Water-absorbent resin composition, method for producing water-absorbent resin composition, and method for slowing the water absorption rate of water-absorbent resin particles

Abstract

Provided is a water-absorbent resin composition which slows the water absorption rate without increasing the particle diameter.　A water-absorbent resin composition which contains water-absorbent resin particles having a median particle diameter of 200-600μm and an acidic compound having a median particle diameter of 20-600μm.

Description

[Technical Field]

[0001] The present invention relates to a water-absorbent resin composition, a method for producing a water-absorbent resin composition, and a method for slowing the water absorption rate of water-absorbent resin particles, and more particularly to a water-absorbent resin composition constituting an absorbent suitable for use in hygiene materials such as disposable diapers, sanitary napkins, and incontinence pads, a method for producing a water-absorbent resin composition, and a method for slowing the water absorption rate of water-absorbent resin particles. [Background technology]

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

[0003] As such a water-absorbent resin, a cross-linked polymer of a partially neutralized salt of acrylic acid has excellent water-absorbing ability, and since its raw material, acrylic acid, is easily available industrially, it can be produced at low cost with consistent quality, and is less susceptible to decay or deterioration, and therefore has many other advantages, and is therefore considered to be a preferred water-absorbent resin (see, for example, Patent Document 1).

[0004] On the other hand, absorbent articles such as disposable diapers, sanitary napkins, and incontinence pads are mainly composed of an absorbent core located in the center that absorbs and retains 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 a water-absorbent resin. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 3-227301 Summary of the Invention [Problem to be solved by the invention]

[0006] In such absorbent articles, the water-absorbent resin particles contained in the absorbent body are required to have a high water absorption rate, but if the water absorption rate is too high, when liquid is introduced into the absorbent body, the water-absorbent resin particles will quickly absorb the liquid at the introduced point, making it difficult for the liquid to spread throughout the entire absorbent body, and a wide area of ​​the absorbent body may not be effectively utilized. In such cases, the liquid introduced into the absorbent body multiple times will accumulate around the water-absorbent resin particles around the introduction point, leading to local saturation, and the unabsorbed liquid will cause a backflow phenomenon (i.e., a phenomenon in which liquid returns from the absorbent body, and the absorbent body feels wet when touched with a hand).

[0007] As a method for diffusing the liquid throughout the absorbent body and suppressing the backflow of the absorbent body, there is a method for using water-absorbent resin particles with a slower water absorption rate. Another method for slowing the water absorption rate is to increase the particle size of the water-absorbent resin particles and reduce the surface area of ​​the water-absorbent resin particles. However, when the particle size of the water-absorbent resin particles is increased, problems arise such as a worsening of the feel of the absorbent body due to the increased roughness caused by the large particles.

[0008] Under these circumstances, a main object of the present invention is to provide a water-absorbing resin composition that has a slower water absorption rate without increasing the particle size. [Means for solving the problem]

[0009] The present inventors have conducted extensive research to solve the above-mentioned problems. As a result, they have found that a water-absorbent resin composition containing water-absorbent resin particles having a median particle diameter of 200 to 600 μm and an acidic compound having a median particle diameter of 20 to 600 μm has a slow water absorption rate, even though the particles have a particle diameter of 600 μm or less, which is not large. The present invention has been completed based on this finding and through further extensive research.

[0010] That is, the present invention provides the following configuration. Item 1. A water-absorbent resin composition comprising water-absorbent resin particles having a median particle diameter of 200 to 600 μm and an acidic compound having a median particle diameter of 20 to 600 μm. Item 2. The water-absorbent resin composition according to Item 1, wherein the difference (BA) between the water-absorption rate A of the water-absorbent resin particles and the water-absorption rate B of the water-absorbent resin composition is 1 second or more. Item 3. The water-absorbing resin composition according to Item 1 or 2, wherein the water-absorbing rate B of the water-absorbing resin composition is 4 to 130 seconds. Item 4. The water-absorbing resin composition according to any one of Items 1 to 3, wherein the acidic compound has a first acid dissociation constant of 0.1 to 5.0. Item 5. An absorbent comprising water-absorbent resin particles having a median particle diameter of 200 to 600 μm and an acidic compound having a median particle diameter of 20 to 600 μm. Item 6. An absorbent article comprising the absorbent body according to Item 5. Item 7. A method for producing a water-absorbent resin composition, comprising a step of mixing water-absorbent resin particles having a median particle diameter of 200 to 600 μm with an acidic compound having a median particle diameter of 20 to 600 μm. Item 8. The method for producing a water-absorbing resin composition according to Item 7, wherein the temperature in the mixing step is 0 to 90°C and the relative humidity is 30 to 75%. Item 9. The method for producing a water-absorbing resin composition according to Item 7 or 8, wherein the amount of the acidic compound is 0.05 to 30 parts by mass relative to 100 parts by mass of the water-absorbing resin particles. Item 10. The method for producing a water absorbent resin composition according to any one of Items 7 to 9, wherein a ratio (T / S) of a median particle diameter T (μm) of the water absorbent resin particles to a median particle diameter S (μm) of the acidic compound is 0.1 to 30. Item 11. A method for slowing the water absorption rate of water-absorbent resin particles, comprising a step of mixing an acidic compound having a median particle diameter of 20 to 600 μm with water-absorbent resin particles having a median particle diameter of 200 to 600 μm. [Effects of the Invention]

[0011] According to the present invention, it is possible to provide a water-absorbent resin composition having a slower water absorption rate without increasing the particle size. Furthermore, according to the present invention, it is also possible to provide a method for producing the water-absorbent resin composition and a method for slowing the water absorption rate of water-absorbent resin particles. DETAILED DESCRIPTION OF THE INVENTION

[0012] 1.Water absorbent resin composition The water-absorbent resin composition of the present invention is characterized by containing water-absorbent resin particles having a median particle diameter of 200 to 600 μm and an acidic compound having a median particle diameter of 20 to 600 μm. The water-absorbent resin composition of the present invention having such characteristics has a slow water absorption rate, despite the particle diameter being not large, at 600 μm or less. The mechanism behind this can be considered as follows.

[0013] The water absorption behavior of water-absorbent resin particles is caused by the osmotic pressure generated between the water-absorbent resin particles (e.g., containing partially neutralized sodium polyacrylate) with a high ion concentration and the liquid to be absorbed with a relatively low ion concentration. Therefore, as the ion concentration in the liquid to be absorbed increases, the osmotic pressure decreases, the water absorption ability weakens, and the water absorption properties, such as the amount and rate of water absorption, decrease. That is, in a water-absorbent resin composition in which an acidic compound is present in the vicinity of water-absorbent resin particles, the acidic compound in the vicinity of the water-absorbent resin particles dissolves locally during water absorption, increasing the ion concentration in the vicinity of the surface of the water-absorbent resin particles, and suitably slowing the water absorption properties, particularly the rate of water absorption. Therefore, the rate of water absorption can be slowed without increasing the particle size of the water-absorbent resin composition. The water-absorbent resin composition of the present invention will be described in detail below.

[0014] In order to more effectively exert the effects of the present invention, the acidic compound is preferably solid at room temperature (25°C) and under normal pressure (1 atm). From the same viewpoint, the acidic compound is preferably water-soluble. In the present invention, "water-soluble" means that the compound has a solubility of 0.5% by mass or more in water under normal temperature and pressure.

[0015] From the same viewpoint, the first acid dissociation constant of the acidic compound at 25°C is preferably 0.1 to 5.0, more preferably 0.5 to 4.5, and even more preferably 1.0 to 4.0. If the acidic compound has a second acid dissociation constant, the second acid dissociation constant is preferably 2.0 to 7.0, more preferably 2.5 to 6.5, and even more preferably 3.0 to 6.0. If the acidic compound has a third acid dissociation constant, the third acid dissociation constant is preferably 3.0 to 7.0, more preferably 3.5 to 6.5, and even more preferably 4.0 to 6.0.

[0016] From the same viewpoint, the median particle diameter of the acidic compound is 20 μm or more, preferably 50 μm or more, 80 μm or more, 100 μm or more, or 120 μm or more. On the other hand, the median particle diameter of the acidic compound is 600 μm or less, preferably 500 μm or less, 400 μm or less, 300 μm or less, or 280 μm or less. That is, the median particle diameter of the acidic compound is 20 to 600 μm, preferably 50 to 500 μm, and even more preferably 100 to 300 μm. The median particle diameter of the acidic compound can be measured using a JIS standard sieve, and specifically, is a value measured by the method described in the Examples.

[0017] From the same viewpoint, the ratio (T / S) of the median particle diameter T (μm) of the water absorbent resin particles to the median particle diameter S (μm) of the acidic compound is preferably 0.1 to 30, more preferably 0.5 to 20, even more preferably 0.8 to 15, and still more preferably 1.0 to 10.

[0018] The acidic compound is preferably an organic acid, and among the organic acids, tartaric acid, citric acid, malic acid, fumaric acid, sorbic acid, maleic acid, salicylic acid, succinic acid, adipic acid, glutaric acid, glycolic acid, phthalic acid, mandelic acid, and benzoic acid are particularly preferred, and tartaric acid, citric acid, malic acid, and fumaric acid are more preferred. The acidic compound contained in the water-absorbing resin composition of the present invention may be one type or two or more types.

[0019] From the viewpoint of more suitably exhibiting the effects of the present invention, the content of the acidic compound in the water-absorbent resin composition of the present invention is preferably 0.05 parts by mass or more, more preferably 0.1 parts by mass or more, even more preferably 0.5 parts by mass or more, and even more preferably 1 part by mass, relative to 100 parts by mass of the water-absorbent resin particles. From the viewpoint of ease of industrial production and cost, the upper limit of the content of the acidic compound relative to 100 parts by mass of the water-absorbent resin is preferably 30 parts by mass or less, more preferably 20 parts by mass or less, even more preferably 15 parts by mass or less, and even more preferably 10 parts by mass or less. These lower and upper limits can be arbitrarily combined, and the content of the acidic compound relative to 100 parts by mass of the water-absorbent resin is preferably 0.05 to 30 parts by mass, more preferably 0.1 to 20 parts by mass, even more preferably 0.5 to 15 parts by mass, and even more preferably 1 to 10 parts by mass.

[0020] From the viewpoint of more suitably exhibiting the effects of the present invention, in the water-absorbent resin composition of the present invention, the difference (BA) between the water-absorption rate A (seconds) of the water-absorbent resin particles and the water-absorption rate B (seconds) of the water-absorbent resin composition is preferably 1 second or more, more preferably 2 seconds or more, even more preferably 3 seconds or more, still more preferably 4 seconds or more, and particularly preferably 5 seconds or more. The difference (BA) is preferably 70 seconds or less, more preferably 60 seconds or less, even more preferably 50 seconds or less, still more preferably 40 seconds or less, and particularly preferably 30 seconds or less. The range of the difference (BA) is preferably 1 to 70, more preferably 2 to 60, even more preferably 3 to 50, still more preferably 4 to 40, and particularly preferably 5 to 30.

[0021] From the same viewpoint, the water absorption rate A (seconds) of the water-absorbent resin particles is preferably 3 seconds or more, more preferably 5 seconds or more, even more preferably 10 seconds or more, still more preferably 13 seconds or more, and particularly preferably 16 seconds or more. The water absorption rate A (seconds) is preferably 60 seconds or less, more preferably 50 seconds or less, even more preferably 40 seconds or less, still more preferably 35 seconds or less, and particularly preferably 30 seconds or less. The range of the water absorption rate A (seconds) is preferably 3 to 60 seconds, more preferably 5 to 50 seconds, even more preferably 10 to 40 seconds, still more preferably 13 to 35 seconds, and particularly preferably 15 to 30 seconds.

[0022] Furthermore, from the same viewpoint, the water absorption rate B (seconds) of the water-absorbent resin composition is preferably 4 seconds or more, more preferably 7 seconds or more, even more preferably 13 seconds or more, still more preferably 17 seconds or more, and particularly preferably 21 seconds or more. The water absorption rate B (seconds) is preferably 130 seconds or less, more preferably 110 seconds or less, even more preferably 90 seconds or less, still more preferably 75 seconds or less, and particularly preferably 60 seconds or less. The range of the water absorption rate B (seconds) is preferably 4 to 130 seconds, more preferably 7 to 110 seconds, even more preferably 13 to 90 seconds, still more preferably 17 to 75 seconds, and particularly preferably 21 to 60 seconds.

[0023] The water absorption rate A (seconds) of the water-absorbent resin particle and the water absorption rate B (seconds) of the water-absorbent resin composition are measured in accordance with the method specified in JIS K7224-1996 "Test method for water absorption rate of water-absorbent resin", and specifically, are values ​​measured by the method described in the examples.

[0024] Next, the water-absorbent resin particles contained in the water-absorbent resin composition of the present invention will be described in detail.

[0025] (Water-absorbing resin particles) The water-absorbent resin particles contained in the water-absorbent resin composition of the present invention are constituted by a crosslinked polymer of a water-soluble ethylenically unsaturated monomer, i.e., a crosslinked polymer having a structural unit derived from a water-soluble ethylenically unsaturated monomer.

[0026] The water-absorbent resin is usually in a particulate form. From the viewpoint of avoiding localized absorption in an absorbent article, the median particle diameter of the water-absorbent resin particles is 200 μm or more, and preferably 250 μm or more, 280 μm or more, 300 μm or more, or 350 μm or more. From the viewpoint of providing a comfortable feel to the touch in an absorbent article, the median particle diameter of the water-absorbent resin particles is 600 μm or less, and preferably 550 μm or less, 500 μm or less, 450 μm or less, or 400 μm or less. That is, the median particle diameter is 200 to 600 μm, preferably 250 to 500 μm, more preferably 300 to 450 μm, and even more preferably 350 to 400 μm. Also, the water-absorbing resin composition of the present invention preferably has a median particle size of 200 to 600 μm, more preferably 250 to 500 μm, even more preferably 300 to 450 μm, and even more preferably 350 to 400 μm.

[0027] The water-absorbent resin particles may be in a form consisting of a single particle, or may be in a form (secondary particles) in which fine particles (primary particles) are aggregated. Examples of the shape of the primary particles include a substantially spherical shape, an irregularly crushed shape, a plate shape, etc. In the case of primary particles produced by reverse-phase suspension polymerization, examples include a substantially spherical single particle shape having a smooth surface shape such as a true sphere or an oval sphere. Primary particles having such a shape have high flowability as a powder due to the smooth surface shape, and are not easily broken even when subjected to impact because the aggregated particles are easily packed densely, resulting in water-absorbent resin particles with high particle strength.

[0028] 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.

[0029] Typical polymerization methods for water-soluble ethylenically unsaturated monomers include aqueous solution polymerization, emulsion polymerization, and reversed-phase suspension polymerization. In aqueous solution polymerization, polymerization is carried out by heating an aqueous solution of a water-soluble ethylenically unsaturated monomer while stirring as necessary. In reversed-phase suspension polymerization, polymerization is carried out by heating a water-soluble ethylenically unsaturated monomer in a hydrocarbon dispersion medium while stirring. Reversed-phase suspension polymerization is preferably used from the viewpoint of precise control of the polymerization reaction and wide range of particle size control.

[0030] An example of a method for producing water-absorbent resin particles will be described below.

[0031] A specific example of the method for producing water-absorbent resin particles is a method for producing water-absorbent resin particles by reversed-phase suspension polymerization of a water-soluble ethylenically unsaturated monomer in a hydrocarbon dispersion medium, which method comprises a step of carrying out polymerization in the presence of a radical polymerization initiator and a step of post-crosslinking the hydrogel-like material obtained by the polymerization in the presence of a post-crosslinking agent. Note that, in the method for producing water-absorbent resin particles of the present invention, an internal crosslinking agent may be added to the water-soluble ethylenically unsaturated monomer as needed to form a hydrogel-like material having an internal crosslinked structure.

[0032] <Polymerization process> [Water-soluble ethylenically unsaturated monomers] Examples of water-soluble ethylenically unsaturated monomers include (meth)acrylic acid (herein, "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 and quaternized products thereof such as N,N-diethylaminoethyl(meth)acrylate, N,N-diethylaminopropyl(meth)acrylate, and diethylaminopropyl(meth)acrylamide. Among these water-soluble ethylenically unsaturated monomers, (meth)acrylic acid or salts thereof, (meth)acrylamide, and N,N-dimethylacrylamide are preferred, and (meth)acrylic acid and salts thereof are more preferred, from the viewpoint of industrial availability. These water-soluble ethylenically unsaturated monomers may be used alone or in combination of two or more.

[0033] Among these, acrylic acid and its salts are widely used as raw materials for water-absorbent resins, and these acrylic acid and / or its salts may be copolymerized with the other water-soluble ethylenically unsaturated monomers described above. 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.

[0034] The water-soluble ethylenically unsaturated monomer is preferably 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. 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.

[0035] 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 neutralizer, if necessary. Examples of such alkaline neutralizers include alkali metal salts such as sodium hydroxide, sodium carbonate, sodium bicarbonate, potassium hydroxide, and potassium carbonate; and ammonia. These alkaline neutralizers may be used in the form of an aqueous solution to simplify the neutralization process. The alkaline neutralizers described above may be used alone or in combination of two or more.

[0036] The degree of neutralization of the water-soluble ethylenically unsaturated monomer with the alkaline neutralizing agent is preferably 10 to 100 mol%, more preferably 30 to 90 mol%, even more preferably 40 to 85 mol%, and even more preferably 50 to 80 mol%, in terms of the degree of neutralization of all acid groups possessed by the water-soluble ethylenically unsaturated monomer.

[0037] [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-butyl peroxyisobutyrate, t-butyl peroxypivalate, and hydrogen peroxide, as well as 2,2'-azobis(2-amidinopropane) dihydrochloride and 2,2'-azobis[2-(N-phenyl)propane]. Examples of suitable radical polymerization initiators 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 due to their ease of availability and ease of 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.

[0038] 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.

[0039] [Internal crosslinking agent] The internal crosslinking agent can crosslink the polymer of the water-soluble ethylenically unsaturated monomer used, for example, (poly)ethylene glycol ("(poly)" refers to the agent with or without 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, 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, and divinylbenzene. Examples of the compound include compounds having two or more polymerizable unsaturated groups; diglycidyl compounds such as (poly)ethylene glycol diglycidyl ether, (poly)propylene glycol diglycidyl ether, and (poly)glycerin diglycidyl ether, and polyglycidyl compounds such as triglycidyl compounds; epihalohydrin compounds such as epichlorohydrin, epibromohydrin, 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-oxetanemethanol, 3-ethyl-3-oxetanemethanol, 3-butyl-3-oxetanemethanol, 3-methyl-3-oxetaneethanol, 3-ethyl-3-oxetaneethanol, and 3-butyl-3-oxetaneethanol. Among these internal cross-linking agents, it is preferable to use unsaturated polyesters or polyglycidyl compounds, it is more preferable to use diglycidyl ether compounds, 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.

[0040] The amount of the internal crosslinking agent used is preferably 0.000001 to 0.02 mol, more preferably 0.00001 to 0.01 mol, even more preferably 0.00001 to 0.005 mol, and even more preferably 0.00005 to 0.002 mol, relative to 1 mol of the water-soluble ethylenically unsaturated monomer.

[0041] [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 due to their industrial availability, stable quality, and low cost. These hydrocarbon dispersion media may be used alone or in combination of two or more. Suitable results can also be obtained using a mixture of hydrocarbon dispersion media, such as commercially available Exxol Heptane (manufactured by ExxonMobil Corporation; containing 75 to 85% by mass of heptane and its isomeric hydrocarbons).

[0042] The amount of hydrocarbon dispersion medium used is preferably 100 to 1500 parts by mass, 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 a single stage or a first stage in a multistage polymerization (the same applies hereinafter).

[0043] [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. As the dispersion stabilizer, a surfactant can be used.

[0044] 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, alkylaryl 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, polyoxyethylene alkyl ether phosphate esters, and polyoxyethylene alkyl allyl ether phosphate esters. Among these surfactants, sorbitan fatty acid esters, polyglycerin fatty acid esters, and sucrose fatty acid esters are particularly preferred in terms of dispersion stability of the monomer. These surfactants may be used alone or in combination of two or more.

[0045] 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.

[0046] (polymer dispersant) As a dispersion stabilizer used in the reversed phase suspension polymerization, a polymeric dispersant may be used in combination with the surfactant described above.

[0047] 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.

[0048] The amount of 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.

[0049] [Other ingredients] 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 other components, various additives such as a thickener and a chain transfer agent can be added.

[0050] 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.

[0051] Examples of thickeners that can be used 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.

[0052] [Reverse Phase Suspension Polymerization] In performing 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.

[0053] 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.

[0054] Such reversed-phase suspension polymerization can be carried out in one stage or in multiple stages of two or more stages, and is preferably carried out in two to three stages from the viewpoint of increasing productivity.

[0055] When performing reversed-phase suspension polymerization in two or more stages, after performing the first-stage reversed-phase suspension polymerization, a water-soluble ethylenically unsaturated monomer is added to and mixed with the reaction mixture obtained in the first-stage polymerization reaction, and reversed-phase suspension polymerization in the second and subsequent stages can be performed in the same manner as in the first stage. In the reversed-phase suspension polymerization in each stage from the second stage onwards, it is preferable to perform the reversed-phase suspension polymerization by adding, in addition to the water-soluble ethylenically unsaturated monomer, a radical polymerization initiator within the molar ratio of each component to the water-soluble ethylenically unsaturated monomer as described above, based on the amount of water-soluble ethylenically unsaturated monomer added during the reversed-phase suspension polymerization in each stage from the second stage onwards. Note that, in the second and subsequent polymerization stages, an internal crosslinking agent may also be added to the water-soluble ethylenically unsaturated monomer, if necessary.

[0056] 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 allow the reaction to proceed smoothly.

[0057] <Post-crosslinking process> Next, the water-absorbent resin particles of the present invention are obtained by adding a post-crosslinking agent to a hydrogel-like substance having an internal crosslinked structure obtained by polymerizing a water-soluble ethylenically unsaturated monomer to crosslink the substance (post-crosslinking reaction). This post-crosslinking reaction is preferably carried out in the presence of a post-crosslinking agent after the polymerization of the water-soluble ethylenically unsaturated monomer. In this way, by subjecting a hydrogel-like substance having an internal crosslinked structure to a post-crosslinking reaction after the polymerization, the crosslink density near the surface of the water-absorbent resin particles can be increased, and water-absorbent resin particles with improved properties such as water absorption capacity under load can be obtained.

[0058] Examples of the post-crosslinking agent include compounds having two or more reactive functional groups. For example, polyols such as ethylene glycol, propylene glycol, 1,4-butanediol, trimethylolpropane, glycerin, polyoxyethylene glycol, polyoxypropylene glycol, and polyglycerin; polyglycidyl compounds such as (poly)ethylene glycol diglycidyl ether, (poly)glycerin diglycidyl ether, (poly)glycerin triglycidyl ether, trimethylolpropane triglycidyl ether, (poly)propylene glycol polyglycidyl ether, and (poly)glycerol polyglycidyl ether; epichlorohydrin, epibromohydrin, α-methylepichlorohydrin, isocyanate compounds such as 2,4-tolylene diisocyanate and hexamethylene diisocyanate; oxetane compounds such as 3-methyl-3-oxetanemethanol, 3-ethyl-3-oxetanemethanol, 3-butyl-3-oxetanemethanol, 3-methyl-3-oxetaneethanol, 3-ethyl-3-oxetaneethanol, and 3-butyl-3-oxetaneethanol; oxazoline compounds such as 1,2-ethylenebisoxazoline; carbonate compounds such as ethylene carbonate; and hydroxyalkylamide compounds such as bis[N,N-di(β-hydroxyethyl)]adipamide. Among these post-crosslinking agents, preferred are 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, (poly)glycerol polyglycidyl ether, etc. These post-crosslinking agents may be used alone or in combination of two or more.

[0059] The amount of the post-crosslinking agent used is preferably 0.00001 to 0.01 mol, more preferably 0.00005 to 0.005 mol, and even more preferably 0.0001 to 0.002 mol, per mol of the total amount of water-soluble ethylenically unsaturated monomers used in the polymerization.

[0060] The post-crosslinking agent may be added as it is or as an aqueous solution, or may be added as a solution using a hydrophilic organic solvent as a solvent, if necessary. 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 alone, in combination with two or more, or as a mixed solvent with water.

[0061] The timing of adding the post-crosslinking agent may be after the polymerization reaction of the water-soluble ethylenically unsaturated monomer has almost completely finished, and it is preferably added in the presence of water in the range of 1 to 400 parts by mass, more preferably in the range of 5 to 200 parts by mass, even more preferably in the range of 10 to 100 parts by mass, and even more preferably in the range of 20 to 60 parts by mass, relative to 100 parts by mass of the water-soluble ethylenically unsaturated monomer. Note that the amount of water means the total amount of water contained in the reaction system and water used as needed when adding the post-crosslinking agent.

[0062] The reaction temperature in the post-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 in the post-crosslinking reaction is preferably 1 to 300 minutes, and more preferably 5 to 200 minutes.

[0063] <Drying process> After the above-mentioned reversed-phase suspension polymerization, the method may include a drying step in which water, hydrocarbon dispersion medium, etc. are removed by distillation by applying energy such as heat from the outside. When dehydrating the hydrogel after reversed-phase suspension polymerization, the system in which the hydrogel 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 distilled 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. Subsequently, water and hydrocarbon dispersion medium are distilled off to obtain water-absorbent resin particles. By controlling the processing conditions of the drying step after this polymerization to adjust the amount of dehydration, it is possible to control the various properties of the obtained water-absorbent resin particles.

[0064] In the drying step, the drying treatment by distillation may be carried out under normal pressure or under reduced pressure. Furthermore, from the viewpoint of increasing the drying efficiency, the drying treatment may be carried out under a stream of gas 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. 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.

[0065] In addition, when a post-crosslinking step using a post-crosslinking agent is carried out after the polymerization of the monomer by reversed-phase suspension polymerization, the drying step by distillation described above is carried out after the end of the post-crosslinking step. Alternatively, the post-crosslinking step and the drying step may be carried out simultaneously.

[0066] The water-absorbent resin composition of the present invention may contain additives according to the purpose in addition to the acidic compound. Examples of such additives include inorganic powders, surfactants, oxidizing agents, reducing agents, metal chelating agents, radical chain inhibitors, antioxidants, antibacterial agents, etc. For example, by adding 0.05 to 5 parts by mass of amorphous silica as inorganic powder to 100 parts by mass of water-absorbent resin particles, the fluidity of the water-absorbent resin composition can be further improved.

[0067] In the water-absorbing resin composition of the present invention, the content of water-absorbing resin particles (excluding additives) is preferably 70% by mass or more, more preferably 80% by mass or more, and even more preferably 90% by mass or more.

[0068] The water-absorbent resin composition of the present invention can be suitably produced, for example, by a method including a step of mixing water-absorbent resin particles, which are a crosslinked polymer of the above-mentioned water-soluble ethylenically unsaturated monomer, an internal crosslinking agent, and a post-crosslinking agent, with an acidic compound. The temperature in the mixing step may be 0 to 90°C. The temperature in the mixing step is preferably 15 to 70°C, and the relative humidity is preferably 30 to 75%. For example, by mixing the water-absorbent resin particles and the acidic compound in a solid phase, the acidic compound can be present on the surface of the water-absorbent resin particles to an extent that the effects of the present invention can be exhibited. Alternatively, the acidic compound can be mixed with a liquid medium such as an aqueous liquid. 2 minutes The dispersed state may be mixed with water-absorbent resin particles to prepare the water-absorbent resin composition of the present invention.

[0069] From the viewpoint of more suitably exhibiting the effects of the present invention, the amount of the acidic compound added in the method for producing a water absorbent resin composition of the present invention is preferably 0.05 to 30 parts by mass, more preferably 0.1 to 20 parts by mass, even more preferably 0.5 to 15 parts by mass, and still more preferably 1 to 10 parts by mass, relative to 100 parts by mass of the water absorbent resin particles.

[0070] From the same viewpoint, in the method for producing a water absorbent resin composition of the present invention, the ratio (T / S) of the median particle diameter T (μm) of the water absorbent resin particles to the median particle diameter S (μm) of the acidic compound is preferably 0.1 to 30, more preferably 0.5 to 20, even more preferably 0.8 to 15, and still more preferably 1.0 to 10.

[0071] The method for slowing the water absorption rate of water-absorbent resin particles of the present invention can be said to be a method of mixing, preferably adhering, an acidic compound having a median particle diameter of 20 to 600 μm to water-absorbent resin particles having a median particle diameter of 200 to 600 μm.

[0072] 2. Absorbent materials and absorbent articles The water-absorbent resin composition of the present invention constitutes an absorbent core used in hygiene materials such as sanitary products and disposable diapers, and is suitably used in absorbent articles containing the absorbent core.

[0073] Here, an absorbent using the water-absorbent resin composition of the present invention contains, as essential constituent units, water-absorbent resin particles having a median particle diameter of 200 to 600 μm and an acidic compound having a median particle diameter of 20 to 600 μm. The absorbent may further contain hydrophilic fibers. The absorbent may contain the water-absorbent resin particles and the acidic compound in a form in which the water-absorbent resin particles and the acidic compound are adjacent to each other and form respective layers; the water-absorbent resin particles and hydrophilic fibers may be mixed to form a uniform composition, with the acidic compound adjacent to the outer surface of the mixture; or the water-absorbent resin particles and the acidic compound may be sandwiched between multiple hydrophilic fiber layers. In such a form, the ratio of the water-absorbent resin particles to the acidic compound is preferably 0.05 to 30 parts by mass, more preferably 0.1 to 20 parts by mass, even more preferably 0.5 to 15 parts by mass, and even more preferably 1 to 10 parts by mass, per 100 parts by mass of the water-absorbent resin particles.

[0074] The absorbent using the water-absorbent resin composition of the present invention more preferably contains the water-absorbent resin composition of the present invention. The absorbent may further contain hydrophilic fibers. Examples of the absorbent structure include a sheet-like structure in which the water-absorbent resin composition is fixed on a nonwoven fabric or between multiple nonwoven fabrics, a mixed dispersion obtained by mixing the water-absorbent resin composition and hydrophilic fibers to form a uniform composition, a sandwich structure in which the water-absorbent resin composition is sandwiched between layered hydrophilic fibers, and a structure in which the water-absorbent resin composition 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.

[0075] The content of the water-absorbent resin composition in the absorbent 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.

[0076] Examples of hydrophilic fibers 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 fibers is usually 0.1 to 10 mm, or may be 0.5 to 5 mm.

[0077] The absorbent article of the present invention can be produced by holding an absorbent body using the water-absorbent resin composition 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 side opposite to the side that comes into contact with the body.

[0078] Examples of liquid-permeable sheets include nonwoven fabrics such as air-through, spunbond, chemical-bond, and needle-punched types 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. [Example]

[0079] The present invention will be described in detail below with reference to examples and comparative examples, but the present invention is not limited to these examples.

[0080] The water-absorbent resin compositions obtained in the following Examples and Comparative Examples were evaluated by the following various tests. Unless otherwise specified, the measurements were carried out in an environment of a temperature of 25±2°C and a humidity of 50±10%. Each evaluation test method will be described below.

[0081] <Water absorption rate> The water absorption rates of the water-absorbent resin composition and the water-absorbent resin particles were measured according to the vortex method (JIS K7224-1996) using the following procedure. First, 2,000 parts by mass of ion-exchanged water was mixed with 0.05 parts by mass of Blue No. 1 to prepare colored ion-exchanged water, which was then adjusted to 25±0.2°C in a thermostatic water bath. 50±0.01 g of the colored ion-exchanged water was weighed into a 100 mL beaker. Next, a stirring bar (8 mm diameter × 30 mm, without ring) was placed in the beaker, and a vortex was generated by stirring at 600 rpm using a magnetic stirrer. After adding 0.5±0.0002 g of the water-absorbent resin composition to the beaker, the time (seconds) until the stirring bar was covered with gelled ion-exchanged water was measured. Five measurements were performed, and the average values ​​were used as the water absorption rate B of the water-absorbent resin composition and the water absorption rate A of the water-absorbent resin particles, respectively. Table 1 also shows the difference (BA) between the water absorption rate A of the water-absorbent resin particles and the water absorption rate B of the water-absorbent resin composition.

[0082] <Median particle size of water-absorbent resin particles> JIS standard sieves were combined in the following order from top to bottom: a sieve with an 850 μm mesh size, a 600 μm mesh size, a 500 μm mesh size, a 425 μm mesh size, a 300 μm mesh size, a 250 μm mesh size, a 150 μm mesh size, and a tray. 50 g of water-absorbent resin particles were placed on the top sieve and shaken for 10 minutes using a rotary shaker to classify the particles. After classification, the mass of the water-absorbent resin particles remaining on each sieve was calculated as a mass percentage relative to the total mass, and the particle size distribution was determined. The particles on the sieves were integrated in descending order of particle diameter, and the relationship between the sieve opening size and the integrated value of the mass percentage of the water-absorbent resin particles remaining on the sieves was plotted on logarithmic probability paper. By connecting the plots on the probability paper with a straight line, the particle diameter corresponding to a cumulative mass percentage of 50 mass % was determined as the median particle diameter of the water-absorbent resin particles.

[0083] <Median particle size of acidic compounds> Ten grams of acidic compounds were sieved using a continuous, fully automated ultrasonic sieving analyzer (Robot Sifter RPS-205, Seishin Enterprise Co., Ltd.) with JIS standard sieves with mesh sizes of 850 μm, 500 μm, 425 μm, 300 μm, 212 μm, 106 μm, 75 μm, and 45 μm, and a pan, under sieving conditions of 80 Hz, 1-second pulse interval, and 2-minute classification time. The mass of the acidic compounds remaining on each sieve was calculated as a mass percentage of the total mass. The mass percentages of the acidic compounds remaining on each sieve were integrated in descending order of particle size, and the relationship between the sieve mesh size and the integrated mass percentage of the acidic compounds remaining on the sieves was plotted on logarithmic probability paper. The plots on the probability paper were connected with a straight line to determine the particle size corresponding to a cumulative mass percentage of 50% by mass, and this was taken as the median particle size of the acidic compound.

[0084] <Saline water absorption> 500 g of saline solution and a stirring bar (8 mm diameter x 30 mm, without ring) were placed in a 500 mL plastic beaker and stirred at 600 rpm using a magnetic stirrer. 2.0 g of water-absorbent resin particles were dispersed in the beaker and gently stirred at 600 rpm for 1 hour to allow for sufficient swelling. Meanwhile, the mass (Wa) of a 75 μm standard sieve was measured, and the aqueous solution containing the swollen gel was filtered through the 75 μm standard sieve. The 75 μm standard sieve was left for 30 minutes while tilted at an angle of approximately 30 degrees relative to the horizontal, and excess saline was removed from the water-absorbent resin particles. The mass (Wb) of the sieve containing the swollen gel was measured, and the water absorption was calculated by subtracting the mass (Wa) of the 75 μm standard sieve from the mass (Wb) and dividing the result by the mass (2.0 g) of the water-absorbent resin particles. Water absorption amount = (Wb-Wa) ÷ (water-absorbing resin particle mass)

[0085] <Production Example 1> A 2-L round-bottom cylindrical separable flask with an inner diameter of 11 cm and equipped with a reflux condenser, a dropping funnel, a nitrogen gas inlet, and a stirrer with a two-stage, four-paddle blade arrangement with a blade diameter of 5 cm was prepared. 293 g of n-heptane was added to the flask as a hydrocarbon dispersion medium, and 0.736 g of maleic anhydride-modified ethylene-propylene copolymer (Hiwax 1105A, Mitsui Chemicals, Inc.) was added as a polymeric dispersant. The flask was heated to 80°C with stirring to dissolve the dispersant, and then cooled to 50°C. Separately, 92.0 g (1.03 mol) of an 80.5% by mass acrylic acid aqueous solution as a water-soluble ethylenically unsaturated monomer was placed in a 300 mL beaker, and while cooling with ice water from the outside, 147.7 g of a 20.9% by mass sodium hydroxide aqueous solution was added dropwise to neutralize the mixture to 75 mol %. After that, 0.092 g of hydroxyethyl cellulose (Sumitomo Seika Chemicals Co., Ltd., HECAW-15F) as a thickener, 0.0736 g (0.272 mmol) of potassium persulfate as a water-soluble radical polymerization agent, and 0.010 g (0.057 mmol) of ethylene glycol diglycidyl ether as an internal crosslinking agent were added and dissolved to prepare the first aqueous solution. The aqueous solution prepared above was then added to a separable flask and stirred for 10 minutes. A surfactant solution prepared by heating and dissolving 0.736 g of sucrose stearate with HLB3 (Ryoto Sugar Ester S-370, Mitsubishi Chemical Foods Corporation) as a surfactant in 6.62 g of n-heptane was then added. The system was thoroughly purged with nitrogen while stirring at a stirrer speed of 550 rpm, and the flask was then immersed in a 70°C water bath to raise the temperature. Polymerization was carried out for 60 minutes, thereby obtaining a first-stage polymerization slurry. On the other hand, 128.8 g (1.43 mol) of an 80.5 mass% aqueous acrylic acid solution was placed in another 500 mL beaker as a water-soluble ethylenically unsaturated monomer, and while cooling from the outside, 159.0 g of a 27 mass% aqueous sodium hydroxide solution was added dropwise to carry out 75 mol% neutralization. Thereafter, 0.103 g (0.381 mmol) of potassium persulfate as a water-soluble radical polymerization initiator and 0.0116 g (0.067 mmol) of ethylene glycol diglycidyl ether as an internal crosslinking agent were added and dissolved to prepare a second-stage aqueous liquid.The separable flask system was cooled to 25°C while stirring at a stirrer speed of 1000 rpm. The entire second-stage aqueous solution was then added to the first-stage polymerization slurry. The system was then purged with nitrogen for 30 minutes. The flask was then again immersed in a 70°C water bath and heated. The polymerization reaction was carried out for 60 minutes to obtain a hydrogel polymer. To the hydrogel polymer after the second-stage polymerization, 0.589 g of a 45% by weight aqueous solution of pentasodium diethylenetriamine pentacetate was added with stirring. The flask was then immersed in an oil bath set at 125°C, and 257.7 g of water was extracted 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 weight aqueous solution of ethylene glycol diglycidyl ether as a surface cross-linking agent was added to the flask, and the mixture was maintained at 83°C for 2 hours. Thereafter, n-heptane was evaporated at 125°C to dry the mixture, thereby obtaining polymer particles (dried product). The polymer particles were passed through a sieve with an opening of 850µm to obtain 228.0g of water-absorbent resin particles. The water-absorbent resin particles had a median particle size of 394µm and a water absorption capacity of physiological saline of 61g / g.

[0086] Example 1 To 100 parts by mass of the water absorbent resin particles obtained in Production Example 1, 0.5 parts by mass of L-tartaric acid (manufactured by Fuso Chemical Co., Ltd., product name: purified L-tartaric acid, first acid dissociation constant pKa1=2.87, second acid dissociation constant pKa2=3.97, median particle size 280 μm) was added, and mixed for 30 minutes (conditions, revolution speed 50 rpm, rotation speed 50 rpm) using a cross rotary mixer manufactured by Meiwa Kogyo Co., Ltd. under an environment of a temperature of 25°C and a relative humidity of 50%, to obtain a water absorbent resin composition.

[0087] <Example 2> A water-absorbent resin composition was obtained in the same manner as in Example 1, except that the amount of L-tartaric acid was changed to 1.0 part by mass relative to 100 parts by mass of the water-absorbent resin particles.

[0088] Example 3 A water-absorbent resin composition was obtained in the same manner as in Example 1, except that the amount of L-tartaric acid was changed to 2.0 parts by mass relative to 100 parts by mass of the water-absorbent resin particles.

[0089] Example 4 A water-absorbent resin composition was obtained in the same manner as in Example 1, except that the amount of L-tartaric acid was changed to 3.0 parts by mass relative to 100 parts by mass of the water-absorbent resin particles.

[0090] <Example 5> A water-absorbent resin composition was obtained in the same manner as in Example 1, except that the amount of L-tartaric acid was changed to 5.0 parts by mass relative to 100 parts by mass of the water-absorbent resin particles.

[0091] Example 6 A water-absorbent resin composition was obtained in the same manner as in Example 1, except that the amount of L-tartaric acid was changed to 10.0 parts by mass relative to 100 parts by mass of the water-absorbent resin particles.

[0092] Example 7 To 100 parts by mass of the water absorbent resin particles obtained in Production Example 1, 1.0 part by mass of citric acid (manufactured by Fuso Chemical Co., Ltd., product name: fuso citrate (anhydrous), first acid dissociation constant pKa1=2.90, second acid dissociation constant pKa2=4.35, third acid dissociation constant pKa3=5.69, median particle size 236 μm) was added, and mixed for 30 minutes (conditions, revolution speed 50 rpm, rotation speed 50 rpm) using a cross rotary mixer manufactured by Meiwa Kogyo Co., Ltd. under an environment of a temperature of 25°C and a relative humidity of 50%, to obtain a water absorbent resin composition.

[0093] Example 8 To 100 parts by mass of the water absorbent resin particles obtained in Production Example 1, 1.0 part by mass of DL-malic acid (manufactured by Fuso Chemical Co., Ltd., product name: Fuso Malate, first acid dissociation constant pKa1=3.23, second acid dissociation constant pKa2=4.77, median particle size 156 μm) was added, and mixed for 30 minutes (conditions, revolution speed 50 rpm, rotation speed 50 rpm) using a cross rotary mixer manufactured by Meiwa Kogyo Co., Ltd. under an environment of a temperature of 25°C and a relative humidity of 50%, to obtain a water absorbent resin composition.

[0094] Example 9 To 100 parts by mass of the water absorbent resin particles obtained in Production Example 1, 1.0 part by mass of fumaric acid (manufactured by Fuso Chemical Co., Ltd., product name: fumaric acid, first acid dissociation constant pKa1=3.07, second acid dissociation constant pKa2=4.58, median particle size 161 μm) was added, and the mixture was mixed for 30 minutes (conditions, revolution speed 50 rpm, rotation speed 50 rpm) using a cross rotary mixer manufactured by Meiwa Kogyo Co., Ltd. under an environment of a temperature of 25°C and a relative humidity of 50%, to obtain a water absorbent resin composition.

[0095] <Comparative Example 1> The water-absorbent resin particles obtained in Production Example 1 were used as the water-absorbent resin particles of Comparative Example 1 as they were.

[0096] [Table 1]

Claims

1. The water-absorbent resin particles have a median particle diameter of 200 to 600 μm, and the acidic compound has a median particle diameter of 20 to 600 μm, The water-absorbent resin composition, wherein the acidic compound comprises at least one selected from the group consisting of tartaric acid, citric acid, malic acid, fumaric acid, sorbic acid, maleic acid, salicylic acid, succinic acid, adipic acid, glutaric acid, glycolic acid, phthalic acid, mandelic acid, and benzoic acid.

2. The water-absorbent resin composition according to claim 1, wherein the difference (B-A) between the water-absorption rate A of the water-absorbent resin particles and the water-absorption rate B of the water-absorbent resin composition, measured by the method described in <Absorption rate> below, is 1 second or more. <Water absorption rate> The water absorption rates of the water-absorbent resin composition and the water-absorbent resin particles were measured according to the vortex method (JIS K7224-1996) using the following procedure. First, 2,000 parts by mass of ion-exchanged water was mixed with 0.05 parts by mass of Blue No. 1 to prepare colored ion-exchanged water, which was then adjusted to a temperature of 25±0.2°C in a thermostatic water bath. 50±0.01 g of the colored ion-exchanged water was weighed into a 100 mL beaker. Next, a stirring bar (8 mm diameter x 30 mm, without ring) was placed in the beaker, and a vortex was generated by stirring at 600 rpm using a magnetic stirrer. 0.5±0.0002 g of the water-absorbent resin composition was added to the beaker, and the time (seconds) until the stirring bar was covered with gelled ion-exchanged water was measured. Five measurements were performed, and the average values ​​were designated as the water absorption rate B of the water-absorbent resin composition and the water absorption rate A of the water-absorbent resin particles, respectively.

3. The water-absorbent resin composition according to claim 1 or 2, wherein the water-absorbent resin composition has a water-absorption speed B of 4 to 130 seconds, as measured by the method described in <Absorption speed> below. <Water absorption rate> The water absorption rate of a water-absorbent resin composition is measured according to the vortex method (JIS K7224-1996) using the following procedure. First, 2,000 parts by mass of ion-exchanged water is mixed with 0.05 parts by mass of Blue No. 1 to prepare colored ion-exchanged water, which is then adjusted to a temperature of 25±0.2°C in a thermostatic water bath. 50±0.01 g of the colored ion-exchanged water is weighed into a 100 mL beaker. Next, a stirring bar (8 mm diameter x 30 mm, without ring) is placed in the beaker, and a vortex is generated by stirring at 600 rpm using a magnetic stirrer. 0.5±0.0002 g of the water-absorbent resin composition is added to the beaker, and the time (seconds) until the stirring bar is covered with gelled ion-exchanged water is measured. Five measurements are performed, and the average value is taken as the water absorption rate B of each water-absorbent resin composition.

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

5. An absorbent article comprising the absorbent body according to claim 4.

6. The method comprises a step of mixing water-absorbent resin particles having a median particle diameter of 200 to 600 μm with an acidic compound having a median particle diameter of 20 to 600 μm, wherein the acidic compound comprises at least one selected from the group consisting of tartaric acid, citric acid, malic acid, fumaric acid, sorbic acid, maleic acid, salicylic acid, succinic acid, adipic acid, glutaric acid, glycolic acid, phthalic acid, mandelic acid, and benzoic acid.

7. The method for producing a water-absorbing resin composition according to claim 6, wherein the temperature in the mixing step is 0 to 90°C and the relative humidity is 30 to 75%.

8. 8. The method for producing a water-absorbent resin composition according to claim 6, wherein the amount of the acidic compound is 0.05 to 30 parts by mass relative to 100 parts by mass of the water-absorbent resin particles.

9. 9. The method for producing a water absorbent resin composition according to claim 6, wherein a ratio (T / S) of a median particle diameter T (μm) of the water absorbent resin particles to a median particle diameter S (μm) of the acidic compound is 0.1 to 30.

10. The method comprises a step of mixing an acidic compound having a median particle diameter of 20 to 600 μm with water-absorbent resin particles having a median particle diameter of 200 to 600 μm, wherein the acidic compound comprises at least one selected from the group consisting of tartaric acid, citric acid, malic acid, fumaric acid, sorbic acid, maleic acid, salicylic acid, succinic acid, adipic acid, glutaric acid, glycolic acid, phthalic acid, mandelic acid, and benzoic acid.

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