Water-absorbent resin particles, absorbent article, method for producing water-absorbent resin particles, and method for increasing the permeation rate of physiological saline into an absorbent material

Abstract

Disclosed are water absorbing resin particles having a gel leakage rate of 0 to 20%. The gel leakage rate is determined by a method including: preparing 20g of swollen gel formed from water absorbing resin particles that has absorbed physiological saline, compressing the swollen gel with a weight having a mass of 2,000 g for 30 seconds, and measuring the mass W (g) of the swollen gel which leaked from through-holes in a disk during the compression; and calculating the gel leakage rate (%) using the formula: gel leakage rate (%) = (W / 20) × 100.

Description

[Technical Field]

[0001] The present invention relates to water-absorbent resin particles, absorbent articles, methods for producing water-absorbent resin particles, and methods for increasing the permeation rate of saline into an absorbent body after a load is applied in the water-absorbed state. [Background technology]

[0002] Conventionally, absorbent articles for absorbing liquids whose main component is water, such as urine, have used absorbents containing water-absorbent resin particles. For example, Patent Document 1 discloses a water-absorbent resin that has excellent absorbency in an unpressurized state, excellent water-absorbing properties under high load, and little residual monomer. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2006-199805 Summary of the Invention [Problem to be solved by the invention]

[0004] A load may be applied to an absorbent article including an absorbent body in a water-absorbed state. It is desirable that the absorbent body be able to absorb urine and the like at a high permeation rate even after a load is applied in the water-absorbed state.

[0005] One aspect of the present invention provides water-absorbent resin particles that can increase the rate at which saline solution permeates into an absorbent body after a load is applied in the water-absorbed state. [Means for solving the problem]

[0006] One aspect of the present invention relates to water-absorbent resin particles having a gel outflow ratio of 0 to 20% when measured in an environment of 25±2°C by a method including the following steps (1), (2), (3), (4), (5), (6), and (7) in this order: (1) A tubular body having an inner diameter of 5.0 cm and a bottom portion that closes one end of the tubular body and forms an opening with a diameter of 3.0 cm in the center is fixed in place with the longitudinal direction of the tubular body vertical and the bottom portion facing downwards. (2) Inside the tubular body, on the bottom, a disk having a diameter of 4.9 cm and a thickness of 3.0 mm and having a through hole formed in its center is placed with its main surface oriented horizontally. The through hole has walls inclined relative to the main surface of the disk so that the through hole is narrowest at the center in the thickness direction of the disk, and the diameter of the through hole is a maximum of 8.0 mm at both main surfaces of the disk and a minimum of 5.0 mm at the center in the thickness direction of the disk. (3) 20 g of a swollen gel formed by the water-absorbent resin particles absorbing physiological saline is placed in the tubular body and placed on the disk, and the swollen gel has a mass 30 times the mass of the water-absorbent resin particles before absorbing physiological saline. (4) A cylindrical first weight having a mass of 500 g and a diameter of 4.9 cm is placed on the swollen gel in the tubular body to compress the swollen gel for 30 seconds. (5) A second cylindrical weight having a mass of 1500 g and a diameter of 4.9 cm is placed on the first weight, thereby further compressing the swollen gel with a load of 2000 g. (6) The swollen gel is compressed with a load of 2000 g for 30 seconds, and the mass W (g) of the swollen gel that flows out from the through-holes of the disk is measured. (7) Calculate the gel outflow ratio (%) using the following formula. Gel outflow ratio (%) = (W / 20) x 100

[0007] A small gel outflow ratio means that the swollen gel formed by the water-absorbent resin particles absorbing a large amount of saline is unlikely to move when a load is applied. When the absorbent body absorbs liquid, a swollen gel is formed inside. If the movement of the swollen gel is small when a load is applied, the swollen gel in the area where the load is applied remains unmoved, and it is thought that even if saline is again introduced, it can still absorb the liquid, and the permeation rate will remain high. On the other hand, if the movement of the swollen gel is large when a load is applied, the swollen gel in the area where the load is applied moves outward from the loaded area, and it is thought that even if saline is again introduced, the amount that can be absorbed in the loaded area will be limited, and the permeation rate will be slow.

[0008] Another aspect of the present invention relates to an absorbent article comprising a liquid-impermeable sheet, an absorbent body, and a liquid-permeable sheet, the liquid-impermeable sheet, the absorbent body, and the liquid-permeable sheet being arranged in this order. The absorbent body contains the water-absorbent resin particles.

[0009] Yet another aspect of the present invention relates to a method for producing water-absorbent resin particles, which comprises selecting water-absorbent resin particles having the above-mentioned gel outflow ratio of 0 to 20%.

[0010] Yet another aspect of the present invention relates to a method for increasing the permeation rate of saline into an absorbent body containing water-absorbent resin particles after a load is applied to the absorbent body in a water-absorbent state, the method comprising reducing a gel outflow ratio of the water-absorbent resin particles. [Effects of the Invention]

[0011] According to one aspect of the present invention, there is provided a water-absorbent resin particle that can increase the rate at which saline solution permeates into an absorbent body after a load is applied in the absorbent body. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 2 is a schematic diagram showing a method for measuring the gel outflow ratio of water-absorbent resin particles. [Figure 2]FIG. 2 is a schematic diagram showing a method for measuring the gel outflow ratio of water-absorbent resin particles. [Figure 3] FIG. 2 is a schematic diagram showing a method for measuring the gel outflow ratio of water-absorbent resin particles. [Figure 4] 1 is a cross-sectional view showing one embodiment of an absorbent article. [Figure 5] FIG. 2 is a plan view showing an example of an agitating blade. [Figure 6] FIG. 2 is a schematic diagram showing a method for measuring the amount of water absorption of a water-absorbent resin particle in physiological saline under a load of 4.14 kPa. DETAILED DESCRIPTION OF THE INVENTION

[0013] Hereinafter, several embodiments of the present invention will be described in detail, but the present invention is not limited to the following embodiments.

[0014] In this specification, "acrylic" and "methacrylic" are collectively referred to as "(meth)acrylic." "Acrylate" and "methacrylate" are similarly referred to as "(meth)acrylate." "(Poly)" refers to both cases with and without the prefix "poly." In the numerical ranges described in stages in this specification, the upper or lower limit of a certain numerical range can be arbitrarily combined with the upper or lower limit of another numerical range. In the numerical ranges described in this specification, the upper or lower limit of the numerical range may be replaced with a value shown in the Examples. "A or B" means either A or B, or both A and B. "Water-soluble" means exhibiting a solubility of 5% by mass or more in water at 25°C. The materials exemplified in this specification may be used alone or in combination of two or more. When multiple substances corresponding to each component are present in the composition, the content of each component in the composition refers to the total amount of the multiple substances present in the composition, unless otherwise specified.

[0015] The water-absorbent resin particles according to this embodiment exhibit a gel outflow ratio of 0 to 20%. Figures 1, 2, and 3 are schematic diagrams showing a method for measuring the gel outflow ratio of water-absorbent resin particles. The method shown in Figures 1 to 3 includes the following steps (1), (2), (3), (4), (5), (6), and (7) in this order. The gel outflow ratio is measured in an environment of 25±2°C. <Evaluation of gel outflow ratio> (1) A tubular body 41 having an inner diameter D1 (= 5.0 cm) and a circular cross section, and a bottom 41B that closes one end of the tubular body 41 and forms an opening 41A with a diameter D2 (= 3.0 cm) in the center, is fixed in place with the longitudinal direction of the tubular body 41 vertical and the bottom 41B facing downward (Figure 1). (2) Inside the tubular body 41, on the bottom 41B, a disk 42 having a diameter of 4.9 cm and a thickness of 3.0 mm and having a through hole H formed in its center is placed with the main surface of the disk 42 oriented horizontally (FIG. 1). The through hole H has walls that are inclined with respect to the main surface of the disk 42 so that the through hole H is narrowest at the center in the thickness direction of the disk 42. The diameter of the through hole H is a maximum value d1 (= 8.0 mm) at both main surfaces of the disk 42 and a minimum value d2 (= 5.0 mm) at the center in the thickness direction of the disk 42. (3) 20 g of swollen gel 45 formed by the water-absorbent resin particles absorbing physiological saline is placed in the tubular body 41 and placed on the disk 42. The swollen gel 45 has a mass 30 times the mass of the water-absorbent resin particles before absorbing the physiological saline. (4) A tube 41 is placed on the swollen gel 45, and the tube 41 has a mass of 500 g and a diameter of 4.9 mm. cm The swollen gel 45 is compressed for 30 seconds by placing a cylindrical first weight 46 on it (FIG. 2). (5) A mass of 1500 g and a diameter of 4.9 mm are placed on the first weight 46. cm A cylindrical second weight 47 is placed on the swollen gel 45 to further compress the swollen gel 45 with a load of 2000 g (FIG. 3). (6) The mass W (g) of the swollen gel 45 that flows out of the through-hole H of the disk 42 while the swollen gel 45 is compressed with a load of 2000 g for 30 seconds is measured. (7) Calculate the gel outflow ratio (%) using the following formula. Gel outflow ratio (%) = (W / 20) x 100

[0016] The tubular body 41 may be, for example, a molded body made of acrylic resin. The tubular body 41 is fixed using a fixing device such as a clamp. Each weight is gently placed on the swollen gel 45 to prevent the swollen gel 45 from being subjected to a sudden, strong impact when dropped. The entire amount of swollen gel 45 that flows out of the through-hole H is collected within 30 seconds after the first weight 46 and the second weight 47 start applying a load of 2000 g, and its mass W (g) is measured. The flowed-out swollen gel 45 often falls. However, if some or all of the flowed-out swollen gel 45 is held hanging from the through-hole H, the mass W of the flowed-out swollen gel 45 is measured, including the swollen gel hanging below the lower main surface of the disk 42.

[0017] The gel outflow ratio of the water-absorbent resin particles may be 15% or less, 10% or less, or 5% or less. When no swollen gel 45 flows out from the through-holes H, the gel outflow ratio is 0%, which is the lower limit of the gel outflow ratio.

[0018] The water absorption capacity of the water-absorbent resin particles for saline solution under a load of 4.14 kPa (hereinafter, sometimes referred to as "water absorption capacity under load") may be 12 mL / g or more. When the water-absorbent resin particles have a water absorption capacity under load of 12 mL / g, the gel outflow ratio tends to be small. This can further increase the rate at which saline solution permeates into the absorbent body after a load is applied in the water-absorbed state. In addition, a large water absorption capacity under load is advantageous in that gel blocking is less likely to occur in absorbent articles. The water absorption capacity under load of the water-absorbent resin particles may be 14 mL / g or more, 16 mL / g or more, 18 mL / g or more, or 20 mL / g or more, or may be 35 mL / g or less, 30 mL / g or less, 28 mL / g or less, or 26 mL / g or less. The water absorption capacity under load is measured by the method described in the examples below.

[0019] The water-absorbent resin particles may have a water-retention capacity for physiological saline of 35 g / g or more, 36 g / g or more, 37 g / g or more, 38 g / g or more, 39 g / g or more, or 40 g / g or more. The water-retention capacity for physiological saline of the water-absorbent resin particles may be 50 g / g or less, 49 g / g or less, 48 ​​g / g or less, 47 g / g or less, 46 g / g or less, or 45 g / g or less. The water-retention capacity for physiological saline of the water-absorbent resin particles may be 35 to 50 g / g, 38 to 48 g / g, or 40 to 46 g / g. When the water-retention capacity is within these ranges, the permeation rate of the absorbent body tends to be increased, and liquid leakage from the absorbent body tends to be suppressed. water retention The amount is measured by the method described in the Examples below.

[0020] The shape of the water-absorbent resin particles according to the present embodiment may be substantially spherical, crushed, granular, etc. The median particle diameter of the water-absorbent resin particles according to the present embodiment may be 250 μm or more and 850 μm or less, 700 μm or less, or 600 μm or more, or 300 μm or more and 850 μm or less, 700 μm or less, or 600 μm or less. The water-absorbent resin particles according to the present embodiment may have a desired particle size distribution when obtained by the production method described below, but the particle size distribution may be adjusted by performing an operation such as particle size adjustment using classification with a sieve.

[0021] The water-absorbent resin particles according to the present embodiment may include, for example, a crosslinked polymer formed by polymerization of a monomer including an ethylenically unsaturated monomer. The crosslinked polymer has a monomer unit derived from the ethylenically unsaturated monomer.

[0022] The water-absorbent resin particles can be produced by a method including a step of polymerizing a monomer containing an ethylenically unsaturated monomer. Examples of the polymerization method include a reversed-phase suspension polymerization method, an aqueous solution polymerization method, a bulk polymerization method, and a precipitation polymerization method. From the viewpoint of ensuring good water absorption properties of the obtained water-absorbent resin particles and facilitating control of the polymerization reaction, the polymerization method may be a reversed-phase suspension polymerization method or an aqueous solution polymerization method. In the following, the reversed-phase suspension polymerization method will be described as an example of a method for polymerizing an ethylenically unsaturated monomer.

[0023] The ethylenically unsaturated monomer may be water-soluble. Examples of water-soluble ethylenically unsaturated monomers include (meth)acrylic acid and its salts, 2-(meth)acrylamido-2-methylpropanesulfonic acid and its salts, (meth)acrylamide, N,N-dimethyl(meth)acrylamide, 2-hydroxyethyl(meth)acrylate, N-methylol(meth)acrylamide, polyethylene glycol mono(meth)acrylate, N,N-diethylaminoethyl(meth)acrylate, N,N-diethylaminopropyl(meth)acrylate, and diethylaminopropyl(meth)acrylamide. When the ethylenically unsaturated monomer has an amino group, the amino group may be quaternized. The ethylenically unsaturated monomer may be used alone or in combination of two or more. The functional groups, such as carboxyl groups and amino groups, of the above-mentioned monomers can function as functional groups capable of crosslinking in the surface crosslinking step described below.

[0024] From the viewpoint of industrial availability, the ethylenically unsaturated monomer may include at least one compound selected from the group consisting of (meth)acrylic acid and its salts, acrylamide, methacrylamide, and N,N-dimethylacrylamide, or may include at least one compound selected from the group consisting of (meth)acrylic acid and its salts, and acrylamide. From the viewpoint of further improving water absorption properties, the ethylenically unsaturated monomer may include at least one compound selected from the group consisting of (meth)acrylic acid and its salts.

[0025] The ethylenically unsaturated monomer is usually preferably used as an aqueous solution. The concentration of the ethylenically unsaturated monomer in the aqueous solution containing the ethylenically unsaturated monomer (hereinafter simply referred to as "aqueous monomer solution") may be 20% by mass or more and the saturated concentration or less, 25 to 70% by mass, or 30 to 55% by mass. Examples of water used in the aqueous solution include tap water, distilled water, and ion-exchanged water.

[0026] As a monomer for obtaining water-absorbing resin particles, a monomer other than the above-mentioned ethylenically unsaturated monomer may be used. Such a monomer may be used, for example, by mixing with an aqueous solution containing the above-mentioned ethylenically unsaturated monomer. The amount of the ethylenically unsaturated monomer used may be 70 to 100 mol% based on the total amount of monomers. The proportion of (meth)acrylic acid and its salts may be 70 to 100 mol% based on the total amount of monomers.

[0027] When the ethylenically unsaturated monomer has an acid group, the aqueous monomer solution may be used after neutralizing the acid group with an alkaline neutralizing agent. The degree of neutralization of the ethylenically unsaturated monomer with the alkaline neutralizing agent may be 10 to 100 mol %, 50 to 90 mol %, or 60 to 80 mol % of the acid group in the ethylenically unsaturated monomer, from the viewpoint of increasing the osmotic pressure of the resulting water-absorbent resin particles and further improving the water absorption properties (water absorption amount, etc.). Examples of the alkaline neutralizing agent include alkali metal salts such as sodium hydroxide, sodium carbonate, sodium bicarbonate, potassium hydroxide, and potassium carbonate; and ammonia. The alkaline neutralizing agent may be used alone or in combination of two or more. The alkaline neutralizing agent may be used in the form of an aqueous solution to simplify the neutralization operation. The acid group of the ethylenically unsaturated monomer can be neutralized, for example, by adding dropwise an aqueous solution of sodium hydroxide, potassium hydroxide, or the like to the above-mentioned aqueous monomer solution and mixing them.

[0028] In the reversed-phase suspension polymerization method, an aqueous monomer solution is dispersed in a hydrocarbon dispersion medium in the presence of a surfactant, and the ethylenically unsaturated monomer can be polymerized using a radical polymerization initiator, etc. As the radical polymerization initiator, a water-soluble radical polymerization initiator can be used.

[0029] Examples of surfactants include nonionic surfactants and anionic surfactants. Examples of nonionic surfactants include sorbitan fatty acid esters, polyglycerin fatty acid esters, sucrose fatty acid esters, polyoxyethylene sorbitan fatty acid esters, polyoxyethylene glycerin fatty acid esters, sorbitol fatty acid esters, polyoxyethylene sorbitol fatty acid esters, polyoxyethylene alkyl ethers, polyoxyethylene alkylphenyl ethers, polyoxyethylene castor oil, polyoxyethylene hydrogenated castor oil, alkylaryl formaldehyde condensed polyoxyethylene ethers, polyoxyethylene polyoxypropylene block copolymers, polyoxyethylene polyoxypropyl alkyl ethers, and polyethylene glycol fatty acid esters. Examples of anionic surfactants include fatty acid salts, alkylbenzenesulfonates, alkylmethyltaurates, polyoxyethylene alkylphenyl ether sulfates, polyoxyethylene alkyl ether sulfonates, polyoxyethylene alkyl ether phosphates, and polyoxyethylene alkylallyl ether phosphates. The surfactants may be used alone or in combination of two or more.

[0030] From the viewpoints that the state of the W / O type reverse phase suspension is good, that water absorbent resin particles having a suitable particle size are easily obtained, and that the surfactant is easily available industrially, the surfactant may contain at least one compound selected from the group consisting of a sorbitan fatty acid ester, a polyglycerin fatty acid ester, and a sucrose fatty acid ester. From the viewpoint that the water absorption properties of the obtained water absorbent resin particles are easily improved, the surfactant may contain a sucrose fatty acid ester or a sucrose stearate.

[0031] The amount of surfactant used may be 0.05 to 10 parts by mass, 0.08 to 5 parts by mass, or 0.1 to 3 parts by mass per 100 parts by mass of the aqueous monomer solution, from the viewpoint of obtaining a sufficient effect relative to the amount used and from the viewpoint of economy.

[0032] In reversed-phase suspension polymerization, a polymeric dispersant may be used in combination with the surfactants described above. Examples of polymeric dispersants include maleic anhydride-modified polyethylene, maleic anhydride-modified polypropylene, maleic anhydride-modified ethylene-propylene copolymer, maleic anhydride-modified EPDM (ethylene-propylene-diene terpolymer), maleic anhydride-modified polybutadiene, maleic anhydride-ethylene copolymer, maleic anhydride-propylene copolymer, maleic anhydride-ethylene-propylene copolymer, maleic anhydride-butadiene copolymer, polyethylene, polypropylene, ethylene-propylene copolymer, oxidized polyethylene, oxidized polypropylene, oxidized ethylene-propylene copolymer, ethylene-acrylic acid copolymer, ethyl cellulose, and ethylhydroxyethyl cellulose. Polymeric dispersants may be used alone or in combination of two or more. From the viewpoint of achieving excellent dispersion stability of the monomer, the polymeric dispersant may contain at least one selected from the group consisting of maleic anhydride-modified polyethylene, maleic anhydride-modified polypropylene, maleic anhydride-modified ethylene-propylene copolymer, maleic anhydride-ethylene copolymer, maleic anhydride-propylene copolymer, maleic anhydride-ethylene-propylene copolymer, polyethylene, polypropylene, ethylene-propylene copolymer, oxidized polyethylene, oxidized polypropylene, and oxidized ethylene-propylene copolymer.

[0033] The amount of polymeric dispersant used may be 0.05 to 10 parts by mass, 0.08 to 5 parts by mass, or 0.1 to 3 parts by mass per 100 parts by mass of the aqueous monomer solution, from the viewpoint of obtaining a sufficient effect relative to the amount used and from the viewpoint of economy.

[0034] The hydrocarbon dispersion medium may contain at least one compound selected from the group consisting of chain aliphatic hydrocarbons having 6 to 8 carbon atoms and alicyclic hydrocarbons having 6 to 8 carbon atoms. Examples of hydrocarbon dispersion mediums 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 benzene, toluene, and xylene. The hydrocarbon dispersion medium may be used alone or in combination of two or more.

[0035] From the viewpoints of industrial availability and stable quality, the hydrocarbon dispersion medium may contain at least one selected from the group consisting of n-heptane and cyclohexane. From the same viewpoint, for example, commercially available Exxol Heptane (manufactured by ExxonMobil Corporation: containing 75 to 85% of n-heptane and isomeric hydrocarbons) may be used as the mixture of the hydrocarbon dispersion medium.

[0036] The amount of the hydrocarbon dispersion medium used may be 30 to 1,000 parts by mass, 40 to 500 parts by mass, or 50 to 300 parts by mass per 100 parts by mass of the aqueous monomer solution, from the viewpoint of appropriately removing the heat of polymerization and facilitating control of the polymerization temperature. When the amount of the hydrocarbon dispersion medium used is 30 parts by mass or more, control of the polymerization temperature tends to be easy. When the amount of the hydrocarbon dispersion medium used is 1,000 parts by mass or less, polymerization productivity tends to be improved, which is economical.

[0037] The radical polymerization initiator may be water-soluble. Examples of water-soluble radical polymerization initiators include persulfates such as potassium persulfate, ammonium persulfate, and sodium persulfate; peroxides such as methyl ethyl ketone peroxide, methyl isobutyl ketone peroxide, di-t-butyl peroxide, t-butyl cumyl peroxide, t-butyl peroxyacetate, t-butyl peroxyisobutyrate, t-butyl peroxypivalate, and hydrogen peroxide; 2,2'-azobis(2-amidinopropane) dihydrochloride, 2,2'-azobis[2-(N-phenylamidino)propane] dihydrochloride, and 2,2'-azobis(2-amidinopropane) dihydrochloride. Examples of the radical polymerization initiator include azo compounds such as bis[2-(N-allylamidino)propane]dihydrochloride, 2,2'-azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride, 2,2'-azobis{2-[1-(2-hydroxyethyl)-2-imidazolin-2-yl]propane}dihydrochloride, 2,2'-azobis{2-methyl-N-[1,1-bis(hydroxymethyl)-2-hydroxyethyl]propionamide}, 2,2'-azobis[2-methyl-N-(2-hydroxyethyl)-propionamide], and 4,4'-azobis(4-cyanovaleric acid). The radical polymerization initiator may be used alone or in combination of two or more. The radical polymerization initiator may be at least one selected from the group consisting of potassium persulfate, ammonium persulfate, sodium persulfate, 2,2'-azobis(2-amidinopropane) dihydrochloride, 2,2'-azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride, and 2,2'-azobis{2-[1-(2-hydroxyethyl)-2-imidazolin-2-yl]propane} dihydrochloride.

[0038] The amount of radical polymerization initiator used may be 0.00005 to 0.01 moles per mole of the ethylenically unsaturated monomer. When the amount of radical polymerization initiator used is 0.00005 moles or more, the polymerization reaction does not take a long time and is efficient. When the amount of radical polymerization initiator used is 0.01 moles or less, it is easy to prevent a rapid polymerization reaction from occurring.

[0039] The above-mentioned radical polymerization initiators can also be used as redox polymerization initiators in combination with reducing agents such as sodium sulfite, sodium hydrogen sulfite, ferrous sulfate, and L-ascorbic acid.

[0040] During the polymerization reaction, the aqueous monomer solution used for polymerization may contain a chain transfer agent, such as hypophosphites, thiols, thiolic acids, secondary alcohols, and amines.

[0041] In order to control the particle size of the water-absorbent resin particles, the aqueous monomer solution used for polymerization may contain a thickener. Examples of thickeners include hydroxyethyl cellulose, hydroxypropyl cellulose, methyl cellulose, carboxymethyl cellulose, polyacrylic acid, polyethylene glycol, polyacrylamide, polyethyleneimine, dextrin, sodium alginate, polyvinyl alcohol, polyvinylpyrrolidone, and polyethylene oxide. If the stirring speed during polymerization is the same, the higher the viscosity of the aqueous monomer solution, the larger the median particle size of the resulting particles tends to be.

[0042] During polymerization, crosslinking may occur by self-crosslinking, but crosslinking may also be performed by using an internal crosslinking agent. The use of an internal crosslinking agent makes it easy to control the water absorption properties of the water-absorbent resin particles. The internal crosslinking agent is usually added to the reaction solution during the polymerization reaction. Examples of the internal crosslinking agent include di- or tri(meth)acrylic acid esters of polyols such as ethylene glycol, propylene glycol, trimethylolpropane, glycerin, polyoxyethylene glycol, polyoxypropylene glycol, and polyglycerin; unsaturated polyesters obtained by reacting the above-mentioned polyols with unsaturated acids (maleic acid, fumaric acid, and the like); bis(meth)acrylamides such as N,N'-methylenebis(meth)acrylamide; di- or tri(meth)acrylic acid esters obtained by reacting polyepoxides with (meth)acrylic acid; di(meth)acrylic acid carbamyl esters obtained by reacting polyisocyanates (tolylene diisocyanate, hexamethylene diisocyanate, and the like) with hydroxyethyl (meth)acrylate; compounds having two or more polymerizable unsaturated groups such as allylated starch, allylated cellulose, diallyl phthalate, N,N',N"-triallyl isocyanurate, and divinylbenzene; (poly)ethylene glycol Examples of the internal crosslinking agent include polyglycidyl compounds such as ethanol diglycidyl ether, (poly)propylene glycol diglycidyl ether, (poly)glycerin diglycidyl ether, (poly)glycerin triglycidyl ether, (poly)propylene glycol polyglycidyl ether, and polyglycerol polyglycidyl ether; haloepoxy compounds such as epichlorohydrin, epibromohydrin, and α-methylepichlorohydrin; and compounds having two or more reactive functional groups, such as isocyanate compounds (2,4-tolylene diisocyanate, hexamethylene diisocyanate, etc.). The internal crosslinking agent may be used alone or in combination of two or more. The internal crosslinking agent may include a polyglycidyl compound or a diglycidyl ether compound, and may include at least one selected from the group consisting of (poly)ethylene glycol diglycidyl ether, (poly)propylene glycol diglycidyl ether, and (poly)glycerin diglycidyl ether.

[0043] The amount of the internal crosslinking agent used may be 0 mmol or more, 0.02 mmol or more, 0.03 mmol or more, 0.04 mmol or more, or 0.05 mmol or more, or may be 0.1 mol or less, per mole of the ethylenically unsaturated monomer, from the viewpoint that the water-soluble property is suppressed by moderate crosslinking of the obtained polymer, and a sufficient water absorption amount is easily obtained.

[0044] An aqueous phase containing an ethylenically unsaturated monomer, a radical polymerization initiator, and optionally an internal crosslinking agent, and an oil phase containing a hydrocarbon-based dispersant and optionally a surfactant, a polymer-based dispersant, and the like are mixed and heated under stirring to carry out reverse phase suspension polymerization in a water-in-oil system.

[0045] When performing reversed-phase suspension polymerization, an aqueous monomer solution containing an ethylenically unsaturated monomer is dispersed in a hydrocarbon dispersion medium in the presence of a surfactant (and optionally a polymeric dispersant). In this case, the surfactant, polymeric dispersant, etc. may be added either before or after the addition of the aqueous monomer solution, as long as they are added before the polymerization reaction is started.

[0046] From the viewpoint of easily reducing the amount of hydrocarbon dispersion medium remaining in an obtained water absorbent resin, after dispersing an aqueous monomer solution in a hydrocarbon dispersion medium having a polymeric dispersant dispersed therein, a surfactant may be further dispersed therein, and then polymerization may be carried out.

[0047] The reversed-phase suspension polymerization can be carried out in one stage or in multiple stages of two or more stages, and may be carried out in two or three stages from the viewpoint of increasing productivity.

[0048] When performing reversed-phase suspension polymerization in two or more stages, after performing the first-stage reversed-phase suspension polymerization, an ethylenically unsaturated monomer is added to and mixed with the reaction mixture obtained in the first-stage polymerization reaction, and reversed-phase suspension polymerization in the second and subsequent stages can be performed in the same manner as in the first stage. In the reversed-phase suspension polymerization in each stage from the second stage onward, in addition to the ethylenically unsaturated monomer, the above-mentioned radical polymerization initiator may be added within the molar ratio range of each component to the ethylenically unsaturated monomer, based on the amount of ethylenically unsaturated monomer added during the reversed-phase suspension polymerization in each stage from the second stage onward. In the reversed-phase suspension polymerization in each stage from the second stage onward, an internal crosslinking agent may be used as needed. When an internal crosslinking agent is used, the reversed-phase suspension polymerization may be performed by adding the internal crosslinking agent within the molar ratio range of each component to the ethylenically unsaturated monomer, based on the amount of ethylenically unsaturated monomer used in each stage.

[0049] The temperature of the polymerization reaction varies depending on the radical polymerization initiator used. From the viewpoints of increasing economy by rapidly progressing the polymerization and shortening the polymerization time, and also of easily removing the heat of polymerization to smoothly carry out the reaction, the temperature of the polymerization reaction may be 20 to 150°C or 40 to 120°C. The reaction time is usually 0.5 to 4 hours. The completion of the polymerization reaction can be confirmed, for example, by the cessation of the temperature rise in the reaction system. As a result, a polymer of the ethylenically unsaturated monomer is usually obtained in the form of a hydrogel polymer.

[0050] After the polymerization, a crosslinking agent may be added to the obtained hydrogel polymer and heated to perform post-polymerization crosslinking. By performing post-polymerization crosslinking, the degree of crosslinking of the hydrogel polymer can be increased, thereby further improving the water absorption properties of the water-absorbent resin particles.

[0051] Examples of crosslinking agents for crosslinking after polymerization include polyols such as ethylene glycol, propylene glycol, 1,4-butanediol, trimethylolpropane, glycerin, polyoxyethylene glycol, polyoxypropylene glycol, and polyglycerin; compounds having two or more epoxy groups such as (poly)ethylene glycol diglycidyl ether, (poly)propylene glycol diglycidyl ether, and (poly)glycerin diglycidyl ether; haloepoxy compounds such as epichlorohydrin, epibromohydrin, and α-methylepichlorohydrin; compounds having two or more isocyanate groups such as 2,4-tolylene diisocyanate and hexamethylene diisocyanate; oxazoline compounds such as 1,2-ethylenebisoxazoline; carbonate compounds such as ethylene carbonate; and hydroxyalkylamide compounds such as bis[N,N-di(β-hydroxyethyl)]adipamide. The crosslinking agent for post-polymerization crosslinking may include a polyglycidyl compound such as (poly)ethylene glycol diglycidyl ether, (poly)glycerin diglycidyl ether, (poly)glycerin triglycidyl ether, (poly)propylene glycol polyglycidyl ether, polyglycerol polyglycidyl ether, etc. These crosslinking agents may be used alone or in combination of two or more.

[0052] The amount of the crosslinking agent used for post-polymerization crosslinking may be 0 to 0.03 mol, 0 to 0.01 mol, or 0.00001 to 0.005 mol per mol of the ethylenically unsaturated monomer, from the viewpoint of allowing the resulting hydrogel polymer to be appropriately crosslinked and thereby exhibiting favorable water absorption properties.

[0053] The timing of adding the crosslinking agent for post-polymerization crosslinking may be after the polymerization of the ethylenically unsaturated monomer used in the polymerization. In the case of multi-stage polymerization, the crosslinking agent may be added after the multi-stage polymerization. Considering heat generation during and after polymerization, retention due to process delays, opening of the system when the crosslinking agent is added, and fluctuations in moisture due to the addition of water accompanying the addition of the crosslinking agent, the crosslinking agent for post-polymerization crosslinking may be added in the range of [moisture content immediately after polymerization ±3% by mass] from the viewpoint of moisture content (described below).

[0054] Subsequently, the obtained hydrogel polymer is dried to remove water. By drying, polymer particles containing a polymer of an ethylenically unsaturated monomer are obtained. Examples of drying methods include (a) a method in which the hydrogel polymer dispersed in a hydrocarbon dispersion medium is heated externally to perform azeotropic distillation and the hydrocarbon dispersion medium is refluxed to remove water, (b) a method in which the hydrogel polymer is removed by decantation and dried under reduced pressure, and (c) a method in which the hydrogel polymer is filtered through a filter and dried under reduced pressure. Method (a) may be used because of its simplicity in the production process.

[0055] The particle size of the water-absorbent resin particles can be adjusted by adjusting the rotation speed of the agitator during the polymerization reaction, or by adding a flocculant to the system after the polymerization reaction or at the beginning of drying. The particle size of the resulting water-absorbent resin particles can be increased by adding a flocculant. As the flocculant, an inorganic flocculant can be used. Examples of inorganic flocculants (e.g., powdered inorganic flocculants) include silica, zeolite, bentonite, aluminum oxide, talc, titanium dioxide, kaolin, clay, and hydrotalcite. From the viewpoint of achieving excellent flocculation effect, the flocculant may contain at least one selected from the group consisting of silica, aluminum oxide, talc, and kaolin.

[0056] In the reversed-phase suspension polymerization, the flocculant may be added by a method in which the flocculant is preliminarily dispersed in the same type of hydrocarbon dispersion medium as that used in the polymerization or in water, and then mixed under stirring into the hydrocarbon dispersion medium containing the hydrous gel polymer.

[0057] The amount of the flocculant to be added may be 0.001 to 1 part by mass, 0.005 to 0.5 part by mass, or 0.01 to 0.2 part by mass relative to 100 parts by mass of the ethylenically unsaturated monomer to be used for polymerization. When the amount of the flocculant to be added is within the above-mentioned range, water absorbent resin particles having a target particle size distribution are easily obtained.

[0058] The polymerization reaction can be carried out using various agitators equipped with agitating blades. Examples of the agitating blades that can be used include flat blades, lattice blades, paddle blades, propeller blades, anchor blades, turbine blades, Pfaudle blades, ribbon blades, full zone blades, and Max Blend blades. The flat blades have a shaft (stirring shaft) and a flat plate portion (stirring portion) arranged around the shaft. The flat plate portion may have slits or the like. When flat blades are used as agitating blades, the crosslinking of the polymer in the formed polymer particles tends to be more uniform. High crosslinking uniformity means that there are fewer locally softened portions in the water-absorbent resin particles with low crosslink density, making it easier to control the gel outflow rate to a low level while maintaining water absorption properties such as water retention capacity.

[0059] In the production of water-absorbent resin particles, crosslinking (surface crosslinking) of the surface portion of the hydrogel polymer may be carried out using a crosslinking agent in the drying step or any of the subsequent steps. By carrying out surface crosslinking, it becomes easier to control the water absorption properties of the water-absorbent resin particles. The surface crosslinking may be carried out at a timing when the hydrogel polymer has a specific water content. The timing of surface crosslinking may be when the water content of the hydrogel polymer is 5 to 50% by mass, 10 to 40% by mass, or 15 to 35% by mass. The water content (mass%) of the hydrogel polymer is calculated by the following formula. Moisture content=[Ww / (Ww+Ws)]×100 Ww: The water content of the hydrogel polymer, calculated by subtracting the amount of water discharged outside the system in the drying process from the amount of water contained in the aqueous monomer solution before polymerization in all polymerization processes, and adding the amount of water used as needed when mixing a flocculant, surface cross-linking agent, etc. Ws: The solid content calculated from the amounts of the ethylenically unsaturated monomers, crosslinking agents, initiators, and other materials that constitute the hydrogel polymer.

[0060] Examples of crosslinking agents (surface crosslinking agents) for surface crosslinking include compounds having two or more reactive functional groups. Examples of crosslinking agents include polyols such as ethylene glycol, propylene glycol, 1,4-butanediol, trimethylolpropane, glycerin, polyoxyethylene glycol, polyoxypropylene glycol, and polyglycerin; polyglycidyl compounds such as (poly)ethylene glycol diglycidyl ether, (poly)glycerin diglycidyl ether, (poly)glycerin triglycidyl ether, trimethylolpropane triglycidyl ether, (poly)propylene glycol polyglycidyl ether, and (poly)glycerol polyglycidyl ether; epichlorohydrin, epibromohydrin, and α-methylepichlorohydrin. Examples of crosslinking agents include haloepoxy compounds such as phosphorus; isocyanate compounds such as 2,4-tolylene diisocyanate and hexamethylene diisocyanate; oxetane compounds such as 3-methyl-3-oxetanemethanol, 3-ethyl-3-oxetanemethanol, 3-butyl-3-oxetanemethanol, 3-methyl-3-oxetaneethanol, 3-ethyl-3-oxetaneethanol, and 3-butyl-3-oxetaneethanol; oxazoline compounds such as 1,2-ethylenebisoxazoline; carbonate compounds such as ethylene carbonate; and hydroxyalkylamide compounds such as bis[N,N-di(β-hydroxyethyl)]adipamide. The crosslinking agents may be used alone or in combination of two or more. The crosslinking agent may contain a polyglycidyl compound, and may contain at least one compound selected from the group consisting of (poly)ethylene glycol diglycidyl ether, (poly)glycerin diglycidyl ether, (poly)glycerin triglycidyl ether, (poly)propylene glycol polyglycidyl ether, and polyglycerol polyglycidyl ether.

[0061] The amount of the surface crosslinking agent used may be 0.00001 to 0.02 mol, 0.00005 to 0.01 mol, or 0.0001 to 0.005 mol per mol of the ethylenically unsaturated monomer used in the polymerization, from the viewpoint of allowing the obtained hydrogel polymer to be appropriately crosslinked and thereby exhibiting favorable water absorption properties. When the amount of the surface crosslinking agent used is 0.00001 mol or more, the crosslink density in the surface portion of the water absorbent resin particles is sufficiently increased, and the gel strength of the water absorbent resin particles is likely to be increased. When the amount of the surface crosslinking agent used is 0.02 mol or less, the water absorption capacity of the water absorbent resin particles is likely to be increased.

[0062] After the surface cross-linking, the water and the hydrocarbon dispersion medium are distilled off by a known method, whereby polymer particles, which are a dried product of the surface-cross-linked water-absorbent resin particles, can be obtained.

[0063] The water-absorbent resin particles according to the present embodiment may be composed of polymer particles alone, but may further contain various additional components selected from, for example, a gel stabilizer, a metal chelating agent, and a flowability improver (lubricant). The additional components may be disposed inside the polymer particles, on the surface of the polymer particles, or both. The additional component may be a flowability improver (lubricant). The flowability improver may be inorganic particles. Examples of inorganic particles include silica particles such as amorphous silica.

[0064] The water-absorbent resin particles may contain a plurality of inorganic particles arranged on the surface of the polymer. For example, the inorganic particles can be arranged on the surface of the polymer particles by mixing the polymer particles with the inorganic particles. The inorganic particles may be silica particles such as amorphous silica. When the water-absorbent resin particles contain inorganic particles arranged on the surface of the polymer particles, the ratio of the inorganic particles to the mass of the polymer particles may be 0.2% by mass or more, 0.5% by mass or more, 1.0% by mass or more, or 1.5% by mass or more, and may be 5.0% by mass or less, or 3.5% by mass or less. The inorganic particles here are usually very small compared to the size of the polymer particles. For example, the average particle diameter of the inorganic particles may be 0.1 to 50 μm, 0.5 to 30 μm, or 1 to 20 μm. The average particle diameter here may be a value measured by dynamic light scattering or laser diffraction / scattering.

[0065] The absorbent body according to one embodiment contains the water-absorbent resin particles according to the present embodiment. The absorbent body according to the present embodiment can contain fibrous material, for example, a mixture containing water-absorbent resin particles and fibrous material. The absorbent body may have 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 or layer shape, or another configuration.

[0066] 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. The fibrous materials may be used alone or in combination of two or more. Hydrophilic fibers may be used as the fibrous material.

[0067] The mass proportion of the water-absorbent resin particles in the absorbent body may be 2 to 100 mass %, 10 to 80 mass %, or 20 to 60 mass % relative to the total mass of the water-absorbent resin particles and fibrous materials.

[0068] 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, and adhesive emulsions. The adhesive binders may be used alone or in combination of two or more.

[0069] 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.

[0070] Examples of hot melt adhesives include mixtures of base polymers 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 tackifiers, plasticizers, antioxidants, and the like.

[0071] 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.

[0072] The absorbent body according to the present embodiment may contain inorganic powder (for example, amorphous silica), a deodorant, an antibacterial agent, a fragrance, etc. When the water-absorbent resin particles contain inorganic particles, the absorbent body may contain inorganic powder in addition to the inorganic particles in the water-absorbent resin particles.

[0073] The shape of the absorbent body according to this embodiment is not particularly limited and may be, for example, a sheet. The thickness of the absorbent body (for example, the thickness of a sheet-shaped absorbent body) may be, for example, 0.1 to 20 mm, or 0.3 to 15 mm.

[0074] The absorbent article according to the present embodiment includes the absorbent body according to the present embodiment. Other components of the absorbent article according to the present embodiment include a core wrap that maintains the shape of the absorbent body; a liquid-permeable sheet that is disposed on the outermost side of the side into which the liquid to be absorbed penetrates; and a liquid-impermeable sheet that is disposed on the outermost side of the side opposite to the side into which the liquid to be absorbed penetrates. 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, parts for portable toilets, and animal waste disposal materials.

[0075] Fig. 4 is a cross-sectional view showing an example of an absorbent article. The absorbent article 100 shown in Fig. 4 comprises an absorbent body 10, core wraps 20a and 20b, a liquid-permeable sheet 30, and a liquid-impermeable sheet 40. In the absorbent article 100, the liquid-impermeable sheet 40, the core wrap 20b, the absorbent body 10, the core wrap 20a, and the liquid-permeable sheet 30 are layered in this order. In Fig. 4, some parts are shown as having gaps between the members, but the members may be in close contact with each other without any gaps.

[0076] The absorbent body 10 includes water-absorbent resin particles 10a according to this embodiment and a fiber layer 10b containing fibrous material. The water-absorbent resin particles 10a are dispersed in the fiber layer 10b.

[0077] The core wrap 20a is arranged on one side of the absorbent body 10 (the upper side of the absorbent body 10 in FIG. 4) while in contact with the absorbent body 10. The core wrap 20b is arranged on the other side of the absorbent body 10 (the lower side of the absorbent body 10 in FIG. 4) while in contact with the absorbent body 10. The absorbent body 10 is arranged between the core wrap 20a and the core wrap 20b. Examples of the core wraps 20a and 20b include tissue and nonwoven fabric. The core wrap 20a and the core wrap 20b have, for example, a main surface of the same size as the absorbent body 10.

[0078] The liquid-permeable sheet 30 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 30 is disposed on the core wrap 20a in a state of contact with the core wrap 20a. The liquid-impermeable sheet 40 is disposed on the outermost side of the absorbent article 100, on the side opposite the liquid-permeable sheet 30. The liquid-impermeable sheet 40 is disposed below the core wrap 20b in a state of contact with the core wrap 20b. The liquid-permeable sheet 30 and the liquid-impermeable sheet 40 have, for example, main surfaces that are wider than the main surface of the absorbent body 10, and the outer edges of the liquid-permeable sheet 30 and the liquid-impermeable sheet 40 extend around the absorbent body 10 and the core wraps 20a, 20b.

[0079] The size relationships among the absorbent body 10, core wraps 20a, 20b, liquid-permeable sheet 30, and liquid-impermeable sheet 40 are not particularly limited and may be adjusted as appropriate depending on the intended use of the absorbent article. The absorbent body 10 shown in FIG. 4 is retained in shape by being sandwiched between two core wraps 20a, 20b. The method of retaining the shape of the absorbent body using the core wraps is not limited to this, and for example, the absorbent body may be sandwiched between a single folded core wrap. The core wraps may form a bag, and the absorbent body may be placed inside it.

[0080] The liquid-permeable sheet 30 may be a sheet formed from a resin or fiber commonly used in the art. From the viewpoints of liquid permeability, flexibility, and strength when used in an absorbent article, the liquid-permeable sheet 30 may contain, for example, a synthetic resin such as polyolefins such as polyethylene (PE) and polypropylene (PP), polyesters such as polyethylene terephthalate (PET), polytrimethylene terephthalate (PTT), and polyethylene naphthalate (PEN), polyamides such as nylon, and rayon, or a synthetic fiber containing such a synthetic resin. Alternatively, the liquid-permeable sheet 30 may be made of natural fibers such as cotton, silk, hemp, or pulp (cellulose). From the viewpoint of increasing the strength of the liquid-permeable sheet 30, the liquid-permeable sheet 30 may contain synthetic fibers. The synthetic fibers may be polyolefin fibers, polyester fibers, or a combination thereof. These materials may be used alone, or two or more materials may be used in combination.

[0081] The liquid-permeable sheet 30 may be a nonwoven fabric, a porous sheet, or a combination thereof. A nonwoven fabric is a sheet in which fibers are intertwined without being woven. The nonwoven fabric may be a nonwoven fabric made of short fibers (i.e., staples) (short fiber nonwoven fabric) or a nonwoven fabric made of long fibers (i.e., filaments) (long fiber nonwoven fabric). The staples may generally have a fiber length of several hundred mm or less, although this is not limited thereto.

[0082] The liquid-permeable sheet 30 may be a thermal-bonded nonwoven fabric, an air-through nonwoven fabric, a resin-bonded nonwoven fabric, a spunbonded nonwoven fabric, a melt-blown nonwoven fabric, an air-laid nonwoven fabric, a spunlace nonwoven fabric, a point-bonded nonwoven fabric, or a laminate of two or more nonwoven fabrics selected from these. These nonwoven fabrics may be formed, for example, from the synthetic fibers or natural fibers described above. A laminate of two or more nonwoven fabrics may be, for example, a spunbond / meltblown / spunbonded nonwoven fabric, which is a composite nonwoven fabric having a spunbonded nonwoven fabric, a meltblown nonwoven fabric, and a spunbonded nonwoven fabric laminated in this order. From the viewpoint of suppressing liquid leakage, a thermal-bonded nonwoven fabric, an air-through nonwoven fabric, a spunbonded nonwoven fabric, or a spunbonded / meltblown / spunbonded nonwoven fabric may also be used.

[0083] The nonwoven fabric used as the liquid-permeable sheet 30 may have appropriate hydrophilicity from the viewpoint of the liquid absorption performance of the absorbent article. From that viewpoint, the liquid-permeable sheet 30 may be a nonwoven fabric having a hydrophilicity of 5 to 200 as measured in accordance with the Paper and Pulp Testing Method No. 68 (2000) of the Paper and Pulp Technology Association. The hydrophilicity of the nonwoven fabric may also be 10 to 150. For details of Paper and Pulp Testing Method No. 68, see, for example, WO2011 / 086843.

[0084] The hydrophilic nonwoven fabric described above may be formed from fibers exhibiting moderate hydrophilicity, such as rayon fibers, or from fibers obtained by hydrophilizing hydrophobic chemical fibers, such as polyolefin fibers or polyester fibers. Methods for obtaining nonwoven fabrics containing hydrophilically treated hydrophobic chemical fibers include, for example, a method in which a hydrophilizing agent is mixed with hydrophobic chemical fibers to obtain a nonwoven fabric by the spunbonding method, a method in which a hydrophilizing agent is added when producing a spunbond nonwoven fabric from hydrophobic chemical fibers, and a method in which a hydrophilizing agent is impregnated into a spunbond nonwoven fabric obtained from hydrophobic chemical fibers. Examples of hydrophilizing agents include anionic surfactants such as aliphatic sulfonates and higher alcohol sulfate ester salts, cationic surfactants such as quaternary ammonium salts, nonionic surfactants such as polyethylene glycol fatty acid esters, polyglycerin fatty acid esters, and sorbitan fatty acid esters, silicone surfactants such as polyoxyalkylene-modified silicones, and stain release agents made from polyester, polyamide, acrylic, or urethane resins.

[0085] The liquid-permeable sheet 30 may be a nonwoven fabric that is appropriately bulky and has a large basis weight, from the viewpoint of imparting good liquid permeability, flexibility, strength, and cushioning properties to the absorbent article, and from the viewpoint of increasing the liquid permeation rate of the absorbent article. The basis weight of the nonwoven fabric used for the liquid-permeable sheet 30 is 5 to 200 g / m 2 , 8~150g / m 2 , or 10 to 100 g / m 2 The thickness of the nonwoven fabric used for the liquid-permeable sheet 30 may be 20 to 1400 μm, 50 to 1200 μm, or 80 to 1000 μm.

[0086] The liquid-impermeable sheet 40 is disposed at the outermost side of the absorbent article 100, opposite the liquid-permeable sheet 30. The liquid-impermeable sheet 40 is disposed below the core wrap 20b, in contact with the core wrap 20b. The liquid-impermeable sheet 40 has, for example, a main surface that is wider than the main surface of the absorbent body 10, and the outer edge of the liquid-impermeable sheet 40 extends around the absorbent body 10 and the core wraps 20a, 20b. The liquid-impermeable sheet 40 prevents liquid absorbed by the absorbent body 10 from leaking out from the liquid-impermeable sheet 40 side to the outside.

[0087] Examples of the liquid-impermeable sheet 40 include sheets made of synthetic resins such as polyethylene, polypropylene, and polyvinyl chloride; sheets made of nonwoven fabrics such as spunbond / meltblown / spunbond (SMS) nonwoven fabrics in which a water-resistant meltblown nonwoven fabric is sandwiched between high-strength spunbond nonwoven fabrics; and sheets made of composite materials of these synthetic resins and nonwoven fabrics (e.g., spunbond nonwoven fabrics, spunlace nonwoven fabrics). The liquid-impermeable sheet 40 may be breathable from the viewpoint of reducing stuffiness when worn and alleviating discomfort to the wearer. A sheet made of a synthetic resin mainly composed of low-density polyethylene (LDPE) resin can be used as the liquid-impermeable sheet 40. From the viewpoint of ensuring flexibility so as not to impair the wearing comfort of the absorbent article, the liquid-impermeable sheet 40 has a basis weight of, for example, 10 to 50 g / m. 2 The sheet may be made of a synthetic resin.

[0088] The absorbent article 100 can be manufactured, for example, by a method including placing the absorbent body 10 between core wraps 20a and 20b, and then placing these between a liquid-permeable sheet 30 and a liquid-impermeable sheet 40. A laminate in which the liquid-impermeable sheet 40, core wrap 20b, absorbent body 10, core wrap 20a, and liquid-permeable sheet 30 are laminated in this order is pressurized as necessary.

[0089] The absorbent body 10 is formed by mixing the water-absorbent resin particles 10a with a fibrous material. The absorbent body 10 may be formed by selecting absorbent resin particles having the above-mentioned gel outflow ratio of 0 to 20% and selectively using these.

[0090] The gel outflow ratio of the water-absorbent resin particles 10a can be used as an index for increasing the permeation rate of saline into the absorbent 10 after a load is applied to the absorbent 10 containing the water-absorbent resin particles 10a in a state where the absorbent 10 has absorbed water. A method including reducing the gel outflow ratio of the water-absorbent resin particles 10a can increase the permeation rate of saline into the absorbent 10 after a load is applied to the absorbent 10 in a state where the absorbent 10 has absorbed water. For example, water-absorbent resin particles 10a having a gel outflow ratio of a predetermined value (e.g., 20%) or less may be selected. Alternatively, the gel outflow ratio of the water-absorbent resin particles can be set to a predetermined value (e.g., 20%) or less by selecting production conditions for the water-absorbent resin particles so as to increase the uniformity of crosslinking in the water-absorbent resin particles. Examples of a method for enhancing the uniformity of crosslinking in water absorbent resin particles include using a flat blade as an agitator blade during the polymerization reaction, selecting the number of rotations of the agitator within an appropriate range, adopting conditions that make it easy to provide and remove heat (for example, a reaction in a hydrocarbon dispersion medium), and using a crosslinker with an appropriately high reactivity.

[0091] According to the present embodiment, a method for absorbing liquid using the water-absorbent resin particles, absorbent body, or absorbent article according to the present embodiment can be provided. The method for absorbing liquid according to the present embodiment includes a step of bringing the water-absorbent resin particles, absorbent body, or absorbent article according to the present embodiment into contact with a liquid to be absorbed. [Example]

[0092] The present invention will be described in more detail below with reference to examples, although the present invention is not limited to these examples.

[0093] 1. Preparation of Water-absorbent Resin Particles Example 1 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 dropping funnel, a nitrogen gas inlet tube, and a stirrer. A stirring blade 200, the outline of which is shown in FIG. 5, was attached to the stirrer. The stirring blade 200 includes a shaft 200a and a flat plate portion 200b. The flat plate portion 200b is welded to the shaft 200a and has a curved tip. Four slits S extending along the axial direction of the shaft 200a are formed in the flat plate portion 200b. The four slits S are arranged in the width direction of the flat plate portion 200b, with the inner two slits S being 1 cm wide and the outer two slits S being 0.5 cm wide. The length of the flat plate portion 200b is approximately 10 cm, and the width of the flat plate portion 200b is approximately 6 cm. 293 g of n-heptane and 0.736 g of dispersant (maleic anhydride-modified ethylene-propylene copolymer, manufactured by Mitsui Chemicals, Inc., Hiwax 1105A) were mixed in the prepared separable flask. The mixture in the separable flask was stirred with a stirrer and heated to 80 °C to dissolve the dispersant in the n-heptane. The resulting solution was cooled to 50 °C.

[0094] A 300 mL beaker was charged with 92.0 g (1.03 mol) of an 80.5 wt% aqueous acrylic acid solution. While cooling externally, 147.7 g of a 20.9 wt% aqueous sodium hydroxide solution was added dropwise to the acrylic acid solution in the beaker to neutralize the 75 mol% acrylic acid. Next, 0.092 g of hydroxyethyl cellulose (Sumitomo Seika Chemicals Co., Ltd., HEC AW-15F), 0.092 g (0.339 mmol) of 2,2'-azobis(2-amidinopropane) dihydrochloride as a water-soluble radical polymerization initiator, 0.018 g (0.067 mmol) of potassium persulfate, and 0.0046 g (0.026 mmol) of ethylene glycol diglycidyl ether as an internal crosslinking agent were added and dissolved in the aqueous solution to prepare the first-stage monomer aqueous solution.

[0095] The resulting aqueous monomer solution was placed in a separable flask containing a dispersant solution, and the reaction solution in the separable flask was stirred for 10 minutes. A surfactant solution containing 0.736 g of sucrose stearate (Ryoto Sugar Ester S-370, Mitsubishi Chemical Foods Corporation, HLB value: 3) dissolved in 6.62 g of n-heptane was added to the reaction solution. The reaction solution was stirred at 425 rpm using a stirrer, while the system was thoroughly purged with nitrogen. The first-stage polymerization reaction was allowed to proceed by heating the separable flask in a 70°C hot water bath for 60 minutes while continuing to stir, yielding a first-stage polymerization slurry containing a hydrous gel polymer.

[0096] In a separate 500 mL beaker, 128.8 g (1.44 mol) of an 80.5% by weight acrylic acid aqueous solution was placed. While cooling externally, 159.0 g of a 27% by weight sodium hydroxide aqueous solution was added dropwise to the acrylic acid aqueous solution in the beaker to neutralize the 75 mol% acrylic acid. Next, 0.129 g (0.476 mmol) of 2,2'-azobis(2-amidinopropane) dihydrochloride and 0.026 g (0.096 mmol) of potassium persulfate as water-soluble radical polymerization initiators, and 0.0117 g (0.067 mmol) of ethylene glycol diglycidyl ether as an internal crosslinking agent were added and dissolved in the aqueous solution to prepare the second-stage monomer aqueous solution.

[0097] The first-stage polymerization slurry was cooled to 25°C while stirring at a stirrer speed of 650 rpm. The entire amount of the second-stage aqueous monomer solution was added thereto. After the system was purged with nitrogen for 30 minutes, the separable flask was again heated in a 70°C hot water bath for 60 minutes while continuing stirring, thereby allowing the second-stage polymerization reaction to proceed.

[0098] To the reaction solution after the second-stage polymerization reaction, 0.589 g of a 45% by mass aqueous solution of pentasodium diethylenetriaminepentaacetate was added while stirring. The reaction solution was heated in a 125°C oil bath, and 224.3 g of water was removed from the system by azeotropic distillation. Thereafter, 4.42 g (0.507 mmol) of a 2% by mass aqueous solution 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 allow the surface cross-linking reaction to proceed.

[0099] The reaction solution was heated to 125°C to evaporate n-heptane, and dried polymer particles were obtained. The dried product was passed through a sieve with an opening of 850µm, and 0.2% by mass of amorphous silica (Toxil NP-S, Oriental Silicas Corporation) relative to the mass of the polymer particles was mixed with the polymer particles, and 231.0g of water-absorbent resin particles were obtained. The median particle diameter of the water-absorbent resin particles was 342µm.

[0100] Example 2 Except for the fact that the amount of water extracted by azeotropic distillation from the reaction solution after the second-stage polymerization reaction was 229.2 g, 233.0 g of water-absorbent resin particles containing 0.2 mass % of amorphous silica relative to the mass of the polymer particles were obtained in the same procedure as in Example 1. The median particle diameter of the water-absorbent resin particles was 368 μm.

[0101] Example 3 A first-stage polymerization slurry was obtained in the same manner as in Example 1, except that a first-stage aqueous monomer solution was prepared using only 0.0736 g (0.272 mmol) of potassium persulfate as the water-soluble radical polymerization initiator and changing the amount of ethylene glycol diglyceride ether to 0.010 g (0.057 mmol). A second-stage aqueous monomer solution was prepared in the same manner as in Example 1, except that only 0.090 g (0.333 mmol) of potassium persulfate was used as the water-soluble radical polymerization initiator.

[0102] Except for using the obtained first-stage polymerization slurry liquid and second-stage aqueous monomer solution, and for the amount of water extracted by azeotropic distillation from the reaction liquid after the second-stage polymerization reaction being 264.3 g, the same procedure as in Example 1 was carried out to obtain 221.5 g of water-absorbent resin particles containing 0.2 mass % of amorphous silica relative to the mass of the polymer particles. The median particle diameter of the water-absorbent resin particles was 376 μm.

[0103] Example 4 A first-stage polymerization slurry was obtained in the same manner as in Example 1, except that a first-stage aqueous monomer solution was prepared using only 0.0736 g (0.272 mmol) of potassium persulfate as the water-soluble radical polymerization initiator and changing the amount of ethylene glycol diglyceride ether to 0.010 g (0.057 mmol). A second-stage aqueous monomer solution was prepared in the same manner as in Example 1, except that only 0.090 g (0.333 mmol) of potassium persulfate was used as the water-soluble radical polymerization initiator.

[0104] Except for using the obtained first-stage polymerization slurry liquid and second-stage aqueous monomer solution, and for the amount of water extracted by azeotropic distillation from the reaction liquid after the second-stage polymerization reaction being 257.6 g, the same procedure as in Example 1 was carried out to obtain 229.8 g of water-absorbent resin particles containing 0.2 mass % of amorphous silica relative to the mass of the polymer particles. The median particle diameter of the water-absorbent resin particles was 362 μm.

[0105] ( Reference example ) A round-bottomed, cylindrical, separable flask (baffle width: 7 mm), 11 cm in inner diameter, and 2 L capacity with four sidewall baffles was prepared. It was equipped with a reflux condenser, a dropping funnel, a nitrogen gas inlet tube, and a stirrer. A stirrer with a fluororesin-surface-treated, two-stage, four-paddle blade with a 5 cm blade diameter was used. 451.4 g of n-heptane and 1.288 g of sorbitan monolaurate (Nonion LP-20R, HLB value: 8.6, NOF Corporation) were mixed in the separable flask. The mixture in the separable flask was heated to 50°C while stirring with the stirrer, dissolving the sorbitan monolaurate in the n-heptane. The resulting solution was cooled to 40°C.

[0106] 92.0 g (1.03 mol) of an 80.5% by mass acrylic acid aqueous solution was placed in a 300 mL beaker. While cooling from the outside, 147.7 g of a 20.9% by mass sodium hydroxide aqueous solution was added dropwise to the acrylic acid aqueous solution in the beaker to neutralize the 75 mol% acrylic acid. Next, 0.1012 g (0.374 mmol) of potassium persulfate was added as a water-soluble radical polymerization initiator and dissolved in the aqueous solution to prepare a monomer aqueous solution.

[0107] The resulting aqueous monomer solution was added to the sorbitan monolaurate-containing solution in the separable flask, and the system was thoroughly purged with nitrogen. The polymerization reaction was then allowed to proceed by heating the separable flask in a 70°C hot water bath for 60 minutes while stirring the reaction solution at 700 rpm. The polymerization reaction produced a hydrogel polymer dispersed in the reaction solution.

[0108] A dispersion of 0.092 g of amorphous silica (Toxil NP-S, Oriental Silicas Corporation) previously dispersed in 100 g of n-heptane was added to the reaction solution containing the hydrogel polymer while stirring at 1000 rpm. After 10 minutes of stirring, the separable flask was immersed in a 125°C oil bath, and 91.1 g of water was removed from the system by azeotropic distillation. Next, 4.14 g of a 2% by weight aqueous solution of ethylene glycol diglycidyl ether (ethylene glycol diglycidyl ether: 0.475 mmol) was added as a surface cross-linking agent, and the mixture was maintained at an internal temperature of 83±2°C for 2 hours.

[0109] The reaction solution was heated to 120°C to evaporate water and n-heptane until almost no evaporants were distilled off, thereby obtaining a dried product of polymer particles. This dried product was passed through a sieve with an opening of 850 µm to obtain 91.3 g of water-absorbent resin particles. The median particle diameter of the water-absorbent resin particles was 360 µm.

[0110] (Comparative Example 1) Except that the amount of water extracted from the reaction mixture by azeotropic distillation was 129.0 g. Reference example Water-absorbent resin particles 90.1 were obtained by the same procedure as in 1. The median particle diameter of the water-absorbent resin particles was 358 μm.

[0111] (Comparative Example 2) A round-bottom, cylindrical, separable flask with an inner diameter of 11 cm and an internal volume of 2 L, equipped with four baffles on the sidewall (baffle width: 7 mm), was prepared. The stirrer had a fluororesin-treated, two-stage, four-paddle blade with a 5 cm blade diameter and four inclined blades. 293 g of n-heptane and 0.736 g of dispersant (maleic anhydride-modified ethylene-propylene copolymer, Mitsui Chemicals, Inc., Hiwax 1105A) were mixed in the separable flask. The mixture in the separable flask was heated to 80 °C while stirring with the stirrer, dissolving the dispersant in the n-heptane. The resulting solution was cooled to 50 °C.

[0112] A 300 mL beaker was charged with 92.0 g (1.03 mol) of an 80.5 wt% aqueous acrylic acid solution. While cooling externally, 147.7 g of a 20.9 wt% aqueous sodium hydroxide solution was added dropwise to the acrylic acid solution in the beaker to neutralize the 75 mol% acrylic acid. Next, 0.092 g of hydroxyethyl cellulose (Sumitomo Seika Chemicals Co., Ltd., HEC AW-15F), 0.0736 g (0.272 mmol) of potassium persulfate as a water-soluble radical polymerization initiator, and 0.010 g (0.057 mmol) of ethylene glycol diglycidyl ether as an internal crosslinking agent were added and dissolved in the aqueous solution to prepare the first-stage monomer aqueous solution.

[0113] The resulting aqueous monomer solution was placed in a separable flask containing a dispersant solution, and the reaction mixture in the separable flask was stirred for 10 minutes. A surfactant solution containing 0.736 g of sucrose stearate (Ryoto Sugar Ester S-370, Mitsubishi Chemical Foods Corporation, HLB value: 3) dissolved in 6.62 g of n-heptane was then added. The reaction mixture was stirred at 550 rpm using a stirrer, while the system was thoroughly purged with nitrogen. The first-stage polymerization reaction was allowed to proceed by heating the separable flask in a 70°C hot water bath for 60 minutes while continuing to stir, yielding a first-stage polymerization slurry containing a hydrous gel polymer.

[0114] Into another 500 mL beaker, 128.8 g (1.44 mol) of an 80.5% by mass acrylic acid aqueous solution was placed. While cooling from the outside, 159.0 g of a 27% by mass sodium hydroxide aqueous solution was added dropwise to the acrylic acid aqueous solution in the beaker to neutralize the 75 mol% acrylic acid. Next, 0.090 g (0.333 mmol) of potassium persulfate as a water-soluble radical polymerization initiator and 0.0117 g (0.067 mmol) of ethylene glycol diglycidyl ether as an internal crosslinking agent were added and dissolved in the aqueous solution to prepare the second-stage monomer aqueous solution.

[0115] The first-stage polymerization slurry was cooled to 25°C while stirring at a stirrer speed of 1000 rpm. The entire amount of the second-stage aqueous monomer solution was added thereto. After the system was purged with nitrogen for 30 minutes, the separable flask was again heated in a 70°C hot water bath for 60 minutes while continuing stirring, thereby allowing the second-stage polymerization reaction to proceed.

[0116] To the reaction solution after the second-stage polymerization reaction, 0.265 g of a 45% by mass aqueous solution of pentasodium diethylenetriaminepentaacetate was added while stirring. The reaction solution was heated in a 125°C oil bath, and 278.9 g of water was removed from the system by azeotropic distillation. Then, 4.42 g (0.507 mmol) of a 2% by mass aqueous solution of ethylene glycol diglycidyl ether was added as a surface cross-linking agent, and the mixture was maintained at 83°C for 2 hours.

[0117] The reaction solution was heated to 125°C to evaporate n-heptane, and dried polymer particles were obtained. The dried product was passed through a sieve with an opening of 850µm, and 0.2% by mass of amorphous silica (Toxil NP-S, Oriental Silicas Corporation) relative to the mass of the polymer particles was mixed with the polymer particles, thereby obtaining 230.8g of water-absorbent resin particles. The median particle diameter of the water-absorbent resin particles was 377µm.

[0118] (Comparative Example 3) A first-stage polymerization slurry liquid and a second-stage monomer aqueous solution were obtained in the same procedure as in Example 1, except that a stirrer having stirring blades with two stages of four inclined paddle blades with a blade diameter of 5 cm that had been surface-treated with a fluororesin was used. The second-stage polymerization reaction was allowed to proceed in the same procedure as in Example 1, except that the rotation speed was changed to 1000 rpm. After the second-stage polymerization reaction, 229.6 g of water-absorbent resin particles containing 0.2 mass % of amorphous silica relative to the mass of the polymer particles were obtained in the same procedure as in Example 1. The median particle diameter of the water-absorbent resin particles was 355 μm.

[0119] 2. Evaluation of water-absorbent resin particles 2-1. Gel outflow ratio 58 g of saline solution at 25°C was stirred at 600 rpm in a glass beaker using a stirrer tip (length 3.0 cm, diameter 8 mm), and 2 g of water-absorbent resin particles were added thereto. After confirming that the water-absorbent resin particles had swollen and the vortex on the liquid surface had converged, stirring was stopped. The mixture was then left to stand for 10 minutes, and a swollen gel (30-fold swollen gel) formed by absorbing 58 g of saline solution was obtained. Using the obtained swollen gel, the gel outflow ratio of the water-absorbent resin particles was measured in an environment of 25°C according to the above-described method including steps (1), (2), (3), (4), (5), (6), and (7) shown in Figures 1, 2, and 3.

[0120] 2-2.Water retention capacity A cotton bag (membrane broadcloth No. 60, 100 mm wide x 200 mm long) containing 2.0 g of water-absorbent resin particles was placed in a 500 mL beaker. 500 g of a 0.9% by mass sodium chloride aqueous solution (physiological saline) was poured into the cotton bag all at once, taking care not to allow the bag to swell. The top of the cotton bag was tied with a rubber band, and the cotton bag was left to stand for 30 minutes to allow the water-absorbent resin particles to swell. After 30 minutes, the cotton bag was dehydrated for 1 minute using a dehydrator (manufactured by Kokusan Co., Ltd., product number H-122) set to a centrifugal force of 167 G, and the mass (Wa) of the cotton bag containing the swollen gel after dehydration was measured. The same procedure was performed without adding water-absorbent resin particles, and the empty mass (Wb) of the cotton bag when wet was measured. The water retention capacity under no pressure was calculated using the following formula: Water retention amount (g / g)=[Wa-Wb] / 2.0

[0121] 2-3. Water absorption amount 500 g of saline (0.9% by mass sodium chloride aqueous solution) was stirred at 600 rpm in a 500 mL beaker using a magnetic stir bar (8 mm diameter x 30 mm, no ring). While stirring the saline, 2.0 g of water-absorbent resin particles were added, and the water-absorbent resin particles were dispersed without forming lumps. The saline was left to stand for 60 minutes while stirring, allowing the water-absorbent resin particles to fully swell. The contents of the beaker containing the swollen gel were then filtered using a standard sieve with a mesh size of 75 μm and a mass of Wc (g). The sieve with the swollen gel was tilted at an angle of approximately 30 degrees relative to the horizontal and left for 30 minutes, thereby filtering out excess water. The mass Wd (g) of the sieve with the swollen gel was measured. The water absorption of the water-absorbent resin particles in saline without pressure was calculated using the following formula: Water absorption (g / g)=(Wd-Wc) / 2.0

[0122] 2-4. Water absorption under load The water absorption of water-absorbent resin particles in physiological saline under a load of 4.14 kPa (water absorption under load) was measured using an apparatus outlined in FIG. 6. The water absorption under load was measured twice for one type of water-absorbent resin particle, and the average of the measured values ​​was calculated. The apparatus in FIG. 6 includes a burette unit 1, a clamp 3, a conduit 5, a stand 11, a measurement table 13, and a measurement unit 4 placed on the measurement table 13. The burette unit 1 includes a burette tube 21 with a scale, a rubber stopper 23 that seals the opening at the top of the burette tube 21, a cock 22 connected to the tip of the bottom of the burette tube 21, and an air introduction tube 25 and a cock 24 connected to the bottom of the burette tube 21. The burette unit 1 is fixed with a clamp 3. The flat measurement table 13 has a through-hole 13a with a diameter of 2 mm formed in its center, and is supported by a height-adjustable stand 11. The through hole 13a of the measurement table 13 and the cock 22 of the burette part 1 are connected by a conduit 5. The inner diameter of the conduit 5 is 6 mm.

[0123] The measurement unit 4 has a cylinder 31 made of acrylic resin, a polyamide mesh 32 adhered to one opening of the cylinder 31, and a weight 33 that is movable up and down within the cylinder 31. The cylinder 31 is placed on the measurement table 13 via the polyamide mesh 32. The inner diameter of the cylinder 31 is 20 mm. The opening of the polyamide mesh 32 is 75 μm (200 mesh). The weight 33 has a diameter of 19 mm and a mass of 119.6 g, and can apply a load of 4.14 kPa to the water-absorbent resin particles 10a uniformly arranged on the polyamide mesh 32, as will be described later.

[0124] Measurement of the water absorption of saline solution under a load of 4.14 kPa using the measuring device shown in FIG. 6 was carried out in a room at 25°C. First, the stopcocks 22 and 24 of the burette unit 1 were closed, and 0.9% by mass saline solution adjusted to 25°C was poured into the burette tube 21 through the opening at the top of the burette tube 21. Next, the top opening of the burette tube 21 was sealed with a rubber stopper 23, and then the stopcocks 22 and 24 were opened. The inside of the conduit 5 was filled with 0.9% by mass saline solution 50 while preventing air bubbles from entering. The height of the measurement table 13 was adjusted so that the height of the water surface of the 0.9% by mass saline solution 50 reaching the through-hole 13a was the same as the height of the upper surface of the measurement table 13. After adjustment, the height of the water surface of the 0.9% by mass saline solution 50 in the burette tube 21 was read on the scale of the burette tube 21, and this position was designated as the zero point (the reading at 0 seconds).

[0125] In the measurement unit 4, 0.10 g of water-absorbent resin particles 10a were uniformly arranged on polyamide mesh 32 in a cylinder 31, a weight 33 was placed on the water-absorbent resin particles 10a, and the cylinder 31 was installed so that its center coincided with the conduit opening at the center of the measurement table 13. The amount of reduced saline in the burette tube 21 (i.e., the amount of saline absorbed by the water-absorbent resin particles 10a) We (mL) 60 minutes after the water-absorbent resin particles 10a started to absorb saline from the conduit 5 was read, and the amount of water absorbed by the water-absorbent resin particles 10a for saline under a load of 4.14 kPa was calculated by the following formula. The results are shown in Table 1. Water absorption under load (mL / g) = We (mL) / mass of water-absorbent resin particles (g)

[0126] 2-5. Median particle size The above-mentioned median particle diameter of the water-absorbent resin particles was measured by the following procedure. That is, JIS standard sieves were combined in the following order from top to bottom: a sieve with a mesh size of 600 μm, a sieve with a mesh size of 500 μm, a sieve with a mesh size of 425 μm, a sieve with a mesh size of 300 μm, a sieve with a mesh size of 250 μm, a sieve with a mesh size of 180 μm, a sieve with a mesh size of 150 μm, and a tray. 50 g of water-absorbent resin particles were placed on the top sieve of the combination and classified in accordance with JIS Z 8815 (1994) using a Rotap type shaker (manufactured by Iida Seisakusho Co., Ltd.). After classification, the ratio of the mass of the particles remaining on each sieve was calculated as a mass percentage relative to the total amount to determine the particle size distribution. Regarding this particle size distribution, the ratios of the weight percentages of the particles remaining on the sieves in descending order of particle size were integrated, and the relationship between the sieve opening size and the integrated value of the ratios of the mass percentages of the particles remaining on the sieves was plotted on a logarithmic probability paper. By connecting the plots on the probability paper with a straight line, the particle size corresponding to an integrated mass percentage of 50 mass% was obtained as the median particle size.

[0127] 2-6.Evaluation of absorbent articles 10 g of water-absorbent resin particles and 6.7 g of crushed pulp were uniformly mixed by air-pressing using a flow mixer (Autech Co., Ltd., Pad Former) to prepare a sheet-shaped absorbent body measuring 40 cm x 12 cm. The absorbent body was then cut into a sheet of the same size as the absorbent body, with a basis weight of 16 g / m. 2 The whole was pressed with a load of 196 kPa for 30 seconds. A sheet of tissue paper with the same size as the absorbent body and a basis weight of 22 g / m was placed on top of the upper tissue paper. 2 Then, an air-through type porous liquid-permeable sheet made of polyethylene-polypropylene was placed on the absorbent article, and an absorbent article for evaluation having a structure of liquid-permeable sheet / tissue paper / absorbent body / tissue paper was obtained.

[0128] In a room at 25±2°C, the absorbent article for evaluation was placed on a horizontal table with the liquid-permeable sheet facing up. A liquid-injection cylinder was prepared, equipped with a 100 mL liquid storage compartment and a 3 cm inner diameter inlet tube extending from the bottom of the liquid storage compartment. The cylinder was held in place with the tip of the inlet tube in contact with the center of the absorbent article for evaluation. 80 mL of saline solution, colored with a small amount of Blue No. 1 and adjusted to 25±1°C, was poured into the liquid storage compartment of the cylinder all at once, and the saline solution was absorbed into the absorbent body through the tip of the inlet tube. A stopwatch was used to measure the time until the saline solution completely disappeared from the cylinder. This time was recorded as the first penetration time (seconds). Five minutes after the saline solution was poured, a 5 kg weight with a 10 cm x 10 cm square base was placed on the center of the absorbent article for evaluation, applying a load to the portion of the absorbent article where the saline solution had been absorbed. After 4 minutes of loading, the weight was removed from the absorbent article for evaluation. One minute after the weight was removed (corresponding to 10 minutes after the initial injection of saline), a small amount of saline dyed with Blue No. 1 was injected in the same manner as the initial injection of saline, and the second penetration time (seconds) was measured. Five minutes after the second injection of saline, the weight was placed again on the absorbent article for evaluation, and a load was applied to the absorbent article for evaluation for four minutes, after which the weight was removed. One minute after the weight was removed (corresponding to 20 minutes after the initial injection of saline), a small amount of saline dyed with Blue No. 1 was injected in the same manner as the initial injection of saline, and the third penetration time was measured.

[0129] [Table 1]

[0130] From the results shown in Table 1, it was confirmed that the water-absorbent resin particles of Examples showing a gel outflow ratio of 20% or less significantly improved the permeation rate (third permeation rate) after a load was applied to the absorbent body that had absorbed water, compared with the water-absorbent resin particles of Comparative Examples. [Explanation of symbols]

[0131] 1...burette part, 3...clamp, 4...measuring part, 5...conduit, 10...absorbent body, 10a...water-absorbent resin particles, 10b...fiber layer, 11...frame, 13...measuring stand, 13a...through-hole, 20a, 20b...core wrap, 21...burette tube, 22...cock, 23...rubber stopper, 24...cock, 25...air introduction tube, 30...liquid-permeable sheet, 31...cylinder, 32...polyamide mesh, 33...weight, 40...liquid-impermeable sheet, 41...tubular body, 41A...opening, 41B...bottom, 42...disc, 45...swollen gel, 46...first weight, 47...second weight, 100...absorbent article, 200...stirring blade, 200a...shaft, 200b...flat plate part, H...through-hole.

Claims

1. A water-absorbent resin particle comprising a crosslinked polymer formed by polymerization of a monomer including an ethylenically unsaturated monomer, wherein the ethylenically unsaturated monomer comprises at least one compound selected from the group consisting of (meth)acrylic acid and salts thereof, and the ratio of the (meth)acrylic acid and salts thereof is 70 to 100 mol % based on the total amount of the monomers, The gel outflow ratio measured in an environment of 25±2°C by a method including the following steps (1), (2), (3), (4), (5), (6), and (7) in this order is 0 to 20%, (1) A tubular body having an inner diameter of 5.0 cm and a bottom portion that closes one end of the tubular body and forms an opening with a diameter of 3.0 cm in the center is fixed in an orientation such that the longitudinal direction of the tubular body is vertical and the bottom portion is facing downward. (2) Inside the tubular body, a disk having a diameter of 4.9 cm and a thickness of 3.0 mm and having a through hole formed in the center is placed on the bottom with the main surface of the disk oriented horizontally. The through hole has a wall surface that is inclined with respect to the main surface of the disk so that the through hole is narrowest at the center position in the thickness direction of the disk, and the diameter of the through hole is a maximum of 8.0 mm at the positions of both main surfaces of the disk and a minimum of 5.0 mm at the center position in the thickness direction of the disk. (3) 20 g of swollen gel formed by the water-absorbent resin particles absorbing physiological saline is placed in the tubular body and placed on the disk. The swollen gel has a mass 30 times the mass of the water-absorbent resin particles before absorbing the physiological saline solution. (4) A cylindrical first weight having a mass of 500 g and a diameter of 4.9 cm is placed on the swollen gel in the tubular body to compress the swollen gel for 30 seconds. (5) A second cylindrical weight having a mass of 1500 g and a diameter of 4.9 cm is placed on the first weight to further compress the swollen gel with a load of 2000 g. (6) The swollen gel is compressed with a load of 2000 g for 30 seconds, and the mass W (g) of the swollen gel that flows out from the through-holes of the disk is measured. (7) Calculate the gel outflow ratio (%) using the following formula. Gel outflow ratio (%) = (W / 20) x 100 4. The water absorption amount of the water-absorbent resin particles for physiological saline under a load of 14 kPa is 14 mL / g or more and 18 mL / g or less, The water-retention capacity of the water-absorbent resin particles in physiological saline is 42 g / g or more and 45 g / g or less, The water-absorbent resin particles have a median particle diameter of 250 μm or more and 376 μm or less. Water-absorbing resin particles.

2. An absorbent article comprising a liquid impermeable sheet, an absorbent body, and a liquid permeable sheet, the liquid impermeable sheet, the absorbent body, and the liquid permeable sheet being arranged in this order, An absorbent article, wherein the absorbent body comprises the water-absorbent resin particles according to claim 1.

3. A method for increasing a permeation rate of physiological saline into an absorbent body after a load is applied to the absorbent body, the absorbent body containing water-absorbent resin particles, after the absorbent body has absorbed water, the method comprising: a crosslinked polymer formed by polymerization of a monomer containing an ethylenically unsaturated monomer, the ethylenically unsaturated monomer containing at least one compound selected from the group consisting of (meth)acrylic acid and salts thereof, the proportion of the (meth)acrylic acid and salts thereof being 70 to 100 mol % based on the total amount of the monomers; the crosslinked polymer being crosslinked with an internal crosslinking agent and a surface crosslinking agent, the internal crosslinking agent and the surface crosslinking agent containing ethylene glycol diglycidyl ether, The method comprises reducing a gel outflow ratio of the water-absorbent resin particles, which is measured in an environment of 25±2°C, by a method comprising the following steps (1), (2), (3), (4), (5), (6) and (7) in this order: (1) A tubular body having an inner diameter of 5.0 cm and a bottom portion that closes one end of the tubular body and forms an opening with a diameter of 3.0 cm in the center is fixed in an orientation such that the longitudinal direction of the tubular body is vertical and the bottom portion is facing downward. (2) Inside the tubular body, a disk having a diameter of 4.9 cm and a thickness of 3.0 mm and having a through hole formed in the center is placed on the bottom with the main surface of the disk oriented horizontally. The through hole has a wall surface that is inclined with respect to the main surface of the disk so that the through hole is narrowest at the center position in the thickness direction of the disk, and the diameter of the through hole is a maximum of 8.0 mm at the positions of both main surfaces of the disk and a minimum of 5.0 mm at the center position in the thickness direction of the disk. (3) 20 g of swollen gel formed by the water-absorbent resin particles absorbing physiological saline is placed in the tubular body and placed on the disk. The swollen gel has a mass 30 times the mass of the water-absorbent resin particles before absorbing the physiological saline solution. (4) A cylindrical first weight having a mass of 500 g and a diameter of 4.9 cm is placed on the swollen gel in the tubular body to compress the swollen gel for 30 seconds. (5) A second cylindrical weight having a mass of 1500 g and a diameter of 4.9 cm is placed on the first weight to further compress the swollen gel with a load of 2000 g. (6) The swollen gel is compressed with a load of 2000 g for 30 seconds, and the mass W (g) of the swollen gel that flows out from the through-holes of the disk is measured. (7) Calculate the gel outflow ratio (%) using the following formula. Gel outflow ratio (%) = (W / 20) x 100

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