Water-absorbent resin particles, absorber, and absorbent article

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

[0001] The present invention relates to water-absorbent resin particles, an absorbent body, and an absorbent article, and more particularly to water-absorbent resin particles constituting an absorbent body suitable for use in hygiene materials such as disposable diapers, sanitary napkins, and incontinence pads, and an absorbent body and an absorbent article using the water-absorbent resin particles.

[0002] BACKGROUND ART In recent years, water-absorbent resin particles have been widely used in the field of sanitary materials such as disposable diapers, sanitary napkins, and incontinence pads.

[0003] As such water-absorbent resin particles, a crosslinked product of a polymer of a water-soluble ethylenically unsaturated monomer, more specifically a crosslinked product of a polymer of a partially neutralized polyacrylic acid, has excellent water-absorbing ability, and since acrylic acid, which is a raw material thereof, is easily available industrially, it can be produced at low cost with constant quality, and is less susceptible to putrefaction and deterioration, and therefore, it is considered to be a preferable water-absorbent resin particle (see, for example, Patent Document 1).

[0004] Absorbent articles such as disposable diapers, sanitary napkins, and incontinence pads are mainly composed of an absorbent core located in the center for absorbing and retaining body 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. The absorbent core is usually composed of hydrophilic fibers such as pulp and water-absorbent resin particles.

[0005] Japanese Patent Application Publication No. 3-227301

[0006] In such absorbent articles, the absorbent body that has absorbed liquid may be subjected to a load due to deformation such as bending, and such deformation may easily cause damage such as cracks in the absorbent body that has absorbed water. Therefore, it is desirable for the absorbent body that has absorbed water to have high resistance to deformation.

[0007] Under these circumstances, a main object of the present invention is to provide water-absorbent resin particles that can have higher resistance to deformation of an absorbent body that has absorbed water. Another object of the present invention is to provide an absorbent body and an absorbent article that utilize the water-absorbent resin particles.

[0008] The present inventors have conducted extensive research to solve the above-mentioned problems. In the course of their research, the present inventors have focused on the gel diffusion distance, which is an index of the binding strength of absorbed water-absorbent resin particles, and have found 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. That is, the present inventors have conducted extensive research into means for imparting higher resistance to deformation of an absorbent body containing water-absorbent resin particles, and have found that water-absorbent resin particles having a gel diffusion distance set within a specific range have suitable resistance to deformation of the absorbent body when the absorbent body containing water-absorbent resin particles absorbs water. The present invention has been completed based on this finding and through further extensive research.

[0009] That is, the present invention provides an invention having the following configuration. Item 1. Water-absorbent resin particles having a gel diffusion distance of more than 0.0 cm and 2.0 cm or less, measured by the following method. (Method for measuring gel diffusion distance) A stainless steel petri dish is placed on a movable base. A cylindrical container (A) having a stainless steel wire mesh with a mesh size of 38 μm covering an upper opening is placed in the stainless steel petri dish. Next, a cylindrical container (B) having open ends at both axial ends 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 is uniformly scattered inside the cylindrical container (B), and the water-absorbent resin particles are arranged on the stainless steel wire mesh. Next, 30 mL of 0.9% by mass physiological saline solution is poured into the cylindrical container (B) at a constant rate of 20 mL / min from a height of 5.0 cm above the bottom end and the center of the inner diameter. 20 seconds after the end of the pouring, the movable platen is lowered 5.0 cm at a speed of 1.2 cm / s while the cylindrical container (B) is fixed. At this time, the gel diffusion distance is measured for the portion where the swollen gel formed by the water-absorbent resin particles absorbs water and the stainless steel wire mesh contacts. The gel diffusion distance is measured by measuring the maximum distance of the 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 platen. The gel diffusion distance (cm) is calculated by subtracting the inner diameter (diameter) of the cylindrical container (B), 2.0 cm, from the maximum distance Wa. Gel diffusion distance (cm) = Maximum distance Wa - 2.0 Item 2. Saline flow conductivity (SFC) of the water-absorbent resin particles (×10 -7 cm 3Item 1. A water-absorbent resin particle according to Item 1, wherein a 3-minute no-pressure DW value of the water-absorbent resin particles is 15 mL / g or more. Item 2. An absorbent body comprising the water-absorbent resin particles according to any one of Items 1 to 3. Item 3. An absorbent article comprising the absorbent body according to Item 4.

[0010] According to the present invention, it is possible to provide water-absorbent resin particles that have higher resistance to deformation of an absorbent body when the absorbent body containing the water-absorbent resin particles absorbs water. Furthermore, the present invention can provide an absorbent body and an absorbent article that use the water-absorbent resin particles.

[0011] Fig. 4(a) and (b) are schematic diagrams illustrating a method for evaluating the deformation resistance of an absorbent body that has absorbed water. Fig. 4(a) is a schematic diagram illustrating a device for measuring the gel diffusion distance of water-absorbent resin particles. Fig. 4(b) is a schematic diagram illustrating a device for measuring the saline flow conductivity (SFC) of water-absorbent resin particles. Fig. 4(b) is a schematic diagram illustrating a device for measuring the 3-minute value of the no-pressure DW of water-absorbent resin particles. Fig. 4(a) is a schematic diagram illustrating a method for evaluating the deformation resistance of an absorbent body that has absorbed water.

[0012] As used herein, the term "comprising" includes "consisting essentially of" and "consisting of." Furthermore, as used herein, "(meth)acrylic" means "acrylic or methacrylic," "(meth)acrylate" means "acrylate or methacrylate," and "(poly)" refers to the presence or absence of the prefix "poly." Furthermore, as used herein, "water-soluble" means exhibiting a solubility of 5% by mass or more in water at 25°C.

[0013] In this specification, a numerical value connected with "~" means a numerical range that includes the numerical values ​​before and after "~" as the lower limit and upper limit. When multiple lower limit values ​​and multiple upper limit values ​​are listed separately, any lower limit value and upper limit value can be selected and connected with "~".

[0014] 1. Water-absorbent resin particles The water-absorbent resin particles of the present invention have a gel diffusion distance, measured by the following method, of more than 0.0 cm and not more than 2.0 cm. Due to these characteristics, 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) at a height of 5.0 cm above the bottom end and from the top of the center of the inner diameter at a constant rate of 20 mL / min. 20 seconds after the end of the pouring, the movable platen is lowered 5.0 cm at a speed of 1.2 cm / s while the cylindrical container (B) is fixed. At this time, the gel diffusion distance is measured for the portion where the swollen gel formed by the water-absorbent resin particles absorbs water and contacts the stainless steel wire mesh. The gel diffusion distance was determined by measuring the maximum distance of the 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 that spread on the stainless steel mesh by lowering the movable plate, and subtracting the inner diameter (diameter) of the cylindrical container (B), 2.0 cm, from the maximum distance Wa to obtain the gel diffusion distance (cm). Gel diffusion distance (cm) = Maximum distance Wa - 2.0

[0015] The gel diffusion distance measured for water-absorbent resin particles can be said to be an index of the binding strength of water-absorbent resin particles that have absorbed water. As described above, the present inventors have found that there is a correlation between the gel diffusion distance, which is an index of the binding strength of water-absorbent resin particles that have absorbed water, and the resistance of an absorbent body containing water-absorbent resin particles to deformation when the absorbent body absorbs water. That is, 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 suitable 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 the absorbent resin particles that have absorbed water, so that even when bent in the absorbent body, the water-absorbent resin particles are bound to each other and to hydrophilic fibers, and are less likely to crack due to deformation, which is thought to be why such excellent effects are exhibited.

[0016] 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 not more than 2.0 cm, but from the viewpoint of more suitably exhibiting 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. 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.

[0017] 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 step such as reverse-phase suspension polymerization described below. Also, 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 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 absorbing a liquid.

[0018] From the viewpoint of more suitably exhibiting 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 cm 3 s / g or more, more preferably 0.3 × 10 -7 cm 3 s / g or more, more preferably 1.0 × 10 -7 cm 3 s / g or more, particularly preferably 5.0 × 10 -7 cm 3 s / g or more, more particularly preferably 10.0 × 10 -7 cm 3 s / g or more. In order to suppress 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 body, the saline flow conductivity (SFC) of the water-absorbent resin particles is preferably 50.0 × 10 -7 cm 3 s / g or less, more preferably 30.0 × 10 -7 cm 3 s / g or less, more preferably 25.0 × 10 -7 cm 3 s / g or less, particularly preferably 20.0 × 10 -7 cm 3 s / g or less. The preferred range is 0.0 × 10 -7 cm 3 ・s / g~50.0×10 -7 cm 3 ・s / g, 0.0×10 -7 cm 3 ・s / g~30.0×10 -7 cm 3 ・s / g, 0.0×10 -7cm 3 ・s / g~25.0×10 -7 cm 3 ・s / g、0.0×10 -7 cm 3 ・s / g~20.0×10 -7 cm 3 ・s / g、0.3×10 -7 cm 3 ・s / g~50.0×10 -7 cm 3 ・s / g、0.3×10 -7 cm 3 ・s / g~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 can be mentioned.

[0019] The saline flow conductivity (SFC) of the water-absorbent resin particles is measured by the method described in the examples.

[0020] In addition, from the viewpoint of shortening the diffusion length of the absorbent body, 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, more particularly preferably 30 mL / g or more, and even particularly preferably 32 mL / g or more. In addition, 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 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 to 60 mL / g, 28 to 55 mL / g, 28 to 50 mL / g, 28 to 48 mL / g, 28 to 45 mL / g, 30 to 60 mL / g, 30 to 55 mL / g, 30 to 50 mL / g, 30 to 48 mL / g, 30 to 45 mL / g, 32 to 60 mL / g, 32 to 55 mL / g, 32 to 50 mL / g, 32 to 48 mL / g, and 32 to 45 mL / g.

[0021] The no-pressure DW 3-minute value of the water-absorbent resin particles is measured by the method described in the examples.

[0022] The water absorption speed of the water-absorbent resin particles for physiological 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. From the viewpoint of shortening the diffusion length of the absorbent body, the water absorption speed of the water-absorbent resin particles for physiological 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, still 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.

[0023] The water absorption rate of the water-absorbent resin particles for physiological saline is measured by the method described in the Examples.

[0024] 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, and 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.

[0025] The water retention capacity of the water-absorbent resin particles in physiological saline solution is measured by the method described in the Examples.

[0026] The water-absorbent resin particles of the present invention are constituted by crosslinking a polymer of a water-soluble ethylenically unsaturated monomer, that is, by a crosslinked polymer having structural units derived from a water-soluble ethylenically unsaturated monomer.

[0027] 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, approximately spherical, irregularly pulverized, plate-like, fibrous, flake-like, and aggregated shapes of these resins.

[0028] 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 an irregularly crushed shape or a shape in which irregularly crushed particles are aggregated, and more preferably an irregularly crushed shape. As described above, for example, by drying and then crushing polymer particles obtained through a polymerization step such as reverse-phase suspension polymerization described below, the gel diffusion distance of the obtained 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.

[0029] From the viewpoint of more suitably exhibiting 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 include a median particle size 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, and 350 to 450 μm.

[0030] The median particle size of the water-absorbent resin particles can be measured using a JIS standard sieve, and specifically, it is a value measured by the method described in the examples.

[0031]

[0033] 2. Manufacturing Method of Water-Absorbent Resin Particles

[0034] 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.

[0032] As described above, examples of a method 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 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 (e.g., a thickener such as polyethylene glycol) to the water-absorbent resin particles to increase the viscosity of the water-absorbent resin particles after absorbing a liquid. By adopting these methods, the binding force of the water-absorbent resin particles after absorbing a liquid can be increased, thereby shortening the gel diffusion distance. Hereinafter, the method for producing water-absorbent resin particles of the present invention will be described in detail.

[0033] <Polymerization Step> 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.

[0034] [Water-soluble ethylenically unsaturated monomer] Examples of the water-soluble ethylenically unsaturated monomer include (meth)acrylic acid (in this specification, "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 such as N,N-diethylaminoethyl(meth)acrylate, N,N-diethylaminopropyl(meth)acrylate, and diethylaminopropyl(meth)acrylamide, and quaternized products thereof. Among these water-soluble ethylenically unsaturated monomers, (meth)acrylic acid or a salt thereof, (meth)acrylamide, and N,N-dimethylacrylamide are preferred, and (meth)acrylic acid and a salt thereof are more preferred, from the viewpoint of industrial ease of availability, etc. These water-soluble ethylenically unsaturated monomers may be used alone or in combination of two or more.

[0035] Among these, acrylic acid and its salts are widely used as raw materials for water-absorbent resin particles, and these acrylic acid and / or salts thereof may be copolymerized with the other water-soluble ethylenically unsaturated monomers described above for use. In this case, it is preferable that acrylic acid and / or its salts are used as the main water-soluble ethylenically unsaturated monomer in an amount of 70 to 100 mol % based on the total amount of water-soluble ethylenically unsaturated monomers.

[0036] The water-soluble ethylenically unsaturated monomer may be dispersed in a hydrocarbon dispersion medium in the form of an aqueous solution and subjected to reversed-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. 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.

[0037] 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 neutralizing agent, as necessary. Examples of such alkaline neutralizing agents include alkali metal salts such as sodium hydroxide, sodium carbonate, sodium bicarbonate, potassium hydroxide, and potassium carbonate; and ammonia. These alkaline neutralizing agents may be used in the form of an aqueous solution to simplify the neutralization operation. The alkaline neutralizing agents described above may be used alone or in combination of two or more.

[0038] 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 still more preferably 70 to 80 mol %, as the degree of neutralization with respect to all acid groups possessed by the water-soluble ethylenically unsaturated monomer.

[0039] [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 to be 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.

[0040] The amount of radical polymerization initiator used is, for example, 0.00005 to 0.01 mole per mole of the 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.

[0041] [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 crosslinking agents may be used alone or in combination of two or more.

[0042]

[0113] From the viewpoint of adjusting the gel diffusion distance, SFC, no-pressure DW 3-minute value, water absorption rate for physiological saline and water retention capacity of physiological saline of the water-absorbent resin particle within suitable ranges, the amount of the internal crosslinking agent to be used is preferably 0.02 mol or less, more preferably 0.000001 to 0.01 mol, still more preferably 0.000005 to 0.005 mol, and still more preferably 0.00001 to 0.00005 mol, relative to 1 mol of the water-soluble ethylenically unsaturated monomer.

[0043] [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 they are easily available industrially, have stable quality, and are inexpensive. These hydrocarbon dispersion media may be used alone or in combination of two or more. As an example of a mixture of hydrocarbon dispersion media, a commercially available product such as Exxol Heptane (manufactured by ExxonMobil Corporation; contains 75 to 85% by mass of hydrocarbons such as heptane and its isomers) can also be used to obtain favorable results.

[0044] 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 viewpoints of uniformly dispersing the water-soluble ethylenically unsaturated monomer and facilitating control of the polymerization temperature. As will be described later, the reversed-phase suspension polymerization is carried out in one stage (single stage) or in multiple stages of two or more stages, and the above-mentioned first stage polymerization refers to the polymerization reaction in the first stage of single-stage polymerization or multi-stage polymerization (the same applies hereinafter).

[0045] [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. A surfactant can be used as the dispersion stabilizer.

[0046] Examples of surfactants that can be used include 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, alkylallyl 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, phosphate esters of polyoxyethylene alkyl ethers, and phosphate esters of polyoxyethylene alkyl allyl ethers. Among these surfactants, sorbitan fatty acid esters, polyglycerin fatty acid esters, and sucrose fatty acid esters are particularly preferred from the standpoint of dispersion stability of the monomer. These surfactants may be used alone or in combination of two or more.

[0047] The amount of surfactant used is preferably 0.1 to 30 parts by mass, more preferably 0.3 to 20 parts by mass, per 100 parts by mass of the first stage water-soluble ethylenically unsaturated monomer.

[0048] (Polymer-Based Dispersant) As a dispersion stabilizer used in reversed-phase suspension polymerization, a polymer-based dispersant may be used in combination with the surfactant described above.

[0049] 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, and ethylhydroxyethyl cellulose. Among these polymeric dispersants, it is particularly preferable to use 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 from the viewpoint of dispersion stability of the monomer. These polymeric dispersants may be used alone or in combination of two or more.

[0050] The amount of the polymeric dispersant used is preferably 0.1 to 30 parts by mass, more preferably 0.3 to 20 parts by mass, per 100 parts by mass of the first stage water-soluble ethylenically unsaturated monomer.

[0051] [Other Components] In the method for producing water-absorbent resin particles, if desired, other components may be added to an aqueous solution containing a 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.

[0052] For example, a thickener can be added to an aqueous solution containing a water-soluble ethylenically unsaturated monomer to carry out reversed-phase suspension polymerization. By adjusting the viscosity of the aqueous solution by adding a thickener in this way, it is possible to control the median particle size obtained in the reversed-phase suspension polymerization.

[0053] Examples of usable thickeners 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, polyethylene oxide, etc. If the stirring speed during polymerization is the same, the higher the viscosity of the water-soluble ethylenically unsaturated monomer aqueous solution, the larger the primary particles and / or secondary particles of the resulting particles tend to be.

[0054]

[0033] In performing the 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.

[0055] Among these, from the viewpoint of easily reducing 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 having a polymeric dispersant dispersed therein, and then further disperse a surfactant therein, and then carry out polymerization.

[0056] Such reversed-phase suspension polymerization can be carried out in one stage or in two or more stages, and is preferably carried out in two or three stages from the viewpoint of increasing productivity.

[0057] When carrying out reversed-phase suspension polymerization in two or more stages, after carrying out reversed-phase suspension polymerization in the first stage, a water-soluble ethylenically unsaturated monomer is added to the reaction mixture obtained in the polymerization reaction in the first stage and mixed, and reversed-phase suspension polymerization in the second and subsequent stages can be carried out in the same manner as in the first stage.In the reversed-phase suspension polymerization in each stage from the second stage onwards, it is preferable to carry out reversed-phase suspension polymerization by adding a radical polymerization initiator in addition to the water-soluble ethylenically unsaturated monomer within the molar ratio of each component to the water-soluble ethylenically unsaturated monomer as described above, based on the amount of the water-soluble ethylenically unsaturated monomer added during the reversed-phase suspension polymerization in each stage from the second stage onwards.In addition, in the polymerization in the second and subsequent stages, an internal crosslinking agent may be added to the water-soluble ethylenically unsaturated monomer as needed.

[0058] The reaction temperature of the polymerization reaction is preferably 20 to 110°C, more preferably 40 to 90°C, from the viewpoints of rapidly progressing the polymerization, shortening the polymerization time, thereby improving economic efficiency, and easily removing the heat of polymerization to smoothly carry out the reaction.

[0059] <Dehydration Step> After the above-described reversed-phase suspension polymerization, a dehydration step may be included in which water, hydrocarbon dispersion medium, and the like are removed by distillation by externally applying energy such as heat. 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 treatment 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.

[0060] In the dehydration step, dehydration treatment by distillation may be carried out under normal pressure. When dehydration treatment 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.

[0061] <Surface cross-linking step> 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 (aqueous gel), the surface cross-linking step is a step of adding a surface cross-linking agent to the aqueous gel 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.

[0062] Examples of the surface cross-linking agent include compounds having two or more reactive functional groups. For example, 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 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.

[0063] The amount of the surface crosslinking agent used is preferably 0.00001 to 0.01 mol, more preferably 0.00005 to 0.005 mol, still more preferably 0.0001 to 0.001 mol, and still more preferably 0.0002 to 0.0009 mol, relative to 1 mol of the total amount of the water-soluble ethylenically unsaturated monomers used in the polymerization.

[0064] As a method for adding the surface crosslinking agent, the surface crosslinking agent may be added as it is or as an aqueous solution, or, if necessary, may be added as a solution using a hydrophilic organic solvent as a solvent. Examples of the hydrophilic organic solvent include lower alcohols such as methyl alcohol, ethyl alcohol, n-propyl alcohol, isopropyl alcohol, etc.; ketones such as acetone, methyl ethyl ketone, etc.; ethers such as diethyl ether, dioxane, tetrahydrofuran, etc.; amides such as N,N-dimethylformamide, etc.; sulfoxides such as dimethyl sulfoxide, etc. These hydrophilic organic solvents may be used alone, or two or more types may be used in combination, or as a mixed solvent with water.

[0065] From the viewpoint of adjusting the gel diffusion distance, SFC, no-pressure DW 3-minute value, water absorption rate for physiological saline, and water retention capacity of physiological saline of the water-absorbent resin particles within suitable ranges, the timing of addition of the surface crosslinking agent may be after the polymerization reaction of the water-soluble ethylenically unsaturated monomer has almost completely finished, and the surface crosslinking agent is added in the presence of water in a range of preferably 1 to 400 parts by mass, more preferably 5 to 200 parts by mass, still more preferably 10 to 100 parts by mass, still more preferably 15 to 60 parts by mass, and particularly preferably 35 to 50 parts by mass, relative to 100 parts by mass of the water-soluble ethylenically unsaturated monomer used for polymerization. Note that the amount of water means the total amount of water contained in the reaction system and water used as needed when adding the surface crosslinking agent.

[0066] The reaction temperature in the surface crosslinking reaction is preferably 50 to 250° C., more preferably 60 to 180° C., further preferably 60 to 140° C., and even more preferably 70 to 120° C. The reaction time of the surface crosslinking reaction is preferably 1 to 300 minutes, and more preferably 5 to 200 minutes.

[0067] <Drying step> After the above-described 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.

[0068] In the drying step, the drying treatment by distillation may be carried out under normal pressure or under reduced pressure. Moreover, from the viewpoint of increasing the drying efficiency, it may be carried out under a gas flow such as nitrogen. When the drying treatment 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. Moreover, when the drying treatment is carried out under reduced pressure, the drying temperature is preferably 40 to 160°C, more preferably 50 to 110°C.

[0069] In addition, when the 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.

[0070] After the drying step, the polymer particles may be crushed to obtain crushed particles having an angular shape, thereby increasing the binding force of the water-absorbent resin particles that have absorbed water, and thereby adjusting the gel diffusion distance of the obtained water-absorbent resin particles to a range of more than 0.0 cm and not more than 2.0 cm. The method for crushing the polymer particles is not particularly limited, and the polymer particles can be crushed using a crusher such as a centrifugal crusher, a roller mill, a stamp mill, a jet mill, a high-speed rotary crusher, and a container-driven mill.

[0071] From the viewpoint of adjusting the median particle diameter of the water-absorbent resin particles to a suitable range, the polymer particles obtained by pulverization may be classified. Classification refers to an operation of dividing a particle group (powder) into two or more particle groups having 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, and 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 pass through 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 by utilizing air flow.

[0072] The water-absorbent resin particles of the present invention may contain additives according to the purpose. Examples of such additives include surfactants, oxidizing agents, reducing agents, metal chelating agents, radical chain inhibitors, antioxidants, antibacterial agents, etc. The additives are preferably hydrophilic or water-soluble.

[0073] From the viewpoint of adjusting the no-pressure DW 3-minute value and the water absorption rate for physiological saline solution to a suitable range, the water-absorbent resin particles may contain an inorganic powder. The inorganic powder is preferably hydrophilic or water-soluble, and for example, amorphous silica can be used. The inorganic powder is preferably contained in an amount of 0.05 to 5 parts by mass, more preferably 0.1 to 2.5 parts by mass, per 100 parts by mass of the water-absorbent resin particles.

[0074] By including a thickener in the water-absorbent resin particles, the viscosity of the water-absorbent resin particles after absorbing a liquid is increased, thereby increasing the binding strength of the absorbed water-absorbent resin particles, and the gel diffusion distance of the water-absorbent resin particles may be adjusted to a range of more than 0.0 cm and 2.0 cm or less. The thickener may be water-soluble, 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 the thickener per 100 parts by mass of the water-absorbent resin particles. It is also preferable to include the thickener in solid form relative to the water-absorbent resin particles.

[0075] 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.

[0076] 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 the absorbent's configuration 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 layered 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.

[0077] The basis weight of the water-absorbent resin particles in the absorbent body of the present invention is 30 g / m 2 More than 500g / m 2 The basis weight is preferably 100 g / m or less.2 More preferably, 120 g / m 2 More preferably, 140 g / m 2 and preferably 400 g / m 2 or less, more preferably 350 g / m 2 More preferably, 300 g / m or less 2 The following is the result.

[0078] The hydrophilic fiber may be at least one selected from the group consisting of finely ground wood pulp, cotton, cotton linter, 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 of synthetic resins such as hydrophilically treated polyamide, polyester, and polyolefin. The average fiber length of the hydrophilic fiber is usually 0.1 to 10 mm, or may be 0.5 to 5 mm.

[0079] The basis weight of the hydrophilic fiber in the absorbent body of the present invention is 0 g / m 2 800g / m or more 2 The basis weight is preferably 50 g / m or less. 2 More preferably, 70 g / m 2 More preferably, 90 g / m 2 More preferably, 100 g / m 2 and preferably 700 g / m 2 or less, more preferably 600 g / m 2 More preferably 500 g / m or less 2 The following is the result.

[0080] The content of the 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.

[0081] 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 arranged on the side that comes into contact with the body, and the liquid-impermeable sheet is arranged on the opposite side that comes into contact with the body.

[0082] Examples of liquid-permeable sheets include air-through, spunbond, chemical-bond, and needle-punched nonwoven fabrics made of fibers such as polyethylene, polypropylene, and polyester, as well as porous synthetic resin sheets. Examples of liquid-impermeable sheets include synthetic resin films made of resins such as polyethylene, polypropylene, and polyvinyl chloride. The liquid-permeable sheet is preferably at least one selected from the group consisting of thermal-bonded nonwoven fabrics, air-through nonwoven fabrics, spunbonded nonwoven fabrics, and spunbonded / meltblown / spunbonded nonwoven fabrics.

[0083] The basis weight of the liquid permeable sheet is 5 g / m 2 More than 100g / m 2 Preferably, it is 10 g / m or less. 2 60g / m or more 2 It is more preferable that the liquid-permeable sheet has a surface embossed or perforated to improve the liquid diffusibility. The embossing or perforation can be carried out by a known method.

[0084] 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 (for example, spunbond nonwoven fabrics, spunlace nonwoven fabrics). As the liquid-impermeable sheet, a sheet made of a synthetic resin mainly composed of low-density polyethylene (LDPE) resin can also be used. The liquid-impermeable sheet has, for example, a basis weight of 10 to 50 g / m. 2The sheet may be made of a synthetic resin.

[0085] The absorbent article preferably comprises a laminate having an absorbent body containing water-absorbent resin particles and a core wrap 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 to the liquid-permeable sheet side.

[0086] 4. Additional Notes This specification includes at least the inventions shown 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) A saline flow conductivity (SFC) of the water-absorbent resin particles is 0.0×10 -7 cm 3 ・s / g~50.0×10 -7 cm 3 ・s / g, 0.0×10 -7 cm 3 ・s / g~30.0×10 -7 cm 3 ・s / g, 0.0×10 -7 cm 3 ・s / g~25.0×10 -7 cm 3 ・s / g, 0.0×10 -7 cm 3 ・s / g~20.0×10 -7 cm 3 ・s / g, 0.3×10 -7 cm 3 ・s / g~50.0×10 -7 cm 3 ・s / g, 0.3×10 -7 cm 3 ・s / g~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 -7cm 3 ・s / g~20.0×10 -7 cm 3(3) The water-absorbent resin particles according to the above (1), wherein the DW value at 3 minutes without pressure in physiological saline of the water-absorbent resin particles is 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 4 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-absorbent resin particles according to any one of (1) to (3) above, wherein the water-absorbent resin particles have a water-absorption speed for physiological saline of 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, or 24 to 42 seconds. (5) The water-absorbent resin particles according to any one of (1) to (4), wherein the water-absorbent resin particles have a physiological saline water retention capacity of 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 according to any one of (1) to (5), wherein the median particle diameter of the water-absorbent resin particles is 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. (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.

[0087] The present invention will be described in detail below with reference to 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%.

[0088] <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, thereby dissolving the dispersant in the n-heptane. The resulting n-heptane solution was cooled to 50°C.

[0089] A 300 mL beaker was charged with 92.0 g (1.03 mol) of an 80.5 wt% aqueous acrylic acid solution as the water-soluble ethylenically unsaturated monomer, and while cooling from the outside, 147.7 g of a 20.9 wt% aqueous sodium hydroxide 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 a first-stage aqueous solution.

[0090] 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 a surfactant, sucrose stearate ester (Ryoto Sugar Ester S-370, HLB: 3, manufactured by 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.

[0091] [Second-stage polymerization step] 128.8 g (1.44 mol) of an 80.5% by mass aqueous acrylic acid solution was placed in a 500 mL beaker, and while cooling from the outside, 159.0 g of a 27% by 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 acrylic acid solution and dissolved therein to prepare a second-stage aqueous solution.

[0092] 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 thereto. After the atmosphere in the separable flask was replaced with nitrogen for 30 minutes, the separable flask was again immersed in a water bath at 70°C to raise the temperature of the reaction solution, and a hydrous gel polymer was formed by a second-stage polymerization reaction for 60 minutes.

[0093] [Surface crosslinking] 0.589 g of a 45% by mass aqueous solution of pentasodium diethylenetriamine pentacetate was added to the reaction solution containing the hydrous gel polymer 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 crosslinking agent was placed in the separable flask, and the temperature was maintained at 83°C for 2 hours.

[0094] Thereafter, the n-heptane was removed by drying at 125° C., thereby obtaining 232.06 g of polymer particles (1).

[0095] (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, thereby dissolving the dispersant in the n-heptane. The formed n-heptane solution was cooled to 50 °C.

[0096] A 300 mL beaker was charged with 92.0 g (1.03 mol) of an 80.5 wt% aqueous acrylic acid solution as a water-soluble ethylenically unsaturated monomer. While cooling from the outside, 147.7 g of a 20.9 wt% aqueous sodium hydroxide solution was added dropwise to the beaker to neutralize the 75 mol% acrylic acid. To the solution, 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 a first-stage aqueous solution.

[0097] 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 a surfactant, sucrose stearate ester (Ryoto Sugar Ester S-370, HLB: 3, manufactured by 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.

[0098] [Second-stage polymerization step] 128.8 g (1.44 mol) of an 80.5 wt% aqueous acrylic acid solution was placed in a 500 mL beaker, and while cooling from the outside, 159.9 g of a 27 wt% aqueous sodium hydroxide solution was added dropwise to neutralize 75 mol% of the acrylic acid. 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 to the beaker and dissolved to prepare a second-stage aqueous solution.

[0099] 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 thereto. After the atmosphere in the separable flask was replaced with nitrogen for 30 minutes, the separable flask was again immersed in a water bath at 70°C to raise the temperature of the reaction solution, and a hydrous gel polymer was formed by a second-stage polymerization reaction for 60 minutes.

[0100] [Surface crosslinking] 0.589 g of a 45% by mass aqueous solution of pentasodium diethylenetriamine pentacetate was added to the reaction solution containing the hydrous gel polymer 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 crosslinking agent was placed in the separable flask, and the temperature was maintained at 83°C for 2 hours.

[0101] Thereafter, the n-heptane was removed by drying at 125° C., thereby obtaining 247.6 g of polymer particles (2).

[0102] <Production of Water-Absorbent Resin Particles> (Example 1) The polymer particles (1) produced 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.

[0103] [Pulverization] 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.

[0104] (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 into 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.

[0105] (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.

[0106] (Example 4) 50.25 g of irregularly pulverized water absorbent resin particles of Example 4 was 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.

[0107] (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.

[0108] [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 was determined by measuring the maximum distance of a straight line passing through the center point of the swollen gel when it was located inside the cylindrical container (B) and connecting the ends of the swollen gel that spread on the stainless steel 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

[0109] [Saline Flow Conductivity (SFC) of Water-Absorbent Resin Particles] (a) Preparation of Synthetic Urine Into a 1 L container, 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 and completely dissolved. Further distilled water was added to adjust the total volume of the aqueous solution to 1 L.

[0110] (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-shaped 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.

[0111] (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 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 for 10 minutes at 20-second intervals. The flow rate F passing through the swollen gel 35 (mainly between the particles) was s The time (t) was determined in units of g / s by dividing the increase in mass (g) of the liquid passing through the gel layer by the time (s) it took to pass through the gel layer. The time at which a constant hydrostatic pressure and a stable flow rate were obtained was defined as ts Let t s Only data obtained between t and 10 minutes are used for flow velocity calculation. s Using the flow rate obtained during 10 minutes, F s The value at (t = 0), i.e., the initial flow rate through the gel layer, was calculated. F s (t=0) is F s (t) was calculated by extrapolating the results of a least squares fit of time to t = 0. The results are shown in Table 1.

[0112] SFC = (F s (t=0)×L0) / (ρ×A×ΔP) = (F s (t=0)×L0) / 139506 where, F s (t = 0): flow rate in g / s L: initial height of the gel layer in cm ρ: density of 0.69% by mass 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 "x 10 -7 cm 3 .s / g".

[0113] [Measurement of 3-minute value of no-pressure DW (Demand Wetability)] The 3-minute value of no-pressure DW of 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.

[0114] 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 the 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 the 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.

[0115] First, the stopcocks 22 and 24 of the burette part 1 were closed, and 0.9% by mass saline solution 50 adjusted to 25°C was poured into the burette tube 21 through the opening at the top of the burette tube 21. The concentration of the saline solution, 0.9% by mass, is based on the mass of the saline solution. After sealing the opening of the burette tube 21 with a rubber stopper 23, the stopcocks 22 and 24 were opened. The inside of the conduit 5 was filled with 0.9% by mass saline solution 50 while preventing air bubbles from entering. The height of the measurement table 13 was adjusted so that the height of the water surface of the 0.9% by mass saline solution 50 that had reached the through-hole 13a was the same as the height of the upper surface of the measurement table 13. After adjustment, the height of the water surface of the 0.9% by mass saline solution 50 in the burette tube 21 was read on the scale of the burette tube 21, and this position was designated as the zero point (the reading at 0 seconds).

[0116] A nylon mesh sheet 15 (100 mm x 100 mm, 250 mesh, approximately 50 μm thick) was laid near the through-hole 13 a 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 10 a was uniformly dispersed in the cylinder. Thereafter, the cylinder was carefully removed, and a sample in which the water-absorbent resin particles 10 a were dispersed in a circular pattern was obtained in the center of the nylon mesh sheet 15. Next, the nylon mesh sheet 15 on which the water-absorbent resin particles 10 a were placed was quickly moved so that its center was positioned at the position of the through-hole 13 a, without dissipating the water-absorbent resin particles 10 a, 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).

[0117] 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 0.1 mL increments, and the amount of the lost amount Wb (mL) of the 0.9% by mass saline solution 50 lost 3 minutes after the water-absorbent resin particles 10a started absorbing water 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. The 3-minute value of the no-pressure DW value (mL / g) = Wb / 1.00

[0118] [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 a temperature of 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.

[0119] [Saline Water Retention Capacity] A cotton bag (membrane broadcloth No. 60, 100 mm wide x 200 mm long) containing 2.0 g of water-absorbent resin particles was placed in a 500 mL beaker. 500 g of a 0.9% by mass aqueous sodium chloride solution (saline) was poured into the cotton bag containing the water-absorbent resin particles all at once, taking care not to allow the bag to swell. The top of the cotton bag was tied with a rubber band and allowed 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 saline water retention capacity was calculated using the following formula. The results are shown in Table 1. Saline water retention capacity (g / g) = [Wc - Wd] / 2.0

[0120] [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.), sieves with JIS standard 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 order from the largest 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 size corresponding to an integrated mass percentage of 50 mass% was determined, and this was taken as the median particle size (μm).

[0121] [Preparation of absorbent core] 12.0 g of water-absorbent resin particles and 6.4 g of pulverized pulp were uniformly mixed by air papermaking using an air flow type mixer (Pad Former, manufactured by Autech Co., Ltd.) to prepare an absorbent core measuring 12 cm x 40 cm. Next, two sheets of tissue paper (basis weight: 16 g / m) of the same size as the absorbent core were placed on the paper. 2 The absorbent core was sandwiched between the sheets of paper and pressed under a load of 141 kPa for 30 seconds, after which 5.0 cm wide sections were cut off from both ends in the longitudinal direction to prepare an absorbent body.

[0122] [Method for measuring deformation resistance and diffusion length] A hydrophilic top sheet (12 cm x 30 cm, basis weight 21 g / m) was attached to the upper surface of the prepared absorbent body. 2 , Rengo Co., Ltd.) was placed on a horizontal table 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 defined 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 5919.6 g - NaCl 60.0 g - CaCl2.H2O 1.8 g - MgCl2.6H2O 3.6 g - Food Blue No. 1 (for coloring) - 15.0 g of 1%-Triton X-100

[0123] FIG. 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 and completely covered with a liquid-impermeable resin sheet were prepared. The cardboard 71, 72 were fixed with adhesive tape with their long sides in contact with each other to prepare a test plate 73 that could be folded at the central joint 75. The absorbent 70 was fixed to the test plate 73 with its longitudinal direction perpendicular to the joint 75, the side on which the artificial urine was poured facing the test plate 73, and the center of the absorbent 70 overlapping the joint 75 with the adhesive tape. Next, the test plate 73 was slowly and evenly folded at a speed of 10 degrees per second so that the joint 75 formed a mountain fold. The folding was stopped when a crack occurred in the absorbent 70, and the angle θ (degrees) at which the test plate 73 was folded up to that point was measured. A larger angle θ indicates better deformation resistance. The results are shown in Table 1.

[0124]

[0125] REFERENCE SIGNS LIST 1 burette part 3 clamp 4 measuring part 5 conduit 10a water-absorbent resin particles 11 stand 13 measuring table 13a through-hole 15 nylon mesh sheet 21 burette tube 22 cock 23 rubber stopper 24 cock 25 air inlet 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 glass tube for adjusting static pressure 39 tank 40 0.69 mass% sodium chloride aqueous solution 41 funnel 42 support base 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

1. 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 following method. (Method for measuring gel diffusion distance) A stainless steel petri dish is placed on a movable base. A cylindrical container (A) having 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) having open ends at both axial ends 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 is uniformly scattered inside the cylindrical container (B), and the water-absorbent resin particles are arranged on the stainless steel wire mesh. Next, 30 mL of 0.9% by mass physiological saline solution is poured into the cylindrical container (B) at a constant rate of 20 mL / min from a height of 5.0 cm above the bottom end and the center of the inner diameter. 20 seconds after the end of the pouring, the movable platen is lowered 5.0 cm at a speed of 1.2 cm / s while the cylindrical container (B) is fixed. At this time, the gel diffusion distance is measured for the portion where the swollen gel formed by the water-absorbent resin particles absorbs water and the stainless steel wire mesh contacts. The gel diffusion distance is measured by measuring the maximum distance of the 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 platen. The gel diffusion distance (cm) is calculated by subtracting the inner diameter (diameter) of the cylindrical container (B), 2.0 cm, from the maximum distance Wa. Gel diffusion distance (cm) = maximum distance Wa - 2.0 2. Saline flow conductivity (SFC, ×10 -7 cm 3 2. The water-absorbent resin particles according to claim 1, wherein the water-absorbent resin particles have a surface roughness (s / g) of 50.0 or less.

3. The water-absorbent resin particles according to claim 1, wherein the water-absorbent resin particles have a no-pressure DW value of 15 mL / g or more in 3 minutes.

4. An absorbent body comprising the water-absorbent resin particles according to any one of claims 1 to 3.

5. An absorbent article comprising the absorbent body according to claim 4.

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