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; forming a concentrate by extracting a portion of the water; 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. The amount of the surfactant is 1.15 mass% or less based on the water-soluble ethylenically unsaturated monomer.

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 solution 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 solution, 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 solution; and surface-crosslinking the agglomerated particles in a mixture containing the concentrate and a surface crosslinking agent, the reaction solution does not contain an internal crosslinking agent that crosslinks the polymer, or the amount of the internal crosslinking agent in the reaction solution is 0.093 mmol or less per 1 mol of the water-soluble ethylenically unsaturated monomer, Based on the water-soluble olefinic unsaturated monomer, the amount of the surfactant is less than 1.15% by mass. [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] According to the method described in [1] or [2], wherein, 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. [4] A water-absorbing resin particle exhibits the following characteristics: A contact angle of 50° or more and 70° or less for a 25% by mass saline solution; 0.05m 2 / g or more and 0.22m 2 Specific surface area below / g; and The water absorption capacity of physiological saline is more than 60g / g. [5] According to [4], the water-absorbing resin particles exhibit a dry powder throughput of more than 2.0 g for physiological saline. [6] An absorbent comprising the water-absorbing resin particles described in [4] or [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 2 This is a top view showing an example of a stirring blade.

[0017] Figure 3is a schematic view showing a device for measuring the amount of water absorption under a load of physiological saline.

[0018] Figure 4 is a schematic view showing a method for measuring the amount of liquid permeation of a dry powder.

[0019] Figure 5 is a schematic view showing a method of a locking test.

[0020] Figure 6 is a plan view showing an example of a state in which a test liquid diffuses in an absorbent article in the measurement of a diffusion area.

[0021] Figure 7 is a photograph showing an example of an absorbent article to which a test liquid is added in the measurement of whitening time. DETAILED DESCRIPTION

[0022] The present application is not limited to the following examples. In the present specification, "room temperature" means 25 ± 2°C. "Layer" is used as a term including a structure of a shape formed locally in the in-plane direction in addition to a structure of a shape formed continuously in the in-plane direction. "Physiological saline" means an aqueous sodium chloride solution having a concentration of 0.9 mass% of sodium chloride per 1000 mL of water at room temperature.

[0023] Figure 1 is a partial cross-sectional view showing an example of an absorbent article. Figure 1 The absorbent article 50 shown includes a sheet-shaped absorbent body 10, a first shape-retaining member 21, a second shape-retaining member 22, a liquid-permeable sheet 30, and an adhesive 35. The absorbent body 10 is a laminate composed of one layer of a water-absorbing resin layer 11 containing a plurality of water-absorbing resin particles 11a and one layer of a hydrophilic fiber layer 12 containing hydrophilic fibers, and is disposed on the inner side of the liquid-permeable sheet 30. The absorbent body 10 is arranged between the sheet-shaped first shape-retaining member 21 and the sheet-shaped second shape-retaining member 22. The absorbent body 10 can be composed of only the water-absorbing resin layer 11 containing the water-absorbing resin particles 11a. The entire absorbent body 10 can be surrounded by the first shape-retaining member 21 and the second shape-retaining member 22. The first shape-retaining member 21 and the second shape-retaining member 22 can be one sheet or two separate sheets. The first shape-retaining member 21 and the second shape-retaining member 22 can be, for example, tissue paper. The adhesive 35 is present between the liquid-permeable sheet 30 and the second shape-retaining member 22 and adheres them. The adhesive 35 can be, for example, a hot-melt adhesive. In the case of the example of the absorbent article 50 shown, the water-absorbing resin layer 11 and the hydrophilic fiber layer 12 are disposed in this order from the liquid-permeable sheet 30 side. The absorbent article can further be provided with a liquid-impermeable sheet disposed on the outer side of the first shape-retaining member 21. Figure 1

[0024] ​According to the present inventors' insight, the wettability of the water-absorbent resin particles with respect to a 25 mass% salt water solution is related to the time until a water-absorbent article that has absorbed a liquid forms a dry surface. From the viewpoint of the correlation, the contact angle of the water-absorbent resin particles with respect to a 25 mass% salt water solution can be 50° or more and 70° or less. It is considered that if the contact angle is within these ranges, the liquid easily passes rapidly in the thickness direction of the water-absorbent resin layer, as a result of which the time until a water-absorbent article that has absorbed a liquid forms a dry surface is shortened. For example, in the case of a water-absorbent article that has absorbed a liquid, the time until a dry surface is formed can be shortened by 10 seconds or more, 20 seconds or more, or 30 seconds or more. 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 contact angle in the above range also contributes to the suppression of the spread area of the liquid in the absorbent article. The contact angle of the water-absorbent resin particles to the saline having a concentration of 25 mass% can be 50° or more and 69° or less, 68° or less, 67° or less, 66° or less, 65° or less, 64° or less, 63° or less, 62° or less, 61° or less, 60° or less, or 59° or less. The contact angle of the water-absorbent resin particles to the saline having a concentration of 25 mass% can be 51° or more and 70° or less, 69° or less, 68° or less, 67° or less, 66° or less, 65° or less, 64° or less, 63° or less, 62° or less, 61° or less, 60° or less, or 59° or less. The contact angle of the water-absorbent resin particles to the saline having a concentration of 25 mass% can be 52° or more and 70° or less, 69° or less, 68° or less, 67° or less, 66° or less, 65° or less, 64° or less, 63° or less, 62° or less, 61° or less, 60° or less, or 59° or less. The contact angle of the water-absorbent resin particles to the saline having a concentration of 25 mass% can be 53° or more and 70° or less, 69° or less, 68° or less, 67° or less, 66° or less, 65° or less, 64° or less, 63° or less, 62° or less, 61° or less, 60° or less, or 59° or less. The contact angle of the water-absorbent resin particles to the saline having a concentration of 25 mass% can be 54° or more and 70° or less, 69° or less, 68° or less, 67° or less, 66° or less, 65° or less, 64° or less, 63° or less, 62° or less, 61° or less, 60° or less, or 59° or less. The contact angle of the water-absorbent resin particles to the saline having a concentration of 25 mass% can be 55° or more and 70° or less, 69° or less, 68° or less, 67° or less, 66° or less, 65° or less, 64° or less, 63° or less, 62° or less, 61° or less, 60° or less, or 59° or less. The contact angle of the water-absorbent resin particles to the saline having a concentration of 25 mass% can be 56° or more and 70° or less, 69° or less, 68° or less, 67° or less, 66° or less, 65° or less, 64° or less, 63° or less, 62° or less, 61° or less, 60° or less, or 59° or less.

[0025] As described in the following examples, the contact angle of the absorbent resin particles with a 25% by mass saline solution was determined by a method comprising the following steps: distributing 1.0 g of absorbent resin particles onto an adhesive surface having dimensions of 25 mm × 70 mm to form an absorbent resin layer; vertically erecting the adhesive surface and removing any remaining absorbent resin particles; and, under an environment of 25 ± 2 °C and 50 ± 10% humidity, aligning the absorbent resin layer with the surface of the absorbent resin layer with the surface of the absorbent resin layer in a horizontal orientation. A spherical droplet with a diameter of 3.0±0.1 mm in a 25% (w / w) saline solution at 25±2℃ was brought into contact with the absorbent resin layer. The droplet was observed horizontally, and the angle between the left and right ends of the line connecting the droplet and the absorbent resin layer at the moment of contact with the droplet and the highest point of the droplet relative to the horizontal plane was defined as θ / 2. The contact angle θ was calculated by multiplying θ / 2 by 2. This was repeated five times, and the average of the three measurements after removing the maximum and minimum values ​​was recorded as the contact angle of the absorbent resin particle. This method was used to determine the contact angle. A 25% (w / w) saline solution is defined as 25 g of saline solution in 1000 mL of water at 25±2℃.

[0026] The specific surface area of ​​the water-absorbing resin particles constituting the absorbent can be 0.05 m². 2 / g or more and 0.22m 2 / g or less. If the specific surface area of ​​the absorbent resin particles is within this range, the absorbent resin layer in contact with the liquid can rapidly absorb the liquid in the initial short period of time. As a result, the diffusion area of ​​the liquid in the absorbent article is suppressed. Having a specific surface area of ​​absorbent resin particles within the above range also helps to shorten the time until a dry surface forms on the absorbent article that has absorbed liquid. From the same point of view, the specific surface area of ​​the absorbent resin particles constituting the absorbent body can be 0.05m². 2 / g or more, and 0.21m 2 / g or less, 0.20m 2 / g or less, 0.19m 2 / g or less, 0.18m 2 / g or less, 0.17m 2 / g or less or 0.16m 2 The specific surface area of ​​the water-absorbing resin particles can be less than 0.06 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, 0.18m 2 / g or less, 0.17m 2 / g or less or 0.16m 2The specific surface area of ​​the water-absorbing resin particles can be less than 0.07 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, 0.18m 2 / g or less, 0.17m 2 / g or less or 0.16m 2 The specific surface area of ​​the water-absorbing resin particles can be less than 0.08 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, 0.18m 2 / g or less, 0.17m 2 / g or less or 0.16m 2 The specific surface area of ​​the water-absorbing resin particles can be less than 0.09 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, 0.18m 2 / g or less, 0.17m 2 / g or less or 0.16m 2 The specific surface area of ​​the water-absorbing resin particles 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, 0.18m 2 / g or less, 0.17m 2 / g or less or 0.16m 2 The specific surface area of ​​the water-absorbing resin particles 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, 0.18m 2 / g or less, 0.17m 2 / g or less or 0.16m2 / g or less. The specific surface area of the water-absorbent resin particles can be 0.12 m 2 / g or less, and 0.22 m 2 / g or less, 0.21 m 2 / g or less, 0.20 m 2 / g or less, 0.19 m 2 / g or less, 0.18 m 2 / g or less, 0.17 m 2 / g or less, or 0.16 m 2 / g or less.

[0027] 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 degassing condition of vacuum exhaust at 100°C for 16 hours; measuring an 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 water-absorbent resin particles, the specific surface area is measured using a sample of the water-absorbent resin particles before the lubricant is attached from the viewpoint of improving the measurement accuracy.

[0028] If the water-absorbent resin particles that constitute the absorbent have a large water absorption amount for physiological saline, the amount of liquid that the water-absorbent resin particles can hold increases when the absorbent of the absorbent article absorbs liquid. Therefore, there is a tendency that, for example, the backflow of liquid when the absorbent article absorbs liquid is less. From the relevant viewpoint, the water-absorbent resin particles can have a water absorption amount for physiological saline of 60 g / g or more, 60 g / g or more and 80 g / g or less, 75 g / g or less, or 70 g / g or less.

[0029] The water absorption amount for physiological saline of the water-absorbent resin particles is measured by a method including the following steps: under a room temperature environment, while stirring a 500 mL beaker containing 100 mL of physiological saline using a magnetic stirrer (8 mm A 500 g of physiological saline at 25 ± 2°C was stirred at 600 rpm, and 2.0 g of the water-absorbing resin particles were dispersed in the physiological saline in such a manner that no clumps were generated, while the physiological saline was stirred, and then left to stand for 60 minutes, whereby a dispersion liquid containing a swollen gel formed by swelling of the water-absorbing resin particles was obtained. The dispersion liquid was passed through a JIS Z 8801-1 standard sieve having a mesh size of 75 μm with a mass of Wa [g], and the sieve on which the swollen gel remained was left to stand at an inclination angle of about 30 degrees with respect to the horizontal for 30 minutes, whereby the remaining moisture was removed. The mass Wb [g] of the sieve on which the swollen gel remained was measured, and the water absorption amount [g / g] of the water-absorbing resin particles with respect to the physiological saline was calculated by the following formula.

[0030] Water absorption amount [g / g] = (Wb - Wa) / 2.0 A high dry powder liquid throughput of absorbent resin particles indicates that liquid can easily and quickly pass through the thickness of the absorbent resin layer. If the dry powder liquid throughput of the absorbent resin particles constituting the absorbent body is high, a dry surface can be quickly formed when liquid is supplied to the absorbent article. A high dry powder liquid throughput of absorbent resin particles also helps to suppress the diffusion area of ​​liquid in the absorbent article. Based on these points, the dry powder liquid throughput of absorbent resin particles can be 1.0g or more, 1.1g or more, 1.2g or more, 1.3g or more, 1.4g or more, 1.5g or more, 1.6g or more, 2.0g or more, 2.1g or more, or 2.2g or more. The dry powder liquid throughput of absorbent resin particles can be 1.0g or more, 1.1g or more, 1.2g or more, 1.3g or more, 1.4g or more, 1.5g or more, 1.6g or more, 2.0g or more, 2.1g or more, or 2.2g or more, and less than 15g. The dry powder liquid throughput of the water-absorbing resin particles can be 1.0g or more, 1.1g or more, 1.2g or more, 1.3g or more, 1.4g or more, 1.5g or more, 1.6g or more, 2.0g or more, 2.1g or more, or 2.2g or more, and is less than 14g. The dry powder liquid throughput of the water-absorbing resin particles can be 1.0g or more, 1.1g or more, 1.2g or more, 1.3g or more, 1.4g or more, 1.5g or more, 1.6g or more, 2.0g or more, 2.1g or more, or 2.2g or more, and is less than 13g. The dry powder liquid throughput of the water-absorbing resin particles can be 1.0g or more, 1.1g or more, 1.2g or more, 1.3g or more, 1.4g or more, 1.5g or more, 1.6g or more, 2.0g or more, 2.1g or more, or 2.2g or more, and is less than 12g. The dry powder liquid throughput of the water-absorbing resin particles can be 1.0g or more, 1.1g or more, 1.2g or more, 1.3g or more, 1.4g or more, 1.5g or more, 1.6g or more, 2.0g or more, 2.1g or more, or 2.2g or more, and is less than 11g. The dry powder liquid throughput of the water-absorbing resin particles can be 1.0g or more, 1.1g or more, 1.2g or more, 1.3g or more, 1.4g or more, 1.5g or more, 1.6g or more, 2.0g or more, 2.1g or more, or 2.2g or more, and is less than 10g. The dry powder liquid throughput of the water-absorbing resin particles can be 1.0g or more, 1.1g or more, 1.2g or more, 1.3g or more, 1.4g or more, 1.5g or more, 1.6g or more, 2.0g or more, 2.1g or more, or 2.2g or more, and is less than 9.0g. The dry powder liquid throughput of the water-absorbing resin particles can be more than 1.0g, more than 1.1g, more than 1.2g, more than 1.3g, more than 1.4g, more than 1.5g, more than 1.6g, more than 2.0g, more than 2.1g, or more than 2.2g, and less than 8.0g.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, or 2.2 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, or 2.2 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, or 2.2 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, or 2.2 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, or 2.2 g or more, and 3.0 g or less.

[0031] The dry powder permeation amount of the water-absorbent resin particles is measured by a method including the following steps: at room temperature, 0.450 g of the water-absorbent resin particles are disposed on a 250-μm mesh that plugs the opening of a cylinder having an inner diameter of 26 mm; in a state in which the cylinder is vertically erected with the mesh on the lower side, 20 g of physiological saline at 25 ± 2°C is poured onto the water-absorbent resin particles on the mesh in the cylinder over 2 seconds; and 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 is set as the dry powder permeation amount, as described in the Examples below.

[0032] 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 area of diffusion 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 1.4 cm or greater, 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, or 2.4 cm or greater. The lock 10 seconds value of the water-absorbent resin particles can be 1.4 cm or greater, 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, or 2.4 cm or greater, and 4.5 cm or less. The lock 10 seconds value of the water-absorbent resin particles can be 1.4 cm or greater, 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, or 2.4 cm or greater, and 4.0 cm or less. The lock 10 seconds value of the water-absorbent resin particles can be 1.4 cm or greater, 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, or 2.4 cm or greater, and 3.5 cm or less. The lock 10 seconds value of the water-absorbent resin particles can be 1.4 cm or greater, 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, or 2.4 cm or greater, and 3.0 cm or less.

[0033] The lock 20 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 20 seconds after the start of liquid absorption in contact with a large amount of liquid. If the amount of water 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 liquid absorption. 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, 3.7 cm or less, 3.6 cm or less, 3.5 cm or less, 3.4 cm or less, or 3.3 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, 3.7 cm or less, 3.6 cm or less, 3.5 cm or less, 3.4 cm or less, or 3.3 cm or less, and 1.5 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, 3.7 cm or less, 3.6 cm or less, 3.5 cm or less, 3.4 cm or less, or 3.3 cm or less, and 1.6 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, 3.7 cm or less, 3.6 cm or less, 3.5 cm or less, 3.4 cm or less, or 3.3 cm or less, and 1.7 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, 3.7 cm or less, 3.6 cm or less, 3.5 cm or less, 3.4 cm or less, or 3.3 cm or less, and 1.8 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, 3.7 cm or less, 3.6 cm or less, 3.5 cm or less, 3.4 cm or less, or 3.3 cm or less, and 1.9 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, 3.7 cm or less, 3.6 cm or less, 3.5 cm or less, 3.4 cm or less, or 3.3 cm or less, and 2.0 cm or more.

[0034] 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.

[0035] The water-absorbent resin particles can exhibit a contact angle of 50° or more and 70° or less against a 25 mass% salt water, a specific surface area of 0.05 m2 / g or more and 0.22 m2 / g or less, and a water absorption amount of 60 g / g or more against physiological saline. 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. 2 2 / g or more and 0.22 m2 / g or less, and a water absorption amount of 60 g / g or more against physiological saline. 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.

[0036] ​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.

[0037] The water retention amount of the water-absorbent resin particles 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, 45 g / g or less, or 40 g / g or less, can be 30 g / g or more, and 60 g / g or less, or 55 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, 45 g / g or less, or 40 g / g or less, can be 35 g / g or more, and 60 g / g or less, or 55 g / g or less, or can be 35 g / g or more, and 60 g / g or less, 55 g / g or less, 50 g / g or less, 45 g / g or less, or 40 g / g or less. The method for measuring the water retention amount of the water-absorbent resin particles with respect to physiological saline is described later in the Examples.

[0038] The water absorption speed of the water-absorbent resin particles against 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, or 5.0 seconds or less, can be 3.0 seconds or more and 15.0 seconds or less, 10 seconds or less, 9.0 seconds or less, 8.0 seconds or less, 7.0 seconds or less, 6.0 seconds or less, or 5.0 seconds or less, or can be 4.0 seconds or more and 15.0 seconds or less, 10 seconds or less, 9.0 seconds or less, 8.0 seconds or less, 7.0 seconds or less, 6.0 seconds or less, or 5.0 seconds or less. The water absorption speed of the water-absorbent resin particles against physiological saline herein is a value measured by the Vortex method as described in the Examples below.

[0039] The water absorption amount of the water-absorbent resin particles constituting the absorbent at a load of 2.07 kPa can be 10 mL / g or more and 45 mL / g or less, 40 mL / g or less, 35 mL / g or less, or 30 mL / g or less, can be 15 mL / g or more and 45 mL / g or less, 40 mL / g or less, 35 mL / g or less, or 30 mL / g or less, can be 20 mL / g or more and 45 mL / g or less, 40 mL / g or less, 35 mL / g or less, or 30 mL / g or less, or can be 25 mL / g or more and 45 mL / g or less, 40 mL / g or less, 35 mL / g or less, or 30 mL / g or less. The method for measuring the water absorption amount of the water-absorbent resin particles at a load of 2.07 kPa is as described in the Examples below.

[0040] 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 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 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.

[0041] 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 a specific surface area in an appropriate range. 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.

[0042] If the amount of the surfactant in the reaction liquid is small, it is easy to obtain water-absorbent resin particles in which the hydrophilicity is appropriately suppressed in a short time, and which show a contact angle of 50° or more and 70° or less against a 25 mass% salt water. From a related viewpoint, the amount of the surfactant can be 1.15 mass% or less, 1.12 mass% or less, 1.10 mass% or less, 1.08 mass% or less, 1.05 mass% or less, 1.03 mass% or less, 1.00 mass% or less, 0.90 mass% or less, or 0.80 mass% or less, based on the amount of the water-soluble ethylenically unsaturated monomer. The amount of the surfactant can be 0.20 mass% or more and 1.15 mass% or less, 1.12 mass% or less, 1.10 mass% or less, 1.08 mass% or less, 1.05 mass% or less, 1.03 mass% or less, 1.00 mass% or less, 0.90 mass% or less, or 0.80 mass% or less, based on the water-soluble ethylenically unsaturated monomer. The amount of the surfactant can be 0.30 mass% or more and 1.15 mass% or less, 1.12 mass% or less, 1.10 mass% or less, 1.08 mass% or less, 1.05 mass% or less, 1.03 mass% or less, 1.00 mass% or less, 0.90 mass% or less, or 0.80 mass% or less, based on the water-soluble ethylenically unsaturated monomer. The amount of the surfactant can be 0.40 mass% or more and 1.15 mass% or less, 1.12 mass% or less, 1.10 mass% or less, 1.08 mass% or less, 1.05 mass% or less, 1.03 mass% or less, 1.00 mass% or less, 0.90 mass% or less, or 0.80 mass% or less, based on the water-soluble ethylenically unsaturated monomer. The amount of the surfactant can be 0.50 mass% or more and 1.15 mass% or less, 1.12 mass% or less, 1.10 mass% or less, 1.08 mass% or less, 1.05 mass% or less, 1.03 mass% or less, 1.00 mass% or less, 0.90 mass% or less, or 0.80 mass% or less, based on the water-soluble ethylenically unsaturated monomer. The amount of the surfactant can be 0.60 mass% or more and 1.15 mass% or less, 1.12 mass% or less, 1.10 mass% or less, 1.08 mass% or less, 1.05 mass% or less, 1.03 mass% or less, 1.00 mass% or less, 0.90 mass% or less, or 0.80 mass% or less, based on the water-soluble ethylenically unsaturated monomer. The amount of the surfactant can be 0.70 mass% or more and 1.15 mass% or less, 1.12 mass% or less, 1.10 mass% or less, 1.08 mass% or less, 1.05 mass% or less, 1.03 mass% or less, 1.00 mass% or less, 0.90 mass% or less, or 0.80 mass% or less, based on the water-soluble ethylenically unsaturated monomer.In the case where the polymerization of the water-soluble ethylenically unsaturated monomer is performed 2 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.

[0043] 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; anionic surfactants such as fatty acid salt, alkylbenzenesulfonic acid salt, alkylmethyl taurine salt, polyoxyethylene alkyl phenyl ether sulfate ester salt, polyoxyethylene alkyl ether sulfonic acid salt, phosphate ester of polyoxyethylene alkyl ether, and phosphate ester of polyoxyethylene alkyl allyl ether.

[0044] The aqueous liquid 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 blowing agent, or a combination thereof.

[0045] 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 the case where the water-soluble ethylenically unsaturated monomer contains an amino group, the amino group can be quaternized.

[0046] 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.

[0047] 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.

[0048] 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.

[0049] 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.

[0050] 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.

[0051] 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.

[0052] The radical polymerization initiator can include, for example, an azo compound, a peroxide, or a combination thereof.

[0053] 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].

[0054] 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.

[0055] 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.

[0056] Examples of the chain transfer agent include hypophosphite, mercaptan, mercapto acid, secondary alcohol, and amine.

[0057] 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.

[0058] 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).

[0059] 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.

[0060] 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.

[0061] 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.

[0062] 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.

[0063] By forming the aqueous gel-like polymer having a suitably small particle diameter through the polymerization reaction, and then going through the step of forming the coagulation particles, it is possible to easily obtain the water-absorbent resin particles having a coagulation particle form and a specific range of the specific surface area. As a result, it is possible to easily obtain the water-absorbent resin particles that exhibit a large dry powder permeation amount and a large 10-second lock value, and exhibit a suitably low 20-second lock value. From a related viewpoint, the polymer particles formed by removing water from the aqueous gel-like polymer before the formation of the coagulation particles can have a median particle diameter of 120 μm or more and 340 μm or less. The median particle diameter of the polymer particles formed by removing water from the aqueous gel-like polymer before the coagulation (hereinafter, sometimes referred to as "median particle diameter before coagulation") can be 120 μm or more, 125 μm or more, or 130 μm or more, and 330 μm or less. The median particle diameter before coagulation can be 120 μm or more, 125 μm or more, or 130 μm or more, and 320 μm or less.

[0064] During the polymerization reaction, the reaction liquid is generally stirred. If the stirring speed is large, there is a tendency for the particle diameter of the aqueous gel-like polymer before coagulation 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, by using a stirring blade having a high stirring efficiency, even if the stirring speed is small, it is possible to form the aqueous gel-like polymer before coagulation having a suitably small particle diameter. In this way, by appropriately controlling the type of the stirring blade and the stirring speed, it is possible to control the particle diameter of the aqueous gel-like polymer before coagulation in the above range.

[0065] By adding a coagulation agent to the reaction liquid containing the aqueous gel-like polymer and stirring the mixed liquid containing the aqueous gel-like polymer and the coagulum, coagulated particles as the coagulum of the aqueous gel-like polymer can be formed. To form the coagulated particles, the reaction liquid can be heated, for example, to 30°C or higher and 80°C or lower. The time for stirring the reaction liquid to form the coagulated particles can be, for example, 1 minute or more and 60 minutes or less.

[0066] The coagulation agent can be an inorganic particle, and as examples thereof, there can be mentioned silica particles (e.g., amorphous silica particles), zeolite, bentonite, alumina, talc, titanium dioxide, kaolin, clay, and hydrotalcite. Of these, at least one selected from the group consisting of amorphous silica, alumina, talc, and kaolin is preferable from the viewpoint of coagulation effect. The amount of the coagulation 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 as the coagulation agent is dispersed in a hydrophobic solvent can be added to the reaction liquid.

[0067] After the formation of the coagulated particles, before the extraction of water from the reaction liquid to form the concentrate, an intermediate crosslinking agent can be added to the reaction liquid and the coagulated particles can be subjected to intermediate crosslinking in the reaction liquid containing the intermediate crosslinking agent. 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.

[0068] The intermediate crosslinking agent can be a compound having two 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 two 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 two or more.

[0069] 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.

[0070] 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.

[0071] 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 these. 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.

[0072] 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.

[0073] 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.

[0074] 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.

[0075] 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.

[0076] 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 until the surface of the absorbent article dries 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.

[0077] For the 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.

[0078] The water and the dispersion medium and the like are removed from the mixture after the surface crosslinking, and dry polymer particles (water-absorbent resin particles) can be obtained. Thus, the mixture can be heated.

[0079] 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.

[0080] 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.

[0081] 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.

[0082] 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.

[0083] 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.

[0084] 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).

[0085] 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.

[0086] 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.

[0087] 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 and 50 g / m 2 or more and 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 and 50 g / m 2 or more and 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 and 50 g / m 2 or more.

[0088] 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.

[0089] 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 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.

[0090] 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.

[0091] 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 10 cm 2 or more and 150 cm 2 or less.

[0092] The 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 and the maximum length d2 in the direction perpendicular to the length direction of the absorbent article with respect to the area in which the test liquid has spread in the absorbent article; and calculating the spread area [cm 2 ] by the following equation.

[0093] 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 25 seconds or less, or 5 seconds or more and 25 seconds or less.

[0094] 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.

[0095] Absorbent items can be, for example, waterproof nursing sheets, diapers, sanitary napkins, tampons, or pet pads.

[0096] Example The present invention is not limited to the following embodiments.

[0097] 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.

[0098] To the prepared split flask, 472.3 g of n-heptane and 0.92 g of sorbitol monolaurate (surfactant, NONION LP-20R, HLB: 8.6, prepared by NOF CORPORATION.) (1.01% 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.

[0099] 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.

[0100] 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.

[0101] 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.

[0102] Intermediate crosslinking To the reaction solution containing the agglomerated particles, an aqueous solution of ethylene glycol diglycidyl ether 0.41 g (ethylene glycol diglycidyl ether (intermediate crosslinking agent): 0.047 mmol) having a concentration of 2 mass% 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.

[0103] 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.

[0104] Surface crosslinking To the concentrate formed by the extraction of water, an aqueous solution of ethylene glycol diglycidyl ether 4.14 g (ethylene glycol diglycidyl ether (surface crosslinking agent): 0.48 mmol) having a concentration of 2 mass% was mixed. Surface crosslinking was performed in the mixture by keeping the internal temperature at 83 ± 2°C for 2 hours.

[0105] Drying The mixture after surface crosslinking was heated to 120°C and water and n-heptane were evaporated until almost no distillate was distilled out from the inside of the system, whereby a powder of dried polymer particles (coagulated particles) was obtained. The powder was passed through a sieve with a mesh size of 850 μm, whereby 83.1 g of water-absorbent resin particles of Example 1 were obtained.

[0106] Example 2 The amount of water extracted by azeotropic distillation before surface crosslinking was 110.7 g, and otherwise, under the same conditions as in Example 1, 87.7 g of water-absorbent resin particles of Example 2 were obtained.

[0107] Example 3 The amount of sorbitan monolaurate (surfactant) was changed to 0.74 g (0.81 mass% with respect to the acrylic acid partial neutralization used in the polymerization reaction), and the amount of water extracted by azeotropic distillation before surface crosslinking was 110.7 g, and otherwise, under the same conditions as in Example 1, 79.7 g of water-absorbent resin particles of Example 3 were obtained.

[0108] Example 4 The stirring blade of the stirred reaction liquid during the polymerization reaction was changed to a 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 700 rpm, and the amount of water extracted by azeotropic distillation before surface crosslinking was 106.1 g, and otherwise, under the same conditions as in Example 3, 76.1 g of water-absorbent resin particles of Example 4 were obtained.

[0109] Example 5 The rotation speed of the stirrer during the polymerization reaction was changed to 400 rpm, the amount of water extracted by azeotropic distillation before surface crosslinking was 107.9 g, and otherwise, under the same conditions as in Example 1, 86.7 g of water-absorbent resin particles of Example 5 were obtained.

[0110] Example 6 The amount of water extracted by azeotropic distillation before surface crosslinking was 109.9 g, and after the water was extracted to the outside of the system, before the addition of the surface crosslinking agent, 0.204 g of a 45 mass% aqueous solution of penta-sodium diethylenetriaminepentaacetate (penta-sodium diethylenetriaminepentaacetate: 0.182 mmol) was added (concentration: 45 mass%), and otherwise, under the same conditions as in Example 1, 86.7 g of water-absorbent resin particles of Example 6 were obtained.

[0111] Comparative Example 1 The amount of sorbitol monolaurate (surfactant) was changed to 1.10 g (1.21% by mass relative to the acrylic acid moiety neutralizer supplied for the polymerization reaction), 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 the water-absorbing resin particles of Comparative Example 1 were obtained.

[0112] Comparative Example 2 The amount of dehydrated sorbitol monolaurate (surfactant) was changed to 1.10 g (1.21% by mass relative to the acrylic acid moiety neutralizer supplied for the polymerization reaction), and otherwise, under the same conditions as in Example 1, 90.4 g of the water-absorbing resin particles of Comparative Example 2 were obtained.

[0113] Comparative Example 3 The amount of water extracted by azeotropic distillation before surface crosslinking was 111.6 g. Otherwise, under the same conditions as Comparative Example 2, 85.9 g of water-absorbing resin particles of Comparative Example 3 were obtained.

[0114] Comparative Example 4 The amount of dehydrated sorbitol monolaurate (surfactant) was changed to 1.10 g (1.21% by mass relative to the acrylic acid fraction neutralized for the polymerization reaction). 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.

[0115] 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.

[0116] 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.

[0117] 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.

[0118] <Second-stage polymerization reaction> 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.

[0119] The first-stage polymerization slurry was stirred at 1000 rpm while the separatory flask was cooled to 25°C. Then, the entire volume of the second-stage monomer aqueous solution was added to the first-stage polymerization slurry, and the system was purged with nitrogen for 30 minutes. Next, the separatory flask was immersed in a 70°C water bath and maintained at this state for 60 minutes to carry out the polymerization reaction. Through the polymerization reaction, a reaction solution containing a granular hydrogel-like polymer was obtained.

[0120] concentrate Following the polymerization reaction, 0.589 g of a 45% by mass aqueous solution of pentasodium diethylenetriaminepentaacetate (0.527 mmol) was added to the reaction solution containing the hydrogel polymer under stirring. Then, a separating flask was immersed in an oil bath set to 125°C, and 256.8 g of water was extracted from the system by azeotropic distillation of n-heptane and water, with the n-heptane refluxed.

[0121] Surface crosslinking 4.42 g of a 2% by mass aqueous solution of ethylene glycol diglycidyl ether (ethylene glycol diglycidyl ether (surface crosslinking agent): 0.507 mmol) was added to the concentrate (containing hydrogel polymer) after water extraction. Surface crosslinking was performed in the mixture by maintaining the internal temperature of the split flask at 83 ± 2 °C for 2 hours.

[0122] dry The cross-linked mixture was heated in an oil bath at 125°C, and water and n-heptane were evaporated until almost no evaporated material distilled from the system, thereby obtaining a dry polymer particle powder. This powder was then passed through a sieve with an 850 μm mesh. 0.5% by mass of amorphous silica particles (lubricant, Oriental Silicas Corporation, Tokusil NP-S) relative to the polymer particles passing through the sieve were mixed with the polymer particles, resulting in 230.2 g of Comparative Example 5, in which amorphous silica particles, acting as a lubricant, were attached to the polymer particles.

[0123] 2. Median particle size 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.

[0124] The conditions of the polymerization step, the median particle diameter before coagulation, the presence or absence of the coagulation step, the amount of surfactant per 1 mole of acrylic acid, the amount of the intermediate crosslinking agent, and the amount of the surface crosslinking agent are shown in Table 1. The water content of the concentrate for surface crosslinking is also shown in Table 1. The water content is calculated by the following formula.

[0125] Water content (mass %) = (Ww / Ws) x 100 Ww = (total mass of water contained in the reaction solution) - (amount of water extracted by azeotropic distillation) Ws = mass of the polymer before surface crosslinking In each of the examples and comparative examples, 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 aqueous solution of the intermediate crosslinking agent. Ws is the theoretical yield of the polymer before surface crosslinking, which is calculated from the amounts of acrylic acid and neutralization product, the radical polymerization initiator, and the intermediate crosslinking agent.

[0126] [Table 1]

[0127] 3. Evaluation of Water-absorbent Resin Particles The water-absorbent resin particles of the examples or comparative examples were evaluated by the following methods. In the absence of a specific description, the measurement was performed in an environment of a temperature of 25 ± 2°C and a humidity of 50 ± 10%. The evaluation results are shown in Tables 2 and 3.

[0128] Water retention amount A cotton bag (broadcloth No. 60, 100 mm in width x 200 mm in length) into which 2.0 g of the water-absorbent resin particles were put was disposed in a beaker having a capacity of 500 mL. The cotton bag into which the water-absorbent resin particles were put was injected with physiological saline 500 g in one go in such a manner that no lump was generated, and the upper portion of the cotton bag was bound with an elastic 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 a dewatering treatment using a dewatering machine (manufactured by KOKUSAN Co., Ltd., product number: H-122) for 1 minute with a centrifugal force 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 into which no water-absorbent resin particles were put, 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 the physiological saline was calculated according to the following formula.

[0129] Water retention amount [g / g] = (Wa - Wb) / 2.0 Water absorption under load The useFigure 3 The measurement device Y shown in the drawing measured the water absorption amount of the water-absorbent resin particles under a load of physiological saline. The measurement device Y was composed of a burette section 61, a conduit 62, a measurement table 63, and a measurement section 64 placed on the measurement table 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 interior of the burette 61a at 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 measurement table 63. The inner diameter of the conduit 62 was 6 mm. A hole with a diameter of 2 mm was formed in the central portion of the measurement table 63, and the conduit 62 was joined to the hole. The measurement 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 of the measurement object were uniformly spread on the nylon mesh 64b. The diameter of the weight 64c was 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.

[0130] In the cylinder 64a of the measurement device Y, 0.100 g of the water-absorbent resin particles 11a was added. The weight 64c was placed on the water-absorbent resin particles 11a, and the measurement 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 interior of the burette 61a through the air inlet tube 61d, and thus the decrease in the water level of the physiological saline in the interior 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 engraved 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 water absorption and the scale Vb of the burette 61a after 60 minutes from the start of water absorption were read, and the water absorption amount under the load was calculated by the following equation.

[0131] Water absorption amount under load [mL / g] = (Vb - Va) / 0.1 Water absorption speed The water absorption speed 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 × 30 mm without a ring). The beaker was immersed in a constant-temperature water bath, and the liquid temperature was adjusted to 25 ± 0.2°C. Next, the beaker was placed on a magnetic stirrer to stir the physiological saline at a rotational speed of 600 rpm to generate a vortex, and 2.0 g of the water-absorbent resin particles were quickly added thereto. The time (seconds) from the time when the water-absorbent resin particles were added to the time when the vortex of the liquid surface converged along with water absorption by the water-absorbent resin particles was measured, and was set as the water absorption rate of the water-absorbent resin particles.

[0132] Median particle diameter The water-absorbent resin particles were passed through a sieve having a mesh size of 250 μm according to the JIS Z 8801-1 standard. 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 sieves according to (A) below, and in the case where it was less than 50% by mass, the median particle diameter was measured using a combination of sieves according to (B) below.

[0133] (A) Sieves having mesh sizes of 710 μm, 600 μm, 500 μm, 425 μm, 300 μm, 250 μm, 150 μm, and a tray, as JIS standard sieves, were sequentially combined from top to bottom.

[0134] (B) Sieves having mesh sizes of 425 μm, 250 μm, 180 μm, 150 μm, 106 μm, 75 μm, 45 μm, and a tray, as JIS standard sieves, were sequentially combined from top to bottom.

[0135] The water-absorbent resin particles were fractionated by adding the water-absorbent resin particles to the uppermost sieve and vibrating for 20 minutes using a continuous full-automatic sonic vibration sieve fractionating apparatus (Robot Shifter RPS-205, manufactured by SEISHIN ENTERPRISE Co., Ltd.). After the fractionation, 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 proportions of the components having large particle diameters from the beginning, the relationship between the mesh size 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 an accumulated mass percentage of 50% by mass was found, and this value was set as the median particle diameter.

[0136] Contact angle A contact angle meter (manufactured by Kyowa Interface Science Co., Ltd.: Dmo-601) having a sample placement stage configured horizontally and movable in the vertical direction, a syringe portion provided on the upper portion thereof, and an observation portion capable of horizontally observing the stage was used, and the contact angle of the water-absorbing resin particles was measured in the following order.

[0137] A double-sided tape (No. 5000NS manufactured by NITTO DENKO CORPORATION: 25 mm x 45 mm) was attached to a metal plate (25 mm x 70 mm), and the adhesive surface of the double-sided tape was exposed. 1.0 g of the water-absorbing resin particles was uniformly scattered on the entire adhesive surface of the double-sided tape. Then, the metal plate was vertically erected, and the remaining water-absorbing resin particles were removed, and a measurement sample having a water-absorbing resin layer formed on the adhesive layer was obtained.

[0138] The measurement sample was placed on the stage portion of the syringe portion (2 mL in volume) of the contact angle meter. Using the observation portion of the contact angle meter, a spherical droplet of 3.0 ± 0.1 mm in diameter of a saline solution having a concentration of 25 mass% was formed at the front end of the syringe portion. By moving the stage portion upward, the surface of the water-absorbing resin layer of the measurement sample was brought into contact with the droplet. The time at which the water-absorbing resin layer and the droplet were brought into contact was set to t = 0 (seconds). The angle of the straight line connecting the left and right ends on the contact surface of the droplet and the water-absorbing resin layer at t = 0 (seconds) with the vertex (highest point) of the droplet with respect to the horizontal plane was read using the observation portion of the contact angle meter, and this angle was set to θ / 2. The contact angle θ was obtained by multiplying the value of θ / 2 by 2. The measurement was repeated 5 times, and the average of 3 times after removing the maximum and minimum values was recorded as the contact angle of the water-absorbing resin particles. The method of reading the angle conformed to JIS R 3257 (1999) "Test method for wettability of glass surface".

[0139] Water absorption amount In a beaker having a volume of 500 mL, 500 g of a physiological saline solution at 25 ± 2°C was added. A magnetic stirrer (8 mm ×Length 30 mm, no ring) while stirring at 600 rpm, 2.0 g of the water-absorbent resin particles were dispersed in the physiological saline in such a manner that no caking occurred. By leaving the physiological saline while stirring for 60 minutes, 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 of mesh 75 μm of mass 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 from the horizontal for 30 minutes, the remaining moisture was removed. Next, 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.

[0140] Water absorption amount Wc [g / g] = (Wb - Wa) / 2.0 Specific surface area By using a sieve of mesh 400 μm and a classification of mesh 300 μm, a component of the water-absorbent resin particles that passed through the sieve of mesh 400 μm and remained on the sieve of mesh 300 μm was obtained, and used as a sample for specific surface area measurement. The sample was dried under a degassing condition of vacuum evacuation for 16 hours at 100°C. The adsorption isotherm after drying was measured by a method of using krypton gas as an adsorbing gas at a temperature of 77 K. A specific surface area measuring device (AUTOSORB-1, manufactured by Quantachrome Corporation) was used for 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 that contained amorphous silica particles as a lubricant, the specific surface area was measured using the polymer particles before the amorphous silica particles were attached.

[0141] Dry powder permeation amount Figure 4 is a schematic view showing a method of measuring a dry powder permeation amount. A polyamide resin-made net 43 (250 mesh, Nippon Tokushu fabric Inc., product number NNO.250T) was plugged into one opening of a cylinder 41 (inner diameter: 26 mm, outer diameter: 39 mm), and 0.450 g of the water-absorbent resin particles 11a was 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 disposed on the net 43. A cylindrical piston 42 (inner diameter: 19 mm, outer diameter: 25.6 mm) whose one opening was plugged by a polyamide resin-made net 44 (250 mesh, Nippon Tokushu fabric Inc., product number NNO.250T) was inserted into the cylinder 41 with the net 44 in contact with the water-absorbent resin particles 11a.

[0142] A metal mesh 46 (8 mesh) having a mass Wc (g) was placed on a petri dish 45 having a mass Wb (g). A cylinder 41 into which the water absorbent resin particles 11a and the piston 42 were inserted 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.

[0143] Dry powder permeation amount [g] = Wa - Wb - Wc Locking test Figure 5 is a schematic diagram showing the method of the locking test. The bottom surface of a cylinder 70 (inner diameter x 2.0 cm, outer diameter 3.0 cm, depth y 8.0 cm) placed on a horizontal table was entirely covered with 0.100 g of the water absorbent resin particles 11a in a layered manner so 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 formed from the bottom surface of the cylinder 70 was read using a vernier caliper (Niigataseiki Co., Ltd. SK Standard Pocket Vernier Caliper 100 mm) 10 seconds or 20 seconds after the time when the entire amount of the physiological saline was poured. 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.

[0144] 4. Production of absorbent article A first shape-retaining member (tissue 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 shape-retaining member, 5.4 g of hydrophilic fibers (pulverized pulp) were 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 shape-retaining member. From the laminate of the first shape-retaining member and the hydrophilic fiber layer, portions 10 cm from both ends in the lengthwise direction were cut off. By halving the remaining laminate, two laminates each having a size of 20 cm x 20 cm were obtained.

[0145] 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, the water absorbent resin particles (3.0 g) were uniformly spread on the hydrophilic fiber layer to form a water absorbent resin layer. A second shape-retaining member (tissue paper) having a size of 20 cm x 20 cm and a unit area weight of 16 g / m2 The second conforming member (tissue paper) was laminated on the water-absorbent resin layer, thereby obtaining a laminate having the first conforming member, the hydrophilic fiber layer, the resin layer, and the second conforming member in this order from the bottom.

[0146] A breathable nonwoven fabric (KNH Enterprise Co., Ltd., size: 20 cm x 20 cm, weight per unit area: 25 g / m 2 ) having a size of 20 cm x 20 cm was coated with a hot-melt adhesive (Henkel Japan Ltd., ME-765E) on the second conforming member of the laminate in contact with the second conforming member, thereby obtaining an absorbent article. The hot-melt adhesive was coated on the breathable nonwoven fabric in such a manner as 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 weight per unit area of the water-absorbent resin particles was 75 g / m 2 , and the weight per unit area of the hydrophilic fiber (pulverized pulp) was 45 g / m 2 .

[0147] 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 evaluations.

[0148] Post-dripping rewet amount An absorbent article was arranged on a horizontal table with the breathable nonwoven fabric on the upper side. From the upper part of the absorbent article 1 cm away toward the center of the absorbent article, 30 mL of the test solution adjusted to 25 ± 1°C was dripped for 10 seconds using a pump (INTEGR A Biosciences Co., Ltd., DOSE IT P910) to which a nozzle having an inner diameter of 0.4 mm was attached. Immediately after the completion of the dripping of the test solution, a filter paper (ADVANTEC No. 51A, formed into 100 mm x 100 mm) of a mass previously measured in an amount of about 75 g was placed on the center of the absorbent article, a weight (100 mm x 100 mm in 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-dripping rewet amount [g].

[0149] 1-minute post-dripping rewet amount A filter paper was placed on the center of the absorbent article 1 minute after the completion of the dripping of the test solution, and otherwise, the 1-minute post-dripping rewet amount [g] was measured in the same order as the post-dripping rewet amount.

[0150] diffusion area An absorbent article was arranged on a horizontal table with the air-permeable 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 the test solution adjusted to 25 ± 1°C was dropped for 10 seconds using a pump (INTEGRA Biosciences, DOSE IT P910) to which a 0.4 mm inner diameter inlet was connected. The dropped test solution diffused in the absorbent article in the horizontal direction. Figure 6 is a plan view showing an example of the state of diffusion of the test solution in the absorbent article. Two minutes after completion of dropping of the test solution, the maximum length dl 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 [cm2] was calculated by the following formula. 2 ].

[0151] diffusion area [cm 2 ] = (dl / 2) x (d2 / 2) x 3.14 whitening time An absorbent article was arranged on a horizontal table with the air-permeable 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 the test solution adjusted to 25 ± 1°C was dropped for 10 seconds using a pump (INTEGRA Biosciences, 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 air-permeable nonwoven fabric immediately after dropping, and then absorbed by the absorbent. With transfer to the absorbent, the color of the air-permeable nonwoven fabric surface of the absorbent article changed from deep blue based on the test solution to the original white color. The time from the start of dropping of 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 dropping of the test solution, and (b) is a photograph after the air-permeable nonwoven fabric whitened.

[0152] [Table 2]

[0153] [Table 3] As shown in Table 3, it was confirmed that the absorbent articles containing the water-absorbent resin particles of each example showed a small diffusion area at the time of water absorption, and less reabsorption and were able to quickly form a dry surface.

[0154] industrial applicability The water-absorbing resin particle production method according to the present application can provide an absorbent article in which the area in which liquid absorbed by the water-absorbing resin particles diffuses is small, the water-absorbing resin particles have a low rate of reabsorption, and a dry surface is quickly formed, and thus the frequency of replacement of the absorbent article (e.g., a pet pad) can be reduced. As a result, the water-absorbing resin particles of the present application can suppress the amount of use of materials other than the water-absorbing resin particles (e.g., pulp, nonwoven fabric, and the like, which are raw materials derived from natural substances (biomass resources)) included in the water-absorbing resin particles and the absorbent article, and thus can contribute to the protection of the global environment.

[0155] Explanation of Reference Signs 10 - Absorbent, 11 - Water-absorbing resin layer, 11a - Water-absorbing 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 manufacturing water-absorbing resin particles, comprising the following steps: In a reaction solution containing a water-soluble olefinic unsaturated monomer, water, a dispersion medium, and a surfactant having an HLB of 7 or more and 16 or less, the water-soluble olefinic unsaturated monomer is polymerized by reverse suspension polymerization, thereby forming a particulate hydrogel polymer containing the water-soluble olefinic unsaturated monomer and water. The aqueous gel polymer is agglomerated in the reaction solution to form aggregated particles containing a plurality of the aqueous gel polymer; A concentrate is formed by extracting a portion of the water from the reaction solution; and The aggregated particles are surface crosslinked in a mixture containing the concentrate and a surface crosslinking agent. The reaction solution does not contain an internal crosslinking agent that crosslinks the polymer, or the amount of the internal crosslinking agent in the reaction solution is less than 0.093 mmol per mole of the water-soluble olefinic unsaturated monomer. Based on the water-soluble olefinic unsaturated monomer, the amount of the surfactant is less than 1.15% by mass.

2. The method according to claim 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. The method according to claim 1 or 2, wherein, 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.

4. A water-absorbing resin particle exhibiting: A contact angle of 50° or more and 70° or less for a 25% by mass saline solution; 0.05m 2 / g or more and 0.22m 2 Specific surface area below / g; and The water absorption capacity of physiological saline is more than 60g / g.

5. The absorbent resin particles according to claim 4, exhibiting a dry powder throughput of 2.0 g or more for physiological saline.

6. An absorbent comprising the absorbent resin particles of claim 4 or 5.

7. An absorbent article comprising: Liquid-permeable sheets; and The absorber of claim 6 is disposed on the inner side of the liquid-permeable sheet.

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