Method for producing water-absorbing resin particles

Abstract

Provided is a method for producing water-absorbing resin particles that makes it possible for a chelating agent to be uniformly contained throughout the entirety of the water-absorbing resin particles, which have a particle size distribution.　A method for producing water-absorbing resin particles according to the present invention comprises a step 1 for mixing flowing polymer particles and a chelating agent, and comprises any of method (A), method (B), method (C), and method (D), as follows. Method (A): A method in which a solution of the chelating agent is added to the polymer particles at a chelating agent addition rate V, defined by formula (1) V=F×A / T, of 30 or less. Method (B): A method in which droplets of a solution of the chelating agent are added to the polymer particles, the droplet diameter of the droplets being 500 μm or less. Method (C): A method in which the polymer particles and a hydrophilic surfactant are mixed and then the chelating agent is added to the polymer particles. Method (D): A method in which a mixture containing a hydrophilic surfactant and the chelating agent is added to the polymer particles.

Description

Method for producing water-absorbent resin particles

[0001] The present invention relates to a method for producing water-absorbent resin particles.

[0002] It is known that water-absorbent resin particles have excellent water absorption capacity and can exert a water-stopping effect, and therefore can be applied to various applications where water-stopping is required. For example, water-absorbent resin particles are used as a water-stopping material for communication cables such as optical cables, power cables, etc.

[0003] For example, Patent Document 1 discloses a technology in which a water-stopping material is used, which is an absorbent sheet containing a water-absorbent resin enclosed in an exterior material, to prevent the gel of the absorbent resin that swells after absorbing water from seeping out.

[0004] JP 2014-147281 A

[0005] It is important that the water-absorbent resin particles used in the above-mentioned water-stopping material (for example, water-stopping tape) have not only swelling performance (water-stopping performance) that enables them to immediately stop water seepage, but also excellent heat resistance even after swelling, that is, a property that makes the gel (swollen water-absorbent resin particles) less susceptible to deterioration due to heat generated by the cable. In this regard, a method for improving the heat resistance of the water-absorbent resin particles after swelling includes a method of incorporating a chelating agent into the water-absorbent resin particles.

[0006] However, when a chelating agent is contained in water-absorbent resin particles, there is a problem that the chelating agent may not be contained uniformly throughout the water-absorbent resin particles depending on the particle size distribution of the water-absorbent resin particles. That is, the present inventors have found that the inclusion of a chelating agent in water-absorbent resin particles strongly depends on the particle size distribution of the water-absorbent resin particles.

[0007] For example, in a region where the median particle size of water-absorbent resin particles is relatively large (300 μm or more), the chelating agent can be uniformly contained throughout the water-absorbent resin particles. On the other hand, in a region where the median particle size is small (250 μm or less), for example, there is a problem that the chelating agent tends to be unevenly distributed toward the large particle size side in the particle size distribution. That is, it can be said that it is difficult to uniformly contain the chelating agent throughout the water-absorbent resin particles with a small median particle size. Therefore, in the water-absorbent resin particles, variations in performance such as heat resistance between particle sizes tend to occur, and desired heat resistance performance cannot be obtained.

[0008] The present invention has been made in view of the above, and an object of the present invention is to provide a method for producing water-absorbent resin particles, which can make it possible to uniformly contain a chelating agent throughout water-absorbent resin particles having a particle size distribution.

[0009] Means for Solving the Problems The present inventors have conducted extensive research to achieve the above object, and as a result have found that the above object can be achieved by mixing polymer particles used for producing water-absorbent resin particles with a chelating agent in a predetermined method, and have thus completed the present invention.

[0010] That is, the present invention encompasses, for example, the subject matter described in the following items. Item 1: A method for producing water-absorbent resin particles, comprising a step 1 of mixing flowing polymer particles with a chelating agent, wherein the step 1 comprises the following methods (A), (B), (C), and (D): Method (A): A method for adding a solution of the chelating agent to the polymer particles at a chelating agent addition rate V defined by the following formula (1): V=F×A / T Formula (1) is 30 or less (in the formula (1), V is the chelating agent addition rate [min －１ ], F is the average linear flow velocity from the nozzle [m / min], A is the cross-sectional area of ​​the nozzle [m ２ ], T is the total amount of chelating agent solution added [m ３Item (B): a method of adjusting the droplet size of the solution of the chelating agent to 500 μm or less and adding the solution to the polymer particles, Method (C): a method of mixing the polymer particles with a hydrophilic surfactant and then adding the chelating agent to the polymer particles, Method (D): a method of adding a mixture containing a hydrophilic surfactant and the chelating agent to the polymer particles. Item 2: A method of producing water-absorbent resin particles according to Item 1, wherein in step 1, 0.01 part by mass or more of the chelating agent is added relative to 100 parts by mass of the polymer particles. Item 3: A method of producing water-absorbent resin particles according to Item 1 or 2, wherein in step 1, 0.01 part by mass or more of the hydrophilic surfactant is added relative to 100 parts by mass of the polymer particles.

[0011] According to the method for producing water-absorbent resin particles of the present invention, the chelating agent can be uniformly contained throughout the water-absorbent resin particles having a particle size distribution.

[0012]

[0023] In the present specification, the terms "contain" and "comprise" include the concepts of "contain," "comprise," "consist essentially of," and "consist only of."

[0013] In the numerical ranges described in stages in this specification, the upper or lower limit of a certain numerical range can be arbitrarily combined with the upper or lower limit of another numerical range. In the numerical ranges described in this specification, the upper or lower limit of the numerical range may be replaced with a value shown in an example or a value that can be unambiguously derived from an example. In addition, in this specification, a numerical value connected with "to" means a numerical range that includes the numbers before and after "to" as the lower and upper limits.

[0014] 1. Method for Producing Water-Absorbent Resin Particles The method for producing water-absorbent resin particles of the present invention comprises at least step 1 of mixing flowing polymer particles with a chelating agent.

[0015] In the production method of the present invention, in step 1, the chelating agent and the polymer particles are mixed by at least one method selected from the group consisting of the following methods (A), (B), (C), and (D):

[0016] Method (A) is a method in which the chelating agent solution is added to the polymer particles at a chelating agent addition rate V defined by the following formula (1): V=F×A / T Formula (1) is 30 or less. In the formula (1), V is the chelating agent addition rate [min －１ ], F is the average linear flow velocity from the nozzle [m / min], A is the cross-sectional area of ​​the nozzle [m ２ ], T is the total amount of chelating agent solution added [m ３ ] is shown.

[0017] Method (B) is a method in which the droplets of the chelating agent solution are adjusted to a diameter of 500 μm or less and added to the polymer particles.

[0018] Method (C) is a method in which the polymer particles and a hydrophilic surfactant are mixed together, and then the chelating agent is added to the polymer particles.

[0019] Method (D) is a method in which a mixture containing a hydrophilic surfactant and the chelating agent is added to the polymer particles.

[0020] According to the method for producing water-absorbent resin particles of the present invention, a chelating agent can be uniformly contained throughout water-absorbent resin particles having a particle size distribution. This reduces, for example, the variation in heat resistance of the gel among particle sizes in a swollen state (gel state) of the water-absorbent resin particles, and as a result, it is expected that the heat resistance of the entire water-absorbent resin particles will be improved.

[0021] The method for producing water-absorbent resin particles of the present invention will be specifically described below, and hereinafter the method for producing water-absorbent resin particles of the present invention will be simply abbreviated as "the production method of the present invention."

[0022] In the production method of the present invention, polymer particles and a chelating agent are used in step 1. Specifically, in step 1, the chelating agent is brought into contact with the polymer particles while the polymer particles are fluidized. As a result, the chelating agent is contained in the polymer particles, and the target water-absorbent resin particles are obtained.

[0023] (Polymer Particles) The polymer particles used in step 1 can be, for example, a wide range of polymer particles that are used in known water-absorbing resin particles. The polymer particles can have, for example, a structure in which a polymer of a water-soluble ethylenically unsaturated monomer is crosslinked. The polymer particles having a crosslinked structure may have, for example, a structure crosslinked by an internal crosslinking agent described below, or may have a structure crosslinked by a post-crosslinking agent described below instead of or in addition to the internal crosslinking agent.

[0024] As the water-soluble ethylenically unsaturated monomer, for example, a wide range of known monomers that can be used in general water-absorbent resins can be applied. Examples of water-soluble ethylenically unsaturated monomers include (meth)acrylic acid (herein, "acry" and "methacry" are collectively referred to as "(meth)acry", the same applies hereinafter) and salts thereof; 2-(meth)acrylamido-2-methylpropanesulfonic acid and salts thereof; nonionic monomers such as (meth)acrylamide, N,N-dimethyl(meth)acrylamide, 2-hydroxyethyl(meth)acrylate, N-methylol(meth)acrylamide, and polyethylene glycol mono(meth)acrylate; and amino group-containing unsaturated monomers such as N,N-diethylaminoethyl(meth)acrylate, N,N-diethylaminopropyl(meth)acrylate, and diethylaminopropyl(meth)acrylamide, as well as quaternized products thereof. These water-soluble ethylenically unsaturated monomers may be used alone, or two or more may be used in combination (copolymerized). Among these, (meth)acrylic acid and salts thereof, (meth)acrylamide, and N,N-dimethylacrylamide are preferred from the viewpoint of industrial availability, and (meth)acrylic acid and salts thereof are more preferred.

[0025] When acrylic acid and its salts are used as the water-soluble ethylenically unsaturated monomer, the acrylic acid and its salts are used as the main water-soluble ethylenically unsaturated monomer, and specifically, it is preferable that they are used in an amount of 70 to 100 mol % based on the total number of moles of the water-soluble ethylenically unsaturated monomer.

[0026] The water-soluble ethylenically unsaturated monomer may be used in the form of an aqueous solution in order to increase the dispersion efficiency in a hydrocarbon dispersion medium when performing the reversed-phase suspension polymerization described below. The concentration of the monomer in such an aqueous solution is usually 20% by mass or more and not more than the saturated concentration, preferably 25 to 90% by mass, and more preferably 30 to 85% by mass.

[0027] When the water-soluble ethylenically unsaturated monomer has an acid group, such as (meth)acrylic acid or 2-(meth)acrylamido-2-methylpropanesulfonic acid, the acid group may be neutralized in advance with an alkaline neutralizing agent, as necessary. Examples of such alkaline neutralizing agents include alkali metal salts such as sodium hydroxide, sodium carbonate, sodium bicarbonate, potassium hydroxide, and potassium carbonate; and ammonia. In particular, these alkaline neutralizing agents may be used in the form of an aqueous solution to simplify the neutralization operation. The alkaline neutralizing agents described above may be used alone or in combination of two or more.

[0028] Regarding the degree of neutralization of the water-soluble ethylenically unsaturated monomer with the alkaline neutralizing agent, in order to increase the osmotic pressure of the obtained water absorbent resin and thereby increase the water absorption performance, the degree of neutralization of all acid groups which the water-soluble ethylenically unsaturated monomer has is usually preferably 10 to 100 mol%, more preferably 30 to 80 mol%.

[0029] The polymer particles can be produced, for example, by a production method including a step of carrying out a polymerization reaction using a raw material containing the water-soluble ethylenically unsaturated monomer (hereinafter referred to as the "polymerization step").

[0030] The polymerization reaction carried out in the polymerization step can be, for example, a reversed-phase suspension polymerization method, an aqueous solution polymerization method, an emulsion polymerization method, etc. In the following, the reversed-phase suspension polymerization method and the aqueous solution polymerization method will be described as examples.

[0031] Inverse-phase suspension polymerization is a method in which, for example, a poorly soluble monomer is suspended in a dispersion medium in the presence of a dispersion stabilizer, a polymerization reaction (polymerization step) is carried out, and if necessary, post-crosslinking as described below is carried out to obtain polymer particles. As is well known, the reverse-phase suspension polymerization method may be a multi-stage polymerization in which the monomer is polymerized in multiple stages, or may be, for example, a two-stage polymerization. In multi-stage polymerization, the first polymerization reaction is referred to as the first stage, and the monomers added stepwise after the first stage polymerization are subsequently referred to as the second stage, third stage, etc.

[0032] A hydrocarbon dispersion medium can be used as the dispersion medium used in reversed-phase suspension polymerization. Examples of hydrocarbon dispersion media include aliphatic hydrocarbons such as n-hexane, n-heptane, n-octane, and ligroin; alicyclic hydrocarbons such as cyclopentane, methylcyclopentane, cyclohexane, and methylcyclohexane; and aromatic hydrocarbons such as benzene, toluene, and xylene. Among these dispersion media, n-hexane, n-heptane, and cyclohexane are preferred because they are easily available industrially, have stable quality, and are inexpensive. These dispersion media may be used alone or in combination of two or more. Examples of dispersion media that can be used include mixed solvents such as Exxsol Heptane (manufactured by ExxonMobil: hydrocarbons of heptane and its isomers) and Nappar 6 (manufactured by ExxonMobil: hydrocarbons of cyclohexane and its isomers).

[0033] Monomers that can be used in reverse suspension polymerization include the water-soluble ethylenically unsaturated monomers described above. In terms of excellent reactivity, the water-soluble ethylenically unsaturated monomers are preferably (meth)acrylic acid and salts thereof, (meth)acrylamide, and N,N-dimethylacrylamide, and more preferably (meth)acrylic acid and salts thereof. The water-soluble ethylenically unsaturated monomers may be neutralized to a degree of neutralization within the above range.

[0034] In the reversed-phase suspension polymerization, a thickener can be used as needed, such as hydroxyethyl cellulose, hydroxypropyl cellulose, methyl cellulose, carboxymethyl cellulose, polyacrylic acid, (partially) neutralized polyacrylic acid, polyethylene glycol, polyacrylamide, polyethyleneimine, dextrin, sodium alginate, polyvinyl alcohol, polyvinylpyrrolidone, polyethylene oxide, etc.

[0035] The dispersion stabilizer used in reverse suspension polymerization may be a surfactant having an HLB value of less than 5. Examples of such surfactants include sucrose fatty acid esters, polyglycerin fatty acid esters, sorbitan fatty acid esters, polyoxyethylene sorbitan fatty acid esters, polyoxyethylene glycerin fatty acid esters, sorbitol fatty acid esters, polyoxyethylene sorbitol fatty acid esters, polyoxyethylene alkyl ethers, polyoxyethylene alkylphenyl ethers, polyoxyethylene castor oil, polyoxyethylene hydrogenated castor oil, alkylallyl formaldehyde condensed polyoxyethylene ethers, polyoxyethylene polyoxypropylene block copolymers, polyoxyethylene polyoxypropyl alkyl ethers, polyethylene glycol fatty acid esters, alkyl glucosides, N-alkyl gluconamides, polyoxyethylene fatty acid amides, polyoxyethylene alkylamines, phosphate esters of polyoxyethylene alkyl ethers, and phosphate esters of polyoxyethylene alkyl allyl ethers. Among these, sorbitan fatty acid esters, polyglycerin fatty acid esters, sucrose fatty acid esters, and the like are preferred from the viewpoint of dispersion stability of the monomer. These surfactants may be used alone or in combination of two or more.

[0036] In order to maintain a good dispersion state of the water-soluble ethylenically unsaturated monomer in the hydrocarbon dispersion medium and to obtain a dispersing effect commensurate with the amount used, the amount of the surfactant used is preferably 0.1 to 30 parts by mass, more preferably 0.3 to 20 parts by mass, per 100 parts by mass of the first-stage water-soluble ethylenically unsaturated monomer.

[0037] Furthermore, a polymeric dispersant may be used in combination with or instead of a surfactant as a dispersion stabilizer. Examples of polymeric dispersants that can be used include maleic anhydride-modified polyethylene, maleic anhydride-modified polypropylene, maleic anhydride-modified ethylene-propylene copolymer, maleic anhydride-modified EPDM (ethylene-propylene-diene terpolymer), maleic anhydride-modified polybutadiene, maleic anhydride-ethylene copolymer, maleic anhydride-propylene copolymer, maleic anhydride-ethylene-propylene copolymer, maleic anhydride-butadiene copolymer, polyethylene, polypropylene, ethylene-propylene copolymer, oxidized polyethylene, oxidized polypropylene, oxidized ethylene-propylene copolymer, ethylene-acrylic acid copolymer, ethyl cellulose, and ethylhydroxyethyl cellulose. Among these, from the viewpoint of dispersion stability of the monomer, preferred are maleic anhydride-modified polyethylene, maleic anhydride-modified polypropylene, maleic anhydride-modified ethylene-propylene copolymer, maleic anhydride-ethylene copolymer, maleic anhydride-propylene copolymer, maleic anhydride-ethylene-propylene copolymer, polyethylene, polypropylene, ethylene-propylene copolymer, oxidized polyethylene, oxidized polypropylene, oxidized ethylene-propylene copolymer, etc. These polymeric dispersants may be used alone or in combination of two or more.

[0038] The amount of polymeric dispersant used is preferably 0.1 to 30 parts by mass, more preferably 0.3 to 20 parts by mass, per 100 parts by mass of the first-stage water-soluble ethylenically unsaturated monomer, in order to maintain a good dispersion state of the water-soluble ethylenically unsaturated monomer in the hydrocarbon dispersion medium and to obtain a dispersing effect commensurate with the amount used.

[0039] On the other hand, the aqueous solution polymerization method is a method in which, for example, a monomer is polymerized in water to produce a gel, and the gel is pulverized to obtain a crushed material, and the crushed material is obtained as polymer particles.

[0040] Monomers that can be used in the aqueous solution polymerization method include the water-soluble ethylenically unsaturated monomers described above. In terms of excellent reactivity, the water-soluble ethylenically unsaturated monomers are preferably (meth)acrylic acid and its salts, (meth)acrylamide, and N,N-dimethylacrylamide, and more preferably (meth)acrylic acid and its salts. Furthermore, the water-soluble ethylenically unsaturated monomers may be neutralized so that the degree of neutralization falls within the above-mentioned range. Monomers that can be used in the aqueous solution polymerization method may contain an internal crosslinking agent, which will be described later.

[0041] The method for obtaining polymer particles by pulverizing the gel produced by aqueous polymerization is not particularly limited, and a wide range of known pulverization conditions can be applied to the present invention. For example, the gel produced by aqueous polymerization can be roughly pulverized, and if necessary, dried to obtain a dried product, which can then be pulverized using, for example, an ultracentrifugal pulverizer to obtain polymer particles. The obtained polymer particles can also be post-crosslinked if necessary. The particle size of the obtained polymer particles can also be adjusted by an appropriate method.

[0042] The polymerization steps carried out to obtain polymer particles, including the reversed-phase suspension polymerization method and the aqueous solution polymerization method, will be described below.

[0043] In the polymerization step, for example, a wide variety of known polymerization initiators can be used. Examples of radical polymerization initiators include persulfates such as potassium persulfate, ammonium persulfate, and sodium persulfate; peroxides such as methyl ethyl ketone peroxide, methyl isobutyl ketone peroxide, di-t-butyl peroxide, t-butylcumyl peroxide, and hydrogen peroxide; and azo compounds such as 2,2'-azobis(2-methylpropionamidine) dihydrochloride, 2,2'-azobis[2-(N-phenylamidino)propane] dihydrochloride, 2,2'-azobis[2-(N-allylamidino)propane] dihydrochloride, and 4,4'-azobis(4-cyanovaleric acid). The radical polymerization initiator can also be used in combination with a reducing agent such as sodium sulfite, sodium hydrogen sulfite, ferrous sulfate, or L-ascorbic acid to form a redox polymerization initiator.

[0044] From the viewpoint of polymerization stability, the lower limit of the amount of radical polymerization initiator used in the polymerization step is preferably 0.01 mmol, more preferably 0.05 mmol, relative to 1 mol of the water-soluble ethylenically unsaturated monomer used. From the viewpoint of polymerization stability, the upper limit of the amount of radical polymerization initiator is preferably 20 mmol, more preferably 10 mmol, relative to 1 mol of the water-soluble ethylenically unsaturated monomer used. By using the radical polymerization initiator within this range, the production of the water-absorbing resin becomes easy.

[0045] In the polymerization step, a chain transfer agent may be used as needed. Examples of the chain transfer agent include hypophosphites, thiols, thiolic acids, secondary alcohols, and amines.

[0046] In the polymerization step, an internal crosslinking agent can be used as needed. As a result, the polymer particles obtained in the polymerization step can have a structure in which the inside is crosslinked by the internal crosslinking agent. That is, the obtained water-absorbent resin particles can have an internal crosslinked structure. In this specification, the crosslinking agent used for crosslinking the inside of the polymer is referred to as an internal crosslinking agent to distinguish it from a post-crosslinking agent.

[0047] The internal cross-linking agent may be a compound having two or more polymerizable unsaturated groups. Specific examples of the internal cross-linking agent include (poly)ethylene glycol (in this specification, for example, "polyethylene glycol" and "ethylene glycol" are collectively referred to as "(poly)ethylene glycol"). the same applies hereinafter)], di- or tri(meth)acrylic acid esters of polyols such as (poly)propylene glycol, trimethylolpropane, glycerin polyoxyethylene glycol, polyoxypropylene glycol, and (poly)glycerin; unsaturated polyesters obtained by reacting the above-mentioned polyols with unsaturated acids such as maleic acid and fumaric acid; bisacrylamides such as N,N'-methylenebis(meth)acrylamide; di- or tri(meth)acrylic acid esters obtained by reacting polyepoxides with (meth)acrylic acid; di(meth)acrylic acid carbamyl esters obtained by reacting polyisocyanates such as tolylene diisocyanate and hexamethylene diisocyanate with hydroxyethyl (meth)acrylate; allylated starch; allylated cellulose; diallyl phthalate; N,N',N''-triallyl isocyanurate; and divinylbenzene.

[0048] Examples of the internal cross-linking agent include compounds having two or more polymerizable unsaturated groups, as well as glycidyl group-containing compounds such as (poly)ethylene glycol diglycidyl ether, (poly)propylene glycol diglycidyl ether, and (poly)glycerin diglycidyl ether; (poly)ethylene glycol, (poly)propylene glycol, (poly)glycerin, pentaerythritol, ethylenediamine, polyethyleneimine, and glycidyl (meth)acrylate. Two or more of these internal cross-linking agents may be used in combination. Among these, (poly)ethylene glycol diglycidyl ether, (poly)propylene glycol diglycidyl ether, and (poly)glycerin diglycidyl ether, and N,N'-methylenebisacrylamide are preferred from the viewpoint of excellent reactivity at low temperatures.

[0049] When an internal crosslinking agent is used, the amount thereof is preferably 0.0001 millimoles, more preferably 0.0005 millimoles, even more preferably 0.001 millimoles, and particularly preferably 0.01 millimoles, relative to 1 mole of the water-soluble ethylenically unsaturated monomer used, from the viewpoint of ease of production of the water-absorbing resin.Furthermore, the upper limit of the amount of the internal crosslinking agent used is preferably 5 millimoles, more preferably 0.5 millimoles, even more preferably 0.1 millimoles, and particularly preferably 0.05 millimoles, relative to 1 mole of the water-soluble ethylenically unsaturated monomer used.One or more types of internal crosslinking agents can be used.

[0050] In the polymerization step, the temperature of the polymerization reaction can be appropriately set depending on the type and amount of the radical polymerization initiator used, and can be, for example, 20 to 110° C., preferably 40 to 90° C. The reaction time can be set, for example, in the range of 0.1 to 4 hours.

[0051] In the case of the reversed-phase suspension polymerization method, the polymerization step can be carried out by, for example, adding an aqueous solution containing a water-soluble ethylenically unsaturated monomer, which has been neutralized as necessary, a thickener, a radical polymerization initiator, and an internal crosslinking agent to a dispersion medium in which a polymer dispersion stabilizer has been dissolved, and then adding a surfactant to create a suspended state.

[0052] In the polymerization step, the water-soluble ethylenically unsaturated monomer is polymerized to produce polymer particles. For example, in reversed-phase suspension polymerization, a slurry in which a polymer of the water-soluble ethylenically unsaturated monomer is dispersed is obtained. In the aqueous solution polymerization method, a gel-like product is obtained, which can be subjected to a pulverization treatment as described above. When an internal crosslinking agent is used, the polymer obtained in the polymerization step has a structure crosslinked by the internal crosslinking agent.

[0053] In the case of the above-mentioned multi-stage polymerization in the polymerization step, after the first-stage reversed-phase suspension polymerization is carried out by the above-mentioned method, the water-soluble ethylenically unsaturated monomer is added to and mixed with the reaction mixture obtained in the first-stage polymerization step, and the second-stage and subsequent reversed-phase suspension polymerizations are carried out by the same method as in the first stage.In the reversed-phase suspension polymerization in each stage from the second stage onwards, in addition to the water-soluble ethylenically unsaturated monomer, a radical polymerization initiator and an internal crosslinking agent added as needed are added within the range of the molar ratio of each component to the water-soluble ethylenically unsaturated monomer mentioned above, based on the amount of the water-soluble ethylenically unsaturated monomer added during the reversed-phase suspension polymerization in each stage from the second stage onwards, and the reversed-phase suspension polymerization is carried out under the same conditions as in the above-mentioned method.In addition, when the reversed-phase suspension polymerization is carried out in multiple stages, from the viewpoint of facilitating the production of the desired water-absorbing resin, it is preferable that the total amount of the polymerization initiator and the total amount of the internal crosslinking agent used as needed per mole of the water-soluble ethylenically unsaturated monomer used in the reversed-phase suspension polymerization are each set to be within the above-mentioned range.

[0054] In the reversed-phase suspension polymerization method, a dispersion of polymer particles is obtained by the polymerization step, and the dispersion can be subjected to step 1. The polymer particles obtained by the reversed-phase suspension polymerization method may be subjected to a drying treatment in a drying step at any stage before or after step 1, and the polymer particles may be subjected to a post-crosslinking treatment in a post-crosslinking step at any stage before or after step 1.

[0055] On the other hand, aqueous solution polymerization produces a gel, which can be pulverized in the pulverization step as described above to produce polymer particles. The polymer particles thus obtained may be subjected to a drying treatment in a drying step at any stage before or after step 1, and the polymer particles may be subjected to a post-crosslinking treatment in a post-crosslinking step at any stage before or after step 1. That is, the production method of the present invention may include a drying step and / or a post-crosslinking step in addition to step 1.

[0056] As described above, in the production method of the present invention, even when the polymer particles are obtained by various methods such as reverse phase suspension polymerization and aqueous solution polymerization, the production method may include a drying step and / or a post-crosslinking step in addition to step 1.

[0057] The drying step may be performed before or after step 1. Also, the drying step may be performed during step 1. The post-crosslinking step may also be performed before or after step 1, or may be performed during step 1. The drying step may be performed before or after the post-crosslinking step.

[0058] The drying step is a step of removing water from the polymer obtained in the polymerization step or the polymer crosslinked with an internal crosslinking agent by applying external energy such as heat to the polymer. For example, in the case of reversed-phase suspension polymerization, in the drying step, the polymer (hydrous gel) obtained in the polymerization step is dispersed in a hydrocarbon dispersion medium and subjected to azeotropic distillation to remove water, hydrocarbon dispersion medium, etc. from the polymer. By carrying out the drying step, the water content of the polymer or the polymer crosslinked with an internal crosslinking agent can be adjusted. The drying step may be carried out simultaneously with the post-crosslinking step.

[0059] The drying step may be carried out under normal pressure or under reduced pressure, and may be carried out under a stream of nitrogen or the like to increase drying efficiency. When the drying step is carried out under normal pressure, the drying temperature is preferably 70 to 250°C, more preferably 80 to 180°C, even more preferably 80 to 140°C, and particularly preferably 90 to 130°C. When the drying step is carried out under reduced pressure, the drying temperature is preferably 40 to 160°C, more preferably 50 to 120°C.

[0060] The post-crosslinking step is a step for treating the polymer particles obtained in the polymerization step, the polymer particles dried in the drying step, or the polymer particles to which a chelating agent has been added in step 1 with a post-crosslinking agent. This allows polymer particles having a crosslinked structure due to the post-crosslinking agent to be obtained, thereby increasing the crosslinking density near the surface. In the case of the reversed-phase suspension polymerization method, the treatment with the post-crosslinking agent can be carried out, for example, in the dispersion medium. In the case of the aqueous solution polymerization method, the treatment with the post-crosslinking agent can be carried out, for example, by adding the post-crosslinking agent to the pulverized polymer particles.

[0061] The types of post-crosslinking agents include, for example, a wide variety of post-crosslinking agents that can be used in conventional water-absorbent resins. A compound having two or more reactive functional groups can be used as the post-crosslinking agent. Specific examples of the post-crosslinking agent include polyols such as ethylene glycol, propylene glycol, 1,4-butanediol, trimethylolpropane, glycerin, polyoxyethylene glycol, polyoxypropylene glycol, and polyglycerin; polyglycidyl compounds such as (poly)ethylene glycol diglycidyl ether, (poly)glycerin diglycidyl ether, (poly)glycerin triglycidyl ether, trimethylolpropane triglycidyl ether, (poly)propylene glycol polyglycidyl ether, and (poly)glycerol polyglycidyl ether; epichlorohydrin, epibromohydrin, α-methylepichlorohydrin, and the like. isocyanate compounds such as 2,4-tolylene diisocyanate and hexamethylene diisocyanate; oxetane compounds such as 3-methyl-3-oxetanemethanol, 3-ethyl-3-oxetanemethanol, 3-butyl-3-oxetanemethanol, 3-methyl-3-oxetaneethanol, 3-ethyl-3-oxetaneethanol and 3-butyl-3-oxetaneethanol; oxazoline compounds such as 1,2-ethylenebisoxazoline; carbonate compounds such as ethylene carbonate; and hydroxyalkylamide compounds such as bis[N,N-di(β-hydroxyethyl)]adipamide. Among these, polyglycidyl compounds such as (poly)ethylene glycol diglycidyl ether, (poly)ethylene glycol triglycidyl ether, (poly)glycerin diglycidyl ether, (poly)glycerin triglycidyl ether, (poly)propylene glycol polyglycidyl ether, (poly)glycerol polyglycidyl ether, etc. are more preferred. The post-crosslinking agents may be used alone or in combination of two or more.

[0062] In view of the tendency to improve water absorption performance, the lower limit of the amount of post-crosslinking agent used is preferably 0.01 mmol, more preferably 0.05 mmol, and even more preferably 0.1 mmol, relative to 1 mol of the water-soluble ethylenically unsaturated monomer constituting the post-crosslinked polymer particles, and the upper limit is preferably 10 mmol, more preferably 5 mmol, and even more preferably 2 mmol, relative to 1 mol of the water-soluble ethylenically unsaturated monomer.

[0063] The reaction temperature in the post-crosslinking step (i.e., the temperature at which the polymer is treated with the post-crosslinking agent) is preferably 50 to 250° C., more preferably 60 to 180° C., and even more preferably 60 to 140° C. The reaction time of the post-crosslinking (i.e., the time at which the polymer is treated with the post-crosslinking agent at the reaction temperature) cannot be determined in general because it differs depending on the reaction temperature, the type and amount of the post-crosslinking agent used, etc., but is usually 1 to 300 minutes, and preferably 5 to 200 minutes.

[0064] The median particle size of the polymer particles can be 10 to 250 μm. In particular, in the production method of the present invention, the chelating agent can be uniformly contained throughout the water-absorbent resin particles having a particle size distribution. In particular, even if the median particle size is 250 μm or less, the chelating agent can be uniformly contained throughout the particle size of the polymer particles.

[0065] (Chelating Agent) The chelating agent used in step 1 can be a wide range of known chelating agents that are used in water-absorbent resin particles.

[0066] Examples of the chelating agent include phosphonic acid-based metal chelating agents and metal chelating agents having an aminocarboxylic acid moiety.

[0067] Phosphonic acid metal chelating agents are metal salts having at least two phosphonic acid moieties in the molecule. Examples of phosphonic acid metal chelating agents include various phosphonic acid compounds such as ethylenediaminetetramethylenephosphonic acid, diethylenetriaminepentamethylenephosphonic acid, nitrotrismethylenephosphonic acid, 1-hydroxyethylidene-1,1-diphosphonic acid, and diethylenetriaminepenta(methylenephosphonic acid), as well as salts thereof.

[0068] Examples of metal chelating agents having an aminocarboxylic acid moiety include compounds having an aminocarboxylic acid moiety such as ethylenediamine-N,N'-disuccinic acid, ethylene glycol bis(β-aminoethyl ether)-N,N,N,N-tetraacetic acid, 3-propanediamine-N,N,N',N'-tetraacetic acid, N-(2-hydroxyethyl)ethylenediamine-N,N',N'-triacetic acid, ethylenediaminetetraacetic acid, nitrilotriacetic acid, hydroxyethylenediaminetriacetic acid, diethylenetriaminepentaacetic acid, and dihydroxyethylglycine, as well as salts thereof.

[0069] Other examples of the chelating agent include citric acid, polyphosphoric acid, phytic acid, pyrocatechol, N,N'-bis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionyl]hydrazine, p-trans-coumaric acid, 1,10-phenanthroline, and α-cyclodextrin.

[0070] When the chelating agent is in the form of a salt, examples thereof include salts of alkali metals such as sodium and potassium, and salts of alkaline earth metals such as magnesium and calcium. All or part of the ligands of the chelating agent can form a salt.

[0071] The chelating agent used in step 1 may be one type or two or more types. The chelating agent used in step 1 may be obtained by a known production method or may be available as a commercially available product.

[0072] (Step 1) Step 1 is a step for mixing the chelating agent and the polymer particles by at least one method selected from the group consisting of the above-mentioned method (A), method (B), method (C), and method (D). By such Step 1, for example, the chelating agent is contained in the polymer particles, and desired water-absorbent resin particles are obtained.

[0073] <<Method (A)>> In method (A), the chelating agent solution is added to the polymer particles at a chelating agent addition rate V defined by the following formula (1): V=F×A / T Formula (1) is 30 or less. In the formula (1), V is the chelating agent addition rate [min －１ ], F is the average linear flow velocity from the nozzle [m / min], A is the cross-sectional area of ​​the nozzle [m ２ ], T is the total amount of chelating agent solution added [m ３ ] is shown.

[0074] F is the average linear flow velocity from the nozzle, and is an index showing the force with which the solution of the chelating agent (chelating agent solution) being added is ejected from its inlet (i.e., the inlet nozzle). When the chelating agent solution is added using a pump or the like, the average linear flow velocity F from the inlet nozzle is calculated from the flow rate (by dividing the volumetric flow rate by the cross-sectional area of ​​the outlet of the inlet nozzle). When a method of adding the chelating agent solution using gravity or the like is used, the average volumetric flow rate can be determined by dividing the amount (volume) of the aqueous monomer solution to be added by the time required for the addition, and the average linear flow velocity F can be calculated in the same manner.

[0075] A is the cross-sectional area of ​​the nozzle, and is related to the size of the droplets of the chelating agent solution that are injected.

[0076] T is the total amount of the chelating agent solution to be added, and is the amount of the chelating agent solution to be charged, which can be determined depending on the size of the polymerization reaction vessel in which the polymer particles are accommodated, the amount of the polymer particles, etc.

[0077] The chelating agent addition rate V is determined by the above formula (1), specifically, F [m / min], A [m ２ ], T [m ３] is used as a basis to eliminate the effect of scale factors on the addition rate.

[0078] Specifically, in method (A), a solution of a chelating agent (chelating agent solution) can be added to a dispersion of polymer particles at a predetermined rate. In method (A), the chelating agent may be added before or after the drying step, and may be added before or after the post-crosslinking step. When the polymer particles are dispersed as a dispersion, examples of the dispersion medium include the hydrocarbon dispersion mediums described above. The dispersion of polymer particles may contain, as needed, for example, a surfactant and a polymeric dispersant used in the polymerization step. For example, the surfactant and polymeric dispersant may each be contained in an amount of 0.01 to 1 part by mass per 100 parts by mass of the polymer particles.

[0079] The chelating agent solution may be, for example, an aqueous solution, and the concentration of the chelating agent solution may be 1.0 to 50% by mass, preferably 15% by mass or more, more preferably 20% by mass or more, and even more preferably 25% by mass or more.

[0080] In the method (A), the chelating agent solution can be added while stirring the polymer particles in the dispersion. As a means for adding the chelating agent solution, a dropping device equipped with a nozzle having the above-mentioned cross-sectional area A can be used.

[0081] In the method (A), the chelating agent solution is added at a chelating agent addition rate V (min －１ ) is added so that the ratio of the chelating agent to the polymer particles is 30 or less. By adding the chelating agent to the polymer particles at such a rate, the chelating agent can be contained uniformly throughout the water-absorbent resin particles having a particle size distribution. In particular, even if the median particle size is 250 μm or less, the chelating agent can be contained uniformly throughout the particle size of the polymer particles.

[0082] In method (A), the chelating agent addition rate V is preferably 10 or less, more preferably 8 or less, even more preferably 5 or less, even more preferably 2 or less, and particularly preferably 1 or less.

[0083] In the method (A), the temperature when the chelating agent is added to the polymer particles is, for example, 20 to 150° C., preferably 80 to 140° C. The mixing time can be set within an appropriate range depending on the temperature and the like, and is, for example, 1 to 150 minutes.

[0084] By mixing a chelating agent and polymer particles by method (A), polymer particles containing a chelating agent can be obtained. After mixing the chelating agent and polymer particles, the dispersion medium can be removed as needed to obtain dry polymer particles. Such polymer particles can be subjected to a post-crosslinking treatment or the like as needed to obtain polymer particles containing a chelating agent. The polymer particles containing a chelating agent can be made into water-absorbent resin particles, or other additives can be added to make water-absorbent resin particles.

[0085] <<Method (B)>> In method (B), the chelating agent solution is adjusted so that the droplet size is 500 μm or less, and then added to the polymer particles.

[0086] In the method (B), the chelating agent may be added before or after the drying step, and may be added before or after the post-crosslinking step.

[0087] In method (B), the same chelating agent solution as used in method (A) can be used. Therefore, the chelating agent solution in method (B) is preferably an aqueous solution. The concentration of the chelating agent solution can be 1.0 to 50% by mass, preferably 15% by mass or more, more preferably 20% by mass or more, and even more preferably 25% by mass or more.

[0088] In method (B), the chelating agent solution can be added while stirring the polymer particles. The chelating agent may be added directly to solid polymer particles such as powder, or, as in method (A), to the dispersion of polymer particles obtained in step 1. When the polymer particles are dispersed as a dispersion, examples of the dispersion medium include the hydrocarbon dispersion medium described above. The dispersion of polymer particles may contain, as necessary, for example, a surfactant and a polymeric dispersant used in the polymerization step. These surfactants and polymeric dispersants may each be contained in an amount of 0.01 to 1 part by mass per 100 parts by mass of the polymer particles.

[0089] As a means for adding the chelating agent solution, for example, a wide variety of known spraying means can be adopted in the present invention. By adding the chelating agent by spraying, the obtained water-absorbent resin particles satisfy the above formula (1).

[0090] In the method (B), by setting the droplet size of the chelating agent solution to be added to the polymer particles to be 500 µm or less, the chelating agent can be contained uniformly throughout the water absorbent resin particles having a particle size distribution, and particularly, even if the median particle size is 250 µm, the chelating agent becomes contained uniformly in the polymer particles over the entire particle size.

[0091] Specifically, the droplet diameter of the chelating agent solution being 500 μm or less means that when the chelating agent solution is observed under a microscope, 10 droplets are randomly selected based on the observed droplet diameter, and the average droplet diameter is 500 μm or less.

[0092] The method for adjusting the droplet size of the chelating agent solution is not particularly limited, and for example, a wide variety of known methods can be adopted. For example, when the chelating agent solution is sprayed, the droplet size of the chelating agent solution can be adjusted to a desired range by adjusting the nozzle size, the flow rate of the chelating agent solution, etc.

[0093] The polymer particles can be stirred (fluidized) by a stirrer equipped with stirring blades, which will be described later.

[0094] In the method (B), the temperature when the chelating agent is added to the polymer particles is, for example, 20 to 150° C., preferably 80 to 140° C. The mixing time can be set within an appropriate range depending on the temperature and the like, and is, for example, 1 to 150 minutes.

[0095] By mixing a chelating agent and polymer particles by method (B), polymer particles containing a chelating agent can be obtained. After mixing the chelating agent and polymer particles, the dispersion medium can be removed as needed to obtain dry polymer particles. Such polymer particles can be subjected to a post-crosslinking treatment as needed to obtain polymer particles containing a chelating agent. The polymer particles containing a chelating agent can be made into water-absorbent resin particles, or other additives can be added to make water-absorbent resin particles.

[0096] <<Method (C)>> In method (C), the polymer particles and a hydrophilic surfactant are mixed together, and then the chelating agent is added to the polymer particles.

[0097] In the method (C), specifically, a chelating agent solution can be added to a dispersion containing the polymer particles and the hydrophilic surfactant. Note that, in the method (C), the chelating agent may be added before or after the drying step, and may be added before or after the post-crosslinking step.

[0098] In method (C), the same chelating agent solution as used in method (A) can be used. Therefore, in method (C), the chelating agent solution is preferably an aqueous solution. The concentration of the chelating agent solution can be 1.0 to 50% by mass, preferably 15% by mass or more, more preferably 20% by mass or more, and even more preferably 25% by mass or more.

[0099] The method for preparing the mixture obtained by mixing the polymer particles and the surfactant is not particularly limited, and for example, a method of adding a hydrophilic surfactant to the dispersion containing the polymer particles obtained in the polymerization step can be mentioned. The dispersion of polymer particles may be newly prepared separately from the polymerization step.

[0100] The hydrophilic surfactant used in method (C) has an HLB of 5 or more. The HLB is preferably 6 or more, more preferably 7 or more, and even more preferably 8 or more. Examples of the hydrophilic surfactant used in method (C) include sucrose fatty acid esters, polyglycerol fatty acid esters, sorbitan fatty acid esters, polyoxyethylene sorbitan fatty acid esters, polyoxyethylene glycerol fatty acid esters, sorbitol fatty acid esters, polyoxyethylene sorbitol fatty acid esters, polyoxyethylene alkyl ethers, polyoxyethylene alkylphenyl ethers, polyoxyethylene castor oil, polyoxyethylene hydrogenated castor oil, alkylaryl formaldehyde condensed polyoxyethylene ethers, polyoxyethylene polyoxypropylene block copolymers, polyoxyethylene polyoxypropyl alkyl ethers, polyethylene glycol fatty acid esters, alkyl glucosides, N-alkyl gluconamides, polyoxyethylene fatty acid amides, polyoxyethylene alkylamines, phosphate esters of polyoxyethylene alkyl ethers, and phosphate esters of polyoxyethylene alkyl allyl ethers. A hydrophilic surfactant that can be preferably used is a sorbitan fatty acid ester (such as sorbitan monolaurate).

[0101] The amount of the hydrophilic surfactant used is not particularly limited. It is preferable that the amount of the hydrophilic surfactant used is 0.01 parts by mass or more per 100 parts by mass of the polymer particles. That is, in method (C), it is preferable that the hydrophilic surfactant is contained in an amount of 0.01 parts by mass or more per 100 parts by mass of the polymer particles. It is more preferable that the amount of the hydrophilic surfactant used is 0.1 to 5.0 parts by mass per 100 parts by mass of the polymer particles.

[0102] In this specification, "100 parts by mass of polymer particles" may include not only the polymer formed by polymerization of the water-soluble ethylenically unsaturated monomer, but also the thickener, surfactant, and polymeric dispersant used in the polymerization step.

[0103] In method (C), the chelating agent solution can be added while stirring the polymer particles. When the polymer particles are dispersed as a dispersion, the dispersion medium can be the hydrocarbon dispersion medium described above. The polymer particle dispersion may contain, as necessary, for example, a surfactant or a polymeric dispersant used in the polymerization step. These surfactants and polymeric dispersants may each be contained in an amount of 0.01 to 1 part by mass per 100 parts by mass of the polymer particles. In method (C), the polymer particle dispersion may not contain a hydrophilic surfactant.

[0104] In the method (C), the chelating agent solution can be added while stirring the polymer particles in the dispersion. As a means for adding the chelating agent solution, a known dropping device or the like can be used.

[0105] In the method (C), the chelating agent addition rate V is not particularly limited. Therefore, the chelating agent addition rate V may exceed 30.

[0106] In the method (C), by adding the chelating agent after mixing the polymer particles with the hydrophilic surfactant, the chelating agent can be contained uniformly throughout the water absorbent resin particles having a particle size distribution, and particularly, even if the median particle size is 250 μm or less, the chelating agent can be contained uniformly in the polymer particles throughout the particle size.

[0107] The temperature when the chelating agent is added to the polymer particles is, for example, 20 to 150° C., preferably 80 to 140° C. The mixing time can be set within an appropriate range depending on the temperature and the like, and is, for example, 1 to 150 minutes.

[0108] By mixing a chelating agent and polymer particles by method (C), polymer particles containing a chelating agent can be obtained. After mixing the chelating agent and polymer particles, the dispersion medium can be removed as needed to obtain dry polymer particles. Such polymer particles can be subjected to a post-crosslinking treatment as needed to obtain polymer particles containing a chelating agent. The polymer particles containing a chelating agent can be made into water-absorbent resin particles, or other additives can be added to make water-absorbent resin particles.

[0109] <<Method D>> In method (D), a mixture containing a hydrophilic surfactant and the chelating agent is added to the polymer particles.

[0110] In the method (D), specifically, a chelating agent solution containing a hydrophilic surfactant can be added to the dispersion containing the polymer particles. In the method (D), the chelating agent may be added before or after the drying step, and may be added before or after the post-crosslinking step.

[0111] In method (D), the same chelating agent solution as used in method (A) can be used, except that it contains a hydrophilic surfactant. Therefore, the chelating agent solution in method (D) is preferably an aqueous solution. The concentration of the chelating agent solution can be 1.0 to 50% by mass, preferably 15% by mass or more, more preferably 20% by mass or more, and even more preferably 25% by mass or more.

[0112] The method for preparing the mixture obtained by mixing the chelating agent and the hydrophilic surfactant is not particularly limited, and for example, a method in which the hydrophilic surfactant is added to a solution of the chelating agent can be mentioned.

[0113] The hydrophilic surfactant used in method (D) has an HLB of 5 or more. The HLB is preferably 6 or more, more preferably 7 or more, and even more preferably 8 or more. Examples of the hydrophilic surfactant used in method (D) include sucrose fatty acid esters, polyglycerol fatty acid esters, sorbitan fatty acid esters, polyoxyethylene sorbitan fatty acid esters, polyoxyethylene glycerol fatty acid esters, sorbitol fatty acid esters, polyoxyethylene sorbitol fatty acid esters, polyoxyethylene alkyl ethers, polyoxyethylene alkylphenyl ethers, polyoxyethylene castor oil, polyoxyethylene hydrogenated castor oil, alkylaryl formaldehyde condensed polyoxyethylene ethers, polyoxyethylene polyoxypropylene block copolymers, polyoxyethylene polyoxypropyl alkyl ethers, polyethylene glycol fatty acid esters, alkyl glucosides, N-alkyl gluconamides, polyoxyethylene fatty acid amides, polyoxyethylene alkylamines, phosphate esters of polyoxyethylene alkyl ethers, and phosphate esters of polyoxyethylene alkyl allyl ethers. The hydrophilic surfactant that can be preferably used is a sorbitan fatty acid ester (such as sorbitan monolaurate).

[0114] The amount of the hydrophilic surfactant used is not particularly limited. It is preferable to use the hydrophilic surfactant so that the amount of the hydrophilic surfactant used is 0.01 parts by mass per 100 parts by mass of the polymer particles. That is, in method (D), a mixture containing a chelating agent and a hydrophilic surfactant is added so that the amount of the hydrophilic surfactant is 0.01 parts by mass or more per 100 parts by mass of the polymer particles. It is more preferable that the amount of the hydrophilic surfactant used is 0.1 to 5.0 parts by mass per 100 parts by mass of the polymer particles.

[0115] In method (D), the chelating agent solution containing the hydrophilic surfactant can be added while stirring the polymer particles. When the polymer particles are dispersed as a dispersion, the dispersion medium can be the hydrocarbon dispersion medium described above. The dispersion of the polymer particles may contain, as necessary, for example, a surfactant (i.e., a surfactant other than a hydrophilic surfactant) or a polymeric dispersant used in the polymerization step. For example, these surfactants and polymeric dispersants can each be contained in an amount of 0.01 to 1 part by mass per 100 parts by mass of the polymer particles.

[0116] In the method (D), the chelating agent solution can be added while stirring the polymer particles in the dispersion. As a means for adding the chelating agent solution, a known dropping device or the like can be used.

[0117] In the method (D), the chelating agent addition rate V is not particularly limited. Therefore, the chelating agent addition rate V may exceed 30.

[0118] In the method (D), by adding a chelating agent containing a hydrophilic surfactant to polymer particles, the chelating agent can be contained uniformly throughout the water absorbent resin particles having a particle size distribution, and particularly, even if the median particle size is 250 μm or less, the chelating agent becomes contained uniformly in the polymer particles over the entire particle size.

[0119] In the method (D), the temperature when the chelating agent is added to the polymer particles is, for example, 20 to 150° C., preferably 80 to 140° C. The mixing time can be set within an appropriate range depending on the temperature and the like, and is, for example, 1 to 150 minutes.

[0120] By mixing a chelating agent and polymer particles by method (D), polymer particles containing a chelating agent can be obtained. After mixing the chelating agent and polymer particles, the dispersion medium can be removed as needed to obtain dry polymer particles. Such polymer particles can be subjected to a post-crosslinking treatment or the like as needed to obtain polymer particles containing a chelating agent. The polymer particles containing a chelating agent can be made into water-absorbent resin particles, or other additives can be added to make water-absorbent resin particles.

[0121] <Step 1> In step 1, the method of adding the chelating agent to the polymer particles may be a combination of two or more of methods (A), (B), (C), and (D), or may be only one of these methods.

[0122] In either method, the polymer particles and the chelating agent can be mixed using various types of stirrers with stirring blades, air flow mixers, or other devices. When various types of stirrers with stirring blades are used, the stirring blades can be, for example, flat blades, lattice blades, paddle blades, propeller blades, anchor blades, turbine blades, Pfaudler blades, ribbon blades, full zone blades, or Max Blend blades. One example is a stirring blade having four inclined paddle blades arranged in two stages.

[0123] When various agitators having agitating blades are used in step 1, the chelating agent can be added while stirring the polymer particles by adjusting the tip peripheral speed of the agitating blade. In this case, the tip peripheral speed of the agitating blade is preferably 1.5 m / s or more, more preferably 2.5 m / s or more, even more preferably 4.5 m / s or more, and even more preferably 6.0 m / s or more. It is also preferably 10 m / s or less, more preferably 8.5 m / s or less, and even more preferably 7.0 m / s or less. Preferred ranges include 1.5 to 10 m / s, 2.5 to 9.0 m / s, 4.5 to 8.0 m / s, and 6.0 to 7.0 m / s.

[0124] When an airflow mixer is used in step 1, the chelating agent can be added while stirring the polymer particles by feeding gas through a nozzle or hole provided inside the container. The mean linear velocity of the airflow is preferably 0.5 m / s or more, more preferably 0.6 m / s or more, even more preferably 0.7 m / s or more, and even more preferably 0.8 m / s or more, and is preferably 1.5 m / s or less, more preferably 1.2 m / s or less, and even more preferably 1.0 m / s or less. Preferred ranges include 0.5 to 1.5 m / s, 0.6 to 1.3 m / s, 0.7 to 1.2 m / s, and 0.8 to 1.0 m / s.

[0125] In step 1, the chelating agent can be added in an amount of 0.01 part by mass or more relative to 100 parts by mass of the polymer particles, that is, the amount of the chelating agent used can be 0.01 part by mass or more relative to 100 parts by mass of the polymer particles in step 1. This makes it easier for the chelating agent to be contained in the water-absorbent resin particles obtained by the production method of the present invention.

[0126] In step 1, the chelating agent is preferably added in an amount of 0.1 parts by mass or more, more preferably 0.2 parts by mass or more, and even more preferably 0.3 parts by mass or more, relative to 100 parts by mass of the polymer particles. In step 1, the chelating agent is preferably added in an amount of 5.0 parts by mass or less, more preferably 3.0 parts by mass or less, even more preferably 2.0 parts by mass or less, and particularly preferably 1.5 parts by mass or less, relative to 100 parts by mass of the polymer particles.

[0127] (Water-absorbent resin particles) By the production method of the present invention including the above-mentioned step 1, water-absorbent resin particles containing a chelating agent can be obtained. The water-absorbent resin particles containing a chelating agent may contain other components in addition to the chelating agent, for example, raw materials used in the polymerization step and other optionally added components. Examples of the optionally added components include inorganic powders, surfactants, oxidizing agents, reducing agents, radical chain inhibitors, antioxidants, antibacterial agents, deodorizers, etc., which can be added during the polymerization step or in a step subsequent to the polymerization step.

[0128] The water-absorbent resin particles may consist of only polymer particles containing a chelating agent, or may contain other materials such as silica, as long as the effects of the present invention are not impaired. The water-absorbent resin particles preferably contain 90% by mass or more of polymer particles containing a chelating agent, more preferably 95% by mass or more, and even more preferably 99% by mass or more.

[0129] The median particle size of the water-absorbent resin particles can be 10 to 250 μm. In particular, the water-absorbent resin particles obtained by the production method of the present invention contain a chelating agent uniformly throughout the entire particle size distribution, and even if the median particle size is 250 μm or less, the chelating agent can be contained uniformly throughout the entire particle size distribution.

[0130] Although not necessarily limiting, the reason why the chelating agent is uniformly contained throughout the particle size distribution in the production method of the present invention is presumed to be as follows. It is considered that while most of the polymer particles in the small median particle size range (e.g., 250 μm or less) are dispersed throughout the system by stirring, some of the large polymer particles settle and exist at the bottom of the system. Since the chelating agent reaches the bottom of the system immediately after addition, it is selectively absorbed by the large polymer particles at the bottom. Therefore, in water-absorbent resin particles with a median particle size of 250 μm or less, the chelating agent is likely to be unevenly distributed on the large particle size side. In this regard, it is presumed that the production method of the present invention, by employing one or more of methods (A), (B), (C), and (D), suppresses uneven distribution of the chelating agent on the large particle size side and allows the chelating agent to be uniformly contained throughout the particle size distribution.

[0131] The content of the chelating agent contained in the water-absorbent resin particles can be measured by UPLC or ICP-AES.

[0132] The water-absorbent resin particles obtained by the method for producing water-absorbent resin particles of the present invention are excellent in heat resistance, swelling height, and water absorption rate. Therefore, the water-absorbent resin particles of the present invention are suitable for various uses, and can be widely applied to various fields, for example, industrial materials such as water-stopping materials and anti-condensation agents, agricultural and horticultural materials such as water retention agents and soil conditioners, and sanitary materials such as disposable diapers and sanitary products. In particular, the water-absorbent resin particles obtained by the production method of the present invention are suitable for use in water-stopping materials because they can maintain their water-stopping effect for a long period of time.

[0133] 2. Water-Stopping Material and Cable The water-absorbent resin particles obtained by the manufacturing method of the present invention can be used in a water-stopping material. As long as such a water-stopping material contains the water-absorbent resin particles obtained by the manufacturing method of the present invention, the other components can be the same as, for example, known water-stopping materials. The water-stopping material can be formed from water-absorbent resin particles alone, or can be formed by molding a mixture of water-absorbent resin particles with rubber and / or a thermoplastic resin. Examples of water-stopping materials include water-stopping tapes and water-stopping yarns.

[0134] A water-stop tape can be obtained, for example, by holding water-absorbent resin particles in a liquid-permeable sheet. Alternatively, a water-stop tape can be obtained by sandwiching water-absorbent resin particles between two or more liquid-permeable sheets. Specifically, a water-stop tape can be obtained by fixing water-absorbent resin particles and a liquid-impermeable sheet using an adhesive to form a sheet. Examples of liquid-permeable sheets that can be used include nonwoven fabrics, woven fabrics, and films made of synthetic resins such as polyolefin, polyester, polyamide, nylon, and acrylic.

[0135] The water-blocking yarn can be obtained, for example, by coating or impregnating a substrate with water-absorbent resin particles. Examples of such a substrate include long synthetic fibers or plastic yarns (substrate yarns), and more specifically, long synthetic resin fibers or plastic yarns such as polyolefin, polyester, polyamide, nylon, and acrylic are used.

[0136] In the production of a water-blocking yarn, the method of applying or impregnating a substrate with water-absorbent resin particles can be, for example, by applying or impregnating the substrate with a dispersion liquid in which water-absorbent resin particles are dispersed in an organic solvent, thereby fixing the water-absorbent resin particles to the substrate. When the fixing strength of the water-absorbent resin particles to the substrate is weak, the fixing strength can be improved by using an adhesive in combination.

[0137] Examples of the adhesive include rubber-based adhesives such as natural rubber, butyl rubber, and polyisoprene; styrene-based elastomer adhesives such as styrene-isoprene block copolymer (SIS) and styrene-butadiene block copolymer (SBS); ethylene-vinyl acetate copolymer (EVA) adhesives; ethylene-acrylic acid derivative copolymer-based adhesives such as ethylene-ethyl acrylate copolymer (EEA); ethylene-acrylic acid copolymer (EAA) adhesives; polyamide-based adhesives such as copolymerized nylon; polyolefin-based adhesives such as polyethylene and polypropylene; polyester-based adhesives such as polyethylene terephthalate (PET) and copolymerized polyester; and acrylic adhesives.

[0138] The water-absorbing material contains water-absorbent resin particles obtained by the manufacturing method of the present invention, and since the water-absorbent resin particles have excellent swelling properties, they can be particularly suitably used for cable applications.

[0139] The waterstop material can be used in a variety of applications, for example, in various cables such as communication cables (e.g., optical cables) and power cables, and more specifically, it can be suitably used in power cables used underground or under the sea. In particular, the above-mentioned waterstop tapes, waterstop yarns, etc. are particularly suitable as waterstop materials for power cables.

[0140] The optical cable may have a configuration similar to that of a known optical cable. Like a typical optical cable, the optical cable may have a structure including an optical fiber unit in which a plurality of optical fiber cores are bundled, an optical fiber assembly in which a plurality of optical fiber units are assembled, and a coating layer provided on the outer periphery of the optical fiber unit assembly. To prevent water entering from the outside from running along the length of the cable, a waterproof tape is disposed between the optical fiber unit assembly and the coating layer. Furthermore, the waterproof tape may be housed inside the optical fiber unit assembly.

[0141] The power cable may have a configuration similar to that of a known power cable. Like a typical power cable, the power cable may have a structure in which an inner semiconductive layer, an insulating layer, an outer semiconductive layer, and a coating layer are sequentially provided around the outer periphery of a central conductor. To prevent water entering from the outside from running along the length of the cable, a waterproof tape is disposed between the outer semiconductive layer and the coating layer. Furthermore, the waterproof tape may be disposed between the inner semiconductive layer and the insulating layer. A waterproof yarn may also be housed in the central conductor.

[0142] In specifying the inventions included in the present disclosure, the components (properties, structures, functions, etc.) described in the embodiments of the present disclosure may be combined in any manner. In other words, the present disclosure includes all subject matter consisting of all combinations of the components that can be combined as described in this specification.

[0143] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the embodiments of these examples. The water-absorbent resin particles obtained in the following examples and comparative examples were evaluated by various test methods described below. Unless otherwise specified, measurements were carried out in an environment of a temperature of 25±2°C and a humidity of 50±10%.

[0144] [Production of Water-Absorbent Resin Particles] (Production Example 1) A stirrer (diameter: 8 mm, length: 40 mm, no ring) was placed in the center of a stainless steel tray (external dimensions of opening: 300 x 220 mm, internal dimensions of bottom: 240 x 190 mm, height: 40 mm) with a fluororesin-coated inner surface. 888.3 g of partially neutralized sodium acrylate solution (monomer used in polymerization, monomer concentration: 45% by mass, neutralization rate of sodium acrylate: 75 mol%), 0.93 g (1.8 mmol) of polyethylene glycol diacrylate (Tokyo Chemical Industry Co., Ltd., polyethylene glycol diacrylate N≒9), and 149.9 g of ion-exchanged water were added, and the mixture was mixed uniformly by rotating the stirrer to obtain a mixture. The top of the stainless steel tray was then covered with polyethylene film. The temperature of the mixture in the stainless steel tray was adjusted to 25°C, and the mixture was purged with nitrogen to adjust the dissolved oxygen content to 0.1 ppm or less. Next, with stirring at 300 rpm, 6.46 g (1.2 mmol) of 5% by mass 2,2'-azobis(2-amidinopropane) dihydrochloride and 3.39 g (0.10 mmol) of 0.5% by mass L-ascorbic acid aqueous solution were added dropwise in that order to prepare an aqueous monomer solution. The concentration of partially neutralized sodium acrylate in the aqueous monomer solution was 38% by mass. Polymerization began 2 minutes after the dropwise addition of the 0.5% by mass L-ascorbic acid aqueous solution. The maximum temperature was reached 22 minutes after the start of polymerization, after which the temperature began to decrease. The resulting product was immersed in a 75°C water bath while still in the container and aged for 20 minutes to obtain a gel (post-polymerization gel).

[0145] After the aging, the entire gel was removed from the container and cut into a grid of 5 cm intervals. The entire cut gel was then placed in a meat chopper (model number: 12VR-750SDX, manufactured by Kiryu Royal Co., Ltd.) to crush the gel. The diameter of the holes (circular) in the plate located at the outlet of the meat chopper was 6.4 mm, and the hole density was 40 holes / 36.30 cm. ２ Crushing was continued until the crushed material (crushed gel, crushed hydrogel) no longer came out from the plate of the meat chopper. The crushed material was then dried with hot air at 180°C for 30 minutes to obtain a dried material (dried crosslinked polymer).

[0146] 50.0 g of the dried product was crushed using an ultracentrifugal crusher (manufactured by Verder Scientific K.K., product name: ZM200, six-blade rotor, rotor rotation speed: 6000 rpm, trapezoidal screen holes: 1.00 mm) to obtain a crushed product (crushed crosslinked polymer product). The above crushing step was repeated four times.

[0147] 50.0 g of the above-mentioned crushed material was weighed into a round-bottomed cylindrical separable flask with an inner diameter of 11 cm and equipped with a fluororesin anchor-shaped stirring blade. Next, while stirring at 400 rpm, an aqueous crosslinking agent solution obtained by mixing 0.1 g (0.57 mmol) of ethylene glycol diglycidyl ether, 6.0 g of water, 2.0 g of propylene glycol, and 2.0 g of isopropyl alcohol was added dropwise to the separable flask with a Pasteur pipette to obtain a mixture. While stirring this mixture for 40 minutes, the separable flask was immersed in an oil bath at 180 °C to heat the mixture, thereby carrying out post-crosslinking. After cooling to room temperature, the mixture was passed through a mesh with an opening of 850 μm to obtain polymer particles. The above-mentioned post-crosslinking was repeated three times.

[0148] The above-mentioned production of polymer particles was repeated 10 times, and the obtained polymer particles were mixed and classified into fractions of 0-75 μm, 75-106 μm, 106-150 μm, 150-180 μm, 180-250 μm, 250-500 μm, and 500-850 μm so that the weight was uniform, and 1000 g of particle size-adjusted polymer particles were obtained.

[0149] Example 1 A round-bottomed cylindrical separable flask with an inner diameter of 11 cm and a capacity of 2 L was prepared, equipped with a reflux condenser, a dropping funnel, a nitrogen gas inlet tube, and a stirrer with a stirring blade having two stages of four inclined paddle blades with a blade diameter of 5 cm. 100 g of the polymer particles described in Production Example 1, 100 g of n-heptane, 0.43 g each of a sucrose stearate ester with an HLB of 3 (Ryoto Sugar Ester S-370, Mitsubishi Chemical Foods Corporation), and a maleic anhydride-modified ethylene-propylene copolymer (Hiwax 1105A, Mitsui Chemicals, Inc.) were added to the separable flask and stirred at 1,000 rpm. The flask was immersed in an oil bath heated to 125°C, and the temperature was raised to 89°C. While stirring the polymer particles in the flask, 1.41 g of a 32 mass% aqueous solution of diethylenetriaminepentaacetic acid (hereinafter abbreviated as DTPA.5Na) was added at a chelating agent addition rate V of 0.6 min. －１ The addition was carried out using a liquid pump (Masterflex L / S manufactured by Yamato Scientific Co., Ltd.). Next, n-heptane was heated in an oil bath at 125°C to evaporate and dry, and the mixture was passed through a sieve with an opening of 850 µm to obtain 90.5 g of water-absorbent resin particles (1).

[0150] (Example 2) A round-bottomed cylindrical separable flask with an inner diameter of 11 cm and a capacity of 2 L was prepared, equipped with a reflux condenser, a dropping funnel, a nitrogen gas inlet tube, and a stirrer with a stirring blade having two stages of four inclined paddle blades with a blade diameter of 5 cm. 292 g of n-heptane was added to this flask as a hydrocarbon dispersion medium, and 0.782 g of maleic anhydride-modified ethylene-propylene copolymer (Mitsui Chemicals, Inc., Hiwax 1105A) was added as a polymeric dispersant. The mixture was heated to 80 ° C. with stirring to dissolve the dispersant, and then cooled to 56 ° C. Meanwhile, in a 300 mL beaker, 92.0 g (1.03 mol) of an 80.5 wt% aqueous acrylic acid solution was added as a water-soluble ethylenically unsaturated monomer, and 102.8 g of a 30 wt% aqueous sodium hydroxide solution was added dropwise to prepare a 75 mol% neutralized product of acrylic acid. Further, 44.7 g of water, 0.0644 g (0.238 mmol) of potassium persulfate as a water-soluble radical polymerization agent, and 0.0102 g (0.059 mmol) of ethylene glycol diglycidyl ether as an internal crosslinking agent were added and dissolved to prepare a first-stage aqueous solution. This aqueous solution was added to a separable flask and stirred for 10 minutes. After this, a surfactant solution was added to a 20 mL vial, in which 0.782 g of sucrose stearate ester with an HLB of 3 (Ryoto Sugar Ester S-370, Mitsubishi Chemical Foods Corporation) was dissolved in 7.04 g of n-heptane under heating. The system was thoroughly purged with nitrogen while stirring at a stirrer speed of 450 rpm. The flask was then immersed in a 70°C water bath to raise the temperature, and polymerization was carried out for 60 minutes to obtain a first-stage polymerization slurry.

[0151] Meanwhile, 101.2 g (1.13 mol) of an 80.5% by mass aqueous solution of acrylic acid (as a water-soluble ethylenically unsaturated monomer) was placed in a separate 500 mL beaker, and while cooling with ice water, 113.1 g of a 30% by mass aqueous solution of sodium hydroxide was added dropwise to prepare a 75 mol % neutralized product of acrylic acid. Further, 13.2 g of water, 0.081 g (0.300 mmol) of potassium persulfate (as a water-soluble radical polymerization agent), and 0.0091 g (0.053 mmol) of ethylene glycol diglycidyl ether (as an internal crosslinking agent) were added and dissolved to prepare a second aqueous solution.

[0152] While stirring at a stirrer speed of 1000 rpm, the separable flask system was cooled to 31 ° C., and the entire amount of the second-stage aqueous liquid was added to the first-stage polymerization slurry liquid. The system was then purged with nitrogen for 30 minutes, and the flask was again immersed in a 70 ° C. water bath to raise the temperature and carry out a polymerization reaction for 60 minutes to obtain a hydrogel polymer. Thereafter, the flask was immersed in an oil bath set at 125 ° C., and 243.2 g of water was extracted from the system by azeotropic distillation of n-heptane and water while refluxing n-heptane. While stirring the resulting polymer particles, 2.41 g of a 32 mass % DTPA.5Na aqueous solution was added at a chelating agent addition rate V of 0.6 min. －１ The addition was carried out so that the total weight of the ethylene glycol diglycidyl ether solution was 100 parts by mass. This addition was carried out using a liquid transfer pump (Masterflex L / S manufactured by Yamato Scientific Co., Ltd.). Thereafter, 7.15 g (0.821 mmol) of a 2% by mass aqueous solution of ethylene glycol diglycidyl ether was added as a post-crosslinking agent to the flask, and the mixture was maintained at 83°C for 2 hours. Next, n-heptane was heated in an oil bath at 125°C to evaporate and dry, and the mixture was further passed through a sieve with an opening of 850 μm, thereby obtaining 196.5 g of a dried product. 0.1 parts by mass of amorphous silica (Toxil NP-S, Oriental Silicas Corporation) was mixed with 100 parts by mass of this dried product, thereby obtaining water-absorbent resin particles (2).

[0153] (Example 3) The amount of 32 mass% DTPA·5Na aqueous solution added was changed to 0.94 g, and the chelating agent addition rate V was 0.1 min －１ 89.1 g of water-absorbent resin particles (3) was obtained in the same manner as in Example 1, except that the water-absorbent resin particles (3) were added so as to satisfy the following formula:

[0154] (Example 4) The amount of 32 mass% DTPA·5Na aqueous solution added was changed to 0.94 g, and the chelating agent addition rate V was 0.07 min －１ 89.5 g of water-absorbent resin particles (4) was obtained in the same manner as in Example 1, except that the amount of water-absorbent resin particles (4) was changed to 89.5 g.

[0155] (Example 5) 89.1 g of water absorbent resin particles (5) was obtained in the same manner as in Example 1, except that the amount of the 32 mass% DTPA.5Na aqueous solution to be added was changed to 0.94 g, and the method of addition was changed to spray atomization using a spray kit (Fine Atomizer (Oral) manufactured by Fuji Medical Co., Ltd.) The average droplet diameter of the droplets produced by spray atomization was 283.7 μm.

[0156] (Example 6) The amount of the 32 mass% DTPA·5Na aqueous solution added was changed to 0.94 g, and a solution prepared by dissolving 1.1 g of a hydrophilic surfactant, sorbitan monolaurate (manufactured by NOF Corporation, trade name: Nonion LP-20R), in 1.1 g of n-heptane was added to the polymer particles before the addition of the aqueous solution. In addition, the chelating agent addition rate V was changed to 60 min －１ 90.1 g of water-absorbent resin particles (6) was obtained in the same manner as in Example 1, except that the water-absorbent resin particles (6) were added so as to satisfy the following formula:

[0157] (Example 7) The 32% by mass DTPA·5Na aqueous solution was changed to contain 1.1 g of sorbitan monolaurate, a hydrophilic surfactant, and the amount of sorbitan monolaurate added was changed to 0.94 g. The chelating agent addition rate V was 60 min －１ 89.9 g of water-absorbent resin particles (7) was obtained in the same manner as in Example 1, except that the amount of water-absorbent resin particles (7) was changed to 89.9 g.

[0158] (Example 8) 89.4 g of water absorbent resin particles (8) was obtained in the same manner as in Example 5, except that the 32 mass % DTPA·5Na aqueous solution was changed to a 32 mass % ethylenediaminetetramethylenephosphonic acid (hereinafter abbreviated as EDTMP·5Na) aqueous solution and the amount added was 1.53 g. The average droplet diameter of the droplets produced by spraying was 238.5 μm.

[0159] Example 9 89.8 g of water-absorbent resin particles (9) were obtained in the same manner as in Example 6, except that the 32 mass % DTPA·5Na aqueous solution was changed to a 32 mass % EDTMP·5Na aqueous solution and the amount added was 1.53 g.

[0160] Example 10 90.7 g of water-absorbent resin particles (10) was obtained in the same manner as in Example 7, except that the 32 mass % DTPA·5Na aqueous solution was changed to a 32 mass % EDTMP·5Na aqueous solution and the amount added was 1.28 g.

[0161] (Comparative Example 1) The amount of 32 mass% DTPA·5Na aqueous solution added was changed to 0.94 g, and the chelating agent addition rate V was 60 min －１ 89.8 g of water-absorbent resin particles (11) were obtained in the same manner as in Example 1, except that the water-absorbent resin particles (11) were added so as to satisfy the following formula:

[0162] (Droplet diameter sprayed by spraying) Using a digital microscope (VHX-5000 manufactured by KEYENCE Corporation), the diameter of droplets formed when the chelating aqueous solution was sprayed was measured. Specifically, using a spray kit (Fine Atomizer (Oral) manufactured by Fuji Medical Co., Ltd.), 0.5 mL of the chelating agent aqueous solution was sprayed onto a glass slide (2.5 cm long, 7.5 cm wide), and the droplets adhering to the glass slide were observed with the digital microscope to measure the droplet diameter. The average value of 10 droplets was taken as the droplet diameter.

[0163] (Chelating Agent Content) The content of the chelating agent in the water-absorbent resin particles was measured by UPLC when the chelating agent was DTPA.5Na. Specifically, the water-absorbent resin particles were fractionated according to particle size distribution, and the content of the chelating agent in each fraction was measured by UPLC. The particle size distribution of the water-absorbent resin particles was classified using a sieve into a 0-180 μm fraction, a 180-250 μm fraction, and a 250-850 μm fraction. 0.4 g of water-absorbent resin particles of each fraction was added to 40 g of physiological saline and allowed to swell under stirring for 1 hour, and the filtrate was collected. The filtrate was filtered through a membrane filter (0.45 μm), and then the content of DTPA.5Na was measured by UPLC. On the other hand, when the chelating agent was EDTMP.5Na, the content was measured by ICP-AES. Specifically, the water-absorbent resin particles were fractionated according to particle size distribution, and the content of the chelating agent in each fraction was measured by ICP-AES. The particle size distribution of the water-absorbent resin particles was classified using a sieve into a 0-180 μm fraction, a 180-250 μm fraction, and a 250-850 μm fraction. 200 mg of water-absorbent resin particles from each fraction was collected in a quartz container, acid was added, the container was sealed, and then microwave irradiation was performed at a maximum temperature of 250°C to perform acid decomposition under pressure. After complete decomposition, ultrapure water was added to the container to make a constant volume of 50 mL, and the P element was quantified using ICP-AES. Thereby, the content of EDTMP.5Na contained in the water-absorbent resin particles was measured.

[0164] (Standard Deviation and Coefficient of Variation for Each Fraction) The standard deviation for each fraction of the chelating agent was calculated using the following formula (2).

[0165]

[0166] In formula (2), x represents the chelating agent content of each fraction, x (bar above) represents the average value of the chelating agent content of each fraction, and n represents the total number of data. The coefficient of variation was calculated using the following formula (3): Coefficient of variation = (standard deviation) / (average value of chelating agent content) (3)

[0167] (Water absorption rate of water-absorbent resin particles) 50±0.1 g of physiological saline was weighed into a 100 mL beaker. A magnetic stirrer bar (8 mmφ×30 mm without ring) was added, and the beaker was immersed in a thermostatic water bath to adjust the liquid temperature to 25±0.2°C. Next, the beaker was placed on a magnetic stirrer, and the rotation speed was set to 600 rpm to generate a vortex in the physiological saline, and then 2.0 g of water-absorbent resin particles were quickly added to the beaker. Using a stopwatch, the time (seconds) from the time the water-absorbent resin particles were added to the time the vortex on the liquid surface converged was measured, and this time (seconds) was taken as the water absorption rate of the water-absorbent resin particles.

[0168] (Pure Water Absorption Capacity) 1500 g of ion-exchanged water was weighed into a 2 L beaker, and 0.5 g of water-absorbent resin particles was dispersed in the beaker while stirring at 600 r / min using a magnetic stirrer (stirring bar: 10 mmφ x 40 mm without ring) to prevent the formation of lumps. The mixture was left in a stirred state for 60 minutes to allow the water-absorbent resin to fully swell. The mass Wc (g) of a 75 μm standard sieve was then measured in advance, and the contents of the beaker were filtered using this. The sieve was tilted at an angle of approximately 30 degrees relative to the horizontal and left for 30 minutes to filter out excess water. The mass Wd (g) of the sieve containing the water-absorbent gel was measured, and the pure water absorption capacity was calculated using the following formula: Pure Water Absorption Capacity (g / g) = [Wd - Wc] (g) / Mass of Water-Absorbent Resin (g).

[0169]

[0123] Table 1 shows conditions (chelating agent addition speed V, average linear flow velocity F from a nozzle, cross-sectional area A of the nozzle, and total amount T of the chelating agent solution) when mixing polymer particles with a chelating agent solution in the production of water-absorbent resin particles carried out in each of Examples and Comparative Examples.

[0170]

[0171] Table 2 shows the chelating agent content (ppm) for each fraction of the water-absorbent resin particles obtained in each Example and Comparative Example, the average value of the chelating agent content (ppm), the standard deviation for each fraction, and the coefficient of variation. Note that the average value of the chelating agent content means the average value of the chelating agent content for the 0-180 μm fraction, the chelating agent content for the 180-250 μm fraction, and the chelating agent content for the 250-850 μm fraction.

[0172] From Table 2, it can be seen that the water-absorbent resin particles obtained by the production method performed in each Example, i.e., the water-absorbent resin particles obtained via Step 1, have a small coefficient of variation. This indicates that the water-absorbent resin particles obtained in the Examples contain a chelating agent uniformly throughout the water-absorbent resin particles having a particle size distribution. Therefore, it was found that the method of adding a chelating agent performed in the Examples can contain a chelating agent uniformly throughout the water-absorbent resin particles having a particle size distribution.

[0173] Table 3 shows the measurement results of the mass proportion (%), median particle size (μm), pure water absorption capacity (g / g), and water absorption rate (sec) of particles belonging to each fraction of the water-absorbent resin particles obtained in each Example and Comparative Example.

[0174]

[0175]

Claims

1. A method for producing water-absorbent resin particles, comprising step 1 of mixing flowing polymer particles with a chelating agent, wherein step 1 comprises the following methods (A), (B), (C), and (D): Method (A): A method of adding a solution of the chelating agent to the polymer particles at a chelating agent addition rate V defined by the following formula (1): V = F × A / T (1) (where V is the chelating agent addition rate [min －１ ], F is the average linear flow velocity from the nozzle [m / min], A is the cross-sectional area of ​​the nozzle [m ２ ], T is the total amount of chelating agent solution added [m ３ [0023] (Method (B): a method of adjusting the droplet size of the solution of the chelating agent to 500 μm or less and adding the solution to the polymer particles; Method (C): a method of mixing the polymer particles with a hydrophilic surfactant, and then adding the chelating agent to the polymer particles; Method (D): a method of adding a mixture containing a hydrophilic surfactant and the chelating agent to the polymer particles.

2. The method for producing water-absorbent resin particles according to claim 1, wherein in step 1, 0.01 parts by mass or more of the chelating agent is added relative to 100 parts by mass of the polymer particles.

3. The method for producing water-absorbent resin particles according to claim 1 or 2, wherein in step 1, 0.01 parts by mass or more of the hydrophilic surfactant is added relative to 100 parts by mass of the polymer particles.

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