Water-absorbent resin particles

Abstract

Disclosed are water-absorbent resin particles having a gel brightness L* of 8-60 which is determined by a method including the following steps (A), (B), and (C) in this order. (A) In a colorless transparent round cell having an inner diameter of 30 mm for a color difference meter, 0.1 g of water-absorbent resin particles are uniformly sprayed. (B) 5.0 g of ion exchange water is added to the round cell and absorbed in the water-absorbent resin particles. (C) After 5 minutes from the addition of the ion exchange water, the obtained gel brightness L* is determined in a black background.

Description

[Technical Field]

[0001] The present invention relates to water-absorbent resin particles. [Background technology]

[0002] Water-absorbent resins are used in the field of sanitary products, specifically as materials for absorbents contained in absorbent articles such as diapers. In the manufacture of absorbents, attempts have been made to improve the absorption efficiency of the absorbent by, for example, adjusting the properties of pulp (e.g., Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2012-200429 Summary of the Invention [Problem to be solved by the invention]

[0004] However, there is still room for improvement in the absorption performance of absorbents. An object of the present invention is to provide water-absorbent resin particles that provide absorbents with excellent absorption capacity. [Means for solving the problem]

[0005] The present inventors have found for the first time that the brightness of the gel when swollen by about 50 times varies depending on the water-absorbent resin particles, and that when the gel brightness is within a predetermined range, the absorbent body has excellent swelling capacity.

[0006] The water-absorbent resin particles of the present invention have a gel lightness L measured by a method comprising the following steps (A), (B) and (C) in this order: * is 8 to 60. (A) 0.1 g of water-absorbent resin particles is uniformly dispersed in a colorless, transparent round cell for a color difference meter having an inner diameter of 30 mm. (B) 5.0 g of ion-exchanged water is added to the round cell, and the water-absorbent resin particles are allowed to absorb the water. (C) The brightness of the resulting gel 5 minutes after adding ion-exchanged water, L * is measured against a black background.

[0007] The water-absorbent resin particles make it possible to obtain an absorbent material with excellent absorption capacity.

[0008] The water-absorbent resin particles preferably have a saline water retention capacity of 30 to 60 g / g.

[0009] The water-absorbent resin particles preferably have an unpressurized DW 5-minute value of 30 ml / g or more.

[0010] The present invention also provides an absorbent material containing the above water-absorbent resin particles.

[0011] The present invention also provides an absorbent article comprising the above-mentioned absorbent body.

[0012] The present invention also provides an absorbent article that is a diaper.

[0013] The present invention also provides a gel lightness L measured by a method comprising the following steps (A), (B), and (C) in this order: * and selecting water-absorbent resin particles having a molecular weight of 8 to 60. (A) 0.1 g of water-absorbent resin particles is uniformly dispersed in a colorless, transparent round cell for a color difference meter having an inner diameter of 30 mm. (B) 5.0 g of ion-exchanged water is added to the round cell, and the water-absorbent resin particles are allowed to absorb the water. (C) The brightness of the resulting gel 5 minutes after adding ion-exchanged water, L * is measured against a black background.

[0014] An absorbent material using the water-absorbent resin particles obtained by the above-mentioned production method can have a high absorption capacity.

[0015] The present invention also provides a method for measuring a gel lightness L of a water-absorbent resin particle, the method comprising the following steps (A), (B), and (C) in this order: *The present invention also provides a method for improving the absorption capacity of an absorbent material containing the water-absorbent resin particles, the method comprising adjusting the above-mentioned formula: (A) 0.1 g of water-absorbent resin particles is uniformly dispersed in a colorless, transparent round cell for a color difference meter having an inner diameter of 30 mm. (B) 5.0 g of ion-exchanged water is added to the round cell, and the water-absorbent resin particles are allowed to absorb the water. (C) The brightness of the resulting gel 5 minutes after adding ion-exchanged water, L * is measured against a black background. [Effects of the Invention]

[0016] The present invention provides water-absorbent resin particles that provide an absorbent material with excellent absorption capacity. [Brief explanation of the drawings]

[0017] [Figure 1] FIG. 1 is a cross-sectional view showing an example of an absorbent article. [Figure 2] FIG. 2 is a plan view showing the general shape of an agitating blade (a flat blade having a slit in the flat plate portion). [Figure 3] FIG. 1 is a diagram showing an outline of a method for measuring gel brightness. [Figure 4] FIG. 1 is a schematic diagram showing a method for measuring non-pressure DW. [Figure 5] Photographs showing the gels of Example 1 (a) and Comparative Example 3 (b) are shown. DETAILED DESCRIPTION OF THE INVENTION

[0018] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail below with reference to the preferred embodiments. However, the present invention is not limited to the following embodiments and can be practiced in various modified forms within the scope of the present invention.

[0019] 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 the 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 the multiple substances present in the composition, unless otherwise specified. "Saline" refers to a 0.9% by mass aqueous solution of sodium chloride.

[0020] The water-absorbent resin particles according to the present embodiment have a gel lightness L measured by a method including the following steps (A), (B), and (C) in this order: * However, it is between 8 and 60. (A) 0.1 g of water-absorbent resin particles is uniformly dispersed in a colorless, transparent round cell for a color difference meter having an inner diameter of 30 mm. (B) 5.0 g of ion-exchanged water is added to the round cell, and the water-absorbent resin particles are allowed to absorb the water. (C) The brightness of the resulting gel 5 minutes after adding ion-exchanged water, L * is measured against a black background.

[0021] The lightness L in this specification * That is, L * a * b * It is a color system and is expressed in the range of 0 to 100. Gel lightness L * A more specific method for measuring this will be shown in the Examples below.

[0022] In this embodiment, the gel lightness is measured using a black background, so the higher the light transmittance of the gel, the lower the gel lightness. * When water-absorbent resin particles having a gel lightness L in the range of 8 to 60 are used in an absorbent body, the absorbent body can have a high absorption capacity. The reason why such an effect is obtained is not clear, but the present inventors speculate as follows. However, the present invention is not limited to the following mechanism. * In the water-absorbent resin particles according to the present embodiment, each particle is more uniformly crosslinked and the crosslinking within the particle is more uniform, so that when the particles are swollen by about 50 times, each particle absorbs liquid uniformly and swells uniformly within the particle, and therefore the brightness of the entire gel is thought to be 60 or less. * By keeping the particle diameter within the specified range, when used in an absorbent body, it is believed that each particle and each particle can absorb water more evenly, thereby improving the absorption capacity of the absorbent body.

[0023] Gel Lightness L * From the viewpoint of further improving the absorption capacity of the absorbent body, the gel lightness L is preferably 58 or less, more preferably 55 or less, even more preferably 50 or less, and even more preferably 45 or less. * may be, for example, 10 or more, 20 or more, 30 or more, or 35 or more.

[0024] The water-absorbent resin particles according to this embodiment have a brightness L * However, the lightness L in a dry state may be, for example, 91 or more or 95 or more. * The lightness L of water-absorbent resin particles having a moisture content of 10% by mass or less * Refers to...

[0025]

[0043] From the viewpoint of easily increasing the 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, 35 g / g or more, 38 g / g or more, 40 g / g or more, 42 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.

[0026] The water-absorbent resin particles according to this embodiment may have a 5-minute no-pressure DW value of, for example, 30 to 80 ml / g. From the viewpoint of easily increasing the absorption capacity of the absorbent body, the 5-minute no-pressure DW value is preferably 37 ml / g or more, more preferably 41 ml / g or more, and even more preferably 44 ml / g or more. The 5-minute no-pressure DW value may be, for example, 70 ml / g or less.

[0027] The pure water absorption capacity (ion-exchanged water absorption capacity) of the water-absorbent resin particles according to this embodiment may be 100 to 1000 g / g, 200 to 800 g / g, or 300 to 700 g / g. The pure water absorption capacity may be a value at 25° C. The pure water absorption capacity of the water-absorbent resin particles can be measured by the method described in the examples below.

[0028] Examples of the shape of the water-absorbent resin particles according to the present embodiment include substantially spherical, crushed, and granular shapes. The water-absorbent resin particles according to the present embodiment may be in a form consisting of a single particle, or may be in a form (secondary particles) in which fine particles (primary particles) are aggregated. The water-absorbent resin particles according to the present embodiment (water-absorbent resin particles before water absorption) may have a median particle diameter of 250 to 850 μm, 300 to 700 μm, 300 to 600 μm, 330 to 500 μm, or 350 to 400 μm. The water-absorbent resin particles according to the present embodiment may have a desired particle size distribution when obtained by the production method described below, but the particle size distribution may be adjusted by performing an operation such as particle size adjustment using classification with a sieve.

[0029] The water-absorbent resin particles according to the present 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 as the polymer particles. That is, the water-absorbent resin particles according to the present 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 (such as water retention capacity) of the resulting water-absorbent resin particles and facilitating control of the polymerization reaction. Hereinafter, reverse-phase suspension polymerization will be described as an example of a method for polymerizing an ethylenically unsaturated monomer.

[0030] 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. The functional groups, such as the carboxyl group and amino group, of the above-mentioned monomers can function as functional groups capable of crosslinking in the surface crosslinking step described below.

[0031] Among these, from the viewpoint of industrial ease of availability, the ethylenically unsaturated monomer preferably contains 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 contains at least one compound selected from the group consisting of (meth)acrylic acid and its salts, and acrylamide. From the viewpoint of further improving water absorption properties (water retention capacity, etc.), the ethylenically unsaturated monomer more preferably contains at least one compound selected from the group consisting of (meth)acrylic acid and its salts. That is, the water absorbent resin particles preferably have structural units derived from at least one selected from the group consisting of (meth)acrylic acid and its salts.

[0032] Monomers other than the above-mentioned 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-mentioned 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% based on the total amount of monomers (the total amount of monomers for obtaining water-absorbent resin particles; for example, the total amount of monomers that provide structural units of a crosslinked polymer; the same applies hereinafter). In particular, the proportion of (meth)acrylic acid and salts thereof may be 70 to 100 mol%, 80 to 100 mol%, 90 to 100 mol%, 95 to 100 mol%, or 100 mol% based on the total amount of monomers. The "proportion of (meth)acrylic acid and salts thereof" means the proportion of the total amount of (meth)acrylic acid and salts thereof.

[0033] According to the present embodiment, as an example of the water-absorbent resin particles, it is possible to provide water-absorbent resin particles containing a crosslinked polymer having a structural unit derived from an ethylenically unsaturated monomer, wherein the ethylenically unsaturated monomer contains at least one compound selected from the group consisting of (meth)acrylic acid and salts thereof, and the ratio of the (meth)acrylic acid and salts thereof is 70 to 100 mol % based on the total amount of monomers for obtaining the water-absorbent resin particles.

[0034] The ethylenically unsaturated monomer is usually preferably used as an aqueous solution. The concentration of the ethylenically unsaturated monomer in the aqueous solution containing the ethylenically unsaturated monomer (hereinafter simply referred to as "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.

[0035] When the ethylenically unsaturated monomer has an acid group, the aqueous monomer solution may be used after neutralizing the acid group with an alkaline neutralizing agent. 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 (water retention capacity, etc.). Examples of alkaline neutralizing agents include alkali metal salts such as sodium hydroxide, sodium carbonate, sodium bicarbonate, potassium hydroxide, and potassium carbonate; and ammonia. 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 operation. The acid group of the ethylenically unsaturated monomer can be neutralized, 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 them.

[0036] In the reversed phase suspension polymerization method, an aqueous solution of a monomer is dispersed in a hydrocarbon dispersion medium in the presence of a surfactant, and an ethylenically unsaturated monomer can be polymerized using a radical polymerization initiator or the like.

[0037] Examples of surfactants include nonionic surfactants and anionic surfactants. Examples of 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. Examples of anionic surfactants include fatty acid salts, alkylbenzene sulfonates, alkylmethyl taurates, polyoxyethylene alkylphenyl ether sulfates, polyoxyethylene alkyl ether sulfonates, polyoxyethylene alkyl ether phosphates, and polyoxyethylene alkylallyl ether phosphates. The surfactants may be used alone or in combination of two or more.

[0038] From the viewpoints of obtaining a good state of W / O type reverse phase suspension, easily obtaining water-absorbent resin particles having a suitable particle size, and being easily available industrially, the surfactant preferably contains at least one compound selected from the group consisting of sorbitan fatty acid esters, polyglycerin fatty acid esters, and sucrose fatty acid esters. From the viewpoints of easily obtaining an appropriate particle size distribution of the water-absorbent resin particles and easily improving the water-absorption properties (water retention capacity, etc.) of the water-absorbent resin particles and the performance of absorbents and absorbent articles using the same, the surfactant preferably contains a sucrose fatty acid ester, more preferably a sucrose stearate ester.

[0039] 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 the aqueous monomer solution, from the viewpoint of obtaining a sufficient effect relative to the amount used and from the viewpoint of economy.

[0040] 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 polymeric 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.

[0041] 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 the aqueous monomer solution, from the viewpoint of obtaining a sufficient effect relative to the amount used and from the viewpoint of economy.

[0042] 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. The hydrocarbon dispersion medium may be used alone or in combination of two or more.

[0043] The hydrocarbon dispersion medium may contain at least one selected from the group consisting of n-heptane and cyclohexane, which are easily available industrially and have stable quality. From the same viewpoint, the mixture of the hydrocarbon dispersion medium may be, for example, commercially available Exxol Heptane (manufactured by ExxonMobil Corporation: containing 75 to 85% of n-heptane and isomeric hydrocarbons).

[0044] 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 the aqueous monomer solution, from the viewpoint of adequately removing the heat of polymerization and facilitating control of the polymerization temperature. When the amount of hydrocarbon dispersion medium used is 30 parts by mass or more, control of the polymerization temperature tends to be easy. When the amount of hydrocarbon dispersion medium used is 1,000 parts by mass or less, polymerization productivity tends to be improved, which is economical.

[0045] The radical polymerization initiator is preferably water-soluble, and examples thereof include persulfates such as potassium persulfate, ammonium persulfate, and sodium persulfate; peroxides such as methyl ethyl ketone peroxide, methyl isobutyl ketone peroxide, di-t-butyl peroxide, t-butylcumyl peroxide, t-butyl peroxyacetate, t-butyl peroxyisobutyrate, t-butyl peroxypivalate, and hydrogen peroxide; 2,2'-azobis(2-amidinopropane) dihydrochloride, 2,2'-azobis[2-(N-phenylamidino)propane] dihydrochloride, 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.

[0046] The amount of radical polymerization initiator used may be 0.05 to 10 millimoles per mole of the ethylenically unsaturated monomer. When the amount of radical polymerization initiator used is 0.05 millimoles or more, the polymerization reaction does not take a long time and is efficient. When the amount of radical polymerization initiator used is 10 millimoles or less, it is easy to suppress the occurrence of a rapid polymerization reaction.

[0047] 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.

[0048] During the polymerization reaction, the aqueous monomer solution used for polymerization may contain a chain transfer agent, such as hypophosphites, thiols, thiolic acids, secondary alcohols, and amines.

[0049] In order to control the particle size of the water-absorbent resin particles, the aqueous monomer solution used for 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, if the stirring speed during polymerization is the same, the higher the viscosity of the aqueous monomer solution, the larger the median particle size of the resulting particles tends to be.

[0050] During polymerization, internal crosslinking may occur due to self-crosslinking, but crosslinking may also be performed by using an internal crosslinking agent. The use of an internal crosslinking agent makes it easy to control the water absorption properties (water retention capacity, etc.) of the water-absorbent resin particles. The internal crosslinking agent is usually added to the reaction solution 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.

[0051] The amount of the internal crosslinking agent used is preferably 30 mmol or less, more preferably 0.01 to 10 mmol, even more preferably 0.012 to 5 mmol, particularly preferably 0.015 to 1 mmol, extremely preferably 0.02 to 0.1 mmol, and very preferably 0.025 to 0.08 mmol, per mole of the ethylenically unsaturated monomer, from the viewpoint of easily obtaining an excellent absorbent swelling capacity in the absorbent article and of easily obtaining a sufficient water absorption capacity by suppressing the water-soluble properties of the resulting polymer through moderate crosslinking.

[0052] An aqueous phase containing an ethylenically unsaturated monomer, a radical polymerization initiator, and optionally an internal crosslinking agent, and an oil phase containing a hydrocarbon dispersion medium, a surfactant, and optionally a polymeric dispersant, etc., are mixed and heated under stirring to carry out reverse phase suspension polymerization in a water-in-oil system.

[0053] 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). In this case, 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 start of the polymerization reaction.

[0054] Among these, from the viewpoint of easily reducing the amount of hydrocarbon dispersion medium remaining in an obtained water absorbent resin, it is preferable to disperse an aqueous monomer solution in a hydrocarbon dispersion medium having a polymeric dispersant dispersed therein, and then further disperse a surfactant therein, and then carry out polymerization.

[0055] The reversed-phase suspension polymerization can be carried out in one stage or in multiple stages of two or more stages, and is preferably carried out in two to three stages from the viewpoint of increasing productivity.

[0056] When performing reversed-phase suspension polymerization in two or more stages, after performing the first-stage reversed-phase suspension polymerization, an ethylenically unsaturated monomer is added to and mixed with the reaction mixture obtained in the first-stage polymerization reaction, and reversed-phase suspension polymerization in the second and subsequent stages is performed in the same manner as in the first stage. In the reversed-phase suspension polymerization in each stage from the second stage onward, in addition to the ethylenically unsaturated monomer, it is preferable to perform the reversed-phase suspension polymerization by adding 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 be used as needed in the reversed-phase suspension polymerization in each stage from the second stage onward. When an internal crosslinking agent is used, it is preferable to perform the reversed-phase suspension polymerization by adding the internal crosslinking 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.

[0057] The temperature of the polymerization reaction varies depending on the radical polymerization initiator used, but is preferably 20 to 150°C, more preferably 40 to 120°C, from the viewpoints of rapidly progressing the polymerization, shortening the polymerization time, thereby improving economy, and easily removing the heat of polymerization to smoothly carry out the reaction. 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 increase in the reaction system. As a result, a polymer of the ethylenically unsaturated monomer is usually obtained in the form of a hydrogel.

[0058] After polymerization, a post-polymerization crosslinking agent may be added to the resulting hydrogel polymer and the resulting polymer may be heated to crosslink the polymer. By crosslinking the polymer after polymerization, the degree of crosslinking of the hydrogel polymer can be increased, thereby further improving the water absorption properties (water retention capacity, etc.).

[0059] Examples of crosslinking agents for crosslinking after polymerization 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.

[0060] The amount of the post-polymerization crosslinking agent may be 30 mmol or less, 10 mmol or less, or 0.01 to 5 mmol per mole of the ethylenically unsaturated monomer, from the viewpoint of easily obtaining suitable water absorption properties (water retention capacity, etc.).

[0061] The timing of adding the post-polymerization crosslinking agent may be after the polymerization of the ethylenically unsaturated monomer used in the polymerization, and in the case of multi-stage polymerization, it is preferably added after the multi-stage polymerization. Note that, taking into consideration heat generation during and after polymerization, retention due to process delay, opening of the system when the crosslinking agent is added, and fluctuations in moisture due to addition of water accompanying the addition of the crosslinking agent, the post-polymerization crosslinking agent is preferably added in the range of [moisture content immediately after polymerization ±3% by mass] from the viewpoint of moisture content (described later).

[0062] Subsequently, the obtained hydrogel polymer is dried to remove water, thereby obtaining 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 dispersed in a hydrocarbon dispersion medium is heated externally to perform azeotropic distillation and the hydrocarbon dispersion medium is refluxed to remove water, (b) a method in which the hydrogel polymer is removed by decantation and dried under reduced pressure, and (c) a method in which the hydrogel polymer is filtered through a filter and dried under reduced pressure. Among these, method (a) is preferred because of its simplicity in the production process.

[0063] 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 can increase the particle size of the resulting water-absorbent resin particles. As the flocculant, an inorganic flocculant can be used. Examples of inorganic flocculants (e.g., powdered inorganic flocculants) include silica, zeolite, bentonite, aluminum oxide, talc, titanium dioxide, kaolin, clay, and hydrotalcite. From the viewpoint of excellent flocculation effect, the flocculant is preferably at least one selected from the group consisting of silica, aluminum oxide, talc, and kaolin.

[0064] In the reversed-phase suspension polymerization, a method of adding a flocculant is preferably a method in which the flocculant is preliminarily dispersed in the same type of hydrocarbon dispersion medium as that used in the polymerization or in water, and then mixed under stirring into the hydrocarbon dispersion medium containing the hydrous gel polymer.

[0065] The amount of the 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, relative to 100 parts by mass of the ethylenically unsaturated monomer used in polymerization. When the amount of the flocculant added is within the above range, water-absorbent resin particles having a target particle size distribution can be easily obtained.

[0066] In the production of water-absorbent resin particles, it is preferable to perform surface cross-linking of the surface portion (surface and near-surface) of the hydrogel polymer using a cross-linking agent in the drying step (water removal step) or a subsequent step. By performing surface cross-linking, it is easy to control the gel brightness and water absorption characteristics (water retention capacity, etc.) of the water-absorbent resin particles. It is preferable to perform surface cross-linking when the hydrogel polymer has a specific water content. The timing of surface cross-linking is preferably when the water 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.

[0067] The water content (mass%) of the hydrogel polymer is calculated by the following formula. Moisture content = [Ww / (Ww+Ws)]×100 Ww: The water content of the hydrogel polymer, calculated by subtracting the amount of water discharged outside the system in the drying process from the amount of water contained in the aqueous monomer solution before polymerization in all polymerization processes, and adding the amount of water used as needed when mixing a flocculant, surface cross-linking agent, etc. Ws: The solid content calculated from the amounts of the ethylenically unsaturated monomer, crosslinking agent, initiator, and other materials that constitute the hydrogel polymer.

[0068] Examples of the crosslinking agent (surface crosslinking agent) for performing surface crosslinking include compounds having two or more reactive functional groups. Examples of the surface crosslinking agent include polyols such as ethylene glycol, propylene glycol, 1,4-butanediol, trimethylolpropane, glycerin, polyoxyethylene glycol, polyoxypropylene glycol, and polyglycerin; polyglycidyl compounds such as (poly)ethylene glycol diglycidyl ether, (poly)glycerin diglycidyl ether, (poly)glycerin triglycidyl ether, trimethylolpropane triglycidyl ether, (poly)propylene glycol polyglycidyl ether, and (poly)glycerol polyglycidyl ether; epichlorohydrin, epibromohydrin, α-methylepichlorohydrin, and the like. Examples of the surface cross-linking agent include haloepoxy compounds such as benzophenone, 2,4-tolylene diisocyanate, and hexamethylene diisocyanate; isocyanate compounds such as 3-methyl-3-oxetanemethanol, 3-ethyl-3-oxetanemethanol, 3-butyl-3-oxetanemethanol, 3-methyl-3-oxetaneethanol, 3-ethyl-3-oxetaneethanol, and 3-butyl-3-oxetaneethanol; oxazoline compounds such as 1,2-ethylenebisoxazoline; carbonate compounds such as ethylene carbonate; and hydroxyalkylamide compounds such as bis[N,N-di(β-hydroxyethyl)]adipamide. The surface cross-linking agent may be used alone or in combination of two or more. 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.

[0069] The amount of the surface cross-linking agent used is preferably 0.01 to 20 mmol, more preferably 0.05 to 10 mmol, still more preferably 0.1 to 5 mmol, particularly preferably 0.15 to 1 mmol, and extremely preferably 0.2 to 0.5 mmol, relative to 1 mol of the ethylenically unsaturated monomer used for polymerization, from the viewpoint of easily obtaining suitable water absorption properties (water retention capacity, etc.).

[0070] After the surface crosslinking, the water and hydrocarbon dispersion medium are distilled off by a known method, and the resulting product is dried under heating and reduced pressure, thereby obtaining polymer particles that are a surface crosslinked dried product.

[0071] The polymerization reaction can be carried out using various agitators equipped with agitating blades. Examples of the 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. The flat blades have a shaft (stirring shaft) and a flat plate portion (stirring portion) arranged around the shaft. The flat plate portion may have slits or the like. When flat blades are used as agitating blades, the crosslinking reaction in the polymer particles is easily carried out uniformly, and the gel brightness can be easily adjusted to the desired range while maintaining water absorption properties such as water retention capacity.

[0072] The water-absorbent resin particles according to the present embodiment may be composed of only 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, for example, pentasodium diethylenetriaminepentaacetate, etc.), 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.

[0073] The water-absorbent resin particles may contain a plurality of inorganic particles arranged on the surface of the polymer particles. For example, the inorganic particles can be arranged on the surface of the polymer particles by mixing the polymer particles with the inorganic particles. The inorganic particles may be silica particles such as amorphous silica.

[0074] 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 in 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.

[0075] The inorganic particles herein are typically very small compared to the size of 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 a capillary electrical resistance method or a laser diffraction / scattering method depending on the particle characteristics.

[0076] The water-absorbent resin particles according to the present embodiment have excellent absorbency of body fluids such as urine and blood, and can be applied to fields such as hygiene products such as disposable diapers, sanitary napkins, and tampons, and animal excrement treatment materials such as pet sheets, and dog or cat litter compositions.

[0077] 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. The content of the water-absorbent resin particles in the absorbent body is set to 100 to 1000 g per square meter of the absorbent body (i.e., 100 to 1000 g / m2) from the viewpoint of obtaining sufficient liquid absorption performance when the absorbent body is used in an absorbent article. 2 ), and more preferably 150 to 800 g / m 2 , and more preferably 200 to 700 g / m 2 In order to ensure that the absorbent article exhibits sufficient liquid absorption performance, the content is 100 g / m 2 From the viewpoint of suppressing the occurrence of the gel blocking phenomenon, the content is preferably 1000 g / m or more. 2 It is preferable that:

[0078] The absorbent may further include, for example, fibrous material in addition to the water-absorbent resin particles. The absorbent may be, for example, a mixture containing the water-absorbent resin particles and the fibrous material. The mass ratio of the water-absorbent resin particles in the absorbent may be 2 to 100 mass%, preferably 10 to 80 mass%, and more preferably 20 to 70 mass%, relative to the total mass of the water-absorbent resin particles and the fibrous material. The absorbent may be configured, for example, in a form in which the water-absorbent resin particles and the 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.

[0079] Examples of fibrous materials include cellulose 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 usually 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.

[0080] To improve the shape retention of the absorbent body before and during use, the fibers may be bonded together by adding an adhesive binder to the fibrous material. Examples of adhesive binders include heat-fusible synthetic fibers, hot-melt adhesives, and adhesive emulsions.

[0081] 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, and the like.

[0082] 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.

[0083] The absorbent according to the present embodiment may further contain additives such as inorganic powder (e.g., amorphous silica), deodorant, pigment, dye, antibacterial agent, fragrance, adhesive, etc. These additives can impart various functions to the absorbent. When the water-absorbent resin particles contain inorganic particles, the absorbent may contain inorganic powder in addition to the inorganic particles in the water-absorbent resin particles. Examples of inorganic powders include silicon dioxide, zeolite, kaolin, clay, etc.

[0084] The shape of the absorbent body according to this embodiment is not particularly limited and may be, for example, a sheet. The thickness of the absorbent body (for example, the thickness of a sheet-shaped absorbent body) may be, for example, 0.1 to 20 mm, or 0.3 to 15 mm.

[0085] The absorbent article according to this embodiment may include, in addition to the absorbent body, a core wrap, a liquid-permeable top sheet, and a liquid-impermeable back sheet, for example. The core wrap maintains the shape of the absorbent body. The liquid-permeable top sheet is disposed on the outermost side on the side from which the liquid to be absorbed penetrates. The liquid-impermeable back sheet is disposed on the outermost side on the side opposite to the side from which the liquid to be absorbed penetrates.

[0086] Examples of 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.

[0087] Fig. 1 is a cross-sectional view showing an example of an absorbent article. The absorbent article 100 shown in Fig. 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, the core wrap 20b, the absorbent body 10, the core wrap 20a, and the liquid-permeable topsheet 30 are layered in this order. In Fig. 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.

[0088] The absorbent body 10 includes water-absorbent resin particles 10a and a fiber layer 10b containing fibrous material. The water-absorbent resin particles 10a are dispersed in the fiber layer 10b.

[0089] The core wrap 20a is disposed on one side of the absorbent body 10 (the upper side of the absorbent body 10 in FIG. 1) while in contact with the absorbent body 10. The core wrap 20b is disposed on the other side of the absorbent body 10 (the lower side of the absorbent body 10 in FIG. 1) while in contact with the absorbent body 10. The absorbent body 10 is disposed between the core wrap 20a and the core wrap 20b.

[0090] The core wraps 20a and 20b have, for example, main surfaces of the same size as the absorbent body 10. By using the core wraps, the shape retention of the absorbent body can be maintained and the water-absorbent resin particles and other components constituting the absorbent body can be prevented from falling off or flowing. Examples of core wraps include nonwoven fabrics, woven fabrics, tissues, synthetic resin films with liquid-permeable holes, and net-like sheets with mesh, and from the viewpoint of economy, tissues made by wet-forming pulp are preferably used.

[0091] The liquid permeable topsheet 30 is disposed 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 disposed on the core wrap 20a in a state of contact with the core wrap 20a. The liquid impermeable backsheet 40 is disposed on the outermost side of the absorbent article 100, opposite the liquid permeable topsheet 30. The liquid impermeable backsheet 40 is disposed below the core wrap 20b in a state of 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.

[0092] 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 preferably used.

[0093] The constituent material of the liquid permeable topsheet 30 can be any resin or fiber known in the art, and 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. From the viewpoint of increasing the strength of the liquid permeable topsheet 30, synthetic fibers are preferably used as the constituent material, and polyolefins and polyesters are particularly preferred. These materials may be used alone or in combination of two or more types.

[0094] From the viewpoint of improving the liquid absorption performance of the absorbent article, it is desirable that the nonwoven fabric used in the liquid permeable top sheet 30 has an appropriate level of hydrophilicity. From this viewpoint, 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 appropriate hydrophilicity, such as rayon fibers, or those made from hydrophobic chemical fibers, such as polyolefin fibers or polyester fibers, that have been hydrophilized by a known method to impart appropriate hydrophilicity.

[0095] Methods for hydrophilizing chemical fibers include, for example, spunbonding a mixture of hydrophobic chemical fibers and a hydrophilizing agent to produce a nonwoven fabric, adding a hydrophilizing agent when preparing a spunbonded nonwoven fabric from hydrophobic chemical fibers, or impregnating a spunbonded nonwoven fabric from hydrophobic chemical fibers with a hydrophilizing agent. Examples of hydrophilizing agents include anionic surfactants such as aliphatic sulfonates and higher alcohol sulfate esters, 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.

[0096] The nonwoven fabric used in the liquid-permeable top sheet 30 is preferably suitably 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 increasing the liquid penetration rate of the absorbent article. The basis weight of the nonwoven fabric is preferably 5 to 200 g / m 2 and more preferably 8 to 150 g / m 2 and more preferably 10 to 100 g / m 2 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.

[0097] The liquid-impermeable backsheet 40 prevents liquid absorbed in the absorber 10 from leaking out from the backsheet 40 side. The liquid-impermeable backsheet 40 can be made of a liquid-impermeable film mainly made of polyolefin resin such as polyethylene (PE) or polypropylene (PP), a breathable resin film, a composite film in which a breathable resin film is bonded to a nonwoven fabric such as spunbond or spunlace, or a spunbond / meltblown / spunbond (SMS) nonwoven fabric in which a water-resistant meltblown nonwoven fabric is sandwiched between high-strength spunbond nonwoven fabrics. From the viewpoint of ensuring flexibility so as not to impair the wearing comfort of the absorbent article, the backsheet 40 is made mainly of low-density polyethylene (LDPE) resin and has a basis weight of 10 to 50 g / m. 2 Furthermore, when a breathable material is used, stuffiness during wearing is reduced, and discomfort to the wearer can also be alleviated.

[0098] The size relationships among the absorbent body 10, core wraps 20a, 20b, liquid-permeable top sheet 30, and liquid-impermeable back sheet 40 are not particularly limited and are adjusted as appropriate depending on the intended use of the absorbent article, etc. Furthermore, the method for maintaining the shape of the absorbent body 10 using the core wraps 20a, 20b is not particularly limited, and the absorbent body may be sandwiched between multiple core wraps as shown in Figure 1, or may be covered by a single core wrap.

[0099] The absorbent body 10 may be bonded to the liquid-permeable top sheet 30. By bonding the absorbent body 10 and the liquid-permeable top sheet 30, liquid is 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, and it is even more preferable that the core wrap and the absorbent body 10 are bonded. Examples of bonding methods include a method in which a hot-melt adhesive is applied to the liquid-permeable top sheet 30 at predetermined intervals across its width in a pattern such as longitudinal stripes or a spiral, and a method in which a water-soluble binder selected from starch, carboxymethyl cellulose, polyvinyl alcohol, polyvinylpyrrolidone, and other water-soluble polymers is used for bonding. Furthermore, when the absorbent body 10 contains heat-fusible synthetic fibers, a method of bonding by heat fusion of the fibers may be used.

[0100] The method for producing water-absorbent resin particles according to the present embodiment is characterized in that the gel lightness L * Specifically, the sorting may include sorting the water-absorbent resin particles based on, for example, gel lightness L * The selection may be carried out by, for example, collecting an in-process product at any stage in the production process, and selecting water-absorbent resin particles having a suitable gel lightness L * In the above-mentioned production method, the water-absorbent resin particles are selected to satisfy the gel lightness L * The properties of the water-absorbent resin particles to be selected may satisfy the above-mentioned aspects of the water-absorbent resin particles (for example, a specific range of water retention capacity of physiological saline, a specific range of 5-minute value of no-pressure DW, etc.).

[0101] One aspect of the present embodiment is a gel lightness L of the water-absorbent resin particles measured by the above-mentioned measurement method. * This can also be said to be a method for improving the absorption capacity of an absorbent body, which includes adjusting the gel lightness L *A more specific method for measuring the absorption amount of the absorbent body is shown in the examples below. * to the range of 8 to 60, the water retention capacity of the water-absorbent resin particles in physiological saline to 30 to 60 g / g, and the 5-minute value of the unpressurized DW of the water-absorbent resin particles to 30 ml / g or more. Specific examples of the method for producing water-absorbent resin particles having these predetermined properties are as described above. The gel lightness L of the water-absorbent resin particles * In order to make the value fall within the range of 8 to 60, for example, the production conditions of the water-absorbent resin particles can be selected so that each particle of the water-absorbent resin particles is uniformly crosslinked and the uniformity of the crosslinking within the particles is further increased.

[0102] According to the present embodiment, it is possible to provide a method for manufacturing an absorbent body using water-absorbent resin particles obtained by the above-mentioned method for manufacturing water-absorbent resin particles. The method for manufacturing an absorbent body according to the present embodiment includes a particle manufacturing step of obtaining water-absorbent resin particles by the above-mentioned method for manufacturing water-absorbent resin particles. The method for manufacturing an absorbent body according to the present embodiment may include a step of mixing water-absorbent resin particles and fibrous material after the particle manufacturing step. According to the present embodiment, it is possible to provide a method for manufacturing an absorbent article using the absorbent body obtained by the above-mentioned method for manufacturing an absorbent body. The method for manufacturing an absorbent article according to the present embodiment includes an absorbent body manufacturing step of obtaining the absorbent body by the above-mentioned method for manufacturing an absorbent body. The method for manufacturing an absorbent article according to the present embodiment may include a step of obtaining the absorbent article using the absorbent body and other constituent members of the absorbent article after the absorbent body manufacturing step, and in this step, the absorbent article is obtained, for example, by stacking the absorbent body and other constituent members of the absorbent article on each other. [Example]

[0103] The present invention will be described in more detail below using examples and comparative examples, but the present invention is not limited to the following examples.

[0104] <Production of water-absorbent resin particles> [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 (flat blade) 200, the outline of which is shown in FIG. 2, was attached to the stirrer. The stirring blade 200 includes a shaft 200a and a flat plate portion 200b. The flat plate portion 200b is welded to the shaft 200a and has a curved tip. Four slits S extending along the axial direction of the shaft 200a are formed in the flat plate portion 200b. The four slits S are arranged in the width direction of the flat plate portion 200b, with the inner two slits S being 1 cm wide and the outer two slits S being 0.5 cm wide. The length of the flat plate portion 200b is approximately 10 cm, and the width of the flat plate portion 200b is approximately 6 cm. In the prepared separable flask, 293 g of n-heptane and 0.736 g of maleic anhydride-modified ethylene-propylene copolymer (Hiwax 1105A, manufactured by Mitsui Chemicals, Inc.) as a dispersant were mixed. The mixture in the separable flask was stirred with a stirrer and heated to 80 °C to dissolve the dispersant in the n-heptane. The resulting solution was cooled to 50 °C.

[0105] Separately, 92.0 g (1.03 mol) of an 80.5 wt% 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 wt% aqueous solution of sodium hydroxide was added dropwise to the beaker to neutralize it to 75 mol%. The first aqueous solution was then prepared by adding and dissolving 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 0.010 g (0.057 mmol) of ethylene glycol diglycidyl ether as an internal crosslinker.

[0106] The first-stage aqueous solution was added to the separable flask and stirred for 10 minutes. Then, a surfactant solution was obtained by dissolving 0.736 g of sucrose stearate (Mitsubishi Chemical Foods Corporation, Ryoto Sugar Ester S-370, HLB: 3) as a surfactant in 6.62 g of n-heptane with heating, and the solution was added to the separable flask. The system was thoroughly purged with nitrogen while stirring at a stirrer speed of 425 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 to obtain a first-stage polymerization slurry.

[0107] In a separate 500 mL beaker, 128.8 g (1.44 mol) of an 80.5 wt% aqueous solution of acrylic acid (as a water-soluble ethylenically unsaturated monomer) was placed. While cooling from the outside, 159.0 g of a 27 wt% aqueous solution of sodium hydroxide was added dropwise to neutralize the solution to 75 mol%. After neutralization, 0.0907 g (0.381 mmol) of sodium persulfate as a water-soluble radical polymerization initiator and 0.0116 g (0.067 mmol) of ethylene glycol diglycidyl ether as an internal crosslinking agent were added and dissolved to prepare the second aqueous solution.

[0108] While stirring at a stirrer speed of 650 rpm, the contents of the separable flask system were cooled to 25°C, and then the entire amount of the second-stage aqueous liquid was added to the first-stage polymerization slurry. After the atmosphere in the separable flask system was replaced with nitrogen for 30 minutes, the separable flask was again immersed in a 70°C water bath to raise the temperature, and a polymerization reaction was carried out for 60 minutes to obtain a hydrogel polymer.

[0109] After the second-stage polymerization, 0.589 g of a 45% by mass aqueous solution of pentasodium diethylenetriamine pentacetate was added to the hydrous gel polymer under stirring. The separable flask was then immersed in an oil bath set at 125°C, and 262.7 g of water was extracted from the system by azeotropic distillation of n-heptane and water while refluxing n-heptane. Then, 4.42 g (0.507 mmol) of a 2% by mass 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.

[0110] Thereafter, n-heptane was evaporated at 125°C to dry the particles, thereby obtaining polymer particles (dried product). The polymer particles were passed through a sieve with an opening of 850µm, 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, thereby obtaining 222.0g of water-absorbent resin particles containing amorphous silica. The water-absorbent resin particles had a median particle size of 372µm and a pure water absorption of 568g / g.

[0111] [Example 2] Except for changing the amount of water extracted to the outside of the system by azeotropic distillation to 271.0 g and changing the amount of amorphous silica mixed with the polymer particles to 0.2 mass % relative to the mass of the polymer particles, the same procedure as in Example 1 was carried out to obtain 229.0 g of water-absorbent resin particles. The water-absorbent resin particles had a median particle diameter of 360 μm and a pure water absorption of 684 g / g.

[0112] [Example 3]

[0113] In the same manner as in Example 1, except that in the preparation of the first-stage polymerization slurry liquid, the stirrer rotation speed during nitrogen substitution was changed to 350 rpm, 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 % relative to the mass of the polymer particles, 231.2 g of water-absorbent resin particles were obtained. The water-absorbent resin particles had a median particle diameter of 359 µm and a pure water absorption of 452 g / g.

[0113] [Example 4] In the preparation of the first-stage aqueous liquid, the amount of ethylene glycol diglycidyl ether added as an internal crosslinking agent was changed to 0.0156 g (0.090 mmol), in the preparation of the first-stage polymerization slurry liquid, the stirrer rotation speed during nitrogen substitution was changed to 350 rpm, in the preparation of the second-stage aqueous liquid, the amount of ethylene glycol diglycidyl ether added as an internal crosslinking agent was changed to 0.0129 g (0.074 mmol), the amount of water extracted from the system by azeotropic distillation was changed to 254.5 g, and the amount of amorphous silica mixed with the polymer particles was changed to 0.2 mass% relative to the polymer particle mass. Except for this, 231.1 g of water-absorbent resin particles were obtained in the same manner as in Example 1. The water-absorbent resin particles had a median particle diameter of 370 μm and a pure water absorption of 334 g / g.

[0114] [Comparative Example 1] The stirring blades were changed to those with four inclined paddle blades with a blade diameter of 5 cm arranged in two stages; in the preparation of the first stage aqueous liquid, the radical polymerization initiator used was changed to 0.092 g (0.339 mmol) of 2,2'-azobis(2-amidinopropane) dihydrochloride and 0.018 g (0.068 mmol) of potassium persulfate; the amount of ethylene glycol diglycidyl ether added as an internal crosslinking agent was changed to 0.0046 g (0.026 mmol); in the preparation of the first stage polymerization slurry, the stirrer rotation speed during nitrogen substitution was changed to 550 rpm; in the preparation of the second stage aqueous liquid, The same procedures as in Example 1 were carried out except that the radical polymerization initiator used was changed to 0.129 g (0.475 mmol) of 2,2'-azobis(2-amidinopropane) dihydrochloride and 0.026 g (0.095 mmol) of potassium persulfate, the stirrer rotation speed when cooling the inside of the separable flask system to 25°C after preparation of the second-stage aqueous liquid was changed to 1000 rpm, the amount of water extracted from the system by azeotropic distillation was changed to 216.7 g, and the amount of amorphous silica mixed with the polymer particles was changed to 0.2 mass% relative to the mass of the polymer particles, thereby obtaining 229.0 g of water-absorbent resin particles. The median particle diameter of the water-absorbent resin particles was 348 μm.

[0115] Comparative Example 2 After the preparation of the second-stage aqueous liquid, except that the cooling temperature in the separable flask system was changed to 28°C and the amount of water extracted to the outside of the system by azeotropic distillation was changed to 204.7 g, 231.5 g of water absorbent resin particles were obtained in the same manner as in Comparative Example 1. The median particle diameter of the water absorbent resin particles was 337 µm.

[0116] Comparative Example 3 A 2-L, 110-mm-inner-diameter, round-bottom, cylindrical separable flask with four sidewall baffles (baffle width: 7 mm) was prepared. The flask was equipped with a fluororesin-surface-treated, 50-mm-diameter, two-stage, four-slant paddle blade. 660 mL of n-heptane and 0.984 g of sorbitan monolaurate (trade name: Nonion LP-20R, HLB value: 8.6, NOF Corporation) were mixed in the separable flask. The mixture in the separable flask was heated to 50°C while stirring with the stirrer, dissolving the sorbitan monolaurate in the n-heptane. The resulting solution was cooled to 40°C.

[0117] 92 g of an 80% by mass acrylic acid aqueous solution (1.02 mol of acrylic acid) was placed in a 500 mL Erlenmeyer flask. While cooling with ice from the outside, 146 g of a 21% by mass sodium hydroxide aqueous solution was added dropwise to the acrylic acid aqueous solution in the flask to neutralize 75 mol% of the acrylic acid. Next, 0.101 g (0.374 mmol) of potassium persulfate was added as a water-soluble radical polymerization initiator and dissolved in the aqueous solution to prepare a monomer aqueous solution.

[0118] The resulting aqueous monomer solution was added to the separable flask containing the sorbitan monolaurate solution, and the system was thoroughly purged with nitrogen. The reaction solution in the separable flask was stirred at a rotation speed of 700 rpm and kept in a hot water bath at 70°C for 60 minutes to allow the polymerization reaction to proceed.

[0119] A dispersion of 0.092 g of amorphous silica (Carplex #80, manufactured by Evonik Degussa Japan Co., Ltd.) dispersed in 100 g of n-heptane was prepared. This dispersion was added to a reaction solution containing a hydrogel polymer produced by a polymerization reaction, and the reaction solution was stirred for 10 minutes. The separable flask was immersed in an oil bath at 125°C, and 104 g of water was removed from the system by azeotropic distillation. Thereafter, 8.28 g of a 2% by mass aqueous solution of ethylene glycol diglycidyl ether (ethylene glycol diglycidyl ether: 0.95 mmol) was added as a surface cross-linking agent, and the mixture was maintained at an internal temperature of 80±2°C for 2 hours to allow the surface cross-linking reaction to proceed.

[0120] The reaction solution was heated to 125°C to evaporate n-heptane, thereby obtaining a dried product of polymer particles. This dried product was passed through a sieve with an opening of 850 µm, thereby obtaining 90.5 g of water-absorbent resin particles. The median particle diameter of the water-absorbent resin particles was 420 µm.

[0121] The obtained water-absorbent resin particles were measured for the no-pressure DW 5-minute value and gel brightness L * , dry particle lightness L * The saline solution used in this example was a 0.9% by mass aqueous solution of NaCl, and the saline water retention capacity, median particle size, and absorbent body swelling capacity were evaluated.

[0122] <Measurement of pressureless DW> The no-pressure DW of the water-absorbent resin particles was measured using a measuring device shown in Fig. 4. 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 opening at the top of the burette tube 21, a cock 22 connected to the tip of the bottom of the burette tube 21, and an air inlet tube 25 and a cock 24 connected to the bottom 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.

[0123] The measurements were performed in an environment with a temperature of 25±2°C and a humidity of 50±10%. First, the stopcocks 22 and 24 of the burette part 1 were closed, and saline solution (0.9% by mass saline solution) 50 adjusted to 25°C was poured into the burette tube 21 through the opening at the top of the burette tube 21. The concentration of saline solution, 0.9% by mass, is based on the mass of the saline solution. After sealing the opening of the burette tube 21 with a rubber stopper 23, the stopcocks 22 and 24 were opened. The inside of the conduit 5 was filled with saline solution 50 while preventing air bubbles from entering. The height of the measurement table 13 was adjusted so that the level of the saline solution reaching the through-hole 13a was the same as the height of the upper surface of the measurement table 13. After adjustment, the level of the saline solution 50 in the burette tube 21 was read on the scale of the burette tube 21, and this position was designated as the zero point (the reading at 0 seconds).

[0124] A nylon mesh sheet 15 (100 mm × 100 mm, 250 mesh, approximately 50 μm thick) was laid near the through-hole 13 a 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 10 a was uniformly dispersed in the cylinder. Thereafter, the cylinder was carefully removed, and a sample in which the water-absorbent resin particles 10 a were dispersed in a circular pattern was obtained in the center of the nylon mesh sheet 15. Next, the nylon mesh sheet 15 on which the water-absorbent resin particles 10 a were placed was quickly moved so that its center was positioned at the position of the through-hole 13 a, without scattering the water-absorbent resin particles 10 a, and measurement was started. 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).

[0125] 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 successively read in increments of 0.1 mL, and the amount of decrease in the amount of saline solution 50 Wa (g) 5 minutes after the start of water absorption by the water-absorbent resin particles 10a was read. From Wa, the 5-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

[0126] <Evaluation of gel brightness> The gel lightness was measured using a colorimeter (ZE6000, manufactured by Nippon Denshoku Co., Ltd.). The measurement was performed under an environment of 25±2°C temperature and 50±10% humidity. Figure 3 is a schematic cross-sectional view of the device X used for measuring the gel lightness. First, a colorimeter round cell 32, which was 30 mm in inner diameter and 13 mm in height and made of colorless and transparent optical glass (Pyrex), was placed in a cylindrical device 31. 0.1 g of water-absorbent resin particles was uniformly dispersed in the round cell 32, and 5.0 g of ion-exchanged water was quickly added using a pipette, and the lid 33 was closed. The cylindrical device 31 and lid 33 were black. The water-absorbent resin particles were allowed to swell by leaving the container for 5 minutes after the addition of the ion-exchanged water, thereby obtaining a gel 34 for measurement. Zero calibration of the colorimeter and standard calibration using a standard white board were performed, and the device X containing the gel 34 was then set in the colorimeter. The lightness L was measured in the reflection measurement mode. *was measured three times and the average value was obtained. The results are shown in Table 1. In both the Examples and Comparative Examples, the entire amount of added ion-exchanged water was absorbed by the water-absorbent resin particles, and no lumps (unswollen small lumps) of the water-absorbent resin particles were generated. When the same measurement was performed with nothing placed in the round cell 32, the lightness L * The value was 7.7. Optical photographs of the gels for measuring the brightness of the water-absorbent resin particles obtained in Example 1 and Comparative Example 3 are shown in Fig. 5(a) and Fig. 5(b), respectively. The photographs in Fig. 5(a) and (b) were taken from below of a round cell 32 containing a gel 34 against a black background.

[0127] <Evaluation of dry particle brightness> 2.0 g of water-absorbent resin particles in a dry state were uniformly dispersed in the round cell 32, and the lightness L of the water-absorbent resin particles in a dry state was measured in the same manner as in the evaluation of the gel lightness, except that ion-exchanged water was not added. * The lightness L of the water-absorbent resin particles of the Examples and Comparative Examples was measured. * All were in the range of 91 to 96.

[0128] <Mean particle size measurement> The median particle diameter of the water-absorbent resin particles was measured in an environment of 25±2°C temperature and 50±10% humidity by the following procedure. Specifically, JIS standard sieves were stacked 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, an 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 particle size corresponding to an integrated mass percentage of 50% by mass was obtained as the median particle size by connecting the plots on the probability paper with a straight line.

[0129] <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 a 0.9 mass % sodium chloride aqueous solution (physiological saline) was poured into the cotton bag containing the water-absorbent resin particles all at once, taking care not to allow the bag to 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 carried out without adding the water-absorbent resin particles, and the empty mass Wc [g] of the wet cotton bag was measured, and 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

[0130] <Measurement of pure water absorption> The pure water absorption capacity of the water-absorbent resin particles (room temperature, 25°C ± 2°C) was measured using the following procedure. 1000 g of ion-exchanged water was weighed into a 2 L beaker, and 0.5 g of water-absorbent resin particles was added while stirring with a spatula to prevent lumps from forming. After stirring for 20 seconds, the mixture was left standing for 30 minutes to allow the water-absorbent resin particles to fully swell. The contents of the beaker were then filtered using a JIS standard sieve with a mesh size of 75 μm. The sieve was tilted at an angle of approximately 30 degrees relative to the horizontal and left for 30 minutes to filter out excess water. The mass Wd (g) of the 75 μm standard sieve alone was measured in advance. The mass We (g) of the sieve containing the water-absorbent gel was measured, and the pure water absorption capacity was calculated using the following formula: Pure water absorption (g / g) = [We-Wd] (g) / mass of water-absorbent resin particles (g)

[0131] <Evaluation of absorbent performance> (Production of evaluation items) Using an airflow type mixer (Autech Co., Ltd., Pad Former), 10 g of water-absorbent resin particles and 10 g of pulverized pulp were uniformly mixed by air papermaking to prepare a sheet-shaped absorbent body measuring 40 cm x 12 cm. Next, a sheet having the same size as the absorbent body and a basis weight of 16 g / m was prepared. 2 After placing the absorbent body on the core wrap (tissue paper), a sheet of paper with the same size as the absorbent body and a basis weight of 16 g / m2 was placed on the top surface of the absorbent body. 2 A core wrap (tissue paper) of 141 kPa was applied to the absorbent body sandwiched between the core wraps for 30 seconds to obtain a laminate. An SMMS nonwoven fabric (basis weight 13 g / m2) measuring 32.5 cm x 45.0 cm was placed on the absorbent body. 2 ) was folded to a size of 45 cm x 16.25 cm, and then the above-mentioned laminate was wrapped in nonwoven fabric. The three open sides of the nonwoven fabric wrapping the laminate were sealed with a heat sealer (Fuji Impulse Sealer, model number: FI-450-5, manufactured by Fuji Impulse) to seal the laminate. This gave an article for evaluation.

[0132] (Measurement of absorbent body swelling capacity) Measurements were performed in an environment with a temperature of 25±2°C and a humidity of 50±10%. A wire mesh (opening size: 20mm x 20mm, wire diameter: 3mm) and 20 L of saline were placed in a tray, and the liquid temperature was adjusted to 25.0±0.2°C. Next, the evaluation article was spread out on the wire mesh and immersed in the saline solution for 10 minutes. After that, the wire mesh was lifted up together with the evaluation article, and the water was drained for 5 minutes before the evaluation article's mass was measured. The absorbent body swelling capacity (unit: g) is the difference in mass of the evaluation article before and after the test, and is calculated using the following formula. A larger absorbent body swelling capacity indicates a larger absorption capacity of the absorbent body. Absorbent body swelling capacity = Mass (g) of evaluation article after test - Mass (g) of evaluation article before test

[0133] [Table 1]

[0134] Gel Lightness L *It was confirmed that the absorbent material using the water-absorbent resin particles of the Examples having a value of 60 or less exhibited a sufficiently high absorption amount. [Explanation of symbols]

[0135] 1...burette part, 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, 31...cylindrical device, 32...round cell, 33...lid, 34...gel, 40...liquid-impermeable back sheet, 50...physiological saline solution, 100...absorbent article, 200...stirring blade, 200a...shaft, 200b...flat plate part, S...slit, X...device.

Claims

1. Gel lightness L measured by a method including the following steps (A), (B), and (C) in this order: ＊ water absorbent resin particles having a water retention capacity of 45 to 50 g / g, and having internal crosslinking by an internal crosslinking agent (however, excluding those obtained by a method including a step of carrying out polymerization using an azo compound and a peroxide, and those obtained by a method for producing a water absorbent resin powder, characterized in that in producing a water absorbent resin powder by polymerizing an aqueous solution of a water-soluble monomer selected from α,β-unsaturated carboxylic acids, derivatives thereof or salts thereof, in a state where the solution is dispersed and suspended in a polymerization dispersion medium containing a polymerization dispersant, under stirring, a rotation shaft extending in a vertical direction is disposed in a polymerization tank, a paddle for stirring a bottom part of the polymerization tank is attached to a lower part of the rotation shaft, and a lattice blade consisting of a horizontal member extending horizontally from the rotation shaft and a member extending in a direction perpendicular to the horizontal member is attached to an upper part of the rotation shaft, and a plurality of baffles are arranged at intervals on a side wall surface of the polymerization tank along the direction of the rotation shaft, and polymerization is carried out with stirring in the polymerization tank). (A) 0.1 g of water-absorbent resin particles is uniformly dispersed in a colorless, transparent round cell for a color difference meter having an inner diameter of 30 mm. (B) 5.0 g of ion-exchanged water is added to the round cell, and the water-absorbent resin particles are allowed to absorb the water. (C) The brightness L of the resulting gel 5 minutes after adding ion-exchanged water ＊ is measured against a black background.

2. 2. The water-absorbent resin particles according to claim 1, wherein the no-pressure DW 5-minute value is 30 ml / g or more.

3. An absorbent material comprising the water-absorbent resin particles according to claim 1 or 2.

4. An absorbent article comprising the absorbent body according to claim 3.

5. The absorbent article of claim 4, which is a diaper.

6. Gel lightness L measured by a method including the following steps (A), (B), and (C) in this order: ＊ and selecting water-absorbent resin particles having a molecular weight of 43 to 58 and a water retention capacity of physiological saline solution of 45 to 50 g / g (however, excluding those including a step of polymerization using an azo-based compound and a peroxide). (A) 0.1 g of water-absorbent resin particles is uniformly dispersed in a colorless, transparent round cell for a color difference meter having an inner diameter of 30 mm. (B) 5.0 g of ion-exchanged water is added to the round cell, and the water-absorbent resin particles are allowed to absorb the water. (C) The brightness L of the resulting gel 5 minutes after adding ion-exchanged water ＊ is measured against a black background.

7. The gel lightness L of the water-absorbent resin particles measured by a method including the following steps (A), (B), and (C) in this order: ＊ A method for improving the absorption amount of an absorbent material containing the water-absorbent resin particles (excluding those obtained by a method including a step of polymerization using an azo compound and a peroxide) having internal crosslinking by an internal crosslinking agent, comprising adjusting the molecular weight of the water-absorbent resin particles to 43 to 58. (A) 0.1 g of water-absorbent resin particles is uniformly dispersed in a colorless, transparent round cell for a color difference meter having an inner diameter of 30 mm. (B) 5.0 g of ion-exchanged water is added to the round cell, and the water-absorbent resin particles are allowed to absorb the water. (C) The brightness L of the resulting gel 5 minutes after adding ion-exchanged water ＊ is measured against a black background.

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