WO2021049451A1

WO2021049451A1 - Method for producing water-absorbable resin particles

Info

Publication number: WO2021049451A1
Application number: PCT/JP2020/033754
Authority: WO
Inventors: 崇志居藤
Original assignee: 住友精化株式会社
Priority date: 2019-09-09
Filing date: 2020-09-07
Publication date: 2021-03-18
Also published as: JPWO2021049451A1

Abstract

Disclosed is a method for producing water-absorbable resin particles, comprising mixing first polymer particles each having a particle diameter of less than 180 μm with second polymer particles each having a particle diameter of 180 μm or more to produce aggregated masses and pulverizing the aggregated masses, wherein the residual monomer amount in the first polymer particles is 250 ppm or more.

Description

Manufacturing method of water-absorbent resin particles

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

Water-absorbent resin particles are widely used in the fields of 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 dew condensation inhibitors. If the particle size of the water-absorbent resin particles is too small, there are problems such as poor operability, rapid water absorption and easy mamaco, poor water absorption characteristics, and a decrease in the yield of products using the water-absorbent resin particles. .. Therefore, a method is adopted in which the produced particles are classified and only particles having a particle diameter of a certain value or more are collected, or particles having a small particle diameter are aggregated to increase the particle diameter to a desired particle diameter. There is.

For example, Patent Document 1 includes a step of mixing fine powder having a particle size of 180 μm or less and a polymer having a particle size of 300 μm or more among the pulverized polymers and re-granulating while adding steam. A method for producing a water-absorbent resin is disclosed.

Special Table 2016-528368

However, the conventional water-absorbent resin particles have a problem that a large amount of fine powder is likely to be generated when the agglomerates obtained by agglutination in the manufacturing process are reground.

An object of the present invention is to provide a method for producing water-absorbent resin particles capable of reducing the amount of fine powder generated.

One aspect of the present invention is to mix the first polymer particles having a particle size of less than 180 μm and the second polymer particles having a particle size of 180 μm or more to obtain an agglomerate, and to obtain the agglomerates. The present invention relates to a method for producing water-absorbent resin particles, which comprises pulverizing and has a residual monomer amount of 250 ppm or more in the first polymer particles.

In the above production method, the first polymer particles and the second polymer particles may contain a crosslinked polymer of acrylic acid or a salt thereof.

In the above production method, the particle size of the second polymer particles may be 180 to 850 μm.

The above production method preferably includes mixing an aqueous solution in addition to the first polymer particles and the second polymer particles in the step of obtaining agglomerates.

Another aspect of the present invention is to obtain an agglomerate by mixing a first polymer particle having a particle size of less than 180 μm and a second polymer particle having a particle size of 180 μm or more to obtain an agglomerate. The present invention relates to a method for suppressing the generation of fine powder of water-absorbent resin particles, which comprises pulverizing and adjusting the amount of residual monomer in the first polymer particles to 250 ppm or more.

The amount of fine powder generated can be reduced by the method for producing water-absorbent resin particles of the present invention.

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

In this specification, "acrylic" and "methacryl" are collectively referred to as "(meth) acrylic". Similarly, "acrylate" and "methacrylate" are also referred to as "(meth) acrylate". "(Poly)" shall mean both with and without the "poly" prefix. In the numerical range described stepwise in the present specification, the upper limit value or the lower limit value of the numerical range of one step can be arbitrarily combined with the upper limit value or the lower limit value of the numerical range of another step. 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. The content of each component in the composition means the total amount of the plurality of substances present in the composition when a plurality of substances corresponding to each component are present in the composition, unless otherwise specified. "Saline" refers to a 0.9% by mass sodium chloride aqueous solution.

The method for producing water-absorbent resin particles according to the present embodiment includes first polymer particles having a particle size of less than 180 μm (hereinafter, also referred to as “raw material fine particles”) and second polymer particles having a particle size of 180 μm or more. (Hereinafter, also referred to as “raw material particles”) are mixed to obtain agglomerates, and the agglomerates are crushed, and the amount of residual monomer in the first polymer particles is 250 ppm or less. .. The production method according to the present embodiment may include a step of preparing raw material fine particles and raw material particles.

<Preparation of polymer particles>
The water-absorbent resin particles obtained by the method according to the present embodiment contain a polymer. The polymer can be obtained, for example, by polymerizing a monomer containing an ethylenically unsaturated monomer. The polymer may be a crosslinked polymer. The water-absorbent resin particles obtained by the method according to the present embodiment can have a structural unit derived from an ethylenically unsaturated monomer. As the ethylenically unsaturated monomer, a water-soluble ethylenically unsaturated monomer can be used.

Examples of the polymerization method include an aqueous solution polymerization method, a reverse phase suspension polymerization method, a bulk polymerization method, a precipitation polymerization method and the like. Among these, the aqueous solution polymerization method or the reverse phase suspension polymerization method is preferable from the viewpoint of ensuring good water absorption characteristics of the obtained water-absorbent resin particles and facilitating control of the polymerization reaction. Hereinafter, examples of the aqueous solution polymerization method and the reverse phase suspension polymerization method will be described as the polymerization method.

[Aqueous solution polymerization method]
The ethylenically unsaturated monomer is preferably water-soluble, and for example, α, β-unsaturated carboxylic acids such as (meth) acrylic acid, maleic acid, maleic anhydride, and fumaric acid, and carboxylic acids such as salts thereof. System monomer; Nonionic singles such as (meth) acrylamide, N, N-dimethyl (meth) acrylamide, 2-hydroxyethyl (meth) acrylate, N-methylol (meth) acrylamide, polyethylene glycol mono (meth) acrylate, etc. Quantities: Amino group-containing unsaturated monomers such as N, N-diethylaminoethyl (meth) acrylate, N, N-diethylaminopropyl (meth) acrylate, diethylaminopropyl (meth) acrylamide, and quaternized products thereof; vinyl sulfonic acid Examples thereof include sulfonic acid-based monomers such as acid, styrene sulfonic acid, 2- (meth) acrylamide-2-methylpropane sulfonic acid, 2- (meth) acryloylethanesulfonic acid and salts thereof. These ethylenically unsaturated monomers may be used alone or in combination of two or more.

The ethylenically unsaturated monomer is preferably at least one selected from the group consisting of (meth) acrylic acid and salts thereof, (meth) acrylamide, and N, N-dimethylacrylamide. Specifically, the ethylenically unsaturated monomer is preferably 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 100 mol% of 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. The raw material fine particles and the raw material particles preferably contain a crosslinked polymer of acrylic acid or 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 neutralizer if necessary. Can be used as neutralized by. Examples of such an alkaline neutralizer include alkali metal salts such as sodium hydroxide, sodium carbonate, sodium hydrogen carbonate, 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 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.

The ethylenically unsaturated monomer is usually preferably used as an aqueous solution. The concentration of the ethylenically unsaturated monomer in the aqueous solution containing the ethylenically unsaturated monomer (hereinafter, simply referred to as "monomeric aqueous solution") may be usually 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 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-methylpropion amidine] 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-methylpropion amidine] tetrahydrate are preferred. One of these azo compounds may be used alone, or two or more thereof may be used in combination.

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, and hydrogen peroxide are preferably used from the viewpoint of obtaining water-absorbent resin particles having good water absorption performance, and potassium persulfate and persulfate are used. It is more preferable to use ammonium sulfate and 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, L-ascorbic acid and the like.

The total amount of the polymerization initiator used is ethylene used in the polymerization from the viewpoint of avoiding a rapid polymerization reaction, shortening the polymerization reaction time, and increasing the amount of residual monomer in the obtained raw material fine particles to an appropriate range. It is preferably 0.001 to 1 mol, more preferably 0.005 to 0.5 mol, still more preferably 0.008 to 0.3 mol, and 0.01 to 0. 2 mol is even more preferred.

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. As the internal cross-linking agent, for example, a compound having two or more polymerizable unsaturated groups is used, and preferably, a compound having two polymerizable unsaturated groups is used. For example, di or tri (meth) acrylic acid esters of polyols such as (poly) ethylene glycol, (poly) propylene glycol, trimethylpropane, glycerin polyoxyethylene glycol, polyoxypropylene glycol, and (poly) glycerin; Unsaturated polyesters obtained by reacting the above polyol with unsaturated acids such as maleic acid and fumaric acid; bisacrylamides such as N, N'-methylenebis (meth) acrylamide; polyepoxide and (meth) acrylic acid. Di or tri (meth) acrylic acid esters obtained by reaction; carbamil di (meth) acrylate obtained by reacting polyisocyanate such as tolylene diisocyanate or hexamethylene diisocyanate with hydroxyethyl (meth) acrylate. Esters; allylated starch; allylated cellulose; diallyl phthalate; N, N', N "-triallyl isocyanurate; divinylbenzene and the like.

Further, in addition to the compound having two or more polymerizable unsaturated groups, a compound having two or more reactive functional groups can be used as an internal cross-linking agent. 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 thereof include glycerin, pentaerythritol, ethylenediamine, polyethyleneimine, and glycidyl (meth) acrylate. Among these, (poly) ethylene glycol diglycidyl ether, (poly) propylene glycol diglycidyl ether, and (poly) glycerin diglycidyl ether are preferable from the viewpoint of excellent reactivity at low temperature. These internal cross-linking agents may be used alone or in combination of two or more.

When an internal cross-linking agent is used, the amount used is 0.0001 mol with respect to 100 mol of the ethylenically unsaturated monomer in order to sufficiently enhance the water absorption performance such as the water absorption capacity of the obtained water-absorbent resin particles. The above is preferable, 0.001 mol or more is more preferable, 0.003 mol or more is further preferable, and 0.01 mol or more is further preferable.

Although the addition of the internal cross-linking agent insolubilizes the cross-linked polymer and brings about a suitable water absorption capacity, an increase in the amount of the internal cross-linking agent added leads to a decrease in the water absorption capacity of the obtained water-absorbent resin particles. The amount of the agent is preferably, for example, 0.50 mol or less, more preferably 0.25 mol or less, still more preferably 0.05 mol or less, based on 100 mol of the ethylenically unsaturated monomer. is there.

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, neutralized polyacrylic acid, polyacrylamide and the like. One of these may be used alone, or two or more thereof may be used in combination. A water-soluble organic solvent other than water may be appropriately added to the monomer aqueous solution.

(polymerization)
Examples of the polymerization method include a static polymerization method in which the monomer aqueous solution is polymerized without stirring (for example, a static state), a stirring polymerization method in which the monomer aqueous solution is polymerized while being stirred in the reaction apparatus, and the like. It's okay. It is preferable to obtain a hydrogel-like polymer by aqueous solution static polymerization, which is a static polymerization method. In the static polymerization method, when the polymerization is completed, a single block-shaped hydrogel polymer that occupies substantially the same volume as the aqueous monomer solution present in the reaction vessel can be obtained.

The form of production may be batch, semi-continuous, continuous, etc. For example, in the aqueous solution static continuous polymerization, a continuous (for example, strip-shaped) hydrogel can be obtained by carrying out the polymerization reaction while continuously supplying the monomeric aqueous solution to the continuous polymerization apparatus.

The polymerization temperature varies depending on the polymerization initiator used, but from the viewpoint of increasing the productivity by rapidly advancing the polymerization and shortening the polymerization time, and more easily removing the heat of polymerization to carry out the reaction smoothly. 0 to 130 ° C. is preferable, and 10 to 110 ° C. is more preferable. The polymerization time is appropriately set according to the type and 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.

The water content of the massive hydrogel polymer obtained by polymerizing the ethylenically unsaturated monomer is preferably 30 to 80% by mass, more preferably 40 to 75% by mass from the viewpoint of easy implementation of the rough crushing step. It is preferable, and more preferably 50 to 70% by mass. The water content is adjusted by operations such as the water content of the aqueous monomer solution, drying after polymerization, and humidification. The water content of the hydrogel polymer is the content of water in the total mass of the hydrogel polymer in% by mass.

(Rough crushing)
The method for producing water-absorbent resin particles (raw material fine particles and raw material particles) according to the present embodiment may include a step of coarsely crushing a hydrogel polymer. By coarse crushing, a hydrogel coarse crushed material is obtained. The hydrogel crushed product may be in the form of particles, or may have an elongated shape in which particles are connected. The size of the minimum side of the hydrogel crushed product may be, for example, about 0.1 to 15 mm, preferably about 1.0 to 10 mm. The size of the maximum side of the hydrogel crushed product may be about 0.1 to 200 mm, preferably about 1.0 to 150 mm.

As the coarse 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. Of these, a double-armed kneader, a meat chopper, and a cutter mill are more preferable. The coarse crushing device may be of the same type as the crushing device for dried gel products described later.

When the hydrogel polymer is roughly crushed, the hydrogel polymer may be pre-cut to an appropriate size (for example, about 5 cm square) before being put into the crusher.

(Dry)
The method for producing water-absorbent resin particles (raw material fine particles and raw material particles) according to the present embodiment may include a step of drying a hydrogel coarse crushed product. By removing the solvent containing water in the hydrogel crushed material by heating and / or blowing air, the hydrogel crushed material can be dried to obtain a gel-dried product. The drying method may be a general method such as natural drying, heat drying, spray drying, freeze drying and 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. For drying, a plurality of methods may be used in combination. When the drying is carried out at 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 crude gel crushed product is 20% by mass or less, preferably 10% by mass or less, and more preferably 5% by mass or less. The drying temperature is preferably 120 ° C. or higher, more preferably 150 ° C. or higher, still more preferably 180 ° C. or higher, from the viewpoint of increasing the amount of residual monomer in the raw material fine particles to an appropriate range.

(Crushing)
The method for producing water-absorbent resin particles (raw material fine particles and raw material particles) according to the present embodiment may include a step of further pulverizing the gel dried product. A gel pulverized product is obtained by pulverizing the gel dried product. A known crusher can be used for crushing the dried gel product, for example, a roller mill (roll mill), a stamp mill, a jet mill, a high-speed rotary crusher (hammer mill, pin mill, rotor beater mill, etc.), container drive. Mold mills (rotary mills, vibration mills, planetary mills, etc.) can be used. Preferably, a high speed rotary grinder is used. The crusher may have an opening on the outlet side, such as a perforated plate, a screen, or a grid, for controlling the maximum particle size of the crushed 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, more preferably 0.5 to 1.5 mm. preferable.

(Classification)
The method for producing water-absorbent resin particles according to the present embodiment may include, if necessary, a step of classifying the pulverized gel product before the granulation step described later. 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. Further, a plurality of classification steps may be performed, such as re-crushing a part of the particles after classification and repeating the crushing step and the classification step, or the classification step may be performed after the surface cross-linking step described later.

A known classification method can be used for the classification of the pulverized gel product, for example, screen classification, wind power classification, etc., and screen classification is preferable. Examples of the screen classification include a vibrating sieve, a rotary shifter, a cylindrical stirring sieve, a blower shifter, and a low-tap type shaker. 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. Wind power classification is a method of classifying particles using the flow of air.

When classification is performed before the granulation step, for example, it is preferable to divide the particles into particles having a particle diameter of 180 μm and particles having a particle diameter of 180 μm or more, and particles having a particle diameter of 180 μm and particles having a particle diameter of 180 μm are separated. It is more preferable to select and recover the particles having a diameter of about 850 μm. However, in the production method according to the present embodiment, the classification before the granulation step does not have to be performed.

(Surface cross-linking)
The method for producing water-absorbent resin particles (raw material fine particles and raw material particles) according to the present embodiment may include a surface cross-linking step. The surface cross-linking can be carried out, for example, by adding a cross-linking agent (surface cross-linking agent) for performing the surface cross-linking to the cross-linked polymer and reacting it. In the aqueous solution polymerization method, the surface cross-linking agent may be added at any time after the pulverization step, and is carried out before or after the classification step, or before or after the granulation step described later. Can be done. When the particle size distribution is further adjusted by classification or the like after the granulation step described later, it is preferable to perform surface cross-linking after adjusting the particle size distribution. By adding a surface cross-linking agent and performing the surface cross-linking treatment, the cross-linking density in the vicinity of the surface of the cross-linked polymer is increased, so that the water absorption performance of the obtained water-absorbent resin particles can be improved.

The surface cross-linking agent can be added, for example, by adding a surface cross-linking agent solution or by spraying the surface cross-linking agent solution. From the viewpoint of uniformly dispersing the surface cross-linking agent, the surface cross-linking agent is preferably added as a surface cross-linking agent solution by dissolving the surface cross-linking agent in a solvent such as water and / or alcohol. Further, the surface cross-linking step may be carried out once or divided into two or more times.

The surface cross-linking agent may contain, for example, two or more functional groups (reactive functional groups) having reactivity with a functional group derived from an ethylenically unsaturated monomer. Examples of the surface cross-linking agent include polyols such as ethylene glycol, propylene glycol, 1,4-butanediol, trimethylolpropane, glycerin, polyoxyethylene glycol, polyoxypropylene glycol, and polyglycerin; (poly) ethylene glycol di. Polyglycidyl compounds such as glycidyl ether, (poly) glycerin diglycidyl ether, (poly) glycerin triglycidyl ether, (poly) propylene glycol diglycidyl ether, (poly) glycerol polyglycidyl ether; epichlorohydrin, epibromhydrin, α- Haloepoxy compounds such as methyl epichlorohydrin; 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-3- Oxetane compounds such as oxetane methanol, 3-butyl-3-oxetane methanol, 3-methyl-3-oxetane ethanol, 3-ethyl-3-oxetane ethanol, 3-butyl-3-oxetane ethanol; 1,2-ethylenebisoxazoline Oxetane compounds such as; carbonate compounds such as ethylene carbonate; hydroxyalkylamide compounds such as bis [N, N-di (β-hydroxyethyl)] adipamide and the like. Among these, (poly) ethylene glycol diglycidyl ether, (poly) ethylene glycol triglycidyl ether, (poly) glycerin diglycidyl ether, (poly) glycerin triglycidyl ether, (poly) propylene glycol diglycidyl 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 preferable, and polyglycidyl compounds are preferable. More preferred. These surface cross-linking agents may be used alone or in combination of two or more. For example, a polyglycidyl compound and polyols may be used in combination.

The amount of the surface cross-linking agent added is preferably 0, with respect to 100 mol of the total amount of the ethylenically unsaturated monomer usually used for the polymerization, from the viewpoint of appropriately increasing the cross-linking density near the surface of the water-absorbent resin particles. It is 0001 to 1 mol, more preferably 0.001 to 0.5 mol.

The surface cross-linking step is preferably carried out in the presence of water in the range of 1 to 200 parts by mass with respect to 100 parts by mass of the ethylenically unsaturated monomer. The amount of water can be adjusted by appropriately using a water-soluble organic solvent such as water and / or alcohol. By adjusting the amount of water in the surface cross-linking step, the water-absorbent resin particles can be more preferably cross-linked in the vicinity of the particle surface.

The treatment temperature of the surface cross-linking agent is appropriately set according to the surface cross-linking 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.

Surface cross-linking may be performed only once or at multiple timings. The surface cross-linking may be carried out after the pulverization step or before the pulverization in addition to the execution after the pulverization step.

[Reverse phase suspension polymerization method]
Next, a case where a reverse phase suspension polymerization method is used as the polymerization method will be described. The ethylenically unsaturated monomer is preferably water-soluble, for example, (meth) acrylic acid and a salt thereof, 2- (meth) acrylamide-2-methylpropanesulfonic acid and a salt thereof, (meth) acrylamide, N. , N-dimethyl (meth) acrylamide, 2-hydroxyethyl (meth) acrylate, N-methylol (meth) acrylamide, polyethylene glycol mono (meth) acrylate, N, N-diethylaminoethyl (meth) acrylate, N, N-diethylamino Examples thereof include propyl (meth) acrylate and diethylaminopropyl (meth) acrylamide. When the ethylenically unsaturated monomer has an amino group, the amino group may be quaternized. The ethylenically unsaturated monomer may be used alone or in combination of two or more. The functional groups such as the carboxyl group and the amino group of the above-mentioned monomer can function as functional groups capable of cross-linking in the surface cross-linking step described later.

Among these, from the viewpoint of industrial availability, the ethylenically unsaturated monomer is a group consisting of (meth) acrylic acid and salts thereof, acrylamide, methacrylamide, and N, N-dimethylacrylamide. It is preferable to contain at least one compound selected from the above, and it is more preferable to contain at least one compound selected from the group consisting of (meth) acrylic acid and salts thereof, and acrylamide. From the viewpoint of further enhancing the water absorption property, the ethylenically unsaturated monomer further preferably contains at least one compound selected from the group consisting of (meth) acrylic acid and salts thereof. That is, the water-absorbent resin particles preferably have a structural unit derived from at least one selected from the group consisting of (meth) acrylic acid and salts thereof.

As the monomer for obtaining the water-absorbent resin particles (raw material fine particles and raw material particles), a monomer other than the above-mentioned ethylenically unsaturated monomer may be used. Such a monomer can be used, for example, by mixing with an aqueous solution containing the above-mentioned ethylenically unsaturated monomer. 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 monomer giving the structural unit of the crosslinked polymer. The same shall apply 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.

According to the present embodiment, as an example of the water-absorbent resin particles, the water-absorbent resin particles containing a crosslinked polymer having a structural unit derived from the ethylenically unsaturated monomer, wherein the ethylenically unsaturated monomer is used. , At least one compound selected from the group consisting of (meth) acrylic acid and salts thereof, and the ratio of (meth) acrylic acid and salts thereof is based on the total amount of monomers for obtaining water-absorbent resin particles. It is possible to provide water-absorbent resin particles in an amount of 70 to 100 mol%.

The ethylenically unsaturated monomer is usually preferably used as an aqueous solution. The concentration of the ethylenically unsaturated monomer in the aqueous solution containing the ethylenically unsaturated monomer (hereinafter, simply referred to as "monomeric aqueous solution") is preferably 20% by mass or more and preferably 25 to 70% by mass. Is more preferable, and 30 to 55% by mass is further preferable. Examples of the water used in the aqueous solution include tap water, distilled water, ion-exchanged water and the like.

When the ethylenically unsaturated monomer has an acid group, the aqueous monomer solution may be used by neutralizing the acid group with an alkaline neutralizer. The degree of neutralization of the ethylenically unsaturated monomer by the alkaline neutralizing agent increases the osmotic pressure of the obtained water-absorbent resin particles and further enhances the water-absorbing characteristics. It is preferably 10 to 100 mol%, more preferably 50 to 90 mol%, and even more preferably 60 to 80 mol% of the group. Examples of the alkaline neutralizing agent include alkali metal salts such as sodium hydroxide, sodium carbonate, sodium hydrogen carbonate, potassium hydroxide and potassium carbonate; ammonia and the like. The alkaline neutralizer may be used alone or in combination of two or more. The alkaline neutralizer may be used in the form of an aqueous solution to simplify the neutralization operation. Neutralization of the acid group of the ethylenically unsaturated monomer can be performed, for example, by adding an aqueous solution of sodium hydroxide, potassium hydroxide or the like to the above-mentioned monomer aqueous solution and mixing them.

In the reverse phase suspension polymerization method, an aqueous monomer solution is dispersed in a hydrocarbon dispersion medium in the presence of a surfactant, and an ethylenically unsaturated monomer is polymerized using a radical polymerization initiator or the like. Can be done.

Examples of the surfactant include nonionic surfactants, anionic surfactants and the like. Nonionic surfactants include sorbitan fatty acid ester, polyglycerin fatty acid ester, sucrose fatty acid ester, polyoxyethylene sorbitan fatty acid ester, polyoxyethylene glycerin fatty acid ester, sorbitol fatty acid ester, polyoxyethylene sorbitol fatty acid ester, and polyoxyethylene. Alkyl ether, polyoxyethylene alkyl phenyl ether, polyoxyethylene castor oil, polyoxyethylene hydrogenated castor oil, alkylallyl formaldehyde condensed polyoxyethylene ether, polyoxyethylene polyoxypropylene block copolymer, polyoxyethylene polyoxypropyl alkyl ether, Examples thereof include polyethylene glycol fatty acid ester. Anionic surfactants include fatty acid salts, alkylbenzene sulfonates, alkylmethyl taurates, polyoxyethylene alkylphenyl ether sulfates, polyoxyethylene alkyl ether sulfonates, and polyoxyethylene alkyl ether phosphates. , Phosphate ester of polyoxyethylene alkyl allyl ether and the like. The surfactant may be used alone or in combination of two or more.

Surfactant from the viewpoint that the W / O type reverse phase suspension is in a good state, water-absorbent resin particles (raw material fine particles and raw material particles) having a suitable particle size can be easily obtained, and are industrially easily available. The agent preferably contains at least one compound selected from the group consisting of sorbitan fatty acid ester, polyglycerin fatty acid ester and sucrose fatty acid ester. From the viewpoint that an appropriate particle size distribution of the water-absorbent resin particles can be easily obtained, and from the viewpoint that the water-absorbing characteristics of the water-absorbent resin particles and the performance of the absorber and the absorbent article using the same can be easily improved, the surfactant is used as a detergent. It preferably contains a sugar fatty acid ester, more preferably a sucrose stearic acid ester.

The amount of the surfactant used is preferably 0.05 to 10 parts by mass with respect to 100 parts by mass of the aqueous monomer solution from the viewpoint of obtaining a sufficient effect on the amount used and economically. .08 to 5 parts by mass is more preferable, and 0.1 to 3 parts by mass is further preferable.

In reverse phase suspension polymerization, a polymer-based dispersant may be used in combination with the above-mentioned surfactant. Examples of the polymer dispersant include maleic anhydride-modified polyethylene, maleic anhydride-modified polypropylene, maleic anhydride-modified ethylene / propylene copolymer, maleic anhydride-modified EPDM (ethylene / propylene / diene / terpolymer), and maleic anhydride. Modified polybutadiene, maleic anhydride / ethylene copolymer, maleic anhydride / propylene copolymer, maleic anhydride / ethylene / propylene copolymer, maleic anhydride / butadiene copolymer, polyethylene, polypropylene, ethylene / propylene copolymer Examples thereof include coalescence, oxidized polyethylene, oxidized polypropylene, oxidized ethylene / propylene copolymer, ethylene / acrylic acid copolymer, ethyl cellulose, ethyl hydroxyethyl cellulose and the like. The polymer-based dispersant may be used alone or in combination of two or more. As the polymer-based dispersant, from the viewpoint of excellent dispersion stability of the monomer, maleic anhydride-modified polyethylene, maleic anhydride-modified polypropylene, maleic anhydride-modified ethylene / propylene copolymer, and maleic anhydride / ethylene copolymer weight. Combined, maleic anhydride / propylene copolymer, maleic anhydride / ethylene / propylene copolymer, polyethylene, polypropylene, ethylene / propylene copolymer, oxidized polyethylene, oxidized polypropylene, and oxidized ethylene / propylene copolymer At least one selected from the group consisting of coalescing is preferable.

The amount of the polymer-based dispersant used is preferably 0.05 to 10 parts by mass with respect to 100 parts by mass of the aqueous monomer solution from the viewpoint of obtaining a sufficient effect on the amount used and from the viewpoint of economic efficiency. , 0.08 to 5 parts by mass is more preferable, and 0.1 to 3 parts by mass is further preferable.

The hydrocarbon dispersion medium may contain at least one compound selected from the group consisting of chain aliphatic hydrocarbons having 6 to 8 carbon atoms and alicyclic hydrocarbons having 6 to 8 carbon atoms. Hydrocarbon dispersion media include chain aliphatic hydrocarbons such as n-hexane, n-heptane, 2-methylhexane, 3-methylhexane, 2,3-dimethylpentane, 3-ethylpentane, and n-octane; cyclohexane. , Methylcyclohexane, cyclopentane, methylcyclopentane, trans-1,2-dimethylcyclopentane, cis-1,3-dimethylcyclopentane, trans-1,3-dimethylcyclopentane and other alicyclic hydrocarbons; benzene, Examples include aromatic hydrocarbons such as toluene and xylene. The hydrocarbon dispersion medium may be used alone or in combination of two or more.

The hydrocarbon dispersion medium may contain at least one selected from the group consisting of n-heptane and cyclohexane from the viewpoint of being industrially easily available and having stable quality. From the same viewpoint, as the mixture of the above-mentioned hydrocarbon dispersion medium, for example, commercially available ExxonHeptane (manufactured by ExxonMobil: containing 75 to 85% of n-heptane and isomeric hydrocarbons) is used. You may.

The amount of the hydrocarbon dispersion medium used is preferably 30 to 1000 parts by mass and 40 to 500 parts by mass with respect to 100 parts by mass of the monomer aqueous solution from the viewpoint of appropriately removing the heat of polymerization and easily controlling the polymerization temperature. Is more preferable, and 50 to 400 parts by mass is further preferable. When the amount of the hydrocarbon dispersion medium used is 30 parts by mass or more, the polymerization temperature tends to be easily controlled. When the amount of the hydrocarbon dispersion medium used is 1000 parts by mass or less, the productivity of polymerization tends to be improved, which is economical.

The radical polymerization initiator is preferably water-soluble, for example, persulfates such as potassium persulfate, ammonium persulfate, sodium persulfate; methyl ethyl ketone peroxide, methyl isobutyl ketone peroxide, di-t-butyl peroxide, t. -Peroxides such as butyl cumylperoxide, t-butylperoxyacetate, t-butylperoxyisobutyrate, t-butylperoxypivalate, hydrogen peroxide; 2,2'-azobis (2-amidinopropane) ) 2 hydrochloride, 2,2'-azobis [2- (N-phenylamidino) propane] 2 hydrochloride, 2,2'-azobis [2- (N-allylamidino) propane] 2 hydrochloride, 2,2 '-Azobis [2- (2-imidazolin-2-yl) propane] dihydrochloride, 2,2'-azobis {2- [1- (2-hydroxyethyl) -2-imidazolin-2-yl] propane} Dihydrochloride, 2,2'-azobis {2-methyl-N- [1,1-bis (hydroxymethyl) -2-hydroxyethyl] propionamide}, 2,2'-azobis [2-methyl-N- (2-Hydroxyethyl) -propionamide], azo compounds such as 4,4'-azobis (4-cyanovaleric acid) and the like can be mentioned. The radical polymerization initiator may be used alone or in combination of two or more. Examples of the radical polymerization initiator include potassium persulfate, ammonium persulfate, sodium persulfate, 2,2'-azobis (2-amidinopropane) dihydrochloride, and 2,2'-azobis [2- (2-imidazolin-2-). Il) Propane] 2 hydrochloride and at least one selected from the group consisting of 2,2'-azobis {2- [1- (2-hydroxyethyl) -2-imidazolin-2-yl] propane} 2 hydrochloride. Is preferable.

The amount of the radical polymerization initiator used may be 0.05 to 10 mmol per 1 mol of the ethylenically unsaturated monomer. When the amount of the radical polymerization initiator used is 0.05 mmol or more, the polymerization reaction does not require a long time and is efficient. When the amount of the radical polymerization initiator used is 10 mmol or less, it is easy to suppress the occurrence of a rapid polymerization reaction.

The above-mentioned radical polymerization initiator can also be used as a redox polymerization initiator in combination with a reducing agent such as sodium sulfite, sodium hydrogen sulfite, ferrous sulfate, and L-ascorbic acid.

At the time of the polymerization reaction, the aqueous monomer solution used for the polymerization may contain a chain transfer agent. Examples of the chain transfer agent include hypophosphates, thiols, thiolic acids, secondary alcohols, amines and the like.

In order to control the particle size of the water-absorbent resin particles (raw material fine particles and raw material particles), the monomer aqueous solution used for the polymerization may contain a thickener. Examples of the thickener include hydroxyethyl cellulose, hydroxypropyl cellulose, methyl cellulose, carboxymethyl cellulose, polyethylene glycol, polyacrylamide, polyethyleneimine, dextrin, sodium alginate, polyvinyl alcohol, polyvinylpyrrolidone, polyethylene oxide and the like. If the stirring speed at the time of polymerization is the same, the higher the viscosity of the aqueous monomer solution, the larger the medium particle size of the obtained particles tends to be.

Internal cross-linking may occur due to self-cross-linking during polymerization, but cross-linking may be performed by using an internal cross-linking agent. When an internal cross-linking agent is used, it is easy to control the water absorption characteristics of the water-absorbent resin particles. The internal cross-linking agent is usually added to the reaction solution during the polymerization reaction. Examples of the internal cross-linking agent include di or tri (meth) acrylic acid esters of polyols such as ethylene glycol, propylene glycol, trimethylpropane, glycerin, polyoxyethylene glycol, polyoxypropylene glycol, and polyglycerin; Unsaturated polyesters obtained by reacting polyols with unsaturated acids (maleic acid, fumaric acid, etc.); bis (meth) acrylamides such as N, N'-methylenebis (meth) acrylamide; polyepoxides and (meth) Di or tri (meth) acrylic acid esters obtained by reacting with acrylic acid; di (meth) obtained by reacting polyisocyanate (tolylene diisocyanate, hexamethylene diisocyanate, etc.) with hydroxyethyl (meth) acrylate. ) Acrylic acid carbamil esters; compounds having two or more polymerizable unsaturated groups such as allylated starch, allylated cellulose, diallyl phthalate, N, N', N "-triallyl isocyanurate, divinylbenzene; Polys such as (poly) ethylene glycol diglycidyl ether, (poly) propylene glycol diglycidyl ether, (poly) glycerin diglycidyl ether, (poly) glycerin triglycidyl ether, (poly) propylene glycol diglycidyl ether, polyglycerol polyglycidyl ether, etc. Glycidyl compounds; haloepoxy compounds such as epichlorohydrin, epibromhydrin, α-methylepichlorohydrin; 2 reactive functional groups such as isocyanate compounds (2,4-tolylene diisocyanate, hexamethylene diisocyanate, etc.) Examples thereof include compounds having more than one. The internal cross-linking agent may be used alone or in combination of two or more. As the internal cross-linking agent, a polyglycidyl compound is preferable, and a diglycidyl ether compound is used. Is more preferable, and at least one selected from the group consisting of (poly) ethylene glycol diglycidyl ether, (poly) propylene glycol diglycidyl ether, and (poly) glycerin diglycidyl ether is further preferable.

The amount of the internal cross-linking agent used is from the viewpoint that an excellent permeation rate can be easily obtained in an absorbent article, and the water-soluble property is suppressed by appropriately cross-linking the obtained polymer, so that a sufficient water absorption amount can be obtained. From the viewpoint of ease, 30 mmol or less is preferable, 0.01 to 10 mmol is more preferable, 0.012 to 5 mmol is further preferable, and 0.015 to 1 mmol is particularly preferable, per 1 mol of the ethylenically unsaturated monomer. , 0.02 to 0.1 mmol is very preferred, and 0.025 to 0.08 mmol is very preferred.

An aqueous phase containing an ethylenically unsaturated monomer, a radical polymerization initiator and an internal cross-linking agent if necessary, and an oil phase containing a hydrocarbon dispersion medium, a surfactant and a polymer-based dispersant if necessary. Can be heated under stirring in a mixed state to carry out reverse phase suspension polymerization in an aqueous system in oil.

When performing reverse phase suspension polymerization, a monomer aqueous solution containing an ethylenically unsaturated monomer is used as a hydrocarbon dispersion medium in the presence of a surfactant (more polymer-based dispersant if necessary). Disperse. At this time, the timing of adding the surfactant, the polymer-based dispersant, or the like may be either before or after the addition of the aqueous monomer solution, as long as it is before the start of the polymerization reaction.

Among them, from the viewpoint of easily reducing the amount of the hydrocarbon dispersion medium remaining in the obtained water-absorbent resin, the surface activity is applied after the monomer aqueous solution is dispersed in the hydrocarbon dispersion medium in which the polymer-based dispersant is dispersed. It is preferable to further disperse the agent before carrying out the polymerization.

Reverse phase suspension polymerization can be carried out in one stage or in multiple stages of two or more stages. Reversed phase suspension polymerization is preferably carried out in 2 to 3 steps from the viewpoint of increasing productivity. By performing the reverse phase suspension polymerization in multiple stages, a water-absorbent resin having a shape in which the particles of the first stage are aggregated (for example, a tuft of grapes formed by a plurality of spherical primary particles) may be obtained. ..

When reverse-phase suspension polymerization is carried out in two or more stages, an ethylenically unsaturated single amount is added to the reaction mixture obtained in the first-step polymerization reaction after the first-step reverse-phase suspension polymerization is carried out. The bodies may be added and mixed, and the reverse phase suspension polymerization of the second and subsequent steps may be carried out in the same manner as in the first step. In the reverse phase suspension polymerization in each stage of the second and subsequent stages, in addition to the ethylenically unsaturated monomer, the above-mentioned radical polymerization initiator and / or internal cross-linking agent is used in the reverse phase of each stage of the second and subsequent stages. Based on the amount of ethylenically unsaturated monomer added during suspension polymerization, reverse-phase suspension polymerization is carried out by adding within the range of the molar ratio of each component to the above-mentioned ethylenically unsaturated monomer. Is preferable. An internal cross-linking agent may be used in the reverse phase suspension polymerization in each of the second and subsequent stages, if necessary. When an internal cross-linking agent is used, it is added within the range of the molar ratio of each component to the above-mentioned ethylenically unsaturated monomer based on the amount of the ethylenically unsaturated monomer provided in each stage, and the suspension is reversed. It is preferable to carry out turbid polymerization.

The temperature of the polymerization reaction varies depending on the radical polymerization initiator used, but by rapidly advancing the polymerization and shortening the polymerization time, the efficiency is improved and the heat of polymerization is easily removed to carry out the reaction smoothly. From the viewpoint, 20 to 150 ° C. is preferable, and 40 to 120 ° C. is more preferable. The reaction time is usually 0.5-4 hours. The completion of the polymerization reaction can be confirmed, for example, by stopping the temperature rise in the reaction system. As a result, the polymer of the ethylenically unsaturated monomer is usually obtained in the state of a hydrogel.

After polymerization, cross-linking may be performed by adding a cross-linking agent to the obtained hydrogel polymer and heating it. By performing cross-linking after the polymerization, the degree of cross-linking of the hydrogel polymer can be increased to further improve the water absorption characteristics.

Examples of the cross-linking agent for performing post-polymerization cross-linking include polyols such as ethylene glycol, propylene glycol, 1,4-butanediol, trimethylolpropane, glycerin, polyoxyethylene glycol, polyoxypropylene glycol, and polyglycerin; Compounds having two or more epoxy groups such as (poly) ethylene glycol diglycidyl ether, (poly) propylene glycol diglycidyl ether, (poly) glycerin diglycidyl ether; Haloepoxide compounds; compounds having two or more isocyanate groups such as 2,4-tolylene diisocyanate, hexamethylene diisocyanate; oxazoline compounds such as 1,2-ethylene bisoxazoline; carbonate compounds such as ethylene carbonate; bis [N, Examples thereof include hydroxyalkylamide compounds such as N-di (β-hydroxyethyl)] adipamide. Among these, polyglycidyl compounds such as (poly) ethylene glycol diglycidyl ether, (poly) glycerin diglycidyl ether, (poly) glycerin triglycidyl ether, (poly) propylene glycol diglycidyl ether, and polyglycerol polyglycidyl ether are available. preferable. The cross-linking agent may be used alone or in combination of two or more.

The amount of the cross-linking agent after polymerization may be 30 mmol or less, 10 mmol or less, or 0.01 to 5 mmol per mole of the ethylenically unsaturated monomer from the viewpoint that suitable water absorption characteristics can be easily obtained.

The timing of adding the cross-linking agent after polymerization may be after the polymerization of the ethylenically unsaturated monomer used for polymerization, and in the case of multi-stage polymerization, it is preferable to add it after multi-stage polymerization. The post-polymerization cross-linking agent contains water in consideration of heat generation during and after polymerization, retention due to process delay, opening of the system when the cross-linking agent is added, and fluctuation of water content due to addition of water accompanying the addition of the cross-linking agent. From the viewpoint of rate (described later), it is preferable to add in the region of [moisture content immediately after polymerization ± 3% by mass].

Subsequently, by carrying out drying to remove water from the obtained hydrogel-like polymer, polymer particles (for example, polymer particles having a structural unit derived from an ethylenically unsaturated monomer) can be obtained. As a drying method, for example, (a) in a state where the hydrogel polymer is dispersed in a hydrocarbon dispersion medium, azeotropic distillation is performed by heating from the outside, and the hydrocarbon dispersion medium is refluxed to remove water. Examples thereof include (b) a method of taking out the hydrogel polymer by decantation and drying under reduced pressure, and (c) a method of filtering the hydrogel polymer with a filter and drying under reduced pressure. When the method (b) or (c) is performed, the amount of residual monomers in the raw material fine particles increases, and tends to be within an appropriate range for carrying out the production method according to the present embodiment.

In the production of water-absorbent resin particles (raw material fine particles and raw material particles), in the drying step (moisture removing step) or a subsequent step, a cross-linking agent is used to cover the surface portion (surface and vicinity of the surface) of the hydrogel polymer. It is preferable that surface cross-linking is performed. By performing surface cross-linking, it is easy to control water absorption characteristics. The surface cross-linking is preferably performed at a timing when the water-containing gel polymer has a specific water content. The time of surface cross-linking is preferably when the water content of the hydrogel polymer is 5 to 50% by mass, more preferably 10 to 40% by mass, and even more preferably 15 to 35% by mass.

The water content (mass%) of the water-containing gel polymer is calculated by the following formula.
Moisture content = [Ww / (Ww + Ws)] x 100
Ww: If necessary when mixing a surface cross-linking agent or the like, the amount of water contained in the monomer aqueous solution before polymerization in the entire polymerization step minus the amount of water discharged to the outside of the system in the drying step. The amount of water in the hydrogel polymer to which the amount of water used is added.
Ws: A solid content calculated from the amount of materials such as an ethylenically unsaturated monomer, a cross-linking agent, and an initiator that constitute a hydrogel polymer.

Examples of the cross-linking agent (surface cross-linking agent) for performing surface cross-linking include compounds having two or more reactive functional groups. Examples of the surface cross-linking agent include polyols such as ethylene glycol, propylene glycol, 1,4-butanediol, trimethylolpropane, glycerin, polyoxyethylene glycol, polyoxypropylene glycol, and polyglycerin; (poly) ethylene glycol diglycidyl ether. , (Poly) Glycerin diglycidyl ether, (Poly) Glycerin triglycidyl ether, Trimethylol propantriglycidyl ether, (Poly) propylene glycol diglycidyl ether, (Poly) glycerol Polyglycidyl compounds and other polyglycidyl compounds; Epichlorohydrin , Epibrom hydrin, α-methyl epichlorohydrin and other haloepoxy compounds; 2,4-tolylene diisocyanate, hexamethylene diisocyanate and other isocyanate compounds; 3-methyl-3-oxetane methanol, 3-ethyl-3-oxetane Oxetane compounds such as methanol, 3-butyl-3-oxetane methanol, 3-methyl-3-oxetane ethanol, 3-ethyl-3-oxetane ethanol, 3-butyl-3-oxetane ethanol; 1,2-ethylenebisoxazoline and the like. Oxetane compounds; carbonate compounds such as ethylene carbonate; hydroxyalkylamide compounds such as bis [N, N-di (β-hydroxyethyl)] adipamide and the like. The surface cross-linking agent may be used alone or in combination of two or more. As the surface cross-linking agent, a polyglycidyl compound is preferable, and (poly) ethylene glycol diglycidyl ether, (poly) glycerin diglycidyl ether, (poly) glycerin triglycidyl ether, (poly) propylene glycol diglycidyl ether, and polyglycerol are used. At least one selected from the group consisting of polyglycidyl ether is more preferable.

The amount of the surface cross-linking agent used is preferably 0.01 to 20 mmol, preferably 0.05 to 10 to 1 mol of the ethylenically unsaturated monomer used for polymerization, from the viewpoint that suitable water absorption characteristics can be easily obtained. Millimole is more preferable, 0.1 to 5 mmol is further preferable, 0.15 to 1 mmol is particularly preferable, and 0.2 to 0.5 mmol is extremely preferable.

After surface cross-linking, polymer particles that have been surface-crosslinked and dried can be obtained by distilling water and a hydrocarbon dispersion medium by a known method, drying under heating and reduced pressure, and the like.

The polymerization reaction can be carried out using various stirrers having stirring blades. As the stirring blade, flat plate blades, lattice blades, paddle blades, propeller blades, anchor blades, turbine blades, Faudler blades, ribbon blades, full zone blades, max blend blades and the like can be used.

The polymer particles may be pulverized with a part or all of the particle size distribution, if necessary. A known crusher can be used for crushing, for example, a roller mill (roll mill), a stamp mill, a jet mill, a high-speed rotary crusher (hammer mill, pin mill, rotor beater mill, etc.), and a container-driven mill (rotary). Mills, vibration mills, planetary mills, etc.) can be used. Preferably, a high speed rotary grinder is used. The crusher may have an opening on the outlet side, such as a perforated plate, a screen, or a grid, for controlling the maximum particle size of the crushed 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, more preferably 0.5 to 1.5 mm. preferable.

<Granulation>
In the production method according to the present embodiment, the first polymer particles (raw material fine particles) having a particle size of less than 180 μm and the second polymer particles (raw material particles) having a particle size of 180 μm or more are mixed and aggregated. (Aggregation step) and crushing the agglomerate (re-grinding step). Hereinafter, the agglomeration step and the regrinding step are collectively referred to as "granulation".

As the raw material fine particles and the raw material particles, the polymer particles obtained by the above-mentioned method can be used, and may be classified so as to keep the particle size within a predetermined range, if necessary. At the time of aggregation, the raw material fine particles and the raw material particles may be mixed separately, or may be used as they are when the raw material fine particles and the raw material particles are mixed at the time of producing the particles. Good. The raw material fine particles and the raw material particles may have the same or different component compositions. The particle size of the raw material particles is preferably in the range of 180 to 850 μm.

Aggregation can be performed by a known method. Aggregation can be performed, for example, by mixing raw material fine particles, raw material particles, and an aqueous liquid. The aqueous solution may be, for example, water, or may be an aqueous solution containing a component such as a water-soluble salt or a hydrophilic organic solvent. The proportion of water in the aqueous solution may be, for example, 90 to 100% by mass. The temperature at which the raw material fine particles and the raw material particles are mixed is preferably 40 to 150 ° C, more preferably 60 to 100 ° C.

The mixing ratio of the raw material fine particles and the raw material particles may be any ratio. For example, the amount of the raw material fine particles is 1% by mass or more, 5% by mass or more, and 10% by mass with respect to the total amount of the raw material fine particles and the raw material particles. % Or more, 20% by mass or more, 30% by mass or more, 40% by mass or more, 50% by mass or more, 60% by mass or more, 70% by mass or more, 80% by mass or more, or 90% by mass or more, 99% by mass. Below, 95% by mass or less, 90% by mass or less, 80% by mass or less, 70% by mass or less, 60% by mass or less, 50% by mass or less, 40% by mass or more, 30% by mass or less, 20% by mass or less, 10% by mass. Hereinafter, it may be 5% by mass or less.

The amount of the aqueous liquid used for mixing may be, for example, 10 parts by mass or more, 20 parts by mass or more, 30 parts by mass or more, or 40 parts by mass or more with respect to a total of 100 parts by mass of the raw material fine particles and the raw material particles. It may be 70 parts by mass or less, 60 parts by mass or less, or 50 parts by mass or less.

The order of mixing at the time of aggregation is not particularly limited. For example, the raw material fine particles and the raw material particles may be mixed in advance and then the aqueous liquid may be added, or the raw material fine particles and the aqueous liquid may be mixed in advance and then the raw material. Particles may be added. The method for mixing the raw material fine particles and / or the raw material particles with the aqueous liquid is not particularly limited, but for example, the aqueous liquid may be dropped little by little, sprayed, or mixed in the state of steam. Raw material fine particles and / or raw material particles may be added to the aqueous solution.

Next, the agglomerated mass obtained by agglutination is crushed. A known crusher can be used for crushing during granulation, for example, a roller mill (roll mill), a stamp mill, a jet mill, a high-speed rotary crusher (hammer mill, pin mill, rotor beater mill, etc.), container drive. Mold mills (rotary mills, vibration mills, planetary mills, etc.) can be used. Preferably, a high speed rotary grinder is used. The crusher may have an opening on the outlet side, such as a perforated plate, a screen, or a grid, for controlling the maximum particle size of the crushed 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, more preferably 0.5 to 1.5 mm. preferable.

It is preferable to dry the polymer particles after agglutination during granulation and before pulverization. Subsequent pulverization can be performed more easily by drying. The drying can be performed by, for example, heating, blowing air, or the like, and it is preferable to dry by heating. In the case of heat drying, the heating temperature is preferably 75 ° C. or higher. The heating temperature may be, for example, 100 ° C. or lower.

The method for producing water-absorbent resin particles according to the present embodiment may include a step of surface-crosslinking the particles obtained by granulation.

In the production method according to the present embodiment, the amount of residual monomer in the raw material fine particles is 250 ppm or more. The residual monomer is a monomer that does not contribute to the polymerization, and is measured by the measuring method described later. Residual monomers include those that remain unreacted during polymerization, and monomers produced by the decomposition of part of the polymer particles in the post-polymerization step.

According to the production method according to the present embodiment, it is possible to reduce the amount of fine particles, for example, particles having a particle diameter of 180 μm or less, generated in the polymer particles obtained by granulation. By adopting the production method according to the present embodiment, the particle size distribution of the obtained granulated polymer particles becomes narrower and sharper, the generation of fine powders that need to be discarded or re-granulated is suppressed, and more efficiently. Water-absorbent resin particles can be produced. In addition, it is possible to suppress the generation of dust during the production of products using water-absorbent resin particles such as absorbers and absorbent articles.

The amount of residual monomer in the raw material fine particles may be 270 ppm or more, 290 ppm or more, 350 ppm or more, 400 ppm or more, 500 ppm or more, 600 ppm or more, 800 ppm or more, or 1000 ppm or more. The amount of residual monomer in the raw material fine particles may be, for example, 3000 ppm or less, 2500 ppm or less, or 2300 ppm or less. On the other hand, the amount of residual monomer in the raw material particles may be the same as or different from the amount of residual monomer in the raw material fine particles.

Theoretically, the amount of residual monomer in the raw material fine particles can be increased by adding a monomer to the polymerized particles from the outside. However, in the production method according to the present embodiment, from the viewpoint of more reliably reducing the amount of fine powder generated, the amount of residual monomer in the raw material fine particles is not added from the outside but exists inside the particles. That is, it is preferably derived from the monomer used during the polymerization of the particles.

The amount of residual monomer in the raw material fine particles can be determined by measuring the amount of monomer dissolved in the solution after stirring for 1 hour in 250 times the amount of the particles to be measured in physiological saline. The measurement can be performed by high performance liquid chromatography. The monomer to be measured is the monomer used for the polymerization of the raw material fine particles. The amount of residual monomer in the raw material fine particles is preferably measured on the particles in the state immediately before being subjected to granulation, that is, the particles after all the steps performed before being subjected to aggregation.

The CRC (centrifuge holding capacity) of the raw material fine particles used for granulation may be, for example, 25 to 60 g / g, preferably 27 to 55 g / g. A specific method for measuring CRC will be shown in Examples described later.

<Water-absorbent resin particles>
The water-absorbent resin particles according to the present embodiment may be composed only of the crosslinked polymer obtained by granulation, and may be composed of, for example, a gel stabilizer, a metal chelating agent (ethylenediaminetetraacetic acid and a salt thereof, diethylenetriamine 5). Additional components such as acetic acid and salts thereof, such as diethylenetriamine-5 sodium acetate), fluidity improvers (lubricants), etc. can be further included. Additional components may be placed inside, on the surface, or both of the particles of the crosslinked polymer.

The water-absorbent resin particles may contain a plurality of inorganic particles arranged on the surface of the crosslinked polymer. For example, by mixing the crosslinked polymer and the inorganic particles, the inorganic particles can be arranged on the surface of the crosslinked polymer. The inorganic particles may be silica particles such as amorphous silica.

When the water-absorbent resin particles include inorganic particles arranged on the surface, the content of the inorganic particles may be in the following range based on 100 parts of the total mass of the crosslinked polymer. The content of the inorganic particles may be 0.05 parts by mass or more, 0.1 parts by mass or more, 0.15 parts by mass or more, or 0.2 parts by mass or more. The content of the inorganic particles may be 5.0 parts by mass or less, 3.0 parts by mass or less, 1.0 part by mass or less, 0.5 parts by mass or less, or 0.3 parts by mass or less.

Examples of the shape of the water-absorbent resin particles according to the present embodiment include a substantially spherical shape, a crushed shape, a granular shape, and a shape formed by aggregating primary particles having these shapes. The medium particle size of the water-absorbent resin particles according to the present embodiment may be 130 to 800 μm, 200 to 850 μm, 250 to 700 μm, 300 to 600 μm, or 300 to 450 μm. The water-absorbent resin particles according to the present embodiment may be finally adjusted in particle size distribution by performing an operation such as particle size adjustment using classification by a sieve.

The water-absorbent resin particles obtained by the production method according to the present embodiment include, for example, 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 dew condensation inhibitors. It can be used in such fields.

One aspect of this embodiment is to mix first polymer particles (raw material fine particles) having a particle size of less than 180 μm and second polymer particles (raw material particles) having a particle size of 180 μm or more to form agglomerates. The present invention relates to a method for suppressing the generation of fine particles of water-absorbent resin particles, which comprises obtaining, crushing the agglomerates, and adjusting the amount of residual monomers in the first polymer particles to 250 ppm or more. By adjusting the amount of residual monomer in the raw material fine particles used for granulation to 250 ppm or more, it is possible to suppress the generation of fine particles in the obtained particles. As described above, the method for adjusting the amount of residual monomer in the raw material fine particles to 250 ppm or more can be carried out, for example, by increasing the amount of the polymerization initiator used, raising the drying temperature, or the like.

Hereinafter, the content of the present invention will be described in more detail with reference to Examples and Comparative Examples, but the present invention is not limited to the following Examples.

Manufacturing example 1
[Preparation of aqueous monomer solution]
130.0 g (1.80 mol) of 100% acrylic acid was placed in a 2 L separable flask. After adding 112.3 g of ion-exchanged water while stirring the inside of the flask, 112.85 g of 48% by mass sodium hydroxide was added dropwise under an ice bath to obtain a monomer concentration of 45% by mass and a neutralization rate of 75 mol. % Sodium acrylate partially neutralized solution was prepared.

[polymerization]
A stirrer (diameter 8 mm, length 40 mm) is placed in a stainless steel bat (φ20 cm), and the sodium acrylate partially neutralized solution (monomer concentration 45% by mass, acrylate neutralization rate) is used as a monomer for polymerization. (75 mol%) 340.0 g, ion-exchanged water 58.5 g, and ethylene glycol diglycidyl ether 0.0541 g (0.311 mmol) as an internal cross-linking agent were added. Then, the stirrer was rotated to uniformly disperse the components to obtain a mixture (sodium acrylate partial neutralization solution concentration 38% by mass). Then, the upper part of the stainless steel vat was covered with a polyethylene film. After adjusting the temperature of the mixture in the stainless steel vat to 25 ° C., the mixture was replaced with nitrogen to reduce the amount of dissolved oxygen to 0.1 ppm or less.

Next, in the above stainless steel bat, 3.09 g (0.228 mmol) of a 2 mass% 2,2'-azobis (2-amidinopropane) dihydrochloride aqueous solution, 0.49 g of a 0.5 mass% L-ascorbic acid aqueous solution, And 0.53 g of 0.35 mass% hydrochloric acid aqueous solution were sequentially added dropwise under stirring at 300 rpm to prepare a monomer aqueous solution. Polymerization started immediately after the addition of 0.35% by mass hydrogen peroxide solution. Twelve minutes after the start of the polymerization, the obtained hydrogel polymer was immersed in a water bath at 75 ° C. in a stainless steel vat and aged for 20 minutes. The thickness of the obtained hydrogel polymer was 1.3 cm.

[Rough crushing of hydrogel polymer]
The entire amount of the hydrogel-like polymer after aging was taken out from the stainless steel vat, and cuts were made in a grid pattern at 5 cm intervals and cut. The entire amount of the hydrogel polymer cut at 5 cm intervals was sequentially charged into a meat chopper 12VR-750SDX manufactured by Kiren Royal Co., Ltd., and coarsely crushed. The diameter of the hole in the plate located at the outlet of the meat chopper was 6.4 mm. The coarse crushing was carried out until no coarsely crushed gel (hydrous gel coarse crushed material) came out from the plate of the meat chopper. At this time, the amount of the hydrogel-like polymer charged into the meat chopper was 348 g, and the amount of the hydrogel coarse product discharged from the meat chopper and recovered was 235 g.

[Drying hydrogel crushed material]
The hydrogel coarse crushed material was spread on a sieve having an opening of 850 μm and dried in a hot air dryer (manufactured by ADVANTEC, model number: DRE320DB) set at 180 ° C. for 30 minutes to obtain a gel-dried product.

[Crushing dried gel]
The above-mentioned gel dried product is crushed by an ultracentrifugation crusher (ZM200 manufactured by Verder Scientific Co., Ltd., 6-blade rotor, rotor rotation speed: 6000 rpm, screen trapezoidal hole: 1.00 mm) to crush polymer particles. I got something.

[Classification of crushed polymer particles]
The pulverized polymer particles were classified using a sieve having an opening of 850 μm, a sieve having a size of 180 μm, and a receiver. Those that passed through a sieve having a mesh size of 850 μm and remained on the sieve of 180 μm were recovered as polymer particles 1A, and those that passed through the sieve of 180 μm were recovered as polymer particles 1B.

Manufacturing example 2
[Preparation of aqueous monomer solution]
A partial neutralizing solution of sodium acrylate having a monomer concentration of 45% by mass and a neutralization rate of 75 mol% was prepared in the same manner as in Production Example 1.

[polymerization]
A stirrer (diameter 8 mm, length 40 mm) is placed in a stainless steel bat (outer dimensions: 210 mm x 170 mm, height 30 mm) whose inner surface is coated with fluororesin, and sodium acrylate is partially neutralized as a monomer used for polymerization. Liquid (monomer concentration 45% by mass, neutralization rate of acrylic acid 75 mol%) 340.0 g, ion-exchanged water 58.9 g, and ethylene glycol diglycidyl ether 0.0077 g (0.044 mmol) as an internal cross-linking agent. Was added. Then, the stirrer was rotated to uniformly disperse the components to obtain a mixture (sodium acrylate partial neutralization solution concentration 38% by mass). Then, the upper part of the stainless steel vat was covered with a polyethylene film. After adjusting the temperature of the mixture in the stainless steel vat to 25 ° C., the mixture was replaced with nitrogen to reduce the amount of dissolved oxygen to 0.1 ppm or less.

Then, 3.09 g (0.229 mmol) of a 2 mass% potassium persulfate aqueous solution and 0.65 g of a 0.5 mass% L-ascorbic acid aqueous solution were sequentially added dropwise to the stainless steel bat under stirring at 300 rpm. , A monomeric aqueous solution was prepared. Polymerization started immediately after adding 0.65 g of a 0.5 mass% L-ascorbic acid aqueous solution. Twenty-five minutes after the start of the polymerization, the obtained hydrogel polymer was immersed in a water bath at 75 ° C. while still in a stainless steel vat and aged for 20 minutes. The thickness of the obtained hydrogel polymer was 1.3 cm.

The hydrogel polymer was coarsely crushed in the same manner as in Production Example 1. The amount of the hydrogel-like polymer charged into the meat chopper was 347 g, and the amount of the hydrogel coarse crushed product discharged from the meat chopper and recovered was 188 g. The gel was dried, pulverized, and classified in the same manner as in Production Example 1, passed through a sieve having an opening of 850 μm, and what remained on the sieve with a mesh size of 180 μm was passed through a sieve of polymer particles 2A and 180 μm and was weighted. It was recovered as coalesced particles 2B.

Manufacturing example 3
[Preparation of aqueous monomer solution]
A partial neutralizing solution of sodium acrylate having a monomer concentration of 45% by mass and a neutralization rate of 75 mol% was prepared in the same manner as in Production Example 1.

[polymerization]
A stirrer (diameter 8 mm, length 40 mm) is placed in a stainless steel bat (φ20 cm), and a partial neutralizing solution of sodium acrylate (monomer concentration 45% by mass, neutralization rate of acrylic acid 75) is used as a monomer for polymerization. 340.0 g (molar%), 47.7 g of ion-exchanged water, and 0.0541 g (0.311 mmol) of ethylene glycol diglycidyl ether as an internal cross-linking agent were added. Then, the stirrer was rotated to uniformly disperse the components to obtain a mixture (sodium acrylate partial neutralization solution concentration 38% by mass). Then, the upper part of the stainless steel vat was covered with a polyethylene film. After adjusting the temperature of the mixture in the stainless steel vat to 25 ° C., the mixture was replaced with nitrogen to reduce the amount of dissolved oxygen to 0.1 ppm or less.

Then, 12.37 g (0.915 mmol) of a 2 mass% potassium persulfate aqueous solution and 2.54 g of a 0.5 mass% L-ascorbic acid aqueous solution were sequentially added dropwise to the stainless steel bat under stirring at 300 rpm. , Aqueous monomer solution was prepared. Polymerization started immediately after dropping the 0.5 mass% L-ascorbic acid aqueous solution. Twelve minutes after the start of the polymerization, the obtained hydrogel polymer was immersed in a water bath at 75 ° C. in a stainless steel vat and aged for 20 minutes. The thickness of the obtained hydrogel polymer was 1.3 cm.

[Rough crushing of hydrogel polymer]
The hydrogel polymer was coarsely crushed in the same manner as in Production Example 1. The amount of the hydrogel-like polymer charged into the meat chopper was 345 g, and the amount of the hydrogel coarse crushed product discharged from the meat chopper and recovered was 198 g.

The gel was dried, pulverized, and classified in the same manner as in Production Example 1, passed through a sieve having a mesh size of 850 μm, and what remained on the sieve having a mesh size of 180 μm was passed through a sieve having polymer particles 3A and 180 μm. It was recovered as polymer particles 3B.

Manufacturing example 4
[Preparation of aqueous monomer solution]
A partial neutralizing solution of sodium acrylate having a monomer concentration of 45% by mass and a neutralization rate of 75 mol% was prepared in the same manner as in Production Example 1.

[polymerization]
A stirrer (diameter 8 mm, length 40 mm) is placed in a stainless steel bat (φ20 cm), and a partial neutralizing solution of sodium acrylate (monomer concentration 45% by mass, neutralization rate of acrylic acid 75) is used as a monomer for polymerization. 340.0 g (molar%), 32.6 g of ion-exchanged water, and 0.0541 g (0.311 mmol) of ethylene glycol diglycidyl ether as an internal cross-linking agent were added. Then, the stirrer was rotated to uniformly disperse the components to obtain a mixture (sodium acrylate partial neutralization solution concentration 38% by mass). Then, the upper part of the stainless steel vat was covered with a polyethylene film. After adjusting the temperature of the mixture in the stainless steel vat to 25 ° C., the mixture was replaced with nitrogen to reduce the amount of dissolved oxygen to 0.1 ppm or less.

Then, 24.75 g (1.831 mmol) of a 2 mass% potassium persulfate aqueous solution and 5.20 g of a 0.5 mass% L-ascorbic acid aqueous solution were sequentially added dropwise to the stainless steel bat under stirring at 300 rpm. , A monomeric aqueous solution was prepared. Polymerization started immediately after dropping the 0.5 mass% L-ascorbic acid aqueous solution. After 10 minutes from the start of the polymerization, the obtained hydrogel polymer was immersed in a water bath at 75 ° C. in a stainless steel vat and aged for 20 minutes. The thickness of the obtained hydrogel polymer was 1.3 cm.

[Rough crushing of hydrogel polymer]
The hydrogel polymer was coarsely crushed in the same manner as in Production Example 1. The amount of the hydrogel-like polymer charged into the meat chopper was 352 g, and the amount of the hydrogel coarse crushed product discharged from the meat chopper and recovered was 196 g.

In the same manner as in Production Example 1, the gel was dried, pulverized and classified, passed through a sieve having an opening of 850 μm, and what remained on the sieve with a mesh size of 180 μm was passed through a sieve of polymer particles 4A and 180 μm and was weighted. It was recovered as coalesced particles 4B.

Production example 5
The steps up to pulverization of the dried gel product were carried out in the same manner as in Production Example 4 to obtain a pulverized polymer particle product.

[Surface cross-linking]
25.0 g of pulverized polymer particles was placed in a round-bottomed cylindrical separable flask having an inner diameter of 11 cm equipped with a fluororesin-made anchor-shaped stirring blade. Separately, 0.060 g of ethylene glycol diglycidyl ether, 3.6 g of water, 1.20 g of propylene glycol, and 1.20 g of isopropyl alcohol were mixed to obtain an aqueous cross-linking agent. The aqueous cross-linking agent was added to the flask with stirring at 500 rpm. The mixture obtained by adding was uniformly spread on a fluororesin-coated tray, and then heated at 180 ° C. for 40 minutes. After cooling to room temperature, a 850 μm mesh was passed through the mixture to obtain a surface-crosslinked polymer particle pulverized product.

20 g of the pulverized polymer particles after surface cross-linking were classified using a sieve having an opening of 850 μm, a sieve having a mesh size of 180 μm, and a receiver. Those that passed through a sieve having a mesh size of 850 μm and remained on the sieve of 180 μm were recovered as polymer particles 5A, and those that passed through the sieve of 180 μm were recovered as polymer particles 5B.

Production example 6
A round-bottomed cylindrical separable flask with an inner diameter of 11 cm and an internal volume of 2 L equipped with a reflux condenser, a dropping funnel, a nitrogen gas introduction pipe, and a stirring blade having four inclined paddle blades with a blade diameter of 5 cm in two stages as a stirrer. Prepared. To this flask, 293 g of n-heptane as a hydrocarbon dispersion medium was added, 0.736 g of a maleic anhydride-modified ethylene-propylene copolymer (Mitsui Chemicals Co., Ltd., High Wax 1105A) was added as a polymer-based dispersant, and the mixture was stirred. The dispersant was dissolved by raising the temperature to 80 ° C. The formed solution was cooled to 50 ° C.

On the other hand, 92.0 g (1.03 mol) of an 80.5 mass% acrylic acid aqueous solution as a water-soluble ethylenically unsaturated monomer was placed in a beaker having an internal volume of 300 mL, and cooled from the outside by 20.9 mass. 147.7 g of a% aqueous sodium hydroxide solution was added dropwise to neutralize 75 mol%. After that, 0.092 g of hydroxylethyl cellulose (Sumitomo Seika Co., Ltd., HEC AW-15F) as a thickener, 0.0736 g (0.272 mmol) of potassium persulfate as a water-soluble radical polymerization agent, and ethylene glycol as an internal cross-linking agent. An aqueous solution of the first stage was prepared by adding 0.011 g (0.063 mmol) of diglycidyl ether to a beaker and dissolving it.

A surfactant solution is prepared by heating and dissolving 0.736 g of sucrose stearic acid ester (Mitsubishi Chemical Foods Co., Ltd., Ryoto Sugar Ester S-370, HLB: 3) as a surfactant in 6.62 g of n-heptane. Obtained. After adding the first-stage aqueous solution to the flask and stirring for 10 minutes, the surfactant solution is added, the rotation speed of the stirrer is increased to 550 rpm, and the inside of the system is sufficiently nitrogen while stirring. Replaced with. Then, the flask was immersed in a water bath at 70 ° C. to raise the temperature, and polymerization was carried out for 60 minutes to obtain a first-stage polymerization slurry solution.

In another beaker with an internal volume of 500 mL, 128.8 g (1.44 mol) of an 80.5 mass% acrylic acid aqueous solution as a water-soluble ethylenically unsaturated monomer was placed, and 27 mass% of water was added while cooling from the outside. 159.0 g of an aqueous sodium oxide solution was added dropwise to neutralize 75 mol%. Then, 0.090 g (0.333 mmol) of potassium persulfate as a water-soluble radical polymerization initiator and 0.013 g (0.075 mmol) of ethylene glycol diglycidyl ether as an internal cross-linking agent are added to the beaker to dissolve them. , The second stage aqueous solution was prepared.

After cooling the inside of the flask system to 25 ° C. while stirring at a stirring speed of 1000 rpm, the entire amount of the aqueous solution of the second stage is added to the polymerized slurry liquid of the first stage to add the inside of the system. It was replaced with nitrogen for 30 minutes. Then, the flask was immersed in a water bath at 70 ° C. again to raise the temperature, and the polymerization reaction was carried out for 60 minutes to obtain a slurry containing a hydrogel polymer.

After that, the flask was immersed in an oil bath set at 125 ° C., and 176.6 g of water was extracted from the system while refluxing n-heptane by azeotropic distillation of n-heptane and water. Then, 4.42 g (1.05 mmol) of a 3 mass% sodium sulfite aqueous solution and 5.89 g of a 4.5 mass% diethylenetriamine 5-sodium acetate aqueous solution were added to the flask as an inorganic reducing agent under stirring. A slurry containing polymer particles was obtained.

Then, the flask was immersed again in an oil bath set at 125 ° C., and 67.2 g (243.8 g in total) of water was added to the system while refluxing n-heptane by azeotropic distillation of n-heptane and water. I pulled it out. Then, n-heptane and water were evaporated at 125 ° C. and dried to obtain polymer particles.

[Classification of dried products]
A pulverized polymer particle was obtained by pulverizing 30 g of the polymer particles with a small pulverizer (Wonder Blender WB-1) for 10 seconds. The pulverized polymer particles were classified using a sieve having a mesh size of 850 μm, a sieve having a mesh size of 180 μm, and a receiver. Those that passed through a sieve having a mesh size of 850 μm and remained on the sieve of 180 μm were recovered as polymer particles 6A, and those that passed through the sieve of 180 μm were recovered as polymer particles 6B.

Production example 7
[Preparation of aqueous monomer solution]
340.0 g (4.72 mol) of 100% acrylic acid was placed in a separable flask having an internal volume of 2 L. After adding 293.6 g of ion-exchanged water while stirring the inside of the flask, 295.1 g of 48% by mass sodium hydroxide was added dropwise under an ice bath to obtain a monomer concentration of 45% by mass and a neutralization rate of 75 mol. % Sodium acrylate partially neutralized solution was prepared.

[polymerization]
Acrylic acid as a monomer used for polymerization in a fluororesin-coated 18-8 stainless steel container (outer dimensions: 297 mm x 232 mm x height 50 mm) equipped with two stirrers (diameter 8 mm, length 45 mm). 980.9 g of sodium partial neutralization solution (monogenic concentration 45% by mass, neutralization rate of acrylic acid 75 mol%), 94.5 g of ion-exchanged water, and 0.141 g (0) of ethylene glycol diglycidyl ether as an internal cross-linking agent. .809 mmol) was added. Then, the stirrer was rotated to uniformly disperse the components to obtain a mixture (sodium acrylate partial neutralization solution concentration 38% by mass). Then, the upper part of the stainless steel vat was covered with a polyethylene film. After adjusting the temperature of the mixture in the stainless steel vat to 25 ° C., the mixture was replaced with nitrogen to reduce the amount of dissolved oxygen to 0.1 ppm or less.

Then, 40.44 g (2.992 mmol) of a 2 mass% potassium persulfate aqueous solution and 8.68 g of a 0.5 mass% L-ascorbic acid aqueous solution were sequentially added dropwise to the stainless steel bat under stirring at 300 rpm. , A monomeric aqueous solution was prepared. Polymerization started immediately after dropping the 0.5 mass% L-ascorbic acid aqueous solution. After 9 minutes from the start of the polymerization, the obtained hydrogel polymer was immersed in a water bath at 75 ° C. in a container and aged for 20 minutes. The thickness of the obtained hydrogel polymer was 1.7 cm.

The hydrogel polymer was coarsely crushed in the same manner as in Production Example 1. The amount of the hydrogel-like polymer charged into the meat chopper was 792 g, and the amount of the hydrogel coarse crushed product discharged from the meat chopper and recovered was 690 g.

[Drying hydrogel crushed material]
The hydrogel coarse crushed material was spread on a sieve having an opening of 850 μm and dried in a hot air dryer (manufactured by ADVANTEC, model number: DRE320DB) set at 105 ° C. for 3 hours to obtain a gel-dried product.

In the same manner as in Production Example 1, the dried gel product was pulverized and classified, passed through a sieve having a mesh size of 850 μm, and the residue remaining on the sieve having a mesh size of 180 μm was passed through a sieve having polymer particles 7A and 180 μm. It was recovered as coalesced particles 7B.

Production Example 8
A slurry containing polymer particles was obtained in the same manner as in Production Example 6.

Then, the flask containing the slurry was immersed again in an oil bath set at 125 ° C., and 89.1 g (total 265.7 g) was further immersed while refluxing n-heptane by azeotropic distillation of n-heptane and water. ) Water was extracted from the system. Then, 4.42 g (0.507 mmol) of a 2% by mass ethylene glycol diglycidyl ether aqueous solution was added to the flask as a surface cross-linking agent, and the flask was kept at 83 ° C. for 2 hours. Then, n-heptane and water were evaporated at 125 ° C. and dried to obtain polymer particles.

30 g of the polymer particles were pulverized with a small crusher (Wonder Blender WB-1) for 10 seconds to obtain a pulverized polymer particle. The pulverized polymer particles were classified using a sieve having a mesh size of 850 μm, a sieve having a mesh size of 180 μm, and a receiver. Those that passed through a sieve having a mesh size of 850 μm and remained on the sieve of 180 μm were recovered as polymer particles 8A, and those that passed through the sieve of 180 μm were recovered as polymer particles 8B.

<Example 1>
3.75 g of polymer particles 1B were added as raw material fine particles to a 100 ml new disposable cup (made by AS ONE Corporation, CODE1-4620-01). While stirring the raw material fine particles at 600 r / min using a magnetic stirrer bar (8 mmφ x 45 mm without ring), use a macropipettor (manufactured by Shibata Scientific Technology Co., Ltd., 10 mL capacity) to remove 5.63 g of ion-exchanged water. The mixture was added dropwise at a rate of 1 g / sec for 1 minute.

After that, 8.75 g of polymer particles 1A were put into the cup as raw material particles, and the mixture was further stirred at 600 r / min for 1 minute to obtain agglomerates. Dry the agglomerates by transferring the entire amount of the agglomerates to a glass petri dish (inner diameter 5.8 cm) using a spatula and drying with a hot air dryer (manufactured by ADVANTEC, model number: DRE320DB) set at 80 ° C. for 150 minutes. I got a body.

Collect the agglutinated agglomerates that do not exceed φ3 cm, and use an ultracentrifuge crusher (manufactured by Verder Scientific Co., Ltd., ZM200, 6-flute rotor, rotor speed: 6000 rpm, screen trapezoidal hole: 1) It was put into 0.00 mm) and crushed. The particles obtained by pulverizing the dried agglomerates were classified using a sieve having an opening of 850 μm, a sieve having a mesh size of 180 μm, and a receiver. The classification results are shown in Table 2.

<Examples 2 to 8 and Comparative Examples 1 to 4>
The same operation as in Example 1 was carried out using the raw material fine particles and the polymer particles shown in Table 1 as the raw material particles.

<Particle particle size distribution>
The particle size distribution of the particles was measured in an environment of room temperature (25 ± 2 ° C.) and humidity of 50 ± 10% according to the following procedure. That is, the JIS standard sieves were 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. The particles to be measured were placed in the best combined sieve and classified according to JIS Z 8815 (1994) using a low-tap shaker (manufactured by Iida Seisakusho Co., Ltd.).

<Residual monomer content of water-absorbent resin particles>
The residual monomer content of the water-absorbent resin particles was measured by the following procedure in an environment of room temperature (25 ± 2 ° C.) and humidity of 50 ± 10%. 500 g of physiological saline was placed in a beaker having an internal volume of 500 mL, and the mixture was stirred at 600 rpm using a stirrer (stirrer stand: model M-16GM manufactured by Koike Precision Instruments Mfg. Co., Ltd., stirrer tip: diameter 0.7 cm, length 3 cm). 2.0 g of particles to be measured was added to the stirred physiological saline, and the mixture was stirred for 60 minutes. The contents of the beaker were filtered through a JIS standard sieve having an opening of 75 μm and a filter paper (filter paper No. 5C manufactured by ADVANTEC), and separated into a water-absorbing gel and a filtrate (extract). The content of the monomer dissolved in the obtained filtrate was measured by high performance liquid chromatography. The residual monomer to be measured is acrylic acid and its alkali metal salt. The measured value was converted into a value per measured particle mass and used as the residual monomer content (ppm).
[High Performance Liquid Chromatography]
Product name: Japan Waters Corp., ACQUITY (registered trademark) UPLC
Column: UPLC® HSST-3 (1.8 μm, 2.1 mm × 50 mm)
Detector: Photodiode array (PDA) Detector carrier: 0.1% aqueous phosphoric acid solution

<CRC measurement>
With reference to the EDANA method (NWSP 241.0.R2 (15), page.769-778), the CRC of the raw material fine particles was measured in an environment of room temperature (25 ± 2 ° C) and humidity of 50 ± 10% by the following procedure. did.

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 above-mentioned raw material fine particles was 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 non-woven fabric bag was put into the physiological saline solution, the non-woven fabric bag was immersed in the physiological saline solution with a spatula to obtain a non-woven 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, it was weighed mass M _a nonwoven bag containing the mass of gel. Subjected to the same operation as the aforementioned operation on the woven bags without containing ingredients microparticles was measured mass M _b of the nonwoven fabric bag. The CRC was calculated based on the following formula. M _c is the mass of raw material particles used for the measurement is 0.2 g. The results are shown in Table 1.
CRC [g / g] = {(M _a- M _b ) -M _c } / M _c

In the water-absorbent resin particles obtained in the examples, the amount of fine powder having a particle diameter of less than 180 μm was suppressed after re-pulverization.

Claims

The present invention comprises mixing the first polymer particles having a particle size of less than 180 μm and the second polymer particles having a particle size of 180 μm or more to obtain agglomerates, and crushing the agglomerates. A method for producing water-absorbent resin particles, wherein the amount of residual monomers in the first polymer particles is 250 ppm or more.
The method according to claim 1, wherein the first polymer particles and the second polymer particles contain a crosslinked polymer of acrylic acid or a salt thereof.
The method according to claim 1 or 2, wherein the second polymer particles have a particle size of 180 to 850 μm.
The method according to any one of claims 1 to 3, further comprising mixing an aqueous liquid in addition to the first polymer particles and the second polymer particles in the step of obtaining agglomerates.
The first polymer particles having a particle size of less than 180 μm and the second polymer particles having a particle size of 180 μm or more are mixed to obtain an agglomerate, the agglomerates are crushed, and the first A method for suppressing the generation of fine powder of water-absorbent resin particles, which comprises adjusting the amount of residual monomer in the polymer particles to 250 ppm or more.