JP6737571B2 - Water absorbent resin and absorbent article - Google Patents

Description

The present invention relates to a water-absorbent resin and an absorbent article, and more specifically, to a water-absorbent resin that constitutes an absorber that is preferably used for sanitary materials such as paper diapers, sanitary napkins, incontinence pads, and the like. It relates to absorbent articles.

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

As such a water-absorbent resin, acrylic acid partially neutralized salt polymer cross-linked product has excellent water absorption capacity, and since the raw material acrylic acid is easily available industrially, the quality is constant. It is said to be a preferable water-absorbent resin because it can be manufactured at low cost and has many advantages such as deterioration and deterioration.

On the other hand, absorbent articles such as paper diapers, sanitary napkins, incontinence pads, etc., are mainly placed in the center and absorb and retain body fluids such as urine and menstrual blood excreted from the body. It is composed of a liquid-permeable top sheet (top sheet) arranged on the side in contact with the liquid and a liquid-impermeable back sheet (back sheet) arranged on the side opposite to the body. The absorber is composed of hydrophilic fibers such as pulp and a water absorbent resin.

In recent years, from the viewpoints of designability, convenience during carrying, efficiency during distribution, and the like, there is an increasing demand for thinner and lighter absorbent articles. Further, from the viewpoint of environmental protection, there is a growing demand for what is called an eco-friendly orientation, which uses resources effectively and avoids the use of natural materials such as trees that take a long time to grow as much as possible. Conventionally, as a method for thinning generally performed in absorbent articles, for example, reducing hydrophilic fibers such as crushed pulp of wood which is a role of fixing the water-absorbent resin in the absorber, There was a way to increase the water absorbent resin.

An absorber in which the proportion of hydrophilic fibers is low and a large amount of water absorbent resin is used is preferable for thinning from the viewpoint of reducing bulky hydrophilic fibers and retaining liquid. However, when the absorbent body containing the water-absorbent resin is subjected to a load due to deformation or pressure, such as a state in which a baby wearing a thin absorbent article is sitting, the liquid to be absorbed reverts (liquid return). May not be fully prevented. Furthermore, in the case of such an absorbent article, it may not be able to endure urination multiple times, which may cause discomfort to the wearer.

In addition, a large amount of water-absorbent resin becomes a soft gel due to the absorption of liquid, and when a load is applied to this gel, a so-called "gel blocking phenomenon" occurs, liquid diffusivity is significantly reduced, and The rate of penetration of body fluids may be slow. This "gel blocking phenomenon", especially when the absorbent body which is densely packed with water-absorbent resin absorbs the liquid, the water-absorbent resin existing near the surface layer absorbs the liquid, and a soft gel is formed near the surface layer. This is a phenomenon in which the gel becomes dense and the penetration of the liquid into the absorber is hindered, and the water-absorbent resin inside cannot absorb the liquid efficiently.

Therefore, as a means to reduce the hydrophilic fiber and prevent problems that occur when a large amount of water-absorbent resin is used, for example, a specific saline flow-inducing property, a hydrogel absorbent heavy having performance under pressure, etc. A method using a coalescence (see Patent Document 1), a method using a water-absorbent resin obtained by heating a specific water-absorbent resin precursor with a specific surface-crosslinking agent (see Patent Document 2), and the like have been proposed.

However, these methods do not always satisfy the absorption performance as an absorber using a large amount of water-absorbent resin, and there is a tendency that a problem may occur in that the liquid to be absorbed cannot be captured and liquid leakage occurs. ..

Japanese Patent Publication No. 9-510889 JP-A-8-57311

The present invention has been proposed in view of the circumstances as described above, excellent water-absorbing performance, water-absorbent resin and its water-absorbent resin that can improve the absorption performance under load when used in the absorber An object is to provide an absorbent article using an absorbent body containing a resin.

The present inventors have conducted extensive studies to solve the above-mentioned problems. As a result, if a water absorbent resin having an absorption capacity elastic index represented by the product of the storage elastic modulus of the water absorbent resin and the centrifugal retention rate is a predetermined value or more is used for the absorbent article, absorption under excellent load is achieved. It has been found to perform well. That is, the present invention provides the following.

(1) The present invention is a water-absorbent resin obtained by polymerizing a water-soluble ethylenically unsaturated monomer in the presence of an internal cross-linking agent and post-crosslinking with a post-crosslinking agent, which has a load of 4.14 kPa. The physiological saline water absorption capacity below is 16 mL/g or more, and the mass ratio of the particles of 150 to 850 μm in the ratio of the entire water absorbent resin is 85% by mass or more, and further the mass of the particles of 300 to 400 μm. The water absorbent resin has a ratio of 20% by mass or more and an absorption capacity elastic index represented by the following formula (I) of 68000 or more.
Absorption capacity elastic index=storage elastic modulus [Pa]×centrifugal retention [g/g] (I)

(2) Further, the present invention is the water absorbent resin according to the above (1), wherein tan δ measured by classifying particles of 300 to 400 μm of the water absorbent resin is 2.00×10 ⁻² or less. ..

(3) The present invention is an absorbent article comprising the absorbent body containing the water absorbent resin according to (1) or (2).

The absorbent resin according to the present invention is excellent in water absorption performance under load and can form a gel having appropriate elasticity when absorbing a liquid. Therefore, the absorbent article including the absorbent body using the water-absorbent resin, effectively suppresses the reversion of the liquid to be absorbed with time even in a state where a load is applied due to deformation or pressure, and under an excellent load. The absorption performance of can be exhibited.

It is a schematic block diagram which shows the structure of the apparatus for measuring the physiological saline water absorption capacity under a 4.14 kPa load of a water absorbent resin.

Hereinafter, the present invention will be described in detail.

<<1. Water absorbent resin ≫
The water absorbent resin according to the present invention has the following properties.

The water-absorbent resin according to the present invention has a physiological saline absorption capacity under a load of 4.14 kPa of 16 mL/g or more, and the mass ratio of particles of 150 to 850 μm in the total ratio of the water-absorbent resin is 85% by mass. That is, the mass ratio of the particles of 300 to 400 μm is 20 mass% or more, and the absorption capacity elastic index represented by the following formula (I) is 68000 or more.
Absorption capacity elastic index=storage elastic modulus [Pa]×centrifugal retention [g/g] (I)

Regarding the particle size distribution of the water absorbent resin, the mass ratio of the particles of 150 to 850 μm in the total ratio is 85 mass% or more, and more preferably 90 mass% or more. Furthermore, the mass ratio of the particles of 300 to 400 μm in the total ratio is 20 mass% or more, more preferably 25 mass% or more, and further preferably 30 mass% or more.

The particles of the water-absorbent resin may have a form in which fine particles (primary particles) are aggregated (secondary particles), in addition to a form in which each particle is a single particle. Examples of the shape of the primary particles include a substantially spherical shape, an irregular crushed shape, and a plate shape. In the case of the primary particles produced by reverse phase suspension polymerization, a substantially spherical single particle shape having a smooth surface shape such as a true sphere or an elliptic sphere may be mentioned. Is a water-absorbent resin having a high particle strength because the surface shape is smooth, the fluidity as a powder is high, and the aggregated particles are likely to be densely packed so that they are not easily broken even when shocked. Tends to become.

In addition, the water-absorbent resin according to the present invention preferably has a median particle diameter of 200 to 600 μm, more preferably 250 to 500 μm, and further preferably 300 to 400 μm.

And, as described above, the water absorbent resin according to the present invention is characterized in that the absorption capacity elastic index represented by the formula (I) is 68000 or more. It has been found that a water-absorbent resin having a higher index is more excellent in absorption performance under load of the absorbent article used therein (permeation rate, liquid reversion amount, etc.). The absorption capacity elastic index is preferably 70,000 or more, more preferably 72,000 or more, and further preferably 74,000 or more. The absorption capacity elastic index is preferably 200,000 or less, more preferably 150,000 or less, and further preferably 100,000 or less.

Here, the "storage elastic modulus" is a numerical value measured using a dynamic viscoelasticity measuring device for a swollen gel produced by swelling a 300-400 µm classified sample of a water-absorbent resin with physiological saline 50 times. Say. This storage elastic modulus is an index showing the ratio of strain to stress of gel, that is, the shape retention (hardness of deformation) of gel.

The "centrifugal retention rate" means that the water-absorbent resin sample is swollen (absorbed) with physiological saline for 60 minutes under stirring and further dehydrated for 1 minute by a centrifugal force of 167G, and then the water-absorbent resin has its own weight. This is a parameter indicating how many times the swelling ratio can be maintained.

In the water-absorbent resin according to the present invention, the absorption capacity elastic index represented by the formula (I) is 68000 or more, and the water-absorbent resin having such a property has a suitable centrifugal retention rate while maintaining a suitable level. By forming the gel having elasticity, it is possible to effectively suppress the backflow of the liquid to be absorbed with time due to deformation or pressure in the absorber using the gel. For example, when an absorbent article such as a paper diaper is formed using this absorbent resin, it is extremely advantageous for urinating a plurality of times, reducing discomfort to the wearer, and more comfortable use is possible. Further, the gel blocking phenomenon can be suppressed, and the absorbing performance of the absorbent article under a load can be improved.

Further, the water absorbent resin according to the present invention has a physiological saline water absorption capacity of 16 ml/g or more under a load of 4.14 kPa. The physiological saline absorption capacity under a load of 4.14 kPa is preferably 18 ml/g or more, more preferably 20 ml/g or more. The physiological saline water absorption capacity under a load of 4.14 kPa is preferably 50 ml/g or less, and more preferably 40 ml/g or less.

Further, in the water absorbent resin according to the present invention, as described above, tan δ measured by classifying particles of 300 to 400 μm is preferably 2.00×10 ⁻² or less. Here, the details of tan δ are described, for example, in “Viscoelasticity of Polymer” (John D. Ferry, translated by Hiroshi Sobue, translated by Jokichi Murakami, Masao Takahashi, Tokyo Kagaku Dojin, October 1964). ..

Generally, in viscoelasticity evaluation, a polymer material is represented by a model consisting of an elastic component and a viscous component, and the elastic component is a component that converts impact energy into repulsive energy, and the latter converts impact energy into dissipative energy. It is the component to be converted. In the dynamic viscoelasticity measurement by vibration strain, a complex elastic modulus G ^* =G′+iG″ (i is an imaginary unit) is physically shown. Here, G′ (storage elastic modulus) and G″ (loss elastic modulus) represent the sizes of the elastic component and the viscous component of the polymer material, respectively. Then, tan δ (loss coefficient)=G″/G′ is an index of energy lost when the material is deformed.

In the water-absorbent resin according to the present invention, the tan δ represented by “tan δ=loss elastic modulus/storage elastic modulus” is preferably 2.00×10 ⁻² or less, which means that it exhibits a high storage elastic modulus. However, it is possible to suppress the gel blocking phenomenon that occurs when a liquid is absorbed. Note that tan δ is more preferably 1.00×10 ⁻² or more.

Further, the water-absorbent resin according to the present invention preferably has a storage elastic modulus of 1000 Pa or more, more preferably 1200 Pa or more, still more preferably 1500 Pa or more. The upper limit of the storage elastic modulus is preferably 10000 Pa or less, more preferably 5000 Pa or less, further preferably 2500 Pa or less, and further preferably 2000 Pa or less.

Further, the water absorbent resin according to the present invention preferably has a centrifugal retention rate of 30 g/g or more. The centrifugal retention rate represents the degree of liquid absorption capacity of the water absorbent resin. In the water absorbent resin according to the present invention, the centrifugal retention rate is preferably 30 g/g or more, more preferably 36 g/g or more, even more preferably 38 g/g or more, even more preferably 40 g/g or more. The upper limit of the centrifugal retention rate is preferably about 60 g/g or less, more preferably 55 g/g or less, and further preferably 50 g/g or less.

The centrifugal retention rate, physiological saline absorption capacity under a load of 4.14 kPa, medium particle size (particle size distribution), storage elastic modulus, and tan δ of the water-absorbent resin described above are all described in Examples described later. It can be measured by a measuring method.

In addition, in order to give various properties to the obtained water-absorbent resin, additives depending on the purpose can be blended to obtain a water-absorbent resin composition. Examples of such an additive include an inorganic powder, a surfactant, an oxidizing agent, a reducing agent, a metal chelating agent, a radical chain inhibitor, an antioxidant, an antibacterial agent, and a deodorant. For example, by adding 0.05 to 5 parts by mass of amorphous silica as an inorganic powder to 100 parts by mass of the water absorbent resin, the fluidity of the water absorbent resin can be improved.

<<2. Manufacturing method of water absorbent resin >>
The water absorbent resin according to the present invention can be produced by polymerizing a water-soluble ethylenically unsaturated monomer in the presence of an internal crosslinking agent.

As a method for polymerizing the water-soluble ethylenically unsaturated monomer, an aqueous solution polymerization method, an emulsion polymerization method, a reverse phase suspension polymerization method, or the like, which is a typical polymerization method, is used. In the aqueous solution polymerization method, the water-soluble ethylenically unsaturated monomer aqueous solution is heated with stirring as necessary to carry out polymerization. In the reverse phase suspension polymerization method, a water-soluble ethylenically unsaturated monomer is heated in a hydrocarbon dispersion medium with stirring to perform polymerization. In the present invention, the reversed-phase suspension polymerization method is preferable from the viewpoint of enabling precise control of polymerization reaction and control of a wide range of particle diameters.

An example of a method for producing the water absorbent resin according to the present invention will be described below.

As a specific example of the method for producing a water-absorbent resin, in the method for producing a water-absorbent resin by reverse-phase suspension polymerization of a water-soluble ethylenically unsaturated monomer in a hydrocarbon dispersion medium, in the presence of an internal crosslinking agent And a step of carrying out polymerization in the presence of at least an azo compound and a peroxide, and a step of post-crosslinking a hydrogel having an internal cross-linking structure obtained by the polymerization with a post-crosslinking agent.

<Polymerization process>
[Water-soluble ethylenically unsaturated monomer]
Examples of the water-soluble ethylenically unsaturated monomer include (meth)acrylic acid (in the present specification, "acry" and "methacryl" are collectively referred to as "(meth)acry". 2-(meth)acrylamide-2-methylpropanesulfonic acid and salts thereof; (meth)acrylamide, N,N-dimethyl(meth)acrylamide, 2-hydroxyethyl(meth)acrylate, N-methylol(meth). Nonionic monomers such as acrylamide and polyethylene glycol mono(meth)acrylate; amino groups such as N,N-diethylaminoethyl (meth)acrylate, N,N-diethylaminopropyl (meth)acrylate and diethylaminopropyl (meth)acrylamide Examples thereof include unsaturated monomers and quaternary compounds thereof. Among these water-soluble ethylenically unsaturated monomers, (meth)acrylic acid or a salt thereof, (meth)acrylamide, and N,N-dimethylacrylamide are preferable from the viewpoint of industrial availability. , (Meth)acrylic acid and salts thereof are more preferable. These water-soluble ethylenically unsaturated monomers may be used alone or in combination of two or more.

Among these, acrylic acid and its salts are widely used as raw materials for water-absorbent resins, and these partially neutralized acrylic acid salts are used by copolymerizing the other water-soluble ethylenically unsaturated monomer described above. In some cases. In this case, the partially neutralized acrylic acid salt is preferably 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 monomer.

The water-soluble ethylenically unsaturated monomer is preferably dispersed in a hydrocarbon dispersion medium in the state of an aqueous solution and subjected to reverse phase suspension polymerization. By making the water-soluble ethylenically unsaturated monomer into an aqueous solution, the dispersion efficiency in the hydrocarbon dispersion medium can be increased. The concentration of the water-soluble ethylenically unsaturated monomer in this aqueous solution is preferably in the range of 20% by mass to the saturated concentration or less. Further, the concentration of the water-soluble ethylenically unsaturated monomer is more preferably 55% by mass or less, further preferably 50% by mass or less, and further preferably 45% by mass or less. On the other hand, the concentration of the water-soluble ethylenically unsaturated monomer is more preferably 25% by mass or more, further preferably 28% by mass or more, and further preferably 30% by mass or more.

When the water-soluble ethylenically unsaturated monomer has an acid group such as (meth)acrylic acid and 2-(meth)acrylamido-2-methylpropanesulfonic acid, the acid group is preliminarily alkaline as necessary. You may use what was neutralized with the neutralizer. Examples of such an alkaline neutralizing agent include alkali metal salts such as sodium hydroxide, sodium carbonate, sodium hydrogen carbonate, potassium hydroxide and potassium carbonate; ammonia and the like. In addition, these alkaline neutralizing agents may be used in the form of an aqueous solution in order to simplify the neutralizing operation. The above-mentioned alkaline neutralizing agents may be used alone or in combination of two or more kinds.

The degree of neutralization of the water-soluble ethylenically unsaturated monomer with the alkaline neutralizing agent is 10 to 100 mol% as the degree of neutralization with respect to all acid groups of the water-soluble ethylenically unsaturated monomer. Is more preferable, 30 to 90 mol% is more preferable, 40 to 85 mol% is still more preferable, and 50 to 80 mol% is still more preferable.

[Internal cross-linking agent]
Examples of the internal cross-linking agent include those capable of cross-linking the polymer of the water-soluble ethylenically unsaturated monomer to be used. For example, (poly)ethylene glycol [“(poly)” means “poly” prefix It means the case with and without it. The same shall apply hereinafter], diols such as (poly)propylene glycol, 1,4-butanediol, trimethylolpropane, (poly)glycerin, and polyols such as triol, and unsaturated compounds such as (meth)acrylic acid, maleic acid, and fumaric acid. Unsaturated polyesters obtained by reacting with an acid; bisacrylamides such as N,N-methylenebisacrylamide; di(meth)acrylic acid esters or tris obtained by reacting a polyepoxide with (meth)acrylic acid (Meth)acrylic acid ester; di(meth)acrylic acid carbamyl ester obtained by reacting polyisocyanate such as tolylene diisocyanate and hexamethylene diisocyanate with hydroxyethyl (meth)acrylate; allylated starch, allyl Compounds having two or more polymerizable unsaturated groups such as derivatized cellulose, diallyl phthalate, N,N',N''-triallyl isocyanate, divinylbenzene; (poly)ethylene glycol diglycidyl ether, (poly)propylene glycol di Diglycidyl compounds such as glycidyl ether, (poly)glycerin diglycidyl ether, polyglycidyl compounds such as triglycidyl compounds; epihalohydrin compounds such as epichlorohydrin, epibromhydrin, α-methylepichlorohydrin; 2,4-tolylene diisocyanate, hexamethylene A compound having two or more reactive functional groups such as an isocyanate compound such as diisocyanate; 3-methyl-3-oxetanemethanol, 3-ethyl-3-oxetanemethanol, 3-butyl-3-oxetanemethanol, 3-methyl-3 -Oxetane compounds, such as oxetane ethanol, 3-ethyl-3-oxetane ethanol, and 3-butyl-3-oxetane ethanol. Among these internal cross-linking agents, it is preferable to use a polyglycidyl compound, more preferable to use a diglycidyl ether compound, (poly)ethylene glycol diglycidyl ether, (poly)propylene glycol diglycidyl ether, and (poly)glycerin. It is particularly preferred to use diglycidyl ether. These internal cross-linking agents may be used alone or in combination of two or more.

The amount of the internal cross-linking agent used is preferably 0.000001 to 0.02 mol, and preferably 0.00001 to 0.01 mol, based on 1 mol of the water-soluble ethylenically unsaturated monomer. It is more preferably 0.00001 to 0.005 mol, still more preferably 0.00005 to 0.002 mol.

[Hydrocarbon dispersion medium]
As the hydrocarbon dispersion medium, for example, n-hexane, n-heptane, 2-methylhexane, 3-methylhexane, 2,3-dimethylpentane, 3-ethylpentane, n-octane and the like having 6 to 8 carbon atoms. Aliphatic hydrocarbons; alicycles such as cyclohexane, methylcyclohexane, cyclopentane, methylcyclopentane, trans-1,2-dimethylcyclopentane, cis-1,3-dimethylcyclopentane, trans-1,3-dimethylcyclopentane Group hydrocarbons: aromatic hydrocarbons such as benzene, toluene, xylene and the like can be mentioned. Among these hydrocarbon dispersion media, n-hexane, n-heptane, and cyclohexane are preferably used because they are industrially easily available, have stable quality, and are inexpensive. These hydrocarbon dispersion media may be used alone or in combination of two or more. In addition, as an example of the mixture of the hydrocarbon dispersion medium, a commercially available product such as exol heptane (manufactured by Exxon Mobil: heptane and its isomer hydrocarbon 75 to 85% by mass) may be used to obtain suitable results. be able to.

The amount of the hydrocarbon dispersion medium used is from the viewpoint of uniformly dispersing the water-soluble ethylenically unsaturated monomer and facilitating the control of the polymerization temperature. It is preferably 100 to 1500 parts by mass, and more preferably 200 to 1400 parts by mass with respect to 100 parts by mass. As will be described later, the reverse phase suspension polymerization is carried out in one stage (single stage) or in multiple stages of two or more stages, and the above-mentioned first stage polymerization means the first stage in single stage polymerization or multistage polymerization. Means the polymerization reaction of (the same applies below).

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

Examples of the surfactant include sucrose fatty acid ester, polyglycerin fatty acid ester, sorbitan fatty acid ester, polyoxyethylene sorbitan fatty acid ester, polyoxyethylene glycerin fatty acid ester, sorbitol fatty acid ester, polyoxyethylene sorbitol fatty acid ester, polyoxyethylene. Alkyl ether, polyoxyethylene alkyl phenyl ether, polyoxyethylene castor oil, polyoxyethylene hydrogenated castor oil, alkylallyl formaldehyde condensed polyoxyethylene ether, polyoxyethylene polyoxypropylene block copolymer, polyoxyethylene polyoxypropyl alkyl ether, Use polyethylene glycol fatty acid ester, alkyl glucoside, N-alkyl gluconamide, polyoxyethylene fatty acid amide, polyoxyethylene alkyl amine, polyoxyethylene alkyl ether phosphate, polyoxyethylene alkyl allyl ether phosphate, etc. You can Among these surfactants, sorbitan fatty acid ester, polyglycerin fatty acid ester, and sucrose fatty acid ester are preferably used from the viewpoint of dispersion stability of the monomer. These surfactants may be used alone or in combination of two or more.

The amount of the surfactant used is preferably 0.1 to 30 parts by mass, and 0.3 to 20 parts by mass, based on 100 parts by mass of the first-stage water-soluble ethylenically unsaturated monomer. More preferably, it is parts by mass.

(Polymer type dispersant)
Further, as the dispersion stabilizer used in the inverse suspension polymerization, a polymeric dispersant may be used together with the above-mentioned surfactant.

Examples of the polymeric dispersant include maleic anhydride-modified polyethylene, maleic anhydride-modified polypropylene, maleic anhydride-modified ethylene/propylene copolymer, maleic anhydride-modified EPDM (ethylene/propylene/diene/terpolymer), and anhydrous. Maleic acid-modified polybutadiene, maleic anhydride/ethylene copolymer, maleic anhydride/propylene copolymer, maleic anhydride/ethylene/propylene copolymer, maleic anhydride/butadiene copolymer, polyethylene, polypropylene, ethylene/propylene Examples thereof include copolymers, oxidized polyethylene, oxidized polypropylene, oxidized ethylene/propylene copolymers, ethylene/acrylic acid copolymers, ethyl cellulose and ethyl hydroxyethyl cellulose. Among these polymeric dispersants, in particular, from the viewpoint of dispersion stability of the monomer, maleic anhydride-modified polyethylene, maleic anhydride-modified polypropylene, maleic anhydride-modified ethylene/propylene copolymer, maleic anhydride/ Ethylene copolymer, maleic anhydride/propylene copolymer, maleic anhydride/ethylene/propylene copolymer, polyethylene, polypropylene, ethylene/propylene copolymer, oxidized polyethylene, oxidized polypropylene, oxidized ethylene/propylene copolymer It is preferable to use a polymer. These polymeric dispersants may be used alone or in combination of two or more.

The amount of the polymeric dispersant used is preferably 0.1 to 30 parts by mass, and 0.3 to 20 parts by mass, relative to 100 parts by mass of the first-stage water-soluble ethylenically unsaturated monomer. More preferably, it is parts by mass.

[Azo compounds and peroxides]
In one example of the method for producing a water-absorbent resin, reverse phase suspension polymerization can be performed on a water-soluble ethylenically unsaturated monomer in the presence of an azo compound and a peroxide.

Here, “in the presence of an azo compound and a peroxide” does not necessarily mean that the azo compound and the peroxide coexist at the initiation of polymerization reaction (at the time of radical cleavage of the compound). It means a state in which the other compound is present while the monomer conversion rate due to radical cleavage of one compound is less than 10%. It is preferable that they coexist. Further, the azo compound and the peroxide may be added to the polymerization reaction system through separate channels, or may be sequentially added to the polymerization reaction system through the same channel.

The azo compound and the peroxide used may be in the form of powder or an aqueous solution.

(Azo compounds)
Specifically, examples of the azo compound include 1-{(1-cyano-1-methylethyl)azo}formamide, 2,2′-azobis[2-(N-phenylamidino)propane]dihydrochloride, 2,2'-azobis{2-[N-(4-chlorophenyl)amidino]propane} dihydrochloride, 2,2'-azobis{2-[N-(4-hydroxyphenyl)amidino]propane} dihydrochloride , 2,2'-azobis[2-(N-benzylamidino)propane] dihydrochloride, 2,2'-azobis[2-(N-allylamidino)propane] dihydrochloride, 2,2'-azobis( 2-amidinopropane) dihydrochloride, 2,2'-azobis{2-[N-(2-hydroxyethyl)amidino]propane} dihydrochloride, 2,2'-azobis[2-(5-methyl-2) -Imidazolin-2-yl)propane] dihydrochloride, 2,2'-azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride, 2,2'-azobis[2-(4,5) ,6,7-Tetrahydro-1H-1,3-diazepin-2-yl)propane] dihydrochloride, 2,2′-azobis[2-(5-hydroxy-3,4,5,6-tetrahydropyrimidine- 2-yl)propane] dihydrochloride, 2,2'-azobis{2-[1-(2-hydroxyethyl)-2-imidazolin-2-yl]propane} dihydrochloride, 2,2'-azobis[ 2-(2-imidazolin-2-yl)propane], 2,2′-azobis{2-methyl-N-[1,1-bis(hydroxymethyl)-2-hydroxyethyl]propionamide}, 2,2 '-Azobis{2-methyl-N-[1,1-bis(hydroxymethyl)ethyl]propionamide},2,2'-azobis[2-methyl-N-(2-hydroxyethyl)propionamide], 2 ,2'-azobis(2-methylpropionamide) dihydrochloride, 4,4'-azobis-4-cyanovaleic acid, 2,2'-azobis[2-(hydroxymethyl)propionitrile], 2,2' -Azobis[2-(2-imidazolin-2-yl)propane]disulfate dihydrate, 2,2'-azobis[N-(2-carboxyethyl)-2-methylpropionamidine]tetrahydrate , 2,2′-azobis[2-methyl-N-(2-hydroxyethyl)propionamide] and the like. Among these, 2,2′-azobis(2-amidinopropane)dihydrochloride, 2,2′-azobis{2-[1-(2-hydroxyethyl)-2-imidazolin-2-yl]propane}dihydrochloride Hydrochloride, 2,2'-azobis[N-(2-carboxyethyl)-2-methylpropionamidine] tetrahydrate is easy to adjust the polymerization reaction such as polymerization temperature, has a high centrifugal retention rate, and a load. It is particularly preferable from the viewpoint that a water-absorbent resin having a high water-absorbing capacity below can be easily obtained. These azo compounds may be used alone or in combination of two or more.

(Peroxide)
Examples of the peroxide include persulfates such as potassium persulfate, ammonium persulfate and sodium persulfate; methyl ethyl ketone peroxide, methyl isobutyl ketone peroxide, di-t-butyl peroxide, t-butyl cumyl peroxide, t. Examples include peroxides such as -butyl peroxyacetate, t-butyl peroxyisobutyrate, t-butyl peroxypivalate, and hydrogen peroxide. Among these peroxides, potassium persulfate, ammonium persulfate, sodium persulfate, it is preferable to use hydrogen peroxide, from the viewpoint of easily obtaining a water absorbent resin having a high storage elastic modulus, further potassium persulfate, It is more preferable to use ammonium persulfate or sodium persulfate. These peroxides may be used alone or in combination of two or more kinds.

(Amount and ratio of azo compounds and peroxides used)
The amount of the azo compound and peroxide used is preferably 0.00005 mol or more, and more preferably 0.0001 mol or more, relative to 1 mol of the water-soluble ethylenically unsaturated monomer. Further, it is preferably 0.005 mol or less, and more preferably 0.001 mol or less, relative to 1 mol of the water-soluble ethylenically unsaturated monomer.

The proportion of the azo compound and the peroxide used is preferably 40 mass% or more, and 50 mass% or more, of the total amount of the azo compound and the peroxide used. The proportion is more preferable, the proportion is more preferably 60 mass% or more, still more preferably the proportion is 70 mass% or more. On the other hand, the ratio of the azo compound to the total amount of the azo compound and the peroxide used is preferably 95% by mass or less, more preferably 90% by mass or less, and 85% by mass or less. Is more preferable, and a ratio of 80% by mass or less is even more preferable. The mass ratio range (azo compound:peroxide) is preferably 8:12 to 19:1.

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

(Thickener)
As an example, in this method for producing a water absorbent resin, a thickener may be added to an aqueous solution containing a water-soluble ethylenically unsaturated monomer to carry out polymerization. By thus adding the thickener to adjust the viscosity of the aqueous solution, it is possible to control the medium particle size obtained by the polymerization.

Specific examples of the thickener include hydroxyethyl cellulose, hydroxypropyl cellulose, methyl cellulose, carboxymethyl cellulose, polyacrylic acid, polyacrylic acid (partial) neutralized product, polyethylene glycol, polyacrylamide, polyethyleneimine, dextrin, alginic acid. Sodium, polyvinyl alcohol, polyvinylpyrrolidone, polyethylene oxide or the like can be used. 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 obtained particles.

[Reverse phase suspension polymerization]
As an example of a method for producing a water absorbent resin, a water-soluble ethylenically unsaturated monomer can be subjected to reverse phase suspension polymerization. In carrying out reverse phase suspension polymerization, for example, a monomer aqueous solution containing a water-soluble ethylenically unsaturated monomer is added to a hydrocarbon dispersion medium in the presence of a surfactant and/or a polymer dispersant. Disperse. At this time, the surfactant or the polymeric dispersant may be added before or after the dispersion of the aqueous monomer solution before the polymerization reaction is started.

Among them, from the viewpoint of easily reducing the amount of hydrocarbon dispersion medium remaining in the resulting water-absorbent resin, after dispersing the monomer aqueous solution in the hydrocarbon dispersion medium in which the polymer dispersant is dispersed, It is preferable to carry out the polymerization after adding and dispersing a surfactant.

It is possible to carry out such reverse phase suspension polymerization in one stage or in multiple stages of two or more stages. In addition, it is preferable to carry out in 2 to 3 steps from the viewpoint of improving productivity.

When performing reverse phase suspension polymerization in multiple stages of two or more stages, after performing the first stage reverse phase suspension polymerization, the reaction mixture obtained in the first stage polymerization reaction is water-soluble ethylenically unsaturated. The monomers may be added and mixed, and the reverse phase suspension polymerization of the second and subsequent steps may be carried out in the same manner as in the first step. In the reverse phase suspension polymerization in each of the second and subsequent stages, in addition to the water-soluble ethylenically unsaturated monomer, an internal cross-linking agent, the above-mentioned azo compound and peroxide are used in the second and subsequent stages Based on the amount of the water-soluble ethylenically unsaturated monomer to be added during the reverse phase suspension polymerization in the stage, it is added within the range of the molar ratio of each component to the water-soluble ethylenically unsaturated monomer described above. Reverse phase suspension polymerization is preferably carried out.

The reaction temperature of the polymerization reaction is from 20 to 110° C., from the viewpoint of promoting the polymerization rapidly and shortening the polymerization time to improve the economical efficiency and to easily remove the heat of polymerization to carry out the reaction smoothly. Is preferable, and 40 to 90° C. is more preferable. The reaction time is preferably 0.5 to 4 hours.

<Post-crosslinking step>
Next, the water-absorbent resin according to the present invention is to be post-crosslinked with a post-crosslinking agent to a hydrogel having an internal crosslinked structure obtained by polymerizing a water-soluble ethylenically unsaturated monomer ( Post-crosslinking reaction). This post-crosslinking reaction is preferably performed in the presence of a post-crosslinking agent after the polymerization of the water-soluble ethylenically unsaturated monomer. Thus, after the polymerization, by performing a post-crosslinking reaction on the hydrogel having an internal crosslinked structure, the crosslink density near the surface of the water-absorbent resin is increased, and the water-absorbing ability and gel under load are obtained. It is possible to obtain those with improved various properties such as elasticity.

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

The amount of the post-crosslinking agent used is preferably 0.00001 to 0.01 mol, and 0.00005 to 0. 0 mol, based on 1 mol of the total amount of the water-soluble ethylenically unsaturated monomer used for the polymerization. It is more preferably 005 mol, and particularly preferably 0.0001 to 0.002 mol.

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

The post-crosslinking agent may be added at a time after the polymerization reaction of the water-soluble ethylenically unsaturated monomer is almost completed, and 1 to 100 parts by mass of the water-soluble ethylenically unsaturated monomer may be added. It is preferably added in the presence of water in the range of 400 parts by mass, more preferably added in the presence of water in the range of 5 to 200 parts by mass, and added in the presence of water in the range of 10 to 100 parts by mass. Is more preferable, and it is even more preferable to add in the presence of water in the range of 20 to 60 parts by mass. As a result, it is possible to increase the water absorption capacity and the like under load. The amount of water means the total amount of water contained in the polymerization reaction system and water used as necessary when adding the post-crosslinking agent.

The reaction temperature in the post-crosslinking reaction is preferably 50 to 250°C, more preferably 60 to 180°C, further preferably 60 to 140°C, and particularly preferably 70 to 120°C. preferable. The reaction time of the post-crosslinking reaction is preferably 1 to 300 minutes, more preferably 5 to 200 minutes.

<Drying process>
After performing the above-mentioned reverse phase suspension polymerization, a drying step of removing water, a hydrocarbon dispersion medium and the like by distillation by externally applying energy such as heat may be included. When dehydration is performed from the hydrous gel after reverse phase suspension polymerization, the system in which the hydrous gel is dispersed in the hydrocarbon dispersion medium is heated so that the water and the hydrocarbon dispersion medium are once removed from the system by azeotropic distillation. Distill off. At this time, continuous azeotropic distillation is possible by returning only the distilled hydrocarbon dispersion medium into the system. In that case, the temperature in the system during drying is maintained at the azeotropic temperature of the hydrocarbon dispersion medium or lower, which is preferable from the viewpoint that the resin is less likely to deteriorate. Subsequently, the water and the hydrocarbon dispersion medium are distilled off to obtain particles of the water absorbent resin. By controlling the treatment conditions in the drying step after the polymerization to adjust the dehydration amount, it is possible to control the centrifugal retention rate and the like of the resulting water absorbent resin.

In the drying step, the drying treatment by distillation may be performed under normal pressure or under reduced pressure. Further, from the viewpoint of improving the drying efficiency, the drying may be performed under a stream of nitrogen or the like. When the drying treatment is performed under normal pressure, the drying temperature is preferably 70 to 250° C., more preferably 80 to 180° C., further preferably 80 to 140° C., and 90 to It is particularly preferably 130°C. When the drying treatment is performed under reduced pressure, the drying temperature is preferably 40 to 160°C, more preferably 50 to 110°C.

When the post-crosslinking step is performed with the post-crosslinking agent after the polymerization of the monomer by the reverse phase suspension polymerization, the drying step by distillation described above is performed after the completion of the crosslinking step. Alternatively, the post-crosslinking step and the drying step may be performed simultaneously.

Further, if necessary, various additives such as a chelating agent, a reducing agent, an oxidizing agent, an antibacterial agent, and a deodorant may be added to the water absorbent resin after polymerization, during drying, or after drying. ..

<<3. Absorbers, absorbent articles >>
The water-absorbent resin according to the present invention constitutes an absorbent body used for sanitary materials such as paper diapers, sanitary napkins and incontinence pads, and is suitably used for absorbent articles including the absorbent body.

Here, the absorber using the water absorbent resin is composed of, for example, a water absorbent resin and hydrophilic fibers. As the constitution of the absorber, a mixed dispersion obtained by mixing the water-absorbent resin and the hydrophilic fiber so as to have a uniform composition, a sandwich in which the water-absorbent resin is sandwiched between the layered hydrophilic fibers Examples thereof include a structure, a structure in which a water-absorbent resin and hydrophilic fibers are wrapped in a tissue. In addition, the absorbent body may be blended with other components, for example, an adhesive binder such as a heat-fusible synthetic fiber for enhancing the shape retention of the absorbent body, a hot melt adhesive, or an adhesive emulsion. ..

The content of the water absorbent resin in the absorber is preferably 5 to 95% by mass, more preferably 20 to 90% by mass, and further preferably 30 to 80% by mass. When the content of the water absorbent resin is less than 5% by mass, the absorption capacity of the absorber becomes low, which may cause liquid leakage and reversion. On the other hand, when the content of the water-absorbent resin exceeds 95% by mass, the cost of the absorber becomes high and the feel of the absorber becomes hard.

As the hydrophilic fibers, cotton-like pulp obtained from wood, cellulose fibers such as mechanical pulp, chemical pulp, and semi-chemical pulp, artificial cellulose fibers such as rayon and acetate, synthetic hydrophilized polyamide, polyester, polyolefin and the like. Fibers made of resin can be used.

In addition, by holding an absorber using a water-absorbent resin between a liquid-permeable sheet (top sheet) through which liquid can pass and a liquid-impermeable sheet (back sheet) through which liquid cannot pass , Can be an absorbent article. The liquid-permeable sheet is disposed on the side that contacts the body, and the liquid-impermeable sheet is disposed on the opposite side that contacts the body.

Examples of the liquid permeable sheet include air-through type, spun bond type, chemical bond type, needle punch type non-woven fabrics made of fibers such as polyethylene, polypropylene and polyester, and porous synthetic resin sheets. Examples of the liquid impermeable sheet include a synthetic resin film made of a resin such as polyethylene, polypropylene and polyvinyl chloride.

<<4. Example ≫
Hereinafter, the present invention will be described in more detail with reference to Examples and Comparative Examples, but the present invention is not limited to the following Examples and the like.

<4-1. Evaluation test method>
[Evaluation test of water absorbent resin]
The water absorbent resins obtained in the following Examples 1 to 4 and Comparative Examples 1 to 3 were subjected to various tests shown below and evaluated.

(1) Centrifugal retention rate In a beaker with a volume of 500 mL, 500 g of 0.9 mass% sodium chloride aqueous solution (physiological saline) is weighed and stirred at 600 r/min, and 2.0 g of the water-absorbent resin does not generate mamako. Dispersed. The water-absorbent resin was sufficiently swelled by leaving it for 60 minutes with stirring. After that, it was poured into a cotton bag (Membroad No. 60, width 100 mm x length 200 mm), the upper part of the cotton bag was tied with a rubber band, and a dehydrator (manufactured by Domestic Centrifuge Co., Ltd., product number: The cotton bag was dehydrated for 1 minute using H-122), and the mass Wa (g) of the cotton bag containing the swollen gel after dehydration was measured. The same operation was performed without adding the water absorbent resin, the empty mass Wb(g) of the cotton bag when wet was measured, and the centrifugal retention rate was calculated from the following formula.

Centrifugal retention (g/g)=[Wa-Wb] (g)/mass of water-absorbent resin (g)

(2) Water absorption capacity of physiological saline under a load of 4.14 kPa The water absorption capacity of the water absorbent resin under a load of 4.14 kPa was measured using the measuring apparatus X whose schematic configuration is shown in FIG.

The measuring device X shown in FIG. 1 includes a buret part 1, a conduit 2, a measuring table 3, and a measuring part 4 placed on the measuring table 3. In the buret part 1, a rubber stopper 14 is connected to the upper part of the buret 10, an air introducing pipe 11 and a cock 12 are connected to the lower part, and a cock 13 is attached to the upper part of the air introducing pipe 11. A conduit 2 is attached from the burette portion 1 to the measuring table 3, and the diameter of the conduit 2 is 6 mm. A hole having a diameter of 2 mm is opened at the center of the measuring table 3 and the conduit 2 is connected to the hole. The measuring unit 4 includes 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 in 200 mesh (opening 75 μm). Then, a predetermined amount of the water absorbent resin 5 is evenly dispersed on the nylon mesh 41. The weight 42 has a diameter of 1.9 cm and a mass of 119.6 g. This weight 42 is placed on the water-absorbent resin 5, and a load of 4.14 kPa can be uniformly applied to the water-absorbent resin 5.

Using the measuring device X having such a configuration, first, the cock 12 and the cock 13 of the buret part 1 are closed, physiological saline adjusted to 25° C. is put into the buret 10 from above, and a rubber stopper 14 is used to plug the buret upper part. After that, the cock 12 and the cock 13 of the buret part 1 were opened. Next, the height of the measurement table 3 was adjusted so that the tip of the conduit 2 and the air introduction port of the air introduction tube 11 in the center of the measurement table 3 were at the same height.

On the other hand, 0.10 g of the water-absorbent resin 5 was evenly dispersed on the nylon mesh 41 of the cylinder 40, and the weight 42 was placed on the water-absorbent resin 5. The measuring part 4 was placed so that its central part coincided with the conduit port at the central part of the measuring table 3.

From the time when the water absorbent resin 5 started to absorb water, the amount of decrease in physiological saline solution in the buret 10 (the amount of physiological saline solution absorbed by the water absorbent resin 5) Wc (mL) was continuously read. The water absorption capacity of the water absorbent resin under a load of 4.14 kPa after 60 minutes from the start of water absorption was determined by the following formula.

Saline water absorption capacity under a load of 4.14 kPa (mL/g)=Wc/0.10 (g)

(3) Storage elastic modulus, loss elastic modulus, and tan δ
The water-absorbent resin to be measured is adjusted to one that passes through a sieve with an opening of 400 μm and is retained on a sieve with an opening of 300 μm, and the classified sample is swollen 50 times with physiological saline to give a 50 times swollen gel. It was made. Specifically, first, in a 100 mL beaker, 49.0 g of physiological saline was weighed, and a magnetic stirrer bar (without a ring of 8 mmφ×30 mm) was charged to the magnetic stirrer (HS-30D manufactured by iuchi). ), and the magnetic stirrer was adjusted to rotate at 600 r/min. Next, 1.0 g of the classified sample was placed in a stirring beaker, and stirring was continued until the rotating vortex disappeared and the liquid surface became horizontal, to prepare a 50 times swollen gel. This 50 times swollen gel was transferred to a centrifuge tube, degassed for 4 minutes in a centrifuge (manufactured by Domestic Centrifuge Co., Ltd., product number H-103NA SERIES) set so that the centrifugal force was 671 G, and used as a measurement sample. ..

For the measurement, a dynamic viscoelasticity measuring device Rheometer (manufactured by TA Instruments Japan Co., Ltd., product number AR2000eZ) was used to set the prepared measurement sample, and the storage elastic modulus G'(Pa) and loss elastic modulus G were set. The frequency ω (rad/sec) dispersion of ″ (Pa) was measured. A parallel plate having a diameter of 60 mm was used as the sample holder, and the distance between the plates was 3 mm. The thickness of the gel was 3000 μm. The measurement temperature was 25° C., and the measurement was performed under the conditions of frequency ω=0.1 to 300 rad/sec and strain=0.1% strain.

Then, the storage elastic modulus G'(Pa) and the loss elastic modulus G''(Pa) at 10 rad/sec are obtained, and then the value of tan δ is calculated from the ratio of G'and G'' (G''/G'). Calculation was performed, and the value was defined as tan δ of the water absorbent resin.

(4) Medium particle size (particle size distribution)
As a lubricant, 0.25 g of amorphous silica (manufactured by Evonik Degussa Japan Ltd., Carplex #80) was mixed with 50 g of the water absorbent resin. The medium particle size of this was measured using the combination of the sieves of [A] below.

[A] JIS standard sieve, from the top, a sieve having an opening of 850 μm, a sieve having an opening of 600 μm, a sieve having an opening of 500 μm, a sieve having an opening of 400 μm, a sieve having an opening of 300 μm, a sieve having an opening of 250 μm, and an opening of 150 μm. The sieve and the saucer were combined in this order.

The water-absorbent resin was put into the combined uppermost sieve and shaken for 20 minutes using a low-tap shaker to perform classification. After the classification, the mass of the water-absorbent resin remaining on each sieve was calculated as a mass percentage with respect to the total amount to obtain a particle size distribution. With respect to this particle size distribution, the relationship between the mesh opening of the sieve and the integrated value of the mass percentage of the water-absorbent resin remaining on the sieve was plotted on a logarithmic probability paper by sequentially accumulating on the sieve from the largest particle diameter. By connecting the plots on the probability paper with a straight line, the particle size corresponding to an integrated mass percentage of 50 mass% was defined as the median particle size.

The existence ratio of the water absorbent resin having a particle diameter of 300 to 400 μm is the ratio of the water absorbent resin remaining on the sieve having an opening of 300 μm, and the existence ratio of the water absorbent resin having a particle diameter of 150 to 850 μm is also present. The ratio is a numerical value obtained by adding all the ratios of the water-absorbent resin remaining on the sieves having openings of 150 μm, 250 μm, 300 μm, 400 μm, 500 μm and 600 μm.

[Evaluation test of absorber and absorbent article using water-absorbent resin]
(1) Preparation of Absorbent Body and Absorbent Article 12 g of water-absorbent resin and 12 g of crushed pulp (Rayflock manufactured by Leonia Co., Ltd.) were uniformly mixed by air papermaking to obtain a sheet-shaped absorption of 40 cm×12 cm. A body core was made. Next, the upper and lower sides of the absorbent core are sandwiched by two pieces of tissue paper having the same size as the absorbent core and having a basis weight of 16 g/m ² , and a load of 196 kPa is applied to the whole for 30 seconds and pressed. Thus, an absorber was produced. Further, an air-through type porous liquid permeable sheet made of polyethylene-polypropylene having the same size as the absorbent body and having a basis weight of 22 g/m ² is arranged on the upper surface of the absorbent body, and a polyethylene liquid having the same size and the same basis weight is provided. An absorbent article (size: 40×12 cm) was obtained by disposing an impermeable sheet on the lower surface of the absorbent body and sandwiching the absorbent body.

(2) Preparation of artificial urine As artificial urine, compounded and dissolved in ion-exchanged water such that NaCl: 0.780% by mass, CaCl ₂ : 0.022% by mass, MgSO ₄ : 0.038% by mass. A mixture containing a small amount of Blue No. 1 was prepared.

(3) Permeation rate The absorbent article was placed on a horizontal table. At the center of the absorbent article, a measuring instrument having a mass of 2 kg equipped with a liquid injection cylinder having an inner diameter of 3 cm was placed in the center of a plate having a bottom of 10 cm×10 cm, and the absorbent article was put in a state of being loaded. Next, while 80 mL of artificial urine was put into the cylinder at once, the time until the artificial urine completely disappeared from the cylinder was measured using a stopwatch, and the first permeation rate (second) was calculated. did. Next, remove the cylinder, store the absorbent article as it is, and perform the same operation using the measuring instrument at the same position as the first time 30 minutes and 60 minutes after the start of the first artificial urine introduction. The second and third permeation rates (seconds) were measured. The total of the first to third times was defined as the total permeation rate. It can be said that the shorter the permeation rate, the more preferable as the absorbent article.

(4) Liquid return amount 120 minutes after the start of the first artificial urine introduction in the above-described permeation rate measurement, the mass (Wd (g), about 50 g) is measured in advance near the artificial urine introduction position on the absorbent article. The 10 cm square filter paper was placed and a weight of 5 kg having a bottom of 10 cm×10 cm was placed on the filter paper. After loading for 5 minutes, the mass (We (g)) of the filter paper was measured, and the increased mass was defined as the liquid reversion amount (g). It can be said that the smaller the amount of liquid reversion is, the more preferable it is as an absorbent article.
Liquid return amount (g) = We-Wd

<4-2. About Examples and Comparative Examples>
[Example 1]
As a reflux condenser, a dropping funnel, a nitrogen gas introducing pipe, and a stirrer, a round bottom cylindrical separable flask having an inner diameter of 110 mm and a volume of 2 L equipped with a stirring blade having two inclined paddle blades with a blade diameter of 50 mm in two stages was used. Got ready. In this flask, 300 g of n-heptane was used as a hydrocarbon dispersion medium, 0.74 g of sucrose stearate ester of HLB3 (Mitsubishi Chemical Foods Co., Ltd., Ryoto Sugar Ester S-370) as a surfactant, and a polymer dispersant. As an additive, 0.74 g of a maleic anhydride-modified ethylene/propylene copolymer (Mitsui Chemicals, Inc., Hiwax 1105A) was added, the temperature was raised to 80°C with stirring to dissolve the surfactant, and then cooled to 50°C. did.

On the other hand, 92 g (1.02 mol) of 80% by mass acrylic acid aqueous solution was placed in a 500 mL Erlenmeyer flask, and 102.2 g of 30% by mass sodium hydroxide aqueous solution was added dropwise while cooling from the outside, and 75% by mol of 75% by mol. After the neutralization, 0.092 g of hydroxylethyl cellulose (Sumitomo Seika Chemicals Co., Ltd., HEC AW-15F) was used as a thickener, and 2,2′-azobis(2-amidinopropane) dihydrochloride was added as an azo compound. 092 g (0.339 mmol), potassium persulfate 0.037 g (0.137 mmol) as a peroxide, ethylene glycol diglycidyl ether 0.010 g (0.058 mmol) as an internal cross-linking agent, and deionized water 43.8 g. Was added and dissolved to prepare a monomer aqueous solution.

Then, the monomer aqueous solution prepared as described above is added to the separable flask, the system is sufficiently replaced with nitrogen, and the flask is immersed in a water bath at 70° C. to raise the temperature, and polymerization is carried out for 60 minutes. Thus, a first stage polymerization slurry liquid was obtained.

On the other hand, 128.8 g (1.43 mol) of 80% by mass aqueous acrylic acid solution was placed in another 500 mL Erlenmeyer flask, and 143.1 g of 30% by mass aqueous sodium hydroxide solution was added dropwise while cooling from the outside. After neutralization to 75 mol %, 2,2′-azobis(2-amidinopropane) dihydrochloride 0.129 g (0.475 mmol) as an azo compound and potassium persulfate 0.052 g as a peroxide. (0.191 mmol), 0.012 g (0.067 mmol) of ethylene glycol diglycidyl ether as an internal cross-linking agent and 15.9 g of ion-exchanged water were added and dissolved to prepare a second-stage monomer aqueous solution.

After cooling the inside of the separable flask system to 25° C., the total amount of the second stage monomer aqueous solution was added to the first stage polymerization slurry liquid, and the system was sufficiently replaced with nitrogen. The flask was again immersed in a water bath at 70° C. to raise the temperature, and the second stage polymerization was carried out for 30 minutes.

After the second-stage polymerization, the reaction solution was heated in an oil bath at 125° C., and 236 g of water was extracted from the system while refluxing n-heptane by azeotropic distillation of n-heptane and water. As a post-crosslinking agent, 4.42 g (0.51 mmol) of a 2% by mass aqueous solution of ethylene glycol diglycidyl ether was added, and the mixture was kept at 80° C. for 2 hours. Then, n-heptane was evaporated and dried to obtain a dried product. The dried product was passed through a sieve with an opening of 1000 μm to obtain 234.1 g of a water absorbent resin in which spherical particles were aggregated. The water absorbent resin thus obtained was evaluated according to the various test methods described above.

In the obtained water-absorbent resin, the mass ratio of particles having a particle size of 150 to 850 μm was 94 mass %, and the mass ratio of particles having a particle size of 300 to 400 μm was 36 mass %.

[Example 2]
Example 2 was the same as Example 1 except that after the second-stage polymerization, 239 g of water was extracted out of the system while refluxing the n-heptane by azeotropic distillation of n-heptane and water. did. As a result, 231.2 g of a water absorbent resin having a centrifugal retention rate and the like different from those of the water absorbent resin obtained in Example 1 was obtained. The water absorbent resin thus obtained was evaluated according to the various test methods described above.

In the obtained water-absorbent resin, the mass ratio of particles having a particle size of 150 to 850 μm was 92 mass %, and the mass ratio of particles having a particle size of 300 to 400 μm was 32 mass %.

[Example 3]
In Example 3, the amount of ethylene glycol diglycidyl ether added as an internal cross-linking agent to be dissolved in the first-stage monomer aqueous solution was 0.020 g (0.116 mmol), and n-heptane and water were added after the second-stage polymerization. Example 1 was repeated except that 254 g of water was extracted out of the system while refluxing n-heptane by azeotropic distillation with. As a result, 232.9 g of a water-absorbent resin having an internal crosslinking agent content different from that of the water-absorbent resin obtained in Example 1 was obtained. The water absorbent resin thus obtained was evaluated according to the various test methods described above.

In the obtained water-absorbent resin, the mass ratio of the particles of 150 to 850 μm was 95 mass% and the mass ratio of the particles of 300 to 400 μm was 33 mass% in the whole ratio.

[Example 4]
Example 4 is the same as Example 3 except that after the second-stage polymerization, 258 g of water was extracted out of the system while refluxing n-heptane by azeotropic distillation of n-heptane and water. did. As a result, 226.0 g of a water absorbent resin having a centrifugal retention rate and the like different from that of the water absorbent resin obtained in Example 1 was obtained. The water absorbent resin thus obtained was evaluated according to the various test methods described above.

In the obtained water-absorbent resin, the mass ratio of the particles of 150 to 850 μm was 95 mass% and the mass ratio of the particles of 300 to 400 μm was 33 mass% in the whole ratio.

[Comparative Example 1]
In Comparative Example 1, a reflux bottom cooler, a dropping funnel, a nitrogen gas introducing pipe, and a stirrer equipped with a stirring blade having two stages of four inclined paddle blades with a blade diameter of 50 mm, an inner diameter of 110 mm, a 2 L round bottom cylinder Type separable flask was prepared, and 300 g of n-heptane was taken as a hydrocarbon dispersion medium in this flask, and sucrose stearate ester of HLB3 as a surfactant (Mitsubishi Chemical Foods Co., Ltd., Ryoto Sugar Ester S-370). 0.74 g, and 0.74 g of a maleic anhydride-modified ethylene/propylene copolymer (Mitsui Chemicals, Inc., Hiwax 1105A) as a polymer dispersant were added, and the temperature was raised to 80°C with stirring to obtain a surfactant. After dissolution, it was cooled to 50°C.

On the other hand, 92 g (1.02 mol) of 80% by mass acrylic acid aqueous solution was placed in a 500 mL Erlenmeyer flask, and 102.2 g of 30% by mass sodium hydroxide aqueous solution was added dropwise while cooling from the outside, and 75% by mol of 75% by mol. After neutralizing, 0.092 g of hydroxylethyl cellulose (Sumitomo Seika Chemicals Co., Ltd., HEC AW-15F) as a thickener, 0.074 g (0.274 mmol) of potassium persulfate as a peroxide, and an internal crosslinking agent. 0.010 g (0.058 mmol) of ethylene glycol diglycidyl ether and 43.8 g of ion-exchanged water were added and dissolved to prepare a monomer aqueous solution.

Then, the monomer aqueous solution prepared as described above is added to the separable flask, the system is sufficiently replaced with nitrogen, and the flask is immersed in a water bath at 70° C. to raise the temperature, and polymerization is carried out for 60 minutes. Thus, a first stage polymerization slurry liquid was obtained.

On the other hand, 128.8 g (1.43 mol) of 80% by mass aqueous acrylic acid solution was placed in another 500 mL Erlenmeyer flask, and 143.1 g of 30% by mass aqueous sodium hydroxide solution was added dropwise while cooling from the outside. After neutralization to 75 mol%, potassium persulfate 0.104 g (0.382 mmol) as a peroxide and ethylene glycol diglycidyl ether 0.012 g (0.067 mmol) and 15.9 g as an internal cross-linking agent. Was added and dissolved to prepare a second-stage monomer aqueous solution.

After cooling the inside of the separable flask system to 25° C., the entire amount of the second-stage monomer aqueous solution was added to the first-stage polymerization slurry liquid, and the system was sufficiently replaced with nitrogen. Then, the flask was again immersed in a water bath at 70° C. to raise the temperature, and the second stage polymerization was carried out for 30 minutes.

After the second-stage polymerization, the temperature of the reaction solution was raised in an oil bath at 125° C., and 257 g of water was extracted out of the system while refluxing n-heptane by azeotropic distillation of n-heptane and water. As a post-crosslinking agent, 4.42 g (0.51 mmol) of a 2% by mass aqueous solution of ethylene glycol diglycidyl ether was added, and the mixture was kept at 80° C. for 2 hours. Then, n-heptane was evaporated and dried to obtain a dried product. The dried product was passed through a sieve having an opening of 1000 μm to obtain 228.2 g of a water absorbent resin in which spherical particles were aggregated. The water absorbent resin thus obtained was evaluated according to the various test methods described above. In the obtained water-absorbent resin, the mass ratio of particles having a particle size of 150 to 850 μm was 94% by mass, and the mass ratio of particles having a particle size of 300 to 400 μm was 33% by mass.

[Comparative example 2]
Comparative Example 2 was the same as Comparative Example 1 except that after the second-stage polymerization, 259 g of water was extracted from the system while refluxing the n-heptane by azeotropic distillation of n-heptane and water. did. As a result, 228.2 g of a water absorbent resin having a centrifugal retention rate and the like different from that of the water absorbent resin obtained in Comparative Example 1 was obtained. The water absorbent resin thus obtained was evaluated according to the various test methods described above.

In the obtained water-absorbent resin, the mass ratio of particles having a particle size of 150 to 850 μm was 94 mass %, and the mass ratio of particles having a particle size of 300 to 400 μm was 31 mass %.

[Comparative Example 3]
In Comparative Example 3, as compared with the water absorbent resin obtained in Comparative Example 1, a water absorbent resin containing more crosslinking agent was obtained.

Specifically, first, a reflux condenser, a dropping funnel, a nitrogen gas introducing pipe, and a stirrer equipped with a stirrer having two inclined paddle blades with a blade diameter of 50 mm in two stages having an inner diameter of 110 mm and a 2 L volume circle. A bottom cylindrical separable flask was prepared, and 300 g of n-heptane was used as a hydrocarbon dispersion medium in this flask, and sucrose stearate ester of HLB3 was used as a surfactant (Mitsubishi Chemical Foods Corporation, Ryoto Sugar Ester S- 370) 0.74 g, and maleic anhydride-modified ethylene/propylene copolymer (Mitsui Chemicals, Inc., Hiwax 1105A) 0.74 g as a polymer dispersant are added, and the temperature is raised to 80° C. with stirring to form an interface. After the activator was dissolved, it was cooled to 50°C.

On the other hand, 92 g (1.02 mol) of 80% by mass acrylic acid aqueous solution was placed in a 500 mL Erlenmeyer flask, and 102.2 g of 30% by mass aqueous sodium hydroxide solution was added dropwise while cooling from the outside, and 75% by mol of 75% by mol. After neutralizing, 0.092 g of hydroxylethyl cellulose (Sumitomo Seika Chemicals Co., Ltd., HEC AW-15F) as a thickener, 0.074 g (0.274 mmol) of potassium persulfate as a peroxide, and an internal crosslinking agent. 0.018 g (0.106 mmol) of ethylene glycol diglycidyl ether and 43.8 g were added and dissolved to prepare a monomer aqueous solution.

Then, the monomer aqueous solution prepared as described above is added to the separable flask, the system is sufficiently replaced with nitrogen, and the flask is immersed in a water bath at 70° C. to raise the temperature, and polymerization is carried out for 60 minutes. Thus, a first stage polymerization slurry liquid was obtained.

On the other hand, 128.8 g (1.43 mol) of 80% by mass aqueous acrylic acid solution was placed in another 500 mL Erlenmeyer flask, and 143.1 g of 30% by mass aqueous sodium hydroxide solution was added dropwise while cooling from the outside. After neutralization to 75 mol %, potassium persulfate 0.104 g (0.382 mmol) as a peroxide, ethylene glycol diglycidyl ether 0.039 g (0.222 mmol) as an internal cross-linking agent and ion-exchanged water. 15.9 g was added and dissolved to prepare a second stage monomer aqueous solution.

After cooling the inside of the separable flask system to 25° C., the entire amount of the second-stage monomer aqueous solution was added to the first-stage polymerization slurry liquid, and the system was sufficiently replaced with nitrogen. Then, the flask was again immersed in a water bath at 70° C. to raise the temperature, and the second stage polymerization was carried out for 30 minutes.

After the second-stage polymerization, the reaction solution was heated in an oil bath at 125° C., and 273 g of water was extracted from the system while refluxing n-heptane by azeotropic distillation of n-heptane and water. As a post-crosslinking agent, 6.63 g (0.761 mmol) of a 2 mass% aqueous solution of ethylene glycol diglycidyl ether was added, and the mixture was kept at 80° C. for 2 hours. Then, n-heptane was evaporated and dried to obtain a dried product. This dried product was passed through a sieve having an opening of 1000 μm to obtain 231.2 g of a water absorbent resin in which spherical particles were aggregated. The water absorbent resin thus obtained was evaluated according to the various test methods described above. In the obtained water-absorbent resin, the mass ratio of particles having a particle size of 150 to 850 μm was 94% by mass, and the mass ratio of particles having a particle size of 300 to 400 μm was 39% by mass.

<4-3. About evaluation results>
[Evaluation result of water absorbent resin]
Table 1 below shows the evaluation results of the water absorbent resin. In addition, in Table 1, the absorption capacity elastic index represented by the following formula (I) is also shown.
Absorption capacity elastic index=storage elastic modulus [Pa]×centrifugal retention [g/g] (I)

As can be seen from the results shown in Table 1, the water absorbent resins obtained in Examples 1 to 4 were water absorbent resins having the desired performance.

[Evaluation result of absorbent article]
Next, in Table 2 below, regarding the absorbent articles produced using the water absorbent resins obtained in Examples 1 to 3 and Comparative Examples 1 and 3 described above, the permeation rate of artificial urine of the absorbent articles is shown. The results of measurement of liquid reversion amount and diffusion length are shown.

As shown in the results of Table 2, the absorbent articles obtained in Examples 1 to 3 using the water absorbent resin having an absorption capacity elastic index of 68000 or more are the water absorbent resins obtained in Comparative Examples. It was demonstrated that the absorbent article produced by using the above-mentioned method was superior in the absorption performance under load and the liquid reversion amount.

X Measuring device 1 Burette part 2 Conduit 3 Measuring stand 4 Measuring part 5 Water absorbent resin

Claims (5)

Acrylic acid and/or salt thereof in the presence of a polyglycidyl compound as an internal cross-linking agent, wherein the azo compound is 40% by mass or more of the total amount of the azo compound and the peroxide used. And a water-absorbent resin obtained by polymerizing in the presence of the peroxide, and by post-crosslinking with a polyglycidyl compound which is a post-crosslinking agent,
The physiological saline absorption capacity under a load of 4.14 kPa is 16 mL/g or more,
The amount of liquid reversion is 1.3 g or less,
The mass ratio of the particles of 150 to 850 μm in the total mass of the water absorbent resin is 85% by mass or more, and the mass ratio of the particles of 300 to 400 μm is 20% by mass or more,
Tan δ measured by classifying particles of 300 to 400 μm of the water absorbent resin is 2.00×10 ⁻² or less,
A water absorbent resin represented by the following formula (I) having an absorption capacity elastic index of 68000 or more (excluding a water absorbent resin having a water-soluble content of 12% by mass or less).
Absorption capacity elastic index=storage elastic modulus [Pa]×centrifugal retention [g/g] (I)
[Measuring method of the amount of liquid reversion]
12 g of the water-absorbent resin and 12 g of crushed pulp (Rayflock manufactured by Leonia Co., Ltd.) are uniformly mixed by air papermaking to produce a sheet-shaped absorbent core having a size of 40 cm×12 cm. Next, the upper and lower sides of the absorbent core are sandwiched by two pieces of tissue paper having the same size as the absorbent core and having a basis weight of 16 g/m ² , and a load of 196 kPa is applied to the whole for 30 seconds and pressed. By doing so, an absorber is produced. Further, an air-through type porous liquid permeable sheet made of polyethylene-polypropylene having the same size as the absorbent body and having a basis weight of 22 g/m ² is arranged on the upper surface of the absorbent body, and a polyethylene liquid having the same size and the same basis weight is provided. An impermeable sheet (size: 40×12 cm) is obtained by disposing an impermeable sheet on the lower surface of the absorbent body and sandwiching the absorbent body.
Place the absorbent article on a horizontal table. At the center of the absorbent article, by placing a measuring instrument with a mass of 2 kg equipped with a liquid injection cylinder having an inner diameter of 3 cm at the center of a plate having a bottom of 10 cm×10 cm, a state in which a load is applied to the absorbent article is obtained. To do. Next, 80 mL of artificial urine is put into the cylinder at once. Next, the cylinder is removed, the absorbent article is stored as it is, and after 30 minutes and 60 minutes from the start of the first artificial urine charging, the same measurement device is used at the same position as the first time. Perform the operation.
120 minutes after the start of the first artificial urine charging, a 10 cm square filter paper whose mass (Wd (g), about 50 g) was previously measured was placed near the artificial urine charging position on the absorbent article, A weight having a bottom of 10 cm×10 cm and a mass of 5 kg is placed thereon. After loading for 5 minutes, the mass (We (g)) of the filter paper is measured, and the increased mass represented by the following formula (II) is defined as the liquid reversion amount (g).
Liquid return amount (g)=We−Wd (II)
[Measuring method of tan δ above]
First, 49.0 g of physiological saline is weighed in a 100 mL beaker, a magnetic stirrer bar is put therein, the magnetic stirrer is placed on the magnetic stirrer, and the magnetic stirrer is adjusted to rotate at 600 r/min. Next, 1.0 g of the classified sample is put into a stirring beaker, and stirring is continued until the rotating vortex disappears and the liquid surface becomes horizontal, to prepare a 50 times swollen gel. The 50 times swollen gel is transferred to a centrifuge tube and degassed for 4 minutes in a centrifuge set so that the centrifugal force is 671 G to obtain a measurement sample.
Then, the measurement sample is Se Tsu preparative the dynamic viscoelasticity measuring apparatus rheometer. Using a parallel plate having a diameter of 60 mm as a sample holder, the distance between the plates was 3 mm, the thickness of the gel was 3000 μm, the measurement temperature was 25° C., the frequency was ω=0.1 to 300 rad/sec, and the strain was 0.1% strain. The storage elastic modulus and loss elastic modulus are measured with. Of these, the ratio of the storage elastic modulus to the loss elastic modulus at 10 rad/sec (loss elastic modulus/storage elastic modulus) is tan δ.
[Measuring method of storage elastic modulus]
First, 49.0 g of physiological saline is weighed in a 100 mL beaker, a magnetic stirrer bar is put therein, the magnetic stirrer is placed on the magnetic stirrer, and the magnetic stirrer is adjusted to rotate at 600 r/min. Next, 1.0 g of the classified sample is put into a stirring beaker, and stirring is continued until the rotating vortex disappears and the liquid surface becomes horizontal, to prepare a 50 times swollen gel. The 50 times swollen gel is transferred to a centrifuge tube and degassed for 4 minutes in a centrifuge set so that the centrifugal force is 671 G to obtain a measurement sample.
Then, the measurement sample is set in the prepared dynamic rheometer Rheometer. Using a parallel plate having a diameter of 60 mm as a sample holder, the distance between the plates was 3 mm, the thickness of the gel was 3000 μm, the measurement temperature was 25° C., the frequency was ω=0.1 to 300 rad/sec, and the strain was 0.1% strain. To measure the storage modulus. Of these, the storage elastic modulus at 10 rad/sec is used in the above formula (I).
[Measuring method of the centrifugal retention rate]
While stirring 500 g of 0.9 mass% sodium chloride aqueous solution (physiological saline), 2.0 g of the water absorbent resin was dispersed and allowed to stand for 60 minutes, then poured into a cotton bag and the centrifugal force was 167 G using a dehydrator. It is dehydrated for 1 minute under the condition of 1), and the mass Wa (g) of the cotton bag containing the swollen gel after dehydration is measured. Further, the same operation is performed without adding the water absorbent resin, and the empty mass Wb (g) of the cotton bag when wet is measured. The centrifugal retention rate is calculated from the following formula (III).
Centrifugal retention (g/g)=[Wa-Wb] (g)/mass of water-absorbent resin (g) (III)
[Measurement method of water absorption capacity of physiological saline under a load of 4.14 kPa]
After spraying 0.10 g of the water-absorbent resin inside a cylinder having an inner diameter of 2.0 cm with a 200-mesh nylon mesh attached to the bottom, place a weight on the water-absorbent resin and apply a load of 4.14 kPa. Add. The cylinder is placed on the hole of a measuring table having a hole with a diameter of 2 mm so that the center of the cylinder coincides with the hole, the hole, and a burette having an air inlet and containing physiological saline. Are connected via a conduit, and the tip of the conduit on the measuring stand and the air inlet are arranged at the same height. The water absorption of the water-absorbent resin is started by opening the buret and the air inlet, and the decrease amount Wc (mL) of physiological saline after 60 minutes has elapsed from the start is measured, and a load of 4.14 kPa is calculated from the following formula (IV). Calculate the saline absorption capacity underneath.
Saline water absorption capacity under a load of 4.14 kPa (mL/g)=Wc/0.10 (g)
...(IV)
[Measurement method of the water-soluble content]
In a 500 mL beaker, 500 g of physiological saline is weighed, a magnetic stirrer bar (8 mmφ×30 mm without ring) is charged, and the beaker is placed on the magnetic stirrer (HS-30D manufactured by iuchi). Subsequently, the magnetic stirrer bar is adjusted to rotate at 600 rpm, and further, the bottom of the vortex generated by the rotation of the magnetic stirrer bar is adjusted to be near the upper part of the magnetic stirrer bar.
Next, 2.0 g of the water-absorbent resin particles are quickly poured and dispersed between the center of the vortex in the beaker and the side surface of the beaker, and the mixture is stirred for 3 hours. The dispersed water of the aqueous resin particles after stirring is filtered through a JIS standard sieve (opening 75 μm), and the obtained filtrate is suction filtered using a Kiriyama funnel (filter paper No. 6).
80 ± 0.01 g of the obtained filtrate was weighed in advance in a 100 ml beaker that had been dried at 140°C to a constant weight and cooled to room temperature, and the inner temperature was adjusted to 140°C using a hot air dryer (ADVANTEC) to make the weight constant. It is dried until it becomes, and the mass Wf (g) of the filtrate solid content is measured.
On the other hand, the blank mass Wg (g) is measured in the same manner as above without using the particles of the water absorbent resin, and the water-soluble content is calculated from the following formula (V).
Water-soluble content (mass %)=[(Wf-Wg)(500/80)]/2×100 (V) The water absorbent resin according to claim 1, wherein the amount of the azo compound used is 95% by mass or less based on the total amount of the azo compound and the peroxide used. The water absorbent resin according to claim 1, wherein the internal crosslinking agent and the post-crosslinking agent are ethylene glycol diglycidyl ether. An absorbent article comprising the absorbent body containing the water absorbent resin according to claim 1. Acrylic acid and/or salt thereof in the presence of a polyglycidyl compound as an internal cross-linking agent, wherein the azo compound is 40% by mass or more of the total amount of the azo compound and the peroxide used. And a method of producing a water absorbent resin by polymerizing in the presence of the peroxide, and by post-crosslinking with a polyglycidyl compound which is a post-crosslinking agent,
The water-absorbent resin has a saline water-absorption capacity of 16 mL/g or more under a load of 4.14 kPa, and the mass ratio of particles of 150 to 850 μm in the total mass of the water-absorbent resin is 85% by mass or more. In addition, the mass ratio of the particles having a particle size of 300 to 400 μm is 20% by mass or more, and the absorption capacity elastic index represented by the following formula (I) is 68000 or more.
Absorption capacity elastic index=storage elastic modulus [Pa]×centrifugal retention [g/g] (I)
[Measuring method of storage elastic modulus]
First, 49.0 g of physiological saline is weighed in a 100 mL beaker, a magnetic stirrer bar is put therein, the magnetic stirrer is placed on the magnetic stirrer, and the magnetic stirrer is adjusted to rotate at 600 r/min. Next, 1.0 g of the classified sample is put into a stirring beaker, and stirring is continued until the rotating vortex disappears and the liquid surface becomes horizontal, to prepare a 50 times swollen gel. The 50 times swollen gel is transferred to a centrifuge tube and degassed for 4 minutes in a centrifuge set so that the centrifugal force is 671 G to obtain a measurement sample.
Then, the measurement sample is Se Tsu preparative the dynamic viscoelasticity measuring apparatus rheometer. Using a parallel plate having a diameter of 60 mm as a sample holder, the distance between the plates was 3 mm, the thickness of the gel was 3000 μm, the measurement temperature was 25° C., the frequency was ω=0.1 to 300 rad/sec, and the strain was 0.1% strain. To measure the storage modulus. Of these, the storage elastic modulus at 10 rad/sec is used in the above formula (I).
[Measuring method of the centrifugal retention rate]
While stirring 500 g of 0.9 mass% sodium chloride aqueous solution (physiological saline), 2.0 g of the water absorbent resin was dispersed and allowed to stand for 60 minutes, then poured into a cotton bag and the centrifugal force was 167 G using a dehydrator. It is dehydrated for 1 minute under the condition of 1), and the mass Wa (g) of the cotton bag containing the swollen gel after dehydration is measured. Further, the same operation is performed without adding the water absorbent resin, and the empty mass Wb (g) of the cotton bag when wet is measured. The centrifugal retention rate is calculated from the following formula (II).
Centrifugal retention (g/g)=[Wa-Wb](g)/mass of water-absorbent resin (g) (II)
[Measurement method of water absorption capacity of physiological saline under a load of 4.14 kPa]
After spraying 0.10 g of the water-absorbent resin inside a cylinder having an inner diameter of 2.0 cm with a 200-mesh nylon mesh attached to the bottom, place a weight on the water-absorbent resin and apply a load of 4.14 kPa. Add. The cylinder is placed on the hole of a measuring table having a hole with a diameter of 2 mm so that the center of the cylinder coincides with the hole, the hole, and a burette having an air inlet and containing physiological saline. Are connected via a conduit, and the tip of the conduit on the measuring stand and the air inlet are arranged at the same height. The water absorption of the water-absorbent resin is started by opening the buret and the air introduction port, and the decrease amount Wc (mL) of physiological saline after 60 minutes has elapsed from the start is measured, and a load of 4.14 kPa is calculated from the following formula (III). Calculate the saline absorption capacity underneath.
Saline water absorption capacity under a load of 4.14 kPa (mL/g)=Wc/0.10 (g)
...(III)

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