JP2017155194A - Manufacturing method of water-absorbing resin and water-absorbing resin - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a manufacturing method of a water-absorbing resin and a water-absorbing resin, and in more detail, a manufacturing method of a water-absorbing resin that has an excellent water absorption property, that is, a high water retention capacity, a high water absorption power under load, a slight residual monomer, and a slight migration amount of the residual monomer in an early stage of use and is preferably used for sanitary material, and a water-absorbing resin having a particular performance.SOLUTION: The present invention provides a manufacturing method of a water-absorbing resin which includes a polymerization step of obtaining a reaction system including a water-absorbing resin precursor by polymerizing an aqueous ethylenic unsaturated monomer under the presence of an aqueous azo-based compound, and a dehydration step of removing water from the reaction system in a petroleum-based hydrocarbon dispersant, and, in the dehydration step, an oil-soluble radical generator is added to the reaction system in the residual water rate calculated from the following formula (1) of 5 to 75%. Residual water rate (%)=(mass of water remained in the reaction system)/(mass of aqueous ethylenic unsaturated monomer used in the polymerization step)x100 (1).SELECTED DRAWING: None

Description

  The present invention relates to a method for producing a water absorbent resin and a water absorbent resin. More specifically, the present invention relates to a method for producing a water-absorbing resin that has excellent water-absorbing properties and is suitably used for sanitary materials, and a water-absorbing resin having specific performance.

  Water-absorbing resins are widely used in industrial materials such as sanitary materials such as disposable diapers and sanitary napkins, daily necessities such as pet sheets, and waterproofing materials for cables. Many types of water-absorbing resins are known depending on the application. In sanitary materials such as paper diapers and sanitary napkins, water-absorbing resins made of a polymer of a water-soluble ethylenically unsaturated monomer are mainly used. The water-absorbent resin used for sanitary materials is generally used in close proximity to the human body, so high safety is required, and a large amount of bodily fluids can be quickly removed when in contact with bodily fluids such as urine and blood. Water absorption characteristics that can be absorbed and retained are required. In particular, sanitary materials in recent years tend to be thinner due to demands for comfort when worn and carrying convenience, and there is a demand for an improvement in the water absorption amount of the water absorbent resin itself.

  It is considered that a water-absorbing resin comprising a polymer of a water-soluble ethylenically unsaturated monomer generally achieves a high water absorption by lowering the degree of crosslinking. In the production of a water-absorbent resin, a monomer is often polymerized using a persulfate having advantages such as easy polymerization control as a polymerization initiator. However, when a persulfate is used, so-called self-crosslinking is likely to proceed, and a reduction in the degree of crosslinking is restricted, so that it is difficult to obtain a water-absorbing resin having a high water absorption amount.

  In order to solve such a problem, Patent Document 1 describes that an azo compound is used as a polymerization initiator instead of a persulfate. However, when an azo compound is used as the polymerization initiator, it is insufficient in terms of the polymerization rate of the water-soluble ethylenically unsaturated monomer. These are referred to as “residual monomers”). Further, in the dehydration process or drying process in which water is removed from the reaction system containing the produced water-absorbing resin by heating or the like, a residual monomer may be generated even if a part of the water-absorbing resin is decomposed. When such a water-absorbent resin containing a large amount of residual monomer is used as a sanitary material, it migrates to the outside after liquid absorption, which causes rashes and rough skin on the user's skin. In particular, the transfer of the residual monomer to the outside in the initial stage of use with a small amount of liquid absorption tends to be a problem.

  Therefore, a method for suppressing the content of residual monomer in the water absorbent resin has been proposed. For example, in Patent Document 2, a radical polymerization initiator is added before or during drying of a slurry containing a water-absorbing resin obtained by polymerizing a water-soluble ethylenically unsaturated monomer by a reverse phase suspension polymerization method. How to do is described. Patent Document 3 describes a method of adding a reducing substance such as sulfite after polymerization of a water-soluble ethylenically unsaturated monomer.

JP 2006-176570 A JP 2002-105125 A JP-A 64-62217

  However, in these conventional techniques, the properties required for the water-absorbent resin, that is, “high water absorption characteristics (high water retention ability, high water absorption ability under load)”, little residual monomer, and residual monomer at the initial stage of use. From the viewpoint of having the characteristic that the amount of migration to the outside is small, it is still not sufficient.

  An object of the present invention is to provide a water-absorbent resin having characteristics such as high water retention ability, water absorption ability under a high load, extremely low residual monomer, and low external migration amount of residual monomer at the initial stage of use. It is an object of the present invention to provide a water-absorbing resin having a production method and specific performance, and a sanitary material using the water-absorbing resin.

  The present inventor has intensively studied to achieve the above-described object. As a result, in a method for producing a water-absorbent resin by polymerizing a water-soluble ethylenically unsaturated monomer in the presence of a water-soluble azo compound, an oil-soluble radical generator is added to the reaction system at a specific time in the dehydration step. By adding, it has been found that a water-absorbing resin having a high water retention capacity, a high water absorption capacity under load, a low residual monomer content, and a low residual monomer external migration amount in the initial use can be obtained. The present invention has been completed.

  That is, this invention provides the manufacturing method of the following water absorbing resin, the water absorbing resin which has specific performance, and the sanitary material in which this water absorbing resin is used.

（１） [Item 1] A polymerization step of polymerizing a water-soluble ethylenically unsaturated monomer in the presence of a water-soluble azo compound to obtain a reaction system containing a water-absorbing resin precursor, and the reaction in a petroleum hydrocarbon dispersion medium A dehydration step of removing water from the system, and in the dehydration step, the residual water ratio calculated by the following formula (1) is 5 to 75% to the reaction system,
2,2′-azobis (4-methoxy-2,4-dimethylvaleronitrile), 2,2′-azobis-isobutyronitrile, 1,1′-azobis (cyclohexane-1-carbonitrile), 2,2 At least one selected from the group consisting of '-azobis (2,4-dimethylvaleronitrile), 2,2'-azobis (methyl isobutyrate) and 2,2'-azobis (2-methylbutyronitrile) A method for producing a water-absorbent resin, comprising adding an oil-soluble radical generator.

(1)

  [Item 2] The item according to Item 1, wherein the addition amount of the oil-soluble radical generator is 0.0001 to 0.0020 mol per mol of the water-soluble ethylenically unsaturated monomer used in the polymerization step. A method for producing a water-absorbent resin.

  [Item 3] The water-soluble azo compound is 2,2′-azobis (2-amidinopropane) dihydrochloride, 2,2′-azobis [2- [1- (2-hydroxyethyl) -2-imidazoline- 2-yl] propane] dihydrochloride and 2,2′-azobis [N- (2-carboxyethyl) -2-methylpropionamidine] tetrahydrate are at least one selected from the group consisting of: A method for producing the water absorbent resin according to any one of 1 and 2.

  [Item 4] The method for producing a water-absorbent resin according to any one of Items 1 to 3, wherein the water-soluble ethylenically unsaturated monomer is polymerized by a reverse phase suspension polymerization method in the polymerization step.

  [Item 5] The method for producing a water-absorbent resin according to Item 4, wherein in the polymerization step, the reverse phase suspension polymerization method is performed in two or more stages.

  CLAIM | ITEM 6 The manufacturing method of the water absorbing resin of any one of claim | item 1 to 5 which adds a crosslinking agent and performs a post-crosslinking reaction after a superposition | polymerization process.

[Item 7] A water-absorbent resin formed by using a crosslinked polymer of a water-soluble ethylenically unsaturated monomer as an essential component, which satisfies all of the following (A) to (D).
(A) The physiological saline water retention capacity is 38 to 65 g / g.
(B) Saline water absorption capacity under load of 4.14 kPa is 18 ml / g or more (C) Residual monomer content is 150 ppm or less (D) External migration amount of residual monomer at the initial stage of use is 30 ppm or less

  [Item 8] The water-absorbent resin according to Item 7, wherein the median particle diameter is 100 to 600 μm.

  [Item 9] A liquid-permeable sheet, a liquid-impermeable sheet, and an absorbent body held between these sheets, wherein the absorbent body comprises the water-absorbent resin according to any one of Items 7 and 8. Including sanitary materials.

  According to the present invention, a water-absorbing resin having characteristics such as high water retention ability, water absorption ability under a high load, little residual monomer, and little external migration amount of residual monomer at the initial stage of use is produced. A method is provided, thereby providing a water-absorbent resin that combines certain properties suitable for sanitary materials, as well as sanitary materials in which it is used.

It is a schematic diagram which shows schematic structure of the apparatus for measuring the physiological saline water absorptivity under 4.14kPa load of water absorbing resin.

  In the method for producing a water-absorbent resin according to the present invention, first, a water-soluble ethylenically unsaturated monomer is polymerized in the presence of a water-soluble azo compound to prepare a reaction system containing a gel-like product of the water-absorbent resin. (Hereinafter referred to as “polymerization step”). The gel-like product of the water-absorbing resin prepared here is referred to as a “water-absorbing resin precursor” for convenience in order to distinguish it from the finally obtained water-absorbing resin.

  Examples of the water-soluble ethylenically unsaturated monomer used in the present invention include (meth) acrylic acid (in the present specification, “acryl” and “methacryl” are collectively referred to as “(meth) acryl”). The same) and salts thereof; 2- (meth) acrylamide-2-methylpropanesulfonic acid and salts thereof; (meth) acrylamide, N, N-dimethyl (meth) acrylamide, 2-hydroxyethyl (meth) acrylate, N-methylol Nonionic monomers such as (meth) acrylamide and polyethylene glycol mono (meth) acrylate; N, N-diethylaminoethyl (meth) acrylate, N, N-diethylaminopropyl (meth) acrylate, diethylaminopropyl (meth) acrylamide, etc. And amino group-containing unsaturated monomers and quaternized products thereof. These water-soluble ethylenically unsaturated monomers may be used alone or in combination of two or more.

  Of these, (meth) acrylic acid and salts thereof, (meth) acrylamide, and N, N-dimethylacrylamide are preferable as raw materials for producing the water-absorbent resin according to the present invention because they are easily available. Acrylic acid and its salts are more preferred.

  Further, the other water-soluble ethylenically unsaturated monomer may be added to acrylic acid and its salt for copolymerization. In that case, it is preferable that the usage-amount of acrylic acid and its salt is 70-100 mol% with respect to the total amount (mass) of the water-soluble ethylenically unsaturated monomer used at a superposition | polymerization process.

  In addition, the said water-soluble ethylenically unsaturated monomer is normally used as aqueous solution from a viewpoint of the ease of reaction heat removal during superposition | polymerization. The concentration of the monomer in such an aqueous solution is usually preferably 20% by mass or more and a saturated concentration or less, more preferably 25 to 70% by mass, and still more preferably 30 to 55% by mass.

  When the water-soluble ethylenically unsaturated monomer has an acid group such as (meth) acrylic acid or 2- (meth) acrylamido-2-methylpropanesulfonic acid, the acid group is previously alkaline if necessary. You may use what was neutralized with the summing agent. Examples of such an alkaline neutralizer include alkali metal salts such as sodium hydroxide, sodium carbonate, sodium hydrogen carbonate, potassium hydroxide, potassium carbonate; ammonia and the like. These alkaline neutralizers are usually used as an aqueous solution in order to simplify the neutralization operation. The alkaline neutralizers may be used alone or in combination of two or more.

  The degree of neutralization of the water-soluble ethylenically unsaturated monomer by the alkaline neutralizing agent is from the viewpoint of enhancing the water absorption characteristics of the resulting water-absorbent resin and preventing the generation of excessive alkaline neutralizing agent. It is preferably 10 to 90 mol%, more preferably 30 to 85 mol%, still more preferably 40 to 82 mol%, based on all the acid groups of the polymerizable unsaturated monomer. More preferably, it is 50-80 mol%.

  If necessary, a crosslinking agent may be added to the water-soluble ethylenically unsaturated monomer and subjected to a polymerization reaction. As a crosslinking agent (referred to as an internal crosslinking agent) to be added to the water-soluble ethylenically unsaturated monomer before the polymerization reaction, for example, (poly) ethylene glycol (in this specification, “polyethylene glycol” and “polyethylene glycol” "Ethylene glycol" is collectively referred to as "(poly) ethylene glycol. The same applies hereinafter.), (Poly) propylene glycol, 1,4-butanediol, trimethylolpropane, diols such as (poly) glycerin, triols, etc. Unsaturated polyesters obtained by reacting polyols with unsaturated acids such as (meth) acrylic acid, maleic acid, fumaric acid; bisacrylamides such as N, N′-methylenebisacrylamide; polyepoxides and (meth) Di- or tri (meth) acrylic esters obtained by reacting with acrylic acid; Di (meth) acrylic acid carbamyl esters obtained by reacting polyisocyanates such as diisocyanate and hexamethylene diisocyanate with hydroxyethyl (meth) acrylate; allylated starch, allylated cellulose, diallyl phthalate, N, N ′ , N ″ -triallyl isocyanurate, compounds having two or more polymerizable unsaturated groups such as divinylbenzene; (poly) ethylene glycol diglycidyl ether, (poly) propylene glycol diglycidyl ether, (poly) glycerin diglycidyl Polyglycidyl compounds such as diglycidyl compounds such as ether and triglycidyl compounds; Epihalohydrin compounds such as epichlorohydrin, epibromhydrin and α-methylepichlorohydrin; 2,4-tolylene diisocyanate, Isocyanate compounds such as xamethylene diisocyanate; 3-methyl-3-oxetanemethanol, 3-ethyl-3-oxetanemethanol, 3-butyl-3-oxetanemethanol, 3-methyl-3-oxetaneethanol, 3-ethyl-3- Examples thereof include compounds having two or more reactive functional groups such as oxetane compounds such as oxetane ethanol and 3-butyl-3-oxetane ethanol. Among these, (poly) ethylene glycol diglycidyl ether, (poly) propylene glycol diglycidyl ether, (poly) glycerin diglycidyl ether, and N, N′-methylenebisacrylamide are preferable from the viewpoint of crosslinking efficiency. These internal crosslinking agents may be used alone or in combination of two or more.

  The amount of the internal cross-linking agent used is usually 0.000001 to 0.01 with respect to 1 mol of the water-soluble ethylenically unsaturated monomer used in the polymerization step, from the viewpoint of enhancing the water absorption characteristics of the resulting water absorbent resin. It is preferable that it is a mol, and it is more preferable that it is 0.00001-0.005 mol.

  The internal cross-linking agent may be added to the aqueous solution of the water-soluble ethylenically unsaturated monomer, or may be added in the polymerization step separately from the water-soluble ethylenically unsaturated monomer. Further, a chain transfer agent may be added in order to control the water absorption characteristics of the water absorbent resin. Examples of such chain transfer agents include hypophosphites, thiols, thiolic acids, and secondary grades. Examples include alcohols and amines.

  Examples of the water-soluble azo compound used in the present invention include 1-[(1-cyano-1-methylethyl) azo] formamide, 2,2′-azobis [2- (N-phenylamidino) propane] 2 Hydrochloride, 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-imida Rin-2-yl) propane] dihydrochloride, 2,2'-azobis [2- (2-imidazolin-2-yl) propane] dihydrochloride, 2,2'-azobis [2- (4,5,5) 6,7-tetrahydro-1H-1,3-diazepin-2-yl) dihydrochloride, 2,2′-azobis [2- (5-hydroxy-3,4,5,6-tetrahydropyrimidin-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 (1-Imino-1-pyrrolidino-2-methylpropyl Pan) dihydrochloride, 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) dihydrate, 4,4′-azobis-4-cyanovaleric acid, 2,2′-azobis [2- (hydroxymethyl) ) Propionitrile], 2,2′-azobis [2- (2-imidazolin-2-yl) propane] disulfate, 2,2′-azobis [N- (2-carboxyethyl) -2-methylpropion Amidine] tetrahydrate and 2,2′-azobis [2-methyl-N- (2-hydroxyethyl) propionamide] and the like. Among these, from the viewpoint of water absorption properties, 2,2′-azobis (2-amidinopropane) dihydrochloride, 2,2′-azobis [2- [1- (2-hydroxyethyl) -2-imidazoline-2- The use of yl] propane] dihydrochloride and 2,2′-azobis [N- (2-carboxyethyl) -2-methylpropionamidine] tetrahydrate is preferred.

  The water-soluble azo compounds may be used alone or in combination of two or more. In addition, the water solubility in the manufacturing method which concerns on this invention means showing the solubility of 5 mass% or more in water at 25 degreeC. These water-soluble azo compounds generate radical species containing nitrogen atoms by a denitrification reaction, and the radical species are introduced into the end group of the polymer chain as a functional group containing an amino group, imino group, cyano group or the like. Is done.

  The amount of the water-soluble azo compound used is preferably 0.00005 to 0.01 mol with respect to 1 mol of the water-soluble ethylenically unsaturated monomer used in the polymerization step, from the viewpoint of reaction time. More preferably, it is 0.0001 to 0.008 mol.

  The polymerization method in the polymerization step of the water-soluble ethylenically unsaturated monomer can be selected from typical polymerization methods such as an aqueous solution polymerization method, an emulsion polymerization method and a reverse phase suspension polymerization method. For example, in the case of an aqueous solution polymerization method, an aqueous solution of a water-soluble ethylenically unsaturated monomer, a water-soluble azo compound and, if necessary, an internal cross-linking agent are put into a reaction vessel and polymerized by heating with stirring and mixing. Can be advanced. In the case of the reverse phase suspension polymerization method, a surfactant and / or polymer protective colloid is introduced into a petroleum hydrocarbon dispersion medium and dissolved, and then a water-soluble ethylenically unsaturated monomer and a water-soluble Polymerization can be advanced by further adding an aqueous solution in which a water-soluble azo compound and, if necessary, an internal crosslinking agent are mixed, and heating with stirring. As the polymerization method, the reverse phase suspension polymerization method is preferable because the particle diameter of the water-absorbent resin obtained at the time of the polymerization reaction can be widely controlled.

  In the reverse phase suspension polymerization method, a water-soluble ethylenically unsaturated monomer is further added to the water-absorbent resin precursor obtained by the reverse phase suspension polymerization, and the polymerization is performed in two or more stages. It can also be done. In such a multistage reverse phase suspension polymerization method, the particle diameter of the water absorbent resin obtained can be appropriately adjusted by aggregating the water absorbent resin precursor obtained in the first phase reverse phase suspension polymerization. It becomes easier to obtain a desired particle size for sanitary materials such as disposable diapers.

  In the multistage reversed-phase suspension polymerization method, the water-soluble ethylenically unsaturated monomer used in the second and subsequent stages, the water-soluble azo polymerization initiator and the type of internal cross-linking agent used as necessary, The amount of the azo polymerization initiator and the internal cross-linking agent used as necessary with respect to the water-soluble ethylenically unsaturated monomer are the same as described above.

  Hereinafter, the case where the polymerization step is performed by the reverse phase suspension polymerization method will be described in more detail.

  Examples of the petroleum hydrocarbon dispersion medium used in the reverse phase suspension polymerization method include n-hexane, n-heptane, 2-methylhexane, 3-methylhexane, 2,3-dimethylpentane, 3-ethylpentane, C6-C8 aliphatic hydrocarbons such as n-octane; cyclohexane, methylcyclohexane, cyclopentane, methylcyclopentane, trans-1,2-dimethylcyclopentane, cis-1,3-dimethylcyclopentane, trans- Examples include alicyclic hydrocarbons such as 1,3-dimethylcyclopentane; aromatic hydrocarbons such as benzene, toluene and xylene. These petroleum hydrocarbon dispersion media may be used alone or in combination of two or more. Among these petroleum hydrocarbon dispersion media, n-hexane, n-heptane, and cyclohexane are preferably used because they are easily available industrially, have stable quality, and are inexpensive. Moreover, as an example of the mixture of the petroleum hydrocarbon dispersion medium, commercially available Exol heptane (manufactured by ExxonMobil Co., Ltd .: containing 75 to 85% by mass of hydrocarbon of heptane and its isomer) may be used.

  Since the amount of the petroleum hydrocarbon dispersion medium is easy to control the polymerization temperature by removing the polymerization heat, it is usually based on 100 parts by mass of the water-soluble ethylenically unsaturated monomer used for the first stage polymerization. It is preferably 100 to 1500 parts by mass, and more preferably 200 to 1400 parts by mass. The first stage polymerization means a single stage polymerization process and a first stage polymerization process in multistage polymerization.

  Examples of the surfactant used in the reverse phase suspension polymerization method include nonionic surfactants such as sorbitan fatty acid ester, polyglycerin fatty acid ester, sucrose fatty acid ester, sorbitol fatty acid ester and polyoxyethylene alkylphenyl ether, fatty acid And anionic surfactants such as salts, alkylbenzene sulfonates, alkylmethyl taurates, polyoxyethylene alkylphenyl ether sulfates and polyoxyethylene alkyl ether sulfonates. Among these, it is preferable to use a nonionic surfactant, especially sorbitan fatty acid ester, polyglycerin fatty acid ester or sucrose fatty acid ester.

  Examples of polymer protective colloids include ethyl cellulose, ethyl hydroxyethyl cellulose, polyethylene oxide, anhydrous maleated polyethylene, anhydrous maleated polybutadiene, and anhydrous maleated EPDM (ethylene / propylene / diene / terpolymer). Is mentioned.

  From the viewpoint of the stability of the reverse phase suspension polymerization method, the amount of the surfactant and the polymer protective colloid used is 100 parts by mass of the aqueous solution of the water-soluble ethylenically unsaturated monomer used for the first stage polymerization. It is preferable that it is 0.1-5 mass parts with respect to it, and it is more preferable that it is 0.2-3 mass parts.

  The reaction temperature in the polymerization step is preferably 20 to 110 ° C, and more preferably 40 to 100 ° C. The polymerization reaction time is preferably 0.1 to 4 hours.

  Next, water is removed from the reaction system in which the polymerization process is completed, that is, the reaction system including the water-absorbing resin precursor (hereinafter referred to as “dehydration process”). As a method for the dehydration step, a method in which a reaction system including a water-absorbing resin precursor is dispersed in a petroleum hydrocarbon dispersion medium, energy such as heat is applied from the outside, and water is removed by azeotropic distillation is used. In the phase suspension polymerization method, a reaction system containing a water-absorbent resin precursor and a petroleum hydrocarbon dispersion medium is obtained after the polymerization step, and can be transferred to the dehydration step as it is, which is preferable from the viewpoint of work efficiency.

  As the petroleum hydrocarbon dispersion medium used in the dehydration step, the same petroleum hydrocarbon dispersion medium as that used in the reverse phase suspension polymerization can be used.

  The oil-soluble radical generator added in the dehydration step may be added as it is to the reaction system, or may be separately dissolved in a petroleum hydrocarbon dispersion medium and then added to the reaction system.

  The addition timing of the oil-soluble radical generator is determined based on the residual water ratio of the reaction system calculated by the above formula (1). In the formula (1), “mass of water remaining in the reaction system” means the mass of the total amount of water present in the reaction system when calculating the residual water ratio, and is included in the reaction system before the start of the polymerization reaction. It is calculated by subtracting the mass of water removed from the reaction system from the total amount of water and water introduced as necessary until the residual water ratio is calculated.

  The upper limit of the residual water rate when the oil-soluble radical generator is added is 75% or less, preferably 70% or less, and more preferably 65% or less, at any dehydration stage. Further, the lower limit of the residual water ratio when the oil-soluble radical generator is added is 5% or more, preferably 10% or more, more preferably 15% or more. If the oil-soluble radical generator is added at a stage where the residual water ratio of the reaction system is higher than 75% or lower than 5%, the reaction of the oil-soluble radical generator with the water-absorbing resin precursor tends to be insufficient. It is difficult to have both high water absorption characteristics and a small residual monomer content.

  The reaction temperature after addition of the oil-soluble radical generator to the reaction system is preferably 40 to 120 ° C, and more preferably 50 to 100 ° C. The reaction time with the water-absorbing resin precursor after addition of the oil-soluble radical generator is usually 10 minutes to 3 hours.

  The oil-soluble radical generator added to the reaction system in the dehydration step is 2,2′-azobis (4-methoxy-2,4-dimethylvaleronitrile), 2,2′-azobis-isobutyronitrile, 1'-azobis (cyclohexane-1-carbonitrile), 2,2'-azobis (2,4-dimethylvaleronitrile), 2,2'-azobis (methyl isobutyrate), 2,2'-azobis (2- And an oil-soluble azo compound of methylbutyronitrile). These oil-soluble radical generators may be used alone or in combination of two or more. In addition, the oil solubility in the manufacturing method which concerns on this invention means showing the solubility of 5 mass% or more in either toluene, a cyclohexane, or ethanol in 25 degreeC.

  The amount of the oil-soluble radical generator added to the reaction system is preferably 0.0001 to 0.0020 mol with respect to 1 mol of the water-soluble ethylenically unsaturated monomer used in the polymerization step. More preferred is 0.0005 to 0.0015 mol.

  In the production method of the present invention, it is preferable to subject the water-absorbent resin precursor obtained in the polymerization step to a post-crosslinking reaction in which a cross-linking agent is added and reacted. By performing a post-crosslinking reaction after the polymerization step, water absorption characteristics can be further enhanced.

  The post-crosslinking reaction can usually be carried out at any time after the polymerization step, but it is preferably performed after adding and reacting an oil-soluble radical generator in the dehydration step. Examples of the crosslinking agent (post-crosslinking agent) used for the post-crosslinking reaction 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, (poly) propylene glycol polyglycidyl ether, (poly) glycerol polyglycidyl ether, trimethylolpropane triglycidyl ether; epichlorohydrin, epibromohydrin, α -Haloepoxy compounds such as methyl epichlorohydrin; isocyanates such as 2,4-tolylene diisocyanate and hexamethylene diisocyanate Nate compounds; 3-methyl-3-oxetanemethanol, 3-ethyl-3-oxetanemethanol, 3-butyl-3-oxetanemethanol, 3-methyl-3-oxetaneethanol, 3-ethyl-3-oxetaneethanol, 3- Oxetane compounds such as butyl-3-oxetaneethanol; oxazoline compounds such as 1,2-ethylenebisoxazoline; carbonate compounds such as ethylene carbonate; hydroxyalkylamides such as bis [N, N-di (β-hydroxyethyl)] adipamide Compounds. Among these post-crosslinking agents, (poly) ethylene glycol diglycidyl ether, (poly) glycerin diglycidyl ether, (poly) glycerin triglycidyl ether, (poly) propylene glycol polyglycidyl ether, (poly) glycerol polyglycidyl ether, etc. These polyglycidyl compounds are preferably used. These post-crosslinking agents may be used alone or in combination of two or more.

  The amount of the post-crosslinking agent used varies depending on the type of the post-crosslinking agent, but is usually 0.00001 to 0.01 mol with respect to 1 mol of the total amount of the water-soluble ethylenically unsaturated monomer used in the polymerization step. It is preferable that it is 0.00005-0.005 mol, and it is still more preferable that it is 0.0001-0.002 mol.

  The reaction between the water absorbent resin precursor and the post-crosslinking agent is preferably performed in the presence of water. For this reason, when the post-crosslinking agent is added, it is preferable that an appropriate amount of water remains in the reaction system, and the post-crosslinking agent is preferably added to the reaction system as an aqueous solution. The amount of water in the reaction system when the post-crosslinking agent is added to the reaction system (when the post-crosslinking agent is added as an aqueous solution, the amount of water derived from the aqueous solution is included) is usually used in the polymerization step. The total amount of the water-soluble ethylenically unsaturated monomer is preferably 5 to 300 parts by mass, more preferably 10 to 100 parts by mass, and more preferably 10 to 50 parts by mass. Further preferred. The amount of water in the reaction system means the total amount of water remaining in the reaction system during dehydration and water added to the reaction system as necessary for post-crosslinking.

  Thus, after reacting the oil-soluble radical generator and preferably the post-crosslinking agent with the water-absorbent resin precursor, and then subjecting it to a drying treatment to remove water, petroleum hydrocarbon dispersion medium, etc. The water-absorbent resin is obtained. Examples of the drying treatment include a method of applying energy such as heat from the outside and using distillation or hot air feeding. Such drying treatment may be performed under normal pressure or under reduced pressure. In order to increase the drying efficiency, it may be performed under an air stream such as nitrogen, or a combination of these methods may be used. The drying temperature when the drying treatment is normal pressure is preferably 70 to 250 ° C, more preferably 80 to 180 ° C, and further preferably 80 to 140 ° C. Further, the drying temperature when the drying treatment is reduced pressure is preferably 60 to 100 ° C, more preferably 70 to 90 ° C.

The water-absorbent resin according to the present invention obtained by the above examples includes sanitary materials such as disposable diapers and sanitary products, daily necessities such as pet sheets, water-holding materials, agricultural and horticultural materials such as soil conditioners, and water / water-proof cables for power and communication cables. It can be used in various fields such as materials and industrial materials such as anti-condensation materials, but has the following characteristics (A) to (D), and is particularly safe for the human body, particularly the skin, It is particularly suitable as a sanitary material because it can reduce the influence on the skin (skin roughness).
(A) The physiological saline water retention capacity is 38 to 65 g / g.
(B) Saline water absorption capacity under load of 4.14 kPa is 18 ml / g or more (C) Residual monomer content is 150 ppm or less (D) External migration amount of residual monomer at the initial stage of use is 30 ppm or less

  In the usage scene of the water-absorbent resin according to the present invention as a sanitary material, the physiological saline water retention capacity is preferably 40 to 55 g / g from the viewpoint of increasing the absorption capacity and reducing the amount of liquid returned. .

  Further, the physiological saline water absorption capacity under a load of 4.14 kPa is preferably 20 ml / g or more from the viewpoint of reducing the amount of liquid returned when pressure is applied to the sanitary material after liquid absorption, / G or more is more preferable.

  The residual monomer content is preferably 100 ppm or less, and more preferably 80 ppm or less, from the viewpoint of the effect on the skin (reduction of rough skin).

  The water-absorbing resin according to the present invention has an excellent property that the amount of residual monomer transferred to the outside is extremely small when the swelling ratio of the water-absorbing resin is low as in the initial stage of use. When used, the influence on the skin (rough skin) can be significantly reduced. The external migration amount of the residual monomer at the initial stage of use is preferably 20 ppm or less, and more preferably 15 ppm or less.

  Moreover, as a particle diameter, it is preferable that a median particle diameter is 100-600 micrometers from a viewpoint of the handleability as a powder, It is more preferable that it is 200-550 micrometers, It is more preferable that it is 250-500 micrometers. More preferably, it is 300-450 micrometers.

  The sanitary material using the water absorbent resin of the present invention preferably has a form in which an absorbent body containing the water absorbent resin is held between a liquid permeable sheet and a liquid impermeable sheet. The liquid permeable sheet used here is, for example, a nonwoven fabric or a porous sheet made of polyethylene resin, polypropylene resin, polyester resin, polyamide resin, or the like. On the other hand, the liquid impermeable sheet is, for example, a film made of a synthetic resin such as polyethylene resin, polypropylene resin, and polyvinyl chloride resin, or a film made of a composite material of the synthetic resin and a nonwoven fabric.

  Moreover, the absorber used here is preferably a composite of a water-absorbent resin and hydrophilic fibers. This composite includes, for example, a mixing structure in which a water-absorbing resin and a hydrophilic fiber are uniformly blended, a sandwich structure in which the water-absorbing resin is held between layered hydrophilic fibers, and a mixture of the water-absorbing resin and the hydrophilic fiber. It is preferable to use a packaging structure in which the sheet is wrapped with a liquid-permeable packaging sheet such as tissue paper. The hydrophilic fibers used in the composite are, for example, cellulose fibers such as cotton-like pulp, mechanical pulp, chemical pulp and semi-chemical pulp obtained from wood, and artificial cellulose fibers such as rayon and acetate. The hydrophilic fibers may contain synthetic fibers such as polyamide resin fibers, polyester resin fibers, and polyolefin resin fibers.

  Hereinafter, the present invention will be described in detail based on Examples and Comparative Examples, but the present invention is not limited to these Examples and the like.

Example 1
<Polymerization process>
300 g of n-heptane was charged into a 2 L five-necked cylindrical round bottom flask equipped with a stirrer, reflux condenser, dropping funnel, thermometer and nitrogen gas inlet tube, and sucrose fatty acid having an HLB of 3.0 0.74 g of ester (trade name “S-370”, manufactured by Mitsubishi Chemical Corporation) was added, and the mixture was heated to 80 ° C. with dissolution while stirring and then cooled to 55 ° C.

  Separately, 92 g (1.03 mol) of an 80.5% by mass acrylic acid aqueous solution was placed in a 500 mL Erlenmeyer flask, and 147.6 g (0.77 mol) of a 20.9% by mass sodium hydroxide aqueous solution was cooled thereto. Was added dropwise to neutralize the acrylic acid. To this neutralized solution, 0.055 g (0.0002 mol) of 2,2′-azobis (2-amidinopropane) dihydrochloride as a water-soluble azo compound and ethylene glycol diglycidyl ether 0 as an internal cross-linking agent were added. 0.012 g (0.00007 mol) was added to prepare a monomer aqueous solution for the first stage polymerization.

  Moreover, 88.8 mass% acrylic acid aqueous solution 128.8g (1.44 mol) was put into another 500 mL Erlenmeyer flask, and 26.9 mass% sodium hydroxide aqueous solution 160.6g (1. 08 mol) was added dropwise to neutralize 75 mol% of acrylic acid. Furthermore, 0.077 g (0.0003 mol) of 2,2′-azobis (2-amidinopropane) dihydrochloride which is a water-soluble azo compound and 0.008 g (0.003 g) of ethylene glycol diglycidyl ether which is an internal crosslinking agent. 00005 mol) was added to prepare a monomer aqueous solution for the second stage polymerization.

  In the five-necked cylindrical round bottom flask, the entire amount of the monomer aqueous solution for the first stage polymerization obtained above is added and dispersed with stirring, and the inside of the system is sufficiently replaced with nitrogen. The mixture was stirred for 1 hour while immersed in a 70 ° C. water bath. And after cooling the obtained polymerization slurry liquid to 26 degreeC, the whole quantity of monomer aqueous solution for 2nd step | paragraph polymerization was added with stirring. After sufficiently replacing the interior with nitrogen again, the flask was stirred for 2 hours while immersed in a 70 ° C. water bath to obtain a reaction system containing a water-absorbing resin precursor.

<Dehydration process>
Next, the five-necked cylindrical round bottom flask is heated in an oil bath at 120 ° C. to azeotrope the water in the reaction system and n-heptane, and 177 g of water is removed from the system while refluxing n-heptane. It was extracted (residual water ratio: 61%). Here, as an oil-soluble radical generator, a solution prepared by dissolving 0.575 g (0.0035 mol) of 2,2′-azobis-isobutyronitrile in 28 g of n-heptane was added and held at 80 ° C. for 20 minutes. . Subsequently, water and n-heptane were azeotroped, and 79 g of water was withdrawn from the system while refluxing n-heptane, and then a 2% aqueous solution of ethylene glycol diglycidyl ether as a post-crosslinking agent. 42 g (0.0005 mol) was added and held at 80 ° C. for 2 hours. Thereafter, n-heptane and water were distilled off to obtain 229.0 g of a water-absorbing resin as aggregates of spherical particles.

Example 2
In Example 1, the same operation as in Example 1 was performed except that the oil-soluble radical generator was changed to 0.806 g (0.0035 mol) of 2,2′-azobis (methyl isobutyrate). As the aggregate, 228.9 g of a water absorbent resin was obtained.

Example 3
In Example 2, the amount of dehydration when the oil-soluble radical generator was added was changed from 177 g to 214 g (residual water ratio: 44%), and removed from the system after the addition of the oil-soluble radical generator and before the addition of the post-crosslinking agent. Except for changing the amount of water to 42 g, the same operation as in Example 2 was performed to obtain 228.4 g of a water absorbent resin as an aggregate of spherical particles.

Example 4
In Example 2, the amount of dehydration when the oil-soluble radical generator was added was changed from 177 g to 240 g (residual water ratio: 32%) and removed from the system after the addition of the oil-soluble radical generator and before the addition of the post-crosslinking agent. Except for changing the amount of water to 16 g, the same operation as in Example 2 was performed to obtain 228.1 g of a water absorbent resin as an aggregate of spherical particles.

Example 5
In Example 1, except that the oil-soluble radical generator was changed to 0.855 g (0.0035 mol) of 1,1′-azobis (cyclohexane-1-carbonitrile), the same operation as in Example 1 was performed, As an aggregate of spherical particles, 228.2 g of a water absorbent resin was obtained.

Example 6
The same operation as in Example 1 was performed except that the oil-soluble radical generator in Example 1 was changed to 0.497 g (0.002 mol) of 2,2′-azobis (2,4-dimethylvaleronitrile). Thus, 228.5 g of a water-absorbing resin was obtained as an aggregate of spherical particles.

Comparative Example 1
In Example 1, except that an oil-soluble radical generator was not added during the dehydration step, the same operation as in Example 1 was performed to obtain 228.3 g of a water-absorbing resin as an aggregate of spherical particles.

Comparative Example 2
In Example 1, instead of the oil-soluble radical generator added during the dehydration step, 0.949 g (0.0035) of 2,2′-azobis (2-amidinopropane) dihydrochloride, which is a water-soluble azo compound, is used. Except for adding an aqueous solution in which 8 mol of water was dissolved in 8 g of water, the same operation as in Example 1 was performed to obtain 229.5 g of a water-absorbent resin as an aggregate of spherical particles.

Comparative Example 3
In Example 2, the same operation as in Example 2 was performed except that the amount of dehydration at the time of addition of the oil-soluble radical generator was changed from 177 g to 90 g (residual water ratio: 101%). 228.8 g of water absorbent resin was obtained.

Comparative Example 4
In Example 2, instead of 2,2′-azobis (2-amidinopropane) dihydrochloride, which is a water-soluble azo compound, added to the monomer aqueous solution for the first stage polymerization, a water-soluble peroxide is used. 2,2′-azobis (2-amidino) which is a water-soluble azo compound that is added to 0.054 g (0.0002 mol) of potassium persulfate, which is added to the monomer aqueous solution for the second stage polymerization. Propane) Instead of dihydrochloride, use 0.076 g (0.0003 mol) of potassium persulfate, a water-soluble peroxide, and remove it outside the system from the addition of the oil-soluble radical generator to before the addition of the post-crosslinking agent. The same operation as in Example 2 was performed except that the amount of water was changed to 87 g, to obtain 228.4 g of a water-absorbent resin as an aggregate of spherical particles.

Evaluation method About the water-absorbent resin obtained in each Example and Comparative Example, physiological saline water retention capacity, physiological saline water absorption capacity under 4.14 kPa load, residual monomer content, external migration amount of residual monomer at the initial stage of use and The median particle size was measured by the following method. The results are shown in Table 1.

(Saline retention capacity)
Add 500 g of 0.9 mass% sodium chloride aqueous solution (saline) in a 500 mL beaker, and add 2.0 g of the water-absorbing resin while stirring at 600 r / min so as not to generate spatter. , Dispersed. Stirring was continued for 30 minutes to swell the water-absorbent resin, and then all of the contents of the beaker were poured into a cotton bag (Menbroad # 60, width 100 mm × length 200 mm). And after dehydrating this cotton bag, the upper part of which is bound with a rubber band, for 1 minute using a dehydrator (part number “H-122” manufactured by Kokusan Centrifuge Co., Ltd.) set to have a centrifugal force of 167G. The weight Wa (g) of the cotton bag containing the swollen gel was measured. Further, as a comparative control, the same operation was performed without adding a water-absorbing resin to physiological saline, and the mass Wb (g) of a wet cotton bag was measured. And the physiological saline water retention capacity was computed from the following formula.
Saline retention capacity (g / g) = [Wa-Wb] (g) / mass of water absorbent resin (g)

(Saline water absorption capacity under 4.14 kPa load)
The measurement was performed using the measuring apparatus 100 schematically shown in FIG. In the figure, a measuring apparatus 100 includes a burette unit 1, a conduit 2, a measuring table 3, and a measuring unit 4 placed on the measuring table 3. The bullet part 1 has a bullet 10. The upper part of the burette 10 can be closed with a rubber stopper 14, and the air introduction pipe 11 and the cock 12 are connected to the lower part. The air introduction tube 11 has a cock 13 at the tip. The conduit 2 has an inner diameter of 6 mm and connects the cock 12 of the burette unit 1 and the measuring table 3. The measuring table 3 is prepared so as to be movable in the vertical direction, and a hole (conduit port) having a diameter of 2 mm is provided at the center thereof, and one end of the conduit 2 is connected to the hole. The measuring unit 4 includes a cylinder 40 made of Plexiglas, a polyamide mesh 41 bonded to one opening of the cylinder 40, and a weight 42 that is movable in the vertical direction within the cylinder 40. The cylinder 40 is placed on the measurement table 3 through the mesh 41 and has an inner diameter of 20 mm. The opening of the polyamide mesh 41 is 75 μm (200 mesh). The weight 42 has a diameter of 19 mm and a mass of 119.6 g, and a load of 4.14 kPa can be applied to the water absorbent resin 5 uniformly arranged on the polyamide mesh 41 as will be described later.

  The measurement of the physiological saline water absorption capacity under a load of 4.14 kPa by the measuring apparatus 100 was performed in a room at 25 ° C. The specific measurement procedure was as follows. First, the cock 12 and the cock 13 of the burette unit 1 were closed, and physiological saline adjusted to 25 ° C. was added from the top of the burette 10. Next, after closing the upper part of the burette 10 with the rubber stopper 14, the cock 12 and the cock 13 are opened, and the surface of the physiological saline solution coming out from the conduit port of the measuring table 3 through the conduit 2 and the upper surface of the measuring table 3 are formed. The height of the measuring table 3 was adjusted so as to be the same height. On the other hand, in the measurement unit 4, 0.10 g of the water absorbent resin 5 is uniformly arranged on the polyamide mesh 41 in the cylinder 40, the weight 42 is arranged on the water absorbent resin 5, and the cylinder 40 is arranged at its center part. Was installed so as to coincide with the conduit port at the center of the measuring table 3.

  A decrease amount of physiological saline in the burette 10 60 minutes after the water absorbent resin 5 starts to absorb physiological saline from the conduit 2 (that is, the amount of physiological saline absorbed by the water absorbent resin 5) Wc (ml ), And the physiological saline water-absorbing ability under a 4.14 kPa load of the water-absorbing resin 5 was calculated by the following formula.

  4. Saline water absorption capacity under load of 14 kPa (ml / g) = Wc (ml) / mass of water absorbent resin (g)

(Residual monomer content)
In a 500 mL beaker, 500 g of physiological saline was added, and 2.0 g of a water absorbent resin was added thereto, followed by stirring for 60 minutes. The contents of the beaker were passed through a JIS standard sieve having an opening of 75 μm and then filtered through a filter paper (No. 3 manufactured by ADVANTEC) to separate the water-absorbing gel-like resin and physiological saline as a filtrate. . Then, the amount of residual monomer dissolved in the obtained filtrate was measured by high performance liquid chromatography. Here, the residual monomer to be measured is acrylic acid and its alkali metal salt. This measured value was converted to a value per mass of the water-absorbent resin and used as the residual monomer content (ppm).

(External transfer amount of residual monomer at the beginning of use)
In a vacuum suction filtration apparatus (ULVAC KIKOH Co., Ltd., Portable Aspirator MDA-015) having a filter holder whose inner diameter of the liquid passage part is 35 mm, a filter paper (ADVANTEC, No. 3) is placed on the filter holder part, and the filter paper is placed on the filter paper. Water-absorbing resin 0.5g was arrange | positioned uniformly. With the suction pressure adjustment valve closed (that is, the maximum degree of vacuum), 50 g of physiological saline was poured from the top of the filter paper at a time. Then, the amount of residual monomer dissolved in the obtained filtrate was measured by high performance liquid chromatography. This measured value was converted to a value per mass of the water-absorbent resin and used as the external migration amount (ppm) of the residual monomer at the initial stage of use.

(Medium particle size)
As a lubricant, 0.25 g of amorphous silica (Degussa Japan Co., Ltd., Sipernat 200) was mixed with 50 g of the water absorbent resin. When this is passed using a JIS standard sieve having a sieve opening of 250 μm and the amount remaining on the sieve is 50% by mass or more, a combination of the following JIS standard sieves (combination A) Was less than 50% by mass, the median particle size was measured using the combination (combination B) of JIS standard sieves shown in (B) below.

  (A) From top to bottom, a sieve having an opening of 710 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 a sieve having an opening of 150 μm And saucer order.

  (B) From top to bottom, a sieve having an opening of 400 μm, a sieve having an opening of 250 μm, a sieve having an opening of 180 μm, a sieve having an opening of 150 μm, a sieve having an opening of 106 μm, a sieve having an opening of 75 μm, and a sieve having an opening of 45 μm And saucer order.

  The water-absorbing resin was placed on the combined uppermost sieve and classified by shaking for 10 minutes using a low-tap shaker. After classification, the mass of the water-absorbent resin remaining on each sieve is calculated as a percentage by mass with respect to the total amount, and the mass of the water-absorbent resin remaining on the sieve opening and the sieve is calculated by integrating in order from the larger particle size. The relationship between percentage and integrated value was plotted on a logarithmic probability paper. By connecting the plots on the probability paper with a straight line, the particle diameter corresponding to an integrated mass percentage of 50 mass% was defined as the median particle diameter.

  According to Table 1, the water-absorbent resins obtained in the examples all have a high physiological saline water retention ability, a high physiological saline water absorption capacity under a 4.14 kPa load, and an extremely low residual monomer content. In addition, it can be seen that the amount of the external monomer remaining in the initial stage of use is small.

DESCRIPTION OF SYMBOLS 100 Measuring apparatus 1 Bullet part 10 Bullet 11 Air introducing pipe 12 Cock 13 Cock 14 Rubber stopper 2 Conduit 3 Measuring table 4 Measuring part 40 Cylinder 41 Nylon mesh 42 Weight 5 Water absorbing resin

Claims (9)

A polymerization step of polymerizing a water-soluble ethylenically unsaturated monomer in the presence of a water-soluble azo compound to obtain a reaction system containing a water-absorbent resin precursor; and water from the reaction system in a petroleum hydrocarbon dispersion medium. In the dehydration step, the remaining water ratio calculated by the following formula (1) is 5 to 75% to the reaction system,
2,2′-azobis (4-methoxy-2,4-dimethylvaleronitrile), 2,2′-azobis-isobutyronitrile, 1,1′-azobis (cyclohexane-1-carbonitrile), 2,2 At least one selected from the group consisting of '-azobis (2,4-dimethylvaleronitrile), 2,2'-azobis (methyl isobutyrate) and 2,2'-azobis (2-methylbutyronitrile) A method for producing a water-absorbent resin, comprising adding an oil-soluble radical generator.

(1)   The water-absorbent resin according to claim 1, wherein the addition amount of the oil-soluble radical generator is 0.0001 to 0.0020 mol with respect to 1 mol of the water-soluble ethylenically unsaturated monomer used in the polymerization step. Manufacturing method.   The water-soluble azo compound is 2,2′-azobis (2-amidinopropane) dihydrochloride, 2,2′-azobis [2- [1- (2-hydroxyethyl) -2-imidazolin-2-yl]. 3. Propane] dihydrochloride and 2,2′-azobis [N- (2-carboxyethyl) -2-methylpropionamidine] tetrahydrate are at least one selected from the group consisting of The manufacturing method of the water absorbing resin in any one.   The method for producing a water-absorbent resin according to any one of claims 1 to 3, wherein the water-soluble ethylenically unsaturated monomer is polymerized by a reverse phase suspension polymerization method in a polymerization step.   The method for producing a water-absorbent resin according to claim 4, wherein the reversed-phase suspension polymerization method is performed in two or more stages in the polymerization step.   The method for producing a water absorbent resin according to any one of claims 1 to 5, wherein after the polymerization step, a crosslinking agent is added to carry out a post-crosslinking reaction. A water-absorbing resin formed by using a crosslinked polymer of a water-soluble ethylenically unsaturated monomer as an essential component, and satisfying all of the following (A) to (D).
(A) The physiological saline water retention capacity is 38 to 65 g / g.
(B) Saline water absorption capacity under load of 4.14 kPa is 18 ml / g or more (C) Residual monomer content is 150 ppm or less (D) External migration amount of residual monomer at the initial stage of use is 30 ppm or less   The water absorbent resin according to claim 7, wherein the median particle diameter is 100 to 600 μm.   A hygiene comprising a liquid-permeable sheet, a liquid-impermeable sheet, and an absorbent body held between these sheets, wherein the absorbent body comprises the water-absorbent resin according to any one of claims 7 and 8. material.

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