Method for producing water-absorbent resin particles, water-absorbing resin particles, absorber, and absorbent article

Abstract

Disclosed is a method for producing water-absorbent resin particles, the method comprising: forming a hydrogel polymer by polymerizing a water-soluble ethylenically unsaturated monomer through reverse-phase suspension polymerization in a reaction solution containing a surfactant having an HLB of 7-16; forming aggregated particles; extracting some water to form a concentrate; and surface cross-linking the aggregated particles in a mixture containing the concentrate and a surface crosslinking agent. The amount of an internal cross-linking agent is 0.093 mmol or less per mole of the water-soluble ethylenically unsaturated monomer. The polymer particles formed from the hydrogel polymer before forming the aggregated particles have a median particle size of 150 μｍ or more. The mixture is formed by mixing a surface crosslinking agent with a concentrate containing water at a moisture content of 15-50 mass% on the basis of the mass of the polymer.

Description

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

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

[0002] Patent Document 1 discloses a water-absorbent composite sheet, which is an absorbent article for nursing care, having a substrate and substantially spherical water-absorbent resin particles intermittently fixed to the substrate.

[0003] Japanese Patent Application Laid-Open No. 2005-323842

[0004] When an absorbent article absorbs a water-containing liquid, it is desirable that the area over which the liquid spreads within the absorbent article is small. It is also often desirable that the absorbed liquid not return to the surface of the absorbent article, i.e., so-called backflow, is small, and that a dry surface is quickly formed. However, if the liquid diffusion area in the absorbent article is small, backflow tends to be large and it tends to take a long time for a dry surface to be formed.

[0005] The present disclosure relates to an absorbent article that has a small area for the absorbed liquid to spread over and can quickly form a dry surface with little return.

[0006] The present disclosure includes the following: [1] A method for producing a particulate hydrogel polymer containing a polymer of the water-soluble ethylenically unsaturated monomer and water by polymerizing the water-soluble ethylenically unsaturated monomer by reverse phase suspension polymerization in a reaction liquid containing the water-soluble ethylenically unsaturated monomer, water, a dispersion medium, and a surfactant having an HLB of 7 or more and 16 or less, aggregating the hydrogel polymer in the reaction liquid to form a plurality of aggregated particles containing the hydrogel polymer, forming a concentrate by extracting a portion of the water from the reaction liquid, and surface-crosslinking the aggregated particles in a mixture containing the concentrate and a surface-crosslinking agent, wherein the reaction liquid does not contain an internal crosslinking agent that crosslinks the polymer, or the amount of the internal crosslinking agent in the reaction liquid is 0.093 millimoles or less per mole of the water-soluble ethylenically unsaturated monomer, and polymer particles formed by removing water from the hydrogel polymer before forming the aggregated particles have a median particle size of 150 μm or more, A method for producing water-absorbent resin particles, wherein the mixture is formed by mixing the concentrate containing water at a moisture content of 15% by mass or more and 50% by mass or less, based on the mass of the polymer, with the surface cross-linking agent. [2] The method according to [1], wherein the amount of the surface cross-linking agent is 0.15 mmol or more and 5.14 mmol or less per mole of the water-soluble ethylenically unsaturated monomer. [3] Water-absorbent resin particles exhibiting a water absorption amount for physiological saline of 60 g / g or more, a dry powder liquid passing rate of 1.0 g or more, a lock-up value of 10 seconds of 1.5 cm or more, and a lock-up value of 20 seconds of 4.0 cm or less. [4] The water-absorbent resin particles according to [3], having a median particle size of 300 μm or more and 600 μm or less. [5] A water-absorbent resin particle having a median particle size of 0.05 m ２ / g or more 0.22m ２ [6] An absorbent body comprising the water-absorbent resin particles according to any one of [3] to [5], which have a specific surface area of ​​1 / g or less. [7] An absorbent article comprising: a liquid-permeable sheet; and the absorbent body according to [6] provided on the inner side of the liquid-permeable sheet.

[0007] According to the present disclosure, an absorbent article can be provided that has a small area over which absorbed liquid spreads and can quickly form a dry surface with little return. Also provided are water-absorbent resin particles that can constitute such an absorbent article, and a method for producing the same. The absorbent article of the present disclosure is useful, for example, as a waterproof sheet for nursing care.

[0008] FIG. 1 is a partial cross-sectional view showing an example of an absorbent article; FIG. 2 is a plan view showing an example of an agitating impeller; FIG. 3 is a schematic view showing an apparatus for measuring water absorption under load for physiological saline; FIG. 4 is a schematic view showing a method for measuring dry powder liquid passage; FIG. 5 is a schematic view showing a method for a lock-up test; FIG. 6 is a plan view showing an example of a state in which a test liquid has diffused through an absorbent article in measuring a diffusion area; and FIG. 7 is a photograph showing an example of an absorbent article onto which a test liquid has been dropped in measuring a whitening time.

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

[0010] FIG. 1 is a partial cross-sectional view showing an example of an absorbent article. The absorbent article 50 shown in FIG. 1 includes a sheet-like absorbent body 10, a first shape-retaining member 21, a second shape-retaining member 22, a liquid-permeable sheet 30, and an adhesive 35. The absorbent body 10 is a laminate composed of one water-absorbent resin layer 11 containing a plurality of water-absorbent resin particles 11a and one hydrophilic fiber layer 12 containing hydrophilic fibers, and is provided inside the liquid-permeable sheet 30. The absorbent body 10 is disposed between the sheet-like first shape-retaining member 21 and the sheet-like second shape-retaining member 22. The absorbent body 10 may be composed only of the water-absorbent resin layer 11 containing the water-absorbent resin particles 11a. The entire absorbent body 10 may be enclosed by the first shape-retaining member 21 and the second shape-retaining member 22. The first shape-retaining member 21 and the second shape-retaining member 22 may be a single sheet or two separate sheets. The first shape-retaining member 21 and the second shape-retaining member 22 may be, for example, tissue. An adhesive 35 is interposed between the liquid-permeable sheet 30 and the second shape-retaining member 22 to bond them together. The adhesive 35 may be, for example, a hot-melt adhesive. In the example of Fig. 1, a water-absorbent resin layer 11 and a hydrophilic fiber layer 12 are provided in this order from the liquid-permeable sheet 30 side. The absorbent article may further include a liquid-impermeable sheet provided on the outside of the first shape-retaining member 21.

[0011] When the water-absorbent resin particles constituting the absorbent body have a high water absorption capacity for saline solution, the amount of liquid that the water-absorbent resin particles can hold increases when the absorbent body of an absorbent article absorbs liquid. Therefore, for example, there is a tendency for backflow when the absorbent article absorbs liquid to be reduced. From this perspective, the water-absorbent resin particles may have a water absorption capacity for saline solution of 60 g / g or more, 61 g / g or more, 62 g / g or more, 63 g / g or more, 64 g / g or more, 65 g / g or more, 66 g / g or more, 67 g / g or more, 68 g / g or more, 69 g / g or more, or 70 g / g or more. The water absorption capacity of the water-absorbent resin particles for physiological saline may be 60 g / g or more, 61 g / g or more, 62 g / g or more, 63 g / g or more, 64 g / g or more, 65 g / g or more, 66 g / g or more, 67 g / g or more, 68 g / g or more, 69 g / g or more, or 70 g / g or more, and may be 80 g / g or less.

[0012] As described in Examples below, the water absorption amount of water-absorbent resin particles in physiological saline can be measured by dispersing 2.0 g of water-absorbent resin particles in physiological saline while stirring 500 g of physiological saline at 25±2°C in a 500 mL volume beaker at room temperature using a magnetic stir bar (8 mmφ×30 mm length, without ring) at 600 rpm so as not to cause lumps, leaving the physiological saline to stand for 60 minutes while stirring, to obtain a dispersion liquid containing a swollen gel formed by the swelling of the water-absorbent resin particles, and mixing the dispersion liquid with a JIS Z 1000 sieve having a mass Wa [g] and an opening of 75 μm. The water absorption amount is measured by a method including the steps of passing the swollen gel through a 8801-1 standard sieve, leaving the sieve with the swollen gel remaining thereon for 30 minutes in a state inclined at an angle of about 30 degrees from the horizontal to remove excess water, measuring the mass Wb [g] of the sieve with the swollen gel remaining thereon, and calculating the water absorption amount [g / g] of the water-absorbent resin particles in physiological saline solution using the following formula: Water absorption amount [g / g] = (Wb - Wa) / 2.0

[0013] A high dry powder liquid permeability of the water-absorbent resin particles means that liquid can easily pass through quickly in the thickness direction of the water-absorbent resin layer. If the dry powder liquid permeability of the water-absorbent resin particles constituting the absorbent body is high, a dry surface is likely to be formed quickly when liquid is supplied onto the absorbent article. For example, when a water-containing liquid is supplied to the liquid-permeable sheet 30 side of the absorbent article having the configuration illustrated in Figure 1, the liquid quickly moves from the liquid-permeable sheet 30 to the absorbent body 10, and the outer surface of the liquid-permeable sheet 30 is likely to return to a dry state. A high dry powder liquid permeability of the water-absorbent resin particles can also contribute to suppressing the diffusion area of ​​the liquid in the absorbent article. From these viewpoints, the dry powder liquid permeability of the water-absorbent resin particles may be 1.0 g or more, 1.1 g or more, 1.2 g or more, 1.3 g or more, 1.4 g or more, 1.5 g or more, 1.6 g or more, 2.0 g or more, 2.1 g or more, 2.2 g or more, 2.3 g or more, 2.4 g or more, or 2.5 g or more. The dry powder liquid permeability of the water-absorbent resin particles may be 1.0 g or more, 1.1 g or more, 1.2 g or more, 1.3 g or more, 1.4 g or more, 1.5 g or more, 1.6 g or more, 2.0 g or more, 2.1 g or more, 2.2 g or more, 2.3 g or more, 2.4 g or more, or 2.5 g or more, and 15 g or less. The water-absorbent resin particles may have a dry powder liquid permeability of 1.0 g or more, 1.1 g or more, 1.2 g or more, 1.3 g or more, 1.4 g or more, 1.5 g or more, 1.6 g or more, 2.0 g or more, 2.1 g or more, 2.2 g or more, 2.3 g or more, 2.4 g or more, or 2.5 g or more, and 14 g or less. The water-absorbent resin particles may have a dry powder liquid permeability of 1.0 g or more, 1.1 g or more, 1.2 g or more, 1.3 g or more, 1.4 g or more, 1.5 g or more, 1.6 g or more, 2.0 g or more, 2.1 g or more, 2.2 g or more, 2.3 g or more, 2.4 g or more, or 2.5 g or more, and 13 g or less. The dry powder liquid passing amount of the water-absorbent resin particles may be 1.0 g or more, 1.1 g or more, 1.2 g or more, 1.3 g or more, 1.4 g or more, 1.5 g or more, 1.6 g or more, 2.0 g or more, 2.1 g or more, 2.2 g or more, 2.3 g or more, 2.4 g or more, or 2.5 g or more, and may be 12 g or less.The dry powder liquid permeability of the water-absorbent resin particles may be 1.0 g or more, 1.1 g or more, 1.2 g or more, 1.3 g or more, 1.4 g or more, 1.5 g or more, 1.6 g or more, 2.0 g or more, 2.1 g or more, 2.2 g or more, 2.3 g or more, 2.4 g or more, or 2.5 g or more, and may be 11 g or less. The dry powder liquid permeability of the water-absorbent resin particles may be 1.0 g or more, 1.1 g or more, 1.2 g or more, 1.3 g or more, 1.4 g or more, 1.5 g or more, 1.6 g or more, 2.0 g or more, 2.1 g or more, 2.2 g or more, 2.3 g or more, 2.4 g or more, or 2.5 g or more, and may be 10 g or less. The water-absorbent resin particles may have a dry powder liquid permeability of 1.0 g or more, 1.1 g or more, 1.2 g or more, 1.3 g or more, 1.4 g or more, 1.5 g or more, 1.6 g or more, 2.0 g or more, 2.1 g or more, 2.2 g or more, 2.3 g or more, 2.4 g or more, or 2.5 g or more, and 9.0 g or less. The water-absorbent resin particles may have a dry powder liquid permeability of 1.0 g or more, 1.1 g or more, 1.2 g or more, 1.3 g or more, 1.4 g or more, 1.5 g or more, 1.6 g or more, 2.0 g or more, 2.1 g or more, 2.2 g or more, 2.3 g or more, 2.4 g or more, or 2.5 g or more, and 8.0 g or less. The water-absorbent resin particles may have a dry powder liquid permeability of 1.0 g or more, 1.1 g or more, 1.2 g or more, 1.3 g or more, 1.4 g or more, 1.5 g or more, 1.6 g or more, 2.0 g or more, 2.1 g or more, 2.2 g or more, 2.3 g or more, 2.4 g or more, or 2.5 g or more, and 7.0 g or less. The water-absorbent resin particles may have a dry powder liquid permeability of 1.0 g or more, 1.1 g or more, 1.2 g or more, 1.3 g or more, 1.4 g or more, 1.5 g or more, 1.6 g or more, 2.0 g or more, 2.1 g or more, 2.2 g or more, 2.3 g or more, 2.4 g or more, or 2.5 g or more, and 6.0 g or less. The water-absorbent resin particles may have a dry powder liquid permeability of 1.0 g or more, 1.1 g or more, 1.2 g or more, 1.3 g or more, 1.4 g or more, 1.5 g or more, 1.6 g or more, 2.0 g or more, 2.1 g or more, 2.2 g or more, 2.3 g or more, 2.4 g or more, or 2.5 g or more, and 5.0 g or less. The water-absorbent resin particles may have a dry powder liquid permeability of 1.0 g or more, 1.1 g or more, 1.2 g or more, 1.3 g or more, 1.4 g or more, 1.5 g or more, 1.6 g or more, 2.0 g or more, 2.1 g or more, 2.2 g or more, 2.3 g or more, 2.4 g or more, or 2.5 g or more, and 4.0 g or less.The dry powder liquid permeability of the water-absorbent resin particles may be 1.0 g or more, 1.1 g or more, 1.2 g or more, 1.3 g or more, 1.4 g or more, 1.5 g or more, 1.6 g or more, 2.0 g or more, 2.1 g or more, 2.2 g or more, 2.3 g or more, 2.4 g or more, or 2.5 g or more, and may be 3.0 g or less.

[0014] As will be described in Examples below, the dry powder liquid passage amount of a water absorbent resin particle is measured by a method comprising the steps of: placing 0.450 g of water absorbent resin particles on a 250 mesh screen that closes an opening of a cylinder having an inner diameter of 26 mm in a room temperature environment; pouring 20 g of physiological saline at 25±2°C into the water absorbent resin particles on the screen in the cylinder over a period of 2 seconds in a state in which the cylinder is stood upright with the screen facing downwards; and determining the amount of physiological saline that has passed through the water absorbent resin particles until 10 seconds have elapsed from the time when the entire amount of physiological saline has been poured as the dry powder liquid passage amount.

[0015] The lock-up 10-second value of water-absorbent resin particles is a value that reflects the amount of liquid that the water-absorbent resin particles can absorb within 10 seconds after coming into contact with a large amount of liquid and starting to absorb the liquid. Therefore, a large lock-up 10-second value of water-absorbent resin particles indicates that the water-absorbent resin particles can absorb a large amount of liquid within an initial short period of time after coming into contact with the liquid. When the lock-up 10-second value of water-absorbent resin particles is large, the liquid diffusion area in the absorbent article tends to be small. Furthermore, a large lock-up 10-second value of water-absorbent resin particles can also contribute to shortening the time until a dry surface is formed when liquid is supplied onto the absorbent article. From these viewpoints, the lock-up 10-second value of the water-absorbent resin particles may be 1.5 cm or more, 1.6 cm or more, 1.7 cm or more, 1.8 cm or more, 1.9 cm or more, 2.0 cm or more, 2.1 cm or more, 2.2 cm or more, 2.3 cm or more, 2.4 cm or more, 2.5 cm or more, 2.6 cm or more, or 2.7 cm or more. The lock-up value of the water-absorbent resin particles may be 1.5 cm or more, 1.6 cm or more, 1.7 cm or more, 1.8 cm or more, 1.9 cm or more, 2.0 cm or more, 2.1 cm or more, 2.2 cm or more, 2.3 cm or more, 2.4 cm or more, 2.5 cm or more, 2.6 cm or more, or 2.7 cm or more, and 4.5 cm or less. The lock-up value of the water-absorbent resin particles may be 1.5 cm or more, 1.6 cm or more, 1.7 cm or more, 1.8 cm or more, 1.9 cm or more, 2.0 cm or more, 2.1 cm or more, 2.2 cm or more, 2.3 cm or more, 2.4 cm or more, 2.5 cm or more, 2.6 cm or more, or 2.7 cm or more, and 4.4 cm or less. The lock-up value of the water-absorbent resin particles may be 1.5 cm or more, 1.6 cm or more, 1.7 cm or more, 1.8 cm or more, 1.9 cm or more, 2.0 cm or more, 2.1 cm or more, 2.2 cm or more, 2.3 cm or more, 2.4 cm or more, 2.5 cm or more, 2.6 cm or more, or 2.7 cm or more, and 4.3 cm or less. The lock-up value of the water-absorbent resin particles may be 1.5 cm or more, 1.6 cm or more, 1.7 cm or more, 1.8 cm or more, 1.9 cm or more, 2.0 cm or more, 2.1 cm or more, 2.2 cm or more, 2.3 cm or more, 2.4 cm or more, 2.5 cm or more, 2.6 cm or more, or 2.7 cm or more, and 4.2 cm or less.The lock-up value of the water-absorbent resin particles may be 1.5 cm or more, 1.6 cm or more, 1.7 cm or more, 1.8 cm or more, 1.9 cm or more, 2.0 cm or more, 2.1 cm or more, 2.2 cm or more, 2.3 cm or more, 2.4 cm or more, 2.5 cm or more, 2.6 cm or more, or 2.7 cm or more, and 4.1 cm or less. The lock-up value of the water-absorbent resin particles may be 1.5 cm or more, 1.6 cm or more, 1.7 cm or more, 1.8 cm or more, 1.9 cm or more, 2.0 cm or more, 2.1 cm or more, 2.2 cm or more, 2.3 cm or more, 2.4 cm or more, 2.5 cm or more, 2.6 cm or more, or 2.7 cm or more, and 4.0 cm or less.

[0016] The 20-second lockup value of water-absorbent resin particles is a value reflecting the amount of liquid that the water-absorbent resin particles can absorb within 20 seconds after coming into contact with a large amount of liquid and starting to absorb the liquid. If the water absorption amount is relatively large and the lockup value of water-absorbent resin particles is small, when the absorbent body of an absorbent article absorbs liquid, the water-absorbent resin particles do not completely swell and tend to maintain a state in which they have some water-absorbing capacity for several tens of seconds from the start of liquid absorption. Therefore, for example, there is a tendency for the absorbent article to absorb less liquid by returning to its original state. From these viewpoints, the lockup value of water-absorbent resin particles constituting the absorbent body may be 4.0 cm or less, 3.9 cm or less, 3.8 cm or less, or 3.7 cm or less. The lockup value of water-absorbent resin particles may be 4.0 cm or less, 3.9 cm or less, 3.8 cm or less, or 3.7 cm or less, and may be 2.0 cm or more. The lock-up 20-second value of the water-absorbent resin particles may be 4.0 cm or less, 3.9 cm or less, 3.8 cm or less, or 3.7 cm or less, and may be 2.1 cm or more. The lock-up 20-second value of the water-absorbent resin particles may be 4.0 cm or less, 3.9 cm or less, 3.8 cm or less, or 3.7 cm or less, and may be 2.2 cm or more. The lock-up 20-second value of the water-absorbent resin particles may be 4.0 cm or less, 3.9 cm or less, 3.8 cm or less, or 3.7 cm or less, and may be 2.3 cm or more. The lock-up 20-second value of the water-absorbent resin particles may be 4.0 cm or less, 3.9 cm or less, 3.8 cm or less, or 3.7 cm or less, and may be 2.4 cm or more. The lock-up 20-second value of the water-absorbent resin particles may be 4.0 cm or less, 3.9 cm or less, 3.8 cm or less, or 3.7 cm or less, and may be 2.5 cm or more.

[0017] As described in Examples below, the lock-up 10 second value and the lock-up 20 second value of a water-absorbent resin particle are measured by a method including: spreading 0.100 g of water-absorbent resin particles on a bottom surface inside a cylinder having an inner diameter of 2.0 cm and a depth of 8.0 cm in a room temperature environment; pouring 20 g of physiological saline at 25±2°C into the cylinder over 2 seconds; and determining the maximum height from the bottom surface inside the cylinder of the water-absorbent resin particles swelled in the cylinder 10 seconds or 20 seconds after the entire amount of the physiological saline has been poured as the lock-up 10 second value or the lock-up 20 second value.

[0018] The water-absorbent resin particles may exhibit a water absorption amount of 60 g / g or more for physiological saline, a dry powder liquid permeability of 1.0 g or more, a lock-up value of 10 seconds of 1.5 cm or more, and a lock-up value of 20 seconds of 4.0 cm or less. When the water-absorbent resin particles satisfy all of these requirements, a particularly excellent effect is likely to be obtained in that the area over which the absorbed liquid diffuses is small and a dry surface can be quickly formed with little backflow.

[0019] The median particle diameter of the water-absorbent resin particles may 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 may 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 may 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 may 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 may 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 may 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 may 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 may 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 may 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 may 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 as described in the examples below.

[0020] When the specific surface area of ​​the water-absorbent resin particles is appropriately large, the lock-up 10-second value tends to be large. When the specific surface area of ​​the water-absorbent resin particles is appropriately small, the lock-up 20-second value tends to be small. From this viewpoint, the specific surface area of ​​the water-absorbent resin particles constituting the absorbent body is set to 0.05 m ２ / g or more and 0.22m ２ / g or less, 0.21m ２ / g or less, 0.20m ２ / g or less, 0.19m ２ / g or less, or 0.18m ２ The specific surface area of ​​the water-absorbent resin particles constituting the absorbent body may be 0.10 m / g or less. ２ / g or more and 0.22m ２ / g or less, 0.21m ２ / g or less, 0.20m ２ / g or less, 0.19m ２ / g or less, or 0.18m ２ The specific surface area of ​​the water-absorbent resin particles constituting the absorbent body may be 0.11 m ２ / g or more and 0.22m ２ / g or less, 0.21m ２ / g or less, 0.20m ２ / g or less, 0.19m ２ / g or less, or 0.18m ２ The specific surface area of ​​the water-absorbent resin particles constituting the absorbent body may be 0.12 m / g or less. ２ / g or more and 0.22m ２ / g or less, 0.21m ２ / g or less, 0.20m ２ / g or less, 0.19m ２ / g or less, or 0.18m ２ The specific surface area of ​​the water-absorbent resin particles constituting the absorbent body may be 0.13 m / g or less. ２ / g or more and 0.22m ２ / g or less, 0.21m ２ / g or less, 0.20m ２ / g or less, 0.19m ２ / g or less, or 0.18m ２ The specific surface area of ​​the water-absorbent resin particles constituting the absorbent body may be 0.14 m / g or less.２ / g or more and 0.22m ２ / g or less, 0.21m ２ / g or less, 0.20m ２ / g or less, 0.19m ２ / g or less, or 0.18m ２ / g or less.

[0021] As described in the Examples below, the specific surface area of ​​water-absorbent resin particles is measured by a method including: obtaining a fraction of water-absorbent resin particles that have passed through a sieve with a mesh size of 400 μm and remained on a sieve with a mesh size of 300 μm as a sample; drying the sample under degassing conditions of vacuum evacuation at 100° C. for 16 hours; measuring the adsorption isotherm after drying at a temperature of 77 K using krypton gas as the adsorption gas; and determining the specific surface area of ​​the water-absorbent resin particles from a multi-point BET plot. When a lubricant such as silica particles is attached to the surface of the absorbent resin particles, the specific surface area is measured using a sample of the absorbent resin particles before the lubricant is placed thereon, from the viewpoint of improving measurement accuracy.

[0022] The water retention capacity of the water-absorbent resin particles constituting the absorbent body in physiological saline may be, for example, 25 g / g or more and 60 g / g or less, 55 g / g or less, 50 g / g or less, or 45 g / g or less, 30 g / g or more and 60 g / g or less, 55 g / g or less, 50 g / g or less, or 45 g / g or less, 35 g / g or more and 60 g / g or less, 55 g / g or less, 50 g / g or less, or 45 g / g or less, or 40 g / g or more and 60 g / g or less, 55 g / g or less, 50 g / g or less, or 45 g / g or less. The method for measuring the water retention capacity of the water-absorbent resin particles in physiological saline is as described in the examples described later.

[0023] The water absorption rate of the water-absorbent resin particles for physiological saline may be 2.0 seconds or more and 15 seconds or less, 10 seconds or less, 9.0 seconds or less, 8.0 seconds or less, 7.0 seconds or less, 6.0 seconds or less, 5.0 seconds or less, 4.0 seconds or less, or 3.0 seconds or less, or 2.5 seconds or more and 15 seconds or less, 10 seconds or less, 9.0 seconds or less, 8.0 seconds or less, 7.0 seconds or less, 6.0 seconds or less, 5.0 seconds or less, 4.0 seconds or less, or 3.0 seconds or more and 15 seconds or less, 10 seconds or less, 9.0 seconds or less, 8.0 seconds or less, 7.0 seconds or less, 6.0 seconds or less, 5.0 seconds or less, or 4.0 seconds or less. The water absorption rate of the water-absorbent resin particles for physiological saline here is a value measured by the Vortex method, as described in the examples described later.

[0024] The water absorption capacity of the water-absorbent resin particles constituting the absorbent body under a load of 2.07 kPa may be 10 mL / g or more and 30 mL / g or less, 25 mL / g or less, or 20 mL / g or less, or 15 mL / g or more and 30 mL / g or less, 25 mL / g or less, or 20 mL / g or more and 30 mL / g or less, or 25 mL / g or more. The method for measuring the water absorption capacity of the water-absorbent resin particles under a load of 2.07 kPa is as described in the examples below.

[0025] The water-absorbent resin particles can be obtained by a method including the following steps: polymerizing the water-soluble ethylenically unsaturated monomer by reverse phase suspension polymerization in a reaction liquid containing the water-soluble ethylenically unsaturated monomer, water, a dispersion medium, and a surfactant to form a particulate hydrogel polymer containing a polymer of the water-soluble ethylenically unsaturated monomer and water; aggregating the hydrogel polymer in the reaction liquid to form aggregated particles containing a plurality of hydrogel polymers; extracting a portion of the water from the reaction liquid to form a concentrate; and surface-crosslinking the aggregated particles in a mixture containing the concentrate and a surface-crosslinking agent. The reaction liquid for reverse phase suspension polymerization may include an oily liquid, which is a hydrophobic liquid mainly 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.

[0026] The HLB of the surfactant contained in the reaction liquid may be 7 or more and 16 or less. The surfactant may be contained in the oily liquid. When the HLB of the surfactant is within this range, water-absorbent resin particles having a specific surface area within an appropriate range are easily obtained. The HLB of the surfactant may be 7 or more and 15 or less, 14 or less, 13 or less, 12 or less, 11 or less, or 10 or less.

[0027] When the amount of surfactant in the reaction solution is small, water absorption in a short period of time is appropriately suppressed, and water-absorbent resin particles that provide absorbent articles that are easy to quickly form a dry surface are easily obtained. From this viewpoint, the amount of surfactant may be 2.0% by mass or less, 1.7% by mass or less, 1.5% by mass or less, or 1.3% by mass or less based on the water-soluble ethylenically unsaturated monomer. The amount of surfactant may be 0.5% by mass or more and 2.0% by mass or less, 1.7% by mass or less, 1.5% by mass or less, or 1.3% by mass or less based on the water-soluble ethylenically unsaturated monomer. The amount of surfactant may be 0.8% by mass or more and 2.0% by mass or less, 1.7% by mass or less, 1.5% by mass or less, or 1.3% by mass or less based on the water-soluble ethylenically unsaturated monomer. The amount of surfactant may be 1.0% by mass or more and 2.0% by mass or less, 1.7% by mass or less, 1.5% by mass or less, or 1.3% by mass or less based on the water-soluble ethylenically unsaturated monomer. When the polymerization of the water-soluble ethylenically unsaturated monomer is carried out two or more times, the amount of the surfactant relative to the amount of the water-soluble ethylenically unsaturated monomer introduced into the reaction liquid in each polymerization may be within these ranges.

[0028] Examples of surfactants having an HLB of 7 or more and 16 or less include nonionic surfactants such as sorbitan fatty acid esters, (poly)glycerin fatty acid esters, sucrose fatty acid esters, polyoxyethylene sorbitan fatty acid esters, polyoxyethylene glycerin fatty acid esters, sorbitol fatty acid esters, polyoxyethylene sorbitol fatty acid esters, polyoxyethylene alkyl ethers, polyoxyethylene alkylphenyl ethers, polyoxyethylene castor oil, polyoxyethylene hydrogenated castor oil, alkylallyl formaldehyde condensed polyoxyethylene ethers, polyoxyethylene polyoxypropylene block copolymers, polyoxyethylene polyoxypropyl alkyl ethers, and polyethylene glycol fatty acid esters; and anionic surfactants such as fatty acid salts, alkylbenzenesulfonates, alkylmethyltaurates, polyoxyethylene alkylphenyl ether sulfates, polyoxyethylene alkyl ether sulfonates, polyoxyethylene alkyl ether phosphates, and polyoxyethylene alkylallyl ether phosphates.

[0029] The aqueous liquid in the reaction solution contains water and a water-soluble ethylenically unsaturated monomer, and may further contain any additives, such as a thickener, a hydrophilic polymer dispersant, a radical polymerization initiator, a chain transfer agent, a foaming agent, or a combination thereof.

[0030] 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 amide group, an amino group, etc. When the water-soluble ethylenically unsaturated monomer contains an amino group, the amino group may be quaternized.

[0031] The water-soluble ethylenically unsaturated monomer may include, for example, at least one selected from the group consisting of (meth)acrylic acid (hereinafter, "acrylic" and "methacrylic" will be collectively referred to as "(meth)acrylic") and its alkali salt, 2-(meth)acrylamido-2-methylpropanesulfonic acid and its alkali salt, (meth)acrylamide, N,N-dimethyl(meth)acrylamide, 2-hydroxyethyl(meth)acrylate, N-methylol(meth)acrylamide, polyethylene glycol mono(meth)acrylate, N,N-diethylaminoethyl(meth)acrylate, N,N-diethylaminopropyl(meth)acrylate, and diethylaminopropyl(meth)acrylamide. The water-soluble ethylenically unsaturated monomer may include (meth)acrylic acid and its alkali metal salt, or may include acrylic acid and its alkali metal salt. The proportion of (meth)acrylic acid and its alkali metal salts in the water-soluble ethylenically unsaturated monomers in the reaction solution may be 60 mol% or more, 70 mol% or more, 80 mol% or more, 90 mol% or more, or 95 mol% or more, or may be substantially 100 mol%. The proportion of monomer units derived from (meth)acrylic acid or its alkali metal salts in the total amount of monomer units constituting the polymer formed by polymerization may be within the above range.

[0032] The reaction solution may be substantially free of an internal crosslinking agent that crosslinks the polymer. In other words, the amount of the internal crosslinking agent in the reaction solution may be 0 mmol or more and 0.093 mmol or less, 0 mmol or more and 0.070 mmol or less, 0 mmol or more and 0.050 mmol or less, or 0 mmol or more and 0.030 mmol or less per mole of the water-soluble ethylenically unsaturated monomer. When the internal crosslinking agent is absent or present in the reaction solution in a small amount, water-absorbent resin particles exhibiting a high water absorption capacity are easily obtained. When the reaction solution contains an internal crosslinking agent, the internal crosslinking agent may contain the same compound as the examples of the intermediate crosslinking agent described below.

[0033] Examples of thickeners include hydroxyalkyl celluloses such as hydroxyethyl cellulose (HEC) and hydroxypropyl cellulose (HPC), hydroxyalkyl alkyl celluloses such as hydroxyethyl methyl cellulose, hydroxypropyl methyl cellulose, and hydroxyethyl ethyl cellulose, carboxyalkyl celluloses such as carboxymethyl cellulose, and carboxyalkyl hydroxyalkyl celluloses such as carboxymethyl hydroxyethyl cellulose. The thickeners may be used alone or in combination of two or more.

[0034] The amount of the thickener may be 0.05 parts by mass or more and 20 parts by mass or less, 0.2 parts by mass or more and 10 parts by mass or less, or 0.4 parts by mass or more and 5 parts by mass or less, relative to 100 parts by mass of the water-soluble ethylenically unsaturated monomer.

[0035] Examples of hydrophilic polymer dispersants include polyvinyl alcohol (PVA), polyvinylpyrrolidone (PVP), polyethylene glycol (PEG), polypropylene glycol, polyethylene glycol-polypropylene glycol block copolymer, polyglycerin, polyoxyethylene glycerin, polyoxypropylene glycerin, polyoxyethylene-polyoxypropylene glycerin copolymer, and polyoxyethylene sorbitan fatty acid ester. The hydrophilic polymer dispersants may be used alone or in combination of two or more.

[0036] The amount of the hydrophilic polymer dispersant may 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.

[0037] The radical polymerization initiator may include, for example, an azo-based compound, a peroxide, or a combination thereof.

[0038] Examples of the azo compounds include 2,2'-azobis[2-(N-phenylamidino)propane]dihydrochloride, 2,2'-azobis{2-[N-(4-chlorophenyl)amidino]propane}dihydrochloride, 2,2'-azobis{2-[N-(4-hydroxyphenyl)amidino]propane}dihydrochloride, 2,2'-azobis[2-(N-benzylamidino)propane]dihydrochloride, 2,2' -Azobis[2-(N-allylamidino)propane]dihydrochloride, 2,2'-azobis(2-amidinopropane)dihydrochloride, 2,2'-azobis{2-[N-(2-hydroxyethyl)amidino]propane}dihydrochloride, 2,2'-azobis[2-(5-methyl-2-imidazolin-2-yl)propane]dihydrochloride, 2,2'-azobis[2-(2-imidazolin-2-yl) propane] dihydrochloride, 2,2'-azobis[2-(4,5,6,7-tetrahydro-1H-1,3-diazepin-2-yl)propane] dihydrochloride, 2,2'-azobis[2-(5-hydroxy-3,4,5,6-tetrahydropyrimidin-2-yl)propane] dihydrochloride, 2,2'-azobis{2-[1-(2-hydroxyethyl)-2-imidazolin-2-yl]propane] dihydrochloride 2,2'-azobis[N-(2-carboxyethyl)-2-methylpropionamidine]tetrahydrate, and 2,2'-azobis[2-methyl-N-(2-hydroxyethyl)propionamide].

[0039] Examples of peroxides include persulfates such as potassium persulfate, ammonium persulfate, and sodium persulfate; organic peroxides such as methyl ethyl ketone peroxide, methyl isobutyl ketone peroxide, di-t-butyl peroxide, t-butyl cumyl peroxide, t-butyl peroxyacetate, t-butyl peroxyisobutyrate, and t-butyl peroxypivalate; and hydrogen peroxide.

[0040] The amount of the radical polymerization initiator may be, for example, 0.005 to 1 mole per 100 moles of the water-soluble ethylenically unsaturated monomer.

[0041] Examples of chain transfer agents include hypophosphites, thiols, thiolic acids, secondary alcohols, and amines.

[0042] Examples of the blowing agent include inorganic blowing agents such as ammonia carbonate, sodium bicarbonate, and ammonium bicarbonate; nitroso compounds such as dinitrosopentamethylenetetramine; azo compounds such as azodicarbonamide and azobisisobutyronitrile; and organic blowing agents such as sulfonylhydrazide compounds such as 4,4′-oxybisbenzenesulfonylhydrazide and p-toluenesulfonylhydrazide.

[0043] The oily liquid in the reaction mixture is a hydrophobic liquid mainly composed of a hydrophobic dispersion medium. The dispersion medium may be a hydrocarbon dispersion medium. The oily liquid may further contain optional additives (e.g., a hydrophobic polymer dispersant and / or a surfactant, as described below).

[0044] Examples of hydrocarbon dispersion media include chain aliphatic hydrocarbons such as n-hexane, n-heptane, 2-methylhexane, 3-methylhexane, 2,3-dimethylpentane, 3-ethylpentane, and n-octane; alicyclic hydrocarbons such as cyclohexane, methylcyclohexane, cyclopentane, methylcyclopentane, trans-1,2-dimethylcyclopentane, cis-1,3-dimethylcyclopentane, and trans-1,3-dimethylcyclopentane; and aromatic hydrocarbons such as benzene, toluene, and xylene. The hydrocarbon dispersion media may be used alone or in combination of two or more.

[0045] The amount of the dispersion medium contained in the oily liquid may be 30 parts by mass or more and 1,000 parts by mass or less, 50 parts by mass or more and 650 parts by mass or less, 70 parts by mass or more and 550 parts by mass or less, or 100 parts by mass or more and 450 parts by mass or less, relative to 100 parts by mass of the water-soluble ethylenically unsaturated monomer.

[0046] Examples of hydrophobic polymer dispersants include maleic anhydride-modified polyethylene, maleic anhydride-modified polypropylene, maleic anhydride-modified ethylene-propylene copolymer, maleic anhydride-modified EPDM (ethylene-propylene-diene terpolymer), maleic anhydride-modified polybutadiene, maleic anhydride-ethylene copolymer, maleic anhydride-propylene copolymer, maleic anhydride-ethylene-propylene copolymer, maleic anhydride-butadiene copolymer, polyethylene, polypropylene, ethylene-propylene copolymer, oxidized polyethylene, oxidized polypropylene, oxidized ethylene-propylene copolymer, ethylene-acrylic acid copolymer, ethyl cellulose, and ethylhydroxyethyl cellulose. The hydrophobic polymer dispersants may be used alone or in combination of two or more.

[0047] The amount of the hydrophobic polymer dispersant may be 0.05 parts by mass or more and 10 parts by mass or less, 0.08 parts by mass or more and 5 parts by mass or less, or 0.1 parts by mass or more and 3 parts by mass or less, relative to 100 parts by mass of the aqueous liquid.

[0048] By forming a hydrogel polymer having an appropriate particle size by a polymerization reaction and then forming agglomerated particles from the polymer, water-absorbent resin particles having the form of agglomerated particles and a specific surface area within an appropriate range can be easily obtained. As a result, water-absorbent resin particles exhibiting a large dry powder liquid passing rate and a large lock-up value at 10 seconds, and a moderately low lock-up value at 20 seconds, can be easily obtained. From this viewpoint, the polymer particles formed by removing water from the hydrogel polymer before forming the agglomerated particles may have a median particle size of 150 μm or more. The median particle size of the polymer particles formed by removing water from the hydrogel polymer before agglomeration (hereinafter sometimes referred to as the "median particle size before agglomeration") may be 150 μm or more, 155 μm or more, 160 μm or more, 165 μm or more, or 170 μm or more but 350 μm or less. The median particle size before aggregation may be 150 μm or more, 155 μm or more, 160 μm or more, 165 μm or more, or 170 μm or more but 300 μm or less. The median particle size before aggregation may be 150 μm or more, 155 μm or more, 160 μm or more, 165 μm or more, or 170 μm or more but 290 μm or less. The median particle size before aggregation may be 150 μm or more, 155 μm or more, 160 μm or more, 165 μm or more, or 170 μm or more but 280 μm or less. The median particle size before aggregation may be 150 μm or more, 155 μm or more, 160 μm or more, 165 μm or more, or 170 μm or more but 270 μm or less. The median particle size before aggregation may be 150 μm or more, 155 μm or more, 160 μm or more, 165 μm or more, or 170 μm or more but 260 μm or less. The median particle size before aggregation may be 150 μm or more, 155 μm or more, 160 μm or more, 165 μm or more, or 170 μm or more but 250 μm or less. The median particle size before aggregation may be 150 μm or more, 155 μm or more, 160 μm or more, 165 μm or more, or 170 μm or more but 240 μm or less. The median particle size before aggregation may be 150 μm or more, 155 μm or more, 160 μm or more, 165 μm or more, or 170 μm or more but 230 μm or less. The median particle size before aggregation may be 150 μm or more, 155 μm or more, 160 μm or more, 165 μm or more, or 170 μm or more to 220 μm or less.The median particle size before aggregation may be 150 μm or more, 155 μm or more, 160 μm or more, 165 μm or more, or 170 μm or more and 210 μm or less.

[0049] During the polymerization reaction, the reaction solution is usually stirred. High stirring speeds tend to decrease the particle size of the hydrogel polymer before aggregation. The stirring speed can be adjusted, for example, within a range of 200 rpm or more and 1000 rpm or less, 900 rpm or less, 800 rpm or less, or less than 700 rpm. By using a stirring blade with high stirring efficiency, a hydrogel polymer before aggregation exhibiting a moderately small particle size can be formed even at a low stirring speed. Thus, by appropriately controlling the type of stirring blade and the stirring speed, the particle size of the hydrogel polymer before aggregation can be controlled within the above range.

[0050] A flocculant is added to a reaction solution containing a hydrogel polymer, and the mixture containing the hydrogel polymer and the flocculants is stirred to form flocculated particles, which are flocculants of the hydrogel polymer. To form the flocculated particles, the reaction solution may be heated to, for example, 30° C. or higher and 80° C. or lower. The reaction solution may be stirred for, for example, 1 minute or higher and 60 minutes or lower.

[0051] The flocculant may be inorganic particles, examples of which include silica particles (e.g., amorphous silica particles), zeolite, bentonite, aluminum oxide, talc, titanium dioxide, kaolin, clay, and hydrotalcite. Among these, at least one selected from the group consisting of amorphous silica, aluminum oxide, talc, and kaolin is preferred in terms of flocculation effect. The amount of the flocculant may be, for example, 0.0001% by mass or more and 1.0% by mass or less, or 0.001% by mass or more and 0.5% by mass or less, based on the amount of the water-soluble ethylenically unsaturated monomer. A dispersion in which inorganic particles serving as the flocculant are dispersed in a hydrophobic solvent may be added to the reaction liquid.

[0052] After the formation of the aggregated particles, an intermediate crosslinking agent may be added to the reaction solution before removing water from the reaction solution to form a concentrate, and the aggregated particles may be intermediately crosslinked in the reaction solution containing the intermediate crosslinking agent. The reaction solution may be heated for the intermediate crosslinking. The intermediate crosslinking agent may be added to the reaction solution as an aqueous solution.

[0053] The intermediate crosslinking agent may be a compound having two or more reactive functional groups, and examples thereof include di- or tri(meth)acrylic acid esters of polyols such as ethylene glycol, propylene glycol, trimethylolpropane, glycerin, polyoxyethylene glycol, polyoxypropylene glycol, and polyglycerin; unsaturated polyesters obtained by reacting the above-mentioned polyols with unsaturated acids (maleic acid, fumaric acid, etc.); bis(meth)acrylamides such as N,N'-methylenebis(meth)acrylamide; di- or tri(meth)acrylic acid esters obtained by reacting polyepoxides with (meth)acrylic acid; di(meth)acrylic acid carbamyl esters obtained by reacting polyisocyanates (tolylene diisocyanate, hexamethylene diisocyanate, etc.) with hydroxyethyl (meth)acrylate; aryl acrylates such as aryl acrylates; Compounds having two or more polymerizable unsaturated groups, such as allylated starch, allylated cellulose, diallyl phthalate, N,N',N"-triallyl isocyanurate, and divinylbenzene; polyglycidyl compounds, such as (poly)ethylene glycol diglycidyl ether, (poly)propylene glycol diglycidyl ether, (poly)glycerin diglycidyl ether, (poly)glycerin triglycidyl ether, (poly)propylene glycol polyglycidyl ether, and polyglycerol polyglycidyl ether; haloepoxy compounds, such as epichlorohydrin, epibromohydrin, and α-methylepichlorohydrin; and isocyanate compounds (2,4-tolylene diisocyanate, hexamethylene diisocyanate, and the like). The intermediate crosslinking agents may be used alone or in combination of two or more.

[0054] The intermediate crosslinking agent may be a polyglycidyl compound or a diglycidyl ether compound, and may contain at least one compound selected from the group consisting of (poly)ethylene glycol diglycidyl ether, (poly)propylene glycol diglycidyl ether, and (poly)glycerin diglycidyl ether.

[0055] The amount of the intermediate crosslinking agent may be 0.001 mmol or more and 0.3 mmol or less, 0.005 mmol or more and 0.2 mmol or less, or 0.01 mmol or more and 0.1 mmol or less, per mole of the water-soluble ethylenically unsaturated monomer used to form the hydrogel polymer.

[0056] A concentrate is formed by extracting a portion of water from a reaction solution containing aggregated particles. Water can be extracted, for example, by azeotropic distillation of water and a dispersion medium. A surface cross-linking agent is added to the concentrate, and the aggregated particles are surface-cross-linked in a mixture containing them. If the moisture content of the concentrate to be surface-cross-linked is appropriately small, water-absorbent resin particles exhibiting a large water absorption capacity are easily obtained. From this perspective, the moisture content of the concentrate to be surface-cross-linked may be 15% by mass or more and 50% by mass or less, based on the mass of the polymer. In other words, a mixture containing a concentrate and a surface cross-linking agent may be formed by mixing a concentrate containing water at a moisture content of 15% by mass or more and 50% by mass or less, based on the mass of the polymer, with the surface cross-linking agent. Here, the moisture content is calculated by the following formula: Moisture content (mass %) = (Ww / Ws) × 100 Ww = total mass of water contained in the reaction solution Ws = mass of the polymer before surface cross-linking

[0057] Ww is the amount determined by subtracting the amount of water extracted to form a concentrate, including the mass of water contained in the aqueous solution in the reaction solution and, when each raw material, such as the intermediate crosslinking agent, is introduced as an aqueous solution, the amount of water contained in the aqueous solution. The mass of the polymer used to calculate Ws is the theoretical yield calculated from the amount of each raw material, such as the amount of water-soluble ethylenically unsaturated monomer charged, the intermediate crosslinking agent, and the radical polymerization initiator charged.

[0058] The moisture content of the concentrate to be subjected to surface crosslinking may be 15% by mass or more, 20% by mass or more, 25% by mass or more, 30% by mass or more, or 35% by mass or more and 50% by mass or less; may be 15% by mass or more, 20% by mass or more, 25% by mass or more, 30% by mass or more, or 35% by mass or more and 45% by mass or less; or may be 15% by mass or more, 20% by mass or more, 25% by mass or more, 30% by mass or more, or 35% by mass or more and 40% by mass or less.

[0059] The surface cross-linking agent may be a compound having two or more reactive functional groups, and examples thereof include polyols such as ethylene glycol, propylene glycol, 1,4-butanediol, trimethylolpropane, glycerin, polyoxyethylene glycol, polyoxypropylene glycol, and polyglycerin; polyglycidyl compounds such as (poly)ethylene glycol diglycidyl ether, (poly)glycerin diglycidyl ether, (poly)glycerin triglycidyl ether, trimethylolpropane triglycidyl ether, (poly)propylene glycol polyglycidyl ether, and (poly)glycerol polyglycidyl ether; epichlorohydrin, epibromohydrin, haloepoxy compounds such as 2,4-tolylene diisocyanate and α-methylepichlorohydrin; isocyanate compounds such as 2,4-tolylene diisocyanate and hexamethylene diisocyanate; oxetane compounds such as 3-methyl-3-oxetanemethanol, 3-ethyl-3-oxetanemethanol, 3-butyl-3-oxetanemethanol, 3-methyl-3-oxetaneethanol, 3-ethyl-3-oxetaneethanol and 3-butyl-3-oxetaneethanol; oxazoline compounds such as 1,2-ethylenebisoxazoline; carbonate compounds such as ethylene carbonate; and hydroxyalkylamide compounds such as bis[N,N-di(β-hydroxyethyl)]adipamide. The surface cross-linking agent may contain a polyglycidyl compound such as (poly)ethylene glycol diglycidyl ether, (poly)glycerin diglycidyl ether, (poly)glycerin triglycidyl ether, (poly)propylene glycol polyglycidyl ether, polyglycerol polyglycidyl ether, etc. These surface cross-linking agents may be used alone or in combination of two or more.

[0060] The surface crosslinking agent can be the same compound as the intermediate crosslinking agent described above. The surface crosslinking agent may be mixed with the concentrate as a solution. The solvent for the surface crosslinking agent solution can be water, a hydrophilic organic solvent, or a combination thereof. Examples of hydrophilic organic solvents include lower alcohols such as methyl alcohol, ethyl alcohol, and isopropyl alcohol, ketones such as acetone and methyl ethyl ketone, ethers such as dioxane and tetrahydrofuran, amides such as N,N-dimethylformamide, and sulfoxides such as dimethyl sulfoxide. The solvent for the surface crosslinking agent may be used alone or in combination of two or more.

[0061] A large amount of surface cross-linking agent tends to improve the strength of the swollen gel formed by the water absorption of the water-absorbent resin particles. This suppresses gel blocking and increases the dry powder liquid permeability. As a result, backflow and the time required for the surface of the absorbent article to dry are likely to be improved. For example, the amount of surface cross-linking agent may be 0.15 mmol or more and 5.14 mmol or less, 5.0 mmol or less, 4.5 mmol or less, 4.0 mmol or less, 3.5 mmol or less, 3.0 mmol or less, 2.5 mmol or less, or 2.0 mmol or less per mole of water-soluble ethylenically unsaturated monomer. The amount of surface cross-linking agent may be 0.20 mmol or more and 5.14 mmol or less, 5.0 mmol or less, 4.5 mmol or less, 4.0 mmol or less, 3.5 mmol or less, 3.0 mmol or less, 2.5 mmol or less, or 2.0 mmol or less per mole of water-soluble ethylenically unsaturated monomer. The amount of the surface cross-linking agent may be 0.25 mmol or more and 5.14 mmol or less, 5.0 mmol or less, 4.5 mmol or less, 4.0 mmol or less, 3.5 mmol or less, 3.0 mmol or less, 2.5 mmol or less, or 2.0 mmol or less per mole of the water-soluble ethylenically unsaturated monomer. The amount of the surface cross-linking agent may be 0.30 mmol or more and 5.14 mmol or less, 5.0 mmol or less, 4.5 mmol or less, 4.0 mmol or less, 3.5 mmol or less, 3.0 mmol or less, 2.5 mmol or less, or 2.0 mmol or less per mole of the water-soluble ethylenically unsaturated monomer.

[0062] For the surface cross-linking, the mixture may be heated. The heating temperature may be, for example, 60° C. or higher and 200° C. or lower, or 80° C. or higher and 150° C. or lower. The reaction time for the surface cross-linking reaction may be, for example, 1 minute or higher and 300 minutes or lower, or 5 minutes or higher and 200 minutes or lower.

[0063] After the surface cross-linking, water, the dispersion medium, etc. can be removed from the mixture to obtain dried polymer particles (water-absorbent resin particles). For this purpose, the mixture may be heated.

[0064] Various additives may be further added to the dried polymer particles, examples of which include lubricants, metal chelating agents, surface modifiers, heat stabilizers, antioxidants, and antibacterial agents.

[0065] The lubricant may be, for example, amorphous silica particles. Examples of metal chelating agents include ethylenediaminetetraacetic acid and its salts (such as disodium ethylenediaminetetraacetic acid) and diethylenetriaminepentaacetic acid and its salts (such as pentasodium diethylenetriaminepentaacetic acid). Examples of surface modifiers include polyvalent metal compounds such as aluminum sulfate, potassium alum, ammonium alum, sodium alum, (poly)aluminum chloride, and hydrates thereof; and polycation compounds such as polyethyleneimine, polyvinylamine, and polyallylamine.

[0066] The additive may be attached to the surface of the polymer particles or may penetrate into the interior of the polymer particles. The additive (e.g., a lubricant) can be added to the polymer particles after drying to attach the additive to the surface of the polymer particles. The additive may be added to the reaction liquid for polymerization or to the concentrate after water removal.

[0067] The amount of the additive (e.g., lubricant) may be, for example, 0.001 to 10 parts by mass, 0.01 to 5 parts by mass, or 0.1 to 2 parts by mass relative to 100 parts by mass of the polymer particles.

[0068] The structure of the absorbent body 10 is not limited to the structure exemplified in Fig. 1 and can be modified as appropriate. Other examples of absorbent body structures include a mixed structure in which water-absorbent resin particles and hydrophilic fibers are mixed throughout the absorbent body, and a sandwich structure in which a water-absorbent resin layer containing water-absorbent resin particles is held between multiple hydrophilic fiber layers. When the water-absorbent resin layer and the hydrophilic fiber layer are provided separately, the water-absorbent resin particles and the hydrophilic fibers may be mixed near the boundary between them.

[0069] The amount of water-absorbent resin particles in the absorbent body may be, for example, 5% by mass or more and 100% by mass or less, 90% by mass or less, 80% by mass or less, or 70% by mass or less, based on the mass of the absorbent body (or the total mass of the hydrophilic fibers and the water-absorbent resin particles). The amount of water-absorbent resin particles in the absorbent body may be 10% by mass or more and 100% by mass or less, 90% by mass or less, 80% by mass or less, or 70% by mass or less, based on the mass of the absorbent body (or the total mass of the hydrophilic fibers and the water-absorbent resin particles). The amount of water-absorbent resin particles in the absorbent body may be 15% by mass or more and 100% by mass or less, 90% by mass or less, 80% by mass or less, or 70% by mass or less, based on the mass of the absorbent body (or the total mass of the hydrophilic fibers and the water-absorbent resin particles). The amount of water-absorbent resin particles in the absorbent may be 20% by mass or more and 100% by mass or less, 90% by mass or less, 80% by mass or less, or 70% by mass or less, based on the mass of the absorbent (or the total mass of the hydrophilic fibers and the water-absorbent resin particles). The amount of water-absorbent resin particles in the absorbent may be 25% by mass or more and 100% by mass or less, 90% by mass or less, 80% by mass or less, or 70% by mass or less, based on the mass of the absorbent (or the total mass of the hydrophilic fibers and the water-absorbent resin particles). The amount of water-absorbent resin particles in the absorbent may be 30% by mass or more and 100% by mass or less, 90% by mass or less, 80% by mass or less, or 70% by mass or less, based on the mass of the absorbent (or the total mass of the hydrophilic fibers and the water-absorbent resin particles). The amount of water-absorbent resin particles in the absorbent may be 35% by mass or more and 100% by mass or less, 90% by mass or less, 80% by mass or less, or 70% by mass or less, based on the mass of the absorbent (or the total mass of the hydrophilic fibers and the water-absorbent resin particles). The amount of water-absorbent resin particles in the absorbent may be 40% by mass or more and 100% by mass or less, 90% by mass or less, 80% by mass or less, or 70% by mass or less, based on the mass of the absorbent (or the total mass of the hydrophilic fibers and the water-absorbent resin particles). The amount of water-absorbent resin particles in the absorbent may be 45% by mass or more and 100% by mass or less, 90% by mass or less, 80% by mass or less, or 70% by mass or less, based on the mass of the absorbent (or the total mass of the hydrophilic fibers and the water-absorbent resin particles).The amount of water-absorbent resin particles in the absorbent may be 50% by mass or more and 100% by mass or less, 90% by mass or less, 80% by mass or less, or 70% by mass or less, based on the mass of the absorbent (or the total mass of the hydrophilic fibers and the water-absorbent resin particles). The amount of water-absorbent resin particles in the absorbent may be 55% by mass or more and 100% by mass or less, 90% by mass or less, 80% by mass or less, or 70% by mass or less, based on the mass of the absorbent (or the total mass of the hydrophilic fibers and the water-absorbent resin particles). The amount of water-absorbent resin particles in the absorbent may be 60% by mass or more and 100% by mass or less, 90% by mass or less, 80% by mass or less, or 70% by mass or less, based on the mass of the absorbent (or the total mass of the hydrophilic fibers and the water-absorbent resin particles).

[0070] The basis weight of the water-absorbent resin particles in the absorbent article is 10 g / m ２ 300g / m or more ２ Below, 200g / m ２ Below, 150g / m ２ Below, 100g / m ２ Below, 90g / m ２ or less than 80 g / m ２ The weight per unit area of ​​the water-absorbent resin particles may be 50 g / m or less. ２ 300g / m or more ２ Below, 200g / m ２ Below, 150g / m ２ Below, 100g / m ２ Below, 90g / m ２ or less than 80 g / m ２ The weight of the water-absorbent resin particles may be 60 g / m or less. ２ 300g / m or more ２ Below, 200g / m ２ Below, 150g / m ２ Below, 100g / m ２ Below, 90g / m ２ or less than 80 g / m ２ The weight per unit area of ​​the water-absorbent resin particles may be 70 g / m or less. ２ 300g / m or more ２ Below, 200g / m ２ Below, 150g / m ２ Below, 100g / m ２ Below, 90g / m ２or less than 80 g / m ２ In this specification, the basis weight means the mass per unit area of ​​the absorbent article when viewed in the thickness direction of the absorbent article.

[0071] Examples of hydrophilic fibers include cellulose fibers such as cotton pulp and chemical pulp, and artificial cellulose fibers such as rayon and acetate. The absorbent body may further contain hydrophobic fibers made of synthetic resins such as polyamide, polyester, and polyolefin as a reinforcing agent.

[0072] The basis weight of the hydrophilic fiber in the absorbent article is 20 g / m ２ 300g / m or more ２ Below, 200g / m ２ Below, 100g / m ２ Below, 70g / m ２ Below, 60g / m ２ or less, or 50 g / m ２ or less, and ２ 300g / m or more ２ Below, 200g / m ２ Below, 100g / m ２ Below, 70g / m ２ Below, 60g / m ２ or less, or 50 g / m ２ or less, and ２ 300g / m or more ２ Below, 200g / m ２ Below, 100g / m ２ Below, 70g / m ２ Below, 60g / m ２ or less, or 50 g / m ２ It may be the following:

[0073] The liquid-permeable sheet 30 may be, for example, a nonwoven fabric, a porous resin sheet, tissue, or a combination thereof. The nonwoven fabric may contain resin fibers such as polyethylene, polypropylene, polyester, polyamide, or the like.

[0074] The post-drop backflow amount of the absorbent article may be 7.0 g or less, or may be 0.1 g or more and 7.0 g or less. The post-drop backflow amount is measured by a method including placing the absorbent article on a horizontal table with the liquid-permeable sheet facing up, dripping 30 mL of test liquid adjusted to 25 ± 1 °C from a 0.4 mm inner diameter inlet 1 cm above the absorbent article toward the center of the absorbent article over 10 seconds, immediately after the test liquid dripping is completed, placing a 100 mm x 100 mm filter paper whose mass has been measured in advance at the location where the test liquid was dripped, applying a 0.7 psi weight to the filter paper for 3 seconds, removing the weight and filter paper, and recording the mass of the test liquid absorbed by the filter paper as the post-drop backflow amount [g]. The test liquid is prepared by dissolving 45.0 g of NaCl in 4955.0 g of ion-exchanged water and adding a small amount of Blue No. 1. The amount of backflow after 1 minute, the diffusion area and the whitening time, which will be described below, are also measured using the same test liquid.

[0075] The amount of return of the absorbent article after one minute may be 2.5 g or less, or may be 0.1 g or more and 2.5 g or less. The amount of return of the absorbent article after one minute is measured in the same manner as the amount of return after dropping, except that one minute after the completion of dropping the test liquid, a filter paper is placed at the position where the test liquid was dropped.

[0076] When the test liquid is dropped onto the liquid-permeable sheet side of the absorbent article, the spread area of ​​the test liquid is 150 cm ２ It may be less than 10 cm ２ More than 150cm ２ It may be the following:

[0077] The diffusion area was determined by placing the absorbent article on a horizontal table with the liquid-permeable sheet facing upward, dropping 30 mL of test liquid adjusted to 25±1°C from an inlet with an inner diameter of 0.4 mm from 1 cm above the absorbent article toward the center of the absorbent article over 10 seconds, and measuring the maximum length d1 in the longitudinal direction of the absorbent article and the maximum length d2 in the direction perpendicular to the longitudinal direction for the area where the test liquid has diffused within the absorbent article 50, two minutes after the completion of dropping the test liquid. The diffusion area [cm ２ ] is measured by calculating the diffusion area [cm２ ]=(d1 / 2)×(d2 / 2)×3.14

[0078] The whitening time when the test liquid is dropped onto the liquid-permeable sheet side of the absorbent article may be 45 seconds or less, or may be 5 seconds or more and 45 seconds or less.

[0079] The whitening time is the time it takes for the absorbent article to re-form a dry surface after the test liquid has been dropped. The whitening time is measured by placing the absorbent article on a horizontal table with the liquid-permeable sheet facing up, dropping 30 mL of test liquid adjusted to 25±1°C from a 0.4 mm inner diameter opening toward the center of the absorbent article from a position 1 cm above the absorbent article over 10 seconds, and recording the time from the completion of dropping the test liquid until the color of the liquid-permeable sheet changes to a color close to its original color as the whitening time. The term "whitening time" is used here because liquid-permeable sheets are often white, but the whitening time can be measured in a similar manner even when the liquid-permeable sheet is of another color.

[0080] The absorbent article may be, for example, a waterproof nursing sheet, a disposable diaper, a sanitary napkin, a tampon, or a pet sheet.

[0081] The present invention is not limited to the following examples.

[0082] 1. Preparation of Water-Absorbent Resin Particles Example 1 Polymerization Step A round-bottomed cylindrical separable flask (baffle width: 7 mm, baffle length: 10 cm) with an inner diameter of 11 cm and a capacity of 2 L and equipped with four side wall baffles (baffle width: 7 mm, baffle length: 10 cm) was prepared, equipped with a reflux condenser, a dropping funnel, a nitrogen gas inlet tube, and a stirrer. A stirring blade A, the outline of which is shown in FIG. 2, was attached to the stirrer. The stirring blade A had a shaft 200a and a flat plate portion 200b. The flat plate portion 200b was welded to the shaft 200a and had a curved tip. Four slits S extending along the axial direction of the shaft 200a were formed in the flat plate portion 200b. The four slits S were arranged in the width direction of the flat plate portion 200b. The width of the two inner slits S was 1 cm. The width of the two outer slits S was 0.5 cm. The length of the flat plate portion 200b was approximately 10 cm, and the width of the flat plate portion 200b was approximately 6 cm.

[0083] The prepared separable flask was charged with 472.3 g of n-heptane and 1.10 g (1.21% by mass based on the partially neutralized acrylic acid to be subjected to the polymerization reaction) of sorbitan monolaurate (surfactant, Nonion LP-20R, HLB: 8.6, manufactured by NOF Corporation). The mixture in the separable flask was heated to 50°C while being stirred with a stirrer at a rotation speed of 300 rpm, thereby dissolving the sorbitan monolaurate in the n-heptane. The mixture was then cooled to 45°C.

[0084] A 500 mL Erlenmeyer flask was charged with 92.0 g of an 80.5 wt% aqueous acrylic acid solution (1.03 mol of acrylic acid). While cooling with ice from the outside, 147.7 g of a 20.9 wt% aqueous sodium hydroxide solution was added dropwise to neutralize 75 mol% of the acrylic acid. 0.101 g (0.374 mmol) of potassium persulfate was dissolved in the partially neutralized acrylic acid solution formed by the neutralization to form a monomer aqueous solution.

[0085] The resulting monomer aqueous solution was added to the mixture in the separable flask, and the system containing the resulting reaction solution was thoroughly purged with nitrogen. The reaction solution was then stirred with a stirrer at 300 rpm, while the separable flask was immersed in a water bath at 70°C and held in that state for 60 minutes to allow the polymerization reaction to proceed. As the polymerization reaction proceeded, a particulate hydrogel polymer was formed in the reaction solution.

[0086] Aggregation step: The agitator blade A was replaced with agitator blade B having two stages of four inclined paddle blades with a blade diameter of 5 cm, and while stirring at a rotation speed of 1000 rpm, a dispersion containing 0.014 g of amorphous silica particles (flocculant, Oriental Silicas Corporation, Toxil NP-S) and 100 g of n-heptane was added to the reaction solution containing the produced hydrous gel polymer, n-heptane, and surfactant. The separable flask was then immersed in a water bath at 75°C, and the reaction solution was stirred for 10 minutes. Aggregated particles were formed by aggregation of the hydrous gel polymer in the reaction solution.

[0087] To the reaction solution containing the aggregated particles, 0.41 g of an aqueous solution of ethylene glycol diglycidyl ether (ethylene glycol diglycidyl ether (intermediate crosslinking agent): 0.047 mmol) with a concentration of 2% by mass was added. Thereafter, the reaction solution in the separable flask was stirred for 30 minutes while being heated in a water bath at 75°C, thereby allowing intermediate crosslinking to proceed.

[0088] Concentration The reaction solution in the separable flask was heated in an oil bath at 125° C., and 106.1 g of water was extracted from the system by azeotropic distillation of n-heptane and water while refluxing n-heptane.

[0089] Surface crosslinking 4.14 g of a 2 mass% aqueous solution of ethylene glycol diglycidyl ether (ethylene glycol diglycidyl ether (surface crosslinking agent): 0.48 mmol) was mixed with the concentrate formed by removing water. The mixture was kept at an internal temperature of 83±2°C for 2 hours to allow surface crosslinking to proceed in the mixture.

[0090] Drying The mixture after surface cross-linking was heated to 120°C, and water and n-heptane were evaporated until almost no evaporants were distilled from the system, thereby obtaining a powder of dried polymer particles (aggregated particles). This powder was passed through a sieve with an opening of 850 µm, thereby obtaining 90.4 g of water-absorbent resin particles of Example 1.

[0091] Example 2

[0123] 84.9 g of water absorbent resin particles of Example 2 were obtained under the same conditions as those of Example 1 except that the rotation speed of the stirrer during the polymerization reaction was changed to 250 rpm and the amount of water extracted by azeotropic distillation before surface crosslinking was 108.8 g.

[0092] Example 3

[0123] 73.0 g of water absorbent resin particles of Example 3 were obtained under the same conditions as those of Example 1 except that the stirring blade used to stir a reaction liquid during polymerization reaction was changed to stirring blade B having two stages of four inclined paddle blades each having a blade diameter of 5 cm which had been surface-treated with a fluororesin, that the rotation speed of the stirrer during polymerization reaction was changed to 600 rpm, and that the amount of water extracted by azeotropic distillation before surface crosslinking was 107.9 g.

[0093] Example 4 88.5 g of water-absorbent resin particles of Example 4 were obtained under the same conditions as in Example 1, except that the amount of water extracted by azeotropic distillation before surface crosslinking was 108.8 g.

[0094] Example 5 85.9 g of water-absorbent resin particles of Example 5 were obtained under the same conditions as in Example 1, except that the amount of water extracted by azeotropic distillation before surface crosslinking was 111.6 g.

[0095] Example 6 88.9 g of water-absorbent resin particles of Example 6 were obtained under the same conditions as in Example 1, except that the amount of water extracted by azeotropic distillation before surface crosslinking was 113.4 g.

[0096] Example 7

[0123] 82.7 g of water absorbent resin particles of Example 7 was obtained under the same conditions as those of Example 1, except that the amount of an aqueous solution of ethylene glycol diglycidyl ether having a concentration of 2 mass%, as a surface crosslinking agent, was changed to 2.76 g (ethylene glycol diglycidyl ether: 0.32 mmol).

[0097] Example 8

[0123] 80.1 g of water absorbent resin particles of Example 8 was obtained under the same conditions as those of Example 1 except that the amount of water extracted by azeotropic distillation before surface crosslinking was 107.9 g, and the surface crosslinking agent was changed to 4.09 g of an aqueous ethylene glycol diglycidyl ether solution having a concentration of 2.7 mass% (ethylene glycol diglycidyl ether: 0.63 mmol).

[0098] Example 9

[0123] 86.0 g of water absorbent resin particles of Example 9 were obtained under the same conditions as those of Example 1, except that the amount of water extracted by azeotropic distillation before surface crosslinking was 109.8 g, and the surface crosslinking agent was changed to 4.14 g of an aqueous ethylene glycol diglycidyl ether solution having a concentration of 4 mass% (ethylene glycol diglycidyl ether: 0.95 mmol).

[0099] Example 10

[0222] 79.3 g of water absorbent resin particles of Example 10 were obtained under the same conditions as those of Example 1, except that the amount of water extracted by azeotropic distillation before surface crosslinking was 117.0 g, and the surface crosslinking agent was changed to 4.12 g of an aqueous ethylene glycol diglycidyl ether solution having a concentration of 6.7 mass% (ethylene glycol diglycidyl ether: 1.59 mmol).

[0100] Example 11

[0123] 90.3 g of water absorbent resin particles of Example 11 was obtained under the same conditions as those of Example 1, except that after the aggregation step, intermediate crosslinking was not performed, and the amount of water extracted by azeotropic distillation before surface crosslinking was 107.0 g.

[0101] Example 12

[0123] 86.7 g of water absorbent resin particles of Example 12 were obtained under the same conditions as those of Example 1, except that 106.1 g of water was extracted out of the system by azeotropic distillation before surface crosslinking, and then 0.204 g of an aqueous solution of pentasodium diethylenetriaminepentaacetate having a concentration of 45 mass % (pentasodium diethylenetriaminepentaacetate: 0.182 mmol) was added before the addition of a surface crosslinking agent.

[0102] Example 13

[0123] 87.7 g of water absorbent resin particles of Example 13 were obtained under the same conditions as those of Example 1, except that 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, 0.204 g of an aqueous solution of pentasodium diethylenetriaminepentaacetate having a concentration of 45 mass % (pentasodium diethylenetriaminepentaacetate: 0.182 mmol) was added before the addition of a surface crosslinking agent.

[0103] Comparative Example 1

[0223] 86.1 g of water absorbent resin particles of Comparative Example 1 were obtained under the same conditions as in Example 3 except that the rotation speed of the stirrer during polymerization reaction was changed to 700 rpm, and the amount of water extracted by azeotropic distillation before surface crosslinking was 106.1 g.

[0104] Comparative Example 2 81.9 g of water absorbent resin particles of Comparative Example 2 were obtained under the same conditions as in Example 3 except that the rotation speed of the stirrer during the polymerization reaction was changed to 1000 rpm, and the amount of water extracted by azeotropic distillation before surface crosslinking was 110.7 g.

[0105] Comparative Example 3

[0123] 86.0 g of water absorbent resin particles of Comparative Example 3 were obtained under the same conditions as those of Example 1, except that 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 ethylene glycol diglycidyl ether solution having a concentration of 4 mass% (ethylene glycol diglycidyl ether: 0.95 mmol).

[0106] Comparative Example 4 77.2 g of water absorbent resin particles of Comparative Example 4 were obtained under the same conditions as those of Example 1, except that amorphous silica particles for forming aggregated particles were not added after the polymerization reaction, and 107.9 g of water was extracted out of the system while refluxing n-heptane by azeotropic distillation of n-heptane and water.

[0107] Comparative Example 5 Polymerization Step <First-Stage Polymerization Reaction> A round-bottomed, cylindrical, separable flask with an inner diameter of 11 cm and a capacity of 2 L was prepared, equipped with a reflux condenser, a dropping funnel, a nitrogen gas inlet tube, and a stirrer. A stirring blade B was attached to the stirrer. 293 g of n-heptane and 0.736 g of maleic anhydride-modified ethylene-propylene copolymer (hydrophobic polymer dispersant, Mitsui Chemicals, Inc., Hiwax 1105A) were placed in the separable flask. The mixture in the separable flask was heated to 80°C while stirring with the stirrer at a rotation speed of 300 rpm, thereby dissolving the hydrophobic polymer dispersant in n-heptane. The mixture was then cooled to 50°C.

[0108] A 300 mL beaker was charged with 92.0 g of an 80.5 wt% aqueous acrylic acid solution (1.03 mol of acrylic acid). 147.7 g of a 20.9 wt% aqueous sodium hydroxide solution was added dropwise to the beaker while cooling with ice from the outside, thereby neutralizing 75 mol% of the acrylic acid. 0.092 g of hydroxyethyl cellulose (thickener, Sumitomo Seika Chemicals Co., Ltd., HECAW-15F), 0.0736 g of potassium persulfate (water-soluble radical polymerization initiator, 0.272 mmol), and 0.010 g of ethylene glycol diglycidyl ether (internal crosslinking agent, 0.057 mmol) were dissolved in the partially neutralized acrylic acid solution formed by neutralization to prepare a first-stage monomer aqueous solution.

[0109] The first-stage monomer aqueous solution was added to the mixture in the separable flask, and the resulting reaction solution was stirred for 10 minutes using a stirrer at 300 rpm. Subsequently, a surfactant solution containing 6.62 g of n-heptane and 0.736 g of sucrose stearate (surfactant, HLB: 3, Mitsubishi Chemical Foods Corporation, Ryoto Sugar Ester S-370) (0.81% by mass relative to the partially neutralized acrylic acid used in the first-stage polymerization reaction, and 0.58% by mass relative to the partially neutralized acrylic acid used in the second-stage polymerization reaction) was further added. While stirring the reaction solution using a stirrer at 550 rpm, the system was thoroughly purged with nitrogen. The separable flask was then immersed in a 70°C water bath and maintained there for 60 minutes to allow the polymerization reaction to proceed. A first-stage polymerization slurry was formed as a result of the polymerization reaction.

[0110] <Second-stage polymerization reaction> 128.8 g of an 80.5 wt% aqueous acrylic acid solution (acrylic acid: 1.44 mol) was placed in a 500 mL beaker. While cooling with ice from the outside, 159.0 g of a 27 wt% aqueous sodium hydroxide solution was added dropwise to neutralize 75 mol% of the acrylic acid. 0.090 g (0.333 mmol) of potassium persulfate and 0.0116 g of ethylene glycol diglycidyl ether (internal crosslinking agent, 0.067 mmol) were dissolved in the partially neutralized acrylic acid solution formed by neutralization to prepare a second-stage aqueous monomer solution.

[0111] The separable flask was cooled to 25°C while stirring the first-stage polymerization slurry with a stirrer at a rotation speed of 1000 rpm. Thereafter, the entire amount of the second-stage aqueous monomer solution was added to the first-stage polymerization slurry, and the system was purged with nitrogen over 30 minutes. The separable flask was again immersed in a water bath at 70°C, and this state was maintained for 60 minutes to allow the polymerization reaction to proceed. A reaction liquid containing a particulate hydrogel polymer in which polymer particles were aggregated was obtained by the polymerization reaction.

[0112] After the polymerization reaction, 0.589 g of a 45% by mass aqueous solution of pentasodium diethylenetriaminepentaacetate (pentasodium diethylenetriaminepentaacetate: 0.527 mmol) was added to the reaction solution containing the hydrous gel polymer under stirring. The separable flask was then immersed in an oil bath set at 125°C, and 256.8 g of water was extracted from the system by azeotropic distillation of n-heptane and water while refluxing the n-heptane.

[0113] To the concentrate (hydrous gel polymer) after water removal, 4.42 g of a 2 mass% aqueous solution of ethylene glycol diglycidyl ether (ethylene glycol diglycidyl ether (surface cross-linking agent): 0.507 mmol) was added. The internal temperature of the separable flask was maintained at 83±2°C for 2 hours to allow surface cross-linking to proceed in the mixture.

[0114] Drying The surface-crosslinked mixture was heated in an oil bath at 125°C, and water and n-heptane were evaporated until almost no evaporants were distilled from the system, thereby obtaining a powder of dried polymer particles. This powder was passed through a sieve with an opening of 850 μm. The polymer particles that had passed through the sieve were mixed with 0.5% by mass of amorphous silica particles (lubricant, Oriental Silicas Corporation, Toxil NP-S) relative to the mass of the polymer particles, thereby obtaining 230.2 g of water-absorbent resin particles of Comparative Example 5 in which amorphous silica particles as a lubricant were attached to the polymer particles.

[0115] Reference Example 1 39.5 g of water-absorbent resin particles of Reference Example 1 was obtained under the same conditions as in Example 1, except that polymerization was performed at a rotation speed of the stirrer of 200 rpm. 50 mass % or more of particles that did not pass through a sieve with an opening of 850 μm were present in the obtained water-absorbent resin particles. Therefore, it was determined that the polymerization reaction was unstable, and no evaluation was performed.

[0116] 2. Median particle diameter before aggregation A reaction liquid containing a hydrogel polymer was formed by polymerization reaction under the same conditions as in each Example or Comparative Example. The reaction liquid after the polymerization reaction was heated in an oil bath at 125°C, and water was extracted from the system by azeotropic distillation of n-heptane and water while refluxing the n-heptane until the temperature inside the system reached 89°C. The remaining n-heptane was evaporated at 125°C to obtain a powder of dried polymer particles before aggregation. The median particle diameter of this polymer particle was measured by the method described below, and this value was taken as the median particle diameter before aggregation.

[0117] Table 1 shows the conditions of the polymerization step, the median particle size before aggregation, whether or not an aggregation step was performed, and the amounts of surfactant, intermediate crosslinking agent, and surface crosslinking agent per mole of charged acrylic acid. Table 1 also shows the moisture content of the concentrate subjected to surface crosslinking. The moisture content was calculated using the following formula: Moisture 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 polymer before surface crosslinking. In each example and comparative example, Ww was determined by subtracting the mass of water extracted by azeotropic distillation from the sum of the mass of water contained in the aqueous monomer 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, calculated from the charged amounts of acrylic acid and its neutralized product, radical polymerization initiator, and intermediate crosslinking agent.

[0118]

[0119] 3. Evaluation of Water-Absorbent Resin Particles The water-absorbent resin particles of Examples and Comparative Examples were evaluated by the following method. Unless otherwise specified, measurements were carried out in an environment of a temperature of 25±2°C and a humidity of 50±10%. The evaluation results are shown in Tables 2 and 3.

[0120] Water Retention Capacity A cotton bag (membrane broadcloth No. 60, 100 mm wide x 200 mm long) containing 2.0 g of water-absorbent resin particles was placed in a 500 mL beaker. 500 g of saline solution was poured into the cotton bag containing the water-absorbent resin particles all at once, taking care not to allow the bag to become lumpy, and the top of the cotton bag was tied with a rubber band. The cotton bag was left to stand for 30 minutes to allow the water-absorbent resin particles in the cotton bag to swell. The swollen gel in the cotton bag was then dehydrated for 1 minute using a dehydrator (manufactured by Kokusan Co., Ltd., product number: H-122) set to a centrifugal force of 167 G. The mass Wa (g) of the cotton bag containing the swollen gel after dehydration was measured. The same procedure was performed on a cotton bag containing no water-absorbent resin particles, and the empty mass Wb (g) of the cotton bag when wet was measured. The water retention capacity of the water-absorbent resin particles in saline solution was calculated using the following formula: Water retention capacity [g / g] = (Wa - Wb) / 2.0

[0121] Water Absorption Under Load The water absorption under load of the water-absorbent resin particles in physiological saline was measured using a measuring device Y shown in FIG. 3. The measuring device Y is composed of a burette unit 61, a conduit 62, a measurement table 63, and a measurement unit 64 placed on the measurement table 63. The burette unit 61 has a burette 61a extending vertically, a rubber stopper 61b arranged at the upper end of the burette 61a, a cock 61c arranged at the lower end of the burette 61a, an air introduction tube 61d having one end extending into the burette 61a near the cock 61c, and a cock 61e arranged at the other end of the air introduction tube 61d. The conduit 62 is attached between the burette unit 61 and the measurement table 63. The inner diameter of the conduit 62 is 6 mm. A hole with a diameter of 2 mm is drilled in the center of the measurement table 63, and the conduit 62 is connected to it. The measuring unit 64 has a cylinder 64a (made of acrylic resin (Plexiglas)), a nylon mesh 64b adhered to the bottom of the cylinder 64a, and a weight 64c. The inner diameter of the cylinder 64a is 20 mm. The openings of the nylon mesh 64b are 75 μm (200 mesh). During measurement, the water-absorbent resin particles 11a to be measured are uniformly scattered on the nylon mesh 64b. The diameter of the weight 64c is 19 mm, and the mass of the weight 64c is 59.8 g. The weight 64c is placed on the water-absorbent resin particles 11a, and can apply a load of 2.07 kPa to the water-absorbent resin particles 11a.

[0122] 0.100 g of water-absorbent resin particles 11a were placed in the cylinder 64a of the measuring device Y. A weight 64c was placed on the water-absorbent resin particles 11a, and the cocks 61c and 61e were opened to start the measurement. Air of the same volume as the physiological saline solution absorbed by the water-absorbent resin particles 11a was quickly and smoothly supplied into the burette 61a through the air inlet tube 61d. Therefore, the decrease in the level of the physiological saline solution inside the burette 61a corresponds to the amount of physiological saline solution absorbed by the water-absorbent resin particles 11a. The scale of the burette 61a is marked from top to bottom in increments of 0 mL to 0.5 mL. The scale Va of the burette 61a before the start of water absorption and the scale Vb of the burette 61a 60 minutes after the start of water absorption were read as the physiological saline solution level, and the water absorption amount under load was calculated using the following formula: Water absorption amount under load [mL / g] = (Vb - Va) / 0.1

[0123] Water absorption rate The water absorption rate of the water-absorbent resin particles was measured by the Vortex method. 50±0.1 g of saline solution and a magnetic stirrer bar (8 mmφ×30 mm without ring) were placed in a 100 mL beaker. The beaker was immersed in a thermostatic water bath to adjust the liquid temperature to 25±0.2 ° C. Next, the beaker was placed on a magnetic stirrer, and the saline solution was stirred at a rotation speed of 600 rpm to generate a vortex, and 2.0 g of water-absorbent resin particles were quickly added thereto. The time (seconds) from the time the water-absorbent resin particles were added to the time the vortex on the liquid surface converged due to water absorption by the water-absorbent resin particles was measured, and this was taken as the water absorption rate of the water-absorbent resin particles.

[0124] Median Particle Diameter The water-absorbent resin particles were passed through a JIS Z 8801-1 standard sieve having a mesh size of 250 μm. When the amount remaining on the sieve relative to the total amount was 50 mass% or more, the median particle diameter was measured using the following combination of sieves (A); when the amount remaining on the sieve relative to the total amount was less than 50 mass%, the median particle diameter was measured using the following combination of sieves (B). (A) The JIS standard sieves were combined in the following order from top to bottom: a sieve with a mesh size of 710 μm, a sieve with a mesh size of 600 μm, a sieve with a mesh size of 500 μm, a sieve with a mesh size of 425 μm, a sieve with a mesh size of 300 μm, a sieve with a mesh size of 250 μm, a sieve with a mesh size of 150 μm, and a tray. (B) JIS standard sieves were arranged in the following order from top to bottom: a sieve with a mesh size of 425 μm, a sieve with a mesh size of 250 μm, a sieve with a mesh size of 180 μm, a sieve with a mesh size of 150 μm, a sieve with a mesh size of 106 μm, a sieve with a mesh size of 75 μm, a sieve with a mesh size of 45 μm, and a tray.

[0125] Water-absorbent resin particles were placed in the sieve located at the top stage, and the water-absorbent resin particles were classified by shaking for 20 minutes using a continuous fully automatic ultrasonic vibration sieving measuring instrument (Robot Sifter RPS-205, manufactured by Seishin Enterprise Co., Ltd.). After classification, the ratio (mass percentage) of the mass of the water-absorbent resin particles remaining on each sieve to the total amount was calculated. By integrating the proportions of the fractions in order from the largest particle size, the relationship between the sieve openings and the integrated value of the proportion of the water-absorbent resin particles remaining on the sieve was plotted on logarithmic probability paper. By connecting the plots on the probability paper with a straight line, the particle size corresponding to an integrated mass percentage of 50% by mass was determined, and this value was taken as the median particle size.

[0126] Specific Surface Area By classification using a sieve with a 400 μm opening and a 300 μm opening, a fraction of the water-absorbent resin particles that passed through the 400 μm opening sieve and remained on the 300 μm opening sieve was obtained, and this fraction was used as a sample for measuring the specific surface area. This sample was dried under degassing conditions of vacuum evacuation at 100° C. for 16 hours. The adsorption isotherm after drying was measured at a temperature of 77 K by a method using krypton gas as the adsorption gas. A specific surface area measuring device (AUTOSORB-1, manufactured by Quantachrome) was used to measure the adsorption isotherm. The specific surface area (BET specific surface area) of the water-absorbent resin particles was determined from a multi-point BET plot. In the case of the water-absorbent resin particles of Comparative Example 4 containing amorphous silica particles as a lubricant, the specific surface area was measured using polymer particles before the amorphous silica particles were attached.

[0127] Water Absorption Capacity 500 g of physiological saline solution at 25±2°C was placed in a 500 mL beaker. While stirring at 600 rpm using a magnetic stir bar (8 mm diameter x 30 mm length, without ring), 2.0 g of water-absorbent resin particles were dispersed in the physiological saline solution to prevent the formation of lumps. The physiological saline solution was left to stand for 60 minutes while stirring, thereby obtaining a dispersion containing a swollen gel formed by the swelling of the water-absorbent resin particles. This dispersion was passed through a JIS Z 8801-1 standard sieve with a mass Wa [g] and a mesh size of 75 μm. The sieve on which the swollen gel remained was tilted at an angle of approximately 30 degrees relative to the horizontal and left to stand for 30 minutes to remove excess water. Next, the mass Wb [g] of the sieve on which the swollen gel remained was measured. The water absorption capacity Wc [g / g] of the water-absorbent resin particles in physiological saline solution was calculated using the following formula: Water absorption amount Wc [g / g] = (Wb-Wa) / 2.0

[0128] Dry Powder Liquid Passage Figure 4 is a schematic diagram showing a method for measuring dry powder liquid passage. One opening of a cylinder 41 (inner diameter: 26 mm, outer diameter: 39 mm) was blocked with a polyamide resin net 43 (250 mesh, Nippon Tokushu Orimono, product number NNO. 250T), and 0.450 g of water-absorbent resin particles 11a was placed through the other opening. The cylinder 41 was stood vertically with the opening blocked with the net 43 facing downward, and the water-absorbent resin particles 11a were uniformly arranged on the net 43. A cylindrical piston 42 (inner diameter: 19 mm, outer diameter: 25.6 mm) with one opening blocked with a polyamide resin net 44 (250 mesh, Nippon Tokushu Orimono, product number NNO. 250T) was inserted into the cylinder 41 with the net 44 in contact with the water-absorbent resin particles 11a.

[0129] A wire 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 water-absorbent resin particles 11a and a piston 42 were inserted was placed on the wire mesh 46. Next, 20 g of physiological saline was poured into the cylinder 41 from above the piston 42 over 2 seconds. 10 seconds after the entire amount of physiological saline was poured, the cylinder 41 was quickly removed. The total mass Wa (g) of the physiological saline 65 that had passed through the water-absorbent resin particles 11a, the petri dish 45, and the wire mesh 46 was measured. The dry powder liquid passing amount was calculated from the following formula: Dry powder liquid passing amount [g]=Wa-Wb-Wc

[0130] Lock-up Test FIG. 5 is a schematic diagram showing a method for a lock-up test. 0.100 g of water-absorbent resin particles 11 a were spread in a layer on the bottom surface of an acrylic resin cylinder 70 (inner diameter x 2.0 cm, outer diameter 3.0 cm, depth y 8.0 cm) placed on a horizontal table, so that the surface was parallel to the bottom surface of the cylinder 70. Thereafter, 20 g of physiological saline was poured into the cylinder 70 over 2 seconds. 10 seconds or 20 seconds after the entire amount of physiological saline 65 was poured, the maximum height h (cm) of the formed swollen water-absorbent resin particles 11 a from the bottom surface of the cylinder 70 was read using a vernier caliper (Niigata Seiki SK standard pocket caliper, 100 mm). The maximum height h after 10 seconds was taken as the 10-second lock-up value, and the maximum height h after 20 seconds was taken as the 20-second lock-up value.

[0131] 4. Preparation of absorbent article: A 20 cm x 60 cm sized absorbent article with a basis weight of 16 g / m ２ A first shape-retaining member (tissue) of 10 cm was prepared. 5.4 g of hydrophilic fibers (ground pulp) were deposited on this first shape-retaining member by air-pressure forming using an airflow mixer (Autech Co., Ltd., Pad Former) to form a hydrophilic fiber layer covering the entire upper surface of the first shape-retaining member. 10 cm sections were cut off from both longitudinal ends of the laminate of the first shape-retaining member and the hydrophilic fiber layer. The remaining laminate was divided into two equal parts, yielding two laminates measuring 20 cm x 20 cm.

[0132] Each of the two laminates was sprayed with 1.0 g of water evenly using a spray bottle, and then a load of 500 kPa was applied for 30 seconds. Thereafter, water-absorbent resin particles (3.0 g) were evenly spread on the hydrophilic fiber layer to form a water-absorbent resin layer. ２ The second shape-retaining member (tissue) was layered on top of the water-absorbent resin layer to obtain a laminate having, from the bottom, the first shape-retaining member, the hydrophilic fiber layer, the water-absorbent resin layer, and the second shape-retaining member.

[0133] On the second shape-retaining member of this laminate, an air-through nonwoven fabric (KNH Enterprise Co., Ltd., basis weight: 25 g / m) having a size of 20 cm x 20 cm and coated with a hot melt adhesive (ME-765E, Henkel Japan Co., Ltd.) was placed. ２ ) was laminated in a direction in which the hot melt adhesive was in contact with the second shape-retaining member, thereby obtaining an absorbent article. 0.1 g of the hot melt adhesive was applied to the air-through nonwoven fabric so as to form a spiral stripe pattern of 20 stripes arranged at 10 mm intervals. In the obtained sheet-like absorbent article (20 cm x 20 cm), the basis weight of the water-absorbent resin particles was 75 g / m ２ The weight of the hydrophilic fiber (crushed pulp) is 45 g / m ２ It was.

[0134] 5. Evaluation of absorbent articles Test solution A test solution was prepared by dissolving 45.0 g of NaCl in 4955.0 g of ion-exchanged water and further adding a small amount of Blue No. 1. This test solution was used for the following evaluations.

[0135] Amount of backflow after dripping: The absorbent article was placed on a horizontal table with the air-through nonwoven fabric facing upward. 30 mL of test liquid adjusted to 25±1°C was dripped from 1 cm above the absorbent article toward the center of the absorbent article over 10 seconds using a pump (INTEGRA Biosciences, DOSE IT P910) connected to an inlet with an inner diameter of 0.4 mm. Immediately after dripping of the test liquid, a pre-measured mass of approximately 75 g of filter paper (ADVANTEC No. 51A, 100 mm x 100 mm) was placed on the center of the absorbent article, and a weight (100 mm x 100 mm base, 5.0 kg) equivalent to a pressure of approximately 0.7 psi (4.8 kPa) was quickly placed on top of it, and the load was applied for 3 seconds. Thereafter, the weight and the filter paper were removed, and the mass of the test liquid absorbed by the filter paper was measured and recorded as the amount of backflow after dropping [g].

[0136] Amount of backflow after 1 minute The amount of backflow [g] after 1 minute was measured in the same manner as for backflow after dropping, except that the filter paper was placed on the center of the absorbent article 1 minute after the completion of dropping the test liquid.

[0137] Spreading Area The absorbent article was placed on a horizontal table with the air-through nonwoven fabric facing upward. 30 mL of test liquid adjusted to 25±1°C was dripped from 1 cm above the absorbent article toward the center of the absorbent article over 10 seconds using a pump (DOSE IT P910, manufactured by INTEGRA Biosciences) connected to an inlet with an inner diameter of 0.4 mm. The dripped test liquid diffused horizontally within the absorbent article. Figure 6 is a plan view showing an example of the state in which the test liquid diffused within the absorbent article. Two minutes after the completion of dripping of the test liquid, the maximum length d1 in the longitudinal direction of the absorbent article and the maximum length d2 in the lateral direction of the absorbent article were measured for the region DA where the test liquid diffused within the absorbent article 50. The diffusion area [cm ] was calculated using the following formula: ２ ] was calculated. ２ ]=(d1 / 2)×(d2 / 2)×3.14

[0138] Whitening Time: The absorbent article was placed on a horizontal table with the air-through nonwoven fabric facing upward. 30 mL of test liquid adjusted to 25±1°C was dripped from 1 cm above the absorbent article toward the center of the absorbent article over 10 seconds using a pump (DOSE IT P910, manufactured by INTEGRA Biosciences) connected to an inlet with an inner diameter of 0.4 mm. A portion of the test liquid was temporarily retained in the air-through nonwoven fabric immediately after dripping and then absorbed into the absorbent body. As the test liquid migrated to the absorbent body, the color of the surface of the air-through nonwoven fabric of the absorbent article changed from the dark blue color caused by the test liquid to its original white color. The time from the completion of dripping of the test liquid until the surface color changed to a color close to white was recorded as the whitening time. Figure 7 is a photograph showing an example of an absorbent article to which the test liquid had been dripped. (a) is a photograph taken immediately after the test liquid was dropped, and (b) is a photograph taken after the air-through nonwoven fabric had turned white.

[0139]

[0140]

[0141] As shown in Table 3, it was confirmed that the absorbent articles containing the water-absorbent resin particles of each Example exhibited a narrow diffusion area when absorbing water, and were able to quickly form a dry surface with little backflow. The amount of backflow after 1 minute in Comparative Example 2 could not be measured because the whitening time exceeded 1 minute and the measurement of the amount of backflow after 1 minute was performed after the whitening time was confirmed.

[0142] According to the method for producing water-absorbent resin particles of the present disclosure, an absorbent article can be provided which has a small area for diffusing absorbed liquid and can quickly form a dry surface with little backflow, thereby reducing the frequency of replacing the absorbent article (e.g., pet sheets).As a result, the water-absorbent resin particles of the present disclosure can reduce the amount of materials (e.g., natural raw materials (biomass resources) such as pulp and nonwoven fabric) used other than the water-absorbent resin particles and the water-absorbent resin particles contained in the absorbent article, thereby contributing to the conservation of the global environment.

[0143] 10...absorbent body, 11...water-absorbent resin layer, 11a...water-absorbent resin particles, 12...hydrophilic fiber layer, 21...first shape-retaining member, 22...second shape-retaining member, 30...liquid-permeable sheet, 35...adhesive, 50...absorbent article, 65...physiological saline solution

Claims

1. In a reaction solution containing a water-soluble ethylenically unsaturated monomer, water, a dispersion medium, and a surfactant having an HLB of 7 to 16, the water-soluble ethylenically unsaturated monomer is polymerized by reverse-phase suspension polymerization to form a polymer of the water-soluble ethylenically unsaturated monomer and a particulate, water-containing, water-containing, hydrated gel polymer. The water-containing gel polymer is aggregated in the reaction solution to form aggregated particles containing a plurality of the water-containing gel polymers, A concentrate is formed by removing a portion of the water from the reaction solution. The aggregated particles are surface-crosslinked in a mixture containing the concentrate and the surface crosslinking agent, Includes, The reaction solution does not contain an internal crosslinking agent for crosslinking the polymer, or the amount of the internal crosslinking agent in the reaction solution is 0.093 mmol or less per mole of the water-soluble ethylenically unsaturated monomer. The polymer particles formed by removing water from the water-containing gel-like polymer before the formation of the aggregated particles have a medium particle size of 150 μm or more. The mixture is formed by mixing the concentrate containing water at a moisture content of 15% by mass or more and 50% by mass or less based on the mass of the polymer with the surface crosslinking agent. A method for producing superabsorbent polymer particles.

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

3. The method according to claim 1 or 2, wherein the amount of the surfactant is 2.0% by mass or less based on the water-soluble ethylenically unsaturated monomer.

4. Absorption amount of 60 g / g or more relative to physiological saline, A dry powder volume of 1.0 g or more relative to physiological saline, A lock-up value of 10 seconds for 1.5 cm or more, This indicates a lock-up value of 20 seconds for a length of 4.0 cm or less. Water-absorbing resin particles.

5. The water-absorbing resin particles according to claim 4, having an intermediate particle size of 300 μm or more and 600 μm or less.

6. 0.05m ２ / g or more 0.22m ２ The water-absorbing resin particles according to claim 4, having a specific surface area of ​​less than or equal to / g.

7. An absorbent body comprising water-absorbing resin particles according to any one of claims 4 to 6.

8. Liquid permeable sheet, The absorbent according to claim 7 is provided on the inside of the liquid permeable sheet, An absorbent article equipped with [a specific feature].

Images