Water-absorbent resin particles, absorbent body, and absorbent article

Abstract

Provided are water-absorbent resin particles that, when applied to an absorbent body, reduce the amount of liquid remaining between gel particles without being absorbed by the water-absorbent resin particles (excess liquid).　Water-absorbent resin particles having a median particle diameter of 200 μm or more and an aspect ratio of 0.8 to 1.0.

Description

Superabsorbent resin particles, absorbents, and absorbent articles

[0001] The present invention relates to water-absorbent resin particles, absorbents, and absorbent articles, and more specifically, to water-absorbent resin particles constituting absorbents suitably used in sanitary materials such as disposable diapers, sanitary napkins, and incontinence pads, absorbents using said water-absorbent resin particles, and absorbent articles.

[0002] Superabsorbent polymer particles have recently been widely used in the field of sanitary materials such as disposable diapers, sanitary napkins, and incontinence pads.

[0003] Such water-absorbing resin particles include crosslinked polymers of water-soluble ethylenically unsaturated monomers, and more specifically, crosslinked polymers of partially neutralized polyacrylic acid. These are considered preferable water-absorbing resin particles because they possess excellent water absorption capabilities, and because acrylic acid, their raw material, is readily available industrially, they can be manufactured with consistent quality and at low cost, and they have numerous advantages such as being resistant to spoilage and deterioration (see, for example, Patent Document 1).

[0004] Absorbent products such as disposable diapers, sanitary napkins, and incontinence pads are primarily composed of an absorbent core located in the center that absorbs and retains bodily fluids such as urine and menstrual blood excreted from the body, a liquid-permeable top sheet located on the side that comes into contact with the body, and a liquid-impermeable 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 Publication No. 3-227301, International Publication No. 2020 / 184393

[0006] In such absorbent materials, when the water-absorbent resin particles contained in the absorbent material absorb liquid, they swell and form gel particles. Within the absorbent material, the water-absorbent resin particles absorb liquid, but the gel particles formed by the absorption of liquid hinder the movement of liquid within the absorbent material, resulting in the problem of liquid remaining between the gel particles. When an absorbent material is applied to the body, the liquid remaining between the gel particles in the absorbent material (excess liquid such as excess water) can cause skin irritation. Therefore, it is necessary to reduce the amount of excess liquid in the absorbent material.

[0007] For example, Patent Document 2 discloses a technology that reduces excess water and suppresses stuffiness and skin irritation by using water-absorbent resin particles with a gel association degree of 15 or higher as an absorbent. The technology in Patent Document 2 utilizes aggregates of water-absorbent resin particles (gel association degree of 15 or higher).

[0008] The primary objective of this invention is to provide superabsorbent resin particles in which, when applied to an absorbent material, the amount of liquid remaining between gel particles that is not absorbed by the superabsorbent resin particles (excess liquid) is reduced. Furthermore, the invention also aims to provide an absorbent material and an absorbent article utilizing these superabsorbent resin particles.

[0009] The inventors diligently studied to solve the above problems. As a result, they found that by setting the median particle size of the water-absorbent resin particles to a predetermined value or higher, and controlling the aspect ratio within a predetermined range, the amount of liquid remaining between the gel particles (excess liquid) that is not absorbed by the water-absorbent resin particles is reduced when the water-absorbent resin particles are applied to an absorbent material. The present invention was completed based on this finding and further diligent studies.

[0010] In other words, the present invention provides an invention having the following configuration: water-absorbing resin particles having a median particle diameter of 200 μm or more and an aspect ratio of 0.8 or more and 1.0 or less.

[0011] According to the present invention, when the water-absorbent resin particles are applied to an absorbent material, the amount of liquid remaining between the gel particles without being absorbed (excess liquid) is reduced. Furthermore, the present invention can provide an absorbent material and an absorbent article utilizing these water-absorbent resin particles.

[0012] This is a schematic diagram of a device for measuring water absorption under a 4.14 kPa load.

[0013] In this specification, “composing” includes “consisting essentially of” and “consisting of.” In this specification, “(meth)acrylic” means “acrylic or methacrylic,” “(meth)acrylate” means “acrylate or methacrylate,” and “(poly)” means with or without the “poly” prefix. In this specification, “water-soluble” means solubility of 5% by mass or more in water at 25°C.

[0014] In this specification, numbers enclosed in "~" represent a numerical range that includes the numbers before and after "~" as the lower and upper limits, respectively. If multiple lower and upper limits are listed separately, any lower and upper limits may be selected and enclosed in "~".

[0015] 1. Superabsorbent Resin Particles The superabsorbent resin particles of the present invention have a median particle diameter of 200 μm or more and an aspect ratio of 0.8 or more and 1.0 or less. Because the superabsorbent resin particles of the present invention possess these characteristics, when applied to an absorbent material, the amount of liquid remaining between the gel particles without being absorbed by the superabsorbent resin particles (excess liquid) is reduced. The superabsorbent resin particles of the present invention will be described in detail below.

[0016] In the present invention, the median particle size of the water-absorbing resin particles is 200 μm or more, and from the viewpoint of more favorably exhibiting the effects of the present invention, it is preferably 250 μm or more, more preferably 300 μm or more, and also preferably 600 μm or less, more preferably 500 μm or less, and even more preferably 450 μm or less, with a preferred range being 250 to 600 μm.

[0017] In this invention, the median particle size of the water-absorbent resin particles is a value measured using a continuous fully automatic ultrasonic vibration sieving analyzer. The particle size distribution of 5 g of water-absorbent resin particles is measured using sieves of JIS standards of 710 μm, 600 μm, 500 μm, 425 μm, 300 μm, 250 μm, 150 μm, and 75 μm, as well as a receiving tray. The particle size distribution is calculated by accumulating the particles on the sieve in descending order of particle size, plotting the relationship between the sieve opening and the accumulated mass percentage of the particles remaining on the sieve on logarithmic probability paper, and connecting the plots on the probability paper with a straight line. The particle size corresponding to an accumulated mass percentage of 50% is defined as the median particle size. In this method of measuring the median particle size of water-absorbent resin particles, for example, if primary particles of water-absorbent resin particles aggregate to form secondary particles, the median particle size is measured based on the particle size of the secondary particles. The specific measurement method is as described in the examples.

[0018] Furthermore, in the present invention, the aspect ratio of the water-absorbing resin particles is 0.8 or more and 1.0 or less, and from the viewpoint of more favorably exhibiting the effects of the present invention, it is preferably 0.90 or more, more preferably 0.95 or more, and even more preferably 0.97 or more. The preferred upper limit of the aspect ratio is, for example, 1.0.

[0019] In this invention, the aspect ratio of the water-absorbent resin particles is determined by passing the water-absorbent resin particles through a JIS Z 8801-1 standard sieve with a mesh size of 36 (mesh opening of 425 μm) to adjust the particle size to be held on a standard sieve with a mesh size of 50 (mesh opening of 300 μm), then photographing each of the 50 particles taken from this sample using a scanning electron microscope (SEM), and measuring the value for each of the 50 photographed particles. At this time, the longest length in the longitudinal direction of each particle is defined as the major axis, and the longest length perpendicular to the major axis is defined as the minor axis. The ratio of the minor axis to the major axis (minor axis / major axis) is calculated, and the average value of these particles is calculated to obtain the aspect ratio. In this method of measuring the aspect ratio of water-absorbent resin particles, for example, if primary water-absorbent resin particles aggregate to form secondary particles, the aspect ratio of the secondary particles is measured. The specific measurement method is as described in the examples.

[0020] Furthermore, when primary water-absorbing resin particles aggregate to form secondary particles, it is technically impossible for the aspect ratio of the secondary particles to be close to a perfect sphere, such as between 0.8 and 1.0. Therefore, it is preferable that the water-absorbing resin particles of the present invention substantially do not contain aggregates of water-absorbing resin particles. The fact that the aspect ratio of the water-absorbing resin particles of the present invention is in the range of 0.8 to 1.0 also means that the water-absorbing resin particles of the present invention substantially do not contain aggregates of water-absorbing resin particles.

[0021] The superabsorbent resin particles of the present invention have a large median particle diameter of 200 μm or more, and a shape close to a perfect sphere with an aspect ratio of 0.8 to 1.0. When superabsorbent resin particles of this shape are applied to an absorbent, and the superabsorbent resin particles absorb liquid and swell to become gel particles, the area in contact between the large, close-spherical gel particles is small, and gaps through which liquid flows are easily formed. When the superabsorbent resin particles aggregate to form secondary particles, liquid tends to remain between the swollen gels. For this reason, it is presumed that the amount of liquid remaining between the gel particles that is not absorbed by the superabsorbent resin particles (excess liquid) is reduced in the superabsorbent resin particles of the present invention.

[0022] From the viewpoint of more favorably exhibiting the effects of the present invention, the amount of saline solution that the superabsorbent resin particles can hold is preferably 30 g / g or more, more preferably 38 g / g or more, even more preferably 45 g / g or more, and even more preferably 48 g / g or more. Furthermore, it is preferably 80 g / g or less, more preferably 70 g / g or less, and even more preferably 65 g / g or less. Preferred ranges include 30-80 g / g, 38-70 g / g, 45-65 g / g, and 48-65 g / g. It is presumed that a higher saline solution capacity allows the superabsorbent resin particles to absorb a larger amount of liquid, thus making it less likely for liquid to remain between the swollen gel. The measurement of the saline solution capacity of the superabsorbent resin particles is as described in the examples.

[0023] Furthermore, from the viewpoint of more favorably exhibiting the effects of the present invention, the amount of physiological saline absorbed by the water-absorbent resin particles under a 4.14 kPa load is preferably 20 g / g or more, more preferably 25 g / g or more, even more preferably 27 g / g or more, and even more preferably 30 g / g or more, and also preferably 50 g / g or less, more preferably 40 g / g or less, and even more preferably 35 g / g or less, with preferred ranges including 20 to 50 g / g, 25 to 40 g / g, 27 g / g to 35 g / g, and 30 to 35 g / g. It is presumed that if the amount of physiological saline absorbed under a 4.14 kPa load is high, the swollen gel particles become less likely to collapse, so gaps are more easily formed between the gel particles, and less liquid remains between the swollen gels. The measurement of the amount of physiological saline absorbed by the water-absorbent resin particles under a 4.14 kPa load is as described in the examples.

[0024] Furthermore, from the viewpoint of more favorably exhibiting the effects of the present invention, the amount of saline solution absorbed by the superabsorbent resin particles is preferably 45 g / g or more, more preferably 50 g / g or more, even more preferably 55 g / g or more, and even more preferably 57 g / g or more. Also, it is preferably 80 g / g or less, more preferably 70 g / g or less, and even more preferably 65 g / g or less. Preferred ranges include 45-80 g / g, 45-70 g / g, 45-65 g / g, 50-80 g / g, 50-70 g / g, 50-65 g / g, 55-80 g / g, 55-70 g / g, 55-65 g / g, 57-80 g / g, 57-70 g / g, and 57-65 g / g. It is presumed that a higher saline solution absorption rate means that a larger amount of liquid can be absorbed by the superabsorbent resin particles, making it less likely for liquid to remain between the swollen gel particles. The amount of saline solution absorbed by the superabsorbent polymer particles is measured according to the description in the examples.

[0025] Furthermore, from the viewpoint of more favorably exhibiting the effects of the present invention, the solubility of the superabsorbent resin particles in physiological saline is preferably 40% or less, more preferably 25% or less, and even more preferably 15% or less, and the preferred lower limit of the solubility in physiological saline is, for example, 0. A preferred range is 0 to 40%. If the solubility in physiological saline is low, the tackiness of the swollen gel will be low, gaps between gel particles will be easily formed, and it is presumed that liquid will not easily remain between the swollen gel. The solubility of the superabsorbent resin particles in physiological saline can be reduced by performing internal crosslinking in the manufacturing process of the superabsorbent resin particles. The solubility of the superabsorbent resin particles in physiological saline is measured by adding 2,000 g of superabsorbent resin particles to 500 g of physiological saline and stirring for 3 hours in a 25°C environment, and the specific measurement is described in the examples.

[0026] Furthermore, from the viewpoint of more favorably exhibiting the effects of the present invention, the bulk density of the water-absorbing resin particles is preferably 0.75 g / mL or more, more preferably 0.85 g / mL or more, even more preferably 0.90 g / mL or more, and also preferably 1.20 g / mL or less, more preferably 1.10 g / mL or less, even more preferably 1.05 g / mL or less, with preferred ranges including 0.75 to 1.20 g / mL, 0.85 to 1.10 g / mL, and 0.90 to 1.05 g / mL. In an absorbent using a predetermined amount of water-absorbing resin particles, if the bulk density of the water-absorbing resin particles is high, the distance between particles arranged in the absorbent becomes longer, so it is presumed that gaps are easily formed between swollen gel particles, and liquid is less likely to remain between the swollen gels. The bulk density of the water-absorbing resin particles is measured as described in the examples.

[0027] The water-absorbing resin particles of the present invention are polymers of water-soluble ethylenically unsaturated monomers. Preferably, these polymers are crosslinked, i.e., composed of crosslinked polymers having structural units derived from water-soluble ethylenically unsaturated monomers.

[0028] The water-absorbing resin particles of the present invention are in the form of particles with a shape close to a perfect sphere (primary particles), and as described above, it is preferable that they are substantially free of aggregated primary particles (secondary particles). The shape of the primary particles is approximately spherical, with an aspect ratio within a predetermined range.

[0029] 2. Method for Producing Water-Absorbing Resin Particles The method for producing water-absorbing resin particles of the present invention is not particularly limited as long as water-absorbing resin particles having a median particle diameter of 200 μm or more and an aspect ratio of 0.8 or more and 1.0 or less are obtained. The method for producing water-absorbing resin particles of the present invention comprises, for example, a step of polymerizing a water-soluble ethylenically unsaturated monomer to obtain polymer particles. If the polymer particles are to be crosslinked, the method further comprises a surface crosslinking step of applying surface crosslinking to the polymer particles.

[0030] In order to ensure that the median particle size of the water-absorbing resin particles of the present invention is 200 μm or larger, and that the aspect ratio is in the range of 0.8 to 1.0, it is desirable to manufacture the water-absorbing resin particles in a way that prevents aggregation. The method for manufacturing the water-absorbing resin particles of the present invention will be described in detail below.

[0031] <Polymerization Process> The polymerization process is a process of polymerizing water-soluble ethylenically unsaturated monomers to obtain polymer particles. Typical methods for polymerizing water-soluble ethylenically unsaturated monomers include aqueous solution polymerization, spray droplet polymerization, emulsion polymerization, and reversed-phase suspension polymerization. In aqueous solution polymerization, polymerization is carried out by heating an aqueous solution of water-soluble ethylenically unsaturated monomers while stirring as needed. In reversed-phase suspension polymerization, polymerization is carried out by heating water-soluble ethylenically unsaturated monomers in a hydrocarbon dispersion medium while stirring. Among these, reversed-phase suspension polymerization is preferred from the viewpoint of improving general water absorption performance (such as the amount of water retained by physiological saline and the amount of water absorbed by physiological saline) while setting the median particle size of the water-absorbing resin particles to 200 μm or more and the aspect ratio to a range of 0.8 to 1.0.

[0032] In performing inverse 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. At this time, before starting the polymerization reaction, the addition timing of the dispersion stabilizer (surfactant or polymer-based dispersant) may be either before or after the addition of the aqueous monomer solution.

[0033] Among them, from the viewpoint of easily reducing the amount of the hydrocarbon dispersion medium remaining in the obtained water-absorbing resin particles, after dispersing the aqueous monomer solution in the hydrocarbon dispersion medium in which the polymer-based dispersant is dispersed, it is preferable to further disperse a surfactant and then carry out the polymerization.

[0034] As described above, in the present invention, such inverse phase suspension polymerization is preferably carried out in one stage.

[0035] As the reaction temperature of the polymerization reaction, from the viewpoints of promoting the polymerization rapidly, shortening the polymerization time to enhance the economy, and easily removing the polymerization heat to carry out the reaction smoothly, it is preferably 20 to 110°C, and more preferably 40 to 90°C.

[0036] [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 shall apply hereinafter) and its salts; 2-(meth)acrylamido-2-methylpropanesulfonic acid and its salts; nonionic monomers such as (meth)acrylamide, N,N-dimethyl(meth)acrylamide, 2-hydroxyethyl (meth)acrylate, N-methylol(meth)acrylamide, polyethylene glycol mono(meth)acrylate; amino group-containing unsaturated monomers such as N,N-diethylaminoethyl (meth)acrylate, N,N-diethylaminopropyl (meth)acrylate, diethylaminopropyl (meth)acrylamide, and their quaternized products. Among these water-soluble ethylenically unsaturated monomers, from the viewpoint of easy industrial availability, etc., (meth)acrylic acid or its salts, (meth)acrylamide, and N,N-dimethylacrylamide are preferred, and (meth)acrylic acid and its salts are more preferred. These water-soluble ethylenically unsaturated monomers may be used alone or in combination of two or more.

[0037] Among these, acrylic acid and its salts are widely used as raw materials for water-absorbent resin particles, and in some cases, the above-mentioned other water-soluble ethylenically unsaturated monomers may be copolymerized with these acrylic acid and / or its salts and used. In this case, acrylic acid and / or its salts are preferably used at 70 to 100 mol% based on the total water-soluble ethylenically unsaturated monomers as the main water-soluble ethylenically unsaturated monomer.

[0038] The water-soluble ethylenically unsaturated monomer may be dispersed in a hydrocarbon dispersion medium in aqueous solution and subjected to reverse-phase suspension polymerization. By making the water-soluble ethylenically unsaturated monomer 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 saturation concentration or less. Furthermore, 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. On the other hand, 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.

[0039] When the water-soluble ethylenically unsaturated monomer has an acidic group, such as (meth)acrylic acid or 2-(meth)acrylamide-2-methylpropanesulfonic acid, the acidic group may be neutralized beforehand with an alkaline neutralizing agent as needed. 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 also be used in aqueous solution form to simplify the neutralization process. The above-mentioned alkaline neutralizing agents may be used individually or in combination of two or more types.

[0040] The degree of neutralization of a water-soluble ethylenically unsaturated monomer by an alkaline neutralizing agent is preferably 40 to 100 mol%, more preferably 50 to 90 mol%, even more preferably 60 to 85 mol%, and even more preferably 70 to 80 mol% as the degree of neutralization with respect to all acid groups of the water-soluble ethylenically unsaturated monomer.

[0041] [Radical polymerization initiators] Examples of radical polymerization initiators added to the polymerization process 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-butyl peroxyisobutyrate, t-butyl peroxypivalate, and hydrogen peroxide; and 2,2'-azobis(2-amidinopropane) dihydrochloride, 2,2'-azobis[2-(N-Fe]. Examples of azo compounds include 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 viewpoint of being readily available and easy to handle. These radical polymerization initiators may be used alone or in combination of two or more. Furthermore, the radical polymerization initiator can also be used as a redox polymerization initiator in combination with reducing agents such as sodium sulfite, sodium bisulfite, ferrous sulfate, and L-ascorbic acid.

[0042] For example, the amount of radical polymerization initiator used is 0.00005 to 0.01 moles per mole of water-soluble ethylenically unsaturated monomer. By using such an amount, it is possible to avoid rapid polymerization reactions and complete the polymerization reaction within an appropriate time.

[0043] [Internal Crosslinking Agent] Examples of internal crosslinking agents include those that can crosslink the polymer of the water-soluble ethylenically unsaturated monomer used, such as (poly)ethylene glycol ["(poly)" means whether or not the prefix "poly" is present.] [The same applies hereafter], unsaturated polyesters obtained by reacting polyols 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; allyl starch, allyl cellulose, diallyl phthalate, N,N',N''-triallyl isocyanurate, divinyl Examples include compounds having two or more polymerizable unsaturated groups such as benzene; diglycidyl compounds such as (poly)ethylene glycol diglycidyl ether, (poly)propylene glycol diglycidyl ether, and (poly)glycerin diglycidyl ether; polyglycidyl compounds such as triglycidyl compounds; epihalohydrin compounds such as epichlorohydrin, epibromuhydrin, and α-methylepichlorohydrin; compounds having two or more reactive functional groups such as isocyanate compounds such as 2,4-tolylene diisocyanate and hexamethylene diisocyanate; and oxetane compounds 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 crosslinking agents, polyglycidyl compounds are preferred, diglycidyl ether compounds are more preferred, and (poly)ethylene glycol diglycidyl ether, (poly)propylene glycol diglycidyl ether, and (poly)glycerin diglycidyl ether are particularly preferred.These internal crosslinking agents may be used individually or in combination of two or more types.

[0044] The amount of internal crosslinking agent used in the monomer aqueous solution is preferably 0.000001 to 0.005 moles, more preferably 0.00001 to 0.002 moles, even more preferably 0.00001 to 0.001 moles, and still more preferably 0.00005 to 0.0005 moles per mole of water-soluble ethylenically unsaturated monomer.

[0045] [Hydrogen Dispersion Medium] Examples of hydrocarbon dispersion mediums 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 mediums, n-hexane, n-heptane, and cyclohexane are particularly suitable because they are readily available industrially, have stable quality, and are inexpensive. These hydrocarbon dispersion mediums may be used individually or in combination of two or more types. Furthermore, suitable results can also be obtained by using commercially available hydrocarbon dispersion media such as exolheptane (manufactured by ExxonMobil: containing 75-85% by mass of heptane and its isomers).

[0046] 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 water-soluble ethylenically unsaturated monomers, from the viewpoint of uniformly dispersing the water-soluble ethylenically unsaturated monomers and facilitating control of the polymerization temperature. Reverse-phase suspension polymerization is preferably carried out in one stage. If carried out in two or more stages, aggregates of polymer particles are formed, and the aspect ratio of the water-absorbing resin particles falls outside the range of 0.8 to 1.0.

[0047] [Dispersion Stabilizers] (Surfactants) In reverse-phase suspension polymerization, dispersion stabilizers can be used to improve the dispersion stability of water-soluble ethylenically unsaturated monomers in hydrocarbon dispersion media. Surfactants can be used as such dispersion stabilizers.

[0048] 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, alkylallylformaldehyde condensed polyoxyethylene ethers, polyoxyethylene polyoxypropylene block copolymers, polyoxyethylene polyoxypropyl alkyl ethers, polyethylene glycol fatty acid esters, alkyl glucosides, N-alkylgluconamides, polyoxyethylene fatty acid amides, polyoxyethylene alkylamines, phosphate esters of polyoxyethylene alkyl ethers, and phosphate esters of polyoxyethylene alkylallyl ethers. Among these surfactants, sorbitan fatty acid esters, polyglycerin fatty acid esters, and sucrose fatty acid esters are particularly preferred in terms of monomer dispersion stability. These surfactants may be used individually or in combination of two or more types.

[0049] The amount of surfactant used is preferably 0.1 to 30 parts by mass, and more preferably 0.3 to 20 parts by mass, per 100 parts by mass of the water-soluble ethylenically unsaturated monomer.

[0050] (Polymer-based dispersants) In addition, polymer-based dispersants may be used together with the surfactants mentioned above as dispersion stabilizers in reverse-phase suspension polymerization.

[0051] 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, ethylcellulose, and ethyl hydroxyethylcellulose. 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 monomer dispersion stability. These polymeric dispersants may be used individually or in combination of two or more types.

[0052] The amount of polymeric dispersant used is preferably 0.1 to 30 parts by mass, and more preferably 0.3 to 20 parts by mass, per 100 parts by mass of water-soluble ethylenically unsaturated monomer.

[0053] [Other Components] In the method for producing water-absorbent polymer particles, other components may be added to an aqueous solution containing a water-soluble ethylenically unsaturated monomer to carry out reverse-phase suspension polymerization, if desired. Other components may include various additives such as thickeners and chain transfer agents.

[0054] For example, reverse-phase suspension polymerization can be carried out by adding a thickening agent to an aqueous solution containing a water-soluble ethylenically unsaturated monomer. By adjusting the viscosity of the aqueous solution by adding a thickening agent in this way, it is possible to control the intermediate particle size obtained in reverse-phase suspension polymerization.

[0055] Examples of thickeners that can be used include hydroxyethylcellulose, hydroxypropylcellulose, methylcellulose, carboxymethylcellulose, polyacrylic acid, partially neutralized polyacrylic acid, polyethylene glycol, polyacrylamide, polyethyleneimine, dextrin, sodium alginate, polyvinyl alcohol, polyvinylpyrrolidone, and polyethylene oxide. It should be noted that, assuming the same stirring speed during polymerization, the higher the viscosity of the aqueous solution of the water-soluble ethylenically unsaturated monomer, the larger the primary particles of the resulting particles tend to be.

[0056] <Dehydration Step> After the reverse-phase suspension polymerization described above, a dehydration step may be included in which water, hydrocarbon dispersion medium, etc. are removed by distillation by applying energy such as heat from an external source. When dehydrating from a water-containing gel-like substance after reverse-phase suspension polymerization, the system in which the water-containing gel-like substance is dispersed in the hydrocarbon dispersion medium is heated, and the water and hydrocarbon dispersion medium are temporarily removed from the system by azeotropic distillation. At this time, if only the removed hydrocarbon dispersion medium is returned to the system, continuous azeotropic distillation becomes possible. In this case, the temperature inside the system during drying is maintained below the azeotropic temperature with the hydrocarbon dispersion medium, which is preferable from the viewpoint of preventing resin degradation. By controlling the processing conditions of this dehydration step after polymerization and adjusting the amount of dehydration (i.e., adjusting the water content of the polymer particles), it is possible to control the various properties of the resulting water-absorbing resin particles.

[0057] In the dehydration process, dehydration by distillation may be carried out under atmospheric pressure. When dehydration is carried out under atmospheric 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.

[0058] <Surface Crosslinking Process> The surface crosslinking process is a process in which surface crosslinking is applied to the polymer particles obtained in the polymerization process. When the polymer particles are crosslinked polymer particles (water-containing gel-like material), this process involves adding a surface crosslinking agent to the water-containing gel-like material having an internal crosslinking structure obtained by polymerizing water-soluble ethylenically unsaturated monomers to crosslink it (surface crosslinking reaction). It is preferable to carry out this surface crosslinking reaction in the presence of the surface crosslinking agent after the polymerization of the water-soluble ethylenically unsaturated monomers. In this way, by applying a surface crosslinking reaction to the water-containing gel-like material having an internal crosslinking structure after polymerization, the crosslinking density near the surface of the water-absorbing resin particles can be set to a specific range, thereby obtaining water-absorbing resin particles with enhanced properties such as water absorption capacity under load.

[0059] Examples of surface crosslinking agents 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; halo-epoxy compounds such as epichlorohydrin, epibromhydrin, and α-methylepichlorohydrin; isocyanate compounds such as 2,4-tolylene diisocyanate and hexamethylene diisocyanate; and 3-methyl-3-oxetane methanol and 3-ethyl-3-oxetane. Oxetane compounds such as methanol, 3-butyl-3-oxetane methanol, 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, and 4-ethyl Examples of surface crosslinking agents include carbonate compounds (e.g., alkylene carbonates) such as -1,3-dioxolan-2-one, 4-hydroxymethyl-1,3-dioxolan-2-one, 1,3-dioxan-2-one, 4-methyl-1,3-dioxan-2-one, 4,6-dimethyl-1,3-dioxan-2-one, and 1,3-dioxolan-2-one; and hydroxyalkylamide compounds such as bis[N,N-di(β-hydroxyethyl)]adipamide. Among these surface crosslinking 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 crosslinking agents may be used individually or in combination of two or more types.

[0060] The amount of surface crosslinking agent used is preferably 0.00001 to 0.01 moles, more preferably 0.00005 to 0.005 moles, more preferably 0.0001 to 0.001 moles, and even more preferably 0.0004 to 0.0009 moles, per mole of the total amount of water-soluble ethylenically unsaturated monomers used in polymerization.

[0061] Regarding the method of adding the surface crosslinking agent, it may be added as is, as an aqueous solution, or, if necessary, as a solution using a hydrophilic organic solvent. Examples of hydrophilic organic solvents include lower alcohols such as methyl alcohol, ethyl alcohol, n-propyl alcohol, and isopropyl alcohol; ketones such as acetone and methyl ethyl ketone; ethers such as diethyl ether, dioxane, and tetrahydrofuran; amides such as N,N-dimethylformamide; and sulfoxides such as dimethyl sulfoxide. These hydrophilic organic solvents may be used individually, in combination of two or more, or as a mixed solvent with water.

[0062] The surface crosslinking agent can be added after the polymerization reaction of the water-soluble ethylenically unsaturated monomer has been almost completely finished. It is preferable to add the surface crosslinking agent in the presence of 1 to 400 parts by mass of water per 100 parts by mass of the water-soluble ethylenically unsaturated monomer used in polymerization, more preferably in the presence of 5 to 200 parts by mass of water, even more preferably in the presence of 10 to 100 parts by mass of water, and still more preferably in the presence of 15 to 60 parts by mass of water. The amount of water refers to the total amount of water contained in the reaction system and the water used as needed when adding the surface crosslinking agent.

[0063] The reaction temperature for the surface crosslinking reaction is preferably 50 to 250°C, more preferably 60 to 180°C, even more preferably 60 to 140°C, and even more preferably 70 to 120°C. The reaction time for the surface crosslinking reaction is preferably 1 to 300 minutes, and more preferably 5 to 200 minutes.

[0064] <Drying Process> After the surface crosslinking described above, a drying process may be included in which water, hydrocarbon dispersion medium, etc. are removed by distillation by applying energy such as heat from an external source. By drying the polymer particles after surface crosslinking and removing water and hydrocarbon dispersion medium by distillation, water-absorbing resin particles are obtained.

[0065] In the drying process, the drying treatment by distillation may be carried out under atmospheric pressure or under reduced pressure. Furthermore, from the viewpoint of improving drying efficiency, it may be carried out under a stream of gas such as nitrogen. When the drying treatment is carried out under atmospheric 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. When the drying treatment is carried out under reduced pressure, the drying temperature is preferably 40 to 160°C, and more preferably 50 to 110°C.

[0066] Furthermore, if monomer polymerization is performed by reverse-phase suspension polymerization followed by a surface crosslinking step using a surface crosslinking agent, the drying step by distillation described above should be performed after the completion of the surface crosslinking step. Alternatively, the surface crosslinking step and the drying step may be performed simultaneously.

[0067] The water-absorbing resin particles of the present invention may contain additives depending on the purpose. Examples of such additives include inorganic powders, surfactants, oxidizing agents, reducing agents, metal chelating agents, radical chain inhibitors, antioxidants, and antibacterial agents. For example, the fluidity of the water-absorbing resin particles can be further improved by adding 0.05 to 5 parts by mass of amorphous silica as inorganic powder to 100 parts by mass of water-absorbing resin particles. It is preferable that the additives are hydrophilic or water-soluble.

[0068] 3. Absorbent material, absorbent article The water-absorbing resin particles of the present invention constitute an absorbent material used in sanitary materials such as sanitary napkins and disposable diapers, and are suitably used in absorbent articles containing the absorbent material.

[0069] The absorbent material of the present invention contains the water-absorbent resin particles of the present invention. The absorbent material may further contain hydrophilic fibers. Examples of the structure of the absorbent material include a sheet-like structure in which water-absorbent resin particles are fixed on or between multiple nonwoven fabrics, a mixed dispersion obtained by mixing water-absorbent resin particles and hydrophilic fibers to 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 material may also contain other components, such as adhesive binders such as heat-fusible synthetic fibers, hot-melt adhesives, and adhesive emulsions to enhance the shape retention of the absorbent material.

[0070] The basis weight of the water-absorbing resin particles in the absorbent material of the present invention is 30 g / m². 2 More than 500g / m 2 The following applies. The basis weight is preferably 100 g / m². 2 More comfortably, 120 g / m² 2 More preferably 140 g / m² 2 The above applies, and preferably 300 g / m². 2 More preferably, 250 g / m² 2 More preferably, 200 g / m 2 The following applies:

[0071] Examples of hydrophilic fibers include 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, mechanical pulp, chemical pulp, and semi-chemical pulp obtained from wood, artificial cellulose fibers such as rayon and acetate, and fibers made from synthetic resins such as hydrophilized polyamide, polyester, and polyolefin. The average fiber length of hydrophilic fibers is usually 0.1 to 10 mm, or may be 0.5 to 5 mm.

[0072] The basis weight of the hydrophilic fibers in the absorber of the present invention is 0 g / m 2 or more and 800 g / m 2 or less. The basis weight is preferably 50 g / m 2 or more, more preferably 100 g / m 2 or more, even more preferably 120 g / m 2 or more, still more preferably 140 g / m 2 or more, and preferably is also at most 700 g / m 2 or less, more preferably 600 g / m 2 or less, even more preferably 500 g / m 2 or less.

[0073] As the content of the water-absorbing resin particles in the absorber, it is preferably 5 to 100% by mass, more preferably 10 to 95% by mass, even more preferably 20 to 90% by mass, and still more preferably 30 to 80% by mass.

[0074] By holding the absorber using the water-absorbing 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 absorbent article of the present invention can be obtained. The liquid-permeable sheet is disposed on the side in contact with the body, and the liquid-impermeable sheet is disposed on the opposite side not in contact with the body.

[0075] Examples of the liquid-permeable sheet include nonwoven fabrics such as air-through type, spunbond type, chemical bond type, needle punch type, etc. made of fibers such as polyethylene, polypropylene, polyester, and porous synthetic resin sheets. Examples of the liquid-impermeable sheet 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 bond nonwoven fabric, air-through nonwoven fabric, spunbond nonwoven fabric, and spunbond / meltblown / spunbond nonwoven fabric.

[0076] The basis weight of the liquid-permeable sheet is preferably 5 g / m 2 or more and 100 g / m 2 or less, and preferably 10 g / m2 60g / m or more 2 The following is more preferable. Furthermore, the liquid-permeable sheet may be embossed or perforated on its surface to improve the diffusion of the liquid. The embossing or perforation can be carried out by known methods.

[0077] Examples of liquid-impermeable sheets include sheets made from synthetic resins such as polyethylene, polypropylene, and polyvinyl chloride; sheets made from nonwoven fabrics such as spunbond / meltblown / spunbond (SMS) nonwoven fabrics, which consist of a water-resistant meltblown nonwoven fabric sandwiched between high-strength spunbond nonwoven fabrics; and sheets made from composite materials of these synthetic resins and nonwoven fabrics (e.g., spunbond nonwoven fabrics, spunlace nonwoven fabrics). Liquid-impermeable sheets can also be made from synthetic resins primarily composed of low-density polyethylene (LDPE) resin. Liquid-impermeable sheets, for example, have a basis weight of 10 to 50 g / m². 2 It may be a sheet made of synthetic resin.

[0078] 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 side of the laminate opposite to the liquid-permeable sheet.

[0079] 4. Additional Notes This specification includes at least the inventions shown in (1) to (10) below: (1) Superabsorbent resin particles having a median particle diameter of 200 μm or more and an aspect ratio of 0.8 or more and 1.0 or less. (2) Superabsorbent resin particles according to (1) above, having an aspect ratio of 0.85 or more and 1.00 or less, 0.90 or more and 1.00 or less, 0.95 or more and 1.00 or less, or 0.97 or more and 1.00 or less. (3) Superabsorbent resin particles according to (1) or (2) above, having a median particle diameter of 250 μm or more and 600 μm or less, 300 μm or more and 500 μm or less, or 350 μm or more and 450 μm or less. (4) Absorbent resin particles according to any one of (1) to (3), wherein the amount of saline solution retained is 30 g / g or more and 80 g / g or less, 38 g / g or more and 70 g / g or less, 45 g / g or more and 65 g / g or less, or 48 g / g or more and 65 g / g or less. (5) Absorbent resin particles according to any one of (1) to (4), wherein the amount of saline solution absorbed under a 4.14 kPa load is 20 g / g or more and 50 g / g or less, 25 g / g or more and 40 g / g or less, 27 g / g or more and 40 g / g or less, or 30 g / g or more and 35 g / g or less. (6) Absorbent resin particles according to any one of (1) to (5), wherein the amount of saline solution absorbed is 50 g / g or more and 80 g / g or less, 55 g / g or more and 70 g / g or less, or 57 g / g or more and 65 g / g or less. (7) Superabsorbent polymer particles according to any one of (1) to (6), wherein the solubility in physiological saline is 40% or less, 25% or less, or 15% or less. (8) Superabsorbent polymer particles according to any one of (1) to (7), wherein the bulk density is 0.75 g / mL or more and 1.20 g / mL or less, 0.85 g / mL or more and 1.10 g / mL or less, or 0.90 g / L or more and 1.05 g / mL or less. (9) An absorbent body containing the superabsorbent polymer particles according to any one of (1) to (8) above. (10) An absorbent article containing the absorbent body according to (9) above.

[0080] The present invention will be described in detail below with reference to examples and comparative examples. However, the present invention is not limited to the examples. Unless otherwise specified, measurements were performed in an environment with a temperature of 25 ± 2°C and a humidity of 50 ± 10%.

[0081] <Production of superabsorbent polymer particles> (Example 1) A reflux condenser, a dropping funnel, a nitrogen gas inlet tube, and a round-bottom cylindrical separable flask with an inner diameter of 110 mm and a capacity of 2 L were prepared. The flask was equipped with a stirring blade having a blade diameter of 58 mm, a maximum height of 104 mm, and a grid-like blade with a total of four slits of 65 mm in the vertical direction. 293 g of n-heptane was taken into this flask as a hydrocarbon dispersion medium, and 0.736 g of maleic anhydride-modified ethylene-propylene copolymer (Mitsui Chemicals, Inc., High Wax 1105A) was added as a polymeric dispersant. The temperature was raised to 80°C while stirring to dissolve the dispersant, and then cooled to 60°C.

[0082] Meanwhile, 92.0 g (1.03 mol) of an 80.5% by mass acrylic acid aqueous solution was taken into a 300 mL beaker and, while cooling with ice water, 147.5 g of a 20.9% by mass sodium hydroxide aqueous solution was added dropwise to neutralize to 75 mol%. Then, 0.0276 g (0.102 mmol) of potassium persulfate as a water-soluble radical polymerization agent, 0.092 g (0.339 mmol) of 2,2'-azobis(2-aziminopropane) dihydrochloride, and 0.0046 g (0.0264 mmol) of ethylene glycol diglycidyl ether as an internal crosslinking agent were added and dissolved to prepare a water-soluble ethylene unsaturated monomer aqueous solution.

[0083] The prepared monomer aqueous solution was added to the reaction mixture in the separable flask, and the mixture was stirred for 10 minutes at a stirrer speed of 176 rpm. Next, a surfactant solution prepared by heating and dissolving 0.736 g of sucrose stearate (Mitsubishi Chemical Foods Corporation, Ryoto Sugar Ester S-370) in 6.62 g of n-heptane was added to the reaction mixture, and the system was thoroughly purged with nitrogen while stirring at a stirrer speed of 129 rpm. After that, the flask was immersed in a 70°C water bath and the temperature was raised, and polymerization was carried out for 60 minutes to obtain a hydrated gel polymer.

[0084] Subsequently, the flask was immersed in an oil bath set at 125°C, and 102.9 g of water was removed from the system by azeotropic distillation of n-heptane and water while refluxing the n-heptane. Then, 4.60 g (0.528 mmol) of a 2% by mass aqueous solution of ethylene glycol diglycidyl ether was added to the flask, and the mixture was held at 83°C for 2 hours.

[0085] Subsequently, the n-heptane was evaporated at 125°C and dried, and then passed through a sieve with an opening of 850 μm to obtain 88.9 g of water-absorbing resin particles.

[0086] (Example 2) The same procedure as in Example 1 was performed, except that the amount of water removed from the system by azeotropic distillation was changed to 99.3 g, and 93.2 g of water-absorbing resin particles were obtained.

[0087] (Example 3) The same procedure as in Example 1 was performed, except that the rotation speed of the stirrer after adding sucrose stearate was changed to 210 rpm and the amount of water removed from the system by azeotropic distillation was changed to 108.4 g, and 95.7 g of water-absorbing resin particles were obtained.

[0088] (Comparative Example 1) A reflux condenser, a dropping funnel, a nitrogen gas inlet tube, and a round-bottom cylindrical separable flask with an inner diameter of 110 mm and a capacity of 2 L, equipped with a stirring blade having four inclined paddle blades with a blade diameter of 50 mm in two stages, were prepared. 252 g of n-heptane was taken into this flask as a hydrocarbon dispersion medium, and 0.736 g of maleic anhydride-modified ethylene-propylene copolymer was added as a polymeric dispersant. The mixture was heated to 80°C while stirring to dissolve the dispersant, and then cooled to 60°C.

[0089] Meanwhile, 92.0 g (1.03 mol) of an 80.5% by mass acrylic acid aqueous solution as an ethylenically unsaturated monomer was taken into a 300 ml beaker, and while cooling from the outside, 147.5 g of a 20.9% by mass sodium hydroxide aqueous solution was added dropwise to neutralize to 75 mol%, and then 0.0920 g of hydroxyethylcellulose as a thickener, 0.0920 g (0.339 mmol) of 2,2'-azobis(2-amidinopropane) dihydrochloride as an azo compound, 0.0276 g (0.102 mmol) of potassium persulfate as a peroxide, and 0.00460 g (0.0264 mmol) of ethylene glycol diglycidyl ether as an internal crosslinking agent were added and dissolved to prepare the first stage monomer aqueous solution.

[0090] The prepared first-stage monomer aqueous solution was added to the reaction mixture in the separable flask, and the mixture was stirred for 10 minutes at a stirrer speed of 300 rpm. Next, a surfactant solution prepared by heating and dissolving 0.736 g of sucrose stearate as a surfactant in 6.62 g of n-heptane was added to the reaction mixture, and the system was thoroughly purged with nitrogen while stirring at a stirrer speed of 500 rpm. After that, the flask was immersed in a 70°C water bath and the temperature was raised, and polymerization was carried out for 60 minutes to obtain the first-stage polymerization slurry.

[0091] Next, 128.8 g (1.44 mol) of an 80.5% by mass acrylic acid aqueous solution was taken into another 500 ml beaker as an ethylenically unsaturated monomer, and while cooling from the outside, 143.89 g of a 30% by mass sodium hydroxide aqueous solution was added dropwise to neutralize it to 75 mol%. Then, 0.129 g (0.475 mmol) of 2,2'-azobis(2-amidinopropane) dihydrochloride as an azo compound, 0.0386 g (0.143 mmol) of potassium persulfate as a peroxide, and 0.0116 g (0.0665 mmol) of ethylene glycol diglycidyl ether and 11.2 g of deionized water were added and dissolved to prepare the second monomer aqueous solution.

[0092] The separable flask system was cooled to 25°C while stirring at a stirrer speed of 1000 rpm. Next, the entire volume of the second stage monomer aqueous solution was added to the first stage polymerization slurry in the separable flask, and the system was purged with nitrogen for 30 minutes. After that, the flask was again immersed in a 70°C water bath to raise the temperature, and the polymerization reaction was carried out for 60 minutes to obtain a hydrated gel polymer.

[0093] Subsequently, the flask was immersed in an oil bath set at 125°C, and 227.2 g of water was removed from the system by azeotropic distillation of n-heptane and water while refluxing the n-heptane. Then, 4.42 g (0.507 mmol) of a 2% by mass aqueous solution of ethylene glycol diglycidyl ether was added to the flask, and the mixture was held at 83°C for 2 hours.

[0094] Subsequently, the n-heptane was evaporated at 125°C and dried, and then passed through a sieve with an opening of 850 μm to obtain 225.1 g of water-absorbing resin particles.

[0095] (Comparative Example 2) The same procedure as in Example 1 was performed, except that the rotation speed of the stirrer after adding sucrose stearate was changed to 225 rpm and the amount of water removed from the system by azeotropic distillation was changed to 125.7 g, and 95.7 g of water-absorbing resin particles were obtained.

[0096] For each example and comparative example, the water-absorbing polymer particles obtained were measured using the following methods: saline solution water retention, median particle size, aspect ratio, saline solution water absorption, dissolved matter, bulk density, water absorption under a 4.14 kPa load, and excess water volume. Unless otherwise specified, measurements were performed at 25°C ± 2°C and 50 ± 10% humidity. The results are shown in Table 1.

[0097] <Water Retention Capacity of Physiological Saline Solution> The water retention capacity of the superabsorbent resin particles in physiological saline solution (at room temperature, 25±2℃) was measured using the following procedure. First, a cotton bag (membrane no. 60, 100mm wide x 200mm long) containing 2.0g of superabsorbent resin particles was placed in a 500mL beaker. 500g of physiological saline solution was poured into the cotton bag containing the superabsorbent resin particles in one go, taking care not to cause spillage. The top of the cotton bag was then tied with a rubber band and left to stand for 30 minutes to allow the superabsorbent resin particles to swell. After 30 minutes, the cotton bag was dewatered for 1 minute using a dehydrator (manufactured by Kokusan Co., Ltd., model number: H-122) set to a centrifugal force of 167G. The mass Wa [g] of the cotton bag containing the swollen gel after dewatering was measured. The same procedure was performed without adding superabsorbent resin particles, and the empty mass Wb [g] of the cotton bag when wet was measured. The water retention capacity of the superabsorbent resin particles in physiological saline solution was calculated using the following formula. Water retention amount [g / g] = (Wa-Wb) / 2.0

[0098] <Median Particle Size> The median particle size of the superabsorbent polymer particles was measured under room temperature (25±2℃) and humidity of 50±10% using the following procedure. A continuous fully automatic ultrasonic vibration sieving analyzer (Robot Shifter RPS-205, manufactured by Seishin Corporation) was used to measure the particle size distribution of 5g of superabsorbent polymer particles using JIS standard sieves of 710μm, 600μm, 500μm, 425μm, 300μm, 250μm, 150μm, and 75μm, as well as a receiving tray. The relationship between the sieve opening and the cumulative mass percentage of the particles remaining on the sieve was plotted on logarithmic probability paper by accumulating the particles on the sieve in descending order of particle size. By connecting the plots on the probability paper with a straight line, the particle size corresponding to a cumulative mass percentage of 50% was obtained as the median particle size.

[0099] <Aspect Ratio> The superabsorbent polymer particles used to measure the aspect ratio were passed through a JIS Z 8801-1 standard sieve with a 36 mesh (425 μm opening) to adjust the particle size so that it could be held on a standard sieve with a 50 mesh (300 μm opening). Next, each of the 50 particles in this sample was photographed individually using a scanning electron microscope (SEM), and the values ​​were measured for each of the 50 photographed particles. The longest length in the longitudinal direction of each particle was defined as the major axis, and the longest length perpendicular to the major axis was defined as the minor axis. The ratio of the minor axis to the major axis (minor axis / major axis) was calculated, and the average value of these particles was calculated to obtain the aspect ratio. In this method of measuring the aspect ratio of superabsorbent polymer particles, if the primary particles of the superabsorbent polymer particles aggregated to form secondary particles, the aspect ratio of the secondary particles was measured.

[0100] <Amount of water absorbed by physiological saline> 500 g of physiological saline was weighed into a 500 mL beaker. Next, 2.0 g of superabsorbent resin particles were dispersed in the physiological saline while stirring at 600 rpm using a magnetic stirrer bar (8 mm diameter x 30 mm length, without ring) to prevent the formation of lumps. The mixture was left to stand for 60 minutes while stirring to allow the particles to swell sufficiently, thereby obtaining a dispersion containing swollen gel. Subsequently, the mass Wc [g] of a 75 μm mesh standard sieve was measured, and the dispersion was passed through this standard sieve. Then, excess water was removed by leaving the sieve tilted at an angle of approximately 30 degrees to the horizontal for 30 minutes. The mass Wd [g] of the sieve containing the swollen gel was measured, and the amount of water absorbed by physiological saline [g / g] was calculated using the following formula: Amount of water absorbed by physiological saline [g / g] = (Wd - Wc) / 2.0

[0101] <Dissolved Components> 500 g of physiological saline in a 500 mL beaker was stirred with a stirring bar (cylindrical, 8 mm diameter x 30 mm length, without a ring) rotating at 600 rpm. The temperature of the physiological saline was 25°C. 2,000 g of superabsorbent polymer particles were added, and the dispersion containing the polymer particles was stirred for 3 hours. The dispersion was filtered through a 75 μm mesh standard sieve, and the filtrate was collected. 80 g of the obtained filtrate was weighed into a 100 mL beaker that had been pre-weighed at 140°C. The filtrate in the beaker was heated in a forced-air dryer (ADVANTEC, FV-320) at 140°C for 15 hours to remove water, and the mass We (g) of the remaining solid component was measured. A blank test was performed using the same procedure as above, without adding the polymer particles to the physiological saline, and the mass Wf (g) of the remaining solid component in the beaker was measured. The dissolved components were calculated according to the following formula. Dissolved content [%] = (((We-Wf) / 80) x 500 / 2.000) x 100

[0102] <Bulk Density> The bulk density of the water-absorbent resin particles was measured using the "bulk density measuring device" described in JIS-K-6720-2. Approximately 12 mL of water-absorbent resin particles were placed in the funnel portion of the device, with the bottom sealed by a damper. A 10 mL receiving container (20 mm inner diameter, cylindrical) was placed 38 mm below the damper. The damper of the device was then quickly removed, and the water-absorbent resin particles fell into the receiving container. After scraping off the water-absorbent resin particles that had risen from the receiving container with a flat plate, the mass Wg of the container and particles together was measured. Separately, the mass Wh of the empty receiving container was measured, and the mass of the water-absorbent resin particles (Wg minus Wg) was divided by the volume V mL of the receiving container (receiving container V = 10 mL) to obtain the value S (g / mL). S (g / mL) = [Wg - Wh] (g) / V (mL) S was measured a total of three times in the same manner as above, and the average value was taken as the bulk density.

[0103] <Water Absorption under 4.14 kPa Load> The amount of water absorbed by superabsorbent polymer particles in physiological saline under a load of 4.14 kPa (water absorption under 4.14 kPa load) was measured using the apparatus schematically shown in Figure 1. The water absorption under load was measured twice for one type of superabsorbent polymer particle, and the average value of the measured values ​​was calculated. The apparatus in Figure 2 comprises a burette section 1, a clamp 3, a conduit 5, a stand 11, a measuring table 13, and a measuring unit 4 placed on the measuring table 13. The burette section 1 has a burette tube 21 with markings, a rubber stopper 23 that seals the opening at the top of the burette tube 21, a cock 22 connected to the lower end of the burette tube 21, and an air inlet tube 25 and a cock 24 connected to the lower part of the burette tube 21. The burette section 1 is fixed by the clamp 3. The flat measuring platform 13 has a through hole 13a with a diameter of 2 mm formed in its center and is supported by a height-adjustable frame 11. The through hole 13a of the measuring platform 13 and the cock 22 of the burette section 1 are connected by a conduit 5. The inner diameter of the conduit 5 is 6 mm.

[0104] The measuring unit 4 comprises an acrylic resin cylinder 31, a polyamide mesh 32 bonded to one opening of the cylinder 31, and a weight 33 that is movable vertically within the cylinder 31. The cylinder 31 is placed on the measuring stand 13 via the polyamide mesh 32. The inner diameter of the cylinder 31 is 20 mm. The mesh opening of the polyamide mesh 32 is 75 μm (200 mesh). The weight 33 has a diameter of 19 mm and a mass of 119.6 g, and can apply a load of 4.14 kPa to water-absorbing resin particles 10a uniformly arranged on the polyamide mesh 32, as described later.

[0105] First, the stopcocks 22 and 24 of the burette section 1 were closed, and 0.9 mass% physiological 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 tightly with the rubber stopper 23, and then the stopcocks 22 and 24 were opened. The inside of the conduit 5 was filled with 0.9 mass% saline solution 50, taking care not to let air bubbles in. The height of the measuring platform 13 was adjusted so that the water level of the 0.9 mass% saline solution that reached the through-hole 13a was the same as the height of the top surface of the measuring platform 13. After the adjustment, the water level of the 0.9 mass% saline solution 50 inside the burette tube 21 was read on the scale of the burette tube 21, and that position was set as the zero point (reading at 0 seconds).

[0106] In the measurement unit 4, 0.10 g of superabsorbent resin particles 10a were uniformly arranged on the polyamide mesh 32 inside the cylinder 31, a weight 33 was placed on the superabsorbent resin particles 10a, and the cylinder 31 was positioned so that its center coincided with the conduit opening in the center of the measurement stage 13. The amount of physiological saline in the burette tube 21 decreased 60 minutes after the superabsorbent resin particles 10a began absorbing physiological saline from the conduit 5 (i.e., the amount of physiological saline absorbed by the superabsorbent resin particles 10a) Wi (mL) was read, and the amount of physiological saline absorbed by the superabsorbent resin particles 10a under a 4.14 kPa load was calculated using the following formula. The results are shown in Table 1. Amount of physiological saline absorbed under a 4.14 kPa load (g / g) = Wi (mL) × 1.0028 [g / mL] (specific gravity of physiological saline) / 0.10

[0107] <Measurement of excess water volume> (Preparation of evaluation items) 0.24 g (5 cm x 5 cm) of crushed pulp was layered on a 5 cm x 5 cm tissue, and 1.06 g of superabsorbent resin particles were uniformly scattered on top of it. Furthermore, 0.24 g (5 cm x 5 cm) of pulp was layered on top of the scattered superabsorbent resin particles, and a tissue (5 cm x 5 cm) was placed on top of that to create a laminate. A 10 cm x 12 cm polypropylene SMMS nonwoven fabric (basis weight 12.5 gsm) was folded to a size of 10 cm x 6 cm, and then the above laminate was wrapped in the nonwoven fabric. The three unsealed sides of the nonwoven fabric wrapping the laminate were pressed together with a heat sealer (Fuji Impulse FI-450-5 model) to seal the laminate and create an evaluation item. In this embodiment, the basis weight (weight per unit area) of the water-absorbing resin particles was increased to about twice the amount used in the evaluation described in Patent Document 2, and the evaluation was performed under harsher conditions that tend to increase excess water.

[0108] (Mass measurement of evaluation items containing excess water) A wire mesh (27 cm x 46 cm, mesh size: 20 mm x 20 mm, wire diameter 3 mm) and 10 L of physiological saline solution were placed in a tray (28.5 cm x 48.0 cm, depth 12.0 cm), and the liquid temperature was adjusted to 25.0 ± 0.2 °C. Next, the evaluation items were placed on the wire mesh and immersed in the physiological saline solution for 30 minutes. After that, the wire mesh with the evaluation items was lifted horizontally, the wire mesh was placed on the edge of the tray, and the water was drained by letting it stand for 5 minutes, after which the mass Wj (g) of the evaluation items was measured.

[0109] (Mass measurement of the evaluation item after removing excess water) The evaluation item, whose Wj was measured, was placed back on the wire mesh, and a weight (10 cm x 6 cm, 2.07 kPa) was placed on top of the evaluation item and left to stand for 5 minutes. Next, the base of the separator funnel (55Z-144-4, manufactured by Kiriyama Seisakusho Co., Ltd.) was attached to the Kiriyama funnel suction bell (VKU-500, manufactured by Kiriyama Seisakusho Co., Ltd.). After adjusting the pressure of the decompression pump (ULVAC MDA-015) to 0.08 kPa, the evaluation item was placed on the base of the separator funnel, and after suction for 2 minutes, the mass Wk (g) of the evaluation item was measured.

[0110] (Calculation of excess water volume) The excess water volume contained in the evaluation sample was calculated using the following formula: Excess water volume (g) = Wj - Wk

[0111]

[0112] 1. Burette section 3. Clamp 4. Measuring section 5. Conduit 10a. Absorbent resin particles 11. Stand 13. Measuring platform 13a. Through hole 21. Burette tube 22. Cock 23. Rubber stopper 24. Cock 25. Air inlet tube 31. Cylinder 32. Polyamide mesh 33. Weight 50. Saltwater

Claims

1. Superabsorbent polymer particles having a median particle size of 200 μm or more and an aspect ratio of 0.8 or more and 1.0 or less.

2. The water-absorbing resin particles according to claim 1, wherein the water retention capacity of physiological saline solution is 30 g / g or more.

3. The superabsorbent resin particles according to claim 2, wherein the amount of physiological saline absorbed under a 4.14 kPa load is 20 g / g or more.

4. An absorbent comprising water-absorbing resin particles according to any one of claims 1 to 3.

5. An absorbent article comprising the absorbent material described in claim 4.

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