Coated resin particles and method for producing the coated resin particles

Abstract

One aspect of the present invention relates to coated resin particles, each of which comprises a water absorbent resin particle and a coating layer that covers at least a part of the surface of the water absorbent resin particle, wherein the coating layer contains a water-soluble component that has a solubility within the range of from 1.0 g to 150 g at 25°C with respect to 100 g of water.

Description

Technical Field

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

Background Art

[0002] Water-absorbing resin particles are widely used in various fields such as sanitary materials for disposable diapers, sanitary products, and portable toilets, agro-horticultural materials such as water retention agents and soil improvers, and industrial materials such as water-stopping agents and dew condensation preventives. In addition to performance such as high water absorption capacity and gel strength, control of the water absorption rate is required for water-absorbing resin particles. The water absorption rate can be controlled, for example, by varying the specific surface area of the water-absorbing resin particles or the amount of cross-linking agent used. For example, in paragraph

[0062] of Patent Document 1, it is disclosed that "by subjecting a water-containing gel-like substance having an internal cross-linked structure to a post-cross-linking reaction, the cross-linking density near the surface of the water-absorbing resin is increased to increase the water absorption rate."

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] Conventional water-absorbing resin particles reach a swollen state (a state where further water absorption is not possible) in a relatively short time after coming into contact with a liquid such as water or urine that is the object of water absorption (hereinafter simply referred to as "liquid"). When the water-absorbing resin particles swell, the gaps that originally existed between the water-absorbing resin particles are filled with the swollen gel-like water-absorbing resin particles, making it difficult for the liquid to pass through the gaps. This is generally referred to as the gel blocking phenomenon. As a result, the diffusion of the liquid through the gaps becomes difficult, which contributes to liquid leakage. To suppress the gel blocking phenomenon, for example, the water absorption rate can be slowed down (for example, the time for the water-absorbing resin particles to reach the swollen state and / or the time to start water absorption is delayed) by changing the specific surface area of the water-absorbing resin particles or the amount of cross-linking agent used. However, these measures require changing the polymerization conditions of the water-absorbing resin particles, and it is complicated to find the optimal conditions.

[0005] The present invention has been made in view of such problems, and an object thereof is to provide coated resin particles whose water absorption rate is controlled to be slow without changing the polymerization conditions of the water-absorbing resin particles, and a method for producing the same.

Means for Solving the Problems

[0006] One aspect of the present invention provides coated resin particles having water-absorbing resin particles and a coating layer that coats at least a part of the surface of the water-absorbing resin particles, wherein the coating layer contains a water-soluble component having a solubility in 100 g of water in the range of 1.0 g or more and 150 g or less at 25°C.

[0007] Another aspect of the present invention provides a method for producing the above-mentioned coated resin particles, comprising a step of mixing water-absorbing resin particles and a coating material containing a water-soluble component having a solubility in 100 g of water in the range of 1.0 g or more and 150 g or less at 25°C to form a coating layer on at least a part of the surface of the water-absorbing resin particles.

Effects of the Invention

[0008] According to the present invention, it is possible to provide coated resin particles whose water absorption rate is controlled to be slow without changing the polymerization conditions of the water-absorbing resin particles, and a method for producing the same.

Brief Description of the Drawings

[0009]

Figure 1

Figure 2

Modes for Carrying Out the Invention

[0010] Hereinafter, some embodiments of the present invention will be described in detail. However, the present invention is not limited to the following embodiments.

[0011] In this specification, “acrylic” and “methacrylic” are collectively referred to as “(meth)acrylic”. Similarly, “acrylate” and “methacrylate” are also referred to as “(meth)acrylate”. In the numerical ranges described step by step in this specification, the upper limit value or lower limit value of a certain step's numerical range can be arbitrarily combined with the upper limit value or lower limit value of another step's numerical range. In the numerical ranges described in this specification, the upper limit value or lower limit value of the numerical range may be replaced with the values shown in the examples. The materials exemplified in this specification may be used alone or in combination of two or more. The content of each component in the composition means the total amount of the plurality of substances corresponding to each component in the composition when there are a plurality of substances corresponding to each component in the composition, unless otherwise specified.

[0012] [Coated Resin Particles] (Basic Configuration of Coated Resin Particles) The coated resin particles of the present invention have water-absorbing resin particles and a coating layer that coats at least a part of the surface of the water-absorbing resin particles. The coating layer contains a water-soluble component having a solubility in 100 g of water in the range of 1.0 g or more and 150 g or less at 25°C.

[0013] The coating layer is preferably chemically and / or physically bonded to the surface of the water-absorbing resin particles so that the water-absorbing resin particles in the state before water absorption do not easily fall off. The physical bonding is realized, for example, by an anchor effect caused by the coating layer entering into fine recesses present on the surface of the water-absorbing resin particles. Hereinafter, with reference to FIGS. 1 and 2, the presumed mechanism by which the water absorption rate can be controlled by the coated resin particles of the present invention will be described.

[0014] FIG. 1 is a schematic cross-sectional view showing an embodiment of the coated resin particles. As shown in FIG. 1(a), the coated resin particles 1 according to the present embodiment include water-absorbing resin particles 10 and a coating layer 20 that coats at least a part of the surface of the water-absorbing resin particles 10. In FIG. 1(a), the entire surface of the water-absorbing resin particles 10 is coated with the coating layer 20.

[0015] In the portion of the water-absorbing resin particles 10 coated with the coating layer 20, the contact with the liquid is blocked. Therefore, when the entire surface of the water-absorbing resin particles 10 is coated with the coating layer 20, the water-absorbing resin particles 10 cannot absorb the liquid. On the other hand, although not particularly shown, when a part of the surface of the water-absorbing resin particles 10 is coated with the coating layer 20, the water-absorbing resin particles 10 can absorb the liquid at the portion where the surface is exposed (the portion not coated with the coating layer 20), but the coating layer 20 functions as a binder that inhibits the expansion of the water-absorbing resin particles 10. Therefore, when the coating layer 20 is provided on at least a part of the surface of the water-absorbing resin particles 10, the water-absorbing resin particles 10 cannot exhibit their original water absorption ability. However, as described above, since the coating layer 20 contains a water-soluble component having a solubility in 100 g of water in the range of 1.0 g or more and 150 g or less at 25°C, when the coated resin particles 1 come into contact with the liquid, the coating layer 20 gradually dissolves and disappears. As the coating layer 20 disappears, the water-absorbing resin particles 10 gradually exhibit their original water absorption ability, and finally, they become the swollen water-absorbing resin particles 10a shown in FIG. 1(b).

[0016] As described above, since the coated resin particles of the present invention have the water-absorbing resin particles coated with a specific coating layer, the water-absorbing ability of the water-absorbing resin particles is suppressed. That is, the time until the coated resin particles reach the swollen state is slower than the case where only the water-absorbing resin particles constituting the coated resin particles are used. Hereinafter, the change in the water absorption amount that the coated resin particles can exhibit will be described with reference to FIG. 2.

[0017] FIG. 2 is a graph showing the change in the water absorption amount over time (hereinafter simply referred to as "water absorption behavior") after contact with a liquid for each of the coated resin particles and the water-absorbing resin particles constituting the coated resin particles. Note that FIG. 2 is not a graph obtained by actually measuring specific coated resin particles and water-absorbing resin particles, but a conceptual diagram showing the concept of the present invention.

[0018] The water absorption behavior of the coated resin particles may be, for example, as shown in FIG. 2(a), "the time to start water absorption is the same as that of the water-absorbing resin particles (the moment the coated resin particles come into contact with the liquid), but the amount of water absorbed is substantially constant and small, so the time until the swollen state is reached is slow", or as shown in FIG. 2(b), "the time to start water absorption and the time until the swollen state is reached are both slower than those of the water-absorbing resin particles", or as shown in FIG. 2(c), "the time to start water absorption is the same as that of the water-absorbing resin particles (the moment the coated resin particles come into contact with the liquid), but the initial water absorption amount is extremely small, so the time until the swollen state is reached is slow".

[0019] Thus, the coated resin particles have a slower water absorption rate compared to the case where only the water-absorbing resin particles are used. Therefore, by using the coated resin particles of the present invention, the occurrence of the gel blocking phenomenon can be effectively suppressed compared to the case where only the water-absorbing resin particles are used. The coated resin particles preferably exhibit a significant increase in the water absorption amount after a certain period of time, as shown in (b) or (c) of FIG. 2, and more preferably do not absorb liquid until a certain period of time has elapsed, as shown in (b) of FIG. 2. In particular, the coated resin particles showing the water absorption behavior as shown in (b) of FIG. 2 do not absorb liquid until a certain period of time has elapsed, so that the occurrence of the gel blocking phenomenon can be more effectively suppressed.

[0020] When the coated resin particles exhibit the water absorption behavior as shown in (b) of FIG. 2, the time to start water absorption after the coated resin particles come into contact with a 0.9 mass% sodium chloride aqueous solution (hereinafter simply referred to as "physiological saline") may be, for example, 3 to 120 minutes, 5 to 90 minutes, or 10 to 60 minutes. When the coated resin particles exhibit the water absorption behavior as shown in (c) of FIG. 2, the time until the coated resin particles exhibit 10% of the water absorption capacity after coming into contact with physiological saline may be, for example, 3 to 120 minutes, 5 to 90 minutes, or 10 to 60 minutes. Note that "exhibiting 10% of the water absorption capacity" means absorbing physiological saline corresponding to 10 mass% of the total water absorption amount in the swollen state.

[0021] The water absorption behavior that is easy to achieve will be described according to the mode of the coating layer. When the coating layer covers the entire surface of the water-absorbing resin particles, even when the coated resin particles come into contact with a liquid, water absorption does not start until the coating layer dissolves and the surface of the water-absorbing resin particles is exposed. As a result, the coated resin particles are likely to exhibit the water absorption behavior as shown in (b) of FIG. 2. In this case, the time until the coated resin particles start water absorption can be appropriately controlled by the forming material and / or thickness of the coating layer, etc.

[0022] When the coating layer covers a part of the surface of the water-absorbing resin particles, although water absorption starts immediately when the coated resin particles come into contact with a liquid, the swelling of the water-absorbing resin particles is suppressed by the coating layer. Therefore, the coated resin particles cannot exhibit their original water absorption ability until the coating layer is sufficiently dissolved. As a result, it becomes easy for the coated resin particles to show a water absorption behavior as shown in Fig. 2(a) or (c). Whether the coated resin particles show a water absorption behavior as shown in Fig. 2(a) (the amount of water absorbed per unit time (water absorption rate) is relatively constant until reaching the swollen state) or a water absorption behavior as shown in Fig. 2(c) (the amount of water absorbed increases rapidly after a certain period of time) can be appropriately controlled by factors such as the forming material, thickness, and / or coverage rate of the coating layer.

[0023] (Water-absorbing resin particles) The water-absorbing resin particles are not particularly limited as long as they are composed of a resin having water absorption. The water-absorbing resin particles may contain, for example, a crosslinked polymer formed by polymerization of a monomer containing an ethylenically unsaturated monomer. The crosslinked polymer can have monomer units derived from the ethylenically unsaturated monomer. The water-absorbing resin particles can be produced, for example, by a method including a step of polymerizing a monomer containing an ethylenically unsaturated monomer. Examples of the polymerization method include inverse phase suspension polymerization method, aqueous solution polymerization method, bulk polymerization method, precipitation polymerization method, etc.

[0024] 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 water at 25°C). Examples of the water-soluble ethylenically unsaturated monomer include (meth)acrylic acid and its salts, 2-(meth)acrylamido-2-methylpropanesulfonic acid and its salts, (meth)acrylamide, N,N-dimethyl(meth)acrylamide, 2-hydroxyethyl (meth)acrylate, N-methylol(meth)acrylamide, polyethylene glycol mono(meth)acrylate, N,N-diethylaminoethyl (meth)acrylate, N,N-diethylaminopropyl (meth)acrylate, and diethylaminopropyl (meth)acrylamide. When the ethylenically unsaturated monomer has an amino group, the amino group may be quaternized. The ethylenically unsaturated monomer may be used alone or in combination of two or more kinds.

[0025] When the ethylenically unsaturated monomer has an acid group, the acid group may be neutralized with an alkaline neutralizing agent and then used in the polymerization reaction. The degree of neutralization of the ethylenically unsaturated monomer with the alkaline neutralizing agent may be, for example, 10 to 100 mol%, 50 to 90 mol%, or 60 to 80 mol% of the acidic groups in the ethylenically unsaturated monomer.

[0026] From the viewpoint of easy industrial availability, the ethylenically unsaturated monomer may contain at least one compound selected from the group consisting of (meth)acrylic acid and its salts, acrylamide, methacrylamide, and N,N-dimethylacrylamide. The ethylenically unsaturated monomer may contain at least one compound selected from the group consisting of (meth)acrylic acid and its salts, and acrylamide.

[0027] As the monomer for obtaining the water-absorbing resin particles, monomers other than the above-described ethylenically unsaturated monomers may be used. Such monomers can be used, for example, by mixing them into an aqueous solution containing the above-described ethylenically unsaturated monomers. The usage amount of the ethylenically unsaturated monomer may be 70 to 100 mol% based on the total amount of the monomers. The proportion of (meth)acrylic acid and its salts may be 70 to 100 mol% based on the total amount of the monomers.

[0028] Crosslinking by self-crosslinking occurs during polymerization, but crosslinking may be promoted by using an internal crosslinking agent. When an internal crosslinking agent is used, it is easy to control the water absorption characteristics (such as water retention amount) of the water-absorbing resin particles. The internal crosslinking agent is usually added to the reaction solution during the polymerization reaction.

[0029] The water-absorbing resin particles may be those in which crosslinking (surface crosslinking) near the surface has been performed. Also, the water-absorbing resin particles may be composed only of polymer particles (crosslinked polymers), but may further contain various additional components selected from, for example, a gel stabilizer, a metal chelating agent, and a fluidity improver (lubricant), etc. The additional components can be disposed inside the polymer particles, on the surface of the polymer particles, or both. The additional component is preferably a fluidity improver (lubricant). The fluidity improver may contain inorganic particles. Examples of the inorganic particles include silica particles such as amorphous silica.

[0030] The shape of the water-absorbing resin particles is not particularly limited, and may be, for example, substantially spherical, crushed, or granular, or may be a shape in which primary particles having these shapes are aggregated.

[0031] The median particle diameter of the water-absorbing resin particles may be 100 to 800 μm, 150 to 700 μm, 200 to 600 μm, or 250 to 500 μm. The median particle diameter is measured by the method described in the examples.

[0032] The water absorption amount of the water-absorbing resin particles in physiological saline may be, for example, 10 to 100 g / g, 20 to 90 g / g, or 30 to 80 g / g at 25°C. The water absorption amount is measured by the method described in the examples.

[0033] (Coating layer) The coating layer contains a water-soluble component (hereinafter simply referred to as "water-soluble component") whose solubility in 100 g of water is in the range of 1.0 g or more and 150 g or less at 25°C. By having such a coating layer, it becomes easier to control the water absorption behavior of the coated resin particles. When the solubility of the water-soluble component is less than 1 g, it takes a long time for the coating layer to dissolve, making it difficult to produce practical coated resin particles. Also, when the solubility of the water-soluble component exceeds 150 g, depending on the thickness of the coating layer, after the coated resin particles come into contact with a liquid, the entire coating layer may dissolve within several seconds, and there is a risk that the water absorption rate cannot be substantially controlled.

[0034] The lower limit of the solubility of the water-soluble component in 100 g of water at 25°C may be 1.1 g or more, 1.2 g or more, 1.5 g or more, or 2.0 g or more. Also, the upper limit of the solubility of the water-soluble component in 100 g of water at 25°C is preferably 90 g or less, more preferably 80 g or less, still more preferably 70 g or less, even more preferably 60 g or less, and particularly preferably 50 g or less. The upper limit of the solubility may be 40 g or less, 30 g or less, 20 g or less, 10 g or less, or 5 g or less. The solubility is measured by the method described in the examples.

[0035] The viscosity of the saturated aqueous solution of the water-soluble component at 25°C is preferably in the range of 1 Pa·s or more and 2000 Pa·s or less. The lower limit of the viscosity is more preferably 1.5 Pa·s or more, still more preferably 2 Pa·s or more, even more preferably 2.5 Pa·s or more, and particularly preferably 3 Pa·s or more. Also, the upper limit of the viscosity is more preferably 1500 Pa·s or less, still more preferably 1000 Pa·s or less, even more preferably 750 Pa·s or less, and particularly preferably 500 Pa·s or less.

[0036] As the viscosity of the water-soluble component increases, it becomes easier to bind to the surface of the water-absorbing resin particles, but its handleability deteriorates and it tends to be difficult to form a coating layer with a uniform thickness. On the other hand, as the viscosity of the water-soluble component decreases, its handleability improves and it becomes easier to form a coating layer with a uniform thickness, but it tends to be difficult to bind to the surface of the water-absorbing resin particles. If the viscosity of the water-soluble component is within the above range, it becomes easier to easily adhere the water-soluble component to the surface of the water-absorbing resin particles while maintaining appropriate handleability. The viscosity of the water-soluble component is measured by the following procedure.

[0037] <Method for Measuring Viscosity (Pa·s)> Mix 600 g of distilled water and a specific amount according to solubility (300 g when the solubility is 1 to 50 g, 600 g when the solubility is 50 to 100 g, 900 g when the solubility is 100 to 150 g) of the water-soluble component thoroughly at 90°C for 1 hour to prepare an aqueous solution. After cooling the aqueous solution to 25°C, filter it using a JIS Z8801 standard sieve with an opening of 1410 μm to remove insolubles, and use the filtrate as a saturated aqueous solution. Pour the saturated aqueous solution into a 500 mL beaker with an inner diameter of 85 mmφ up to a height of 85 mm, adjust the temperature to 25 ± 0.5°C, and then measure the viscosity using a B-type viscometer. The value after 60 seconds is converted to the viscosity [Pa·s] of the saturated aqueous solution using a multiplier according to the rotor and rotation speed used. As the B-type viscometer, use the Bismetron VS-H1 type manufactured by Shibaura Semtech Co., Ltd. (former Shibaura System Co., Ltd.). Also, use the rotor of the same company.

[0038] Since the water-soluble component easily satisfies the above solubility and viscosity, it preferably contains a compound having a hydrophilic group (hereinafter simply referred to as "hydrophilic group-containing compound"), and more preferably consists only of the hydrophilic group-containing compound. The hydrophilic group that the hydrophilic group-containing compound can have is, for example, at least one group selected from the group consisting of an anionic group, a cationic group, an amphoteric group, and a nonionic group. By appropriately adjusting the type and number of the hydrophilic groups, the solubility and viscosity of the water-soluble component can be adjusted, and as a result, the water absorption behavior of the coated resin particles can be appropriately controlled.

[0039] Examples of the anionic group include a carboxyl group, a sulfonic acid group, and a phosphoric acid 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 cyclic lactam group such as a pyrrolidone group and a caprolactam group; an alkoxy group; a (poly)oxyalkylene group such as a (poly)oxyethylene group and a (poly)oxypropylene group.

[0040] The repeating number of the (poly)oxyalkylene group may be, for example, 1 to 150,000, or may be 150 to 100,000. When the repeating number of the (poly)oxyalkylene group is 1, it is simply referred to as an oxyalkylene group, and when the repeating number is 2 or more, it is referred to as a polyoxyalkylene group. The same applies to the various functional groups listed above contained in the (poly)oxyalkylene group. By appropriately adjusting the repeating number, the solubility and viscosity of the water-soluble component can be appropriately adjusted. The number of carbon atoms of the (poly)oxyalkylene group may be, for example, 1 to 4, or may be 2 to 3. By appropriately adjusting this number of carbon atoms, the solubility of the water-soluble component can be appropriately adjusted.

[0041] Examples of the compound having a hydroxyl group include polyvinyl alcohol. Examples of the compound having an amide group include polyacrylamide. Examples of the compound having a (poly)oxyalkylene group include polyalkylene oxide, polyalkylene glycol, and polyoxyalkylene alkyl ether.

[0042] The hydrophilic group-containing compound is preferably at least one selected from the group consisting of polyvinyl alcohol, polyacrylamide, polyalkylene oxide, polyalkylene glycol, polyoxyalkylene alkyl ether, and copolymers of monomers constituting these polymers, and more preferably polyvinyl alcohol, polyethylene oxide, polyethylene glycol, or polyoxyethylene alkyl ether.

[0043] The coating layer is preferably composed of substantially only water-soluble components, but may contain components other than water-soluble components (hereinafter simply referred to as "other components") on the condition that the solubility in 100 g of water is 1.0 g or more and 150 g or less at 25°C for the entire coating layer. The other components are compounds having a solubility in 100 g of water of less than 1.0 g or exceeding 150 g at 25°C. When other components are included, the ratio of the water-soluble components in the entire coating layer may be, for example, 20% by mass or more, 30% by mass or more, or 50% by mass or more, preferably 80% by mass or more, more preferably 90% by mass or more, and still more preferably 95% by mass or more. By including other components in the coating layer, the water absorption behavior of the coated resin particles can be appropriately controlled. Examples of the other components include inorganic substances such as silica and talc, and organic water-insoluble components (organic compounds having a solubility in 100 g of water of less than 1.0 g at 25°C).

[0044] Examples of the water-insoluble organic component include polyesters such as polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, and polybutylene naphthalate; polyamides such as nylon 6 and nylon 66; polyolefins such as polyethylene, polypropylene, ethylene-butene copolymer, ethylene-propylene copolymer, and copolymer of alkene and water-soluble ethylenically unsaturated monomer; polyurethanes such as ether-based polyurethane, ester-based polyurethane, and carbonate-based polyurethane; polystyrenes such as poly-α-methylstyrene and syndiotactic polystyrene; polycarbonates such as bisphenol A and polyhexamethylene carbonate; polyacrylates such as trimethylolpropane tri(meth)acrylate, pentaerythritol tri(meth)acrylate, and dipentaerythritol hexa(meth)acrylate; polyacetals such as polyoxymethylene, polyacetaldehyde, polypropionaldehyde, and polybutyraldehyde; halogen-based polymers such as polyvinyl chloride, polyvinyl fluoride, and polyvinylidene fluoride; and polysiloxane. The water-insoluble organic component may be used alone or in combination of two or more. Further, since the water absorption behavior of the coated resin particles can be more easily controlled, the water-insoluble organic component may be acid-modified.

[0045] As the water-insoluble organic component, polyolefin is preferable, and copolymer of alkene and water-soluble ethylenically unsaturated monomer is more preferable. When using the copolymer of alkene and water-soluble ethylenically unsaturated monomer as the water-insoluble organic component, as the alkene, it is preferable to use at least one alkene selected from ethylene, propylene, and butene, and more preferably to use ethylene. In this case, as the water-soluble ethylenically unsaturated monomer, the above-described compounds can be used, but preferably (meth)acrylic acid and / or its salt is used.

[0046] The coating layer may have a single-layer structure or a multilayer structure having two or more layers. For example, the coating layer may have a first layer containing a first water-soluble component and a second layer containing a second water-soluble component that covers at least a part of the surface of the first layer. By making the coating layer into a multilayer structure, coated resin particles showing a more complex water absorption behavior can be produced.

[0047] From the viewpoint of controlling the water absorption behavior of the coated resin particles, the thickness of the coating layer (when the coating layer has a multilayer structure, it refers to the total thickness obtained by summing the thicknesses of each layer) may be 0.001 to 100 μm, 0.01 to 50 μm, or 0.1 to 30 μm. The thickness of the coating layer can be calculated by observing the cross-section of the coated resin particles using an optical microscope. Specifically, after cross-section processing of the coated resin particles with an ultramicrotome, the cross-section is observed using an optical microscope "SZX16" (manufactured by Olympus) and a confocal microscope OPTELEICS HYBRID (manufactured by Lasertec) to calculate it.

[0048] The coating layer only needs to cover at least a part of the surface of the water-absorbing resin particles, and the water absorption behavior of the coated resin particles can be controlled according to the coating rate. The coating rate of the coating layer on the surface of the water-absorbing resin particles may be 30% or more, 40% or more, or 50% or more, and may also be 100% or less, 90% or less, or 80% or less. The coating rate is calculated by RAMAN touch (manufactured by Nanophoton).

[0049] The coated resin particles of the present invention have at least a part of the surface of the water-absorbing resin particles coated with a coating layer containing a water-soluble component. Therefore, the water-absorbing resin particles cannot exhibit their original water-absorbing ability until most or all of the water-soluble component dissolves in the liquid. Accordingly, the coated resin particles have a slower time to reach the swollen state compared to the case where the water-absorbing resin particles, which are the constituent materials thereof, are used alone, and as a result, the occurrence of the gel blocking phenomenon can be suppressed. In particular, the coated resin particles of the present invention can be easily produced by providing a coating layer on the surface of the water-absorbing resin particles. Therefore, it is not necessary to use a complicated method of changing the polymerization conditions of the water-absorbing resin particles as in the prior art.

[0050] The water absorption rate (cm) of the coated resin particles in physiological saline can be measured by the method described in the examples. The water absorption rate of the coated resin particles after 1 minute at 25°C is preferably 1.7 cm or less, and may be 1.6 cm or less, 1.5 cm or less, or 1.4 cm or less. The water absorption rate of the coated resin particles after 5 minutes at 25°C is preferably 4.5 cm or less, and may be 4.4 cm or less, 4.3 cm or less, or 4.2 cm or less.

[0051] The coated resin particles of the present invention can be used alone, but can also be used as mixed particles by mixing with water-absorbing resin particles other than the coated resin particles (hereinafter simply referred to as "other water-absorbing resin particles"). By using the mixed particles, the time to reach the swollen state can be made slower compared to the case where other water-absorbing resin particles are used alone, and as a result, the occurrence of the gel blocking phenomenon can be suppressed. Further, when using the mixed particles, arbitrary water absorption behavior can be realized by appropriately changing the type of the coated resin particles, the type of the other water-absorbing resin particles, the mixing ratio of the coated resin particles and the other water-absorbing resin particles, and the like.

[0052] [Method for producing coated resin particles] The method for producing the coated resin particles of the present invention comprises a step of mixing water-absorbing resin particles and a coating material containing a water-soluble component having a solubility in 100 g of water in the range of 1.0 to 150 g at 25°C to form a coating layer on at least a part of the surface of the water-absorbing resin particles.

[0053] The coating material is a compound containing a water-soluble component capable of forming the above-described coating layer. The coating material may be in a solid state and used for mixing with the water-absorbing resin particles, or may be in a liquid state and used for mixing with the water-absorbing resin. Hereinafter, specific production methods of the coated resin particles will be described according to the state of the coating material.

[0054] <When using a solid coating material> When using a solid coating material, the coating material can be pressure-bonded to the surface of the water-absorbing resin particles using a particle composite device to form a coating layer. Specifically, a predetermined amount of water-absorbing resin particles and a solid (for example, powder) coating material are introduced into the particle composite device. Then, by rotating the stirring blades provided in the device, stress (compressive stress and shear stress) is applied to the water-absorbing resin particles and the coating material, and the coating material is pressure-bonded to the surface of the water-absorbing resin particles by the stress to produce coated resin particles.

[0055] In this case, by appropriately adjusting the amounts of the water-absorbing resin particles and the coating material introduced into the particle composite device, the thickness and coating rate of the coating layer can be arbitrarily adjusted. Note that the water-absorbing resin particles and the coating material may be separately introduced into the particle composite device, but it is preferable to introduce them into the particle composite device in a state where the water-absorbing resin particles and the coating material are previously mixed because more uniform coating can be expected. When using a particle composite device, coated resin particles with a coating layer covering a part of the surface of the water-absorbing resin particles are easily obtained. Therefore, it is considered that the coated resin particles are likely to exhibit a water absorption behavior as shown in (a) or (c) of FIG. 2. As the particle composite device, for example, the particle composite device Nobilta MINI (manufactured by Sugino Machine Limited) can be used.

[0056] <When using a liquid coating material> The liquid coating material (hereinafter simply referred to as "coating liquid") can be obtained, for example, by melting the coating material or by dissolving or dispersing the coating material in an arbitrary solvent or dispersion medium. Since it is easy to form a coating layer with a uniform thickness, it is preferable that the coating liquid is obtained by dissolving or dispersing the coating material in an arbitrary solvent or dispersion medium.

[0057] Examples of the solvent include water, hydrophilic solvents, and mixed solvents of water and hydrophilic solvents. A hydrophilic solvent is a solvent that dissolves substantially uniformly in water. Examples of the hydrophilic solvent 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. The hydrophilic solvents may be used alone or in combination of two or more.

[0058] As the dispersion medium, a hydrocarbon dispersion medium is preferably used. Examples of the hydrocarbon dispersion medium 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 benzene, toluene, and xylene. The hydrocarbon dispersion medium may be used alone or in combination of two or more.

[0059] The concentration of the coating material in the coating liquid is not particularly limited and can be appropriately adjusted in consideration of the amount of the water - absorbent resin particles to be coated in order to obtain a coating layer with a desired thickness. For example, it may be 1 to 50% by mass, 3 to 30% by mass, or 5 to 20% by mass.

[0060] When using a coating liquid, the coating layer can be formed, for example, by (1) a method of adding the coating liquid to a hydrocarbon dispersion medium in which water-absorbing resin particles are dispersed, (2) a method of adding the coating liquid and the water-absorbing resin particles to the hydrocarbon dispersion medium substantially simultaneously, or (3) a method of bringing the coating liquid into contact with the water-absorbing resin particles in a dry state. Hereinafter, each method will be specifically described.

[0061] An example of the above method (1) will be described. First, a separable flask equipped with a reflux condenser, a dropping funnel, a nitrogen gas inlet tube, and a stirrer is prepared. Subsequently, a hydrocarbon dispersion medium and water-absorbing resin particles are charged into the separable flask, and the mixture is sufficiently stirred while maintaining a high temperature (for example, 60 to 80 °C). On the other hand, a solvent or dispersion medium and a coating material are added to a beaker and mixed to prepare a coating liquid. After adding the coating liquid into the above separable flask and sufficiently stirring, the separable flask is immersed in an oil bath set at a high temperature (for example, 100 to 125 °C), and water that may be contained in the reaction system is withdrawn outside the system while refluxing the hydrocarbon dispersion medium by azeotropic distillation of the hydrocarbon dispersion medium and water. Thereafter, by evaporating the hydrocarbon dispersion medium, coated resin particles in which the coating material is coated on the surface of the water-absorbing resin particles are obtained.

[0062] An example of the above method (2) will be described. First, a separable flask equipped with a reflux condenser, a dropping funnel, a nitrogen gas inlet tube, and a stirrer is prepared. Subsequently, a hydrocarbon dispersion medium, water-absorbing resin particles, and a coating liquid are charged into the separable flask, and the mixture is sufficiently stirred while maintaining a high temperature (for example, 60 to 80 °C). Thereafter, by evaporating the hydrocarbon dispersion medium, coated resin particles in which the coating material is coated on the surface of the water-absorbing resin particles are obtained.

[0063] The above method (3) is various. Hereinafter, as its representative examples, (3-1) a method using an eggplant flask, (3-2) a method using a sprayer, and (3-3) a method using various granulators will be described.

[0064] (3-1) Pour the coating liquid into an eggplant flask, and then add water-absorbing resin particles. Attach the eggplant flask to an evaporator, heat it while rotating, and distill off the solvent or dispersion medium contained in the coating liquid under reduced pressure conditions. Thereby, coated resin particles in which the coating material is coated on the surface of the water-absorbing resin particles can be obtained.

[0065] (3-2) Add water-absorbing resin particles to a separable flask equipped with a stirring blade and stir. Spray the coating liquid onto the water-absorbing resin particles lifted by the stirring by the stirring blade. The spraying of the coating liquid can be performed, for example, using a two-fluid type nozzle. Since a uniform coating can be expected, it is desirable that the coating liquid is atomized and sprayed by an air stream of an inert gas such as nitrogen. Thereafter, the content of the separable flask is taken out, heated in a hot air dryer, and then cooled to room temperature to obtain coated resin particles.

[0066] (3-3) Examples of the granulator used for producing the coated resin particles include a rolling granulator, a stirring granulator, and a fluidized bed granulator.

[0067] When using a rolling granulator, rotate the inclined shallow circular container provided in the rolling granulator, supply water-absorbing resin particles to the circular container, and add an appropriate amount of the coating liquid. Then, due to the solvent or dispersion medium contained in the coating liquid, a coating layer is formed on the surface while a part of the water-absorbing resin particles during rolling aggregates. Note that the addition steps of the water-absorbing resin particles and the coating liquid can be performed a plurality of times as necessary.

[0068] When using a stirring granulator, water-absorbing resin particles are put into a mixer provided in the stirring granulator, and mixing is carried out by stirring while adding a coating liquid. Then, due to the solvent or dispersion medium contained in the coating liquid, a part of the water-absorbing resin particles being stirred aggregates while a coating layer is formed on its surface. The addition steps of the water-absorbing resin particles and the coating liquid can be carried out a plurality of times as necessary. Note that excessive aggregation of the water-absorbing resin particles can be suppressed by controlling the shearing force of the mixer.

[0069] When using a fluidized bed granulator, first, water-absorbing resin particles are put into a container provided in the fluidized bed granulator that can send out hot air from the lower part, and the water-absorbing resin particles are fluidized in advance. Then, when the coating liquid is sprayed from a nozzle provided in the container, due to the solvent or dispersion medium contained in the coating liquid, a part of the water-absorbing resin particles being stirred aggregates while a coating layer is formed on its surface. The spraying of the coating liquid can be carried out a plurality of times as necessary. Note that excessive aggregation of the water-absorbing resin particles can be suppressed by adjusting the spraying amount and spraying frequency of the coating liquid. As the fluidized bed granulator, for example, a fluidized bed granulator FBD / SG (manufactured by YENCHEN MACHINERY) can be used.

[0070] When a coating layer is formed using the coating liquid, since the coating material can easily come into contact with the water-absorbing resin particles evenly, it is considered that the coating layer is easily formed on the entire surface thereof. In particular, the method using the fluidized bed granulator of the above (1), (2), and (3) is considered to be more likely to obtain a coating layer with a more uniform thickness compared to other methods.

Examples

[0071] Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited to these examples.

[0072] (Production of water-absorbing resin particles) A round-bottomed cylindrical separable flask with an inner diameter of 11 cm and an internal volume of 2 L, equipped with a reflux condenser, a dropping funnel, a nitrogen gas inlet tube, and a stirrer (a stirring blade having four inclined paddle blades with a blade diameter of 5 cm in two stages), was prepared. 293 g of n-heptane and 0.736 g of maleic anhydride-modified ethylene-propylene copolymer (dispersant, Mitsui Chemicals, Inc., HiWax 1105A) were added to this flask to obtain a mixture. While stirring this mixture, the temperature was raised to 80 °C to dissolve the dispersant in n-heptane, and then the mixture was cooled to 50 °C.

[0073] Next, 92.0 g (1.03 mol of acrylic acid) of an 80.5 mass% aqueous acrylic acid solution as a water-soluble ethylenically unsaturated monomer was placed in a beaker with an internal volume of 300 mL. Subsequently, while cooling from the outside, 147.7 g of a 20.9 mass% aqueous sodium hydroxide solution was dropped into the beaker to neutralize 75 mol% of acrylic acid. Thereafter, 0.092 g of hydroxyethyl cellulose (manufactured by Sumitomo Seika Chemicals Co., Ltd., HEC AW-15F) as a thickener, 0.0736 g (0.272 mmol) of potassium persulfate as a water-soluble radical polymerization initiator, and 0.010 g (0.057 mmol) of ethylene glycol diglycidyl ether as an internal crosslinking agent were added and then dissolved to prepare the first-stage aqueous solution.

[0074] After adding the above-mentioned first-stage aqueous solution to the above-mentioned separable flask, it was stirred for 10 minutes. Thereafter, a surfactant solution obtained by dissolving 0.736 g of sucrose stearate (surfactant, manufactured by Mitsubishi Chemical Foods Co., Ltd., Ryoto Sugar Ester S-370, HLB: 3) in 6.62 g of n-heptane was added to the separable flask to obtain a reaction solution. Then, while stirring the reaction solution at a rotation speed of 550 rpm of the stirrer, the inside of the system was sufficiently replaced with nitrogen. Thereafter, the separable flask was immersed in a water bath at 70 °C to raise the temperature of the reaction solution, and the polymerization reaction was allowed to proceed for 60 minutes to obtain the first-stage polymerization slurry solution.

[0075] Next, 128.8 g (1.43 mol of acrylic acid) of an aqueous acrylic acid solution of 80.5% by mass as a water-soluble ethylenically unsaturated monomer was placed in another beaker with an inner volume of 500 mL. Subsequently, while cooling from the outside, 159.0 g of a 27% by mass aqueous sodium hydroxide solution was added dropwise into the beaker to neutralize 75 mol% of the acrylic acid. Then, 0.103 g (0.381 mmol) of potassium persulfate as a water-soluble radical polymerization initiator and 0.0116 g (0.067 mmol) of ethylene glycol diglycidyl ether as an internal crosslinking agent were added to the beaker containing the aqueous acrylic acid solution, and then dissolved to prepare the second-stage aqueous solution.

[0076] While stirring with the rotation speed of the stirrer set at 1000 rpm, the first-stage polymerization slurry liquid in the above flask was cooled to 25°C, and the total amount of the second-stage aqueous solution was added. After replacing the inside of the flask with nitrogen for 30 minutes, the flask was immersed again in a water bath at 70°C to raise the temperature of the reaction solution, and the second-stage polymerization reaction was carried out for 60 minutes to obtain a water-containing gel-like polymer. Then, the flask was immersed in an oil bath set at 125°C, and 257.7 g of water was withdrawn from the system by azeotropic distillation of n-heptane and water. Next, 4.42 g (0.507 mmol) of a 2% by mass aqueous ethylene glycol diglycidyl ether solution as a surface crosslinking agent was added to the flask and held at 83°C for 2 hours.

[0077] Thereafter, the temperature of the second-stage reaction mixture was raised in an oil bath at 125°C, and 245 g of water was withdrawn from the system by azeotropic distillation of n-heptane and water while refluxing n-heptane. Then, the dry product (polymer) was obtained by evaporating and drying n-heptane at 125°C. By passing this dry product through a sieve with an opening size of 850 μm, 236.8 g of water-absorbing resin particles in the form of aggregated spherical particles were obtained.

[0078] [Example 1] As a coating material, polyethylene oxide (Sumitomo Seika Chemical Co., Ltd., PEO-1) was prepared. 7.5 g of polyethylene oxide was mixed with 150 g of distilled water to prepare a coating solution.

[0079] A round-bottom cylindrical separable flask with an inner diameter of 11 cm and an internal volume of 2 L, equipped with a reflux condenser, a nitrogen gas inlet tube, and a stirrer (a stirring blade having four inclined paddle blades with a blade diameter of 5 cm in two stages) was prepared. 300 g of n-heptane and 25 g of water-absorbing resin particles were charged into the flask, and while stirring at 1000 rpm, the temperature was raised to 80 °C to disperse the water-absorbing resin particles in n-heptane. A coating solution was added to the dispersion and stirred for 10 minutes.

[0080] Next, the flask was immersed in an oil bath set at 125 °C, and 140 g of water was withdrawn from the system while refluxing n-heptane by azeotropic distillation of n-heptane and water. Thereafter, the precursor of the coated resin particles was obtained by removing n-heptane at 125 °C. This precursor was passed through a sieve with an opening of 850 μm to obtain 5 g of coated resin particles.

[0081] [Example 2] As a coating material, polyoxyethylene stearyl ether (Nikko Emulsion Co., Ltd., EMALEX 625) was prepared.

[0082] 250 g of n-heptane, 100 g of water-absorbing resin particles, and 10 g of polyoxyethylene stearyl ether were charged into the same flask as in Example 1, and stirred at 1000 rpm and 85 °C for 10 minutes. Next, the flask was immersed in an oil bath set at 125 °C, and the precursor of the coated resin particles was obtained by removing n-heptane at 125 °C. This precursor was passed through a sieve with an opening of 850 μm to obtain 88 g of coated resin particles.

[0083] [Example 3] As a coating material, polyvinyl alcohol (Kuraray Co., Ltd., Kuraray Poval 3-98) was prepared. 150 g of polyvinyl alcohol was mixed with 1995 g of distilled water and 855 g of ethanol to prepare a coating solution.

[0084] 500 g of water-absorbing resin particles were put into the container of a fluidized bed granulator (Powrex Corporation, FD-MP-01), and hot air at 60°C was blown from the bottom of the container. While drying, 3000 g of a coating liquid was sprayed onto the water-absorbing resin particles being lifted by the blowing. After spraying the coating liquid, it was dried at 60°C for 30 minutes to obtain a precursor of coated resin particles. This precursor was passed through a sieve with an opening of 850 μm to obtain 575 g of coated resin particles.

[0085] [Example 4] As a coating material, polyethylene glycol (Tokyo Chemical Industry Co., Ltd., PEG6000) was prepared. A liquid obtained by mixing 600 g of polyethylene glycol with 2700 g of distilled water was put into a spray tank equipped with a stirrer, and 2700 g of ethanol was further added to prepare a coating liquid.

[0086] 3000 g of water-absorbing resin particles were put into the container of a fluidized bed granulator (Powrex Corporation, MP-01mini), and air was blown from the bottom of the container. While drying, 6000 g of a coating liquid was sprayed onto the water-absorbing resin particles being lifted by the hot air at 50°C. After spraying the coating liquid, it was dried at 50°C for 30 minutes to obtain a precursor of coated resin particles. This precursor was passed through a sieve with an opening of 850 μm to obtain 2998 g of coated resin particles.

[0087] [Example 5] As a coating material, an ethylene-sodium acrylate copolymer (Sumitomo Seika Chemicals Co., Ltd., Zeicen N) and polyethylene glycol (Tokyo Chemical Industry Co., Ltd., PEG6000) were prepared. 525 g of distilled water, 200 g of a 25% by mass aqueous dispersion emulsion of an ethylene-sodium acrylate copolymer, and 25 g of polyethylene glycol were mixed to prepare a coating liquid (containing an ethylene-sodium acrylate copolymer and polyethylene glycol at a mass ratio of 2:1).

[0088] 500 g of water-absorbing resin particles were put into a container of a fluidized bed granulator (Paurec Co., Ltd., FD-MP-01), and hot air at 50°C was blown from the bottom of the container. While drying 750 g of a coating liquid, it was sprayed onto the water-absorbing resin particles being lifted up by the blowing. After spraying the coating liquid, it was dried at 50°C for 30 minutes to obtain a precursor of coated resin particles. This precursor was passed through a sieve with an opening of 850 μm to obtain 506 g of coated resin particles.

[0089] [Comparative Example 1] The water-absorbing resin particles were used as they were without forming a coating layer.

[0090] The following evaluations were performed on the water-absorbing resin particles and the coated resin particles. The results are shown in Table 1.

[0091] [Calculation method for the ratio of the coating material] The ratio of the coating material in the production of the coated resin particles was calculated by the following formula. Ratio of the coating material (mass%) = {Mass of the coating material used for forming the coating layer / (Mass of the water-absorbing resin particles used for forming the coating layer + Mass of the coating material used for forming the coating layer)} × 100

[0092] [Measurement method for the solubility of the coating material] When measuring the solubility of the coating material, 5 g of a solid coating material of an appropriate size was prepared for easy measurement, and a measurement solution was prepared by adding this to 100 g of distilled water.

[0093] (Measurement solutions of Examples 1 to 4) 100 g of distilled water at 25°C was put into a 200 mL beaker and stirred at 600 rpm using a rotor (8 mm × 30 mm, without rings). The coating material was classified, and 5 g of the coating material that passed through a sieve with an opening of 850 μm and remained on a sieve with an opening of 75 μm was put into the beaker and stirred for 1 hour to obtain a mixed solution. The mixed solution was suction-filtered using a 34 μm stainless steel wire mesh. The filtrate was collected and used as the measurement solution.

[0094] (Measurement solution of Example 5) 100 g of a 25% aqueous dispersion emulsion of an ethylene-sodium acrylate copolymer was placed in a Teflon-coated vat (bottom dimensions 250 × 185 mm), covered with aluminum foil, and capped. The aluminum foil was perforated and dried at 60 °C for 1 hour and then at 80 °C for 1 hour using a hot air dryer (ADVANTEC, FV-320) to obtain a polymer film of the ethylene-sodium acrylate copolymer. The polymer film was finely cut with scissors, placed in a Teflon-coated vat, covered with aluminum foil, and capped. The aluminum foil was perforated and heated at 105 °C for 2 hours using a hot air dryer (FV-320) for complete drying to obtain 22.5 g of a solid ethylene-sodium acrylate copolymer. Polyethylene glycol and the ethylene-sodium acrylate copolymer were each classified, and 1.67 g of polyethylene glycol and 3.33 g of the ethylene-sodium acrylate copolymer that passed through a sieve with an opening of 850 μm and remained on a sieve with an opening of 75 μm were mixed to obtain 5 g of a coating material (containing the ethylene-sodium acrylate copolymer and polyethylene glycol in a mass ratio of 2:1). Using 5 g of the coating material, a measurement solution was obtained in the same procedure as the preparation of the measurement solutions in Examples 1 to 4.

[0095] (Calculation of solubility) 60 g of the measurement solution was placed in a weighed 100 mL beaker and dried at 140 °C using a hot air dryer (FV-320) for 15 hours to measure the mass (Ws) of the solid content contained in the measurement solution. The solubility of the coating material in 100 g of water was calculated using the following formula. Solubility (g) = (Ws / 60) × 100

[0096] (Physiological saline saturation water absorption) 2.0 g of the water-absorbing resin particles were dispersed in 500 g of physiological saline in a 500 mL beaker and stirred at 600 rpm for 1 hour to swell them. Then, the mass (Wa) of a JIS standard metal sieve with an opening of 75 μm was measured, and the aqueous solution containing the swollen gel was filtered through the metal sieve. The metal sieve was left to stand for 30 minutes in an inclined state such that the angle formed with the horizontal was about 30 degrees to remove the excess physiological saline. The mass (Wb) of the metal sieve containing the swollen gel was measured, and the water absorption was calculated from the following formula. Water absorption (g / g) = [Wb - Wa] / 2.0

[0097] (Retention capacity of physiological saline) 2.0 g of the water-absorbing resin particles or coated resin particles were dispersed in 500 g of physiological saline in a 500 mL beaker and stirred at 600 rpm for 30 minutes to swell them. The swollen gel was poured into a cotton bag (membrane filter No. 60, 100 mm wide × 200 mm long), the upper part of the cotton bag was tied with a rubber band, and dehydration was carried out for 1 minute using a dehydrator (manufactured by Kokusan Co., Ltd., product number: H-122) set so that the centrifugal force was 167 G. The mass Wc (g) of the cotton bag containing the swollen gel after dehydration was measured. The same operation was carried out without adding the water-absorbing resin particles or coated resin particles, the empty mass Wd (g) of the cotton bag when wet was measured, and the retention capacity of the physiological saline was calculated from the following formula. Retention capacity (g / g) = [Wc - Wd] / 2.0

[0098] (Median particle size) Using a continuous fully automatic ultrasonic vibration sieve shaker (Robot Shifter RPS-205, manufactured by Seishin Enterprise Co., Ltd.), sieves with JIS standard openings of 710 μm, 600 μm, 500 μm, 425 μm, 300 μm, 250 μm, and 150 μm, and a tray, the particle size distributions of 5 g of the water-absorbing resin particles and 5 g of the coated resin particles were measured. Regarding this particle size distribution, by integrating the oversize in order from the larger particle size, the relationship between the sieve opening and the integrated value of the mass percentage of the particles remaining on the sieve was plotted on logarithmic probability paper. By connecting the plots on the probability paper with a straight line, the particle size corresponding to an integrated mass percentage of 50 mass% was obtained as the median particle size.

[0099] (Water absorption rate) Weigh accurately 0.200 g of the water-absorbing resin particles or coated resin particles, spread them at the bottom of an acrylic cylinder with an inner diameter of 2.0 cm and a depth of 8.0 cm, and measure the height H0 of the layer of the water-absorbing resin particles or coated resin particles at 25°C. Then, pour 20 g of physiological saline from the upper part of the acrylic cylinder. Measure the height Hn of the layer of the water-absorbing resin particles or coated resin particles n minutes (1 minute and 5 minutes) after the total amount of physiological saline has been added. Calculate the water absorption rates at 1 minute and 5 minutes from the following formula. Water absorption rate (cm) = Hn - H0

[0100] [Table 1] [Explanation of Symbols]

[0101] 1... Coated resin particles, 10, 10a... Water-absorbing resin particles, 20... Coating layer.

Claims

1. It has water-absorbing resin particles and a coating layer that coats at least a part of the surface of the water-absorbing resin particles, the water-absorbing resin particles contain a crosslinked polymer having monomer units derived from an ethylenically unsaturated monomer, and the ethylenically unsaturated monomer is at least one selected from the group consisting of (meth)acrylic acid and its salts, (meth)acrylamide, and N,N-dimethyl(meth)acrylamide, the coating layer contains a water-soluble component having a solubility in 100 g of water in the range of 1.0 g or more and 5 g or less at 25°C and an organic water-insoluble component having a solubility in 100 g of water of less than 1.0 g at 25°C, the water-soluble component contains a compound having a hydrophilic group, and the compound having a hydrophilic group is at least one selected from the group consisting of polyvinyl alcohol, polyacrylamide, polyalkylene oxide, polyalkylene glycol, polyoxyalkylene alkyl ether, and copolymers of monomers constituting these polymers, the organic water-insoluble component is a coating resin particle that is a copolymer of an alkene and a water-soluble ethylenically unsaturated monomer.

2. The coating resin particles according to Claim 1, wherein the ratio of the water-soluble component in the entire coating layer is 20% by mass or more.

3. The coating resin particles according to Claim 1 or 2, wherein the water absorption of the water-absorbing resin particles is 10 to 100 g / g at 25°C.

4. A method for producing the coating resin particles according to any one of Claims 1 to 3, comprising a step of mixing water-absorbing resin particles and a coating material containing a water-soluble component having a solubility in 100 g of water in the range of 1.0 g or more and 5 g or less at 25°C and an organic water-insoluble component having a solubility in 100 g of water of less than 1.0 g at 25°C to form a coating layer on at least a part of the surface of the water-absorbing resin particles.

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