JP2019072653A - Manufacturing method of absorbent, and absorbent - Google Patents

Abstract

To provide a manufacturing method of an absorbent having an excellent water absorption performance while capable of easily separating from a recycled resource when collecting the recycled resource, and also to provide the absorbent.SOLUTION: A manufacturing method of a water-absorbing resin includes a step of lyophilizing water-absorbing resin swelling with an aqueous solvent containing alcohol and water. The water-absorbing resin has a gel degradation index of 0.75 or less expressed by a gel degradation index=B/A (here, A is an initial gel strength (Pa), and B is a gel strength (Pa) three days later).SELECTED DRAWING: None

Description

  The present invention relates to a method of producing a water absorbing agent and a water absorbing agent.

Water-absorbent resins are widely used in various fields such as sanitary materials such as disposable diapers and sanitary products, agricultural and horticultural materials such as water retention agents and soil conditioners, and industrial materials such as water sealants and anti-condensing agents. In particular, water absorbent resins are often used in sanitary materials such as disposable diapers, sanitary products and simple toilets.
In recent years, from the viewpoint of environmental protection, attempts have been made to recover recycled resources (for example, pulp) contained in used sanitary materials (for example, disposable diapers) and reuse them. In order to recover reclaimed resources from sanitary materials, it is first necessary to separate the reclaimed resources from the water absorbent resin. However, in general, a water-absorbent resin (hereinafter referred to as a "spent water-absorbent resin") that has become a gel by absorbing water is difficult to separate from the reclaimed resources, and therefore, it can be used hygienic materials to absorb used water. It is difficult to recover only recycled resources except for resin.

  In order to solve such a problem, a method of decomposing a used water absorbent resin and then recovering a reclaimed resource has been studied. For example, in the techniques disclosed in Patent Documents 1 and 2, after the used disposable diaper is broken, the used water-absorbent resin contained in the disposable diaper is decomposed into monomers with water mixed with a polymer decomposing agent, and then the recycled resource is used. Pulp components are separated and collected.

Japanese Patent Laid-Open No. 2000-084533 JP, 2006-289154, A

  However, in the techniques disclosed in Patent Documents 1 and 2, since the polymer decomposing agent is required for decomposing the used water absorbent resin, not only the recovery operation of the recycled resources is complicated, but the polymer decomposing agent is always stored. There is a problem of having to

  The present invention has been made in view of the above, and provides a method for producing a water-absorbing agent that has excellent water-absorbing performance and can be easily separated from the reclaimed resource when recovering the reclaimed resource, and the water-absorbing agent. The purpose is to

The present inventors can easily separate the used water absorbent from the recycled resource without using any external means such as a polymer decomposing agent by using a water absorbent resin that exhibits excellent self-decomposition characteristics after water absorption. As a result of intensive research, the present invention has been completed.
Specifically, the present inventors have repeatedly conducted researches on water-absorbent resins having excellent water absorption performance equal to or higher than conventional water-absorbent agents and having excellent self-decomposition characteristics after water absorption, and as a result, It has been found that the above object can be achieved by lyophilizing a water-absorbent resin swollen with an aqueous solvent of a specific composition, and the present invention has been accomplished.

That is, the present invention includes, for example, the inventions described in the following sections.
Item 1
A method for producing a water-absorbing agent, comprising the step of freeze-drying a water-absorbent resin swollen with an aqueous solvent containing alcohol and water.
Item 2
Item 2. The method for producing a water-absorbing agent according to Item 1, wherein the aqueous solvent contains 1 to 25% by mass of an alcohol.
Item 3
Item 3. The method for producing a water-absorbent agent according to item 1 or 2, wherein the alcohol is a lower alcohol having 5 or less carbon atoms.
Item 4
Item 4. The method for producing a water-absorbing agent according to any one of Items 1 to 3, wherein the alcohol is a secondary alcohol or a tertiary alcohol.
Item 5
Item 3. The method for producing a water-absorbing agent according to Item 1 or 2, wherein the alcohol is a tertiary alcohol having 5 or less carbon atoms.
Item 6
Has a porous structure,
A water absorbing agent whose gel degradation index represented by the following formula (1) is 0.75 or less.
Gel degradation index = B / A (1)
(Here, A is the initial gel strength (Pa), B is the gel strength after 3 days (Pa))
Item 7
The water-absorbing agent according to claim 6, which has a water-holding capacity of 35 to 50 g / g or more, a water-absorbing capacity under pressure of 24 to 30 ml / g, and a water-absorbing speed of 35 to 42 seconds with respect to physiological saline.

According to the method of producing a water absorbing agent of the present invention, it is possible to easily produce a water absorbing agent that can be easily separated from regenerated resources, while having excellent water absorption performance equal to or greater than that of the conventional water absorbing agent.
The water absorbing agent of the present invention has a rapid decomposition after water absorption while maintaining excellent water absorbing performance.

It is a schematic diagram which shows schematic structure of the apparatus for measuring water absorption ability under pressure. The image which observed the water absorption agent obtained by each Example and the comparative example by 200 times the magnification of this using a scanning electron microscope (SEM) is shown.

Hereinafter, embodiments of the present invention will be described in detail.
In addition, in this specification, the numerical range connected by "-" means the numerical range which includes the numerical value before and behind "-" as a lower limit and an upper limit. When a plurality of lower limits and a plurality of upper limits are described separately, a specific numerical range can be obtained by selecting an arbitrary lower limit and an arbitrary upper limit and connecting them with "-". .
Further, in the present specification, in order to distinguish between the water-absorbent resin before and after being freeze-dried, the freeze-dried water-absorbent resin is referred to as a "water-absorbent agent".

1. Method for Producing Water-Absorbing Agent The method for producing a water-absorbing agent of the present invention comprises the steps of freeze-drying a water-absorbent resin swollen with an aqueous solvent containing alcohol and water (hereinafter sometimes abbreviated as "lyophilization step") Including.
Hereinafter, the freeze-drying step will be described after the water-absorbent resin used in the present invention is described.
Water-absorbent resin
The type of water-absorbent resin used in the present invention is not particularly limited. For example, known water absorbent resins can be widely used. For example, the water-absorbent resin may be a hydrolyzate of a starch-acrylonitrile graft copolymer, a neutralized product of a starch-acrylic acid graft polymer, a saponified vinyl acetate-acrylic acid ester copolymer, and an acrylic acid polymer portion. Japanese salt cross-linked products etc. may be mentioned. Among these, partially cross-linked acrylic acid polymer neutralized salt is preferable from the viewpoint of production amount and production cost.

The shape of the water-absorbent resin is not particularly limited, and may be, for example, various shapes such as spherical, crushed, granular, elliptical, scaly, rod-like, and massive.
The median particle diameter of the water-absorbent resin is also not particularly limited, and can be in the same range as a known water-absorbent resin. For example, the median particle diameter can be 100 to 800 μm, preferably 200 to 600 μm, more preferably 250 to 550 μm, and particularly preferably 300 to 500 μm.

  In the production method of the present invention, for example, a commercially available water absorbent resin can be used. Alternatively, in the production method of the present invention, the water absorbent resin can be separately produced. That is, the production method of the present invention can also include the step of producing a water absorbent resin prior to the lyophilization step.

  Examples of the method for producing the water absorbent resin include reverse phase suspension polymerization and aqueous solution polymerization. Among them, the reverse phase suspension polymerization method is preferable in that it is easy to obtain a water absorbent resin having the simplicity of the production process, the excellent water absorption performance and the like. The reverse phase suspension polymerization is a method of suspending and polymerizing a poorly soluble polymerizable monomer in the dispersion medium in the presence of a dispersion stabilizer in a dispersion medium.

  The polymerization reaction can be carried out in a dispersion medium. As the dispersion medium, a hydrocarbon dispersion medium can be used. The hydrocarbon dispersion medium includes aliphatic hydrocarbons such as n-hexane, n-heptane, n-octane and ligroin; alicyclic hydrocarbons such as cyclopentane, methylcyclopentane, cyclohexane and methylcyclohexane; and benzene and toluene And aromatic hydrocarbons such as xylene. Among these dispersion media, n-hexane, n-heptane, and cyclohexane are preferably used because they are industrially easily available, stable in quality, and inexpensive. These dispersion media may be used alone or in combination of two or more. As the dispersion medium, for example, Exxsol Heptane (manufactured by Exxon Mobil: heptane and isomeric hydrocarbons) and Nappar 6 (manufactured by Exxon Mobil: cyclohexane and isomeric hydrocarbons) which are known as mixed solvents can be used.

As the polymerizable monomer, for example, a water-soluble ethylenically unsaturated monomer is used. Examples of the water-soluble ethylenically unsaturated monomer include (meth) acrylic acid and a salt thereof, 2- (meth) acrylamide-2-methylpropane sulfonic acid and a salt thereof, (meth) acrylamide, N, N-dimethyl Nonionic monomers such as (meth) acrylamide, 2-hydroxyethyl (meth) acrylate, N-methylol (meth) acrylamide, polyethylene glycol mono (meth) acrylate; and N, N-diethylaminoethyl (meth) acrylate And amino group-containing unsaturated monomers such as N, N-diethylaminopropyl (meth) acrylate and diethylaminopropyl (meth) acrylamide, or quaternary compounds thereof. The water-soluble ethylenically unsaturated monomers may be used alone or in combination of two or more. As the water-soluble ethylenically unsaturated monomer, acrylic acid, methacrylic acid and salts thereof, acrylamide, methacrylamide and N, N-dimethyl acrylamide are preferably used.
In the present specification, "(meth) acrylic" means "acrylic or methacrylic". That is, for example, the description of "(meth) acrylic acid" is the same as the description of "acrylic acid or methacrylic acid".

When a monomer having an acid group such as (meth) acrylic acid or 2- (meth) acrylamido-2-methylpropanesulfonic acid is used as the water-soluble ethylenic unsaturated monomer, the acid group is previously made alkaline You may neutralize by a Japanese agent. Examples of such an alkaline neutralizer include alkali metal compounds such as sodium hydroxide and potassium hydroxide, and ammonia, and may be used in the form of an aqueous solution. The degree of neutralization is also not particularly limited, and can be, for example, similar to known reverse phase suspension polymerization.
The water-soluble ethylenically unsaturated monomer to be used can also be an aqueous solution. The concentration of the aqueous solution is not particularly limited, but may be usually 20% by mass or more and saturating concentration or less, preferably 25 to 70% by mass, and more preferably 30 to 55% by mass.

  Thickeners can also be used in the polymerization reaction, if necessary. As a thickener, for example, hydroxyethyl cellulose, hydroxypropyl cellulose, methyl cellulose, carboxymethyl cellulose, polyacrylic acid, polyacrylic acid (partially) neutralized product, polyethylene glycol, polyacrylamide, polyethylene imine, dextrin, sodium alginate, polyvinyl alcohol , Polyvinyl pyrrolidone, polyethylene oxide and the like can be used.

  As a dispersion stabilizer used in the polymerization reaction, for example, a surfactant can be used. As the surfactant, for example, 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, alkyl allyl formaldehyde condensed polyoxyethylene ether, polyoxyethylene polyoxypropylene block copolymer, polyoxyethylene polyoxypropyl alkyl ether, Polyethylene glycol fatty acid ester, alkyl glucoside, N-alkyl gluconamide Polyoxyethylene fatty acid amides, polyoxyethylene alkyl amines, phosphoric esters of polyoxyethylene alkyl ethers, can be used phosphoric acid ester of polyoxyethylene alkyl aryl ether. Among them, sorbitan fatty acid ester, polyglycerin fatty acid ester, sucrose fatty acid ester and the like are preferable 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 surfactant used is a water soluble ethylenically unsaturated single amount used in the polymerization reaction in order to maintain the dispersion state of the monomer well in the hydrocarbon dispersion medium and to obtain the dispersion effect corresponding to the amount used. Preferably it is 0.1-30 mass parts with respect to 100 mass parts of bodies, More preferably, it is 0.3-20 mass parts.

  Further, as a dispersion stabilizer, a polymeric dispersant may be used in combination with the surfactant. Examples of polymer 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), anhydride 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 Copolymers, oxidized polyethylene, oxidized polypropylene, oxidized ethylene / propylene copolymer, ethylene / acrylic acid copolymer, ethyl cellulose, ethyl hydroxyethyl cellulose and the like can be mentioned. Among them, maleic anhydride-modified polyethylene, maleic anhydride-modified polypropylene, maleic anhydride-modified ethylene-propylene copolymer, maleic anhydride-ethylene copolymer, maleic anhydride Preferred are propylene copolymer, maleic anhydride / ethylene / propylene copolymer, polyethylene, polypropylene, ethylene / propylene copolymer, oxidized polyethylene, oxidized polypropylene, oxidized ethylene / propylene copolymer and the like. These polymeric dispersants may be used alone or in combination of two or more.

  The amount of the polymeric dispersant used is a water-soluble ethylenic unsaturation to be used in the polymerization reaction in order to maintain the dispersed state of the monomer well in the hydrocarbon dispersion medium and to obtain the dispersion effect corresponding to the amount used. Preferably it is 0.1-30 mass parts with respect to 100 mass parts of monomers, More preferably, it is 0.3-20 mass parts.

  In the polymerization reaction, for example, known polymerization initiators can be widely used. As a radical polymerization initiator, for example, persulfates such as potassium persulfate, ammonium persulfate and sodium persulfate; methyl ethyl ketone peroxide, methyl isobutyl ketone peroxide, di-t-butyl peroxide, t-butylcumyl peroxide, Peroxides such as hydrogen peroxide; and 2,2'-azobis (2-methylpropionamidine) dihydrochloride, 2,2'-azobis [2- (N-phenylamidino) propane] dihydrochloride, Examples include azo compounds such as 2,2'-azobis [2- (N-allylamidino) propane] dihydrochloride, 4,4'-azobis (4-cyanovaleric acid) and the like.

The radical polymerization initiators may be used alone or in combination of two or more. For example, persulfates such as potassium persulfate and azo compounds such as 2,2'-azobis (2-amidinopropane) dihydrochloride can be used in combination.
The radical polymerization initiator can be used as a redox polymerization initiator in combination with a reducing agent such as sodium sulfite, sodium bisulfite, ferrous sulfate, L-ascorbic acid and the like.
The amount of the radical polymerization initiator used in the polymerization reaction can be, for example, 0.005 to 1 mol% with respect to the total amount of water-soluble ethylenically unsaturated monomers used. In this case, a rapid polymerization reaction is unlikely to occur, and the reaction time is also unlikely to be longer than necessary.

In the polymerization reaction, a chain transfer agent can also be used if necessary. As a chain transfer agent, hypophosphites, thiols, thiol acids, secondary alcohols, amines etc. are illustrated.
In the polymerization reaction, a crosslinking agent can be used as needed.

  As a crosslinking agent used by a polymerization reaction, the compound which has 2 or more of polymerizable unsaturated groups is mentioned. Specific examples of the crosslinking agent include di- or tri (meth) acrylic acid esters of polyols; unsaturated polyesters obtained by reacting polyols with unsaturated acids such as maleic acid and fumaric acid; N, N'-methylenebis (meth ) Bisacrylamides such as acrylamide; Di or tri (meth) acrylic esters obtained by reacting polyepoxide with (meth) acrylic acid; Polyisocyanates such as tolylene diisocyanate or hexamethylene diisocyanate and hydroxyethyl (meth) acrylate And di (meth) acrylic acid carbamyl ester obtained by Polyol di or di Examples of the polyol in (meth) acrylic acid ester include (poly) ethylene glycol [in this specification, for example, “polyethylene glycol” and “ethylene glycol” are combined and denoted as “(poly) ethylene glycol”], (poly And the like) propylene glycol, trimethylolpropane, glycerine polyoxyethylene glycol, polyoxypropylene glycol, (poly) glycerin and the like.

  Moreover, as a crosslinking agent, glycidyl groups such as (poly) ethylene glycol diglycidyl ether, (poly) propylene glycol diglycidyl ether, and (poly) glycerin diglycidyl ether, in addition to the compound having two or more of the above-mentioned polymerizable unsaturated groups Also included are compounds containing, (poly) ethylene glycol, (poly) propylene glycol, (poly) glycerin, pentaerythritol, ethylenediamine, polyethyleneimine, glycidyl (meth) acrylate and the like.

  The amount of the crosslinking agent used can be 1 mol% or less, preferably 0.5 mol% or less, and more preferably 0.001 mol% or less based on the total amount of the water-soluble ethylenically unsaturated monomer used. It is -0.25 mol%. If the amount of the crosslinking agent used is in this range, the water absorption performance of the resulting water absorbent resin is likely to be enhanced.

  The temperature of the polymerization reaction can be appropriately set according to the type and amount of use of the radical polymerization initiator, and may be, for example, 20 to 110 ° C., and preferably 40 to 90 ° C. The reaction time can be set, for example, in the range of 0.1 hours to 4 hours.

  For example, water in which a water-soluble ethylenically unsaturated monomer, a thickener, a radical polymerization initiator, and a crosslinking agent, which are optionally neutralized, is added to a dispersion medium in which a polymer dispersion stabilizer is dissolved. The polymerization reaction can be carried out by adding and then adding a surfactant. In addition, the order which adds each raw material is not limited to this.

The water soluble ethylenically unsaturated monomer is polymerized to form a polymer as primary particles. In this step, for example, a slurry in which a polymer of a water-soluble ethylenically unsaturated monomer is dispersed is obtained.
The reverse phase suspension polymerization may be performed in one stage, or may be performed in two or more stages. The number of stages is preferably two to three from the viewpoint of enhancing the productivity.

When performing two or more stages of reverse phase suspension polymerization, after performing the first stage reverse phase suspension polymerization by the method described above, water-soluble ethylenicity is obtained in the reaction mixture obtained by the first stage polymerization reaction. Unsaturated monomers can be added and mixed, and reverse phase suspension polymerization in the second and subsequent stages can be performed in the same manner as in the first stage.
In the reverse phase suspension polymerization in each of the second and subsequent stages, in addition to the water-soluble ethylenic unsaturated monomer, a radical polymerization initiator and a crosslinking agent to be added as needed are each of the second and subsequent stages. Based on the amount of the water-soluble ethylenically unsaturated monomer added in the reverse phase suspension polymerization in the stage, added within the range of the molar ratio of each component to the water-soluble ethylenically unsaturated monomer described above The reverse phase suspension polymerization may be performed under the same conditions as those described above.

  A water absorbing resin can be obtained by the above polymerization reaction.

  After the polymerization reaction, a step of post-crosslinking the water-absorbent resin using a crosslinking agent (post-crosslinking step) can also be provided. In addition, the crosslinking agent for post-crosslinking is called "post-crosslinking agent". By post-crosslinking the water-absorbent resin with the post-crosslinking agent, the vicinity of the surface of the water-absorbent resin is further crosslinked, and the crosslink density in the vicinity of the surface becomes higher than the inside. Thereby, the strength and the water absorption performance of the water absorbent resin can be particularly enhanced.

  The post-crosslinking agent can be added, for example, to the reaction solution in which the polymer of the water-soluble ethylenically unsaturated monomer is dispersed after the completion of the polymerization reaction. The addition time of the post-crosslinking agent is not particularly limited, and may be, for example, the same conditions as known reverse phase suspension polymerization methods.

  As post-crosslinking agents, compounds having two or more reactive functional groups can be used. Specific examples of the post-crosslinking agent include diglycidyl group-containing compounds such as (poly) ethylene glycol diglycidyl ether, (poly) glycerol (poly) glycidyl ether, (poly) propylene glycol diglycidyl ether, and (poly) glycerin diglycidyl ether. And (poly) ethylene glycol, (poly) propylene glycol, (poly) glycerin, pentaerythritol, ethylene diamine, polyethylene imine and the like. Among these, (poly) ethylene glycol diglycidyl ether, (poly) propylene glycol diglycidyl ether, and (poly) glycerin diglycidyl ether are particularly preferable. The post-crosslinking agents may be used alone or in combination of two or more.

  The amount of post-crosslinking agent used can not be determined indiscriminately because it varies depending on the type thereof, but generally it is 0.00001 relative to 1 mol of the total of water-soluble ethylenically unsaturated monomers used in the polymerization reaction. It is -0.01 mol, preferably 0.00005-0.005 mol, more preferably 0.0001-0.002 mol. From the viewpoint of sufficiently increasing the surface crosslink density of the water-absorbent resin and increasing the gel strength of the water-absorbent resin, the amount of post-crosslinking agent used is preferably 0.00001 mol or more, and from the viewpoint of increasing the water retention capacity of the water-absorbent resin 0 .01 mol or less is preferable.

  The addition time of the post-crosslinking agent is after the completion of the polymerization. The post-crosslinking agent is preferably added in a state in which the polymer contains water in the range of 1 to 400 parts by mass with respect to 100 parts by mass of the water-soluble ethylenically unsaturated monomer constituting the polymer, It is more preferable to add the water in the range of 200 parts by mass, and it is particularly preferable to add the water in the range of 10 to 100 parts by mass. The amount of water is the total amount of the water contained in the polymerization reaction system and the water used as needed when adding the post-crosslinking agent.

  The reaction temperature of the post-crosslinking reaction can be, for example, 50 to 250 ° C., preferably 60 to 180 ° C., more preferably 60 to 140 ° C., particularly preferably 70 to 120 ° C. preferable. The reaction time can not be generally determined because it varies depending on the reaction temperature, the type and amount of the post-crosslinking agent, etc., but it is usually 1 to 300 minutes, preferably 5 to 200 minutes.

After the polymerization reaction or after the post-crosslinking reaction, a drying step may be performed to dry the obtained water-absorbent resin. In the drying step, energy such as heat is externally applied to the reaction system to remove water, a hydrocarbon dispersion medium and the like from the reaction system by distillation. The drying step may be performed under normal pressure or under reduced pressure, or may be performed under a stream of nitrogen or the like to enhance the drying efficiency. When a drying process is a normal pressure, 70-250 degreeC is preferable, 80-180 degreeC is more preferable, 80-140 degreeC is still more preferable, and 90-130 degreeC is especially preferable. In the case of under reduced pressure, the drying temperature is preferably 40 to 160 ° C, and more preferably 50 to 120 ° C. In addition, the above-mentioned post-crosslinking step and the drying step can be carried out simultaneously.
The water absorbing resin obtained by drying can also be subjected to particle size adjustment by classification treatment using a sieve or the like as appropriate.

  In addition, after the polymerization reaction, in order to impart various properties to the water-absorbent resin, various additives according to the purpose may be blended. Examples of such additives include inorganic powders, surfactants, oxidizing agents, reducing agents, metal chelating agents, radical chain inhibitors, antioxidants, antibacterial agents, deodorants and the like. 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 flowability of the water absorbent resin can be improved.

<Lyophilization process>
A water absorbing agent can be produced by lyophilizing the above-described water absorbent resin with an aqueous solvent containing alcohol and water. The water-absorbing agent thus obtained has excellent water-absorbing performance, and also has a rapid decomposition after water absorption (that is, excellent self-degrading properties).

(Aqueous solvent)
The aqueous solvent used in the lyophilization process contains alcohol and water as essential components. The proportion of water contained in the aqueous solvent is preferably 60% by mass or more, more preferably 70% by mass or more. The aqueous solvent may contain an additive such as a surfactant or a solvent other than water, but preferably contains substantially only alcohol and water. The phrase "contains essentially only alcohol and water" includes not only cases where the aqueous solvent consists only of alcohol and water, but also cases where the aqueous solvent contains a trace amount of additives and / or solvents other than water. Specifically, it includes the case where the additive and / or the solvent other than water is more than 0 and 1% by mass or less, preferably the additive and / or the solvent other than water is more than 0 and 0.5 Including the case of containing at most mass%.

The lower limit value of the alcohol content in the aqueous solvent is preferably 1% by mass, more preferably 3% by mass, and still more preferably 5% by mass. The upper limit thereof is preferably 25% by mass, more preferably 20% by mass, and still more preferably 18% by mass.
The water-absorbing agent obtained by using such an aqueous solvent in the lyophilization step is excellent in water absorption performance and autolysis characteristics.

As the alcohol used in the present invention, conventionally known alcohols can be used as long as they can impart autolysis characteristics to the water absorbing agent.
In this respect, the reason why the water-absorbing agent obtained by the production method of the present invention is excellent in the self-decomposition properties is not clear, but the present inventors speculate as follows.

  The water-absorbent resin is swollen with an aqueous solvent containing alcohol and water, and then it is freeze-dried to obtain a water-absorbent agent having a porous structure having a plurality of micropores, which results from this porous structure. It is considered that the water absorbing agent is easily decomposed after absorbing water. In particular, the alcohol contained in the aqueous solvent is considered to inhibit the formation of large ice crystals inside the water-absorbent resin when it is frozen. Therefore, in the present invention, it is considered that a water absorbing agent having a porous structure having smaller pores can be obtained as compared with the case where the water absorbent resin is freeze-dried using only water, and as a result, in the production method of the present invention The water-absorbing agent obtained is presumed to be excellent in the autolysis properties.

The alcohol used in the present invention may be a higher alcohol having 6 or more carbon atoms, or a lower alcohol having 5 or less carbon atoms. The alcohol may be a polyhydric alcohol having two or more hydroxyl groups, or a monohydric alcohol having one hydroxyl group.
Among these, it is preferable to use an alcohol that satisfies one of the two conditions of (1) high solubility in water (2) high vapor pressure, and use an alcohol that satisfies all the two conditions. Is more preferred. Alcohols with high water solubility tend to inhibit the formation of large ice crystals in the water-absorbent resin, and alcohols with high vapor pressure tend to sublime when the water-absorbent resin is freeze-dried. Therefore, it is considered that a water absorbing agent having a porous structure having more minute pores can be obtained by using an alcohol satisfying the conditions (1) and / or (2).

  From the viewpoint of the above (1), the alcohol is preferably a lower alcohol having 5 or less carbon atoms. Lower alcohols having 5 or less carbon atoms have high solubility in water because the amount of hydrophobic components is reduced. In addition, the lower the valence of alcohol (the smaller the number of hydroxyl groups), the lower the boiling point (the higher the vapor pressure). Similarly, the higher the alcohol series, the lower the boiling point because the steric hindrance causes the hydrogen bond to be more prejudicial. Therefore, from the viewpoint of the above (2), the alcohol is preferably a monohydric alcohol, and among monohydric alcohols, in particular, tertiary or secondary alcohols are preferable, and tertiary alcohols are more preferable. preferable.

  Considering these conditions comprehensively, alcohols such as methanol, ethanol, n-propanol, propan-2-ol, 2-butanol, tert-butyl alcohol, n-butanol, n-pentanol, isopentanol, etc. A monohydric alcohol having 5 or less carbon atoms is preferable, and a secondary or tertiary alcohol having 5 or less carbon atoms such as propan-2-ol, butan-2-ol, tert-butyl alcohol, 2-pentanol and the like. It is more preferable that it is a tertiary alcohol having 5 or less carbon atoms such as tert-butyl alcohol and 2-methyl-2-butanol.

  In particular, tert-butyl alcohol is particularly preferably used. By using tert-butyl alcohol, it is possible to obtain a water absorbing agent having both excellent water absorption performance and excellent self-decomposition characteristics.

  The alcohol may be used singly or in combination of two or more different ones.

(Swelling method)
By absorbing the aqueous solvent in the water-absorbent resin (that is, swelling the water-absorbent resin with the aqueous solvent), a water-absorbent resin whose volume is expanded by the aqueous solvent can be obtained. The method for swelling the water-absorbent resin with an aqueous solvent is not particularly limited, and, for example, known methods can be widely adopted.

  For example, the water-absorbent resin can be swollen with the aqueous solvent by mixing the aqueous solvent and the water-absorbent resin. Specifically, the water absorbent resin can be swollen with the aqueous solvent by stirring the water absorbent resin in the aqueous solvent.

  When the water absorbing resin is allowed to absorb water into the aqueous solvent, the swelling ratio of the water absorbing resin can be appropriately set in accordance with the type of aqueous solvent to be used and the type of water absorbing resin. For example, the lower limit value of the swelling ratio is 1.5 times, preferably 2 times, more preferably 3 times. The upper limit value of the swelling ratio is, for example, 1000 times, preferably 100 times, and more preferably 10 times. The swelling ratio can be calculated by dividing the total mass of the aqueous solvent and the water absorbent resin used for the swelling treatment by the used mass of the water absorbent resin.

(freeze drying)
In the lyophilization step, as described above, the water-absorbent resin swollen with the aqueous solvent containing alcohol and water is lyophilized. "Lyophilization" is the freezing of a water-absorbent resin swollen with an aqueous solvent, and the sublimation of substantially all the aqueous solvent from the frozen water-absorbent resin.
The method for freezing the swollen water-absorbent resin is not particularly limited, and it is preferable to freeze the swollen water-absorbent resin in a refrigerator kept at a low temperature atmosphere for a predetermined time.

  The temperature in the refrigerator may be lower than the freezing point of the aqueous solvent used, for example, the upper limit thereof is -10 ° C or less, preferably -20 ° C or less, more preferably -30 ° C or less is there. Moreover, the lower limit is -100 degreeC, for example, Preferably it is -80 degreeC.

  The method of drying the frozen water-absorbent resin (method of sublimating substantially all of the frozen aqueous solvent) is not particularly limited, and the frozen water-absorbent resin is kept in the dryer for a predetermined time under a reduced pressure atmosphere. It is preferable to leave still.

The pressure inside the dryer may be lower than the atmospheric pressure, and the upper limit thereof is preferably, for example, 100 Pa, more preferably 50 Pa, and still more preferably 20 Pa. Further, the lower limit thereof is preferably 1 Pa, and more preferably 5 Pa.
The temperature in the dryer may be higher than the freezing point of the aqueous solvent, for example, -20 ° C to 10 ° C, preferably -10 ° C to 0 ° C. The temperature in the dryer may be constant or may be raised stepwise. Specifically, the initially frozen water-absorbent resin can be primarily dried, and then secondary drying can be performed at a higher temperature than primary drying.
In addition, although a refrigerator and a dryer may be another apparatus, one apparatus which has a freezing function and a drying function can also be used. As such an apparatus, the dry chamber mentioned later can be illustrated.

  In the present specification, “lyophilization (sublimate substantially all the aqueous solvent from the frozen water-absorbent resin)” means to completely sublime the aqueous solvent contained in the frozen water-absorbent resin. In addition, it includes cases where the aqueous solvent slightly remains in the water absorbing agent, and specifically includes cases where the aqueous solvent remains 10% by mass or less based on the total mass of the dried water absorbing agent, These include cases where 5% by weight or less remain, more preferably 1% by weight or less.

  The median particle diameter of the water absorbing agent obtained by the production method of the present invention can be 100 to 2000 μm, preferably 200 to 1500 μm, more preferably 250 to 1200 μm, and particularly preferably 300 to 1000 μm.

  The water-absorbing agent obtained by the production method of the present invention has a porous structure in which pores smaller than those of the prior art are formed, so that it has excellent water-absorbing performance while rapidly decomposing the water-absorbent resin after water absorption. It progresses and is excellent in the autolysis characteristic.

2. Water Absorbent Next, the water absorbent of the present invention will be described. The water absorbing agent of the present invention has the following formula (1)
Gel degradation index = B / A (1)
(Here, A is the initial gel strength (Pa), B is the gel strength after 3 days (Pa))
The gel degradation index represented by is less than 0.75.

  The initial gel strength of the water absorbing agent is preferably 700 Pa or more. When the initial gel strength is 700 Pa or more, the water-absorbing agent has sufficient gel strength, so application to various applications is facilitated. The initial gel strength of the water absorbing agent is more preferably 800 Pa or more, and still more preferably 900 Pa or more, since it is easy to impart excellent use feeling and absorption performance to the absorbent and absorbent article produced using the water absorbing agent.

  When the gel degradation index is 0.75 or less, the water absorbing agent has a rapid decomposition after water absorption, and is excellent also in the autolysis characteristics. Absorbent articles and the like containing such a water absorbing agent are easy to recycle and discard, and are advantageous from the viewpoint of eco-friendliness. The gel degradation index is more preferably 0.7 or less. Further, the lower limit value of the gel deterioration index can be, for example, 0.3.

  In addition, from the viewpoint that the absorbent or the absorbent article can easily obtain better absorption performance, the water retention ability of the water absorbing agent to physiological saline (0.9% sodium chloride aqueous solution) is preferably 35 to 50 g / g. The water absorption capacity under pressure is preferably 24 to 30 ml / g, and the water absorption speed to physiological saline is preferably 35 to 42 seconds.

The water absorbing agent of the present invention has a porous structure, and the manufacturing method is not particularly limited as long as the gel degradation index satisfies 0.75 or less. For example, as described above, the water-absorbent resin is synthesized by reverse phase suspension polymerization, and the obtained water-absorbent resin is freeze-dried under the same conditions as the above-mentioned freeze-drying step, so that the gel degradation index is 0. A water-absorbing agent having a porous structure satisfying 75 or less can be produced.
The other configurations of the water absorbing agent, for example, the shape and the median particle diameter can be all the same as those of the water absorbing agent described in the section of the method of manufacturing the water absorbing agent described above.

3. Absorbent Articles Various absorbent articles can be formed using the water absorbing agent obtained by the production method of the present invention or the water absorbing agent of the present invention. An absorbent article is provided with the absorber containing a water absorption agent. Specifically, an absorbent article can be formed of a water absorbing agent and a hydrophilic fiber.
Examples of hydrophilic fibers include cellulose fibers and artificial cellulose fibers. The hydrophilic fiber may contain a synthetic fiber having hydrophobicity, as long as the object of the present invention is not inhibited.

  The content of the water-absorbing agent in the absorber may be generally about 40% by mass or more, preferably 50% by mass or more, and more preferably from the viewpoint of sufficiently absorbing body fluid such as urine and providing a comfortable wearing feeling. Is 60% by mass or more, particularly preferably 70% by mass or more. In addition, the content of the water absorbing agent in the absorber may be generally about 98% by mass or less, in consideration of including an appropriate amount of hydrophilic fiber or the like, in order to enhance the form retention of the obtained absorber. Preferably it is 95 mass% or less, More preferably, it is 90 mass% or less.

The structure of the absorber is not particularly limited, and may be, for example, the same structure as known. An absorbent article can be obtained by, for example, holding the absorber between the liquid-permeable sheet and the liquid-impermeable sheet.
Examples of the liquid permeable sheet include non-woven fabrics of air through type, spun bond type, chemical bond type, needle punch type and the like made of fibers such as polyethylene, polypropylene and polyester.
As a liquid impervious sheet, the synthetic resin film etc. which consist of resin, such as polyethylene, a polypropylene, polyvinyl chloride, etc. are mentioned, for example.

  The type of absorbent article is not particularly limited. For example, sanitary materials such as disposable diapers, sanitary napkins, incontinence pads, urine absorbing materials for pets, materials for civil engineering and construction such as packing materials, materials for maintaining food freshness such as drip absorbents, cold insulators, soil Agricultural and horticultural articles such as water-retaining materials, water-stopping materials and the like can be mentioned.

EXAMPLES Hereinafter, the present invention will be more specifically described by way of examples. However, the present invention is not limited to the embodiments of these examples.
(Production Example 1; Production of Water Absorbent Resin)
A 2 L round bottom cylindrical separable flask with an inner diameter of 110 mm was prepared, equipped with a reflux condenser, a dropping funnel, a nitrogen gas inlet pipe, and a stirring blade having two stages of four inclined paddle blades with a blade diameter of 50 mm. In this flask, 300 g of n-heptane is taken as a hydrocarbon dispersion medium, and 0.74 g of maleic anhydride-modified ethylene-propylene copolymer (Mitsui Chemical Co., Ltd., Hi Wax 1105A) as a polymeric dispersant, and surface activity As an agent, 0.74 g of sucrose stearic acid ester of HLB3 (Mitsubishi Chemical Foods Co., Ltd., Ryoto Sugar Ester S-370) was added, and while heating and dissolving while stirring, it was cooled to 55 ° C.

  On the other hand, take 92 g (1.02 mol) of an 80% by weight aqueous solution of acrylic acid in a 500 mL Erlenmeyer flask, add 102.2 g of a 30% by weight aqueous solution of sodium hydroxide dropwise while cooling from the outside, The neutralization was done. Thereto, 0.092 g of hydroxyethyl cellulose as a thickener (Sumitomo Seika Co., Ltd., HECAW-15F), 0.055 g of 0.22 g of 2,2'-azobis (2-amidinopropane) dihydrochloride as an azo compound (0. Dissolve with 204 mmol), 0.009 g (0.034 mmol) of potassium persulfate as persulfate, 0.006 g (0.037 mmol) of ethylene glycol diglycidyl ether as internal crosslinking agent and 48.0 g of ion exchanged water To prepare an aqueous monomer solution.

  The aqueous monomer solution was added to the separable flask into which a hydrocarbon dispersant in which a polymeric dispersant and a surfactant were dissolved was charged. After the inside of the system is sufficiently replaced with nitrogen while stirring in the separable flask, the flask is immersed in a 70 ° C. water bath to raise the temperature, and the polymerization reaction is carried out for 10 minutes to obtain the first-stage reaction mixture. Obtained.

  On the other hand, take 128.8 g (1.43 mol) of an 80% by weight aqueous solution of acrylic acid in another 500 mL Erlenmeyer flask, and while cooling from the outside, add 143.1 g of a 30% by weight aqueous solution of sodium hydroxide dropwise Neutralization was performed at 75 mol%. There, 0.077 g (0.285 mmol) of 2,2'-azobis (2-amidinopropane) dihydrochloride as an azo compound, 0.013 g (0.048 mmol) of potassium persulfate as a peroxide, internal As a crosslinking agent, 0.009 g (0.052 mmol) of ethylene glycol diglycidyl ether and 12.5 g of ion exchange water were added and dissolved to prepare an aqueous monomer solution for the second stage polymerization reaction.

After cooling the first stage reaction mixture to 25 ° C., the entire amount of the second stage monomer aqueous solution was added to the first stage reaction mixture to sufficiently replace the inside of the system with nitrogen. Thereafter, the flask was again immersed in a 70 ° C. water bath to raise the temperature, and the second stage polymerization reaction was carried out for 5 minutes to obtain a second stage reaction mixture.
After the second stage polymerization, the reaction solution was heated in a 125 ° C. oil bath, and 274 g of water was extracted out of the system while refluxing n-heptane by azeotropic distillation of n-heptane and water. Then, 5.89 g (0.53 mmol) of a 4.5% aqueous solution of diethylenetriamine pentaacetic acid pentasodium salt as a chelating agent, and 4.42 g (0.51 mmol) of a 2% aqueous solution of ethylene glycol diglycidyl ether as a post-crosslinking agent ) Was added and kept at 80 ° C. for 2 hours, then n-heptane was evaporated to dryness. The obtained polymer was passed through a sieve of 850 μm, to obtain 231.2 g of a water absorbent resin in the form of aggregated spherical particles. The water-absorbent resin had a median particle size of 320 μm and a water content of 7.3%. With respect to the water-absorbent resin thus obtained, 0.2% by mass of amorphous silica (Carplex # 80, manufactured by Evonik Degussa Japan Co., Ltd.) was mixed and used in the following Examples and Comparative Examples.

Example 1
In a 300 mL beaker with a rotor, 76 g of water and 4 g of tert-butyl alcohol are added to prepare 80 g of an aqueous solvent, and then a magnetic stirrer (M-20G manufactured by Koike Precision Instruments Mfg. Co., Ltd.) (stirler chip: 8 mm × 30 mm, 20 g of the water-absorbent resin produced in Production Example 1 was charged into an aqueous solvent (80 g) at 25 ° C. while stirring at 600 rpm with no ring. The stirring of the aqueous solvent was stopped immediately before the vortex disappeared and the liquid level became horizontal, the contents were taken out, and 100 g of a water-absorbent resin swollen with an aqueous solvent five times was obtained. At this time, it took 12 seconds for the water absorbent resin to completely absorb the aqueous solvent. The water-absorbent resin was freeze-dried as follows.

First, the swollen water-absorbent resin was uniformly spread on a SUS vat (24 cm × 24 cm), and frozen for 20 minutes in a dry chamber at −40 ° C. (EDRA “DRC-1000”). Then, after making the inside of a dry chamber into 10 Pa or less using a vacuum pump (ULVAC "OMC-200"), the dry chamber was set to -10 degreeC.
One hour after the temperature of the water absorbent resin in the dry chamber reached -10 ° C, the inside of the dry chamber was kept at 20 ° C, and secondary drying was performed for 5 hours. Thus, 20 g of a water absorbing agent having a porous structure was obtained.

(Example 2)
As in Example 1, 20 g of a water absorbing agent having a porous structure was obtained except that the aqueous solvent was prepared using 72 g of water and 8 g of tert-butyl alcohol.

(Example 3)
As in Example 1, 20 g of a water absorbing agent having a porous structure was obtained except that the aqueous solvent was prepared using 68 g of water and 12 g of tert-butyl alcohol.

(Example 4)
As in Example 1, 20 g of a water absorbing agent having a porous structure was obtained except that the aqueous solvent was prepared using 64 g of water and 16 g of tert-butyl alcohol. In addition, since the gel strength of the water absorbing agent obtained in Example 4 was less than the measurement limit value of the neocard meter, it was impossible to measure. Therefore, the gel strength of the water absorbing agent was estimated to be less than 500 Pa, and the gel degradation index was also calculated based on this estimated value.

(Comparative example 1)
100 g of the water absorbent resin obtained in Production Example 1 was used as it was without swelling and lyophilization.

(Comparative example 2)
As in Example 1, 20 g of a water absorbing agent having a porous structure was obtained, except that the aqueous solvent was changed to only 80 g of water without using tert-butyl alcohol.

(Comparative example 3)
As in Example 1, 20 g of a water absorbing agent having a porous structure was obtained, except that it was prepared using 72 g of water and 8 g of dioxane.

(Evaluation method)
With respect to the water absorbing agent obtained in each Example and Comparative Example, measurement of gel strength (initial, one day and three days later), water retention ability, water absorption ability under pressure, water absorption rate and median particle diameter are performed as follows. The
[Measurement of gel strength and calculation of gel deterioration index]
In a 100 ml beaker, 49 g of physiological saline (0.9% sodium chloride aqueous solution) containing 0.02% vitamin C is charged, and a magnetic stirrer (M-20G manufactured by Koike Precision Instruments Mfg. Co., Ltd.) (stirler chip: 8 mm × 30 mm, Stir at 600 rpm using the ring). After 1.00 g of the water absorbing agent was introduced into the vortex generated by the stirring, the vortex disappeared and the mixture was stirred until the liquid level became horizontal. The 5-fold swelling gel prepared in this manner is 1 hour (initial value) at room temperature (25 ° C.) and 3 in a thermo-hygrostat (ESPEC LHU-113) (temperature: 40 ° C., humidity: 60%). After standing for one day (72 hours), gel strength measurement was performed using a neocard meter (ME-303 manufactured by I Techno Engineering Co., Ltd.). The gel strength of the gel left at room temperature (25 ° C.) for 1 hour corresponds to A (initial gel strength (Pa)) of the above formula (1), and the gel strength of the gel left for 3 days in the constant temperature and humidity chamber is It corresponds to B (gel strength (Pa) after 3 days) of the formula (1). The gel degradation index was calculated from the above-described equation (1) based on the measurement results.

  The neocard meter uses a pressure sensitive shaft with a diameter of 16 mm. The gel strength can be measured by measuring the viscosity property at a speed of 1 inch / 21 sec while applying a load of 400 g with a neocard meter.

[Holding ability measurement]
After putting 2.00 g of a water-absorbing agent for evaluation into a cotton bag, 500 g of physiological saline was poured into the cotton bag and the whole was immersed at the same time. Tied top of the cotton bag with a rubber band, after immersion for 30 minutes, dehydrated for 1 minute in a centrifuge (Kokusan Co. H-122) (167G), and weighed _{W A.} Also, the same operation was carried out without putting the sample in the cotton bag, and the empty weight W _B of the cotton bag was measured. And water retention ability was computed from the following formulas.
Water retention capacity _{(g / g) = (W} A -W B) / 2

[Measurement of water absorption capacity under pressure]
FIG. 1 is a schematic view of a measuring apparatus used for measuring the water absorption capacity under pressure of a water absorbing agent. It measured using the measuring apparatus Y shown in FIG. The measuring device Y shown in FIG. 1 comprises a bullet 1 and a conduit 2, a measuring stand 3, and a measuring portion 4 placed on the measuring stand 3. The bullet portion 1 has a rubber plug 14 connected to the upper portion of the burette 10, an air introducing pipe 11 and a cock 12 connected to the lower portion, and the air introducing pipe 11 has a cock 13 at the tip. A conduit 2 is attached between the bullet 1 and the measuring stand 3. The inner diameter of the conduit 2 is 6 mm. A hole of 2 mm in diameter is provided at the central part of the measurement stand 3 and the conduit 2 is connected. The measuring unit 4 has a cylinder 40 (made of plexiglass), a nylon mesh 41 adhered to the bottom of the cylinder 40, and a weight 42. The inner diameter of the cylinder 40 is 20 mm. The opening of the nylon mesh 41 is 75 μm (200 mesh). At the time of measurement, the water absorbing agent 5 is uniformly distributed on the nylon mesh 41. The weight 42 has a diameter of 19 mm and a mass of 119.6 g. The weight 42 is placed on the water absorbing agent 5 so that a load of 4.14 kPa can be applied to the water absorbing agent 5.

In the measuring apparatus Y having such a configuration, the cock 12 and the cock 13 of the burette 1 are first closed, the physiological saline adjusted to 25 ° C. is put in from the upper part of the burette 10, and the upper part of the burette is plugged with the rubber plug 14 , Open the cock 12 and the cock 13 of the burette 1. Next, the height of the measurement stand 3 is adjusted so that the tip of the conduit 2 and the air introduction port of the air introduction pipe 11 at the center of the measurement stand 3 are at the same height.
On the other hand, 0.10 g of the water absorbing agent 5 is uniformly dispersed on the nylon mesh 41 of the cylinder 40, and the weight 42 is placed on the water absorbing agent 5. The measuring unit 4 is placed so that its center coincides with the mouth of the center of the measuring stand 3.
From the time when the water absorbing agent 5 starts to absorb water, the amount of decrease in the physiological saline in the burette 10 (the amount of physiological saline solution in which the water absorbing agent 5 absorbs water) W _E (mL) is read. The water absorption amount after 60 minutes from the start of water absorption was determined by the following equation as the water absorption capacity to physiological saline under pressure of the water absorbing agent 5.
Water absorption capacity under pressure (mL / g) = W _E /0.1

[Water absorption rate measurement (Vortex method)]
50 g of physiological saline was added to a 100 ml beaker containing a rotor (8 mm × 30 mm, no ring), and kept at 25 ° C. in a thermostat. Next, 2.00 g of a water-absorbing agent for evaluation was introduced into a vortex of physiological saline stirred at 600 rpm, and at the same time, measurement by a stopwatch was started. The time (seconds) until that point was taken as the water absorption rate, with the end point when the vortex disappears and the liquid level becomes horizontal.

[Particle size measurement]
The water-absorbing agent for evaluation, 5.00 g, was measured using a robot shifter ("RPS-205" manufactured by Seishin Enterprise Co., Ltd.) (frequency 80 Hz, 2 minutes). The sieves used were 850 μm, 710 μm, 600 μm, 500 μm, 425 μm, 300 μm, 180 μm and 75 μm.

  Table 1 shows the evaluation results of the water absorbing agent obtained in each Example and Comparative Example. In Table 1, TBA represents tert-butyl alcohol, and DO represents dioxane.

[Evaluation]
As shown in Table 1, the water absorbing agents obtained in Examples 1 to 4 have excellent water absorbing performance equal to or greater than the water absorbing resin (Comparative Example 1) before freeze-drying and have excellent self-decomposition characteristics. Have.
2 (a) and 2 (c) show SEM images obtained by observing the water-absorbing agent obtained in Example 3 and Comparative Example 2 at a magnification of 200 times, and FIG. The SEM image which observed the obtained water absorbing resin by 200 times of magnification is shown. From these figures, it can be seen that the water-absorbing agent of Example 3 is a porous structure having pores of a minute size.

Y measuring device 1 burette part 2 conduit 3 measurement stand 4 measuring part 5 water absorbing agent 10 burette 11 air introduction pipe 12 faucet 13 faucet 14 rubber plug 40 cylinder 41 cylinder 41 nylon mesh 42 weight

Claims (7)

A method for producing a water-absorbing agent, comprising the step of freeze-drying a water-absorbent resin swollen with an aqueous solvent containing alcohol and water. The method for producing a water absorbent according to claim 1, wherein the aqueous solvent contains 1 to 25% by mass of an alcohol. The method for producing a water-absorbing agent according to claim 1, wherein the alcohol is a lower alcohol having 5 or less carbon atoms. The method for producing a water-absorbent agent according to any one of claims 1 to 3, wherein the alcohol is a secondary alcohol or a tertiary alcohol. The method for producing a water-absorbing agent according to claim 1, wherein the alcohol is a tertiary alcohol having 5 or less carbon atoms. Has a porous structure,
A water absorbing agent whose gel degradation index represented by the following formula (1) is 0.75 or less.
Gel degradation index = B / A (1)
(Here, A is the initial gel strength (Pa), B is the gel strength after 3 days (Pa)) The water-absorbing agent according to claim 6, which has a water-holding capacity of 35 to 50 g / g or more, a water-absorbing capacity under pressure of 24 to 30 ml / g, and a water-absorbing speed of 35 to 42 seconds with respect to physiological saline.