absorbent articles

Abstract

To provide an absorbent article capable of reducing the amount of liquid returned after liquid absorption for the second time.SOLUTION: An absorbent article includes, in this order, a permeable top sheet, an absorber containing poly(meth)acrylic acid (salt)-based resin and a hydrophilic fiber material, having specific properties, and a liquid impermeable back sheet.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to an absorbent article, and more specifically to an absorbent article having an absorbent body containing a poly(meth)acrylic acid (salt)-based water-absorbent resin. [Background technology]

[0002] Known absorbent articles include sanitary materials that absorb body fluids, such as disposable diapers, sanitary napkins, and incontinence pads. In recent years, water-absorbent resins have been used as constituent materials (absorbents) of such sanitary materials, particularly from the viewpoint of absorbing body fluids.

[0003] Known examples of such water-absorbent resins include hydrolysates of starch-acrylonitrile graft copolymers, neutralized starch-acrylic acid graft polymers, saponified vinyl acetate-acrylic acid ester copolymers, and crosslinked products of partially neutralized acrylic acid polymers. From the viewpoint of water-absorbing performance, however, poly(meth)acrylic acid (salt)-based water-absorbent resins using (meth)acrylic acid and / or its salts as the main monomer component are most widely produced industrially.

[0004] However, absorbent articles may experience liquid pooling or liquid backflow during use. This can cause discomfort when wearing the absorbent article, and therefore there is a demand for absorbent articles that are less susceptible to liquid pooling or liquid backflow.

[0005] One of the known causes of the liquid pooling or return of liquid in absorbent articles is a decrease in water absorption performance due to gel blocking of the water-absorbent resin. Patent Documents 1 to 3 disclose techniques for suppressing gel blocking and improving the water absorption performance of absorbent articles by using a water-absorbent resin with excellent elasticity (gel elasticity) after absorbing water in the absorbent body. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] WO2020 / 218160 [Patent Document 2] WO2020 / 218161 [Patent Document 3] WO2020 / 218162 Summary of the Invention [Problem to be solved by the invention]

[0007] However, although gel booking is suppressed in the absorbent article (absorbent body) using the above-mentioned conventional water-absorbent resin, there is a problem that liquid returns after the second absorption when a load (e.g., the weight of the user) is applied locally, and in this respect, there is room for further improvement.

[0008] An object of one aspect of the present invention is to provide an absorbent article in which the amount of liquid wetting back after the second liquid absorption is reduced. [Means for solving the problem]

[0009] The present inventors discovered that in an absorbent article having an absorbent body containing a water-absorbent resin with a small gel migration index, which indicates the change in gel shape retention when a load is applied to the absorbent resin under different conditions, the amount of liquid returning after the second absorption can be suppressed when a load is applied locally to the absorbent article, and thus completed the present invention.

[0010] That is, one embodiment of the present invention is as follows. <1> An absorbent article comprising, in this order, a liquid-permeable top sheet, an absorbent body containing a water-absorbent resin and a hydrophilic fibrous material, and a liquid-impermeable back sheet, wherein the absorbent body has a layer of water-absorbent resin and a layer of hydrophilic fibrous material, the layer of water-absorbent resin being disposed on the liquid-permeable top sheet side, and the layer of hydrophilic fibrous material being disposed on the liquid-impermeable back sheet side, and the basis weight of the layer of hydrophilic fibrous material being 100 g / m 2 is as follows: The water-absorbent resin is a particulate water-absorbent resin containing a crosslinked polymer including a structural unit derived from a poly(meth)acrylic acid (salt)-based monomer, and has a gel mobility index of 0.20 or less, as determined by the following procedure, and a water absorption rate (Vortex method) of 50 seconds or less: (1) 38 g of a 0.9% by mass sodium chloride aqueous solution at 25°C and 2.00 g of a water-absorbent resin are placed in a 100 mL beaker with a body diameter of 55 mm and a height of 70 mm to form a swollen gel.

[0011] (2) A compression tool having a disk portion and a rod-shaped portion with one end connected to the center of the disk portion (the disk portion has flat surfaces on both sides and is disk-shaped with a diameter of 2.5 cm and a thickness of 1 cm, and the length of the rod-shaped portion is 5.5 cm) is placed so that it is located in the center of the beaker containing the swollen gel of (1).After the underside of the disk portion of the compression tool is brought into contact with the surface of the swollen gel, the compression tool is placed at the measurement start position so that the underside of the disk portion is 0.05 mm vertically elevated from the surface of the swollen gel.

[0012] (3) The compression jig placed at the measurement start position is pressed into the swollen gel vertically by 10.0 mm at a speed of 10 cm / min.

[0013] (4) In (3), the compression jig is pressed vertically to a position 8.0 mm from the measurement start position, and the load (N) applied to the compression jig is observed and taken as the gel shape retention force A.

[0014] (5) The same operations as (1) to (4) were carried out except that the speed at which the compression jig was pressed was changed to 1 mm / min, and the load (N) applied to the compression jig was observed and taken as the gel shape retention force B.

[0015] (6) Calculate the gel migration index according to the following formula (1): Gel migration index = (gel shape retention A / gel shape retention B) - 1 (1). <2> The water-absorbing resin is an aggregate of spherical particles. <1> The absorbent article according to claim 1. <3> The water-absorbing resin contains water-insoluble inorganic fine particles. <1> or <2> The absorbent article according to claim 1. <4> The water-absorbent resin has a gel shape retention capacity A of 10.0 N or more. <1> ~ <3> 10. An absorbent article according to any one of the preceding claims. <5> The water-absorbent resin has an AAP of 15 g / g or more. <1> ~ <4> 10. An absorbent article according to any one of the preceding claims. <6> The mass of the water-absorbent resin contained in the absorbent body is 50% by mass or more and less than 100% by mass of the total mass of the water-absorbent resin and the hydrophilic fibrous material. <1> ~ <5> 10. An absorbent article according to any one of the preceding claims. [Effects of the Invention]

[0016] The water-absorbent resin according to the present invention can provide an absorbent article that reduces the amount of liquid returning after the second absorption when a load is locally applied. [Brief explanation of the drawings]

[0017] [Figure 1] FIG. 1 is a schematic diagram of a compression jig used to measure the gel migration index of a water-absorbent resin. [Figure 2] 1A to 1C are plan views showing a process for producing an absorbent sheet for reference evaluation of a water-absorbent resin. [Figure 3] FIG. 2 is a cross-sectional view of an absorbent sheet for reference evaluation of a water-absorbent resin. DETAILED DESCRIPTION OF THE INVENTION

[0018] The present invention will now be described with reference to the best mode thereof. It should be understood that the terms used in this specification are used in the same manner as commonly used in the art unless otherwise specified. Therefore, unless otherwise defined, all technical and scientific terms used in this specification have the same meaning as commonly understood by those skilled in the art to which this invention belongs. In the event of any conflict, the present specification (including definitions) shall prevail. The present invention is not limited to the following embodiments, and various modifications may be made within the scope of the claims.

[0019] An absorbent article according to one embodiment of the present invention is an absorbent article comprising, in this order, a liquid-permeable top sheet, an absorbent body containing a water-absorbent resin and a hydrophilic fibrous material, and a liquid-impermeable back sheet, wherein the absorbent body has a layer of water-absorbent resin and a layer of hydrophilic fibrous material, the layer of water-absorbent resin being disposed on the liquid-permeable top sheet side, and the layer of hydrophilic fibrous material being disposed on the liquid-impermeable back sheet side, and the layer of hydrophilic fibrous material having a basis weight of 100 g / m 2 The water-absorbent resin is a particulate water-absorbent resin containing a crosslinked polymer containing a structural unit derived from a poly(meth)acrylic acid (salt)-based monomer, and has a gel mobility index of 0.20 or less and a water absorption speed (Vortex method) of 50 seconds or less.

[0020] Based on the following novel findings, the inventors have invented an absorbent article according to one embodiment of the present invention, which is an absorbent article that reduces the amount of liquid returning during the second absorption when a load is applied locally. A water-absorbent resin with a small gel migration index can suppress the migration of the swollen gel (gel bias) within the absorbent body even after absorbing liquid (this can also be said to have excellent deformation resistance). Therefore, an absorbent body containing such a water-absorbent resin can maintain sufficient water absorption performance even after absorbing liquid for the second time. Furthermore, by arranging such a water-absorbent resin having a small gel migration index on the layer of hydrophilic fiber material, i.e., on the liquid-permeable top sheet side (skin side), the effect of improving the deformation resistance of the water-absorbent resin layer is more pronounced, and the migration of the swollen gel can be further reduced.

[0021] In addition, in the field of conventional absorbent articles, it has been believed that the greater the basis weight of the hydrophilic fibrous material layer, the greater the absorbency and cushioning properties of the absorbent article against local loads, and the less the amount of liquid rewet after the second absorption. However, the present inventors have surprisingly discovered that the amount of liquid rewet after the second absorption of an absorbent article can be further reduced by lowering the basis weight of the hydrophilic fibrous material layer. The present invention is based on the many new findings discovered by the present inventors.

[0022] [1. Definitions of Terms] [1-1. Water-absorbing resin] The term "water-absorbent resin" as used herein refers to a water-swellable, water-insoluble polymer gelling agent that satisfies the following physical properties: "Water-swellable" refers to a polymer gelling agent that has a CRC of 5 g / g or more as defined in ERT441.2-02, and "water-insoluble" refers to a polymer gelling agent that has an Ext of 50 wt% or less as defined in ERT470.2-02.

[0023] The water-absorbent resin can be appropriately designed depending on its application, and is not particularly limited, but is preferably a hydrophilic cross-linked polymer obtained by cross-linking an unsaturated monomer having a carboxyl group. In addition, the water-absorbent resin is not limited to a form in which the entire amount (100% by weight) of the water-absorbent resin is a polymer, and may be a water-absorbent resin composition containing additives and the like within a range that satisfies the above physical properties (CRC, Ext).

[0024] Furthermore, the water-absorbent resin in the present invention is not limited to a final product, but may also refer to an intermediate in the manufacturing process of the water-absorbent resin (for example, a hydrogel-like crosslinked polymer after polymerization, a dried polymer after drying, a water-absorbent resin powder before surface crosslinking, etc.), and all of these, together with the water-absorbent resin composition, are collectively referred to as "water-absorbent resin". Note that, examples of the shape of the water-absorbent resin include sheet-like, fibrous, film-like, particulate, gel-like, etc., but particulate water-absorbent resins are preferred in the present invention.

[0025] [1-2. "EDANA" and "ERT"] "EDANA" is an abbreviation for European Disposables and Nonwovens Associations, and "ERT" is an abbreviation for EDANA Recommended Test Methods, a European standard (almost a global standard) for measuring the properties of water-absorbent resins. In the present invention, unless otherwise specified, the physical properties of a water-absorbent resin are measured in accordance with the original ERT (revised in 2002 / publicly known document).

[0026] [1-3.Other] In this specification, the range "X to Y" means "X or more and Y or less."

[0027] In this specification, unless otherwise specified, "ppm" means "ppm by mass."

[0028] In this specification, "acid (salt)" means "acid and / or its salt." "(Meth)acrylic" means "acrylic and / or methacrylic." "Poly(meth)acrylic acid (salt)-based water-absorbing resin" means a water-absorbing resin containing (meth)acrylic acid (salt) as a repeating unit as a main component, and specifically refers to a water-absorbing resin containing preferably 50 to 100 mol%, more preferably 70 to 100 mol%, even more preferably 90 to 100 mol%, and particularly preferably substantially 100 mol% of (meth)acrylic acid (salt) in all monomers (excluding crosslinking agents) used in polymerization.

[0029] In this specification, the unit of volume "liter" may be written as "l" or "L".

[0030] In this specification, "weight" and "mass," "wt %" and "mass %," and "parts by weight" and "parts by mass" are treated as synonyms.

[0031] [2. Absorbent Articles] An absorbent article according to one embodiment of the present invention comprises, in this order, a liquid-permeable top sheet, an absorbent body containing a water-absorbent resin and a hydrophilic fibrous material, and a liquid-impermeable back sheet. Hereinafter, "the absorbent article according to one embodiment of the present invention" may be referred to as "the present absorbent article."

[0032] Because the absorbent article has the above-described configuration, when a load is applied locally, the amount of liquid returning after the second absorption is suppressed, thereby reducing discomfort when worn.

[0033] <2-1. Liquid-permeable top sheet> The liquid-permeable top sheet of the absorbent article (the liquid-permeable top sheet according to one embodiment of the present invention) is disposed at the outermost side of the absorbent article, on the side into which the liquid to be absorbed penetrates. When the absorbent article is worn, the liquid-permeable top sheet is disposed on the inside of the absorbent body (i.e., on the user's side), and can also be said to be disposed at the outermost side opposite the liquid-impermeable back sheet.

[0034] The liquid-permeable top sheet according to one embodiment of the present invention is not particularly limited as long as it is a sheet-like member having appropriate hydrophilicity, and may be, for example, a sheet-like member formed from resin or fiber. The material of the liquid-permeable top sheet can be appropriately selected from the viewpoints of the liquid permeability, flexibility, and strength of the absorbent article. More specifically, the liquid-permeable top sheet may be made of hydrophilic fibers such as pulp fiber, cotton linter crosslinked cellulose fiber, rayon, cotton, wool, acetate, and vinylon; nonwoven fabrics such as thermal-bonded nonwoven fabrics, air-through nonwoven fabrics, resin-bonded nonwoven fabrics, spunbonded nonwoven fabrics, melt-blown nonwoven fabrics, air-laid nonwoven fabrics, spunlace nonwoven fabrics, and point-bonded nonwoven fabrics; polyolefins such as polyethylene (PE) and polypropylene (PP); polyesters such as polyethylene terephthalate (PET), polytrimethylene terephthalate (PTT), and polyethylene naphthalate (PEN); polyamides such as nylon; synthetic resins such as rayon, or synthetic fibers containing these synthetic resins; or natural fibers such as cotton, silk, and hemp. These materials may be used alone or in combination of two or more. Among these, nonwoven fabrics are preferred because they are inexpensive and highly flexible. Furthermore, composite nonwoven fabrics can also be used. Examples of composite nonwoven fabrics include spunbond / meltblown / meltblown / spunbond nonwoven fabrics (SMMS nonwoven fabrics).

[0035] The basis weight and thickness of the liquid-permeable top sheet are selected from the viewpoints of imparting good liquid permeability, flexibility, strength, and cushioning properties to the absorbent article, increasing the liquid penetration rate of the absorbent article, and preventing the top sheet itself from retaining excessive liquid and returning to the skin side when a load is applied.

[0036] The basis weight of the liquid-permeable top sheet is preferably 5 to 70 g / m 2 and more preferably 10 to 50 g / m 2 and more preferably 10 to 30 g / m 2 The thickness of the liquid-permeable top sheet is preferably 50 to 500 μm, more preferably 100 to 300 μm, and even more preferably 150 to 250 μm.

[0037] From the viewpoint of the liquid absorption performance of the absorbent article, the liquid-permeable topsheet preferably has a moderate hydrophilicity. From this viewpoint, the liquid-permeable topsheet preferably has a hydrophilicity measured in accordance with Pulp and Paper Testing Method No. 68 (2000) by the Pulp and Paper Technology Association of Japan of 5 to 200, more preferably 10 to 150. For details of Pulp and Paper Testing Method No. 68, see, for example, WO2011 / 086843.

[0038] Methods for providing a liquid-permeable topsheet with the above-described appropriate hydrophilicity include, for example, forming the liquid-permeable topsheet from fibers exhibiting appropriate hydrophilicity, such as rayon fibers, or forming the topsheet by subjecting hydrophobic chemical fibers, such as polyolefin fibers or polyester fibers, to a hydrophilization treatment. Examples of such hydrophilization treatments include the use of a hydrophilizing agent. Examples of hydrophilizing agents that can be used include anionic surfactants such as aliphatic sulfonates and higher alcohol sulfate ester salts; cationic surfactants such as quaternary ammonium salts; nonionic surfactants such as polyethylene glycol fatty acid esters, polyglycerin fatty acid esters, and sorbitan fatty acid esters; silicone surfactants such as polyoxyalkylene-modified silicones; and stain release agents made from polyester, polyamide, acrylic, or urethane resins.

[0039] <2-2.Absorbent> The absorbent body of the absorbent article (which is an absorbent body according to one embodiment of the present invention and may be referred to as "the absorbent body" hereinafter) contains a water-absorbent resin and a hydrophilic fibrous material. First, the water-absorbent resin contained in the absorbent body (which is a water-absorbent resin according to one embodiment of the present invention and may be referred to as "the water-absorbent resin" hereinafter) will be described in detail.

[0040] (Shape of water-absorbent resin) In the present invention, the shape of the water-absorbent resin is preferably particulate. More specifically, examples of particulate shapes include irregularly crushed shapes, spheres, football shapes, and aggregate shapes. Among these, agglomerated particles of primary particles (spherical particles) are particularly preferred, since a spherical particle shape, or particularly an aggregate shape, allows the water-absorbent resin to absorb liquid at a high rate and suppresses the amount of liquid returning. Such a water-absorbent resin of agglomerated particles of primary particles can be obtained, for example, by sizing spherical particles of a water-absorbent resin obtained by reverse-phase suspension polymerization (for example, by sizing using a gel sizing device having an extrusion section and a perforated plate). In this specification, "spherical" includes not only a perfect sphere but also an approximately spherical shape with an aspect ratio of 1.0 to 1.2.

[0041] (Additives contained in water-absorbing resin) In one embodiment of the present invention, the water-absorbent resin may contain additives to exhibit various functions. Specific examples of such additives include surfactants, compounds containing phosphorus atoms, oxidizing agents, organic reducing agents, water-insoluble inorganic fine particles, organic powders such as metal soaps, deodorants, antibacterial agents, pulp, and thermoplastic fibers. Among these, it is preferable to contain water-insoluble inorganic fine particles. These additives may be used alone or in combination of two or more. The compounds disclosed in "(5) Water-insoluble inorganic fine particles" in International Patent Publication No. 2011 / 040530 are applicable to the present invention as the water-insoluble inorganic fine particles. Among these water-insoluble inorganic fine particles, hydrophilic fine particles (those having a high hydrophilicity (expressed as the proportion of fine particles suspended in a colloidal state in a mixed liquid of water / methanol = 70 / 30) (e.g., 70% or more) as described in European Patent No. 0629411, and / or those having a low contact angle with water (e.g., 10° or less) as described in Japanese Patent No. 6837139), for example, containing silica (silicon dioxide) or hydrotalcite, cause the swollen water-absorbent resin (swollen gel) to adequately catch on each other, making it difficult for the swollen gel to move within the absorbent body even when a load is applied locally (i.e., the swollen gel is unlikely to become biased), thereby exhibiting excellent absorption performance; in other words, the water-absorbent resin has a low amount of liquid return after the second absorption, and is therefore preferred.

[0042] The content of the water-insoluble inorganic fine particles in the water absorbent resin of the present invention is, from the viewpoint of improving the liquid compatibility of the water absorbent resin (for example, water absorbent resin particles), 0.01 to 5 parts by mass, more preferably 0.05 to 3 parts by mass, still more preferably 0.1 to 1 part by mass, and particularly preferably 0.2 to 0.5 parts by mass, relative to 100 parts by mass of the water absorbent resin.

[0043] Here, the water-insoluble inorganic fine particles usually have a size smaller than that of the water-absorbent resin. For example, the average particle size of the water-insoluble inorganic fine particles may be 0.01 μm to 50 μm, 0.1 μm to 30 μm, or 1 μm to 20 μm. The average particle size of the water-insoluble inorganic fine particles can be measured by a pore electrical resistance method or a laser diffraction / scattering method depending on the particle characteristics.

[0044] (CRC) "CRC" is an abbreviation for Centrifuge Retention Capacity, which means the water absorption capacity of a water-absorbent resin under no pressure.

[0045] The CRC of the water-absorbent resin of the present invention is preferably 30 g / g or more, more preferably 32 g / g or more. There is no particular limitation on the upper limit, and a higher CRC is preferable, but from the viewpoint of balance with other physical properties, it is preferably 50 g / g or less, more preferably 48 g / g or less, 46 g / g or less, 44 g / g or less, 42 g / g or less, 40 g / g or less, 38 g / g or less, or 36 g / g or less.

[0046] If the CRC of the water-absorbent resin is 30 g / g or more, the amount of liquid absorption is sufficient, and therefore, an absorbent article including an absorbent core containing the water-absorbent resin is suitable as an absorbent core for absorbent articles such as disposable diapers. Also, if the CRC is 50 g / g or less, a decrease in the rate of absorption of body fluids such as urine and blood is prevented, and the resin is suitable for use in high-absorption-rate disposable diapers, etc. The CRC value of the water-absorbent resin can be controlled by changing the type and amount of an internal cross-linking agent, a surface cross-linking agent, etc.

[0047] (Ext) "Ext" is an abbreviation for Extractables (water-soluble content) and refers to the amount of extractable content extracted from the water-absorbent resin. Ext is measured in accordance with the EDANA method (ERT470.2-02).

[0048] The Ext of the present water-absorbent resin is preferably 30% by mass or less, more preferably 20% by mass or less, and further preferably 10% by mass or less. The lower limit is not particularly limited and is, for example, 0% by mass or more, but from the viewpoint of the balance with other physical properties, it is preferably 2% by mass or more, more preferably 4% by mass or more.

[0049] When the Ext of the water-absorbent resin is 30% by mass or less, a decrease in the rate at which the absorbent resin absorbs body fluids such as urine and blood is prevented, and therefore, absorbent articles including an absorbent core containing the water-absorbent resin are suitable for use in high-absorption-rate disposable diapers, etc. Furthermore, this is preferable because the swollen gel becomes less slippery and the change in shape retention of the swollen gel when different loads are applied is small. The Ext value of the water-absorbent resin can be controlled by changing the type and amount of the polymerization initiator, internal crosslinking agent, surface crosslinking agent, etc., and can also be controlled by using a chain transfer agent in the polymerization process.

[0050] (AAP) "AAP" is an abbreviation for Absorption Against Pressure, and means the water absorption capacity of a water-absorbent resin under pressure. In this specification, AAP (absorption capacity against pressure) is measured in accordance with the EDANA method (ERT442.2-02) except that the load condition is changed to 4.83 kPa (0.7 psi). Specifically, 0.9 g of a water-absorbent resin is swelled under a pressure of 4.83 kPa for 1 hour using a 0.9 mass % sodium chloride aqueous solution, and then AAP (absorption capacity against pressure) (g / g) is measured.

[0051] From the viewpoint of excellent water absorption properties when used in sanitary materials, the AAP of the water-absorbent resin of the present invention is preferably 15 g / g or more, more preferably 16 g / g or more, and even more preferably 17 g / g or more. The upper limit of the AAP of the water-absorbent resin is not particularly limited, but is preferably 40 g / g or less.

[0052] (moisture content) The "moisture content" is measured in accordance with the EDANA method (ERT430.2-02), except that the sample amount is changed to 1.0 g and the drying temperature is changed to 180°C.

[0053] The water content of the water-absorbent resin of the present invention is not particularly limited, but is preferably 1% by mass to 20% by mass, more preferably 1% by mass to 15% by mass, even more preferably 2% by mass to 13% by mass, even more preferably 5% by mass to 13% by mass, even more preferably 8% by mass to 13% by mass, and particularly preferably 10% by mass to 13% by mass. If the water-absorbent resin has a water content of 1% by mass to 20% by mass, a decrease in the rate at which it absorbs body fluids such as urine and blood is prevented, and the resin is suitable for use in high-absorption-rate disposable diapers and the like.

[0054] (Mass average particle diameter (D50)) The "mass-average particle diameter (D50)" of a water-absorbent resin is measured in accordance with "(3) Mass-Average Particle Diameter (D50) and Logarithmic Standard Deviation (σζ) of Particle Diameter Distribution" described in columns 27 and 28 of U.S. Pat. No. 7,638,570.

[0055] The mass average particle diameter (D50) of the present water-absorbent resin is preferably 200 μm to 700 μm, more preferably 250 μm to 600 μm, even more preferably 250 μm to 500 μm, and particularly preferably 300 μm to 450 μm. The proportion of particles with a particle diameter of less than 150 μm is preferably 20 mass % or less, more preferably 10 mass % or less. When the mass average particle diameter of the water-absorbent resin is 200 μm or more, dust is small and handling is easy. When the mass average particle diameter of the water-absorbent resin is 700 μm or less, a decrease in the rate of absorption of body fluids such as urine and blood is prevented, and therefore, absorbent articles including an absorbent core containing the water-absorbent resin are suitable for use in high-absorption-rate disposable diapers and the like.

[0056] (number average particle size) When the water-absorbent resin is in the form of an aggregate of primary particles, the number-average particle size of the water-absorbent resin is the number-average particle size of the primary particles constituting the aggregate, and is a value measured using an electron microscope. The number-average particle size of the primary particles of the water-absorbent resin is preferably 5 μm to 1000 μm, more preferably 5 μm to 800 μm, even more preferably 8 μm to 500 μm, still more preferably 10 μm to 300 μm, still more preferably 10 μm to 200 μm, and particularly preferably 30 μm to 100 μm.

[0057] (bulk density) The "bulk density" of the water-absorbent resin is measured in accordance with the EDANA method (ERT460.2-02).

[0058] The bulk density of the water-absorbent resin is preferably 0.68 g / cm 3 ~1.00g / cm 3 , more preferably 0.69 g / cm 3 ~0.95g / cm 3 , and most preferably 0.68 g / cm 3 ~0.90g / cm 3 The bulk density of the water-absorbent resin is 0.68 to 1.00 g / cm 3 This prevents a decrease in the rate at which the absorbent absorbs body fluids such as urine and blood, making it suitable for use in high-absorption-rate disposable diapers, etc. Furthermore, the swollen gels are appropriately caught together, and there is little change in the shape retention of the gel when different loads are applied, which is preferable because the gel is less likely to move within the absorbent core and exhibits excellent absorption performance.

[0059] (surface tension) The "surface tension" of the water-absorbent resin in this specification refers to the surface tension of the aqueous solution when the water-absorbent resin is dispersed in a 0.9 mass % sodium chloride aqueous solution, and is measured by the method described in WO2015 / 129917.

[0060] The surface tension of the present water-absorbent resin may be 74 mN / m or less, or 55 mN / m or more, preferably 60 mN / m or more, more preferably 65 mN / m or more, even more preferably 67 mN / m or more, and particularly preferably 70 mN / m or more. When the surface tension of the water-absorbent resin is in the above range, slippage of the swollen gel is suppressed, the gel is less likely to move within the absorbent body, and excellent absorption performance is exhibited, which is preferable.

[0061] (Water absorption rate) In this specification, the "water absorption rate" of a water-absorbent resin is measured in accordance with the Vortex method of Japanese Industrial Standards JIS K 7224 (1996). That is, the water absorption rate (Vortex method) refers to the time it takes for 2.0 g of a water-absorbent resin to be added to 50 g of a 0.9 mass % aqueous sodium chloride solution stirred at 600 rpm with a stirrer tip, and for the stirrer tip to be covered with the test liquid.

[0062] A water-absorbent resin having an excellent "water absorption rate" is preferable because, when used in an absorbent article, it quickly absorbs liquid and suppresses the amount of liquid returning. The "water absorption rate" is 50 seconds or less, preferably 45 seconds or less, and more preferably 40 seconds or less. There is no particular lower limit, but it may be 10 seconds or more, 15 seconds or more, or 20 seconds or more.

[0063] (gel shape retention) In this specification, the "gel shape retention" of a water-absorbent resin is a new physical property value that represents the load (gel shape retention) when a predetermined load is applied to a water-absorbent resin in a gel state that has absorbed water (swollen gel). The "gel shape retention" is measured by the method described in the examples below.

[0064] In this specification, a compression jig comprising a disk portion and a rod-shaped portion with one end connected to the center of the disk portion (the disk portion has flat surfaces on both sides and is disk-shaped with a diameter of 2.5 cm and a thickness of 1 cm, and the length of the rod-shaped portion is 5.5 cm) is pressed 10.0 mm at a speed of 10 cm / min into a gel swollen 20 times with a 0.9% by mass aqueous sodium chloride solution (20-fold swollen gel), and the load (N) applied to the compression jig when the compression jig is pressed to a position of 8.0 mm is defined as gel shape retention force A, and the load (N) applied to the compression jig when the pressing speed is changed to 1 mm / min and measurement is made in the same manner is defined as gel shape retention force B.

[0065] The gel shape retention force A is preferably 6.0 N or more, more preferably 8.0 N or more, even more preferably 10.0 N or more, even more preferably 10.4 N or more, and particularly preferably 10.6 N or more. There is no particular upper limit, but it may be 15.0 N or less, 14.0 N or less, or 13.0 N or less. The gel shape retention force B is preferably 5.0 N or more, more preferably 7.0 N or more, even more preferably 9.0 N or more, even more preferably 9.1 N, and particularly preferably 9.2 N. There is no particular upper limit, but it may be 14.0 N or less, 13.0 N or less, or 12.0 N or less.

[0066] A water-absorbent resin with high "gel shape retention" (for example, a water-absorbent resin with a gel shape retention A of 10.0 N or more) is preferred when used in an absorbent article, particularly in a thin absorbent article, because the gel is less likely to deform or move even when a load (body weight) is applied after absorbing water, and therefore the gel is less likely to become unevenly distributed within the absorbent body, and absorption performance is not impaired.

[0067] (Gel migration index) While conducting research into the above-mentioned problem of the present application, the inventors discovered for the first time that when a load is applied locally to an absorbent article, the gel inside the absorbent body becomes uneven, and this unevenness also causes an uneven absorption performance of the absorbent body, which is the cause of liquid return after the second absorption.

[0068] The present inventors have further conducted intensive studies based on the above findings, and as a result, have newly found that the amount of liquid return after the second liquid absorption when a load (body weight) is applied locally to a water-absorbent resin can be evaluated by using the "gel mobility index," which is a new physical property value that represents the change in shape retention of a gel when different loads are applied to a swollen gel.

[0069] As used herein, the term "gel migration index" refers to a value defined by the following formula (1): Gel migration index = (gel shape retention A / gel shape retention B) - 1 (1) The "gel mobility index" is a new physical property value that represents the ease of movement of a water-absorbent resin when it swells. Conventionally, a method for measuring the physical properties of a swollen gel by pressing a compression tool into the swollen gel has been known. However, in this conventional method, the pressing speed of the compression tool is only one point, and this method can only measure the strength of the swollen gel. On the other hand, in measuring the "gel mobility index," the pressing speed of the compression tool is measured at two different points. Since the state of rearrangement of the swollen gel changes when the pressing speed is changed, by comparing data from different pressing speeds, the ease of movement of the water-absorbent resin when it swells (swollen gel), which could not be quantified by conventional methods, can be quantified as the "gel mobility index." The specific method for measuring the gel mobility index is as described in the Examples.

[0070] A water-absorbent resin with a small "gel migration index" is preferred when used in an absorbent article, particularly a thin absorbent article, because deformation of the gel layer is suppressed even when a load (body weight) is applied after absorbing water, and the gel is less likely to become unevenly distributed within the absorbent core, so that absorption performance is not impaired. The gel migration index of the water-absorbent resin of the present invention is 0.20 or less, more preferably 0.18 or less, and even more preferably 0.16 or less. There is no particular lower limit, but it may be a negative value or may be 0.00 or more.

[0071] The "gel migration index" of the water-absorbent resin can be controlled by adjusting, for example, the raw materials, additives, polymerization method, classification, and / or shape of the water-absorbent resin.

[0072] (Amount of liquid returning to the absorbent resin) From the viewpoint of providing an absorbent article in which the amount of liquid returned after the second absorption is further reduced, the amount of liquid returned by the water-absorbent resin is preferably 1.0 g or less, more preferably 0.8 g or less, and even more preferably 0.7 g or less. In this specification, the amount of liquid returned by the water-absorbent resin is a value measured using an absorbent sheet for evaluation. The amount of liquid returned by the water-absorbent resin can also be said to be the amount of liquid returned by the absorbent sheet for evaluation. The amount of liquid returned by the water-absorbent resin (the amount of liquid returned by the absorbent sheet for evaluation) is a value measured by the method described in the Examples.

[0073] <Method of manufacturing water-absorbent resin> The method for producing the water-absorbent resin of the present invention is not particularly limited as long as it can obtain a water-absorbent resin having desired physical properties, and any of aqueous solution polymerization, reversed-phase suspension polymerization, gas-phase droplet polymerization, and other polymerization methods may be used, but reversed-phase suspension polymerization is preferred from the viewpoint of ease of controlling the physical properties of the water-absorbent resin. Hereinafter, reversed-phase suspension polymerization will be described as an example.

[0074] A method for producing a water-absorbent resin according to one embodiment of the present invention includes a polymerization step of polymerizing a monomer in a state in which droplets containing the monomer are dispersed or suspended in a hydrophobic organic solvent to obtain a hydrogel polymer, and a drying step of drying the hydrogel polymer (for example, by using an agitator dryer) to obtain a particulate dried polymer. In this specification, the "hydrogel polymer" may be simply referred to as "hydrogel".

[0075] The polymerization method in the method for producing a water-absorbent resin of the present invention may be reversed-phase suspension polymerization, in which a hydrogel polymer is obtained by polymerizing droplets containing a monomer dispersed or suspended in a liquid phase consisting of a hydrophobic organic solvent. The polymerization method may be batch or continuous. A batch production method involves adding or dropping an aqueous monomer solution into a hydrophobic organic solvent in a reactor, dispersing or suspending the aqueous monomer solution, and then polymerizing the solution to obtain a hydrogel polymer. A continuous production method involves continuously pumping an aqueous monomer solution into a hydrophobic organic solvent in a reactor, dispersing or suspending the aqueous monomer solution, and polymerizing the resulting aqueous gel polymer. The hydrophobic organic solvent and the resulting hydrogel polymer are continuously discharged from the reactor. A preferred embodiment of the present invention is continuous reversed-phase suspension polymerization (continuous polymerization), and more preferably liquid-phase droplet continuous polymerization, in which an aqueous monomer solution is continuously dispersed in a hydrophobic organic solvent and polymerized.

[0076] Such a continuous production process is preferable in that each step and each operation between steps can be carried out continuously, enabling mass production through long-term operation. Continuous reversed-phase suspension polymerization is also a preferable mode in terms of the physical properties of the water-absorbent resin. A method for producing a water-absorbent resin according to one embodiment of the present invention may include a separation step for separating the hydrophobic organic solvent from the hydrogel polymer obtained in the polymerization step. In the continuous production process, it is preferable to recover the hydrophobic organic solvent separated from the hydrogel polymer in the separation step and reuse it as the hydrophobic organic solvent in the polymerization step. Such a circulatory production process can reduce the amount of organic solvent used, which is preferable in terms of production costs and waste liquid treatment.

[0077] Continuous polymerization is a mode in which an aqueous monomer solution is continuously suspended or dispersed as droplets in a hydrophobic organic solvent in a dispersing device, and the resulting dispersion / suspension is continuously supplied to a reactor. This mode is clearly distinguishable from a mode in which dispersion and polymerization are carried out in a single device (batch operation, batch method). When continuous operation is carried out, the operating time is preferably 1 hour or more, more preferably 3 hours or more, even more preferably 8 hours or more, and even more preferably 24 hours or more. Furthermore, the operating time is usually 1 year or less.

[0078] A method for producing a water-absorbent resin according to one embodiment of the present invention includes an optional step of preparing an aqueous monomer solution; an optional dispersing step; a polymerization step; an optional step of separating a reversed-phase suspension polymerization gel (hydrogel) (separation step); an optional gel sizing step; and a drying step. After the drying step, the method may optionally include a cooling step, a pulverizing step, a classification step, a surface cross-linking step, a hydration (rewetting) step, a step of adding other additives, a sizing step, a fine powder removal step, a granulation step, and a fine powder recycling step. The method may further include a transporting step, a storing step, a packaging step, a storage step, and the like.

[0079] In particular, the method for producing a water absorbent resin of the present invention preferably comprises a separation step, a gel granulation step, a drying step (preferably hot air drying), and a surface cross-linking step (preferably powder surface treatment). The above-mentioned configuration has an advantage that the gel shape retention of the obtained water absorbent resin is enhanced, compared with general reverse phase suspension polymerization which comprises an azeotropic dehydration step in a hydrophobic organic solvent after polymerization and a surface cross-linking step in a dispersion system.

[0080] Hereinafter, each step that may be included in the method for producing a water-absorbent resin according to one embodiment of the present invention will be described.

[0081] [Monomer aqueous solution preparation process] The aqueous monomer solution is an aqueous solution containing a (meth)acrylic acid (salt)-based monomer that is a raw material for the water-absorbent resin, and is a solution that is dispersed or suspended in a hydrophobic organic solvent to carry out reversed-phase suspension polymerization.

[0082] The solvent for the aqueous monomer solution is preferably water or a mixture of water and a water-soluble organic solvent (e.g., alcohol), and more preferably water. When the solvent for the aqueous monomer solution is a mixture of water and a water-soluble organic solvent, the content of the water-soluble organic solvent (e.g., alcohol) is preferably 30% by mass or less, and more preferably 5% by mass or less.

[0083] The aqueous monomer solution may contain, in addition to (meth)acrylic acid, a water-soluble ethylenically unsaturated monomer other than (meth)acrylic acid (other water-soluble ethylenically unsaturated monomer). Examples of other water-soluble ethylenically unsaturated monomers include acid group-containing unsaturated monomers such as (anhydride) maleic acid, itaconic acid, cinnamic acid, vinyl sulfonic acid, allyl toluene sulfonic acid, vinyl toluene sulfonic acid, styrene sulfonic acid, 2-(meth)acrylamido-2-methylpropanesulfonic acid, 2-(meth)acryloylethanesulfonic acid, 2-(meth)acryloylpropanesulfonic acid, 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 2-hydroxyethyl (meth)acryloyl phosphate, methoxypolyethylene glycol (meth)acrylate, and polyethylene glycol mono(meth)acrylate; (meth)acrylamide; N-ethyl (meth)acrylate; Examples of the unsaturated monomers include amide group-containing unsaturated monomers such as vinylpyridine, N,N-dimethyl(meth)acrylamide, Nn-propyl(meth)acrylamide, N-isopropyl(meth)acrylamide, vinylpyridine, N-vinylpyrrolidone, N-acryloylpiperidine, N-acryloylpyrrolidine, and N-vinylacetamide; amino group-containing unsaturated monomers such as N,N-dimethylaminoethyl(meth)acrylate, N,N-dimethylaminopropyl(meth)acrylate, N,N-dimethylaminopropyl(meth)acrylamide, and N,N-diethylaminoethyl(meth)acrylate; mercapto group-containing unsaturated monomers; phenolic hydroxyl group-containing unsaturated monomers; and lactam group-containing unsaturated monomers such as N-vinylpyrrolidone.

[0084] In addition, in consideration of the stability of the (meth)acrylic acid-based monomer and other water-soluble ethylenically unsaturated monomers (also referred to as monomer components), a polymerization inhibitor may be added to the aqueous monomer solution as needed.

[0085] Among the monomer components, the (meth)acrylic acid monomer and other water-soluble ethylenically unsaturated monomers having an acid group such as a carboxyl group (hereinafter, the monomer component having an acid group may be referred to as an acid group-containing unsaturated monomer) can be used as neutralized salts in which the acid group of the monomer has been neutralized. In this case, the neutralized salt of the acid group-containing unsaturated monomer is preferably a salt with a monovalent cation, more preferably at least one selected from alkali metal salts, ammonium salts, and amine salts, even more preferably an alkali metal salt, still more preferably at least one selected from sodium salts, lithium salts, and potassium salts, and particularly preferably a sodium salt.

[0086] From the viewpoint of the water absorption performance of the obtained water absorbent resin, the monomer component is preferably an acid group-containing unsaturated monomer and / or a salt thereof, more preferably at least one selected from (meth)acrylic acid (salt), (anhydrous) maleic acid (salt), itaconic acid (salt), and cinnamic acid (salt), further preferably (meth)acrylic acid (salt), and particularly preferably acrylic acid (salt).

[0087] When an acid group-containing unsaturated monomer is used as a monomer, it is preferable to use it in combination with a neutralized salt of the acid group-containing unsaturated monomer from the viewpoint of the water absorption performance of the resulting water absorbent resin. From the viewpoint of water absorption performance, the number of moles of the neutralized salt relative to the total number of moles of the acid group-containing unsaturated monomer and its neutralized salt (hereinafter referred to as "neutralization rate") is preferably 40 mol% or more, more preferably 40 mol% to 95 mol%, even more preferably 50 mol% to 90 mol%, even more preferably 55 mol% to 85 mol%, and particularly preferably 60 mol% to 80 mol%.

[0088] In the method for producing a water-absorbent resin according to one embodiment of the present invention, in the preparation of the aqueous monomer solution, any one of the above-exemplified monomers may be used alone, or any two or more kinds of monomers may be used in appropriate mixture. Moreover, as long as the object of the present invention is achieved, a monomer other than the water-soluble ethylenically unsaturated monomer may also be mixed.

[0089] In the preparation of the above-mentioned aqueous monomer solution, when two or more kinds of monomers are used in combination, from the viewpoint of the water absorption performance of the obtained water absorbent resin, the monomer component preferably contains (meth)acrylic acid (salt) as a main component. "Containing (meth)acrylic acid (salt) as a main component" means that the ratio of (meth)acrylic acid (salt) to the total amount of monomers used in polymerization is, for example, 50 mol% or more, preferably 70 mol% or more, more preferably 80 mol% or more, and even more preferably 90 mol% or more (upper limit: 100 mol%).

[0090] In preparing the above-mentioned aqueous monomer solution, an internal crosslinking agent can be used as needed. Examples of the internal crosslinking agent include conventionally known internal crosslinking agents having two or more polymerizable unsaturated groups or two or more reactive groups in one molecule. Examples of the internal crosslinking agent include N,N'-methylenebis(meth)acrylamide, (poly)ethylene glycol di(meth)acrylate, (poly)propylene glycol di(meth)acrylate, trimethylolpropane tri(meth)acrylate, trimethylolpropane di(meth)acrylate, glycerin tri(meth)acrylate, glycerin acrylate methacrylate, ethylene oxide-modified trimethylolpropane tri(meth)acrylate, pentaerythritol tetra(meth)acrylate, dipentaerythritol hexa(meth)acrylate, and the like. Examples of the internal crosslinking agent include (meth)acrylate, triallyl cyanurate, triallyl isocyanurate, triallyl phosphate, triallylamine, poly(meth)allyloxyalkane, (poly)ethylene glycol diglycidyl ether, glycerol diglycidyl ether, ethylene glycol, polyethylene glycol, propylene glycol, glycerin, 1,4-butanediol, pentaerythritol, ethylenediamine, ethylene carbonate, propylene carbonate, polyethyleneimine, glycidyl (meth)acrylate, etc. These internal crosslinking agents may be used alone or in combination of two or more.

[0091] In the method for producing a water absorbent resin according to one embodiment of the present invention, the amount of the internal crosslinking agent used may be appropriately determined depending on the desired physical properties of the water absorbent resin, and is, for example, 0.0001 mol % to 5 mol %, more preferably 0.001 mol % to 3 mol %, and even more preferably 0.005 mol % to 1.5 mol %, based on the monomer contained in the aqueous monomer solution.

[0092] In addition, the following substances (hereinafter referred to as "other substances") may be added to the aqueous monomer solution.

[0093] Specific examples of other substances include chain transfer agents such as thiols, thiolic acids, secondary alcohols, amines, and hypophosphites; blowing agents such as carbonates, bicarbonates, azo compounds, and bubbles; chelating agents such as metal salts of ethylenediaminetetraacetic acid and metal salts of diethylenetriaminepentaacetic acid; polyacrylic acid (salts) and crosslinked products thereof, starch, cellulose, starch-cellulose derivatives, polyvinyl alcohol, etc. The other substances may be used alone or in combination of two or more.

[0094] The amount of other substances used is not particularly limited, but the total concentration of other substances is preferably 10% by mass or less, more preferably 1% by mass or less, and even more preferably 0.1% by mass or less, based on the total amount of monomers contained in the aqueous monomer solution. However, the total concentration (total amount) of polyacrylic acid (salts) and crosslinked products thereof, starch, cellulose, starch-cellulose derivatives, and polyvinyl alcohol is 30% by mass or less, more preferably 20% by mass or less, and even more preferably 10% by mass or less, based on the total amount of monomers contained in the aqueous monomer solution.

[0095] In one embodiment of the present invention, the dissolved oxygen in the aqueous monomer solution may be reduced by increasing the temperature or by replacing the oxygen with an inert gas.

[0096] (Polymerization initiator) A polymerization initiator may be used in preparing the aqueous monomer solution. When a polymerization initiator is used in preparing the aqueous monomer solution, gelation or an increase in viscosity of the aqueous monomer solution may occur. Therefore, the polymerization initiator is preferably added by the following methods: (i) immediately before dispersing / suspending the aqueous monomer solution in the hydrophobic organic solvent; (ii) cooling the aqueous monomer solution and mixing it with the polymerization initiator at a temperature lower than room temperature (20°C or lower, preferably around 0°C); or (iii) subjecting the aqueous monomer solution and the polymerization initiator to the dispersion step while line mixing. A thermally decomposable polymerization initiator is preferably used as the polymerization initiator. The thermally decomposable polymerization initiator refers to a compound that decomposes when heated to generate radicals. The 10-hour half-life temperature of the thermally decomposable polymerization initiator is preferably 0°C to 120°C, more preferably 30°C to 100°C, and even more preferably 50°C to 80°C, from the viewpoints of storage stability and production efficiency of the water-absorbent resin.

[0097] From the viewpoint of ease of handling and the physical properties of the water-absorbing resin, the polymerization initiator is preferably a water-soluble polymerization initiator, and more preferably a water-soluble radical polymerization initiator.

[0098] Examples of water-soluble radical polymerization initiators include persulfates such as potassium persulfate, ammonium persulfate, and sodium persulfate; peroxides such as methyl ethyl ketone peroxide, methyl isobutyl ketone peroxide, di-t-butyl peroxide, t-butylcumyl peroxide, t-butyl peroxyacetate, t-butylperoxyisobutyrate, t-butylperoxypivalate, and hydrogen peroxide; and water-soluble azo compounds such as 2,2'-azobis(2-amidinopropane) dihydrochloride, 2,2'-azobis[2-(N-phenylamidino)propane] dihydrochloride, and 2,2'-azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride. These polymerization initiators may be used alone or in combination of two or more. Among these, preferably used are persulfates or water-soluble azo compounds, more preferably sodium persulfate, potassium persulfate, ammonium persulfate, or 2,2'-azobis(2-amidinopropane) dihydrochloride, and even more preferably sodium persulfate.

[0099] The amount of the polymerization initiator used is appropriately set depending on the types of the monomer and the polymerization initiator, and is not particularly limited, but from the viewpoint of production efficiency, it is preferably 0.001 g / mol or more, more preferably 0.005 g / mol or more, and even more preferably 0.01 g / mol or more, relative to the total amount of the monomers contained in the aqueous monomer solution. Moreover, from the viewpoint of improving the water absorption performance of the water absorbent resin, it is preferably 2 g / mol or less, more preferably 1 g / mol or less. If necessary, the thermally decomposable polymerization initiator can be used in combination with other polymerization initiators such as a photodecomposable polymerization initiator. Specific examples of the photodecomposable polymerization initiator include benzoin derivatives, benzyl derivatives, acetophenone derivatives, and benzophenone derivatives. Furthermore, the thermally decomposable polymerization initiator can be used in combination with a reducing agent to form a redox polymerization initiator. In the redox polymerization initiator, the thermally decomposable polymerization initiator functions as an oxidizing agent. The reducing agent to be used is not particularly limited, but examples thereof include (bis)sulfites such as sodium sulfite and sodium hydrogen sulfite; reducing metal salts such as ferrous salts; L-ascorbic acid (salts), and amines.

[0100] (Monomer concentration in aqueous monomer solution) In the present invention, the concentration of the monomer in the aqueous monomer solution is selected depending on the selected monomer and the type of hydrophobic organic solvent, but in terms of production efficiency, the lower limit is preferably 10% by mass or more, more preferably 20% by mass or more, and even more preferably 30% by mass or more, and the upper limit is preferably 100% by mass or less, more preferably 90% by mass or less, even more preferably 80% by mass or less, and even more preferably 70% by mass or less.

[0101] As long as the object of the present invention is not impaired, it is also possible to blend additives such as an internal crosslinking agent, a surfactant, a density adjuster, a thickener, a chelating agent, etc. The type and amount of the additive can be appropriately selected depending on the combination of the monomer and the hydrophobic organic solvent used.

[0102] [Dispersion process] The dispersion process is a process of dispersing or suspending a monomer aqueous solution (droplets containing a monomer) in a hydrophobic organic solvent. Hereinafter, the term "dispersion" is intended to encompass suspension. More specifically, the dispersion process involves adding the monomer aqueous solution to a hydrophobic organic solvent and mixing and stirring to disperse the monomers. For example, a stirrer equipped with a stirring blade (such as a propeller blade, paddle blade, anchor blade, turbine blade, Pfaudle blade, ribbon blade, or flat blade) may be used. When using a stirrer equipped with such a stirring blade, the size of the dispersed droplets can be adjusted by the type, blade diameter, and rotation speed of the stirring blade, making it particularly suitable for use in batch-type reverse-phase suspension polymerization. Alternatively, a dispersion can be obtained by methods described in International Publication Nos. 2009 / 025235 and 2013 / 018571. When continuous reverse phase suspension polymerization is carried out, the dispersing step preferably comprises continuously supplying an aqueous monomer solution and a hydrophobic organic solvent to a dispersing device separately to produce droplets containing the monomer dispersed in the hydrophobic organic solvent.

[0103] When continuous reversed-phase suspension polymerization is carried out, examples of the dispersing device used in the dispersing step include, but are not particularly limited to, a spray nozzle, a high-speed rotary shear type agitator (rotary mixer type, turbo mixer type, disk type, double cylinder type, etc.), a cylindrical nozzle such as a needle, an orifice plate having a large number of holes directly formed in a plate, a spray nozzle, and a centrifugal atomizer such as a rotating wheel.

[0104] (hydrophobic organic solvent) Preferred hydrophobic organic solvents include at least one organic solvent selected from the group consisting of aliphatic hydrocarbons, alicyclic hydrocarbons, aromatic hydrocarbons, and halogenated hydrocarbons. Specific examples include aliphatic hydrocarbons such as n-pentane, n-hexane, n-heptane, and n-octane; alicyclic hydrocarbons such as cyclohexane, methylcyclohexane, cyclooctane, and decalin; aromatic hydrocarbons such as benzene, toluene, and xylene; and halogenated hydrocarbons such as chlorobenzene, bromobenzene, carbon tetrachloride, and 1,2-dichloroethane. Among these, n-hexane, n-heptane, and cyclohexane are preferred from the viewpoints of availability and quality stability. As the hydrophobic organic solvent, only one of these may be used alone, or a mixed solvent of two or more of them may be used.

[0105] In one embodiment of the present invention, a dispersing aid such as a surfactant or a polymer additive may be added to the hydrophobic organic solvent as needed, as long as the object of the present invention is not impaired. The type of dispersing aid is appropriately selected depending on the combination of the hydrophobic organic solvent and the monomer used, and examples of dispersing aids that can be used include the following surfactants and polymer additives.

[0106] Specific examples of the surfactant 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, alkylaryl formaldehyde condensed polyoxyethylene ethers, polyoxyethylene polyoxypropylene block copolymers, polyoxyethylene polyoxypropyl alkyl ethers, polyethylene glycol fatty acid esters, alkyl glucosides, N-alkyl gluconamides, polyoxyethylene fatty acid amides, polyoxyethylene alkylamines, phosphate esters of polyoxyethylene alkyl ethers, and phosphate esters of polyoxyethylene alkyl allyl ethers. Of these, only one type may be used alone, or two or more types may be used in combination. Furthermore, polymerizable surfactants having polymerizability may also be used as the surfactant. Specific examples of the polymerizable surfactant include compounds having the following structure:

[0107] [ka]

[0108] In the formula, R 1 and R 2 are each independently hydrogen, methyl or ethyl, and n is an integer of 3 to 20.

[0109] Among the above surfactants, fatty acid esters such as sucrose fatty acid esters, polyglycerol fatty acid esters, sorbitan fatty acid esters, polyoxyethylene sorbitan fatty acid esters, polyoxyethylene glycerol fatty acid esters, sorbitol fatty acid esters, polyoxyethylene sorbitol fatty acid esters, and polyethylene glycol fatty acid esters are preferred, and among these, sucrose fatty acid esters having an HLB value preferably in the range of 1 to 20, more preferably 1 to 10, and even more preferably 3 to 6 are preferred.

[0110] Specific examples of the polymer additive include maleic anhydride-modified polyethylene, maleic anhydride-modified polypropylene, maleic anhydride-modified ethylene-propylene copolymer, maleic anhydride-modified ethylene-propylene-diene terpolymer (EPDM), maleic anhydride-modified polybutadiene, maleic anhydride-ethylene copolymer, maleic anhydride-propylene copolymer, maleic anhydride-ethylene-propylene copolymer, maleic anhydride-butadiene copolymer, polyethylene, polypropylene, ethylene-propylene copolymer, oxidized polyethylene, oxidized polypropylene, oxidized ethylene-propylene copolymer, ethylene-acrylic acid copolymer, ethyl cellulose, hydroxyethyl cellulose, etc. Among these, from the viewpoint of dispersion stability of the aqueous monomer solution, maleic anhydride-modified polyethylene, maleic anhydride-modified polypropylene, maleic anhydride-modified ethylene-propylene copolymer, maleic anhydride-ethylene copolymer, maleic anhydride-propylene copolymer, maleic anhydride-ethylene-propylene copolymer, polyethylene, polypropylene, ethylene-propylene copolymer, oxidized polyethylene, oxidized polypropylene, and oxidized ethylene-propylene copolymer are preferred. Of these, one type may be used alone, or two or more types may be used in combination.

[0111] In one embodiment of the present invention, the above polymer additives are preferably used as dispersing aids, and maleic anhydride-modified ethylene-propylene copolymers are more preferably used. The above polymer additives may also be used in combination with the above surfactants, or the polymer additives may be used alone without surfactants.

[0112] The amount of the dispersing aid used is appropriately set depending on the polymerization form, the type of aqueous monomer solution and the hydrophobic organic solvent, etc. Specifically, the concentration of the dispersing aid in the hydrophobic organic solvent is preferably 0.0001 to 2% by mass, and more preferably 0.0005 to 1% by mass.

[0113] [Polymerization process] The polymerization step is a step in which the droplets containing the monomer obtained in the dispersion step are polymerized to obtain a hydrogel polymer (hereinafter also simply referred to as a hydrogel).

[0114] (Reaction Apparatus) The reaction apparatus used in the polymerization step may be the same as the dispersing apparatus used in the dispersing step, or may be a different apparatus. In the case of batch-type reversed-phase suspension polymerization, the apparatus used in the dispersing step can be used as the reaction apparatus as is, which is preferable in terms of workability. When the reaction apparatus is a different apparatus from the dispersing apparatus, the monomer dispersion obtained in the dispersing step is supplied to the reaction apparatus.

[0115] The shape of the reactor in which the polymerization reaction is carried out is not particularly limited, and known reactors can be used. As described above, a stirring device that can be suitably used in the dispersion step can also be suitably used in the polymerization reaction. In the case of a continuous production method, the reactor is preferably configured so that the monomer (aqueous solution) can undergo the polymerization reaction while moving as a droplet-like dispersed phase in the hydrophobic organic solvent that forms a continuous phase within the reactor. Examples of such reactors include reactors in which tubular reaction tubes are arranged vertically, horizontally, or spirally. In this embodiment, the monomer (aqueous solution) is supplied into the hydrophobic organic solvent moving within the reaction section, so that droplets of the monomer aqueous solution move together with the hydrophobic organic solvent without stagnating. This prevents contact between monomer reactants with different polymerization rates.

[0116] Furthermore, the reaction apparatus may be equipped with a temperature control means, if necessary, so that the continuous phase inside the reaction apparatus can be heated or cooled from the outside.

[0117] (Polymerization temperature) The polymerization temperature, which is the reaction temperature in the polymerization step, may be set appropriately depending on the type and amount of the polymerization initiator used, but is preferably 20°C to 100°C, and more preferably 40°C to 90°C. If the polymerization temperature is 100°C or less, a rapid polymerization reaction can be suppressed. In this specification, the polymerization temperature refers to the temperature of the hydrophobic organic solvent, which is the dispersion medium (hereinafter referred to as "Td").

[0118] In the polymerization step, since the monomer (aqueous solution) is dispersed in the hydrophobic organic solvent in the form of droplets, the temperature of the aqueous monomer solution rises rapidly due to heat transfer from the hydrophobic organic solvent. If the polymerization initiator contained in the droplets is a thermally decomposable polymerization initiator, the thermally decomposable polymerization initiator decomposes as the temperature rises, generating radicals. The generated radicals then initiate a polymerization reaction, and as the polymerization reaction progresses, a hydrogel polymer is formed.

[0119] In the case of a continuous production method, the formed hydrogel moves inside the reactor with the moving continuous phase (hydrophobic organic solvent) and is discharged from the reactor together with the hydrophobic organic solvent that forms the continuous phase.

[0120] When the aqueous monomer solution contains a thermally decomposable polymerization initiator, the Td is, from the viewpoint of the polymerization rate, preferably 70° C. or higher, more preferably 75° C. or higher, and even more preferably 80° C. or higher. There are no particular limitations on the upper limit of Td, but from the viewpoint of safety, it is appropriately selected within a range that does not exceed the boiling point of the hydrophobic organic solvent that forms the continuous phase.

[0121] (Multi-stage reversed-phase suspension polymerization) In the method for producing an absorbent resin according to one embodiment of the present invention, multi-stage polymerization may be performed in order to obtain an appropriate aggregate particle size. Specifically, after the first polymerization step is completed, a monomer aqueous solution is added again to perform a polymerization reaction, and the multi-stage polymerization can be performed.

[0122] (Inorganic fine particles) In the method for producing an absorbent resin according to one embodiment of the present invention, inorganic fine particles may be added to the hydrogel polymer during or after the polymerization in order to obtain an appropriate aggregate particle size. The inorganic fine particles can also be called a powdered inorganic flocculant.

[0123] Examples of inorganic fine particles that can be used in the present invention include silicon dioxide, aluminum oxide, titanium dioxide, calcium phosphate, calcium carbonate, magnesium phosphate, calcium sulfate, diatomaceous earth, bentonite, zeolite, and other metal oxides. Silicon dioxide, aluminum oxide, and titanium dioxide are particularly preferred. Favorable results can be obtained by adding inorganic fine particles in an amount of, for example, 0.001 to 1 part by mass, preferably 0.001 to 0.5 parts by mass, per 100 parts by mass of the hydrogel polymer. The amount of inorganic fine particles added within the above range is preferred because the effects of adding the inorganic fine particles are efficiently realized and there is little effect on water absorption performance.

[0124] [Separation process] The separation step is a step of separating the hydrogel polymer obtained in the polymerization step from the hydrophobic organic solvent. The type and structure of the apparatus used in the separation step are not particularly limited, and known apparatuses such as filtration, sedimentation, centrifugation, and squeezing can be used. Alternatively, the hydrophobic organic solvent may be separated by heating under normal or reduced pressure using the stirring device having the stirring blades used in the polymerization step and distilling. In batch-type reversed-phase suspension polymerization, distillation under normal or reduced pressure is preferably performed.

[0125] [Gel granulation process] The method for producing an absorbent resin according to one embodiment of the present invention may include a gel sizing step. In the gel sizing step, the hydrogel polymer separated from the hydrophobic organic solvent in the separation step is sized using a gel sizing device having an extrusion section and a perforated plate. This results in a sized hydrogel polymer (hereinafter, the hydrogel polymer after gel sizing will be referred to as a sized gel). The inclusion of the gel sizing step makes it easier to control the shape retention of the swollen gel of the water-absorbent resin.

[0126] In one embodiment of the present invention, the hydrogel polymer subjected to the gel sizing step is in the form of a single spherical gel particle (primary particle) or an aggregate of spherical gel particles (secondary particles formed by agglomeration of primary particles). There is no particular lower limit to the average particle size of the hydrogel polymer, but it is preferably 0.01 mm or more, more preferably 0.03 mm or more, even more preferably 0.05 mm or more, and even more preferably 0.1 mm or more. There is also no particular upper limit, but it is preferably 20 mm or less, more preferably 10 mm or less. Furthermore, the primary particle size refers to the particle diameter in the case of a single particle, and to the particle diameter of each spherical gel (primary particle) constituting the aggregate in the case of an aggregate. In the present invention, the average primary particle size of the primary particles constituting the hydrogel polymer is not particularly limited, but from the viewpoint of being able to suppress the generation of fine powder in the drying step, it is preferably 1 to 2000 μm, more preferably 1 to 1000 μm, even more preferably 5 to 800 μm, still more preferably 8 to 500 μm, still more preferably 10 to 300 μm, and particularly preferably 10 to 200 μm.

[0127] Incidentally, a device having a cutter may be installed before the gel particle size adjusting device having the extrusion section and the perforated plate to break down large aggregates contained in the hydrogel polymer.

[0128] (Hydrated gel temperature) The lower limit of the temperature of the hydrogel polymer fed to the gel sizing device is not particularly limited, but from the viewpoint of granulation efficiency and suppression of damage to the hydrogel, it is preferably 80° C. or higher, more preferably 90° C. or higher. The upper limit of the temperature of the hydrogel polymer when fed into the gel sizing device is not particularly limited, but is, for example, 100° C. or lower.

[0129] (Gel granulation device) In this specification, "gel sizing" refers to the process of producing granular gel (sized gel) having a substantially uniform shape and size from a wet powder raw material by extruding a hydrogel polymer (a wet mass of powder) into a cylindrical shape through small holes in a perforated plate. In other words, by using a perforated plate, hydrogel that has become excessively aggregated in the separation step and has taken the form of large aggregates is broken down, and hydrogel in the form of small, single particles is appropriately aggregated. Therefore, this process makes it possible to obtain hydrogel (sized gel) in a granular shape with a relatively uniform particle size. The sized gel may contain hydrogel in the form of single particles.

[0130] The "gel sizing device having an extrusion section and a perforated plate" used in the gel sizing step is not particularly limited as long as it has an extrusion section and a perforated plate (die or screen), the extrusion section usually has an extrusion member that extrudes and supplies the contents toward the perforated plate, and is an apparatus that can produce particles of a certain size by extruding the material through the perforated plate, for example, an extruder with a perforated plate. These devices may also be used in series.

[0131] Furthermore, the shape of the holes in this perforated plate (die or screen) is not particularly limited and can be arbitrarily selected from shapes suitable for use, such as perfect circles, ellipses, polygons such as hexagons, and triangles. However, perfect circles and ellipses are preferred from the viewpoint of sizing strength. The hole diameter is also not particularly limited, but is preferably 1.5 mm or less, more preferably 1.0 mm or less, and even more preferably 0.8 mm or less. By having the hole diameter of the perforated plate be 1.5 mm or less, the size of the resulting sieved gel is prevented from increasing more than necessary, and the amount of fine powder generated during drying using an agitator dryer in the downstream process can be reduced. Furthermore, the hole diameter of the perforated plate is preferably 0.3 to 1.5 mm, more preferably 0.3 to 0.8 mm. A hole diameter of 0.3 mm or more in the perforated plate allows for efficient extrusion during the extrusion operation.

[0132] The hole diameter of the perforated plate is defined as follows: First, if the holes are not perfectly round, the geometric mean value of the minor and major axes of the holes is used as the hole diameter. Furthermore, if the hole diameters of the perforated plate are different, the hole diameters of all the holes are calculated, and the arithmetic mean value is used as the hole diameter of the perforated plate. Furthermore, if the hole diameter of the perforated plate changes from the extrusion action side to the opposite side (the hole diameter changes in the thickness direction of the perforated plate), the value that gives the smallest hole diameter among them is used.

[0133] In the gel sizing step, additives may be added to the hydrogel polymer to be subjected to the gel sizing step. Examples of additives that can be added in this step include polymerization initiators, oxidizing agents, reducing agents, chelating agents, thickeners, surfactants, crosslinking agents, acids, bases, foaming agents, organic or inorganic fine particles, and polyvalent metal salts. Among these, additives that can control the degree of aggregation, such as thickeners such as starch, cellulose, starch-cellulose derivatives, and polyvinyl alcohol, surfactants, fine powders of water-absorbent resins, crosslinking agents, and polyvalent metal salts, are preferred.

[0134] [Drying process] The drying step is a step of drying the hydrogel. In the present invention, the drying method in the drying step is not particularly limited, and a conventionally known method (for example, azeotropic dehydration in a hydrophobic dispersion solvent) may be used. However, from the viewpoint of obtaining the water-absorbent resin of the present invention by reversed-phase suspension polymerization, a more suitable method is stirring drying or standing drying. Among these, stirring drying is preferred in order to maintain the particle size of the gel controlled in the gel sizing step.

[0135] The dried polymer made of particles obtained in this drying step can be directly used as a water-absorbent resin for various applications. In addition, in the method for producing an absorbent resin according to one embodiment of the present invention, the dried polymer obtained in the drying step can also be used in a surface cross-linking step described later. In this case, the dried polymer used in the surface cross-linking step described later is also referred to as a "water-absorbent resin powder" for convenience.

[0136] (additives) As long as the effects of the present invention are not impaired, additives may be added to the hydrogel to be subjected to the drying step. The additives may be added during stirring (rotation) in a rotating container and / or heating by a heating means, or may be added before the drying step (before stirring (rotation) in a rotating container and heating by a heating means). Furthermore, the additives may be added at any step before the drying step. The additives can reduce excessive adhesion between hydrogel particles during drying, and a water-absorbent resin with an excellent water absorption rate can be obtained.

[0137] Examples of additives that can be added to the hydrogel include drying aids.

[0138] Specifically, from the viewpoint of industrial efficiency, it is preferable to add a drying aid, particularly when handling particulate hydrogels having a diameter of 1 mm or less. In particular, by adding a drying aid before the drying step of the present invention, a water-absorbent resin having an excellent water absorption rate can be obtained. That is, a preferred embodiment of the present invention has a step of adding a drying aid to a hydrogel polymer.

[0139] (drying aid) The drying aid is added for the purpose of maintaining fluidity during agitation and drying in the drying step. Specific examples of the drying aid include surfactants and polymer lubricants. As the drying aid, a polymer lubricant and a surfactant may be used in combination. In the method for producing an absorbent resin according to one embodiment of the present invention, it is preferable to use a drying aid from the viewpoint of reducing adhesion between hydrogel particles to dry uniformly, preventing bias in the particle strength of the hydrogel, and obtaining excellent shape retention.

[0140] The amount of the drying aid to be added is appropriately determined depending on the water content of the drying aid and the type and amount of each component (e.g., gel fluidizing agent, etc.) contained in the drying aid. The total amount of the drying aid to be added in the drying step is preferably 0.001% by mass to 0.5% by mass, more preferably 0.01% by mass to 0.3% by mass, and even more preferably 0.02% by mass to 0.2% by mass, based on the solid content of the hydrogel.

[0141] Furthermore, when the obtained hydrogel is dried using a rotary dryer, the hydrogel is less likely to fuse and the particle size can be easily adjusted by crushing, etc., so that the amount of drying aid used can be reduced. In a preferred embodiment of the present invention, in the drying step, the hydrogel polymer contains less than 0.08 mass % of the drying aid relative to the solid content of the hydrogel polymer.

[0142] The addition of the drying aid to the hydrogel polymer is preferably carried out in a step prior to the drying step, such as (i) adding it to the hydrogel separated from the hydrophobic organic solvent in the separation step, (ii) adding it to the sized gel prior to the drying step, (iii) adding it to the aqueous monomer solution in the aqueous monomer solution preparation step, or (iv) adding it to the hydrophobic organic solvent in the dispersion step. Among these, addition in a step immediately prior to the drying step is preferred, and more specifically, addition after a step prior to the drying step (e.g., a gel sizing step) and before the drying step is more preferred. Furthermore, it is also preferred to add the drying aid in a step prior to the drying step (e.g., a gel sizing step), and then add the drying aid after a step prior to the drying step (e.g., a gel sizing step) and before the drying step (a total of two additions of the drying aid). Examples of ways of adding the drying aid before the drying step include adding the hydrogel polymer and the drying aid to a dryer, or adding the drying aid to the hydrogel polymer before the dryer. The drying aid may overlap with the surfactant or polymer additive used as a dispersion aid in the dispersion step.

[0143] Specific examples of surfactants used in drying aids include: (1) sucrose fatty acid esters, polyglycerin fatty acid esters, sorbitan fatty acid esters, polyoxyethylene sorbitan fatty acid esters, polyoxyethylene glycerin fatty acid esters, sorbitol fatty acid esters, polyoxyethylene sorbitol fatty acid esters, polyoxyethylene alkyl ethers, polyoxyethylene alkylphenyl ethers, polyoxyethylene castor oil, polyoxyethylene hydrogenated castor oil, alkylaryl formaldehyde condensed polyoxyethylene ethers, polyoxyethylene polyoxypropylene block copolymers, polyoxyethylene polyoxypropyl alkyl ethers, polyethylene glycol fatty acid esters, alkyl glucosides, N-alkyl gluconamides, polyoxyethylene fatty acid amides, polyoxyethylene alkylamines, phosphate esters of polyoxyethylene alkyl ethers, and phosphate esters of polyoxyethylene alkyl allyl ethers. (2) alkyldialkylaminoacetic acid betaines such as capryldimethylaminoacetic acid betaine, lauryldimethylaminoacetic acid betaine, myristyldimethylaminoacetic acid betaine, and stearyldimethylaminoacetic acid betaine; alkylamidopropyl betaines such as lauric acid amidopropyl betaine, coconut oil fatty acid amidopropyl betaine, and palm kernel oil fatty acid amidopropyl betaine; alkylhydroxysulfobetaines such as laurylhydroxysulfobetaine; and amphoteric surfactants such as alkylcarboxymethylhydroxyethylimidazolinium betaine and 2-alkyl-N-carboxymethyl-N-hydroxyethylimidazolinium betaine; (3) anionic surfactants such as monoalkali metal alkylaminodiacetates such as monosodium laurylaminodiacetate, potassium laurylaminodiacetate, and sodium myristylaminodiacetate; and (4) cationic surfactants such as long-chain alkyldimethylaminoethyl quaternary salts. Of these, only one type may be used alone, or two or more types may be used in combination.

[0144] Specific examples of polymeric lubricants used in drying aids include maleic anhydride-modified polyethylene, maleic anhydride-modified polypropylene, maleic anhydride-modified ethylene-propylene copolymer, maleic anhydride-modified ethylene-propylene-diene terpolymer (EPDM), maleic anhydride-modified polybutadiene, maleic anhydride-ethylene copolymer, maleic anhydride-propylene copolymer, maleic anhydride-ethylene-propylene copolymer, maleic anhydride-butadiene copolymer, polyethylene, polypropylene, ethylene-propylene copolymer, oxidized polyethylene, oxidized polypropylene, oxidized ethylene-propylene copolymer, ethylene-acrylic acid copolymer, ethyl cellulose, hydroxyethyl cellulose, and polyalkylene oxides such as polyethylene glycol. The molecular weight (weight-average molecular weight) of these lubricants is preferably selected from the range of 2 to 2,000,000, more preferably 4 to 1,000,000. These lubricants may be used alone or in combination of two or more.

[0145] (Particle size distribution of dried polymer) The particle size distribution of the dried polymer obtained in the drying step is such that the proportion of particles with a diameter of 850 μm or more (the proportion of particles that did not pass through a sieve with an opening of 850 μm) is preferably 50% by mass or less, more preferably 45% by mass or less, and even more preferably 40% by mass or less. According to the method of this embodiment, by combining drying using an agitator dryer, the formation of coarse particles is significantly suppressed, making it possible to achieve the above-mentioned preferred range. Furthermore, the proportion of particles with a diameter of 1400 μm or more (the proportion of particles that did not pass through a sieve with an opening of 1400 μm) of the dried polymer is preferably 40% by mass or less, more preferably 35% by mass or less, and even more preferably 30% by mass or less.

[0146] [Surface crosslinking process] The water absorbent resin powder obtained through the above drying step (and any subsequent step) is preferably surface-crosslinked with a surface crosslinking agent. This surface crosslinking is a treatment to provide a portion with high crosslink density in the surface layer of the water absorbent resin powder (a portion several tens of μm from the surface of the water absorbent resin powder). By carrying out the surface crosslinking treatment, various water absorption properties of the water absorbent resin can be improved. In particular, by appropriately adjusting the crosslink density of the water absorbent resin (powder), it is possible to obtain a water absorbent resin having an excellent absorption capacity under load. Note that in the present invention, conventionally known surface crosslinking techniques such as surface crosslinking in a state where the water absorbent resin is dispersed in a hydrophobic organic solvent or surface crosslinking of a powder in a dry state can be appropriately applied, but from the viewpoint of improving the absorption capacity under load of the water absorbent resin obtained, it is preferable to subject a water absorbent resin that has been subjected to a gel separation step and a drying step to the surface crosslinking step. Note that the surface crosslinking agent used in the surface crosslinking step in the method for producing an absorbent resin according to one embodiment of the present invention is also referred to as a "post-crosslinking agent" in known techniques in order to distinguish it from the internal crosslinking agent used in the step of preparing an aqueous monomer solution.

[0147] In the method for producing an absorbent resin according to one embodiment of the present invention, the surface cross-linking step may be carried out after the drying step or during the drying step. In known surface cross-linking steps, a surface cross-linking agent is generally mixed with a hydrogel cross-linked polymer or a cross-linked polymer of the dried product thereof, and the mixture is heated to carry out a cross-linking reaction. However, in the present invention, these steps may be separately provided after the drying step, or a surface cross-linking agent may be added in the drying step to carry out the surface cross-linking reaction and drying simultaneously. Furthermore, when a water-absorbent resin is produced by a batch-type reversed-phase suspension polymerization method, the solvent and the hydrogel polymer can be separated by distillation in the separation step after the polymerization reaction, but surface-cross-linked water-absorbent resin particles can be obtained by adding a surface cross-linking agent even during the separation step.

[0148] [Other processes] The method for producing a water-absorbent resin according to one embodiment of the present invention may include, in addition to the above-mentioned steps, a cooling step, a pulverizing step, a water-containing (rewetting) step, a step of adding other additives, a classification step, a sizing step, and a fine powder recycling step, as needed. In addition, the method may further include a transporting step, a storing step, a packaging step, a preservation step, etc.

[0149] (cooling process) In the method for producing an absorbent resin according to one embodiment of the present invention, the cooling step, which is optionally performed, involves cooling the particulate dry polymer obtained in the drying step using a known cooling means, thereby obtaining a particulate dry polymer cooled to the desired temperature.

[0150] (Crushing process) The method for producing an absorbent resin according to one embodiment of the present invention preferably includes a pulverization step of pulverizing the particulate dried polymer obtained in the drying step (and any subsequent cooling step). By passing through the pulverization step, it is possible to obtain a water-absorbent resin powder having a controlled particle size or particle size distribution.

[0151] In the above-mentioned pulverization step, a suitable pulverization means may be selected from high-speed rotary pulverizers such as roll mills, hammer mills, screw mills, and pin mills, vibration mills, knuckle-type pulverizers, cylindrical mixers, and the like.

[0152] (Rewetting process) In the method for producing an absorbent resin according to one embodiment of the present invention, the rewetting step, which is optionally carried out, is a step of adding at least one kind of additive selected from the group consisting of a polyvalent metal salt, a cationic polymer, a chelating agent, an inorganic reducing agent, and an α-hydroxycarboxylic acid compound to the water-absorbent resin particles obtained in the surface-crosslinking step.

[0153] In the rewetting step, the additive is preferably added to the water absorbent resin in the form of an aqueous solution or a dispersion (slurry). The additive may be added and mixed simultaneously with the above-mentioned surface cross-linking agent solution.

[0154] Specifically, the rewetting step described in International Patent Publication No. 2015 / 053372, "(2-7) Rewetting step," is also applicable to the present invention.

[0155] (Other additive addition process) In the method for producing an absorbent resin according to one embodiment of the present invention, additives other than those mentioned above (other additives) can be added to impart various functions to the water-absorbent resin. Specific examples of such additives include surfactants, compounds containing phosphorus atoms, oxidizing agents, organic reducing agents, water-insoluble inorganic fine particles, organic powders such as metal soaps, deodorants, antibacterial agents, pulp, and thermoplastic fibers. The water-insoluble inorganic fine particles disclosed in "[5] Water-insoluble inorganic fine particles" in International Patent Publication No. 2011 / 040530 are applicable to the present invention. Addition of these water-insoluble inorganic fine particles is preferred because it improves the friction between water-absorbent resin particles, suppresses gel migration within the absorbent core, and reduces deformation of the absorbent core. Silica (silicon dioxide) is particularly preferred. The water-insoluble inorganic fine particles may also serve as inorganic fine particles (powdered inorganic flocculant) added during the polymerization process or may be added separately.

[0156] (Water absorbent resin powder sizing process) The "water-absorbent resin powder sizing step" means a step of loosening the water-absorbent resin powder that has been loosely aggregated through the surface-crosslinking step, and adjusting the particle size. This sizing step includes a fine powder removal step and a classification step that follow the surface-crosslinking step. A manufacturing method of an absorbent resin according to one embodiment of the present invention preferably includes a water-absorbent resin powder sizing step, from the viewpoint of adjusting the particle size of the water-absorbent resin and obtaining stable water absorption properties. A specific aspect of the water-absorbent resin powder sizing step is not particularly limited, but examples thereof include a method of passing the water-absorbent resin powder through a sieve (for example, a JIS standard sieve) and the like.

[0157] (Fine powder reuse process) The "fine powder recycling step" means a step of supplying the fine powder generated by sieve classification or the like in each of the above steps to any of the steps as is or after granulating the fine powder. From the viewpoint of reducing production loss of the water-absorbent resin, the manufacturing method of the absorbent resin according to one embodiment of the present invention preferably includes a fine powder recycling step.

[0158] (hydrophilic fiber material) The absorbent body contains a hydrophilic fiber material. Examples of such hydrophilic fiber materials include pulp fiber, cotton linter cross-linked cellulose fiber, rayon, cotton, wool, acetate, and vinylon. The hydrophilic fiber material may be one of the above materials or a combination of two or more materials. However, in this specification, cloth-like materials (woven or nonwoven) are not considered to be hydrophilic fibers.

[0159] Therefore, the amount of liquid returning the second time can be reduced, and an absorbent article with a good feeling of use can be provided. Therefore, the basis weight of the hydrophilic fiber material (layer of the hydrophilic fiber material) is set to 100 g / m 2 Preferably, it is 90 g / m or less. 2 Preferably, it is 80 g / m or less. 2 The lower limit of the weight is preferably 1 g / m or less. 2 It is preferable that the content is 10 g / m or more. 2 The basis weight of the hydrophilic fibrous material (layer of hydrophilic fibrous material) is measured by the method described in the examples.

[0160] The content of the water-absorbent resin in 100% by mass of the present absorbent body (core concentration) is preferably 30% by mass or more and less than 100% by mass, more preferably 40% by mass or more and less than 100% by mass, even more preferably 50% by mass or more and less than 100% by mass, still more preferably 60% by mass or more and less than 100% by mass, particularly preferably 70% by mass or more and less than 100% by mass, and most preferably 75% by mass to 95% by mass.

[0161] Furthermore, in the present absorbent body, the content of the water-absorbent resin in the total of 100% by mass of the water-absorbent resin and the hydrophilic fiber material is preferably 50% by mass or more and less than 100% by mass, more preferably 60% by mass or more and less than 100% by mass, more preferably 70% by mass or more and less than 100% by mass, more preferably 80% by mass or more and less than 100% by mass, even more preferably 90% by mass or more and less than 100% by mass, and particularly preferably 90% by mass to 95% by mass.

[0162] By setting the content of the water-absorbent resin within the above range, (i) the absorbent article including the absorbent can be maintained in a clean white state, and (ii) since the absorbent has excellent diffusibility of body fluids such as urine and blood, efficient liquid distribution can be achieved, which is expected to improve the absorption capacity.

[0163] <Additives> In addition to the water-absorbent resin and hydrophilic fibrous material, the absorbent may optionally contain additives. Examples of such additives include additives that are commonly contained in absorbent bodies in absorbent articles, such as inorganic powders (e.g., amorphous silica), deodorants, antibacterial agents, and fragrances. Furthermore, when the water-absorbent resin contained in the absorbent body contains inorganic particles, the absorbent body may contain the inorganic powder separately from the inorganic particles contained in the water-absorbent resin. The inorganic powder that may be contained in the absorbent body and the inorganic particles contained in the water-absorbent resin contained in the absorbent body may be the same or different. Examples of such inorganic powders include silicon dioxide, zeolite, kaolin, and clay.

[0164] (Method of manufacturing absorbent body) The method for producing the absorbent body is not particularly limited, but for example, the absorbent body can be produced by layering a water-absorbent resin on a layer of hydrophilic fibrous material. The absorbent body produced in this way can also be said to have a layer of water-absorbent resin and a layer of hydrophilic fibrous material.

[0165] When the absorbent body has a layer of water-absorbent resin and a layer of hydrophilic fibrous material, the layer of water-absorbent resin is preferably positioned on the liquid-permeable top sheet side, in order to improve the liquid absorption performance of the absorbent body and provide an absorbent article with superior liquid absorption capacity from the second time onwards, and in this case, the layer of hydrophilic fibrous material is preferably positioned on the liquid-impermeable back sheet side.

[0166] The absorbent body may also be covered with a core wrap. By covering the absorbent body with a core wrap, the shape of the absorbent body can be maintained. Tissue, nonwoven fabric, etc. can be suitably used as the core wrap. The shape of the absorbent body may also be maintained by covering (sandwiching) the surface of the absorbent resin layer side and the surface of the hydrophilic fiber material layer side with two core wraps, respectively, or by covering both sides with a single folded core wrap, or by storing the absorbent body in a bag-shaped core wrap.

[0167] <Liquid-impermeable backsheet> The liquid-impermeable backsheet is opposite to the liquid-permeable topsheet in the absorbent article. The liquid-impermeable backsheet is positioned on the outermost side of the absorbent body when the absorbent article is worn. The liquid-impermeable backsheet prevents liquid absorbed into the absorbent body via the liquid-permeable topsheet from leaking out of the absorbent article from the liquid-impermeable backsheet side.

[0168] Examples of the liquid-impermeable backsheet provided in the absorbent article include sheets made of synthetic resins such as polyethylene, polypropylene, polyvinyl chloride, etc., sheets made of nonwoven fabrics such as spunbond / meltblown / spunbond (SMS) nonwoven fabrics in which a water-resistant meltblown nonwoven fabric is sandwiched between high-strength spunbond nonwoven fabrics, and sheets made of composite materials of these synthetic resins and nonwoven fabrics (e.g., spunbond nonwoven fabrics, spunlace nonwoven fabrics).The liquid-impermeable backsheet is preferably breathable, as this reduces stuffiness when worn and can alleviate discomfort to the wearer.More specifically, it is preferable to use a sheet made of a synthetic resin mainly composed of low-density polyethylene (LDPE) resin.

[0169] In order to maintain the comfort of wearing the absorbent article and ensure flexibility, the basis weight of the liquid-impermeable back sheet is 10 g / m 2 ~50g / m 2 It is preferable that:

[0170] (Amount of liquid returned to the absorbent article the second time (Rewet)) The amount of liquid wetback of the absorbent article after the second washout is preferably less than 0.6 g, and more preferably 0.5 g or less. An absorbent article with a second wetback amount of less than 0.6 g is preferred because it reduces discomfort when worn. The amount of liquid wetback of the absorbent article after the second washout can be measured by the method described in the Examples.

[0171] 3. Method for manufacturing absorbent articles The method for manufacturing the absorbent article is not particularly limited, but for example, the absorbent body can be manufactured by laminating a liquid-permeable top sheet, an absorbent body, and a liquid-impermeable back sheet in this order. The absorbent body can also be said to be a laminate formed by laminating a liquid-permeable top sheet, an absorbent body, and a liquid-impermeable back sheet in this order. Furthermore, if necessary, the obtained laminate may be pressurized, and the outer edges may be bonded with an adhesive or the like.

[0172] [4. Uses of absorbent articles] The uses of this absorbent article are not particularly limited, but because it reduces the amount of liquid return after the second absorption and reduces discomfort when worn, it can be suitably used as hygiene products such as disposable diapers (for infants and adults), sanitary napkins, and incontinence pads. In particular, it can be suitably used as thin, high-concentration disposable diapers, which have a problem in that when a load (body weight) is applied locally, the gel in that area shifts and becomes uneven, resulting in a decrease in the efficiency of liquid absorption over multiple times. The absorbent article can also be used as other absorbent articles. Other absorbent articles include, for example, soil water retention agents, seedling raising sheets, anti-condensation sheets, drip absorbents, freshness-preserving materials, cooling bandanas, ice packs, medical waste liquid solidifying agents, leftover soil solidifying materials, waste liquid gelling agents to prevent water damage, water-absorbing sandbags, portable toilets for disasters, compresses, water-stopping materials for electrical and electronic material communication cables, gasket packing, pet sheets, wound protection dressing materials, additives for resins such as anti-condensation building materials, etc. [Example]

[0173] The present invention will be described in more detail with reference to the following examples and comparative examples. However, the present invention is not limited to these examples. Examples obtained by appropriately combining the technical means disclosed in each example are also within the scope of the present invention. Although the terms "parts" and "%" may be used in the examples, they represent "parts by mass" or "% by mass" unless otherwise specified. Unless otherwise specified, each operation was performed at room temperature (25°C) and a relative humidity of 40 to 55%. Electrical equipment used in the examples and comparative examples (including those used to measure the physical properties of the water-absorbent resin) used a 200V or 100V 60Hz power supply unless otherwise noted.

[0174] (number average particle size) A scanning electron microscope (SEM) photograph of the water-absorbent resin or water-absorbent resin powder was taken. 50 primary particles on the front surface of the aggregate-like particles were randomly selected from the photograph, and the major axis and minor axis of each particle were measured and averaged to obtain the primary particle diameter. The average value of the primary particle diameters of each particle was calculated, and the average value was used as the number-average particle diameter of the water-absorbent resin.

[0175] (moisture content) The moisture content of the water-absorbent resin was measured in accordance with the EDANA method (ERT430.2-02). Note that, during the measurement, the sample mass was changed to 1.0 g, the drying temperature to 180°C, and the drying time to 3 hours. Specifically, 1.0 g of the hydrogel or the water-absorbent resin was placed in an aluminum cup with a bottom diameter of 50 mm, and the total mass W1 (g) of the sample (hydrogel or water-absorbent resin) and the aluminum cup was accurately weighed. Next, the sample placed in the aluminum cup was placed in an oven set at an ambient temperature of 180°C. After 3 hours, the sample and the aluminum cup were removed from the oven, and the total mass W2 (g) of the dried sample and the aluminum cup was accurately weighed. When the mass of the sample used in this measurement was M (1.0 g), the moisture content α (mass%) of the sample was calculated according to the following formula (1):

[0176] Moisture content α (mass%) = {(W1-W2) / M}×100 Formula (1) (Mass average particle diameter (D50)) The mass-average particle diameter (D50) of the water-absorbent resin was measured according to the method described in “(3) Mass-Average Particle Diameter (D50) and Logarithmic Standard Deviation (σζ) of Particle Diameter Distribution” in columns 27 and 28 of U.S. Pat. No. 7,638,570.

[0177] (CRC) The CRC (centrifuge retention capacity) of the water-absorbent resin was measured in accordance with the EDANA method (ERT441.2-02). Specifically, 0.2 g of the water-absorbent resin was placed in a nonwoven bag, and then immersed in a large excess of 0.9% by mass sodium chloride aqueous solution for 30 minutes to allow free swelling. Thereafter, the bag was centrifuged (250 G) for 3 minutes, and the water absorption capacity (g / g) was determined after draining.

[0178] (AAP) The absorbency under load (AAP) of the water-absorbent resin was measured in accordance with the EDANA method (ERT442.2-02), except that the load condition was changed to 4.83 kPa (0.7 psi).

[0179] (Ext) The Ext (water-soluble content) of the water-absorbent resin was measured in accordance with the EDANA method (ERT470.2-02).

[0180] (bulk density) The bulk density of the water-absorbent resin was measured in accordance with the EDANA method (ERT460.2-02).

[0181] (surface tension) The surface tension of the water-absorbent resin was measured by the method described in WO2015 / 129917.

[0182] (Water absorption rate (Vortex method)) The water absorption rate of the water-absorbent resin was measured in accordance with Japanese Industrial Standard JIS K 7224 (1996).

[0183] (gel shape retention) The gel shape retention of the water-absorbent resin was measured by the following method.

[0184] (1) Preparation of swollen gel Into a 100 ml capacity beaker (for example, a beaker conforming to JIS R-3503 sold by Sogo Rikagaku Glass Works) with a body diameter of 55 mm and a height of 70 mm, which was placed on a flat measurement table and contained 38 g of a 0.9 mass % sodium chloride aqueous solution at room temperature (25°C), 2.00 g of a water-absorbent resin was added under stirring at 400 rpm.

[0185] A cylindrical magnetic stirrer made of Teflon (registered trademark) (length: 25 mm, diameter: 8 mm, no ring) was used for stirring. After it was confirmed that the added water-absorbent resin had swelled and the vortex on the liquid surface had converged, stirring was stopped. The mixture was then left to stand for 10 minutes, yielding a gel that had swollen 20 times its original size.

[0186] (2) Measurement of shape retention of swollen gel (Measurement of gel shape retention A) To measure the gel shape retention A, an Autograph AG-X (manufactured by Shimadzu Corporation, product name: Autograph AG-X) was used. When a compression tool was brought into contact with the surface of the gel swollen 20 times and placed in the beaker described above under specific conditions, a load cell observed the response of the load applied to the compression tool, and the value of the load displayed on the measurement screen as the test force was taken as the gel shape retention A (N). A more specific measurement method was as follows.

[0187] A load cell (SLBL-50N, manufactured by Shimadzu Corporation) and a compression jig 100 (shown in FIG. 1 ) including a disk 1 and a rod 2 connected at one end to the center of the disk 1 were attached to the Autograph AG-X. (The disk 1 has flat surfaces on both sides and is disk-shaped, measuring 2.5 cm in diameter and 1 cm thick. The rod 2 is 5.5 cm long and circular, measuring 0.52 cm in cross section.) The flat surface of the disk 1 was attached downward. The vertical position of the attached compression jig 100 was then adjusted. Specifically, the user clicked "Create new test conditions" on the operation software (Shimadzu Trapezium X software for the Autograph, manufactured by Shimadzu Corporation) and set the test conditions as follows:

[0188] (Test conditions) "system" Test mode: Single Test type: Compression Load Cell Polarity: Compression Movement direction: Down Units: SI Format: Rounded "sensor" Test Force Channel: Test force amplifier Name: Test force Full scale: 50N Limit: 50N (check) Stress name: Stress Use true stress: Uncheck stroke Name: Stroke Limit: 500mm (check) Stroke (strain) Name: Stroke (strain) Use true strain: Uncheck Perform deflection compensation: Uncheck Displacement gauge Displacement meter 1 Channel: None Width meter Channel: None others Other 1 Channel: None "Test Control" Stretch origin: From the beginning Area 1 Control action: Load Control: Stroke V1: 10cm / min Setting between Area 1 and Area 2 Target Value Channel stroke 10mm Area 2 Control action: OFF Termination Condition Break Settings: Uncheck all Post-test action: Stop Break detection start point: 0.035% sampling Time: 10msec Preload Uncheck all "Test piece" Material: Plastic Shape: Flat plate Number of batches: 1 Number of sub-batches: 1 Dimensions in mm "Data Processing Items" No special setting is required "graph" Can be set arbitrarily Reports Can be set arbitrarily (Observation of test force) The test force was observed in an environment with a temperature of 25±2°C and a humidity of 50±10%.

[0189] The compression jig 100 was placed on the measurement table so as to be positioned in the center of the beaker containing the 20-fold swollen gel.

[0190] Next, the compression jig 100 was lowered until the load cell connected to the compression jig 100 detected a test force of 0.01 N, so that the underside of the disk portion 1 of the compression jig 100 came into contact with the surface of the swollen gel. Next, the compression jig 100 was raised 0.05 mm, and the compression jig 100 was placed at the measurement start position (a position 0.05 mm elevated vertically from the surface of the swollen gel).

[0191] Subsequently, the compression jig 100 was pressed vertically into the swollen gel by 10.0 mm at a speed of 10 cm / min.

[0192] The test force (load applied to the compression jig 100) observed when the compression jig 100 was pressed vertically to a position 8.0 mm from the measurement start position was taken as the gel shape retention force A (N).

[0193] (Measurement of gel shape retention B) The same operation as above (measurement of gel shape retention A) was performed, except that the pressing speed of the compression jig 100 was changed to 1 mm / min, and the test force (load applied to the compression jig 100) observed when the compression jig 100 was pressed into the 20-fold swollen gel at a position 8.0 mm vertically from the measurement start position was defined as gel shape retention B (N). The changes in the test conditions on the operation software were as follows:

[0194] (Changes) "Test Control" Stretch origin: From the beginning Area 1 Control action: Load Control: Stroke V1: 1mm / min sampling Time: 1sec (Gel migration index) The gel migration index was calculated using the following formula (1). Gel migration index = (gel shape retention A / gel shape retention B) - 1 (1).

[0195] (Basis weight of hydrophilic fiber material layer) Using the area and weight of the hydrophilic fibrous material layer in the absorbent core used in the Examples and Comparative Examples described below, the basis weight of the hydrophilic fibrous material layer in the absorbent core was calculated based on the following formula: The weight of the hydrophilic fiber layer (g / m 2 ) = Weight of the hydrophilic fiber material layer (g) / Area of ​​the hydrophilic fiber material layer (m 2 ).

[0196] (Thickness of liquid-permeable topsheet, core wrap, and liquid-impermeable backsheet) The thicknesses of the liquid-permeable top sheet, core wrap, and liquid-impermeable back sheet were measured using a large-type dial thickness gauge (thickness measuring device) (manufactured by Ozaki Seisakusho Co., Ltd., model number: JB, measuring probe: anvil upper and lower φ50 mm) by bringing the upper measuring probe of the thickness measuring device close to a height of 2 to 3 mm from the liquid-permeable top sheet and core wrap, and then slowly releasing the handle.

[0197] (Measurement of the amount of liquid returning to the absorbent sheet for reference evaluation) (1) Preparation of reference evaluation absorbent sheet 2 and 3, the amount of liquid return (i.e., the amount of liquid return of the water-absorbent resin) of the reference evaluation absorbent sheet 200 (hereinafter sometimes referred to as the absorbent sheet 200) was measured. The specific procedure for producing the absorbent sheet 200 is as follows.

[0198] First, 0.50 g of water-absorbent resin 23 was sprayed on the adhesive surface of adhesive tape 20 (vinyl tape No. 21S manufactured by NITTO DENKO) cut to a size of 10 cm x 10 cm in an area (water-absorbent resin sprayed area 21) 2.75 cm inward from the edge of each of the four sides of the adhesive tape 20, and then a 10 cm x 10 cm nonwoven fabric 22 (air-laid nonwoven fabric, basis weight 46.6 g / m) was sprayed on top of the adhesive surface. 2) was placed on the nonwoven fabric 20 and attached to the adhesive surfaces of the four sides of the adhesive tape 20, to produce an absorbent sheet 200 for evaluation having the laminated structure shown in FIG. 3 (a structure in which the adhesive tape 20, the water-absorbent resin 23, and the nonwoven fabric 22 were laminated in this order).

[0199] (2) Measurement of the amount of liquid returning The prepared absorbent sheet 200 was placed on a flat table with the nonwoven fabric surface 22 facing up, and 10 mL of a 0.9% by mass aqueous sodium chloride solution at 20°C was added over 1 minute using a syringe (syringe needle gauge: 21G) with the tip of the syringe needle in contact with the center of the absorbent sheet 200. 10 minutes after the start of addition of the 0.9% by mass aqueous sodium chloride solution, a 25 mm diameter, 500 g weight weight was placed on the center of the absorbent sheet 200, and the weight was removed after another 1 minute. Next, using the syringe, with the tip of the syringe in contact with the center of the absorbent sheet 200, an additional 5 mL of a 0.9% by mass aqueous sodium chloride solution at 20°C was added over 10 seconds. Immediately after the addition of the 0.9% by mass aqueous solution of sodium chloride was completed, 36 sheets of kitchen paper (kitchen towels manufactured by Oji Nepia Co., Ltd.) cut to a size of 3 cm x 3 cm and having a total weight of K1 (g) were stacked and placed on the absorbent sheet 200, and a weight of 25 mm in diameter and 100 g in weight was placed on top of the kitchen paper. 30 seconds after the kitchen paper and the weight were placed on top of the kitchen paper, the kitchen paper and the weight were removed, and the total weight K2 (g) of the 36 sheets of kitchen paper was measured. The amount of liquid return was calculated as the amount of liquid absorbed by the kitchen paper based on the following formula: Amount of liquid returned (g) = K2 - K1.

[0200] (Amount of liquid returned to the absorbent article the second time (Rewet)) The second rewet of the absorbent article was measured according to the following procedure: The absorbent article was placed on a flat table, and 25 g of a 0.9% by mass aqueous sodium chloride solution was added to the center of the absorbent article from a height of 5 mm via a liquid delivery tube (inner diameter 1 mm) over a period of 40 seconds from the start of liquid addition. Immediately after the addition was completed, a 380 g cylindrical weight with a diameter of 30 mm was placed on the center of the absorbent article.

[0201] Ten minutes after the start of liquid addition, the weight was removed, and 20 g of a 0.9% by mass sodium chloride aqueous solution was added to the center of the absorbent article from a height of 5 mm using a syringe (Terumo Syringe® 30 mL, horizontal opening) over 20 seconds. Thirty seconds after the start of liquid addition, 20 sheets of kitchen paper (Super Absorbent Kitchen Towel, manufactured by Oji Nepia Co., Ltd.) cut to 4 cm x 4 cm and having a total weight of K3 (g) were placed in the center of the absorbent article, and at the same time, a 380 g cylindrical weight with a diameter of 30 mm was placed on top of the kitchen paper. Thirty seconds after the kitchen paper and weight were placed, the kitchen paper and weight were removed, and the total weight K4 (g) of the 20 sheets of kitchen paper was measured. The second return of liquid (Rewet) of the absorbent article was calculated as the amount of liquid absorbed by the kitchen paper according to the following formula: Amount of liquid returned to the absorbent article the second time (Rewet) (g) = K4 - K3 [Manufacturing Example 1] According to the manufacturing process shown in FIG. 1 of WO 2020 / 067310, a hydrogel polymer (1) was obtained from the separation device 16. Here, as the dispersing device 12, the double-cylinder high-speed rotary shear agitator (dispersing device 12G) shown in FIG. 8 of WO 2020 / 067310 and used in Example 1 was used. Specifically, the process was as follows.

[0202] As a preparation step for the polymerization reaction, 21,000 g of n-heptane, a hydrophobic organic solvent, was charged into the dispersion device, the reaction device 14, the separation device 16, and the piping (including joints) connecting these devices.

[0203] Next, the liquid pump 18 was operated to start circulating the hydrophobic organic solvent at a flow rate of 300 mL / min. Furthermore, the heat exchanger 20 was operated to heat the circulating hydrophobic organic solvent so that the set temperature (the temperature of the hydrophobic organic solvent present in the region of the reactor where the aqueous monomer solution (1) described below is introduced) reached 90°C. Furthermore, a maleic anhydride-modified ethylene-propylene copolymer (trade name: Hiwax (registered trademark) 2203A / Mitsui Chemicals, Inc.) was added as a dispersing aid in an amount of 0.11% by mass relative to 100% by mass of the hydrophobic organic solvent.

[0204] Next, sodium acrylate, acrylic acid, and ion-exchanged water were mixed, and then polyethylene glycol diacrylate (average degree of polymerization: 9) and pentasodium diethylenetriaminepentaacetate, which are internal cross-linking agents, were added to prepare a solution containing the monomer (acrylic acid (salt)) (monomer aqueous solution preparation process).

[0205] Next, the monomer-containing solution obtained by the above operation and sodium persulfate were supplied to a mixer 10 and mixed to prepare an aqueous monomer solution (1). The monomer concentration (monomer concentration) of the aqueous monomer solution (1) was 43 mass %, and the neutralization rate of the monomer was 73 mol %. The amount of polyethylene glycol diacrylate was 0.008 mol % relative to 100 mol % of the monomer (acrylic acid (salt)), the amount of diethylenetriaminepentaacetic acid pentasodium was 50 ppm relative to the monomer (acrylic acid (salt)), and the amount of sodium persulfate was 0.12 g / mol relative to the monomer (acrylic acid (salt)).

[0206] The rotor of the dispersing device 12G (double cylindrical high-speed rotary shear agitator) was rotated at a rotation speed of 3600 rpm (shear rate 2765 [1 / s]). Next, the aqueous monomer solution (1) was fed into the piping of the dispersing device at a flow rate of 40 mL / min (47.2 g / min). The supplied aqueous monomer solution (1) was dispersed in droplets in the hydrophobic organic solvent by the dispersing device to obtain a dispersion (dispersion step).

[0207] The dispersion thus obtained was then supplied to a reactor 14. The droplets of the aqueous monomer solution (1) in the dispersion polymerized while falling through the reactor filled with the hydrophobic organic solvent, which was the continuous phase, to obtain a fine spherical hydrogel polymer (1) near the outlet of the reactor (polymerization step). The hydrogel polymer (1) obtained by the above series of operations was continuously supplied together with the hydrophobic organic solvent from the reactor via a junction to a separator 16, where the hydrogel polymer (1) and the organic solvent were separated (separation step). The hydrogel polymer (1) obtained in the separator 16 was an aggregate of fine spherical particles. The average particle size of the aggregate was 5 to 10 mm, and the average primary particle size was 80 μm.

[0208] A hydrogel polymer (1) (gel temperature: 90°C) previously mixed with a 3.5% by mass aqueous solution of lauryldimethylaminoacetic acid betaine (0.20% by mass relative to the solids content of the obtained hydrogel polymer (1)) as a drying aid was placed in a gel sizing device equipped with a screw and a perforated plate with a hole diameter of 0.8 mm, and the polymer was discharged from the gel sizing device to obtain a sizing gel (1) (gel sizing step). Then, 700 ppm of lauryldimethylaminoacetic acid betaine was added to the sizing gel (1).

[0209] Next, the granulated gel (1) was dried using a rotary dryer equipped with heating tubes (drying step). The rotary dryer was equipped with a cylindrical rotating vessel (volume 100 L) having ten heating tubes extending in the direction of the rotation axis and two partitions (doughnut-shaped partition plates with one circular opening in the center, opening ratio 50%). The ten heating tubes were arranged at intervals on a circumference centered on the rotation axis of the rotating vessel. Furthermore, the end of the rotating vessel on the outlet side had a doughnut-shaped partition plate (also known as a discharge weir) with one circular opening (opening ratio 24%) in the center.

[0210] First, steam at 2.7 MPa (temperature 228.1°C) was introduced into each heating tube, and the inside of the rotating vessel (as determined by a contact thermometer) was preheated to over 200°C. The outer wall of the rotating vessel was then thoroughly heated (to 200°C) by steam tracing. The rotating vessel was then rotated at 30 rpm to a Froude number of 0.15, and the granulated gel (1) heated to 90°C was added. Drying was carried out for 50 minutes to obtain a dried polymer (1). During drying, the supply and exhaust rates of nitrogen (carrier gas; 140°C) were adjusted so that the pressure difference between the inside and outside air of the rotating vessel was -20 Pa and the exhaust dew point was 85 to 90°C.

[0211] Subsequently, the obtained dried polymer (1) was fed to a roll mill (pulverizer) and pulverized to adjust the particle size (pulverization step), and further classified using a sieve with an opening of 150 μm to obtain a water absorbent resin powder (1). The average particle diameter of the water absorbent resin powder (1) was 340 μm.

[0212]

[0122] A surface crosslinking agent solution composed of 0.03 part by mass of ethylene glycol diglycidyl ether, 0.385 part by mass of ethylene carbonate, 0.644 part by mass of propylene glycol and 2.6 parts by mass of ion-exchanged water was sprayed with a sprayer onto 100 parts by mass of the obtained water absorbent resin powder (1), and the mixture was mixed uniformly using a high-speed continuous mixer, thereby obtaining a mixture (surface crosslinking step).

[0213] The obtained mixture was introduced into a heat treatment machine whose atmospheric temperature was adjusted to 195°C ± 2°C, and subjected to heat treatment for 25 minutes, and then the powder temperature (temperature of the mixture) was forcibly cooled to 60°C (cooling step), thereby obtaining a surface-crosslinked water absorbent resin powder (1).

[0214] The obtained surface-crosslinked water absorbent resin powder (1) was hydrated to a moisture content of 11.8% by mass, and sized by passing through a JIS standard sieve having an opening of 1000 μm (water absorbent resin powder sizing step), to obtain water absorbent resin particles (1).

[0215] To 100 parts by mass of the obtained water absorbent resin particles (1), 0.3 parts by mass of finely divided silicon dioxide (trade name: Aerosil (registered trademark) 200, manufactured by Nippon Aerosil Co., Ltd.) as water-insoluble inorganic fine particles was added and mixed (other additives addition step), thereby obtaining a water absorbent resin (1) that is a particulate water absorbent resin. The physical properties of the obtained water absorbent resin (1) are shown in Table 1. The particle shape of the obtained water absorbent resin (1) was an aggregate of spherical particles.

[0216] [Manufacturing Example 2] The same manufacturing process as in Manufacturing Example 1 was carried out. Specifically, the process was as follows.

[0217] As a preparation step for the polymerization reaction, 21,000 g of n-heptane, a hydrophobic organic solvent, was charged into the dispersing device, the reactor, the separator, and the piping (including joints) connecting these devices.

[0218] Next, the liquid pump was operated to start circulating the hydrophobic organic solvent at a flow rate of 240 mL / min. Furthermore, the heat exchanger was operated to heat the circulating hydrophobic organic solvent so that the set temperature (the temperature of the hydrophobic organic solvent present in the region of the reactor where the aqueous monomer solution (2) below is introduced) reached 85°C. Furthermore, a sucrose fatty acid ester (trade name: S-370, manufactured by Mitsubishi Chemical Corporation) was added as a dispersing aid in an amount of 0.11% by mass relative to 100% by mass of the hydrophobic organic solvent.

[0219] Subsequently, sodium acrylate, acrylic acid, and ion-exchanged water were mixed, and polyethylene glycol diacrylate (average degree of polymerization: 9) serving as an internal cross-linking agent was further added to prepare a solution containing a monomer.

[0220] Next, the monomer-containing solution obtained by the above operation and potassium persulfate were fed into a mixer and mixed to prepare an aqueous monomer solution (2). The monomer concentration of the aqueous monomer solution (2) was 43 mass %, and the neutralization rate of the monomer was 75 mol %. The amount of polyethylene glycol diacrylate was 0.02 mol % relative to 100 mol % of the monomer (acrylic acid (salt)), and the amount of potassium persulfate was 0.10 g / mol relative to the monomer (acrylic acid (salt)).

[0221] The rotor of the dispersing device (double cylindrical high-speed rotary shear agitator) was rotated at a rotation speed of 3600 rpm (shear rate 2765 [1 / s]). Next, the aqueous monomer solution (2) was fed into the piping of the dispersing device at a flow rate of 40 mL / min (47.2 g / min). The supplied aqueous monomer solution (2) was dispersed in droplets in the hydrophobic organic solvent by the dispersing device to obtain a dispersion.

[0222] The dispersion thus obtained was then supplied to a reactor. The droplets of the aqueous monomer solution (2) in the dispersion polymerized while falling through the reactor filled with the hydrophobic organic solvent, which was the continuous phase, to obtain a fine spherical hydrogel polymer (2) near the outlet of the reactor. The hydrogel polymer (2) obtained by the above series of operations was continuously supplied together with the hydrophobic organic solvent from the reactor via a junction to a separator, where the hydrogel polymer (2) and the organic solvent were separated. The hydrogel polymer (2) obtained in the separator was an aggregate of fine spherical particles, and the spherical particles (primary particles) constituting the aggregates had an average particle size of 5 to 10 mm and an average primary particle size of 65 μm.

[0223] A 3.5% by mass aqueous solution of lauryldimethylaminoacetic acid betaine (0.20% by mass relative to the solids content of the resulting hydrogel polymer (2)) was added as a drying aid to a gel sizing device equipped with a screw and a perforated plate with a hole diameter of 0.8 mm, and the mixed hydrogel polymer (2) (gel temperature: 90°C) was charged and discharged from the gel sizing device to obtain a sizing gel (2). Thereafter, 700 ppm of lauryldimethylaminoacetic acid betaine was added to the sizing gel (2).

[0224] Subsequently, a drying step was carried out in the same manner as in Production Example 1, to obtain a dried polymer (2). The obtained dried polymer (2) was fed to a roll mill (pulverizer) and pulverized to adjust the particle size, and further classified using a sieve with an opening of 150 μm, to obtain a water absorbent resin powder (2). The average particle size of the water absorbent resin powder (2) was 340 μm.

[0225]

[0223] A surface crosslinking agent solution composed of 0.015 part by mass of ethylene glycol diglycidyl ether, 1.0 part by mass of propylene glycol and 3.0 parts by mass of ion exchanged water was sprayed with a sprayer onto 100 parts by mass of the obtained water absorbent resin powder (2), and the mixture was mixed uniformly using a high-speed continuous mixer, thereby obtaining a mixture.

[0226] The obtained mixture was introduced into a heat treatment machine whose atmospheric temperature was adjusted to 195°C ± 2°C, and subjected to heat treatment for 40 minutes, and then the powder temperature (temperature of the mixture) was forcibly cooled to 60°C, thereby obtaining a surface-crosslinked water absorbent resin powder (2).

[0227] The obtained surface-crosslinked water absorbent resin powder (2) was hydrated to a moisture content of 11.0% by mass, and sized by passing through a JIS standard sieve having an opening of 1000 μm (water absorbent resin powder sizing step), to obtain water absorbent resin particles (2).

[0228] To 100 parts by mass of the obtained water absorbent resin particles (2), 0.1 part by mass of finely divided silicon dioxide (trade name: Aerosil (registered trademark) 200, manufactured by Nippon Aerosil Co., Ltd.) as water-insoluble inorganic fine particles was added and mixed, thereby obtaining a water absorbent resin (2). The physical properties of the obtained water absorbent resin (2) are shown in Table 1. The particle shape of the obtained water absorbent resin (2) was an aggregate of spherical particles.

[0229] [Comparative Manufacturing Example 1] In Production Example 1, except that the amount of polyethylene glycol diacrylate as a crosslinking agent was changed to 0.007 mol% relative to 100 mol% of the monomer (acrylic acid (salt)), a polymerization step, a subsequent separation step, a gel sizing step, and a drying step were carried out in the same manner as in Production Example 1 to obtain a comparative dried polymer (1), which was then fed to a roll mill (pulverizer) and pulverized to adjust the particle size, and further classified using a sieve with an opening of 150 μm to obtain a comparative water absorbent resin powder (1). The average particle size of the comparative water absorbent resin powder (1) was 390 μm.

[0230]

[0223] A surface crosslinking agent solution composed of 0.03 part by mass of ethylene glycol diglycidyl ether, 0.385 part by mass of ethylene carbonate, 0.644 part by mass of propylene glycol and 2.6 parts by mass of ion exchanged water was sprayed with a sprayer onto 100 parts by mass of the obtained comparative water absorbent resin powder (1), and the mixture was mixed uniformly using a high-speed continuous mixer, thereby obtaining a mixture.

[0231] The obtained mixture was introduced into a heat treatment machine whose atmospheric temperature was adjusted to 195°C ± 2°C, and subjected to heat treatment for 10 minutes, and then the powder temperature (temperature of the mixture) was forcibly cooled to 60°C, thereby obtaining a surface-crosslinked comparative water absorbent resin powder (1).

[0232] The obtained comparative water absorbent resin powder (1) was hydrated to a moisture content of 12.0% by mass, and sized by passing through a JIS standard sieve with a mesh size of 1000 μm to obtain a comparative water absorbent resin (1). The physical properties of the obtained comparative water absorbent resin (1) are shown in Table 1. The particle shape of the obtained comparative water absorbent resin (2) was an aggregate of spherical particles.

[0233] [Comparative Manufacturing Example 2] In Production Example 1, a polymerization step, a subsequent separation step, a gel sizing step and a drying step were carried out in the same manner as in Production Example 1, except that 0.11 mass % of sucrose fatty acid ester (trade name: S-370, manufactured by Mitsubishi Chemical Corporation) was used in place of the maleic anhydride-modified ethylene-propylene copolymer serving as a dispersing aid, and a comparative water absorbent resin powder (2) was obtained, and the polymer was then supplied to a roll mill (pulverizer) and pulverized to adjust the particle size, and further classified using a sieve having an opening of 150 μm, whereby a comparative water absorbent resin powder (2) was obtained.

[0234]

[0223] A surface crosslinking agent solution composed of 0.03 part by mass of ethylene glycol diglycidyl ether, 0.385 part by mass of ethylene carbonate, 0.644 part by mass of propylene glycol and 2.6 parts by mass of ion exchanged water was sprayed with a sprayer onto 100 parts by mass of the obtained comparative water absorbent resin powder (2), and the mixture was mixed uniformly using a high-speed continuous mixer, thereby obtaining a mixture.

[0235] The obtained mixture was introduced into a heat treatment machine whose atmospheric temperature was adjusted to 195°C ± 2°C, and subjected to heat treatment for 20 minutes, and then the powder temperature (temperature of the mixture) was forcibly cooled to 60°C, thereby obtaining a surface-crosslinked comparative water absorbent resin powder (2).

[0236] The obtained surface-crosslinked comparative water absorbent resin powder (2) was allowed to absorb water so that the water content was 13.6% by mass, and the particles were sized by passing through a JIS standard sieve with an opening of 1000 μm, thereby obtaining comparative water absorbent resin particles (2).

[0237] To 100 parts by mass of the obtained comparative water absorbent resin particles (2), 0.1 part by mass of finely divided silicon dioxide (trade name: Aerosil (registered trademark) 200, manufactured by Nippon Aerosil Co., Ltd.) as water-insoluble inorganic fine particles was added and mixed, thereby obtaining a comparative water absorbent resin (2). The physical properties of the obtained comparative water absorbent resin (2) are shown in Table 1. The particle shape of the obtained comparative water absorbent resin (2) was an aggregate of spherical particles.

[0238] [Table 1]

[0239] As is clear from Table 1, the water-absorbent resins (1) and (2) obtained in Production Examples 1 and 2 have a gel migration index of 0.20 or less and a water absorption rate of 50 seconds or less, and are water-absorbent resins according to one embodiment of the present invention. On the other hand, the comparative water-absorbent resins (1) and (2) obtained in Comparative Production Examples 1 and 2 have a gel migration index of 0.20 or more, and do not correspond to the water-absorbent resin according to one embodiment of the present invention.

[0240] [Example 1] (Production of absorbent articles) Polypropylene nonwoven fabric 1 (spunbond, equivalent to core wrap, thickness: 0.18 mm, basis weight: 13 g / m) cut into a length of 12 cm and a width of 12 cm. 2 In a square area 9cm long and 9cm wide, 1.5cm inward from the edges of each side of the paper, crushed pulp (equivalent to a hydrophilic fiber material) was applied in a weight of 15g / m 2 Then, 0.12 g of the water-absorbent resin (2) was uniformly spread on the ground pulp to form a layer of hydrophilic fibrous material. Next, 1.65 g of the water-absorbent resin (2) was uniformly spread on the ground pulp. Next, on top of the water-absorbent resin (2), a polypropylene nonwoven fabric (spunbond, equivalent to a liquid-permeable top sheet, thickness: 0.18 mm, basis weight: 13 g / m) cut into a length of 12 cm and a width of 12 cm is placed. 2 ) were stacked on top of each other, and the layer of water-absorbent resin and the layer of hydrophilic fibrous material were sandwiched between spunbond nonwoven fabrics (liquid-permeable top sheet and core wrap). The inside of each of the four edges of the spunbond woven fabric, 1.0 cm apart, was heated and pressed with a heat sealer to obtain an absorbent body sandwiched between nonwoven fabrics (liquid-permeable top sheet and core wrap) on both sides.

[0241] Furthermore, as a liquid-impermeable backsheet, OK bag No. 7 (thickness 0.03 mm) manufactured by Okura Kogyo Co., Ltd. was cut into a length of 16 cm and a width of 14 cm, and the absorbent body sandwiched between the nonwoven fabrics was placed in the center of the cut sheet so that the nonwoven fabric on the core wrap surface was in contact with the liquid-impermeable backsheet (i.e., so that the layer of hydrophilic fibrous material was placed on the backsheet side), thereby obtaining absorbent article (1). The physical properties of the obtained absorbent article are shown in Table 2.

[0242] [Example 2] In Example 1, the amount of hydrophilic fiber material used was changed to 0.26 g, and the basis weight of the hydrophilic fiber material layer was changed to 32 g / m 2 An absorbent article (2) was obtained in the same manner as in Example 1, except for the above change. The physical properties of the obtained absorbent article are shown in Table 2.

[0243] [Example 3] In Example 1, the amount of hydrophilic fiber material used was changed to 0.45 g, and the basis weight of the hydrophilic fiber material layer was changed to 55 g / m 2 Except for the above change, an absorbent article (3) was obtained in the same manner as in Example 1. The physical properties of the obtained absorbent article are shown in Table 2.

[0244] [Example 4] In Example 1, the amount of hydrophilic fiber material used was changed to 0.62 g, and the basis weight of the hydrophilic fiber material layer was changed to 77 g / m 2 Except for the above change, an absorbent article (4) was obtained in the same manner as in Example 1. The physical properties of the obtained absorbent article are shown in Table 2.

[0245] [Example 5] A comparative absorbent article (5) was obtained in the same manner as in Example 1, except that the water-absorbent resin (2) was changed to the water-absorbent resin (1). The physical properties of the obtained absorbent article are shown in Table 2.

[0246] [Comparative Example 1] In Example 1, the amount of hydrophilic fiber material used was changed to 1.00 g, and the basis weight of the hydrophilic fiber material layer was changed to 123 g / m 2 A comparative absorbent article (1) was obtained in the same manner as in Example 1, except for the above change. The physical properties of the obtained absorbent article are shown in Table 2.

[0247] Comparative Example 2 A comparative absorbent article (2) was obtained in the same manner as in Example 1, except that no pulverized pulp was used. The physical properties of the obtained absorbent article are shown in Table 2.

[0248] Comparative Example 3 In Example 1, air-laid nonwoven fabric (basis weight 46.6 g / m) was used instead of crushed pulp. 2 A comparative absorbent article (3) was obtained in the same manner as in Example 1, except that the composition was changed to ). The physical properties of the obtained absorbent article are shown in Table 2.

[0249] Comparative Example 4 A comparative absorbent article (4) was obtained in the same manner as in Example 1, except that the order of the hydrophilic fiber material layer and the water-absorbent resin layer was reversed. Specifically, a polypropylene nonwoven fabric 1 (spunbond, equivalent to core wrap, thickness: 0.18 mm, basis weight: 13 g / m) was cut into a length of 12 cm and a width of 12 cm. 2 1.65 g of water-absorbent resin (2) was evenly spread on a square area of ​​9 cm length and 9 cm width, 1.5 cm inward from the ends of each of the four sides of the paper. Next, ground pulp (corresponding to a hydrophilic fibrous material) was spread on top of the water-absorbent resin (2) in an amount of 15 g / m². 2 A comparative absorbent article (4) was obtained in the same manner as in Example 1, except that the powder was uniformly dispersed so that the powder was in the range of 0.1 to 1.0 μm. The physical properties of the obtained absorbent article are shown in Table 2.

[0250] Comparative Example 5 A comparative absorbent article (5) was obtained in the same manner as in Example 1, except that the water-absorbent resin (2) was changed to the comparative water-absorbent resin (1). The physical properties of the obtained absorbent article are shown in Table 2.

[0251] Comparative Example 6 A comparative absorbent article (6) was obtained in the same manner as in Example 1, except that the water-absorbent resin (2) in Example 1 was changed to the comparative water-absorbent resin (2). The physical properties of the obtained absorbent article are shown in Table 2.

[0252] [Table 2]

[0253] [summary] As is clear from Table 2, by comparing Examples 1 to 4 with Comparative Examples 1 and 2, it is clear that the fibers containing no hydrophilic fiber material or having a layer of hydrophilic fiber material with a basis weight of 100 g / m 2 It can be seen that when the gel transfer index is greater than 0.20, the amount of liquid weeping back the second time is large, resulting in an absorbent article that feels uncomfortable when used. Furthermore, Comparative Example 3 shows that hydrophilic fibrous materials cannot be substituted with nonwoven fabrics. Comparative Example 4 shows that when the position of the hydrophilic fibrous material is changed (placed on the top sheet side), the amount of liquid weeping back the second time is large, resulting in an absorbent article that feels uncomfortable when used. Furthermore, a comparison of Examples 1 and 5 with Comparative Examples 5 and 6 shows that by using a water-absorbent resin with a gel transfer index of 0.20 or less and a water absorption rate of 50 seconds or less, it is possible to provide an absorbent article that has a small amount of liquid weeping back the second time, does not cause the user to feel liquid weeping back, and is comfortable to use. [Industrial Applicability]

[0254] The absorbent article according to one embodiment of the present invention can be suitably used for a variety of absorbent articles, such as disposable diapers, sanitary napkins, adult incontinence products (incontinence pads), hygiene materials (hygienic products) such as pet sheets, etc. [Explanation of symbols]

[0255] 1: Disc part 2: Rod-shaped part 20: Adhesive tape 21: Water absorbent resin spray area 22: Nonwoven fabric 23: Water-absorbing resin 100: Compression jig 200: Reference evaluation absorption sheet

Claims

1. An absorbent article comprising, in this order, a liquid-permeable top sheet, an absorbent body containing a water-absorbent resin and a hydrophilic fiber material, and a liquid-impermeable back sheet, the absorbent body has a layer of a water-absorbent resin and a layer of a hydrophilic fiber material, the water-absorbent resin layer is disposed on the liquid-permeable top sheet side, the hydrophilic fiber material layer is disposed on the liquid-impermeable backsheet side; The weight of the layer of the hydrophilic fiber material is 100 g / m ２ is as follows: The water-absorbing resin is a particulate water-absorbing resin containing a crosslinked polymer containing a structural unit derived from a poly(meth)acrylic acid (salt)-based monomer, Contains water-insoluble inorganic fine particles, The surface tension is 62 mN / m or more, and The gel shape retention force A determined by the following procedure is 9.6 N or more, the gel migration index determined by the following procedure is 0.20 or less, and the water absorption rate (Vortex method) is 50 seconds or less: (1) 38 g of a 0.9% by mass sodium chloride aqueous solution at 25°C and 2.00 g of a water-absorbent resin are placed in a 100 mL beaker having a body diameter of 55 mm and a height of 70 mm to form a swollen gel. (2) A compression tool having a disk portion and a rod-shaped portion with one end connected to the center of the disk portion (the disk portion has flat surfaces on both sides and is disk-shaped with a diameter of 2.5 cm and a thickness of 1 cm, and the length of the rod-shaped portion is 5.5 cm) is placed so that it is located in the center of the beaker containing the swollen gel of (1).After the underside of the disk portion of the compression tool is brought into contact with the surface of the swollen gel, the compression tool is placed at the measurement start position so that the underside of the disk portion is 0.05 mm vertically elevated from the surface of the swollen gel. (3) The compression jig placed at the measurement start position is pressed into the swollen gel by 10.0 mm in the vertical direction at a speed of 10 cm / min. (4) In (3), when the compression jig is pressed vertically to a position 8.0 mm from the measurement start position, the load (N) applied to the compression jig is observed and taken as the gel shape retention force A. (5) The same operations as in (1) to (4) are carried out except that the speed at which the compression jig is pressed is changed to 1 mm / min, and the load (N) applied to the compression jig is observed and taken as the gel shape retention force B. (6) Calculate the gel migration index according to the following formula (1): Gel migration index=(gel shape retention A / gel shape retention B)−1 (1).

2. The absorbent article according to claim 1 , wherein the water-absorbing resin is an aggregate of spherical particles.

3. The absorbent article according to claim 1 or 2, wherein the water-absorbent resin has a gel shape retention capacity A of 10.0 N or more.

4. The absorbent article according to any one of claims 1 to 3, wherein the water-absorbent resin has an AAP of 15 g / g or more.

5. The absorbent article according to any one of claims 1 to 4, wherein the mass of the water-absorbent resin contained in the absorbent body is 50% by mass or more and less than 100% by mass of the total mass of the water-absorbent resin and the hydrophilic fiber.

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