JP7355646B2 - Manufacturing method of water absorbent resin - Google Patents

Description

The present invention relates to a method for producing a water absorbent resin.

In recent years, water-absorbing resins have been widely used in the field of various absorbent products, such as sanitary materials such as disposable diapers and sanitary products, agricultural and horticultural materials such as water-retaining agents and soil conditioners, and industrial materials such as water-stopping agents and anti-condensation agents. It is used. Many types of water-absorbing resins are known depending on their uses, but among them, polymers with a crosslinked structure obtained by polymerizing water-soluble ethylenically unsaturated monomers (hereinafter referred to as Water-absorbing resins consisting of cross-linked polymers (referred to as cross-linked polymers) are mainly used. Examples of the manufacturing method include a method in which a hydrogel of a crosslinked polymer is manufactured by aqueous solution polymerization, suspension polymerization, etc., and then dried.

In Patent Document 1, as a manufacturing method, for example, a hydrous gel of such a crosslinked polymer (hereinafter referred to as a hydrogel of a crosslinked polymer) is prepared by an aqueous solution polymerization method, and a hydrous gel of the crosslinked polymer obtained as a block is prepared. It is disclosed that a water-absorbing resin as a final product can be obtained by heating the gel to a temperature of 45 to 90°C, crushing it, drying it, crushing the dried product, and subjecting it to particle size adjustment and surface crosslinking if necessary. ing.

Patent Document 2 discloses a method for producing a particulate hydrogel polymer in which a shearing force is applied to a hydrogel polymer having a crosslinked structure to subdivide it into particles in a container at a temperature of 40 to 110°C. A method for producing a particulate hydrogel polymer is disclosed, which is characterized in that shearing force is repeatedly applied to a heated hydrogel polymer while applying a load of 0.01 to 1.5 kg/cm ² .

Patent Document 3 discloses a method for producing a particulate hydrogel polymer by extruding a hydrogel polymer having a crosslinked structure through a porous plate having pores with a diameter of 3 to 20 mm and a thickness of 1 to 20 mm. , discloses a method for producing a particulate hydrogel polymer, in which the temperature of the hydrogel polymer is 35 to 90°C.

Patent Document 4 discloses a gel crushing device used for producing a water absorbent resin, which is equipped with a screw, a supply port, an extrusion port, a perforated plate, and a barrel, and is used at 40°C to 120°C in the gel crushing process. It is disclosed that a water-absorbing resin is produced using a gel crushing device characterized by:

In all of these methods, the temperature of the hydrogel is set at a temperature higher than 35°C or 40°C, and the hydrogel is crushed using a specific device such as a perforated plate or a pressure lid. .

Japanese Patent Application Publication No. 9-309916 Japanese Patent Application Publication No. 5-112654 Japanese Patent Application Publication No. 6-41319 International Publication No. 2015/030129

Although it depends on the type of polymerization reactor, the crosslinked polymer obtained by polymerizing the water-soluble ethylenically unsaturated monomer is obtained as a block of hydrogel. Usually, the obtained hydrogel of the crosslinked polymer is crushed and dried, and then the dried product is crushed, and if necessary, the particle size is adjusted and the surface crosslinked to obtain the final product of the water-absorbing resin. However, as a result of studies conducted by the present inventors, it was found that the obtained water-absorbing resin has a problem in that, depending on the manufacturing conditions, variations tend to occur between manufacturing lots, and in particular, the reproducibility of the particle size distribution is low.

An object of the present invention is to provide a method for producing a water-absorbing resin in which the reproducibility of the particle size distribution of the water-absorbing resin is improved and there is less variation between production lots.

As a result of extensive research to solve the above problems, the present inventors have found that in a method for producing a water-absorbent resin, the temperature of the hydrogel of a crosslinked polymer is set to 35°C or less, and the hydrogel is coarsely crushed. It has been found that including this step increases the reproducibility of the particle size distribution of the resulting water absorbent resin.

That is, the present invention provides a method for producing a water-absorbing resin, which includes a step of coarsely crushing a hydrogel of a crosslinked polymer, wherein the temperature of the hydrogel in the crushing step is 35° C. or lower. Provides a manufacturing method.

By using the production method of the present invention, it is possible to improve the reproducibility of the particle size distribution of the obtained water absorbent resin.

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

<Crosslinked polymer hydrogel manufacturing process>
The hydrogel of the crosslinked polymer in the present invention is, for example, a water-soluble ethylenically unsaturated monomer in an aqueous solution (hereinafter also referred to as "monomer aqueous solution") containing the water-soluble ethylenically unsaturated monomer. It can be obtained by polymerization.

[Water-soluble ethylenically unsaturated monomer]
Examples of the water-soluble ethylenically unsaturated monomer used in the present invention include (meth)acrylic acid (in this specification, "acrylic" and "methacrylic" are collectively referred to as "(meth)acrylic"). ), carboxylic acid monomers such as α,β-unsaturated carboxylic acids and their salts such as maleic acid, maleic anhydride, and fumaric acid; (meth)acrylamide, N,N-dimethyl(meth)acrylamide, Nonionic monomers such as 2-hydroxyethyl (meth)acrylate, N-methylol (meth)acrylamide, polyethylene glycol mono(meth)acrylate; N,N-diethylaminoethyl (meth)acrylate, N,N-diethylaminopropyl Amino group-containing unsaturated monomers such as (meth)acrylate, diethylaminopropyl (meth)acrylamide, and their quaternized products; vinylsulfonic acid, styrenesulfonic acid, 2-(meth)acrylamido-2-methylpropanesulfonic acid, Examples include sulfonic acid monomers such as 2-(meth)acryloylethanesulfonic acid and salts thereof. These water-soluble ethylenically unsaturated monomers may be used alone or in combination of two or more.

Specifically, the water-soluble ethylenically unsaturated monomer is preferably at least one selected from (meth)acrylic acid and salts thereof. Other water-soluble ethylenically unsaturated monomers may be copolymerized with (meth)acrylic acid and its salt. In this case, it is preferable to use 70 to 100 mol%, more preferably 80 to 100 mol%, and even more preferably 90 to 100 mol% of the total amount of water-soluble ethylenically unsaturated monomers. More preferred.

When the water-soluble ethylenically unsaturated monomer has an acid group, such as (meth)acrylic acid, 2-(meth)acrylamido-2-methylpropanesulfonic acid, etc., the acid group may be prepared in an alkaline solution as necessary. Those that have been neutralized with a compensating agent can be used. Examples of such alkaline neutralizing agents include alkali metal salts such as sodium hydroxide, sodium carbonate, sodium hydrogen carbonate, potassium hydroxide, and potassium carbonate; ammonia, and the like. These alkaline neutralizing agents may be used in the form of an aqueous solution in order to simplify the neutralization operation. The above alkaline neutralizing agents may be used alone or in combination of two or more. Note that the acid group may be neutralized before, during or after the polymerization of the water-soluble ethylenically unsaturated monomer as a raw material.

The degree of neutralization of the water-soluble ethylenically unsaturated monomer by the alkaline neutralizer increases the water absorption performance by increasing the osmotic pressure of the resulting water-absorbent resin, and also improves safety due to the presence of excess alkaline neutralizer. From the viewpoint of avoiding problems with properties, etc., it is usually preferably 10 to 100 mol%, more preferably 30 to 90 mol%, even more preferably 40 to 85 mol%, Even more preferably, it is 50 to 80 mol%. Here, the degree of neutralization is defined as the degree of neutralization for all acid groups possessed by the water-soluble ethylenically unsaturated monomer.

The concentration of the water-soluble ethylenically unsaturated monomer in the monomer aqueous solution may be generally 20% by mass or more and below the saturated concentration, preferably 25 to 70% by mass, more preferably 30 to 50% by mass. .

[Internal crosslinking agent]
In the method of the present invention, the crosslinked polymer preferably has an internal crosslinking structure that is not only self-crosslinked by a polymerization reaction but also crosslinked by an internal crosslinking agent. As the internal crosslinking agent, for example, a compound having two or more polymerizable unsaturated groups is used. For example, (poly)ethylene glycol (in this specification, for example, "polyethylene glycol" and "ethylene glycol" are collectively referred to as "(poly)ethylene glycol". The same applies hereinafter), (poly)propylene glycol, trimethylolpropane , di- or tri(meth)acrylic acid esters of polyols such as glycerin, polyoxyethylene glycol, polyoxypropylene glycol, and (poly)glycerin; reaction of the above polyols with unsaturated acids such as maleic acid and fumaric acid. unsaturated polyesters obtained by reacting; bisacrylamides such as N,N'-methylenebis(meth)acrylamide; di- or tri(meth)acrylic acid esters obtained by reacting polyepoxide with (meth)acrylic acid; Di(meth)acrylic acid carbamyl esters obtained by reacting polyisocyanates such as tolylene diisocyanate and hexamethylene diisocyanate with hydroxyethyl (meth)acrylate; Allylated starch; Allylated cellulose; Diallyl phthalate; N, Examples include N',N''-triallylisocyanurate; divinylbenzene, and the like. Among these, di- or tri(meth)acrylic acid esters of the above-mentioned polyols are preferred.

Furthermore, in addition to the compounds having two or more polymerizable unsaturated groups, compounds having two or more reactive functional groups can be used as internal crosslinking agents. For example, glycidyl group-containing compounds such as (poly)ethylene glycol diglycidyl ether, (poly)propylene glycol diglycidyl ether, and (poly)glycerin diglycidyl ether; (poly)ethylene glycol, (poly)propylene glycol, (poly) Examples include glycerin, pentaerythritol, ethylenediamine, polyethyleneimine, glycidyl (meth)acrylate, and the like. Among these, (poly)ethylene glycol diglycidyl ether, (poly)propylene glycol diglycidyl ether, and (poly)glycerin diglycidyl ether are preferred from the viewpoint of excellent reactivity at low temperatures. These internal crosslinking agents may be used alone or in combination of two or more types.

When using an internal crosslinking agent, the amount used is 0.0001 mol per 100 moles of the water-soluble ethylenically unsaturated monomer in order to sufficiently increase the water absorption performance such as the water retention capacity of the resulting water absorbent resin. It is preferably mol or more, more preferably 0.001 mol or more, even more preferably 0.003 mol or more, even more preferably 0.01 mol or more.

Although the addition of an internal crosslinking agent insolubilizes the crosslinked polymer and provides suitable water absorption capacity, increasing the amount of internal crosslinking agent added leads to a decrease in the water absorption capacity of the resulting water absorbent resin. The amount is preferably, for example, 0.50 mol or less, more preferably 0.25 mol or less, and even more preferably 0.05 mol or less, per 100 mol of the water-soluble ethylenically unsaturated monomer. It is.

[Other ingredients]
The monomer aqueous solution may contain additives such as a chain transfer agent, a thickener, and an inorganic filler, if necessary. Examples of chain transfer agents include thiols, thiol acids, secondary alcohols, hypophosphorous acid, and phosphorous acid. Examples of the thickener include carboxymethylcellulose, hydroxyethylcellulose, hydroxypropylcellulose, methylcellulose, polyethylene glycol, polyacrylic acid, neutralized polyacrylic acid, and polyacrylamide. Examples of the inorganic filler include metal oxides, ceramics, clay minerals, and the like. These may be used alone or in combination of two or more.

〔polymerization〕
In the hydrogel manufacturing process of the present invention, a water-soluble ethylenically unsaturated monomer in an aqueous solution is polymerized using a polymerization initiator in the presence of a crosslinking agent if necessary. As the polymerization of the water-soluble ethylenically unsaturated monomer in the present invention, aqueous solution polymerization, emulsion polymerization, reverse phase suspension polymerization, etc. are used. In the present invention, it is preferable to obtain a hydrogel of a crosslinked polymer by aqueous solution polymerization.

The polymerization method may be a stationary polymerization method in which the monomer aqueous solution is polymerized without stirring (for example, in a stationary state), a stirring polymerization method in which the monomer aqueous solution is polymerized while stirring in a reaction apparatus, etc. . In the present invention, in the aqueous solution polymerization, it is preferable to obtain a hydrogel of a crosslinked polymer by aqueous solution stationary polymerization, which is a stationary polymerization method. In the stationary polymerization method, upon completion of polymerization, a hydrogel of the crosslinked polymer is obtained as a single block-shaped hydrogel occupying approximately the same volume as the aqueous monomer solution present in the reaction vessel. Note that the monomer aqueous solution may contain a water-soluble organic solvent other than water as appropriate.

The mode of production may be batch, semi-continuous, continuous, etc. For example, in aqueous solution polymerization, which is a stationary polymerization method and a continuous form, the polymerization reaction is carried out while continuously supplying a monomer aqueous solution to a continuous polymerization device. A hydrogel (for example in the form of a band) can be obtained.

[Polymerization initiator]
Polymerization is initiated by adding a polymerization initiator to the monomer aqueous solution, and performing heating, light irradiation, etc., if necessary. As the polymerization initiator, a water-soluble radical polymerization initiator is preferably used.

Peroxides used as polymerization initiators include, for example, persulfates such as potassium persulfate, ammonium persulfate, and sodium persulfate; methyl ethyl ketone peroxide, methyl isobutyl ketone peroxide, di-t-butyl peroxide, and t-butyl peroxide. -Organic peroxides such as butylcumyl peroxide, t-butylperoxyacetate, t-butylperoxyisobutyrate, and t-butylperoxypivalate; and peroxides such as hydrogen peroxide. Among these peroxides, it is preferable to use potassium persulfate, ammonium persulfate, sodium persulfate, and hydrogen peroxide from the viewpoint of obtaining a water-absorbent resin having good water absorption performance. It is more preferable to use ammonium sulfate or sodium persulfate. These peroxides may be used alone or in combination of two or more. For example, a combination of persulfate and hydrogen peroxide can be used.

Examples of the azo compound used as a polymerization initiator 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-benzyl) amidino)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, 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-tetrahydropyrimidin-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-imidazolin-2-yl)propane]disulfate dihydrate, 2,2'-azobis[N-(2-carboxyethyl)-2-methylpropionamidine]tetrahydrate , 2,2'-azobis[2-methyl-N-(2-hydroxyethyl)propionamide] and other water-soluble azo compounds. 2,2'-azobis(2-amidinopropane) dihydrochloride, 2,2'-azobis{2-[1-(2-hydroxyethyl) )-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 types.

From the viewpoint of easily obtaining a water-absorbing resin having good water-absorbing performance, it is preferable to use a combination of two or more of the above-mentioned polymerization initiators, and in particular, a combination of a peroxide and a water-soluble azo compound is preferred. is more preferable. For example, a combination of sodium persulfate and 2,2'-azobis(2-amidinopropane) dihydrochloride, or a combination of sodium persulfate, hydrogen peroxide, and 2,2'-azobis(2-amidinopropane) dihydrochloride. etc. Furthermore, the above polymerization initiator can be used in combination with a reducing agent such as sodium sulfite, sodium hydrogen sulfite, ferrous sulfate, and L-ascorbic acid to serve as a redox polymerization initiator.

When using a combination of multiple polymerization initiators, the amount of the azo compound to be used relative to the total polymerization initiator may be 10 to 100 mol%, more preferably 25 to 90 mol%, from the viewpoint of improving water absorption performance. It is more preferably 30 to 85 mol%, even more preferably 35 to 80 mol%.

The total amount of the polymerization initiator used is 0.005 to 0.005 moles per 100 moles of the water-soluble ethylenically unsaturated monomer used in the polymerization, from the viewpoint of avoiding rapid polymerization reactions and shortening the polymerization reaction time. It is preferably 1 mol, more preferably 0.01 to 0.5 mol, even more preferably 0.0125 to 0.1 mol, even more preferably 0.015 to 0.05 mol.

The polymerization temperature cannot be determined unconditionally because it varies depending on the polymerization initiator used, but it is possible to increase productivity by speeding up the polymerization and shortening the polymerization time, and to remove the polymerization heat more easily. From the viewpoint of smoothly carrying out the reaction, the temperature is preferably 0 to 130°C, more preferably 10 to 110°C. The polymerization time is appropriately set depending on the type and amount of the polymerization initiator used, the reaction temperature, etc., but is preferably 1 to 200 minutes, more preferably 5 to 100 minutes.

[Hydrogel of crosslinked polymer]
In the present invention, the crosslinked polymer obtained by polymerizing the water-soluble ethylenically unsaturated monomer is in the form of a hydrogel. The water content of the hydrogel of the crosslinked polymer is preferably 30 to 80% by mass, more preferably 40 to 75% by mass, and even more preferably 50 to 70% by mass from the viewpoint of ease of carrying out the crushing step. The water content is adjusted by the amount of water in the monomer aqueous solution or by operations such as drying and humidification after polymerization. In the present invention, the water content of the hydrogel polymer is the content of water in the total mass of the hydrogel polymer expressed in mass %.

<Crushing step of hydrogel>
The method for producing a water absorbent resin of the present invention includes a step of crushing a hydrogel of a crosslinked polymer. The size of the hydrogel after the coarse crushing treatment is, for example, about 0.1 to 10 mm, preferably about 0.1 to 5 mm. The size here means the maximum size per hydrogel.

The temperature of the hydrogel in the crushing step is 35°C or lower, preferably 33°C or lower, more preferably 30°C or lower, even more preferably 25°C or lower.

The temperature of the hydrogel in the crushing step may be 0°C or higher, for example 5°C or higher, preferably 10°C or higher, more preferably 15°C or higher.

The temperature of the water-containing gel in the coarse crushing process may vary during the crushing process as long as it is within the above-mentioned range, and if it fluctuates, the temperature range is, for example, within plus or minus 10 degrees Celsius with respect to the predetermined temperature, preferably It is within plus or minus 5°C, more preferably within plus or minus 3°C.

The temperature of the hydrous gel can be adjusted to the above temperature by leaving the high-temperature hydrogel still for a predetermined time, by contacting the hydrogel with a low-temperature gas for a predetermined time, or by placing the container holding the hydrogel in a refrigerant bath, etc. Examples include a method of adjusting the temperature to the above temperature by soaking it for a predetermined period of time. Since the hydrogel after polymerization is usually at a high temperature, it is preferable to achieve temperature control by cooling.

As a crushing device for the hydrogel, a crushing device such as a kneader (for example, a pressure kneader, a double-arm kneader, etc.), a meat chopper, a cutter mill, a pharma mill, etc. can be used. Among them, a double-arm kneader, a meat chopper, and a cutter mill are more preferable. The crushing device may be of the same type as the crushing device for dry hydrogel gel described below.

<Drying process of crushed hydrogel>
The method for producing a water absorbent resin of the present invention includes a drying step of drying the crushed hydrogel obtained in the crushing step. The crushed material is dried by removing the water-containing solvent in the crushed material by heating or blowing air. As for the drying method, the solvent may be removed from the hydrogel by a general method such as natural drying, heat drying, spray drying, or freeze drying. Drying can be performed under normal pressure, under reduced pressure, under a stream of nitrogen or the like to increase drying efficiency, and a combination of these methods may be used. When drying is carried out under normal pressure, the drying temperature is preferably 70 to 250°C, more preferably 80 to 200°C. The drying step is carried out until the water content of the crosslinked polymer is 20% by mass or less, preferably 10% by mass or less, more preferably 5% by mass or less.

<Dry material grinding process>
The method for producing a water absorbent resin of the present invention includes a pulverizing step of obtaining a pulverized product by pulverizing the dried material obtained in the drying step.

A known pulverizer can be used to pulverize the dried material, such as a roller mill, a stamp mill, a jet mill, a high-speed rotary pulverizer (hammer mill, pin mill, rotor beater mill, etc.), a container-driven mill, etc. Type mills (rotary mills, vibration mills, planetary mills, etc.) can be used. Preferably, a high speed rotary mill is used. The pulverizer may have openings on the outlet side, such as perforated plates, screens, grids, etc., for controlling the maximum particle size of the pulverized particles. The shape of the opening may be polygonal, circular, etc., and the maximum diameter of the opening may be 0.1 to 5 mm, preferably 0.3 to 3.0 mm, and more preferably 0.5 to 1.5 mm. preferable.

The manufacturing method of the present invention may further include other steps, such as a classification step for particle size adjustment, a surface crosslinking step, and the like.

<Classification process of crushed material>
The method for producing a water absorbent resin of the present invention may include a classification step of classifying the pulverized product obtained in the pulverization step. In addition, there may be a plurality of classification steps, such as pulverizing the particles after classification again and repeating the pulverization step and the classification step, or there may be a classification step after the surface crosslinking step described below. Here, classification refers to the operation of dividing a certain particle group into two or more particle groups having different particle size distributions according to the particle size.

For classification of the pulverized material, a known classification method may be used, such as screen classification, air classification, etc., and screen classification is preferable. Examples of screen classification include vibrating sieves, rotary sifters, cylindrical stirring sieves, blower sifters, and low-tap shakers. 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 mesh of the screen by vibrating the screen. Wind classification is a method of classifying particles using air flow.

In the present invention, 90% or more of the total weight of the water-absorbing resin has a particle size of preferably 850 μm or less, more preferably 700 μm or less, and even more preferably 600 μm or less. Note that in this specification, the particle size means a value measured by a sieving method.

In the present invention, the water-absorbing resin preferably has a median particle size of 100 to 800 μm, more preferably 150 to 600 μm, even more preferably 200 to 500 μm, and even more preferably 240 to 450 μm. Even more preferred. The median particle size is measured according to the method described in the Examples below.

The production method of the present invention can obtain a pulverized product with a desired particle size distribution with good reproducibility even when the number of times of pulverization of the dried material is small or the time of pulverization of the dried material is short, and it can be used for industrial purposes. It is advantageous. Furthermore, by reducing the number or time of pulverization, it is also possible to avoid deterioration in the performance of the water-absorbing resin due to exposure to long-term mechanical loads.

<Surface crosslinking process>
In the production method of the present invention, a crosslinking agent (referred to as a surface crosslinking agent) containing two or more functional groups that are reactive with a functional group derived from a water-soluble ethylenically unsaturated monomer is added to the crosslinked polymer. ) may be added and reacted (referred to as surface crosslinking treatment). By adding a surface crosslinking agent and performing surface crosslinking treatment, the crosslinking density near the surface of the water absorbent resin increases, thereby increasing the water absorption performance of the resulting water absorbent resin such as water absorption capacity under load, gel strength, and liquid permeability. be able to.

The surface cross-linking agent may be added at any time after the crushing step, and can be carried out at any time after the crushing step, such as by adding a hydrous gel of a crosslinked polymer, a crushed product of a hydrogel of a crosslinked polymer, a dried product of the crushed product, or a pulverization of the dried product. It is possible to perform this on a dried product, but it is particularly preferable to perform this on a dried product of a coarsely crushed product or a pulverized product of a dried product. As for the method of adding the surface crosslinking agent, for example, the surface crosslinking agent solution may be added to the water absorbent resin, or the surface crosslinking agent solution may be sprayed and added to the water absorbent resin. Regarding the addition form of the surface crosslinking agent, from the viewpoint of uniformly dispersing the surface crosslinking agent, it is preferable that the surface crosslinking agent is dissolved in a solvent such as water and/or alcohol, and added as a surface crosslinking agent solution. Further, the surface crosslinking step may be performed once or divided into two or more times.

Examples of the surface crosslinking agent include compounds having two or more reactive functional groups. For example, polyols such as ethylene glycol, propylene glycol, 1,4-butanediol, trimethylolpropane, glycerin, polyoxyethylene glycol, polyoxypropylene glycol, polyglycerin; (poly)ethylene glycol diglycidyl ether, (poly) Polyglycidyl compounds such as ethylene glycol triglycidyl ether, (poly)glycerin diglycidyl ether, (poly)glycerin triglycidyl ether, (poly)propylene glycol polyglycidyl ether, (poly)glycerol polyglycidyl ether; epichlorohydrin, epibromohydrin , haloepoxy compounds such as α-methylepichlorohydrin; compounds having two or more reactive functional groups such as isocyanate compounds such as 2,4-tolylene diisocyanate and hexamethylene diisocyanate; 3-methyl-3-oxetane methanol, 3-ethyl -Oxetane compounds such as 3-oxetanemethanol, 3-butyl-3-oxetanemethanol, 3-methyl-3-oxetaneethanol, 3-ethyl-3-oxetaneethanol, 3-butyl-3-oxetaneethanol; 1,2- Examples include oxazoline compounds such as ethylenebisoxazoline; carbonate compounds such as ethylene carbonate; and hydroxyalkylamide compounds such as bis[N,N-di(β-hydroxyethyl)]adipamide. Among these, (poly)ethylene glycol diglycidyl ether, (poly)ethylene glycol triglycidyl ether, (poly)glycerin diglycidyl ether, (poly)glycerin triglycidyl ether, (poly)propylene glycol polyglycidyl ether, (poly) Polyglycidyl compounds such as glycerol polyglycidyl ether and/or polyols such as ethylene glycol, propylene glycol, 1,4-butanediol, trimethylolpropane, polyoxyethylene glycol, and polyoxypropylene glycol are preferred, and polyglycidyl compounds are more preferred. preferable. These surface crosslinking agents may be used alone or in combination of two or more types. For example, polyglycidyl compounds and polyols may be used in combination.

From the viewpoint of appropriately increasing the crosslinking density near the surface of the water-absorbing resin, the amount of the surface crosslinking agent added is usually preferably 0 to 100 moles of the total amount of water-soluble ethylenically unsaturated monomers used in the polymerization. The amount is 0.0001 to 1 mol, more preferably 0.001 to 0.5 mol.

The surface crosslinking step is preferably carried out in the presence of 1 to 200 parts by weight of water based on 100 parts by weight of the water-soluble ethylenically unsaturated monomer. The moisture content can be adjusted by appropriately using water and/or a water-soluble organic solvent such as alcohol. Note that the solid content of the water-absorbing resin in the hydrogel can be calculated from the charged amount of the water-soluble ethylenically unsaturated monomer used in the polymerization reaction, and at the same time, the amount of water contained in the aqueous monomer solution can also be calculated. That is, the amount of water contained in the hydrogel in each step after the hydrogel production step can be calculated by subtracting the amount of water removed from the hydrogel after polymerization from the amount of water contained in the aqueous monomer solution. In this way, by adjusting the amount of water during the surface crosslinking step, crosslinking can be more suitably carried out in the vicinity of the particle surface of the water absorbent resin.

The treatment temperature of the surface crosslinking agent is appropriately set depending on the surface crosslinking agent used, and may be 20 to 250° C., and the treatment time is preferably 1 to 200 minutes, more preferably 5 to 100 minutes.

EXAMPLES The present invention will be explained in more detail below based on Examples and Comparative Examples, but the present invention is not limited only to these Examples.

The water content of the hydrogel of the crosslinked polymer in each Example and Comparative Example was evaluated by the method shown below.

(moisture content)
2.0 g of the crosslinked polymer was placed in an aluminum wheel case (No. 8) that had been made to have a constant weight (Wa (g)) in advance and was accurately weighed (Wb (g)). The sample was dried for 2 hours in a hot air dryer (manufactured by ADVANTEC) whose internal temperature was set to 105°C, and then allowed to cool in a desiccator, and the mass after drying (Wc (g)) was measured. The moisture content of the crosslinked polymer was calculated from the following formula.
Moisture content (mass%) = [(Wb-Wa)-(Wc-Wa)]/(Wb-Wa) x 100

(Particle size distribution measurement of water absorbent resin)
JIS standard sieves were assembled in the following order from above: a sieve with an opening of 850 μm, a sieve with an opening of 500 μm, a sieve with an opening of 250 μm, a sieve with an opening of 180 μm, a sieve with an opening of 150 μm, and a saucer.

50 g of a water-absorbing resin was placed in the uppermost sieve of the combination, and the mixture was classified by shaking for 20 minutes using a low-tap shaker. After classification, the water absorbent resin on the 850 μm sieve and the water absorbent resin on the saucer were removed. Thereafter, the mass of the water absorbent resin remaining on each sieve was calculated as a mass percentage with respect to the total amount, and the particle size distribution was determined.

(medium particle size)
According to the above (measurement of particle size distribution of water absorbent resin), the mass of the water absorbent resin remaining on each sieve was calculated as a mass percentage with respect to the total amount, and the particle size distribution was determined. Regarding the particle size distribution, by integrating the mass percentage on each sieve in descending order of particle size, the relationship between the sieve opening and the integrated value of the mass percentage of the water absorbent resin remaining on the sieve is plotted on log probability paper. Plotted. By connecting the plots on the probability paper with a straight line, the particle size corresponding to an integrated mass percentage of 50% by mass was defined as the median particle size.

[Example 1]
A 2 L separable flask was charged with 195.36 g of 100% acrylic acid. While stirring the inside of the separable flask, 135.13 g of ion-exchanged water was added, and further, 357.93 g of 30% sodium hydroxide was added dropwise in an ice bath. Thereafter, 104.73 g of 100% acrylic acid was added to prepare a partially neutralized acrylic acid solution.
In a 2L plastic bottle, 780g of the above-mentioned partially neutralized acrylic acid solution, 43.78g of a 2% polyethylene glycol diacrylate (average repeating unit of ethylene oxide: 9) aqueous solution as an internal crosslinking agent solution, 180.7g of ion-exchanged water, were mixed to prepare an aqueous monomer solution. This aqueous monomer solution was placed in a stainless steel circular vat (inner diameter 200 mm, depth 60 mm), sealed from above with a film, and then nitrogen was blown into the vat to reduce the dissolved oxygen in the solution to 0.1 ppm or less. Subsequently, the temperature of the monomer aqueous solution was adjusted to 18° C. under a nitrogen atmosphere, and then 1.58 g of a 5% aqueous sodium persulfate solution and 1.58 g of a 5% aqueous 2,2'-azobis(2-amidinopropane) dihydrochloride solution were added. 58 g of L-ascorbic acid, 1.50 g of a 0.5% aqueous L-ascorbic acid solution, and 1.70 g of a 0.35% aqueous hydrogen peroxide solution were sequentially added dropwise with stirring. After dropping hydrogen peroxide, stirring was stopped. Polymerization started immediately, and the temperature of the aqueous monomer solution reached a peak temperature of 86°C after 9 minutes. Subsequently, the bat was immersed in a hot water bath at 80° C. and maintained for 10 minutes to obtain a hydrogel of the crosslinked polymer.
Subsequently, a thermometer was inserted into the hydrogel taken out from the vat, and the gel was allowed to stand in a room at 23° C. for 50 minutes to adjust the temperature to a predetermined temperature (30° C.). The water content of the hydrogel of the crosslinked polymer after cooling was 63%. The hydrous gel of the crosslinked polymer was crushed using a 1 L double-arm kneader, and then dried at 180° C. for 30 minutes to obtain a dried product. Thereafter, the dried product was pulverized using a pulverizer (Rotabee Mill) with a screen hole size of 1 mm. After pulverization, a water-absorbing resin (1) having a median particle size of 280 μm was obtained by removing the pulverized materials having a size of 850 μm or more and the pulverized materials having a size of less than 150 μm.
The same operation as above was performed five times, and the average particle size distribution and coefficient of variation CV (=standard deviation of particle size distribution/average of particle size distribution x 100) for each sieve of the water absorbent resin (1) were determined. The results are shown in Tables 1 and 2.

[Example 2]
A hydrogel of a crosslinked polymer was obtained in the same manner as in Example 1. The obtained hydrogel of the crosslinked polymer was allowed to stand in a room at 23°C for 50 minutes, and then placed on an ice water bath for 5 minutes to adjust the temperature to a predetermined temperature (20°C). The water content of the hydrogel of the crosslinked polymer after cooling was 63%. Thereafter, it was crushed, dried, and crushed in the same manner as in Example 1. After pulverization, the pulverized materials with a particle size of 850 μm or more and the pulverized particles with a particle size of less than 150 μm were removed to obtain a water absorbent resin (2) having a median particle size of 250 μm.
The same operation as above was performed five times, and the average particle size distribution and coefficient of variation CV of the water absorbent resin (2) for each sieve were determined. The results are shown in Tables 1 and 2.

[Comparative example 1]
A hydrogel of a crosslinked polymer was obtained in the same manner as in Example 1. The obtained hydrogel of the crosslinked polymer was allowed to stand in a room at 23° C. for 1 minute to adjust the temperature to a predetermined temperature (70° C.). The water content of the hydrogel of the crosslinked polymer after cooling was 64%. Thereafter, it was crushed, dried, and crushed in the same manner as in Example 1. After the pulverization, the pulverized materials with a particle size of 850 μm or more and the pulverized particles with a particle size of less than 150 μm were removed to obtain a comparative water absorbent resin (1) having a median particle size of 330 μm.
The same operation as above was performed five times, and the average particle size distribution and coefficient of variation CV for each sieve of the comparative water absorbent resin (1) were determined. The results are shown in Tables 1 and 2.

[Comparative example 2]
A hydrogel of a crosslinked polymer was obtained in the same manner as in Example 1. The obtained hydrogel of the crosslinked polymer was left standing in a room at 23°C for 20 minutes to adjust the temperature to a predetermined temperature (45°C). The water content of the hydrogel of the crosslinked polymer after cooling was 64%. Thereafter, it was crushed, dried, and crushed in the same manner as in Example 1. After pulverization, the pulverized materials with a particle size of 850 μm or more and the pulverized particles with a particle size of less than 150 μm were removed to obtain a comparative water absorbent resin (2) having a median particle size of 250 μm.
The same operation as above was performed five times, and the average particle size distribution and coefficient of variation CV for each sieve of the comparative water absorbent resin (2) were determined. The results are shown in Tables 1 and 2.

It can be suitably used in various absorbent articles, such as sanitary 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 water stop agents and anti-condensation agents.

Claims (8)

A hydrogel production step of producing a hydrogel of a crosslinked polymer by polymerizing a water-soluble ethylenically unsaturated monomer using a polymerization initiator containing a persulfate and a water-soluble azo compound; and the hydrogel. A method for producing a water-absorbing resin comprising a coarse crushing step of coarsely crushing
The temperature of the hydrogel in the coarse crushing step is 35°C or less (excluding 24°C) ,
A method for producing a water-absorbing resin, wherein the proportion of the water-soluble azo compound to the total polymerization initiator is 35 to 80 mol%. Production of the water absorbent resin according to claim 1, wherein 70 to 100 mol% of the total amount of the water-soluble ethylenically unsaturated monomer is at least one selected from (meth)acrylic acid and its salts. Method. The method for producing a water absorbent resin according to claim 1 or 2, wherein the water content of the hydrogel is 30 to 70% by mass. The method for producing a water-absorbing resin according to any one of claims 1 to 3, wherein the temperature of the hydrogel in the coarse crushing step is 25° C. or lower. The water-absorbing resin according to any one of claims 1 to 4, comprising a drying step of drying the crushed material obtained in the crushing step to obtain a dried product, and a pulverizing step of pulverizing the dried product. Production method. A method for producing a water-absorbing resin, comprising a crushing step of crushing a hydrous gel,
A method for producing a water absorbent resin, wherein the temperature of the hydrogel in the coarse crushing step is 35°C or less (excluding 24°C) .
However, this excludes cases where the coarse crushing step satisfies any of the following (A) to (C).
(A) The hydrous gel is cut into short pieces by inserting it into a pair of roller-type cutters that rotate in a direction that engages with each other, and then the resulting short pieces of hydrogel are cut into small pieces using a rotating blade and a fixed blade. Cut into shapes.
(B) The hydrogel is sheared by being sandwiched between a pair of spiral rotary blades facing each other and having different feed speeds.
(C) Cutting the hydrogel with scissors. The method for producing a water absorbent resin according to claim 6, wherein the temperature of the hydrogel in the crushing step is 35° C. or lower (excluding 20° C. to 25° C.). The method for producing a water absorbent resin according to claim 6, wherein the temperature of the hydrogel in the coarse crushing step is 20° C. or lower.