylamide compounds such as bis[N,N-di(β-hydroxyethyl)]adipamide, and the like. The surface crosslinking agent may be used alone or in combination of two or more thereof. The surface cross-linking agent is preferably a polyglycidyl compound, and more preferably 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.

From the viewpoint of easily obtaining favorable water absorption properties, the amount of surface cross-linking agent used is preferably 0.01 to 20 millimoles, more preferably 0.05 to 10 millimoles, even more preferably 0.1 to 5 millimoles, particularly preferably 0.15 to 1 millimoles, and extremely preferably 0.2 to 0.5 millimoles, per mole of ethylenically unsaturated monomer used in polymerization.

After surface cross-linking, the water and hydrocarbon dispersion medium can be removed by distillation using known methods, or the particles can be dried under heat and reduced pressure to obtain surface-cross-linked, dried polymer particles.

The polymerization reaction can be carried out using various agitators equipped with agitating blades. Agitating blades that can be used include flat blades, lattice blades, paddle blades, propeller blades, anchor blades, turbine blades, Pfaudler blades, ribbon blades, full zone blades, and Max Blend blades. Flat blades have a shaft (agitation shaft) and a flat plate portion (agitation portion) arranged around the shaft. The flat plate portion may have slits, etc. When flat blades are used as agitating blades, it is easier to carry out the crosslinking reaction in the polymer particles uniformly and to adjust the gel settling rate to the desired range while maintaining water absorption properties.

The water-absorbent resin particles according to this embodiment may be composed solely of polymer particles, but may further contain additional components such as a gel stabilizer, a metal chelating agent (ethylenediaminetetraacetic acid and its salts, diethylenetriaminepentaacetic acid and its salts, such as pentasodium diethylenetriaminepentaacetate), a flowability improver (lubricant) for the polymer particles, etc. The additional components may be located inside the polymer particles, on the surface of the polymer particles, or both.

The water-absorbent resin particles may contain a plurality of inorganic particles disposed on the surface of the polymer particles. For example, by mixing the polymer particles with the inorganic particles, the inorganic particles can be disposed on the surface of the polymer particles. The inorganic particles may be silica particles such as amorphous silica.

When the water-absorbent resin particles contain inorganic particles disposed on the surfaces of the polymer particles, the content of the inorganic particles may be within the following ranges based on the total mass of the polymer particles. The content of the inorganic particles may be 0.05% by mass or more, 0.1% by mass or more, 0.15% by mass or more, or 0.2% by mass or more. The content of the inorganic particles may be 5.0% by mass or less, 3.0% by mass or less, 1.0% by mass or less, 0.5% by mass or less, or 0.3% by mass or less.

The inorganic particles here typically have a size that is very small compared to the size of the polymer particles. For example, the average particle size of the inorganic particles may be 0.1 to 50 μm, 0.5 to 30 μm, or 1 to 20 μm. The average particle size can be measured by the pore electrical resistance method or the laser diffraction/scattering method, depending on the particle characteristics.

The water-absorbent resin particles of this embodiment have excellent absorption properties for bodily fluids such as urine and blood, and can be used in fields such as hygiene products such as disposable diapers, sanitary napkins, and tampons, and animal waste treatment materials such as pet sheets and dog or cat litter compounds.

The water-absorbent resin particles according to this embodiment can be suitably used in an absorbent body. The absorbent body according to this embodiment contains the water-absorbent resin particles according to this embodiment. From the viewpoint of obtaining sufficient liquid absorption performance when the absorbent body is used in an absorbent article, the content of the water-absorbent resin particles in the absorbent body is preferably 100 to 1000 g per square meter of the absorbent body (i.e., 100 to 1000 g/m ² ), more preferably 150 to 800 g/m ² , and even more preferably 200 to 700 g/m ² . From the viewpoint of exhibiting sufficient liquid absorption performance as an absorbent article, the content is preferably 100 g/m ² or more. From the viewpoint of suppressing the occurrence of gel blocking, the content is preferably 1000 g/m ² or less.

In addition to the water-absorbent resin particles, the absorbent may further comprise, for example, fibrous material. The absorbent may be, for example, a mixture containing water-absorbent resin particles and fibrous material. The mass proportion of the water-absorbent resin particles in the absorbent may be 2 to 100% by mass, preferably 10 to 80% by mass, and more preferably 20 to 70% by mass, relative to the total mass of the water-absorbent resin particles and fibrous material. The absorbent may be configured, for example, in a form in which the water-absorbent resin particles and fibrous material are uniformly mixed, or in a form in which the water-absorbent resin particles are sandwiched between fibrous material formed in a sheet or layer, or in other forms.

Examples of fibrous materials include cellulose-based fibers such as finely ground wood pulp, cotton, cotton linters, rayon, and cellulose acetate, and synthetic fibers such as polyamide, polyester, and polyolefin. The average fiber length of the fibrous material is typically 0.1 to 10 mm, and may be 0.5 to 5 mm. The fibrous material may also be a mixture of the above-mentioned fibers.

To improve the shape retention of the absorbent body before and during use, an adhesive binder may be added to the fibrous material to bond the fibers together. Examples of adhesive binders include heat-fusible synthetic fibers, hot-melt adhesives, and adhesive emulsions.

Examples of heat-fusible synthetic fibers include full-melt binders such as polyethylene, polypropylene, and ethylene-propylene copolymers, as well as non-full-melt binders consisting of a side-by-side or core-sheath structure of polypropylene and polyethylene. In the non-full-melt binders mentioned above, only the polyethylene portion is heat-fused. Examples of hot-melt adhesives include blends 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 with tackifiers, plasticizers, antioxidants, etc.

Examples of adhesive emulsions 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. These adhesive binders may be used alone or in combination of two or more.

The absorbent body according to this embodiment may further contain additives such as inorganic powders (e.g., amorphous silica), deodorants, pigments, dyes, antibacterial agents, fragrances, adhesives, etc. These additives can impart various functions to the absorbent body. When the water-absorbent resin particles contain inorganic particles, the absorbent body may contain inorganic powders in addition to the inorganic particles in the water-absorbent resin particles. Examples of inorganic powders include silicon dioxide, zeolite, kaolin, clay, etc.

The shape of the absorbent body according to this embodiment is not particularly limited and may be, for example, sheet-like. The thickness of the absorbent body (for example, the thickness of a sheet-like absorbent body) may be, for example, 0.1 to 20 mm, or 0.3 to 15 mm.

The absorbent article according to this embodiment may comprise, in addition to the absorbent body, for example, a core wrap, a liquid-permeable top sheet, and a liquid-impermeable back sheet. The core wrap maintains the shape of the absorbent body. The liquid-permeable top sheet is placed on the outermost side on the side from which the liquid to be absorbed penetrates. The liquid-impermeable back sheet is placed on the outermost side on the side opposite to the side from which the liquid to be absorbed penetrates.

Absorbent articles include diapers (e.g., disposable diapers), toilet training pants, incontinence pads, hygiene products (sanitary napkins, tampons, etc.), sweat pads, pet sheets, parts for portable toilets, and animal waste disposal materials.

Figure 1 is a cross-sectional view showing an example of an absorbent article. The absorbent article 100 shown in Figure 1 comprises an absorbent body 10, core wraps 20a and 20b, a liquid-permeable topsheet 30, and a liquid-impermeable backsheet 40. In the absorbent article 100, the liquid-impermeable backsheet 40, core wrap 20b, absorbent body 10, core wrap 20a, and liquid-permeable topsheet 30 are layered in this order. In Figure 1, some parts are shown as having gaps between the components, but the components may be in close contact with each other without any gaps.

The absorbent body 10 comprises water-absorbent resin particles 10a and a fiber layer 10b containing fibrous material. The water-absorbent resin particles 10a are dispersed within the fiber layer 10b.

The core wrap 20a is arranged on one side of the absorbent body 10 (the upper side of the absorbent body 10 in Figure 1) while in contact with the absorbent body 10. The core wrap 20b is arranged on the other side of the absorbent body 10 (the lower side of the absorbent body 10 in Figure 1) while in contact with the absorbent body 10. The absorbent body 10 is arranged between the core wraps 20a and 20b.

The core wraps 20a and 20b have, for example, a main surface of the same size as the absorbent body 10. The use of core wraps helps maintain the shape of the absorbent body and prevents the water-absorbent resin particles and other components that make up the absorbent body from falling off or flowing. Examples of core wraps include nonwoven fabrics, woven fabrics, tissues, synthetic resin films with liquid-permeable holes, and mesh-like sheets. From an economical standpoint, tissues made by wet-forming pulp are preferred.

The liquid-permeable topsheet 30 is positioned on the outermost side of the absorbent article 100, on the side into which the liquid to be absorbed penetrates. The liquid-permeable topsheet 30 is positioned on top of the core wrap 20a, in contact with the core wrap 20a. The liquid-impermeable backsheet 40 is positioned on the outermost side of the absorbent article 100, opposite the liquid-permeable topsheet 30. The liquid-impermeable backsheet 40 is positioned below the core wrap 20b, in contact with the core wrap 20b. The liquid-permeable topsheet 30 and the liquid-impermeable backsheet 40 have, for example, major surfaces that are wider than the major surface of the absorbent body 10, and the outer edges of the liquid-permeable topsheet 30 and the liquid-impermeable backsheet 40 extend around the absorbent body 10 and the core wraps 20a, 20b.

Examples of the liquid-permeable top sheet 30 include nonwoven fabrics and porous sheets. Examples of nonwoven fabrics include thermal-bonded nonwoven fabrics, air-through nonwoven fabrics, resin-bonded nonwoven fabrics, spunbonded nonwoven fabrics, melt-blown nonwoven fabrics, spunbonded/melt-blown/spunbonded nonwoven fabrics, air-laid nonwoven fabrics, spunlace nonwoven fabrics, and point-bonded nonwoven fabrics. Of these, thermal-bonded nonwoven fabrics, air-through nonwoven fabrics, spunbonded nonwoven fabrics, and spunbonded/melt-blown/spunbonded nonwoven fabrics are preferred.

The liquid-permeable topsheet 30 may be made from any resin or fiber known in the art. From the viewpoints of liquid permeability, flexibility, and strength when used in an absorbent article, examples include polyolefins such as polyethylene (PE) and polypropylene (PP), polyesters such as polyethylene terephthalate (PET), polytrimethylene terephthalate (PTT), and polyethylene naphthalate (PEN), polyamides such as nylon, rayon, other synthetic resins or fibers, cotton, silk, hemp, and pulp (cellulose) fibers. Synthetic fibers are preferred as constituent materials from the viewpoint of increasing the strength of the liquid-permeable topsheet 30, with polyolefins and polyesters being particularly preferred. These materials may be used alone or in combination of two or more materials.

The nonwoven fabric used in the liquid-permeable top sheet 30 desirably has a moderate hydrophilicity in order to improve the liquid absorption performance of the absorbent article. From this perspective, the hydrophilicity, as measured according to the "Hydrophilicity of Nonwoven Fabrics" (based on Paper Pulp Test Method No. 68 (2000)) described in WO 2011/086843, is preferably 5 to 200, and more preferably 10 to 150. Such hydrophilic nonwoven fabrics may be those made from the above-mentioned nonwoven fabrics that themselves exhibit a moderate hydrophilicity, such as rayon fibers, or may be hydrophobic chemical fibers, such as polyolefin fibers or polyester fibers, that have been hydrophilized using known methods to impart a moderate hydrophilicity.

Methods for hydrophilizing chemical fibers include, for example, spunbonding nonwoven fabrics by mixing hydrophobic chemical fibers with a hydrophilizing agent, adding a hydrophilizing agent when producing spunbond nonwoven fabrics from hydrophobic chemical fibers, or impregnating hydrophilizing agents into spunbond nonwoven fabrics from hydrophobic chemical fibers. Examples of hydrophilizing agents include anionic surfactants such as aliphatic sulfonates and higher alcohol sulfate ester salts, cationic surfactants such as quaternary ammonium salts, nonionic surfactants such as polyethylene glycol fatty acid esters, polyglycerin fatty acid esters, and sorbitan fatty acid esters, silicone surfactants such as polyoxyalkylene-modified silicones, and stain release agents made from polyester, polyamide, acrylic, or urethane resins.

The nonwoven fabric used in the liquid-permeable top sheet 30 is preferably moderately bulky and has a large basis weight, from the viewpoints of imparting good liquid permeability, flexibility, strength, and cushioning properties to the absorbent article, and of accelerating the liquid penetration rate of the absorbent article. The basis weight of the nonwoven fabric is preferably 5 to 200 g/ ^m2 , more preferably 8 to 150 g/ ^m2 , and even more preferably 10 to 100 g/ ^m2 . The thickness of the nonwoven fabric is preferably 20 to 1400 μm, more preferably 50 to 1200 μm, and even more preferably 80 to 1000 μm.

The liquid-impermeable backsheet 40 prevents liquid absorbed by the absorbent body 10 from leaking out from the backsheet 40 side. Examples of materials that can be used for the liquid-impermeable backsheet 40 include liquid-impermeable films primarily made of polyolefin resins such as polyethylene (PE) and polypropylene (PP), breathable resin films, composite films in which a breathable resin film is bonded to a nonwoven fabric such as spunbond or spunlace, and spunbond/meltblown/spunbond (SMS) nonwoven fabrics in which a water-resistant meltblown nonwoven fabric is sandwiched between high-strength spunbond nonwoven fabrics. To ensure flexibility and maintain the comfort of the absorbent article, the backsheet 40 can be made of a resin film primarily made of low-density polyethylene (LDPE) resin with a basis weight of 10 to 50 g/ ^m² . Furthermore, using a breathable material can reduce stuffiness during wear, thereby reducing discomfort to the wearer.

The relative sizes of the absorbent body 10, core wraps 20a, 20b, liquid-permeable top sheet 30, and liquid-impermeable back sheet 40 are not particularly limited and may be adjusted as appropriate depending on the intended use of the absorbent article. Furthermore, the method for maintaining the shape of the absorbent body 10 using the core wraps 20a, 20b is not particularly limited. As shown in Figure 1, the absorbent body may be sandwiched between multiple core wraps, or may be covered by a single core wrap.

The absorbent body 10 may be bonded to the liquid-permeable top sheet 30. Bonding the absorbent body 10 and the liquid-permeable top sheet 30 allows liquid to be more smoothly guided to the absorbent body, making it easier to obtain an absorbent article with better liquid leakage prevention. When the absorbent body 10 is sandwiched or covered by a core wrap, it is preferable that at least the core wrap and the liquid-permeable top sheet 30 are bonded together, and it is even more preferable that the core wrap and the absorbent body 10 are bonded together. Examples of bonding methods include applying a hot-melt adhesive to the liquid-permeable top sheet 30 at predetermined intervals across its width in a pattern such as longitudinal stripes or a spiral, or bonding using a water-soluble binder selected from starch, carboxymethyl cellulose, polyvinyl alcohol, polyvinylpyrrolidone, and other water-soluble polymers. Furthermore, if the absorbent body 10 contains heat-sealable synthetic fibers, bonding by heat fusion may also be used.

The method for producing water-absorbent resin particles according to this embodiment may include selecting water-absorbent resin particles based on the gel settling velocity measured by the above-described method. Specifically, the selection may involve selecting water-absorbent resin particles having a gel settling velocity of 45 seconds or more. The above-described production method may include a step of measuring the gel settling velocity of the water-absorbent resin particles. The properties of the selected water-absorbent resin particles may further satisfy the above-described aspects of the water-absorbent resin particles (e.g., a bulk density within a specific range, a water retention capacity of physiological saline within a specific range, etc.).

One aspect of this embodiment can be described as a method for suppressing liquid leakage from an absorbent article containing water-absorbent resin particles, which includes adjusting the gel settling rate of the water-absorbent resin particles measured by the above-described measurement method. A more specific method for measuring the gel settling rate is shown in the Examples below. The method for suppressing liquid leakage from an absorbent article may further include, for example, adjusting the gel settling rate of the water-absorbent resin particles to 45 seconds or more, adjusting the bulk density of the water-absorbent resin particles to 0.5 g/ml or more, or adjusting the saline water retention capacity to 30 g/g or more. Specific examples of manufacturing methods for water-absorbent resin particles having these predetermined properties are as described above. The gel settling rate of the water-absorbent resin particles can be increased to 45 seconds or more by, for example, selecting manufacturing conditions for the water-absorbent resin particles so as to increase the uniformity of crosslinking within the water-absorbent resin particles.

According to this embodiment, a method for manufacturing an absorbent body can be provided, using water-absorbent resin particles obtained by the above-described method for manufacturing water-absorbent resin particles. The method for manufacturing an absorbent body according to this embodiment includes a particle manufacturing step of obtaining water-absorbent resin particles by the above-described method for manufacturing water-absorbent resin particles. The method for manufacturing an absorbent body according to this embodiment may include a step of mixing water-absorbent resin particles and fibrous material after the particle manufacturing step. According to this embodiment, a method for manufacturing an absorbent article can be provided, using an absorbent body obtained by the above-described method for manufacturing an absorbent body. The method for manufacturing an absorbent article according to this embodiment includes an absorbent body manufacturing step of obtaining an absorbent body by the above-described method for manufacturing an absorbent body. The method for manufacturing an absorbent article according to this embodiment may include a step of obtaining an absorbent article using the absorbent body and other components of the absorbent article after the absorbent body manufacturing step. In this step, the absorbent article is obtained, for example, by stacking the absorbent body and other components of the absorbent article together.

The present invention will be explained in more detail below using examples and comparative examples, but the present invention is not limited to the following examples.

[Example 1]
A round-bottomed cylindrical separable flask with an inner diameter of 11 cm and an internal volume of 2 L was prepared, equipped with a reflux condenser, a dropping funnel, a nitrogen gas inlet tube, and a stirrer. A stirring blade 200, the outline of which is shown in FIG. 2, was attached to the stirrer. The stirring blade 200 had a shaft 200a and a flat plate 200b. The flat plate 200b was welded to the shaft 200a and had a curved tip. Four slits S extending along the axial direction of the shaft 200a were formed in the flat plate 200b. The four slits S were arranged in the width direction of the flat plate 200b, with the two inner slits S being 1 cm wide and the two outer slits S being 0.5 cm wide. The length of the flat plate 200b was approximately 10 cm, and the width of the flat plate 200b was approximately 6 cm. 293 g of n-heptane and 0.736 g of dispersant (maleic anhydride-modified ethylene-propylene copolymer, manufactured by Mitsui Chemicals, Inc., Hiwax 1105A) were mixed in the prepared separable flask. The mixture in the separable flask was stirred with a stirrer while being heated to 80°C, thereby dissolving the dispersant in the n-heptane. The resulting solution was cooled to 50°C.

92.0 g (1.03 mol) of an 80.5% by weight aqueous solution of acrylic acid (as a water-soluble ethylenically unsaturated monomer) was placed in a 300 mL beaker. While cooling from the outside, 147.7 g of a 20.9% by weight aqueous solution of sodium hydroxide was added dropwise to the beaker to achieve 75 mol% neutralization. Next, 0.092 g of hydroxyethyl cellulose (Sumitomo Seika Chemicals Co., Ltd., HEC AW-15F) as a thickener, 0.0648 g (0.272 mmol) of sodium persulfate as a water-soluble radical polymerization agent, and 0.0156 g (0.090 mmol) of ethylene glycol diglycidyl ether as an internal crosslinking agent were added and dissolved to prepare the first aqueous solution.

The first-stage aqueous solution was added to the separable flask and stirred for 10 minutes. Subsequently, a surfactant solution was obtained by dissolving 0.736 g of sucrose stearate (Ryoto Sugar Ester S-370, HLB: 3, manufactured by Mitsubishi Chemical Foods Corporation) as a surfactant in 6.62 g of n-heptane under heat. This surfactant solution was then added to the separable flask, and the system was thoroughly purged with nitrogen while stirring at a stirrer speed of 350 rpm. The separable flask was then immersed in a 70°C water bath to raise the temperature, and polymerization was carried out for 60 minutes, yielding a first-stage polymerization slurry.

128.8 g (1.44 mol) of an 80.5% by weight aqueous solution of acrylic acid (as a water-soluble ethylenically unsaturated monomer) was placed in a separate 500 mL beaker. While cooling externally, 159.0 g of a 27% by weight aqueous solution of sodium hydroxide was added dropwise to achieve 75 mol% neutralization. Subsequently, 0.0907 g (0.381 mmol) of sodium persulfate as a water-soluble radical polymerization initiator and 0.0129 g (0.074 mmol) of ethylene glycol diglycidyl ether as an internal crosslinking agent were added and dissolved to prepare the second aqueous solution.

While stirring at a stirrer speed of 650 rpm, the contents of the separable flask system were cooled to 25°C, and the entire second-stage aqueous liquid was added to the first-stage polymerization slurry, and the system was purged with nitrogen for 30 minutes. The flask was then immersed again in a 70°C water bath to raise the temperature, and the polymerization reaction was carried out for 60 minutes to obtain a hydrogel polymer.

0.589 g of a 45% by weight aqueous solution of pentasodium diethylenetriaminepentaacetate was added to the hydrous gel polymer after the second-stage polymerization while stirring. The separable flask was then immersed in an oil bath set at 125°C, and 255.2 g of water was extracted from the system by azeotropic distillation of n-heptane and water while refluxing the n-heptane. Next, 4.42 g (0.507 mmol) of a 2% by weight aqueous solution of ethylene glycol diglycidyl ether was added as a surface cross-linking agent to the separable flask, and the mixture was maintained at 83°C for 2 hours.

Then, the n-heptane was evaporated at 125°C and the mixture was dried to obtain polymer particles (dried product).The polymer particles were passed through a sieve with an opening of 850 μm, and 0.5% by mass of amorphous silica (Toxil NP-S, Oriental Silicas Corporation) relative to the mass of the polymer particles was mixed with the polymer particles to obtain 230.1 g of water-absorbent resin particles containing amorphous silica.The median particle diameter of the water-absorbent resin particles was 357 μm.

[Example 2]
[0111] In the first-stage aqueous liquid preparation, the amount of ethylene glycol diglycidyl ether used as an internal crosslinking agent was changed to 0.010 g (0.057 mmol), in the second-stage aqueous liquid preparation, the amount of ethylene glycol diglycidyl ether used as an internal crosslinking agent was changed to 0.0116 g (0.067 mmol), the amount of water extracted from the system by azeotropic distillation was changed to 257.2 g, and the amount of amorphous silica mixed with the polymer particles was changed to 0.2 mass% with respect to the mass of the polymer particles, and the same procedure as in Example 1 was carried out to obtain 231.2 g of water-absorbent resin particles. The median particle diameter of the water-absorbent resin particles was 359 µm.

[Example 3]
[0113] In the same manner as in Example 2, except that in the preparation of the first-stage polymerization slurry liquid, the rotation speed of the stirrer during nitrogen substitution was changed to 425 rpm, and the amount of water extracted to the outside of the system by azeotropic distillation was changed to 272.5 g, 229.0 g of water absorbent resin particles were obtained. The median particle diameter of the water absorbent resin particles was 349 µm.

[Example 4]
[0113] In the preparation of the first-stage aqueous liquid, except that the cooling temperature in the separable flask system was changed to 31°C after the preparation of the second-stage aqueous liquid, and the amount of water extracted to the outside of the system by azeotropic distillation was changed to 276.2 g, 232.3 g of water-absorbent resin particles were obtained in the same manner as in Example 3. The median particle diameter of the water-absorbent resin particles was 146 µm.

[Example 5]
[0113] Except for changing the amount of water extracted to outside the system by azeotropic distillation in the second-stage aqueous liquid preparation to 263.7 g and changing the amount of amorphous silica mixed with the polymer particles to 0.5 mass % relative to the mass of the polymer particles, the same procedure as in Example 3 was carried out to obtain 230.8 g of water-absorbent resin particles. The median particle diameter of the water-absorbent resin particles was 350 µm.

[Example 6]
Except for changing the amount of water extracted to outside the system by azeotropic distillation to 275.4 g, the same procedure as in Example 5 was carried out to obtain 230.0 g of water-absorbent resin particles. The median particle diameter of the water-absorbent resin particles was 349 μm.

[Example 7]
[0113] Except for changing the amount of water extracted to outside the system by azeotropic distillation to 243.0 g and changing the amount of a 2 mass% ethylene glycol diglycidyl ether aqueous solution used as a surface crosslinking agent to 6.62 g (0.761 mmol), the same procedures as in Example 1 were carried out to obtain 228.5 g of water absorbent resin particles. The median particle diameter of the water absorbent resin particles was 371 µm.

[Comparative Example 1]
The stirring blades were changed to those having four inclined paddle blades with a blade diameter of 5 cm arranged in two stages; in the preparation of the first-stage aqueous liquid, sodium persulfate was not used as the water-soluble radical polymerization initiator, but 0.092 g (0.339 mmol) of 2,2'-azobis(2-amidinopropane) dihydrochloride and 0.018 g (0.068 mmol) of potassium persulfate were used; the amount of ethylene glycol diglycidyl ether used as the internal crosslinking agent in the preparation of the first-stage aqueous liquid was changed to 0.0046 g (0.026 mmol); in the preparation of the first-stage polymerization slurry liquid, the stirrer rotation speed during nitrogen substitution was changed to 550 rpm; in the preparation of the second-stage aqueous liquid, sodium persulfate was not used as the water-soluble radical polymerization initiator, but 2 232.0 g of water-absorbent resin particles were obtained in the same manner as in Example 1, except that 0.129 g (0.475 mmol) of 2'-azobis(2-amidinopropane) dihydrochloride and 0.026 g (0.095 mmol) of potassium persulfate were used, the amount of ethylene glycol diglycidyl ether used as an internal crosslinking agent in the preparation of the second-stage aqueous liquid was changed to 0.0116 g (0.067 mmol), the stirrer rotation speed when cooling the inside of the separable flask system to 25°C after the preparation of the second-stage aqueous liquid was changed to 1000 rpm, the amount of water extracted to the outside of the system by azeotropic distillation was changed to 207.6 g, and the amount of amorphous silica mixed with the polymer particles was changed to 0.2 mass% with respect to the mass of the polymer particles. The median particle diameter of the water-absorbent resin particles was 361 μm.

[Comparative Example 2]
Except for changing the amount of water extracted to outside the system by azeotropic distillation to 224.7 g, the same procedure as in Comparative Example 1 was carried out to obtain 227.7 g of water-absorbent resin particles. The median particle diameter of the water-absorbent resin particles was 342 μm.

[Comparative Example 3]
Except for changing the amount of water extracted to outside the system by azeotropic distillation to 232.8 g, the same procedure as in Comparative Example 1 was carried out to obtain 228.3 g of water-absorbent resin particles. The median particle diameter of the water-absorbent resin particles was 355 μm.

The obtained water-absorbent resin particles were evaluated for the 10-minute unpressurized DW value, median particle size, saline water retention capacity, bulk density, gel settling rate, and liquid leakage during use using the following methods. Note that the saline used in this example was a 0.9% by mass NaCl aqueous solution.

<Measurement of pressureless DW>
The no-pressure DW of the water-absorbent resin particles was measured using a measurement device shown in Fig. 3. The measurement was carried out five times for one type of water-absorbent resin particle, and the average value of the measured values at three points excluding the minimum and maximum values was calculated.
The measuring device comprises a burette unit 1, a conduit 5, a measurement table 13, a nylon mesh sheet 15, a stand 11, and a clamp 3. The burette unit 1 comprises a burette tube 21 with a scale, a rubber stopper 23 that seals the upper opening of the burette tube 21, a cock 22 connected to the lower tip of the burette tube 21, and an air introduction tube 25 and a cock 24 connected to the lower part of the burette tube 21. The burette unit 1 is fixed with the 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 the height-adjustable stand 11. The through-hole 13a of the measurement table 13 and the cock 22 of the burette unit 1 are connected by a conduit 5. The inner diameter of the conduit 5 is 6 mm.

Measurements were performed in an environment with a temperature of 25°C and a humidity of 50±10%. First, stopcocks 22 and 24 of burette part 1 were closed, and saline solution (0.9% by mass saline solution) 50 adjusted to 25°C was poured into burette tube 21 through the opening at the top of burette tube 21. The saline solution concentration of 0.9% by mass is based on the mass of the saline solution. After sealing the opening of burette tube 21 with rubber stopper 23, stopcocks 22 and 24 were opened. Conduit 5 was filled with saline solution 50, taking care to prevent air bubbles from entering. The height of measurement platform 13 was adjusted so that the level of the saline solution reaching through hole 13a was the same as the top surface of measurement platform 13. After adjustment, the level of the saline solution 50 in burette tube 21 was read using the scale on burette tube 21, and this position was designated as the zero point (the reading at 0 seconds).

A nylon mesh sheet 15 (100 mm x 100 mm, 250 mesh, approximately 50 μm thick) was placed near the through-hole 13a on the measurement table 13, and a cylinder with an inner diameter of 30 mm and a height of 20 mm was placed in the center of the sheet. 1.00 g of water-absorbent resin particles 10a was uniformly dispersed in the cylinder. The cylinder was then carefully removed, yielding a sample in which the water-absorbent resin particles 10a were dispersed in a circular pattern in the center of the nylon mesh sheet 15. Next, the nylon mesh sheet 15 with the water-absorbent resin particles 10a placed on it was quickly moved so that its center was aligned with the through-hole 13a, without scattering the water-absorbent resin particles 10a, and measurement was initiated. The time when air bubbles were first introduced into the burette tube 21 from the air inlet tube 25 was defined as the start of water absorption (0 seconds).

The amount of decrease in the saline solution 50 in the burette tube 21 (i.e., the amount of saline solution absorbed by the water-absorbent resin particles 10a) was sequentially read in increments of 0.1 mL, and the amount of decrease in the amount of saline solution 50 Wa (g) 10 minutes after the start of water absorption by the water-absorbent resin particles 10a was read. From Wa, the 10-minute value of the no-pressure DW was calculated using the following formula. The no-pressure DW is the amount of water absorbed per 1.00 g of the water-absorbent resin particles 10a.
No-pressure DW value [mL/g] = Wa/1.00

<Measurement of median particle size>
The median particle diameter of the water-absorbent resin particles was measured by the following procedure. The measurement was performed in an environment of 25±2°C temperature and 50±10% humidity. That is, JIS standard sieves were combined in the following order from top to bottom: a 600 μm mesh sieve, a 500 μm mesh sieve, a 425 μm mesh sieve, a 300 μm mesh sieve, a 250 μm mesh sieve, a 180 μm mesh sieve, a 150 μm mesh sieve, and a tray. 50 g of water-absorbent resin particles were placed on the top sieve and classified in accordance with JIS Z 8815 (1994) using a Rotap shaker (manufactured by Iida Seisakusho Co., Ltd.). After classification, the mass of the particles remaining on each sieve was calculated as a mass percentage relative to the total mass to determine the particle size distribution. The particle size distribution was calculated by integrating the particles remaining on the sieves in descending order of particle size, and the relationship between the sieve opening size and the integrated value of the mass percentage of the particles remaining on the sieve was plotted on a logarithmic probability paper. The plots on the probability paper were connected with a straight line to obtain the particle size corresponding to an integrated mass percentage of 50% by mass as the median particle size.

<Measurement of saline water retention capacity>
The water retention capacity of the water-absorbent resin particles in physiological saline (room temperature, 25°C ± 2°C) was measured using the following procedure. First, a cotton bag (membrane broadcloth No. 60, 100 mm wide x 200 mm long) containing 2.0 g of water-absorbent resin particles was placed in a 500 mL beaker. 500 g of physiological saline was poured into the cotton bag containing the water-absorbent resin particles all at once, taking care not to allow the bag to become lumpy. The top of the cotton bag was then tied with a rubber band and allowed to stand for 30 minutes to allow the water-absorbent resin particles to swell. After 30 minutes had passed, 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 Wb [g] of the cotton bag containing the swollen gel after dehydration was measured. The same procedure was performed without adding water-absorbent resin particles, and the empty mass Wc [g] of the cotton bag when wet was measured. The water retention capacity of the water-absorbent resin particles in physiological saline was calculated using the following formula. The results are shown in Table 1.
Water retention amount [g/g] = (Wb-Wc)/2.0

[Measurement of bulk density]
The bulk density of the water-absorbent resin particles was measured by the following procedure using a "bulk specific gravity measuring device" described in JIS K-6720-2. The measurement was carried out under conditions of a temperature of 25±2°C and a humidity of 50±10%. Approximately 120 mL of water-absorbent resin particles was placed in the funnel portion of the above-mentioned device, the bottom of which was blocked with a damper. Thereafter, with a 90 mL capacity (inner diameter 40 mmφ, cylindrical shape) receiver placed 38 mm below the damper, the damper of the above-mentioned device was quickly pulled out, and the water-absorbent polymer particles were dropped into the receiver.

Next, the water-absorbent resin particles that had risen from the receiver were scraped off horizontally with a glass rod against the opening of the receiver, and then the mass W1 (g) of the receiver containing the water-absorbent resin particles was measured. Separately, the mass W0 (g) of the empty receiver was measured. The mass of the water-absorbent resin particles obtained by subtracting the mass W0 (g) from the mass W1 (g) was divided by the volume of the receiver (V = 90 mL) to obtain a value S (g/mL). The formula for obtaining the value S is shown below.
S (g/mL) = [W1-W0] (g)/V (mL)
The measured water-absorbent resin particles were collected, and the value S was measured three times in total in the same manner as above, and the average value was taken as the bulk density.

[Measurement of gel sedimentation velocity]
300 g of physiological saline and a stirrer tip (stirring bar, diameter 0.8 cm, length 3 cm, no ring) were placed in a 300 ml glass beaker (HARIO, inner diameter 75 mm) and rotated at 600 rpm using a magnetic stirrer (stirring stand: Model M-20G manufactured by Koike Precision Machinery Manufacturing Co., Ltd.). 0.5 g of water-absorbent resin particles were added to the rotating physiological saline, and stirring was continued for 15 minutes. During this time, the water-absorbent resin particles swelled and became a gel. 15 minutes after the water-absorbent resin particles were added to the physiological saline, the magnetic stirrer was switched off to stop the stirrer. Time measurement was started from the point when the switch was turned off, and the time until all of the gel floating in the water precipitated was measured. The precipitation of the gel was confirmed visually. "All precipitated" means that an examiner with near vision as described in the "Guidelines for Certification of Visual Inspection Agencies" (published by the Japan Accreditation Board) observes no visible floating or swirling gel particles on the upper liquid layer side of the already stacked gel layer when viewed from a distance of 30 cm or less from the beaker. Measurements were performed five times, and the average of the three values excluding the maximum and minimum values was taken as the gel settling velocity. The three values excluding the maximum and minimum values and the average value are shown in Table 1.

<Evaluation of liquid leakage during use>
[Preparation of artificial urine]
An artificial urine having the following composition was prepared.
Deionized water 5919.6g
・NaCl 60.0g
・CaCl ₂・H ₂ O 1.8g
・_{MgCl2.6H2O 3.6g}
・Food blue No. 1 (for coloring)
1% Triton X-100 15.0g

[Calibration of the liquid delivery pump]
An INTEGRA liquid delivery pump (DOSEIT P910, inlet diameter: 0.5 cmφ) was assembled. The test conditions were set to a test liquid delivery volume of 40 mL and a delivery rate of 8 mL/sec. The test liquid was delivered to the tip of a tube (inner diameter: 0.5 cm). After the tip of the tube was placed in a measuring cylinder, the test liquid was injected into the measuring cylinder by driving the liquid delivery pump. Calibration was performed by reading the value on the measuring cylinder and confirming that the injection volume of the test liquid was the set value (40 mL).

[Production of absorbent articles]
An airflow type mixer (Pad Former, manufactured by Autech Co., Ltd.) was used to uniformly mix 10 g of water-absorbent resin particles and 8 g of pulverized pulp by air papermaking to produce a sheet-like absorbent body measuring 40 cm x 12 cm. Next, the absorbent body was sandwiched between two sheets of tissue paper with a basis weight of 16 g/ ^m2 and the same size as the sheet-like absorbent body, and pressed under a load of 196 kPa for 30 seconds to obtain a laminate.

A backsheet with a pair of gathers attached to each of the two short edges was removed from a commercially available diaper (manufactured by Procter & Gamble Japan Co., Ltd., product name: Pampers Sarasara Care, tape type, size L, purchased in 2018). The gathers were turned up in the short direction to expose the center of the backsheet. Next, a polyethylene sheet (12 cm x 42 cm) was attached with tape adhesive to the central area of the backsheet, excluding the areas (width: 3 cm) at both longitudinal edges. The absorbent body prepared as described above was then placed in the center of the polyethylene sheet. Next, tape adhesive was applied to the periphery of the absorbent body, and then a 22 g/ ^m² polyethylene-polypropylene air-through porous liquid-permeable sheet (15.5 cm x 47 cm) was attached to the backsheet so as to cover the absorbent body. Then, the central portion of the backsheet of the pair of gathers (the portion that had been turned up) was attached to the liquid-permeable sheet so that it returned to its original position, and an absorbent article (diaper) was obtained in which that portion overlapped the liquid-permeable sheet.

[Measurement of liquid absorption amount of absorbent article]
A Kato Tech doll (a mannequin for girls, a child's anatomical model from the lower abdomen to the thighs, transparent) was prepared in an environment of 25±2°C temperature and 50±10% humidity. The doll's dimensions are as follows. The test liquid was adjusted to a temperature of 25±2°C in a thermostatic chamber. After connecting the tube leading to the doll's urination area to the tube of the liquid supply pump, the test liquid was supplied to the liquid outlet of the doll's urination area.
Belly circumference (around the navel): 475 mm
Hip circumference: 465mm
Length around the base of the leg: 280mm
Crotch length (length between the bases of both legs): 36mm
Length between navel and urination area: 145mm

After measuring the mass Wd of the above-mentioned absorbent article, the absorbent article was fitted to a doll. The fastening tape on the absorbent article was attached at the tape attachment position "1" printed on the surface of the backsheet. At this time, it was confirmed that the gathers of the absorbent article were in contact with the doll's crotch area, that the gathers were standing up, that there was no imbalance in the length of the fitted portion between the front and back of the absorbent article, and that there was no looseness around the back or stomach.

The doll was placed face up in the tray. 40 mL of test liquid was pumped into the tray every 5 minutes at a rate of 8 mL/sec. The pumping was repeated until leakage from the absorbent article (leakage from the back or crotch) occurred. After leakage occurred, the absorbent article was removed from the doll and the mass We of the absorbent article was measured. The amount of test liquid absorbed (We - Wd) was then calculated. The results are shown in Table 1. The greater the amount of liquid absorbed, the greater the amount of liquid that can be absorbed before leakage occurs.

It was confirmed that absorbent articles using the water-absorbent resin particles of the examples, which have a gel settling rate of a specific value, can absorb more liquid before leakage occurs from the absorbent article when worn.

1...burette portion, 3...clamp, 5...conduit, 10...absorbent body, 10a...water-absorbent resin particles, 10b...fiber layer, 11...stand, 13...measuring table, 13a...through-hole, 15...nylon mesh sheet, 20a, 20b...core wrap, 21...burette tube, 22...cock, 23...rubber stopper, 24...cock, 25...air introduction tube, 30...liquid-permeable top sheet, 40...liquid-impermeable back sheet, 50...physiological saline solution, 100...absorbent article, 200...stirring blade, 200a...shaft, 200b...flat plate portion, S...slit.

Claims (9)

a polymer particle including a crosslinked polymer having structural units derived from an ethylenically unsaturated monomer, and a plurality of inorganic particles disposed on the surface of the polymer particle;
the crosslinked polymer has internal crosslinks derived from an internal crosslinking agent,
the amount of the internal crosslinking agent used is 0.05 to 10 mmol per 1 mole of the ethylenically unsaturated monomer;
The gel sedimentation rate measured by a method including the following (A), (B), and (C) in this order is 46 seconds or more and 100 seconds or less,
Water-absorbent resin particles having a bulk density of 0.65 g/ml or more.
(A) 300 g of saline is stirred in a 300 ml beaker at 600 rpm using a magnetic stirrer.
(B) 0.5 g of water-absorbent aqueous resin particles is added to the physiological saline solution.
(C) The stirring bar is stopped 15 minutes after the addition of the physiological saline solution, and the time from the time when the stirring bar is stopped until the gel formed by the swelling of the water-absorbent resin particles in the water is completely precipitated is defined as the gel settling rate. The water-absorbent resin particles according to claim 1, having a saline water retention capacity of 20 g/g or more. 3. The water-absorbent resin particle according to claim 1, wherein the crosslinked polymer is a polymer obtained by polymerizing the ethylenically unsaturated monomer in the presence of a radical polymerization initiator (excluding azo compounds). a polymer particle including a crosslinked polymer having structural units derived from an ethylenically unsaturated monomer, and a plurality of inorganic particles disposed on the surface of the polymer particle;
the crosslinked polymer is obtained by polymerizing the ethylenically unsaturated monomer in the presence of a radical polymerization initiator (excluding azo compounds), and has internal crosslinks derived from an internal crosslinking agent;
the amount of the internal crosslinking agent used is 0.05 to 10 mmol per 1 mole of the ethylenically unsaturated monomer;
A water-absorbent resin particle having a gel sedimentation rate of 46 seconds or more and 100 seconds or less, measured by a method including the following (A), (B), and (C) in this order (however, excluding a porous water-absorbent polymer obtained by polymerizing an O/W emulsion obtained by dispersing a hydrophobic organic compound in an aqueous monomer solution containing a water-soluble surfactant and/or a water-soluble polymer dispersant in a state where the emulsion is dispersed and suspended in a polymerization dispersion medium containing a polymerization dispersant under stirring):
(A) 300 g of saline is stirred in a 300 ml beaker at 600 rpm using a magnetic stirrer.
(B) 0.5 g of water-absorbent aqueous resin particles is added to the physiological saline solution.
(C) The stirring bar is stopped 15 minutes after the addition of the physiological saline solution, and the time from the time when the stirring bar is stopped until the gel formed by the swelling of the water-absorbent resin particles in the water is completely precipitated is defined as the gel settling rate. An absorbent material comprising the water-absorbent resin particles according to any one of claims 1 to 4 . An absorbent article comprising the absorbent body according to claim 5 . The absorbent article of claim 6 , which is a diaper. A method for producing water-absorbent resin particles, comprising:
the water-absorbing resin particles include polymer particles and a plurality of inorganic particles disposed on the surfaces of the polymer particles,
a method for selecting the water-absorbent resin particles having a gel settling velocity of 46 seconds or more and 100 seconds or less, measured by a method comprising the following (A), (B), and (C) in this order:
(A) 300 g of saline is stirred in a 300 ml beaker at 600 rpm using a magnetic stirrer.
(B) 0.5 g of water-absorbent aqueous resin particles is added to the physiological saline solution.
(C) The stirring bar is stopped 15 minutes after the addition of the physiological saline solution, and the time from the time when the stirring bar is stopped until the gel formed by the swelling of the water-absorbent resin particles in the water is completely precipitated is defined as the gel settling rate. A method for suppressing liquid leakage from an absorbent article containing water-absorbent resin particles, comprising:
the water-absorbing resin particles include polymer particles and a plurality of inorganic particles disposed on the surfaces of the polymer particles,
A method comprising adjusting a gel settling rate of the water-absorbent resin particles, measured by a method comprising the following steps (A), (B), and (C) in this order, to 46 seconds or more and 100 seconds or less:
(A) 300 g of saline is stirred in a 300 ml beaker at 600 rpm using a magnetic stirrer.
(B) 0.5 g of water-absorbent aqueous resin particles is added to the physiological saline solution.
(C) The stirring bar is stopped 15 minutes after the addition of the physiological saline solution, and the time from the time when the stirring bar is stopped until the gel formed by the swelling of the water-absorbent resin particles in the water is completely precipitated is defined as the gel settling rate.