JP7585210B2 - Water-absorbent sheets and absorbent articles - Google Patents

Description

The present invention relates to an absorbent sheet and an absorbent article.

Recently, thin absorbents (or absorbent sheets) equipped with an absorbent layer capable of absorbing liquids have been used in absorbent articles for absorbing liquids mainly composed of water, such as urine. In such absorbents or absorbent sheets, multiple absorbent layers may be provided (for example, Patent Documents 1 to 3).

Japanese Patent Application Publication No. 10-118117 JP 2002-113800 A JP 2012-183175 A

Absorbent articles can leak liquid when in use, and the thinner they are, the more likely they are to leak liquid. There was room for further improvement in this regard.

Therefore, one aspect of the present invention aims to provide an absorbent sheet that can suppress liquid leakage from absorbent articles.

One aspect of the present invention relates to a water-absorbent sheet having a plurality of absorbent layers, one of which contains water-absorbent resin particles having a water-absorption capacity of 10.5 g/g or more and a permeation water absorption capacity of 12.5 g/g or more. Here, the water-absorption capacity is the mass of saline per 1 g of water-absorbent resin particles that is absorbed by the water-absorbent resin particles within 10 minutes after the start of absorption when 0.5 g of the water-absorbent resin particles placed in a container consisting of a cylinder with an inner diameter of 25 mm and a mesh sheet attached to the bottom of the cylinder are allowed to absorb saline according to the DW method. In addition, the permeation water absorption capacity is the amount of saline absorbed per 1 g of water-absorbent resin particles, measured 1 minute after the start of the introduction, after 0.5 g of the water-absorbent resin particles are placed on a mesh sheet in a container consisting of a cylinder with an inner diameter of 60 mm and a mesh sheet attached to the bottom of the cylinder.

Another aspect of the present invention relates to an absorbent article comprising a liquid-impermeable sheet, a water-absorbent sheet having a plurality of absorbent layers, and a liquid-permeable sheet, the liquid-impermeable sheet, the water-absorbent sheet, and the liquid-permeable sheet being arranged in this order, the outermost absorbent layer on the liquid-impermeable sheet side containing water-absorbent resin particles having a water-absorption capacity of 10.5 g/g or more and a water-permeation capacity of 12.5 g/g or more. Here, the water-absorption capacity is the mass of saline solution per 1 g of the water-absorbent resin particles that is absorbed by the water-absorbent resin particles within 10 minutes after the water-absorbent resin particles start absorbing saline solution when the water-absorbent resin particles are placed in a container consisting of a cylinder with an inner diameter of 25 mm and a mesh sheet attached to the bottom of the cylinder according to the DW method. In addition, the permeate absorption amount is an absorption amount of physiological saline per 1 g of water absorbent resin particles, which is measured 1 minute after 30 g of physiological saline is poured in after 0.5 g of water absorbent resin particles is placed on a mesh sheet in a container consisting of a cylinder with an inner diameter of 60 mm and a mesh sheet adhered to the bottom of the cylinder.

The present invention provides an absorbent sheet that can suppress liquid leakage from absorbent articles.

FIG. 2 is a cross-sectional view showing an example of a water-absorbent sheet. 4 is a plan view showing an example of an adhesive application pattern formed on a core wrap sheet. FIG. FIG. 1 is a cross-sectional view showing an example of an absorbent article. FIG. 2 is a schematic diagram showing a method for measuring the amount of water absorption. FIG. 2 is a schematic diagram showing a method for measuring water absorption under load. FIG. 2 is a schematic diagram showing a method for measuring a liquid leakage test.

Several embodiments of the present invention are described in detail below. However, the present invention is not limited to the following embodiments.

In this specification, "acrylic" and "methacrylic" are collectively referred to as "(meth)acrylic". "Acrylate" and "methacrylate" are also collectively referred to as "(meth)acrylate". 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 the numerical range of another stage. In the numerical ranges described in this specification, the upper or lower limit of the numerical range may be replaced with the value shown in the examples. "A or B" may contain either A or B, or may contain both. "Water-soluble" means that the material has 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 a plurality of substances corresponding to each component are present in the composition, the content of each component in the composition means the total amount of the plurality of substances present in the composition, unless otherwise specified.

The absorbent sheet according to one embodiment has multiple absorbent layers. One of the multiple absorbent layers, the outermost absorbent layer, contains absorbent resin particles having a water uptake capacity of 10.5 g/g or more and a permeate water absorption capacity of 12.5 g/g or more.

The amount of water absorption is the mass of saline per 1 g of water absorbent resin particles that is absorbed within 10 minutes after the water absorbent resin particles start absorbing saline, when 0.5 g of water absorbent resin particles are placed in a container consisting of a cylinder with an inner diameter of 25 mm and a mesh sheet attached to the bottom of the cylinder according to the DW method. The amount of water absorption is specifically measured by the method described in the examples below. In this specification, saline is an aqueous sodium chloride solution with a concentration of 0.9% by mass. The concentration of 0.9% by mass is based on the mass of saline.

The permeate absorption is the amount of saline absorbed per gram of water-absorbent resin particles, measured 1 minute after 30 g of saline is poured in after 0.5 g of water-absorbent resin particles are placed on a mesh sheet in a container consisting of a cylinder with an inner diameter of 60 mm and a mesh sheet attached to the bottom of the cylinder. Specifically, the permeate absorption is measured by the method described in the Examples below.

According to the water-absorbent sheet of this embodiment, it is possible to suppress liquid leakage in absorbent articles. The reason for obtaining such an effect is not clear, but the inventor speculates as follows. However, the cause is not limited to the following. According to the water-absorbent resin particles having the suction absorption amount and the permeation liquid absorption amount within the above range, in addition to enabling rapid liquid absorption, it is considered that the occurrence of a phenomenon in which the liquid passage is blocked by the concentration of absorbed water-absorbent resin particles (blocking) is suppressed, and it is possible to absorb more liquid. It is considered that liquid leakage is effectively suppressed by using an absorbent layer containing such water-absorbent resin particles as an absorbent layer located on the outermost layer opposite to the side where the liquid penetrates in the absorbent article.

In a plurality of absorbent layers, a layer other than the outermost layer containing the first water-absorbent resin particles (hereinafter sometimes referred to as the "other layer") may contain water-absorbent resin particles of the same type or a different type as the first water-absorbent resin particles (hereinafter sometimes referred to as the "second water-absorbent resin particles").

The multiple absorbent layers may have an outermost layer containing first water-absorbent resin particles and one or more layers other than the outermost layer (other layers) containing second water-absorbent resin particles. The absorbent sheet is arranged so that the outermost layer containing the first water-absorbent resin particles is on the opposite side to the side into which liquid penetrates. In the absorbent sheet, the number of absorbent layers may be 2 or more, and may be, for example, 2 to 5, 2 to 4, 2 to 3, or 2.

Figure 4 is a schematic diagram showing a method for measuring the amount of water suction. The measuring device shown in Figure 4 has the same configuration as the device for measuring the DW (Demand Wetability) value, and has a burette part 1, a conduit 5, a measuring stand 13, a stand 14, and a clamp 3. The burette part 1 has a burette tube 26 with a scale, a rubber plug 23 that seals the opening at the top of the burette tube 26, a cock 22 connected to the tip of the bottom of the burette tube 26, and an air introduction tube 25 and a cock 24 connected to the bottom of the burette tube 26. The burette part 1 is fixed with a clamp 3. The flat measuring stand 13 has a through hole 13a with a diameter of 2 mm formed in its center, and is supported by a stand 14 whose height is variable. The through hole 13a of the measuring stand 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.

The water absorption amount is measured by a method including the following steps in an environment of a temperature of 25° C. and a humidity of 60±10%.
(1) The stopcocks 22 and 24 of the burette part 1 are closed, and physiological saline solution 27 at 25° C. is poured into the burette tube 26 through the opening at the top of the burette tube 26.
(2) After sealing the opening of the burette tube 26 with the rubber plug 23, the cocks 22 and 24 are opened. The inside of the conduit 5 is filled with physiological saline solution 27 to prevent air bubbles from entering.
(3) The height of the measurement table 13 is adjusted so that the height of the water surface of the physiological saline solution that has reached the through hole 13a is the same as the height of the upper surface of the measurement table 13. After the adjustment, the height of the water surface of the physiological saline solution 27 in the burette tube 26 is read on the scale of the burette tube 26, and this position is set as the zero point (the reading at 0 seconds).
(4) 0.5 g of water-absorbent resin particles is uniformly scattered in a container for measuring the amount of water absorption, which is composed of a cylinder 16 (inner diameter 25 mmφ×height 150 mm) and a nylon mesh sheet 15 (250 mesh, thickness approximately 60 μm) adhered to the bottom of the cylinder.
(5) The nylon mesh sheet 15 on which the water-absorbent resin particles 11a are arranged is placed so that its center is located at the position of the through-hole 13a, and the measurement is started. The time when air bubbles are first introduced from the air inlet tube into the burette tube 26 is regarded as the start of absorption (0 seconds).
(6) The amount of reduction in the saline solution 27 in the burette tube 26 (i.e., the amount of saline solution absorbed by the water-absorbent resin particles 11a) is read in increments of 0.1 mL. The amount of water absorption A (amount of reduction) (mL) of the saline solution 27 10 minutes after the water-absorbent resin particles 11a start absorbing water is read. The amount of water absorption is calculated from the amount of water absorption A and the density of the saline solution (1.0028 g/mL) according to the following formula. The amount of water absorption is the mass of the absorbed saline solution per gram of the water-absorbent resin particles.
Water absorption amount [g/g] = A [mL] x 1.0028 (density of saline solution) [g/mL] / 0.5 [g]

The water absorption capacity of the first water-absorbent resin particles may be 12.0 g/g or more, 14.0 g/g or more, 16.0 g/g or more, 18.0 g/g or more, 20.0 g/g or more, 22.0 g/g or more, 24.0 g/g or more, 26.0 g/g or more, or 28.0 g/g or more, from the viewpoint of a more excellent liquid leakage suppression effect. The water absorption capacity of the first water-absorbent resin particles may be, for example, 50.0 g/g or less, 40.0 g/g or less, 35.0 g/g or less, or 30.0 g/g or less. The water absorption capacity of the second water-absorbent resin particles may be within the above range or may be outside the above range.

Water-absorbent resin particles having a water absorption capacity of 10.5 g/g or more can be obtained, for example, by subjecting the polymer particles described below to surface cross-linking and then surface modification after surface cross-linking, by increasing the surface area of the water-absorbent resin particles, or by a combination of these.

The permeate absorption is measured by a method including the following steps.
(1) 0.5 g of water-absorbent resin particles is uniformly scattered on a mesh sheet in a container consisting of an acrylic resin cylinder having an inner diameter of 60 mmφ and a height of 70 mm and a mesh sheet adhered to the bottom of the cylinder, and a total mass Wc of the container and the water-absorbent resin particles is measured.
(2) After the container in which the water-absorbent resin particles are arranged is placed on a wire mesh (mesh size: 1.4 mm, 100 mm x 100 mm) placed on an empty petri dish, 30 g of physiological saline (salt solution with a concentration of 0.9 wt %) is poured into the container in which the water-absorbent resin particles are arranged all at once, and after 1 minute, the total mass Wd of the container and its contents is measured.
(3) The permeate absorption amount is calculated from the total mass Wc of the container and the water-absorbent resin particles and the total mass Wd of the container and its contents by the following formula.
Permeate water absorption amount [g/g] = (Wd [g] - Wc [g]) / 0.5 [g]

From the viewpoint of achieving a more excellent liquid leakage suppression effect, the permeate absorption amount of the first water-absorbent resin particles may be 15.0 g/g or more, 17.0 g/g or more, 20.0 g/g or more, 25.0 g/g or more, 30.0 g/g or more, 35.0 g/g or more, or 40.0 g/g or more. The permeate absorption amount of the first water-absorbent resin particles may be, for example, 60 g/g or less, 55 g/g or less, or 50 g/g or less. The permeate absorption amount of the second water-absorbent resin particles may be within the above range or may be outside the above range.

Water-absorbent resin particles having a permeate absorption capacity of 12.5 g/g or more can be obtained, for example, by subjecting the polymer particles described below to surface cross-linking and surface modification after surface cross-linking, by increasing the surface area of the water-absorbent resin particles, or by a combination of these.

From the viewpoint of further suppressing liquid leakage, the saline water absorption capacity of the first water-absorbent resin particles and the second water-absorbent resin particles may be 40 g/g or more, 50 g/g or more, or 55 g/g or more, respectively, and may be 80 g/g or less, 70 g/g or less, or 65 g/g or less. The saline water absorption capacity is measured by the method described in the examples below.

The shapes of the first and second water-absorbent resin particles may be, for example, approximately spherical, crushed, or granular, or may be aggregates of primary particles having these shapes. The median particle diameters of the first and second water-absorbent resin particles may be 800 μm or less, 600 μm or less, 500 μm or less, 450 μm or less, or 400 μm or less, and may be 100 μm or more, 150 μm or more, 200 μm or more, 250 μm or more, 300 μm or more, or 350 μm or more. The median particle diameters of the first and second water-absorbent resin particles may be 100 to 800 μm, 150 to 600 μm, 200 to 500 μm, or 250 to 450 μm, respectively. The first and second water-absorbent resin particles may have a desired particle size distribution when obtained by a production method described later, but the particle size distribution may be adjusted by carrying out an operation such as particle size adjustment using classification with a sieve.

When the water-absorbent resin particles are used for sanitary materials, it is generally considered desirable for the water-absorbent resin particles to have a relatively large median particle diameter from the viewpoint of ease of handling during the manufacture of an absorbent or water-absorbent sheet. However, the water-absorbent resin particles (first water-absorbent resin particles) contained in the outermost layer of a water-absorbent sheet having multiple absorbent layers may be water-absorbent resin particles having a relatively small median particle diameter (for example, 450 μm or less or 300 μm or less) from the viewpoint of achieving an even more excellent liquid leakage suppression effect.

In the first water-absorbent resin particles, the content of particles having a particle diameter of 150 μm or less may be 21.0 mass% or less, 15.0 mass% or less, 10.0 mass% or less, 8.0 mass% or less, or 6.0 mass% or less, and may be 0.5 mass% or more, 1.0 mass% or more, or 1.5 mass% or more, based on the total mass of the water-absorbent resin particles, from the viewpoint of being more excellent in the liquid leakage suppression effect. In the first water-absorbent resin particles, the content of particles having a particle diameter of 150 μm or less may be 0.5 to 21.0 mass%, 1.0 to 15.0 mass%, or 1.5 to 10.0 mass%. The content of water-absorbent resin particles having a particle diameter of 150 μm or less is the content of water-absorbent resin particles that pass through a sieve with a mesh size of 150 μm based on the total mass of the water-absorbent resin particles.

When the water-absorbent resin particles are used for sanitary materials, it is generally considered desirable that the content of particles having a particle diameter of 150 μm or less is relatively low (e.g., less than 0.5 mass%) in order to reduce gaps (loss) during the production of the absorbent body or water-absorbent sheet. However, the water-absorbent resin particles (first water-absorbent resin particles) contained in the outermost layer of a water-absorbent sheet having multiple absorbent layers may be water-absorbent resin particles having a relatively high content of particles having a particle diameter of 150 μm or less (e.g., 0.5 mass% or more and 21.0 mass% or less) in order to achieve an even more excellent effect of suppressing liquid leakage.

The water absorption of physiological saline solution under a load of 2.07 kPa of the first and second water-absorbent resin particles (hereinafter sometimes simply referred to as "water absorption under load") may be 15 mL/g or more, 18 mL/g or more, 20 mL/g or more, 25 mL/g or more, or 30 mL/g or more, and may be 50 mL/g or less, 45 mL/g or less, 40 mL/g or less, or 34 mL/g or less. If the water absorption under load is large, the absorbent article can maintain a high water absorption capacity even when subjected to a load from a wearer. The water absorption under load is measured by the method described in the examples below.

The water absorption rate of the first water-absorbent resin particles in physiological saline by the DW method may be, for example, less than 5.0 g/30 sec/0.3 g, 1.0 g/30 sec/0.3 g or more, or 2.5 g/30 sec/0.3 g or more. The water absorption rate of physiological saline by the DW method is measured by the method described in the examples below.

The absorption speed of saline solution by the first water-absorbent resin particles using the Vortex method may be, for example, 2 seconds or more, 3 seconds or more, or 4 seconds or more, and may be 30 seconds or less, 25 seconds or less, 20 seconds or less, 16 seconds or less, or 13 seconds or less. The absorption speed of saline solution by the Vortex method is measured by the method described in the examples below.

The liquid permeation time of the second water-absorbent resin particles may be, for example, more than 20 seconds, and may be 60 seconds or less, 50 seconds or less, 40 seconds or less, or 35 seconds or less. The liquid permeation time is measured by the method described in the examples below.

The first and second water-absorbent resin particles may include, for example, a crosslinked polymer formed by polymerization of a monomer including an ethylenically unsaturated monomer. The crosslinked polymer has monomer units derived from the ethylenically unsaturated monomer.

The first and second water-absorbent resin particles can be manufactured by a method including a step of polymerizing a monomer including 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 easy control of the polymerization reaction, a reversed-phase suspension polymerization method or an aqueous solution polymerization method may be applied. In the following, a method of polymerizing an ethylenically unsaturated monomer will be described by taking a reversed-phase suspension polymerization method as an example.

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, diethylaminopropyl(meth)acrylamide, and the like. When the ethylenically unsaturated monomer has an amino group, the amino group may be quaternized. The ethylenically unsaturated monomer may be used alone or in combination of two or more kinds. The functional groups such as the carboxyl group and amino group of the above-mentioned monomers can function as functional groups capable of crosslinking in the surface crosslinking step described below.

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

The ethylenically unsaturated monomer can be used in the polymerization reaction 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 the "monomer aqueous solution") may be 20% by mass or more and less than the saturated concentration, 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.

As a monomer for obtaining water-absorbent 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 ratio of (meth)acrylic acid and its salts to the total amount of monomers may be 70 to 100 mol%.

When the ethylenically unsaturated monomer has an acid group, the acid group may be neutralized with an alkaline neutralizing agent before the monomer solution is used in the polymerization reaction. 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-absorbing resin particles and further increasing 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; ammonia, etc. The alkaline neutralizing agent may be used alone or in combination of two or more kinds. The alkaline neutralizing agent may be used in the form of an aqueous solution to simplify the neutralization operation. The neutralization of the acid group of the ethylenically unsaturated monomer can be carried out, for example, by dropping an aqueous solution of sodium hydroxide, potassium hydroxide, etc. into the above-mentioned aqueous monomer solution and mixing.

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 ethylenically unsaturated monomers can be polymerized using a radical polymerization initiator or the like. A water-soluble radical polymerization initiator can be used as the radical polymerization initiator.

Examples of surfactants include nonionic surfactants and anionic surfactants. Examples of nonionic surfactants include sorbitan fatty acid esters, (poly)glycerin fatty acid esters ("(poly)" means both cases with and without the "poly" prefix. The same applies below), 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 alkyl phenyl ethers, polyoxyethylene castor oil, polyoxyethylene hydrogenated castor oil, alkyl allyl 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, alkyl benzene sulfonates, alkyl methyl taurate salts, polyoxyethylene alkyl phenyl ether sulfate salts, polyoxyethylene alkyl ether sulfonates, polyoxyethylene alkyl ether phosphates, and polyoxyethylene alkyl allyl ether phosphates. The surfactants may be used alone or in combination of two or more kinds.

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 it is easy to obtain industrially, the surfactant may contain at least one compound selected from the group consisting of sorbitan fatty acid esters, polyglycerin fatty acid esters, and sucrose fatty acid esters. From the viewpoints that the water absorption properties of the obtained water-absorbent resin particles are easily improved, and that it is suitable for increasing the surface area of the water-absorbent resin particles, the surfactant may be a sorbitan fatty acid ester (e.g., sorbitan monolaurate) and/or a sucrose fatty acid ester (e.g., sucrose stearate). These surfactants may be used alone or in combination of two or more.

The amount of surfactant may be 0.05 to 10 parts by weight, 0.08 to 5 parts by weight, or 0.1 to 3 parts by weight per 100 parts by weight of the aqueous monomer solution.

In the reversed-phase suspension polymerization, a polymeric dispersant may be used in combination with the above-mentioned surfactant. 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, ethyl hydroxyethyl cellulose, etc. The polymeric dispersant may be used alone or in combination of two or more. From the viewpoint of excellent dispersion stability of the monomer, the polymer dispersant may be 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.

The amount of polymeric dispersant may be 0.05 to 10 parts by weight, 0.08 to 5 parts by weight, or 0.1 to 3 parts by weight per 100 parts by weight of the aqueous monomer solution.

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 the hydrocarbon dispersion medium 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.

From the viewpoint 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 Exxolheptane (manufactured by ExxonMobil Corp.: contains 75-85% of n-heptane and its isomers) may be used as the mixture of the above-mentioned hydrocarbon dispersion medium.

The amount of the hydrocarbon dispersion medium may be 30 to 1000 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 making it easy to control the polymerization temperature. When the amount of the hydrocarbon dispersion medium is 30 parts by mass or more, it tends to be easier to control the polymerization temperature. When the amount of the hydrocarbon dispersion medium is 1000 parts by mass or less, polymerization productivity tends to improve, which is economical.

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-(N-phenylamidino)propane] dihydrochloride. Examples of the azo compounds include 2,2'-azobis[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 kinds. 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.

The amount of radical polymerization initiator may be 0.00005 to 0.01 moles per mole of ethylenically unsaturated monomer. If 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. If the amount of radical polymerization initiator is 0.01 moles or less, it is easy to prevent a rapid polymerization reaction from occurring.

The exemplified 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.

During the polymerization reaction, the aqueous monomer solution may contain a chain transfer agent. Examples of chain transfer agents include hypophosphites, thiols, thiolic acids, secondary alcohols, and amines.

In order to control the particle size of the water-absorbent resin particles, the aqueous monomer solution used in the 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.

During polymerization, crosslinking can occur by self-crosslinking, but further crosslinking can also be achieved by using an internal crosslinking agent. Using an internal crosslinking agent makes it easier to control the water absorption properties of the water-absorbent resin particles. The internal crosslinking agent is usually added to the reaction liquid 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 diglycerides, Examples of the internal crosslinking agent include polyglycidyl compounds such as polyglycidyl 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 be a polyglycidyl compound or a diglycidyl ether compound. The internal crosslinking agent may include at least one selected from the group consisting of (poly)ethylene glycol diglycidyl ether, (poly)propylene glycol diglycidyl ether, and (poly)glycerin diglycidyl ether.

The amount of the internal crosslinking agent may be 0 millimoles or more, 0.01 millimoles or more, 0.015 millimoles or more, or 0.020 millimoles or more, or may be 0.1 moles or less, per mole of ethylenically unsaturated monomer, from the viewpoint that the polymer obtained by polymerization of the above-mentioned aqueous monomer solution is appropriately crosslinked to suppress the water-soluble properties and make it easier to obtain a sufficient amount of water absorption.

Reverse phase suspension polymerization can be carried out in a water-in-oil system by mixing an aqueous phase containing an ethylenically unsaturated monomer, a radical polymerization initiator, and, if necessary, an internal crosslinking agent, with an oil phase containing a hydrocarbon-based dispersant and, if necessary, a surfactant, a polymer-based dispersant, and the like, and heating the mixture with stirring.

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, if necessary, 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, so long as it is before the start of the polymerization reaction.

From the viewpoint of making it easier to reduce the amount of hydrocarbon dispersion medium remaining in the obtained water absorbent resin, it is also possible to disperse an aqueous monomer solution in a hydrocarbon dispersion medium in which a polymeric dispersant has been dispersed, and then further disperse a surfactant before carrying out polymerization.

Reverse phase suspension polymerization can be carried out in one stage or in two or more stages. From the viewpoint of increasing productivity, reverse phase suspension polymerization may be carried out in two or three stages.

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

The temperature of the polymerization reaction varies depending on the radical polymerization initiator used, but may be 20 to 150°C or 40 to 120°C from the viewpoint of rapidly progressing the polymerization and shortening the polymerization time, thereby improving economy, and easily removing the polymerization heat to carry out the reaction smoothly. The reaction time is usually 0.5 to 4 hours. The end 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 hydrous gel polymer.

After polymerization, a crosslinking agent may be added to the resulting 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.

Examples of crosslinking agents for post-polymerization crosslinking 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 be 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.

The amount of 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 water-soluble ethylenically unsaturated monomer, from the viewpoint of ensuring that the resulting hydrous gel polymer is appropriately crosslinked and exhibits favorable water absorption properties.

The crosslinking agent for post-polymerization crosslinking is added to the reaction solution after the polymerization reaction of the ethylenically unsaturated monomer. In the case of multi-stage polymerization, the crosslinking agent for post-polymerization crosslinking may be added after the multi-stage polymerization. Taking into consideration 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).

Next, water is removed from the obtained hydrous gel polymer. By drying to remove water, polymer particles containing a polymer of an ethylenically unsaturated monomer are obtained. Examples of drying methods include (a) a method in which water is removed by azeotropic distillation while the hydrous gel polymer is dispersed in a hydrocarbon dispersion medium, (b) a method in which the hydrous gel polymer is removed by decantation and dried under reduced pressure, and (c) a method in which the hydrous gel polymer is filtered through a filter and dried under reduced pressure.

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. By adding a flocculant, the particle size of the obtained water-absorbent resin particles can be increased. 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 excellent flocculation effect, the flocculant may be at least one selected from the group consisting of silica, aluminum oxide, talc, and kaolin.

In reversed-phase suspension polymerization, the coagulant may be pre-dispersed in the same type of hydrocarbon dispersion medium as that used in the polymerization or in water, and then this may be mixed under stirring into the hydrocarbon dispersion medium containing the hydrous gel polymer.

The amount of the flocculant may be 0.001 to 1 part by mass, 0.005 to 0.5 parts by mass, or 0.01 to 0.2 parts by mass, relative to 100 parts by mass of the ethylenically unsaturated monomer used in the polymerization. When the amount of the flocculant is within these ranges, water-absorbent resin particles having the desired particle size distribution are easily obtained.

The polymerization reaction can be carried out using various agitators having agitating blades. Examples of the agitating blades that can be used include flat blades, lattice blades, paddle blades, propeller blades, anchor blades, turbine blades, Pfaudler blades, ribbon blades, full zone blades, and Max Blend blades. The flat blades have a shaft (agitation shaft) and a flat plate portion (agitation portion) arranged around the shaft. Furthermore, the flat plate portion may have slits, etc.

In the production of water-absorbent resin particles, crosslinking (surface crosslinking) of the surface portion of the hydrous gel polymer may be performed using a crosslinking agent in the drying step (water removal step) or any of the subsequent steps. By performing surface crosslinking, the water absorption properties of the water-absorbent resin particles can be easily controlled. The water content of the hydrous gel polymer to be surface crosslinked may be 5 to 50% by mass, 10 to 40% by mass, or 15 to 35% by mass. The water content (mass%) of the hydrous gel polymer is calculated by the following formula.
Moisture content = [Ww/(Ww+Ws)]×100
Ww: the water content of the hydrous gel polymer calculated by subtracting the water content discharged outside the system in a drying step or the like from the water content contained in the aqueous monomer solution before polymerization in all polymerization steps, and adding the water content used as necessary when mixing a flocculant, a surface crosslinking agent, or the like.

Ws: Solid content calculated from the charged amounts of ethylenically unsaturated monomers, crosslinking agents, initiators, and other materials that make up the hydrous gel polymer.

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, α-methylepichlorohydrin, and the like. Examples of the surface crosslinking agent include haloepoxy compounds such as hydrin, 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 surface crosslinking agent may be used alone or in combination of two or more. The surface cross-linking agent may be a polyglycidyl compound, and may contain at least one 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.

The amount of the surface cross-linking agent 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. When the amount of the surface cross-linking agent is 0.00001 mol or more, the cross-linking 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 easily increased. When the amount of the surface cross-linking agent is 0.02 mol or less, the water absorption capacity of the water-absorbent resin particles is easily increased.

In the production of water-absorbent resin particles, the surface portion of the hydrous gel polymer may be treated (surface modified) using a surface modifier in the drying step (water removal step) or any of the steps thereafter. The surface modification may be performed, for example, before, during, or after the surface cross-linking step. The surface modification may be performed especially after the surface cross-linking. In particular, in the case of hydrous gel polymers obtained by reversed-phase suspension polymerization, aqueous solution polymerization, or other polymerization methods, when the hydrous gel polymer is treated with a surface modifier after surface cross-linking, there is a tendency for water-absorbent resin particles to be formed whose wicking water absorption and permeation water absorption are within the above ranges.

The surface modifier may be, for example, a surfactant such as a nonionic surfactant, an anionic surfactant, a cationic surfactant, or an amphoteric surfactant. For example, the HLB value of the nonionic surfactant used as the surface modifier may be, for example, 3 to 12, or 6 to 10. Examples of nonionic surfactants include sorbitan fatty acid esters such as sorbitan monolaurate. In particular, when the surface modifier is a nonionic surfactant with an HLB value within the above range, it tends to form water-absorbent resin particles whose suction water absorption amount and permeate water absorption amount are within the above range. The HLB value is measured by the Griffin method.

The amount of surface modifier may be 0.01 to 0.50 parts by weight, 0.02 to 0.40 parts by weight, or 0.04 to 0.30 parts by weight per 100 parts by weight of ethylenically unsaturated monomer used in the polymerization.

If necessary, surface cross-linking and/or surface modification can be performed, and then the water and hydrocarbon dispersion medium can be distilled off from the hydrous gel polymer to obtain polymer particles as a dry product.

The water-absorbent resin particles according to the present embodiment may be composed of only polymer particles, but may further contain various additional components selected from, for example, a gel stabilizer, a metal chelating agent, and a flow 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 flow improver (lubricant). The flow improver may contain inorganic particles. Examples of inorganic particles include silica particles such as amorphous silica.

The water-absorbent resin particles may contain a plurality of inorganic particles arranged on the surface of the polymer particles. 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 amount of the inorganic particles to the mass of the polymer particles may be 0.2 mass% or more, 0.5 mass% or more, 1.0 mass% or more, or 1.5 mass% or more, and may be 5.0 mass% or less, or 3.5 mass% or less. The inorganic particles here usually have a minute size compared to the size of the polymer particles. For example, the average particle size of the inorganic particles may be 0.1 to 50 μm, 0.5 to 30 μm, or 1 to 20 μm. The average particle size here can be a value measured by a dynamic light scattering method or a laser diffraction/scattering method.

1 is a cross-sectional view showing an example of an absorbent sheet. The absorbent sheet 50 shown in FIG. 1 has a first absorbent layer 10a, a second absorbent layer 10b, and three core wrap sheets 20a, 20b, and 20c. The core wrap sheets 20a and 20c are arranged on both sides of the first absorbent layer 10a, and the core wrap sheets 20b and 20c are arranged on both sides of the second absorbent layer 10b. In other words, the first absorbent layer 10a is arranged inside the core wrap sheets 20a and 20c, and the second absorbent layer 10b is arranged inside the core wrap sheets 20b and 20c. The first absorbent layer 10a and the second absorbent layer 10b are sandwiched between the two core wrap sheets to maintain their shape. The core wrap sheets 20a and 20b may be separate sheets, a folded sheet, or a bag.

The absorbent sheet 50 may further have an adhesive 21 interposed between the core wrap sheet 20a and the first absorbent layer 10a, and between the core wrap sheet 20b and the second absorbent layer 10b. FIG. 2 is a plan view showing an example of an adhesive application pattern formed on the core wrap sheet. The adhesive 21 shown in FIG. 2 forms an application pattern consisting of a plurality of linear portions arranged at intervals on the core wrap sheet 20a. The application pattern of the adhesive 21 may be linear, curved, dotted, or a combination thereof. The adhesive 21 may be interposed only between the core wrap sheet 20a and the first absorbent layer 10a, or between the core wrap sheet 20b and the second absorbent layer 10b. The absorbent sheet 50 may not have the adhesive 21. The adhesive 21 may be, for example, a water-based adhesive, a solvent-based adhesive, an elastic adhesive, an aerosol adhesive, a hot melt adhesive, or the like.

The first absorbent layer 10a and the second absorbent layer 10b are absorbent layers each containing the above-mentioned water-absorbent resin particles. The first absorbent layer 10a contains the first water-absorbent resin particles 11a. The first water-absorbent resin particles 11a have a water-absorption capacity of 10.5 g/g or more, and the first water-absorbent resin particles 11a have a water-absorption capacity of 12.5 g/g or more. The first absorbent layer 10a has a fiber layer 12a containing fibrous material in addition to the first water-absorbent resin particles 11a. The second absorbent layer 10b has the second water-absorbent resin particles 11b and a fiber layer 12b containing fibrous material. The first absorbent layer 10a may not have the fiber layer 12a. Similarly, the second absorbent layer 10b may not have the fiber layer 12b. The second water-absorbent resin particles 11b may be the same type as or different from the first water-absorbent resin particles 11a.

The content of water-absorbent resin particles in the absorbent layer may be 70 to 100% by mass, 80 to 100% by mass, or 90 to 100% by mass, based on the mass of the absorbent layer.

The thickness of the first absorbent layer 10a and the thickness of the second absorbent layer 10b may be, for example, 20 mm or less, 15 mm or less, 10 mm or less, 5 mm or less, 4 mm or less, or 3 mm or less, or 0.1 mm or more, or 0.3 mm or more, in a dry state. The thickness of the first absorbent layer 10a and the thickness of the second absorbent layer 10b may be the same or different.

The mass per unit area of the first absorbent layer 10a and the second absorbent layer 10b may be 800 g/m2 or ^less , 600 g/m2 or ^less , or 400 g/m2 ^or less, and 50 g/ ^m2 or more, 80 g/m2 ^or more, or 100 g/m2 or ^more . The mass per unit area of the first absorbent layer 10a and the mass per unit area of the second absorbent layer 10b may be the same or different.

The fibrous material constituting the fiber layers 12a, 12b can be, for example, cellulosic fibers, synthetic fibers, or a combination thereof. Examples of cellulosic fibers include comminuted wood pulp, cotton, cotton linters, rayon, and cellulose acetate. Examples of synthetic fibers include polyamide fibers, polyester fibers, and polyolefin fibers. The fibrous material may be a hydrophilic fiber (e.g., pulp).

The first absorbent layer 10a and the second absorbent layer 10b may further contain inorganic particles (e.g., amorphous silica), a deodorant, an antibacterial agent, a fragrance, etc. When the first water-absorbing resin particles 11a contain inorganic particles, the first absorbent layer 10a may contain inorganic particles other than the inorganic particles in the first water-absorbing resin particles 11a. Similarly, when the second water-absorbing resin particles 11b contain inorganic particles, the second absorbent layer 10b may contain inorganic particles other than the inorganic particles in the second water-absorbing resin particles 11b.

The core wrap sheets 20a, 20b, and 20c may be, for example, nonwoven fabrics. The core wrap sheets 20a, 20b, and 20c may be the same or different nonwoven fabrics. For example, the core wrap sheets 20a and 20b may be the same type of nonwoven fabric, and the core wrap sheet 20c may be a different type of nonwoven fabric. 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.

The core wrap sheets 20a, 20b, and 20c may be a thermally 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 containing two or more types of nonwoven fabric selected from these.

The nonwoven fabric used as the core wrap sheets 20a, 20b, and 20c can be a nonwoven fabric formed of synthetic fibers, natural fibers, or a combination thereof. Examples of synthetic fibers include fibers containing synthetic resins selected from 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. Examples of natural fibers include fibers containing cotton, silk, hemp, or pulp (cellulose). The fibers forming the nonwoven fabric may be polyolefin fibers, polyester fibers, or a combination thereof. The core wrap sheets 20a, 20b, and 20c may be tissue.

The absorbent sheet 50 can be obtained, for example, by the following method. First, the first absorbent resin particles 11a or a mixture containing the first absorbent resin particles 11a and fibrous material is sandwiched between the core wrap sheets 20a and 20c. Next, the second absorbent resin particles 11b or a mixture containing the second absorbent resin particles 11b and fibrous material is sandwiched between the core wrap sheets 20b and 20c. The absorbent sheet 50 in which the core wrap sheet 20a, the first absorbent layer 10a, the core wrap sheet 20c, the second absorbent layer 10b, and the core wrap sheet 20b are arranged in this order can be obtained by a method in which the structure formed by the above procedure is pressurized while being heated as necessary. If necessary, an adhesive 21 is arranged between the core wrap sheet 20a and the first absorbent resin particles 11a or a mixture containing the same, and/or between the core wrap sheet 20b and the second absorbent resin particles 11b or a mixture containing the same.

The absorbent sheet 50 is used, for example, to manufacture various absorbent articles. Examples of absorbent articles include diapers (e.g., paper diapers), toilet training pants, incontinence pads, sanitary materials (sanitary napkins, tampons, etc.), sweat pads, pet sheets, parts for simple toilets, and animal waste treatment materials.

Figure 3 is a cross-sectional view showing an example of an absorbent article. The absorbent article 100 shown in Figure 3 comprises an absorbent sheet 50 having a first absorbent layer 10a and a second absorbent layer 10b, a liquid-permeable sheet 30, and a liquid-impermeable sheet 40. In other words, the absorbent sheet 50 is sandwiched between the liquid-permeable sheet 30 and the liquid-impermeable sheet 40.

The absorbent sheet 50 is arranged so that the first absorbent layer 10a faces the liquid-impermeable sheet 40. In other words, the outermost absorbent layer facing the liquid-impermeable sheet 40 contains the first water-absorbent resin particles 11a.

The liquid-permeable sheet 30 is disposed at the outermost layer position on the side into which the liquid to be absorbed penetrates. The liquid-permeable sheet 30 is disposed outside the core wrap sheet 20b in contact with the core wrap sheet 20b. The liquid-impermeable sheet 40 is disposed at the outermost layer position on the opposite side of the liquid-permeable sheet 30 in the absorbent article 100. The liquid-impermeable sheet 40 is disposed outside the core wrap sheet 20a in contact with the core wrap sheet 20a. The liquid-permeable sheet 30 and the liquid-impermeable sheet 40 have a main surface that is wider than the main surface of the absorbent sheet 50, and the outer edge portions of the liquid-permeable sheet 30 and the liquid-impermeable sheet 40 extend around the first absorbent layer 10a, the second absorbent layer 10b, and the core wrap sheets 20a, 20b. However, the size relationships among the absorbent layers 10a, 10b, the core wrap sheets 20a, 20b, the liquid permeable sheet 30, and the liquid impermeable sheet 40 are appropriately adjusted depending on the application of the absorbent article, etc.

The liquid permeable sheet 30 may be a nonwoven fabric. The nonwoven fabric used as the liquid permeable sheet 30 may have a suitable 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 according to the measurement method of Paper and Pulp Test Method No. 68 (2000) by the Paper and Pulp Technology Association. The hydrophilicity of the nonwoven fabric may be 10 to 150. For details of Paper and Pulp Test Method No. 68, refer to, for example, WO2011/086843.

A hydrophilic nonwoven fabric may be formed from fibers that exhibit a moderate degree of hydrophilicity, such as rayon fibers, or from fibers obtained by hydrophilizing hydrophobic chemical fibers, such as polyolefin fibers and polyester fibers. Methods for obtaining a nonwoven fabric containing hydrophobic chemical fibers that have been hydrophilized include, for example, a method of obtaining a nonwoven fabric by a spunbond method using a mixture of hydrophobic chemical fibers and a hydrophilizing agent, a method of accompanying a hydrophilizing agent when producing a spunbond nonwoven fabric with hydrophobic chemical fibers, and a method of impregnating a spunbond nonwoven fabric obtained using hydrophobic chemical fibers with a hydrophilizing agent. Examples of hydrophilizing agents include anionic surfactants such as aliphatic sulfonates and higher alcohol sulfate 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 of polyester, polyamide, acrylic, and urethane resins.

The basis weight (mass per unit area) of the nonwoven fabric used as the liquid permeable sheet 30 may be 5 to 200 g/m ² , 8 to 150 g/m ² , or 10 to 100 g/m 2 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 thickness of the liquid permeable sheet 30 may be 20 to 1400 μm, 50 to 1200 μm, or 80 to 1000 μm.

The liquid-impermeable sheet 40 prevents the liquid absorbed in the absorbent layer 10 from leaking out from the liquid-impermeable sheet 40 side to the outside. The liquid-impermeable sheet 40 may be a resin sheet or a nonwoven fabric. The resin sheet may be a sheet made of a synthetic resin such as polyethylene, polypropylene, or polyvinyl chloride. The nonwoven fabric may be a spunbond/meltblown/spunbond (SMS) nonwoven fabric in which a water-resistant meltblown nonwoven fabric is sandwiched between high-strength spunbond nonwoven fabric. The liquid-impermeable sheet 40 may also be a composite sheet of a resin sheet and a nonwoven fabric (e.g., a spunbond nonwoven fabric, a spunlace nonwoven fabric). The liquid-impermeable sheet 40 may have breathability from the viewpoint of reducing stuffiness when worn and reducing discomfort to the wearer. For example, a low-density polyethylene (LDPE) resin sheet can be used as the breathable liquid-impermeable sheet 40.

From the viewpoint of ensuring flexibility so as not to impair the wearing comfort of the absorbent article, the basis weight (mass per unit area) of the liquid impermeable sheet 40 may be 5 to 100 g/m ² , or 10 to 50 g/m ² .

The absorbent article 100 can be manufactured, for example, by a method including arranging the water-absorbent sheet 50 between the liquid-permeable sheet 30 and the liquid-impermeable sheet 40. The laminate in which the liquid-impermeable sheet 40, the water-absorbent sheet 50, and the liquid-permeable sheet 30 are laminated in this order is pressurized as necessary. Alternatively, the absorbent article 100 can be obtained by arranging the liquid-permeable sheet 30, the core wrap sheet 20b, the first water-absorbent resin particles 11a or a mixture containing the first water-absorbent resin particles 11a and a fibrous material, the core wrap sheet 20c, the second water-absorbent resin particles 11b or a mixture containing the second water-absorbent resin particles 11b and a fibrous material, the core wrap sheet 20a, and the liquid-impermeable sheet 40 in this order, and pressurizing the formed structure while heating it as necessary. The respective constituent units may be bonded with an adhesive.

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

1. Preparation of water-absorbent resin particles Production Example 1
A round-bottomed cylindrical separable flask (baffle width: 7 mm, baffle length: 10 cm) with an inner diameter of 11 cm and an internal volume of 2 L and equipped with a reflux condenser, a dropping funnel, a nitrogen gas inlet tube, and a stirrer (a stirrer blade having two stages of four inclined paddle blades (surface-treated with fluororesin) with a blade diameter of 5 cm) and four side wall baffles was prepared. 451.4 g of n-heptane was added to this flask as a hydrocarbon dispersion medium, and 1.104 g of sorbitan monolaurate (Nonion LP-20R, HLB value: 8.6, manufactured by NOF Corporation) was added as a surfactant to obtain a mixture. The mixture was heated to 50° C. while stirring at a stirrer speed of 300 rpm to dissolve sorbitan monolaurate in n-heptane, and then the mixture was cooled to 40° C.

Next, 92.0 g of 80.5% by mass acrylic acid aqueous solution (acrylic acid: 1.03 mol) was placed in a 500 mL Erlenmeyer flask. Next, 147.7 g of 20.9% by mass sodium hydroxide aqueous solution was added dropwise while cooling with ice from the outside to neutralize the acrylic acid, thereby obtaining an aqueous solution of partially neutralized acrylic acid. Next, 0.1012 g (0.374 mmol) of potassium persulfate was added as a water-soluble radical polymerization initiator to the aqueous solution of partially neutralized acrylic acid and then dissolved to prepare an aqueous monomer solution.

After the monomer aqueous solution was added to the separable flask, the system was thoroughly replaced with nitrogen. The flask was then immersed in a water bath at 70°C while stirring at a stirrer speed of 700 rpm, and held there for 60 minutes to complete the polymerization, yielding a hydrous gel polymer.

After that, while stirring at 1000 rpm, a dispersion obtained by dispersing 0.092 g of amorphous silica (Oriental Silicas Corporation, Toxil NP-S) in 100 g of n-heptane as a powdered inorganic flocculant was added to the polymerization liquid containing the hydrogel polymer, n-heptane, and surfactant, and then mixed for 10 minutes. The flask containing the reaction liquid was then immersed in an oil bath at 125°C, and 129.0 g of water was extracted from the system while refluxing n-heptane by azeotropic distillation of n-heptane and water. After that, 4.14 g of a 2% by mass aqueous solution of ethylene glycol diglycidyl ether (ethylene glycol diglycidyl ether: 0.475 mmol) was added as a surface crosslinking agent, and the internal temperature was kept at 83±2°C for 2 hours.

Next, a surfactant solution containing 0.074 g of the surfactant sorbitan monolaurate (product name: Nonion LP-20R, HLB value 8.6, manufactured by NOF Corporation) dissolved in 6.62 g of n-heptane was added.

Then, the water and n-heptane were evaporated at 120°C, and the mixture was dried until almost no evaporants were distilled from the system, yielding a dried product. The dried product was passed through a sieve with an opening of 850 μm to obtain 90.1 g of water-absorbent resin particles. The median particle diameter of the water-absorbent resin particles was 382 μm.

The water absorption capacity of the water-absorbent resin particles of Production Example 1 was 60 g/g, and the water absorption capacity under load of the water-absorbent resin particles was 32.5 mL/g.

Production Example 2
Except for changing the amount of sorbitan monolaurate added to the separable flask before the addition of the aqueous monomer solution from 1.104 g to 0.97 g, and changing the amount of water extracted by azeotropic distillation of n-heptane and water from 129.0 g to 131.0 g, water absorbent resin particles were obtained in the same manner as in Production Example 1. The median particle diameter of the water absorbent resin particles was 384 μm.

The water absorption capacity of the water-absorbent resin particles of Production Example 2 was 63 g/g, and the water absorption capacity under load of the water-absorbent resin particles was 30.5 mL/g.

Production Example 3
A round-bottomed cylindrical separable flask (baffle width: 7 mm) with an inner diameter of 11 cm and a capacity of 2 L and equipped with four side wall baffles, was prepared, which was equipped with a reflux condenser, a dropping funnel, a nitrogen gas inlet tube, and a stirring blade having two stages of four inclined paddle blades (surface-treated with fluororesin) with a blade diameter of 5 cm as a stirrer. 451.4 g of n-heptane was placed in this flask as a petroleum-based hydrocarbon dispersion medium, and 0.984 g of sorbitan monolaurate (Nonion LP-20R, HLB value 8.6, manufactured by NOF Corporation) was added as a surfactant, and the stirrer was rotated at 300 rpm and heated to 50°C. After dissolving the sorbitan monolaurate in n-heptane by heating, the internal temperature was cooled to 40°C.

92.0 g (1.03 mol) of 80.5% by mass acrylic acid aqueous solution was placed in a 500 mL Erlenmeyer flask, and 147.7 g of 20.9% by mass sodium hydroxide aqueous solution was added dropwise while cooling with ice from the outside, to perform 75 mol % neutralization. Next, 0.101 g (0.374 mmol) of potassium persulfate was added and dissolved as a radical polymerization initiator in the resulting partially neutralized acrylic acid aqueous solution to prepare a monomer aqueous solution.

The above monomer aqueous solution was added to the separable flask, and the system was thoroughly replaced with nitrogen. The stirrer speed was then set to 700 rpm, and the flask was immersed in a water bath at 70°C and held there for 60 minutes.

After that, the stirrer was rotated at 1000 rpm, and 0.092 g of amorphous silica (Oriental Silicas Corporation, Toxil NP-S) as a powdered inorganic flocculant, which had been dispersed in 100 g of n-heptane in advance, was added to the polymerization liquid containing the produced hydrogel, n-heptane, and surfactant, and mixed for 10 minutes. After that, the flask containing the reaction liquid was immersed in an oil bath at 125°C, and 112 g of water was extracted from the system while refluxing n-heptane by azeotropic distillation of n-heptane and water, after which 4.97 g (2.85 mmol) of a 10% by weight aqueous solution of ethylene glycol diglycidyl ether was added as a post-crosslinking agent, and the internal temperature was kept at 83±2°C for 2 hours.

Next, a surfactant solution containing 0.074 g of the surfactant sorbitan monolaurate (product name: Nonion LP-20R, HLB value 8.6, manufactured by NOF Corporation) dissolved in 6.62 g of n-heptane was added.

Then, the water and n-heptane were evaporated and the system was dried until almost no evaporants were distilled out. The flask was then removed from the oil bath and 13.8 g of water was sprayed at a flow rate of 0.3 g per second using a sprayer. The system was then maintained at 80°C for 30 minutes while nitrogen was blown into it at a flow rate of 200 mL per minute to obtain a dried product. The dried product was passed through a sieve with an opening of 850 μm to obtain 90.5 g of water-absorbent resin particles. The median particle diameter of the water-absorbent resin particles was 368 μm.

The water absorption capacity of the water-absorbent resin particles of Production Example 3 was 63 g/g, and the water absorption capacity under load of the water-absorbent resin particles was 32.0 mL/g.

Production Example 4
A round-bottomed cylindrical separable flask with an inner diameter of 110 mm was prepared, which was equipped with a reflux condenser, a nitrogen gas inlet tube, and a stirring blade (with a fluororesin surface coating) having two stages of four inclined paddle blades with a blade diameter of 50 mm as a stirrer. 451.4 g of n-heptane was placed in this flask as a hydrocarbon dispersion medium, and 1.104 g of sorbitan monolaurate (NOF Corp., Nonion LP-20R) with an HLB of 8.6 was added as a surfactant, and the flask was immersed in a water bath at 80° C. while stirring to raise the temperature. Next, the inside of the system was thoroughly replaced with nitrogen.

92 g (1.03 mol) of 80.5% by mass acrylic acid aqueous solution was placed in a 500 mL Erlenmeyer flask, and neutralization was performed by dropping 147.7 g of 20.9% by mass sodium hydroxide aqueous solution while cooling it from the outside. 0.1104 g (0.407 mmol) of 2,2'-azobis(2-amidinopropane) dihydrochloride as an azo compound and 0.0092 g (0.0528 mmol) of ethylene glycol diglycidyl ether as an internal crosslinking agent were added and dissolved in the neutralized solution to prepare a monomer aqueous solution.

Next, after sufficiently aerating the monomer aqueous solution with nitrogen, the stirring speed was set to 1000 rpm, and the monomer aqueous solution was dripped into the separable flask at a constant rate so that the drip time for the entire monomer aqueous solution was 7 minutes. After the dripping was completed, 0.41 g (0.047 mmol) of a 2% by mass aqueous solution of ethylene glycol diglycidyl ether was added as an intermediate crosslinking agent to the separable flask, and the mixture was kept at 80°C for 20 minutes to obtain a hydrous gel polymer.

Next, the reaction liquid in the separable flask was heated in an oil bath at 125°C, and 105 g of water was extracted from the system while refluxing n-heptane into the system by azeotropic distillation of n-heptane and water. After that, 4.14 g (0.475 mmol) of a 2% by mass aqueous solution of ethylene glycol diglycidyl ether was added to the separable flask as a post-crosslinking agent, and the mixture was maintained at 83°C for 120 minutes.

Next, a surfactant solution containing 0.074 g of the surfactant sorbitan monolaurate (product name: Nonion LP-20R, HLB value 8.6, manufactured by NOF Corporation) dissolved in 6.62 g of n-heptane was added to the separable flask.

Then, the water and n-heptane were evaporated and the system was dried until almost no evaporants flowed out, yielding a dried polymer particle product. The dried product was passed through a sieve with an opening of 850 μm to obtain 70.0 g of granular water-absorbent resin particles. The median particle diameter of the water-absorbent resin particles was 448 μm.

The water absorption capacity of the water-absorbent resin particles of Production Example 4 was 63 g/g, and the water absorption capacity under load of the water-absorbent resin particles was 33.1 mL/g.

Production Example 5
[Preparation of Aqueous Monomer Solution]
A 2 L separable flask was charged with 130.0 g (1.80 mol) of 100% acrylic acid. 112.3 g of ion-exchanged water was added while stirring the contents of the separable flask, and then 112.85 g of 48% by mass sodium hydroxide was added dropwise in an ice bath to prepare a partially neutralized acrylic acid solution having a monomer concentration of 45% by mass and a neutralization rate of 75 mol%.

[polymerization]
A stirrer (diameter 8 mm, length 40 mm) was placed in a stainless steel tray (φ20 cm), and 340.0 g of partially neutralized acrylic acid solution (monomer concentration 45% by mass, neutralization rate of acrylic acid 75 mol%) as a monomer to be used for polymerization, 32.6 g of ion-exchanged water, and 0.0541 g (0.311 mmol) of ethylene glycol diglycidyl ether as an internal crosslinking agent were added, and then the stirrer was rotated and uniformly dispersed to obtain a mixture (concentration of partially neutralized acrylic acid solution 38% by mass). Then, the top of the stainless steel tray was sealed with a polyethylene film to cover the inside of the stainless steel tray. After adjusting the temperature of the mixture in the stainless steel tray to 25° C., the mixture was replaced with nitrogen to reduce the amount of dissolved oxygen to 0.1 ppm or less. Next, 24.75 g (1.831 mmol) of a 2% by mass aqueous solution of potassium persulfate and 5.20 g of a 0.5% by mass aqueous solution of L-ascorbic acid were added dropwise in this order under stirring at 300 rpm to prepare an aqueous monomer solution. Polymerization started immediately after the addition of the 0.5% by mass aqueous solution of L-ascorbic acid. 10 minutes after the start of polymerization, the resulting polymerized gel was immersed in a water bath at 75° C. while still in the container and aged for 20 minutes. The thickness of the resulting polymerized gel was 1.3 cm.

[Crushing of the polymerized gel]
The entire amount of the hydrogel polymer after aging was taken out of the container and cut into a grid pattern with 5 cm intervals. The entire amount of the hydrogel polymer cut into 5 cm intervals was sequentially put into a meat chopper 12VR-750SDX manufactured by Kire Royal Co., Ltd. and crushed. The diameter of the hole in the plate located at the outlet of the meat chopper was 6.4 mm. Crushing was performed until the crushed gel (crushed hydrogel) no longer came out from the plate of the meat chopper. At this time, the amount of hydrogel polymer put into the meat chopper was 390 g, and the amount of the crushed hydrogel that was discharged and recovered was 217 g.

Drying of Crushed Gel
The coarsely crushed gel was spread on a sieve with 850 μm mesh and dried for 30 minutes in a hot air dryer (manufactured by ADVANTEC, model number: DRE320DB) set at 180° C. to obtain a dried product.

[Crushing of dried material]
The above-mentioned dried product was pulverized in an ultracentrifugal pulverizer (Verder Scientific, ZM200, 6-blade rotor, rotor rotation speed: 6000 rpm, trapezoidal screen hole: 1.00 mm) to obtain 101 g of crosslinked polymer particles. The same operations from polymerization to pulverization were repeated to obtain a total of 198 g of crosslinked polymer.

[Surface cross-linking]
25.0 g of the crushed particles was weighed into a round-bottomed cylindrical separable flask with an inner diameter of 11 cm equipped with an anchor-shaped stirring blade made of fluororesin. Next, while stirring at 500 rpm, an aqueous crosslinking agent solution obtained by mixing 0.060 g of ethylene glycol diglycidyl ether, 3.6 g of water, 1.20 g of propylene glycol, and 1.20 g of isopropyl alcohol was added to the separable flask. The mixture obtained by this addition was uniformly spread on a tray coated with fluororesin, and then heated at 180° C. for 40 minutes. After cooling to room temperature, the particles were passed through a 850 μm mesh to obtain crosslinked polymer particles subjected to surface crosslinking. The surface crosslinking operation was performed a total of seven times to obtain 165 g of crosslinked polymer particles.

[Classification of crosslinked polymer particles]
The crosslinked polymer particles after the above-mentioned surface crosslinking were classified using a sieve with an opening of 850 μm, a sieve with an opening of 180 μm, and a receiver. Those that passed through the sieve with an opening of 850 μm and remained on the sieve with an opening of 180 μm were collected as raw material particles, and those that passed through the sieve with an opening of 180 μm were collected as raw material fine particles.

[Granulation of crosslinked polymer particles]
3.75 g of the above-mentioned raw material fine particles were added to a 100 mL new disposable cup (manufactured by AS ONE Corporation, CODE1-4620-01). While stirring at 600 rpm with a magnetic stir bar (8 mmφ×45 mm without ring), 5.63 g of ion-exchanged water was dropped at 0.1 g/sec using a macropipettor (manufactured by Shibata Scientific Co., Ltd., 10 mL volume) and stirred for 1 minute. Then, 8.75 g of the above-mentioned raw material particles were added and further stirred at 600 rpm for 1 minute to obtain an aggregate.

The entire amount of the above-mentioned aggregate was transferred to a glass petri dish (inner diameter 5.8 cm) using a spatula, and dried for 150 minutes in a hot air dryer (manufactured by ADVANTEC, model number: DRE320DB) set at 80°C to obtain a dried aggregate. This granulation operation was repeated five times.

The dried aggregate obtained above was put into an ultracentrifugal grinder (Verder Scientific, ZM200, 6-blade rotor, rotor rotation speed: 6000 rpm, trapezoidal screen hole: 1.00 mm, inlet diameter φ3 cm) and ground to obtain 60 g of polymer particles. The polymer particles were passed through a sieve with a mesh size of 500 μm. Then, 0.5% by mass of amorphous silica (Oriental Silicas Corporation, Toxil NP-S) relative to the mass of the polymer particles was mixed with the polymer particles to obtain 43.3 g of water-absorbent resin particles containing amorphous silica. The median particle size was 208 μm.

The water absorption capacity of the water-absorbent resin particles of Production Example 5 was 50 g/g, and the water absorption capacity under load of the water-absorbent resin particles was 27.0 mL/g.

Production Example 6
[First stage polymerization reaction]
A round-bottomed cylindrical separable flask with an inner diameter of 11 cm and a volume of 2 L was prepared, which was equipped with a reflux condenser, a dropping funnel, a nitrogen gas inlet tube, and a stirring blade having two stages of four inclined paddle blades with a blade diameter of 5 cm. 293.0 g of n-heptane was added to this flask as a hydrocarbon dispersion medium, and 0.736 g of maleic anhydride-modified ethylene-propylene copolymer (Mitsui Chemicals, Inc., Hiwax 1105A) was added as a polymeric dispersant. The mixture was heated to 80°C while stirring to dissolve the dispersant, and then cooled to 50°C.

92.0 g (1.03 mol) of 80.5% by mass acrylic acid aqueous solution was placed in a 300 mL beaker as a water-soluble ethylenically unsaturated monomer, and while cooling from the outside, 147.7 g of 20.9% by mass sodium hydroxide aqueous solution was added dropwise to neutralize 75 mol%, after which 0.092 g of hydroxyethyl cellulose (Sumitomo Seika Chemicals Co., Ltd., HECAW-15F) as a thickener, 0.092 g (0.339 mmol) of 2,2'-azobis(2-amidinopropane) dihydrochloride as a water-soluble radical polymerization initiator, and 0.018 g (0.067 mmol) of potassium persulfate, and 0.010 g (0.057 mmol) of ethylene glycol diglycidyl ether as an internal crosslinking agent were added and dissolved to prepare the first stage monomer aqueous solution.

The aqueous liquid prepared above was then added to a separable flask and stirred for 10 minutes, after which a surfactant solution prepared by heating and dissolving 0.736 g of sucrose stearate with an HLB of 3 (Ryoto Sugar Ester S-370, Mitsubishi Chemical Foods Corporation) in 6.62 g of n-heptane was further added. The system was thoroughly replaced with nitrogen while stirring at a stirrer speed of 550 rpm, and the flask was then immersed in a water bath at 70°C to raise the temperature, and polymerization was carried out for 60 minutes to obtain a first-stage polymerization slurry.

[Second-stage polymerization reaction]
In a 500 mL beaker was placed 128.8 g (1.44 mol) of an 80.5 mass% aqueous acrylic acid solution as a water-soluble ethylenically unsaturated monomer, and while cooling from the outside, 159.0 g of a 27 mass% aqueous sodium hydroxide solution was added dropwise to carry out 75 mol% neutralization, and then 0.129 g (0.476 mmol) of 2,2'-azobis(2-amidinopropane) dihydrochloride as a water-soluble radical polymerization initiator, 0.026 g (0.096 mmol) of potassium persulfate, and 0.0116 g (0.067 mmol) of ethylene glycol diglycidyl ether as an internal crosslinking agent were added and dissolved to prepare a second-stage aqueous monomer solution.

While stirring at a stirrer speed of 1000 rpm, the separable flask system was cooled to 25°C, and the entire amount of the second-stage monomer aqueous solution was added to the first-stage polymerization slurry liquid, and the system was replaced with nitrogen for 30 minutes. After that, the flask was again immersed in a 70°C water bath to raise the temperature, and the polymerization reaction was carried out for 60 minutes. This resulted in a hydrous gel polymer.

After the second polymerization step, 0.589 g of 45% by weight aqueous solution of pentasodium diethylenetriaminepentaacetate was added to the hydrous gel polymer under stirring. The reaction solution was then heated in a 125°C oil bath, and 239.2 g of water was extracted from the system by azeotropic distillation of n-heptane and water while refluxing n-heptane. Then, 4.42 g (0.507 mmol) of 2% by weight aqueous solution of ethylene glycol diglycidyl ether was added to the flask as a post-crosslinking agent, and the mixture was kept at 83°C for 2 hours.

Thereafter, the water and n-heptane were evaporated at 125°C to dry the polymer particles, yielding a dried product. The polymer particles were passed through a sieve with an opening of 850 μm, and 0.2% by mass of amorphous silica (Toxil NP-S, Oriental Silicas Corporation) was mixed with the polymer particles, yielding 228.1 g of water-absorbent resin particles containing amorphous silica. The median particle diameter of the water-absorbent resin particles was 362 μm.

The water absorption capacity of the water-absorbent resin particles of Production Example 6 was 61 g/g, and the water absorption capacity under load of the water-absorbent resin particles was 39.5 mL/g. The liquid permeation time of the water-absorbent resin particles was 30.8 seconds.

Production Example 7
[First stage polymerization reaction]
A round-bottomed cylindrical separable flask with an inner diameter of 11 cm and a volume of 2 L was prepared, which was equipped with a reflux condenser, a dropping funnel, a nitrogen gas inlet tube, and a stirring blade having two stages of four inclined paddle blades with a blade diameter of 5 cm. 293 g of n-heptane was added to this flask as a hydrocarbon dispersion medium, and 0.736 g of maleic anhydride-modified ethylene-propylene copolymer (Mitsui Chemicals, Inc., Hiwax 1105A) was added as a polymeric dispersant. The mixture was heated to 80°C with stirring to dissolve the dispersant, and then cooled to 50°C.

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

The aqueous liquid prepared above was then added to a separable flask and stirred for 10 minutes, after which a surfactant solution prepared by heating and dissolving 0.736 g of sucrose stearate with an HLB of 3 (Ryoto Sugar Ester S-370, Mitsubishi Chemical Foods Corporation) in 6.62 g of n-heptane was further added. The system was thoroughly replaced with nitrogen while stirring at a stirrer speed of 500 rpm, and the flask was then immersed in a water bath at 70°C to raise the temperature, and polymerization was carried out for 60 minutes to obtain a first-stage polymerization slurry.

[Second-stage polymerization reaction]
In a 500 mL beaker was placed 128.8 g (1.44 mol) of an 80.5 mass% aqueous acrylic acid solution as a water-soluble ethylenically unsaturated monomer, and while cooling from the outside, 159.0 g of a 27 mass% aqueous sodium hydroxide solution was added dropwise to carry out 75 mol% neutralization, and then 0.090 g (0.333 mmol) of potassium persulfate as a water-soluble radical polymerization initiator and 0.0116 g (0.067 mmol) of ethylene glycol diglycidyl ether as an internal crosslinking agent were added and dissolved to prepare a second-stage aqueous monomer solution.

While stirring at a stirrer speed of 1000 rpm, the contents of the separable flask system were cooled to 44°C, and the entire amount of the second-stage aqueous liquid was added to the first-stage polymerization slurry liquid. The system was then purged with nitrogen for 30 minutes, after which the flask was again immersed in a 70°C water bath to raise the temperature, and the polymerization reaction was carried out for 60 minutes to obtain a hydrous gel polymer.

The flask was then immersed in an oil bath set at 125°C, and 260.1 g of water was extracted from the system by azeotropic distillation of n-heptane and water while refluxing n-heptane. Then, 4.42 g (0.507 mmol) of a 2% by mass aqueous solution of ethylene glycol diglycidyl ether was added to the flask as a surface cross-linking agent, and the flask was kept at 83°C for 2 hours.

Thereafter, the water and n-heptane were evaporated at 125°C and the mixture was dried to obtain polymer particles (dried product). The polymer particles were passed through a sieve with an opening of 850 μm, 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 to obtain 231.0 g of water-absorbent resin particles containing amorphous silica. The median particle diameter of the water-absorbent resin particles was 76 μm.

The water absorption capacity of the water-absorbent resin particles of Production Example 7 was 67 g/g, and the water absorption capacity under load of the water-absorbent resin particles was 3.3 mL/g.

Production Example 8
[First stage polymerization reaction]
A round-bottomed cylindrical separable flask with an inner diameter of 11 cm and a volume of 2 L was prepared, which was equipped with a reflux condenser, a dropping funnel, a nitrogen gas inlet tube, and a stirring blade having two stages of four inclined paddle blades with a blade diameter of 5 cm. 293.0 g of n-heptane was added to this flask as a hydrocarbon dispersion medium, and 0.736 g of maleic anhydride-modified ethylene-propylene copolymer (Mitsui Chemicals, Inc., Hiwax 1105A) was added as a polymeric dispersant. The mixture was heated to 80°C while stirring to dissolve the dispersant, and then cooled to 50°C.

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

The aqueous liquid prepared above was then added to a separable flask and stirred for 10 minutes, after which a surfactant solution prepared by heating and dissolving 0.736 g of sucrose stearate with an HLB of 3 (Ryoto Sugar Ester S-370, Mitsubishi Chemical Foods Corporation) in 6.62 g of n-heptane was further added. The system was thoroughly replaced with nitrogen while stirring at a stirrer speed of 550 rpm, and the flask was then immersed in a water bath at 70°C to raise the temperature, and polymerization was carried out for 60 minutes to obtain a first-stage polymerization slurry.

[Second-stage polymerization reaction]
In a 500 mL beaker was placed 128.8 g (1.44 mol) of an 80.5 mass% aqueous acrylic acid solution as a water-soluble ethylenically unsaturated monomer, and while cooling from the outside, 159.0 g of a 27 mass% aqueous sodium hydroxide solution was added dropwise to carry out 75 mol% neutralization, and then 0.090 g (0.333 mmol) of potassium persulfate as a water-soluble radical polymerization initiator and 0.0116 g (0.067 mmol) of ethylene glycol diglycidyl ether as an internal crosslinking agent were added and dissolved to prepare a second-stage aqueous liquid.

While stirring at a stirrer speed of 1000 rpm, the separable flask system was cooled to 25°C, and the entire amount of the second-stage aqueous liquid was added to the first-stage polymerization slurry liquid. The system was replaced with nitrogen for 30 minutes, and the flask was again immersed in a 70°C water bath to raise the temperature, and the polymerization reaction was carried out for 60 minutes. This resulted in a hydrous gel polymer.

0.265 g of 45% by mass aqueous solution of pentasodium diethylenetriaminepentaacetate was added to the hydrous gel after the second-stage polymerization under stirring. The reaction solution was then heated in a 125°C oil bath, and 256.1 g of water was extracted from the system by azeotropic distillation of n-heptane and water while refluxing n-heptane. 4.42 g (0.507 mmol) of 2% by mass aqueous solution of ethylene glycol diglycidyl ether was then added to the flask as a post-crosslinking agent, and the solution was kept at 83°C for 2 hours.

Then, the water and n-heptane were evaporated at 125°C to dry the product, yielding a dried product. The dried product was passed through a sieve with an opening of 850 μm, and 0.1% by mass of amorphous silica (Toxil NP-S, Oriental Silicas Corporation) was mixed with the dried product to obtain 230.8 g of water-absorbent resin particles. The median particle size of the water-absorbent resin particles was 402 μm.

The water absorption capacity of the water-absorbent resin particles of Production Example 8 was 60 g/g, and the water absorption capacity under load of the water-absorbent resin particles was 34.5 mL/g. The liquid permeation time of the water-absorbent resin particles was 32.2 seconds.

Production Example 9
[First stage polymerization reaction]
A round-bottomed cylindrical separable flask with an inner diameter of 11 cm and a volume of 2 L was prepared, which was equipped with a reflux condenser, a dropping funnel, a nitrogen gas inlet tube, and a stirring blade having two stages of four inclined paddle blades with a blade diameter of 5 cm. 293 g of n-heptane was added to this flask as a hydrocarbon dispersion medium, and 0.736 g of maleic anhydride-modified ethylene-propylene copolymer (Mitsui Chemicals, Inc., Hiwax 1105A) was added as a polymeric dispersant. The mixture was heated to 80°C with stirring to dissolve the dispersant, and then cooled to 50°C.

92.0 g (1.03 mol) of 80.5% by mass acrylic acid aqueous solution was placed in a 300 mL beaker as a water-soluble ethylenically unsaturated monomer, and while cooling from the outside, 147.7 g of 20.9% by mass sodium hydroxide aqueous solution was added dropwise to neutralize 75 mol%, after which 0.092 g of hydroxyethyl cellulose (Sumitomo Seika Chemicals Co., Ltd., HECAW-15F) as a thickener, 0.092 g (0.339 mmol) of 2,2'-azobis(2-amidinopropane) dihydrochloride as a water-soluble radical polymerization initiator, and 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 to prepare the first stage monomer aqueous solution.

The aqueous liquid prepared above was then added to a separable flask and stirred for 10 minutes, after which a surfactant solution prepared by heating and dissolving 0.736 g of sucrose stearate with an HLB of 3 (Ryoto Sugar Ester S-370, Mitsubishi Chemical Foods Corporation) in 6.62 g of n-heptane was further added. The system was thoroughly replaced with nitrogen while stirring at a stirrer speed of 550 rpm, and the flask was then immersed in a water bath at 70°C to raise the temperature, and polymerization was carried out for 60 minutes to obtain a first-stage polymerization slurry.

[Second-stage polymerization reaction]
In a 500 mL beaker was placed 128.8 g (1.44 mol) of an 80.5 mass% aqueous acrylic acid solution as a water-soluble ethylenically unsaturated monomer, and while cooling from the outside, 159.0 g of a 27 mass% aqueous sodium hydroxide solution was added dropwise to carry out 75 mol% neutralization, and then 0.129 g (0.476 mmol) of 2,2'-azobis(2-amidinopropane) dihydrochloride as a water-soluble radical polymerization initiator, 0.026 g (0.096 mmol) of potassium persulfate, and 0.0116 g (0.067 mmol) of ethylene glycol diglycidyl ether as an internal crosslinking agent were added and dissolved to prepare a second-stage aqueous monomer solution.

While stirring at a stirrer speed of 1000 rpm, the contents of the separable flask system were cooled to 25°C, and the entire amount of the second-stage aqueous liquid was added to the first-stage polymerization slurry liquid. The system was then purged with nitrogen for 30 minutes, after which the flask was again immersed in a 70°C water bath to raise the temperature, and the polymerization reaction was carried out for 60 minutes.

0.589 g of 45% by mass aqueous solution of pentasodium diethylenetriaminepentaacetate was added to the hydrous gel after the second-stage polymerization under stirring. The reaction solution was then heated in a 125°C oil bath, and 216.7 g of water was extracted from the system by azeotropic distillation of n-heptane and water while refluxing n-heptane. 4.42 g (0.507 mmol) of 2% by mass aqueous solution of ethylene glycol diglycidyl ether was then added to the flask as a surface crosslinking agent, and the mixture was kept at 83°C for 2 hours.

Then, the n-heptane was evaporated at 125°C to dry the product, yielding a dried product. The dried product was passed through a sieve with an opening of 850 μm, and 0.2% by mass of amorphous silica (Toxil NP-S, Oriental Silicas Corporation) was mixed with the dried product, yielding 229.0 g of water-absorbent resin particles. The median particle size of the water-absorbent resin particles was 351 μm.

The water absorption capacity of the water-absorbent resin particles of Production Example 9 was 56 g/g, and the water absorption capacity under load of the water-absorbent resin particles was 35.3 mL/g.

2. Evaluation of Water-Absorbent Resin Particles <Water Absorption Amount of Physiological Saline (g/g)>
500 g of physiological saline (0.9% by weight sodium chloride aqueous solution) was weighed out into a 500 mL beaker, and 2.0 g of water-absorbent resin particles were dispersed in a manner that did not cause lumps while stirring at 600 rpm with a magnetic stir bar (8 mmφ×30 mm without ring). The mixture was left in a stirred state for 60 minutes to allow the water-absorbent resin particles to swell sufficiently. Thereafter, the mass Wa (g) of a standard sieve with a mesh size of 75 μm was measured in advance, and the contents of the beaker were filtered using this, and the sieve was tilted at an angle of about 30 degrees to the horizontal and left for 30 minutes to filter out excess water. The mass Wb (g) of the sieve containing the swollen gel was measured, and the amount of water absorbed by the physiological saline was calculated using the following formula.
Water absorption amount of physiological saline solution [g/g] = (Wb [g] - Wa [g]) / 2.0 [g]

<Water absorption amount (g/g)>
Using the measuring device shown in Fig. 4, the amount of water absorption was measured under an environment of 25 ± 2 ° C. temperature and 60 ± 10% humidity. First, the cocks 22 and 24 of the burette part 1 were closed, and physiological saline (0.9 mass% saline) 27 adjusted to 25 ° C. was poured into the burette tube 26 from the opening at the top of the burette tube 26. After sealing the opening of the burette tube 26 with a rubber stopper 23, the cocks 22 and 24 were opened. The inside of the conduit 5 was filled with physiological saline 27 so that no air bubbles were introduced. The height of the measurement table 13 was adjusted so that the height of the water surface of the physiological saline reaching the through hole 13a was the same as the height of the upper surface of the measurement table 13. After the adjustment, the height of the water surface of the physiological saline 27 in the burette tube 26 was read on the scale of the burette tube 26, and that position was set as the zero point (the reading at 0 seconds).

A container for measuring the amount of water absorption was prepared, consisting of an acrylic resin cylinder 16 (inner diameter 25 mmφ × height 150 mm) and a nylon mesh sheet 15 (250 mesh, thickness approximately 60 μm) attached to the bottom of the cylinder. 0.5 g of water-absorbent resin particles were evenly spread in the container for measuring the amount of water absorption. The nylon mesh sheet 15 on which the water-absorbent resin particles 11a were placed was positioned so that its center was at the position of the through-hole 13a, and the measurement was started. The time when air bubbles were first introduced from the air introduction tube 25 into the burette tube 26 was defined as the start of water absorption (0 seconds).

The amount of water absorbed by the water-absorbent resin particles 11a within 10 minutes after the start of absorption was measured as water absorption amount A. The water uptake amount was calculated using formula (1).
Formula (1): Water absorption amount [g/g] = Water absorption amount A [mL] × 1.0028 (density of saline solution) [g/mL] / 0.5 [g]

<Permeate water absorption amount (g/g)>
A vessel for measuring the permeated liquid was prepared, which consisted of an acrylic resin cylinder (inner diameter 60 mmφ×height 70 mm) and a mesh sheet (SUS 400 mesh) attached to the bottom of the cylinder. 0.5 g of water-absorbent resin particles were uniformly spread on the mesh sheet, and the total mass Wc of the container for measuring the permeate liquid and the water-absorbent resin particles was measured. Physiological saline (concentration 0. A wire mesh (1.4 mm mesh, 100 mm x 100 mm) was placed on an empty petri dish, and a permeate measurement container with water-absorbent resin particles placed on the wire mesh was placed on the petri dish. The physiological saline solution in the beaker was poured into the permeate measurement container all at once, and the total mass Wd of the permeate measurement container and its contents was measured one minute after the start of the pouring. 2) was used to calculate the amount of water absorbed by the permeate.
Formula (2): Permeate water absorption amount [g/g] = (Wd [g] - Wc [g]) / 0.5 [g]

<Water absorption under load>
The water absorption amount of the water-absorbent resin particles in physiological saline under a load of 2.07 kPa was measured in an environment of 25°C ± 2°C using the measuring device Y shown in Figure 5. The measuring device Y is composed of a burette part 71, a conduit 72, a measuring table 73, and a measuring part 74 placed on the measuring table 73. The burette part 71 has a burette 71a extending vertically, a rubber stopper 71b arranged at the upper end of the burette 71a, a cock 71c arranged at the lower end of the burette 71a, an air introduction tube 71d whose one end extends into the burette 71a near the cock 71c, and a cock 71e arranged at the other end side of the air introduction tube 71d. The conduit 72 is attached between the burette part 71 and the measuring table 73. The inner diameter of the conduit 72 is 6 mm. A hole with a diameter of 2 mm is opened in the center of the measuring table 73, and the conduit 72 is connected to it. The measuring section 74 has a cylinder 74a (made of acrylic resin (Plexiglas)), a nylon mesh 74b bonded to the bottom of the cylinder 74a, and a weight 74c. The inner diameter of the cylinder 74a is 20 mm. The opening of the nylon mesh 74b is 75 μm (200 mesh). During measurement, the water-absorbent resin particles 75 to be measured are uniformly spread on the nylon mesh 74b. The diameter of the weight 74c is 19 mm, and the mass of the weight 74c is 59.8 g. The weight 74c is placed on the water-absorbent resin particles 75, and can apply a load of 2.07 kPa to the water-absorbent resin particles 75.

After 0.100 g of water-absorbent resin particles 75 was placed in the cylinder 74a of the measuring device Y, a weight 74c was placed on the cylinder 74a, and measurement was started. Since air of the same volume as the physiological saline solution absorbed by the water-absorbent resin particles 75 was rapidly and smoothly supplied to the inside of the burette 71a from the air inlet tube, the reduction in the water level of the physiological saline solution inside the burette 71a corresponds to the amount of physiological saline solution absorbed by the water-absorbent resin particles 75. The scale of the burette 71a is marked from top to bottom in increments of 0 mL to 0.5 mL. As the water level of the physiological saline solution, the scale Va of the burette 71a before the start of water absorption and the scale Vb of the burette 71a 60 minutes after the start of water absorption were read, and the water absorption amount under load (water absorption amount of physiological saline solution under a load of 2.07 kPa) was calculated from the following formula.
Water absorption under load [mL/g] = (Vb [mL] - Va [mL]) / 0.1 [g]

<Median particle size>
The above-mentioned median particle diameter of the water-absorbent resin particles was measured by the following procedure. That is, the JIS standard sieves were combined in the following order from the top: 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 according to JIS Z 8815 (1994) using a Rotap type shaker (manufactured by Iida Seisakusho Co., Ltd.). After classification, the mass of the particles remaining on each sieve was calculated as a mass percentage relative to the total amount to obtain the particle size distribution. The particle size distribution was calculated by accumulating the particles remaining on the sieve in order of particle size from the largest to the smallest, and the relationship between the sieve opening and the accumulated mass percentage of the particles remaining on the sieve was plotted on a logarithmic probability paper. The plots on the probability paper were connected with a straight line to obtain the particle diameter corresponding to an accumulated mass percentage of 50% by mass as the median particle diameter. Table 1 shows the median particle diameter and the content (mass%) of water-absorbent resin particles having a particle diameter of 150 μm or less (water-absorbent resin particles passing through a sieve with an opening of 150 μm) based on the total mass of the water-absorbent resin particles.

<Liquid permeation time>
The liquid permeation time (seconds) was measured according to the methods described in JP-A-10-118117, JP-A-9-276391, and Japanese Patent No. 3311753. Specifically, 0.5 g of water-absorbent resin particles were first filled into an acrylic resin cylinder having a cross-sectional area of 4.91 cm ² (inner diameter 25 mmφ) and a cock (inner diameter 4 mmφ) that could be opened and closed at the bottom, with the cock closed, together with physiological saline, and the water-absorbent resin particles were swollen with the physiological saline until they reached a saturated state. The swelling time of the water-absorbent resin particles with the physiological saline was 1 hour. After the swollen water-absorbent resin particles settled onto a filter (glass filter G1), the cock was opened, 50 mL of physiological saline was passed through, and the time required for the 50 mL of physiological saline to pass through was obtained as the liquid permeation time.

<Water absorption rate of physiological saline solution by DW method>
The water absorption rate (g/30 seconds/0.3 g) of physiological saline by the DW method was measured based on the method described in JP-A-10-118117. Specifically, using a device (Demand Wetability Tester) generally known as a device for implementing the DW method, 0.3 g of water-absorbent resin particles were sprayed on a polymer spraying table (70 mmφ, a table in which filter paper No. 2 (55 mmφ) was placed on glass filter G1 (60 mmφ)) in which the liquid level of physiological saline was set to an equal water level, the water absorption amount at the time when the water-absorbent resin particles were sprayed was set to 0, and the water absorption amount after 30 seconds (this water absorption amount was measured by the scale of a burette indicating the amount of decrease in the water level of physiological saline) was measured, and this value was taken as the water absorption rate (unit: g/30 seconds/0.3 g).

<Water absorption rate by Vortex method>
The water absorption rate (seconds) by the Vortex method was measured according to the method described in JP 2012-183175 A. The water absorption rate test by the Vortex method was carried out in a room adjusted to 25°C ± 1°C. 50 ± 0.1 g of physiological saline was weighed out into a 100 mL beaker, a magnetic stirrer bar (without a ring of 8 mmφ x 30 mm) was added, and the beaker was immersed in a thermostatic water bath to adjust the liquid temperature to 25 ± 0.2°C. Next, the beaker was placed on a magnetic stirrer, and the rotation speed was set to 600 rpm to generate a vortex in the physiological saline, and then 2.0 ± 0.002 g of water-absorbent resin particles were quickly added to the beaker, and the time (seconds) from the addition of the water-absorbent resin particles to the time when the vortex on the liquid surface converged was measured using a stopwatch, and was taken as the water absorption rate of the water-absorbent resin particles.

3. Preparation and evaluation of water-absorbent sheet Three sheets of nonwoven fabric selected from spunlace nonwoven fabric and air-through nonwoven fabric were laminated to prepare a water-absorbent sheet having an absorbent layer containing water-absorbent resin particles between the upper and lower layers of the middle nonwoven fabric. Details are shown in Table 2.

Example 1
A spunlace nonwoven fabric (Kuraray Co., Ltd., 70% rayon, 20% PET, 10% PP/PE) with a basis weight of 35 g/ ^m2 was cut into two pieces with a size of 42 cm x 14 cm to give spunlace nonwoven fabrics-1 and 2. A 45 g/ ^m2 air-through nonwoven fabric (Hualong (Nanjing) Co., Ltd.) cut into a size of 42 cm x 14 cm was placed on the spunlace nonwoven fabric-1, and 7.2 g of the water-absorbent resin particles obtained in Production Example 1 was uniformly sprayed over a 12 cm x 40 cm area in the center of the nonwoven fabric using an airflow type mixer (Autech Co., Ltd., pad former).

0.2 g of hot melt adhesive (Henkel Japan Co., Ltd., ME-765E) was applied to the spunlace nonwoven fabric-2 using a hot melt coater (Harries Co., Ltd., pump: Marshal 150, table: XA-DT, tank set temperature: 150°C, hose inside set temperature: 165°C, gun head set temperature: 170°C) (pattern: spiral stripe, 12 stripes at 10 mm intervals). The surface of the spunlace nonwoven fabric-2 to which the hot melt adhesive had been applied was aligned with the surface of the above-mentioned air-through nonwoven fabric to which the water-absorbent resin particles had been sprayed, and the two were sandwiched with release paper and inverted upside down. At this time, the spunlace nonwoven fabric-1 placed below the air-through nonwoven fabric was removed. By the above procedure, a laminate was obtained in which the spunlace nonwoven fabric (lower layer core wrap sheet), the absorbent layer (lower layer absorbent layer) made of the water-absorbent resin particles obtained in Production Example 1, and the air-through nonwoven fabric (middle layer core wrap sheet) were arranged in this order. The removed spunlace nonwoven fabric-1 was cut to 6 cm on both ends in the longitudinal direction and 1 cm on both ends in the lateral direction, and used as an upper core wrap sheet described later.

In the air-through nonwoven fabric of the obtained laminate, 7.2 g of the water-absorbent resin particles obtained in Manufacturing Example 6 was uniformly spread over the surface opposite to the surface on which the water-absorbent resin particles were spread, in a range of 12 cm x 40 cm at the center of the nonwoven fabric, using an airflow type mixer. The surface on which the water-absorbent resin particles obtained in Manufacturing Example 6 were spread was placed up, and the laminate was cut into 6 cm at both ends in the longitudinal direction and 2 cm at both ends in the lateral direction, and cut into 30 cm x 10 cm. As the upper core wrap sheet, 0.12 g of hot melt adhesive was applied to spunlace nonwoven fabric-1 (30 cm x 12 cm) using a hot melt coater as described above. The center and both ends in the longitudinal direction of the spunlace nonwoven fabric-1 were aligned from above against the surface on which the water-absorbent resin particles obtained in Manufacturing Example 6 were spread, and the both ends in the lateral direction of the upper core wrap sheet were folded back to match the laminate. The laminate with the upper core wrap sheet was sandwiched between release paper and pressed with a laminator (Hashima Co., Ltd., Straight Linear Fussing Press, model HP-600LFS) at 110 ° C. and 0.1 MPa to produce a water-absorbent sheet. The obtained water-absorbent sheet has a spunlace nonwoven fabric (lower core wrap sheet), an absorbent layer (lower absorbent layer) made of the water-absorbent resin particles obtained in Production Example 1, an air-through nonwoven fabric (middle core wrap sheet), an absorbent layer (upper absorbent layer) made of the water-absorbent resin particles obtained in Production Example 6, and a spunlace nonwoven fabric (upper core wrap sheet) arranged in this order.

Example 2
A water-absorbent sheet was obtained in the same manner as in Example 1, except that the water-absorbent resin particles used in the lower absorbent layer were changed to those prepared in Production Example 2.

Example 3
A water-absorbent sheet was obtained in the same manner as in Example 1, except that the water-absorbent resin particles used in the lower absorbent layer were changed to those prepared in Production Example 3.

Example 4
A water-absorbent sheet was obtained in the same manner as in Example 1, except that the water-absorbent resin particles used in the lower absorbent layer were changed to those prepared in Production Example 4.

Example 5
A water-absorbent sheet was obtained in the same manner as in Example 1, except that the water-absorbent resin particles used in the lower absorbent layer were changed to those prepared in Production Example 5.

Comparative Example 1
A water-absorbent sheet was obtained in the same manner as in Example 1, except that the water-absorbent resin particles used in the lower absorbent layer were changed to those prepared in Production Example 6.

Comparative Example 2
A water-absorbent sheet was obtained in the same manner as in Example 1, except that the water-absorbent resin particles used in the lower absorbent layer were changed to those prepared in Production Example 7.

Comparative Example 3
A water-absorbent sheet was obtained in the same manner as in Example 1, except that the water-absorbent resin particles used in the lower absorbent layer were changed to those prepared in Production Example 8.

Comparative Example 4
A water-absorbent sheet was obtained in the same manner as in Example 1, except that the water-absorbent resin particles used in the lower absorbent layer were changed to those prepared in Production Example 9.

Example 6
A water-absorbent sheet was obtained in the same manner as in Example 1, except that the water-absorbent resin particles used in the upper absorbent layer were changed to those prepared in Production Example 8.

Example 7
A water-absorbent sheet was obtained in the same manner as in Example 1, except that the air-through nonwoven fabric used as the intermediate core wrap sheet was changed to a spunlace nonwoven fabric having a basis weight of 35 g/ ^m2 (manufactured by Kuraray Co., Ltd., 70% rayon, 20% PET, 10% PP/PE).

Example 8
A water-absorbent sheet was obtained in the same manner as in Example 1, except that the spunlace nonwoven fabric used as the upper core wrap sheet and the lower core wrap sheet was changed to an air-through nonwoven fabric having a basis weight of 45 g/ ^m2 (manufactured by Hualong (Nanjing)).

<Preparation of artificial urine (test liquid)>
The following inorganic salts were mixed and dissolved in ion-exchanged water to prepare artificial urine, and a small amount of Blue No. 1 was added to the mixture. The concentrations shown below are based on the total mass of the artificial urine.
NaCl: 0.777% by mass
_CaCl2 : 0.022% by mass
_MgSO4 : 0.038% by mass
Urea: 1.933% by mass
Blue No. 1: 0.002% by mass

<45 degree leak test>
The liquid leakage was evaluated by a 45-degree leakage test. FIG. 6 is a schematic diagram showing a method for evaluating the leakage of an absorbent article. A support plate 45 (here, an acrylic resin plate, hereinafter also referred to as the inclined surface S ₁ ) having a length of 45 cm and a flat main surface was fixed by a stand 41 in a state inclined at 45±2 degrees with respect to a horizontal plane S _0. At room temperature of 25±2° C., a test absorbent sheet 50 was attached to the inclined surface S ₁ of the fixed support plate 45 with its longitudinal direction aligned with the longitudinal direction of the support plate 45. Here, the test absorbent sheet 50 was arranged so that the lower absorbent layer was on the support plate side. Next, a test liquid 46 (artificial urine) adjusted to 25±1° C. was dripped from a drip funnel 42 arranged vertically above the absorbent sheet 50 toward a position 2 cm above the center of the absorbent sheet. 80 mL of the test liquid 46 was poured at a rate of 8 mL/sec per time. The distance between the tip of the dropping funnel 42 and the water-absorbent sheet was 10±1 mm. Ten minutes after the start of the first test liquid pouring, the test liquid was poured under the same conditions.

When the test liquid that was not absorbed by the test absorbent sheet leaked out from the bottom of the support plate 45, the leaked test liquid was collected in a metal tray 44 placed below the support plate 45. The weight (g) of the collected test liquid was measured using a balance 43, and this value was recorded as the leakage amount (g). The measurement results are shown in Table 2.

The evaluation results are shown in Table 2. The absorbent sheet produced using absorbent resin particles with specific values of wicking and permeation absorption in the lower absorbent layer had improved liquid leakage.

REFERENCE SIGNS LIST 1...burette part, 3...clamp, 5...conduit, 10a...first absorbent layer, 10b...second absorbent layer, 11a...first water-absorbent resin particles, 11b...second water-absorbent resin particles, 12a, 12b...fiber layer, 13...measurement table, 13a...through hole, 14...frame, 15...nylon mesh sheet, 16...cylinder, 20a...core wrap sheet, 20b...core wrap sheet, 21...adhesive, 22...cock, 23...rubber stopper, 24...cock, 25...air introduction tube, 26...burette tube, 27...life Saline solution, 30...liquid-permeable sheet, 40...liquid-impermeable sheet, 41...stand, 42...dropping funnel, 43...balance, 44...tray, 45...support plate, 46...test liquid, 50...absorbent sheet, 71...burette part, 71a...burette, 71b...rubber stopper, 71c...cock, 71d...air inlet tube, 71e...cock, 72...conduit, 73...measuring table, 74...measuring part, 74a...cylinder, 74b...nylon mesh, 74c...weight, 75...water-absorbent resin particles, 100...absorbent article, _S0 ...horizontal surface, _S1 ...inclined surface.

Claims (6)

A water-absorbent sheet comprising a plurality of absorbent layers each containing water-absorbent resin particles,
one outermost absorbent layer contains water-absorbent resin particles having a water uptake capacity of 10.5 g/g or more, a water permeation capacity of 12.5 g/g or more, and a median particle size of 250 to 450 μm ;
The water uptake amount is a mass of physiological saline per 1 g of water absorbent resin particles, which is absorbed by the water absorbent resin particles within 10 minutes after the water absorbent resin particles start absorbing physiological saline, when 0.5 g of water absorbent resin particles are placed in a container consisting of a cylinder with an inner diameter of 25 mm and a mesh sheet attached to the bottom of the cylinder in accordance with the DW method,
The permeate absorption amount is an absorption amount of physiological saline per 1 g of the water-absorbent resin particles, which is measured 1 minute after 30 g of physiological saline is poured into the mesh sheet in a container consisting of a cylinder with an inner diameter of 60 mm and a mesh sheet adhered to a bottom of the cylinder, and is the absorption amount of physiological saline per 1 g of the water-absorbent resin particles. 2. The water-absorbent sheet according to claim 1 , wherein the content of said water-absorbent resin particles having a particle diameter of 150 μm or less is 21.0 mass % or less based on the total mass of said water-absorbent resin particles. 3. The water-absorbent sheet according to claim 1, wherein the water-absorbent resin particles have a water absorption capacity of 15 mL/g or more when they absorb physiological saline under a load of 2.07 kPa. An absorbent article comprising a liquid impermeable sheet, a water absorbent sheet having a plurality of absorbent layers, and a liquid permeable sheet, the liquid impermeable sheet, the water absorbent sheet, and the liquid permeable sheet being arranged in this order,
the outermost absorbent layer on the liquid-impermeable sheet side has a water-pickup absorption of 10.5 g/g or more, a permeation absorption of 12.5 g/g or more , and contains water-absorbent resin particles having a median particle diameter of 250 to 450 μm ;
The water uptake amount is a mass of physiological saline per 1 g of water absorbent resin particles, which is absorbed by the water absorbent resin particles within 10 minutes after the water absorbent resin particles start absorbing physiological saline, when 0.5 g of water absorbent resin particles are placed in a container consisting of a cylinder with an inner diameter of 25 mm and a mesh sheet attached to the bottom of the cylinder in accordance with the DW method,
an absorbent article, wherein the permeate absorption amount is an absorption amount of physiological saline per 1 g of the water absorbent resin particles, which is measured 1 minute after 30 g of physiological saline is poured into the mesh sheet in a container including a cylinder having an inner diameter of 60 mm and a mesh sheet bonded to a bottom of the cylinder, and the absorption amount of physiological saline per 1 g of the water absorbent resin particles is measured. The absorbent article according to claim 4 , wherein the content of the water-absorbent resin particles having a particle diameter of 150 μm or less is 21.0 mass % or less based on the total mass of the water-absorbent resin particles. 6. The absorbent article according to claim 4, wherein the water-absorbent resin particles have a water absorption capacity of 15 mL/g or more when they absorb physiological saline under a load of 2.07 kPa.

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