Water-absorbing resin particle production method

Abstract

Provided is a water-absorbing resin particle production method that uses an inverse phase suspension polymerization method and that reduces the amount a hydrocarbon dispersion medium in obtained water-absorbing resin particles.　This water-absorbing resin particle production method comprises: (A) a primary dispersion step for mixing, through stirring in the absence of a surfactant, an aqueous solution of a water-soluble ethylenically-unsaturated monomer and a hydrocarbon dispersion medium in which a hydrophobic polymer-based dispersant is dispersed or dissolved, to prepare a first dispersion liquid; (B) a secondary dispersion step for mixing, through stirring, a surfactant and the first dispersion liquid obtained in said step (A) to prepare a second dispersion liquid; and (C) a polymerization step for subjecting, after said step (B), the water-soluble ethylenically-unsaturated monomer to inverse phase suspension polymerization. The ratio (the tip circumferential speed of stirring blades in the primary dispersion step / the tip circumferential speed of stirring blades in the secondary dispersion step) of the tip circumferential speed of stirring blades in the primary dispersion step with respect to the tip circumferential speed of stirring blades in the secondary dispersion step is set to more than 0.10 but less than 0.60, and the time for the primary dispersion step is set in a range of more than 1 minute but less than 90 minutes.

Description

Method for producing water-absorbent resin particles

[0001] The present invention relates to a method for producing water-absorbent resin particles, and more particularly to a method for producing water-absorbent resin particles that constitute absorbents suitable for use in sanitary materials such as disposable diapers, sanitary napkins, and incontinence pads.

[0002] BACKGROUND ART In recent years, water-absorbent resins have been widely used in the field of sanitary materials such as disposable diapers, sanitary napkins, and incontinence pads.

[0003] As such a water-absorbent resin, a crosslinked polymer of a partially neutralized salt of acrylic acid has excellent water-absorbing ability, and since acrylic acid, which is a raw material thereof, is easily available industrially, it can be produced at low cost with constant quality, and is less susceptible to putrefaction and deterioration, and therefore, it is considered to be a preferred water-absorbent resin.

[0004] Japanese Patent Application Laid-Open No. 2005-29751

[0005] A reversed-phase suspension polymerization method is known as a method for producing water-absorbent resin particles. The reversed-phase suspension polymerization method is a method for producing water-absorbent resin particles, in which a hydrocarbon dispersion medium and an aqueous solution of a water-soluble ethylenically unsaturated monomer are stirred and mixed to prepare a dispersion, and the water-soluble ethylenically unsaturated monomer is polymerized in the aqueous solution of the water-soluble ethylenically unsaturated monomer in the dispersion, thereby obtaining water-absorbent resin particles.

[0006] In such a reverse phase suspension polymerization method, a water-soluble ethylenically unsaturated monomer is polymerized in an aqueous solution, but a hydrocarbon dispersion medium present around the aqueous solution is taken into the aqueous solution, and the hydrocarbon dispersion medium is sometimes contained in produced water absorbent resin particles.

[0007] If the water-absorbent resin particles contain a hydrocarbon dispersion medium, the water-absorbent resin particles may cause odor when applied to an absorbent body and absorb liquid.

[0008] A main object of the present invention is to provide a method for producing water-absorbent resin particles by utilizing a reverse phase suspension polymerization method, which method reduces the amount of a hydrocarbon dispersion medium in the obtained water-absorbent resin particles.

[0009] The present inventors have conducted extensive research to solve the above-mentioned problems. As a result, they have found that in a method for producing water-absorbent resin particles comprising the following steps (A) to (C), the amount of hydrocarbon dispersion medium in the obtained water-absorbent resin particles can be reduced by setting the ratio of the tip peripheral speed of the stirring blade in the primary dispersion step to the tip peripheral speed of the stirring blade in the secondary dispersion step within a specific range, and further setting the time of the primary dispersion step within a specific range. The present invention is an invention that was completed based on such findings and through further extensive research.

[0010] Step (A): A primary dispersion step in which a hydrocarbon dispersion medium in which a hydrophobic polymer-based dispersant has been dispersed or dissolved and an aqueous solution of a water-soluble ethylenically unsaturated monomer are stirred and mixed in the absence of a surfactant to prepare a first dispersion; Step (B): A secondary dispersion step in which the first dispersion obtained in Step (A) is stirred and mixed with a surfactant to prepare a second dispersion; Step (C): A polymerization step in which the water-soluble ethylenically unsaturated monomer is subjected to reverse phase suspension polymerization after Step (B).

[0011] That is, the present invention provides an invention having the following features. Item 1. A method for producing water-absorbent resin particles, comprising the following steps: (A) a primary dispersion step of stirring and mixing, in the absence of a surfactant, a hydrocarbon dispersion medium having a hydrophobic polymer-based dispersant dispersed or dissolved therein and an aqueous solution of a water-soluble ethylenically unsaturated monomer to prepare a first dispersion; (B) a secondary dispersion step of stirring and mixing the first dispersion obtained in step (A) with a surfactant to prepare a second dispersion; and (C) a polymerization step of performing reverse suspension polymerization of the water-soluble ethylenically unsaturated monomer after step (B), wherein a ratio of the tip peripheral speed of the agitator blade in the primary dispersion step to the tip peripheral speed of the agitator blade in the secondary dispersion step (tip peripheral speed of the agitator blade in the primary dispersion step / tip peripheral speed of the agitator blade in the secondary dispersion step) is more than 0.10 and less than 0.60, and the duration of the primary dispersion step is in the range of more than 1 minute and less than 90 minutes. Item 2. Item 1. A method for producing water-absorbent resin particles according to Item 1, wherein a tip peripheral speed of the stirring blade during the primary dispersion step is 0.15 m / s or more and 1.7 m / s or less. Item 3. A method for producing water-absorbent resin particles according to Item 1, wherein a tip peripheral speed of the stirring blade during the secondary dispersion step is 0.7 m / s or more and 3.5 m / s or less. Item 4. A method for producing water-absorbent resin particles according to any one of Items 1 to 3, wherein in the polymerization step, reverse phase suspension polymerization is performed in two or more stages.

[0012] According to the present invention, it is possible to provide a method for producing water-absorbent resin particles by utilizing a reverse phase suspension polymerization method, which method reduces the amount of a hydrocarbon dispersion medium in the obtained water-absorbent resin particles.

[0013] In this specification, the term "comprising" includes "consisting essentially of" and "consisting of". Furthermore, in this specification, "(meth)acrylic" means "acrylic or methacrylic", and "(meth)acrylate" means "acrylate or methacrylate". Furthermore, "water-soluble" means exhibiting a solubility of 5% by mass or more in water at 25°C.

[0014] In addition, in this specification, a numerical value connected with "~" means a numerical range that includes the numerical values ​​before and after "~" as the lower limit and upper limit. When multiple lower limit values ​​and multiple upper limit values ​​are listed separately, any lower limit value and upper limit value can be selected and connected with "~".

[0015] 1. Method for Producing Water-Absorbent Resin Particles The method for producing water-absorbent resin particles of the present invention comprises the following steps (A) to (C): step (A): a primary dispersion step of preparing a first dispersion by stirring and mixing a hydrocarbon dispersion medium in which a hydrophobic polymer-based dispersant has been dispersed or dissolved and an aqueous solution of a water-soluble ethylenically unsaturated monomer in the absence of a surfactant; step (B): a secondary dispersion step of preparing a second dispersion by stirring and mixing the first dispersion obtained in step (A) and a surfactant; and step (C): a polymerization step of performing reverse-phase suspension polymerization of the water-soluble ethylenically unsaturated monomer after step (B).

[0016]

[0023] Furthermore, the method for producing water-absorbent resin particles of the present invention is characterized in that the ratio of the tip peripheral speed of the agitator blade during the primary dispersion step of step (A) to the tip peripheral speed of the agitator blade during the secondary dispersion step of step (B) (tip peripheral speed of the agitator blade during the primary dispersion step / tip peripheral speed of the agitator blade during the secondary dispersion step) is more than 0.10 and less than 0.60, and the duration of the primary dispersion step of step (A) is in the range of more than 1 minute and less than 90 minutes. By virtue of having such characteristics, the production method of the present invention reduces the amount of hydrocarbon dispersion medium in the obtained water-absorbent resin particles. The method for producing water-absorbent resin particles of the present invention will be described in detail below.

[0017] As will be described later, the reversed-phase suspension polymerization method is carried out in one stage (single stage) or in two or more stages, and the above-mentioned first-stage polymerization step means the polymerization reaction in the first stage of single-stage polymerization or multi-stage polymerization (the same applies hereinafter). In the method for producing water-absorbent resin particles of the present invention, when the reversed-phase suspension polymerization method is carried out in two or more stages, it is preferable to carry out steps (A) to (C) in the first stage. That is, it is preferable that the entire following steps (A) to (C) be the first stage of the reversed-phase suspension polymerization method.

[0018] <Step (A)> Step (A) is a primary dispersion step in which a hydrocarbon dispersion medium in which a hydrophobic polymer dispersant has been dispersed or dissolved and an aqueous solution of a water-soluble ethylenically unsaturated monomer are stirred and mixed in the absence of a surfactant to prepare a first dispersion. That is, the amount of surfactant used in step (A) is 0 parts by mass relative to 100 parts by mass of the water-soluble ethylenically unsaturated monomer.

[0019] [Water-soluble ethylenically unsaturated monomer] Examples of the water-soluble ethylenically unsaturated monomer include (meth)acrylic acid (in this specification, "acrylic" and "methacrylic" are collectively referred to as "(meth)acrylic", 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, and quaternized products thereof. Among these water-soluble ethylenically unsaturated monomers, (meth)acrylic acid or a salt thereof, (meth)acrylamide, and N,N-dimethylacrylamide are preferred, and (meth)acrylic acid and a salt thereof are more preferred, from the viewpoint of industrial ease of availability, etc. These water-soluble ethylenically unsaturated monomers may be used alone or in combination of two or more.

[0020] Among these, acrylic acid and its salts are widely used as raw materials for water-absorbent resins, and these acrylic acid and / or salts thereof may be copolymerized with the other water-soluble ethylenically unsaturated monomers described above. In this case, it is preferable that acrylic acid and / or its salts are used as the main water-soluble ethylenically unsaturated monomer in an amount of 70 to 100 mol % based on the total amount of water-soluble ethylenically unsaturated monomers.

[0021] The water-soluble ethylenically unsaturated monomer may be dispersed in a hydrocarbon dispersion medium in the form of an aqueous solution and subjected to reversed-phase suspension polymerization. By preparing the water-soluble ethylenically unsaturated monomer as an aqueous solution, the dispersion efficiency in the hydrocarbon dispersion medium can be increased. The concentration of the water-soluble ethylenically unsaturated monomer in this aqueous solution is preferably in the range of 20% by mass to the saturated concentration or less. The concentration of the water-soluble ethylenically unsaturated monomer is more preferably 60% by mass or less, even more preferably 55% by mass or less, and even more preferably 50% by mass or less. Meanwhile, the concentration of the water-soluble ethylenically unsaturated monomer is more preferably 25% by mass or more, even more preferably 28% by mass or more, and even more preferably 30% by mass or more.

[0022] 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. 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.

[0023] The degree of neutralization of the water-soluble ethylenically unsaturated monomer with the alkaline neutralizing agent is preferably 30 to 100 mol%, more preferably 50 to 90 mol%, even more preferably 70 to 80 mol%, and still more preferably 73 to 80 mol%, as the degree of neutralization with respect to all acid groups possessed by the water-soluble ethylenically unsaturated monomer.

[0024] In the step (A), the aqueous solution of the water-soluble ethylenically unsaturated monomer may contain a chain transfer agent, etc., if necessary.

[0025] Examples of the chain transfer agent include compounds such as thiols, thiolic acids, secondary alcohols, hypophosphorous acid, phosphorous acid, etc. These may be used alone or in combination of two or more.

[0026] [Hydrocarbon Dispersion Medium] Examples of hydrocarbon dispersion media include aliphatic hydrocarbons having 6 to 8 carbon atoms, such as n-hexane, n-heptane, 2-methylhexane, 3-methylhexane, 2,3-dimethylpentane, 3-ethylpentane, and n-octane; alicyclic hydrocarbons, such as cyclohexane, methylcyclohexane, cyclopentane, methylcyclopentane, trans-1,2-dimethylcyclopentane, cis-1,3-dimethylcyclopentane, and trans-1,3-dimethylcyclopentane; and aromatic hydrocarbons, such as benzene, toluene, and xylene. Among these hydrocarbon dispersion media, n-hexane, n-heptane, and cyclohexane are particularly preferred because they are easily available industrially, have stable quality, and are inexpensive. These hydrocarbon dispersion media may be used alone or in combination of two or more. As an example of a mixture of hydrocarbon dispersion media, a commercially available product such as Exxol Heptane (manufactured by ExxonMobil Corporation; contains 75 to 85% by mass of hydrocarbons such as heptane and its isomers) can also be used to obtain favorable results.

[0027] The amount of the hydrocarbon dispersion medium used is preferably 100 to 1500 parts by mass, and more preferably 200 to 1400 parts by mass, per 100 parts by mass of the water-soluble ethylenically unsaturated monomer, from the viewpoints of uniformly dispersing the water-soluble ethylenically unsaturated monomer and facilitating control of the polymerization temperature.

[0028] In the step (A), in the absence of a surfactant, an aqueous solution of a water-soluble ethylenically unsaturated monomer and a hydrocarbon dispersion medium in which a hydrophobic polymer-based dispersant is dispersed or dissolved are stirred and mixed to effect primary dispersion, thereby reducing the amount of the hydrocarbon dispersion medium remaining in the obtained water absorbent resin particles.

[0029] It is preferable to select and use a hydrophobic polymer dispersant that dissolves or disperses in the hydrocarbon dispersion medium to be used, and examples of such a dispersant include those having a viscosity average molecular weight of 20,000 or less, preferably 10,000 or less, and more preferably 5,000 or less.

[0030] Specific examples of hydrophobic polymer dispersants include 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, ethylene-acrylic acid copolymer, ethyl cellulose, ethylhydroxyethyl cellulose, maleic anhydride polybutadiene, maleic anhydride EPDM (ethylene / propylene / diene terpolymer), etc. Only one type of hydrophobic polymer dispersant may be used, or two or more types may be used.

[0031] Of these, the hydrophobic polymer dispersant is preferably at least one selected from the group consisting of 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, and oxidized ethylene-propylene copolymer.

[0032] From the viewpoint of more suitably exerting the effects of the present invention, the amount of the hydrophobic polymer dispersant added is preferably 5 parts by mass or less, more preferably 3 parts by mass or less, and is also preferably 0.01 part by mass or more, more preferably 0.05 part by mass or more, relative to 100 parts by mass of the water-soluble ethylenically unsaturated monomer aqueous solution. Preferred ranges include 0.01 to 5 parts by mass, 0.01 to 3 parts by mass, 0.05 to 5 parts by mass, and 0.05 to 3 parts by mass.

[0033] It is preferable that after adding the hydrophobic polymer dispersant to the hydrocarbon dispersion medium, the dispersion medium is heated once to dissolve or disperse part or all of the hydrophobic polymer dispersant, and then the aqueous monomer solution is added. After heating, the dispersion medium is cooled, and even if part or all of the hydrophobic polymer dispersant precipitates and becomes a cloudy dispersion, there is no problem in adding the aqueous monomer solution.

[0034] In the present invention, the viscosity of the water-soluble ethylenically unsaturated monomer aqueous solution in step (A) is preferably 15.0 mPa s or less, more preferably 14.5 mPa s or less, even more preferably 12.0 mPa s or less, and still more preferably 10.0 mPa s or less. The lower limit is preferably 5.0 mPa s or more, more preferably 8.0 mPa s or more. Preferred ranges include 5.0 to 15.0 mPa s, 5.0 to 14.5 mPa s, 5.0 to 12.0 mPa s, 5.0 to 10.0 mPa s, 8.0 to 15.0 mPa s, 8.0 to 14.5 mPa s, 8.0 to 12.0 mPa s, and 8.0 to 10.0 mPa s. The viscosity is a value measured by adjusting the temperature of the water-soluble ethylenically unsaturated monomer aqueous solution to 20±0.5°C and using a BII type viscometer (manufactured by Toki Sangyo Co., Ltd., model: BLII rotor No. 1, rotation speed: 60 rpm, measurement time: 180 seconds), and specifically, the value measured by the method described in the examples.

[0035] In the present invention, the aqueous water-soluble ethylenically unsaturated monomer solution in step (A) may contain a hydrophilic polymer dispersant, but from the viewpoint of, for example, satisfying the above-mentioned viscosity, it is preferable that it does not contain a hydrophilic polymer dispersant (the amount of hydrophilic polymer dispersant is 0 parts by mass per 100 parts by mass of the water-soluble ethylenically unsaturated monomer). Examples of hydrophilic polymer dispersants include polyvinylpyrrolidone (abbreviated as "PVP"), polyvinyl alcohol (abbreviated as "PVA"), polyglycerin, and polyacrylates. Even if the aqueous water-soluble ethylenically unsaturated monomer solution in step (A) contains a hydrophilic polymer dispersant, the amount of the hydrophilic polymer dispersant is preferably less than 0.1 parts by mass per 100 parts by mass of the water-soluble ethylenically unsaturated monomer.

[0036] In step (A), a hydrocarbon dispersion medium in which a hydrophobic polymer dispersant is dispersed or dissolved and an aqueous solution of a water-soluble ethylenically unsaturated monomer are stirred and mixed under predetermined stirring conditions to prepare a first dispersion. In the present invention, the stirring conditions are important, and the ratio of the tip peripheral speed of the agitator blade during the primary dispersion step to the tip peripheral speed of the agitator blade during the secondary dispersion step described below (tip peripheral speed of the agitator blade during the primary dispersion step / tip peripheral speed of the agitator blade during the secondary dispersion step) is set to more than 0.10 and less than 0.60, and the duration of the primary dispersion step is set to more than 1 minute and less than 90 minutes. In the present invention, the amount of hydrocarbon dispersion medium in the water-absorbent resin particles is suitably reduced by adjusting the stirring conditions in steps (A) and (B). The tip peripheral speed ratio is a value calculated by the method described in the Examples.

[0037] From the viewpoint of more suitably exerting the effects of the present invention, the ratio of the tip peripheral speed of the stirring blade in the primary dispersion step to the tip peripheral speed of the stirring blade in the secondary dispersion step is preferably 0.12 or more, more preferably 0.15 or more, even more preferably 0.18 or more, and is preferably 0.57 or less, more preferably 0.50 or less, even more preferably 0.45 or less, and preferred ranges include 0.12 to 0.57, 0.12 to 0.50, 0.12 to 0.45, 0.15 to 0.57, 0.15 to 0.50, 0.15 to 0.45, 0.18 to 0.57, 0.18 to 0.50, and 0.18 to 0.45.

[0038] Furthermore, from the viewpoint of more suitably exerting the effects of the present invention, the time for the primary dispersion step (the stirring time under the stirring conditions in the primary dispersion step) is preferably more than 1 minute, more preferably 3 minutes or more, even more preferably 4 minutes or more, and is preferably 80 minutes or less, more preferably 65 minutes or less, even more preferably 50 minutes or less, particularly preferably 40 minutes or less, and preferred ranges include 2 to 80 minutes, 2 to 65 minutes, 2 to 50 minutes, 2 to 40 minutes, 3 to 80 minutes, 3 to 65 minutes, 3 to 50 minutes, 3 to 40 minutes, 4 to 80 minutes, 4 to 65 minutes, 4 to 50 minutes, and 4 to 40 minutes.

[0039] In order to more suitably exert the effects of the present invention, the peripheral speed of the tip of the stirring blade during the primary dispersion step is preferably 0.15 m / s or more, more preferably 0.20 m / s or more, even more preferably 0.25 m / s or more, and is preferably 1.7 m / s or less, more preferably 1.2 m / s or less, even more preferably 1.0 m / s or less, and particularly preferably 0.75 m / s or less. Preferred ranges include 0.15 to 1.7 m / s, 0.15 to 1.2 m / s, 0.15 to 1.0 m / s, 0.15 to 0.75 m / s, 0.20 to 1.7 m / s, 0.20 to 1.2 m / s, 0.20 to 1.0 m / s, 0.20 to 0.75 m / s, 0.25 to 1.7 m / s, 0.25 to 1.2 m / s, 0.25 to 1.0 m / s, and 0.25 to 0.75 m / s.

[0040] The shape of the stirring blade is not particularly limited. Examples of the stirring blade that can be used include paddle blades, flat blades, propeller blades, anchor blades, turbine blades, Pfaudle blades, ribbon blades, Fullzone blades (manufactured by Shinko Pantech Co., Ltd.), Max Blend blades (manufactured by Sumitomo Heavy Industries, Ltd.), and Supermix (manufactured by Satake Chemical Machinery Co., Ltd.).

[0041] <Step (B)> Step (B) is a secondary dispersion step in which the first dispersion obtained in step (A) and a surfactant are stirred and mixed to prepare a second dispersion.

[0042] [Surfactant] In step (B), a surfactant is used as a dispersion stabilizer to improve the dispersion stability of the water-soluble ethylenically unsaturated monomer in the hydrocarbon dispersion medium.

[0043] Examples of surfactants that can be used 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 surfactants, sorbitan fatty acid esters, polyglycerin fatty acid esters, and sucrose fatty acid esters are particularly preferred from the standpoint of dispersion stability of the monomer. These surfactants may be used alone or in combination of two or more.

[0044] The amount of the surfactant used in step (B) is preferably 0.1 to 30 parts by mass, more preferably 0.3 to 20 parts by mass, relative to 100 parts by mass of the water-soluble ethylenically unsaturated monomer in step (A).

[0045] [Polymer-based dispersant] Furthermore, as a dispersion stabilizer, a polymer-based dispersant may be used in combination with the surfactant described above.

[0046] Examples of polymeric dispersants 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 polymeric dispersants, it is particularly preferable to use 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, and oxidized ethylene-propylene copolymer from the viewpoint of dispersion stability of the monomer. These polymeric dispersants may be used alone or in combination of two or more.

[0047] 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 water-soluble ethylenically unsaturated monomer in step (A).

[0048] The form of the surfactant mixed in step (B) is not particularly limited, but a method in which the surfactant is diluted or dissolved in advance in a small amount of dispersion medium is preferred, as this allows for dispersion stabilization in a short period of time.

[0049] As described above, in the present invention, the stirring conditions for each dispersion in the steps (A) and (B) are important, and the amount of hydrocarbon dispersion medium in the water absorbent resin particles is reduced by setting the ratio of the tip peripheral speed of the stirring blade in the primary dispersion step to the tip peripheral speed of the stirring blade in the secondary dispersion step (tip peripheral speed of the stirring blade in the primary dispersion step / tip peripheral speed of the stirring blade in the secondary dispersion step) and the duration of the primary dispersion step within the above-mentioned predetermined ranges, respectively.

[0050] From the viewpoint of more suitably exerting the effects of the present invention, the tip peripheral speed of the stirring blade during the secondary dispersion step is preferably 0.7 m / s or more, more preferably 1.0 m / s or more, even more preferably 1.2 m / s or more, and is preferably 3.5 m / s or less, more preferably 3.0 m / s or less, even more preferably 2.5 m / s or less, and preferred ranges include 0.7 to 3.5 m / s, 1.0 to 3.0 m / s, 1.2 to 3.0 m / s, and 1.2 to 2.5 m / s.

[0051] Furthermore, from the viewpoint of more suitably exerting the effects of the present invention, the time for the secondary dispersion step (stirring time under the stirring conditions in the secondary dispersion step) is preferably 5 minutes or more, more preferably 10 minutes or more, even more preferably 15 minutes or more, and particularly preferably 20 minutes or more, and is preferably 60 minutes or less, more preferably 50 minutes or less, and even more preferably 40 minutes or less; preferred ranges include 5 to 60 minutes, 5 to 50 minutes, 5 to 40 minutes, 10 to 60 minutes, 10 to 50 minutes, 10 to 40 minutes, 15 to 60 minutes, 15 to 50 minutes, 15 to 40 minutes, 20 to 60 minutes, 20 to 50 minutes, and 20 to 40 minutes.

[0052] In step (B), the shape of the stirring blade is not particularly limited, and examples thereof include the same as those exemplified in step (A).

[0053] <Step (C)> Step (C) is a polymerization step in which the water-soluble ethylenically unsaturated monomer is subjected to reverse suspension polymerization after step (B). The polymerization step is initiated by blowing an inert gas (such as nitrogen) into a polymerization tank.

[0054] Specifically, the water-soluble ethylenically unsaturated monomer in the dispersion obtained through step (B) is subjected to reversed-phase suspension polymerization in a hydrocarbon dispersion medium containing a hydrophobic polymeric dispersant and a surfactant to obtain hydrogel particles. In the reversed-phase suspension polymerization, polymerization is carried out in the presence of a radical polymerization initiator. Furthermore, as described below, an internal crosslinking agent may be added to the water-soluble ethylenically unsaturated monomer, if necessary, to form hydrogel particles having an internal crosslinked structure.

[0055]

[0113] In the case of performing reverse phase suspension polymerization in two or more stages, from the viewpoint of easily controlling the median particle diameter of a water absorbent resin particle, the tip peripheral speed of the stirring blade during the polymerization step is preferably 0.7 m / s or more, more preferably 1.0 m / s or more, still more preferably 1.2 m / s or more, and is also preferably 3.5 m / s or less, more preferably 3.0 m / s or less, still more preferably 2.5 m / s or less, and preferred ranges include 0.7 to 3.5 m / s, 1.0 to 3.0 m / s, 1.2 to 3.0 m / s, and 1.2 to 2.5 m / s.

[0056] In step (C), the shape of the stirring blade is not particularly limited, and examples thereof include the same as those exemplified in step (A).

[0057] In each of steps (A), (B), and (C), from the viewpoint of the dispersion efficiency of the first dispersion and the second dispersion, d / D, which is the ratio of the blade diameter d [m] of the stirring blade to the inner diameter D [m] of the polymerization tank into which each dispersion is charged, is preferably 0.30 or more, more preferably 0.35 or more, even more preferably 0.40 or more, and is preferably 1.00 or less, more preferably 0.95 or less, even more preferably 0.90 or less. Preferred ranges include 0.30 to 1.00, 0.30 to 0.95, 0.30 to 0.90, 0.35 to 1.00, 0.35 to 0.95, 0.35 to 0.90, 0.40 to 1.00, 0.40 to 0.95, and 0.40 to 0.90.

[0058] [Radical Polymerization Initiator] Examples of the radical polymerization initiator added to the polymerization step 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, t-butyl peroxyacetate, t-butylperoxyisobutyrate, t-butylperoxypivalate, and hydrogen peroxide, as well as 2,2'-azobis(2-amidinopropane) dihydrochloride and 2,2'-azobis[2-(N-phenyl)propane]. Examples of the radical polymerization initiator include azo compounds such as 2,2'-azobis[2-(N-allylamidino)propane] dihydrochloride, 2,2'-azobis{2-[1-(2-hydroxyethyl)-2-imidazolin-2-yl]propane} dihydrochloride, 2,2'-azobis{2-methyl-N-[1,1-bis(hydroxymethyl)-2-hydroxyethyl]propionamide}, 2,2'-azobis[2-methyl-N-(2-hydroxyethyl)-propionamide], and 4,4'-azobis(4-cyanovaleric acid). Among these radical polymerization initiators, potassium persulfate, ammonium persulfate, sodium persulfate, and 2,2'-azobis(2-amidinopropane) dihydrochloride are preferred from the viewpoints of ease of availability and handling. These radical polymerization initiators may be used alone or in combination of two or more. The radical polymerization initiator can also be used as a redox polymerization initiator in combination with a reducing agent such as sodium sulfite, sodium hydrogen sulfite, ferrous sulfate, or L-ascorbic acid.

[0059] The amount of radical polymerization initiator used is, for example, 0.00005 to 0.01 mole per mole of the water-soluble ethylenically unsaturated monomer. By using such an amount, it is possible to avoid a rapid polymerization reaction and complete the polymerization reaction within an appropriate time.

[0060] [Internal Crosslinking Agent] The internal crosslinking agent can be one that can crosslink the polymer of the water-soluble ethylenically unsaturated monomer used, such as (poly)ethylene glycol ("(poly)" refers to both the presence and absence of the prefix "poly"). the same applies hereinafter)], unsaturated polyesters obtained by reacting polyols such as diols and triols, such as (poly)propylene glycol, 1,4-butanediol, 1,6-hexanediol, trimethylolpropane, and (poly)glycerin, with unsaturated acids, such as (meth)acrylic acid, maleic acid, and fumaric acid; bisacrylamides such as N,N-methylenebisacrylamide; di(meth)acrylic acid esters 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, divinyl Examples of the compound include a compound having two or more polymerizable unsaturated groups such as benzene; a diglycidyl compound such as (poly)ethylene glycol diglycidyl ether, (poly)propylene glycol diglycidyl ether, (poly)glycerin diglycidyl ether, and the like, and a polyglycidyl compound such as a triglycidyl compound; an epihalohydrin compound such as epichlorohydrin, epibromohydrin, and α-methylepichlorohydrin; a compound having two or more reactive functional groups such as an isocyanate compound such as 2,4-tolylene diisocyanate and hexamethylene diisocyanate; and an oxetane compound such as 3-methyl-3-oxetane methanol, 3-ethyl-3-oxetane methanol, 3-butyl-3-oxetane methanol, 3-methyl-3-oxetane ethanol, 3-ethyl-3-oxetane ethanol, and 3-butyl-3-oxetane ethanol. Among these internal cross-linking agents, it is preferable to use a polyglycidyl compound, it is more preferable to use a diglycidyl ether compound, and it is preferable to use (poly)ethylene glycol diglycidyl ether, (poly)propylene glycol diglycidyl ether, or (poly)glycerin diglycidyl ether.These internal crosslinking agents may be used alone or in combination of two or more.

[0061] The amount of the internal crosslinking agent used is preferably 0.000001 to 0.02 mol, more preferably 0.00001 to 0.01 mol, even more preferably 0.00001 to 0.005 mol, and still more preferably 0.00005 to 0.002 mol, relative to 1 mol of the water-soluble ethylenically unsaturated monomer.

[0062] [Other Components] If desired, other components may be added to the aqueous solution containing the water-soluble ethylenically unsaturated monomer to carry out reverse phase suspension polymerization. As other components, various additives such as a thickener and a chain transfer agent may be added.

[0063] As an example, a thickener can be added to an aqueous solution containing a water-soluble ethylenically unsaturated monomer to carry out reversed-phase suspension polymerization. By adjusting the viscosity of the aqueous solution by adding a thickener in this way, it is possible to control the median particle size obtained in reversed-phase suspension polymerization. However, as mentioned above, in step (A) of the present invention, the aqueous solution of the water-soluble ethylenically unsaturated monomer may contain a hydrophilic polymer dispersant that functions as a thickener, but in order to satisfy the above-mentioned viscosity, it is preferable that the aqueous solution does not contain a hydrophilic polymer dispersant, and the amount of the hydrophilic polymer dispersant is preferably 0 parts by mass or more and less than 0.1 parts by mass per 100 parts by mass of the water-soluble ethylenically unsaturated monomer.

[0064] Examples of usable thickeners include 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. If the stirring speed during polymerization is the same, the higher the viscosity of the water-soluble ethylenically unsaturated monomer aqueous solution, the larger the primary particles and / or secondary particles of the resulting particles tend to be.

[0065] The reversed-phase suspension polymerization can be carried out in one stage or in multiple stages of two or more stages, and is preferably carried out in two or three stages from the viewpoints of increasing productivity and reducing the amount of residual dispersion medium.

[0066] When performing reverse phase suspension polymerization in two or more stages, after performing the first stage of reverse phase suspension polymerization, a water-soluble ethylenically unsaturated monomer is added to and mixed with the reaction mixture obtained in the first stage of polymerization, and the second and subsequent stages of reverse phase suspension polymerization can be performed in the same manner as the first stage. As described above, in the method for producing water-absorbent resin particles of the present invention, it is preferable that the entire process (A)-(C) is the first stage of the reverse phase suspension polymerization method, and when performing the second and subsequent stages of reverse phase suspension polymerization, it is preferable to perform the entire process (A)-(C) as the first stage of reverse phase suspension polymerization. In the reverse phase suspension polymerization in each stage from the second stage onwards, it is preferable to perform the reverse phase suspension polymerization by adding, in addition to the water-soluble ethylenically unsaturated monomer, a radical polymerization initiator within the molar ratio of each component to the water-soluble ethylenically unsaturated monomer as described above, based on the amount of water-soluble ethylenically unsaturated monomer added during the reverse phase suspension polymerization in each stage from the second stage onwards. Note that, in the second and subsequent stages of polymerization, an internal crosslinking agent may also be added to the water-soluble ethylenically unsaturated monomer as needed.

[0067] The reaction temperature of the polymerization reaction is preferably 20 to 110°C, more preferably 40 to 90°C, from the viewpoints of rapidly progressing the polymerization, shortening the polymerization time, thereby improving economic efficiency, and easily removing the heat of polymerization to smoothly carry out the reaction.

[0068] <Dehydration Step> After the reversed-phase suspension polymerization in step (C), a dehydration step may be included in which water, hydrocarbon dispersion medium, etc. are removed by distillation by applying energy such as heat from the outside. When dehydrating the hydrous gel particles after the reversed-phase suspension polymerization, the system in which the hydrous gel particles are dispersed in the hydrocarbon dispersion medium is heated, and the water and hydrocarbon dispersion medium are temporarily distilled out of the system by azeotropic distillation. In this case, if only the evaporated hydrocarbon dispersion medium is returned to the system, continuous azeotropic distillation is possible. In this case, the temperature in the system during drying is maintained below the azeotropic temperature with the hydrocarbon dispersion medium, which is preferable from the viewpoint of preventing deterioration of the resin. By controlling the processing conditions of this dehydration step after polymerization to adjust the amount of dehydration (i.e., adjusting the water content of the polymer particles), it is possible to control the various properties of the obtained water-absorbent resin particles.

[0069] In the dehydration step, dehydration treatment by distillation may be carried out under normal pressure. When dehydration treatment is carried out under normal pressure, the dehydration temperature is preferably 70 to 250°C, more preferably 80 to 180°C, even more preferably 80 to 140°C, and even more preferably 90 to 130°C.

[0070] <Surface cross-linking step> The surface cross-linking step is a step of subjecting the polymer particles obtained in the polymerization step of step (C) to surface cross-linking.When the polymer particles are cross-linked polymer particles (hydrogel particles), this is a step of adding a surface cross-linking agent to the hydrogel particles having an internal cross-linked structure obtained by polymerizing a water-soluble ethylenically unsaturated monomer to perform cross-linking (surface cross-linking reaction).This surface cross-linking reaction is preferably carried out in the presence of a surface cross-linking agent after the polymerization of the water-soluble ethylenically unsaturated monomer.In this way, by subjecting the hydrogel particles having an internal cross-linked structure after polymerization to a surface cross-linking reaction, the cross-linking density near the surface of the water-absorbent resin particles can be set to a specific range, thereby obtaining water-absorbent resin particles with improved performance such as water absorption capacity under load.

[0071] Examples of the surface cross-linking agent include compounds having two or more reactive functional groups. For example, polyols such as ethylene glycol, propylene glycol, 1,4-butanediol, diethylene glycol, triethylene glycol, 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; haloepoxy compounds such as epichlorohydrin, epibromohydrin, and α-methylepichlorohydrin; isocyanate compounds such as 2,4-tolylene diisocyanate and hexamethylene diisocyanate; 3-methyl-3-oxetanemethanol and 3-ethyl-3-oxetane Oxetane compounds such as methanol, 3-butyl-3-oxetanemethanol, 3-methyl-3-oxetaneethanol, 3-ethyl-3-oxetaneethanol, and 3-butyl-3-oxetaneethanol; oxazoline compounds such as 1,2-ethylenebisoxazoline; ethylene carbonate, propylene carbonate, 4,5-dimethyl-1,3-dioxolan-2-one, 4,4-dimethyl-1,3-dioxolan-2-one, 4-ethyl and hydroxyalkylamide compounds such as bis[N,N-di(β-hydroxyethyl)]adipamide. Among these surface cross-linking agents, 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 are preferred.These surface cross-linking agents may be used alone or in combination of two or more.

[0072] The amount of the surface crosslinking agent used is preferably 0.00001 to 0.01 mol, more preferably 0.00005 to 0.005 mol, still more preferably 0.0001 to 0.001 mol, and still more preferably 0.0004 to 0.0009 mol, relative to 1 mol of the total amount of the water-soluble ethylenically unsaturated monomers used in the polymerization.

[0073] As a method for adding the surface crosslinking agent, the surface crosslinking agent may be added as it is or as an aqueous solution, or, if necessary, may be added as a solution using a hydrophilic organic solvent as a solvent. Examples of the hydrophilic organic solvent include lower alcohols such as methyl alcohol, ethyl alcohol, n-propyl alcohol, isopropyl alcohol, etc.; ketones such as acetone, methyl ethyl ketone, etc.; ethers such as diethyl ether, dioxane, tetrahydrofuran, etc.; amides such as N,N-dimethylformamide, etc.; sulfoxides such as dimethyl sulfoxide, etc. These hydrophilic organic solvents may be used alone, or two or more types may be used in combination, or as a mixed solvent with water.

[0074] The timing of addition of the surface crosslinking agent may be after the polymerization reaction of the water-soluble ethylenically unsaturated monomer has almost completely finished, and the surface crosslinking agent is added in the presence of water in a range of preferably 1 to 400 parts by mass, more preferably 5 to 200 parts by mass, still more preferably 10 to 100 parts by mass, and still more preferably 15 to 60 parts by mass, relative to 100 parts by mass of the water-soluble ethylenically unsaturated monomer used for polymerization. Note that the amount of water means the total amount of water contained in the reaction system and water used as necessary when adding the surface crosslinking agent.

[0075] The reaction temperature in the surface crosslinking reaction is preferably 50 to 250° C., more preferably 60 to 180° C., further preferably 60 to 140° C., and even more preferably 70 to 120° C. The reaction time of the surface crosslinking reaction is preferably 1 to 300 minutes, and more preferably 5 to 200 minutes.

[0076] <Drying Step> After the above-described surface cross-linking, a drying step may be included in which water, hydrocarbon dispersion medium, etc. are removed by distillation by applying energy such as heat from the outside. The surface-cross-linked hydrogel particles are dried and the water and hydrocarbon dispersion medium are distilled off to obtain polymer particles.

[0077] In the drying step, the drying treatment by distillation may be carried out under normal pressure or under reduced pressure. Moreover, from the viewpoint of increasing the drying efficiency, it may be carried out under a gas flow such as nitrogen. When the drying treatment 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 even more preferably 90 to 130°C. Moreover, when the drying treatment is carried out under reduced pressure, the drying temperature is preferably 40 to 160°C, more preferably 50 to 110°C.

[0078] In addition, in the case where the surface cross-linking step using a surface cross-linking agent is carried out after carrying out the polymerization of the monomer by reverse phase suspension polymerization in the step (C), the drying step by distillation described above is carried out after the completion of the surface cross-linking step. Alternatively, the surface cross-linking step and the drying step may be carried out simultaneously.

[0079] The water-absorbent resin particles of the present invention may be composed of polymer particles alone, or may contain additives according to the purpose. Examples of such additives include inorganic powders, surfactants, oxidizing agents, reducing agents, metal chelating agents, radical chain inhibitors, antioxidants, antibacterial agents, etc. For example, the fluidity of the water-absorbent resin particles can be further improved by adding 0.05 to 5 parts by mass of amorphous silica as an inorganic powder relative to 100 parts by mass of the water-absorbent resin particles. The additives are preferably hydrophilic or water-soluble.

[0080] 2. Water-absorbent resin particles The amount of residual dispersion medium in the water-absorbent resin particles obtained by the production method of the present invention is preferably 200 ppm or less, more preferably 150 ppm or less, still more preferably 120 ppm or less, and more preferably 100 ppm or less, and the lower limit is 0 ppm. The amount of residual dispersion medium in the water-absorbent resin particles is a value measured by the method described in the examples.

[0081] The median particle diameter of the water-absorbent resin particles obtained by the production method of the present invention is not particularly limited, but is preferably, for example, 200 μm or more, 250 μm or more, 300 μm or more, 320 μm or more, or 350 μm or more. From the same viewpoint, the median particle diameter is preferably 700 μm or less, 600 μm or less, 550 μm or less, 500 μm or less, or 450 μm or less. Preferred ranges include a median particle size of 200 to 700 μm, 200 to 600 μm, 200 to 550 μm, 200 to 500 μm, 200 to 450 μm, 250 to 700 μm, 250 to 600 μm, 250 to 550 μm, 250 to 500 μm, 250 to 450 μm, 300 to 700 μm, 300 to 600 μm, 300 to 550 μm, 300 to 500 μm, 300 to 450 μm, 350 to 700 μm, 350 to 600 μm, 350 to 550 μm, 350 to 500 μm, and 350 to 450 μm.

[0082] The median particle size of the water-absorbent resin particles can be measured using a JIS standard sieve, and specifically, it is a value measured by the method described in the examples.

[0083]

[0033] 3. Absorbent body, absorbent article The water-absorbent resin particles obtained by the production method of the present invention constitute an absorbent body used in, for example, sanitary materials such as sanitary products and disposable diapers, and are suitably used for absorbent articles containing the absorbent body. In particular, the water-absorbent resin particles obtained by the production method of the present invention are preferably used for the purpose of constituting an absorbent body, in that the odor of the hydrocarbon dispersion medium used in the production process of the water-absorbent resin particles is reduced.

[0084] Here, the absorbent using the water-absorbent resin particles of the present invention contains the water-absorbent resin particles of the present invention. The absorbent may further contain hydrophilic fibers. Examples of the configuration of the absorbent include a sheet-like structure in which water-absorbent resin particles are fixed on a nonwoven fabric or between multiple nonwoven fabrics, a mixed dispersion obtained by mixing water-absorbent resin particles and hydrophilic fibers to form a uniform composition, a sandwich structure in which water-absorbent resin particles are sandwiched between layered hydrophilic fibers, and a structure in which water-absorbent resin particles and hydrophilic fibers are wrapped in tissue. Note that the absorbent may contain other components, such as adhesive binders such as heat-fusible synthetic fibers, hot-melt adhesives, and adhesive emulsions, to improve the shape retention of the absorbent.

[0085] The content of the water-absorbent resin particles in the absorbent body is preferably 5 to 100% by mass, more preferably 10 to 95% by mass, even more preferably 20 to 90% by mass, and even more preferably 30 to 80% by mass.

[0086] Examples of hydrophilic fibers include cellulose fibers such as cotton-like pulp obtained from wood, mechanical pulp, chemical pulp, and semi-chemical pulp, artificial cellulose fibers such as rayon and acetate, and fibers made of synthetic resins such as hydrophilically treated polyamide, polyester, and polyolefin. The average fiber length of the hydrophilic fibers is usually 0.1 to 10 mm, or may be 0.5 to 5 mm.

[0087] The absorbent article of the present invention can be produced by holding an absorbent body using the water-absorbent resin particles of the present invention between a liquid-permeable sheet (top sheet) through which liquid can pass and a liquid-impermeable sheet (back sheet) through which liquid cannot pass. The liquid-permeable sheet is arranged on the side that comes into contact with the body, and the liquid-impermeable sheet is arranged on the opposite side that comes into contact with the body.

[0088] Examples of liquid-permeable sheets include nonwoven fabrics and porous synthetic resin sheets, such as air-through, spunbond, chemical-bond, and needle-punched types, made of fibers such as polyethylene, polypropylene, and polyester. Examples of liquid-impermeable sheets include synthetic resin films made of resins such as polyethylene, polypropylene, and polyvinyl chloride.

[0089] 4. Additional Notes This specification includes at least the inventions shown in (1) to (6) below. (1) A method for producing water-absorbent resin particles, comprising the following steps: (A) a primary dispersion step of stirring and mixing, in the absence of a surfactant, a hydrocarbon dispersion medium having a hydrophobic polymer-based dispersant dispersed or dissolved therein and an aqueous solution of a water-soluble ethylenically unsaturated monomer to prepare a first dispersion, (B) a secondary dispersion step of stirring and mixing the first dispersion obtained in the step (A) with a surfactant to prepare a second dispersion, and (C) a polymerization step of performing reverse suspension polymerization of the water-soluble ethylenically unsaturated monomer after the step (B), wherein a ratio of the tip peripheral speed of the agitator blade in the primary dispersion step to the tip peripheral speed of the agitator blade in the secondary dispersion step (tip peripheral speed of the agitator blade in the primary dispersion step / tip peripheral speed of the agitator blade in the secondary dispersion step) is more than 0.10 and less than 0.60, and the duration of the primary dispersion step is in the range of more than 1 minute and less than 90 minutes. (2) The method for producing water-absorbent resin particles according to the above (1), wherein a ratio of the tip peripheral speed of the stirring blade during the primary dispersion step to the tip peripheral speed of the stirring blade during the secondary dispersion step (tip peripheral speed of the stirring blade during the primary dispersion step / tip peripheral speed of the stirring blade during the secondary dispersion step) is set to 0.12 to 0.57, 0.15 to 0.50, 0.18 to 0.50, or 0.18 to 0.45. (3) The method for producing water-absorbent resin particles according to the above (1) or (2), wherein a time period for the primary dispersion step is set to a range of 2 to 80 minutes, 2 to 65 minutes, 2 to 50 minutes, 2 to 40 minutes, 3 to 40 minutes, or 4 to 40 minutes. (4) The method for producing water-absorbent resin particles according to any one of (1) to (3) above, wherein a tip peripheral speed of the stirring blade during the primary dispersion step is 0.15 to 1.7 m / s, 0.20 to 1.2 m / s, 0.20 to 1.0 m / s, 0.20 to 0.75 m / s, or 0.25 to 0.75 m / s. (5) The method for producing water-absorbent resin particles according to any one of (1) to (4) above, wherein a tip peripheral speed of the stirring blade during the secondary dispersion step is 0.7 to 3.5 m / s, 1.0 to 3.0 m / s, 1.2 to 3.0 m / s, or 1.2 to 2.5 m / s. (6) The method for producing water-absorbent resin particles according to any one of (1) to (5) above, wherein in the polymerization step, reversed-phase suspension polymerization is performed in two or more stages.

[0090] The present invention will be described in detail below with reference to examples and comparative examples, but the present invention is not limited to these examples.

[0091] The water-absorbent resin particles obtained in the following Production Examples and the water-absorbent resin particles obtained in the Examples and Comparative Examples were evaluated by the following various tests. Unless otherwise specified, the measurements were carried out in an environment of a temperature of 25±2°C and a humidity of 50±10%.

[0092]

[0133] Regarding the water absorbent resin particles obtained in each of Examples and Comparative Examples, the median particle size, the amount of residual dispersion medium (the amount of hydrocarbon dispersion medium remaining inside the water absorbent resin particle), the viscosity of the water-soluble ethylenically unsaturated monomer aqueous solution, and the ratio of the tip peripheral speed of the stirring blade ((the tip peripheral speed of the stirring blade in the primary dispersion step / the tip peripheral speed of the stirring blade in the secondary dispersion step)) were evaluated by the methods shown below.

[0093] [Median particle size] JIS standard sieves were combined in the following order from top to bottom: a sieve with a mesh size of 710 μm, a sieve with a mesh size of 600 μm, a sieve with a mesh size of 500 μm, a sieve with a mesh size of 425 μm, a sieve with a mesh size of 300 μm, a sieve with a mesh size of 250 μm, a sieve with a mesh size of 150 μm, and a tray.

[0094] 50 g of water-absorbent resin particles were placed on the top sieve of the combination, and the particles were shaken for 20 minutes using a rotary shaker to classify them. After classification, the mass of the water-absorbent resin particles remaining on each sieve was calculated as a mass percentage relative to the total amount, and the particle size distribution was determined. By integrating the particles on the sieves in descending order of particle size in this particle size distribution, the relationship between the sieve openings and the integrated value of the mass percentage of the water-absorbent resin particles remaining on the sieves was plotted on a logarithmic probability paper. By connecting the plots on the probability paper with a straight line, the particle size corresponding to an integrated mass percentage of 50% by mass was determined as the median particle size.

[0095] [Amount of remaining dispersion medium] (a) Preparation of a calibration curve The hydrocarbon dispersion medium (hereinafter referred to as "dispersion medium") used in polymerization of water absorbent resin particles for which the amount of remaining dispersion medium is to be measured, and DMF (dimethylformamide) and a 25 mass % aqueous phosphoric acid solution to be used in the measurement were cooled in an ice-water bath (in order to suppress evaporation of the dispersion medium during charging, charging is carried out after sufficient cooling).

[0096] 0.1 g of the dispersion medium was precisely weighed into a 100 mL measuring flask, and the cooled DMF was added thereto to make a total of 100 mL, followed by stirring to prepare standard sample solution 1. This standard sample solution 1 was also cooled in an ice-water bath.

[0097] Next, 10 mL of standard sample solution 1 was precisely weighed into a 20 mL measuring flask, and the cooled DMF was added thereto to make a total of 20 mL, followed by stirring to prepare standard sample solution 2. This standard sample solution 2 was also cooled in an ice-water bath.

[0098] 2 mL of standard sample solution 2 was accurately weighed into a 20 mL measuring flask, and the cooled DMF was added thereto to make a total of 20 mL, followed by stirring to prepare standard sample solution 3. This standard sample solution 3 was also cooled in an ice-water bath.

[0099] 5 mL of standard sample solution 3 was accurately weighed into a 20 mL measuring flask, and the cooled DMF was added thereto to make a total of 20 mL, followed by stirring to prepare standard sample solution 4. This standard sample solution 4 was also cooled in an ice-water bath.

[0100] 4 mL of standard sample solution 4 was accurately weighed into a 20 mL measuring flask, and the cooled DMF was added thereto to make a total of 20 mL, followed by stirring to prepare standard sample solution 5. This standard sample solution 5 was also cooled in an ice-water bath.

[0101] 10 mL of standard sample solution 5 was accurately weighed into a 20 mL measuring flask, and the cooled DMF was added thereto to make a total of 20 mL, followed by stirring to prepare standard sample solution 6. This standard sample solution 6 was also cooled in an ice-water bath.

[0102] 4 mL of each of the cooled standard sample solutions 1 to 6 was precisely weighed into a 20 mL vial (manufactured by Alpha Moss, model number: 202-0050). 5 mL of the cooled 25% by mass aqueous phosphoric acid solution was added to each vial, and the vial was then stoppered with a rubber septum and an aluminum cap and crimped, after which the vial was shaken to mix.

[0103] The process from placing the sample in the 20 mL vial to tightening the stopper was carried out quickly to prevent evaporation of the dispersion medium as much as possible. Also, to prevent evaporation of the dispersion medium due to heat generated when mixing DMF and 25% by mass aqueous phosphoric acid, care was taken to thoroughly cool both reagents and seal them with an aluminum cap or the like before thorough mixing.

[0104] The vial was heated and shaken at 110° C. for 2 hours, and 1 mL of the gas phase was sampled without cooling and injected into a gas chromatograph to obtain a chromatogram (using a headspace autosampler).

[0105] The concentration of the standard sample solution was determined based on the amount charged, and the amount of dispersion medium charged in each vial was calculated. A calibration curve was then created from the amount charged and the peak area of ​​the chromatogram. When a hydrocarbon mixture was used as the dispersion medium, multiple peaks appeared, so the calibration curve was created from the sum of the peak areas and the amount charged.

[0106] (b) Measurement of the amount of residual dispersion medium in water absorbent resin particles Approximately 2 g of water absorbent resin particles to be measured was placed in an aluminum cup, and the contained moisture amount was adjusted by drying for 2 hours with a hot air dryer at 105° C. Required amounts of DMF and a 25 mass% aqueous phosphoric acid solution to be used for measurement were placed in a screw bottle, and cooled in an ice-water bath.

[0107] 0.10 g of the water-absorbent resin particles was precisely weighed into a 20 mL vial (manufactured by Alpha Moss, model: 202-0050), and the bottom of the vial was immersed in an ice bath to cool the vial and the water-absorbent resin particles. 4 mL of the cooled DMF was added to this vial, and 5 mL of the cooled 25% by mass aqueous phosphoric acid solution was further added. The vial was then stoppered with a rubber septum and an aluminum cap and crimped, and the vial was lightly shaken and mixed. Note that the process from charging the 20 mL vial to crimping the stopper was carried out quickly, and care was taken to prevent the dispersion medium from evaporating from the vial as much as possible. In addition, to prevent the dispersion medium from evaporating due to heat generated when mixing DMF and the 25% by mass aqueous phosphoric acid solution, both reagents were sufficiently cooled, and the vial was sealed with an aluminum cap or the like before being thoroughly mixed.

[0108] The vial was heated and shaken at 110° C. for 2 hours, and 1 mL of the gas phase was sampled without cooling and injected into a gas chromatograph to obtain a chromatogram (using a headspace autosampler).

[0109]

[0133] Based on the peak area of ​​the obtained chromatogram, the amount of the dispersion medium contained in the charged water absorbent resin particles (actual measured value of 0.10 g) was calculated from the calibration curve created previously, and converted into the amount [ppm] of the dispersion medium contained per 1 g of the water absorbent resin particles.

[0110] The gas chromatograph conditions used in the present invention to measure the amount of residual dispersion medium are as follows: Model: Shimadzu GC-2014 (with headspace autosampler) Packing: Carrier - Shimalite (NAW) 80-100 mesh, liquid phase - Squalane 25% Column: inner diameter 3.2 mmΦ x column length 2.1 m Column temperature: 80°C Inlet temperature: 180°C Detector temperature: 180°C Detector: FID Gas carrier: nitrogen gas Vial bottle heating temperature: 110°C Syringe set temperature: 130°C

[0111] [Measurement of Viscosity of Water-Soluble Ethylenically Unsaturated Monomer Aqueous Solution] The viscosity [mPa s] of the water-soluble ethylenically unsaturated monomer aqueous solution was measured using a BII type viscometer (manufactured by Toki Sangyo Co., Ltd., model: BLII rotor No. 1, rotation speed: 60 rpm, measurement time: 180 seconds) by adjusting the temperature of the water-soluble ethylenically unsaturated monomer aqueous solution to 20±0.5° C. This viscosity measurement was carried out in a state where the water-soluble ethylenically unsaturated monomer aqueous solution was filled to a height of 94 mm in a glass container having an inner diameter of 45 mm and a height of 110 mm.

[0112] [Method for calculating the ratio of the tip peripheral speed of the stirring blade] The tip peripheral speed ratio refers to the ratio of the tip peripheral speed of the stirring blade during the primary dispersion step to the tip peripheral speed of the stirring blade during the secondary dispersion step (tip peripheral speed of the stirring blade during the primary dispersion step / tip peripheral speed of the stirring blade during the secondary dispersion step), and was calculated as follows: The tip peripheral speed of the stirring blade during each dispersion step was calculated using the stirring rotation speed X1 [rpm] during the primary dispersion step, the stirring rotation speed X2 [rpm] during the secondary dispersion step, and the blade diameter r [m] of the stirring blade, and the tip peripheral speed ratio of the stirring blade was derived from the ratio. Tip peripheral speed of stirring impeller during primary dispersion process [m / s] = (r × (X1 / 60) × π) Tip peripheral speed of stirring impeller during secondary dispersion process [m / s] = (r × (X2 / 60) × π) Tip peripheral speed ratio [-] = (tip peripheral speed of stirring impeller during primary dispersion process [m / s]) / (tip peripheral speed of stirring impeller during secondary dispersion process [m / s])

[0113] [Method for calculating the dimensional ratio between the polymerization vessel and the stirring blade] The dimensional ratio between the polymerization vessel and the stirring blade (ratio of the stirring blade diameter to the vessel diameter of the polymerization vessel during stirring) was calculated using the following formula. The stirring blade diameter d [m] refers to the diameter of the blade attached to the stirring shaft that extends in the out-of-plane direction relative to the stirring shaft and is the farthest from the stirring shaft, and the polymerization vessel diameter D [m] refers to the inner diameter of the polymerization vessel at the same height as the blade with the blade diameter d [m]. Dimension ratio between the polymerization vessel and the stirring blade = (stirring blade blade diameter d [m]) / (vessel diameter D [m] of the polymerization vessel)

[0114] Comparative Example 1: A 110 mm inner diameter, 2 L round-bottomed cylindrical separable flask (polymerization vessel) equipped with a reflux condenser, a dropping funnel, a nitrogen gas inlet tube, and a stirring blade with two stages of four 50 mm inclined paddle blades was prepared. The dimensional ratio of the polymerization vessel to the stirring blade was 0.45. 293 g of n-heptane was added to the flask as a hydrocarbon dispersion medium, and 0.736 g of maleic anhydride-modified ethylene-propylene copolymer (Hiwax 1105A, Mitsui Chemicals, Inc.) was added as a hydrophobic polymer dispersant. The mixture was heated to 80°C with stirring to dissolve the dispersant, and then cooled to 62°C. Separately, 92.0 g (1.03 mol) of an 80.5 mass% aqueous acrylic acid solution was placed in a 300 mL beaker as the water-soluble ethylenically unsaturated monomer, and while cooling with ice water, 147.7 g of a 20.9 mass% aqueous sodium hydroxide solution was added dropwise to carry out 75 mol% neutralization, followed by the addition and dissolution of 0.0736 g (0.272 mmol) of potassium persulfate as a water-soluble radical polymerization agent and 0.0101 g (0.058 mmol) of ethylene glycol diglycidyl ether as an internal crosslinking agent to prepare a first-stage water-soluble ethylenically unsaturated monomer aqueous solution. The viscosity of the first-stage water-soluble ethylenically unsaturated monomer aqueous solution prepared at this time was 8.7 mPa s. The resulting aqueous solution of water-soluble ethylenically unsaturated monomer was added to the separable flask, and the mixture was stirred and mixed for 10 minutes at a rotation speed of 300 rpm (four-blade inclined, two-stage paddle blade, tip peripheral speed: 0.79 [m / s]) to carry out a primary dispersion step. Meanwhile, in a 20 mL vial, 0.736 g of sucrose stearate (Ryoto Sugar Ester S-370, Mitsubishi Chemical Foods Corporation) as a surfactant was dissolved in 6.62 g of n-heptane by heating to prepare a surfactant solution. This surfactant solution was added to the resulting primary dispersion, and then the mixture was stirred and mixed at a rotation speed of 500 rpm (four-blade inclined, two-stage paddle blade, tip peripheral speed: 1.31 [m / s]) to carry out a secondary dispersion step. The ratio of the tip peripheral speeds of the stirring blades was 0.60. Thereafter, nitrogen was blown into the system to thoroughly replace the atmosphere, and the flask was immersed in a water bath at 70°C to raise the temperature, and the polymerization step was carried out for 60 minutes to obtain a first-stage polymerization slurry. The secondary dispersion step was carried out for 20 minutes.

[0115] On the other hand, 128.8 g (1.44 mol) of an 80.5 mass% aqueous acrylic acid solution was placed as a water-soluble ethylenically unsaturated monomer in another 500 mL beaker, and while cooling with ice water, 160.0 g of a 27 mass% aqueous sodium hydroxide solution was added dropwise to carry out 75 mol% neutralization, and then 0.103 g (0.381 mmol) of potassium persulfate as a water-soluble radical polymerization initiator and 0.0116 g (0.067 mmol) of ethylene glycol diglycidyl ether as an internal crosslinking agent were added and dissolved to prepare a second-stage aqueous water-soluble ethylenically unsaturated monomer solution.

[0116] While stirring at a stirrer rotation speed of 1000 rpm (tip peripheral speed: 2.62 [m / s]), the contents of the separable flask system were cooled to 28.5°C, and then the entire amount of the second-stage water-soluble ethylenically unsaturated monomer aqueous solution was added to the first-stage polymerization slurry liquid, and nitrogen was blown into the system to replace it with nitrogen for 30 minutes.The flask was then again immersed in a water bath at 70°C to raise the temperature, and a polymerization reaction was carried out for 60 minutes to obtain hydrous gel-like particles.

[0117] Thereafter, the flask was immersed in an oil bath set at 125°C, and 245.3 g of water was extracted from the system by azeotropic distillation of n-heptane and water while refluxing n-heptane. Thereafter, n-heptane was evaporated at 125°C to dry the mixture, and the mixture was passed through a sieve with an opening of 850 μm to obtain polymer particles. 0.5 parts by mass of amorphous silica (Toxil NP-S, Oriental Silicas Corporation) was mixed with 100 parts by mass of the polymer particles to obtain 220.3 g of water-absorbent resin particles (1). The median particle size of the water-absorbent resin particles (1) was 372 μm, and the amount of residual dispersion medium was 332 ppm.

[0118] <Comparative Example 2> The same operation as in Comparative Example 1 was carried out, except that the rotation speed of the stirrer in the primary dispersion step was changed from 300 rpm (tip peripheral speed: 0.79 [m / s]) and the time period of the primary dispersion step was changed from 10 minutes to 50 rpm (tip peripheral speed: 0.13 [m / s]) and the time period of the primary dispersion step was changed to 5 minutes, to obtain 238.8 g of water-absorbent resin particles (2). In Comparative Example 2, the viscosity of the water-soluble ethylenically unsaturated monomer aqueous solution in the first stage was 8.7 mPa s, the ratio of the tip peripheral speed of the stirring blade was 0.10, and the dimensional ratio of the polymerization tank and the stirring blade was 0.45. The median particle diameter of the water-absorbent resin particles (2) was 359 μm, and the amount of residual dispersion medium was 33,932 ppm.

[0119] Example 1 The rotation speed of the stirrer in the primary dispersion step was changed from 300 rpm (tip peripheral speed: 0.79 [m / s]) and the time period of the primary dispersion step was changed from 10 minutes to 100 rpm (tip peripheral speed: 0.26 [m / s]) and the time period of the primary dispersion step was changed to 5 minutes in Comparative Example 1, and the same operation as in Comparative Example 1 was performed to obtain 230.4 g of water-absorbent resin particles (3). The viscosity of the water-soluble ethylenically unsaturated monomer aqueous solution in the first stage in Example 1 was 8.7 mPa s, the ratio of the tip peripheral speed of the stirring blade was 0.20, and the dimensional ratio of the polymerization tank and the stirring blade was 0.45. The median particle diameter of the water-absorbent resin particles (3) was 358 μm, and the amount of residual dispersion medium was 62 ppm.

[0120] Example 2 The same operation as in Comparative Example 1 was carried out, except that the rotation speed of the stirrer in the primary dispersion step was changed from 300 rpm (tip peripheral speed: 0.79 [m / s]) and the time period of the primary dispersion step was changed from 10 minutes to 200 rpm (tip peripheral speed: 0.52 [m / s]) and the time period of the primary dispersion step was changed to 5 minutes, to obtain 229.8 g of water-absorbent resin particles (4). In Example 2, the viscosity of the water-soluble ethylenically unsaturated monomer aqueous solution in the first stage was 8.7 mPa s, the ratio of the tip peripheral speed of the stirring blade was 0.40, and the dimensional ratio of the polymerization tank and the stirring blade was 0.45. The median particle diameter of the water-absorbent resin particles (4) was 362 μm, and the amount of residual dispersion medium was 90 ppm.

[0121] Example 3 The same operation as in Comparative Example 1 was performed, except that the rotation speed of the stirrer in the primary dispersion step was changed from 300 rpm (tip peripheral speed: 0.79 [m / s]) and the time period of the primary dispersion step was changed from 10 minutes to 275 rpm (tip peripheral speed: 0.72 [m / s]) and the time period of the primary dispersion step was changed to 5 minutes, to obtain 230.2 g of water-absorbent resin particles (5). In Example 3, the viscosity of the water-soluble ethylenically unsaturated monomer aqueous solution in the first stage was 8.7 mPa s, the ratio of the tip peripheral speed of the stirring blade was 0.55, and the dimensional ratio of the polymerization tank and the stirring blade was 0.45. The median particle diameter of the water-absorbent resin particles (5) was 360 μm, and the amount of residual dispersion medium was 130 ppm.

[0122] Example 4 The same operation as in Comparative Example 1 was performed, except that the rotation speed of the stirrer during the primary dispersion step was changed from 300 rpm (tip peripheral speed: 0.79 [m / s]) to 100 rpm (tip peripheral speed: 0.26 [m / s]) in Comparative Example 1, to obtain 235.2 g of water-absorbent resin particles (6). In Example 4, the viscosity of the water-soluble ethylenically unsaturated monomer aqueous solution in the first stage was 8.7 mPa s, the ratio of the tip peripheral speed of the stirring blade was 0.20, and the dimensional ratio of the polymerization tank and the stirring blade was 0.45. The median particle diameter of the water-absorbent resin particles (6) was 350 μm, and the amount of residual dispersion medium was 55 ppm.

[0123] <Comparative Example 3> The same operation as in Comparative Example 1 was performed, except that the rotation speed of the stirrer in the primary dispersion step was changed from 300 rpm (tip peripheral speed: 0.79 [m / s]) and the time period of the primary dispersion step was changed from 10 minutes to 200 rpm (tip peripheral speed: 0.52 [m / s]) and the time period of the primary dispersion step was changed to 1 minute, to obtain 225.2 g of water-absorbent resin particles (7). In Comparative Example 3, the viscosity of the water-soluble ethylenically unsaturated monomer aqueous solution in the first stage was 8.7 mPa s, the ratio of the tip peripheral speed of the stirring blade was 0.40, and the dimensional ratio of the polymerization tank and the stirring blade was 0.45. The median particle diameter of the water-absorbent resin particles (7) was 313 μm, and the amount of residual dispersion medium was 208 ppm.

[0124] Example 5 The same operation as in Comparative Example 1 was carried out, except that the rotation speed of the stirrer in the primary dispersion step, which was set to 300 rpm (tip peripheral speed: 0.79 [m / s]), was changed to 200 rpm (tip peripheral speed: 0.52 [m / s]) in Comparative Example 1, to obtain 222.2 g of water-absorbent resin particles (8). In Example 5, the viscosity of the water-soluble ethylenically unsaturated monomer aqueous solution in the first stage was 8.7 mPa s, the ratio of the tip peripheral speed of the stirring blade was 0.40, and the dimensional ratio of the polymerization tank and the stirring blade was 0.45. The median particle diameter of the water-absorbent resin particles (8) was 334 μm, and the amount of residual dispersion medium was 63 ppm.

[0125] Example 6 The same operation as in Comparative Example 1 was performed, except that the rotation speed of the stirrer in the primary dispersion step was changed from 300 rpm (tip peripheral speed: 0.79 [m / s]) and the time period of the primary dispersion step was changed from 10 minutes to 200 rpm (tip peripheral speed: 0.52 [m / s]) and the time period of the primary dispersion step was changed to 30 minutes, to obtain 220.2 g of water-absorbent resin particles (9). In Example 6, the viscosity of the water-soluble ethylenically unsaturated monomer aqueous solution in the first stage was 8.7 mPa s, the ratio of the tip peripheral speed of the stirring blade was 0.40, and the dimensional ratio of the polymerization tank and the stirring blade was 0.45. The median particle diameter of the water-absorbent resin particles (9) was 354 μm, and the amount of residual dispersion medium was 60 ppm.

[0126] Example 7 The same operation as in Comparative Example 1 was performed, except that the rotation speed of the stirrer in the primary dispersion step was changed from 300 rpm (tip peripheral speed: 0.79 [m / s]) and the time period of the primary dispersion step was changed from 10 minutes to 200 rpm (tip peripheral speed: 0.52 [m / s]) and the time period of the primary dispersion step was changed to 60 minutes, to obtain 229.8 g of water-absorbent resin particles (10). In Example 7, the viscosity of the water-soluble ethylenically unsaturated monomer aqueous solution in the first stage was 8.7 mPa s, the ratio of the tip peripheral speed of the stirring blade was 0.40, and the dimensional ratio of the polymerization tank and the stirring blade was 0.45. The median particle diameter of the water-absorbent resin particles (10) was 304 μm, and the amount of residual dispersion medium was 82 ppm.

[0127] Example 8 A reflux condenser, a dropping funnel, a nitrogen gas inlet tube, and a 2 L round-bottomed cylindrical separable flask (polymerization vessel) with an inner diameter of 110 mm and a stirring blade with a lattice-shaped blade having a blade diameter of 58 mm, a maximum height of 104 mm, and a total of four 65 mm slits in the vertical direction were prepared. The dimensional ratio of the polymerization vessel to the stirring blade was 0.53. 293 g of n-heptane was added to the flask as a hydrocarbon dispersion medium, and 0.736 g of maleic anhydride-modified ethylene-propylene copolymer (Hiwax 1105A, Mitsui Chemicals, Inc.) was added as a polymeric dispersant. The mixture was heated to 80 ° C with stirring to dissolve the dispersant, and then cooled to 60 ° C. Separately, 92.0 g (1.03 mol) of an 80.5 mass% aqueous acrylic acid solution was placed in a 300 mL beaker as the water-soluble ethylenically unsaturated monomer, and while cooling with ice water, 147.7 g of a 20.9 mass% aqueous sodium hydroxide solution was added dropwise to carry out 75 mol% neutralization, followed by the addition and dissolution of 0.0736 g (0.272 mmol) of potassium persulfate as a water-soluble radical polymerization agent and 0.0101 g (0.058 mmol) of ethylene glycol diglycidyl ether as an internal crosslinking agent to prepare a first-stage water-soluble ethylenically unsaturated monomer aqueous solution. The viscosity of the first-stage water-soluble ethylenically unsaturated monomer aqueous solution prepared at this time was 8.7 mPa s. The aqueous solution prepared above was added to a separable flask with the stirrer rotation speed set to 140 rpm (flat blade (a lattice-shaped blade having a width of 58 mm, a maximum height of 104 mm, and a total of four 65 mm slits in the vertical direction) tip peripheral speed: 0.49 [m / s]), and the mixture was stirred for 10 minutes to carry out a primary dispersion step. Meanwhile, in a 20 mL vial, 0.736 g of sucrose stearate (Ryoto Sugar Ester S-370, Mitsubishi Chemical Foods Corporation) as a surfactant was dissolved in 6.62 g of n-heptane by heating to prepare a surfactant solution. This surfactant solution was added to the obtained primary dispersion, and the mixture was stirred at a stirrer rotation speed set to 300 rpm (flat blade (a Max Blend blade (a lattice-shaped blade having a width of 58 mm, a maximum height of 104 mm, and a total of four 65 mm slits in the vertical direction)) tip peripheral speed: 0.91 [m / s]). The ratio of the tip peripheral speed of the stirring blade at this time was 0.47.Thereafter, nitrogen was blown into the system to thoroughly replace the atmosphere, and the flask was immersed in a water bath at 70°C to raise the temperature, and the polymerization step was carried out for 60 minutes to obtain a first-stage polymerization slurry. The secondary dispersion step was carried out for 20 minutes.

[0128] On the other hand, 128.8 g (1.44 mol) of an 80.5 mass% aqueous acrylic acid solution was placed as a water-soluble ethylenically unsaturated monomer in another 500 mL beaker, and while cooling with ice water, 160.0 g of a 27 mass% aqueous sodium hydroxide solution was added dropwise to carry out 75 mol% neutralization, and then 0.103 g (0.381 mmol) of potassium persulfate as a water-soluble radical polymerization initiator and 0.0116 g (0.067 mmol) of ethylene glycol diglycidyl ether as an internal crosslinking agent were added and dissolved to prepare a second-stage aqueous water-soluble ethylenically unsaturated monomer solution.

[0129] While stirring at a stirrer rotation speed of 650 rpm (tip peripheral speed: 1.97 [m / s]), the contents of the separable flask system were cooled to 28.5°C, and then the entire amount of the second-stage water-soluble ethylenically unsaturated monomer aqueous solution was added to the first-stage polymerization slurry liquid, and nitrogen was blown into the system to replace the atmosphere with nitrogen for 30 minutes. After that, the flask was again immersed in a water bath at 70°C to raise the temperature, and a polymerization reaction was carried out for 60 minutes to obtain hydrous gel-like particles.

[0130] Thereafter, the flask was immersed in an oil bath set at 125°C, and 245.3 g of water was extracted from the system by azeotropic distillation of n-heptane and water while refluxing n-heptane. Thereafter, n-heptane was evaporated at 125°C to dry the mixture, and the mixture was passed through a sieve with an opening of 850 μm to obtain polymer particles. 0.5 parts by mass of amorphous silica (Toxil NP-S, Oriental Silicas Corporation) was mixed with 100 parts by mass of the polymer particles to obtain 217.4 g of water-absorbent resin particles (11). The water-absorbent resin particles (11) had a median particle size of 340 μm and a residual dispersion medium amount of 139 ppm.

[0131] <Comparative Example 4> The same operation as in Example 8 was carried out, except that the rotation speed of the stirrer during the primary dispersion step was changed from 140 rpm (tip peripheral speed: 0.49 [m / s]) to 300 rpm (tip peripheral speed: 0.91 [m / s]) during the primary dispersion step, to obtain 211.3 g of water-absorbent resin particles (12). In Comparative Example 4, the viscosity of the water-soluble ethylenically unsaturated monomer aqueous solution in the first stage was 8.7 mPa s, the ratio of the tip peripheral speed of the stirring blade was 0.47, and the dimensional ratio of the polymerization tank and the stirring blade was 0.53. The median particle diameter of the water-absorbent resin particles (12) was 350 μm, and the amount of residual dispersion medium was 473 ppm.

[0132]

Claims

1. A method for producing water-absorbent resin particles, comprising the following steps: (A) a primary dispersion step of stirring and mixing, in the absence of a surfactant, a hydrocarbon dispersion medium in which a hydrophobic polymer-based dispersant has been dispersed or dissolved, and an aqueous solution of a water-soluble ethylenically unsaturated monomer to prepare a first dispersion; (B) a secondary dispersion step of stirring and mixing the first dispersion obtained in the step (A) with a surfactant to prepare a second dispersion; and (C) a polymerization step of performing reverse suspension polymerization of the water-soluble ethylenically unsaturated monomer after the step (B), wherein a ratio of the tip peripheral speed of the agitator blade in the primary dispersion step to the tip peripheral speed of the agitator blade in the secondary dispersion step (tip peripheral speed of the agitator blade in the primary dispersion step / tip peripheral speed of the agitator blade in the secondary dispersion step) is greater than 0.10 and less than 0.60, and the duration of the primary dispersion step is within the range of greater than 1 minute and less than 90 minutes.

2. The method for producing water-absorbent resin particles according to claim 1, wherein the peripheral speed of the tip of the stirring blade during the primary dispersion step is 0.15 m / s or more and 1.7 m / s or less.

3. The method for producing water-absorbent resin particles according to claim 1, wherein the peripheral speed of the tip of the stirring blade during the secondary dispersion step is 0.7 m / s or more and 3.5 m / s or less.

4. The method for producing water-absorbent resin particles according to any one of claims 1 to 3, wherein in the polymerization step, reversed-phase suspension polymerization is carried out in two or more stages.

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