Water absorbent resin particles, water absorbent body, water absorbent article, and method for producing water absorbent resin particles

Abstract

Disclosed are water absorbent resin particles comprising: polymer particles that include a polymer containing a water-soluble ethylenically unsaturated monomer as a monomer unit; and a phosphonic acid-based chelating agent. The water absorbent resin particles have a water absorption rate for a physiological saline solution of 30 seconds or less, and the proportion of particles having a particle diameter of 150 μm or less in the water absorbent resin particles is 20 mass% or less with respect to the total amount of the water absorbent resin particles.

Description

Water-absorbent resin particles, absorbent body, absorbent article, and method for producing water-absorbent resin particles

[0001] The present disclosure relates to water-absorbent resin particles, an absorbent body, an absorbent article, and a method for producing water-absorbent resin particles.

[0002] With respect to water-absorbent resin particles constituting absorbent articles used in the field of sanitary materials and the like, metal chelating agents such as organic phosphorus compounds and carboxylic acid compounds may be introduced in order to suppress coloration over time and the like (for example, Patent Documents 1 to 5).

[0003] JP 2003-206305 A, International Publication No. 2008 / 090961, International Publication No. 2009 / 005114, JP 2016-28118 A, International Publication No. 2020 / 059871

[0004] Although the introduction of a metal chelating agent can suppress the coloring of water-absorbent resin particles, it has not been expected to sufficiently improve the absorption properties of absorbent articles. In sheet-like absorbent articles used in nursing care products or pet sheets, the diffusion area when absorbing urine tends to be emphasized as an absorption property from the viewpoint of reducing the frequency of replacement, etc. In particular, when an absorbent article absorbs urine that is high in iron and vitamin C, the diffusion area tends to increase, and therefore improvement in this area is desired.

[0005] The present disclosure relates to suppressing coloration of water-absorbent resin particles over time and reducing the area over which urine spreads in an absorbent article.

[0006] The present disclosure includes the following: [1] Water-absorbent resin particles comprising: polymer particles containing a polymer containing a water-soluble ethylenically unsaturated monomer as a monomer unit; and a phosphonic acid chelating agent, wherein the water-absorbent resin particles have a water-absorption speed for physiological saline solution of 30 seconds or less, and a ratio of particles having a particle diameter of 150 μm or less in the water-absorbent resin particles is 20 mass% or less with respect to the total amount of the water-absorbent resin particles. [2] The water-absorbent resin particles according to [1], wherein the phosphonic acid chelating agent contains a compound having 3 to 8 phosphoryl groups. [3] The water-absorbent resin particles according to [1] or [2], wherein the amount of the phosphonic acid chelating agent is 200 ppm by mass or more and 15,000 ppm by mass or less with respect to the amount of the polymer particles. [4] The water-absorbent resin particles according to [1] or [2], wherein the amount of the phosphonic acid chelating agent is 400 ppm by mass or more and 12,000 ppm by mass or less with respect to the amount of the polymer particles. [5] The water-absorbing resin particles according to any one of [1] to [4], wherein the water-soluble ethylenically unsaturated monomer comprises (meth)acrylic acid and an alkali metal salt thereof. [6] An absorbent body comprising the water-absorbing resin particles according to any one of [1] to [5]. [7] An absorbent article comprising the absorbent body according to [6]. [8] A method for producing water-absorbent resin particles, the method comprising: forming a particulate hydrogel polymer in a reaction liquid containing a water-soluble ethylenically unsaturated monomer, water, a dispersion medium, and a surfactant having an HLB of 7 or more and 16 or less, by polymerizing the water-soluble ethylenically unsaturated monomer by reverse phase suspension polymerization in the reaction liquid; aggregating the hydrogel polymer in the reaction liquid to form a plurality of aggregated particles containing the hydrogel polymer; extracting a portion of water from the reaction liquid containing the aggregated particles and water to form a concentrate; and forming a powder containing polymer particles containing the polymer from the concentrate, the method further comprising mixing at least one selected from the group consisting of the reaction liquid, the concentrate, and the powder with a phosphonic acid-based chelating agent.

[0007] The discoloration of the water-absorbent resin particles over time can be suppressed, and the area over which urine spreads in the absorbent article can be reduced.The absorbent article of the present disclosure is useful as, for example, a nursing care product (waterproof sheet) or a pet sheet.

[0008] Fig. 2 is a partial cross-sectional view showing an example of an absorbent article, and Fig. 3 is a plan view showing an example of a state in which artificial urine has been diffused in the absorbent article.

[0009] The present invention is not limited to the following examples. In this specification, "room temperature" means 25±2°C. "Layer" is used as a term that encompasses not only a shaped structure continuously formed in the in-plane direction, but also a shaped structure partially formed in the in-plane direction. "Saline" means an aqueous sodium chloride solution with a concentration of 0.9% by mass, containing 9 g of sodium chloride per 1000 mL of water at room temperature. "(Meth)acrylic" means both "acrylic" and "methacrylic".

[0010] An example of the water-absorbent resin particles includes polymer particles containing a polymer and a phosphonic acid-based chelating agent. The polymer includes a water-soluble ethylenically unsaturated monomer as a monomer unit. The water-absorbent resin particles have a water-absorbing speed for physiological saline solution of 30 seconds or less. The proportion of particles having a particle diameter of 150 μm or less in the water-absorbent resin particles is 20 mass% or less with respect to the total amount of the water-absorbent resin particles.

[0011] The polymer particles include a polymer formed by polymerization of a monomer including a water-soluble ethylenically unsaturated monomer. The water-soluble ethylenically unsaturated monomer can be any monomer capable of forming water-absorbing polymer particles. The polymer constituting the polymer particles may include, as a monomer unit, at least one water-soluble ethylenically unsaturated monomer selected from the group consisting of (meth)acrylic acid and its alkali salts, 2-(meth)acrylamido-2-methylpropanesulfonic acid and its alkali 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. The water-soluble ethylenically unsaturated monomer may include (meth)acrylic acid and its alkali metal salt, or acrylic acid and its alkali metal salt. The alkali metal salt may be a sodium salt. Of the total amount of monomer units constituting the polymer, the proportion of monomer units derived from (meth)acrylic acid or an alkali metal salt thereof may be 60 mol% or more, 70 mol% or more, 80 mol% or more, 90 mol% or more, or 95 mol% or more, or may be substantially 100 mol%.

[0012] The polymer constituting the polymer particles may be a crosslinked polymer having a crosslinked structure. The crosslinked polymer may include a crosslinked structure due to self-crosslinking of a water-soluble ethylenically unsaturated monomer, a crosslinked structure formed by a reaction between a crosslinking agent and an ethylenically unsaturated monomer, or both of these. The crosslinked polymer may include a crosslinked structure formed by an internal crosslinking agent that reacts with the water-soluble ethylenically unsaturated monomer during polymerization of the water-soluble ethylenically unsaturated monomer, a surface crosslinking agent that reacts with the polymer mainly in the surface layer portion of the polymer particles, or both of these. The polymer chains constituting the crosslinked polymer may be entangled with each other to form a pseudo-crosslinked structure.

[0013] The phosphonic acid chelating agent forms an integral particle with the polymer particle. For example, the phosphonic acid chelating agent may penetrate into the interior of the polymer particle or may be attached to the surface of the polymer particle.

[0014] The phosphonic acid chelating agent is composed of one or more compounds having a phosphoryl group and capable of forming a chelate complex with a metal ion through a coordinate bond. The phosphonic acid chelating agent may also contain a compound having a coordinating group other than a phosphoryl group that can form a coordinate bond with a metal ion together with the phosphoryl group. In the present disclosure, the phosphoryl group refers to a group represented by the following formula (1): In formula (1), X １ and X ２ are each independently —OH (hydroxy group) or —O － M ＋ (M ＋ Ha-O － ) represents the counter cation of M ＋ is an alkali metal ion (e.g., Na ＋ ) may also be used.

[0015] The phosphonic acid chelating agent may include a compound having multiple phosphoryl groups, and in particular may include a compound having 3 to 8 phosphoryl groups. A compound having 3 to 8 phosphoryl groups may contribute to suppressing discoloration over time and further improving the reduction of the diffusion area in the absorbent article. From a similar perspective, the phosphonic acid chelating agent may include a compound having 3 to 7, 6 to 5 phosphoryl groups. The phosphonic acid chelating agent may include a compound having 4 to 8, 7 to 6, or 5 phosphoryl groups.

[0016] The phosphonic acid chelating agent may include a polyalkyleneamine phosphonic acid compound having a group in which two or more hydrogen atoms have been removed from a polyalkyleneamine and two or more phosphoryl groups bonded to the alkylene group of the polyalkyleneamine. Examples of the polyalkyleneamine polyphosphonic acid compound include ethylenediaminetetramethylenephosphonic acid (EDTMP), diethylenetriaminepentamethylenephosphonic acid (DTPMP), 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetra(methylenephosphonic acid) (DOTMP), nitrilotri(methylenephosphonic acid) (NTMP), hydroxyethylethylenediaminetri(methylenephosphonic acid) (HEEDTMP), tris(2-aminomethyl-2-methyl-1,4-diamino ... Examples of suitable phosphonic acid chelating agents include ethylenediamine-N,N'-di(methylene phosphonic acid), cyclohexanediaminetetra(methylene phosphonic acid), ethylenediamine-N,N'-diacetic acid-N,N'-di(methylene phosphonic acid), polymethylenediaminetetra(methylene phosphonic acid), nitriloacetic acid-di(methylene phosphinic acid), and salts thereof. From the viewpoints of suppressing coloration over time and further improving the reduction of the diffusion area in absorbent articles, the phosphonic acid chelating agent may include one or more compounds selected from ethylenediaminetetramethylenephosphonic acid-pentasodium (EDTMP-5Na), ethylenediaminetetramethylenephosphonic acid hydrate (EDTMP-8H), and diethylenetriaminepentamethylenephosphonic acid-hexasodium (DTPMP-7Na).

[0017] The amount of the phosphonic acid chelating agent may be 200 ppm by mass or more and 15,000 ppm by mass or less relative to the amount of the polymer particles. The amount of the phosphonic acid chelating agent of 200 ppm by mass or more and 15,000 ppm by mass or less relative to the amount of the polymer particles can contribute to suppressing discoloration over time and further improving the reduction of the diffusion area in the absorbent article. From the same perspective, the amount of the phosphonic acid chelating agent may be 250 ppm by mass or more, 300 ppm by mass or more, 350 ppm by mass or more, 400 ppm by mass or more, 450 ppm by mass or more, 500 ppm by mass or more, 550 ppm by mass or more, or 600 ppm by mass or more and 15,000 ppm by mass or less relative to the amount of the polymer particles. The amount of the phosphonic acid chelating agent may be 200 ppm by mass or more, 250 ppm by mass or more, 300 ppm by mass or more, 350 ppm by mass or more, 400 ppm by mass or more, 450 ppm by mass or more, 500 ppm by mass or more, 550 ppm by mass or more, or 600 ppm by mass or more and 14000 ppm by mass or less, relative to the amount of the polymer particles. The amount of the phosphonic acid chelating agent may be 200 ppm by mass or more, 250 ppm by mass or more, 300 ppm by mass or more, 350 ppm by mass or more, 400 ppm by mass or more, 450 ppm by mass or more, 500 ppm by mass or more, 550 ppm by mass or more, or 600 ppm by mass or more and 13000 ppm by mass or less, relative to the amount of the polymer particles. The amount of the phosphonic acid chelating agent may be 200 ppm by mass or more, 250 ppm by mass or more, 300 ppm by mass or more, 350 ppm by mass or more, 400 ppm by mass or more, 450 ppm by mass or more, 500 ppm by mass or more, 550 ppm by mass or more, or 600 ppm by mass or more and 12000 ppm by mass or less, relative to the amount of the polymer particles. The amount of the phosphonic acid chelating agent may be 200 ppm by mass or more, 250 ppm by mass or more, 300 ppm by mass or more, 350 ppm by mass or more, 400 ppm by mass or more, 450 ppm by mass or more, 500 ppm by mass or more, 550 ppm by mass or more, or 600 ppm by mass or more and 11000 ppm by mass or less, relative to the amount of the polymer particles.The amount of the phosphonic acid chelating agent may be 200 ppm by mass or more, 250 ppm by mass or more, 300 ppm by mass or more, 350 ppm by mass or more, 400 ppm by mass or more, 450 ppm by mass or more, 500 ppm by mass or more, 550 ppm by mass or more, or 600 ppm by mass or more and 10,000 ppm by mass or less, relative to the amount of the polymer particles. The amount of the phosphonic acid chelating agent may be 200 ppm by mass or more, 250 ppm by mass or more, 300 ppm by mass or more, 350 ppm by mass or more, 400 ppm by mass or more, 450 ppm by mass or more, 500 ppm by mass or more, 550 ppm by mass or more, or 600 ppm by mass or more and 900 ppm by mass or less, relative to the amount of the polymer particles. The amount of the phosphonic acid chelating agent may be 200 ppm by mass or more, 250 ppm by mass or more, 300 ppm by mass or more, 350 ppm by mass or more, 400 ppm by mass or more, 450 ppm by mass or more, 500 ppm by mass or more, 550 ppm by mass or more, or 600 ppm by mass or more and 800 ppm by mass or less, relative to the amount of the polymer particles.

[0018] The water-absorbent resin particles have a water-absorption rate for physiological saline of 30 seconds or less. The water-absorption rate for physiological saline of the water-absorbent resin particles here is a value measured by the Vortex method, as described in the examples described later, and a smaller value means a higher water-absorption rate. The high water-absorption rate for physiological saline of the water-absorbent resin particles can contribute to a reduction in the diffusion area, particularly immediately after the absorbent article has absorbed urine. From the same viewpoint, the water absorption speed of the water-absorbent resin particles for physiological saline may be 29 seconds or less, 28 seconds or less, 27 seconds or less, 26 seconds or less, 25 seconds or less, 24 seconds or less, 23 seconds or less, 22 seconds or less, 21 seconds or less, 20 seconds or less, 19 seconds or less, 18 seconds or less, 17 seconds or less, 16 seconds or less, 15 seconds or less, 14 seconds or less, 13 seconds or less, 12 seconds or less, 11 seconds or less, 10 seconds or less, 9.0 seconds or less, 8.0 seconds or less, 7.0 seconds or less, 6.0 seconds or less, or 5.0 seconds or less. The water absorption speed of the water-absorbent resin particles for physiological saline may be 1.0 second or more. The water absorption speed of the water-absorbent resin particles with respect to physiological saline may be 1.0 seconds or more and 30 seconds or less, 29 seconds or less, 28 seconds or less, 27 seconds or less, 26 seconds or less, 25 seconds or less, 24 seconds or less, 23 seconds or less, 22 seconds or less, 21 seconds or less, 20 seconds or less, 19 seconds or less, 18 seconds or less, 17 seconds or less, 16 seconds or less, 15 seconds or less, 14 seconds or less, 13 seconds or less, 12 seconds or less, 11 seconds or less, 10 seconds or less, 9.0 seconds or less, 8.0 seconds or less, 7.0 seconds or less, 6.0 seconds or less, or 5.0 seconds or less.

[0019] The water retention capacity of the water-absorbent resin particles in physiological saline may be, for example, 25 g / g or more and 60 g / g or less, 55 g / g or less, 50 g / g or less, or 45 g / g or less. The water retention capacity of the water-absorbent resin particles in physiological saline may be 30 g / g or more and 60 g / g or less, 55 g / g or less, 50 g / g or less, or 45 g / g or less. The water retention capacity of the water-absorbent resin particles in physiological saline may be 35 g / g or more and 60 g / g or less, 55 g / g or less, 50 g / g or less, or 45 g / g or less. The water retention capacity of the water-absorbent resin particles in physiological saline may be 40 g / g or more and 60 g / g or less, 55 g / g or less, 50 g / g or less, or 45 g / g or less. The method for measuring the water retention capacity of the water-absorbent resin particles in physiological saline is as described in the examples below.

[0020] The water absorption capacity of the water-absorbent resin particles in physiological saline may be, for example, 50 g / g or more, 51 g / g or more, 52 g / g or more, 53 g / g or more, 54 g / g or more, 55 g / g or more, 56 g / g or more, 57 g / g or more, 58 g / g or more, 59 g / g or more, or 60 g / g or more. The water absorption capacity of the water-absorbent resin particles in physiological saline may be 50 g / g or more, 51 g / g or more, 52 g / g or more, 53 g / g or more, 54 g / g or more, 55 g / g or more, 56 g / g or more, 57 g / g or more, 58 g / g or more, 59 g / g or more, or 60 g / g or more, and 80 g / g or less. The method for measuring the water absorption capacity of the water-absorbent resin particles in physiological saline is as described in the examples below.

[0021] A small proportion of water-absorbent resin particles having a relatively small particle size of 150 μm or less can contribute to a reduction in the diffusion area, particularly immediately after the absorbent article absorbs urine. This is thought to be because a small proportion of particles having a small particle size makes it difficult for gel blocking due to swollen water-absorbent resin particles to occur, thereby facilitating urine penetration in the thickness direction within the absorbent body. From this perspective, the proportion of particles having a particle size of 150 μm or less may be 20% by mass or less, 19% by mass or less, 18% by mass or less, 17% by mass or less, 16% by mass or less, 15% by mass or less, 14% by mass or less, 13% by mass or less, 12% by mass or less, 11% by mass or less, 10% by mass or less, 9% by mass or less, 8% by mass or less, 7% by mass or less, 6% by mass or less, 5% by mass or less, 4% by mass or less, or 3% by mass or less, and 0% by mass or more, relative to the total amount of water-absorbent resin particles. The proportion of particles having a particle size of 150 μm or less in the water-absorbent resin particles can be measured using a JIS standard sieve, as described in the examples below.

[0022] The median particle diameter of the water-absorbent resin particles may be 250 μm or more, 260 μm or more, 270 μm or more, 280 μm or more, 290 μm or more, 300 μm or more, 310 μm or more, 320 μm or more, 330 μm or more, or 340 μm or more and 600 μm or less. The median particle diameter of the water-absorbent resin particles may be 250 μm or more, 260 μm or more, 270 μm or more, 280 μm or more, 290 μm or more, 300 μm or more, 310 μm or more, 320 μm or more, 330 μm or more, or 340 μm or more and 550 μm or less. The median particle diameter of the water-absorbent resin particles may be 250 μm or more, 260 μm or more, 270 μm or more, 280 μm or more, 290 μm or more, 300 μm or more, 310 μm or more, 320 μm or more, 330 μm or more, or 340 μm or more and 500 μm or less. The median particle diameter of the water-absorbent resin particles may be 250 μm or more, 260 μm or more, 270 μm or more, 280 μm or more, 290 μm or more, 300 μm or more, 310 μm or more, 320 μm or more, 330 μm or more, or 340 μm or more and 450 μm or less. The median particle diameter of the water-absorbent resin particles may be 250 μm or more, 260 μm or more, 270 μm or more, 280 μm or more, 290 μm or more, 300 μm or more, 310 μm or more, 320 μm or more, 330 μm or more, or 340 μm or more and 440 μm or less. The median particle diameter of the water-absorbent resin particles may be 250 μm or more, 260 μm or more, 270 μm or more, 280 μm or more, 290 μm or more, 300 μm or more, 310 μm or more, 320 μm or more, 330 μm or more, or 340 μm or more and 430 μm or less. The median particle diameter of the water-absorbent resin particles may be 250 μm or more, 260 μm or more, 270 μm or more, 280 μm or more, 290 μm or more, 300 μm or more, 310 μm or more, 320 μm or more, 330 μm or more, or 340 μm or more and 420 μm or less. The median particle diameter of the water-absorbent resin particles may be 250 μm or more, 260 μm or more, 270 μm or more, 280 μm or more, 290 μm or more, 300 μm or more, 310 μm or more, 320 μm or more, 330 μm or more, or 340 μm or more and 410 μm or less.The median particle diameter of the water-absorbent resin particles may be 250 μm or more, 260 μm or more, 270 μm or more, 280 μm or more, 290 μm or more, 300 μm or more, 310 μm or more, 320 μm or more, 330 μm or more, or 340 μm or more and 400 μm or less. The median particle diameter of the water-absorbent resin particles can be measured using a JIS standard sieve, as described in the examples described later.

[0023] The water-absorbent resin particles according to the present disclosure are unlikely to discolor over time. For example, the yellowness index of the water-absorbent resin particles after being left for 5 days in an environment at a temperature of 70±2°C and a relative humidity of 90±2% may be 18 or less, 15 or less, 12 or less, 10 or less, 9 or less, or 8 or less, or may be 1 or more. The yellowness index of the water-absorbent resin particles after being left for 5 days in an environment at a temperature of 70±2°C and a relative humidity of 90±2% may be 1 or more and may be 18 or less, 15 or less, 12 or less, 10 or less, 9 or less, or 8 or less. The yellowness index here means a value calculated by the following formula using a standard white board and tristimulus values ​​X, Y, and Z of a colorimeter. Yellowness index=100(1.28X-1.06Z) / Y

[0024] The water-absorbent resin particles may further contain other components such as a lubricant, a metal chelating agent other than a phosphonic acid-based agent, a surface modifier, a heat resistance stabilizer, an antioxidant, an antibacterial agent, etc. The other components may be attached to the surface of the polymer particles or may penetrate into the inside of the polymer particles.

[0025] The lubricant may be, for example, silica particles (e.g., amorphous silica particles). The amount of the lubricant (e.g., silica particles) may be, for example, 0.001 parts by mass or more and 10 parts by mass or less, 0.01 parts by mass or more and 5 parts by mass or less, or 0.1 parts by mass or more and 2 parts by mass or less, relative to 100 parts by mass of the polymer particles.

[0026] Examples of surface modifiers include polyvalent metal compounds such as aluminum sulfate, potassium alum, ammonium alum, sodium alum, (poly)aluminum chloride, and hydrates thereof; and polycation compounds such as polyethyleneimine, polyvinylamine, and polyallylamine.

[0027] The water-absorbent resin particles according to the present disclosure can be obtained by a method including: polymerizing the water-soluble ethylenically unsaturated monomer by reverse phase suspension polymerization in a reaction liquid containing the water-soluble ethylenically unsaturated monomer, water, a dispersion medium, and a surfactant, to form a particulate hydrogel polymer in the reaction liquid, the particulate hydrogel polymer containing a polymer containing the water-soluble ethylenically unsaturated monomer as a monomer unit, and water; aggregating the hydrogel polymer in the reaction liquid to form aggregated particles containing a plurality of hydrogel polymers; extracting a portion of the water from the reaction liquid containing the aggregated particles and water to form a concentrate; and forming a powder containing polymer particles containing a polymer from the concentrate.

[0028] In the above method, for example, by mixing at least one selected from the group consisting of a reaction liquid, a concentrate containing aggregated particles, and a powder containing polymer particles with a phosphonic acid chelating agent, water-absorbent resin particles containing polymer particles and a phosphonic acid chelating agent can be obtained. The reaction liquid to be mixed with the phosphonic acid chelating agent may be the reaction liquid before the formation of a hydrogel polymer by polymerization reaction, or may be the reaction liquid after the formation of a hydrogel polymer by polymerization reaction. However, by adding the phosphonic acid chelating agent after the formation of the hydrogel polymer, the phosphonic acid chelating agent can be easily introduced while avoiding its influence on the polymerization reaction. The reaction liquid or a concentrate thereof may be mixed with the phosphonic acid chelating agent, and then the powder may be further mixed with the phosphonic acid chelating agent.

[0029] When mixing a reaction liquid or a concentrate thereof with a phosphonic acid chelating agent, a mixture containing an aqueous solution containing the phosphonic acid chelating agent and the reaction liquid or a concentrate thereof may be stirred. When mixing a powder with a phosphonic acid chelating agent, an aqueous solution containing the phosphonic acid chelating agent may be sprayed onto the powder, or a mixture containing a powder containing the phosphonic acid chelating agent and a powder containing polymer particles may be stirred. The mixture of the reaction liquid, concentrate, or powder with the phosphonic acid chelating agent may also be heated. Examples of methods for spraying an aqueous solution of a phosphonic acid chelating agent include methods using a spray nozzle device, and methods using an ultrasonic device with an ultrasonic vibrator or a rotary atomization centrifugal spray device. The spray nozzle device may be a one-fluid or two-fluid spray device having a spray pattern such as a flat spray, hollow cone, or full cone.

[0030] The reaction liquid for reverse phase suspension polymerization may mainly contain an oily liquid, which is a hydrophobic liquid composed of a hydrophobic dispersion medium, and a particulate aqueous liquid containing water and a water-soluble ethylenically unsaturated monomer and dispersed in the oily liquid.

[0031] During the polymerization reaction, the reaction solution is usually stirred. During the polymerization reaction, the reaction solution may be stirred using various stirrers having stirring blades. The stirring blades may be flat blades, lattice blades, paddle blades, propeller blades, anchor blades, turbine blades, Pfaudler blades, ribbon blades, Fullzone blades, or Max Blend blades. The stirring speed may be, for example, within a range of 200 rpm or more and 1000 rpm or less, 900 rpm or less, 800 rpm or less, or less than 700 rpm. A hydrogel polymer may be formed by two or more polymerization reactions.

[0032] The HLB of the surfactant contained in the reaction liquid may be 7 or more and 16 or less. When the HLB of the surfactant is in this range, water-absorbent resin particles having a moderately large specific surface area are easily obtained. When the specific surface area of ​​the water-absorbent resin particles is moderately large, the water absorption rate of the water-absorbent resin particles tends to be easily increased. From the same viewpoint, the HLB of the surfactant may be 7 or more and 15 or less, 14 or less, 13 or less, 12 or less, 11 or less, or 10 or less. The surfactant may be contained in the oily liquid.

[0033] Examples of surfactants having an HLB of 7 or more and 16 or less include nonionic surfactants such as sorbitan fatty acid esters, (poly)glycerin 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; and anionic surfactants such as fatty acid salts, alkylbenzenesulfonates, alkylmethyltaurates, polyoxyethylene alkylphenyl ether sulfates, polyoxyethylene alkyl ether sulfonates, polyoxyethylene alkyl ether phosphates, and polyoxyethylene alkylallyl ether phosphates.

[0034] The amount of the surfactant may be 2.0% by mass or less, 1.7% by mass or less, 1.5% by mass or less, or 1.3% by mass or less, or may be 0.5% by mass or more, based on the amount of the water-soluble ethylenically unsaturated monomer. The amount of the surfactant may be 0.5% by mass or more, and may be 2.0% by mass or less, 1.7% by mass or less, 1.5% by mass or less, or 1.3% by mass or less, based on the amount of the water-soluble ethylenically unsaturated monomer. When the hydrogel polymer is formed by two or more polymerization reactions, the amount of the surfactant relative to the water-soluble ethylenically unsaturated monomer introduced into the reaction solution in each polymerization reaction may be within the above range.

[0035] The aqueous liquid in the reaction solution may further contain an internal crosslinking agent that reacts with the monomer or monomer unit to form a crosslinked polymer. The internal crosslinking agent may be the same compound as the examples of the intermediate crosslinking agent described below. The reaction solution may be substantially free of an internal crosslinking agent. The amount of the internal crosslinking agent in the reaction solution may be 0 mmol or more and 0.093 mmol or less, 0 mmol or more and 0.070 mmol or less, 0 mmol or more and 0.050 mmol or less, or 0 mmol or more and 0.030 mmol or less per mole of the water-soluble ethylenically unsaturated monomer.

[0036] The reaction liquid may further contain a radical polymerization initiator. The radical polymerization initiator may be water-soluble and may contain, for example, an azo compound, a peroxide, or a combination thereof.

[0037] Examples of the azo compounds include 2,2'-azobis[2-(N-phenylamidino)propane]dihydrochloride, 2,2'-azobis{2-[N-(4-chlorophenyl)amidino]propane}dihydrochloride, 2,2'-azobis{2-[N-(4-hydroxyphenyl)amidino]propane}dihydrochloride, 2,2'-azobis[2-(N-benzylamidino)propane]dihydrochloride, 2,2' -Azobis[2-(N-allylamidino)propane]dihydrochloride, 2,2'-azobis(2-amidinopropane)dihydrochloride, 2,2'-azobis{2-[N-(2-hydroxyethyl)amidino]propane}dihydrochloride, 2,2'-azobis[2-(5-methyl-2-imidazolin-2-yl)propane]dihydrochloride, 2,2'-azobis[2-(2-imidazolin-2-yl) propane] dihydrochloride, 2,2'-azobis[2-(4,5,6,7-tetrahydro-1H-1,3-diazepin-2-yl)propane] dihydrochloride, 2,2'-azobis[2-(5-hydroxy-3,4,5,6-tetrahydropyrimidin-2-yl)propane] dihydrochloride, 2,2'-azobis{2-[1-(2-hydroxyethyl)-2-imidazolin-2-yl]propane] dihydrochloride 2,2'-azobis[N-(2-carboxyethyl)-2-methylpropionamidine]tetrahydrate, and 2,2'-azobis[2-methyl-N-(2-hydroxyethyl)propionamide].

[0038] Examples of peroxides include persulfates such as potassium persulfate, ammonium persulfate, and sodium persulfate; organic peroxides such as methyl ethyl ketone peroxide, methyl isobutyl ketone peroxide, di-t-butyl peroxide, t-butyl cumyl peroxide, t-butyl peroxyacetate, t-butyl peroxyisobutyrate, and t-butyl peroxypivalate; and hydrogen peroxide.

[0039] The amount of the radical polymerization initiator may be, for example, 0.005 moles or more and 1 mole or less per 100 moles of the water-soluble ethylenically unsaturated monomer.

[0040] The aqueous liquid in the reaction solution may further contain a thickener. Examples of thickeners include hydroxyalkyl celluloses such as hydroxyethyl cellulose (HEC) and hydroxypropyl cellulose (HPC); hydroxyalkyl alkyl celluloses such as hydroxyethyl methyl cellulose, hydroxypropyl methyl cellulose, and hydroxyethyl ethyl cellulose; carboxyalkyl celluloses such as carboxymethyl cellulose; and carboxyalkyl hydroxyalkyl celluloses such as carboxymethyl hydroxyethyl cellulose. The thickener may be a single compound or a combination of two or more compounds.

[0041] The amount of the thickener may be 0.05 parts by mass or more and 20 parts by mass or less, 0.2 parts by mass or more and 10 parts by mass or less, or 0.4 parts by mass or more and 5 parts by mass or less, relative to 100 parts by mass of the water-soluble ethylenically unsaturated monomer.

[0042] The aqueous liquid in the reaction solution may further contain a hydrophilic polymer agent. Examples of hydrophilic polymer dispersants include polyvinyl alcohol (PVA), polyvinylpyrrolidone (PVP), polyethylene glycol (PEG), polypropylene glycol, polyethylene glycol-polypropylene glycol block copolymer, polyglycerin, polyoxyethylene glycerin, polyoxypropylene glycerin, polyoxyethylene-polyoxypropylene glycerin copolymer, and polyoxyethylene sorbitan fatty acid ester. The hydrophilic polymer dispersants may be used alone or in combination of two or more.

[0043] The aqueous liquid in the reaction mixture may further contain a chain transfer agent. Examples of the chain transfer agent include hypophosphites, thiols, thiolic acids, secondary alcohols, and amines.

[0044] The aqueous liquid in the reaction solution may further contain a blowing agent. Examples of the blowing agent include inorganic blowing agents such as ammonium carbonate, sodium bicarbonate, and ammonium bicarbonate; nitroso compounds such as dinitrosopentamethylenetetramine; azo compounds such as azodicarbonamide and azobisisobutyronitrile; and organic blowing agents such as sulfonylhydrazide compounds such as 4,4'-oxybisbenzenesulfonylhydrazide and p-toluenesulfonylhydrazide.

[0045] The oily liquid in the reaction mixture is a hydrophobic liquid mainly composed of a hydrophobic dispersion medium, which may be a hydrocarbon dispersion medium.

[0046] Examples of hydrocarbon dispersion media 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 media may be used alone or in combination of two or more.

[0047] The amount of the dispersion medium contained in the oily liquid may be 30 parts by mass or more and 1,000 parts by mass or less, 50 parts by mass or more and 650 parts by mass or less, 70 parts by mass or more and 550 parts by mass or less, or 100 parts by mass or more and 500 parts by mass or less, relative to 100 parts by mass of the water-soluble ethylenically unsaturated monomer.

[0048] The oily liquid in the reaction solution may further contain a hydrophobic polymer dispersant. Examples of hydrophobic polymer 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. The hydrophobic polymer dispersants may be used alone or in combination of two or more.

[0049] The amount of the hydrophobic polymer dispersant may be 0.05 parts by mass or more and 10 parts by mass or less, 0.08 parts by mass or more and 5 parts by mass or less, or 0.1 parts by mass or more and 3 parts by mass or less, relative to 100 parts by mass of the aqueous liquid.

[0050] By forming aggregated particles from a hydrogel polymer formed by a polymerization reaction, it is easy to obtain water-absorbent resin particles having a form of aggregated particles and a small proportion of particles having a small particle diameter (for example, 150 μm or less). Also, it is possible to adjust the median particle diameter of the water-absorbent resin particles.

[0051] The aggregated particles can be formed, for example, by a method including adding an aggregating agent to a reaction solution containing a hydrogel polymer and stirring a mixture containing the hydrogel polymer and the aggregating agent. To form the aggregated particles, the mixture may be heated, for example, to a temperature of 30° C. or higher and 80° C. or lower. The time for stirring the mixture to form the aggregated particles may be, for example, 1 minute or higher and 60 minutes or lower.

[0052] The flocculant may be inorganic particles, examples of which include silica particles (e.g., amorphous silica particles), zeolite, bentonite, aluminum oxide, talc, titanium dioxide, kaolin, clay, and hydrotalcite. In terms of flocculation effect, the flocculant may contain at least one selected from the group consisting of amorphous silica, aluminum oxide, talc, and kaolin. The amount of the flocculant may be, for example, 0.0001% by mass or more and 1.0% by mass or less, or 0.001% by mass or more and 0.5% by mass or less, based on the amount of the water-soluble ethylenically unsaturated monomer. A dispersion in which inorganic particles serving as the flocculant are dispersed in a hydrophobic solvent may be added to the reaction liquid.

[0053] After the formation of the agglomerated particles, an intermediate crosslinking agent may be added to the reaction solution before water is removed from the reaction solution to form a concentrate, and the polymer in the agglomerated particles may be crosslinked in the reaction solution containing the intermediate crosslinking agent. The reaction solution may be heated for the intermediate crosslinking. An aqueous solution containing the intermediate crosslinking agent may be added to the reaction solution.

[0054] The intermediate crosslinking agent may be a compound having two or more reactive functional groups capable of reacting with a monomer unit derived from a water-soluble ethylenically unsaturated monomer. Examples of compounds that can be used as the intermediate 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 which are reaction products of polyols such as ethylene glycol, propylene glycol, trimethylolpropane, glycerin, polyoxyethylene glycol, polyoxypropylene glycol, and polyglycerin with unsaturated acids (maleic acid, fumaric acid, etc.); bis(meth)acrylamides such as N,N'methylenebis(meth)acrylamide; di- or tri(meth)acrylic acid esters which are reaction products of polyepoxides and (meth)acrylic acid; polyisocyanates (tolylene diisocyanate, hexamethylene diisocyanate, etc.) with hydroxyethoxy ... compounds having two or more polymerizable unsaturated groups, such as allylated starch, allylated cellulose, diallyl phthalate, N,N',N"-triallyl isocyanurate, and divinylbenzene; polyglycidyl compounds, such as (poly)ethylene glycol diglycidyl ether, (poly)propylene glycol diglycidyl ether, (poly)glycerin diglycidyl ether, (poly)glycerin triglycidyl ether, (poly)propylene glycol polyglycidyl ether, and polyglycerol polyglycidyl ether; haloepoxy compounds, such as epichlorohydrin, epibromohydrin, and α-methylepichlorohydrin; and isocyanate compounds (2,4-tolylene diisocyanate, hexamethylene diisocyanate, etc.). The intermediate crosslinking agent may contain one or more compounds.

[0055] The intermediate crosslinking agent may include a polyglycidyl compound or a diglycidyl ether compound, and may include at least one compound selected from the group consisting of (poly)ethylene glycol diglycidyl ether, (poly)propylene glycol diglycidyl ether, and (poly)glycerin diglycidyl ether.

[0056] The amount of the intermediate crosslinking agent may be, for example, 0.001 mmol or more and 0.3 mmol or less, 0.005 mmol or more and 0.2 mmol or less, or 0.01 mmol or more and 0.1 mmol or less per mole of the water-soluble ethylenically unsaturated monomer used to form the hydrogel polymer.

[0057] A concentrate is formed by extracting a portion of water from a reaction solution containing aggregated particles. Water can be extracted, for example, by azeotropic distillation of water and a dispersion medium. A surface cross-linking agent may be added to the concentrate, and the aggregated particles may be surface-crosslinked in a mixture containing them. When the moisture content of the concentrate to be surface-crosslinked is appropriately small, the water retention capacity and water absorption capacity of the water-absorbent resin particles tend to be large. From this perspective, the moisture content of the concentrate to be surface-crosslinked may be 20% by mass or more and 70% by mass or less, based on the mass of the polymer. In other words, a mixture containing a concentrate and a surface cross-linking agent may be formed by mixing a concentrate containing water at a moisture content of 20% by mass or more and 70% by mass or less, based on the mass of the polymer, with the surface cross-linking agent. Here, the moisture content is calculated by the following formula: Moisture content (mass %) = (Ww / Ws) × 100 Ww = total mass of water contained in the reaction solution Ws = mass of the polymer before surface cross-linking

[0058] Ww is a quantity determined by subtracting the mass of water extracted to form a concentrate from the mass including the mass of water contained in the aqueous solution in the reaction solution and, when each raw material such as the intermediate crosslinking agent is introduced as an aqueous solution, the mass of water contained in the aqueous solution. The mass of the polymer used to calculate Ws is a theoretical yield calculated from the charged amount of each raw material, such as the water-soluble ethylenically unsaturated monomer, intermediate crosslinking agent, and radical polymerization initiator.

[0059] The moisture content of the concentrate to be subjected to surface crosslinking may be 20% by mass or more, 25% by mass or more, 30% by mass or more, or 35% by mass or more and 70% by mass or less. The moisture content of the concentrate to be subjected to surface crosslinking may be 20% by mass or more, 25% by mass or more, 30% by mass or more, or 35% by mass or more and 65% by mass or less. The moisture content of the concentrate to be subjected to surface crosslinking may be 20% by mass or more, 25% by mass or more, 30% by mass or more, or 35% by mass or more and 60% by mass or less. The moisture content of the concentrate to be subjected to surface crosslinking may be 20% by mass or more, 25% by mass or more, 30% by mass or more, or 35% by mass or more and 55% by mass or less.

[0060] The surface cross-linking agent may be a compound having two or more reactive functional groups, examples of which include polyols such as ethylene glycol, propylene glycol, 1,4-butanediol, trimethylolpropane, glycerin, polyoxyethylene glycol, polyoxypropylene glycol, and polyglycerin; polyglycidyl compounds such as (poly)ethylene glycol diglycidyl ether, (poly)glycerin diglycidyl ether, (poly)glycerin triglycidyl ether, trimethylolpropane triglycidyl ether, (poly)propylene glycol polyglycidyl ether, and (poly)glycerol polyglycidyl ether; epichlorohydrin, epibromohydrin and α-methylepichlorohydrin; isocyanate compounds such as 2,4-tolylene diisocyanate and hexamethylene diisocyanate; oxetane compounds such as 3-methyl-3-oxetanemethanol, 3-ethyl-3-oxetanemethanol, 3-butyl-3-oxetanemethanol, 3-methyl-3-oxetaneethanol, 3-ethyl-3-oxetaneethanol, and 3-butyl-3-oxetaneethanol; oxazoline compounds such as 1,2-ethylenebisoxazoline; carbonate compounds such as ethylene carbonate; and hydroxyalkylamide compounds such as bis[N,N-di(β-hydroxyethyl)]adipamide. The surface cross-linking agent may include a polyglycidyl compound such as (poly)ethylene glycol diglycidyl ether, (poly)glycerin diglycidyl ether, (poly)glycerin triglycidyl ether, (poly)propylene glycol polyglycidyl ether, polyglycerol polyglycidyl ether, etc. These surface cross-linking agents may be used alone or in combination of two or more.

[0061] A solution containing a surface crosslinking agent may be mixed with the concentrate. The solvent of the solution containing the surface crosslinking agent can be water, a hydrophilic organic solvent, or a combination thereof. Examples of the hydrophilic organic solvent include lower alcohols such as methyl alcohol, ethyl alcohol, and isopropyl alcohol, ketones such as acetone and methyl ethyl ketone, ethers such as dioxane and tetrahydrofuran, amides such as N,N-dimethylformamide, and sulfoxides such as dimethyl sulfoxide. The solvent for the surface crosslinking agent may be used alone or in combination of two or more.

[0062] The amount of the surface cross-linking agent may be, for example, 0.01 mmol or more and 10 mmol or less, 0.03 mmol or more and 3 mmol or less, or 0.05 mmol or more and 1 mmol or less, per 1 mole of the water-soluble ethylenically unsaturated monomer used to form the hydrogel polymer.

[0063] For the surface cross-linking, the mixture may be heated. The heating temperature may be, for example, 60° C. or higher and 200° C. or lower, or 80° C. or higher and 150° C. or lower. The reaction time for the surface cross-linking reaction may be, for example, 1 minute or higher and 300 minutes or lower, or 5 minutes or higher and 200 minutes or lower.

[0064] By removing water, a dispersion medium, and the like from the surface-crosslinked mixture (concentrate), a powder containing agglomerated particles (polymer particles) can be formed from the mixture (concentrate). For this purpose, the mixture (concentrate) may be heated. The obtained powder of agglomerated particles (polymer particles) may be classified by sieving or the like, if necessary.

[0065] FIG. 1 is a partial cross-sectional view showing an example of an absorbent article. The absorbent article 50 shown in FIG. 1 includes a sheet-like absorbent body 10, a first shape-retaining member 21, a second shape-retaining member 22, a liquid-permeable sheet 30, and an adhesive 35. The absorbent body 10 is a laminate composed of one water-absorbent resin layer 11 containing a plurality of water-absorbent resin particles 11a and one hydrophilic fiber layer 12 containing hydrophilic fibers, and is provided inside the liquid-permeable sheet 30. The absorbent body 10 is disposed between the sheet-like first shape-retaining member 21 and the sheet-like second shape-retaining member 22. The absorbent body 10 may be composed only of the water-absorbent resin layer 11 containing the water-absorbent resin particles 11a. The water-absorbent resin particles 11a may be the water-absorbent resin particles described above. The entire absorbent body 10 may be enclosed by the first shape-retaining member 21 and the second shape-retaining member 22. The first shape-retaining member 21 and the second shape-retaining member 22 may be a single sheet or two separate sheets. The first shape-retaining member 21 and the second shape-retaining member 22 may be, for example, tissue. An adhesive 35 is interposed between the liquid-permeable sheet 30 and the second shape-retaining member 22 to bond them together. The adhesive 35 may be, for example, a hot-melt adhesive. In the example of FIG. 1 , the water-absorbent resin layer 11 and the hydrophilic fiber layer 12 are provided in this order from the liquid-permeable sheet 30 side. The absorbent article may further include a liquid-impermeable sheet provided on the outside of the first shape-retaining member 21.

[0066] The structure of the absorbent body 10 is not limited to the structure exemplified in Fig. 1 and can be modified as appropriate. Other examples of absorbent body structures include a mixed structure in which water-absorbent resin particles and hydrophilic fibers are mixed throughout the absorbent body, and a sandwich structure in which a water-absorbent resin layer containing water-absorbent resin particles is held between multiple hydrophilic fiber layers. When the water-absorbent resin layer and the hydrophilic fiber layer are provided separately, the water-absorbent resin particles and the hydrophilic fibers may be mixed near the boundary between them.

[0067] The amount of water-absorbent resin particles in the absorbent body may be, for example, 5% by mass or more and 100% by mass or less, 90% by mass or less, 80% by mass or less, or 70% by mass or less, based on the mass of the absorbent body (or the total mass of the hydrophilic fibers and the water-absorbent resin particles). The amount of water-absorbent resin particles in the absorbent body may be 10% by mass or more and 100% by mass or less, 90% by mass or less, 80% by mass or less, or 70% by mass or less, based on the mass of the absorbent body (or the total mass of the hydrophilic fibers and the water-absorbent resin particles). The amount of water-absorbent resin particles in the absorbent body may be 15% by mass or more and 100% by mass or less, 90% by mass or less, 80% by mass or less, or 70% by mass or less, based on the mass of the absorbent body (or the total mass of the hydrophilic fibers and the water-absorbent resin particles). The amount of water-absorbent resin particles in the absorbent may be 20% by mass or more and 100% by mass or less, 90% by mass or less, 80% by mass or less, or 70% by mass or less, based on the mass of the absorbent (or the total mass of the hydrophilic fibers and the water-absorbent resin particles). The amount of water-absorbent resin particles in the absorbent may be 25% by mass or more and 100% by mass or less, 90% by mass or less, 80% by mass or less, or 70% by mass or less, based on the mass of the absorbent (or the total mass of the hydrophilic fibers and the water-absorbent resin particles). The amount of water-absorbent resin particles in the absorbent may be 30% by mass or more and 100% by mass or less, 90% by mass or less, 80% by mass or less, or 70% by mass or less, based on the mass of the absorbent (or the total mass of the hydrophilic fibers and the water-absorbent resin particles). The amount of water-absorbent resin particles in the absorbent may be 35% by mass or more and 100% by mass or less, 90% by mass or less, 80% by mass or less, or 70% by mass or less, based on the mass of the absorbent (or the total mass of the hydrophilic fibers and the water-absorbent resin particles). The amount of water-absorbent resin particles in the absorbent may be 40% by mass or more and 100% by mass or less, 90% by mass or less, 80% by mass or less, or 70% by mass or less, based on the mass of the absorbent (or the total mass of the hydrophilic fibers and the water-absorbent resin particles). The amount of water-absorbent resin particles in the absorbent may be 45% by mass or more and 100% by mass or less, 90% by mass or less, 80% by mass or less, or 70% by mass or less, based on the mass of the absorbent (or the total mass of the hydrophilic fibers and the water-absorbent resin particles).The amount of water-absorbent resin particles in the absorbent may be 50% by mass or more and 100% by mass or less, 90% by mass or less, 80% by mass or less, or 70% by mass or less, based on the mass of the absorbent (or the total mass of the hydrophilic fibers and the water-absorbent resin particles). The amount of water-absorbent resin particles in the absorbent may be 55% by mass or more and 100% by mass or less, 90% by mass or less, 80% by mass or less, or 70% by mass or less, based on the mass of the absorbent (or the total mass of the hydrophilic fibers and the water-absorbent resin particles). The amount of water-absorbent resin particles in the absorbent may be 60% by mass or more and 100% by mass or less, 90% by mass or less, 80% by mass or less, or 70% by mass or less, based on the mass of the absorbent (or the total mass of the hydrophilic fibers and the water-absorbent resin particles).

[0068] The basis weight of the water-absorbent resin particles in the absorbent article is 10 g / m ２ or more and 300 g / m ２ Below, 200g / m ２ Below, 150g / m ２ Below, 100g / m ２ Below, 90g / m ２ or less, or 80 g / m ２ The basis weight of the water-absorbent resin particles in the absorbent article may be 50 g / m or less. ２ or more and 300 g / m ２ Below, 200g / m ２ Below, 150g / m ２ Below, 100g / m ２ Below, 90g / m ２ or less, or 80 g / m ２ The basis weight of the water-absorbent resin particles in the absorbent article may be 60 g / m or less. ２ or more and 300 g / m ２ Below, 200g / m ２ Below, 150g / m ２ Below, 100g / m ２ Below, 90g / m ２ or less, or 80 g / m ２ The basis weight of the water-absorbent resin particles in the absorbent article may be 70 g / m or less. ２ or more and 300 g / m ２ Below, 200g / m ２ Below, 150g / m ２Below, 100g / m ２ Below, 90g / m ２ or less, or 80 g / m ２ In this specification, the basis weight means the mass per unit area of ​​the absorbent article when viewed in the thickness direction of the absorbent article.

[0069] Examples of hydrophilic fibers include cellulose fibers such as cotton pulp and chemical pulp, and artificial cellulose fibers such as rayon and acetate. The absorbent body may further contain hydrophobic fibers made of synthetic resins such as polyamide, polyester, and polyolefin as a reinforcing agent.

[0070] The basis weight of the hydrophilic fiber in the absorbent article is 20 g / m ２ or more and 300 g / m ２ Below, 200g / m ２ Below, 100g / m ２ Below, 70g / m ２ Below, 60g / m ２ or less, or 50 g / m ２ The basis weight of the hydrophilic fibers in the absorbent article may be 30 g / m or less. ２ or more and 300 g / m ２ Below, 200g / m ２ Below, 100g / m ２ Below, 70g / m ２ Below, 60g / m ２ or less, or 50 g / m ２ The basis weight of the hydrophilic fibers in the absorbent article may be 40 g / m or less. ２ or more and 300 g / m ２ Below, 200g / m ２ Below, 100g / m ２ Below, 70g / m ２ Below, 60g / m ２ or less, or 50 g / m ２ It may be the following:

[0071] The liquid-permeable sheet 30 may be, for example, a nonwoven fabric, a porous resin sheet, tissue, or a combination thereof. The nonwoven fabric may contain resin fibers such as polyethylene, polypropylene, polyester, and polyamide.

[0072] The absorbent article is particularly useful as, for example, a waterproof nursing sheet, a disposable diaper, a sanitary napkin, a tampon, or a pet sheet, due to its small coloring and diffusion area.

[0073] The present invention is not limited to the following examples.

[0074] 1. Preparation of Water-Absorbent Resin Particles Example 1 Polymerization Step A round-bottomed cylindrical separable flask (baffle width: 7 mm, baffle length: 10 cm) with an inner diameter of 11 cm and a capacity of 2 L and equipped with four side wall baffles (baffle width: 7 mm, baffle length: 10 cm) was prepared, equipped with a reflux condenser, a dropping funnel, a nitrogen gas inlet tube, and a stirrer. A stirring blade A having a shaft and a flat plate portion was attached to the stirrer. The flat plate portion was welded to the shaft and had a curved tip. Four slits extending along the axial direction of the shaft were formed in the flat plate portion. The four slits were arranged in the width direction of the flat plate portion. The width of the two inner slits was 1 cm, and the width of the two outer slits was 0.5 cm. The length of the flat plate portion was approximately 10 cm, and the width of the flat plate portion was approximately 6 cm.

[0075] 472.3 g of n-heptane and 1.10 g (3.17 mmol) of sorbitan monolaurate (surfactant, Nonion LP-20R, HLB: 8.6, manufactured by NOF Corporation) were placed in the prepared separable flask. The mixture in the separable flask was heated to 50°C while being stirred with a stirrer at a rotation speed of 300 rpm, thereby dissolving the sorbitan monolaurate in the n-heptane. The mixture was then cooled to 45°C.

[0076] A 500 mL Erlenmeyer flask was charged with 92.0 g of an 80.5 wt% aqueous acrylic acid solution (1.03 mol of acrylic acid). While cooling with ice from the outside, 147.7 g of a 20.9 wt% aqueous sodium hydroxide solution (0.77 mol of sodium hydroxide) was added dropwise to neutralize 75 mol% of the acrylic acid. 0.101 g of potassium persulfate (0.374 mmol of water-soluble radical polymerization initiator) was dissolved in the partially neutralized acrylic acid solution formed by the neutralization to form a monomer aqueous solution.

[0077] The resulting aqueous monomer solution was added to the mixture in the separable flask, and the atmosphere in the system containing the resulting reaction solution was thoroughly purged with nitrogen. The reaction solution was then stirred with a stirrer at 300 rpm, while the separable flask was immersed in a 70°C water bath to raise the temperature, and the temperature was maintained for 60 minutes to allow the polymerization reaction to proceed. As the polymerization reaction proceeded, a particulate hydrogel polymer was formed in the reaction solution.

[0078] Formation of aggregated particles: The agitator blade A was replaced with agitator blade B, which had two stages of four inclined paddle blades with a blade diameter of 5 cm. While stirring at a rotation speed of 1,000 rpm, a dispersion containing 0.014 g of amorphous silica particles (flocculant, Oriental Silicas Corporation, Toxil NP-S) and 100 g of n-heptane was added to the reaction solution containing the produced hydrogel polymer, n-heptane, and surfactant. The separable flask was then immersed in a water bath at 75°C, and the reaction solution was stirred for 10 minutes. Aggregated particles were formed by aggregation of the hydrogel polymer in the reaction solution.

[0079] To the reaction solution containing the aggregated particles, 0.41 g of an aqueous solution of ethylene glycol diglycidyl ether (ethylene glycol diglycidyl ether (intermediate crosslinking agent): 0.048 mmol) with a concentration of 2% by mass was added. Thereafter, the reaction solution in the separable flask was stirred for 30 minutes while being heated in a water bath at 75°C, thereby allowing intermediate crosslinking to proceed.

[0080] Concentration The reaction solution in the separable flask was heated in an oil bath at 125° C., and 110.6 g of water was extracted from the system by azeotropic distillation of n-heptane and water while refluxing n-heptane.

[0081] Addition of Chelating Agent: 1.02 g (chelating agent: 0.101 mmol) of an aqueous solution of pentasodium ethylenediaminetetramethylenephosphonate (chelating agent, EDTMP.5Na) with a concentration of 5.4% by mass was mixed with the concentrate formed by removing water.

[0082] To the concentrate after the addition of the chelating agent, 3.31 g of an aqueous solution of ethylene glycol diglycidyl ether having a concentration of 2.5 mass % (ethylene glycol diglycidyl ether (surface crosslinking agent): 0.48 mmol) was mixed. The mixture was kept at an internal temperature of 83±2°C for 2 hours to allow surface crosslinking to proceed in the mixture.

[0083] Drying The mixture after surface cross-linking was heated to 120°C, and water and n-heptane were evaporated until almost no evaporants were distilled out from the system, thereby obtaining a powder of dried polymer particles (aggregated particles). This powder was passed through a sieve with an opening of 850 µm, thereby obtaining 86.2 g of water-absorbent resin particles of Example 1.

[0084] Example 2 A concentrate containing aggregated particles was formed in a separable flask using the same procedure as in Example 1, except that the amount of water extracted from the system by azeotropic distillation for concentration was changed to 108.8 g. 4.14 g of a 2 mass% aqueous solution of ethylene glycol diglycidyl ether (ethylene glycol diglycidyl ether (surface cross-linking agent): 0.48 mmol) was mixed with the formed concentrate. Surface cross-linking was allowed to proceed in the mixture by maintaining the internal temperature at 83±2°C for 2 hours.

[0085] The surface-crosslinked mixture was heated to 120°C, and water and n-heptane were evaporated until almost no evaporants were distilled from the system, thereby obtaining a powder of dried polymer particles (aggregated particles). This powder was passed through a sieve with an opening of 850 µm, thereby obtaining 87.1 g of polymer particles.

[0086] 20 g of the obtained polymer particles were placed in a round-bottomed cylindrical separable flask with an inner diameter of 11 cm and equipped with an anchor-shaped stirring blade made of fluororesin. While stirring the polymer particles in the separable flask at 300 rpm, 0.20 g of a 6.0 wt% aqueous solution of pentasodium ethylenediaminetetramethylenephosphonate (chelating agent) (chelating agent: 0.022 mmol) was sprayed into the separable flask over 1 second using a 3 mL syringe equipped with a Fine Atomizer Oral (Yoshikawa Chemical Co., Ltd.). After stirring for 10 minutes, the mixture containing the polymer particle powder and the chelating agent was heated at 100°C for 30 minutes to obtain water-absorbent resin particles of Example 2.

[0087] Example 3 20 g of polymer particles before addition of a chelating agent obtained in the same manner as in Example 2 and 0.012 g (0.028 mmol) of ethylenediaminetetramethylenephosphonic acid hydrate (chelating agent, EDTMP 8H) powder were sealed in a resin bag having a size of 85 mm x 120 mm (Unipack D-4, Seisan Nippon Sha, 85 mm x 120 mm). The bag was shaken for 1 minute so that the mixture in the bag became uniform, thereby obtaining water-absorbent resin particles of Example 3.

[0088] Example 4 86.8 g of water-absorbent resin particles of Example 4 were obtained under the same conditions as in Example 1, except that the amount of water extracted by azeotropic distillation for concentration was changed to 107.1 g.

[0089] Example 5 Water-absorbent resin particles of Example 5 were obtained under the same conditions as in Example 2, except that the amount of water extracted by azeotropic distillation for concentration was changed to 105.2 g, and 87.1 g of polymer particles were obtained.

[0090] Example 6 Water-absorbent resin particles of Example 6 were obtained under the same conditions as in Example 3, except that the amount of water extracted by azeotropic distillation for concentration was changed to 105.2 g, and 87.1 g of polymer particles were obtained.

[0091] Example 7 Water absorbent resin particles of Example 7 were obtained under the same conditions as in Example 2, except that the aqueous solution to be sprayed for adding a chelating agent was changed to 0.62 g of an aqueous solution of pentasodium ethylenediaminetetramethylenephosphonate (chelating agent) having a concentration of 32.1 mass % (chelating agent: 0.364 mmol).

[0092] Example 8 Water absorbent resin particles of Example 8 were obtained under the same conditions as in Example 2, except that the aqueous solution to be sprayed for adding a chelating agent was changed to 0.2 g of an aqueous solution of pentasodium ethylenediaminetetramethylenephosphonate (chelating agent) having a concentration of 4.0 mass % (chelating agent: 0.015 mmol).

[0093] Example 9 Water-absorbent resin particles of Example 9 were obtained under the same conditions as in Example 3, except that the amount of ethylenediaminetetramethylenephosphonic acid hydrate was changed to 0.3 g (chelating agent: 0.549 mmol).

[0094] Example 10 Water-absorbent resin particles of Example 10 were obtained under the same conditions as in Example 2, except that the amount of water extracted by azeotropic distillation for concentration was changed to 111.6 g, and 85.9 g of polymer particles were obtained.

[0095] Example 11 Water-absorbent resin particles of Example 11 were obtained under the same conditions as in Example 2, except that the amount of water extracted by azeotropic distillation for concentration was changed to 113.4 g, and 88.9 g of polymer particles were obtained.

[0096] Example 12 Water-absorbent resin particles of Example 12 were obtained under the same conditions as in Example 2, except that the amount of sorbitan monolaurate used in polymerization was changed to 0.74 g (2.12 mmol), the amount of water extracted by azeotropic distillation for concentration was changed to 110.7 g, and 79.7 g of polymer particles were obtained.

[0097] Example 13 Water absorbent resin particles of Example 13 were obtained under the same conditions as in Example 2, except that the aqueous solution sprayed for adding a chelating agent was changed to 0.20 g (chelating agent: 0.021 mmol) of an aqueous solution of diethylenetriaminepentamethylenephosphonic acid·7sodium salt (chelating agent, DTPMP·7Na) having a concentration of 6.0 mass %.

[0098] Comparative Example 1 Water absorbent resin particles of Comparative Example 1 were obtained under the same conditions as in Example 2, except that the aqueous solution sprayed for adding a chelating agent was changed to 0.20 g of an aqueous solution of pentasodium diethylenetriaminepentaacetic acid (chelating agent, DTPA.5Na) having a concentration of 6.0 mass % (chelating agent: 0.024 mmol).

[0099] Comparative Example 2 Water absorbent resin particles of Comparative Example 2 were obtained under the same conditions as in Example 5, except that the aqueous solution sprayed for adding a chelating agent was changed to 0.20 g of an aqueous solution of pentasodium diethylenetriaminepentaacetic acid (chelating agent, DTPA.5Na) having a concentration of 6.0 mass % (chelating agent: 0.024 mmol).

[0100] Comparative Example 3 Water-absorbent resin particles of Comparative Example 3 were obtained under the same conditions as in Example 2, except that the aggregation step was not performed, the amount of water extracted by azeotropic distillation for concentration was changed to 110.3 g, 80.0 g of polymer particles were obtained, and the amount of the aqueous ethylene glycol diglycidyl ether solution having a concentration of 2.0 mass % for surface crosslinking was changed to 2.76 g (0.32 mmol).

[0101] Comparative Example 4 Polymerization Step <First-Stage Polymerization Reaction> A round-bottomed cylindrical separable flask with an inner diameter of 11 cm and a capacity of 2 L was prepared, equipped with a reflux condenser, a dropping funnel, a nitrogen gas inlet tube, and a stirrer. A stirring blade B having four inclined paddle blades with a blade diameter of 5 cm arranged in two stages was attached to the stirrer.

[0102] 293 g of n-heptane and 0.736 g of maleic anhydride-modified ethylene-propylene copolymer (polymer dispersant, Mitsui Chemicals, Inc., Hiwax 1105A) were placed in the prepared separable flask. The mixture in the separable flask was heated to 80°C while being stirred with a stirrer, thereby dissolving the maleic anhydride-modified ethylene-propylene copolymer in the n-heptane. The mixture was then cooled to 50°C.

[0103] A 300 mL beaker was charged with 92.0 g of an 80.5 wt% aqueous acrylic acid solution (1.03 mol of acrylic acid). While cooling with ice from the outside, 147.7 g of a 20.9 wt% aqueous sodium hydroxide solution was added dropwise to neutralize 75 mol% of the acrylic acid. 0.0736 g of potassium persulfate (water-soluble radical polymerization initiator, 0.272 mmol) and 0.010 g of ethylene glycol diglycidyl ether (internal crosslinking agent, 0.057 mmol) were dissolved in the partially neutralized acrylic acid solution formed by neutralization to form a first-stage aqueous monomer solution.

[0104] The first-stage monomer aqueous solution was added to the mixture in the separable flask, and the resulting reaction solution was stirred for 10 minutes with a stirrer. Subsequently, a surfactant solution containing 6.62 g of n-heptane and 0.736 g of sucrose stearate (surfactant, HLB: 3, Mitsubishi Chemical Foods Corporation, Ryoto Sugar Ester S-370) was further added. The reaction solution was stirred with a stirrer at 500 rpm while the system was thoroughly purged with nitrogen. The separable flask was then immersed in a 70°C water bath and maintained in that state for 60 minutes to allow the polymerization reaction to proceed. A first-stage polymerization slurry was formed as a result of the polymerization reaction.

[0105] <Second-Stage Polymerization Reaction> 128.8 g of an 80.5 wt% aqueous acrylic acid solution (acrylic acid: 1.44 mol) was placed in a 500 mL beaker. While cooling with ice from the outside, 160.0 g of a 27 wt% aqueous sodium hydroxide solution was added dropwise to neutralize 75 mol% of the acrylic acid. 0.103 g of potassium persulfate (water-soluble radical polymerization initiator, 0.381 mmol) and 0.0116 g of ethylene glycol diglycidyl ether (internal crosslinking agent, 0.067 mmol) were dissolved in the partially neutralized acrylic acid solution formed by neutralization to form a second-stage aqueous monomer solution.

[0106] The separable flask was cooled to 25°C while stirring the first-stage polymerization slurry with a stirrer at a rotation speed of 1000 rpm. Thereafter, the entire amount of the second-stage aqueous monomer solution was added to the first-stage polymerization slurry, and the system was purged with nitrogen over 30 minutes. The separable flask was again immersed in a 70°C water bath to raise the temperature, and the polymerization reaction was carried out for 60 minutes to obtain a hydrous gel polymer.

[0107] Concentration The reaction solution in the separable flask was heated in an oil bath at 125° C., and 183.8 g of water was extracted from the system by azeotropic distillation of n-heptane and water while refluxing n-heptane.

[0108] Addition of chelating agent: 4.41 g of an aqueous solution of pentasodium ethylenediaminetetramethylenephosphonate (chelating agent) with a concentration of 3.0 mass % (chelating agent: 0.242 mmol) was mixed with the concentrate formed by the removal of water under stirring. Thereafter, 61.0 g of water was again removed from the system by azeotropic distillation of n-heptane and water while refluxing n-heptane.

[0109] To the concentrate after the addition of the chelating agent, 4.42 g of an aqueous solution of ethylene glycol diglycidyl ether having a concentration of 2% by mass (ethylene glycol diglycidyl ether (surface crosslinking agent): 0.507 mmol) was mixed. The mixture was kept at an internal temperature of 83±2°C for 2 hours to allow surface crosslinking to proceed in the mixture.

[0110] Drying The surface-crosslinked mixture was heated in an oil bath at 125°C, and water and n-heptane were evaporated until almost no evaporants were distilled from the system, thereby obtaining a powder of dried polymer particles (aggregated particles). This powder was passed through a sieve with an opening of 850 μm to obtain 221.3 g of polymer particles. The polymer particles that had passed through the sieve were mixed with 0.5% by mass of amorphous silica (Toxil NP-S, Oriental Silicas Corporation) relative to the mass of the polymer particles, to obtain 221.3 g of water-absorbent resin particles of Comparative Example 4.

[0111] Comparative Example 5 The polymer particles to which the chelating agent had not yet been added in Example 2 were used as the water-absorbent resin particles in Comparative Example 5.

[0112] Comparative Example 6 The polymer particles to which the chelating agent had not yet been added in Example 5 were used as the water-absorbent resin particles in Comparative Example 6.

[0113] 2. Evaluation of Water-Absorbent Resin Particles The water-absorbent resin particles of each Example or Comparative Example were evaluated by the following method. Unless otherwise specified, measurements were carried out in an environment of a temperature of 25±2°C and a humidity of 50±10%. The evaluation results are shown in Tables 1 and 2. The amount of chelating agent shown in each table is the ratio (ppm by mass) to the amount of polymer particles. The methods of adding the chelating agent shown in each table are as follows: (i) Mixing an aqueous solution of the chelating agent with the concentrate before drying. (ii) After drying, spraying the aqueous solution of the chelating agent onto polymer particles after passing through an 850 μm sieve. (iii) After drying, mixing the powder of the chelating agent with polymer particles after passing through an 850 μm sieve.

[0114] Water Retention Capacity 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 saline solution at 25 ± 2 °C 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, and the top of the cotton bag was tied with a rubber band. The cotton bag was left to stand for 30 minutes to allow the water-absorbent resin particles in the cotton bag to swell. The swollen gel in the cotton bag was then dehydrated for 1 minute using a dehydrator (manufactured by Kokusan Co., Ltd., product number: H-122) set to a centrifugal force of 167 G. The mass Wa [g] of the cotton bag containing the swollen gel after dehydration was measured. The same operation was performed on a cotton bag not containing water-absorbent resin particles, and the empty mass Wb [g] of the cotton bag when wet was measured. The water retention capacity of the water-absorbent resin particles in saline solution was calculated using the following formula. Water retention amount [g / g] = (Wa-Wb) / 2.0

[0115] Water Absorption Capacity 500 g of physiological saline solution at 25±2°C was placed in a 500 mL beaker. While stirring at 600 rpm using a magnetic stir bar (8 mm diameter x 30 mm length, no ring), 2.0 g of water-absorbent resin particles were dispersed in the physiological saline solution, taking care not to generate lumps. The physiological saline solution was left for 60 minutes with stirring, to obtain a dispersion containing a swollen gel formed by the swelling of the water-absorbent resin particles. This dispersion was passed through a JIS Z 8801-1 standard sieve with a mass Wc [g] and a mesh size of 75 μm. The sieve on which the swollen gel remained was tilted at an angle of approximately 30 degrees relative to the horizontal and left for 30 minutes to remove excess water. Next, the mass Wd [g] of the sieve on which the swollen gel remained was measured. The water absorption capacity [g / g] of the water-absorbent resin particles in physiological saline solution was calculated using the following formula: Water absorption capacity [g / g] = (Wd - Wc) / 2.0

[0116] Water absorption rate (Vortex method) 50±0.1 g of saline solution and a magnetic stirrer bar (8 mmφ×30 mm without ring) were placed in a 100 mL beaker. The beaker was immersed in a thermostatic water bath to adjust the liquid temperature to 25±0.2 ° C. Next, the beaker was placed on a magnetic stirrer, and the saline solution was stirred at a rotation speed of 600 rpm to generate a vortex, and 2.0 g of water-absorbent resin particles were quickly added thereto. The time [seconds] from the time the water-absorbent resin particles were added to the time the vortex on the liquid surface converged due to water absorption by the water-absorbent resin particles was measured, and this was taken as the water absorption rate of the water-absorbent resin particles.

[0117] Median Particle Diameter and Particle Size Distribution Water-absorbent resin particles were passed through a JIS Z 8801-1 standard sieve having a mesh size of 250 μm. When the amount remaining on the sieve relative to the total amount was 50 mass% or more, the median particle diameter was measured using the following combination of sieves (A); when the amount remaining on the sieve relative to the total amount was less than 50 mass%, the median particle diameter was measured using the following combination of sieves (B). (A) The JIS standard sieves were combined in the following order from top to bottom: a sieve with a mesh size of 710 μm, a sieve with a mesh size of 600 μm, a sieve with a mesh size of 500 μm, a sieve with a mesh size of 425 μm, a sieve with a mesh size of 300 μm, a sieve with a mesh size of 250 μm, a sieve with a mesh size of 150 μm, and a tray. (B) JIS standard sieves were arranged in the following order from top to bottom: a sieve with a mesh size of 500 μm, a sieve with a mesh size of 425 μm, a sieve with a mesh size of 250 μm, a sieve with a mesh size of 180 μm, a sieve with a mesh size of 150 μm, a sieve with a mesh size of 106 μm, a sieve with a mesh size of 75 μm, and a tray.

[0118] The water-absorbent resin particles were placed in the sieve located at the top stage, and the particles were shaken for 20 minutes using a continuous fully automatic ultrasonic vibration sieving measuring instrument (Robot Sifter RPS-205, manufactured by Seishin Enterprise Co., Ltd.) to classify the water-absorbent resin particles. After classification, the proportion (mass percentage) of the water-absorbent resin particles remaining on each sieve relative to the total amount of the water-absorbent resin particles was calculated. By integrating the proportions of the fractions in order from the largest particle diameter, the relationship between the sieve opening and the integrated value of the proportion of the water-absorbent resin particles remaining on the sieve was plotted on logarithmic probability paper. By connecting the plots on the probability paper with a straight line, the particle diameter corresponding to an integrated mass percentage of 50% by mass was determined, and this value was taken as the median particle diameter.

[0119] The total mass of the water-absorbent resin particles remaining on the tray in the combination of sieves (A) and the mass of the water-absorbent resin particles remaining on the sieve with a mesh size of 106 μm, the sieve with a mesh size of 75 μm, and the tray in the combination of sieves (B) was calculated as the total mass of particles with a particle size of 150 μm or less. From this and the total mass of the water-absorbent resin particles, the proportion (mass percentage) of particles with a particle size of 150 μm or less to the total amount of water-absorbent resin particles was calculated.

[0120] Yellowness test under high temperature and high humidity 2.0 g of water-absorbent resin particles were uniformly placed in a glass petri dish with an inner diameter of 3 cm and a depth of 1 cm, and the container was stored for 5 days in a thermo-hygrostat (manufactured by Espec Corporation, LHU-113) set at a temperature of 70 ± 2 ° C. and a relative humidity of 90 ± 2%. Thereafter, the container was removed from the thermo-hygrostat and left to cool to room temperature for a while. The entire amount of water-absorbent resin particles in the container was placed in a glass measurement container with an inner diameter of 3 cm, and the yellowness of the water-absorbent resin particles was measured using a color difference meter (Color Meter ZE6000, manufactured by Nippon Denshoku Industries Co., Ltd.) in which X, Y, and Z, which are the tristimulus values ​​of the colorimetric color difference meter, were corrected using a standard white board. The yellowness after leaving for 5 days in a high temperature and high humidity environment was calculated from X, Y, and Z (tristimulus values) using the following formula: Yellowness = 100 (1.28X - 1.06Z) / Y

[0121] 3. Preparation of absorbent article: A 20 cm x 60 cm sized absorbent article with a basis weight of 16 g / m ２ A first shape-retaining member (tissue) of 10 cm was prepared. 5.4 g of hydrophilic fibers (ground pulp) were deposited on this first shape-retaining member by air-pressure forming using an airflow mixer (Autech Co., Ltd., Pad Former) to form a hydrophilic fiber layer covering the entire upper surface of the first shape-retaining member. 10 cm sections were cut off from both longitudinal ends of the laminate of the first shape-retaining member and the hydrophilic fiber layer. The remaining laminate was divided into two equal parts, yielding two laminates measuring 20 cm x 20 cm.

[0122] 1.0 g of water was sprayed evenly onto each of the two laminates using a spray bottle, and then a load of 500 kPa was applied for 30 seconds. Then, 3.0 g of water-absorbent resin particles were evenly spread on the hydrophilic fiber layer to form a water-absorbent resin particle layer. A 20 cm x 20 cm sheet with a basis weight of 16 g / m was used. ２ The second shape-retaining member (tissue) was layered on top of the water-absorbent resin particle layer to obtain a laminate having, from the bottom, the first shape-retaining member, the hydrophilic fiber layer, the water-absorbent resin particle layer, and the second shape-retaining member.

[0123] On the second shape-retaining member of this laminate, an air-through nonwoven fabric (KNH Enterprise Co., Ltd., basis weight: 25 g / m) having a size of 20 cm x 20 cm and coated with a hot melt adhesive (ME-765E, Henkel Japan Co., Ltd.) was placed. ２ ) was laminated in a direction in which the hot melt adhesive contacted the second shape-retaining member, thereby obtaining an absorbent article. 0.1 g of the hot melt adhesive was applied to the air-through nonwoven fabric so as to form a spiral stripe pattern of 20 stripes arranged at 10 mm intervals. In the absorbent body of the obtained sheet-like absorbent article, the basis weight of the water-absorbent resin particles was 75 g / m ２ The weight of the hydrophilic fiber (crushed pulp) is 45 g / m ２ It was.

[0124] 4. Evaluation of absorbent articles Artificial urine The following ingredients were mixed and dissolved in ion-exchanged water, and a small amount of Food Blue No. 1 was added to prepare artificial urine. Composition of artificial urine: Deionized water: 970.9 g, Urea: 20.0 g, Sodium chloride: 8.0 g, Calcium chloride dihydrate: 0.3 g, Magnesium sulfate heptahydrate: 0.8 g, L(+)-ascorbic acid: 0.2 g, Iron (II) sulfate heptahydrate: 0.01 g

[0125] Initial diffusion area: An acrylic plate (25 cm x 25 cm, 3 mm thick) was placed on a horizontal platform, and the absorbent article was placed on top of it with the air-through nonwoven fabric facing upward. 30 mL of artificial urine adjusted to 25±1°C was dripped toward the center of the absorbent article from 1 cm above the absorbent article using a pump (DOSE IT P910, manufactured by INTEGRA Biosciences) connected to an inlet with an inner diameter of 0.4 mm over 10 seconds. The dripped artificial urine diffused horizontally within the absorbent article. Figure 2 is a plan view showing an example of the state in which the artificial urine has diffused through the absorbent article. Immediately after the completion of dripping the artificial urine, the length d1 of the region DA in the vertical direction of the absorbent article at a position passing through the center C where the artificial urine was dripped, and the length d2 of the region DA in the horizontal direction of the absorbent article at a position passing through the center C, were measured for the region DA where the artificial urine had diffused within the absorbent article 50. The initial diffusion area [cm ２The initial diffusion area [cm ２ ]=(d1 / 2)×(d2 / 2)×3.14

[0126] Diffusion Area Change Rate After the artificial urine was dripped, the absorbent article was sealed together with an acrylic plate in a zipper bag (Unipack L-4, manufactured by Seisan Nippon Sha, Ltd., size 400 mm x 280 mm, made of polyethylene) and placed on a horizontal table. Four hours after the dripping of the artificial urine, the length d1' of the region DA in the vertical direction of the absorbent article at a position passing through the center C and the length d2' of the region DA in the horizontal direction of the absorbent article at a position passing through the center C were measured. The ratio of the diffusion area after 4 hours to the initial diffusion area (diffusion area change rate) was calculated using the following formula: Diffusion Area Change Rate After 4 Hours [%] = [{(d1' / 2) x (d2' / 2) x 3.14} / Initial Diffusion Area] x 100

[0127] Thereafter, the article was allowed to stand for a further period of time, and 20 hours after the dropping of the artificial urine, the length d1'' of the region DA in the longitudinal direction of the absorbent article at a position passing through the center C and the length d2'' of the region DA in the lateral direction of the absorbent article at a position passing through the center C were measured for the region DA in which the artificial urine had diffused within the absorbent article 50. The rate of change in the diffusion area after 20 hours relative to the initial diffusion area was calculated using the following formula: Diffusion area change rate after 20 hours [%] = [{(d1'' / 2) x (d2'' / 2) x 3.14} / initial diffusion area] x 100

[0128]

[0129]

[0130] As shown in Table 1, it was confirmed that the water-absorbent resin particles of each Example, which contained a phosphonic acid-based chelating agent, had a water-absorption speed for physiological saline of 30 seconds or less, and had a ratio of particles having a particle diameter of 150 μm or less of 20 mass % or less relative to the total amount of water-absorbent resin particles, showed low yellowness after being left in a high-temperature, high-humidity environment for 5 days. Furthermore, it was confirmed that the absorbent articles containing these water-absorbent resin particles showed a small initial diffusion area, and also showed small rates of change in the diffusion area after 4 hours and 20 hours.

[0131] A small diffusion area when an absorbent article absorbs urine is advantageous for reducing discomfort to the user and extending the usage time, and as a result, the frequency of replacing the absorbent article can be reduced. If the frequency of replacing the absorbent article is reduced, not only the amount of water-absorbent resin particles constituting the absorbent article but also the amount of other materials (for example, natural raw materials (biomass resources) such as pulp and nonwoven fabric) used can be reduced, which can contribute to the conservation of the global environment.

[0132] 10...absorbent body, 11...water-absorbent resin layer, 11a...water-absorbent resin particles, 12...hydrophilic fiber layer, 21...first shape-retaining member, 22...second shape-retaining member, 30...liquid-permeable sheet, 35...adhesive, 50...absorbent article.

Claims

1. Water-absorbent resin particles comprising: polymer particles containing a polymer containing a water-soluble ethylenically unsaturated monomer as a monomer unit; and a phosphonic acid-based chelating agent, wherein the water-absorbent resin particles have a water-absorption speed for physiological saline of 30 seconds or less, and the proportion of particles having a particle diameter of 150 μm or less in the water-absorbent resin particles is 20 mass% or less relative to the total amount of the water-absorbent resin particles.

2. The water-absorbent resin particles according to claim 1, wherein the phosphonic acid chelating agent comprises a compound having 3 to 8 phosphoryl groups.

3. The water-absorbent resin particles according to claim 1, wherein the amount of the phosphonic acid chelating agent is 200 ppm by mass or more and 15,000 ppm by mass or less based on the amount of the polymer particles.

4. The water-absorbent resin particles according to claim 1, wherein the amount of the phosphonic acid chelating agent is 400 ppm by mass or more and 12,000 ppm by mass or less based on the amount of the polymer particles.

5. The water-absorbent resin particles according to claim 1, wherein the water-soluble ethylenically unsaturated monomer comprises (meth)acrylic acid and an alkali metal salt thereof.

6. An absorbent comprising the water-absorbent resin particles according to any one of claims 1 to 5.

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

8. A method for producing water-absorbent resin particles, comprising: forming a particulate hydrogel polymer in a reaction liquid containing a water-soluble ethylenically unsaturated monomer, water, a dispersion medium, and a surfactant having an HLB of 7 or more and 16 or less, by polymerizing the water-soluble ethylenically unsaturated monomer by reverse phase suspension polymerization in the reaction liquid; forming a plurality of agglomerated particles containing the hydrogel polymer by agglomerating the hydrogel polymer in the reaction liquid; extracting a portion of the water from the reaction liquid containing the agglomerated particles and water to form a concentrate; and forming a powder containing polymer particles containing the polymer from the concentrate, the method further comprising mixing at least one selected from the group consisting of the reaction liquid, the concentrate, and the powder with a phosphonic acid-based chelating agent.

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