Method for producing water-absorbent resin particles and water-absorbent resin particles

Abstract

Provided is a method for producing water-absorbent resin particles, which includes a step of obtaining polymer particles by performing reverse-phase suspension polymerization of a water-soluble ethylenically unsaturated monomer in a hydrocarbon dispersion medium, and in the method for producing water-absorbent resin particles, water-absorbent resin particles having a narrow particle size distribution can be obtained. A method for producing water-absorbent resin particles, which includes a step of obtaining polymer particles by performing reverse-phase suspension polymerization of a water-soluble ethylenically unsaturated monomer in a hydrocarbon dispersion medium, the reverse-phase suspension polymerization being performed in multiple stages of two or more stages, and the method for producing water-absorbent resin particles includes a step of aggregating the polymer particles in the presence of a dispersion stabilizer, and two or more kinds of (poly)glycerol fatty acid esters, which differ in precipitation temperature when a 0.46 mass% heptane solution is prepared, are used as the dispersion stabilizer.

Description

Technical Field

[0001] This invention relates to a method for manufacturing absorbent resin particles, absorbent resin particles, absorbent bodies, and absorbent articles. More specifically, it relates to a method for manufacturing absorbent resin particles that constitute absorbent bodies suitable for sanitary materials such as diapers, sanitary napkins, and incontinence pads, and to absorbent resin particles themselves. Background Technology

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

[0003] As such absorbent resin particles, cross-linked polymers of water-soluble olefinic unsaturated monomers, and more specifically, cross-linked polymers of partially neutralized polyacrylic acid, have excellent water absorption capacity. Acrylic acid, as their raw material, is readily available industrially. Therefore, they have many advantages such as stable quality, low cost, and resistance to spoilage and deterioration, and are thus considered preferred absorbent resin particles (see, for example, Patent Document 1).

[0004] Absorbent products such as diapers, sanitary napkins, and incontinence pads mainly consist of an absorbent core, a liquid-permeable surface sheet (top sheet), and a liquid-impermeable back sheet (back sheet). The absorbent core is located in the center to absorb and retain bodily fluids such as urine and menstrual blood. The surface sheet is located on the side that contacts the body, and the back sheet is located on the opposite side. Furthermore, the absorbent core is typically composed of hydrophilic fibers such as pulp and absorbent resin particles.

[0005] Existing technical documents

[0006] Patent documents

[0007] Patent Document 1: Japanese Patent Application Publication No. 3-227301 Summary of the Invention

[0008] The technical problem that the invention aims to solve

[0009] Water-absorbing resin particles can be manufactured, for example, by reverse suspension polymerization of water-soluble olefinic unsaturated monomers in a hydrocarbon dispersion medium to obtain polymer particles.

[0010] The main objective of this invention is to provide a method for manufacturing water-absorbing resin particles, comprising a step of reverse-phase suspension polymerization of a water-soluble olefinic unsaturated monomer in a hydrocarbon dispersion medium to obtain polymer particles. This method yields water-absorbing resin particles with a narrow particle size distribution. Furthermore, another objective of this invention is to provide water-absorbing resin particles with a narrow particle size distribution.

[0011] Technical solutions for solving technical problems

[0012] To solve the aforementioned problems, the inventors conducted in-depth research. The results showed that in a method for manufacturing absorbent resin particles, including a step of reverse suspension polymerization of water-soluble olefinically unsaturated monomers in a hydrocarbon dispersion medium to obtain polymer particles, performing reverse suspension polymerization in two or more stages, including a step of agglomerating polymer particles in the presence of a dispersion stabilizer, and using two or more (poly)glycerol fatty acid esters with different precipitation temperatures when preparing a 0.46% by mass heptane solution as dispersion stabilizers, absorbent resin particles with a narrow particle size distribution can be obtained. This invention was completed based on further repeated and in-depth research based on this insight.

[0013] That is, the present invention provides an invention having the following structure.

[0014] Item 1. A method for manufacturing water-absorbing resin particles, comprising the step of reverse-phase suspension polymerization of a water-soluble olefinic unsaturated monomer in a hydrocarbon dispersion medium to obtain polymer particles.

[0015] The reverse suspension polymerization is carried out in multiple stages, consisting of two or more phases.

[0016] The method for manufacturing the water-absorbing resin particles includes a step of aggregating the polymer particles in the presence of a dispersing stabilizer.

[0017] Two or more (poly)glycerol fatty acid esters with different precipitation temperatures when preparing a 0.46% by mass heptane solution are used as the dispersion stabilizer.

[0018] Item 2. The method for manufacturing water-absorbing resin particles according to Item 1, wherein,

[0019] Among two or more of the aforementioned (poly)glycerol fatty acid esters, the difference between the precipitation temperature of the (poly)glycerol fatty acid ester with the highest precipitation temperature and the precipitation temperature of the (poly)glycerol fatty acid ester with the lowest precipitation temperature is less than 15°C.

[0020] Item 3. The method for manufacturing water-absorbing resin particles according to Item 1 or 2, wherein,

[0021] At least one of the (poly)glycerol fatty acid esters is tristearate hexaglyceride.

[0022] The amount of the tristearate hexaglyceride used is in the range of 0.2 parts by mass or more and 4.0 parts by mass or less, relative to 100 parts by mass of the water-soluble olefinic unsaturated monomer used in the first stage of polymerization.

[0023] Item 4. The method for manufacturing water-absorbing resin particles according to Item 3, wherein,

[0024] The dispersant stabilizer comprises at least one of the following: the tristearate hexaglyceride, the (poly)glycerol fatty acid ester with a higher precipitation temperature than the tristearate hexaglyceride, and the (poly)glycerol fatty acid ester with a lower precipitation temperature than the tristearate hexaglyceride.

[0025] The amounts of the (poly)glycerol fatty acid esters with a precipitation temperature higher than that of the tristearate hexaglyceride and the (poly)glycerol fatty acid esters with a precipitation temperature lower than that of the tristearate hexaglyceride are respectively in the range of more than 0 parts by mass and less than 3.0 parts by mass relative to 1 part by mass of the tristearate hexaglyceride.

[0026] Item 5. The method for manufacturing water-absorbing resin particles according to Item 4, wherein,

[0027] The dispersion stabilizer comprises the tristearate hexaglyceride, and the (poly)glycerol fatty acid ester with a precipitation temperature higher than that of the tristearate hexaglyceride, and the (poly)glycerol fatty acid ester with a precipitation temperature lower than that of the tristearate hexaglyceride.

[0028] The amounts of the (poly)glycerol fatty acid esters with a precipitation temperature higher than that of the tristearate hexaglyceride and the (poly)glycerol fatty acid esters with a precipitation temperature lower than that of the tristearate hexaglyceride are respectively more than 0 parts by mass and less than 2.0 parts by mass relative to 1 part by mass of the tristearate hexaglyceride.

[0029] Item 6. A water-absorbing resin particle comprising polymer particles having water-soluble olefinically unsaturated monomers as monomer units.

[0030] The absorbent resin particles have a particle size distribution uniformity of less than 2.5 and contain (poly)glycerol fatty acid esters near their surface.

[0031] Invention Effects

[0032] According to the present invention, a method for manufacturing water-absorbing resin particles can be provided, comprising a step of performing reverse suspension polymerization of a water-soluble olefinically unsaturated monomer in a hydrocarbon dispersion medium to obtain polymer particles. In this method for manufacturing water-absorbing resin particles, water-absorbing resin particles with a narrow particle size distribution can be obtained. Furthermore, according to the present invention, water-absorbing resin particles with a narrow particle size distribution can also be provided. Detailed Implementation

[0033] In this specification, "comprising" means "consisting essentially of" and "consisting of". Additionally, in this specification, "(meth)acrylic acid" means "acrylic acid or methacrylic acid", "(meth)acrylate" means "acrylate or methacrylate", and "(poly)" refers to both the presence and absence of the prefix "poly". Furthermore, in this specification, "water-soluble" means having a solubility of 5% by mass or more in water at 25°C.

[0034] In addition, in this invention, the area near the surface of the water-absorbing resin particles refers to the region from the outermost surface of the water-absorbing resin particles to a depth of approximately 30 μm from the outermost surface toward the center of the particles.

[0035] In this specification, the values ​​connected by "~" refer to the range of values ​​before and after "~" as the lower and upper limits. When multiple lower and upper limits are recorded separately, any lower and upper limits can be selected and connected by "~".

[0036] 1. Method for manufacturing water-absorbing resin particles

[0037] The method for manufacturing water-absorbing resin particles of the present invention includes a step (polymerization step) of reverse suspension polymerization of water-soluble olefinic unsaturated monomers in a hydrocarbon dispersion medium to obtain polymer particles.

[0038] In the method for manufacturing the water-absorbing resin particles of the present invention, the reverse suspension polymerization is carried out in multiple stages with two or more stages. Furthermore, the manufacturing method of the present invention includes a step of agglomerating the polymer particles in the presence of a dispersing stabilizer. Moreover, in the manufacturing method of the present invention, two or more (poly)glycerol fatty acid esters with different precipitation temperatures when forming a 0.46% by mass heptane solution are used as dispersing stabilizers.

[0039] The method for manufacturing the absorbent resin particles of the present invention, by having these structures, enables the production of absorbent resin particles with a suitable narrow particle size distribution. Hereinafter, the method for manufacturing the absorbent resin particles of the present invention will be described in detail.

[0040] <Polymerization Process>

[0041] The polymerization process is a procedure in which water-soluble olefinically unsaturated monomers are polymerized to obtain polymer particles via reverse suspension polymerization. Reverse suspension polymerization involves heating water-soluble olefinically unsaturated monomers in a hydrocarbon dispersion medium under stirring. During the polymerization process, an internal crosslinking agent can also be added to the water-soluble olefinically unsaturated monomers as needed to produce crosslinked polymer particles with an internally crosslinked structure. An example of the polymerization process is described below.

[0042] [Water-soluble olefinic unsaturated monomer]

[0043] Examples of water-soluble olefinically unsaturated monomers include (meth)acrylic acid (in this specification, "acrylic acid" and "methacrylic acid" are collectively referred to as "(meth)acrylic acid"; the same applies hereinafter) and its salts; 2-(meth)acrylamide-2-methylpropanesulfonic acid and its salts; nonionic monomers such as (meth)acrylamide, N,N-dimethyl(meth)acrylamide, (meth)acrylic acid-2-hydroxyethyl ester, N-hydroxymethyl(meth)acrylamide, and polyethylene glycol mono(meth)acrylic acid ester; and amino-containing unsaturated monomers and their quaternary ammonium compounds such as N,N-diethylaminoethyl(meth)acrylic acid ester, N,N-diethylaminopropyl(meth)acrylic acid ester, and diethylaminopropyl(meth)acrylamide. From the viewpoint of easy industrial availability, (meth)acrylic acid or its salts, (meth)acrylamide, and N,N-dimethylacrylamide are preferred, and (meth)acrylic acid and its salts are more preferred. It should be noted that these water-soluble olefinic unsaturated monomers can be used alone or in combination of two or more.

[0044] Acrylic acid and its salts are widely used as raw materials for water-absorbing resin particles. Sometimes, these acrylic acids and / or their salts are used in combination with other water-soluble olefinic unsaturated monomers. In this case, acrylic acid and / or its salts are preferably used at 70 mol% to 100 mol% of the total water-soluble olefinic unsaturated monomers as the main water-soluble olefinic unsaturated monomer.

[0045] Water-soluble olefinic unsaturated monomers can also be dispersed in a hydrocarbon dispersion medium in an aqueous solution for reverse-phase suspension polymerization. By forming an aqueous solution, the dispersion efficiency of the water-soluble olefinic unsaturated monomers in the hydrocarbon dispersion medium can be improved. The concentration of the water-soluble olefinic unsaturated monomer in this aqueous solution is preferably in the range of 20% by mass to below the saturation concentration. Furthermore, the concentration of the water-soluble olefinic unsaturated monomer is more preferably 55% by mass or less, further 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 olefinic unsaturated monomer is more preferably 25% by mass or more, further preferably 28% by mass or more, and even more preferably 30% by mass or more.

[0046] Water-soluble olefinically unsaturated monomers, such as (meth)acrylic acid and 2-(meth)acrylamide-2-methylpropanesulfonic acid, which contain acid groups, can also be used if their acid groups have been pre-neutralized with an alkaline neutralizing agent. 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. Furthermore, to simplify the neutralization process, these alkaline neutralizing agents can also be used in aqueous solution form. It should be noted that the above-mentioned alkaline neutralizing agents can be used alone or in combination of two or more.

[0047] The degree of neutralization of the water-soluble olefinic unsaturated monomer using an alkaline neutralizing agent, relative to the degree of neutralization of all acid groups in the water-soluble olefinic unsaturated monomer, is preferably 40 mol% to 100 mol%, more preferably 50 mol% to 90 mol%, even more preferably 60 mol% to 85 mol%, and even more preferably 70 mol% to 80 mol%.

[0048] [Free radical polymerization initiator]

[0049] Examples of free radical polymerization initiators added in this 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-tert-butyl peroxide, tert-butyl cumene peroxide, tert-butyl peracetate, tert-butyl perisobutyrate, tert-butyl perpentyl peroxide, and hydrogen peroxide; and 2,2'-azobis(2-amidinylpropane) dihydrochloride and 2,2'-azobis[2-(N-phenylamidinyl)propane]. Azo compounds such as dihydrochlorides, 2,2'-azobis[2-(N-allylamidinyl)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-cyanopentanoic acid) are among these free radical polymerization initiators. From the viewpoint of ease of acquisition and operation, potassium persulfate, ammonium persulfate, sodium persulfate, and 2,2'-azobis(2-amidinylpropane) dihydrochloride are preferred. These free radical polymerization initiators can be used alone or in combination of two or more. In addition, the above-mentioned free radical polymerization initiators can also be used in combination with reducing agents such as sodium sulfite, sodium bisulfite, ferrous sulfate, and L-ascorbic acid as redox polymerization initiators.

[0050] The amount used as a free radical polymerization initiator can be, for example, 0.00005 mol to 0.01 mol relative to 1 mol of a water-soluble olefinic unsaturated monomer. By meeting such a dosage, rapid polymerization can be avoided, and the polymerization reaction can be terminated at an appropriate time.

[0051] [Internal cross-linking agent]

[0052] As an internal crosslinking agent, examples include crosslinking agents that can crosslink polymers of water-soluble olefinic unsaturated monomers, such as (poly) glycol [the expression "(poly)" indicates both the case with and without the prefix "poly"]. [The following are the same]: unsaturated polyesters obtained by reacting diols and triols such as poly(poly)propylene glycol, 1,4-butanediol, 1,6-hexanediol, trimethylolpropane, and poly(poly)glycerol with unsaturated acids such as methacrylic acid, maleic acid, and fumaric acid; bisacrylamides such as N,N-methylenebisacrylamide; di(meth)acrylates or tri(meth)acrylates obtained by reacting polyepoxides with methacrylic acid; carbamoyl dimethacrylates obtained by reacting polyisocyanates such as toluene diisocyanate and hexamethylene diisocyanate with hydroxyethyl methacrylate; allylated starch, allylated cellulose, diallyl phthalate, N,N',N''-triallyl isocyanurate, divinylbenzene, etc. Compounds having two or more polymerizable unsaturated groups; diglycidyl compounds such as (poly)ethylene glycol diglycidyl ether, (poly)propylene glycol diglycidyl ether, and (poly)glycerol diglycidyl ether; polyglycidyl compounds such as triglycidyl compounds; epihalohydrins such as epichlorohydrin, epibromohydrin, and α-methylepiochlorohydrin; isocyanate compounds such as 2,4-toluene diisocyanate and hexamethylene diisocyanate; and compounds having two or more reactive functional groups 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, etc. Among these internal crosslinking agents, polyethylene glycol diacrylate, trimethylolpropane triacrylate, (poly)ethylene glycol diglycidyl ether, (poly)propylene glycol diglycidyl ether, and (poly)glycerol diglycidyl ether are preferred. These internal crosslinking agents can be used alone or in combination of two or more.

[0053] The amount of the internal crosslinking agent used is preferably 0.000001 mol to 0.02 mol relative to 1 mol of the water-soluble olefinic unsaturated monomer, more preferably 0.00001 mol to 0.01 mol, even more preferably 0.00001 mol to 0.005 mol, and even more preferably 0.00005 mol to 0.002 mol.

[0054] [Hydrocarbon Dispersion Medium]

[0055] Examples of hydrocarbon dispersion media include: aliphatic hydrocarbons with 6 to 8 carbon atoms, such as n-hexane, n-heptane, 2-methylhexane, 3-methylhexane, 2,3-dimethylpentane, 3-hexylpentane, 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 suitable from the perspective of being readily available industrially, having stable quality, and being inexpensive. These hydrocarbon dispersion media can be used alone or in combination of two or more. It should be noted that, as an example of a mixture of hydrocarbon dispersion media, commercially available products such as Exxsol heptane (manufactured by Exxon Mobil: containing 75% to 85% by mass of heptane and its isomers) also yield suitable results.

[0056] From the viewpoint of uniformly dispersing the water-soluble olefinic unsaturated monomer and easily controlling the polymerization temperature, the amount of water-soluble olefinic unsaturated monomer used as the hydrocarbon dispersion medium is preferably 100 to 1500 parts by mass, and more preferably 200 to 1400 parts by mass, relative to 100 parts by mass of the water-soluble olefinic unsaturated monomer in the first stage. It should be noted that, as described below, in this invention, the reverse suspension polymerization is carried out in multiple stages (two or more stages), and the aforementioned first-stage polymerization refers to the polymerization reaction in the first stage of the multi-stage polymerization (the same applies hereinafter).

[0057] [Dispersion stabilizer]

[0058] In the manufacturing method of the present invention, a step of agglomerating polymer particles in the presence of a dispersion stabilizer is included, and two or more (poly)glycerol fatty acid esters with different precipitation temperatures when preparing a 0.46% by mass heptane solution are used as dispersion stabilizers.

[0059] The method for determining the precipitation temperature (hereinafter sometimes simply referred to as "precipitation temperature") of a 0.46% by mass solution of (poly)glycerol fatty acid esters in heptane is based on the method described in the examples.

[0060] From the viewpoint of further and appropriately maximizing the effects of the present invention, among the two or more of the above-mentioned (poly)glycerol fatty acid esters, the difference between the precipitation temperature of the (poly)glycerol fatty acid ester with the highest precipitation temperature and the precipitation temperature of the (poly)glycerol fatty acid ester with the lowest precipitation temperature is preferably 15°C or less, more preferably 10°C or less, even more preferably 6°C or less, and preferably 1°C or more, more preferably 2°C or more, even more preferably 3°C or more. As preferred ranges, examples include 1°C to 15°C, 1°C to 10°C, 1°C to 6°C, 2°C to 15°C, 2°C to 10°C, 2°C to 6°C, 3°C to 15°C, 3°C to 10°C, and 3°C to 6°C.

[0061] Furthermore, the precipitation temperature of (poly)glycerol fatty acid esters is preferably below 55°C, more preferably below 45°C, even more preferably below 40°C, and even more preferably below 35°C. In addition, it is preferably above 0°C, more preferably above 10°C, even more preferably above 15°C, and even more preferably above 20°C. As preferred ranges, examples include 0°C~55°C, 0°C~45°C, 0°C~40°C, 0°C~35°C, 10°C~55°C, 10°C~45°C, 10°C~40°C, 10°C~35°C, 15°C~55°C, 15°C~45°C, 15°C~40°C, 15°C~35°C, 20°C~55°C, 20°C~45°C, 20°C~40°C, and 20°C~35°C.

[0062] Polyglycerol fatty acid esters can reduce the discoloration of absorbent resin particles caused by heating. Specific examples of polyglycerol fatty acid esters include monoglyceride monostearate, tetraglyceride monostearate, tetraglyceride tristearate, tetraglyceride pentastearate, hexaglyceride tristearate, decaglyceride tristearate, decaglyceride decaglyceride, decaglyceride pentastearate, decahexaglyceride dodecanoic acid decaglyceride, pentaisostearate decaglyceride, pentaoleic acid decaglyceride, pentapalmitic acid decaglyceride, pentalauric acid decaglyceride, pentahydroxystearate decaglyceride, and condensed ricinoleic acid hexaglyceride. In this invention, it is preferable to use two or more of these polyglycerol fatty acid esters. Furthermore, among the two or more polyglycerol fatty acid esters, hexaglyceride tristearate is more preferably used.

[0063] Compared to 100 parts by mass of water-soluble olefinic unsaturated monomers used in the first stage of reverse suspension polymerization, the total amount of two or more (poly)glycerol fatty acid esters used is preferably 0.2 parts by mass or more, more preferably 0.9 parts by mass or more, even more preferably 1.3 parts by mass or more, even more preferably 1.4 parts by mass or more, and preferably 12.0 parts by mass or less, more preferably 4.0 parts by mass or less, even more preferably 2.0 parts by mass or less, even more preferably 1.7 parts by mass or less. Examples of preferred ranges include 0.2 parts by mass to 12.0 parts by mass, and 0.2 parts by mass to... 4.0 parts by weight, 0.2 to 2.0 parts by weight, 0.2 to 1.7 parts by weight, 0.9 to 12.0 parts by weight, 0.9 to 4.0 parts by weight, 0.9 to 2.0 parts by weight, 0.9 to 1.7 parts by weight, 1.3 to 12.0 parts by weight, 1.3 to 4.0 parts by weight, 1.3 to 2.0 parts by weight, 1.3 to 1.7 parts by weight, etc.

[0064] Furthermore, among the two or more polyglycerol fatty acid esters, the amount of the polyglycerol fatty acid ester with the highest precipitation temperature used is preferably 0.1 parts by mass or more, more preferably 0.2 parts by mass or more, and even more preferably 0.3 parts by mass or more, and preferably 12.0 parts by mass or less, more preferably 6.0 parts by mass or less, even more preferably 4.0 parts by mass or less, even more preferably 1.2 parts by mass or less, even more preferably 1.0 parts by mass or less, and even more preferably 0.5 parts by mass or less. Examples of preferred ranges include 0.1 parts by mass to 12.0 parts by mass, and 0.1 parts by mass... The quantities are as follows: 6.0 parts by weight, 0.1 parts by weight to 4.0 parts by weight, 0.1 parts by weight to 1.2 parts by weight, 0.1 parts by weight to 1.0 parts by weight, 0.1 parts by weight to 0.5 parts by weight, 0.2 parts by weight to 12.0 parts by weight, 0.2 parts by weight to 6.0 parts by weight, 0.2 parts by weight to 4.0 parts by weight, 0.2 parts by weight to 1.2 parts by weight, 0.2 parts by weight to 1.0 parts by weight, 0.2 parts by weight to 0.5 parts by weight, etc.

[0065] Furthermore, among the two or more polyglycerol fatty acid esters, the amount of the polyglycerol fatty acid ester with the lowest precipitation temperature used is preferably 0.2 parts by mass or more, more preferably 0.4 parts by mass or more, and preferably 12.0 parts by mass or less, more preferably 4.0 parts by mass or less, even more preferably 1.2 parts by mass or less, and even more preferably 0.8 parts by mass or less. Examples of preferred ranges include 0.2 parts by mass to 12.0 parts by mass, 0.2 parts by mass to 4.0 parts by mass, 0.2 parts by mass to 1.2 parts by mass, 0.2 parts by mass to 0.8 parts by mass, 0.4 parts by mass to 12.0 parts by mass, 0.4 parts by mass to 4.0 parts by mass, 0.4 parts by mass to 1.2 parts by mass, and 0.4 parts by mass to 0.8 parts by mass.

[0066] When at least one of the (poly)glycerol fatty acid esters is tristearate hexaglyceride, the amount of tristearate hexaglyceride used is preferably 0.2 parts by mass or more, more preferably 0.5 parts by mass or more, and preferably 9.0 parts by mass or less, more preferably 4.0 parts by mass or less, more preferably 0.9 parts by mass or less, and even more preferably 0.7 parts by mass or less. Examples of preferred ranges include 0.2 parts by mass to 9.0 parts by mass, 0.2 parts by mass to 4.0 parts by mass, 0.2 parts by mass to 0.9 parts by mass, 0.2 parts by mass to 0.7 parts by mass, 0.5 parts by mass to 9.0 parts by mass, 0.5 parts by mass to 4.0 parts by mass, 0.5 parts by mass to 0.9 parts by mass, and 0.5 parts by mass to 0.7 parts by mass.

[0067] At least one of the (poly)glycerol fatty acid esters is tristearate hexaglyceride. When the product comprises at least one of tristearate hexaglyceride, a (poly)glycerol fatty acid ester with a precipitation temperature higher than tristearate hexaglyceride, and a (poly)glycerol fatty acid ester with a precipitation temperature lower than tristearate hexaglyceride, one or more of the tristearate fatty acid esters with a precipitation temperature higher than tristearate hexaglyceride may be used. Alternatively, one or more of the (poly)glycerol fatty acid esters with a precipitation temperature lower than tristearate hexaglyceride may be used.

[0068] Furthermore, when at least one of the (poly)glycerol fatty acid esters is tristearate hexaglyceride, the amount of each of the following (poly)glycerol fatty acid esters is preferably 0.0 parts by mass or more, more preferably 0.3 parts by mass or more, and even more preferably 1 part by mass of tristearate hexaglyceride. The (poly)glycerol fatty acid ester with a higher precipitation temperature than tristearate hexaglyceride and the (poly)glycerol fatty acid ester with a lower precipitation temperature than tristearate hexaglyceride are used in amounts that are respectively 1 part by mass of tristearate hexaglyceride. The preferred amounts are 0.5 parts by weight or more, preferably 3 parts by weight or less, more preferably 2.0 parts by weight or less, even more preferably 1.3 parts by weight or less, and even more preferably 1.0 parts by weight or less. Examples of preferred ranges include 0.0 parts by weight to 3.0 parts by weight, 0.0 parts by weight to 2.0 parts by weight, 0.0 parts by weight to 1.3 parts by weight, 0.3 parts by weight to 3.0 parts by weight, 0.3 parts by weight to 2.0 parts by weight, 0.3 parts by weight to 1.3 parts by weight, 0.5 parts by weight to 3.0 parts by weight, 0.5 parts by weight to 2.0 parts by weight, and 0.5 parts by weight to 1.3 parts by weight.

[0069] In cases where at least one of the (poly)glycerol fatty acid esters is tristearate hexaglyceride, and the product comprises tristearate hexaglyceride, a (poly)glycerol fatty acid ester with a precipitation temperature higher than that of tristearate hexaglyceride, and a (poly)glycerol fatty acid ester with a precipitation temperature lower than that of tristearate, one or more (poly)glycerol fatty acid esters with a precipitation temperature higher than that of tristearate hexaglyceride may be used. Alternatively, one or more (poly)glycerol fatty acid esters with a precipitation temperature lower than that of tristearate hexaglyceride may be used.

[0070] Furthermore, when at least one of the (poly)glycerol fatty acid esters is tristearate hexaglyceride, the amount of each of the following esters used is preferably more than 0.0 parts by mass, more preferably 0.3 parts by mass or more, and even more preferably 0. More than 5 parts by weight, preferably 2.0 parts by weight or less, more preferably 1.3 parts by weight or less, and even more preferably 1.0 parts by weight or less. Examples of preferred ranges include more than 0.0 parts by weight and less than 2.0 parts by weight, more than 0.0 parts by weight and less than 1.3 parts by weight, more than 0.0 parts by weight and less than 2.0 parts by weight, 0.3 parts by weight to 2.0 parts by weight, 0.3 parts by weight to 1.3 parts by weight, 0.3 parts by weight to 1.0 parts by weight, 0.5 parts by weight to 2.0 parts by weight, 0.5 parts by weight to 1.3 parts by weight, and 0.5 parts by weight to 1.0 parts by weight.

[0071] Furthermore, when at least one of the (poly)glycerol fatty acid esters is tristearate hexaglyceride, and when the product includes tristearate hexaglyceride, a (poly)glycerol fatty acid ester with a higher precipitation temperature than tristearate hexaglyceride, and a (poly)glycerol fatty acid ester with a lower precipitation temperature than tristearate hexaglyceride, the amount of the (poly)glycerol fatty acid ester with a higher precipitation temperature than tristearate hexaglyceride used is preferably more than 0.0 parts by mass, more preferably 0.2 parts by mass or more, and even more preferably 0.4 parts by mass or more. Additionally, it is preferable that... Less than 2.0 parts by weight, more preferably less than 1.0 parts by weight, and even more preferably less than 0.8 parts by weight. Examples of preferred ranges include more than 0.0 parts by weight and less than 2.0 parts by weight, more than 0.0 parts by weight and less than 1.0 parts by weight, more than 0.0 parts by weight and less than 0.8 parts by weight, 0.2 parts by weight to 2.0 parts by weight, 0.2 parts by weight to 1.0 parts by weight, 0.2 parts by weight to 0.8 parts by weight, 0.4 parts by weight to 2.0 parts by weight, 0.4 parts by weight to 1.0 parts by weight, and 0.4 parts by weight to 0.8 parts by weight.

[0072] Furthermore, when at least one of the (poly)glycerol fatty acid esters is tristearate hexaglyceride, and when the product comprises tristearate hexaglyceride, a (poly)glycerol fatty acid ester with a higher precipitation temperature than tristearate hexaglyceride, and a (poly)glycerol fatty acid ester with a lower precipitation temperature than tristearate hexaglyceride, the amount of the (poly)glycerol fatty acid ester with a lower precipitation temperature than tristearate hexaglyceride used is preferably more than 0.0 parts by mass, more preferably 0.2 parts by mass or more, and even more preferably 0.5 parts by mass or more, relative to 1 part by mass of tristearate hexaglyceride. The preferred range is 2.0 parts by weight or less, more preferably 1.5 parts by weight or less, and even more preferably 0.9 parts by weight or less. Examples of preferred ranges include more than 0.0 parts by weight and less than 2.0 parts by weight, more than 0 parts by weight and less than 1.5 parts by weight, more than 0 parts by weight and less than 0.9 parts by weight, 0.2 parts by weight to 2.0 parts by weight, 0.2 parts by weight to 1.5 parts by weight, 0.2 parts by weight to 0.9 parts by weight, 0.5 parts by weight to 2.0 parts by weight, 0.5 parts by weight to 1.5 parts by weight, and 0.5 parts by weight to 0.9 parts by weight.

[0073] The use of two or more (poly)glycerol fatty acid esters with different precipitation temperatures when preparing a 0.46% by mass heptane solution is permitted, preferably two to five, more preferably two to four, even more preferably two to three, and particularly preferably three.

[0074] In the aggregation step of the manufacturing method of the present invention, in addition to the two or more (poly)glycerol fatty acid esters mentioned above, other dispersing stabilizers different from (poly)glycerol fatty acid esters may also be present. From the viewpoint of narrowing the particle size distribution, polymeric dispersants are preferred as other dispersing stabilizers. In the aggregation step, the total proportion of the two or more (poly)glycerol fatty acid esters in all dispersing stabilizers is preferably 50% by mass or more, more preferably 60% by mass or more, further preferably 80% by mass or more, and may also be 90% by mass or more, 95% by mass or more, 99% by mass, etc.

[0075] As a dispersion stabilizer used in the process of agglomerating polymer particles in the presence of a dispersion stabilizer, surfactants, polymeric dispersants, and (poly)glycerol fatty acid esters can be used as surfactants. Examples of dispersion stabilizers different from (poly)glycerol fatty acid esters include the following surfactants and polymeric dispersants.

[0076] (surfactant)

[0077] In reverse suspension polymerization, dispersion stabilizers can be used to improve the dispersion stability of water-soluble olefinic unsaturated monomers in hydrocarbon dispersion media. Surfactants can be used as such dispersion stabilizers.

[0078] As surfactants, examples include sucrose fatty acid esters, dehydrated sorbitan fatty acid esters, polyoxyethylene dehydrated sorbitan fatty acid esters, polyoxyethylene glycerol fatty acid esters, sorbitan fatty acid esters, polyoxyethylene sorbitan fatty acid esters, polyoxyethylene alkyl ethers, polyoxyethylene alkylphenyl ethers, polyoxyethylene castor oil, polyoxyethylene hydrogenated castor oil, alkylallyl formaldehyde condensation polyoxyethylene ethers, polyoxyethylene polyoxypropylene block copolymers, polyoxyethylene polyoxypropylene alkyl ethers, polyethylene glycol fatty acid esters, alkyl glucosides, N-alkyl glucosamides, polyoxyethylene fatty acid amides, polyoxyethylene alkylamines, phosphate esters of polyoxyethylene alkyl ethers, and phosphate esters of polyoxyethylene alkyl allyl ethers. These surfactants can be used alone or in combination of two or more.

[0079] (Polymer dispersant)

[0080] In addition, as a dispersant stabilizer used in reverse suspension polymerization, it can also be used together with the surfactants mentioned above and with a polymeric dispersant.

[0081] 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 ethyl hydroxyethyl cellulose. Among these polymeric dispersants, particularly from the perspective of monomer dispersion stability, maleic anhydride-modified polyethylene, maleic anhydride-modified polypropylene, 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 are preferred. These polymeric dispersants can be used alone or in combination of two or more.

[0082] [Other ingredients]

[0083] In the manufacturing method of water-absorbent resin particles, other components can be added to an aqueous solution containing water-soluble olefinically unsaturated monomers as needed to carry out reverse-phase suspension polymerization. Various additives such as thickeners and chain transfer agents can be added as these other components.

[0084] As an example, a thickener can be added to an aqueous solution containing a water-soluble olefinically unsaturated monomer to induce reverse suspension polymerization. By adjusting the viscosity of the aqueous solution through the addition of a thickener, the median particle size obtained in the reverse suspension polymerization can be controlled.

[0085] As thickeners, hydroxyethyl cellulose, hydroxypropyl cellulose, methyl cellulose, carboxymethyl cellulose, polyacrylic acid, partially neutralized polyacrylic acid, polyethylene glycol, polyacrylamide, polyethyleneimine, dextrin, sodium alginate, polyvinyl alcohol, polyvinylpyrrolidone, and polyethylene oxide can be used, for example. It should be noted that if the stirring speed during polymerization is the same, there is a tendency that the higher the viscosity of the aqueous solution of the water-soluble olefinically unsaturated monomer, the larger the primary and / or secondary particles will be.

[0086] [Reverse suspension polymerization]

[0087] In reverse suspension polymerization, for example, an aqueous monomer solution containing a water-soluble olefinically unsaturated monomer is dispersed in a hydrocarbon dispersion medium in the presence of two or more (poly)glycerol fatty acid esters (and other dispersion stabilizers as needed) as dispersion stabilizers. In this case, the dispersion stabilizer can be added either before or after the monomer aqueous solution is added, as long as it is before the start of the polymerization reaction.

[0088] From the viewpoint of easily reducing the amount of residual hydrocarbon dispersion medium in the obtained water-absorbing resin particles, it is preferable to disperse the monomer aqueous solution in a hydrocarbon dispersion medium in which a polymeric dispersant is dispersed, and then disperse the surfactant before polymerization.

[0089] Such reverse suspension polymerization can be carried out in multiple stages, with two or more stages. Furthermore, from the viewpoint of improving productivity, it is preferable to carry out the process in two to three stages.

[0090] Multi-stage reverse suspension polymerization involving two or more stages can be performed by adding a water-soluble olefinically unsaturated monomer to the reaction mixture obtained from the first stage of reverse suspension polymerization and mixing it, then carrying out subsequent reverse suspension polymerizations using the same method as the first stage. In each subsequent stage of reverse suspension polymerization, preferably, in addition to the water-soluble olefinically unsaturated monomer, the free radical polymerization initiator is added based on the amount of water-soluble olefinically unsaturated monomer added in each subsequent stage of reverse suspension polymerization, within the range of the aforementioned molar ratios of each component relative to the water-soluble olefinically unsaturated monomer. It should be noted that, in subsequent stages of polymerization, an internal crosslinking agent may also be added to the water-soluble olefinically unsaturated monomer, if necessary.

[0091] From the viewpoint of improving economy by enabling rapid polymerization and shortening polymerization time, and facilitating smooth reaction by easily removing the heat of polymerization, the reaction temperature is preferably 20°C to 110°C, and more preferably 40°C to 90°C.

[0092] If the polymer particle aggregation process is carried out after the first stage of reverse suspension polymerization, the aggregation process can be performed between the first stage and the second stage of reverse suspension polymerization, or it can be performed after the second stage of reverse suspension polymerization. From the viewpoint of further and appropriately maximizing the effects of the present invention, the polymer particle aggregation process is preferably carried out between the first stage and the second stage of reverse suspension polymerization. Furthermore, in the polymer particle aggregation process, the polymerization reaction of the water-soluble olefinic unsaturated monomer may or may not be carried out.

[0093] In this invention, during the aggregation process, when polymer particles obtained through the first stage of reverse suspension polymerization are aggregated, the degree of aggregation of polymer particles can be controlled by adjusting the temperature of the slurry to allow two or more (poly)glycerol fatty acid esters with different precipitation temperatures to precipitate in stages. For example, before adding the aqueous solution of the water-soluble olefinic unsaturated monomer in the second stage of reverse suspension polymerization, the temperature of the slurry is lowered, causing at least one of the two or more (poly)glycerol fatty acid esters to precipitate. Consequently, the droplets of the subsequently added water-soluble olefinic unsaturated monomer become unstable in the hydrocarbon dispersion medium and are absorbed by the polymer particles (gel-like primary particles), thereby promoting the aggregation of polymer particles. On the other hand, when the temperature of the slurry is increased, the (poly)glycerol fatty acid esters dissolve, the droplets of the water-soluble olefinic unsaturated monomer become stable in the hydrocarbon dispersion medium, and the aggregation of polymer particles is suppressed. In this invention, by adjusting the promotion / suppression of the aggregation of polymer particles, it is possible to produce water-absorbing resin particles with a suitable narrow particle size distribution.

[0094] As for the temperature range in the aggregation process, there are no particular limitations as long as it is a temperature range that can adjust the aggregation of polymer particles. For example, 5°C to 50°C is an example, preferably 10°C to 40°C, and more preferably 15°C to 30°C.

[0095] <Dehydration Process>

[0096] Alternatively, after the aforementioned reverse suspension polymerization, dehydration treatment can be performed, including removing water and hydrocarbon dispersion media by distillation through external application of energy such as heat. In the case of dehydration of the aqueous gel-like material after reverse suspension polymerization, the system in which the aqueous gel-like material is dispersed in a 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 distilled hydrocarbon dispersion medium is returned to the system, continuous azeotropic distillation can be performed. In this case, the temperature within the drying system is maintained below the azeotropic temperature with the hydrocarbon dispersion medium, which is preferable from the viewpoint of minimizing resin degradation. By controlling the dehydration treatment conditions after polymerization to adjust the amount of water removed (i.e., adjusting the moisture content of the polymer particles), various properties of the obtained water-absorbing resin particles can be controlled.

[0097] In the dehydration process, dehydration treatment by distillation can also be carried out under normal pressure. When dehydration treatment is carried out under normal pressure, the dehydration temperature is preferably 70°C to 250°C, more preferably 80°C to 180°C, even more preferably 80°C to 140°C, and even more preferably 90°C to 130°C.

[0098] <Surface Crosslinking Process>

[0099] The surface crosslinking process is a process of performing surface crosslinking on the polymer particles obtained in the polymerization process. When the polymer particles are crosslinked polymer particles (including hydrogel-like substances), it becomes a process of adding a surface crosslinking agent to the hydrogel-like substance with an internal crosslinked structure obtained by polymerizing water-soluble olefinic unsaturated monomers to perform a crosslinking reaction (surface crosslinking reaction). This surface crosslinking reaction is preferably carried out in the presence of a surface crosslinking agent after the polymerization of the water-soluble olefinic unsaturated monomers is completed. In this way, by performing a surface crosslinking reaction on the hydrogel-like substance with an internal crosslinked structure after polymerization, the crosslinking density near the surface of the water-absorbing resin particles can be increased, resulting in water-absorbing resin particles with improved water absorption capacity under load and other properties.

[0100] Compounds having two or more reactive functional groups can be cited as surface crosslinking agents. Examples include: polyols such as ethylene glycol, propylene glycol, 1,4-butanediol, diethylene glycol, triethylene glycol, trimethylolpropane, glycerol, polyoxyethylene glycol, polyoxypropylene glycol, and polyglycerol; polyglycidyl compounds such as (poly)ethylene glycol diglycidyl ether, (poly)glycerol diglycidyl ether, (poly)glycerol triglycidyl ether, trimethylolpropane triglycidyl ether, (poly)propylene glycol polyglycidyl ether, and (poly)glycerol polyglycidyl ether; halogenated epoxy compounds such as epichlorohydrin, epibromohydrin, and α-methylepiochlorohydrin; isocyanate compounds such as 2,4-toluene diisocyanate and hexamethylene diisocyanate; 3-methyl-3-oxetane methanol, 3-ethyl-3-oxetane methanol, 3-butyl-3-oxetane methanol, and 3-methyl-3-oxetane methanol. Oxybutane compounds such as alcohols, 3-ethyl-3-oxetane ethanol, and 3-butyl-3-oxetane ethanol; oxazoline compounds such as 1,2-ethylidene bisoxazoline; ethylene carbonate; propylene carbonate, carbonates such as 4,5-dimethyl-1,3-dioxolane-2-one, 4,4-dimethyl-1,3-dioxolane-2-one, 4-ethyl-1,3-dioxolane-2-one, 4-hydroxymethyl-1,3-dioxolane-2-one, 1,3-dioxane-2-one, 4-methyl-1,3-dioxane-2-one, 4,6-dimethyl-1,3-dioxane-2-one, and 1,3-dioxolane-2-one (e.g., alkylene carbonates); hydroxyalkylamide compounds such as bis[N,N-di(β-hydroxyethyl)]hexamethylenediamide. Among these surface crosslinking agents, polyglycidyl compounds such as (poly)ethylene glycol diglycidyl ether, (poly)glycerol diglycidyl ether, (poly)glycerol triglycidyl ether, trimethylolpropane triglycidyl ether, (poly)propylene glycol polyglycidyl ether, and (poly)glycerol polyglycidyl ether are preferred. These surface crosslinking agents can be used alone or in combination of two or more.

[0101] The amount of surface crosslinking agent used is preferably 0.00001 mol to 0.01 mol, more preferably 0.00005 mol to 0.005 mol, and even more preferably 0.0001 mol to 0.002 mol, relative to 1 mol of the total amount of water-soluble olefinic unsaturated monomers used in the polymerization.

[0102] As a method of adding a surface crosslinking agent, it can be added directly, as an aqueous solution, or, as needed, as a solution using a hydrophilic organic solvent. Examples of hydrophilic organic solvents include: lower alcohols such as methanol, ethanol, n-propanol, and isopropanol; 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 can be used alone, in combination of two or more, or as a mixed solvent with water.

[0103] The surface crosslinking agent can be added once the polymerization reaction of the water-soluble olefinic unsaturated monomer is almost completely completed. It is preferably added in the presence of water in the range of 1 to 400 parts by mass relative to 100 parts by mass of the water-soluble olefinic unsaturated monomer; more preferably, in the presence of water in the range of 5 to 200 parts by mass; even more preferably, in the presence of water in the range of 10 to 100 parts by mass; and even more preferably, in the presence of water in the range of 20 to 60 parts by mass. It should be noted that the amount of water refers to the total amount of water contained in the reaction system and the water required for adding the surface crosslinking agent.

[0104] From the viewpoint of further and appropriately maximizing the effects of the present invention, the water content of the polymer particles when the surface crosslinking agent is added is preferably 1% by mass or more, more preferably 10% by mass or more, even more preferably 20% by mass or more, and preferably 60% by mass or less, more preferably 40% by mass or less, even more preferably 35% by mass or less. Examples of preferred ranges include 1% to 60% by mass, 1% to 40% by mass, 1% to 35% by mass, 10% to 60% by mass, 10% to 40% by mass, 10% to 35% by mass, 20% to 60% by mass, 20% to 40% by mass, and 20% to 35% by mass.

[0105] The preferred reaction temperature for the surface crosslinking reaction is 50°C to 250°C, more preferably 60°C to 180°C, even more preferably 60°C to 140°C, and even more preferably 70°C to 120°C. The preferred reaction time for the surface crosslinking reaction is 1 minute to 300 minutes, more preferably 5 minutes to 200 minutes.

[0106] <Drying Process>

[0107] Alternatively, after the above-mentioned surface crosslinking, the following drying process can be performed: water, hydrocarbon dispersion medium, etc., are removed by distillation through the application of energy such as heat from the outside. The surface-crosslinked polymer particles are dried, and water and hydrocarbon dispersion medium are removed by distillation, thereby obtaining water-absorbing resin particles.

[0108] In the drying process, the drying treatment by distillation can be carried out under atmospheric pressure or under reduced pressure. Alternatively, from the viewpoint of improving drying efficiency, it can also be carried out under a gas stream such as nitrogen. When drying is carried out under atmospheric pressure, the preferred drying temperature is 70°C to 250°C, more preferably 80°C to 180°C, even more preferably 80°C to 140°C, and even more preferably 90°C to 130°C. When drying is carried out under reduced pressure, the preferred drying temperature is 40°C to 160°C, more preferably 50°C to 110°C.

[0109] It should be noted that when a surface crosslinking step using a surface crosslinking agent is performed after monomer polymerization via reverse suspension polymerization, the aforementioned drying step by distillation is performed after the surface crosslinking step is completed. Alternatively, the surface crosslinking step and the drying step can be performed simultaneously.

[0110] The absorbent resin particles of the present invention may also contain additives appropriate to the intended purpose. Examples of such additives include inorganic powders, surfactants, oxidizing agents, reducing agents, metal chelating agents, free radical chain reaction terminators, antioxidants, and antibacterial agents. For example, by adding 0.05 to 5 parts by weight of amorphous silica as an inorganic powder relative to 100 parts by weight of the absorbent resin particles, the flowability of the absorbent resin particles can be further improved. It should be noted that the above-mentioned additives are preferably hydrophilic or water-soluble.

[0111] 2. Water-absorbing resin particles

[0112] By employing the above-described method for manufacturing water-absorbing resin particles of the present invention, water-absorbing resin particles with a narrow particle size distribution can be suitably manufactured. More specifically, by employing the method for manufacturing water-absorbing resin particles of the present invention, for example, water-absorbing resin particles with a particle size distribution uniformity of 2.5 or less and containing (poly)glycerol fatty acid esters near the surface can be suitably manufactured.

[0113] The uniformity of the particle size distribution of the absorbent resin particles of the present invention is preferably 2.5 or less, more preferably 2.3 or less, more preferably 1.8 or less, and even more preferably 1.6 or less, with a lower limit of, for example, 1.

[0114] The uniformity of the particle size distribution of the absorbent resin particles was determined based on the determination method described in the examples.

[0115] Furthermore, the absorbent resin particles of the present invention contain (poly)glycerol fatty acid esters near their surface. In the method for manufacturing the absorbent resin particles of the present invention, as described above, (poly)glycerol fatty acid esters are used. Therefore, the absorbent resin particles contain (poly)glycerol fatty acid esters near their surface. Specific examples of (poly)glycerol fatty acid esters can be given, such as those exemplified in the method for manufacturing the absorbent resin particles of the present invention. Preferably, two or more (poly)glycerol fatty acid esters are contained near the surface of the absorbent resin particles, and preferably, tristearate hexaglyceride is included.

[0116] The amount of (poly)glycerol fatty acid ester contained near the surface of the absorbent resin particles of the present invention is preferably 1 part by weight or less, more preferably 0.65 parts by weight or less, and even more preferably 0.55 parts by weight or less, relative to 100 parts by weight of the absorbent resin particles. It should be noted that the lower limit is preferably 0.1 parts by weight or more, more preferably 0.2 parts by weight or more, and even more preferably 0.3 parts by weight or more.

[0117] Furthermore, the yellowness of the absorbent resin particles of the present invention after heating at 200°C for 2 hours is preferably 20 or less, more preferably 15 or less, and even more preferably 12 or less. It should be noted that the lower limit of this yellowness is, for example, 0.

[0118] Furthermore, the initial value of the yellowness of the absorbent resin particles of the present invention (yellowness before heating at 200°C for 2 hours) is preferably 15 or less, more preferably 12 or less, and even more preferably 10 or less. It should be noted that the lower limit of this yellowness is, for example, 0.

[0119] The yellowness of the absorbent resin particles before and after heating at 200°C for 2 hours was determined based on the determination methods described in the examples.

[0120] The water-absorbing resin particles of the present invention are preferably composed of a substance formed by cross-linking a polymer of a water-soluble olefinic unsaturated monomer, that is, a cross-linked polymer having structural units derived from the water-soluble olefinic unsaturated monomer.

[0121] The absorbent resin particles of the present invention are in the form of aggregates of fine particles (primary particles) (secondary particles). Examples of primary particle shapes include approximately spherical, irregularly broken, and plate-like shapes. The absorbent resin particles of the present invention, as secondary particles, can have various shapes. Examples of absorbent resin particle shapes include granular, approximately spherical, irregularly broken, plate-like, fibrous, flake-like, or aggregated forms of these resins. Preferably, the absorbent resin particles are granular, approximately spherical, irregularly broken, fibrous, or aggregated forms of these resins.

[0122] The median particle size of the water-absorbing resin particles is preferably 100 μm or more, 150 μm or more, 200 μm or more, 250 μm or more, 280 μm or more, 300 μm or more, or 320 μm or more. Furthermore, from the same viewpoint, the median particle size is preferably 700 μm or less, 600 μm or less, 550 μm or less, 500 μm or less, 450 μm or less, or 400 μm or less. That is, the median particle size is preferably 150 μm to 700 μm, preferably 200 μm to 600 μm, more preferably 250 μm to 500 μm, further preferably 250 μm to 450 μm, and even more preferably 250 μm to 400 μm.

[0123] The median particle size of the water-absorbing resin particles can be determined using JIS standard sieves, specifically, by the method described in the examples.

[0124] The absorption rate of physiological saline by the absorbent resin particles is preferably 20 seconds or more, more preferably 25 seconds or more, even more preferably 30 seconds or more, and preferably 65 seconds or less, more preferably 60 seconds or less, even more preferably 55 seconds or less. Examples of more preferred ranges include 20 seconds to 65 seconds and 25 seconds to 60 seconds.

[0125] The saline water retention capacity of the absorbent resin particles of the present invention is preferably 20 g / g or more, more preferably 30 g / g or more, and preferably 80 g / g or less, more preferably 60 g / g or less, even more preferably 55 g / g or less, and even more preferably 53 g / g or less. Examples of preferred ranges include 20 g / g to 80 g / g, 20 g / g to 60 g / g, 20 g / g to 55 g / g, 20 g / g to 53 g / g, 30 g / g to 80 g / g, 30 g / g to 60 g / g, 30 g / g to 55 g / g, and 30 g / g to 53 g / g.

[0126] 3. Absorbent materials

[0127] The absorbent resin particles of the present invention constitute an absorbent material used in sanitary materials such as menstrual products and diapers, and are suitable for absorbent articles containing the above-mentioned absorbent material.

[0128] The absorbent of the present invention comprises the absorbent resin particles of the present invention. The absorbent may further comprise hydrophilic fibers. Examples of absorbent structures include sheet-like structures in which absorbent resin particles are fixed on or between multiple nonwoven fabrics; mixed dispersions obtained by mixing absorbent resin particles and hydrophilic fibers into a uniform composition; sandwich structures in which absorbent resin particles are sandwiched between layered hydrophilic fibers; and structures formed by wrapping absorbent resin particles and hydrophilic fibers in thin paper. It should be noted that other components may be incorporated into the absorbent, such as heat-melt synthetic fibers, heat-melt adhesives, adhesive emulsions, and other adhesives to improve the shape retention performance of the absorbent.

[0129] The absorbent resin particles in the absorbent body of this invention have a unit area weight of 50 g / m³. 2 Above and 400g / m 2 The following is a preferred weight per unit area: 100 g / m². 2 The above, more preferably 120g / m 2 The above is further optimized to 140g / m 2 In addition, the preferred value is 300g / m³. 2 The following is more preferably 250g / m 2 The following is a further preferred value: 200g / m 2 the following.

[0130] Examples of hydrophilic fibers include at least one selected from the group consisting of finely ground wood pulp, cotton, cotton linters, rayon, cellulose acetate, polyamide, polyester, and polyolefins. Examples include cellulose fibers such as cotton-like pulp, mechanical pulp, chemical pulp, and semi-chemical pulp obtained from wood; man-made cellulose fibers such as rayon and acetate; and fibers made from synthetic resins such as polyamide, polyester, and polyolefins that have undergone hydrophilic treatment. The average fiber length of hydrophilic fibers is typically 0.1 mm to 10 mm, or 0.5 mm to 5 mm.

[0131] The hydrophilic fibers in the absorbent of this invention have a surface area weight of 50 g / m². 2 Above and 800g / m 2 The following is a preferred weight per unit area: 100 g / m². 2 The above, more preferably 120g / m 2 The above is further optimized to 140g / m 2 The above; additionally, 700g / m³ is preferred. 2 The following is more preferably 600g / m 2 The following is a further preferred value: 500g / m 2 the following.

[0132] The content of water-absorbing resin particles 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.

[0133] The absorbent article of the present invention can be manufactured by holding an absorbent body using the absorbent resin particles of the present invention between a liquid-permeable sheet (top sheet) through which liquid can pass and a liquid-impermeable sheet (back sheet) through which liquid cannot pass. The liquid-permeable sheet is disposed on the side in contact with the body, and the liquid-impermeable sheet is disposed on the opposite side in contact with the body.

[0134] Examples of liquid-permeable sheets include nonwoven fabrics such as hot-air type, spunbond type, chemically bonded type, and needle-punched type 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 containing resins such as polyethylene, polypropylene, and polyvinyl chloride. The liquid-permeable sheet is preferably selected from at least one of the following groups: thermally bonded nonwoven fabric, hot-air nonwoven fabric, spunbond nonwoven fabric, and spunbond / meltblown / spunbond nonwoven fabric.

[0135] The preferred weight per unit area of ​​the liquid-permeable sheet is 5 g / m². 2 Above and 100g / m 2 The following is more preferably 10g / m 2 Above and 60g / m 2 Furthermore, to improve the diffusion properties of the liquid, the surface of the liquid-permeable sheet can be embossed or perforated. These embossing and perforation processes can be performed using known methods.

[0136] Examples of liquid-impermeable sheets include: sheets made of synthetic resins such as polyethylene, polypropylene, and polyvinyl chloride; sheets made of nonwoven fabrics such as spunbond / meltblown / spunbond (SMS) nonwoven fabrics made by sandwiching water-resistant meltblown nonwoven fabric with high-strength spunbond nonwoven fabric; and sheets made of composite materials of these synthetic resins and nonwoven fabrics (e.g., spunbond nonwoven fabrics, spunlace nonwoven fabrics). Sheets made of synthetic resins primarily composed of low-density polyethylene (LDPE) resin can also be used as liquid-impermeable sheets. For example, a liquid-impermeable sheet may also be made of a material with a weight per unit area of ​​10 g / m². 2 ~50g / m 2 Sheets made of synthetic resin.

[0137] The absorbent article preferably has a laminate, 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 side of the liquid-permeable sheet. The laminate has an absorbent containing absorbent resin particles and upper and lower core-packing materials (core-packing sheets) that hold the absorbent.

[0138] Example

[0139] The present invention will now be described in detail with reference to embodiments and comparative examples. However, the present invention is not limited to the embodiments.

[0140] It should be noted that the water-absorbing resin particles obtained in the following examples and comparative examples were evaluated through various tests. It should be noted that, unless otherwise specified, the measurements were conducted at a temperature of 25±2℃ and a humidity of 50±10%.

[0141] [Manufacturing of water-absorbing resin particles]

[0142] (Example 1)

[0143] Prepare a round-bottomed cylindrical flask with an inner diameter of 11 cm and a volume of 2 L, equipped with a reflux condenser, dropping funnel, nitrogen inlet tube, and a stirrer (with two sections of four inclined blades with a blade diameter of 5 cm). Add 300 g of n-heptane (hydrocarbon dispersion medium), 0.737 g of tristearate tetraglyceride (dispersion stabilizer, manufactured by Sakamoto Pharmaceutical Co., Ltd., SY GLYSTAR TS-3S), and 0.737 g of tristearate hexaglyceride (dispersion stabilizer, manufactured by Sakamoto Pharmaceutical Co., Ltd., SY GLYSTAR TS-5S) to the flask. While stirring, heat to 80°C to dissolve the dispersant, then cool to 55°C.

[0144] Next, 92.0 g of an 80.5% by mass aqueous solution of acrylic acid (acrylic acid: 1.03 mol) was added to a 500 mL Erlenmeyer flask. Then, while cooling from the outside, 102.78 g of a 30% by mass aqueous solution of sodium hydroxide was added dropwise, resulting in a 75 mol% neutralization. Afterward, 0.0736 g (0.272 mmol) of potassium persulfate (a water-soluble free radical polymerization initiator), 0.0101 g (0.0580 mmol) of ethylene glycol diglycidyl ether (an internal crosslinking agent), and 44.62 g of deionized water were added and dissolved to prepare the first-stage aqueous solution.

[0145] The aqueous solution from the first stage was added to the separable flask, and the mixture was stirred at 400 rpm while the system was fully purged with nitrogen. The separable flask was then immersed in a 70°C water bath to heat the reaction solution and allow the first stage of polymerization to proceed for 60 minutes, thus obtaining the first stage slurry.

[0146] Next, 128.8 g of an 80.5% by mass aqueous solution of acrylic acid (acrylic acid: 1.44 mol) was added to another 500 mL Erlenmeyer flask. Then, while cooling from the outside, 143.89 g of a 30% by mass aqueous solution of sodium hydroxide was added dropwise to achieve 75 mol% neutralization. Then, 0.1030 g (0.3810 mmol) of potassium persulfate, 0.0116 g (0.0666 mmol) of ethylene glycol diglycidyl ether (internal crosslinking agent) and 16.75 g of deionized water were added and dissolved to prepare the aqueous solution for the second stage.

[0147] While stirring the first-stage slurry at 1000 rpm, the system in the detachable flask was cooled to 20°C. Then, all the aqueous solution from the second stage was added to the first-stage slurry for the polymerization process. After purging the system with nitrogen for 30 minutes, the detachable flask was immersed again in a 70°C water bath for 60 minutes to obtain the second-stage slurry.

[0148] Following the second-stage polymerization, the slurry was heated in an oil bath at 125°C. Azeotropic distillation of n-heptane and water was performed, with 248g of water being discharged from the system while the n-heptane was refluxed. The n-heptane was then evaporated and dried to obtain a dried product. This dried product was passed through an 850μm sieve, yielding 203.8g of approximately spherical aggregates of absorbent resin particles.

[0149] (Example 2)

[0150] Using penta-stearic acid decyl ester (dispersion stabilizer, manufactured by Nippon Surfactant Industries, Ltd., NIKKOLDecaglyn 5-SV) instead of tristearate tetraglyceride in Example 1, the same procedure as in Example 1 was performed to obtain 200.3 g of water-absorbing resin particles.

[0151] (Example 3)

[0152] 0.552 g of tristearate tetraglyceride, 0.552 g of tristearate hexaglyceride, and 0.184 g of pentastearate decaglyceride were used instead of 0.737 g of tristearate tetraglyceride and 0.737 g of tristearate hexaglyceride in Example 1. Otherwise, the procedure was the same as in Example 1, and 194.5 g of water-absorbing resin particles were obtained.

[0153] (Example 4)

[0154] 0.276 g of tristearate tetraglyceride, 0.460 g of tristearate hexaglyceride, and 0.368 g of pentastearate decaglyceride were used instead of 0.737 g of tristearate tetraglyceride and 0.737 g of tristearate hexaglyceride in Example 1. Otherwise, the procedure was the same as in Example 1, and 215.5 g of water-absorbing resin particles were obtained.

[0155] (Example 5)

[0156] Prepare a round-bottomed cylindrical flask with an inner diameter of 11 cm and a volume of 2 L, equipped with a reflux condenser, a dropping funnel, a nitrogen inlet tube, and a stirrer (with two sections of four inclined blades with a blade diameter of 5 cm). Add 293 g of n-heptane (hydrocarbon dispersion medium) and 0.276 g of maleic anhydride-modified ethylene-propylene copolymer (polymer dispersant, Mitsui Chemicals, HI-WAX 1105A) to the flask. While stirring, heat to 80 °C to dissolve the copolymer, then cool to 55 °C.

[0157] Next, 92.0 g of an 80.5% by mass aqueous solution of acrylic acid (acrylic acid: 1.03 mol) was added to a 500 mL Erlenmeyer flask. Then, while cooling from the outside, 102.78 g of a 30% by mass aqueous solution of sodium hydroxide was added dropwise to achieve 75 mol% neutralization. After this, 0.0736 g (0.272 mmol) of potassium persulfate (a water-soluble free radical polymerization initiator), 0.0558 g of tetra(ethylene glycol) diacrylate (an internal crosslinking agent), and 44.62 g of deionized water were added and dissolved to prepare the first-stage aqueous solution.

[0158] The aqueous solution from the first stage was added to the separable flask. After stirring for 10 minutes, 0.276 g of tristearate tetraglyceride, 0.460 g of tristearate hexaglyceride, and 0.368 g of pentastearate decaglyceride were dissolved in 6.62 g of n-heptane by heating, thereby obtaining a dispersion stabilizer solution. 7.724 g of the obtained dispersion stabilizer solution was added to the separable flask, and the system was stirred at 400 rpm while the system was fully purged with nitrogen. Then, the separable flask was immersed in a 70°C water bath to heat the reaction solution and carry out the first stage of polymerization for 60 minutes, thereby obtaining the first stage slurry.

[0159] Next, 128.8 g of an 80.5% by mass aqueous solution of acrylic acid (acrylic acid: 1.44 mol) was added to another 500 mL Erlenmeyer flask. Then, while cooling from the outside, 143.89 g of a 30% by mass aqueous solution of sodium hydroxide was added dropwise to achieve 75 mol% neutralization. After that, 0.1030 g (0.3810 mmol) of potassium persulfate, 0.0447 g of tetra(ethylene glycol) diacrylate (internal crosslinking agent), and 16.75 g of deionized water were added and dissolved to prepare the aqueous solution for the second stage.

[0160] While stirring the first-stage slurry at 1000 rpm, the system in the separable flask was cooled to 20°C. Then, all the aqueous solution from the second stage was added to the first-stage slurry for the polymerization process. After purging the system with nitrogen for 30 minutes, the flask was immersed again in a 70°C water bath for 60 minutes to obtain the second-stage slurry.

[0161] Following the second-stage polymerization, the slurry was heated in an oil bath at 125°C. Azeotropic distillation of n-heptane and water was performed, with 248g of water being discharged from the system while the n-heptane was refluxed. The n-heptane was then evaporated and dried to obtain a dried product. This dried product was passed through an 850μm sieve, yielding 205.4g of approximately spherical aggregates of absorbent resin particles.

[0162] (Comparative Example 1)

[0163] 1.289 g of tristearate was used instead of 0.737 g of tristearate and 0.737 g of tristearate hexaglyceride in Example 1. Otherwise, the procedure was the same as in Example 1, and 103.1 g of water-absorbing resin particles were obtained.

[0164] Evaluation of Water-Absorbent Resin Particles

[0165] (Median particle size)

[0166] A water-absorbing resin composition was prepared by mixing 0.5 g of amorphous silica (manufactured by Oriental Silicas Corporation, Tokusil NP-S) as a lubricant into 100 g of water-absorbing resin particles. Seven JIS standard sieves (mesh sizes of 600 μm, 500 μm, 425 μm, 300 μm, 250 μm, 150 μm, and 75 μm) were used in this determination. The water-absorbing resin composition was placed on top of a sieve consisting of the selected JIS standard sieves and a receiving tray, and vibrated for 20 minutes using a Ro-Tap type vibrator. The mass of the water-absorbing resin composition remaining on each sieve was then calculated as a percentage of the total mass, starting with the smallest particle size. The relationship between the sieve mesh size and the cumulative percentage of the mass remaining on the sieve was plotted on logarithmic probability paper. By connecting the plots on the probability paper with straight lines, the particle size corresponding to 50% of the cumulative mass percentage was taken as the median particle size. The results are shown in Table 3.

[0167] (Uniformity)

[0168] In the above median particle size determination, the particle size (A1) and the particle size (A2) corresponding to 10% mass percentage of the cumulative mass under sieve were calculated, and the uniformity was calculated using the following formula. The results are shown in Table 3.

[0169] Uniformity = A2 / A1

[0170] That is, when the particle size distribution is narrow, the uniformity is close to 1, and if the particle size distribution becomes wider, the uniformity is greater than 1.

[0171] (Precipitation temperature of the dispersion stabilizer)

[0172] The precipitation temperature of the dispersion stabilizer was determined as follows: a mixed solution obtained by dissolving the dispersion stabilizer in a solvent through heating was cooled, and the turbidity of the mixed solution at each temperature was measured. Specifically, 100 g of n-heptane (hydrocarbon dispersion medium) and 0.46 g of dispersion stabilizer were added to a gai flask. The solution was heated to 80°C with stirring to obtain a mixed solution containing the dispersion stabilizer. Then, the mixed solution was cooled, and the turbidity was measured every 1°C using an integrating sphere turbidimeter SEP-PT-706D (manufactured by Nitto Seiko Analytech Co., Ltd., 10 mm path length) using a standard curve prepared with a turbidity standard solution (kaolin 1000°C). The precipitation temperature of the dispersion stabilizer was defined as the temperature at which the turbidity increased by more than 10 ppm compared to the turbidity at a temperature 1°C higher. The results are shown in Table 1.

[0173] <Yellow coloring test (determination of yellowness) before and after heating at 200℃ for 2 hours>

[0174] 2.0 g of absorbent resin particles were evenly placed into a glass measuring container with an inner diameter of 3 cm. The yellowness of the absorbent resin particles was measured using a colorimeter (Color Meter ZE 6000, manufactured by Nippon Denshoku Kogyo Co., Ltd.) that had been calibrated for the tristimulus values ​​(X, Y, Z) using a standard white plate. Based on the obtained X, Y, Z (tristimulus values) of the absorbent resin particles, the yellowness was calculated using the following formula and was taken as the yellowness before heating. The results are shown in Table 3.

[0175] Yellowness = 100 (1.28X - 1.06Z) / Y

[0176] In addition, the yellowing test of the heated water-absorbing resin particles was conducted as follows: 2.0 g of water-absorbing resin particles were evenly placed into a glass petri dish with an inner diameter of 3 cm and a depth of 1 cm. Nitrogen gas was introduced at a flow rate of 400 mL / min from the outlet. A vacuum dryer (manufactured by AS ONE Co., Ltd., AVO-310N) preheated to 200±5°C was prepared, with a 3 cm thick layer of glass wool insulation material at the bottom. A stainless steel tank was placed on the insulation material inside the vacuum dryer, and the glass petri dish containing the aforementioned water-absorbing resin particles and a surface thermometer were placed on the stainless steel tank. Heating was started for 2 hours from the moment the surface thermometer reached 200±5°C. Then, the glass petri dish containing the water-absorbing resin particles, along with the stainless steel tank, was removed from the vacuum dryer and allowed to stand in the dryer for 30 minutes to cool to room temperature. All the absorbent resin particles from the glass petri dish were placed in a glass measuring container with an inner diameter of 3 cm. The yellowness of the absorbent resin particles was measured using a colorimeter (Color Meter ZE6000, manufactured by Nippon Denshoku Kogyo Co., Ltd.). Based on the obtained X, Y, and Z (tristimulus values) of the absorbent resin particles, the yellowness was calculated using the above formula and used as the yellowness after heating. The results are shown in Table 3.

[0177] [Table 1]

[0178]

[0179] [Table 2]

[0180]

[0181] In Table 2, the amount of dispersant stabilizer added is based on 100 parts by mass of acrylic acid, a water-soluble olefinic unsaturated monomer used in the first stage of polymerization.

[0182] [Table 3]

[0183]

Claims

1. A method for manufacturing water-absorbing resin particles, characterized in that, The process includes the step of reverse suspension polymerization of water-soluble olefinic unsaturated monomers in a hydrocarbon dispersion medium to obtain polymer particles. The reverse suspension polymerization is carried out in multiple stages, consisting of two or more phases. The method for manufacturing the water-absorbing resin particles includes a step of aggregating the polymer particles in the presence of a dispersing stabilizer. Two or more (poly)glycerol fatty acid esters with different precipitation temperatures when preparing a 0.46% by mass heptane solution are used as the dispersion stabilizer.

2. The method for manufacturing water-absorbing resin particles according to claim 1, wherein, Among two or more of the aforementioned (poly)glycerol fatty acid esters, the difference between the precipitation temperature of the (poly)glycerol fatty acid ester with the highest precipitation temperature and the precipitation temperature of the (poly)glycerol fatty acid ester with the lowest precipitation temperature is less than 15°C.

3. The method for manufacturing water-absorbing resin particles according to claim 1 or 2, wherein, At least one of the (poly)glycerol fatty acid esters is tristearate hexaglyceride. The amount of the tristearate hexaglyceride used is in the range of 0.2 parts by mass or more and 4.0 parts by mass or less, relative to 100 parts by mass of the water-soluble olefinic unsaturated monomer used in the first stage of polymerization.

4. The method for manufacturing water-absorbing resin particles according to claim 3, wherein, The dispersant stabilizer comprises at least one of the following: the tristearate hexaglyceride, and a (poly)glycerol fatty acid ester with a higher precipitation temperature than the tristearate hexaglyceride and a (poly)glycerol fatty acid ester with a lower precipitation temperature than the tristearate hexaglyceride. The amounts of the (poly)glycerol fatty acid esters with a precipitation temperature higher than that of the tristearate hexaglyceride and the (poly)glycerol fatty acid esters with a precipitation temperature lower than that of the tristearate hexaglyceride are respectively in the range of more than 0 parts by mass and less than 3.0 parts by mass relative to 1 part by mass of the tristearate hexaglyceride.

5. The method for manufacturing water-absorbing resin particles according to claim 4, wherein, The dispersion stabilizer comprises the tristearate hexaglyceride, a (poly)glycerol fatty acid ester with a higher precipitation temperature than the tristearate hexaglyceride, and a (poly)glycerol fatty acid ester with a lower precipitation temperature than the tristearate hexaglyceride. The amount of the (poly)glycerol fatty acid ester with a precipitation temperature higher than that of the tristearate hexaglyceride and the amount of the (poly)glycerol fatty acid ester with a precipitation temperature lower than that of the tristearate hexaglyceride used are more than 0 parts by weight and less than 2.0 parts by weight relative to 1 part by weight of the tristearate hexaglyceride.

6. A water-absorbing resin particle, characterized in that, Polymer particles containing water-soluble olefinically unsaturated monomers as monomer units. The absorbent resin particles have a particle size distribution uniformity of less than 2.5 and contain (poly)glycerol fatty acid esters near their surface.

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