Method for producing coated resin particles

Abstract

One aspect of the present invention relates to a method for producing coated resin particles which includes a coating step in which water-absorbing resin particles and a coating material are brought into contact with each other to obtain coated resin particles that have coating portions coating at least part of the water-absorbing resin particles. In the coating step, the proportion of a time period over which the water-absorbing resin particles and the coating material are in contact with each other at ambient temperatures which are not lower by at least 27°C than the glass transition temperature of a main component of the coating material and not higher by at least 4°C than the melting point of the main component of the coating material is 30% or higher.

Description

Method for producing coated resin particles

[0001] The present invention relates to a method for producing coated resin particles.

[0002] Water-absorbent resin particles are widely used in various fields, such as sanitary materials such as disposable diapers, sanitary products, and portable toilets; agricultural and horticultural materials such as water retention agents and soil conditioners; and industrial materials such as waterproofing agents and anti-condensation agents. The water-absorbent resin particles are required to have high performance such as a high water absorption capacity and gel strength, as well as control of the water absorption rate (see, for example, Patent Document 1 listed below).

[0003] JP 2016-28117 A

[0004] Upon contact with a liquid to be absorbed (aqueous liquid containing water), water-absorbent resin particles begin absorbing water immediately and reach a swollen state (a state in which they can no longer absorb any more water) in a short time. Therefore, water-absorbent resin particles may not be effectively utilized in fields where it is required that they begin absorbing water after a certain time has passed since contact with the liquid, or that they do not immediately absorb a large amount of liquid. For example, in sanitary materials such as disposable diapers, sanitary products, and portable toilets, when a liquid to be absorbed comes into contact with an absorbent core containing water-absorbent resin particles, the absorbent core may immediately begin absorbing a large amount of liquid, resulting in gel blocking (a phenomenon in which voids existing between water-absorbent resin particles are filled with water-absorbent resin particles that have become gel-like due to water absorption, thereby worsening the permeability of the liquid), and the liquid may not be sufficiently diffused within the absorbent core. In this case, liquid may pool at the point of contact with the liquid, and the sanitary material may be discarded with a large number of water-absorbent resin particles that have not sufficiently absorbed water present in areas other than the point of contact with the liquid. Therefore, if the water-absorption behavior of the water-absorbent resin particles can be adjusted, the occurrence of gel blocking may be suppressed. In this case, the liquid is sufficiently dispersed within the absorbent body, and the water-absorbent resin particles can be effectively utilized.

[0005] An object of one aspect of the present invention is to provide a method for producing coated resin particles that can be adjusted to have a slow water absorption rate.

[0006] In some aspects, the present invention relates to the following [1] to

[10] , etc. [1] A method for producing coated resin particles, comprising a coating step of contacting water-absorbent resin particles and a coating material to obtain coated resin particles having a coating portion that coats at least a portion of the water-absorbent resin particles, wherein in the coating step, the water-absorbent resin particles and the coating material are contacted with each other at an ambient temperature that is at least 27°C lower than the glass transition temperature of a main component of the coating material and at most 4°C higher than the melting point of the main component of the coating material for 30% or more of the time. [2] A method for producing coated resin particles according to [1], wherein the glass transition temperature of the main component of the coating material is 20 to 140°C. [3] A method for producing coated resin particles according to [1] or [2], wherein the melting point of the main component of the coating material is 30 to 150°C. [4] A method for producing coated resin particles according to any one of [1] to [3], wherein the main component of the coating material is a copolymer of an olefin and an ethylenically unsaturated monomer. [5] The method for producing coated resin particles according to any one of [1] to [4], wherein the water-absorbent resin particles and the coating material are brought into contact with each other while supplying an airflow from a vertically downward side into an internal space of an apparatus having an internal space in which the water-absorbent resin particles are accommodated. [6] In at least a part of the coating step, the amount of heat X represented by the following formula (1) is 1.000 [MJ / (min m ２ 5. The method for producing coated resin particles according to [4], wherein the water-absorbent resin particles and the coating material are brought into contact with each other in a state where the water-absorbent resin particles and the coating material are ... ３ / min] SG: specific gravity of the gas constituting the airflow [kg / m ３ ] SH: specific heat of the gas constituting the airflow [kJ / (kg K)] TD: temperature difference of the airflow temperature at the time of supply to the device with respect to 25°C [K] NA: area of ​​the narrowest cross section perpendicular to the vertical direction of the internal space [m ２ [7] The heat quantity X is 3.000 [MJ / (min m ２[8] In at least a part of the coating step, the moisture content Y [kg / (min m )] is at least equal to or greater than the moisture content Y [kg / (min m )] expressed by the following formula (2): ２ ) )] to the amount of heat X [MJ / (min m ２ )】 is 2.00 to 35.00 [MJ / kg], Y = [(AL × RW) / 100] / (NA × 1000) ... (2) AL: amount of the coating liquid supplied to the internal space [g / min] RW: content of water in the coating liquid [mass %] NA: area of ​​the narrowest cross section perpendicular to the vertical direction of the internal space [m ２ [9] The method for producing coated resin particles according to [8], wherein the ratio X / Y is 7.00 to 30.00 [MJ / kg].

[10] The water content Y is 0.200 [kg / (min m ２ ))] or more.

[0007] According to one aspect of the present invention, there is provided a method for producing coated resin particles that can be adjusted to have a slow water absorption rate.

[0008] 1 is a schematic cross-sectional view showing a treatment device for bringing water-absorbent resin particles and a coating material into contact with each other.

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

[0010] As used herein, "(meth)acrylic" refers to at least one of acrylic and its corresponding methacrylic. The same applies to other similar expressions such as "(meth)acrylate." "(Poly)" refers to both cases with and without the "poly" prefix. A numerical range of "A or greater" means a range exceeding A and A. A numerical range of "A or less" means a range of A and less than A. 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 a numerical range of another stage. In the numerical ranges described in this specification, the upper or lower limit of the numerical range may be replaced with the value shown in the examples. The materials exemplified in this specification may be used alone or in combination of two or more. "A or B" may include either A or B, or may include both. When multiple substances corresponding to each component are present in a composition, the content of each component refers to the total amount of those multiple substances present in the composition unless otherwise specified. "Room temperature" means 25°C ± 2°C. "Sieve" refers to a test sieve (metal mesh sieve) as specified in JIS Z 8801-1:2019. The term "process" includes not only independent processes but also processes that cannot be clearly distinguished from other processes as long as the intended function of the process is achieved. "Physiological saline" means a 0.9% by mass aqueous sodium chloride solution. The phrase "A contains B" includes all forms in which A contains B, and also forms in which A contains only B (i.e., forms in which A consists only of B) as well as forms in which B constitutes only a part of A.

[0011] The method for producing coated resin particles according to this embodiment includes a coating step in which water-absorbent resin particles and a coating material are brought into contact with each other to obtain coated resin particles having a coating portion that coats at least a portion of the water-absorbent resin particles. In the method for producing coated resin particles according to this embodiment, the proportion of time during which the water-absorbent resin particles and the coating material are brought into contact with each other is 30% or more at an ambient temperature that is at least 27°C lower than the glass transition temperature of the main component of the coating material and at most 4°C higher than the melting point of the main component of the coating material. The ambient temperature is the temperature of the space in which the water-absorbent resin particles and the coating material are brought into contact with each other.

[0012] According to the method for producing coated resin particles of this embodiment, coated resin particles can be obtained that can be adjusted to have a slow water absorption rate. According to the method for producing coated resin particles of this embodiment, the 5-minute value of the lock-up height can be reduced to, for example, 2.1 cm or less (preferably, 1.9 cm or less, 1.5 cm or less, 1.2 cm or less, 1.0 cm or less, etc.) in the evaluation described in the Examples below.

[0013] The reason why coated resin particles adjusted to have a slow water absorption rate are obtained is presumed to be as follows. However, the reason why such coated resin particles are obtained is not limited to the following. That is, the coating portion of the coated resin particles obtained by contacting the water-absorbent resin particles and the coating material can prevent the water-absorbent resin particles from coming into contact with the liquid to be absorbed. However, if the water-absorbent resin particles and the coating material are kept in contact with each other for a long time at a temperature excessively lower than the glass transition temperature of the main component of the coating material, the coating material is likely to not soften sufficiently, resulting in defects (portions where the water-absorbent resin particles are exposed) in the coating portion of the coated resin particles, making it difficult to adjust the water absorption rate to be slow. Furthermore, if the water-absorbent resin particles and the coating material are kept in contact with each other for a long time at a temperature excessively higher than the melting point of the main component of the coating material, the coating material will melt excessively, causing the water-absorbent resin particles to adhere to each other via the coating material, making it difficult to obtain coated resin particles. On the other hand, in the method for producing coated resin particles according to this embodiment, the proportion of time during the coating step during which the water-absorbent resin particles and the coating material are in contact with each other at an ambient temperature that is at least 27°C lower than the glass transition temperature of the main component of the coating material and at most 4°C higher than the melting point of the main component of the coating material is 30% or more. This makes it easy for the coating material to soften, making it less likely for defects (portions where the water-absorbent resin particles are exposed) to occur in the coating portion of the coated resin particles. Furthermore, because the coating material is less likely to melt excessively, it is easy to prevent adhesion between water-absorbent resin particles via the coating material. As described above, the method for producing coated resin particles according to this embodiment can obtain coated resin particles that are adjusted to have a slow water absorption rate.

[0014] Typically, to slow the water absorption rate of coated resin particles, it is necessary to increase the amount of coating material used in the coating process. However, using a large amount of coating material may reduce productivity due to an increase in coating time, etc. On the other hand, according to one aspect of the method for producing coated resin particles of this embodiment, even if the coating portion is made thin, defects are unlikely to occur, so the water absorption rate of the coated resin particles can be adjusted to be slow without increasing the amount of coating material used.

[0015] The coated resin particles obtained by the method for producing coated resin particles according to this embodiment can be used, for example, by mixing them with water-absorbent resin particles that do not have a coating portion. Using a mixture (mixed particles) of coated resin particles and water-absorbent resin particles that do not have a coating portion in an absorbent body of a sanitary material (such as disposable diapers, sanitary products, or portable toilets) tends to prevent gel blocking from occurring when the absorbent body comes into contact with a liquid to be absorbed. Specifically, when the mixed particles come into contact with a liquid, the water-absorbent resin particles that do not have a coating portion gel upon absorption, but because the absorption of the coated resin particles is suppressed (i.e., the coated resin particles do not gel immediately), the voids between the particles are not filled for a predetermined period of time, ensuring liquid permeability. This allows for sufficient diffusion of liquid within the absorbent body, facilitating effective use of the water-absorbent resin particles throughout the absorbent body. The time during which gel blocking can be suppressed can be adjusted by appropriately changing the type or amount of the constituent material of the coating portion; the type of water-absorbent resin particles that constitute the coated resin particles; the type of water-absorbent resin particles that do not have a coating portion; the mixing ratio of the coated resin particles and the water-absorbent resin particles that do not have a coating portion, etc.

[0016] The water-absorbent resin particles constituting the coated resin particles may be polymer particles. The polymer particles may be obtained by polymerizing a monomer including an ethylenically unsaturated monomer (a compound having an ethylenically unsaturated bond), or may be obtained by polymerizing only an ethylenically unsaturated monomer, may have an ethylenically unsaturated monomer as a monomer unit (a monomer unit derived from an ethylenically unsaturated monomer), or may have only an ethylenically unsaturated monomer as a monomer unit. The polymer particles may be polymer particles having a crosslinked structure (crosslinked polymer particles). The method for producing coated resin particles according to this embodiment may include a polymerization step of polymerizing an ethylenically unsaturated monomer to obtain water-absorbent resin particles before the coating step. Examples of polymerization methods of an ethylenically unsaturated monomer to obtain polymer particles include reverse-phase suspension polymerization, aqueous solution polymerization, bulk polymerization, and precipitation polymerization.

[0017] The ethylenically unsaturated monomer is a compound having at least one carbon-carbon double bond in the molecule and having radical polymerizability, preferably a compound having one carbon-carbon double bond in the molecule and having radical polymerizability. The ethylenically unsaturated monomer may be a water-soluble ethylenically unsaturated monomer (an ethylenically unsaturated monomer having a solubility of 1.0 g or more in 100 g of ion-exchanged water at 25°C). The solubility of the water-soluble ethylenically unsaturated monomer in 100 g of ion-exchanged water at 25°C may be 5.0 g or more, 10 g or more, 50 g or more, or 100 g or more. Examples of the ethylenically unsaturated monomer include (meth)acrylic acid and salts thereof, 2-(meth)acrylamido-2-methylpropanesulfonic acid and salts thereof, (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.

[0018] When the ethylenically unsaturated monomer has an acidic group (e.g., a carboxy group), the acidic group may be neutralized with an alkaline neutralizing agent before being used in the polymerization reaction. The degree of neutralization of the ethylenically unsaturated monomer (neutralization degree with the alkaline neutralizing agent) may be 10 to 100 mol %, 50 to 90 mol %, or 60 to 80 mol % of the acidic group in the ethylenically unsaturated monomer.

[0019] From the viewpoint of industrial availability, the ethylenically unsaturated monomer may include at least one compound selected from the group consisting of (meth)acrylic acid, (meth)acrylates, (meth)acrylamide, and N,N-dimethylacrylamide, or may include at least one compound selected from the group consisting of (meth)acrylic acid, (meth)acrylates, and (meth)acrylamide.

[0020] Monomers other than the above-mentioned ethylenically unsaturated monomers may be used as monomers for obtaining water-absorbent resin particles. Such monomers can be used, for example, by mixing them with an aqueous solution containing the ethylenically unsaturated monomer. The proportion of the amount of the ethylenically unsaturated monomer used may be 70 to 100 mol%, 80 to 100 mol%, 90 to 100 mol%, 95 to 100 mol%, or 99.5 to 100 mol% relative to the total amount of monomers (total amount of monomers for obtaining water-absorbent resin particles). The proportion of the total amount of (meth)acrylic acid and (meth)acrylate salts may be 70 to 100 mol%, 80 to 100 mol%, 90 to 100 mol%, 95 to 100 mol%, or 99.5 to 100 mol% relative to the total amount of monomers (total amount of monomers for obtaining water-absorbent resin particles).

[0021] The water-absorbent resin particles may be crosslinked polymer particles. Crosslinking may occur due to self-crosslinking during polymerization, 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 (water retention capacity, etc.) of the water-absorbent resin particles. The polymerization step may be a step of obtaining water-absorbent resin particles by polymerizing an ethylenically unsaturated monomer in the presence of an internal crosslinking agent.

[0022] Examples of the internal crosslinking agent include compounds having two or more reactive functional groups (for example, polymerizable unsaturated groups). Examples of the internal crosslinking agent include di- or tri(meth)acrylic acid esters of polyols (polyethylene glycol di(meth)acrylate, etc.), unsaturated polyesters obtained by reacting polyols with unsaturated acids, glycidyl group-containing compounds ((poly)ethylene glycol diglycidyl ether, (poly)propylene glycol diglycidyl ether, (poly)glycerin diglycidyl ether, etc.), bisacrylamides, di- or tri(meth)acrylic acid esters obtained by reacting polyepoxides with (meth)acrylic acid, and hydroxy groups of polyisocyanates and (meth)acrylic acid.

[0123] Examples of the internal crosslinking agent include di(meth)acrylic acid carbamyl esters obtained by reacting di(meth)acrylic acid with diethyl, allylated starch, allylated cellulose, diallyl phthalate, N,N',N"-triallyl isocyanurate, divinylbenzene, pentaerythritol, ethylenediamine, polyethyleneimine, and the like. From the viewpoint of easily adjusting the water absorption properties of the water absorbent resin particles, the internal crosslinking agent may contain at least one selected from the group consisting of (poly)ethylene glycol diglycidyl ether, (poly)propylene glycol diglycidyl ether, and (poly)glycerin diglycidyl ether.

[0023] The amount of the internal crosslinking agent can be appropriately adjusted in order to adjust the water absorption properties of the water absorbent resin particles, and may be, for example, within the following ranges relative to 1 mole of the ethylenically unsaturated monomer (for example, 1 mole of (meth)acrylic acid and a salt thereof in total). The amount of the internal crosslinking agent may be 0.001 mmol or more, 0.005 mmol or more, 0.010 mmol or more, 0.015 mmol or more, 0.020 mmol or more, 0.025 mmol or more, 0.030 mmol or more, 0.035 mmol or more, 0.040 mmol or more, 0.045 mmol or more, or 0.050 mmol or more. The amount of internal crosslinker may be 0.300 mmol or less, 0.200 mmol or less, 0.100 mmol or less, 0.090 mmol or less, 0.080 mmol or less, 0.070 mmol or less, 0.060 mmol or less, 0.055 mmol or less, 0.050 mmol or less, 0.045 mmol or less, or 0.040 mmol or less. From these viewpoints, the amount of the internal crosslinking agent may be 0.001 to 0.300 mmol, 0.001 to 0.100 mmol, 0.001 to 0.080 mmol, 0.001 to 0.045 mmol, 0.030 to 0.300 mmol, 0.030 to 0.100 mmol, 0.030 to 0.080 mmol, 0.030 to 0.045 mmol, 0.045 to 0.300 mmol, 0.045 to 0.100 mmol, or 0.045 to 0.080 mmol.

[0024] In the water-absorbent resin particles, the crosslink density in the vicinity of the surface of the polymer particle may be increased (surface crosslinking may be performed). The water absorption properties (water retention capacity, etc.) of the water-absorbent resin particles can be easily adjusted by surface crosslinking. Specifically, the water absorption properties can be adjusted by adjusting the crosslink density in the vicinity of the surface of the polymer particle depending on the moisture content of the polymer particle subjected to surface crosslinking, the type or amount of the surface crosslinking agent used in surface crosslinking, etc.

[0025] The water-absorbent resin particles may contain components such as gel stabilizers, metal chelating agents, flow improvers (lubricants), etc. These components may be located inside the water-absorbent resin particles, on the surface of the water-absorbent resin particles, or both.

[0026] The shape of the water-absorbent resin particles or coated resin particles may be, for example, substantially spherical, crushed, or granular, or may be a shape formed by aggregation of primary particles having these shapes. The particle diameter (median particle diameter determined by the measurement method described in Patent Document 1) of the water-absorbent resin particles or coated resin particles may be 100 to 800 μm, 150 to 700 μm, 200 to 600 μm, 250 to 500 μm, 300 to 400 μm, or 250 to 850 μm.

[0027] The coating portion of the coated resin particle covers at least a part (part or all) of the water-absorbent resin particle, and can cover at least a part (part or all) of the surface of the water-absorbent resin particle. The coating portion may be a layered coating layer. The coating layer may have a single-layer structure or a multi-layer structure having two or more layers.

[0028] The coating portion can be obtained by contacting a coating material with the water-absorbent resin particles. The coating material may contain a polymer component. The coating material may be water-soluble, water-insoluble, or poorly water-soluble. The coating material may contain a water-soluble component or a poorly water-soluble component. "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 more than 0 g but less than 1 g in 100 g of ion-exchanged water at 25°C.

[0029] The water-soluble component may contain a compound having a hydrophilic group. Examples of the hydrophilic group include an anionic group, a cationic group, an amphoteric group, and a nonionic group. Examples of the anionic group include a carboxyl group, a sulfonic acid group, and a phosphate group. Examples of the cationic group include an amino group, an imino group, and a quaternary ammonium group. Examples of the amphoteric group include a carbobetaine group, a sulfobetaine group, and a phosphobetaine group. Examples of the nonionic group include a hydroxyl group, an amide group, a pyrrolidone group, a lactam group, an alkoxy group, and a (poly)oxyalkylene group.

[0030] The water-soluble component may contain at least one selected from the group consisting of compounds having a hydroxyl group, compounds having an amide group, compounds having a quaternary ammonium group, and compounds having a (poly)oxyalkylene group. Examples of compounds having a hydroxyl group include polyvinyl alcohol, monosaccharides, polysaccharides, phenyl diglycol, etc. Examples of compounds having an amide group include polyacrylamide, polyvinylpyrrolidone, etc. Examples of compounds having a quaternary ammonium group include ammonium chloride, etc. Examples of compounds having a (poly)oxyalkylene group include polyalkylene oxides (e.g., polyethylene oxide), polyalkylene glycols (e.g., polyethylene glycol), etc.

[0031] 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 (e.g., nylon 6 and nylon 66); polyolefins such as polyurethane, polyethylene, polypropylene, polyisoprene, ethylene / butene copolymer, and ethylene / propylene copolymer; poly-α-methylstyrene, and syndiotactic polystyrene. Examples of the poorly water-soluble component include polystyrenes such as polyhexamethylene carbonate; polycarbonates such as polyhexamethylene carbonate; 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, polyvinyl acetate, and polyvinyl fluoride; polyvinylidene fluoride; and polysiloxanes. The poorly water-soluble component may be acid-modified. The poorly water-soluble component may be acid-modified with, for example, an acid anhydride (maleic anhydride, succinic anhydride, phthalic anhydride, etc.).

[0032] The coating material may contain a polymer having an ethylenically unsaturated monomer as a monomer unit (a polymer having a monomer unit derived from an ethylenically unsaturated monomer). The ethylenically unsaturated monomer is a compound having at least one carbon-carbon double bond in the molecule and having radical polymerizability, and preferably a compound having one carbon-carbon double bond in the molecule and having radical polymerizability. Examples of the ethylenically unsaturated monomer include (meth)acrylic acid and salts thereof, (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-based monomers ((meth)acrylamide, N-isopropyl(meth)acrylamide, 2-(meth)acrylamido-2-methylpropanesulfonic acid and salts thereof, N,N-dimethyl(meth)acrylamide, N-methylol(meth)acrylamide, diethylaminopropyl(meth)acrylamide, etc.), polyethylene glycol mono(meth)acrylate, styrene, α-alkylstyrene, butadiene, etc. From the viewpoint of easily adjusting the water absorption rate of the coated resin particles to be slow, the ethylenically unsaturated monomer may include a (meth)acrylic compound (a compound having a (meth)acryloyl group) and may include at least one selected from the group consisting of (meth)acrylic acid and salts thereof.

[0033] Examples of coating materials that can be used include 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 urethane resins (e.g., condensates of polyol and polyisocyanate), phenolic resins (e.g., condensates of phenolic compounds and aldehydes), polyesters, polyamides, and polycarbonates.

[0034] From the viewpoint of easily adjusting the water absorption rate of the coated resin particles to be slow, the coating material may contain at least one selected from the group consisting of polyvinyl alcohol, polyacrylamide, polyalkylene oxide, polyalkylene glycol, phenyl diglycol, polyoxyalkylene alkyl ether, polyalkyl(meth)acrylate, polyamide, polyolefin, and olefin / ethylenically unsaturated monomer copolymer (copolymer having an olefin and an ethylenically unsaturated monomer as monomer units), or may contain at least one selected from the group consisting of polyvinyl alcohol, polyalkylene glycol, polyalkyl(meth)acrylate, and olefin / ethylenically unsaturated monomer copolymer. The polymer component contained in the coating material may be composed of at least one selected from the group consisting of polyvinyl alcohol, polyacrylamide, polyalkylene oxide, polyalkylene glycol, phenyl diglycol, polyoxyalkylene alkyl ether, polyalkyl(meth)acrylate, polyamide, polyolefin, and copolymer of olefin / ethylenically unsaturated monomer, from the viewpoint of easily adjusting the water absorption rate of the coated resin particles to be slow, or may be composed of at least one selected from the group consisting of polyvinyl alcohol, polyalkylene glycol, polyalkyl(meth)acrylate, and copolymer of olefin / ethylenically unsaturated monomer. From the viewpoint of easily adjusting the water absorption rate of the coated resin particles to be slow, the coating material may contain copolymer of olefin / ethylenically unsaturated monomer and polyalkylene glycol, or copolymer of olefin / ethylenically unsaturated monomer and polyethylene glycol. The polymer component contained in the coating material may be composed only of an olefin / ethylenically unsaturated monomer copolymer and polyalkylene glycol, or may be composed only of an olefin / ethylenically unsaturated monomer copolymer and polyethylene glycol, from the viewpoint of easily adjusting the water absorption rate of the coated resin particles to be slow.

[0035] When the ethylenically unsaturated monomer constituting the polymer component contained in the coating material has an acidic group (e.g., a carboxy group), the acidic group may be neutralized with an alkaline neutralizing agent. In this case, the degree of neutralization of the ethylenically unsaturated monomer may be greater than 0 mol% to 100 mol%, 5 to 100 mol%, 10 to 100 mol%, 20 to 100 mol%, 30 to 100 mol%, 40 to 100 mol%, or 50 to 100 mol% of the acidic groups in the ethylenically unsaturated monomer. Increasing the degree of neutralization facilitates stabilization of the coating liquid (e.g., emulsion) described below, making it easier to obtain uniformly coated resin particles. From this perspective, the degree of neutralization of the ethylenically unsaturated monomer may be 10 to 100 mol%, 20 to 100 mol%, 30 to 100 mol%, 40 to 100 mol%, or 50 to 100 mol%. Examples of the ethylenically unsaturated monomers whose acidic groups have been neutralized in this way include sodium acrylate and ammonium acrylate with a degree of neutralization of 5 to 100 mol %.

[0036] The olefin, which is a monomer unit of polyolefin, or the olefin, which is a monomer unit of an olefin / ethylenically unsaturated monomer copolymer, may contain at least one selected from the group consisting of ethylene, propylene, and butene, or may contain ethylene, from the viewpoint of easily adjusting the water absorption rate of the coated resin particles to be slow. The ethylenically unsaturated monomer in the olefin / ethylenically unsaturated monomer copolymer may contain the ethylenically unsaturated monomer listed as the constituent material of the water-absorbent resin particles described above, may contain a (meth)acrylic compound, or may contain at least one selected from the group consisting of (meth)acrylic acid and salts thereof, from the viewpoint of easily adjusting the water absorption rate of the coated resin particles to be slow.

[0037] In an olefin / ethylenically unsaturated monomer copolymer, the water absorption rate of the coated resin particles can be easily adjusted by adjusting the ratio of olefin monomer units (monomer units derived from an olefin) to ethylenically unsaturated monomer units (monomer units derived from an ethylenically unsaturated monomer). That is, by increasing the ratio of highly hydrophobic monomer units, the water absorption rate of the coated resin particles can be adjusted to be slower. For example, in an olefin / ethylenically unsaturated monomer copolymer, the ratio of olefin monomer units may be 72.0 to 98.0 mol% or 80.0 to 97.0 mol%.

[0038] The coating material may be used alone or in combination of two or more. The main component of the coating material is the component of the coating material that is used in the largest amount (mass). When one coating material is used alone, that one coating material is the main component of the coating material. When two or more coating materials are used in combination, the component of the coating materials that is used in the largest amount is the main component of the coating material. From the viewpoint of easily adjusting the water absorption rate of the coated resin particles to be slow, the main component of the coating material may be an olefin / ethylenically unsaturated monomer copolymer or an ethylene / ethylenically unsaturated monomer copolymer, and the ethylenically unsaturated monomer in these copolymers may include a (meth)acrylic compound and may include at least one selected from the group consisting of (meth)acrylic acid and salts thereof.

[0039] The proportion of the main component of the coating material may be within the following ranges based on the total mass of the coating material, from the viewpoint of easily adjusting the water absorption rate of the coated resin particles to be slow. The proportion of the main component of the coating material may be 50% by mass or more, more than 50% by mass, 60% by mass or more, 70% by mass or more, 80% by mass or more, 85% by mass or more, 90% by mass or more, 91% by mass or more, 92% by mass or more, 95% by mass or more, or 96% by mass or more. The proportion of the main component of the coating material may be 100% by mass or less, less than 100% by mass, 99% by mass or less, 98% by mass or less, 97% by mass or less, 96% by mass or less, 95% by mass or less, 92% by mass or less, or 91% by mass or less. From these viewpoints, the proportion of the main component of the coating material may be 50 to 100% by mass, 70% by mass or more but less than 100% by mass, 80 to 98% by mass, or 85 to 97% by mass. When two or more main coating materials are present (when the amounts (mass) of the two or more coating materials used are the same and each of them is used in the largest amount), the content of the main component of the coating material is the total amount of those two or more coating materials.

[0040] The glass transition temperature (Tg) of the main component of the coating material may be in the following ranges, from the viewpoint of easily adjusting the water absorption rate of the coated resin particles to be slow. The glass transition temperature may be 20°C or higher, 30°C or higher, 40°C or higher, 45°C or higher, 50°C or higher, 52°C or higher, 55°C or higher, 56°C or higher, 58°C or higher, 60°C or higher, 62°C or higher, or 63°C or higher. The glass transition temperature may be 140°C or lower, 120°C or lower, 100°C or lower, 90°C or lower, 80°C or lower, 75°C or lower, 70°C or lower, 65°C or lower, 63°C or lower, 62°C or lower, 60°C or lower, 58°C or lower, or 56°C or lower. From these viewpoints, the glass transition temperature may be 20 to 140°C, 30 to 120°C, 40 to 100°C, 50 to 100°C, 55 to 100°C, 60 to 100°C, 40 to 70°C, 50 to 70°C, 55 to 70°C, 60 to 70°C, 40 to 60°C, 50 to 60°C, or 55 to 60°C. When two or more main coating materials are present, the glass transition temperature of each coating material may be in the above-mentioned range. The glass transition temperature is measured by the method described in the Examples below.

[0041] The melting point (MP) of the main component of the coating material may be within the following ranges, from the viewpoint of easily adjusting the water absorption rate of the coated resin particles. The melting point may be 30°C or higher, 40°C or higher, 50°C or higher, 60°C or higher, 65°C or higher, 70°C or higher, 75°C or higher, 80°C or higher, 85°C or higher, 88°C or higher, 90°C or higher, 95°C or higher, or 99°C or higher. The melting point may be 150°C or lower, 140°C or lower, 130°C or lower, 120°C or lower, 110°C or lower, 105°C or lower, 100°C or lower, 99°C or lower, 95°C or lower, 90°C or lower, or 88°C or lower. From these viewpoints, the melting point may be 30 to 150°C, 40 to 140°C, 50 to 130°C, 60 to 120°C, 60 to 100°C, 60 to 95°C, 80 to 130°C, 80 to 100°C, 80 to 95°C, 85 to 130°C, 85 to 100°C, 85 to 95°C, 95 to 130°C, or 95 to 100°C. When two or more main coating materials are present, the melting point of each coating material may be in the above-mentioned range. The melting point is measured by the method described in the Examples below.

[0042] The proportion of the coating material in the coating step 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 rate of the coated resin particles to be slow, the proportion of the coating material 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, 8.00 parts by mass or more, 9.00 parts by mass or more, 10.00 parts by mass or more, or 11.00 parts by mass or more. From the viewpoint of improving productivity, such as shortening the coating time, the proportion of the coating material 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, 9.00 parts by mass or less, 8.00 parts by mass or less, 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 proportion of the coating material may be 0.10 to 50.00 parts by mass, 0.10 to 15.00 parts by mass, 0.10 to 10.00 parts by mass, 4.00 to 50.00 parts by mass, 4.00 to 15.00 parts by mass, 4.00 to 10.00 parts by mass, 6.00 to 50.00 parts by mass, 6.00 to 15.00 parts by mass, 6.00 to 10.00 parts by mass, 10.00 to 50.00 parts by mass, or 10.00 to 15.00 parts by mass.

[0043] The ratio of the main component of the coating material in the coating step (when two or more types of main component coating materials are present, the ratio of the total amount thereof) may be in the following range relative to 100 parts by mass of the water-absorbent resin particles. From the viewpoint of easily adjusting the water absorption rate of the coated resin particles to be slow, the ratio of the main component of the coating material 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, 8.00 parts by mass or more, 9.00 parts by mass or more, or 10.00 parts by mass or more. The proportion of the main component of the coating material 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, or 10.00 parts by mass or less. The proportion of the main component of the coating material may be 9.00 parts by mass or less, 8.00 parts by mass or less, 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 proportion of the main component of the coating material may be 0.10 to 50.00 parts by mass, 0.10 to 15.00 parts by mass, 0.10 to 10.00 parts by mass, 0.10 to 8.00 parts by mass, 4.00 to 50.00 parts by mass, 4.00 to 15.00 parts by mass, 4.00 to 10.00 parts by mass, 4.00 to 8.00 parts by mass, 6.00 to 50.00 parts by mass, 6.00 to 15.00 parts by mass, 6.00 to 10.00 parts by mass, 6.00 to 8.00 parts by mass, 8.00 to 50.00 parts by mass, 8.00 to 15.00 parts by mass, or 8.00 to 10.00 parts by mass.

[0044] In the coating step of the method for producing coated resin particles according to this embodiment, water-absorbent resin particles (subject to be coated) and a coating material are brought into contact with each other to obtain coated resin particles having a coating portion that coats at least a portion of the water-absorbent resin particles. That is, the coating step is a step in which the coating material is supplied to the water-absorbent resin particles all at once, continuously, or intermittently to bring the water-absorbent resin particles and the coating material into contact with each other. The coating step ends when the water-absorbent resin particles and the coating material reach a state in which they cannot contact each other (typically, when the entire amount of the planned coating material has been supplied). In this specification, the time required for the coating step is referred to as the "coating time." However, if the coating step is performed in multiple steps (e.g., by temporarily stopping the supply of the coating material during the coating step to allow an interval, and then restarting the supply of the coating material), the interval is not included in the coating time. The coating step includes a step X in which the water-absorbent resin particles and the coating material are brought into contact with each other at an ambient temperature "not less than a specific temperature A1 and not more than a specific temperature A2." That is, the step X is a step of bringing the water-absorbent resin particles and the coating material into contact with each other at an ambient temperature that satisfies "temperature A1≦ambient temperature≦temperature A2."

[0045] In the step X, from the viewpoint of obtaining coated resin particles that can be adjusted to have a slower water absorption rate, the water-absorbent resin particles and the coating material are brought into contact with each other at an atmospheric temperature of temperature A1 or higher, which is equal to or higher than a temperature that is 27° C. lower than the glass transition temperature of the main component of the coating material, and the water-absorbent resin particles and the coating material are brought into contact with each other at an atmospheric temperature of temperature A2 or lower, which is equal to or lower than a temperature that is 4° C. higher than the melting point of the main component of the coating material. When two or more main components are present in the coating material, the temperatures A1 and A2 are set based on the main component having the highest glass transition temperature among the two or more main components.

[0046] From the viewpoint of easily adjusting the water absorption rate of the coated resin particles, temperature A1 may be a temperature that is 25°C, 20°C, 15°C, 10°C, 5°C, 3°C, or 1°C lower than the glass transition temperature of the main component of the coating material, a temperature that is equivalent to the glass transition temperature of the main component of the coating material, or a temperature that is 1°C, 2°C, 3°C, 4°C, 5°C, 10°C, 15°C, 20°C, or 25°C higher than the glass transition temperature of the main component of the coating material.

[0047] From the viewpoint of easily adjusting the water absorption rate of the coated resin particles, temperature A2 may be a temperature that is 3°C, 2°C, or 1°C higher than the melting point of the main component of the coating material, or may be a temperature equivalent to the melting point of the main component of the coating material, or may be a temperature that is 1°C, 2°C, 3°C, 4°C, 5°C, 10°C, 15°C, 20°C, 25°C, 30°C, 35°C, 40°C, 45°C, 50°C, or 55°C lower than the melting point of the main component of the coating material.

[0048] The atmospheric temperature at which the water-absorbent resin particles and the coating material are brought into contact with each other in the step X may be, from the viewpoint of easily adjusting the water absorption rate of the coated resin particles, an atmospheric temperature that is not less than a temperature that is 26°C lower than the glass transition temperature of the main component of the coating material and not more than a melting point of the main component of the coating material, may be an atmospheric temperature that is not less than a temperature that is 10°C lower than the glass transition temperature of the main component of the coating material and not more than a temperature that is 5°C lower than the melting point of the main component of the coating material, may be an atmospheric temperature that is not less ... not more than the melting point of the main component of the coating material, or may be an atmospheric temperature that is not less than the glass transition temperature of the main component of the coating material and not more than a temperature that is 20°C lower than the melting point of the main component of the coating material.

[0049] From the viewpoint of easily adjusting the water absorption rate of the coated resin particles, the ambient temperature in step X may be in the following ranges: 20° C. or higher, 23° C. or higher, 25° C. or higher, 30° C. or higher, 35° C. or higher, 40° C. or higher, 45° C. or higher, 50° C. or higher, 55° C. or higher, 60° C. or higher, 65° C. or higher, 70° C. or higher, 75° C. or higher, or 80° C. or higher. The ambient temperature in step X may be 150° C. or lower, 140° C. or lower, 130° C. or lower, 120° C. or lower, 110° C. or lower, 100° C. or lower, 95° C. or lower, 90° C. or lower, 85° C. or lower, 80° C. or lower, 75° C. or lower, 70° C. or lower, 65° C. or lower, 60° C. or lower, 55° C. or lower, 50° C. or lower, 45° C. or lower, 40° C. or lower, 35° C. or lower, or 30° C. or lower. From these viewpoints, the atmospheric temperature in step X may be 20 to 150°C, 20 to 140°C, 23 to 130°C, 30 to 120°C, 30 to 90°C, 30 to 60°C, 40 to 120°C, 40 to 90°C, 40 to 60°C, 60 to 120°C, or 60 to 90°C.

[0050] The coating step may have, in addition to step X, step Y of bringing the water-absorbent resin particles and the coating material into contact with each other at an ambient temperature lower than temperature A1, and / or step Z of bringing the water-absorbent resin particles and the coating material into contact with each other at an ambient temperature higher than temperature A2. For example, step Y may be a step performed before step X until the ambient temperature of step X is reached, and step Z may be a step performed after step X. The coating step may include a plurality of steps X, a plurality of steps Y, a plurality of steps Z, or may include steps X and Y alternately, or may include steps X and Z alternately. The ambient temperature of the coating steps (steps X, Y, and Z) may be a constant temperature or may change over time.

[0051] In the coating step, the time (when the coating step is performed in multiple steps, the total time for multiple steps) during which the water-absorbent resin particles and the coating material are in contact with each other at an ambient temperature that is at least 27°C lower than the glass transition temperature of the main component of the coating material and at most 4°C higher than the melting point of the main component of the coating material) is 30% or more based on the total time of the coating step (coating time), from the viewpoint of adjusting the water absorption rate of the coated resin particles to be slow. From the viewpoint of easily adjusting the water absorption rate of the coated resin particles to be slow, the ratio TR may be 35% or more, 40% or more, 45% or more, 50% or more, 55% or more, 60% or more, 65% or more, 70% or more, 75% or more, 80% or more, 85% or more, 90% or more, or 95% or more. The ratio TR may be 100% or less, 95% or less, 90% or less, 85% or less, 80% or less, 75% or less, 70% or less, or 65% or less. From these perspectives, the ratio TR may be 30 to 100%, 50 to 100%, 60 to 100%, 65 to 100%, 70 to 100%, 80 to 100%, 85 to 100%, 90 to 100%, or 95 to 100%.

[0052] In the coating step, for example, a coating material can be brought into contact with water-absorbent resin particles present in an atmosphere of air, an inert gas (e.g., nitrogen gas), a mixed gas thereof, etc. The coating material brought into contact with the water-absorbent resin particles may be a dry coating material, a liquid or gel-like coating material (e.g., a molten coating material), or a coating material in a coating liquid (a solution of the coating material, a dispersion of the coating material (e.g., an emulsion), etc.) containing the coating material and a liquid medium (e.g., water).

[0053] The coating material may be brought into contact with the water-absorbent resin particles by bringing a coating liquid containing the coating material and a liquid medium (e.g., water) into contact with the water-absorbent resin particles. This makes it easier to obtain a coating portion with a uniform thickness. The liquid medium may be a solvent or a dispersion medium. The coating liquid containing the coating material and the liquid medium may be obtained by dissolving the coating material in a solvent or by dispersing the coating material in a dispersion medium.

[0054] Examples of solvents or dispersion media include water, hydrophilic compounds, and hydrocarbon compounds. A single solvent or dispersion medium may be used, or a mixture of two or more solvents or dispersion media (e.g., a mixture of water and a hydrophilic compound) may be used. A hydrophilic compound is a compound that dissolves substantially uniformly in water. Examples of hydrophilic compounds include alcohols such as methanol 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.

[0055] From the viewpoint of easily adjusting the water absorption rate of the coated resin particles to be slow, the content of the coating material in the coating liquid may be within the following ranges based on the total mass of the coating liquid. The content of the coating material may be 1.00% by mass or more, 3.00% by mass or more, 5.00% by mass or more, 8.00% by mass or more, 10.00% by mass or more, 10.50% by mass or more, 11.00% by mass or more, 12.00% by mass or more, or 15.00% by mass or more. The content of the coating material may be 50.00% by mass or less, 40.00% by mass or less, 30.00% by mass or less, 20.00% by mass or less, 15.00% by mass or less, 12.00% by mass or less, 11.00% by mass or less, 10.50% by mass or less, or 10.00% by mass or less. From these viewpoints, the content of the coating material may be 1.00 to 50.00 mass%, 1.00 to 20.00 mass%, 1.00 to 15.00 mass%, 5.00 to 50.00 mass%, 5.00 to 20.00 mass%, 5.00 to 15.00 mass%, 10.00 to 50.00 mass%, 10.00 to 20.00 mass%, 10.00 to 15.00 mass%, 12.00 to 50.00 mass%, or 12.00 to 20.00 mass%.

[0056] The content of the liquid medium in the coating liquid may be within the following ranges, based on the total mass of the coating liquid, from the viewpoint of easily adjusting the water absorption rate of the coated resin particles to be slow. The content of the liquid medium may be 10.00% by mass or more, 30.00% by mass or more, 50.00% by mass or more, 60.00% by mass or more, 70.00% by mass or more, 80.00% by mass or more, 85.00% by mass or more, 88.00% by mass or more, 89.00% by mass or more, 89.50% by mass or more, or 90.00% by mass or more. The content of the liquid medium may be 99.00% by mass or less, 97.00% by mass or less, 95.00% by mass or less, 92.00% by mass or less, 90.00% by mass or less, 89.50% by mass or less, 89.00% by mass or less, 88.00% by mass or less, or 85.00% by mass or less. From these viewpoints, the content of the liquid medium may be 10.00 to 99.00 mass%, 30.00 to 99.00 mass%, 50.00 to 99.00 mass%, 80.00 to 99.00 mass%, 85.00 to 99.00 mass%, 50.00 to 95.00 mass%, 80.00 to 95.00 mass%, 85.00 to 95.00 mass%, 50.00 to 90.00 mass%, 80.00 to 90.00 mass%, 85.00 to 90.00 mass%, 50.00 to 88.00 mass%, or 80.00 to 88.00 mass%.

[0057] The water content in the coating liquid (the water content RW described below) may be within the following ranges, based on the total mass of the coating liquid, from the viewpoint of easily adjusting the water absorption rate of the coated resin particles to be slow. The water content may be 10.00% by mass or more, 30.00% by mass or more, 50.00% by mass or more, 60.00% by mass or more, 70.00% by mass or more, 80.00% by mass or more, 85.00% by mass or more, 88.00% by mass or more, 89.00% by mass or more, 89.50% by mass or more, or 90.00% by mass or more. The water content may be 99.00% by mass or less, 97.00% by mass or less, 95.00% by mass or less, 92.00% by mass or less, 90.00% by mass or less, 89.50% by mass or less, 89.00% by mass or less, 88.00% by mass or less, or 85.00% by mass or less. From these viewpoints, the water content may be 10.00 to 99.00 mass%, 30.00 to 99.00 mass%, 50.00 to 99.00 mass%, 80.00 to 99.00 mass%, 85.00 to 99.00 mass%, 50.00 to 95.00 mass%, 80.00 to 95.00 mass%, 85.00 to 95.00 mass%, 50.00 to 90.00 mass%, 80.00 to 90.00 mass%, 85.00 to 90.00 mass%, 50.00 to 88.00 mass%, or 80.00 to 88.00 mass%.

[0058] The method for bringing the water-absorbent resin particles and the coating material into contact with each other in the coating step is not particularly limited, and various methods can be used. For example, in the coating step, the water-absorbent resin particles and the coating material (e.g., the coating material of a coating liquid) may be mixed in a container such as a flask to bring the water-absorbent resin particles and the coating material into contact with each other. In the coating step, the coating material may be brought into contact with the water-absorbent resin particles in an airflow, or the water-absorbent resin particles blown up by the airflow may be brought into contact with the coating material, or the water-absorbent resin particles and the coating material may be brought into contact with each other by supplying the coating material to a fluidized bed of the water-absorbent resin particles. The gas constituting the airflow may be air, an inert gas (e.g., nitrogen gas), a mixed gas thereof, or the like. The temperature of the airflow may exceed 25°C. In the coating step, the coating material may be brought into contact with the water-absorbent resin particles in a state of being stirred using a stirring means. The stirring means may be a stirring means having a stirring blade, or may be an air supply means capable of supplying an airflow different from the airflow for blowing up the water-absorbent resin particles. In the coating step, the water-absorbent resin particles may be agitated only by an airflow for blowing the water-absorbent resin particles upward, and no agitation means other than the airflow for blowing the water-absorbent resin particles upward may be used. In the coating step, the water-absorbent resin particles and the coating material may be brought into contact with each other by spraying a coating liquid onto the water-absorbent resin particles. The nozzle for spraying the coating liquid is not particularly limited, and, for example, a tangential spray or a two-fluid nozzle (e.g., a nozzle for spraying the coating liquid and an inert gas) may be used. In the coating step, the water-absorbent resin particles and the coating material may be brought into contact with each other using various devices such as a tumbling granulator, an agitation granulator, or a fluidized bed granulator.

[0059] The method for producing coated resin particles according to this embodiment may include a step of adjusting the ambient temperature of the space in which the water-absorbent resin particles and the coating material are brought into contact with each other before the coating step. The ambient temperature of the space in which the water-absorbent resin particles and the coating material are brought into contact with each other before the coating step may be 25°C or higher, 35°C or higher, 45°C or higher, or 55°C or higher, from the viewpoint of easily adjusting the temperature so as to slow the water absorption rate of the coated resin particles. The ambient temperature may be 140°C or lower, 120°C or lower, 100°C or lower, or 80°C or lower. From these viewpoints, the ambient temperature may be 25 to 140°C, 25 to 120°C, 25 to 100°C, or 25 to 80°C.

[0060] The method for producing coated resin particles according to this embodiment may include a drying step of drying the coated resin particles after the coating step. The ambient temperature and time of the drying step are appropriately adjusted depending on the type and amount of coating material, etc. The ambient temperature of the drying step may be 20°C or higher, 30°C or higher, 40°C or higher, or 50°C or higher, from the viewpoint of easily adjusting the temperature so as to slow the water absorption rate of the coated resin particles. The ambient temperature of the drying step may be 140°C or lower, 120°C or lower, 100°C or lower, or 80°C or lower. From these viewpoints, the ambient temperature of the drying step may be 20 to 140°C, 30 to 120°C, 40 to 100°C, or 50 to 80°C. The duration of the drying step may be 10 minutes or more, 20 minutes or more, or 30 minutes or more. The duration of the drying step may be 180 minutes or less, 120 minutes or less, or 60 minutes or less. From these viewpoints, the drying time may be 10 to 180 minutes, 20 to 120 minutes, or 30 to 60 minutes.

[0061] In the method for producing coated resin particles according to the present embodiment, in the coating step, the water-absorbent resin particles and the coating material may be brought into contact with each other while supplying an airflow from a vertically downward side into the internal space of an apparatus (e.g., a fluidized bed granulator) having an internal space in which the water-absorbent resin particles are accommodated. The internal space is a space into which the coating material is supplied and into which the water-absorbent resin particles and the coating material can come into contact with each other. The airflow may be supplied vertically from a vertically downward side, or may be supplied vertically from a vertically downward side in a direction intersecting the vertical direction.

[0062] 1 is a schematic cross-sectional view showing a treatment apparatus for bringing water-absorbent resin particles and a coating material into contact with each other. The treatment apparatus 1 includes a treatment section 10 and an air supply section (not shown) that supplies an air flow G to the treatment section 10.

[0063] The processing unit 10 is a substantially cylindrical member extending in the vertical direction (height direction of the processing device 1; the same applies below). The processing unit 10 has a substantially cylindrical internal space 11 in which the water-absorbent resin particles 20 are accommodated. The internal space 11 has a cylindrical space 11a, a space 11b located vertically above the space 11a, and a cylindrical space 11c located vertically above the space 11b. The spaces 11a, 11b, and 11c are continuous from below to above in the vertical direction, thereby constituting the internal space 11. The space 11a has the narrowest cross section (narrowest cross section) in the internal space 11. The space 11b tapers upward in the vertical direction.

[0064] An air inlet 12 is formed in the center of the bottom surface of the processing section 10. The air inlet 12 is connected to the air supply section, and an air flow G is supplied to the internal space 11 through the air inlet 12. The ambient temperature of the internal space 11 can be adjusted by adjusting the temperature of the air flow G.

[0065] The processing unit 10 includes an agitation means 13 (e.g., a rotor blade) for the water-absorbent resin particles 20. The agitation means 13 includes a disk-shaped disk portion 13a whose center protrudes vertically upward and a support portion 13b that supports the disk portion 13a. The disk portion 13a extends horizontally below the vertical direction in the space 11a, and an annular opening 14 is formed in the space 11a at a position on the outer periphery of the disk portion 13a. The disk portion 13a has an annular upper surface around the central protrusion, and horizontally extending blades (not shown: for example, three blades extending radially from the center of the disk portion 13a and arranged at equal intervals) are formed on the upper surface. The support portion 13b extends vertically and extends from the outside of the internal space 11 into the interior of the internal space 11 via the air inlet 12. The support portion 13b rotates in the horizontal direction (rotates around the axis of the support portion 13b), thereby allowing the disk portion 13a to rotate in the horizontal direction.

[0066] The air flow G supplied from the air intake port 12 to the internal space 11 is blocked by the disk portion 13a of the stirring means 13 and supplied to the outer periphery of the processing section 10, and then supplied vertically upward through the opening 14 located on the outer periphery of the space 11a.

[0067] The water-absorbent resin particles 20 are deposited on the upper surface of the disk portion 13a, etc., and are agitated as the disk portion 13a rotates, and are blown vertically upward by the airflow G supplied through the opening 14. After being blown vertically upward by the airflow G, the water-absorbent resin particles 20 fall vertically downward by gravity. An exhaust filter (e.g., a bag filter) 15 is disposed in the space 11c of the internal space 11, and the airflow G supplied to the internal space 11 is discharged from the exhaust filter 15 to the outside of the processing device 1.

[0068] The processing unit 10 includes a liquid supply unit 16 (e.g., a nozzle) that supplies the coating liquid L to the space 11a. The liquid supply unit 16 supplies the coating liquid L from the outer periphery to the inner periphery of the space 11a at a position vertically above the disk unit 13a of the stirring means 13 in the space 11a. The coating liquid L supplied from the liquid supply unit 16 is blown up by the airflow G and comes into contact with the water-absorbent resin particles 20 that are descending due to gravity. The water-absorbent resin particles 20 and the coating material of the coating liquid L come into contact with each other to form a coating portion, thereby obtaining coated resin particles. Volatile components (water, etc.) in the coating liquid L are volatilized by the airflow G, heat in the internal space 11, etc.

[0069] The processing unit 10 is provided with a thermometer (not shown) that measures the atmospheric temperature of the internal space 11. The thermometer may be arranged at a position in the internal space 11 where the water-absorbent resin particles 20 and the coating liquid L come into contact with each other, or may be arranged in the vicinity of the liquid supply unit 16 at a height equivalent to that of the liquid supply unit 16.

[0070] The configuration of the treatment device for bringing the water-absorbent resin particles and the coating material into contact with each other is not limited to the configuration of the treatment device 1. For example, the treatment device may be provided with an agitation device that supplies an airflow from the side of the treatment device 10 to agitate the water-absorbent resin particles 20, instead of or in addition to the agitation device of the treatment device 1. The treatment device may be provided with a means (e.g., a heater) for heating the treatment device 10 as a means for adjusting the atmospheric temperature of the internal space 11 of the treatment device 10 in the treatment device 1.

[0071] In the method for producing coated resin particles according to this embodiment, in at least a part (part or all) of the coating step or at least a part (part or all) of the step X, the amount of heat X [unit: MJ / (min m ２ ))] is within a specific range, the water-absorbent resin particles and the coating material may be brought into contact with each other while supplying an airflow from the vertically downward side into the internal space of the device having an internal space containing the water-absorbent resin particles. By adjusting the amount of heat X, the coating material is easily softened and the uniformity of the coating portion is easily adjusted, so that the water absorption rate of the coated resin particles can be adjusted. The temperature of the airflow may exceed 25°C. The narrowest cross-sectional area of ​​the internal space is the narrowest cross-sectional area of ​​the internal space in the horizontal direction. The area occupied by a device (e.g., agitation means 13) placed in the internal space is included in the narrowest cross-sectional area. X = (AG x SG x SH x TD) / (NA x 1000) ... (1) AG: Amount of airflow supplied to the internal space [m ３ / min] SG: specific gravity of gas constituting the airflow [kg / m ３ ] SH: specific heat of the gas constituting the airflow [kJ / (kg K)] TD: temperature difference of the airflow temperature at the time of supply to the device relative to 25°C [K] NA: area of ​​the narrowest cross section perpendicular to the vertical direction of the internal space [m ２ ]

[0072] The calorific value X is within the following range (unit: MJ / (min m ２From the viewpoint of easily adjusting the water absorption rate of the coated resin particles to be slow, the calorific value X may be 1.000 or more, 2.000 or more, 3.000 or more, 4.000 or more, 5.000 or more, 6.000 or more, 7.000 or more, 8.000 or more, 9.000 or more, 10.000 or more, or 11.000 or more. From the viewpoint of easily adjusting the water absorption rate of the coated resin particles, the calorific value X may be 15.000 or less, 14.000 or less, 13.000 or less, 12.000 or less, 11.000 or less, 10.000 or less, 9.000 or less, 8.000 or less, 7.000 or less, 6.000 or less, 5.000 or less, 4.000 or less, 3.000 or less, or 2.000 or less. From these viewpoints, the calorific value X may be 1.000 to 15.000, 1.000 to 10.000, 1.000 to 7.000, 5.000 to 15.000, 5.000 to 10.000, 5.000 to 7.000, 7.000 to 15.000, or 7.000 to 10.000.

[0073] The supply amount AG of the airflow to the internal space can be appropriately adjusted depending on the volume of the internal space, etc. From the viewpoint of easily adjusting the water absorption rate of the coated resin particles, the supply amount AG is set to the following range (unit: m ３ / min" may be omitted). The supply rate AG may be 0.1 or more, 0.3 or more, 0.5 or more, 0.6 or more, 0.7 or more, 0.8 or more, 0.9 or more, or 1.0 or more. The supply rate AG may be 1000 or less, 500 or less, 300 or less, 100 or less, 75 or less, 50 or less, 25 or less, 10 or less, 3.0 or less, 2.5 or less, 2.0 or less, 1.5 or less, 1.2 or less, 1.0 or less, 0.9 or less, or 0.8 or less. From these viewpoints, the supply rate AG may be 0.1 to 1000, 0.3 to 500, 0.5 to 300, 0.6 to 100, 0.7 to 75, 0.8 to 50, 0.9 to 25, or 1.0 to 10.

[0074] In the method for producing coated resin particles according to this embodiment, in at least a part (part or all) of the coating step or at least a part (part or all) of the step X, the moisture content Y [unit: kg / (min m ２) )] is in a specific range, the water-absorbent resin particles may be brought into contact with a coating liquid containing a coating material and water. By adjusting the water content Y, the coating material is easily softened and the uniformity of the coating layer can be improved, making it easy to adjust the water absorption rate of the coated resin particles. Y = [(AL x RW) / 100] / (NA x 1000) ... (2) AL: Amount of coating liquid supplied to the internal space [g / min] RW: Water content in the coating liquid [mass %] NA: Area of ​​the narrowest cross section perpendicular to the vertical direction of the internal space [m ２ ]

[0075] The moisture content Y is within the following range (unit: kg / (min m ２ The moisture content Y may be 0.100 or more, 0.200 or more, 0.300 or more, 0.400 or more, or 0.500 or more, from the viewpoint of easily adjusting the moisture absorption rate of the coated resin particles to be slow. The moisture content Y may be 0.600 or more, 0.800 or more, 1.000 or more, 1.200 or more, or 1.400 or more, from the viewpoint of easily adjusting the moisture absorption rate of the coated resin particles to be slow. The moisture content Y may be 2.000 or less, 1.800 or less, 1.500 or less, 1.400 or less, 1.200 or less, 1.000 or less, 0.800 or less, 0.600 or less, or 0.500 or less, from the viewpoint of easily adjusting the moisture absorption rate of the coated resin particles to be slow. From the viewpoint of easily adjusting the water absorption rate of the coated resin particles, the water content Y may be 0.400 or less, 0.300 or less, or 0.200 or less. From these viewpoints, the water content Y may be 0.100 to 2.000, 0.100 to 1.500, 0.100 to 0.600, 0.200 to 2.000, 0.200 to 1.500, 0.200 to 0.600, 0.400 to 2.000, 0.400 to 1.500, or 0.400 to 0.600.

[0076] The supply amount AL of the coating liquid to the internal space can be appropriately adjusted depending on the amount of water-absorbent resin particles supplied to the treatment section, etc. From the viewpoint of easily adjusting the water absorption rate of the coated resin particles, the supply amount AL may be within the following range (the unit "g / min" is omitted). The supply amount AL may be 1.0 or more, 2.0 or more, 3.0 or more, 4.0 or more, 5.0 or more, 6.0 or more, 7.0 or more, 8.0 or more, 10.0 or more, 12.0 or more, 15.0 or more, or 18.0 or more. The supply amount AL may be 10,000 or less, 1,000 or less, 100 or less, 50.0 or less, 30.0 or less, 25.0 or less, 21.0 or less, 20.0 or less, 18.0 or less, 15.0 or less, 12.0 or less, 10.0 or less, 8.0 or less, 7.0 or less, 6.0 or less, 5.0 or less, 4.0 or less, or 3.0 or less. From these viewpoints, the supply amount AL may be 1.0 to 10,000, 1.0 to 1000, 1.0 to 100, 1.0 to 50.0, 1.0 to 30.0, 1.0 to 20.0, 1.0 to 10.0, 1.0 to 5.0, 5.0 to 30.0, 5.0 to 20.0, 5.0 to 10.0, 10.0 to 30.0, or 10.0 to 20.0.

[0077] In the method for producing coated resin particles according to this embodiment, the moisture content Y [kg / (min m ２ ) )] to the above-mentioned heat quantity X [MJ / (min m ２ The water-absorbent resin particles and the coating liquid containing the coating material and water may be brought into contact with each other in a state where the ratio X / Y [unit: MJ / kg] of the ratio X / Y is in a specific range. By adjusting the ratio X / Y, it is possible to easily adjust the degree of softening of the coating material to a more suitable level, and the uniformity of the coating layer can be improved, so that the water absorption rate of the coated resin particles can be easily adjusted.

[0078] The ratio X / Y may be within the following ranges (the unit "MJ / kg" is omitted): From the viewpoint of easily adjusting the water absorption rate of the coated resin particles to be slow, the ratio X / Y may be 1.000 or more, 2.000 or more, 3.000 or more, 4.000 or more, 5.000 or more, 6.000 or more, 7.000 or more, 8.000 or more, 9.000 or more, 10.000 or more, 11.000 or more, 12.000 or more, 15.000 or more, 17.000 or more, or 19.000 or more. From the viewpoint of easily adjusting the water absorption rate of the coated resin particles, the ratio X / Y may be 40,000 or less, 35,000 or less, 30,000 or less, 25,000 or less, 20,000 or less, 19,000 or less, 17,000 or less, 15,000 or less, or 12,000 or less. From the viewpoint of easily adjusting the water absorption rate of the coated resin particles, the ratio X / Y may be 11,000 or less, 10,000 or less, 9,000 or less, 8,000 or less, 7,000 or less, 6,000 or less, 5,000 or less, or 4,000 or less. From these viewpoints, the ratio X / Y is 1.000 to 40.000, 1.000 to 30.000, 1.000 to 20.000, 1.000 to 15.000, 2.000 to 40.000, 2.000 to 35.000, 2.000 to 30.000, 2.000 to 20.000, 2.000 to 15.000, 7.000 to 40.000, It may be 7,000 to 30,000, 7,000 to 20,000, 7,000 to 15,000, 10,000 to 40,000, 10,000 to 30,000, 10,000 to 20,000, 10,000 to 15,000, 15,000 to 40,000, 15,000 to 30,000, or 15,000 to 20,000.

[0079] The coated resin particles according to the present embodiment are coated resin particles obtained by the method for producing coated resin particles according to the present embodiment. The coated resin particles according to the present embodiment include water-absorbent resin particles and a coating that coats at least a portion (partially or entirely) of the water-absorbent resin particles. The absorbent according to the present embodiment contains the coated resin particles according to the present embodiment, and may contain the coated resin particles according to the present embodiment and water-absorbent resin particles without a coating, or may contain the coated resin particles according to the present embodiment and fibrous material, or may contain the coated resin particles according to the present embodiment, water-absorbent resin particles without a coating, and fibrous material. Examples of fibrous material include finely pulverized wood pulp; cotton; cotton linter; rayon; cellulosic fibers such as cellulose acetate; synthetic fibers such as polyamide, polyester, and polyolefin; and mixtures of these fibers. The absorbent article according to the present embodiment includes the absorbent according to the present embodiment and may include the absorbent according to the present embodiment and other components. Other components include a core wrap (e.g., a core wrap that maintains the shape of the absorbent body), a liquid-permeable sheet (e.g., a liquid-permeable sheet that is placed on the outermost side of the side where the liquid to be absorbed penetrates), and a liquid-impermeable sheet (e.g., a liquid-impermeable sheet that is placed on the outermost side opposite the side where the liquid to be absorbed penetrates).

[0080] The present invention will be described in more detail below with reference to examples. However, the present invention is not limited to these examples. Unless the temperature during the experimental procedure is specified below, the experimental procedure was carried out at room temperature.

[0081] <Preparation of water-absorbent resin particles> (Water-absorbent resin particles (1)) 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 tube, and a stirrer (a stirring blade having 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 (polymer dispersant, manufactured by Mitsui Chemicals, Inc., Hiwax 1105A) were added to the separable flask to obtain a mixture. While stirring this mixture at a rotation speed of 300 rpm, the separable flask was immersed in an 85 ° C. water bath and heated to 80 ° C. to dissolve the polymer dispersant. Thereafter, the water bath was removed, and the mixture was allowed to cool to 55 ° C. at room temperature.

[0082] Next, 92.0 g of an 80.5% by mass acrylic acid aqueous solution (acrylic acid: 1.03 mol) and a stirrer chip were placed in a 500 mL Erlenmeyer flask, and stirring was initiated. Subsequently, while cooling externally, 102.2 g of a 30% by mass aqueous sodium hydroxide solution was added dropwise to the Erlenmeyer flask to neutralize the 75 mol% acrylic acid. Thereafter, 0.092 g of hydroxyethyl cellulose (thickener, manufactured by Sumitomo Seika Chemicals Co., Ltd., HEC AW-15F), 0.0736 g (0.272 mmol) of potassium persulfate (water-soluble radical polymerization initiator), 0.0101 g (0.0580 mmol) of ethylene glycol diglycidyl ether (internal crosslinking agent), and 34.66 g of ion-exchanged water were added, and the mixture was stirred until all components other than water were fully dissolved to prepare a first-stage monomer aqueous solution.

[0083] The first-stage aqueous monomer solution was added to the separable flask and stirred for 10 minutes. A surfactant solution was obtained by dissolving 0.736 g of sucrose stearate (surfactant, manufactured by Mitsubishi Chemical Foods Corporation, Ryoto Sugar Ester S-370, HLB: 3) in 6.62 g of n-heptane under heating. 7.356 g of this surfactant solution was added to the separable flask to obtain a reaction solution. The rotation speed was then changed to 550 rpm, and the atmosphere inside the separable flask was thoroughly purged with nitrogen while stirring the reaction solution. 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 from the point at which the internal temperature reached its maximum (maximum temperature of 82°C), yielding a first-stage polymerization product.

[0084] Next, 128.8 g of an 80.5% by mass acrylic acid aqueous solution (1.44 mol of acrylic acid) and a stirrer chip were placed in a separate 500 mL Erlenmeyer flask, and stirring was initiated. Subsequently, while cooling externally, 143.1 g of a 30% by mass aqueous sodium hydroxide solution was added dropwise to the Erlenmeyer flask to neutralize the 75 mol% acrylic acid. Subsequently, 0.1030 g (0.3810 mmol) of potassium persulfate, 0.0116 g (0.0666 mmol) of ethylene glycol diglycidyl ether (internal crosslinking agent), and 3.13 g of ion-exchanged water were added, and the mixture was stirred until all components other than water were fully dissolved, to prepare a second-stage monomer aqueous solution.

[0085] The rotation speed was changed to 1000 rpm, and the first-stage polymerization product was cooled to 25°C while stirring. The entire second-stage aqueous monomer solution was then added to the first-stage polymerization product to obtain a reaction mixture. The atmosphere in the separable flask was thoroughly purged with nitrogen while stirring the reaction mixture. The separable flask was then immersed in a 70°C water bath to heat the reaction mixture. Once the polymerization reaction had progressed and the internal temperature had reached its maximum (maximum temperature of 82°C), heating was continued for an additional 5 minutes to obtain a second-stage polymerization product (a slurry of polymer particles before surface crosslinking).

[0086] After the second-stage polymerization, the temperature of the second-stage polymerization product was increased in an oil bath at 125°C, and 252 g of water was extracted from the system by azeotropic distillation of n-heptane and water while refluxing n-heptane. Subsequently, 0.0884 g (0.5075 mmol) of ethylene glycol diglycidyl ether was added as a surface cross-linking agent, and the mixture was maintained at 83°C for 2 hours to obtain a slurry of surface-cross-linked polymer particles.

[0087] Thereafter, the slurry of the polymer particles after the surface crosslinking described above was heated in an oil bath at 125°C, and n-heptane was evaporated and dried to obtain a dried product. Of this dried product, the polymer particles that had passed through a sieve with an opening of 850µm and remained on the surface of the sieve with an opening of 250µm were collected to obtain 210.1g of water-absorbent resin particles (1) in the form of agglomerated spherical particles (particles in a state in which no coating portion was formed: particle diameter 250 to 850µm). Further, the same operation was performed to prepare a total of 500.0g or more of water-absorbent resin particles (1).

[0088] (Water-absorbent resin particles (2)) The same procedure as for the water-absorbent resin particles (1) was carried out except that the amount of ethylene glycol diglycidyl ether (internal crosslinking agent) used when preparing the first-stage aqueous monomer solution was changed to 0.00534 g (0.0306 mmol), thereby obtaining 211.1 g of water-absorbent resin particles (2). Further, the same procedure was carried out to prepare 500.0 g or more of water-absorbent resin particles (2) in total.

[0089] <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 placed on a magnetic stirrer, and a stirrer tip was placed in the beaker, and stirring was initiated.

[0090] 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% by mass aqueous sodium hydroxide solution.

[0091] A round-bottomed, cylindrical, separable flask with an inner diameter of 11 cm and an internal volume of 2 L was prepared. It was equipped with a reflux condenser, a thermometer, and a stirrer (a stirring blade with four inclined paddle blades and a blade diameter of 5 cm). 100 g of ethylene / acrylic acid copolymer (ethylene monomer: 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 sodium hydroxide aqueous solution was added. The beaker used to prepare the sodium hydroxide aqueous 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.

[0092] While stirring the reaction solution with a stirrer at a rotation 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.

[0093] Thereafter, the separable flask was 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 lower, 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)").

[0094] <Preparation of Ethylene / Methacrylic 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 889.8 g of ion-exchanged water was then added to the beaker. The ice bath was placed on a magnetic stirrer, and a stirrer tip was placed in the beaker to stir the ion-exchanged water in the beaker.

[0095] 8.83 g (0.221 mol) of sodium hydroxide (granules, manufactured by Nacalai Tesque, Inc.) was added little by little to the above beaker to prepare a 0.98% by mass aqueous sodium hydroxide solution.

[0096] A round-bottomed, cylindrical, separable flask with an inner diameter of 11 cm and an internal volume of 2 L was prepared, equipped with a reflux condenser, a thermometer, and a stirrer (a stirring blade with four inclined paddle blades with a blade diameter of 5 cm). 100 g of ethylene / methacrylic acid copolymer (ethylene monomer to methacrylic acid monomer molar ratio = 12.3:1, manufactured by Mitsui-Dow Polychemicals Co., Ltd., Nuclel 2060) was added to the separable flask. Subsequently, the entire amount of the 0.98% by mass sodium hydroxide aqueous solution was added. The beaker used to prepare the sodium hydroxide aqueous 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.

[0097] While stirring the reaction solution with a stirrer at a rotation 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.

[0098] Thereafter, the separable flask was 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 / methacrylic acid copolymer (P(E / MAA), an aqueous dispersion of a partially neutralized ethylene / methacrylic acid copolymer, nonvolatile content 10% by mass, degree of neutralization 92%; hereinafter referred to as "aqueous dispersion (B)").

[0099] <Measurement of Glass Transition Temperature and Melting Point of Main Component of Coating Material> 7.00 g of the above-mentioned 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 to seal the lid. After perforating the aluminum foil, it was heated to 40°C in a hot air dryer and dried to obtain 1.08 g of polymer film (A). In addition, 1.04 g of polymer film (B) was obtained in the same manner, except that 7.00 g of aqueous dispersion (A) was replaced with 10.0 g of the above-mentioned aqueous dispersion (B).

[0100] 3.0 mg of the above-mentioned polymer film (polymer film (A) or polymer film (B)) 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 and cooling 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 of the partially neutralized ethylene / acrylic acid copolymer (neutralization degree 90 mol%) obtained from the aqueous dispersion (A) was 56°C and the melting point was 88°C. The partially neutralized ethylene / methacrylic acid copolymer (degree of neutralization: 92 mol%) obtained from the aqueous dispersion (B) had a glass transition temperature of 63°C and a melting point of 99°C.

[0101] <Preparation of Coated Resin Particles> (Comparative Example 1) In a 1 L beaker (made of polypropylene), 333.33 g of the above-mentioned aqueous dispersion (A) and 5.00 g of polyethylene glycol (manufactured by Tokyo Chemical Industry Co., Ltd., PEG 6000, number average molecular weight: 7300 to 9300) were mixed with 161.67 g of ion-exchanged water as coating materials to prepare 500.00 g of a coating liquid. The contents of the coating materials in the 500.00 g coating liquid were 50.00 g (10.00 mass%) of partially neutralized ethylene / acrylic acid copolymer (P(E / AA), degree of neutralization 90%) and 5.00 g (1.00 mass%) of polyethylene glycol.

[0102] A fluidized bed granulator having the configuration shown in Figure 1 was prepared. The area of ​​the narrowest cross section perpendicular to the vertical direction of the internal space in the processing section of the fluidized bed granulator was 0.011 m ２ It was.

[0103] 500.0 g of the water-absorbent resin particles (1) were introduced into the processing section of the fluidized bed granulator. Next, the inside of the processing section was stirred with a stirring means (rotor blade, rotation speed: 250 rpm), and the supply air temperature was 27°C and the supply amount was 0.8 m ３ An airflow (air) supplied to the fluidized bed granulator at a rate of 6.0 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 6.0 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 11.00 parts by mass. Thereafter, the coating material was sprayed onto the fluidized bed granulator at room temperature at a supply rate of 0.8 m ３ An air flow (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 particles, thereby obtaining particles (A).

[0104] 50.0 g of the particles (A) were spread on a metal tray measuring 26 cm in length and 20 cm in width, and then covered with aluminum foil. After holes were perforated in the aluminum foil, the particles (A) were heated for 30 minutes in a hot air dryer (FV-320, manufactured by ADVANTEC) set at 120°C to obtain 50.0 g of coated resin particles.

[0105] Example 1 The same procedure as in Comparative Example 1 was carried out except that the temperature of the supply air was changed to 45°C, to obtain 50.0 g of coated resin particles.

[0106] Example 2 The same procedure as in Comparative Example 1 was carried out except that the temperature of the supply air was changed to 50°C, to obtain 50.0 g of coated resin particles.

[0107] (Example 3) The same procedure as in Comparative Example 1 was carried out, except that the supply air temperature of the airflow was changed to 90°C, and that after spraying the coating liquid, the airflow was supplied until the ambient temperature dropped to 50°C or less, thereby obtaining 50.0 g of coated resin particles.

[0108] (Comparative Example 2) 250.00 g of the above-mentioned aqueous dispersion (A) and 1.25 g of polyethylene glycol (manufactured by Tokyo Chemical Industry Co., Ltd., PEG6000, number average molecular weight: 7300 to 9300) were mixed with 123.75 g of ion-exchanged water to prepare 375.00 g of a coating liquid. The temperature of the airflow supply was changed to 130°C, and the airflow supply volume was 1.0 m ３ / min, the supply rate of the total amount of the coating liquid was changed to 3.0 g / min, and after spraying the coating liquid, the airflow was supplied until the atmospheric temperature decreased to 50°C or less. As the contents of the coating material in 375.00 g of the coating liquid, the content of partially neutralized ethylene / acrylic acid copolymer (P(E / AA), neutralization degree 90%) was 37.50 g (10.00 mass%), and the content of polyethylene glycol was 1.25 g (0.33 mass%). The supply amount of the coating material per 100 parts by mass of the water-absorbent resin particles was 7.75 parts by mass. During spraying of the coating liquid, the water-absorbent resin particles adhered to the inner wall and bottom surface of the processing section of the fluidized bed granulator and became agglomerated, so that coated resin particles could not be obtained.

[0109] (Example 4) The temperature of the airflow supply was changed to 50°C, and the airflow supply volume was 0.8 m ３ The same procedure as in Comparative Example 2 was carried out except that the speed was changed to / min, and 50.0 g of coated resin particles were obtained.

[0110] Example 5 The same procedure as in Comparative Example 2 was carried out except that the total supply rate of the coating liquid was changed to 4.5 g / min, to obtain 50.0 g of coated resin particles.

[0111] (Example 6) The temperature of the airflow supply was changed to 90°C, and the airflow supply volume was 0.8 m ３ The same procedure as in Comparative Example 2 was carried out except that the supply rate of the coating liquid was changed to 6.0 g / min and the total supply rate of the coating liquid was changed to 6.0 g / min, and 50.0 g of coated resin particles were obtained.

[0112] (Example 7) In preparing the coating solution, 251.25 g of the coating solution was prepared without mixing ion-exchanged water, and the amount of airflow was 0.8 m ３ The same procedure as in Comparative Example 2 was carried out except that the feed rate was changed to 6.0 g / min and the total supply rate of the coating liquid was changed to 6.0 g / min, thereby obtaining 50.0 g of coated resin particles. As the content of the coating material in 251.25 g of the coating liquid, the content of partially neutralized ethylene / acrylic acid copolymer (P(E / AA), neutralization degree 90%) was 37.50 g (14.93 mass%), and the content of polyethylene glycol was 1.25 g (0.50 mass%). The supply amount of the coating material per 100 parts by mass of the water-absorbent resin particles was 7.75 parts by mass.

[0113] Example 8 The same procedure as in Comparative Example 2 was carried out except that the total supply rate of the coating liquid was changed to 18.0 g / min, to obtain 50.0 g of coated resin particles.

[0114] (Example 9) Air flow supply volume: 0.8 m ３ The same procedure as in Comparative Example 2 was carried out except that the feed rate was changed to 18.0 g / min and the total supply rate of the coating liquid was changed to 18.0 g / min, thereby obtaining 50.0 g of coated resin particles.

[0115] (Example 10) 50.0 g of coated resin particles were obtained in the same manner as in Comparative Example 2, except that 250.00 g of a coating liquid was prepared by mixing 166.70 g of the above-mentioned aqueous dispersion (A) with 83.30 g of ion-exchanged water as the coating material without using polyethylene glycol, and that the total supply rate of the coating liquid was changed to 18.0 g / min. As the content of the coating material in the 250.00 g coating liquid, the content of a partially neutralized ethylene / acrylic acid copolymer (P(E / AA), neutralization degree 90%) was 25.00 g (10.00 mass%). The supply amount of the coating material per 100 mass parts of the water-absorbent resin particles was 5.00 mass parts.

[0116] (Comparative Example 3) 50.0 g of coated resin particles were obtained in the same manner as in Comparative Example 1, except that 376.25 g of a coating liquid was prepared by dissolving 1.25 g of polyethylene glycol in 375.00 g of the above-mentioned aqueous dispersion (B). As the contents of the coating materials in the 376.25 g of coating liquid, the content of a partially neutralized ethylene / methacrylic acid copolymer (P(E / MAA), neutralization degree 92%) was 37.50 g (9.97 mass%), and the content of polyethylene glycol was 1.25 g (0.33 mass%). The amount of the coating material supplied per 100 parts by mass of the water-absorbent resin particles was 7.75 parts by mass.

[0117] (Example 11) The same procedure as in Comparative Example 3 was carried out, except that the supply air temperature of the airflow was changed to 90°C, and that after spraying the coating liquid, the airflow was supplied until the ambient temperature dropped to 50°C or less, thereby obtaining 50.0 g of coated resin particles.

[0118] (Comparative Example 4) The temperature of the airflow supply was changed to 140°C, and the airflow supply amount was 1.0 m ３ / min, the supply rate of the total amount of the coating liquid was changed to 3.0 g / min, and after spraying the coating liquid, the air flow was supplied until the atmospheric temperature was reduced to 50° C. or less. During spraying of the coating liquid, the water-absorbent resin particles adhered to the inner wall and bottom surface of the processing section of the fluidized bed granulator and were agglomerated, and therefore coated resin particles could not be obtained.

[0119] Example 12 The same procedure as in Example 2 was carried out except that the water-absorbent resin particles (1) were changed to the water-absorbent resin particles (2), to obtain 50.0 g of coated resin particles.

[0120] Example 13 The same procedure as in Example 3 was carried out except that the water-absorbent resin particles (1) were changed to the water-absorbent resin particles (2), to obtain 50.0 g of coated resin particles.

[0121] <Measurement of Ambient Temperature> A thermometer was placed at a position where the water-absorbent resin particles and the coating liquid contact each other in the internal space of the processing section of the above-mentioned fluidized bed granulator, where the water-absorbent resin particles were blown up by an air current, to measure the ambient temperature. The ambient temperature rose from the start of spraying of the coating liquid, and maintained at a constant value after a certain time. Table 2 shows "the proportion of time during which the water-absorbent resin particles and the coating material were in contact with each other at an ambient temperature that was at least 27°C lower than the glass transition temperature of the main component of the coating material and at most 4°C higher than the melting point of the main component of the coating material" (proportion of time within a specific temperature range) during spraying of the coating liquid (from the start of spraying to the end of spraying). Table 2 also shows the ambient temperature at the end of spraying of the coating liquid (the temperature at the time when the entire amount of the coating liquid had been sprayed), and the difference between the ambient temperature at the end of spraying and the glass transition temperature (Tg) or melting point (MP) of the coating material (the main component of the coating material).

[0122] <Calculation of the amount of heat X, the amount of moisture Y, and the ratio X / Y> From the above formula (1), the amount of heat X [MJ / (min m ２ ) was calculated. The specific gravity SG was calculated based on the specific gravity of air, 1.1845 [kg / m ３ The specific heat SH was calculated using the specific heat of air, 1.0063 kJ / (kg·K). The calculation results for the amount of heat X are shown in Table 2.

[0123] From the above formula (2), the moisture content Y [kg / (min m ２ ) was calculated. The difference between the total amount of coating liquid and the total amount of coating material (non-volatile content) was calculated as the water content RW in the coating liquid. In addition, the ratio X / Y [MJ / kg] of the calorie amount X to the water content Y was calculated. The calculation results of the water content Y and the ratio X / Y are shown in Table 2.

[0124] <Measurement of Lock-Up Height> 0.200 g of the above-described coated resin particles (excluding Comparative Examples 2 and 4) was precisely weighed. Next, the coated resin particles were spread in a layer on the bottom of an acrylic cylinder with an inner diameter of 2.0 cm and a depth of 8.0 cm to form a particle layer with a flat top surface. The height H0 from the top of the bottom of the acrylic cylinder to the top of the particle layer was then measured. 20 g of saline solution at 25°C was then poured into the acrylic cylinder from the top all at once. Measurement was started once the entire amount of saline solution had been poured, and after 5 minutes, the height H5 from the top of the bottom of the acrylic cylinder to the top of the water-absorbed particle layer was measured. The 5-minute lock-up height of the coated resin particles was calculated using the formula "5-minute lock-up height [cm] = H5 - H0." When the top surface of the particle layer after water absorption was not flat, the height of the highest point was recorded as H5. The results are shown in Table 2.

[0125]

[0126]

[0127] DESCRIPTION OF SYMBOLS 1... processing device, 10... processing section, 11... internal space, 11a, 11b, 11c... space, 12... air inlet, 13... stirring means, 13a... disk section, 13b... support section, 14... opening, 15... exhaust filter, 16... liquid supply section, 20... water-absorbent resin particles, G... air flow, L... coating liquid

Claims

1. A method for producing coated resin particles, comprising a coating step of obtaining coated resin particles having a coating portion that covers at least a portion of the water-absorbent resin particles by contacting water-absorbent resin particles and a coating material with each other, wherein in the coating step, the water-absorbent resin particles and the coating material are in contact with each other at an ambient temperature that is at least 27°C lower than the glass transition temperature of the main component of the coating material and at most 4°C higher than the melting point of the main component of the coating material for 30% or more of the time.

2. The method for producing coated resin particles according to claim 1, wherein the glass transition temperature of the main component of the coating material is 20 to 140°C.

3. The method for producing coated resin particles according to claim 1, wherein the melting point of the main component of the coating material is 30 to 150°C.

4. The method for producing coated resin particles according to claim 1, wherein the main component of the coating material is a copolymer of an olefin and an ethylenically unsaturated monomer.

5. A method for producing coated resin particles according to any one of claims 1 to 4, comprising contacting the water-absorbent resin particles and the coating material with each other while supplying an airflow from a vertically downward side into the internal space of a device having an internal space in which the water-absorbent resin particles are housed.

6. In at least a part of the coating process, the heat quantity X represented by the following formula (1) is 1.000 [MJ / (min m ２ 6. The method for producing coated resin particles according to claim 5, wherein the water-absorbent resin particles and the coating material are brought into contact with each other in a state where the water-absorbent resin particles and the coating material are ... ３ / min] SG: specific gravity of the gas constituting the airflow [kg / m ３ ] SH: specific heat of the gas constituting the airflow [kJ / (kg·K)] TD: temperature difference of the airflow temperature at the time of supply to the device with respect to 25° C. [K] NA: area of ​​the narrowest cross section perpendicular to the vertical direction of the internal space [m ２ ] 7. The heat quantity X is 3.000 [MJ / (min m ２ 7. The method for producing coated resin particles according to claim 6, wherein the total amount of the resin particles is equal to or greater than 100%.

8. In at least a part of the coating process, the moisture content Y [kg / (min m ２ The amount of heat X [MJ / (min m ２ 7. The method for producing coated resin particles according to claim 6, wherein the water-absorbing resin particles are brought into contact with the coating liquid containing the coating material and water in a state where a ratio X / Y of the ratio X / Y of the coating material and the water-absorbing resin particles is 2.00 to 35.00 [MJ / kg]. Y=[(AL×RW) / 100] / (NA×1000) (2) AL: supply amount of the coating liquid to the internal space [g / min] RW: content of water in the coating liquid [mass %] NA: area of ​​the narrowest cross section perpendicular to the vertical direction of the internal space [m ２ ] 9. The method for producing coated resin particles according to claim 8, wherein the ratio X / Y is 7.00 to 30.00 [MJ / kg].

10. The moisture content Y is 0.200 [kg / (min m ２ 9. The method for producing coated resin particles according to claim 8, wherein the total amount of the resin particles is equal to or greater than 100%.

Images