WO2025204969A1 - Water-absorbent resin particles, absorber, and absorbent article - Google Patents

Abstract

Provided are water-absorbent resin particles that, when an absorber containing the water-absorbent resin particles absorbs water, have high resistance to deformation of the absorber.　These water-absorbent resin particles have a gel diffusion distance greater than 0.0 cm and no greater than 2.0 cm as measured using the following method. (Method for measuring gel diffusion distance) A stainless-steel petri dish is installed on a movable base plate. A cylindrical container (A) provided with a stainless-steel mesh that has a mesh opening of 38 μm and covers an upper opening is installed within the stainless-steel petri dish. Next, a cylindrical container (B) of which both axial-direction ends are open is installed at the center of the stainless-steel mesh, and the cylindrical container (B) is fixed by a clamp. The stainless-steel petri dish has an inside diameter of 75 mm and a height of 20 mm. The cylindrical container (A) has an inside diameter of 60 mm, an outside diameter of 70 mm, and a height of 60 mm. The cylindrical container (B) has an inside diameter of 20 mm, an outside diameter of 30 mm, and a height of 60 mm. Next, 0.20 g of the water-absorbent resin particles is uniformly sprayed inside the cylindrical container (B), and the water-absorbent resin particles are arranged on the stainless-steel mesh. Then, from the lower end of the cylindrical container (B) to a height of 5.0 cm or greater, and from the upper part of the center position of the inside diameter, 30 mL of physiological saline at 0.9 mass% is charged at a constant rate of 20 mL/min. After 20 seconds from the completion of the charging, the movable base plate is lowered by 5.0 cm at a speed of 1.2 cm/s while the cylindrical container (B) remains fixed. At this time, a gel diffusion distance is measured for a portion where the stainless-steel mesh is in contact with a swollen gel generated through absorption of water by the water-absorbent resin particles. The gel diffusion diameter is obtained by measuring the maximum distance along a straight line passing through the center point of the swollen gel on the stainless-steel mesh when the swollen gel is positioned within the cylindrical container (B), said straight line connecting the end parts of the swollen gel that is spread onto the stainless-steel mesh by lowering the movable base plate, and the gel diffusion diameter (cm) is set to a value obtained by subtracting 2.0 cm of the inside diameter (diameter) of the cylindrical container (B) from the maximum distance Wa. Gel diffusion distance (cm) = maximum distance Wa − 2.0

Description

Water-absorbent resin particles, absorbent body and absorbent article

The present invention relates to water-absorbent resin particles, absorbents, and absorbent articles; more specifically, it relates to water-absorbent resin particles that constitute absorbents suitable for use in sanitary materials such as disposable diapers, sanitary napkins, and incontinence pads, as well as absorbents and absorbent articles that use these water-absorbent resin particles.

In recent years, water-absorbent resin particles have been widely used in hygiene materials such as disposable diapers, sanitary napkins, and incontinence pads.

Among such water-absorbent resin particles, cross-linked polymers of water-soluble ethylenically unsaturated monomers, more specifically cross-linked polymers of partially neutralized polyacrylic acid, have excellent water absorption capabilities, and because the raw material, acrylic acid, is easily available industrially, they can be produced at low cost with consistent quality, and are less susceptible to spoilage and deterioration. These advantages make them preferred water-absorbent resin particles (see, for example, Patent Document 1).

Absorbent articles such as disposable diapers, sanitary napkins, and incontinence pads are primarily composed of an absorbent core located in the center that absorbs and retains bodily fluids such as urine and menstrual blood excreted from the body, a liquid-permeable surface sheet (top sheet) located on the side that comes into contact with the body, and a liquid-impermeable back sheet (back sheet) located on the opposite side that comes into contact with the body. Furthermore, the absorbent core is typically composed of hydrophilic fibers such as pulp and water-absorbent resin particles.

Japanese Patent Application Publication No. 3-227301

In such absorbent articles, the absorbent body that has absorbed liquid may be subjected to loads due to deformations such as bending, and this deformation can easily cause damage such as cracks in the absorbent body. Therefore, it is desirable for the absorbent body to be highly resistant to deformation.

Under these circumstances, the primary objective of the present invention is to provide water-absorbent resin particles that can provide higher resistance to deformation of absorbent bodies that have absorbed water. The present invention also aims to provide absorbent bodies and absorbent articles that utilize these water-absorbent resin particles.

The present inventors have conducted extensive research to solve the above-mentioned problems. During their research, the inventors focused on the gel diffusion distance, which is an indicator of the binding strength of absorbed water-absorbent resin particles, and discovered that there is a correlation between the gel diffusion distance and the resistance of an absorbent body containing water-absorbent resin particles to deformation when the absorbent body absorbs water. In other words, the inventors have conducted extensive research into means for providing an absorbent body with higher resistance to deformation when it absorbs water, and have found that water-absorbent resin particles with a gel diffusion distance set within a specific range have suitable resistance to deformation when the absorbent body containing the water-absorbent resin particles absorbs water. The present invention was completed based on this knowledge and through further extensive research.

That is, the present invention provides the following configuration.
Item 1. Water-absorbent resin particles having a gel diffusion distance, measured by the following method, of more than 0.0 cm and 2.0 cm or less.
(Method for measuring gel diffusion distance)
A stainless steel petri dish is placed on a movable base. A cylindrical container (A) equipped with a stainless steel wire mesh with a mesh size of 38 μm covering the upper opening is placed in the stainless steel petri dish. Next, a cylindrical container (B) with both axial ends open is placed in the center of the stainless steel wire mesh, and the cylindrical container (B) is fixed with a clamp. The stainless steel petri dish has an inner diameter of 75 mm and a height of 20 mm. The cylindrical container (A) has an inner diameter of 60 mm, an outer diameter of 70 mm, and a height of 60 mm. The cylindrical container (B) has an inner diameter of 20 mm, an outer diameter of 30 mm, and a height of 60 mm. Next, 0.20 g of water-absorbent resin particles are uniformly dispersed inside the cylindrical container (B), and the water-absorbent resin particles are placed on the stainless steel wire mesh. Next, 30 mL of 0.9% by mass physiological saline is poured into the cylindrical container (B) from above the center of the inner diameter at a height of 5.0 cm above the bottom end at a constant rate of 20 mL/min. 20 seconds after the end of the addition, while keeping the cylindrical container (B) fixed, the movable plate is lowered 5.0 cm at a speed of 1.2 cm/s. At this time, the gel diffusion distance is measured for the portion where the swollen gel formed by the water-absorbent resin particles absorbing water and the stainless steel wire mesh are in contact. The gel diffusion distance is measured by measuring the maximum distance of a straight line passing through the center point of the swollen gel when it was located in the cylindrical container (B) and connecting the ends of the swollen gel spread on the stainless steel wire mesh by lowering the movable plate, and the gel diffusion distance (cm) is calculated by subtracting the inner diameter (diameter) of the cylindrical container (B) of 2.0 cm from the maximum distance Wa.
Gel diffusion distance (cm) = maximum distance Wa - 2.0
Item 2. The water-absorbent resin particles according to Item 1, wherein the water-absorbent resin particles have a saline flow conductivity (SFC, ×10 ⁻⁷ cm ³ ·s/g) of 50.0 or less.
Item 3. The water-absorbent resin particles according to Item 1, wherein the water-absorbent resin particles have an unpressurized DW 3-minute value of 15 mL/g or more.
Item 4. An absorbent body comprising the water-absorbent resin particles according to any one of Items 1 to 3.
Item 5. An absorbent article comprising the absorbent body according to Item 4.

The present invention provides water-absorbent resin particles that are highly resistant to deformation of an absorbent body when the absorbent body containing the water-absorbent resin particles absorbs water. The present invention also provides absorbent bodies and absorbent articles that utilize the water-absorbent resin particles.

FIG. 1 is a schematic diagram of an apparatus for measuring the gel diffusion distance of water-absorbent resin particles. 1 is a schematic diagram of an apparatus for measuring saline flow conductivity (SFC) of water-absorbent resin particles. FIG. 1 is a schematic diagram of an apparatus used to measure the 3-minute value of the no-pressure DW of water-absorbent resin particles. 4(a) and 4(b) are schematic diagrams for explaining a method for evaluating the deformation resistance of an absorbent body that has absorbed water.

In this specification, the term "comprising" includes "consisting essentially of" and "consisting of". In addition, in this specification, "(meth)acrylic" means "acrylic or methacrylic", "(meth)acrylate" means "acrylate or methacrylate", and "(poly)" means with or without the prefix "poly". In addition, in this specification, "water-soluble" means exhibiting a solubility of 5% by mass or more in water at 25°C.

In this specification, numbers connected by "~" mean a numerical range that includes the numbers before and after "~" as the lower and upper limits. When multiple lower limits and multiple upper limits are listed separately, any lower limit and upper limit may be selected and connected by "~".

1. Water-absorbent resin particles The water-absorbent resin particles of the present invention have a gel diffusion length, measured by the following method, of more than 0.0 cm and not more than 2.0 cm. By virtue of having such a characteristic, the water-absorbent resin particles of the present invention have high resistance to deformation of an absorbent body containing the water-absorbent resin particles when the absorbent body absorbs water. The water-absorbent resin particles of the present invention will be described in detail below.
(Method for measuring gel diffusion distance)
A stainless steel petri dish is placed on a movable base. A cylindrical container (A) equipped with a stainless steel wire mesh with a mesh size of 38 μm covering the upper opening is placed in the stainless steel petri dish. Next, a cylindrical container (B) with both axial ends open is placed in the center of the stainless steel wire mesh, and the cylindrical container (B) is fixed with a clamp. The stainless steel petri dish has an inner diameter of 75 mm and a height of 20 mm. The cylindrical container (A) has an inner diameter of 60 mm, an outer diameter of 70 mm, and a height of 60 mm. The cylindrical container (B) has an inner diameter of 20 mm, an outer diameter of 30 mm, and a height of 60 mm. Next, 0.20 g of water-absorbent resin particles are uniformly dispersed inside the cylindrical container (B), and the water-absorbent resin particles are placed on the stainless steel wire mesh. Next, 30 mL of 0.9% by mass physiological saline is poured into the cylindrical container (B) from above the center of the inner diameter at a height of 5.0 cm above the bottom end at a constant rate of 20 mL/min. 20 seconds after the end of the addition, while keeping the cylindrical container (B) fixed, the movable plate is lowered 5.0 cm at a speed of 1.2 cm/s. At this time, the gel diffusion distance is measured for the portion where the swollen gel formed by the water-absorbent resin particles absorbing water and the stainless steel wire mesh are in contact. The gel diffusion distance is measured by measuring the maximum distance of a straight line passing through the center point of the swollen gel when it was located in the cylindrical container (B) and connecting the ends of the swollen gel spread on the stainless steel wire mesh by lowering the movable plate, and the gel diffusion distance (cm) is calculated by subtracting the inner diameter (diameter) of the cylindrical container (B) of 2.0 cm from the maximum distance Wa.
Gel diffusion distance (cm) = maximum distance Wa - 2.0

The gel diffusion distance measured for water-absorbent resin particles can be said to be an indicator of the binding strength of absorbed water-absorbent resin particles. As described above, the present inventors have discovered a correlation between the gel diffusion distance, which is an indicator of the binding strength of absorbed water-absorbent resin particles, and the resistance of an absorbent body containing water-absorbent resin particles to deformation when the absorbent body absorbs water. In other words, water-absorbent resin particles whose gel diffusion distance is set in a specific range of more than 0.0 cm and not more than 2.0 cm have favorable resistance to deformation of the absorbent body when the absorbent body containing water-absorbent resin particles absorbs water. Water-absorbent resin particles whose gel diffusion distance is more than 0.0 cm and not more than 2.0 cm have a strong binding strength between absorbed water-absorbent resin particles, so even when bent in the absorbent body, the water-absorbent resin particles bind to each other and to hydrophilic fibers, making them less likely to crack due to deformation, and this is thought to be why they exhibit such excellent effects.

The gel diffusion distance of the water-absorbent resin particles of the present invention may be in the range of more than 0.0 cm and less than 2.0 cm, but from the viewpoint of more suitably exerting the effects of the present invention, it is preferably more than 0.0 cm, more preferably 0.1 cm or more, and even more preferably 0.2 cm or more. Furthermore, from the same viewpoint, the gel diffusion distance of the water-absorbent resin particles is preferably 2.0 cm or less, more preferably 1.5 cm or less, even more preferably 1.0 cm or less, particularly preferably 0.7 cm or less, and even more particularly preferably 0.5 cm or less. Preferred ranges include greater than 0.0 cm to 2.0 cm, greater than 0.0 cm to 1.5 cm, greater than 0.0 cm to 1.0 cm, greater than 0.0 cm to 0.7 cm, greater than 0.0 cm to 0.5 cm, 0.1 cm to 2.0 cm, 0.1 cm to 1.5 cm, 0.1 cm to 1.0 cm, 0.1 cm to 0.7 cm, 0.1 cm to 0.5 cm, 0.2 cm to 2.0 cm, 0.2 cm to 1.5 cm, 0.2 cm to 1.0 cm, 0.2 cm to 0.7 cm, and 0.2 cm to 0.5 cm.

The gel diffusion distance of the water-absorbent resin particles can be adjusted to a range of more than 0.0 cm and not more than 2.0 cm, for example, by drying and then crushing polymer particles obtained through a polymerization process such as reverse-phase suspension polymerization, which will be described later. The gel diffusion distance of the water-absorbent resin particles can also be adjusted to a range of more than 0.0 cm and not more than 2.0 cm by adding a water-soluble polymer (for example, a thickener such as polyethylene glycol) to the water-absorbent resin particles to increase the viscosity of the water-absorbent resin particles after they have absorbed liquid.

From the viewpoint of more suitably exerting the effects of the present invention, the saline flow conductivity (SFC) of the water-absorbent resin particles of the present invention is preferably 0.0× ^{10-7 cm3} ·s/g or more, more preferably ^0.3 × ^10-7 cm3·s/g or more, even more preferably 1.0× ^{10-7 cm3} ·s/g or more, particularly preferably 5.0× ^10-7 cm3· ^s /g or more, and more particularly preferably 10.0× ^{10-7 cm3} ·s/g or more. Furthermore, from the viewpoint of suppressing the occurrence of liquid leakage from the absorbent body, an absorbent body having a short diffusion length is desired. From the viewpoint of shortening the diffusion length of the absorbent, the saline flow conductivity (SFC) of the water-absorbent resin particles is preferably 50.0× ^{10-7 cm3} ·s/g or less, more preferably 30.0× ^{10-7 cm3} ·s/g or less, even more preferably 25.0× ^{10-7 cm3} ·s/g or less, and particularly preferably 20.0× ^{10-7 cm3} ·s/g or less. Preferred ranges are 0.0×10 ⁻⁷ cm ³ ·s/g to 50.0×10 ⁻⁷ cm ³ ·s/g, 0.0×10 ⁻⁷ cm ³ ·s/g to 30.0×10 ⁻⁷ cm ³ ·s/g, 0.0×10 ⁻⁷ cm ³ ·s/g to 25.0×10 ⁻⁷ cm ³ ·s/g, 0.0×10 ⁻⁷ cm ³ ·s/g to 20.0×10 ⁻⁷ cm ³ ·s/g, 0.3×10 ⁻⁷ cm ³ ·s/g to 50.0×10 ⁻⁷ cm ³ ·s/g, 0.3×10 ⁻⁷ cm ³ ·s/g to 30.0×10 ⁻⁷ cm ³ ·s/g, and 0.3×10 ⁻⁷ cm ³ ·s/g to 25.0×10 ⁻⁷ cm ^3.s /g, 0.3×10 ^-7 cm ^3.s /g to 20.0×10 ^-7 cm 3.s/g, 1.0×10 -7 cm ^3.s /g to 50.0×10 ^-7 cm ^3.s /g, 1.0× ^{10 -7 cm 3.s /} g to 30.0×10 ^-7 cm ³・s/g, 1.0×10 ^-7 cm ³・s/g to 25.0×10 ^-7 cm ³・s/g, 1.0×10 ^-7 cm ³・s/g to 20.0×10 ^-7 cm ³・s/g, 5.0×10 ^-7 cm ³・s/g to 50.0×10 ^-7 cm ³・s/g, 5.0×10 ^-7 cm ³・s/g ~ 30.0×10 ^-7 cm ³ ·s/g, 5.0×10 ⁻⁷ cm ³ ·s/g to 25.0×10 ⁻⁷ cm ³ ·s/g, 5.0×10 ⁻⁷ cm ³ ·s/g to 20.0×10 ⁻⁷ cm ³ ·s/g, 10.0×10 ⁻⁷ cm ³ ·s/g to 50.0×10 ⁻⁷ cm ³ ·s/g, 10.0×10 ⁻⁷ cm ³ ·s/g to 30.0×10 ⁻⁷ cm ³ ·s/g, 10.0×10 ⁻⁷ cm ³ ·s/g to 25.0×10 ⁻⁷ cm ³ ·s/g, 10.0×10 ⁻⁷ cm ³ ·s/g to 20.0×10 ⁻⁷ cm ³ ·s/g.

The saline flow conductivity (SFC) of water-absorbent resin particles is measured according to the method described in the Examples.

Furthermore, from the viewpoint of shortening the diffusion length of the absorbent, the no-pressure DW 3-minute value of the water-absorbent resin particles is preferably 15 mL/g or more, more preferably 20 mL/g or more, even more preferably 25 mL/g or more, particularly preferably 28 mL/g or more, even more particularly preferably 30 mL/g or more, and even more particularly preferably 32 mL/g or more. Furthermore, the no-pressure DW 3-minute value of the water-absorbent resin particles is preferably 60 mL/g or less, more preferably 55 mL/g or less, even more preferably 50 mL/g or less, particularly preferably 48 mL/g or less, and even more particularly preferably 45 mL/g or less. Preferred ranges are 15 to 60 mL/g, 15 to 55 mL/g, 15 to 50 mL/g, 15 to 48 mL/g, 15 to 45 mL/g, 20 to 60 mL/g, 20 to 55 mL/g, 20 to 50 mL/g, 20 to 48 mL/g, 20 to 45 mL/g, 25 to 60 mL/g, 25 to 55 mL/g, 25 to 50 mL/g, 25 to 48 mL/g, and 25 to 45 mL. /g, 28-60 mL/g, 28-55 mL/g, 28-50 mL/g, 28-48 mL/g, 28-45 mL/g, 30-60 mL/g, 30-55 mL/g, 30-50 mL/g, 30-48 mL/g, 30-45 mL/g, 32-60 mL/g, 32-55 mL/g, 32-50 mL/g, 32-48 mL/g, and 32-45 mL/g.

The no-pressure DW 3-minute value of water-absorbent resin particles is measured according to the method described in the Examples.

Furthermore, the water absorption rate of the water-absorbent resin particles for saline is preferably 10 seconds or more, more preferably 15 seconds or more, even more preferably 20 seconds or more, particularly preferably 22 seconds or more, and even more particularly preferably 24 seconds or more. Furthermore, from the viewpoint of shortening the diffusion length of the absorbent body, the water absorption rate of the water-absorbent resin particles for saline is preferably 70 seconds or less, more preferably 60 seconds or less, even more preferably 55 seconds or less, particularly preferably 50 seconds or less, even more particularly preferably 45 seconds or less, and even more particularly preferably 42 seconds or less. Preferred ranges include 10 to 70 seconds, 10 to 60 seconds, 10 to 55 seconds, 10 to 50 seconds, 10 to 45 seconds, 10 to 42 seconds, 15 to 70 seconds, 15 to 60 seconds, 15 to 55 seconds, 15 to 50 seconds, 15 to 45 seconds, 15 to 42 seconds, 20 to 70 seconds, 20 to 60 seconds, 20 to 55 seconds, 20 to 50 seconds, 20 to 45 seconds, 20 to 42 seconds, 22 to 70 seconds, 22 to 60 seconds, 22 to 55 seconds, 22 to 50 seconds, 22 to 45 seconds, 22 to 42 seconds, 24 to 70 seconds, 24 to 60 seconds, 24 to 55 seconds, 24 to 50 seconds, 24 to 45 seconds, and 24 to 42 seconds.

The water absorption rate of water-absorbent resin particles in physiological saline solution was measured using the method described in the Examples.

Furthermore, from the viewpoint of shortening the diffusion length of the absorbent body, the saline water retention capacity of the water-absorbent resin particles is preferably 15 g/g or more, more preferably 20 g/g or more, even more preferably 25 g/g or more, and particularly preferably 27 g/g or more. Also, it is preferably 80 g/g or less, more preferably 60 g/g or less, even more preferably 50 g/g or less, particularly preferably 45 g/g or less, and even particularly preferably 40 g/g or less. Preferred ranges include 15 to 80 g/g, 15 to 60 g/g, 15 to 50 g/g, 15 to 45 g/g, 15 to 40 g/g, 20 to 80 g/g, 20 to 60 g/g, 20 to 50 g/g, 20 to 45 g/g, 20 to 40 g/g, 25 to 80 g/g, 25 to 60 g/g, 25 to 50 g/g, 25 to 45 g/g, 25 to 40 g/g, 27 to 80 g/g, 27 to 60 g/g, 27 to 50 g/g, 27 to 45 g/g, and 27 to 40 g/g.

The saline water retention capacity of water-absorbent resin particles was measured according to the method described in the Examples.

The water-absorbent resin particles of the present invention are composed of a crosslinked polymer of a water-soluble ethylenically unsaturated monomer, i.e., a crosslinked polymer having structural units derived from a water-soluble ethylenically unsaturated monomer.

The water-absorbent resin particles of the present invention are irregularly pulverized particles. Examples of the shape of the water-absorbent resin particles include granular, roughly spherical, irregularly pulverized, plate-like, fibrous, flake-like, and aggregates of these resins.

From the viewpoint of setting the gel diffusion distance of the water-absorbent resin particles of the present invention in the range of more than 0.0 cm and not more than 2.0 cm, the shape of the water-absorbent resin particles is preferably irregularly crushed or a shape formed by agglomeration of irregularly crushed particles, and is more preferably irregularly crushed. As described above, for example, by drying and then crushing polymer particles obtained through a polymerization process such as reverse-phase suspension polymerization described below, the gel diffusion distance of the resulting irregularly crushed water-absorbent resin particles can be adjusted to the range of more than 0.0 cm and not more than 2.0 cm.

From the viewpoint of more optimally exerting the effects of the present invention, the median particle diameter of the water-absorbent resin particles is preferably 200 μm or more, 250 μm or more, 300 μm or more, 320 μm or more, or 350 μm or more. From the same viewpoint, the median particle diameter is preferably 700 μm or less, 600 μm or less, 550 μm or less, 500 μm or less, or 450 μm or less. Preferred ranges for the median particle size include 200 to 700 μm, 200 to 600 μm, 200 to 550 μm, 200 to 500 μm, 200 to 450 μm, 250 to 700 μm, 250 to 600 μm, 250 to 550 μm, 250 to 500 μm, 250 to 450 μm, 300 to 700 μm, 300 to 600 μm, 300 to 550 μm, 300 to 500 μm, 300 to 450 μm, 350 to 700 μm, 350 to 600 μm, 350 to 550 μm, 350 to 500 μm, and 350 to 450 μm.

The median particle diameter of the water-absorbent resin particles can be measured using a JIS standard sieve, and specifically, is the value measured using the method described in the Examples.

2. Manufacturing Method of Water-Absorbent Resin Particles The manufacturing method of water-absorbent resin particles of the present invention is not particularly limited as long as it can obtain water-absorbent resin particles having a gel diffusion distance of more than 0.0 cm and not more than 2.0 cm, as measured by the above-mentioned method. The manufacturing method of water-absorbent resin particles of the present invention, for example, comprises, in this order, a step of polymerizing a water-soluble ethylenically unsaturated monomer to obtain polymer particles, a surface cross-linking step of surface-cross-linking the polymer particles, a step of pulverizing the polymer particles, and a step of classifying the polymer particles.

As mentioned above, methods for adjusting the gel diffusion distance of the water-absorbent resin particles of the present invention to a range of more than 0.0 cm and not more than 2.0 cm include, for example, a method of drying and then crushing polymer particles obtained through a polymerization process such as reverse-phase suspension polymerization, which will be described later, and a method of adding a water-soluble polymer (for example, a thickener such as polyethylene glycol) to the water-absorbent resin particles to increase the viscosity of the water-absorbent resin particles after they have absorbed liquid. By employing these methods, the binding strength of the water-absorbent resin particles after absorbing liquid can be increased, thereby shortening the gel diffusion distance. Below, a detailed description is given of the method for producing water-absorbent resin particles of the present invention.

<Polymerization process>
The polymerization step is a step of polymerizing a water-soluble ethylenically unsaturated monomer to obtain polymer particles. Typical polymerization methods for polymerizing a water-soluble ethylenically unsaturated monomer include aqueous solution polymerization, spray-droplet polymerization, emulsion polymerization, and reverse-phase suspension polymerization. In the aqueous solution polymerization method, polymerization is carried out by heating an aqueous solution of the water-soluble ethylenically unsaturated monomer while stirring as necessary. In the reverse-phase suspension polymerization method, polymerization is carried out by heating the water-soluble ethylenically unsaturated monomer in a hydrocarbon dispersion medium while stirring. Among these, reverse-phase suspension polymerization is preferred from the viewpoint of suitably producing water-absorbent resin particles having a gel diffusion distance of more than 0.0 cm and not more than 2.0 cm. In the polymerization step, an internal crosslinking agent may be added to the water-soluble ethylenically unsaturated monomer as needed to form crosslinked polymer particles (aqueous gel-like material) having an internal crosslinked structure. An example of the polymerization step is described below.

[Water-soluble ethylenically unsaturated monomer]
Examples of water-soluble ethylenically unsaturated monomers include (meth)acrylic acid (herein, "acrylic" and "methacrylic" are collectively referred to as "(meth)acrylic", the same applies hereinafter) and salts thereof; 2-(meth)acrylamido-2-methylpropanesulfonic acid and salts thereof; nonionic monomers such as (meth)acrylamide, N,N-dimethyl(meth)acrylamide, 2-hydroxyethyl(meth)acrylate, N-methylol(meth)acrylamide, and polyethylene glycol mono(meth)acrylate; and amino group-containing unsaturated monomers and quaternized products thereof such as N,N-diethylaminoethyl(meth)acrylate, N,N-diethylaminopropyl(meth)acrylate, and diethylaminopropyl(meth)acrylamide. Among these water-soluble ethylenically unsaturated monomers, (meth)acrylic acid or salts thereof, (meth)acrylamide, and N,N-dimethylacrylamide are preferred, and (meth)acrylic acid and salts thereof are more preferred, from the viewpoint of ease of industrial availability. These water-soluble ethylenically unsaturated monomers may be used alone or in combination of two or more.

Among these, acrylic acid and its salts are widely used as raw materials for water-absorbent resin particles, and these acrylic acids and/or their salts may be copolymerized with the other water-soluble ethylenically unsaturated monomers mentioned above. In this case, it is preferable that acrylic acid and/or its salts be used as the main water-soluble ethylenically unsaturated monomer in an amount of 70 to 100 mol % relative to the total water-soluble ethylenically unsaturated monomers.

The water-soluble ethylenically unsaturated monomer may be dispersed in a hydrocarbon dispersion medium in the form of an aqueous solution and subjected to reverse-phase suspension polymerization. By forming the water-soluble ethylenically unsaturated monomer into an aqueous solution, the dispersion efficiency in the hydrocarbon dispersion medium can be increased. The concentration of the water-soluble ethylenically unsaturated monomer in this aqueous solution is preferably in the range of 20% by mass to the saturated concentration or less. Furthermore, the concentration of the water-soluble ethylenically unsaturated monomer is more preferably 55% by mass or less, even more preferably 50% by mass or less, and even more preferably 45% by mass or less. Meanwhile, the concentration of the water-soluble ethylenically unsaturated monomer is more preferably 25% by mass or more, even more preferably 28% by mass or more, and even more preferably 30% by mass or more.

When the water-soluble ethylenically unsaturated monomer has an acid group, such as (meth)acrylic acid or 2-(meth)acrylamido-2-methylpropanesulfonic acid, the acid group may be neutralized in advance with an alkaline neutralizer, if necessary. Examples of such alkaline neutralizers include alkali metal salts such as sodium hydroxide, sodium carbonate, sodium bicarbonate, potassium hydroxide, and potassium carbonate; ammonia, etc. These alkaline neutralizers may also be used in the form of an aqueous solution to simplify the neutralization process. The alkaline neutralizers mentioned above may be used alone or in combination of two or more types.

The degree of neutralization of the water-soluble ethylenically unsaturated monomer with the alkaline neutralizing agent is preferably 40 to 100 mol%, more preferably 50 to 90 mol%, even more preferably 60 to 85 mol%, and even more preferably 70 to 80 mol%, in terms of the degree of neutralization of all acid groups possessed by the water-soluble ethylenically unsaturated monomer.

[Radical polymerization initiator]
From the viewpoint of adjusting the gel diffusion distance, SFC, no-pressure DW 3-minute value, water absorption rate of physiological saline solution, and water retention capacity of physiological saline solution of the water absorbent resin particles within suitable ranges, examples of the radical polymerization initiator added to the polymerization step include persulfates such as potassium persulfate, ammonium persulfate, and sodium persulfate, peroxides such as methyl ethyl ketone peroxide, methyl isobutyl ketone peroxide, di-t-butyl peroxide, t-butyl cumyl peroxide, t-butyl peroxyacetate, t-butyl peroxyisobutyrate, t-butyl peroxypivalate, and hydrogen peroxide, and 2,2′-azobis(2- azo compounds such as 2,2'-azobis[2-(N-phenylamidino)propane]dihydrochloride, 2,2'-azobis[2-(N-allylamidino)propane]dihydrochloride, 2,2'-azobis{2-[1-(2-hydroxyethyl)-2-imidazolin-2-yl]propane}dihydrochloride, 2,2'-azobis{2-[1-(2-hydroxyethyl)-2-imidazolin-2-yl]propane}dihydrochloride, 2,2'-azobis{2-methyl-N-[1,1-bis(hydroxymethyl)-2-hydroxyethyl]propionamide}, 2,2'-azobis[2-methyl-N-(2-hydroxyethyl)-propionamide], and 4,4'-azobis(4-cyanovaleric acid). Among these radical polymerization initiators, potassium persulfate, ammonium persulfate, sodium persulfate, and 2,2'-azobis(2-amidinopropane) dihydrochloride are preferred from the viewpoints of easy availability and ease of handling. These radical polymerization initiators may be used alone or in combination of two or more. Furthermore, the radical polymerization initiators may also be used as redox polymerization initiators in combination with reducing agents such as sodium sulfite, sodium bisulfite, ferrous sulfate, and L-ascorbic acid.

The amount of radical polymerization initiator used is, for example, 0.00005 to 0.01 moles per mole of water-soluble ethylenically unsaturated monomer. By using such an amount, it is possible to avoid a rapid polymerization reaction and complete the polymerization reaction within an appropriate time.

[Internal crosslinking agent]
The internal crosslinking agent can be one that can crosslink the polymer of the water-soluble ethylenically unsaturated monomer used, such as (poly)ethylene glycol ("(poly)" refers to both the presence and absence of the prefix "poly"). the same applies hereinafter)], unsaturated polyesters obtained by reacting polyols such as diols and triols, such as (poly)propylene glycol, 1,4-butanediol, 1,6-hexanediol, trimethylolpropane, and (poly)glycerin, with unsaturated acids, such as (meth)acrylic acid, maleic acid, and fumaric acid; bisacrylamides such as N,N-methylenebisacrylamide; di(meth)acrylic acid esters or tri(meth)acrylic acid esters obtained by reacting polyepoxides with (meth)acrylic acid; di(meth)acrylic acid carbamyl esters obtained by reacting polyisocyanates, such as tolylene diisocyanate and hexamethylene diisocyanate, with hydroxyethyl (meth)acrylate; allylated starch, allylated cellulose, diallyl phthalate, N,N',N''-triallyl isocyanurate, divinyl Examples of the compound include a compound having two or more polymerizable unsaturated groups such as benzene; a diglycidyl compound such as (poly)ethylene glycol diglycidyl ether, (poly)propylene glycol diglycidyl ether, (poly)glycerin diglycidyl ether, and the like, and a polyglycidyl compound such as a triglycidyl compound; an epihalohydrin compound such as epichlorohydrin, epibromohydrin, and α-methylepichlorohydrin; a compound having two or more reactive functional groups such as an isocyanate compound such as 2,4-tolylene diisocyanate and hexamethylene diisocyanate; and an oxetane compound such as 3-methyl-3-oxetane methanol, 3-ethyl-3-oxetane methanol, 3-butyl-3-oxetane methanol, 3-methyl-3-oxetane ethanol, 3-ethyl-3-oxetane ethanol, and 3-butyl-3-oxetane ethanol. Among these internal cross-linking agents, it is preferable to use a polyglycidyl compound, it is more preferable to use a diglycidyl ether compound, and it is preferable to use (poly)ethylene glycol diglycidyl ether, (poly)propylene glycol diglycidyl ether, or (poly)glycerin diglycidyl ether. These internal cross-linking agents may be used alone or in combination of two or more.

From the viewpoint of adjusting the gel diffusion distance, SFC, no-pressure DW 3-minute value, water absorption rate for saline solution, and saline water retention capacity of the water-absorbent resin particles within suitable ranges, the amount of internal crosslinking agent used is preferably 0.02 mol or less, more preferably 0.000001 to 0.01 mol, even more preferably 0.000005 to 0.005 mol, and even more preferably 0.00001 to 0.00005 mol, per 1 mol of water-soluble ethylenically unsaturated monomer.

[Hydrocarbon dispersion medium]
Examples of hydrocarbon dispersion media include aliphatic hydrocarbons having 6 to 8 carbon atoms, 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. Among these hydrocarbon dispersion media, n-hexane, n-heptane, and cyclohexane are particularly preferred because of their industrial availability, stable quality, and low cost. These hydrocarbon dispersion media may be used alone or in combination of two or more. Suitable results can also be obtained using a commercially available product, such as Exxol Heptane (manufactured by ExxonMobil Corporation; containing 75 to 85% by mass of hydrocarbons such as heptane and its isomers).

The amount of hydrocarbon dispersion medium used is preferably 100 to 1500 parts by mass, and more preferably 200 to 1400 parts by mass, per 100 parts by mass of the water-soluble ethylenically unsaturated monomer in the first stage, from the viewpoint of uniformly dispersing the water-soluble ethylenically unsaturated monomer and facilitating control of the polymerization temperature. As will be described later, reversed-phase suspension polymerization is carried out in one stage (single stage) or in multiple stages (two or more stages), and the above-mentioned first stage polymerization refers to the polymerization reaction in a single stage or multiple stage polymerization (the same applies hereinafter).

[Dispersion stabilizer]
(Surfactant)
In the reversed-phase suspension polymerization, a dispersion stabilizer can be used to improve the dispersion stability of the water-soluble ethylenically unsaturated monomer in the hydrocarbon dispersion medium. As the dispersion stabilizer, a surfactant can be used.

Surfactants that can be used include, for example, sucrose fatty acid esters, polyglycerin fatty acid esters, sorbitan fatty acid esters, polyoxyethylene sorbitan fatty acid esters, polyoxyethylene glycerin fatty acid esters, sorbitol fatty acid esters, polyoxyethylene sorbitol fatty acid esters, polyoxyethylene alkyl ethers, polyoxyethylene alkylphenyl ethers, polyoxyethylene castor oil, polyoxyethylene hydrogenated castor oil, alkylaryl formaldehyde condensed polyoxyethylene ethers, polyoxyethylene polyoxypropylene block copolymers, polyoxyethylene polyoxypropyl alkyl ethers, polyethylene glycol fatty acid esters, alkyl glucosides, N-alkyl gluconamides, polyoxyethylene fatty acid amides, polyoxyethylene alkylamines, polyoxyethylene alkyl ether phosphate esters, and polyoxyethylene alkyl allyl ether phosphate esters. Among these surfactants, sorbitan fatty acid esters, polyglycerin fatty acid esters, and sucrose fatty acid esters are particularly preferred in terms of dispersion stability of the monomer. These surfactants may be used alone or in combinations of two or more.

The amount of surfactant used is preferably 0.1 to 30 parts by mass, and more preferably 0.3 to 20 parts by mass, per 100 parts by mass of the first-stage water-soluble ethylenically unsaturated monomer.

(polymeric dispersant)
As a dispersion stabilizer used in the reversed phase suspension polymerization, a polymeric dispersant may be used in combination with the surfactant described above.

Examples of polymeric dispersants include maleic anhydride-modified polyethylene, maleic anhydride-modified polypropylene, maleic anhydride-modified ethylene-propylene copolymer, maleic anhydride-modified EPDM (ethylene-propylene-diene terpolymer), maleic anhydride-modified polybutadiene, maleic anhydride-ethylene copolymer, maleic anhydride-propylene copolymer, maleic anhydride-ethylene-propylene copolymer, maleic anhydride-butadiene copolymer, polyethylene, polypropylene, ethylene-propylene copolymer, oxidized polyethylene, oxidized polypropylene, oxidized ethylene-propylene copolymer, ethylene-acrylic acid copolymer, ethyl cellulose, ethylhydroxyethyl cellulose, etc. Among these polymeric dispersants, maleic anhydride-modified polyethylene, maleic anhydride-modified polypropylene, maleic anhydride-modified ethylene-propylene copolymer, maleic anhydride-ethylene copolymer, maleic anhydride-propylene copolymer, maleic anhydride-ethylene-propylene copolymer, polyethylene, polypropylene, ethylene-propylene copolymer, oxidized polyethylene, oxidized polypropylene, and oxidized ethylene-propylene copolymer are particularly preferred from the viewpoint of dispersion stability of the monomer. These polymeric dispersants may be used alone or in combination of two or more.

The amount of polymeric dispersant used is preferably 0.1 to 30 parts by mass, and more preferably 0.3 to 20 parts by mass, per 100 parts by mass of the first-stage water-soluble ethylenically unsaturated monomer.

[Other ingredients]
In the method for producing water-absorbent resin particles, if desired, other components may be added to the aqueous solution containing the water-soluble ethylenically unsaturated monomer to carry out reverse phase suspension polymerization. As the other components, various additives such as a thickener and a chain transfer agent can be added.

As an example, reverse-phase suspension polymerization can be carried out by adding a thickener to an aqueous solution containing a water-soluble ethylenically unsaturated monomer. By adding a thickener in this way to adjust the viscosity of the aqueous solution, it is possible to control the median particle size obtained in reverse-phase suspension polymerization.

Examples of thickeners that can be used include hydroxyethyl cellulose, hydroxypropyl cellulose, methyl cellulose, carboxymethyl cellulose, polyacrylic acid, partially neutralized polyacrylic acid, polyethylene glycol, polyacrylamide, polyethyleneimine, dextrin, sodium alginate, polyvinyl alcohol, polyvinylpyrrolidone, and polyethylene oxide. Furthermore, if the stirring speed during polymerization is the same, the higher the viscosity of the aqueous solution of the water-soluble ethylenically unsaturated monomer, the larger the primary and/or secondary particles that are obtained tend to be.

[Reverse Phase Suspension Polymerization]
In carrying out reversed-phase suspension polymerization, for example, an aqueous monomer solution containing a water-soluble ethylenically unsaturated monomer is dispersed in a hydrocarbon dispersion medium in the presence of a dispersion stabilizer. In this case, the dispersion stabilizer (surfactant or polymeric dispersant) may be added either before or after the addition of the aqueous monomer solution, as long as it is before the start of the polymerization reaction.

Among these, from the viewpoint of facilitating a reduction in the amount of hydrocarbon dispersion medium remaining in the obtained water-absorbent resin particles, it is preferable to disperse an aqueous monomer solution in a hydrocarbon dispersion medium in which a polymeric dispersant has been dispersed, and then further disperse a surfactant therein before carrying out polymerization.

Such reverse phase suspension polymerization can be carried out in one stage or in two or more stages. From the standpoint of increasing productivity, it is preferable to carry it out in two to three stages.

When performing reversed-phase suspension polymerization in two or more stages, after the first stage of reversed-phase suspension polymerization, the water-soluble ethylenically unsaturated monomer is added to and mixed with the reaction mixture obtained in the first polymerization stage, and the second and subsequent stages of reversed-phase suspension polymerization are performed in the same manner as the first stage. In the reversed-phase suspension polymerization in each stage from the second stage onwards, it is preferable to perform the reversed-phase suspension polymerization by adding, in addition to the water-soluble ethylenically unsaturated monomer, a radical polymerization initiator within the molar ratio of each component to the water-soluble ethylenically unsaturated monomer as described above, based on the amount of water-soluble ethylenically unsaturated monomer added during the reversed-phase suspension polymerization in each stage from the second stage onwards. Note that, in the second and subsequent stages of polymerization, an internal crosslinking agent may also be added to the water-soluble ethylenically unsaturated monomer, if necessary.

The reaction temperature for the polymerization reaction is preferably 20 to 110°C, and more preferably 40 to 90°C, from the perspective of improving economic efficiency by rapidly progressing the polymerization and shortening the polymerization time, as well as easily removing the heat of polymerization to ensure a smooth reaction.

<Dehydration process>
After the above-mentioned reversed-phase suspension polymerization, the method may include a dehydration step in which water, hydrocarbon dispersion medium, etc. are removed by distillation by applying energy such as heat from the outside. When dehydrating the hydrous gel-like material after reversed-phase suspension polymerization, the system in which the hydrous gel-like material is dispersed in the hydrocarbon dispersion medium is heated, and the water and hydrocarbon dispersion medium are temporarily distilled out of the system by azeotropic distillation. In this case, if only the evaporated hydrocarbon dispersion medium is returned to the system, continuous azeotropic distillation is possible. In this case, the temperature in the system during drying is maintained below the azeotropic temperature with the hydrocarbon dispersion medium, which is preferable from the viewpoint of preventing deterioration of the resin. By controlling the processing conditions of this dehydration step after polymerization to adjust the amount of dehydration (i.e., adjusting the water content of the polymer particles), it is possible to control the various properties of the obtained water-absorbent resin particles.

In the dehydration step, dehydration by distillation may be carried out under normal pressure. When dehydration is carried out under normal pressure, the dehydration temperature is preferably 70 to 250°C, more preferably 80 to 180°C, even more preferably 80 to 140°C, and even more preferably 90 to 130°C.

<Surface crosslinking process>
The surface cross-linking step is a step of subjecting the polymer particles obtained in the polymerization step to surface cross-linking. When the polymer particles are cross-linked polymer particles (hydrogel-like material), it is a step of adding a surface cross-linking agent to the hydrogel-like material having an internal cross-linked structure obtained by polymerizing a water-soluble ethylenically unsaturated monomer to perform cross-linking (surface cross-linking reaction). This surface cross-linking reaction is preferably carried out in the presence of a surface cross-linking agent after the polymerization of the water-soluble ethylenically unsaturated monomer.

Surface cross-linking agents include compounds with two or more reactive functional groups. Examples include polyols such as ethylene glycol, propylene glycol, 1,4-butanediol, diethylene glycol, triethylene glycol, 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; haloepoxy compounds such as epichlorohydrin, epibromohydrin, and α-methylepichlorohydrin; isocyanate compounds such as 2,4-tolylene diisocyanate and hexamethylene diisocyanate; 3-methyl-3-oxetanemethanol and 3-ethyl-3-oxetane Oxetane compounds such as methanol, 3-butyl-3-oxetanemethanol, 3-methyl-3-oxetaneethanol, 3-ethyl-3-oxetaneethanol, and 3-butyl-3-oxetaneethanol; oxazoline compounds such as 1,2-ethylenebisoxazoline; ethylene carbonate, propylene carbonate, 4,5-dimethyl-1,3-dioxolan-2-one, 4,4-dimethyl-1,3-dioxolan-2-one, 4-ethyl Examples of suitable surface cross-linking agents include carbonate compounds (e.g., alkylene carbonates) such as 1,3-dioxolan-2-one, 4-hydroxymethyl-1,3-dioxolan-2-one, 1,3-dioxan-2-one, 4-methyl-1,3-dioxan-2-one, 4,6-dimethyl-1,3-dioxan-2-one, and 1,3-dioxolan-2-one; and hydroxyalkylamide compounds such as bis[N,N-di(β-hydroxyethyl)]adipamide. Among these surface cross-linking agents, 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 are preferred. These surface cross-linking agents may be used alone or in combination of two or more.

The amount of surface cross-linking agent used is preferably 0.00001 to 0.01 mol, more preferably 0.00005 to 0.005 mol, even more preferably 0.0001 to 0.001 mol, and even more preferably 0.0002 to 0.0009 mol per mol of the total amount of water-soluble ethylenically unsaturated monomers used in the polymerization.

As a method for adding the surface cross-linking agent, the surface cross-linking agent may be added as is or as an aqueous solution, but if necessary, it may be added as a solution using a hydrophilic organic solvent as the solvent. Examples of hydrophilic organic solvents include lower alcohols such as methyl alcohol, ethyl alcohol, n-propyl alcohol, and isopropyl alcohol; ketones such as acetone and methyl ethyl ketone; ethers such as diethyl ether, dioxane, and tetrahydrofuran; amides such as N,N-dimethylformamide; and sulfoxides such as dimethyl sulfoxide. These hydrophilic organic solvents may be used alone, in combination with two or more types, or as a mixed solvent with water.

From the viewpoint of adjusting the gel diffusion distance, SFC, no-pressure DW 3-minute value, water absorption rate in physiological saline, and physiological saline water retention capacity of the water-absorbent resin particles within suitable ranges, the timing of adding the surface cross-linking agent may be after the polymerization reaction of the water-soluble ethylenically unsaturated monomer has almost completely completed. The surface cross-linking agent is preferably added in the presence of water in a range of 1 to 400 parts by mass, more preferably in the presence of water in a range of 5 to 200 parts by mass, even more preferably in the presence of water in a range of 10 to 100 parts by mass, even more preferably in the presence of water in a range of 15 to 60 parts by mass, and particularly preferably in the presence of water in a range of 35 to 50 parts by mass, relative to 100 parts by mass of the water-soluble ethylenically unsaturated monomer used in the polymerization. The amount of water means the total amount of water contained in the reaction system and water used as needed when adding the surface cross-linking agent.

The reaction temperature for the surface cross-linking reaction is preferably 50 to 250°C, more preferably 60 to 180°C, even more preferably 60 to 140°C, and even more preferably 70 to 120°C. Furthermore, the reaction time for the surface cross-linking reaction is preferably 1 to 300 minutes, and more preferably 5 to 200 minutes.

<Drying process>
After the above-mentioned surface cross-linking is performed, a drying step may be included in which water, a hydrocarbon dispersion medium, etc. are removed by distillation by applying energy such as heat from the outside. The polymer particles after surface cross-linking are dried and the water and the hydrocarbon dispersion medium are distilled off, thereby obtaining water-absorbent resin particles.

In the drying step, the drying process by distillation may be carried out under normal pressure or under reduced pressure. Furthermore, from the viewpoint of increasing drying efficiency, it may also be carried out under a stream of gas such as nitrogen. When the drying process is carried out under normal pressure, the drying temperature is preferably 70 to 250°C, more preferably 80 to 180°C, even more preferably 80 to 140°C, and even more preferably 90 to 130°C. When the drying process is carried out under reduced pressure, the drying temperature is preferably 40 to 160°C, and more preferably 50 to 110°C.

If a surface cross-linking step using a surface cross-linking agent is carried out after the polymerization of monomers by reverse phase suspension polymerization, the drying step by distillation described above is carried out after the surface cross-linking step is completed. Alternatively, the surface cross-linking step and the drying step may be carried out simultaneously.

After the drying step, the polymer particles may be crushed to obtain crushed particles with an angular shape, thereby increasing the binding strength of the absorbed water-absorbent resin particles, and adjusting the gel diffusion distance of the resulting water-absorbent resin particles to a range of more than 0.0 cm and not more than 2.0 cm. There are no particular limitations on the method for crushing the polymer particles, and the polymer particles can be crushed using a crusher such as a centrifugal crusher, roller mill, stamp mill, jet mill, high-speed rotary crusher, or container-driven mill.

In order to adjust the median particle size of the water-absorbent resin particles to a suitable range, the polymer particles obtained by pulverization may be classified. Classification refers to the process of dividing a particle group (powder) into two or more particle groups with different particle size distributions. A portion of the polymer particles after classification may be pulverized and classified again. The classification method is not particularly limited, but may be, for example, screen classification or air classification. Screen classification is a method of classifying particles on a screen into particles that pass through the meshes of the screen and particles that do not by vibrating the screen. Screen classification can be performed using, for example, a vibrating sieve, a rotary sifter, a cylindrical stirring sieve, a blower sifter, or a rotary shaker. Air classification is a method of classifying particles using air flow.

The water-absorbent resin particles of the present invention may contain additives according to the purpose. Such additives include surfactants, oxidizing agents, reducing agents, metal chelating agents, radical chain inhibitors, antioxidants, antibacterial agents, etc. It is preferable that the additives are hydrophilic or water-soluble.

In order to adjust the no-pressure DW 3-minute value and the water absorption rate for physiological saline solution within a suitable range, the water-absorbent resin particles may contain an inorganic powder. The inorganic powder is preferably hydrophilic or water-soluble, and amorphous silica, for example, may be used. The inorganic powder is preferably contained in an amount of 0.05 to 5 parts by mass, and more preferably 0.1 to 2.5 parts by mass, per 100 parts by mass of the water-absorbent resin particles.

By including a thickener in the water-absorbent resin particles, the viscosity of the water-absorbent resin particles after absorbing liquid is increased, thereby increasing the binding strength of the absorbent resin particles and adjusting the gel diffusion distance of the water-absorbent resin particles to a range of more than 0.0 cm and less than 2.0 cm. Any water-soluble thickener is sufficient, and a water-soluble polymer is preferable. For example, polyethylene glycol, hydroxyethyl cellulose, hydroxypropyl cellulose, hydroxyethyl methyl cellulose, hydroxypropyl methyl cellulose, hydroxyethyl ethyl cellulose, methyl cellulose, carboxymethyl cellulose, polyacrylamide, polyethyleneimine, dextrin, sodium alginate, polyvinyl alcohol, polyvinylpyrrolidone, polyethylene oxide, etc. are preferably used, with polyethylene glycol being more preferable. It is preferable to include 0.01 to 5 parts by mass of thickener per 100 parts by mass of water-absorbent resin particles. It is also preferable to include the thickener in solid form relative to the water-absorbent resin particles.

3. Absorbent Material, Absorbent Articles The water-absorbing resin particles of the present invention constitute an absorbent material used in hygiene materials such as sanitary products and disposable diapers, and are suitably used in absorbent articles containing the absorbent material.

The absorbent of the present invention contains the water-absorbent resin particles of the present invention. The absorbent may further contain hydrophilic fibers. Examples of absorbent configurations include a sheet-like structure in which water-absorbent resin particles are fixed on a nonwoven fabric or between multiple nonwoven fabrics, a mixed dispersion obtained by mixing water-absorbent resin particles and hydrophilic fibers to form a uniform composition, a sandwich structure in which water-absorbent resin particles are sandwiched between layers of hydrophilic fibers, and a structure in which water-absorbent resin particles and hydrophilic fibers are wrapped in tissue. The absorbent may also contain other components, such as adhesive binders such as heat-fusible synthetic fibers, hot-melt adhesives, and adhesive emulsions to improve the shape retention of the absorbent.

The basis weight of the water-absorbent resin particles in the absorbent body of the present invention is 30 g/ ^m2 or more and 500 g/ ^m2 or less. The basis weight is preferably 100 g/ ^m2 or more, more preferably 120 g/ ^m2 or more, even more preferably 140 g/ ^m2 or more, and is preferably 400 g/ ^m2 or less, more preferably 350 g/m2 or ^less , even more preferably 300 g/ ^m2 or less.

Hydrophilic fibers include at least one selected from the group consisting of finely ground wood pulp, cotton, cotton linters, rayon, cellulose acetate, polyamide, polyester, and polyolefin. Examples include cellulose fibers such as cotton-like pulp obtained from wood, mechanical pulp, chemical pulp, and semi-chemical pulp; artificial cellulose fibers such as rayon and acetate; and fibers made from synthetic resins such as hydrophilically treated polyamide, polyester, and polyolefin. The average fiber length of the hydrophilic fibers is typically 0.1 to 10 mm, or may be 0.5 to 5 mm.

The basis weight of the hydrophilic fibers in the absorbent body of the present invention is 0 g/ ^m2 or more and 800 g/ ^m2 or less. The basis weight is preferably 50 g/ ^m2 or more, more preferably 70 g/ ^m2 or more, even more preferably 90 g/ ^m2 or more, and still more preferably 100 g/ ^m2 or more, and is preferably 700 g/m2 or ^less , more preferably 600 g/ ^m2 or less, and even more preferably 500 g/ ^m2 or less.

The content of water-absorbent resin particles in the absorbent body is preferably 5 to 100% by mass, more preferably 10 to 95% by mass, even more preferably 20 to 90% by mass, and even more preferably 30 to 80% by mass.

The absorbent article of the present invention can be produced by holding an absorbent body using the water-absorbent resin particles of the present invention between a liquid-permeable sheet (top sheet) through which liquid can pass and a liquid-impermeable sheet (back sheet) through which liquid cannot pass. The liquid-permeable sheet is placed on the side that comes into contact with the body, and the liquid-impermeable sheet is placed on the opposite side that comes into contact with the body.

Liquid-permeable sheets include nonwoven fabrics such as air-through, spunbond, chemical-bond, and needle-punched types made from fibers such as polyethylene, polypropylene, and polyester, as well as porous synthetic resin sheets. Liquid-impermeable sheets include synthetic resin films made from resins such as polyethylene, polypropylene, and polyvinyl chloride. The liquid-permeable sheet is preferably at least one type selected from the group consisting of thermal-bonded nonwoven fabrics, air-through nonwoven fabrics, spunbonded nonwoven fabrics, and spunbonded/meltblown/spunbonded nonwoven fabrics.

The basis weight of the liquid-permeable sheet is preferably 5 g/m ^{or more and} 100 g/m ^{or less} , and more preferably 10 g/m or ^more and 60 g/m or ^less . Furthermore, the liquid-permeable sheet may be embossed or perforated on its surface to improve liquid diffusibility. The embossing or perforation can be carried out by a known method.

Examples of liquid-impermeable sheets include sheets made of synthetic resins such as polyethylene, polypropylene, and polyvinyl chloride; sheets made of nonwoven fabrics such as spunbond/meltblown/spunbond (SMS) nonwoven fabrics, in which a water-resistant meltblown nonwoven fabric is sandwiched between high-strength spunbond nonwoven fabrics; and sheets made of composite materials of these synthetic resins and nonwoven fabrics (e.g., spunbond nonwoven fabrics, spunlace nonwoven fabrics). A sheet made of a synthetic resin primarily composed of low-density polyethylene (LDPE) resin can also be used as the liquid-impermeable sheet. The liquid-impermeable sheet may be, for example, a sheet made of a synthetic resin with a basis weight of 10 to 50 g/m ² .

The absorbent article preferably comprises a laminate having an absorbent body containing water-absorbent resin particles and core wraps sandwiching the absorbent body from above and below, a liquid-permeable sheet disposed on the upper surface of the laminate, and a liquid-impermeable sheet disposed on the surface of the laminate opposite the liquid-permeable sheet side.

4. Supplementary Notes This specification includes at least the inventions set forth in (1) to (6) below.
(1)
Water-absorbent resin particles having a gel diffusion distance of more than 0.0 cm to 2.0 cm, more than 0.0 cm to 1.5 cm, more than 0.0 cm to 1.0 cm, more than 0.0 cm to 0.7 cm, more than 0.0 cm to 0.5 cm, 0.1 cm to 2.0 cm, 0.1 cm to 1.5 cm, 0.1 cm to 1.0 cm, 0.1 cm to 0.7 cm, 0.1 cm to 0.5 cm, 0.2 cm to 2.0 cm, 0.2 cm to 1.5 cm, 0.2 cm to 1.0 cm, 0.2 cm to 0.7 cm, or 0.2 cm to 0.5 cm.
(2)
The saline flow conductivity (SFC) of the water-absorbent resin particles is 0.0×10 ⁻⁷ cm ³ ·s/g to 50.0×10 ⁻⁷ cm ³ ·s/g, 0.0×10 ⁻⁷ cm ³ ·s/g to 30.0×10 ⁻⁷ cm ³ ·s/g, 0.0×10 ⁻⁷ cm ³ ·s/g to 25.0×10 ⁻⁷ cm ³ ·s/g, 0.0×10 ⁻⁷ cm ³ ·s/g to 20.0×10 ⁻⁷ cm ³ ·s/g, 0.3×10 ⁻⁷ cm ³ ·s/g to 50.0×10 ⁻⁷ cm ³ ·s/g, 0.3×10 ⁻⁷ cm ³ ·s/g to 30.0×10 ⁻⁷ cm ³ ·s/g, 0.3×10 ⁻⁷ cm ³・s/g～25.0×10 ^-7 cm ³・s/g, 0.3×10 ^-7 cm ³・s/g～20.0×10 ^-7 cm ³・s/g, 1.0×10 ^-7 cm ³・s/g～50.0×10 ^-7 cm ³・s/g, 1.0×10 ^-7 cm ³・s/g～30.0×10 ^-7 cm ³・s/g, 1.0×10 ^-7 cm ³・s/g～25.0×10 ^-7 cm ³・s/g, 1.0×10 ^-7 cm ³・s/g～20.0×10 ^-7 cm ³・s/g, 5.0×10 ^-7 cm ³・s/g～50.0×10 ^-7 cm ³・s/g, 5.0×10 ^-7 cm ³・s/g～30.0×10 ^-7 cm ³・s/g, 5.0×10 ^-7 cm ³・s/g～25.0×10 ^-7 cm ³・s/g, 5.0×10 ^-7 cm ³・s/g～20.0×10 ^-7 cm ³・s/g, 10.0×10 ^-7 cm ³・s/g～50.0×10 ^-7 cm ³・s/g, 10.0×10 ^-7 cm ³・s/g～30.0×10 ^-7 cm ³・s/g, 10.0×10 ^-7 cm ³・s/g～25.0×10 ^-7 cm ³・s/g, 10.0×10 ^-7 cm ³・s/g～20.0×10 ^-7 cm ³ ·s/g.
(3)
The water-absorbent resin particles have a DW value of 3 minutes without pressure in physiological saline of 15 mL/g or more, 15 to 60 mL/g, 15 to 55 mL/g, 15 to 50 mL/g, 15 to 48 mL/g, 15 to 45 mL/g, 20 to 60 mL/g, 20 to 55 mL/g, 20 to 50 mL/g, 20 to 48 mL/g, 20 to 45 mL/g, 25 to 60 mL/g, 25 to 55 mL/g, 25 to 50 mL/g, 25 to 48 mL/g, 25 to 45 mL/g, 5mL/g, 28 to 60mL/g, 28 to 55mL/g, 28 to 50mL/g, 28 to 48mL/g, 28 to 45mL/g, 30 to 60mL/g, 30 to 55mL/g, 30 to 50mL/g, 30 to 48mL/g, 30 to 45mL/g, 32 to 60mL/g, 32 to 55mL/g, 32 to 50mL/g, 32 to 48mL/g, 32 to 45mL/g. The water-absorbent resin particles according to (1) or (2) above,
(4)
The water-absorbing resin particles according to any one of (1) to (3), wherein the water-absorbing speed of the water-absorbing resin particles in physiological saline is 10 to 70 seconds, 10 to 60 seconds, 10 to 55 seconds, 10 to 50 seconds, 10 to 45 seconds, 10 to 42 seconds, 15 to 70 seconds, 15 to 60 seconds, 15 to 55 seconds, 15 to 50 seconds, 15 to 45 seconds, 15 to 42 seconds, 20 to 70 seconds, 20 to 60 seconds, 20 to 55 seconds, 20 to 50 seconds, 20 to 45 seconds, 20 to 42 seconds, 22 to 70 seconds, 22 to 60 seconds, 22 to 55 seconds, 22 to 50 seconds, 22 to 45 seconds, 22 to 42 seconds, 24 to 70 seconds, 24 to 60 seconds, 24 to 55 seconds, 24 to 50 seconds, 24 to 45 seconds, 24 to 42 seconds.
(5)
The water-absorbent resin particles according to any one of the above (1) to (4), wherein the physiological saline water retention capacity of the water-absorbent resin particles is 15 to 80 g/g, 15 to 60 g/g, 15 to 50 g/g, 15 to 45 g/g, 15 to 40 g/g, 20 to 80 g/g, 20 to 60 g/g, 20 to 50 g/g, 20 to 45 g/g, 20 to 40 g/g, 25 to 80 g/g, 25 to 60 g/g, 25 to 50 g/g, 25 to 45 g/g, 25 to 40 g/g, 27 to 80 g/g, 27 to 60 g/g, 27 to 50 g/g, 27 to 45 g/g, or 27 to 40 g/g.
(6)
The water-absorbent resin particles have a median particle diameter of 200 to 700 μm, 200 to 600 μm, 200 to 550 μm, 200 to 500 μm, 200 to 450 μm, 250 to 700 μm, 250 to 600 μm, 250 to 550 μm, 250 to 500 μm, 250 to 450 μm, 300 to 700 μm, 300 to 600 μm, 300 to 550 μm, 300 to 500 μm, 300 to 450 μm, 350 to 700 μm, 350 to 600 μm, 350 to 550 μm, 350 to 500 μm, 350 to 450 μm, (1) to (5). The water-absorbent resin particles according to any one of the above.
(7)
The water-absorbent resin particles according to any one of (1) to (6) above, wherein the shape of the water-absorbent resin particles is irregularly crushed, or a shape in which irregularly crushed particles are aggregated, or irregularly crushed.

The present invention will be explained in detail below using examples and comparative examples. However, the present invention is not limited to these examples. Unless otherwise specified, the production examples, examples, comparative examples, and measurements were carried out in an environment of a temperature of 25±2°C and a humidity of 50±10%.

<Production of polymer particles>
(Production Example 1)
[First-stage polymerization step]
A round-bottomed, cylindrical, separable flask with an inner diameter of 11 cm and an internal volume of 2 L was prepared, equipped with a reflux condenser, a dropping funnel, a nitrogen gas inlet tube, and a stirrer. The stirrer used had a stirring blade with two stages of four inclined paddle blades with a blade diameter of 5 cm. 293 g of n-heptane and 0.736 g of maleic anhydride-modified ethylene-propylene copolymer (Hiwax 1105A, manufactured by Mitsui Chemicals, Inc.) as a dispersant were added to the separable flask and mixed. The mixture in the flask was stirred with the stirrer and heated to 80 °C, dissolving the dispersant in the n-heptane. The resulting n-heptane solution was cooled to 50 °C.

92.0 g (1.03 mol) of an 80.5% by mass acrylic acid aqueous solution (water-soluble ethylenically unsaturated monomer) was placed in a 300 mL beaker. While cooling from the outside, 147.7 g of a 20.9% by mass sodium hydroxide aqueous solution was added dropwise to the beaker to neutralize the 75 mol% acrylic acid. 0.092 g of hydroxyethyl cellulose (Sumitomo Seika Chemicals Co., Ltd., HEC AW-15F) as a thickener, 0.0736 g (0.272 mmol) of potassium persulfate as a radical polymerization initiator, and 0.0156 g (0.090 mmol) of ethylene glycol diglycidyl ether as an internal crosslinking agent were added and dissolved to prepare the first-stage aqueous solution.

The first-stage aqueous solution was added to the n-heptane solution in the separable flask, and the resulting reaction solution was stirred for 10 minutes. Separately, a surfactant solution was prepared by dissolving 0.736 g of surfactant sucrose stearate (Ryoto Sugar Ester S-370, HLB: 3, Mitsubishi Chemical Foods Corporation) in 6.62 g of n-heptane. This surfactant solution was added to the reaction solution, and while stirring the reaction solution at a stirrer speed of 550 rpm, the system was thoroughly purged with nitrogen. The separable flask was then immersed in a 70°C water bath to raise the temperature of the reaction solution, and the polymerization reaction was allowed to proceed for 60 minutes, yielding a first-stage polymerization slurry.

[Second-stage polymerization step]
A 500 mL beaker was charged with 128.8 g (1.44 mol) of an 80.5 mass % aqueous acrylic acid solution, and while cooling from the outside, 159.0 g of a 27 mass % aqueous sodium hydroxide solution was added dropwise to neutralize 75 mol % of the acrylic acid. 0.103 g (0.381 mmol) of potassium persulfate and 0.0116 g (0.067 mmol) of ethylene glycol diglycidyl ether were added to the beaker containing the neutralized aqueous acrylic acid solution and dissolved therein to prepare a second-stage aqueous solution.

The first-stage polymerization slurry in the separable flask was cooled to 25°C while stirring at a stirrer speed of 1000 rpm, and the entire amount of the second-stage aqueous solution was added to it. After replacing the atmosphere in the separable flask with nitrogen for 30 minutes, the separable flask was again immersed in a 70°C water bath to raise the temperature of the reaction liquid, and a hydrous gel polymer was formed through a 60-minute second-stage polymerization reaction.

[Surface crosslinking]
To the reaction solution containing the hydrous gel polymer, 0.589 g of a 45% by mass aqueous solution of pentasodium diethylenetriaminepentaacetate was added under stirring. Subsequently, the separable flask was immersed in an oil bath set at 125°C, and 256.1 g of water was extracted from the system by azeotropic distillation of n-heptane and water. 4.42 g of a 2% by mass aqueous solution containing ethylene glycol diglycidyl ether (0.508 mmol) as a surface cross-linking agent was placed in the separable flask, and the temperature was maintained at 83°C for 2 hours.

Then, the n-heptane was removed by drying at 125°C, yielding 232.06 g of polymer particles (1).

(Production Example 2)
[First-stage polymerization step]
A round-bottomed, cylindrical, separable flask with an inner diameter of 11 cm and an internal volume of 2 L was prepared, equipped with a reflux condenser, a dropping funnel, a nitrogen gas inlet tube, and a stirrer. The stirrer used had a stirring blade with two stages of four inclined paddle blades with a blade diameter of 5 cm. 293 g of n-heptane and 0.736 g of maleic anhydride-modified ethylene-propylene copolymer (Hiwax 1105A, manufactured by Mitsui Chemicals, Inc.) as a dispersant were added to the separable flask and mixed. The mixture in the flask was stirred with the stirrer and heated to 80 °C, dissolving the dispersant in the n-heptane. The resulting n-heptane solution was cooled to 50 °C.

92.0 g (1.03 mol) of 80.5% by mass acrylic acid aqueous solution (water-soluble ethylenically unsaturated monomer) was placed in a 300 mL beaker. While cooling externally, 147.7 g of 20.9% by mass sodium hydroxide aqueous solution was added dropwise to the beaker to neutralize the 75 mol% acrylic acid. 0.092 g of hydroxyethyl cellulose (Sumitomo Seika Chemicals Co., Ltd., HEC AW-15F) as a thickener, 0.092 g (0.339 mmol) of 2,2'-azobis(2-amidinopropane) dihydrochloride as a radical polymerization initiator, 0.028 g (0.102 mmol) of potassium persulfate, and 0.0046 g (0.026 mmol) of ethylene glycol diglycidyl ether as an internal crosslinking agent were added and dissolved to prepare the first-stage aqueous solution.

The first-stage aqueous solution was added to the n-heptane solution in the separable flask, and the resulting reaction solution was stirred for 10 minutes. Separately, a surfactant solution was prepared by dissolving 0.736 g of surfactant sucrose stearate (Ryoto Sugar Ester S-370, HLB: 3, Mitsubishi Chemical Foods Corporation) in 6.62 g of n-heptane. This surfactant solution was added to the reaction solution, and while stirring the reaction solution at a stirrer speed of 550 rpm, the system was thoroughly purged with nitrogen. The separable flask was then immersed in a 70°C water bath to raise the temperature of the reaction solution, and the polymerization reaction was allowed to proceed for 60 minutes, yielding a first-stage polymerization slurry.

[Second-stage polymerization step]
A 500 mL beaker was charged with 128.8 g (1.44 mol) of an 80.5% by mass aqueous acrylic acid solution, and while cooling from the outside, 159.9 g of a 27% by mass aqueous sodium hydroxide solution was added dropwise to neutralize 75 mol % of the acrylic acid. To the beaker containing the neutralized acrylic acid solution, 0.129 g (0.476 mmol) of 2,2'-azobis(2-amidinopropane) dihydrochloride, 0.039 g (0.143 mmol) of potassium persulfate, and 0.0116 g (0.067 mmol) of ethylene glycol diglycidyl ether as an internal crosslinking agent were added and dissolved to prepare a second-stage aqueous solution.

The first-stage polymerization slurry in the separable flask was cooled to 25°C while stirring at a stirrer speed of 1000 rpm, and the entire amount of the second-stage aqueous solution was added to it. After replacing the atmosphere in the separable flask with nitrogen for 30 minutes, the separable flask was again immersed in a 70°C water bath to raise the temperature of the reaction liquid, and a hydrous gel polymer was formed through a 60-minute second-stage polymerization reaction.

[Surface crosslinking]
To the reaction solution containing the hydrous gel polymer, 0.589 g of a 45% by mass aqueous solution of pentasodium diethylenetriaminepentaacetate was added under stirring. Subsequently, the separable flask was immersed in an oil bath set at 125°C, and 218.2 g of water was extracted from the system by azeotropic distillation of n-butane and water. 4.42 g of a 2% by mass aqueous solution containing ethylene glycol diglycidyl ether (0.508 mmol) as a surface cross-linking agent was placed in the separable flask, and the temperature was maintained at 83°C for 2 hours.

Then, the n-heptane was removed by drying at 125°C, yielding 247.6 g of polymer particles (2).

<Production of Water-Absorbent Resin Particles>
Example 1
The polymer particles (1) prepared in Production Example 1 were classified using a sieve with an opening of 500 μm to obtain polymer particles having a particle size of 500 μm or more.

[Crushing]
100 g of the polymer particles were pulverized using a centrifugal pulverizer (ZM200 manufactured by Retsch, screen diameter 1.5 μm, rotation speed 12,000 rpm) to obtain polymer particles. Subsequently, the obtained polymer particles were classified using a sieve with a mesh size of 150 μm to obtain polymer particles having a particle diameter of 150 μm or more. 0.5% by mass of amorphous silica (Toxil NP-S, Oriental Silicas Corporation) was mixed with the obtained polymer particles to obtain 50.71 g of irregularly pulverized water-absorbent resin particles of Example 1. The median particle diameter of the water-absorbent resin particles of Example 1 was 399 μm.

Example 2
50.0 g of the water-absorbent resin particles of Example 1 and polyethylene glycol (Tokyo Chemical Industry Co., Ltd., PEG20000) were crushed in a mortar and then passed through a sieve with a mesh size of 400 μm, and the resulting polyethylene glycol was added in an amount of 0.5 mass % relative to the mass of the water-absorbent resin particles of Example 1 to a 1.0 L SUS bottle, and the mixture was mixed for 30 minutes (revolution speed 50 rpm, rotation speed 50 rpm) using a cross rotary mixer (manufactured by Meiwa Kogyo Co., Ltd.) to obtain 50.25 g of irregularly crushed water-absorbent resin particles of Example 2. The median particle diameter of the water-absorbent resin particles of Example 2 was 382 μm.

Example 3
55.20 g of irregularly pulverized water absorbent resin particles of Example 3 was obtained in the same manner as in Example 1, except that the polymer particles (1) used were changed to the polymer particles (2) produced in Production Example 2. The median particle diameter of the water absorbent resin particles of Example 3 was 385 μm.

Example 4
[0123] 50.25 g of irregularly pulverized water absorbent resin particles of Example 4 were obtained in the same manner as in Example 2, except that the polymer particles (1) used were changed to the polymer particles (2) produced in Production Example 2. The median particle diameter of the water absorbent resin particles of Example 4 was 393 µm.

(Comparative Example 1)
The polymer particles (1) of Production Example 1 were passed through a sieve with an opening of 850 μm, and 0.5% by mass of amorphous silica (Toxil NP-S, Oriental Silicas Corporation) relative to the mass of the polymer particles (1) was mixed therewith, thereby obtaining 215.6 g of water-absorbent resin particles of Comparative Example 1 having a shape in which substantially spherical resin aggregates were formed. The water-absorbent resin particles of Comparative Example 1 had a median particle diameter of 351 μm.

[Measurement of Gel Diffusion Distance]
The gel diffusion distance of the water-absorbent resin particles was measured using an apparatus schematically shown in FIG. 1. As the apparatus, a stainless steel petri dish 61 was placed on a movable base plate 60 of a Curdmeter-MAX ME-500 (manufactured by Asuka Kikai). A cylindrical container (A) 62 equipped with a stainless steel wire mesh 63 having a mesh size of 38 μm covering the upper opening was placed in the stainless steel petri dish 61. Next, a cylindrical container (B) 64 having open ends at both axial ends was placed in the center of the stainless steel wire mesh 63, and the cylindrical container (B) 64 was fixed with a clamp 65. The stainless steel petri dish 61 had an inner diameter of 75 mm and a height of 20 mm. The cylindrical container (A) 62 had an inner diameter of 60 mm, an outer diameter of 70 mm, and a height of 60 mm. The cylindrical container (B) 64 had an inner diameter of 20 mm, an outer diameter of 30 mm, and a height of 60 mm. 0.20 g of water-absorbent resin particles (SAP) was uniformly dispersed inside the cylindrical container (B) 64, and a silicone hose with an inner diameter of 2.0 mm and a length of 8.0 cm, connected to a liquid pump (INTEGRA Biosciences, DOSE IT P910) was vertically installed at the center of the inner diameter of the cylindrical container (B) 64. 30 mL of 0.9% by mass saline solution was added at a constant rate of 20 mL/min. 20 seconds after the end of addition, the movable base plate 60 was lowered 5.0 cm at a speed of 1.2 cm/s. At this time, the gel diffusion distance was measured for the portion where the swollen gel formed by the water-absorbent resin particles absorbed water and contacted the stainless steel wire mesh. The gel diffusion distance (cm) was determined by measuring the maximum distance of a straight line passing through the center of the swollen gel on the stainless steel wire mesh when the swollen gel was located inside the cylindrical container (B) and connecting the ends of the swollen gel that had spread on the stainless steel wire mesh by lowering the movable plate, and the gel diffusion distance (cm) was determined by subtracting the inner diameter (diameter) of the cylindrical container (B), 2.0 cm, from the maximum distance Wa. The results are shown in Table 1.
Gel diffusion distance (cm) = maximum distance Wa - 2.0

[Saline Flow Conductivity (SFC) of Water-Absorbent Resin Particles]
(a) Preparation of synthetic urine: 2.0 g of potassium chloride, 2.0 g of anhydrous sodium sulfate, 0.25 g of calcium chloride dihydrate, 0.50 g of magnesium chloride hexahydrate, 0.85 g of ammonium dihydrogen phosphate, 0.15 g of ammonium monohydrogen phosphate, and an appropriate amount of distilled water were placed in a 1 L container and completely dissolved. Further distilled water was added to adjust the total volume of the aqueous solution to 1 L.

(b) Installation of the Measuring Apparatus The measuring apparatus used was one whose schematic configuration is shown in Figure 2. The apparatus consisted of a tank 39 equipped with a static pressure adjusting glass tube 38, and the lower end of the glass tube 38 was positioned so that the liquid level of a 0.69% by mass sodium chloride aqueous solution 40 in the cylinder 32 could be maintained at a height of 5 cm above the bottom of the swollen gel 35. The 0.69% by mass sodium chloride aqueous solution 40 in the tank 39 was supplied to the cylinder 32 through an L-shaped pipe 37 with a cock. A container 43 for collecting the liquid that passed through was placed below the cylinder 32, and the collection container 43 was placed on a top-loading balance 44. The cylinder 32 had an inner diameter of 6 cm, and a No. 400 stainless steel wire mesh (mesh opening: 38 µm) 36 was attached to the bottom of the lower part. The piston-type weight 31 had a hole 33 at the bottom that was large enough for the liquid to pass through, and a highly permeable glass filter 34 was attached to the bottom to prevent the water-absorbent resin or its swollen gel from entering the hole 33.

(c) Measurement of Saline Flow Conductivity (SFC) Water-absorbent resin particles (0.90 g) uniformly placed in a cylindrical container 30 were swollen in the above-mentioned synthetic urine under a load of 2.07 kPa for 60 minutes, and the height of the gel layer of the swollen gel 35 was recorded. Next, a 0.69 mass % sodium chloride aqueous solution 40 was supplied to the swollen gel layer from a tank 39 at a constant hydrostatic pressure under a load of 2.07 kPa. This SFC test was carried out at room temperature (20 to 25°C). Using a computer and a balance, the amount of liquid passing through the gel layer as a function of time was recorded at 20-second intervals for 10 minutes. The flow rate F _s (t) passing through the swollen gel 35 (mainly between the particles) was determined in units of g/s by dividing the increase in mass (g) of the liquid amount passing through the gel layer by the increase in time (s). The time when constant hydrostatic pressure and stable flow rate were obtained was designated as _ts , and only data obtained between _ts and 10 minutes were used for the flow rate calculation. The flow rates obtained between _ts and 10 minutes were used to calculate the value of _Fs (t=0), i.e., the initial flow rate through the gel layer. _Fs (t=0) was calculated by extrapolating the results of a least-squares fit of _Fs (t) versus time to t=0. The results are shown in Table 1.

SFC=(F _s (t=0)×L ₀ )/(ρ×A×ΔP)
=(F _s (t=0)×L ₀ )/139506
where:
F _s (t=0): flow rate in g/s L ₀ : initial height of the gel layer in cm ρ: density of 0.69 wt % sodium chloride aqueous solution = 1.003 g/cm ³
A: Area above the gel layer in the cylinder 32 = 28.27 cm ²
ΔP: hydrostatic pressure applied to the gel layer = 4920 dyne/cm ²
The unit of SFC is "×10 ⁻⁷ cm ³ ·s/g".

[Measurement of 3-minute value of no-pressure DW (Demand Wetability)]
The 3-minute value of the no-pressure DW of the water-absorbent resin particles was measured using a measuring device shown in Fig. 3. The measurement was carried out five times for one type of water-absorbent resin particle, and the average value of the measured values at three points excluding the minimum and maximum values was calculated. The results are shown in Table 1.

The measuring device comprises a burette unit 1, a conduit 5, a measurement table 13, a nylon mesh sheet 15, a stand 11, and a clamp 3. The burette unit 1 comprises a burette tube 21 with a scale, a rubber stopper 23 that seals the upper opening of the burette tube 21, a cock 22 connected to the lower tip of the burette tube 21, and an air introduction tube 25 and a cock 24 connected to the lower part of the burette tube 21. The burette unit 1 is fixed with a clamp 3. The flat measurement table 13 has a through-hole 13a with a diameter of 2 mm formed in its center and is supported by a height-adjustable stand 11. The through-hole 13a of the measurement table 13 and the cock 22 of the burette unit 1 are connected by a conduit 5. The inner diameter of the conduit 5 is 6 mm.

First, cocks 22 and 24 of burette part 1 were closed, and 0.9% by mass saline solution 50 adjusted to 25°C was poured into burette tube 21 through the opening at the top of burette tube 21. The 0.9% by mass concentration of saline solution is based on the mass of the saline solution. After sealing the opening of burette tube 21 with rubber stopper 23, cocks 22 and 24 were opened. The inside of conduit 5 was filled with 0.9% by mass saline solution 50, taking care to prevent air bubbles from entering. The height of measurement platform 13 was adjusted so that the height of the water surface of 0.9% by mass saline solution 50 that reached through hole 13a was the same as the height of the upper surface of measurement platform 13. After adjustment, the height of the water surface of 0.9% by mass saline solution 50 in burette tube 21 was read on the scale of burette tube 21, and this position was designated as the zero point (the reading at 0 seconds).

A nylon mesh sheet 15 (100 mm x 100 mm, 250 mesh, approximately 50 μm thick) was laid near the through-hole 13a on the measurement table 13, and a cylinder with an inner diameter of 30 mm and a height of 20 mm was placed in the center of the sheet. 1.00 g of water-absorbent resin particles 10a was uniformly dispersed into this cylinder. The cylinder was then carefully removed, yielding a sample in which the water-absorbent resin particles 10a were dispersed in a circular pattern in the center of the nylon mesh sheet 15. Next, the nylon mesh sheet 15 with the water-absorbent resin particles 10a placed on it was quickly moved so that its center was positioned at the position of the through-hole 13a, without allowing the water-absorbent resin particles 10a to dissipate, and measurement was started. The time when air bubbles were first introduced into the burette tube 21 from the air inlet tube 25 was defined as the start of water absorption (0 seconds).

The amount of the 0.9% by mass saline solution 50 lost in the burette tube 21 (i.e., the amount of 0.9% by mass saline solution absorbed by the water-absorbent resin particles 10a) was sequentially read in increments of 0.1 mL, and the amount of the lost weight Wb (mL) of the 0.9% by mass saline solution 50 lost 3 minutes after the start of water absorption by the water-absorbent resin particles 10a was read. From Wb, the 3-minute value of the no-pressure DW was calculated using the following formula. The no-pressure DW is the amount of water absorbed per 1.00 g of the water-absorbent resin particles 10a.
3-minute DW value without pressure (mL/g) = Wb/1.00

[Water absorption rate (Vortex method)]
The water absorption rate of saline solution by water-absorbent resin particles was measured using the Vortex method according to the following procedure. First, 50±0.1 g of saline solution adjusted to 25±0.2°C in a thermostatic water bath was weighed into a 100 mL beaker. Next, a vortex was generated by stirring at 600 rpm using a magnetic stir bar (8 mmφ×30 mm, without ring). 2.0±0.002 g of water-absorbent resin particles were added to the saline solution at once. The time (seconds) from the addition of the water-absorbent resin particles to the point at which the vortex on the liquid surface converged was measured, and this time was taken as the water absorption rate of the water-absorbent resin particles. The results are shown in Table 1.

[Saline water retention capacity]
A cotton bag (membrane broadcloth No. 60, width 100 mm x length 200 mm) containing 2.0 g of water-absorbent resin particles was placed in a 500 mL beaker. 500 g of a 0.9% by mass aqueous sodium chloride solution (physiological saline) was poured into the cotton bag containing the water-absorbent resin particles all at once to prevent the bag from becoming lumpy. The top of the cotton bag was tied with a rubber band and left to stand for 30 minutes to allow the water-absorbent resin particles to swell. After 30 minutes, the cotton bag was dehydrated for 1 minute using a dehydrator (manufactured by Kokusan Co., Ltd., product number: H-122) set to a centrifugal force of 167 G, and the mass Wc (g) of the cotton bag containing the swollen gel after dehydration was measured. The same procedure was performed without adding water-absorbent resin particles, and the empty mass Wd (g) of the cotton bag when wet was measured. The water retention capacity of the physiological saline solution was calculated using the following formula. The results are shown in Table 1.
Saline water retention capacity (g/g) = [Wc - Wd]/2.0

[Method for measuring median particle size]
10 g of water-absorbent resin particles were sieved using a continuous fully automatic ultrasonic vibration sieving measuring instrument (Robot Sifter RPS-205, manufactured by Seishin Enterprise Co., Ltd.), JIS standard sieves with openings of 850 μm, 710 μm, 600 μm, 500 μm, 425 μm, 300 μm, 250 μm, and 150 μm, and a tray. The mass of the particles remaining on each sieve was calculated as a mass percentage relative to the total amount. The mass percentages of the particles remaining on each sieve were integrated in descending order of particle size, and the relationship between the sieve opening and the integrated value of the mass percentage of the particles remaining on the sieve was plotted on logarithmic probability paper. By connecting the plots on the probability paper with a straight line, the particle diameter corresponding to an integrated mass percentage of 50% by mass was determined, and this was taken as the median particle diameter (μm).

[Preparation of absorbent body]
An airflow mixer (Autech Co., Ltd., Pad Former) was used to uniformly mix 12.0 g of water-absorbent resin particles and 6.4 g of pulverized pulp by air papermaking to prepare an absorbent core measuring 12 cm x 40 cm. Next, the absorbent core was sandwiched between two sheets of tissue paper (basis weight: 16 g/ ^m2 ) of the same size as the absorbent core, and pressed under a load of 141 kPa for 30 seconds. After that, 5.0 cm wide sections were cut off from both ends in the longitudinal direction to prepare an absorbent body.

[Method for measuring deformation resistance and diffusion length]
A hydrophilic top sheet (12 cm x 30 cm, basis weight 21 g/ ^m2 , Rengo Co., Ltd.) was placed on the upper surface of the prepared absorbent body to obtain an absorbent article. The absorbent article was placed on a horizontal table in a room at a temperature of 25±2°C, and 150 mL of artificial urine was poured all at once toward the center of the absorbent article from a liquid-feeding cylinder with an opening of 3 cm inner diameter. Five minutes after the artificial urine was poured, the hydrophilic top sheet was removed from the absorbent article, and the length of the absorbent body in the longitudinal direction passing through the center of the absorbent body of the blue region wetted with the test liquid was measured and recorded as the diffusion length (cm). Ten minutes after the artificial urine was poured, the absorbent body that had absorbed the artificial urine was used to evaluate its deformation resistance. The artificial urine composition used was as follows.
(Artificial urine composition)
Deionized water 5919.6g
・NaCl 60.0g
・CaCl ₂・H ₂ O 1.8g
・MgCl ₂・6H ₂ O 3.6g
・Food blue No. 1 (for coloring)
1% Triton X-100 15.0g

Figure 4 is a schematic diagram showing a method for evaluating the deformation resistance of an absorbent body. Two pieces of cardboard 71, 72 measuring 40 cm x 20 cm were prepared and completely covered with a liquid-impermeable resin sheet. The cardboard 71, 72 were secured with adhesive tape so that their long sides were in contact with each other, creating a test plate 73 that could be folded at the central joint 75. The absorbent body 70 was secured to the test plate 73 with its longitudinal direction perpendicular to the joint 75, the side on which the artificial urine had been poured facing the test plate 73, and the center of the absorbent body 70 overlapping the joint 75. Next, the test plate 73 was slowly and evenly folded at a rate of 10 degrees per second, forming a mountain fold with the joint 75 as the apex. Bending was stopped when a crack appeared in the absorbent body 70, and the angle θ (degrees) at which the test plate 73 had been folded up to that point was measured. A larger angle θ indicates better deformation resistance. The results are shown in Table 1.

1 Burette part 3 Clamp 4 Measurement part 5 Conduit 10a Water-absorbent resin particles 11 Stand 13 Measurement table 13a Through-hole 15 Nylon mesh sheet 21 Burette tube 22 Cock 23 Rubber stopper 24 Cock 25 Air introduction tube 30 Cylindrical container 31 Piston-type weight 32 Cylinder 33 Hole 34 Glass filter 35 Swollen gel 36 Stainless steel wire mesh 37 L-shaped tube with cock 38 Static pressure adjusting glass tube 39 Tank 40 0.69 mass% sodium chloride aqueous solution 41 Funnel 42 Support table 43 Container 44 Top-pan balance 60 Movable base plate 61 Stainless steel Petri dish 62 Cylindrical container (A)
63 Stainless steel wire mesh 64 Cylindrical container (B)
65 Clamp 70 Absorber 71 Corrugated cardboard 72 Corrugated cardboard 73 Test plate 75 Joint

Claims (5)

A water-absorbent resin particle having a gel diffusion distance, measured by the following method, of more than 0.0 cm and 2.0 cm or less.
(Method for measuring gel diffusion distance)
A stainless steel petri dish is placed on a movable base. A cylindrical container (A) equipped with a stainless steel wire mesh with a mesh size of 38 μm covering the upper opening is placed in the stainless steel petri dish. Next, a cylindrical container (B) with both axial ends open is placed in the center of the stainless steel wire mesh, and the cylindrical container (B) is fixed with a clamp. The stainless steel petri dish has an inner diameter of 75 mm and a height of 20 mm. The cylindrical container (A) has an inner diameter of 60 mm, an outer diameter of 70 mm, and a height of 60 mm. The cylindrical container (B) has an inner diameter of 20 mm, an outer diameter of 30 mm, and a height of 60 mm. Next, 0.20 g of water-absorbent resin particles are uniformly dispersed inside the cylindrical container (B), and the water-absorbent resin particles are placed on the stainless steel wire mesh. Next, 30 mL of 0.9% by mass physiological saline is poured into the cylindrical container (B) from above the center of the inner diameter at a height of 5.0 cm above the bottom end at a constant rate of 20 mL/min. 20 seconds after the end of the addition, while keeping the cylindrical container (B) fixed, the movable plate is lowered 5.0 cm at a speed of 1.2 cm/s. At this time, the gel diffusion distance is measured for the portion where the swollen gel formed by the water-absorbent resin particles absorbing water and the stainless steel wire mesh are in contact. The gel diffusion distance is measured by measuring the maximum distance of a straight line passing through the center point of the swollen gel when it was located in the cylindrical container (B) and connecting the ends of the swollen gel spread on the stainless steel wire mesh by lowering the movable plate, and the gel diffusion distance (cm) is calculated by subtracting the inner diameter (diameter) of the cylindrical container (B) of 2.0 cm from the maximum distance Wa.
Gel diffusion distance (cm) = maximum distance Wa - 2.0 2. The water-absorbent resin particles according to claim 1, wherein the water-absorbent resin particles have a saline flow conductivity (SFC, ×10 ⁻⁷ cm ³ ·s/g) of 50.0 or less. The water-absorbent resin particles according to claim 1, wherein the water-absorbent resin particles have an unpressurized DW value of 15 mL/g or more at 3 minutes. An absorbent material comprising the water-absorbent resin particles described in any one of claims 1 to 3. An absorbent article comprising the absorbent body according to claim 4.