JP7793530B2 - Method for producing water-absorbent resin particles - Google Patents

Description

The present disclosure relates to a method for producing water-absorbent resin particles.

In the production of water-absorbent resin particles, a process is carried out in which block-shaped or coarse-particle polymer obtained by polymerization is crushed to form particles.

Japanese Patent Application Laid-Open No. 2005-54151

For use as water-absorbent resin particles, there is an appropriate particle size range, such as 180 to 850 μm. However, when pulverizing the polymer in the production of water-absorbent resin particles, not only particles with the desired particle size are generated, but also fine powder that does not meet the desired particle size. The fine powder is increased in particle size by granulation and used as granulated particles (see, for example, Patent Document 1). When producing granulated particles, the fine powder is removed by further pulverization and classification as necessary, and particles with the desired particle size are extracted. When producing granulated particles, it is necessary to minimize the generation of fine powder again.

One aspect of the present disclosure relates to a manufacturing method that, when agglomerating and pulverizing fine powder generated during the production of water-absorbent resin particles to obtain granulated particles, can increase the proportion of granulated particles having a particle diameter of 180 to 850 μm compared to granulation using only fine powder that has not undergone an agglomeration process.

The inventors have newly discovered that by using a certain proportion of fine powder (granulated fine powder) that is regenerated when granulating fine powder generated in the production of water-absorbent resin particles, the particle size distribution of the resulting granulated particles can be adjusted to a more suitable range than when granulating using only fine powder that has not undergone an agglomeration process. Furthermore, they have discovered that this effect is observed when the CRC (absorbency without load) of the fine powder (which may not have undergone an agglomeration process) used to obtain the granulated fine powder is below a certain level.

That is, one aspect of the present disclosure relates to a method for producing water-absorbent resin particles containing granulated particles, the method comprising: preparing a polymer fine powder that passes through a sieve with 180 μm openings and has a CRC of 55 g/g or less; mixing the polymer fine powder with water to agglomerate the polymer fine powder to form first agglomerates; drying and pulverizing the first agglomerates to obtain granulated fine powder that passes through a sieve with 180 μm openings; mixing particle groups containing the polymer fine powder before forming the first agglomerates and the granulated fine powder with water to agglomerate the particle groups to form second agglomerates; and drying and pulverizing the second agglomerates to form granulated particles, wherein the content of the granulated fine powder in the particle groups is more than 0% by mass and 30% by mass or less with respect to the total amount of the particle groups.

In the above-mentioned manufacturing method, the polymer fine powder may be a fine powder other than 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. In other words, the polymer fine powder may be a fine powder that has not undergone an agglomeration process.

In the above manufacturing method, the content of the polymer fine powder in the particle group may be 60% by mass or more and less than 100% by mass relative to the total amount of the particle group.

In the above manufacturing method, the total amount of the polymer fine powder and the granulated fine powder may be 90% by mass or more of the total amount of the particle group.

The above manufacturing method may further include classifying the granulated particles.

In the above manufacturing method, the granulated particles may include particles that do not pass through a sieve with an opening of 180 μm.

In the above manufacturing method, 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 30 mass% or less relative to the total amount of the particle group.

According to one aspect of the present disclosure, a manufacturing method can be provided in which, when agglomerating and pulverizing fine powder generated during the production of water-absorbent resin particles to obtain granulated particles, the proportion of granulated particles having a particle diameter of 180 to 850 μm can be increased compared to when granulating using only fine powder that has not undergone an agglomeration process.

As used herein, "(meth)acrylic" refers to both acrylic and methacrylic. "Acrylate" and "methacrylate" are also referred to 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 herein, the upper or lower limit of a certain range can be arbitrarily combined with the upper or lower limit of another range. In the numerical ranges described herein, the upper or lower limit may be replaced with a value shown in the examples. "Water-soluble" refers to a solubility of 5% by weight or more in water at 25°C. The materials exemplified herein 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 substances present in the composition, unless otherwise specified. "Saline" refers to a 0.9% by weight aqueous sodium chloride solution. The sieve used means a JIS standard sieve.

The method for producing water-absorbent resin particles including granulated particles according to this embodiment includes: preparing a polymer fine powder that passes through a sieve with a mesh size of 180 μm and has a CRC of 55 g/g or less; mixing the polymer fine powder with water to agglomerate the polymer fine powder and form first agglomerates; drying and pulverizing the first agglomerates to obtain granulated fine powder that passes through a sieve with a mesh size of 180 μm; mixing a particle group containing the polymer fine powder and the granulated fine powder before forming the first agglomerates with water to agglomerate the particle group and form second agglomerates; and drying and pulverizing the second agglomerates to form granulated particles. In other words, a method for producing water-absorbent resin particles may include the steps of: preparing a polymer fine powder that has not been subjected to an agglomeration step and that passes through a sieve with an opening of 180 μm and has a CRC of 55 g/g or less; mixing the polymer fine powder that passes through the sieve with water to agglomerate the polymer fine powder and obtain agglomerates (first agglomerates); drying and pulverizing the agglomerates to obtain granulated fine powder that passes through the sieve with an opening of 180 μm; mixing particle groups containing the polymer fine powder that has not been subjected to the agglomeration step and the granulated fine powder with water to agglomerate the particle groups again and obtain agglomerates (second agglomerates); and drying and pulverizing the agglomerates again to obtain granulated particles. The content of the granulated fine powder in the particle groups is more than 0% by mass and 30% by mass or less with respect to the total amount of the particle groups.

[Primary fine powder]
In the production method according to this embodiment, the first agglomerates are formed using polymer fine powder that passes through a sieve with 180 μm openings. The polymer fine powder used to form the first agglomerates may be polymer fine powder that has not undergone an aggregation process (hereinafter also referred to as "primary fine powder"). The CRC of the primary fine powder used in this embodiment is 55 g/g or less. When primary fine powder with a CRC of 55 g/g or less is used, the proportion of particles having a particle size in the range of 180 to 850 μm in the resulting granulated particles can be increased compared to granulated particles obtained by granulating the primary fine powder alone. In this specification, granulation refers to agglomerating particles to obtain particles with a larger particle size than the original particles.

Primary fine powder is a fine powder generated during the production of polymer particles used in water-absorbent resin particles, and can be, for example, a fine powder generated when a block-shaped or coarse-particle polymer is pulverized into particles. Primary fine powder may be a fine powder excluding fine powder formed by a method including mixing the polymer fine powder with water to agglomerate the polymer fine powder and forming agglomerates, and then drying and pulverizing the agglomerates. 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. Examples of methods for obtaining primary fine powder are described in detail below.

(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.

Polymerization can be carried out, for example, by aqueous solution polymerization. The polymerization of monomers by aqueous solution polymerization is described below.

The ethylenically unsaturated monomer may be water-soluble. Examples of ethylenically unsaturated monomers include carboxylic acid monomers such as (meth)acrylic acid, maleic acid, maleic anhydride, fumaric acid, and their salts; 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 their salts. Ethylenically unsaturated monomers may be used alone or in combination.

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 of 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.

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; ammonia, etc. These alkaline neutralizers may be used in the form of an aqueous solution to simplify the neutralization process. 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 the polymerization of the ethylenically unsaturated monomer raw material.

The degree of neutralization of the ethylenically unsaturated monomer with the alkaline neutralizing agent may be 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 resulting water-absorbent resin particles to thereby enhance their water absorption performance, and preventing safety and other issues caused by the presence of excess alkaline neutralizing agent. Here, the degree of neutralization refers to the degree of neutralization of all acid groups possessed by the ethylenically unsaturated monomer.

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 the "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.

The amount of 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% relative to the total amount of monomers (total amount of monomers used to obtain water-absorbent resin particles; for example, the total amount of monomers that provide the structural units of a crosslinked polymer; the same applies below). In particular, 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% relative to the total amount of monomers. "Proportion of (meth)acrylic acid and salts thereof" refers to the proportion of the total amount of (meth)acrylic acid and salts thereof.

The water-absorbent resin particles may be, for example, water-absorbent resin particles containing a cross-linked polymer having structural units derived from an ethylenically unsaturated monomer, in which the ethylenically unsaturated monomer contains at least one compound selected from the group consisting of (meth)acrylic acid and salts thereof, and the proportion of (meth)acrylic acid and salts thereof is 70 to 100 mol % relative to the total amount of monomers used to obtain the water-absorbent resin particles.

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 or irradiating the solution with light. Examples of the polymerization initiator include a photopolymerization initiator or a radical polymerization initiator. The polymerization initiator may also be a water-soluble radical polymerization initiator. The polymerization initiator may be, for example, an azo compound, a peroxide, or the like.

Examples of 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'-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.

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-butylperoxyisobutyrate, and t-butylperoxypivalate; and peroxides such as hydrogen peroxide. Among these peroxides, potassium persulfate, ammonium persulfate, sodium persulfate, or hydrogen peroxide may be used, or potassium persulfate, ammonium persulfate, or sodium persulfate may be used, from the viewpoint of obtaining water-absorbent resin particles with good water absorption performance. These peroxides may be used alone or in combination of two or more.

A polymerization initiator and a reducing agent can also be used in combination as a redox polymerization initiator. Examples of reducing agents include sodium sulfite, sodium bisulfite, ferrous sulfate, and L-ascorbic acid.

From the viewpoint of the balance of performance such as CRC in the primary fine powder, crosslinked polymer particles, and water-absorbent resin particles, the amount of polymerization initiator may be 0.05 to 1 mmol, 0.08 to 0.8 mmol, or 0.1 to 0.7 mmol per mole of monomer.

The aqueous monomer solution may contain an internal cross-linking agent. By including an internal cross-linking agent, the resulting cross-linked polymer can have an internal cross-linking structure that includes cross-linking due to the internal cross-linking agent in addition to self-cross-linking due to the polymerization reaction.

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.

When an internal crosslinking agent is used, the amount used may be 0.02 to 1.0 millimoles, 0.05 to 0.8 millimoles, or 0.1 to 0.6 millimoles per mole of monomer, in order to adjust the performance balance of the primary fine powder, crosslinked polymer particles, and CRC of the water-absorbent resin particles.

The aqueous monomer solution may contain additives such as chain transfer agents and thickeners, 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 also contain solvents other than water, such as water-soluble organic solvents, as appropriate.

The polymerization method may be, for example, a static polymerization method in which the aqueous monomer solution is polymerized without stirring (e.g., 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 that occupies approximately the same volume as the aqueous monomer solution present in the reaction vessel upon completion of polymerization can be obtained.

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

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

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 with a chain of particles. The minimum side size of the crushed material may be, for example, approximately 0.1 to 15 mm, or approximately 1.0 to 10 mm. The maximum side size of the crushed material may be, for example, approximately 0.1 to 200 mm, or approximately 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.

(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, reduced pressure drying, or the like. Drying may be performed, for example, under normal pressure or reduced pressure, or may be performed under a nitrogen or other gas stream to increase 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.

(Crushing process)
The dried product is then pulverized to obtain particle groups containing fine powder. For pulverization, for example, a pulverizer such as a roller mill (roll mill), stamp mill, jet mill, high-speed rotary pulverizer (ultracentrifugal pulverizer, hammer mill, pin mill, rotor beater mill, etc.), or container-driven mill (rotary mill, vibration mill, planetary mill, etc.) can be used. The pulverizer 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.

Pulverization can be carried out so that at least a portion of the particle group becomes a fine powder having a particle size that will pass through a sieve with 180 μm openings. For example, pulverization can be carried out in a manner that produces a portion of the fine powder having a particle size that will pass through a sieve with 180 μm openings, while the primary purpose is to obtain polymer particles having an appropriate particle size of 180 μm or more but less than 850 μm.

(Classification process)
In the production method according to this embodiment, among the particles obtained in the above-described polymer particle production process, fine powders that pass through a sieve with a mesh size of 180 μm can be used as primary fine powders. For example, when the particles obtained in the above-described pulverization process include particles that do not pass through a sieve with a mesh size of 180 μm, the particle group can be classified using the sieve to obtain primary fine powders that pass through a sieve with a mesh size of 180 μm. 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.

The classification method can be any 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 using air flow. In this specification, "particles that pass through a sieve with 180 μm openings" refers to particles of that size, and is not limited to classifications that use sieves.

The CRC of the primary fine powder is 55 g/g or less, and may be 53 g/g or less, 51 g/g or less, 49 g/g or less, 47 g/g or less, 45 g/g or less, 43 g/g or less, 41 g/g or less, or 39 g/g or less.The CRC of the primary fine powder may be, for example, 30 g/g or more, 33 g/g or more, 35 g/g or more, 37 g/g or more, 39 g/g or more, 41 g/g or more, or 43 g/g or more. The CRC of the primary fines may be 30 g/g or more and 55 g/g or less, 53 g/g or less, 51 g/g or less, 49 g/g or less, 47 g/g or less, 45 g/g or less, 43 g/g or less, 41 g/g or less, or 39 g/g or less; 33 g/g or more and 55 g/g or less, 53 g/g or less, 51 g/g or less, 49 g/g or less, 47 g/g or less, 45 g/g or less, 43 g/g or less, 41 g/g or less, or 39 g/g or less; 35 g/g or more and 55 g/g or less, 53 g/g or less, 51 g/g or less, 49 g/g or less, 47 g/g or less, 45 g/g or less, 43 g/g or less, 41 g/g or less, or 39 g/g or less; and 37 g/g or more and 55 g/g or less. , 53 g/g or less, 51 g/g or less, 49 g/g or less, 47 g/g or less, 45 g/g or less, 43 g/g or less, 41 g/g or less, or 39 g/g or less; 39 g/g or more and 55 g/g or less, 53 g/g or less, 51 g/g or less, 49 g/g or less, 47 g/g or less, 45 g/g or less, 43 g/g or less, or 41 g/g or less; 41 g/g or more and 55 g/g or less, 53 g/g or less, 51 g/g or less, 49 g/g or less, 47 g/g or less, 45 g/g or less, or 43 g/g or less; 43 g/g or more and 55 g/g or less, 53 g/g or less, 51 g/g or less, 49 g/g or less, 47 g/g or less, or 45 g/g or less. 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).

[Granulated fine powder]
The granulated fine powder used in the production method of 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 of this embodiment is granulated particles that pass through a 180 μm sieve. In this specification, the steps from mixing the fine powder to agglomerating the fine powder (cutting as necessary), drying and pulverizing the first lumps may also be collectively referred to as the granulation step. The production method of the granulated fine powder will be described in detail below.

(Agglomeration 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.

The water mixed with the polymer fine powder does not need to be pure water; it may contain components such as water-soluble salts, water-soluble polymerizable monomers such as ethylenically unsaturated monomers, crosslinking agents, or hydrophilic organic solvents. In other words, 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. Examples of crosslinking agents that can be used include the internal crosslinking agents described above or crosslinking agents similar to the surface crosslinking agents described below.

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, or 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, per 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, per 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 all at once, water may be sprayed, or the water may be mixed in the form of steam.

Mixing of polymer fine powder and water can be carried out using, for example, various agitators equipped with agitating blades. 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. Flat blades have a shaft (agitation shaft) and a flat plate portion (agitation 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.

In order to achieve more uniform mixing, the mixing time for the polymer fine powder and water may be 30 to 150 seconds, or 60 to 120 seconds, after the entire amount of polymer fine powder and water is added to the same container.

By mixing the polymer fine powder with water, the polymer fine powder aggregates to obtain a first mass.

(Drying and crushing of lumps)
Next, the obtained first lumps are dried. 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. The first lumps are dried and pulverized, and if necessary, classified to obtain a fine powder that passes through a sieve with 180 μm openings, i.e., a granulated fine powder.

The size of the granulated fine powder may be such that it passes through a 180 μm sieve, but the content of fine powder that passes through a 180 μm sieve but not a 150 μm sieve may be 30% by mass or less, or less than 25% by mass, of the total amount of granulated fine powder. It is believed that when the content of fine powder that does not pass through a 150 μm sieve in the granulated fine powder is below a certain level, the particle size distribution of the resulting granulated particles tends to be within a more preferable range. The content of fine powder that passes through a 180 μm sieve but not a 150 μm sieve in the granulated fine powder may be, for example, greater than 0% by mass, 5% by mass or more, 10% by mass or more, or 15% by mass or more, of the total amount of granulated fine powder. Among the granulated fine powders, the content of fine powders that pass through a sieve with an opening of 180 μm but do not pass through a sieve with an opening of 150 μm may be more than 0% by mass and not more than 30% by mass or not more than 25% by mass, may be 5% by mass or more and not more than 30% by mass or not more than 25% by mass, may be 10% by mass or more and not more than 30% by mass or not more than 25% by mass, or may be 15% by mass or more and not more than 30% by mass or not more than 25% by mass, relative to the total amount of granulated particles.

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 more than 75% by mass, and 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 more than 75% by mass but not more than 85% by mass, relative to the total amount of the granulated fine powder.

[Agglomeration of primary fine powder and 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 performed using particle groups containing the above-mentioned primary fine powder and granulated fine powder. The production method according to this embodiment includes mixing the particle groups containing the primary fine powder (primary fine powder before forming first aggregates) and granulated fine powder with water to aggregate the particle groups and obtain second aggregates. The content of the granulated fine powder in the particle groups used for main aggregation is more than 0% by mass and 30% by mass or less with respect to the total amount of the particle groups.

The content of granulated fine powder in the particle groups used for main aggregation may be 1% by mass or more, 2% by mass or more, 3% by mass or more, 4% by mass or more, 5% by mass or more, 8% by mass or more, or 10% by mass or more, relative to the total amount of the particle groups. The content of granulated fine powder in the particle groups used for main aggregation may be 28% by mass or less, 25% by mass or less, 20% by mass or less, 15% by mass or less, 12% by mass or less, or 10% by mass or less, relative to the total amount of the particle groups. The content of the granulated fine powder in the particle groups used for main aggregation may be 1% by mass or more and 28% by mass or less, 25% by mass or less, 20% by mass or less, 15% by mass or less, 12% by mass or less, or 10% by mass or less, or 2% by mass or more and 28% by mass or less, 25% by mass or less, 20% by mass or less, 15% by mass or less, 12% by mass or less, or 10% by mass or less, or 3% by mass or more and 28% by mass or less, 25% by mass or less, 20% by mass or less, 15% by mass or less, 12% by mass or less, or 10% by mass or less, or 4% by mass or more and 28% by mass or less , 25% by mass or less, 20% by mass or less, 15% by mass or less, 12% by mass or less, or 10% by mass or less; 5% by mass or more and 28% by mass or less, 25% by mass or less, 20% by mass or less, 15% by mass or less, 12% by mass or less, or 10% by mass or less; 8% by mass or more and 28% by mass or less, 25% by mass or less, 20% by mass or less, 15% by mass or less, 12% by mass or less, or 10% by mass or more and 28% by mass or less, 25% by mass or less, 20% by mass or less, 15% by mass or less, 12% by mass or less, or 10% by mass or less.

The content of primary fine powder (polymer fine powder) in the particle groups used for main aggregation may be 60% by mass or more, 63% by mass or more, 65% by mass or more, 67% by mass or more, 70% by mass or more, 73% by mass or more, 75% by mass or more, 77% by mass or more, 80% by mass or more, 83% by mass or more, 85% by mass or more, 88% by mass or more, 90% by mass or more, 93% by mass or more, or 95% by mass or more, based on the total amount of the particle groups. The content of primary fine powder in the particle groups used for main aggregation may be, for example, less than 100% by mass, 99% by mass or less, 98% by mass or less, 97% by mass or less, 96% by mass or less, 95% by mass or less, 92% by mass or less, or 90% by mass or less, based on the total amount of the particle groups. The content of primary fine powder (polymer fine powder) in the particle groups used for main aggregation may be 60% by mass or more and less than 100% by mass, 99% by mass or less, 98% by mass or less, 97% by mass or less, 96% by mass or less, 95% by mass or less, 92% by mass or less, or 90% by mass or less, or 63% by mass or more and less than 100% by mass, 99% by mass or less, 98% by mass or less, 97% by mass or less, 96% by mass or less, 95% by mass or less, 92% by mass or less, or 90% by mass or less, or 65% by mass or more and less than 100% by mass, 99% by mass or less, 98% by mass or less, 97% by mass or less, 96% by mass or less, 95% by mass or less, 92% by mass or less, or 90% by mass or less, or 67% by mass or more and less than 100% by mass, It may be 99% by mass or less, 98% by mass or less, 97% by mass or less, 96% by mass or less, 95% by mass or less, 92% by mass or less, or 90% by mass or less; it may be 70% by mass or more and less than 100% by mass, 99% by mass or less, 98% by mass or less, 97% by mass or less, 96% by mass or less, 95% by mass or less, 92% by mass or less, or 90% by mass or less; it may be 73% by mass or more and less than 100% by mass, 99% by mass or less, 98% by mass or less, 97% by mass or less, 96% by mass or less, 95% by mass or less, 92% by mass or less, or 90% by mass or less; it may be 75% by mass or more and less than 100% by mass, 99% by mass or less, 98% by mass or less, 97% by mass or less, 96% by mass or less, 95% by mass or less, 92% by mass or less, or 90% by mass or less. It may be 77% by mass or more and less than 100% by mass, 99% by mass or less, 98% by mass or less, 97% by mass or less, 96% by mass or less, 95% by mass or less, 92% by mass or less, or 90% by mass or less, it may be 80% by mass or more and less than 100% by mass, 99% by mass or less, 98% by mass or less, 97% by mass or less, 96% by mass or less, 95% by mass or less, 92% by mass or less, or 90% by mass or less, it may be 83% by mass or more and less than 100% by mass, 99% by mass or less, 98% by mass or less, 97% by mass or less, 96% by mass or less, 95% by mass or less, 92% by mass or less, or 90% by mass or less, it may be 85% by mass or more and less than 100% by mass, 99% by mass or less, 98% by mass or less, 97% by mass or less, 96% by mass or less, 95% by mass or less It may be 88% by mass or more and less than 100% by mass, 99% by mass or less, 98% by mass or less, 97% by mass or less, 96% by mass or less, 95% by mass or less, 92% by mass or less, or 90% by mass or less, it may be 88% by mass or more and less than 100% by mass, 99% by mass or less, 98% by mass or less, 97% by mass or less, 96% by mass or less, 95% by mass or less, or 92% by mass or less, it may be 90% by mass or more and less than 100% by mass, 99% by mass or less, 98% by mass or less, 97% by mass or less, 96% by mass or less, 95% by mass or less, it may be 93% by mass or more and less than 100% by mass, 99% by mass or less, 98% by mass or less, 97% by mass or less, 96% by mass or less, or 95% by mass or less, it may be 95% by mass or more and less than 100% by mass, 99% by mass or less, 98% by mass or less, 97% by mass or less, or 96% by mass or less.

The particle groups 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 particle groups 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 particle groups 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 particle groups used for main aggregation.

The method of mixing particle groups containing primary fine powder and granulated fine powder with water to agglomerate them, and the method of cutting the resulting second agglomerates as needed, followed by drying and pulverization, can be similar to the method for producing the granulated fine powder described above. Multiple types of primary fine powder and granulated fine powder may be used, for example, a combination of fine powders obtained under different manufacturing conditions, or fine powders obtained in different manufacturing lots under similar manufacturing conditions. The particle groups containing primary fine powder and granulated fine powder may be uniformly mixed in advance before being mixed with water.

The manufacturing method according to this embodiment increases the particle size distribution of the granulated particles obtained by pulverization, compared to granulation using only primary fine powder. By mixing the granulated fine powder obtained through the agglomeration process in a predetermined ratio and agglomerating it again, it is possible to increase the granulation strength and reduce the rate of fine powder passing through a 180 μm sieve when the powder is subsequently re-pulverized.

[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.

In the particle size distribution of the granulated particles obtained by this embodiment before classification, the proportion of particles that pass through a sieve with an 850 μm opening but do not pass through a sieve with an 180 μm opening may be, for example, 40% by mass or more, 45% by mass or more, 50% by mass or more, 55% by mass or more, or 60% by mass or more, and may be 70% by mass or less, 65% by mass or less, 63% by mass or less, or 61% by mass or less. In the particle size distribution of the granulated particles before classification, the proportion of particles that pass through a sieve with an 850 μm opening but do not pass through a sieve with an 180 μm opening may be 40 to 70% by mass, 45 to 65% by mass, 50 to 63% by mass, or 55 to 61% by mass. In the particle size distribution of the granulated particles before classification, the proportion of particles passing through a sieve with an opening of 180 μm may be 60% by mass or less, 55% by mass or less, 50% by mass or less, 45% by mass or less, or 40% by mass or less, or may be 25% by mass or more, 30% by mass or more, or 35% by mass or more. In the particle size distribution of the granulated particles before classification, the proportion of particles passing through a sieve with an opening of 180 μm may be 25 to 60% by mass, 30 to 55% by mass, or 35 to 50% by mass.

[Surface crosslinking]
The method for producing water-absorbent resin particles may include a step of surface-crosslinking granulated particles (surface-crosslinking step). 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.

The surface cross-linking agent can be added to the granulated particles, for example, by adding a surface cross-linking agent solution or by spraying the surface cross-linking agent solution. To ensure uniform dispersion of 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.

The surface cross-linking agent may contain, for example, two or more functional groups (reactive functional groups) that are reactive with functional groups derived from ethylenically unsaturated monomers. Examples of surface cross-linking agents 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 -tolylene diisocyanate, hexamethylene diisocyanate, and other isocyanate compounds; 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.

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

The surface cross-linking step may be carried out in the presence of water in the range of 1 to 200 parts by mass per 100 parts by mass of the ethylenically unsaturated monomer. The amount of water can be adjusted as appropriate by 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 near the particle surfaces of the water-absorbent resin particles (granulated particles).

The treatment temperature for the surface cross-linking agent is set appropriately depending on the surface cross-linking agent used and may be between 20 and 250°C. The treatment time with the surface cross-linking agent may be between 1 and 200 minutes or between 5 and 100 minutes. Surface cross-linking may be performed once or multiple times.

[Water-absorbing resin particles]
The water-absorbent resin particles obtained by the production method according to the present 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, pentasodium diethylenetriaminepentaacetate, etc.), a flowability improver (lubricant), etc. The additional components may be disposed inside the granulated particles, on the surface thereof, or both.

The water-absorbent resin particles may include a plurality of inorganic particles arranged on the surface of the granulated particles. The manufacturing method according to this embodiment may further include a step of attaching inorganic particles to the surface of the granulated particles.

The shape of the granulated particles or water-absorbent resin particles obtained by the manufacturing 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.

The CRC of the granulated particles or water-absorbent resin particles obtained by the manufacturing method according to this embodiment may be, for example, 30 g/g or more, 35 g/g or more, 38 g/g or more, or 40 g/g or more, and may be 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 65 g/g, 35 to 60 g/g, 38 to 55 g/g, or 40 to 50 g/g.

The water-absorbent resin particles obtained by the manufacturing method of this embodiment have excellent water absorption properties and can be used in fields such as hygiene materials such as disposable diapers and sanitary products, agricultural and horticultural materials such as water retention agents and soil conditioners, and industrial materials such as waterproofing agents and anti-condensation agents.

The present invention will be explained in more detail below with reference to examples. However, the present invention is not limited to these examples.

The centrifuge retention capacity (CRC), median particle size, particle size distribution of particles after agglomeration and grinding, and particle size distribution of fine powder were measured for the particles produced in the following examples.

(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). The measurement was carried out 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 opening of 850 μm but did not pass through a sieve with an opening of 180 μm. The results are shown in Tables 1 to 3.

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

The nonwoven fabric bag was floated without being folded over on 1,000 g of saline solution contained in a stainless steel tray (240 mm x 320 mm x 45 mm) to completely wet the entire nonwoven fabric bag. One minute after placing the nonwoven fabric bag in the saline solution, the nonwoven fabric bag was immersed in the saline solution with a spatula to obtain a nonwoven fabric bag containing gel.

The nonwoven fabric bag was removed from the saline 30 minutes after being 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 (manufactured by 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 operation 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 based on 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

(median particle size)
The median particle size of the particles was measured in an environment of 25±2°C and 50±10% humidity 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. 10 g of the particle powder 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 tray. The mass of the particles remaining on each sieve was calculated as a mass percentage of the total mass. The mass percentage of the 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 the particles remaining on the sieve was plotted on a logarithmic probability paper. The plots on the probability paper were connected with a straight line to determine the particle size corresponding to an integrated mass percentage of 50% by mass, which was taken as the median particle size. The results are shown in Tables 1 to 3.

(Particle size distribution after agglomeration and crushing)
The particle size distribution of the particles after agglomeration and pulverization (granulated particles before classification) was measured by the following procedure. JIS standard sieves were assembled in the following order from top to bottom: a sieve with an 850 μm mesh, a sieve with an 180 μm mesh, and a tray. The pulverized polymer particles were placed on the top sieve and classified in accordance with JIS Z 8815 (1994).

After classification, the ratio of the total amount of particles remaining on the sieve with each mesh size to the total amount of particles used in the measurement was calculated using the following formula. The results are shown in Tables 1 to 3.
Content of 180 μm passing fraction [mass %]=[(amount of particles passing through a sieve with 180 μm openings)/(amount of particles used for measurement)]×100
Content of 180 to 850 μm fraction [mass %] = [(amount of particles that passed through a sieve with an 850 μm opening and remained on a sieve with an 180 μm opening) / (amount of particles used in measurement)] × 100
Content of 850 μm non-passing fraction=[(amount of granulated particles remaining on a sieve with 850 μm openings [g])/(amount of granulated particles used in measurement)]×100

(Particle size distribution of fine powder)
The particle size distribution of the granulated fine powder passing through the 180 μm mesh sieve was further measured. JIS standard sieves (diameter 20 cm) were assembled from top to bottom, consisting of 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 to 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 to 180 μm fraction [mass %]=[(amount of fine powder that passed through a sieve with 180 μm openings and remained on a sieve with 150 μm openings)/(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 measured fine powder)]×100

(Production Example 1)
[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 a partially neutralized sodium acrylate solution.

406.25 g of ion-exchanged water and 3.55 g of polyethylene glycol diacrylate (n≒9, internal crosslinking agent, NOF Corporation, product name: Blemmer 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 for 30 minutes under a nitrogen gas atmosphere to adjust the dissolved oxygen content to 0.1 ppm or less.

Next, a 5-liter stainless steel, double-arm kneader (manufactured by Irie Shokai Co., Ltd.) with a jacket, two sigma-type blades, a thermometer, and a nitrogen inlet tube was prepared. The reaction solution was fed into the kneader, and the atmosphere inside the kneader was replaced with nitrogen gas while maintaining the reaction solution at 25°C. Next, while stirring the reaction solution, 92.63 g (7.78 mmol) of a 2.0% by weight aqueous solution of sodium persulfate and 15.85 g of a 0.5% by weight aqueous solution of L-ascorbic acid were added. After approximately 1 minute, the temperature began to rise and polymerization began. After 6 minutes, the thermometer indicated a maximum temperature of 85°C. Stirring was continued while maintaining the jacket temperature at 60°C. 60 minutes after the start of polymerization, the resulting hydrogel polymer was removed.

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

[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).

[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 a fraction that passed through the 850 μm sieve but did not pass through the 180 μm sieve, and crosslinked polymer fine powder (primary fine powder (a1)), which is a fraction that passed through the 180 μm sieve, were obtained. The CRC of the primary fine powder (a1) 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%.

[Pelletization 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 portion. The flat plate portion was welded to the shaft and had a curved tip. Four slits were formed in the flat plate portion extending along the axial direction of the shaft. The four slits were arranged in the width direction of the flat plate portion, 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 portion was approximately 10 cm, and the width of the flat plate portion was approximately 6 cm.

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 mass of aggregates of the fine powder. The entire contents (masses) 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 mass of lumps, including the cut pieces, was dried with hot air at 150°C for 60 minutes to obtain a dried mass (first mass).

The dried material 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. Classification yielded 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. The content of the 150-180 μm fraction of the granulated fine powder (a2) was 22% by mass, and the content of the fraction that passed through the 150 μm sieve was 78% by mass.

(Production Example 2)
[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 sodium acrylate solution having a monomer concentration of 45% by mass.

887.31 g of the partially neutralized sodium acrylate solution was mixed with 158.02 g of ion-exchanged water and 0.525 g of polyethylene glycol diacrylate (Blemmer 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 uniform 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 then purged with nitrogen to adjust the dissolved oxygen content to 0.1 ppm or less.

Next, while stirring the mixture at 300 rpm, 3.23 g (0.596 mmol) of a 5% by weight 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 weight aqueous solution of L-ascorbic acid, and 1.85 g of a 0.35% by weight aqueous solution of hydrogen peroxide were added dropwise in that order using a syringe (10 mL disposable syringe manufactured by Terumo Corporation, 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. Six minutes after the completion of the addition of the hydrogen peroxide solution, the thermometer indicated 82°C, and the temperature then began to decrease. The stainless steel tray containing the hydrogel polymer containing water and a polymer, which had been formed by the gelation of the reaction solution, was immersed in a water bath at 75°C, and the hydrogel polymer was allowed to age in that state for 20 minutes.

After aging, the hydrogel polymer was cut into pieces of appropriate size and then subjected to the same drying, pulverizing, and classification steps as those used in the production of the primary fine powder in Production Example 1. This resulted in crosslinked polymer particles (B1), which are the fraction that passed through an 850 μm sieve but not an 180 μm sieve, and crosslinked polymer fine powder (primary fine powder (b1)), which is the fraction that passed through an 180 μm sieve. The CRC of the primary fine powder (b1) was 45 g/g, the content of the 150-180 μm fraction was 25% by mass, and the content of the 150 μm-passing fraction was 75% by mass.

[Pelletization 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 a fraction that passed through an 850 μm sieve but not a 180 μm sieve, and a crosslinked polymer fine powder (granulated fine powder (b2)), which is a fraction that passed through a 180 μm sieve. The content of the 150 to 180 μm fraction in the granulated fine powder (b2) was 19 mass%, and the content of the fraction that passed through a 150 μm sieve was 81 mass%.

(Production Example 3)
[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 sodium acrylate solution having a monomer concentration of 45% by mass.

158.24 g of ion-exchanged water and 0.309 g of polyethylene glycol diacrylate (Blemmer ADE-400A) were added to 887.31 g of the partially neutralized sodium acrylate solution to obtain a reaction solution (aqueous monomer solution). Using this reaction solution, a polymerization reaction was initiated in the same manner as in Production Example 2. As the polymerization reaction progressed, the viscosity of the reaction solution increased, and then the reaction solution gelled. Four minutes after the dropwise addition of hydrogen peroxide was completed, the thermometer indicated 103°C, after which the temperature began to decrease. The stainless steel tray containing the hydrogel polymer containing water and polymer formed by the gelation of the reaction solution was immersed in a 75°C water bath, and the hydrogel polymer was allowed to age in this state for 20 minutes.

After aging, the hydrogel polymer was cut into pieces of appropriate size and then subjected to the same drying, pulverizing, and classification steps as in Production Example 1 to obtain crosslinked polymer particles (C1), which are the fraction that passed through an 850 μm sieve but not an 180 μm sieve, and crosslinked polymer fine powder (primary fine powder (c1)), which is the fraction that passed through the 180 μm sieve. The CRC of the primary fine powder (c1) was 56 g/g, the content of the 150-180 μm fraction was 30% by mass, and the content of the fraction that passed through a 150 μm sieve was 70% by mass.

[Pelletization process]
Using 40 g of the primary fine powder (c1) obtained above, the same operation as in the granulation step of Production Example 1 was carried out to obtain crosslinked polymer particles (granulated particles (C2)), which are a fraction that passed through an 850 μm sieve but not a 180 μm sieve, and crosslinked polymer fine powder (granulated fine powder (c2)), which is a fraction that passed through a 180 μm sieve. The content of the 150 to 180 μm fraction in the granulated fine powder (c2) was 17 mass%, and the content of the fraction that passed through a 150 μm sieve was 83 mass%.

<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.

Example 1
[Preparation of fine powder group]
45 g of the primary fine powder (a1) and 5 g of the granulated fine powder (a2) were placed in a 100 mL mayonnaise bottle, and then the bottle was shaken for 30 minutes in a paint shaker (manufactured by Toyo Seiki Seisakusho, Ltd.) to mix uniformly and obtain a fine powder group (particle group).

[Pelletization 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 in 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 a 180 μm sieve.

Example 2
Crosslinked polymer particles (granulated particles (Y2)), which are a fraction that passed through an 850 μm sieve but did not pass through a 180 μm sieve, were obtained in the same manner as in Example 1, except that the amount of primary fine powder (a1) was changed to 37.5 g and the amount of granulated fine powder (a2) was changed to 12.5 g.

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

Example 3
Crosslinked polymer particles (granulated particles (Y3)), which are a fraction that passed through an 850 μm sieve but did not pass through a 180 μm sieve, were obtained in the same manner as in Example 1, except that the amount of primary fine powder (a1) was changed to 47.5 g and 2.5 g of granulated fine powder (b2) was used instead of granulated fine powder (a1).

Example 4
[Preparation of fine powder group]
Crosslinked polymer particles (granulated particles (Y4)), which are a fraction that passed through an 850 μm sieve but did not pass through a 180 μm sieve, were obtained in the same manner as in Example 1, except that the amount of primary fine powder (a1) was changed to 45 g and 5 g of granulated fine powder (c2) was used instead of granulated fine powder (a1).

In an example in which granulated fine powder was mixed with primary fine powder in a specified ratio and granulated, the particle size distribution of the particles obtained by agglomeration and pulverization (granulated particles before classification) had a higher proportion of particles with particle diameters of 180 to 850 μm than in Comparative Example 1, in which primary fine powder was agglomerated alone. On the other hand, when the mixing ratio of granulated fine powder was too high, the above-mentioned effect was not obtained (Comparative Example 2).

<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.

Example 5
[Preparation of fine powder group]
Crosslinked polymer particles (granulated particles (Y5)), which are a fraction that passed through an 850 μm sieve but did not pass through a 180 μm sieve, were obtained in the same manner as in Example 1, except that 45 g of primary fine powder (b1) was used instead of the primary fine powder (a1) and 5 g of granulated fine powder (b2) was used instead of the granulated fine powder (a1).

(Comparative Example 4)
[Preparation of fine powder group]
Crosslinked polymer particles (granulated particles (BA2)), which are a fraction that passed through an 850 μm sieve but did not pass through a 180 μm sieve, were obtained in the same manner as in Example 5, except that the amount of primary fine powder (b1) was changed to 37.5 g and 12.5 g of primary fine powder (a1) was used instead of primary fine powder (b2).

Even when a different primary fine powder from Test System 1 was used, in the examples in which granulated fine powder was mixed with primary fine powder in a specified ratio and granulated, the proportion of particles with particle diameters of 180 to 850 μm in the particle size distribution of the particles obtained by agglomeration and grinding (granulated particles before classification) was higher than in the comparative examples in which primary fine powder alone was granulated.

<Test system 3>
(Comparative Example 5)
The granulated particles (C2) obtained in the granulation step of Production Example 3 were used as Comparative Example 5.

(Comparative Example 6)
[Preparation of fine powder group]
Crosslinked polymer particles (granulated particles (Y6)), which are a fraction that passed through an 850 μm sieve but did not pass through a 180 μm sieve, were obtained in the same manner as in Example 1, except that 37.5 g of primary fine powder (c1) was used instead of the primary fine powder (a1) and 12.5 g of granulated fine powder (c2) was used instead of the granulated fine powder (a1).

(Comparative Example 7)
[Pelletization process]
Using 40 g of the granulated fine powder (c2), the same operation as in the granulation step of Production Example 1 was carried out to obtain crosslinked polymer particles (granulated particles (C3)), which are a fraction that passed through an 850 μm sieve but did not pass through a 180 μm sieve.

When the CRC of the primary fine powder was a high value of 56 g/g, even when the primary fine powder and the granulated fine powder were mixed in a certain ratio and granulated, the particle size distribution of the particles obtained after granulation and pulverization was not improved compared to when the primary fine powder was granulated alone.

Claims (7)

A method for producing water-absorbent resin particles, comprising:
Preparing a polymer fine powder that passes through a sieve with an opening of 180 μm and has a CRC of 55 g/g or less;
agglomerating the polymer fines by mixing the polymer fines with water to form first agglomerates;
drying and pulverizing the first agglomerates to obtain a granulated fine powder that passes through a sieve with openings of 180 μm;
mixing the polymer fine powder before forming the first agglomerates and the particle groups including the granulated fine powder with water to aggregate the particle groups to form second agglomerates;
drying and grinding the second agglomerates to form granulated particles;
the content of the granulated fine powder in the particle groups is more than 0% by mass and 30% 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. The method of claim 1, wherein the polymer fines are fines other than those formed by a process comprising mixing the polymer fines with water to agglomerate the polymer fines to form agglomerates, and drying and grinding the agglomerates. The method of claim 1 or 2, wherein the content of the polymer fine powder in the particle group is 60% by mass or more and less than 100% by mass of the total amount of the particle group. The method according to any one of claims 1 to 3, wherein the total amount of the polymer fine powder and the granulated fine powder is 90% by mass or more of the total amount of the particle group. The method according to any one of claims 1 to 4, further comprising classifying the granulated particles. The method according to any one of claims 1 to 5, wherein the granulated particles include particles that do not pass through a sieve with an opening of 180 μm. The method according to any one of claims 1 to 6, 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.