Method for producing water-absorbent resin particles

Abstract

This method for producing water-absorbing resin particles, which contain granulated particles, comprises: forming a first mass by agglomerating a polymer fine powder, which passes through a sieve having a 180μm mesh, by mixing the same with water; drying the first mass and forming a granulated fine powder that passes through a sieve having a 180μm mesh; forming a second mass by agglomerating particle groups, which contain the granulated fine powder, by mixing the same with water; and forming granulated particles by drying and granulating the second mass. The content ratio of the granulated fine powder in the particle groups is 80-100mass% with respect to the total particle groups.

Description

[Technical Field]

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

[0002] In the production of water-absorbent resin particles, a step is carried out in which a block-shaped or coarse-particle polymer obtained by polymerization is pulverized to form particles. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2005-54151 Summary of the Invention [Problem to be solved by the invention]

[0004] For use as water-absorbent resin particles, there is a suitable particle size range, for example, 180 to 850 μm. However, in the production of water-absorbent resin particles, when a polymer is pulverized, not only particles having the desired particle size but also fine powder having a particle size smaller than the desired size is generated. The fine powder is used as granulated particles by increasing the particle size through granulation (for example, Patent Document 1). Granulated particles produced using the fine powder are also required to have a sufficiently high water absorption rate.

[0005] One aspect of the present disclosure relates to providing a production method capable of obtaining water-absorbent resin particles containing granulated particles having a superior water absorption rate compared to a case where only fine powder that has not undergone an aggregation step is granulated. [Means for solving the problem]

[0006] One aspect of the present disclosure relates to a method for producing water-absorbent resin particles containing granulated particles, the method comprising: mixing a polymer fine powder that passes through a sieve with 180 μm openings with water to agglomerate the polymer fine powder and form a first agglomerate; drying and pulverizing the first agglomerate to form a granulated fine powder that passes through a sieve with 180 μm openings; mixing a particle group containing the granulated fine powder with water to agglomerate the particle group and form a second agglomerate; and drying and pulverizing the second agglomerate to form granulated particles, wherein the content of the granulated fine powder in the particle group is 80 mass % or more and 100 mass % or less with respect to the total amount of the particle group.

[0007] In the above production method, the particle groups may further contain more than 0 mass % and 20 mass % or less of polymer fine powder that passes through a sieve with 180 μm openings, based on the total amount of the particle groups.

[0008] In the above production method, the polymer fine powder contained in the particle group may be a fine powder excluding a fine powder formed by a method including mixing the polymer fine powder with water to agglomerate the polymer fine powder to form lumps, and drying and pulverizing the lumps.

[0009] The production method may further include classifying the granulated particles.

[0010] In the above production method, the content of fine powder that passes through a sieve with 180 μm openings but does not pass through a sieve with 150 μm openings in the granulated fine powder may be 30 mass % or less based on the total amount of the granulated fine powder. [Effects of the Invention]

[0011] According to one aspect of the present disclosure, it is possible to provide a production method capable of obtaining water-absorbent resin particles containing granulated particles having a superior water absorption rate compared to a case where only fine powder that has not undergone an aggregation step is granulated. DETAILED DESCRIPTION OF THE INVENTION

[0012] In this specification, "(meth)acrylic" refers to both acrylic and methacrylic. "Acrylate" and "methacrylate" are also written as "(meth)acrylate." The same applies to other similar terms. "(Poly)" refers to both cases with and without the prefix "poly." In the numerical ranges described 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 that numerical range may be replaced with a value shown in the Examples. "Water-soluble" refers to a solubility of 5% by mass or more in water at 25°C. The materials exemplified in this specification may be used alone or in combination of two or more. When multiple substances corresponding to each component are present in the composition, the content of each component in the composition refers to the total amount of those multiple substances present in the composition, unless otherwise specified. "Saline" refers to a 0.9% by mass aqueous sodium chloride solution. The sieve refers to a JIS standard sieve.

[0013] The method for producing water-absorbent resin particles containing granulated particles according to this embodiment includes: mixing a polymer fine powder that passes through a sieve with 180 μm openings with water to agglomerate the polymer fine powder and form a first agglomerate; drying and pulverizing the first agglomerate to form a granulated fine powder that passes through a sieve with 180 μm openings; mixing a particle group containing the granulated fine powder in a content of 80 mass % or more and 100 mass % or less with respect to the total amount of the particle group with water to agglomerate the particle group and form a second agglomerate; and drying and pulverizing the second agglomerate to form granulated particles. In other words, a method for producing water-absorbent resin particles may include the steps of mixing a polymer fine powder that passes through a sieve with 180 μm openings with water to agglomerate the polymer fine powder to obtain agglomerates (first agglomerates), drying and pulverizing the agglomerates to obtain granulated fine powder that passes through a sieve with 180 μm openings, mixing a particle group containing the granulated fine powder at a content of 80 mass % or more and 100 mass % or less with water to agglomerate the particle group again to obtain agglomerates (second agglomerates), and drying and pulverizing the agglomerates obtained again to obtain granulated particles.

[0014] The manufacturing method of the present disclosure is characterized in that granulated fine powder generated when granulating fine powder to form particles is used again for granulation, and the particle group used during regranulation contains granulated fine powder at a high ratio. In this specification, granulation refers to agglomerating particles to obtain particles having a larger particle diameter than the original particles. According to the manufacturing method of the present invention, water-absorbent resin particles having a faster water absorption rate can be manufactured compared to a case where fine powder (primary fine powder) generated in the manufacturing process of water-absorbent resin particles and not subjected to an aggregation step is granulated alone.

[0015] [Polymer fine powder] An example of a method for obtaining polymer fine powder, which is the raw material for the granulated fine powder, will be described. The polymer fine powder is fine powder that passes through a sieve with 180 μm mesh, and is used to form the first agglomerates. The polymer fine powder used to form the first agglomerates may be, for example, a fine powder that has not undergone an aggregation process (hereinafter also referred to as "primary fine powder"), or may be a granulated fine powder that has undergone the aggregation process described below. The primary fine powder is a fine powder generated during the production of polymer particles used in water-absorbent resin particles, and may be, for example, a fine powder generated when a block-shaped or coarse-particle polymer is pulverized and granulated. The primary fine powder may be a fine powder other than that formed by a method comprising mixing the polymer fine powder with water to agglomerate the polymer fine powder to form agglomerates, and then drying and pulverizing the agglomerates. The primary fine powder can be obtained, for example, by polymerizing a monomer to obtain a hydrogel polymer, followed by drying, pulverizing, and classifying the hydrogel polymer. An example of a method for obtaining the primary fine powder will be described in detail below.

[0016] (Polymerization process) First, a monomer containing an ethylenically unsaturated monomer is polymerized to obtain a hydrogel polymer. The hydrogel polymer may be a crosslinked polymer formed by polymerization of a monomer containing an ethylenically unsaturated monomer, which has absorbed water and become gel-like. The water-absorbent resin particles obtained by the production method according to this embodiment may contain a crosslinked polymer formed by polymerization of a monomer containing an ethylenically unsaturated monomer. The crosslinked polymer has a monomer unit derived from the ethylenically unsaturated monomer. That is, the water-absorbent resin particles according to this embodiment may have a structural unit derived from the ethylenically unsaturated monomer.

[0017] The polymerization can be carried out by, for example, aqueous solution polymerization. Polymerization of the monomer by aqueous solution polymerization will be described below.

[0018] The ethylenically unsaturated monomer may be water-soluble. Examples of the ethylenically unsaturated monomer include carboxylic acid monomers such as (meth)acrylic acid, maleic acid, maleic anhydride, fumaric 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; amino group-containing unsaturated monomers and quaternized products thereof such as N,N-diethylaminoethyl(meth)acrylate, N,N-diethylaminopropyl(meth)acrylate, and diethylaminopropyl(meth)acrylamide; and sulfonic acid monomers such as vinyl sulfonic acid, styrene sulfonic acid, 2-(meth)acrylamido-2-methylpropanesulfonic acid, 2-(meth)acryloylethanesulfonic acid, and salts thereof. The ethylenically unsaturated monomers may be used alone or in combination of two or more.

[0019] The ethylenically unsaturated monomer may include at least one selected from the group consisting of (meth)acrylic acid and its salts, maleic acid, fumaric acid, (meth)acrylamide, and N,N-dimethylacrylamide, or at least one selected from (meth)acrylic acid and its salts. (Meth)acrylic acid and its salts may be copolymerized with other ethylenically unsaturated monomers. In this case, the proportion of the (meth)acrylic acid and its salts in the total amount of ethylenically unsaturated monomers may be 70 to 100 mol%, 80 to 100 mol%, or 90 to 100 mol%. The ethylenically unsaturated monomer may include at least one of (meth)acrylic acid and its salts.

[0020] When the 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 neutralizer, if necessary. Examples of such alkaline neutralizers include alkali metal salts such as sodium hydroxide, sodium carbonate, sodium bicarbonate, potassium hydroxide, and potassium carbonate; and ammonia. These alkaline neutralizers may be used in the form of an aqueous solution to simplify the neutralization procedure. One type of alkaline neutralizer may be used alone, or two or more types may be used in combination. Neutralization of the acid group may be carried out before, during, or after polymerization of the ethylenically unsaturated monomer, which is the raw material.

[0021] The degree of neutralization of the ethylenically unsaturated monomer with the alkaline neutralizing agent may be usually 10 to 100 mol%, 30 to 90 mol%, 40 to 85 mol%, or 50 to 80 mol%, from the viewpoint of increasing the osmotic pressure of the obtained water-absorbent resin particles to thereby enhance the water absorption performance and preventing problems such as safety caused by the presence of an excess alkaline neutralizing agent. Here, the neutralization degree refers to the degree of neutralization of all acid groups possessed by the ethylenically unsaturated monomer.

[0022] The ethylenically unsaturated monomer can usually be used in the form of an aqueous solution. The concentration of the ethylenically unsaturated monomer in the aqueous solution containing the ethylenically unsaturated monomer (hereinafter simply referred to as "aqueous monomer solution") may be 20% by mass or more and the saturated concentration or less, and may be 25 to 70% by mass, or 30 to 50% by mass.

[0023] The amount of the ethylenically unsaturated monomer used may be 70 to 100 mol%, 80 to 100 mol%, 90 to 100 mol%, 95 to 100 mol%, or 100 mol% based on the total amount of monomers (the total amount of monomers for obtaining water-absorbent resin particles; for example, the total amount of monomers that provide structural units of a crosslinked polymer; the same applies hereinafter). Among these, the proportion of (meth)acrylic acid and salts thereof may be 70 to 100 mol%, 80 to 100 mol%, 90 to 100 mol%, 95 to 100 mol%, or 100 mol% based on the total amount of monomers. The "proportion of (meth)acrylic acid and salts thereof" means the proportion of the total amount of (meth)acrylic acid and salts thereof.

[0024] The water-absorbent resin particles may be, for example, water-absorbent resin particles containing a crosslinked polymer having a structural unit derived from an ethylenically unsaturated monomer, wherein the ethylenically unsaturated monomer contains at least one compound selected from the group consisting of (meth)acrylic acid and salts thereof, and the ratio of the (meth)acrylic acid and salts thereof is 70 to 100 mol % based on the total amount of monomers for obtaining the water-absorbent resin particles.

[0025] The aqueous monomer solution may contain a polymerization initiator. Polymerization of the monomers contained in the aqueous monomer solution is initiated by adding the polymerization initiator to the aqueous monomer solution and, if necessary, heating, irradiating with light, or the like. Examples of the polymerization initiator include a photopolymerization initiator or a radical polymerization initiator. The polymerization initiator may be a water-soluble radical polymerization initiator. The polymerization initiator may be, for example, an azo compound, a peroxide, or the like.

[0026] Examples of the azo compounds include 2,2'-azobis[2-(N-phenylamidino)propane]dihydrochloride, 2,2'-azobis{2-[N-(4-chlorophenyl)amidino]propane}dihydrochloride, 2,2'-azobis{2-[N-(4-hydroxyphenyl)amidino]propane}dihydrochloride, 2,2'-azobis[2-(N-benzylamidino)propane]dihydrochloride, 2,2' -Azobis[2-(N-allylamidino)propane] dihydrochloride, 2,2'-azobis(2-amidinopropane) dihydrochloride, 2,2'-azobis{2-[N-(2-hydroxyethyl)amidino]propane} dihydrochloride, 2,2'-azobis[2-(5-methyl-2-imidazolin-2-yl)propane] dihydrochloride, 2,2'-azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride ] dihydrochloride, 2,2'-azobis[2-(4,5,6,7-tetrahydro-1H-1,3-diazepin-2-yl)propane] dihydrochloride, 2,2'-azobis[2-(5-hydroxy-3,4,5,6-tetrahydropyrimidin-2-yl)propane] dihydrochloride, 2,2'-azobis{2-[1-(2-hydroxyethyl)-2-imidazolin-2-yl]propane} dihydrochloride, 2 Examples of azo compounds include 2,2'-azobis(2-methylpropionamide) dihydrochloride, 2,2'-azobis[2-(2-imidazolin-2-yl)propane]disulfate dihydrate, 2,2'-azobis[N-(2-carboxyethyl)-2-methylpropionamidine]tetrahydrate, and 2,2'-azobis[2-methyl-N-(2-hydroxyethyl)propionamide]. From the viewpoint of facilitating the production of water-absorbent resin particles having good water absorption performance, the azo compound may be 2,2'-azobis(2-amidinopropane) dihydrochloride, 2,2'-azobis{2-[1-(2-hydroxyethyl)-2-imidazolin-2-yl]propane}dihydrochloride, or 2,2'-azobis[N-(2-carboxyethyl)-2-methylpropionamidine]tetrahydrate. These azo compounds may be used alone or in combination of two or more.

[0027] Examples of peroxides include persulfates such as potassium persulfate, ammonium persulfate, and sodium persulfate; organic peroxides such as methyl ethyl ketone peroxide, methyl isobutyl ketone peroxide, di-t-butyl peroxide, t-butylcumyl peroxide, t-butyl peroxyacetate, t-butyl peroxyisobutyrate, and t-butyl peroxypivalate; and peroxides such as hydrogen peroxide. Among these peroxides, from the viewpoint of obtaining water-absorbent resin particles having good water absorption performance, potassium persulfate, ammonium persulfate, sodium persulfate, or hydrogen peroxide may be used, or potassium persulfate, ammonium persulfate, or sodium persulfate may be used. These peroxides may be used alone or in combination of two or more.

[0028] A polymerization initiator and a reducing agent may be used in combination as a redox polymerization initiator. Examples of reducing agents include sodium sulfite, sodium hydrogen sulfite, ferrous sulfate, and L-ascorbic acid.

[0029] From the viewpoint of the balance of performance such as CRC (absorption capacity without load) of the polymer fine powder, the crosslinked polymer particles, and the water-absorbent resin particles, the amount of the polymerization initiator may be 0.05 to 1 mmol, 0.08 to 0.8 mmol, or 0.1 to 0.7 mmol relative to 1 mol of the monomer.

[0030] The aqueous monomer solution may contain an internal crosslinking agent. By containing the internal crosslinking agent, the resulting crosslinked polymer can have, as its internal crosslinked structure, crosslinks caused by the internal crosslinking agent in addition to self-crosslinks caused by the polymerization reaction.

[0031] The internal crosslinking agent may include a compound having a (meth)acrylic group, an allyl group, an epoxy group, or an amino group. Compounds having two or more of these reactive functional groups can be used as the internal crosslinking agent. Examples of compounds having a (meth)acrylic group include (poly)ethylene glycol di(meth)acrylate, trimethylolpropane tri(meth)acrylate (poly)propylene glycol di(meth)acrylate, glycerol tri(meth)acrylate, trimethylolpropane di(meth)acrylate, and N,N'-methylenebis(meth)acrylamide. Examples of compounds having an allyl group include triallylamine. Examples of compounds having an epoxy group include (poly)ethylene glycol diglycidyl ether, (poly)propylene glycol diglycidyl ether, (poly)glycerin diglycidyl ether, and epichlorohydrin. Examples of compounds having an amino group include triethylenetetramine, ethylenediamine, and hexamethylenediamine. The internal crosslinking agent may be used alone or in combination of two or more kinds.

[0032] When an internal crosslinking agent is used, the amount used may be 0.02 to 1.0 mmol, 0.05 to 0.8 mmol, or 0.1 to 0.6 mmol per mol of the monomer, from the viewpoint of adjusting the balance of performance such as CRC in the polymer fine powder, crosslinked polymer particles, and water-absorbent resin particles.

[0033] The aqueous monomer solution may contain additives such as a chain transfer agent and a thickener, as needed. Examples of chain transfer agents include thiols, thiolic acids, secondary alcohols, hypophosphorous acid, and phosphorous acid. Examples of thickeners include carboxymethyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, methyl cellulose, polyethylene glycol, polyacrylic acid, neutralized polyacrylic acid, and polyacrylamide. These may be used alone or in combination of two or more. The aqueous monomer solution may contain, as appropriate, a solvent other than water, such as a water-soluble organic solvent.

[0034] The polymerization method may be, for example, a static polymerization method in which the aqueous monomer solution is polymerized without stirring (for example, in a static state), or an agitation polymerization method in which the aqueous monomer solution is polymerized while being stirred in a reaction vessel. A hydrogel polymer may also be obtained by static polymerization of an aqueous solution, which is a static polymerization method. In the static polymerization method, a single block-shaped hydrogel polymer is obtained that occupies approximately the same volume as the aqueous monomer solution present in the reaction vessel upon completion of polymerization.

[0035] The production form may be batchwise, semi-continuous, continuous, etc. For example, in the case of continuous static polymerization of an aqueous solution, a polymerization reaction is carried out while continuously supplying an aqueous monomer solution to a belt conveyor-type continuous polymerization apparatus, and a hydrogel having a continuous shape, for example, a strip shape, can be obtained.

[0036] The polymerization temperature varies depending on the polymerization initiator used, but from the viewpoint of rapidly progressing the polymerization and shortening the polymerization time to increase productivity, it may be 0 to 130°C or 10 to 110°C. The polymerization time is appropriately set depending on the type or amount of the polymerization initiator used, the reaction temperature, etc., but may be 1 to 200 minutes or 5 to 100 minutes.

[0037] The hydrogel polymer may be crushed before drying. The crushed material obtained by crushing the hydrogel polymer may be in the form of particles, or may have an elongated shape like a series of particles. The minimum side size of the crushed material may be, for example, about 0.1 to 15 mm, or about 1.0 to 10 mm. The maximum side size of the crushed material may be about 0.1 to 200 mm, or about 1.0 to 150 mm. The crushing device may be, for example, a kneader (e.g., a pressure kneader, a double-arm kneader, etc.), a meat chopper, a cutter mill, a farmer mill, etc., and may be a double-arm kneader, a meat chopper, or a cutter mill.

[0038] (drying process) A dried product can be obtained by removing the water-containing solvent from the bulk hydrogel polymer or its crushed product by heating and / or blowing air. The drying method may be natural drying, heat drying, or reduced-pressure drying. Drying may be performed, for example, under normal pressure or reduced pressure, or may be performed under a nitrogen or other gas stream to improve drying efficiency. A combination of drying methods may be used. The heating temperature for drying under normal pressure may be, for example, 70 to 250°C, or 80 to 200°C. The moisture content of the dried product obtained by drying may be, for example, 20% by mass or less, 10% by mass or less, or 5% by mass or less, or 0% by mass or more, or 1% by mass or more. The moisture content of the dried product obtained by drying may be, for example, 0 to 20% by mass, or 1 to 10% by mass.

[0039] (Crushing process) The dried product is then pulverized to obtain particle groups containing fine powder. Pulverization can be performed using a mill such as a roller mill (roll mill), stamp mill, jet mill, high-speed rotary mill (ultracentrifugal mill, hammer mill, pin mill, rotor beater mill, etc.), or container-driven mill (rotary mill, vibration mill, planetary mill, etc.). The mill may have an opening on the outlet side, such as a perforated plate, screen, or grid, that controls the maximum particle size of the pulverized particles. The shape of the opening may be polygonal, circular, or the like, and the maximum diameter of the opening may be 0.1 to 5 mm, 0.3 to 3.0 mm, or 0.5 to 1.5 mm.

[0040] The pulverization may be carried out so that at least a portion of the particle group becomes fine powder having a particle size that can pass through a sieve with an opening of 180 μm. For example, the pulverization may be carried out in a manner that produces fine powder having a particle size that can pass through a sieve with an opening of 180 μm while pulverizing mainly to obtain polymer particles having an appropriate particle size of about 180 μm or more and less than 850 μm.

[0041] (Classification process) Of the particles obtained in the above-mentioned polymer particle production process, fine powder that passes through a sieve with 180 μm openings can be used as primary fine powder. For example, if the particles obtained in the above-mentioned pulverization process include particles that do not pass through a sieve with 180 μm openings, the particle group can be classified using the sieve to obtain primary fine powder that passes through a sieve with 180 μm openings. Classification refers to the operation of dividing a particle group into two or more particle groups with different particle size distributions according to particle diameter.

[0042] The classification method can be a known classification method, such as screen classification or air classification. Screen classification is a method of classifying particles on a screen into particles that pass through the meshes of the screen and particles that do not by vibrating the screen. Screen classification can be performed using, for example, a vibrating sieve, a rotary sifter, a cylindrical stirring sieve, a blower sifter, or a rotary shaker. Air classification is a method of classifying particles by utilizing air flow. In this specification, "particles that pass through a sieve with an opening of 180 μm" refers to particles having such a size, and is not limited to classification using a sieve.

[0043] The CRC of the primary fine powder may be, for example, 65 g / g or less, 60 g / g or less, or 58 g / g or less, or 30 g / g or more, 33 g / g or more, 35 g / g or more, 37 g / g or more, 41 g / g or more, 45 g / g or more, or 50 g / g or more. The CRC of the primary fine powder may be 30 to 65 g / g, 33 to 60 g / g, or 35 to 58 g / g. The CRC is measured by the method described in the Examples below, with reference to the EDANA method (NWSP 241.0.R2(15), pages 769-778).

[0044] [Granulated fine powder] The granulated fine powder used in the production method according to this embodiment is included as part of granulated particles obtained by mixing polymer fine powder that passes through a 180 μm sieve with water to agglomerate the polymer fine powder to form first lumps, and then drying and pulverizing the first lumps. The granulated fine powder used in the production method according to this embodiment is granulated particles that pass through a 180 μm sieve. In this specification, the steps from mixing the fine powder, agglomerating the fine powder (and cutting it as necessary), and drying and pulverizing the first lumps are sometimes collectively referred to as the granulation step. The method for producing the granulated fine powder will be described in detail below.

[0045] (Agglutination process) The agglomeration step is a step in which the polymer fine powder is mixed with water to agglomerate the polymer fine powder and form agglomerates. The formed agglomerates are dried and pulverized to form a granulated fine powder. The polymer fine powder used to produce the granulated fine powder may be, for example, the above-mentioned primary fine powder, a granulated fine powder obtained by subjecting the primary fine powder to an agglomeration step, or a mixture thereof. The granulated fine powder formed by drying and pulverizing the first agglomerates may be a granulated fine powder obtained by using the primary fine powder.

[0046] The water to be mixed with the polymer fine powder may contain components such as water-soluble salts, water-soluble polymerizable monomers such as ethylenically unsaturated monomers, crosslinking agents, or hydrophilic organic solvents. That is, the water may be mixed with the polymer fine powder in the form of an aqueous liquid containing other components added to the water. The proportion of water in the aqueous liquid may be, for example, 80 to 100% by mass. As the crosslinking agent, for example, the same crosslinking agents as the internal crosslinking agents described above or the surface crosslinking agents described below can be used.

[0047] The temperature when mixing the polymer fine powder and water may be, for example, 30 to 150°C, 60 to 110°C, or 80 to 100°C. The amount of water mixed with the polymer fine powder may be, for example, 10 parts by mass or more, 30 parts by mass or more, 50 parts by mass or more, 80 parts by mass or more, or 90 parts by mass or more, and 200 parts by mass or less, 150 parts by mass or less, 130 parts by mass or less, or 100 parts by mass or less, relative to 100 parts by mass of the raw polymer fine powder. The amount of water mixed with the polymer fine powder may be 10 to 200 parts by mass, 30 to 150 parts by mass, 50 to 130 parts by mass, or 80 to 100 parts by mass, relative to 100 parts by mass of the raw polymer fine powder. When mixing the polymer fine powder and water, for example, water may be added dropwise to the polymer fine powder, the entire amount of water may be added at once, water may be sprayed, or the water may be mixed in the form of steam.

[0048] The polymer fine powder and water can be mixed, for example, using various agitators equipped with agitating blades. Examples of agitating blades that can be used include flat blades, lattice blades, paddle blades, propeller blades, anchor blades, turbine blades, Pfaudler blades, ribbon blades, full zone blades, and Max Blend blades. A flat blade has a shaft (stirring shaft) and a flat plate portion (stirring portion) arranged around the shaft. Furthermore, the flat plate portion may have slits or the like. Using flat blades as agitating blades allows for more uniform granulation, and tends to produce relatively large granulated particle groups. Examples of agitating mixers include mortar mixers, double-arm kneaders, continuous kneaders, and Lödige mixers.

[0049] From the viewpoint of more uniform mixing, the time for mixing the polymer fine powder and water may be 30 to 150 seconds or 60 to 120 seconds after the total amounts of the polymer fine powder and water are charged into the same container.

[0050] By mixing the polymer fine powder with water, the polymer fine powder aggregates with itself, and a first agglomerate can be obtained.

[0051] (Drying and crushing of lumps) Next, the obtained first lumps are dried. In order to improve drying efficiency, the first lumps may be cut into pieces of about 3 to 10 mm before drying. The drying, pulverization, and classification of the first lumps can be carried out in the same manner as the drying, pulverization, and classification steps in the production of the primary fine powder described above. After granulation, drying and pulverization, and classification as necessary, can be carried out to obtain a fine powder that passes through a sieve with 180 μm openings, i.e., a granulated fine powder.

[0052] The size of the granulated fine powder may be such that it passes through a sieve with 180 μm openings, but the content of fine powder that passes through a sieve with 180 μm openings but does not pass through a sieve with 150 μm openings may be 30% by mass or less, or 25% by mass or less, relative to the total amount of the granulated fine powder. When the content of fine powder that does not pass through a sieve with 150 μm openings in the granulated fine powder is below a certain level, it is thought that the water absorption rate of the obtained granulated particles or water-absorbent resin particles tends to be increased. The content of fine powder that passes through a sieve with 180 μm openings but does not pass through a sieve with 150 μm openings in the granulated fine powder may be, for example, 10% by mass or more, or 15% by mass or more, relative to the total amount of the granulated fine powder. The content of fine powder in the granulated fine powder that passes through a sieve with an opening of 180 μm but does not pass through a sieve with an opening of 150 μm may be 10% by mass or more and 30% by mass or less, or 25% by mass or less, or 15% by mass or more and 30% by mass or less, or 25% by mass or less, relative to the total amount of the granulated fine powder.

[0053] The proportion of particles in the granulated fine powder that pass through a sieve with 150 μm openings may be, for example, 70% by mass or more, or 75% by mass or more, or 90% by mass or less, or 85% by mass or less, relative to the total amount of the granulated fine powder. The proportion of particles in the granulated fine powder that pass through a sieve with 150 μm openings may be 70 to 90% by mass or 75 to 85% by mass, relative to the total amount of the granulated fine powder.

[0054] [Agglomeration of granulated fine powder] In the method for producing water-absorbent resin particles according to this embodiment, aggregation (hereinafter also referred to as "main aggregation") is carried out using particle groups containing the above-mentioned granulated fine powder. The production method according to this embodiment includes mixing the particle groups containing the granulated fine powder with water to aggregate the particle groups, thereby obtaining a second aggregate. The content of the granulated fine powder in the particle groups used for the main aggregation is 80% by mass or more and 100% by mass or less with respect to the total amount of the particle groups.

[0055] The content of the granulated fine powder in the particle groups used for main aggregation may be 83 mass% or more, 85 mass% or more, 88 mass% or more, 90 mass% or more, 93 mass% or more, 95 mass% or more, or 98 mass% or more, based on the total amount of the particle groups. The content of the granulated fine powder in the particle groups used for main aggregation may be 99 mass% or less, 98 mass% or less, or 95 mass% or less, based on the total amount of the particle groups. The content of the granulated fine powder in the particle groups used for main aggregation may be 83% by mass or more and 99% by mass or less, 98% by mass or less, or 95% by mass or less, 85% by mass or more and 99% by mass or less, 98% by mass or less, or 95% by mass or less, 88% by mass or more and 99% by mass or less, 98% by mass or less, or 95% by mass or less, 90% by mass or more and 99% by mass or less, 98% by mass or less, or 95% by mass or less, 93% by mass or more and 99% by mass or less, 98% by mass or less, 95% by mass or more and 99% by mass or less, or 98% by mass or less, or 98% by mass or more and 99% by mass or less.

[0056] The particle groups used for the primary aggregation may contain the above-mentioned primary fine powder (polymer fine powder that has not undergone an aggregation process and passes through a sieve with an opening of 180 μm). When the particle groups contain primary fine powder, the content of the primary fine powder relative to the total amount of the particle groups may be, for example, more than 0% by mass, 3% by mass or more, 5% by mass or more, 8% by mass or more, 10% by mass or more, or 15% by mass or more. The content of the primary fine powder relative to the total amount of the particle groups may be, for example, 20% by mass or less, 18% by mass or less, 15% by mass or less, 13% by mass or less, 10% by mass or less, 8% by mass or less, or 5% by mass or less. The content of primary fine powder relative to the total amount of particle groups may be greater than 0% by mass and less than 20%, 18%, 15%, 13%, 10%, 8%, or 5% by mass; may be 3% by mass or more and less than 20%, 18%, 15%, 13%, 10%, 8%, or 5% by mass; may be 5% by mass or more and less than 20%, 18%, 15%, 13%, 10%, or 8% by mass; may be 8% by mass or more and less than 20%, 18%, 15%, 13%, or 10% by mass; may be 10% by mass or more and less than 20%, 18%, 15%, or 13% by mass; may be 15% by mass or more and less than 20%, 15%, 13 ...8%, 15%, or 13% by mass; may be 15% by mass or more and less than 20%, 15%, 15%, or 13% by mass; may be 15% by mass or more and less than 20%, 18%, 15%, or 13% by mass;

[0057] The particles used for main aggregation may contain polymer particles other than the primary fine powder and granulated fine powder. The total amount of the primary fine powder and granulated fine powder may be 90% by mass or more, 92% by mass or more, 95% by mass or more, 98% by mass or more, 99% by mass or more, or 100% by mass, based on the total amount of the particles used for main aggregation. The total amount of the primary fine powder and granulated fine powder may be 100% by mass or less, or 99% by mass or less, based on the total amount of the particles used for main aggregation. The total amount of the primary fine powder and granulated fine powder may be 90 to 100% by mass, based on the total amount of the particles used for main aggregation.

[0058] The method of mixing particle groups containing granulated fine powder with water to agglomerate the particle groups to obtain second lumps, and the method of optionally cutting the second lumps, drying, and pulverizing them can be similar to the method for producing the granulated fine powder described above. The granulated fine powder may be a combination of different primary particles, for example, fine powders obtained under different production conditions, or fine powders obtained in different production lots under similar production conditions. When the particle groups contain primary fine powder or other polymer particles in addition to the granulated fine powder, or when a mixture of multiple types of granulated fine powders is used, the particle groups may be uniformly mixed in advance before mixing with water.

[0059]

[0043] The production method according to the present embodiment can increase the water absorption rate of the obtained granulated particles or water-absorbent resin particles, compared with the case of granulation using only a primary fine powder. Generally, the water absorption rate tends to decrease as the median particle diameter of water-absorbent resin particles increases, but the water-absorbent resin particles obtained by the production method according to the present embodiment have a high water absorption rate relative to the size of the median particle diameter.

[0060] [Classification process] The granulated particles obtained by pulverization may be classified as needed. That is, the method for producing water-absorbent resin particles according to the present embodiment may include a step of classifying the granulated particles. If needed, the particles after classification may be pulverized again, and a plurality of classification steps may be performed, such as repeating the pulverization step and the classification step, or the classification step may be performed after the surface cross-linking step described below. The classification may be performed in the same manner as the classification method in the method for producing the primary fine powder or the granulated fine powder described above.

[0061] [Surface crosslinking] The method for producing water-absorbent resin particles may include a step of surface-crosslinking the granulated particles. That is, the granulated particles contained in the water-absorbent resin particles obtained by the production method according to this embodiment may be surface-crosslinked. Surface crosslinking can be performed, for example, by adding a crosslinking agent (surface crosslinking agent) for surface crosslinking to the granulated particles and allowing them to react. The addition of the surface crosslinking agent may be performed at any timing after pulverizing the second agglomerates obtained by primary aggregation, and may be performed before or after classification. By adding the surface crosslinking agent and performing the surface crosslinking treatment, the crosslink density in the vicinity of the surface of the granulated particles is increased, and therefore the water absorption performance of the obtained water-absorbent resin particles can be improved.

[0062] The addition of the surface cross-linking agent to the granulated particles can be carried out, for example, by adding a surface cross-linking agent solution or by spraying the surface cross-linking agent solution. From the viewpoint of uniformly dispersing the surface cross-linking agent on the surface of the granulated particles, the surface cross-linking agent may be dissolved in a solvent such as water and / or alcohol, and the surface cross-linking agent solution may be added to the granulated particles. The surface cross-linking step may be carried out once or in multiple divided steps of two or more.

[0063] The surface cross-linking agent may contain, for example, two or more functional groups (reactive functional groups) having reactivity with a functional group derived from an ethylenically unsaturated monomer. Examples of the surface cross-linking 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, (poly)propylene glycol polyglycidyl ether, and (poly)glycerol polyglycidyl ether; haloepoxy compounds such as epichlorohydrin, epibromohydrin, and α-methylepichlorohydrin; 2,4 -Compounds having two or more reactive functional groups, such as isocyanate compounds such as 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. The surface cross-linking agent may contain a polyglycidyl compound 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, or (poly)glycerol polyglycidyl ether, and / or a polyol such as ethylene glycol, propylene glycol, 1,4-butanediol, trimethylolpropane, polyoxyethylene glycol, or polyoxypropylene glycol, or may contain a polyglycidyl compound. These surface cross-linking agents may be used alone or in combination of two or more. For example, a polyglycidyl compound may be used in combination with a polyol.

[0064] The amount of the surface cross-linking agent added may be usually 0.0001 to 4.0 mol, or 0.001 to 2.0 mol, relative to 100 mol of the total amount of the ethylenically unsaturated monomers used in the polymerization, from the viewpoint of appropriately increasing the cross-linking density in the vicinity of the surface of the water absorbent resin particles (granulated particles).

[0065] The surface cross-linking step may be carried out in the presence of water in the range of 1 to 200 parts by mass relative to 100 parts by mass of the ethylenically unsaturated monomer. The amount of water can be adjusted by appropriately using water and / or a water-soluble organic solvent such as alcohol. By adjusting the amount of water during the surface cross-linking step, cross-linking can be more appropriately carried out in the vicinity of the particle surface of the water-absorbent resin particles (granulated particles).

[0066] The treatment temperature with the surface cross-linking agent is appropriately set depending on the surface cross-linking agent used, and may be 20 to 250° C. The treatment time with the surface cross-linking agent may be 1 to 200 minutes or 5 to 100 minutes. The surface cross-linking may be carried out only once or at multiple times.

[0067] [Water-absorbing resin particles] The water-absorbent resin particles obtained by the production method according to this embodiment include the above-mentioned granulated particles. The water-absorbent resin particles may consist of only granulated particles, or may further include additional components such as a gel stabilizer, a metal chelating agent (ethylenediaminetetraacetic acid and its salts, diethylenetriaminepentaacetic acid and its salts, for example, diethylenetriaminepentaacetic acid pentasodium, etc.), a flowability improver (lubricant), etc. The additional components may be disposed inside the granulated particles, on the surface thereof, or both.

[0068] The water-absorbent resin particles may include a plurality of inorganic particles arranged on the surfaces of the granulated particles. The production method according to the present embodiment may further include a step of attaching the inorganic particles to the surfaces of the granulated particles.

[0069] The shape of the granulated particles or water-absorbent resin particles obtained by the production method according to this embodiment may be, for example, crushed or formed by agglomeration of crushed particles. The median particle diameter of the granulated particles or water-absorbent resin particles may be 250 μm or more, 280 μm or more, 300 μm or more, 320 μm or more, or 340 μm or more, and may be 850 μm or less, 800 μm or less, 750 μm or less, 700 μm or less, 650 μm or less, 600 μm or less, 550 μm or less, 500 μm or less, 450 μm or less, 420 μm or less, 400 μm or less, or 380 μm or less. The median particle diameter of the granulated particles or water-absorbent resin particles may be 250 to 850 μm, 280 to 750 μm, 300 to 650 μm, 320 to 550 μm, or 340 to 450 μm.

[0070] The water absorption rate of the water-absorbent resin particles obtained by the production method according to this embodiment may be, for example, 42 seconds or less, 40 seconds or less, 38 seconds or less, 37 seconds or less, 36 seconds or less, 35 seconds or less, 33 seconds or less, or 31 seconds or less, or 27 seconds or more, 29 seconds or more, 31 seconds or more, or 33 seconds or more. The water absorption rate of the water-absorbent resin particles may be 27 to 42 seconds or 29 to 40 seconds. A method for measuring the water absorption rate will be shown in the Examples described later.

[0071] The CRC of the granulated particles or water-absorbent resin particles obtained by the production method according to the present embodiment may be, for example, 30 g / g or more, 35 g / g or more, 38 g / g or more, 40 g / g or more, 43 g / g or more, 45 g / g or more, 48 g / g or more, 50 g / g or more, 55 g / g or more, or 60 g / g or more, and may be 68 g / g or less, 65 g / g or less, 62 g / g or less, 60 g / g or less, 58 g / g or less, 55 g / g or less, 53 g / g or less, or 50 g / g or less. The CRC of the granulated particles or water-absorbent resin particles may be 30 to 68 g / g, 35 to 68 g / g, 38 to 65 g / g, or 40 to 65 g / g.

[0072]

[0113] The water absorbent resin particles obtained by the production method according to the present embodiment have excellent water absorbency, and can be used in the fields of, for example, hygiene materials such as disposable diapers and sanitary products, agricultural and horticultural materials such as water retention agents and soil conditioners, industrial materials such as water stopping agents and anti-condensation agents, etc. [Example]

[0073] The present invention will be described in more detail below with reference to examples, although the present invention is not limited to these examples.

[0074] The particles produced in the following examples were measured for centrifuge retention capacity (CRC), water absorption rate, median particle size, and fines particle size distribution.

[0075] (Centrifuge holding capacity) The centrifuge retention capacity (CRC) was measured using the following procedure, with reference to the EDANA method (NWSP 241.0.R2(15), pages 769-778). Measurements were performed in an environment with a temperature of 25°C ± 2°C and a humidity of 50% ± 10%. The measurement targets were the primary fine powder and the fraction of granulated particles that passed through a sieve with an 850 μm opening but did not pass through a sieve with an 180 μm opening. The results are shown in Tables 1 to 3.

[0076] A nonwoven fabric measuring 60 mm x 170 mm (product name: Heat Pack MWA-18, manufactured by Nippon Paper Papylia Co., Ltd.) was folded in half lengthwise to reduce the size to 60 mm x 85 mm. A 60 mm x 85 mm nonwoven fabric bag was produced by heat-sealing the nonwoven fabric together on both longitudinal edges (5 mm-wide crimped sections were formed on both longitudinal edges). 0.2 g of particles to be measured were precisely weighed and placed inside the nonwoven fabric bag. The remaining short edge was then heat-sealed to close the nonwoven fabric bag.

[0077] The nonwoven fabric bag was floated without being folded over on 1000 g of saline contained in a stainless steel tray (240 mm × 320 mm × 45 mm) to completely wet the entire nonwoven fabric bag. One minute after placing the nonwoven fabric bag in the saline, the nonwoven fabric bag was immersed in the saline with a spatula to obtain a nonwoven fabric bag containing the gel.

[0078] The nonwoven fabric bag was removed from the saline solution 30 minutes after it was placed in the saline solution (a total of 1 minute of floating time and 29 minutes of immersion time). The nonwoven fabric bag was then placed in a centrifuge (Kokusan Co., Ltd., model number: H-122). After the centrifugal force in the centrifuge reached 250 G, the nonwoven fabric bag was dehydrated for 3 minutes. After dehydration, the mass Ma [g] of the nonwoven fabric bag, including the mass of the gel, was weighed. The same procedure as described above was performed on the nonwoven fabric bag without containing the target particles, and the mass Mb [g] of the nonwoven fabric bag after dehydration was measured. The CRC [g / g] was calculated using the following formula: Mc [g] is the precisely weighed value of the mass of the target particles, 0.2 g, used in the measurement. CRC=[(Ma-Mb)-Mc] / Mc

[0079] (Water absorption rate) The water absorption rate of the particles in physiological saline was measured using the Vortex method according to the following procedure. The measurement target was the fraction of the obtained granulated particles that passed through a sieve with an 850 μm mesh size but did not pass through a sieve with an 180 μm mesh size. First, 50±0.1 g of physiological saline adjusted to 25±0.2°C in a thermostatic water bath was weighed into a 100 mL beaker. Next, a vortex was generated by stirring at 600 rpm using a magnetic stir bar (8 mmφ×30 mm, without ring). 2.0±0.002 g of water-absorbent resin particles were added all at once to the sodium chloride aqueous solution. The time (seconds) from the addition of the particles to the point at which the vortex on the liquid surface converged was measured, and this time was taken as the water absorption rate of the particles. The results are shown in Tables 1 to 3.

[0080] (median particle size) The median particle size of the particles was measured using the following procedure under an environment of 25±2°C and 50±10% humidity. The measurement target was the fraction of the obtained granulated particles that passed through a sieve with an 850 μm mesh size but did not pass through a sieve with an 180 μm mesh size. 10 g of the powder particles was sieved using a continuous, fully automatic ultrasonic vibration sieving measuring device (Robot Sifter RPS-205, manufactured by Seishin Enterprise Co., Ltd.) and JIS standard sieves with mesh sizes of 850 μm, 600 μm, 500 μm, 425 μm, 300 μm, 250 μm, and 180 μm, and a pan. The mass of the particles remaining on each sieve was calculated as a mass percentage of the total mass. The mass percentage of particles remaining on each sieve was integrated in descending order of particle size, and the relationship between the sieve opening and the integrated value of the mass percentage of particles remaining on the sieve was plotted on logarithmic probability paper. The particle size corresponding to an integrated mass percentage of 50% by mass was determined by connecting the plots on the probability paper with a straight line, and this was taken as the median particle size. The results are shown in Tables 1 to 3.

[0081] (Particle size distribution of fine powder) The particle size distribution of the granulated fine powder that passed through the 180 μm mesh sieve was further measured. JIS standard sieves (diameter 20 cm) were stacked in the following order from top to bottom: a 180 μm mesh sieve, a 150 μm mesh sieve, and a tray. The fine powder was placed on the top sieve and classified at 25°C ± 2°C in accordance with JIS Z 8815 (1994). After classification, the content (mass%) of the 150-180 μm fraction and the 150 μm passing fraction relative to the total amount of fine powder used in the measurement was calculated using the following formula based on the total amount of particles remaining on the sieves with each mesh size. Content of 150-180 μm fraction [mass%] = [(amount of fine powder that passed through a 180 μm sieve and remained on a 150 μm sieve) / (total amount of measured fine powder)] × 100 Content of 150 μm passing fraction [mass%] = [(amount of fine powder that passed through a 150 μm sieve and landed on the tray) / (total amount of fine powder measured)] × 100

[0082] (Production Example 1) [Polymerization process] 340.00 g (4.72 mol) of acrylic acid was placed in a round-bottomed cylindrical separable flask equipped with a stirrer, having an inner diameter of 11 cm and an internal volume of 2 L. While stirring the acrylic acid, 291.70 g of ion-exchanged water was added to the separable flask, and then 297.80 g of 48% by mass sodium hydroxide was added dropwise in an ice bath to prepare 929.50 g of a partially neutralized acrylic sodium acid solution with a monomer concentration of 45% by mass.

[0083] 887.31 g of the partially neutralized sodium acrylate solution was mixed with 158.24 g of ion-exchanged water and 0.309 g of polyethylene glycol diacrylate (n≒9, internal crosslinking agent, NOF Corporation, product name: Blenmer ADE-400A) to obtain a reaction solution (monomer aqueous solution). The reaction solution was placed in a fluororesin-coated 18-8 stainless steel tray (external dimensions: 297 mm x 232 mm x 50 mm height) and stirred with two stir bars (8 mm diameter, 45 mm length) to form a homogeneous mixture inside the stainless steel tray. The top of the stainless steel tray was then covered with polyethylene film. The temperature of the mixture inside 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.

[0084] Next, while the mixture was stirred at 300 rpm, 3.23 g (0.596 mmol) of a 5% by mass aqueous solution of 2,2'-azobis(2-amidinopropane) dihydrochloride (V-50) (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.), 1.70 g of a 0.5% by mass aqueous solution of L-ascorbic acid, and 1.85 g of a 0.35% by mass aqueous solution of hydrogen peroxide were added dropwise in this order using a syringe (a 10 mL disposable syringe manufactured by Terumo Corporation, with a syringe needle manufactured by Terumo Corporation). After the hydrogen peroxide solution was added dropwise, the polymerization reaction started immediately. As the polymerization reaction progressed, the viscosity of the reaction solution increased, and then the reaction solution gelled. Four minutes after the hydrogen peroxide solution was added dropwise, the thermometer indicated 103°C, and the temperature then began to drop. The stainless steel tray containing the hydrogel polymer containing water and polymer, which had formed as the reaction solution gelled, was immersed in a 75°C water bath, and the hydrogel polymer was allowed to age in that state for 20 minutes.

[0085] [Drying process] The aged hydrogel polymer was then placed in a meat chopper (12VR-750SDX) manufactured by Kiryu Royal Co., Ltd., and crushed. The diameter of the holes in the plate located at the outlet of the meat chopper was 6.4 mm. The resulting hydrogel polymer was spread on a wire mesh with 0.8 cm x 0.8 cm openings and dried with hot air at 180°C for 30 minutes to obtain a dried product.

[0086] [Crushing process] The dried product was pulverized using a centrifugal pulverizer (ZM200 manufactured by Retsch, screen diameter 1 mm, 6000 rpm) to obtain irregularly pulverized particle groups (A).

[0087] [Classification process] The particle group (A) was classified using a sieve with an opening of 850 μm and a sieve with an opening of 180 μm. By the classification, crosslinked polymer particles (A1), which are the fraction that passed through the 850 μm sieve but not the 180 μm sieve, and crosslinked polymer fine powder (primary fine powder (a1)), which is the fraction that passed through the 180 μm sieve, were obtained. The CRC of the primary fine powder (a1) was 56 g / g, the content of the 150 to 180 μm fraction was 30 mass%, and the content of the fraction that passed through the 150 μm sieve was 70 mass%.

[0088] [Granulation process] 40 g of the primary fine powder (a1) obtained above was placed in a round-bottomed cylindrical separable flask with an inner diameter of 11 cm and an internal volume of 2 L, equipped with a stirrer. The flask was used while being kept warm in a bath at 85°C. A flat blade with slits was attached to the stirrer as a stirring blade. This stirring blade had a shaft and a flat plate. The flat plate was welded to the shaft and had a curved tip. Four slits were formed in the flat plate, extending along the axial direction of the shaft. The four slits were arranged in the width direction of the flat plate, with the two inner slits being 1 cm wide and the two outer slits being 0.5 cm wide. The length of the flat plate was approximately 10 cm, and the width of the flat plate was approximately 6 cm.

[0089] While the stirring blades of the stirrer were rotating at 284 rpm, 40 g of ion-exchanged water heated to 90°C was added all at once to the flask. The primary fine powder (a1) and water were stirred in the flask for 90 seconds to obtain a lump, which was an agglomeration of the fine powder. The entire contents (lumps) were removed from the flask. Larger lumps were cut into pieces of 3 to 10 mm in size, so that the entire contents were approximately 10 mm or less in size. The entire lump, including the cut pieces, was dried with hot air at 150°C for 60 minutes to obtain a coarsely crushed and dried lump (first lump).

[0090] The coarsely crushed and dried product was pulverized using a centrifugal mill (Retsch ZM200, screen diameter 1 mm, 6000 rpm) and further classified using a sieve with an 850 μm mesh size and a sieve with an 180 μm mesh size. By classification, crosslinked polymer particles (granulated particles (A2)), which are the fraction that passed through the 850 μm sieve but not the 180 μm sieve, and crosslinked polymer fine powder (granulated fine powder (a2)), which is the fraction that passed through the 180 μm sieve, were obtained. The content of the 150 to 180 μm fraction of the granulated fine powder (a2) was 17% by mass, and the content of the fraction that passed through the 150 μm sieve was 83% by mass.

[0091] (Production Example 2) [Polymerization process] 549.71 g (7.63 mol) of acrylic acid was placed in a round-bottomed, cylindrical, separable flask equipped with a stirrer, having an inner diameter of 11 cm and an internal volume of 2 L. While stirring the acrylic acid, 470.72 g of ion-exchanged water was added to the separable flask, and then 479.57 g of 48% by mass sodium hydroxide was added dropwise in an ice bath to prepare 1500.00 g of a partially neutralized sodium acrylate solution with a monomer concentration of 45% by mass. This procedure was repeated twice to obtain the required amount of partially neutralized sodium acrylate solution.

[0092] 406.25 g of ion-exchanged water and 3.55 g of polyethylene glycol diacrylate (Blenmer ADE-400A) were added to 2781.72 g of the partially neutralized sodium acrylate solution to obtain a reaction solution (monomer aqueous solution). The reaction solution was purged with nitrogen gas under a nitrogen atmosphere for 30 minutes to adjust the dissolved oxygen content to 0.1 ppm or less. Next, a 5-L stainless steel double-arm kneader (manufactured by Irie Shokai Co., Ltd.) with a jacket, two sigma-type blades, an openable lid, a thermometer, and a nitrogen inlet tube was prepared. The reaction solution was fed into the kneader, and the atmosphere inside the kneader was purged with nitrogen gas while maintaining the reaction solution at 25°C.

[0093] Next, 92.63 g (7.78 mmol) of a 2.0% by mass aqueous solution of sodium persulfate and 15.85 g of a 0.5% by mass aqueous solution of L-ascorbic acid were added to the reaction solution while stirring. After about 1 minute, the temperature began to rise and polymerization began. After 6 minutes, the thermometer indicated a maximum temperature of 85°C. Thereafter, stirring was continued while maintaining the jacket temperature at 60°C. 60 minutes after the start of polymerization, the resulting hydrogel polymer was removed.

[0094] The extracted hydrogel polymer was cut into pieces of appropriate size, and then subjected to the same drying, pulverizing, and classification steps as in the production of the primary fine powder in Production Example 1 to obtain crosslinked polymer particles (B1), which are the fraction that passed through the 850 μm sieve but not the 180 μm sieve, and crosslinked polymer fine powder (primary fine powder (b1)), which is the fraction that passed through the 180 μm sieve. The CRC of the primary fine powder (b1) was 37 g / g, the content of the 150 to 180 μm fraction was 26 mass%, and the content of the 150 μm-passing fraction was 74 mass%.

[0095] [Granulation process] Using 40 g of the primary fine powder (b1) obtained above, the same operation as in the granulation step of Production Example 1 was carried out to obtain crosslinked polymer particles (granulated particles (B2)), which are the fraction that passed through an 850 μm sieve but not an 180 μm sieve, and crosslinked polymer fine powder (granulated fine powder (b2)), which is the fraction that passed through an 180 μm sieve. The content of the 150 to 180 μm fraction in the granulated fine powder (b2) was 22 mass%, and the content of the fraction that passed through a 150 μm sieve was 78 mass%.

[0096] <Test system 1> (Comparative Example 1) The granulated particles (A2) obtained in the granulation step of Production Example 1 were used as Comparative Example 1.

[0097] (Comparative Example 2) [Preparation of fine powders] 37.5 g of primary fine powder (a1) and 12.5 g of granulated fine powder (a2) were placed in a 100 mL mayonnaise bottle, and then the mixture was shaken for 30 minutes in a paint shaker (manufactured by Toyo Seiki Seisakusho Co., Ltd.) to mix uniformly and obtain a fine powder group (particle group).

[0098] [Granulation process] Using 40 g of the fine powder group (particle group) obtained above, second lumps were formed from the particle group by the same operation as the granulation step of Production Example 1, the second lumps were dried and pulverized, and the pulverized dried product was classified to obtain crosslinked polymer particles (granulated particles (Y1)) which were a fraction that passed through an 850 μm sieve but did not pass through an 180 μm sieve.

[0099] Example 1 The same procedure as in Comparative Example 2 was repeated except that the amount of primary fine powder (a1) used was changed to 5 g and the amount of granulated fine powder (a2) used was changed to 45 g, and crosslinked polymer particles (granulated particles (Y2)) were obtained, which were the fraction that passed through an 850 μm sieve but did not pass through an 180 μm sieve.

[0100] Example 2 [Granulation process] Using 40 g of granulated fine powder (a2), the same operation as in the granulation process of Production Example 1 was carried out to obtain crosslinked polymer particles (granulated particles (A3)), which are the fraction that passed through an 850 μm sieve but did not pass through an 180 μm sieve.

[0101] [Table 1]

[0102] In the Example in which particle groups containing a certain proportion of granulated fine powder were agglomerated, the water absorption rate of the resulting granulated particles was faster than that of Comparative Example 1 in which only primary fine powder was agglomerated. On the other hand, in Comparative Example 2 in which the content of granulated fine powder in the particle groups was low, no clear improvement in the water absorption rate compared to Comparative Example 1 was observed.

[0103] <Test System 2> (Comparative Example 3) The granulated particles (B2) obtained in the granulation step of Production Example 2 were used as Comparative Example 3.

[0104] Example 3 [Granulation process] Using 40 g of granulated fine powder (b2), the same operation as in the granulation process of Production Example 1 was carried out to obtain crosslinked polymer particles (granulated particles (B3)), which are the fraction that passed through an 850 μm sieve but did not pass through an 180 μm sieve.

[0105] [Table 2]

[0106] Even when using granulated fine powder obtained in a manufacturing example other than Test System 1, it was confirmed that by agglomerating particle groups containing a certain proportion or more of granulated fine powder, the water absorption rate of the resulting granulated particles can be increased compared to when primary fine powder is agglomerated alone.

[0107] <Test System 3> Comparative Example 4 The same procedure as in Comparative Example 2 was repeated except that the amount of primary fine powder (a1) used was changed to 12.5 g and 37.5 g of primary fine powder (b1) was used instead of primary fine powder (a1), to obtain crosslinked polymer particles (granulated particles (AB2)), which are the fraction that passed through an 850 μm sieve but did not pass through an 180 μm sieve.

[0108] Example 4 In the same manner as in Comparative Example 2, except that no primary fine powder was used, and 12.5 g of granulated fine powder (a2) and 37.5 g of granulated fine powder (b2) were used, crosslinked polymer particles (granulated particles (AB3)) were obtained, which were the fraction that passed through an 850 μm sieve but did not pass through an 180 μm sieve.

[0109] [Table 3]

[0110] It was confirmed that even when granulated fine powders from different manufacturing examples are mixed and agglomerated, by agglomerating particle groups in which the total proportion of granulated fine powder is above a certain level, the water absorption rate of the resulting granulated particles can be increased compared to when primary fine powders are agglomerated alone.

Claims

1. A method for producing water-absorbent resin particles, comprising: mixing the polymer fine powder passing through a sieve with 180 μm openings with water to agglomerate the polymer fine powder and form a first agglomerate; drying and pulverizing the first agglomerates to form a granulated fine powder that passes through a sieve with 180 μm openings; mixing the particle group containing the granulated fine powder with water to aggregate the particle group to form a second agglomerate; drying and grinding the second agglomerates to form granulated particles; Including, the content of the granulated fine powder in the particle groups is 80% by mass or more and 100% by mass or less with respect to the total amount of the particle groups, The water-absorbing resin particles have a structural unit derived from an ethylenically unsaturated monomer. A method for producing water-absorbent resin particles including granulated particles.

2. The content of the granulated fine powder in the particle group is 99 mass% or less with respect to the total amount of the particle group, The method according to claim 1, wherein the particle group further contains more than 0% by mass and 20% by mass or less of the polymer fine powder before forming the first agglomerates, based on the total amount of the particle group.

3. 3. The method according to claim 2, wherein the polymer fine powder contained in the particle groups is a fine powder excluding fine powder formed by a method comprising mixing the polymer fine powder with water to agglomerate the polymer fine powder to form agglomerates, and drying and pulverizing the agglomerates.

4. The method according to any one of claims 1 to 3, further comprising classifying the granulated particles.

5. The method according to any one of claims 1 to 4, wherein the content of fine powder in the granulated fine powder that passes through a sieve having an opening of 180 μm and does not pass through a sieve having an opening of 150 μm is 30 mass% or less with respect to the total amount of the granulated fine powder.

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