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

Abstract

Disclosed is a method for producing water-absorbent resin particles, which comprises the steps of: polymerizing a water-soluble ethylenically unsaturated monomer by reversed-phase suspension polymerization in a reaction solution containing a surfactant having an HLB of 7-16 (inclusive), thereby forming a water-containing gel-like polymer; forming agglomerated particles; extracting a portion of the water to form a concentrate; and surface-crosslinking the agglomerated particles in a mixture containing the concentrate and a surface-crosslinking agent. The amount of the internal crosslinking agent is 0.093 mmol or less per 1 mol of the water-soluble ethylenically unsaturated monomer. Polymer particles formed from the water-containing gel-like polymer before the formation of the agglomerated particles have a median diameter of 150 [mu] m or more. The mixture is formed by mixing a surface cross-linking agent and a concentrate containing water at a moisture content of 15-50% by mass (inclusive) based on the mass of the polymer.

Description

TECHNICAL FIELD

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

[0002] Patent Document 1 discloses a care absorbent article, i.e., a water-absorbent composite sheet, having a substrate and water-absorbent resin particles of substantially spherical shape intermittently fixed to the substrate.

[0003] PRIOR ART DOCUMENTS PATENT DOCUMENT Patent Document 1: Japanese Patent Application Publication No. 2005-323842 SUMMARY

[0004] PROBLEMS TO BE SOLVED BY THE INVENTION When an absorbent article absorbs a liquid containing moisture, it is desirable that the area in which the liquid spreads within the absorbent article be small. Also, it is generally desirable that the so-called rewet of the absorbed liquid to the surface of the absorbent article be small, and further that a dry surface be quickly formed. However, if the area in which the liquid spreads within the absorbent article is small, there is a tendency that the rewet becomes large and the time until a dry surface is formed becomes long.

[0005] The present application relates to an absorbent article in which the area in which a liquid absorbed by the article spreads is small, and in which the rewet is small and a dry surface can be quickly formed.

[0006] MEANS FOR SOLVING THE PROBLEMS The present application includes the following. [1] A method for producing water-absorbent resin particles, comprising the steps of: polymerizing a water-soluble ethylenically unsaturated monomer by inverse suspension polymerization in a reaction liquid containing the water-soluble ethylenically unsaturated monomer, water, a dispersion medium, and a surfactant having an HLB of 7 or more and 16 or less, thereby forming a particulate hydrogel-like polymer containing the polymer of the water-soluble ethylenically unsaturated monomer and water; agglomerating the hydrogel-like polymer in the reaction liquid, thereby forming agglomerated particles containing a plurality of the hydrogel-like polymer; forming a concentrate by extracting a portion of the water from the reaction liquid; and surface-crosslinking the agglomerated particles in a mixture containing the concentrate and a surface crosslinking agent, the reaction liquid does not contain an internal crosslinking agent that crosslinks the polymer, or the amount of the internal crosslinking agent in the reaction liquid is 0.093 mmol or less per 1 mol of the water-soluble ethylenically unsaturated monomer, The polymer particles formed by removing water from the hydrogel-like polymer prior to the formation of the aggregated particles have a median particle size of 150 μm or more. The mixture is formed by mixing the concentrate containing water at a moisture content of 15% to 50% by mass based on the mass of the polymer with the surface crosslinking agent. [2] According to the method described in [1], wherein, The amount of the surface crosslinking agent is more than 0.15 mmol and less than 5.14 mmol per mole of the water-soluble olefinic unsaturated monomer. [3] A water-absorbing resin particle exhibits the following characteristics: The water absorption capacity of physiological saline is above 60g / g; A flow rate of 1.0g or more of dry powder; Locking for 10 seconds at a distance of 1.5cm or more; and Locking time for 20 seconds for lengths below 4.0cm. [4] The water-absorbing resin particles according to [3] have a median particle size of more than 300 μm and less than 600 μm. [5] The water-absorbing resin particles according to [3] or [4] have a density of 0.05 μm. 2 / g or more and 0.22m 2 Specific surface area below / g. [6] An absorbent comprising any one of the water-absorbing resin particles described in [3] to [5]. [7] An absorbent material having: Liquid-permeable sheets; and [6] The absorber is disposed on the inner side of the liquid-permeable sheet.

[0014] Invention Effects According to the present invention, an absorbent article can be provided that absorbs liquid over a small area, exhibits minimal backflow, and can quickly form a dry surface. Furthermore, absorbent resin particles capable of constituting such an absorbent article and a method for manufacturing the same can also be provided. The absorbent article of the present invention is useful, for example, as a waterproof bed sheet for nursing care. Attached Figure Description

[0015] Figure 1 This is a partial cross-sectional view showing an example of an absorbent article.

[0016] Figure 2This is a top view showing an example of a stirring blade.

[0017] Figure 3 This is a schematic diagram of a device for measuring the amount of water absorbed by physiological saline under load.

[0018] Figure 4 This is a schematic diagram illustrating the method for determining the flow rate of dry powder.

[0019] Figure 5 This is a schematic diagram illustrating the method of lockout testing.

[0020] Figure 6 This is a top view illustrating an example of the diffusion state of a test solution in an absorbent material during a diffusion area determination.

[0021] Figure 7 This is a photograph showing an example of an absorbent material to which the test solution was added during the determination of whitening time. Detailed Implementation

[0022] This invention is not limited to the following examples. In this specification, "room temperature" means 25±2°C. "Layer" is used as a term to include structures with shapes that are partially formed in the in-plane direction, in addition to structures with shapes that are continuously formed in the in-plane direction. "Physiological saline" means an aqueous solution of sodium chloride with a concentration of 0.9% by mass, containing 9g of sodium chloride per 1000mL of water at room temperature.

[0023] Figure 1 This is a partial cross-sectional view showing an example of an absorbent article. Figure 1 The absorbent article 50 shown includes a sheet-like absorbent 10, a first conformal member 21, a second conformal member 22, a liquid-permeable sheet 30, and an adhesive 35. The absorbent 10 is a laminate consisting of a single layer of absorbent resin 11 containing a plurality of absorbent resin particles 11a and a single layer of hydrophilic fibers 12 containing hydrophilic fibers, disposed inside the liquid-permeable sheet 30. The absorbent 10 is disposed between the sheet-like first conformal member 21 and the sheet-like second conformal member 22. The absorbent 10 may consist solely of the absorbent resin layer 11 containing the absorbent resin particles 11a. The entire absorbent 10 may be surrounded by the first conformal member 21 and the second conformal member 22. The first conformal member 21 and the second conformal member 22 may be a single sheet or two separate sheets. The first conformal member 21 and the second conformal member 22 may, for example, be tissue paper. Adhesive 35 is present between the liquid-permeable sheet 30 and the second conformal member 22, and bonds them together. Adhesive 35 can be, for example, a hot-melt adhesive. Figure 1In the case of the example of the absorbent article 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 can further have a liquid-impermeable sheet provided on the outer side of the first shape-retaining member 21.

[0024] If the water-absorption amount of the water-absorbent resin particles with respect to physiological saline is large, the amount of liquid that the water-absorbent resin particles can hold increases when the absorbent body of the absorbent article absorbs liquid. Therefore, there is a tendency for, for example, the rewet to be less when the absorbent article absorbs liquid. From the viewpoint of the related art, the water-absorption amount of the water-absorbent resin particles with respect to physiological saline can be 60 g / g or greater, 61 g / g or greater, 62 g / g or greater, 63 g / g or greater, 64 g / g or greater, 65 g / g or greater, 66 g / g or greater, 67 g / g or greater, 68 g / g or greater, 69 g / g or greater, or 70 g / g or greater. The water-absorption amount of the water-absorbent resin particles with respect to physiological saline can be 60 g / g or greater, 61 g / g or greater, 62 g / g or greater, 63 g / g or greater, 64 g / g or greater, 65 g / g or greater, 66 g / g or greater, 67 g / g or greater, 68 g / g or greater, 69 g / g or greater, or 70 g / g or greater, and 80 g / g or less.

[0025] As described in the examples described later, the water-absorption amount of the water-absorbent resin particles with respect to physiological saline is measured by a method including the following steps: while stirring 500 mL of physiological saline in a beaker with a magnetic stirrer at room temperature, 2.0 g of the water-absorbent resin particles are dispersed in the physiological saline in a manner that does not cause caking; while stirring the physiological saline, the mixture is left to stand for 60 minutes, thereby obtaining a dispersion liquid containing a swollen gel formed by swelling of the water-absorbent resin particles; the dispersion liquid is passed through a JIS Z 8801-1 standard sieve with a mesh size of 75 pm having a mass of Wa [g]; the sieve with the swollen gel remaining thereon is left to stand for 30 minutes in a state in which the sieve is inclined at an inclination angle of about 30 degrees with respect to the horizontal, thereby removing the remaining water; the mass Wb [g] of the sieve with the swollen gel remaining thereon is measured; and the water-absorption amount of the water-absorbent resin particles with respect to physiological saline [g / g] is calculated by the following formula.

[0026] Water-absorption amount [g / g] = (Wb - Wa) / 2.0 A large liquid permeation amount of the dry powder of the water-absorbent resin particles indicates that liquid easily passes quickly in the thickness direction of the water-absorbent resin layer. If the liquid permeation amount of the dry powder of the water-absorbent resin particles that constitute the absorbent body is large, a dry surface is easily formed quickly when liquid is supplied to the absorbent article. For example, in the case of the example of the absorbent article 1, the liquid permeation amount of the dry powder of the water-absorbent resin particles is 0.5 mL / g or greater, 0.6 mL / g or greater, 0.7 mL / g or greater, 0.8 mL / g or greater, 0.9 mL / g or greater, 1.0 mL / g or greater, 1.1 mL / g or greater, 1.2 mL / g or greater, 1.3 mL / g or greater, 1.4 mL / g or greater, 1.5 mL / g or greater, 1.6 mL / g or greater, 1.7 mL / g or greater, 1.8 mL / g or greater, 1.9 mL / g or greater, or 2.0 mL / g or greater. Figure 1When the liquid containing water is supplied to the liquid permeable sheet 30 side of the absorbent article of the structure exemplified in the middle, the liquid quickly moves from the liquid permeable sheet 30 to the absorbent body 10, and the outer side surface of the liquid permeable sheet 30 easily returns to a dry state. The liquid permeable amount of the dry powder of the water-absorbent resin particles is also large, and can contribute to the suppression of the spread area of the liquid in the absorbent article. From these viewpoints, the liquid permeable amount of the dry powder of the water-absorbent resin particles can be 1.0 g or greater, 1.1 g or greater, 1.2 g or greater, 1.3 g or greater, 1.4 g or greater, 1.5 g or greater, 1.6 g or greater, 2.0 g or greater, 2.1 g or greater, 2.2 g or greater, 2.3 g or greater, 2.4 g or greater, or 2.5 g or greater. The liquid permeable amount of the dry powder of the water-absorbent resin particles can be 1.0 g or greater, 1.1 g or greater, 1.2 g or greater, 1.3 g or greater, 1.4 g or greater, 1.5 g or greater, 1.6 g or greater, 2.0 g or greater, 2.1 g or greater, 2.2 g or greater, 2.3 g or greater, 2.4 g or greater, or 2.5 g or greater, and 15 g or less. The liquid permeable amount of the dry powder of the water-absorbent resin particles can be 1.0 g or greater, 1.1 g or greater, 1.2 g or greater, 1.3 g or greater, 1.4 g or greater, 1.5 g or greater, 1.6 g or greater, 2.0 g or greater, 2.1 g or greater, 2.2 g or greater, 2.3 g or greater, 2.4 g or greater, or 2.5 g or greater, and 14 g or less. The liquid permeable amount of the dry powder of the water-absorbent resin particles can be 1.0 g or greater, 1.1 g or greater, 1.2 g or greater, 1.3 g or greater, 1.4 g or greater, 1.5 g or greater, 1.6 g or greater, 2.0 g or greater, 2.1 g or greater, 2.2 g or greater, 2.3 g or greater, 2.4 g or greater, or 2.5 g or greater, and 13 g or less. The liquid permeable amount of the dry powder of the water-absorbent resin particles can be 1.0 g or greater, 1.1 g or greater, 1.2 g or greater, 1.3 g or greater, 1.4 g or greater, 1.5 g or greater, 1.6 g or greater, 2.0 g or greater, 2.1 g or greater, 2.2 g or greater, 2.3 g or greater, 2.4 g or greater, or 2.5 g or greater, and 12 g or less. The liquid permeable amount of the dry powder of the water-absorbent resin particles can be 1.0 g or greater, 1.1 g or greater, 1.2 g or greater, 1.3 g or greater, 1.4 g or greater, 1.5 g or greater, 1.6 g or greater, 2.0 g or greater, 2.1 g or greater, 2.2 g or greater, 2.3 g or greater, 2.4 g or greater, or 2.5 g or greater, and 11 g or less. The liquid permeable amount of the dry powder of the water-absorbent resin particles can be 1.0 g or greater, 1.1 g or greater, 1.2 g or greater, 1.3 g or greater, 1.4 g or greater, 1.5 g or greater, 1.6 g or greater, 2.0 g or greater, 2.1 g or greater, 2.2 g or greater, 2.3 g or greater, 2.4 g or greater, or 2.5 g or greater, and 10 g or less.The dry powder permeation amount of the water-absorbent resin particles can be 1.0 g or more, 1.1 g or more, 1.2 g or more, 1.3 g or more, 1.4 g or more, 1.5 g or more, 1.6 g or more, 2.0 g or more, 2.1 g or more, 2.2 g or more, 2.3 g or more, 2.4 g or more, or 2.5 g or more, and 9.0 g or less. The dry powder permeation amount of the water-absorbent resin particles can be 1.0 g or more, 1.1 g or more, 1.2 g or more, 1.3 g or more, 1.4 g or more, 1.5 g or more, 1.6 g or more, 2.0 g or more, 2.1 g or more, 2.2 g or more, 2.3 g or more, 2.4 g or more, or 2.5 g or more, and 8.0 g or less. The dry powder permeation amount of the water-absorbent resin particles can be 1.0 g or more, 1.1 g or more, 1.2 g or more, 1.3 g or more, 1.4 g or more, 1.5 g or more, 1.6 g or more, 2.0 g or more, 2.1 g or more, 2.2 g or more, 2.3 g or more, 2.4 g or more, or 2.5 g or more, and 7.0 g or less. The dry powder permeation amount of the water-absorbent resin particles can be 1.0 g or more, 1.1 g or more, 1.2 g or more, 1.3 g or more, 1.4 g or more, 1.5 g or more, 1.6 g or more, 2.0 g or more, 2.1 g or more, 2.2 g or more, 2.3 g or more, 2.4 g or more, or 2.5 g or more, and 6.0 g or less. The dry powder permeation amount of the water-absorbent resin particles can be 1.0 g or more, 1.1 g or more, 1.2 g or more, 1.3 g or more, 1.4 g or more, 1.5 g or more, 1.6 g or more, 2.0 g or more, 2.1 g or more, 2.2 g or more, 2.3 g or more, 2.4 g or more, or 2.5 g or more, and 5.0 g or less. The dry powder permeation amount of the water-absorbent resin particles can be 1.0 g or more, 1.1 g or more, 1.2 g or more, 1.3 g or more, 1.4 g or more, 1.5 g or more, 1.6 g or more, 2.0 g or more, 2.1 g or more, 2.2 g or more, 2.3 g or more, 2.4 g or more, or 2.5 g or more, and 4.0 g or less. The dry powder permeation amount of the water-absorbent resin particles can be 1.0 g or more, 1.1 g or more, 1.2 g or more, 1.3 g or more, 1.4 g or more, 1.5 g or more, 1.6 g or more, 2.0 g or more, 2.1 g or more, 2.2 g or more, 2.3 g or more, 2.4 g or more, or 2.5 g or more, and 3.0 g or less.

[0027] As described in the examples described later, the dry powder permeation amount of the water-absorbent resin particles is measured by a method including the steps of: arranging 0.450 g of the water-absorbent resin particles on a 250-μm mesh that plugs the opening of a cylinder having an inner diameter of 26 mm in a room temperature environment; in a state in which the cylinder is vertically erected with the mesh on the lower side, pouring 20 g of physiological saline at 25 ± 2°C onto the water-absorbent resin particles on the mesh in the cylinder over 2 seconds; and setting the amount of the physiological saline that has passed through the water-absorbent resin particles within 10 seconds from the time at which the entire amount of the physiological saline is poured as the dry powder permeation amount.

[0028] The lock 10 seconds value of the water-absorbent resin particles is a value reflecting the extent of the amount of liquid that the water-absorbent resin particles can absorb during 10 seconds after the start of liquid absorption upon contact with a large amount of liquid. Thus, a large lock 10 seconds value of the water-absorbent resin particles indicates that the water-absorbent resin particles can absorb a large amount of liquid in a short time immediately after contact with liquid. If the lock 10 seconds value of the water-absorbent resin particles is large, there is a tendency for the spread area of liquid in the absorbent article to be small. Also, a large lock 10 seconds value of the water-absorbent resin particles can contribute to a reduction in the time until a dry surface is formed when liquid is supplied to the absorbent article. From these viewpoints, the lock 10 seconds value of the water-absorbent resin particles can be 1.5 cm or greater, 1.6 cm or greater, 1.7 cm or greater, 1.8 cm or greater, 1.9 cm or greater, 2.0 cm or greater, 2.1 cm or greater, 2.2 cm or greater, 2.3 cm or greater, 2.4 cm or greater, 2.5 cm or greater, 2.6 cm or greater, or 2.7 cm or greater. The lock 10 seconds value of the water-absorbent resin particles can be 1.5 cm or greater, 1.6 cm or greater, 1.7 cm or greater, 1.8 cm or greater, 1.9 cm or greater, 2.0 cm or greater, 2.1 cm or greater, 2.2 cm or greater, 2.3 cm or greater, 2.4 cm or greater, 2.5 cm or greater, 2.6 cm or greater, or 2.7 cm or greater, and 4.5 cm or less. The lock 10 seconds value of the water-absorbent resin particles can be 1.5 cm or greater, 1.6 cm or greater, 1.7 cm or greater, 1.8 cm or greater, 1.9 cm or greater, 2.0 cm or greater, 2.1 cm or greater, 2.2 cm or greater, 2.3 cm or greater, 2.4 cm or greater, 2.5 cm or greater, 2.6 cm or greater, or 2.7 cm or greater, and 4.4 cm or less. The lock 10 seconds value of the water-absorbent resin particles can be 1.5 cm or greater, 1.6 cm or greater, 1.7 cm or greater, 1.8 cm or greater, 1.9 cm or greater, 2.0 cm or greater, 2.1 cm or greater, 2.2 cm or greater, 2.3 cm or greater, 2.4 cm or greater, 2.5 cm or greater, 2.6 cm or greater, or 2.7 cm or greater, and 4.3 cm or less. The lock 10 seconds value of the water-absorbent resin particles can be 1.5 cm or greater, 1.6 cm or greater, 1.7 cm or greater, 1.8 cm or greater, 1.9 cm or greater, 2.0 cm or greater, 2.1 cm or greater, 2.2 cm or greater, 2.3 cm or greater, 2.4 cm or greater, 2.5 cm or greater, 2.6 cm or greater, or 2.7 cm or greater, and 4.2 cm or less. The lock 10 seconds value of the water-absorbent resin particles can be 1.5 cm or greater, 1.6 cm or greater, 1.7 cm or greater, 1.8 cm or greater, 1.9 cm or greater, 2.0 cm or greater, 2.1 cm or greater, 2.2 cm or greater, 2.3 cm or greater, 2.4 cm or greater, 2.5 cm or greater, 2.6 cm or greater, or 2.7 cm or greater, and 4.1 cm or less.The lock 10 seconds value of the water-absorbent resin particles can be 1.5 cm or more, 1.6 cm or more, 1.7 cm or more, 1.8 cm or more, 1.9 cm or more, 2.0 cm or more, 2.1 cm or more, 2.2 cm or more, 2.3 cm or more, 2.4 cm or more, 2.5 cm or more, 2.6 cm or more, or 2.7 cm or more, and 4.0 cm or less.

[0029] The lock 20 seconds value of the water-absorbent resin particles is a value reflecting the degree of the amount of liquid that the water-absorbent resin particles can absorb during 20 seconds after the start of absorption of liquid in contact with a large amount of liquid. If the amount of absorption is large to some extent and the lock 20 seconds value of the water-absorbent resin particles is small, when the absorbent body of the absorbent article absorbs liquid, the state in which the water-absorbent resin particles do not completely swell and leave a margin in the water-absorption force is easily maintained during several tens of seconds or so from the start of absorption of liquid. Therefore, there is a tendency that, for example, the back penetration when the absorbent article absorbs liquid is less. From these viewpoints, the lock 20 seconds value of the water-absorbent resin particles constituting the absorbent body can be 4.0 cm or less, 3.9 cm or less, 3.8 cm or less, or 3.7 cm or less. The lock 20 seconds value of the water-absorbent resin particles can be 4.0 cm or less, 3.9 cm or less, 3.8 cm or less, or 3.7 cm or less, and 2.0 cm or more. The lock 20 seconds value of the water-absorbent resin particles can be 4.0 cm or less, 3.9 cm or less, 3.8 cm or less, or 3.7 cm or less, and 2.1 cm or more. The lock 20 seconds value of the water-absorbent resin particles can be 4.0 cm or less, 3.9 cm or less, 3.8 cm or less, or 3.7 cm or less, and 2.2 cm or more. The lock 20 seconds value of the water-absorbent resin particles can be 4.0 cm or less, 3.9 cm or less, 3.8 cm or less, or 3.7 cm or less, and 2.3 cm or more. The lock 20 seconds value of the water-absorbent resin particles can be 4.0 cm or less, 3.9 cm or less, 3.8 cm or less, or 3.7 cm or less, and 2.4 cm or more. The lock 20 seconds value of the water-absorbent resin particles can be 4.0 cm or less, 3.9 cm or less, 3.8 cm or less, or 3.7 cm or less, and 2.5 cm or more.

[0030] As described in the Examples described later, the lock 10 seconds value and the lock 20 seconds value of the water-absorbent resin particles are measured by a method including the steps of: spreading 0.100 g of the water-absorbent resin particles on the bottom surface in a cylinder having an inner diameter of 2.0 cm and a depth of 8.0 cm in a room temperature environment; injecting 20 g of physiological saline at 25 ± 2°C into the cylinder over 2 seconds; and setting the maximum height of the water-absorbent resin particles swollen in the cylinder from the bottom surface in the cylinder as the lock 10 seconds value or the lock 20 seconds value after 10 seconds or 20 seconds from the time when the entire amount of the physiological saline is injected.

[0031] The water-absorbent resin particles can exhibit a water absorption amount of 60 g / g or more against physiological saline, a dry powder permeation amount of 1.0 g or more, a lock 10 seconds value of 1.5 cm or more, and a lock 20 seconds value of 4.0 cm or less. If the water-absorbent resin particles satisfy all of these requirements, particularly excellent effects are easily obtained in terms of the area in which the absorbed liquid diffuses being small and the infiltration being less and a dry surface being able to be quickly formed.

[0032] The median particle diameter of the water-absorbent resin particles can be 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 can be 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 can be 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 can be 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 can be 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 can be 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 can be 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 can be 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 300 μm or more, 310 μm or more, 320 μm or more, 330 μm or more, or 340 μm or more, and 390 μm or less. The median particle diameter of the water-absorbent resin particles can be 300 μm or more, 310 μm or more, 320 μm or more, 330 μm or more, or 340 μm or more, and 380 μm or less. The method for measuring the median particle diameter is described later in the Examples.

[0033] If the specific surface area of the water-absorbent resin particles is appropriately large, the lock 10 seconds value tends to easily become large. If the specific surface area of the water-absorbent resin particles is appropriately small, the lock 20 seconds value tends to easily become small. From a related viewpoint, the specific surface area of the water-absorbent resin particles constituting the absorbent body can be 0.05 m 2 / g or more, and 0.22m 2 / g or less, 0.21m 2 / g or less, 0.20m 2 / g or less, 0.19m 2 / g or less or 0.18m 2 The specific surface area of ​​the water-absorbing resin particles constituting the absorbent can be less than 0.10 m² / g. 2 / g or more, and 0.22m 2 / g or less, 0.21m 2 / g or less, 0.20m 2 / g or less, 0.19m 2 / g or less or 0.18m 2 The specific surface area of ​​the water-absorbing resin particles constituting the absorbent can be less than 0.11 m² / g. 2 / g or more, and 0.22m 2 / g or less, 0.21m 2 / g or less, 0.20m 2 / g or less, 0.19m 2 / g or less or 0.18m 2 The specific surface area of ​​the water-absorbing resin particles constituting the absorbent can be less than 0.12 m² / g. 2 / g or more, and 0.22m 2 / g or less, 0.21m 2 / g or less, 0.20m 2 / g or less, 0.19m 2 / g or less or 0.18m 2 The specific surface area of ​​the water-absorbing resin particles constituting the absorbent can be less than 0.13 m² / g. 2 / g or more, and 0.22m 2 / g or less, 0.21m 2 / g or less, 0.20m 2 / g or less, 0.19m 2 / g or less or 0.18m 2 The specific surface area of ​​the water-absorbing resin particles constituting the absorbent can be less than 0.14 m² / g. 2 / g or more, and 0.22m 2 / g or less, 0.21m 2 / g or less, 0.20m 2 / g or less, 0.19m 2 / g or less or 0.18m 2 / g or less.

[0034] The specific surface area of the water-absorbent resin particles is measured by a method including the following steps: obtaining a component in the water-absorbent resin particles that passes through a sieve with a mesh size of 400 μm and remains on a sieve with a mesh size of 300 μm as a sample; drying the sample under a vacuum degassing condition at 100°C for 16 hours; measuring the adsorption isotherm of the dried sample at a temperature of 77 K using krypton gas as an adsorption gas; and obtaining the specific surface area of the water-absorbent resin particles from a multipoint BET plot. In the case where a lubricant such as a silica particle is attached to the surface of the absorbent resin particles, the specific surface area is measured using a sample of the absorbent resin particles before the lubricant is attached thereto from the viewpoint of improving the measurement accuracy.

[0035] The water retention amount of the water-absorbent resin particles that constitute the absorbent with respect to physiological saline can 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, can 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, can 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, or can 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 amount of the water-absorbent resin particles with respect to physiological saline is as described in the examples below.

[0036] The water absorption speed of the water-absorbent resin particles with respect to physiological saline can be 2.0 seconds or more and 15 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, 5.0 seconds or less, 4.0 seconds or less, or 3.0 seconds or less, can be 2.5 seconds or more and 15 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, 5.0 seconds or less, 4.0 seconds or less, or 3.0 seconds or less, or can be 3.0 seconds or more and 15 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, 5.0 seconds or less, or 4.0 seconds or less. The water absorption speed of the water-absorbent resin particles with respect to physiological saline herein is a value measured by the Vortex method as described in the examples below.

[0037] The water absorption amount of the water-absorbent resin particles constituting the absorbent under a load of 2.07 kPa can be 10 mL / g or more and 30 mL / g or less, 25 mL / g or less, or 20 mL / g or less, can be 15 mL / g or more and 30 mL / g or less, 25 mL / g or less, or 20 mL / g or less, or can be 20 mL / g or more and 30 mL / g or less, or 25 mL / g or less. The method for measuring the water absorption amount of the water-absorbent resin particles under a load of 2.07 kPa is described later in the Examples.

[0038] The water-absorbent resin particles can be obtained, for example, by a method including: polymerizing a water-soluble ethylenically unsaturated monomer by inverse suspension polymerization in a reaction liquid containing the water-soluble ethylenically unsaturated monomer, water, a dispersion medium, and a surfactant, thereby forming a particulate hydrogel-like polymer containing the water-soluble ethylenically unsaturated monomer and water; causing the hydrogel-like polymer to coagulate in the reaction liquid, thereby forming coagulated particles containing a plurality of the hydrogel-like polymer; forming a concentrate by extracting a part of the water from the reaction liquid; and surface-crosslinking the coagulated particles in a mixture containing the concentrate and a surface crosslinking agent. The reaction liquid for the inverse suspension polymerization can mainly contain a hydrophobic liquid, i.e., an oily liquid, composed of a hydrophobic dispersion medium, and a particulate aqueous liquid containing water and the water-soluble ethylenically unsaturated monomer and dispersed in the oily liquid.

[0039] The HLB of the surfactant contained in the reaction liquid can be 7 or more and 16 or less. The surfactant can be contained in the oily liquid. If the HLB of the surfactant is in this range, it is easy to obtain water-absorbent resin particles having an appropriate range of specific surface area. The HLB of the surfactant can be 7 or more and 15 or less, 14 or less, 13 or less, 12 or less, 11 or less, or 10 or less.

[0040] If the amount of the surfactant in the reaction solution is small, the water-absorbent resin particles that provide a dry surface that is easily formed quickly and that provide appropriate inhibition of hydrophilicity in a short time can be easily obtained. From a related viewpoint, the amount of the surfactant can 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 water-soluble ethylenically unsaturated monomer. The amount of the surfactant can be 0.5% by mass or more and 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 water-soluble ethylenically unsaturated monomer. The amount of the surfactant can be 0.8% by mass or more and 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 water-soluble ethylenically unsaturated monomer. The amount of the surfactant can be 1.0% by mass or more and 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 water-soluble ethylenically unsaturated monomer. In a case where the polymerization of the water-soluble ethylenically unsaturated monomer is performed two or more times, the amount of the surfactant with respect to the water-soluble ethylenically unsaturated monomer introduced into the reaction solution in each polymerization can be within these ranges.

[0041] Examples of the surfactant having an HLB of 7 or more and 16 or less include nonionic surfactants such as sorbitan fatty acid ester, (poly)glycerin fatty acid ester, sucrose fatty acid ester, polyoxyethylene sorbitan fatty acid ester, polyoxyethylene glycerin fatty acid ester, sorbitol fatty acid ester, polyoxyethylene sorbitol fatty acid ester, polyoxyethylene alkyl ether, polyoxyethylene alkyl phenyl ether, polyoxyethylene castor oil, polyoxyethylene hardened castor oil, alkyl allyl formaldehyde condensation polyoxyethylene ether, polyoxyethylene polyoxypropylene block copolymer, polyoxyethylene polyoxypropylene alkyl ether, and polyethylene glycol fatty acid ester; and anionic surfactants such as fatty acid salt, alkylbenzenesulfonic acid salt, alkylmethyl taurine salt, polyoxyethylene alkyl phenyl ether sulfate salt, polyoxyethylene alkyl ether sulfonic acid salt, phosphate ester of polyoxyethylene alkyl ether, and phosphate ester of polyoxyethylene alkyl allyl ether.

[0042] The aqueous solution in the reaction solution contains water and the water-soluble ethylenically unsaturated monomer, and can also contain an arbitrary additive. The additive can include, for example, a thickening agent, a hydrophilic polymer dispersant, a radical polymerization initiator, a chain transfer agent, a foaming agent, or a combination thereof.

[0043] Examples of the water-soluble ethylenically unsaturated monomer include an ethylenically unsaturated monomer having at least one functional group selected from the group consisting of a carboxyl group, a sulfo group, an amido group, and an amino group. In a case where the water-soluble ethylenically unsaturated monomer contains an amino group, the amino group can be quaternized.

[0044] As the water-soluble ethylenically unsaturated monomer, for example, at least one selected from the group consisting of (meth)acrylic acid (hereinafter, "acrylic acid" and "methacrylic acid" are collectively expressed as "(meth)acrylic acid"), alkali salts thereof, 2-(meth)acrylamido-2-methylpropane sulfonic acid and alkali salts thereof, (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 can be contained. The water-soluble ethylenically unsaturated monomer can contain (meth)acrylic acid and alkali metal salts thereof, or can contain acrylic acid and alkali metal salts thereof. The proportion of (meth)acrylic acid and alkali metal salts thereof in the water-soluble ethylenically unsaturated monomer in the reaction solution can be 60 mol% or more, 70 mol% or more, 80 mol% or more, 90 mol% or more, or 95 mol% or more, or can be substantially 100 mol%. The proportion of monomer units derived from (meth)acrylic acid or alkali metal salts thereof in the total amount of monomer units constituting the polymer formed by polymerization can be within the above range.

[0045] The reaction solution can substantially not contain an internal crosslinking agent that crosslinks the polymer. In other words, the amount of the internal crosslinking agent in the reaction solution can 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 1 mol of the water-soluble ethylenically unsaturated monomer. If the internal crosslinking agent is not present or is present in a small amount in the reaction solution, it is easy to obtain water-absorbent resin particles that exhibit a high water absorption amount. In the case where the reaction solution contains the internal crosslinking agent, the internal crosslinking agent can contain the same compounds as the examples of the intermediate crosslinking agent described later.

[0046] Examples of the thickening agent 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 thickening agent can be used alone as one kind, or two or more kinds can be used in combination.

[0047] The amount of the thickening agent can 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.

[0048] Examples of the hydrophilic high-molecular dispersant include polyvinyl alcohol (PVA), polyvinylpyrrolidone (PVP), polyethylene glycol (PEG), polypropylene glycol, polyethylene glycol-polypropylene glycol block copolymer, polyglycerol, polyoxyethylene glycerol, polyoxypropylene glycerol, polyoxyethylene-polyoxypropylene glycerol copolymer, and polyoxyethylene sorbitan fatty acid ester. The hydrophilic high-molecular dispersant can be used alone or in combination of two or more.

[0049] The amount of the hydrophilic high-molecular dispersant can be 0.001 parts by mass or more and 10 parts by mass or less, 0.005 parts by mass or more and 5 parts by mass or less, 0.01 parts by mass or more and 3 parts by mass or less, or 0.01 parts by mass or more and 1.5 parts by mass or less, relative to 100 parts by mass of the water-soluble ethylenically unsaturated monomer.

[0050] The radical polymerization initiator can include, for example, an azo compound, a peroxide, or a combination thereof.

[0051] Examples of the azo compound 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-amidino propane) 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(2-methylpropanamide) dihydrochloride, 2,2'-azobis[2-(2-imidazolin-2-yl)propane] disulfate dihydrate, 2,2'-azobis[N-(2-carboxyethyl)-2-methylpropanamidine] tetrahydrate, and 2,2'-azobis[2-methyl-N-(2-hydroxyethyl)propanamide].

[0052] Examples of the peroxide include potassium persulfate, ammonium persulfate, sodium persulfate, and the like; 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, t-butyl peroxyneopentanoate, and the like; and hydrogen peroxide.

[0053] The amount of the radical polymerization initiator can be, for example, 0.005 to 1 mole, relative to 100 moles of the water-soluble ethylenically unsaturated monomer.

[0054] Examples of the chain transfer agent include hypophosphite, mercaptan, mercapto acid, secondary alcohol, and amine.

[0055] Examples of the foaming agent include inorganic foaming agents such as ammonia carbonate, sodium bicarbonate, ammonium bicarbonate, nitroso compounds such as di-nitrosopentamethylene tetramine, azo compounds such as azodicarboxamide and azobisisobutyronitrile, sulfonyl hydrazide compounds such as 4,4'-oxybisbenzenesulfonyl hydrazide and p-toluenesulfonyl hydrazide, and organic foaming agents.

[0056] The oily liquid in the reaction liquid is a hydrophobic liquid mainly composed of a hydrophobic dispersion medium. The dispersion medium can be a hydrocarbon dispersion medium. The oily liquid can also contain any additive (for example, the hydrophobic polymer dispersant and / or the surfactant described later).

[0057] Examples of the hydrocarbon dispersion medium 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 can be used alone or in combination with two or more kinds.

[0058] The amount of the dispersion medium contained in the oily liquid can be 30 mass parts or more and 1000 mass parts or less, 50 mass parts or more and 650 mass parts or less, 70 mass parts or more and 550 mass parts or less, or 100 mass parts or more and 450 mass parts or less, relative to 100 mass parts of the water-soluble ethylenically unsaturated monomer.

[0059] Examples of the hydrophobic polymer dispersant include maleic anhydride-modified polyethylene, maleic anhydride-modified polypropylene, maleic anhydride-modified ethylene-propylene copolymer, maleic anhydride-modified EPDM (ethylene-propylene-diene terpolymer), maleic anhydride-modified polybutadiene, maleic anhydride-ethylene copolymer, maleic anhydride-propylene copolymer, maleic anhydride-ethylene-propylene copolymer, maleic anhydride-butadiene copolymer, polyethylene, polypropylene, ethylene-propylene copolymer, oxidized polyethylene, oxidized polypropylene, oxidized ethylene-propylene copolymer, ethylene-acrylic acid copolymer, ethyl cellulose, and ethyl hydroxyethyl cellulose. The hydrophobic polymer dispersant can be used alone or in combination of two or more.

[0060] The amount of the hydrophobic polymer dispersant can 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, with respect to 100 parts by mass of the aqueous liquid 100.

[0061] The water-absorbent resin particles having a proper range of specific surface area in a form of agglomerated particles can be easily obtained by forming an aqueous gel-like polymer having a proper particle size through a polymerization reaction, and then passing through a step of forming agglomerated particles. As a result, the water-absorbent resin particles showing a large dry powder permeation amount and a large 10-second value and showing a proper low 20-second value can be easily obtained. From a related viewpoint, the polymer particles formed by removing water from the aqueous gel-like polymer before the formation of the agglomerated particles can have a median particle diameter of 150 μm or more. The median particle diameter of the polymer particles formed by removing water from the aqueous gel-like polymer before the agglomeration (hereinafter, sometimes referred to as "median particle diameter before agglomeration") can be 150 μm or more, 155 μm or more, 160 μm or more, 165 μm or more, or 170 μm or more, and 350 μm or less. The median particle diameter before agglomeration can be 150 μm or more, 155 μm or more, 160 μm or more, 165 μm or more, or 170 μm or more, and 300 μm or less. The median particle diameter before agglomeration can be 150 μm or more, 155 μm or more, 160 μm or more, 165 μm or more, or 170 μm or more, and 290 μm or less. The median particle diameter before agglomeration can be 150 μm or more, 155 μm or more, 160 μm or more, 165 μm or more, or 170 μm or more, and 280 μm or less. The median particle diameter before agglomeration can be 150 μm or more, 155 μm or more, 160 μm or more, 165 μm or more, or 170 μm or more, and 270 μm or less. The median particle diameter before agglomeration can be 150 μm or more, 155 μm or more, 160 μm or more, 165 μm or more, or 170 μm or more, and 260 μm or less. The median particle diameter before agglomeration can be 150 μm or more, 155 μm or more, 160 μm or more, 165 μm or more, or 170 μm or more, and 250 μm or less. The median particle diameter before agglomeration can be 150 μm or more, 155 μm or more, 160 μm or more, 165 μm or more, or 170 μm or more, and 240 μm or less. The median particle diameter before agglomeration can be 150 μm or more, 155 μm or more, 160 μm or more, 165 μm or more, or 170 μm or more, and 230 μm or less. The median particle diameter before agglomeration can be 150 μm or more, 155 μm or more, 160 μm or more, 165 μm or more, or 170 μm or more, and 220 μm or less. The median particle diameter before agglomeration can be 150 μm or more, 155 μm or more, 160 μm or more, 165 μm or more, or 170 μm or more, and 210 μm or less.

[0062] During the polymerization reaction, the reaction liquid is generally stirred. If the stirring speed is high, there is a tendency for the particle size of the aqueous gel-like polymer before aggregation to be small. The stirring speed can be adjusted, for example, in 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. In addition, even if the stirring speed is low, an aqueous gel-like polymer before aggregation that exhibits an appropriately small particle size can be formed by using a stirring blade that has a high stirring efficiency. In this way, by appropriately controlling the type of stirring blade and the stirring speed, the particle size of the aqueous gel-like polymer before aggregation can be controlled within the above-described range.

[0063] By adding an aggregating agent to the reaction liquid containing the aqueous gel-like polymer and stirring the mixture liquid containing the aqueous gel-like polymer and the aggregates, an aggregated particle that is an aggregate of the aqueous gel-like polymer can be formed. In order to form the aggregated particle, the reaction liquid can be heated, for example, to 30°C or more and 80°C or less. The time for which the reaction liquid is stirred for the formation of the aggregated particle can be, for example, 1 minute or more and 60 minutes or less.

[0064] The aggregating agent can be an inorganic particle, and as examples thereof, there can be mentioned silica particles (for example, amorphous silica particles), zeolite, bentonite, alumina, talc, titanium dioxide, kaolin, clay, and hydrotalcite. Among these, from the viewpoint of the aggregation effect, at least one selected from the group consisting of amorphous silica, alumina, talc, and kaolin is preferable. The amount of the aggregating agent can be, for example, 0.0001 mass% or more and 1.0 mass% or less, or 0.001 mass% or more and 0.5 mass% or less, based on the amount of the water-soluble ethylenically unsaturated monomer. A dispersion liquid in which the inorganic particle of the aggregating agent is dispersed in a hydrophobic solvent can be added to the reaction liquid.

[0065] After the formation of the aggregated particle, before the extraction of water from the reaction liquid to form a concentrate, an intermediate crosslinking agent can be added to the reaction liquid and the aggregated particle can be subjected to intermediate crosslinking in the reaction liquid containing the intermediate crosslinking agent. In order to perform the intermediate crosslinking, the reaction liquid can be heated. The intermediate crosslinking agent can be added to the reaction liquid as an aqueous solution.

[0066] The intermediate crosslinking agent can be a compound having 2 or more reactive functional groups, examples of which include di- or tri-(meth)acrylates of polyhydric alcohols such as ethylene glycol, propylene glycol, trimethylolpropane, glycerol, polyoxyethylene glycol, polyoxypropylene glycol, polyglycerol, and the like; unsaturated polyesters obtained by reacting the above polyhydric alcohols with unsaturated acids (maleic acid, fumaric acid, and the like); bis(meth)acrylamides such as N,N'-methylenebis(meth)acrylamide; di- or tri-(meth)acrylates obtained by reacting polyepoxide with (meth)acrylic acid; di(meth)acrylamides obtained by reacting polyisocyanate (toluene diisocyanate, hexamethylene diisocyanate, and the like) with hydroxyethyl (meth)acrylate; compounds having 2 or more polymerizable unsaturated groups such as allyl starch, allyl cellulose, diallyl phthalate, N,N',N"-triallylisocyanurate, divinylbenzene, and the like; polyglycidyl compounds such as (poly)ethylene glycol diglycidyl ether, (poly)propylene glycol diglycidyl ether, (poly)glycerol diglycidyl ether, (poly)glycerol triglycidyl ether, (poly)propylene glycol polyglycidyl ether, polyglycerol polyglycidyl ether, and the like; halogenated epoxide compounds such as epichlorohydrin, epibromohydrin, a-methyl epichlorohydrin, and the like; and isocyanate compounds (2,4-toluene diisocyanate, hexamethylene diisocyanate, and the like). The intermediate crosslinking agent can be used alone or in combination with 2 or more.

[0067] The intermediate crosslinking agent can be a polyglycidyl compound or a diglycidyl ether compound. The intermediate crosslinking agent can include at least one compound selected from the group consisting of (poly)ethylene glycol diglycidyl ether, (poly)propylene glycol diglycidyl ether, and (poly)glycerol diglycidyl ether.

[0068] The amount of the intermediate crosslinking agent can be 0.001 millimole or more and 0.3 millimole or less, 0.005 millimole or more and 0.2 millimole or less, or 0.01 millimole or more and 0.1 millimole or less, per 1 mole of the water-soluble ethylenically unsaturated monomer used to form the hydrogel-like polymer.

[0069] The concentrate is formed by extracting a portion of the water from the reaction liquid containing the coagulated particles. The water can be extracted, for example, by azeotropic distillation of the water and the dispersion medium. The surface crosslinking agent is added to the concentrate, and the coagulated particles are surface-crosslinked in the mixture containing them. If the water content of the concentrate supplied to the surface crosslinking is suitably small, it is easy to obtain the water-absorbent resin particles that exhibit a large water absorption amount. From the relevant viewpoint, the water content of the concentrate supplied to the surface crosslinking can be 15 mass% or more and 50 mass% or less, based on the mass of the polymer. In other words, the mixture containing the concentrate and the surface crosslinking agent can be formed by mixing the concentrate containing water at a water content of 15 mass% or more and 50 mass% or less, based on the mass of the polymer, and the surface crosslinking agent. Here, the water content is calculated by the following formula.

[0070] Water content (mass%) = (Ww / Ws) x 100 Ww = total mass of water contained in the reaction liquid Ws = mass of the polymer before surface crosslinking Ww is the amount of water contained in the aqueous solutions of the respective raw materials such as the intermediate crosslinking agent, etc., when these aqueous solutions are introduced as aqueous solutions, in addition to the mass of water contained in the aqueous liquid in the reaction liquid, and is an amount obtained by subtracting the amount of water extracted in order to form the concentrate. The mass of the polymer used to calculate Ws is the theoretical yield calculated from the amounts of the respective raw materials such as the water-soluble ethylenically unsaturated monomer, the intermediate crosslinking agent, and the radical polymerization initiator, in addition to the amounts of the respective raw materials.

[0071] The water content of the concentrate supplied to the surface crosslinking can be 15 mass% or more, 20 mass% or more, 25 mass% or more, 30 mass% or more, or 35 mass% or more, and 50 mass% or less, can be 15 mass% or more, 20 mass% or more, 25 mass% or more, 30 mass% or more, or 35 mass% or more, and 45 mass% or less, or can be 15 mass% or more, 20 mass% or more, 25 mass% or more, 30 mass% or more, or 35 mass% or more, and 40 mass% or less.

[0072] The surface cross-linking agent can be a compound having two or more reactive functional groups, examples of which include polyhydric alcohols such as ethylene glycol, propylene glycol, 1,4-butanediol, trimethylolpropane, glycerol, polyoxyethylene glycol, polyoxypropylene glycol, polyglycerol, and the like; polyglycidyl compounds such as (poly)ethylene glycol diglycidyl ether, (poly)glycerol diglycidyl ether, (poly)glycerol triglycidyl ether, trimethylolpropane triglycidyl ether, (poly)propylene glycol polyglycidyl ether, (poly)glycerol polyglycidyl ether, and the like; halogenated epoxide compounds such as epichlorohydrin, epibromohydrin, a-methyl epichlorohydrin, and the like; isocyanate compounds such as 2,4-toluene diisocyanate, hexamethylene diisocyanate, and the like; oxetane compounds such as 3-methyl-3-oxetanemethanol, 3-ethyl-3-oxetanemethanol, 3-butyl-3-oxetanemethanol, 3-methyl-3-oxetaneethanol, 3-ethyl-3-oxetaneethanol, 3-butyl-3-oxetaneethanol, and the like; oxazoline compounds such as 1,2-vinylbisoxazoline, and the like; carbonate compounds such as vinyl carbonate, and the like; and hydroxyalkylamide compounds such as bis[N,N-di(β-hydroxyethyl)]adipamide, and the like. The surface cross-linking agent can include polyglycidyl compounds such as (poly)ethylene glycol diglycidyl ether, (poly)glycerol diglycidyl ether, (poly)glycerol triglycidyl ether, (poly)propylene glycol polyglycidyl ether, (poly)glycerol polyglycidyl ether, and the like. These surface cross-linking agents can be used alone or in combination with two or more.

[0073] The surface cross-linking agent can be the same compound as the above-mentioned intermediate cross-linking agent. The surface cross-linking agent can be mixed as a solution with the concentrate. The solvent of the solution of the surface cross-linking agent can be water, a hydrophilic organic solvent, or a combination thereof. Examples of the hydrophilic organic solvent include lower alcohols such as methanol, ethanol, and isopropanol, 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 of the surface cross-linking agent can be used alone or in combination with two or more.

[0074] If the amount of the surface crosslinking agent is large, there is a tendency that the strength of the swollen gel formed by water absorption of the water-absorbent resin particles increases. Thus, gel blocking is suppressed, and the dry powder permeation amount is easily increased. As a result, the rewet and the time required for drying the surface of the absorbent article are easily further improved. For example, the amount of the surface crosslinking agent can be 0.15 millimoles or more and 5.14 millimoles or less, 5.0 millimoles or less, 4.5 millimoles or less, 4.0 millimoles or less, 3.5 millimoles or less, 3.0 millimoles or less, 2.5 millimoles or less, or 2.0 millimoles or less per 1 mole of the water-soluble ethylenically unsaturated monomer. The amount of the surface crosslinking agent can be 0.20 millimoles or more and 5.14 millimoles or less, 5.0 millimoles or less, 4.5 millimoles or less, 4.0 millimoles or less, 3.5 millimoles or less, 3.0 millimoles or less, 2.5 millimoles or less, or 2.0 millimoles or less per 1 mole of the water-soluble ethylenically unsaturated monomer. The amount of the surface crosslinking agent can be 0.25 millimoles or more and 5.14 millimoles or less, 5.0 millimoles or less, 4.5 millimoles or less, 4.0 millimoles or less, 3.5 millimoles or less, 3.0 millimoles or less, 2.5 millimoles or less, or 2.0 millimoles or less per 1 mole of the water-soluble ethylenically unsaturated monomer. The amount of the surface crosslinking agent can be 0.30 millimoles or more and 5.14 millimoles or less, 5.0 millimoles or less, 4.5 millimoles or less, 4.0 millimoles or less, 3.5 millimoles or less, 3.0 millimoles or less, 2.5 millimoles or less, or 2.0 millimoles or less per 1 mole of the water-soluble ethylenically unsaturated monomer.

[0075] For surface crosslinking, the mixture can be heated. The heating temperature can 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 of the surface crosslinking reaction can be, for example, 1 minute or more and 300 minutes or less, or 5 minutes or more and 200 minutes or less.

[0076] The water and the dispersion medium and the like are removed from the mixture after surface crosslinking, and dry polymer particles (water-absorbent resin particles) can be obtained. Thus, the mixture can be heated.

[0077] Various additives can be further added to the polymer particles after drying. Examples of the additives include lubricants, metal chelating agents, surface modifiers, heat resistance stabilizers, antioxidants, and antibacterial agents.

[0078] The lubricant can be, for example, amorphous silica particles. Examples of the metal chelating agent include ethylenediaminetetraacetic acid and salts thereof (disodium ethylenediaminetetraacetate, etc.), diethylenetriaminepentaacetic acid and salts thereof (penta-sodium diethylenetriaminepentaacetate, etc.). Examples of the surface modifier include polyvalent metal compounds such as aluminum sulfate, potassium alum, ammonium alum, sodium alum, (poly)aluminum chloride, and hydrates thereof; and polycationic compounds such as polyethyleneimine, polyvinylamine, and polyallylamine.

[0079] The additive can be attached to the surface of the polymer particles or can be impregnated in the interior of the polymer particles. By adding the additive (e.g., lubricant) to the polymer particles after drying, the additive can be attached to the surface of the polymer particles. The additive can also be added to the reaction solution used for polymerization or the concentrate after extraction with water.

[0080] The amount of the additive (e.g., lubricant) can 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.

[0081] The structure of the absorbent 10 is not limited to Figure 1 the structure exemplified above and can be appropriately changed. Other examples of the structure of the absorbent include a mixed structure in which the water-absorbent resin particles and the hydrophilic fibers are mixed throughout the entire absorbent and a sandwich structure in which a water-absorbent resin layer containing the water-absorbent resin particles is held between a plurality of hydrophilic fiber layers. In the case where the water-absorbent resin layer and the hydrophilic fiber layer are provided separately, the water-absorbent resin particles and the hydrophilic fibers can be mixed near the boundary between the two.

[0082] The amount of the water-absorbent resin particles in the absorbent can 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 (or the total mass of the hydrophilic fibers and the water-absorbent resin particles). The amount of the water-absorbent resin particles in the absorbent can 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 (or the total mass of the hydrophilic fibers and the water-absorbent resin particles). The amount of the water-absorbent resin particles in the absorbent can 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 (or the total mass of the hydrophilic fibers and the water-absorbent resin particles). The amount of the water-absorbent resin particles in the absorbent can 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 the water-absorbent resin particles in the absorbent can 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 the water-absorbent resin particles in the absorbent can 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 the water-absorbent resin particles in the absorbent can 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 the water-absorbent resin particles in the absorbent can 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 the water-absorbent resin particles in the absorbent can 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 the water-absorbent resin particles in the absorbent can 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 the water-absorbent resin particles in the absorbent body can 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 body (or the total mass of the hydrophilic fibers and the water-absorbent resin particles). The amount of the water-absorbent resin particles in the absorbent body can 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 body (or the total mass of the hydrophilic fibers and the water-absorbent resin particles).

[0083] The unit area weight of the water-absorbent resin particles in the absorbent article can be 10 g / m 2 or more and 300 g / m 2 or less, 200 g / m 2 or less, 150 g / m 2 or less, 100 g / m 2 or less, 90 g / m 2 or less, or 80 g / m 2 or less. The unit area weight of the water-absorbent resin particles can be 50 g / m 2 or more and 300 g / m 2 or less, 200 g / m 2 or less, 150 g / m 2 or less, 100 g / m 2 or less, 90 g / m 2 or less, or 80 g / m 2 or less. The unit area weight of the water-absorbent resin particles can be 60 g / m 2 or more and 300 g / m 2 or less, 200 g / m 2 or less, 150 g / m 2 or less, 100 g / m 2 or less, 90 g / m 2 or less, or 80 g / m 2 or less. The unit area weight of the water-absorbent resin particles can be 70 g / m 2 or more and 300 g / m 2 or less, 200 g / m 2 or less, 150 g / m 2 or less, 100 g / m 2 or less, 90 g / m 2 or less, or 80 g / m 2 or less. In the present specification, the unit area weight indicates the mass per unit area of the absorbent article when viewed in the thickness direction of the absorbent article.

[0084] Examples of the hydrophilic fibers include cotton pulp, cellulose fibers such as chemical pulp, rayon, and acetate rayon. The absorbent body can further include hydrophobic fibers composed of synthetic resins such as polyamide, polyester, and polyolefin as a reinforcing agent.

[0085] The unit area weight of the hydrophilic fibers in the absorbent article can be 20 g / m 2 or more and 300 g / m 2 or more and 200 g / m 2 or more and 100 g / m 2 or more and 70 g / m 2 or more and 60 g / m 2 or more or 50 g / m 2 or more, and can also be 30 g / m 2 or more and 300 g / m 2 or more and 200 g / m 2 or more and 100 g / m 2 or more and 70 g / m 2 or more and 60 g / m 2 or more or 50 g / m 2 or more, and can also be 40 g / m 2 or more and 300 g / m 2 or more and 200 g / m 2 or more and 100 g / m 2 or more and 70 g / m 2 or more and 60 g / m 2 or more or 50 g / m 2 or more.

[0086] The liquid permeable sheet 30 can be, for example, a nonwoven fabric, a porous resin sheet, tissue paper, or a combination thereof. The nonwoven fabric can include resin fibers of polyethylene, polypropylene, polyester, polyamide, or the like.

[0087] The post-dripping rewet amount of the absorbent article can be 7.0 g or less, or 0.1 g or more and 7.0 g or less. The post-dripping rewet amount is measured by a method including the steps of: arranging the absorbent article on a horizontal table with the liquid-permeable sheet on the upper side; dripping 30 mL of a test liquid adjusted to 25 ± 1°C from a 0.4-mm inner diameter nozzle for 10 seconds toward the center portion of the absorbent article from 1 cm above the upper portion of the absorbent article; immediately after completion of the dripping of the test liquid, placing a filter paper having a size of 100 mm x 100 mm and a mass measured in advance on the position where the test liquid was dripped, and applying a load of 0.7 psi for 3 seconds from above the filter paper; and removing the load and the filter paper, and setting the mass of the test liquid absorbed by the filter paper as the post-dripping rewet amount [g]. The test liquid is prepared by dissolving 45.0 g of NaCl in 4955.0 g of ion exchange water, and further adding a small amount of blue No. 1. The 1-minute post-dripping rewet amount, the spread area, and the whitening time described below are also measured using the same test liquid.

[0088] The 1-minute post-dripping rewet amount of the absorbent article can be 2.5 g or less, or 0.1 g or more and 2.5 g or less. The measurement is performed in the same manner as the post-dripping rewet amount, except that the filter paper is placed on the position where the test liquid was dripped 1 minute after completion of the dripping of the test liquid.

[0089] The spread area of the test liquid when the test liquid is dripped on the liquid-permeable sheet side of the absorbent article can be 150 cm 2 or more and 150 cm 2 or less. 2 .

[0090] The above-described spread area is measured by the steps of: arranging the absorbent article on a horizontal table with the liquid-permeable sheet on the upper side; dripping 30 mL of a test liquid adjusted to 25 ± 1°C from a 0.4-mm inner diameter nozzle for 10 seconds toward the center portion of the absorbent article from 1 cm above the upper portion of the absorbent article; 2 minutes after completion of the dripping of the test liquid, measuring the maximum length d1 in the length direction of the absorbent article and the maximum length d2 in the direction perpendicular to the length direction, with respect to the region in which the test liquid has spread within the absorbent article; and calculating the spread area [cm 2 ] by the following equation.

[0091] Spread area [cm 2 ] = (d1 / 2) x (d2 / 2) x 3.14 The whitening time when the test liquid is dripped on the liquid-permeable sheet side of the absorbent article can be 45 seconds or less, or 5 seconds or more and 45 seconds or less.

[0092] The whitening time mentioned above refers to the time until the absorbent article re-dries after the test solution is added. The whitening time is determined by the following steps: The absorbent article is positioned on a horizontal table with the liquid-permeable sheet facing upwards; 30 mL of test solution, adjusted to 25 ± 1 °C, is added dropwise over 10 seconds from the top of the absorbent article (1 cm above the center) through a 0.4 mm inner diameter inlet; the time from the completion of the test solution addition until the color of the liquid-permeable sheet changes to a color close to its original color is recorded as the whitening time. Since the liquid-permeable sheet is mostly white, the term "whitening time" is used here; however, the same method is used to determine the whitening time even when the liquid-permeable sheet has other colors.

[0093] Absorbent items can be, for example, waterproof nursing sheets, diapers, sanitary napkins, tampons, or pet pads.

[0094] Example The present invention is not limited to the following embodiments.

[0095] 1. Preparation of water-absorbing resin particles Example 1 Polymerization process A round-bottomed cylindrical split flask with four sidewall baffles (baffle width: 7mm, baffle length: 10cm) was prepared, equipped with a reflux condenser, dropping funnel, nitrogen inlet tube, and stirrer. The flask has an inner diameter of 11cm and a capacity of 2L. The stirrer was equipped with a reflux condenser, dropping funnel, nitrogen inlet tube, and stirrer. Figure 2 The diagram shows a schematically shaped stirring blade A. The stirring blade A 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 front end. Four slits S extending axially along 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. The width of the two inner slits S is 1 cm. The width of the two outer slits S is 0.5 cm. 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.

[0096] To the prepared split flask, 472.3 g of n-heptane and 1.10 g of sorbitol monolaurate (surfactant, NONION LP-20R, HLB: 8.6, prepared by NOF CORPORATION.) (1.21% by mass relative to the acrylic acid moiety neutralizer used in the polymerization reaction) were added. The mixture in the split flask was stirred at 300 rpm while the temperature was raised to 50°C, thereby dissolving the sorbitol monolaurate in the n-heptane. The mixture was then cooled to 45°C.

[0097] A 500 mL capacity separable flask was charged with an aqueous solution of acrylic acid 92.0 g (acrylic acid: 1.03 moles) having a concentration of 80.5 mass%. While ice-cooling from the outside, an aqueous solution of sodium hydroxide 147.7 g having a concentration of 20.9 mass% was added dropwise thereto, whereby 75 mole% of the acrylic acid was neutralized. Potassium persulfate 0.101 g (0.374 mmol) was dissolved in the aqueous solution of the acrylic acid partial neutralization product formed by the neutralization, thereby forming a monomer aqueous solution.

[0098] The monomer aqueous solution formed was added to the mixed solution in the separable flask described above, and the inside of the system containing the reaction solution formed was sufficiently replaced with nitrogen gas. Then, while stirring the reaction solution with a stirrer at a rotation speed of 300 rpm, the separable flask was immersed in a water bath at 70°C, and the polymerization reaction was performed in this state for 60 minutes. As the polymerization reaction proceeded, a particulate water-containing gel-like polymer was formed in the reaction solution.

[0099] Agglomeration step The stirring blade A was changed to a stirring blade B having 4 pieces of inclined paddle blade with a 2-stage blade diameter of 5 cm, and while stirring at a rotation speed of 1000 rpm, a dispersion liquid containing amorphous silica particles (agglomerating agent, Oriental Silicas Corporation, Tokusil NP-S) 0.014 g and n-heptane 100 g was added to the reaction solution containing the water-containing gel-like polymer formed, n-heptane, and a surfactant. Subsequently, the separable flask was immersed in a water bath at 75°C, and the reaction solution was stirred for 10 minutes. Agglomerated particles were formed by agglomeration of the water-containing gel-like polymer in the reaction solution.

[0100] Intermediate crosslinking To the reaction solution containing the agglomerated particles, an aqueous solution of ethylene glycol diglycidyl ether having a concentration of 2 mass% 0.41 g (ethylene glycol diglycidyl ether (intermediate crosslinking agent): 0.047 mmol) was added. Then, while heating the reaction solution in the separable flask in a water bath at 75°C, stirring was performed for 30 minutes, whereby intermediate crosslinking was performed.

[0101] Concentration The reaction solution in the separable flask was heated in an oil bath at 125°C, and by azeotropic distillation of n-heptane and water, 106.1 g of water was extracted to the outside of the system while refluxing n-heptane.

[0102] Surface crosslinking To the concentrate formed by the extraction of water, an aqueous solution of ethylene glycol diglycidyl ether having a concentration of 2 mass% 4.14 g (ethylene glycol diglycidyl ether (surface crosslinking agent): 0.48 mmol) was mixed. Surface crosslinking was performed in the mixture by keeping the internal temperature at 83 ± 2°C for 2 hours.

[0103] Drying The mixture after surface crosslinking was heated to 120°C and water and n-heptane were evaporated until almost no distillate was distilled from the inside of the system, whereby a powder of dry polymer particles (coagulated particles) was obtained. The powder was passed through a sieve with a mesh size of 850 μm, whereby the water-absorbent resin particles of Example 1 were obtained in an amount of 90.4 g.

[0104] Example 2 The rotation speed of the stirrer during the polymerization reaction was changed to 250 rpm, the amount of water extracted by azeotropic distillation before surface crosslinking was 108.8 g, and except for this, under the same conditions as in Example 1, the water-absorbent resin particles of Example 2 were obtained in an amount of 84.9 g.

[0105] Example 3 The stirring blade of the stirred reaction solution during the polymerization reaction was changed to stirring blade B of 4 pieces of inclined paddle blades with a 2-stage blade diameter of 5 cm, which were surface-treated with a fluorine resin, the rotation speed of the stirrer during the polymerization reaction was changed to 600 rpm, and the amount of water extracted by azeotropic distillation before surface crosslinking was 107.9 g, and except for this, under the same conditions as in Example 1, the water-absorbent resin particles of Example 3 were obtained in an amount of 73.0 g.

[0106] Example 4 The amount of water extracted by azeotropic distillation before surface crosslinking was 108.8 g, and except for this, under the same conditions as in Example 1, the water-absorbent resin particles of Example 4 were obtained in an amount of 88.5 g.

[0107] Example 5 The amount of water extracted by azeotropic distillation before surface crosslinking was 111.6 g, and except for this, under the same conditions as in Example 1, the water-absorbent resin particles of Example 5 were obtained in an amount of 85.9 g.

[0108] Example 6 The amount of water extracted by azeotropic distillation before surface crosslinking was 113.4 g, and except for this, under the same conditions as in Example 1, the water-absorbent resin particles of Example 6 were obtained in an amount of 88.9 g.

[0109] Example 7 The amount of an aqueous solution of ethylene glycol diglycidyl ether as a surface crosslinking agent with a concentration of 2 mass% was changed to 2.76 g (ethylene glycol diglycidyl ether: 0.32 mmol), and except for this, under the same conditions as in Example 1, the water-absorbent resin particles of Example 7 were obtained in an amount of 82.7 g.

[0110] Example 8 The amount of water extracted by azeotropic distillation before surface crosslinking was 107.9 g, the surface crosslinking agent was changed to a 2.7 mass% ethylene glycol diglycidyl ether aqueous solution 4.09 g (ethylene glycol diglycidyl ether: 0.63 mmol), and otherwise, under the same conditions as in Example 1, the water-absorbent resin particles of Example 8, 80.1 g, were obtained.

[0111] Example 9 The amount of water extracted by azeotropic distillation before surface crosslinking was 109.8 g, the surface crosslinking agent was changed to a 4 mass% ethylene glycol diglycidyl ether aqueous solution 4.14 g (ethylene glycol diglycidyl ether: 0.95 mmol), and otherwise, under the same conditions as in Example 1, the water-absorbent resin particles of Example 9, 86.0 g, were obtained.

[0112] Example 10 The amount of water extracted by azeotropic distillation before surface crosslinking was 117.0 g, the surface crosslinking agent was changed to a 6.7 mass% ethylene glycol diglycidyl ether aqueous solution 4.12 g (ethylene glycol diglycidyl ether: 1.59 mmol), and otherwise, under the same conditions as in Example 1, the water-absorbent resin particles of Example 10, 79.3 g, were obtained.

[0113] Example 11 After the coagulation step, no intermediate crosslinking was performed, and the amount of water extracted by azeotropic distillation before surface crosslinking was 107.0 g, and otherwise, under the same conditions as in Example 1, the water-absorbent resin particles of Example 11, 90.3 g, were obtained.

[0114] Example 12 After 106.1 g of water was extracted outside the system by azeotropic distillation before surface crosslinking, a 45 mass% aqueous solution of penta-sodium diethylenetriaminepentaacetate 0.204 g (penta-sodium diethylenetriaminepentaacetate: 0.182 mmol) was added before the addition of the surface crosslinking agent, and otherwise, under the same conditions as in Example 1, the water-absorbent resin particles of Example 12, 86.7 g, were obtained.

[0115] Example 13 The amount of water extracted by azeotropic distillation before surface crosslinking was 109.9 g, and after the water was extracted outside the system, a 45 mass% aqueous solution of penta-sodium diethylenetriaminepentaacetate 0.204 g (penta-sodium diethylenetriaminepentaacetate: 0.182 mmol) was added before the addition of the surface crosslinking agent, and otherwise, under the same conditions as in Example 1, the water-absorbent resin particles of Example 13, 87.7 g, were obtained.

[0116] Comparative Example 1 The stirring speed during the polymerization reaction was changed to 700 rpm, and the amount of water extracted by azeotropic distillation before surface crosslinking was 106.1 g. Otherwise, under the same conditions as in Example 3, 86.1 g of water-absorbing resin particles of Comparative Example 1 were obtained.

[0117] Comparative Example 2 The stirring speed during the polymerization reaction was changed to 1000 rpm, and the amount of water extracted by azeotropic distillation before surface crosslinking was 110.7 g. Otherwise, under the same conditions as in Example 3, 81.9 g of water-absorbing resin particles of Comparative Example 2 were obtained.

[0118] Comparative Example 3 The amount of water extracted by azeotropic distillation before surface crosslinking was 97.9 g, and the surface crosslinking agent was changed to 4.14 g of an aqueous solution of ethylene glycol diglycidyl ether with a concentration of 4% by mass (ethylene glycol diglycidyl ether: 0.95 mmol). Otherwise, under the same conditions as in Example 1, 86.0 g of water-absorbing resin particles of Comparative Example 3 were obtained.

[0119] Comparative Example 4 No amorphous silica particles for forming aggregated particles were added after the polymerization reaction. 107.9 g of water was extracted from the system by azeotropic distillation of n-heptane and water while refluxing n-heptane. Otherwise, under the same conditions as in Example 1, 77.2 g of water-absorbing resin particles of Comparative Example 4 were obtained.

[0120] Comparative Example 5 Polymerization process <Population reaction in stage 1> A round-bottomed cylindrical flask with an inner diameter of 11 cm and a capacity of 2 L, equipped with a reflux condenser, dropping funnel, nitrogen inlet tube, and stirrer, was prepared. Stirring blade B was installed on the stirrer. 293 g of n-heptane and 0.736 g of maleic anhydride-modified ethylene-propylene copolymer (hydrophobic polymeric dispersant, Mitsui Chemicals, Inc., Hi-WAX 1105A) were added to the flask. The mixture in the flask was stirred at 300 rpm while the temperature was raised to 80 °C, thereby dissolving the hydrophobic polymeric dispersant in the n-heptane. The mixture was then cooled to 50 °C.

[0121] In a beaker with a capacity of 300 mL, an aqueous solution of acrylic acid with a concentration of 80.5 mass% was added at 92.0 g (acrylic acid: 1.03 mol). While ice cooling was performed from the outside, an aqueous solution of sodium hydroxide with a concentration of 20.9 mass% was added dropwise at 147.7 g, whereby 75 mol% of the acrylic acid was neutralized. Hydroxyethyl cellulose 0.092 g (thickening agent, SUMITOMO SEIKA CHEMICALS CO., LTD., HECAW-15F), potassium persulfate 0.0736 g (water-soluble radical polymerization initiator, 0.272 mmol), and ethylene glycol diglycidyl ether 0.010 g (internal crosslinking agent, 0.057 mmol) were dissolved in the aqueous solution of the acrylic acid partial neutralization product formed by the neutralization, and a monomer aqueous solution for the first stage was prepared.

[0122] The monomer aqueous solution for the first stage was added to the mixed solution in the separation flask described above, and the reaction solution formed was stirred with a stirrer at 300 rpm for 10 minutes. Then, a surfactant solution containing n-heptane 6.62 g and sucrose stearate (surfactant, HLB: 3, MITSUBISHI-CHEMICAL FOODS CORPORATION, RYOTO SUGAR ESTER S-370) 0.736 g (0.81 mass% with respect to the acrylic acid partial neutralization product for the polymerization reaction in the first stage, and 0.58 mass% with respect to the acrylic acid partial neutralization product for the polymerization reaction in the second stage) was further added. While the reaction solution was stirred with a stirrer at 550 rpm, the inside of the system was sufficiently replaced with nitrogen. Subsequently, the separation flask was immersed in a water bath at 70°C, and the polymerization reaction was performed while maintaining this state for 60 minutes. By the polymerization reaction, a polymer slurry for the first stage was formed.

[0123] <Second-stage polymerization reaction> In a beaker with a capacity of 500 mL, an aqueous solution of acrylic acid with a concentration of 80.5 mass% was added at 128.8 g (acrylic acid: 1.44 mol). While ice cooling was performed from the outside, an aqueous solution of sodium hydroxide with a concentration of 27 mass% was added dropwise at 159.0 g, whereby 75 mol% of the acrylic acid was neutralized. Potassium persulfate 0.090 g (0.333 mmol) and ethylene glycol diglycidyl ether 0.0116 g (internal crosslinking agent, 0.067 mmol) were dissolved in the aqueous solution of the acrylic acid partial neutralization product formed by the neutralization, and a monomer aqueous solution for the second stage was prepared.

[0124] While the polymerization slurry of the first stage was stirred with a stirrer at 1000 rpm, the separable flask was cooled to 25°C. Then, the entire amount of the aqueous monomer solution of the second stage was added to the polymerization slurry of the first stage, and the inside of the system was replaced with nitrogen for 30 minutes. Again, the separable flask was immersed in a water bath at 70°C, and the polymerization reaction was performed for 60 minutes in this state. By the polymerization reaction, a reaction liquid containing a particulate, hydrogel-like polymer in which polymer particles were aggregated was obtained.

[0125] Concentration After the polymerization reaction, 0.589 g of an aqueous solution of penta-sodium diethylenetriaminepentaacetate (penta-sodium diethylenetriaminepentaacetate: 0.527 mmol) having a concentration of 45 mass% was added to the reaction liquid containing the hydrogel-like polymer under stirring. Then, the separable flask was immersed in an oil bath set to 125°C, and 256.8 g of water was extracted outside the system by azeotropic distillation of n-heptane and water while refluxing n-heptane.

[0126] Surface cross-linking To the concentrate (hydrogel-like polymer) after the extraction of water, 4.42 g of an aqueous solution of ethyleneglycol diglycidyl ether (surface cross-linking agent) having a concentration of 2 mass% (ethyleneglycol diglycidyl ether (surface cross-linking agent): 0.507 mmol) was added. Surface cross-linking was performed in the mixture by keeping the inside temperature of the separable flask at 83 ± 2°C for 2 hours.

[0127] Drying The mixture after the surface cross-linking was heated in an oil bath at 125°C, and water and n-heptane were evaporated until almost no distillate was distilled from the inside of the system, thereby obtaining a powder of dried polymer particles. The powder was passed through a sieve having a mesh size of 850 μm. To the polymer particles that passed through the sieve, 0.5 mass% of amorphous silica particles (lubricant, Oriental Silicas Corporation, Tokusil NP-S) relative to the mass of the polymer particles were mixed, and water-absorbent resin particles 230.2 g of Comparative Example 5 in which the amorphous silica particles were attached to the polymer particles as a lubricant were obtained.

[0128] Reference Example 1 The polymerization was performed with the rotation speed of the stirrer set to 200 rpm, and otherwise, under the same conditions as in Example 1, to obtain water-absorbent resin particles 39.5 g of Reference Example 1. In the obtained water-absorbent resin particles, there were more than 50 mass% of particles that did not pass through a sieve having a mesh size of 850 μm. Therefore, it was judged that the polymerization reaction was unstable, and evaluation was not performed.

[0129] 2. Median particle diameter before aggregation A reaction solution containing a hydrogel-like polymer was formed by polymerization under the same conditions as in the examples or comparative examples. The reaction solution after polymerization was heated in an oil bath at 125°C, and water was extracted to the outside of the system by azeotropic distillation of n-heptane and water, while the n-heptane was refluxed, until the internal temperature of the system reached 89°C. By evaporating the remaining n-heptane at 125°C, dry powder of polymer particles before agglomeration was obtained. The median particle size of this polymer particle was determined by the method described later, and this value was set as the median particle size before agglomeration.

[0130] Table 1 shows the conditions of the polymerization process, the median particle size before coagulation, the presence or absence of the coagulation process, and the amounts of surfactant, intermediate crosslinking agent, and surface crosslinking agent relative to 1 mole of acrylic acid. Table 1 also shows the moisture content of the concentrate supplied for surface crosslinking. The moisture content is calculated using the following formula.

[0131] Moisture content (mass%) = (Ww / Ws) × 100 Ww = (Total mass of water in the reaction solution) - (Amount of water extracted by azeotropic distillation) Ws = Mass of the polymer before surface crosslinking In each embodiment and comparative example, Ww is calculated by subtracting the mass of water extracted by azeotropic distillation from the total mass of water contained in the monomer aqueous solution (including the mass of water generated by the neutralization reaction) and the mass of water contained in the intermediate crosslinking agent aqueous solution. Ws is the theoretical yield of the polymer before surface crosslinking, calculated based on the amount of acrylic acid and its neutralization product, free radical polymerization initiator, and intermediate crosslinking agent added.

[0132] [Table 1]

[0133] 3. Evaluation of water-absorbing resin particles The water-absorbing resin particles of the examples or comparative examples were evaluated using the following methods. Unless otherwise specified, the measurements were performed at a temperature of 25±2°C and a humidity of 50±10%. The evaluation results are shown in Tables 2 and 3.

[0134] Water retention capacity A cotton bag (cotton cloth No. 60, width 100 mm x length 200 mm) to which 2.0 g of the water absorbent resin particles were added was disposed in a beaker having a capacity of 500 mL. The cotton bag was filled with 500 g of physiological saline at one time in such a manner that no lump was generated, and the upper portion of the cotton bag was bound with a rubber band. The water absorbent resin particles in the cotton bag were allowed to swell by leaving the cotton bag for 30 minutes. Subsequently, the swollen gel in the cotton bag was subjected to dewatering treatment for 1 minute using a dewatering machine (manufactured by KOKUSAN Co., Ltd., product number: H-122) in which the centrifugal force was set to 167 G. The mass Wa (g) of the cotton bag containing the dewatered swollen gel was measured. The same operation was performed on the cotton bag to which no water absorbent resin particles were added, and the mass Wb (g) of the cotton bag at the time of wetting was measured. The water retention amount of the water absorbent resin particles with respect to physiological saline was calculated according to the following formula.

[0135] Water retention amount [g / g] = (Wa - Wb) / 2.0 Water absorption under load Using Figure 3 The water absorption under load of the water absorbent resin particles with respect to physiological saline was measured using a measuring device Y shown in the drawing. The measuring device Y was composed of a burette section 61, a conduit 62, a measuring stage 63, and a measuring section 64 disposed on the measuring stage 63. The burette section 61 had a burette 61a extending in the vertical direction, a rubber stopper 61b disposed at the upper end of the burette 61a, a stopcock 61c disposed at the lower end of the burette 61a, an air inlet tube 61d extending into the burette 61a at one end in the vicinity of the stopcock 61c, and a stopcock 61e disposed at the other end side of the air inlet tube 61d. The conduit 62 was installed between the burette section 61 and the measuring stage 63. The inner diameter of the conduit 62 was 6 mm. A hole having a diameter of 2 mm was formed in the central portion of the measuring stage 63, and the conduit 62 was joined to the hole. The measuring section 64 had a cylinder 64a (made of acrylic resin (organic glass)), a nylon mesh 64b bonded to the bottom of the cylinder 64a, and a weight 64c. The inner diameter of the cylinder 64a was 20 mm. The mesh of the nylon mesh 64b was 75 μm (200 mesh). At the time of measurement, the water absorbent resin particles 11a to be measured were uniformly spread on the nylon mesh 64b. The weight 64c had a diameter of 19 mm, and the mass of the weight 64c was 59.8 g. The weight 64c was placed on the water absorbent resin particles 11a, and a load of 2.07 kPa could be applied to the water absorbent resin particles 11a.

[0136] In the cylinder 64a of the measuring device Y, 0.100 g of the water-absorbent resin particles 11a was added. A weight 64c was placed on the water-absorbent resin particles 11a, and the measuring was started by opening the stopcock 61c and the stopcock 61e. The same volume of air as the physiological saline absorbed by the water-absorbent resin particles 11a was supplied rapidly and smoothly into the inside of the burette 61a through the air inlet tube 61d, and thus the decrease in the water level of the physiological saline in the inside of the burette 61a became the amount of the physiological saline absorbed by the water-absorbent resin particles 11a. The scale of the burette 61a was marked at intervals of 0.5 mL from 0 mL in the upward direction, and as the water level of the physiological saline, the scale Va of the burette 61a before the start of the water absorption and the scale Vb of the burette 61a after 60 minutes from the start of the water absorption were read, and the water absorption amount under the load was calculated by the following equation.

[0137] Water absorption amount under load [mL / g] = (Vb - Va) / 0.1 Water absorption rate The water absorption rate of the water-absorbent resin particles was measured by the Vortex method. In a beaker with a capacity of 100 mL, 50 ± 0.1 g of physiological saline and a magnetic stirrer (8 mm x 30 mm without a ring) were added. The beaker was immersed in a constant-temperature water tank, and the liquid temperature was adjusted to 25 ± 0.2°C. Subsequently, the beaker was placed on a magnetic stirrer, and the physiological saline was stirred at a rotation speed of 600 rpm to generate a vortex, and 2.0 g of the water-absorbent resin particles was rapidly added thereto. The time (seconds) from the time when the water-absorbent resin particles were added to the time when the vortex converged along with the water absorption of the water-absorbent resin particles was measured, and was set as the water absorption rate of the water-absorbent resin particles.

[0138] Median particle diameter The water-absorbent resin particles were passed through a sieve with a mesh size of 250 μm of the JIS Z 8801-1 standard sieve. In the case where the amount remaining on the sieve was 50% by mass or more of the total amount, the median particle diameter was measured using a combination of the sieves of (A) below, and in the case where it was less than 50% by mass, the median particle diameter was measured using a combination of the sieves of (B) below.

[0139] (A) Sieves with mesh sizes of 710 μm, 600 μm, 500 μm, 425 μm, 300 μm, 250 μm, 150 μm, and a tray were combined in this order from the top downward as JIS standard sieves.

[0140] (B) Sieves with mesh sizes of 425 μm, 250 μm, 180 μm, 150 μm, 106 μm, 75 μm, 45 μm, and a tray were combined in this order from the top downward as JIS standard sieves.​

[0141] The water-absorbent resin particles were classified by vibrating for 20 minutes using a continuous full-automatic sonic vibration type sieve classifier (Robot Shifter RPS-205, manufactured by SEISHIN ENTERPRISE Co., Ltd.) in sieves located in the uppermost stage. After the classification, the proportion (mass percentage) of the water-absorbent resin particles remaining on each sieve with respect to the total amount was calculated. By sequentially accumulating the proportion of the component with a large particle diameter, the relationship between the mesh of the sieve and the accumulated value of the proportion of the water-absorbent resin particles remaining on the sieve was plotted on a log probability paper. By connecting the plotted points on the probability paper with a straight line, the particle diameter corresponding to the accumulated mass percentage of 50 mass% was found, and this value was set as the median particle diameter.

[0142] Specific surface area By using a sieve with a mesh of 400 pm and a classification with a mesh of 300 pm, a component of the water-absorbent resin particles that passed through the sieve with a mesh of 400 pm and remained on the sieve with a mesh of 300 pm was obtained, and this was used as a sample for specific surface area measurement. This sample was dried under a degassing condition of vacuum degassing for 16 hours at 100°C. The adsorption isotherm after drying was measured at a temperature of 77 K by a method using krypton gas as an adsorbing gas. A specific surface area measuring device (AUTOSORB-1, manufactured by Quantachrome Corporation) was used for the measurement of the adsorption isotherm. The specific surface area (BET specific surface area) of the water-absorbent resin particles was found from a multipoint BET plot. In the case of the water-absorbent resin particles of Comparative Example 4 containing amorphous silica particles as a lubricant, the specific surface area was measured using the polymer particles before the amorphous silica particles were attached.

[0143] Water absorption amount A beaker with a volume of 500 mL was charged with 500 g of physiological saline at 25 ± 2°C. A magnetic stirrer was dispersed in the physiological saline in such a manner that no clumps were generated. While the physiological saline was being stirred, it was left to stand for 60 minutes, whereby a dispersion liquid containing a swollen gel formed by swelling of the water-absorbent resin particles was obtained. The dispersion liquid was passed through a JIS Z 8801-1 standard sieve with a mesh of 75 pm and a mass of Wa [g]. By leaving the sieve on which the swollen gel remained in a state of being inclined at an inclination angle of about 30 degrees with respect to the horizontal for 30 minutes, the remaining moisture was removed. Subsequently, the mass Wb [g] of the sieve on which the swollen gel remained was measured. The water absorption amount Wc [g / g] of the water-absorbent resin particles with respect to the physiological saline was found by the following equation.

[0144] Water absorption amount Wc [g / g] = (Wb - Wa) / 2.0 Liquid permeation amount of dry powder Figure 4 is a schematic diagram showing a method for measuring the dry powder permeation amount. A net 43 (250 mesh, Nippon Tokushu fabric Inc., product number NNO.250T) made of polyamide resin was plugged into one opening of a cylinder 41 (inner diameter: 26 mm, outer diameter: 39 mm), and 0.450 g of water-absorbent resin particles 11a were added from the other opening. The cylinder 41 was vertically erected with the opening plugged by the net 43 on the lower side, and the water-absorbent resin particles 11a were uniformly arranged on the net 43. A cylindrical piston 42 (inner diameter: 19 mm, outer diameter: 25.6 mm) having one opening plugged by a net 44 (250 mesh, Nippon Tokushu fabric Inc., product number NNO.250T) made of polyamide resin was inserted into the cylinder 41 with the net 44 in contact with the water-absorbent resin particles 11a.

[0145] A metal mesh 46 (8 mesh) having a mass Wc (g) was placed on a petri dish 45 having a mass Wb (g). The cylinder 41 having the water-absorbent resin particles 11a and the piston 42 inserted therein was placed on the metal mesh 46. Then, 20 g of physiological saline was poured into the cylinder 41 from the upper portion of the piston 42 over 2 seconds. The cylinder 41 was removed rapidly 10 seconds after the time when the entire amount of the physiological saline was poured. The total mass Wa (g) of the physiological saline 65 that passed through the water-absorbent resin particles 11a, the petri dish 45, and the metal mesh 46 was measured. The dry powder permeation amount was calculated according to the following equation.

[0146] Dry powder permeation amount [g] = Wa - Wb - Wc Locking test Figure 5 is a schematic diagram showing a method for the locking test. The bottom surface of an acrylic cylinder 70 (inner diameter x 2.0 cm, outer diameter 3.0 cm, depth y 8.0 cm) placed on a horizontal table was filled with 0.100 g of water-absorbent resin particles 11a in a layer such that the surface was parallel to the bottom surface of the cylinder 70. Then, 20 g of physiological saline was poured into the cylinder 70 over 2 seconds. The maximum height h (cm) of the swelled water-absorbent resin particles 11a from the bottom surface of the cylinder 70 was measured 10 seconds or 20 seconds after the time when the entire amount of the physiological saline was poured using a vernier caliper (Niigataseiki Co., Ltd. SK-100 mm). The maximum height h after 10 seconds was set as the locking 10-second value, and the maximum height h after 20 seconds was set as the locking 20-second value.

[0147] 4. Production of absorbent article A first shaped member (thin paper) having a size of 20 cm x 60 cm and a unit area weight of 16 g / m 2 was prepared. On the first shaped member, 5.4 g of hydrophilic fibers (pulverized pulp) was air-laid by using an air flow type mixing device (pad former) to form a hydrophilic fiber layer that covered the entire upper surface of the first shaped member. From the laminate of the first shaped member and the hydrophilic fiber layer, a portion of 10 cm from both ends in the length direction was cut off. By halving the remaining laminate, two laminates having a size of 20 cm x 20 cm were obtained.

[0148] After the two laminates were each uniformly sprayed with 1.0 g of water using a sprayer, a load of 500 kPa was applied for 30 seconds. Then, water-absorbent resin particles (3.0 g) were uniformly spread on the hydrophilic fiber layer to form a water-absorbent resin layer. A second shaped member (thin paper) having a size of 20 cm x 20 cm and a unit area weight of 16 g / m 2 was laminated on the water-absorbent resin layer to obtain a laminate having, in order from the bottom, the first shaped member, the hydrophilic fiber layer, the water-absorbent resin layer, and the second shaped member.

[0149] On the second shaped member of the laminate, a gas-permeable nonwoven fabric (KNH Enterprise Co., Ltd., unit area weight: 25 g / m 2 ) having a size of 20 cm x 20 cm was laminated with a hot melt adhesive in contact with the second shaped member to obtain an absorbent article. On the gas-permeable nonwoven fabric, 0.1 g of the hot melt adhesive was applied to form a spiral stripe pattern of 20 stripes arranged at intervals of 10 mm. In the obtained sheet-shaped absorbent article (20 cm x 20 cm), the unit area weight of the water-absorbent resin particles was 75 g / m 2 , and the unit area weight of the hydrophilic fibers (pulverized pulp) was 45 g / m 2 .

[0150] 5. Evaluation of absorbent article Test solution A test solution was prepared by dissolving 45.0 g of NaCl in 4955.0 g of ion exchange water, further adding a small amount of blue No. 1. The test solution was used for the following evaluation.

[0151] Backflow amount after dripping An absorbent article was arranged on a horizontal table with a breathable nonwoven fabric on the upper side. From 1 cm above the upper portion of the absorbent article toward the center portion of the absorbent article, 30 mL of a test solution adjusted to 25 ± 1°C was dropped for 10 seconds using a pump (DOSE IT P910, manufactured by INTEGRA Biosciences) to which a nozzle with an inner diameter of 0.4 mm was attached. Immediately after the dropping of the test solution was completed, a filter paper (ADVANTEC No. 51A, formed into 100 mm x 100 mm) of which the mass was previously measured in an amount of about 75 g was placed on the center portion of the absorbent article, a weight (100 mm x 100 mm in the bottom surface, 5.0 kg) corresponding to a pressure of about 0.7 psi (4.8 kPa) was quickly placed thereon, and a load was applied for 3 seconds. Then, the weight and the filter paper were removed, the mass of the test solution absorbed by the filter paper was measured, and was set as the post-dropping rewet amount [g].

[0152] 1-minute post-dropping rewet amount The filter paper was placed on the center portion of the absorbent article 1 minute after the dropping of the test solution was completed, and otherwise, the 1-minute post-dropping rewet amount [g] was measured in the same order as the post-dropping rewet.

[0153] Diffusion area An absorbent article was arranged on a horizontal table with a breathable nonwoven fabric on the upper side. From 1 cm above the upper portion of the absorbent article toward the center portion of the absorbent article, 30 mL of a test solution adjusted to 25 ± 1°C was dropped for 10 seconds using a pump (DOSE IT P910, manufactured by INTEGRA Biosciences) to which a nozzle with an inner diameter of 0.4 mm was attached. The dropped test solution diffused in the horizontal direction within the absorbent article. Figure 6 is a plan view showing an example of the state in which the test solution diffused in the absorbent article 50. Two minutes after the dropping of the test solution was completed, the maximum length d1 in the longitudinal direction of the absorbent article and the maximum length d2 in the lateral direction of the absorbent article were measured with respect to the region DA in which the test solution diffused in the absorbent article 50. The diffusion area [cm 2 ] was calculated by the following formula.

[0154] Diffusion area [cm 2 ]= (d1 / 2) x (d2 / 2) x 3.14 Whitening time An absorbent article was arranged on a horizontal table with a breathable nonwoven fabric on the upper side. From 1 cm above the upper side toward the center of the absorbent article, 30 mL of the test solution adjusted to 25 ± 1°C was dropped for 10 seconds using a pump (INTEGRAL Biosciences Co., Ltd., DOSE IT P910) to which a 0.4 mm inner diameter inlet was connected. A part of the test solution was temporarily held on the breathable nonwoven fabric immediately after dropping, and then was absorbed by the absorbent body. The color of the surface of the breathable nonwoven fabric of the absorbent article changed from dark blue based on the test solution to the original white color with the transfer to the absorbent body. The time from the start of dropping the test solution until the color of the surface changed to a color close to white was recorded as the whitening time. Figure 7 is a photograph showing an example of the absorbent article to which the test solution was dropped. (a) is a photograph immediately after the test solution was dropped, and (b) is a photograph after the breathable nonwoven fabric was whitened.

[0155] [Table 2]

[0156] [Table 3] As shown in Table 3, it was confirmed that the absorbent articles of each of the examples containing the water-absorbent resin particles showed a small diffusion area at the time of water absorption, and a small amount of re-uptake to be able to quickly form a dry surface. For the re-uptake amount after 1 minute of Comparative Example 2, since the whitening time exceeded 1 minute and the measurement of the re-uptake amount after 1 minute was performed after the whitening time was confirmed, the measurement could not be performed.

[0157] Industrial Applicability The manufacturing method of the water-absorbent resin particles according to the present application can provide an absorbent article in which the area of diffusion of the absorbed liquid is small and the amount of re-uptake is small to be able to quickly form a dry surface, and thus the frequency of replacement of the absorbent article (for example, a pet pad) can be reduced. As a result, the water-absorbent resin particles of the present application can suppress the use amount of materials other than the water-absorbent resin particles (for example, pulp, nonwoven fabric, and the like, which are raw materials derived from natural substances (biomass resources)) contained in the water-absorbent resin particles and the absorbent article, and thus can contribute to the protection of the global environment.

[0158] Explanation of Reference Numerals 10 - absorbent body, 11 - water-absorbent resin layer, 11a - water-absorbent resin particle, 12 - hydrophilic fiber layer, 21 - first shape-retaining member, 22 - second shape-retaining member, 30 - liquid-permeable sheet, 35 - adhesive, 50 - absorbent article, 65 - physiological saline.

Claims

1. A method for producing water-absorbent resin particles, comprising the steps of: polymerizing a water-soluble ethylenically unsaturated monomer by inverse suspension polymerization in a reaction liquid containing the water-soluble ethylenically unsaturated monomer, water, a dispersion medium, and a surfactant having an HLB of 7 or more and 16 or less, thereby forming a particulate hydrogel-like polymer containing the polymer of the water-soluble ethylenically unsaturated monomer and water; causing the hydrogel-like polymer to coagulate in the reaction liquid, thereby forming coagulated particles containing a plurality of the hydrogel-like polymer; forming a concentrate by extracting a portion of the water from the reaction liquid; and surface-crosslinking the coagulated particles in a mixture containing the concentrate and a surface-crosslinking agent, wherein the reaction liquid does not contain an internal crosslinking agent that crosslinks the polymer, or the amount of the internal crosslinking agent in the reaction liquid is 0.093 mmol or less per 1 mol of the water-soluble ethylenically unsaturated monomer, the polymer particles formed by removing water from the hydrogel-like polymer before the coagulated particles are formed have a median particle diameter of 150 μm or more, and the mixture is formed by mixing the concentrate containing water at a moisture content of 15 mass% or more and 50 mass% or less based on the mass of the polymer and the surface-crosslinking agent.

2. The method according to claim 1, wherein the amount of the surface-crosslinking agent is 0.15 mmol or more and 5.14 mmol or less per 1 mol of the water-soluble ethylenically unsaturated monomer.

3. A water-absorbent resin particle that exhibits: a water absorption amount of 60 g / g or more against physiological saline; a dry powder permeation amount of 1.0 g or more against physiological saline; a 10-second lock time of 1.5 cm or more; and a 20-second lock time of 4.0 cm or less.

4. The water-absorbent resin particle according to claim 3, having a median particle diameter of 300 μm or more and 600 μm or less.

6. An absorbent body comprising the water-absorbent resin particle according to any one of claims 3 to 5.

7. An absorbent article provided with: a liquid-permeable sheet; and the absorbent body according to claim 6 disposed on the inner side of the liquid-permeable sheet. ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ 5. The water-absorbent resin particles according to claim 3, which have a specific surface area of 0.05 m2 / g or more and 0.22 m2 / g or less. 2 / g or more and 0.22 m2 / g or less. 2 / g or more and 0.22 m2 / g or less. ​ ​ ​ ​

Images