Method for producing water absorbent

Abstract

To provide a method for producing a water absorbent that allows suppression of agglomeration in a heating step of coated resin particles.SOLUTION: One aspect of the present invention relates to a method for producing a water absorbent, comprising a heating step of providing the water absorbent by heating, at a temperature at or above the glass transition temperature of a polymer component, mixed particles containing coated resin particles, which are water-absorbent resin particles with their surfaces at least partially coated with a coating material containing the polymer component, and a lubricant.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for producing a water-absorbing agent. [Background technology]

[0002] Conventionally, water-absorbent resin particles have been used in a wide range of fields, such as sanitary materials such as disposable diapers, sanitary napkins, and pet sheets; industrial materials such as water-stopping materials for cables and water-stopping materials for civil engineering; and daily commodities such as cooling gels, body warmers, air fresheners, and portable toilets.

[0003] In general, when water-absorbent resin particles are used as a water-absorbing agent in sanitary materials such as disposable diapers, sanitary napkins, and pet sheets, it is required to control the water-absorbing properties of the water-absorbing agent, such as the water retention capacity, the water absorption capacity under load, the amount of water-soluble components, and the water-absorption rate (for example, Patent Document 1). For example, if the water-absorption rate is too fast, the water-absorbent resin particles reach a swollen state in a short time when they come into contact with a liquid to be absorbed, such as urine, and this causes gel blocking (a phenomenon in which swollen gel-like water-absorbent resin particles hinder the diffusion of the liquid, thereby slowing down the penetration rate of the liquid into the diaper, etc.), which leads to a poor feeling in use and the occurrence of liquid pooling. Therefore, in order to adjust the water-absorption properties of the water-absorbing agent (for example, to slow down the water-absorption rate), a method has been proposed in which water-absorbent resin particles are coated with a coating material and then heat-treated at a predetermined temperature (Patent Document 2). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2006-176570 [Patent Document 2] WO2022 / 209536 publication Summary of the Invention [Problem to be solved by the invention]

[0005] The present inventors have found that when water-absorbent resin particles coated with a coating material containing a polymer component are heat-treated as a method for adjusting the water absorption properties of a water-absorbing agent, the coating material softens depending on the heating temperature (particularly when the heating temperature is equal to or higher than the glass transition temperature of the polymer component contained in the coating material), and the coated resin particles may adhere to each other via the softened coating material (hereinafter referred to as aggregation).

[0006] Therefore, an object of the present invention is to provide a method for producing a water-absorbing agent that can suppress aggregation in a heat treatment step of coated resin particles. [Means for solving the problem]

[0007] This specification includes at least the following inventions [1] to [7]. [1] A method for producing a water-absorbing agent, comprising a heat treatment step of heating mixed particles containing a lubricant and coated resin particles, which are water-absorbing resin particles at least partly coated on the surface with a coating material containing a polymer component, at a temperature equal to or higher than the glass transition temperature of the polymer component to obtain a water-absorbing agent. [2] The method for producing a water-absorbing agent according to [1], wherein the heat treatment step is carried out at a temperature lower than the thermal decomposition temperature of the polymer component. [3] The method for producing a water-absorbing agent according to [1] or [2], wherein the heat treatment step is carried out at a temperature 5°C to 180°C higher than the glass transition temperature of the polymer component. [4] The method for producing a water-absorbing agent according to any one of [1] to [3], wherein the content of the lubricant in the mixed particles is 0.05% by mass to 3% by mass. [5] The method for producing a water-absorbing agent according to any one of [1] to [4], wherein the lubricant contains an inorganic compound. [6] The method for producing a water-absorbing agent according to [5], wherein the inorganic compound is silica. [7] The method for producing a water-absorbing agent according to any one of [1] to [6], wherein the lubricant contains particles having a volume average particle size of 1 μm to 100 μm. [Effects of the Invention]

[0008] According to the present invention, it is possible to provide a method for producing a water-absorbing agent that can suppress aggregation in the heat treatment step of coated resin particles. DETAILED DESCRIPTION OF THE INVENTION

[0009] Hereinafter, embodiments of the present invention will be described in detail. However, the present invention is not limited to the following embodiments and can be practiced in various modifications within the scope of the present invention.

[0010] In this specification, "acrylic" and "methacrylic" are collectively referred to as "(meth)acrylic." "Acrylate" and "methacrylate" are similarly referred to as "(meth)acrylate." "(Poly)" refers to both cases with and without the prefix "poly." In the numerical ranges described in stages in this specification, the upper or lower limit of a certain numerical range can be arbitrarily combined with the upper or lower limit of another numerical range. In the numerical ranges described in this specification, the upper or lower limit of that numerical range may be replaced with a value shown in the Examples. The various materials and agents exemplified in this specification may be used alone or in combination of two or more. When multiple substances corresponding to each component are present in the composition, the content of each component in the composition refers to the total amount of those multiple substances present in the composition, unless otherwise specified. "Water-soluble" means a solubility of 1 g or more (e.g., 1 to 150 g) in 100 g of ion-exchanged water at 25°C. "Poorly water-soluble" means a solubility of less than 1 g in 100 g of ion-exchanged water at 25°C. Unless otherwise specified, the examples and comparative examples were carried out in an environment of 1 atmosphere, normal temperature and humidity, and the various parameters disclosed in this specification were also measured in the same environment, with the temperature of the various samples being normal temperature. "1 atmosphere" is 101,325 Pa, "normal temperature" is 25°C, and "normal humidity" is 50% RH.

[0011] The method for producing a water-absorbing agent of the present invention includes a heat treatment step of heating mixed particles containing a lubricant and coated resin particles, which are water-absorbing resin particles at least partly coated on the surface with a coating material containing a polymer component, at a temperature equal to or higher than the glass transition temperature of the polymer component to obtain a water-absorbing agent.

[0012] According to the method for producing a water-absorbing agent of the present invention, it is possible to suppress the aggregation of coated resin particles. As a result, it is possible to reduce agglomerated out-of-specification products and waste. The present inventors speculate that this is because the lubricant prevents the coated resin particles from adhering to each other, thereby suppressing aggregation even when heated at a temperature equal to or higher than the glass transition temperature of the polymer component contained in the coating material (i.e., even when the coating material is softened).

[0013] <1. Water-absorbent resin particles and their manufacturing method> The water-absorbent resin particles (subject to be coated) used in the present invention are particles containing a resin with water absorption properties, and are not particularly limited as long as they have water absorption properties. The water-absorbent resin particles may be, for example, polymer particles formed by polymerization of a monomer containing an ethylenically unsaturated monomer, or particles containing a component derived from cellulose, starch, or the like, and polymer particles are preferred from the viewpoint of ease of mass production. The polymer particles can be produced, for example, by a method including a step of polymerizing a monomer containing an ethylenically unsaturated monomer. Examples of the polymerization method include reverse-phase suspension polymerization, aqueous solution polymerization, bulk polymerization, and precipitation polymerization.

[0014] The ethylenically unsaturated monomer may be a water-soluble ethylenically unsaturated monomer. Examples of water-soluble ethylenically unsaturated monomers include (meth)acrylic acid and its salts, 2-(meth)acrylamido-2-methylpropanesulfonic acid and its salts, (meth)acrylamide, N,N-dimethyl(meth)acrylamide, 2-hydroxyethyl(meth)acrylate, N-methylol(meth)acrylamide, polyethylene glycol mono(meth)acrylate, N,N-diethylaminoethyl(meth)acrylate, N,N-diethylaminopropyl(meth)acrylate, and diethylaminopropyl(meth)acrylamide. When the ethylenically unsaturated monomer has an amino group, the amino group may be quaternized. The ethylenically unsaturated monomer may be used alone or in combination of two or more.

[0015] When the ethylenically unsaturated monomer has an acidic group or a basic group, the functional group may be neutralized with a neutralizing agent before use in the polymerization reaction, or may be neutralized after the polymerization reaction. The degree of neutralization of the ethylenically unsaturated monomer with the neutralizing agent may be, for example, 10 to 100 mol %, 50 to 90 mol %, or 60 to 80 mol % of the acidic group in the ethylenically unsaturated monomer.

[0016] From the viewpoint of industrial ease of availability, the ethylenically unsaturated monomer may include at least one compound selected from the group consisting of (meth)acrylic acid and its salts, acrylamide, methacrylamide, and N,N-dimethylacrylamide. The ethylenically unsaturated monomer may include at least one compound selected from the group consisting of (meth)acrylic acid and its salts, and acrylamide.

[0017] Monomers other than the above-mentioned ethylenically unsaturated monomers may be used as monomers for obtaining polymer particles. Such monomers may be used by mixing them with an aqueous solution containing the above-mentioned ethylenically unsaturated monomer. The amount of the ethylenically unsaturated monomer used may be 70 to 100 mol% based on the total amount of monomers. The proportion of (meth)acrylic acid and its salts may be 70 to 100 mol% based on the total amount of monomers.

[0018] During polymerization, crosslinking occurs by self-crosslinking or physical crosslinking (e.g., formation of an interpenetrating polymer network), but crosslinking may be promoted by using an internal crosslinking agent. The use of an internal crosslinking agent makes it easy to control the water absorption properties of the polymer particles (water retention capacity, water absorption rate, etc.). The internal crosslinking agent is usually added to the reaction solution used in the polymerization reaction.

[0019] The polymer particles may be crosslinked near the surface (surface crosslinked). The polymer particles may further contain a component selected from, for example, a gel stabilizer, a metal chelating agent, and a flowability improver. These components may be located inside the polymer particles, on the surface of the polymer particles, or both.

[0020] The polymer particles may be substantially spherical, crushed, or granular, or may be in the form of an aggregate of primary particles having these shapes. The median particle size of the polymer particles may be 100 μm to 800 μm, 150 μm to 700 μm, 200 μm to 600 μm, or 250 μm to 500 μm.

[0021] <2. Coated Resin Particles and Their Manufacturing Method> The coated resin particles used in the present invention have a coating layer formed on the surface of the water-absorbent resin particle by a coating material. The coating layer may cover at least a part of the surface of the water-absorbent resin particle, or may cover the entire surface of the water-absorbent resin particle. The coating layer may have a single-layer structure or a multi-layer structure having two or more layers.

[0022] The coating layer may contain a poorly water-soluble component (for example, a poorly water-soluble polymer component described later), or may contain a water-soluble component (for example, a water-soluble polymer component described later), or may contain both of them. For example, by coating with a coating material containing a poorly water-soluble component, the water absorption rate of the water-absorbing agent can be adjusted to be slow, and further, moisture absorption of the water-absorbing agent during the production of diapers and the like can be easily reduced. Furthermore, for example, by coating with a coating material containing a water-soluble component, the hydrophilicity of the surface of the water-absorbing agent can be increased. One type of coating material may be used alone, or two or more types may be used in combination.

[0023] With regard to coated resin particles, the content of the coating layer or the amount of the coating material used for coating may be in the following ranges relative to 100 parts by mass of the water-absorbent resin particles. From the viewpoint of easily adjusting the water absorption properties of the coated resin particles, the content or amount may be 0.10 parts by mass or more, 0.50 parts by mass or more, 1.00 parts by mass or more, 2.00 parts by mass or more, 3.00 parts by mass or more, 4.00 parts by mass or more, 5.00 parts by mass or more, 6.00 parts by mass or more, 7.00 parts by mass or more, 7.50 parts by mass or more, 8.00 parts by mass or more, 9.00 parts by mass or more, 10.00 parts by mass or more, 11.00 parts by mass or more, or 12.00 parts by mass or more. From the viewpoint of improving productivity, such as shortening the coating time, the content or amount used may be 50.00 parts by mass or less, 40.00 parts by mass or less, 30.00 parts by mass or less, 25.00 parts by mass or less, 20.00 parts by mass or less, 18.00 parts by mass or less, 15.00 parts by mass or less, 12.00 parts by mass or less, 11.00 parts by mass or less, 10.00 parts by mass or less, less than 10.00 parts by mass, 9.00 parts by mass or less, 8.00 parts by mass or less, 7.50, 7.00 parts by mass or less, 6.00 parts by mass or less, or 5.00 parts by mass or less. From these viewpoints, the content or amount used may be 0.10 to 50.00 parts by mass, 0.10 to 20.00 parts by mass, 0.10 to 15.00 parts by mass, 0.10 to 10.00 parts by mass, 0.50 to 50.00 parts by mass, 0.50 to 20.00 parts by mass, 0.50 to 10.00 parts by mass, 1.00 to 50.00 parts by mass, 1.00 to 20.00 parts by mass, 1.00 to 15.00 parts by mass, 1.00 to 10.00 parts by mass, 1.00 to 9.00 parts by mass, 3.00 to 50.00 parts by mass, 3.00 to 20.00 parts by mass, 3.00 to 10.00 parts by mass, 5.00 to 50.00 parts by mass, 5.00 to 20.00 parts by mass, or 5.00 to 10.00 parts by mass.

[0024] The coating layer or coating material contains a polymer component, and preferably contains the polymer component as the main component (the component with the largest content (by mass)). The glass transition temperature (Tg) of the polymer component contained in the coating layer or coating material may be, for example, −30° C. to 200° C., 50 to 100° C., 52 to 90° C., 55 to 80° C., or 60 to 70° C., from the viewpoint of easily forming a coating layer with a uniform thickness.

[0025] When multiple polymer components are present, the "Tg of the polymer component" refers to the Tg of the main polymer component (the polymer component with the highest content (by mass)) contained in the coating layer or coating material. When multiple main polymer components are present (for example, when the coating layer or coating material contains equal amounts of two polymer components), the "Tg of the polymer component" refers to the polymer component with the lowest Tg among the multiple main polymer components. The same applies to the "thermal decomposition temperature of the polymer component" described below. Furthermore, when the main polymer component has multiple Tgs, the lowest value is used as the Tg in this application.

[0026] The proportion of the polymer component in the coating material or coating layer may be, for example, 10 to 100 mass %, 30 to 100 mass %, 50 to 100 mass %, 70 to 100 mass %, or 90 to 100 mass %.

[0027] The polymer component may be water-soluble or poorly water-soluble. The polymer component may include a water-soluble component or a poorly water-soluble component. Typically, the water-soluble (or poorly water-soluble) polymer component includes a water-soluble component (or a poorly water-soluble component).

[0028] The water-soluble component may contain a compound having a hydrophilic group. Examples of hydrophilic groups include anionic groups, cationic groups, amphoteric groups, and nonionic groups. Examples of anionic groups include carboxyl groups, sulfonic acid groups, and phosphate groups. Examples of cationic groups include amino groups, imino groups, and quaternary ammonium groups. Examples of amphoteric groups include carbobetaine groups, sulfobetaine groups, and phosphobetaine groups. Examples of nonionic groups include hydroxyl groups, amide groups, pyrrolidone groups, lactam groups, alkoxy groups, and (poly)oxyalkylene groups.

[0029] Examples of compounds having a hydroxyl group include polyvinyl alcohol, etc. Examples of compounds having an amide group include polyacrylamide, etc. Examples of compounds having a (poly)oxyalkylene group include polyalkylene oxides (e.g., polyethylene oxide) and polyalkylene glycols (e.g., polyethylene glycol).

[0030] Examples of poorly water-soluble components include polyoxyalkylene alkyl ethers such as polyoxyethylene oleyl ether, polyoxyethylene lauryl ether, and polyoxyethylene stearyl ether; polyesters such as polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, and polybutylene naphthalate; polyamides such as nylon 6 and nylon 66; polyolefins such as polyethylene, polypropylene, ethylene-butene copolymers, and ethylene-propylene copolymers; and polystyrenes such as poly-α-methylstyrene and syndiotactic polystyrene. poly(meth)acrylates such as trimethylolpropane tri(meth)acrylate, pentaerythritol tri(meth)acrylate, and dipentaerythritol hexa(meth)acrylate; poly(alkyl(meth)acrylates) such as polymethyl(meth)acrylate; polyacetals such as polyoxymethylene, polyacetaldehyde, polypropionaldehyde, and polybutyraldehyde; halogenated vinyl polymers such as polyvinyl chloride and polyvinyl fluoride; polyvinylidene fluoride; and polysiloxanes.

[0031] The polymer component may contain a polymer having an ethylenically unsaturated monomer as a monomer unit (a polymer having a structural unit derived from an ethylenically unsaturated monomer). Examples of the ethylenically unsaturated monomer include (meth)acrylic acid and its salts, (meth)acrylic acid esters (methyl (meth)acrylate, ethyl (meth)acrylate, 2-hydroxyethyl (meth)acrylate, 2-(diethylamino)ethyl (meth)acrylate, 2-(diethylamino)propyl (meth)acrylate, etc.), (meth)acrylamide monomers ((meth)acrylamide, N-isopropyl(meth)acrylamide, 2-(meth)acrylamido-2-methylpropanesulfonic acid and its salts, N,N-dimethyl(meth)acrylamide, N-methylol(meth)acrylamide, diethylaminopropyl(meth)acrylamide, etc.), and polyethylene glycol mono(meth)acrylate.

[0032] As the polymer component, chain polymerization products such as poly(meth)acrylic acid, poly(meth)acrylamide, polyvinyl alcohol, polyalkylene oxide, and polyalkylene glycol; and step-growth polymerization products such as phenolic resins (for example, condensates of phenolic compounds and aldehydes), polyesters, polyamides, and polycarbonates may be used.

[0033] The polymer component may be acid-modified, for example, with an acid anhydride (maleic anhydride, succinic anhydride, phthalic anhydride, etc.).

[0034] From the viewpoint of slowing down the water absorption rate and making it easier to adjust, the polymer component may contain at least one selected from the group consisting of polyvinyl alcohol, polyacrylamide, polyalkylene oxide, polyalkylene glycol, polyoxyalkylene alkyl ether, poly(alkyl(meth)acrylate), polyolefin, olefin-water-soluble ethylenically unsaturated monomer copolymer, and copolymers of the monomers that constitute these polymers, or may contain at least one selected from the group consisting of polyvinyl alcohol, polyalkylene glycol, poly(alkyl(meth)acrylate, and olefin-water-soluble ethylenically unsaturated monomer copolymer. When the polymer component contains an olefin-water-soluble ethylenically unsaturated monomer copolymer, it is preferable to use a mixed composition in combination with polyalkylene glycol as the polymer component, and it is more preferable to use a mixed composition in combination with polyethylene glycol as the polymer component.

[0035] As the olefin, which is a structural unit of the polyolefin, and as the olefin, which is a structural unit of the olefin-water-soluble ethylenically unsaturated monomer copolymer, for example, at least one selected from ethylene, propylene, and butene is preferably used, and ethylene is more preferably used. As the water-soluble ethylenically unsaturated monomer, the compounds listed as the structural materials of the polymer particles above can be used, and preferably (meth)acrylic acid and / or a salt thereof is used.

[0036] The coating layer can be formed by bringing a liquid or gel-like coating material (hereinafter simply referred to as "coating liquid") into contact with water-absorbent resin particles. The coating liquid can be obtained by melting the coating material, or by dissolving or dispersing the coating material in an arbitrary solvent or dispersion medium. Since it is easy to form a coating layer with a uniform thickness, it is preferable to obtain the coating liquid by dissolving or dispersing the coating material in an arbitrary solvent or dispersion medium.

[0037] Examples of solvents or dispersion media include water, hydrophilic compounds, mixtures of water and hydrophilic compounds, and hydrocarbon compounds. Hydrophilic compounds are compounds that dissolve substantially uniformly in water. Examples of hydrophilic compounds include alcohols such as ethanol and isopropyl alcohol; glycols such as ethylene glycol; cellosolves such as methyl cellosolve and ethyl cellosolve; ketones such as acetone and methyl ethyl ketone; esters such as ethyl acetate; and ethers such as tetrahydrofuran. Examples of hydrocarbon compounds include chain aliphatic hydrocarbons such as n-hexane, n-heptane, 2-methylhexane, 3-methylhexane, 2,3-dimethylpentane, 3-ethylpentane, and n-octane; alicyclic hydrocarbons such as cyclohexane, methylcyclohexane, cyclopentane, methylcyclopentane, trans-1,2-dimethylcyclopentane, cis-1,3-dimethylcyclopentane, and trans-1,3-dimethylcyclopentane; and aromatic hydrocarbons such as toluene and xylene. These compounds may be used alone or in combination of two or more.

[0038] The concentration of the coating material in the coating liquid is not particularly limited, and can be appropriately adjusted in consideration of the amount of the water-absorbent resin particles to be coated in order to obtain a coating layer of a desired thickness, and may be, for example, 1 to 50 mass %, 3 to 30 mass %, or 5 to 20 mass %.

[0039] Since it is easy to form a coating layer with a uniform thickness, the coated resin particles may be produced by (1) a method of bringing a coating liquid and / or a coating material into contact with water-absorbent resin particles in a dry state, (2) a method of adding a coating liquid and / or a coating material to a hydrocarbon dispersion medium in which water-absorbent resin particles are dispersed, or (3) a method of adding a coating liquid and water-absorbent resin particles to a hydrocarbon dispersion medium at approximately the same time, with method (1) being more preferred, and method (1) in which a coating liquid is used being even more preferred.

[0040] Examples of the above method (1) using a coating liquid include (1-1) a method using an eggplant flask, (1-2) a method using a sprayer, and (1-3) a method using various granulators.

[0041] In the method (1-1), a coating liquid is poured into an eggplant flask, followed by the addition of water-absorbent resin particles. The eggplant flask is attached to an evaporator and heated while rotating, and the solvent or dispersion medium contained in the coating liquid is evaporated under reduced pressure to obtain coated resin particles. The heating for evaporating the solvent or dispersion medium contained in the coating liquid and the heat treatment step described below may be carried out separately (i.e., after obtaining the coated resin particles, the particles are transferred to a separate device as needed and then subjected to heat treatment), or may be carried out simultaneously (i.e., the heating for evaporating the solvent or dispersion medium contained in the coating liquid is carried out within the temperature range of the heat treatment step described below). Because this facilitates the formation of a coating layer with a uniform thickness, it is preferable to carry out the heating for obtaining the coated resin particles and the heat treatment step separately.

[0042] In the method (1-2), water-absorbent resin particles are added to a separable flask equipped with a stirring blade and stirred, and a coating liquid is sprayed onto the water-absorbent resin particles that have been stirred up by the stirring with the stirring blade. The coating liquid can be sprayed using, for example, a two-fluid nozzle. Since uniform coating can be expected, it is desirable that the coating liquid is atomized and sprayed using a current of inert gas such as nitrogen. Thereafter, the solvent or dispersion medium contained in the coating liquid is evaporated by heating, thereby obtaining coated resin particles. For the same reason as in the above (1-1), the heating for evaporating the solvent or dispersion medium contained in the coating liquid and the heat treatment step described below may be carried out separately or simultaneously, but are preferably carried out separately.

[0043] Examples of the granulator used in the method (1-3) include a tumbling granulator, an agitation granulator, and a fluidized bed granulator.

[0044] When a tumbling granulator is used, an inclined shallow circular container attached to the tumbling granulator is rotated, and water-absorbent resin particles are supplied to the circular container, and an appropriate amount of coating liquid is added. The process of adding the water-absorbent resin particles and the coating liquid can be performed multiple times as necessary. Thereafter, the coating liquid is heated by any heating means (for example, hot air or heating the circular container itself), thereby distilling off the solvent or dispersion medium, and coated resin particles are obtained.

[0045] When using an agitation granulator, water-absorbent resin particles are charged into a mixer attached to the agitation granulator, and the coating liquid is added while the particles are mixed by agitation. Then, a coating layer is formed on the surface of the water-absorbent resin particles while some of the particles are aggregated by the solvent or dispersion medium contained in the coating liquid during the agitation. The step of adding the water-absorbent resin particles and the coating liquid can be carried out multiple times as necessary. Thereafter, the coating liquid is heated by any heating means (for example, hot air or heat of the mixer) to evaporate the solvent or dispersion medium, thereby obtaining coated resin particles.

[0046] When a fluidized bed granulator is used, first, water-absorbent resin particles are charged into a container equipped in the fluidized bed granulator and capable of blowing air from the bottom, and the water-absorbent resin particles are fluidized in advance. Thereafter, when a coating liquid is sprayed from a nozzle equipped in the container, a coating layer is formed on the surface of the water-absorbent resin particles while some of the particles are agglomerated by the solvent or dispersion medium contained in the coating liquid during stirring. The spraying of the coating liquid can be carried out multiple times as necessary.

[0047] Among these devices, a fluidized bed granulator is suitably used for forming a coating layer. When a fluidized bed granulator is used, contact between the coating liquid and the water-absorbent resin and evaporation of the coating liquid are carried out simultaneously, so that it is possible to form a coating layer while suppressing aggregation of a part of the water-absorbent resin particles due to the solvent or dispersion medium contained in the coating liquid. Furthermore, by using hot air as the air blown from the bottom of the fluidized bed granulator, it is possible to hasten evaporation of the solvent or dispersion medium contained in the coating liquid, and it is possible to further suppress aggregation.

[0048] <3. Mixed particles and their manufacturing method> The mixed particles used in the present invention contain the coated resin particles and a lubricant. The lubricant may be disposed on the surface of the coated resin particles (in other words, present without being fixed to the coated resin particles), or at least a part of the surface of the lubricant may be embedded in or permeate the coating layer of the coated resin particles (in other words, fixed to the coated resin particles).

[0049] The lubricant may be partially melted during the heat treatment step described below. The melting point of the lubricant may be at least 20°C lower, equal to, 20°C higher, 40°C higher, or 60°C higher than the Tg of the polymer component contained in the coating material.

[0050] The water content of the lubricant, measured in accordance with ISO787-2, may be, for example, 0.01 to 50%, 0.1 to 30%, 1 to 15%, or 5 to 10%.

[0051] The loss on ignition of the lubricant, measured in accordance with ISO3262-19, may be, for example, 0.1 to 50%, 1 to 20%, 5 to 18%, or 8 to 15%.

[0052] The lubricant may be a particulate lubricant. The shape of the particulate lubricant may be, for example, spherical, approximately spherical, crushed, or granular. From the viewpoint of easier suppression of aggregation, the volume average particle diameter of the particulate lubricant may be, for example, 0.001 μm to 1000 μm, 0.01 μm to 1000 μm, 0.1 μm to 1000 μm, 1 μm to 1000 μm, 1 μm to 500 μm, 1 μm to 300 μm, 1 μm to 150 μm, 1 μm to 100 μm, or 1 μm to 50 μm. From the above viewpoint, it is preferably 0.01 μm to 500 μm, more preferably 0.1 μm to 300 μm, even more preferably 1 μm to 150 μm, even more preferably 1 μm to 100 μm, and most preferably 1 μm to 50 μm. The volume average particle size can be measured, for example, by a laser diffraction / scattering particle size measurement method using a particle size distribution analyzer.

[0053] The specific surface area of ​​a particulate lubricant, measured in accordance with ISO 5794-1, is, for example, 10 m 2 / g~800m 2 / g, 30m 2 / g~500m 2 / g or 50m 2 / g~200m 2 / g.

[0054] The proportion of the lubricant in the mixed particles may be 0.01% by mass or more, 0.05% by mass or more, 0.1% by mass or more, 0.2% by mass or more, 0.5% by mass or more, 1.0% by mass or more, 1.5% by mass or more, 2.0% by mass or more, or 5% by mass or more, from the viewpoint of easier suppression of aggregation. From the viewpoint of easier control of water absorption characteristics, the proportion may be 50% by mass or less, 40% by mass or less, 30% by mass or less, 25% by mass or less, 20% by mass or less, 15% by mass or less, 10% by mass or less, 5% by mass or less, 3% by mass or less, or 2% by mass or less. From the above viewpoints, the proportion is preferably 0.01% by mass to 15% by mass, more preferably 0.01% by mass to 10% by mass, even more preferably 0.05% by mass to 5% by mass, even more preferably 0.05% by mass to 3% by mass, and most preferably 0.05% by mass to 2% by mass.

[0055] The lubricant may contain an organic compound and / or an inorganic compound. The lubricant may be one type of compound or a combination of two or more types. From the viewpoint of more easily suppressing aggregation of the mixed particles in the heat treatment step described below, it is preferable that the lubricant contains an inorganic compound.

[0056] Examples of organic compounds contained in lubricants include general-purpose resins such as polypropylene, polyethylene, ABS resin, polyvinyl chloride, and acrylic resin; general-purpose engineering resins such as nylon-polyamide, polyacetal, polycarbonate, polyethylene terephthalate, polybutylene terephthalate, and modified polyphenylene ether; special engineering resins such as polysulfone, polyether-sulfone, polyphenylene-sulfide, and polyarylate; natural waxes derived from animals such as beeswax, shellac wax, and privet wax; natural waxes derived from plants such as carnauba wax, candelilla wax, rice wax, and Japan wax; fossil-based natural waxes derived from crude oil such as paraffin wax, microcrystalline wax, and slack wax; fossil-based natural waxes derived from minerals such as montan wax, ceresin, and ozokerite; and synthetic waxes such as polyethylene wax, functional wax, polypropylene wax, and Fischer-Tropsch wax.

[0057] Examples of inorganic compounds contained in the lubricant include water-insoluble inorganic salts such as silica (silicon dioxide), titanium oxide, aluminum oxide, magnesium oxide, zeolite, silicic acid (salt), kaolin, talc, mica, hydrotalcite, clay, bentonite, calcium phosphate, barium phosphate, fluorite, and alumina; and water-soluble inorganic salts such as aluminum chloride, aluminum sulfate, aluminum nitrate, alum, magnesium chloride, magnesium sulfate, zinc chloride, zinc sulfate, and zirconium chloride. From the viewpoint of easily adjusting the water absorption characteristics, the inorganic compound contained in the lubricant is preferably a water-insoluble inorganic salt, more preferably contains at least one member selected from the group consisting of silica, silicic acid (salt), and alumina, and even more preferably is silica.

[0058] As a method for obtaining mixed particles, for example, a method of mixing coated resin particles with a lubricant can be mentioned. For example, the lubricant may be added to the coated resin particles and mixed, the coated resin particles may be added to the lubricant and mixed, or the lubricant and the coated resin particles may be added and mixed approximately simultaneously. According to these methods, mixed particles in which the lubricant is disposed on the surface of the coated resin particles can be obtained. Another method for obtaining mixed particles is, for example, to add a lubricant to the coating liquid to obtain coated resin particles (i.e., obtain mixed particles at the same time as obtaining coated resin particles). This method obtains mixed particles in which at least a portion of the surface of the lubricant is embedded in the coating layer of the coated resin particles. When this method is adopted, the heating for distilling off the solvent or dispersion medium contained in the coating liquid and the heat treatment step described below can be performed simultaneously.

[0059] The lubricant may be added as a dry compound or as a dispersion liquid dispersed in any dispersion medium. The dispersion medium may be a hydrophilic compound or a hydrophobic compound, for example, water. The dispersion medium may be used alone or in combination of two or more. At least a part of the lubricant may be dissolved in the dispersion medium.

[0060] The concentration of the lubricant in the dispersion is not particularly limited, and may be, for example, 1 to 50% by mass, 3 to 30% by mass, or 5 to 20% by mass.

[0061] Examples of devices used to mix the coated resin particles and the lubricant include container-rotating mixers such as W-type mixers, V-type mixers, and drum-type mixers; stirring mixers such as ribbon mixers and conical screw mixers; fluidized bed dryers, and groove dryers.

[0062] <4. Heat treatment process> The heat treatment step is a step of obtaining a water-absorbing agent by heat-treating the mixed particles at a temperature higher than the Tg of the polymer component contained in the coating layer. The heat treatment step facilitates reducing defects in the coating layer (portions where water-absorbent resin particles are exposed on the surface of the coated resin particles), thereby enabling favorable control of water absorption properties. Furthermore, when the coating layer contains multiple types of polymer components, the heat treatment step can adjust the phase separation structure of the multiple types of polymer components, thereby forming, for example, a sea-island structure consisting of the multiple types of polymer components. From the above viewpoints, the heating temperature of the mixed particles in the heat treatment step is preferably 5°C to 180°C higher than the Tg of the polymer component, more preferably 8°C to 160°C higher than the Tg of the polymer component, even more preferably 10°C to 155°C higher than the Tg of the polymer component, and even more preferably 20°C to 150°C higher than the Tg of the polymer component.

[0063] The water-absorbing agent can be more suitably obtained by heat-treating the mixed particles at a temperature lower than the thermal decomposition temperature of the polymer component contained in the coating layer. The thermal decomposition temperature of the polymer component is the temperature at which the weight loss is confirmed when the polymer component is heated using a high-temperature thermogravimetric and differential thermal analyzer under the following measurement conditions: nitrogen flow rate: 200 mL / min, heating rate: 20°C / min, measurement temperature: 30°C to 500°C, and hold time: 5 minutes.

[0064] In the heat treatment step, examples of the device used to heat the mixed particles include radiant heat transfer dryers such as hot air dryers, conduction heat transfer dryers, and infrared dryers; dielectric heating dryers such as hot air heat transfer dryers and microwave dryers; groove-type agitator dryers, and fluidized bed dryers. [Example]

[0065] The present invention will be described in more detail below with reference to examples, although the present invention is not limited to these examples.

[0066] [Preparation of water-absorbent resin particles] <First step polymerization reaction> A round-bottomed, cylindrical, separable flask with an inner diameter of 11 cm and a volume of 2 L was prepared, equipped with a reflux condenser, a dropping funnel, a nitrogen gas inlet, and a stirrer (a stirrer with two stages of four inclined paddle blades with a blade diameter of 5 cm). 293 g of n-heptane (hydrocarbon dispersion medium) and 0.736 g of maleic anhydride-modified ethylene-propylene copolymer (polymeric dispersant, Mitsui Chemicals, Inc., Hiwax 1105A) were added to the separable flask to obtain a mixture. The mixture was heated to 80 °C while stirring at 300 rpm to dissolve the dispersant. The mixture was then allowed to cool to 55 °C.

[0067] Next, 92.0 g of an 80.5 wt% acrylic acid aqueous solution (1.03 mol of acrylic acid) was placed in a 500 mL Erlenmeyer flask. Subsequently, while cooling externally, 147.7 g of a 20.9 wt% sodium hydroxide aqueous solution was added dropwise to the Erlenmeyer flask to neutralize the 75 mol% acrylic acid. Next, 0.092 g of hydroxyethyl cellulose (thickener, Sumitomo Seika Chemicals Co., Ltd., HEC AW-15F), 0.0736 g (0.272 mmol) of potassium persulfate as a radical polymerization initiator, and 0.0101 g (0.0580 mmol) of ethylene glycol diglycidyl ether (internal crosslinking agent) were added, and the remaining components except for water were dissolved to prepare the first-stage monomer aqueous solution.

[0068] The first-stage monomer aqueous solution was added to the separable flask and stirred for 10 minutes. Separately, 0.736 g of sucrose stearate (surfactant, manufactured by Mitsubishi Chemical Foods Corporation, Ryoto Sugar Ester S-370, HLB: 3) was dissolved in 6.62 g of n-heptane with heating to obtain a surfactant solution. 7.356 g of the resulting surfactant solution was added to the separable flask to obtain a reaction solution. The atmosphere inside the separable flask was then thoroughly purged with nitrogen while stirring the reaction solution at a rotation speed of 550 rpm. The separable flask was then immersed in a 70°C water bath to heat the reaction solution. As the polymerization reaction progressed, heating was continued for another 10 minutes after the internal temperature reached its maximum (maximum temperature of 82°C), yielding the first-stage reaction mixture.

[0069] <Second-stage polymerization reaction> Next, 128.8 g of an 80.5% by mass acrylic acid aqueous solution (1.44 mol of acrylic acid) was placed in a 500 mL Erlenmeyer flask. Subsequently, while cooling externally, 154.2 g of a 28% by mass sodium hydroxide aqueous solution was added dropwise to the Erlenmeyer flask to neutralize the 75 mol% acrylic acid. After that, 0.1030 g (0.3810 mmol) of potassium persulfate as a radical polymerization initiator, 0.0116 g (0.0666 mmol) of ethylene glycol diglycidyl ether (internal crosslinking agent), and 6.48 g of ion-exchanged water were added, and the remaining components except for water were dissolved to prepare a second-stage monomer aqueous solution.

[0070] The first-stage reaction mixture was cooled to 25°C while stirring at 1000 rpm, and then the entire amount of the second-stage aqueous monomer solution was added to the first-stage reaction mixture to obtain a reaction solution. The reaction mixture was thoroughly purged with nitrogen while stirring. The separable flask was then immersed in a 70°C water bath to heat the reaction mixture. Once the polymerization reaction progressed and the internal temperature reached its maximum (maximum temperature of 82°C), heating was continued for an additional 5 minutes to obtain the second-stage reaction mixture (polymer particles before surface crosslinking).

[0071] After the second-stage polymerization, the reaction mixture was heated in a 125°C oil bath, and 251 g of water was extracted from the system by azeotropic distillation of n-heptane and water while refluxing n-heptane. Subsequently, 4.42 g of a 2% by mass aqueous solution containing ethylene glycol diglycidyl ether (0.5075 mmol) as a surface cross-linking agent was added, and the mixture was maintained at 83°C for 2 hours to obtain a dispersion of surface-cross-linked polymer particles.

[0072] Thereafter, the temperature of the dispersion liquid of the surface-crosslinked polymer particles was raised in an oil bath at 125°C, and n-heptane was evaporated and dried to obtain polymer particles. Of these polymer particles, those that passed through a sieve with an opening of 850µm and remained on the top of a sieve with an opening of 250µm were collected to obtain 210.1g of water-absorbent resin particles with a particle diameter of 250µm to 850µm. Furthermore, the same operation was carried out to prepare a total of 500.0g or more of water-absorbent resin particles (particle diameter of 250µm to 850µm).

[0073] [Preparation of Ethylene / Acrylic Acid Copolymer Partially Neutralized Emulsion] An ice bath at 3°C ​​was prepared by adding water and ice to a plastic tray measuring 27 cm in length, 38 cm in width, and 7 cm in depth. A glass beaker with an internal volume of 1 L was placed in the ice bath, and 545.78 g of ion-exchanged water was then added to the beaker. The ice bath was then placed on a magnetic stirrer, and a stirrer tip was placed in the beaker to begin stirring.

[0074] 10.55 g (0.264 mol) of sodium hydroxide (granules, manufactured by Nacalai Tesque, Inc.) was added little by little to the above beaker to prepare a 1.9 mass % aqueous sodium hydroxide solution.

[0075] A round-bottomed, cylindrical, separable flask with an inner diameter of 11 cm and a volume of 2 L was prepared, equipped with a reflux condenser, a thermometer, and a stirrer (a stirrer with four inclined paddle blades and a blade diameter of 5 cm). 100 g of ethylene / acrylic acid copolymer (ethylene monomer to acrylic acid monomer molar ratio = 10:1, Primacol 5980i, manufactured by SK Global Chemical) was added to the separable flask. Subsequently, the entire amount of the 1.9% by mass aqueous sodium hydroxide solution was added. The beaker used to prepare the aqueous sodium hydroxide solution was then washed with 50.0 g of ion-exchanged water, and the wash water was added to the separable flask to obtain a reaction solution.

[0076] While stirring the reaction solution at a stirrer speed of 500 rpm, the separable flask was immersed in an oil bath at 103° C., and the internal temperature of the separable flask was raised to 95° C. Thereafter, the internal temperature of the separable flask was maintained at 95 to 97° C. for 4 hours while appropriately adjusting the temperature of the oil bath.

[0077] The separable flask was then removed from the oil bath and allowed to cool at room temperature until the internal temperature of the separable flask reached 35°C. After confirming that the internal temperature of the separable flask had reached 35°C or below, the product in the separable flask was filtered through a nylon mesh with a mesh size of 108 µm. The filtrate was collected to obtain an emulsion of a partially neutralized ethylene / acrylic acid copolymer (P(E / AA), an aqueous dispersion of a partially neutralized ethylene / acrylic acid copolymer, nonvolatile content 15% by mass, degree of neutralization 90%; hereinafter referred to as "aqueous dispersion (A)").

[0078] <Measurement of glass transition temperature and melting point> 7.00 g of the aqueous dispersion (A) was added to a beaker (inner diameter: 6.2 cm) with an inner surface coated with a fluororesin, and then the beaker was covered with aluminum foil. After perforating the aluminum foil, it was heated to 40°C and dried in a hot air dryer to obtain 1.08 g of polymer film (A).

[0079] 3.0 mg of the polymer film (A) was sealed in an aluminum sealed sample container (Hitachi High-Tech Science Corporation, GCA-0017), and the glass transition temperature (Tg) and melting point (MP) of the polymer film were measured using a high-sensitivity differential scanning calorimeter (Yamato Scientific Co., Ltd., DSC7020). An empty aluminum sealed sample container was used as a reference. Differential scanning calorimetry (DSC) was performed by repeating two cycles of heating and cooling from -20°C to 150°C (heating rate: 20°C / min, nitrogen flow rate: 40 mL / min). The glass transition temperature and melting point were calculated from the change in baseline during the second heating cycle. The glass transition temperature and melting point of the partially neutralized ethylene / acrylic acid copolymer (neutralization degree 90 mol%) obtained from aqueous dispersion (A) were 56°C and 88°C, respectively.

[0080] [Preparation of coated resin particles] In a 1 L beaker (made of polypropylene), 250 g of the above-mentioned aqueous dispersion (A) was mixed with 1.25 g of polyethylene glycol (manufactured by Tokyo Chemical Industry Co., Ltd., PEG6000, number average molecular weight: 7300-9300) as a coating material to prepare 251.25 g of coating liquid. The coating material contents in the 251.25 g of coating liquid were 37.50 g (14.93 mass%) of partially neutralized ethylene / acrylic acid copolymer (P(E / AA), neutralization degree 90%) and 1.25 g (0.50 mass%) of polyethylene glycol.

[0081] 500.0 g of the above-mentioned water-absorbent resin particles were put into a fluidized bed granulator. Next, while stirring the inside of the treatment section with a stirring means (rotor blade, rotation speed: 250 rpm), the supply air temperature was 130°C and the supply amount was 1.0 m 3An airflow (air) supplied to the fluidized bed granulator at a rate of 18.0 g / min was supplied to the processing section from the air inlet. Then, the entire amount of the above-mentioned coating liquid was sprayed onto the water-absorbent resin particles being blown up by the airflow using a tangential spray at a supply rate of 18.0 g / min, thereby bringing the water-absorbent resin particles into contact with the coating material of the coating liquid. At this time, the supply amount of the coating material per 100 parts by mass of the water-absorbent resin particles was 7.75 parts by mass. Thereafter, the water-absorbent resin particles were sprayed at room temperature with a supply rate of 1.0 m 3 The airflow (air) supplied to the fluidized bed granulator at a rate of 1 / min was supplied from the air inlet to the treatment section for 30 minutes to dry the particles, thereby obtaining coated resin particles.

[0082] (Comparative Example 1) 12.0 g of these coated resin particles were spread on a metal dish (stainless steel SUS304) with a diameter of 12.0 cm and a height of 2.5 cm, and then covered with aluminum foil. After perforating the aluminum foil, the particles (A) were heated for 30 minutes in a hot air dryer (Advantec, FV-320) set at 120°C. The metal dish was removed from the hot air dryer and allowed to stand in a desiccator (temperature 20°C, humidity 35% RH) until it reached room temperature. JIS sieves with mesh sizes of 850 μm, 710 μm, 600 μm, 500 μm, 425 μm, 300 μm, and 150 μm and a tray were prepared. 10.0 g of the coated resin particles were rubbed by hand against the sieves, starting with the largest mesh size, until no particles passed through. The coated resin particles remaining on each sieve and tray were then collected to obtain a water-absorbing agent.

[0083] (Comparative Example 2) A water-absorbing agent was obtained in the same manner as in Comparative Example 1, except that 0.4 parts by mass of silica (Toxil NP-S, Oriental Silicas Corporation) was mixed with the coated resin particles after the heat treatment for 10 minutes using a cross rotary mixer (Tsukasa Kogyo Co., Ltd., Model 3LCM-3).

[0084] Example 1 A water-absorbing agent was obtained in the same manner as in Comparative Example 1, except that 0.1 parts by mass of silica (Toxil NP-S, Oriental Silicas Corporation) was mixed with the coated resin particles before heat treatment for 10 minutes using a cross rotary mixer.

[0085] Example 2 A water-absorbing agent was obtained in the same manner as in Example 1, except that the amount of silica mixed was changed to 0.4 parts by mass.

[0086] (Comparative Example 3) A water-absorbing agent was obtained in the same manner as in Comparative Example 1, except that the heating time of the hot air dryer was changed to 60 minutes.

[0087] Comparative Example 4 A water-absorbing agent was obtained in the same manner as in Comparative Example 2, except that the heating time of the hot air dryer was changed to 60 minutes.

[0088] Example 3 A water-absorbing agent was obtained in the same manner as in Example 1, except that the heating time of the hot air dryer was changed to 60 minutes.

[0089] Example 4 A water-absorbing agent was obtained in the same manner as in Example 2, except that the heating time of the hot air dryer was changed to 60 minutes.

[0090] Example 5 A water-absorbing agent was obtained in the same manner as in Example 3, except that the amount of silica mixed was changed to 1.0 part by mass.

[0091] Example 6 A water-absorbing agent was obtained in the same manner as in Example 3, except that the amount of silica mixed was changed to 2.0 parts by mass.

[0092] Example 7 In a 100 ml beaker (made of polypropylene), 2.0 g of silica (Toxil NP-S, Oriental Silicas Corporation) and 48.0 g of ion-exchanged water were mixed to prepare 50.0 g of a silica dispersion.

[0093] 500.0 g of the above-mentioned water-absorbent resin particles were put into a fluidized bed granulator. Next, while stirring the inside of the treatment section with a stirring means (rotor blade, rotation speed: 250 rpm), the supply air temperature was 130°C and the supply amount was 1.0 m 3 An airflow (air) supplied to the fluidized bed granulator at a rate of 18.0 g / min was supplied to a processing section from an air inlet. Then, the entire amount of the above-mentioned coating liquid was sprayed onto the water-absorbent resin particles being blown up by the airflow at a supply rate of 18.0 g / min using a tangential spray, thereby bringing the water-absorbent resin particles into contact with the coating material of the coating liquid. At this time, the supply amount of the coating material per 100 parts by mass of the water-absorbent resin particles was 7.75 parts by mass. Subsequently, 50.0 g of the prepared silica dispersion was fed into the fluidized bed granulator while stirring with a stirrer, and sprayed into the fluidized bed granulator at a feed rate of 18.0 g / min. After the entire amount of the silica dispersion was sprayed, the mixture was stirred at room temperature with a feed rate of 1.0 m 3 The airflow (air) supplied to the fluidized bed granulator at a rate of 1 / min was supplied from the air inlet to the processing section for 30 minutes to dry the mixture, thereby obtaining mixed particles. 12.0 g of the mixed particles were spread on a metal dish (SUS304 stainless steel) with a diameter of 12.0 cm and a height of 2.5 cm, and then covered with aluminum foil. After perforating the aluminum foil, the particles (A) were heated for 60 minutes in a hot air dryer (Advantec, FV-320) set at 120°C. The metal dish was removed from the hot air dryer and allowed to stand in a desiccator (temperature 20°C, humidity 35% RH) until it reached room temperature. JIS sieves with mesh sizes of 850 μm, 710 μm, 600 μm, 500 μm, 425 μm, 300 μm, and 150 μm and a tray were prepared, and 10.0 g of the mixed particles were rubbed by hand against the sieves, starting with the largest mesh size, until no particles passed through. The mixed particles remaining on each sieve and tray were then collected to obtain a water-absorbing agent.

[0094] Example 8 A water-absorbing agent was obtained in the same manner as in Example 2, except that the heating time of the hot air dryer was changed to 120 minutes.

[0095] Example 9 A water-absorbing agent was obtained in the same manner as in Example 5, except that the heating time of the hot air dryer was changed to 120 minutes.

[0096] Example 10 A water-absorbing agent was obtained in the same manner as in Example 6, except that the heating time of the hot air dryer was changed to 120 minutes.

[0097] Example 11 A water-absorbing agent was obtained in the same manner as in Example 7, except that the heating time of the hot air dryer was changed to 120 minutes.

[0098] <Evaluation of aggregation inhibition> In each example and comparative example, immediately after being allowed to cool to room temperature after heat treatment, the bottom of a metal petri dish was tilted 45° relative to the horizontal and lightly impacted 10 times against a horizontal laboratory bench, and the weight W1 of the particles that spilled out of the metal petri dish was measured. Next, all particles were recovered from the metal petri dish, and the weight W2 of the recovered particles was measured. If the degree of aggregation inhibition calculated by the following formula (1) was 0.7 or more, the evaluation was "Good." If it was 0.05 or more and less than 0.7, the evaluation was "Average." If it was less than 0.05, the evaluation was "Poor." The results are shown in Table 1. A higher degree of aggregation inhibition indicates greater inhibition of aggregation. Aggregation inhibition degree = W1 / (W1+W2)...Equation (1)

[0099] [Table 1]

Claims

1. A method for producing a water-absorbing agent, comprising a heat treatment step of heating mixed particles containing a lubricant and coated resin particles, which are water-absorbing resin particles at least partly coated on the surface with a coating material containing a polymer component, at a temperature equal to or higher than the glass transition temperature of the polymer component, to obtain a water-absorbing agent.

2. The method for producing a water-absorbing agent according to claim 1, wherein the heat treatment step is carried out at a temperature lower than the thermal decomposition temperature of the polymer component.

3. 3. The method for producing a water-absorbing agent according to claim 1, wherein the heat treatment step is carried out at a temperature that is 5° C. to 180° C. higher than the glass transition temperature of the polymer component.

4. 3. The method for producing a water-absorbing agent according to claim 1, wherein the content of the lubricant in the mixed particles is 0.05% by mass to 3% by mass.

5. The method for producing a water-absorbing agent according to claim 1 or 2, wherein the lubricant contains an inorganic compound.

6. The method for producing a water-absorbing agent according to claim 5, wherein the inorganic compound is silica.

7. 3. The method for producing a water-absorbing agent according to claim 1, wherein the lubricant contains particles having a volume average particle size of 1 μm to 100 μm.

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