Water-absorbent resin particles, water-absorbent resin particle production method, absorber, and absorbent article

Abstract

Provided are water-absorbent resin particles with which, in an absorbent article comprising an absorber containing the water-absorbent resin particles, when the absorbent article is deformed in a state in which a load is applied to the absorber containing a gel swollen due to the water-absorbent resin particles absorbing water, misalignment of the swollen gel in the absorber is inhibited.　The water-absorbent resin particles have a saline solution suction distance of 4.2 cm or more.

Description

Superabsorbent resin particles, method for producing superabsorbent resin particles, absorbent material, and absorbent article 【0001】 The present invention relates to water-absorbent resin particles, absorbents, and absorbent articles, and more specifically, to water-absorbent resin particles constituting absorbents suitably used in sanitary materials such as disposable diapers, sanitary napkins, and incontinence pads, a method for producing water-absorbent resin particles, and absorbents and absorbent articles using said water-absorbent resin particles. 【0002】 Superabsorbent polymer particles have recently been widely used in the field of sanitary materials such as disposable diapers, sanitary napkins, and incontinence pads. 【0003】 Such water-absorbing resin particles include crosslinked polymers of water-soluble ethylenically unsaturated monomers, and more specifically, crosslinked polymers of partially neutralized polyacrylic acid. These are considered preferable water-absorbing resin particles because they possess excellent water absorption capabilities, and because acrylic acid, their raw material, is readily available industrially, they can be manufactured with consistent quality and at low cost, and they have numerous advantages such as being resistant to spoilage and deterioration (see, for example, Patent Document 1). 【0004】 Absorbent products such as disposable diapers, sanitary napkins, and incontinence pads are primarily composed of an absorbent core located in the center that absorbs and retains bodily fluids such as urine and menstrual blood excreted from the body, a liquid-permeable top sheet located on the side that comes into contact with the body, and a liquid-impermeable 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】 For example, when an absorbent item is worn on the body, and a load is applied to the absorbent material containing a gel formed by the absorption of water-absorbing resin particles and subsequent swelling, a problem has been found where the swollen gel within the absorbent material is prone to displacement when the absorbent item deforms. 【0007】Under these circumstances, the main objective of the present invention is to provide water-absorbent resin particles that, in an absorbent article equipped with an absorbent body containing water-absorbent resin particles, suppress displacement of the swollen gel within the absorbent body when the absorbent article deforms under load, particularly when a load is applied to the absorbent body containing the swollen gel. The present invention also aims to provide an absorbent body and an absorbent article utilizing these water-absorbent resin particles, as well as a novel method for manufacturing water-absorbent resin particles. 【0008】 The inventors diligently studied to solve the above problems. In their studies, the inventors focused on the suction distance when water-absorbent resin particles absorb liquid. They found that when the suction distance of the water-absorbent resin particles under predetermined conditions is greater than or equal to a predetermined value, the displacement of the swollen gel within the absorbent material under load is suitably suppressed. The present invention was completed based on these findings and further diligent studies. 【0009】In other words, the present invention provides an invention having the following configuration. Item 1. Superabsorbent resin particles having a saline absorption distance of 4.2 cm to 7.6 cm, as measured by the following method. The measurement of the saline absorption distance is performed in an environment with a temperature of 25 ± 2°C and a humidity of 50 ± 10%. <Preparation of the measurement sample> A glass plate (26 mm × 76 mm, 1.2 mm thick) is prepared by aligning the center of a piece of double-sided tape (20 mm × 76 mm) and attaching it to the center, exposing the adhesive surface of the tape. The glass plate is placed in a stainless steel petri dish (120 mm inner diameter, 25 mm height), and 0.35 g of superabsorbent resin particles are uniformly scattered over the entire adhesive surface of the tape attached to the glass plate. Then, the glass plate is stood upright, the excess superabsorbent resin particles are collected, and the collected superabsorbent resin particles are again scattered over the entire adhesive surface of the tape attached to the glass plate. This operation is repeated until the excess absorbent resin particles are 0.04 g or less, adjusting the amount of absorbent resin particles on the tape to 0.33 ± 0.02 g. A nylon mesh (250 mesh, 26 mm x 100 mm) is placed in the center of the front surface (the surface on which the absorbent resin particles are scattered) of the glass plate, with its center aligned. Both ends of the nylon mesh are folded along the longitudinal ends of the glass plate and attached to the back of the glass plate using masking tape to secure them. Furthermore, a nylon mesh (250 mesh, 50 mm x 76 mm) is placed in the center of the front surface of the glass plate, with its center aligned. Both ends of the nylon mesh are folded along the short ends of the glass plate and attached to the back of the glass plate using masking tape to secure them, thereby obtaining a sample for measurement. <Measurement of the absorption distance of physiological saline> A support plate (acrylic resin plate) with a length of 45 cm and a flat inclined surface is fixed to a stand at an angle of 45 degrees to the horizontal plane. Place the sample to be measured on the inclined surface of the fixed support plate, with its longitudinal direction aligned with the longitudinal direction of the support plate, and the center of the sample to be measured aligning with the lower end of the support plate. Attach the upper end of the sample to the support plate with masking tape. Next, weigh out 15 g of physiological saline solution colored with food coloring blue no. 1 at 25±1°C into a stainless steel petri dish (inner diameter 75 mm, height 20 mm).Using a lab jack, the stainless steel petri dish is raised at a constant speed so that the sample to be measured comes into contact with the liquid surface in the stainless steel petri dish, and then, 3 seconds later, comes into contact with the bottom surface of the stainless steel petri dish. Thirty minutes after the sample to be measured comes into contact with the bottom surface of the stainless steel petri dish, the stainless steel petri dish is lowered using the lab jack to remove the sample to be measured from the liquid surface in the stainless steel petri dish. After that, the suction distance (cm) of the physiological saline absorbed by the sample to be measured is measured. The suction distance (cm) of physiological saline absorbed is the value measured by drawing a straight line parallel to the longitudinal direction of the sample to connect the edge of the glass plate that was immersed on the sample to the furthest point reached by the physiological saline with the longitudinal direction of the sample. Item 2. The water-absorbing resin particles according to Item 1, wherein the amount of physiological saline water retained by the water-absorbing resin particles is 35 g / g or more. Item 3. The water-absorbing resin particles according to Item 1 or 2, wherein the amount of water absorbed under load by the water-absorbing resin particles is 10 mL / g or more. Item 4. 1. Absorbent resin particles according to item 1 or 2, wherein the unpressurized DW3 min value of the absorbent resin particles is 30 mL / g or more. 5. Absorbent resin particles according to item 1 or 2, wherein the unpressurized DW5 min value of the absorbent resin particles is 40 mL / g or more. 6. An absorbent body containing the absorbent resin particles according to item 1 or 2. 7. An absorbent article containing the absorbent body according to item 6. 8. A method for producing absorbent resin particles by reverse-phase suspension polymerization of a water-soluble ethylenically unsaturated monomer in a hydrocarbon dispersion medium, wherein the reverse-phase suspension polymerization is carried out in two or more stages, in the first stage polymerization step of the reverse-phase suspension polymerization, the concentration of the water-soluble ethylenically unsaturated monomer in the aqueous solution containing the water-soluble ethylenically unsaturated monomer is 40% by mass or more, and in the second stage polymerization step of the reverse-phase suspension polymerization, an internal crosslinking agent is used in an amount of 0 mmol or more and 0.042 mmol or less per mole of the water-soluble ethylenically unsaturated monomer used in the second stage polymerization. 【0010】According to the present invention, in an absorbent article equipped with an absorbent body containing water-absorbent resin particles, when a load is applied to the absorbent body containing a swollen gel and the absorbent article deforms, the displacement of the swollen gel within the absorbent body is suppressed. Furthermore, the present invention can provide an absorbent body and an absorbent article utilizing these water-absorbent resin particles, as well as a novel method for manufacturing water-absorbent resin particles. 【0011】 This is a schematic diagram of an apparatus used to measure the water absorption capacity of superabsorbent polymer particles under load. This is a schematic diagram of an apparatus used to measure the unpressurized DW value of superabsorbent polymer particles. This is a schematic diagram illustrating the sample used for measuring the absorption distance of physiological saline for superabsorbent polymer particles. This is a schematic diagram illustrating the sample used for measuring the absorption distance of physiological saline for superabsorbent polymer particles. This is a schematic diagram illustrating the method for measuring the absorption distance of physiological saline for superabsorbent polymer particles. 【0012】 In this specification, "comprising" includes "consisting essentially of" and "consisting of." In this specification, "(meth)acrylic" means "acrylic or methacrylic," "(meth)acrylate" means "acrylate or methacrylate," and "(poly)" means with or without the prefix "poly." In this specification, "water-soluble" means solubility of 5% by mass or more in water at 25°C. In this specification, "room temperature" means 25±2°C. In addition, the term "layer" includes not only structures that are formed over the entire surface when observed in a plan view, but also structures that are formed in part. 【0013】 In this specification, numbers enclosed in "~" represent a numerical range that includes the numbers before and after "~" as the lower and upper limits, respectively. If multiple lower and upper limits are listed separately, any lower and upper limits may be selected and enclosed in "~". 【0014】[Absorbent Resin Particles] The absorbent resin particles of the present invention have an absorption distance of physiological saline solution of 4.2 cm to 7.6 cm, as measured by the following method. Because the absorbent resin particles of the present invention possess these characteristics, when an absorbent article containing absorbent resin particles is deformed under load, the displacement of the swollen gel within the absorbent material is suppressed. The absorbent resin particles of the present invention will be described in detail below. 【0015】 In this invention, the absorption distance of physiological saline solution measured for superabsorbent resin particles is the absorption distance of the liquid while the superabsorbent resin particles are fixed. A long absorption distance indicates good liquid diffusion when the superabsorbent resin particles are fixed. Through repeated investigations by the inventors of this invention, it became clear that good liquid diffusion when the superabsorbent resin particles are fixed effectively suppresses displacement of the gel formed by the swelling of the superabsorbent resin particles within the absorbent material. This is thought to be because the amount of liquid present around the gel formed by the swelling of the superabsorbent resin particles is reduced, thereby suppressing the movement of the gel. 【0016】 From the viewpoint of exhibiting the effects of the present invention more favorably, the suction distance of the water-absorbing resin particles of the present invention is preferably 4.2 cm or more, more preferably 4.4 cm or more, and even more preferably 4.5 cm or more. Furthermore, the suction distance of the water-absorbing resin particles of the present invention may be in the range of 7.6 cm or less, but from the same viewpoint, it is preferably 7.0 cm or less, more preferably 6.0 cm or less, even more preferably 5.0 cm or less, and particularly preferably 4.5 cm or less. The preferred ranges for the suction distance of the superabsorbent resin particles of the present invention include 4.2 to 7.6 cm, 4.2 to 7.0 cm, 4.2 to 6.0 cm, 4.2 to 5.0 cm, 4.2 to 4.5 cm, 4.4 to 7.6 cm, 4.4 to 7.0 cm, 4.4 to 6.0 cm, 4.4 to 5.0 cm, 4.4 to 4.5 cm, 4.5 to 7.6 cm, 4.5 to 7.0 cm, 4.5 to 6.0 cm, 4.5 to 5.0 cm, and 4.5 cm. The suction distance is measured by the following method, more specifically by the method described in the examples. 【0017】The measurement of the saline solution absorption distance is performed in an environment with a temperature of 25±2°C and a humidity of 50±10%. <Preparation of the measurement sample> A glass plate (26 mm × 76 mm, thickness 1.2 mm) is prepared by aligning the center of a piece of double-sided tape (20 mm × 76 mm) and attaching it to the center, exposing the adhesive surface of the tape. The glass plate is placed in a stainless steel petri dish (inner diameter 120 mm, height 25 mm), and 0.35 g of superabsorbent resin particles are uniformly sprinkled over the entire adhesive surface of the tape attached to the glass plate. Then, the glass plate is stood upright, and the excess superabsorbent resin particles are collected. The collected superabsorbent resin particles are then sprinkled over the entire adhesive surface of the tape attached to the glass plate again. This operation is repeated until the amount of excess superabsorbent resin particles is 0.04 g or less, and the amount of superabsorbent resin particles on the tape is adjusted to 0.33 ± 0.02 g. A nylon mesh (250 mesh, 26 mm x 100 mm) is placed in the center of the front surface (the surface on which the water-absorbing resin particles are scattered) of the glass plate, with its center aligned to the center. The ends of the nylon mesh are folded along the ends of the glass plate in the longitudinal direction, and then attached and secured to the back of the glass plate using masking tape. Furthermore, a nylon mesh (250 mesh, 50 mm x 76 mm) is placed in the center of the front surface of the glass plate, with its center aligned to the center. The ends of the nylon mesh are folded along the ends of the glass plate in the short direction, and then attached and secured to the back of the glass plate using masking tape to obtain a sample for measurement. <Measurement of the absorption distance of physiological saline solution> A support plate (acrylic resin plate) with a length of 45 cm and a flat inclined surface is fixed by a stand at an angle of 45 degrees to the horizontal plane. The sample to be measured is placed on the inclined surface of a fixed support plate, with its longitudinal direction aligned with the longitudinal direction of the support plate, and the center of the sample aligning with the lower end of the support plate. The upper end of the sample is then attached to the support plate with masking tape. Next, 15 g of physiological saline solution colored with food coloring blue no. 1 at 25 ± 1°C is weighed into a stainless steel petri dish (inner diameter 75 mm, height 20 mm). The stainless steel petri dish is raised at a constant speed using a lab jack, so that the sample to be measured comes into contact with the liquid surface in the stainless steel petri dish, and then, 3 seconds later, comes into contact with the bottom surface of the stainless steel petri dish.Thirty minutes after the sample for measurement comes into contact with the bottom surface of the stainless steel petri dish, the stainless steel petri dish is lowered using a lab jack to remove the sample from the liquid surface within the petri dish. Subsequently, the distance (cm) that the sample has drawn up with saline solution is measured. The distance (cm) that the sample has drawn up with saline solution is measured by drawing a straight line parallel to the longitudinal direction of the sample, connecting the edge of the glass plate that was immersed on the sample and the furthest point reached by the saline solution. 【0018】 Furthermore, from the viewpoint of more favorably exhibiting the effects of the present invention, the saline water retention capacity of the superabsorbent resin particles of the present invention is preferably 35 g / g or more, more preferably 40 g / g or more, even more preferably 45 g / g or more, particularly preferably 47 g / g or more, preferably 60 g / g or less, more preferably 55 g / g or less, and even more preferably 49 g / g or less. Preferred ranges for the saline water retention capacity of the superabsorbent resin particles of the present invention include 35-60 g / g, 35-55 g / g, 35-49 g / g, 40-60 g / g, 40-55 g / g, 40-49 g / g, 45-60 g / g, 45-55 g / g, 45-49 g / g, 47-60 g / g, 47-55 g / g, and 47-49 g / g. The saline water retention capacity is measured by the method described in the examples. 【0019】 From the viewpoint of more favorably exhibiting the effects of the present invention, the water absorption under load of the water-absorbing resin particles of the present invention is preferably 10 mL / g or more, more preferably 12 mL / g or more, even more preferably 15 mL / g or more, preferably 30 mL / g or less, more preferably 24 mL / g or less, even more preferably 20 mL / g or less, and particularly preferably 18 mL / g or less. Preferred ranges for the water absorption under load of the water-absorbing resin particles of the present invention include 10 to 40 mL / g, 10 to 35 mL / g, 10 to 30 mL / g, 12 to 40 mL / g, 12 to 35 mL / g, 12 to 30 mL / g, 15 to 40 mL / g, 15 to 35 mL / g, and 15 to 30 mL / g. The water absorption under load is measured by the method described in the examples. 【0020】From the viewpoint of more favorably exhibiting the effects of the present invention, the unpressurized DW3 value of the water-absorbing resin particles of the present invention is preferably 30 mL / g or more, more preferably 36 mL / g or more, even more preferably 42 mL / g or more, particularly preferably 47 mL / g or more, even more preferably 51 mL / g or more, preferably 70 mL / g or less, more preferably 65 mL / g or less, and even more preferably 60 mL / g or less. Preferred ranges for the unpressurized DW3 min values ​​of the superabsorbent resin particles of the present invention include 30-70 mL / g, 30-65 mL / g, 30-60 mL / g, 36-70 mL / g, 36-65 mL / g, 36-60 mL / g, 42-70 mL / g, 42-65 mL / g, 42-60 mL / g, 47-70 mL / g, 47-65 mL / g, 47-60 mL / g, 51-70 mL / g, 51-65 mL / g, and 51-60 mL / g. The unpressurized DW3 min values ​​are measured by the method described in the examples. 【0021】 From the viewpoint of more favorably exhibiting the effects of the present invention, the unpressurized DW5 min value of the water-absorbent resin particles of the present invention is preferably 40 mL / g or more, more preferably 50 mL / g or more, even more preferably 60 mL / g or more, particularly preferably 66 mL / g or more, and also preferably 80 mL / g or less, more preferably 75 mL / g or less, and even more preferably 70 mL / g or less. Preferred ranges for the unpressurized DW5 min value of the water-absorbent resin particles of the present invention include 40-80 mL / g, 40-75 mL / g, 40-70 mL / g, 50-80 mL / g, 50-75 mL / g, 50-70 mL / g, 60-80 mL / g, 60-75 mL / g, 60-70 mL / g, 66-80 mL / g, 66-75 mL / g, and 66-70 mL / g. The unpressurized DW5 min value is measured by the method described in the examples. 【0022】 The water-absorbing resin particles of the present invention can be composed, for example, of a polymer of a water-soluble ethylenically unsaturated monomer. In the water-absorbing resin particles of the present invention, the polymer may be crosslinked, that is, composed of a crosslinked polymer having structural units derived from a water-soluble ethylenically unsaturated monomer. The polymer of the water-soluble ethylenically unsaturated monomer and the crosslinked polymer will be described in detail in the section on [Method for producing water-absorbing resin particles] below. 【0023】 Examples of the shapes of the water-absorbing resin particles of the present invention include granular, substantially spherical, irregularly shaped crushed, plate-like, fibrous, flake-like, or aggregated forms of these resins. 【0024】 From the viewpoint of more favorably exhibiting the effects of the present invention, the median particle size of the water-absorbing 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. Also, from the same viewpoint, the median particle size 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 medium particle sizes of 200-700 μm, 200-600 μm, 200-550 μm, 200-500 μm, 200-450 μm, 250-700 μm, 250-600 μm, 250-550 μm, 250-500 μm, 250-450 μm, 300-700 μm, 300-600 μm, 300-550 μm, 300-500 μm, 300-450 μm, 350-700 μm, 350-600 μm, 350-550 μm, 350-500 μm, and 350-450 μm. 【0025】 The median particle size of the water-absorbent resin particles can be measured using a JIS standard sieve, and specifically, the value is measured by the method described in the examples. 【0026】 The method for producing the water-absorbent resin particles of the present invention is not particularly limited as long as it produces water-absorbent resin particles that satisfy the above-mentioned absorption distance, but they are preferably produced by the method described in [Method for producing water-absorbent resin particles] below. 【0027】 [Method for producing water-absorbent resin particles] The present invention provides a method for producing water-absorbent resin particles by reverse-phase suspension polymerization of a water-soluble ethylenically unsaturated monomer in a hydrocarbon dispersion medium. 【0028】The present invention provides a method for producing superabsorbent resin particles, characterized in that the reverse-phase suspension polymerization is carried out in two or more stages, in the first stage polymerization step of the reverse-phase suspension polymerization, the concentration of the water-soluble ethylenically unsaturated monomer in the aqueous solution containing the water-soluble ethylenically unsaturated monomer is 40% by mass or more, and in the second stage polymerization step of the reverse-phase suspension polymerization, an internal crosslinking agent is used in an amount of 0 mmol or more and 0.042 mmol or less per mole of the water-soluble ethylenically unsaturated monomer used in the second stage polymerization. By having these characteristics, the present invention provides a method for producing superabsorbent resin particles that can suitably produce, for example, superabsorbent resin particles having the predetermined suction distance described in the [Superabsorbent Resin Particles] section above. The method for producing superabsorbent resin particles of the present invention will be described in detail below. 【0029】 <Reverse-Phase Suspension Polymerization> Reverse-phase suspension polymerization is a method for producing superabsorbent resin particles by polymerizing water-soluble ethylenically unsaturated monomers in a hydrocarbon dispersion medium under stirring. In the method for producing superabsorbent resin particles of the present invention, reverse-phase suspension polymerization is carried out in two or more stages, and the first stage polymerization and the second stage polymerization refer to the first and second stage polymerization reactions in the multi-stage polymerization, respectively (the same applies hereinafter). 【0030】 In reverse-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. At this time, as long as it is before the polymerization reaction begins, the timing of adding the dispersion stabilizer (surfactant or polymer-based dispersant) does not matter; it can be added before or after the addition of the aqueous monomer solution. 【0031】 Among these, from the viewpoint of easily reducing the amount of hydrocarbon dispersion medium remaining in the resulting water-absorbing resin particles, it is preferable to disperse a monomer aqueous solution in a hydrocarbon dispersion medium containing a polymer-based dispersant, and then further disperse a surfactant before carrying out polymerization. 【0032】 In the method for producing water-absorbent resin particles of the present invention, such reverse-phase suspension polymerization is carried out in two or more stages. From the viewpoint of increasing the productivity of water-absorbent resin particles, it is preferable to carry it out in two to three stages. 【0033】In the case of inverse suspension polymerization in two or more stages, after the inverse suspension polymerization in the first stage, a water-soluble ethylenically unsaturated monomer is added to and mixed with the reaction mixture obtained in the polymerization reaction of the first stage, and the inverse suspension polymerization from the second stage onward is carried out in the same manner as in the first stage. In the inverse suspension polymerization in each stage from the second stage onward, in addition to the water-soluble ethylenically unsaturated monomer, a radical polymerization initiator is added within the range of the molar ratio of each component to the water-soluble ethylenically unsaturated monomer described below, based on the amount of the water-soluble ethylenically unsaturated monomer added during the inverse suspension polymerization in each stage from the second stage onward, and it is preferable to carry out the inverse suspension polymerization. 【0034】 As the reaction temperature of the polymerization reaction in each stage, from the viewpoint of enhancing economy by allowing the polymerization to proceed rapidly and shortening the polymerization time, and easily removing the polymerization heat and allowing the reaction to proceed smoothly, it is preferably 20 to 110°C, and more preferably 40 to 90°C. 【0035】 (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 its salts; 2-(meth)acrylamido-2-methylpropanesulfonic acid and its salts; nonionic monomers such as (meth)acrylamide, N,N-dimethyl(meth)acrylamide, 2-hydroxyethyl (meth)acrylate, N-methylol(meth)acrylamide, polyethylene glycol mono(meth)acrylate; amino group-containing unsaturated monomers such as N,N-diethylaminoethyl (meth)acrylate, N,N-diethylaminopropyl (meth)acrylate, diethylaminopropyl (meth)acrylamide, and their quaternized products, etc. Among these water-soluble ethylenically unsaturated monomers, from the viewpoints of easy industrial availability, etc., (meth)acrylic acid or its salts, (meth)acrylamide, N,N-dimethylacrylamide are preferable, and (meth)acrylic acid and its salts are more preferable. These water-soluble ethylenically unsaturated monomers may be used alone or in combination of two or more. 【0036】Among these, acrylic acid and its salts are widely used as raw materials for the water-absorbing resin particles, and in some cases, these acrylic acid and / or its salts may be copolymerized with the above-mentioned other water-soluble ethylenically unsaturated monomers for use. In this case, acrylic acid and / or its salts are preferably used in an amount of 70 to 100 mol% based on the total water-soluble ethylenically unsaturated monomers as the main water-soluble ethylenically unsaturated monomers. 【0037】 The water-soluble ethylenically unsaturated monomer may be dispersed in a hydrocarbon dispersion medium in the form of an aqueous solution and subjected to inverse phase suspension polymerization. By using the water-soluble ethylenically unsaturated monomer as an aqueous solution, the dispersion efficiency in the hydrocarbon dispersion medium can be increased. 【0038】 The concentration of the water-soluble ethylenically unsaturated monomer in the aqueous solution containing the water-soluble ethylenically unsaturated monomer in the entire polymerization step is preferably 30% by mass or more, more preferably 35% by mass or more, still more preferably 40% by mass or more, and may be below the saturation concentration, preferably 70% by mass or less, more preferably 60% by mass or less, still more preferably 55% by mass or less. Preferred ranges include 30 to 70% by mass, 30 to 60% by mass, 30 to 55% by mass, 35 to 70% by mass, 35 to 60% by mass, 35 to 55% by mass, 40 to 70% by mass, 40 to 60% by mass, and 40 to 55% by mass. 【0039】Furthermore, as described above, in the method for producing water-absorbent resin particles of the present invention, inverse-phase suspension polymerization is carried out in two or more stages, and in the first stage polymerization step of inverse-phase suspension polymerization, the concentration of water-soluble ethylenically unsaturated monomers in the aqueous solution containing water-soluble ethylenically unsaturated monomers is 40% by mass or more. If the concentration of water-soluble ethylenically unsaturated monomers in the aqueous solution containing water-soluble ethylenically unsaturated monomers in the first stage polymerization step of inverse-phase suspension polymerization is high, the suction distance becomes longer, and the unpressurized DW3 min value and unpressurized DW5 min value become higher. From the viewpoint of adjusting the suction distance, unpressurized DW3 min value and unpressurized DW5 min value of the water-absorbent resin particles to a suitable range, in the first stage polymerization step of inverse-phase suspension polymerization, the concentration of water-soluble ethylenically unsaturated monomers in the aqueous solution containing water-soluble ethylenically unsaturated monomers is preferably 41% by mass or more, more preferably 42% by mass or more, and even more preferably 43% by mass or more. Furthermore, from the viewpoint of adjusting the amount of water absorbed by the water-absorbent resin particles in physiological saline solution and the amount of water absorbed under load to a suitable range, the concentration of the water-soluble ethylenically unsaturated monomer in the aqueous solution containing the water-soluble ethylenically unsaturated monomer is preferably 50% by mass or less, more preferably 48% by mass or less, and even more preferably 46% by mass or less. Preferred ranges for the concentration of the water-soluble ethylenically unsaturated monomer in the aqueous solution containing the water-soluble ethylenically unsaturated monomer include 40-50% by mass, 40-48% by mass, 40-46% by mass, 42-50% by mass, 42-48% by mass, 42-46% by mass, 43-50% by mass, 43-48% by mass, and 43-46% by mass. 【0040】 When the water-soluble ethylenically unsaturated monomer has an acidic group, such as (meth)acrylic acid or 2-(meth)acrylamide-2-methylpropanesulfonic acid, the acidic group may be neutralized beforehand with an alkaline neutralizing agent as needed. 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 also be used in aqueous solution form to simplify the neutralization process. The above-mentioned alkaline neutralizing agents may be used individually or in combination of two or more types. 【0041】The degree of neutralization of a water-soluble ethylenically unsaturated monomer by an alkaline neutralizing agent is preferably 40 to 100 mol%, more preferably 50 to 90 mol%, even more preferably 60 to 85 mol%, and even more preferably 70 to 80 mol% as the degree of neutralization with respect to all acid groups of the water-soluble ethylenically unsaturated monomer. 【0042】 (Radical polymerization initiator) From the viewpoint of adjusting the suction distance of the water-absorbing resin particles, the amount of water retained in physiological saline, the amount of water absorbed under load, the value at 3 minutes DW and 5 minutes DW without pressure to a suitable range, the radical polymerization initiator added to the polymerization step may be, for example, 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-butylcumyl peroxide, t-butyl peroxyacetate, t-butyl peroxyisobutyrate, t-butyl peroxypivalate, and hydrogen peroxide; and 2,2'-azobis( Examples of azo compounds include 2-amidinopropane) dihydrochloride, 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-methyl-N-[1,1-bis(hydroxymethyl)-2-hydroxyethyl]propionamide}, 2,2'-azobis[2-methyl-N-(2-hydroxyethyl)-propionamide], 4,4'-azobis(4-cyanovaleric acid), and other azo compounds. Among these radical polymerization initiators, potassium persulfate, ammonium persulfate, sodium persulfate, and 2,2'-azobis(2-amidinopropane) dihydrochloride are preferred from the viewpoint of being readily available and easy to handle. These radical polymerization initiators may be used alone or in combination of two or more. Furthermore, the radical polymerization initiators can also be used as redox polymerization initiators in combination with reducing agents such as sodium sulfite, sodium bisulfite, ferrous sulfate, and L-ascorbic acid. 【0043】For example, the amount of radical polymerization initiator used is 0.00005 to 0.01 moles per mole of water-soluble ethylenically unsaturated monomer. By using such an amount, it is possible to avoid rapid polymerization reactions and complete the polymerization reaction within an appropriate time. 【0044】(Internal Crosslinking Agent) Examples of internal crosslinking agents include those that can crosslink the polymer of the water-soluble ethylenically unsaturated monomer used, such as (poly)ethylene glycol ["(poly)" means whether or not the prefix "poly" is present.] [The same applies hereafter], unsaturated polyesters obtained by reacting polyols 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; allyl starch, allyl cellulose, diallyl phthalate, N,N',N''-triallyl isocyanurate, divinyl Examples include compounds having two or more polymerizable unsaturated groups such as benzene; diglycidyl compounds such as (poly)ethylene glycol diglycidyl ether, (poly)propylene glycol diglycidyl ether, and (poly)glycerin diglycidyl ether; polyglycidyl compounds such as triglycidyl compounds; epihalohydrin compounds such as epichlorohydrin, epibromuhydrin, and α-methylepichlorohydrin; compounds having two or more reactive functional groups such as isocyanate compounds such as 2,4-tolylene diisocyanate and hexamethylene diisocyanate; and oxetane compounds 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 crosslinking agents, polyglycidyl compounds are preferred, diglycidyl ether compounds are more preferred, and (poly)ethylene glycol diglycidyl ether, (poly)propylene glycol diglycidyl ether, and (poly)glycerin diglycidyl ether are particularly preferred.These internal crosslinking agents may be used individually or in combination of two or more types. 【0045】 The total amount of internal crosslinking agent used is preferably 20 mmol or less, more preferably 0.001 to 10 mmol, even more preferably 0.005 to 5 mmol, and still more preferably 0.01 to 0.05 mmol, per mole of water-soluble ethylenically unsaturated monomer. 【0046】 In the method for producing superabsorbent resin particles of the present invention, reverse-phase suspension polymerization is carried out in two or more stages, and in the first stage polymerization step of reverse-phase suspension polymerization, the amount of internal crosslinking agent used per mole of water-soluble ethylenically unsaturated monomer used in the first stage polymerization is preferably 0.001 mmol or more, more preferably 0.01 mmol or more, and also preferably 5 mmol or less, more preferably 2 mmol or less, even more preferably 0.5 mmol or less, and particularly preferably 0.2 mmol or less. Preferred ranges include 0.001 to 5 mmol, 0.01 to 2 mmol, and 0.01 to 0.5 mmol. 【0047】 As described above, in the method for producing superabsorbent resin particles of the present invention, reverse-phase suspension polymerization is carried out in two or more stages, and in the second stage polymerization step of reverse-phase suspension polymerization, an internal crosslinking agent of 0 mmol or more and 0.042 mmol or less is used per mole of water-soluble ethylenically unsaturated monomer used in the second stage polymerization. If the amount of internal crosslinking agent used in the second stage polymerization is small, the suction distance will be longer and the amount of saline solution that can be retained will be higher. From the viewpoint of adjusting the suction distance and the amount of saline solution that can be retained by the superabsorbent resin particles to a suitable range, the amount of internal crosslinking agent used is preferably 0 mmol or more, more preferably 0.042 mmol or less, more preferably 0.035 mmol or less, even more preferably 0.028 mmol or less, and particularly preferably 0.021 mmol or less. Preferred ranges include 0 to 0.042 mmol, 0 to 0.035 mmol, 0 to 0.028 mmol, and 0 to 0.021 mmol. 【0048】(Hydrogen Dispersion Medium) Examples of hydrocarbon dispersion mediums 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 mediums, n-hexane, n-heptane, and cyclohexane are particularly suitable because they are readily available industrially, have stable quality, and are inexpensive. These hydrocarbon dispersion mediums may be used individually or in combination of two or more types. Furthermore, suitable results can also be obtained by using commercially available hydrocarbon dispersion media such as exolheptane (manufactured by ExxonMobil: containing 75-85% by mass of heptane and its isomers). 【0049】 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 first-stage water-soluble ethylenically unsaturated monomer, from the viewpoint of uniformly dispersing the water-soluble ethylenically unsaturated monomer and facilitating control of the polymerization temperature. 【0050】 (Dispersion stabilizers) (Surfactants) In reverse-phase suspension polymerization, dispersion stabilizers can be used to improve the dispersion stability of water-soluble ethylenically unsaturated monomers in hydrocarbon dispersion media. Surfactants can be used as dispersion stabilizers. 【0051】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, alkylallylformaldehyde condensed polyoxyethylene ethers, polyoxyethylene polyoxypropylene block copolymers, polyoxyethylene polyoxypropyl alkyl ethers, polyethylene glycol fatty acid esters, alkyl glucosides, N-alkylgluconamides, polyoxyethylene fatty acid amides, polyoxyethylene alkylamines, phosphate esters of polyoxyethylene alkyl ethers, and phosphate esters of polyoxyethylene alkylallyl ethers. Among these surfactants, sorbitan fatty acid esters, polyglycerin fatty acid esters, and sucrose fatty acid esters are particularly preferred in terms of monomer dispersion stability. These surfactants may be used individually or in combination of two or more types. 【0052】 The amount of surfactant used is preferably 0.1 to 30 parts by mass, and more preferably 0.3 to 20 parts by mass, per 100 parts by mass of the first-stage water-soluble ethylenically unsaturated monomer. 【0053】 (Polymer-based dispersants) In addition, polymer-based dispersants may be used together with the surfactants mentioned above as dispersion stabilizers in reverse-phase suspension polymerization. 【0054】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, ethylcellulose, and ethyl hydroxyethylcellulose. 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 monomer dispersion stability. These polymeric dispersants may be used individually or in combination of two or more types. 【0055】 The amount of polymeric dispersant used is preferably 0.1 to 30 parts by mass, and more preferably 0.3 to 20 parts by mass, per 100 parts by mass of the first-stage water-soluble ethylenically unsaturated monomer. 【0056】 (Other components) In the method for producing water-absorbent polymer particles, other components may be added to an aqueous solution containing a water-soluble ethylenically unsaturated monomer and reverse-phase suspension polymerization may be carried out. Other components may include various additives such as thickeners and chain transfer agents. 【0057】For example, reverse-phase suspension polymerization can be carried out by adding a thickening agent to an aqueous solution containing a water-soluble ethylenically unsaturated monomer. By adjusting the viscosity of the aqueous solution by adding a thickening agent in this way, it is possible to control the intermediate particle size obtained in reverse-phase suspension polymerization. 【0058】 Examples of thickening agents that can be used include hydroxyethylcellulose, hydroxypropylcellulose, methylcellulose, carboxymethylcellulose, polyacrylic acid, partially neutralized polyacrylic acid, polyethylene glycol, polyacrylamide, polyethyleneimine, dextrin, sodium alginate, polyvinyl alcohol, polyvinylpyrrolidone, and polyethylene oxide. It should be noted that, assuming the same stirring speed during polymerization, the higher the viscosity of the aqueous solution of the water-soluble ethylenically unsaturated monomer, the larger the primary and / or secondary particles obtained tend to be. 【0059】 <Dehydration Step> After the reverse-phase suspension polymerization described above, a dehydration step may be included in which water, hydrocarbon dispersion medium, etc. are removed by distillation by applying energy such as heat from an external source. When dehydrating from a water-containing gel-like substance after reverse-phase suspension polymerization, the system in which the water-containing gel-like substance is dispersed in the hydrocarbon dispersion medium is heated, and the water and hydrocarbon dispersion medium are temporarily removed from the system by azeotropic distillation. At this time, if only the removed hydrocarbon dispersion medium is returned to the system, continuous azeotropic distillation becomes possible. In this case, the temperature inside the system during drying is maintained below the azeotropic temperature with the hydrocarbon dispersion medium, which is preferable from the viewpoint of preventing resin degradation. By controlling the processing conditions of this dehydration step after polymerization and adjusting the amount of dehydration (i.e., adjusting the water content of the polymer particles), it is possible to control the various properties of the resulting water-absorbing resin particles. 【0060】 In the dehydration process, dehydration by distillation may be carried out under atmospheric pressure. When dehydration is carried out under atmospheric 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 Crosslinking Step> The surface crosslinking step is a step in which surface crosslinking is applied to the polymer particles obtained in the polymerization step. The method for producing water-absorbent resin particles of the present invention may include a surface crosslinking step as needed. When the polymer particles are crosslinked polymer particles (water-containing gel-like substance), the step involves adding a surface crosslinking agent to the water-containing gel-like substance having an internal crosslinking structure obtained by polymerizing water-soluble ethylenically unsaturated monomers to crosslink it (surface crosslinking reaction). It is preferable that this surface crosslinking reaction be carried out in the presence of the surface crosslinking agent after the polymerization of the water-soluble ethylenically unsaturated monomers. 【0062】Examples of surface crosslinking agents 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; halo-epoxy compounds such as epichlorohydrin, epibromhydrin, and α-methylepichlorohydrin; isocyanate compounds such as 2,4-tolylene diisocyanate and hexamethylene diisocyanate; and 3-methyl-3-oxetane methanol and 3-ethyl-3-oxetane. Oxetane compounds such as methanol, 3-butyl-3-oxetane methanol, 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, and 4-ethyl Examples of surface crosslinking agents include carbonate compounds (e.g., alkylene carbonates) such as -1,3-dioxolan-2-one, 4-hydroxymethyl-1,3-dioxolan-2-one, 1,3-dioxan-2-one, 4-methyl-1,3-dioxan-2-one, 4,6-dimethyl-1,3-dioxan-2-one, and 1,3-dioxopan-2-one; and hydroxyalkylamide compounds such as bis[N,N-di(β-hydroxyethyl)]adipamide. Among these surface crosslinking 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 crosslinking agents may be used individually or in combination of two or more types. 【0063】 The amount of surface crosslinking agent used is preferably 0.00001 to 0.01 moles, more preferably 0.00005 to 0.005 moles, more preferably 0.0001 to 0.001 moles, and even more preferably 0.0002 to 0.0009 moles, per mole of the total amount of water-soluble ethylenically unsaturated monomers used in polymerization. 【0064】 Regarding the method of adding the surface crosslinking agent, it may be added as is, as an aqueous solution, or, if necessary, as a solution using a hydrophilic organic solvent. Examples of hydrophilic organic solvents include lower alcohols such as methyl alcohol, ethyl alcohol, n-propyl alcohol, and isopropyl alcohol; ketones such as acetone and methyl ethyl ketone; ethers such as diethyl ether, dioxane, and tetrahydrofuran; amides such as N,N-dimethylformamide; and sulfoxides such as dimethyl sulfoxide. These hydrophilic organic solvents may be used individually, in combination of two or more, or as a mixed solvent with water. 【0065】 The surface crosslinking agent can be added after the polymerization reaction of the water-soluble ethylenically unsaturated monomer has been almost completely finished. It is preferable to add the surface crosslinking agent in the presence of 1 to 400 parts by mass of water per 100 parts by mass of the water-soluble ethylenically unsaturated monomer used in polymerization, more preferably in the presence of 5 to 200 parts by mass of water, even more preferably in the presence of 10 to 100 parts by mass of water, and still more preferably in the presence of 15 to 60 parts by mass of water. The amount of water refers to the total amount of water contained in the reaction system and the water used as needed when adding the surface crosslinking agent. 【0066】The reaction temperature for the surface crosslinking reaction is preferably 50 to 250°C, more preferably 60 to 180°C, even more preferably 60 to 140°C, and even more preferably 70 to 120°C. The reaction time for the surface crosslinking reaction is preferably 1 to 300 minutes, and more preferably 5 to 200 minutes. 【0067】 <Drying Process> After the surface crosslinking described above, a drying process may be included in which water, hydrocarbon dispersion medium, etc. are removed by distillation by applying energy such as heat from an external source. By drying the polymer particles after surface crosslinking and removing water and hydrocarbon dispersion medium by distillation, water-absorbing resin particles are obtained. 【0068】 In the drying process, the drying treatment by distillation may be carried out under atmospheric pressure or under reduced pressure. Furthermore, from the viewpoint of improving drying efficiency, it may be carried out under a stream of gas such as nitrogen. When the drying treatment is carried out under atmospheric pressure, the drying temperature is preferably 70 to 250°C, more preferably 80 to 180°C, even more preferably 80 to 140°C, and even more preferably 90 to 130°C. When the drying treatment is carried out under reduced pressure, the drying temperature is preferably 40 to 160°C, and more preferably 50 to 110°C. 【0069】 Furthermore, if monomer polymerization is performed by reverse-phase suspension polymerization followed by a surface crosslinking step using a surface crosslinking agent, the drying step by distillation described above should be performed after the completion of the surface crosslinking step. Alternatively, the surface crosslinking step and the drying step may be performed simultaneously. 【0070】 The water-absorbing resin particles of the present invention may contain additives depending on the purpose. Examples of such additives include surfactants, oxidizing agents, reducing agents, metal chelating agents, radical chain inhibitors, antioxidants, and antibacterial agents. It is preferable that the additives are hydrophilic or water-soluble. 【0071】From the viewpoint of adjusting the water absorption performance of the water-absorbing resin particles to a suitable range, the water-absorbing resin particles may contain inorganic powder. The inorganic powder is preferably hydrophilic or water-soluble, and for example, amorphous silica can be used. It is preferable to include 0.05 to 5 parts by mass of inorganic powder, and more preferably 0.1 to 2.5 parts by mass, per 100 parts by mass of water-absorbing resin particles. 【0072】 [Absorbent material, absorbent article] The water-absorbing resin particles of the present invention constitute an absorbent material used in sanitary materials such as sanitary napkins and disposable diapers, and are suitably used in absorbent articles containing the absorbent material. 【0073】 The absorbent material of the present invention contains the water-absorbent resin particles of the present invention. The absorbent material may further contain hydrophilic fibers. Examples of the structure of the absorbent material include a sheet-like structure in which water-absorbent resin particles are fixed on or between multiple nonwoven fabrics, a mixed dispersion obtained by mixing water-absorbent resin particles and hydrophilic fibers to 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 material may also contain other components, such as adhesive binders such as heat-fusible synthetic fibers, hot-melt adhesives, and adhesive emulsions to enhance the shape retention of the absorbent material. 【0074】 The basis weight of the water-absorbing resin particles in the absorbent material of the present invention is 30 g / m². 2 More than 500g / m 2 The following applies. The basis weight is preferably 100 g / m². 2 More comfortably, 120 g / m² 2 More preferably 140 g / m² 2 The above applies, and preferably 400 g / m². 2 More preferably, 350 g / m 2 More preferably, 300 g / m 2 The following applies: 【0075】As the hydrophilic fiber, at least one selected from the group consisting of finely pulverized wood pulp, cotton, cotton linter, rayon, cellulose acetate, polyamide, polyester, and polyolefin can be mentioned. Examples include cellulose fibers such as cotton-like pulp, mechanical pulp, chemical pulp, and semi-chemical pulp obtained from wood, artificial cellulose fibers such as rayon and acetate, and fibers made of synthetic resins such as hydrophilized 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. 【0076】 The basis weight of the hydrophilic fiber in the absorber of the present invention is 0 g / m 2 or more and 800 g / m 2 or less. The basis weight is preferably 0 g / m 2 or more and 80 g / m 2 or less, more preferably 0 g / m 2 or more and 50 g / m 2 or less, and even more preferably 0 g / m 2 or less. 【0077】 The content of the water-absorbing resin particles in the absorber is preferably 5 to 100% by mass, more preferably 10 to 95% by mass, even more preferably 20 to 90% by mass, and still more preferably 30 to 80% by mass. 【0078】 By holding the absorber using the water-absorbing 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 absorbent article of the present invention can be obtained. The liquid-permeable sheet is arranged on the side contacting the body, and the liquid-impermeable sheet is arranged on the opposite side contacting the body. 【0079】Examples of liquid-permeable sheets include nonwoven fabrics such as air-through type, spunbond type, chemical bond type, and needle-punched type, and porous synthetic resin sheets made of fibers such as polyethylene, polypropylene, and polyester. Examples of liquid-impermeable sheets include synthetic resin films made of resins such as polyethylene, polypropylene, and polyvinyl chloride. Preferably, the liquid-permeable sheet is at least one selected from the group consisting of thermal bond nonwoven fabric, air-through nonwoven fabric, spunbond nonwoven fabric, and spunbond / meltblown / spunbond nonwoven fabric. 【0080】 The basis weight of the liquid permeable sheet is 5 g / m². 2 More than 100g / m 2 Preferably, it is 10 g / m 2 60g / m or more 2 The following is more preferable. Furthermore, the liquid-permeable sheet may be embossed or perforated on its surface to improve the diffusion of the liquid. The embossing or perforation can be carried out by known methods. 【0081】 Examples of liquid-impermeable sheets include sheets made from synthetic resins such as polyethylene, polypropylene, and polyvinyl chloride; sheets made from nonwoven fabrics such as spunbond / meltblown / spunbond (SMS) nonwoven fabrics, which consist of a water-resistant meltblown nonwoven fabric sandwiched between high-strength spunbond nonwoven fabrics; and sheets made from composite materials of these synthetic resins and nonwoven fabrics (e.g., spunbond nonwoven fabrics, spunlace nonwoven fabrics). Liquid-impermeable sheets can also be made from synthetic resins primarily composed of low-density polyethylene (LDPE) resin. Liquid-impermeable sheets, for example, have a basis weight of 10 to 50 g / m². 2 It may be a sheet made of synthetic resin. 【0082】 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 side of the laminate opposite to the liquid-permeable sheet. 【0083】[Additional Notes] This specification includes at least the inventions shown in (1) to (9) below. (1) Superabsorbent resin particles having a saline solution absorption distance of 4.2 to 7.6 cm, 4.2 to 7.0 cm, 4.2 to 6.0 cm, 4.2 to 5.0 cm, 4.2 to 4.5 cm, 4.4 to 7.6 cm, 4.4 to 7.0 cm, 4.4 to 6.0 cm, 4.4 to 5.0 cm, 4.4 to 4.5 cm, 4.5 to 7.6 cm, 4.5 to 7.0 cm, 4.5 to 6.0 cm, 4.5 to 5.0 cm, and 4.5 cm. (2) The superabsorbent resin particles according to (1) above, wherein the amount of saline solution the superabsorbent resin particles hold is 35 g / g or more, 35 to 60 g / g, 35 to 55 g / g, 35 to 49 g / g, 40 to 60 g / g, 40 to 55 g / g, 40 to 49 g / g, 45 to 60 g / g, 45 to 55 g / g, 45 to 49 g / g, 47 to 60 g / g, 47 to 55 g / g, or 47 to 49 g / g. (3) The water-absorbing resin particles according to (1) or (2) above, wherein the amount of water absorbed under load of the water-absorbing resin particles is 10 mL / g or more, 10 to 40 mL / g, 10 to 35 mL / g, 10 to 30 mL / g, 12 to 40 mL / g, 12 to 35 mL / g, 12 to 30 mL / g, 15 to 40 mL / g, 15 to 35 mL / g, or 15 to 30 mL / g. (4) The water-absorbing resin particles according to (1) to (3) above, wherein the unpressurized DW3 value of the water-absorbing resin particles is 30 mL / g or more, 30 to 70 mL / g, 30 to 65 mL / g, 30 to 60 mL / g, 36 to 70 mL / g, 36 to 65 mL / g, 36 to 60 mL / g, 42 to 70 mL / g, 42 to 65 mL / g, 42 to 60 mL / g, 47 to 70 mL / g, 47 to 65 mL / g, 47 to 60 mL / g, 51 to 70 mL / g, 51 to 65 mL / g, or 51 to 60 mL / g. (5) The water-absorbing resin particles according to (1) to (4) above, wherein the unpressurized DW5 min value of the water-absorbing resin particles is 40 mL / g or more, 40 to 80 mL / g, 40 to 75 mL / g, 40 to 70 mL / g, 50 to 80 mL / g, 50 to 75 mL / g, 50 to 70 mL / g, 60 to 80 mL / g, 60 to 75 mL / g, 60 to 70 mL / g, 66 to 80 mL / g, 66 to 75 mL / g, or 66 to 70 mL / g.(6) The water-absorbing resin particles according to (1) to (5) above, wherein the median particle size of the water-absorbing 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. 【0084】 The present invention will be described in detail below with reference to examples and comparative examples. However, the present invention is not limited to the examples. Unless otherwise specified, the manufacturing examples, examples, comparative examples, and measurements were carried out under room temperature (temperature 25 ± 2°C) and humidity 50 ± 10%. "Physiological saline" refers to a 0.9% by mass aqueous solution of sodium chloride. 【0085】 <Production of superabsorbent polymer particles> (Example 1) A round-bottom 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 four inclined paddle blades with a blade diameter of 5 cm arranged in two stages. 272 ​​g of n-heptane as a hydrocarbon dispersion medium and 0.782 g of maleic anhydride-modified ethylene-propylene copolymer (manufactured by Mitsui Chemicals, Inc., High Wax 1105A) as a dispersant were added to the flask and mixed. The mixture in the flask was heated to 80°C while stirring with the stirrer at a rotation speed of 300 rpm, thereby dissolving the dispersant in n-heptane. The formed solution was cooled to 50°C. 【0086】In a 300 mL beaker, 92.0 g (1.03 mol) of an 80.5% by mass acrylic acid aqueous solution was added as a water-soluble ethylenically unsaturated monomer. While cooling from the outside, 102.8 g of a 30% by mass sodium hydroxide aqueous solution was added dropwise to the beaker to neutralize it to 75 mol%. Subsequently, 0.074 g (0.271 mmol) of 2,2'-azobis(2-amidinopropane) dihydrochloride as an azo compound, 0.028 g (0.104 mmol) of potassium persulfate as a peroxide, 0.0074 g (0.042 mmol) of ethylene glycol diglycidyl ether as an internal crosslinking agent, and 32.8 g of deionized water were added and dissolved to prepare the first monomer aqueous solution. At this time, the concentration of the water-soluble ethylenically unsaturated monomer was 40% by mass. 【0087】 The prepared first-stage monomer aqueous solution was added to the n-heptane solution containing the dispersant in the separable flask and stirred for 10 minutes. Next, a surfactant solution prepared by heating and dissolving 0.828 g of sucrose stearate (HLB: 3, Mitsubishi Chemical Foods Corporation, Ryoto Sugar Ester S-370) in 6.62 g of n-heptane was added to the reaction mixture, and the system was thoroughly purged with nitrogen while stirring at a stirrer speed of 500 rpm. After that, the flask was immersed in a 70°C water bath and the temperature was raised, and polymerization was carried out for 60 minutes to obtain the first-stage polymerization slurry. 【0088】 Next, 128.8 g (1.44 mol) of an 80.5% by mass acrylic acid aqueous solution was placed in another 500 ml beaker as an ethylenically unsaturated monomer. While cooling from the outside, 143.89 g of a 30% by mass sodium hydroxide aqueous solution was added dropwise to neutralize to 75 mol%. Subsequently, 0.064 g (0.236 mmol) of 2,2'-azobis(2-amidinopropane) dihydrochloride as an azo compound, 0.039 g (0.144 mmol) of potassium persulfate as a peroxide, and 17.0 g of deionized water were added and dissolved to prepare the second monomer aqueous solution. 【0089】The separable flask system was cooled to 25°C while stirring at a stirrer speed of 1000 rpm. Next, the entire volume of the second stage monomer aqueous solution was added to the first stage polymerization slurry in the separable flask, and the system was purged with nitrogen for 30 minutes. After that, the flask was again immersed in a 70°C water bath to raise the temperature, and the polymerization reaction was carried out for 60 minutes. 【0090】 To the reaction solution containing the hydrated gel polymer after the second polymerization stage, 0.491 g of a 45% by mass aqueous solution of sodium diethylenetriaminepentaacetate was added under stirring. The flask was then immersed in an oil bath set at 125°C, and 293.4 g of water was removed from the system by azeotropic distillation of n-heptane and water. Subsequently, 5.52 g (0.634 mmol) of a 2% by mass aqueous solution of ethylene glycol diglycidyl ether was added to the flask as a surface crosslinking agent, and the temperature inside the separable flask was maintained at 83°C for 2 hours. 【0091】 Subsequently, the separable flask was immersed in an oil bath set at 125°C to remove n-heptane, thereby obtaining polymer particles (dried). These polymer particles were passed through a sieve with a mesh size of 850 μm, and 0.2% by mass of amorphous silica (Oriental Silicas Corporation, Toxil NP-S, hydrophilic) relative to the mass of the polymer particles was mixed with the polymer particles to obtain 230.3 g of water-absorbing resin particles containing amorphous silica. The median particle size of these water-absorbing resin particles was 377 μm. 【0092】 (Example 2) A round-bottom 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 four inclined paddle blades with a blade diameter of 5 cm arranged in two stages. 252 g of n-heptane as a hydrocarbon dispersion medium and 0.736 g of maleic anhydride-modified ethylene-propylene copolymer (Mitsui Chemicals, Inc., High Wax 1105A) as a dispersant were added to the flask and mixed. The mixture in the flask was heated to 80°C while stirring with the stirrer at a rotation speed of 300 rpm, thereby dissolving the dispersant in n-heptane. The formed solution was cooled to 50°C. 【0093】In a 300 mL beaker, 92.0 g (1.03 mol) of an 80.5% by mass acrylic acid aqueous solution was added as a water-soluble ethylenically unsaturated monomer. While cooling from the outside, 102.8 g of a 30% by mass sodium hydroxide aqueous solution was added dropwise to the beaker to neutralize it to 75 mol%. Subsequently, 0.092 g of hydroxylethylcellulose (Sumitomo Seika Co., Ltd., HEC AW-15F) as a thickener, 0.0920 g (0.339 mmol) of 2,2'-azobis(2-amidinopropane) dihydrochloride as an azo compound, 0.0276 g (0.102 mmol) of potassium persulfate as a peroxide, and 0.00276 g (0.0158 mmol) of ethylene glycol diglycidyl ether as an internal crosslinking agent, along with 12.1 g of deionized water, were added and dissolved to prepare the first monomer aqueous solution. At this time, the concentration of the water-soluble ethylenically unsaturated monomer was 44% by mass. 【0094】 The prepared first-stage monomer aqueous solution was added to the n-heptane solution containing the dispersant in the separable flask and stirred for 10 minutes. Next, a surfactant solution prepared by heating and dissolving 0.736 g of sucrose stearate (HLB: 3, Mitsubishi Chemical Foods Corporation, Ryoto Sugar Ester S-370) in 6.62 g of n-heptane was added to the reaction mixture, and the system was thoroughly purged with nitrogen while stirring at a stirrer speed of 600 rpm. After that, the flask was immersed in a 70°C water bath and the temperature was raised, and polymerization was carried out for 60 minutes to obtain the first-stage polymerization slurry. 【0095】 Next, 128.8 g (1.44 mol) of an 80.5% by mass acrylic acid aqueous solution was placed in another 500 ml beaker as an ethylenically unsaturated monomer. While cooling from the outside, 143.89 g of a 30% by mass sodium hydroxide aqueous solution was added dropwise to neutralize to 75 mol%. Subsequently, 0.129 g (0.476 mmol) of 2,2'-azobis(2-amidinopropane) dihydrochloride as an azo compound, 0.0386 g (0.143 mmol) of potassium persulfate as a peroxide, and 17.0 g of deionized water were added and dissolved to prepare the second monomer aqueous solution. 【0096】The separable flask system was cooled to 25°C while stirring at a stirrer speed of 1000 rpm. Next, the entire volume of the second stage monomer aqueous solution was added to the first stage polymerization slurry in the separable flask, and the system was purged with nitrogen for 30 minutes. After that, the flask was again immersed in a 70°C water bath to raise the temperature, and the polymerization reaction was carried out for 60 minutes. 【0097】 To the reaction solution containing the hydrated gel polymer after the second polymerization stage, 0.392 g of a 45% by mass aqueous solution of sodium diethylenetriaminepentaacetate was added under stirring. The flask was then immersed in an oil bath set at 125°C, and 282.3 g of water was removed from the system by azeotropic distillation of n-heptane and water. Subsequently, 4.42 g (0.507 mmol) of a 2% by mass aqueous solution of ethylene glycol diglycidyl ether was added to the flask as a surface crosslinking agent, and the temperature inside the separable flask was maintained at 83°C for 2 hours. 【0098】 Subsequently, the separable flask was immersed in an oil bath set at 125°C to remove n-heptane, thereby obtaining polymer particles (dried). These polymer particles were passed through a sieve with a mesh size of 850 μm, and 0.2% by mass of amorphous silica (Oriental Silicas Corporation, Toxil NP-S, hydrophilic) relative to the mass of the polymer particles was mixed with the polymer particles to obtain 217.2 g of water-absorbing resin particles containing amorphous silica. The median particle size of these water-absorbing resin particles was 381 μm. 【0099】 (Comparative Example 1) A round-bottom 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 four inclined paddle blades with a blade diameter of 5 cm arranged in two stages. 293 g of n-heptane as a hydrocarbon dispersion medium and 0.736 g of maleic anhydride-modified ethylene-propylene copolymer (Mitsui Chemicals, Inc., High Wax 1105A) as a dispersant were added to the flask and mixed. The mixture in the flask was heated to 80°C while stirring with the stirrer at a rotation speed of 300 rpm, thereby dissolving the dispersant in n-heptane. The formed solution was cooled to 50°C. 【0100】In a 300 mL beaker, 92.0 g (1.03 mol) of an 80.5% by mass acrylic acid aqueous solution was added as a water-soluble ethylenically unsaturated monomer. While cooling from the outside, 147.5 g of a 20.9% by mass sodium hydroxide aqueous solution was added dropwise to the beaker to neutralize it to 75 mol%. Subsequently, 0.092 g of hydroxyethylcellulose (Sumitomo Seika Co., Ltd., HEC AW-15F) was added as a thickener, 0.0736 g (0.272 mmol) of potassium persulfate was added as a peroxide, and 0.00460 g (0.0264 mmol) of ethylene glycol diglycidyl ether was added as an internal crosslinking agent and dissolved to prepare the first monomer aqueous solution. At this time, the concentration of the water-soluble ethylenically unsaturated monomer was 38% by mass. 【0101】 The prepared first-stage monomer aqueous solution was added to the n-heptane solution containing the dispersant in the separable flask and stirred for 10 minutes. Next, a surfactant solution prepared by heating and dissolving 0.736 g of sucrose stearate (HLB: 3, Mitsubishi Chemical Foods Corporation, Ryoto Sugar Ester S-370) in 6.62 g of n-heptane was added to the reaction mixture, and the system was thoroughly purged with nitrogen while stirring at a stirrer speed of 550 rpm. After that, the flask was immersed in a 70°C water bath and the temperature was raised, and polymerization was carried out for 60 minutes to obtain the first-stage polymerization slurry. 【0102】 Next, 128.8 g (1.44 mol) of an 80.5% by mass acrylic acid aqueous solution was placed in another 500 ml beaker as an ethylenically unsaturated monomer. While cooling from the outside, 160.7 g of a 27% by mass sodium hydroxide aqueous solution was added dropwise to neutralize it to 75 mol%. Subsequently, 0.090 g (0.333 mmol) of potassium persulfate was added as a peroxide and 0.0116 g (0.067 mmol) of ethylene glycol diglycidyl ether was added as an internal crosslinking agent and dissolved to prepare the second monomer aqueous solution. 【0103】The separable flask system was cooled to 25°C while stirring at a stirrer speed of 1000 rpm. Next, the entire volume of the second stage monomer aqueous solution was added to the first stage polymerization slurry in the separable flask, and the system was purged with nitrogen for 30 minutes. After that, the flask was again immersed in a 70°C water bath to raise the temperature, and the polymerization reaction was carried out for 60 minutes. 【0104】 To the reaction solution containing the hydrated gel polymer after the second polymerization stage, 0.392 g of a 45% by mass aqueous solution of sodium diethylenetriaminepentaacetate was added under stirring. The flask was then immersed in an oil bath set at 125°C, and 257.3 g of water was removed from the system by azeotropic distillation of n-heptane and water. Subsequently, 4.42 g (0.507 mmol) of a 2% by mass aqueous solution of ethylene glycol diglycidyl ether was added to the flask as a surface crosslinking agent, and the temperature inside the separable flask was maintained at 83°C for 2 hours. 【0105】 Subsequently, the separable flask was immersed in an oil bath set at 125°C to remove n-heptane, thereby obtaining polymer particles (dried). These polymer particles were passed through a sieve with an opening of 850 μm, and 0.2% by mass of amorphous silica (Oriental Silicas Corporation, Toxil NP-S, hydrophilic) relative to the mass of the polymer particles was mixed with the polymer particles to obtain 228.8 g of water-absorbing resin particles containing amorphous silica. The median particle size of these water-absorbing resin particles was 364 μm. 【0106】 (Comparative Example 2) A round-bottom 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 four inclined paddle blades with a blade diameter of 5 cm arranged in two stages. 272 ​​g of n-heptane as a hydrocarbon dispersion medium and 0.782 g of maleic anhydride-modified ethylene-propylene copolymer (Mitsui Chemicals, Inc., High Wax 1105A) as a dispersant were added to the flask and mixed. The mixture in the flask was heated to 80°C while stirring with the stirrer at a rotation speed of 300 rpm, thereby dissolving the dispersant in n-heptane. The formed solution was cooled to 50°C. 【0107】In a 300 mL beaker, 92.0 g (1.03 mol) of an 80.5% by mass acrylic acid aqueous solution was added as a water-soluble ethylenically unsaturated monomer. While cooling from the outside, 102.8 g of a 30% by mass sodium hydroxide aqueous solution was added dropwise to the beaker to neutralize it to 75 mol%. Subsequently, 0.074 g (0.271 mmol) of 2,2'-azobis(2-amidinopropane) dihydrochloride as an azo compound, 0.028 g (0.104 mmol) of potassium persulfate as a peroxide, 0.00460 g (0.0264 mmol) of ethylene glycol diglycidyl ether as an internal crosslinking agent, and 44.8 g of deionized water were added and dissolved to prepare the first monomer aqueous solution. At this time, the concentration of the water-soluble ethylenically unsaturated monomer was 38% by mass. 【0108】 The prepared first-stage monomer aqueous solution was added to the n-heptane solution containing the dispersant in the separable flask and stirred for 10 minutes. Next, a surfactant solution prepared by heating and dissolving 0.828 g of sucrose stearate (HLB: 3, Mitsubishi Chemical Foods Corporation, Ryoto Sugar Ester S-370) in 6.62 g of n-heptane was added to the reaction mixture, and the system was thoroughly purged with nitrogen while stirring at a stirrer speed of 500 rpm. After that, the flask was immersed in a 70°C water bath and the temperature was raised, and polymerization was carried out for 60 minutes to obtain the first-stage polymerization slurry. 【0109】 Next, 128.8 g (1.44 mol) of an 80.5% by mass acrylic acid aqueous solution was placed in another 500 ml beaker as an ethylenically unsaturated monomer. While cooling from the outside, 143.89 g of a 30% by mass sodium hydroxide aqueous solution was added dropwise to neutralize to 75 mol%. Subsequently, 0.064 g (0.236 mmol) of 2,2'-azobis(2-amidinopropane) dihydrochloride as an azo compound, 0.039 g (0.144 mmol) of potassium persulfate as a peroxide, and 17.0 g of deionized water were added and dissolved to prepare the second monomer aqueous solution. 【0110】The separable flask system was cooled to 25°C while stirring at a stirrer speed of 1000 rpm. Next, the entire volume of the second stage monomer aqueous solution was added to the first stage polymerization slurry in the separable flask, and the system was purged with nitrogen for 30 minutes. After that, the flask was again immersed in a 70°C water bath to raise the temperature, and the polymerization reaction was carried out for 60 minutes. 【0111】 To the reaction solution containing the hydrated gel polymer after the second polymerization stage, 0.491 g of a 45% by mass aqueous solution of sodium diethylenetriaminepentaacetate was added under stirring. The flask was then immersed in an oil bath set at 125°C, and 253.74 g of water was removed from the system by azeotropic distillation of n-heptane and water. Subsequently, 5.52 g (0.634 mmol) of a 2% by mass aqueous solution of ethylene glycol diglycidyl ether was added to the flask as a surface crosslinking agent, and the temperature inside the separable flask was maintained at 83°C for 2 hours. 【0112】 Subsequently, the separable flask was immersed in an oil bath set to 125°C to remove n-heptane, thereby obtaining polymer particles (dried). These polymer particles were passed through a sieve with an opening of 850 μm, and 0.2% by mass of amorphous silica (Oriental Silicas Corporation, Toxil NP-S, hydrophilic) relative to the mass of the polymer particles was mixed with the polymer particles to obtain 227.7 g of water-absorbing resin particles containing amorphous silica. The median particle size of these water-absorbing resin particles was 350 μm. 【0113】 [Measurement of the physical properties of water-absorbent resin particles] The physical properties of each water-absorbent resin particle obtained in the examples and comparative examples were evaluated using the following methods. The results are shown in Table 1. 【0114】(Saline Solution Water Retention Capacity) The physiological saline solution water retention capacity test was conducted at room temperature. A cotton bag (Membrane No. 60, 100 mm wide x 200 mm long) containing 2.0 g of superabsorbent polymer particles was placed in a 500 mL beaker. 500 g of physiological saline solution was poured into the cotton bag containing the superabsorbent polymer particles in one go, taking care not to let it spill. The top of the cotton bag was tied with a rubber band and left to stand for 30 minutes to allow the superabsorbent polymer particles to swell. After 30 minutes, the cotton bag was dehydrated for 1 minute using a dehydrator (manufactured by Kokusan Co., Ltd., model number: H-122) set to a centrifugal force of 167 G, and the mass Wa (g) of the cotton bag containing the swollen gel after dehydration was measured. The same procedure was performed without adding superabsorbent polymer particles, and the empty mass Wb (g) of the cotton bag when wet was measured. The physiological saline solution water retention capacity was calculated from the following formula: Physiological saline solution water retention capacity [g / g] = (Wa - Wb) / 2.0 【0115】 (Water absorption under load) The amount of physiological saline absorbed by superabsorbent resin particles under load (pressure) at room temperature was measured using the measuring device Y shown in Figure 1. The measuring device Y consists of a burette section 61, a conduit 62, a measuring stand 63, and a measuring unit 64 placed on the measuring stand 63. The burette section 61 has a burette 61a extending vertically, a rubber stopper 61b positioned at the upper end of the burette 61a, a cock 61c positioned at the lower end of the burette 61a, an air inlet pipe 61d with one end extending into the burette 61a near the cock 61c, and a cock 61e positioned at the other end of the air inlet pipe 61d. The conduit 62 is installed between the burette section 61 and the measuring stand 63. The inner diameter of the conduit 62 is 6 mm. There is a hole with a diameter of 2 mm in the center of the measuring stand 63, to which the conduit 62 is connected. The measuring unit 64 comprises a cylinder 64a (made of acrylic resin (plexiglass)), a nylon mesh 64b bonded to the bottom of the cylinder 64a, and a weight 64c. The inner diameter of the cylinder 64a is 20 mm. The mesh opening of the nylon mesh 64b is 75 μm (200 mesh). During measurement, the water-absorbing resin particles 65 to be measured are uniformly scattered on the nylon mesh 64b. The diameter of the weight 64c is 19 mm, and the mass of the weight 64c is 59.8 g. The weight 64c is placed on the water-absorbing resin particles 65, and can apply a load of 4.14 kPa to the water-absorbing resin particles 65. 【0116】 After placing 0.100 g of superabsorbent resin particles 65 into the cylinder 64a of the measuring device Y, a weight 64c was placed on top and measurement was started. Since the same volume of air as the saline solution absorbed by the superabsorbent resin particles 65 is quickly and smoothly supplied into the burette 61a through the air inlet tube, the decrease in the saline solution level inside the burette 61a represents the amount of saline solution absorbed by the superabsorbent resin particles 65. The burette 61a is marked from top to bottom in increments of 0.5 mL from 0 mL. The saline solution level was measured by reading the Va mark on the burette 61a before absorption began and the Vb mark on the burette 61a 60 minutes after absorption began, and the amount of absorbed solution under load was calculated using the following formula: Amount of absorbed solution under load [mL / g] = (Vb - Va) / 0.1 【0117】 (Unpressurized DW 3-minute and 5-minute values) The unpressurized DW of superabsorbent polymer particles was measured using the measuring device shown in Figure 2. Measurements were performed five times for one type of superabsorbent polymer particle, and the average of the three measured values ​​excluding the lowest and highest values ​​was calculated. 【0118】The measuring device comprises a burette section 1, a conduit 5, a measuring platform 13, a nylon mesh sheet 15, a stand 11, and a clamp 3. The burette section 1 includes a burette tube 21 with markings, a rubber stopper 23 that seals the opening at the top of the burette tube 21, a cock 22 connected to the lower end of the burette tube 21, and an air inlet tube 25 and a cock 24 connected to the lower part of the burette tube 21. The burette section 1 is fixed with a clamp 3. The flat measuring platform 13 has a through hole 13a with a diameter of 2 mm formed in its center and is supported by a stand 11 with adjustable height. The through hole 13a of the measuring platform 13 and the cock 22 of the burette section 1 are connected by a conduit 5. The inner diameter of the conduit 5 is 6 mm. First, the cocks 22 and 24 of the burette section 1 were closed, and physiological saline solution 50 was poured into the burette tube 21 through the opening at the top of the burette tube 21. After sealing the opening of the burette tube 21 with the rubber stopper 23, the stopcocks 22 and 24 were opened. The inside of the conduit 5 was filled with physiological saline solution 50 to prevent air bubbles from entering. The height of the measuring platform 13 was adjusted so that the level of the physiological saline solution that reached the through-hole 13a was the same as the height of the top surface of the measuring platform 13. After the adjustment, the level of the physiological saline solution 50 inside the burette tube 21 was read using the scale on the burette tube 21, and that position was set as the zero point (reading at 0 seconds). 【0119】 A nylon mesh sheet 15 (100 mm x 100 mm, 250 mesh, approximately 50 μm thick) was laid near the through-hole 13a on the measuring platform 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-absorbing resin particles 10a were uniformly scattered into this cylinder. The cylinder was then carefully removed, and a sample was obtained in which the water-absorbing resin particles 10a were dispersed in a circular pattern in the center of the nylon mesh sheet 15. Next, the nylon mesh sheet 15 on which the water-absorbing resin particles 10a were placed was quickly moved so that its center was at the position of the through-hole 13a, without causing the water-absorbing resin particles 10a to dissipate, and the measurement was started. The start of water absorption (0 seconds) was defined as the moment when the first air bubble was introduced into the burette tube 21 from the air inlet tube 25. 【0120】The decrease in physiological saline 50 in the burette tube 21 (i.e., the amount of physiological saline absorbed by the superabsorbent resin particles 10a) was read sequentially in 0.1 mL units, and the decrease in physiological saline 50 Wc (g) was read 3 minutes and 5 minutes after the start of absorption by the superabsorbent resin particles 10a. From Wc, the 3-minute and 5-minute values ​​of unpressurized DW were calculated using the following formula. Unpressurized DW is the amount of water absorbed per 1.00 g of superabsorbent resin particles 10a. The results are shown in Table 1. Unpressurized DW value (mL / g) = Wc / 1.00 【0121】 (Medium particle size) JIS standard sieves were arranged from top to bottom in the following order: sieve with a mesh size of 710 μm, sieve with a mesh size of 600 μm, sieve with a mesh size of 500 μm, sieve with a mesh size of 425 μm, sieve with a mesh size of 300 μm, sieve with a mesh size of 250 μm, sieve with a mesh size of 150 μm, and a receiving tray. 【0122】 The water-absorbing resin particles were placed in the sieve located at the top and classified by shaking for 20 minutes using a continuous fully automatic ultrasonic vibration sieving analyzer (Robot Shifter RPS-205, manufactured by Seishin Corporation). 【0123】 After classification, the mass of the absorbent resin particles remaining on each sieve was calculated as a mass percentage of the total amount. By accumulating these values ​​in order from the largest particle size, the relationship between the sieve opening and the accumulated mass percentage of the absorbent resin particles remaining on the sieve was plotted on logarithmic probability paper. By connecting the plots on the probability paper with a straight line, the particle size corresponding to an accumulated mass percentage of 50% was defined as the median particle size. 【0124】(Method for measuring the absorption distance of physiological saline solution) <Preparation of the sample for measurement> A glass plate (26 mm x 76 mm, 1.2 mm thick) was prepared by aligning the center of a double-sided tape (Nitto Denko No. 5000NS: 20 mm x 76 mm) and attaching it to the center, exposing the adhesive surface of the tape. The glass plate was placed in a stainless steel petri dish (φ120 mm, height 25 mm), and 0.35 g of superabsorbent resin particles were uniformly sprinkled over the entire adhesive surface of the tape attached to the glass plate. Then, the glass plate was stood upright, and the excess superabsorbent resin particles were collected. The collected superabsorbent resin particles were then sprinkled over the entire adhesive surface of the tape attached to the glass plate again. This operation was repeated until the amount of excess superabsorbent resin particles was 0.04 g or less, and the amount of superabsorbent resin particles on the tape was adjusted to 0.33 ± 0.02 g. 【0125】 A nylon mesh (250 mesh, Nippon Tokushu Orimono, product number NNO.250T: 26 mm x 100 mm) was placed center-aligned on the front surface (the surface on which the water-absorbing resin particles were scattered) of the aforementioned glass plate. Both ends of the nylon mesh were folded along the longitudinal edges of the glass plate and attached and secured to the back of the glass plate using masking tape (Paiolan tape (Diatex Co., Ltd. Y-06-WH, width 13 mm)). Furthermore, a nylon mesh (250 mesh, Nippon Tokushu Orimono, product number NNO.250T: 50 mm x 76 mm) was placed center-aligned on the front surface of the aforementioned glass plate. Both ends of the nylon mesh were folded along the short edges of the glass plate and attached and secured to the back of the glass plate using masking tape to obtain a sample for measurement. 【0126】<Measurement of the suction distance of physiological saline solution> Figure 5 is a schematic diagram showing the method for measuring the suction distance of physiological saline solution. A support plate (in this case, an acrylic resin plate) with a length of 45 cm and a flat inclined surface S1 was fixed by a frame 41 at an angle of 45 degrees with respect to the horizontal plane S0. A measurement sample 100 was placed on the inclined surface S1 of the fixed support plate with its longitudinal direction aligned with the longitudinal direction of the support plate, and the center of the measurement sample 100 overlapping with the lower end of the support plate. The upper end of the measurement sample 100 was attached to the support plate with masking tape (Paiolan tape (Y-06-WH, manufactured by Diatex Co., Ltd.), 50 mm wide). 【0127】 Next, 15 g of physiological saline solution colored with food coloring blue no. 1 at 25 ± 1°C was weighed into a stainless steel petri dish S2 (inner diameter 75 mm, height 20 mm). Using a lab jack S3, the stainless steel petri dish S2 was raised at a constant speed so that the sample 100 for measurement came into contact with the liquid surface in the stainless steel petri dish S2, and then, 3 seconds later, came into contact with the bottom surface of the stainless steel petri dish S2. Thirty minutes after the sample 100 for measurement came into contact with the bottom surface of the stainless steel petri dish S2, the stainless steel petri dish S2 was lowered using the lab jack S3 to remove the sample 100 for measurement from the liquid surface in the stainless steel petri dish S2. After that, the distance (cm) of physiological saline solution absorbed by the sample 100 for measurement was measured. The suction distance (cm) of the physiological saline solution is the distance measured by drawing a straight line parallel to the longitudinal direction of the sample 100, connecting the edge of the glass plate that was immersed on the sample 100 and the point to which the physiological saline solution reached. 【0128】 [Evaluation of displacement of swollen gel] <Preparation of absorbent material> As the first and second sheets, spunlace nonwoven fabric (manufactured by Zhejiang Wangjin Nonwoven Fabric Co., Ltd.) cut to 42 cm x 14 cm (basis weight: 35 g / m²) 2A hot melt adhesive (Henkel Japan Ltd., ME-765E) was applied to the spunlace nonwoven fabric of the second sheet in 13 lines at 10 mm intervals along the longitudinal direction using a hot melt coating machine (Harries Co., Ltd., pump: Marshall 150, table: XA-DT, tank setting temperature: 150°C, hose setting temperature: 165°C, gun head setting temperature: 170°C). The adhesive application pattern was a spiral stripe. Subsequently, 8.4 g of superabsorbent polymer particles were uniformly scattered on the adhesive-coated surface of the second sheet, excluding the short direction and the outer 1 cm area at both ends of the longitudinal direction. 【0129】 Hot melt adhesive was applied to the first sheet using the same procedure as described above. The side of the first sheet coated with hot melt adhesive and the side of the second sheet scattered with water-absorbing resin particles were aligned at both ends, then sandwiched from above and below with two sheets of release paper, and pressed together using a laminating machine (Hashima Co., Ltd., Straight Linear Fussing Press, model HP-600LFS) at 110°C and 0.1 MPa. The sheets were then removed to obtain a sheet-like absorbent structure. In the absorbent structure, the basis weight of the water-absorbing resin particles was 175 g / m². 2 That was the case. 【0130】 Furthermore, a hydrophilic air-through nonwoven fabric of the same size as the absorbent sheet is placed on the top surface of the absorbent sheet (Rengo Nonwoven Products Co., Ltd., basis weight: 21 g / m²). 2 An absorbent article for evaluation was obtained by arranging the materials and removing the original top sheet and absorbent from a diaper manufactured by Daio Paper Corporation (product name: Goo.n Plus Sensitive Skin Design, tape type, L size, purchased in 2024), and placing it on a back sheet having a pair of side gathers bonded to each of the short ends, with the central parts of each overlapping. The obtained absorbent article has the following components arranged in this order: hydrophilic air-through nonwoven fabric, first sheet, hot melt adhesive, absorbent layer consisting of water-absorbent resin particles, hot melt adhesive, and second sheet, and liquid-impermeable sheet. 【0131】(Test Solution) The test solution was prepared by dissolving the inorganic salts in deionized water as described below, and then adding a small amount of Blue No. 1. Test Solution Composition: Deionized water: 4955.0 g, NaCl: 45.0 g, Food Blue No. 1 (for coloring) 【0132】 (Amount of displacement and return of swollen gel) At room temperature, the absorbent material described above was placed on a horizontal table on a U-shaped sample stand (B2 L136mm, W135mm) and a sample stand (T2) conforming to the GB standard in China, "GB / T 28004.1-2021 NATIONAL STANDARD OF THE PEOPLE'S REPUBLIC OF CHINA, Disposable diapers - Part 1: Disposable diapers for baby". 【0133】 Next, 240 mL of test solution, adjusted to 25 ± 1°C, was dripped onto the absorbent material from 3 cm above the material to the center of the material using a burette over 24 seconds, allowing the absorbent resin particles to absorb the solution. Five minutes after the completion of the test solution dripping, the absorbent material was removed from the U-shaped sample stand and spread out on a horizontal table. 【0134】 Next, the top and bottom edges of the absorbent material were secured with masking tape (Paiolan Tape (Y-06-WH, manufactured by Diatex Co., Ltd.)). Approximately 23 g of filter paper (ADVANTEC No. 51A, formed to 200 mm x 100 mm), whose mass had been measured beforehand, was placed near the center of the absorbent material. A load of approximately 4.0 kg was applied by moving a roller (φ105 mm, width 60 mm) up and down three times. After that, the filter paper was removed, and the mass of the test liquid absorbed by the filter paper was measured and recorded as the amount of backflow [g]. Subsequently, the absorbent material was divided equally into five parts along its length, and the weight Xa of the central part was measured. On the other hand, the weight Xb of the central part of the absorbent material divided equally into five parts along its length without the roller treatment was measured, and the displacement amount [g] was calculated from the following formula. The measurement results for the amount of backflow [g] and displacement amount [g] are shown in Table 1. Positional displacement [g] = (Xb - Xa) 【0135】 【0136】 1 Burette section 3 Clamp 4 Measurement section 5 Conduit 10a Absorbent resin particles 11 Stand 13 Measurement platform 13a Through hole 15 Nylon mesh sheet 21 Burette tube 22 Stopcock 23 Rubber stopper 24 Stopcock 25 Air inlet tube 41 Stand 50 Physiological saline 61 Burette section 61a Burette 61b Rubber stopper 61c Stopcock 61d Air inlet tube 61e Stopcock 62 Conduit 63 Measurement platform 64 Measurement section 64a Cylinder 64b Nylon mesh 64c Weight 100 Sample for measurement S0 Horizontal surface S1 Inclined surface S2 Stainless steel petri dish S3 Lab jack

Claims

1. Superabsorbent polymer particles having a saline solution absorption distance of 4.2 cm to 7.6 cm, as measured by the following methods (1) to (6). (1) Attach a 20 mm x 76 mm piece of double-sided tape to the center of a 26 mm x 76 mm glass plate, aligning the centers. (2) Place the water-absorbing resin particles over the entire adhesive surface of the tape. At this time, adjust the amount of water-absorbing resin particles attached to the adhesive surface of the tape to be 0.33 ± 0.02 g. (2) Place a 26 mm x 100 mm 250 mesh nylon mesh in the center of the front surface of the glass plate (the surface on which the water-absorbing resin particles are scattered). (3) Place a 50 mm x 76 mm 250 mesh nylon mesh in front of the nylon mesh. Fold the nylon mesh at both ends of the glass plate and fix it to the back of the glass plate to obtain a sample for measurement. (4) Place 15 g of physiological saline solution colored with food color blue No. 1 into a petri dish with an inner diameter of 75 mm. (5) Immerse the sample for measurement in the petri dish with the sample fixed at a 45-degree angle. (6) After 30 minutes of immersion, read the maximum distance traveled by the physiological saline solution drawn up from the edge of the glass plate of the sample for measurement, and measure the distance (cm) the physiological saline solution was drawn up.

2. The water-absorbing resin particles according to claim 1, wherein the amount of physiological saline solution the water-absorbing resin particles hold is 35 g / g or more.

3. The water-absorbing resin particles according to claim 1 or 2, wherein the water absorption capacity under load of the water-absorbing resin particles is 10 mL / g or more.

4. The water-absorbing resin particles according to claim 1 or 2, wherein the unpressurized DW3 value of the water-absorbing resin particles is 30 mL / g or more.

5. The water-absorbing resin particles according to claim 1 or 2, wherein the unpressurized DW5 minute value of the water-absorbing resin particles is 40 mL / g or more.

6. An absorbent body comprising water-absorbent resin particles as described in claim 1 or 2.

7. An absorbent article comprising the absorbent material described in claim 6.

8. A method for producing water-absorbent resin particles by reverse-phase suspension polymerization of a water-soluble ethylenically unsaturated monomer in a hydrocarbon dispersion medium, wherein the reverse-phase suspension polymerization is carried out in two or more stages, in the first stage polymerization step of the reverse-phase suspension polymerization, the concentration of the water-soluble ethylenically unsaturated monomer in the aqueous solution containing the water-soluble ethylenically unsaturated monomer is 40% by mass or more, and in the second stage polymerization step of the reverse-phase suspension polymerization, an internal crosslinking agent is used in an amount of 0 mmol or more and 0.042 mmol or less per mole of the water-soluble ethylenically unsaturated monomer used in the second stage polymerization.

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