Method for manufacturing coated resin particles, and coated resin particles

Abstract

To provide a method for producing coated resin particles in which water absorption is delayed while suppressing a decrease in water retention capacity, and to provide coated resin particles. [Solution] A method for producing coated resin particles, comprising the steps of: preparing mixed particles containing water-insoluble particles and water-absorbent resin particles; and bringing the mixed particles into contact with a coating material containing a polymer component to obtain coated resin particles having a coating portion that covers at least a part of the surface of the mixed particles.

Description

[Technical Field]

[0001] This invention relates to a method for producing coated resin particles and to coated resin particles. [Background technology]

[0002] Superabsorbent polymers are widely used in various fields, including sanitary materials such as disposable diapers and sanitary products, agricultural and horticultural materials such as water-retaining agents and soil conditioners, and industrial materials such as water-stopping agents and condensation-preventing agents. Among these fields, they are particularly often used in sanitary materials such as disposable diapers, sanitary products, and portable toilets. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] International Publication No. 2022 / 209536 [Overview of the project] [Problems that the invention aims to solve]

[0004] In some cases, it is necessary to control the water absorption rate of superabsorbent resin particles. One method of controlling the water absorption rate is to coat the surface of the superabsorbent resin particles with an arbitrary coating layer, thereby creating coated resin particles, which can be used to adjust the water absorption rate to a slower level (for example, Patent Document 1). However, in this case, there is a problem that the amount of water that can be retained decreases.

[0005] The present invention aims to provide a method for producing coated resin particles in which water absorption is delayed while suppressing a decrease in water retention capacity, and to provide coated resin particles. [Means for solving the problem]

[0006] The present invention relates, for example, to the following [1] to [9]. [1] A method for producing coated resin particles, comprising the steps of preparing mixed particles containing water-insoluble particles and water-absorbent resin particles, and bringing the mixed particles into contact with a coating material containing a polymer component to obtain coated resin particles having a coating portion that covers at least a part of the surface of the mixed particles. [2] The manufacturing method according to [1], wherein the median particle size of the water-insoluble particles is 1.0 to 100.0 μm. [3] A method for producing water-insoluble particles containing an inorganic compound, as described in [1] or [2]. [4] The method for producing the inorganic compound according to [3], wherein the inorganic compound is at least one selected from the group consisting of silica, activated carbon, alkaline earth metal halides, alkaline earth metal salts, and metal oxides. [5] The manufacturing method according to any one of [1] to [4], wherein the content or amount of water-insoluble particles added is greater than 0 parts by mass and 3.0 parts by mass or less per 100 parts by mass of water-absorbent resin particles. [6] A manufacturing method according to any one of [1] to [5], further comprising a heat treatment step of heating the coated resin particles at a temperature above the glass transition temperature of the polymer component. [7] A method of manufacturing according to any one of [1] to [6], wherein the coating material comprises at least one from the group consisting of an olefin / ethylenically unsaturated monomer copolymer and a styrene / ethylenically unsaturated monomer copolymer. [8] The manufacturing method according to any one of [1] to [7], wherein the amount of coating material added is 3.0 to 8.0 parts by mass per 100 parts by mass of water-absorbing resin particles. [9] Coated resin particles comprising water-absorbent resin particles and water-insoluble particles disposed on the surface of the water-absorbent resin particles, and a coating portion that covers at least a portion of the surface of the mixed particles. [Effects of the Invention]

[0007] The present invention provides a method for producing coated resin particles in which water absorption is delayed while suppressing a decrease in water retention capacity, and also provides coated resin particles. [Brief explanation of the drawing]

[0008] [Figure 1]This is a schematic cross-sectional view showing a processing apparatus for bringing mixed particles and a coating material into contact with each other. [Figure 2] This is a schematic cross-sectional view showing a device for measuring water absorption under a load of 4.14 kPa. [Figure 3] This is a scanning electron microscope image showing the coated resin particles of Comparative Example 1. [Figure 4] This is a scanning electron microscope image showing the coated resin particles of Example 4. [Figure 5] This is a scanning electron microscope image showing the coated resin particles of Comparative Example 3. [Modes for carrying out the invention]

[0009] Embodiments of the present invention will be described below. However, the present invention is not limited to the embodiments described below, and can be implemented with various modifications within the scope of its gist.

[0010] As used herein, “(meth)acryl” means at least one of acrylic and the corresponding methacrylic. The same applies to other similar expressions such as “(meth)acrylate”. “(poly)” means both cases with and without the prefix “poly”. “A or more” in a numerical range means A and a range exceeding A. “A or less” in a numerical range means A and a range less than A. In the numerical ranges described stepwise in this specification, the upper limit value or the lower limit value of a numerical range at a certain step can be arbitrarily combined with the upper limit value or the lower limit value of a numerical range at another step. In the numerical ranges described in this specification, the upper limit value or the lower limit value of the numerical range may be replaced with the value shown in the experimental examples. The materials exemplified in this specification may be used alone or in combination of two or more. “A or B” means that either A or B may be included, and both may be included. The content of each component in the composition means the total amount of the plurality of substances present in the composition when there are a plurality of substances corresponding to each component in the composition, unless otherwise specified. “Room temperature” means 25°C ± 2°C. The term “step” includes not only an independent step but also a step in which, even if it cannot be clearly distinguished from other steps, the intended action of the step is achieved. “Physiological saline” means an aqueous solution of 0.9% by mass sodium chloride. “Water-soluble” means a solubility of 1.0 g or more (for example, 1.0 to 150.0 g) with respect to 100 g of ion-exchanged water at 25°C. “Water-insoluble” means a solubility of less than 1.0 g with respect to 100 g of ion-exchanged water at 25°C.

[0011] The method for producing coated resin particles according to this embodiment includes a step of preparing mixed particles containing water-insoluble particles and water-absorbing resin particles, and a step of obtaining coated resin particles having a coating portion that coats at least a part of the surface of the mixed particles by bringing the mixed particles into contact with a coating material containing a polymer component.

[0012] According to the method for producing coated resin particles according to this embodiment, coated resin particles can be obtained in which the decrease in the water retention amount is suppressed compared to the case where water-insoluble particles are not added while delaying water absorption. The mechanism is not particularly limited to the following reasons, but the present inventor speculates as follows. When the water-absorbing resin particles absorb water and swell, the swelling of the internal water-absorbing resin particles is suppressed by the coating portion remaining adhered to the surface of the water-absorbing resin particles without falling off, thereby reducing the water retention amount. On the other hand, by previously mixing water-insoluble particles with the water-absorbing resin particles, arranging the water-insoluble particles on the surface of the water-absorbing resin particles, and then coating with a coating material, water-insoluble particles can be interposed between the surface of the water-absorbing particles and the coating layer, and the degree of adhesion between the surface of the water-absorbing resin particles and the coating portion becomes low. Therefore, when the water-absorbing resin particles swell, the coating portion is likely to fall off, and it is considered that a decrease in the water retention amount can be suppressed.

[0013] In the method for producing coated resin particles according to this embodiment, the mixed particles containing water-insoluble particles and water-absorbing resin particles may be prepared by purchasing the mixed particles in a mixed state, or may be prepared by a mixing step of mixing the water-insoluble particles and the water-absorbing resin particles. Hereinafter, an embodiment including a mixing step of mixing water-insoluble particles and water-absorbing resin particles will be described.

[0014] The mixing of the water-insoluble particles and the water-absorbing resin particles is preferably performed before the coating step. By performing the mixing of the water-insoluble particles and the water-absorbing resin particles before the coating step, the water-insoluble particles can be directly arranged on the surface of the water-absorbing resin particles, and it is considered that the suppression of the decrease in the water retention amount and the delay of water absorption can be performed more efficiently. When mixing the water-insoluble particles and the water-absorbing resin particles, it is preferable to mix both particles in a dry state without adding a liquid such as water. After forming a coating portion on the mixed particles containing the water-insoluble particles and the water-absorbing resin particles, water-insoluble particles may be further added.

[0015] The water-insoluble particles may be organic compounds or inorganic compounds. From the viewpoint of easily reducing the degree of adhesion between the surface of the water-absorbent resin particles and the coating, the water-insoluble particles are preferably inorganic compounds. Examples of organic compounds include synthetic polymers such as polyethylene and natural polymers such as cellulose.

[0016] Examples of inorganic compounds include silica, activated carbon, alkaline earth metal halides, alkaline earth metal salts, metal oxides, zeolites, kaolin, talc, mica, hydrotalcite, clay, bentonite, calcium phosphate, barium phosphate, fluorite, and alumina. The inorganic compound may be at least one selected from the group consisting of silica, activated carbon, alkaline earth metal halides, alkaline earth metal salts, and metal oxides. The silica may be amorphous silica. The amorphous silica may be wet silica such as silica gel, or dry silica such as fumed silica. The alkaline earth metal halides may be calcium fluoride or magnesium fluoride. The alkaline earth metal salts may be carbonates such as beryllium carbonate, magnesium carbonate, calcium carbonate, strontium carbonate, and barium carbonate, or sulfates such as calcium sulfate, strontium sulfate, and barium sulfate. The metal oxides may be titanium oxide, aluminum oxide, or magnesium oxide.

[0017] The medium particle size of water-insoluble particles tends to improve fluidity as it increases, and from the viewpoint of ease of handling, it may be 1.0 μm or larger, 2.0 μm or larger, 3.0 μm or larger, 4.0 μm or larger, 5.0 μm or larger, 6.0 μm or larger, 7.0 μm or larger, 8.0 μm or larger, 9.0 μm or larger, 10.0 μm or larger, 12.0 μm or larger, 14.0 μm or larger, or 16.0 μm or larger. The medium particle size of water-insoluble particles may be 100.0 μm or less, 90.0 μm or less, 80.0 μm or less, 70.0 μm or less, 60.0 μm or less, 50.0 μm or less, 40.0 μm or less, 30.0 μm or less, 25.0 μm or less, 22.0 μm or less, 20.0 μm or less, 18.0 μm or less, 16.0 μm or less, 14.0 μm or less, 12.0 μm or less, or 10.0 μm or less, from the viewpoint that defects are less likely to occur in the coating and the water absorption rate can be easily adjusted to be slow. The median particle size of water-insoluble particles refers to the value obtained by measuring with a laser diffraction particle size distribution analyzer. The SALD2300 (manufactured by Shimadzu Corporation) can be used as the laser diffraction particle size distribution analyzer.

[0018] The water-insoluble particles may be hydrophilic or hydrophobic. The contact angle of the water-insoluble particles may be 10 degrees or more, 12 degrees or more, 14 degrees or more, 16 degrees or more, 17 degrees or more, 20 degrees or more, 30 degrees or more, 40 degrees or more, 50 degrees or more, 60 degrees or more, 65 degrees or more, 70 degrees or more, 75 degrees or more, 78 degrees or more, or 80 degrees or more, and may be 85 degrees or less, 83 degrees or less, 80 degrees or less, 78 degrees or less, 76 degrees or less, 74 degrees or less, 72 degrees or less, 70 degrees or less, 68 degrees or less, 66 degrees or less, 64 degrees or less, 63 degrees or less, 60 degrees or less, 50 degrees or less, 40 degrees or less, 30 degrees or less, 25 degrees or less, 20 degrees or less, 18 degrees or less, or 17 degrees or less. The contact angle of the water-insoluble particles is measured by the method described in the examples below.

[0019] The amount of water-insoluble particles in the mixed particles, or the amount of water-insoluble particles added in the mixing process, is greater than 0 parts by mass per 100 parts by mass of water-absorbing resin particles, and from the viewpoint of easily suppressing a decrease in water retention capacity, it may be 0.01 parts by mass or more, 0.015 parts by mass or more, 0.02 parts by mass or more, 0.05 parts by mass or more, 0.1 parts by mass or more, 0.15 parts by mass or more, 0.2 parts by mass or more, 0.3 parts by mass or more, 0.4 parts by mass or more, 0.5 parts by mass or more, 0.6 parts by mass or more, 0.7 parts by mass or more, 0.8 parts by mass or more, 0.9 parts by mass or more, or 1.0 part by mass or more. The amount of water-insoluble particles added may be 3.0 parts by mass or less, 2.8 parts by mass or less, 2.6 parts by mass or less, 2.4 parts by mass or less, 2.2 parts by mass or less, 2.0 parts by mass or less, 1.8 parts by mass or less, 1.6 parts by mass or less, 1.4 parts by mass or less, 1.2 parts by mass or less, 1.0 parts by mass or less, 0.8 parts by mass or less, 0.6 parts by mass or less, 0.5 parts by mass or less, 0.4 parts by mass or less, 0.3 parts by mass or less, 0.2 parts by mass or less, 0.15 parts by mass or less, 0.1 parts by mass or less, 0.05 parts by mass or less, 0.03 parts by mass or less, or 0.02 parts by mass or less, per 100 parts by mass of water-absorbing resin particles, from the viewpoint of being able to easily adjust the rate of water absorption.

[0020] Mixing water-insoluble particles and water-absorbent resin particles can be done using known methods, such as mixing dried particles or dispersing dried particles in a dispersion medium and then mixing them. Mixing water-insoluble particles and water-absorbent resin particles can be done using, for example, container-rotating mixers such as W-type mixers, V-type mixers, and drum-type mixers; stirring-type mixers such as ribbon mixers and conical screw-type mixers; and fluidized bed dryers, groove dryers, etc.

[0021] The water content of the superabsorbent resin particles in the mixing process (the amount of water in the total mass of the superabsorbent resin particles) may be, for example, 50% by mass or less, preferably 30% by mass or less, and more preferably 20% by mass or less. Adjusting the water content of the superabsorbent resin particles in the mixing process to a low level is preferable because it suppresses aggregation of water-containing gels (superabsorbent resin particles swollen with water) during the mixing process.

[0022] After the mixing step, the mixed particles (to be coated) containing water-absorbent resin particles and water-insoluble particles are brought into contact with a coating material containing polymer components to obtain coated resin particles having a coating portion that covers at least a portion of the surface of the mixed particles. The coating portion of the coated resin particles is formed to cover part or all of the surface of the mixed particles. The coating portion may be a layered coating layer. The coating layer may be a single-layer structure or a multilayer structure having two or more layers.

[0023] The coating material contains a polymer component. The glass transition temperature (Tg) of the polymer component may be, for example, 40-110°C, 45-105°C, 50-100°C, 52-90°C, 55-80°C, 55-70°C, or 55-65°C. The 10% weight loss temperature (10%Td) of the polymer component may be, for example, 200-500°C, 250-490°C, 280-480°C, 300-470°C, 350-460°C, 300-480°C, or 350-480°C. The glass transition temperature (Tg) and 10% weight loss temperature (10%Td) may be within the above ranges from the viewpoint of easily forming a coating layer of uniform thickness and easily controlling the water absorption rate to be slow.

[0024] The coating material may be water-soluble or water-insoluble. The coating material may contain water-soluble components or water-insoluble components.

[0025] The water-soluble component may contain a compound having a hydrophilic group. Examples of hydrophilic groups include anionic groups, cationic groups, amphoteric groups, and nonionic groups. Examples of anionic groups include carboxyl groups, sulfonic acid groups, and phosphate groups. Examples of cationic groups include amino groups, imino groups, and quaternary ammonium groups. Examples of amphoteric groups include carbobetaine groups, sulfobetaine groups, and phosphobetaine groups. Examples of nonionic groups include hydroxyl groups, amide groups, pyrrolidone groups, lactam groups, alkoxy groups, and (poly)oxyalkylene groups.

[0026] The water-soluble component may include 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, and phenyldiglycol. Examples of compounds having an amide group include polyacrylamide and polyvinylpyrrolidone. Examples of compounds having a quaternary ammonium group include ammonium chloride. Examples of compounds having a (poly)oxyalkylene group include polyalkylene oxide (e.g., polyethylene oxide) and polyalkylene glycol (e.g., polyethylene glycol).

[0027] Water-insoluble 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; and poly-α-methylstyrene and syndiotactic polystyrene. Examples include polystyrene 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; alkyl poly(meth)acrylates such as poly(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. Water-insoluble components may be acid-modified. Water-insoluble components may be acid-modified with, for example, acid anhydrides (maleic anhydride, succinic anhydride, phthalic anhydride, etc.).

[0028] The coating material may contain a polymer having an ethylenically unsaturated monomer as a monomer unit (a polymer having monomer units derived from an ethylenically unsaturated monomer). An ethylenically unsaturated monomer is a compound having at least one carbon-carbon double bond in its molecule and exhibiting radical polymerizability. Examples of ethylenically unsaturated monomers include (meth)acrylic acid and its salts, (meth)acrylic acid esters ((meth)acrylic acid methyl, (meth)acrylic acid ethyl, (meth)acrylic acid 2-hydroxyethyl, (meth)acrylic acid 2-(diethylamino)ethyl, (meth)acrylic acid 2-(diethylamino)propyl, etc.), (meth)acrylamide monomers ((meth)acrylamide, N-isopropyl(meth)acrylamide, 2-(meth)acrylamide-2-methylpropanesulfonic acid and its salts, N,N-dimethyl(meth)acrylamide, N-methylol(meth)acrylamide, diethylaminopropyl(meth)acrylamide, etc.), polyethylene glycol mono(meth)acrylate, styrene, α-alkylstyrene, and butadiene. The ethylenically unsaturated monomer may contain (meth)acrylic compounds (compounds having a (meth)acryloyl group), may contain at least one selected from the group consisting of (meth)acrylic acid and its salts, may contain styrene, may contain (meth)acrylic compounds and styrene, or may contain at least one selected from the group consisting of (meth)acrylic acid and its salts, and styrene.

[0029] As coating materials, chain polymerization reaction products such as poly(meth)acrylic acid, poly(meth)acrylamide, polyvinyl alcohol, polyalkylene oxide, and polyalkylene glycol may be used; and step polymerization reaction products such as urethane resins (e.g., condensates of polyols and polyisocyanates), phenolic resins (e.g., condensates of phenolic compounds and aldehydes), polyesters, polyamides, and polycarbonates may be used.

[0030] 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, alkyl poly(meth)acrylate, polyamide, polyolefin, olefin / ethylenically unsaturated monomer copolymer, styrene / ethylenically unsaturated monomer copolymer, and siloxane compound (compound having a siloxane bond) / ethylenically unsaturated monomer copolymer, from the viewpoint of easily delaying water absorption of the coating resin particles. The coating material may also contain styrene / ethylenically unsaturated monomer copolymer. The coating material may contain an olefin / ethylenically unsaturated monomer copolymer and polyalkylene glycol, or an olefin / ethylenically unsaturated monomer copolymer and polyethylene glycol, from the viewpoint of easily delaying the water absorption of the coated resin particles. The ethylenically unsaturated monomer in the olefin / ethylenically unsaturated monomer copolymer, styrene / ethylenically unsaturated monomer copolymer, or siloxane compound / ethylenically unsaturated monomer copolymer may contain a (meth)acrylic compound, at least one selected from the group consisting of (meth)acrylic acid and its salts, and may contain butadiene, from the viewpoint of easily delaying the water absorption of the coated resin particles.

[0031] If the ethylenically unsaturated monomer has an acidic group, the acidic group may be neutralized by an alkaline neutralizing agent. In this case, the degree of neutralization of the ethylenically unsaturated monomer may be greater than 0 mol% and less than or equal to 100 mol%, 5-100 mol%, 10-100 mol%, 20-100 mol%, 30-100 mol%, 40-100 mol%, or 50-100 mol% of the acidic group in the ethylenically unsaturated monomer. Increasing the degree of neutralization makes the coating liquid (e.g., emulsion) described later more stable and makes it easier to obtain uniform coated resin particles. From this viewpoint, the degree of neutralization of the ethylenically unsaturated monomer may be 10-100 mol%, 30-100 mol%, or 50-100 mol%. Examples of ethylenically unsaturated monomers in which the acidic group has been neutralized in this way include sodium acrylate and ammonium acrylate, which have a degree of neutralization of 5-100 mol%.

[0032] The olefin, which is the monomer unit of a polyolefin, or the olefin, which is the monomer unit of an olefin / ethylenically unsaturated monomer copolymer, may contain at least one selected from the group consisting of ethylene, propylene, and butene, and may contain ethylene, from the viewpoint of easily delaying the water absorption of the coated resin particles. The ethylenically unsaturated monomer in the olefin / ethylenically unsaturated monomer copolymer may contain the ethylenically unsaturated monomers listed later as constituent materials of water-absorbing resin particles, may contain (meth)acrylic compounds, and may contain at least one selected from the group consisting of (meth)acrylic acid and its salts, from the viewpoint of easily delaying the water absorption of the coated resin particles.

[0033] In an olefin / ethylenically unsaturated monomer copolymer, the water absorption behavior of the coated resin particles can be easily adjusted by adjusting the ratio of olefin monomer units (monomer units derived from olefins) to ethylenically unsaturated monomer units (monomer units derived from ethylenically unsaturated monomers). That is, by increasing the proportion of monomer units with high hydrophobicity, the water absorption of the coated resin particles can be adjusted to be slower. For example, in an olefin / ethylenically unsaturated monomer copolymer, the proportion of olefin monomer units may be 72.0 to 98.0 mol% or 80.0 to 97.0 mol%. The coating material may be used alone or in combination of two or more types.

[0034] Normally, to slow down 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 can worsen productivity due to increased coating time, etc. On the other hand, according to one embodiment of the method for manufacturing coated resin particles according to this embodiment, even if the coating is made thin, defects are less likely to occur, so the water absorption rate of the coated resin particles can be adjusted to be slower without increasing the amount of coating material used.

[0035] The amount of coating material added in the coating process, or the content of the coating portion in the coated resin particles obtained by the manufacturing method according to this embodiment, may be within the following ranges per 100 parts by mass of water-absorbing resin particles. The amount of coating material added or the content of the coating portion may be 0.1 parts by mass or less, or 0.5 parts by mass or more, 1.0 parts by mass or more, 2.0 parts by mass or more, or 3.0 parts by mass or more, from the viewpoint of easily delaying water absorption by the coated resin particles. The amount of coating material added or the content of the coating portion may be 4.0 parts by mass or more, 5.0 parts by mass or more, 5.5 parts by mass or more, 6.0 parts by mass or more, 6.5 parts by mass or more, 7.0 parts by mass or more, 7.5 parts by mass or more, 8.0 parts by mass or more, 9.0 parts by mass or more, 10.0 parts by mass or more, 11.0 parts by mass or more, or 12.0 parts by mass or more.

[0036] The amount of coating material added or the content of the coated portion may be 50.0 parts by mass or less, 40.0 parts by mass or less, 30.0 parts by mass or less, 25.0 parts by mass or less, 20.0 parts by mass or less, 18.0 parts by mass or less, or 15.0 parts by mass or less, from the viewpoint of improving productivity such as shortening the coating time. The amount of coating material added or the content of the coated portion may be 13.0 parts by mass or less, 12.0 parts by mass or less, 11.0 parts by mass or less, 10.0 parts by mass or less, 9.0 parts by mass or less, 8.0 parts by mass or less, 7.0 parts by mass or less, 6.0 parts by mass or less, 5.5 parts by mass or less, or 5.0 parts by mass or less, from the viewpoint of suppressing a decrease in water retention and suppressing aggregation of coating resin particles. From these perspectives, the amount of coating material added or the content of the coated portion is 0.1 to 50.0 parts by mass, 0.1 to 20.0 parts by mass, 0.1 to 15.0 parts by mass, 0.1 to 10.0 parts by mass, 0.5 to 50.0 parts by mass, 0.5 to 20.0 parts by mass, 0.5 to 10.0 parts by mass, 1.0 to 50.0 parts by mass, 1.0 to 20.0 parts by mass, 1.0 to 15.0 parts by mass, 1.0 to 10.0 parts by mass, The amount may be 1.0 to 9.0 parts by mass, 3.0 to 50.0 parts by mass, 3.0 to 20.0 parts by mass, 3.0 to 15.0 parts by mass, 3.0 to 10.0 parts by mass, 3.0 to 8.0 parts by mass, 5.0 to 50.0 parts by mass, 5.0 to 20.0 parts by mass, 5.0 to 15.0 parts by mass, 5.0 to 10.0 parts by mass, 5.0 to 8.0 parts by mass, 10.0 to 15.0 parts by mass, or 11.0 to 15.0 parts by mass.

[0037] The coating process involves supplying a coating material to a mixed particle containing water-absorbent resin particles and water-insoluble particles, either in a single step or continuously, to bring the mixed particle and the coating material into contact with each other.

[0038] The ambient temperature during the coating process may be within the following ranges, from the viewpoint of easily adjusting the water absorption behavior of the coated resin particles. The ambient temperature during the coating process may be 20°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, 80°C or higher, 85°C or higher, 90°C or higher, 95°C or higher, 100°C or higher, 110°C or higher, 120°C or higher, or 130°C or higher. The ambient temperature during the coating process 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, or 50°C or lower. From these perspectives, the ambient temperature during the coating process may be 20-150°C, 50-140°C, 50-100°C, 70-140°C, 70-100°C, 100-150°C, or 100-140°C. Furthermore, the percentage of time during the coating process in which the ambient temperature is within the above range may be 30-100%, 50-100%, 60-100%, 65-100%, 70-100%, 80-100%, 85-100%, 90-100%, or 95-100%, based on the total time of the coating process.

[0039] In the coating process, for example, the coating material may be brought into contact with mixed particles present in a dispersion medium, or with mixed particles present in an atmosphere such as air, an inert gas (e.g., nitrogen gas), or a mixture thereof. The coating material brought into contact with the mixed 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 solution (a solution of the coating material, a dispersion of the coating material (e.g., an emulsion)) containing the coating material and a liquid medium (e.g., water).

[0040] By bringing a coating solution containing a coating material and a liquid medium (e.g., water) into contact with mixed particles, the coating material can be brought into contact with the mixed particles. This makes it easier to obtain a coating of uniform thickness. The liquid medium may be a solvent or a dispersion medium. The coating solution 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.

[0041] Examples of solvents or dispersion media include water, hydrophilic compounds, and hydrocarbon compounds. The solvent or dispersion media may be used alone or as a mixture of two or more (for example, a mixture of water and a hydrophilic compound). Hydrophilic compounds are compounds that dissolve 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 methyl acetate; and ethers such as tetrahydrofuran. Examples of hydrocarbon compounds include linear 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.

[0042] 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, from the viewpoint of easily delaying the water absorption of the coated resin particles: 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, 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 perspectives, the content of the coating material may be 1.00-50.00% by mass, 1.00-20.00% by mass, 1.00-15.00% by mass, 5.00-50.00% by mass, 5.00-20.00% by mass, 5.00-15.00% by mass, 10.00-50.00% by mass, 10.00-20.00% by mass, 10.00-15.00% by mass, or 15.00-20.00% by mass.

[0043] The content of the liquid medium in the coating solution may be within the following ranges, based on the total mass of the coating solution, from the viewpoint of easily delaying the water absorption of the coating resin particles. The content of the liquid medium may be 10.0% by mass or more, 30.0% by mass or more, 50.0% by mass or more, 60.0% by mass or more, 70.0% by mass or more, 75.0% by mass or more, 80.0% by mass or more, 85.0% by mass or more, 88.0% by mass or more, 89.0% by mass or more, 89.5% by mass or more, or 90.0% by mass or more. The content of the liquid medium may be 99.0% by mass or less, 97.0% by mass or less, 95.0% by mass or less, 92.0% by mass or less, 90.0% by mass or less, 88.0% by mass or less, or 85.0% by mass or less. From these perspectives, the liquid medium content may be 10.0-99.0% by mass, 30.0-99.0% by mass, 50.0-99.0% by mass, 80.0-99.0% by mass, 85.0-99.0% by mass, 50.0-95.0% by mass, 80.0-95.0% by mass, 85.0-95.0% by mass, 50.0-90.0% by mass, 80.0-90.0% by mass, or 85.0-90.0% by mass.

[0044] The water content in the coating solution may be within the following ranges, based on the total mass of the coating solution, from the viewpoint of delaying water absorption by the coating resin particles: The water content may be 10.0% by mass or more, 30.0% by mass or more, 50.0% by mass or more, 60.0% by mass or more, 70.0% by mass or more, 75.0% by mass or more, 80.0% by mass or more, 85.0% by mass or more, 88.0% by mass or more, 89.0% by mass or more, 89.5% by mass or more, or 90.0% by mass or more. The water content may be 99.0% by mass or less, 97.0% by mass or less, 95.0% by mass or less, 92.0% by mass or less, 90.0% by mass or less, 88.0% by mass or less, or 85.0% by mass or less. From these perspectives, the water content may be 10.0-99.0% by mass, 30.0-99.0% by mass, 50.0-99.0% by mass, 80.0-99.0% by mass, 85.0-99.0% by mass, 50.0-95.0% by mass, 80.0-95.0% by mass, 85.0-95.0% by mass, 50.0-90.0% by mass, 80.0-90.0% by mass, or 85.0-90.0% by mass.

[0045] Various methods can be used to bring the mixed particles and coating material into contact with each other during the coating process. For example, during the coating process, the mixed particles and coating material (e.g., the coating material of a coating solution) may be mixed in a container such as a flask to bring the mixed particles and coating material into contact with each other. During the coating process, the coating material may be brought into contact with the mixed particles in an airflow, or with the mixed particles blown up by the airflow, or with the mixed particles and coating material may be brought into contact with each other by supplying the coating material to a fluidized bed of mixed particles. The gas constituting the airflow may be air, an inert gas (e.g., nitrogen gas), or a mixture of these gases. The temperature of the airflow may exceed 25°C.

[0046] In the coating process, the coating material may be brought into contact with the mixed particles, which have been stirred using a stirring means. The stirring means may be a stirring means having stirring blades, or an air supply means capable of supplying an airflow different from the airflow used to blow up the mixed particles. In the coating process, the mixed particles may be stirred only by the airflow used to blow up the mixed particles, and it is not necessary to use a stirring means different from the airflow used to blow up the mixed particles. In the coating process, the mixed particles and the coating material may be brought into contact with each other by spraying the coating liquid onto the mixed particles. The nozzle for spraying the coating liquid is not particularly limited, and for example, a tangential spray may be used, or a two-fluid type nozzle (for example, a nozzle that sprays the coating liquid and an inert gas) may be used. In the coating process, the mixed particles and the coating material may be brought into contact with each other using various devices such as a rolling granulator, a stirring granulator, or a fluidized bed granulator.

[0047] 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 mixed particles and the coating material are in contact before the coating step. The ambient temperature of the space in which the mixed particles and the coating material are in contact 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 delaying the water absorption 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, or 25 to 100°C.

[0048] In the method for producing coated resin particles according to this embodiment, during the coating process, an airflow may be supplied from vertically downwards to the internal space of a device (e.g., a fluid bed granulator) that contains a mixed particle containing water-absorbing resin particles, while bringing the mixed particle and the coating material into contact with each other. The internal space is the space to which the coating material is supplied, and is a space in which the mixed particle and the coating material can come into contact with each other. The airflow may be supplied vertically from vertically downwards, or vertically downwards in a direction intersecting the vertical direction.

[0049] Figure 1 is a schematic cross-sectional view showing a processing apparatus for bringing mixed particles and a coating material into contact. The processing apparatus 1 comprises a processing unit 10 and an air supply unit (not shown) that supplies an airflow G to the processing unit 10.

[0050] The processing unit 10 is a substantially cylindrical member extending in the vertical direction (the height direction of the processing unit 1; the same applies hereinafter). The processing unit 10 has a substantially cylindrical internal space 11 in which the mixed particles 20 are contained. The internal space 11 has a cylindrical space 11a, a space 11b located vertically above space 11a, and a cylindrical space 11c located vertically above space 11b. Spaces 11a, 11b, and 11c are continuous from the bottom to the top in the vertical direction, thereby constituting the internal space 11. Space 11a has the narrowest cross-section (the narrowest cross-section) in the internal space 11. Space 11b tapers upward in the vertical direction.

[0051] An air inlet 12 is formed in the center of the bottom surface of the processing unit 10. The air inlet 12 is connected to the air supply unit, and an airflow 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 airflow G.

[0052] The processing unit 10 includes a stirring means 13 (e.g., rotor blades) for stirring the mixed particles 20. The stirring means 13 has a disc-shaped disc portion 13a with its center projecting vertically upward, and a support portion 13b that supports the disc portion 13a. The disc portion 13a extends horizontally at the lower vertical part of the space 11a, and an annular opening 14 is formed on the outer circumference of the space 11a relative to the disc portion 13a. The disc portion 13a has an annular upper surface around the central projection, and horizontally extending blades (not shown: for example, three blades that extend radially from the center of the disc portion 13a and are arranged at equal intervals from one another) are formed on the upper surface. The support portion 13b extends vertically and extends from outside the internal space 11 to inside the internal space 11 via an air intake 12. The disc portion 13a can rotate horizontally as the support portion 13b rotates horizontally (rotating around the axis of the support portion 13b).

[0053] The airflow G supplied to the internal space 11 from the air intake port 12 is shielded by the disc portion 13a of the stirring means 13 and supplied to the outer circumference of the processing unit 10, and then supplied vertically upward through the opening 14 located on the outer circumference of the space 11a.

[0054] The mixed particles 20 accumulate on the upper surface of the disc portion 13a, and are agitated as the disc portion 13a rotates, and are also blown vertically upward by the airflow G supplied through the opening 14. After being blown vertically upward by the airflow G, the mixed particles 20 descend vertically downward by gravity. An exhaust filter (e.g., a bag filter) 15 is placed in the space 11c of the internal space 11, and the airflow G supplied to the internal space 11 is discharged to the outside of the processing device 1 from the exhaust filter 15.

[0055] The processing unit 10 includes a liquid supply unit 16 (e.g., a nozzle) that supplies coating liquid L to the space 11a. The liquid supply unit 16 supplies the coating liquid L from the outer circumference to the inner circumference of the space 11a, vertically above the disc portion 13a of the stirring means 13 in the space 11a. The coating liquid L supplied from the liquid supply unit 16 comes into contact with the mixed particles 20 that are blown upward by the airflow G and descend by gravity. A coating is formed when the mixed particles 20 and the coating material of the coating liquid L come into contact with each other, and coated resin particles are obtained. Volatile components (water, etc.) in the coating liquid L volatilize due to the airflow G, heat in the internal space 11, etc.

[0056] The processing unit 10 is equipped with a thermometer (not shown) for measuring the ambient temperature of the internal space 11. The thermometer may be placed in the internal space 11 at a position where the mixed particles 20 and the coating liquid L are in contact with each other, or it may be placed near the liquid supply unit 16 at the same height as the liquid supply unit 16.

[0057] The configuration of the apparatus for bringing the mixed particles and coating material into contact with each other is not limited to the configuration of the apparatus 1 described above. For example, the apparatus may be equipped with stirring means to stir the mixed particles 20 by supplying an airflow from the side of the processing unit 10, in place of or in addition to the stirring means of the apparatus 1 described above. The apparatus may be equipped with means for heating the processing unit 10 (e.g., a heater) as means for adjusting the ambient temperature of the internal space 11 of the processing unit 10 in the apparatus 1.

[0058] The amount of airflow supplied to the internal space can be adjusted as appropriate depending on the volume of the internal space, etc. The amount of airflow supplied is within the following range (unit: m 3The " / min" display may be omitted. The airflow supply rate 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 airflow supply rate 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 airflow supply rate 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.

[0059] The amount of coating liquid supplied to the internal space can be appropriately adjusted depending on the amount of mixed particles supplied to the processing area. The amount of coating liquid supplied may be within the following ranges (the unit "g / min" is omitted): The amount of coating liquid supplied 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, 18.0 or more, 20.0 or more, or 21.0 or more. The amount of coating liquid supplied may be 10000 or less, 1000 or less, 500 or less, 300 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, or 6.0 or less. From these perspectives, the supply amount of coating liquid may be 1.0-10000, 1.0-1000, 1.0-500, 1.0-300, 1.0-100, 1.0-500, 1.0-300, 1.0-200, 1.0-100, 1.0-500, 5.0-3000, 5.0-2000, 5.0-1000, 10.0-3000, or 10.0-2000.

[0060] In this embodiment, the method for producing coated resin particles preferably includes a heat treatment step after the coating step. The heat treatment dries the coated resin particles. Including a heat treatment step after the coating step is preferable because it softens the polymer components contained in the coating, improving film formation and making it easier to form a more uniform coating, and thus making it easier to obtain a delayed water absorption effect. Furthermore, since performing a heat treatment step after the coating step tends to reduce the water retention capacity of the resulting coated resin particles, the method for producing coated resin particles according to this embodiment, particularly when it includes a heat treatment step after the coating step, has the effect of suppressing the reduction in water retention capacity.

[0061] The ambient temperature and time of the heat treatment process are adjusted as appropriate according to the type and amount of coating material used. From the viewpoint of delaying the water absorption of the coating resin particles, the ambient temperature of the heat treatment process is preferably above the glass transition temperature of the polymer component contained in the coating material used. The ambient temperature of the heat treatment process is preferably lower than the 10% weight loss temperature of the coating material used. The ambient temperature of the heat treatment process may be, for example, 20°C or higher, 30°C or higher, 40°C or higher, 50°C or higher, 60°C or higher, 70°C or higher, 80°C or higher, 90°C or higher, or 95°C or higher. The ambient temperature of the heat treatment process may be 140°C or lower, 120°C or lower, 110°C or lower, or 100°C or lower. From these viewpoints, the ambient temperature of the heat treatment process may be 20-140°C, 40-120°C, 60-120°C, 60-110°C, 90-120°C, or 90-110°C.

[0062] The heating process may last for 10 minutes or more, 20 minutes or more, 30 minutes or more, 45 minutes or more, or 55 minutes or more. The heating process may last for 180 minutes or less, 120 minutes or less, 90 minutes or less, 70 minutes or less, or 65 minutes or less.

[0063] The water-absorbing resin particles constituting the coated resin particles may contain ethylenically unsaturated monomers (compounds having ethylenically unsaturated bonds) as monomer units (monomer units derived from ethylenically unsaturated monomers). The method for producing coated resin particles according to this embodiment may include a polymerization step to obtain water-absorbing resin particles by polymerizing ethylenically unsaturated monomers before the coating step. Examples of polymerization methods for ethylenically unsaturated monomers include reverse-phase suspension polymerization, aqueous solution polymerization, bulk polymerization, and precipitation polymerization.

[0064] Ethylene-unsaturated monomers are compounds that have at least one carbon-carbon double bond in their molecule and are radically polymerizable. Ethylene-unsaturated monomers may be water-soluble ethylenically unsaturated monomers (ethylenically unsaturated monomers whose solubility in 100 g of deionized water at 25°C is 1.0 g or more). Examples of ethylenically unsaturated monomers include (meth)acrylic acid and its salts, 2-(meth)acrylamide-2-methylpropanesulfonic acid and its salts, (meth)acrylamide, N,N-dimethyl(meth)acrylamide, 2-hydroxyethyl(meth)acrylate, N-methylol(meth)acrylamide, polyethylene glycol mono(meth)acrylate, N,N-diethylaminoethyl(meth)acrylate, N,N-diethylaminopropyl(meth)acrylate, and diethylaminopropyl(meth)acrylamide. If the ethylenically unsaturated monomer has an amino group, the amino group may be quaternized. Ethylene-unsaturated monomers may be used individually or in combination of two or more.

[0065] If the ethylenically unsaturated monomer has acidic groups, these acidic groups may be neutralized with an alkaline neutralizing agent before being used in the polymerization reaction. The degree of neutralization by the alkaline neutralizing agent in the ethylenically unsaturated monomer may be 10-100 mol%, 50-90 mol%, or 60-80 mol% of the acidic groups in the ethylenically unsaturated monomer.

[0066] The ethylenically unsaturated monomer may include at least one compound selected from the group consisting of (meth)acrylic acid, (meth)acrylic acid salts, (meth)acrylamide, and N,N-dimethylacrylamide, from the viewpoint of industrial availability, and may include at least one compound selected from the group consisting of (meth)acrylic acid, (meth)acrylic acid salts, and (meth)acrylamide.

[0067] Other monomers besides the ethylenically unsaturated monomers mentioned above may be used as monomers to obtain superabsorbent resin particles. Such monomers can be used, for example, by mixing them with an aqueous solution containing ethylenically unsaturated monomers. The amount of ethylenically unsaturated monomer used may be 70 to 100 mol% of the total amount of monomers (total amount of monomers to obtain superabsorbent resin particles). 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% of the total amount of monomers (total amount of monomers to obtain superabsorbent resin particles).

[0068] The water-absorbing resin particles may be crosslinked polymers. Crosslinking may occur due to self-crosslinking during polymerization, but crosslinking may be promoted by using an internal crosslinking agent. Using an internal crosslinking agent makes it easier to control the water absorption properties (water retention capacity, etc.) of the water-absorbing resin particles. The polymerization process may be a process of obtaining water-absorbing resin particles by polymerizing ethylenically unsaturated monomers in the presence of an internal crosslinking agent.

[0069] Examples of internal crosslinking agents include compounds having two or more reactive functional groups (e.g., polymerizable unsaturated groups). Examples of internal crosslinking agents include di or tri(meth)acrylic acid esters of polyols, 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 compounds obtained by reacting polyisocyanates with hydroxyethyl (meth)acrylate. Examples include di(meth)acrylate carbamyl esters, allylated starch, allylated cellulose, diallyl phthalate, N,N',N''-triallyl isocyanurate, divinylbenzene, pentaerythritol, ethylenediamine, and polyethyleneimine. The internal crosslinking agent may include at least one selected from the group consisting of (poly)ethylene glycol diglycidyl ether, (poly)propylene glycol diglycidyl ether, and (poly)glycerin diglycidyl ether, from the viewpoint of easily adjusting the water absorption properties of the water-absorbing resin particles.

[0070] The amount of internal crosslinking agent can be adjusted as appropriate to control the water absorption properties of the water-absorbing resin particles, but for example, it may be within the following ranges per mole of ethylenically unsaturated monomer (e.g., 1 mole total of (meth)acrylic acid and its salts): The amount of 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 crosslinking agent 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 perspectives, the amount of 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.

[0071] In superabsorbent polymer particles, the crosslinking density near the surface of the particles may be increased (surface crosslinking may be applied). Surface crosslinking makes it easier to adjust the water absorption properties (water retention capacity, etc.) of the superabsorbent polymer particles. Specifically, the water absorption properties can be adjusted by adjusting the crosslinking density near the surface of the polymer particles by controlling the moisture content of the polymer particles used for surface crosslinking, the type or amount of surface crosslinking agent used for surface crosslinking, etc.

[0072] The water-absorbing resin particles may contain components such as gel stabilizers, metal chelating agents, and fluidity enhancers (lubricants). These components may be arranged inside the water-absorbing resin particles, on the surface of the water-absorbing resin particles, or both.

[0073] The shape of the water-absorbing resin particles may be, for example, approximately spherical, crushed, or granular, and may be an aggregate of primary particles having these shapes. The median particle size of the water-absorbing resin particles may be 100-800 μm, 150-700 μm, 200-600 μm, 250-500 μm, 300-400 μm, or 250-850 μm.

[0074] Another embodiment of the present invention is a coated resin particle comprising a mixed particle containing water-absorbent resin particles and water-insoluble particles disposed on the surface of the water-absorbent resin particles, and a coating portion covering at least a portion of the surface of the mixed particle. The coated resin particle can be obtained by the manufacturing method described above.

[0075] In the coated resin particles according to this embodiment, water-insoluble particles are mainly located between the water-absorbent resin particles and the coating portion. Some water-insoluble particles may be located on the surface of the water-absorbent resin particles where the coating portion is not formed, or they may be located inside the coating portion that is not in direct contact with the water-absorbent resin particles.

[0076] The shape of the coated resin particles according to this embodiment may be, for example, substantially spherical, crushed, or granular, or it may be a shape formed by aggregation of primary particles having these shapes. The median particle size of the coated resin particles may be 100-800 μm, 150-700 μm, 200-600 μm, 250-500 μm, 300-400 μm, or 250-850 μm.

[0077] The coated resin particles according to this embodiment can have a higher saline water retention capacity compared to those manufactured under the same conditions without placing water-insoluble particles on the surface of the water-absorbent resin particles. The saline water retention capacity of the coated resin particles according to this embodiment is 20 g / g or more, 25 g / g or more, 26 g / g or more, 27 g / g or more, 28 g / g or more, 29 g / g or more, 30 g / g or more, 31 g / g or more, 32 g / g or more, 33 g / g or more, 34 g / g or more, 35 g / g or more, 36 g / g or more, 37 g / g or more, 38 g / g or more, 39 g / g or more, 40 g / g or more, 41 g / g or more, and 42 g / g or more. The amount may be 43 g / g or more, 44 g / g or more, or 45 g / g or more, and may be 48 g / g or less, 47 g / g or less, 46 g / g or less, 45 g / g or less, 44 g / g or less, 43 g / g or less, 42 g / g or less, 41 g / g or less, 40 g / g or less, 39 g / g or less, 38 g / g or less, 37 g / g or less, 36 g / g or less, 35 g / g or less, 34 g / g or less, 33 g / g or less, or 32 g / g or less. The amount of saline solution that the coated resin particles can hold can be measured by the method described in the examples below.

[0078] The water absorption rate of the coated resin particles according to this embodiment may be 100 seconds or more, 110 seconds or more, 120 seconds or more, 130 seconds or more, 140 seconds or more, 150 seconds or more, 180 seconds or more, 200 seconds or more, 250 seconds or more, 300 seconds or more, 350 seconds or more, 400 seconds or more, 450 seconds or more, 500 seconds or more, 550 seconds or more, 600 seconds or more, 650 seconds or more, or 700 seconds or more, and may be 1000 seconds or less, 900 seconds or less, 800 seconds or less, 750 seconds or less, 700 seconds or less, 650 seconds or less, 600 seconds or less, 550 seconds or less, 500 seconds or less, 450 seconds or less, 400 seconds or less, 350 seconds or less, 300 seconds or less, 250 seconds or less, 200 seconds or less, or 150 seconds or less. The water absorption rate of the coated resin particles can be measured by the method described in the examples below. The water absorption rate can be adjusted by the type and amount of coating material added.

[0079] The water absorption of the coated resin particles under a 4.14 kPa load according to this embodiment may be 8 mL / g or more, 9 mL / g or more, 10 mL / g or more, 12 mL / g or more, 14 mL / g or more, 16 mL / g or more, 18 mL / g or more, 20 mL / g or more, 22 mL / g or more, or 23 mL / g or more, and may be 30 mL / g or less, 28 mL / g or less, 26 mL / g or less, 24 mL / g or less, 23 mL / g or less, 22 mL / g or less, 20 mL / g or less, 18 mL / g or less, 16 mL / g or less, 14 mL / g or less, or 12 mL / g or less. The water absorption of the coated resin particles under a 4.14 kPa load can be measured by the method described in the examples below. [Examples]

[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.

[0081] [Manufacturing Example 1: Production of superabsorbent polymer particles] <First stage polymerization reaction> A round-bottom 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 (with a stirring blade having two stages of four inclined paddle blades with a blade diameter of 5 cm). A mixture was obtained by adding 293 g of n-heptane (hydrocarbon dispersion medium) and 0.736 g of maleic anhydride-modified ethylene-propylene copolymer (polymeric dispersant, manufactured by Mitsui Chemicals, Inc., High Wax 1105A) to this separable flask. The dispersant was dissolved by heating this mixture to 80°C while stirring at a rotation speed of 300 rpm. The mixture was then allowed to cool to 55°C.

[0082] Next, 92.0 g of an 80.5% by mass acrylic acid aqueous solution (acrylic acid: 1.03 moles) was placed in a 500 mL Erlenmeyer flask. Subsequently, while cooling from the outside, 147.7 g of a 20.9% by mass sodium hydroxide aqueous solution was added dropwise to the Erlenmeyer flask to neutralize the 75 moles of acrylic acid. After that, 0.092 g of hydroxyethyl cellulose (thickener, manufactured by Sumitomo Seika Co., Ltd., HEC AW-15F), 0.0736 g (0.272 mmol) of potassium persulfate as a radical polymerization initiator, and 0.0101 g (0.0580 mmol) of ethylene glycol diglycidyl ether (internal crosslinking agent) were added, and the first stage monomer aqueous solution was prepared by dissolving all components except water.

[0083] The above-mentioned first-stage monomer aqueous solution was added to the above-mentioned separable flask and stirred for 10 minutes. On the other hand, a surfactant solution was obtained by heating and 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. The reaction solution was obtained by adding 7.356 g of the obtained surfactant solution to the above-mentioned separable flask. The reaction solution was then stirred at a rotation speed of 550 rpm while the separable flask system was thoroughly purged with nitrogen. Subsequently, the separable flask was immersed in a 70°C water bath to raise the temperature of the reaction solution, and heating was continued for another 10 minutes from the point when the polymerization reaction had progressed and the internal temperature reached its maximum (maximum temperature reached 82°C), thereby obtaining the first-stage reaction mixture.

[0084] <Second stage polymerization reaction> Next, 128.8 g of an 80.5% by mass acrylic acid aqueous solution (acrylic acid: 1.44 moles) was placed in another 500 mL Erlenmeyer flask. Subsequently, while cooling from the outside, 154.2 g of a 28% by mass sodium hydroxide aqueous solution was added dropwise to the Erlenmeyer flask to neutralize the 75 moles of acrylic acid. Then, 0.1030 g (0.3810 mmol) of potassium persulfate as a radical polymerization initiator, 0.0116 g (0.0666 mmol) of ethylene glycol diglycidyl ether (internal crosslinking agent), and 6.48 g of deionized water were added, and the components other than water were dissolved to prepare the second stage monomer aqueous solution.

[0085] The reaction mixture from the first stage was cooled to 25°C while being stirred at 1000 rpm, and then the entire amount of the monomer aqueous solution from the second stage was added to the reaction mixture from the first stage to obtain the reaction solution. The reaction solution was stirred while the system was thoroughly purged with nitrogen. Then, the reaction solution was heated by immersing the separable flask in a 70°C water bath, and heating was continued for a further 5 minutes from the point when the polymerization reaction had progressed and the internal temperature reached its maximum (maximum temperature reached 82°C) to obtain the reaction mixture from the second stage (polymer particles before surface crosslinking).

[0086] After the second stage of polymerization, the reaction mixture from the second stage was heated in an oil bath at 125°C, and 251 g of water was removed from the system by azeotropic distillation of n-heptane and water while refluxing the n-heptane. Subsequently, 4.42 g of a 2% by mass aqueous solution containing ethylene glycol diglycidyl ether (0.5075 mmol) as a surface crosslinking agent was added, and the mixture was held at 83°C for 2 hours to obtain a dispersion of polymer particles after surface crosslinking.

[0087] Subsequently, the dispersion of the polymer particles after surface crosslinking was heated in an oil bath at 125°C, and the n-heptane was evaporated to dry the mixture and obtain dried product (A). JIS standard sieves were arranged in the following order from top to bottom: a sieve with an 850 μm mesh opening, a sieve with a 250 μm mesh opening, and a receiving tray. The dried product (A) was placed on the top sieve and classified. By obtaining the particles remaining on the 250 μm mesh opening sieve, 210.1 g of superabsorbent polymer particles (A1) in a form of aggregated spherical particles (particles without a coating: particle size 250-850 μm) was obtained. A total of 500.0 g or more of superabsorbent polymer particles (A1) were prepared by performing the same procedure. The measurements described later were performed on Production Example 1 (superabsorbent polymer particles (A1)), and the measurement results are shown in Table 2.

[0088] [Preparation of partially neutralized ethylene / acrylic acid copolymer emulsion] A 3°C ice bath was prepared by adding water and ice to a plastic tray measuring 27 cm long, 38 cm wide, and 7 cm deep. A 1 L glass beaker was placed inside this ice bath, and 544.94 g of deionized water was added to the beaker. After placing the ice bath on top of a magnetic stirrer, a stirrer tip was placed inside the beaker, and stirring was started.

[0089] A 1.8% by mass aqueous solution of sodium hydroxide was prepared by gradually adding 10.24 g (0.256 mol) of sodium hydroxide (granules, manufactured by Nacalai Tesque Co., Ltd.) to the beaker described above.

[0090] A round-bottom cylindrical separable flask with an inner diameter of 11 cm and an internal volume of 2 L was prepared, equipped with a reflux condenser, a thermometer, and a stirrer (with a stirring blade having four inclined paddle blades with a blade diameter of 5 cm). 100 g of ethylene / acrylic acid copolymer (P(E / AA), molar ratio of ethylene monomer to acrylic acid monomer = 10:1, manufactured by SKglobal chemical, Primacol 5980i) was added to this separable flask. Subsequently, the entire amount of the aforementioned 1.8 mass% sodium hydroxide aqueous solution was added. After that, the beaker used to prepare the sodium hydroxide aqueous solution was washed with 50.0 g of deionized water, and the wash water was added to the separable flask to obtain the reaction solution.

[0091] While stirring the reaction mixture with a stirrer at 500 rpm, the aforementioned separable flask was immersed in an oil bath at 103°C, raising the internal temperature of the separable flask to 95°C. Subsequently, the internal temperature of the separable flask was maintained at 95-97°C for 4 hours while adjusting the oil bath temperature as needed.

[0092] Subsequently, 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 was below 35°C, 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 ethylene / acrylic acid copolymer partially neutralized emulsion (P(E / AA), ethylene / acrylic acid copolymer partially neutralized aqueous dispersion, non-volatile content 15% by mass, degree of neutralization 90%: hereinafter referred to as "aqueous dispersion (A)").

[0093] [Preparation of coated resin particles] (Comparative Example 1) <Coating Process> A fluidized bed granulator having the configuration shown in Figure 1 was prepared. 500.0 g of water-absorbing resin particles (A1) were placed into the processing section of the fluidized bed granulator. Next, while stirring the contents of the processing section with a stirring device (rotor blade, rotation speed: 250 rpm), the supply air temperature was 130°C and the supply rate was 1.0 m³. 3The airflow (air) supplied to the fluid bed granulator at a rate of / min was supplied to the processing unit through the air inlet. Then, 166.67 g of aqueous dispersion (A) was sprayed as a coating material at a supply rate of 18.0 g / min using a tangential sprayer onto the water-absorbing resin particles (A1) being blown up by the airflow, thereby bringing the water-absorbing resin particles and the coating material into contact. At this time, the supply amount of coating material per 100 parts by mass of water-absorbing resin particles was 5.0 parts by mass. Subsequently, at room temperature and a supply rate of 1.0 m³ 3 By supplying air (a stream of air) from the fluid bed granulator at a rate of / min to the processing unit through the air inlet for 30 minutes to dry the particles, we obtained particles coated with a coating material.

[0094] <Heat treatment process> 50.0 g of the coated particles described above were spread on a metal tray measuring 26 cm in length and 20 cm in width, and covered with aluminum foil. Perforations were made in the aluminum foil, and the particles were heated for 60 minutes in a hot air dryer (ADVANTEC, model: FV-320) set to 100°C. The metal tray was removed from the hot air dryer and left to stand in a desiccator (room temperature, 20% RH) until it reached room temperature to obtain the coated resin particles of Comparative Example 1.

[0095] (Example 1) 0.02 parts by mass of amorphous silica 1 (water-insoluble particles, manufactured by Oriental Silicas Corporation, Toxil NP-S) was mixed with the water-absorbing resin particles (A1) before the coating process by mixing for 30 minutes using a cross-rotary mixer (Tsukasa Industries Co., Ltd., model 3LCM-3). The resulting mixed particles were subjected to a coating process and a heat treatment process in the same manner as in Comparative Example 1 to obtain the coated resin particles of Example 1.

[0096] (Example 2) The coated resin particles of Example 2 were obtained in the same manner as in Example 1, except that the amount of amorphous silica 1 added was changed to 0.1 parts by mass.

[0097] (Example 3) The coated resin particles of Example 3 were obtained in the same manner as in Example 1, except that the amount of amorphous silica 1 added was changed to 0.2 parts by mass.

[0098] (Example 4) The coated resin particles of Example 4 were obtained in the same manner as in Example 1, except that the amount of amorphous silica 1 added was changed to 0.5 parts by mass.

[0099] (Example 5) The coated resin particles of Example 5 were obtained in the same manner as in Example 1, except that the amount of amorphous silica 1 added was changed to 1.0 part by mass.

[0100] (Comparative Example 2) The coated resin particles of Comparative Example 2 were obtained in the same manner as in Example 1, except that the mixing timing of 0.5 parts by mass of amorphous silica 1 was changed to after the coating process and before the heat treatment process.

[0101] (Comparative Example 3) The coated resin particles of Comparative Example 3 were obtained in the same manner as in Example 1, except that the mixing timing of 0.5 parts by mass of amorphous silica 1 was changed to after the heat treatment step.

[0102] (Example 6) The coated resin particles of Example 6 were obtained in the same manner as in Example 1, except that the amount of amorphous silica 1 added to the particles before the coating process was changed to 0.1 parts by mass, and 0.4 parts by mass of amorphous silica 1 was further mixed into the particles after the coating process and before the heat treatment process.

[0103] (Example 7) The coated resin particles of Example 7 were obtained in the same manner as in Example 1, except that the water-insoluble particles to be mixed were changed to 0.2 parts by mass of activated carbon (Osaka Gas Chemical Co., Ltd., Granular Shirasagi FPG-1).

[0104] (Example 8) The coated resin particles of Example 8 were obtained in the same manner as in Example 1, except that the water-insoluble particles to be mixed were changed to 0.2 parts by mass of amorphous silica 2 (manufactured by Nippon Aerosil Co., Ltd., AEROSIL® 90G).

[0105] (Example 9) The coated resin particles of Example 9 were obtained in the same manner as in Example 1, except that the water-insoluble particles to be mixed were changed to 0.2 parts by mass of calcium fluoride (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.).

[0106] (Comparative Example 4) Comparative Example 4 coated resin particles were obtained in the same manner as Comparative Example 1, except that 400.00 g of aqueous dispersion (A) and 0.75 g of polyethylene glycol (manufactured by Tokyo Chemical Industry Co., Ltd., PEG6000, number average molecular weight: 7300-9300) were mixed in a 1 L beaker (made of polypropylene) to obtain 400.75 g of aqueous dispersion (B), which was used as the coating solution instead of aqueous dispersion (A), and that the setting temperature of the hot air dryer in the heat treatment step was set to 120°C and the heating time to 30 minutes. In the 400.75 g coating solution, the content of the coating material was 60.00 g (14.97 mass%) of partially neutralized ethylene / acrylic acid copolymer (P(E / AA), degree of neutralization 90%) and 0.75 g (0.19 mass%) of polyethylene glycol. The amount of coating material supplied per 100 parts by mass of water-absorbing resin particles was 12.15 parts by mass.

[0107] (Example 10) The coated resin particles of Example 10 were obtained in the same manner as in Comparative Example 4, except that 0.1 parts by mass of amorphous silica 1 was mixed with the water-absorbing resin particles (A1) before the coating process in the same manner as in Example 1.

[0108] <Measurement of medium particle size 1> The median particle size 1 was measured under conditions of 25°C ± 2°C and 50 ± 10% humidity. The median particle size 1 (D50 (median diameter), volume-based) of the water-insoluble particles used was measured using a laser diffraction particle size distribution analyzer (Shimadzu Corporation, SALD2300). The refractive index was measured at 1.6–0.1i. The results are shown in Table 1.

[0109] <Measuring Contact Angle> The contact angle was measured under the environment of 25°C ± 2°C and humidity of 50 ± 10%. A double-sided tape (manufactured by Nitto Denko: 20 mm × 70 mm) was attached to a metal plate (30 mm × 70 mm), and one with the adhesive surface exposed was prepared. First, water-insoluble particles were uniformly sprayed onto the double-sided tape attached to the plate so that the adhesive surface was not exposed. Then, the plate was set upright to remove excess water-insoluble particles, and a measurement sample was prepared.

[0110] Using a contact angle meter (manufactured by Kyowa Interface Science, DMo-601), the ion-exchanged water contact angle of each water-insoluble powder was measured. The contact angle was determined using the θ / 2 method at the time point 144 ms after a 3 μL droplet of ion-exchanged water was brought into contact with a smooth area on the surface of the above measurement sample. The results are shown in Table 〈1〉.

[0111] 〈Loss on drying〉 The measurement of loss on drying was carried out under the environment of 25°C ± 2°C and humidity of 50 ± 10%. 2.0 g of each of the water-absorbing resin particles (A1) of Production Example 1, the coated resin particles of Comparative Example 1, or Comparative Example 4 was placed in an aluminum wheel case (No. 8) that had been pre-weighed to a constant weight (W A [g]), and its mass W B [g] was accurately weighed. The accurately weighed powder was dried in a hot air dryer (manufactured by ADVANTEC, FV-320) with the internal temperature set at 105°C for 2 hours. After the powder was allowed to cool in a desiccator, its mass W C [g] was measured as the dry mass. The loss on drying of each particle was calculated from the following formula. The results are shown in Table 〈2〉. Loss on drying (mass%) = [{(W B - W A ) - (W C - W A )} / (W B - W A )] × 100

[0112] 〈Water retention capacity〉 The water retention capacity was measured under conditions of 25°C ± 2°C and 50 ± 10% humidity. First, 500 g of physiological saline solution at 25°C was added to a 500 mL polyethylene beaker. Then, 2.0 g of particles were added in small increments using a stirrer with a stirring bar (8 mm × 30 mm, without a ring) rotating at 600 rpm. After all the particles had been added, the mixture was stirred for 30 minutes to obtain the mixture. Next, this mixture was transferred to a cotton bag (membrane No. 60, 100 mm wide × 200 mm long), and the top of the cotton bag was closed with a rubber band. Then, the mixture was dehydrated for 1 minute using a centrifuge (manufactured by Kokusan Co., Ltd., H-122) set to a centrifugal force of 167 G. The mass of the cotton bag containing the swollen gel after dehydration was W. D [g] was measured. The same procedure was performed without putting particles in the cotton bag, and the empty mass W of the wet cotton bag was measured. E The amount [g] was measured. Then, the water content [g / g] of the particles in physiological saline was calculated using the following formula. The results are shown in Table 2. Water retention capacity = (W) D -W E ) / particle mass

[0113] <Water absorption rate: Vortex method> The water absorption rate was measured under conditions of 25°C ± 2°C and 50 ± 10% humidity. 50.0 g of physiological saline was added to a 100 mL beaker, and the saline was maintained at 25°C in a constant temperature bath. A vortex was generated in the physiological saline by stirring with a stirring bar (8 mmφ × 30 mm, without a ring) at a rotation speed of 600 rpm. 2.00 g of particles (absorbent resin particles or coated resin particles) were added to the vortex of the physiological saline, and measurement was started with a stopwatch simultaneously. The endpoint was defined as the point when the vortex disappeared and the liquid surface became horizontal, and the time to the endpoint was obtained as the water absorption rate of the particles [seconds]. The results are shown in Table 2.

[0114] <Measurement of medium particle size 2> Measurement of the medium particle size 2 was performed under conditions of 25°C ± 2°C and 50 ± 10% humidity. 5.0 g of particles (absorbent resin particles or coated resin particles) were sieved using an ultrasonic sieving automatic particle size distribution analyzer (Robot Shifter RPS-02, manufactured by Seishin Corporation) and sieves with JIS standard mesh sizes of 850 μm, 710 μm, 600 μm, 500 μm, 425 μm, 300 μm, 250 μm, and 150 μm, along with a receiving tray. The mass of the particles remaining on each sieve was calculated as a mass percentage of the total amount. The mass percentages of the particles remaining on each sieve were accumulated in order from the largest particle size, and the relationship between the sieve mesh size and the accumulated mass percentage of the particles remaining on the sieve was plotted on logarithmic probability paper. By connecting the plots on probability paper with straight lines, the particle size corresponding to a cumulative mass percentage of 50% was determined, and this was defined as the median particle size of 2 [μm]. The results are shown in Table 2.

[0115] <Amount of saline solution absorbed under a 4.14 kPa load> The amount of water absorbed by superabsorbent polymer particles in physiological saline solution under a load of 4.14 kPa (water absorption under 4.14 kPa load) was measured using the apparatus schematically shown in Figure 2. The measurements were performed in an environment of 25°C ± 2°C and 50 ± 10% humidity. The apparatus in Figure 2 comprises a burette section 21, a clamp 23, a conduit 25, a stand 31, a measuring platform 33, and a measuring unit 24 placed on the measuring platform 33. The burette section 21 has a burette tube 41 with markings, a rubber stopper 43 that seals the opening at the top of the burette tube 41, a cock 42 connected to the lower end of the burette tube 41, and an air inlet tube 45 and a cock 44 connected to the lower part of the burette tube 41. The burette section 21 is fixed by the clamp 23. The flat measuring platform 33 has a through hole 33a with a diameter of 2 mm formed in its center and is supported by a stand 31 with adjustable height. The through-hole 33a of the measuring table 33 and the stopcock 42 of the burette section 21 are connected by a conduit 25. The inner diameter of the conduit 25 is 6 mm.

[0116] The measuring unit 24 includes an acrylic resin cylinder 51, a polyamide mesh 52 bonded to one opening of the cylinder 51, and a weight 53 that is movable vertically within the cylinder 51. The cylinder 51 is placed on the measuring stand 33 via the polyamide mesh 52. The inner diameter of the cylinder 51 is 20 mm. The mesh opening of the polyamide mesh 52 is 75 μm (200 mesh). The weight 53 has a diameter of 19 mm and a mass of 119.6 g, and can apply a load of 4.14 kPa to resin particles 30a uniformly arranged on the polyamide mesh 52, as described later.

[0117] First, the stopcocks 42 and 44 of the burette section 21 were closed, and physiological saline solution adjusted to 25°C was poured into the burette tube 41 through the opening at the top of the burette tube 41. Next, the top opening of the burette tube 41 was sealed tightly with the rubber stopper 43, and then the stopcocks 42 and 44 were opened. The inside of the conduit 25 was filled with physiological saline solution 70 to prevent air bubbles from entering. The height of the measuring platform 33 was adjusted so that the level of the physiological saline solution that reached the through-hole 33a was the same as the height of the top surface of the measuring platform 33. After the adjustment, the level of the physiological saline solution 70 inside the burette tube 41 was read on the scale of the burette tube 41, and that position was set as the zero point (reading at 0 seconds).

[0118] In the measurement unit 24, 0.10 g of resin particles 30a were uniformly arranged on the polyamide mesh 52 inside the cylinder 51, a weight 53 was placed on the resin particles 30a, and the cylinder 51 was positioned so that its center coincided with the conduit opening in the center of the measurement stage 33. The amount of physiological saline in the burette tube 41 decreased 60 minutes after the resin particles 30a began to absorb physiological saline from the conduit 25 (i.e., the amount of physiological saline absorbed by the resin particles 30a) W K The volume [mL] was read, and the amount of saline solution absorbed by resin particle 30a under a 4.14 kPa load was calculated using the following formula. The results are shown in Table 2. 4.14 kPa load: Physiological saline absorption [mL / g] = W K [mL] / Mass of resin particles [g]

[0119] [Table 1]

[0120] [Table 2]

[0121] The coated resin particles of the comparative examples and examples, which had a coated portion, all showed a significantly reduced water absorption rate compared to the water-absorbing resin particles of Production Example 1. In Examples 1 to 9, the water retention capacity increased compared to Comparative Example 1 by mixing water-insoluble particles with the water-absorbing resin particles before performing the coating process. Even in Example 10, where the amount of coating material was increased, the water retention capacity increased compared to Comparative Example 4, which used the same amount of coating material.

[0122] In Example 7, the water retention capacity is higher than in Manufacturing Example 1. This is presumed to be because the heat treatment in the coating and heat treatment processes reduces the drying loss (≒ water content in the particles), and increases the polyacrylic acid (sodium) content per 2g of water-absorbing resin particles at the time of water retention measurement.

[0123] <Taking scanning electron microscope images> Scanning electron microscope images were taken of the coated resin particles obtained in Comparative Examples 1 and 3 and Example 4. The coated resin particles were surface-coated using a Pt coating machine (JEOL, JFC-1600 AUTO FINE COATER), and then photographed at 200x magnification using a scanning microscope (JEOL, JSM-IT200LA).

[0124] Scanning electron microscope images of the coated resin particles of Comparative Example 1, Example 4, and Comparative Example 3 are shown in Figures 3, 4, and 5, respectively. In the photograph of Example 4 (Figure 4), in which water-insoluble particles were added before the coating process, it can be confirmed that the surface is coated and the water-insoluble particles are not exposed on the surface. On the other hand, in Comparative Example 3, in which water-insoluble particles were added after the coating heat treatment process, the surface is rougher compared to Comparative Example 1 (Figure 3) and Example 4 (Figure 4), in which no water-insoluble particles were added, and it can be confirmed that the water-insoluble particles are located outside the coated area. [Explanation of Symbols]

[0125] 1... Processing device, 10... Processing unit, 11... Internal space, 11a, 11b, 11c... Space, 12... Air inlet, 13... Stirring means, 13a... Disc part, 13b... Support part, 14... Opening, 15... Exhaust filter, 16... Liquid supply part, 20... Mixed particles, G... Airflow, L... Coating liquid, 21... Burette part, 23... Clamp, 24... Measuring part, 25... Conduit, 30a... Resin particles, 31... Stand, 33... Measuring stand, 33a... Through hole, 41... Burette tube, 42, 44... Cock, 43... Rubber stopper, 45... Air inlet tube, 51... Cylinder, 52... Polyamide mesh, 53... Weight.

Claims

1. A step of preparing mixed particles containing water-insoluble particles and water-absorbent resin particles, and A method for producing coated resin particles, comprising the step of bringing the mixed particles into contact with a coating material containing a polymer component to obtain coated resin particles having a coating portion that covers at least a part of the surface of the mixed particles.

2. The manufacturing method according to claim 1, wherein the median particle size of the water-insoluble particles is 1.0 to 100.0 μm.

3. The manufacturing method according to claim 1 or 2, wherein the water-insoluble particles include an inorganic compound.

4. The manufacturing method according to claim 3, wherein the inorganic compound is at least one selected from the group consisting of silica, activated carbon, alkaline earth metal halides, alkaline earth metal salts, and metal oxides.

5. The manufacturing method according to claim 1 or 2, wherein the content or amount of the water-insoluble particles is greater than 0 parts by mass and less than or equal to 3.0 parts by mass per 100 parts by mass of the water-absorbent resin particles.

6. The manufacturing method according to claim 1 or 2, further comprising a heat treatment step of heating the coated resin particles at a temperature above the glass transition temperature of the polymer component.

7. The manufacturing method according to claim 1 or 2, wherein the coating material comprises at least one from the group consisting of an olefin / ethylenically unsaturated monomer copolymer and a styrene / ethylenically unsaturated monomer copolymer.

8. The manufacturing method according to claim 1 or 2, wherein the amount of the coating material added is 3.0 to 8.0 parts by mass per 100 parts by mass of the water-absorbing resin particles.

9. Coated resin particles comprising water-absorbent resin particles, mixed particles containing water-insoluble particles arranged on the surface of the water-absorbent resin particles, and a coating portion that covers at least a portion of the surface of the mixed particles.

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