Water absorptive resin particles

Abstract

Water absorptive resin particles according to the present invention exhibit a water absorption rate of 70 seconds or more based on a Vortex method using 2.0 g of the water absorptive resin particles and 100 g of physiological saline in a container having a volume of 150 mL, and have an average aspect ratio of 1.37 or less.

Description

water absorbent resin particles

[0001] The present invention relates to water-absorbent resin particles and the like.

[0002] Water-absorbent resin particles are widely used in various fields such as sanitary materials such as disposable diapers, sanitary products, portable toilets, etc.; agricultural and horticultural materials such as water retention agents and soil conditioners; industrial materials such as waterproofing agents and anti-condensation agents, etc. For example, Patent Documents 1 and 2 listed below disclose water-absorbent resin particles having a predetermined water absorption rate based on the conventional Vortex method.

[0003] JP 2013-132433 A JP 2008-178667 A

[0004] If the liquid supplied to the absorbent article does not sufficiently penetrate the absorbent article, the excess liquid may flow along the surface and leak out of the absorbent article, which may cause problems. Therefore, absorbent articles are required to allow the liquid to penetrate at an excellent penetration rate.

[0005] Furthermore, with respect to absorbent articles, when the absorbent member is brought into contact with the liquid supply position of the absorbent article under load while the absorbent article is in a state where liquid has permeated the absorbent article, it is required that the liquid that has permeated the absorbent article is not easily absorbed by the absorbent member, i.e., that backflow under load is suppressed.

[0006] An object of one aspect of the present invention is to provide water-absorbent resin particles that can provide an absorbent article having an excellent permeation rate and suppressed backflow under load.

[0007] The present invention relates to the following [1] to [5], etc. [1] Water-absorbent resin particles having a water absorption rate of 70 seconds or more and an average aspect ratio of 1.37 or less, as determined by a Vortex method using 2.0 g of water-absorbent resin particles and 100 g of physiological saline in a container having a volume of 150 mL. [2] The water-absorbent resin particles according to [1], having a water absorption rate of 70 to 1,500 seconds. [3] The water-absorbent resin particles according to [1] or [2], having a water retention capacity in physiological saline of 30 to 70 g / g. [4] The water-absorbent resin particles according to any one of [1] to [3], comprising first particles having no coating portion covering at least a part of their surfaces, and second particles having a coating portion covering at least a part of their surfaces. [5] The water-absorbent resin particles according to [4], wherein a ratio of the second particles to the total of the first particles and the second particles is 5 to 60 mass%.

[0008] According to one aspect of the present invention, it is possible to provide water-absorbent resin particles that can be used to obtain absorbent articles that have an excellent permeation rate and are suppressed from returning under load.

[0009] 1 is a schematic cross-sectional view showing a processing apparatus for obtaining coated resin particles. 2 is a schematic cross-sectional view showing an example of an absorbent article.

[0010] Hereinafter, embodiments of the present invention will be described. However, the present invention is not limited to the following embodiments, and various modifications can be made within the scope of the present invention.

[0011] As used herein, "(meth)acrylic" refers to at least one of acrylic and its corresponding methacrylic. The same applies to other similar expressions such as "(meth)acrylate." "(Poly)" refers to both cases with and without the "poly" prefix. A numerical range of "A or greater" means a range exceeding A and A. A numerical range of "A or less" means a range of A and less than A. In the numerical ranges described in stages in this specification, the upper or lower limit of a certain numerical range can be arbitrarily combined with the upper or lower limit of a numerical range of another stage. In the numerical ranges described in this specification, the upper or lower limit of the numerical range may be replaced with the value shown in the examples. The materials exemplified in this specification may be used alone or in combination of two or more. "A or B" may include either A or B, or may include both. When multiple substances corresponding to each component are present in a composition, the content of each component refers to the total amount of those multiple substances present in the composition unless otherwise specified. "Physiological saline" refers to a 0.9% by mass aqueous solution of sodium chloride. "Sieve" refers to a test sieve (metal mesh sieve) specified in JIS Z 8801-1:2019. Unless otherwise specified, the examples and comparative examples were carried out in an environment of 1 atmosphere, room temperature, and normal humidity, and the various parameters disclosed herein were also measured in the same environment, with the temperature of the various samples being room temperature. "1 atmosphere" is 101,325 Pa, "room temperature" is 25°C, and "room humidity" is 50% RH. The term "process" includes not only independent processes but also processes that cannot be clearly distinguished from other processes as long as the intended effect of the process is achieved. The phrase "A contains B" includes all forms in which A contains B, and includes forms in which A contains only B (forms in which A consists only of B) as well as forms in which B constitutes only a part of A.

[0012] In the water-absorbent resin particles according to the present embodiment, the water-absorption speed (hereinafter referred to as "water-absorption speed A") based on a Vortex method using 2.0 g of the water-absorbent resin particles and 100 g of physiological saline in a container having a volume of 150 mL is 70 seconds or more, and the average aspect ratio is 1.37 or less.

[0013] The water absorption rate based on the conventional Vortex method, established as a JIS standard in 1996, was useful in the water-absorbent resin industry, where technological developments were diverse, as a simple indicator that allowed easy comparison of resins produced by various manufacturing methods. However, as further advances in manufacturing methods have led to higher water absorption capacities or the addition of various modifiers to impart water absorption properties, this indicator has sometimes been found to be insufficient for applications such as absorbent articles (e.g., diapers). In the conventional Vortex method, 2.0 g of water-absorbent resin particles and 50 g of physiological saline are used in a 100 mL container. In the conventional measurement method, particles with a relatively fast water absorption rate swell sufficiently, but the measurement ends when particles with a slow water absorption rate have not yet swelled sufficiently. Therefore, this method is not sufficient to evaluate the water absorption rate of the entire particles that make up the water-absorbent resin particles. In contrast to this, the present inventors have found that by measuring the water absorption rate of water-absorbent resin particles under conditions in which the relative amount of saline to the water-absorbent resin particles is greater than that in the conventional Vortex method, and using 2.0 g of water-absorbent resin particles and 100 g of saline in a container with a volume of 150 mL, the water absorption performance of the entire particles constituting the water-absorbent resin particles (water-absorbent resin particle group) can be easily evaluated, and therefore adjusting the water absorption rate A of the water-absorbent resin particles is effective for improving the permeation rate in absorbent articles. Furthermore, the present inventors have found that adjusting the average aspect ratio of the water-absorbent resin particles is effective for suppressing backflow under load in absorbent articles.

[0014] The water-absorbent resin particles according to this embodiment can provide an absorbent article having an excellent permeation rate (fast permeation rate) and suppressed backflow under load. The water-absorbent resin particles according to this embodiment can provide a permeation rate of, for example, less than 298 seconds (permeation time: preferably, 297 seconds or less, 290 seconds or less, 280 seconds or less, 270 seconds or less, etc.) in the evaluation described in the examples described later. The water-absorbent resin particles according to this embodiment can provide a backflow amount of, for example, less than 59 g (preferably, 58 g or less, 55 g or less, 50 g or less, 45 g or less, 40 g or less, etc.) in the evaluation described in the examples described later.

[0015] The reason why an excellent permeation rate is obtained in an absorbent article and backflow under load is suppressed is presumed to be as follows. However, the reason for obtaining such an effect is not limited to the following. That is, when a liquid to be absorbed comes into contact with the water-absorbent resin particles of an absorbent article, if the particles immediately start absorbing a large amount of liquid, gel blocking (a phenomenon in which voids existing between water-absorbent resin particles are filled with water-absorbent resin particles that have become gel-like due to water absorption, thereby reducing the permeability of the liquid) may occur, making it difficult for the liquid to diffuse within the absorbent article and resulting in a decrease in the permeation rate. On the other hand, the water-absorbent resin particles according to this embodiment have a slow water absorption rate A (long water absorption time), and therefore are likely to suppress the occurrence of gel blocking when a liquid to be absorbed comes into contact with the water-absorbent resin particles of the absorbent article, thereby achieving an excellent permeation rate in the absorbent article. Furthermore, if the average aspect ratio of the water-absorbent resin particles is small, when pressure is applied to the water-absorbent resin particles that have become gel-like due to water absorption, the contact area between an object applying pressure to the gel and the gel that has deformed due to the load applied thereto becomes smaller. In other words, since the area over which the load is applied to the gel (gel-like water-absorbent resin particles) is reduced, the liquid held in the gel is less likely to be released from the gel, thereby suppressing backflow under load in the absorbent article.

[0016] From the viewpoint of obtaining an excellent permeation rate in the absorbent article, the water absorption rate A is 70 seconds or more. From the viewpoint of easily obtaining an excellent permeation rate in the absorbent article or from the viewpoint of adjusting the permeation rate in the absorbent article, the water absorption rate A may be in the following ranges. The water absorption rate A may be 75 seconds or more, 80 seconds or more, 85 seconds or more, 90 seconds or more, 92 seconds or more, 95 seconds or more, 100 seconds or more, 103 seconds or more, 105 seconds or more, 110 seconds or more, 115 seconds or more, 120 seconds or more, 122 seconds or more, 125 seconds or more, 130 seconds or more, 150 seconds or more, 180 seconds or more, 200 seconds or more, or 220 seconds or more. The water absorption rate A may be 1800 seconds or less, 1500 seconds or less, 1200 seconds or less, 1100 seconds or less, 1000 seconds or less, 800 seconds or less, 500 seconds or less, 300 seconds or less, 250 seconds or less, 220 seconds or less, 200 seconds or less, 180 seconds or less, 150 seconds or less, 130 seconds or less, 125 seconds or less, 122 seconds or less, 120 seconds or less, 115 seconds or less, 110 seconds or less, 105 seconds or less, 103 seconds or less, 100 seconds or less, 95 seconds or less, or 92 seconds or less. From these viewpoints, the water absorption rate A may be 70 to 1800 seconds, 70 to 1500 seconds, 70 to 1200 seconds, 70 to 1100 seconds, 70 to 250 seconds, 70 to 150 seconds, 90 to 1500 seconds, 90 to 1100 seconds, 90 to 250 seconds, 90 to 150 seconds, 100 to 1500 seconds, 100 to 1100 seconds, 100 to 250 seconds, 100 to 150 seconds, 120 to 1500 seconds, 120 to 1100 seconds, 120 to 250 seconds, or 120 to 150 seconds.

[0017] The water absorption rate A is obtained based on the conventional Vortex method (Vortex method in accordance with Japanese Industrial Standards JIS K 7224 (1996)), except that the volume of the container in the conventional Vortex method was changed from 100 mL to 150 mL, the amount of saline used was changed from 50 g to 100 g, and the measurement endpoint was changed from the point at which the vortex disappeared and the liquid surface became flat to the point at which the gel on the liquid surface stopped rotating (i.e., the liquid surface stopped). The water absorption rate A can be obtained by the method described in the Examples below. 2.0 g of water-absorbent resin particles are added to 100 g of saline stirred at 600 rpm in a 150 mL container, and the water absorption rate A is calculated as the time [seconds] from the addition of the water-absorbent resin particles to the time at which the liquid surface stops. The water absorption rate A is measured at 25°C. The water absorption rate A can be adjusted by the particle size of the water-absorbent resin particles; the type or content of a polymerization initiator or a crosslinking agent used to obtain the water-absorbent resin particles; the constituent material or content of a coating portion of the second particles described below; the type or content of inorganic particles contained in the water-absorbent resin particles; the mixing ratio of the first particles and the second particles described below; and the like.

[0018] The average aspect ratio of the water-absorbent resin particles according to this embodiment is 1.37 or less from the viewpoint of suppressing backflow under load in an absorbent article. The average aspect ratio may be in the following ranges from the viewpoint of easily suppressing backflow under load in an absorbent article or from the viewpoint of adjusting the amount of backflow under load in an absorbent article. The average aspect ratio may be 1.35 or less, 1.34 or less, 1.33 or less, 1.32 or less, 1.31 or less, or 1.30 or less. The average aspect ratio may be 1.00 or more, 1.05 or more, 1.10 or more, 1.15 or more, 1.20 or more, 1.25 or more, 1.26 or more, 1.27 or more, 1.28 or more, 1.29 or more, or 1.30 or more. From these viewpoints, the average aspect ratio may be 1.00 to 1.37, 1.00 to 1.35, 1.00 to 1.32, 1.00 to 1.30, 1.20 to 1.37, 1.20 to 1.35, 1.20 to 1.32, 1.20 to 1.30, 1.25 to 1.37, 1.25 to 1.35, 1.25 to 1.32, 1.25 to 1.30, 1.28 to 1.37, 1.28 to 1.35, 1.28 to 1.32, or 1.28 to 1.30.

[0019] The average aspect ratio of the water-absorbent resin particles according to this embodiment is the average value of the ratio of the major axis to the minor axis (major axis / minor axis) in a photograph obtained by photographing the water-absorbent resin particles. The major axis is the maximum diameter of the water-absorbent resin particle, and the minor axis is the longest diameter of the water-absorbent resin particle that is perpendicular to the major axis. The average aspect ratio can be obtained by the method described in the Examples below, and is obtained by measuring the major axis and minor axis of 50 particles in a photograph obtained by photographing 0.1 g of water-absorbent resin particles. The average aspect ratio is the average aspect ratio at 25°C. The average aspect ratio can be adjusted by the particle diameter of the water-absorbent resin particles; the constituent material or content of the coating portion of the second particle described below; the viscosity of the aqueous solution of the monomer used to obtain the particles; the mixing of multiple types of particles, etc.

[0020] The water retention capacity of the water-absorbent resin particles for physiological saline may be in the following ranges, from the viewpoint of easily achieving a superior permeation rate in an absorbent article or more easily suppressing backflow under load in an absorbent article: The water retention capacity may be 10 g / g or more, 15 g / g or more, 20 g / g or more, 25 g / g or more, more than 25 g / g, 28 g / g or more, 30 g / g or more, more than 30 g / g, 32 g / g or more, 35 g / g or more, more than 35 g / g, 38 g / g or more, 40 g / g or more, more than 40 g / g, 42 g / g or more, 43 g / g or more, 44 g / g or more, or 45 g / g or more. The water retention may be 80 g / g or less, 75 g / g or less, 70 g / g or less, 65 g / g or less, 60 g / g or less, 55 g / g or less, 50 g / g or less, 45 g / g or less, 44 g / g or less, 43 g / g or less, 42 g / g or less, 40 g / g or less, less than 40 g / g, or 38 g / g or less. From these viewpoints, the water retention capacity may be 10 to 80 g / g, 10 to 55 g / g, 10 to 45 g / g, 10 to 40 g / g, 25 to 80 g / g, 25 to 55 g / g, 25 to 45 g / g, 25 to 40 g / g, 30 to 80 g / g, 30 to 70 g / g, 30 to 55 g / g, 30 to 45 g / g, 30 to 40 g / g, 40 to 80 g / g, 40 to 55 g / g, or 40 to 45 g / g. The water retention capacity can be obtained by the method described in the Examples below. The water retention capacity is measured at 25°C. The water retention capacity can be adjusted by the type or content of the polymerization initiator or crosslinking agent used to obtain the water-absorbent resin particles; the mixing ratio of the first particles and the second particles described below; the degree of neutralization of the acidic groups of the polymer particles, or the composition of the monomer units.

[0021] The median particle diameter (25°C) of the water-absorbent resin particles according to this embodiment may be in the following ranges from the viewpoint of easily adjusting the water absorption rate A and the aspect ratio. The median particle diameter may be 100 μm or more, 150 μm or more, 200 μm or more, 250 μm or more, 300 μm or more, 330 μm or more, 340 μm or more, 350 μm or more, 360 μm or more, 370 μm or more, or 380 μm or more. The median particle diameter may be 800 μm or less, 700 μm or less, 600 μm or less, 500 μm or less, 450 μm or less, 420 μm or less, 400 μm or less, 390 μm or less, 380 μm or less, 370 μm or less, 360 μm or less, 350 μm or less, or 340 μm or less. From these viewpoints, the median particle size may be 100 to 800 μm, 100 to 600 μm, 100 to 400 μm, 200 to 800 μm, 200 to 600 μm, 200 to 400 μm, 300 to 800 μm, 300 to 600 μm, or 300 to 400 μm. The median particle size may be a particle size based on mass. The median particle size can be obtained by the method described in the Examples below.

[0022] The water-absorbent resin particles according to this embodiment may contain polymer particles or may be polymer particles.

[0023] As the water-absorbent resin particles according to the present embodiment, a water-absorbent resin particle group containing a plurality of particles can be used. By mixing a plurality of particles, the water absorption rate A and the aspect ratio can be easily adjusted.

[0024] The water-absorbent resin particles according to the present embodiment may comprise at least one particle selected from the group consisting of first particles not having a coating portion covering at least a portion of their surface, and second particles (coated resin particles) having a coating portion covering at least a portion of their surface. The water-absorbent resin particles according to the present embodiment may be composed only of first particles, may be composed only of second particles, or may comprise first particles and second particles. The water-absorbent resin particles according to the present embodiment may comprise a plurality of first particles, or may comprise a plurality of second particles. The shape of the water-absorbent resin particles, first particles, or second particles according to the present embodiment may be, for example, substantially spherical, crushed, or granular, or may be an aggregate shape of primary particles having these shapes.

[0025] The first particles may be polymer particles. The second particles may have polymer particles and a coating portion that coats at least a portion (partially or entirely) of the polymer particles. When the water-absorbent resin particles according to the present embodiment include first particles and second particles, the polymer particles of the first particles and the polymer particles of the second particles may be of the same type or different types.

[0026] The use of the first particles and the second particles in absorbent articles tends to suppress gel blocking, thereby enabling the absorbent article to achieve a superior permeation rate. Specifically, the coating of the second particles prevents contact between the liquid to be absorbed and the polymer particles, suppressing water absorption (i.e., the second particles do not gel immediately). This prevents the voids between the particles from filling for a predetermined period of time, ensuring liquid permeability and facilitating sufficient diffusion of the liquid within the absorbent article. The time period during which gel blocking can be suppressed can be adjusted by appropriately changing the type or amount of material constituting the coating of the second particles; the type of polymer particles constituting the second particles; the type of first particles; the mixing ratio of the first particles and the second particles, etc.

[0027] The ratio of the second particles to the total of the first particles and the second particles may be in the following ranges from the viewpoint of easily obtaining an excellent permeation rate in the absorbent article, easily suppressing backflow under load in the absorbent article, or easily adjusting the water absorption rate A of the water-absorbent resin particles. The ratio of the second particles may be more than 0% by mass, 5% by mass or more, 10% by mass or more, 15% by mass or more, 18% by mass or more, 20% by mass or more, 22% by mass or more, 25% by mass or more, 28% by mass or more, or 30% by mass or more. The proportion of the second particles is less than 100 mass%, 95 mass% or less, 90 mass% or less, 85 mass% or less, 80 mass% or less, 75 mass% or less, 70 mass% or less, 65 mass% or less, 60 mass% or less, 55 mass% or less , 50% by mass or less, 45% by mass or less, 40% by mass or less, 35% by mass or less, 30% by mass or less, 25% by mass or less, 20% by mass or less, 18% by mass or less, 15% by mass or less, or 10% by mass or less. From these viewpoints, the ratio of the second particles is more than 0% by mass and less than 100% by mass, more than 0% by mass and less than 50% by mass, more than 0% by mass and less than 40% by mass, more than 0% by mass and less than 30% by mass, more than 0% by mass and less than 25% by mass, 10% by mass or more and less than 100% by mass, 5 to 95% by mass, 5 to 80% by mass, 5 to 60% by mass, 5 to 50% by mass, 5 to 40% by mass, 5 to 30% by mass, 5 to 25% by mass, 10 to 60% by mass, 10 to 5 ... 0% by mass, 10-40% by mass, 10-30% by mass, 10-25% by mass, 20% by mass or more and less than 100% by mass, 20-50% by mass, 20-40% by mass, 20-30% by mass, 20-25% by mass, 2 It may be 5% by mass or more and less than 100% by mass, 25-50% by mass, 25-40% by mass, 25-30% by mass, 30% by mass or more and less than 100% by mass, 30-50% by mass, or 30-40% by mass.

[0028] The polymer particles may be obtained by polymerizing a monomer containing an ethylenically unsaturated monomer (a compound having an ethylenically unsaturated bond), or may be obtained by polymerizing only an ethylenically unsaturated monomer, and may have an ethylenically unsaturated monomer as a monomer unit (a monomer unit derived from an ethylenically unsaturated monomer), or may have only an ethylenically unsaturated monomer as a monomer unit. Examples of the method for polymerizing a monomer to obtain the polymer particles include reverse phase suspension polymerization, aqueous solution polymerization, bulk polymerization, precipitation polymerization, and gas phase polymerization.

[0029] The ethylenically unsaturated monomer is a compound having at least one carbon-carbon double bond in the molecule and having radical polymerizability, and may be a compound having one carbon-carbon double bond in the molecule and having radical polymerizability. The ethylenically unsaturated monomer may be a water-soluble ethylenically unsaturated monomer (an ethylenically unsaturated monomer having a solubility of 1.0 g or more in 100 g of ion-exchanged water at 25°C). The solubility of the water-soluble ethylenically unsaturated monomer in 100 g of ion-exchanged water at 25°C may be 5.0 g or more, 10 g or more, 50 g or more, or 100 g or more. Examples of the ethylenically unsaturated monomer include (meth)acrylic acid and salts thereof, 2-(meth)acrylamido-2-methylpropanesulfonic acid and salts thereof, (meth)acrylamide, N,N-dimethyl(meth)acrylamide, 2-hydroxyethyl(meth)acrylate, N-methylol(meth)acrylamide, polyethylene glycol mono(meth)acrylate, N,N-diethylaminoethyl(meth)acrylate, N,N-diethylaminopropyl(meth)acrylate, and diethylaminopropyl(meth)acrylamide. When the ethylenically unsaturated monomer has an amino group, the amino group may be quaternized. The ethylenically unsaturated monomer may be used alone or in combination of two or more.

[0030] When an ethylenically unsaturated monomer is used, the ratio of the ethylenically unsaturated monomer to the total amount of the aqueous solution (the aqueous solution containing the ethylenically unsaturated monomer) may be 10 to 60% by mass. From the viewpoint of obtaining an aqueous solution with a suitable viscosity and facilitating adjustment of the aspect ratio of particles obtained by polymerization, the ratio of the ethylenically unsaturated monomer to the total amount of the aqueous solution is preferably 20 to 50% by mass, and more preferably 30 to 50% by mass.

[0031] When the ethylenically unsaturated monomer has an acidic group (e.g., a carboxy group), the acidic group may be neutralized with an alkaline neutralizing agent before use in the polymerization reaction. From the viewpoint of obtaining an aqueous solution with a suitable viscosity and facilitating adjustment of the aspect ratio, or from the viewpoint of facilitating adjustment of the water absorption properties (water absorption rate A, water retention capacity, etc.; the same applies to the "water absorption properties" below), the degree of neutralization of the ethylenically unsaturated monomer may be 10 to 100 mol %, preferably 50 to 90 mol %, and more preferably 60 to 80 mol %, of the acidic group (e.g., a carboxy group) in the ethylenically unsaturated monomer.

[0032] From the viewpoint of easily adjusting the water absorption characteristics, the ethylenically unsaturated monomer may include at least one compound selected from the group consisting of (meth)acrylic acid, (meth)acrylates, (meth)acrylamide, and N,N-dimethylacrylamide, or may include at least one compound selected from the group consisting of (meth)acrylic acid, (meth)acrylates, and (meth)acrylamide.

[0033] Monomers other than the above-mentioned ethylenically unsaturated monomers may be used as monomers for obtaining polymer particles. Such monomers may be used, for example, by mixing with an aqueous solution containing the ethylenically unsaturated monomer. The proportion of the amount of the ethylenically unsaturated monomer used may be 70 to 100 mol%, 80 to 100 mol%, 90 to 100 mol%, 95 to 100 mol%, or 99.5 to 100 mol% relative to the total amount of monomers (total amount of monomers for obtaining polymer particles). The proportion of the total amount of (meth)acrylic acid and (meth)acrylate salts may be 70 to 100 mol%, 80 to 100 mol%, 90 to 100 mol%, 95 to 100 mol%, or 99.5 to 100 mol% relative to the total amount of monomers (total amount of monomers for obtaining polymer particles).

[0034] The polymer particles may be crosslinked polymer particles having a crosslinked structure. Crosslinking may occur due to self-crosslinking during polymerization, but crosslinking may be promoted by using an internal crosslinking agent. The use of an internal crosslinking agent makes it easier to control the water absorption characteristics. The polymer particles may be obtained by polymerizing an ethylenically unsaturated monomer in the presence of an internal crosslinking agent.

[0035] Examples of the internal crosslinking agent include compounds having two or more reactive functional groups (e.g., polymerizable unsaturated groups) that are reactive with the functional groups of the monomer. Examples of the internal crosslinking agent include di- or tri(meth)acrylic acid esters of polyols (polyethylene glycol diacrylate, etc.), unsaturated polyesters obtained by reacting polyols with unsaturated acids, glycidyl group-containing compounds ((poly)ethylene glycol diglycidyl ether, (poly)propylene glycol diglycidyl ether, (poly)glycerin diglycidyl ether, etc.), bisacrylamides, di- or tri(meth)acrylic acid esters obtained by reacting polyepoxides with (meth)acrylic acid, and compounds of polyisocyanates and (meth)acrylic acid hydrides. Examples of the internal crosslinking agent include di(meth)acrylic acid carbamyl esters obtained by reacting hydroxyethyl with allylated starch, allylated cellulose, diallyl phthalate, N,N',N"-triallyl isocyanurate, divinylbenzene, pentaerythritol, ethylenediamine, and polyethyleneimine. From the viewpoint of easy adjustment of water absorption properties, the internal crosslinking agent may contain at least one selected from the group consisting of (poly)ethylene glycol diglycidyl ether, (poly)propylene glycol diglycidyl ether, and (poly)glycerin diglycidyl ether.

[0036] The amount of the internal crosslinking agent can be adjusted as appropriate to adjust the water absorption properties, and may be, for example, within the following ranges per mole of ethylenically unsaturated monomer (e.g., 1 mole of (meth)acrylic acid and its salts in total): The amount of the internal crosslinking agent may be 0.001 mmol or more, 0.005 mmol or more, 0.010 mmol or more, 0.015 mmol or more, 0.020 mmol or more, 0.025 mmol or more, 0.030 mmol or more, 0.035 mmol or more, 0.040 mmol or more, 0.045 mmol or more, or 0.050 mmol or more. The amount of internal crosslinker may be 0.300 mmol or less, 0.200 mmol or less, 0.100 mmol or less, 0.090 mmol or less, 0.080 mmol or less, 0.070 mmol or less, 0.060 mmol or less, 0.055 mmol or less, 0.050 mmol or less, 0.045 mmol or less, or 0.040 mmol or less. From these viewpoints, the amount of the internal crosslinking agent may be 0.001 to 0.300 mmol, 0.001 to 0.100 mmol, 0.001 to 0.080 mmol, 0.001 to 0.045 mmol, 0.030 to 0.300 mmol, 0.030 to 0.100 mmol, 0.030 to 0.080 mmol, 0.030 to 0.045 mmol, 0.045 to 0.300 mmol, 0.045 to 0.100 mmol, or 0.045 to 0.080 mmol.

[0037] The crosslink density near the surface of the polymer particles may be increased (surface crosslinking may be performed). The water absorption properties can be easily adjusted by surface crosslinking. Specifically, the water absorption properties can be adjusted by adjusting the crosslink density near the surface of the polymer particles depending on the moisture content of the polymer particles subjected to surface crosslinking, the type or amount of the surface crosslinking agent used in the surface crosslinking, etc.

[0038] Examples of the surface cross-linking agent include a compound having a reactive functional group that is reactive with the functional group of the monomer unit of the polymer particle, or a precursor thereof. The surface cross-linking agent may be a compound having two or more of the reactive functional groups, or may be a compound that generates two or more of the reactive functional groups during or before the surface cross-linking step. Examples of such a compound having a reactive functional group include a glycidyl group-containing compound, an oxetane group-containing compound, a hydroxyl group-containing compound, and a carbonate compound.

[0039] The polymer particles may contain components such as gel stabilizers, metal chelating agents, and flow improvers (lubricants). These components may be located inside the polymer particles, on the surface of the polymer particles, or both. The polymer particles may contain inorganic particles (e.g., amorphous silica) as these components. When inorganic particles are present on the surface of the polymer particles, the water absorption rate can be easily adjusted to be slow by reducing the proportion of inorganic particles.

[0040] The water absorption rate A of the polymer particles of the first particles or the second particles may be in the following ranges, from the viewpoint of easily achieving an excellent penetration rate in an absorbent article: The water absorption rate A may be 30 seconds or more, 50 seconds or more, 70 seconds or more, 75 seconds or more, 80 seconds or more, 85 seconds or more, 90 seconds or more, 92 seconds or more, 95 seconds or more, 100 seconds or more, 103 seconds or more, 105 seconds or more, 110 seconds or more, 115 seconds or more, 120 seconds or more, 122 seconds or more, 125 seconds or more, 130 seconds or more, 150 seconds or more, 180 seconds or more, 200 seconds or more, or 220 seconds or more. The water absorption rate A may be 1800 seconds or less, 1500 seconds or less, 1200 seconds or less, 1100 seconds or less, 1000 seconds or less, 800 seconds or less, 500 seconds or less, 300 seconds or less, 250 seconds or less, 220 seconds or less, 200 seconds or less, 180 seconds or less, 150 seconds or less, 130 seconds or less, 125 seconds or less, 122 seconds or less, 120 seconds or less, 115 seconds or less, 110 seconds or less, 105 seconds or less, 103 seconds or less, 100 seconds or less, 95 seconds or less, or 92 seconds or less. From these viewpoints, the water absorption rate A may be 30 to 1800 seconds, 50 to 1800 seconds, 70 to 1800 seconds, 70 to 1500 seconds, 70 to 1200 seconds, 70 to 1100 seconds, 70 to 250 seconds, 70 to 150 seconds, 90 to 1500 seconds, 90 to 1100 seconds, 90 to 250 seconds, 90 to 150 seconds, 100 to 1500 seconds, 100 to 1100 seconds, 100 to 250 seconds, 100 to 150 seconds, 120 to 1500 seconds, 120 to 1100 seconds, 120 to 250 seconds, or 120 to 150 seconds.

[0041] The average aspect ratio of the polymer particles of the first particles or the second particles may be in the following ranges, from the viewpoint of easily suppressing reversion under load in an absorbent article. The average aspect ratio may be 1.50 or less, 1.45 or less, 1.40 or less, 1.37 or less, 1.35 or less, 1.34 or less, 1.33 or less, 1.32 or less, 1.31 or less, or 1.30 or less. The average aspect ratio may be 1.00 or more, 1.05 or more, 1.10 or more, 1.15 or more, 1.20 or more, 1.25 or more, 1.26 or more, 1.27 or more, 1.28 or more, 1.29 or more, or 1.30 or more. From these viewpoints, the average aspect ratio may be 1.00 to 1.50, 1.00 to 1.45, 1.00 to 1.40, 1.00 to 1.37, 1.00 to 1.35, 1.00 to 1.32, 1.00 to 1.30, 1.20 to 1.37, 1.20 to 1.35, 1.20 to 1.32, 1.20 to 1.30, 1.25 to 1.37, 1.25 to 1.35, 1.25 to 1.32, 1.25 to 1.30, 1.28 to 1.37, 1.28 to 1.35, 1.28 to 1.32, or 1.28 to 1.30.

[0042] The water retention capacity of the polymer particles of the second particles in physiological saline may be in the following ranges, from the viewpoint of easily achieving a superior permeation rate in an absorbent article or more easily suppressing backflow under load in an absorbent article: The water retention capacity may be 10 g / g or more, 15 g / g or more, 20 g / g or more, 25 g / g or more, more than 25 g / g, 28 g / g or more, 30 g / g or more, more than 30 g / g, 32 g / g or more, 35 g / g or more, more than 35 g / g, 38 g / g or more, 40 g / g or more, more than 40 g / g, 42 g / g or more, or 43 g / g or more. The water retention capacity may be 80 g / g or less, 75 g / g or less, 70 g / g or less, 65 g / g or less, 60 g / g or less, 55 g / g or less, 50 g / g or less, 45 g / g or less, 43 g / g or less, or 42 g / g or less. From these viewpoints, the water retention capacity may be 10 to 80 g / g, 10 to 55 g / g, 10 to 45 g / g, 10 to 40 g / g, 25 to 80 g / g, 25 to 55 g / g, 25 to 45 g / g, 25 to 40 g / g, 30 to 80 g / g, 30 to 55 g / g, 30 to 45 g / g, 30 to 40 g / g, 40 to 80 g / g, 40 to 55 g / g, or 40 to 45 g / g. The water retention capacity can be obtained by the method described in the examples below. The water retention capacity is measured at 25° C. The water retention capacity can be adjusted by the type or content of a polymerization initiator or a crosslinking agent used to obtain the water-absorbent resin particles; the degree of neutralization of acidic groups of the polymer particles; or the composition of the monomer units.

[0043] The coating portion of the second particle covers at least a portion of the polymer particle, and can cover at least a portion of the surface of the polymer particle. The coating portion may be a layered coating layer. The coating layer may have a single-layer structure or a multi-layer structure having two or more layers.

[0044] The coating portion contains a coating material and can be obtained by bringing the coating material into contact with the polymer particles. The coating material may contain a polymer component. The coating material may be water-soluble or poorly water-soluble. The coating material may contain a water-soluble component or poorly water-soluble component. "Water-soluble" means a solubility of 1 g or more (1 to 150 g, 5 to 150 g, 10 to 150 g, etc.) in 100 g of ion-exchanged water at 25°C. "Poorly water-soluble" means a solubility of less than 1 g in 100 g of ion-exchanged water at 25°C.

[0045] The water-soluble component may contain a compound having a hydrophilic group. Examples of the hydrophilic group include an anionic group, a cationic group, an amphoteric group, and a nonionic group. Examples of the anionic group include a carboxyl group, a sulfonic acid group, and a phosphate group. Examples of the cationic group include an amino group, an imino group, and a quaternary ammonium group. Examples of the amphoteric group include a carbobetaine group, a sulfobetaine group, and a phosphobetaine group. Examples of the nonionic group include a hydroxyl group, an amide group, a pyrrolidone group, a lactam group, an alkoxy group, and a (poly)oxyalkylene group.

[0046] The water-soluble component may contain at least one selected from the group consisting of compounds having a hydroxyl group, compounds having an amide group, compounds having a quaternary ammonium group, and compounds having a (poly)oxyalkylene group. Examples of compounds having a hydroxyl group include polyvinyl alcohol, monosaccharides, polysaccharides, phenyl diglycol, etc. Examples of compounds having an amide group include polyacrylamide, polyvinylpyrrolidone, etc. Examples of compounds having a quaternary ammonium group include ammonium chloride, etc. Examples of compounds having a (poly)oxyalkylene group include polyalkylene oxides (e.g., polyethylene oxide), polyalkylene glycols (e.g., polyethylene glycol), etc.

[0047] Examples of poorly water-soluble components include polyoxyalkylene alkyl ethers such as polyoxyethylene oleyl ether, polyoxyethylene lauryl ether, and polyoxyethylene stearyl ether; polyesters such as polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, and polybutylene naphthalate; polyamides such as nylon (e.g., nylon 6 and nylon 66); polyolefins such as polyurethane, polyethylene, polypropylene, polyisoprene, ethylene / butene copolymer, and ethylene / propylene copolymer; poly-α-methylstyrene, and syndiotactic polystyrene. Examples of the poorly water-soluble component include polystyrenes such as polyhexamethylene carbonate; polycarbonates such as polyhexamethylene carbonate; poly(meth)acrylates such as trimethylolpropane tri(meth)acrylate, pentaerythritol tri(meth)acrylate, and dipentaerythritol hexa(meth)acrylate; poly(alkyl(meth)acrylates) such as polymethyl(meth)acrylate; polyacetals such as polyoxymethylene, polyacetaldehyde, polypropionaldehyde, and polybutyraldehyde; halogenated vinyl polymers such as polyvinyl chloride, polyvinyl acetate, and polyvinyl fluoride; polyvinylidene fluoride; and polysiloxanes. The poorly water-soluble component may be acid-modified. The poorly water-soluble component may be acid-modified with, for example, an acid anhydride (maleic anhydride, succinic anhydride, phthalic anhydride, etc.).

[0048] The coating material may include a polymer having an ethylenically unsaturated monomer as a monomer unit (a polymer having a monomer unit derived from an ethylenically unsaturated monomer). The ethylenically unsaturated monomer is a compound having at least one carbon-carbon double bond in the molecule and having radical polymerizability, and may be a compound having one carbon-carbon double bond in the molecule and having radical polymerizability. Examples of the ethylenically unsaturated monomer include (meth)acrylic acid and salts thereof, (meth)acrylic acid esters (methyl (meth)acrylate, ethyl (meth)acrylate, 2-hydroxyethyl (meth)acrylate, 2-(diethylamino)ethyl (meth)acrylate, 2-(diethylamino)propyl (meth)acrylate, etc.), (meth)acrylamide-based monomers ((meth)acrylamide, N-isopropyl(meth)acrylamide, 2-(meth)acrylamido-2-methylpropanesulfonic acid and salts thereof, N,N-dimethyl(meth)acrylamide, N-methylol(meth)acrylamide, diethylaminopropyl(meth)acrylamide, etc.), polyethylene glycol mono(meth)acrylate, styrene, α-alkylstyrene, butadiene, etc. From the viewpoint of easily adjusting the water absorption rate of the second particles to be slow, the ethylenically unsaturated monomer may include a (meth)acrylic compound (a compound having a (meth)acryloyl group) and may include at least one selected from the group consisting of (meth)acrylic acid and salts thereof.

[0049] Examples of coating materials that can be used include chain polymerization products such as poly(meth)acrylic acid, poly(meth)acrylamide, polyvinyl alcohol, polyalkylene oxide, and polyalkylene glycol; and step-growth polymerization products such as urethane resins (e.g., condensates of polyol and polyisocyanate), phenolic resins (e.g., condensates of phenolic compounds and aldehydes), polyesters, polyamides, and polycarbonates.

[0050] From the viewpoint of easily adjusting the water absorption rate of the second particles to be slow, the coating material may contain at least one selected from the group consisting of polyvinyl alcohol, polyacrylamide, polyalkylene oxide, polyalkylene glycol, phenyl diglycol, polyoxyalkylene alkyl ether, polyalkyl(meth)acrylate, polyamide, polyolefin, and olefin / ethylenically unsaturated monomer copolymer (copolymer having an olefin and an ethylenically unsaturated monomer as monomer units), or may contain at least one selected from the group consisting of polyvinyl alcohol, polyalkylene glycol, polyalkyl(meth)acrylate, and olefin / ethylenically unsaturated monomer copolymer. The polymer component contained in the coating material may be composed of at least one selected from the group consisting of polyvinyl alcohol, polyacrylamide, polyalkylene oxide, polyalkylene glycol, phenyl diglycol, polyoxyalkylene alkyl ether, polyalkyl(meth)acrylate, polyamide, polyolefin, and copolymer of an olefin and an ethylenically unsaturated monomer, from the viewpoint of easily adjusting the water absorption rate of the second particles to be slow, or may be composed of at least one selected from the group consisting of polyvinyl alcohol, polyalkylene glycol, polyalkyl(meth)acrylate, and copolymer of an olefin and an ethylenically unsaturated monomer. From the viewpoint of easily adjusting the water absorption rate of the second particles to be slow, the coating material may contain copolymer of an olefin and an ethylenically unsaturated monomer and polyalkylene glycol, or copolymer of an olefin and an ethylenically unsaturated monomer and polyethylene glycol. The polymer component contained in the coating material may be composed only of an olefin / ethylenically unsaturated monomer copolymer and polyalkylene glycol, or may be composed only of an olefin / ethylenically unsaturated monomer copolymer and polyethylene glycol, from the viewpoint of making it easier to adjust the water absorption rate of the second particles to be slow.

[0051] When the ethylenically unsaturated monomer constituting the polymer component contained in the coating material has an acidic group (e.g., a carboxy group), the acidic group may be neutralized. In this case, the degree of neutralization of the ethylenically unsaturated monomer may be greater than 0 mol% to 100 mol%, 5 to 100 mol%, 10 to 100 mol%, 20 to 100 mol%, 30 to 100 mol%, 40 to 100 mol%, or 50 to 100 mol% of the acidic groups in the ethylenically unsaturated monomer. Increasing the degree of neutralization facilitates stabilization of the coating liquid (e.g., emulsion) described below, making it easier to obtain second particles having a uniform coating portion. From this perspective, the degree of neutralization of the ethylenically unsaturated monomer may be 10 to 100 mol%, 20 to 100 mol%, 30 to 100 mol%, 40 to 100 mol%, or 50 to 100 mol%. Examples of the ethylenically unsaturated monomers whose acidic groups have been neutralized in this way include sodium acrylate and ammonium acrylate with a degree of neutralization of 5 to 100 mol %.

[0052] The olefin, which is a monomer unit of a polyolefin, or the olefin, which is a monomer unit of an olefin / ethylenically unsaturated monomer copolymer, may contain at least one selected from the group consisting of ethylene, propylene, and butene, or may contain ethylene, from the viewpoint of easily adjusting the water absorption rate of the second particles to be slow. The ethylenically unsaturated monomer in the olefin / ethylenically unsaturated monomer copolymer may contain the ethylenically unsaturated monomer listed as the constituent material of the polymer particles described above, may contain a (meth)acrylic compound, or may contain at least one selected from the group consisting of (meth)acrylic acid and salts thereof, from the viewpoint of easily adjusting the water absorption rate of the second particles to be slow.

[0053] In an olefin / ethylenically unsaturated monomer copolymer, the water absorption rate of the second particles can be easily adjusted by adjusting the ratio of olefin monomer units (monomer units derived from an olefin) to ethylenically unsaturated monomer units (monomer units derived from an ethylenically unsaturated monomer). That is, by increasing the ratio of highly hydrophobic monomer units, the water absorption rate of the second particles can be adjusted to be slower. For example, in an olefin / ethylenically unsaturated monomer copolymer, the ratio of olefin monomer units may be 72.0 to 98.0 mol% or 80.0 to 97.0 mol%.

[0054] The coating portion may contain one type of coating material alone, or may contain two or more types of coating materials. The main component of the coating material is the component of the coating material that is used in the largest amount (by mass). When one type of coating material alone is used, that one type of coating material is the main component of the coating material. When two or more types of coating materials are used in combination, the component of the multiple coating materials that is used in the largest amount is the main component of the coating material. From the viewpoint of easily adjusting the water absorption rate of the second particles to be slow, the main component of the coating material may be an olefin / ethylenically unsaturated monomer copolymer or an ethylene / ethylenically unsaturated monomer copolymer, and the ethylenically unsaturated monomer in these copolymers may include a (meth)acrylic compound and may include at least one selected from the group consisting of (meth)acrylic acid and salts thereof.

[0055] The proportion of the main component of the coating material may be in the following ranges based on the total mass of the coating material, from the viewpoint of easily adjusting the water absorption rate of the second particles to be slow. The proportion of the main component of the coating material may be 50% by mass or more, more than 50% by mass, 60% by mass or more, 70% by mass or more, 80% by mass or more, 85% by mass or more, 90% by mass or more, 91% by mass or more, 92% by mass or more, 95% by mass or more, or 96% by mass or more. The proportion of the main component of the coating material may be 100% by mass or less, less than 100% by mass, 99% by mass or less, 98% by mass or less, 97% by mass or less, 96% by mass or less, 95% by mass or less, 92% by mass or less, or 91% by mass or less. From these viewpoints, the proportion of the main component of the coating material may be 50 to 100% by mass, 70% by mass or more but less than 100% by mass, 80 to 98% by mass, or 85 to 97% by mass. When two or more main coating materials are present (when the amounts (mass) of the two or more coating materials used are the same and each of them is used in the largest amount), the content of the main component of the coating material is the total amount of those two or more coating materials.

[0056] The proportion of the coating portion in the second particles may be in the following ranges relative to 100 parts by mass of the polymer particles in the second particles, from the viewpoint of easily adjusting the water absorption rate A of the water-absorbent resin particles to be slow: The proportion of the coating portion may be 0.10 parts by mass or more, 0.50 parts by mass or more, 1.00 parts by mass or more, 2.00 parts by mass or more, 3.00 parts by mass or more, 4.00 parts by mass or more, 5.00 parts by mass or more, 6.00 parts by mass or more, 7.00 parts by mass or more, 8.00 parts by mass or more, 9.00 parts by mass or more, 10.00 parts by mass or more, or 11.00 parts by mass or more. The ratio of the coating portion is 50.00 parts by mass or less, 40.00 parts by mass or less, 30.00 parts by mass or less, 25.00 parts by mass or less, 20.00 parts by mass or less, 18.00 parts by mass or less, 15.00 parts by mass or less, 12. 00 parts by mass or less, 11.00 parts by mass or less, 10.00 parts by mass or less, 9.00 parts by mass or less, 8.00 parts by mass or less, 7.00 parts by mass or less, 6.00 parts by mass or less, or 5.00 parts by mass or less. From these viewpoints, the coating portion may be 0.10 to 50.00 parts by mass, 0.10 to 15.00 parts by mass, 0.10 to 10.00 parts by mass, 4.00 to 50.00 parts by mass, 4.00 to 15.00 parts by mass, 4.00 to 10.00 parts by mass, 6.00 to 50.00 parts by mass, 6.00 to 15.00 parts by mass, 6.00 to 10.00 parts by mass, 10.00 to 50.00 parts by mass, or 10.00 to 15.00 parts by mass.

[0057] The ratio of the main component of the coating material in the second particles (when two or more types of main component coating materials are present, the ratio of the total amount thereof) may be in the following range relative to 100 parts by mass of the polymer particles in the second particles, from the viewpoint of easily adjusting the water absorption rate A of the water-absorbent resin particles to be slow. The ratio of the main component of the coating material may be 0.10 parts by mass or more, 0.50 parts by mass or more, 1.00 parts by mass or more, 2.00 parts by mass or more, 3.00 parts by mass or more, 4.00 parts by mass or more, 5.00 parts by mass or more, 6.00 parts by mass or more, 7.00 parts by mass or more, 8.00 parts by mass or more, 9.00 parts by mass or more, or 10.00 parts by mass or more. The proportion of the main component of the coating material may be 50.00 parts by mass or less, 40.00 parts by mass or less, 30.00 parts by mass or less, 25.00 parts by mass or less, 20.00 parts by mass or less, 18.00 parts by mass or less, 15.00 parts by mass or less, 12.00 parts by mass or less, 11.00 parts by mass or less, 10.00 parts by mass or less, 9.00 parts by mass or less, 8.00 parts by mass or less, 7.00 parts by mass or less, 6.00 parts by mass or less, or 5.00 parts by mass or less. From these viewpoints, the proportion of the main component of the coating material may be 0.10 to 50.00 parts by mass, 0.10 to 15.00 parts by mass, 0.10 to 10.00 parts by mass, 0.10 to 8.00 parts by mass, 4.00 to 50.00 parts by mass, 4.00 to 15.00 parts by mass, 4.00 to 10.00 parts by mass, 4.00 to 8.00 parts by mass, 6.00 to 50.00 parts by mass, 6.00 to 15.00 parts by mass, 6.00 to 10.00 parts by mass, 6.00 to 8.00 parts by mass, 8.00 to 50.00 parts by mass, 8.00 to 15.00 parts by mass, or 8.00 to 10.00 parts by mass.

[0058] The method for producing the second particles includes a coating step of contacting the polymer particles (subject to be coated) with a coating material to obtain the second particles having a coating portion that coats at least a portion of the polymer particles. The ambient temperature of the coating step may be 20 to 150°C, 20 to 140°C, or 20 to 130°C.

[0059] In the coating step, a coating material can be brought into contact with polymer particles present in an atmosphere or a dispersion medium such as air, an inert gas (e.g., nitrogen gas), a mixed gas thereof, etc. The coating material brought into contact with the polymer particles may be a dry coating material, a liquid or gel-like coating material (e.g., a molten coating material), or a coating material in a coating liquid containing the coating material and a liquid medium (e.g., water) (a solution of the coating material, a dispersion of the coating material (e.g., an emulsion), etc.).

[0060] The coating material may be brought into contact with the polymer particles by bringing a coating liquid containing the coating material and a liquid medium (e.g., water) into contact with the polymer particles. This makes it easier to obtain a coating portion with a uniform thickness. The liquid medium may be a solvent or a dispersion medium. The coating liquid containing the coating material and the liquid medium may be obtained by dissolving the coating material in a solvent or by dispersing the coating material in a dispersion medium.

[0061] Examples of solvents or dispersion media include water, hydrophilic compounds, and hydrocarbon compounds. A single solvent or dispersion medium may be used, or a mixture of two or more solvents or dispersion media (e.g., a mixture of water and a hydrophilic compound) may be used. A hydrophilic compound is a compound that dissolves substantially uniformly in water. Examples of hydrophilic compounds include alcohols such as methanol and isopropyl alcohol; glycols such as ethylene glycol; cellosolves such as methyl cellosolve and ethyl cellosolve; ketones such as acetone and methyl ethyl ketone; esters such as ethyl acetate; and ethers such as tetrahydrofuran. Examples of hydrocarbon compounds include chain aliphatic hydrocarbons such as n-hexane, n-heptane, 2-methylhexane, 3-methylhexane, 2,3-dimethylpentane, 3-ethylpentane, and n-octane; alicyclic hydrocarbons such as cyclohexane, methylcyclohexane, cyclopentane, methylcyclopentane, trans-1,2-dimethylcyclopentane, cis-1,3-dimethylcyclopentane, and trans-1,3-dimethylcyclopentane; and aromatic hydrocarbons such as toluene and xylene.

[0062] The method for bringing the polymer particles and the coating material into contact with each other in the coating step is not particularly limited, and various methods can be used. For example, in the coating step, the polymer particles and the coating material (e.g., the coating material of a coating liquid) may be mixed in a container such as a flask to bring the polymer particles and the coating material into contact with each other. In the coating step, the coating material may be brought into contact with the polymer particles in an airflow, the polymer particles blown up by the airflow, or the polymer particles and the coating material may be brought into contact with each other by supplying the coating material to a fluidized bed of polymer particles. The gas constituting the airflow may be air, an inert gas (e.g., nitrogen gas), a mixture thereof, or the like. In the coating step, the coating material may be brought into contact with the polymer particles in a state of being stirred using a stirring means. The stirring means may be a stirring means having a stirring blade, or may be an air supply means capable of supplying an airflow different from the airflow used to blow the polymer particles up. In the coating step, the polymer particles may be stirred only by the airflow used to blow the polymer particles up, and a stirring means different from the airflow used to blow the polymer particles up may not be used. In the coating step, the polymer particles and the coating material may be brought into contact with each other by spraying the coating liquid onto the polymer particles. The nozzle for spraying the coating liquid is not particularly limited, and for example, a tangential spray or a two-fluid nozzle (e.g., a nozzle that sprays the coating liquid and an inert gas) may be used. In the coating step, the polymer particles and the coating material may be brought into contact with each other using various devices such as a tumbling granulator, an agitation granulator, or a fluidized bed granulator.

[0063] In the coating step, the polymer particles and the coating material may be brought into contact with each other while supplying an airflow from the vertically downward side into the internal space of an apparatus (e.g., a fluidized bed granulator) having an internal space in which the polymer particles are accommodated. The internal space is a space into which the coating material is supplied and into which the polymer particles and the coating material can come into contact with each other. The airflow may be supplied vertically from the vertically downward side, or may be supplied vertically from the vertically downward side in a direction intersecting the vertical direction.

[0064] 1 is a schematic cross-sectional view showing a processing apparatus for bringing polymer particles and a coating material into contact with each other. The processing apparatus 1 includes a processing section 10 and an air supply section (not shown) that supplies an air flow G to the processing section 10.

[0065] The processing unit 10 is a substantially cylindrical member extending in the vertical direction (height direction of the processing device 1; the same applies below). The processing unit 10 has a substantially cylindrical internal space 11 in which polymer particles 20 are accommodated. 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. Space 11a, space 11b, and space 11c are continuous from below to above in the vertical direction, thereby constituting the internal space 11. Space 11b tapers upward in the vertical direction.

[0066] An air inlet 12 is formed in the center of the bottom surface of the processing section 10. The air inlet 12 is connected to the air supply section, and an air flow G is supplied to the internal space 11 through the air inlet 12. The ambient temperature of the internal space 11 can be adjusted by adjusting the temperature of the air flow G.

[0067] The processing section 10 includes a stirring means 13 (e.g., a rotor blade) for stirring the polymer particles 20. The stirring means 13 includes a disk-shaped disk portion 13a whose center protrudes vertically upward and a support portion 13b that supports the disk portion 13a. The disk portion 13a extends horizontally below the vertical direction in the space 11a, and an annular opening 14 is formed in the space 11a at a position closer to the outer periphery of the disk portion 13a. The disk portion 13a has an annular upper surface around the central protrusion, and horizontally extending blades (not shown: for example, three blades extending radially from the center of the disk portion 13a and arranged at equal intervals) are formed on the upper surface. The support portion 13b extends vertically and extends from the outside of the internal space 11 into the interior of the internal space 11 through the air inlet 12. The support portion 13b rotates in the horizontal direction (rotates around the axis of the support portion 13b), thereby allowing the disk portion 13a to rotate in the horizontal direction.

[0068] The air flow G supplied from the air intake port 12 to the internal space 11 is blocked by the disk portion 13a of the stirring means 13 and supplied to the outer periphery of the processing section 10, and then supplied vertically upward through the opening 14 located on the outer periphery of the space 11a.

[0069] The polymer particles 20 are deposited on the upper surface of the disk portion 13a, and are agitated as the disk portion 13a rotates, and are blown vertically upward by the airflow G supplied through the opening 14. After being blown vertically upward by the airflow G, the polymer particles 20 fall vertically downward by gravity. An exhaust filter (e.g., a bag filter) 15 is disposed in the space 11c of the internal space 11, and the airflow G supplied to the internal space 11 is discharged from the exhaust filter 15 to the outside of the processing apparatus 1.

[0070] The processing unit 10 includes a liquid supply unit 16 (e.g., a nozzle) that supplies the coating liquid L to the space 11a. The liquid supply unit 16 supplies the coating liquid L from the outer periphery to the inner periphery of the space 11a, vertically above the disk unit 13a of the stirring means 13. The coating liquid L supplied from the liquid supply unit 16 is blown up by the airflow G and contacts the polymer particles 20 that are descending due to gravity. The polymer particles 20 and the coating material of the coating liquid L come into contact with each other to form a coating portion, thereby obtaining second particles (coated resin particles). Volatile components (water, etc.) in the coating liquid L volatilize due to the airflow G, heat in the internal space 11, etc.

[0071] The processing section 10 is equipped with a thermometer (not shown) that measures the atmospheric temperature of the internal space 11. The thermometer may be disposed at a position in the internal space 11 where the polymer particles 20 and the coating liquid L come into contact with each other, or may be disposed near the liquid supply section 16 at a height equivalent to that of the liquid supply section 16.

[0072] The configuration of the processing apparatus for bringing the polymer particles and the coating material into contact with each other is not limited to the configuration of the processing apparatus 1. For example, the processing apparatus may be provided with a stirring means for supplying an airflow from the side of the processing section 10 to stir the polymer particles 20, instead of or in addition to the stirring means of the processing apparatus 1. The processing apparatus may be provided with a means for heating the processing section 10 (e.g., a heater) as a means for adjusting the atmospheric temperature of the internal space 11 of the processing section 10 in the processing apparatus 1.

[0073] The absorbent body according to the present embodiment contains the water-absorbent resin particles according to the present embodiment. The absorbent body according to the present embodiment may contain the water-absorbent resin particles according to the present embodiment and fibrous material, and may be, for example, a mixture containing the water-absorbent resin particles and fibrous material. The configuration of the absorbent body may be, for example, a configuration in which the water-absorbent resin particles and fibrous material are uniformly mixed, a configuration in which the water-absorbent resin particles are sandwiched between fibrous material formed in a sheet shape or layer shape, or another configuration.

[0074] Examples of fibrous materials include finely ground wood pulp, cotton, cotton linters, rayon, cellulosic fibers such as cellulose acetate, synthetic fibers such as polyamide, polyester, and polyolefin, and mixtures of these fibers. One type of fibrous material may be used alone, or two or more types may be used in combination. Hydrophilic fibers can be used as the fibrous material.

[0075] In order to improve the shape retention of the absorbent body before and during use, an adhesive binder may be added to the fibrous material to bond the fibers together. Examples of adhesive binders include heat-fusible synthetic fibers, hot-melt adhesives, adhesive emulsions, etc. The adhesive binders may be used alone or in combination of two or more.

[0076] Examples of heat-fusible synthetic fibers include full-melt binders such as polyethylene, polypropylene, and ethylene-propylene copolymers; and non-full-melt binders having a side-by-side or core-sheath structure of polypropylene and polyethylene. In the above-mentioned non-full-melt binders, only the polyethylene portion can be heat-fused.

[0077] Examples of hot melt adhesives include mixtures of a base polymer such as ethylene-vinyl acetate copolymer, styrene-isoprene-styrene block copolymer, styrene-butadiene-styrene block copolymer, styrene-ethylene-butylene-styrene block copolymer, styrene-ethylene-propylene-styrene block copolymer, and amorphous polypropylene with a tackifier, a plasticizer, an antioxidant, and the like.

[0078] The adhesive emulsion may be, for example, a polymer of at least one monomer selected from the group consisting of methyl methacrylate, styrene, acrylonitrile, 2-ethylhexyl acrylate, butyl acrylate, butadiene, ethylene, and vinyl acetate.

[0079] The absorbent body according to the present embodiment may contain additives such as inorganic powder (e.g., amorphous silica), deodorants, antibacterial agents, pigments, dyes, fragrances, adhesives, etc. When the water-absorbent resin particles contain these various additives, the absorbent body may contain various additives different from the additives in the water-absorbent resin particles.

[0080] The absorbent body according to this embodiment may be, for example, in the form of a sheet, and the thickness of the absorbent body (for example, the thickness of the sheet-shaped absorbent body) may be 0.1 to 50 mm, 0.1 to 30 mm, 0.1 to 20 mm, or 0.3 to 15 mm.

[0081] The content of water-absorbent resin particles in the absorbent body may be within the following ranges based on the total mass of the absorbent body or the sum of the water-absorbent resin particles and fibrous material. From the viewpoint of easily obtaining sufficient water absorption properties, the content of water-absorbent resin particles may be 2% by mass or more, 10% by mass or more, 20% by mass or more, or 50% by mass or more. The content of water-absorbent resin particles is 100% by mass or less, and from the viewpoint of easily obtaining sufficient water absorption properties, it may be 80% by mass or less, 70% by mass or less, or 60% by mass or less. From these viewpoints, the content of water-absorbent resin particles may be 2 to 100% by mass, 10 to 80% by mass, 20 to 70% by mass, or 50 to 60% by mass.

[0082] The absorbent article according to the present embodiment includes the absorbent body according to the present embodiment. The absorbent article according to the present embodiment may include the absorbent body according to the present embodiment and other components. Examples of components other than the absorbent body include a core wrap (e.g., a core wrap that maintains the shape of the absorbent body), a liquid-permeable sheet (e.g., a liquid-permeable sheet arranged on the outermost side of the side where the liquid to be absorbed penetrates), and a liquid-impermeable sheet (e.g., a liquid-impermeable sheet arranged on the outermost side of the side opposite the side where the liquid to be absorbed penetrates). The absorbent article according to the present embodiment may include the absorbent body according to the present embodiment, a core wrap, and at least one of a liquid-permeable sheet and a liquid-impermeable sheet. Examples of absorbent articles include diapers (e.g., disposable diapers), toilet training pants, incontinence pads, sanitary materials (sanitary napkins, tampons, etc.), sweat pads, pet sheets, portable toilet components, and animal waste disposal materials.

[0083] Fig. 2 is a schematic cross-sectional view showing an example of an absorbent article. The absorbent article 100 shown in Fig. 2 comprises an absorbent body 110, core wraps 120a and 120b, a liquid-permeable sheet 130, and a liquid-impermeable sheet 140. In the absorbent article 100, the liquid-impermeable sheet 140, the core wrap 120b, the absorbent body 110, the core wrap 120a, and the liquid-permeable sheet 130 are layered in this order. In Fig. 2, some parts are shown as if there are gaps between the members, but the members may be in close contact with each other without any gaps.

[0084] The absorbent body 110 includes water-absorbent resin particles 110a according to this embodiment and a fiber layer 110b containing fibrous material. The water-absorbent resin particles 110a are dispersed in the fiber layer 110b.

[0085] The core wrap 120a is disposed on one side of the absorbent body 110 (the upper side of the absorbent body 110 in FIG. 2 ) while in contact with the absorbent body 110. The core wrap 120b is disposed on the other side of the absorbent body 110 (the lower side of the absorbent body 110 in FIG. 2 ) while in contact with the absorbent body 110. The absorbent body 110 is disposed between the core wraps 120a and 120b. Examples of the core wraps 120a and 120b include tissue, nonwoven fabric, woven fabric, synthetic resin films with liquid-permeable holes, and net-like sheets with mesh. The core wraps 120a and 120b have, for example, a main surface of the same size as the absorbent body 110.

[0086] The liquid-permeable sheet 130 is disposed on the outermost side of the absorbent article 100, on the side into which the liquid to be absorbed penetrates. The liquid-permeable sheet 130 is disposed on the core wrap 120a in a state of contact with the core wrap 120a. Examples of the liquid-permeable sheet 130 include nonwoven fabrics and porous sheets made of synthetic resins such as polyethylene, polypropylene, polyester, and polyamide. The liquid-impermeable sheet 140 is disposed on the outermost side of the absorbent article 100, opposite the liquid-permeable sheet 130. The liquid-impermeable sheet 140 is disposed below the core wrap 120b in a state of contact with the core wrap 120b. Examples of the liquid-impermeable sheet 140 include sheets made of synthetic resins such as polyethylene, polypropylene, and polyvinyl chloride, and sheets made of composite materials of these synthetic resins and nonwoven fabric. The liquid-permeable sheet 130 and the liquid-impermeable sheet 140 have, for example, a main surface that is wider than the main surface of the absorbent body 110, and the outer edges of the liquid-permeable sheet 130 and the liquid-impermeable sheet 140 extend around the absorbent body 110 and the core wraps 120a, 120b.

[0087] The size relationships among the absorbent body 110, the core wraps 120a, 120b, the liquid-permeable sheet 130, and the liquid-impermeable sheet 140 are not particularly limited and are adjusted as appropriate depending on the intended use of the absorbent article, etc. The method for maintaining the shape of the absorbent body 110 using the core wraps 120a, 120b is not particularly limited, and the absorbent body may be wrapped with multiple core wraps as shown in Figure 2, or may be wrapped with a single core wrap.

[0088] The absorbent body may be adhered to the top sheet. When the absorbent body is sandwiched or covered by a core wrap, at least the core wrap and the top sheet may be adhered, or the core wrap and the top sheet may be adhered together and the core wrap and the absorbent body may be adhered. Examples of methods for adhering the absorbent body include a method in which a hot melt adhesive is applied to the top sheet in a striped or spiral pattern at predetermined intervals in the width direction, and a method in which a water-soluble binder such as starch, carboxymethyl cellulose, polyvinyl alcohol, polyvinylpyrrolidone, or other water-soluble polymer is used for adhesion. Furthermore, when the absorbent body contains heat-fusible synthetic fibers, a method of adhesion by heat-fusible bonding of the heat-fusible synthetic fibers may be used.

[0089] The present invention will be described in more detail below with reference to examples. However, the present invention is not limited to these examples. Unless the temperature during an experimental procedure is specified below, the experimental procedure was performed at room temperature.

[0090] <Preparation of Particle A> (Particle A1) A round-bottomed cylindrical separable flask with an inner diameter of 11 cm and a volume of 2 L was prepared, equipped with a reflux condenser, a dropping funnel, a nitrogen gas inlet tube, and a stirrer (a stirring blade with two stages of four inclined paddle blades with a blade diameter of 5 cm). 293 g of n-heptane (hydrocarbon dispersion medium) and 0.736 g of maleic anhydride-modified ethylene-propylene copolymer (polymer dispersant, manufactured by Mitsui Chemicals, Inc., product name: Hiwax 1105A) were added to the separable flask to obtain a mixture. While stirring the mixture at 300 rpm, the separable flask was immersed in an 85°C water bath and heated to 80°C to dissolve the polymer dispersant. The water bath was then removed, and the mixture was cooled to 55°C at room temperature.

[0091] Next, 92.0 g of an 80.5% by mass acrylic acid aqueous solution (acrylic acid: 1.03 mol) and a stirrer chip were placed in a 500 mL Erlenmeyer flask, and stirring was initiated. Subsequently, while cooling 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 mol% acrylic acid. Thereafter, 0.092 g of hydroxyethyl cellulose (thickener, manufactured by Sumitomo Seika Chemicals Co., Ltd., trade name: HEC AW-15F), 0.0736 g (0.272 mmol) of potassium persulfate (radical polymerization initiator), and 0.0101 g (0.0580 mmol) of ethylene glycol diglycidyl ether (internal crosslinking agent) were added, and the mixture was stirred until all components other than water were fully dissolved to prepare a first-stage monomer aqueous solution.

[0092] The first-stage aqueous monomer solution was added to the separable flask and stirred for 10 minutes. A surfactant solution was obtained by dissolving 0.736 g of sucrose stearate (surfactant, manufactured by Mitsubishi Chemical Foods Corporation, product name: Ryoto Sugar Ester S-370, HLB: 3) in 6.62 g of n-heptane under heating. 7.356 g of this surfactant solution was added to the separable flask to obtain a reaction solution. The rotation speed was then changed to 550 rpm, and the atmosphere inside the separable flask was thoroughly purged with nitrogen while stirring the reaction solution. The separable flask was then immersed in a 70°C water bath to heat the reaction solution. As the polymerization reaction progressed, heating was continued for another 10 minutes from the point at which the internal temperature reached its maximum (maximum temperature of 82°C), yielding a first-stage polymerization product.

[0093] Next, 128.8 g of an 80.5% by weight aqueous acrylic acid solution (1.44 mol of acrylic acid) and a stirrer tip were placed in a 500 mL Erlenmeyer flask, and stirring was initiated. Subsequently, while cooling externally, 154.2 g of a 28% by weight aqueous sodium hydroxide solution was added dropwise to the Erlenmeyer flask to neutralize the 75 mol% acrylic acid. Subsequently, 0.1030 g (0.3810 mmol) of potassium persulfate (radical polymerization initiator), 0.0116 g (0.0666 mmol) of ethylene glycol diglycidyl ether (internal crosslinking agent), and 6.48 g of ion-exchanged water were added, and the mixture was stirred until all components other than water were fully dissolved, to prepare a second-stage aqueous monomer solution.

[0094] The rotation speed was changed to 1000 rpm, and the first-stage polymerization product was cooled to 25°C while stirring. The entire second-stage aqueous monomer solution was then added to the first-stage polymerization product to obtain a reaction mixture. The atmosphere in the separable flask was thoroughly purged with nitrogen while stirring the reaction mixture. The separable flask was then immersed in a 70°C water bath to heat the reaction mixture. As the polymerization reaction progressed, heating was continued for an additional 5 minutes from the point at which the internal temperature reached its maximum (maximum temperature of 82°C), thereby obtaining a second-stage polymerization product (a slurry of polymer particles before surface crosslinking).

[0095] After the second-stage polymerization, the second-stage polymerization product was heated in an oil bath at 125°C, and 252 g of water was extracted from the system by azeotropic distillation of n-heptane and water while refluxing n-heptane. Subsequently, the degree of immersion of the separable flask in the oil bath was adjusted so that the internal temperature was 83°C. 0.0884 g (0.5075 mmol) of ethylene glycol diglycidyl ether was added as a surface cross-linking agent, and the mixture was maintained at 83°C for 2 hours to obtain a slurry of surface-cross-linked polymer particles.

[0096] Thereafter, the slurry of the surface-crosslinked polymer particles was heated in an oil bath at 125°C, and the n-heptane was evaporated and dried to obtain a dried product. This dried product was passed through a sieve with an opening of 850 μm, and the particles that passed through the sieve were collected to obtain 210.1 g of particles A1 (particles in a state where no coating portion was formed) in the form of agglomerated spherical particles. Further, the same operation was repeated to obtain a total of 500.0 g or more of particles A1.

[0097] (Particles A2) The particles A1 were classified using a sieve with a mesh size of 250 μm, and the particles that did not pass through the sieve were collected to obtain 500.0 g or more of particles A2 (particles without a coating portion formed: particle diameter 250 to 850 μm).

[0098] (Particles A3) Particles A1 were mixed with 0.2 mass% (based on the total amount of particles A1) of amorphous silica (Toxil NP-S, hydrophilic, manufactured by Oriental Silicas Corporation) to obtain 500.0 g or more of particles A3 (particles without a coating portion formed thereon).

[0099] (Particles A4) 211.1 g of particles A4 (particles in a state in which no coating portion was formed) was obtained by performing the same procedure as for particles A1, except that the amount of ethylene glycol diglycidyl ether (internal crosslinking agent) used during the preparation of the first-stage monomer aqueous solution was changed to 0.00534 g (0.0306 mmol). Further, by performing the same procedure, a total of 500.0 g or more of particles A4 was obtained.

[0100] (Particles A5) Particles A4 were classified using a sieve with a mesh size of 250 μm, and particles that did not pass through the sieve were collected to obtain 500.0 g or more of particles A5 (particles without a coating portion formed: particle diameter 250 to 850 μm).

[0101] (Particles A6) Particles A4 were mixed with 0.5% by mass (based on the total amount of particles A4) of amorphous silica (Toxil NP-S, hydrophilic, manufactured by Oriental Silicas Corporation) to obtain 500.0 g or more of particles A6 (particles without a coating portion formed thereon).

[0102] (Particles A7) 227.2 g of particles A7 (particles in a state where no coating portion was formed) was obtained in the same manner as for particles A1, except that the radical polymerization initiator added during the preparation of the first-stage aqueous monomer solution was changed to 0.092 g (0.339 mmol) of 2,2′-azobis(2-amidinopropane) dihydrochloride and 0.028 g (0.102 mmol) of potassium persulfate, the amount of ethylene glycol diglycidyl ether added during the preparation of the first-stage aqueous monomer solution was changed to 0.0046 g (0.026 mmol), the radical polymerization initiator added during the preparation of the second-stage aqueous monomer solution was changed to 0.129 g (0.475 mmol) of 2,2′-azobis(2-amidinopropane) dihydrochloride and 0.039 g (0.143 mmol) of potassium persulfate, and the amount of water withdrawn from the system by azeotropic distillation was changed to 226.5 g. The same procedure was repeated to obtain a total of 500.0 g or more of particles A7.

[0103] (Particles A8) Particles A7 were classified using a sieve with a mesh size of 250 μm, and particles that did not pass through the sieve were collected to obtain 500.0 g or more of particles A8 (particles without a coating portion formed: particle diameter 250 to 850 μm).

[0104] (Particles A9) Particles A7 were mixed with 0.2 mass% (based on the total amount of particles A7) of amorphous silica (Toxil NP-S, hydrophilic, manufactured by Oriental Silicas Corporation) to obtain 500.0 g or more of particles A9 (particles without a coating portion formed thereon).

[0105] (Particles A10) 236.3 g of particles A10 (particles without a coating portion formed thereon) were obtained in the same manner as particles A1, except that the radical polymerization initiator added during the preparation of the first-stage aqueous monomer solution was changed to 0.092 g (0.339 mmol) of 2,2′-azobis(2-amidinopropane) dihydrochloride and 0.028 g (0.102 mmol) of potassium persulfate, the amount of ethylene glycol diglycidyl ether added during the preparation of the first-stage aqueous monomer solution was changed to 0.0046 g (0.026 mmol), the radical polymerization initiator added during the preparation of the second-stage aqueous monomer solution was changed to 0.129 g (0.475 mmol) of 2,2′-azobis(2-amidinopropane) dihydrochloride and 0.039 g (0.143 mmol) of potassium persulfate, and the amount of water withdrawn from the system by azeotropic distillation was changed to 219.8 g. The same procedure was repeated to obtain a total of 500.0 g or more of particles A10.

[0106] (Particles A11) Particles A10 were mixed with 0.2 mass% (based on the total mass of particles A10) of amorphous silica (Toxil NP-S, Oriental Silicas Corporation, hydrophilic) to obtain 500.0 g or more of particles A11 (particles without a coating portion formed thereon).

[0107] (Particles A12) 500.0 g or more of particles A12 were obtained by collecting particles from commercially available diapers (manufactured by The Honest Company, trade name: Honest Overnight Diapers, tape type, size 4).

[0108] <Preparation of Ethylene / Acrylic Acid Copolymer Emulsion> An ice bath at 3°C ​​was prepared by adding water and ice to a plastic tray measuring 27 cm in length, 38 cm in width, and 7 cm in depth. A glass beaker with an internal volume of 1 L was placed in the ice bath, and 545.78 g of ion-exchanged water was then added to the beaker. The ice bath was placed on a magnetic stirrer, and a stirrer tip was placed in the beaker to begin stirring.

[0109] 10.55 g (0.264 mol) of sodium hydroxide (granules, manufactured by Nacalai Tesque, Inc.) was added little by little to the above beaker to prepare a 1.9% by mass aqueous sodium hydroxide solution.

[0110] A round-bottomed, cylindrical, separable flask with an inner diameter of 11 cm and an internal volume of 2 L was prepared, equipped with a reflux condenser, a thermometer, and a stirrer (a stirring blade with four inclined paddle blades and a blade diameter of 5 cm). 100 g of ethylene / acrylic acid copolymer (SK Global Chemical, Primacol 5980i) with a molar ratio of ethylene monomer to acrylic acid monomer of 10:1 was added to the separable flask. Subsequently, the entire amount of the 1.9% by mass aqueous sodium hydroxide solution was added. The beaker used to prepare the aqueous sodium hydroxide solution was then washed with 50.0 g of ion-exchanged water, and the wash water was added to the separable flask to obtain a reaction solution.

[0111] Stirring of the reaction solution was started at a stirrer rotation speed of 500 rpm, and the separable flask was immersed in an oil bath at 103° C., and the internal temperature of the separable flask was raised to 95° C. Thereafter, the internal temperature of the separable flask was maintained at 95 to 97° C. for 4 hours while appropriately adjusting the temperature of the oil bath.

[0112] Thereafter, the separable flask was removed from the oil bath and allowed to cool at room temperature until the internal temperature of the separable flask reached 35°C. The product in the separable flask was filtered through a nylon mesh with a mesh size of 108 µm. The filtrate was collected to obtain an emulsion of a partially neutralized ethylene / acrylic acid copolymer (P(E / AA), an aqueous dispersion of a partially neutralized ethylene / acrylic acid copolymer, nonvolatile content 15% by mass, degree of neutralization 90%; hereinafter referred to as "aqueous dispersion (X)").

[0113] <Preparation of Particles B> (Particles B1) In a 1 L beaker (made of polypropylene), 250.0 g of the above-mentioned aqueous dispersion (X) and 1.25 g of polyethylene glycol (manufactured by Tokyo Chemical Industry Co., Ltd., trade name: PEG6000, number average molecular weight: 7300 to 9300) were mixed as coating materials to prepare 251.25 g of a coating liquid. The contents of the coating materials in the 251.25 g of coating liquid were 37.5 g (14.9% by mass) of partially neutralized ethylene / acrylic acid copolymer (P(E / AA), degree of neutralization 90%) and 1.25 g (0.5% by mass) of polyethylene glycol.

[0114] A fluidized bed granulator having the configuration shown in Fig. 1 was prepared. 500.0 g of the above-described particles A2 were introduced into the processing section of the fluidized bed granulator. Next, while stirring the inside of the processing section with a stirring means (rotor blade, rotation speed: 250 rpm), the inlet air temperature was 130°C and the supply amount was 1.0 m ３ An airflow (air) supplied to the fluidized bed granulator at a rate of 18.0 g / min was supplied to the processing section through the air inlet. Then, the entire amount of the coating liquid was sprayed onto the particles A2 being blown up by the airflow using a tangential spray at a supply rate of 18.0 g / min, thereby bringing the particles A2 into contact with the coating material of the coating liquid. Thereafter, the particles A2 were sprayed at room temperature with a supply rate of 1.0 m ３ The airflow (air) supplied to the fluidized bed granulator at a rate of 1 / min was supplied to the processing section through an air inlet, whereby the particles were cooled until their temperature reached 50°C or less, thereby obtaining particles (Y).

[0115] 50.0 g of the particles (Y) were spread on a metal tray measuring 26 cm in length and 20 cm in width, and then covered with aluminum foil. After holes were perforated in the aluminum foil, the particles (Y) were heated for 30 minutes in a hot air dryer (FV-320, manufactured by ADVANTEC) set to 120°C, thereby obtaining 50.0 g of particles B1 (coated resin particles).

[0116] (Particles B2) 50.0 g of particles B2 (coated resin particles) were obtained in the same manner as for particles B1, except that polyethylene glycol was not used and 166.7 g of the above-mentioned aqueous dispersion (X) was used as the coating material, and the set temperature of the hot air dryer for heating particles (Y) was changed to 100° C. The content of the partially neutralized ethylene / acrylic acid copolymer (P(E / AA), neutralization degree 90%) in the 166.7 g coating liquid was 25.0 g (15.0 mass%).

[0117] (Particles B3) 50.0 g of particles B3 (coated resin particles) were obtained in the same manner as for particles B1, except that particles A2 were changed to particles A5.

[0118] (Particles B4) 400.0 g of the above-mentioned aqueous dispersion (X) and 0.75 g of polyethylene glycol (manufactured by Tokyo Chemical Industry Co., Ltd., trade name: PEG6000, number average molecular weight: 7300-9300) were mixed as coating materials to prepare 400.75 g of a coating liquid. 50.0 g of particles B4 (coated resin particles) were obtained in the same manner as for particles B1, except that particles A2 were replaced with particles A5. The coating material contents in the 400.75 g of coating liquid were 60.0 g (15.0% by mass) of partially neutralized ethylene / acrylic acid copolymer (P(E / AA), degree of neutralization 90%) and 0.75 g (0.19% by mass) of polyethylene glycol.

[0119] (Particles B5) 50.0 g of particles B5 (coated resin particles) were obtained in the same manner as particles B4, except that particles A5 were changed to particles A8 and the set temperature of the hot air dryer for heating particles (Y) was changed to 100°C.

[0120] <Preparation of Particles for Evaluation> Particles A3, A6, and A12 were prepared as particles for evaluation (water-absorbent resin particles) for Comparative Examples 1 to 3. Particles A and B described above were mixed to prepare particles for evaluation (water-absorbent resin particles) for Examples 1 to 16. The types and mixing ratios of particles A and particles B in the particles for evaluation for Examples 1 to 16 are shown in Table 1.

[0121] <Median Particle Diameter> The particle size distribution of 5 g of evaluation particles was measured using a continuous, fully automatic ultrasonic vibration sieving measuring instrument (manufactured by Seishin Enterprise Co., Ltd., product name: Robot Sifter RPS-205), JIS standard sieves with openings of 850 μm, 710 μm, 600 μm, 500 μm, 400 μm, 300 μm, 250 μm, and 150 μm, and a tray. The particle sizes remaining on the sieves were integrated in descending order of particle size, and the relationship between the sieve opening and the integrated value of the mass percentage of the particles remaining on the sieves was plotted on a logarithmic probability paper. The particle diameter corresponding to an integrated mass percentage of 50% by mass was determined as the median particle diameter (25°C) by connecting the plots on the probability paper with a straight line. The results are shown in Table 1.

[0122] <Water absorption rate A> 100 g of saline was added to a 150 mL beaker (inner diameter: 50 mm), and the saline was then maintained at 25°C in a thermostatic bath. A vortex was generated in the saline by stirring at 600 rpm using a stirring bar (8 mmφ x 30 mm, without ring). 2.0 g of the above-mentioned particles for evaluation were added to the vortex of the saline at 25°C, and simultaneously measurement was started with a stopwatch. The time when the liquid level stopped falling was defined as the end point, and the time until the end point was obtained as water absorption rate A (25°C, unit: seconds). The results are shown in Table 1.

[0123] <Average Aspect Ratio> 0.1 g of the above-mentioned evaluation particles was photographed with a microscope (manufacturer: KEYENCE, model number: VHX-5000) in a room adjusted to 25°C and 50% humidity (RH) to obtain a photograph containing 50 or more particles. Fifty particles were randomly selected from the photograph, and the maximum diameter of the 50 particles was measured as the major diameter to obtain the major diameters of the 50 particles. Furthermore, the longest diameter of the diameters perpendicular to the major diameter of the 50 particles was measured as the minor diameter to obtain the minor diameters of the 50 particles. The average value of the aspect ratios (ratio of major diameter to minor diameter (major diameter / minor diameter)) obtained from the 50 particles was obtained as the average aspect ratio (25°C). The results are shown in Table 1.

[0124] <Water Retention Capacity> The water retention capacity was measured in a room adjusted to 25°C and 50% humidity (RH). First, 500 g of physiological saline was added to a 500 mL polyethylene beaker, and then 2.0 g of the above-mentioned particles for evaluation was added little by little while rotating a stirring bar (8 mm x 30 mm, without ring) at 600 rpm using a stirrer. After the entire amount of particles for evaluation had been added, the mixture was stirred for 30 minutes to obtain a swollen gel. Subsequently, the entire amount of this swollen gel was transferred into a cotton bag (membrane No. 60, width 100 mm x length 200 mm), and the top of the cotton bag was closed with a rubber band. Next, the mixture was dehydrated for 1 minute using a centrifuge (manufactured by Kokusan Co., Ltd., product number: H-122) set to a centrifugal force of 167 G. The mass W of the cotton bag containing the swollen gel after dehydration was Ａ The same procedure was carried out without placing the particles for evaluation in the cotton bag, and the wet empty mass W Ｂ The water retention capacity of the particles for evaluation in physiological saline (25°C, unit: g / g) was then calculated using the following formula. The results are shown in Table 1. Water retention capacity = (W Ａ -W Ｂ ) / Mass of particles for evaluation

[0125] <Evaluation> (Preparation of mixing core type absorbent article) 12.0 g of the above-mentioned particles for evaluation and 8.0 g of crushed pulp (manufactured by Leonia, product name: Rayflock) were uniformly mixed by air papermaking to prepare an absorbent article measuring 40 cm x 12 cm. Next, two sheets of tissue paper (basis weight: 16 g / m) of the same size as this absorbent article were placed between the mixed particles. ２ The absorbent body was sandwiched between two sheets of paper and pressed under a load of 141 kPa for 30 seconds to produce a laminate. Next, a polyethylene air-through porous liquid-permeable sheet (basis weight: 22 g / m) of the same size as the laminate was placed on the top surface of the laminate. ２ ) was placed in the absorbent article to obtain a mixing core type absorbent article.

[0126] (Penetration Rate) A test solution was prepared by mixing 9,866.0 g of distilled water, 100.0 g of sodium chloride, 3.0 g of calcium chloride dihydrate, 6.0 g of magnesium chloride hexahydrate, 25.0 g of a 1 mass % Triton X solution (a mixture of Triton X-100 and water manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.), and 0.25 g of Food Blue No. 1 (for coloring).

[0127] The above-mentioned absorbent article was placed on a horizontal table in a room conditioned to 25°C and 50% humidity (RH). A liquid-injection cylinder (mass: 59.80 g) with a circular opening (outer diameter 3.5 cm, inner diameter 3 cm) was placed at the center of the absorbent article, and 80 mL of the above-mentioned test liquid was injected into the cylinder from the vertical direction all at once. The time from the start of injection of the test liquid to the time the test liquid was completely absorbed into the absorbent article was measured as the first penetration time [seconds]. Immediately after the test liquid was completely absorbed into the absorbent article, the liquid-injection cylinder was removed from the absorbent article.

[0128] Next, 30 minutes after the start of the test liquid addition, the second penetration time [seconds] was measured using the same procedure as the first penetration time measurement. The third, fourth, and fifth penetration times [seconds] were measured using the same procedure. The total [seconds] of the five penetration times was calculated as the penetration rate. The results of the penetration rate are shown in Table 1.

[0129] (Returnover Amount) After the permeation rate evaluation was performed, the absorbent article was left to stand for 60 minutes in a room conditioned at 25°C and 50% RH. Subsequently, a 10 cm square filter paper (manufactured by ADVANTEC, No. 51A) whose mass (approximately 75 g) had been measured in advance was placed on the absorbent article at the test liquid application position. A weight (bottom: 10 cm x 10 cm, mass: 5.0 kg (approximately 0.7 psi, 4.8 kPa)) was then placed on the filter paper. After 5 minutes of loading, the weight and filter paper were removed. The mass of the filter paper was measured, and the mass of the test liquid absorbed by the filter paper was obtained as the returnover amount [g]. The results are shown in Table 1.

[0130]

[0131] 1... processing device, 10... processing section, 11... internal space, 11a, 11b, 11c... space, 12... air inlet, 13... stirring means, 13a... disk section, 13b... support section, 14... opening, 15... exhaust filter, 16... liquid supply section, 20... polymer particles, 100... absorbent article, 110... absorbent body, 110a... water-absorbent resin particles, 110b... fiber layer, 120a, 120b... core wrap, 130... liquid-permeable sheet, 140... liquid-impermeable sheet, G... air flow, L... coating liquid.

Claims

1. Water-absorbent resin particles having a water absorption speed of 70 seconds or more based on the Vortex method using 2.0 g of water-absorbent resin particles and 100 g of physiological saline in a container with a volume of 150 mL, and an average aspect ratio of 1.37 or less.

2. The water-absorbent resin particles according to claim 1, wherein the water absorption rate is 70 to 1500 seconds.

3. The water-absorbent resin particles according to claim 1, which have a water-retention capacity of physiological saline of 30 to 70 g / g.

4. The water-absorbent resin particles according to any one of claims 1 to 3, comprising first particles having no coating portion covering at least a part of the surface, and second particles having a coating portion covering at least a part of the surface.

5. The water-absorbent resin particles according to claim 4, wherein the proportion of the second particles to the total of the first particles and the second particles is 5 to 60% by mass.

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