WO2022025003A1

WO2022025003A1 - Water-absorbent resin particles, and method for producing water-absorbent resin particles

Info

Publication number: WO2022025003A1
Application number: PCT/JP2021/027576
Authority: WO
Inventors: 直矢淡路
Original assignee: 住友精化株式会社
Priority date: 2020-07-28
Filing date: 2021-07-26
Publication date: 2022-02-03
Also published as: JPWO2022025003A1

Abstract

One aspect of the present invention relates to water-absorbent resin particles that have been surface crosslinked, the water-absorbent resin particles having a median particle diameter of 200-430 μm and a gel dissociation degree of 20-60%.

Description

Method for producing water-absorbent resin particles and water-absorbent resin particles

The present invention relates to water-absorbent resin particles and a method for producing the water-absorbent resin particles.

Conventionally, an absorber containing water-absorbent resin particles has been used as an absorbent article for absorbing a liquid containing water as a main component such as urine. In the production of the water-absorbent resin particles, a step of pulverizing the block-shaped or coarse-particle-shaped polymer obtained by the polymerization into particles by a treatment such as pulverization is performed. Small particles such as particles having a particle diameter of less than 180 μm generated by pulverization are used by increasing the particle diameter by granulation (see, for example, Patent Document 1).

Japanese Unexamined Patent Publication No. 2008-526502

In order to improve the performance of the absorbent article, it is required to increase the water absorption rate of the water-absorbent resin particles contained therein. Further, the water-absorbent resin particles are also required to improve the performance balance between the centrifuge holding capacity (CRC) and the water absorption ratio under pressure (AAP).

The present invention provides a method for producing water-absorbent resin particles and water-absorbent resin particles having an excellent water absorption rate and an excellent performance balance between the centrifuge holding capacity (CRC) and the water absorption ratio under pressure (AAP). The purpose is.

The present invention relates to surface-crosslinked water-absorbent resin particles having a medium particle size of 200 to 430 μm and a gel dissociation degree of 20 to 60%. The present invention also includes a step of adding a cross-linking agent solution containing a cross-linking agent and water to the polymer fine powder passing through a sieve having an opening of 180 μm or less to simultaneously perform adhesion of the polymer fine powder and surface cross-linking. , The present invention relates to a method for producing water-absorbent resin particles.

According to the present invention, there are methods for producing water-absorbent resin particles and water-absorbent resin particles having an excellent water absorption rate and an excellent balance between the centrifuge holding capacity (CRC) and the water absorption ratio under pressure (AAP). Can be provided.

It is a schematic diagram which shows the measuring method of the absorption magnification under pressure.

Hereinafter, some embodiments of the present invention will be described in detail. However, the present invention is not limited to the following embodiments.

In the present specification, "(meth) acrylic" means both acrylic and methacrylic. Similarly, "acrylate" and "methacrylate" are also referred to as "(meth) acrylate". The same is true for other similar terms. "(Poly)" shall mean both with and without the "poly" prefix. Within the numerical range described stepwise herein, the upper or lower limit of the numerical range at one stage may be optionally combined with the upper or lower limit of the numerical range at another stage. In the numerical range described in the present specification, the upper limit value or the lower limit value of the numerical range may be replaced with the value shown in the examples. "Water-soluble" means that it exhibits a solubility in water of 5% by mass or more at 25 ° C. The materials exemplified in the present specification may be used alone or in combination of two or more. "Saline" means a 0.9% by mass sodium chloride aqueous solution. "Room temperature" means 25 ° C.

The water-absorbent resin particles according to the present embodiment are surface-crosslinked water-absorbent resin particles having a medium particle size of 200 to 430 μm and a gel dissociation degree of 20 to 60%. The water-absorbent resin particles have an excellent water absorption rate and are also excellent in the performance balance between CRC and AAP.

In one embodiment of the method for producing the water-absorbent resin particles, a cross-linking agent and a cross-linking agent solution containing water are added to the polymer fine powder passing through a sieve having an opening of 180 μm or less, and the polymer fine powder is adhered. And surface cross-linking are performed at the same time. By adhering and surface-crosslinking the polymer fine powder, surface-crosslinked adhered particles can be obtained.

The adherent particles according to the present embodiment are particles obtained by weakly granulating (weakly granulating) the polymer fine powder with a small amount of water, and are aggregated as compared with the particles obtained through the conventional granulation step. Since the strength is weak and the particles crumble when water is absorbed, the surface area is increased and the water absorption rate can be increased. In addition, the adhered particles do not return to fine powder at once by absorbing water, so that they do not become mamaco. Further, by performing surface cross-linking at the same time as adhesion, the performance balance between CRC and AAP can be improved.

The polymer fine powder can be produced, for example, by polymerizing a monomer to obtain a hydrogel-like polymer, and then pulverizing and classifying the hydrogel-like polymer. Hereinafter, each step of producing the polymer fine powder and the water-absorbent resin particles according to the present embodiment will be described in detail.

[polymerization]
First, a monomer containing an ethylenically unsaturated monomer is polymerized to obtain a hydrogel-like polymer. The water-containing gel-like polymer may be a cross-linked polymer formed by polymerization of a monomer containing an ethylenically unsaturated monomer and formed into a gel-like state containing water. The water-absorbent resin particles obtained by the production method according to the present embodiment can contain a crosslinked polymer formed by polymerizing a monomer containing an ethylenically unsaturated monomer. The crosslinked polymer has a monomeric unit derived from an ethylenically unsaturated monomer. That is, the water-absorbent resin particles according to the present embodiment can have a structural unit derived from an ethylenically unsaturated monomer.

Polymerization can be performed, for example, by an aqueous solution polymerization method. Hereinafter, the polymerization of the monomer by the aqueous solution polymerization method will be described.

The ethylenically unsaturated monomer is preferably water-soluble. Examples of the ethylenically unsaturated monomer include carboxylic acid-based monomers such as (meth) acrylic acid, maleic acid, maleic anhydride, fumaric acid and salts thereof; (meth) acrylamide, N, N-dimethyl. Nonionic monomers such as (meth) acrylamide, 2-hydroxyethyl (meth) acrylate, N-methylol (meth) acrylamide, polyethylene glycol mono (meth) acrylate; N, N-diethylaminoethyl (meth) acrylate, N , N-diethylaminopropyl (meth) acrylate, diethylaminopropyl (meth) acrylamide and other amino group-containing unsaturated monomers and quaternized products thereof; vinyl sulfonic acid, styrene sulfonic acid, 2- (meth) acrylamide-2-methyl Examples thereof include sulfonic acid-based monomers such as propanesulfonic acid, 2- (meth) acryloylethanesulfonic acid and salts thereof. As the ethylenically unsaturated monomer, one type may be used alone, or two or more types may be used in combination.

The ethylenically unsaturated monomer preferably contains at least one selected from the group consisting of (meth) acrylic acid and salts thereof, maleic acid, fumaric acid, (meth) acrylamide, and N, N-dimethylacrylamide. , (Meta) It is more preferable to contain at least one selected from (meth) acrylic acid and salts thereof. Further, (meth) acrylic acid and a salt thereof may be copolymerized with another ethylenically unsaturated monomer. In this case, 70 to 100 mol% of the above (meth) acrylic acid and a salt thereof are preferably used, more preferably 80 to 100 mol%, and 90 to 90 to the total amount of the ethylenically unsaturated monomer. It is more preferable to use 100 mol%. The ethylenically unsaturated monomer preferably contains at least one of acrylic acid and a salt thereof.

When the ethylenically unsaturated monomer has an acid group such as (meth) acrylic acid and 2- (meth) acrylamide-2-methylpropanesulfonic acid, the acid group is previously an alkaline neutralizing agent if necessary. Can be used after being neutralized by. Examples of such an alkaline neutralizer include alkali metal salts such as sodium hydroxide, sodium carbonate, sodium hydrogencarbonate, potassium hydroxide and potassium carbonate; ammonia and the like. These alkaline neutralizers may be used in the form of an aqueous solution in order to simplify the neutralization operation. One type of alkaline neutralizer may be used alone, or two or more types may be used in combination. The acid group may be neutralized before the polymerization of the ethylenically unsaturated monomer as a raw material, or may be performed during or after the polymerization.

The degree of neutralization of the ethylenically unsaturated monomer by the alkaline neutralizer enhances the water absorption performance by increasing the osmotic pressure of the obtained water-absorbent resin particles, and is safe due to the presence of the excess alkaline neutralizer. From the viewpoint of preventing problems such as, usually, it is preferably 10 to 100 mol%, more preferably 30 to 90 mol%, further preferably 40 to 85 mol%, and 50. It is even more preferably ~ 80 mol%. Here, the degree of neutralization is the degree of neutralization for all the acid groups of the ethylenically unsaturated monomer.

It is usually preferable to use the ethylenically unsaturated monomer in the state 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 “monomer aqueous solution”) may be 20% by mass or more and the saturation concentration or less, and is 25 to 70. The mass% is preferable, and 30 to 50% by mass is more preferable.

The amount of the ethylenically unsaturated monomer used is the total amount of the monomer (the total amount of the monomer for obtaining the water-absorbent resin particles. For example, the total amount of the monomers giving the structural unit of the crosslinked polymer. The same applies hereinafter). It may be 70 to 100 mol%, 80 to 100 mol%, 90 to 100 mol%, 95 to 100 mol%, or 100 mol%. Among them, the ratio of (meth) acrylic acid and its salt may be 70 to 100 mol% with respect to the total amount of the monomer, 80 to 100 mol%, 90 to 100 mol%, 95 to 100 mol%, or It may be 100 mol%. "Ratio of (meth) acrylic acid and its salt" means the ratio of the total amount of (meth) acrylic acid and its salt.

The water-absorbent resin particles are, for example, water-absorbent resin particles containing a crosslinked polymer having a structural unit derived from an ethylenically unsaturated monomer, and the ethylenically unsaturated monomer includes (meth) acrylic acid and It contains at least one compound selected from the group consisting of the salts, and the ratio of (meth) acrylic acid and its salts is 70 to 100 mol% with respect to the total amount of monomers for obtaining water-absorbent resin particles. It may be a thing.

The monomer aqueous solution may contain a polymerization initiator. The polymerization of the monomer contained in the aqueous monomer solution is started by adding a 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 and a radical polymerization initiator, and among them, a water-soluble radical polymerization initiator is preferably used. The polymerization initiator may be, for example, an azo compound, a peroxide or the like.

Examples of the azo compound include 2,2'-azobis [2- (N-phenylamidino) propane] dihydrochloride and 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-Imidazoline-2-yl) propane] dihydrochloride, 2,2'-azobis [2- (4,5,6,7-tetrahydro-1H-1,3-diazepine-2-yl) propane] Dihydrochloride, 2,2'-azobis [2- (5-hydroxy-3,4,5,6-tetrahydropyrimidine-2-yl) propane] dihydrochloride, 2,2'-azobis {2- [1 -(2-Hydroxyethyl) -2-imidazolin-2-yl] propane} dihydrochloride, 2,2'-azobis (2-methylpropionamide) dihydrochloride, 2,2'-azobis [2- (2) -Imidazoline-2-yl) propane] disulfate dihydrate, 2,2'-azobis [N- (2-carboxyethyl) -2-methylpropionamidine] tetrahydrate, 2,2'-azobis Examples thereof include azo compounds such as [2-methyl-N- (2-hydroxyethyl) propionamide]. From the viewpoint that water-absorbent resin particles having good water-absorbing performance can be easily obtained, 2,2'-azobis (2-amidinopropane) dihydrochloride, 2,2'-azobis {2- [1- (2-hydroxy) Ethyl) -2-imidazolin-2-yl] propane} dihydrochloride and 2,2'-azobis [N- (2-carboxyethyl) -2-methylpropionamidine] tetrahydrate are preferred. These azo compounds may be used alone or in combination of two or more.

Examples of the peroxide include persulfates such as potassium persulfate, ammonium persulfate, and sodium persulfate; methyl ethyl ketone peroxide, methyl isobutyl ketone peroxide, di-t-butyl peroxide, t-butyl cumyl peroxide, t. -Organic peroxides such as butylperoxyacetate, t-butylperoxyisobutyrate, t-butylperoxypivalate; peroxides such as hydrogen peroxide can be mentioned. Among these peroxides, potassium persulfate, ammonium persulfate, sodium persulfate, or hydrogen peroxide is preferably used, preferably potassium persulfate, from the viewpoint of obtaining water-absorbent resin particles having good water absorption performance. It is more preferable to use ammonium persulfate or sodium persulfate. One of these peroxides may be used alone, or two or more thereof may be used in combination.

It can also be used as a redox polymerization initiator by using a polymerization initiator and a reducing agent in combination. Examples of the reducing agent include sodium sulfite, sodium hydrogen sulfite, ferrous sulfate, and L-ascorbic acid.

From the viewpoint of the performance balance of CRC and AAP of the water-absorbent resin particles, the amount of the polymerization initiator is 0.05 to 1 mmol, 0.08 to 0.8 mmol, or 0.1 per 1 mol of the monomer. It may be up to 0.7 mmol.

The monomer aqueous solution preferably contains an internal cross-linking agent. By including the internal cross-linking agent, the obtained cross-linked polymer can have cross-linking by the internal cross-linking agent in addition to self-cross-linking by the polymerization reaction as its internal cross-linking structure.

The internal cross-linking agent may contain a compound having a (meth) acrylic group, an allyl group, an epoxy group, or an amino group. A compound having two or more of these reactive functional groups can be used as an internal cross-linking 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, and glycerol tri (meth) acrylate. Examples thereof include 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. As the internal cross-linking agent, one type may be used alone, or two or more types may be used in combination.

When the internal cross-linking agent is used, the amount used is 0.02 to 1.0 mmol or 0.05 to 1 mol of the monomer from the viewpoint of adjusting the performance balance of CRC and AAP of the water-absorbent resin particles. It may be 0.8 mmol, or 0.1 to 0.6 mmol.

The monomer aqueous solution may contain additives such as a chain transfer agent and a thickener, if necessary. Examples of the chain transfer agent include thiols, thiol acids, secondary alcohols, hypophosphorous acid, phosphorous acid and the like. Examples of the thickener include carboxymethyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, methyl cellulose, polyethylene glycol, polyacrylic acid, polyacrylic acid neutralizer, polyacrylamide and the like. These may be used alone or in combination of two or more. A solvent other than water, such as a water-soluble organic solvent, may be appropriately added to the monomer aqueous solution.

The polymerization method is, for example, a static polymerization method in which the monomer aqueous solution is polymerized without stirring (for example, a static state), or a stirring polymerization method in which the monomer aqueous solution is polymerized while stirring in the reaction apparatus. It's okay. It is preferable to obtain a hydrogel-like polymer by static polymerization of an aqueous solution, which is a static polymerization method. In the static polymerization method, it is possible to obtain a single block-shaped hydrogel-like polymer that occupies substantially the same volume as the monomer aqueous solution present in the reaction vessel when the polymerization is completed.

The form of production may be batch, semi-continuous, continuous, etc. For example, in the aqueous solution static continuous polymerization, a polymerization reaction can be carried out while continuously supplying a monomer aqueous solution to a belt conveyor-shaped continuous polymerization apparatus to obtain a water-containing gel having a continuous shape such as a band shape. ..

The polymerization temperature varies depending on the polymerization initiator used, but is preferably 0 to 130 ° C., more preferably 10 to 110 ° C. from the viewpoint of increasing the productivity by rapidly advancing the polymerization and shortening the polymerization time. The polymerization time is appropriately set according to the type or amount of the polymerization initiator used, the reaction temperature and the like, but is preferably 1 to 200 minutes, more preferably 5 to 100 minutes.

[Particle]
Next, the hydrogel-like polymer is atomized to obtain a particle group containing fine particles having a particle diameter of less than 180 μm. The particle-forming step may include, for example, a step of coarsely crushing the hydrogel-like polymer to obtain a coarsely crushed product, and a step of further crushing the coarsely crushed material to obtain a particle group. The particle formation step may include a step of drying the hydrogel-like polymer, the crude product and / or the particle group. The coarsely crushed product is preferably subjected to pulverization after undergoing a drying step. The particle formation step may further include a step of classifying the particle group obtained by pulverization. The particle group obtained by the particle formation step may consist only of fine particles having a particle diameter of less than 180 μm. Hereinafter, each step in the particle formation step will be described in detail.

(Coarse)
The particle formation step can include a step of coarsely crushing the hydrogel-like polymer to obtain a coarsely crushed product. The coarsely crushed product obtained by coarsely crushing the hydrogel-like polymer may be in the form of a hydrogel. The coarsely crushed product may be in the form of particles, or may have an elongated shape such as a series of particles. The size of the minimum side of the coarsely crushed product may be, for example, about 0.1 to 15 mm or about 1.0 to 10 mm. The size of the maximum side of the coarsely crushed product may be about 0.1 to 200 mm, or about 1.0 to 150 mm.

As the crushing device, for example, a kneader (for example, a pressurized kneader, a double-armed kneader, etc.), a meat chopper, a cutter mill, a pharma mill, or the like can be used, and a double-armed kneader, a meat chopper, a cutter mill, or the like can be used. ..

(Dry)
The particleization step can include the step of drying the hydrogel polymer, the coarsely crushed product and / or the pulverized product. These dried products can be obtained by removing the solvent containing water in the water-containing gel polymer, the coarsely crushed product, or the crushed product by heating and / or sending. The drying method may be a method of natural drying, heat drying, vacuum drying or the like. The drying may be performed under normal pressure or reduced pressure, for example, and may be performed under an air flow such as nitrogen in order to improve the drying efficiency. Drying may be performed by combining a plurality of methods. The crushed product may be dried under normal pressure or reduced pressure. The heating temperature for drying under normal pressure may be 70 to 250 ° C. or 80 to 200 ° C. The water 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.

(Crushing)
The particleization step can include a step of pulverizing a hydrogel-like polymer, a coarsely crushed product, and / or a dried product thereof. For pulverization, for example, a roller mill (roll mill), a stamp mill, a jet mill, or a high-speed rotary pulverization can be used to obtain a particle group containing fine powder by pulverizing a hydrogel polymer, a coarse pulverized product, and / or a dried product thereof. A crusher such as a machine (ultra-centrifugal crusher, hammer mill, pin mill, rotor beater mill, etc.), container-driven mill (rotary mill, vibration mill, planetary mill, etc.) can be used. The crusher may have an opening on the outlet side, such as a perforated plate, a screen, a grid, etc., for controlling the maximum particle size of the crushed 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.

The pulverization may be performed so that at least a part of the particle group becomes fine powder having a particle diameter of less than 180 μm. In the pulverization, for example, while pulverizing for the main purpose of obtaining polymer particles having a particle diameter of less than 850 μm and having an appropriate particle diameter that can be used without granulation, some fine particles having a particle diameter of less than 180 μm are generated. It can be done in such a way.

The abundance of fine particles having a particle diameter of less than 180 μm in the total amount of the particle group obtained by the particle formation step may be, for example, 1 to 100% by mass, preferably 30 to 60% by mass. A particle size of less than 180 μm means a particle that can pass through a JIS standard sieve having an opening of 180 μm.

The particle group obtained by the particle formation step may include particles having a particle diameter of 180 μm or more and less than 850 μm (particles that pass through a JIS standard sieve having a mesh size of 850 μm and do not pass through a JIS standard sieve having a mesh size of 180 μm). ..

(Classification)
The particle formation step may include a step of classifying the particle group obtained by pulverization. Classification refers to an operation of dividing a certain particle group into two or more particle groups having different particle size distributions according to the particle size. By classification, a particle group consisting only of fine particles having a particle size of less than 180 μm may be separated, or the abundance of fine particles having a particle size of less than 180 μm in the particle group may be increased. If necessary, a part of the classified particles may be pulverized again, or the pulverization step and the classification step may be repeated. By classification, the polymer fine powder according to this embodiment can be obtained.

As the classification method, a known classification method can be used, and for example, screen classification or wind power classification may be used. Screen classification is a method of classifying particles on a screen into particles that pass through the mesh of the screen and particles that do not pass through the screen by vibrating the screen. Screen classification can be performed using, for example, a vibrating sieve, a rotary shifter, a cylindrical stirring sieve, a blower shifter, or a low-tap shaker. Wind power classification is a method of classifying particles using the flow of air.

The CRC (Centrifuge Retention Capacity) of the polymer fine powder may be, for example, 38 g / g or more, 41 g / g or more, 44 g / g or more, or 47 g / g or more, and 65 g / g or less, 63 g / g or less, 60 g. It may be / g or less, 55 g / g or less, or 50 g / g or less. CRC is measured by the method described in Examples described later with reference to the EDANA method (NWSP 241.0.R2 (15), pages 769 to 778).

[Adhesion / surface cross-linking]
In the method according to the present embodiment, a cross-linking agent solution containing a cross-linking agent and water is added to the polymer fine powder after classification to simultaneously perform adhesion of the polymer fine powder and surface cross-linking, and surface-crosslinked coagulation. The landing particles can be obtained. The surface-crosslinked adhered particles are weakly formed into particles due to the presence of a small amount of water when a cross-linking agent (surface cross-linking agent) for performing surface cross-linking of the polymer fine powder is added and reacted. It can be formed by doing. The water absorption performance of the water-absorbent resin particles can be improved by performing the adhesion and surface cross-linking treatment of the polymer fine powder.

The water content of the cross-linking agent solution may be 10 parts by mass or less, 9 parts by mass or less, 8.5 parts by mass or less, or 8 parts by mass or less with respect to 100 parts by mass of the polymer fine powder, and is 1 part by mass or more. , 1.5 parts by mass or more, 1.8 parts by mass or more, or 2 parts by mass or more.

The cross-linking agent solution may further contain alcohol. Examples of the alcohol include monohydric alcohols having 1 to 5 carbon atoms. The alcohol content of the cross-linking agent solution may be 5 parts by mass or less, 4 parts by mass or less, or 3.5 parts by mass or less, and 0.5 parts by mass or more and 1 part by mass with respect to 100 parts by mass of the polymer fine powder. It may be 10 parts or more, or 1.5 parts by mass or more.

As the surface cross-linking agent, for example, a compound containing two or more functional groups (reactive functional groups) having reactivity with a functional group derived from an ethylenically unsaturated monomer can be used. Examples of the surface cross-linking agent include alkylene carbonate compounds such as ethylene carbonate; ethylene glycol, propylene glycol, 1,4-butanediol, trimethylolpropane, glycerin, polyoxyethylene glycol, polyoxypropylene glycol, polyglycerin and the like. Polypoly compounds; (poly) ethylene glycol diglycidyl ether, (poly) glycerin diglycidyl ether, (poly) glycerin triglycidyl ether, trimethylolpropane triglycidyl ether (poly) propylene glycol polyglycidyl ether, and (poly) glycerol polyglycidyl. Polyglycidyl compounds such as ether; haloepoxy compounds such as epichlorohydrin, epibromhydrin, and α-methylepicrolhydrin; isocyanate compounds such as 2,4-tolylene diisocyanate and hexamethylenediisocyanate; 3-methyl-3-oxetanemethanol, Oxetane compounds such as 3-ethyl-3-oxetan methanol, 3-butyl-3-oxetan methanol, 3-methyl-3-oxetan ethanol, 3-ethyl-3-oxetan ethanol, and 3-butyl-3-oxetane ethanol; Oxazoline compounds such as 1,2-ethylenebisoxazoline; hydroxyalkylamide compounds such as bis [N, N-di (β-hydroxyethyl)] adipamide can be mentioned. These surface cross-linking agents may be used alone or in combination of two or more. The surface cross-linking agent may contain an alkylene carbonate compound, a polyol compound, or a combination thereof. The ratio of the alkylene carbonate compound in the surface cross-linking agent is 50 to 100% by mass, 60 to 100% by mass, 70 to 100% by mass, 80 to 100% by mass, or 90 to 100% by mass based on the total mass of the surface cross-linking agent. May be.

From the viewpoint of increasing the CRC and AAP of the water-absorbent resin particles, the amount of the surface cross-linking agent was 0.3 to 5.0 parts by mass and 0.5 to 3.0 parts by mass with respect to 100 parts by mass of the polymer fine powder. It may be 0.8 to 2.5 parts by mass or 1.0 to 2.0 parts by mass.

The heating temperature and heating time for surface cross-linking are adjusted so that the cross-linking reaction proceeds appropriately in consideration of the type of surface cross-linking agent and the like. For example, the heating temperature for surface cross-linking may be 80 ° C. or higher, 100 ° C. or higher, 120 ° C. or higher, 150 ° C. or higher, or 180 ° C. or higher, or 190 ° C. or higher. The heating temperature for surface cross-linking may be 250 ° C. or lower, 230 ° C. or lower, or 210 ° C. or lower. The heating time for surface cross-linking may be, for example, 5 to 90 minutes, 10 to 80 minutes, or 15 to 60 minutes.

The surface-crosslinked adhered particles may be further dried or classified if necessary. The adhered particles may be used as they are as the water-absorbent resin particles, or the inorganic particles may be attached to the surface of the polymer particles obtained by classifying the surface-crosslinked adhered particles. That is, the water-absorbent resin particles may contain a plurality of inorganic particles arranged on the surface of the polymer particles. For example, by mixing the polymer particles and the inorganic particles, the inorganic particles can be arranged on the surface of the polymer particles.

The inorganic particles may be silica particles such as amorphous silica. Inorganic particles usually have a small size as compared with the size of polymer particles. For example, the primary average particle size of the inorganic particles may be 1 to 500 nm, 2 to 100 nm, 5 to 50 nm, or 10 to 20 nm. The average particle size can be measured by a transmission electron microscope, a pore electric resistance method, or a laser diffraction / scattering method, depending on the characteristics of the particles.

When the water-absorbent resin particles include the inorganic particles arranged on the surface, the content of the inorganic particles is 0.05 parts by mass or more and 0.08 parts by mass or more based on the total mass of 100 parts of the polymer particles. , 0.1 parts by mass or more, 0.15 parts by mass or more, or 0.2 parts by mass or more, 5.0 parts by mass or less, 3.0 parts by mass or less, 1.0 part by mass or less, 0. It may be 5 parts by mass or less, or 0.3 parts by mass or less.

The shape of the water-absorbent resin particles according to the present embodiment may be, for example, a crushed shape, an amorphous shape, an amorphous crushed shape, or a shape formed by aggregating these particles. The medium particle size of the water-absorbent resin particles is 200 to 430 μm from the viewpoint of further improving the performance balance between CRC and AAP, even if it is 205 to 400 μm, 210 to 350 μm, 220 to 350 μm, or 250 to 320 μm. good.

The degree of gel dissociation of the water-absorbent resin particles may be 25 to 60%, 30 to 60%, or 32 to 60% from the viewpoint of increasing the water absorption rate. The degree of gel dissociation of the water-absorbent resin particles is measured by the method described in Examples described later.

The CRC of the water-absorbent resin particles according to the present embodiment may be, for example, 22 g / g or more, 24 g / g or more, 25 g / g or more, or 26 g / g or more, and 70 g / g or less, 60 g / g or less, It may be 50 g / g or less, or 45 g / g or less.

The absorption ratio (AAP, Absorption Gainst Pressure) of the water-absorbent resin particles according to the present embodiment under a pressure of 2.07 kPa (0.3 psi) is, for example, 10 g / g or more, 11 g / g or more, or 12 g / g or more. It may be 38 g / g or less, 34 g / g or less, 30 g / g or less, or 26 g / g or less. The absorption ratio of the water-absorbent resin particles under a pressure of 2.07 kPa is measured by the method described in Examples described later.

The water absorption rate of the water-absorbent resin particles according to the present embodiment by the Vortex method may be, for example, 2 to 45 seconds, 5 to 40 seconds, 8 to 35 seconds, or 10 to 30 seconds. The absorption rate is measured by the method described in Examples below.

The water-absorbent resin particles according to the present embodiment are used to form an absorbent body constituting an absorbent article such as a diaper, for example.

Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited to these examples.

1. 1. Measurement method The water content, centrifuge holding capacity (CRC), absorption ratio under pressure (AAP), water absorption rate by the Vortex method, medium particle size, and gel dissociation degree were measured by the following procedures.

(Moisture content)
1.0 g of a sample (polymer powder or polymer fine powder) is placed in an aluminum foil case (No. 8) having a constant mass (W1 (g)) in advance, and the mouth of the aluminum foil case is lightly closed. The total mass W2 (g) of the above was precisely weighed. The above-mentioned aluminum foil case containing a sample is dried for 2 hours in a hot air dryer (manufactured by ADVANTEC, model: FV-320) whose internal temperature is set to 200 ° C. The dried aluminum foil case containing the sample was allowed to cool to room temperature in a desiccator. The total mass W3 (g) of the aluminum foil case containing the sample after allowing to cool was measured, and the water content of the sample was calculated from the following formula.
Moisture content (% by mass) = [{(W2-W1)-(W3-W1)} / (W2-W1)] × 100

(Centrifuge holding capacity)
The centrifuge holding capacity (CRC) was measured by the following procedure with reference to the EDANA method (NWSP 241.0.R2 (15), pages 769 to 778). The measurement was performed in an environment where the temperature was 25 ° C. ± 2 ° C. and the humidity was 50% ± 10%.

A non-woven fabric with a size of 60 mm x 170 mm (product name: Heat Pack MWA-18, manufactured by Nippon Paper Papylia Co., Ltd.) was folded in half in the longitudinal direction to adjust the size to 60 mm x 85 mm. A 60 mm × 85 mm non-woven fabric bag was produced by crimping the non-woven fabrics to each other on both sides extending in the longitudinal direction with a heat seal (a crimped portion having a width of 5 mm was formed on both sides along the longitudinal direction). 0.2 g of the particles to be measured were precisely weighed and contained in the non-woven fabric bag. Then, the non-woven fabric bag was closed by crimping the remaining one side extending in the lateral direction with a heat seal.

The entire non-woven fabric bag was completely moistened by floating the non-woven fabric bag on 1000 g of physiological saline contained in a stainless steel vat (240 mm × 320 mm × 45 mm) without folding the non-woven fabric bag. One minute after the nonwoven fabric bag was put into the physiological saline solution, the nonwoven fabric bag was immersed in the physiological saline solution with a spatula to obtain a nonwoven fabric bag containing the gel.

Thirty minutes after the non-woven fabric bag was put into the physiological saline solution (a total of 1 minute of floating time and 29 minutes of immersion time), the non-woven fabric bag was taken out from the physiological saline solution. Then, the non-woven fabric bag was put in a centrifuge (manufactured by Kokusan Co., Ltd., model number: H-122). After the centrifugal force in the centrifuge reached 250 G, the non-woven fabric bag was dehydrated for 3 minutes. After dehydration, the mass Ma [g] of the non-woven fabric bag containing the mass of the gel was weighed. The non-woven fabric bag was subjected to the same operation as described above without accommodating the particles to be measured, and the mass Mb [g] of the non-woven fabric bag after dehydration was measured. CRC [g / g] was calculated based on the following formula. Mc [g] is a precise value of 0.2 g of the mass of the particle to be measured used for the measurement.
CRC [g / g] = {(Ma [g] -Mb [g])-Mc [g]} / Mc [g]

(Absorption rate under pressure)
Using the measuring device 110 shown in FIG. 1, the absorption magnification (AAP) under pressure of 2.07 kPa (0.3 psi) was measured at 25 ° C. ± 2 ° C. First, a weight 112 (cross section: circular, total weight: 577.38 g) adjusted to a pressure of 2.07 kPa, a plastic cylinder 114 having an inner diameter of 60 mm, and one end (bottom surface) of the cylinder 114 were arranged. A measuring device 110 provided with a wire mesh 116 having a 400 mesh (opening 38 μm) was prepared. The weight 112 is a cylinder having a disc portion 112a (diameter 59 mm), a rod-shaped portion 112b extending from the center of the disc portion 112a in a direction perpendicular to the disc portion 112a, and a through hole inserted into the rod-shaped portion 112b in the center. It has a portion 112c and. The disk portion 112a of the weight 112 has a diameter substantially equal to the inner diameter of the cylinder 114 so that it can move in the longitudinal direction of the cylinder 114 inside the cylinder 114. The diameter of the cylindrical portion 112c is smaller than the diameter of the disc portion 112a. One end of the cylinder 114 is open but shielded by the wire mesh 116, and the other end of the cylinder 114 is open so that the weight 112 can be inserted. Inside the cylinder 114, 0.90 g of the particles 120 to be measured were uniformly sprayed on the wire mesh 116. Then, after the weight 112 is inserted inside the cylinder 114 and the weight 112 is placed on the measurement target particle 120, the total mass of the measuring device 110 (the total mass of the measuring device 110 and the measurement target particle 120 before liquid absorption). Wa [g] was measured.

After placing a glass filter 140 (ISO4793 P-250) with a diameter of 90 mm and a thickness of 7 mm in the center of the bottom surface of the recess of the stainless petri dish 130 with a diameter of 150 mm, 0 so that the water surface is at the same height as the upper surface of the glass filter 140. .90% by mass sodium chloride aqueous solution (25 ° C ± 2 ° C) was added. A sheet of filter paper 150 with a diameter of 90 mm (manufactured by ADVANTEC Toyo Co., Ltd., product name: (No. 3), thickness 0.23 mm, reserved particle diameter 5 μm) was placed on the glass filter 140 so that the entire surface was wet. And the excess liquid was removed. Then, the above-mentioned measuring device 110 was placed on the filter paper 150, and the liquid was absorbed under a load. After 1 hour, the measuring device 110 was lifted, and the total mass of the measuring device 110 (total mass of the measuring device 110 and the measurement target particle 120 after liquid absorption) Wb [g] was measured.

From Wa and Wb, the absorption rate (AAP) [g / g] under 2.07 kPa pressurization was calculated based on the following formula.
AAP [g / g] = (Wb [g] -Wa [g]) /0.90 [g]

(Water absorption rate)
The water absorption rate of the physiological saline of the water-absorbent resin particles was measured by the following procedure based on the Vortex method. First, 50 ± 0.1 g of physiological saline adjusted to a temperature of 25 ± 0.2 ° C. in a constant temperature water tank was weighed in a beaker having an internal volume of 100 mL. Next, a vortex was generated by stirring at a rotation speed of 600 rpm using a magnetic stirrer bar (8 mmφ × 30 mm, without ring). 2.0 ± 0.002 g of water-absorbent resin particles were added to the aqueous sodium chloride solution at one time. The time [seconds] from the addition of the water-absorbent resin particles to the time when the vortex on the liquid surface converged was measured, and the time was obtained as the water absorption rate of the water-absorbent resin particles.

(Medium particle size)
10 g of powder of water-absorbent resin particles is mixed with a continuous fully automatic sonic vibration type sieving measuring instrument (robot shifter RPS-205, manufactured by Seishin Enterprise Co., Ltd.) and JIS standard opening 850 μm, 600 μm, 500 μm, 425 μm, 300 μm, 250 μm. And 180 μm sieve and a saucer were used for sieving. The mass of the particles remaining on each sieve was calculated as a mass percentage to the total amount. The mass percentages of the particles remaining on each sieve were integrated in order from the one with the largest particle size, and the relationship between the mesh opening of the sieve and the integrated value of the mass percentages of the particles remaining on the sieve was plotted on a logarithmic probability paper. .. By connecting the plots on the probability paper with a straight line, the particle size corresponding to the integrated mass percentage of 50% by mass was obtained, and this was defined as the medium particle size.

(Degree of gel dissociation)
A JIS standard sieve (diameter 20 cm) was combined in this order from the top, a sieve with an opening of 850 μm, a sieve with an opening of 180 μm, and a saucer. Water-absorbent resin particles were placed in the combined top sieve and classified at 25 ° C. ± 2 ° C. according to JIS Z 8815 (1994). The water-absorbent resin particles of the fraction that passed through the 850 μm sieve and did not pass through the 180 μm sieve were used for the subsequent measurements.

1000 g of saline was weighed in a beaker with an internal volume of 2000 mL. Add 1.0 g of the water-absorbent resin particles (180 to 850 μm) obtained above to the sodium chloride aqueous solution little by little without stirring and let stand for 60 minutes to swell the water-absorbent resin particles. I let you. Subsequently, a JIS standard sieve (diameter 20 cm) was combined in this order from the top with a sieve having a mesh opening of 600 μm and a sieve having a mesh opening of 75 μm, and placed in a plastic vat. The above-mentioned contents after being allowed to stand for 60 minutes were placed in the combined top sieve, and the saline solution that had passed through the bottom sieve was collected in a plastic vat. The entire amount of recovered saline was put back into the best combined sieve and flushed with saline to classify the swollen gel. At this time, the total amount of the recovered physiological saline solution was poured at once from a height of 5 cm above the gel surface so as to hit the swollen gel on the sieve. After repeating the above recovery operation and classification operation 10 times, the combined sieves are separated, and the swollen gel is placed on the sieve with each sieve tilted so as to have an inclination angle of about 30 degrees with respect to the horizontal. It was left for 30 minutes to remove excess water. After 30 minutes, the degree of gel dissociation was calculated by the following formula based on the total amount of swollen gel remaining on the sieve of each opening.
Gel dissociation degree [mass%] = (amount of swelling gel that has passed through a sieve with an opening of 600 μm and remained on a sieve with an opening of 75 μm [g]) / {(a swelling gel remaining on a sieve with an opening of 600 μm Amount [g]) + (Amount of swelling gel that has passed through a sieve with a mesh opening of 600 μm and remains on a sieve with a mesh opening of 75 μm [g])} × 100

2. 2. Preparation of polymer fine powder (Production Example 1)
[polymerization]
425.65 g (5.91 mol) of acrylic acid was placed in a round bottom cylindrical separable flask equipped with a stirrer and having an inner diameter of 11 cm and an internal volume of 2 L. After adding 387.22 g of ion-exchanged water into the separable flask while stirring this acrylic acid, 320.09 g of 48% by mass sodium hydroxide was added dropwise under an ice bath to obtain a monomer concentration of 45% by mass. Acrylic acid partial neutralizing solution (1132.96 g) was prepared. This operation was repeated twice to obtain a required amount of acrylic acid partial neutralizing solution.

Ion-exchanged water 1019.31 g and polyethylene glycol diacrylate (n≈9, internal cross-linking agent, manufactured by Nichiyu Co., Ltd., product name: Blemmer ADE-400A) 1.34 g are added to 2039.32 g of the acrylic acid partial neutralizing solution. In addition, a reaction solution (monomer aqueous solution) was obtained. The amount of dissolved oxygen was adjusted to 0.1 ppm or less by substituting the reaction solution with nitrogen gas for 30 minutes in a nitrogen gas atmosphere. Next, a 5 L stainless steel double-armed kneader (manufactured by Irie Shokai Co., Ltd.) with a jacket equipped with a thermometer and a nitrogen blow tube and having two sigma-shaped blades with a lid that can be opened and closed was prepared. The above reaction solution was supplied to the kneader, and the inside of the kneader was replaced with nitrogen gas while keeping the reaction solution at 25 ° C. Subsequently, while stirring the reaction solution, 2.55 g (2.14 mmol) of a 20 mass% sodium persulfate aqueous solution and 10 mass% (2,2'-azobis (2-amidinopropane) dihydrochloride (V). -50) Aqueous solution (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) 10.22 g (3.77 mmol), 0.1% by mass L-ascorbic acid aqueous solution 4.47 g, and 0.35% by mass hydrogen peroxide solution. When 2.55 g was added, the temperature began to rise after about 1 minute and polymerization started. After 20 minutes, the jacket temperature was raised to 80 ° C., at which time the thermometer showed 42 ° C. Stirring was continued while maintaining the jacket temperature at 80 ° C., the polymerization reaction was sufficiently completed, and the aqueous gel-like polymer produced 60 minutes after the temperature rise was taken out.

[Drying]
The hydrogel polymer was spread on a wire mesh having an opening of 0.8 cm × 0.8 cm and dried with hot air at 180 ° C. for 30 minutes to obtain a dried product.

[Crush]
The dried product was pulverized using a centrifugal pulverizer (ZM200 manufactured by Retsch, screen diameter 1 mm, 6000 rpm) to obtain an amorphous crushed particle group (A).

[Classification]
The particle group (A) was classified using a sieve having an opening of 850 μm and a sieve having a size of 180 μm. By classification, a polymer powder (A1), which was a fraction that passed through an 850 μm sieve and did not pass through a 180 μm sieve, and a polymer fine powder (a1), which was a fraction that passed through a 180 μm sieve, were obtained.

(Manufacturing Example 2)
[polymerization]
548.38 g (7.61 mol) of acrylic acid was placed in a round bottom cylindrical separable flask equipped with a stirrer and having an inner diameter of 11 cm and an internal volume of 2 L. After adding 469.71 g of ion-exchanged water into the separable flask while stirring this acrylic acid, 481.92 g of 48% by mass sodium hydroxide was added dropwise under an ice bath to give a monomer concentration of 45% by mass. Acrylic acid partial neutralizing solution (1500.01 g) was prepared. This operation was repeated twice to obtain a required amount of acrylic acid partial neutralizing solution.

Ion-exchanged water 406.49 g and polyethylene glycol diacrylate (Blemmer ADE-400A) 0.97 g were added to 2784.19 g of the acrylic acid partial neutralizing solution to obtain a reaction solution (monomer aqueous solution). The reaction solution was replaced with nitrogen gas for 30 minutes under a nitrogen gas atmosphere. Next, the above reaction solution was supplied to a 5 L stainless steel double-armed kneader (manufactured by Irie Shokai Co., Ltd.) with a jacket having two sigma-type blades with openable and closable lids, and the reaction solution was kept at 30 ° C. Was replaced with nitrogen gas to adjust the amount of dissolved oxygen to 0.1 ppm or less. Subsequently, while stirring the reaction solution, 92.52 g (7.77 mmol) of a 2.0 mass% sodium persulfate aqueous solution and 15.83 g of a 0.5 mass% L-ascorbic acid aqueous solution were added. After 1 minute, the temperature began to rise and polymerization started. After 7 minutes, the thermometer showed a maximum temperature of 76 ° C., then stirring was continued while keeping the jacket temperature at 60 ° C., and 60 minutes after the start of the polymerization, the produced hydrogel-like polymer was taken out.

[Drying]
The hydrogel polymer was sequentially charged into a meat chopper 12VR-750SDX manufactured by Kiren Royal Co., Ltd. and coarsely crushed. The diameter of the hole of the plate located at the outlet of the meat chopper was 6.4 mm. The coarsely crushed product of the hydrogel-like polymer was spread on a wire mesh having an opening of 0.8 cm × 0.8 cm and dried with hot air at 160 ° C. for 60 minutes to obtain a dried product.

[Crush]
The dried product was pulverized using a centrifugal pulverizer (ZM200 manufactured by Retsch, screen diameter 1 mm, 12000 rpm) to obtain an amorphous crushed particle group (B).
[Classification]
The particle group (B) was classified using a sieve having an opening of 850 μm and a sieve having a size of 180 μm. By classification, a polymer powder (B1), which was a fraction that passed through an 850 μm sieve and did not pass through a 180 μm sieve, and a polymer fine powder (b1), which was a fraction that passed through a 180 μm sieve, were obtained.

(Manufacturing Example 3)
[polymerization]
549.71 g (7.63 mol) of acrylic acid was placed in a round bottom cylindrical separable flask equipped with a stirrer and having an inner diameter of 11 cm and an internal volume of 2 L. After adding 470.72 g of ion-exchanged water into the separable flask while stirring this acrylic acid, 479.57 g of 48% by mass sodium hydroxide was added dropwise under an ice bath to obtain a monomer concentration of 45% by mass. Acrylic acid partial neutralizing solution (1500.00 g) was prepared. This operation was repeated twice to obtain a required amount of acrylic acid partial neutralizing solution.

Ion-exchanged water 406.89 g and polyethylene glycol diacrylate (Blemmer ADE-400A) 2.90 g were added to 2781.72 g of the acrylic acid partial neutralizing solution to obtain a reaction solution (monomer aqueous solution). The reaction solution was replaced with nitrogen gas for 30 minutes under a nitrogen gas atmosphere. Next, the above reaction solution was supplied to a 5 L stainless steel double-armed kneader (manufactured by Irie Shokai Co., Ltd.) with a jacket having two sigma-type blades with openable and closable lids, and the reaction solution was kept at 30 ° C. Was replaced with nitrogen gas to adjust the amount of dissolved oxygen to 0.1 ppm or less. Subsequently, while stirring the reaction solution, 92.63 g (7.78 mmol) of a 2.0 mass% sodium persulfate aqueous solution and 15.85 g of a 0.5 mass% L-ascorbic acid aqueous solution were added. After 1 minute, the temperature began to rise and polymerization started. After 6 minutes, the thermometer showed a maximum temperature of 93 ° C., then stirring was continued while keeping the jacket temperature at 60 ° C., and 60 minutes after the start of the polymerization, the produced hydrogel-like polymer was taken out.

[Crush]
The dried product was pulverized using a centrifugal pulverizer (ZM200 manufactured by Retsch, screen diameter 1 mm, 12000 rpm) to obtain an amorphous crushed particle group (C).

[Classification]
The particle group (C) was classified using a sieve having an opening of 850 μm and a sieve having a size of 180 μm. By classification, a polymer powder (C1), which was a fraction that passed through an 850 μm sieve and did not pass through a 180 μm sieve, and a polymer fine powder (c1), which was a fraction that passed through a 180 μm sieve, were obtained.

Table 1 shows the particle size, water content, CRC, and AAP of the polymer fine powder and the polymer powder produced in the production example.

3. 3. Preparation of water-absorbent resin particles (Example 1)
[Adhesion / surface cross-linking]
20 g of the polymer fine powder (a1) was placed in a round-bottomed cylindrical separable flask having an inner diameter of 11 cm and equipped with a fluororesin-made anchor-shaped stirring blade having an outer diameter of 9 cm. Next, while stirring at 300 rpm, a cross-linking agent solution in which 0.356 g of ethylene carbonate (EC) and 1.6 g of deionized water were mixed was dropped into a separable flask with a pasteur pipette, and the mixture was stirred for 5 minutes. A mixture of the surface cross-linking agent solution and the polymer fine powder was obtained. The mixture was classified on a sieve with an opening of 850 μm, and the fraction passed through the sieve with an opening of 850 μm was heated at 200 ° C. for 30 minutes. After cooling to room temperature, the mixture containing the surface-crosslinked adhered particles was classified using a sieve having an opening of 850 μm and a sieve having a size of 180 μm. By classification, surface-crosslinked polymer particles were obtained as fractions that passed through an 850 μm sieve and did not pass through a 180 μm sieve. Silicon dioxide (product name: Aerosil 200, manufactured by Nippon Aerosil Co., Ltd.) was sufficiently mixed with 10 g of the polymer particles to obtain water-absorbent resin particles.

(Example 2)
[Adhesion / surface cross-linking]
Water-absorbent resin particles were obtained in the same manner as in Example 1 except that a cross-linking agent solution in which 0.356 g of ethylene carbonate (EC), 0.312 g of isopropanol (IPA) and 1.2 g of deionized water were mixed was used. ..

(Example 3)
[Adhesion / surface cross-linking]
Water-absorbent resin particles were obtained in the same manner as in Example 1 except that a cross-linking agent solution in which 0.356 g of ethylene carbonate (EC), 0.624 g of isopropanol (IPA) and 0.4 g of deionized water were mixed was used. ..

(Example 4)
[Adhesion / surface cross-linking]
Water-absorbent resin particles were obtained in the same manner as in Example 1 except that a cross-linking agent solution in which 0.356 g of ethylene carbonate (EC), 0.624 g of isopropanol (IPA) and 0.8 g of deionized water were mixed was used. ..

(Example 5)
[Adhesion / surface cross-linking]
Water-absorbent resin particles were obtained in the same manner as in Example 1 except that a cross-linking agent solution in which 0.356 g of ethylene carbonate (EC), 0.624 g of isopropanol (IPA) and 1.2 g of deionized water were mixed was used. ..

(Example 6)
[Adhesion / surface cross-linking]
Water-absorbent resin particles were obtained in the same manner as in Example 1 except that a cross-linking agent solution obtained by mixing 0.308 g of propylene glycol (PG) and 1.6 g of deionized water was used.

(Example 7)
[Adhesion / surface cross-linking]
Water-absorbent resin particles were obtained in the same manner as in Example 1 except that a cross-linking agent solution in which 0.308 g of propylene glycol (PG), 0.624 g of isopropanol (IPA) and 0.8 g of deionized water were mixed was used. ..

(Comparative Example 1)
[Granulation]
40 g of the polymer fine powder (b1) was placed in a round-bottomed cylindrical separable flask having an inner diameter of 11 cm and an internal volume of 2 L equipped with a stirrer (the 2 L container is kept warm in a bath at 85 ° C.). As the stirrer, a flat plate slit blade having a length of about 10 cm and a width of about 6 cm was used.

While rotating the stirring blade of the stirrer at 284 rpm, 40 g of ion-exchanged water heated to 90 ° C. was put into the flask at one time. Granulated particles were obtained by stirring the fine powder and water in the flask for 90 seconds. The entire amount of the contents (granulated particles) was taken out from the flask. The lumps contained in the contents were cut to a size of 3 to 10 mm. The entire amount of the granulated particles including the cut particles was dried with hot air at 150 ° C. for 60 minutes to obtain a coarsely crushed dried product of the granulated particles. The coarsely crushed dried product was pulverized using a centrifugal pulverizer (ZM200 manufactured by Retsch, screen diameter 1 mm, 6000 rpm), and further classified by a sieve having an opening of 850 μm and a sieve having a mesh size of 180 μm. By classification, polymer particles (P1), which are fractions that passed through a sieve of 850 μm and did not pass through a sieve of 180 μm, were obtained.

[Surface cross-linking]
20 g of the polymer particles (P1) were placed in a round-bottomed cylindrical separable flask having an inner diameter of 11 cm and equipped with a fluororesin-made Ikari-shaped stirring blade having an outer diameter of 9 cm. Next, a cross-linking agent solution containing 0.0626 g of ethylene carbonate (EC), 0.100 g of propylene glycol (PG), and 0.4 g of deionized water was mixed in a separable flask with a pasteur pipette while stirring at 300 rpm. The mixture was obtained by dropping into a mixture and stirring for 35 minutes. The mixture was heated at 200 ° C. for 35 minutes. After cooling to room temperature, the mixture was classified using a sieve with an opening of 850 μm and a sieve of 180 μm. By classification, surface-crosslinked polymer particles were obtained as fractions that passed through an 850 μm sieve and did not pass through a 180 μm sieve. Silicon dioxide (product name: Aerosil 200, manufactured by Nippon Aerosil Co., Ltd.) was sufficiently mixed with 10 g of the polymer particles to obtain water-absorbent resin particles.

(Comparative Example 2)
The granulation step was carried out in the same manner as in Comparative Example 1 except that 40 g of the polymer fine powder (c1) and 10 g of ion-exchanged water were used to obtain polymer particles (P2). The same surface cross-linking step as in Comparative Example 1 was carried out using the polymer particles (P2) to obtain water-absorbent resin particles.

(Comparative Example 3)
[Adhesion / surface cross-linking]
Water-absorbent resin particles were obtained in the same manner as in Example 1 except that the polymer powder (A1) was used instead of the polymer fine powder (a1).

(Comparative Example 4)
[Adhesion / surface cross-linking]
Water-absorbent resin particles were obtained in the same manner as in Example 2 except that the polymer powder (A1) was used instead of the polymer fine powder (a1).

Table 2 shows the evaluation results of the parts by mass of each component contained in the cross-linking agent solution and the water-absorbent resin particles with respect to 100 parts by mass of the polymer fine powder (a1) in the examples. In Comparative Examples 1 and 2, 100 parts by mass of each component contained in the cross-linking agent solution with respect to 100 parts by mass of the polymer particles (P1) or (P2), and each component contained in the cross-linking agent solution with respect to 100 parts by mass of the polymer powder (A1). Table 3 shows the evaluation results of the parts by mass and the water-absorbent resin particles. Table 4 shows the evaluation results of the parts by mass of each component contained in the cross-linking agent solution and the water-absorbent resin particles with respect to 100 parts by mass of the polymer powder (A1) in Comparative Examples 3 and 4.

114 ... Cylinder, 110 ... Measuring device, 112 ... Weight, 112a ... Disk part, 112b ... Rod-shaped part, 112c ... Cylinder part, 116 ... Wire mesh, 120 ... Particles to be measured, 130 ... Stainless petri dish, 140 ... Glass filter, 150 ... filter paper.

Claims

Surface-crosslinked water-absorbent resin particles having a medium particle size of 200 to 430 μm and a gel dissociation degree of 20 to 60%.
A water-absorbent resin comprising a step of adding a cross-linking agent solution containing a cross-linking agent and water to a polymer fine powder passing through a sieve having a mesh size of 180 μm or less to simultaneously perform adhesion and surface cross-linking of the polymer fine powder. How to make particles.
The method according to claim 2, wherein the water content of the cross-linking agent solution with respect to 100 parts by mass of the polymer fine powder is 10 parts by mass or less.
The method according to claim 2 or 3, wherein the cross-linking agent solution further contains alcohol.
The method according to claim 4, wherein the alcohol content of the cross-linking agent solution with respect to 100 parts by mass of the polymer fine powder is 5 parts by mass or less.
The method according to any one of claims 2 to 5, wherein the cross-linking agent solution contains at least one surface cross-linking agent selected from the group consisting of an alkylene carbonate compound and a polyol compound.