Water absorbent

Abstract

To provide a water absorber having high solubility in alkali aqueous solution.SOLUTION: A water absorber is composed of a polyacrylic acid (salt)-based water-absorbing resin that is obtained by aqueous solution polymerization and is internally crosslinked with an epoxy compound and / or epoxy acrylate, and has a dissolution rate in 0.5% sodium hydroxide aqueous solution of 50% or more.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a water-absorbing agent that is highly soluble in an alkaline aqueous solution. [Background technology]

[0002] Water-absorbent resins are widely used in a variety of fields, including sanitary products, food, agriculture, forestry, and civil engineering. Taking advantage of their water-absorbing properties, they are particularly popular in sanitary products such as disposable diapers and sanitary napkins. These sanitary products retain a large amount of moisture after use, making them flame-retardant. Furthermore, because they contain a large amount of organic polymers, they tend to reach high temperatures when they begin to burn, placing a heavy burden on incineration and landfills. As the aging population is expected to continue to increase the amount of discarded sanitary products, there is a demand for water-absorbent resins with a low environmental impact.

[0003] Partially neutralized salts of polyacrylic acid or polymethacrylic acid are generally used as water-absorbent resins for sanitary products. Furthermore, to retain the absorbed water molecules, the main chains of the polyacrylic acid are internally crosslinked. Known compounds used for crosslinking include polyhydric alcohol compounds such as ethylene glycol and propylene glycol (Patent Document 1), mixtures of epoxy compounds such as ethylene glycol diglycidyl ether and polyalkylene glycol diacrylate (Patent Document 2), and epoxy compounds (Patent Documents 3 and 4). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-open No. 61-16903 [Patent Document 2] Japanese Patent Application Laid-Open No. 2017-185485 [Patent Document 3] Japanese Patent Application Laid-Open No. 2002-284892 [Patent Document 4] Japanese Patent Application Laid-Open No. 2010-59254 Summary of the Invention [Problem to be solved by the invention]

[0005] In Patent Document 1, crosslinking is formed by an ester, and in Patent Document 2, crosslinking is present by alkylene glycol, and therefore the decomposability of the water-absorbing agent is not sufficient. In Patent Documents 3 and 4, internal crosslinking is present by an epoxy compound, but the water-absorbing resin is polymerized by reversed-phase suspension polymerization, and therefore the decomposability of the water-absorbing agent made of this water-absorbing resin is not sufficient. The object of the present invention is to provide a water-absorbing agent with high decomposability. [Means for solving the problem]

[0006] The present inventors have found that when a polyacrylic acid (salt)-based water-absorbing resin obtained by an aqueous solution polymerization method has internal crosslinking with an epoxy compound and / or an epoxy acrylate, the solubility in an alkaline aqueous solution can be improved, and have completed the present invention.

[0007] That is, the present invention relates to a water-absorbing agent comprising a polyacrylic acid (salt)-based water-absorbing resin containing internal crosslinking with an epoxy compound and / or an epoxy acrylate, obtained by an aqueous solution polymerization method, and having a solubility of 50% or more in a 0.5% aqueous sodium hydroxide solution.

[0008] The CRC (centrifuge retention capacity) of the physiological saline is preferably 5 to 55 g / g.

[0009] It is preferable that 80 mol % or more of the internal crosslinks are internal crosslinks caused by the epoxy compound and / or epoxy acrylate.

[0010] It is preferable that 60% by weight or more of the crosslinking agent used for internal crosslinking is the above-mentioned epoxy compound and / or epoxy acrylate.

[0011] The epoxy compound and / or epoxy acrylate preferably contains an ester bond.

[0012] The present invention also relates to a method for producing the water-absorbing agent by adiabatic polymerization. [Effects of the Invention]

[0013] The water-absorbing agent of the present invention has high solubility in alkaline aqueous solutions because the polyacrylic acid (salt)-based water-absorbing resin contains internal crosslinks mainly due to epoxy compounds and / or epoxy acrylates. Such a water-absorbing agent is easily decomposed after use, reducing the environmental impact. DETAILED DESCRIPTION OF THE INVENTION

[0014] <<Water absorbent>> The water-absorbing agent of the present invention is characterized by comprising a polyacrylic acid (salt)-based water-absorbing resin containing internal crosslinking with an epoxy compound and / or epoxy acrylate, obtained by aqueous solution polymerization, and having a solubility of 50% or more in a 0.5% aqueous sodium hydroxide solution.

[0015] <Water absorbent resin> Examples of the polyacrylic acid (salt)-based water-absorbing resin used in the present invention include partially neutralized crosslinked polyacrylic acid, self-crosslinking partially neutralized polyacrylic acid, and starch-acrylic acid graft polymer.

[0016] The main constituent units of the water-absorbing resin are acrylic acid and acrylic acid (salt). Of the total amount of acrylic acid and acrylic acid (salt), it is preferable that acrylic acid accounts for 10 to 40 mol % and acrylic acid (salt) accounts for 90 to 60 mol %. Examples of the salt of acrylic acid include sodium salt, potassium salt, and ammonium salt, with sodium salt being particularly preferred.

[0017] When obtaining a water-absorbing resin, a monomer other than acrylic acid (salt) may be used in combination, if necessary. Examples of the monomer other than acrylic acid (salt) include anionic unsaturated monomers and salts thereof, such as methacrylic acid, maleic acid, vinyl sulfonic acid, styrene sulfonic acid, 2-(meth)acrylamide-2-methylpropanesulfonic acid, 2-(meth)acryloylethanesulfonic acid, and 2-(meth)acryloylpropanesulfonic acid; acrylamide, methacrylamide, N-ethyl(meth)acrylamide, Nn-propyl(meth)acrylamide, N-isopropyl(meth)acrylamide, N,N-dimethyl(meth)acrylamide, 2-hydroxyethyl(meth)acrylate, 2-hydroxyethyl(meth)acrylate, and 2-hydroxyethyl(meth)acrylate. Examples of suitable unsaturated monomers include nonionic hydrophilic group-containing unsaturated monomers such as dimethylpropyl (meth)acrylate, methoxypolyethylene glycol (meth)acrylate, polyethylene glycol mono(meth)acrylate, vinylpyridine, N-vinylpyrrolidone, N-acryloylpiperidine, and N-acryloylpyrrolidine; and cationic unsaturated monomers such as N,N-dimethylaminoethyl (meth)acrylate, N,N-diethylaminoethyl (meth)acrylate, N,N-dimethylaminopropyl (meth)acrylate, N,N-dimethylaminopropyl (meth)acrylamide, and quaternary salts thereof. These may be used alone or in combination of two or more.

[0018] When a monomer other than acrylic acid (salt) is used, the amount used is preferably 30 mol % or less, more preferably 10 mol % or less, based on the total amount including the acrylic acid (salt) used as the main component. By using the monomer in this range, the water absorption properties of the obtained water absorbent resin are further improved, and the water absorbent resin can be obtained at a further reduced cost.

[0019] <Aqueous solution polymerization method> The polyacrylic acid (salt)-based water-absorbing resin used in the present invention is obtained by aqueous solution polymerization using an aqueous solution of acrylic acid (salt) as a monomer. The concentration of acrylic acid (salt) in the aqueous solution in the aqueous solution polymerization is preferably 10 to 70% by weight, more preferably 20 to 40% by weight. When performing aqueous solution polymerization, solvents such as methanol, ethanol, n-propyl alcohol, isopropyl alcohol, n-butanol, isobutyl alcohol, t-butyl alcohol, isopentyl alcohol, acetone, methyl ethyl ketone, N,N-dimethylformamide, and dimethyl sulfoxide may be used in combination with water. When a solvent other than water is used in combination, the amount of the solvent other than water is preferably 30% by weight or less of the total solvent. A mixture of these solvents other than water may be used.

[0020] Examples of polymerization initiators that can be used include radical polymerization initiators such as potassium persulfate, ammonium persulfate, sodium persulfate, t-butyl hydroperoxide, hydrogen peroxide, and 2,2'-azobis(2-aminodipropane) dihydrochloride. Furthermore, by using a reducing agent that promotes the decomposition of these polymerization initiators, they can be used as a redox initiator that combines the two. Examples of the reducing agent include (bis)sulfites (salts) such as sodium sulfite and sodium hydrogensulfite, L-ascorbic acid (salts), reducing metals (salts) such as ferrous salts, and amines, but are not limited to these.

[0021] The amount of the polymerization initiator used is preferably 0.0001 to 5 mol % relative to the acrylic acid (salt) as a monomer, and more preferably 0.001 to 2 mol %. If the amount is less than 0.0001 mol %, the amount of unreacted monomer increases, and the amount of residual monomer in the obtained water absorbent resin may increase. On the other hand, if the amount of the polymerization initiator used exceeds 5 mol %, the amount of water-soluble components in the obtained water absorbent resin may increase.

[0022] Alternatively, the polymerization reaction may be initiated by irradiating the reaction system with active energy rays such as radiation, electron beams, or ultraviolet rays. The reaction time is not particularly limited and can be appropriately set depending on the types of hydrophilic monomer and polymerization initiator, the reaction temperature, etc. The reaction temperature in the polymerization reaction is not particularly limited, but is preferably 0°C to 90°C. However, since the polymerization reaction is easily controlled, it is preferable to carry out the polymerization by adiabatic polymerization.

[0023] A gel polymer is produced by aqueous solution polymerization. This gel polymer can be subjected to drying, pulverization, classification, etc., as needed, to obtain a water-absorbent resin. Drying, pulverization, classification, etc. can be performed by known methods.

[0024] <Internal crosslinking> Polyacrylic acid (salt)-based water-absorbing resins contain internal crosslinks formed by epoxy compounds and / or epoxy acrylates. The reaction between the carboxyl groups of polyacrylic acid (salt) and the epoxy groups of the epoxy compounds forms crosslinks containing two or more epoxy esters between the main chains of polyacrylic acid (salt). Similar crosslinks are also formed by synthesizing polyacrylic acid (salt) through a polymerization reaction of acrylic acid (salt) in the presence of epoxy acrylate.

[0025] Examples of epoxy compounds include succinic acid glycidyl ester, sorbitol polyglycidyl ether, trimethylolpropane polyglycidyl ether, polyethylene glycol diglycidyl ether, glycerol polyglycidyl ether, diglycerol polyglycidyl ether, polyglycerol polyglycidyl ether, propylene glycol diglycidyl ether, polypropylene glycol diglycidyl ether, glycidol, etc. Among these, epoxy compounds containing an ester bond are preferred from the viewpoint of containing an abundance of hydrolyzable functional groups, and succinic acid glycidyl ester is more preferred.

[0026] As the epoxy acrylate, a compound obtained by reacting the above epoxy compound with (meth)acrylic acid can be used.

[0027] The polyacrylic acid (salt)-based water-absorbing resin may contain internal crosslinks by an internal crosslinking agent other than the epoxy compound and / or epoxy acrylate, in addition to internal crosslinks by the epoxy compound and / or epoxy acrylate. However, it is preferable that 80 mol % or more of the internal crosslinks are internal crosslinks by the epoxy compound and / or epoxy acrylate, more preferably 90 mol % or more, and even more preferably 95 mol % or more. If the proportion of internal crosslinks by the epoxy compound and / or epoxy acrylate is less than 80 mol %, the solubility of the water-absorbing agent in an alkaline aqueous solution tends to be insufficient. Here, the mol % refers to the molar ratio of the epoxy compound and / or epoxy acrylate to the total number of moles of the internal crosslinking agent charged.

[0028] Furthermore, the crosslinking agent used for internal crosslinking preferably comprises the epoxy compound and / or epoxy acrylate in an amount of 60% by weight or more, more preferably 70% by weight or more, and even more preferably 80% by weight or more, where the weight % refers to the weight ratio of the epoxy compound and / or epoxy acrylate to the total charged weight of the internal crosslinking agent.

[0029] Examples of the internal crosslinking agent other than the epoxy compound and / or epoxy acrylate include polyhydric alcohol compounds, polyvalent amine compounds, polyisocyanate compounds, alkylene carbonate compounds, haloepoxy compounds, halohydrin compounds, polyvalent oxazoline compounds, silane coupling agents, polyvalent metal compounds, etc. These may be used alone or in combination of two or more.

[0030] Examples of the polyhydric alcohol compound include ethylene glycol, diethylene glycol, propylene glycol, triethylene glycol, tetraethylene glycol, polyethylene glycol, 1,3-propanediol, dipropylene glycol, 2,2,4-trimethyl-1,3-pentanediol, polypropylene glycol, glycerin, polyglycerin, 2-butene-1,4-diol, 1,3-butanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 1,2-cyclohexanedimethanol, 1,2-cyclohexanediol, trimethylolpropane, diethanolamine, triethanolamine, polyoxypropylene, oxyethylene-oxypropylene block copolymer, pentaerythritol, and sorbitol.

[0031] Examples of the polyvalent amine compound include ethylenediamine, diethylenetriamine, triethylenetetramine, tetraethylenepentamine, pentaethylenehexamine, polyethyleneimine, inorganic salts or organic salts (such as azithinium salts) of these polyvalent amine compounds, and the like.

[0032] Examples of the polyisocyanate compound include 2,4-tolylene diisocyanate and hexamethylene diisocyanate, and examples of the polyoxazoline compound include 1,2-ethylenebisoxazoline.

[0033] Examples of the alkylene carbonate compound include 1,3-dioxolan-2-one, 4-methyl-1,3-dioxolan-2-one, 4,5-dimethyl-1,3-dioxolan-2-one, 4,4-dimethyl-1,3-dioxolan-2-one, 4-ethyl-1,3-dioxolan-2-one, 4-hydroxymethyl-1,3-dioxolan-2-one, 1,3-dioxan-2-one, 4-methyl-1,3-dioxan-2-one, and 4,6-dimethyl-1,3-dioxan-2-one.

[0034] Examples of the haloepoxy compound include epichlorohydrin, epibromohydrin, α-methylepichlorohydrin, and polyamine adducts thereof (eg, Kaimen (registered trademark) manufactured by Hercules).

[0035] Other known crosslinking agents that can be used include silane coupling agents such as γ-glycidoxypropyltrimethoxysilane and γ-aminopropyltriethoxysilane, and polyvalent metal compounds such as hydroxides and chlorides of zinc, calcium, magnesium, aluminum, iron, zirconium, etc.

[0036] The amount of epoxy compound and / or epoxy acrylate used for internal crosslinking is preferably 0.01 to 10 mol % of epoxy groups and / or acrylate groups relative to 1 mol of carboxylic acid groups and / or carboxylate salt groups contained in the water-absorbent resin, more preferably 0.05 to 7 mol %, and even more preferably 0.1 to 5 mol %. If the epoxy group or acrylate group is less than 0.01 mol %, crosslinking of the water-absorbent resin cannot be effectively carried out, and a sufficient effect of improving water absorbency under pressure may not be obtained. If it exceeds 10 mol %, the crosslink density becomes too high, and the water-absorbing capacity and water-absorbing speed of the obtained water-absorbing agent may decrease.

[0037] When internal crosslinking is carried out, the epoxy compound and / or epoxy acrylate may be added to the reaction system during polymerization of the monomer polyacrylic acid (salt), or may be added after polymerization.

[0038] When an epoxy compound and / or an epoxy acrylate is added after polymerization of polyacrylic acid (salt), internal crosslinking can be formed by heating after the addition of the epoxy compound and / or the epoxy acrylate. The heating temperature is not particularly limited, but is preferably 40 to 250°C. The heating time is also not particularly limited, but is preferably 0.2 to 3 hours.

[0039] When performing internal crosslinking, various foaming agents such as (hydrogen) carbonates, carbon dioxide, azo compounds, and inert organic solvents; hydrophilic polymers such as starch, cellulose, starch, cellulose derivatives, polyvinyl alcohol, polyacrylic acid (salts), and crosslinked polyacrylic acid (salts); various surfactants; and chain transfer agents such as hypophosphorous acid (salts) may be added to the reaction system.

[0040] When performing internal crosslinking, solvents other than water may be used, such as lower aliphatic alcohols such as methanol, ethanol, n-propyl alcohol, and isopropyl alcohol, ketones such as acetone, ethers such as dioxane, tetrahydrofuran, and methoxy(poly)ethylene glycol, and amides such as ε-caprolactam and N,N-dimethylformamide.

[0041] <Surface crosslinking> The water-absorbing agent of the present invention may be one which has been internally crosslinked and then further surface-crosslinked, which can improve the strength of the water-absorbing agent.

[0042] The surface cross-linking agent can be the same as the above-mentioned epoxy compound and / or epoxy acrylate, and the internal cross-linking agent other than the epoxy compound and / or epoxy acrylate. These may be used alone or in combination of two or more. Among these, epoxy compounds are preferred from the viewpoint of easy control of CRC, and ethylene glycol diglycidyl ether and succinic acid glycidyl ester are more preferred.

[0043] The surface crosslinking can be formed by spraying a surface crosslinking agent onto a water absorbent resin, mixing the mixture by a known method using a cylindrical mixer, a V-shaped mixer, a ribbon mixer, a screw mixer, a double-arm mixer, a pulverizing kneader, etc., and then crosslinking the mixture. During spraying and mixing, a surfactant can be added as necessary.

[0044] When surface cross-linking is carried out, the amount of the surface cross-linking agent used is preferably 0.001 to 10 parts by weight, more preferably 0.005 to 5 parts by weight, and even more preferably 0.01 to 3 parts by weight, relative to 100 parts by weight of the water absorbent resin before surface cross-linking.

[0045] When surface crosslinking is performed, a solvent consisting of water, a hydrophilic organic solvent, or a mixture thereof may be added. Examples of such hydrophilic organic solvents include the solvents other than water that are mentioned above as the solvents used when internal crosslinking is performed.

[0046] The heating temperature during surface crosslinking is not particularly limited, but is preferably 40 to 250° C. The heating time is also not particularly limited, but is preferably 0.2 to 3 hours.

[0047] Furthermore, for the purpose of imparting various functions, the water-absorbing agent may contain other additives such as disinfectants, deodorants, antibacterial agents, fragrances, various inorganic powders, foaming agents, pigments, dyes, hydrophilic short fibers, fertilizers, oxidizing agents, reducing agents, water, and salts. The amounts of these other additives to be added are appropriately selected by those skilled in the art.

[0048] <Physical properties of water absorbent> The water-absorbing agent of the present invention contains internal crosslinking by an epoxy compound and / or an epoxy acrylate, and has an improved solubility in an alkaline aqueous solution. The solubility in an alkaline aqueous solution is evaluated by immersing the water-absorbing agent in a 0.5% aqueous sodium hydroxide solution and leaving it at 40°C for 24 hours, and then measuring the proportion of dissolved resin. The water-absorbing agent of the present invention has a solubility of 50% or more, preferably 60% or more, and more preferably 70% or more.

[0049] The CRC (centrifuge retention capacity) is determined by making the water-absorbing agent absorb physiological saline (0.9% aqueous sodium chloride solution) without applying a load, and then centrifuging the agent at 150 G to remove the water, and then measuring the water absorbency after that by the method described in the Examples. The water-absorbing agent of the present invention preferably has a CRC of 5 to 55 g / g, and more preferably 25 to 40 g / g.

[0050] The AAP (absorbency under pressure) is determined by measuring the absorbency of physiological saline when a load of 0.7 psi is applied to the water-absorbing agent by the method described in Examples. The water-absorbing agent of the present invention preferably has an AAP of 10 to 30 g / g, more preferably 11 to 25 g / g.

[0051] FSC (absorbency under no load) is determined by measuring the absorbency of physiological saline solution when no load is applied to the water-absorbing agent by the method described in Examples. The water-absorbing agent of the present invention preferably has an FSC of 30 to 100 g / g, more preferably 40 to 70 g / g.

[0052] <<Hygiene supplies>> The water-absorbing agent of the present invention can be suitably used in sanitary products such as disposable diapers and sanitary products. Examples of the structure of the sanitary products include a laminate in which a back sheet, an absorbent body, and a top sheet are laminated in this order. The absorbent body contains the water-absorbing agent of the present invention, and may further contain water-absorbent paper or pulp, as necessary. [Example]

[0053] The present invention will be described below with reference to examples, but is not limited to the following examples. Hereinafter, "parts" and "%" mean "parts by weight" and "% by weight", respectively, unless otherwise specified.

[0054] (1) Materials used (1-1) Raw material monomer for water-absorbent resin Acrylic acid (Mitsubishi Chemical Corporation) (1-2) Crosslinking agent Acrylic ester (Nippon Kayaku Co., Ltd., PEG400DA-D) Ethylene glycol diglycidyl ether (Nagase ChemteX Corporation, EX-810) Glycidyl succinate (Nagase ChemteX Corporation, GSR-105) Ethylene carbonate (Fujifilm Wako Pure Chemical Industries, Ltd.) (1-3) Hydrophilic solvent Propylene glycol (Fujifilm Wako Pure Chemical Industries, Ltd.)

[0055] (2) Polymerization and internal crosslinking of water-absorbent resin (Synthesis Examples 1 to 5, Comparative Synthesis Example 1) (Synthesis Example 1) A 2L separable flask is charged with 242g of acrylic acid and 903g of ion-exchanged water cooled to below 10°C. 206g of 48.5wt% sodium hydroxide aqueous solution is charged to the dropping funnel and added dropwise to the above acrylic acid aqueous solution to prepare a 75mol% neutralized acrylic acid aqueous solution. The flask is cooled in an ice bath, and the rate of dropping the sodium hydroxide aqueous solution is adjusted so that the liquid temperature does not exceed 20°C. After the sodium hydroxide addition is complete, the liquid temperature is cooled to below 10°C and transferred to a 2L glass Dewar flask that has been pre-cooled in a freezer. Nitrogen gas is bubbled into the 75mol% neutralized acrylic acid aqueous solution in the Dewar flask for at least 30 minutes to replace the atmosphere with nitrogen, and the liquid temperature is confirmed to be below 10°C. After nitrogen substitution, 0.71 g of ethylene glycol diglycidyl ether (Denacol EX-810, Nagase ChemteX Corporation) was added as an internal crosslinking agent, 3.2 g of a 0.3 wt% hydrogen peroxide solution, 3.6 g of a 1.0 wt% ascorbic acid solution, and a 0.01 wt% iron(II) sulfate solution were added. Polymerization began, and when the internal temperature exceeded 10°C due to the heat of polymerization, the introduction of nitrogen gas was stopped and the Dewar vessel was sealed with a silicone stopper. One hour after sealing, the Dewar vessel was placed in a blast oven set at 70°C and polymerization was continued for 12 hours. After polymerization was completed, the vessel was cooled to room temperature, and the gel-like polymer was removed from the Dewar vessel. The gel was then finely shredded using scissors or a mincer and dried in a blast oven set at 105°C for at least 12 hours. The dried gel was crushed in a food mixer to a particle size of 150-850 μm, yielding water-absorbent resin 1.

[0056] (Synthesis Example 2) Water absorbent resin 2 was obtained in the same manner as in Synthesis Example 1, except that the internal crosslinking agent was changed from 0.71 g of ethylene glycol diglycidyl ether to 0.71 g of succinic acid diglycidyl ester (GSR-105, manufactured by Nagase ChemteX Corporation).

[0057] (Synthesis Example 3) Water absorbent resin 3 was obtained in the same manner as in Synthesis Example 1, except that the internal crosslinking agent was changed from 0.71 g of ethylene glycol diglycidyl ether to 1.06 g of succinic acid diglycidyl ester (GSR-105 manufactured by Nagase ChemteX Corporation).

[0058] (Synthesis Example 4) Water absorbent resin 4 was obtained by the same operation as in Synthesis example 1, except that the internal crosslinking agent was changed from 0.71 g of ethylene glycol diglycidyl ether to 0.63 g of ethylene glycol diglycidyl ether (Denacol EX-810, manufactured by Nagase Chemtec Corporation) and 0.07 g of acrylic ester (PEG400 diacrylate, manufactured by Nippon Kayaku Co., Ltd.).

[0059] (Synthesis Example 5) Water absorbent resin 5 was obtained by the same operation as in Synthesis example 1, except that the internal crosslinking agent was changed from 0.71 g of ethylene glycol diglycidyl ether to 0.63 g of succinic acid diglycidyl ester (manufactured by Nagase ChemteX Corporation, GSR-105) and 0.07 g of acrylic ester (manufactured by Nippon Kayaku Co., Ltd., PEG400 diacrylate).

[0060] (Comparative Synthesis Example 1) 145.4 parts of acrylic acid was diluted with 9.4 parts of water and neutralized with 242.3 parts of 225 wt.% aqueous sodium hydroxide solution while cooling to 30°C. 0.08 parts of ethylene glycol diglycidyl ether (Denacol EX-810, manufactured by Nagase ChemteX Corporation), 0.015 parts of sodium hypophosphite monohydrate, and 0.07 parts of potassium persulfate were added and dissolved to prepare a monomer aqueous solution. 624 parts of cyclohexane was placed in a four-neck round-bottom flask equipped with a stirrer, reflux condenser, thermometer, and nitrogen gas inlet tube. 1.56 parts of polyoxyethylene octylphenyl ether phosphate (Brysurf A210G, manufactured by Daiichi Kogyo Seiyaku Co., Ltd.) as a dispersant was added and dispersed by stirring at 350 rpm. After replacing the atmosphere in the flask with nitrogen, the temperature was raised to 75°C and the cyclohexane was refluxed. The aqueous monomer solution prepared above was added dropwise to this over 90 minutes, and after completion of the dropwise addition, the temperature was maintained at 75°C for 60 minutes, and the water content of the reaction mixture was reduced to 10% (weight basis), and thereafter, when stirring was stopped, a water absorbent resin cake settled to the bottom of the flask, and this was separated by decantation.

[0061] The separated water-absorbent resin cake was dried at a drying temperature of 130°C for 60 minutes to obtain water-absorbent resin particles. The obtained water-absorbent resin particles were adjusted to a particle size of 150 to 850 μm. 100 parts by weight of this powder was mixed with 4.0 g of a 10 wt% water / methanol mixed solution of ethylene glycol diglycidyl ether (water / methanol (weight ratio) = 60 / 40) while stirring, and then heated at 120°C for 60 minutes to crosslink the powder, thereby obtaining a powdery surface-crosslinked water-absorbent resin.

[0062] (3) Surface crosslinking To 100 parts by weight of the water-absorbing resins of Synthesis Examples 1 to 5 after polymerization and internal cross-linking, 7.5 parts by weight of each internal-cross-linking agent solution blended at the blending ratio shown in Table 1 was sprayed and thoroughly mixed. The mixture was heated at 120°C for 30 minutes to obtain a surface-cross-linked water-absorbing agent. The measurement results of AUL, FSC, and CRC of this water-absorbing agent are shown in Table 1.

[0063] (4) Evaluation of the physical properties of water absorbents (4-1) AAP (absorbency under pressure) A crucible-type glass filter (inner diameter 40 mm, height 70 mm) was set upright, and 1 g of a water-absorbing agent was uniformly placed therein. A PET film (thickness 100 μm) was placed on the water-absorbing agent, and the initial weight Wa (g) was measured. A weight with an outer diameter of 38 mm was then placed on top of the PET film so that a load of 0.7 psi was applied. Next, the crucible-type glass filter containing the water-absorbing agent was immersed, bottom-side down, in a tray (length 210 mm, width 170 mm) containing approximately 630 g of physiological saline (concentration 0.9%) for 30 minutes, and then pulled out, and the weight Wb (g) after water absorption was measured. The water absorption capacity under pressure was calculated from Wa and Wb according to the following formula: AAP = (Wb(g) - Wa(g)) / Weight of water absorbent (g)

[0064] (4-2) FSC (water absorption capacity without pressure) 0.2 g of the water-absorbing agent was evenly placed in a nylon mesh bag (60 mm x 60 mm) and immersed in a beaker containing approximately 200 g of physiological saline (concentration 0.9%). After 30 minutes, the bag was taken out and centrifuged at 250 x 9.81 m / sec. 2 After draining the water at 250 G for 3 minutes, the weight of the bag, W1 (g), was measured. The weight, W0 (g), was also measured in the same manner except that no water-absorbing agent was used. The water absorption capacity without pressure was calculated from W1 and W0 according to the following formula: FSC = (W1(g) - W0(g)) / Weight of water absorbent (g)

[0065] (4-3)CRC (centrifuge retention capacity) After measuring the FSC, the tea bag was placed in a centrifuge and centrifuged at 150G for 90 seconds to remove excess saline. The weight including the tea bag (W2) was measured and the water retention capacity was calculated using the following formula. CRC (g / g) = ((W2)-(W0)) / weight of absorbent (g)

[0066] (4-4) Dissolution rate in alkaline aqueous solution 0.5 g of water-absorbent resin was weighed out (Ws) into a 30 ml glass screw tube, and 25 g of 0.5 wt % sodium hydroxide aqueous solution was added. The lid was then closed. The screw tube was placed in a dryer set at 40°C and left to stand for 24 hours. After 24 hours, the screw tube was removed from the dryer and allowed to cool to room temperature. The contents of the screw tube were then emptied onto a 1 mm mesh sieve and washed with ion-exchanged water. The gel-like water-absorbent resin remaining on the sieve was transferred to a Petri dish whose weight (W0) had been measured in advance. The Petri dish was placed in a blast dryer set at 120°C and dried for 12 hours. After drying, the weight (W1) of the Petri dish was measured, and the dissolution rate was calculated using the following formula: Dissolution rate (%)= (1-(W1-W0) / Ws)×100

[0067] [Table 1]

[0068] In Comparative Example 1, the water absorbing agent was produced by reversed-phase suspension polymerization, and therefore the surface of the obtained water absorbent resin was coated with the dispersant used in the production, and the solubility in an alkaline aqueous solution was low. In Examples 1 to 5, the water absorbing agents were produced by aqueous solution polymerization, and had an internal crosslinked structure mainly due to an epoxy compound and / or an epoxy acrylate, and therefore the solubility in an alkaline aqueous solution exceeded 50%.

Claims

1. The water-absorbing resin is a polyacrylic acid (salt)-based water-absorbing resin containing internal crosslinks with an epoxy compound and / or an epoxy acrylate, which is obtained by aqueous solution polymerization of acrylic acid (salt), the epoxy group and / or the acrylate group of the epoxy compound and / or the epoxy acrylate used in the aqueous solution polymerization method is 0.163 to 0.274 mol % relative to 1 mol of the carboxylic acid group and / or the carboxylate group of the acrylic acid (salt), A water-absorbing agent having a solubility of 50% or more in a 0.5% aqueous solution of sodium hydroxide. (However, the dissolution rate is calculated by the following method. 0.5 g of a water-absorbent resin is accurately weighed (Ws) into a 30 ml glass screw tube, 25 g of a 0.5 wt % aqueous sodium hydroxide solution is added, and the lid is closed. This screw tube is placed in a dryer set at 40°C and left to stand for 24 hours. After 24 hours, the screw tube is taken out from the dryer and cooled to room temperature, and then the content of the screw tube is emptied onto a sieve with 1 mm openings and washed with ion-exchanged water. The gel-like water-absorbent resin remaining on the sieve is transferred to a petri dish whose weight (W0) has been measured in advance. The petri dish is placed in a blower dryer set at 120°C and dried for 12 hours. After drying, the weight (W1) of the petri dish is measured, and the dissolution rate is calculated using the following formula. Dissolution rate (%) = (1-(W1-W0) / Ws) x 100)

2. 2. The water-absorbing agent according to claim 1, wherein the CRC (centrifuge retention capacity) of physiological saline is 5 to 55 g / g.

3. 3. The water-absorbing agent according to claim 1, wherein 80 mol % or more of the internal crosslinks are internal crosslinks caused by the epoxy compound and / or epoxy acrylate.

4. 3. The water-absorbing agent according to claim 1, wherein 60% by weight or more of the crosslinking agent used for the internal crosslinking is the epoxy compound and / or epoxy acrylate.

5. The water-absorbing agent according to claim 4, wherein the epoxy compound and / or the epoxy acrylate contains an ester bond.

6. A method for producing the water-absorbing agent according to any one of claims 1 to 5 by adiabatic polymerization.

7. A method for dissolving a water-absorbing agent, comprising the step of immersing the water-absorbing agent according to any one of claims 1 to 5 in an alkaline aqueous solution.

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