Water absorbent resin particles, absorber, and absorbent article

Abstract

The present invention provides water absorbent resin particles with which, if the water absorbent resin particles are applied to an absorber, back-flow of liquid after the liquid is absorbed by the absorber under pressure and in a state where the absorber is curved is suppressed.　The water absorbent resin particles have a 15-minute value of a physiological saline solution absorption amount under a load of 0.69 kPa of 30 mL / g to 60 mL / g inclusive.

Description

Water-absorbent resin particles, absorbent body and absorbent article

[0001] The present invention relates to water-absorbent resin particles, an absorbent body, and an absorbent article, and more particularly to water-absorbent resin particles constituting an absorbent body suitable for use in hygiene materials such as disposable diapers, sanitary napkins, and incontinence pads, and an absorbent body and an absorbent article using the water-absorbent resin particles.

[0002] BACKGROUND ART In recent years, water-absorbent resin particles have been widely used in the field of sanitary materials such as disposable diapers, sanitary napkins, and incontinence pads.

[0003] As such water-absorbent resin particles, a crosslinked product of a polymer of a water-soluble ethylenically unsaturated monomer, more specifically a crosslinked product of a polymer of a partially neutralized polyacrylic acid, has excellent water-absorbing ability, and since acrylic acid, which is a raw material thereof, is easily available industrially, it can be produced at low cost with constant quality, and is less susceptible to putrefaction and deterioration, and therefore, it is considered to be a preferable water-absorbent resin particle (see, for example, Patent Document 1).

[0004] Absorbent articles such as disposable diapers, sanitary napkins, and incontinence pads are mainly composed of an absorbent core located in the center for absorbing and retaining body fluids such as urine and menstrual blood excreted from the body, a liquid-permeable surface sheet (top sheet) located on the side that comes into contact with the body, and a liquid-impermeable back sheet (back sheet) located on the opposite side that comes into contact with the body. The absorbent core is usually composed of hydrophilic fibers such as pulp and water-absorbent resin particles.

[0005] Japanese Patent Application Publication No. 3-227301

[0006] In such absorbent articles, when the absorbent body is curved or pressurized, after the absorbent body has absorbed liquid, the liquid may return from the absorbent body to the wearer's side of the diaper, a phenomenon known as backflow.

[0007] The present inventors have repeatedly studied technologies for suppressing backflow after an absorbent body has absorbed liquid in a curved state, and technologies for suppressing backflow after an absorbent body has absorbed liquid in a pressurized state. Specifically, with reference to conventional technologies, the inventors have studied technologies for suppressing backflow by adjusting various properties of water-absorbent resin particles, such as the amount of saline solution absorbed under load, the no-pressure DW value, the absorption rate for saline solution, and the saline solution water retention capacity. However, even when these technologies are applied to an absorbent body, the present inventors have found that it is difficult to sufficiently suppress backflow when the absorbent body is caused to absorb liquid in a curved state and a pressurized state.

[0008] Under these circumstances, a main object of the present invention is to provide water-absorbent resin particles that, when applied to an absorbent body, allow the absorbent body to be curved and suppress backflow of liquid after absorbing liquid under pressure. Another object of the present invention is to provide an absorbent body and an absorbent article using the water-absorbent resin particles.

[0009] The present inventors have conducted extensive research to solve the above-mentioned problems. As a result, they have found that, when the water-absorbent resin particles are applied to an absorbent body, by setting the 15-minute value of the water absorption of physiological saline solution under a load of 0.69 kPa to a specific range of 30 mL / g to 60 mL / g, the absorbent body is curved and backflow of liquid after absorbing liquid under pressure is suppressed. The present invention has been completed based on this finding and through further extensive research.

[0010] That is, the present invention provides inventions having the following configurations. Item 1. Water-absorbent resin particles having a 15-minute water absorption value of physiological saline solution under a load of 0.69 kPa of 30 mL / g or more and 60 mL / g or less. Item 2. Water-absorbent resin particles according to Item 1, wherein the water-absorbent resin particles have a 3-minute no-pressure DW value of 20 mL / g or more and 50 mL / g or less. Item 3. Water-absorbent resin particles according to Item 2, wherein the water-absorbent resin particles have a water-absorption speed for physiological saline solution of 60 seconds or less. Item 4. Water-absorbent resin particles according to Item 2, wherein the water-absorbent resin particles have a water-retention capacity for physiological saline solution of 30 g / g or more. Item 5. An absorbent body comprising the water-absorbent resin particles according to any one of Items 1 to 4. Item 6. A water-absorbent article comprising the absorbent body according to Item 5.

[0011] According to the present invention, it is possible to provide water-absorbent resin particles that, when applied to an absorbent body, keep the absorbent body in a curved state and suppress backflow of liquid after absorbing liquid under pressure. Furthermore, the present invention can provide an absorbent body and an absorbent article using the water-absorbent resin particles.

[0012] 1 is a schematic diagram of a measuring device for measuring the water absorption of physiological saline under a load of 0.69 kPa. It is a schematic diagram of a measuring device used for measuring the 3-minute value and the 10-minute value of the DW of the water-absorbent resin particle without pressure.

[0013] As used herein, the term "comprising" includes "consisting essentially of" and "consisting of." Furthermore, as used herein, "(meth)acrylic" means "acrylic or methacrylic," "(meth)acrylate" means "acrylate or methacrylate," and "(poly)" refers to the presence or absence of the prefix "poly." Furthermore, as used herein, "water-soluble" means exhibiting a solubility of 5% by mass or more in water at 25°C.

[0014] In this specification, a numerical value connected with "~" means a numerical range that includes the numerical values ​​before and after "~" as the lower limit and upper limit. When multiple lower limit values ​​and multiple upper limit values ​​are listed separately, any lower limit value and upper limit value can be selected and connected with "~".

[0015] 1. Water-absorbent resin particles The water-absorbent resin particles of the present invention have a 15-minute water absorption value of physiological saline solution under a load of 0.69 kPa of 30 mL / g or more and 60 mL / g or less. Because the water-absorbent resin particles of the present invention have such characteristics, when the water-absorbent resin particles are applied to an absorbent body, the absorbent body is curved and backflow of liquid after absorbing liquid under pressure is suppressed. The water-absorbent resin particles of the present invention will be described in detail below.

[0016] As mentioned above, the present inventors, with reference to the prior art, have investigated techniques for suppressing backflow of liquid by adjusting various properties of water-absorbent resin particles, such as the saline water absorption capacity under load, the no-pressure DW value, the water absorption rate for saline, and the saline water retention capacity of the water-absorbent resin particles. However, even when these techniques are applied to an absorbent body, it is difficult to sufficiently suppress backflow when the absorbent body is in a curved state and is pressurized and is allowed to absorb liquid. In response to this, the present inventors focused on the 15-minute value of the saline water absorption capacity of water-absorbent resin particles under a load of 0.69 kPa and found that when this value is set to a specific range of 30 mL / g or more and 60 mL / g or less, when the water-absorbent resin particles are applied to an absorbent body, backflow of liquid after the absorbent body is in a curved state and has absorbed liquid under pressure is suppressed. The mechanism behind this can be considered, for example, as follows. That is, in an absorbent article in a curved state under pressure, the load actually applied to the water-absorbent resin particles is lighter than the load applied to the absorbent article due to the influence of the liquid-permeable surface sheet (top sheet), hydrophilic fibers, etc. For this reason, it is considered that the backflow is suppressed by setting the 15-minute value of the water-absorbent resin particles' water absorption amount of physiological saline solution under a relatively light load condition of 0.69 kPa load to within a predetermined range.

[0017] From the viewpoint of more suitably exhibiting the effects of the present invention, the 15-minute value of the physiological saline water absorption amount under a load of 0.69 kPa of the water-absorbent resin particle is preferably 30 mL / g or more, more preferably 32 mL / g or more, even more preferably 34 mL / g or more, particularly preferably 35 mL / g or more, and is preferably 60 mL / g or less, more preferably 58 mL / g or less, even more preferably 55 mL / g or less, and preferred ranges include 30 to 60 mL / g, 30 to 58 mL / g, 30 to 55 mL / g, 32 to 60 mL / g, 32 to 58 mL / g, 32 to 55 mL / g, 34 to 60 mL / g, 34 to 58 mL / g, 34 to 55 mL / g, 35 to 60 mL / g, 35 to 58 mL / g, and 35 to 55 mL / g.

[0018] The 15-minute value of the physiological saline water absorption amount of the water-absorbent resin particles under a load of 0.69 kPa is measured by the method described in the Examples.

[0019]

[0044] Examples of a method for adjusting the 15-minute value of the physiological saline water absorption amount of a water-absorbent resin particle under a load of 0.69 kPa include increasing the amount of a surface cross-linking agent used in the production method of water-absorbent resin particles described below, and increasing the ratio of the water-soluble ethylenically unsaturated monomer in the second-stage aqueous monomer solution to the water-soluble ethylenically unsaturated monomer in the first-stage aqueous monomer solution. These methods strengthen the surface cross-linked layer of the water-absorbent resin particle, and make it possible to suitably absorb and retain liquid even under a condition of a small load such as under a load of 0.69 kPa and for a short time such as a 15-minute value.

[0020] Furthermore, from the viewpoint of accelerating the water absorption rate of the absorbent body (shortening the absorption time) while more suitably exerting the effects of the present invention, the no-pressure DW 3-minute value of the water-absorbent resin particles is preferably 20 mL / g or more, more preferably 30 mL / g or more, even more preferably 33 mL / g or more, particularly preferably 36 mL / g or more, and even more particularly preferably 39 mL / g or more, and is also preferably 50 mL / g or less, more preferably 48 mL / g or less. , and more preferably 46 mL / g or less, and preferred ranges include 20 to 50 mL / g, 20 to 48 mL / g, 20 to 46 mL / g, 30 to 50 mL / g, 30 to 48 mL / g, 30 to 46 mL / g, 33 to 50 mL / g, 33 to 48 mL / g, 33 to 46 mL / g, 36 to 50 mL / g, 36 to 48 mL / g, 36 to 46 mL / g, 39 to 50 mL / g, 39 to 48 mL / g, and 39 to 46 mL / g.

[0021] Further, from the viewpoint of more suitably exhibiting the effects of the present invention, the no-pressure DW 10-minute value of the water-absorbent resin particles is preferably 30 mL / g or more, more preferably 45 mL / g or more, even more preferably 50 mL / g or more, particularly preferably 54 mL / g or more, more particularly preferably 56 mL / g or more, and also preferably 80 mL / g or less, more preferably 75 mL / g or less, even more preferably 72 mL / g or less, and preferred ranges include 30 to 80 mL / g, 30 to 75 mL / g, 30 to 72 mL / g, 45 to 80 mL / g, 45 to 75 mL / g, 45 to 72 mL / g, 50 to 80 mL / g, 50 to 75 mL / g, 50 to 72 mL / g, 54 to 80 mL / g, 54 to 75 mL / g, 54 to 72 mL / g, 56 to 80 mL / g, 56 to 75 mL / g, and 56 to 72 mL / g.

[0022] The no-pressure DW 3-minute value and no-pressure DW 10-minute value of the water-absorbent resin particles are measured by the methods described in the Examples.

[0023]

[0113] Furthermore, from the viewpoint of increasing the water absorption rate of the absorbent body (shortening the absorption time) while more suitably exhibiting the effects of the present invention, the water absorption rate of the water-absorbent resin particles with respect to physiological saline is preferably 60 seconds or less, more preferably 55 seconds or less, even more preferably 50 seconds or less, and is preferably 20 seconds or more, more preferably 25 seconds or more, even more preferably 30 seconds or more, and preferred ranges include 20 to 60 seconds, 20 to 55 seconds, 20 to 50 seconds, 25 to 60 seconds, 25 to 55 seconds, 25 to 50 seconds, 30 to 60 seconds, 30 to 55 seconds, and 30 to 50 seconds.

[0024] The water absorption rate of the water-absorbent resin particles for physiological saline is measured by the method described in the examples.

[0025]

[0113] Moreover, from the viewpoint of more suitably exhibiting the effects of the present invention, the water retention capacity of the physiological saline solution of the water-absorbent resin particles is preferably 30 g / g or more, more preferably 36 g / g or more, still more preferably 41 g / g or more, and is preferably 70 g / g or less, more preferably 65 g / g or less, still more preferably 60 g / g or less, and preferred ranges include 30 to 70 g / g, 30 to 65 g / g, 30 to 60 g / g, 36 to 70 g / g, 36 to 65 g / g, 36 to 60 g / g, 41 to 70 g / g, 41 to 65 g / g, and 41 to 60 g / g.

[0026] The water retention capacity of the water-absorbent resin particles in physiological saline solution is measured by the method described in the examples.

[0027] Furthermore, from the viewpoint of more suitably exhibiting the effects of the present invention, the physiological saline water absorption capacity of the water-absorbent resin particles is preferably 45 g / g or more, 52 g / g or more, or 59 g / g or more, and is preferably 80 g / g or less, 74 g / g or less, or 69 g / g or less. Preferred ranges of the physiological saline water absorption capacity of the water-absorbent resin particles include 45 to 80 g / g, 45 to 74 g / g, 45 to 69 g / g, 52 to 80 g / g, 52 to 74 g / g, 52 to 69 g / g, 59 to 80 g / g, 59 to 74 g / g, and 59 to 69 g / g.

[0028] The physiological saline water absorption amount of the water-absorbent resin particles is measured by the method described in the examples.

[0029] Furthermore, from the viewpoint of more suitably exerting the effects of the present invention, the difference between the physiological saline water absorption capacity and the physiological saline water retention capacity is preferably 5 or more, or 10 or more, and is preferably 30 or less, 25 or less, or 20 or less, and preferred ranges include 5 to 30, 5 to 25, 5 to 20, 10 to 30, 10 to 25, and 10 to 20.

[0030] Furthermore, from the viewpoint of more suitably exerting the effects of the present invention, the water-absorbent resin particles of the present invention have a reversion amount under bending and pressure, measured in accordance with the provisions of "GB / T 28004.1-2021 NATIONAL STANDARD OF THE PEOPLE'S REPUBLIC OF CHINA, Disposable diapers - Part 1: Disposable diapers for baby", of preferably 10 g or less, more preferably 8 g or less, and the lower limit is preferably 0 g, and preferred ranges include 0 to 10 g and 0 to 8 g.

[0031] Furthermore, from the viewpoint of more suitably exerting the effects of the present invention, the water-absorbent resin particles of the present invention have a total absorption time (the sum of the first absorption time and the second absorption time of the test liquid), measured in accordance with the provisions of "GB / T 28004.1-2021 NATIONAL STANDARD OF THE PEOPLE'S REPUBLIC OF CHINA, Disposable diapers - Part 1: Disposable diapers for baby", of preferably 90 seconds or less, more preferably 80 seconds or less, still more preferably 75 seconds or less, and also preferably 20 seconds or more, and preferred ranges include 20 to 90 seconds, 20 to 80 seconds, and 20 to 75 seconds.

[0032] The amount of return under bending and pressure, measured in accordance with the provisions of "GB / T 28004.1-2021 NATIONAL STANDARD OF THE PEOPLE'S REPUBLIC OF CHINA, Disposable diapers - Part 1: Disposable diapers for baby," and the total absorption time (the sum of the first absorption time (seconds) and the second absorption time (seconds) of the test liquid) were measured according to the methods described in the examples.

[0033] The water-absorbent resin particles of the present invention are constituted by crosslinking a polymer of a water-soluble ethylenically unsaturated monomer, that is, by a crosslinked polymer having structural units derived from a water-soluble ethylenically unsaturated monomer.

[0034] The water-absorbent resin particles of the present invention are in the form (secondary particles) of aggregates of fine particles (primary particles). Examples of the shape of the primary particles include substantially spherical, irregularly crushed, and plate-like shapes. The water-absorbent resin particles of the present invention, which are secondary particles, may have various shapes. Examples of the shape of the water-absorbent resin particles include granular, substantially spherical, irregularly crushed, plate-like, fibrous, flake-like shapes, and shapes obtained by aggregating these resins. The water-absorbent resin particles are preferably granular, substantially spherical, irregularly crushed, fibrous, or shapes obtained by aggregating these resins.

[0035] From the viewpoint of accelerating the water absorption rate of the absorbent (shortening the absorption time) while more suitably exhibiting the effects of the present invention, the median particle diameter of the water-absorbent resin particles is preferably 200 μm or more, 250 μm or more, 300 μm or more, 320 μm or more, or 350 μm or more. From the same viewpoint, the median particle diameter is preferably 700 μm or less, 600 μm or less, 550 μm or less, 500 μm or less, or 450 μm or less. Preferred ranges include a median particle size of 200 to 700 μm, 200 to 600 μm, 200 to 550 μm, 200 to 500 μm, 200 to 450 μm, 250 to 700 μm, 250 to 600 μm, 250 to 550 μm, 250 to 500 μm, 250 to 450 μm, 300 to 700 μm, 300 to 600 μm, 300 to 550 μm, 300 to 500 μm, 300 to 450 μm, 350 to 700 μm, 350 to 600 μm, 350 to 550 μm, 350 to 500 μm, and 350 to 450 μm.

[0036] The median particle size of the water-absorbent resin particles can be measured using a JIS standard sieve, and specifically, it is a value measured by the method described in the examples.

[0037]

[0033] 2. Method for Producing Water-Absorbent Resin Particles

[0034] The method for producing water-absorbent resin particles of the present invention is not particularly limited as long as it can obtain water-absorbent resin particles having a 15-minute water absorption value of physiological saline solution under a load of 0.69 kPa of 30 mL / g or more and 60 mL / g or less. The method for producing water-absorbent resin particles of the present invention, for example, comprises, in this order, a step of polymerizing a water-soluble ethylenically unsaturated monomer to obtain polymer particles, and a surface cross-linking step of surface-crosslinking the polymer particles.

[0038] As described above, examples of a method for adjusting the 15-minute value of the physiological saline water absorption capacity of the water-absorbent resin particles of the present invention under a load of 0.69 kPa include increasing the amount of surface cross-linking agent used in the method for producing water-absorbent resin particles, and increasing the ratio of the water-soluble ethylenically unsaturated monomer in the second-stage aqueous monomer solution to the water-soluble ethylenically unsaturated monomer in the first-stage aqueous monomer solution. These methods strengthen the surface cross-linked layer of the water-absorbent resin particles, making it possible to suitably absorb and retain liquid even under a low load condition of 0.69 kPa under a load and in a short time such as 15 minutes. Hereinafter, the method for producing water-absorbent resin particles of the present invention will be described in detail.

[0039] <Polymerization Step> The polymerization step is a step in which a water-soluble ethylenically unsaturated monomer is polymerized to obtain polymer particles. Typical methods for polymerizing a water-soluble ethylenically unsaturated monomer include aqueous solution polymerization, spray-droplet polymerization, emulsion polymerization, and reverse-phase suspension polymerization. In aqueous solution polymerization, polymerization is carried out by heating an aqueous solution of the water-soluble ethylenically unsaturated monomer while stirring as needed. In reverse-phase suspension polymerization, polymerization is carried out by heating the water-soluble ethylenically unsaturated monomer in a hydrocarbon dispersion medium while stirring. Among these, reverse-phase suspension polymerization is preferred from the viewpoint of improving general water absorption performance (e.g., saline water retention capacity, saline water absorption capacity, etc.) while setting the 15-minute value of the physiological saline water absorption capacity of the water-absorbent resin particles under a load of 0.69 kPa in the range of 30 to 60 mL / g. In the polymerization step, an internal crosslinking agent may be added to the water-soluble ethylenically unsaturated monomer to form crosslinked polymer particles (a hydrogel-like substance) having an internal crosslinked structure. An example of the polymerization step is described below.

[0040] [Water-soluble ethylenically unsaturated monomer] Examples of the water-soluble ethylenically unsaturated monomer include (meth)acrylic acid (in this specification, "acrylic" and "methacrylic" are collectively referred to as "(meth)acrylic", the same applies hereinafter) and salts thereof; 2-(meth)acrylamido-2-methylpropanesulfonic acid and salts thereof; nonionic monomers such as (meth)acrylamide, N,N-dimethyl(meth)acrylamide, 2-hydroxyethyl(meth)acrylate, N-methylol(meth)acrylamide, and polyethylene glycol mono(meth)acrylate; and amino group-containing unsaturated monomers such as N,N-diethylaminoethyl(meth)acrylate, N,N-diethylaminopropyl(meth)acrylate, and diethylaminopropyl(meth)acrylamide, and quaternized products thereof. Among these water-soluble ethylenically unsaturated monomers, (meth)acrylic acid or a salt thereof, (meth)acrylamide, and N,N-dimethylacrylamide are preferred, and (meth)acrylic acid and a salt thereof are more preferred, from the viewpoint of industrial ease of availability, etc. These water-soluble ethylenically unsaturated monomers may be used alone or in combination of two or more.

[0041] Among these, acrylic acid and its salts are widely used as raw materials for water-absorbent resin particles, and these acrylic acid and / or salts thereof may be copolymerized with the other water-soluble ethylenically unsaturated monomers described above for use. In this case, it is preferable that acrylic acid and / or its salts are used as the main water-soluble ethylenically unsaturated monomer in an amount of 70 to 100 mol % based on the total amount of water-soluble ethylenically unsaturated monomers.

[0042] The water-soluble ethylenically unsaturated monomer may be dispersed in a hydrocarbon dispersion medium in the form of an aqueous solution and subjected to reversed-phase suspension polymerization. By forming the water-soluble ethylenically unsaturated monomer into an aqueous solution, the dispersion efficiency in the hydrocarbon dispersion medium can be increased. The concentration of the water-soluble ethylenically unsaturated monomer in this aqueous solution is preferably in the range of 20% by mass to the saturated concentration or less. The concentration of the water-soluble ethylenically unsaturated monomer is more preferably 55% by mass or less, even more preferably 50% by mass or less, and even more preferably 45% by mass or less. Meanwhile, the concentration of the water-soluble ethylenically unsaturated monomer is more preferably 25% by mass or more, even more preferably 28% by mass or more, and even more preferably 30% by mass or more.

[0043] The ratio of the water-soluble ethylenically unsaturated monomer in the second-stage aqueous monomer solution to the water-soluble ethylenically unsaturated monomer in the first-stage aqueous monomer solution (second-stage water-soluble ethylenically unsaturated monomer / first-stage water-soluble ethylenically unsaturated monomer) is preferably 0.1 to 3.0, more preferably 0.5 to 2.5, even more preferably 1.0 to 2.0, and still more preferably 1.5 to 1.8, from the viewpoint of adjusting the 15-minute value of the water absorption amount in physiological saline under a load of 0.69 kPa within a preferred range.

[0044] When the water-soluble ethylenically unsaturated monomer has an acid group, such as (meth)acrylic acid or 2-(meth)acrylamido-2-methylpropanesulfonic acid, the acid group may be neutralized in advance with an alkaline neutralizing agent, as necessary. Examples of such alkaline neutralizing agents include alkali metal salts such as sodium hydroxide, sodium carbonate, sodium bicarbonate, potassium hydroxide, and potassium carbonate; and ammonia. These alkaline neutralizing agents may be used in the form of an aqueous solution to simplify the neutralization operation. The alkaline neutralizing agents described above may be used alone or in combination of two or more.

[0045] The degree of neutralization of the water-soluble ethylenically unsaturated monomer with the alkaline neutralizing agent is preferably 40 to 100 mol %, more preferably 50 to 90 mol %, even more preferably 60 to 85 mol %, and still more preferably 70 to 80 mol %, as the degree of neutralization with respect to all acid groups possessed by the water-soluble ethylenically unsaturated monomer.

[0046] [Radical Polymerization Initiator] Examples of the radical polymerization initiator added to the polymerization step 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-butylcumyl peroxide, t-butyl peroxyacetate, t-butylperoxyisobutyrate, t-butylperoxypivalate, and hydrogen peroxide, as well as 2,2'-azobis(2-amidinopropane) dihydrochloride and 2,2'-azobis[2-(N-phenyl)propane]. Examples of the radical polymerization initiator include azo compounds such as 2,2'-azobis[2-(N-allylamidino)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). Among these radical polymerization initiators, potassium persulfate, ammonium persulfate, sodium persulfate, and 2,2'-azobis(2-amidinopropane) dihydrochloride are preferred from the viewpoints of ease of availability and handling. These radical polymerization initiators may be used alone or in combination of two or more. The radical polymerization initiator can also be used as a redox polymerization initiator in combination with a reducing agent such as sodium sulfite, sodium hydrogen sulfite, ferrous sulfate, or L-ascorbic acid.

[0047] The amount of radical polymerization initiator used is, for example, 0.00005 to 0.01 mole per mole of the water-soluble ethylenically unsaturated monomer. By using such an amount, it is possible to avoid a rapid polymerization reaction and complete the polymerization reaction within an appropriate time.

[0048] [Internal Crosslinking Agent] The internal crosslinking agent can be one that can crosslink the polymer of the water-soluble ethylenically unsaturated monomer used, such as (poly)ethylene glycol ("(poly)" refers to both the presence and absence of the prefix "poly"). the same applies hereinafter)], unsaturated polyesters obtained by reacting polyols such as diols and triols, such as (poly)propylene glycol, 1,4-butanediol, 1,6-hexanediol, trimethylolpropane, and (poly)glycerin, with unsaturated acids, such as (meth)acrylic acid, maleic acid, and fumaric acid; bisacrylamides such as N,N-methylenebisacrylamide; di(meth)acrylic acid esters or tri(meth)acrylic acid esters obtained by reacting polyepoxides with (meth)acrylic acid; di(meth)acrylic acid carbamyl esters obtained by reacting polyisocyanates, such as tolylene diisocyanate and hexamethylene diisocyanate, with hydroxyethyl (meth)acrylate; allylated starch, allylated cellulose, diallyl phthalate, N,N',N''-triallyl isocyanurate, divinyl Examples of the compound include a compound having two or more polymerizable unsaturated groups such as benzene; a diglycidyl compound such as (poly)ethylene glycol diglycidyl ether, (poly)propylene glycol diglycidyl ether, (poly)glycerin diglycidyl ether, and the like, and a polyglycidyl compound such as a triglycidyl compound; an epihalohydrin compound such as epichlorohydrin, epibromohydrin, and α-methylepichlorohydrin; a compound having two or more reactive functional groups such as an isocyanate compound such as 2,4-tolylene diisocyanate and hexamethylene diisocyanate; and an oxetane compound such as 3-methyl-3-oxetane methanol, 3-ethyl-3-oxetane methanol, 3-butyl-3-oxetane methanol, 3-methyl-3-oxetane ethanol, 3-ethyl-3-oxetane ethanol, and 3-butyl-3-oxetane ethanol. Among these internal cross-linking agents, it is preferable to use a polyglycidyl compound, it is more preferable to use a diglycidyl ether compound, and it is preferable to use (poly)ethylene glycol diglycidyl ether, (poly)propylene glycol diglycidyl ether, or (poly)glycerin diglycidyl ether.These internal crosslinking agents may be used alone or in combination of two or more.

[0049] The amount of the internal crosslinking agent used in the first-stage aqueous monomer solution is preferably 0.000001 to 0.005 mol, more preferably 0.00001 to 0.002 mol, even more preferably 0.00001 to 0.001 mol, and still more preferably 0.00005 to 0.0005 mol, relative to 1 mol of the water-soluble ethylenically unsaturated monomer.

[0050] The amount of the internal crosslinking agent used in the second-stage aqueous monomer solution is preferably 0 to 0.005 mol, more preferably 0 to 0.0005 mol, still more preferably 0 to 0.0001 mol, and particularly preferably 0 to 0.00004 mol, relative to 1 mol of the water-soluble ethylenically unsaturated monomer, from the viewpoint of improving general water absorption performance (such as physiological saline water retention capacity and physiological saline water absorption capacity).

[0051] [Hydrocarbon Dispersion Medium] Examples of hydrocarbon dispersion media include aliphatic hydrocarbons having 6 to 8 carbon atoms, 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. Among these hydrocarbon dispersion media, n-hexane, n-heptane, and cyclohexane are particularly preferred because they are easily available industrially, have stable quality, and are inexpensive. These hydrocarbon dispersion media may be used alone or in combination of two or more. As an example of a mixture of hydrocarbon dispersion media, a commercially available product such as Exxol Heptane (manufactured by ExxonMobil Corporation; contains 75 to 85% by mass of hydrocarbons such as heptane and its isomers) can also be used to obtain favorable results.

[0052] The amount of hydrocarbon dispersion medium used is preferably 100 to 1500 parts by mass, and more preferably 200 to 1400 parts by mass, per 100 parts by mass of the water-soluble ethylenically unsaturated monomer in the first stage, from the viewpoints of uniformly dispersing the water-soluble ethylenically unsaturated monomer and facilitating control of the polymerization temperature. As will be described later, the reversed-phase suspension polymerization is carried out in one stage (single stage) or in multiple stages of two or more stages, and the above-mentioned first stage polymerization refers to the polymerization reaction in the first stage of single-stage polymerization or multi-stage polymerization (the same applies hereinafter).

[0053] [Dispersion stabilizer] (Surfactant) In the reversed-phase suspension polymerization, a dispersion stabilizer can be used to improve the dispersion stability of the water-soluble ethylenically unsaturated monomer in the hydrocarbon dispersion medium. A surfactant can be used as the dispersion stabilizer.

[0054] Examples of surfactants that can be used include sucrose fatty acid esters, polyglycerin fatty acid esters, sorbitan fatty acid esters, polyoxyethylene sorbitan fatty acid esters, polyoxyethylene glycerin fatty acid esters, sorbitol fatty acid esters, polyoxyethylene sorbitol fatty acid esters, polyoxyethylene alkyl ethers, polyoxyethylene alkylphenyl ethers, polyoxyethylene castor oil, polyoxyethylene hydrogenated castor oil, alkylallyl formaldehyde condensed polyoxyethylene ethers, polyoxyethylene polyoxypropylene block copolymers, polyoxyethylene polyoxypropyl alkyl ethers, polyethylene glycol fatty acid esters, alkyl glucosides, N-alkyl gluconamides, polyoxyethylene fatty acid amides, polyoxyethylene alkylamines, phosphate esters of polyoxyethylene alkyl ethers, and phosphate esters of polyoxyethylene alkyl allyl ethers. Among these surfactants, sorbitan fatty acid esters, polyglycerin fatty acid esters, and sucrose fatty acid esters are particularly preferred from the standpoint of dispersion stability of the monomer. These surfactants may be used alone or in combination of two or more.

[0055] The amount of surfactant used is preferably 0.1 to 30 parts by mass, more preferably 0.3 to 20 parts by mass, per 100 parts by mass of the first stage water-soluble ethylenically unsaturated monomer.

[0056] (Polymer-Based Dispersant) As a dispersion stabilizer used in reversed-phase suspension polymerization, a polymer-based dispersant may be used in combination with the surfactant described above.

[0057] Examples of polymeric dispersants include maleic anhydride-modified polyethylene, maleic anhydride-modified polypropylene, maleic anhydride-modified ethylene-propylene copolymer, maleic anhydride-modified EPDM (ethylene-propylene-diene terpolymer), maleic anhydride-modified polybutadiene, maleic anhydride-ethylene copolymer, maleic anhydride-propylene copolymer, maleic anhydride-ethylene-propylene copolymer, maleic anhydride-butadiene copolymer, polyethylene, polypropylene, ethylene-propylene copolymer, oxidized polyethylene, oxidized polypropylene, oxidized ethylene-propylene copolymer, ethylene-acrylic acid copolymer, ethyl cellulose, and ethylhydroxyethyl cellulose. Among these polymeric dispersants, it is particularly preferable to use 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 from the viewpoint of dispersion stability of the monomer. These polymeric dispersants may be used alone or in combination of two or more.

[0058] The amount of the polymeric dispersant used is preferably 0.1 to 30 parts by mass, more preferably 0.3 to 20 parts by mass, per 100 parts by mass of the first stage water-soluble ethylenically unsaturated monomer.

[0059] [Other Components] In the method for producing water-absorbent resin particles, if desired, other components may be added to an aqueous solution containing a water-soluble ethylenically unsaturated monomer to carry out reverse phase suspension polymerization. As the other components, various additives such as a thickener and a chain transfer agent can be added.

[0060] For example, a thickener can be added to an aqueous solution containing a water-soluble ethylenically unsaturated monomer to carry out reversed-phase suspension polymerization. By adjusting the viscosity of the aqueous solution by adding a thickener in this way, it is possible to control the median particle size obtained in the reversed-phase suspension polymerization.

[0061] Examples of usable thickeners include hydroxyethyl cellulose, hydroxypropyl cellulose, methyl cellulose, carboxymethyl cellulose, polyacrylic acid, (partially) neutralized polyacrylic acid, polyethylene glycol, polyacrylamide, polyethyleneimine, dextrin, sodium alginate, polyvinyl alcohol, polyvinylpyrrolidone, polyethylene oxide, etc. If the stirring speed during polymerization is the same, the higher the viscosity of the water-soluble ethylenically unsaturated monomer aqueous solution, the larger the primary particles and / or secondary particles of the resulting particles tend to be.

[0062]

[0033] In performing the reversed-phase suspension polymerization, for example, an aqueous monomer solution containing a water-soluble ethylenically unsaturated monomer is dispersed in a hydrocarbon dispersion medium in the presence of a dispersion stabilizer. In this case, the dispersion stabilizer (surfactant or polymeric dispersant) may be added either before or after the addition of the aqueous monomer solution, as long as it is before the start of the polymerization reaction.

[0063] Among these, from the viewpoint of easily reducing the amount of hydrocarbon dispersion medium remaining in the obtained water absorbent resin particles, it is preferable to disperse an aqueous monomer solution in a hydrocarbon dispersion medium having a polymeric dispersant dispersed therein, and then further disperse a surfactant therein, and then carry out polymerization.

[0064] Such reversed-phase suspension polymerization can be carried out in one stage or in two or more stages, and is preferably carried out in two or three stages from the viewpoint of increasing productivity.

[0065] When carrying out reversed-phase suspension polymerization in two or more stages, after carrying out reversed-phase suspension polymerization in the first stage, a water-soluble 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 and subsequent stages can be carried out in the same manner as in the first stage.In the reversed-phase suspension polymerization in each stage from the second stage onwards, it is preferable to carry out reversed-phase suspension polymerization by adding a radical polymerization initiator in addition to the water-soluble ethylenically unsaturated monomer within the molar ratio of each component to the water-soluble ethylenically unsaturated monomer as described above, based on the amount of the water-soluble ethylenically unsaturated monomer added during the reversed-phase suspension polymerization in each stage from the second stage onwards.In addition, in the polymerization in the second and subsequent stages, an internal crosslinking agent may be added to the water-soluble ethylenically unsaturated monomer as needed.

[0066] The reaction temperature of the polymerization reaction is preferably 20 to 110°C, more preferably 40 to 90°C, from the viewpoints of rapidly progressing the polymerization, shortening the polymerization time, thereby improving economic efficiency, and easily removing the heat of polymerization to smoothly carry out the reaction.

[0067] <Dehydration Step> After the above-described reversed-phase suspension polymerization, a dehydration step may be included in which water, hydrocarbon dispersion medium, and the like are removed by distillation by externally applying energy such as heat. When dehydrating the hydrous gel-like material after reversed-phase suspension polymerization, the system in which the hydrous gel-like material is dispersed in the hydrocarbon dispersion medium is heated, and the water and hydrocarbon dispersion medium are temporarily distilled out of the system by azeotropic distillation. In this case, if only the evaporated hydrocarbon dispersion medium is returned to the system, continuous azeotropic distillation is possible. In this case, the temperature in the system during drying is maintained below the azeotropic temperature with the hydrocarbon dispersion medium, which is preferable from the viewpoint of preventing deterioration of the resin. By controlling the treatment conditions of this dehydration step after polymerization to adjust the amount of dehydration (i.e., adjusting the water content of the polymer particles), it is possible to control the various properties of the obtained water-absorbent resin particles.

[0068] In the dehydration step, dehydration treatment by distillation may be carried out under normal pressure. When dehydration treatment is carried out under normal pressure, the dehydration temperature is preferably 70 to 250°C, more preferably 80 to 180°C, even more preferably 80 to 140°C, and even more preferably 90 to 130°C.

[0069] <Surface cross-linking step> The surface cross-linking step is a step of subjecting the polymer particles obtained in the polymerization step to surface cross-linking. When the polymer particles are cross-linked polymer particles (hydrogel-like material), this is a step of adding a surface cross-linking agent to a hydrogel-like material having an internal cross-linked structure obtained by polymerizing a water-soluble ethylenically unsaturated monomer to perform cross-linking (surface cross-linking reaction). This surface cross-linking reaction is preferably carried out in the presence of a surface cross-linking agent after the polymerization of the water-soluble ethylenically unsaturated monomer. In this way, by subjecting a hydrogel-like material having an internal cross-linked structure after polymerization to a surface cross-linking reaction, the cross-linking density near the surface of the water-absorbent resin particles can be set within a specific range, thereby obtaining water-absorbent resin particles with improved performance such as water absorption capacity under load.

[0070] Examples of the surface cross-linking agent include compounds having two or more reactive functional groups. For example, polyols such as ethylene glycol, propylene glycol, 1,4-butanediol, diethylene glycol, triethylene glycol, 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; haloepoxy compounds such as epichlorohydrin, epibromohydrin, and α-methylepichlorohydrin; isocyanate compounds such as 2,4-tolylene diisocyanate and hexamethylene diisocyanate; 3-methyl-3-oxetanemethanol and 3-ethyl-3-oxetane Oxetane compounds such as methanol, 3-butyl-3-oxetanemethanol, 3-methyl-3-oxetaneethanol, 3-ethyl-3-oxetaneethanol, and 3-butyl-3-oxetaneethanol; oxazoline compounds such as 1,2-ethylenebisoxazoline; ethylene carbonate, propylene carbonate, 4,5-dimethyl-1,3-dioxolan-2-one, 4,4-dimethyl-1,3-dioxolan-2-one, 4-ethyl and hydroxyalkylamide compounds such as bis[N,N-di(β-hydroxyethyl)]adipamide. Among these surface cross-linking agents, 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 are preferred.These surface cross-linking agents may be used alone or in combination of two or more.

[0071] From the viewpoint of adjusting the 15-minute value of the water absorption amount of physiological saline under a load of 0.69 kPa within a preferred range, the amount of the surface crosslinking agent to be used is preferably 0.00001 to 0.01 mol, more preferably 0.00005 to 0.005 mol, still more preferably 0.0001 to 0.001 mol, and still more preferably 0.0004 to 0.0009 mol, relative to 1 mol of the total amount of the water-soluble ethylenically unsaturated monomers used in the polymerization.

[0072] As a method for adding the surface crosslinking agent, the surface crosslinking agent may be added as it is or as an aqueous solution, or, if necessary, may be added as a solution using a hydrophilic organic solvent as a solvent. Examples of the hydrophilic organic solvent include lower alcohols such as methyl alcohol, ethyl alcohol, n-propyl alcohol, isopropyl alcohol, etc.; ketones such as acetone, methyl ethyl ketone, etc.; ethers such as diethyl ether, dioxane, tetrahydrofuran, etc.; amides such as N,N-dimethylformamide, etc.; sulfoxides such as dimethyl sulfoxide, etc. These hydrophilic organic solvents may be used alone, or two or more types may be used in combination, or as a mixed solvent with water.

[0073] The timing of addition of the surface crosslinking agent may be after the polymerization reaction of the water-soluble ethylenically unsaturated monomer has almost completely finished, and the surface crosslinking agent is added in the presence of water in a range of preferably 1 to 400 parts by mass, more preferably 5 to 200 parts by mass, still more preferably 10 to 100 parts by mass, and still more preferably 15 to 60 parts by mass, relative to 100 parts by mass of the water-soluble ethylenically unsaturated monomer used for polymerization. Note that the amount of water means the total amount of water contained in the reaction system and water used as necessary when adding the surface crosslinking agent.

[0074] The reaction temperature in the surface crosslinking reaction is preferably 50 to 250° C., more preferably 60 to 180° C., further preferably 60 to 140° C., and even more preferably 70 to 120° C. The reaction time of the surface crosslinking reaction is preferably 1 to 300 minutes, and more preferably 5 to 200 minutes.

[0075] <Drying step> After the above-described surface cross-linking is performed, a drying step may be included in which water, a hydrocarbon dispersion medium, etc. are removed by distillation by applying energy such as heat from the outside. The polymer particles after surface cross-linking are dried and the water and the hydrocarbon dispersion medium are distilled off, thereby obtaining water-absorbent resin particles.

[0076] In the drying step, the drying treatment by distillation may be carried out under normal pressure or under reduced pressure. Moreover, from the viewpoint of increasing the drying efficiency, it may be carried out under a gas flow such as nitrogen. When the drying treatment is carried out under normal pressure, the drying temperature is preferably 70 to 250°C, more preferably 80 to 180°C, even more preferably 80 to 140°C, and even more preferably 90 to 130°C. Moreover, when the drying treatment is carried out under reduced pressure, the drying temperature is preferably 40 to 160°C, more preferably 50 to 110°C.

[0077] In addition, when the surface cross-linking step using a surface cross-linking agent is carried out after the polymerization of monomers by reverse phase suspension polymerization, the drying step by distillation described above is carried out after the completion of the surface cross-linking step. Alternatively, the surface cross-linking step and the drying step may be carried out simultaneously.

[0078] The water-absorbent resin particles of the present invention may contain additives according to the purpose. Examples of such additives include inorganic powders, surfactants, oxidizing agents, reducing agents, metal chelating agents, radical chain inhibitors, antioxidants, antibacterial agents, etc. For example, the fluidity of the water-absorbent resin particles can be further improved by adding 0.05 to 5 parts by mass of amorphous silica as inorganic powder relative to 100 parts by mass of the water-absorbent resin particles. The additives are preferably hydrophilic or water-soluble.

[0079] 3. Absorbent Material, Absorbent Articles The water-absorbing resin particles of the present invention constitute an absorbent material used in hygiene materials such as sanitary products and disposable diapers, and are suitably used in absorbent articles containing the absorbent material.

[0080] The absorbent of the present invention contains the water-absorbent resin particles of the present invention. The absorbent may further contain hydrophilic fibers. Examples of the absorbent's configuration include a sheet-like structure in which water-absorbent resin particles are fixed on a nonwoven fabric or between multiple nonwoven fabrics, a mixed dispersion obtained by mixing water-absorbent resin particles and hydrophilic fibers to form a uniform composition, a sandwich structure in which water-absorbent resin particles are sandwiched between layered hydrophilic fibers, and a structure in which water-absorbent resin particles and hydrophilic fibers are wrapped in tissue. The absorbent may also contain other components, such as adhesive binders such as heat-fusible synthetic fibers, hot-melt adhesives, and adhesive emulsions, to improve the shape retention of the absorbent.

[0081] The basis weight of the water-absorbent resin particles in the absorbent body of the present invention is 30 g / m 2 More than 500g / m 2 The basis weight is preferably 100 g / m or less. 2 More than 400g / m 2 The following is the result.

[0082] The basis weight of the water-absorbent resin particles in the mixed dispersion of the present invention is 30 g / m 2 More than 500g / m 2 The basis weight is preferably 100 g / m or less. 2 More preferably, 120 g / m 2 More preferably, 140 g / m 2 or more, and preferably 300 g / m 2 or less, more preferably 250 g / m 2 More preferably 200 g / m or less 2 The following is the result.

[0083] The basis weight of the water-absorbent resin particles in the sheet-like structure of the present invention is 150 g / m 2 More than 500g / m 2 The basis weight is preferably 250 g / m or less. 2 More preferably, 300 g / m2 More preferably, 350 g / m 2 or more, and preferably 450 g / m 2 or less, more preferably 400 g / m 2 More preferably 350 g / m or less 2 The following is the result.

[0084] The hydrophilic fiber may be at least one selected from the group consisting of finely ground wood pulp, cotton, cotton linter, rayon, cellulose acetate, polyamide, polyester, and polyolefin. Examples include cellulose fibers such as cotton-like pulp obtained from wood, mechanical pulp, chemical pulp, and semi-chemical pulp; artificial cellulose fibers such as rayon and acetate; and fibers made of synthetic resins such as hydrophilically treated polyamide, polyester, and polyolefin. The average fiber length of the hydrophilic fiber is usually 0.1 to 10 mm, or may be 0.5 to 5 mm.

[0085] The basis weight of the hydrophilic fiber in the absorbent body of the present invention is 0 g / m 2 800g / m or more 2 The basis weight is preferably 0 g / m 2 More than 500g / m 2 The following is the result.

[0086] The basis weight of the hydrophilic fiber in the mixed dispersion of the present invention is 50 g / m 2 800g / m or more 2 The basis weight is preferably 100 g / m or less. 2 More preferably, 120 g / m 2 More preferably, 140 g / m 2 and preferably 700 g / m 2 or less, more preferably 600 g / m 2 More preferably 500 g / m or less 2 The following is the result.

[0087] The basis weight of the hydrophilic fiber in the sheet-like structure of the present invention is 0 g / m 2 More than 100g / m 2 The basis weight is preferably 0 g / m 2 80g / m or more2 or less, more preferably 0 g / m 2 50g / m or more 2 More preferably, 0 g / m 2 is.

[0088] The content of the water-absorbent resin particles in the absorbent body is preferably 5 to 100% by mass, more preferably 10 to 95% by mass, even more preferably 20 to 90% by mass, and even more preferably 30 to 80% by mass.

[0089] The absorbent article of the present invention can be produced by holding an absorbent body using the water-absorbent resin particles of the present invention between a liquid-permeable sheet (top sheet) through which liquid can pass and a liquid-impermeable sheet (back sheet) through which liquid cannot pass. The liquid-permeable sheet is arranged on the side that comes into contact with the body, and the liquid-impermeable sheet is arranged on the opposite side that comes into contact with the body.

[0090] Examples of liquid-permeable sheets include air-through, spunbond, chemical-bond, and needle-punched nonwoven fabrics made of fibers such as polyethylene, polypropylene, and polyester, as well as porous synthetic resin sheets. Examples of liquid-impermeable sheets include synthetic resin films made of resins such as polyethylene, polypropylene, and polyvinyl chloride. The liquid-permeable sheet is preferably at least one selected from the group consisting of thermal-bonded nonwoven fabrics, air-through nonwoven fabrics, spunbonded nonwoven fabrics, and spunbonded / meltblown / spunbonded nonwoven fabrics.

[0091] The basis weight of the liquid permeable sheet is 5 g / m 2 More than 100g / m 2 Preferably, it is 10 g / m or less. 2 60g / m or more 2 It is more preferable that the liquid-permeable sheet has a surface embossed or perforated to improve the liquid diffusibility. The embossing or perforation can be carried out by a known method.

[0092] Examples of liquid-impermeable sheets include sheets made of synthetic resins such as polyethylene, polypropylene, and polyvinyl chloride; sheets made of nonwoven fabrics such as spunbond / meltblown / spunbond (SMS) nonwoven fabrics in which a water-resistant meltblown nonwoven fabric is sandwiched between high-strength spunbond nonwoven fabrics; and sheets made of composite materials of these synthetic resins and nonwoven fabrics (for example, spunbond nonwoven fabrics, spunlace nonwoven fabrics). As the liquid-impermeable sheet, a sheet made of a synthetic resin mainly composed of low-density polyethylene (LDPE) resin can also be used. The liquid-impermeable sheet has, for example, a basis weight of 10 to 50 g / m. 2 The sheet may be made of a synthetic resin.

[0093] The absorbent article preferably comprises a laminate having an absorbent body containing water-absorbent resin particles and a core wrap sandwiching the absorbent body from above and below, a liquid-permeable sheet disposed on the upper surface of the laminate, and a liquid-impermeable sheet disposed on the surface of the laminate opposite to the liquid-permeable sheet side.

[0094] 4. Additional Notes This specification includes at least the inventions shown in (1) to (8) below: (1) Water-absorbent resin particles, the 15-minute water absorption value of physiological saline solution under a load of 0.69 kPa being 30 to 60 mL / g, 30 to 58 mL / g, 30 to 55 mL / g, 32 to 60 mL / g, 32 to 58 mL / g, 32 to 55 mL / g, 34 to 60 mL / g, 34 to 58 mL / g, 34 to 55 mL / g, 35 to 60 mL / g, 35 to 58 mL / g, or 35 to 55 mL / g. (2) The water-absorbent resin particles have a no-pressure DW 3-minute value of 20 to 50 mL / g, 20 to 48 mL / g, 20 to 46 mL / g, 30 to 50 mL / g, 30 to 48 mL / g, 30 to 46 mL / g, 33 to 50 mL / g, 33 to 48 mL / g, 33 to 46 mL / g, 36 to 50 mL / g, 36 to 48 mL / g, 36 to 46 mL / g, 39 to 50 mL / g, 39 to 48 mL / g, or 39 to 46 mL / g. The water-absorbent resin particles according to (1) above. (3) The water-absorbent resin particles according to (1) or (2) above, wherein the water-absorbent resin particles have a water-absorption speed for physiological saline of 60 seconds or less, 20 to 60 seconds, 20 to 55 seconds, 20 to 50 seconds, 25 to 60 seconds, 25 to 55 seconds, 25 to 50 seconds, 30 to 60 seconds, 30 to 55 seconds, or 30 to 50 seconds. (4) The water-absorbent resin particles according to any one of (1) to (3) above, wherein the water-absorbent resin particles have a water-retention capacity for physiological saline of 30 g / g or more, 30 to 70 g / g, 30 to 65 g / g, 30 to 60 g / g, 36 to 70 g / g, 36 to 65 g / g, 36 to 60 g / g, 41 to 70 g / g, 41 to 65 g / g, or 41 to 60 g / g. (5) The water-absorbent resin particles have a 10-minute no-pressure DW value of 30 to 80 mL / g, 30 to 75 mL / g, 30 to 72 mL / g, 45 to 80 mL / g, 45 to 75 mL / g, 45 to 72 mL / g, 50 to 80 mL / g, 50 to 75 mL / g, 50 to 72 mL / g, 54 to 80 mL / g, 54 to 75 mL / g, 54 to 72 mL / g, 56 to 80 mL / g, 56 to 75 mL / g, or 56 to 72 mL / g. The water-absorbent resin particles according to any one of (1) to (4) above.(6) The water-absorbent resin particles according to any one of (1) to (5) above, wherein the water-absorbent resin particles have a physiological saline water absorption capacity of 30 g / g or more, 45 to 80 g / g, 45 to 74 g / g, 45 to 69 g / g, 52 to 80 g / g, 52 to 74 g / g, 52 to 69 g / g, 59 to 80 g / g, 59 to 74 g / g, or 59 to 69 g / g. (7) The water-absorbent resin particles according to any one of (1) to (6) above, wherein the difference between the physiological saline water absorption capacity and the physiological saline water retention capacity of the water-absorbent resin particles is 5 to 30, 5 to 25, 5 to 20, 10 to 30, 10 to 25, or 10 to 20. (8) The water-absorbent resin particles according to any one of (1) to (7), wherein the median particle diameter of the water-absorbent resin particles is 200 to 700 μm, 200 to 600 μm, 200 to 550 μm, 200 to 500 μm, 200 to 450 μm, 250 to 700 μm, 250 to 600 μm, 250 to 550 μm, 250 to 500 μm, 250 to 450 μm, 300 to 700 μm, 300 to 600 μm, 300 to 550 μm, 300 to 500 μm, 300 to 450 μm, 350 to 700 μm, 350 to 600 μm, 350 to 550 μm, 350 to 500 μm, 350 to 450 μm.

[0095] The present invention will be described in detail below with reference to examples and comparative examples. However, the present invention is not limited to these examples. Unless otherwise specified, measurements were carried out in an environment of a temperature of 25±2°C and a humidity of 50±10%.

[0096] <Production of water-absorbent resin particles> (Example 1) [First-stage polymerization step] A round-bottomed cylindrical separable flask with an inner diameter of 11 cm and a capacity of 2 L was prepared, 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 as a stirrer. 292 g of n-heptane was placed in this flask as a hydrocarbon dispersion medium, and 0.782 g of maleic anhydride-modified ethylene-propylene copolymer (Hiwax 1105A, Mitsui Chemicals, Inc.) was added as a polymer dispersant. The mixture was heated to 80°C with stirring to dissolve the dispersant, and then cooled to 50°C.

[0097] In a 300 mL beaker was placed 92.0 g (1.03 mol) of an 80.5 mass% aqueous acrylic acid solution as a water-soluble ethylenically unsaturated monomer, and while cooling from the outside, 110.1 g of a 28.0 mass% aqueous sodium hydroxide solution was added dropwise to carry out 75 mol% neutralization, followed by the addition and dissolution of 0.092 g (0.339 mmol) of 2,2′-azobis(2-amidinopropane) dihydrochloride and 0.028 g (0.102 mmol) of potassium persulfate as water-soluble radical polymerization initiators, 0.0101 g (0.058 mmol) of ethylene glycol diglycidyl ether as an internal crosslinking agent, and 37.2 g of ion-exchanged water, to prepare a first-stage aqueous monomer solution.

[0098] The aqueous solution prepared above was then added to a separable flask and stirred for 10 minutes, after which a surfactant solution prepared by heating and dissolving 0.828 g of a sucrose stearate ester with an HLB of 3 (Ryoto Sugar Ester S-370, Mitsubishi-Kagaku Foods Corporation) as a surfactant in 7.45 g of n-heptane was further added, and the system was thoroughly purged with nitrogen while stirring at a stirrer rotation speed of 500 rpm.The flask was then immersed in a water bath at 70°C to raise the temperature, and polymerization was carried out for 60 minutes, thereby obtaining a first-stage polymerization slurry liquid containing primary particles.

[0099] [Second-Stage Polymerization Step] 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, 154.2 g of a 28.0 mass % aqueous sodium hydroxide solution was added dropwise to carry out 75 mol % neutralization. Thereafter, 0.064 g (0.237 mmol) of 2,2′-azobis(2-amidinopropane) dihydrochloride as a water-soluble radical polymerization initiator, 0.039 g (0.143 mmol) of potassium persulfate, and 6.49 g of ion-exchanged water were added and dissolved to prepare a second-stage aqueous monomer solution.

[0100] The contents of the separable flask were cooled to 25°C while stirring at a stirrer speed of 1000 rpm, and then the entire amount of the second-stage aqueous solution was added to the first-stage polymerization slurry. The atmosphere in the system was replaced with nitrogen for 30 minutes, and the flask was again immersed in a water bath at 70°C to raise the temperature, and the polymerization reaction was carried out for 60 minutes. By the above operations, a hydrogel after second-stage polymerization was obtained.

[0101] [Surface Crosslinking] The flask was then immersed in an oil bath set at 125°C, and 176.6 g of water was extracted from the system by azeotropic distillation of n-heptane and water while refluxing n-heptane. Then, 4.91 g of a 4.5 mass% aqueous solution of pentasodium diethylenetriaminepentaacetate and 2.58 g of a 3 mass% aqueous solution of sodium sulfite were added to the flask with stirring to obtain a second-stage polymerization slurry containing secondary particles.

[0102] Thereafter, the flask was again immersed in the oil bath set to 125°C, and 99.5 g (total 276.1 g) of water was further extracted from the system by azeotropic distillation of n-heptane and water while refluxing n-heptane. Thereafter, 16.6 g (1.90 mmol) of a 2 mass% aqueous solution of ethylene glycol diglycidyl ether was added as a surface crosslinking agent to the flask, and the mixture was maintained at 83°C for 2 hours.

[0103] Thereafter, n-heptane was evaporated at 125° C. to dry the product, thereby obtaining a dried product. This dried product was passed through a sieve with an opening of 850 μm, and 0.2 mass % of amorphous silica (Toxil NP-S, manufactured by Oriental Silicas Corporation) was mixed with the dried product, thereby obtaining 210.4 g of water absorbent resin particles of Example 1.

[0104] Example 2 218.6 g of water-absorbent resin particles of Example 2 were obtained in the same manner as in Example 1, except that the amount of water extracted from the system by the second azeotropic distillation was changed to 100.9 g (total 277.5 g).

[0105] Example 3 [First-stage polymerization step] A round-bottomed cylindrical separable flask with an inner diameter of 11 cm and a capacity of 2 L was prepared, equipped with a reflux condenser, a dropping funnel, a nitrogen gas inlet tube, and a stirrer with a stirring blade having two stages of four inclined paddle blades with a blade diameter of 5 cm. 315 g of n-heptane was added to this flask as a hydrocarbon dispersion medium, and 0.782 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.

[0106] In a 300 mL beaker was placed 92.0 g (1.03 mol) of an 80.5 mass% aqueous acrylic acid solution as a water-soluble ethylenically unsaturated monomer, and while cooling from the outside, 110.1 g of a 28.0 mass% aqueous sodium hydroxide solution was added dropwise to carry out 75 mol% neutralization, followed by the addition and dissolution of 0.092 g (0.339 mmol) of 2,2′-azobis(2-amidinopropane) dihydrochloride and 0.028 g (0.102 mmol) of potassium persulfate as water-soluble radical polymerization initiators, 0.0101 g (0.058 mmol) of ethylene glycol diglycidyl ether as an internal crosslinking agent, and 37.2 g of ion-exchanged water, to prepare a first-stage aqueous monomer solution.

[0107] The aqueous solution prepared above was then added to a separable flask and stirred for 10 minutes, after which a surfactant solution prepared by heating and dissolving 0.828 g of a sucrose stearate ester with an HLB of 3 (Ryoto Sugar Ester S-370, Mitsubishi-Kagaku Foods Corporation) as a surfactant in 7.45 g of n-heptane was further added, and the system was thoroughly purged with nitrogen while stirring at a stirrer rotation speed of 500 rpm.The flask was then immersed in a water bath at 70°C to raise the temperature, and polymerization was carried out for 60 minutes, thereby obtaining a first-stage polymerization slurry liquid containing primary particles.

[0108] [Second-Stage Polymerization Step] In a 500 mL beaker was placed 147.2 g (1.64 mol) of an 80.5 mass % aqueous acrylic acid solution as a water-soluble ethylenically unsaturated monomer, and while cooling from the outside, 176.2 g of a 28.0 mass % aqueous sodium hydroxide solution was added dropwise to carry out 75 mol % neutralization. Thereafter, 0.074 g (0.271 mmol) of 2,2′-azobis(2-amidinopropane) dihydrochloride as a water-soluble radical polymerization initiator, 0.044 g (0.163 mmol) of potassium persulfate, and 7.41 g of ion-exchanged water were added and dissolved to prepare a second-stage aqueous monomer solution.

[0109] The contents of the separable flask were cooled to 25°C while stirring at a stirrer speed of 1000 rpm, and then the entire amount of the second-stage aqueous solution was added to the first-stage polymerization slurry. The atmosphere in the system was replaced with nitrogen for 30 minutes, and the flask was again immersed in a water bath at 70°C to raise the temperature, and the polymerization reaction was carried out for 60 minutes. By the above operations, a hydrogel after second-stage polymerization was obtained.

[0110] [Surface Crosslinking] The flask was then immersed in an oil bath set at 125°C, and 191.4 g of water was extracted from the system by azeotropic distillation of n-heptane and water while refluxing n-heptane. Then, 5.32 g of a 4.5 mass% aqueous solution of pentasodium diethylenetriaminepentaacetate and 2.79 g of a 3 mass% aqueous solution of sodium sulfite were added to the flask with stirring to obtain a second-stage polymerization slurry containing secondary particles.

[0111] Thereafter, the flask was again immersed in the oil bath set to 125°C, and 84.6 g (276.0 g in total) of water was further extracted from the system by azeotropic distillation of n-heptane and water while refluxing n-heptane. Thereafter, 5.98 g (0.69 mmol) of a 2 mass% aqueous solution of ethylene glycol diglycidyl ether was added as a surface crosslinking agent to the flask, and the mixture was maintained at 83°C for 2 hours.

[0112] Thereafter, n-heptane was evaporated at 125° C. to dry the product, thereby obtaining a dried product. This dried product was passed through a sieve with an opening of 850 μm, and 0.2 mass % of amorphous silica (Toxil NP-S, manufactured by Oriental Silicas Corporation) was mixed with the dried product, thereby obtaining 223.3 g of water-absorbent resin particles of Example 3.

[0113] (Comparative Example 1) [First-stage polymerization step] A round-bottomed cylindrical separable flask with an inner diameter of 11 cm and a capacity of 2 L was prepared, equipped with a reflux condenser, a dropping funnel, a nitrogen gas inlet tube, and a stirrer with a stirring blade having two stages of four inclined paddle blades with a blade diameter of 5 cm. 292 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.

[0114] In a 300 mL beaker was placed 92.0 g (1.03 mol) of an 80.5 mass % aqueous acrylic acid solution as a water-soluble ethylenically unsaturated monomer, and while cooling from the outside, 110.1 g of a 28.0 mass % aqueous sodium hydroxide solution was added dropwise to carry out 75 mol % neutralization, followed by adding and dissolving 0.074 g (0.272 mmol) of potassium persulfate as a water-soluble radical polymerization initiator, 0.0101 g (0.058 mmol) of ethylene glycol diglycidyl ether as an internal crosslinking agent, and 37.2 g of ion-exchanged water, to prepare a first-stage aqueous monomer solution.

[0115] The aqueous solution 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 a sucrose stearate ester with an HLB of 3 (Ryoto Sugar Ester S-370, Mitsubishi-Kagaku Foods Corporation) as a surfactant in 7.45 g of n-heptane was further added, and the system was thoroughly purged with nitrogen while stirring at a stirrer rotation speed of 500 rpm.The flask was then immersed in a water bath at 70°C to raise the temperature, and polymerization was carried out for 60 minutes, thereby obtaining a first-stage polymerization slurry liquid containing primary particles.

[0116] [Second-stage polymerization step] 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, 154.2 g of a 28.0 mass% aqueous sodium hydroxide solution was added dropwise to carry out 75 mol% neutralization. Thereafter, 0.103 g (0.381 mmol) of potassium persulfate as a water-soluble radical polymerization initiator, 0.012 g (0.067 mmol) of ethylene glycol diglycidyl ether as an internal crosslinking agent, and 6.48 g of ion-exchanged water were added and dissolved to prepare a second-stage aqueous monomer solution.

[0117] The contents of the separable flask were cooled to 25°C while stirring at a stirrer speed of 1000 rpm, and then the entire amount of the second-stage aqueous solution was added to the first-stage polymerization slurry. The atmosphere in the system was replaced with nitrogen for 30 minutes, and the flask was again immersed in a water bath at 70°C to raise the temperature, and the polymerization reaction was carried out for 60 minutes. By the above operations, a hydrogel after second-stage polymerization was obtained.

[0118] [Surface Crosslinking] The flask was then immersed in an oil bath set at 125°C, and 176.6 g of water was extracted from the system by azeotropic distillation of n-heptane and water while refluxing n-heptane. Then, 4.91 g of a 4.5 mass% aqueous solution of pentasodium diethylenetriaminepentaacetate and 2.58 g of a 3 mass% aqueous solution of sodium sulfite were added to the flask with stirring to obtain a second-stage polymerization slurry containing secondary particles.

[0119] Thereafter, the flask was again immersed in the oil bath set to 125°C, and 89.6 g (266.3 g in total) of water was further extracted from the system by azeotropic distillation of n-heptane and water while refluxing n-heptane. Thereafter, 4.42 g (0.51 mmol) of a 2 mass% aqueous solution of ethylene glycol diglycidyl ether was added as a surface crosslinking agent to the flask, and the mixture was maintained at 83°C for 2 hours.

[0120] Thereafter, n-heptane was evaporated at 125° C. to dry the product, thereby obtaining a dried product. This dried product was passed through a sieve having an opening of 850 μm, and 0.5% by mass of amorphous silica (Toxil NP-S, manufactured by Oriental Silicas Corporation) was mixed with the dried product, thereby obtaining 227.3 g of water-absorbent resin particles of Comparative Example 1.

[0121] (Comparative Example 2) In the second-stage polymerization step, the amount of ethylene glycol diglycidyl ether to be added as an internal crosslinking agent was changed to 0.026 g (0.148 millimoles), the amount of ion-exchanged water to be added was changed to 6.46 g, the amount of water to be extracted to the outside of the system by the second azeotropic distillation was changed to 97.4 g (total 274.1 g), and the amount of 2 mass % aqueous ethylene glycol diglycidyl ether solution to be added as a surface crosslinking agent was changed to 5.52 g (0.63 millimoles), except for these, 225.8 g of water absorbent resin particles of Comparative Example 2 was obtained in the same manner as in Example 1.

[0122] For the water-absorbent resin particles obtained in each Example and Comparative Example, the saline water absorption capacity under a load of 0.69 kPa, saline water retention capacity, saline water absorption capacity, no-pressure DW 3-minute and 10-minute values, water absorption rate (Vortex method), median particle size, absorption time of the absorbent article, and backflow amount were measured by the following methods. Unless otherwise specified, the measurements were performed in an environment of 25°C ± 2°C and humidity 50 ± 10%. The results are shown in Table 1.

[0123] [Water absorption amount of physiological saline solution under a load of 0.69 kPa] The water absorption amount of water-absorbent resin particles for physiological saline solution under a load of 0.69 kPa (water absorption amount under a load of 0.69 kPa) was measured using an apparatus outlined in FIG. 1 . The water absorption amount under load was measured twice for one type of water-absorbent resin particle, and the average value of the measured values ​​was calculated. The apparatus in FIG. 1 includes a burette unit 1, a clamp 3, a conduit 5, a stand 11, a measurement table 13, and a measurement unit 4 placed on the measurement table 13. The burette unit 1 includes a burette tube 21 having a scale, a rubber stopper 23 that seals the opening at the top of the burette tube 21, a cock 22 connected to the tip of the bottom of the burette tube 21, and an air introduction tube 25 and a cock 24 connected to the bottom of the burette tube 21. The burette unit 1 is fixed with a clamp 3. The flat measuring table 13 has a through-hole 13a with a diameter of 2 mm formed in its center, and is supported by a height-adjustable stand 11. The through-hole 13a of the measuring table 13 and the cock 22 of the burette part 1 are connected by a conduit 5. The inner diameter of the conduit 5 is 6 mm.

[0124] The measurement unit 4 has a cylinder 31 made of acrylic resin, a polyamide mesh 32 adhered to one opening of the cylinder 31, and a weight 33 that is movable up and down within the cylinder 31. The cylinder 31 is placed on the measurement table 13 via the polyamide mesh 32. The inner diameter of the cylinder 31 is 20 mm. The opening of the polyamide mesh 32 is 75 μm (200 mesh). The weight 33 has a diameter of 19 mm and a mass of 20.5 g, and can apply a load of 0.69 kPa to the water-absorbent resin particles 10a uniformly arranged on the polyamide mesh 32, as will be described later.

[0125] First, the stopcocks 22 and 24 of the burette part 1 were closed, and 0.9% by mass saline solution adjusted to 25°C was poured into the burette tube 21 through the opening at the top of the burette tube 21. Next, the top opening of the burette tube 21 was sealed with a rubber stopper 23, and then the stopcocks 22 and 24 were opened. The inside of the conduit 5 was filled with 0.9% by mass saline solution 50 while preventing air bubbles from entering. The height of the measurement table 13 was adjusted so that the height of the water surface of the 0.9% by mass saline solution 50 that had reached 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 0.9% by mass saline solution 50 in the burette tube 21 was read on the scale of the burette tube 21, and this position was designated as the zero point (the reading at 0 seconds).

[0126] In the measurement unit 4, 0.10 g of water-absorbent resin particles 10a were uniformly arranged on the polyamide mesh 32 in the cylinder 31, a weight 33 was placed on the water-absorbent resin particles 10a, and the cylinder 31 was installed so that its center coincided with the conduit opening at the center of the measurement table 13. The amount of saline solution reduced in the burette tube 21 (i.e., the amount of saline solution absorbed by the water-absorbent resin particles 10a) Wa (mL) 15 minutes after the water-absorbent resin particles 10a started to absorb saline solution from the conduit 5 was read, and the amount of saline solution absorbed by the water-absorbent resin particles 10a under a load of 0.69 kPa was calculated using the following formula. The results are shown in Table 1. Amount of saline solution absorbed under a load of 0.69 kPa (mL / g) = Wa (mL) / mass of water-absorbent resin particles (g)

[0127] [Saline Water Retention Capacity] A cotton bag (membrane broadcloth No. 60, 100 mm wide x 200 mm long) containing 2.0 g of water-absorbent resin particles was placed in a 500 mL beaker. 500 g of a 0.9% by mass aqueous sodium chloride solution (saline) was poured into the cotton bag containing the water-absorbent resin particles all at once, taking care not to allow the bag to swell. The top of the cotton bag was tied with a rubber band and allowed to stand for 30 minutes to allow the water-absorbent resin particles to swell. After 30 minutes, the cotton bag was dehydrated for 1 minute using a dehydrator (manufactured by Kokusan Co., Ltd., product number: H-122) set to a centrifugal force of 167 G, and the mass Wb (g) of the cotton bag containing the swollen gel after dehydration was measured. The same procedure was performed without adding water-absorbent resin particles, and the empty mass Wc (g) of the cotton bag when wet was measured. The saline water retention capacity was calculated using the following formula. The results are shown in Table 1. Saline water retention capacity (g / g) = [Wb - Wc] / 2.0

[0128] [Saline Water Absorption] 500 g of 0.9% saline was weighed into a 500 mL beaker. Next, 2.0 g of water-absorbent resin particles were dispersed in the saline solution while stirring at 600 rpm using a magnetic stir bar (8 mm diameter x 30 mm length, no ring) to prevent lumps from forming. The mixture was left to stand for 60 minutes while stirring to allow the particles to swell sufficiently, yielding a dispersion containing a swollen gel. Next, the mass Wd [g] of a 75 μm standard sieve was measured, and the dispersion was passed through the standard sieve. The sieve was then tilted at an angle of approximately 30 degrees relative to the horizontal and left for 30 minutes to remove excess water. The mass We [g] of the sieve with the swollen gel remaining was measured, and the saline water absorption (g / g) was calculated using the following formula: Saline Water Absorption (g / g) = (We - Wd) / 2.0

[0129] [Measurement of 3-minute and 10-minute values ​​of no-pressure DW (Demand Wetability)] The 3-minute and 10-minute values ​​of no-pressure DW of water-absorbent resin particles were measured using a measuring device shown in Fig. 2. The measurement was carried out five times for one type of water-absorbent resin particle, and an average value of three measurement values ​​excluding the minimum and maximum values ​​was calculated.

[0130] The measuring device comprises a burette unit 1, a conduit 5, a measurement table 13, a nylon mesh sheet 15, a stand 11, and a clamp 3. The burette unit 1 comprises a burette tube 21 with a scale, a rubber stopper 23 that seals the upper opening of the burette tube 21, a cock 22 connected to the lower tip of the burette tube 21, and an air introduction tube 25 and a cock 24 connected to the lower part of the burette tube 21. The burette unit 1 is fixed with the clamp 3. The flat measurement table 13 has a through-hole 13a with a diameter of 2 mm formed in its center and is supported by the height-adjustable stand 11. The through-hole 13a of the measurement table 13 and the cock 22 of the burette unit 1 are connected by a conduit 5. The inner diameter of the conduit 5 is 6 mm.

[0131] First, the stopcocks 22 and 24 of the burette part 1 were closed, and 0.9% by mass saline solution 50 adjusted to 25°C was poured into the burette tube 21 through the opening at the top of the burette tube 21. The concentration of the saline solution, 0.9% by mass, is based on the mass of the saline solution. After sealing the opening of the burette tube 21 with a rubber stopper 23, the stopcocks 22 and 24 were opened. The inside of the conduit 5 was filled with 0.9% by mass saline solution 50 while preventing air bubbles from entering. The height of the measurement table 13 was adjusted so that the height of the water surface of the 0.9% by mass saline solution 50 that had reached the through-hole 13a was the same as the height of the upper surface of the measurement table 13. After adjustment, the height of the water surface of the 0.9% by mass saline solution 50 in the burette tube 21 was read on the scale of the burette tube 21, and this position was designated as the zero point (the reading at 0 seconds).

[0132] A nylon mesh sheet 15 (100 mm x 100 mm, 250 mesh, approximately 50 μm thick) was laid near the through-hole 13 a on the measurement table 13, and a cylinder with an inner diameter of 30 mm and a height of 20 mm was placed in the center of the sheet. 1.00 g of water-absorbent resin particles 10 a was uniformly dispersed in the cylinder. Thereafter, the cylinder was carefully removed, and a sample in which the water-absorbent resin particles 10 a were dispersed in a circular pattern was obtained in the center of the nylon mesh sheet 15. Next, the nylon mesh sheet 15 on which the water-absorbent resin particles 10 a were placed was quickly moved so that its center was positioned at the position of the through-hole 13 a, without dissipating the water-absorbent resin particles 10 a, and measurement was started. The time when air bubbles were first introduced into the burette tube 21 from the air inlet tube 25 was defined as the start of water absorption (0 seconds).

[0133] The amount of reduction in the 0.9% by mass saline solution 50 in the burette tube 21 (i.e., the amount of 0.9% by mass saline solution absorbed by the water-absorbent resin particles 10a) was sequentially read in 0.1 mL increments, and the amount of reduction in the amount of 0.9% by mass saline solution 50 Wf (mL) 3 minutes after the start of water absorption by the water-absorbent resin particles 10a, and the amount of reduction in the amount of 0.9% by mass saline solution 50 Wg (mL) 10 minutes after the start of water absorption by the water-absorbent resin particles 10a were read. From Wf, the 3-minute value of the no-pressure DW was calculated using the following formula. Furthermore, from Wg, the 10-minute value of the no-pressure DW was calculated using the following formula. The no-pressure DW is the amount of water absorbed per 1.00 g of the water-absorbent resin particles 10a. 3-minute value of the no-pressure DW value (mL / g) = Wf / 1.00 10-minute value of the no-pressure DW value (mL / g) = Wg / 1.00

[0134] [Water absorption rate (Vortex method)] The water absorption rate of saline solution by water-absorbent resin particles was measured by the Vortex method using the following procedure. First, 50±0.1 g of saline solution adjusted to a temperature of 25±0.2°C in a thermostatic water bath was weighed into a 100 mL beaker. Next, a vortex was generated by stirring at a rotation speed of 600 rpm using a magnetic stir bar (8 mmφ×30 mm, without ring). 2.0±0.002 g of water-absorbent resin particles were added to the saline solution at once. 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, and this time was obtained as the water absorption rate of the water-absorbent resin particles.

[0135] [Method for measuring median particle size] 10 g of water-absorbent resin particles were sieved using a continuous fully automatic ultrasonic vibration sieving measuring instrument (Robot Sifter RPS-205, manufactured by Seishin Enterprise Co., Ltd.), sieves with JIS standard openings of 850 μm, 710 μm, 600 μm, 500 μm, 425 μm, 300 μm, 250 μm, and 150 μm, and a tray. The mass of the particles remaining on each sieve was calculated as a mass percentage relative to the total amount. The mass percentages of the particles remaining on each sieve were integrated in order from the largest particle size, and the relationship between the sieve opening and the integrated value of the mass percentage of the particles remaining on the sieve was plotted on logarithmic probability paper. By connecting the plots on the probability paper with a straight line, the particle size corresponding to an integrated mass percentage of 50 mass% was determined, and this was taken as the median particle size (μm).

[0136] [Preparation of absorbent body and absorbent article] (Absorbent body and absorbent article of mixed dispersion) Using an air flow type mixer (Pad Former, manufactured by Autech Co., Ltd.), 8.11 g of water-absorbent resin particles and 12.38 g of pulverized pulp were uniformly mixed by air papermaking to prepare an absorbent core measuring 12 cm x 40 cm. Next, two tissue papers (basis weight: 16 g / m) of the same size as the absorbent core were placed on the paper. 2 The absorbent core was sandwiched between two sheets of paper and pressed under a load of 141 kPa for 30 seconds to prepare an absorbent body. Furthermore, a hydrophilic air-through nonwoven fabric (Rengo Nonwoven Products) (basis weight: 21 g / m) of the same size as the absorbent body was placed on the upper surface of the absorbent body. 2 ) was placed on the absorbent article. The original top sheet and absorbent body were removed from a diaper manufactured by Daio Paper Corporation (product name: Goon Plus Sensitive Skin Design, tape type, L size, purchased in 2022) to obtain a back sheet having a pair of side gathers bonded to both ends in the short direction. A hydrophilic air-through nonwoven fabric and an absorbent body were placed on the back sheet so that their centers overlapped, to obtain an absorbent article for evaluation. In the absorbent article, the basis weight of the water-absorbent resin particles was 169 g / m 2 The weight of the crushed pulp (hydrophilic fiber) is 258 g / m 2 It was.

[0137] (Absorbent body and absorbent article having a sheet-like structure) Spunlace nonwoven fabric (Zhejiang Wangjin Nonwoven Fabric Co., Ltd., basis weight: 35 g / m 2 ) was cut into two pieces measuring 14 cm x 42 cm, which were designated as spunlace nonwoven fabrics 1 and 2, respectively.

[0138] Using a hot melt coater (Harries Co., Ltd., pump: Marshal 150, table: XA-DT, tank set temperature: 150°C, hose internal set temperature: 165°C, gun head set temperature: 170°C), a total amount of 0.2 g of hot melt adhesive (Henkel Japan Co., Ltd., ME-765E) was applied to the spunlace nonwoven fabric 1 in 14 straight lines spaced 10 mm apart along the longitudinal direction of the spunlace nonwoven fabric. The adhesive application pattern was a spiral stripe. During application, the four outer periphery sides (1 cm wide) of the spunlace nonwoven fabric 1 were masked to prevent the adhesive from being applied.

[0139] 6.5 g of water-absorbent resin particles were uniformly spread over an area of ​​12 cm x 40 cm in the center of the surface of the spunlace nonwoven fabric 1 to which the hot melt adhesive had been applied, to obtain a water-absorbent layer A.

[0140] A total of 0.2 g of hot melt adhesive was applied in 14 straight lines spaced 10 mm apart to the spunlace nonwoven fabric 2. The adhesive was applied in a spiral stripe pattern. 13.5 g of water-absorbent resin particles were uniformly scattered over the central portion (a 12 cm × 40 cm area) of the adhesive-coated surface of the spunlace nonwoven fabric 2 to obtain a water-absorbing layer B.

[0141] A 12 cm x 40 cm air-through nonwoven fabric (Guangzhou Jinhan Nonwoven Fabric Co., Ltd., basis weight: 45 g / m) was placed on the surface of the spunlace nonwoven fabric 1 on which the water-absorbent resin particles had been sprayed. 2 ), spunlace nonwoven fabric 1 and spunlace nonwoven fabric 2 were aligned at both ends so that the adhesive-coated surfaces were on the inside, sandwiched between release papers, and pressed together using a laminating machine (Hashima Corporation, Straight Linear Fussing Press, model HP-600LFS) at 110°C and 0.1 MPa, and the release paper was peeled off to obtain an absorbent body. The area in which the water-absorbent resin particles were dispersed in the absorbent body was 12 cm x 40 cm.

[0142] Furthermore, a 12 cm x 40 cm hydrophilic air-through nonwoven fabric (Rengo Nonwoven Products) (basis weight: 21 g / m) was placed on the top surface of the absorbent body so that the center of each was overlapped. 2 ) was placed on the backsheet. The original top sheet and absorbent body were removed from a diaper manufactured by P&G Japan LLC (product name: Pampers Sarasara Care, tape type, L size, purchased in 2022) to obtain a backsheet having a pair of side gathers bonded to both ends in the short direction. A hydrophilic air-through nonwoven fabric and an absorbent body were placed on the backsheet so that their centers overlapped, to obtain an absorbent article for evaluation. In the absorbent article, the basis weight of the water-absorbent resin particles was 340 g / m 2 The absorbent article had a hydrophilic air-through nonwoven fabric, a water-absorbing layer A, an air-through nonwoven fabric, and a water-absorbing layer B arranged in this order.

[0143] [Measurement of absorption time and return amount] The measurement of the return and absorption time of the liquid of the absorbent article was carried out in accordance with the GB standard in China, "GB / T 28004.1-2021 NATIONAL STANDARD OF THE PEOPLE'S REPUBLIC OF CHINA, Disposable diapers - Part 1: Disposable diapers for baby". The specific method is shown below.

[0144] A U-shaped sample base (Figure A.1 of the GB Standard), a sample stand (Figure A.2 of the GB Standard), a test liquid injection module (Figure A.3 of the GB Standard), and a pressure module (Figure A.4 of the GB Standard) were prepared. A test liquid was obtained by mixing 90.0 g of sodium chloride, 9910 g of deionized water, and Food Blue No. 1 (for coloring), and the temperature of this test liquid was adjusted to 36±1°C in a thermostatic bath. The above-mentioned absorbent article was placed on the U-shaped sample base and sample stand, which were placed on a horizontal table. The sizes of each tool were as follows:

[0145] U-shaped sample base: B2 L 136 mm, W 135 mm Sample stand: T2 Test liquid injection module: M2 W 85 mm, L 108 mm Pressure module: Y2 W 85 mm, L 108 mm

[0146] Using a liquid feed pump and the test liquid injection module shown in Figure A.3 of the GB standard, 80 mL of test liquid was injected at a rate of 480 mL / min. The first absorption time (seconds) was measured from the time the test liquid was injected until the liquid level at the lowest point in the absorption area disappeared. The test liquid injection module was adjusted so that a pressure of 2.00 kPa was applied to the sample surface, and pressure was continued for one minute after the liquid was injected. After one minute, the test liquid injection module was removed. If the absorption time was longer than one minute, the first absorption test was terminated by waiting until absorption was complete and then removing the test liquid injection module.

[0147] Eight minutes after the first test solution was added, the second absorption time (seconds) was measured using the same procedure as the first absorption time. The sum of the first and second absorption times was taken as the total absorption time (seconds). 15 minutes after the first test solution was added, 10 sheets of 11.0 cm x 7.0 cm filter paper were stacked and placed so that the center of the filter paper overlapped the center of the sample surface. 16 minutes after the first test solution was added, the pressure module was pressed against the filter paper, applying a pressure of 4.00 kPa. After 1 minute of pressure application, the pressure module and filter paper were removed, and the weight of the filter paper was measured using a balance. The difference in weight of the filter paper before and after the test was calculated as the mass of the test solution absorbed by the filter paper, and this was taken as the return amount (g).

[0148]

[0149] REFERENCE SIGNS LIST 1 burette part 3 clamp 4 measuring part 5 conduit 10a water-absorbent resin particles 11 stand 13 measuring table 13a through-hole 15 nylon mesh sheet 21 burette tube 22 cock 23 rubber stopper 24 cock 25 air introduction tube 31 cylinder 32 polyamide mesh 33 weight 50 saline solution

Claims

1. Water-absorbent resin particles having a 15-minute water absorption value of physiological saline solution under a load of 0.69 kPa of 30 mL / g or more and 60 mL / g or less.

2. The water-absorbent resin particles according to claim 1, wherein the water-absorbent resin particles have a no-pressure DW value of 3 minutes of 20 mL / g or more and 50 mL / g or less.

3. The water-absorbent resin particles according to claim 2, wherein the water-absorbent resin particles have a water-absorption rate for physiological saline of 60 seconds or less.

4. The water-absorbent resin particles according to claim 2, wherein the water-absorbent resin particles have a saline water-holding capacity of 30 g / g or more.

5. An absorbent body comprising the water-absorbent resin particles according to any one of claims 1 to 4.

6. A water-absorbent article comprising the absorbent body according to claim 5.

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