Water-absorbing resin

Abstract

Provided are a water-absorbent resin that can reduce the amount of backflow of absorbed liquid even when the water-absorbent resin is used in a small content in an absorbent body, and an absorbent article that uses an absorbent body containing the water-absorbent resin. The water-absorbent resin is a cross-linked polymer of a water-soluble ethylenically unsaturated monomer, and has a dry-up index of 1.85 or more, as shown in the following formula (1). Here, the overall absorption capacity term α and the water absorption rate term β are calculated by the following equations (2) and (3).

Description

[Technical Field]

[0001] The present disclosure relates to a water-absorbent resin and a method for producing the same. More specifically, the present disclosure relates to a water-absorbent resin constituting an absorbent body suitable for use in absorbent articles for hygiene material applications such as sanitary products and disposable diapers, and a method for producing the same. [Background technology]

[0002] Absorbent articles for hygiene material applications, such as sanitary napkins and disposable diapers, usually comprise an absorbent body whose main constituent units are hydrophilic fibers and a water-absorbent resin. Known examples of water-absorbent resins include hydrolyzates of starch-acrylonitrile graft copolymers, neutralized starch-acrylic acid graft copolymers, saponified vinyl acetate-acrylic acid ester copolymers, and partially neutralized polyacrylic acids.

[0003] Generally, when the content of a water-absorbent resin in an absorbent body is low, the texture (softness) of the absorbent body is excellent, but the absorption performance, as typified by the amount of return, is unsatisfactory. Therefore, increasing the content of the water-absorbent resin tends to improve the absorption performance after absorbing body fluids, but the texture of the absorbent body is impaired. Furthermore, it is difficult to uniformly disperse a large amount of water-absorbent resin in the absorbent body, and an improvement in performance commensurate with the amount of water-absorbent resin is not necessarily guaranteed. Thus, it is considered difficult to satisfy both the absorption performance and texture requirements in an absorbent body. Therefore, there is a demand for a water-absorbent resin that has properties that can improve the absorption performance of the absorbent body even when the content in the absorbent body is small.

[0004] In order to improve the absorption performance of absorbent articles for sanitary materials, desirable properties of water-absorbent resins include high water retention capacity and high water absorption capacity under load. Research has been conducted to obtain water-absorbent resins with such properties. For example, the following proposals have been made. That is, there are known a method for producing a water-absorbent resin in which, by carrying out reverse phase suspension polymerization of a water-soluble ethylenically unsaturated monomer in multiple stages, the particle size of the resulting water-absorbent resin is large, there is little fine powder, the particle size distribution is sharp, and the water-wetability to water is high, in addition to excellent water absorbency (see Patent Document 1); a method for producing water-absorbent resin particles in which, by carrying out reverse phase suspension polymerization of a water-soluble ethylenically unsaturated monomer to obtain primary particles having a specific median particle size, a water-soluble ethylenically unsaturated monomer in a second stage is added to carry out a reverse phase suspension polymerization reaction to obtain secondary particles having a specific median particle size (see Patent Document 2); and a water-absorbent resin formed by polymerizing a water-soluble ethylenically unsaturated monomer by a reverse phase suspension polymerization method, and further aggregating, by a reverse phase suspension polymerization method, primary particles obtained by polymerizing the water-soluble ethylenically unsaturated monomer, the primary particles have a median particle size of 100 to 250 μm, and the water-retention capacity of the water-absorbent resin in physiological saline is 30 g / g or less (see Patent Document 3). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 3-227301 [Patent Document 2] International Publication No. 2007 / 123188 [Patent Document 3] International Publication No. 2012 / 023433 Summary of the Invention [Problem to be solved by the invention]

[0006] A main object of the present disclosure is to provide a water-absorbent resin that can reduce the amount of absorbed liquid returning from an absorbent body even when the absorbent body has a low content of the water-absorbent resin. [Means for solving the problem]

[0007] The present inventors have found that by using a water-absorbent resin that satisfies a specific index, the amount of absorbed liquid returning from the absorbent may be reduced even in an absorbent having a low content of water-absorbent resin, and have made further improvements.

[0008] The present disclosure includes, for example, the subject matter described in the following sections:

[0009] Item 1. A water-absorbent resin which is a cross-linked polymer of a water-soluble ethylenically unsaturated monomer, and which has a dry-up index of 1.85 or more as shown in the following formula (1).

[0010]

number

[0011] Here, the overall absorption capacity term α and the water absorption rate term β are calculated by the following equations (2) and (3).

[0012]

number

[0013]

number

[0014] Item 2. The water-absorbent resin according to Item 1, wherein the overall absorption capacity term α is 0.95 or more.

[0015] Item 3. The water-absorbent resin according to Item 1 or 2, wherein the water absorption rate β is 1.56 or more.

[0016] Item 4. The water-absorbent resin according to any one of Items 1 to 3, wherein the difference between the physiological saline water absorption capacity and the physiological saline water retention capacity is 18 or less.

[0017] Item 5. The water-absorbent resin according to any one of Items 1 to 4, wherein the difference between the physiological saline water-absorption capacity under a load of 2.07 kPa and the physiological saline water-absorption capacity under a load of 4.82 kPa is 17 to 36.

[0018] Item 6. An absorbent material containing 5 to 50 mass % of the water-absorbent resin according to any one of items 1 to 5 or the following items A to F. Section A. A water-absorbent resin which is a crosslinked polymer of a water-soluble ethylenically unsaturated monomer, prepared by a polymerization method which satisfies at least two of the following conditions (i), (ii), and (iii): (i) The molar ratio of the water-soluble ethylenically unsaturated monomer to the internal crosslinking agent used in the first polymerization step (water-soluble ethylenically unsaturated monomer / internal crosslinking agent) is 10 × 10 3 ~15×10 3 is. (ii) The molar ratio of the water-soluble ethylenically unsaturated monomer to the internal crosslinking agent used in the second polymerization (water-soluble ethylenically unsaturated monomer / internal crosslinking agent) is 15 × 10 3 ~25×10 3 is. (iii) The molar ratio of the total amount of water-soluble ethylenically unsaturated monomers used in the polymerization for preparing the resin used in the post-crosslinking reaction to the post-crosslinking agent (water-soluble ethylenically unsaturated monomer / post-crosslinking agent) is 2.5 × 10 3 ~4.5×10 3 is. Section B. 6. The water-absorbing resin according to any one of items 1 to 5, which is prepared by a polymerization method that satisfies at least two of the following conditions (i), (ii), and (iii): (i) The molar ratio of the water-soluble ethylenically unsaturated monomer to the internal crosslinking agent used in the first polymerization step (water-soluble ethylenically unsaturated monomer / internal crosslinking agent) is 10 × 10 3 ~15×10 3 is. (ii) The molar ratio of the water-soluble ethylenically unsaturated monomer to the internal crosslinking agent used in the second polymerization (water-soluble ethylenically unsaturated monomer / internal crosslinking agent) is 15 × 10 3 ~25×10 3 is. (iii) The molar ratio of the total amount of water-soluble ethylenically unsaturated monomers used in the polymerization for preparing the resin used in the post-crosslinking reaction to the post-crosslinking agent (water-soluble ethylenically unsaturated monomer / post-crosslinking agent) is 2.5 × 10 3 ~4.5×10 3 is. Section C. Item A or B: The water-absorbing resin according to item A or B, wherein the water-soluble ethylenically unsaturated monomer is (meth)acrylic acid. Section D. The water-absorbent resin according to any one of Items A to C, which satisfies at least either (i) or (ii), and the internal cross-linking agent is at least one selected from the group consisting of (poly)ethylene glycol diglycidyl ether, (poly)propylene glycol diglycidyl ether, and (poly)glycerin diglycidyl ether. Section E. The water-absorbing resin according to any one of Items A to C, which satisfies at least (iii), and the post-crosslinking agent is at least one selected from the group consisting of (poly)ethylene glycol diglycidyl ether, (poly)propylene glycol diglycidyl ether, and (poly)glycerin diglycidyl ether. Section F. The water absorbent resin according to any one of Items A to E, wherein the polymerization method that satisfies at least two of the conditions (i), (ii), and (iii) is reversed-phase suspension polymerization (preferably one-stage or two-stage reversed-phase suspension polymerization, more preferably two-stage reversed-phase suspension polymerization). Section G. The water-absorbing resin according to any one of items A to F, wherein the polymerization method satisfies all of the conditions (i), (ii), and (iii). [Effects of the Invention]

[0019] Provided are a water-absorbent resin that can reduce the amount of backflow even when the water-absorbent resin is used in an absorbent body at a low content, and a method for producing the same. [Brief explanation of the drawings]

[0020] [Figure 1]FIG. 1 is a schematic diagram showing the configuration of an apparatus X for measuring the saline water absorption capacity of a water-absorbent resin under a load of 2.07 kPa. [Figure 2] FIG. 1 is a schematic diagram showing the configuration of an apparatus Y for measuring the saline water absorption capacity of a water-absorbent resin under a load of 4.82 kPa. DETAILED DESCRIPTION OF THE INVENTION

[0021] Hereinafter, each embodiment included in the present disclosure will be described in more detail. The present disclosure preferably includes a specific water-absorbing resin and a manufacturing method thereof, but is not limited thereto, and the present disclosure includes all that is disclosed in the present specification and that can be recognized by a person skilled in the art.

[0022] 1.Water absorbent resin The water-absorbent resin encompassed by the present disclosure is a water-absorbent resin constituted by a polymer of a water-soluble ethylenically unsaturated monomer, and has a dry-up index of 1.85 or more, which is expressed by the product of the overall absorption capacity term α and the water absorption rate term β shown in the following formula (1). This water-absorbent resin may be referred to as "the water-absorbent resin of the present disclosure."

[0023]

number

[0024] The water absorbent resin of the present disclosure preferably has a dry-up index of 1.9 to 5.0. The lower limit of this numerical range may be, for example, 2.0, 2.1, 2.2, 2.3, 2.4, 2.5, 2.6, 2.7, 2.8, or 2.9. The upper limit of this numerical range may be, for example, 4.9, 4.8, 4.7, 4.6, 4.5, 4.4, 4.3, 4.2, 4.1, 4.0, 3.9, 3.8, 3.7, 3.6, 3.5, 3.4, 3.3, 3.2, 3.1, or 3.0. The numerical range is more preferably, for example, 2.0 to 4.0, and even more preferably 2.1 to 3.0.

[0025] The overall absorption capacity term α of the water-absorbent resin is a value calculated by the following formula (2).

[0026]

number

[0027] Here, in this specification, "water absorption capacity of physiological saline solution" is a value obtained by stirring 500 g of physiological saline solution at 600 r / min while stirring 2.0 g of a water-absorbent resin for 60 minutes, filtering the resultant solution using a standard sieve with an opening of 75 μm, leaving the resultant solution to stand for 30 minutes with the sieve tilted at an angle of about 30 degrees with respect to the horizontal, and then measuring the mass of the swollen gel.

[0028] In this specification, the "water retention capacity of physiological saline" is a value determined by stirring 500 g of physiological saline at 600 r / min while stirring 2.0 g of a water-absorbent resin for 30 minutes, pouring the mixture into a cotton bag (membrane No. 60), dehydrating the cotton bag for 1 minute using a dehydrator set to a centrifugal force of 167 G, and measuring the mass of the swollen gel after dehydration.

[0029] In this specification, "water absorption capacity of physiological saline solution under a load of 2.07 kPa" is determined by making the water absorbent resin absorb water in a state where a load of 2.07 kPa is uniformly applied by a weight to 0.1 g of a water absorbent resin that is uniformly spread on a cylinder having an inner diameter of 2.0 cm and equipped with a 200-mesh nylon mesh, and measuring the amount of physiological saline solution 60 minutes after the start of water absorption. For this measurement, a measuring device X whose schematic configuration is shown in Fig. 1 can be suitably used.

[0030] As mentioned above, in this specification, a clear distinction is made between the simple expression "saline water absorption capacity" and the explicit expression "saline water absorption capacity under a load of 2.07 kPa."

[0031] In this specification, "saline solution water absorption capacity under a load of 4.82 kPa" is determined by causing the water absorbent resin to absorb water in a state in which a load of 4.82 kPa is uniformly applied by a weight to 0.9 g of a water absorbent resin placed in a support cylinder having an inner diameter of 60 mm and fitted with a 400-mesh wire netting, and measuring the amount of saline solution after 60 minutes have elapsed since the start of water absorption. For this measurement, a measuring apparatus Y whose schematic configuration is shown in Fig. 2 can be suitably used.

[0032] As mentioned above, in this specification, a clear distinction is made between the simple expression "saline water absorption capacity" and the explicit expression "saline water absorption capacity under a load of 4.82 kPa."

[0033] The water-absorbent resin of the present disclosure preferably has an overall absorption capacity term α of 0.95 or more, more preferably 1.00 to 4.00, and even more preferably 1.05 to 2.00. The upper limit may be 4.00, 3.00, 2.00, 1.80, 1.65, or 1.55.

[0034] In the water-absorbent resin of the present disclosure, the difference between the water-absorption capacity of physiological saline solution and the water-retention capacity of physiological saline solution is preferably 18 or less, more preferably 17 or less, and even more preferably 16 or less. In addition, the difference between the water-absorption capacity of physiological saline solution and the water-retention capacity of physiological saline solution is preferably 5 or more, more preferably 8 or more, and even more preferably 10 or more.

[0035] The difference between the physiological saline water absorption capacity of the water-absorbent resin according to the present disclosure under a load of 2.07 kPa and the physiological saline water absorption capacity under a load of 4.82 kPa is preferably 17-36, more preferably 17-33, and even more preferably 17-30.

[0036] The water absorption rate term β of the water-absorbent resin is expressed as the ratio of the dynamic water absorption rate to the static water absorption rate, and is calculated by the following formula (3).

[0037]

number

[0038] Here, the dynamic water absorption rate is determined by stirring 50 g of physiological saline at a temperature of 25°C with a magnetic stirrer bar of 8 mmφ×30 mm to generate a vortex at a rotation speed of 600 r / min, adding 2.0 g of a water-absorbent resin all at once, and measuring the time from the addition of the water-absorbent resin to the time when the vortex on the liquid surface converges. More specifically, it is a value measured by the method described in the examples described later.

[0039] The static water absorption rate can be determined by measuring the amount of physiological saline solution absorbed by 0.1 g of water-absorbent resin, which is uniformly spread on a cylinder with an inner diameter of 2.0 cm and fitted with a 200-mesh nylon mesh, without applying a load by a weight, and measuring the time (seconds) required for 1 g of the water-absorbent resin to absorb 25 g of physiological saline solution. More specifically, this is a value measured by the method described in the Examples below. For this measurement, a measuring device X whose schematic configuration is shown in Figure 1 can be suitably used.

[0040] The water-absorbent resin of the present disclosure has a water absorption rate term β of preferably 1.56 or more, more preferably 1.60 or more, and even more preferably 1.65 or more. The upper limit is not particularly limited, but is preferably 3 or less, and may be 2.95, 2.9, 2.85, 2.8, 2.75, or 2.7 or less.

[0041] Furthermore, when used in hygiene materials such as sanitary products and disposable diapers, from the viewpoint of reducing the feeling of a foreign body when worn, the water-absorbing resin according to the present disclosure preferably has a median particle size of 200 to 600 μm, more preferably 250 to 550 μm, and even more preferably 300 to 500 μm.

[0042] The obtained water absorbent resin may be blended with additives according to the purpose. Examples of such additives include inorganic powders, surfactants, oxidizing agents, reducing agents, metal chelating agents, radical chain inhibitors, antioxidants, antibacterial agents, deodorants, etc. For example, the fluidity of the water absorbent resin can be improved by adding 0.05 to 5 parts by mass of amorphous silica as inorganic powder to 100 parts by mass of the water absorbent resin.

[0043] 2. Manufacturing method of water-absorbent resin Examples of methods for obtaining the water absorbent resin of the present disclosure include reversed-phase suspension polymerization, aqueous solution polymerization, etc. Hereinafter, the water absorbent resin of the present disclosure will be described in more detail with reference to the reversed-phase suspension polymerization as an example of the production method thereof.

[0044] <2-1. Polymerization process> The polymerization can be carried out by a method known in the field of water absorbent resin preparation or a method that can be derived from a known method. Among them, reversed-phase suspension polymerization is preferred. In the reversed-phase suspension polymerization method, polymerization is carried out by stirring and mixing an aqueous solution of a water-soluble ethylenically unsaturated monomer containing a radical polymerization initiator and, if necessary, a crosslinking agent (internal crosslinking agent) in a hydrocarbon dispersion medium in the presence of a dispersion stabilizer, and heating the mixture.

[0045] The polymerization reaction may be carried out in one stage, or in two or more stages. In the case of a multi-stage polymerization, the number of stages is preferably two or three from the viewpoint of increasing productivity. When carrying out multi-stage polymerization, after carrying out the first stage polymerization by the method described below, an aqueous solution of a water-soluble ethylenically unsaturated monomer may be added to and mixed with the reaction mixture obtained in the first stage polymerization reaction, and the second stage polymerization may be carried out by the same method as the first stage. Furthermore, multi-stage polymerization can also be carried out by repeating the same operation. Furthermore, when carrying out two or more stage polymerizations, the same polymerization method or different polymerization methods may be used, and it is more preferable to use the same polymerization method. In either polymerization, it is more preferable to use reverse phase suspension polymerization.

[0046] In the polymerization in each stage from the second stage onwards, in addition to the water-soluble ethylenically unsaturated monomer, a radical polymerization initiator, an internal crosslinking agent and the like can be added within the range of the molar ratio of each component to the water-soluble ethylenically unsaturated monomer described below, based on the amount of the water-soluble ethylenically unsaturated monomer added during the polymerization in each stage from the second stage onwards, to carry out the polymerization.

[0047] [Water-soluble ethylenically unsaturated monomers] Examples of water-soluble ethylenically unsaturated monomers include (meth)acrylic acid (herein, "acry" and "methacry" are collectively referred to as "(meth)acry", the same applies below) and salts thereof; 2-(meth)acrylamido-2-methylpropanesulfonic acid and salts thereof; nonionic monomers such as (meth)acrylamide, N,N-dimethyl(meth)acrylamide, 2-hydroxyethyl(meth)acrylate, N-methylol(meth)acrylamide, and polyethylene glycol mono(meth)acrylate; and amino group-containing unsaturated monomers such as N,N-diethylaminoethyl(meth)acrylate, N,N-diethylaminopropyl(meth)acrylate, and diethylaminopropyl(meth)acrylamide, as well as quaternized products thereof. These water-soluble ethylenically unsaturated monomers may be used alone or in combination of two or more. Among these, (meth)acrylic acid and its salts, (meth)acrylamide, and N,N-dimethylacrylamide are preferred from the viewpoint of industrial availability, and (meth)acrylic acid and its salts are more preferred. Among these, for example, acrylic acid and its salts are widely used as raw materials for water-absorbent resins, and these acrylic acids and their salts can also be used by copolymerizing them with the other water-soluble ethylenically unsaturated monomers described above. In this case, it is preferred that acrylic acid and its salts are used as the main water-soluble ethylenically unsaturated monomer in an amount of 70 to 100 mol% based on the total water-soluble ethylenically unsaturated monomers.

[0048] The water-soluble ethylenically unsaturated monomer may be used in the form of an aqueous solution in order to improve dispersibility in a hydrocarbon dispersion medium during reversed-phase suspension polymerization. The concentration of the water-soluble ethylenically unsaturated monomer in such an aqueous solution is usually 20% by mass to a saturated concentration or less, but from the viewpoint of improving the water absorption performance of the obtained water absorbent resin while ensuring productivity, the concentration of the water-soluble ethylenically unsaturated monomer is preferably 20 to 50% by mass, more preferably 22 to 45% by mass, and even more preferably 24 to 36% by mass.

[0049] When the water-soluble ethylenically unsaturated monomer has an acid group, such as (meth)acrylic acid or 2-(meth)acrylamido-2-methylpropanesulfonic acid, the acid group may be neutralized in advance with an alkaline neutralizing agent, if necessary. Examples of such alkaline neutralizing agents include alkali metal salts such as sodium hydroxide, sodium carbonate, sodium bicarbonate, potassium hydroxide, and potassium carbonate; ammonia, etc. In particular, these alkaline neutralizing agents may be used in the form of an aqueous solution to simplify the neutralization procedure.

[0050] The alkaline neutralizing agent may be used alone or in combination of two or more kinds. Regarding the degree of neutralization of the water-soluble ethylenically unsaturated monomer by the alkaline neutralizing agent, from the viewpoint of increasing the osmotic pressure of the obtained water absorbent resin to thereby improve the water absorption performance, and preventing problems such as safety caused by the presence of an excess alkaline neutralizing agent, the degree of neutralization of all acid groups possessed by the water-soluble ethylenically unsaturated monomer is preferably 40 to 90 mol%, more preferably 70 to 88 mol%, even more preferably 75 to 85 mol%, and still more preferably 77 to 80 mol%.

[0051] [Hydrocarbon dispersion medium] Examples of hydrocarbon dispersion media include aliphatic hydrocarbons having 6 to 8 carbon atoms, such as n-hexane, n-heptane, 2-methylhexane, 3-methylhexane, 2,3-dimethylpentane, 3-ethylpentane, and n-octane; alicyclic hydrocarbons, such as cyclohexane, methylcyclohexane, cyclopentane, methylcyclopentane, trans-1,2-dimethylcyclopentane, cis-1,3-dimethylcyclopentane, and trans-1,3-dimethylcyclopentane; and aromatic hydrocarbons, such as benzene, toluene, and xylene. These hydrocarbon dispersion media may be used alone or in combination of two or more. Among these hydrocarbon dispersion media, n-hexane, n-heptane, and cyclohexane are preferred because of their industrial availability, stable quality, and low cost. Examples of mixtures of the above hydrocarbon dispersion media include commercially available Exxol Heptane (manufactured by ExxonMobil Corporation; containing 75 to 85% by mass of heptane and its isomeric hydrocarbons), and such commercially available products can also be used.

[0052] The amount of the hydrocarbon dispersion medium used is usually preferably 80 to 1500 parts by mass, more preferably 120 to 1200 parts by mass, per 100 parts by mass of the water-soluble ethylenically unsaturated monomer in the first stage, from the viewpoint of uniformly dispersing the aqueous solution of the water-soluble ethylenically unsaturated monomer and facilitating control of the polymerization temperature.

[0053] [Dispersion stabilizer] A surfactant can be used as the dispersion stabilizer, and examples thereof 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, and polyoxyethylene alkylamines. Among these, sorbitan fatty acid esters, polyglycerin fatty acid esters, and sucrose fatty acid esters are preferred from the viewpoint of dispersion stability of the aqueous monomer solution. These surfactants may be used alone or in combination of two or more.

[0054] The amount of the surfactant used is preferably 0.05 to 30 parts by mass, more preferably 0.1 to 20 parts by mass, relative to 100 parts by mass of the water-soluble ethylenically unsaturated monomer used for polymerization, from the viewpoint of maintaining a good dispersion state of the aqueous monomer solution in the hydrocarbon dispersion medium and obtaining a dispersing effect commensurate with the amount used.

[0055] Furthermore, a polymeric dispersant may be used in combination with the surfactant as a dispersion stabilizer. Examples of polymeric dispersants that can be used 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. These polymeric dispersants may be used alone or in combination of two or more.

[0056] The amount of polymeric dispersant used is preferably 0.05 to 30 parts by mass, more preferably 0.1 to 20 parts by mass, per 100 parts by mass of the water-soluble ethylenically unsaturated monomer used for polymerization, from the viewpoint of maintaining a good dispersion state of the aqueous monomer solution in the hydrocarbon dispersion medium and obtaining a dispersing effect commensurate with the amount used.

[0057] The timing of addition of the surfactant used as a dispersion stabilizer may be either before or after the addition of the aqueous monomer solution, as long as it is before the start of the polymerization reaction. In particular, from the viewpoint of being able to reduce the amount of hydrocarbon dispersion medium remaining in the obtained water absorbent resin, it is preferable to disperse the aqueous monomer solution, and then further disperse the surfactant, and then perform polymerization. Furthermore, the timing of addition of the polymeric dispersant used in combination with the surfactant as a dispersion stabilizer may be either before or after the addition of the aqueous monomer solution, but from the viewpoint of dispersion stability of the aqueous monomer solution and reducing the amount of hydrocarbon dispersion medium remaining in the water absorbent resin, it is preferable to add it before dispersing the aqueous monomer solution. That is, it is more preferable to disperse the aqueous monomer solution in the hydrocarbon dispersion medium in which the polymeric dispersant has been dispersed, and then further disperse the surfactant, and then perform polymerization.

[0058] [Radical polymerization initiator] Examples of the radical polymerization initiator 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; 2,2'-azobis(2-amidinopropane) dihydrochloride, 2,2'-azobis[2-(N-phenylamidino) 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 availability and ease of handling. These radical polymerization initiators may be used alone or in combination of two or more.

[0059] From the viewpoint of avoiding a rapid polymerization reaction and shortening the polymerization reaction time, the amount of the radical polymerization initiator used is usually preferably 0.005 to 1 mol, more preferably 0.01 to 0.5 mol, even more preferably 0.0125 to 0.1 mol, and even more preferably 0.015 to 0.05 mol, relative to 100 mol of the water-soluble ethylenically unsaturated monomer used in the polymerization.

[0060] The reaction temperature of the polymerization reaction varies depending on the radical polymerization initiator used, but from the viewpoints of rapidly progressing the polymerization to increase productivity and more smoothly removing the heat of polymerization, it is usually preferably 20 to 110° C., more preferably 40 to 90° C. The reaction time is usually preferably about 0.1 to 4 hours.

[0061] [Internal crosslinking agent] When polymerizing the water-soluble ethylenically unsaturated monomer, a crosslinking agent may be used as needed. When the polymerization is multistage, a crosslinking agent may be used in all stages, or there may be stages in which no crosslinking agent is used. In addition, in the case of multistage polymerization, the type of crosslinking agent used in each stage may be the same or different, and it is preferable that the same agent is used. Examples of such crosslinking agents (hereinafter referred to as "internal crosslinking agents") include (poly)ethylene glycol ("(poly)" refers to the presence or absence of the prefix "poly"). the same applies hereinafter)], unsaturated polyesters obtained by reacting polyols such as diols and triols, such as (poly)propylene glycol, 1,4-butanediol, trimethylolpropane, and (poly)glycerin, with unsaturated acids, such as (meth)acrylic acid, maleic acid, and fumaric acid; bisacrylamides such as N,N-methylenebisacrylamide; di- or tri(meth)acrylic acid esters obtained by reacting polyepoxides with (meth)acrylic acid; di(meth)acrylic acid carbamyl esters obtained by reacting polyisocyanates, such as tolylene diisocyanate and hexamethylene diisocyanate, with hydroxyethyl (meth)acrylate; allylated precipitates Examples of the internal crosslinking agent include compounds having two or more polymerizable unsaturated groups, such as cellulose ether, allylated cellulose, diallyl phthalate, N,N',N''-triallyl isocyanurate, and divinylbenzene; diglycidyl compounds, such as (poly)ethylene glycol diglycidyl ether, (poly)propylene glycol diglycidyl ether, and (poly)glycerin diglycidyl ether; polyglycidyl compounds, such as triglycidyl compounds; epihalohydrin compounds, such as epichlorohydrin, epibromohydrin, and α-methylepichlorohydrin; and compounds having two or more reactive functional groups, such as isocyanate compounds, such as 2,4-tolylene diisocyanate and hexamethylene diisocyanate. Among these internal crosslinking agents, preferred are polyglycidyl compounds, more preferred are diglycidyl ether compounds, and particularly preferred are (poly)ethylene glycol diglycidyl ether. These internal crosslinking agents may be used alone or in combination of two or more. The internal crosslinking agent is preferably used by adding it to the above-mentioned aqueous monomer solution.

[0062] When an internal crosslinking agent is used, the amount thereof is preferably 0.00001 to 1 mol, more preferably 0.0001 to 0.5 mol, relative to 100 mol of the water-soluble ethylenically unsaturated monomer, in order to sufficiently enhance the water absorption performance of the resulting water absorbent resin.

[0063] [Other ingredients] In one example of the method for producing this water-absorbing resin, when performing reverse phase suspension polymerization, other components may be added to the aqueous solution of the water-soluble ethylenically unsaturated monomer. As other components, various additives such as a thickener and a chain transfer agent can be added.

[0064] (thickener) When carrying out the polymerization reaction, a thickener may be added to the aqueous solution of the water-soluble ethylenically unsaturated monomer. By adjusting the viscosity of the aqueous solution by adding the thickener in this way, it is possible to control the median particle size of the obtained water absorbent resin.

[0065] 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 median particle size of the resulting particles tends to be.

[0066] <2-2. Post-crosslinking step> After the polymerization step, a hydrous gel-like substance (a water-absorbent resin obtained by polymerization and containing water) is subjected to a post-crosslinking reaction to increase the crosslink density in the vicinity of the surface of the water-absorbent resin, thereby making it possible to improve various properties such as water absorption capacity under load. In the production of the water-absorbent resin of the present disclosure, post-crosslinking may be performed using a post-crosslinking agent.

[0067] Examples of post-crosslinking agents include those capable of reacting with the carboxyl groups of the water-absorbent resin. Representative examples of post-crosslinking agents include polyols such as (poly)ethylene glycol, (poly)propylene glycol, 1,4-butanediol, trimethylolpropane, and (poly)glycerin; diglycidyl ether compounds such as (poly)ethylene glycol diglycidyl ether, (poly)propylene glycol diglycidyl ether, and (poly)glycerin diglycidyl ether; epihalohydrin compounds such as epichlorohydrin, epibromohydrin, and α-methylepichlorohydrin; and compounds having two or more reactive functional groups, such as isocyanate compounds such as 2,4-tolylene diisocyanate and hexamethylene diisocyanate. Among these, (poly)ethylene glycol diglycidyl ether is preferred. These may be used alone or in combination of two or more. The post-crosslinking agent may also be used by dissolving it in water, an organic solvent, or the like.

[0068] The amount of the post-crosslinking agent varies depending on the type of the post-crosslinking agent and cannot be determined in general, but if the amount of the post-crosslinking agent used is small, the crosslinking density of the surface layer of the water-absorbent resin tends to be insufficient, resulting in a decrease in the water absorption capacity under load, while if the amount of the post-crosslinking agent used is large, the water retention capacity of the water-absorbent resin tends to decrease. Therefore, the amount of the post-crosslinking agent used is usually 0.00001 to 0.01 mol, preferably 0.00005 to 0.005 mol, and more preferably 0.0001 to 0.002 mol, relative to 1 mol of the total amount of the water-soluble ethylenically unsaturated monomers used in the polymerization.

[0069] The post-crosslinking agent is preferably added to a system containing 1 to 400 parts by mass of water, more preferably 5 to 200 parts by mass, and even more preferably 10 to 100 parts by mass of water, relative to 100 parts by mass of the total amount of water-soluble ethylenically unsaturated monomers. 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.

[0070] The reaction temperature in the post-crosslinking reaction is preferably 50 to 250° C., more preferably 60 to 180° C. The reaction time for post-crosslinking cannot be determined in general because it varies depending on the reaction temperature, the type and amount of the post-crosslinking agent, etc., but is usually 1 to 300 minutes, preferably 5 to 200 minutes.

[0071] <2-3. Drying process> The method may include a drying step in which water, hydrocarbon dispersion medium, etc. are removed by distillation by applying external energy such as heat to the hydrous gel-like material. For example, when dehydrating the hydrous gel-like material after reversed-phase suspension polymerization, the system in which the hydrous gel-like material is dispersed in the hydrocarbon dispersion medium is heated, and the water and hydrocarbon dispersion medium are temporarily removed from the system by azeotropic distillation. In this case, continuous azeotropic distillation is possible by recycling only the removed hydrocarbon dispersion medium back into the system. By adopting such a method, the temperature in the system can be maintained below the azeotropic temperature, which is preferable from the viewpoint of preventing resin degradation. Next, the water and hydrocarbon dispersion medium are removed by distillation to obtain water-absorbent resin particles.

[0072] In the drying step, the drying treatment may be carried out under normal pressure or under reduced pressure. Furthermore, from the viewpoint of improving drying efficiency, the drying treatment may be carried out under a gas stream 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. When the drying treatment is carried out under reduced pressure, the drying temperature is preferably 40 to 160°C, more preferably 50 to 110°C.

[0073] A particularly preferred embodiment of the water-absorbing resin of the present disclosure is a water-absorbing resin produced through a polymerization reaction of a water-soluble ethylenically unsaturated monomer, which satisfies at least two of the following three conditions (i) to (iii). More preferred is a water-absorbing resin which satisfies all three conditions.

[0074] (i) The molar ratio of the water-soluble ethylenically unsaturated monomer to the internal crosslinking agent used in the first polymerization step (water-soluble ethylenically unsaturated monomer / internal crosslinking agent) is 10 × 10 3 ~15×10 3 is.

[0075] (ii) The molar ratio of the water-soluble ethylenically unsaturated monomer to the internal crosslinking agent used in the second polymerization (water-soluble ethylenically unsaturated monomer / internal crosslinking agent) is 15 × 10 3 ~25×10 3 is.

[0076] (iii) The molar ratio of the total amount of water-soluble ethylenically unsaturated monomers used in the polymerization for preparing the resin used in the post-crosslinking reaction to the post-crosslinking agent (water-soluble ethylenically unsaturated monomer / post-crosslinking agent) is 2.5 × 10 3 ~4.5×10 3 is.

[0077] By preparing a water-absorbent resin so as to satisfy at least two or three of these conditions, it may be possible to prepare a water-absorbent resin that exhibits the above-mentioned preferred overall absorption capacity term α and water absorption rate term β, and furthermore a preferred dry-up index. In other words, the water-absorbent resin of the present disclosure may be preferably prepared using these conditions (i) to (iii) as indicators.

[0078] In the condition (i), the molar ratio of the water-soluble ethylenically unsaturated monomer to the internal crosslinking agent used in the first polymerization step (water-soluble ethylenically unsaturated monomer / internal crosslinking agent) is more preferably 10×10 3 ~14×10 3 and more preferably 10×10 3 ~13×10 3 and even more preferably 11×10 3 ~12×10 3 is.

[0079] Under the condition (ii), the molar ratio of the water-soluble ethylenically unsaturated monomer to the internal crosslinking agent used in the second-stage polymerization (water-soluble ethylenically unsaturated monomer / internal crosslinking agent) is more preferably 17.5 × 10 3 ~24×103 and more preferably 20×10 3 ~23×10 3 and even more preferably 21×10 3 ~22×10 3 is.

[0080] Under the condition (iii), the molar ratio of the total amount of the water-soluble ethylenically unsaturated monomers used in the polymerization for preparing the resin to be used in the post-crosslinking reaction to the post-crosslinking agent (water-soluble ethylenically unsaturated monomer / post-crosslinking agent) is more preferably 3 × 10 3 ~4×10 3 and more preferably 3 × 10 3 ~3.5×10 3 is.

[0081] The present disclosure also preferably encompasses a method for producing a water-absorbent resin that satisfies these conditions.

[0082] 3. Absorbent materials and absorbent articles The water-absorbent resin of the present disclosure is used, for example, in conjunction with hydrophilic fibers to form an absorbent body. Such an absorbent body is suitable for use in absorbent articles used in hygiene products such as sanitary products and disposable diapers.

[0083] The absorbent body is composed of, for example, a water-absorbent resin and hydrophilic fibers. Examples of the absorbent body configuration include a mixed dispersion obtained by mixing the water-absorbent resin and hydrophilic fibers to a uniform composition, a sandwich structure in which the water-absorbent resin is sandwiched between layers of hydrophilic fibers, and a structure in which the water-absorbent resin and hydrophilic fibers are wrapped in tissue or the like. The absorbent body may also contain other components, such as adhesive binders such as heat-fusible synthetic fibers, hot-melt adhesives, and adhesive emulsions, in order to improve the shape retention of the absorbent body.

[0084] The content of the water-absorbent resin of the present disclosure in the absorbent body is preferably 5 to 50 mass %, more preferably 10 to 45 mass %, and even more preferably 15 to 40 mass %, relative to the total mass of the absorbent body.

[0085] 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.

[0086] The absorbent article can be made by holding the absorbent body 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 contacts the body, and the liquid-impermeable sheet is arranged on the opposite side that contacts the body.

[0087] 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.

[0088] In this specification, the term "comprising" includes "consisting essentially of" and "consisting of." Furthermore, the present disclosure encompasses all arbitrary combinations of the constituent elements described in this specification.

[0089] Furthermore, the various characteristics (properties, structures, functions, etc.) described in each embodiment of the present disclosure above may be combined in any way to specify the subject matter encompassed by the present disclosure, i.e., the present disclosure encompasses all subject matter consisting of any combination of the combinable characteristics described herein. [Example]

[0090] The present disclosure will be described in more detail below with reference to examples, but the present disclosure is not limited to the following examples.

[0091] <Evaluation test method> [Evaluation test of water-absorbent resin] The water-absorbent resins obtained in the following Examples and Comparative Examples were evaluated by the following various tests. Each test method will be explained below.

[0092] 1) Absorbency of physiological saline solution 500 g of 0.9% by mass sodium chloride aqueous solution (physiological saline) was weighed into a 500 mL beaker, and 2.0 g of water-absorbent resin was dispersed therein while stirring at 600 r / min, taking care not to generate lumps. Stirring was continued for 60 minutes to allow the water-absorbent resin to fully swell. The mass (Wa) (g) of a 200 mm diameter standard sieve with 75 μm openings was then measured in advance, and the contents of the beaker were filtered using this. The sieve was tilted at an angle of approximately 30 degrees relative to the horizontal, and the sieve was left to stand for 30 minutes to filter out excess water. The mass (Wb) of the sieve containing the swollen gel was measured, and the physiological saline water absorption capacity was calculated using the following formula:

[0093] Saline water absorption capacity (g / g) = [Wb-Wa] (g) / mass of water-absorbent resin (g)

[0094] 2) Saline water retention capacity 500 g of 0.9% by mass sodium chloride aqueous solution (physiological saline) was weighed into a 500 mL beaker and dispersed with 2.0 g of water-absorbent resin while stirring at 600 rpm, taking care not to cause lumps. Stirring was continued for 30 minutes to allow the water-absorbent resin to fully swell. The swollen gel and saline solution in the beaker were then poured into a cotton bag (membrane broadcloth No. 60, 100 mm wide x 200 mm long). The top of the cotton bag was tied with a rubber band and dehydrated for 1 minute using a dehydrator (Kokusan Centrifuge Co., Ltd., product number H-122) set to a centrifugal force of 167 G. The mass (Wc) of the cotton bag containing the swollen gel after dehydration was measured. The same procedure was repeated without the addition of the water-absorbent resin. The empty mass (Wd) of the wet cotton bag was measured, and the saline water retention capacity was calculated using the following equation:

[0095] Water retention capacity of physiological saline solution (g / g) = [Wc-Wd] (g) / mass of water-absorbent resin (g)

[0096] 3) Absorption capacity of saline solution under a load of 2.07 kPa Using a measuring device X whose schematic configuration is shown in FIG. 1, the saline water absorption capacity of the water-absorbent resin was measured under a load of 2.07 kPa.

[0097] The measuring device X shown in FIG. 1 comprises a burette unit 1, a conduit 2, a measuring table 3, and a measuring unit 4 placed on the measuring table 3. The burette unit 1 has a rubber stopper 14 at the top of the burette 10, an air inlet tube 11 and a cock 12 connected to the bottom, and a cock 13 at the top of the air inlet tube 11. A conduit 2 is attached from the burette unit 1 to the measuring table 3, and the conduit 2 has a diameter of 6 mm. A hole with a diameter of 2 mm is drilled in the center of the measuring table 3, and the conduit 2 is connected to it. The measuring unit 4 comprises a cylinder 40, a nylon mesh 41 attached to the bottom of the cylinder 40, and a weight 42. The inner diameter of the cylinder 40 is 2.0 cm. The nylon mesh 41 is formed to 200 mesh (openings of 75 μm). A predetermined amount of water-absorbent resin 5 is uniformly spread on the nylon mesh 41. The weight 42 has a diameter of 1.9 cm and a mass of 59.8 g. The weight 42 is placed on the water-absorbent resin 5 so that a load of 2.07 kPa can be applied to the water-absorbent resin 5 uniformly.

[0098] In the measuring device X configured as above, first, the cocks 12 and 13 of the burette part 1 are closed, a 0.9% by mass sodium chloride aqueous solution (physiological saline) adjusted to 25°C is poured into the top of the burette 10, the top of the burette is plugged with a rubber stopper 14, and then the cocks 12 and 13 of the burette part 1 are opened. Next, the height of the measuring table 3 is adjusted so that the tip of the conduit 2 in the center of the measuring table 3 and the air inlet of the air inlet tube 11 are at the same height.

[0099] Meanwhile, 0.10 g of water-absorbent resin 5 is evenly spread on nylon mesh 41 of cylinder 40, and weight 42 is placed on this water-absorbent resin 5. The measuring part 4 is placed so that its center coincides with the conduit opening at the center of measuring table 3.

[0100] From the time when the water-absorbent resin 5 started to absorb water, the amount of decrease in the saline solution in the burette 10 (the amount of saline solution absorbed by the water-absorbent resin 5) We (mL) was continuously read. The saline solution absorption capacity of the water-absorbent resin 5 under load 60 minutes after the start of water absorption was calculated by the following formula using the specific gravity of saline solution of 1.0 (g / mL).

[0101] Absorbency of saline solution under a load of 2.07 kPa (g / g) = We (mL) × 1.0 (g / mL) / mass of absorbent resin (g)

[0102] 4) Absorption capacity of saline solution under a load of 4.82 kPa Using a measuring device Y whose schematic configuration is shown in FIG. 2, the saline water absorption capacity of the water-absorbent resin was measured under a load of 4.82 kPa.

[0103] The measuring device Y shown in Fig. 2 comprises a measuring section mainly consisting of a weight 90, a support cylinder 91, and a piston 92, and a liquid supply section mainly consisting of a Petri dish 6 and a glass filter 7. The measuring section has a 400 mesh (38 µm mesh) stainless steel wire mesh 93 adhered to one side (bottom surface) of a cylindrical plastic support cylinder 91 with an inner diameter of 60 mm, and the cylinder is provided with a piston 92 with a diameter slightly smaller than 60 mm, which does not create a gap on the wall surface with the support cylinder 91 and does not hinder up and down movement, and a weight 90 on top of the piston 92, and the piston 92 and weight 90 are configured to be able to uniformly apply a load of 4.82 kPa to the water absorbent resin 5. Using such a measuring section, 0.90 g of water absorbent resin 5 was uniformly sprayed on the wire mesh of the support cylinder 91, and then the piston 92 and weight 90 were placed, and the mass Wf (g) of the measuring section was measured. The mass Wf (g) is the total of the masses of the support cylinder 91, the water-absorbent resin 5, the piston 92, and the weight 90.

[0104] Separately, a 90 mm diameter, 5 mm thick glass filter 7 (manufactured by Shibata Scientific Co., Ltd.) was placed inside a 150 mm diameter Petri dish 6, and a 0.9 mass % sodium chloride aqueous solution (physiological saline) adjusted to 25±1°C was added to the Petri dish 6 so that it was flush with the upper surface of the glass filter 7. A sheet of filter paper 8 (Advantec, No. 2) with a diameter of 9 cm was then placed on top of the glass filter 7 to prepare a liquid supply section. It was confirmed that the entire surface of the filter paper 8 was wet, and any excess liquid was appropriately absorbed with tissue.

[0105] The measuring unit was placed on the liquid supply unit, and physiological saline was absorbed into the water-absorbent resin 5 under a load of 4.82 kPa. When the liquid level in the liquid supply unit became lower than the upper surface of the glass filter 7, physiological saline was added appropriately to keep the liquid level constant. 60 minutes after placing the measuring unit on the liquid supply unit, the measuring unit was removed from the liquid supply unit, and the mass Wg (g) was measured.

[0106] The saline water absorption capacity (g / g) under a load of 4.82 kPa was calculated using the following formula.

[0107] Absorbency of saline solution under a load of 4.82 kPa (g / g) = [Wg-Wf] (g) / mass of absorbent resin (g)

[0108] 5) Dynamic water absorption rate The dynamic water absorption rate was measured in a room regulated at 25°C ± 1°C. 50 ± 0.1 g of physiological saline was weighed into a 100 mL beaker and adjusted to a temperature of 25 ± 0.2°C in a thermostatic water bath. The solution was then stirred with a magnetic stir bar (8 mmφ × 30 mm, no ring) to generate a vortex at a rotation speed of 600 r / min. 2.0 ± 0.002 g of water-absorbent resin was added to the physiological saline solution all at once, and the time (seconds) from the addition of the water-absorbent resin to the point at which the vortex on the liquid surface converged was measured. This was converted to minutes and used as the dynamic water absorption time (minutes).

[0109] The dynamic water absorption rate (1 / min) was calculated as follows:

[0110] Dynamic water absorption rate (1 / min) = (amount of saline solution ÷ amount of absorbent resin) ÷ (dynamic water absorption time (min))

[0111] 6) Static water absorption rate 1, the static water absorption rate of the water-absorbent resin was measured in the same manner as in the measurement of the water-absorbency capacity of physiological saline under a load of 2.07 kPa described above, except that the measurement was carried out under no load without using the weight 42. In this measurement method, as described above, the weight 42 was not used, and therefore the water absorption rate of the water-absorbent resin 5 under no load (in other words, under no load) was measured.

[0112] Similarly to the above, the amount of decrease in the saline solution in the burette 10 (the amount of saline solution absorbed by the water absorbent resin 5) Wi (mL) was continuously read from the time when the water absorbent resin 5 started to absorb water under no load, and the amount of saline solution absorbed by the water absorbent resin 5 up to a specific time was calculated by the following formula. The specific gravity of the saline solution was set to 1.0 g / mL.

[0113] Amount of saline solution absorbed by the water-absorbent resin 5 (g / g) = Wi (mL) × 1.0 (g / mL) / mass of the water-absorbent resin (g)

[0114] When measuring the amount of saline solution (g / g) absorbed by the water-absorbent resin 5 in this manner, the time (seconds) required for 1 g of the water-absorbent resin 5 to absorb 25 g of saline solution was measured with a stopwatch as the elapsed time from the point when the water-absorbent resin 5 started to absorb water, and the time converted into minutes was taken as the static water-absorption time (minutes). The static water-absorption rate (1 / min) was calculated by the following formula.

[0115] Static water absorption rate (1 / min) = 25 (g / g) ÷ static water absorption time (min)

[0116] 7) Median particle size (particle size distribution) The JIS standard sieves were arranged in the following order from top to bottom: a sieve with an opening of 850 μm, a sieve with an opening of 600 μm, a sieve with an opening of 500 μm, a sieve with an opening of 425 μm, a sieve with an opening of 300 μm, a sieve with an opening of 250 μm, a sieve with an opening of 150 μm, and a tray.

[0117] 50 g of water-absorbent resin was placed on the top sieve of the combination, and the resin was shaken for 20 minutes using a Rotap shaker (manufactured by Iida Seisakusho Co., Ltd.) for classification. After classification, the mass of the water-absorbent resin remaining on each sieve was calculated as a mass percentage relative to the total amount, and the particle size distribution was determined. By integrating the particles on the sieves in descending order of particle size in this particle size distribution, the relationship between the sieve openings and the integrated value of the mass percentage of the water-absorbent resin remaining on the sieves was plotted on logarithmic probability paper. By connecting the plots on the probability paper with a straight line, the particle size corresponding to an integrated mass percentage of 50% by mass was determined to be the median particle size.

[0118] [Evaluation test of absorbent materials using water-absorbent resin] 8) Backflow amount (a) Preparation of test solution 2.5g of calcium chloride dihydrate, 5.0g of magnesium chloride hexahydrate, 20g of potassium chloride, 20g of sodium sulfate, 8.5g of ammonium dihydrogen phosphate, 1.5g of diammonium hydrogen phosphate, and an appropriate amount of distilled water were placed in a 10L container and completely dissolved. The remaining distilled water was added to dilute the mixture, and a small amount of Blue No. 1 was added to color the mixture to prepare a test solution.

[0119] (b) Fabrication of absorbent bodies and absorbent articles 6.6 g of water-absorbent resin and 10 g of crushed pulp (Leonia Rayflock) were mixed uniformly by air-pressing to prepare an absorbent core measuring 40 cm x 12 cm. Next, a 16 g / m2 sheet of the same size as the absorbent core was prepared. 2 The two tissue papers were placed on top and bottom of the absorbent core, and a load of 196 kPa was applied to the entire absorbent core for 30 seconds to press it, thereby producing an absorbent core with a water-absorbent resin content of 40% by mass. 2 An air-through type porous liquid-permeable polyethylene sheet was placed on the bottom of the absorbent body, and a liquid-impermeable polyethylene sheet of the same size and basis weight was placed on the bottom of the absorbent body, sandwiching the absorbent body to prepare an absorbent article for study.

[0120] (c) Measurement of backflow amount First, the absorbent article was placed on a horizontal platform. A liquid-injection cylinder with an opening of 3 cm inner diameter was placed in the center of the absorbent article, and 50 mL of test liquid was poured into the cylinder all at once. After the test liquid had penetrated, the cylinder was removed, and the absorbent article was stored as is. The same procedure was repeated 30 and 60 minutes after the first test liquid injection, using the cylinder in the same position as the first injection. Sixty minutes after the third test liquid injection, a 10 cm square piece of filter paper, the mass of which (Wk (g), approximately 70 g) had been measured, was placed at the test liquid injection position of the absorbent article, aligned with the center of the absorbent article, and a 5 kg weight with a 10 cm x 10 cm base was placed on top of it. After 5 minutes of loading, the mass of the filter paper after the test (Wl (g)) was measured, and the increase in mass was calculated using the following formula, which was used as the backflow amount (g).

[0121] Backflow amount (g) = Wl - Wk

[0122] <Examples and Comparative Examples> [Example 1] A 2-L round-bottom cylindrical separable flask with an inner diameter of 110 mm 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 50 mm was prepared. 300 g of n-heptane was added to the flask as a hydrocarbon dispersion medium, and 0.62 g of maleic anhydride-modified ethylene-propylene copolymer (Hiwax 1105A, Mitsui Chemicals, Inc.) was added as a polymeric dispersant. The mixture was heated and dissolved with stirring, and then cooled to 50°C.

[0123] Separately, 78 g (0.87 mol) of an 80% by weight aqueous solution of acrylic acid was placed in a 500 mL Erlenmeyer flask, and while cooling externally, 120.6 g of a 22.4% by weight aqueous solution of sodium hydroxide was added dropwise to neutralize the solution. Then, 1.170 g of hydroxyethyl cellulose (Sumitomo Seika Chemicals Co., Ltd., HEC AW-15F) as a thickener, 0.055 g (0.203 mmol) of potassium persulfate as a radical polymerization initiator, and 59.0 g of ion-exchanged water were added and dissolved to prepare a monomer aqueous solution with a monomer concentration of 30% by weight. (Note that the monomer concentration refers to the mass ratio of water-soluble ethylenically unsaturated monomers and their salts to the total amount of the aqueous monomer solution, and this is expressed in the same way hereinafter in this specification.)

[0124] The aqueous monomer solution prepared as described above was then added to a separable flask and stirred for 10 minutes. 6.2 g of a surfactant solution prepared by dissolving 0.62 g of a sucrose stearate ester with an HLB of 3 (Ryoto Sugar Ester S-370, Mitsubishi Chemical Foods Corporation) as a surfactant in 5.62 g of n-heptane under heating was then added. The system was thoroughly purged with nitrogen while stirring, and the flask was then immersed in a 70°C water bath to raise the temperature. Polymerization was carried out for 60 minutes, yielding a first-stage polymerization slurry.

[0125] After the first polymerization stage, the reaction mixture was heated in an oil bath at 125°C. While refluxing n-heptane, 155.3 g of water was removed from the system by azeotropic distillation of n-heptane and water. Then, 3.90 g (0.45 mmol) of a 2% by weight aqueous solution of ethylene glycol diglycidyl ether was added as a post-crosslinking agent, and the mixture was maintained at 80°C for 2 hours. The n-heptane was then evaporated and dried to obtain a dried product. This dried product was mixed with 0.2% by weight of amorphous silica (Evonik Degussa Japan Co., Ltd., Carplex #80), and the mixture was passed through a 1000 μm mesh sieve to obtain 82.1 g of a spherical water-absorbent resin with a median particle size of 350 μm. This water-absorbent resin and absorbents using it were evaluated according to the various test methods described above.

[0126] [Example 2] A 2-L round-bottom cylindrical separable flask with an inner diameter of 110 mm 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 50 mm was prepared. 300 g of n-heptane was added to the flask as a hydrocarbon dispersion medium, and 0.52 g of maleic anhydride-modified ethylene-propylene copolymer (Hiwax 1105A, Mitsui Chemicals, Inc.) was added as a polymeric dispersant. The mixture was heated and dissolved with stirring, and then cooled to 50°C.

[0127] Separately, 65 g (0.72 mol) of an 80% by mass acrylic acid aqueous solution was placed in a 500 mL Erlenmeyer flask, and while cooling from the outside, 100.5 g of a 22.4% by mass sodium hydroxide aqueous solution was added dropwise to neutralize the solution. After that, 0.065 g of hydroxyethyl cellulose (Sumitomo Seika Chemicals Co., Ltd., HEC AW-15F) as a thickener, 0.046 g (0.170 mmol) of potassium persulfate as a radical polymerization initiator, 0.011 g (0.063 mmol) of ethylene glycol diglycidyl ether as an internal crosslinking agent, and 92.0 g of ion-exchanged water were added and dissolved to prepare a monomer aqueous solution with a monomer concentration of 25% by mass.

[0128] The aqueous monomer solution prepared as described above was then added to a separable flask and stirred for 10 minutes. 5.2 g of a surfactant solution prepared by dissolving 0.52 g of a sucrose stearate ester with an HLB of 3 (Ryoto Sugar Ester S-370, Mitsubishi Chemical Foods Corporation) as a surfactant in 4.68 g of n-heptane under heating was then added. The system was thoroughly purged with nitrogen while stirring, and the flask was then immersed in a 70°C water bath to raise the temperature. Polymerization was carried out for 60 minutes, yielding a first-stage polymerization slurry.

[0129] Meanwhile, 94.3 g (1.05 mol) of an 80 mass% aqueous acrylic acid solution was placed in another 500 mL Erlenmeyer flask, and while cooling from the outside, 116.2 g of a 28.1 mass% aqueous sodium hydroxide solution was added dropwise to neutralize it, and then 0.066 g (0.244 mmol) of potassium persulfate as a radical polymerization initiator, 0.009 g (0.052 mmol) of ethylene glycol diglycidyl ether as an internal crosslinking agent, and 1.9 g of ion-exchanged water were added and dissolved to prepare a second-stage aqueous monomer solution with a monomer concentration of 44 mass%.

[0130] After the separable flask system was cooled to 27°C, the entire amount of the second-stage aqueous monomer solution was added to the first-stage polymerization slurry, and the system was thoroughly purged with nitrogen. After that, the flask was again immersed in a 70°C water bath to raise the temperature, and the second-stage polymerization was carried out for 30 minutes.

[0131] After the second polymerization stage, the reaction mixture was heated in an oil bath at 125°C. While refluxing n-heptane, 259.3 g of water was removed from the system by azeotropic distillation of n-heptane and water. Then, 4.78 g (0.55 mmol) of a 2% by weight aqueous solution of ethylene glycol diglycidyl ether was added as a post-crosslinking agent, and the mixture was maintained at 80°C for 2 hours. The n-heptane was then evaporated and dried to obtain a dried product. This dried product was mixed with 0.2% by weight of amorphous silica (Evonik Degussa Japan Co., Ltd., Carplex #80), and the mixture was passed through a 1000 μm mesh sieve to obtain 173.3 g of a water-absorbent resin in the form of spherical particle aggregates with a median particle diameter of 332 μm. This water-absorbent resin and absorbents using it were evaluated according to the various test methods described above.

[0132] [Example 3] A 2-L round-bottom cylindrical separable flask with an inner diameter of 110 mm 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 50 mm was prepared. 300 g of n-heptane was added to the flask as a hydrocarbon dispersion medium, and 0.62 g of maleic anhydride-modified ethylene-propylene copolymer (Hiwax 1105A, Mitsui Chemicals, Inc.) was added as a polymeric dispersant. The mixture was heated and dissolved with stirring, and then cooled to 50°C.

[0133] Separately, 78 g (0.87 mol) of an 80% by mass acrylic acid aqueous solution was placed in a 500 mL Erlenmeyer flask, and while cooling from the outside, 120.6 g of a 22.4% by mass sodium hydroxide aqueous solution was added dropwise to neutralize the solution. After that, 0.078 g of hydroxyethyl cellulose (Sumitomo Seika Chemicals Co., Ltd., HEC AW-15F) as a thickener, 0.055 g (0.203 mmol) of potassium persulfate as a radical polymerization initiator, 0.013 g (0.075 mmol) of ethylene glycol diglycidyl ether as an internal crosslinking agent, and 59.0 g of ion-exchanged water were added and dissolved to prepare a monomer aqueous solution with a monomer concentration of 30% by mass.

[0134] The aqueous monomer solution prepared as described above was then added to a separable flask and stirred for 10 minutes. 6.2 g of a surfactant solution prepared by dissolving 0.62 g of a sucrose stearate ester with an HLB of 3 (Ryoto Sugar Ester S-370, Mitsubishi Chemical Foods Corporation) as a surfactant in 5.62 g of n-heptane under heating was then added. The system was thoroughly purged with nitrogen while stirring, and the flask was then immersed in a 70°C water bath to raise the temperature. Polymerization was carried out for 60 minutes, yielding a first-stage polymerization slurry.

[0135] Separately, 110.8 g (1.23 mol) of an 80% by mass acrylic acid aqueous solution was placed in a 500 mL Erlenmeyer flask, and while cooling from the outside, 136.5 g of a 28.1% by mass sodium hydroxide aqueous solution was added dropwise to neutralize it, followed by the addition and dissolution of 0.078 g (0.289 mmol) of potassium persulfate as a radical polymerization initiator and 0.010 g (0.057 mmol) of ethylene glycol diglycidyl ether as an internal crosslinking agent to prepare a second-stage monomer aqueous solution with a monomer concentration of 44% by mass. After preparing the second-stage monomer aqueous solution, the second-stage polymerization was carried out in the same manner as in Example 2.

[0136] After the second polymerization stage, the reaction mixture was heated in an oil bath at 125°C. 263.8 g of water was extracted from the system by azeotropic distillation of n-heptane and water while refluxing the n-heptane. 5.66 g (0.65 mmol) of a 2% by weight aqueous solution of ethylene glycol diglycidyl ether was added as a post-crosslinking agent, and the mixture was maintained at 80°C for 2 hours. The n-heptane was then evaporated and dried to obtain a dried product. This dried product was mixed with 0.2% by weight of amorphous silica (Evonik Degussa Japan Co., Ltd., Carplex #80), and the mixture was passed through a 1000 μm mesh sieve to obtain 205.5 g of a water-absorbent resin in the form of spherical particle aggregates with a median particle size of 350 μm. This water-absorbent resin and absorbents using it were evaluated according to the various test methods described above.

[0137] [Example 4] A 2-L round-bottom cylindrical separable flask with an inner diameter of 110 mm 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 50 mm was prepared. 300 g of n-heptane was added to the flask as a hydrocarbon dispersion medium, and 0.74 g of maleic anhydride-modified ethylene-propylene copolymer (Hiwax 1105A, Mitsui Chemicals, Inc.) was added as a polymeric dispersant. The mixture was heated and dissolved with stirring, and then cooled to 50°C.

[0138] Separately, 92 g (1.02 mol) of an 80% by mass acrylic acid aqueous solution was placed in a 500 mL Erlenmeyer flask, and while cooling from the outside, 142.3 g of a 22.4% by mass sodium hydroxide aqueous solution was added dropwise to neutralize the solution. After that, 0.092 g of hydroxyethyl cellulose (Sumitomo Seika Chemicals Co., Ltd., HEC AW-15F) as a thickener, 0.064 g (0.237 mmol) of potassium persulfate as a radical polymerization initiator, 0.016 g (0.092 mmol) of ethylene glycol diglycidyl ether as an internal crosslinking agent, and 26.0 g of ion-exchanged water were added and dissolved to prepare a monomer aqueous solution with a monomer concentration of 35% by mass.

[0139] The aqueous monomer solution prepared as described above was then added to a separable flask and stirred for 10 minutes. 7.3 g of a surfactant solution prepared by dissolving 0.74 g of a sucrose stearate ester with an HLB of 3 (Ryoto Sugar Ester S-370, Mitsubishi Chemical Foods Corporation) as a surfactant in 6.62 g of n-heptane under heating was then added. The system was thoroughly purged with nitrogen while stirring, and the flask was then immersed in a 70°C water bath to raise the temperature. Polymerization was carried out for 60 minutes, yielding a first-stage polymerization slurry.

[0140] Meanwhile, 128.8 g (1.43 mol) of an 80 mass% aqueous acrylic acid solution was placed in another 500 mL Erlenmeyer flask, and while cooling from the outside, 158.8 g of a 28.1 mass% aqueous sodium hydroxide solution was added dropwise to neutralize it, and then 0.090 g (0.333 mmol) of potassium persulfate as a radical polymerization initiator and 0.012 g (0.069 mmol) of ethylene glycol diglycidyl ether as an internal crosslinking agent were added and dissolved to prepare a second-stage aqueous monomer solution with a monomer concentration of 44 mass%.

[0141] After the separable flask system was cooled to 27°C, the entire amount of the second-stage aqueous monomer solution was added to the first-stage polymerization slurry, and the system was thoroughly purged with nitrogen. After that, the flask was again immersed in a 70°C water bath to raise the temperature, and the second-stage polymerization was carried out for 30 minutes.

[0142] After the second polymerization stage, the reaction mixture was heated in an oil bath at 125°C. 283.6 g of water was extracted from the system by azeotropic distillation of n-heptane and water while refluxing n-heptane. 6.62 g (0.76 mmol) of a 2% by weight aqueous solution of ethylene glycol diglycidyl ether was added as a post-crosslinking agent, and the mixture was maintained at 80°C for 2 hours. The n-heptane was then evaporated and dried to obtain a dried product. This dried product was mixed with 0.2% by weight of amorphous silica (Evonik Degussa Japan Co., Ltd., Carplex #80), and the mixture was passed through a 1000 μm mesh sieve to obtain 230.7 g of a water-absorbent resin in the form of spherical particle aggregates with a median particle diameter of 361 μm. This water-absorbent resin and absorbents using it were evaluated according to the various test methods described above.

[0143] [Comparative Example 1] A 2-L round-bottomed cylindrical separable flask with an inner diameter of 110 mm and equipped with a reflux condenser, a dropping funnel, a nitrogen gas inlet tube, and a stirrer with a two-stage stirring blade consisting of four 50-mm-diameter inclined paddle blades was prepared. 300 g of n-heptane was placed in the flask as a hydrocarbon dispersion medium, and 0.74 g of maleic anhydride-modified ethylene-propylene copolymer (Hiwax 1105A, Mitsui Chemicals, Inc.) was added as a polymeric dispersant. The mixture was heated to dissolve with stirring and then cooled to 50°C.

[0144] Separately, 92 g (1.02 mol) of an 80% by mass acrylic acid aqueous solution was placed in a 500 mL Erlenmeyer flask, and while cooling from the outside, 142.5 g of a 21.5% by mass sodium hydroxide aqueous solution was added dropwise to neutralize the solution. After that, 0.092 g of hydroxyethyl cellulose (Sumitomo Seika Chemicals Co., Ltd., HEC AW-15F) as a thickener, 0.064 g (0.237 mmol) of potassium persulfate as a radical polymerization initiator, 0.010 g (0.057 mmol) of ethylene glycol diglycidyl ether as an internal crosslinking agent, and 10.0 g of ion-exchanged water were added and dissolved to prepare a monomer aqueous solution with a monomer concentration of 37% by mass.

[0145] The aqueous monomer solution prepared as described above was then added to a separable flask and stirred for 10 minutes. 7.4 g of a surfactant solution prepared by dissolving 0.74 g of a sucrose stearate ester with an HLB of 3 (Ryoto Sugar Ester S-370, Mitsubishi Chemical Foods Corporation) as a surfactant in 6.62 g of n-heptane under heating was then added. The system was thoroughly purged with nitrogen while stirring, and the flask was then immersed in a 70°C water bath to raise the temperature. Polymerization was carried out for 60 minutes, yielding a first-stage polymerization slurry.

[0146] On the other hand, 128.8 g (1.43 mol) of an 80 mass% aqueous acrylic acid solution was placed in another 500 mL Erlenmeyer flask, and while cooling from the outside, 142.5 g of a 27.0 mass% aqueous sodium hydroxide solution was added dropwise to carry out 75 mol% neutralization, followed by adding and dissolving 0.090 g (0.333 mmol) of potassium persulfate as a radical polymerization initiator and 0.012 g (0.069 mmol) of ethylene glycol diglycidyl ether as an internal crosslinking agent to prepare a second-stage aqueous monomer solution with a monomer concentration of 44 mass%.

[0147] After the separable flask system was cooled to 27°C, the entire amount of the second-stage aqueous monomer solution was added to the first-stage polymerization slurry, and the system was thoroughly purged with nitrogen. After that, the flask was again immersed in a 70°C water bath to raise the temperature, and the second-stage polymerization was carried out for 30 minutes.

[0148] After the second polymerization stage, the reaction mixture was heated in an oil bath at 125°C. 266.2 g of water was extracted from the system by azeotropic distillation of n-heptane and water while refluxing n-heptane. 4.42 g (0.51 mmol) of a 2% by weight aqueous solution of ethylene glycol diglycidyl ether was added as a post-crosslinking agent, and the mixture was maintained at 80°C for 2 hours. The n-heptane was then evaporated and dried to obtain a dried product. This dried product was mixed with 0.2% by weight of amorphous silica (Evonik Degussa Japan Co., Ltd., Carplex #80), and the mixture was passed through a 1000 μm mesh sieve to obtain 229.7 g of a water-absorbent resin in the form of spherical particle aggregates with a median particle size of 345 μm. This water-absorbent resin and absorbents using it were evaluated according to the various test methods described above.

[0149] Comparative Example 2 A 2-L round-bottomed cylindrical separable flask with an inner diameter of 110 mm and equipped with a reflux condenser, a dropping funnel, a nitrogen gas inlet tube, and a stirrer with a two-stage stirring blade consisting of four 50-mm-diameter inclined paddle blades was prepared. 300 g of n-heptane was placed in the flask as a hydrocarbon dispersion medium, and 0.74 g of maleic anhydride-modified ethylene-propylene copolymer (Hiwax 1105A, Mitsui Chemicals, Inc.) was added as a polymeric dispersant. The mixture was heated to dissolve with stirring and then cooled to 50°C.

[0150] Separately, 92 g (1.02 mol) of an 80% by mass acrylic acid aqueous solution was placed in a 500 mL Erlenmeyer flask, and while cooling from the outside, 142.5 g of a 21.5% by mass sodium hydroxide aqueous solution was added dropwise to neutralize the solution. After that, 0.092 g of hydroxyethyl cellulose (Sumitomo Seika Chemicals Co., Ltd., HEC AW-15F) as a thickener, 0.064 g (0.237 mmol) of potassium persulfate as a radical polymerization initiator, 0.010 g (0.057 mmol) of ethylene glycol diglycidyl ether as an internal crosslinking agent, and 10.0 g of ion-exchanged water were added and dissolved to prepare a monomer aqueous solution with a monomer concentration of 37% by mass.

[0151] The aqueous monomer solution prepared as described above was then added to a separable flask and stirred for 10 minutes. 7.4 g of a surfactant solution prepared by dissolving 0.74 g of a sucrose stearate ester with an HLB of 3 (Ryoto Sugar Ester S-370, Mitsubishi Chemical Foods Corporation) as a surfactant in 6.66 g of n-heptane under heating was then added. The system was thoroughly purged with nitrogen while stirring, and the flask was then immersed in a 70°C water bath to raise the temperature. Polymerization was carried out for 60 minutes, yielding a first-stage polymerization slurry.

[0152] On the other hand, 128.8 g (1.43 mol) of an 80 mass% aqueous acrylic acid solution was placed in another 500 mL Erlenmeyer flask, and while cooling from the outside, 142.5 g of a 27.0 mass% aqueous sodium hydroxide solution was added dropwise to carry out 75 mol% neutralization, followed by adding and dissolving 0.090 g (0.333 mmol) of potassium persulfate as a radical polymerization initiator and 0.012 g (0.069 mmol) of ethylene glycol diglycidyl ether as an internal crosslinking agent to prepare a second-stage aqueous monomer solution with a monomer concentration of 44 mass%.

[0153] After the separable flask system was cooled to 27°C, the entire amount of the second-stage aqueous monomer solution was added to the first-stage polymerization slurry, and the system was thoroughly purged with nitrogen. After that, the flask was again immersed in a 70°C water bath to raise the temperature, and the second-stage polymerization was carried out for 30 minutes.

[0154] After the second polymerization stage, the reaction mixture was heated in an oil bath at 125°C. While refluxing n-heptane, 261.8 g of water was removed from the system by azeotropic distillation of n-heptane and water. Then, 4.42 g (0.51 mmol) of a 2% by weight aqueous solution of ethylene glycol diglycidyl ether was added as a post-crosslinking agent, and the mixture was maintained at 80°C for 2 hours. The n-heptane was then evaporated and dried to obtain a dried product. This dried product was mixed with 0.5% by weight of amorphous silica (Evonik Degussa Japan Co., Ltd., Carplex #80), and the mixture was passed through a 1000 μm mesh sieve to obtain 234.2 g of a water-absorbent resin in the form of spherical particle aggregates with a median particle size of 380 μm. This water-absorbent resin and absorbents using it were evaluated according to the various test methods described above.

[0155] Comparative Example 3 A 2-L round-bottomed cylindrical separable flask with an inner diameter of 110 mm and equipped with a reflux condenser, a dropping funnel, a nitrogen gas inlet tube, and a stirrer with a two-stage stirring blade consisting of four 50-mm-diameter inclined paddle blades was prepared. 300 g of n-heptane was placed in the flask as a hydrocarbon dispersion medium, and 0.74 g of maleic anhydride-modified ethylene-propylene copolymer (Hiwax 1105A, Mitsui Chemicals, Inc.) was added as a polymeric dispersant. The mixture was heated to dissolve with stirring and then cooled to 50°C.

[0156] Separately, 92 g (1.02 mol) of an 80% by mass acrylic acid aqueous solution was placed in a 500 mL Erlenmeyer flask, and while cooling from the outside, 142.5 g of a 21.5% by mass sodium hydroxide aqueous solution was added dropwise to neutralize the solution. After that, 0.092 g of hydroxyethyl cellulose (Sumitomo Seika Chemicals Co., Ltd., HEC AW-15F) as a thickener, 0.064 g (0.237 mmol) of potassium persulfate as a radical polymerization initiator, 0.010 g (0.057 mmol) of ethylene glycol diglycidyl ether as an internal crosslinking agent, and 10.0 g of ion-exchanged water were added and dissolved to prepare a monomer aqueous solution with a monomer concentration of 37% by mass.

[0157] The aqueous monomer solution prepared as described above was then added to a separable flask and stirred for 10 minutes. 7.4 g of a surfactant solution prepared by dissolving 0.74 g of a sucrose stearate ester with an HLB of 3 (Ryoto Sugar Ester S-370, Mitsubishi Chemical Foods Corporation) as a surfactant in 6.66 g of n-heptane under heating was then added. The system was thoroughly purged with nitrogen while stirring, and the flask was then immersed in a 70°C water bath to raise the temperature. Polymerization was carried out for 60 minutes, yielding a first-stage polymerization slurry.

[0158] On the other hand, 128.8 g (1.43 mol) of an 80 mass% aqueous acrylic acid solution was placed in another 500 mL Erlenmeyer flask, and while cooling from the outside, 142.5 g of a 27.0 mass% aqueous sodium hydroxide solution was added dropwise to carry out 75 mol% neutralization, followed by adding and dissolving 0.090 g (0.333 mmol) of potassium persulfate as a radical polymerization initiator and 0.012 g (0.069 mmol) of ethylene glycol diglycidyl ether as an internal crosslinking agent to prepare a second-stage aqueous monomer solution with a monomer concentration of 44 mass%.

[0159] After the separable flask system was cooled to 27°C, the entire amount of the second-stage aqueous monomer solution was added to the first-stage polymerization slurry, and the system was thoroughly purged with nitrogen. After that, the flask was again immersed in a 70°C water bath to raise the temperature, and the second-stage polymerization was carried out for 30 minutes.

[0160] After the second polymerization stage, the reaction mixture was heated in an oil bath at 125°C. 259.7 g of water was extracted from the system by azeotropic distillation of n-heptane and water while refluxing n-heptane. 4.42 g (0.51 mmol) of a 2% by weight aqueous solution of ethylene glycol diglycidyl ether was added as a post-crosslinking agent, and the mixture was maintained at 80°C for 2 hours. The n-heptane was then evaporated and dried to obtain a dried product. This dried product was mixed with 0.5% by weight of amorphous silica (Evonik Degussa Japan Co., Ltd., Carplex #80), and the mixture was passed through a 1000 μm mesh sieve to obtain 236.0 g of a water-absorbent resin in the form of spherical particle aggregates with a median particle size of 356 μm. This water-absorbent resin and absorbents using it were evaluated according to the various test methods described above.

[0161] Comparative Example 4 A 2-L round-bottomed cylindrical separable flask with an inner diameter of 110 mm and equipped with a reflux condenser, a dropping funnel, a nitrogen gas inlet tube, and a stirrer with a two-stage stirring blade consisting of four 50-mm-diameter inclined paddle blades was prepared. 300 g of n-heptane was placed in the flask as a hydrocarbon dispersion medium, and 0.74 g of maleic anhydride-modified ethylene-propylene copolymer (Hiwax 1105A, Mitsui Chemicals, Inc.) was added as a polymeric dispersant. The mixture was heated to dissolve with stirring and then cooled to 50°C.

[0162] Separately, 92 g (1.02 mol) of an 80% by mass acrylic acid aqueous solution was placed in a 500 mL Erlenmeyer flask, and while cooling from the outside, 142.5 g of a 21.5% by mass sodium hydroxide aqueous solution was added dropwise to neutralize the solution. After that, 0.092 g of hydroxyethyl cellulose (Sumitomo Seika Chemicals Co., Ltd., HEC AW-15F) as a thickener, 0.064 g (0.237 mmol) of potassium persulfate as a radical polymerization initiator, 0.018 g (0.103 mmol) of ethylene glycol diglycidyl ether as an internal crosslinking agent, and 10.0 g of ion-exchanged water were added and dissolved to prepare a monomer aqueous solution with a monomer concentration of 37% by mass.

[0163] The aqueous monomer solution prepared as described above was then added to a separable flask and stirred for 10 minutes. 7.4 g of a surfactant solution prepared by dissolving 0.74 g of a sucrose stearate ester with an HLB of 3 (Ryoto Sugar Ester S-370, Mitsubishi Chemical Foods Corporation) as a surfactant in 6.66 g of n-heptane under heating was then added. The system was thoroughly purged with nitrogen while stirring, and the flask was then immersed in a 70°C water bath to raise the temperature. Polymerization was carried out for 60 minutes, yielding a first-stage polymerization slurry.

[0164] Meanwhile, 128.8 g (1.43 mol) of an 80 mass% aqueous acrylic acid solution was placed in another 500 mL Erlenmeyer flask, and while cooling from the outside, 142.5 g of a 27.0 mass% aqueous sodium hydroxide solution was added dropwise to carry out 75 mol% neutralization, followed by the addition and dissolution of 0.090 g (0.333 mmol) of potassium persulfate as a radical polymerization initiator and 0.039 g (0.224 mmol) of ethylene glycol diglycidyl ether as an internal crosslinking agent, to prepare a second-stage aqueous monomer solution with a monomer concentration of 44 mass%.

[0165] After the separable flask system was cooled to 27°C, the entire amount of the second-stage aqueous monomer solution was added to the first-stage polymerization slurry, and the system was thoroughly purged with nitrogen. After that, the flask was again immersed in a 70°C water bath to raise the temperature, and the second-stage polymerization was carried out for 30 minutes.

[0166] After the second polymerization stage, the reaction mixture was heated in an oil bath at 125°C. 260.8 g of water was extracted from the system by azeotropic distillation of n-heptane and water while refluxing the n-heptane. 6.62 g (0.76 mmol) of a 2% by weight aqueous solution of ethylene glycol diglycidyl ether was then added as a post-crosslinking agent, and the mixture was maintained at 80°C for 2 hours. The n-heptane was then evaporated and dried to obtain a dried product. This dried product was mixed with 0.5% by weight of amorphous silica (Evonik Degussa Japan Co., Ltd., Carplex #80), and the mixture was passed through a 1000 μm mesh sieve to obtain 235.1 g of a water-absorbent resin in the form of spherical particle aggregates with a median particle size of 343 μm. This water-absorbent resin and absorbents using it were evaluated according to the various test methods described above.

[0167] <About the evaluation results> [Evaluation results of water-absorbent resin and absorbent material] The evaluation test results of the water-absorbent resin are shown in Table 1 below. In addition, the dry-up index, overall absorption capacity term α, water absorption speed term β, and evaluation results (return amount) of the water-absorbent resin are shown in Table 2 below.

[0168]

number

[0169] Here, the overall absorption capacity term α and the water absorption rate term β are calculated by the following equations (2) and (3).

[0170]

number

[0171]

number

[0172] [Table 1]

[0173] [Table 2] [Explanation of symbols]

[0174] 1 Burette section 10 Burettes 11 Air intake pipe 12 Cook 13 Cook 14 Rubber stopper 2 conduit 3 Measuring table 4 Measuring part 40 Cylinder 41 Nylon Mesh 42 weight 5 Water-absorbing resin particles 6 Petri dishes 7. Glass Filter 8 Filter paper 90 weights 91 Plastic support cylinder 92 Piston 93 Stainless steel wire mesh X Measuring device Y measuring device

Claims

1. A water-absorbent resin which is a cross-linked polymer of a water-soluble ethylenically unsaturated monomer, The water-absorbent resin has a median particle size of 300 to 500 μm, the water-absorbing resin is a polymer cross-linked product of a water-soluble ethylenically unsaturated monomer and an internal cross-linking agent, which is further cross-linked with a post-cross-linking agent, The water-soluble ethylenically unsaturated monomer contains 70 to 100 mol % of acrylic acid and its salts based on the total amount of the water-soluble ethylenically unsaturated monomers, The dry-up index represented by the following formula (1) is 2.1 to 3.0, The physiological saline water absorption capacity under a load of 2.07 kPa is 28.5 to 34.9 (g / g), The difference between the physiological saline water absorption capacity under a load of 2.07 kPa and the physiological saline water absorption capacity under a load of 4.82 kPa is 17 to 36, The overall absorption capacity term α is 1.05 to 2.00, The dynamic water absorption rate is 9.3 to 29.4, and the static water absorption rate is 4.8 to 13.9, The water absorption rate term β is 1.65 to 2.

7. Water-absorbing resin. [Equation 1] Here, the overall absorption capacity term α and the water absorption rate term β are calculated by the following equations (2) and (3). [Equation 2] [Equation 3]

2. 2. The water-absorbent resin according to claim 1, wherein the difference between the physiological saline water absorption capacity and the physiological saline water retention capacity is 18 or less.

3. The absorbent resin according to claim 1 or 2, having a saline water absorption capacity of 62 to 65 (g / g).

4. An absorbent material comprising 5 to 50 mass % of the water-absorbent resin according to any one of claims 1 to 3.

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