Poly(METH)acrylic acid (SALT)-based particle-like water absorbent and method for producing same

Abstract

The purpose of the present invention is to provide a disposable diaper that exhibits an excellent liquid uptake speed under pressure regardless of the concentration and constitution of a water absorbent in an absorber. The present disclosure pertains to a poly(meth)acrylic acid (salt)-based particle-like water absorbent that contains poly(meth)acrylic acid (salt)-based water-absorbing resin particles as the main component. The particle-like water absorbent includes cavities that are spaces connected to the outside and voids that are enclosed spaces not connected to the outside. The total volume fraction of the cavities is 18 vol% or more. The void / cavity ratio which is the ratio of the total volume fraction of the voids and the total volume fraction of the cavities is 0.015-0.025. The mass-average particle diameter (D50) of the particle-like water absorbent is 300-450 µm.

Description

Poly(meth)acrylic acid (salt)-based particulate water absorbent and method for producing same

[0001] The present invention relates to a particulate water-absorbing agent containing poly(meth)acrylic acid (salt)-based water-absorbing resin particles as a main component and a method for producing the same, and more particularly to a particulate water-absorbing agent capable of improving the performance of absorbent articles such as disposable diapers and a method for producing the same.

[0002] A water-absorbent resin (SAP / Super Absorbent Polymer) is a water-swellable, water-insoluble polymer gelling agent that exhibits excellent absorbency for aqueous liquids such as body fluids. Therefore, water-absorbing agents containing water-absorbent resins as their main component are used in a wide range of applications, including absorbent articles such as disposable diapers and sanitary napkins, agricultural and horticultural water retention agents, and industrial water-stopping materials. Many monomers and hydrophilic polymers are available as raw materials for the water-absorbent resins that constitute such water-absorbing agents. Among these, poly(meth)acrylic acid (salt)-based water-absorbent resins, primarily composed of (meth)acrylic acid and / or its salt, are most widely used industrially from the standpoints of price and performance. Such water-absorbent resins are produced through various processes, including a polymerization process, a drying process, an optional process for removing undried material, a pulverization process, a classification process, and a surface crosslinking process (Modern Superabsorbent Polymer Technology (1998)).

[0003]

[0003] Along with the improvement in performance of disposable diapers, which are the main application of the water-absorbing agent, many functions (physical properties) are required of the water-absorbing agent. Specific examples of the functions (physical properties) include water absorption rate, water absorption capacity, water absorption capacity under pressure, liquid permeability, resorption amount (return), gel strength, water extractable content, particle size distribution, urine resistance, antibacterial property, impact resistance (damage resistance), powder flowability, deodorizing property, coloring resistance (whiteness), low dust, etc.

[0004] Furthermore, when using disposable diapers, which are the main application of the water absorbing agent, there is a demand for improvements in urine leakage and skin rash. It is presumed that such urine leakage and skin rash occur when urine is not easily absorbed into the disposable diaper, or when urine is absorbed but is slowly absorbed by the water absorbing agent. For this reason, it is believed that by improving the absorbency of the water absorbing agent while also improving its water absorption rate, the amount of resorption and the water absorption time in the disposable diaper can be reduced, which will result in the elimination of urine leakage and skin rash in the disposable diaper.

[0005] Therefore, in order to improve the absorbency and water absorption rate of the water-absorbing agent, a technology for improving the particle shape of the water-absorbing resin has been proposed. International Publication No. 2013 / 002387 (corresponding US specification: US Patent Application Publication No. 2014 / 0193641) discloses that a water-absorbent resin powder obtained by foam polymerization and having open cells (communicating holes) (also called cavities) that are spaces communicating with the outside and closed cells (also called voids) that are closed spaces that do not communicate with the outside, has excellent liquid permeability and water absorption rate. Furthermore, International Publication No. 2022 / 181771 (corresponding US specification: US Patent Application Publication No. 2024 / 0299907) discloses that by adjusting the communicating holes and closed cells within a predetermined range, it is possible to provide a particulate water-absorbing agent that is excellent in the liquid uptake rate and return amount of the absorbent body. WO 2016 / 204302 (corresponding US specification: US Patent Application Publication No. 2018 / 0298132) discloses that by adding an adhesion control agent and a liquid permeability improver, it is possible to provide a particulate water-absorbing agent that is excellent in the liquid uptake rate and return amount of an absorbent.

[0006] The present inventors have found that further improvement in the speed of liquid uptake is necessary in order to achieve a more comfortable feel when used.

[0007] Therefore, an object of the present invention is to solve at least one of the following problems.

[0008] The object of the present invention is to provide a poly(meth)acrylic acid (salt)-based particulate water-absorbing agent that has an excellent liquid uptake rate under pressure.

[0009] Another object of the present invention is to provide a method for producing a poly(meth)acrylic acid (salt)-based particulate water-absorbing agent that has an excellent liquid uptake rate under pressure.

[0010] The present invention provides a sanitary material (for example, a disposable diaper) that has an excellent liquid uptake rate under pressure.

[0011] The present inventors have conducted extensive research in view of the above-mentioned problems, and as a result have found that the above-mentioned problems can be solved by a poly(meth)acrylic acid (salt)-based particulate water-absorbing agent containing poly(meth)acrylic acid (salt)-based water-absorbing resin particles as a main component, the particulate water-absorbing agent including cavities which are spaces communicating with the outside and voids which are closed spaces not communicating with the outside, wherein a total volume ratio of the cavities is 18% by volume or more, a void / cavity ratio which is a ratio of the total volume ratio of the cavities to the total volume ratio of the voids is 0.015 or more and 0.025 or less, and a mass median particle diameter (D50) of the particulate water-absorbing agent is 300 μm or more and 450 μm or less, thereby completing the present invention.

[0012] The present inventors have also found that the poly(meth)acrylic acid (salt)-based particulate water-absorbing agent can be produced by a specific production method, that is, the above-mentioned problems can be solved by a specific production method for the poly(meth)acrylic acid (salt)-based particulate water-absorbing agent.

[0013] That is, according to the present invention, there is provided a method for producing a poly(meth)acrylic acid (salt)-based particulate water-absorbing agent containing poly(meth)acrylic acid (salt)-based water-absorbing resin particles as a main component, the method comprising: (i) a step of preparing a (meth)acrylic acid (salt)-based monomer aqueous solution; (ii) a step of polymerizing the (meth)acrylic acid (salt)-based monomer aqueous solution to obtain a hydrogel-like crosslinked polymer; (iii) a step of gel-crushing the hydrogel-like crosslinked polymer during or after polymerization to obtain hydrogel particles; (iv) a step of drying the hydrogel particles to obtain a dried product; (v) a step of pulverizing and / or classifying the dried product to obtain a water-absorbent resin powder; and (vi) a step of surface-crosslinking the water-absorbent resin powder to obtain water-absorbent resin particles, wherein in at least one of the step (i) and the step (ii), a polymerization additive is added to the (meth)acrylic acid (salt)-based monomer aqueous solution, and the polymerization additive is an amphoteric and the hydrogel particles obtained in the step (iii) are one or more compounds selected from the group consisting of a polyoxyethylene (3) lauryl ether sodium sulfate (0.02 mass %) and a cationic substance, and the hydrogel particles are obtained by crushing the hydrogel particles in a gel state of the crosslinked polymer with a gel crushing energy (GGE) (2) of 30 J / g or more, and the hydrogel particles are obtained by crushing the hydrogel particles in 1000 parts by mass of an aqueous solution containing 0.02 mass % of sodium polyoxyethylene (3) lauryl ether sulfate and 20 mass % of sodium chloride. Also provided is a method for producing a poly(meth)acrylic acid (salt)-based particulate water-absorbing agent, in which, when 50 parts by mass of poly(meth)acrylic acid (salt) particles are immersed in water, the mass average particle diameter after 1 hour of immersion is defined as D50 (1 hr), and the mass average particle diameter after 16 hours of immersion is defined as D50 (16 hr), the ratio of D50 (1 hr) to D50 (16 hr), (D50 (16 hr) / D50 (1 hr)), is 0.20 or more and 0.85 or less, and D50 (1 hr) is 350 μm or more.

[0014] Further, according to the present invention, there is provided a poly(meth)acrylic acid (salt)-based particulate water-absorbing agent containing poly(meth)acrylic acid (salt)-based water-absorbing resin particles as a main component, the agent having the following properties (1) to (4): (1) an absorbency under pressure (AAP) of 0.7 psi (4.83 kPa) of 24 g / g or more, (2) a saline flow conductivity (SFC) of 15×10 －７ cm ３(2) A poly(meth)acrylic acid (salt)-based particulate water-absorbing agent is provided, which satisfies at least one of the following: (1) a water absorption rate under pressure (CRC) of 25 g / g or more; (2) a water absorption rate under pressure index (AAP) of 0.7 psi (4.83 kPa) of 460 or more; (3) a water absorption capacity under no load (CRC) of 25 g / g or more; and (4) a water absorption rate under load index (AAP) of 460 or more, which is the product of the total volume fraction of the cavity and the water absorption capacity under load (AAP) of 0.7 psi (4.83 kPa).

[0015] Fig. 1 is a schematic front view illustrating cavities and voids formed in a particulate water-absorbing agent according to one embodiment of the present invention. Fig. 2 is a schematic view of an apparatus used for evaluating an absorbent body according to the present invention.

[0016] Hereinafter, a particulate water-absorbing agent and a manufacturing method thereof according to the present invention will be described in detail, but the scope of the present invention is not limited to these descriptions, and besides the following examples, appropriate modifications can be made without departing from the spirit of the present invention. Therefore, all other possible embodiments, methods of use, and operational techniques that can be conceived by those skilled in the art without departing from the gist of the present invention are included in the scope and gist of the present invention, and are also included in the inventions described in the claims and their equivalents. The embodiments described in this specification can be combined in any way to form other embodiments.

[0017] [1] Definition of Terms [1-1] "Water-absorbent resin" The water-absorbent resin in the present invention refers to a water-swellable, water-insoluble polymer gelling agent, and indicates one having the following physical properties. That is, as "water-swellable," it is a polymer gelling agent that satisfies the CRC (centrifuge retention capacity) defined by NWSP 241.0. R2(15) of 5 g / g or more, and as "water-insoluble," it is a polymer gelling agent that satisfies the Ext (water-soluble component) defined by NWSP 270.0. R2(15) of 50 mass% or less. The term "NWSP" will be described later.

[0018] The water-absorbent resin can be designed according to its application and purpose, and is not particularly limited. Among them, a hydrophilic cross-linked polymer obtained by cross-linking polymerization of an unsaturated monomer having a carboxyl group is preferable. The shape of the water-absorbent resin is preferably particulate. Particulate water-absorbent resins having the particle size described below are particularly preferable.

[0019] The "water-absorbent resin" in the present invention may be surface-crosslinked or may not be surface-crosslinked. Preferably, it is surface-crosslinked. In this specification, the "water-absorbent resin" may also be referred to as "water-absorbent resin powder" or "water-absorbent resin particles." The "water-absorbent resin powder" refers to a "water-absorbent resin" that is a dried product obtained by drying a hydrogel-like crosslinked polymer (hereinafter referred to as "hydrogel") obtained by polymerization, and the "water-absorbent resin particles" refer to the above-mentioned water-absorbent resin powder that has been surface-crosslinked.

[0020] The "water absorbent resin" in the present invention is not limited to a final product, and may refer to an intermediate in a production process of a water absorbent resin (for example, a hydrogel after polymerization, a dried product after drying, a water absorbent resin powder before surface cross-linking, etc.), and all of these are collectively referred to as a "water absorbent resin".

[0021] [1-2] "Particulate water absorbing agent" In this specification, the particulate water absorbing agent means a water absorbing agent in a particulate form. Therefore, whether it is a single particle of the particulate water absorbing agent or a plurality of particles of the particulate water absorbing agent, it is referred to as a "particulate water absorbing agent." "Particulate" means having the form of particles, and a particle refers to a small granular solid or liquid body having a measurable size (JIS Industrial Terminology Dictionary, 4th edition, p. 2002). Note that in this specification, the particulate water absorbing agent may also be simply referred to as a water absorbing agent.

[0022] The particulate water-absorbing agent according to the present invention is primarily composed of poly(meth)acrylic acid (salt)-based water-absorbent resin particles. That is, the poly(meth)acrylic acid (salt)-based water-absorbent resin particles are preferably contained in the particulate water-absorbing agent in an amount of 60% by mass to 100% by mass, 70% by mass to 100% by mass, 80% by mass to 100% by mass, or 90% by mass to 100% by mass. The particulate water-absorbing agent may contain one or more of other water-absorbent resins, water, and the additives described below. Examples of the additives include inorganic fine particles, moisture-absorbing blocking inhibitors, cationic polymer compounds, water-soluble polyvalent metal cation-containing compounds, surfactants, dust-generation inhibitors, coloring inhibitors, urine-resistance improvers, deodorizers, fragrances, antibacterial agents, foaming agents, pigments, dyes, fertilizers, oxidizing agents, and reducing agents. These additives may be contained in an amount of more than 0% by mass to 10% by mass, preferably 0.1% by mass to 1% by mass, based on the total mass (100% by mass) of the particulate water-absorbing agent.

[0023] The particulate water-absorbing agent according to the present invention is used as a sanitary material for absorbing aqueous liquid. The aqueous liquid is not limited to water, but may be urine, blood, sweat, feces, waste liquid, moisture, steam, ice, a mixture of water and an organic solvent and / or an inorganic solvent, rainwater, groundwater, etc., and is not particularly limited as long as it contains water. Preferred examples include urine, menstrual blood, sweat, and other body fluids.

[0024] The particulate water-absorbing agent according to the present invention preferably has a moisture content of 0.2% by mass or more and 30% by mass or less. That is, a water-absorbing resin composition in which these components are integrated also falls within the category of the particulate water-absorbing agent. Furthermore, the particulate water-absorbing agent used in the present invention preferably has excellent handleability under high humidity conditions (specifically, the particulate water-absorbing agent has a moisture-absorbing fluidity (moisture-absorbing blocking rate) described in WO2017 / 170605 of preferably 30% or less, more preferably 20% or less, even more preferably 10% or less, and most preferably 5% or less).

[0025] Furthermore, the particulate water-absorbing agent according to the present invention is mainly composed of poly(meth)acrylic acid (salt)-based water-absorbent resin particles, but may also contain other water-absorbent resin particles. Examples of the other water-absorbent resin particles include polysulfonic acid (salt)-based water-absorbent resin particles, maleic anhydride (salt)-based water-absorbent resin particles, polyacrylamide-based water-absorbent resin particles, polyvinyl alcohol-based water-absorbent resin particles, polyethylene oxide-based water-absorbent resin particles, polyaspartic acid (salt)-based water-absorbent resin particles, polyglutamic acid (salt)-based water-absorbent resin particles, polyalginic acid (salt)-based water-absorbent resin particles, starch-based water-absorbent resin particles, and cellulose-based water-absorbent resin particles.

[0026] [1-3] "Poly(meth)acrylic acid (salt)" In the present invention, "poly(meth)acrylic acid (salt)" means poly(meth)acrylic acid and / or a salt thereof. Further, "poly(meth)acrylic acid (salt)-based water absorbent resin" means a crosslinked polymer containing, as a repeating unit, a structure derived from (meth)acrylic acid and / or a salt thereof (hereinafter also referred to as "(meth)acrylic acid (salt)") as a main component, and containing, as an optional component, a structure derived from an internal crosslinking agent.

[0027] The above-mentioned "main component" means that the amount (content) of (meth)acrylic acid (salt) used is preferably 50 mol % or more and 100 mol % or less, more preferably 70 mol % or more and 100 mol % or less, even more preferably 90 mol % or more and 100 mol % or less, and particularly preferably substantially 100 mol %, based on the total amount of monomers used in the polymerization (100 mol % of all monomers excluding the crosslinking agent).

[0028] Here, the above-mentioned "poly(meth)acrylic acid (salt)" may be unneutralized, but is preferably partially or completely neutralized poly(meth)acrylic acid, more preferably a monovalent salt, even more preferably an alkali metal salt or ammonium salt, particularly preferably an alkali metal salt, and most preferably a sodium salt.

[0029] In this specification, particulate poly(meth)acrylic acid (salt)-based water absorbent resins are referred to as "poly(meth)acrylic acid (salt)-based water absorbent resin particles", but may also be simply referred to as "poly(meth)acrylic acid (salt)-based water absorbent resin". Specifically, a "poly(meth)acrylic acid (salt)-based water absorbent resin", which is a dried product obtained by drying a hydrogel obtained by polymerization, is referred to as a "poly(meth)acrylic acid (salt)-based water absorbent resin powder", and a surface-crosslinked "poly(meth)acrylic acid (salt)-based water absorbent resin powder" is sometimes referred to as a "poly(meth)acrylic acid (salt)-based water absorbent resin particle".

[0030] [1-4] "Absorbent" In the present invention, the term "absorbent" refers to a structure comprising a first substrate having a liquid-absorbing surface formed on its front side that directly absorbs liquid, and a water-absorbing layer disposed on the back side of the first substrate. The water-absorbing layer is a structure in which a particulate water-absorbing agent (water-absorbent resin) is supported and / or sandwiched between one or more substrates and / or fibers (preferably hydrophilic fibers). The water-absorbing layer may contain a fibrous material such as hydrophilic fibers in addition to the particulate water-absorbing agent. When the water-absorbing layer is composed of a particulate water-absorbing agent and hydrophilic fibers, examples of the absorbent structure include a structure in which the particulate water-absorbing agent and hydrophilic fibers are uniformly mixed, and / or a structure in which the particulate water-absorbing agent is sandwiched between layers of hydrophilic fibers. Note that the structure of the absorbent structure is not limited to these examples, and may be formed by further sandwiching the water-absorbing layer containing the particulate water-absorbing agent and hydrophilic fibers between nonwoven fabrics, tissue paper, or the like. Furthermore, a water-absorbing layer formed by directly fixing a particulate water-absorbing agent to a sheet-like substrate such as a nonwoven fabric or tissue paper without containing hydrophilic fibers (i.e., a water-absorbing layer including a particulate water-absorbing agent and a sheet-like substrate) can also be the absorbent of the present invention.

[0031] [1-5] Definition of Evaluation Method "NWSP" stands for "Non-Woven Standard Procedures-Edition 2015", which was jointly issued by EDANA (European Disposables and Nonwovens Association) and INDA (Association of the Nonwoven Fabrics Industry) to standardize evaluation methods for nonwoven fabrics and their products in the United States and Europe, and indicates a standard measurement method for water-absorbent resins. Unless otherwise specified, in the present invention, the physical properties of the water-absorbent resin are measured in accordance with "Non-Woven Standard Procedures-Edition 2015". Regarding evaluation methods not described in the NWSP, measurements are made using the methods and conditions described in the Examples.

[0032] "Ext" (NWSP 270.0.R2(15)) "Ext" described in the above section [1-1] "Water-absorbent resin" is an abbreviation for Extractables, and means the water-soluble content (amount of water-soluble component) of the water-absorbent resin. "Ext" was measured in accordance with NWSP 241.0.R2(15). Specifically, it refers to the amount of dissolved polymer (unit: mass%) after adding 1.0 g of water-absorbent resin to 200 ml of a 0.9 mass% sodium chloride aqueous solution and stirring at 500 rpm for 16 hours. The amount of dissolved polymer is measured using pH titration.

[0033] [1-6] Others In this specification, "acid (salt)" means "acid and / or its salt," and "(meth)acrylic" means "acrylic and / or methacrylic."

[0034] [2] "Particulate Water Absorbing Agent" The particulate water absorbing agent according to the present invention is a particulate water absorbing agent whose main component is poly(meth)acrylic acid (salt)-based water absorbent resin particles, and the particulate water absorbing agent includes cavities, which are spaces that communicate with the outside, and voids, which are closed spaces that do not communicate with the outside, wherein the total volume fraction of the cavities is 18% by volume or more, the void / cavity ratio, which is the ratio of the total volume fraction of the cavities to the total volume fraction of the voids, is 0.015 or more and 0.025 or less, and the mass median particle diameter (D50) of the particulate water absorbing agent is 300 μm or more and 450 μm or less. Hereinafter, the poly(meth)acrylic acid (salt)-based water absorbent resin particles contained as the main component of the particulate water absorbing agent may be simply referred to as "water absorbent resin particles."

[0035] Incidentally, the content explained in the above [1-2] can be applied to the explanation of "containing poly(meth)acrylic acid (salt)-based water-absorbent resin particles as a main component", and the content explained in the below-mentioned [3] Manufacturing method of a particulate water-absorbing agent can be applied to the manufacturing method of poly(meth)acrylic acid (salt)-based water-absorbent resin particles. Details of the components constituting the particulate water-absorbing agent of the present invention, for example, the components constituting the poly(meth)acrylic acid (salt)-based water-absorbent resin particles as a main component, will be explained in the below-mentioned manufacturing method.

[0036] Here, in the present invention, a "cavity" (also referred to as a "communicating hole") refers to a cavity that is connected to the outside of the particulate water absorbing agent 1 (exposed on the surface of the particulate water absorbing agent 1) among cavities formed in the particulate water absorbing agent 1, as shown as a cavity 1a in Fig. 1. That is, a "cavity" is a space (void) that exists in the particulate water absorbing agent and communicates with the outside. The cavity also includes depressions, grooves, and the like formed on the surface of the particulate water absorbing agent 1. Specifically, the cavity refers to a hole, a through-hole, a depression, a groove, or the like, the presence of which can be confirmed on the surface of the particulate water-absorbing agent 1 when three-dimensional image data is acquired under the following conditions using a microfocus X-ray CT system (inspexio SMX-100CT / manufactured by Shimadzu Corporation) as described later, and the three-dimensional image data is analyzed under the conditions described in Examples using high-speed three-dimensional analysis software (TRI / 3D-VOL-FCS64 / manufactured by Ratoc System Engineering Co., Ltd.).

[0037] In the present invention, the term "void" (also referred to as "closed cells") refers to a cavity such as an air bubble that is not connected to the outside of the particulate water-absorbing agent 1 (existing inside the particulate water-absorbing agent 1), among cavities formed in the particulate water-absorbing agent 1, as shown as void 1b in FIG. 1. That is, the "void" is a closed space (cavity) that exists in the particulate water-absorbing agent and does not communicate with the outside. Specifically, the term "void" refers to a cavity such as an air bubble whose presence can be confirmed inside the particulate water-absorbing agent 1 when three-dimensional image data is acquired under the following conditions using a microfocus X-ray CT system (inspexio SMX-100CT / manufactured by Shimadzu Corporation) as described later, and the three-dimensional image data is analyzed under the conditions described in the examples using high-speed three-dimensional analysis software (TRI / 3D-VOL-FCS64 / manufactured by Ratoc System Engineering Co., Ltd.).

[0038] In the particulate water absorbing agent according to the present invention, particles containing cavities and voids are mainly water-absorbent resin particles, but other particles contained in the particulate water absorbing agent may also contain cavities and voids. In the particulate water absorbing agent (in all components constituting the particulate water absorbing agent), the effects of the present invention are exhibited when the cavities and voids occupy a predetermined range of volume as a whole. In addition, in the examples described later, the total volume ratio of cavities and the total volume ratio of voids are measured using the particulate water absorbing agent, but in the method for measuring the total volume ratio of cavities and the total volume ratio of voids according to the present invention, the numerical values ​​of the total volume ratio of cavities and the total volume ratio of voids are almost the same whether the measurement object is only water-absorbent resin particles or the particulate water absorbing agent.

[0039] In the following description of cavities and voids, they will be described as cavities and voids of a particulate water-absorbing agent, but they can also be understood as cavities and voids of water-absorbent resin particles.

[0040] The "total volume ratio of cavities" means the ratio of the total volume of cavities to the total volume of the particulate water-absorbing agent, and is specifically calculated by the method described later in Examples. The "total volume ratio of voids" means the ratio of the total volume of voids to the total volume of the particulate water-absorbing agent, and is specifically calculated by the method described later in Examples.

[0041] The particulate water absorbing agent according to the present invention has a total volume fraction of cavities of 18% by volume or more, a void / cavity ratio, which is a ratio of the total volume fraction of cavities to the total volume fraction of voids, of 0.015 to 0.025, and a mass median particle diameter (D50) of the particulate water absorbing agent of 300 μm to 450 μm, and is therefore excellent in liquid uptake speed. That is, the particulate water absorbing agent according to the present invention can quickly absorb liquid when used as an absorbent (absorbent article).

[0042] When assuming actual use of an absorbent article containing a particulate water-absorbing agent, various usage situations are assumed, for example, when urination occurs while weight is applied, when the absorbent article swells without pressure during urination but is pressurized with movement, when blood is absorbed, etc. In the task of "an absorbent body capable of swiftly absorbing liquid", the present inventors have found that the shape of the particulate water-absorbing agent, i.e., the particle diameter of the particulate water-absorbing agent and the ratio of cavities to voids in the particulate water-absorbing agent are important, and have focused on the particle diameter of the particulate water-absorbing agent, the total volume ratio of cavities, and the ratio of the total volume ratio of cavities to the total volume ratio of voids (void / cavity ratio).

[0043] The reason why the ratio of the cavities and voids of the particulate water absorbing agent is important when the mass average particle diameter (D50) of the particulate water absorbing agent is 300 μm or more and 450 μm or less is considered as follows: A particulate water absorbing agent having a large total volume ratio of cavities tends to have a large specific surface area. However, the total volume ratio of cavities cannot be sufficiently evaluated by measuring only the specific surface area, and is evaluated by measuring the volume of recessed portions on the particle surface.

[0044] When the total volume ratio of the cavities is 18% by volume or more, the speed at which the particulate water-absorbing agent itself absorbs liquid increases due to capillary action, and in addition, the gaps between gel particles when the particulate water-absorbing agent swells also increase, and the amount of liquid held in the cavity volume portions of the gel particles and the gaps between the gel particles increases. As a result, the absorbency of the particulate water-absorbing agent is fully exhibited in evaluation of the absorbent body, and the liquid uptake speed increases.

[0045] In a particulate water-absorbing agent having a void / cavity ratio of 0.015 or more, the volume change during swelling proceeds toward the inside of the particle so as to fill the voids, compared with a particulate water-absorbing agent having a void / cavity ratio of less than 0.015, so that the apparent volume change is small and the particulate water-absorbing agent can absorb liquid without filling the gaps between particles, thereby improving the liquid absorption speed, particularly in the initial stage of water absorption.

[0046] The presence of voids in a particulate water-absorbing agent may inhibit the movement of liquid, resulting in the voids hindering liquid absorption. In a particulate water-absorbing agent having a void / cavity ratio of 0.025 or less, when liquid is absorbed into the particulate water-absorbing agent, no voids hinder liquid absorption, improving the absorbency of the particulate water-absorbing agent and increasing the liquid uptake rate. In addition, the intrusion of absorbed liquid into the voids is reduced, making it easier for the particulate water-absorbing agent itself to absorb the liquid.

[0047] If the mass median particle diameter (D50) of the particulate water absorbing agent is less than 300 μm, the liquid uptake rate becomes too high, and before the liquid passes through the whole of the particulate water absorbing agent to be used, only a part of the particulate water absorbing agent that has come into contact with the liquid takes up the liquid, which deteriorates the liquid uptake efficiency and the uptake rate.

[0048] When the mass median particle diameter (D50) of the particulate water-absorbing agent exceeds 450 μm, even if the total volume ratio of the cavities is 18% by volume or more, the specific surface area of ​​the particles itself decreases, and the liquid uptake rate deteriorates.

[0049] As described above, it is believed that by setting the ratio of cavities and voids in the particulate water-absorbing agent within a predetermined range, the absorbent body was able to quickly absorb liquid.

[0050] As described above, when cavities and voids are present at a predetermined ratio in a particulate water absorbing agent having a predetermined particle size, the particulate water absorbing agent can rapidly absorb a liquid. Such a form of particulate water absorbing agent can be obtained by a method for producing a poly(meth)acrylic acid (salt)-based particulate water absorbing agent, the method comprising the following steps: (i) a step of preparing a (meth)acrylic acid (salt)-based monomer aqueous solution; (ii) a step of polymerizing the (meth)acrylic acid (salt)-based monomer aqueous solution to obtain a hydrogel-like crosslinked polymer; (iii) a step of gel-crushing the hydrogel-like crosslinked polymer during or after polymerization to obtain hydrogel particles; (iv) a step of drying the hydrogel particles to obtain a dried product; (v) a step of pulverizing and / or classifying the dried product to obtain a water-absorbent resin powder; and (vi) a step of surface-crosslinking the water-absorbent resin powder to obtain water-absorbent resin particles, and in at least one of the step (i) and the step (ii), a polymerization additive is added to an aqueous solution of a carboxylic acid (salt)-based monomer, the polymerization additive being one or more compounds selected from amphoteric substances, anionic substances, and cationic substances; in step (iii), a gel-crushing energy (GGE)(2) for gel-crushing the hydrogel-like crosslinked polymer is 30 J / g or more; and the hydrogel particles obtained in step (iii) contain polyoxyethylene(3) lauryl ether sodium sulfate. When 50 parts by mass of the hydrogel particles are immersed in 1,000 parts by mass of an aqueous solution containing 0.02% by mass of sodium chloride and 20% by mass of sodium chloride, and the mass average particle diameter after 1 hour of immersion is defined as D50 (1 hr), and the mass average particle diameter after 16 hours of immersion is defined as D50 (16 hr), the ratio of D50 (1 hr) to D50 (16 hr), (D50 (16 hr) / D50 (1 hr)), is 0.20 or more and 0.85 or less, and D50 (1 hr) is 350 μm or more.

[0051] That is, an aqueous solution of a (meth)acrylic acid (salt)-based monomer is polymerized in the presence of a specific polymerization additive, and then the obtained hydrogel is crushed under predetermined conditions, thereby obtaining a particulate water-absorbing agent having a predetermined particle size and in which cavities and voids exist in a predetermined ratio.

[0052] The present inventors have found that when the hydrogel obtained by polymerization under the above conditions is crushed under specific conditions, hydrogel particles that become particles having specific cavities and voids after drying can be obtained. For example, they have found that if the energy given to the hydrogel during gel crushing is too small, the total volume ratio of the cavities in the obtained particulate water-absorbing agent becomes small, and the liquid uptake rate decreases.

[0053] The reason why the particulate water-absorbing agent of the present invention has a specific particle size and can have a specific ratio of cavities and voids is believed to be that the specific polymerization additive added during polymerization affects the hardness of the hydrogel (a certain level of hardness) and the tribology of the gel particle surface when the hydrogel is crushed. In other words, it is believed that the specific polymerization additive adjusts the hardness of the hydrogel and the friction between the hydrogel particles, and can adjust the particle shape (particle size and the ratio of cavities and voids) of the particulate water-absorbing agent of the present invention.

[0054] It should be noted that this mechanism is merely a hypothesis, and the present invention is not limited to this mechanism in any way.

[0055] Therefore, according to the present invention, it is possible to provide a poly(meth)acrylic acid (salt)-based particulate water-absorbing agent having an excellent liquid uptake rate under pressure. Also, according to the present invention, it is possible to provide a method for producing a poly(meth)acrylic acid (salt)-based particulate water-absorbing agent having an excellent liquid uptake rate under pressure. Furthermore, according to the present invention, it is possible to provide a sanitary material (e.g., disposable diaper) having an excellent liquid uptake rate under pressure.

[0056] The lower limit of the total volume ratio of the cavities of the particulate water-absorbing agent according to the present invention is preferably more than 18 volume %, more preferably 18.3 volume % or more, even more preferably 18.5 volume % or more, particularly preferably 18.7 volume % or more, and most preferably 19.0 volume % or more. The upper limit of the total volume ratio of the cavities of the particulate water-absorbing agent is preferably 30 volume % or less, more preferably 28 volume % or less, even more preferably 26 volume % or less, particularly preferably 25 volume % or less, and most preferably 22 volume % or less. According to one embodiment, the total volume ratio of the cavities of the particulate water-absorbing agent according to the present invention is more than 18 volume % and 30 volume % or less, 18.3 volume % or more and 28 volume % or less, 18.5 volume % or more and 26 volume % or less, 18.7 volume % or more and 25 volume % or less, or 19.0 volume % or more and 22 volume % or less. When the total volume ratio of the cavities of the particulate water-absorbing agent is within the above range, the liquid absorption speed is further improved, and the particulate water-absorbing agent itself and the particulate water-absorbing agent in a gel state after absorbing liquid can sufficiently maintain their mechanical strength.

[0057] The total volume ratio of voids in the particulate water-absorbing agent according to the present invention is preferably 0.3% by volume or more and 0.5% by volume or less. Compared with a particulate water-absorbing agent having a total void volume ratio of less than 0.3% by volume, a particulate water-absorbing agent having a total void volume ratio of 0.3% by volume or more undergoes volume change during swelling in a direction toward the inside of the particles to fill the voids, resulting in a smaller apparent volume change. This allows liquid to be absorbed without filling the gaps between particles, thereby improving the liquid absorption rate, particularly in the initial stage of water absorption. A particulate water-absorbing agent having a total void volume ratio of 0.5% by volume or less does not experience any void interference when absorbing liquid into the particulate water-absorbing agent, improving the absorbency of the particulate water-absorbing agent and increasing the liquid absorption rate. In addition, the infiltration of absorbed liquid into the voids is reduced, making it easier for the particulate water-absorbing agent itself to absorb liquid.

[0058] The upper limit of the total volume ratio of voids in the particulate water-absorbing agent according to the present invention is preferably 0.48 vol% or less, more preferably 0.47 vol% or less, even more preferably 0.46 vol% or less, particularly preferably 0.45 vol% or less, and most preferably 0.44 vol% or less. The lower limit of the total volume ratio of voids in the particulate water-absorbing agent is preferably 0.31 vol% or more, more preferably 0.32 vol% or more, even more preferably 0.33 vol% or more, particularly preferably 0.35 vol% or more, and most preferably 0.38 vol% or more. According to one embodiment, the total volume ratio of voids in the particulate water-absorbing agent according to the present invention is 0.31 vol% or more and 0.48 vol% or less, 0.32 vol% or more and 0.47 vol% or less, 0.33 vol% or more and 0.46 vol% or more, 0.35 vol% or more and 0.45 vol% or less, or 0.38 vol% or more and 0.44 vol% or less. When the total volume ratio of the voids of the particulate water-absorbing agent is within the above range, the liquid absorption rate is further improved.

[0059] The lower limit of the void / cavity ratio of the particulate water absorbing agent according to the present invention is preferably 0.016 or more, more preferably 0.017 or more. The upper limit of the void / cavity ratio of the particulate water absorbing agent is preferably less than 0.025, more preferably 0.024 or less. According to one embodiment, the void / cavity ratio of the particulate water absorbing agent according to the present invention is 0.016 or more but less than 0.025, or 0.017 or more but 0.024 or less. When the void / cavity ratio of the particulate water absorbing agent is within the above range, the liquid absorption speed is further improved, and the mechanical strength of the particulate water absorbing agent itself and in a gel state after absorbing liquid can be sufficiently maintained. The void / cavity ratio is the ratio of the total volume fraction of cavities to the total volume fraction of voids, and is calculated by dividing the total volume fraction of voids by the total volume fraction of cavities.

[0060] According to one embodiment, the particulate water absorbing agent according to the present invention has a total volume ratio of cavities of 18% by volume or more, a total volume ratio of voids of 0.3% by volume or more and 0.5% by volume or less, and a void / cavity ratio of 0.015 to 0.025 or less. When the ratios of cavities and voids of the particulate water absorbing agent are within the above ranges, the liquid absorption speed is further improved, and the particulate water absorbing agent itself and the particulate water absorbing agent in a gel state after absorbing liquid can be sufficiently maintained in their mechanical strength.

[0061] It should be noted that in the present invention, "capable of quickly absorbing liquid" refers to an evaluation of an absorbent body (absorbent sheet) in which an absorbent layer containing a particulate water-absorbing agent, pulp, or the like is laminated with a nonwoven fabric, or the like, and is not an evaluation of the particulate water-absorbing agent itself.

[0062] The particulate water-absorbing agent according to the present invention has an irregularly pulverized particle shape. Here, "irregularly pulverized" means particles in a pulverized state that do not have a uniform shape. The particulate water-absorbing agent according to the present invention is preferably a pulverized product obtained by aqueous solution polymerization. On the other hand, in reversed-phase suspension polymerization or spray-droplet polymerization in which an aqueous monomer solution is sprayed or dropped to polymerize, no pulverization step is performed, and therefore the obtained spherical particles or granulated products of spherical particles are not irregularly pulverized.

[0063] The particulate water-absorbing agent according to the present invention has a mass-average particle diameter (D50) of 300 μm or more and 450 μm or less. If the mass-average particle diameter (D50) of the particulate water-absorbing agent is less than 300 μm, the liquid uptake rate becomes too high, and only a portion of the particulate water-absorbing agent that comes into contact with the liquid takes in the liquid before the liquid passes through the entire particulate water-absorbing agent. This reduces the liquid uptake efficiency and the uptake rate. If the mass-average particle diameter (D50) of the particulate water-absorbing agent exceeds 450 μm, the specific surface area of ​​the particles itself decreases, and therefore the liquid uptake rate deteriorates, even if the total volume ratio of the cavities is 18% by volume or more.

[0064] The lower limit of the mass average particle diameter (D50) of the particulate water-absorbing agent is preferably more than 300 μm, more preferably 310 μm or more, even more preferably 320 μm or more, particularly preferably 330 μm or more, and most preferably 340 μm or more. The lower limit of the mass average particle diameter (D50) of the particulate water-absorbing agent is preferably 440 μm or less, more preferably 420 μm or less, even more preferably 400 μm or less, particularly preferably 380 μm or less, and most preferably 370 μm or less. That is, the mass average particle diameter (D50) of the particulate water-absorbing agent is preferably more than 300 μm and 440 μm or less, more preferably 310 μm or more and 420 μm or less, even more preferably 320 μm or more and 400 μm or less, particularly preferably 330 μm or more and 380 μm or less, and most preferably 340 μm or more and 370 μm or less. According to one embodiment, the mass average particle diameter (D50) of the particulate water-absorbing agent is 300 μm or more and 400 μm or less, 300 μm or more and 390 μm or less, 320 μm or more and 400 μm or less, 320 μm or more and 390 μm or less, 330 μm or more and 400 μm or less, 330 μm or more and 390 μm or less, or 340 μm or more and 380 μm or less. When the mass average particle diameter (D50) of the particulate water-absorbing agent is within the above range, the expected effect of the present invention is further exhibited.

[0065] Furthermore, the upper limit of the mass ratio of particles having a particle diameter of less than 150 μm in the particulate water absorbing agent is preferably 10 mass% or less, more preferably 8 mass% or less, and even more preferably 6 mass% or less. Furthermore, the upper limit of the mass ratio of particles having a particle diameter of 850 μm or more in the particulate water absorbing agent is preferably 5 mass% or less, more preferably 3 mass% or less, and even more preferably 1 mass% or less. This particulate water absorbing agent contains particles having a particle diameter of 150 μm or more and less than 850 μm in an amount of preferably 90 mass% or more, more preferably 95 mass% or more, even more preferably 97 mass% or more, and particularly preferably 99 mass% or more. Ideally, the particulate water absorbing agent contains 100 mass% of particles having a particle diameter of 150 μm or more and less than 850 μm.

[0066] The particulate water-absorbing agent according to the present invention contains particles having a particle diameter of 300 μm or more and less than 425 μm at preferably 20% by mass or more, more preferably 25% by mass or more, even more preferably 28% by mass or more, and particularly preferably 30% by mass or more. The upper limit of the proportion of particles having a particle diameter of 300 μm or more and less than 425 μm in the particulate water-absorbing agent is not particularly limited, but in practical terms, in one embodiment, it is 90% by mass or less, 85% by mass or less, 80% by mass or less, 75% by mass or less, 70% by mass or less, or 65% by mass or less. By having the proportion of particles having a particle diameter of 300 μm or more and less than 425 μm within the above range, the effects of the present invention are more effectively exhibited.

[0067] The particulate water absorbing agent according to the present invention has a logarithmic standard deviation (σζ) of particle size distribution of preferably 0.20 or more and 0.50 or less, more preferably 0.25 or more and 0.45 or less, and further preferably 0.30 or more and 0.42 or less. Note that, also in the case of water-absorbent resin particles contained as a main component of the particulate water absorbing agent, the particle size is preferably in the same numerical range as that of the particulate water absorbing agent.

[0068] The lower limit of the CRC of the particulate water-absorbing agent according to the present invention is preferably 20 g / g or more, more preferably 23 g / g or more, even more preferably 25 g / g or more, particularly preferably 26 g / g or more, and most preferably 27 g / g or more. When the CRC is 20 g / g or more, the absorption capacity becomes appropriate, and the performance as an absorbent for sanitary materials such as disposable diapers is ensured. Furthermore, the upper limit of the CRC of the particulate water-absorbing agent is preferably 45 g / g or less, more preferably 40 g / g or less, even more preferably 38 g / g or less, particularly preferably 35 g / g or less, and most preferably 32 g / g or less. When the CRC is 45 g / g or less, the absorption rate of body fluids such as urine and blood is maintained, and therefore the particulate water-absorbing agent is also suitable for use in high-absorption-rate disposable diapers. The CRC can be controlled by the type and amount of the internal cross-linking agent. Furthermore, the CRC of the water-absorbing resin particles contained as a main component of the particulate water-absorbing agent is preferably in the same numerical range as that of the particulate water-absorbing agent. Furthermore, "CRC" is an abbreviation for Centrifuge Retention Capacity, and means the water absorption capacity of the particulate water-absorbing agent under no pressure (sometimes referred to as "water absorption capacity"). The CRC is measured by the method described in Examples.

[0069] The lower limit of the AAP of 0.7 psi (4.83 kPa) of the particulate water-absorbing agent according to the present invention is preferably 15 g / g or more, more preferably 20 g / g or more, even more preferably 22 g / g or more, still more preferably 23 g / g or more, particularly preferably 24 g / g or more, and most preferably 25 g / g or more. The upper limit of the AAP of 0.7 psi (4.83 kPa) of the particulate water-absorbing agent is preferably 50 g / g or less, more preferably 45 g / g or less, even more preferably 40 g / g or less, and particularly preferably 38 g / g or less. When the AAP of 0.7 psi (4.83 kPa) is within the above range, the effects of the present invention are more effectively exhibited. Furthermore, disposable diapers manufactured using a particulate water-absorbing agent having an AAP of 0.7 psi within the above range have excellent liquid absorption ability and can suppress skin rash and urinary leakage. The AAP can be controlled by adjusting the particle size, changing the surface cross-linking agent, or the like. Also, in the case of the water-absorbent resin particles contained as a main component of the particulate water-absorbing agent, the AAP of 0.7 psi (4.83 kPa) is preferably in the same numerical range as that of the particulate water-absorbing agent. Furthermore, "AAP" is an abbreviation for Absorption Against Pressure, and means the water absorption capacity of the particulate water-absorbing agent under pressure (0.7 psi (4.83 kPa)). The AAP is measured by the method described in the examples.

[0070] The lower limit of the SFC of the particulate water absorbing agent according to the present invention is preferably 15×10 －７ cm ３ sec / g or more, more preferably 18×10 －７ cm ３ sec / g or more, more preferably 20 × 10 －７ cm ３ sec / g or more, and even more preferably 25×10 －７ cm ３ sec / g or more, particularly preferably 28 × 10 －７ cm ３ sec / g or more, most preferably 30 x 10 －７ cm ３ The upper limit of the SFC of the particulate water-absorbing agent is preferably 60×10 －７ cm ３ sec / g or less, more preferably 55 × 10 －７cm ３ sec / g or less, more preferably 50 × 10 －７ cm ３ sec / g or less, particularly preferably 45 × 10 －７ cm ３ According to one embodiment, the SFC of the particulate water-absorbing agent according to the present invention is 15×10 －７ ~60 x 10 －７ cm ３ ・sec / g, 18×10 －７ ~55 x 10 －７ cm ３ ・sec / g, 20×10 －７ ~50 x 10 －７ cm ３ ・sec / g, 25×10 －７ ~50 x 10 －７ cm ３ ・sec / g, 28×10 －７ ~50 x 10 －７ cm ３ sec / g, or 30 x 10 －７ ~45 x 10 －７ cm ３ sec / g. When the SFC of the particulate water-absorbing agent is within the above range, the effects of the present invention are more pronounced. Furthermore, a disposable diaper manufactured using a particulate water-absorbing agent having an SFC within the above range has excellent liquid absorption ability, and can suppress skin rash and urinary leakage. The SFC can be controlled by adjusting the particle size or changing the amount of the internal cross-linking agent. Furthermore, the SFC of the water-absorbent resin particles contained as a main component of the particulate water-absorbing agent is preferably in the same numerical range as that of the particulate water-absorbing agent. Furthermore, "SFC" is an abbreviation for Saline Flow Conductivity, and refers to the liquid permeability of a water-absorbent resin to an aqueous sodium chloride solution under pressure (load of 2.07 kPa (0.3 psi)) of the particulate water-absorbing agent. The SFC is measured by the method described in the Examples.

[0071] The lower limit of the moisture content of the particulate water absorbing agent according to the present invention is preferably 0.1 mass% or more, more preferably 0.5 mass% or more, even more preferably 0.6 mass% or more, particularly preferably 0.7 mass% or more, and most preferably 0.8 mass% or more. The upper limit of the moisture content of the particulate water absorbing agent is preferably 10 mass% or less, more preferably 8 mass% or less, even more preferably 5 mass% or less, and particularly preferably 3 mass% or less. By setting the moisture content within the above range, a water absorbing agent having excellent powder properties (e.g., fluidity, transportability, damage resistance, etc.) can be obtained. The lower limit of the solid content of the particulate water absorbing agent according to the present invention is preferably 85.0 mass% or more, more preferably 87.0 mass% or more, even more preferably 90.0 mass% or more, particularly preferably 92.0 mass% or more, and most preferably 95.0 mass% or more. The upper limit of the solid content of the particulate water absorbing agent is preferably 99.9 mass % or less, more preferably 99.5 mass % or less, further preferably 99.3 mass % or less, particularly preferably 99.2 mass % or less, and most preferably 99.0 mass % or less. Note that, also in the case of the water-absorbent resin particles contained as a main component of the particulate water absorbing agent, the water content and solid content are preferably in the same numerical ranges as those of the particulate water absorbing agent.

[0072] The upper limit of the water absorption speed (Vortex) of the particulate water absorbing agent according to the present invention is preferably 40 seconds or less, and in the following order, it is preferred to set it to 35 seconds or less, 30 seconds or less, 28 seconds or less, 26 seconds or less, 24 seconds or less, 22 seconds or less, 20 seconds or less, and less than 20 seconds, and most preferably 19 seconds or less. The lower limit of the water absorption speed (Vortex) of the particulate water absorbing agent according to the present invention is preferably 5 seconds or more.

[0073] When the water absorption rate (Vortex) is in the above range, it is possible to absorb a predetermined amount of liquid in a short time. Furthermore, when used in an absorbent body of an absorbent article such as a disposable diaper, the time for which the user feels wet on the skin is reduced, which reduces discomfort and also reduces the amount of leakage. Note that the water absorption rate (Vortex) of the water-absorbent resin particles contained as a main component of the particulate water-absorbing agent is also preferably in the same numerical range as that of the particulate water-absorbing agent.

[0074] The particulate water-absorbing agent according to the present invention is characterized by a high water absorption rate even under high pressure. Conventionally, the water absorption rate (Vortex) has often been used for evaluation. However, in order to obtain a particulate water-absorbing agent having an excellent liquid uptake rate under pressure, which is the object of the present invention, evaluation using the water absorption rate (Vortex) has often been found to be insufficient because it is measured under no pressure. Therefore, the present inventors have found that the product of the AAP of 0.7 psi (4.83 kPa) and the total cavity volume fraction, i.e., the water absorption rate index under pressure, is appropriate as a new index for evaluating the characteristic of a high water absorption rate even under high pressure. That is, when the AAP of 0.7 psi (4.83 kPa) is in the above range or more, the absorption capacity under high pressure is high, and when the total cavity volume ratio is in the above range or more, the specific surface area is high. By combining these, it is possible to maintain a high specific surface area even under high pressure, and it can be used as an index for evaluating the performance of a particulate water-absorbing agent that can quickly absorb a sufficient amount of liquid.

[0075] The index (the product of the total cavity volume ratio and AAP 0.7 psi (4.83 kPa)) (referred to as the water absorption rate index under pressure in this specification) is preferably 460 or more, more preferably 465 or more, even more preferably 470 or more, and particularly preferably 475 or more. In the particulate water-absorbing agent according to the present invention, the upper limit of the water absorption rate index under pressure is preferably 800 or less, more preferably 700 or less, even more preferably 650 or less, and particularly preferably 600 or less. By setting the water absorption rate index under pressure to 460 or more, the liquid uptake rate of the absorbent body under pressure becomes high, which is preferable. In addition, by setting the water absorption rate index under pressure to 800 or less, the liquid is dispersed throughout the absorbent body, which reduces the amount of return, which is preferable.

[0076] [3] Manufacturing Method of Particulate Water Absorbing Agent As described above, the manufacturing method of the particulate water absorbing agent according to the present invention includes: (i) a step of preparing a (meth)acrylic acid (salt)-based monomer aqueous solution; (ii) a step of polymerizing the (meth)acrylic acid (salt)-based monomer aqueous solution to obtain a hydrogel-like crosslinked polymer; (iii) a step of gel-crushing the hydrogel-like crosslinked polymer during or after polymerization to obtain hydrogel particles; (iv) a step of drying the hydrogel particles to obtain a dried product; (v) a step of pulverizing and / or classifying the dried product to obtain a water-absorbent resin powder; and (vi) a step of surface-crosslinking the water-absorbent resin powder to obtain water-absorbent resin particles, wherein in at least one of the step (i) and the step (ii), a polymerization additive is added to the (meth)acrylic acid (salt)-based monomer aqueous solution, and the polymerization additive is selected from amphoteric substances, anionic substances, and cationic substances. and in step (iii), the hydrous gel crosslinked polymer has a gel-crushing energy (GGE)(2) of 30 J / g or more, and the hydrous gel particles obtained in step (iii) are immersed in 1000 parts by mass of an aqueous solution containing 0.02 mass% of sodium polyoxyethylene (3) lauryl ether sulfate and 20 mass% of sodium chloride, and the mass average particle diameter after 1 hour of immersion is defined as D50(1 hr) and the mass average particle diameter after 16 hours of immersion is defined as D50(16 hr), where D50(1 hr) is a ratio of D50(1 hr) to D50(16 hr), i.e., D50(16 hr) / D50(1 hr) is 0.20 or more and 0.85 or less, and D50(1 hr) is 350 μm or more.

[0077] In this way, by polymerizing an aqueous solution of a (meth)acrylic acid (salt)-based monomer in the presence of a specific polymerization additive, and then pulverizing the obtained hydrogel under specific conditions, the obtained hydrogel particles have a specific aggregation state, and as a result, a particulate water-absorbing agent having a specific particle size and having cavities and voids present in a specific ratio can be obtained, and a water-absorbing agent with excellent liquid uptake speed can be obtained, thereby solving the intended problem of the present invention.

[0078] Furthermore, according to the present invention, there is also provided a method for producing a poly(meth)acrylic acid (salt)-based particulate water-absorbing agent that satisfies at least one of the following (1) to (5): (1) a saline flow conductivity (SFC) of the poly(meth)acrylic acid (salt)-based particulate water-absorbing agent is 15×10 －７ cm ３ (2) the mass average particle diameter (D50) of the hydrogel particles is 150 μm or more and 400 μm or less; (3) the amphoteric substance is (a) an alkylbetaine or (b) an alkylamine oxide, the anionic substance is (c) a sulfate ester salt of a higher alcohol alkylene oxide adduct, or (d) a dicarboxylate, (e) a sulfonate, (f) an alkyl sulfate ester salt, (g) an alkylamine diacetate, (h) a phosphate ester salt of a higher alcohol alkylene oxide adduct, or (i) a carboxylate salt of a higher alcohol alkylene oxide adduct, and the cationic substance is (j) an ammonium salt; (4) the amount of the polymerization additive added is 0.01% by mass or more and 5% by mass or less with respect to the total amount of monomers contained in the (meth)acrylic acid (salt)-based monomer aqueous solution (100% by mass of the raw material monomers).

[0079] According to the present invention, there is also provided (5) a method for producing a poly(meth)acrylic acid (salt)-based particulate water-absorbing agent, which comprises adding an internal crosslinking agent and a polymerization additive to the (meth)acrylic acid (salt)-based monomer aqueous solution in at least one of the step (i) and the step (ii), and the amount of the internal crosslinking agent is 0.070 mol % or more and 0.100 mol % or less with respect to the total amount of monomers contained in the (meth)acrylic acid (salt)-based monomer aqueous solution (100 mol % of raw material monomers).

[0080] In the above manufacturing method, it is preferable that steps (i) to (vi) are carried out "sequentially" in this order, but each step may be carried out simultaneously with the preceding or succeeding step.

[0081] The time between each of the above steps is appropriately determined, including transportation time and storage time, and is preferably from 0 second to 2 hours, more preferably from 1 second to 1 hour.

[0082] Hereinafter, the manufacturing method of the particulate water-absorbing agent according to the present invention will be explained mainly in chronological order, but the manufacturing method only needs to have each of the essential steps described above, and may further include other steps within the scope that does not deviate from the gist of each manufacturing method.

[0083] [3-1] Step of preparing aqueous (meth)acrylic acid (salt)-based monomer solution (step (i)) In this specification, the term "aqueous (meth)acrylic acid (salt)-based monomer solution" refers to an aqueous solution of a monomer containing (meth)acrylic acid (salt) as a main component, into which components constituting a water-absorbent resin such as a crosslinking agent, a graft component, and trace components (chelating agent, surfactant, dispersant, etc.) are blended as necessary, and which is subjected to polymerization in an as-is state by adding a polymerization initiator.

[0084] In the method for producing a particulate water-absorbing agent according to the present invention, the (meth)acrylic acid (salt)-based monomer aqueous solution is polymerized in the presence of an internal crosslinking agent and a polymerization additive, which will be described later. The polymerization additive may be added to the (meth)acrylic acid (salt)-based monomer aqueous solution in the step of preparing the (meth)acrylic acid (salt)-based monomer aqueous solution (step (i)), or may be added after the step (i) and before the step of polymerizing the (meth)acrylic acid (salt)-based monomer aqueous solution, which will be described later (step (ii)).

[0085] According to a preferred embodiment, in the method for producing a particulate water-absorbing agent of the present invention, the (meth)acrylic acid (salt)-based monomer aqueous solution is polymerized in the presence of a polymerization additive and an internal crosslinking agent, which will be described later. That is, according to a preferred embodiment, in the method for producing a particulate water-absorbing agent of the present invention, the (meth)acrylic acid (salt)-based monomer aqueous solution is polymerized in the presence of a polymerization additive and an internal crosslinking agent. Like the polymerization additive, the internal crosslinking agent may be added to the (meth)acrylic acid (salt)-based monomer aqueous solution in the step (i) of preparing the (meth)acrylic acid (salt)-based monomer aqueous solution, or may be added after the step (i) and before the step (ii) of polymerizing the (meth)acrylic acid (salt)-based monomer aqueous solution, which will be described later. The polymerization additive and the internal crosslinking agent may be added simultaneously to the (meth)acrylic acid (salt)-based monomer aqueous solution, or may be added separately to the (meth)acrylic acid (salt)-based monomer aqueous solution. For example, either the polymerization additive or the internal crosslinking agent may be added in step (i), and then the other of the polymerization additive or the internal crosslinking agent may be added in step (ii) after step (i). Preferably, the polymerization additive and the internal crosslinking agent are added to the aqueous (meth)acrylic acid (salt)-based monomer solution in the same step.

[0086] The internal crosslinking agent and polymerization additive will be explained in step (i).

[0087] The (meth)acrylic acid (salt) may be unneutralized or may be a salt type (fully neutralized type or partially neutralized type), and the aqueous monomer solution may have a concentration exceeding the saturated concentration, and even a supersaturated aqueous solution or an aqueous slurry solution (aqueous dispersion) of (meth)acrylic acid (salt) is treated as the aqueous (meth)acrylic acid (salt)-based monomer solution of the present invention. Note that, from the viewpoint of the physical properties of the obtained particulate water-absorbing agent, it is preferable to use an aqueous (meth)acrylic acid (salt)-based monomer solution having a saturated concentration or less.

[0088] Furthermore, water is preferred as a solvent for dissolving the monomer, and the (meth)acrylic acid (salt)-based monomer is treated as an aqueous solution. Here, the "aqueous solution" is not limited to a case where the solvent is 100% by mass of water, and a water-soluble organic solvent (e.g., alcohol, etc.) may be used in combination with water in an amount of more than 0% by mass and 30% by mass or less, preferably more than 0% by mass and 5% by mass or less, when the total amount of the solvent is taken as 100% by mass, and these are treated as aqueous solutions in the present invention.

[0089] In this specification, the term "aqueous (meth)acrylic acid (salt)-based monomer solution in the process of preparation" as described later refers to an aqueous solution of (meth)acrylic acid (salt) before all constituent components are mixed into the aqueous monomer solution containing (meth)acrylic acid (salt) as the main component, and specifically refers to an aqueous (meth)acrylic acid solution and a completely neutralized or partially neutralized aqueous (meth)acrylic acid (salt) solution.

[0090] The (meth)acrylic acid (salt)-based monomer aqueous solution being prepared is further neutralized, mixed with water as a solvent, mixed with the above-mentioned trace components, etc., to obtain a final (meth)acrylic acid (salt)-based monomer aqueous solution. Note that with regard to this final (meth)acrylic acid (salt)-based monomer aqueous solution, the state before being charged into a polymerization apparatus or before the start of polymerization after being charged into a polymerization apparatus is referred to as the "prepared (meth)acrylic acid (salt)-based monomer aqueous solution before the polymerization step."

[0091] [3-1-1] Monomer The water absorbent resin of the present invention uses a monomer containing (meth)acrylic acid (salt) as a main component. The main component refers to a state in which (meth)acrylic acid (salt) is contained in an amount of usually 50 mol % or more, preferably 70 mol % or more, more preferably 80 mol % or more, even more preferably 90 mol % or more, and particularly preferably 95 mol % or more (upper limit 100 mol %), based on the entire monomer (excluding the internal crosslinking agent).

[0092] In the present invention, the poly(meth)acrylic acid (salt) is not limited to unneutralized (neutralization rate 0 mol%), but includes partially neutralized and completely neutralized (neutralization rate 100 mol%).

[0093] As long as (meth)acrylic acid (salt) is contained as the main monomer component, other monomers that become a water-absorbing resin by polymerization may be contained, 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-methylpropane sulfonic acid, 2-(meth)acryloylethane sulfonic acid, 2-(meth)acryloylpropane sulfonic acid, 2-hydroxyethyl (meth)acryloyl Examples of the water-absorbing resin include anionic unsaturated monomers (salts) such as ammonium phosphate, mercapto group-containing unsaturated monomers, phenolic hydroxyl group-containing unsaturated monomers, amide group-containing unsaturated monomers such as (meth)acrylamide, N-ethyl(meth)acrylamide, and N,N-dimethyl(meth)acrylamide, and amino group-containing unsaturated monomers such as N,N-dimethylaminoethyl(meth)acrylate, N,N-dimethylaminopropyl(meth)acrylate, and N,N-dimethylaminopropyl(meth)acrylamide. The water-absorbing resin may also contain other monomers as copolymerization components.

[0094] In the present invention, the neutralization rate of the (meth)acrylic acid (salt)-based monomer or the hydrogel after polymerization is not particularly limited, but from the viewpoint of the physical properties of the obtained particulate water-absorbing agent and the reactivity of the surface cross-linking agent, it is preferably 40 mol% or more and 90 mol% or less, more preferably 50 mol% or more and 85 mol% or less, and even more preferably 65 mol% or more and 80 mol% or less. Therefore, according to a preferred embodiment of the present invention, the neutralization rate of the aqueous solution of the acrylic acid (salt)-based monomer in the step (i) is 40 mol% or more and 90 mol% or less.

[0095] When the neutralization rate is low, the water absorption rate (Vortex) tends to decrease, and conversely, when the neutralization rate is high, the reactivity between the poly(meth)acrylic acid (salt)-based water absorbent resin and a surface crosslinking agent (particularly, a dehydration reactive surface crosslinking agent described later) decreases, which tends to decrease productivity or decrease water absorption capacity under load (for example, AAP), and therefore, a neutralization rate within the above range is preferable.

[0096] The neutralization may be carried out on the monomer and / or the aqueous monomer solution before polymerization, on the hydrogel after polymerization, or both. When the neutralization is carried out multiple times, it is preferable to adjust the neutralization rate within the range taking into consideration the amounts of all basic compounds added.

[0097]

[0133] In particular, when unneutralized polymerization with a neutralization rate of 0 mol% is performed and neutralization is performed in a later step, neutralization becomes non-uniform, and unevenness occurs in the degree of progress of surface crosslinking for each water absorbent resin powder in a surface crosslinking step, and therefore there is a concern that the water absorption capacity under pressure is significantly reduced, and therefore neutralized polymerization is preferred.

[0098] In addition, from the viewpoint of the water absorption capacity under no load (CRC) and water absorption rate of the obtained particulate water-absorbing agent, the (meth)acrylic acid (salt)-based monomer or the hydrogel may be partly or entirely in the form of a salt, and one or more kinds of monovalent salts such as sodium salt, lithium salt, potassium salt, ammonium salt, and amines are preferable, among which one or more kinds of alkali metal salts are more preferable, and sodium salt and / or potassium salt are further preferable, and from the viewpoint of cost and physical properties, sodium salt is particularly preferable.

[0099] [3-1-2] Polymerization Additive In the method for producing a particulate water-absorbing agent according to the present invention, a polymerization additive is used in the polymerization. The polymerization additive may be present during the polymerization of the aqueous solution of a (meth)acrylic acid (salt)-based monomer, and specifically, is added before step (ii). The polymerization additive may be added to the aqueous solution of a (meth)acrylic acid (salt)-based monomer all at once or in portions.

[0100] Furthermore, the addition of a polymerization additive to the aqueous (meth)acrylic acid (salt)-based monomer solution during polymerization in the step (ii) is also included in the concepts of "adding a polymerization additive in the step (ii)" and "polymerizing the aqueous (meth)acrylic acid (salt)-based monomer solution in the presence of a polymerization additive."

[0101] The polymerization additive may be either a liquid or a solid, and may be added as is, or in the form of a solution or suspension.

[0102] The solvent when the polymerization additive is added in the form of a solution, and the dispersion medium when the polymerization additive is added in the form of a suspension are not particularly limited, but water and alcohol are preferred, and water is particularly preferred.

[0103] When the polymerization additive is added in the form of a solution or suspension, the concentration of the polymerization additive is preferably 0.1% by mass or more and 99% by mass or less, more preferably 0.1% by mass or more and 75% by mass or less, and even more preferably 0.1% by mass or more and 50% by mass or less.

[0104] When the solution is added, the temperature is from the melting point to the boiling point, preferably from 0° C. to 100° C., and more preferably from 20° C. to 50° C., and heating may be carried out as necessary to improve solubility.

[0105] The amount of the polymerization additive to be added may be determined in consideration of the type of the polymerization additive to be added.

[0106] The amount of the polymerization additive to be added varies depending on the type of the polymerization additive to be added, but is preferably 0.001% by mass or more and 5% by mass or less, more preferably 0.005% by mass or more and 3% by mass or less, even more preferably 0.01% by mass or more and 2% by mass or less, still more preferably 0.05% by mass or more and 1% by mass or less, and particularly preferably 0.08% by mass or more and 1% by mass or less, relative to the total amount of monomers contained in the aqueous (meth)acrylic acid (salt)-based monomer solution (100% by mass of raw material monomers (excluding the internal crosslinking agent)).

[0107] In addition, when the amount of the polymerization additive to be added in the step (ii) is the amount to be added relative to the total amount of monomers (100 mol% of the raw material monomers (excluding the internal crosslinking agent)), the total amount of monomers does not refer to the amount of the remaining raw material monomers, but refers to the amount of the raw material monomers used when preparing in the step (i) (the total amount of all monomers).

[0108] Therefore, according to a preferred embodiment of the present invention, the amount of the polymerization additive added is 0.001% by mass or more and 5% by mass or less with respect to the total amount of monomers contained in the (meth)acrylic acid (salt)-based monomer aqueous solution (100% by mass of raw material monomers (excluding the internal crosslinking agent)). If the amount is below these lower limits, it becomes difficult to confirm the effect of controlling the hardness of the hydrogel and the frictional force between the hydrogel particles, and if the amount is above these upper limits, the improvement in the effect of adding the polymerization additive does not correspond to the amount added, making it uneconomical.

[0109] In this example, the raw material monomers are acrylic acid and sodium acrylate.

[0110] The polymer additive of the present invention is present on the surface of the hydrogel and / or the hydrogel particles newly generated by the pulverization during gel pulverization, and thereby the tribology of the gel particle surface can be appropriately adjusted, thereby making it possible to obtain a particulate water-absorbing agent having a specific particle size and a specific total volume ratio of cavities and a specific total volume ratio of voids, thereby obtaining desired physical properties.

[0111] On the other hand, when no polymerization additive is added, the hardness of the hydrogel and the tribology of the surface of the hydrogel particles are not appropriate, and it becomes difficult to obtain a particulate water-absorbing agent in which the total volume ratio of the cavities and the total volume ratio of the voids are present at a desired specific ratio.

[0112] In the present invention, as shown in the Examples, when measuring the mass-average particle size (D50) of hydrogel particles, the hydrogel particles were immersed in an aqueous solution containing 0.02% by mass of sodium polyoxyethylene (3) lauryl ether sulfate (0.08% by mass of EMAL (registered trademark, the same applies hereinafter) 20C; surfactant, manufactured by Kao Corporation) and 20% by mass of sodium chloride for 1 hour and 16 hours, and the change in D50 was observed to confirm the tribology occurring on the hydrogel particle surface. Generally, hydrogels obtained after gel pulverization of poly(meth)acrylic acid (salt) are adhesive and aggregated due to their nature, but the hydrogel particles obtained after pulverization by the production method of the present invention maintain relatively large masses when the immersion time is short (e.g., 1 hour), and after a certain period of immersion (e.g., 16 hours of immersion), the adhesiveness is sufficiently released and the particle size is reduced to form granulated particles.

[0113] The present inventors believe that a hydrogel having an appropriate hardness can be obtained by the production method of the present invention during polymerization (i.e., a specific polymerization additive), and that by gel-crushing the hydrogel with an appropriate force (i.e., a gel-crushing energy (GGE)(2) of 30 J / g or more), the hydrogel particles obtained by gel-crushing have a granulated state, and as a result, the shape (particle size and proportion of cavities and voids) of the particulate water-absorbing agent of the present invention can be achieved. In other words, it is believed that the hardness of the hydrogel and the granulation strength of the hydrogel particles after pulverization are adjusted by the specific polymerization additive (preferably a specific amount of an internal cross-linking agent and a specific polymerization additive) and appropriate strength of gel-crushing, and that a particulate water-absorbing agent having a specific particle size and a specific proportion of cavities and voids can be obtained.

[0114] In the method for producing a particulate water-absorbing agent according to the present invention, the polymerization additive is one or more compounds selected from amphoteric substances, anionic substances, and cationic substances. By using the compound as the polymerization additive, a balance can be achieved between adhesion between hydrogel particles and grinding energy during gel grinding, and the desired effects of the present invention can be achieved.

[0115] Specific examples of the polymerization additives used in the present invention include: amphoteric substances such as (a) alkyl betaine, (b) alkyl amine oxide; anionic substances such as (c) sulfate ester salts of higher alcohol alkylene oxide adducts, (d) dicarboxylate salts, (e) sulfonates, (f) alkyl sulfate ester salts, (g) alkyl amine diacetate salts, (h) phosphate ester salts of higher alcohol alkylene oxide adducts, and (i) carboxylate salts of higher alcohol alkylene oxide adducts; and cationic substances such as (j) ammonium salts.

[0116] That is, the polymerization additive is one or more ionic substances selected from (a) alkylbetaines and (b) alkylamine oxides, one or more anionic substances selected from (c) sulfate salts of higher alcohol alkylene oxide adducts, (d) dicarboxylate salts, (e) sulfonates, (f) alkyl sulfate salts, (g) alkylamine diacetates, (h) phosphate salts of higher alcohol alkylene oxide adducts, and (i) carboxylate salts of higher alcohol alkylene oxide adducts, and one or more cationic substances selected from (j) ammonium salts. This configuration allows the intended effects of the present invention to be efficiently achieved.

[0117] (a) Alkylbetaine Alkylbetaine refers to a compound having a cationic group and an anionic group at non-adjacent positions within the same molecule, the cationic group being a secondary to quaternary ammonium group, and at least one of the secondary to quaternary ammonium groups being modified with a substituent having a hydrocarbon group having 1 to 30 carbon atoms. When the polymerization additive is an alkylbetaine, the adhesion between pulverized hydrogel particles can be controlled, and a balance can be achieved between the adhesion between hydrogel particles during gel pulverization and the pulverization energy, thereby better achieving the desired effects of the present invention. In one embodiment, the alkylbetaine is preferably a compound represented by the following general formula "Chemical Formula 1."

[0118]

[0119] R １The hydrocarbon group is a hydrocarbon group having 1 to 30 carbon atoms. Here, the hydrocarbon group having 1 to 30 carbon atoms is not limited to a linear group, but may be a branched or cyclic saturated hydrocarbon group and / or unsaturated hydrocarbon group, an aromatic hydrocarbon group such as an alkylphenyl group or an alkylbenzyl group, or a polycyclic aromatic hydrocarbon such as a naphthyl group. Furthermore, the hydrocarbon group may have a reactive functional group such as a hydroxy group, an amino group, or a glycidyl group, and may have an ether bond, an ester bond, a urethane bond, or an amide bond. The number of carbon atoms in the hydrocarbon group is preferably 1 to 30, more preferably 2 to 28, even more preferably 3 to 26, even more preferably 4 to 24, particularly preferably 6 to 22, and most preferably 12 to 22. When the hydrocarbon group has 1 to 30 carbon atoms, a proper balance between hydrophobicity and hydrophilicity is achieved, and a balance is achieved between adhesion between hydrogel particles and the grinding energy during gel grinding, thereby better achieving the desired effects of the present invention.

[0120] R ２ , R ３ are each independently hydrogen or a hydrocarbon group having 1 to 30 carbon atoms. The hydrocarbon group having 1 to 30 carbon atoms is not limited to a linear group, but may be a branched or cyclic saturated and / or unsaturated hydrocarbon group, an aromatic hydrocarbon group such as an alkylphenyl group or an alkylbenzyl group, or a polycyclic aromatic hydrocarbon such as a naphthyl group. Furthermore, the hydrocarbon group may have a reactive functional group such as a hydroxy group, an amino group, or a glycidyl group, and may have an ether bond, an ester bond, a urethane bond, or an amide bond. The number of carbon atoms in the hydrocarbon group is preferably 1 to 30, more preferably 1 to 25, even more preferably 1 to 20, even more preferably 1 to 12, particularly preferably 1 to 8, and most preferably 1 to 3. When the hydrocarbon group has 1 to 30 carbon atoms, a proper balance between hydrophobicity and hydrophilicity is achieved, and a balance is achieved between adhesion between hydrogel particles and the grinding energy during gel grinding, thereby better achieving the desired effects of the present invention. R １ , R ２ , R ３ may be different or the same.

[0121] The structure of X is not particularly limited, except that it contains at least 1 carbon atom. X is, for example, a hydrocarbon group having 1 to 30 carbon atoms, and the number of carbon atoms in the hydrocarbon group may be 1 to 30, 1 to 25, 1 to 20, 1 to 12, or 1 to 8 or 1 to 3.

[0122] Anion part (Z － ) is a carboxylate anion (COO － ), sulfonate anion (SO ４ － ), phosphate anion (PO ４ － ) are listed.

[0123] As the alkylbetaine, for example, aminoacetic acid betaine type and sulfobetaine type are preferred.

[0124] However, in addition to those represented by the general formula "Chemical Formula 1," there are also compounds having a cationic group on the imidazolium ring, such as Amphitol 20YB (manufactured by Kao Corporation) shown in the following "Chemical Formula 2."

[0125]

[0126] R １ is a hydrocarbon group having 1 to 30 carbon atoms. Here, the hydrocarbon group having 1 to 30 carbon atoms is not limited to a linear group, but may be a branched or cyclic saturated and / or unsaturated hydrocarbon group, an aromatic hydrocarbon group such as an alkylphenyl group or an alkylbenzyl group, or a polycyclic aromatic hydrocarbon such as a naphthyl group. Furthermore, the hydrocarbon group may have a reactive functional group such as a hydroxy group, an amino group, or a glycidyl group, and may have an ether bond, an ester bond, a urethane bond, or an amide bond. The number of carbon atoms in the hydrocarbon group is preferably 1 to 30, more preferably 2 to 28, even more preferably 3 to 26, particularly preferably 4 to 24, and most preferably 6 to 22. When the hydrocarbon group has 1 to 30 carbon atoms, a proper balance between hydrophobicity and hydrophilicity is achieved, and a balance is achieved between adhesion between hydrogel particles and the grinding energy during gel grinding, thereby better achieving the desired effects of the present invention.

[0127] R ２is hydrogen or a hydrocarbon group having 1 to 30 carbon atoms. Here, the hydrocarbon group having 1 to 30 carbon atoms is not limited to a straight chain, and may be a branched or cyclic saturated hydrocarbon group and / or unsaturated hydrocarbon group, an aromatic hydrocarbon group such as an alkylphenyl group or an alkylbenzyl group, or a polycyclic aromatic hydrocarbon such as a naphthyl group. Furthermore, the hydrocarbon group may have a reactive functional group such as a hydroxy group, an amino group, or a glycidyl group, and may have an ether bond, an ester bond, a urethane bond, or an amide bond. The number of carbon atoms in the hydrocarbon group is preferably 1 to 30, more preferably 1 to 25, and even more preferably 1 to 20. When the hydrocarbon group has 1 to 30 carbon atoms, a balance between hydrophobicity and hydrophilicity is appropriate, and a balance is achieved between adhesion between hydrogel particles and the grinding energy during gel grinding, thereby better achieving the desired effects of the present invention. R １ , R ２ may be different or the same.

[0128] The structure of X is not particularly limited, except that it contains one or more carbon atoms.

[0129] Examples of the anion moiety (Z) include carboxylates, sulfonates, and phosphates.

[0130] (a) Alkylbetaines are readily available on the market, and preferred examples thereof include the following products:

[0131] Manufactured by Kao Corporation: Anhithol 20BS, Anhithol 24B (desalted product of 20BS), Anhithol 86B, Anhithol 20N, Anhithol 20YB, Anhithol 20AB, Anhithol 55AB, Anhithol 20HD Manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd.: Amogen (registered trademark, the same applies hereinafter) S-H, Amogen K, Amogen LB-C, Amogen CB-H, Amogen HB-C Manufactured by Adeka Corporation: Adeka Anhort (registered trademark, the same applies hereinafter) PB-30L, Adeka Anhort AB-35L Manufactured by NOF Corporation: Nissan Anon (registered trademark, the same applies hereinafter) BF, Nissan Anon BL, Nissan Anon BL-SF, Nissan Anon BDF-R, Nissan Anon BDF-SF, Nissan Anon BDC-SF, Nissan Anon BDL-SF, Nissan Anon GLM-R Manufactured by Nippon Nyukazai: Teksnor (registered trademark, the same applies hereinafter) R2 Manufactured by Toho Chemical Industry Co., Ltd.: Obazoline (registered trademark, the same applies hereinafter) LB-SF.

[0132] (b) Alkylamine oxide) Alkylamine oxide refers to a compound having a cationic group and an anionic group at adjacent positions in the same molecule, the cationic group being a secondary to quaternary ammonium group, and at least one of the secondary to quaternary ammonium groups being modified with a substituent having a hydrocarbon group having 1 to 30 carbon atoms. When the polymerization additive is an alkylamine oxide, the adhesion between the pulverized hydrogel particles can be controlled, and a balance can be achieved between the adhesion between the hydrogel particles during gel pulverization and the pulverization energy, thereby better achieving the desired effects of the present invention. The alkylamine oxide is preferably a compound represented by the following general formula "Chemical Formula 3."

[0133]

[0134] R １is a hydrocarbon group having 1 to 30 carbon atoms. Here, the hydrocarbon group having 1 to 30 carbon atoms is not limited to a linear group, but may be a branched or cyclic saturated and / or unsaturated hydrocarbon group, an aromatic hydrocarbon group such as an alkylphenyl group or an alkylbenzyl group, or a polycyclic aromatic hydrocarbon such as a naphthyl group. Furthermore, the hydrocarbon group may have a reactive functional group such as a hydroxy group, an amino group, or a glycidyl group, and may have an ether bond, an ester bond, a urethane bond, or an amide bond. The number of carbon atoms in the hydrocarbon group is preferably 1 to 30, more preferably 2 to 28, even more preferably 3 to 26, particularly preferably 4 to 24, and most preferably 6 to 22. When the hydrocarbon group has 1 to 30 carbon atoms, a proper balance between hydrophobicity and hydrophilicity is achieved, and a balance is achieved between adhesion between hydrogel particles and the grinding energy during gel grinding, thereby better achieving the desired effects of the present invention.

[0135] R ２ , R ３ are each independently hydrogen or a hydrocarbon group having 1 to 30 carbon atoms. Here, the hydrocarbon group having 1 to 30 carbon atoms is not limited to a straight chain, and may be a branched or cyclic saturated hydrocarbon group and / or unsaturated hydrocarbon group, an aromatic hydrocarbon group such as an alkylphenyl group or an alkylbenzyl group, or a polycyclic aromatic hydrocarbon such as a naphthyl group. Furthermore, the hydrocarbon group may have a reactive functional group such as a hydroxy group, an amino group, or a glycidyl group, and may have an ether bond, an ester bond, a urethane bond, or an amide bond. The number of carbon atoms in the hydrocarbon group is preferably 1 to 30, more preferably 1 to 25, and even more preferably 1 to 20. When the hydrocarbon group has 1 to 30 carbon atoms, a balance between hydrophobicity and hydrophilicity is appropriate, and a balance is achieved between adhesion between hydrogel particles and the grinding energy during gel grinding, thereby better achieving the desired effects of the present invention. R １ , R ２ , R ３ may be different or the same.

[0136] (b) Alkylamine oxides are readily available on the market, and preferred examples thereof include the following products:

[0137] Manufactured by Daiichi Kogyo Seiyaku Co., Ltd.: Amogene AOL Manufactured by NOF Corporation: Unisafe (registered trademark, same hereinafter) A-LM, Unisafe A-SM, Unisafe A-LE Manufactured by New Japan Chemical Co., Ltd.: Wondamin (registered trademark, same hereinafter) OX-300.

[0138] (c) Sulfate of a higher alcohol alkylene oxide adduct The sulfate of a higher alcohol alkylene oxide adduct refers to a compound in which one end of a (poly)alkylene glycol is modified with a substituent having a hydrocarbon of 1 to 30 carbon atoms and the other end is a sulfate. When the polymerization additive is a sulfate of a higher alcohol alkylene oxide adduct, the adhesion between the pulverized hydrogel particles can be controlled, and a balance can be achieved between the adhesion between the hydrogel particles and the pulverization energy during gel pulverization, thereby better achieving the desired effects of the present invention.

[0139] In one embodiment, the sulfate salt of a higher alcohol alkylene oxide adduct is preferably a (poly)alkylene glycol in which one side is modified with a substituent having a hydrocarbon having 1 to 30 carbon atoms, and the other side is a sulfate salt. In one embodiment, the sulfate salt of a higher alcohol alkylene oxide adduct is preferably a compound represented by the following general formula "Chemical Formula 4".

[0140]

[0141] R is a hydrocarbon group having 1 to 30 carbon atoms. The hydrocarbon group having 1 to 30 carbon atoms is not limited to a linear group, and may be a branched or cyclic saturated and / or unsaturated hydrocarbon group, an aromatic hydrocarbon group such as an alkylphenyl group or an alkylbenzyl group, or a polycyclic aromatic hydrocarbon such as a naphthyl group. Furthermore, the hydrocarbon group may have a reactive functional group such as a hydroxy group, an amino group, or a glycidyl group, and may have an ether bond, an ester bond, a urethane bond, or an amide bond. The number of carbon atoms in the hydrocarbon group is preferably 1 to 30, more preferably 2 to 28, even more preferably 3 to 26, particularly preferably 4 to 24, and most preferably 6 to 22. When the hydrocarbon group has 1 to 30 carbon atoms, a proper balance between hydrophobicity and hydrophilicity is achieved, and a balance between adhesion between hydrogel particles and grinding energy during gel grinding is achieved, thereby better achieving the desired effects of the present invention.

[0142] AO is C ｎ H ２ｎ The repeating unit may be represented by the formula: CH 2 O (n is a natural number), and the number of carbon atoms is preferably 1 to 6, more preferably 1 to 3, and even more preferably 2 to 3. ２ CH ２ O is particularly preferred.

[0143] The repeating units may be polymers of units having the same number of carbon atoms, or may be block polymers or random polymers of units having different carbon atoms.

[0144] a is the number of repeating units of AO, and is preferably 1 to 1000, more preferably 2 to 500, and even more preferably 2 to 300. When the number of repeating units is 1000 or less, the viscosity does not become too high, and it becomes easy to add the copolymer uniformly to an aqueous solution of a (meth)acrylic acid (salt)-based monomer.

[0145] M ＋ As the cation, alkali metal ions (Li ＋ , Na ＋ , K. ＋ ), and ammonium ion.

[0146] (c) Sulfate salts of alkylene oxide adducts of higher alcohols are readily available on the market, and preferred examples thereof include the following products:

[0147] Manufactured by Kao Corporation: Sodium polyoxyethylene lauryl ether sulfate EMAL (registered trademark, same hereinafter) 20C, EMAL E-27C, EMAL 270J, EMAL 20CM Manufactured by Nippon Nyukazai Co., Ltd.: Polyoxyethylene alkyl ether sulfate Newcol (registered trademark, same hereinafter) 1020-SN, Newcol 2308-SF, Newcol 2320-SN, Newcol 2360-SN, Newcol 1305-SN, Newcol 1330-SF, Newcol 1703-SFD, Newcol 1525-SFC Manufactured by NOF Corporation: Sodium polyoxyethylene alkyl ether sulfate Nissantrax (registered trademark, same hereinafter) K-40, Nissantrax K-300, Persoft (registered trademark, same hereinafter) EP, Persoft EF, Persoft EDO, Persoft EL, Persoft EK.

[0148] (d) Dicarboxylates Dicarboxylates are compounds that contain a carboxylic acid group (carboxyl group: -CH) in the same molecule. ３ COO － ) is a compound having two of the above groups. When the polymerization additive is a dicarboxylate, the adhesion between the pulverized hydrogel particles can be controlled, and the adhesion between the hydrogel particles during gel pulverization and the pulverization energy can be balanced, thereby better achieving the desired effects of the present invention. Note that the term "dicarboxylate" here refers to compounds other than amine compounds and does not include "(g) alkylamine diacetate" described below. Examples of dicarboxylates include alkenyl succinates and acylaspartates. In one embodiment, the alkenyl succinate is preferably a compound represented by the following general formula "Chemical Formula 5." Furthermore, in one embodiment, the acylaspartate is preferably a compound represented by the following general formula "Chemical Formula 6."

[0149]

[0150]

[0151] R is a hydrocarbon group having 1 to 30 carbon atoms. The hydrocarbon group having 1 to 30 carbon atoms is not limited to a linear group, and may be a branched or cyclic saturated and / or unsaturated hydrocarbon group, an aromatic hydrocarbon group such as an alkylphenyl group or an alkylbenzyl group, or a polycyclic aromatic hydrocarbon such as a naphthyl group. Furthermore, the hydrocarbon group may have a reactive functional group such as a hydroxy group, an amino group, or a glycidyl group, and may have an ether bond, an ester bond, a urethane bond, or an amide bond. The number of carbon atoms in the hydrocarbon group is preferably 1 to 30, more preferably 2 to 28, even more preferably 3 to 26, particularly preferably 4 to 24, and most preferably 6 to 22. When the hydrocarbon group has 1 to 30 carbon atoms, a proper balance between hydrophobicity and hydrophilicity is achieved, and a balance between adhesion between hydrogel particles and grinding energy during gel grinding is achieved, thereby better achieving the desired effects of the present invention.

[0152] M ＋ As the cation, alkali metal ions (Li ＋ , Na ＋ , K. ＋ ), ammonium ion, proton (H ＋ In addition, the present invention is not limited to a case where two carboxylic acid groups form a salt in one molecule (a case where two carboxylate moieties are present: a disalt), and may also include a case where only one carboxylic acid group forms a salt (a case where one carboxylate moiety is present: a monosalt).

[0153] (d) Dicarboxylates are readily available on the market, and preferred examples thereof include the following products:

[0154] Manufactured by Kao Corporation: Dipotassium alkenyl succinate Latemul (registered trademark, same below) ASK Manufactured by Asahi Kasei Fine Chemical Corporation: Sodium cocoyl glutamate Aminosurfact (registered trademark, same below) ACDS-L Sodium lauroyl aspartate Aminoformer (registered trademark, same below) FLDS-L.

[0155] (e) Sulfonate A sulfonate is a compound having a sulfonic acid group (-SO ) in the same molecule. ３ －When the polymerization additive is a sulfonate, the adhesion between the pulverized hydrogel particles can be controlled, and the adhesion between the hydrogel particles during gel pulverization and the pulverization energy can be balanced, so that the desired effect of the present invention can be more effectively achieved. In one embodiment, the sulfonate is a compound represented by the general formula: "R-SO ３ － M ＋ " is preferably a compound represented by the formula:

[0156] In the above general formula, R is a hydrocarbon group having 1 to 70 carbon atoms. The hydrocarbon group may be a linear, branched, or cyclic saturated and / or unsaturated hydrocarbon group, an aromatic hydrocarbon group (such as an alkylphenyl group or an alkylbenzyl group), or a polycyclic aromatic hydrocarbon group (such as a naphthyl group). Furthermore, the hydrocarbon group may have a reactive functional group such as a hydroxy group, an amino group, or a glycidyl group, and may have an ether bond, an ester bond, a urethane bond, or an amide bond. The number of carbon atoms in the hydrocarbon group is preferably 1 to 70, more preferably 2 to 56, even more preferably 3 to 52, particularly preferably 4 to 48, and most preferably 6 to 44. When the hydrocarbon group has 1 to 30 carbon atoms, a suitable balance between hydrophobicity and hydrophilicity is achieved, and a balance is achieved between adhesion between hydrogel particles and the grinding energy during gel grinding, thereby better achieving the desired effects of the present invention.

[0157] In the above formula, M ＋ is an alkali metal ion (Li ＋ , Na ＋ , K. ＋ ), which may be an ammonium ion.

[0158] According to one embodiment, the sulfonate is preferably a dialkyl sulfosuccinate. A dialkyl sulfosuccinate is a compound having two carboxylic acid groups and one sulfonic acid group in the same molecule. When the polymerization additive is a dialkyl sulfosuccinate, the adhesion between the pulverized hydrogel particles can be controlled, and the adhesion between the hydrogel particles during gel pulverization and the pulverization energy can be balanced, thereby further achieving the desired effects of the present invention. In one embodiment, the dialkyl sulfosuccinate is a compound represented by the following general formula "Chemical Formula 7."

[0159]

[0160] R １ , R ２ are each independently a hydrocarbon group having 1 to 30 carbon atoms. Here, the hydrocarbon group having 1 to 30 carbon atoms is not limited to a linear group, but may be a branched or cyclic saturated and / or unsaturated hydrocarbon group, an aromatic hydrocarbon group such as an alkylphenyl group or an alkylbenzyl group, or a polycyclic aromatic hydrocarbon such as a naphthyl group. Furthermore, the hydrocarbon group may have a reactive functional group such as a hydroxy group, an amino group, or a glycidyl group, and may have an ether bond, an ester bond, a urethane bond, or an amide bond. The number of carbon atoms in the hydrocarbon group is preferably 1 to 30, more preferably 2 to 28, even more preferably 3 to 26, particularly preferably 4 to 24, and most preferably 6 to 22. When the hydrocarbon group has 1 to 30 carbon atoms, a suitable balance between hydrophobicity and hydrophilicity is achieved, and a balance is achieved between adhesion between hydrogel particles and the grinding energy during gel grinding, thereby better achieving the desired effects of the present invention.

[0161] M ＋ As the cation, alkali metal ions (Li ＋ , Na ＋ , K. ＋ ), and ammonium ion.

[0162] (e) Sulfonates are readily available on the market, and preferred examples thereof include the following products:

[0163] Manufactured by FUJIFILM Wako Pure Chemical Industries, Ltd.: Sodium di-2-ethylhexyl sulfosuccinate Aerosol OT Manufactured by Lion Specialty Chemicals Co., Ltd.: Sodium di-2-ethylhexyl sulfosuccinate Ripearl 870P, Ripearl 860K, Ripearl 835I Manufactured by NOF Corporation: Sodium di-2-ethylhexyl sulfosuccinate Rapisol A-30, Rapisol A-80 Manufactured by Daiichi Kogyo Seiyaku Co., Ltd.: Sodium dioctyl sulfosuccinate Neocoal P, Neocoal SW, Neocoal SW-C, Neocoal YSK Manufactured by Kao Corporation: Alkylbenzenesulfonate Neoperex (registered trademark, the same applies hereinafter) GS Sodium dodecylbenzenesulfonate Neoperex G-15, Neoperex G-25, Neoperex G-65 Sodium alkylnaphthalenesulfonate Pelex (registered trademark, the same applies hereinafter) NB-L - Sodium dialkyl sulfosuccinate Pelex OT-P, Pelex TR - Disodium alkyl monoamide sulfosuccinate Pelex TA - Sodium alkyl diphenyl ether disulfonate Pelex SS-L, Pelex SS-H - Sodium alkane sulfonate Latemul PS manufactured by Takemoto Oil & Fat Co., Ltd. - Sodium alkyl diphenyl ether disulfonate Pionin A-43-D, Takesurf (registered trademark, the same applies hereinafter) A-43-NQ.

[0164] (f) Alkyl sulfate ester salts Alkyl sulfate ester salts have a sulfate group (-SO ) in the same molecule. ４ － ) is a compound having the formula: When the polymerization additive is an alkyl sulfate, the adhesion between the pulverized hydrogel particles can be controlled, and the adhesion between the hydrogel particles during gel pulverization and the pulverization energy can be balanced, so that the desired effect of the present invention can be more effectively achieved. In one embodiment, the alkyl sulfate is preferably a compound represented by the following general formula "Chemical Formula 8".

[0165]

[0166] R is a hydrocarbon group having 1 to 30 carbon atoms. The hydrocarbon group having 1 to 30 carbon atoms is not limited to a linear group, and may be a branched or cyclic saturated and / or unsaturated hydrocarbon group, an aromatic hydrocarbon group such as an alkylphenyl group or an alkylbenzyl group, or a polycyclic aromatic hydrocarbon such as a naphthyl group. Furthermore, the hydrocarbon group may have a reactive functional group such as a hydroxy group, an amino group, or a glycidyl group, and may have an ether bond, an ester bond, a urethane bond, or an amide bond. The number of carbon atoms in the hydrocarbon group is preferably 1 to 30, more preferably 2 to 28, even more preferably 3 to 26, particularly preferably 4 to 24, and most preferably 6 to 22. When the hydrocarbon group has 1 to 30 carbon atoms, a proper balance between hydrophobicity and hydrophilicity is achieved, and a balance between adhesion between hydrogel particles and grinding energy during gel grinding is achieved, thereby better achieving the desired effects of the present invention.

[0167] M ＋ As the cation, alkali metal ions (Li ＋ , Na ＋ , K. ＋ ), and ammonium ion.

[0168] (f) Alkyl sulfate ester salts are readily available on the market, and preferred examples thereof include the following products:

[0169] Manufactured by Kao Corporation: Emar 2F, Latemul AD-25 Manufactured by Takemoto Oil & Fat Co., Ltd.: Takesurf A-24 Manufactured by NOF Corporation: Scintrex (registered trademark, same below) EH-R, Persoft SK Manufactured by Lion Specialty Chemicals Co., Ltd.: Sanol (registered trademark, same below) LM-1130.

[0170] ((g) Alkylamine diacetate) Alkylamine diacetate is a compound having an alkyl group and two acetate groups (-CH ３ COO －) is an amine compound having the above structure. When the polymerization additive is an alkyldiamine diacetate, adhesion between pulverized hydrogel particles can be controlled, and a balance between adhesion between hydrogel particles and pulverization energy during gel pulverization can be achieved, thereby further achieving the desired effects of the present invention. In one embodiment, the alkylamine diacetate is preferably a compound represented by the following general formula "Chemical Formula 9."

[0171]

[0172] R is a hydrocarbon group having 1 to 30 carbon atoms. The hydrocarbon group having 1 to 30 carbon atoms is not limited to a linear group, and may be a branched or cyclic saturated and / or unsaturated hydrocarbon group, an aromatic hydrocarbon group such as an alkylphenyl group or an alkylbenzyl group, or a polycyclic aromatic hydrocarbon such as a naphthyl group. Furthermore, the hydrocarbon group may have a reactive functional group such as a hydroxy group, an amino group, or a glycidyl group, and may have an ether bond, an ester bond, a urethane bond, or an amide bond. The number of carbon atoms in the hydrocarbon group is preferably 1 to 30, more preferably 2 to 28, even more preferably 3 to 26, particularly preferably 4 to 24, and most preferably 6 to 22. When the hydrocarbon group has 1 to 30 carbon atoms, a proper balance between hydrophobicity and hydrophilicity is achieved, and a balance between adhesion between hydrogel particles and grinding energy during gel grinding is achieved, thereby better achieving the desired effects of the present invention.

[0173] M ＋ As the cation, alkali metal ions (Li ＋ , Na ＋ , K. ＋ ), and ammonium ion.

[0174] (g) Alkylamine diacetate is readily available on the market, and preferred examples thereof include the following products:

[0175] Manufactured by NOF Corporation: Nissan Anon LA.

[0176] (h) Phosphate Salt of Higher Alcohol Alkylene Oxide Adduct) The phosphate salt of a higher alcohol alkylene oxide adduct has, within the same molecule, one end of a (poly)alkylene glycol modified with a substituent having a hydrocarbon of 1 to 30 carbon atoms, and the other end being a phosphate salt. When the polymerization additive is a phosphate salt of a higher alcohol alkylene oxide adduct, the adhesion between pulverized hydrogel particles can be controlled, and a balance can be achieved between the adhesion between hydrogel particles and the pulverization energy during gel pulverization, thereby better achieving the desired effects of the present invention.

[0177] In one embodiment, the phosphate salt of a higher alcohol alkylene oxide adduct is a compound represented by the following general formula "Chemical Formula 10," in which one of the (poly)alkylene glycols is modified with a substituent having a hydrocarbon having 1 to 30 carbon atoms and the other is a sulfate salt.

[0178]

[0179] R １ , R ２ are each independently a hydrocarbon group having 1 to 30 carbon atoms. Here, the hydrocarbon group having 1 to 30 carbon atoms is not limited to a linear group, but may be a branched or cyclic saturated and / or unsaturated hydrocarbon group, an aromatic hydrocarbon group such as an alkylphenyl group or an alkylbenzyl group, or a polycyclic aromatic hydrocarbon such as a naphthyl group. Furthermore, the hydrocarbon group may have a reactive functional group such as a hydroxy group, an amino group, or a glycidyl group, and may have an ether bond, an ester bond, a urethane bond, or an amide bond. The number of carbon atoms in the hydrocarbon group is preferably 1 to 30, more preferably 2 to 28, even more preferably 3 to 26, particularly preferably 4 to 24, and most preferably 6 to 22. When the hydrocarbon group has 1 to 30 carbon atoms, a suitable balance between hydrophobicity and hydrophilicity is achieved, and a balance is achieved between adhesion between hydrogel particles and the grinding energy during gel grinding, thereby better achieving the desired effects of the present invention.

[0180] a is CH ２ CH２ The repeating number of the repeating unit of O is preferably 1 to 1000, more preferably 2 to 500, and even more preferably 2 to 300. When the repeating unit is 1000 or less, the viscosity does not become too high, and it becomes easy to add the compound uniformly to the aqueous solution of the (meth)acrylic acid (salt)-based monomer.

[0181] M ＋ As the cation, alkali metal ions (Li ＋ , Na ＋ , K. ＋ ), and ammonium ion.

[0182] (h) Phosphate salts of higher alcohol alkylene oxide adducts are readily available on the market, and preferred examples include the following products:

[0183] Manufactured by Daiichi Kogyo Seiyaku Co., Ltd.: Plysurf (registered trademark, the same applies hereinafter) A212C, Plysurf A207H, Plysurf A208S Manufactured by Takemoto Yushi Co., Ltd.: Takesurf A-72TK65, Takesurf A-7004.

[0184] (i) Carboxylate of a higher alcohol alkylene oxide adduct The carboxylate of a higher alcohol alkylene oxide adduct refers to a compound in which one end of a (poly)alkylene glycol is modified with a substituent having a hydrocarbon group having 1 to 30 carbon atoms, and the other end is a carboxylate. When the polymerization additive is a carboxylate of a higher alcohol alkylene oxide adduct, the adhesion between pulverized hydrogel particles can be controlled, and the adhesion between hydrogel particles during gel pulverization and the pulverization energy can be balanced, thereby better achieving the desired effects of the present invention. In one embodiment, the carboxylate of a higher alcohol alkylene oxide adduct is preferably a compound represented by the following general formula "Chemical Formula 11."

[0185]

[0186] In the above formula (Chemical Formula 11), R is a hydrocarbon group having 1 to 30 carbon atoms. The hydrocarbon group may be a linear, branched, or cyclic saturated and / or unsaturated hydrocarbon group, an aromatic hydrocarbon group (such as an alkylphenyl group or an alkylbenzyl group), or a polycyclic aromatic hydrocarbon group (such as a naphthyl group). Furthermore, the hydrocarbon group may have a reactive functional group such as a hydroxy group, an amino group, or a glycidyl group, and may have an ether bond, an ester bond, a urethane bond, or an amide bond. The hydrocarbon group preferably has 1 to 30 carbon atoms, more preferably 2 to 28, even more preferably 3 to 26, particularly preferably 4 to 24, and most preferably 6 to 22. When the hydrocarbon group has 1 to 30 carbon atoms, a suitable balance between hydrophobicity and hydrophilicity is achieved, and a balance is achieved between adhesion between hydrogel particles and the grinding energy during gel grinding, thereby better achieving the desired effects of the present invention.

[0187] In the above formula, (AO) is C ｎ H ２ｎ The number of carbon atoms constituting AO (the above n) is preferably 1 to 6, more preferably 1 to 3, even more preferably 2 to 3, and particularly preferably 2. That is, the repeating unit (AO) in the above general formula "Chemical Formula 11" can be represented by [CH ２ CH ２ O], and the repeating unit may be a structure derived from ethylene oxide addition or ethylene glycol condensation.

[0188] In the above formula, a represents the number of the repeating units (AO), and when a is 2 or more, the number of carbon atoms constituting each repeating unit may be the same or different. That is, the alkylene oxide moiety contained in the above general formula (Chemical Formula 11) may be a polymer having the same repeating unit (AO), or may be a block polymer or random polymer having different repeating units (AO).

[0189] In the above formula, a is preferably 1 to 1000, more preferably 2 to 500, and even more preferably 2 to 300. When the number of repeating units is 1000 or less, the viscosity does not become too high, and it becomes easy to add the compound uniformly to an aqueous solution of a (meth)acrylic acid (salt)-based monomer.

[0190] In the above formula, M ＋ is an alkali metal ion (Li ＋ , Na ＋ , K. ＋ ), which may be an ammonium ion.

[0191] (i) Carboxylate salts of higher alcohol alkylene oxide adducts are readily available on the market, and preferred examples thereof include the following products:

[0192] Manufactured by Kao Corporation: Polyoxyethylene lauryl ether sodium acetate Kao Akipo RLM-100NV, Kao Akipo RLM-100, Kao Akipo RLM-45NV, Kao Akipo RLM-45.

[0193] (j) Ammonium Salt The ammonium salt is a compound in which at least one hydrogen atom of the ammonium salt is modified with a substituent having a hydrocarbon having 1 to 30 carbon atoms. This configuration makes it possible to control the adhesion between pulverized hydrogel particles, and balances the adhesion between hydrogel particles during gel pulverization with the pulverization energy, thereby better achieving the desired effects of the present invention. In one embodiment, the ammonium salt is preferably a compound represented by the general formula "Chemical Formula 12."

[0194]

[0195] R １is a hydrocarbon group having 1 to 30 carbon atoms. Here, the hydrocarbon group having 1 to 30 carbon atoms is not limited to a linear group, but may be a branched or cyclic saturated and / or unsaturated hydrocarbon group, an aromatic hydrocarbon group such as an alkylphenyl group or an alkylbenzyl group, or a polycyclic aromatic hydrocarbon such as a naphthyl group. Furthermore, the hydrocarbon group may have a reactive functional group such as a hydroxy group, an amino group, or a glycidyl group, and may have an ether bond, an ester bond, a urethane bond, or an amide bond. The number of carbon atoms in the hydrocarbon group is preferably 1 to 30, more preferably 2 to 28, even more preferably 3 to 26, particularly preferably 4 to 24, and most preferably 6 to 22. When the hydrocarbon group has 1 to 30 carbon atoms, a proper balance between hydrophobicity and hydrophilicity is achieved, and a balance is achieved between adhesion between hydrogel particles and the grinding energy during gel grinding, thereby better achieving the desired effects of the present invention.

[0196] R ２ , R ３ , R ４ are each independently hydrogen or a hydrocarbon group having 1 to 30 carbon atoms. Here, the hydrocarbon group having 1 to 30 carbon atoms is not limited to a straight chain, and may be a branched or cyclic saturated hydrocarbon group and / or unsaturated hydrocarbon group, an aromatic hydrocarbon group such as an alkylphenyl group or an alkylbenzyl group, or a polycyclic aromatic hydrocarbon such as a naphthyl group. Furthermore, the hydrocarbon group may have a reactive functional group such as a hydroxy group, an amino group, or a glycidyl group, and may have an ether bond, an ester bond, a urethane bond, or an amide bond. The number of carbon atoms in the hydrocarbon group is preferably 1 to 30, more preferably 1 to 25, and even more preferably 1 to 20. When the hydrocarbon group has 1 to 30 carbon atoms, a balance between hydrophobicity and hydrophilicity is appropriate, and a balance is achieved between adhesion between hydrogel particles and the grinding energy during gel grinding, thereby better achieving the desired effects of the present invention. R １ , R ２ , R ３ , R ４ may be different or the same.

[0197] X －is a counter anion of the ammonium cation, such as a halogen ion, a carboxylate ion, a sulfonate ion, a hydroxyl ion, or BF ４－ , P.F. ６－ , ClO ４－ , AsF ６－ , SbF ６－ Examples include:

[0198] (j) Ammonium salts are readily available on the market, and preferred examples thereof include the following products:

[0199] Manufactured by Kao Corporation: Coconutamine acetate Acetamine (registered trademark, the same applies hereinafter) 24, Stearylamine acetate Acetamine 86, Lauryltrimethylammonium chloride Courtamine (registered trademark, the same applies hereinafter) 24P, Stearyltrimethylammonium chloride Courtamine 86W, Cetyltrimethylammonium chloride Courtamine 60W, Distearyldimethylammonium chloride Courtamine D86P, Alkylbenzyldimethylammonium chloride Sanizol (registered trademark, the same applies hereinafter) C, Sanizol B-50 Manufactured by NOF Corporation: Tetradecylamine acetate Nissan Cation (registered trademark, the same applies hereinafter) MA, Dodecyltrimethylammonium chloride Nissan Cation BB, Coconut alkyltrimethylammonium chloride Nissan Cation FB, Hexadecyltrimethylammonium chloride Nissan Cation PB-300, Beef tallow alkyltrimethylammonium chloride Nissan Cation ABT2-500 - Octadecyltrimethylammonium chloride Nissan Cation AB, Nissan Cation AB-600 - Behenyltrimethylammonium chloride Nissan Cation VB-M Flake, Nissan Cation VB-F - Didecyldimethylammonium chloride Nissan Cation 2-DB-500E - Dioleyldimethylammonium chloride Nissan Cation 2-OLR - Coconut alkyldimethylbenzylammonium chloride Nissan Cation F2-50R - Tetradecyldimethylbenzylammonium chloride Nissan Cation M2-100R.

[0200] The polymerization additive used in the present application preferably has a hydrophilic unit (a cationic group such as a quaternary ammonium salt, a sulfonate, a carboxylate, an amine (amino group), or a polyethylene glycol chain) and a hydrophobic unit (a hydrocarbon group) in the same compound. As the hydrophilic unit, a quaternary ammonium salt, a sulfonate, a carboxylate, or an amino group is particularly preferred, a quaternary ammonium salt, a carboxylate, or an amino group is more preferred, and a polymerizable additive having two or more groups selected from the group consisting of a quaternary ammonium salt and a carboxylate is even more preferred.

[0201] The effect of using the polymer additive is to adjust the particle size of the hydrogel and the degree of particle-particle bonding (especially the degree of particle-particle bonding). The hydrophilic unit of the polymer additive is thought to interact with the inside and / or surface of the hydrophilic particulate water-absorbing agent, making it difficult for the polymer additive to be eluted from the particulate water-absorbing agent. Therefore, it is thought that the polymer additive exists inside the particulate water-absorbing agent, inhibiting the hydrogel cross-linked copolymer from strongly re-bonding with itself during the gel crushing process, thereby maintaining an appropriate particle size.

[0202] Due to its hydrophobic properties, the hydrophobic unit of the polymer additive is thought to have the effect of controlling the adhesion of the crushed gel particles to each other on the surface of the hydrogel particles after gel crushing, and guiding the shape of the particulate water-absorbing agent of the present invention (particle diameter, and proportion of cavities and voids).

[0203] The molecular weight (mass average molecular weight) of the polymerization additive is not particularly limited, but in order to achieve the desired effect with a smaller amount added and to avoid adverse effects such as a decrease in water absorption capacity, the molecular weight (hereinafter, when the polymerization additive is a polymer, the mass average molecular weight) is preferably in the range of 100 to 500,000, more preferably 150 to 100,000, even more preferably 180 to 10,000, particularly preferably 200 to 5,000, and most preferably 250 to 2,000. According to one embodiment, the molecular weight (mass average molecular weight) of the polymerization additive may be 250 to 1,000, 250 to 800, or 250 to 500.

[0204] If the molecular weight is below the above range, the compound tends to volatilize and the effect is reduced, which is not preferred, whereas if the molecular weight is above the above range, the compound tends to have a high viscosity and the compound is not added uniformly, which is not preferred.

[0205] The mass average molecular weight of the polymerization additive may be a value calculated from the atomic weight of each constituent atom, or may be a value measured by size exclusion chromatography (GPC) in terms of polyethylene glycol.

[0206] When the weight average molecular weight is calculated by size exclusion chromatography (GPC) in terms of polyethylene glycol, the measurement is carried out, for example, under the following measurement conditions.

[0207] Measurement conditions Apparatus: Waters Alliance (2695) Analysis software: Waters Empower Professional + GPC option Column used: Tosoh TSK guard column SWXL + TSKgel G4000SWXL + G3000SWXL + G2000SWXL Detector: Differential refractometer (RI) detector (Waters 2414) Eluent: A solution prepared by dissolving 115.6 g of sodium acetate trihydrate in a mixed solvent of 10,999 g of water and 6,001 g of acetonitrile, and further adjusting the pH to 6.0 with acetic acid. Standard substance for preparing calibration curve: polyethylene glycol [peak top molecular weight (Mp) 300,000, 200,000, 107,000, 50,000, 27,700, 11,840, 6,450, 1,470, 472]. Calibration curve: prepared using a cubic equation based on the Mp value and elution time of the above polyethylene glycol. Flow rate: 1.0 mL / min. Column temperature: 40°C. Measurement time: 45 minutes. Sample solution injection volume: 100 μL (eluent preparation solution with a sample concentration of 0.5 wt %).

[0208] [3-1-3] Internal Crosslinking Agent In the method for producing a particulate water-absorbing agent according to the present invention, it is preferable to use an internal crosslinking agent in the polymerization. The internal crosslinking agent may be added during or after the step of preparing the aqueous solution of a (meth)acrylic acid (salt)-based monomer, and specifically, it is added before step (ii). By using the internal crosslinking agent, a crosslinked structure is introduced into the polymer. The internal crosslinking agent may be added to the aqueous solution of a (meth)acrylic acid (salt)-based monomer all at once or in portions.

[0209] As the internal crosslinking agent, known agents can be used, and examples thereof include N,N'-methylenebis(meth)acrylamide, (poly)ethylene glycol di(meth)acrylate, (poly)propylene glycol di(meth)acrylate, trimethylolpropane tri(meth)acrylate, glycerin tri(meth)acrylate, glycerin acrylate methacrylate, ethylene oxide-modified trimethylolpropane tri(meth)acrylate, pentaerythritol hexa(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, and glycidyl (meth)acrylate. Of these, one or more can be used in consideration of reactivity. Among them, it is preferable to use a compound having two or more polymerizable unsaturated groups, it is more preferable to use a compound having a (poly)alkylene structural unit in the molecule, and it is even more preferable to use a compound having a polyethylene glycol structural unit.

[0210] In the method for producing a particulate water-absorbing agent according to the present invention, the internal crosslinking agent is preferably used in an amount of 0.070 mol% or more and 0.100 mol% or less relative to 100 mol% of the (meth)acrylic acid (salt)-based monomer. When the amount of the internal crosslinking agent used is 0.070 mol% or more, the AAP and SFC values ​​of the obtained particulate water-absorbing agent are also good, and the liquid uptake rate is improved. On the other hand, when the amount of the internal crosslinking agent used is 0.100 mol% or less, the crosslinking density of the obtained particulate water-absorbing agent is appropriate, and the particle diameter and the proportion of cavities and voids in the particulate water-absorbing agent can be adjusted to more suitable ranges, and the liquid uptake rate of the absorbent body is improved.

[0211] In the method for producing a particulate water absorbing agent according to the present invention, the amount of the internal crosslinking agent relative to 100 mol % of the (meth)acrylic acid (salt)-based monomer is preferably 0.072 mol % or more and 0.100 mol % or less, more preferably 0.075 mol % or more and 0.100 mol % or less, even more preferably 0.078 mol % or more and 0.100 mol % or less, particularly preferably 0.080 mol % or more and 0.100 mol % or less, and most preferably 0.082 mol % or more and 0.099 mol % or less. When the amount of the internal crosslinking agent used is within the above range, the particulate water absorbing agent can achieve desired physical properties (for example, a total volume fraction of cavities in the particulate water absorbing agent of 18 vol % or more), and a particulate water absorbing agent excellent in liquid absorption speed can be obtained.

[0212] (Substances to be added to aqueous monomer solution) In the present invention, from the viewpoint of improving the physical properties of the particulate water-absorbing agent to be obtained, the following substances can be added to the aqueous monomer solution at one or more points of when the aqueous monomer solution is prepared, during the polymerization reaction and the crosslinking reaction, or after the polymerization reaction and the crosslinking reaction.

[0213] Specific examples of such substances include hydrophilic polymers such as starch, starch derivatives, cellulose, cellulose derivatives, polyvinyl alcohol (PVA), polyacrylic acid (salts), and crosslinked polyacrylic acid (salts); and compounds such as carbonates, azo compounds, various types of foam-generating blowing agents, chelating agents, and chain transfer agents.

[0214] [3-2] Polymerization step (step (ii)) Examples of polymerization methods for obtaining the hydrogel of the present invention include spray polymerization, droplet polymerization, bulk polymerization, precipitation polymerization, aqueous solution polymerization, and reversed-phase suspension polymerization. To solve the problems of the present invention, aqueous solution polymerization or reversed-phase suspension polymerization in which a monomer is dissolved in an aqueous solution is preferred, more preferably aqueous solution polymerization, and even more preferably continuous aqueous solution polymerization is selected.

[0215] The aqueous solution polymerization is a method of polymerizing an aqueous monomer solution without using a dispersion solvent, and is disclosed, for example, in U.S. Pat. Nos. 4,625,001, 4,873,299, 4,286,082, 4,973,632, 4,985,518, 5,124,416, 5,250,640, 5,264,495, 5,145,906, 5,380,808, European Patent Nos. 0,811,636, 0,955,086, and 0,922,717.

[0216] Further, continuous aqueous solution polymerization can produce water-absorbing resins with high productivity, U.S. Patent No. 4,893,999, U.S. Patent No. 6,241,928, U.S. Patent No. 6,906,159, U.S. Patent No. 7,091,253, U.S. Patent No. 7,741,400, U.S. Patent No. 8,519,212, U.S. Patent Application Publication No. 2005 / 215,734, continuous belt polymerization described in JP-A-2005-36100, U.S. Patent No. 6,987,151, U.S. Patent No. 6,710,141, etc., continuous kneader polymerization described in U.S. Patent No. 6,987,151, U.S. Patent No. 6,710,141, etc., batch kneader polymerization described in U.S. Patent No. 6,710,141, etc. Among these aqueous solution polymerizations, the method described in JP-A-2002-212,204, by setting the polymerization initiation temperature at a high temperature of 50 ° C. or more, can shorten the polymerization process. Furthermore, by utilizing the latent heat of vaporization of water, the maximum temperature can be suppressed, and as a result, a water-absorbing resin having a high main chain molecular weight and a narrow molecular weight distribution can be obtained, which is particularly preferable.

[0217] Preferred forms of the continuous aqueous solution polymerization include high-temperature initiation polymerization, high-concentration polymerization, and foam polymerization. The term "high-temperature initiation polymerization" refers to a polymerization form in which the temperature of the aqueous monomer solution at the start of polymerization is preferably 35°C or higher, more preferably 40°C or higher, even more preferably 45°C or higher, and particularly preferably 50°C or higher, and is preferably below the boiling point of the aqueous monomer solution. The term "high-concentration polymerization" refers to a polymerization form in which the monomer concentration at the start of polymerization is preferably 30% by mass or higher, more preferably 35% by mass or higher, even more preferably 40% by mass or higher, and particularly preferably 45% by mass or higher, and is preferably below the saturated concentration of the aqueous monomer solution. The term "foam polymerization" refers to a polymerization form in which an aqueous monomer solution containing a foaming agent or bubbles is polymerized. These polymerization forms may be carried out alone or in combination of two or more.

[0218] Examples of a method for dispersing bubbles in foaming polymerization include a method for dispersing a gas dissolved in a monomer aqueous solution as bubbles by reducing the solubility, a method for introducing a gas from the outside and dispersing it as bubbles, a method for adding a foaming agent to a monomer aqueous solution to foam it, etc. Furthermore, the dispersion methods may be used in combination as appropriate depending on the physical properties of the target particulate water-absorbing agent.

[0219] When a gas is introduced from the outside, examples of the gas include oxygen, air, nitrogen, carbon dioxide, ozone, etc., and a mixture of these gases. From the viewpoints of polymerizability and cost, an inert gas such as nitrogen or carbon dioxide is preferably used, and nitrogen is more preferably used.

[0220] Examples of usable foaming agents include azo compounds, organic or inorganic carbonate solutions, dispersions, and powders with particle sizes of 0.1 μm or more and 1000 μm or less. Among these, inorganic carbonates are preferred, and specifically, carbonates and hydrogencarbonates such as sodium carbonate, ammonium carbonate, and magnesium carbonate can be used.

[0221] Drying of the foamed hydrogel obtained by foaming polymerization is facilitated by gel crushing. Furthermore, by forming the particulate water-absorbing agent into a foamed form, the water absorption rate can be improved and furthermore, immobilization to an absorbent article becomes easy. The foamed form can be determined by observing pores on the surface of the particulate water-absorbing agent using an electron microscope, for example, pores with a diameter of 1 μm or more and 100 μm or less. The number of pores is preferably 1 or more, more preferably 10 or more, and preferably 10,000 or less, more preferably 1,000 or less per particle of the particulate water-absorbing agent, and can be controlled by foaming polymerization.

[0222] Furthermore, the reversed-phase suspension polymerization is a method of suspending an aqueous monomer solution in a hydrophobic organic solvent and polymerizing it, and is disclosed, for example, in U.S. Pat. Nos. 4,093,776, 4,367,323, 4,446,261, 4,683,274, and 5,244,735. The monomers, polymerization initiators, etc. disclosed in these patent documents can also be applied to the present invention. The concentration of the aqueous monomer solution during the polymerization is not particularly limited, but is preferably 20% by mass or more and less than the saturated concentration, more preferably 25% by mass or more and less than 80% by mass, and even more preferably 30% by mass or more and less than 70% by mass. A concentration of 20% by mass or more is preferable because high productivity can be achieved. Note that polymerization in a monomer slurry (aqueous dispersion of (meth)acrylate salt) is observed to result in a decrease in physical properties, so it is preferable to carry out the polymerization at a concentration below the saturated concentration (see JP-A-1-318021).

[0223] The polymerization step in the present invention can be carried out under normal pressure, reduced pressure, or increased pressure, but is preferably carried out at normal pressure (or near normal pressure, usually ±10 mmHg). Furthermore, in order to promote polymerization and improve physical properties, a degassing step of dissolved oxygen (e.g., a substitution step with an inert gas) may be carried out during polymerization, if necessary. The temperature at the start of polymerization varies depending on the type of polymerization initiator used, but is preferably 15°C or higher and 130°C or lower, more preferably 20°C or higher and 120°C or lower.

[0224] (Polymerization initiator) The polymerization initiator used in the present invention is appropriately determined depending on the polymerization form and is not particularly limited, but examples thereof include photodegradable polymerization initiators, thermally degradable polymerization initiators, redox-based polymerization initiators, etc. These polymerization initiators initiate the polymerization of the present invention.

[0225] Examples of the photodecomposition type polymerization initiator include benzoin derivatives, benzyl derivatives, acetophenone derivatives, benzophenone derivatives, and azo compounds.

[0226] Examples of the thermally decomposable polymerization initiator include persulfates such as sodium persulfate, potassium persulfate, and ammonium persulfate; peroxides such as hydrogen peroxide, t-butyl peroxide, and methyl ethyl ketone peroxide; and azo compounds such as 2,2′-azobis(2-amidinopropane)dihydrochloride and 2,2′-azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride.

[0227] Furthermore, examples of the redox-based polymerization initiator include a system in which the persulfate or peroxide is used in combination with a reducing compound such as L-ascorbic acid or sodium hydrogen sulfite. It is also a preferred embodiment to use the photodecomposition type polymerization initiator in combination with a thermal decomposition type polymerization initiator. Among these polymerization initiators, the one that can be thermally decomposed to form N ２ Foaming may be promoted by using an azo-based polymerization initiator that generates: Furthermore, active energy rays such as ultraviolet rays, electron beams, and γ rays may be used alone or in combination with the above polymerization initiators.

[0228] The amount of the polymerization initiator used is preferably 0.0001 mol% or more and 1 mol% or less, and more preferably 0.0005 mol% or more and 0.5 mol% or less, relative to 100 mol% of the monomer. When the amount used is 1 mol% or less, deterioration of color tone of the particulate water-absorbing agent is suppressed, which is preferable, and when the amount used is 0.5 mol% or less, deterioration of color tone is further suppressed, which is more preferable. Furthermore, when the amount used is 0.0001 mol% or more, increase of the remaining monomer is suppressed, which is preferable, and when the amount used is 0.0005 mol% or more, increase of the remaining monomer is further suppressed, which is more preferable.

[0229] (Polymerization Additives) In order to achieve the object of the present invention, polymerization is carried out in the presence of the polymerization additives described in detail in [3-1-2]. The polymerization additives have been described in [3-1-2] and will not be described here.

[0230] (Internal Crosslinking Agent) According to a preferred embodiment of the present invention, polymerization is carried out in the presence of the internal crosslinking agent described in detail in [3-1-3]. The internal crosslinking agent has been described in [3-1-3] and will not be described here.

[0231] (More Preferred Polymerization Method) In the present invention, as a method for polymerizing an aqueous solution of a (meth)acrylic acid (salt)-based monomer, aqueous solution polymerization is adopted from the viewpoint of the physical properties (e.g., water absorption rate and liquid permeability) of the particulate water-absorbing agent, ease of polymerization control, etc. Among them, kneader polymerization or belt polymerization is preferred, and continuous aqueous solution polymerization is more preferably adopted. Furthermore, in the continuous aqueous solution polymerization, it is more preferred to carry out high-temperature initiated polymerization, high-concentration polymerization, and foaming polymerization.

[0232] As an example of a preferred embodiment of the aqueous solution polymerization, the monomer concentration in the aqueous (meth)acrylic acid (salt)-based monomer solution is preferably 10% by mass or more and 80% by mass or less, more preferably 20% by mass or more and 60% by mass or less, and even more preferably 30% by mass or more and 50% by mass or less.

[0233] Within these ranges, the load on the machine in the subsequent gel crushing step will not be excessively high, and the mass-average particle size of the hydrogel particles can be efficiently controlled within a desired range.

[0234] Furthermore, during the polymerization, if necessary, a chain transfer agent such as hypophosphorous acid (salt) or a chelating agent such as trisodium diethylenetriaminepentaacetate or pentasodium diethylenetriaminepentaacetate may be added to the reaction system before or during the polymerization in an amount preferably greater than 0% by mass and not more than 3% by mass, more preferably 0.001% by mass or more and not more than 1% by mass, relative to 100% by mass of the monomer.

[0235] [3-3] Gel Crushing Step (Step (iii)) This step is carried out during and / or after the polymerization step, and is a step of obtaining a particulate hydrogel (hereinafter, sometimes referred to as "hydrogel particles") by gel-crushing the hydrogel. According to a preferred embodiment of the present invention, the gel-crushing step is carried out after the polymerization step. By gel-crushing the hydrogel, particularly gel-crushing by kneading, the hydrogel particles are refined to a specific particle size range, thereby improving the CRC, AAP, and vortex. Note that this step is referred to as "gel crushing" to distinguish it from the "crushing" in the crushing step described below.

[0236] Here, from the viewpoint of ease of gel crushing and drying of the hydrogel particles after gel crushing, the solid content of the hydrogel is preferably 10% by mass or more and 80% by mass or less, more preferably 20% by mass or more and 70% by mass or less, even more preferably 20% by mass or more and 60% by mass or less, particularly preferably 30% by mass or more and 55% by mass or less, and most preferably 35% by mass or more and 50% by mass or less.

[0237] If the solid content is lower than 10% by mass, the moisture content of the hydrogel particles after pulverization will be high, which will increase the energy required for drying, resulting in increased costs, and the drying time will be longer, resulting in reduced production efficiency.If the solid content exceeds 80% by mass, the gel will become hard, which will increase the load on the pulverizer and may cause the pulverizer to break.

[0238] There are no particular limitations on the method for adjusting the solid content of the hydrogel to 10% by mass or more and 80% by mass or less, and it may be achieved, for example, by restricting the total amount of monomers during polymerization.

[0239] (Particle diameter D50 of hydrogel particles) The "mass average particle diameter (D50) of the hydrogel particles" after this gel crushing step is, for example, 150 μm or more and 500 μm or less, preferably 150 μm or more and 400 μm or less, more preferably 150 μm or more and 380 μm or less, even more preferably 160 μm or more and 370 μm or less, particularly preferably 160 μm or more and 350 μm or less, and may be 160 μm or more and 340 μm or less, 160 μm or more and 320 μm or less, 170 μm or more and 300 μm or less, or 170 μm or more and 280 μm or less. Here, the particle diameter D50 of the hydrogel particles corresponds to D50 (16 hr) described later.

[0240] The present inventors believe that a hydrogel having an appropriate hardness can be obtained by the production method of the present invention during polymerization (i.e., a specific polymerization additive), and that by gel-crushing the hydrogel with an appropriate force (i.e., a gel-crushing energy (GGE)(2) of 30 J / g or more), the hydrogel particles obtained by gel-crushing have a granulated state, and as a result, the shape (particle size and proportion of cavities and voids) of the particulate water-absorbing agent of the present invention can be achieved. In other words, it is believed that the hardness of the hydrogel and the granulation strength of the hydrogel particles after pulverization are adjusted by the specific polymerization additive (preferably a specific amount of an internal cross-linking agent and a specific polymerization additive) and appropriate intensity of gel-crushing, and that a particulate water-absorbing agent having a specific particle size and a specific proportion of cavities and voids can be obtained.

[0241] According to the production method of the present invention, for the hydrogel particles obtained after gel pulverization in step (iii), 50 parts by mass of the hydrogel particles are immersed in 1,000 parts by mass of an aqueous solution containing 0.02% by mass of sodium polyoxyethylene (3) lauryl ether sulfate and 20% by mass of sodium chloride, and the mass-average particle size of the hydrogel particles after 1 hour of immersion is defined as D50(1 hr) (μm), and the mass-average particle size of the hydrogel particles after 16 hours of immersion is defined as D50(16 hr) (μm). The ratio of D50(1 hr) to D50(16 hr) is 0.20 or more and 0.85 or less, and D50(1 hr) (μm) is 350 μm or more. The hydrogel particles obtained by the production method of the present invention maintain relatively large agglomerates when the immersion time is short (e.g., 1 hour), but become sufficiently detackified and reduced in size after a certain period of immersion (e.g., 16 hours). That is, D50(16hr)(μm) is regarded as D50 of the hydrogel particles, but D50(1hr)(μm) is an index of the ease of collapse of the hydrogel granulated particles. That is, when D50(1hr)(μm) is a value equivalent to D50(16hr)(μm), it means that the granulation state of the hydrogel particles is not appropriate, and it becomes difficult to obtain a particulate water-absorbing agent in which the desired total volume ratio of cavities and the total volume ratio of voids are present at specific ratios.

[0242] In the present invention, as shown in the Examples, the hydrogel particles obtained by the production method of the present invention maintain relatively large masses when the immersion time in water is short (for example, 1 hour), but after a certain period of immersion in water (for example, 16 hours), the adhesiveness is sufficiently released and the particles are reduced to small particle sizes.

[0243] In the present invention, the ratio of D50(1 hr) to D50(16 hr) (D50(16 hr) / D50(1 hr)) is 0.20 or more and 0.85 or less. When D50(16 hr) / D50(1 hr) is less than 0.20, the hydrogel particles are pulverized in a state where the frictional force on the surface is large, and the proportion of cavities and voids in the obtained particulate water-absorbing agent is not suitable, and the liquid uptake efficiency of the particulate water-absorbing agent is deteriorated, and the uptake speed is deteriorated. When D50(16 hr) / D50(1 hr) is more than 0.85, the frictional force on the surface of the hydrogel particles is too small, and the granulation properties of the hydrogel granulated particles are changed, and the proportion of cavities and voids in the particulate water-absorbing agent is not suitable, and the liquid uptake efficiency of the particulate water-absorbing agent is deteriorated, and the uptake speed is deteriorated.

[0244] In the present invention, D50(16hr) (μm) corresponds to the "mass average particle diameter (D50) of hydrogel particles" as described above. Therefore, the preferred range of D50(16hr) is the same as the "mass average particle diameter (D50) of hydrogel particles." Specifically, D50(16hr) is, for example, 150 μm or more and 500 μm or less, preferably 150 μm or more and 400 μm or less, more preferably 150 μm or more and 380 μm or less, even more preferably 160 μm or more and 370 μm or less, particularly preferably 160 μm or more and 350 μm or less, and may be 160 μm or more and 340 μm or less, 160 μm or more and 320 μm or less, 170 μm or more and 300 μm or less, or 170 μm or more and 280 μm or less.

[0245] In the present invention, D50 (1 hr) (μm) is 350 μm or more. When D50 (1 hr) is less than 350 μm, it is considered that excessive pulverization is performed by gel pulverization, and the hydrogel particles (granulated particles) continue to be disintegrated in the subsequent steps, resulting in a particulate water-absorbing agent having a small particle size, and the presence ratio of cavities and voids in the particulate water-absorbing agent is not suitable, which deteriorates the liquid uptake efficiency of the particulate water-absorbing agent and deteriorates the liquid uptake speed. D50 (1 hr) is preferably 350 μm or more and 900 μm or less, more preferably more than 350 μm and 850 μm or less, even more preferably 360 μm or more and 800 μm or less, particularly preferably 370 μm or more and 750 μm or less, and may be 380 μm or more and 700 μm or less, 400 μm or more and 650 μm or less, 420 μm or more and 620 μm or less, or 430 μm or more and 550 μm or less.

[0246] In the present invention, D50(16 hr) / D50(1 hr) is preferably 0.25 or more and 0.82 or less, more preferably 0.30 or more and 0.80 or less, even more preferably 0.35 or more and 0.78 or less, particularly preferably 0.40 or more and 0.75 or less, and most preferably 0.45 or more and 0.70 or less.

[0247] (Gel Crushing Apparatus) In the present invention, it is important that the mass average particle size of the hydrogel particles is controlled within the above range, and any means for achieving this may be used, but examples thereof include a batch or continuous double-arm kneader or other gel crusher equipped with multiple rotating stirring blades, a single-screw extruder, a twin-screw extruder, a meat chopper, etc. Among these, a screw-type extruder having a perforated plate at the tip (for example, a meat chopper) is preferred, and an example thereof includes the screw-type extruder disclosed in JP 2000-063527 A.

[0248] Among these, the gel crushing device used in this step is more preferably a screw-type extruder, and even more preferably a screw-type extruder with a perforated plate installed at one end of the casing (e.g., a meat chopper), and examples thereof include the screw-type extruders disclosed in JP 2000-63527 A, WO 2015 / 030129 A, and WO 2015 / 030130 A. An example of a screw-type extruder used in this step will be described below. A similar device is also used in this example.

[0249] The screw extruder used in this process is composed of, for example, a casing, a base, a screw, a feed port, a hopper, an extrusion port, a perforated plate, a rotary blade, a ring, a backflow prevention member, a motor, streaky protrusions, etc. The casing is cylindrical, and a screw is disposed therein. One end of the casing has an extrusion port through which the hydrogel is extruded and crushed, and a perforated plate is disposed in front of it, while the other end has a motor and a drive system for rotating the screw. A base is provided below the casing, allowing the screw extruder to be installed stably. Meanwhile, the upper part of the casing has a feed port through which the hydrogel is supplied, and a hopper is provided to facilitate the supply of the hydrogel. The shape and size of the casing are not particularly limited as long as it has a cylindrical inner surface corresponding to the shape of the screw. The screw rotation speed is not particularly limited because it varies depending on the shape of the screw extruder, but it is preferable that the screw rotation speed be variable, as described below. A backflow prevention member, streaky protrusions disposed on the screw, etc. may also be provided near the extrusion port. These configurations, materials of the members, sizes, materials of the various rotary blades attached to the return members and screws, and all other configurations related to the screw extruder can be selected in accordance with the methods disclosed in JP 2000-63527 A, WO 2015 / 030129 and WO 2015 / 030130.

[0250] For example, the backflow prevention member is not particularly limited as long as it has a structure that can prevent the hydrogel from returning near the extrusion port, and examples thereof include spiral or concentric band-like protrusions provided on the inner wall of the casing, or streak-like, granular, spherical, or angular protrusions provided parallel to the screw. When the pressure near the extrusion port increases as gel crushing progresses, the hydrogel tries to return toward the supply port, but by providing a backflow prevention member, the hydrogel can be crushed while preventing backflow.

[0251] In the present invention, in order to adjust the energy for pulverizing the hydrogel, the hole diameter (die diameter) of the perforated plate is preferably in the range of 1.0 mm or more and 50 mm or less, more preferably 2.0 mm or more and 30 mm or less, and may be 2.0 mm or more and 5.0 mm or less.

[0252] The rate of addition of the hydrogel is preferably 0.10 kg / min or more and 550 kg / min or less, and more preferably 0.12 kg / min or more and 500 kg / min or less.

[0253] When the feeding speed of the hydrogel is within the above range, the hydrogel can be crushed into hydrogel particles having a desired particle size, and the ratio of cavities and voids in the obtained particulate water absorbing agent can be suitably adjusted.

[0254] Furthermore, as described above, according to the present invention, the particle size after gel crushing is significantly small. There are no particular limitations on the method for achieving this, and it may be achieved by setting the gel crushing energy to an appropriate value.

[0255] The gel crushing energy (GGE(1)) is preferably 10 J / g or more and 300 J / g or less, more preferably 15 J / g or more and 250 J / g or less, even more preferably 20 J / g or more and 200 J / g or less, particularly preferably 45 J / g or more and 180 J / g or less, and most preferably 50 J / g or more and 150 J / g or less.

[0256] When the gel crushing energy (GGE(1)) is within the above range, the hydrogel can be crushed into hydrogel particles having a desired particle size, and the ratio of cavities and voids in the obtained particulate water-absorbing agent can be suitably adjusted.

[0257] The gel crushing energy (GGE(2)) is 30 J / g or more. If the gel crushing energy (GGE(2)) is less than 30 J / g, the hydrogel may not be crushed into hydrogel particles having the desired particle size. The gel crushing energy (GGE(2)) is preferably 32 J / g or more and 200 J / g or less, more preferably 35 J / g or more and 180 J / g or less, even more preferably 36 J / g or more and 150 J / g or less, particularly preferably 37 J / g or more and 100 J / g or less, and most preferably 37 J / g or more and 70 J / g or less.

[0258] When the gel crushing energy (GGE(2)) is within the above range, the hydrogel can be crushed into hydrogel particles having a desired particle size, and the ratio of cavities and voids in the obtained particulate water-absorbing agent can be suitably adjusted.

[0259] GGE Calculation Method "Gel Grinding Energy" (GGE (1), GGE (2)) In the present invention, "gel grinding energy" refers to the mechanical energy per unit mass (unit mass of hydrous gel) required by the gel grinding device when grinding hydrous gel, and does not include the energy for heating and cooling the jacket or the energy of the water and steam introduced. Note that "gel grinding energy" is abbreviated as "GGE (1)" from the English term "Gel Grinding Energy." When the gel grinding device is driven by three-phase AC power, GGE is calculated by the following (Equation 1).

[0260]

[0261] The above "power factor" and "motor efficiency" are values ​​specific to the device that change depending on the operating conditions of the gel grinding device, and take values ​​between 0 and 1. These values ​​can be obtained by contacting the device manufacturer, etc. When the gel grinding device is driven by single-phase AC power, GGE can be calculated by changing "√3" in the above formula to "1." The unit of voltage is [V], the unit of current is [A], and the unit of hydrogel mass is [g / s].

[0262] In the present invention, since the mechanical energy applied to the hydrogel is important, it is preferable to calculate the gel crushing energy by subtracting the current value when the gel crushing device is running idle. In particular, when gel crushing is performed using multiple devices, the total current value during idle operation becomes large, so a method of calculating by subtracting the current value during idle operation is preferable. In this case, the gel crushing energy is calculated using the following (Equation 2). In addition, to distinguish it from the above GGE (1), it is denoted as GGE (2).

[0263]

[0264] According to a preferred embodiment of the present invention, a hydrogel is crushed to obtain hydrogel particles, the hydrogel particles are dried to obtain a dried product, and then the dried product is "crushed." Since "crushing" is performed in this manner in a later step, it is thought that there is no need to control the particle size after gel crushing by setting the gel crushing energy to a predetermined value or more.

[0265] In contrast to this, the present invention employs a configuration in which a hydrogel cross-linked polymer obtained by polymerization in the presence of a predetermined amount of an internal cross-linking agent and a predetermined polymerization additive is crushed with a gel crushing energy of a predetermined value or more, thereby controlling the particle size after gel crushing, thereby making it possible to provide a poly(meth)acrylic acid (salt)-based particulate water-absorbing agent having an excellent liquid uptake rate and a method for producing the same.

[0266] (Operating conditions of gel crushing device) When the gel crushing device used in the gel crushing step of the present invention is a screw extruder (e.g., a meat chopper), the screw shaft rotation speed of the screw extruder may be appropriately adjusted depending on the inner diameter of the casing, the outer diameter of the screw shaft, etc., and the shaft rotation speed is preferably 80 rpm or more and 500 rpm or less, more preferably 90 rpm or more and 400 rpm or less, and even more preferably 100 rpm or more and 300 rpm or less.

[0267] The inner diameter of the casing is preferably about 15 mm or more and 2500 mm or less, and more preferably about 25 mm or more and 1500 mm or less.

[0268] When the shaft rotation speed is 80 rpm or more, the shear and compression forces necessary for gel crushing are obtained, and when the shaft rotation speed is 500 rpm or less, the shear and compression forces applied to the hydrogel are not excessive, so that deterioration of physical properties is unlikely to occur, and the load on the gel crushing device is small, eliminating the risk of damage to the device.

[0269] The peripheral speed of the rotary blades at this time is preferably 0.5 m / s or more and 10 m / s or less, and more preferably 0.5 m / s or more and 8 m / s or less.

[0270] The temperature of the gel crusher in the present invention is preferably heated or maintained at 30° C. or higher and 120° C. or lower, more preferably 40° C. or higher and 100° C. or lower, in order to prevent adhesion of the hydrogel.

[0271] It is also preferable to set the temperature of the gel grinding device in the present invention within the gel temperature range described below.

[0272] (Number of times of gel crushing treatment) In the present invention, the number of times of gel crushing treatment is not particularly limited, and may be one time or multiple times, as long as the particle size after gel crushing is significantly small.

[0273] (Gel Temperature) From the viewpoint of particle size control and physical properties, the gel temperature, i.e., the temperature of the hydrogel before gel pulverization, is preferably from 40° C. to 120° C., more preferably from 50° C. to 120° C., even more preferably from 52° C. to 110° C., still more preferably from 48° C. to 80° C., and particularly preferably from 56° C. to 70° C. In addition, it may be from 65° C. to 110° C.

[0274] The gel temperature values ​​may also be applied to the temperature of the gel crusher.

[0275] When the gel temperature is within the above range, the hydrogel can be pulverized into hydrogel particles having a desired particle size, and the ratio of cavities and voids in the obtained particulate water-absorbing agent can be suitably adjusted.

[0276] (Gel CRC) The CRC of the hydrogel particles after gel pulverization is 25 g / g or more and 50 g / g or less, more preferably 26 g / g or more and 45 g / g or less, even more preferably 27 g / g or more and 40 g / g or less, particularly preferably 27 g / g or more and 38 g / g or less, and most preferably 27 g / g or more and 35 g / g or less. When the gel CRC is within the above range, it is preferable because it is easy to control the particle shape and particle size distribution during gel pulverization. Such gel CRC can be appropriately controlled by the amount of crosslinking agent added during polymerization, the polymerization concentration, etc. It is well known that water-absorbent resins with high CRC are preferred, but when the gel CRC is higher than the above range, it may be difficult to control the particle shape and particle size distribution.

[0277] The CRC of the hydrogel particles after gel crushing does not need to be cut or pulverized, and can be determined by the measurement method described in the Examples below.

[0278] (Use of Water) In the gel-crushing step of the present invention, water may be added to the hydrogel to crush the gel. In the present invention, the term "water" includes at least one of solid, liquid, and gas forms.

[0279] There are no limitations on the method or timing of adding the water, as long as water is supplied to the gel crusher while the hydrogel is retained therein. Alternatively, a hydrogel to which water has been added in advance may be introduced into the gel crusher. Furthermore, the water is not limited to "water alone," and other additives (e.g., surfactants, neutralizing bases, etc.) or solvents other than water may be added. However, in this case, the water content is preferably 90% by mass or more and 100% by mass or less, more preferably 99% by mass or more and 100% by mass or less, and even more preferably substantially 100% by mass.

[0280] In the present invention, the water can be used in at least one form of solid, liquid, and gas, but from the viewpoint of handleability, liquid and / or gas is preferred. The amount of water supplied is preferably 0.1 parts by mass or more and 30 parts by mass or less, more preferably 1 part by mass or more and 25 parts by mass or less, even more preferably 2 parts by mass or more and 20 parts by mass or less, particularly preferably 3 parts by mass or more and 25 parts by mass or less, and most preferably 4 parts by mass or more and 20 parts by mass or less, relative to 100 parts by mass of the hydrogel. When the amount of water supplied is within the above range, the degree of adhesion between particles can be adjusted and the particles can be crushed in a balanced manner by gel crushing, and the expected effects of the present invention can be further exerted.

[0281] When the water is supplied as a liquid, the temperature of the water when supplied is preferably 10°C or higher and 100°C or lower, more preferably 40°C or higher and 100°C or lower. Liquid water is added as needed by spraying, misting, showering, droplets, straight pipe, or the like. When water is supplied as a gas, the temperature of the water when supplied is preferably 100°C or higher and 220°C or lower, more preferably 100°C or higher and 160°C or lower, and even more preferably 100°C or higher and 130°C or lower. When water is supplied as a gas, the preparation method is not particularly limited, and examples include a method using steam generated by heating a boiler, and a method using gaseous water generated from the water surface by vibrating water with ultrasound. In the present invention, when water is supplied as a gas, steam at a pressure higher than atmospheric pressure is preferred, and steam generated in a boiler is more preferred.

[0282] In order to solve the problems of the present invention, it is preferable to employ aqueous solution polymerization rather than reverse-phase suspension polymerization, which does not require gel crushing, and it is particularly preferable to employ aqueous solution polymerization in which gel crushing is performed during polymerization (e.g., kneader polymerization) or after polymerization (e.g., belt polymerization, and further, kneader polymerization as necessary).

[0283] (More Suitable Gel-Crushing Method) According to a preferred embodiment of the present invention, the gel-crushing step preferably uses the gel-crushing method described in WO 2011 / 126079. Furthermore, by combining the gel-crushing method with the foaming polymerization described in the polymerization step, the desired effects of the present invention can be further exhibited.

[0284] [3-4] Drying step (step (iv)) This step is a step of drying the hydrogel or hydrogel particles obtained through the polymerization step or the like to obtain a dried product. When the polymerization step is aqueous solution polymerization, gel pulverization is performed before drying the hydrogel. Alternatively, the dried product obtained in the drying step may be supplied as is to the pulverization step.

[0285] In one embodiment of the present invention, a hydrogel or hydrogel particles are dried to a desired resin solid content to obtain a dried product. The resin solid content is a value determined from loss on drying (the change in mass when 1 g of a sample is heated at 180°C for 3 hours), and is preferably 80% by mass or more, more preferably 85% by mass to 99% by mass, even more preferably 90% by mass to 99% by mass, and particularly preferably 92% by mass to 99% by mass.

[0286] In one embodiment of the present invention, the drying method is a method that can dry the hydrogel or the hydrogel particles until they become the above-mentioned resin solid content. In one embodiment of the present invention, the drying method is not particularly limited, and various methods can be used. Specific examples include heat drying, hot air drying, reduced pressure drying, fluidized bed drying, infrared drying, microwave drying, drum dryer drying, azeotropic dehydration drying with a hydrophobic organic solvent, and high-humidity drying using high-temperature water vapor, and the like, and one or two of these can be used in combination. The drying step may be performed by one type of drying method. The drying step may be performed by two appropriately selected types of drying methods.

[0287] In one embodiment of the present invention, among the above-mentioned drying methods, hot air drying is preferred from the viewpoint of drying efficiency. In one embodiment of the present invention, band drying, in which hot air drying is performed on a belt, is more preferred. In one embodiment of the present invention, drying is preferably performed using hot air at a temperature of 120°C or higher and 250°C or lower, more preferably 140°C or higher and 220°C or lower, and even more preferably 160°C or higher and 200°C or lower. According to this embodiment, the drying time can be shortened.

[0288] In one embodiment of the present invention, the drying time is preferably 120 minutes or less, more preferably 80 minutes or less, and even more preferably 60 minutes or less. This embodiment has the technical effect of improving the initial color tone. In one embodiment of the present invention, the drying time is practically 1 minute or more, 3 minutes or more, or even 5 minutes or more.

[0289] In one embodiment of the present invention, the drying time until the solid content reaches 80% by mass is preferably 30 minutes or less, more preferably 20 minutes or less, and even more preferably 15 minutes or less. This embodiment has the technical effect of improving the chelating agent residual rate. In one embodiment of the present invention, the drying time until the solid content reaches 80% by mass is practically 1 minute or more, and 3 minutes or more.

[0290] In one embodiment of the present invention, when band drying is performed, the band drying conditions described in WO 2006 / 100300, WO 2011 / 025012, WO 2011 / 025013, WO 2011 / 111657, etc. are preferably applied.

[0291] In one embodiment of the present invention, the drying is carried out in a dryer selected from a static dryer and a band dryer, which has the technical effect of enabling drying in a short time even in mass production.

[0292] By drying in this drying step, the finely pulverized hydrogel particles adhere to each other, forming irregularly pulverized granulated particles.

[0293] In the present invention, granulation refers to forming particles larger than the original particles (primary particles) by adhering particles to each other by physical or chemical means, and the adhesion between the primary particles is loose or in point contact. The degree of adhesion of the granulated product or granulated particles of the present invention can be controlled by the amount and type of polymerization additive, and the AAP, SFC, vortex, etc. can be controlled.

[0294] [3-5] Pulverization and Classification Step (Step (v)) This step is a step in which the dried substance obtained in the drying step is pulverized and / or classified, to preferably obtain a water absorbent resin powder having a specific particle size. Note that this step differs from the above [3-3] Gel-pulverization step in that the substance to be pulverized has been subjected to a drying step.

[0295] This step is carried out before and / or after the surface cross-linking step [3-6], preferably before the surface cross-linking step [3-6], and may be carried out at least twice, before and after the surface cross-linking step [3-6].

[0296] Examples of equipment (pulverizers) that can be used in the pulverization step of the present invention include high-speed rotary pulverizers such as roll mills, hammer mills, screw mills, and pin mills, vibration mills, knuckle-type pulverizers, and cylindrical mixers, which can be used in combination as needed.

[0297]

[0111] In one embodiment of the present invention, a mass average particle diameter (D50) of a water absorbent resin powder is preferably 300 µm or more and 500 µm or less, more preferably 310 µm or more and 480 µm or less, and still more preferably 320 µm or more and 450 µm or less, from the viewpoint of handleability (particularly handleability under moisture absorption), AAP, SFC, vortex, and the like.

[0298]

[0133] In addition, the content of fine particles having a particle diameter of less than 150 µm defined by standard sieve classification is preferably as small as possible, and is preferably more than 0 mass % and 5 mass % or less, more preferably more than 0 mass % and 3 mass % or less, and further preferably more than 0 mass % and 2 mass % or less, with respect to the entire water absorbent resin powder.

[0299]

[0133] Furthermore, the content of coarse particles having a particle diameter of 850 µm or more defined by standard sieve classification is preferably as small as possible, and from the viewpoint of water absorption rate and the like, is preferably more than 0 mass % and 5 mass % or less, more preferably more than 0 mass % and 3 mass % or less, and further preferably more than 0 mass % and 1 mass % or less, based on the entire water absorbent resin powder.

[0300]

[0122] Furthermore, a mass proportion of particles having a particle diameter of 150 µm or more and less than 850 µm is preferably 90% by mass or more, more preferably 95% by mass or more, further preferably 98% by mass or more, and particularly preferably 99% by mass or more (upper limit is 100% by mass), with respect to the entire water absorbent resin powder, from the viewpoint of FGBP, water absorption rate, water absorption capacity under pressure, and the like.

[0301] The logarithmic standard deviation (σζ) of the particle size distribution is preferably 0.20 or more and 0.50 or less, more preferably 0.25 or more and 0.45 or less, and even more preferably 0.30 or more and 0.40 or less.

[0302] By adjusting the content of the fine particles having a particle size of less than 150 μm to a small amount within the above range, dust generation is reduced, the material becomes easier to handle, and AAP and SFC are improved.

[0303] The particle size can be controlled during polymerization, gel crushing, or crushing and classification after drying, but is preferably controlled during crushing and / or classification after drying. The particle size is measured using JIS standard sieves in accordance with the method specified in NWSP 220.0.R2(15).

[0304] In order to further solve the problems of the present invention, the above particle size can be applied to the water-absorbent resin particles after surface cross-linking, and also to the particulate water-absorbing agent which is the final product.

[0305] The fine particles generated by the above particle size control (for example, particles passing through a sieve with an opening of 150 μm) may be discarded, or may be recovered by a conventionally known recovery method into an aqueous monomer solution before polymerization (WO 92 / 001008, WO 92 / 020723) or a recovery method into a hydrogel during polymerization (WO 2007 / 074167, WO 2009 / 109563, WO 2009 / 153196, WO 2010 / 006937).

[0306] Furthermore, the shape of the water absorbent resin powder of the present invention is not particularly limited, and may be spherical, fibrous, rod-like, approximately spherical, flat, irregular, granulated particles, particles having a porous structure, etc.

[0307] [3-6] Surface Cross-Linking Step (Step (vi)) This step is more specifically comprised of a surface cross-linking agent addition step shown in [3-6-1] below and a heat treatment step shown in [3-6-2] below.

[0308] [3-6-1] Surface crosslinking agent adding step This step is a step of mixing the water absorbent resin powder with a surface crosslinking agent, thereby preparing a water absorbent resin powder containing a surface crosslinking agent to be subjected to a surface crosslinking step.

[0309]

[0113] In general, surface crosslinking is carried out by adding an organic surface crosslinking agent described later, polymerizing a monomer (polymerizable surface crosslinking agent) on the surface of a water absorbent resin powder, or adding a radical polymerization initiator (surface crosslinking agent in a broad sense) such as persulfate and heating / ultraviolet irradiation, etc. In the present invention, it is preferable to add an organic surface crosslinking agent to the water absorbent resin powder obtained above.

[0310] (Organic Surface Crosslinking Agent) As the organic surface crosslinking agent that can be used in the present invention, from the viewpoint of physical properties of the obtained particulate water-absorbing agent, an organic compound having a reactive group such as a hydroxy group and / or an amino group that undergoes a dehydration esterification reaction or a dehydration amidation reaction with a carboxyl group that is a functional group of the poly(meth)acrylic acid (salt)-based water-absorbing resin particle is preferred.

[0311] The organic compound is not limited to an alcohol compound or an amine compound that directly has a hydroxy group or an amino group, but also includes a compound that has a reactive group that generates a hydroxy group or an amino group and / or a reactive group that directly reacts with a carboxyl group, even if it is a cyclic compound such as an alkylene carbonate compound or an oxazolidinone compound.

[0312] Examples of the organic surface cross-linking agent include polyhydric alcohol compounds, epoxy compounds, polyamine compounds or condensates thereof with haloepoxy compounds, oxazoline compounds, (mono-, di-, or poly)oxazolidinone compounds, oxetane compounds, and alkylene carbonate compounds, and polyhydric alcohol compounds, alkylene carbonate compounds, and oxazolidinone compounds are more preferred.

[0313] Specific examples of the organic surface crosslinking agent include polyalcohol compounds (polyhydric alcohols) such as (di-, tri-, tetra-, poly)ethylene glycol, (di-, poly-)propylene glycol, 1,3-propanediol, 2,2,4-trimethyl-1,3-pentanediol, (poly)glycerin, 2-butene-1,4-diol, 1,4-butanediol, 1,3-butanediol, 1,5-pentanediol, 1,6-hexanediol, trimethylolpropane, di- or triethanolamine, pentaerythritol, and sorbitol; epoxy compounds such as (poly)ethylene glycol diglycidyl ether, (di-, poly-)glycerol polyglycidyl ether, and glycidol; oxazoline compounds such as 2-oxazolidone, N-hydroxyethyl-2-oxazolidone, and 1,2-ethylenebisoxazoline; Alkylene carbonate compounds such as 1,3-dioxolan-2-one (i.e., ethylene carbonate), 4-methyl-1,3-dioxolan-2-one, 4,5-dimethyl-1,3-dioxolan-2-one, 4,4-dimethyl-1,3-dioxolan-2-one, 4-ethyl-1,3-dioxolan-2-one, 4-hydroxymethyl-1,3-dioxolan-2-one, 1,3-dioxan-2-one, 4-methyl-1,3-dioxan-2-one, 4,6-dimethyl-1,3-dioxan-2-one, and 1,3-dioxopan-2-one; haloepoxy compounds such as epichlorohydrin, epibromohydrin, and α-methylepichlorohydrin, and polyamine adducts thereof (e.g., Hercules Kaimen; registered trademark); Examples of the silane coupling agent include γ-glycidoxypropyltrimethoxysilane and γ-aminopropyltriethoxysilane; oxetane compounds such as 3-methyl-3-oxetanemethanol, 3-ethyl-3-oxetanemethanol, 3-butyl-3-oxetanemethanol, 3-methyl-3-oxetaneethanol, 3-ethyl-3-oxetaneethanol, 3-butyl-3-oxetaneethanol, 3-chloromethyl-3-methyloxetane, 3-chloromethyl-3-ethyloxetane, and polyvalent oxetane compounds; and cyclic urea compounds such as 2-imidazolidinone.

[0314] Furthermore, it is preferably selected from polyhydric alcohol compounds, epoxy compounds, oxazoline compounds, and alkylene carbonate compounds, and two or more compounds may be used in combination.

[0315] (Solvent and Concentration) The total amount of the organic surface crosslinking agent to be added is preferably 0.001 parts by mass or more and 15 parts by mass or less, and more preferably 0.01 parts by mass or more and 5 parts by mass or less, relative to 100 parts by mass of the water absorbent resin powder before addition.

[0316]

[0044] The organic surface crosslinking agent is preferably added as an aqueous solution. The amount of water used in the aqueous solution is preferably 0.5 parts by mass or more and 20 parts by mass or less, more preferably 0.5 parts by mass or more and 10 parts by mass or less, in total relative to 100 parts by mass of the water absorbent resin powder before the addition treatment. Note that, the amount of water also includes water of crystallization and water of hydration of the surface crosslinking agent.

[0317]

[0063] Furthermore, a hydrophilic organic solvent may be added to an aqueous solution of an organic surface crosslinking agent, and in this case, the amount of the hydrophilic organic solvent is preferably more than 0 part by mass and not more than 10 parts by mass, and more preferably more than 0 part by mass and not more than 5 parts by mass, relative to 100 parts by mass of a water absorbent resin powder before addition treatment. Examples of the hydrophilic organic solvent include primary alcohols having 1 or more and 4 or less carbon atoms, further having 2 or more and 3 or less carbon atoms, and other lower ketones having 4 or less carbon atoms such as acetone, and in particular, volatile alcohols having a boiling point of less than 150°C, more preferably less than 100°C, are more preferred because they volatilize during surface crosslinking treatment and do not leave any residue.

[0318] Specific examples include lower alcohols such as methyl alcohol, ethyl alcohol, n-propyl alcohol, isopropyl alcohol, n-butyl alcohol, isobutyl alcohol, and t-butyl alcohol; ketones such as acetone; ethers such as dioxane, tetrahydrofuran, and methoxy(poly)ethylene glycol; amides such as ε-caprolactam and N,N-dimethylformamide; sulfoxides such as dimethyl sulfoxide; and polyhydric alcohols such as polyoxypropylene and oxyethylene-oxypropylene block copolymers.

[0319]

[0113] Furthermore, further, when mixing a surface crosslinking agent solution into a water absorbent resin powder, a water-insoluble fine particle or a surfactant can also be co-present in an amount of more than 0 part by mass and not more than 10 parts by mass, preferably more than 0 part by mass and not more than 5 parts by mass, more preferably more than 0 part by mass and not more than 1 part by mass, relative to 100 parts by mass of a water absorbent resin powder before addition treatment, within a range not impeding the effects of the present invention. In this case, the surfactant etc. to be used are disclosed in U.S. Patent No. 7,473,739 etc.

[0320] The concentration of the surface cross-linking agent in the aqueous solution of the surface cross-linking agent is determined appropriately, but is set to 1 mass % or more and 80 mass % or less, further 5 mass % or more and 60 mass % or less, 10 mass % or more and 40 mass % or less, or 15 mass % or more and 30 mass % or less in view of the physical properties of the particulate water-absorbing agent to be obtained, with the remainder containing a hydrophilic organic solvent and other components.

[0321] The temperature of the aqueous solution of an organic surface crosslinking agent is appropriately determined depending on the solubility of the organic surface crosslinking agent to be used, the viscosity of the aqueous solution, and the like, and is preferably −10° C. or higher and 100° C. or lower, more preferably 5° C. or higher and 70° C. or lower, still more preferably 10° C. or higher and 65° C. or lower, and particularly preferably in the range of 25° C. or higher and 50° C. or lower.

[0322] If the temperature is high, it is not preferable because the cyclic compound may be hydrolyzed (for example, decomposition from ethylene carbonate to ethylene glycol, decomposition from oxazolidinone to ethanolamine) before being mixed or reacted with a water absorbent resin powder, or the miscibility may be reduced due to evaporation of water or a hydrophilic organic solvent, etc. Furthermore, if the temperature is too low, it is not preferable because there is a risk that the surface crosslinking agent solution may be solidified or the surface crosslinking agent may be precipitated.

[0323] (Use of Acid or Base in Surface Crosslinking Agent Solution) The surface crosslinking agent solution may contain an acid or a base in addition to the organic surface crosslinking agent, the hydrophilic organic solvent, the surfactant, and the water-insoluble fine particles, in order to promote reaction of the surface crosslinking agent or uniform mixing.

[0324]

[0044] As the acid or base, an organic acid or a salt thereof, an inorganic acid or a salt thereof, or an inorganic base is used, and is appropriately used in an amount of more than 0 part by mass and not more than 10 parts by mass, more preferably 0.001 part by mass or more and 5 parts by mass or less, and further preferably 0.01 part by mass or more and 3 parts by mass or less, relative to 100 parts by mass of a water absorbent resin powder before addition treatment. The organic acid is a water-soluble organic acid having a carbon number of 1 or more and 6 or less, more preferably 2 or more and 4 or less, a water-soluble saturated organic acid, and particularly a hydroxy group-containing saturated organic acid.

[0325] Other examples include non-crosslinkable water-soluble inorganic bases (preferably alkali metal salts, ammonium salts, alkali metal hydroxides, and ammonia or its hydroxide), and non-reducing alkali metal salt pH buffers (preferably hydrogen carbonates, dihydrogen phosphates, hydrogen phosphates, etc.).

[0326]

[0062] (Method of adding organic surface crosslinking agent solution) An organic surface crosslinking agent is added to a water absorbent resin powder by an adding treatment. The method of the adding treatment is not particularly limited, and for example, a method of immersing a water absorbent resin powder in a hydrophilic organic solvent to adsorb an added crosslinking agent, a method of directly adding a crosslinking agent to a water absorbent resin powder and mixing by spraying or dropping a crosslinking agent solution, etc. can be exemplified, and from the viewpoint of uniformly adding a predetermined amount, the latter is preferred. Furthermore, in order to add uniformly, it is preferable to perform the adding treatment while stirring the water absorbent resin powder, and further it is preferable to spray the organic surface crosslinking agent solution.

[0327] In the addition treatment, two or more kinds of additive crosslinking agents having different compositions may be added simultaneously using, for example, different spray nozzles, but a single composition is preferable in terms of uniformity, etc. Furthermore, if a single composition is used, multiple spray nozzles may be used, taking into consideration the size of the addition treatment device, the amount of treatment, the spray angle of the spray nozzle, etc.

[0328] Suitable devices used in the addition process (hereinafter sometimes referred to as "mixing devices") include, for example, cylindrical mixers, double-walled conical mixers, V-shaped mixers, ribbon mixers, screw mixers, fluidized bed furnaces, rotary disc mixers, airflow mixers, twin-arm kneaders, internal mixers, grinding kneaders, rotary mixers, screw extruders, turbulent mixers, and ploshear mixers. Furthermore, for large-scale production such as commercial production, devices capable of continuous mixing are preferred. The same device or different devices may be used for each addition process.

[0329]

[0123] It is preferable that a water absorbent resin powder to be subjected to this step is heated and kept warm, and the temperature is preferably in a range of 10°C or higher and 100°C or lower, more preferably 15°C or higher and 80°C or lower, and further preferably 20°C or higher and 70°C or lower.

[0330] If this temperature is 10° C. or more, precipitation of the surface crosslinking agent, moisture absorption of the water absorbent resin powder, etc. are suppressed, and the surface treatment is carried out sufficiently and uniformly, which is preferable. Also, if this temperature is 100° C. or less, evaporation of water from the aqueous solution of the surface crosslinking agent is suppressed, and the risk of precipitation of the surface crosslinking agent, etc. is reduced, which is preferable.

[0331] [3-6-2] Heat Treatment Step This step is a step of performing heat treatment to crosslink the surface or the vicinity of the surface of a water absorbent resin powder in order to improve the AAP and SFC of the particulate water absorbing agent. However, since excessive surface crosslinking treatment may excessively lower the CRC, it is preferable to have a step of performing surface crosslinking treatment until the CRC becomes 25 g / g or more.

[0332] A preferable degree of surface crosslinking can be confirmed by the degree of decrease in CRC before and after surface crosslinking, and the amount of the surface crosslinking agent and the reaction temperature and time may be appropriately selected so that the decrease in CRC due to surface crosslinking is preferably 0.5 g / g or more, more preferably 1 g / g or more and 20 g / g or less, and further preferably 2 g / g or more and 15 g / g or less.

[0333] The heat treatment step may be carried out simultaneously with the surface cross-linking agent addition step or may be carried out after the surface cross-linking agent addition step, but is preferably carried out after the surface cross-linking agent addition step. In addition, this step may be carried out once, or may be carried out multiple times under the same conditions or different conditions.

[0334]

[0123] The dried water absorbent resin powder obtained up to the above-mentioned step [3-5] has a granulated shape, but before surface crosslinking, primary particles constituting granulated particles are in a state of physically adhering to each other, and the granulated shape is broken and broken up upon swelling, and there are cases in which water absorption performance and liquid permeability are deteriorated.

[0335] By carrying out this surface cross-linking step, not only is the cross-linking density in the vicinity of the surface of the granulated particles increased, but also the primary particles (gel particles obtained in the gel crushing step) that form the granulated particles are chemically bonded to each other, and inter-particle cross-linking is achieved even though they are in loose or point contact, which has the effect of making the surface-cross-linked particles less likely to collapse when swollen.

[0336] As described above, in the present invention, it is significant not only to add a specific amount of an internal crosslinking agent and a specific polymerization additive during polymerization, but also to carry out a surface crosslinking step in combination.

[0337] (Liquid Permeability Improver) The liquid permeability improver refers to an additive selected from an insoluble particulate compound and a polyvalent cationic compound, or an additive that improves FGBP (Free Gel Bed Permeability) compared to when no liquid permeability improver is used. For FGBP, see the contents disclosed in WO 2016 / 204302.

[0338] According to a preferred embodiment of the present invention, the particulate water-absorbing agent of the present invention does not contain a liquid-permeability improver for improving FGBP. That is, according to a preferred embodiment of the present invention, the method for producing a particulate water-absorbing agent of the present invention does not include a step of adding a liquid-permeability improver for improving FGBP. The step of adding the liquid-permeability improver is carried out during or after the surface-crosslinking step.

[0339] The liquid permeability improver is selected from water-insoluble inorganic fine particles and polyvalent cationic compounds (cationic polymer compounds or water-soluble polyvalent metal cation-containing compounds).

[0340] In this specification, a "water-soluble" compound refers to a compound that dissolves in an amount of 1 g or more, or even 5 g or more, per 100 g of water at 25°C, and a "water-insoluble" compound refers to a compound that dissolves in an amount of less than 1 g, or even less than 0.5 g, or even less than 0.1 g, per 100 g of water at 25°C.

[0341] (Inorganic fine particles) Examples of the inorganic fine particles include water-insoluble fine inorganic powders such as silicon dioxide, titanium dioxide, aluminum oxide, magnesium oxide, zinc oxide, talc, metal phosphates (e.g., calcium phosphate, barium phosphate, aluminum phosphate), metal borates (e.g., titanium borate, aluminum borate, iron borate, magnesium borate, manganese borate, and calcium borate), silicic acid or its salts, clay, diatomaceous earth, zeolite, bentonite, kaolin, hydrotalcite, and activated clay, and organic fine powders such as calcium lactate, aluminum lactate, and metal soaps (polyvalent metal salts of long-chain fatty acids). The inorganic fine particles have a volume average particle diameter of, for example, 10 μm or less, or 1 μm or less.

[0342] (Cationic Polymer Compound) The cationic polymer compound is not particularly limited, but polyethyleneimine, polyvinylamine, polyallylamine, dimethylamine / ammonia / epichlorohydrin condensates, as well as the cationic polymer compounds described in U.S. Patent Nos. 5,382,610 and 7,098,284, WO 2009 / 110645, WO 2009 / 041731, and WO 2009 / 041727 can be suitably used.

[0343] The molecular weight of the cationic polymer compound is, for example, preferably 1,000 or more and 5,000,000 or less, more preferably 2,000 or more and 1,000,000 or less, and even more preferably 10,000 or more and 500,000 or less, in terms of mass average molecular weight.

[0344] (Water-soluble polyvalent metal cation-containing compound) The water-soluble polyvalent metal cation-containing compound refers to a compound containing a divalent or higher, preferably trivalent or higher, metal cation. Examples of the trivalent or higher metal cation include aluminum, zirconium, and titanium.

[0345] Examples of the polyvalent metal cation-containing compound include inorganic surface crosslinking agents such as inorganic salts of polyvalent metals, such as aluminum sulfate, aluminum chloride, zirconium oxide chloride, ammonium zirconium carbonate, potassium zirconium carbonate, potassium zirconium carbonate, zirconium sulfate, zirconium acetate, and zirconium nitrate; and polyvalent metal compounds such as organic salts of polyvalent metals, such as aluminum acetate, aluminum lactate, zirconium hydroxychloride, titanium triethanolaminate, and titanium lactate.

[0346] Further examples of the liquid permeability improver other than those mentioned above include the water-soluble polysiloxanes described in WO 2009 / 093708 and the primary to tertiary amine compounds described in WO 2008 / 108343.

[0347] The amount of the liquid permeability improver is preferably 0.001 parts by mass or more and 5 parts by mass or less, more preferably 0.002 parts by mass or more and 2 parts by mass or less, and further preferably 0.005 parts by mass or more and 1 part by mass or less, relative to 100 parts by mass of the water absorbent resin particles to be added.

[0348] In the case of a water-soluble polyvalent metal cation-containing compound, the value is calculated as the amount of polyvalent metal cation (for example, in the case of aluminum sulfate, Al ３＋ (The value is specified by the amount of

[0349] [3-7] Step of Adding Other Additives This step is a step of adding other additives to impart various functions to a water absorbent resin powder or surface-crosslinked water absorbent resin particles, and is composed of one or more steps. The additives are agents that can impart or enhance functions, such as deodorants, fragrances, antibacterial agents, foaming agents, chelating agents such as trisodium diethylenetriaminepentaacetate and pentasodium diethylenetriaminepentaacetate, surfactants, coloring inhibitors, pigments, dyes, fertilizers, oxidizing agents, and reducing agents. The additives may be added in the form of a solution or by dry blending.

[0350] This step may be carried out between any of steps (i) to (vii), or may be carried out simultaneously with any of steps (i) to (vii), preferably during or after step (vi).

[0351] The usage ratio of these additives is less than 10 mass %, preferably less than 5 mass %, more preferably less than 1 mass % of the water absorbent resin powder or the surface cross-linked water absorbent resin particles. Furthermore, these additives may be added simultaneously with the above surface cross-linking step or may be added separately.

[0352] In the above-described manufacturing method of the present invention, the SFC of the obtained poly(meth)acrylic acid (salt)-based water-absorbing agent is 15×10 －７ cm ３ -Adjust so that it is sec / g or more.

[0353] SFC 15x10 －７ cm ３ In order to make it sec / g or more, it is preferable to control the crosslink density by controlling the amount of an internal crosslinking agent, the drying temperature and time of the hydrogel, or by controlling the strength of surface crosslinking of the water absorbent resin powder.

[0354] According to a preferred embodiment of the present invention, the SFC of the water-absorbing agent is adjusted to fall within a specific range, thereby making it possible to efficiently solve the intended problem.

[0355] [4] Absorbent Articles The use of the particulate water-absorbing agent of the present invention is not particularly limited, but it is preferably used in absorbents used in disposable diapers and sanitary napkins.

[0356] The absorbent body in the present invention refers to an absorbent material molded with the particulate water-absorbing agent of the present invention and hydrophilic fibers as the main components, and in the absorbent body of the present invention, the content of the particulate water-absorbing agent (core concentration) relative to the total mass of the particulate water-absorbing agent and hydrophilic fibers is preferably 20% by mass or more and 100% by mass or less, more preferably 30% by mass or more and 95% by mass or less, and even more preferably 50% by mass or more and 90% by mass or less.

[0357] Furthermore, when the absorbent body of the present invention is thin, it is preferable that the thickness of the absorbent body is 1 mm or more and 5 mm or less. Such a thin absorbent body can be used to make a thin absorbent article. For example, the absorbent article may include the above-mentioned thin absorbent body of the present invention, a liquid-permeable top sheet, and a liquid-impermeable back sheet.

[0358] The method for producing a thin absorbent article of the present invention may be, for example, to prepare an absorbent body (absorbent core) by blending or sandwiching a fibrous substrate and a particulate water-absorbing agent, and then sandwiching the absorbent body between a substrate such as a liquid-permeable top sheet and a substrate such as a liquid-impermeable back sheet, and then providing an elastic member, a diffusion layer, an adhesive tape, etc., as necessary, to produce an absorbent article, particularly a disposable diaper or a sanitary napkin. Such an absorbent article has a density of 0.06 g / cc or more and 0.50 g / cc or less and a basis weight of 0.01 g / cm. ２ 0.20g / cm or more ２ The fiber substrate is compression molded to the following range. Examples of the fiber substrate include hydrophilic fibers such as pulverized wood pulp, cotton linters, crosslinked cellulose fibers, rayon, cotton, wool, acetate, vinylon, etc. Air-laid fibers are preferred.

[0359] The particulate water-absorbing agent of the present invention exhibits excellent absorption properties. Therefore, specific examples of the absorbent article of the present invention include sanitary materials such as disposable diapers for adults, which have seen remarkable growth in recent years, disposable diapers for babies, sanitary napkins, and so-called incontinence pads. The particulate water-absorbing agent of the present invention present in the absorbent article reduces leakage and skin rashes, thereby significantly reducing the burden on the wearer and caregivers.

[0360] According to a preferred embodiment of the present invention, the sanitary material contains the above-mentioned poly(meth)acrylic acid (salt)-based particulate water-absorbing agent.

[0361] Although the embodiments of the present invention have been described in detail, it is clear that this is by way of illustration and example only and not of limitation, and that the scope of the present invention should be interpreted by the appended claims.

[0362] The present invention encompasses the following aspects and configurations.

[0363] [1] A poly(meth)acrylic acid (salt)-based particulate water-absorbing agent containing poly(meth)acrylic acid (salt)-based water-absorbing resin particles as a main component, the particulate water-absorbing agent including cavities which are spaces communicating with the outside and voids which are closed spaces not communicating with the outside, the total volume ratio of the cavities being 18% by volume or more, a void / cavity ratio which is a ratio of the total volume ratio of the cavities to the total volume ratio of the voids being 0.015 or more and 0.025 or less, and a mass median particle diameter (D50) of the particulate water-absorbing agent being 300 μm or more and 450 μm or less.

[0364] [2] The poly(meth)acrylic acid (salt)-based particulate water-absorbing agent according to the above [1], which has an absorbency against pressure (AAP) of 0.7 psi (4.83 kPa) of 24 g / g or more.

[0365] [3] Saline flow conductivity (SFC) is 15 x 10 －７ cm ３ The poly(meth)acrylic acid (salt)-based particulate water-absorbing agent according to the above [1] or [2], which has a water-absorbent capacity of 1000 kJ / g or more.

[0366] [4] The poly(meth)acrylic acid (salt)-based particulate water-absorbing agent according to any one of the above [1] to [3], which has a water absorption capacity without load (CRC) of 25 g / g or more.

[0367] [5] The poly(meth)acrylic acid (salt)-based particulate water-absorbing agent according to any one of the above [1] to [4], wherein a water absorption rate index under pressure, which is the product of the total volume ratio of the cavities and an absorption capacity under pressure (AAP) of 0.7 psi (4.83 kPa), is 460 or more.

[0368] [6] A sanitary material comprising the poly(meth)acrylic acid (salt)-based particulate water-absorbing agent according to any one of [1] to [5] above.

[0369] [7] A method for producing a poly(meth)acrylic acid (salt)-based particulate water-absorbing agent containing poly(meth)acrylic acid (salt)-based water-absorbing resin particles as a main component, the method comprising: (i) a step of preparing a (meth)acrylic acid (salt)-based monomer aqueous solution; (ii) a step of polymerizing the (meth)acrylic acid (salt)-based monomer aqueous solution to obtain a hydrogel-like crosslinked polymer; (iii) a step of gel-crushing the hydrogel-like crosslinked polymer during or after polymerization to obtain hydrogel particles; (iv) a step of drying the hydrogel particles to obtain a dried product; (v) a step of pulverizing and / or classifying the dried product to obtain a water-absorbent resin powder; and (vi) a step of surface-crosslinking the water-absorbent resin powder to obtain water-absorbent resin particles, wherein in at least one of the step (i) and the step (ii), a polymerization additive is added to the (meth)acrylic acid (salt)-based monomer aqueous solution, and the polymerization additive is selected from the group consisting of an amphoteric substance, an anionic surfactant ... and (iii) a gel crushing energy (GGE)(2) for crushing the hydrogel crosslinked polymer to a gel of 30 J / g or more, and (iii) a hydrogel particle obtained in the step (iii) is obtained by mixing 500 parts by mass of the hydrogel particle with 1000 parts by mass of an aqueous solution containing 0.02% by mass of sodium polyoxyethylene (3) lauryl ether sulfate and 20% by mass of sodium chloride. a method for producing a poly(meth)acrylic acid (salt)-based particulate water-absorbing agent, wherein, when 0 parts by mass of the poly(meth)acrylic acid (salt)-based particulate water-absorbing agent is immersed in water, the mass average particle diameter after 1 hour of immersion is defined as D50(1 hr) (μm), and the mass average particle diameter after 16 hours of immersion is defined as D50(16 hr) (μm), the ratio of D50(1 hr) to D50(16 hr), i.e., D50(16 hr) / D50(1 hr), is 0.20 or more and 0.85 or less, and D50(1 hr) is 350 μm or more.

[0370] [8] The saline flow conductivity (SFC) of the poly(meth)acrylic acid (salt)-based particulate water-absorbing agent is 15×10 －７ cm ３ sec / g or more, in the method for producing a poly(meth)acrylic acid (salt)-based particulate water-absorbing agent according to [7] above.

[0371] [9] The method for producing a poly(meth)acrylic acid (salt)-based particulate water-absorbing agent according to the above [7] or [8], wherein the mass average particle diameter (D50) of the hydrogel particles is 150 μm or more and 500 μm or less.

[0372]

[10] The method for producing a poly(meth)acrylic acid (salt)-based particulate water-absorbing agent according to any one of the above [7] to [9], wherein the amphoteric substance is at least one selected from (a) alkyl betaine and (b) alkyl amine oxide, the anionic substance is at least one selected from (c) sulfate ester salts of higher alcohol alkylene oxide adducts, (d) dicarboxylate salts, (e) sulfonates, (f) alkyl sulfate ester salts, (g) alkyl amine diacetate salts, (h) phosphate ester salts of higher alcohol alkylene oxide adducts, and (i) carboxylate salts of higher alcohol alkylene oxide adducts, and the cationic substance is (j) ammonium salt.

[0373]

[11] The method for producing a poly(meth)acrylic acid (salt)-based particulate water-absorbing agent according to any one of the above [7] to

[10] , wherein an amount of the polymerization additive added is 0.01 mass % or more and 5 mass % or less with respect to a total amount of monomers contained in the (meth)acrylic acid (salt)-based monomer aqueous solution (100 mass % of raw material monomers).

[0374]

[12] The method for producing a poly(meth)acrylic acid (salt)-based particulate water-absorbing agent according to any one of the above [7] to

[11] , wherein the polymerization additive is at least one selected from the group consisting of (a) alkyl betaine and (d) dicarboxylate.

[0375]

[13] The method for producing a poly(meth)acrylic acid (salt)-based particulate water-absorbing agent according to the above

[10] , wherein the (e) dicarboxylate is at least one selected from the group consisting of alkenyl succinates and acylaspartates.

[0376]

[14] A sanitary material comprising a poly(meth)acrylic acid (salt)-based particulate water-absorbing agent obtained by the method for producing a poly(meth)acrylic acid (salt)-based particulate water-absorbing agent according to any one of [7] to

[13] above.

[0377] The present invention will be described below with reference to examples, but the present invention should not be construed as being limited to these examples. The physical properties described in the claims and examples of the present invention were determined using the following measurement methods (a) to (h). Unless otherwise specified, each step in each example was carried out at essentially normal pressure (within ±5%, more preferably 1%, of atmospheric pressure), without intentionally increasing or decreasing the pressure in any of the steps. Furthermore, measurements of physical properties, etc. were carried out at room temperature (20°C to 25°C) and at a relative humidity of 40% RH to 50% RH, unless otherwise specified.

[0378] (a) Water absorption capacity without load (CRC) (NWSP 241.0.R2(15)) The water absorption capacity without load (CRC) of the particulate water absorbing agent according to the present invention was measured in accordance with NWSP 241.0.R2(15). That is, 0.200 g (mass W0 (g)) of the particulate water absorbing agent was weighed, uniformly placed in a nonwoven bag (60 × 85 mm), heat-sealed, and then immersed in 500 mL of a 0.90 mass% sodium chloride aqueous solution adjusted to 23 ± 2 ° C. After 30 minutes, the bag was pulled out and drained at 250 G for 3 minutes using a centrifuge (Kokusan Co., Ltd. Centrifuge: Model H-122). Thereafter, the mass (W1 (g)) of the bag was measured. The same operation was performed without adding the particulate water absorbing agent, and the mass (W2 (g)) of the bag at that time was measured. The water absorption capacity without load (CRC) of the particulate water absorbing agent was calculated from the obtained W0 (g), W1 (g), and W2 (g) according to the following (Equation 3).

[0379]

[0380] The water absorption capacity without load (CRC) of the hydrogel or hydrogel particles was measured as gel CRC. The gel CRC was measured using 0.4 g of hydrogel or hydrogel particles as a sample, and the same procedure as above was carried out, except that the free swelling time was 24 hours. Furthermore, the resin solid content of the hydrogel or hydrogel particles was measured separately, and the mass of the water-absorbent resin in the 0.4 g of hydrogel or hydrogel particles was determined, and the gel CRC was calculated according to the following (Equation 4). Each sample was measured three times, and the average value was used.

[0381]

[0382] Here, msi: mass (g) of the hydrogel or hydrogel particles before measurement; mb: mass (g) of the blank (nonwoven fabric only) after free swelling and draining; mwi: total mass (g) of the hydrogel or hydrogel particles and nonwoven fabric after free swelling and draining; Wn: solid content (mass%) of the hydrogel or hydrogel particles.

[0383] (b) Absorption Capacity Under Pressure (AAP) (NWSP 242.0.R2(15)) The absorption capacity under pressure (AAP) of the particulate water absorbing agent according to the present invention was measured in accordance with NWSP 242.0.R2(15). That is, 0.900 g (mass W3(g)) of the particulate water absorbing agent was put into a measuring device, and the mass (W4(g)) of the measuring device set was measured. Next, a 0.90 mass % sodium chloride aqueous solution adjusted to 23±2°C was added to the measuring device at 4.83 kPa (0.7 psi, 49 g / cm). ２ After one hour had passed, the mass (W5 (g)) of the set of the measuring device was measured, and the water absorption capacity against pressure (AAP) of the particulate water-absorbing agent was calculated from the obtained W3 (g), W4 (g), and W5 (g) according to the following (Equation 5).

[0384]

[0385] (c) Water Absorption Rate (Vortex) The water absorption rate (Vortex) of the particulate water absorbing agent according to the present invention was measured by the following method. That is, 0.02 parts by mass of a food additive, Food Blue No. 1, was added to 1,000 parts by mass of a previously prepared 0.90% by mass sodium chloride aqueous solution, and the liquid temperature was adjusted to 30°C. 50 ml of the blue-colored 0.90% by mass sodium chloride aqueous solution was weighed into a 100 ml beaker, and 2.00 g of the particulate water absorbing agent was added while stirring at 600 rpm with a cylindrical stirrer having a length of 40 mm and a diameter of 8 mm, and the water absorption rate was measured. The endpoint was measured in accordance with the standard described in JIS K 7224-1996 "Explanation of the Water Absorption Rate Test Method for Super Absorbent Polymers," and the time until the particulate water absorbing agent absorbed the sodium chloride aqueous solution and the test liquid covered the stirrer tip was measured as the water absorption rate (unit: seconds).

[0386] (d) Particle Size Distribution (PSD), Mass Median Particle Diameter (D50) and Logarithmic Standard Deviation of Particle Size Distribution (σζ) The particle size distribution (PSD) and the logarithmic standard deviation of the particle size distribution (σζ) of the particulate water-absorbing agent according to the present invention were measured in accordance with the measurement method disclosed in U.S. Patent Application Publication No. 2006 / 204755.

[0387] That is, 10.00 g of the sample was classified using JIS standard sieves (The IIDA TESTING SIEVE: inner diameter 80 mm; JIS Z8801-1 (2000)) having mesh sizes of 850 μm, 600 μm, 500 μm, 425 μm, 300 μm, 150 μm, and 45 μm, or sieves equivalent to the JIS standard sieves. After classification, the mass of each sieve was measured, and the mass percentage (mass%) of particles with a particle size of less than 150 μm was calculated. Note that the "mass percentage of particles with a particle size of less than 150 μm" refers to the mass ratio (%) of particles passing through a JIS standard sieve with a mesh size of 150 μm relative to the entire sample.

[0388] The mass-average particle diameter (D50) was determined by plotting the residual percentage R of each particle size on a logarithmic probability chart, and reading the particle diameter corresponding to R=50% by mass from the graph as the mass-average particle diameter (D50). The mass-average particle diameter (D50) refers to the particle diameter corresponding to 50% by mass of the entire particulate water-absorbing agent (sample). The logarithmic standard deviation (σζ) of the particle size distribution is expressed by the following (Equation 6), and a smaller σζ value means a narrower particle size distribution.

[0389]

[0390] (X1 means the particle size when R=84.1%, X2 means the particle size when R=15.9%, and ln means the natural logarithm.) (e) Solid Content and Moisture Content The solid content and moisture content of the particulate water-absorbing agent according to the present invention were measured by the following method. That is, for the particulate water-absorbing agent in a dry state, the solid content and moisture content are defined by the loss on drying of 1.0 g of the particulate water-absorbing agent at 180° C. for 3 hours, and for the hydrogel or hydrogel particles before drying, the solid content and moisture content are defined by the loss on drying of 2.0 g of the hydrogel or hydrogel particles at 180° C. for 24 hours.

[0391] Specifically, in the case of a particulate water absorbing agent, 1.0 g of the particulate water absorbing agent (mass W9 (g)) was weighed out into an aluminum cup (mass W8 (g)) having a bottom diameter of approximately 5 cm, and the cup was left to stand in a windless dryer at 180°C for 3 hours to dry. The total mass (W10 (g)) of the aluminum cup and the particulate water absorbing agent after drying was measured, and the solid content was calculated using the following (Equation 7). The moisture content was calculated using the following (Equation 8).

[0392] In the case of hydrogel or hydrogel particles, 2.0 g of hydrogel or hydrogel particles (mass W9 (g)) was weighed into an aluminum cup (mass W8 (g)) with a bottom diameter of approximately 5 cm, and left to stand in a windless dryer at 180°C for 24 hours to dry. The total mass (W10 (g)) of the aluminum cup and water-absorbent resin after drying was measured, and the solid content was calculated using the following (Equation 7). The moisture content was also calculated using the following (Equation 8).

[0393]

[0394]

[0395] (f) Particle size distribution, mass average particle diameter (D50), D50 (1 hr) and D50 (16 hr) of hydrogel or hydrogel particles The particle size distribution, mass average particle diameter (D50), D50 (1 hr) and D50 (16 hr) of hydrogel or hydrogel particles were measured by the following methods. That is, 50 parts by mass of hydrogel or hydrogel particles (solid content α% by mass) at a temperature of 20°C to 25°C was added to 1,000 parts by mass of an aqueous solution containing 0.02% by mass of polyoxyethylene (3) sodium lauryl ether sulfate (0.08% by mass of EMAL 20C (surfactant, manufactured by Kao Corporation)) and 20% by mass of sodium chloride (hereinafter referred to as "aqueous sodium chloride solution") to prepare a dispersion, and the mixture was stirred at 300 rpm using a stirrer tip having a length of 50 mm and a diameter of 7 mm for 1 hour or 16 hours (using a cylindrical polypropylene container having a height of 21 cm and a diameter of 8 cm, approximately 1.14 L).

[0396] After stirring, the dispersion was poured into the center of a JIS standard sieve (diameter 21 cm, sieve openings: 8 mm / 4 mm / 2 mm / 1 mm / 0.60 mm / 0.30 mm / 0.15 mm / 0.075 mm) placed on a rotating plate. After washing out all the hydrous gel or all the hydrous gel particles onto the sieve using 100 parts by mass of an aqueous sodium chloride solution, 1000 parts by mass of an aqueous sodium chloride solution was poured from above into the water injection range (50 cm) using a shower (72 holes, liquid volume: 6.0 L / min) from a height of 30 cm while rotating the sieve by hand (20 rpm). ２ The water-containing gel or water-containing gel particles were classified by repeating the process of pouring the water evenly over the entire sieve 10 times. The water-containing gel or water-containing gel particles on the classified first sieve were drained for about 2 minutes and then weighed. The second and subsequent sieves were also classified in the same manner, and the water-containing gel or water-containing gel particles remaining on each sieve after draining were weighed. Note that when the diameter of the water-containing gel or water-containing gel particles became small and clogging occurred with sieve openings of 0.15 mm and 0.075 mm, measurements were performed using JIS standard sieves with larger diameters (diameter 30 cm, sieve openings: 0.15 mm / 0.075 mm).

[0397] The mass percent of the hydrogel or hydrogel particles remaining on each sieve was calculated using the following formula (9): The mesh size of the sieve after draining was calculated using the following formula (10), and the particle size distribution of the hydrogel or hydrogel particles was plotted on logarithmic probability paper. From this graph, the particle size corresponding to a residual percentage of 50% by mass was read as the mass-average particle size (D50) of the hydrogel or hydrogel particles.

[0398]

[0399]

[0400] Here, X is the mass % (%) of the hydrogel or hydrogel particles remaining on each sieve after classification and draining, w is the mass (g) of each of the hydrogel or hydrogel particles remaining on each sieve after classification and draining, W is the total mass (g) of the hydrogel or hydrogel particles remaining on each sieve after classification and draining, R(α) is the sieve opening (mm) when converted into hydrogel or hydrogel particles with a solids content of α% by mass, and r is the sieve opening (mm) through which the hydrogel or hydrogel particles swollen in a 20% by mass aqueous sodium chloride solution were classified.

[0401] (g) SFC (Saline Flow Conductivity) The SFC (Saline Flow Conductivity) of the particulate water-absorbing agent according to the present invention was measured in accordance with the SFC test described in the specification of U.S. Pat. No. 5,849,405. Specifically, 1.5 g of the particulate water-absorbing agent was uniformly placed in a container and swelled in artificial urine (1) under a pressure of 0.3 psi (2.07 kPa) for 60 minutes, and the height of the gel layer was recorded. Next, 0.69 mass % saline was passed through the swollen gel layer at a constant hydrostatic pressure under a pressure of 0.3 psi (2.07 kPa). This SFC test was conducted at room temperature (20°C to 25°C). Using a computer and a balance, the amount of liquid passing through the gel layer as a function of time was recorded at 20-second intervals for 10 minutes. The flow velocity Fs(t) passing through the swollen gel (mainly between particles) was determined in units of g / s by dividing the increased mass (g) by the increased time (s). The time when constant hydrostatic pressure and stable flow rate were obtained was designated as ts. Only data obtained between ts and 10 min were used for flow rate calculations. The flow rates obtained between ts and 10 min were used to calculate the value of Fs(t=0), i.e., the initial flow rate through the gel layer. Fs(t=0) was calculated by extrapolating the results of a least squares fit of Fs(t) versus time to t=0. The SFC value has units of (10 －７ cm ３ ·sec / g).

[0402] Flow conductivity of 0.69 mass% saline solution = (F ｓ (t=0)×L ０ ) / (ρ×A×ΔP) = (F ｓ (t=0)×L ０ ) / 139506 where, F ｓ (t = 0): Flow rate in g / s L０ : initial thickness of the gel layer in cm ρ: density of the NaCl solution (1.003 g / cm ３ ) A: Area above the gel layer in cell 41 (28.27 cm ２ ) ΔP: hydrostatic pressure applied to the gel layer (4920 dyne / cm ２ ).

[0403] The artificial urine (1) was a mixture of 0.25 g of calcium chloride dihydrate, 2.0 g of potassium chloride, 0.50 g of magnesium chloride hexahydrate, 2.0 g of sodium sulfate, 0.85 g of ammonium dihydrogen phosphate, 0.15 g of diammonium hydrogen phosphate, and 994.25 g of pure water.

[0404] (h) Total Volume Ratio of Cavities and Total Volume Ratio of Voids The total volume ratio of cavities (unit: volume %) and the total volume ratio of voids (unit: volume %) relative to the total volume of the particulate water absorbing agent according to the present invention were measured by the following method. That is, three-dimensional image data of the particulate water absorbing agent was obtained using a microfocus X-ray CT system (inspexio SMX-100CT / manufactured by Shimadzu Corporation), the three-dimensional image data was analyzed using high-speed three-dimensional analysis software (TRI / 3D-VOL-FCS64 / manufactured by Ratoc System Engineering Co., Ltd.), and calculations were performed using Microsoft Excel to determine the total volume ratio of cavities and the total volume ratio of voids. The total volume ratio of cavities and the total volume ratio of voids were determined using a particulate water absorbing agent having a particle diameter of 300 μm or more and less than 600 μm. Specifically, three-dimensional image data of a particulate water-absorbing agent having a particle diameter of 300 μm or more and less than 600 μm was acquired using the microfocus X-ray CT system, and analyzed using the high-speed three-dimensional analysis software, and then the analysis results were calculated using Excel.

[0405] Next, a specific calculation method will be described. First, a method for extracting a particulate water-absorbing agent having a particle diameter of 300 μm or more and less than 600 μm will be described. First, JIS standard sieves (The IIDA TESTING SIEVE / manufactured by Iida Seisakusho Co., Ltd., diameter: 8 cm, sieve openings: 600 μm / 300 μm) were stacked from the top in descending order of opening size, and a receiver (manufactured by Iida Seisakusho Co., Ltd., diameter: 8 cm) was placed on the bottom. Subsequently, 10.0 g of the particulate water-absorbing agent was placed on the top sieve (sieve openings: 600 μm), and a lid (manufactured by Iida Seisakusho Co., Ltd., diameter: 8 cm) was placed on it. Subsequently, the set of the overlapping lid, the two sieves, and the receiver was shaken for 5 minutes using an IIDA SIEVE SHAKER (TYPE: ES-65, SER. No. 0632). After shaking, only the material remaining on the sieve with a mesh size of 300 μm was extracted, and this was used as a particulate water-absorbing agent with a particle diameter of 300 μm or more and less than 600 μm. The above operation was carried out in a room adjusted to a temperature of 20.0°C to 25.0°C and a humidity of 35% to 50%.

[0406] Next, 0.3 g of thermosetting spherical microparticles (Eposter MV1002 / manufactured by Nippon Shokubai Co., Ltd.) was placed in a plastic cylindrical container with a lid (inner diameter approximately 1 cm, height approximately 5 cm), and then 0.1 g of a particulate water-absorbing agent with a particle diameter of 300 μm or more and less than 600 μm was placed in the container, and the mixture was thoroughly mixed by shaking or tapping to uniformly disperse the particulate water-absorbing agent in the thermosetting spherical microparticles, thereby preparing a sample. Next, double-sided tape was attached to the bottom of the cylindrical container, and the container was fixed to the sample stage of the microfocus X-ray CT system, and then 3D image data was obtained under the following conditions.

[0407]

[0408] Using the above-mentioned high-speed three-dimensional analysis software, the analysis was carried out according to the following procedure.

[0409] 1. From the menu, select Particle Measurement > 3D Particles > Particle Separation > Giant Particle Separation.

[0410] 2. Select L-W in the Binaryize tab on the EVC panel, leave the W value at its default value, and increase the L value by 1 to extract a circular measurement region. This process was then applied to all slice images. The image data extracted by this operation was designated (A) and stored in bin5ch (b5) on the BC panel.

[0411] 3. L-W was selected in the Binaryize tab on the EVC panel, and the W value was left at the initial value, while the L value was changed from the initial value to "37580," and all particulate water-absorbing agents in the measurement target area were extracted. Subsequently, this process was applied to all slice images. The particle image data extracted by this operation was designated as (B), and stored in binDch (bD) on the BC panel.

[0412] 4. Based on the particle image data (B), Ers Sml was first selected in the Binary tab on the EVC panel to remove particles with a particle size of 10 voxels or less, which were considered to be noise. Next, Invert was selected in the Binary tab on the EVC panel to invert the areas where particles were extracted from the areas where they were not. Next, Ers Sml was selected in the Binary tab on the EVC panel to remove particles with a particle size of 1 voxel or less, which were considered to be noise. Next, Labeling was selected in the 3D tab on the EVC panel, and Volume and Max were selected to extract only the area with the largest volume. After confirming that the Label Count was displayed as 1, Invert was again selected in the Binary tab on the EVC panel to remove noise in the measurement area and extract all particles with voids filled. The particle image data extracted by these operations was designated as (C) and stored in bin2ch (b2) on the BC panel.

[0413] 5. In the LOp tab (inter-channel logical operation processing), select "2" for object 1 and "D" for object 2, then select "SUB" and press Execute to subtract particle image data (B) from particle image data (C). Then, select Ers Sml in the Binary tab on the EVC panel to extract voids by removing particles with a particle size of 1 voxel or less that are considered noise. The particle image data extracted by these operations was named (D) and stored in bin6ch (b6) on the BC panel.

[0414] 6. Based on the particle image data (C), the 8-connected periphery was selected in the 3D tab on the EVC panel, and dilation processing (dilation) was performed twice, followed by erosion processing (erosion) twice. The particle image data extracted by this operation was named (E) and stored in bin1h (b1) on the BC panel.

[0415] 7. In the LOp tab (inter-channel logical operation processing), select "1" for target 1 and "2" for target 2, then select "SUB" and press Execute to subtract particle image data (C) from particle image data (E). This operation extracted the cavity. The resulting particle image data was named (F) and stored in bin7ch (b7) on the BC panel.

[0416] 8. Based on the particle image data (E), small particle extraction was selected (large particle extraction was not selected) on the large particle separation panel, and the constriction ratio, Repair Filter Size, and Repair Mrg Sml Diameter were all set to "0," and the particles were separated and color-coded.

[0417] 9. Labeling was selected in the 3D tab on the EVC panel, and the coordinate values ​​(cycle) were selected, and the microparticle size was set to "100" to separate the particles. The particle image data extracted by these operations was designated as (G) and stored in bin1ch (b1) on the BC panel.

[0418] 10. From the menu, select Particle Measurement > 3D Voids in Particles > Measurement after Separation.

[0419] 11. After separation, on the measurement panel, voxel was selected as the unit, then edge particle removal was selected, and surface area calculation and void calculation were selected as measurement items. Binary 5ch was selected as the measurement ROI specification, and calculation processing was performed.

[0420] 12. The data obtained by the calculation process in 10 above was extracted in Excel CSV format.

[0421] By the above operation, the particle volume (unit: mm ３ ), void volume (unit: mm ３ ), and cavity volume (unit: mm ３ ) data was obtained. The particle volume is a value calculated in a state where the voids and cavities in the particulate water absorbing agent are filled. In addition, the particulate water absorbing agent containing about 200 to 500 particles exists in the measurement target area.

[0422] (Calculation of total volume ratio (volume %) of cavities) Next, the particle volume (unit: mm ３ ), void volume (unit: mm ３ ), and cavity volume (unit: mm ３ The total volume ratio (volume %) of cavities was calculated using the data of the particle volume per particle (unit: mm ３ ), void volume (unit: mm ３ ), and cavity volume (unit: mm ３ ) were added together to obtain the total particle volume (H) (unit: mm ３ ), total volume of voids (I) (unit: mm ３ ), and the total volume of the cavity (J) (unit: mm ３ ) was calculated. The total particle volume of all particles is a value calculated in a state where the voids and cavities in the particulate water absorbing agent are filled. Next, based on the following formula (i), a total volume ratio (unit: volume %) of cavities, which is an average value of all the particulate water absorbing agents present in the measurement target area, was calculated.

[0423] Total volume ratio of cavities = J / (H-I) x 100 (Equation (i)) where, H: total particle volume of all particles (unit: mm ３ ) I: total volume of voids in all particles (unit: mm ３ ) J: total volume of all particle cavities (unit: mm ３ )

[0424] (Calculation of Total Volume Ratio (Volume %) of Voids) Based on the following formula (ii), the total volume ratio (unit: volume %) of voids, which is the average value of all the particulate water-absorbing agents present in the measurement target area, was calculated.

[0425] Total volume ratio of voids = I / (H-J) x 100 (Equation (ii)) where, H: total particle volume of all particles (unit: mm ３ ) I: total volume of voids in all particles (unit: mm ３ ) J: total volume of all particle cavities (unit: mm ３ )

[0426] Example 1 (Process for Preparing Aqueous Monomer Solution) A 2 L polypropylene container was charged with 422.0 parts by mass of acrylic acid, 173.9 parts by mass of a 48.5% by mass aqueous sodium hydroxide solution, 2.45 parts by mass (0.080 mol % relative to the total amount of monomers) of polyethylene glycol diacrylate (average molecular weight: 523) as an internal crosslinking agent, 1.3 parts by mass of a 2.0% by mass aqueous solution of trisodium diethylenetriaminepentaacetic acid, 1.474 parts by mass (1223 ppm by mass (0.12% by mass) relative to the total amount of monomers) of Obazoline LB-SF (manufactured by Toho Chemical Industry Co., Ltd., lauryldimethylaminoacetic acid betaine (35% by mass concentration)) as a polymerization additive, and 401.9 parts by mass of deionized water, and mixed to prepare an aqueous monomer solution. The temperature of the aqueous monomer solution exceeded 40°C due to the heat of neutralization and heat of dissolution generated during the mixing process.

[0427] (Polymerization Step) Next, the aqueous monomer solution was cooled with stirring. When the liquid temperature reached 40°C, 178.7 parts by mass of a 48.5% by mass aqueous sodium hydroxide solution adjusted to 40°C was added to the aqueous monomer solution over approximately 20 seconds in an open-to-air state and mixed (starting the second stage of neutralization). This prepared aqueous monomer solution (1). At this time, the liquid temperature of the aqueous monomer solution (1) rose to approximately 78°C due to the heat of neutralization and heat of dissolution generated during the mixing process. Although precipitates were observed immediately after starting to mix the aqueous sodium hydroxide solution with the aqueous monomer solution, they gradually dissolved, and the prepared aqueous monomer solution (1) became a transparent, homogeneous solution.

[0428] Nitrogen gas was then introduced into the stirred aqueous monomer solution (1) using a Kinoshita glass bowl filter (filter particle No. 4, manufactured by Kinoshita Rika Kogyo Co., Ltd.) at a pressure of 0.1 MPa and a flow rate of 0.1 L / min for 5 seconds. Subsequently, 18.4 parts by mass of a 4.5% by mass aqueous sodium persulfate solution was added to the aqueous monomer solution (1). Thereafter, the aqueous monomer solution (1) was immediately poured into a stainless steel bat-shaped container (bottom 340 x 340 mm, height 25 mm, inner surface: Teflon (registered trademark) coated) in an open-to-air state. The time from the start of the second-stage neutralization to the pouring of the aqueous monomer solution (1) into the bat-shaped container was 65 seconds. The bat-shaped container was heated using a hot plate (NEO HOTPLATE HI-1000, manufactured by Iuchi Seieido Co., Ltd.) until the surface temperature reached 50°C.

[0429] The polymerization reaction started within 1 minute after the aqueous monomer solution (1) was poured into the bat-shaped container. During the polymerization reaction, the polymerization of the aqueous monomer solution (1) proceeded while expanding and foaming in all directions and generating steam. The resulting polymer then shrunk to a size slightly larger than the bottom of the bat-shaped container. Two minutes after the start of the polymerization reaction, the hydrogel (1) was removed from the bat-shaped container. This series of operations was carried out in an open-to-air state.

[0430] (Gel Crushing Step) Next, the hydrogel (1) obtained by the polymerization reaction was cut into pieces with a mass of about 60 g per piece, and then the gel was crushed using a meat chopper (22VR-1500SDX, die diameter (hole diameter of the plate) 3.2 mm / manufactured by Alpha Royal Co., Ltd.) to obtain hydrogel particles (1). The amount of the hydrogel (1) introduced was approximately 360 g / min, the temperature of the hydrogel (1) at the time of introduction was 60 ° C., the temperature of the gel crusher was set to 60 ° C., and in parallel with the introduction of the hydrogel (1), deionized water adjusted to 90 ° C. was added to the meat chopper at 50 g / min, and the screw shaft rotation speed of the meat chopper was set to 172 rpm, and gel crushing was performed. The GGE (1) was 87 J / g, and the GGE (2) was 38 J / g. The inner diameter of the casing of the meat chopper was 80 mm, and the peripheral speed of the rotating blades was 0.62 m / s.

[0431] The physical properties of the hydrogel particles (1) are shown in Table 2 below. In this example, when calculating D50, 50 parts by mass of hydrogel particles were immersed in 1,000 parts by mass of a 20% by mass aqueous sodium chloride solution (sodium chloride solution) containing 0.02% by mass of polyoxyethylene (3) lauryl ether sodium sulfate (0.08% by mass of EMAL 20C (surfactant, manufactured by Kao Corporation)), allowed to stand, and then the particle diameter was measured. In this case, the particle diameter after 1 hour of immersion is shown as D50 (1 hr), and the particle diameter after 16 hours of immersion is shown as D50 (16 hr) in Table 2. D50 (16 hr) is considered to be the D50 of the hydrogel particles. D50 (1 hr) is an index of the ease of disintegration of the hydrogel particles, and when D50 (1 hr) is equal to D50 (16 hr), it means that the hydrogel particles have weak adhesion to each other.

[0432] The resulting hydrogel particles (1) had a solid content of 48.6% by mass (water content of 51.4% by mass).

[0433] (Drying step) Next, the hydrogel particles (1) were spread on a wire mesh with a mesh size of 300 μm and placed in a hot air dryer. The hydrogel particles (1) were then dried by passing hot air at 190° C. for 30 minutes to obtain a dried product (1). The resin solid content of the dried product (1) was 97.1% by mass.

[0434] (Classification step) Next, the dried product (1) was charged into a roll mill (WML type roll crusher / manufactured by Inokuchi Giken Co., Ltd.) and crushed, and then classified using two types of JIS standard sieves having openings of 710 μm and 150 μm. By this operation, an irregularly crushed water absorbent resin powder (1) which passed through the sieve having an opening of 710 μm and remained on the sieve having an opening of 150 μm was obtained.

[0435] (Surface Cross-Linking Step) Next, a surface cross-linking agent aqueous solution consisting of 0.4 parts by mass of ethylene carbonate, 0.7 parts by mass of propylene glycol, and 2.9 parts by mass of deionized water was sprayed and added to 100 parts by mass of the water absorbent resin powder (1) and mixed uniformly. Thereafter, the obtained mixture was heat-treated at 206°C for 90 minutes to perform surface cross-linking, thereby obtaining water absorbent resin particles (1). Next, the water absorbent resin particles (1) were classified using two types of JIS standard sieves with mesh sizes of 710 μm and 150 μm. By this operation, a particulate water absorbing agent (1) was obtained that passed through the sieve with a mesh size of 710 μm and remained on the sieve with a mesh size of 150 μm. The physical properties of the particulate water absorbing agent (1) are shown in Table 2.

[0436] Furthermore, the particulate water-absorbing agent (1) was passed through sieves with mesh sizes of 850 μm, 425 μm, 300 μm, and 150 μm in order, and based on the proportion of the particulate water-absorbing agent that passed through each sieve and the proportion of the particulate water-absorbing agent that did not pass through each sieve, the proportion (mass%) of particles having a particle diameter of more than 850 μm in the particulate water-absorbing agent (1); the proportion (mass%) of particles having a particle diameter of less than 150 μm; and the proportion (mass%) of particles having a particle diameter of 300 μm or more and less than 425 μm were calculated. From these results, the standard deviation (σ) was calculated and shown in Table 3 as particle size distribution data. Table 3 also shows the solid content of the hydrogel particles (1), the resin solid content of the dried product (1), and the moisture content of the particulate water-absorbing agent (1).

[0437] [Example 2] A particulate water-absorbing agent (2) was obtained by performing the same operation as in Example 1, except that the amount of the polymerization additive: Obazoline LB-SF (lauryldimethylaminoacetic acid betaine) used in the step of preparing the aqueous monomer solution in Example 1 was changed to 2.949 parts by mass (2447 ppm by mass (0.24% by mass) with respect to the total amount of the monomers) and the heat treatment time in the surface cross-linking step was changed to 80 minutes.

[0438] During the gel crushing step, GGE (1) was 100 J / g, GGE (2) was 51 J / g, the solid content of the hydrous gel particles (2) was 47.1% by mass, and the resin solid content of the dried product (2) was 96.9% by mass. The physical properties of the hydrous gel particles (2) and the particulate water-absorbing agent (2) obtained by the gel crushing step are shown in Table 2. Table 3 also shows the solid content of the hydrous gel particles (2), the resin solid content of the dried product (2) obtained by drying the hydrous gel particles (2), the moisture content of the particulate water-absorbing agent (2), and the particle size distribution data of the particulate water-absorbing agent (2).

[0439] [Example 3] A particulate water-absorbing agent (3) was obtained by performing the same operation as in Example 1, except that the polymerization additive used in the step of preparing an aqueous monomer solution was changed to 1.474 parts by mass (1223 ppm by mass (0.12% by mass) of Latemul ASK (dipotassium alkenyl succinate, manufactured by Kao Corporation)) and that the heat treatment time in the surface cross-linking step was changed to 80 minutes.

[0440] During the gel crushing step, GGE (1) was 101 J / g, GGE (2) was 51 J / g, the solid content of the hydrous gel particles (3) was 47.1% by mass, and the resin solid content of the dried product (3) was 96.7% by mass. The physical properties of the hydrous gel particles (3) and the physical properties of the particulate water-absorbing agent (3) are shown in Table 2. Table 3 also shows the solid content of the hydrous gel particles (3), the resin solid content of the dried product (3) obtained by drying the hydrous gel particles (3), the moisture content of the particulate water-absorbing agent (3), and particle size distribution data of the particulate water-absorbing agent (3).

[0441] [Example 4] A particulate water-absorbing agent (4) was obtained by performing the same operation as in Example 1, except that the amount of the internal cross-linking agent: polyethylene glycol diacrylate (average molecular weight: 523) used in the preparation step of the aqueous monomer solution in Example 1 was changed to 2.76 parts by mass (0.090 mol % with respect to the total amount of the monomers).

[0442] During the gel crushing step, GGE (1) was 102 J / g, GGE (2) was 52 J / g, the solid content of the hydrous gel particles (4) was 47.0% by mass, and the resin solid content of the dried product (4) was 98.5% by mass. The physical properties of the hydrous gel particles (4) and the physical properties of the particulate water-absorbing agent (4) are shown in Table 2. Table 3 also shows the solid content of the hydrous gel particles (4), the resin solid content of the dried product (4) obtained by drying the hydrous gel particles (4), the moisture content of the particulate water-absorbing agent (4), and the particle size distribution data of the particulate water-absorbing agent (4).

[0443] [Example 5] A particulate water-absorbing agent (5) was obtained by performing the same operation as in Example 1, except that the amount of the internal cross-linking agent: polyethylene glycol diacrylate (average molecular weight: 523) used in the preparation step of the aqueous monomer solution in Example 1 was changed to 3.06 parts by mass (0.100 mol % with respect to the total amount of the monomers).

[0444] During the gel crushing step, GGE (1) was 102 J / g, GGE (2) was 52 J / g, the solid content of the hydrous gel particles (5) was 46.7% by mass, and the resin solid content of the dried product (5) was 98.3% by mass. The physical properties of the hydrous gel particles (5) and the physical properties of the particulate water-absorbing agent (5) are shown in Table 2. Table 3 also shows the solid content of the hydrous gel particles (5), the resin solid content of the dried product (5) obtained by drying the hydrous gel particles (5), the moisture content of the particulate water-absorbing agent (5), and the particle size distribution data of the particulate water-absorbing agent (5).

[0445] [Example 6] A particulate water-absorbing agent (6) was obtained by performing the same operation as in Example 1, except that the amount of the internal cross-linking agent: polyethylene glycol diacrylate (average molecular weight: 523) used in the preparation step of the aqueous monomer solution in Example 1 was changed to 2.14 parts by mass (0.070 mol % with respect to the total amount of the monomers).

[0446] During the gel crushing step, GGE (1) was 102 J / g, GGE (2) was 52 J / g, the solid content of the hydrous gel particles (6) was 46.0% by mass, and the resin solid content of the dried product (6) was 98.3% by mass. The physical properties of the hydrous gel particles (6) and the physical properties of the particulate water-absorbing agent (6) are shown in Table 2. Table 3 also shows the solid content of the hydrous gel particles (6), the resin solid content of the dried product (6) obtained by drying the hydrous gel particles (6), the moisture content of the particulate water-absorbing agent (6), and the particle size distribution data of the particulate water-absorbing agent (6).

[0447] [Comparative Example 1] A particulate water-absorbing agent (C1) was obtained by performing the same operation as in Example 1, except that the amount of the internal crosslinking agent: polyethylene glycol diacrylate (average molecular weight: 523) used in the preparation step of the aqueous monomer solution was changed to 1.23 parts by mass (0.040 mol % with respect to the total amount of the monomers).

[0448] During the gel crushing step, GGE (1) was 100 J / g, GGE (2) was 51 J / g, the solid content of the hydrous gel particles (C1) was 47.2% by mass, and the resin solid content of the dried product (C1) was 97.1% by mass. The physical properties of the hydrous gel particles (C1) and the particulate water-absorbing agent (C1) are shown in Table 2. Table 3 also shows the solid content of the hydrous gel particles (C1), the resin solid content of the dried product (C1) obtained by drying the hydrous gel particles (C1), the water content of the particulate water-absorbing agent (C1), and the particle size distribution data of the particulate water-absorbing agent (C1).

[0449] [Comparative Example 2] A particulate water-absorbing agent (C2) was obtained by performing the same operation as in Example 1, except that the amount of the internal cross-linking agent: polyethylene glycol diacrylate (average molecular weight: 523) used in the preparation step of the aqueous monomer solution was changed to 1.83 parts by mass (0.060 mol % with respect to the total amount of the monomers).

[0450] During the gel crushing step, GGE (1) was 89 J / g, GGE (2) was 39 J / g, the solid content of the hydrous gel particles (C2) was 46.6% by mass, and the resin solid content of the dried product (C2) was 98.2% by mass. The physical properties of the hydrous gel particles (C2) and the physical properties of the particulate water-absorbing agent (C2) are shown in Table 2. Table 3 also shows the solid content of the hydrous gel particles (C2), the resin solid content of the dried product (C2) obtained by drying the hydrous gel particles (C2), the moisture content of the particulate water-absorbing agent (C2), and the particle size distribution data of the particulate water-absorbing agent (C2).

[0451] [Comparative Example 3] In Example 1, the polymerization additive used in the preparation step of the aqueous monomer solution was changed to 1.032 parts by mass of polyethylene glycol 600 (mass average molecular weight 600) (2446 mass ppm (0.24 mass%) with respect to the total amount of monomers), and the same operation as in Example 1 was performed to obtain a particulate water-absorbing agent (C3).

[0452] During the gel crushing step, GGE (1) was 90 J / g, GGE (2) was 41 J / g, the solid content of the hydrous gel particles (C3) was 49.4% by mass, and the resin solid content of the dried product (C3) was 97.2% by mass. The physical properties of the hydrous gel particles (C3) and the physical properties of the particulate water-absorbing agent (C3) are shown in Table 2. Table 3 also shows the solid content of the hydrous gel particles (C3), the resin solid content of the dried product (C3) obtained by drying the hydrous gel particles (C3), the moisture content of the particulate water-absorbing agent (C3), and particle size distribution data of the particulate water-absorbing agent (C3).

[0453] [Comparative Example 4] A particulate water-absorbing agent (C4) was obtained by performing the same operation as in Example 1, except that the polymerization additive: Obazoline LB-SF (lauryldimethylaminoacetic acid betaine) used in the preparation step of the aqueous monomer solution in Example 1 was not used.

[0454] During the gel crushing step, GGE (1) was 71 J / g, GGE (2) was 22 J / g, the solid content of the hydrous gel particles (C4) was 46.9% by mass, and the resin solid content of the dried product (C4) was 97.0% by mass. The physical properties of the hydrous gel particles (C4) and the physical properties of the particulate water-absorbing agent (C4) are shown in Table 2. Table 3 also shows the solid content of the hydrous gel particles (C4), the resin solid content of the dried product (C4) obtained by drying the hydrous gel particles (C4), the moisture content of the particulate water-absorbing agent (C4), and the particle size distribution data of the particulate water-absorbing agent (C4).

[0455] Comparative Example 5 A particulate water-absorbing agent (C5) was obtained by carrying out the same operations as in Example 1, except that the die diameter of the meat chopper in the gel crushing step was changed to 6.2 mm.

[0456] During the gel crushing step, GGE (1) was 78 J / g, GGE (2) was 27 J / g, the solid content of the hydrous gel particles (C5) was 47.2% by mass, and the resin solid content of the dried product (C5) was 97.1% by mass. The physical properties of the hydrous gel particles (C5) and the particulate water-absorbing agent (C5) are shown in Table 2. Table 3 also shows the solid content of the hydrous gel particles (C5), the resin solid content of the dried product (C5) obtained by drying the hydrous gel particles (C5), the moisture content of the particulate water-absorbing agent (C5), and the particle size distribution data of the particulate water-absorbing agent (C5).

[0457] [Comparative Example 6] A particulate water-absorbing agent (C6) was obtained by performing the same operation as in Example 1, except that in Example 1, a polymerization additive: Obazoline LB-SF (lauryldimethylaminoacetic acid betaine) was not used in the step of preparing an aqueous monomer solution, and the die diameter of the meat chopper was changed to 6.2 mm in the gel-crushing step.

[0458] During the gel crushing step, GGE (1) was 58 J / g, GGE (2) was 8 J / g, the solid content of the hydrous gel particles (C6) was 46.5% by mass, and the resin solid content of the dried product (C6) was 95.8% by mass. The physical properties of the hydrous gel particles (C6) and the particulate water-absorbing agent (C6) are shown in Table 2. Table 3 also shows the solid content of the hydrous gel particles (C6), the resin solid content of the dried product (C6) obtained by drying the hydrous gel particles (C6), the moisture content of the particulate water-absorbing agent (C6), and particle size distribution data of the particulate water-absorbing agent (C6).

[0459] [Comparative Example 7] The same operation as in Example 1 was performed, except that the amount of an internal crosslinking agent: polyethylene glycol diacrylate (average molecular weight: 523) used in the preparation step of an aqueous monomer solution was changed to 1.68 parts by mass (0.055 mol% with respect to the total amount of monomers), the amount of a polymerization additive was changed to 0.317 parts by mass (750 ppm by mass (0.075% by mass) with respect to the total amount of monomers), and the amount of deionized water was changed to 525 parts by mass, to obtain surface-crosslinked water absorbent resin particles (C7).

[0460] Next, 1.0 mass parts of a 1.0 mass% aqueous solution of diethylenetriaminepentaacetic acid trisodium was added as a chelating agent to 100 mass parts of the surface-crosslinked water-absorbent resin particles (C7) while stirring, and mixed for 1 minute. After that, the mixture was left in a hot air dryer at 60 ° C for 30 minutes, and then passed through a wire mesh with an opening of 850 μm. 30 g of the mixture obtained by this operation was placed in a 225 mL mayonnaise bottle together with 0.18 g of fumed silica (Aerosil 200, manufactured by Nippon Aerosil Co., Ltd.), and shaken for 3 minutes using a paint shaker to obtain a particulate water-absorbing agent (C7).

[0461] During the gel crushing step, GGE (1) was 66 J / g, GGE (2) was 24 J / g, the solid content of the hydrous gel particles (C7) was 41.9% by mass, and the resin solid content of the dried product (C7) was 95.8% by mass. The physical properties of the hydrous gel particles (C7) and the particulate water-absorbing agent (C7) are shown in Table 2. Table 3 also shows the solid content of the hydrous gel particles (C7), the resin solid content of the dried product (C7) obtained by drying the hydrous gel particles (C7), the moisture content of the particulate water-absorbing agent (C7), and the particle size distribution data of the particulate water-absorbing agent (C7).

[0462] [Example 7] In Example 1, except that the polymerization additive used in the preparation step of the aqueous monomer solution was changed to 1.474 parts by mass of EMAL 20C (manufactured by Kao Corporation, polyoxyethylene lauryl ether sodium sulfate) (1223 mass ppm (0.12 mass%) with respect to the total amount of monomers), and the die diameter of the meat chopper in the gel crushing step was changed to 3.5 mm, the same operation as in Example 1 was performed to obtain a particulate water-absorbing agent (7).

[0463] During the gel crushing step, GGE (1) was 97 J / g, GGE (2) was 47 J / g, the solid content of the hydrous gel particles (7) was 47.0% by mass, and the resin solid content of the dried product (7) was 98.2% by mass. The physical properties of the hydrous gel particles (7) and the physical properties of the particulate water-absorbing agent (7) are shown in Table 2. Table 3 also shows the solid content of the hydrous gel particles (7), the resin solid content of the dried product (7) obtained by drying the hydrous gel particles (7), the moisture content of the particulate water-absorbing agent (7), and the particle size distribution data of the particulate water-absorbing agent (7).

[0464] [Example 8] In Example 1, except that the polymerization additive used in the preparation step of the aqueous monomer solution was changed to 1.474 parts by mass of EMAL 20F (manufactured by Kao Corporation, alkyl sulfate ester salt) (1223 mass ppm (0.12 mass%) with respect to the total amount of monomers), and the die diameter of the meat chopper in the gel crushing step was changed to 3.5 mm, the same operation as in Example 1 was performed to obtain a particulate water-absorbing agent (8).

[0465] During the gel crushing step, GGE (1) was 100 J / g, GGE (2) was 51 J / g, the solid content of the hydrous gel particles (8) was 46.8% by mass, and the resin solid content of the dried product (8) was 98.1% by mass. The physical properties of the hydrous gel particles (8) and the physical properties of the particulate water-absorbing agent (8) are shown in Table 2. Table 3 also shows the solid content of the hydrous gel particles (8), the resin solid content of the dried product (8) obtained by drying the hydrous gel particles (8), the moisture content of the particulate water-absorbing agent (8), and particle size distribution data of the particulate water-absorbing agent (8).

[0466] [Example 9] In Example 1, except that the polymerization additive used in the preparation step of the aqueous monomer solution was changed to Nissan Anon LA (manufactured by NOF Corporation, sodium laurylaminodiacetate) 1.474 parts by mass (1223 ppm by mass (0.12% by mass) with respect to the total amount of the monomers), and the die diameter of the meat chopper in the gel crushing step was changed to 3.5 mm, a particulate water-absorbing agent (9) was obtained by performing the same operation as in Example 1.

[0467] During the gel crushing step, GGE (1) was 99 J / g, GGE (2) was 50 J / g, the solid content of the hydrous gel particles (9) was 46.7% by mass, and the resin solid content of the dried product (9) was 98.3% by mass. The physical properties of the hydrous gel particles (9) and the physical properties of the particulate water-absorbing agent (9) are shown in Table 2. Table 3 also shows the solid content of the hydrous gel particles (9), the resin solid content of the dried product (9) obtained by drying the hydrous gel particles (9), the moisture content of the particulate water-absorbing agent (9), and the particle size distribution data of the particulate water-absorbing agent (9).

[0468] [Example 10] In Example 1, except that the polymerization additive used in the preparation step of the aqueous monomer solution was changed to 1.474 parts by mass (1223 ppm by mass (0.12% by mass) with respect to the total amount of monomers) of Plysurf A212C (manufactured by Daiichi Kogyo Seiyaku Co., Ltd., phosphoric acid ester salt of higher alcohol alkylene oxide adduct), and the die diameter of the meat chopper in the gel crushing step was changed to 3.5 mm, the same operation as in Example 1 was performed to obtain a particulate water-absorbing agent (10).

[0469] In addition, during the gel crushing step, GGE (1) was 99 J / g, GGE (2) was 50 J / g, the solid content of the hydrous gel particles (10) was 47.0% by mass, and the resin solid content of the dried product (10) was 98.4% by mass. The physical properties of the hydrous gel particles (10) and the physical properties of the particulate water-absorbing agent (10) are shown in Table 2. In addition, the solid content of the hydrous gel particles (10), the resin solid content of the dried product (10) obtained by drying the hydrous gel particles (10), the moisture content of the particulate water-absorbing agent (10), and particle size distribution data of the particulate water-absorbing agent (10) are shown in Table 3.

[0470] [Example 11] In Example 1, except that the polymerization additive used in the preparation step of the aqueous monomer solution was changed to 1.474 parts by mass of KOTAMIN 86W (manufactured by Kao Corporation, stearyl trimethyl ammonium chloride) (1223 mass ppm (0.12 mass%) with respect to the total amount of monomers), and the die diameter of the meat chopper in the gel crushing step was changed to 3.5 mm, the same operation as in Example 1 was performed to obtain a particulate water-absorbing agent (11).

[0471] In addition, the GGE (1) at the time of the gel crushing step was 97 J / g, the GGE (2) was 47 J / g, the solid content of the hydrous gel particles (11) was 47.3 mass%, and the resin solid content of the dried product (11) was 98.0 mass%. The physical properties of the hydrous gel particles (11) and the physical properties of the particulate water-absorbing agent (11) are shown in Table 2. In addition, the solid content of the hydrous gel particles (11), the resin solid content of the dried product (11) obtained by drying the hydrous gel particles (11), the moisture content of the particulate water-absorbing agent (11), and the particle size distribution data of the particulate water-absorbing agent (11) are shown in Table 3.

[0472] [Evaluation of Particulate Water Absorbing Agent] (Water Absorption Rate Index Under Pressure) To evaluate the performance of the particulate water absorbing agent, the water absorption rate index under pressure, which is "total volume ratio of cavities x (AAP) 0.7 psi (4.83 kPa)", was calculated and shown in Table 1.

[0473] [Evaluation of Absorbent Body Performance] (Method for Evaluating the Liquid Uptake Rate of the Absorbent Body) The liquid uptake rate of the absorbent body was measured using the device shown in FIG.

[0474] The absorbent body to be measured was prepared by the following method. First, a 160 mm x 80 mm, approximately 0.1 mm thick water-absorbent paper 12 was placed in an acrylic resin container 11 (inner dimensions 160 mm x 80 mm, height 30 mm), and then 2.0 g of a particulate water-absorbing agent 13 was uniformly spread thereon. A liquid-permeable surface sheet 14 (Pampers activebaby, serial number 3041-45-1812, 2013 / 2 / 10 18:57 made in Poland, cut to 160 mm x 80 mm, approximately 0.1 mm thick) taken from a commercially available paper diaper was placed thereon, and a SUS mesh (160 mm x 80 mm, opening 1 mm, thickness 0.1 mm) was placed thereon to prepare a model diaper absorbent body 18.

[0475] Then, the liquid injection device 16 (weight: 80 g, load on the absorbent body: 0.63 g / cm ) was set so that a load was evenly applied on the absorbent body 18 of the model diaper. ２ (0.06 kPa), having a cylinder with a diameter of 20 mm and a height of 120 mm, into whose center a liquid can be poured. ２Two 750 g weights 17 were placed around the cylinder of the liquid injection device 16 so that a load of (1.2 kPa) was evenly applied, and then 20 g of 25°C saline (0.90% sodium chloride aqueous solution) was quickly (in one go) poured into the cylinder. The time from the start of pouring the saline until the saline was completely absorbed by the absorbent body was measured and defined as the first saline uptake rate (liquid uptake rate 1) (seconds). 60 seconds after the first injection of saline, 20 g of the 25°C saline was quickly (in one go) poured into the cylinder. The time from the start of pouring the saline until the saline was completely absorbed by the absorbent body 18 was measured and defined as the second saline uptake rate (liquid uptake rate 2) (seconds).

[0476] (Absorbent Body Evaluation Results) For each of the particulate water absorbing agents (1) to (11) of Examples 1 to 6 and the particulate water absorbing agents (C1) to (C7) of Comparative Examples 1 to 7, the absorbent body 18 was produced as the particulate water absorbing agent 13, and measurements were made to evaluate the liquid uptake rate 1 and the liquid uptake rate 2. The total time of the liquid uptake rate 1 and the liquid uptake rate 2 was defined as the "total liquid uptake rate," and the results are shown in Table 4 below.

[0477]

[0478]

[0479]

[0480] As shown in Examples 1 to 11, it is understood that the particulate water-absorbing agents (1) to (11) having specific particle diameters and specific shapes (i.e., the total volume ratio of cavities and the void / cavity ratio are in specific ranges) exhibit excellent AAP and SFC, and also exhibit excellent water absorption speed (Vortex), and exhibit excellent absorption characteristics. It is also understood that the water-absorbing bodies using the particulate water-absorbing agents (1) to (11) having excellent absorption characteristics exhibit excellent uptake speeds.

[0481] On the other hand, as shown in Comparative Examples 1 to 7, the particulate water-absorbing agents (C1) to (C7) whose total volume ratio of cavities and void / cavity ratio are out of the specific ranges have a slow water absorption rate (Vortex) and are inferior in absorption characteristics compared to the particulate water-absorbing agents (1) to (11) of Examples 1 to 11. Furthermore, it is understood that the water-absorbing bodies using the particulate water-absorbing agents (C1) to (C7) also have a slow liquid uptake rate.

[0482] It should be noted that published research results show that the urination time of mammals is not proportional to the mammal's body size, and that the urination time of almost all mammals weighing 3 kg or more is approximately 21 seconds on average (Patricia J Yang, et al., "Duration of urination does not change with body size", Proceedings of the National Academy of Sciences, , June 26, 2014, 111 (33) 11932-11937). Therefore, the inventors determined that if the urination time is within 3 seconds of the average urination time, i.e., if the total liquid uptake rate of the absorbent body is 24 seconds or less, the degree of discomfort caused by urination is low, whereas if the urination time is 5 seconds or more from the average urination time, i.e., if the total liquid uptake rate of the absorbent body is 27 seconds or more, the degree of discomfort increases. That is, in the present invention, an absorbent body with a total liquid uptake rate of 24 seconds or less is set as a suitable example of the present invention. Furthermore, the water absorption rate (Vortex) was 16 seconds for Comparative Example 2 and 15 seconds for Comparative Example 7, which were almost the same as those of the Examples, but the total liquid uptake rate of the absorbent body was 27 seconds and 31 seconds, respectively, which were unfavorable results. From this, it can be seen that evaluation of the water absorption rate (Vortex) measured under no pressure alone is insufficient, and that the void / cavity ratio and water absorption rate index under pressure (product of total cavity volume fraction and AAP 0.7 psi (4.83 kPa)) newly introduced in the present invention have a higher correlation with the evaluation of the absorbent body.

[0483] This application is based on Japanese Patent Application No. 2024-105054, filed on June 28, 2024, the disclosure of which is incorporated by reference in its entirety.

[0484] REFERENCE SIGNS LIST 1...particulate water-absorbing agent 1a...cavity (void connected to the outside) 1b...void (void not connected to the outside) 11...acrylic resin container, 12...absorbent paper, 13...particulate water-absorbing agent, 14...absorbent paper, 15...surface sheet, 16...liquid injection device, 17...weight, 18...absorbent body.

Claims

1. A poly(meth)acrylic acid (salt)-based particulate water-absorbing agent whose main component is poly(meth)acrylic acid (salt)-based water-absorbing resin particles, wherein the particulate water-absorbing agent includes cavities, which are spaces that communicate with the outside, and voids, which are closed spaces that do not communicate with the outside, wherein the total volume ratio of the cavities is 18% by volume or more, wherein the void / cavity ratio, which is the ratio of the total volume ratio of the cavities to the total volume ratio of the voids, is 0.015 or more and 0.025 or less, and wherein the mass average particle diameter (D50) of the particulate water-absorbing agent is 300 μm or more and 450 μm or less.

2. The poly(meth)acrylic acid (salt)-based particulate water-absorbing agent according to claim 1, which has an absorbency against pressure (AAP) of 24 g / g or more at 0.7 psi (4.83 kPa).

3. Saline flow conductivity (SFC) is 15 x 10 －７ cm ３ 3. The poly(meth)acrylic acid (salt)-based particulate water-absorbing agent according to claim 1 or 2, wherein the water-absorbing capacity is 1 / 3 sec / g or more.

4. The poly(meth)acrylic acid (salt)-based particulate water-absorbing agent according to claim 1 or 2, which has a water absorption capacity without load (CRC) of 25 g / g or more.

5. The poly(meth)acrylic acid (salt)-based particulate water-absorbing agent according to claim 1 or 2, wherein a water absorption rate index under pressure, which is the product of the total volume ratio of the cavities and an absorption capacity under pressure (AAP) of 0.7 psi (4.83 kPa), is 460 or more.

6. A sanitary material comprising the poly(meth)acrylic acid (salt)-based particulate water-absorbing agent according to claim 1 or 2.

7. A method for producing a poly(meth)acrylic acid (salt)-based particulate water-absorbing agent containing poly(meth)acrylic acid (salt)-based water-absorbing resin particles as a main component, the method comprising: (i) a step of preparing a (meth)acrylic acid (salt)-based monomer aqueous solution; (ii) a step of polymerizing the (meth)acrylic acid (salt)-based monomer aqueous solution to obtain a hydrogel-like crosslinked polymer; (iii) a step of gel-crushing the hydrogel-like crosslinked polymer during or after polymerization to obtain hydrogel particles; (iv) a step of drying the hydrogel particles to obtain a dried product; (v) a step of pulverizing and / or classifying the dried product to obtain a water-absorbent resin powder; and (vi) a step of surface-crosslinking the water-absorbent resin powder to obtain water-absorbent resin particles; and in at least one of the step (i) and the step (ii), a polymerization additive is added to the (meth)acrylic acid (salt)-based monomer aqueous solution. the polymerization additive is one or more compounds selected from amphoteric substances, anionic substances, and cationic substances; and in the step (iii), the gel-crushing energy (GGE) (2) of the hydrogel crosslinked polymer is 30 J / g or more; A method for producing a poly(meth)acrylic acid (salt)-based particulate water-absorbing agent, wherein, for the hydrous gel particles obtained in step (iii), 50 parts by mass of the hydrous gel particles are immersed in 1,000 parts by mass of an aqueous solution containing 0.02 mass% of sodium polyoxyethylene (3) lauryl ether sulfate and 20 mass% of sodium chloride, and the mass average particle diameter after 1 hour of immersion is defined as D50 (1 hr) and the mass average particle diameter after 16 hours of immersion is defined as D50 (16 hr), the ratio of D50 (1 hr) to D50 (16 hr), i.e., D50 (16 hr) / D50 (1 hr), is 0.20 or more and 0.85 or less, and D50 (1 hr) is 350 μm or more.

8. The saline flow conductivity (SFC) of the poly(meth)acrylic acid (salt)-based particulate water-absorbing agent is 15 x 10 －７ cm ３ 8. The method for producing a poly(meth)acrylic acid (salt)-based particulate water-absorbing agent according to claim 7, wherein the water-absorbing capacity is adjusted to be .0 sec / g or more.

9. The method for producing a poly(meth)acrylic acid (salt)-based particulate water-absorbing agent according to claim 7 or 8, wherein the mass-average particle diameter (D50) of the hydrogel particles is 150 μm or more and 500 μm or less.

10. The method for producing a poly(meth)acrylic acid (salt)-based particulate water absorbent according to claim 7 or 8, wherein the amphoteric substance is at least one selected from (a) alkylbetaine and (b) alkylamine oxide, the anionic substance is at least one selected from (c) sulfate ester salts of higher alcohol alkylene oxide adducts, (d) dicarboxylate salts, (e) sulfonates, (f) alkyl sulfate ester salts, (g) alkylamine diacetate salts, (h) phosphate ester salts of higher alcohol alkylene oxide adducts, and (i) carboxylate salts of higher alcohol alkylene oxide adducts, and the cationic substance is (j) ammonium salt.

11. A method for producing a poly(meth)acrylic acid (salt)-based particulate water absorbent according to claim 7 or 8, wherein the amount of the polymerization additive added is 0.01 mass % or more and 5 mass % or less relative to the total amount of monomers contained in the (meth)acrylic acid (salt)-based monomer aqueous solution (100 mass % of raw material monomers).

12. A method for producing a poly(meth)acrylic acid (salt)-based particulate water-absorbing agent according to claim 7 or 8, wherein the polymerization additive is one or more selected from the group consisting of (a) alkyl betaine and (d) dicarboxylate salt.

13. The method for producing a poly(meth)acrylic acid (salt)-based particulate water absorbent according to claim 10, wherein the (d) dicarboxylate is at least one selected from the group consisting of alkenyl succinates and acylaspartates.

14. A sanitary material comprising a poly(meth)acrylic acid (salt)-based particulate water-absorbing agent obtained by the method for producing a poly(meth)acrylic acid (salt)-based particulate water-absorbing agent according to claim 7 or 8.

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